the effect of dual injection on combustor emissions

The Effect of Dual Injection on Combustor Emissions

A Research Paper

Presented to the

Science Department

Eleanor Roosevelt High School

In Partial Fulfillment

Of the Requirements for

Research Practicum

By

Miles Robinson

May, 2013

i

Abstract: Effect of Dual Injection on Combustor Emissions

Miles Robinson May, 2013

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 carbon and nitrogen oxides. The mixing of

fuel and air that takes place in a combustor is not adequate resulting in hotspots (local

spot of high temperature). This increases the amount of pollutants formed. The standard

combustor has 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 combustor was run with methane used as fuel at air-fuel equivalence ratios of

0.8, 0.7, 0.6, 0.5, and 0.4. Dual injection proved to be unsuccessful in lowering the

combustion emissions when tested using methane as fuel. A regression line was used to

test the correlation between the NO and CO emissions of single injection versus dual

injection. Strong R2 values indicate there is a significant increase in emissions of dual

injection.

ii

Acknowledgements

I would like to thank Dr. Ashwani K. Gupta for the internship opportunity at the

University of Maryland Combustion Laboratory. This internship has provided me with

valuable hands-on experience as well as an introduction into the broad field of

engineering. I would also like to thank Ahmed E. E. Khalil for his willing assistance

with anything and everything related to my project.

iii

Biographical Outline

Personal Data:

Name: Miles Robinson

Date of Birth: October 30, 1995

Place of Birth: Holy Cross Hospital, Silver Spring, MD

City of Residence: Bowie

College Attending: University of Maryland, College Park

Major: Aerospace Engineering

Academic Achievements:

Full Banneker/Key Scholarship to the University of Maryland

AP Scholar with Honor

Outstanding Participant in the National Achievement Scholarship Program

Activities:

National Honor Society

Varsity Wrestling Team

Varsity Football Team

STEMS Mentoring Society

iv

Table of Contents

Abstract ................................................................................................................................ i

Acknowledgements ............................................................................................................. ii

Biographical Outline .......................................................................................................... iii

Table of Contents ............................................................................................................... iv

List of Tables and Figures................................................................................................... v

Chapter One ...................................................................................................................... 1

Chapter Two ...................................................................................................................... 5

Introduction ..................................................................................................................... 5

Emissions ........................................................................................................................ 5

Carbon Monoxide ....................................................................................................... 5

Carbon Dioxide ........................................................................................................... 6

Nitrogen Oxides .......................................................................................................... 6

Gas Turbines ................................................................................................................... 7

The Combustor................................................................................................................ 7

Improving Combustion ................................................................................................... 8

Air and Fuel Injection ..................................................................................................... 8

Swirl Flow ....................................................................................................................... 9

Exit Arrangements .......................................................................................................... 9

Summary ......................................................................................................................... 9

Chapter Three ................................................................................................................. 11

Chapter Four ................................................................................................................... 13

Chapter Five .................................................................................................................... 18

Literature Cited ................................................................................................................. 21

v

List of Tables and Figures

Figure 3.1: Dual Injection Combustor Diagram ............................................................... 11

Table 4.1: Trial #1 Emissions ........................................................................................... 13

Table 4.2: Trial #2 Emissions ........................................................................................... 14

Figure 4.1: NO and CO Emissions ................................................................................... 14

Figure 4.2: Single vs Dual Injection ................................................................................. 15

Figure 4.3: NO Emissions Single vs Dual ........................................................................ 16

Figure 4.4: CO Emissions: Single vs Dual ....................................................................... 17

Chapter One

The Problem and Its Setting

Introduction to the Problem


inside an engine to provide power to a system. In the process, combustion also emits

harmful gases into the environment such as carbon and nitrogen oxides (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 not adequate and it results in hotspots (local

spot of high temperature). This increases the amount of pollutants formed, especially

nitrogen oxides which are strong functions of the temperature. (Beer, 2012)

The emission of harmful pollutants is a big problem of the combustion process.

