MCS 7 Chia Laguna, Cagliari, Sardinia, Italy, September 11-15, 2011 Experimental investigation of the effect of nitrogen dilution on NO x emission in propane-air non-premixed flame Abdolrasoul Rangrazi* Hamid Niazmand* and Hamid Momahedi Heravi** rasoul_rangrazi@yahoo.com * Department of Mechanical Engineering, Ferdowsi University of Mashhad, Mashhad, Iran ** Department of Mechanical Engineering, Islamic Azad University, Mashhad Branch, Iran Abstract NO x emission, as one the main pollutants resulting from fossil fuel combustion, have been always studied by researchers in the field of combustion. One of the methods to reduce the NO x emission is dilution. Dilution increases the heat capacity of the mixture in the combustion chamber, which in turn, decreases the temperature of the combustion chamber and consequently, the NO x emission decreases. In this paper experimental investigation of nitrogen dilution effects on NO x emission production in propane-air non-premixed flame in a cylindrical combustion chamber is conducted. Dilution is performed over a wide range of dilution and equivalence ratios. Results show nitrogen dilution decreases NO x emission with an average of 64%. INTRODUCTION NO x emission, as one the main pollutants resulting from fossil fuel combustion, have been always studied by researchers in the field of combustion. NO x is the reason of ground-level ozone formation. This pollutant has harmful effects on the respiratory system of human beings and plant growth. It also causes acid rain and photochemical smog. NO and NO 2 are the most important oxides of nitrogen and these kind of nitrogen oxides are generally called NO x emissions. NO is a toxic gas which is formed during combustion at high temperature zones in the combustion chamber (i.e. the automobile engines, power plants and furnaces) While entering into the atmosphere NO changes into NO 2. Generally three mechanisms for production of NO in combustion are considered: Thermal NO, Fuel NO and Prompt NO. Thermal NO is formed at very high temperatures and is a result of the oxidation of the diatomic nitrogen found in combustion air. Thermal NO is a function of the temperature and the residence time of the nitrogen at that temperature. Fuel NO is formed when the nitrogen in fuels combines with the oxygen in the air. Prompt NO is formed in the earliest stage of combustion. Prompt NO is made by the reaction of atmospheric nitrogen with radicals in the air[1]. One of the methods to reduce the NO x emission is dilution. Adding a diluent with a high heat capacity increases the total heat capacity of the fuel-air-diluent mixture and as a result the mixture absorbs more heat during the combustion and reduces the temperature of combustion chamber. This leads to reduction of NO x emission production which is related to temperature decrease[2]. Until now there have been various types of diluents which have been studied in the field of combustion such as CO 2, N 2, H 2, H 2 O and their effects on the combustion parameters have been studied. In 2009 Kobayashi et al.[3] investigated dilution effects of superheated water vapor on turbulent premixed flames at high pressure and high temperature. Their studies showed that water vapor plays an important role in reducing of NO x emission.in 2006 Giles et al.[4] numerically investigated the effect of dilution on non-premixed flame using H 2 O, CO 2 and N 2 as diluent. Their results showed that the H 2 O and CO 2 due to higher heat capacities are more
effective than N 2 to reduce NO x emission and temperature of the combustion chamber. In 2005 Cho et al.[5] numerically investigated the effect of N 2 and CO 2 diluents on the NO x reduction. They concluded that diluting with CO 2 is more effective in NO reduction compared to N 2 because of the large temperature drop due to the larger specific heat of CO 2. In 2005 Prathap et al.[6] studied effects of nitrogen dilution on the laminar burning velocity and flame stability of syngas fuel. Their results showed that the nitrogen dilution decreases laminar burning velocity due to reduction in heat release and increased heat capacity of unburned gas mixture and hence the flame temperature. In 2004 Salvador et al.[7] investigated the effect of dilution using N 2 on NO x emission in a furnace with natural gas fuel. Their results showed N 2 dilution reduces NO x emission more than 60%. In this study experimental investigation of nitrogen dilution on NO x emission production in propane-air non-premixed flame is conducted. Non-premixed flames have many applications in jet engines, diesel engines, boilers, furnaces, etc. In this kind of flame, fuel and air are mixed after entering the combustion chamber[8]. Results show for non-premixed combustion, fuel dilution is more effective in reducing NO x emission than air dilution[9]. In this paper the fuel which has been tested is propane. Propane is one of the gases which are widely used in domestic and industrial applications. This gas easily changes to liquid under pressure which makes it easy for transportation. Wide applications of propane in furnaces, cooking stoves, water heaters, etc. have resulted in many investigations for studying its combustion characteristics and controlling the emission production of this fuel[10]. Experimental setup As shown in Figures 1 and 2 the studied furnace in this investigation has a cylindrical combustion chamber with a 1m length and inner radius of 0.105m made of Steel AISI316 which is completely isolated during the experiments. In order to see the flame 15 holes with 0.02m diameter at the top of the combustion chamber are generated. The distance between each of the holes is 0.07m. Propane enters to the combustion chamber with a pipe of 0.004m diameter which is surrounded by the entrance of air with a diameter of 0.035m. Propane and air enter the combustion chamber in axial direction. A collector is used to mix propane and nitrogen. Figure 1. Experimental set up of the combustion chamber
A gas analyzer (TESTO350XL) with an accuracy of 0.05 ppm is used for measuring NO x emission amount in the exhaust gases. Two Rotameters with an accuracy of 0.02 lit/min were used to measure the flow rate of nitrogen and propane. A flow meter with an accuracy of 0.01 to 0.05 m/s was used to measure flow rate of air. Figure 2. Schematic of experimental setup Experimental Results In this experimental study dilution is conducted over a wide range of dilution and equivalence ratios and the amount of NO x emission is measured in the outlet of the combustion chamber. Dilution ratio is defined as the fraction of diluent (nitrogen) moles to the fuel (propane) moles: n Diluent (1) n Fuel Figure 3 shows the variation of NO x emission in different equivalence ratios and without any dilution. As it can be seen in the figure, with increasing equivalence ratio from 0.7 to 1 and approaching to the Stochiometry mode, the NO x emission and temperature increase. With increasing the equivalence ratio from 1 to 1.3 NO x emission decreases which is due to the increase in the ratio of fuel to air and the fact that we are away from the Stoichiometry state. This also reduces the temperature in the combustion chamber.
