水热生物炭燃烧特性与动力学分析

Combustion characteristic and kinetics analysis of hydrochars

采用热重法对锯末、玉米秸秆水热生物炭燃烧特性及动力学进行了研究,考察了不同升温速率(10、20、40℃/min)对燃烧特性的影响,分析了它们的燃烧特性及动力学参数。结果表明:1)水热炭化前后生物质燃烧质量损失集中在挥发分和固定碳燃烧阶段,升温速率快,着火温度、燃尽温度高,整体向高温区转移,综合燃烧特性指数越大;2)40℃/min时,锯末水热生物炭综合燃烧特性指数远大于玉米秸秆,在其余升温速率下区别不明显;3)以20℃/min相同升温速率时,锯末、玉米秸秆水热生物综合炭燃烧特性相对于未炭化生物质下降27%、13%;4)采用一级反应动力学模型和积分法对水热生物炭燃烧动力学进行了研究,一级反应动力学能很好的描述2种生物炭的燃烧动力学,相关系数(R2)均高于0.9,挥发分阶段活化能大于固定碳阶段的活化能。研究结果可为水热生物炭的燃烧应用能提供理论指导。

China is rich in agricultural and forestry resources, and thus biomass energy has broad prospects for development. Biomass energy utilization technologies are mainly divided into curing molding, thermo- chemical conversion and biochemical conversion. Hydrothermal carbonization（HTC） is one of the thermo- chemical conversion methods. HTC is very suitable to transform wet biomass feedstocks into carbon- rich solid fuels without drying the biomass, commonly referred to as hydrochars, whose physicochemical properties are comparable to that of coal.Sawdust and corn straw, as the representative of woody biomass and herbaceous biomass, respectively, were selected to produce hydrochars accordingly through hydrothermal carbonization under the condition of 230 ℃, 30 min, and gas flow rate of 60 m L/min（the volume ratio of N2 to O2was 4∶1）. Result indicated that the energy density of hydrochars was greatly improved, and could be used as solid fuel. Combustion characteristics of the hydrochar are different from biomass and biochar when produced at high temperature. Thermogravimetric（TG） analysis was used to investigate the combustion characteristics and kinetics of sawdust hydrochars and corn straw hydrochars. The influences of different heating rates（10, 20 and 40 ℃/min） on hydrochar combustion behavior were analyzed, while raw material was investigated for the comparison at heating rate of 20 ℃/min, too. The combustion characteristic parameters and the kinetic parameters were determined. The results indicated that： 1） The combustion weight loss of biomass and the hydrochar was focused on the burning process of the volatile matter and coke respectively, the ignition temperature and burnout temperature moved to the high temperature zone, and the integrated combustion characteristic index increased when the heating rate increased; 2） Integrated combustion characteristic index of sawdust hydrochars and corn straw hydrochars was increased by about 9 times and 7 times at 40 ℃/min respectively compared to raw biomass, and integrated combustion characteristic index of sawdust hydrochars was much bigger than corn straw hydrochars; 3） At the heating rate of 20 ℃/min, integrated combustion characteristic index of sawdust hydrochars and corn straw hydrochars was decreased a little; 4） The combustion kinetics analysis of hydrochars was performed using the first order reaction kinetics model and the integral method; all correlation coefficients of the fitting curves were greater than 0.9, and most of them were over 0.95 at both high and low heating rate; the activation energy of the volatile combustion phase was greater than that of the fixed carbon combustion stage, which were 43.0- 97.7 and 29.6- 41.6 k J/mol respectively. In summary, biomass is suitable for the preparation of solid fuels through hydrothermal carbonization, and the fuel has good combustion characteristics. Combustion characteristics of woody biomass hydrochars are superior to that of herbaceous biomass hydrochars.