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.

2

Statement of the Problem

The purpose of this experiment is to test how multiple inlets affect emissions from

a combustor. This research will aid in obtaining combustion characteristics for gas

turbine application. The performance of the combustor will be tested by experimentation

using methane as fuel. Flow rates of the fuel will be adjusted to determine the best ratio

of flow speed between the two injection points. The ultimate goal of this research is to

develop a combustor with ultra-low emissions to make the combustion process less

harmful to the environment.

Hypothesis

It is predicted that the presence of two separate injection points in the combustor

will enhance fuel-air mixing and reduce hotspots, therefore reducing the amount of CO

and NO that is released from the system. It is unclear whether adjusting the ratio of flow

rates between the injection points will have any effect on the system.

Variables and Limitations

Independent Variables

1. Combustor geometry: dual injection

2. Flow rates at injection points

Dependent Variables

1. Amount of gases emitted

a. CO

b. CO2

3

c. NO

d. O2

2. Temperature inside the combustion chamber

Control Treatments

1. Methane will be used as the fuel.

Regulated Conditions

Research will be conducted in the Combustion Laboratory in the J.M. Patterson

building; University of Maryland, 7950 Baltimore Avenue, College Park, MD 20742.

Limitations

1. The flow controllers used are not very precise, so measured flow rates may be slightly

different from actual flow rates.

Assumptions

1. Flow controllers are calibrated correctly to give fairly accurate measurements of flow

rates.

Statistical Analysis

A regression line will be used to compare emissions of carbon and nitrogen

oxides of single injection combustion to those of dual injection combustion.

Definition of Terms and Abbreviations

1. CO: carbon monoxide

2. CO2: carbon dioxide

4

3. Hotspot: local spot of high temperature inside a combustor that causes an increase in

harmful emissions

4. Injection point: the place at which the fuel-air mixing inside the combustor is initiated

5. NO: nitric oxide

6. NOx: This includes nitric oxide as well as any variations to the bonding structure of

the molecule. (NO, NO2, NO3)

7. Tex: Temperature inside the combustor

Chapter Two

The Review of the Related Literature

Introduction


inside an engine to provide power to a system. For example, one of the reactions that

take place in the burning of methane is:

CH4 + 2 O2 → CO2 + 2 H2O + energy

However, no combustion process is one hundred percent efficient so some reactants are

left over, including harmful pollutants such as carbon dioxide, carbon monoxide, and

various nitrogen oxides. The mixing of fuel and air that takes place in a combustor is not

entirely adequate, resulting in hotspots (local spot of high temperature). This increases

the amount of pollutants formed, especially nitrogen oxides which are strong functions of

the temperature. Engineers and researchers are always looking for ways to make the

combustion process more efficient and less harmful to the environment. (Beer, 2012)

Emissions

Carbon Monoxide

One of the harmful gases emitted during combustion is carbon monoxide (CO). It

is odorless, tasteless, and invisible, making it nearly impossible to detect without a carbon

monoxide alarm. Exposure to this gas can lead to carbon

6

monoxide poisoning with symptoms such as headache, nausea, drowsiness, and even

death if exposed to high levels. As there are about 150 deaths per year in the United

States due to carbon monoxide exposure, it is very important to limit CO emissions in

combustion. (U.S. Fire Administration, n.d.)

Carbon Dioxide

Carbon dioxide (CO2) is the primary gas emitted during combustion. Carbon

dioxide is naturally present in the air; however, human activities have greatly increased

the amount of CO2 in the atmosphere. These human activities include burning fossil

fuels for electricity generation and transportation. Since 1990, CO2 emissions have

increased by about 10% due to fossil fuel combustion. Plants naturally remove carbon

dioxide from the atmosphere, but with increased CO2 emissions, they cannot remove the

gas as fast as it is being emitted. It is important to reduce carbon dioxide emissions from

combustion because high levels of CO2 can cause climate change and lead to global

warming. (U.S. Environmental Protection Agency, n.d.)

