(209c) On the Role of Sodium Content of Fine Ash Particles in Determining Deposition Rates Under Air- and Oxy-Coal Combustion Conditions

During the combustion of coal, the sodium content in the ash is commonly thought to be the â??bad actorâ? in determining ash deposition rates on boiler heat transfer surfaces. We have conducted 16 tests in which deposition rates of three coals, burned under air fired and oxy-fired combustion conditions, were measured. A 100kW down-fired laboratory combustor, coupled with a specially designed deposition probe was employed. Emphasis was on deposition rates of the tightly bonded â??insideâ? deposits, rather than on the loosely bound â??outsideâ? deposits, which could not be weighed precisely. Surprisingly, the measured deposit rates correlate neither with the sodium content of the coal, nor with the sodium content of the fine ash aerosol (PM1 or PM0.6). Instead they correlate well with the overall flue gas concentration of PM1, the alkali composition of which varied significantly over the 16 tests. This suggests a mechanism of deposit adhesion that depends simply on the presence of submicron particles of any composition. Additional research is needed to confirm this, because this conclusion disagrees with nearly all existing models for ash deposition and has implications for simulations predicting boiler performance for coal or biomass fired fuels.