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  • 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.

  • 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.

  • 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.

  • 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: ; and ii: cathode ). 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.

  • 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.

  • 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.

  • State-of-the-art in premixed combustion modeling using flamelet generated manifolds
    Prog. Energy Combust. Sci. (IF 17.382) Pub Date : 2016-10-13
    J.A. van Oijen, A. Donini, R.J.M. Bastiaans, J.H.M. ten Thije Boonkkamp, L.P.H. de Goey

    Flamelet based chemical reduction techniques are very promising methods for efficient and accurate modeling of premixed flames. Over the years the Flamelet Generated Manifold (FGM) technique has been developed by the Combustion Technology Group of Eindhoven University of Technology. Current state-of-the-art of FGM for the modeling of premixed and partially-premixed flames is reviewed. The fundamental basis of FGM consists of a generalized description of the flame front in a (possibly moving) flame-adapted coordinate system. The basic nature of the generalized flamelet model is that effects of strong stretch in turbulent flames are taken into account by resolving the detailed structure of flame stretch and curvature inside the flame front. The generalized flamelet model, which forms the basis on which FGM is built, is derived in Part I. To be able to validate numerical results of flames obtained with full chemistry and obtained from FGM, it is important that the generalized flamelet model is analyzed further. This is done by investigating the impact of strong stretch, curvature and preferential diffusion effects on the flame dynamics as described by the local mass burning rate. This so-called strong stretch theory is derived and analyzed in Part I, as well as multiple simplifications of it, to compare the strong stretch theory with existing stretch theories. The results compare well with numerical results for flames with thin reaction layers, but described by multiple-species transport and chemistry. This opens the way to use the generalized flamelet model as a firm basis for applying FGM in strongly stretched laminar and turbulent flames in Part II. The complete FGM model is derived first and the use of FGM in practice is reviewed. The FGM model is then validated by studying effects of flame stretch, heat loss, and changes in elements, as well as NO formation. The application to direct numerical simulations of turbulent flames is subsequently studied and validated using the strong stretch theory. It is shown that the generalized flamelet model still holds even in case of strong stretch and curvature effects, at least as long as the reaction layer is dominated by reaction and diffusion phenomena and not perturbed too much by stretch related perturbations. The FGM model then still performs very well with a low number of control variables. Turbulent flames with strong preferential diffusion effects can also be modeled efficiently with an FGM model using a single additional control variable for the changes in element mass fractions and enthalpy. Finally FGM is applied to the modeling of turbulent flames using LES and RANS flow solvers. For these cases, the flame front structure is not resolved anymore and unresolved terms need to be modeled. A common approach to include unresolved turbulent fluctuations is the presumed probability density function (PDF) approach. The validity of this FGM-PDF approach is discussed for a few test cases with increasing level of complexity.

  • Catalytic effects of nano additives on decomposition and combustion of RDX-, HMX-, and AP-based energetic compositions**
    Prog. Energy Combust. Sci. (IF 17.382) Pub Date : 2016-10-14
    Qi-Long Yan, Feng-Qi Zhao, Kenneth K. Kuo, Xiao-Hong Zhang, Svatopluk Zeman, Luigi T. DeLuca

    The RDX, HMX, and AP are currently the most widely used energetic ingredients in composite solid propellants, since the newly developed energetic compounds are still unable to replace them due to various bottleneck technical problems. In order to improve their combustion efficiency and performance, a common alternative way is to utilize novel nano-sized energetic additives. There are a great many nanomaterials that have been developed in the past decades, which include nanometal particles, metal oxides, metal salts, metallic composites, organometallic compounds, energetic nanocatalysts, and carbon nanomaterials. These additives could increase both the decomposition and the burning rate as well as enhance the combustion efficiency of the corresponding solid propellants by changing the thermal conductivity, energy barrier of thermolysis, heat of reaction, and gas-phase reaction mechanisms of the main ingredients such as RDX, HMX, and AP. This review paper discusses and summarizes the effects of abovementioned nano additives on decomposition kinetics, reaction models, decomposition mechanisms and burning rates, pressure exponents, combustion wave structures, and flame propagation of RDX-, HMX-, and AP-based energetic compositions. The catalytic mechanisms associated with different types of nanomaterials are explained and clarified. Owing to their extremely large specific surface areas, nano-sized energetic additives have significant catalytic effects in both condensed and gas phases during decomposition and subsequent combustion via activation of the reactants and acceleration of their transition state formations. The flame structures of AP-based composite propellants under the effect of some nanoadditives are presented showing the enhanced burning characteristics and stabilized combustion process.

