Hydrogen production by non-catalytic partial oxidation of coal in supercritical water: Explore the way to complete gasification of lignite and bituminous coal

Supercritical water gasification (SCWG) of coal is a promising technology for clean coal utilization. In this paper, hydrogen production by non-catalytic partial oxidation of coal was systematically investigated in supercritical water (SCW) with quartz batch reactors for the first time. The influences of the main operating parameters including residence time, temperature, oxidant equivalent ratio (ER) and feed concentration on the gasification characteristics of coal were investigated. The experimental results showed that H₂ yield and carbon gasification efficiency (CE) increased with increasing temperature and decreasing feed concentration. CE increased with increasing ER, and H₂ yield peaked when ER equaled 0.1. CE increased quickly within 1 min and then tended to be stable between 2 and 3 min. In particular, complete gasification of lignite was obtained at 950 °C when ER equaled 0.1, as for bituminous coal, at a higher temperature of 980 °C when ER equaled 0.2.     

Optimization and modeling of process parameters during hydrothermal gasification of biomass model compounds to generate hydrogen-rich gas products

Hydrothermal gasification in subcritical and supercritical water is gaining attention as an attractive option to produce hydrogen from lignocellulosic biomass. However, for process optimization, it is important to understand the fundamental phenomenon involved in hydrothermal gasification of synthetic biomass or biomass model compounds, namely cellulose, hemicellulose and lignin. In this study, the response surface methodology using the Box-Behnken design was applied for the first time to optimize the process parameters during hydrothermal (subcritical and supercritical water) gasification of cellulose. The process parameters investigated include temperature (300–500 °C), reaction time (30–60 min) and feedstock concentration (10–30 wt%). Temperature was found to be the most significant factor that influenced the yields of hydrogen and total gases. Furthermore, negligible interaction was found between lower temperatures and reaction time while the interaction became dominant at higher temperatures. Hydrogen yield remained at about 0.8 mmol/g with an increase in the reaction time from 30 min to 60 min at the temperature range of 300–400 °C. When the temperature was raised to 500 °C, hydrogen yield started to elevate at longer reaction time. Maximum hydrogen yield of 1.95 mmol/g was obtained from supercritical water gasification of cellulose alone at 500 °C with 12.5 wt% feedstock concentration in 60 min. Using these optimal reaction conditions, a comparative evaluation of the gas yields and product distribution of cellulose, hemicellulose (xylose) and lignin was performed. Among the three model compounds, hydrogen yields increased in the order of lignin (0.73 mmol/g) < cellulose (1.95 mmol/g) < xylose (2.26 mmol/g). Based on the gas yields from these model compounds, a possible reaction pathway of model lignocellulosic biomass decomposition in supercritical water was proposed.     

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.     

Noncatalytic gasification of isooctane in supercritical water: A Strategy for high-yield hydrogen production

Continuous supercritical water gasification of isooctane, a model gasoline compound, is investigated using an updraft gasification system. A new reactor material, Haynes^® 230^® alloy, is employed to run gasification reactions at high temperature and pressure (763 ± 2 °C; 25 MPa). A large-volume reactor is used (170 mL) to enable the gasification to be run at a long residence time, up to 120 s. Various gasification experiments are performed by changing the residence time (60-120 s), the isooctane concentration (6.3-14.7 wt%), and the oxidant concentration (equivalent oxidant ratio 0-0.3). The total gas yield and the hydrogen gas yield increase with increasing residence time. At 106 s and an isooctane concentration of 6.3 wt%, a very high hydrogen gas yield of 12.4 mol/mol isooctane, which is 50% of the theoretical maximum hydrogen gas yield and 92% of the equilibrium hydrogen gas yield under the given conditions, is achieved. Under these conditions, supercritical water partial oxidation does not increase the hydrogen gas yield significantly. The produced gases are hydrogen (68 mol%), carbon dioxide (20 mol%), methane (9.8 mol%), carbon monoxide (1.3 mol%), and ethane (0.9 mol%). The carbon gasification efficiency is in the range 75-91%, depending on the oxidant concentration. A comparison of supercritical water gasification with other conventional methods, including steam reforming, autothermal reforming, and partial oxidation, is also presented.     