Nitrogen Oxides

Combustion emits many different forms of nitrogen oxides including nitrous

oxide (N2O), nitric oxide (NO), and nitrogen dioxide (NO2). Nitrous oxide is very

harmful to the environment because there is no natural way to remove it from the

atmosphere. N2O stays in the air for about 120 years and has an impact on warming the

atmosphere that is nearly 300 times as great as that of carbon dioxide. (U.S.

Environmental Protection Agency, n.d.) NO and NO2 are more common products of

combustion. These forms of nitrogen oxide are highly reactive and can lead to smog,

acid rain, and deterioration in water quality. Reducing emissions of nitrogen oxides from

7

the combustion process is extremely vital because of their harmful environmental effects.

(Nitrogen oxide (nox) emissions, n.d.)

Gas Turbines

Combustion mechanics can be directly applied to gas turbines. A gas turbine is a

machine that converts fuel into mechanical energy or thrust, depending on the type of

energy needed. There are two main types of gas turbines: jet engines and industrial

turbines. Both of these types have three main parts: the compressor, the combustion

chamber, and the turbine. The gas is compressed in the compressor, heated in the

combustion chamber, and finally converted into mechanical work by the turbine. (Basics

of gas turbines, n.d.)

The Combustor

A combustor, or combustion chamber, is the part of a gas engine in which the

combustion takes place. The air and fuel mix together, allowing for combustion to occur.

The combustor is generally a cylindrical body with several openings to allow for air and

fuel to enter and products to exit. However, the combustor design is not limited to the

traditional cylindrical shape, as researchers have developed combustors with rectangular

prism geometries that do not have any negative drawbacks (Arghode, Gupta, Bryden,

2012).

8

Improving Combustion

Emissions can be controlled in three separate phases of the combustion process:

the fuel can be treated before it is burned, the actual combustion process can be modified,

or the process can be cleaned up after the combustion takes place. The most promising

method is modifying the combustion process. Different techniques include altering the

injection points of air and fuel to the combustor, the use of fuel-gas recirculation, and

premixing the air and fuel before they are injected into the combustor. Altering the

combustion process has led to significant results. Using various methods to enhance the

combustor design, nitrogen oxide emissions have been reduced by up to 95% (Beér,

2012). Other ways to reduce emissions and enhance combustor performance include

developing sensors to monitor the reactions inside the combustor as well as developing

techniques to capture carbon dioxide and prevent it from being emitted to the

environment (Lieuwen, 2006).

Air and Fuel Injection

The injection of air and fuel into a combustor significantly affects the efficiency

of the combustion. Several methods of injection have been thoroughly tested. These

methods include premixing, in which air and fuel are mixed together prior to injection,

coaxial injection, in which the fuel line is inserted inside the air inlet, creating a coaxial

nozzle, and perpendicular injection, in which air and fuel are injected separately and

pointed perpendicular to each other. It was found that coaxial injection produces the

highest levels of nitrogen oxides. Separate air and fuel injection produced lower levels of

emissions. (Khalil, Gupta, Bryden, 2012) From experimentation, it has been concluded

9

that separate air and fuel injection allows for proper fuel-air mixing and produces the

lowest pollutants.

Swirl Flow

Swirl flow is a technique used to circulate the combustion products around and

back to the flame in the combustor for more efficient energy conversion. The more

reactants converted to energy, the lower the combustor emissions will be. The air is

injected tangent to the combustor so that it forms a swirling motion when it travels

through the cylinder and mixes with the fuel (Khalil, Gupta, 2011).

