A comparative analysis of hydrogen production from the thermochemical conversion of algal biomass

Gasification has the potential to convert biomass into gaseous mixtures that can be used for hydrogen production. Thermal gasification and supercritical water gasification are commonly used thermochemical methods for conversion of biomass to hydrogen. Supercritical water gasification handles wet biomass, thus eliminating the capital cost-intensive drying step. Thermal gasification is considered as an alternative means of producing hydrogen from microalgae where biomass has to be dried before gasification. The authors developed techno-economic models for assessment of the production of hydrogen through supercritical gasification and thermal gasification processes. Techno-economic assessment was based on developed process models. Equipment was sized and costs were estimated using the developed process models, and the product value was determined assuming 20 years of plant life. The economic assessment of supercritical water and thermal gasification show that 2000 dry tonnes/day plant requires total capital investments of 277.8 M$ and 215.3 M$ for hydrogen product values of $4.59 ± 0.10/kg and $5.66 ± 0.10/kg, respectively. The relatively higher yield obtained in supercritical water gasification compared to thermal gasification results in lower product value of hydrogen for supercritical water gasification, thereby making it more desirable. This cost of hydrogen is about 4 times the cost of hydrogen from natural gas. The sensitivity analysis indicates that biomass cost and yield are the most sensitive parameters in the economics of the supercritical or thermal gasification process; this signifies the importance of algal biomass availability. The techno-economic assessment helps to identify options for the production of hydrogen fuel through these novel technologies.     

Supercritical water gasification of biomass for hydrogen production

Hydrogen from waste biomass is considered to be a clean gaseous fuel and efficient for heat and power generation due to its high energy content. Supercritical water gasification is found promising in hydrogen production by avoiding biomass drying and allowing maximum conversion. Waste biomass contains cellulose, hemicellulose and lignin; hence it is essential to understand their degradation mechanisms to engineer hydrogen production in high-pressure systems. Process conditions higher than 374 °C and 22.1 MPa are required for biomass conversion to gases. Reaction temperature, pressure, feed concentration, residence time and catalyst have prominent roles in gasification. This review focuses on the degradation routes of biomass model compounds such as cellulose and lignin at near and supercritical conditions. Some homogenous and heterogeneous catalysts leading to water–gas shift, methanation and other sub-reactions during supercritical water gasification are highlighted. The parametric impacts along with some reactor configurations for maximum hydrogen production and technical challenges encountered during hydrothermal gasification processes are also discussed.     

Biomass-based hydrogen production: A review and analysis

In this study, various processes for conversion of biomass into hydrogen gas are comprehensively reviewed in terms of two main groups, namely (i) thermo-chemical processes (pyrolysis, conventional gasification, supercritical water gasification (SCWG)), and (ii) biological conversions (fermentative hydrogen production, photosynthesis, biological water gas shift reactions (BWGS)). Biomass-based hydrogen production systems are discussed in terms of their energetic and exergetic aspects. Literature studies and potential methods are then summarized for comparison purposes. In addition, a biomass gasification process via oxygen and steam in a downdraft gasifier is exergetically studied for performance assessment as a case study. The operating conditions and strategies are really important for better performance of the system for hydrogen production. A distinct range of temperatures and pressures is used, such as that the temperatures may vary from 480 to 1400 °C, while the pressures are in the range of 0.1–50 MPa in various thermo-chemical processes reviewed. For the operating conditions considered the data for steam biomass ratio (SBR) and equivalence ratio (ER) range from 0.6 to 10 and 0.1 to 0.4, respectively. In the study considered, steam is used as the gasifying agent with a product gas heating value of about 10–15 MJ/Nm³, compared to an air gasification of biomass process with 3–6 MJ/Nm³. The exergy efficiency value for the case study system is calculated to be 56.8%, while irreversibility and improvement potential rates are found to be 670.43 and 288.28 kW, respectively. Also, exergetic fuel and product rates of the downdraft gasifier are calculated as 1572.08 and 901.64 kW, while fuel depletion and productivity lack ratios are 43% and 74.3%, respectively.     

