Full Report Rql Combustor
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Transcript of Full Report Rql Combustor
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
8 RQL Combustor
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
9 RQL Combustor
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
10 RQL Combustor
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.
11 RQL Combustor
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.
13 RQL Combustor
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
14 RQL Combustor
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
15 RQL Combustor
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.
16 RQL Combustor
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.
17 RQL Combustor
Figure 4.3.1: Ultra Low NOx Coal Burner
Figure 4.3.2: Ultra Low NOx Coal Burner Performance
18 RQL Combustor
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
19 RQL Combustor
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.
20 RQL Combustor
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