Mathematical Modelling of Biomass Gasification in a Circulating Fluidized Bed CFB Reactor

The scope of the present paper is to investigate the suitability of a mathematical model for Circulating Fluidized Bed (CFB) coal combustion (developed by the International Energy Agency), to predict and simulate the performance of the 100 kWth CFB for air-blown biomass gasification. The development of a mathematical model allows to simulate the operative conditions during biomass gasification, control the quality of the synthesis gas and improve the gasifier design. The geometrical, mechanical, hydro dynamical and thermo chemical features were introduced in the model by properly setting the input file and, some changes have been made in the code to assure the final convergence. A sensitivity analysis has been performed to study the variation in the input parameters of the program, and it has been finally verified by comparing the results with the empirical data collected during coal and wood combustion tests. The program, in the same case, could not successfully run;probably depending on wood char density value. For these reason the influence of char density will be investigated. The model predicts the development of tar and other hydrocarbons, valuating the agreement between the measured and calculated efficiency. A further development, to consider solid biomass, with a certain volatile percentages (20% - 40%), as a fuel has been previewed and analyzed. Finally some investigations have been carried out to provide some useful indications for future developments of the code, in the biomass gasification

Chemical-looping gasification of biomass in a 10 kW_(th) interconnected fluidized bed reactor using Fe_2O_3/Al_2O_3 oxygen carrier

Abstract:The aim of this research is to design and operate a 10 kW hot chemical-looping gasification(CLG)unit using Fe2O3/Al2O3as an oxygen carrier and saw dust as a fuel.The effect of the operation temperature on gas composition in the air reactor and the fuel reactor,and the carbon conversion of biomass to CO2and CO in the fuel reactor have been experimentally studied.A total60 h run has been obtained with the same batch of oxygen carrier of iron oxide supported with alumina.The results show that CO and H2concentrations are increased with increasing temperature in the fuel reactor.It is also found that with increasing fuel reactor temperature,both the amount of residual char in the fuel reactor and CO2concentration of the exit gas from the air reactor are degreased.Carbon conversion rate and gasification efficiency are increased by increasing temperature and H2production at 870℃reaches the highest rate.Scanning electron microscopy(SEM),X-ray diffraction(XRD)and BET-surface area tests have been used to characterize fresh and reacted oxygen carrier particles.The results display that the oxygen carrier activity is not declined and the specific surface area of the oxygen carrier particles is not decreased significantly.

Simulation Study on Influence of Steam on Co-gasification of Biomass and Coal in a Fluidized Bed

生物质与煤共气化可解决生物质不易稳定流化以及气化气焦油含量高的问题。文中利用Aspen Plus软件对流化床生物质与煤共气化进行模拟。为了更真实地反映流化床内的实际反应情况，采用嵌入 FORTRAN 气化反应动力学及流体动力学子程序的连续搅拌釜式反应器来代替惯用的Gibbs反应器，结果表明该模型的模拟值与试验值吻合较好。利用灵敏度分析功能研究了水蒸气/物料质量比（S/F）对产气组分、热值和气化效率等参数的影响，结果表明：增大水蒸气与物料质量比有利于氢气产率的提高，当S/F值取0.35~0.45时，气化效率和产气热值较高。

An Experimental Investigation of Fluidized Bed Gasification of Biomass Blended from Wood, Miscanthus, Straw and other Industrial Bioresidues

In order to identify potential wood substitutes for the production of energy by gasification, binary blends （wood/miscanthus, miscanthus/straw and wood/straw） and ternary blends （wood, miscanthus and organic residue） were systematic tested in a laboratory bubbling fluidized bed gasification system. The results of experiments were compared with results of wood gasification. Of the binary blends, wood and miscanthus exhibited great potential as a wood substitute in fluidized bed gasification in terms of process stability and product gas quality. Adding 10 wt. % of organic residues to form ternary blends further improved the product gas quality. Gasification of fuels blended with straw tended to agglomerate in the fluidized bed because of straw＇s low ash melting temperature. This can be counteracted by adding Ca（OH）2 to fuels. Nonetheless, fuels blended with straw with higher percentages of Ca（OH）2 need further study to establish the optimal additive ratio.

