A parametric investigation of tar release in coal devolatilization

Abstract

A parametric investigation of the effects of coal characteristics and devolatilization conditionson the tar component of volatile matter has been performed. A heated grid apparatus is employed with programmed heating rates of ∼102–103 C/sec to final temperatures of 500–1500 C. Fourteen different coals were utilized in the investigation, ranging from lignite to anthracite. The maximum tar fraction of the total volatile yield is a function of the chemical characteristics of the parent coal, amounting to ∼20% of the total volatile yield for a lignite to ∼80% for a medium volatile bituminous coal. The sensitivity of tar yields to changes in devolatilization conditions is also observed to be a function of coal characteristics. Tar formation and release is observed to be endothermic and to perturb the temperature of the grid in immediate contact with the sample. The fuel nitrogen in the parent coal is distributed proportionally in the pyrolysis products. Specifically, the mass fraction of fuel nitrogen in the tars is similar to the mass fraction of the parent coal that evolves as tar. Tar yields are reduced with increases in ambient pressure in the 10−3 to 760 torr range, with greatest reductions occurring in the 10−3 to 200 torr range. The higher pressures increase the secondary reactions of the tars resulting in an increase in light hydrocarbons in the gaseous products. Models of coal devolatilization therefore must explicitly include tar evolution and its dependence on coal characteristics, heating rate and pressure. A model should also deal with the endothermicity of the tar release process.