Hydrogen generation via supercritical water gasification of lignin using Ni‐Co/Mg‐Al catalysts

In this study, a group of Ni‐Co/Mg‐Al catalysts was prepared for hydrogen production via supercritical water gasification of lignin. The effects of different supports and preparation methods were examined. All catalysts were evaluated under the operation conditions of 650 °C, 26 MPa, and water to biomass mass ratio of 5 in a batch reactor. The Cop.2.6Ni‐5.2Co/2.6Mg‐Al catalyst showed the best performance with highest gas yield (12.9 wt%) and hydrogen yield (2.36 mmol·g^?1). The results from catalyst characterization suggest that the properties of this type of catalyst are dependent on multiple factors including support Mg‐Al molar ratio and preparation method, and better coke resistance of the catalyst could be obtained by the preparation method of coprecipitation. Therefore, coprecipitation method should be applied for the preparation of Ni‐Co/Mg‐Al catalysts for hydrogen production via supercritical water gasification of lignin. Copyright © 2017 John Wiley & Sons, Ltd.     

Hydrogen-rich gas from fruit shells via supercritical water extraction

In this study, the aqueous conversion of whole fruit shell to hydrogen rich gas under low temperature, but supercritical conditions was investigated. The yields of total extraction products from supercritical water extraction increase with increasing temperature for all runs. The yields of hydrogen (YHs) increase with increasing temperature and pressure for all runs and the increase of YHs with pressure are higher than those with temperature. Compared with other biomass thermochemical processes such as pyrolysis, gasification, air gasification or steam gasification, the supercritical water gasification can directly deal with the wet biomass without drying, and have high gasification efficiency at lower temperature.     

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。     

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.     

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.     

Hydrogen production by coal gasification in supercritical water with a fluidized bed reactor

The technology of supercritical water gasification of coal can converse coal to hydrogen-rich gaseous products effectively and cleanly. However, the slugging problem in the tubular reactor is the bottleneck of the development of continuous large-scale hydrogen production from coal. The reaction of coal gasification in supercritical water was analyzed from the point of view of thermodynamics. A chemical equilibrium model based on Gibbs free energy minimization was adopted to predict the yield of gaseous products and their fractions. The gasification reaction was calculated to be complete. A supercritical water gasification system with a fluidized bed reactor was applied to investigate the gasification of coal in supercritical water. 24 wt% coal-water-slurry was continuously transported and stably gasified without plugging problems; a hydrogen yield of 32.26  mol/kg was obtained and the hydrogen fraction was 69.78%. The effects of operational parameters upon the gasification characteristics were investigated. The recycle of the liquid residual from the gasification system was also studied.     

Recent progress in thermochemical techniques to produce hydrogen gas from biomass: A state of the art review

The present work comprehensively covers the literature that describes the thermochemical techniques of hydrogen production from biomass. This survey highlights the current approaches, relevant methods, technologies and resources adopted for high yield hydrogen production. Prominent thermochemical methods i.e. pyrolysis, gasification, supercritical water gasification, hydrothermal upgrading followed by steam gasification, bio-oil reforming, and pyrolysis inline reforming have been discussed thoroughly in view of the current research trend and latest emerging technologies. Influences of important factors and parameters on hydrogen yield, such as biomass type, temperature, steam to biomass ratio, retention time, biomass particle size, heating rate, etc. have also been extensively studied. Catalyst is a vital integrant that has received enough attention due to its encouraging influence on hydrogen production. Literature confirms that hydrogen obtained from biomass has high-energy efficiency and potential to reduce greenhouse gases hence, it deserves versatile applications in the coming future. The study also reveals that hydrogen production through steam reforming, pyrolysis, and in-line reforming deliver a considerable amount of hydrogen from biomass with higher process efficiency. It has been identified that higher temperature, suitable steam to biomass ratio and catalyst type favor useful hydrogen yield. Nevertheless, hydrogen is not readily available in the sufficient amount and production cost is still high. Tar generation during thermochemical processing of biomass is also a concern and requires consistent efforts to minimize it.     

Industrial emergy evaluation for hydrogen production systems from biomass and natural gas

Fossil fuel resources are the main source for hydrogen production, and hydrogen production by renewable energy, such as biomass, is under development. To compare the performance in natural resource utilization for different hydrogen production systems, in this paper, two laboratorial hydrogen production systems from biomass and one industrial hydrogen production system from natural gas are analyzed by using industrial emergy evaluation indices. One of the laboratorial systems is a continuous supercritical water gasification system from glucose, and the other is a batch supercritical water gasification system from sawdust. The industrial system adopts American Brown technology. The evaluation results show that although the industrial emergy efficiency (IEE) of the industrial system from natural gas is higher than that of the laboratorial systems from biomass, the industrial emergy index of sustainability (IEIS) of the two laboratorial systems are higher than that of the industrial system. To make the laboratorial biomass system become an industrial system, the system should improve its yield, and reduce its capital investment.     

