沼气组分比例对其燃烧稳定性及层流燃烧特性的影响

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

为了获得沼气主要组分比例所占比例对其燃烧稳定性与层流燃烧特性的影响规律,试验测量了当量比范围为0.8~1.3、初始压力范围为0.1~0.3 MPa、初始温度为320 K、甲烷体积分数分别为47%、55.5%、59%条件下沼气的火焰发展特性。同时,分析了沼气主要组分比例所占比例对沼气层流燃烧时的火焰稳定性与燃烧速度的影响规律。结果表明,当初始温度为320 K、当量比为1.0、初始压力为0.1 MPa时,在不同甲烷体积分数下火焰前锋面均较为光滑,并呈准球形向外发展;当初始压力升高至0.2 MPa时,不同甲烷体积分数下火焰前锋面均出现了裂纹与火焰突起,优先扩散不稳定性初步显示;当初始压力继续升高至0.3 MPa时,优先扩散不稳定性逐步明显,同时,浮力不稳定性将初步显示。随着当量比的增加、初始压力的降低或沼气中甲烷体积分数的增加,马克斯坦长度逐渐增大,火焰稳定性逐步增强。在各工况下无拉伸火焰传播速度与层流燃烧速度随当量比的升高呈现先增加后降低的趋势,在当量比为1.1时达到最大;同时,随着初始压力的降低或甲烷体积分数的升高,无拉伸火焰传播速度与层流燃烧速度逐渐增大。

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.