The cofiring of other fuels with coal, especially biomass represents the one of the biggest challenges to power plant
operation in the last fifteen years. A large number of fuels have been studied at laboratory, pilot and full scale and many are now in regular use in operating plants. The ash chemistry of these alternative fuels is often very different to that of the coals and has given rise to serious problems, particularly in the early days of cocombustion. The alkali metals are particularly important in deposit initiation and growth and care has to be taken when selecting biofuels and establishing the appropriate coal replacement levels for cocombustion. However, the widespread practice of cofiring has demonstrated that these problems can be overcome, with minimum ash-related impact on plant operation. Although predictive models for slagging and fouling are now in widespread use, they are not thought to be capable of predicting ash impacts with sufficient reliability in all cases. The sub-models are thought to be at an advanced stage of development, but overall, model integration is still thought to require development. The prediction of ash impacts is generally thought to be more reliably achieved through a combination of predictive indices, models, historical information on coal and boiler performance and experience.