Detection of onset of agglomeration in a bubbling fluidized bed biomass combustor using reactive Eulerian computational fluid dynamics

The choice of a type of combustion technology to be used for heat or power generation depends on economic,technical,operational and fuel availability constraints.The benefits associated with the evolving market driven by the fluidised bed combustion(FBC)technology cannot be overlooked especially when gauged at 65 GWth of worldwide installed capacity alongside added benefits of handling fuel variation,low pollutant emissions and high combustion efficiency.Biomass or biomass waste will continue to have a vital role to play in the future FBC technology-based power generation.Biomass often contains high levels of inorganic species that can form sticky agglomerates posing a significant risk to boiler operation resulting in unscheduled outages.This added complexity of the behaviour of the fuel and bed material mix highlights the requirement for simulation models to identify agglomeration to help improve the overall performance and reliability of FBC technology.To resolve this problem,this research devised a simulation strategy for the detection of agglomeration using the Eulerian–Eulerian approach.The developed modelling strategy is validated with the experimental data available in literature for two-dimensional simplified geometry of a pilot-scale fluidised bed combustor.The model results were found promising and robust to predict bed defluidisation times and other parameters consistent with the experimental data.

Study of biomass gasification in an industrial-scale dual circulating fluidized bed(DCFB)using the Eulerian-Lagrangian method

Dual circulating fluidized bed(DCFB)has emerged as an efficient reactor for biomass gasification due to its unique feature of high gas-solid contact efficiency and separated reactions in two reactors,yet the understanding of complex in-furnace phenomena is still lacking.In this study,biomass gasification in an industrial-scale DCFB system was numerically studied using a multiphase particle-in-cell(MP-PIC)method featuring thermochemical sub-models(e.g.,heat transfer,heterogeneous reactions,and homogeneous reactions)under the Eulerian-Lagrangian framework.After model validation,the hydrodynamics and thermochemical characteristics(i.e.,pressure,temperature,and species)in the DCFB are comprehensively investigated.The results show that size-/density-induced segregation makes solid fuels concentrate on the bed surface.Interphase momentum exchange leads to the continuous decrease of the gas pressure axially.In the gasifier and combustor,the lower heating value(LHV)of the gas products is 5.56 MJ/Nm^(3)and 0.2 MJ/Nm^(3)and the combustible gas concentration(CGC)is 65.5%and 1.86%,respectively.The temperature in the combustor is about 100 K higher than that in the gasifier.A higher solid concentration results in a smaller value of particle heat transfer coefficient(HTC).The HTCs range from 50 to 150 W/(m^(2) K)for a solid concentration larger than 0.3 in the combustor while the HTCs range from 100 to 200 W/(m^(2 )K)in the gasifier.The Reynolds number of biomass particles is two orders of magnitude larger than that of the sand particle.The numerical results shed light on the reactor design and process optimization of biomass gasification in DCFBs.

Euler-Lagrange/DEM simulation of wood gasification in a bubbling fluidized bed reactor

We present an Euler-Lagrange method for the simulation of wood gasification in a bubbling fluidized bed. The gas phase is modeled as a continuum using the 2D Navier-Stokes equations and the solid phase is modeled by a Discrete Element Method （DEM） using a soft-sphere approach for the particle collision dynamic. Turbulence is included via a Large-Eddy approach using the Smagorinsky sub-grid model. The model takes into account detailed gas phase chemistry, zero-dimensional modeling of the pyrolysis and gasification of each individual particle, particle shrinkage, and heat and mass transfer between the gas phase and the particulate phase. We investigate the influence of wood feeding rate and compare exhaust gas compositions and temperature results obtained with the model against experimental data of a laboratory scale bubbling fiuidized bed reactor.

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 twophase 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,CO_(2),H_(2),H_(2)O,CH_(4) 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.

