A comprehensive review of measurements and data analysis of laminar burning velocities for various fuel+air mixtures Prog. Energy Combust. Sci. (IF 17.382) Pub Date : 2018-06-20 Alexander A. Konnov, Akram Mohammad, Velamati Ratna Kishore, Nam Il Kim, Chockalingam Prathap, Sudarshan Kumar
Accurate measurement and prediction of laminar burning velocity is important for characterization of premixed combustion properties of a fuel, development and validation of new kinetic models, and calibration of turbulent combustion models. Understanding the variation of laminar burning velocity with thermodynamic conditions is important from the perspective of practical applications in industrial furnaces, gas turbine combustors and rocket engines as operating temperatures and pressures are significantly higher than ambient conditions. With this perspective, a brief review of spherical flame propagation method, counterflow/stagnation burner method, heat-flux method, annular stepwise method, externally heated diverging channel method, and Bunsen method is presented. A direct comparison of power exponents for temperature (α) and pressure (β) obtained from different experiments and derived from various kinetic mechanisms is reported to provide an independent tool for detailed validation of kinetic schemes. Accurate prediction of laminar burning velocities at higher temperatures and pressures for individual fuels will help in closer scrutiny of the existing experimental data for various uncertainties due to inherent challenges in individual measurement techniques. Laminar burning velocity data for hydrogen (H2), gaseous alkane fuels (methane, ethane, propane, n-butane, n-pentane), liquid alkane fuels (n-heptane, isooctane, n-decane), alcohols (CH3OH, C2H5OH, n-propanol, n-butanol, n-pentanol) and di-methyl ether (DME) are obtained from literature of last three decades for a wide range of pressures (1–10 bar), temperatures (300–700 K), equivalence ratios and mixture dilutions. The available experimental and numerical data for H2 and methane fuels compares well for various pressures and temperatures. However, more experimental and kinetic model development studies are required for other fuels. Comparison of laminar burning velocity data obtained from different measurement techniques at higher initial pressures and temperatures showed significant deviations for all fuels. This suggests to conduct focused measurements at elevated pressure and temperature conditions for different fuels to enable the development of accurate kinetic models for wider range of mixtures and thermodynamic conditions.
Recyclable metal fuels for clean and compact zero-carbon power Prog. Energy Combust. Sci. (IF 17.382) Pub Date : 2018-06-18 Jeffrey M. Bergthorson
Metal fuels, as recyclable carriers of clean energy, are promising alternatives to fossil fuels in a future low-carbon economy. Fossil fuels are a convenient and widely-available source of stored solar energy that have enabled our modern society; however, fossil-fuel production cannot perpetually keep up with increasing energy demand, while carbon dioxide emissions from fossil-fuel combustion cause climate change. Low-carbon energy carriers, with high energy density, are needed to replace the multiple indispensable roles of fossil fuels, including for electrical and thermal power generation, for powering transportation fleets, and for global energy trade. Metals have high energy densities and metals are, therefore, fuels within many batteries, energetic materials, and propellants. Metal fuels can be burned with air or reacted with water to release their chemical energy at a range of power-generation scales. The metal-oxide combustion products are solids that can be captured and then be recycled using zero-carbon electrolysis processes powered by clean energy, enabling metals to be used as recyclable zero-carbon solar fuels or electrofuels. A key technological barrier to the increased use of metal fuels is the current lack of clean and efficient combustor/reactor/engine technologies to convert the chemical energy in metal fuels into motive or electrical power (energy). This paper overviews the concept of low-carbon metal fuels and summarizes the current state of our knowledge regarding the reaction of metal fuels with water, to produce hot hydrogen on demand, and the combustion of metal fuels with air in laminar and turbulent flames. Many important questions regarding metal-fuel combustion processes remain unanswered, as do questions concerning the energy-cycle efficiency and life-cycle environmental impacts and economics of metals as recyclable fuels. Metal fuels can be an important technology option within a future low-carbon society and deserve focused attention to address these open questions.
Foundational techniques for catalyst design in the upgrading of biomass-derived multifunctional molecules Prog. Energy Combust. Sci. (IF 17.382) Pub Date : 2018-02-20 Brian M. Murphy, Bingjun Xu
Modeling nitrogen chemistry in combustion Prog. Energy Combust. Sci. (IF 17.382) Pub Date : 2018-02-22 Peter Glarborg, James A. Miller, Branko Ruscic, Stephen J. Klippenstein
Understanding of the chemical processes that govern formation and destruction of nitrogen oxides (NOx) in combustion processes continues to be a challenge. Even though this area has been the subject of extensive research over the last four decades, there are still unresolved issues that may limit the accuracy of engineering calculations and thereby the potential of primary measures for NOx control. In this review our current understanding of the mechanisms that are responsible for combustion-generated nitrogen-containing air pollutants is discussed. The thermochemistry of the relevant nitrogen compounds is updated, using the Active Thermochemical Tables (ATcT) approach. Rate parameters for the key gas-phase reactions of the nitrogen species are surveyed, based on available information from experiments and high-level theory. The mechanisms for thermal and prompt-NO, for fuel-NO, and NO formation via NNH or N2O are discussed, along with the chemistry of NO removal processes such as reburning and Selective Non-Catalytic Reduction of NO. Each subset of the mechanism is evaluated against experimental data and the accuracy of modeling predictions is discussed.
Advanced heat transfer fluids for direct molten salt line-focusing CSP plants Prog. Energy Combust. Sci. (IF 17.382) Pub Date : 2018-03-02 Alexander Bonk, Salvatore Sau, Nerea Uranga, Marta Hernaiz, Thomas Bauer
Concentrating solar power coupled to thermal energy storage (TES) is a vastly growing industrial process allowing for the generation of dispatchable and green electricity. This paper focuses on direct molten salt line-focusing technology using linear Fresnel and parabolic trough collector systems. Direct molten salt technology utilizes molten salt as heat transfer fluid in solar field and TES medium. Nitrate salts can be applied since they cover a wide temperature range. As storage medium Solar Salt, a binary NaNO3−KNO3 (60-40 wt%) mixture, is most commonly used but variations of this system have promising thermal properties in terms of a lower melting temperature to minimize the risk of undesired salt freezing events. These modified salts are typically ternary, ternary reciprocal or higher order systems formed by adding additional cations, anions or both. In this study five molten salt systems Solar Salt, HitecXL (CaKNa//NO3), LiNaK-Nitrate, Hitec (NaK//NO23) and CaLiNaK//NO23 are both investigated and critically reviewed. Their thermo-physical properties including phase diagrams, composition, melting ranges, melting temperature, minimum operation temperature, thermal stability, maximum operation temperature, density, heat capacity, thermal conductivity, viscosity and handling are evaluated and the most recommended values are discussed and highlighted. This review contributes to a better understanding of how the listed properties can be determined in terms of measurement conditions and provides temperature dependent data useful for future simulations of direct molten salt LF CSP plants.
Fuel reforming in internal combustion engines Prog. Energy Combust. Sci. (IF 17.382) Pub Date : 2018-03-16 L. Tartakovsky, M. Sheintuch
This article offers a comprehensive overview of research on fuel reforming in internal combustion engines (ICE). It includes a historical perspective of research in this field, a discussion on the considerations to be made prior to choosing a primary fuel for reforming purposes, and the main processes in fuel reforming. Steam reforming offers a moderate degree of thermochemical recuperation and is applicable to methanol and ethanol feeding. Reforming with air reduces the degree of recuperation, but opens up the use of heavier fuels such as gasoline and diesel fuel. Dry reforming (with CO2) offers the best recuperation but is prone to fast coking. The choice of catalyst and the expected side reactions for each fuel are also discussed. While there is extensive literature on steam reforming catalysts and kinetics at atmospheric pressure, studies at higher pressures and/or on decomposition reactions are very few. The thermodynamics of fuel reforming in ICE and simulation approaches are also discussed. The paper also reports on engineering aspects of fuel reformer design and provides an overview of engines with thermo-chemical recuperation (TCR), fuel supply, and load control strategies in ICE with TCR. In-cylinder fuel reforming as well as application of fuel reforming for performance improvement of emission aftertreatment systems are subsequently discussed. This overview reveals ongoing diverse research activities in the field of onboard fuel reforming. However, several problems, including reformate burning velocity at typical for ICE conditions, in-cylinder behavior of directly injected reformates and particle formation still need to be addressed. A discussion on some of these unresolved issues is attempted herein.
Recent advances in the synthesis and modification of carbon-based 2D materials for application in energy conversion and storage Prog. Energy Combust. Sci. (IF 17.382) Pub Date : 2018-03-16 Rajesh Kumar, Ednan Joanni, Rajesh K. Singh, Dinesh P. Singh, Stanislav A. Moshkalev
Graphene, reduced graphene oxide (rGO) and derived materials have emerged as promising solutions for applications in renewable energy storage/conversion devices. No alternatives are known to simultaneously exhibit large specific surface area, high electrical conductivity, good chemical stability, high mechanical strength and flexibility. This review article is a collection of some of the most relevant research efforts published in the last few years focusing on the synthesis and modification of graphene/rGO as well as doped and hybrid bi-dimensional carbon materials. For research on graphene growth, the choice of precursor and physical state (gas, solid or liquid) has been proved to be as important as the environment and synthesis approach. On the other hand, research focused on graphene oxide reduction has relied on the development of simple and efficient techniques for rGO conversion and device structuring. Modifications applied to graphene (during synthesis) or rGO (during reduction) have included doping, decoration with nanoparticles and the formation of composite microstructures. Fabrication of electrodes based on graphene/rGO for application in energy storage and conversion has been reported, including relevant performance data from real devices (supercapacitors, lithium ion batteries, fuel cells or solar cells). This review concludes with a brief discussion of some of the possible directions for promising research in the area of graphene/rGO fabrication for energy conversion and storage devices.
Underground coal gasification – Part I: Field demonstrations and process performance Prog. Energy Combust. Sci. (IF 17.382) Pub Date : 2018-03-16 Greg Perkins
Underground coal gasification can convert deep coal resources into synthesis gas for use in the production of electricity, fuels and chemicals. This paper provides a review of the various methods of undertaking underground coal gasification and observations from demonstrations of the process in the field. A general representation of the underground process is presented, along with an identification of the various zones and associated governing phenomena. The main factors affecting the performance of underground coal gasification, such as coal rank, depth and thickness and oxidant composition and injection rate are examined in detail. A brief assessment of the economic and environmental considerations relevant to underground coal gasification projects is presented. Finally, guidelines for site and oxidant selection are provided based on the learnings from prior demonstration projects.
Two-stage electrostatic precipitators for the reduction of PM2.5 particle emission Prog. Energy Combust. Sci. (IF 17.382) Pub Date : 2018-03-27 A. Jaworek, A. Marchewicz, A.T. Sobczyk, A. Krupa, T. Czech
Electrostatic precipitator is the most efficient device used for the removal of fly ash particles from the flue gases produced by coal-fired boilers in power plants. However, the fractional collection efficiency of electrostatic precipitators steeply decreases for particles smaller than 1 μm, and particles in the size range of 200–500 nm are removed with lower collection efficiency than those outside this range. These particles are dangerous to humans and have detrimental environmental effects, so there is a need for novel more efficient technologies for their control. One of the answers to this challenge is the two-stage electrostatic precipitator, in which the electrostatic charging and precipitation processes have been separated. The PM2.5 particles (of a size < 2.5 μm) are electrically charged in a separate device (precharger) to a maximal possible electric charge, and then precipitated in a parallel plate collector, free of corona discharge. The electric field in the collection stage can be higher than in an electrostatic precipitator due to the lack of sharp discharge points. A higher electric field allows an increase of the collection efficiency for PM2.5 particles. Another solution is the agglomeration of submicron particles to form larger particles before their precipitation by a parallel-plate collector, conventional electrostatic precipitator or any other gas cleaning device. In some of the reviewed devices, both of these processes were combined in a single device that allowed further increases in the collection efficiency for submicron particles. Devices of this type have been tested in a lab- or semi-industrial scale for the removal of PM2.5 particles from flue gases or diesel engine exhausts. In this paper, various constructions of two-stage electrostatic precipitators, comprising a precharger and/or agglomerator in the first stage, and an electrostatic collector in the second stage, have been reviewed. Some of these devices were able to increase the mass collection efficiency above 95% for PM2.5 particles.
