coal selection criteria for industrial pfbc firing project 3.2 - CCSD

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“Coal Selection Criteria for Industrial PFBC Firing” was mainly for the purpose of bed maintenance instead of sulfur retention as its ash contains desulfurizing components (Andersson, Bergqvist et al. 1999). Modifications in sorbent size distribution or type had proved to improve the desulfurization significantly. Other factors which influenced SO2 emissions were bed temperature and excess air. Increases in bed temperature and excess air improved desulfurization (Andersson, Bergqvist et al. 1999). 4.3 NOx Emissions NOx emissions depended on the fuel type, fuel nitrogen content, bed material composition, temperature, etc. However, the parameter which had the strongest influence was excess air ratio during combustion. An increase in excess air produced more NOx. The formation of NOx involves complicated interactions between all the parameters stated previously (Andersson, Bergqvist et al. 1999). 4.4 N2O Emissions N2O emissions were mainly influenced by bed, freeboard and gas path temperatures. The higher the temperature, the less N2O was emitted. There was some influence of coal type but this effect was inferior to the temperature effect. At an operating temperature of 860 o C, the emissions of N2O were less than 20 ppm (Andersson, Bergqvist et al. 1999). Page 22
5. CONCLUSIONS “Coal Selection Criteria for Industrial PFBC Firing” Combustion inefficiency was one of the potential problems faced by PFBC plants. It was mainly caused by unburnt char elutriation. For the relatively narrow range of coal rank of Australian export coals, unburnt char elutriation from PFBC correlated with the coal’s petrographic composition, specifically with the ratio Telovitrinite : Inertinite. This effect was attributed to the highly swelling Telovitrinite generating larger diameter pores in the devolatilised char, allowing greater combustion-enhanced attrition from the pore mouths on the char surface. A Telovitrinite : Inertinite ratio below 0.200 would be satisfactory and a Telovitrinite : Inertinite ratio above 1.871 would indicate an unsuitable coal for PFBC firing. Other factors reported to affect PFBC combustion efficiency include Coal reactivity, volatile content, swelling, fragmentation and calorific value. These factors were studied over a wider range of coal rank, indicating that combustion inefficiency increased with coal rank. However, the general correlation with coal rank did not always predict commercial-scale PFBC performance, so the correlation (Eq. 1) with petrographic analysis is recommended for assessing sub-bituminous and bituminous coals. Bed agglomeration or sinter egg formation occurred at Escatrón, Värtan, Tidd, Tomatoh- Atsuma, Wakamatsu and Karita. The coal-related factor which caused bed agglomeration was the ash fusion temperature. Low ash fusion temperature generated agglomeration. Despite their high combustion efficiencies, low rank coals contain high levels of alkali that caused agglomeration problems. Two of the Japanese commercial plants firing Australian export coals specify < 7% Fe2O3 in the coal ash and one also specifies an ash fusion temperature > 1200 o C. However, since this problem still limits the maximum output from the Karita plant, it warrants the further research being conducted in CCSD. Another problem in PFBC plants was fouling and deposit formation. The key element responsible for this was iron, which decomposed and oxidized during combustion. Coals with low iron content are advised to minimize this problem. Page 23
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coal selection criteria for industrial pfbc firing project 3.2 - CCSD

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