Hydrogen production by biomass gasification in supercritical water using concentrated solar energy: System development and proof of concept

A novel system of hydrogen production by biomass gasification in supercritical water using concentrated solar energy has been constructed, installed and tested at the State Key Laboratory of Multiphase Flow in Power Engineering (SKLMF). The “proof of concept” tests for solar-thermal gasification of biomass in supercritical water (SCW) were successfully carried out. Biomass model compounds (glucose) and real biomass (corn meal, wheat stalk) were gasified continuously with the novel system to produce hydrogen-rich gas. The effect of direct normal solar irradiation (DNI) and catalyst on gasification of biomass was also investigated. The results showed that the maximal gasification efficiency (the mass of product gas/the mass of feedstock) in excess of 110% were reached, hydrogen fraction in the gas product also approached to 50%. The experimental results confirmed the feasibility of the system and the advantage of the process, which supports future work to address the technical issues and develop the technology of solar-thermal hydrogen production by gasification of biomass in supercritical water.     

Performance simulation and thermodynamics analysis of hydrogen production based on supercritical water gasification of coal

The heating method of SCWG reactor is critical to system construction, and almost all existing reactors rely on external heat sources. In this article, the thermodynamic equilibrium model is established to predict the distribution of gasification products from supercritical water gasification of coal. The transformation rule of gas components in the SCWG process of coal and oxidation process of gasification products is explored. Especially, the influence of key parameters such as feedstock concentration, gasification temperature and pressure on the hydrogen yield during the gasification and oxidation processes is also discussed. Based on the above research, the autothermal gasification system for hydrogen production integrated supercritical water gasification of coal and oxidation of gasification products is proposed. The flow matching of supercritical water, coal slurry, and oxygen and its effect on the autothermal hydrogen yield are discussed. By optimizing the flow rate of the reactants, 80% of the hydrogen production efficiency is achieved.     

Thermochemical conversion of sewage sludge: A critical review

The increasing levels of sewage sludge production demands research and development to introduce more commercially feasible options for reducing socio-economic and environmental problems associated with its current treatment. Sewage sludge may be processed to produce useful products or as a feedstock for energy generation. Initially, the characteristics of sewage sludge are discussed in terms of composition and the current options for its treatment with the associated environmental impacts. Processes to valorize sewage sludge are discussed, including heavy metal removal from sewage sludge, production of bio-char, production and use of activated carbon and use of sewage sludge combustion ash in cement and concrete. Thermochemical processes i.e., pyrolysis, co-pyrolysis and catalytic pyrolysis, also gasification and combustion for process intensification, energy and resource recovery from sewage sludge are then critically reviewed in detail. The pyrolysis of sewage sludge to produce a bio-oil is covered in relation to product bio-oil composition, reactor type and the use of catalysts. Gasification of sewage sludge focusses on the characteristics of the different available reactor types and the influence of a range of process parameters and catalysts on gas yield and composition. The selection and design of catalysts are of vital importance to enhance the selectivity of the selected thermochemical pyrolysis or gasification process. The catalysts used for sewage sludge treatment need more research to enable selectivity towards the targeted desired end-products along with optimization of parametric conditions and development of innovative reactor technologies. The combustion of sewage sludge is reviewed in terms of reactor technologies, flue gas cleaning systems and pollutant emissions. In addition, reactor technologies in terms of technological strength and market competitiveness with the particular application to sewage sludge are compared for the first time for thermochemical conversion. A critical comparison is made of the drying techniques, co-feedstocks and catalytic processes, reaction kinetics, reactor technologies, operating conditions to be optimized, removal of impurities, fuel properties, their constraints and required improvements. The emphasis of this review is to promote environmental sustainability for process intensification, energy and resource recovery from pyrolysis, gasification and combustion involving the use of catalysts.     

Comparison of thermochemical conversion processes of biomass to hydrogen-rich gas mixtures

Hydrogen can be produced from biomass materials via thermochemical conversion processes such as pyrolysis, gasification, steam gasification, steam-reforming, and supercritical water gasification (SCWG) of biomass. In general, the total hydrogen-rich gaseous products increased with increasing pyrolysis temperature for the biomass sample. The aim of gasification is to obtain a synthesis gas (bio-syngas) including mainly H₂ and CO. Steam reforming is a method of producing hydrogen-rich gas from biomass. Hydrothermal gasification in supercritical water medium has become a promising technique to produce hydrogen from biomass with high efficiency. Hydrogen production by biomass gasification in the supercritical water (SCW) is a promising technology for utilizing wet biomass. The effect of initial moisture content of biomass on the yields of hydrogen is good.     

