Effect Of Pre-desulfurization And Heating Course On The Transformation Of Sulfur And Nitrogen During Coal Pyrolysis

The sulfur and nitrogen in coal is one of the main source polluting environment, and have become the important constraint factor of the feasibility to effectually develop and economically utilize coal. Studying on the formation mechanism and influence factors of sulfur and nitrogen during coal processing, will provide an important valuable reference to co-control the sulfur and nitrogen. Ximeng, Yima, Yanzhou and Zunyi coals with different ranks in China were selected in this paper. In order to remove minerals and different forms of sulfurs in coal, HC1/HF acid demineralization, flotation, HNO3 acid pickling and ultrasound on extracting methods were selected respectively. The raw and pretreatment coals were investigated subsequently in the fixed-bed pyrolysis reactor. The effects of heating course, the mineral matters and the different forms of sulfur on the formation of S-, N-containing gases from the raw and pretreatment coals pyrolysis were investigated. The main results are as follows.The main temperature range of H2S and COS released from raw coals programmed pyrolysis is 300-700℃, and the peak temperatures are 40℃and 600℃respectively, corresponding to the decomposition of organic and pyrite sulfur. The main temperature range of NH3 and HCN release is 500-900℃. The peak temperature of NH3 is 700℃, and that of HCN is related to the coal ranks, distributed between the temperature of 600-800℃.The features of coal pyrolysis is dominated by the heating course, and the releases of S-, N-containing gases in this process are affected by both the pyrolized temperature and the holding time. The stable organic sulfur can be decomposed to form H2S and COS in the holding stage, and the release of S-containing gases from coal pyrolysis is raised in the lower temperature periods. At the temperature of> 700℃, the formation of S-containing gases tends to finish in the period of programmed temperature. The formation processes of volatile affects the releases of NH3 and HCN from coal pyrolysis. H radicals mainly reacts with N functional groups to form NH3 and HCN in the period of programmed temperature, while attacks the N heterocyclics exposed to nascent char in the holding stage. The release contents of NH3 and HCN from the holding stage at lower temperature are much more than that at higher temperature. H radicals formed by condensation reaction among aromatic rings attacks char-N heterocyclics at high temperature and promotes the formation of NH3 in the holding stage.The mineral matter can promote the decomposition and transformation of pyrite in coal and change the emission rules of H2S and COS. The minerals have the function of sulfur retention and suppressing the release of H2S and COS. Minerals can inhibit the formation of HCN and promote that of NH3. The transformation of HCN into NH3 is caused by minerals in coal and this is more obvious at high pyrolysis temperature. Minerals remained in float coal mainly promote the formation of NH3, but have no effect on HCN.The release amounts and peak types of S-, N-containing gases are changed when pyrite in coal is removed by the methods of flotation and HNO3 acid pickling. The yields of H2S and NH3 is increased, but that of COS and HCN is decreased with the removel of residual pyrite and minerals in the float coal. When coal is treated by the method of HNO3 acid pickling, the yields of S-N-containing gases are increased and it is easier sulfur radicals to form COS during pyrolysis at low temperature.The ultrasonic extraction can remove the organic sulfur in coal and this will affect the formation of S-, N-containing gases. The sulfur in Yanzhou coal mainly forms H2S and COS, in whch H2S is primary. The remove of sulfur from Yanzhou coal shows the tendency of coal-S conversion to COS, but this effect is insignificant. The yield of H2S is basically unchanged, but COS is significantly reduced in the process of pyrolyzing Ximeng coal treated by ultrasonic extraction. Moreover, the yields of H2S and COS all are increased in Zunyi coal. The release amounts of NH3 and HCN are increased as the organic sulfur is removed in Yanzhou coal, but the yield ratio of HCN and NH3 was basically similar to that in Yanzhou raw coal. However, NH3 yield is increased and HCN yield is decreased in Zunyi and Ximeng coals treated by ultrasonic extraction.The competing ability of the nitrogen functional groups reacts with H radicals to form N-containing gases is significantly lower than that of the fatty organic sulfur to form H2S. H radicals in volatile tends to react with Char-N to form NH3 and HCN at high pyrolysis temperature.