Applications of Ultrafine Limestone Sorbents for the Desulfurization Process in CFB Boilers

With the stringent emission regulation taking effect, it is difficult for the conventional desulfurization technology in circulating fluidized bed (CFB) boilers to meet the requirements of ultralow SO2 emission. Therefore, in this paper, the application of natural ultrafine limestone, with a Sauter mean diameter of less than 20 μm, was tested by conducting bench-scale, pilot-scale, and commercial-scale experiments to realize highly efficient desulfurization in CFB furnaces. In the past, such small-size limestone was considered unsuitable for CFB boilers. However, as demonstrated by bench-scale results, the desulfurization performance was clearly superior to that of coarse limestone, especially at low SO2 concentrations. In a 3 MWth pilot-scale CFB boiler, the ultrafine limestone exhibited competent desulfurization efficiency to that of the coarse limestone but clearly less significant catalytic effects on NOx formation. As revealed by field tests in four commercial-scale CFB boilers, when high-efficiency cyclones were applied to CFB boilers, the mass inventory of ultrafine particles was significantly increased and the residence time would be extended accordingly; thus, the ultrafine limestone can be used to achieve high desulfurization efficiency and even ultralow SO2 emission with a favorable Ca/S ratio. Furthermore, a technical roadmap was drawn for the cost-effective control of SO2 emission.