Underground coal gasification – Part II: Fundamental phenomena and modeling Prog. Energy Combust. Sci. (IF 17.382) Pub Date : 2018-03-27 Greg Perkins
Underground coal gasification can convert deep coal into synthesis gas for use in the production of electricity, fuels and chemicals. This paper provides a review of the fundamental physical phenomena in underground coal gasification and associated modeling efforts. The relevant fundamentals of coal gasification are described and the phenomena of cavity growth at the sidewall and roof of the underground cavity are examined in detail. The transport phenomena and chemical reactions occurring in the permeable bed of char and ash and the void space are reviewed. The modeling of the transport of heat and mass, including contaminants, in the near- and far-fields surrounding an underground coal gasifier are also summarized. An overview of the geomechanical phenomena and the coupled interactions between transport and mechanical phenomena are provided. Finally, integrated UCG models are reviewed and recommendations for future model development are provided.
Thermally stable polymers for advanced high-performance gas separation membranes Prog. Energy Combust. Sci. (IF 17.382) Pub Date : 2017-12-08 Mashallah Rezakazemi, Mohtada Sadrzadeh, Takeshi Matsuura
Polymeric membranes can be used for the energy-efficient and low-cost gas separation. However, their inability to resist high temperatures limits their use in certain industries. Some polymer membranes can be operated at some levels of the unpleasant environments, but the energy- and cost-efficiencies are offset by the necessity to severely cool hot streams. In many cases, such implementation is impossible or altogether impractical. Therefore, numerous studies have been focused on modifying polymers to create synthetic polymeric membranes which survive at high temperatures. Polymer scientists introduced many thermally stable polymers mostly based on carbocyclic and heterocyclic aromatic polymers, which exhibit enhanced thermal stability. However, the major problem with these polymers is their low processability, which is mainly due to their insolubility or high phase transition temperatures. So far, there has been little success in processing the high-temperature-resistant polymers to make membranes with acceptable separation and performance characteristics. The aim of this study is to review efforts which have been made to produce high-performance polymeric membranes with a focus on the preparation procedure; whereby, the main limitations and challenges to be faced were explored. The key factors discussed include the type of polymer, membrane preparation method, thermal analysis results and application of the prepared membranes. The primary purpose of this review is to lay out the basics for selecting polymer, solvent, additives and the appropriate preparation method to produce thermally stable polymeric membranes for gas separation. The future direction of research and development to fully exploit the potential usage of thermally stable polymeric membranes to achieve commercially viable processes was also shown.
A review of cavity-based trapped vortex, ultra-compact, high-g, inter-turbine combustors Prog. Energy Combust. Sci. (IF 17.382) Pub Date : 2017-12-16 Dan Zhao, Ephraim Gutmark, Philip de Goey
Trapped vortex combustor (TVC) is different from conventional swirl-stabilized combustors. It takes advantages of a cavity to stabilize the flame. When the cavity size of a TVC is well designed, a large rotating vortex can be formed in the cavity. The vortex cannot shed out the cavity and is thus named a “locked” or “stable” vortex. One of the main challenges for TVC design is fuel injection. Typically, fuel can be injected directly into the cavity or from the diffuser upstream. Injecting from the diffuser leads to the fuel being mixed with the air before it enters the combustor. When the fuel is injected directly into the cavity, it is desirable to supply the fuel in such way that the locked vortex in the cavity is reinforced. Furthermore, the fuel-air mixing in the cavity will be promoted, as the bypass air is directly added into the cavity. Since the recirculation zone anchored in the cavity is not exposed to the main incoming flow, stable combustion is achieved, even in the presence of a high speed main flow as typically expected in Ramjets and Scramjets. A well-designed trapped vortex combustor (TVC) enables a better fuel-air mixing, a better stabilized flame, lower emission, ultra-compact and high efficient combustion to be achievable. As a promising combustion concept, intensive scientific research has been conducted on TVC in the application areas of aerospace propulsion, power generation and waste incineration. In this work, we will firstly introduce the fundamental concepts, the development and evolution history of TVCs. The combustion, aerodynamics, and aeroacoustics features of trapped vortex combustion are then described. This includes reviewing and discussing the cavity flow/aerodynamics, fuel-air injection and mixing, trapped vortex combustion, emission and combustion of alternative fuels, and aeroacoustics characteristics. The ‘spin-off’ application of trapped vortex combustion concept for the design of ultra-compact and high-g combustors, inter-turbine burners, in-Situ and flameless TVC reheat combustors are then reviewed and discussed. Various practical applications of trapped vortex combustion concept in gas turbines, ramjets, scramjets and waste incinerators are discussed and summarized. Finally, the challenges and future directions of the design and implementation of TVCs are provided.
Macroscopic modeling of solid oxide fuel cell (SOFC) and model-based control of SOFC and gas turbine hybrid system Prog. Energy Combust. Sci. (IF 17.382) Pub Date : 2017-12-19 Cheng Bao, Ying Wang, Daili Feng, Zeyi Jiang, Xinxin Zhang
In addition to high energy conversion efficiency and considerable fuel flexibility, the advantage of high-quality exhaust energy of solid oxide fuel cells (SOFC) allows it to be combined with gas turbines (GT) to form a SOFC/GT hybrid generation system. This article makes a comprehensive review on the state of the art of macroscopic SOFC models and model-based control of the SOFC/GT hybrid system. Several topics in modeling of electrochemistry and transport in SOFC are first presented, including multi-component mass transfer, internal reforming, detailed radiative heat transfer, surface diffusion, and our new viewpoints on H2/CO electrochemical co-oxidation and physical resolution of electrochemical impedance spectra, etc. Then the SOFC models are summarized in different dimensions and from steady-state to transient. After a basic summary of the balancing unit models, the issues of system layout, safe operation region, off-design and part-load operation, load following, control strategies, decentralized/centralized controllers, and hardware-in-the-loop simulation of the SOFC/GT hybrid system are investigated. Some useful data have been sorted out in tabular form for easy access. The analytical solutions can play an important role in bridging the gap between mechanistic and system-level models. Some perspectives on the advanced models and model-based control are finally presented.
Advances and challenges in alkaline anion exchange membrane fuel cells Prog. Energy Combust. Sci. (IF 17.382) Pub Date : 2018-02-03 Z.F. Pan, L. An, T.S. Zhao, Z.K. Tang
The last several decades have witnessed the rapid development of alkaline anion exchange membrane fuel cells (AAEMFCs) that possess a series of advantages as compared to acid proton exchange membrane fuel cells, such as the enhanced electrochemical kinetics of oxygen reduction reaction and the use of inexpensive non-platinum electrocatalysts, both of which are rendered by the alkaline medium. As an emerging power generation technology, the significant progress has been made in developing the alkaline anion exchange membrane fuel cells in recent years. This review article starts with a general description of the setup of AAEMFCs running on hydrogen and physical and chemical processes occurring in multi-layered porous structure. Then, the electrocatalytic materials and mechanisms for both hydrogen oxidation and oxygen reduction are introduced, including metal-based, metal oxide-based, and non-metal based electrocatalysts. In addition, the chemistries of alkaline anion exchange membranes (AAEMs), e.g. polymer backbone and function groups, are reviewed. The effects of pre-treatment, carbonate, and radiation on the performance of AAEMs are concluded as well. The effects of anode and cathode ionomers, structural designs, and water flooding on the performance of the single-cell are explained, and the durability and power output of a single-cell are summarized. Afterwards, two innovative system designs that are hybrid fuel cells and regenerative fuel cells are presented and mathematical modeling on mass transport phenomenon in AAEMFCs are highlighted. Finally, the challenges and perspectives for the future development of the AAEMFCs are discussed.
The combustion mitigation of methane as a non-CO2 greenhouse gas Prog. Energy Combust. Sci. (IF 17.382) Pub Date : 2016-08-12 X. Jiang, D. Mira, D.L. Cluff
Anthropogenic emissions of non-CO2 greenhouse gases such as fugitive methane contribute significantly to global warming. A review of fugitive methane combustion mitigation and utilisation technologies, which are primarily aimed at methane emissions from coal mining activities, with a focus on modelling and simulation of ultra-lean methane oxidation/combustion is presented. The challenges associated with ultra-lean methane oxidation are on the ignition of the ultra-lean mixture and sustainability of the combustion process. There is a lack of fundamental studies on chemical kinetics of ultra-lean methane combustion and reliable kinetic schemes that can be used together with computational fluid dynamics studies to design and develop advanced mitigation systems. Mitigation of methane as a greenhouse gas calls for more efforts on understanding ultra-lean combustion. Recuperative combustion provides a promising means for mitigating ultra-lean methane emissions. Progress is needed on effective methods to ignite and to recuperate and retain heat for oxidation/combustion of the ultra-lean mixtures. Catalysts can be very effective in reducing the temperatures required for oxidation while plasmas may be utilised to assist the ignition, but thermodynamic/aerodynamic limits of burning ultra-lean methane remain unexplored. Further technological developments may be focussed on developing innovative capturing technology as well as technological innovations to achieve effective ignition and sustainable oxidation/combustion.
Chemical looping combustion of solid fuels Prog. Energy Combust. Sci. (IF 17.382) Pub Date : 2017-12-06 J. Adánez, A. Abad, T. Mendiara, P. Gayán, L.F. de Diego, F. García-Labiano
Chemical Looping Combustion (CLC) has arisen during last years as a very promising combustion technology for power plants and industrial applications, with inherent CO2 capture which reduces the energy penalty imposed on other competing technologies. The use of solid fuels in CLC has been highly developed in the last decade and currently stands at a technical readiness level (TRL) of 6. In this paper, experience gained during CLC operation in continuous units is reviewed and appraised, focusing mainly on technical and environmental issues relating to the use of solid fuels. Up to now, more than 2700 h of operational experience has been reported in 19 pilot plants ranging from 0.5 kWth to 4 MWth. When designing a CLC unit of solid fuels, the preferred configuration for the scale-up is a two circulating fluidized beds (CFB) system. Coal has been the most commonly used solid fuel in CLC, but biomass has recently emerged as a very promising option to achieve negative emissions using bioenergy with carbon dioxide capture and storage (BECCS). Mostly low cost iron and manganese materials have been used as oxygen carriers in the so called in-situ gasification CLC (iG-CLC). The development of Chemical Looping with Oxygen Uncoupling (CLOU) makes a qualitative step forward in the solid fuel combustion, due to the use of materials able to release oxygen. The performance and environmental issues of CLC of solid fuels is evaluated here. Regarding environmental aspects, the pollutant emissions (SO2, NOx, etc.) released into the atmosphere from the air reactor are no cause of concern for the environment. However, the presence of SO2, NOx and Hg at the exit of the fuel reactor affects CO2 quality, which must be taken into account during the later compression and purification stages. The effect of the main variables affecting CLC performance is evaluated for fuel conversion, CO2 capture rate, and combustion efficiency obtained in different CLC units. Solid fuel conversion is normally not complete during operation, due to the undesired loss of char. A methodology is presented to extrapolate the current information to what could be expected in a larger CLC system. CO2 capture near 100% has been reported using a highly efficient carbon stripper, highly reactive fuels (such as lignites and biomass, etc.) or by the CLOU process. Operational experience in iG-CLC has showed that it is not possible to reach complete fuel combustion, making an additional oxygen polishing step necessary. For the further scale-up, it is essential to reduce the unburnt compounds at the fuel reactor outlet. Proposals to achieve this reduction already exist and include both improvement to the gas-oxygen carrier contact, or new design concepts based on the current scheme for iG-CLC. In addition, CLOU based on copper materials has shown that complete fuel combustion could be achieved. Main challenges for the future development and scale-up of CLC technology have been also identified. A breakthrough in the future development of CLC technology for solid fuels will come from developing long-life materials for CLOU that are easy to recover from the ash purge stream.
