LABORATORY STUDIES OF ASH DEPOSIT FORMATION DURING THE COMBUSTION OF WESTERN U.S. COALS (UNITED STATES)

A laboratory-scale laminar-flow vertical tube furnace was used to study ash deposit formation during the combustion of selected Western U.S. coals under conditions simulating a utility boiler. Deposits were formed by the impingement of fly ash and products of combustion on a cooled boiler steel substrate. The deposits consisted of several layers. The initiating layer was comprised of particles strongly bonded to the oxide layer of the substrates. The next layer consisted of very small particles that covered the entire substrate. The third layer or the base layer of the deposit contained very lightly sintered fly ash particles. The final layer grew out of the base layer in the direction of the oncoming gas stream as a result of higher temperatures which caused melting of the deposited material.; The most significant findings include the following: (1) The adhesive strengths and composition were determined for individual particles which had formed a strong bond with the oxide layer of the substrate. The results showed that the particles producing the strongest bond with the oxide can be ranked as follows: iron and silicon particles > iron and sulfur particles > calcium and sulfur particles. (2) The deposit growth rates for a series of Western coals were based on the rate of deposit formation divided by the rate of ash fired. Most Western coals exhibited an even growth rate. (3) A device was developed to measure strengths of the deposits produced in the vertical tube furnace. The use of the vertical tube furnace to form deposits and the measurement of deposit strength provided a very sensitive method to assess the ability of a coal to produce a strong deposit on a laboratory scale. (4) Many of the deposits produced from the Western coals were highly crystalline. The degree of crystallinity appears to increase with deposit strength. (5) The viscosity of the glass phase was calculated based on its composition. The results showed an inverse relationship between the viscosity of the glass and deposit strength. (6) Preliminary studies were conducted on the application of complex thermochemical equilibrium calculations to determine the behavior of inorganic constituents during combustion.