3. Combustor - Engsoft · 2015-05-18 · The resulting expanded and accelerated high temperature exhaust gas is used to turn the power turbine. Initially, mixing of fuel and air is

Combined Cycle Power Plants 3. Combustor 1 / 110

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Combined Cycle Power Plants

3. Combustor


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Fuel and Combustion Theory 2 1

Factors Affecting Combustor Design 20 2

Combustor Type 27 3

NOx Formation and Its Control 44 4

Diffusion Combustor 59 5

Dry Low NOx Combustor 65 6

Catalytic Combustor 87 7

Combustor Cooling 104 8


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The function of the combustor is to add heat energy to the flowing gases, thereby expanding

and accelerating the gases into the turbine section.

From the viewpoint of thermodynamics, if the fuel heat is added at constant pressure, the

volume of the gas is increased and, with flow area remaining the same, this causes an

acceleration of gas to occur.

Air Inlet Compressor Combustors Turbine Exhaust

Cold Section Hot Section

Combustor


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The fuel is combined with high pressure air and burned in a combustor.

The combustion chamber has the difficult task of burning large quantities of fuel, supplied through fuel spray

nozzles, with extensive volumes of air, and releasing the resulting heat in such a manner that the air is

expanded and accelerated to give a smooth stream of uniformly heated gas.

This task must be accomplished with the minimum loss in pressure and with the maximum heat release

within the limited space available.

The resulting expanded and accelerated high temperature exhaust gas is used to turn the power turbine.

Initially, mixing of fuel and air is occurred under condition that the resulting flame is self-sustaining.

The mixing should be done as uniform as possible for minimum emission and flame stabilization.

The liner is perforated to enhance mixing of the fuel and air.

The details of mixing and burning the fuel are quite complex and require extensive testing to develop a new

burner.

Because the TIT cannot be directly measured, reading are taken of the turbine pressure ratio and exhaust

gas temperature, from which the TIT is calculated.

Generals for Combustor [1/2]


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The amount of fuel added to the air will depend upon the temperature rise required. For example, if the

required TIT is 1500°C that is determined by the materials for turbine blades and nozzles, and the air has

already been heated to 500°C by the work done in the compressor, temperature rise of 1000°C is given from

the combustion process.

Since the gas temperature determines the engine power, the combustion chamber must be capable of

maintaining stable and efficient combustion over a wide range of engine operating conditions.

The temperature of the gas after combustion is about 1800 to 2000°C, which is far too hot for entry to the

nozzle guide vanes of the turbine.

The air not used for combustion, which amounts to about 60 percent of the total airflow, is therefore

introduced progressively into the flame tube.

Approximately one third of this gas is used to lower the temperature inside the combustor; the remainder is

used for cooling the walls of the flame tube.

There are three types of combustors, the can-type, the annular, and the silo combustor.

The central shaft that connects the turbine and compressor passes through the center hole.

Burners are made from materials that can withstand the high temperatures of combustion.

Generals for Combustor [2/2]


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연소의 정의

• 연료 중의 가연성분(C,H,S)이 공기 중의 산소와 결합하여 산화되는 발열반응

• 연소반응의 세 가지 요소: 가연성 연료, 산소, 점화원(불씨)

완전연소

• 산소가 충분한 상태에서 가연분이 완전히 산화되는 반응

C + O2 = CO2 + 33.9 MJ/kg

H2 + 1/2O2 = H2O(water) + 143 MJ/kg HHV(Higher Heating Value)

H2 + 1/2O2 = H2O(vapor) + 120.6 MJ/kg LHV(Lower Heating Value)

S + O2 = SO2 + 9.28 MJ/kg

• 탄소는 연소하면서 일차적으로 CO가 된 후에 이차적으로 CO2 가 됨. 따라서 탄소는 수소에 비해

연소에 더 많은 시간이 소요됨

불완전연소

• 산소가 불충분한 상태에서 가연분이 불완전하게 산화되는 반응

C + O = CO + 10.3 MJ/kg

[CO + 1/2O2 = CO2 + 10.1 MJ/kg (CO는 다시 산소와 반응하여 완전연소될 수 있음)]

Combustion [1/6]


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Combustion [2/6]

Combustion, in its most basic sense, is the process whereby the hydrogen and carbon in fuels are

combined with oxygen from the air to release heat.

