Characterization of new sub-bituminous coal using a scaled-down single burner combustion test facility

• Main Author: Muhamad Shazarizul Haziq Mohd Samsuri
• Format: text::Thesis
• Language: English
• Published: 2023

Coal degradation is becoming a leading issue for coal-fired power plant (energy) industry [1-2]. Numerous methods have been practiced in overcoming the issue, but a concrete solution is still nowhere to be found. Various types of coals have been introduced by the coal suppliers, however there is yet a reliable method to assess the combustion behaviour of the new coals. Thus, this work attempt to establish method to evaluate coal blend characterization and determine suitability of a given coal blend to be fed as fuel in conventional boiler. The main objective of this study is to investigate the suitability of adopting a newly-introduced sub-bituminous coal in an existing boiler furnace commonly firing standard design coals. In order to ensure that the new coal will not give an adverse effect to the boiler, detail analytical and combustion performance of the new coal is investigated, together with design and other limiting coals which has been recommended by power plant operator. The analytical fuel test is required to get an insight of the combustion behaviour, which involves analyses of gross calorific value (GCV), Ash Fusion Temperature (AFT), elemental oxides, chemical and thermal properties of coals. In addition, combustion test was performed in a scaled down, 150kW, single swirl burner combustion test facility available in TNB Research Sdn. Bhd. [3]. In the study, combustion gas temperature at different sectors downstream of burner region was measured to analyze the combustion behaviour, i.e. temperature profile, ash deposition and gas emissions for all the tested coals. Based on the investigation, it was noted that the design and limiting coal with the highest fixed carbon content (45.32 %) gives the highest temperature at all sectors (peak temperature at 856 °C). On the other hand, coal with the lowest fixed carbon (40.50 %) gives the lowest temperature (peak temperature at 743 °C) due to low calorific value (5422.60 kcal/kg) [4]. As for the newly-introduced coal, the temperature profile was found to be comparable [3] to the design and limiting value of other tested coals e.g. peak temperature within 743-856 °C (typical temperature generated by sub-bituminous coal in this combustion test rig). Even though it was observed that the temperature generated by the new coal is increased 3 % compared to the designed coal at downstream burner, the temperature was observed to be decreasing about 18 % from Section-T3 to Section T4 as combustion gas flow downstream of the combustor rig. Meanwhile, coal with high ash content (6.83 %) and low ash fusion temperature (1079 °C) produces high deposition of ash and vice versa [3]. Higher fouling deposition (4.66 g) is produced by Coal X as compared to other limiting coals due to high ash content (6.83 %). The CO (105.0 mg/m3), NOx (176.0 mg/m3) and SO2 (97.0 mg/m3) emissions released by the new coal were found to be within the acceptable limit; below 200 mg/m3 for CO and 500 mg/m3for both NOx and SO2. To conclude, the new coal is recommended to be used in the existing boiler due to acceptable temperature trends and emissions released; within temperature limit generated by other tested coals and below emission standard. However, proper mitigations are necessary i.e. optimizing soot blowing sequence [5- 7], frequent monitoring of ash deposition at convective area, and optimal scheduling of power plant operational tuning in order to maintain Furnace Exit Gas Temperature (FEGT) at boiler nose area [8] and reduce the propensity of fouling deposition at boiler tubes.