Recent progress in gasoline surrogate fuels Prog. Energy Combust. Sci. (IF 17.382) Pub Date : 2017-12-06 S.Mani Sarathy, Aamir Farooq, Gautam T. Kalghatgi
Petroleum-derived gasoline is currently the most widely used fuel for transportation propulsion. The design and operation of gasoline fuels is governed by specific physical and chemical kinetic fuel properties. These must be thoroughly understood in order to improve sustainable gasoline fuel technologies in the face of economical, technological, and societal challenges. For this reason, surrogate mixtures are formulated to emulate the thermophysical, thermochemical, and chemical kinetic properties of the real fuel, so that fundamental experiments and predictive simulations can be conducted. Early studies on gasoline combustion typically adopted single component or binary mixtures (n-heptane/isooctane) as surrogates. However, the last decade has seen rapid progress in the formulation and utilization of ternary mixtures (n-heptane/isooctane/toluene), as well as multicomponent mixtures that span the entire carbon number range of gasoline fuels (C4–C10). The increased use of oxygenated fuels (ethanol, butanol, MTBE, etc.) as blending components/additives has also motivated studies on their addition to gasoline fuels. This comprehensive review presents the available experimental and chemical kinetic studies which have been performed to better understand the combustion properties of gasoline fuels and their surrogates. Focus is on the development and use of surrogate fuels that emulate real fuel properties governing the design and operation of engines. A detailed analysis is presented for the various classes of compounds used in formulating gasoline surrogate fuels, including n-paraffins, isoparaffins, olefins, naphthenes, and aromatics. Chemical kinetic models for individual molecules and mixtures of molecules to emulate gasoline surrogate fuels are presented. Despite the recent progress in gasoline surrogate fuel combustion research, there are still major gaps remaining; these are critically discussed, as well as their implications on fuel formulation and engine design.
State-of-the-art applications of fly ash from coal and biomass: A focus on zeolite synthesis processes and issues Prog. Energy Combust. Sci. (IF 17.382) Pub Date : 2017-11-10 Claudia Belviso
Over the years, the production of waste ash from many sources (e.g. coal, biomass, industrial, animal, and municipal solid waste) and from conventional and renewable energy technologies has increased, generating environmental problems due to the increasing amount of material disposed of in landfills.Fly ash from coal and biomass represent the foremost waste products produced by fossil fuel combustion and alternative natural energy sources, respectively. These waste materials are most widely utilized in agricultural applications, soil stabilization, and the cement and concrete industries. Among the various methods proposed for the reuse of fly ash, conversion to zeolite offers the greatest benefits; the process diverts ash waste materials from disposal sites and transforms them into useful secondary products for applications ranging from environmental mitigation to catalysis. The vast amount of literature on fly ash application is the fruit of growing waste production and the consequent need to find innovative methods to reduce the amount of waste deposited in landfills.This article summarizes studies concerning both coal and biomass fly ash. The characterization and potential applications of these materials are analysed in detail through reference to the numerous studies published on fly ash worldwide over the last number of decades. A considerable number of experiments have been conducted using ash as a raw material for zeolite synthesis, and many others concern the utilization of the newly-formed mineral. This paper discusses the key factors affecting zeolite synthesis, primarily from coal fly ash; the drawbacks of each approach are also analysed.
Recent advances in sulfonated resin catalysts for efficient biodiesel and bio-derived additives production Prog. Energy Combust. Sci. (IF 17.382) Pub Date : 2017-11-21 Valeria Trombettoni, Daniela Lanari, Pepijn Prinsen, Rafael Luque, Assunta Marrocchi, Luigi Vaccaro
The drive toward greener process chemistry has stimulated the development of strong solid acids to replace traditional catalysts for efficient biodiesel production from free fatty acid (FFA) enriched biomass feedstocks. Notably, heterogeneous systems enable simple product isolation procedures and improve the catalyst recyclability. They may also be used in continuous reactors. Increasing interest is directed toward organic polymer-supported solid acid catalysts, holding the promise of easy incorporation of the catalyst into the support with high density and easy tuning of the support microstructure/morphology. In the present review, the focus is set on the most widely employed members of this class, including cation-exchange resins, micro- and mesoporous acidic resins, as well as supported acidic ionic liquids and ionomeric membranes. Moreover, we present the use of alternative organic polymer-based acidic catalysts (hybrid systems). Attention is paid to correlations between parameters such as catalyst morphology, excess of alcohol required, FFA content in oil feedstock, presence of impurities and the performance of resin-supported acid catalysts, as well as to the catalyst recyclability. Finally, a brief survey illustrates the use of resin-supported acid catalysts for the preparation of biofuel additives alkyl levulinates – structurally quite similar to the biodiesel – starting from biomass derived levulinic acid.
Cold start of proton exchange membrane fuel cell Prog. Energy Combust. Sci. (IF 17.382) Pub Date : 2017-10-25 Yueqi Luo, Kui Jiao
In this review, “cold start” is defined as the startup of proton exchange membrane (PEM) fuel cells from subfreezing temperatures. Problems occurring during the cold start pose some of the remaining barriers to commercial applications of PEM fuel cells in transportation, stationary, auxiliary and portable systems. Fundamental studies of transport phenomena are critical to a better understanding of the mechanisms of cold start and offer ultimate solutions to resolving cold-start issues. In this review, experimental studies are discussed, focusing on output performance degradation, water and ice visualization, and component damages during a cold start. Analytical, numerical, and microscopic models and their results are also discussed. One of the emphases is on transport phenomena relevant to cold starts, including supercooling, phase change and transport of water in the membrane, catalyst layer, microporous layer, and gas diffusion layer. Another emphasis is placed on the strategies utilized to optimize cold-start processes for improved performance. The strategies include material designs of the components, cell/stack structures, and startup mode/load controls. It is shown that all of the effective strategies to mitigating cold-start problems derive from a basic understanding of the transport mechanisms during a cold start. It is also suggested that future models for this problem should place a great deal of attention in supercooling phenomena and water phase-change and transport in multilayer porous media. Lastly, more advanced experimental methods, such as real-time water/ice visualization and cryogenic microscopy, are needed to validate emerging theories and models.
Heatlines: Modeling, visualization, mixing and thermal management Prog. Energy Combust. Sci. (IF 17.382) Pub Date : 2017-11-06 Tanmay Basak, Debayan Das, Pratibha Biswal
The bulk motion of fluid and diffusive transport within fluid are two processes during natural or forced convection. The complexity of the convective heat flow is realized since last few decades and the analysis of the heat flow as well as thermal characteristics gradually becomes cumbersome. Although earlier researchers studied convective heat flow via velocity profiles, streamlines and isotherms, these tools were not enough for the efficient visualization of the unique features of convection heat flow. An efficient tool, termed as ‘heatline’ (mathematically represented as heatfunction) was first proposed by Kimura and Bejan in 1983 for the heat flow visualization during convective heat flow. The aim of this article is to review existing works on ‘heatline’ involving various physical systems. The mathematical implications of heatfunctions based on derivations of governing equations and boundary conditions for heatfunctions are presented in detail. The non-homogeneous boundary conditions for heatfunctions arise due to hot or cold or adiabatic walls as well as the junction between the walls and these conditions vary with the location of the reference or datum of the heatfunction. The physics on the heat flow via ‘heatlines’ are found to be invariant with the locations of the reference value of the heatfunction. The heat flow visualization is analyzed for various test cases from simple one dimensional boundary layer problem to convection in two dimensional complex cavities. The detailed explanations of earlier works on ‘heatlines’ during one dimensional flow involving forced or natural convection with various applications are discussed. Further, applications of ‘heatlines’ during convective heat flow within enclosed cavities involving uniform or non uniform heating of walls, discrete heating or cooling, conjugate convection and mixed convection are discussed and ‘heatlines’ are found to be successful to demonstrate various complex heat flow paths and multiple heat flow circulation cells. Overall, the analysis of convective heat flow from simple to complicated geometries via ‘heatline’ is crucial for the visualization of the thermal transport, mixing and efficient thermal management.
Carbon nanotubes: A potential material for energy conversion and storage Prog. Energy Combust. Sci. (IF 17.382) Pub Date : 2017-11-07 Sandeep Kumar, Monika Nehra, Deepak Kedia, Neeraj Dilbaghi, K Tankeshwar, Ki-Hyun Kim
Carbon nanotube-based materials are gaining considerable attention as novel materials for renewable energy conversion and storage. The novel optoelectronic properties of CNTs (e.g., exceptionally high surface area, thermal conductivity, electron mobility, and mechanical strength) can be advantageous for applications toward energy conversion and storage. Although many nanomaterials are well known for the unique structure-property relations, such relations have been sought most intensively from CNTs due to their extreme diversity and richness in structures. For the development of energy-related devices (like photovoltaic cells, supercapacitors, and lithium ion batteries), it is critical to conduct pre-evaluation of their design, operation, and performance in terms of cost, life time, performance, and environmental impact. This critical review was organized to address the recent developments in the use of CNT-based materials as working/counter electrodes and electrolytes in photovoltaic devices and as building blocks in next-generation flexible energy storage devices. The most promising research in the applications of CNTs toward energy conversion and storage is highlighted based on both computational and experimental studies along with the challenges for developing breakthrough products.
Solar thermal hybrids for combustion power plant: A growing opportunity Prog. Energy Combust. Sci. (IF 17.382) Pub Date : 2017-10-04 G.J. Nathan, M. Jafarian, B.B. Dally, W.L. Saw, P.J. Ashman, E. Hu, A. Steinfeld
The development of technologies to hybridise concentrating solar thermal energy (CST) and combustion technologies, is driven by the potential to provide both cost-effective CO2 mitigation and firm supply. Hybridisation, which involves combining the two energy sources within a single plant, offers these benefits over the stand-alone counterparts through the use of shared infrastructure and increased efficiency. In the near-term, hybrids between solar and fossil fuelled systems without carbon capture offer potential to lower the use of fossil fuels, while in the longer term they offer potential for low-cost carbon-neutral or carbon-negative energy. The integration of CST into CO2 capture technologies such as oxy-fuel combustion and chemical looping combustion is potentially attractive because the same components can be used for both CO2 capture and the storage of solar energy, to reduce total infrastructure and cost. The use of these hybrids with biomass and/or renewable fuels, offers the additional potential for carbon-negative energy with relatively low cost. In addition to reviewing these technologies, we propose a methodology for classifying solar-combustion hybrid technologies and assess the progress and challenges of each. Particular attention is paid to “direct hybrids”, which harness the two energy sources in a common solar receiver or reactor to reduce total infrastructure and losses.
