Recent developments on sewage sludge pyrolysis and its kinetics: Resources recovery, thermogravimetric platforms, and innovative prospects
Introduction
The global population is increasing exponentially and the population of cities is expanding rapidly in developing countries at a rate of around 3% per year. Natural resources are stressed and are at risk of depletion in the coming future due to increasing industrial development and standard of living (Chan et al., 2019; Wang et al., 2017a). Fossil fuel consumption continues to increase driven by emerging economies, resulting in further stress on fossil fuels reserves (Al-Ansari, et al., 2019; Loy et al., 2018; a) and generating increasing attention on alternative energy sources (Hill et al., 2006; Rezk et al., 2019; Elkhalifa et al., 2019a). These include the utilization of biomass and domestic waste to recover energy and value-added products (AlNouss, et al., 2019; Ghauri et al., 2011; Abdul Raheem et al., 2018;(Elkhalifa et al., 2019b). These systems and processes have gained increased attention during the previous decades, in which various semi-commercial and commercial-scale production have been demonstrated (Manara & Zabaniotou, 2012; Rulkens, 2007).
Sewage sludge is the by-product of the wastewater treatment process, where various categories of sewage sludge are obtained during wastewater treatment (Buonocore et al., 2018). The quantity and quality of sewage sludge depend on the source of wastewater and its treatment stage during wastewater treatment. Wastewater treatment is an energy and resource-intensive process (Morgano et al., 2018). Traditionally, sewage sludge has been considered a liability, the safe management of which is achieved through landfilling, which endures vast land requirements and operational costs prior to dumping. A paradigm shift in wastewater treatment is underway where wastewater treatment is considered as a resource of energy and other value-added products (Wilk et al., 2019). The utilization of sewage sludge as a renewable energy resource is considered a desirable option as it can provide approximately 10% of the global energy supply (Abdur Raheem et al., 2016). The production of sewage sludge is expected to continue to increase as populations grow, accompanied by the desire for a better quality of life in developing countries (Fytili & Zabaniotou, 2008). Moreover, water sustainability requires constant treatment, reuse, and recycling of wastewater to meet future requirements.
The quantity of produced sewage sludge largely depends on the level and method of treatment applied to wastewater, the population growth, and the volume and characteristics of the wastewater stream. The annual production of sewage sludge in different countries is illustrated in Table 1 (Christodoulou & Stamatelatou, 2015; Eurostat, 2018; Grobelak et al., 2019; LeBlanc et al., 2009). The management of sewage sludge requires a significant amount of resources as a percentage of total wastewater operational costs. The contribution of research conducted on various associated issues is exhibited in Fig. 1(a-b). Zhang et al. (2017) analysed 51 review papers on sludge and revealed that most investigations were on topics related to sludge generation and costs (12%), anaerobic digestion (17%), sludge contaminants (20%), resources recovery (11%) and treatment/pre-treatments (15%) as shown in Fig. 1b. Gherghel et al. (2019) analysed the reported literature for characterization, disposal and resources recovery practices of sewage sludge and technology readiness level (TRL) discussed for circular economy.
Non-lignocellulosic biomass such as sewage sludge is a highly suitable alternative to fossil fuels. It can serve as a source of energy as it can produce both bio-oil, biogas, and biochar. Various methods have been proposed for energy extraction from sewage sludge. Options used for MSW treatment include landfill, anaerobic digestion, agriculture use, incineration, burning in cement kilns, and thermochemical conversion (Huang & Yuan, 2015). Agricultural use of sewage sludge is limited due to the presence of heavy metals and pathogens, while landfill requires more land and contributes to global warming due to the emission of gases into the environment (Dabros et al., 2018). The thermochemical conversion of sewage sludge to energy production might dominate in the future (Huang & Yuan, 2015; Wang et al., 2017a). Therefore, thermochemical conversion is considered the most promising alternate technology to reduce the amount of waste and associated harmful environmental impact, supporting sustainability and energy self-sufficiency in the wastewater treatment process (Racz & Goel, 2010; Verlicchi & Zambello, 2015).
Sewage sludge is essentially a solid waste consisting of a heterogeneous mixture of proteins, carbohydrates, lipids or fats, organic and inorganic matters. Due to rapid industrialization and increasing population, the production of sewage sludge has dramatically increased. It contains various harmful components such as bacteria, viruses, dioxins, non-biodegradable organic compounds, heavy metals, etc. (Fytili & Zabaniotou, 2008). Nevertheless, sewage sludge can be used as an energy source because it can produce biofuels by using thermochemical conversion processes, which can also minimize the hazardous environmental impact. Furthermore, it has a high calorific value and reasonable volatile content (Manara & Zabaniotou, 2012). Generally, the volatile content of sewage sludge is in the range of 30-88%, and the calorific value typically 11-25.5 MJ/kg (Fytili & Zabaniotou, 2008; Li et al., 2013; Zhuo et al., 2012).