Investigation on gasification of glycerol and palm shell mixed wastes by fluidization system

In this paper, gasification was utilized in order to produce syngas from crude glycerol and palm shell waste which are by-product of biodiesel production. Experiments were carried out in a fluidized bed quartz reactor using alumina ball with 1 mm diameter as fluidizing medium with equivalent ration of 0.05 with raw materials （mixed crude glycerol and palm shell wastes） at 10 g/min feed rate under 700℃ and 900℃. Glycerol and palm shell powder were fed separately to gasifier at different weight ratio varied from 100：0, 70：30, 50：50, 30：70, 0：100. Decomposition of crude glycerol resulted in much less char when compared with other biomass. From the results, it could be found that combustible gas productions increased with the increasing of crude glycerol fraction and temperature; syngas production was highest at 900℃with only glycerol in feed; gas production rate yields under optimum condition were 4.29% CO2, 8.70% CO, 10.48% H2, and 8.24% CH4 L/min; LHV and H2/CO at optimum condition were 4.87 MJ/m^3 and 1.20, respectively, which were sufficient for power utilization. Obtained H2/CO ratio also indicated that syngas from gasification of crude glycerol and palm shell waste should be suitable for further conversion to methanol and other chemical reagents, and thus closing the chemical recovery cycle of biodiesel production process to ensure the sustainahility status for the use of biodiesel as a prominent renewable energy source.

Conceptual Design of a Sugar Cane Biomass Gasifier Using CSFMB Model

Since 1978, Brazil has invested into bio-fuels alternatives, especially ethanol from sugar-cane processing and that has increased the bagasse production, which requires proper destination and a potential use of such biomass is the gasification process. In the present study, a conceptual design of a gasification system to convert sugar cane bagasse into syngas is presented, which considers air as gasification agent in a flexible configuration of bubbling fluidized bed reactor operating from atmospheric pressure up to 2 MPa, providing a net power output （referred at gas cold conditions） of 3 MWt and 66 MWt, respectively. In this last case, the gas may be used not just as a fuel for gas turbines and internal combustion engines for power generation but also to feed Fischer-Tropsch processes. The optimized conceptual design of the gasifier is described here and was achieved using the CSFMB （Comprehensive Simulator of Fluidized and Moving Bed Equipment）. Simulations predicted the production of gas with high heating value as well stable operations at both conditions. The conceptual design would be followed by the detailed one and construction. Tests would be carried in the near future and would allow direct comparison between the calculated and experimental results.

成型生物质流化床气化及结渣影响因素研究

为提高生物质气化产物的品质,采用成型桉树皮和成型玉米秸秆2种典型农林废弃物,并选取稻壳和木屑作为对比,在中试规模的流化床实验台上进行气化实验,得到成型桉树皮、成型玉米秸秆、稻壳和木屑的最佳空气当量系数,分析了成型生物质在气化中出现结渣现象的原因。结果表明:在实验条件下,成型桉树皮、稻壳和木屑的最佳空气当量系数为0.20,其燃气热值分别为5.5 MJ/m^(3)、5.5 MJ/m^(3)、6 MJ/m^(3),气化效率分别为60%、45%、52%;成型玉米秸秆由于其高灰分、低热值,所需空气量更大,最佳空气当量系数为0.24,燃气热值为4 MJ/m^(3),气化效率为35%;气化温度提高可促进不同生物质的气化反应,碱金属、碱土金属含量较多的成型生物质在气化过程中更易结渣。

松木颗粒流态化两段气化制备清洁燃气的工艺稳定性验证

为了验证流态化两段气化工艺的长时间运行稳定性,本实验采用商用果木炭半焦作为松木流态化两段气化制备清洁燃气的催化床料。实验前期通过形成稳定的有效催化半焦床层,减少半焦在系统内的积累周期,以实现系统快速达到高效脱除焦油的目的。实验中,保持过量空气系数(ER)为0.35,提升管气速为3m/s,并逐渐调整流化介质中的氧气浓度,逐步增大生物质处理量。结果显示,随着松木颗粒的进料量从1kg/h增大到3kg/h,产品气中CO和H2体积分数分别从7.71%和3.13%增长至16.15%和12.60%,CH4的体积分数略微下降。气体收率(标准)也从0.90m^(3)/kg增长至1.36m^(3)/kg,相应的产品气热值(标准)从3.26MJ/m^(3)增加至4.80MJ/m^(3)。通过多次人为启停和改变运行参数等操作,在整个12h运行周期内,流态化两段气化产生的半焦与消耗的半焦速度相匹配,温度和压力表现出良好的运行稳定性。这一结果证明了流态化两段气化工艺在生物质制气中具有技术先进性和长时间运行稳定性。