The role of natural gas and its infrastructure in mitigating greenhouse gas emissions, improving regional air quality, and renewable resource integration Prog. Energy Combust. Sci. (IF 17.382) Pub Date : 2017-11-02 Michael A. Mac Kinnon, Jacob Brouwer, Scott Samuelsen
The pursuit of future energy systems that can meet electricity demands while supporting the attainment of societal environment goals, including mitigating climate change and reducing pollution in the air, has led to questions regarding the viability of continued use of natural gas. Natural gas use, particularly for electricity generation, has increased in recent years due to enhanced resource availability from non-traditional reserves and pressure to reduce greenhouse gasses (GHG) from higher-emitting sources, including coal generation. While lower than coal emissions, current natural gas power generation strategies primarily utilize combustion with higher emissions of GHG and criteria pollutants than other low-carbon generation options, including renewable resources. Furthermore, emissions from life cycle stages of natural gas production and distribution can have additional detrimental GHG and air quality (AQ) impacts. On the other hand, natural gas power generation can play an important role in supporting renewable resource integration by (1) providing essential load balancing services, and (2) supporting the use of gaseous renewable fuels through the existing infrastructure of the natural gas system. Additionally, advanced technologies and strategies including fuel cells and combined cooling heating and power (CCHP) systems can facilitate natural gas generation with low emissions and high efficiencies. Thus, the role of natural gas generation in the context of GHG mitigation and AQ improvement is complex and multi-faceted, requiring consideration of more than simple quantification of total or net emissions. If appropriately constructed and managed, natural gas generation could support and advance sustainable and renewable energy. In this paper, a review of the literature regarding emissions from natural gas with a focus on power generation is conducted and discussed in the context of GHG and AQ impacts. In addition, a pathway forward is proposed for natural gas generation and infrastructure to maximize environmental benefits and support renewable resources in the attainment of emission reductions.
Temperature measurement techniques for gas and liquid flows using thermographic phosphor tracer particles Prog. Energy Combust. Sci. (IF 17.382) Pub Date : 2017-11-02 Christopher Abram, Benoît Fond, Frank Beyrau
Optical diagnostics for fluid temperature measurements continue to further our understanding of flows involving heat transfer and/or chemical reactions, which are intrinsic to key areas including energy production, the process industries, transportation, heating/cooling systems and naturally-occurring thermal convection. Besides temperature, all flows must also be described by their velocity. As these flows are often turbulent, an important capability is to measure both velocity and temperature at the same time to capture, for example, the turbulent heat flux term appearing in the energy conservation equation.This paper reviews temperature measurement techniques for fluid flows that are based on thermographic phosphors, which are materials that possess temperature-dependent luminescence properties. Phosphor particles are seeded into the fluid flow of interest. Following laser excitation, the luminescence of the particles is detected, and the temperature measurement is derived using either the spectral intensity ratio or the lifetime. The same particles can also be used for velocity measurements using well-established particle-based approaches, such as laser Doppler velocimetry (LDV) or particle image velocimetry (PIV), producing instantaneously correlated vector-scalar data. First introduced over a decade ago, this concept has since evolved and is currently capable of two-dimensional measurements in the temperature range 200–900 K. At lower temperatures a single-shot spatial precision better than 4 K is possible, as is imaging at sampling rates in the multi-kHz range. The approach is flexible, allowing, for example, techniques which probe single particles for point measurements with a 200 µm spatial resolution. Besides many validation experiments, the method has been applied in internal combustion engines, a falling film absorber, a high-pressure reaction vessel and in enclosed wind tunnels to study various turbulent heat transfer and reactive flow phenomena.The objective of this article is to provide the first review of this emerging field. The focus is on 1) the method: how has the principle of phosphor thermometry been used for flow measurements, and what instrumentation and processing steps were implemented; 2) how phosphor particles were characterised, and which phosphors are best-suited to temperature measurements in flows; and 3) the applications of the technique. Throughout, and with a detailed analysis of various sources of error, the review endeavours to compare the work and identify common aspects, advantages and limitations of the studies that led to successful flow measurements, and therefore should serve as a guide for researchers using the method. The article also briefly summarises the various challenges which the authors consider are key to the future development of these diagnostics.
Advances in rapid compression machine studies of low- and intermediate-temperature autoignition phenomena Prog. Energy Combust. Sci. (IF 17.382) Pub Date : 2017-07-10 S. Scott Goldsborough, Simone Hochgreb, Guillaume Vanhove, Margaret S. Wooldridge, Henry J. Curran, Chih-Jen Sung
Rapid compression machines (RCMs) are widely used to acquire experimental insights into fuel autoignition and pollutant formation chemistry, especially at conditions relevant to current and future combustion technologies. RCM studies emphasize important experimental regimes, characterized by low- to intermediate-temperatures (600–1200 K) and moderate to high pressures (5–80 bar). At these conditions, which are directly relevant to modern combustion schemes including low temperature combustion (LTC) for internal combustion engines and dry low emissions (DLE) for gas turbine engines, combustion chemistry exhibits complex and experimentally challenging behaviors such as the chemistry attributed to cool flame behavior and the negative temperature coefficient regime. Challenges for studying this regime include that experimental observations can be more sensitive to coupled physical-chemical processes leading to phenomena such as mixed deflagrative/autoignitive combustion. Experimental strategies which leverage the strengths of RCMs have been developed in recent years to make RCMs particularly well suited for elucidating LTC and DLE chemistry, as well as convolved physical-chemical processes.Specifically, this work presents a review of experimental and computational efforts applying RCMs to study autoignition phenomena, and the insights gained through these efforts. A brief history of RCM development is presented towards the steady improvement in design, characterization, instrumentation and data analysis. Novel experimental approaches and measurement techniques, coordinated with computational methods are described which have expanded the utility of RCMs beyond empirical studies of explosion limits to increasingly detailed understanding of autoignition chemistry and the role of physical-chemical interactions. Fundamental insight into the autoignition chemistry of specific fuels is described, demonstrating the extent of knowledge of low-temperature chemistry derived from RCM studies, from simple hydrocarbons to multi-component blends and full-boiling range fuels. Emerging needs and further opportunities are suggested, including investigations of under-explored fuels and the implementation of increasingly higher fidelity diagnostics.
Combustion synthesis of zero-, one-, two- and three-dimensional nanostructures: Current trends and future perspectives Prog. Energy Combust. Sci. (IF 17.382) Pub Date : 2017-07-17 Hayk H. Nersisyan, Jong Hyeon Lee, Jin-Rui Ding, Kyo-Seon Kim, Khachatur V. Manukyan, Alexander S. Mukasyan
The combustion phenomenon is characterized by rapid self-sustaining reactions, which can occur in the solid, liquid, or gas phase. Specific types of these reactions are used to produce valuable materials by different combustion synthesis (CS) routes. In this article, all three CS approaches, i.e. solid-phase, solution, and gas-phase flame, are reviewed to demonstrate their attractiveness for fabrication of zero-, one-, two-, and three-dimensional nanostructures of a large variety of inorganic compounds. The review involves five sections. First, a brief classification of combustion synthesis methods is given along with the scope of the article. Second, the state of art in the field of solid-phase combustion synthesis is described. Special attention is paid to the relationships between combustion parameters and structure/properties of the produced nanomaterials. The third and fourth sections describe details for controlling material structures through solution combustion synthesis and gas-phase flame synthesis, respectively. A variety of properties (e.g., thermal, electronic, electrochemical, and catalytic) associated with different types of CS nanoscale materials are discussed. The conclusion focuses on the most promising directions for future research in the field of advanced nanomaterial combustion synthesis.
Microbial electrolysis cell platform for simultaneous waste biorefinery and clean electrofuels generation: Current situation, challenges and future perspectives Prog. Energy Combust. Sci. (IF 17.382) Pub Date : 2017-07-17 Guangyin Zhen, Xueqin Lu, Gopalakrishnan Kumar, Péter Bakonyi, Kaiqin Xu, Youcai Zhao
Microbial electrolysis cell (MEC) holds the flexible potentials for waste biorefinery, pollutants removal, CO2 capture, and bioelectrosynthesis of clean and renewable electrofuels or valuable chemical commodities, dealing with the depletion of fossil fuels and environmental deterioration issues. Although substantial advances in process design and mechanisms exploration have greatly promoted the development of MEC platform from a concept to a technology, how to virtually utilize it in real-world scenario remains a big challenge. There are numerous technical issues ahead for MEC to be tackled towards up-scaling and real implementations. This review article presents a state-of-the-art overview of the fundamental aspects and the latest breakthrough results and accomplishments obtained from the MEC platform, with a special emphasis on mapping the key extracellular electron transfer (EET) mechanisms between electroactive microorganisms and electrode surface (including i: cells→e−anode; and ii: cathode→e−cells). A unified discussion of different process design: inoculation methods for rapid start-up, role of membranes, modification of cathode materials, cathodic catalysts (i.e. noble, un-noble metal catalysts and biocatalysts) as well as designs and configurations of versatile bioelectrochemical cells, is also involved. Finally, the major challenges and technical problems encountered throughout MEC researches are analyzed, and recommendations and future needs for the virtual utilization of MEC technology in real waste treatment are elaborated.
CO2 capture from the industry sector Prog. Energy Combust. Sci. (IF 17.382) Pub Date : 2017-08-17 Praveen Bains, Peter Psarras, Jennifer Wilcox
It is widely accepted that greenhouse gas emissions, especially CO2, must be significantly reduced to prevent catastrophic global warming. Carbon capture and reliable storage (CCS) is one path towards controlling emissions, and serves as a key component to climate change mitigation and will serve as a bridge between the fossil fuel energy of today and the renewable energy of the future. Although fossil-fueled power plants emit the vast majority of stationary CO2, there are many industries that emit purer streams of CO2, which result in reduced cost for separation. Moreover, many industries outside of electricity generation do not have ready alternatives for becoming low-carbon and CCS may be their only option. The thermodynamic minimum work for separation was calculated for a variety of CO2 emissions streams from various industries, followed by a Sherwood analysis of capture cost. The Sherwood plot correlates the relationship between concentrations of a target substance with the cost to separate it from the remaining components. As the target concentration increases, the cost to separate decreases on a molar basis. Furthermore, the lowest cost opportunities for deploying first-of-a-kind CCS technology were found to be in the Midwest and along the Gulf Coast. Many high purity industries, such as ethanol production, ammonia production and natural gas processing, are located in these regions. The southern Midwest and Gulf Coast are also co-located with potential geologic sequestration sites and enhanced oil recovery opportunities. As a starting point, these sites may provide the demonstration and knowledge necessary for reducing carbon capture technology costs across all industries, and improving the economic viability for CCS and climate change mitigation. The various industries considered in this review were examined from a dilution and impact perspective to determine the best path forward in terms of prioritizing for carbon capture. A possible implementation pathway is presented that initially focuses on CO2 capture from ethanol production, followed by the cement industry, ammonia, and then natural gas processing and ethylene oxide production. While natural gas processing and ethylene oxide production produce high purity streams, they only account for relatively small portions of industrial process CO2. Finally, petroleum refineries account for almost a fifth of industrial process CO2, but are comprised of numerous low-purity CO2 streams. These qualities make these three industries less attractive for initial CC implementation, and better suited for consideration towards the end of the industrial CC pathway.