Different thermochemical conversion techniques such as torrefaction, pyrolysis, liquefaction, gasification and combustion can be used to produce energy in various forms such as bio-oil, bio-char and gaseous products (Chan et al., 2019; Yusup et al., 2019), all of which contain different operational routes to obtain various products (Shahbaz et al., 2020). For example, torrefaction, and pyrolysis can be used to obtain biochar and bio-oil, while pyrolysis and liquefaction can be used to obtain bio-oil (Chen et al., 2015; Naqvi et al., 2018a; Naqvi et al., 2019a).
Pyrolysis is a process of converting different types of sewage sludge in the absence of air or oxygen to three product types, namely, solid (biochar), liquid (bio-oil) and volatile gaseous components by thermal degradation (Aysu et al., 2016; Stolarek et al., 2019). Pyrolysis oil or bio-oil is a dark brown organic liquid fuel obtained from the pyrolysis process, which can be used in various applications (Loy et al., 2018; Naqvi et al., 2015). It can also be utilized as a feedstock to produce hydrocarbons as a feedstock source for present petroleum or future bio-refineries (Aysu & Sanna, 2015). Although it is environmentally friendly and has many advantages, the liquid fuel is faced with technical challenges on a commercial-scale due to its high water content of 15–30 wt.%, and high composition of oxygenated compounds of 35–60 wt.% such as acids, ketones, ethers, and alcohols, resulting in a low caloric value, and low combustion efficiency (Abnisa & Daud, 2015). The improvement of fuel quality and combustion efficiency can be achieved by eliminating O2 content using new pre-treatment and thermal processing (Inguanzo et al., 2002; Lu et al., 2012). Amongst other thermochemical conversion techniques; pyrolysis is the most beneficial process to produce bio-char and bio-oil depending on operational parameters such as heating rate, reaction temperature, residence time and feedstock size (Bach & Chen, 2017b; Fonts et al., 2008; Kim & Parker, 2008).
An extensive amount of literature is available on converting sewage sludge to bio-oil, biochar and gas using pyrolysis (Naqvi et al., 2019b; Abdul Raheem et al., 2018; Stolarek et al., 2019). However, a knowledge gap remains with respect to the optimisation of the pyrolysis process to produce large quantities of fuel and energy. Incidentally, the optimisation of the pyrolysis process has been studied using the thermogravimetric analysis (TGA) technique, which is important in understanding the fundamental mechanisms of the sewage sludge pyrolysis process. Several variables such as temperature, time, mass loss rate and percentage conversion can be monitored and measured (Zhang et al., 2016b). Chemical kinetics such as reaction rate, order of reactions and activation energy can be calculated (Liew et al., 2021; Patrick et al., 2020), which is needed for reactor design and practical pyrolysis reactor operation (Bach & Chen, 2017b). In previous years, several kinetic theories and models using TGA have been developed to forecast the thermal disintegration rate of sewage sludge during pyrolysis and to provide assistance with the reactor design and optimisation (Damartzis et al., 2011; Flynn & Wall, 1966; Lopez-Velazquez et al., 2013; Worzakowska & Torres-Garcia, 2015; Naqvi et al., 2018b). Considering that the importance of chemical kinetics in the conversion of sewage sludge to value-added products, and the lack of a comprehensive review discussing the selection of critical process parameters, this review will focus on the optimisation of process parameters through the study of TGA. A particular focus is placed on experimental and process design by examining the kinetics of sewage sludge pyrolysis from TGA data. It will include a summary of the composition of sewage sludge reported, and the recent developments in the pyrolysis process conditions and kinetic parameters using sewage sludge obtained from municipal wastewater treatment plants. In addition, an evaluation of the diverse models, such as single reaction, multiple parallel reactions, series reactions, Kissinger-Akahira-Sunose (KAS), Coats and Redfern, and distributed activation energy models is provided. Moreover, machine learning approaches are also discussed, as they can support the examination of the thermal degradation behaviour of the components in sewage sludge. Ultimately, the aforementioned discussion can optimise the pyrolysis process as it produces biochar and bio-oil; sewage sludge thermal degradation calculations; and reactor designs for optimum production yield.
