制焦条件对煤焦燃烧反应性的影响

煤的反应中绝大部分化学能在焦炭反应阶段释放或转化,相关研究中采用多种方法在不同环境条件下制焦以进行后续研究,而煤焦反应性又受热解条件影响很大。为研究制焦条件对煤焦燃烧反应性的影响,本文在不同热解温度下分别采用马弗炉、管式炉和鼓泡床3种方法制焦,利用热重分析法(TGA)比较各焦样的燃烧反应性,并借助化学渗透析出(Chemical Percolation Devolatilization,CPD)模型和单颗粒传热模型,对各条件下煤颗粒的热解进程,如颗粒温度、挥发分析出总量、残余旁链结构份额等随时间变化规律进行了模拟。主要内容包括:①实验结果表明,热解环境温度越高,煤焦着火延迟,燃烧反应性越低,即存在"热失活"现象。而从CPD模型计算结果可以看出,热解温度越高,最终析出的挥发分总量越高(煤样工业分析结果也验证了残留在焦炭中的挥发分物质含量随热解温度升高而减小),且残留在碳网中的旁链结构份额越少,反映出酚羟基、羰基、脂肪侧链等的裂解加剧,减少了焦炭表面反应活性位点数量,可能是导致煤焦"热失活"的主要原因之一。另外,对各焦样的孔隙结构测量表明,热解温度越高,微孔与中大孔的相对比例逐渐减小,大量微...

Influence of coal pyrolysis conditions on its char combustion reactivity

In coal reaction,most chemical energy is released or transformed during the period of char reaction.Many related studies use several methods to prepare char samples under different conditions,while the pyrolysis conditions have great effects on char reactivity.In order to study the influence of coal pyrolysis conditions on char combustion reactivity,three kinds of laboratory-scale pyrolysis reactors,i.e.muffle,tube furnace and bubbling fluidized bed(BFB),were used to prepare chars under different pyrolysis temperatures.The combustion reactivity of each char sample was investigated using Thermogravimetric Analysis(TGA).Besides,the Chemical Percolation Devolatilization model(CPD) was also utilized to analyze the pyrolysis process of a single coal particle under different conditions,including the coal particle heating process,the total volatiles yield and the side chain population.The study reaches two main conclusions.Firstly,the results show that the higher pyrolysis temperature,the later the ignition of chars,and the lower the combustion reactivity,namely,thermal annealing associated with heat treatment of coal chars affects char reactivity.The CPD model calculation results show that with the increase of pyrolysis temperature,the total volatiles yield fraction increases,which is also proved by the proximate analysis of char samples,and the population of side chain remained in chars decreases.Thus,it indicates that the cracking reaction of phenol hydroxyl,carbonyl compounds or aliphatic side chain may be enhanced under higher pyrolysis temperature,which leads to lower number of active sites on the surface of chars.It might be one of the main reasons for thermal annealing of chars.In addition,the test of the pore structure of char samples also shows that the ratio of microporous plus mesoporous decreases and the macroporous ratio increases with the increase of pyrolysis temperature,which means that a large number of microporous contract with each other resulting in lower pore connectivity.Therefore,the gas diffusion resistance increases,which is also against the char combustion.Secondly,the reactivity of coal chars prepared by different methods is also obviously discrepant.For the chars prepared in muffle,tube furnace and bubbling fluidized bed with same pyrolysis temperature,their combustion reactivities successively decrease in order.Calculation results show that the heating rates of coal particles in muffle,tube furnace and bubbling fluidized bed significantly increase in order.The discrepancy of char reactivity may be related to the particle heating rate and pyrolysis atmosphere.Unfortunately,though the CPD model can reflect the difference during pyrolysis process for different heating rates,the final states are similar.Therefore the CPD theory can not explain the effects of char preparing method on char reactivity shown in this paper yet,which needs further study.