State of the art of biodiesel production under supercritical conditions Prog. Energy Combust. Sci. (IF 17.382) Pub Date : 2017-08-29 Obie Farobie, Yukihiko Matsumura
This paper reviews the current status of biodiesel production mainly under supercritical conditions. Various methods such as homogeneous acid- and alkali-catalyzed transesterification, heterogeneous acid and alkali-catalyzed transesterification, enzyme-catalyzed transesterification, and supercritical reactions have been employed so far to synthesize biodiesel. Herein, we review the reaction mechanisms and experimental results for these approaches. Recently, supercritical biodiesel production has undergone a vigorous development as the technology offers several advantages over other methods, including the fact that it does not require a catalyst, short residence time, high reaction rate, no pretreatment requirement, and applicability to a wide variety of feedstock. This technology was first designed for biodiesel production using methanol and ethanol. Biodiesel production without glycerol as a byproduct is attractive and has been achieved using supercritical methyl acetate and dimethyl carbonate (DMC). Most recently, biodiesel production in supercritical tert-butyl methyl ether (MTBE) has been developed also. In this review, supercritical biodiesel production will be discussed in detail. Empirical rate expressions are derived for biodiesel production in supercritical methanol, ethanol, methyl acetate, DMC, and MTBE in this study for the first time. These rate equations are critical to predicting biodiesel yields and to comparing the reaction behaviors in different solvents. Lastly challenges for improving energy recovery in supercritical biodiesel production and recommendations for future work are provided.
Numerical models for thermochemical degradation of thermally thick woody biomass, and their application in domestic wood heating appliances and grate furnaces Prog. Energy Combust. Sci. (IF 17.382) Pub Date : 2017-09-12 Inge Haberle, Øyvind Skreiberg, Joanna Łazar, Nils Erland L. Haugen
This paper reviews the current state-of-the-art of numerical models used for thermochemical degradation and combustion of thermally thick woody biomass particles. The focus is on the theory of drying, devolatilization and char conversion with respect to their implementation in numerical simulation tools. An introduction to wood chemistry, as well as the physical characteristics of wood, is also given in order to facilitate the discussion of simplifying assumptions in current models. Current research on single, densified or non-compressed, wood particle modeling is presented, and modeling approaches are compared. The different modeling approaches are categorized by the dimensionality of the model (1D, 2D or 3D), and the one-dimensional models are separated into mesh-based and interface-based models. Additionally, the applicability of the models for wood stoves is discussed, and an overview of the existing literature on numerical simulations of small-scale wood stoves and domestic boilers is given. Furthermore, current bed modeling approaches in large-scale grate furnaces are presented and compared against single particle models.
Underground in situ coal thermal treatment for synthetic fuels production Prog. Energy Combust. Sci. (IF 17.382) Pub Date : 2017-05-19 Hongzhi R. Zhang, Suhui Li, Kerry E. Kelly, Eric G. Eddings
Underground coal thermal treatment (UCTT) is a promising concept that was recently proposed for extracting high-value hydrocarbon fuels from deep coal seams, which are economically unattractive for mining. UCTT is essentially an in situ pyrolysis process that converts underground coals into synthetic liquid and gaseous fuels, while leaving most of the carbon underground as a char matrix. The produced synthetic fuels have higher H/C ratios than coals. The remaining char matrix is an ideal reservoir for CO2 sequestration because pyrolysis significantly increases the surface area of the char. The UCTT concept is relatively new, and there is little research in this area. However, underground oil shale retorting, which is also an in-situ hydrocarbon fuels conversion process, shares key features with UCTT and has gained momentum in demonstration and commercial development. As such, there is a large body of literature available in this area. A review of the studies on underground oil shale retorting that are closely related to UCTT will shed light on the UCTT process. This paper presents a review of the recent literature on underground oil shale retorting that are most relevant to UCTT process. The review provides a background to the reader by comparing the properties of coal with oil shale, with an emphasis on the feasibility of applying oil shale retorting techniques to UCTT process. The review further discusses the coal and oil shale conversion issues and uses the knowledge of the latter as guidance for the development of UCTT. Published data on pyrolysis of large coal blocks at conditions relevant to UCTT process is scarce. Therefore, literature on conventional coal pyrolysis is reviewed for optimization of the UCTT process. Despite the abundant studies on pulverized coal pyrolysis, there are still many open questions on whether they can be directly applied to UCTT. A comparison of the unique environment of UCTT with conditions of conventional pulverized coal pyrolysis clearly shows there are knowledge gaps. Future research needs are then proposed to close these gaps.
Lignocellulosic biomass pyrolysis mechanism: A state-of-the-art review Prog. Energy Combust. Sci. (IF 17.382) Pub Date : 2017-06-07 Shurong Wang, Gongxin Dai, Haiping Yang, Zhongyang Luo
The past decades have seen increasing interest in developing pyrolysis pathways to produce biofuels and bio-based chemicals from lignocellulosic biomass. Pyrolysis is a key stage in other thermochemical conversion processes, such as combustion and gasification. Understanding the reaction mechanisms of biomass pyrolysis will facilitate the process optimization and reactor design of commercial-scale biorefineries. However, the multiscale complexity of the biomass structures and reactions involved in pyrolysis make it challenging to elucidate the mechanism. This article provides a broad review of the state-of-art biomass pyrolysis research. Considering the complexity of the biomass structure, the pyrolysis characteristics of its three major individual components (cellulose, hemicellulose and lignin) are discussed in detail. Recently developed experimental technologies, such as Py-GC–MS/FID, TG-MS/TG-FTIR, in situ spectroscopy, 2D-PCIS, isotopic labeling method, in situ EPR and PIMS have been employed for biomass pyrolysis research, including online monitoring of the evolution of key intermediate products and the qualitative and quantitative measurement of the pyrolysis products. Based on experimental results, many macroscopic kinetic modeling methods with comprehensive mechanism schemes, such as the distributed activation energy model (DAEM), isoconversional method, detailed lumped kinetic model, kinetic Monte Carlo model, have been developed to simulate the mass loss behavior during biomass pyrolysis and to predict the resulting product distribution. Combined with molecular simulations of the elemental reaction routes, an in-depth understanding of the biomass pyrolysis mechanism may be obtained. Aiming to further improve the quality of pyrolysis products, the effects of various catalytic methods and feedstock pretreatment technologies on the pyrolysis behavior are also reviewed. At last, a brief conclusion for the challenge and perspectives of biomass pyrolysis is provided.
Stratified turbulent flames: Recent advances in understanding the influence of mixture inhomogeneities on premixed combustion and modeling challenges Prog. Energy Combust. Sci. (IF 17.382) Pub Date : 2017-06-09 Andrei N. Lipatnikov
The goals of the present review paper are; (i) to introduce experimental facilities and numerical tools applied to investigating inhomogeneously premixed flames, (ii) to summarize recent progress in revealing and understanding local phenomena (e.g. back-supported combustion or generation of flame surface area) that stem from the influence of mixture inhomogeneities on flame propagation through flammable reactants, (iii) to show state-of-the-art of unsteady multidimensional RANS and LES research into inhomogeneously premixed turbulent flames and to discuss models invoked for this purpose, and (iv) to highlight issues that still challenge researchers who develop such models.
Advances in modeling and simulation of Li–air batteries Prog. Energy Combust. Sci. (IF 17.382) Pub Date : 2017-06-22 Peng Tan, Wei Kong, Zongping Shao, Meilin Liu, Meng Ni
Li–air batteries have potential to be the next generation power sources for various applications, from portable devices to electric vehicles and microgrids, due largely to their significantly higher theoretical energy densities than those of the existing batteries. The commercialization of this technology, however, is hindered by a variety of technical hurdles, including low obtainable capacity, poor energy efficiency, and limited cycle life. Breakthrough to these barriers requires a fundamental understanding of the complex electrochemical and transport behaviors inside the batteries. Mathematical modeling and simulation are imperative in gaining important insight into the mechanisms of these complex phenomena, which is vital to achieving rational designs of better materials for high-performance batteries. In this paper, we present a comprehensive review of the latest advances in modeling and simulation of Li–air batteries and offer our perspectives on new directions of future development. Unlike previous reviews that centered mainly on continuum modeling of non-aqueous Li–air batteries, the present paper focuses on mathematical descriptions of the detailed transport and electrochemical processes in different types of Li–air batteries. We start with a brief introduction to the working principles of Li–air batteries. Then, the governing equations for mass transport and electrochemical reactions in non-aqueous Li–air batteries are formulated, including lithium ion and oxygen transport in the porous air electrode, the formation of solid discharge products, the kinetics of electrode reactions, the evolution of electrode structure, the distribution of active sites, the effect of the side reactions during cycling, the phenomena of the volume change, and the charge process. In addition, the mo\deling and simulations of aqueous and hybrid Li–air batteries are reviewed, highlighting the phenomena that are different from those in the non-aqueous ones. Finally, the challenges facing the modeling and simulation of Li–air batteries are discussed and perspectives for the development of a new generation of Li–air batteries are outlined.
Continuous-flow electroreduction of carbon dioxide Prog. Energy Combust. Sci. (IF 17.382) Pub Date : 2017-06-13 B. Endrődi, G. Bencsik, F. Darvas, R. Jones, K. Rajeshwar, C. Janáky
Solar fuel generation through electrochemical CO2 conversion offers an attractive avenue to store the energy of sunlight in the form of chemical bonds, with the simultaneous remediation of a greenhouse gas. While impressive progress has been achieved in developing novel nanostructured catalysts and understanding the mechanistic details of this process, limited knowledge has been gathered on continuous-flow electrochemical reactors for CO2 electroreduction. This is indeed surprising considering that this might be the only way to scale-up this fledgling technology for future industrial application. In this review article, we discuss the parameters that influence the performance of flow CO2 electrolyzers. This analysis spans the overall design of the electrochemical cell (microfluidic or membrane-based), the employed materials (catalyst, support, etc.), and the operational conditions (electrolyte, pressure, temperature, etc.). We highlight R&D avenues offering particularly promising development opportunities together with the intrinsic limitations of the different approaches. By collecting the most relevant characterization methods (together with the relevant descriptive parameters), we also present an assessment framework for benchmarking CO2 electrolyzers. Finally, we give a brief outlook on photoelectrochemical reactors where solar energy input is directly utilized.
Evolution, challenges and path forward for low temperature combustion engines Prog. Energy Combust. Sci. (IF 17.382) Pub Date : 2017-04-02 Avinash Kumar Agarwal, Akhilendra Pratap Singh, Rakesh Kumar Maurya
Universal concerns about degradation in ambient environment, stringent emission legislations, depletion of petroleum reserves, security of fuel supply and global warming have motivated research and development of engines operating on alternative combustion concepts, which also have capability of using renewable as well as conventional fuels. Low temperature combustion (LTC) is an advanced combustion concept for internal combustion (IC) engines, which has attracted global attention in recent years. LTC concept is different from the conventional spark ignition (SI) combustion as well as compression ignition (CI) diffusion combustion concepts. LTC technology offers prominent benefits in terms of simultaneous reduction of both oxides of nitrogen (NOx) and particulate matter (PM), in addition to reduction in specific fuel consumption (SFC). However, controlling ignition timing and combustion rate are primary challenges to be tackled before LTC technology can be implemented in automotive engines commercially. This review covers fundamental aspects of development of LTC engines and its evolution, historical background and origin of LTC concept, encompassing LTC principle, its advantages, challenges and prospects. Detailed insights into preparation of homogeneous charge by external and internal measures for mineral diesel and gasoline like fuels are covered. Fuel requirements and fuel induction system design aspect for LTC engines are also discussed. Combustion characteristics of LTC engines including combustion chemistry, heat release rate (HRR), combustion duration, knock characteristics, high load limit, fuel conversion efficiencies and combustion instability are summarized. Emission characteristics are reviewed along with insights into PM and NOx emissions from LTC engines. Finally, different strategies for controlling combustion rate and combustion timings for gasoline and mineral diesel like fuels are discussed, showing the way forward for this technology in future towards its commercialization.