In general, common fuels may be classified as hydrocarbons. This means that they are predominantly

composed of carbon and hydrogen.

The source of oxygen is called the oxidizer. The oxidizer, likewise, could be a solid, liquid, or gas, but is

usually a gas (air) for gas turbines.

During combustion, new chemical substances are created from the fuel and the oxidizer. These

substances are called exhaust.

Most of the exhaust comes from chemical combinations of the fuel and oxygen.

When a hydrogen-carbon-based fuel (like gasoline) burns, the exhaust includes water (hydrogen +

oxygen) and carbon dioxide (carbon + oxygen).

But the exhaust could also include chemical combinations from the oxidizer alone.

If the gasoline were burned in air, which contains 21% oxygen and 78% nitrogen, the exhaust could also

include nitrous oxides (NOx, nitrogen + oxygen).


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During the combustion process, as the fuel and oxidizer are turned into exhaust products, heat is

generated. Therefore, the temperature of the exhaust is high and the exhaust usually occurs as a gas.

Interestingly, some source of heat is also necessary to start combustion. (Gasoline and air are both

present in the fuel tank; but combustion does not occur because there is no source of heat.)

Since heat is both required to start combustion and is itself a product of combustion, we can see why

combustion takes place very rapidly.

Also, once combustion gets started, we don't have to provide a heat source because the heat of

combustion will keep things going. (Example: We don't have to keep lighting a campfire.)

To summarize, for combustion to occur three things must be present: a fuel to be burned, a source of

oxygen, and a source of heat.

In addition, sufficient temperature, sufficient residence time, and sufficient mixing are required for complete

combustion.

Combustion [3/6]


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Gas Chemical

symbol

Ratio compared to dry air (%) Molecular

mass

(kg/kmol)

Boiling

point

(C) By volume By weight Weight Ratio

Oxygen O2 20.95 23.20 1 32.00 -182.95

Nitrogen N2 78.09 75.47 3.312 28.02 -195.79

Carbon dioxide CO2 0.03 0.046 44.01 -78.5

Hydrogen H2 0.00005 ~0 2.02 -252.87

Argon Ar 0.933 1.28 39.94 -186

Neon Ne 0.0018 0.0012 20.18 -246

Helium He 0.0005 0.00007 4.00 -269

Krypton Kr 0.0001 0.0003 83.8 -153.4

Xenon Xe 9x10-6 0.00004 131.29 -108.1

Table: Composition of air

공기 조성

Combustion [4/6]


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탄화수소계 연료에 대한 연소 기초식

CnHm + (n +m/4)O2 = nCO2 + (m/2)H2O

위 식은 연료 1분자와 산소 (n +m/4) 분자가 반응하면 연료와 산소 모두 연소에 과부족이 생기지 않는 것을 나타낸다. 공연비는 질량비이므로 원자량을 탄소 12, 수소 1, 산소 16으로 하면, 연료와 산소의 질량은

연료: 12n + 1m

산소: 32(n +m/4)

그런데 공기조성에서 산소 질량이 1.0일 때 질소 질량이 3.312이므로

Air 4.31232(n +m/4)

Fuel 12n + 1m

1) 메탄 (CH4)의 공연비 = 17.25 : 1

2) 옥탄(C8H18)의 공연비 = 15.13 : 1

3) 경유 (C17H36)의 공연비 = 14.95 : 1

Fuel-Air Ratio

Combustion [5/6]


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Species Formula Adiabatic Flame Temp. (K)

Methane CH4 2223

Propane C3H8 2261

Carbon Monoxide CO 2381

Hydrogen H2 2370

Another important combustion parameter is the flame temperature.

Flame temperatures are determined by a balance of energy between reactants and products.

In principal, the highest flame temperatures would be produced at = 1, because all of the fuel and oxygen

would be consumed.

Fuel type is important in determining the flame temperature.

