Formation Mechanism And Analysis Of Tar From An Integrated Process Of Coal Pyrolysis With CO₂ Reforming Of Methane

China is rich in coal; however, the use of coal is the main source of air pollution. Meanwhile, the dependence of crude oil in China increase greatly, it has reached over 50% in 2009. So it is important to investigate new technology of coal to liquid oil and chemicals. For improving the tar yield of coal pyrolysis, in this work, Yima, Shendong, Lingwu, Hami and Pingshuo coals were investigated to understand the mechanism and composition of tar formation in various processes of methane catalytic conversion coupling with coal pyrolysis process. The main research works and results are as following:1. Yima coal was used to study the tar formation in the integrated process coal pyrolysis with partial oxidation of methane (POMP), with CO2 reforming of methane (CRMP) and coal catalytic pyrolysis with CO2 reforming of methane (CRMCP). The results showed that, a higher tar yield can be obtained in the hydropyrolysis process than in pyrolysis under N2 atmosphere. And higher tar yield can be obtained in POMP process than in pyrolysis under N2 and H2 atmosphere. At temperature higher than 700℃, higher tar yield can be obtained in CRMP process than in pyrolysis under N2 and H2 atmosphere. CRMCP process can further enhance the tar yield compared with CRMP process. The amount of Fe added in coal affected the coal tar yield. Among these processes, CRMCP process has the highest desulfurisation efficiency. Higher desulfurisation efficiency than that in pyrolysis under N2 atmosphere can also be obtained in pyrolysis under H2 atmosphere and CRMP process.2. The pyrolysis behavious in CRMP and CRMCP processes of three western coals and Pingshuo coal were investigated. Higher tar yield in CRMP process than that in pyrolysis under N2 and H2 atmosphere can be obtained for all four coals investigated. However, for western coals, tar yield was enhanced over 3 times as that in N2 atmosphere, which is only 1.7 times for Pingshuo coal. The tar yield can be further enhanced for all four coals in CRMCP process compared with CRMP process. However, when Fe added into the coal, Pingshuo coal has a decrease of tar yield in N2 and H2 atmosphere.3. The GC and GC/MS of tar fractions showed that the phenol and its alkyl-substituted homologues from C1 to C3 are the major products in the volatile fractions of the tars, other minor aromatic compounds such as naphthalene and its alkyl-substituted homologues, anthracene, xanthene, phenanthrene and dibenzofuran were also identified. It can be inferred from GC analysis of phenolic and naphthalic compounds that the composition of coal tar derived from Yima and Shendong coal pyrolysis under different atmospheres were not vividly different. However, it has a higher total amount of phenolic compounds in CRMP process. The NMR analysis of tars from Shendong coal pyrolysis under different atmospheres indicated that the amount and length of alkly-substitutent attached to aromatic rings were enhanced in CRMP process. GC and NMR analysis of tars from CRMP process of different coals represents that the tar yield was related with the alkly-substitutent attached to aromatic rings.4. The researches on the tar formation in different atmospheres inferred that the coal tar formed in CRMP process includes organic compounds formed in the CO2 reforming of methane process, coal tar formed from the coal pyrolysis process itself, and the tar formed in the coupling process between CH4/CO2 reforming process and coal pyrolysis process.5. The CD4 was used instead of CH4 to investigate the mechanism of tar formation during the CRMP process. The results showed that the deuterium can be detected in the coal tar compounds with the form of [·D] and [·CD3] indicated that part of the increase of coal tar yield in the CRMP process can be explained as the interaction of free radicals formed from CO2 reforming of CH4 and the cracking of coal chemical structure during pyrolysis.