Slagging in PC boilers and developing mitigation strategies

Large reductions in furnace exit gas temperature when a group of sootblowers is activated correspond to an effective surface cleaning. It also means that the deposition rate is very high on the surface that is covered by particular sootblowers. If a sootblower group is activated and there is a negligible impact on the furnace exit gas temperature this means either the surface is already clean or the deposit on the surface cannot be removed by sootblowing. The latter is a serious concern that may result in load reductions or forced unit outages. As can be seen from the following figure, the impact of each sootblower group on the FEGT varies from almost negligible to as high as 120°F. The sootblower subgroups 4 and 6 did not have any impact on the FEGT while the subgroups 1, 2, 3, and 5 had quite large reduction FEGT. [Display omitted] •Off-design coal having a low ash fusion temperature caused excessive slagging.•Impacts of boiler operating settings on FEGT and slagging were studied.•Sootblowers were characterized and optimized for mitigating slagging.•A strategy was developed for slowing the deposition rate down. 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°F range, with an average ash softening temperature in an oxidizing atmosphere of 2360°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 exces