  • Heat and fluid flow in high-power LED packaging and applications
    Prog. Energy Combust. Sci. (IF 17.382) Pub Date : 2016-06-07
    Xiaobing Luo, Run Hu, Sheng Liu, Kai Wang

    Light-emitting diodes (LEDs) are widely used in our daily lives. Both light and heat are generated from LED chips and then transmitted or conducted through multiple packaging materials and interfaces. Part of the transmitted light converts into heat along the light propagation; in return, the accumulation of heat leads to the degradation of light output. The accumulated heat negatively influences the reliability and longevity of LEDs, and thus thermal management is critical for LED packaging and applications. On the other hand, in LED packaging processes, many fluid flow problems exist, such as phosphor coating, silicone injection, chip bonding, solder reflow, etc. Among them, phosphor coating is the most important process which is essential for LED performance. Phosphor gel is a kind of non-Newton fluid and its coating process is a typical fluid-flow problem. Overall, since LED packaging and applications present many heat and fluid flow problems, obtaining a full understanding of these problems enables advancements in the development of LED processes and designs. In this review, the emphasis is placed on heat generation in chips, heat flow in packages and application products, fluid flow in phosphor coating process, etc. This is a domain in which significant progress has been achieved in the last decade, and reporting on these advances will facilitate state-of-the-art LED packaging and application technologies.

  • Methane emissions as energy reservoir: Context, scope, causes and mitigation strategies
    Prog. Energy Combust. Sci. (IF 17.382) Pub Date : 2016-06-22
    Xiaoli Chai, David J. Tonjes, Devinder Mahajan

    Methane (CH4) is now considered a bridge fuel between present fossil (carbon) economy and desired renewables and this energy molecule is projected to play an important role in the global energy mix well beyond 2035. The atmospheric warming potential of CH4 is 28–36 times, when averaged over a 100-year period, that of carbon dioxide (CO2) and this necessitates a close scrutiny of global CH4 emissions inventory. As the second most abundant greenhouse gas (GHG), the annual global CH4 emissions were 645 million metric tons (MMT), accounting for 14.3% of the global anthropogenic GHG emissions. Of this, five key anthropogenic sources: agriculture, coal, landfills, oil and gas operations and wastewater together emitted 68% of all CH4 emissions. Landfills are ranked as the third highest anthropogenic CH4 emission source, behind agriculture and coal mines, and emissions from the waste sector are expected to reach almost 800 million metric tons CO2 equivalent (MMTCO2e) in 2015. The two largest economies spewed out 42% (14% (US) and 28% (China)) of the world's total greenhouse gas (GHG) emissions; these two countries are also the largest producers of municipal solid waste (MSW). The United States averages 250 MMT of MSW annually, of which about 63% enters landfills. In 2015, there were 2434 landfills in the United States and CH4 from these landfills accounted for 138 MMTCO2e released into the atmosphere and represents 17.7% of all US CH4 emissions. China had 580 landfills and treated 105 MMT of MSW in 2013. Methane produced from landfills contributes about 13% of total CH4 emissions in China. Almost 50% of landfills in China did not install efficient LFG collection and utilization systems to make them manageable so a great deal of CH4 and CO2 are emitted without intervention. Recent data show that globally, 45 billion cubic meters (bcm) of CH4 or 282 million barrels of oil equivalent (boe) was annually released from landfills into the atmosphere. Managing methane emission from landfills is a global challenge, though China lags behind in managed landfills that contribute to adverse health effects on the population. Moreover, the rich organic content of MSW in China indicates that CH4 emissions there may be underestimated. The China unmanaged landfill scenario is further duplicated in developing as well as in least-developed countries. This review starts with a dialog on CH4 emissions and climate change and the chemical changes the CH4 molecule undergoes in the atmosphere (Section 1). Section 2 deals with identification of global CH4 emissions from key sources, particularly anthropogenic, among those are agriculture, coal mines, landfills, oil and gas operations and wastewater. Although each of these sources is descriptive on their own, the focus of Section 3 is on landfills with particular emphasis on the United States and China, two largest producers of waste. The quantitative measurement of CH4 emissions is still uncertain so Section 4 is devoted to various CH4 estimation models, such as United States Environmental Protection Agency (US EPA) LandGEM, the United Nations Intergovernmental Panel on Climate Change (IPCC) and others that are under development. The key landfill emissions data bases and the collection methodologies such as those used in the United States and recently released by the Chinese government are highlighted. Section 5 describes chemistry of pathways that produce CH4 from landfills, and how landfills can control those emissions. Section 6 reviews potential of CH4 as an energy source for combined heat and power (CHP) production as well as pathways for conversion of CH4 into renewable gaseous fuel for use as compressed natural gas (CNG) and clean liquids that could be used as either drop-in replacement (gasoline, diesel, jet fuel hydrocarbons) or advanced oxygenated fuels such as methanol, a versatile precursor to fuels and chemicals, and dimethylether (DME), a clean diesel substitute. Section 7 describes in-place government policies to deal with CH4 emissions from specific sectors. These policies vary from country to country but the Unites States and the European Union (EU) countries are well ahead in curbing methane emissions while China is now playing close attention to its increasing global share of emissions. The last section (Section 8) identifies science and technology and needed policy challenges to manage fugitive methane; this includes identification of technological intervention that China and other countries would need to capitalize on this wasted resource by efficiently harvesting this energy source, needed government policies and science and technology issues that researchers have to deal with to help combat climate change. The overall review provides a comprehensive description that could lead a coherent picture to harvest global CH4 emissions for useful energy, a sensible solution. In 2014, a milestone was reached in US and China relations when the White House announced that the United States intends to achieve an economy-wide target of reducing its emissions by 26%–28% below its 2005 level in 2025 while China intends to achieve the peaking of CO2 emissions around 2030 and intends to increase the share of non-fossil fuels in primary energy consumption to around 20% by 2030. In another 2014 initiative, the United States also identified fugitive methane from oil and gas operations, agriculture, and landfills to maintain respective post-2020 actions on climate change, recognizing that these actions are part of the longer term efforts to transition to low-carbon economies, mindful of containing the global temperature increase goal of 2 °C, also known as two-degree scenario (2DS). These commitments by the United States and China were evident in the successful agreement at the culmination of the recently concluded COP21 event in Paris. This review is written to start a dialog among researchers that tetrahedral CH4, the simplest among all organic compounds, plays such a complex role in climate change that as its use increases, it will rival carbon dioxide (CO2) in GHG effect in the coming decades if no attempt is made to contain its emissions.

