Effect Of CO₂ On Products Distribution And Phenols Release During Yining Coal Pressurized Pyrolysis

CO2 can be used as the co-gasifiying agent to replace part of steam for reducing the emission of phenolic wastewater during pressurized fixed bed dry bottom gasification process. At the same time, CO2 will affect inevitably the coal pyrolysis process in gasifier. Coal pyrolysis reaction mainly occurred in the devolatilization zone. Phenolic compounds are coal pyrolysis byproducts with high additional value. CO2 will affect the formation and release of phenolic compounds, therefore, it is important to study the effect of CO2 on products distribution and phenolic compounds release during the coal pyrolysis process, which is of crucial importance for subsequent processing of coal tar and the treatment of phenolic wastewater.Yining coal, which has been used for pressurized fixed bed dry bottom gasification, was crushed to 5—8 mm and pyrolyzed in a fixed bed reactor using N2 atmosphere or CO2 atmosphere under ambient pressure or elevated pressures. The pyrolysis products distribution was studied under different temperature and pressure. Eight species of phenols in tar were identified and quantified by GC/MS. Effect of pyrolysis temperature, pressure, and atmosphere on pyrolysis products distribution and phenolic compounds release were investigated.The experiment results showed that CO2 atmosphere can deepen coal pyrolysis degree under ambient pressure, which is beneficial to the sulfur compounds removal and the liquid product formation. Compared with that under N2 atmosphere, higher tar yield, lower char (700,800 ?) and gas yield were obtained under CO2 atmosphere. CO2 atmosphere can promote the phenolic compounds release, therefore, total phenols amount reached 476.36 ng/mg under CO2 atmosphere at 500 ?, which was 12.95% higher than that under N2 atmosphere. Carbon atom in CO2 showed strong electrophilicity, and it might interact with phenoxy group with stronger nucleophilicity. It makes the alkyl phenyl ether transfer hydrogen in intramolecular or intermolecular and then generates the phenolic compounds. Phenol and cresol are the main species of phenolic compounds produced during coal pyrolysis.With the pressure increased, at 700 ?, the char yield almost unchanged under both two atmospheres. Tar yield decreased under CO2 atmosphere, but it increased under N2 atmosphere. Char and gas yield (except 1.5 MPa) under CO2 atmosphere were lower, tar yield (except 1.5 MPa) was higher than that under N2 atmosphere respectively, that is to say, CO2 atmosphere is beneficial to the formation of liquid products. Phenols amount increased under both two atmosphere with the increasing pressure. Phenol and cresol were still the main species of phenolic compounds. With the pressure increased, proportion of cresol and xylenol increased in total phenols under CO2 atmosphere, but it decreased under N2 atmosphere, namely CO2 atmosphere promoted the formation of alkyl-substituted phenols which have 1 to 2 methyl groups.