Interaction Study On Co-Pyrolysis Between Shendong Coal Direct Liquefaction Residue And Coal

Pyrolysis is the initial stage and vital step of coal thermal conversion process such as liquefaction, gasification and combustion, meanwhile, it’s one of the effective ways for grading refining, clean and highly utilization of low rank coal resources. Low rank coal is characterized with high volatile content and low thermal conversion temperature, which determined that it’s the best material for low temperature upgrading. Coal direct liquefaction residue(DLR) is a byproduct of coal liquefaction process, accounting for about 30% of raw material. It’s rich in liquefied heavy oil. Due to the strong bonding and expansibility properties of DLR, pyrolysis of DLR alone is likely to cause coke discharge difficulty and furnace blockage. Co-pyrolysis of coal and DLR is expected to solve the low tar yield and poor quality problems in traditional coal pyrolysis technology and improve the overall economy of liquefaction process.Firstly, co-pyrolysis experiments of Shendong coal(SD) and DLR were conducted on a self-built single-stage fixed bed reactor, combined with TG simulated distillation, GC-MS, NMR characterization of tar to analyze the interaction in co-pyrolysis process. Meanwhile, the carbon/ hydrogen/ oxygen element distribution in the pyrolysis process was also investigated. Furthermore, two-stage co-pyrolysis experiment(upper layer-U, lower layer-L) of SD and DLR were carried out in order to explore the hydrogen transition mechanism. Meanwhile, DLR was divided into tetrahydrofuran soluble(THFS) and tetrahydrofuran insoluble to study the source of interaction. What’s more, the gasification reactivity of pyrolysis char was investigated by thermo gravimetric analysis(TGA), and the reason of difference was also analyzed based on its physical and chemical properties. The following results and conclusions were obtained:(1) There existed interaction in co-pyrolysis of SD and DLR. Compared with the calculated values, the co-pyrolysis yields of liquid products including tar and water increased, while the char and gas yields decreased. Elemental distribution was changed in co-pyrolysis process. Carbon, hydrogen and oxygen content in co-pyrolysis tar was higher than calculated values. More hydrogen and oxygen migrated to tar and water.(2) SD pyrolysis tar was mainly composed of light components whose boiling point lower than 400 °C, while DLR tar was mostly high boiling point(>400 °C) polycyclic aromatic compounds. The increased tar mainly was the contribution of polycyclic aromatic compounds whose boiling point were above 300 °C. The proportion of condensed aromatic hydrogen in tar was also increased.(3) Hydrogenated aromatic structure like 2-methyl-9H-fluorene generated in hexane soluble of co-pyrolysis tar, and the total phenolic compounds increased little compared with calculated values. Moreover, the addition of DLR inhibited the conversion of complex phenolic compounds to simple ones.(4) The interaction degree of co-pyrolysis of SD and DLR in different ways was in the order of SD(L)+DLR(U)> SD(U)+DLR(U)> SD(U)+DLR(L). Hydrogen utilization ability of THFS was stronger than SD under the same conditions. In co-pyrolysis process, SD interacted with the organic components in DLR-Tetrahydrofuran solubles(THFS), so that part of fragments of THFS which would condense to form char in individual pyrolysis were stabilized by more free radicals from SD and transformed into tar, therefore the interaction occurred.(5) The CO2 gasification reactivity of pyrolysis char was Shen dong char> co-pyrolysis char> residue char. With the increasing addition amount of DLR, the CO2 gasification reactivity of co-pyrolysis char decreased gradually. At the same time, the graphite degree gradually increased and the specific surface area decreased.