双流化床生物质气化的三维全循环数值模拟

基于多相质点网格方法 (multi-phase particle-in-cell,MP-PIC)对工业尺度的双流化床生物质气化过程进行了三维全循环数值模拟。其中,在拉格朗日框架下求解颗粒团运动,采用大涡模拟法(large-eddy simulation, LES)求解气相湍流,同时考虑复杂的气固耦合以及生物质的热解、气化、均相/异相反应。首先,通过独立性检验确定了计算所需的最佳网格数与计算颗粒数,且模拟结果和实验结果对比良好。其次,揭示了流化床内生物质气化过程中的气固流动特性及气体组分分布规律,研究了床内温度、生物质粒径、曳力模型等因素对产物气体组分分布的影响。结果表明:温度升高,出口处的CO摩尔分数增加,而其余组分都减小;较小生物质粒径的气化效果要优于较大的生物质颗粒粒径;曳力模型对各产物气体组分的摩尔分数几乎无影响。     

生物质气化技术和气化炉研究进展

简单介绍了生物质气化的基本原理及生物质气化炉的分类,阐述了不同类型气化炉的特点及技术指标.介绍了国内外生物质气化技术在集中供气、供热和发电方面的发展现状和应用情况,重点介绍了适用于大规模生物质气化合成液体燃料的气化炉;指出了生物质气化技术需要解决的问题,提出了我国在生物质气化领域的重点研究方向.     

国内外生物质气化设备研究进展

生物质是重要的可再生能源,生物质气化技术在国内外得到了广泛应用。本文综述了国内外固定床、鼓泡流化床、外循环流化床、内循环流化床、双循环流化床的结构。固定床安装简单,但焦油较多;外循环流化床燃烧效率高,但回料装置较难控制;内循环流化床不易结焦、氢含量高且不用考虑返料问题;双流化床结构复杂但焦油量少。将对固定床和流化床进行对比,认为固定床安装简单适合农村地区,流化床应不断改进和完善,更适应工业化生产。     

生物质循环流化床气化炉运行特性

我国生物质循环流化床气化炉的商业应用仍处在初始阶段,相关方面的公开报道还极少,因此研究其运行特性和控制方法对该炉型气化炉的商业化发展意义重大.以江苏某米厂5 t/h生物质循环流化床气化炉为例,针对气化炉运行过程中不同阶段的床层压降、床层温度、流化风量、返料风及返料蒸汽量等关键参数的变化规律及控制方法进行了分析讨论.结果...     

生物质气化技术及焦油裂解催化剂的研究进展

本文综述了国内外研究机构在生物质气化技术方面的研究进展,主要是对不同于传统气化炉结构的优化设计和对焦油催化裂解所用的催化剂的研究进行了评述,最后指出了进一步的研究与发展方向:如生物质气化反应器的模拟;焦油催化裂解反应机理的研究和动力学模型的建立。     

生物质气化技术及其研究进展

生物质能源是一种理想的可再生能源 ,由于其在燃烧过程中对大气的二氧化碳净排放量近似于零 ,可有效地减少温室效应 ,因而越来越受到世界各国的关注。对生物质能的概念及其转化方式进行了简单介绍 ,着重介绍了生物质气化技术在国内外的发展现状 ,提出了我国在生物质气化领域的重点研究方向。     

生物质流化床气化炉气化过程的实验研究

在流化床生物质气化炉内 ,采用空气作气化剂 ,对七种农、林废弃物进行了气化实验研究。生成的燃气成分 :CO在 1 4 %～ 1 7%之间 ,H2 含量一般低于 1 0 % ,甲烷含量为 5%～ 1 0 %。燃气热值多数在 53 0 0～ 6 50 0 k J/ Nm3 ,气化效率 72 .6 %。实验结果表明 ,流化床生物质气化炉可用于生物质气化。     

煤燃烧解耦双流化床气化反应基础研究

为确定燃烧解耦双流化床气化的气化反应条件和气化反应器的设计,在直径60 mm和高700 mm的小型流化床反应器中,采用粒径8 mm以下的锅炉烟煤以间歇气化方式在1 133 K的条件下,研究了进料方式、气化剂中水蒸汽和O2含量、以及煤料粒径等因素对煤气化生成燃气反应过程的影响,重点考察了各因素对煤转化速率的作用规律.综合各因素对C转化为燃气的速度、最大C转化率及生成气热值的影响趋势,确定了适宜的煤气化操作条件为;从流化颗粒表面附近加料,气化剂中O2体积分率5%、水蒸汽体积分率35%,煤粒径小于5 mm.在该条件下,实现60%的C转化为燃气所需要的停留时间大致为600s.     

