Study On Conversion Behavior Of Volatiles From Naomaohu Coal Pyrolysis Over Acid/Base Catalysts

Low-rank coal pyrolysis volatiles are highly reactive and prone to reaction,leading to poor tar’s liquidity and high content of heavy components and coke in tar,by which pipeline blockage is caused.At the same time,a large amount of dust is carried in the volatiles due to the thermal crushing of coal particle in the pyrolysis process,which hinders the industrialization of low-rank coal pyrolysis process.It is a promising method to introduce catalysts to regulate and control volatiles’reaction to improve tar quality.In this study,Naomaohu long-flame coal was used due to its rich reserves and high oil content.A metal catalyst（Ni-Al₂O₃）,three acidic catalysts（HZSM-5,Al-MCM-41 and USY）and a base catalyst（Ca-Al）were selected to investigate the effects of different types of catalysts on the distribution of pyrolysis products,tar quality and coke yield during the pyrolysis of NMH coal.It was expected that the conversion behavior of volatiles over the different types of catalysts was revealed.Based on the above results,Al-MCM-41 and Ca-Al with better performance were combined in different ways to further study the composition and yield of volatile products from pyrolysis of NMH coal.In addition,the physical and chemical structures of the coke deposited on the catalysts and suspended in tar were characterized by Raman spectrometer,FT-IR spectrometer and Scanning electron microscope.The formation mechanism of coke during pyrolysis was discussed.The research results are of great guiding significance for promoting the industrialization of low-rank coal pyrolysis process.Relevant research contents and conclusions are as follows:（1）Different types of catalysts have different effects on the composition and distribution of volatile products from pyrolysis of NMH coal.Ni-Al₂O₃ is strong in cracking and dehydrogenation of volatiles.Compared with other catalysts,the tar yield is lowest（5.36 wt.%）and that reduces by 46.08%than that over QB.and the yield of H₂,CO and CO₂ in pyrolysis gas increases obviously.The effects of three acidic catalysts（Al-MCM-41,USY and HZSM-5）on pyrolysis volatiles’reaction are different due to their various acidities,specific surface areas and pore sizes.The acidity of Al-MCM-41 is most suitable for the cracking of heavy components in tar increasing the content of light components,and the reduction in tar yield is least.At the same time,because of its larger specific surface area and pore size,Al-MCM-41 is conducive to the mass transfer process of pyrolysis volatiles,and is superior to USY and HZSM-5 in reducing the content of the oxygen-containing compounds and the yield of coke in tar.The cracking of tar is weak over alkaline catalyst（Ca-Al）and the reduction of tar yield is less.It is worth noting that dehydrogenation of hydrocarbons can promote over Ca-Al catalyst generating more small molecule hydrogen-rich free radicals in the process of pyrolysis,which weakens the polycondensation reaction between macromolecule free radicals.Thus,the coke yield significantly decreases.（2）Based on the characteristics of Al-MCM-41 and Ca-Al,they were combined by different composite methods as the catalysts,and the effects of composite catalyst on volatiles’reaction are studied.The volatiles from coal pyrolysis go through the Al-MCM-41 and Ca-Al catalyst in sequence（denoted as Al/Ca-Al）.Compared with Al-MCM-41 alone,the acidity of composite catalysts reduces due to the addition of Ca-Al,which weakens the catalytic cracking reaction of volatiles resulting in the reduction of tar cracking rate and deoxidation effect.When the pyrolysis volatiles go through Al Ca-Al that is mechanical mixture of Al-MCM-41 and Ca-Al are combined by Al-MCM-41 and Ca-Al blend combination（named as Al Ca-Al）,the coke yield is the highest.When the volatiles first go through Ca-Al with dehydrogenation capability and in turn go through Al-MCM-41 whose cracking effect on volatiles is moderate（denoted as Ca-Al/Al）,the Ca-Al/Al possesses the better performance in the conversion of volatiles.The removal of oxygen-containing compounds is promoted and the content of aromatic hydrocarbon in tar increases.It is superior to Al/Ca-Al and Al Ca-Al in reducing coke yield,and the coke yield is the lowest（0.65%）.（3）The influence of different catalysts on the composition and morphology of coke is studied during the catalytical conversion of pyrolysis volatiles.It is found that the coke deposited on the surface of acid catalyst and metal catalyst has a larger ratio of macrocyclic aromatic hydrocarbons than that deposited on the base catalyst.In addition,the characterization of coke deposited on surface of catalysts and suspended in tar is different.The coke in tar is mainly characterized by large aromatic ring structure,while the coke on catalyst is opposite.Furthermore,the micromorphology of coke in tar is different over the different catalysts.Over Ca-Al,the micromorphology of coke in tar is spherical particles with uniform distribution,and the melting and bonding phenomenon is slight compared with acid catalysts and metal catalyst.Over composite catalyst Ca-Al/Al,the morphology of coke in tar is relatively regular,which results from the fact that Ca-Al/Al weaken the polycondensation reaction of the heavy components in the pyrolysis volatiles.