Influence of Sorbent Characteristics on Fouling and Deposition in Circulating Fluid Bed Boilers Firing High Sulfur Indian Lignite

Abstract

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.

Investigation of the Initial Stage of Ash Deposition during Oxy-Fuel Combustion in a Bench-Scale Fluidized Bed Combustor with Limestone Addition

“…Hence, these elements played an important role in the initial stages of ash deposition. , The viscosity of fine ash was significantly affected by the addition of limestone as a sorbent owing to changes in its chemical composition, which also changed its deposition behavior. Limestone addition also likely changed the structure of ash deposits, since CaO particles deposited on the surface of the probe reacted with CO 2 or SO 2 , resulting in the formation of bonded ash deposits. ,− Based on the above analysis, ash DP showed nonlinear variation as a function of the Ca/S ratio, affecting both the formation of fly ash and ash deposits.…”

“…However, taking the process of ash deposition into consideration, the conditions influencing carbonation reactions are likely to be more complicated. Under air-firing conditions, with about 15% CO 2 in flue gas, agglomeration and fouling of heating surfaces occur owing to carbonation and sulfation. − Previous research results also indicated that the ash deposition rate largely depended on the content of free CaO in the fly ash . Under oxy-fuel firing, with up to 85% CO 2 in the flue gas, potential problems related to carbonation reactions are yet to be evaluated, although there is evidence for concern.…”

Characterization of Microstructure and Thermal property of Ash Deposits on Fire-side Boiler Tube

“…2) Recently, not only the wastes are incinerated, but also diversified efforts to recover the energy produced after combustion are being continued, and diversified studies are in progress to investigate the cause for generation of ash deposits recognized as the major factor for degradation in the boiler efficiency as well as to reduce their generation. [3][4][5] Since not only deposition of fine particles produced in combustion process but also chemical reaction and condensation of diversified components act in a combined way for generation of the ash deposits, the generation mechanism is known to be very complicated. 6) The ash deposits are formed by a combined action of physical and chemical phenomena.…”