Study On Pyrolysis Reaction Characteristics Of Middling

With the national environmental protection requirements of coal production and processing enterprises increasingly strict,China’s raw coal selection rate increased year by year,raw coal selection rate of more than 70%,followed by a large number of inferior wash coal（referred to as coal）.However,most of the middling is directly used as boiler coal,can not make full use of the organic structure of coal,low energy utilization level,poor economic benefits and high carbon emissions.Compared with the traditional way of direct combustion and utilization of middling,coal pyrolysis technology can not only improve the added value of middling,but also have good economic and ecological benefits.It is an effective way of clean and efficient utilization of middling.In this experiment,zichang middling in Yan’an area was used as raw material to study the pyrolysis reaction characteristics of middling with self-built fixed bed pyrolysis furnace and thermogronometric analyzer.Firstly,the yield and composition of semi-coke,tar and pyrolysis gas of pyrolysis products at different pyrolysis temperatures and particle sizes were studied by using fixed bed pyrolysis device combined with GC,GC-MS and industrial analysis methods.Thermogravimetric analyzer was used to study the effects of different heating rates on pyrolysis characteristics.Then,zichang middling was treated with deashing,and four minerals,Na₂CO₃,Ca O,K₂CO₃and Fe₂O₃,were added to deashing coal to investigate their effects on the yield and composition of pyrolysis products,providing theoretical basis for the regulation of pyrolysis reaction parameters of washed middling.The main research contents of this paper are as follows:With the increase of pyrolysis temperature,the yield of tar increases first and then decreases.At 700℃,the yield of tar from coal pyrolysis reaches the maximum10.05%.In the pyrolysis temperature range of 600-800℃,the proportion of alkane compounds in tar products is the highest,followed by aromatic hydrocarbons.This is because zichang coal belongs to bituminous coal with low metamorphic degree,and there are many alkyl side chain structures in its macromolecular structure.When the temperature reaches 750℃,the volatiles in semi-coke decrease significantly and the pyrolysis conversion rate is high,indicating that most of the volatile components have escaped,and further increasing the pyrolysis temperature has little effect on the tar yield.With the increase of coal particle size,the yield of pyrolysis liquid phase products decreases,the yield of semi-coke increases,the proportion of alkane compounds in tar decreases,the proportion of aromatic hydrocarbon compounds increases,and the content of CH₄component in gas phase products increases.This is because with the increase of coal particle size,the separation resistance of substances inside coal macromolecules increases,which promotes the secondary reaction of pyrolysis products,and more CH₄is generated in the secondary pyrolysis process of tar.The effects of different heating rates on pyrolysis reaction characteristics were investigated by thermogravimetric analyzer.The results show that the mass loss rate peak occurs at about 50℃to 100℃for coal samples with different heating rates,which is mainly due to the volatilization of free water attached to the surface and pores of coal samples.The coal sample begins to undergo significant thermal decomposition reaction at 350℃.The maximum pyrolysis rate peak occurs when the pyrolysis temperature reaches about 450℃,and the pyrolysis weight loss rate reaches the maximum at different heating rates,and the pyrolysis weight loss rate reaches8.26%/min at 50℃/min.A small peak of weight loss rate appears at about 720℃,because secondary pyrolysis and polycondensation of coal samples occur at a higher temperature,and more H₂is produced at the same time.The influence of inherent minerals on pyrolysis reaction characteristics of washed coal was investigated by acid cleaning deashing.The results show that the yield of deashing coal tar and pyrolysis gas decreases and the yield of semi-coke increases compared with that of zichang middling,indicating that ash content in coal promotes coal pyrolysis.The addition of four minerals changed the yield of pyrolysis products of deashing coal 2,especially the yield of gas products.For tar yield,the order of promoting effect of these four minerals is Ca O>Na₂CO₃>Fe₂O₃>K₂CO₃.For gas yield,the order of promoting effect of these four minerals is Ca O>Fe₂O₃>Na₂CO₃>K₂CO₃.