coal selection criteria for industrial pfbc firing project 3.2 - CCSD

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“Coal Selection Criteria for Industrial PFBC Firing” Tomatoh-Atsuma was using the coal’s Fuel Ratio (Fixed Carbon/Volatile Matter) as the parameter for predicting coal combustion performance in the furnace. However, occasionally contradictory results had been encountered. The CCSD research discussed above found that the elutriated unburnt carbon correlated with the ratio of Telovitrinite/Inertinite rather than with of Fuel Ratio. This research had helped Tomatoh- Atsuma in solving its problems (Wang 2002). 3.1.2 Other Combustion Efficiency Considerations The major factor causing combustion inefficiency is mostly unburnt carbon elutriation, caused by attrition of the char particles in the fluidized bed and hence affected by the char structure formed during devolatilization. Earlier work reported that other factors affected the combustion efficiency, including coal rank or volatile content, coal reactivity, swelling, fragmentation and calorific value. One previous study concluded that a lower coal rank or a higher volatile content increased the combustion (Laughlin and Sullivan 1997). An increase in pressure resulted in reduction of the volatile transport rate from inner pore to outer surface and thus decreased the coal volatile yield (Laughlin and Sullivan 1997). Char reactivity increased with increasing oxygen and alkaline oxide contents and porosity. It also increased with decreasing rank and mean vitrinite reflectance (Laughlin and Sullivan 1997). An increase in char reactivity increased the combustion efficiency, but char reactivity was not an important consideration for high pressure conditions. Generally, high volatile bituminous coals performance was less sensitive towards changes in chemical kinetics. Lower coal calorific value and higher ash and sulfur contents increased the inefficiencies (Huang, McMullan et al. 2000). Although no correlation between Crucible Swelling Number (CSN) and combustion efficiency was developed, it was shown that an increase in CSN decreased the combustion efficiency for Taiheiyou and Lithgow data in the Wakamatsu plant (Misawa 2000). 3.2 Bed Agglomeration Another major issue in PFBC plant is bed agglomeration or sinter egg formation. These agglomerates are bed particles which are fused together around a hollow core that originated from coal paste lumps (Zando and Bauer 1994). Escatrón, Värtan, Tidd, Tomatoh-Atsuma, Wakamatsu and Karita encountered this problem. At Escatron, Page 8
“Coal Selection Criteria for Industrial PFBC Firing” sintering caused several boiler stops. Tidd experienced bed agglomeration only when it was operating at full load and over 815 o C. Bed agglomerations were indicated by uneven bed temperatures, decaying bed density and reduction in the heat absorbed (Scott and Carpenter 1996). After being analyzed by SEM, EDAX and XRD, it was found that the agglomerate consisted of fine particles of SiO2 and Al2O3 in the ash. These particles stick together in the presence of CaO (from the bed particles) to form Ca2Al2SiO7 glass (Ishom, Harada et al. 2001). The oxides adhered to the surface of the combusting coal. Fine ash and more CaO deposited on the agglomerate forming a bigger agglomerate. Bed agglomerates formed when the temperature was below 1300 o C, possibly around 1100 o C where particles in the agglomerate started to deform even if the whole grain melted at 1300 o C (Ishom, Harada et al. 2001). The causes of these sinter accumulations were poor fuel splitting resulting in large paste lumps in the bed, insufficient fluidizing velocity and localized high feed concentration at full bed height (Zando and Bauer 1994). Failure in the fuel feeding system, e.g. blockage, has also led to an agglomeration problem. To achieve a finer fuel splitting, it was necessary to increase the paste moisture content. However, this could only be done at the expense of reduced thermal efficiency. Installation of more air nozzles improved the bed fluidization. Decreasing the bed particle size and operating in the turbulent regime could also help the fluidization. Inadequate fuel distribution, which was caused by bed defludization, could increase the unburnt carbon elutriation, gas temperature (due to post combustion of unburnt elutriated char) and SOx emission (Wang 2002). Karita’s measures to solve these problems were decreasing the top limestone particle size from 6 mm to 2 mm, adding more fluidizing gas nozzles to improve fluidization in the bottom area and reducing the operating pressure (Wang 2002). Another problem faced by Karita was that it could not operate at pressures above 1.2 MPa, which caused bed agglomeration for some coals. Karita is now operating at about 80% load, with an operating pressure below 1.1 MPa (Wang 2002). Page 9
Page 1 and 2: “Coal Selection Criteria for Indu
Page 5 and 6: LIST OF TABLES “Coal Selection Cr
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Page 23 and 24: 5. CONCLUSIONS “Coal Selection Cr
Page 27 and 28: 7. REFERENCES “Coal Selection Cri
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coal selection criteria for industrial pfbc firing project 3.2 - CCSD

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