ASH DEPOSITION AT LOW-TEMPERATURES IN BOILERS BURNING HIGH-CALCIUM COALS .1. PROBLEM DEFINITION

Utility boilers fired with high-calcium coals from the Western United States often develop massive ash-fouling deposits on steam tubes at mu ch lower temperatures than is observed when low-calcium coals are burn ed, In order to define and develop the capability to predict the probl ems, extensive sampling was performed at five utility boilers, includi ng both pulverized coal- and cyclone-fired units, burning a total of f our different high-calcium subbituminous coals. Four types of low-temp erature deposits were observed, three types forming on the upstream si de of the tubes and one type forming on the downstream side, Upstream deposits include massive ones that form toward the back of the seconda ry superheater and into the reheater at gas temperatures between appro ximately 1700 degrees and 1900 degrees F, enamel-like deposits that fo rm at lower temperatures on primary superheaters and economizers, and double-crested deposits that form in the primary superheater and econo mizer tubes. The deposits forming on the downstream sides of the tube occur from the reheater section to the economizer, The common and defi ning characteristic of the four types of low-temperature deposit is th eir high concentrations of sulfur that was fixed through chemical vapo r deposition after the ash had deposited, In this article the differen t types of low-temperature fouling problems associated with the combus tion of high-calcium coals are defined and the boiler conditions under which they occur are given. In subsequent articles the specific pheno mena affecting the formation of the deposits will be addressed in deta il, including the particular fractions of the ash responsible, the mec hanisms of transport to the heat-exchanger surfaces, and the factors a ffecting the rates of deposition and development of strength in the de posits.