【作者】 费华；

【导师】 向军； 胡松；

【作者基本信息】 华中科技大学 ， 热能工程， 2011， 博士

【摘要】 我国是世界仅有的几个以煤为主要能源的国家之一,煤资源作为我国能源消费的主要支柱的局面必然长期存在。然而它的不可再生性以及附带产生的诸多环境问题,已经严重制约了人类的可持续发展,也逐渐受到全世界的广泛关注与重视。煤的利用带来的污染是我国大气污染的主要来源,因而提高我国煤炭利用效率、减少煤炭利用带来的环境污染是我们急待解决的问题。因此,本文以中国三种典型煤种焦作无烟煤、云浮烟煤和小龙潭褐煤为研究对象,对煤热解和燃烧/气化机理及模型进行了详细系统的研究,对于加深认识煤热解、燃烧和气化过程,揭示其本质具有重要的意义。首先采用气体分析仪开展了煤固定床热解的实验研究,对煤热解过程中气体产物的析出特性进行了实时在线分析,利用Weibull分布函数的特点和阶梯函数的性质,建立了简化的DAEM模型,并对该模型进行修正。研究结果表明,升温速率、温度和煤种是影响煤热分解气体产物释放特性的主要因素。煤热分解过程中气体产物析出浓度的峰值随升温速率的升高而增大,且H2的释放发生在较高温度段。热解温度越高,主要气体产物(CO2、CO和CH4)析出浓度的峰值越大,并且主要气体产物释放特性随煤种的不同而差异较大。通过简化的DAEM模型和修正的DAEM模型,对煤热解过程中气体产物析出特性的模拟结果比较发现,这两个DAEM模型都能较好的模拟实验结果,但修正的DAEM模型的模拟值与实验数据更接近。采用TG/DTG方法对三种典型煤种在化学反应控制条件下O2/C02燃烧特性进行研究,利用分形理论描述颗粒内部孔隙结构,并对随机孔模型中固定的结构参数(?)进行修正,然后建立了分形随机孔模型。研究结果表明,三种典型煤种在低温条件下O2/C02燃烧过程中,在反应起始阶段分形维数变化很小,而后阶段分形维数急剧减少。另外,热解终温对焦结构的影响是不可忽略的,这主要是由于孔隙结构的变化主要受挥发分析出和焦受热变形的影响。在煤焦燃烧过程中(?)的变化可分为两个阶段：在X=0～0.7阶段,(?)基本上保持不变,且起始阶段(X=0～0.3),(?)是略微减少,这主要由于在反应起始阶段大量新孔的产生和微孔的扩容使反应面积明显增加而造成；在后阶段(X=0.7～1),(?)急剧上升,这主要是由于在后阶段孔坍塌造成反应面积的急剧减少。通过五种不同随机孔模型,对三种典型煤焦燃烧过程进行预测的结果比较发现,分形随机孔模型和Struis模型在整个反应阶段均能得到较好的预测结果,而前者更精确。在热重分析仪和固定床上对三种典型煤焦在扩散控制条件下O2/C02燃烧特性进行研究,根据反应气体和焦颗粒的偏微分质量守恒方程,利用逼近的方法简化和求解该偏微分质量守恒方程,建立了在扩散控制条件下预测煤焦反应性的模型。研究结果表明,三种煤焦在不同转换率的吸附等温线虽然在形态上稍有差别,但都呈反“S”形,并且随着转换率的升高其吸附量先增大后减小。在不同转换率条件下煤焦吸附等温线在形态上存在着差异,这意味着在不同转换率条件下焦的孔径分布是不同的。此外,在起始阶段三种典型煤在高温条件下燃烧比表面积都有增加趋势,这主要是由于煤颗粒新孔的产生和微孔的扩容。而微孔孔容积与比表面积的变化规律非常相似,这是由于比表面积主要由微孔来提供。通过不同模型对实验数据的预测结果比较发现,本章中改进的量化模型在整个反应阶段均能得到较好的预测结果,这表明在扩散控制条件下该模型能够作为一种快速合理的方法来预测煤焦反应特性。最后在热重分析仪和沸腾炉上开展了煤与水蒸气气化的实验研究,对煤水蒸气气化特性进行了研究,并利用修正的DAEM模型模拟实验结果。研究结果表明,煤种和温度是影响煤水蒸气气化气体产物释放特性的重要因素。煤水蒸气气化气体产物析出浓度峰值的变化与煤的挥发分含量有关,低挥发分煤与高挥发分煤气化气体析出浓度峰值的变化明显有差异。气化温度越高,气化时气体产物析出浓度的峰值越大,达到其峰值所需的时间越短,且在不同温度下煤气化时气体产物的体积分数的变化也明显不同,高挥发分煤气化时气体产物体积分数的变化相对较小。修正的DAEM能够较好的模拟低挥发分煤水蒸气气化过程中焦的反应特征,但该模型模拟高挥发分煤气化反应特征的效果明显较差,这可能是由于煤的高挥发分使其气化过程中出现第二次反应高峰的现象,从而使其模拟结果产生误差。更多还原