The methane flame temperature is approximately 150 K lower than hydrogen and CO. This distinction

makes it somewhat easier to produce low-emissions from natural gas, which is mostly methane, compared

to syngases containing undiluted H2 and CO.

Flame Temperature

Combustion [6/6]


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비점: 1기압 하에서 액화되는 온도

탄소계 분류 제품 비점(C) 밀도(kg/l) 발열량

MJ/kg (kcal/kg)

C2 이하 천연가스 메탄(CH4)

에탄(C2H6)

-162

-89

0.3

0.37

52 (12425)

48 (11470)

C3~C4 LPG 프로판(C3H8)

부탄(C4H10)

-42

-0.5

0.51

0.58

48

48

C5~C11 나프타 가솔린 35~180 0.6~0.74 44 (10513)

C9~C15 등유 등유 150~250 0.74~0.82 43 (10275)

C12~C22 경유 경유 190~350 0.82~0.88 42 (10036)

C22~ 잔유

A중유

B중유

C중유

윤활유

아스팔트

190~600 0.89 이상 42

원유의 분류와 제품

Petroleum Fuel


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Natural gas is an ideal fuel for gas turbines because it is free from solid residue and has little inherent sulfur

content resulting in low emission of SO2. The sulfur contained in natural gas can be easily removed.

Methane (CH4) and ethane (C2H6) are the principal combustible constituents of natural gas.

Natural gas may contain significant quantities of N2 and CO2.

The lower heating value (LHV) of natural gas is about 1000 Btu/ft3, but can range from 300 to 1,500 Btu/ft3

depending on composition.

It is common for gas turbine manufacturers to specify maximum allowable concentrations of H2S, SO2, and

SO3 in natural gas fuel to ensure that they have taken proper precautions to prevent high temperature

corrosion of turbine blade materials.

The distance sometimes associated with natural gas transmission and pipeline conditions can result in

changes in composition from wellhead and end user. Thus, the as-fired natural gas analysis should be based

on point of use rather than at the wellhead.

Corrosive alkali metals, such as sodium and potassium, are also absent, making natural gas an ideal fuel for

high temperature gas turbines.

Natural Gas [1/3]


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Analysis A B C D E

Constituents,

% by vol.

H2 Hydrogen

CH4 Methane

C2H4 Ethylene

C2H6 Ethane

CO Carbon monoxide

CO2 Carbon dioxide

N2 Nitrogen

O2 Oxygen

H2S Hydrogen sulfide

--

83.40

--

15.80

--

--

0.80

--

--

--

84.00

--

14.80

--

0.70

0.50

--

--

1.82

93.33

0.25

--

0.45

0.22

3.40

0.35

0.18

--

90.00

--

5.00

--

--

5.00

--

--

--

84.10

--

6.70

--

0.80

8.40

--

--

Ultimate,

% by wt.

S Sulfur

H2 Hydrogen

C Carbon

N2 Nitrogen

O2 Oxygen

--

23.53

75.25

1.22

--

--

23.30

74.72

0.76

1.22

0.34

23.20

69.12

5.76

1.58

--

22.68

69.26

8.06

--

--

20.85

64.84

12.90

1.41

Specific gravity (relative to air) 0.636 0.636 0.567 0.600 0.630

HHV, Btu/ft3 @ 60F & 30 in.Hg

Btu/lb of fuel

1,129

23,170

1,116

22,904

964

22,070

1,002

21,824

974

20,160

Source: Steam / Its generation and use, Babcock & Wilcox

The average heat content of natural gas is 1,030 Btu/ft3 on an HHV basis and 930 Btu/ft3 on an LHV basis

– about a 10% difference.

Natural Gas [2/3]


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The Wobbe index (or Wobbe number) is a key parameter for heat capacity of a gaseous fuel because it is

an indicator for the interchangeability of gases.

Wb = Wobbe index, LHV = LHV of the fuel, Sp.Gr. = specific gravity of the fuel, Tamb = ambient temperature

of the fuel in degrees absolute.

Gases with same Wobbe index or within a range of 2 to 5% for premix combustors (15% for diffusion

burners) can be used in the same combustor.

The natural gases have Wobbe index of 1220 10%.

