Experiments On Sulfur Transformation During Coal Pyrolysis Combined With Production Of Tar Char And Gas In Double Fluidized Bed

The hierarchical conversion polygeneration technology based on coal pyrolysis is one of the development directions clean and efficient coal utilization of coal.The coal pyrolysis combined with production of tar,char and gas in doubled fluidized bed developed by Zhejiang University realizes the hierarchical conversion and qualitative utilization of coal rich in volatile matter through the coupling of fluidized bed pyrolysis and fluidized bed char heating process,thereby improving the efficiency and effectiveness of the utilization.In this process,the high-temperature circulating semi-coke enters the fluidized bed and fully mixes with coal to produce pyrolysis gas,tar and char,then the char enters the fluidized bed heating furnace for anoxic combustion heating process.In order to obtain the characteristics of sulfur transformation during the process of dual-fluidized bed coal pyrolysis and co-production of tar char and gas,this paper carried out a typical low-rank coal（Xinjiang runbei coal）experimental study on the conversion characteristics of sulfur during pyrolysis under the condition of semi-coke as the heat carrier,investigating the impact of pyrolysis temperature（600℃,650℃,700℃,750℃）,char/coal（char/coal=0,3,5）and coking temperature（850℃,880℃,900℃）on sulfur transformation.At the same time,an experimental study of sulfur conversion characteristics during anoxic combustion heating process of char in fluidized bed was carried out to investigate the effects of anoxic combustion temperature（850℃,900℃）and semi-coke residence time（2min,4min,6min）on the transformation of sulfur in this process.The research results have good guiding value for the research and development of double fluidized bed coal pyrolysis and co-production of tar and semi-coke gas.First,an experimental study on the characteristics of sulfur transformation during coal pyrolysis using high-temperature char as the heat carrier was carried out on fluidized pyrolysis furnace.The results show that in the experimental pyrolysis temperature range,the precipitation rate of H₂S and COS gradually increase with the increase of pyrolysis temperature,the precipitation rate of CH₃SH gradually decreases,the sulfur content in tar remains basically unchanged,and the sulfur content in semi-coke gradually decreases.After adding the coke carrier（850℃-char,880℃-char,900℃-char）,the precipitation of H₂S,COS,CH₃SH and the sulfur content in the tar during the pyrolysis process can be reduced,and the degree of reduction is related to the pyrolysis temperature.In the experimental pyrolysis temperature range,adding850℃-char with a coking coal mass ratio of 3 can reduce the precipitation rate of H₂S,COS,and CH₃SH by 1.72%～3.94%,0.61%～1.51%and 0.04%～0.4%,respectively,and the ratio of sulfur content in tar to the sulfur content of raw coal is reduced to about 2%.This is mainly because the coke carrier contains Ca O and Fe_1-xS and other minerals,among which Ca O can react with H₂S and COS to form Ca S and stay in the semi-coke,reducing the escape of sulfur-containing gas.The main reasons for the reduction of sulfur content in tar are as follows:on the one hand,Ca O and Fe_1-xS in the coke carrier can promote the decomposition of organic sulfur in the tar;on the other hand,the coke carrier can combine with sulfur-containing groups to inhibit the production of organic sulfur in tar.What’s more,increasing the char/coal mass ratio will increase the inhibition effect of the coke carrier on the sulfur content in the sulfur-containing gas and tar.This is mainly due to the increase in the amount of the coke carrier added,which can increase the degree of reaction between the raw coal and the coke carrier.In addition,the inhibition degree of the three coke carriers prepared at different temperatures on the H₂S precipitation rate,the COS precipitation rate and the sulfur content in the tar during the pyrolysis process is:850℃-char>880℃-char>900℃-char.The main reason is that as the coking temperature increases,the surface sintering degree of the coke carrier increases,which hinders the reaction of minerals in the coke carrier with sulfur-containing gas and organic sulfur in the tar.Therefore,when using the 850℃-char as the heat carrier,the precipitation rate of sulfur-containg gas is the lowest,and the sulfur content in char is the highest,the ratio of total sulfur content in pyrolysis semi-coke to the sulfur content of raw coal is between 55.46%and 66.09%.When 900℃-char is used as the heat carrier,the proportion of total sulfur content in the pyrolysis semi-coke to the total sulfur content of the raw coal is between 52.89%and 61.07%,therefore the sulfur content in the pyrolysis semi-coke is relatively low.At the same time,an experimental study on the characteristics of sulfur migration and transformation during the heating process of semi-coke in anoxic combustion was carried out on a small fluidized bed experimental furnace.The results show that a large proportion of sulfur（about 60%）is distributed in the solid product due to the effect of mineral sulfur fixation in semi-coke.When the coke carrier used for pyrolysis is the same,the semi-coke obtained at a higher pyrolysis temperature will release a lower amount of sulfur gas during the anoxic combustion heating process.This is mainly because the higher temperature pyrolysis process can release more sulfur,so that the sulfur content distributed in the pyrolysis char is relatively low.In addition,during the heating process of anaerobic combustion,with the increase of anaerobic combustion temperature,the precipitation rate of SO₂ and COS gradually increase,and the sulfur content in the obtained solid product gradually decreases.When the heating temperature of 850℃-char-650℃pyrolysis semi-coke（Semi-coke obtained by pyrolysis with 850℃-char as the coke carrier and pyrolysis temperature of 650℃）is 850℃and 900℃,the SO₂precipitation rate is 22.37%and 27.77%,and the COS precipitation rate is 14.43%and 16.39%,respectively.Extending the heating time of anaerobic combustion can increase the escape concentration of SO₂ and COS,and the sulfur-containing gas release rate is proportional to the increase in residence time.For 850℃-char-650℃pyrolysis semi-coke,when the residence time is increased from 2min to 4min,6min,the SO₂precipitation rate rises from 12.73%to 22.37%and 35%,and the COS precipitation rate rises from 5.69%to 12.47%and 16.39%.It indicates that increasing the anoxic combustion temperature and the anoxic combustion time can effectively promote the release of sulfur during the anoxic combustion heating process.Finally,the characteristics of sulfur transformation in the process of the coal pyrolysis combined with production of tar,char and gas in doubled fluidized bed are comprehensively analyzed.In the process of coal pyrolysis,compared with the use of quartz sand as the heat carrier,when 850℃-char is used as the coke carrier,the sulfur precipitation rate in the process can be reduced by 9.2%,the sulfur precipitation rate is reduced by 6.1%during the process of the double fluidized bed coal pyrolysis stage conversion,about 40%of the sulfur migrates into the solid product.Therefore,the coke carrier can reduce the sulfur emission during the coal pyrolysis stage conversion process,and increase the total sulfur content in the semi-coke.