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. 1. Backdrop India with growing energy consumption is looking at utilizing all its potential energy resources in the most economic and environmentally sustainable manner. Coal will continue to be the major energy source in India due to its availability. Per capita consumption of electricity and GDP growth has direct relation, and energy intensity in developing countries like India is comparatively more than the developed world, and the gap between supply and demand is ever increasing. The demand for all forms of energy is expected to increase substantially in the foreseeable future and is expected to get doubled by 2030. Although coal would continue to be a major energy source in India due to its availability, lignite is fast emerging as an alternate source of fuel for electricity generation. In India, the total lignite potential is 4177 million tonnes. Indian lignites have a typical analytical range of ash content of 15 to 35%, sulfur content of 1.0 to 7.0%, and moisture content of 10 to 45%. The varieties found in Gujarat and Rajasthan region have moderate to high sulfur (1 to 7%) content. It has become an economic necessity to use these lignites for power generation in view of spurt in energy demand, with SO2 emission controlled. Circulating fluid bed combustion (CFBC) technology is employed considering the impurities, moisture, ash, and sulfur
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