Structural transformations for a subbituminous coal, impact of temperature on gold-tube pyrolysis chars evaluated using HRTEM

Abstract

Structural transformations accompanying coalification can be explored by gold-tube pyrolysis. There the pyrolysis products are retarded by the high gas pressure and the slow-heating with temperature soak can somewhat simulate in situ conditions in a laboratory setting. Coalification process under geological conditions can be explained. While there has been considerable research expenditure on bituminous and anthracite coals, the subbituminous coals have been less-well explored. Here, the structural transformations accompanying subbituminous coal during heating were followed using HRTEM image analysis. Specifically the distributions of fringe: length, orientation, stacking, and curvature were quantified. Pyrolysis was performed under N2 at a slow heating rate of 20 °C/h with a 50 MPa initial (cold) confining pressure. Temperatures between 325 and 590 °C were explored. Each sample was temperature soaked for 1 h and allowed to cool slowly to room temperature. The structural transformations accompanying pyrolysis could be divided into three stages by the fringe orientation within a 45° bin capturing any thermoplastic transformations. In stage one: 325∼432 °C there is some growth in the lattice fringe lengths and the distribution broadens. In stage two: 456∼504 °C there is an enhanced partial ordering (orientation and stacking) indicating some thermoplastic transformations occur for this subbituminous coal. In stage three: 524∼590 °C the partial ordering extent is reduced. Presumably at higher temperatures the appropriate balance between mass loss, tar viscosity, and other properties are not in balance for significantly improved thermoplastic behavior—although there is a recovery at highest temperatures explored here. These results can provide some structural transformations for a subbituminous coal and could influence the strategy for attempting to produce coke from subbituminous coal.