Hydrogen-rich gas production from biomass air and oxygen/steam gasification in a downdraft gasifier

Biomass gasification is an important method to obtain renewable hydrogen, However, this technology still stagnates in a laboratory scale because of its high-energy consumption. In order to get maximum hydrogen yield and decrease energy consumption, this study applies a self-heated downdraft gasifier as the reactor and uses char as the catalyst to study the characteristics of hydrogen production from biomass gasification. Air and oxygen/steam are utilized as the gasifying agents. The experimental results indicate that compared to biomass air gasification, biomass oxygen/steam gasification improves hydrogen yield depending on the volume of downdraft gasifier, and also nearly doubles the heating value of fuel gas. The maximum lower heating value of fuel gas reaches 11.11 MJ/ N m(3) for biomass oxygen/steam gasification. Over the ranges of operating conditions examined, the maximum hydrogen yield reaches 45.16 g H-2/kg biomass. For biomass oxygen/steam gasification, the content of H-2 and CO reaches 63.27-72.56%, while the content Of H2 and CO gets to 52.19-63.31% for biomass air gasification. The ratio of H-2/CO for biomass oxygen/steam gasification reaches 0.70-0.90, which is lower than that of biomass air gasification, 1.06-1.27. The experimental and comparison results prove that biomass oxygen/steam gasification in a downdraft gasifier is an effective, relatively low energy consumption technology for hydrogen-rich gas production.

Dynamics of collapsing fluidized beds and its application in the simulation of pulsed fluidized beds

A new method to study collapsing process of fluidized bed was proposed. The method is based on the analysis of the pressure variation during collapse. A model is proposed to describe the pressure variation on any location of the bed during collapse. Three kind of particles were fluidized by air and the pressure variation after shutting down the gas supply was measured by pressure transducers and simulated by the proposed model. The simulated results were in good agreement with the experimental data. The parameters of the fluidized bed, such as bubble fraction alpha(b), particle fraction in dense phase alpha(p), bubble velocity u(b) and slip velocity of particle u(i), can be estimated from the measured pressure data by using the proposed model. The model was also used to calculate the pressure variation in pulsed fluidized beds with low frequency.

Dynamics Of Fine Particles In Liquid-Solid Fluidized Beds

On the basis of the Local Equilibrium Model (LEM), fine particles with large Richardson-Zaki exponent n show, under certain conditions during bed expansion and collapse, different dynamic behavior from particles with small n. For an expansion process there may be a concentration discontinuity propagating upward from the distributor, and, on the contrary, for a collapse process there may be a progressively broadening and upward-propagating continuous transition zone instead of discontinuity. The predictions of the bed height variation and the discontinuity trace have been validated experimentally. (c) 2007 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.

Heat-Transfer Study of External Superheater of CFB Incinerator

The heat transfer coefficients for horizontally immersed tubes have been studied in model internally circulating fluidized bed (ICFB) and pilot ICFB incinerators. The characteristics in the ICFB were found to be significantly different from those in a bubbling bed. In ICFB, there is a flowing zone with high velocity, a heat exchange zone, and a moving zone with low velocity. The controllable heat transfer coefficients in ICFB strongly depend on the fluidized velocity in the flowing zone, and also the flow condition in the moving zone. The heat exchange process and suitable bed temperature can be well controlled according to this feature. Based on the results of experiments, a formulation for heat transfer coefficient has been developed. These results were applied to an external superheater of a CFB incinerator with a 450 degreesC steam outlet in a waste-to-energy pilot cogeneration plant of 12 MW in Jiaxing City, China.

Characterizing Particle Dispersion by Image Analysis in ICFB

A new particle image technique was developed to analyze the dispersion of tracer particles in an internally circulating fluidized bed (ICFB). The movement course and the concentration distribution of tracer particles in the bed were imaged and the degree of inhomogeneity of tracer particles was analyzed. The lateral and axial dispersion coefficients of particles were calculated for various zones in ICFB. Results indicate that the lateral diffusion coefficient in the fluidized bed with uneven air distribution is significantly higher than that in uniform bubbling beds with even air distribution. The dispersion coefficients are different along bed length and height.

