1 RQL Combustor

CHAPTER 1

Introduction

Since the emissions from gas turbine engine are highly polluted and harmful to

the environment and to mankind especially, thus clean and stable air transportation is

highly in demand nowadays. The need for clean engine with less or non-emissions

has becoming great concern during the last few decades. There has been continuous

development on the future design of gas turbine to diminish the rate of emission i.e.

CO, UHC, NOx and SOx, yet efficient and meet required mission. All kind of

emission resulted from aeroengine combustion very harmful such that they may lead

to green house effect, global warming problem and depletion of ozone layer. Among

the emissions, oxide of nitrogen NOx is the most dangerous and has been propulsion

engineer and designer main target, due to its radical reaction with the ozone in the

atmosphere. As according to Dr. John R. Richard (2000) [4] in his book Control of

Nitrogen Oxides Emission, aircraft contributes five percent out of total NOx emission

in the category of non-road sources. Airport when an aircraft is about to take-off is

the most polluted area because at that moment, aircraft gas turbine engine is at full

power with maximum NOx formation and emission.

Oxides of nitrogen are produced mainly from high temperature combustion

processes (Nazri, 2005) [2]. Of all nitrogen oxides, nitrogen oxide (NO) and nitrogen

dioxide (NO2) are providing very adverse effect towards environment. NOx gaseous

formation should happen in lean and near stoichiometric fuel and air mixture. At

stoichiometric, the compound contained in the fuel and air is completely burnout. If

the amount of air content is more than the stoichiometric, the combustion is said to

be lean mixture, whereas the mixture of fuel and air under rich condition when the

content of air is less than the stoichiometric. High oxygen concentration enhances

2 RQL Combustor

the formation of NOx. Residence time needed for fuel and air to completely mix is of

significance factor that cause the formation of nitrogen oxides NOx in the

combustion chamber. The higher the rate of mixing residence time, the lower the

potential of NOx formation should be and vice versa.

One method to reduce the formation of NOx emission is through few

modifications on the combustion system of the aircraft gas turbine. The modification

should be focusing on the chemical reaction within the combustion chamber,

followed by the combustor geometry that can sustain to the modified reaction

previously. Since NOx is highly dependent to the temperature, the principles of

combustion modification are aiming on minimizing the peak temperatures and

residence time at the peak temperature [4]. The shorter residence time can avoid near

stoichiometric combustion. John also stated that the modification techniques attempt

to minimise the oxygen concentration at peak temperature. Less availability of

oxygen in fuel rich condition depletes the ability of fuel bound nitrogen to react with

oxygen, thus retard the formation of NOx [3].

Rich-Burn Quick-Quench Lean-Burn (RQL) combustor has been introduced

as one of the way to reduce the formation of oxides of nitrogen in gas turbine

combustor. The RQL concept is that to burn the fuel in the primary zone of the

combustor under rich-fuel condition then quickly mix with secondary air in the lean

condition [2, 3]. The RQL combustor implies quick mixing between secondary air and

rich fuel from primary zone in order to minimise combustion residence time (i.e. rate

of combustion). High rate of combustion avoid the near stoichiometric condition that

can allow the formation of NOx. The RQL combustion has been successful in

reducing the emission of NOx from fuel bound nitrogen, and in avoiding thermal NO

formation [8]. The combustion takes place in two zones [7] which means the

combustor will have physical separation of two chambers (i.e. primary zone and

secondary zone). Where the mixing of fuel and air take place is called quick-

quenching section.

3 RQL Combustor

M. Hideki, N. Tomoyoshi, M. Yoshiaki and I. Mitsuru (2008) in their

technical report [6] stated that the RQL combustor technique features simple structure

yet excellent combustion stability and performance, even during low loads. The

ignitability during mixing of fuel and air take place is great even though the RQL

combustor providing only single fuel path in the fuel nozzle. Particularly, in primary

zone where equivalence ratio is under fuel rich state, large smoke emission might as

well accumulate. By avoiding fuel droplet stagnation caused by the airflow should

diminish the large smoke emission in the combustor [6]. Appropriate number of

dilution holes and their patterns as well play important role since NOx formation

mainly occur near the dilution zone. The RQL combustor differs from conventional

combustor in which it adopted double-wall liner cooling method [6] in order to

sustain metallurgy limitation during high temperature combustion process in fuel

rich condition.

