Developing Sub-Bituminous Coal Sintering Ratio for Predicting Coal Ash Slagging Factors

Fossil fuels such as coal, gas, and distillate are the core fuels for thermal plants. The thermal coal-fired power plant accounts for 40% of global electricity generation. The thermal coal plant is expected to continue generating electricity until 2040, covering roughly 60–75% of the energy demand. Coal is largely used as fuel in thermal power plants in generating electricity. Coal ash causes slagging and fouling at the boiler in the furnace, superheater tube, and pendant tube. Thus, the prediction of coal slagging and fouling is imperative for forecasting boiler repair work and outage. Therefore, this study was conducted in a thermal coal-fired power plant generating 700 megawatts. X-ray diffraction analysis (XRD) was applied to analyze the ash composition. A coal sintering method was developed as an indicator to predict ash slagging. Several coals with different characteristics were selected to conduct the coal ash analysis. Furthermore, sub-bituminous coal sintering indexes for predicting coal ash slagging factors were also developed. From this study, there are minimal ash deposition tendencies for sub-bituminous coal ash with a low sinter ratio of 0.2, whereas the ash deposition tendencies are high for high sinter ratios of 0.8 and above.

Coal ash slagging; Sintering ratio; Slagging factors; Sub-bituminous coal

The global demand of electricity increases by about 1.1% in 2018, after declining for the last three years from 2017. Coal as the main fuel is leading by about 2% from the thermal plant in generating electricity (Samsudin and Abd Aziz, 2020). Over the next five years, global coal demand is forecasted to remain stable, supported by the resilient Chinese market, which accounts for half of global consumption. In fact, thermal coal plants cover about 38% of the power market share as the largest sources. The largest share comes from Asian countries, China, and India, while Europe and North America keep on reducing the consumption of coal thermal generation. In the non-power sectors, despite much coal-to-gas switching in China, the demand remained stable (Tiwary, 2001). The international coal trade grew by 4% in 2018, surpassing 1.4 billion tonnes. In 2019, coal was the major fuel in generating power at a fired power plant for the Asian country. In fact, 75% of power generation in China and Indonesia is generated by coal-fired power plants. Coal resources from nature are categorized as non-renewable resources.

      Coal can be classified into bituminous, sub-bituminous, and lignite (brown) by referring to its calorific value. Besides, coal content includes hydrogen, sulfur, nitrogen, and carbon (Samsudin et al., 2020a). The coal content highly contributes the combustion efficiency and an indicator for formation of slagging and fouling deposition. Coal is heated in the boiler throughout the pulverized.

        Ash slagging is formed on the radiant heat transfer section in the furnace area. (Pozzoli et al., 2015; Abd Aziz et al., 2018). Ash fouling is deposited at the superheated (SH) and reheated (RH) regions from the result of convection. Meanwhile, low-temperature fouling is formed at the economizer region. Consequently, the formation of coal ash slagging decreases plant efficiency and availability (Ahmad et al., 2015). In addition, the accumulation of ash in the boiler tube reduces heat transfer, decreases efficiency, and impacts the reliability of the boiler. Thus, predicting ash formation in the boiler furnace provides an advantage in managing the coal and operation of the boiler (Sushil and Batra, 2006; Abd Aziz et al., 2019). Figure 1 shows the ash deposition area, ash slagging, and ash fouling in the boiler furnace.

Figure 1 Coal ash deposition area in boiler furnace

    From this study, sub-bituminous coal ash with a low sinter ratio of 0.2 had minimal ash deposition tendencies, whereas high sinter ratios of 0.8 and above had high ash deposition tendencies. Furthermore, coal with a high ash content above 3.0% impacted the sinter ratio and consequently increased the tendencies of ash deposition. Besides, this study indicates that the sinter ratio influences the plant heat rate and its performance regarding efficiency.

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