The Study Of Catalyst For The Reforming Of Coke Oven Gas To Hydrogen In An Oxygen Permeation Membrane Reactor

Coke oven gas (COG), a byproduct in the coke-making process, which mainly containing H₂, CH₄, CO, and small amounts of other matters, is a good raw material of H₂ production. The production of COG is huge in China, however only part of produced COG is simply utilized as fuel. A large amount of COG is directly burnt at the end of an opened chimney and then directly discharged into atmospheres, thus resulting in a serious air pollution. Hydrogen as a clean energy has been received a widespread attention around the world. Production of hydrogen by converting COG not only resolves the environment pollution problem but also brings down the cost of the hydrogen production.Partial oxidation of methane (POM) in COG is a promising technology and the most economic way to obtain hydrogen. However, the main difficulty with POM lied in the consumption of large quantities of expensive pure oxygen that was produced through cryogenic air separation process. Coupling POM with a mixed ionic and electronic conductor oxygen permeation membrane could allow for the separation of oxygen and the catalytic oxidation in one unit, therefore not only simplifing the operation unit, but also reducing the production cost. The thermodynamic analysis of the POM in COG is discussed in this paper. Then the performance of several catalysts packed on the BaCo0.7Fe0.2Nb0.1O3-δ(BCFNO) membrane reactor is investigated for catalytic partial oxidation reforming of COG. As well as the reaction pathways of the reforming of COG in BCFNO membrane reactor are proposed according to the investigations in different membrane reactor configurations and different kinds of gas. Finally a Ni-based catalyst for steam reforming of COG was reported.Firstly, the profound thermodynamic analysis of hydrogen production of COG by POM was presented in this paper. The effect of the original hydrogen, steam addition, reaction temperature, energy consumption of steam addition and the ratio of CH₄/O₂ on the conversion of CH₄ and the selectivity of H₂ or CO for COG reforming are carefully examined. The results show the optimization of CH₄/O₂ molar ratio should be 2. The H₂ and CO selectivity would decrease sharply, when the CH₄/O₂ molar ratio <2. However, when CH₄/O₂ molar ratio>2, CH₄ conversion would decrease slowly, while solid carbon would be generated, and CO selectivity would be decreased. With increase of reaction temperature, the CH₄ conversion, H₂ and CO selectivity would increase and the formation of solid carbon could be suppressed. The optimal steam addition could change the enthalpy of reaction, also could enhance the H₂ yields.Ni/Li₂O/REOx/γ-Al₂O₃ catalysts with different Li₂O contents and different REOx (La₂O₃ or CeO₂) contents were prepared by impregnation method and investigated the catalytic performance for POM in COG on BCFNO membrane reactor here. Results showed that the Ni/Li₂O/γ-Al₂O₃ catalysts modified by different CeO₂ contents were benefited to enhance oxygen permeation, and oxygen permeation increased with increasing the CeO₂ contents. For the Ni/Li₂O/γ-Al₂O₃ catalysts modified by different La₂O₃ contents, oxygen permeation increased firstly then decreased with increasing the La₂O₃ contents. And the effect of Li₂O on oxygen permeation was same to the Ni/Li₂O/γ-Al₂O₃ catalysts modified by La₂O₃.The catalytic performance for POM of COG of NiO/MgO solid solution catalyst packed on BCFNO membrane reactor is also investigated here. The reforming process is performed successfully: 94.1% of CH₄ conversion, 75% of H₂ selectivity, 108% of CO selectivity and 17.2 mL/cm2/min of oxygen permeation flux are achieved at 875 oC. The reaction has been steadily carried out for more than 100 h. The results showed that the NiO/MgO solid solution catalyst had a good catalytic activity and stability. Comparison over CoO/MgO solid solution and NiO/MgO solid solution catalyst for the performance of POM in BCFNO membrane reactor are also analyzed.According to the results of the reforming of COG in BCFNO membrane reactor, the reaction scheme was proposed that H₂ could be dissociated on the active Ni surface to H. Then H migrated to the"triphase boundary"reacted with lattice oxygen or oxygen ionic to form H₂O. The cracking of CH₄ also might be existed formed surface Ni ??? Cspecies and H. The H repeated former again. The oxidized product, H₂O can further react with the Ni ??? Cspecies, forming the metal Ni0, H₂ and CO. And also the H₂O formed could react with the residual CH₄ on the catalyst-bed to form H₂ and CO. The catalyst function was achieved in two regions in an oxygen permeation membrane reactor: H₂ dissociated and reacted with lattice oxygen or oxygen ionic to form H₂O near the membrane surface, The Hydrogen might come from the reaction gas or the cracking of CH₄ on catalyst. The H₂O formed could react with the residual CH₄ away from the membrane surface area.At last the steam reforming of COG for hydrogen production was investigated over the NiO/MgO solid solution catalysts reduced at high temperature. It was found that the NiO/MgO catalyst possessed good catalytic activity, and the conversions of CH₄ and CO₂ were greatly affected by the reaction temperature and water to methane (S/C) mole ratio. During the tested period of 100 h under a low S/C ratio of 1.0 at 875 oC, the conversions of CH₄ kept constant values around 97% and the hydrogen volume content was enhanced from 58.2% in the original COG to 77.7% by 1.5 times. These results show that the NiO/MgO catalyst was efficient and stable for the steam reforming of COG to amplify hydrogen in COG.