内旋流流化床内颗粒运动规律的试验与数值研究

通过冷态试验与数值模拟研究了不同布风下装有∧字型布风板的内旋流流化床的流动特性，并进行了对比．气固两相数值模拟结果表明，固相流场产生了大尺度旋流，而且与试验观察到的现象相似．对固相颗粒速度分析表明非均匀布风使内旋流流化床的底部和顶部产生了气、固两相强烈的横向和纵向运动．相比较来说，对称的非均匀布风扩散能力要比均匀布风的扩散能力强．

半焦在流化床燃烧中Cl元素影响CO氧化的试验

在非均匀布风的流化床燃烧系统中进行了半焦中添加PVC/NaCl的混烧试验,研究了Cl元素对燃烧中CO氧化的影响以及混烧时SO_x等污染物的生成规律.试验结果表明,半焦与PVC/NaCl混烧在技术上可行,添加Cl元素抑制CO氧化,加入PVC增加SO_2排放,而加入NaCl则减少SO_2排放.

变截面流化床理论模型及数值模拟

基于双流体模型,在一定假设的条件下推导得到了用于描述变截面床流化过程的局部平衡模型。对于局部平衡模型,空间导数项的离散采用五阶精度的WENO有限差分格式,时间导数项的离散采用TVD Runge-Kutta型的离散格式,对流量突变后的瞬态过程进行了模拟,得到固相体积分数在整个变化过程中沿床高的分布以及床高变化规律和床层表面颗粒速度变化曲线。对于流量突增过程,在床内不同位置形成了一系列的连续波,当所有的连续波到达床面整个变化过程结束。而对于流量突然减小过程,将会有固相体积分数间断在分布板处形成,当所有间断到达床面时,塌落过程结束。

非均匀布风对流化床密相床区燃烧温度的影响

由于采用非均匀布风，内旋流流化床的移动区空气量不足，导致燃烧不充分，温度较低。当移动区未流化时，密相区内存在较明显的温度不均匀性。随着移动区流速的提高，温度差迅速减小。当移动区流速超过2.0 u_(mf)后，密相区温度基本均匀一致。流动区流速对密相区温度均匀有一定的影响，流速越高，温度越均匀。

Influence of HCl on CO and NO emissions in combustion

The influence of HCl on CO and NO emissions was experimentally investigated in an entrained flow reactor (EFR) and an internally circulating fluidized bed (ICFB). The results in EFR show the addition of HCl inhibits CO oxidation and NO formation at 1073 K and 1123 K. At the lower temperature (1073 K) the inhibition of HCl becomes more obvious. In ICFB, chlorine-containing plastic (PVC) was added to increase the concentration of HCl during the combustion of coal or coke. Results show that HCl is likely to enhance the reduction of NO and N2O. HCl greatly increases CO and CH4 emission in the flue gas. A detailed mechanism of CO/NO/HCl/SO2 system was used to model the effect of HCl in combustion. The results indicate that HCl not only promotes the recombination of radicals O, H, and OH, but also accelerates the chemical equilibration of radicals. The influence of HCl on the radicals mainly occurs at 800-1200 K. (C) 2009 Elsevier Ltd. All rights reserved.

内循环流化床生物质气化过程的神经网络模型

基于BP人工神经网络原理，利用MATLAB神经网络工具箱，以实验得到的57组气化实验数据作为样本，建立了一个以加料量和送风量为输入变量，以燃气热值、产气率、碳转化率和气化效率为输出变量，用于描述连续稳定气化过程的内循环流化床生物质气化模型。对模型的隐层节点数和训练周期改变对模拟结果的影响进行了分析，发现当隐层节点数为20，训练步骤为50步，模型的4个输出变量的模拟结果与实验结果相关系数均超过0．95；同时对该模型的预测能力进行了考察，模型预测结果与实验结果吻合良好，证明了该模型具有较强的泛化能力，为生物质内循环流化床气化系统的优化设计和自动控制提供新思路。