生物质发电气化过程的机理建模分析

生物质气化发电的关键技术是生物质气化技术,目前国内外对生物质气化发电技术的研究,还缺乏通用的气化模型和方法来模拟气化过程的特性,不能准确地确定生物质燃气的组分和热值等参数,难以提供气化发电系统的可靠数据.最常用的气化过程建模方法是建立机理模型,文章在重点分析了气化过程机理的基础上,把气化模型划分为平衡模型和动态模型两大类,并比较了各模型的优缺点.     

生物质气化—燃料电池发电系统性能分析

建立了基于热力学平衡的生物质气化模型,利用平衡模型分析了气化过程的特性,研究了气化过程的反应规律及各种因素对气化性能指标的影响,详细分析了当量比及物料湿度对气体产物成分及气化产物热值的影响.同时,建立了以生物质气为燃料的固体氧化物燃料电池的数学模型,该模型考虑了燃料电池的能斯特电动势及各种极化损失.利用建立的模型分析了操作参数以及物料湿度和生物质种类对生物质气化—燃料电池发电系统性能的影响.结果表明,生物质气化—燃料电池发电系统的发电效率可达30％,热电联产效率最高可达95％以上.     

生物质气化过程的可视化研究

生物质燃料种类多,气化过程复杂多变。生物质燃料气化反应条件、规律及反应物的研究,是生物质气化技术研究的重点。文章提出将电容层析成像技术用于生物质气化过程的可视化监测;采用COMSOL建立了生物质气化炉电容层析成像模型;分析了不同生物质介质对电容敏感场的影响。文章通过仿真计算求取灵敏度矩阵及电容值;应用串联的归一化模型对电容值进行归一化处理;应用两种不同算法对气化炉内不规则介质进行成像,对生物质气化过程可视化监测做了有益的探索。     

生物质气化制氢的模拟

以秸秆为研究对象,利用Aspen P lus软件建立气化反应器模型,对生物质气化制氢进行模拟计算.探讨不同反应条件,包括气化温度、生物质与蒸汽质量配比以及催化剂对富氢气体成分的影响.计算结果表明,未加催化剂条件下,采用生物质蒸汽气化技术可获得体积分数为6000/以上的富氢燃料气,增大蒸汽与生物质质量配比有利于氢气产率的提高;添加CaO、MgO催化剂可较大幅度地提高氢气产率,氢气体积分数最大可达到9400/,其中CaO对生物质气化制氢过程的催化作用非常显著.     

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。     

基于多目标遗传算法的生物质气化过程参数优化

生物质气化过程是一个复杂的多目标非线性过程。通过对气化过程的机理分析,针对麦秸和玉米秸这2种软质秸秆类生物质原料特性,建立了气化过程的优化目标函数。在此基础上,采用多目标遗传算法对该目标函数进行优化设计计算。计算结果表明,该目标函数对生物质气化过程参数优化具有良好效果,也验证了该算法对于全局优化以及解决复杂非线性问题的有效性。     

生物质气化焦油脱除过程参数优化方法

焦油是生物质气化过程中的有害产物,它会降低燃气品质,对气化设备及后续用气设备产生危害.本文通过对生物质气化过程中影响焦油生成量的因素进行分析,依据最小二乘曲线拟合原理和目标规划理论建立了生物质料木屑气化过程焦油脱除的参数优化模型,在此基础上采用遗传算法对焦油脱除过程优化模型进行参数寻优.计算结果表明,当气化温度为796.6℃、当量比为0.203时,木屑气化的焦油生成量最小.     

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。     

生物质气化过程最小二乘支持向量机建模探讨

生物质气化过程的最终目标就是尽可能得到更多的高品质可燃气体,而目前的生物质气化过程还存在许多尚待解决的问题,例如气化温度、气化剂当量比、气化效率、燃气热值等参数优化问题.为此建立一种能适应生物质气化过程的模型,用于预测生物质气化气组分、热值、气化效率及碳转化率等指标,以及实现气化过程的参数优化都具有现实意义.本文在对生物质气化过程建模现状分析基础上,初步提出一种基于最小二乘支持向量机的气化过程建模方法,探讨了该方法用于生物质气化过程建模以及对气化过程主要参数进行优化的可行性.     

