Dry Reforming of Biogas to Syngas: An Eco-Friendly Renewable Fuel for I C Engines

Abstract

Abstract Biogas, a promising alternative fuel, a substitute for fossil fuels, is predominantly a mixture of methane and carbon dioxide. Both are major greenhouse gases. Methane has a long-term effect on the environment while carbon dioxide is recycled by the plants. Hence, capture and burning of biogas to consume methane as a fuel is desired both from energy and environmental outlook. The presence of a large amount of carbon dioxide in biogas, however, impairs combustion in engines resulting into slow burning and higher hydrocarbon and carbon monoxide emissions. Dry reforming, a conversion process of biogas to synthesis gas (syngas), a mixture of hydrogen and carbon monoxide, is a catalytic process that has the potential to greatly improve biogas combustion in engines. The researchers’ focus in dry reforming, however, has been for the generation of hydrogen for fuel cells and reactants for Fischer Tropsch process in industry — this approach aims towards maximum conversion of methane and carbon dioxide. The work presented here investigates the possibility of partial conversion of biogas to harness maximum energy and reduce emissions from I.C. Engines. The published research on dry reforming of biogas has examples of high concentrations of methane in the syngas with calorific values suitable for I.C. Engine application. For example, a 50:50 v/v CH4/CO2 composition biogas has calorific value of 13.33 MJ/kg which when converted to a syngas at 550°C results in a gas with 18:42:14:26 v/v CO2/CH4/H2/CO and a calorific value of 19.96 MJ/kg). Such compositions have moderate percentage of hydrogen to act as combustion enhancer and the carbon dioxide present helps to control NOx emissions. The major contributors of energy are methane and carbon monoxide in these cases. The dry reforming reaction is an endothermic reaction, which produces hydrogen. The side reactions that happen are the reverse water gas shift reaction, which reduces hydrogen yield and the Bouduard reaction which results in carbon deposition on the catalyst surface. The reactor conditions need to be chosen appropriately, especially the reactor temperature. Simulation of dry reforming reaction using a process simulation software (Aspen Plus) is carried out to find the extent of conversion and exit syngas composition for different biogas compositions. The endothermic heat for the reactor can be provided by the heat of the engine exhaust — therefore, an opportunity exists to use waste heat recovery from the engine exhaust. However, there is a tradeoff between the reactor temperature, syngas composition going to engine inlet and the engine exhaust heat — which is investigated in this study.