Section snippets
Source and composition of sewage sludge
The primary source of sludge is municipal wastewater treatment plants, termed sewage sludge, though sludge can also be obtained from some industrial wastewater streams. Overall the sludges are similar although slight differences will exist, mostly due to inert and unbiodegradable material present in the wastewater stream. A typical sludge source and treatment process combined with a conventional wastewater treatment plant is presented in Fig. 2. Sewage sludge differs from other biomass
Mechanism of pyrolysis
The pyrolysis process begins with the formation of vapours of volatile components, then primary disintegration of non-volatile substance occurs to produce char, tar, and gases (Anca-Couce, 2016). With an increase of temperature, the secondary decomposition of char occurs and produces hydrocarbons and aromatic compounds in the volatile phase (Khiari et al., 2004). A schematic representation of the sewage sludge pyrolysis mechanism is presented in Fig. 5(Shao et al., 2007). With the increase of
Principles of TGA
Thermogravimetric analysis (TGA) is a technique whose working principle is a controlled temperature program used to measure a sample's weight with temperature change continuously. The physical and chemical properties of a sample based on the variation in sample weight with respect to time and temperature are obtained using the DTG curve resulting from the differentiation of a TGA curve (Bach & Chen, 2017b; Yahiaoui et al., 2015). The thermogravimetric analyser usage has many advantages. For
Potential for process design and up-scaling using kinetic data
The production of bio-oil and bio-char from the pyrolysis of sewage sludge is an important process which has attracted much attention in the literature. A kinetic study is an essential tool that can help design and optimise a thermochemical reactor; therefore, the kinetic parameters can be used in practical design applications. Hakvoort et al. (1989) reported that the thermal conversion curve obtained by calculation could be different from the curve obtained from experimental studies. However,
Machine learning approaches in thermal kinetics of pyrolysis
The pyrolysis process is a complex process affected by the chemical composition of biomass and the propagation of different reactions. In the pyrolysis process the reactions occur simultaneously and it is difficult to identify the main reactions. For this purpose, reaction kinetics and thermodynamic parameters are more important to identify this. In this regard, some mathematical modelling tools are also useful to determine the reaction kinetics that identify the main reactions (Shahbeig &
Challenges and opportunities
Cost-effective and innovative pyrolysis is a prerequisite for environmentally friendly and simultaneous recovery of materials and energy from sewage sludge. A sustainable integrated biorefinery approach could be applied when considering reuse options to produce fuel gases and other by-products, such as chars and fuel oils. This will assist in sewage sludge management to reduce adverse environmental impact and protect public health. However, it is necessary to design an optimised pyrolysis
Conclusion
A critical review on current research accomplishments to recover resources and determine the kinetics of sewage sludge from municipal wastewater treatment plants through TGA pyrolysis data is presented in this review. The data obtained can support the design of the most suitable pyrolysis reactor, optimise the operating conditions and biofuel yields. The isothermal and non-isothermal kinetic data-based models applied in pyrolysis process were analysed in this article. Non-isothermal kinetic
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
The authors would like to thank Hamad bin Khalifa University, Qatar Foundation, Doha Qatar, and National University of Science and Technology, Islamabad, Pakistan, for financial and technical assistance for the completion of this work.
References (193)
- et al.
Optimisation of fuel recovery through the stepwise co-pyrolysis of palm shell and scrap tire
Energy Conversion and Management
(2015) - et al.
Biochar production by sewage sludge pyrolysis
J. Anal. Appl. Pyrol.
(2013) - et al.
Submerged low-cost pyrophyllite ceramic membrane filtration combined with GAC as fluidized particles for industrial wastewater treatment
Chemosphere
(2018) - et al.
Sewage sludge valorization by flash pyrolysis in a conical spouted bed reactor
Chem. Eng. J.
(2015) - et al.
Characterization of the bio-oil obtained by fast pyrolysis of sewage sludge in a conical spouted bed reactor
Fuel Process. Tech.
(2016) Reaction mechanisms and multi-scale modelling of lignocellulosic biomass pyrolysis
Progress in Energy and Combustion Science
(2016)- et al.
Bio-oil production from dry sewage sludge by fast pyrolysis in an electrically-heated fluidized bed reactor
Sustain. Environ. Res.
(2017) - et al.
Anaerobic membrane bioreactors for biohydrogen production: Recent developments, challenges and perspectives
Bioresour. Tech.
(2018) - et al.
Membrane bioreactors for wastewater treatment: a review of mechanical cleaning by scouring agents to control membrane fouling
Chem. Eng. J.
(2017) - et al.