Exit Arrangements

Different exits for the product gases of combustion produce different levels of

emissions. Several types of exits include a normal exit (parallel to the air inlet), axial exit

(perpendicular to the air inlet), and an axial exit with an extended tube inside the

combustion chamber. It was determined that axial exit with an extended tube yields the

lowest emissions because the gases remain in the combustor longer, allowing more time

for the completion of the combustion process (Khalil, Gupta, 2011).

Summary

It has been found that altering combustor geometry is the most practical way to

reduce emissions and enhance the combustion process. Methods for improving

combustion include separate air and fuel injection, swirl flow inside the combustor, and

the use of an axial exit with an extended tube inside the combustor. All of these

10

adjustments have proven to be beneficial to the combustion process. However, the

process is not entirely efficient in all areas of the combustion chamber. 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

with a view to develop a combustor with ultra-low emissions for gas turbine application.

Chapter Three

Materials and Methods

Materials

1. Dual injection combustor

Figure 3.1

2. Methane (Airgas, Inc., Hyattsville, MD)

3. Propane torch

4. Exhaust vent

5. Gas analyzer (HORIBA, Kyoto, Japan)

6. Media gauges (SSI Technologies, Inc., Janesville, WI)

Air Inlet #1

Air Inlet #2

Fuel Inlet #2

Fuel Inlet #1

12

Methods

Prior to experimentation, preliminary data was collected to determine the

performance of a standard, dual injector. Combustor emissions were measured and

compared against this preliminary data. Emissions were measured at different air

temperatures and fuel-air pressure ratios. All data was recorded in the Combustion

Laboratory at the University of Maryland.

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 flow rates were increased to desired values. The combustor was

allowed to operate for thirty minutes before recording any data to ensure the combustor

was in steady state. Once steady state condition was reached, the air-fuel flow rate was

set to 0.8 using the media gauges. 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. This process

was repeated for pressure ratios of 0.7, 0.6, 0.5, and 0.4.

Data Collection & Analysis

After emissions data were collected, a regression line was used to plot emissions

from single injection combustion versus emissions from dual injection combustion.

Carbon and nitrogen oxides were compared separately. CO emissions from dual

injection were statistically compared to those of single injection. NOx emissions were

also statistically compared in the same way.

Chapter Four

Results

Data

Compared to single injection, the dual injection combustor did not produce lower

emissions as expected. 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.

Trial #1 0.8 0.7 0.6 0.5 0.4

NO (ppm) 98.50 36.80 11.90 4.60 2.60

CO (ppm) 179.00 63.00 33.00 61.00 800.00

CO2 (ppm) 11.08 9.56 8.09 6.70 5.30

O2 (ppm) 2.18 4.73 7.23 9.55 11.80

Tex 760K 724K 683K 636K 614K

Table 4.1: Emissions and temperature recorded at each equivalence ratio for the first trial.

14

Trial #2 0.8 0.7 0.6 0.5 0.4

NO (ppm) 96.50 37.10 11.70 4.70 2.40

CO (ppm) 175.00 62.00 36.00 59.00 800.00

CO2 (ppm) 11.03 9.56 8.11 6.70 5.30

O2 (ppm) 2.24 4.72 7.19 9.56 11.85

Tex 762K 729K 683K 640K 617K

Table 4.2: Emissions and temperature recorded at each equivalence ratio for the second

trial.

Figure 4.1: Emissions of NO and CO at each equivalence ratio tested.

1

10

100

1000

0

5

10

15

20

25

0.3 0.4 0.5 0.6 0.7 0.8 0.9

CO

@15%

O2 (

PP

M)

NO

@15%

O2 (

PP

M)

Equivalence Ratio

NO and CO Emissions

NO

CO

15

Figure 4.2: NO emissions using dual injection compared to the results from single

injection.

Data Anaylsis

Dual injection was shown to be unsuccessful in lowering the combustion

emissions when tested using methane as fuel. A regression line was used to test the

correlation between the NO and CO emissions of single injection versus dual injection.

Strong R2 values indicate there is a significant increase in emissions of dual injection.

