An experimental study on biomass air-steam gasification in a fluidized bed

The characteristics of biomass air-steam gasification in a fluidized bed are studied in this paper. A series of experiments have been performed to investigate the effects of reactor temperature, steam to biomass ratio (S/B), equivalence ratio (ER) and biomass particle size on gas composition, gas yield, steam decomposition, low heating value (LHV) and carbon conversion efficiency. Over the ranges of the experimental conditions used, the fuel gas yield varied between 1.43 and 2.57 Nm3/kg biomass and the LHV of the fuel gas was between 6741 and 9143 kJ/Nm3. The results showed that higher temperature contributed to more hydrogen production, but too high a temperature lowered gas heating value. The LHV of fuel gas decreased with ER. Compared with biomass air gasification, the introduction of steam improved gas quality. However, excessive steam would lower gasification temperature and so degrade fuel gas quality. It was also shown that a smaller particle was more favorable for higher gas LHV and yield.     

Biomass as an energy source: thermodynamic constraints on the performance of the conversion process

In the present work an equilibrium model (gas-solid), based on the minimization of the Gibbs energy, has been used in order to estimate the theoretical yield and the equilibrium composition of the reaction products (syngas and char) of biomass thermochemical conversion processes (pyrolysis and gasification). The data obtained from this model have also been used to calculate the heating value of the fuel gas, in order to evaluate the overall energy efficiency of the thermal conversion stage. The proposed model has been applied both to partial oxidation and steam gasification processes with varying air to biomass (ER) and steam to carbon (SC) ratio values and using different feedstocks; the obtained results have been compared with experimental data and with other model predictions obtaining a satisfactory agreement.     

Steam–air fluidized bed gasification of distillers grains: Effects of steam to biomass ratio,equivalence ratio and gasification temperature

In this study, thermochemical biomass gasification was performed on a bench-scale fluidized-bed gasifier with steam and air as fluidizing and oxidizing agents. Distillers grains, a non-fermentable byproduct of ethanol production, were used as the biomass feedstock for the gasification. The goal was to investigate the effects of furnace temperature, steam to biomass ratio and equivalence ratio on gas composition, carbon conversion efficiency and energy conversion efficiency of the product gas. The experiments were conducted using a 3 × 3 × 3 full factorial design with temperatures of 650, 750 and 850 °C, steam to biomass ratios of 0, 7.30 and 14.29 and equivalence ratios of 0.07, 0.15 and 0.29. Gasification temperature was found to be the most influential factor. Increasing the temperature resulted in increases in hydrogen and methane contents, carbon conversion and energy efficiencies. Increasing equivalence ratio decreased the hydrogen content but increased carbon conversion and energy efficiencies. The steam to biomass ratio was optimal in the intermediate levels for maximal carbon conversion and energy efficiencies.     

生物质气化制取代用天然气的模拟

介绍了生物质气化制取代用天然气的技术,并利用Aspen Plus软件建立了串行流化床生物质气化制取代用天然气的模型,并对整个流程进行模拟。着重研究了气化过程中操作参数(气化温度Tg、S/B)对甲烷化反应过程主要指标(包括甲烷产率、碳转化率等)的影响。研究结果表明提高气化温度和S/B有利于提高气化产物中生物质合成气的浓度,并得到较高氢碳比的合成气,从而可以提高甲烷的产率和整个系统的碳转化率;为获得较高的甲烷产率和碳转化率,适宜的气化温度为700～750℃,S/B值在0.4左右。     

Effects of gasifying conditions and bed materials on fluidized bed steam gasification of wood biomass

The effect of steam gasification conditions on products properties was investigated in a bubbling fluidized bed reactor, using larch wood as the starting material. For bed material effect, calcined limestone and calcined waste concrete gave high content of H(2) and CO(2), while silica sand provided the high content of CO. At 650 degrees C, calcined limestone proved to be most effective for tar adsorption and showed high ability to adsorb CO(2) in bed. At 750 degrees C it could not capture CO(2) but still gave the highest cold gas efficiency (% LHV) of 79.61%. Steam gasification gave higher amount of gas product and higher H(2)/CO ratio than those obtained with N(2) pyrolysis. The combined use of calcined limestone and calcined waste concrete with equal proportion contributed relatively the same gas composition, gas yield and cold gas efficiency as those of calcined limestone, but showed less attrition, sintering, and agglomeration propensities similar to the use of calcined waste concrete alone.     