  • Efficient methanol synthesis: Perspectives, technologies and optimization strategies
    Prog. Energy Combust. Sci. (IF 17.382) Pub Date : 2016-07-05
    Giulia Bozzano, Flavio Manenti

    In economy nowadays, methanol is already a key compound widely employed as building block for producing intermediates or synthetic hydrocarbons, solvent, energy storage medium, and fuel. This status is expected to last in the near future or even improve to the point of making this compound a central participant in the worldwide economic landscape. For these reasons, every improvement to its production process, in terms of energy savings, optimization, etc., has potential to promote relevant economic benefits. Methanol production comprises three main steps: preparation of syngas, methanol synthesis and downstream separation. This paper aims at reviewing technologies and procedures for modeling and optimizing the second aforementioned phase: the synthesis reactor. Specifically, we focus on packed-bed units, which represent the most widespread technology. In the manuscript, we are going to describe and compare both steady-state and dynamic reactor models as well as analyze typical assumptions and implementation schemes. The kinetics of methanol synthesis is also reported in detail due to a long debate, present in the literature, concerning the real carbon source for methanol, the nature of the active sites and the effect of their morphology and oxidation state.

  • Exergy analysis of solar thermal collectors and processes
    Prog. Energy Combust. Sci. (IF 17.382) Pub Date : 2016-07-18
    Soteris A. Kalogirou, Sotiriοs Karellas, Konstantinos Braimakis, Camelia Stanciu, Viorel Badescu

    This paper presents a review of exergy analysis of solar thermal collectors and processes. It includes not only various types of solar collectors, but also various applications of solar thermal systems. Initially the fundamentals of second law analysis are briefly presented as well as the exergy of solar radiation, which is the input to any solar system. Concentrating and non-concentrating collectors have been analyzed, including parabolic dish and parabolic trough collectors from the first category, and flat-plate collectors, air solar heaters, and evacuated tube collectors from the second category. Hybrid photovoltaic/ thermal collectors have also been examined. Applications and processes include the use of phase change materials either in the collection or storage of thermal energy, drying, heating, multigeneration, trigeneration, solar cooling, solar assisted heat pumps, domestic cogeneration, hydrogen production, hybridization with other renewables, solar ponds, power plants and desalination/distillation. Through literature review on the above subjects it is shown that exergy analysis, which gives a representative performance evaluation, is emphasized as a valuable method to evaluate and compare possible configurations of these systems.

