Experiments On The Influence Of Pyrolysis Atmosphere On The Fluidized Bed Pyrolysis Characteristics Of Low Rank Coal

Hierarchical transformation based on pyrolysis is one of the development directions of clean and efficient coal utilization.Pyrolysis of solid heat carrier fluidized bed has attracted wide attention due to its advantages of high heat and mass transfer strength,wide adaptability to coal quality,and high quality of pyrolysis gas.The double fluidized bed pyrolysis co-production of semi-tar gas developed by Zhejiang University is a typical solid heat carrier fluidized bed pyrolysis technology.The main features of the fluidized bed pyrolysis process are high temperature semi-coke as the heat carrier and the produced pyrolysis gas?CH₄,CO₂,H₂ and CO?as the fluidizing medium.The composition of fluidized gas and coke carrier have great influence on the pyrolysis characteristics of coal fluidized bed,so further research is needed.This paper relies on National key R&D project to carry out experiments under different carriers?quartz sand,3:1 and 5:1 coke carriers?and different pyrolysis atmosphere conditions with pyrolysis temperature between 500⁸00?on a small-scale fluidized bed,in order to obtain the characteristics of pyrolysis atmosphere affecting pyrolysis in fluidized bed under the condition of coke heat carrier.It can provide a reference for the next technology research and development and design.The experimental results in the CH₄ atmosphere were showed as the following.Due to the methane cracking reaction and the catalysis of semi-coke,when the temperature is not lower than 600°C,the yield of H₂,C2^C3 in the pyrolysis gas increased,and it increased with the proportion of semi-coke and the temperature under the solid heat carrier.And the corresponding pyrolysis water yield also increased.With the increase of temperature,affected by the formation of methyl and hydrogen radicals generated by the cracking of methane and the catalysis of semi-coke,the yield of tar in CH₄ atmosphere increased,and at about 600?the tar yield was the highest,and it increased with the proportion of semi-coke in the solid heat carrier.Under different carrier conditions,the yield and characteristics of semi-coke under CH₄ atmosphere had obvious changes,and the changes were more obvious with increasing temperature.Due to the catalysis of carbon deposition and semi-coke in the methane cracking reaction,the yield of semi-coke under the condition of coke heat carrier increased,and the surface was rougher.When quartz sand was used as the carrier and the temperature was not higher than 600°C,the methane cracking reaction basically did not occur.Therefore,the yield of the semi-coke in the CH₄ atmosphere was lower than the semi-coke yield in the inert atmosphere.When the temperature was higher,the yield of semi coke in CH₄ atmosphere increased due to the effect of the methane cracking and carbon deposition reaction process.The experimental results in the CH₄/CO₂ atmosphere were showed as the following.When the pyrolysis temperature was above 600?,the coal semi-coke promoted the reforming reaction of CH₄ and CO₂.The yields of CO and H₂ increased significantly with the increase of CO₂ concentration and temperature,and the pyrolysis water yield also increased dramatically.Due to reactions such as methane cracking,methane reforming with carbon dioxide,and gasification of carbon dioxide,the yields of tar in CH₄/CO₂ atmosphere increased,and the highest increment occurred at around600°C,while the increase was also significant.In the mixed atmosphere of CH₄/CO₂,the yields and characteristics of semi-coke changed obviously with the increase of the experimental temperature;when the concentration of CO₂ was 6%,CO₂ promoted the cracking and carbon deposition of CH₄,which produced more carbon than the carbon consumed by the gasification reaction of CO₂ and semi-coke.The yields of semi-coke increased and the pores becomed smaller and the surface becomed rougher;and as the concentration of CO₂ increased to 15%,the amount of carbon consumed by the gasification reaction of CO₂ and carbon was greater than the amount of carbon deposits produced by the cracking of methane.So the rate decreased and the pores becomed larger.The experimental results in the CH₄/H₂ atmosphere were showed as the following.The presence of H₂ in the CH₄/H₂ mixed atmosphere inhibited the cracking reaction of CH₄,but it could not completely inhibit the cracking reaction of CH₄.When the pyrolysis temperature was not higher than 600°C,H₂ participated in the pyrolysis process of coal.For the reason of hydrogen radicals enriched by H₂,the yield of each gas component of CO,CO₂,C2^C3 in the pyrolysis gas increased with the increase of the H₂ concentration in the mixed atmosphere,and increased with the increase of the temperature.And the yield of pyrolysis water also increased significantly.In the CH₄/H₂ mixed atmosphere,the presence of H₂ provided abundant hydrogen radicals to promote the stabilization of tar macromolecules,so the tar yield increased.And the tar yield reached a peak at a pyrolysis temperature of about 600°C.The yields and characteristics of semi-coke in CH₄/H₂ atmosphere also changed significantly,and the changes were greater with the increase of pyrolysis temperature.At a lower H₂concentration?15%?,the presence of H₂ inhibited the cracking of CH₄,but it could not completely suppress the cracking of carbon deposits,so the change in semi-coke yield was not obvious.But the porosity of semi-coke became larger,and there was no obvious carbon deposit on the surface.With the increase of H₂ concentration?increased to 25%?,the presence of H₂ strengthened the pyrolysis of coal so that the yield of semi-coke was significantly reduced and the porosity of the semi-coke was larger.The experimental results under pyrolysis gas atmosphere were showed as the following.At lower temperature?500??,H₂ mainly affected the coal pyrolysis in the pyrolysis gas atmosphere,and as the temperature increased,the yields of CO,C2^C3gas components all increased because of the complex reaction of various components in the pyrolysis gas.Affected by reactions such as methane carbon dioxide reforming,the yields of CO₂ and CH₄ always decreased.Since the addition of CO would inhibit the reaction between CH₄ and CO₂,the yields of pyrolysis water in the pyrolysis gas atmosphere was reduced compared with the mixed atmosphere of CH₄/CO₂,but it was significantly increased compared with the inert atmosphere and CH₄/H₂ atmosphere.The simulated pyrolysis gas atmosphere would promote coal pyrolysis,and the tar yields had increased compared with the inert atmosphere,and reached a peak at about600?.Affected by H₂ in pyrolysis gas,the yield of semi-coke in pyrolysis gas at lower temperature?500??was the same as CH₄/H₂.The coke yield was further reduced with the increase of temperature.