Co-pyrolysis Characteristics Between Low-rank Coals And Shenhua Coal Direct Liquefaction Residue

The low-rank coal reserves in China constitute approximately 52.3 billion tons,and account for 42% of the total coal proven reserves.Although the high volatile content and reactivity of these coals,the low yield and poor quality of the tar product are persistent problems that need to be solved.It is widely believed that hydrogenation and condensation reactions of free radicals occur competitively during pyrolysis of low-rank coal.The DLR from the Chinese Shenhua's coal direct liquefaction plant contains an oil-rich fraction(HS 16.6 wt.%,A 28.5 wt.%and PA 2.0 wt.%),carbon-rich substances and minerals(THFIS 52.8 wt.%),which accounts for about 30% of raw coal feed.However,since DLR has a low softening point,poor feeding of the raw material adversely impacts its individual pyrolysis.On the other hand,compared to these coal,DLR has a lower oxygen content,but it has more alkali and alkaline earth metallic(AAEM)species,and solid liquefaction catalysts,which can improve coal pyrolysis.Therefore,co-pyrolysis of low-rank coals together with DLR and has been proposed as a means for improving tar yields by exploiting the properties of DLR.The advantages of this approach are manifest: a)making use of the oil in DLR,b)overcoming the feed problem of DLR pyrolysis,and c)the minerals and liquefaction catalysts in DLR can increase coal conversion.To reasonably utilize the coal direct liquefaction residue(DLR),contrasting research on the co-pyrolysis between different low-rank coals and DLR was investigated using a TGA coupled with an FT-IR spectrophotometer and a fixed-bed reactor.There was an interaction betweent the lignite and the DLR,the weightloss increasing 4.4 wt.% and the yield of tar increasing 1.8 wt.%.Moreover,a kinetic analysis during the co-pyrolysis was conducted using a distributed activation energy model.The co-pyrolysis reactions showed a approximate three-peak distribution,with a decreasing average activation energy(241.8 kJ/mol),which was lower than the average activation energy of lignite(316.7 kJ/mol).GC-MS,FTIR,and XRD were used to explore the reaction mechanisms of the various co-pyrolysis processes.It was experimentally confirmed that the interactive effects between HL coal and DLR existed during their co-pyrolysis.It was found that the THFIS,contained enriched minerals and liquefaction catalysts that were conductive to devolatilization of the HL components,resulting in the higher weight loss(+6.0 wt.%)and the higher tar yield(+3.5 wt.%).The organic components of DLR negatively affected co-pyrolysis,resulting in the lower weight loss(-3.8 wt.%)and the lower tar yield(-2.6 wt.%).Also,the addition of DLR improved the yield of tar in the fixed-bed(+1.8 wt.%),with increasing the content of the polycyclic aromatics in the tar.The type of coal in the co-pyrolysis process was an important factor that affected the co-pyrolysis behavior.In the case of HL,the THFIS fraction favored co-pyrolysis weight loss.The higher H/C ratio of HL in comparison to the subbituminous coal,and the more hydrogen,–COOH and –OH,might make the HL easier to stabilize the tar fragments and to reduce the char yield and to increase the tar yield.The minerals such as Ca and Fe were enriched in the DLR,which would promote hydrogen transfer to the coal.The pyrite might increase the efficiency of hydrogenation of radicals during co-pyrolysis.In the case of the subbituminous coal,the lower H/C ratio make the SD less sensitive to the active components in the DLR.In addition,the positive effect from the THFIS was not sufficient to offset the negative effect of the HS and PA.The Fe-based catalyst might offer no remarkable effect on the SD during the co-pyrolysis.