Effect of Ca- and Mg-bearing minerals on particle agglomeration defluidisation during fluidised-bed combustion of a South Australian lignite

Behaviour of calcium and magnesium during fluidised-bed combustion (FBC) of a South Australian lignite was investigated using a laboratory scale spouted bed combustion system. Combustion experiments were aimed at investigating the effectiveness of Ca- and Mg-bearing minerals (as alternative bed materials) in controlling particle agglomeration and bed defluidisation during FBC combustion of low-rank coals. Additional experiments performed with a Ca-treated coal investigated the role of Ca in agglomeration and defluidisation process. Experimental results indicated that both Ca/Mg-bearing minerals and Ca-treated coal were effective to different extents in reducing bed defluidisation. Tests with calcite (as the bed material) and Ca-treated coal runs (with sand as the bed material) showed trouble free operation for 8–10 h before bed defluidisation incurred. Tests with magnesite (as the bed material) showed no agglomeration and defluidisation tendencies for longer operating periods (∼12 h at 800°C). Mg-bearing compounds have been found to be effective in controlling defluidisation and allowed extended combustion operations. On the other hand, high levels of Ca either in coal or in bed material have been found to delay and decrease the severity of agglomerates formed.