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.