In the gas turbine combustor, increasing the Wobbe index can cause the flame to burn closer to liner, and

decreasing the Wobbe index can cause pulsations in the combustor.

The Wobbe index of a gaseous fuel can be adjusted by diluting it with inert or lean gas (e.g., steam,

nitrogen) or improved by adding rich gases (e.g., evaporated LNG).

Propane, butane, and LPG are usually liquid, however, these will be vaporized to use in gas turbines.

amb

bTGrSp

LHVW

..

Wobbe Index

Natural Gas [3/3]


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Natural gas is the preferred fuel for industrial engines those can use various fuels.

If natural gas is not available, a liquid distillate can be used.

A relatively small number of gas turbines use residual fuels, required pre-treatment which is costly.

Units for base load operation use natural gas, but peak load applications use liquid fuels requiring the

storage of substantial quantities.

Liquid fuels are used in gas turbines are distillates and ash-bearing fuels. (Ash-bearing fuels, by definition,

include any distillate, unrefined crude oil or residual oil containing sufficient quantities of ash to cause

deposit and corrosion problems.)

Light distillates generally do not require preheating because they have sufficiently low pour points under

most ambient conditions. However, heavy distillates are required preheating to prevent filter plugging

because they have high pour points because of the high wax content.

ASTM specifies five grades of liquid fuel for different classes of machines and types of service.

• Grade 0-GT includes naphtha and other light hydrocarbon liquids that have low flash points and low

viscosities as compared to kerosene.

• Grade 1-GT is a light distillate fuel oil suitable for use in nearly all gas turbines.

• Grade 2-GT is a heavier distillate than Grade 1-GT and can be used in gas turbines not requiring the clean

burning characteristics of Grade 1-GT.

• Grade 3-GT may be a heavier distillate than Grade 2-GT, a residual fuel oil that meets the low ash

requirements, or a blend of distillate and residual oils that meets the low ash requirements.

• Grade 4-GT includes most residuals. Because of potentially wide ranging properties, the gas turbine

manufacturer should be consulted on acceptable limits on properties.

Liquid Fuel [1/4]


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Generally, fuel preheating is required for Grade 3-GT and Grade 4-GT. In some cases, however Grade 2-GT

is also required fuel preheating.

Maximum allowable limits are normally set by gas turbine manufacturers for trace metal contaminants such

as sodium, potassium, vanadium, and lead because of their corrosive effects in the hot gas path parts of the

gas turbine.

Vanadium pentoxide (V2O5) an extremely corrosive compound, and sodium vanadate (formed if sodium and

vanadium are present in the fuel) are semi-molten and corrosive at metal temperatures typical of gas turbine

operation.

Typically, the corrosive effects of vanadium are inhibited by adding one of variety of magnesium compounds

to the fuel. The magnesium reacts with vanadium pentoxide and forms magnesium orthovanadate having

melting point higher than gas turbine firing temperature.

If excess magnesium compound is added to the fuel, ash deposits on the turbine blades will increase.

Consequently, some manufacturers recommend that the weight ratio of magnesium to vanadium not exceed

3.5 to minimize ash deposits on hot gas path parts.

Sodium and potassium levels can be reduced to levels in the fuel by providing a system for washing the oil

with water.

Liquid Fuel [2/4]


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A major key to fuel flexibility is the tolerance of the machine to trace metal contaminants.

The five trace metals of most concern are vanadium, sodium, potassium, lead, and calcium. The first four

can cause the corrosion of turbine blade, while all five can cause fouling.

In general, sodium and vanadium are the two most frequently found in petroleum fuels.

The crude oil has been treated by washing to lower the sodium concentration to less than 1 ppm, using a

two-stage electrostatic desalter, and by inhibiting the 3 ppm of vanadium with an oil-soluble magnesium

additive.

Plant output and efficiency can be reduced when the ash bearing fuels (crude oil, residual oil, blends, or

heavy distillate) are used because of fouling occurred in gas turbine and HRSG.

Heavy fuels normally cannot be ignited for gas turbine startup; therefore a startup and shutdown fuel, usually

light distillate, is needed with its own storage, forwarding system, and fuel changeover equipment.

