Ash deposits formed at moderate temperatures (1100-1300 K) in the convective sections of utility boilers firing coals containing lignitic ash are associated with the formation of sticky particles following condensation of sodium sulfate from the vapor phase at 1200-1300 K. The deposits are enriched in calcium, due to the abundance of calcium among the particles impacting and sticking and the low melting temperature of mixtures of calcium, magnesium, and sodium sulfates. Because the formation of these deposits occurs only in a limited range of gas temperatures, approximately from the dew point to the melting temperature of sodium sulfate, they are not observed during experiments in which deposition is accelerated by increasing the temperature to values typical of furnace exit gas or higher. The principal features of the process have only become evident from observations in the field. On startup of a boiler after cleaning, deposits are formed by ash particles made sticky by a layer of condensed sodium sulfate. Although sodium sulfate is most concentrated in the smallest particles, the deposit is formed by particles large enough to impact and small enough to stick. These particles may contain less than the average sodium content of the coal ash. At early times, growth is most rapid at downstream locations, where the thickness of the sticky layer of sodium sulfate on suspended particles is greatest. At later times, the rate of deposition increases when deposits become thick enough that sticky sulfates are present on their surfaces. Growth is then more rapid upstream, where the concentration of sodium sulfate in deposits is greatest, due to direct condensation of vapor. Because deposit accumulation is the difference between the rates of deposition and erosion, the amount and size distribution of erosive particles, such as quartz, are important factors in deposit formation.
