Test on combustion stability of small-sized biomass gas engine

Abstract: By using low temperature gasification, many kinds of biomass materials can be transformed into a gaseous mixture. This gaseous mixture includes combustible and incombustible gases. If the concentration of an individual component could be controlled, there is a possibility of using the gaseous mixture to fuel the internal combustion engine and finally to generate electricity. Furthermore, the utilizing of biomass fuel as an energy resource of internal combustion engine leads to a decrease in the emission of CO2, NOX, SOX, and particulate matters into the atmosphere.The components of biogas vary with the reactants and the reaction conditions. However, they usually include hydrogen (H2), carbon monoxide (CO), CH4, carbon dioxide (CO2), nitrogen (N2), and some other hydrocarbons. Under many conditions, the main components of biogas are CH4 and H2, as well as incombustible component CO2. Some researchers have carried out experimental researches to introduce the design and construction of internally circulating fluidized-bed gasifier (ICFG), to investigate the feasibility of gasifying manure compost using ICFG, and to evaluate the effects of pressure balance, reaction temperature, and steam ratio on the performance of the gasifier. The temperature conditions of the experiments were between 600℃ and 700℃ The results showed that the concentration of the components varied much with the reaction conditions.Different fuel components led to a different influence on combustion. Hydrogen has a much faster combustion velocity than CH4. As a result, combustion happens quickly with the increase in hydrogen composition in the fuel mixture. Meanwhile, carbon dioxide is an inert gas. It could cause an extinguishing of the flame if the composition of CO2 is too high. However, it is very difficult to avoid carbon dioxide in biogas. Moreover, the presence of CO2 leads to less input energy in fuel mixtures. Therefore, the combustion temperature will decrease. As a result, NOx emission will decrease.Since the influences of hydrogen and carbon dioxide on combustion are different, and the presence of CO2 also has benefits on engine performance, it is necessary to find out the balance point between combustion and exhaust emissions of the internal combustion engine which is fueled with the mixture of hydrogen, CH4 and carbon dioxide. And the main objective of the work in this paper is to research the influence of fuel components on engine operation stability.The experimental researches of engine operation stability were carried out based on a small-sized spark ignition engine fueled with simulated biogas, which includes hydrogen, CH4, and carbon dioxide. The influences of equivalence ratio, H2 concentration and CO2 concentration on combustion stability were investigated under wide opening throttle valve (WOT) and maximum brake torque (MBT) conditions. The results showed that engine operation stability was strongly influenced by fuel composition. Under low H2 concentration conditions, CO2 concentration played a dominant effect on combustion stability, and its increasing could induce the increase in cyclic variation and the occurrence of partial burn. However, under high H2 concentration, the effect of CO2 concentration on combustion gradually weakened. By adjusting fuel composition, the operation stability of biogas engines can be improved.