基于能量得率的棉秆热裂解炭化工艺优化

Optimization of pyrolysis carbonization conditions based on energy efficiency for cotton stalk

为了从能源利用角度设计和优化棉花秸秆热裂解制生物炭的热解炭化工艺,该文使用了产率、热值及能量得率3个指标来衡量工艺的优劣。首先,研究了热解温度、保留时间和原料粒径3个工艺条件分别对生物炭产率和热值的影响。结果表明,在3个工艺条件下生物炭产率与热值均呈负相关,即高产率和高热值目标无法同时满足。因此,引入能量得率(单位原料所产生物炭的总能量)作为全面评价生物炭产率和热值的综合指标,重点利用响应面分析法分析了3个工艺条件及其交互作用对能量得率的影响,并经过检验得到优化后的能量得率模型。模型预测结果表明,在炭化温度为429℃,保留时间为1.29 h,原料粒径为0.32 mm时,能量得率达到最大值,为78.95%,通过验证试验证明了模型的有效性。该模型能够用于指导生产高能量得率的生物炭,为生物炭能源高效利用目标的实现提供参考。

With the use and depletion of fossil fuels, environmental pollution and energy crisis have become the biggest obstacles to the survival and development of human beings. Biomass energy utilization is an important way to solve this problem as it has wide source of raw material, and is clean and renewable. Biochar is an important form of biomass energy utilization, and it can not only change the waste straw into biomass energy, but also help reduce the greenhouse effect. The yield is a basic index in the study of pyrolysis technology; while the biochar is used as fuel, its calorific value is the factor directly reflecting the quality. These 2 indices are both necessary during the research. However, the existing research on the pyrolysis process of biochar is limited to all kinds of independent indices, and the optimal pyrolysis process is always not the same. In this paper, the advantages and disadvantages of pyrolysis process for biochar production were evaluated by the yield, calorific value and energy efficiency in order to design and optimize the process of carbonization of cotton stalk. The effects of the temperature, retention time and particle size on the yield and heating value of the biomass were studied; and the typical cotton stalk was taken as raw material to produce biochar under nitrogen gas atmosphere by slow pyrolysis. The results showed that： 1） The effects of pyrolysis temperature on the yield and heating value of the biomass were significant; the biochar yield was negatively correlated with heating value, and with the increase of the temperature, the yield of biochar decreased gradually, while the calorific value increased gradually. 2） The effect of retention time on the yield and heating value of the biochar was obvious, but it was not as obvious as the pyrolysis temperature; at the same time, it could be seen that the yield and heating value were negatively correlated, and with the increase of the retention time, the biochar yield decreased, while the calorific value increased. 3） Compared with the first 2 conditions, there was no significant change in the yield and heating value when the raw material particle size changed, which showed that the effects of particle size on the yield and calorific value were small. In addition, according to the general trend, we could still see a negative correlation between heating value and yield. Energy efficiency （ratio of total energy yield of biochar to energy yield of raw material） was proposed to coordinate the yield and calorific value, and the response surface analysis of the energy efficiency was carried out, from which the energy efficiency model was got. The results showed that when the temperature was 429℃, the retention time was 1.29 h, and the particle size was 0.32 mm, the predicted energy efficiency from the model reached the highest value, which was 78.95%. The model is validated by the experiment. The model can be used to guide the production of biochar with high energy efficiency, and provide the reference for energy efficient utilization.