Fast Pyrolysis Of Coal And Biomass And Upgrading Of Pyrolysis Tar

The clean and efficient utilization of coal is necessary due to the energy structure of rich coal and insufficient oil and gas in China.Biomass,a clean and renewable carbon resource,is also of great significance to deal with energy consumption and combat environmental pollution.The tar from the pyrolysis of coal and biomass at relatively low temperature can be used to produce liquid fuel or high-valued fine chemicals.One of the key technologies for the industrialization of pyrolysis tar production is improving the tar yield and quality.Focused on efficient conversion of coal and biomass for tar,the fast pyrolysis of coal and biomass and the upgrading of tar were studied in this work.In order to optimize the processes of fast coal and biomass pyrolysis and tar upgrading over char at steady state,a novel reactor system containing a free fall pyrolyzer combined with a downstream moving bed upgrading reactor was constructed.Devolatilization during the fast pyrolysis of coal and biomass,formation and evolution of tar and char were studied in the free fall reactor of multi-stage independent temperature-controlled and char level-controlled.The residual volatiles in the nascent char and its effects on pyrolysis product distribution and char characteristics were also studied by prolonging the residence time of the fast pyrolysis char(secondary pyrolysis of nascent char).The volatiles from fast coal and biomass pyrolysis in the free fall reactor were upgraded over the off-situ and in-situ char by adjusting the residence time of additional char bed material(off-situ char)and nascent char bed material(in-situ char)in the downstream moving bed upgrading reactor,respectively.Basing on the tar components yields,light tar(CH2CI2 soluble)compositions and char bed materials properties analysis,the mechanism of tar upgrading over char at steady state was discussed.Using Shenmu bituminous coal(SM)and Inner Mongolia lignite(NM)as raw materials,the fast pyrolysis of coal in the free fall reactor was studied.The fast pyrolysis of the both coals at different particle residence time was focused.At the pyrolysis temperature of 610?and particle size of 0.25-0.38mm,the devolatilizations of the both coals are enhanced with the increase of residence time.The suitable residence time promotes the tar formation and the tar quality improvement,and further increase of residence time leading to polycyclic aromatic hydrocarbons formation.Sufficient evolution of tar components can be achieved by regulating the reaction temperature and residence time during the fast pyrolysis process and the secondary pyrolysis of the nascent char produces more gaseous product,while the tar yield and its composition change little.The results indicate that the fast pyrolysis of coal with suitable residence time of coal at moderate temperatures is advisable for producing high quality tar.Further,using SM as raw material,the upgrading of fast pyrolysis tar with off-situ char and in-situ char were studied,respectively.The results of tar upgrading with off-situ char show that the char promotes the lightening of tar significantly by reducing the asphaltene yield,when using the slow pyrolysis char of SM(SMC)as the upgrading bed material in the moving bed upgrading reactor.The yield of light tar is also decreased while the content of light tar increases significantly.Additionally,the yield of H2 increases a lot.Thus the quality of tar and gas are improved.The contents of heteroatoms(O,N)compounds in the light tar decrease.Characterization of SMC shows that the porous structures of SMC provide the reaction sites for tar upgrading.The asphaltene in the tar adsorbs and condenses on the surface and pores of SMC and then forms coke.Meanwhile,the porous structure of SMC is blocked by the coke,resulting in obviously decrease of the specific surface area.In addition,the coke also leads to the increase of crystalline size,the decrease of Raman peak strength and the increase of aromaticity and 3-5 ring structures of SMC.With the increase of char updating rate,the surface active sites of the fresh SMC increase,as a result,the conversion of asphaltene increases slightly and the removal of heteroatoms(O,N)compounds in the light tar is enhanced.Meanwhile,the coking degree of SMC decreases,resulting in the relatively weak changes of SMC in the specific surface area,crystalline size of crystalline phase structures and Raman structure parameters.The results of tar upgrading with in-situ char show that the nascent char reduces the formation of asphaltene obviously and thus improves the quality of tar.When the char bed temperature is lower than 610?,the yield of light tar remains almost unchanged,the contents of phenol and other aromatic O-containing compounds in the light tar decrease and the contents of benzene and naphthalene aromatic hydrocarbons increase.After reacting with the volatiles,the yield of nascent char increases and the specific surface area of the char decreases.Using sawdust(SD),rice husk(RH)and wheat straw(WS)as raw materials,the fast pyrolysis of biomass in the free fall reactor was studied.The fast pyrolysis of the three kinds of biomass at different particle residence time was focused.At the pyrolysis temperature of 560? and particle size of 0.25-0.38mm,the results show that the devolatilizations of the three kinds of biomass gradually slow down with the increase of residence time.The tar yield increases firstly and then decreases,and longer residence time leads to the intensifying secondary reaction of tar,resulting in the decrease of O-containing compounds contents in the light tar and the increase of polycyclic compounds contents.The sufficient evolution of tar components can be achieved and the secondary pyrolysis of the nascent char produces more gaseous and water,while the tar yield and its composition change little.Further,using SD as raw material,the upgrading of fast pyrolysis tar with off-situ char and in-situ char were studied.The results of tar upgrading with off-situ char show that the tar components yields,the light tar compositions and char properties changes are similar to those with SM as the raw material.These results indicate that tars of significant difference in structure and composition,i.e.,biomass tar and coal tar,undergoes similar upgrading process over the same char.Compared with the tar from SM pyrolysis,the SMC also promotes the decomposition and conversion of water-soluble component in the tar from SD pyrolysis.The results of tar upgrading with in-situ char show that the nascent char not only decrease the yields of asphaltene and water-soluble significantly,but also promotes the formation of light tar in some extent at the optimizing char bed temperatures(<510?).In addition,the contents of catechol and fatty O-containing compounds decrease obviously,and the aromatic compounds of benzene and naphthalene are increased.As a result,the tar quality is improved.