SO2 EMISSIONS CONTROL BY ON-GRATE SORBENT ADDITION ON INDUSTRIAL STOKER-FIRED PLANT

Throughout the world, stoker firing is the predominant method of coal Research Establishment (CRE) to identify and develop suitable methods for such control. Priority was given to identifying cost-effective met hods suitable for application to existing plant with the minimum of mo dification. Addition of sorbent with the coal feed to the stoker grate was examined in the early phases of the programme since, in engineeri ng terms, this was seen as the option which required least plant modif ication. CRE did not consider it likely that this approach would be su ccessful, but considered it worthwhile to undertake a modest programme to investigate the extent of sulphur capture and identify the factors limiting capture. A novel design of fixed grate simulator, installed at the Coal Research Establishment, was used for investigating two met hods of on-grate sorbent addition, viz, the mixing of coarse limestone with the normal coal fed to stokers, and incorporation of fine limest one with fine coal into a pelletized fuel. Under typical industrial st oker conditions, both methods of feeding limestone at a Ca:S molar rat io of 2:1 gave an SO2 abatement of only about 10%. NOx emissions were largely unaffected although ash clinkering was excessive with the pell etized fuel. The poor capture is considered to be caused by the condit ions changing between reducing and oxidizing during burnout of the coa l, and the high temperatures involved which resulted in subsequent rel ease of previously captured sulphur. A marked improvement was achieved under more strongly staged combustion conditions. When coarse limesto ne was premixed with the coal, SO2 abatement was increased to 24% with a Ca:S molar ratio of 2:1, and NOx emissions were reduced by 40% comp ared with normal operation. However, particular care would need to be taken in the disposal of the ash residue since it contained calcium su lphide.