Excessive slagging in coal-fired boilers on heat transfer surfaces such as water wall tubes, lower regions of the finishing superheater and superheater areas on the slope of the nose region was investigated for a pulverized coal-fired boiler that is forced to burn off-design coal. A detailed coal and ash analyses was carried out to understand the root cause problem. Slagging mitigation strategies involving boiler operations and cleaning methods were developed and implemented. Coals from two identical boilers, one with excessive slagging (Boiler A) and the other (Boiler B) with no slagging, were compared to understand the root cause problem. Detailed coal and ash analyses have shown a high iron content in ash as pyrite (FeS2), which has historically been known to promote boiler slagging. CCSEM analyses revealed that the pyrite was present in relatively coarse size fractions. If not pulverized well in the pulverizers, this coarse pyrite can result in low melting phases, because complete oxidation to a higher melting point oxide cannot be achieved. Ash fusion temperatures of ashes were found to be in the 2000-2500 degrees F range, with an average ash softening temperature in an oxidizing atmosphere of 2360 degrees F. Furnace exit gas temperature measurements at the furnace exit plane have shown that typical temperatures at full load around the nose area are in excess of 2300 degrees F. Lower ash fusion temperatures, coupled with higher operational furnace exit temperatures, are probably responsible for the more severe slagging observed at the boiler. A combination of boiler control settings and furnace cleaning schedule was developed and implemented on the boiler for a period of 2 weeks. The field experiments showed very good results with significantly lower ash deposition rates in the most problematic areas of the furnace observed previously. (C) 2013 Elsevier Ltd. All rights reserved.