Catalytic gasification of textile wastewater treatment sludge for hydrogen production in supercritical water with K2CO3/H2O2: Reaction variables,mechanism and kinetics

As the main by-product, sludge was generated during the process of textile wastewater treatment. Due to containing organic compounds, textile sludge has great potential to be effectively reused for production of clean energy. In this work, gasification of textile sludge in supercritical water for hydrogen production was investigated. In order to improve hydrogen production, H₂O₂ and K₂CO₃ were used as catalysts. Effects of reaction variables (including temperature, retention time, oxidation coefficient and alkali catalyst dosage) on hydrogen yield were studied. Experimental results indicate that hydrogen yield increases with rise of temperature. When reaction temperature reaches 500 °C, the maximum value of hydrogen yield being 10.6 mol/kg was obtained. When excessive H₂O₂ was added, decrease of hydrogen yield was achieved. However, the addition of K₂CO₃ is favor to hydrogen yield, which is about 1.5 times as much as that of without catalyst. Meanwhile, reaction mechanism and kinetics of textile sludge gasification in supercritical water were explored. Reaction activation energy and Arrhenius constants were obtained in the established kinetic model.     

Production of useful energy from solid waste materials by steam gasification

A steam gasification processes is an energy conversion pathway through which organic materials are converted to useful energy. In spite of the high energy content in organic waste materials, they have been mostly disposed of in landfills, which causes harmful environmental issues such as methane emissions and ground water pollution and contaminations. In this sense, organic solid waste materials are regarded as alternative resources for conversion to useful energy in the steam gasification process. In this study, three types of waste materials – municipal solid waste (MSW), used tires and sewage sludge – were used to generate syngas through the gasification process in a 1000 °C steam atmosphere. The syngas generation rates and its chemical compositions were measured and evaluated over time to determine the characteristics and dynamics of the gasification process. Also, carbon conversion, and mass and energy balances are presented which demonstrates the feasibility of steam gasification as a waste conversion pathway. The results show that the syngas contains high concentrations of H₂, around 41–55% by volume. The syngas generation rate was found to depend on the carbon content in the feedstock regardless of the types of input materials. Comparing to the hydrogen production from water splitting that requires extremely high temperatures at around 1500 °C, hydrogen production by steam gasification of organic materials can be regarded as equally effective but requires lower system temperatures. Copyright © 2016 John Wiley & Sons, Ltd.     

Influence of sludge properties on the direct gasification of dewatered sewage sludge in supercritical water

In the present study, ten different types of dewatered sewage sludges were treated in supercritical water in a high-pressure autoclave under a given condition (at 400 °C, 60 min and 23 MPa). The feasibility of direct gasification and the effect of sludge properties on the gasification of dewatered sewage sludge with various properties in supercritical water were investigated. The results showed that dewatered sewage sludge with various water contents (73.48–88.51 wt%), organic matter contents (29.25–73.02 wt%, on dry basis) and inorganics can be directly gasified in supercritical water. The total gas and phenol production increased linearly with the increment of organic matter content in dewatered sewage sludge. The difference in hydrogen content in the gaseous product may be related to the content of water and inorganic as well as pH value of the sludge. The char/coke formed in the solid residue increased with decrement of water content, which inhibited the gasification reaction and resulted in the carbonization reaction.     

Systematic analysis of biomass derived fuels for fuel cells

As the demand for energy continuously increases, alternatives to fossil resources must be found to both prevent fossil source depletion and decrease overall environmental impact. One solution is increasing contributions from renewable, biological feedstock, and from wastes. This paper presents an analysis of the current methods of biomass conversion, to extract biofuels and biologically produced gases to then be used in fuel cells. Pathways for converting biomass feedstock into fuel cell fuels selected here were anaerobic digestion, metabolic processing, fermentation, gasification, and supercritical water gasification, which were compared to natural gas and fossil hydrogen reference cases. These thermochemical and biological conversion pathways can also make use of residues from agriculture, forestry, or some household and industry wastes, producing hydrogen and hydrogen-rich gases. Solid oxide fuel cells were also found to be the preferred technology for such bio-derived fuel gases, due to their wide range of fuel options, wide scalability from single kW to multi 100 kW, and high efficiency.     

