Hydrogen production by supercritical water gasification of biomass: Particle and residence time distribution in fluidized bed reactor

Particle distribution and residence time distribution (RTD) in supercritical water fluidized bed reactor (SCWFBR) greatly affect the hydrogen yield through determining the two phase mixing and reaction time. A Eulerian model incorporating the kinetic theory for solid particles was applied to simulate the solid distribution and RTD of feeding materials. The effect of four types of feeding methods and feeding rates on solid distribution and RTD were evaluated based on the simulation results. Results showed that double symmetrical feeding pipe with an feeding mouth angle of 45° provides more uniform solid distribution and longer residence time compared with those of other three types. A nonlinear relationship between feeding rate and RTD was observed, and an optimum feeding rate was found to be related to the best solid-fluid mixing in the study.     

喷动流化床煤气化技术研究的发展

本文论述了喷动化床煤气化技术的原理、优点以及国内外的主要技术进展，并结合当前国内外的研究情况从气化特性试验研究、机理模型研究、工业应用研究三个方面进行了分析，指出了喷动流化床煤气化技术的发展前景以及今后研究方向。     

Supercritical water gasification of real biomass feedstocks in continuous flow system

In this study, supercritical water gasification of the selected five biomass samples (cauliflower residue, acorn, tomatoes residue, extracted acorn and hazelnut shell) was investigated. Lignocellulosic feedstocks were gasified in a continuous flow reactor at 600 °C and 35 MPa. The product gas is composed of hydrogen, carbon dioxide, methane, carbon monoxide and a small amount of C₂ compounds. Quantitative analysis of product gas was performed by Gas chromatography device. Potassium carbonate (K₂CO₃) and Trona (Na₂CO₃·NaHCO₃·2H₂O) were used as catalysts. Carbon gasification efficiencies were improved by addition of these catalysts into the reacting system. Moreover, carbon gasification efficiency changes with type of biomass that includes different ratio of cellulose, hemicellulose and lignin. The H₂ yield (mol gas/kg C in feed) of acorn in the presence of Trona was found to be 7 times higher than that of without catalyst.     

Supercritical gasification of wastewater from updraft wood gasifiers

Supercritical water gasification (SCWG) of the wood tar fraction soluble in water is discussed. The mixture is collected downstream of an updraft wood gasification plant and presents tar compounds typical of low-temperature pyrolysis, with the highest yields attained by acetic acid, levoglucosan and 1-hydroxy-2-propanone. SCWG tests, using a laboratory-scale reactor with a plug-flow behavior, temperatures of 723–821 K, residence times of 46–114 s and initial total organic carbon (TOC) contents of 6.5–31 g/l (pressure equal to 25 MPa), show TOC conversion roughly between 30% and 70%. The corresponding yields of gas (l) with respect to the initial TOC contents (g) vary from 0.4 to 1. Gasification of TOC is well described by an irreversible, first-order, Arrhenius rate reaction with an activation energy of 75.7±22 kJ/mol and a pre-exponential factor of 897±30 s⁻¹. Quantification of 23 tar compounds of the product stream shows the prompt conversion of sugars and complex phenols, with the formation of intermediate products, such as furfurals, which successively decompose, and more thermally resistant species, such as acetic acid, propionic acid, 1,2-ethanediol, ketones and especially cresols and phenols.     

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

In this work, gasification of sewage sludge in supercritical water was investigated in a fluidized bed reactor. Effect of operating parameters such as temperature, concentration of the feedstock, alkali catalysts and catalyst loading on gaseous products and carbon distribution were systematically studied. The results showed that the increase of temperature and the decrease of feedstock concentration were both favorable for gasification, and the addition of catalyst enhanced the formation of hydrogen better. The K₂CO₃ catalyst could better enhance gasification efficiency and the catalytic activity of different catalysts for hydrogen production was in the following order: KOH > K₂CO₃ > NaOH > Na₂CO_3. The maximum molar fraction and yield of hydrogen reached to 55.96% and 15.49 mol/kg respectively with KOH at 540 °C. Most carbon in feedstock existed in gaseous and liquid products, and alkali catalysts mainly promoted the water-gas shift reaction rather than steam reforming.     

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.     

A numerical model of a fluidized bed biomass gasifier

A one-dimensional, steady state, numerical model was developed for a fluidized bed biomass gasifier. The gasifier model consists of a fuel pyrolysis model, an oxidation model, a gasification model and a freeboard model. Given the bed temperature, ambient air flow rate and humidity ratio, fuel moisture content and reactor parameters, the model predicts the fuel feed rate for steady state operation, composition of the producer gas and fuel energy conversion. The gasifier model was validated with experimental results. The effects of major mechanisms (fuel pyrolysis and the chemical and the physical rate processes) were assessed in a sensitivity study of the gasification model. A parametric study was also conducted for the gasifier model. It is concluded that the model can be used for gasifier performance analysis.     

