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 re- activity,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 reactivi- ty. The CPD model calculation results show that with the increase of pyrolysis temperature,the total volatiles yield frac- tion increases,which is also proved by the proximate analysis of char samples,and the population of side chain re- mained in chars decreases. Thus,it indicates that the cracking reaction of phenol hydroxyl,carbonyl compounds or ali- phatic 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 com- bustion 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.