MCS 7
Chia Laguna, Cagliari, Sardinia, Italy, September 11-15, 2011
Experimental investigation of the effect of nitrogen dilution on
NOx emission in propane-air non-premixed flame
Abdolrasoul Rangrazi* Hamid Niazmand* and Hamid Momahedi Heravi**
[email protected]
* Department of Mechanical Engineering, Ferdowsi University of Mashhad, Mashhad, Iran
** Department of Mechanical Engineering, Islamic Azad University, Mashhad Branch, Iran
Abstract
NOx 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
NOx 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 NOx emission decreases. In this paper experimental investigation of
nitrogen dilution effects on NOx 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 NOx emission with
an average of 64%.
INTRODUCTION
NOx emission, as one the main pollutants resulting from fossil fuel combustion, have been
always studied by researchers in the field of combustion. NOx 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 NO2 are the most important oxides of nitrogen and these kind of nitrogen oxides
are generally called NOx 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 NO2.
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 NOx 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 NOx 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 CO2, N2, H2 , H2O 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 NOx emission.In 2006 Giles et al.[4]
numerically investigated the effect of dilution on non-premixed flame using H2O, CO2 and N2
as diluent. Their results showed that the H2O and CO2 due to higher heat capacities are more
effective than N2 to reduce NOx emission and temperature of the combustion chamber. In
2005 Cho et al.[5] numerically investigated the effect of N2 and CO2 diluents on the NOx
reduction. They concluded that diluting with CO2 is more effective in NO reduction compared
to N2 because of the large temperature drop due to the larger specific heat of CO2.
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 N2 on NOx emission in a furnace with natural gas fuel.
Their results showed N2 dilution reduces NOx emission more than 60%.
In this study experimental investigation of nitrogen dilution on NOx 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 NOx 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 NOx
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 NOx 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
n Fuel
(1)
Figure 3 shows the variation of NOx 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 NOx emission and temperature increase. With
increasing the equivalence ratio from 1 to 1.3 NOx 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 NOx 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, NOx
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 NOx emission in various ranges of dilution ratio in
  0.7,0.8,0.9,1
Figure 5. Effect of nitrogen dilution on NOx 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
0.8
0.9
1
1.1
1.2
1.3
1.2
1.4
1.8
2
2
1.7
1.4
CONCLUSIONS
Nitrogen dilution increases thermal capacity of the fuel-air-nitrogen and decreases NOx
emission with an average of 64%. It should be mentioned that dilution causes flame extinction
in particular dilution ratio.
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