流化床富氧气化制取煤气的研究

以一个常压流化床为反应器,采用膜分离技术制氧,对煤富氧-水蒸气气化制取煤气的特性进行实验研究,通过对试验数据的分析,探讨了两种不同煤种的典型运行结果,分析H2O/C对气化温度、煤气成分及热值影响,以及氧浓度对实验结果的影响。结果表明氧浓度的提高,明显增加了煤气的热值,氧浓度从21%提高到30%时,煤气热值提高了1.18 MJ/m3;在温度为920℃,氧浓度30%,H2O/C比为1,O/C比为0.8,煤气热值达到5.95 MJ/m3。     

Supercritical water gasification of wastewater sludge for hydrogen production

The use of hydrogen as clean fuel gas in the power generation sector becomes essential to reduce the environmental issues related to conventional fuel usage. By avoiding biomass drying process, supercritical water gasification is considered the most efficient technology in hydrogen production from wastewater sludge. Wastewater sludge is difficult to disposal in its received form since it is often produced with high moisture content, contribute to numerous environmental issues and direct contact with this waste can result in health concerns. The assessment of the treatment and conversion of this material into fuel gas at condition beyond supercritical state (374°C and 22.1 MPa) is required. This paper is discussed the degradation routes of wastewater sludge in supercritical water. Furthermore, it is reviewed the influence of the main operation parameters role in the hydrogen production, which includes reaction temperature, pressure, residence time, feed concentration and catalysts. The development in reactor design and setup for maximum hydrogen production is highlighted. The technical challenges encountered during the conversion process and its solutions are also discussed. In addition, future prospective to optimal and standardization of the supercritical water gasification process is reviewed.     

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.     

Numerical simulation of hydrogen production by gasification of large biomass particles in high temperature fluidized bed reactor

Biomass as a renewable fuel compared to fossil fuels usually contains high moisture content and volatile release. Hydrogen production by large particle biomass gasification is a promising technology for utilizing high moisture content biomass particle in the high temperature fluidized bed reactor. In the present work, simulation of large particles biomass gasification investigated at high temperature by using the discrete phase model (DPM). Combustible gases with homogeneous gas phase reactions, drying process with a heterogeneous reaction, primary and secondary pyrolysis with independent parallel-reaction by using two-competing-rate model to control a high and low temperature were used. During the thermochemical process of biomass, gaseous products containing of H₂, H₂O, CH₄, CO and CO₂ was obtained. The effects of concentration, mole and mass fraction and hydrodynamics effects on gaseous production during gasification were studied. The results showed that hydrodynamic effect of hot bed is different from cold bed. Concentration and molar fraction of CO and H₂ production by continually and stably state and small amount of CO₂, H₂O, and CH₄ was obtained. The hydrodynamic of bed plays the significant role on the rate of gaseous products.     

Research on the characteristics of biomass gasification in a fluidized bed

The depletion of fossil fuels and the increasing environmental problems, make biomass energy a serious alternative resource of energy. Biomass gasification is one of the major biomass utilization technologies to produce high quality gas. In this paper, biomass gasification was performed in a self-designed fluidized bed. The main factors (equivalence ratio, bed temperature, added catalyst, steam) influenced the gasification process were studied in detail. The results showed that the combustible gas content and the heating value increased with the increase of the temperature, while the CO₂ content decreased. The combustible gas content decreased with the increase of the equivalence ratio (ER), but CO₂ content increased. At the same temperature and at different ratios of CaO (from 0 to 20%), H₂ content was increased significantly, CO content was also increased, CH₄ content increased slightly, but CO₂ content was decreased. With the addition of steam at different temperature, the gas in combustible components increased, the content of H₂ increased obviously. The growth rate was 50% increased. As the bed temperature increased, gas reforming reaction increased, the CO and CH₄ content decreased, but CO₂ and H₂ content increased.     

Process analysis of hydrogen production from biomass gasification in fluidized bed reactor with different separation systems

Gasification is one of the most effective and studied methods for producing energy and fuels from biomass as different biomass feedstock can be handled, with the generation of syngas consisting of H₂, CO, and CH₄, which can be used for several applications. In this study, the gasification of hazelnut shells (biomass) within a circulating bubbling fluidized bed gasifier was analyzed for the first time through a quasi-equilibrium approach developed in the Aspen Plus environment and used to validate and improve an existing bubbling fluidized bed gasifier model. The gasification unit was integrated with a water-gas shift (WGS) reactor to increase the hydrogen content in the outlet stream and with a pressure swing adsorption (PSA) unit for hydrogen separation. The amount of dry H₂ obtained out of the gasifier was 31.3 mol%, and this value increased to 47.5 mol% after the WGS reaction. The simulation results were compared and validated against experimental data reported in the literature. The process model was then modified by replacing the PSA unit with a palladium membrane separation module. The final results of the present work allowed comparison of the effects of the two conditioning systems, PSA and palladium membrane, indicating a comparative increase in the hydrogen recovery ratio of 28.9% with the palladium membrane relative to the PSA configuration.     