  • 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.

  • Flame aerosol synthesis of nanostructured materials and functional devices: Processing, modeling, and diagnostics
    Prog. Energy Combust. Sci. (IF 17.382) Pub Date : 2016-05-18
    Shuiqing Li, Yihua Ren, Pratim Biswas, Stephen D. Tse

    Manufacturing of nanostructured materials and functional devices offers many exciting opportunities for substantial contribution in renewable energy utilization, environmental compliance, and product development. In the past two decades, gas-phase flame synthesis has not only proved to be one of the most scalable and economical technologies for producing well-controlled nanostructured materials, including single metal-oxide, mixed-oxide nanocomposite, and carbon nanostructures, but also has been recognized as robust fabrication method of nano-devices. In this paper, we focus our review mainly on the recent trends in specific applications of flame aerosol synthesis in the last decade, e.g., (a) usage of a substrate in stagnation geometry with controlled particle temperature–time history, (b) application of external fields to control particle characteristics, (c) development of advanced spray technique for doping synthesis of nanocomposites of multicomponent metal oxides or carbon–metal oxides, and (d) fabrication of nanomaterial-based functional devices. For the possibility to improve the design and operation of flame aerosol reactors, we summarize recent advances in: (i) in situ optical diagnostics for either gas phase or particle phase in flame field; (ii) multi-scale modeling and simulation employing gas-phase chemistry, population balance method, molecular dynamics and nanoscale particle dynamics.

  • Progress in the direct catalytic conversion of methane to fuels and chemicals
    Prog. Energy Combust. Sci. (IF 17.382) Pub Date : 2016-05-16
    Canan Karakaya, Robert J. Kee

    This paper reviews the state-of-the-art in catalytic processes to convert methane (a major component of natural gas) to more valuable hydrocarbons as fuels or chemicals. The scope is restricted to “direct” conversion, meaning that processes involving synthesis gas as an intermediate are not considered. Oxygenated products (e.g., alcohols) are also not considered. In all cases, the processes are concerned with catalytic dehydrogenation. The two most widely studied processes are oxidative coupling of methane (OCM) and methane dehydroaromatization (MDA). After reviewing the relevant catalysis literature, the paper goes on to review reactor implementations. Hydrogen- and/or oxygen-permeable membranes can potentially play valuable roles in improving methane conversion and product yields. Despite over 30 years of research, there are still no direct-conversion processes that can compete commercially with methane reforming followed by processes such as Fischer–Tropsch synthesis. Thus, the future practical development and deployment of OCM and MDA will rely on the research and development of advanced catalysts and innovative processes. The present review helps to document the foundation on which the needed development can build.

  • Efficient valorization of biomass to biofuels with bifunctional solid catalytic materials
    Prog. Energy Combust. Sci. (IF 17.382) Pub Date : 2016-05-17
    Hu Li, Zhen Fang, Richard L. Smith, Song Yang
  • Comprehensive review of methane conversion in solid oxide fuel cells: Prospects for efficient electricity generation from natural gas
    Prog. Energy Combust. Sci. (IF 17.382) Pub Date : 2015-12-17
    Turgut M. Gür