Ash formation and deposition in coal and biomass fired combustion systems: Progress and challenges in the field of ash particle sticking and rebound behavior Prog. Energy Combust. Sci. (IF 17.382) Pub Date : 2018-05-24 Ulrich Kleinhans, Christoph Wieland, Flemming J. Frandsen, Hartmut Spliethoff
The purpose of this paper is to review the present knowledge on ash formation, ash particle transport and deposition during solid fuel combustion, with emphasis on particle sticking and rebound behavior. A substantial part of the fuel can be inorganic, forming inorganic vapors and ash particles. The impaction of solid, molten or partially molten particles on surfaces is dependent on the particle and surface characteristics. For instance, a particulate deposit might capture incoming particles or be removed due to erosion, while a molten layer will collect all impacting particles, no matter if they are sticky or not. The main properties affecting the particle stickiness are the viscosity and surface tension for silicate-rich ashes. On the contrary, the stickiness of salt-rich ashes – typical for herbaceous biomass and wood- or waste-based fuels – is often described using the liquid melt fraction. Furthermore, the particle kinetic energy and the angle of impaction, are crucial parameters. If all kinetic energy is dissipated during the impact, the particle will remain on the surface. This review presents an overview of major ash forming elements found in biomass and coal, and discusses the heterogeneity of particles’ inorganic composition. Ash transport and deposition mechanisms as well as their mathematical description are given and discussed, together with composition- and temperature-depended models for the estimation of ash particle and deposit properties. These properties are essential in order to describe the particle sticking and rebound behavior. Ash particle sticking and rebound criteria can be divided into three main groups, based on either: (1) the particle melt fraction, (2) the particle viscosity, or (3) the energy dissipation of a particle, upon impaction. Sticking criteria are presented, their required parameters are discussed and typical particle and surface properties found in combustion systems, are summarized. Eight different sticking criteria are implemented in a computational fluid dynamics code and computations are compared against measurements from an entrained flow reactor. Uniform sized soda-lime glass particles are applied instead of inhomogeneous fly ash particles, since soda-lime glass is known to behave similar to coal fly ash. Best agreement for the deposition rates on a clean tube is achieved using a criterion based on the work of Srinivasachar et al. . In this model, the sticking and rebound threshold, is a function of the particle kinetic energy, the angle of impaction, and, the particle viscosity. Particularly, the particle viscosity is confirmed as a key parameter for silicate-rich ashes. It should be calculated using temperature- and composition-dependent correlations, being aware that there is a significant scattering in the results from such models and that the models are often only valid in narrow compositional ranges, and cannot be used outside these. A mechanistic model is used to explain results from glass particle experiments and their dependence on the particle kinetic energy. Therefore, the impaction process is subdivided in four steps, and the energy dissipation of each step is calculated. These theoretical considerations show that the contact angle of a molten droplet with the substrate is of minor importance, and that the majority of depositing particles are dominated by the work of deformation against viscosity, rather than surface tension effects. This review underlines the importance of the particle viscosity, and its accurate prediction for silicate-rich ashes. The proposed criterion is able to predict the sticking of small, solid particles below 10 µm diameter, as it is often observed in literature. Also, it is crucial to consider the surface structure and stickiness, in order to predict deposition rates in solid fuel-fired systems. Biomass ashes and their stickiness are more difficult, due to a different ash particle chemistry, compared to coal ashes. Salt-rich particles and their stickiness are controlled by the amount of liquid phase. Here, a link between the viscosity and amount of liquid phase is a promising approach, and should be addressed in future work. Furthermore, the viscosity of different ash particles – silicate-, salt- or Ca-rich – should preferentially be modeled from the chemical and physical structure instead of an empirical fitting procedure between fuel chemistry and viscosity measurements.
Progress in O2 separation for oxy-fuel combustion–A promising way for cost-effective CO2 capture: A review Prog. Energy Combust. Sci. (IF 17.382) Pub Date : Fan Wu, Morris D. Argyle, Paul A. Dellenback, Maohong Fan
Refinery co-processing of renewable feeds Prog. Energy Combust. Sci. (IF 17.382) Pub Date : 2018-05-03 Stella Bezergianni, Athanassios Dimitriadis, Oleg Kikhtyanin, David Kubička
Biofuels are called upon to play an important role, not only in reducing the associated greenhouse-gases emissions, but also in enabling the gradual independence from fossil sources, rendering low-carbon-highly-sustainable fuels. Today, the involvement of biomass-derived sources into existing petroleum refineries has a growing interest due to the increasing unpredictability of oil prices, environmental concerns and the necessity to secure an energy supply. Petroleum refineries already have a well-developed infrastructure to produce fuels and base chemicals and, consequently, would not require additional intensive investments for processing of alternative feedstocks. From this point of view, co-processing of biomass-derived feedstocks with petroleum fractions is an attractive option, which has already been industrially demonstrated in some cases. There are two main technologies that could be used for co-processing of biomass feedstocks with petroleum fractions, the first one being catalytic hydroprocessing and the second one being fluid catalytic cracking (FCC). Both technologies are found in virtually any conventional refinery. It is obvious that the co-processing of biomass-based feedstocks with petroleum fractions has the potential to play an important role in the near future. There are several research studies in literature that examine both technologies for co-processing. However, while there are many technological reviews that focus on stand-alone biofuel production (e.g., FAME biodiesel, bioethanol, HVO etc.), a dedicated technological review on co-processing for production of hybrid fuels is still missing. Therefore, this paper is focused on presenting a state-of-the-art review on co-processing bio-based feedstocks with petroleum fractions via hydroprocessing and fluid catalytic cracking, looking at different potential feedstocks, catalysts, operating conditions, products and benefits in detail. As there is no specifically dedicated literature review in this field, the content of this review provides a guideline on co-processing of different bio-based feedstocks with petroleum fractions, aimed at delivering a technological assessment of the existing research efforts.
Review of arsenic behavior during coal combustion: Volatilization, transformation, emission and removal technologies Prog. Energy Combust. Sci. (IF 17.382) Pub Date : 2018-04-17 Chunbo Wang, Huimin Liu, Yue Zhang, Chan Zou, Edward J. Anthony
Growing public awareness of the environmental impact of coal combustion has raised serious concerns about the various hazardous trace elements produced by coal firing. Arsenic deserves special attention due to its toxicity, volatility, bioaccumulation in the environment, and potential carcinogenic properties. As the main anthropogenic source of arsenic is coal combustion, its behavior in power plants is of concern. Unlike mercury, arsenic behavior in coal combustion has not been subjected to systematic, in-depth research. Different researchers have reached opposing conclusions about the behavior of arsenic in combustion systems and, as yet, there is relatively little research on arsenic removal technologies. In this paper, the volatilization, transformation, and emission behavior of arsenic and its removal technologies are discussed in depth. Factors affecting the volatilization characteristics of arsenic are summarized, including temperature, pressure, mode of occurrence of arsenic, coal rank, mineral matter, and the sulfur and chlorine content of the fuel. The behavior of arsenic during oxy-fuel combustion and the effect of combustion atmosphere (O2, CO2, SO2 and H2O(g)) are also reviewed in detail. In order to better understand the pathways of arsenic in a power plant environment, a particular focus in this work is the transformation mechanism of ultra-fine ash particles and the partitioning behavior of arsenic. Finally, the effects of air pollution control devices (APCDs) on arsenic emissions are examined, along with the effectiveness of flue gas arsenic removal technologies with different kinds of adsorbents, including calcium-based adsorbents, metal oxides, activated carbon, and fly ash.
Bioethanol from corn stover – a review and technical assessment of alternative biotechnologies Prog. Energy Combust. Sci. (IF 17.382) Pub Date : 2018-04-07 Yan Zhao, Anders Damgaard, Thomas H. Christensen
Reviewing the literature from the last decade regarding the bioconversion of corn stover into ethanol, 474 references were identified containing 561 datasets. We found 144 datasets which were sufficiently consistent and detailed to address the current state of the art of corn stover conversion to bioethanol, and we were able to categorise 93% of these datasets into eight different technological configurations for the production of bioethanol, based on the pretreatment approaches used. After pretreating, the corn stover is subject to hydrolysis and fermentation, but these two process steps were largely identical in all datasets, albeit a range of operating conditions was reported. The final distillation of the ethanol was very rarely included in the datasets. By parameterising the bioethanol production by 26 parameters, including corn stover compositions, solid loadings, operational conditions, conversion efficiencies and material consumption, we were able to quantify the material flows for each technological configuration and estimate the uncertainty of the flows. The eight technological configurations produced 11–22% ethanol from the dry solid content of the corn stover. Technologies using alkaline-, solvent or ammonia-based pretreatments produced the largest amount of ethanol (19–22%), while fungi-based pretreatment produced much less (11%). All technological configurations resulted in large flows of solid as well as liquid residues, typically containing 60 to 70% of the dry solid corn stover content. Based on the selected datasets, statistical description is provided for all parameters, including mode, median, average and deviation, within each technological configuration. Bivariate correlation analysis across and within all technological configurations indicates that some operational parameters usually considered crucial in laboratory studies, e.g. pretreatment severity, show from a statistical perspective very little correlation with the yields. The review reveals that a great deal of research has addressed the challenge of converting corn stover into bioethanol, but a significant part of these studies is of limited value in terms of scope and documentation when addressing overall material flows and key parameters in a technological context.
Perovskite oxides applications in high temperature oxygen separation, solid oxide fuel cell and membrane reactor: A review Prog. Energy Combust. Sci. (IF 17.382) Pub Date : 2017-04-04 Jaka Sunarso, Siti Salwa Hashim, Na Zhu, Wei Zhou
Perovskite oxides have substantial role in the sustainable energy delivery as reflected by their applicability as oxygen-transporting membranes (OTMs), as electrode/electrolyte components in solid oxide fuel cells (SOFCs), and as OTM-based reactors. These applications represent three major directions that enable the membrane-based oxy-fuel combustion technology, the clean and efficient chemical to electrical energy conversion, and the production of higher value-added chemicals from lower value raw materials. The attractiveness of perovskite oxides arises from the possibility to incorporate different A-site and B-site metal elements into their ABO3-δ lattice to form essentially A1-xA’xB1-yB’yO3-δ compound which allows tailoring of the oxygen non-stoichiometry (and thus the oxygen ionic conductivity), the oxygen reduction reaction activity, and the electronic conductivity to fit a particular application. This paper reviews the basic aspects and progresses in these three directions. The advantages and limitations of perovskites in each application are highlighted and discussed as well as the pertaining aspects.
Potential and challenges for large-scale application of biodiesel in automotive sector Prog. Energy Combust. Sci. (IF 17.382) Pub Date : 2017-04-25 Avinash Kumar Agarwal, Jai Gopal Gupta, Atul Dhar
Biodiesel is receiving serious attention globally as a potential alternative fuel for replacing mineral diesel, partially or fully. In this review paper, most prominent methods of biodiesel production commercially, life-cycle analysis and economic issues related to biodiesel, engine performance, combustion and emission characteristics including particulate, engine compatibility issues and effect of biodiesel usage on engine component wear and lubricating oil are comprehensively discussed. Majority of biodiesel produced globally is via base-catalyzed transesterification process since this is a low temperature and pressure process, having high conversion rates without intermediate steps, and it uses inexpensive materials of construction for the plant. Catalyst types (alkaline, acidic or enzymatic), catalyst concentration, molar ratio of alcohol/oil, reaction temperature, moisture content of reactants, and free fatty acid (FFA) content of oil are the main factors affecting biodiesel (ester) yield from the transesterification process. Substantial reduction in particulate matter (PM), total hydrocarbons (THC) and carbon monoxide (CO) emissions in comparison to mineral diesel, and increased brake specific fuel consumption (BSFC) and oxides of nitrogen (NOX) emissions are reported by most researchers using unmodified compression ignition (CI) engines. This review covers several aspects, which are not covered by previous review articles, such as effect of biodiesel on unregulated emissions, effect of biodiesel on carbon deposits, wear of key engine components, and lubricating oil in long-term endurance studies. It emerges from literature review that even minor blends of biodiesel help control emissions and ease pressure on scarce petroleum resources without sacrificing engine power output, engine performance and fuel economy. This review underscores that future studies should focus on optimization of fuel injection equipment and hardware modifications to develop dedicated biodiesel engines, improve low temperature performance of biodiesel fuelled engines, develop new biodiesel compatible lubricating oil formulations and special materials for engine components before implementing large-scale substitution of mineral diesel by biodiesel globally.
