The influence of alkalis, notably sodium (Na2O) and potassium (K2O), on fusibility and slagging are proportional to the quantity in the coal ash. For sodium-containing coals, the rate of buildup is a function of the sodium concentration. The alkalis can be present in the coal in various forms. Alkalis that vaporize during combustion are classified as active or mobile alkalis and are free to react or condense in the boiler and consist primarily of simple inorganic salts and organically bound alkalis. More stable forms of alkalis exist in impurities such as clays and shales and remain inert during combustion. The mode of occurrence of the alkalis in coal are determined through a coal leaching process via sequential washings using water, acetic acid, and hydrochloric acid. The most active alkalis are those soluble in water and acetic acid. The fouling potential of coals is directly related to the soluble concentration of sodium and has been shown to vary from ~0.001 lb soluble sodium per lb ash per million Btu fired for a low-fouling coal to ~0.044 lb soluble sodium per lb ash per million Btu fired for coals with high or severe fouling potential [8].

Two fouling indices related mainly to sodium have been proposed to predict the extent of fouling of convective heat-transfer surfaces. Both apply to eastern U.S. coals, rather than to the lignites in which the CaO + MgO content of the ash may be greater than the Fe2O3 content [29,30]. The indices are:

• RF = Base/acid x water-soluble Na2O (LTA, low-temperature ash).

Sodium is determined conventionally on the ASTM ash and on the water-soluble portion of the LTA. The fouling characteristics of coals are divided into four categories [29,30]:

Fouling Tendency


0 0

Post a comment