Hydrogen production by biomass gasification in supercritical water: A parametric study

Hydrogen production by biomass gasification in supercritical water is a promising technology for utilizing high moisture content biomass, but reactor plugging is a critical problem when feedstocks with high biomass content are gasified. The objective of this paper is to prevent the plugging problem by studying the effects of the various parameters on biomass gasification in supercritical water. These parameters include pressure, temperature, residence time, reactor geometrical configuration, reactor types, heating rate, reactor wall properties, biomass types, biomass particle size, catalysts and solution concentration. Biomass model compounds (glucose, cellulose) and real biomass are used in this work. All the biomasses have been successfully gasified and the product gas is composed of hydrogen, carbon dioxide, methane, carbon monoxide and a small amount of ethane and ethylene. The results show that the gas yield of biomass gasification in supercritical water is sensitive to some of the parameters and the ways of reducing reactor plugging are obtained.     

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.     

Hydrogen production by supercritical water gasification of biomass: Explore the way to maximum hydrogen yield and high carbon gasification efficiency

Supercritical water gasification (SCWG) is a promising technology for wet biomass utilization. In this paper, orthogonal experimental design method, which can minimize the number of experiments compared with the full factorial experiments, was used to optimize the operation parameters of SCWG with a tubular reactor system. Using this method, the influences of the main parameters including pressure, temperature, residence time and solution concentration on biomass gasification were also investigated. Simultaneously, in order to further improve the gasification efficiency of biomass, acid hydrolysis pretreatment of feedstock, oxidizers addition and increasing reaction temperature were employed. Results from the experiments show that in the range of experimental parameters, the order of the effects of the factors on H₂ yield of corn cob gasification in SCW is temperature > pressure > feedstock concentration > residence time. Temperature and pressure have a significant and complicated effect on biomass gasification. Hydrogen yield increases by the acid hydrolysis pretreatment of feedstock, and oxidizer addition reduces the hydrogen yield but it promotes the increase in carbon gasification efficiency. Biomass feedstock with high concentration was gasified successfully at high reaction temperature.     

Hydrogen production from low temperature supercritical water Co-Gasification of low rank lignites with biomass

Co–gasification of low rank lignite (Çan) with sorghum energy crop was investigated under low temperature conditions with supercritical water (773 K, 26.9 MPa). The effects of the water volume in the reactor, blending ratios of the coal/sorghum mixtures, the use of different catalysts, and the variation of feedstock concentrations on the gasification efficiency, product distribution, and hydrogen yields were evaluated. Synergistic effects were observed for both the gasification efficiency and the hydrogen yield with a coal content of 25 wt% in the coal/biomass mixture. Increasing the initial water volume, decreasing the feedstock concentration, and using the alkali metal catalysts Na₂CO₃ and K₂CO₃ significantly increased the gasification efficiency and the hydrogen yield. In experiments with CaO, almost all the carbon dioxide formed was isolated from the gas product during gasification, and the hydrogen yield was more than 70%. The liquid products were mainly composed of alkylphenols and their derivatives.     

Fundamental design of a continuous biomass gasification process using a supercritical water fluidized bed

A novel biomass gasification (first stage of hydrogen production from biomass) process using a supercritical water fluidized bed was proposed and the fundamental design of the process was conducted. The flow rate was determined by evaluating the minimum fluidization velocity and terminal velocity of alumina particles enabling fluidization with the thermodynamic properties of supercritical water. Three cases were examined: a bubbling fluidized bed in which water was used mainly as the fluidized medium and biomass were added for gasification, a bubbling fluidized bed fluidized by biomass slurry feed alone, and a fast fluidized bed fluidized by biomass slurry feed alone. According to calculations of the residence time and thermal efficiency assuming heat recovery with a heat exchanger efficiency of 0.75, the bubbling fluidized bed fluidized by biomass slurry alone was appropriate for continuous biomass gasification using a fluidized bed.     

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.     

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.     

Exergy analysis of hydrogen production from steam gasification of biomass: A review

Exergy analysis of hydrogen production from steam gasification of biomass was reviewed in this study. The effects of the main parameters (biomass characteristics, particle size, gasification temperature, steam/biomass ratio, steam flow rate, reaction catalyst, and residence time) on the exergy efficiency were presented and discussed. The results show that the exergy efficiency of hydrogen production from steam gasification of biomass is mainly determined by the H₂ yield and the chemical exergy of biomass. Increases in gasification temperatures improve the exergy efficiency whereas increases in particle sizes generally decrease the exergy efficiency. Generally, both steam/biomass ratio and steam flow rate initially increases and finally decreases the exergy efficiency. A reaction catalyst may have positive, negative or negligible effect on the exergy efficiency, whereas residence time generally has slight effect on the exergy efficiency.