Dealing with fuel contaminants in biogas-fed solid oxide fuel cell (SOFC) and molten carbonate fuel cell (MCFC) plants: Degradation of catalytic and electro-catalytic active surfaces and related gas purification methods Prog. Energy Combust. Sci. (IF 17.382) Pub Date : 2017-05-05 Andrea Lanzini, Hossein Madi, Vitaliano Chiodo, Davide Papurello, Susanna Maisano, Massimo Santarelli, Jan Van herle
Fuel cell and hydrogen technologies are re-gaining momentum in a number of sectors including industrial, tertiary and residential ones. Integrated biogas fuel cell plants in wastewater treatment plants and other bioenergy recovery plants are nowadays on the verge of becoming a clear opportunity for the market entry of high-temperature fuel cells in distributed generation (power production from a few kW to the MW scale). High-temperature fuel cell technologies like molten carbonate fuel cells (MCFCs) and solid oxide fuel cells (SOFCs) are especially fit to operate with carbon fuels due to their (direct or indirect) internal reforming capability. Especially, systems based on SOFC technology show the highest conversion efficiency of gaseous carbon fuels (e.g., natural gas, digester gas, and biomass-derived syngas) into electricity when compared to engines or gas turbines. Also, lower CO2 emissions and ultra-low emissions of atmospheric contaminants (SOX, CO, VOC, especially NOX) are generated per unit of electricity output. Nonetheless, stringent requirements apply regarding fuel purity. The presence of contaminants within the anode fuel stream, even at trace levels (sometimes ppb levels) can reduce the lifetime of key components like the fuel cell stack and reformer. In this work, we review the complex matrix (typology and amount) of different contaminants that is found in different biogas types (anaerobic digestion gas and landfill gas). We analyze the impact of contaminants on the fuel reformer and the SOFC stack to identify the threshold limits of the fuel cell system towards specific contaminants. Finally, technological solutions and related adsorbent materials to remove contaminants in a dedicated clean-up unit upstream of the fuel cell plant are also reviewed.
Boundary layer flashback of non-swirling premixed flames: Mechanisms, fundamental research, and recent advances Prog. Energy Combust. Sci. (IF 17.382) Pub Date : 2017-05-18 Alireza Kalantari, Vincent McDonell
Boundary layer flashback in premixed jet flames has been the subject of detailed experimental and numerical investigation since the 1940′s. The traditional approach for characterizing flashback has involved the critical velocity gradient concept, with higher values indicating a higher flashback propensity for a given situation. Recent studies in confined configurations have illustrated that a key assumption underlying the critical velocity gradient concept, namely a lack of interaction between the flame and the approaching flow, is fundamentally incorrect. However, for unconfined configurations, where this interaction is much less important, the critical velocity gradient concept is able to partially capture flashback characteristics. Historically, the critical velocity gradient concept predicts trends of flashback behavior in laminar configurations for a wide range of temperatures, pressures, and fuel compositions more consistently than in turbulent configurations. This is due in part to the fact that many laminar studies establish well behaved velocity conditions in the tube conveying the premixed reactants to the reaction zone. Yet many important practical systems are in the turbulent regime and cannot be approximated by a simplified analysis. Studies to date in either regime, while numerous, generally do not provide a comprehensive methodology for accounting for all parameters. Recent work has attempted to capture the effect of a large number of these parameters in the turbulent regime, with some emphasis on providing design tools that can be used to estimate flashback propensity in more general terms. These approaches have demonstrated reasonable performance for the limited data available at elevated temperature and pressure which are representative of important practical system such as lean premixed combustors for gas turbines. While progress has been made in the last few years relative to predicting flashback for practical systems with high Reynolds numbers, only limited data are available for developing and validating correlations. Open questions remain in terms of using detailed numerical simulations and complex reaction chemistry to predict flashback for unconfined flames. In addition, flame-wall interaction in terms of heat transfer, sensitivity to turbulence levels, the role of general velocity gradients (vs idealized fully developed flow), and the role of high pressure must be further evaluated.
Metal-based nanoenergetic materials: Synthesis, properties, and applications Prog. Energy Combust. Sci. (IF 17.382) Pub Date : 2017-05-18 Dilip Sundaram, Vigor Yang, Richard A. Yetter
Metal particles are attractive candidate fuels for various propulsion and energy-conversion applications, primarily due to their high energy densities. Micron-sized particles present several drawbacks, such as high ignition temperatures and particle agglomeration, resulting in low energy-release rates. Nanoparticles, on the other hand, are quite attractive due to their unique and favorable properties, which are attributed to their high specific surface area and excess energy of surface atoms. As a result, there is a growing interest in employing metal nanoparticles in propulsion and energy-conversion systems. The present work provides a comprehensive review of the advances made over the past few decades in the areas of synthesis, properties, and applications of metal-based energetic nanomaterials. An overview of existing methods to synthesize nanomaterials is first provided. Novel approaches to passivate metal nanoparticles are also discussed. The physicochemical properties of metal nanoparticles are then examined in detail. Low-temperature oxidation processes, and ignition and combustion of metal nanoparticles are investigated. The burning behaviors of different energetic material formulations with metal nanoparticles such as particle-laden dust clouds, solid propellants, liquid fuels and propellants, thermite materials, and inter-metallic systems are reviewed. Finally, deficiencies and uncertainties in our understanding of the field are identified, and directions for future work are suggested.
Knocking combustion in spark-ignition engines Prog. Energy Combust. Sci. (IF 17.382) Pub Date : 2017-05-03 Zhi Wang, Hui Liu, Rolf D Reitz
Knocking combustion research is crucially important because it determines engine durability, fuel consumption, and power density, as well as noise and emission performance. Current spark ignition (SI) engines suffer from both conventional knock and super-knock. Conventional knock limits raising the compression ratio to improve thermal efficiency due to end-gas auto-ignition, while super-knock limits the desired boost to improve the power density of modern gasoline engines due to detonation. Conventional combustion has been widely studied for many years. Although the basic characteristics are clear, the correlation between the knock index and fuel chemistry, pressure oscillations and heat transfer, and auto-ignition front propagation, are still in early stages of understanding. Super-knock combustion in highly boosted spark ignition engines with random pre-ignition events has been intensively studied in the past decade in both academia and industry. These works have mainly focused on the relationship between pre-ignition and super-knock, source analyses of pre-ignition, and the effects of oil/fuel properties on super-knock. The mechanism of super-knock has been recently revealed in rapid compression machines (RCM) under engine-like conditions. It was found that detonation can occur in modern internal combustion engines under high energy density conditions. Thermodynamic conditions and shock waves influence the combustion wave and detonation initiation modes. Three combustion wave modes in the end gas have been visualized as deflagration, sequential auto-ignition and detonation. The most frequently observed detonation initiation mode is shock wave reflection-induced detonation (SWRID). Compared to the effect of shock compression and negative temperature coefficient (NTC) combustion on ignition delay, shock wave reflection is the main cause of near-wall auto-ignition/detonation. Finally, suppression methods for conventional knock and super-knock in SI engines are reviewed, including use of exhaust gas recirculation (EGR), the injection strategy, and the integration of a high tumble - high EGR-Atkinson/Miller cycle. This paper provides deep insights into the processes occurring during knocking combustion in spark ignition engines. Furthermore, knock control strategies and combustion wave modes are summarized, and future research directions, such as turbulence-shock-reaction interaction theory, detonation suppression and utilization, and super-knock solutions, are also discussed.
Progress in biofuel production from gasification Prog. Energy Combust. Sci. (IF 17.382) Pub Date : 2017-05-05 Vineet Singh Sikarwar, Ming Zhao, Paul S. Fennell, Nilay Shah, Edward J. Anthony
Biofuels from biomass gasification are reviewed here, and demonstrated to be an attractive option. Recent progress in gasification techniques and key generation pathways for biofuels production, process design and integration and socio-environmental impacts of biofuel generation are discussed, with the goal of investigating gasification-to-biofuels’ credentials as a sustainable and eco-friendly technology. The synthesis of important biofuels such as bio-methanol, bio-ethanol and higher alcohols, bio-dimethyl ether, Fischer Tropsch fuels, bio-methane, bio-hydrogen and algae-based fuels is reviewed, together with recent technologies, catalysts and reactors. Significant thermodynamic studies for each biofuel are also examined. Syngas cleaning is demonstrated to be a critical issue for biofuel production, and innovative pathways such as those employed by Choren Industrietechnik, Germany, and BioMCN, the Netherlands, are shown to allow efficient methanol generation. The conversion of syngas to FT transportation fuels such as gasoline and diesel over Co or Fe catalysts is reviewed and demonstrated to be a promising option for the future of biofuels. Bio-methane has emerged as a lucrative alternative for conventional transportation fuel with all the advantages of natural gas including a dense distribution, trade and supply network. Routes to produce H2 are discussed, though critical issues such as storage, expensive production routes with low efficiencies remain. Algae-based fuels are in the research and development stage, but are shown to have immense potential to become commercially important because of their capability to fix large amounts of CO2, to rapidly grow in many environments and versatile end uses. However, suitable process configurations resulting in optimal plant designs are crucial, so detailed process integration is a powerful tool to optimize current and develop new processes. LCA and ethical issues are also discussed in brief. It is clear that the use of food crops, as opposed to food wastes represents an area fraught with challenges, which must be resolved on a case by case basis.
Advances and challenges in modeling high-speed turbulent combustion in propulsion systems Prog. Energy Combust. Sci. (IF 17.382) Pub Date : 2017-01-06 Esteban D. Gonzalez-Juez, Alan R. Kerstein, R. Ranjan, S. Menon
Combustion environments in propulsion systems involve the interaction of a variety of physics. In devices such as augmentors, ramjets and scramjets, such environments include the interaction between combustion, high-intensity turbulence, and/or strong flow compressions and expansions, physics which are termed here high-speed combustion. With this motivation in mind, this paper addresses: What are the problems encountered when modeling these interactions, or in other words, what are the problems of turbulent-combustion modeling? Do such interactions need modeling? What are the challenges when going from modeling low-speed- to high-speed-combustion problems? This work addresses these questions by summarizing several modeling studies of gaseous high-speed-combustion problems, and attempts to interpret some predictions in the context of the models’ basic assumptions. Interestingly, the challenges to model high-speed combustion are such that a reader not interested in this topic but in the general one of modeling turbulent combustion may find the present paper useful.
