Formation Of Tar From An Integrated Process Of Coal Pyrolysis With Steam Reforming Of Ethane

Aim at a new process of integrated coal pyrolysis with steam reforming of ethane(SRE)over Ni/Al2O3 catalyst for improving coal tar yield,this work is to investigate the formation and mechanism of tar in the integrated process,and compare the results with those under atmospheres of nitrogen,hydrogen and steam reforming of methane(SRM).Buliangou subbituminous coal was used in this work.The pyrolysis experiments were carried out in a fixed-bed reactor at atmospheric pressure under gas atmospheres of N2,H2,steam reforming with methane(SRM)and ethane(SRE),respectively.The effects of pyrolysis temperature,ethane flow rate and steam to carbon ratio(S/C,the ratio of steam to carbon in feed gas)on tar yields were examined.The results indicated that a higher tar yield can be obtaind from CP-SRE than those from pyrolysis under N2(CP-N2),H2(CP-H2)or SRM(CP-SRM),and the tar yield from CP-SRE at 600 oC is 1.42,1.09 and 1.04 times than that from CP-N2,CP-H2,CP-SRM,respectively.CP-SRE achieve the highest tar yield when S/C ratio is 1.Higher ethane flow rate is beneficial to improve tar yield.When the ethane flow rate increases from 75 mL/min to 125 mL/min,the tar yield increases by 22%.Char yield decreases with the increase of temperature,and CP-SRE has lower char yield than that CP-N2.To understand the fraction distribution and structural information of tar in CP-SRE,the properties of tar were characterized by simulated distillation,gel permeation chromatography,gas chromatography-mass spectrometry(GC/MS)and nuclear magnetic resonance(NMR).The results showed that the tar from CP-SRE presents a lower molecular weight distribution than that from CP-N2,but has no difference with those from CP-H2 and CP-SRM.Moreover,integrated coal pyrolysis with steam reforming of ethane has a positive effect on tar fraction distribution,and the tar from CP-SRE has higher contents of light oil and phenol oil,and lower content of asphaltene.1H-NMR and 13C-NMR analyses of tars showed that the tar from CP-SRE has slightly higher amount of uncondensed aromatic protons and quaternary aromatic carbons than that from CP-N2,which means that some activated species from ethane reforming participate in stabilizing coal cracking fragments and retarding condensation reaction.137 compounds,accounting for about 42 wt.% of volatile fractions of coal tar were identified by GC/MS.Phenol and its alkyl-substituted homologs from C1 to C3 are the major products,which was strongly affected by the integrated process,but small effect on content of naphthalene and its alkyl-substituted homologs was observed.Deuterium oxide(D2O)and deuterated ethane(C2D6)were used to investigate the formation mechanism of coal tar in the integrated process through analyzing the compounds in coal tar with GC/MS and deuterium nuclear magnetic resonance(D-NMR).The results showed that deuterium could be detected both in aromatic rings and aliphatic side chains of the compounds in the tar.The compounds in the tar exist [·D],[·CD3] and [·C2D5],indicating that D2 O and C2D6 take part in the formation of coal tar by being activated into free radicals.These results revealed that atmosphere do play an important role in the formation of coal tar,and we can deduce that the higher coal tar yield in the integrated process is attributed to the free radicals from SRE incorporate into coal radicals and retard the condensation reaction between coal radicals.