纤维素热裂解反应机理及中间产物生成过程模拟研究

Simulation of cellulose pyrolysis mechanism and formation process of intermediate products

基于改进的B-S机理模型,通过求解物料内部和气相空间两段反应过程,对纤维素热裂解过程中一些化合物(活性纤维素、左旋葡聚糖(LG)、乙醇醛、丙酮醇等组分)的生成和演变情况进行了模拟。结果发现,自由水的脱除过程使物料前期升温速率发生了下降,并未影响热解期间温度分布以及反应过程。热裂解过程中,由于一次反应的强烈吸热,物料在长时间内局限于中温范围,其内部各组分质量浓度分布的区别主要体现出一次反应竞争能力的强弱。物料厚度的增加使热裂解时间延长,并加剧物料内部的二次分解。左旋葡聚糖和其竞争产物乙醇醛的生成出现一个大量生成、快速逃逸的过程,相比于左旋葡聚糖,乙醇醛质量浓度的积累具有更快的速度,体现出较高温度下的竞争优势。对于小尺寸反应物,挥发分二次反应主要发生在气相空间,随着气相停留时间的增加,其二次分解的程度提高,该效果随辐射源温度的提高而加剧。相比于LG产率随反应时间的快速下降趋势,高温下生物油产率的降低略显缓和,其变化主要是组分分布的改变,即从大分子结构降解为小分子结构。

Based on the modified Brodio-Shafizadeh mechanism, a two-stage model was proposed to simulate the formation and decomposition of intermemediate products including active cellulose （AC）, levoglucosan （LG）, hydroxyl-acetaldehyde （HAA）, acetol and furfural in the cellulose pyrolysis. Results show that the water evaporation only decreases the heating rate of the pyrolysis prophase, while it has no effects on the temperature distribution and reaction process in the main pyrolysis stage. Cellulose material stays for a long time in the middle temperature due to the endothermic effect of primary cracking. The concentration of LG and other competitive compounds in the fabric structure is primarily determined by its competitive reaction of formation. The rising of material thickness would prolong the decomposition time of cellulose, and enhance the secondary cracking within the fabric structure. The formation process both LG and HAA shows the characteristic of fast formation and rapid escape, but HAA presents a faster accumulating tendency that will be enhanced at high temperature. For small size materials, the secondary reaction of volatile occurs largely in the gaseous phase rather than in the solid phase. The longer residence time of volatile in the high temperature region will induce deep decomposes of LG. Compared with the rapid decrease of LG yield with reaction time, the yield of tar falls slowly at high temperature. The major change exists in the distribution of components, i.e. the large molecule compounds being decomposed into small ones.