生物质直燃和混燃发电环境效益分析

利用生物质代替矿物燃料发电可以减少CO2和SO2的排放量。确定了燃煤机组CO2和SO2排放量基准,建立生物质发电的CO2和SO2的排放量模型及其偏差模型;计算不同发电方式下CO2和SO2的生成量及减排量;分析了气化炉气化效率对生物质发电CO2和SO2生成量的影响。结果表明,提高生物质发电效率和气化效率可以显著降低CO2和SO2的排放;生物质发电的环境效益明显优于燃煤发电,而生物质气化合成气与煤混燃发电的环境效益优于生物质直燃发电。     

基于热化学平衡机理的生物质气化过程模型

生物质气化是生物质能利用的有效方式之一,通过分析气化过程热化学反应机理,基于气化过程的物料平衡、热量平衡和热化学反应平衡关系,建立了一种生物质气化过程的热化学平衡模型.由该模型模拟计算了锯屑、棉花秸和木粉三种生物质料在以空气为气化剂,不同输入参数(物料湿度和气化剂当量比)下的气化气组分及气体产率,考察了当模型输入参数分别变化时气化气组分、气体产率的变化情况,结果表明该模型输出值与文献值的变化趋势基本相符,模拟具有较好的预测效果.     

Theoretical and experimental investigation of biomass gasification process in a fixed bed gasifier

This investigation concerns the process of air biomass gasification in a fixed bed gasifier. Theoretical equilibrium calculations and experimental investigation of the composition of syngas were carried out and compared with findings of other researchers. The influence of excess air ratio (λ) and parameters of biomass on the composition of syngas were investigated. A theoretical model is proposed, based on the equilibrium and thermodynamic balance of the gasification zone.The experimental investigation was carried out at a setup that consists of a gasifier connected by a pipe with a water boiler fired with coal (50 kWth). Syngas obtained in the gasifier is supplied into the coal firing zone of the boiler, and co-combusted with coal. The moisture content in biomass and excess air ratio of the gasification process are crucial parameters, determining the composition of syngas. Another important parameter is the kind of applied biomass. Despite similar compositions and dimensions of the two investigated feedstocks (wood pellets and oats husk pellets), compositions of syngas obtained in the case of these fuels were different. On the basis of tests it may be stated that oats husk pellets are not a suitable fuel for the purpose of gasification.     

Numerical and experimental investigation of hydrogen-rich syngas production via biomass gasification

In this work, the relation between hydrogen-rich syngas production and the gasification parameters such as equivalence ratio (ER), gasification temperature and biomass moisture content were studied. Stoichiometric equilibrium model that developed during this study was used to investigate the optimum hydrogen output generated from woody biomass in a fixed bed downdraft gasifier by considering the thermodynamic equilibrium limit. The mathematical model, based on thermodynamic equilibrium is necessary to understand complicated gasification process that will contribute to obtain maximum attainable hydrogen production. The effects of different oxidizing agents on the hydrogen concentration in the product gas as well as the effect of various air-biomass, oxygen-biomass and steam-biomass ratios were investigated. For validation, the results obtained from the mathematical model were compared with the experimental data obtained from the gasifier that uses air as gasification medium. The validated mathematical model was used to represent the gasifier that uses both oxygen and air-steam mixture as the gasification medium and the theoretical results were obtained for both cases. The theoretical results clearly show that the gasification process specially ones that use the air-steam mixture as the gasification medium can be used for hydrogen production.     

Performance prediction of steam gasification of wood using an ASPEN PLUS thermodynamic equilibrium model

In this article, an equilibrium model based on Gibbs free energy minimisation is presented for steam gasification of biomass in process simulator ASPEN PLUS. Carbon is assumed as fully converted into product gases and no tar content is assumed to be present in gaseous product. The objective is to arrive at the optimum process conditions of gasification. An analysis on the sensitivity of producer gas composition, lower heating value, combustible gas yield, and first and second law efficiencies on gasification process variables including reactor temperature, pressure and steam to biomass mass ratio is also envisaged. Simulations are performed with wood as the biomass material, based on real gas behaviour for product gases and gasifying medium. The predicted results of the model are compared with another Gibbs free energy model formulated using simulated annealing minimisation algorithm. The present ASPEN PLUS model is validated with published experimental results on steam gasification on a fluidised bed gasifier.     

