Successful operation of large Circulating Fluid Bed Combustion boilers firing high sulfur Indian lignite

Over the past decades, designers and operators of fluidized beds have been concentrating on developing the CFBC technology, establishing the optimum operating conditions and troubleshooting associated with refractory, etc. A sorbent, typically limestone or dolomite is used in the fluidized-bed combustion boiler to capture sulfur di-oxides (SO₂) in-situ. Due attention has not been paid to understand the limestone characteristics that are not only important for efficient capture of SO₂ but also greatly influence the deposition. 125 MWe Circulating Fluidized Bed Combustion (CFBC) boiler experienced severe fouling in back pass 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 de-carbonated limestone. It is hypothesized, that loose unreacted calcium oxide that settle on tubes are subjected to re-carbonation and further extended sulfation resulting in hard deposits. Foul probe tests were conducted in selected locations of back pass for five different composition 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. This paper covers the field modifications and remedial measures carried out to dislodge deposits on heat transfer surfaces, to handle the deposits in ash conveying system, and to control sorbent fines from the milling circuit thereby overcoming the deposit formation and clogging.