Assessment of South African coals in a bubbling fluidised bed combustion testing facility

Abstract

South Africa is one of the world’s largest coal producers and exporters. However the quality of coal being mined is declining and hence beneficiating is required to meet export quality. Through beneficiation, waste coal (discard coal and duff coal) is produced and its accumulation is increasing yearly. The storage of waste coal can lead to environmental issues and is an eyesore to local and international sightseers in the highvelds. Therefore utilisation of waste coal, or land reformation of their sites is a necessity. In order to utilise waste coals, international coal consumers have applied fluidised bed combustion (FBC) technology. Fluidised bed combustion is a fuel-flexible technology, capable of burning waste coal with reduced nitrogen oxides and sulphur dioxide emissions. The objective of this research was to evaluate the combustibility of high-ash coals in a bubbling fluidised bed combustor. This study proposed that high ash coal will combust in the BFBC with reduced emissions as well as without ash agglomeration. The research carried out entailed coal and limestone analysis (chemical, elemental and mineral analysis), coal combustion tests in a pilot scale BFBC facility (at different combustion temperatures and different limestone feedrates), fly ash analysis (elemental and mineral quantifications) and data consolidation. On average, fly ashes from the three coals tested had a carbon fraction lower than 0.1%. Coal C had the lowest carbon-in-ash residue at all testing conditions. The addition of limestone without adjustment of the air/fuel ratio impacted negatively on the combustion efficiency at the highest Ca/S ratio. Coal A had the highest sulphur self-capture and retention, and the highest amount of decomposed calcite and dolomite. Overall limestone addition reduces the amount of SO2 emitted. The BFBC test displayed low NOx emissions which were lower than the minimum emission standard for new plants. NOx emission increased with increasing bed temperature and N2O emission decreased with increasing bed temperature. No ash agglomerations or slagging was detected during tests and the post-test reactor observation. Coal B has the lowest slagging propensity. Overall, this study has shown that low quality coals can be combusted in the BFBC. The results showed that the considered coals can be combusted effectively with reduced emissions, without ash agglomeration, and with lower carbon-in-ash.