    Natural gas is an important energy resource for electric power generation and other energy needs. Recent discoveries of vast reserves of shale gas greatly increased its abundance while lowering its cost. Combined with its significantly smaller carbon-footprint than coal, natural gas has increasingly become the preferred choice to generate electrical power even at the expense of converting existing coal fired power plants to run on natural gas. However, most natural gas combustion-based power plants currently operate at efficiencies in the low 30%. Conversion of natural gas in solid oxide fuel cells (SOFC) promises to increase system level conversion efficiencies to above 60%, doubling the current efficiencies and significantly reducing the CO2 emissions by a factor of 2. Such dramatic improvements in conversion efficiency and ease of CO2 capture are currently out of reach for the combustion-based power generation technologies. Equally importantly, the CO2 produced from methane conversion leaves the fuel cell in a highly concentrated form. As nitrogen is blocked off by the impervious ceramic electrolyte of the SOFC from entering the process stream, methane oxidation produces a flue stream that is primarily made of the oxidation products CO2 and steam. The latter can easily be condensed out to capture CO2, thus eliminating the need for expensive and energy intensive post separation operations otherwise required to separate CO2 from N2 for storage purposes. So if successfully developed and deployed widely, natural gas conversion in SOFCs will greatly reduce CO2 emissions, help mitigate climate change, and minimize the environmental impact of power generation. This article organizes and critically reviews the current state of understanding in methane catalysis and oxidation with particular emphasis for electrochemical conversion in SOFCs. It presents a comprehensive review of a vast volume of published work (>600 references) extending from fundamental studies in C single bond H bond activation and methane catalysis to basic concepts of electrochemical conversion in fuel cells, to strategies for effective utilization of methane in SOFCs, and associated technical difficulties such as deactivation due to sulfur and carbon yet to be overcome for realizing natural gas conversion, to impactful opportunities provided by recent theoretical advances in computational catalysis and materials screening studies, and innovative concepts such as strain effects and nanostructuring toward enhancing catalytic rates. It provides tutorial-type information at the appropriate level for the uninitiated but interested reader as well as critical discussions of fundamental phenomena and assessment of recent advances for active researchers in the field. The article is weighted around materials and surface properties and provides an in-depth review with emphasis on electronic structure, charge transport and catalysis. It presents an impartial evaluation of the opportunities as well as the challenges to natural gas utilization in SOFCs. Finally, it concludes that natural gas conversion in SOFCs promises very attractive opportunities for efficient and environmentally friendly power generation, while recognizing and offering in-depth discussions of the challenges facing this technology before it can be considered for large-scale power generation applications.

  • Advances in sulfur chemistry for treatment of acid gases
    Prog. Energy Combust. Sci. (IF 17.382) Pub Date : 2015-12-17
    A.K. Gupta, S. Ibrahim, A. Al Shoaibi

    Increased energy demand worldwide has caused faster depletion of sweet feedstock and increased exploitation of sourer hydrocarbon fuels. These fuels often contain acid gases (H2S and CO2), mercaptans and trace amounts of benzene, toluene and xylene (BTX) that are harmful to human health, the environment and industrial equipment. The US EPA has proposed a reduction of sulfur in gasoline from 30 ppm to 10 ppm by 2017. To reach this goal, crude oil and gas must be subjected to more efficient desulfurization processes in which acid gases are major byproducts. The separated acid gases and associated impurities are further processed for material and energy recovery, as the fuels with high sulfur content are restricted due to their harmful effects. In this paper, a comprehensive review of the treatment of acid gases and associated impurities is given along with an advanced Claus process design that can capture much greater amounts of sulfur in the thermal stage to decrease the burden in catalytic stages and reduce operational costs. Claus process technology, although mature and commonly used for the recovery of sulfur and energy from acid gases, has low thermal stage efficiency that further deteriorates with change in acid gas composition. The non-uniformity of acid gas feed streams poses several technical and operational problems, resulting in higher operational costs and increased toxic gas emissions. Sulfur chemistry provides a path for improved understanding of the complex process in the thermal stage of the Claus reactor with a goal to recover both energy and improve the quality of sulfur produced, so that catalytic stages are minimal. The sulfur chemistry and kinetic models of H2S combustion are reviewed. Practical problems emanating from the presence of acid gas impurities (such as CO2, ammonia, light hydrocarbons, aromatics, COS and CS2) during the acid gas conversion process are evaluated. Reactor conditions that mitigate the impact of impurities are also included. An urgent need exists for the development of comprehensive kinetic models that can capture the combustion chemistry of H2S along with the presence of trace quantities of aromatics, ammonia and other impurities during sulfur recovery in Claus reactors. Our current knowledge lacks a detailed chemistry, so that effective capture of acid gas conversion in the Claus thermal stage remains a challenge. Future studies must focus on a systematic coupling of the available kinetic models for neat H2S, hydrocarbon and ammonia fuels, and subsequent validation under partially oxidizing operating conditions in Claus reactors. Such a mechanism could help to improve the efficiency of sulfur recovery processes and sulfur quality for improved design of advanced Claus reactors with enhanced sulfur capture, energy recovery and mitigated environmental issues.

Some contents have been Reproduced with permission of the American Chemical Society.
Some contents have been Reproduced by permission of The Royal Society of Chemistry.