4 RQL Combustor

CHAPTER 2

RQL CONCEPT

Rich – Burn, Quick – Mix, Lean – Burn (RQL) combustor concept is

predicted from a conclusion that the primary zone of a gas turbine combustor

operates at the most effectively with rich mixture ratios.

Figure 2.1: Rich-Burn, Quick-Mix, Lean-Burn Combustor

First, a “rich - burn” condition in the primary zone generates the stability of

the combustion reaction by producing and maintaining a high concentration of

energetic hydrogen and hydrocarbon radical species. Secondly, the rich burn

5 RQL Combustor

conditions minimize the nitrogen oxides production due to the relative low

temperature and low population of oxygen containing intermediate species.

The liquid waste form the rich primary zone, then, is very high concentrated

of partially oxidized and partially pyrolized (decomposition from high temperatures)

hydrocarbons, hydrogen and carbon monoxides. This waste cannot be exhausted

without further processing. Thus the addition of oxygen is needed to oxidize the high

concentrations of carbon monoxides, hydrogen and hydrocarbon intermediately. This

is done by injecting a considerable amount of air through wall jets to be mixed with

the primary zone effluent. This process creates a “lean – burn” condition at the exit

plane of the combustor.

Generally, this will result in the emission of effluent composed of the major

products of combustion which is carbon dioxide (CO2), water (H2O), nitrogen (N2)

and oxide (O2), and, a non-zero concentration of criteria pollutants such as nitrogen

oxides (NOx), carbon monoxide (CO) and hydrocarbon (HC).

Figure 2.2: Nitric Oxide Formation

6 RQL Combustor

The selection of liner material is important in RQL combustor design. In the

primary zone, the use of air for cooling the liner wall is prevented to avoid the

generation of near-stoichiometric mixture ratios and the production of nitrogen

oxides in the surrounding near the wall. The high temperature and composition of

gases in the primary zone then, create a reducing and demanding environment for the

liner material. Thus the concentration of hydrogen and its demand require a high

quality material.

A good RQL is to be able to mix the air with the waste or effluent that exits

the primary zone. The mixing of the exiting air takes the reaction in which be

exposed to a high production of oxides of nitrogen. This is near the stoichiometric

conditions where the temperature and oxygen atom concentrations are elevated.

Thus, the combustor must be able to continuously and rapidly mix the air into the

rich-burn effluent in order to rapidly produce the lean-burn conditions.

Figure 2.3: RQL Strategy

7 RQL Combustor

Thus, the “quick-mix” label is used to describe the requirement to quickly

mix the air and the primary zone effluent. The Rich-Burn, Quick-Mix, Lean-Burn

combustor concept is basically the following process that takes place in the

combustor in order to reduce the production of NOx.

Figure 2.4: The Process in the RQL Combustor

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CHAPTER 3

NOx Emission in RQL Combustor

Rich-burn, quick-mix, lean burn or RQL combustor is introduced as a

strategy to reduce the emission of nitrogen oxides (NOx). Turbine operates most

effectively at primary zone with rich mixture ratio of 1.8. The high ratio enhances

the stability of the combustion reaction by producing and sustaining a high

concentration of energetic hydrogen and hydrocarbon radical species. Rich burn

condition minimizes production of NOx due to relative low temperature and low

population of oxygen containing intermediate species.

3.1 General NOx Formation in Gas Turbine

Level of pollutants released by gas turbine can be related directly to pressure,

temperature, time and concentration histories of the combustion process. Generally,

gas turbine operates at very high temperate to achieve maximum thermal efficiencies

and to be said the combustion is complete. In fact, the level of CO and UHC is

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decreases at this operating temperature as shown in Figure 3.1.1. However, with

reduced power or at frequent power fluctuation, the flame zone temperatures are

lower than high load temperature, yielding low thermal efficiencies and incomplete

combustion. In other words, CO and UHC levels are lower at high-power setting and

vice versa and in contrast, NOX emission is higher at high-power setting as shown in

Figure 3.1.2.