【Abstract】 China is one of only several countries with coal as the main source of energy in the world, so the situation of coal resources as the main energy consumption will exist for a long time. However, it’s non-renewable and incidental to the many environmental problems, which have seriously hampered the sustainable development of mankind, should have gradually been worldwide attention. Pollution caused by the utilization of coal is main source of air pollution in China, thereby improving the efficiency of the utilization of coal and reducing environmental pollution caused by the utilization of coal is pressing problem. Jiaozuo anthracite, Yunfu bituminous, Xiaolongtan lignite were used in this study as the representatives of three typical Chinese coals, and coal pyrolysis and combustion/gasification mechanism and model were studied in detail, which should understand better coal pyrolysis, combustion and gasification process and reveal its nature is of great significance.First, the pyrolysis of coal was studied using a bench scale fixed bed reactor coupled with gas analyzer. The release properties of gas products during coal pyrolysis were analyzed on-line. Using the characteristics of Weibull distribution function and the nature of step function, simplified DAEM was established and this model to be amended in this chapter. The results show that the heating rate, temperature and coal type are important factors for the gas releasing characteristics during coal pyrolysis. The peak of the gas products concentration increases with the pyrolysis temperature increasing during coal pyrolysis, With increasing pyrolysis temperature, the peak of the main gas products (CO2, CO and CH4) concentration increases, and the release properties of the main gas products during coal pyrolysis due to the difference of coal type is larger distinction. Compared with simplified DAEM and modified DAEM predicting the release properties of gas products during coal pyrolysis, it was found that two DAEM were more accurate to describe the experimental results, especially to the DAEM improved in this chapter.The combustion of Chinese three typical coal chars in O2/CO2 under chemical reaction control was studied using a TG/DTG. The internal pore structure of the coal chars was investigated with the help of fractal theory, and structure parameterΨin random pore model is modified based on pore fractal feature and fractal random pore model was constructed. The results showed that coal chars combustion under O2/CO2 atmosphere at lower temperature fractal dimensions slightly change at the initial stage, and then kept stable in the middle stage before decreasing drastically. On the other hand, the effect of final pyrolysis temperature for pore structure can not be negligible, this phenomenon was mainly caused by the changes of pore structure affected by devolatilization and thermal deformation of the char. The change of parameterΨcan be divided into the following two categories,Ψis almost constant at this stage of reaction (X=0-0.7), thoughΨdecreases slightly during the initial stage of the reaction (X=0-0.3). This phenomenon is mainly caused by generation of a large number of new pores and micropore expansion. At the end of stage (X=0.7-1),Ψvalues increase remarkably. Structure evolution in particles at this stage is mainly caused by pore collapse which also makes reaction surface decrease gradually. Compared to the results calculated by five different random pore models, the prediction results of fractal random pore model and Struis model are better throughout the conversion period, especially to the prediction results of fractal random pore model.The combustion of Chinese three typical coal chars in O2/CO2 under diffusion controlled condition was studied using the thermogravimetric balance and the bench scale fixed bed. The methodology developed is based on an approximate method for decoupling partial differential mass conservation equation, and a mathematical model was created to describe the reaction rates of coal chars under diffusion controlled condition. The results show that the adsorption isotherm curves of three kinds of coal chars in the various conversions slightly distinct in shape, but they were tested against "S" shape, and its adsorption first increases and then decreases with increasing carbon conversions. The adsorption isotherms curves of coal chars prepared at various carbon conversions is diverse, it can be indicated that pore size distribution of coal chars is different at various carbon conversions. In addition, the specific surface area SBET of three typical coal combustion at high temperature has an increasing trend at the initial stage, which is mainly caused by generation of a large number of new pores and micropore expansion. The variation of the micropore volume and surface area is very similar, which mainly due to the specific surface area determined by the micropore. Compared to the results calculated by the models, the prediction results of the model improved in this chapter are better throughout the conversion period, it indicates that this model can be recommended as a quick and reasonable way to predict the reaction rates of coal chars under diffusion controlled condition.Finally, the steam gasification of coal was studied using thermogravimetric and the fluidized bed furnace. The characteristics of coal steam gasification were investigated, and modified DAEM model was used to simulate the experimental results. The results show that coal type and temperature are important factors for the gas products releasing characteristics during coal steam gasification. The change trends of the maximum gas releasing concentration during coal steam gasification is related with volatile content, the change trends of the maximum gas releasing concentration of the low volatile coal and high volatile coal is different. With increasing gasification temperature, the peak of the main gas products concentration increases and reached its peak at the shorter time. The changes of the volume fraction of gas products during coal steam gasification is also significantly different, the changes of the volume fraction of gas products during high volatile coal gasification is smaller. Modified DAEM can simulate better the characteristic of char reaction during the low volatile coal steam gasification. The results simulated by this model during the high volatile coal gasification have rather big difference with experimental results. This phenomenon may be due to the high volatile coal that makes the gasification reaction occur the phenomenon of the second peak, and the errors of simulation results are caused by it.更多还原