Experimental evaluation of process parameters influencing ash deposition in pulverized coal-fired utility boilers and gas turbines

A laboratory test furnace was utilized to investigate ash deposit initiation and build-up as related to pulverized coal-fired combustion systems. Vertical tube furnace investigations were conducted to study the problems of ash deposition on heat transfer surfaces in utility boilers and gas turbine hot gas pathways. The rate of ash deposition and deposit strength were determined for coals ranging in rank from lignite to medium volatile bituminous.;The tube furnace was capable of simulating most of the operating conditions of both utility boilers and gas turbines. A thin stream of pulverized coal was combusted in the furnace and the fly ash was impacted on metal substrates to form deposits. The results of these tests were then compared with a qualitative model involving (1) fly ash generation, (2) particle transport to a surface and (3) particle adhesion to the surface and deposit build-up.;The generation of fly ash from a western U.S. lignite was proposed to occur by partial coalescence of mineral inclusions and organically bound metals in the char. Test results indicated that finer grinding of some low rank coals may reduce the interactions of inorganic constituents and fly ash particle size. For an eastern bituminous coal, it appeared that coalescence of minerals in the char did not occur.;The dominant mechanism of particle transport to surfaces at conditions similar to utility boilers and gas turbines was observed to be by inertial impaction. The tube furnace was a simple system for studying the relationship between coal particle size and resultant fly ash particle size. The fly ash particle size distributions obtained in these tests can be utilized in aerodynamic models which predict collection efficiencies of boiler and gas turbine surfaces.;The retention of fly ash particles which impacted upon a metal surface was enhanced by the presence of a liquid phase on either the fly ash particles or on the metal surface. In tests simulating both boilers and gas turbines, the amount of deposition and strength of deposits were found to be inversely proportional to the viscosity of liquid species predicted to occur at chemical equilibrium.