Ash chemistry issues and efficient combustion research progress in pure coal combustion of high-alkali coal

Coal constitutes a pivotal element of China's energy consumption structure. Xinjiang, which boasts considerable coal deposits, is a crucial energy succession zone and strategic reserve area within China. The coal resources in Xinjiang, particularly the Zhundong variety, typically exhibit elevated concentrations of alkali metals and alkaline earth metals (AAEMs), which are typical high-alkali coal well-suited for prolonged utilisation. The large-scale mining and utilisation of coal resources in Xinjiang has resulted in significant challenges, particularly in the form of slagging, staining and corrosion of high-alkali coal during combustion. These issues have severely limited the large-scale and efficient utilisation of this coal resource. The objective of achieving 100% clean combustion of high-alkali coal represents a significant challenge in the field of high-alkali coal utilisation. However, the primary obstacle lies in the resolution of the ash issue inherent to the clean combustion process. In this paper, about 4401 literatures in the field of high-alkali coal research in the past 20 years were obtained by using CiteSpace software. These literatures were subjected to literature clustering analysis, keyword highlighting analysis and time-series analysis. In addition, the peak interval of the heat of research in different periods was systematically analysed. It is highlighted that the future research in this field should prioritise the investigations into the existence form and migration characteristics of AAEMs, the low-temperature eutectic formation mechanism, and the development of combustion-efficient utilisation regulation measures. Based on a representative high-alkali coal, a summary of the fundamental physicochemical properties was presented, with a particular focus on the impact of the alkaline component of coal ash on the melting temperature of coal ash and the existence form of AAEMs. This study investigates the research progress of two core aspects, namely, AAEMs and low-temperature eutectics, with the objective of exploring the migration characteristics of AAEMs and the generation mechanism of low-temperature eutectics. This is of great significance for the clean and efficient utilisation of coal with low carbon. Subsequently, the regulatory measures for the efficient utilisation of high-alkali coal combustion are categorised into three principal groups: the reduction of AAEMs content, the increase of acidic components, and the optimisation of reactor operation and structure. The principles and effects of the utilisation measures on the mitigation of slagging and staining of high-alkali coals are then analysed, and the shortcomings of the corresponding methods are identified. Furthermore, it is proposed that the coal dust purification-combustion efficient utilisation technology may prove an effective means of solving the problem of efficient utilisation of high-alkali coal combustion. Ultimately, the dual-carbon context underscores the necessity for thermal power units to enhance their variable load regulation capabilities, thereby ensuring a stable power output. This paper puts forward suggestions and proposes avenues for future research in order to achieve the efficient utilisation of high-alkali coal combustion. It is essential that these are combined with local diversified resource advantages and that an overall plan is formulated in order to accurately control the problems of slagging, staining and corrosion throughout the entire process. This will enable the clean, efficient and low-carbon utilisation of the resource to be achieved.