Infrared laser-absorption sensing for combustion gases Prog. Energy Combust. Sci. (IF 17.382) Pub Date : 2017-03-04 Christopher S. Goldenstein, R.Mitchell Spearrin, Jay. B. Jeffries, Ronald K. Hanson
Infrared laser-absorption spectroscopy (IR-LAS) sensors play an important role in diagnosing and characterizing a wide range of combustion systems. Of all the laser-diagnostic techniques, LAS is arguably the most versatile and quantitative, as it has been used extensively to provide quantitative, species-specific measurements of gas temperature, pressure, composition and velocity in both laboratory- and industrial-scale systems. Historically, most IR-LAS work has been conducted using tunable diode lasers; however, today’s researchers have access to a wide range of light sources that provide unique sensing capabilities and convenient access to nearly the entire IR spectrum (≈ 0.8 to 16 µm). In particular, the advent of room-temperature wavelength-tunable mid-infrared semiconductor lasers (e.g., interband- and quantum-cascade lasers) and hyperspectral light sources (e.g., MEMS VCSELs, Fourier-domain mode-locked lasers, dispersed supercontinuum, and frequency combs) has provided a number of unique capabilities that combustion researchers have exploited. The primary goals of this review paper are: (1) to document the recent development, application, and current capabilities of IR-LAS sensors for laboratory- and industrial-scale combustors and propulsion systems, (2) to elucidate the design and use of IR-LAS sensors for combustion gases through a discussion of the modern sensor-design process and state-of-the-art techniques, and (3) to highlight some of the remaining measurement opportunities, challenges, and needs. A thorough review and description of the fundamental spectroscopy governing the accuracy of such sensors, and recent findings and databases that enable improved modeling of molecular absorption spectra will also be provided.
Impact of fuel molecular structure on auto-ignition behavior – Design rules for future high performance gasolines Prog. Energy Combust. Sci. (IF 17.382) Pub Date : 2016-12-29 Michael D. Boot, Miao Tian, Emiel J.M. Hensen, S. Mani Sarathy
At a first glance, ethanol, toluene and methyl tert-butyl ether look nothing alike with respect to their molecular structures. Nevertheless, all share a similarly high octane number. A comprehensive review of the inner workings of such octane boosters has been long overdue, particularly at a time when feedstocks for transport fuels other than crude oil, such as natural gas and biomass, are enjoying a rapidly growing market share. As high octane fuels sell at a considerable premium over gasoline, diesel and jet fuel, new entrants into the refining business should take note and gear their processes towards knock resistant compounds if they are to maximize their respective bottom lines. Starting from crude oil, the route towards this goal is well established. Starting from biomass or natural gas, however, it is less clear what dots on the horizon to aim for. The goal of this paper is to offer insight into the chemistry behind octane boosters and to subsequently distill from this knowledge, taking into account recent advances in engine technology, multiple generic design rules that guarantee good anti-knock performance. Careful analysis of the literature suggests that highly unsaturated (cyclic) compounds are the preferred octane boosters for modern spark-ignition engines. Additional side chains of any variety will dilute this strong performance. Multi-branched paraffins come in distant second place, owing to their negligible sensitivity. Depending on the type and location of functional oxygen groups, oxygenates can have a beneficial, neutral or detrimental impact on anti-knock quality.
Removal of non-CO2 greenhouse gases by large-scale atmospheric solar photocatalysis Prog. Energy Combust. Sci. (IF 17.382) Pub Date : 2017-01-24 Renaud de_Richter, Tingzhen Ming, Philip Davies, Wei Liu, Sylvain Caillol
Fuel consumption and CO2 emissions from passenger cars in Europe – Laboratory versus real-world emissions Prog. Energy Combust. Sci. (IF 17.382) Pub Date : 2017-02-21 Georgios Fontaras, Nikiforos-Georgios Zacharof, Biagio Ciuffo
Tomographic absorption spectroscopy for the study of gas dynamics and reactive flows Prog. Energy Combust. Sci. (IF 17.382) Pub Date : 2016-11-23 Weiwei Cai, Clemens F. Kaminski
Optical imaging techniques are ubiquitous for the resolution of non-uniformities in gas flows. Planar imaging techniques such as laser-induced fluorescence are well established and applied extensively in turbulent reactive flows, offering both high temporal and spatial resolutions. However, planar imaging suffers from a critical disadvantage, the requirement for spatially continuous optical access over large solid angles in both the excitation and detection paths and this precludes their application in many practical situations, for example those encountered in engine testing. Tomographic absorption spectroscopy, TAS, on the other hand, shares many of the advantages of planar imaging techniques but reduces the demands for optical access, because high quality data can be obtained with sparsely sampled volumes. The technique has unrivalled potential for imaging in harsh environments, for example for in-cylinder/in-chamber engine measurements. TAS is beginning to mature as a technique for the simultaneous imaging of temperature and species concentration, and is experiencing a surge of interest due to progress in laser technology, spectroscopy, and theoretical developments of nonlinear tomography techniques. The recent advancements in broad bandwidth, frequency-agile laser sources massively enrich the spectral information obtainable in TAS. Furthermore, nonlinear tomography enables the recovery of multiplexed information from a single tomographic inversion. The utilization of multispectral information improves the immunity of TAS to experimental noise and makes possible the simultaneous imaging of temperature, pressure, and multiple species. Nonlinear tomography can also be used to empower the imaging potential of sensitive and robust absorption techniques, such as wavelength modulation spectroscopy, for use in harsh and even optically dense environments. In combination, this greatly extends the applicability of TAS for more general and harsh scenarios in combustion technology. In this article we review basic concepts and mathematical foundations of classical absorption tomography, proceeding to more advanced recent concepts based on nonlinear tomography, and providing an extensive review of experimental demonstrations and practical applications in the context of state-of-the-art combustion research.
Impacts of additives on performance and emission characteristics of diesel engines during steady state operation Prog. Energy Combust. Sci. (IF 17.382) Pub Date : 2016-12-08 Esmail Khalife, Meisam Tabatabaei, Ayhan Demirbas, Mortaza Aghbashlo
Depletion of fossil fuel resources and stringent emission mandates has spurred the search for improved diesel engines performance and cleaner combustion. One of the best approaches to solve these issues is to use biodiesel/diesel additives. The effects of biodiesel/diesel additives on the performance and emissions of diesel engines were comprehensively reviewed throughout this article. The additives reviewed herein were classified into five categories, i.e., oxygenated additives, metallic and non-metallic based additives, water, antioxidants, and polymeric-based additives. The effects of each category on the engine performance (i.e., brake specific fuel consumption (bsfc) and brake thermal efficiency (bte)) and emissions (i.e., CO, NOx, HC, and PM) were exclusively summarized and discussed. Furthermore, various strategies used for adding water like water-diesel emulsion, direct water injection, and adding water into the inlet manifold were illustrated and their pros and cons were completely scrutinized. Finally, opportunities and limitations of each additive considering both engine performance and combustion benignity were outlined to guide future research and development in the domain.
Progress in dynamic simulation of thermal power plants Prog. Energy Combust. Sci. (IF 17.382) Pub Date : 2016-12-15 Falah Alobaid, Nicolas Mertens, Ralf Starkloff, Thomas Lanz, Christian Heinze, Bernd Epple
While the conventional design of thermal power plants is mainly focused on high process efficiency, market requirements increasingly target operating flexibility due to the continuing shift towards renewables. Dynamic simulation is a cost-efficient tool for improving the flexibility of dispatchable power generation in transient operation such as load changes and start-up procedures. Specific applications include the optimisation of control structures, stress assessment for critical components and plant safety analysis in malfunction cases. This work is a comprehensive review of dynamic simulation, its development and application to various thermal power plants. The required mathematical models and various components for description the basic process, automation and electrical systems of thermal power plants are explained with the support of practical example models. The underlying flow models and their fundamental assumptions are discussed, complemented by an overview of commonly used simulation codes. Relevant studies are summarised and placed in context for different thermal power plant technologies: combined-cycle power, coal-fired power, nuclear power, concentrated solar power, geothermal power, municipal waste incineration and thermal desalination. Particular attention is given to those studies that include measurement validation in order to analyse the influence of model simplifications on simulation results. In conclusion, the study highlights current research efforts and future development potential of dynamic simulation in the field of thermal power generation.
Biodiesel fuels Prog. Energy Combust. Sci. (IF 17.382) Pub Date : 2016-11-09 Gerhard Knothe, Luis F. Razon
The mono-alkyl esters, most commonly the methyl esters, of vegetable oils, animal fats or other materials consisting mainly of triacylglycerols, often referred to as biodiesel, are an alternative to conventional petrodiesel for use in compression-ignition engines. The fatty acid esters that thus comprise biodiesel largely determine many important fuel properties. In turn, the composition of the biodiesel depends on the composition of the parent feedstock because feedstocks with widely varying fatty acid composition can be used for biodiesel production. The use of different feedstocks is also significant under aspects of increasing biodiesel supply and socio-economic issues. In this article, biodiesel production is briefly described, followed by a discussion of biodiesel fuel properties and the influence of varying fatty acid profiles and feedstocks. It is shown that the properties of biodiesel least influenced by minor components can be determined by a straightforward equation in which the properties of the biodiesel fuel are calculated from the amounts of the individual component fatty esters and their properties. Optimizing biodiesel composition is also addressed.
Electrocatalysts for the generation of hydrogen, oxygen and synthesis gas Prog. Energy Combust. Sci. (IF 17.382) Pub Date : 2016-09-22 Foteini M. Sapountzi, Jose M. Gracia, C.J. (Kees-Jan) Weststrate, Hans O.A. Fredriksson, J.W. (Hans) Niemantsverdriet
Water electrolysis is the most promising method for efficient production of high purity hydrogen (and oxygen), while the required power input for the electrolysis process can be provided by renewable sources (e.g. solar or wind). The thus produced hydrogen can be used either directly as a fuel or as a reducing agent in chemical processes, such as in Fischer–Tropsch synthesis. Water splitting can be realized both at low temperatures (typically below 100 °C) and at high temperatures (steam water electrolysis at 500–1000 °C), while different ionic agents can be electrochemically transferred during the electrolysis process (OH−, H+, O2−). Singular requirements apply in each of the electrolysis technologies (alkaline, polymer electrolyte membrane and solid oxide electrolysis) for ensuring high electrocatalytic activity and long-term stability. The aim of the present article is to provide a brief overview on the effect of the nature and structure of the catalyst–electrode materials on the electrolyzer's performance. Past findings and recent progress in the development of efficient anode and cathode materials appropriate for large-scale water electrolysis are presented. The current trends, limitations and perspectives for future developments are summarized for the diverse electrolysis technologies of water splitting, while the case of CO2/H2O co-electrolysis (for synthesis gas production) is also discussed.
Liquid jet in a subsonic gaseous crossflow: Recent progress and remaining challenges Prog. Energy Combust. Sci. (IF 17.382) Pub Date : 2016-10-03 M. Broumand, M. Birouk
This article reviews published literature on the characteristics of a liquid jet injected transversally into a subsonic gaseous crossflow. The review covers the following aspects: (і) liquid jet primary breakup regimes, (іі) liquid jet trajectory and penetration, (ііі) liquid jet breakup length, and (іv) droplets features and formation mechanisms. The focus is on analyzing the role of different prominent parameters which include gaseous and liquid properties, and liquid injector geometry. The review revealed that gas Weber number plays a crucial role in defining non-turbulent primary breakup regimes, while liquid jet Weber number is of great importance for the transition to turbulent primary breakup. Jet-to-crossflow momentum flux ratio is the most important parameter for predicting the trajectory, penetration, and breakup length of a liquid jet in a crossflow. The characteristics of droplets disintegrated during the primary breakup are mostly influenced by the nozzle exit conditions, whereas the characteristics of droplets produced via the secondary breakup are strongly dependent on the velocity of cross airflow. Although the review revealed that substantial progress has been made in understanding this complex two-phase flow phenomenon, there still remain several shortcomings which require further research.
Some contents have been Reproduced by permission of The Royal Society of Chemistry.
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