Study On Directional Conversion Of Hailar Lignite Catalytic Pyrolysis In CO₂ Atmosphere

With the increase in energy demand,low-rank coal has gradually become a precious coal resource.The pyrolysis technology is one of the milder conversion methods for low-rank coal and the selection of a suitable catalyst in pyrolysis can achieve the directional conversion of pyrolysis products.As a greenhouse gas,CO2,if it can be used reasonably,is not only beneficial to improving the environmental pollution,but also reducing the waste of carbon resources.In this paper,the catalytic pyrolysis kinetics and product distribution characteristics will be studied from the aspects of pyrolysis atmosphere and the amount of catalyst.At the same time,the pyrolysis conditions are optimized to improve the gas production rate of low-rank coal pyrolysis and realize the directional conversion of gas products into pyrolysis gas mainly composed of CH4.Mixing Ca(OH)2 and Hailar brown coal with water in proportion,Ca(OH)2 could undergo calcium ion exchange reaction in lignite,and the saturation value of Ca(OH)2 in coal could be reached when the content was more than 7%.According to the thermogravimetric analysis results of coal samples,the pyrolysis process is divided into three stages:the first stage(30 ?-200 ?)is mainly the drying stage;the second stage(200??710?)is mainly the precipitation of volatiles,the maximum weight loss rate of coal samples loaded with Ca(OH)2 shifts to lower temperatures,and the maximum weight loss is also less than that of raw coal;the third stage(710?-850?)is the secondary gasification stage of coal,the thermogravimetric curve shows a new weight loss peak when the content of Ca(OH)2 is greater than 3%,indicating that Ca(OH)2 can effectively promote the gasification reaction of coal in this stage.The Coats-Redfern integration method is used for reaction kinetic analysis,when the content of Ca(OH)2 is 7%,the reaction activation energy is smaller and the reaction is easier.As the temperature increases,the reaction order is different,pyrolysis process could be described as a first-order reaction at the lower temperature stage(553-803 K);the pyrolysis process at the high temperature stage(803-1083 K)is complex,and can be described as second-order or third-order reaction.The coal catalytic pyrolysis experiment in N2 atmosphere showed that the yield of char and tar decreased significantly at higher temperature after Ca(OH)2 was added.At 800 0C,the yield of char is 5.15%less than that of raw coal at the same temperature;when the amount of Ca(OH)2 is 7%,the amount of CO released increases by 13.19 mL/g.The SEM images of char showed that Ca(OH)2 makes the char structure more loose,indicating that Ca(OH)2 can effectively promote the gasification reaction of char.The pyrolysis experiment of coal in CO2 atmosphere shows that CO2 and char undergo a secondary gasification reaction when the temperature reaches 800?,and the yield of char is reduced by 4.02%compared with pyrolysis process in N2 atmosphere.The coal Pyrolysis in a CO2 atmosphere is more conducive to the release of CO,CH4,and C2H6.At 800?,the released amount of CO is as high as 173.64 mL/g,which is 119.33 mL/g more than in the N2 atmosphere.The SEM images of char showed that the gasification reaction of CO2 and char destroy the char structure,which make the the char granular structure smaller.The experiment of coal catalytic pyrolysis in CO2 atmosphere shows that Ca(OH)2 can effectively promote the release of CO in CO2 atmosphere,When Ca(OH)2 addition reaches 5%,the release amount reaches the maximum value,and the release amount of CO reaches 224 mL/g at 800? indicating that Ca(OH)2 promotes the secondary gasification of coal at higher temperatures,and the catalytic effect is most obvious when Ca(OH)2 addition is close to saturation.According to XRD analysis results,the catalyst mainly exists in the form of CaCO3 in the char after the pyrolysis reaction.The SEM images of char showed thatthe granular structure of pyrolysis char is looser after Ca(OH)2 was added,indicating that Ca(OH)2 can promote the gasification reaction of char at high temperature.However,with the continuous increase of Ca(OH)2,the CaCO3 generated in the CO2 atmosphere is enriched on the char surface,which is not conducive to the diffusion of gas products in the late stage of pyrolysis reaction.