Performance correlations for biomass gasifiers using semi‐equilibrium non‐stoichiometric thermodynamic models

Biomass gasification has been a viable alternative for decentralized electricity generation in developing countries. The efficiency of the biomass gasification process for operation of the engine‐generator set is mapped in terms of quality and quantity of the producer gas. In this study, we have attempted to devise generalized correlations for four principal parameters that form the benchmark for the performance of the gasifier. These parameters are lower heating value and net yield (per unit biomass) of producer gas, and volume fractions of CO and CO₂ in the gas resulting from biomass gasification process. The correlations have been constituted using simulations of gasification of three common biomass feedstocks (viz. rice husk, saw dust and corn cobs) using semi‐equilibrium non‐stoichiometric thermodynamic model. The independent variables used in the simulations are air ratio, carbon conversion, gasification temperature and three elemental ratios in the gasification mixture, viz. H/C, O/H and O/C. As many as eight expressions of linear and non‐linear type have been evaluated to best fit the simulations data for each performance parameter. On the basis of statistical indicators, the compatibility of the correlations for best fit of the data has been assessed. Finally, the predictions of the correlation have been tested against experimental data on gasification of different biomass. The best correlation for each performance parameter was chosen on the basis of least average absolute error and highest (absolute) regression coefficient. It was found that the set of best correlations could predict the values of performance parameters within engineering accuracy of ± 10–20%. The correlations proposed in this work are independent of the type of biomass gasifier. These correlations can form a useful tool for design and optimization of fixed or fluidized bed gasifier for any biomass feedstock. Copyright © 2011 John Wiley & Sons, Ltd.     

Energy and exergy analysis of biomass gasification at different temperatures

Biomass is usually gasified above the optimal temperature at the carbon-boundary point, due to the use of different types of gasifiers, gasifying media, clinkering/slagging of bed material, tar cracking, etc. This paper is focused on air gasification of biomass with different moisture at different gasification temperatures. A chemical equilibrium model is developed and analyses are carried out at pressures of 1 and 10 bar with the typical biomass feed represented by CH_1.4O_0.59N_0.0017. At the temperature range 900–1373 K, the increase of moisture in biomass leads to the decrease of efficiencies for the examined processes. The moisture content of biomass may be designated as “optimal” only if the gasification temperature is equal to the carbon-boundary temperature for biomass with that specific moisture content. Compared with the efficiencies based on chemical energy and exergy, biomass feedstock drying with the product gas sensible heat is less beneficial for the efficiency based on total exergy. The gasification process at a given gasification temperature can be improved by the use of dry biomass and by the carbon-boundary temperature approaching the required temperature with the change of gasification pressure or with the addition of heat in the process.     

Hydrogen production by partial oxidative gasification of biomass and its model compounds in supercritical water

Partial oxidative gasification in supercritical water is a new technology for hydrogen production from biomass. Firstly in this paper, supercritical water partial oxidative gasification process was analyzed from the perspective of theory and chemical equilibrium gaseous product was calculated using the thermodynamic model. Secondly, the influence of oxidant equivalent ratio on partial oxidative gasification of model compounds (glucose, lignin) and real biomass (corn cob) in supercritical water was investigated in a fluidized bed system. Experimental results show that oxidant can improve the gasification efficiency, and an appropriate addition of oxidant can improve the yield of hydrogen in certain reaction condition. When ER equaled 0.4, the gasification efficiency of lignin was 3.1 times of that without oxidant. When ER equaled 0.1, the yield of hydrogen from lignin increased by 25.8% compared with that without oxidant. Thirdly, the effects of operation parameters including temperature, pressure, concentration, and flow rate of feedstock on the gasification were investigated. The optimal operation parameters for supercritical water partial oxidative gasification were obtained.