A comprehensive review on efficient utilization methods of High-alkali coals combustion in boilers

High-alkali coals are distributed in many countries all over the world, mainly characterized by contains abundant alkali metals and alkaline earth metals (AAEMs) elements, direct combustion in boilers will bring serious ash deposition, fouling, and slagging (ash-related issues) on the metal heat transfer surface of the boilers. In particular, the Zhundong (ZD) coals in Xinjiang, China, as typical high-alkali coal, have a huge reserve that can effectively ensure China's coal demand in the long term. The main purpose of this paper is to clarify the problems and difficulties faced by the combustion of high-alkali coals in boilers, explore and summarize the effective utilization methods. First, the primary difficulties and problems faced by direct combustion of high-alkali coals in boilers are summarized. Second, the research progress of current efficient utilization methods and solutions of high-alkali coals combustion in boilers are summarized and reviewed, it mainly includes the following aspects: (1) pretreatment technology before combustion; (2) control and optimization of combustion parameters such as temperature and atmosphere; (3) co-combustion with inorganic additives, other coals, biomass fuels, and other fuels, respectively. The above methods have been proved can be effectively reduced and alleviated the ash related issues, but no methods can be eliminated or solved the problems completely at present. Based on the existing research and practice results reviewed in this paper, aiming at the efficient utilization of high-alkali coals combustion in utility boilers, further feasible research directions or key points for the future are proposed.