Figure 3.1.1: CO Emissions in RQL combustor relatives to exit temperature

Figure 3.1.2: NOX Emissions in RQL combustor relatives to exit temperature

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In RQL combustor, reduction in NOx is achieved by preventing

stoichiometric combustion. This is done via three stages. First, fuel is burned in

controlled fuel-rich and fuel-lean regions separated by air quenching. RQL

combustor has excellent operability range. The potential utilization of RQL concept

is limited by the ability of the quench process to rapidly and uniformly dilute the

fuel-rich mixture and to transport in to the lean zone.

In recent research reported by Scott Samuelson, optimization of aerodynamic

mixer may not minimize emission of nitrogen oxides. The results obtained in Figure

3.1.3 shows that NOx increases by fifteen percent when mixer holes are increased

from eight to twelve holes. High concentration of NOx is spotted to occur in the

wakes of the jets adjacent to the wall instead at the centre of the combustor.

In that experiment, three preheat configurations were prepared. First

configuration is for no preheat air wich act as a benchmark. The second

configuration is for jet air preheat which means only the jet air is heated back while

main is not reheated. The third configuration is for both jet air and main air preheat.

Results obtained shown in Figure 3.1.4 indicates that third configuration which is

main and jet air preheat increases the NOx emission while non-preheat air will only

emit small percentage of NOx .

As a result, it can be seen that as the number of holes increases, the number

of preheat air also increases. Thus, the NOx emission also increases.

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Fig 3.1.3: Composite NOx Emission Data Fig 3.1.4: Effect of Preheat on NOx

12 RQL Combustor

CHAPTER 4

Technology for Advanced Low NOx

Advanced low NOx technology (TALON) was deployed commercially by

Pratt & Whitney and this RQL is the anchor combustor technology in aeroengines.

The RQL is preferred the most over lean premixed options in aeroengine

applications due to the safety considerations and overall performance throughout the

duty cycle. In this part, we are going to introduce some of the products that use low

NOx.

4.1 Clayton Steam Generator

Clayton, one of the most respected names in the boiler industry has been a

leader in the development and manufacture of innovative and highly efficient steam

generators since 1930. This steam generator has proven the superiority and ability to

provide reliable, cost effective steam production around the world. This unique

combustion system provides extremely low emissions without sacrificing efficiency

and reliability. Figure 4.1 shows the steam generator.

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Figure 4.1: Steam Generator

Table 4.1 below show us some advantages of the Clayton steam generators

and their description.

Table 4.1: Clayton Steam Generators

Advantages Description

Save fuel The unique counter flow design provides higher fuel-to-

steam efficiency than traditional boilers.

Safe for personnel and

equipment

Inherently safe, the Clayton design eliminates hazardous

steam explosions.

Provide rapid response The Clayton design responds rapidly to sudden or

fluctuating load demands.

Star fast Provide full output from a cold start within fifteen

minutes, without thermal stress.

Compact and lightweight The Clayton design typically occupies one-third of the

floor space and weighs 75% less than a traditional

boiler.

Ensures high quality Clayton provides a 99.5% quality separator to minimize

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steam moisture in the steam.

Offers advanced controls PLC controls, variable speed drives and a linkage-less

servo controlled burner management system are

standard.

Includes outstanding

support

Every steam generator is backed by Clayton factory

direct sales and service plus full service feedwater

treatment.

4.2 FIR Burner

This FIR burner was developed by the Johnston Burner Company. Other than

FIR burner, Johnston Burner products are also AR burner, J burner, A burner and S

burner. All of these burners are using low NOx. In the FIR burner, no efficiency was

lost and consumed the lowest cost premium. The FIR burner was choosen to address

the Under 20 PPM to Under 9 PPM NOx requirement also used for new and retrofit

applications. Figure 4.2.1 and Figure 4.2.2 show the cross section of the FIR burner

and the burner mechanics respectively.

