Improving BTEXN Yield From Coal Pyrolysis Volatile Products Through Catalytic Conversion

A certain amount of BTEXN (benzene, toluene, ethylbenzene, xylene, naphthalene) is produced during coal pyrolysis. In the production, storage and transportation processes, these compounds can release to pollute the environment, damage the ecological system and even endanger the human health. Moreover, BTEXN are currently widely used as industrial chemicals for the production of pesticides, plastic products and synthetic fibres. Therefore, to enrich and extract BTEXN from coal tar have important significances in controlling environmental pollution and developing economy.HZSM-5, Mo-and Ni-modified HZSM-5 were chose to investigate the catalytic effects of the catalysts on the conversion of coal volatile products using Py-GC/MS. By comparing the formation and distribution rules of the BTEXN before and after catalysts added, the most suitable catalyst and operation parameters used to catalyze the pyrolysis volatile of XM lignite were obtained, at which conditions the BTEXN can reach the maximum yield. Moreover, the catalytic effects of these catalysts on other different rank coals were also studied. The main conclusions are as following:(1) The yield of BTEXN from XM lignite pyrolysis increased with raising temperature from 500℃ to 1000℃. At earlier stage of the pyrolysis, the total amounts of BTEXN have a sharp increase for that thermal decomposition is the dominant reactions. But at later stage, only a slow increase was observed because of the occurring of polymerization reaction. The most obvious temperature range of the increasing BTEXN is 600～800 ℃. At the same pyrolysis temperature, the global release trend of BTEXN for different rank coal is as following order:bitumite>lignite>anthracite.(2) The most suitable temperature is 700℃ and the optimum proportion of XM coal and HZSM-5 is 1:0.6 during the catalytic conversion of XM pyrolysis volatile over HZSM-5. The BTEXN yield in the presence of HZSM-5 increased 3 times compared with that from raw coal pyrolysis. The reason is that HZSM-5 has special pore structure and acidic sites which could promote the catalytic cracking, aromatization of hydrocarbons as well as dehydroxylation of phenols.(3) At 700℃, the obvious catalytic effects were observed during the conversion process of pyrolysis volatile to BTEXN over 5%Mo/HZSM-5 and 5%Mo/HZSM-5. In the presence of the two catalysts, the total BTEXN amounts increased 3.8 and 3.6 times compared with the raw coal pyrolysis. Because of the ability of the catalytic cracking, aromatization of hydrocarbons, dehydroxylation of phenols, and aromatization of CH4 over Mo/HZSM-5, Mo/HZSM-5 is beneficial to the production of the BTEXN components, especially the formation of benzene and toluene, the total amounts of benzene and toluene increased 5.3 and 3.7 times respectively. Moreover, Ni/HZSM-5 has a high selectivity on the formation of both benzene and naphthalene, the total amounts of benzene and naphthalene increased 9 and 8.4 times respectively. The reason is that Ni/HZSM-5 could enhance the dehydroxylation of phenols, the cracking of aromatic side chains and the aromatization of hydrocarbons.(4) At 700℃, both 5%Mo/HZSM-5 and 5%Ni/HZSM-5 have promotion effects on the catalytic conversion of pyrolysis volatile from different rank coals. After the catalytic conversion in the presence of 5%Mo/HZSM-5 and 5%Ni/HZSM-5, the BTEXN yields from lignite are higher than young bitumite and nearly the same yields as old bituminous coal. Thus catalytic conversion of coal pyrolysis volatile is an effective way to increase the BTEXN yield during lignite pyrolysis.