Temperature pyrolysis of brown coal and brown coal containing iron hydroxyl complexes

The concentrations profiles of CO2 and CO have been measured at 150-600 degrees C from the pyrolysis, at slow heating rates, of acid- washed brown coal and brown coal containing iron hydroxyl complexes. CO2 formation was greater at low temperatures, but CO increased relative to CO2 with an increasing temperature. The ratio CO2/CO was larger for coal with iron, compared to that from acid- washed coal. The iron species in the chars were Fe2O3 at 200- 400 C, Fe2O3 and Fe3O4 at 400- 600 degrees C, and Fe-0 at 700 degrees C; inorganic carbonate was also detected. Although conventional chemical kinetic simulations could reproduce the total weight loss of coal with temperature, such calculations could not simulate the measured CO2 and CO concentration profiles. Decarboxylation reactions may proceed via a number of reaction routes, including ones involving intermediate species. Semiempirical quantum mechanics modeling (SE-QM) of decarboxylation using three carboxylic compounds provided a relative order of decomposition as carboxylic acid similar to carboxylate >> radical. SE-QM and single-point self-consistent field (1scf) calculations, using 2D and 3D models of brown coal, were conducted to provide changes in the heats of formation for models after the loss of carboxyl groups. Such calculations also indicated that hydrogen transfer from a phenoxyl group to a carbanion was energetically favored. The formation of the various iron oxides in coal was modeled by (i) decarboxylation reactions via an iron- carbonato complex decomposing into CO2 and a mu-oxo iron complex and (ii) decarboxylation of coal via the reduction of iron complexes and the formation of organic radicals.