Continuous supercritical water gasification of isooctane: A promising reactor design

A new design of supercritical water gasification system was developed to achieve high hydrogen gas yield and good gas–liquid flow stability. The apparatus consisted of a reaction zone, an insulation zone and a cooling zone that were directly connected to the reaction zone. The reactor was set up at an inclination of 75° from vertical position, and feed and water were introduced at the bottom of the reactor. The performances of this new system were investigated with gasification of isooctane at various experimental conditions – reaction temperatures of 601–676 °C, residence times of 6–33 s, isooctane concentrations of 5–33 wt%, and oxidant (hydrogen peroxide) concentrations up to 4507 mmol/L without using catalysts. A significant increase in hydrogen gas yield, almost four times higher than that from the previous up-down gasifier configuration (B. Veriansyah, J. Kim, J.D. Kim, Y.W. Lee, Hydrogen Production by Gasification of Isooctane using Supercritical Water, Int. J. Green Energy. 5 (2008) 322–333) was observed with the present gasifier configuration. High hydrogen gas yield (6.13 mol/mol isooctane) was obtained at high reaction temperature of 637 °C, a low feed concentration of 9.9 wt% and a long residence time of 18 s in the presence of 2701.1 mmol/L hydrogen peroxide. At this condition, the produced gases mainly consisted of hydrogen (59.5 mol%), methane (14.8 mol%) and carbon dioxide (22.0 mol%), and a small amount of carbon monoxide (1.6 mol%) and C₂–C₃ species (2.1 mol%). Reaction mechanisms of supercritical water gasification of isooctane were also presented.     

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.     

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.     

流化床生物质气化发电过程动力学建模与验证

为更好地描述生物质气化过程的反应机理,文章从模型采用的反映速率形式出发,对已建立的动力学模型[1]做了进一步修正,并拟和了以松木屑为生物质原料的气化反应动力学参数,建立了包括质量平衡方程、反应动力学方程以及能量平衡方程在内的整体生物质气化动力学模型。最后以MATLB为平台,通过模型仿真,从反应进程以及最终气体组分两个方面验证了模型的可靠性。为进一步应用该模型评价和优化流化床生物质气化过程气化方案和气化参数打下了基础。     

Biomass gasification in supercritical water: II. Effect of catalyst

In this study, the effect of the type of catalyst on hydrothermal gasification of three specifically chosen samples of natural biomass was investigated. Biomass feedstocks, including lignocellulosic materials (cotton stalk and corncob) and the tannery waste, were gasified in supercritical water by the addition of catalyst. The catalysts used were K₂CO₃, Trona (NaHCO₃·Na₂CO₃·2H₂O), red mud (Fe-oxide containing residue from Al-production) and Raney-Ni. The gasification experiments were performed in a batch autoclave at 500 °C. The amounts and compositions of the gases and the amounts of water soluble compounds from gasification were determined. The effect of catalysts on gasification varied with the type of biomass. The catalysts significantly increased the hydrogen yield by supporting the water–gas shift reaction and the methane reformation.     

基于Gibbs自由能最小化原理模拟生物质流化床气化

基于能质平衡和吉布斯(Gibbs)自由能最小化原理,选择松木屑和麦秆两种生物质,利用化工商业化软件ASPEN PLUS模拟生物质流化床气化过程,并结合试验数据验证模拟结果的准确性。在此基础上考察了高温、原料含水率大范围变化等试验中较难实现的操作对气化的影响。模拟结果表明,搭建的气化模型能较好地模拟生物质气化过程,对生物质气化试验与工程放大具有一定的参考价值。     

Experimental investigation of the effect of physical pre-treatment on air-blown fluidized bed biomass gasification

The effect of comminution, drying, and densification on bubbling fluidized bed gasification was investigated by fractionating a forestry residue into a feedstock consisting of different particle sizes, moisture levels, and by densifying to pellets. The gasification performance was evaluated at nominal average bed temperatures of 725°, 800° and 875 °C at a constant fluidizing velocity (0.91 m s⁻¹) with feed input rates between 9 and 24 kg h⁻¹.The gas composition was observed to be influenced by both the particle size and form. Smaller particles led to a gas richer in carbon monoxide and depleted in hydrogen. The gasification of pellets led to a gas with the greatest hydrogen to carbon monoxide ratio. The smallest particles tested resulted in the worst gasification performance, as defined by cold gas efficiency, carbon conversion, and tar production. Despite differences in the gas composition among the larger particles and the pellets, similar carbon conversion and cold gas efficiency was observed.Relative to comparable test conditions with dry feed fractions (having a moisture mass fraction of 7–12%), an average 11% increase in carbon conversion was observed for the wetter feed fractions containing a moisture mass fraction of 24–31%. This increase in carbon conversion offset much of the expected decrease in cold gas efficiency by using a wetter feed material. A slight increase in hydrogen production and negligible change in tar production was observed for the wetter feed fractions relative to the dry feed fraction.