Solar receiver/reactor for hydrogen production with biomass gasification in supercritical water

A novel receiver/reactor driven by concentrating solar energy for hydrogen production by supercritical water gasification (SCWG) of biomass was designed, constructed and tested. Model compound (glucose) and real biomass (corncob) were successfully gasified under SCW conditions to generate hydrogen-rich fuel gas in the apparatus. It is found that the receiver/reactor temperature increased with the increment of the direct normal solar irradiation (DNI). Effects of the DNI, the flow rates and concentration of the feedstocks as well as alkali catalysts addition were investigated. The results showed that DNI and flow rates of reactants have prominent effects on the temperature of reactor wall and gasification results. Higher DNI and lower feed concentrations favor the biomass gasification for hydrogen production. The encouraging results indicate a promising approach for hydrogen production with biomass gasification in supercritical water using concentrated solar energy.     

Hydrogen and syngas production from sewage sludge via steam gasification

High temperature steam gasification is an attractive alternative technology which can allow one to obtain high percentage of hydrogen in the syngas from low-grade fuels. Gasification is considered a clean technology for energy conversion without environmental impact using biomass and solid wastes as feedstock. Sewage sludge is considered a renewable fuel because it is sustainable and has good potential for energy recovery. In this investigation, sewage sludge samples were gasified at various temperatures to determine the evolutionary behavior of syngas characteristics and other properties of the syngas produced. The syngas characteristics were evaluated in terms of syngas yield, hydrogen production, syngas chemical analysis, and efficiency of energy conversion. In addition to gasification experiments, pyrolysis experiments were conducted for evaluating the performance of gasification over pyrolysis. The increase in reactor temperature resulted in increased generation of hydrogen. Hydrogen yield at 1000 °C was found to be 0.076 g_gas g_sample⁻¹. Steam as the gasifying agent increased the hydrogen yield three times as compared to air gasification. Sewage sludge gasification results were compared with other samples, such as, paper, food wastes and plastics. The time duration for sewage sludge gasification was longer as compared to other samples. On the other hand sewage sludge yielded more hydrogen than that from paper and food wastes.     

Characteristics and prediction model of hydrogen production of oily sludge by supercritical water gasification

Harmless treatment and resource utilization of oily sludge are urgent and related to the sustainable green, and low-carbon development of the petroleum industry. Aiming to the supercritical water gasification (SCWG) of oily sludge for hydrogen production, this paper investigated the effects of critical factors, including reaction temperature, initial pressure, retention time, and feed concentration, on the mole fraction, the gas yield, the gasification efficiency, and the hydrogen yield potential. The interaction mechanisms among these four factors were discussed and revealed with a reasonable prediction model of hydrogen production. Results showed that the longer retention time, higher temperature, and lower feed concentration could accelerate hydrogen production from oily sludge by SCWG. The synthetic promotion of the hydrogen yield exists between the temperature and the retention time, while the temperature predominates. A 2.63-fold increase in the H₂ yield was obtained when the condition changed from 135 min to 380 °C to 10 min and 555 °C. The hydrogen production of oily sludge by SCWG, at lower temperature and higher pressure was worse than that at higher temperature and lower pressure.     

Main routes for the thermo-conversion of biomass into fuels and chemicals. Part 1: Pyrolysis systems

Since the energy crises of the 1970s, many countries have become interest in biomass as a fuel source to expand the development of domestic and renewable energy sources and reduce the environmental impacts of energy production. Biomass is used to meet a variety of energy needs, including generating electricity, heating homes, fueling vehicles and providing process heat for industrial facilities. The methods available for energy production from biomass can be divided into two main categories: thermo-chemical and biological conversion routes. There are several thermo-chemical routes for biomass-based energy production, such as direct combustion, liquefaction, pyrolysis, supercritical water extraction, gasification, air–steam gasification and so on. The pyrolysis is thermal degradation of biomass by heat in the absence of oxygen, which results in the production of charcoal (solid), bio-oil (liquid), and fuel gas products. Pyrolysis liquid is referred to in the literature by terms such as pyrolysis oil, bio-oil, bio-crude oil, bio-fuel oil, wood liquid, wood oil, liquid smoke, wood distillates, pyroligneous tar, and pyroligneous acid. Bio-oil can be used as a fuel in boilers, diesel engines or gas turbines for heat and electricity generation.     

