Characteristic And Mechanism Studies On Low Rank Bituminous Coal Pyrolysis With Catalysts

Coal pyrolysis is regarded by many scientists for a long time as the most effective way of coal conversion except coal gasification and liquefaction, and adding catalysts during coal pyrolysis processes can reduce the pyrolytic temperature and improve the efficiency of pyrolysis. Thus, 6 kinds of metal oxides supported catalysts were prepared and characterized in this paper. The influences of them on the pyrolysis processes and products of 3 low rank bituminous coal: Nanhu coal from Xinjiang, Huangling coal from Shaanxi and Hegang coal from Heilongjiang were studied then.The main researches are as follows:(1) 6 kinds of metal oxides supported catalysts MgO/γ-Al2O3, CaO/γ-Al2O3, Fe2O3/γ-Al2O3, Co2O3/γ-Al2O3, NiO/γ-Al2O3 and ZnO/γ-Al2O3 were prepared from γ-Al2O3 and metal nitrate solutions and characterized by physical adsorption, scanning electron microscope and X-ray diffraction techniques. The results showed that the catalytic activities of these catalysts were connected with the properties of metal oxides on the active sites.(2) The effects of heating rates 10.00 ℃ ·min-1, 15.00 ℃ ·min-1, 20.00 ℃ ·min-1, catalysts MgO/γ-Al2O3, CaO/γ-Al2O3, Fe2O3/γ-Al2O3, Co2O3/γ-Al2O3, NiO/γ-Al2O3, ZnO/γ-Al2O3 and catalyst amounts 5 wt.%, 8 wt.%, 10 wt.% on 3 coals pyrolysis procrsses were researched by thermogravimetry. The results showed that the processes of 3 kinds of coals pyrolysis could be divided into three stages generally: stage Ⅰ(dry degassing stage), stage Ⅱ(lively decomposition stage) and stage Ⅲ(secondary degassing stage), of which stage Ⅱ is considered as the main weight loss stage. The best pyrolysis heating rates of Nanhu, Huangling and Hegang coal were 10 ℃ ·min-1, 20 ℃ ·min-1 and 15 ℃ ·min-1 respectively according to the degree of weight losses. 6 catalysts at the amounts of 8 wt.% all could widen the main weight loss peaks and increase coal weight losses, of which the order were decreased as: Nanhu coal, Fe2O3/γ-Al2O3 > Co2O3/γ-Al2O3 > CaO/γ-Al2O3 > NiO/γ-Al2O3 > MgO/γ-Al2O3 > ZnO/γ-Al2O3, the biggest weight loss was increased by 10.05 wt. %; Huangling coal, Fe2O3/γ-Al2O3 > Co2O3/γ-Al2O3 > NiO/γ-Al2O3 > CaO/γ-Al2O3 >MgO/γ-Al2O3 = ZnO/γ-Al2O3, the biggest weight loss was increased by 6.94 wt. %; Hegang coal, Fe2O3/γ-Al2O3 > Co2O3/γ-Al2O3 > NiO/γ-Al2O3 = CaO/γ-Al2O3 > MgO/γ-Al2O3 > ZnO/γ-Al2O3, the biggest weight loss was increased by 4.03 wt. %. The optimal proportion relationships between adding amounts of catalysts and the weight losses of coals were re also received from the studies.(3) The kinetic parameters activation energies E and pre-exponential factors A of 3 coals catalytic pyrolysis were calculated by Coats-Redlfem equations. The excellent catalytic effects was determained by the lower E. It was found that E values of 3 coals during pyrolysis in stage Ⅱ and stage Ⅲ were decreased because of adding catalysts, among which the E values of Nanhu coal with 8 wt.% Fe2O3/γ-Al2O3 decreased the most by 7.55 kJ mol-1 and 51.33 kJ mol-1, E values of Huangling coal addding 8 wt.% Fe2O3/γ-Al2O3 and 8 wt.% NiO3/γ-Al2O3 during these two stages decreased the most by 13.11 k J?mol-1 and 24.22 kJ?mol-1 respectively, E values of Hegang coal addding 5 wt.% Co2O3/γ-Al2O3 decreased the most by 9.62 kJ?mol-1 and 9.37 kJ?mol-1. In addition, a compensation effect was existed between E and A. These changes were agreed with the thermogravimetric experimental of coals catalytic pyrolysis results which better explained the the influences of 6 kinds of catalysts on 3 coals.(4) The products of Nanhu coal catalytic pyrolysis was measured in a atmospheric pressure tubular reactor connected to a gas chromatography. The results showed that catalysts could increase the yields of H2 and C2-C4 hydrocarbons, while decreased the CO and CO2 yields during coals pyrolysis, and the effects on H2 were decreased as Fe2O3/γ-Al2O3 > CaO/γ-Al2O3 > Co2O3/γ-Al2O3 > NiO/γ-Al2O3 > MgO/γ-Al2O3 > ZnO/γ-Al2O3. It was also found that CaO/γ-Al2O3 reduced the liquid yields and improved the carbocoal yields of Nanhu coal pyrolysis, which indicated that it was a good catalyst for tar polycondensation. Another 5 kinds of catalysts showed not only have no selective on solid and liquid products, but even lead to a certain extent reduce of these 2 products yields.