摘要
由于天然气应用的广泛性及其储量有限,生物质低热值燃气的应用日益受到关注。但低位热值的大幅降低和组分的多变性,导致生物质低热值燃气在天然气使用场合的应用受到阻碍和质疑。基于此背景本文利用热通量燃烧器,测量了低热值燃气在常温常压状态下的层流燃烧速度,同时还通过本生燃烧器和同心锥形燃烧器,产生喷射火焰和喷嘴火焰,利用激光手段研究几种典型生物质低热值燃气的火焰稳定性和火焰结构。与此同时使用大涡方法数值计算软件模拟部分实验火焰的火焰结构与流场情况。本研究旨在逐步建立完整的生物质低热值燃气燃烧机理和火焰特性的基础数据库,为将来低热值燃气的广泛运用奠定基础。
研究结果显示使用GRI 3.0模拟生物质-甲烷燃气所得结果与实验数据以及文献中的数据吻合。对GG-S和GG-V燃气当量比低于1的情况,实验数据与GRI 2.11模拟结果吻合,但是GRI 3.0所得结果比实验结果略低;当量比大于1的情况,两种机理所得模拟结果均低于实验结果。GG-S和GG-V燃气的实验数据比文献中数据略高。由于低热值燃气中有氢气和一氧化碳的存在,故层流火焰燃烧速度的最大值出现在燃气较富集的区域。GRI机理可较好的预测GG-S和GG-V层流火焰燃烧速度出现最大值的区域,但不能准确预测LCV1和LCV2层流火焰燃烧速度出现最大值的区域。
对于低热值燃气的火焰稳定性和火焰结构,研究证明喷嘴火焰比喷射火焰更具稳定性。对于喷射火焰,低位热值相同的稀释甲烷气体和LCV1比较,LCV1具有更高的临界起升和吹熄速度,这是因为LCV1中含有氢气。对于喷嘴火焰,火焰稳定性对燃料组分不敏感,这是因为喷嘴火焰与喷射火焰的稳定机理不同。从PLIF图像中可看到不同燃料的喷嘴火焰基本稳定在相同的位置。大涡模拟结果显示,火焰稳定在空气卷吸的回流区,这与锥形喷嘴的角度有关而与燃料组分、燃气出口雷诺数无关。
对于生物质-甲烷燃气的火焰稳定性及局部熄灭,研究证明喷射火焰的火焰稳定性与燃气预混程度与出口雷诺数有关。一般情况下,预混火焰空气预混量的降低会增强火焰的稳定性。OH自由基分布说明,即使是远离吹熄临界线的火焰,仍然存在火焰空穴现象。逐渐增加空气量,火焰空穴会促使火焰的整体熄灭。局部火焰熄灭现象通常发生在局部流体速度高,与火焰发生碰撞的区域。
Biomass derived gases from gasification, pyrolysis, and landfills are renewable and CO_2 neutral fuels that have great potential to be usable in internal combustion engines, gas turbines, and industrial furnaces. It is important to know well about the base characteristics of the gases with low calorific value. Laminar burning velocities of five biomass derived gases have been measured at atmospheric pressure over a range of equivalence ratios, using the heat flux burner. Experimental studies about the stabilization of partially premixed turbulent flames with different biomass derived gases are carried out in a conical burner. Flame stabilization behavior with and without the cone is investigated and significantly different stabilization characteristics are observed in flames. Planar laser induced fluorescence imaging of a fuel-tracer species, acetone, and OH radicals is carried out to characterize the flame structures. Large eddy simulations of the conical flames are conducted to gain further understanding of the flame/flow interaction in the cone.
For measurements of laminar burning velocity, the results of the bio-methane flame are generally in good agreement with data in the literature and the prediction using GRI-Mech 3.0. The measured laminar burning velocity of the industrial gasification gas is generally higher than the predictions from GRI-Mech 3.0 mechanism but agree rather well with the predictions from GRI-Mech 2.11 for lean and moderate rich mixtures. For rich mixtures, the GRI mechanisms underpredict the laminar burning velocities. For the model gasification gas, the measured laminar burning velocity is higher than the data reported in the literature. The peak burning velocities of the gasification gases/air and the co-firing gases/air mixtures are in richer mixtures than the biomethane/air mixtures due to the presence of hydrogen and CO in the gasification gases. The GRI mechanisms could well predict the rich shift for the pure gasification gases but failed for the cofiring gases mixtures. The laminar burning velocities for the bio-methane at elevated initial temperatures are measured and compared with the literature data.
For flame stabilization and flame structure measurement, the data show that the flames with the cone are more stable than those without the cone. Without the cone (i.e. jet burner) the critical jet velocities for blowoff and liftoff of biomass derived gases are higher than that for methane/nitrogen mixture with the same heating values, indicating the enhanced flame stabilization by hydrogen in the mixture. With the cone the stability of flames is not sensitive to the compositions of the fuels, owing to the different flame stabilization mechanism in the conical flames than that in the jet flames. From the PLIF images it is shown that in the conical burner, the flame is stabilized by the cone at nearly the same position for different fuels. From large eddy simulations, the flames are shown to be controlled by the recirculation flows inside cone, which depends on the cone angle, but less sensitive to the fuel compositions and flow speed. The flames tend to be hold in the recirculation zones even at very high flow speed. Flame blowoff occurs when significant local extinction in the main body of the flame appears at high turbulence intensities.
The stabilization characteristics and local extinction structures of partially premixed bio-methane/air flames are studied using simultaneous OH-PLIF/PIV techniques. The stability regime of flames is determined for different degree of partial premixing and Reynolds numbers. It is found that in general partially premixed flames are more stable when the level of partial premixing of air to the fuel stream decreases. For the studied burner configuration at high Reynolds numbers there is an optimal partial premixing level of air to the fuel stream at which the flame is most stable. OH-PLIF images revealed that for the stable flames not very close to the blowout regime, significant local extinction holes appear already. By increasing premixing air to fuel stream successively, local extinction holes develop leading to eventual flame blowout. Local flame extinction is found to frequently attain to locations where locally high velocity flows impinging to the flame. The local flame extinction poses a future challenge for model simulations and the present flames provide a possible test case for such study.
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