双流化床生物质气化动力学建模与分析

为进一步研究双流化床生物质气化器中合成气含量分布,将气化器鼓泡床层分为气泡相和乳化相,依据动力学反应分别进行各相质量和热量衡算,计算结果与实验值吻合较好. 随气化温度升高,CO含量增加,而H2和CO2含量降低;蒸汽与生物质质量比(S/B)增加促进水蒸汽变换和重整反应,消耗CO和CH4,生成H2和CO2,当S/B从0变化到1.2时,CO/H2变化44%,说明S/B增加主要促进了水蒸汽变换反应. 气化温度870℃及S/B=0.75条件下,当气化器高度为0~0.5 m时,H2O含量急剧下降,H2含量急剧上升,CO与CO2含量逐渐上升,当该高度大于0.5 m后,气化反应基本完成.     

Agglomeration of biomass fired fluidized bed gasifier and combustor

Agglomeration is a major problem in biomass fired fluidized bed combustors and gasifiers. Mechanism, reduction options and detection techniques of agglomeration are reviewed. Agglomeration may be classified broadly into three types: defluidization induced agglomeration, melt‐induced agglomeration and coating‐induced agglomeration. Sodium and potassium content of the biomass are the major contributors to the agglomeration in biomass fired fluidized beds. Higher temperature, lower fluidizing velocity and coarser bed particles also increase the risk of agglomeration. Alternative bed materials, additives or the co‐combustion of biomass with other fuels can reduce agglomeration potential of a fluidized bed. Two agglomeration detection techniques are discussed: controlled fluidized bed agglomeration and early agglomeration recognition system.     

A mathematical model of fluidized bed biomass gasification

A mathematical model of biomass gasification in a fluidized bed has been developed. It considers axial variations of concentrations and temperature in the bubble and emulsion phases. The mass balance involves instantaneous oxidation and equilibrium devolatilization of the biomass, kinetics of solid-gas gasification reactions as well as of gaseous phase reactions and interphase mass transfer and gas convection. The energy balance is solved locally for each vertical volume element, and globally on the reactor by iteration on the temperature at the bottom of the bed. Three parameters have been adjusted based on the experimental results: the heat transfer coefficient at the wall, the weighting of the kinetics of the water-gas shift reaction and the fraction of biomass carbon remaining as char after devolatilization. The model is used to simulate a pilot scale (50 kg/h) biomass gasifier, and its predictions compared to experimental measurements. The temperature and gaseous concentrations are estimated with good accuracy for the experiments using a wood feedstock, except for the concentration of hydrogen which is overestimated.     

Three-dimensional numerical modelling of a fast fluidized bed biomass gasifier to generate energy from wastes

The modelling of a biomass fluidized bed gasification system, one of the most effective ways to produce energy from biomass resources and wastes, has been performed in this study. The effect of the turbulence phenomena, including calculations relating to flow turbulence, chemical fuel reactions, and energy and momentum exchange between multiple solid and gas phases, has been taken into account in the current research as a novel approach. A computational fluid dynamics case study model that combines equations with comprehensive geometry has been considered. Results have been compared with published operational records of an existing power plant to validate the model. The solid particle distribution, the velocity of the mixture and gas phase, the turbulent flow viscosity ratio, and the temperature distribution in the model indicated the accuracy of the simulation performance compared with the experimental studies. The production of the molar fraction of the constituent elements of the synthesis gas has been evaluated in transient conditions. Additionally, 35 s after the process began, the system's performance was estimated, and the results indicated the average molecular weights of hydrogen, carbon monoxide, carbon dioxide, and methane are 26%, 23%, 12.5%, and 3.3%, respectively, which presented high precision with the experimental results.     

