Role of Sodium in Coal in Determining Deposition Rates

The alkali content in the ash is commonly thought to be the “bad actor” in determining coal ash deposition rates on boiler heat transfer surfaces. This paper reports results of 16 tests in which ash aerosol deposition rates were measured for three coals, burned under air- and oxy-fired combustion conditions. A 100 kW down-fired laboratory combustor coupled with a specially designed deposition probe was employed. Ash aerosol particle size distributions and size-segregated compositions were measured using electric-mobility, light-scattering, and low-pressure impactor techniques. Net sodium vaporization (assumed to be the fraction of sodium collected as a <0.6 μm fume) was compared to literature data. For ash deposition rates, emphasis was on the tightly bonded “inside” deposits rather than the loosely bound “outside” deposits, which could not be collected or weighed precisely. Over a limited range of these tests, where PM₁ was indeed greatly enriched in sodium, deposition rates did correlate with the sodium content of PM₁ and the ratio of Na/(Al + Si) in the coal, as proposed in the literature. However, over the entire range of deposition rates investigated, the measured rates did not correlate well with the sodium content of the coal or with the sodium content of the fine ash aerosol (PM₁ or PM₀.₆) but did correlate well with the overall flue gas concentration of PM₁, of which the alkali composition varied significantly over the 16 tests. This suggests a mechanism of deposit adhesion that depends simply upon the presence of sub-micrometer particles of any composition. Additional research to confirm this would be useful because this conclusion disagrees with most existing models for ash deposition. This work focuses on the inside fouling deposition, which could be seen as the initial layer of the fouling deposits. Slagging deposition is not within the scope of this work.