Supercritical water gasification of biomass: Thermodynamic constraints

In the present work, the supercritical water gasification (SCWG) of biomass is analyzed with a view to outlining the possible thermodynamic constraints that must be taken into account to develop this new process. In particular, issues concerning the formation of solid carbon and the process heat duty are discussed. The analysis is conducted by means of a two-phase non-stoichiometric thermodynamic model, based on Gibbs free energy minimization. Results show that char formation at equilibrium only occurs at high biomass concentrations, with a strong dependence on biomass composition. As regards the process heat duty, SCWG is mostly endothermic when biomass concentration is low, although a very small amount of oxidizing agent is able to make the process exothermic, with only a small loss in the heating value of the syngas produced.     

Assessment of black liquor gasification in supercritical water

Supercritical water gasification of black liquor (waste pulping chemicals) has been examined. The aim was to evaluate the feasibility of using this technique to convert such bio-based waste to value added fuel products, as well as recovery of pulping materials. Supercritical gasification may improve overall process efficiency by eliminating the energy intensive evaporation step necessary in conventional process and product gas obtained at high pressure may be ready for utilization without any compression requirement. Appropriate operating parameters, including pressure, temperature, feed concentration, and reaction time, which would yield the highest conversion and energy efficiency were determined. Reaction was performed in a quartz capillary heated in a fluidized bed reactor. Results indicated that pressure between 220 and 400 atm has insignificant influence on the gas products and extent of carbon conversion. Increasing temperature and residence time between 375-650 degrees C and 5-120 s resulted in greater gas production, overall carbon conversion, and energy efficiency. Maximum conversion to H(2), CO, CH(4), and C(2)H(X) was achieved at the highest temperature and longest residence time tested showing an overall carbon conversion of 84.8%, gas energy content of 9.4 MJ/m(3) and energy conversion ratio of 1.2. Though higher carbon conversion and energy conversion ratio were obtained with more dilute liquor, energy content was lower than for those with higher solid contents. Due to anticipated complex design and high initial investment cost of this operation, further studies on overall feasibility should be carried out in order to identify the optimum operating window for this novel process.     

Energy and exergy analyses of a biomass-based hydrogen production system

In this paper, a novel biomass-based hydrogen production plant is investigated. The system uses oil palm shell as a feedstock. The main plant processes are biomass gasification, steam methane reforming and shift reaction. The modeling of the gasifier uses the Gibbs free energy minimization approach and chemical equilibrium considerations. The plant, with modifications, is simulated and analyzed thermodynamically using the Aspen Plus process simulation code (version 11.1). Exergy analysis, a useful tool for understanding and improving efficiency, is used throughout the investigation, in addition to energy analysis. The overall performance of the system is evaluated, and its efficiencies become 19% for exergy efficiency and 22% energy efficiency while the gasifier cold gas efficiency is 18%.     

Unconverted chars obtained during biomass gasification on a pilot-scale gasifier as a source of activated carbon production

Biomass gasification was used to produce activated carbon on a pilot-scale fluidised-bed gasifier. The feedstock included both biomass alone and biomass mixed with coal and coal/granulated plastic wastes. This paper reports the results obtained from four different runs undertaken under various conditions of fuel supply, different ratios of steam/air for the gasification and temperature. These conditions were selected because they led to a significant amount of unconverted chars produced during gasification (from 0.72 to 1.4 kg) which then served as raw material for the production of activated carbon whilst the amount of gas obtained was also high enough for its potential use for different end-use applications. From the analysis of the results obtained, it can be concluded that a reasonable porosity development (mainly in the area of narrow micropores) was obtained by gasifying unblended pine wastes with steam for 4 h, producing about 1.4 kg of good-quality activated carbon (micropore volume of 0.263 cm(3)/g). In other runs, chars with a reduced microporosity development (i.e. 0.180 cm(3)/g) were obtained, however, they could be used as a proper starting material for the chemically activated carbon production.     

Effects of the reforming reagents and fuel species on tar reforming reaction

In this study, using wood chips and polyethylene (PE) as fuels, the effects of air and/or steam as reagents on the tar reforming were clarified quantitatively with a simulated gasifier/reformer apparatus of a two-staged gasification process. The results show that when only steam or air was supplied into the reformer, the tar residual rate (defined as the ratio of the tar amount in the reformed gas to the tar amount in the pyrolysis gas) and the carbon particulate concentration in both reformed gases produced from pyrolysis gases of wood chips and PE decreased with the increase of the steam ratio (H₂O/C, 0–1.0) or the air ratio (ER, 0–0.30). Supplying steam into the reformer to suppress carbon particulate formation for PE pyrolysis gas is more effective than for wood chips pyrolysis gas. Comparing with the results of steam only reforming, the effect of air supply on reduction of the tar residual rate was more significant, while that on suppression of carbon particulate formation was smaller.     

