OCR Text |
Show (1981) noted the predominance of calcium-rich sulfates and phosphates in the deposits. The molten state of deposited particles provides adhesion to the surface and may in fact aid in the capture of inertially impacted particles (Bauserman et al., 1980). These adhesion characteristics are hueristically referred to as "glue" by most investigators and are dependent upon the chemical composition of the deposit. The composition, in turn, is affected by such combustion parameters as fuel-air ratio, atomization char-jcteri sties, and residence time. In addition, the concentration levels of these elements in the fuel are also factors but are yet to be correlated to the deposit composition except by empirical standards specified by equipment manufacturers. Rosner and Seshadri (1981) assumed that only a negligible amount of chemical reaction, condensation or coagulation occurs within the thin mass transfer boundary layers adjacent to the surface. In studying the deposition of liquid Ba203, they proposed that, in vapor deposition, the particular species need not be present in the mainstream. Because the growth of "condensible precursors"* can occur as a result of heterogeneous reactions at the outer edge of the boundary layer, which form the condensate. In other words, the BoOo liquid growth can be a result of HBO gas transport. As a result, at the vapor/condensate interface, near the surface, the local thermochemical equilibrium is very much influenced by the transport of species to and from the surface. "Condensate precursors" may be described as compounds existing in the vapor phase (in the mainstream gas) which have a major element in common with the deposited compound. In the case of Na^SO^ (liq) deposit, sodium can be distributed in the external stream among such precursors as NaOH, NaCl, Na (g), etc. 1.5.16 |