Influence Of Gas Residence Time On Product Distribution Of Coal Pyrolysis

Coal pyrolysis is a key step of coal conversion technologies, and its products will affect the design of subsequent processes. Secondary reaction, such as decomposition, hydrogenation and condensation, may happen, and lead to change in final products of coal pyrolysis when coal primary pyrolysis products stay in high-temperature environment. Cracking temperature and gas residence time are two of the most important factors. In this paper, the secondary reactions of volatiles were carried out in a two-stage fixed reactor where primary reaction and secondary reaction can be studied independently. Systematic product analysis was carried out at certain cracking temperatures and gas residence times respectively, when primary pyrolysis condition is same.A two-stage fixed bed experimental apparatus where primary reaction and secondary reaction can be studied independently was established. Gas residence time can be controlled by changing flow rate of the carrier gas and the length of the reactor. The research on short residence time of gaseous products were carried out in a two-stage fixed reactor at cracking temperatures of500～80℃and gas residence times of2-16s respectively. The results show:(1) under600℃, final product changed little; Above600℃, with the increasing of gas residence time, some secondary reaction happened. Higher the cracking temperature was, and longer the gas residence time was, deeper the extent of secondary reaction became. In seconadry reaction, tar yield decreased, and gas yield increased.(2) Increasing cracking temperature and prolonging gas residence time promote secondary reaction. Tar yield decreased, but light components of tar increased. In the secondary reaction, tar tended to form more thermally stable, more simple structure of the liquid components, including benzene, toluene, xylene (BTX) and polycyclic aromatic hydrocarbons (PAH), such as naphthalene, phenanthrene and anthracene.(3) At700℃, with the increasing of gas residence time, tar yield decreased, and gas yield increased. CH4yield increased with a certain rate, which was caused by demethylation of aryl-methyl. The yields of C2～c3hydrocarbons initially increased, then decreased with the increasing of gas residence time. C2～C3hydrocarbons were formed by decomposition of aliphatic side-chain, then decomposed by themselves.(4) At800℃, with the increasing of gas residence time, tar yield declined rapidly, and gas yield increased rapidly. CH4increased rapidly in2s. After10s, the increase of CH4 slowed down. In this condition, demethylation reaction became violent. At the same time, H2and CO yield increased rapidly because of condensation reaction, decomposition of phenolic hydroxyl group and rupture of oxygen-containing heterocyclic. BTX and PAHs inclined, and PCX (phenol, cresol, xylenol) reduced significantly, which indicated demethylation, dehydroxylation and condensation reactions became more intense at800℃.(5) The influences of gas residence time on final products of coal pyrolysis were similar for ZhunNan bituminous coal, HengShan bituminous coal and XiaoLongTan lignite.In order to study the influencing characteristic of longer gas residence time, based on two-stage fixed bed experimental apparatus, the research on long residence time of gaseous products were carried out in a modified two-stage fixed reactor at cracking temperatures of600～800℃and gas residence times of10-50s respectively. The results show:(1) At600℃, only CH and C2H4yield have a slight increase which caused by decomposition of some aliphatic hydrocarbon, and H2, CO and CO2changed little.(2) At700℃, H2and CO yield increased slowly, the former was accompanied by condensation, the latter was the result of decomposition of phenolic hydroxyl and oxygen-containing heterocyclic.(3) At800℃, as the extension of gas residence time, the tar yield decreased rapidly and gradually became stable; the gas yield increased quickly, which was accompanied by CO shift reaction.