Low temperature catalytic gasification of pig compost to produce H2 rich gas

The low temperature catalytic gasification of pig compost before and after acid washing was carried out to produce H2 rich gas using a two-stage fixed-bed reactor. Little effect of the minerals on the manure pyrolysis is determined. Under the presence of Ni/Al2O3 catalyst nearly all the tarry matters were cracked into H2, CO, CO2 and residual carbon. High H2 and CO yields were obtained by low temperature catalytic steam gasification. Acid washing results in the decrease in the content of the ease-hydrolyzed organic components, which volatilize at low temperature. The change in the gas yields from the manure during catalytic decomposition is in accordance with its pyrolysis behavior.     

Experimental study on air-stream gasification of biomass micron fuel (BMF) in a cyclone gasifier

Based on biomass micron fuel (BMF) with particle size of less than 250 microm, a cyclone gasifier concept has been considered in our laboratory for biomass gasification. The concept combines and integrates partial oxidation, fast pyrolysis, gasification, and tar cracking, as well as a shift reaction, with the purpose of producing a high quality of gas. In this paper, experiments of BMF air-stream gasification were carried out by the gasifier, with energy for BMF gasification produced by partial combustion of BMF within the gasifier using a hypostoichiometric amount of air. The effects of ER (0.22-0.37) and S/B (0.15-0.59) and biomass particle size on the performances of BMF gasification and the gasification temperature were studied. Under the experimental conditions, the temperature, gas yields, LHV of the gas fuel, carbon conversion efficiency, stream decomposition and gasification efficiency varied in the range of 586-845 degrees C, 1.42-2.21 N m(3)/kg biomass, 3806-4921 kJ/m(3), 54.44%-85.45%, 37.98%-70.72%, and 36.35%-56.55%, respectively. The experimental results showed that the gasification performance was best with ER being 3.7 and S/B being 0.31 and smaller particle, as well as H(2)-content. And the BMF gasification by air and low temperature stream in the cyclone gasifier with the energy self-sufficiency is reliable.     

Development of a supported tri-metallic catalyst and evaluation of the catalytic activity in biomass steam gasification

A supported tri-metallic catalyst (nano-Ni–La–Fe/γ-Al₂O₃) was developed for tar reduction and enhanced hydrogen production in biomass steam gasification, with focuses on preventing coke deposition and sintering effects to lengthen the lifetime of developed catalysts. The catalyst was prepared by deposition–precipitation method and characterized by various analytical approaches. Following that, the activity of catalysts in biomass steam gasification was investigated in a bench-scale combined fixed bed reactor. With presence of the catalyst, the content of hydrogen in gas products was increased to over 10 vol.%, the tar removal efficiency reached 99% at 1073 K, and more importantly the coke deposition on the catalyst surfaces and sintering effects were avoided, leading to a long lifetime of catalysts.     

Simulation of biomass gasification with a hybrid neural network model

Gasification of several types of biomass has been conducted in a fluidized bed gasifier at atmospheric pressure with steam as the fluidizing medium. In order to obtain the gasification profiles for each type of biomass, an artificial neural network model has been developed to simulate this gasification processes. Model-predicted gas production rates in this biomass gasification processes were consistent with the experimental data. Therefore, the gasification profiles generated by neural networks are considered to have properly reflected the real gasification process of a biomass. Gasification profiles identified by neural network suggest that gasification behavior of arboreal types of biomass is significantly different from that of herbaceous ones.     

Characteristics of rice husk gasification in an entrained flow reactor

Experiments were performed in an entrained flow reactor to better understand the characteristics of biomass gasification. Rice husk was used in this study. Effects of the gasification temperature (700, 800, 900 and 1000 °C) and the equivalence ratio in the range of 0.220.34 on the biomass gasification and the axial gas distribution in the reactor were studied. The results showed that reactions of CnHm were less important in the gasification process except cracking reactions which occurred at higher temperature. In the oxidization zone, reactions between char and oxygen had a more prevailing role. The optimal gasification temperature of the rice husk could be above 900 °C, and the optimal value of ER was 0.25. The gasification process was finished in 1.42 s when the gasification temperature was above 800 °C. A first order kinetic model was developed for describing rice husk air gasification characteristics and the relevant kinetic parameters were determined.     

Economic analysis of biomass power generation schemes under renewable energy initiative with Renewable Portfolio Standards (RPS) in Korea

An economic analysis of biomass power generation was conducted. Two key technologies--direct combustion with a steam turbine and gasification with a syngas engine--were mainly examined. In view of the present domestic biomass infrastructure of Korea, a small and distributed power generation system ranging from 0.5 to 5 MW(e) was considered. It was found that gasification with a syngas engine becomes more economically feasible as the plant size decreases. Changes in the economic feasibilities with and without RPS or heat sales were also investigated. A sensitivity analysis of each system was conducted for representative parameters. Regarding the cost of electricity generation, electrical efficiency and fuel cost significantly affect both direct combustion and gasification systems. Regarding the internal rate of return (IRR), the heat sales price becomes important for obtaining a higher IRR, followed by power generation capacity and electrical efficiency.     

