Transformations and Affinities for Sulfur of Chinese Shenmu Coal Ash in a Pulverized Coal-Fired Boiler

The self-desulfurization efficiency of Shenmu coal with a high initial Ca/S molar ratio of 2.02 was measured in a 1,025 t/h pulverized coal-fired boiler. It increases from 29% to 32% when the power capacity decreases from 100% to 70%. About 60% of the mineral matter and calcium element fed into the furnace is retained in the fly ash, while less than 10% is retained in the bottom ash. About 70% of the sulfur element fed into the furnace is emitted as SO2 in the flue gas, while less than 10% is retained in the fly ash and less than 1% is retained in the bottom ash. The mineralogical compositions of feed coal, fly ash, and bottom ash were obtained by X-ray diffraction analysis. It is found that the initial amorphous phase content is 91.17% and the initial CaCO3 phase content is 2.07% in Shenmu coal. The vitreous phase and sulfation product CaSO4 contents are, respectively, 70.47% and 3.36% in the fly ash obtained at full capacity, while the retained CaCO3 and CaO contents are, respectively, 4.73% and 2.15%. However, the vitreous phase content is only 25.68% and no CaSO4 is detected in the bottom ash obtained at full capacity. When the power capacity decreases from 100% to 70%, the vitreous phase content in fly ash decreases from 70.47% to 67.41% and that in bottom ash increases from 25.68% to 28.10%.