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A moderate temperature dry desulfurization process at 600-800 degrees C was studied in a pilot-scale circulating fluidized bed flue gas desulfurization (CFB-FGD) experimental facility. The desulfurization efficiency was investigated for various operating parameters, such as bed temperature, CO2 concentration, and solids concentration. In addition, structural improvements in key parts of the CFB-FGD system, i.e., the cyclone separator and the distributor, were made to improve the desulfurization efficiency and flow resistance. The experimental results show that the desulfurization efficiency increased rapidly with increasing temperature above 600 degrees C due to enhanced gas diffusion and the shift of the equilibrium for the carbonate reaction. The sorbent sulfated gradually after quick carbonation of the sorbent with a long particle residence time necessary to realize a high desulfurization ratio. A reduced solids concentration in the bed reduced the particle residence time and the desulfurization efficiency. A single-stage cyclone separator produced no improvement in the desulfurization efficiency compared with a two-stage cyclone separator. Compared with a wind cap distributor, a large hole distributor reduced the flow resistance which reduced the desulfurization efficiency due to the reduced bed pressure drop and worsened bed fluidization. The desulfurization efficiency can be improved by increasing the collection efficiency of fine particles to prolong their residence time and by improving the solids concentration distribution to increase the gas-solid contact surface area.

A series of experiments in a circulating fluidized bed (CFB) pilot plant has explored a new dry desulfurization process, using the NO x in the flue gas and a new sorbent that has been prepared from fly ash and lime. Various desulfurization operating parameters were tested for the thermodynamic, chemical, and dynamic states for temperatures of 523−673 K. The NO x increased the calcium conversion ratio in the desulfurization process. In addition, with the NO x , the desulfurization byproduct was determined to be mostly CaSO4, instead of CaSO3, as a result of the chain reaction caused by the NO x . Therefore, the NO x in the flue gas can improve the efficiency of the dry desulfurization process.

A low-cost preparation process of rapidly hydrated sorbent was developed in a pilot-scale preparation system. The process hydrates the lime and the fly ash at ambient temperature for 2 h with drying at 150 °C for 1 h, a greatly reduced hydration temperature and preparation time. The rapidly hydrated sorbent was tested at a medium temperature of 350 °C in a thermogravimetric analyzer (TGA) and a pilot-scale circulating fluidized bed (CFB). The TGA results show that the calcium conversion ratio (η Ca ) of the rapidly hydrated sorbent was ten times higher than that of the original lime and of a dry mixture of lime and fly ash. The η Ca in the CFB reactor was close to the TGA result, verifying the applicability of the rapidly hydrated sorbent in a real desulfurization process. The physical and chemical properties of the sorbent were analyzed to investigate the rapid hydration mechanism. The improved η Ca for the sorbent was mainly due to the greatly improved particle characteristics, including the particle specific surface area and the pore size. Highly active hydration products are not significantly produced at the ambient hydration temperature.

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