Potential approaches to improve gasification of high water content biomass rich in cellulose in dual fluidized bed

Biomass containing water of 30-65 wt.% and rich in cellulose, such as various grounds of drinking materials and the lees of spirit and vinegar, is not suitable for biological digestion, and the thermal conversion approach has to be applied to its conversion into bioenergy. The authors have recently worked on converting such biomass into middle heating-value gas via dual fluidized bed gasification (DFBG) integrated with various process intensification technologies. This article is devoted to highlighting those technical ways, including the choice of the superior technical deployment for a DFBG system, the impregnation of Ca onto fuel in fuel drying, the integration of gas cleaning with fuel gasification via two-stage DFBG (T-DFBG), and the decoupling of fuel drying/pyrolysis and char gasification via the decoupled DFBG (D-DFBG). The attained results demonstrated that the superior deployment of bed combination for the DFBG should be a bubbling/turbulent fluidized bed gasifier integrated with a pneumatic riser combustor. In terms of improving efficiency of fuel conversion into combustible gas and suppressing tar generation during gasification, the impregnation of Ca onto fuel exhibited distinctively high upgrading effect, while both the T-DFBG and D-DFBG were also demonstrated to be effective to a certain degree.     

Statistical assessment of a fluidized bed wood gasifier model

Sensitivity analyses were carried out to assess the behaviour of a fluidized bed wood gasifier simulation model in response to changes in the values of design variables, operating variables, and model coefficients. The model was originally developed using the modified Bubble Assemblage Model. Physical design specifications of the fluidized bed were found to have little influence on the quality of the product gas; in fact the composition of the gas was found to depend almost exclusively on the degree of approach to equilibrium. Overall thermal efficiency could be improved only by increasing the dry wood to oxygen ratio and reducing the moisture content in the wood feed. Because wood gasifiers produce volatiles that greatly enhance the chance of slugging, the influences of the design and operating variables on the occurrence of slugging were also investigated. The onset of slugging was found to be related most strongly to the dynamic bed height.     

Modeling and simulation of biomass air-steam gasification in a fluidized bed

By considering the features of fluidized-bed reactors and the kinetic mechanism of biomass gasification, a steady-state, isothermal, one-dimensional and two-phase mathematical model of biomass gasification kinetics in bubbling fluidized beds was developed. The model assumes the existence of two phases — a bubble and an emulsion phase — with chemical reactions occurring in both phases. The axial gas dispersion in the two phases is accounted for and the pyrolysis of biomass is taken to be instantaneous. The char and gas species CO, CO₂, H₂, H₂O, CH₄ and 8 chemical reactions are included in the model. The mathematical model belongs to a typical boundary value problem of ordinary differential equations and its solution is obtained by a Matlab program. Utilizing wood powder as the feedstock, the calculated data show satisfactory agreement with experimental results and proves the effectiveness and reliability of the model. __________ Translated from Chemical Engineering (China), 2007, 35(10): 23–26 [译自: 化学工程]     

生物质流化床空气-水蒸气气化模型研究

根据流化床反应器特点,结合生物质气化动力学反应机理,建立了生物质在流化床内气化的等温稳态、一维二相动力学模型。该模型所做的主要假定如下:流化床分为气泡相和乳相,在气泡相和乳相内均存在化学反应,考虑二相内的轴向气体扩散,生物质热解过程瞬时完成,主要考虑焦碳以及CO,CO2,H2,H2O,CH4等在流化床内发生的8个主要化学反应。数学模型属于常微分方程组边值问题,利用数值计算软件M atlab7.0进行编程求解。以木粉为原料,将模型结果与实验结果进行了对比,模拟结果与试验数据符合良好,在一定程度上证明了模型的有效性和可靠性。     

Experimental study of a commercial circulated fluidized bed coal gasifier

This paper presented the performance data of a commercial scale circulating fluidized bed gasifier, CFBG-800-I. The operation condition effects on gasifier temperature distribution and gasifier performance were studied. It is found that the external cycle has a critical influence on the gasifier temperature profiles. Both the Air/Coal ratio and Steam/Coal ratio affect the bed temperature. The increasing in Air/Coal ratio decreases the valuable gases content, which is adverse to the gasifier performance but increases the gasifier temperature, which is favorable for the gasification reactions. The best choice of the Air/Coal ratio is a tradeoff of gasifier performance and gasifier temperature. The Steam/Coal ratio could influence gasifier temperature and gasifier performance in several ways. Increasing steam increased the water gas reaction and the CO, H₂ concentrations increase firstly, and then decrease at a Steam/Coal ratio of 0.32. Increased Steam/Coal ratio decreases the bed temperature, which is bad for the gasifier performance, and will decrease the carbon conversion efficiency. The Steam/Coal ratio should be carefully selected by comprehensive evaluation also.