Study On Nickel-based Catalysts For Catalytic Conversion Of Coke Oven Gas To Hydrogen

High efficiency, large scale, and low-cost technologies for producing hydrogen are urgently demanded to meet the needs of the development of proton exchange membrane fuel cells as the power source for power, transportation and other applications. As a by-product generated in the process of producing coke from coal, coke oven gas (COG) is gaining increasing attention as one of the most attractive sources of hydrogen production. The treatment of tar in COG over catalyst is the key issue for the hydrogen amplification and rational utilization of energy. This thesis investigated the catalytic conversion of toluene as a model tar compound over nickel-based catalysts with the mixed MgO-Al₂O₃ as the support. Under the conditions of atmospheric pressure, hydrogen-rich atmosphere and low steam/carbon molar ratio, the influences of calcination temperature, compositions, promoters and the evaluation condition on the catalyst performance of hydrogenation and steam reforming were systematically studied. Also, the hydrogen production by steam reforming and partial oxidation reforming of COG were explored preliminarily. Based on these experimental results, several conclusions were drawn from the investigations.The catalytic performance of the NiO/30%Al₂O₃-MgO catalyst was sensitive to calcination temperature. The characterization results of BET, XRD, H₂-TPR and TEM techniques indicated that the BET surface area dramatically decreased, the (Ni,Mg)Al₂O₄ spinel phase and the interaction between the metals and carrier both increased with increasing the calcination temperature. However, free NiO and those of weak interaction with the carrier appeared when the calcination temperature was higher than 950 oC; and the metals particles agglomerated obviously when the catalyst was calcined at a lower or higher temperature. NiO/30%Al₂O₃-MgO calcined at 850 oC had the best activity and stability. When the content of Al₂O₃ was 30% in the NiO/x%Al₂O₃-MgO catalysts, the precursor of the carrier had the typical hydrotalcite structure and the catalyst showed higher specific surface area and good activity. The addition of CeO₂, La₂O₃ and ZrO₂ enhanced the interaction between the metals and carrier and improved the performance of toluene hydrogenation and resistance to carbon formation.The Ni_0.25-Me_0.25/Mg_2.5(Al)O(Me=Co, Fe, Cu, Zn, Mn) bimetallic catalysts were synthesized with the coprecipitation method. The Ni_0.25Co_0.25/Mg_2.5(Al)O catalyst calcined at 850 oC showed higher specific surface area, stronger interaction between metals and carrier due to the formation of spinel and solid solution phases as well as smaller particulates and highly homogeneous dispersion of active metals. A toluene conversion of 100% had been achieved for 35 h over the catalyst at 800 oC under atmospheric pressure and GHSV of 12,000 mL/(g·h) with a steam/carbon molar ratio of 0.7. After the activity test, a small amount of whisker carbon was observed on the used catalyst, and most of them could be removed in the hydrogen-rich atmosphere, indicting that the carbon deposition on catalysts was a reversible process. Furthermore, with the addition of a small amount of noble metals Pd, the Ni/Mg₃(Al)O catalyst presented excellent catalytic performance for the hydrogenation and steam reforming of toluene. A toluene conversion of 94%, a CH₄ yield of 53% and a CO yield of 41% had been achieved over the 0.5%Pd-12%Ni catalyst at 800 oC and GHSV of 3,000 mL/(g·h) with a steam/carbon molar ratio of 0.42.The Ni_x/Mg_2.33-x(Al)O catalysts were prepared by a homogeneous precipitation method using urea hydrolysis and had good activity in the process of steam reforming of COG. H₂-TPR, SEM and TEM results showed that the catalyst with low Ni content had higher specific surface area, stronger interaction between metals and carrier, and highly homogeneous dispersion of active metals. Reaction temperature had obvious influence on the performance of the catalysts. And when the temperature was higher than 800 oC, the experimental results agreed well with the calculated results. The increase of the steam/carbon molar ratio promoted the conversion of toluene and CH4 to H₂ and CO. The toluene and CH4 could completely be converted to H₂ and CO in the catalytic reforming of COG, and H₂ in the reaction effluent gas was about 4 times more than that in original COG when steam/carbon molar ratio was 1.7. Moreover, the NiO/30%Al₂O₃-MgO catalyst showed excellent sulfur resistant character so that it still had good activity when the H₂S content in the COG was 500 ppm.Hydrogen production by partial oxidation reforming of COG in BaCo_0.7Fe_0.2Nb_0.1O_3-δ(BCFNO) membrane reactor was also investigated in this thesis. The experimental results showed that the increase of air flow rate was beneficial to improve the oxygen permeation flux. However, it had little influence on the oxygen permeation flux when the air flow rate was more than 300 mL/min. The oxygen permeation flux increased with the increase of the reaction temperature. And the oxygen permeation flux was sensitive to the catalysts. The Ni/Mg(Al)O catalyst with the precursor of hydrotalcite structure had good activity and resistance to coking. The rare earth added to the Ni/Mg(Al)O catalysts could improve the activity, resistance to coking, and dispersion of active metals. At optimized reaction conditions, the dense oxygen permeable membrane had an oxygen permeation flux around 15.1 mL/cm²/min and a H₂ yield of 86.8% and a CO yield of 87.2% had been achieved over the La₂O₃ modified Ni/Mg(Al)O catalyst at 875°C. And the conversions of toluene, CH₄ and CO₂ were 100%, 88.5% and 92.7%, respectively. The amount of H₂ in the reaction effluent gas was about 2 times more than that of original H₂ in the reactant gas. The permeation sides of the used membrane were destroyed after the experiments, but the structure changes were only up to several micrometers, which had no effect on the safety work of the membrane reactor.