Combustion, thermal and emission characteristics of gas-fired inverse diffusion flames burning mixed LPG/hydrogen fuel

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Combustion, thermal and emission characteristics of gas-fired inverse diffusion flames burning mixed LPG/hydrogen fuel

 

Author: Miao, Jing
Title: Combustion, thermal and emission characteristics of gas-fired inverse diffusion flames burning mixed LPG/hydrogen fuel
Degree: Ph.D.
Year: 2015
Subject: Jets -- Fluid dynamics.
Heat -- Transmission.
Combustion engineering.
Flame.
Hong Kong Polytechnic University -- Dissertations
Department: Dept. of Mechanical Engineering
Pages: xix, 242 pages : color illustrations
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b2840540
URI: http://theses.lib.polyu.edu.hk/handle/200/8425
Abstract: As a widely applied flame type, gas-fired diffusion flame has the advantage of wide stable operation range. However, the high pollutant emission rate is a noteworthy shortcoming of diffusion flame, and makes diffusion flame not applicable for many industries requiring low air pollutant emissions. Gas-fired premixed flame, however, has very strict stable operation range hence limited application. In the present investigation, an inverse diffusion flame (IDF) burner is used. The burner is featuring with a relatively large air jet in the center of the burner; and the fuel mixture, which is not premixed with air, is supplied to surround the central air jet. When velocity of central air jet is high enough, there will be large pressure difference between the air jet and fuel jets, and the pressure difference will entrain fuel as well as surrounding ambient air into the central air jet to form the flame base. The interaction between air and fuel in the entrainment zone can improve mixing process of air and fuel, and gives IDF the nature of diffusion flame as well as enhanced mixing feature of premixed flame. Therefore, IDF may be regarded as a partially premixed flame, and is expected to have the advantages of both diffusion flame and premixed flame: wide stable operation range and low pollutant emissions. As a compromise of diffusion flame and premixed flame, IDF can be applied to both small and large industrial burners burning gaseous fuels, which are usually hydrocarbon fuels. Although being widely used in combustion, hydrocarbon fuels have the shortcomings of too high fuel-lean flammability limit and inevitable high CO, CO₂ and HC emissions. Liquefied petroleum gas, which is one of the most widely used gaseous fuels in Hong Kong,also possesses the drawbacks of hydrocarbon fuels as mentioned above. In addition to optimization of the structure of IDF burner, improving the fuels used may be a possible method to further enhance the combustion performance and to further widen applications of IDF. Hydrogen fuel (H₂), with high flame temperature, none carbon content and high burning velocity is regarded as a very possible gaseous fuel to improve the properties of hydrocarbon fuels. Although hydrogen addition seems to be a promising solution to enhancing flame temperature and hence thermal performance and flame stability, and to reducing pollutant emissions of hydrocarbon fuels in both premixed flame (Zhang et al., 2008) and normal diffusion flame (Karbasi, 1998), no relevant research work has yet been done on effects of hydrogen addition in partially premixed flame, especially in inverse diffusion flame. The very particular air/fuel mixing mechanism of inverse diffusion flame may result in special phenomenon after adding hydrogen into the gaseous fuel. In fact, there are some contradictory results already reported by different researchers, who have applied various experimental methods and used different gaseous fuels together with hydrogen (Leung and Wierzba, 2008; Miao et al., 2009). Therefore, a comprehensive study is necessary to fully determine the actual influence of hydrogen addition on LPG fuel in inverse diffusion flame.
In this study, experimental investigation has been conducted to fully assess the effects of hydrogen addition on the combustion, thermal and emission characteristics of LPG-H₂ IDF regarding laminar burning velocity, flame stability limits, flame temperature, OH-PLIF distribution and pollutant emission rate. The experimental results show that LPG is dominative in determining laminar burning velocity of LPG-H₂ mixture, and small amount of LPG can successfully impede the burning of H2. In addition, hydrogen addition can significantly expand the fuel-lean flame stability limits in terms of air jet Reynolds number and overall equivalence ratio of the air/fuel jets, and can make LPG IDF more tenacious. Also, LPG-H₂ IDF has wider flame reaction zone marked by applying planar laser-induced fluorescence of hydroxyl radical (OH-PLIF) than LPG IDF does under similar operation conditions, indicating more intensive combustion due to H₂ addition. It is also found that there is a remarkable difference between OH-PLIF results of 100%H₂ IDF and 10%LPG-90%H₂ IDF. The reflux of flame, which forms concentric double flame structure in H₂ IDF has not been seen in 10%LPG-90%H₂ IDF. Another positive effect of H₂ is that H₂ addition actually reduces the carbon-led pollutant emissions of the flame. Experiment result shows that with the use of H₂ addition, the emission rates of HC and CO decrease, while emission rate of CO₂ rises, suggesting that H₂ addition can also facilitate the transformation of emissions from HC and CO to CO₂ and thus perfects the combustion characteristics. In the present study ,there are some results rather different from expectation. Addition of up to 60% H₂ does not show notable effect on flame structure of LPG IDF, and high percentage of H₂ mainly changes the color of outer layer of the flame. The effect of H₂ on flame height of LPG IDF highly depends on percentage of hydrogen. For low H₂%, flame height lengthens with H₂ addition; while for high H₂%, H₂ addition can shorten the flame. The influence of H₂ addition on flame temperature of LPG IDF is almost not notable. Both LPG IDF and 50%LPG-50%H₂ IDF have very similar values of flame temperature, as well as similar temperature distribution profiles. The effect of H₂ on laminar burning velocity of LPG is also not significant when H₂ addition is less than 60%.Conclusively, LPG-H₂ IDF has similar flame structure, flame length, and temperature distribution with those of LPG IDF; however, LPG-H₂ IDF has wider stability range, more intensive and complete combustion, as well as lower pollutant emission rates.

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