Study On The Catalytic Mechanism Of Low Rank Coal Pyrolysis With Solid Heat Carrier

Low rank coals account for more than half in Chinese coal reserves,which have the characteristics of high volatile,high water content and low calorific value.The upgraded char,pyrolysis gas,and tar containing some valuable chemicals can be obtained through pyrolysis.For the purpose of raising tar yield and improving its quality,one method is to strengthen the decomposition of coal in the primary pyrolysis stage,thus producing small fragments as much as possible and making full use of hydrogen and small free radicals within coal particles and in the surroundings.The other is to control the secondary cracking of primary pyrolysis products(tar precursor),and realizing transformation and accumulation of certain tar components under relatively mild conditions.At present,coal pyrolysis upgrading process with char solid heat carrier(SHC)can recover part sensible heat of char and improve thermal efficiency of the system.Volatiles releas ed in pyrolysis process undergo cracking reactions over char,thereby improving the content of light tar fractions.In this paper,the effects of ceramic ball,quartz sand,olivine,and char heat carrier on Hulunbuir lignite and Shendong long-flame coal pyrolysis were investigated using a fixed-bed reactor.Combined with the determination of light tar fractions,the conversion of volatiles with different heat carriers was analyzed.The direct coal liquefaction residue(DCLR)was introduced into Shendong(SD)coal pyrolysis with SHC to study the co-pyrolysis interaction.And the cycling copyrolysis experiment of co-pyrolysis char solid heat carrier to inspect char particle size changes and product distribution in the process.In the mechanism study of SD coal pyrolysis with char heat carrier,based on the determination of tar composition together with physicochemical properties of char heat carrier before and after the pyrolysis,the influence of initial temperature on the cracking of volatiles over char SHC was investigated.Main results obtained in this paper are as follows:1.The experiments of lignite pyrolysis with different SHCs showed that volatiles underwent deep cracking with ceramic ball,and much more water generated.From the product distribution with lignite char SHC,higher light tar fraction was obtained,and the H2 and CO2 in pyrolysis gas increased.The carbon species from cracking deposited on lignite char surface,and it showed a positive effect on its further pyrolysis at higher temperature.2.The reliability of the tar recovery method was verified,which is treating the condensation part with tetrahydrofuran first,then removing the solvent as well as pyrolysis water by rotary evaporator,and extracting with n-hexane to obtain the light tar fraction.GC-MS analysis of light tar of SD coal pyrolyzed with different SHCs was conducted,and the matters detected were classified into four groups.Combined with their changes of relative content,phenols in tar were supposed to convert into aromatics in the process.3.The interaction during SD coal and DCLR co-pyrolysis with co-char solid heat carrier was investigated,and there existed a positive interaction(?Y>0).Cracking effect occurred to the volatiles on co-char,compared with quartz sand in the control group.The 10-cycling simulating experiment of SD coal and DCLR co-pyrolysis with co-char SHC indicated the existence of a counteracting effect between char bulk and supplement of new generated char after certain cycles,the particle size distribution of co-char tended to be stable.4.The changes of SD coal tar yield with SDC heat carrier under different initial temperatures resulted from the comprehensive influence of the thermal field.And it involved the instantaneous heating rate difference caused by the temperature gradient between heat carrier and feeding material,coal pyrolysis extent,cracking of tar precursor species,and volatiles-char reactions.5.The coal pyrolysis volatiles was jointly affected by catalytic cracking and thermal cracking of the hot char,and the former was obvious in the lower temperature range while the latter was obvious in the high temperature range.High temperature was not conducive to the stability of phenolic species,resulting in the decomposition or polycondensation of phenolic hydroxyl,and the content of polycyclic aromatic hydrocarbons increased.The volatiles-char heat carrier reactions had little effect on carbon structure of char.The partial condensation products of volatiles deposited on char caused change of some functional groups on the surface,which would affect its subsequent utilization.6.With the increasing of the initial temperature of the SDC heat carrier,the carbon species deposited on char heat carrier differed not only in quantity but also in structure.The physical adhesion of volatiles,decomposition and condensation of tar on char heat carrier occurred in the pyrolysis process,and whose promotion effect on the CO2 gasification of the char SHC outweighed the adverse effect caused by the decrease in specific surface area.