Mechanisms of Ash Fouling during Low-Rank Coal Combustion

Abstract

Four low-rank coals were investigated for fouling severity using bench and pilot combustion testing and microanalytical examination of fouling deposits. The coals contained varying levels of alkali and alkaline-earth elements that are commonly associated with initiating and accelerating ash fouling, including Na, Mg, K, Ca, and Fe. Combustion testing revealed that fouling deposits generated from these coals shared common chemical and physical properties. Four test coals from western U.S. coal fields were selected, including the Beulah and Gascoyne lignites from western North Dakota and the Colstrip subbituminous coal from Montana, and the Utah Wasatch from Utah. All of these coals contained significant levels of Na, Ca, and Mg, with the Beulah lignite containing the highest levels of sodium. Sodium in the Beulah and Gascoyne lignites was very abundant and was organically bound. The Utah Wasatch coal contained significant levels of sodium, but it was bound in the coal as a zeolite silicate termed analcime. Deposits were ranked from low-fouling to severe-fouling based on deposit build-up rate, deposit strength, and liquid-phase viscosity, which was calculated based on the chemistry and the gas temperature near the deposits at the time of quenching. Deposit build-up rates and crushing strengths were the highest for the Beulah and Gascoyne coals, followed by the Utah Wasatch, during both bench- and pilot-scale fouling deposition simulations. The outer layers of the lignite deposits showed well developed captive liquid surfaces and silicate liquid-phase viscosity distributions that were shifted to much lower values compared with the Utah Wasatch and Colstrip deposits. Microanalysis of the deposits using scanning electron microscopy revealed that the gluing material or phase that was responsible for the cementing of the severe-fouling deposits was a low-melting-point sodium—calcium-rich silicate. The Utah Wasatch coal, which also contained sodium did not form as much of the low-melting-point sodium—calcium silicate, partly because the sodium was locked within an existing coal mineral phase and was not able to interact with the silicate material during combustion.