Remediation of ash problems in pulverised coal-fired boilers

Abstract

This paper reports the control methods for mitigating fouling and slagging in brown coal-fired power utilities. Two control methods were investigated, i.e. wet pretreatment of coal and the use of mineral additives. By the first technique, samples of brown coal were treated with aluminium solutions to adjust the levels of Na, Al and Cl. The effect of cleaning was tested under closely controlled conditions in the range of 1000–1400°C. At a gas temperature of 1250°C, the deposition behaviour of treated and raw coals were compared using a deposition probe maintained at temperatures of 500, 600, 700 and 800°C. The second technique involved experiments with mixtures of candidate mineral additives and sodium compounds at temperatures ranging from 1000 to 1400°C.Experiments showed that both the methods were effective in reducing ash-related problems. Between 1000 and 1400°C, the quantity of condensable salts generated was found to depend only on the amount of sodium in coal. With the wet method, aluminium reduces the sodium level and thus fouling by ion exchange. The stickiness of sodium silicates from this coal was also shown to be reduced by aluminium. Deposition model calculations based on sticking probability also indicated that Al-treated coals result in less ash deposition on boiler surfaces. Experiments with mineral additives showed that sodium could be captured by clay minerals, particularly kaolin. For Victorian brown coal-based power station boilers, 10–20μm kaolin making up 2–3wt.% of the feed was estimated to effectively reduce the ash problems.