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Abstract
The paper presents the research results obtained in the process of oxidative coupling of methane, in which unpurified biogas was used as the feedstock. Biogas obtained from two kinds of biomass materials, i.e., plant materials (potato and beet pulp, Corn-Cob-Mix—biogas 1) and animal waste (waste from fish filleting—biogas 2) was considered. The influence of temperature, the ratio of methane/oxygen and total flows of feedstock on the catalytic performance in oxidative coupling of methane process was investigated. Comparative tests were carried out using pure methane and a mixture of methane-carbon dioxide to simulate the composition of biogas 2. The process was carried out in the presence of an Mn-Na2WO4/SiO2 catalyst. Fresh and used catalysts were characterised by means of powder X-ray diffraction, X-ray photoelectron spectroscopy, and low-temperature nitrogen adsorption techniques. In oxidative coupling of methane, the type of raw material used as the source of methane has a small effect on methane conversion (the differences in methane conversion are below 3%), but a significant effect on the selectivity to C2. Depending on the type of raw material, the differences in selectivity to C2 reach as high as 9%. However, the Mn-Na2WO4/SiO2 catalyst operated steadily in the tested period of time at any feedstock composition. Moreover, it was found that CO2, which is the second main component of biogas in addition to methane, has an effect on catalytic performance. Comparative results of catalytic tests indicate that the CO2 effect varies with temperature. Below 1073 K, CO2 exerts a small poisoning effect on methane conversion, while above this temperature the negative effect of CO2 disappears. In the case of selectivity to C2+, the negative effect of CO2 was observed only at 1023 K. At higher temperatures, CO2 enhances selectivity to C2+. The effect of CO2 was established by correlating the catalytic results with the temperature programmed desorption of CO2 investigation. The poisoning effect of CO2 was connected with the formation of surface Na2CO3, whose concentration depends on temperature.
Highlights
The oxidative coupling of methane has been extensively studied for over 30 years, but the feedstock was usually pure methane; one of the important sources of methane is biogas
The concentrations of volatile organic compounds (VOCs) can be high in biogas, especially if they came from organic materials with compounds originating from the biological degradation process, like aromatics and terpenes [8]
The analysis of low temperature N2 adsorption isotherms reveals that the deposition of manganese on silica support has only a small influence on the total pore volume (Vtotal) and specific surface area (SBET), while the deposition of the Na2WO4 phase leads to the destruction of a porous structure of the support
Summary
The oxidative coupling of methane has been extensively studied for over 30 years, but the feedstock was usually pure methane; one of the important sources of methane is biogas. Different organic feedstocks such as municipal, industrial, agricultural waste, waste from slaughterhouses, food waste products, and energy crops can be used [1,2,3,4,5,6]. Optimization of methane fermentation process of food waste products made it possible to obtain the biogas in an amount 740.4 cm3 · gODM−1 (ODM— Organic Dry Matter) with 68.6% methane, at the substrates mixture containing 50% of meat, 40% dairy products, 10% fruit and vegetables [2]. The high amounts of halogenated and organic silicon compounds in landfill biogas is largely the result from industrial waste [8]. The concentrations of VOCs can be high in biogas, especially if they came from organic materials with compounds originating from the biological degradation process, like aromatics and terpenes [8]
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