Dynamic modeling of syngas fermentation in a continuous stirred-tank reactor: Multi-response parameter estimation and process optimization

Syngas fermentation is one of the bets for the future sustainable biobased economies due to its potential as an intermediate step in the conversion of waste carbon to ethanol fuel and other chemicals. Integrated with gasification and suitable downstream processing, it may constitute an efficient and competitive route for the valorization of various waste materials, especially if systems engineering principles are employed targeting process optimization. In this study, a dynamic multi-response model is presented for syngas fermentation with acetogenic bacteria in a continuous stirred-tank reactor, accounting for gas–liquid mass transfer, substrate (CO, H₂) uptake, biomass growth and death, acetic acid reassimilation, and product selectivity. The unknown parameters were estimated from literature data using the maximum likelihood principle with a multi-response nonlinear modeling framework and metaheuristic optimization, and model adequacy was verified with statistical analysis via generation of confidence intervals as well as parameter significance tests. The model was then used to study the effects of process conditions (gas composition, dilution rate, gas flow rates, and cell recycle) as well as the sensitivity of kinetic parameters, and multiobjective genetic algorithm was used to maximize ethanol productivity and CO conversion. It was observed that these two objectives were clearly conflicting when CO-rich gas was used, but increasing the content of H₂ favored higher productivities while maintaining 100% CO conversion. The maximum productivity predicted with full conversion was 2 g·L⁻¹·hr⁻¹ with a feed gas composition of 54% CO and 46% H₂ and a dilution rate of 0.06 hr⁻¹ with roughly 90% of cell recycle.     

Influence of carbon dioxide solubility on the accuracy of measurements of carbon dioxide production rate by gas balance technique

The total amount of carbon dioxide (CD) produced by a microbial culture is evolved via an outlet gas stream, outflowing broth, and can lead to a rise of the CD content in broth. To analyze relations among the three amounts, the equilibrium is considered between different ionic forms of carbon dioxide at different pH values as well as between the gaseous and disolved CO₂. Dependences of the equilibrium constants on temperature are found by a thermodynamic analysis. Numerical estimation of an error of the CD production rate measurement, originating from the neglect of dissolved CD, is developed. The gas analysis technique alone provides a sufficient accuracy at pH lower than 6. At higher pH the error can be estimated using equations presented below.     

Steam reforming of bio-oil from rice husks fast pyrolysis for hydrogen production

Steam reforming of two kinds of bio-oil from rice husks fast pyrolysis was conducted for hydrogen production at three temperatures (650, 750 and 850 °C) with Ni-based catalyst in a fixed-bed reactor. The gas composition and organic compounds in liquid condensate were detected by gas chromatography (GC) and gas chromatography-mass spectrometry (GC-MS), respectively. In addition, the carbon deposition was also investigated. The results showed that the mole fraction range of hydrogen was within 55.8-61.3% at all temperatures and more hydrogen was produced at the higher temperature. The highest H? efficiency of bio-oil steam reforming was 45.33% when extra water was added. The bio-oil with lower content of chemical compounds has a higher H? efficiency, but its hydrogen volume was less. Analysis of the liquid condensate showed that most of the organic compounds were circularity compounds. The carbon deposition can decrease the bio-oil conversion, and it was easier to form at the temperature of 750 °C.     

Controlling composition of coexisting phases via molecular transitions

Phase separation and transitions among different molecular states are ubiquitous in living cells. Such transitions can be governed by local equilibrium thermodynamics or by active processes controlled by biological fuel. It remains largely unexplored how the behavior of phase-separating systems with molecular transitions differs between thermodynamic equilibrium and cases in which the detailed balance of the molecular transition rates is broken because of the presence of fuel. Here, we present a model of a phase-separating ternary mixture in which two components can convert into each other. At thermodynamic equilibrium, we find that molecular transitions can give rise to a lower dissolution temperature and thus reentrant phase behavior. Moreover, we find a discontinuous thermodynamic phase transition in the composition of the droplet phase if both converting molecules attract themselves with similar interaction strength. Breaking the detailed balance of the molecular transition leads to quasi-discontinuous changes in droplet composition by varying the fuel amount for a larger range of intermolecular interactions. Our findings showcase that phase separation with molecular transitions provides a versatile mechanism to control properties of intracellular and synthetic condensates via discontinuous switches in droplet composition.