Figure 4.2.1: FIR burner cross section

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Figure 4.2.2: FIR burner mechanics

We can see the difference after using the FIR burner and the uncontrolled

burner in the Figure 4.2.3 below. It will reduce NOx emission from the burner if we

installed the FIR burner.

The FIR burner is an efficient solution for boiler low NOx applications which

they are simple to be controlled and has a standard boiler packages.

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Figure 4.2.3: NOx before and after

4.3 Ultra NOx Coal Burner

Fuel Tech’s Ultra Low NOx coal burners provide industrial and utility boiler

owners with the ultimate solution to their NOx compliance needs. Each system

application is specifically designed to maximized NOx reduction without sacrificing

combustion performance or unit operation. Fuels being fired range from sub-

bituminous through low and high sulphur eastern bituminous coals. NOx reductions

exceeding 50% from baseline levels are achieved across the load range with minimal

increases in unburned carbon. Figure 4.3.1 shows the ultra low NOx coal burner and

Figure 4.3.2 show the performance.

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Figure 4.3.1: Ultra Low NOx Coal Burner

Figure 4.3.2: Ultra Low NOx Coal Burner Performance

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CHAPTER 5

CONCLUSION

The RQL combustor is the anchor technology proven today to reduce the

amount of NOx emission from aeroengine. The concept of burning fuel at rich

condition then quickly mix with secondary air to produce lean mixture has proven

the effectiveness of the engine in reducing the emission of NOx without prior

sacrificing the performance of the engine even at low load. Few minor modifications

were made on the conventional combustor in order to diminish the rate of NOx

formation included the separation of the combustor into primary, secondary and

mixing zone. The concentration of NOx in the combustor plays important role

whether the amount of emission would be much or less, thus the design and pattern

of dilution holes is an important consideration in RQL combustor, as well as the

finer droplet of fuel the better. RQL differ from any conventional combustor in

which it implies double-liner geometry to ensure cooling ability of the combustor

during fuel rich condition. Also, the technology did not only stop to gas turbine

engine only but the technology on reducing NOx emission has been further adopted

in other industry too such as manufacturing, power plant and boiler industry. It

appears to us that everybody is giving out their effort in reducing the emission of this

oxide of nitrogen. Everybody is now realizing the importance of reducing the

emission is due to the harmful effects the NOx providing to the environment and

mankind. Although the contribution of aeroengine towards the emission of this oxide

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of nitrogen is considered small, but by mean of RQL combustor, it enhances the

reduction of the harmful emission by significance amount throughout many years

already.

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REFERENCE

1. Scott S. 3.2.1.3-5 Notes: Rich-Burn, Quick-Mix, Lean-Burn (RQL)

Combustor. Irvine.

2. Jermakian V.,McDonell V.G. and Samuelson G.S.. Experiment Study of

the Effects of Elevated Pressure and Temperature on Jet Mixing and

Emission in an RQL Combustor for Stable, Efficient and Low Emissions

Gas Turbine Applications, California. 2012

3. Combustion and Emission Issues in Gas Turbines by Wajid A. Chishty

and Manfred Klein

4. Cozzi F. and Coghe A. .EFFECT OF AIR STAGING ON A COAXIAL SWIRLED NATURAL GAS FLAME, Italy. 2011.

5. Richards J.R. Control of Nitrogen Oxide Emissions: Students Manual

APTI Course 418,US. 2000

6. Moriai H.,Nakae T., Miyake Y. and Inada M.. Research and

Development of a Combustor for and Environmentally Compatible Small

Aero Engine. Mitsubishi Heavy Industries Ltd, 2008. Technical Review

Vol. 45 No. 4

7. Kalogirou I.D., Bakrozis A.G. and Papailiou D.D. TURBULENT MIXING

PROCESSES IN A SWIRLING-MULTIPLE JET CONFINED

CROSSFLOW CONFIGURATION, Greece.

8. Nazri M. Jenis Enjin Pesawat Udara dan Analisis Kitar, Universiti

Teknologi Malaysia; 2005