1. The Effect of Dual Injection
on Combustor Emissions
Introduction
Combustion is a series of chemical reactions that involves the burning of
fuel inside an engine to provide power to a system. In the process,
combustion also emits harmful gases into the environment such as CO
and NO. Engineers and researchers are always looking for ways to make
the combustion process more efficient and less harmful to the
environment. In a combustor, fuel and air are mixed to allow for the
combustion reaction to take place. However, the mixing of fuel and air
that takes place in a combustor is inadequate and it results in hotspots
(local spot of high temperature). This increases the amount of pollutants
formed, especially NOx, which is a strong function of the temperature.
The standard combustor geometry is a cylinder with one air inlet and
one fuel inlet. Altering the combustor design to allow for two sets of air
and fuel inlets has the potential to enhance mixing, reduce the
frequency of these hotspots, and ultimately reduce harmful emissions.
The dual injection combustor has air inlets on the top and
bottom and fuel inlets on each side.
Methodology
The combustor was secured into the test rig and all air and fuel lines
were sealed in order to prevent leakage of any air or fuel.
The combustor was ignited with a propane torch and air and fuel rates
were increased to desired values.
The air-fuel flow rate was set to 0.8 using the flow controllers. The
combustor was allowed to run at this ratio for about three to four
minutes to allow for the reading on the gas analyzer to stabilize.
Emissions of NO, CO, CO2, and O2 were measured using the gas analyzer.
Steps 4 and 5 were repeated for pressure ratios of 0.7, 0.6, 0.5, and 0.4.
Prior to experimentation, preliminary data was collected to determine
the performance of a standard, single-injection combustor. Combustor
emissions were measured and compared against this preliminary data.
Emissions were measured at different air temperatures and fuel-air
pressure ratios.
Conclusion
In this study the effect of dual injection on combustor emissions was
tested. The purpose of this experimentation was to potentially enhance
the combustion process, making it more efficient and environmentally-
friendly. Any positive findings could be applied to gas turbines
specifically in aircrafts. Enhanced combustion could result in better fuel
efficiency for such vehicles and even in automobiles as well.
According to the data, the dual injection system was unsuccessful in
enhancing the mixing of air and fuel inside a combustor. The carbon
oxide emissions of dual injection were generally the same as
demonstrated with single injection, but the emissions of nitrogen oxides
increased significantly.
The results of tests with dual injection can be taken into account when
testing other methods of improving combustion. It is important to note
the negative interaction between the two air-fuel injection points. The
addition of even more inlets in the combustor design can now be
eliminated since this method proved to negatively impact the system.
To avoid such unequal distribution, fuel flow rate distribution can be
modified. Instead of air and fuel being split evenly between both
injection points, the amount of fuel for each injector can be manipulated
to control the strength of each reaction zone. Such fuel variation has the
potential to change the local equivalence ratio of the jet affecting
different flame characteristics such as flame speed.
Varying the fuel amount can impact the resulting emissions and the
strength of the reaction zone. Consequently, fuel variation can be used
as means to control flame characteristics and emissions to produce
enhanced performance and lower emissions than those demonstrated
through single injection. This is of extreme importance for combustion
research as multiple injectors will be required to maintain adequate
residence time and injection velocities within the combustor.
Findings
Dual injection produced significantly higher emissions of nitrogen oxides
than single injection at all equivalence ratios except for 0.5. The dual
injection system recorded its lowest emissions at an air-fuel equivalence
ratio of 0.6: about 5ppm NO and 30ppm CO. Nitrogen oxide emissions
decreased proportionally to the equivalence ratio while carbon oxides
decreased to a minimum at an equivalence ratio of 0.6 and then began
to increase again. Experiments with dual injection demonstrated higher
emissions than those demonstrated with single injection. For the same
equivalence ratio, NO emissions increased by about 20%, with minimal
change in CO emissions.
Dual injection combustor in the process of burning methane.
Miles Robinson
Acknowledgments
I would like to thank Dr. Ashwani K. Gupta for allowing me to
work in the lab and Ahmed E. E. Khalil for working alongside
me on my research.