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

P. Selvakumaran, A. K. Bakthavatsalam
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Abstract

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.
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燃烧高硫印度褐煤的大型循环流化床燃烧锅炉的成功运行
在过去的几十年里,流化床的设计人员和操作人员一直致力于CFBC技术的开发,建立最佳的操作条件以及与耐火材料相关的故障排除等。吸附剂,通常是石灰石或白云石,在流化床燃烧锅炉中用于就地捕获二氧化硫(SO2)。石灰石的特性不仅对有效捕获SO2很重要,而且对沉积也有很大的影响,但对这些特性的了解却没有得到应有的重视。125mwe循环流化床燃烧(CFBC)锅炉在采用高硫褐煤和碳酸钙吸附剂捕集二氧化硫时,锅炉后道结垢严重,导致气体流通通道受阻。硬沉积物的光学显微镜显示主要是硬石膏(CaSO4),没有中间相如氧化钙或在脱碳化石灰石上存在硫酸盐环。据推测,松散的未反应的氧化钙沉淀在管受到再碳酸化和进一步延长的硫酸化导致硬沉积物。对五种不同组成的褐煤在选定的后道位置进行了恶臭探针测试,这些褐煤由矿山提供的高硫和高灰分含量以及控制SO2所需的吸附剂石灰石的比例不同,并确定了沉积物的堆积速度。随着褐煤灰分、吸附剂添加量和石灰石细粉含量的增加,沉积物的堆积也在增加。本文介绍了现场改造和补救措施,以消除传热表面上的沉积物,处理灰输送系统中的沉积物,控制来自磨矿回路的吸收剂细粒,从而克服沉积物的形成和堵塞。
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