## 2 Literature Review  
Industrial device degradation and optimal maintenance plans are predicted using Prognostics and Health Management (PHM) [12; 13]. The PHM of fouling in coal-fired boiler power plants is mainly divided into the following processes, i.e., ash fouling monitoring, ash fouling prediction, and soot blowing optimisation. Ash fouling during coal-fired boiler operations cannot be measured directly based on the complex structure of the heat transfer surfaces, it can be indirectly measured using a heat flux meter, the quantity of heat produced or through acoustic pyrometry [14; 15; 16]. Critical parameters to be considered during a coal-fired boiler construction are the furnace size and height estimation, which depend on the coal ash softening and melting temperature(s). It is estimated that the ash softening temperature must be greater than the temperature of the flue gases leaving the furnace (FEGT) [17]. Other essential factors that influence the changes within the coal constituents and ash formation on heat transfer tubes are the boiler operating conditions, fuel-air mix, soot blowing, and the gas temperature [18]. The severity of ash deposition thickness across the boiler heat transfer sections is different. This variation is dependent on the combustion environment that influences ash deposition. Strongly bonded deposit formation on heat exchanger tubes reduces soot blowing effectiveness [18]. Ash deposition growth during the boiler operation changes the flue gases flow direction and temperature within the boiler [19]. During coal combustion in the boiler furnace, molten ash above the ash fusion temperature (AFT) is then deposited on the furnace wall. Slag deposits seriously affect the performance of coal-fired boiler power plants. A major advantage of the slagging effect is reducing the unused coal ash minerals into the atmosphere [20; 21]. Identifying slag flow, quantity, and position during the boiler operations can be very difficult or almost impossible, this difficulty can be overcome by the development of slag formation mathematical modelling. In recent years,there have been multiple research attempts to develop and utilise computational analysis model in monitoring ash formation and transportation during coal combustion in pulverised coal-fired boilers [22; 23]. Computational modelling enhances a better understanding of the investigation of the slagging performance during the boiler operation and provides a detailed investigation that overcomes experimental work limitations [24]. The following inorganic mineral contents SiO\({}_{2}\), TiO\({}_{2}\), Al\({}_{2}\)O\({}_{3}\), CaO, MgO, Na\({}_{2}\)O, K\({}_{2}\)O, P\({}_{2}\)O\({}_{5}\), Mn\({}_{3}\)O\({}_{4}\), SO\({}_{3}\), Fe\({}_{2}\)O\({}_{3}\) are contained in the coal ash deposited as slag in the boiler furnace wall [25]. A key parameter in estimating slagging and fouling potential during coal-fired boiler operations is the coal ash fusion characteristics. The ash fusion characteristics is mainly affected by the chemical composition of the ash minerals [26; 27; 28; 29]. Higher Ca, Mg, Fe and Na contents in the coal ash lower the AFTs, whereas higher Si, Al, and Ti contents increase the AFTs [28]. The processes of slagging and fouling during the boiler operation are triggered by a higher Fe concentration in the coal ash samples in a reducing environment [30; 31]. Ash produced from coal combustion inside a coal-fired power plant contains approximately 20 wt% bottom ash and 80 wt% fly ash [32]. The fine ash particles, which are the fly ash, are transported by the hot fuse gases with a particle size range of approximately 0.5 \(\upmu\)m to 300 \(\upmu\)m. The fly ash is made up of glassy spheres usually used as a secondary cementitious binder in Portland cement systems and as the main binder in alkali-activated systems.