Low-Temperature Pyrolysis of Brown Coal and Brown Coal Containing Iron Hydroxyl Complexes

The concentrations profiles of CO 2 and CO have been measured at 150−600 °C from the pyrolysis, at slow heating rates, of acid-washed brown coal and brown coal containing iron hydroxyl complexes. CO 2 formation was greater at low temperatures, but CO increased relative to CO 2 with an increasing temperature. The ratio CO 2 /CO was larger for coal with iron, compared to that from acid-washed coal. The iron species in the chars were Fe 2 O 3 at 200−400 °C, Fe 2 O 3 and Fe 3 O 4 at 400−600 °C, and Fe 0 at 700 °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 CO 2 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 ∼ 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 CO 2 and a μ-oxo iron complex and (ii) decarboxylation of coal via the reduction of iron complexes and the formation of organic radicals.