| Low-concentration methane always existed in coal mine and industrial waste gas. When it combusts over transition metal catalyst, the combustion characteristics and kinetic regions should be further investigated for methane. Meanwhile, the obtained low-concentration methane gas always contains a lot of moisture, and it is necessary to discover the water effect on methane catalytic combustion. Thus, it has significant academic meaning and project application value for low-concentration methane combustion over copper catalyst and its kinetic properties.In this paper, Cu/g-Al2O3 particle is used as catalyst for methane combustion. It has investigated catalytic combustion and kinetic characteristics of low-concentration methane in the fixed-bed reactor. Reaction parameters are both studied, such as surface microscopic structure, catalytic activity and surface reactants. This paper mainly studies the effect of surface oxygen for methane oxidation, and discusses the influence of oxygen coverage on methane catalytic activity, which can make the catalytic combustion region transfer as oxygen coverage change. And through the reversible adsorption and combination of surface oxygen with water, it has investigated water effect on methane catalytic activity and change of relative kinetic parameters.The results show that, it can be divided into three reaction regions according to reactant pressure by methane catalytic dehydrogenation and oxidation over Cu cluster: oxygen-enriched combustion(Region 1, O2/CH4 > 2), oxygen- deficiency combustion(Region 2, 0.1< O2/CH4 < 2), oxygen- lean combustion(Region 3, O2/CH4 < 0.1). Oxygen-enriched combustion region( ?+ ?), which presents methane catalytic activity is completely controlled by methane pressure, independent of oxygen pressure. Oxygen-deficiency combustion(?+ ?), which presents reaction rate is controlled by both methane and oxygen pressure. Oxygen- lean combustion( ?+ ?), which presents reaction rate is controlled by oxygen pressure, independent of methane pressure. Reaction order displays an obvious difference in the three regions: Region 1, =0.9, = 0; Region 2, = 0.43, = 0.68; Region 3, = 0, = 1.77. Activation energy is: 146.3 k J/mol, 99.8 k J/mol, and 60.8 k J/mol. From oxygen-enriched region to oxygen- lean region, rate determining step transfer from dehydrogenation of methane on surface oxygen to oxygen activation on metal clusters.As water is added into the feed, methane catalytic activity would be inhibited obviously. During the reaction, water can adsorb onto surface oxygen on the metal surface and form hydroxyl groups. However, each of these forms is not suitable for methane catalytic oxidation. If with additiona l water, methane conversion decrease quickly, and N2 purging almost fully recover methane catalytic activity. In oxygen-enriched conditions, water has no influence on reaction rate as oxygen pressure change. While, if methane pressure change, water has a s ignificant influence on reaction rate. As water pressure increase, methane reaction rate curve move down, and the slope decrease(reaction order decrease). K inetic studies show that, if with water, apparent activation energy of methane catalytic combustion increase quickly, water compete with methane to adsorb on surface oxygen, making methane activation energy increase and water reaction order increase either. At low temperature, water coverage is above 80% with water in the feed, while if without water, the coverage is only 30%. As temperature increase, the surface coverage of both conditions tends to the same. Our research provides theory basis for the efficient conversion and utilization of low-concentration methane. |
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