农林废弃物循环流化床空气气化特性实验研究

【目的】考察生物质种类在不同参数下对循环流化床空气气化特性的影响,为宽燃料适应性的生物质循环流化床气化技术和生物质气化耦合燃煤发电技术提供相关数据参考。【方法】在自行搭建的小型常压循环流化床气化实验装置上,开展了以空气当量比(equivalent ratio,ER)、气化温度为参数,农林废弃物(稻壳、木屑、玉米秸秆和稻草)为原料的空气气化实验研究。【结果】稻壳、木屑、玉米秸秆和稻草气化气组分在不同空气当量比下的变化规律基本一致,随着空气当量比不断增加,稻壳、木屑、玉米秸秆和稻草的气化燃气低位热值和冷煤气效率均呈现先增后减的变化趋势,对于稻壳、木屑和玉米秸秆,ER为0.20时均为最优工况,最高冷煤气效率分别为46.19%、38.07%和37.71%;而对于稻草,ER为0.25时为最优工况,最高冷煤气效率可达39.55%;稻壳、木屑、玉米秸秆和稻草气化气组分中的三大可燃气体(CH4、CO、H2)在不同气化温度下的变化规律也一致,随着气化温度不断升高,稻壳、木屑、玉米秸秆和稻草的气化燃气低位热值和冷煤气效率也均呈现先增后减的变化趋势,其中稻壳和玉米秸秆冷煤气效率在气化温度为750℃时达到峰值,分别为46.19%和37.71%,而木屑和稻草在气化温度为760℃时达到峰值,分别为38.07%和37.56%。【结论】研究结果可为宽燃料适应性的生物质循环流化床气化技术和生物质气化耦合燃煤发电技术提供相关数据参考。

生物质在流化床中的空气-水蒸气气化研究

以流化床为反应器,对生物质的空气 水蒸气气化特性进行了研究。考察了一些主要参变量,如温度(700℃～900℃)、水蒸气/生物质比(0～4 04)、空气当量比(0 19～0 27)以及生物质粒度(0 2mm～0 9mm)等对气化结果的影响。在实验研究的条件范围内,生物质产气率在1 43m3/kg～2 57m3/kg范围内变化,产气的低热值在6741kJ/m3～9143kJ/m3范围内变化。实验结果表明:较高的气化温度有利于氢的产生;但气化温度过高会使气体热值下降;与常规的空气气化相比,水蒸气的加入使生物质气化产气率显著提高,但水蒸气加入量过多使气化温度下降,产气率和产气热值降低;生物质颗粒粒度的大小对产气组分的分布和产气率均有影响,较小颗粒的生物质会产生较多的CH4、CO和较少的CO2。

流化床作为生物质气化反应器试验研究

在流化床生物质气化炉内 ,用空气进行气化生物质 (花生壳 )的试验研究 ,分析的参数是当量比ER 0 .2— 0 .4 5 ,气化床的温度 75 0— 85 0℃和加入二次风。当ER在 0 .2 5— 0 .33,气化燃气热值为 6 .2— 6 .8MJ/m3 ,气体产量在 2 6 0— 390m3 /h ,生物质燃烧时比气化产量在 1.2 8— 2 .0 3m3 /kg之间 ,炭转化率在 5 3%— 80 %。并对 7种农、林废弃物进行了初步气化试验研究 ,生成的燃气体积分数 :CO为 14 %— 18% ,H2 一般低于 6 % ,甲烷 4 %— 12 %。燃气热值在 4 70 0— 710 0kJ/m3 。试验结果表明 ,在流化床生物质气化炉中 ,通过在悬浮空间加入二次风 ,可使燃气热值得到提高。

