Journal of the Southern African Institute of Mining and Metallurgy
On-line version ISSN 2411-9717
TAOLE, R.L.; FALCON, R.M.S. and BADA, S.O.. The impact of coal quality on the efficiency of a spreader stoker boiler. J. S. Afr. Inst. Min. Metall. [online]. 2015, vol.115, n.12, pp. 1159-1165. ISSN 2411-9717. http://dx.doi.org/10.17159/2411-9717/2015/V115n12a3.
This research establishes the combustion characteristics and efficiencies of South African coals of different qualities and their impact on the performance of a grate spreader stoker boiler. Four different coal samples were tested in the particle size range 6.25 x 25 mm. A detailed investigation involving the boiler operating conditions associated with the physicochemical characteristics of the coals, petrographic properties, and temperature profiles from a thermal camera was conducted. The thermal analysis indicates that there is a strong correlation between thermographic data (combustion behaviour and maximum flame temperatures) and petrographic composition of the coals. This association is not reflected in calorific values and proximate analyses of the coals. In terms of combustion efficiencies, all coals yielded relatively high amounts of unburnt carbon in the fly ash (about 36.90%). The highest steam output obtained was 41.76 t/h at the highest combustion efficiency of 79.13%. The thermographic results obtained from this study led to the conclusion that South African low-grade Gondwana coals undergo delayed ignition and burn at unusually high temperatures (1500-1800°C), which is in contrast to the original belief that the combustion temperature is around 1400°C.
Keywords : coal; combustion; macerals; thermographic camera; travelling grate.