Upgrading of Lignite Pyrolysis Tar by Catalytic Cracking over Different Mo-loaded HZSM-5 Catalysts

摘要

Lignite is a low-rank coal with high content of volatile matter, which is beneficial to produce value-added chemicals by way of pyrolysis. However, lignite pyrolysis tar (LPT) often generally has very complex compositions with high fractions of heavy components and heteroatomic compounds especially oxygenates. In many cases, these compositions are undesirable because they have several negative impacts on the qualities of LPT. The catalytic cracking of LPT is perceived as an effective approach to upgrade LPT. In the present work, the experiment was carried out using a two-stage reactor. The volatile matter derived from the lignite pyrolysis in the first reactor was allowed to be subjected to the catalytic cracking in the second reactor. Four different catalysts were used to investigate their catalytic effects, including HZSM-5 without loaded metal (ori-HZSM-5), Mo-loaded HZSM-5 without sulfidation (unsulfided-Mo-HZSM-5), Mo-HZSM-5 sulfided in a stream of H_2S/N_2 gas (half-sulfided Mo-HZSM-5) and Mo-HZSM-5 sulfided in a stream of H_2S/H_2 (sulfided HZSM-5). With HZSM-5, the yields of BTXEN (benzene, toluene, xylene, ethylbenzene and naphthalene) reached a maximum of 1.26 wt.% (dry lignite) at the cracking temperature of 550 °C, in the order of fourfold the yield obtained without catalyst (only thermal cracking at the same temperature). This catalyst also distinctly showed the effect on the deoxygenation of LPT, resulting in increases in the yields of CO_2, CO and H_2O, accompanied by the reductions in the yields of oxygenates such as acetic acid and phenols. The yield of hydrocarbon gases C_2H_4 obtained with ori-HZSM-5 exhibited obvious increase compared to the yield obtained without catalyst. Unsulfided Mo-HZSM-5 and half-sulfided Mo-HZSM-5 showed a close catalytic effect. These two catalysts were somewhat better than ori-HZSM for the production of BTXEN. Sulfided Mo-HZSM-5 had excellent performance in the production of BTXEN and the yields of BTXEN achieved 1.63 wt.% (dry coal) at cracking temperature of 550 °C.