Figure 3. Effect of equivalence ratio on NO x emission without nitrogen dilution Figures 4 and 5 show the effect of nitrogen dilution on reducing Nox emission in different equivalence ratios. As it can be seen in these figures with increasing the dilution ratio, NO x emission decreases with an average of 64% due to the increased heat capacity of fuel-airdiluent mixture. Dilution also reduces the temperature of the combustion. Figure 4. Effect of nitrogen dilution on NO x emission in various ranges of dilution ratio in 0.7,0.8,0.9,1
Figure 5. Effect of nitrogen dilution on NO x emission in various ranges of dilution ratio in 1.1,1.2,1.3 Experimental results show that in 1 nitrogen dilution leads to decreasing of mass fraction of fuel in the mixture which in turn, cause flame extinction. This flame extinction also occurs in 1 due to reduction of air. Table 1 shows the extinction limitation of the flame in different equivalence ratios. Table1. Flame extinction limits in various equivalence ratio 0.7 1.2 0.8 1.4 0.9 1.8 1 2 1.1 2 1.2 1.7 1.3 1.4
CONCLUSIONS Nitrogen dilution increases thermal capacity of the fuel-air-nitrogen and decreases NO x emission with an average of 64%. It should be mentioned that dilution causes flame extinction in particular dilution ratio. REFERENCES [1] Liuzzo, G., Verdone, N., Bravi, M., The Benefits of Flue Gas Recirculation in Waste Incineration, Waste Management, 27 :06-116 (2007) [2] Kim, H. K., Kim, Y., Lee, S. M., Ahn, K. Y., NO Reduction in 0.03-0.2 MW Oxy-Fuel Combustor using Flue Gas Recirculation Technology, Proceedings of the Combustion Institute, 31: 3377-3387 (2007) [3] Kobayashi, H., Yata, S., Ichikawa, Y., Ogami, Y., Dilution Effects of Superheated Water Vapor on Turbulent Premixed Flames at High Pressure and High Temperature, Proceedings of the Combustion Institute, 32: 2607-2614 (2009) [4] Giles, D. E., Som, S., Aggarwal S. K., NOx emission characteristics of counterflow syngasdiffusion flames with airstream dilution, Fuel, 85: 1729-1742 (2006) [5] Cho, E. S., Chung, S. H., Numerical Study on No Emission with Flue Gas Dilution in Air and Fuel Sides, Mechanical Science and Technology, 19:1358-1365 (2005) [6] Prathap, C., Ray, A., Ravi, M. R., Investigation of Nitrogen Dilution Effects on the Laminar Burning Velocity and Flame Stability of Syngas Fuel at Atmospheric Condition, Combustion and Flame, 155: 145-160 (2008). [7] Salvador, S., Kara, Y., Commandre, J. M., Reduction of NO Emissions from a VOC Recuperative Incinerator by Dilution of the Fuel Supply, Applied Thermal Engineering, 24: 245-254 (2004) [8] Warnatz, J.,.Mass, U., Dibble, R. W., Combustion, 3 th edition, Springer-Verlag, Berlin, (2001). [9] McTaggart-Cowan, G. P., Rogak, S. N., Hill, P. G., Munshi, S. R., and Bushe, W. K., Fuel Dilution Effects in a Direct Injection of Natural Gas Engine, Automobile Engineering, 222: 441-453 (2008). [10] Tang, C., Zheng, G., Huang, Z., and Wang, J., Study on Nitrogen Diluted Propane air Premixed Flames at Elevated Pressures and Temperatures, Energy Conversion and Management, 51: 288-295 (2010).