Effects of Secondary Air on So2 Emission in Circulating Fluidized Beds

Coal is a major energy source, and on the other hand coal is a source of pollution. Some of the important environmental issues related to use of coal in energy production are the release of sulphur dioxide (SO2). SO2 is a precursor to acid rain, responsible for atmospheric pollution and damage to human health. Therefore, investigations and development of technologies promising low SO2 emission remains of high interest. Among various kinds of desulfurization technologies, sulfur removal in furnace is very competitive for controlling the SO2 emission derived from coal combustion, due to the low capital and operating costs. One of the advantages of the fluidized bed combustion technology of coal is in situ SO2 capture by added CaO-based sorbents, usually uncalcined limestone (CaCO3). Fluidized beds are operated with excess air. In the case of circulating fluidized beds, air-staging further contributes to a more reducing environment in the bottom bed. Although only the bottom bed is predominantly reducing, these reducing conditions may penetrate to some extent all the way up to the cyclone in streamers of oxygen-deficient gas. As a consequence of the large impact of reducing conditions on the sulphur capture process, the total air ratio and the extent of air-staging become important. From this point of view, in this study, the effect of air-staging on sulphur capture is investigated in a circulating fluidized bed via previously developed 2D model.