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125 MWe circulating fluidized bed combustion (CFBC) boiler experienced severe fouling in backpass of the boiler leading to obstruction of gas flow passage, while using high sulfur lignite with sorbent, calcium carbonate, to capture sulfur dioxide. Optical microscopy of the hard deposits showed mainly anhydrite (CaSO4) and absence of intermediate phases such as calcium oxide or presence of sulfate rims on decarbonated limestone. It is hypothesized that loose unreacted calcium oxides that settle on tubes are subjected to recarbonation and further extended sulfation resulting in hard deposits. Foul probe tests were conducted in selected locations of backpass for five different compositions of lignite, with varied high sulfur and ash contents supplied from the mines along with necessary rates of sorbent limestone to control SO2, and the deposits build-up rate was determined. The deposit build-up was found increasing, with increase in ash content of lignite, sorbent addition, and percentage of fines in limestone. Remedial measures and field modifications to dislodge deposits on heat transfer surfaces, to handle the deposits in ash conveying system, and to control sorbent fines from the milling circuit are explained.

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Fluidized Bed Combustion (FBC) is one of the best suited combustion technologies for petcoke combustion owing to its capability of handling fuels with low volatile and high sulphur content. In this study, petcoke and Indian sub bituminous coal (coal from Singareni mines, Telengana, India) are fired separately in Circulating Fluidized Bed (CFB) test facility and its combustion characteristics are studied and compared. Even though petcoke is a low reactive fuel compared to Indian coal, temperature profile shows petcoke has a stable combustion. Significant post combustion in cyclone is noticed while combusting petcoke. Around 40-60o C rise in the cyclone temperature is noted during combustion of petcoke compared to 20-30o C while burning sub bituminous coal. Compared to Indian sub bituminous coal petcoke has high sulphur content (4-9 %) and low ash content, therefore it requires limestone addition for in situ sulphur capture and for maintaining bed inventory. Owing to the limestone addition, petcoke firing shows a better material distribution among the splash zone. Although CFB boilers are fuel flexible, and petcoke can be fired in CFB boilers designed for Indian coals, test results show that combustion efficiency of petcoke is lower than that of Indian coal. Combustion efficiency can be improved by certain design modifications such as higher furnace height, higher cyclone efficiency and higher refractory zone. The present study explore the opportunities of using 100% petcoke in CFB boiler.