| Summary: | Opportunities exist for effective coal combustion additives that can reduce the carbon content of pulverised fuel ash to below 6%, thereby improving the efficiency of low NOx burners and making the ash saleable for filler/building material applications. However, catalytic performance under pulverised fuel boiler conditions has received relatively little consideration. For the first time, the research set out in this thesis systematically compares the reactivity of catalysed and uncatalysed bituminous coal samples during combustion in a drop tube furnace (DTF) and a thermogravimetric analyser (TGA). All of the inorganic compounds investigated were accordingly found to increase the rate of TGA char oxidation, with certain transition metal halides proving to be extremely successful additives. But these trends did not always translate into analogous carbon burnout improvements on th~ medium-scale DTF. This was due to the inherent variability associated with this experimental setup, the use of physical mixtures for catalyst dispersion and the intrinsic volatility of some of the tested salts. Furthermore several very effective additives on the TGA, the copper halides, seemingly deactivated under the conditions experienced in the DTF, reducing the attained carbon burnouts to levels below those observed with the uncatalysed coal fractions. Complex interactions were subsequently discovered between copper (I) bromide and the coal's mineral matter, implying that a copper-iron association could have been behind this behaviour. In contrast, the thermally labile iron halides were found to be exceptional additives for enhancing DTF carbon burnout, even at temperatures above their predicted boiling points in what could have previously been assumed to be a completely mass transfer controlled reaction regime. The combustion improvements associated with the introduction of iron (Ill) bromide were thus rigorously established and then related to the retention of the additive's cation, as indicated by SEM/EDX and ICP-MS.
|
|---|