0

5

10

15

20

25

30

35

40

0.4 0.5 0.6 0.7 0.8 0.9

NO

@15%

O2 (

PP

M)

Equivalence Ratio

Single vs. Dual Injection

Single Injection

Dual Injection

16

Figure 4.3

y = 2.0191x - 4.2894

R² = 0.9973

0.00

5.00

10.00

15.00

20.00

25.00

30.00

35.00

40.00

0.00 5.00 10.00 15.00 20.00 25.00

Du

al

Inje

ctio

n (

pp

m)

Single Injection (ppm)

NO Emissions: Single vs Dual Injection

The linear regression line

suggests that NO

emissions of dual

injection are twice as

much as single injection.

17

Figure 4.4

y = -0.0423x2 + 4.49x - 43.31

R² = 0.9844

0.00

10.00

20.00

30.00

40.00

50.00

60.00

70.00

80.00

0.00 10.00 20.00 30.00 40.00 50.00 60.00 70.00 80.00

Du

al

Inje

ctio

n (

pp

m)

Single Injection (ppm)

CO Emissions: Single vs Dual Injection

The regression line suggests that

the CO emissions in dual

injection are directly proportional

to the square of the CO emissions

from single injection.

Chapter Five

Conclusions

Summary

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.

Standard combustion involves the injection of air and fuel into a combustion

chamber to be mixed and turned into mechanical energy in order to power a system. Air

and fuel are generally injected at one point in the combustor. The proposed dual injection

system would involve the addition of a second air-fuel injection point which could

potentially enhance the mixing of air and fuel by distributing the reaction more evenly

throughout the combustor. The null hypothesis was that the dual injection system would

produce lower emissions of carbon and nitrogen oxides, thereby resulting in higher

efficiency of the combustion process. The alternative hypothesis was that the adjustment

would negatively affect the combustion process and increase emissions.

To test the performance of the dual injection system, the combustor was run with

methane used as fuel. Emissions of carbon monoxide, carbon dioxide, and nitrogen

oxides were measured using a gas analyzer. These emissions were compared to the

19

results from standard, single combustion in which methane was also used. There was no

statistical test used as the proposed design would either perform absolutely better or

absolutely worse compared to the original design.

Conclusions and Discussion

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; there was about a 20% increase in

nitrogen oxides overall. As a result, the null hypothesis of enhanced combustion due to

the presence of two separate injection points was rejected. This experiment proved that

dual injection negatively impacts the combustion process.

As seen in Figure 4.2, at an air-fuel equivalence ratio of 0.5 the dual injection

combustor produced fewer amounts of nitrogen oxides than single injection. However,

this finding is meaningless because very large amounts of carbon oxides were detected at

this ratio so the low nitrogen oxide levels were insignificant. The increase in overall

emissions suggests that there is an interaction between both injections jets leading to an

un-equal distribution in the flame region of the combustor.

Recommendations

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

20

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.

Future Implications

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.

21

Literature Cited

Arghode, V. K., Gupta, A. K., & Bryden, K. M. (2012). High intensity colorless

distributed combustion for ultra-low emissions and enhanced performance.

Applied Energy, 92, 822-830.

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files/gasTurbines/basicsOfGasTurbines.pdf

Beér, J. M. (2012). Combustion. In Access Science. McGraw-Hill Companies.

Retrieved from http://accessscience.com/

content.aspx?searchStr=combustion&id=150600

Khalil, A. E. E., Gupta, A. K., & Bryden, K. M. (2012). Mixture preparation

effects on distributed combustion for gas turbine applications. Journal of

Energy Resources Technology, 134

Khalil, A. E. E., & Gupta, A. K. (2011). Swirling distributed combustion for clean

energy conversion in gas turbine applications. Applied Energy, 88(11), 3685-3693

Khalil, A. E. E., & Gupta, A. K. (2011). Distributed swirl combustion for gas

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nox.aspx

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