Gasification and char combustion reactivities of Daw Mill coal in wire-mesh and ‘hot-rod’ reactors

Abstract

The pyrolysis and CO 2-gasification behaviour of Daw Mill (UK) coal was investigated using wire-mesh (WMR) and fixed-bed ‘hot-rod’ (HRR) reactors. Experiments were performed at pressures between 0.1 and 3 MPa at 850 and 1000°C. These reaction conditions reflect those of novel power plant technologies (e.g. ABGC) in which coal gasification and char combustion reactivities are relevant to design and operation. Untreated coal samples were used in all experiments rather than pre-prepared chars as appears more usual. CO 2 gasification reaction rates were directly calculated from the difference between mass loss in pyrolysis and during gasification in CO 2. Combustion reactivities of the chars from these experiments were determined in an atmospheric pressure thermogravimetric balance. The partitioning of the original sulfur and nitrogen contents between the volatiles released in the gasifier and the char transferred for combustion in the CFBC were determined. Differences in mass loss between gasification and inert pyrolysis were found to be significantly greater in the wire-mesh reactor, where more efficient gas-solid contact takes place and tar condensation on reacting char is minimized. Whilst combustion reactivities of the CO 2-gasification chars were lower than their pyrolysis counterparts and tended to decline slightly with increasing pressure, reactivities of chars prepared in the HRR were systematically lower than those from the WMR. The reactivities of four other coals were also tested at 1000°C and 2 MPa. A straightforward decline in pyrolysis mass loss was found with increasing rank. However, even within the limits of the small number of samples tested, considerable scatter was found in the reactivities of middle-rank coals, probably due to a combination of structural factors, including differences in char pore structure, catalytic effects of mineral matter components and differences in the inherent reactivity of the carbonaccous matter. The percentages of coal nitrogen and sulfur retained in the residual chars generally declined with increasing pressure; reactive gases appear to remove char nitrogen more efficiently, suggesting that pyrolysis experiments may not be a good guide to char nitrogen partitioning in gasification and combustion.