Analysis of reaction mechanism and kinetics of biomass in pyrolysis and gasification

生物质气化具有较高的热效率，是一种很有前途的气化技术。针对生物质气化过程中产生的大量可凝聚有机物(TAR)堵塞和腐蚀管道的问题，研究了H2和焦油对生物质半焦气化速率的抑制作用。首先，利用热平衡反应器，通过改变H_2分压(P_<H_2>=0、3、10、25kPa)，测定了水蒸气气化过程中焦炭的反应速率。H_2的存在使水蒸气气化速度减慢，当H_2分压较高(P_<H_2>=25kPa)时，抑制作用明显。结果表明，第一阶段发生反向氧交换，第二阶段发生H_2解离吸附。利用自行研制的落管式固定床热天平装置，考察了纤维素热解产物中的主要成分--左旋葡聚糖对焦化反应速率的影响。结果表明，在较高的进料速率下(250 mg/min)，不仅H_2对生物质热解生成的碳氢化合物有明显的抑制作用，而且碱金属和碱土金属(AAEM)对生物质三大组分(纤维素、木聚糖和木质素)之间的相互作用也有一定的影响。研究发现，AAEM能减少纤维素中水溶性焦油的生成。AAEM对纤维素与木聚糖的相互作用没有显著影响。

Biomass gasification is a promising technology because of the high thermal efficiency. However, a large amount of condensable organic compounds (tar) which causes blockages and corrosion of pipes are evolved in biomass gasification.In this study, the inhibition of the gasification rate of biomass char by H_2 and tar was investigated. At first, the reaction rate of char in steam gasification was measured using a thermobalance reactor by varying partial pressure of H_2 (P_<H2>=0, 3, 10, 25kPa). The rate of steam gasification was decelerated by the presence of H_2 and the inhibition was significant when the partial pressure of H_2 was high (P_<H2>=25kPa). It was found that reverse oxygen exchange occurred at first period and dissociative adsorption of H_2 on char occurred at second period. The effect of levoglucosan, which is major component of pyrolysis product from cellulose, on the reaction rate of char was also measured with a newly developed drop-tube/fixed bed thermobalance reactor. It was found that not only H_2 but also other hydrocarbons evolved by pyrolysis of levoglucosan substantially inhibit char gasification when the feed rate of levoglucosan was high (250mg/min).In addition, the effects of alkali and alkaline earth metals (AAEM), which can act as an excellent catalyst for the gasification and combustion reactions, on the interactions between the three major components of biomass (cellulose, xylan and lignin) were investigated. It is found that AAEM decreased the generation of water-soluble tar derived from cellulose. AAEM do not have significant influence on the interaction between cellulose and xylan.