Continuous hydrogen production by biomass gasification in supercritical water heated by molten salt flow: System development and reactor assessment

Hydrogen production by biomass gasification using solar energy is a promising approach for overcoming the drawbacks of fossil fuel utilization, but the storage of discontinuous solar flux is a critical issue for continuous solar hydrogen production. A continuous hydrogen production system by biomass gasification in supercritical water using molten-salts-stored solar energy was proposed and constructed. A novel double tube helical heat exchanger was designed to be molten salts reactor for hydrogen production. Model compounds (glycerol/glucose) and real biomass (corn cob) were successfully gasified in this molten salts reactor for producing hydrogen-rich gas. The unique temperature profiles of biomass slurry in the reactor were observed and compared with that of conventional electrical heating and direct solar heating approaches. Product gases yield, gasification efficiency and exergy conversion efficiency of the reactor were analyzed. The results showed that the performances of reactor were determined by feedstock style, biomass concentration, residence time and biomass slurry temperature profiles.     

Exergetic efficiency and options for improving sewage sludge gasification in supercritical water

The present article deals with an exergy analysis of a process under development for the gasification of biomass in supercritical water (supercritical water gasification, SCWG). This process is aimed at generating hydrogen out of the biogenic feedstock sewage sludge. The principle of the process is based on making use of the modifications of specific physical and chemical properties of water above the critical point (T=374°C, p=221 bar). These properties allow for a nearly complete conversion of the organic substance contained in the feed material into energy-rich fuel gases, containing hydrogen, carbon dioxide and methane. Based on a steady-state model of the process, exergy flow rates are calculated for all components and a detailed exergy analysis is performed. From the exergetic variables, options to improve the individual plant components as well as the overall plant are derived. The components with the highest proportion of exergy destruction in the complete process are identified and possibilities of improving them and the complete system in order to increase the overall efficiency are demonstrated. The combustion chamber necessary for heat supply is found to be the component with the highest proportion of exergy destruction of the complete plant. Moreover, the components of air preheater, reactor contribute significantly to the exergy destruction of the complete system. Copyright © 2006 John Wiley & Sons, Ltd.     

From biogas-to hydrogen – Based integrated urban water,energy and waste solids system - Quest towards decarbonization

Three integrated systems of water and municipal solid waste (MSW) management were evaluated regarding their energy use, production and CO_2eq emissions:(1) Biogas based aerobic treatment of wastewater and waste solids disposal by landfilling wherein codigesting sludge with MSW and landfill gas capture produce electricity by a turbine and generator.(2) Biogas based wastewater treatment with codigestion of sludge with biodegradable solids combined with incineration of combustible sludge and other solids.(3) Hydrogen-based system replacing landfilling by indirect gasification of organic solids followed by hydrogen fuel cells.There are great differences between CO_2eq emissions of biogas and hydrogen-based systems. The first two systems are positive CO₂ and methane emitters. Achieving net zero carbon emissions is unlikely. The H₂ based system is fully decarbonized and in addition to clean water, energy and negative carbon dioxide emissions it produces valuable commodities such as energy, concentrated hydrogen, fertilizers, oxygen/ozone, and concentrated carbon dioxide.     

超临界水中木质素/CMC催化气化制氢

在间歇式高压反应釜中,以碱性化合物K_2CO_3和Ca(OH)_2以及Ru/C为催化剂,对木质素在超临界水中的气化制氢特性进行了实验研究。结果表明:3种催化剂都有较好的催化作用,其中Ru/C的效果最佳,几种催化剂混合使用的效果要比单独一种催化剂使用时好,但是其提高的幅度不很明显。另外,随着温度的升高,H_2和CH_4的产气量以及氢转化率等都相应的升高。     

Recent developments of hydrogen production from sewage sludge by biological and thermochemical process

Hydrogen is a kind of clean effective resource. Sewage sludge is regarded as a promising material for hydrogen production because it owns a wide range of sources and the methods are consistent with the goal of sustainable development. This work reviews existing hydrogen production technologies from sewage sludge, including photo-fermentation, dark-fermentation, sequential dark- and photo-fermentation, pyrolysis, gasification, and supercritical water gasification (SCWG). Overall comparison for the involving approaches is conducted based on their inherent features and current development status along with the technical and environmental aspect. Results show that sequential dark- and photo-fermentation and pyrolysis have improved hydrogen yields, but the emissions of carbon dioxide are also remarkable. Biological processes have an advantage in cost, but the reaction rates are inferior to those of thermochemical method. Enhancing methods and improvements are proposed to guide future research on hydrogen production from sewage sludge and promote the effectiveness both technically and economically.