Study On Sulfur And Nitrogen Migration During Mild Fluidized Bed Coal Gasification

Coal is the most important primary energy in China,for its largest reserves.Moreover,coal will still be the most important energy and chemical materials in the foreseeable future.Coal gasification opens the future perspective for substitute natural gas and fossil fuel production from resources with low economic value.Fluidized bed coal gasification technology is a promising technology with wide application prospect,and it has attracted widely attention due to the broad fuel adaptability and moderate gasification temperature.However,sulfur and nitrogen in coal will be released during fluidized bed gasification,causing a series of problems,such as corrosion of downstream equipment,deactivation of catalyst and environmental pollution,and thus reducing the economic value of coal gas and limiting the application field of coal gas.Moreover,due to the lack of control method,it’s difficult for the downstream gas purification system to meet the higher purification demand that brought by the replacement of upstream coal,and it is difficult to give full play to the technical advantages of fluidized bed gasification.Based on the above problems,the migration of sulfur and nitrogen in the gasification process by using test platforms with different scale,besides,gasification experiments were also carried out in the industrial circulating fluidized bed gasifier to verify the obtained migration behavior of sulfur and nitrogen.Firstly,three chars and graphite were used to investigate the release behavior of gaseous products during CO2 gasification using thermogravimetric-mass spectrometry(TG-MS).Secondly,the effects of operating parameters(reaction atmosphere,gasification temperature,oxygen carbon molar ratio and steam carbon molar ratio)on the migration behaviors of sulfur and nitrogen in the coal gasification process were studied in a vertical tubular furnace,and the influence of operating parameters on the sulfur fixation effect of limestone was also studied influence.Thirdly,the coupling effects of temperature and operating parameters were studied on the circulating fluidized bed coal gasification,and it further improved the migration behaviors of sulfur and nitrogen in the process of fluidized bed gasification.Finally,the data collected from the industrial circulating fluidized bed coal gasification were analyzed to verify the accuracy of the above migration behavior of sulfur and nitrogen.Chars and graphite were used to investigate the release behavior of gaseous products during CO2 gasification using thermogravimetric-mass spectrometry(TG-MS).The results show that SO2 is the main gaseous sulfur,and HCN is the main gaseous nitrogen contaminant.The release temperature of H2S is close to the peak release temperature of H2,and the release temperature range of COS is close to that of CO.Thus,the formation of COS is related to CO.H2S mainly comes from the combination of decomposed sulfur and hydrogen atoms,while SO2 mainly comes from the decomposition of stable organic sulfur and sulfate.The release temperature range of HCN is close to that of weight loss,while few NH3,NO and NO2 are detected.The release of sulfur is mainly influenced by gasification reaction.The release of sulfur in the gasification process was further explored through the correlation between the release temperature ranges of different components.At high temperature,part of the decomposed sulfur will combine with CO or H2 to form COS or H2S and release to the gas phase,while the other part of the sulfur will be recombined with the surrounding char to form organic sulfur with better thermal stability,and remain in the solid phase.When the temperature increases further,the carbon consumption is almost complete,and the organic sulfur is decomposed and released into the gas phase in the form of SO2.The effects of different operation parameters on the migration of sulfur and nitrogen are investigated in the bubbling bed gasification.The results show that the main gaseous sulfur is H2S,followed by COS,and only a small amount of sulfur is released in the form of SO2 and CS2.Compared with the result of TG-MS,the concentration of CO and H2 is higher and the time of gas-solid contact is longer,which promotes the formation of H2S and COS.Few nitrogen is released and thus few NH3 and HCN can be detected.Under different reaction atmospheres,the sulfur release rate is the highest in the process of air gasification,while the content of H2S in gaseous sulfur(H2S/Sg)is the highest in the process of steam gasification.In the process of oxygen rich-steam gasification,the carbon conversion rate and sulfur release rate increase with the increase of temperature,oxygen carbon molar ratio(O2/C)and steam carbon molar ratio(H2O/C).The trend of H2S/Sg is consistent with that of temperature and steam carbon molar ratio,but opposite to that of oxygen carbon molar ratio.The trend of sulfur release rate is consistent with that of carbon conversion rate.The conversion reaction of H2S and COS is far from equilibrium at the end of gasification reaction,and the yield of COS is excess.The increase of H2 yield promoted the conversion of COS to H2S.Under the same condition,the distribution of sulfur and nitrogen species in the bottom char and fly ash is similar,and few pyrite was detected in the fly ash.The content of organic sulfur decreases with the increase of carbon conversion rate.Similarly,with the increase of carbon conversion rate,the content of N-6 with good thermal stability increases.In the process of fluidized bed gasification,the release of sulfur not only related to carbon conversion,but also related to the species of sulfur in coal.When the steam carbon molar ratio(H2O/C)is the same,H2 yield is conducive to the sulfur fixation of limestone.When the steam carbon molar ratio(H2O/C)is changed,the sulfur fixation effect of limestone has the best value.The effects of oxygen carbon molar ratio and steam carbon molar ratio are investigated during circulating fluidized bed coal gasification.The results show that the yield of gaseous sulfur in the process of CFB gasification was as follows:H2S>COS>CS2,and the gaseous nitrogen contaminant mainly releases in the form of NH3.Compared with the bubbling fluidized bed,the gas velocity of CFB is higher,the mix between gas of solid is more intense,and the distribution of char is more uniform,SO2 can be reduced by char,and the contact between char and H2 is increased,which promotes the formation of NH3.The increase of oxygen carbon molar ratio and steam carbon molar ratio promotes the release of sulfur,and promotes the increase of H2 yield and H2S/COS molar ratio.Combined with the results obtained from vertical tubular reactor,the highest point of hydrogen yield is also the highest point of H2S/COS molar ratio in the process of fluidized bed gasification.Within the scope of this study,the increase of oxygen carbon molar ratio promotes the removal rate of gaseous sulfur,while the increase of steam carbon molar ratio reduces the removal rate of gaseous sulfur.Combined with the results obtained from vertical tubular reactor,the increase of H2 yield is beneficial to improve the sulfur fixation capacity of limestone when the steam carbon molar ratio keeps constant.However,the sulfur fixation capacity of limestone is weakened when the steam carbon ratio is increased,in the process of circulating fluidized bed gasification.As for solid sulfur,its migration behavior is basically consistent with that in bubbling fluidized bed gasification.As for nitrogen,the yield of NH3 decreases with the increase of O2/C and increases with the increase of H2O/C;The addition of limestone has little effect on NH3.Carbon conversion rate and the distribution of solid sulfur and nitrogen species can affect the release of sulfur and nitrogen.The obtain migration behavior of sulfur and nitrogen are verified.in the industrial circulating fluidized bed gasification.The results show that with the increase of steam coal molar ratio,the carbon conversion rate and sulfur release rate increase,and the main gaseous sulfur is H2S and COS.With the increase of H2O/C,H2 production increases,the conversion reaction of H2S and COS is closer to equilibrium,and the content of H2S increases.The nitrogen pollutant is mainly in form of NH3,and with the increase of H2O/C,the yield of NH3 increases.