Integrated valorization of waste cooking oil and spent coffee grounds for biodiesel production: Blending with higher alcohols, FT–IR, TGA, DSC, and NMR characterizations
Fuel
(2019)
Bio-oil production via catalytic pyrolysis of Anchusa azurea: Effects of operating conditions on product yields and chromatographic characterization
Bioresour. Tech.
Nannochloropsis algae pyrolysis with ceria-based catalysts for production of high-quality bio-oils
Bioresour. Tech.
A comprehensive study on pyrolysis kinetics of microalgal biomass
Energy Conversion and Management
Pyrolysis characteristics and kinetics of microalgae via thermogravimetric analysis (TGA): a state-of-the-art review
Bioresour. Tech.
Comparative study on the thermal degradation of dry-and wet-torrefied woods
Applied Energy
Kinetic models based in biomass components for the combustion and pyrolysis of sewage sludge and its compost
J. Anal. Appl. Pyrol.
Biomass torrefaction: modeling of volatile and solid product evolution kinetics
Bioresour. Tech.
Thermodynamics, kinetics, gas emissions, and artificial neural network modeling of co-pyrolysis of sewage sludge and peanut shell
Fuel
Influence of torrefaction on the devolatilization and oxidation kinetics of wood
J. Anal. Appl. Pyrol.
Pyrolysis of microalgae residues–a kinetic study
Bioresour. Tech.
Life cycle assessment indicators of urban wastewater and sewage sludge treatment
Ecological Indicators
Characterization of sewage sludges by primary and secondary pyrolysis
J. Anal. Appl. Pyrol.
Nitrogen transformations during fast pyrolysis of sewage sludge
Fuel
Catalytic reforming of volatiles and nitrogen compounds from sewage sludge pyrolysis to clean hydrogen and synthetic gas over a nickel catalyst
Fuel Processing Technology
An overview of biomass thermochemical conversion technologies in Malaysia
Science of The Total Environment
Hydrocarbons fuel upgradation in the presence of modified bi-functional catalyst
J. Clean. Prod.
Co-combustion of sewage sludge and coffee grounds under increased O2/CO2 atmospheres: Thermodynamic characteristics, kinetics and artificial neural network modeling
Bioresour. Tech.
Effect of alkali addition on sulfur transformation during low temperature pyrolysis of sewage sludge
Proceedings of the Combustion Institute
Evolution of gases in the primary pyrolysis of different sewage sludges
Thermochimica Acta
Transportation fuels from biomass fast pyrolysis, catalytic hydrodeoxygenation, and catalytic fast hydropyrolysis
Progress in Energy and Combustion Science
Thermal degradation studies and kinetic modeling of cardoon (Cynara cardunculus) pyrolysis using thermogravimetric analysis (TGA)
Bioresour. Tech.
Calculation of higher heating values of biomass fuels
Fuel
Modeling chemical and physical processes of wood and biomass pyrolysis
Progress in Energy and Combustion Science
Production of bio-fuels by high temperature pyrolysis of sewage sludge using conventional and microwave heating
Bioresour. Tech.
Hydrogen-rich fuel gas production from the pyrolysis of wet sewage sludge at high temperature
J. Anal. Appl. Pyrol.
Food waste to biochars through pyrolysis: A review
Resources, Conservation and Recycling
Pyrolysis of municipal sewage sludges in a slowly heating and gas sweeping fixed-bed reactor
Energy Conversion and Management
Influence of sewage sludge treatment on pyrolysis and combustion of dry sludge
Energy
Kinetic models for the pyrolysis and combustion of two types of sewage sludge
J. Anal. Appl. Pyrol.
Study of the pyrolysis liquids obtained from different sewage sludge
J. Anal. Appl. Pyrol.
Sewage sludge pyrolysis for liquid production: a review
Renewable and Sustainable Energy Reviews
Utilization of sewage sludge in EU application of old and new methods—a review
Renewable and Sustainable Energy Reviews
Thermal analysis and products distribution of dried sewage sludge pyrolysis
J. Anal. Appl. Pyrol.
7 - General considerations on sludge disposal, industrial and municipal sludge
Thermal characterization and syngas production from the pyrolysis of biophysical dried and traditional thermal dried sewage sludge
Bioresour. Tech.
Thermal decomposition of sewage sludge under N2, CO2 and air: Gas characterization and kinetic analysis
J. Environ. Manage.
Co-pyrolysis of sewage sludge and sawdust/rice straw for the production of biochar
J. Anal. Appl. Pyrol.
Recent progress in the direct liquefaction of typical biomass
Progress in Energy and Combustion Science
Microwave co-pyrolysis of sewage sludge and rice straw
Energy
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