基于等转化率法的芒草和玉米秸秆热解特性及动力学研究

摘要

The pyrolysis characteristics of Chinese silvergrass and corn stalk at different heating rates(5,10,20 and 40℃/min) were studied by thermogravimetric analysis. Pyrolysis kinetics were studied using Kissinger-Akahira-Sunose(KAS), Starink and Ozawa integration method. The TG and DTG curves demonstrated that the pyrolysis process of Chinese silvergrass and corn stalk could be divided into 4 stages:dehydration stage,transition stage,main pyrolysis stage and carbonization stage. Along with the increase of heating rate,the pyrolysis curves moved to the high temperature side, and the rate of weight loss increased, which indicated that the increase of heating rate could promote the pyrolysis reaction. The results of pyrolysis of Chinese silvergrass showed that the correlation coefficients of the three methods were more than 0.9,and the values of activation energy calculated by KAS and Starink methods were very close to each other, but the activation energy calculated by Ozawa method was lower. The activation energy of corn stalk pyrolysis obtained by Ozawa method was the highest,followed by that of Starink method and KAS method. The activation energies of Chinese silvergrass pyrolysis obtained by three methods fluctuated obviously with the increase of conversion rate in the whole pyrolysis process, which showed that a series of complex chemical reactions occurred in the pyrolysis process. When the conversion rates were 0.1-0.4,the activation energy of corn stalk pyrolysis increased sharply;but when the conversion rates were 0.4-0.8,the activation energy of corn stalk pyrolysis decreased slowly until it reached a plateau. The conversion rates of 0.1-0.3,0.3-0.7 and 0.7-0.8 corresponded to the main pyrolysis stage of hemicellulose, the main pyrolysis of cellulose, and the main pyrolysis stage of lignin, which indicated that the pyrolysis difficulty degree of the three components in Chinese silvergrass was the order of lignin>cellulose>hemicellulose.%利用热重分析仪对芒草和玉米秸秆在不同升温速率(5、10、20和40 ℃/min)下的热解特性进行了研究,并采用Kissinger-Akahira-Sunose(KAS)、Starink和Ozawa等转化率法研究了其热解动力学特性.结果表明:芒草和玉米秸秆热解过程可分为干燥失水、过渡、主热解和炭化4个阶段;随着升温速率增加,热解各阶段均向高温侧移动,失重率增加,表明升温速率增加可促进热解反应的进行.动力学计算结果表明:3种方法拟合的相关系数均大于0.9,且芒草的相关系数大于玉米秸秆;芒草的活化能,KAS和Starink法计算得到的结果很接近,Ozawa法较低;而玉米秸秆的活化能,Ozawa法得到的最高,Starink法居中,KAS法最低.在整个热解过程中,3种方法求得的芒草的活化能随转化率升高波动明显,表明芒草热解过程发生了一系列复杂的化学反应;转化率为0.1~0.3、0.3~0.7及0.7~0.8时,分别对应半纤维素、纤维素及木质素的主热解阶段,这表明芒草三组分热解难易程度为木质素>纤维素>半纤维素.而玉米秸秆则不太一样,转化率为0.1~0.4时,玉米秸秆活化能急剧增加;转化率为0.4~0.8时,玉米秸秆活化能缓慢下降直至平稳.