Estudio experimental de la inflamabilidad de mezclas gaseosas de CH4, H2 y CO2 en aire y su potencial energético

Abstract

The current environmental crisis has led the European Union to set ambitious decarbonisation targets for 2030 and 2050. Reaching them depends on the ability of researchers and industry to find new energy systems that allow a stable, efficient, and secure supply. In this scenario, renewable gases such as biogas and hydrogen are of great importance, although their integration into conventional energy system still must overcome technological, economic, and social barriers. Particularly it is essential to improve aspects such as biogas combustion efficiency, or hydrogen safety problems. Among all the techniques developed in recent years to improve these conditions, this work focuses on the enrichment of gaseous fuels with hydrogen. This technique has been studied and applied in recent decades to improve the combustion characteristics of natural gas, with up to 5% H2 currently being injected into the gas network. CH4-H2 mixtures therefore have great energy potential. Renewable energy production systems, such as biogas plants, contain this mixture together with other gases such as CO2, as it is the main component of biogas. CO2 is an inert gas with a great impact on combustion and on the flammability characteristics, so the main objective of this research is to establish the conditions for the safe use and optimal energy exploitation of methane-hydrogen mixtures in air, in the presence or not of carbon dioxide. To this end, an exhaustive experimental study has been carried out, focusing firstly on the flammability characteristics of CH4-H2-CO2 mixtures in air. Parameters such as the lower and upper flammability limits, the limiting oxygen concentration and the flammable regions of ternary and quaternary mixtures have been evaluated. The tests have been carried out in the spark test apparatus or STA, for which a new experimental method has been developed based on the graphical representation of the flammability parameters in ternary diagrams. In addition, for a better understanding of the dual effect of hydrogen and carbon dioxide in the quaternary mixture, a new three-dimensional graph is presented in this study. This graph is designed to evaluate the effect of the addition of H2 to the flammable region of CH4 in the presence of CO2. Finally, once the safe operating conditions of the quaternary mixture have been established, its combustion behaviour has been studied. Efficiency parameters such as temperature and flue gas composition, or combustion stability parameters such as ignition behaviour or flame structure have been analysed in a conventional 100 kW natural gas burner. The tests were performed for three different biogas compositions: BG70 (30% CO2), BG60 (40% CO2) and BG50 (50% CO2). To achieve higher flame stability, each biogas was enriched with hydrogen from 5% to 25%. The results reveal a volume of flammable mixtures lower than expected, which is mainly due to the oxygen demand of CH4 and H2 in the mixture. The flammability results obtained are closely related to the experimental combustion results, confirming the difficulty of burning biogas in conventional systems. After analysis of the results, it is observed that the addition of hydrogen leads to improvements in combustion parameters, which also leads to an increase in the flammable regions of methane in air. This effect is counteracted by CO2, destabilising the combustion, and reducing the temperature and therefore the usable energy. The optimal operating points are detailed in this study.