流化床中生物质热解气化的模型研究

对水蒸汽和氮气流化条件下 ,流化床内生物质热解气化生成的纯煤气产率及低位热值随反应温度的变化特性进行了研究。在五种生物质原料实验数据的基础上 ,进一步研究了流化床内生物质热解气化生成气体产物的反应动力学模型 ,得到了纯煤气产率和热值的计算公式 ,并推荐了循环流化床条件下生物质热解的计算方法。

秸秆类生物质与石煤在流化床中的混烧与黏结机理

以玉米秸秆与石煤按不同比例组成的混合物为研究对象,在TG-DTG热分析仪上进行了燃烧特性分析,结果表明玉米秸秆有利于石煤的着火和稳定燃烧,对石煤有一定的助燃作用;在小型鼓泡流化床实验装置上,以石英砂为床料、石煤灰为添加剂,进行了玉米秸秆成型燃料流化床燃烧的床料黏结实验,结果表明:石煤灰能够在生物质流态化燃烧过程中有效地抑制流化床床料黏结现象的发生;通过对实验中形成的结团进行扫描电子显微镜X射线能谱(scanning electron microscopy/Energy-dispersive X-ray-SEM/EDX),对床料进行X射线荧光光谱(X-ray fluorescence,XRF)分析,结果表明石煤灰中的Al和Fe能够与生物质灰中的碱金属化合物以及低熔点共熔物发生化学反应生成高熔点物质,并且覆盖在生物质碳颗粒与石英砂颗粒表面形成隔绝层,从而阻止低熔点物质的生成与迁移。

生物质流态化催化气化技术工程化研究

在研究开发的内循环锥形流态化气化炉内,对稻草、麦草等软秸秆物料粉碎后,或者直接使用木屑等细粉状原料,进行了热解气化和催化气化的工程化应用试验研究。研究结果表明:气化反应在600～820℃的一个较宽温度范围内,均能稳定连续运行。麦草原料气化所产生的煤气热值比稻草和稻壳都高,其热值可达7716kJ/m3。木屑气化所产生煤气热值最高则达9064kJ/m3,远远高于一般生物质气化煤气。对流化床气化来讲,即使在非催化气化条件下,其气化产生的煤气热值比采用下吸式气化炉产生的煤气热值提高40%左右,并且气化温度较固定床(上吸式、下吸式)气化炉低。同时进行的催化气化试验发现,催化剂CaO能明显提高煤气热值、降低CO组分,Na2CO3催化气化能提高气体H2的含量。在800℃试验时,添加催化剂能明显提高气体的热值。

流化床生物质与煤共气化特性的初步研究

在热天平和流化床实验装置中研究了生物质与煤的共气化特性,采用程序升温热重法对稻秆焦、高粱秆焦、玉米秆焦和神木煤焦以及生物质焦与煤焦混合物进行水蒸气气化研究。结果表明,生物质焦和煤焦的反应活性依次增大,其顺序为高粱焦>稻秆焦>玉米焦>神木煤焦。一定温度下,生物质焦与煤焦混合物的气化碳转化率高于各自气化碳转化率的加和。在流化床气化实验中,比较了单独煤气化与稻秆/煤混合物气化的结果,实验结果表明,混合物气化碳转化率、气体中可燃组分的体积分数均高于单独煤气化,气体中CO2的体积分数低于单独煤气化CO2的体积分数。

生物质流化床氧气-水蒸气气化实验研究

在小型流化床气化装置上进行了氧气-水蒸气气化实验,考察了原料、当量比、水蒸气配比、温度、二次风和床料对气化特性的影响。结果表明,原料中C和H含量越高,气化气中H2和CO含量越高,焦油含量越低;当量比为0.27和水蒸气配比为0.6时,H2含量达到最大值;温度的升高可提高H2含量,在840℃以上,可提高CO含量;二次风从进料口偏上且二次风比率为15%通入,气体组分变化较明显,二次风通入点位置越高,焦油含量降低幅度越大;白云石和石灰石裂解焦油和提高H2含量的活性高于橄榄石,但同时明显提高了气体中的灰分含量。