Effects of composition ratios on burning stability and laminar burning characteristics of biogas

Abstract: For the purposes of reducing CO2 emissions during combustion of fossil fuels and removing wastes due to environmental and health concerns, biomass derived gas (biogas), an attractive and potentially renewable energy source from wood, waste, landfill, and alcohol fuels via gasification, pyrolysis or fermentation processes, has been widely studied as a supplement for fossil fuels. At present, the fuel used for the gas turbine for power generation is generally natural gas or light diesel. For the gas turbine using natural gas as fuel, when the fuel is changed from natural gas to biogas, the fuel composition changes, and the calorific value decreases. Moreover, the requirements of environmental protection, as well as the reliability and economy of the gas turbine need to be met. At the same time, the content of each component in biogas varies significantly due to the influence of many factors (such as pyrolysis temperature and feedstock), which have a significant effect on its calorific value, ignition characteristics, and combustion characteristics. Therefore, it is very necessary to have a full understanding of the effects of the main compositions ratios on the combustion characteristics of biogas (such as the flame propagation characteristics, the combustion stability, laminar and turbulent combustion velocities etc.) in order to adjust and improve the combustion chamber structure and operating parameters of the gas turbine. The experimental methods used to measure the combustion characteristics of fuel included the hedging flame, Bunsen burner, and the constant volume bomb method. Compared with other testing methods, the constant volume bomb method has been widely applied because it can conveniently analyze the flame stretching effect and flame stability during flame propagation. In this paper, the effects of the main compositions ratios on the combustion stability and laminar burning characteristics of biogas, the flame propagation characteristics of biogas over the equivalence ratio range of 0.8-1.3, the pressure range of 0.1-0.3 MPa, the temperature of 320 K, and the methane volume fractions of 47%, 55.5% and 59% were measured in the constant volume bomb. Furthermore, the effects of main compositions ratios on the combustion stability and burning velocity of biogas in laminar combustion were investigated. The results showed that the flame front was kept smoothly and quasi-spherical outward was developed at the conditions that the initial temperature was 320 K, the equivalence ratio was 1.0, the initial pressure was 0.1 MPa, and different methane volume fractions. With the initial pressure increasing to 0.2 MPa, cracks and flame bumps appeared on the flame front at different methane volume fractions and the preferential diffusion instability would appear. With the initial pressure increasing to 0.3 MPa, the preferential diffusion instability would become obvious and the buoyancy instability would initial appear. At the same time, increasing the equivalence ratio or decreasing the initial pressure or increasing the methane volume fractions increased the Markstein length and the stability of the flame front was increased. With the equivalence ratio increasing, the unstretched flame propagation speed and the laminar burning velocity illustrated an increase initially and then decreased gradually, and the maximum values were measured at the condition of the equivalence ratio was 1.1 at all conditions. Furthermore, the unstretched flame propagation speed and the laminar burning velocity of biogas increased with the initial pressure decreasing or the methane volume fractions increasing.