Basic Research On Chemical Chain Hydrogen Production From Coke Oven Gas

Coke oven gas(COG),as a byproduct of coal carbonization process,has a large yield but low utilization rate.Many surplus COG is directly burned or discharged into the air,resulting in great waste of resources and serious environmental pollution.The conversion of COG to value-added products has been attracted much attention from both economic and environmental views.In this paper,we propose to use chemical looping hydrogen generation technology to produce pure H₂ via COG as a feedstock.Since oxygen carriers play an important role in the chemical looping hydrogen production process of COG,many different types of oxygen carriers have been studied in this paper.A series of La_1-xSr_xFe O₃(x=0,0.2,0.3,0.4,0.5,0.6)perovskite oxides were prepared by coprecipitation as oxygen carriers.The effect of Sr content on the physicochemical properties of these perovskite-type oxides are studied.The reduction behaviors of La_1-xSr_xFe O₃ perovskite by different reducing gases(H₂,CO,CH₄ and the mixed gases)are also investigated to discuss the competition effect of different components in COG for reacting with the oxygen carriers.The results show that reduction temperature of H₂ and CO is much lower than that of CH₄,and high temperatures(>800°C)are requested for selective oxidation of methane to syngas.The co-existence of CO and H₂ shows weak effect on the equilibrium of methane conversion at high temperatures,but the oxidation of methane to syngas can inhibit the consumption of CO and H₂.The doping of suitable amounts of Sr in La Fe O₃perovskite(e.g.,La_0.5Sr_0.5Fe O₃)significantly promotes the reactivity for selective oxidation of methane to syngas and inhibits the formation of carbon deposition,obtaining high methane conversion in the COG oxidation step and high hydrogen yield in the water splitting step.The La_0.5Sr_0.5Fe O₃ shows the highest methane conversion(67.82%),hydrogen yield(3.34 mmol/g)and the purity of hydrogen(99.85%).The hydrogen yield in the water splitting step is treble as high as the hydrogen consumption in the reduction step.In addition,O₂-assistant chemical looping hydrogen generation process improves the redox stability of oxygen carrier.These results reveal that La_0.5Sr_0.5Fe O₃ can be used to prepare pure H₂ by chemical looping of COG.A series of La Ni_xFe_1-xO₃(x=0,0.01,0.03,0.05,0.07,0.1)perovskite oxides were prepared by sol-gel method as oxygen carriers.The effect of Ni doping on the reactivity of perovskite oxygen carrier was studied,the results showed that Ni doping increased the reactivity of oxygen carrier,However,excessive Ni content in the reaction process is easy to lead to carbon deposition and metal sintering problems.Finally,the optimal reduction time between La Ni_xFe_1-xO₃ perovskite oxygen carrier and COG was 30min,the optimal reaction temperature was 800°C,and the optimal oxygen carrier was La Ni_0.07Fe_0.093O₃.At this time,the highest conversion rate of methane in the reduction stage reached 97.61%,the maximum yield of hydrogen was8.46mmol/g,and the maximum yield of carbon monoxide was 2.64 mmol/g,produces a syngas with H₂/CO greater than 3.The maximum yield of hydrogen produced in the oxidation stage is 3.55 mmol/g,the carbon monoxide content is 0.017 mmol/g,so the hydrogen purity reaches 99.52%.Meawhile,30 chemical looping circulation reactions were performed on La Ni_0.07Fe_0.93O₃ perovskite oxygen carrier,the result showed that La Ni_0.07Fe_0.93O₃ perovskite oxygen carrier has good cycling stability,during the whole cycles,the experimental results remained stable without a significant downward trend,and were further confirmed by XRD and SEM results.In addition,when La Ni_0.07Fe_0.93O₃ perovskite oxygen carrier is used in the chemical looping reaction process,the reduced oxygen carrier can be oxidized back to the original state without additional auxiliary oxidation reactor.The result showed that La Ni_0.07Fe_0.93O₃ also can be used to prepare pure H₂ by chemical looping of COG.