Study On Pyrolysis Characteristics Of Different Rank Coals In LiNaKCO₃ Molten Salt

Coal is widely used to meet energy needs due to its competitive price and high energy density.At present,China's coal resources are mainly used for direct combustion,which is simple,feasible and low in cost.It has the disadvantages of low efficiency and serious environmental pollution.The efficient and high-quality utilization of coal has attracted widespread attention.Pyrolysis is a thermochemical pretreatment step in any coal conversion process(such as gasification or liquefaction),which has been considered a mature low-grade coal upgrade and conversion technology.The catalytic pyrolysis technology of coal in molten salt is conducive to achieving efficient conversion of coal.At present,there are few explorations on the coal pyrolysis with molten salt,which lack in-depth analysis.Therefore,in this study,in order to effectively utlize coal,the effects of temperature and molten salt on the pyrolysis gas,tar and char,and the effect of different coal ranks on molten salt pyrolysis are studied.The main work and results of this paper are as follows:Firstly,pyrolysis of Hulun Buir lignite was studied without and with molten salt(Li Na KCO₃)at various temperatures(500,600,700 and 800?).The pyrolysis product properties,especially char structural changes were tested and compared from these two methods.The results showed that the molten salt pyrolysis helped reduce char yields and enhanced gas(especially CO₂,H₂ and CO)and liquid yields compared to conventional pyrolysis.CO₂,CO and H₂ production increased about 60.43%,103.42%and 65.2%at800?,respectively.Additionally,the presence of molten salt improved the tar quality by exhibiting more hydrocarbon content(maximum increase of 5.8%)and less oxygenated compounds.Secondly,the effect of molten salt(Li Na KCO₃)on the structure and reactivity of char obtained by HL pyrolysis was studied at different temperatures.The results show that the structure and reactivity of char are greatly affected by temperature and the presence of molten salt.The main differences are as follows:the char obtained by pyrolysis with molten salt has lower aromaticity,larger interlayer spacing and smaller crystallite size.Due to the liquid uniformity and fluidity of the molten salt,salt can penetrate the coal structure through macro pores,thereby destroying the crystal structure and increasing the disorder of char.The addition of molten salt improves the char structure and further improves its reactivity.Molten salt can improve the pore structure and increase the microporosity of char(maximum 71.74%).At any temperature studied,the char prepared by molten salt has a higher microporosity than the char obtained by pyrolysis without molten salt.Under the action of molten salt,the structure of char during the pyrolysis process hinders the formation of large aromatic ring structures(more than 6 rings),and promotes its cracking into small aromatic ring structures(3-5 rings).Generally,char produced by molten salt pyrolysis has lower ignition temperature,burn-out temperature and higher reactivity index,which is more active.Finally,the effects of coal ranks(HL lignite,HQH bituminous coal,SHK anthracite coal,and SD coal)on molten salt pyrolysis coal were studied at 600?.The main conclusions are as follows:the char yield increases with the degree of coalification.The yield of pyrolysis gas and tar increases with the increase of volatile content of raw coal.The CO₂ content increases with the increase of the elemental oxygen content in the raw coal,and the H₂ content increases with the increase of coalification.The H/O element ratio of coal is not the only factor affecting the release of H₂,CH₄ and other hydrocarbons.Tar produced by pyrolysis of these coals contains less oxygen compounds(within 5%).As the degree of coalification increases,monocyclic aromatic hydrocarbons and aliphatic hydrocarbons tend to decrease,and polycyclic aromatic hydrocarbons tend to increase.A small amount of sulfide is found in the oil produced by pyrolysis of these three coals HQH,SHK and SD(1.5%-5.1%).The content of sulfide in the oil is positively correlated with the content of sulfur element in the raw coal.