Liquid Fuel [3/4]


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Plant output and efficiency can be reduced when the fuels containing higher sulfur content are used. This is

because higher stack gas temperature is required to prevent condensation of corrosive sulfuric acid.

Sodium sulfates that result during combustion are semi-molten and corrosive at turbine blade metal

temperatures normally associated with gas turbine operation.

The sulfur contained in the fuels reacts with ammonia and produce ammonium bisulfate (NH4HSO4) and

ammonium sulfate ((NH4)2SO4) when the SCR module is employed to reduce NOx emission level.

Both ammonium compounds cause fouling and plugging of the HRSG and increase of PM-10 (particulate

matter smaller in diameter than 10 microns) emissions.

Ammonium bisulfate causes rapid corrosion of HRSG tubes, but ammonium sulfate is not corrosive.

The increase of particulate emissions due to the ammonium salts can be as high as a factor of five due to

conversion of SO2 to SO3.

Some of the SO2 formed from the fuel sulfur is converted to SO3 and it is the SO3 that reacts with water and

ammonia to form ammonium slats, ammonium bisulfate and ammonium sulfate.

The increase is a function of the amount of sulfur in the fuel, the ammonia slip (ammonia that does not react

with NOx, see GER-4249 for details), and the temperature.

Sulfur-Bearing Fuels

Liquid Fuel [4/4]


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Fuel and Combustion Theory 1

Factors Affecting Combustor Design 2

Combustor Type 3

NOx Formation and Its Control 4

Diffusion Combustor 5

Dry Low NOx Combustor 6

Catalytic Combustor 7

Combustor Cooling 8


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1) High combustion efficiency at all operating conditions.

2) Minimized pollutants and emissions: Low levels of unburned hydrocarbons and carbon monoxide, low

oxides of nitrogen at high power.

3) Low pressure drop. (3~4% is common)

4) Combustion must be stable under all operating conditions , including ignition, start-up, and full power.

• The flame should stable in a high velocity stream where sustained combustion is difficult.

• The flame must be self-sustaining and combustion must be stable over a wide range of fuel-air ratio to

avoid ignition loss during transient operation.

• Details for flame stability is quite complex because of various types of combustors.

5) Smooth combustion, with no pulsations or rough burning.

6) A low temperature variation for good turbine life requirements.

• Moderate metal temperatures are necessary to assure long life of the combustor.

• In addition, steep temperature gradients, which distort and crack the combustor liner, must be avoided.

7) Length and diameter compatible with engine envelope (outside dimensions).

8) Designed for minimum cost, repair and maintenance.

9) Carbon deposits must not be formed under any operating conditions.

• Carbon deposits can distort the liner and alter the flow patterns to cause pressure losses.

• Smoke is objectionable as well as a fouler of heat exchangers.

Design Requirement for Combustors


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The size of combustion chamber is determined by the combustion intensity, which is affected by the heat

release rate required, the volume of combustion chamber, and combustion pressure.

The nominal heat release rate = mass flow fuel/air ratio heating value of the fuel.

The lower the combustion intensity, the easier the design of combustor which will meet all design

requirements required.

As the combustor volume increases, it has a lower pressure drop, higher combustion efficiency, better outlet

temperature distribution, and more satisfactory stability characteristics.

When the liquid fuel is used, the evaporation of droplets increases with pressure. Moreover, chemical

reactions are affected by combustion chamber pressure significantly.

The combustion intensity in aviation systems is 2 - 5x104 kW/m3-atm, while industrial gas turbines have

much lower value for this, may be a tenth of this, because they have larger volume of combustion space

available.

It is not appropriate to compare the performance of different combustion systems with quite different orders

of combustion intensity.

combustion intensity = heat release rate

combustor vol. * pressure

[kW / m3-atm]

Combustion Intensity


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Combustor performance is measured by combustion efficiency, the pressure drop in the combustor, and the

evenness of the outlet temperature profile.

Combustion efficiency is a measure of combustion completeness that affects the fuel consumption directly,

since the heating value of any unburned fuel is not used to increase the turbine inlet temperature.

In the past, major goals of combustor design were high combustion efficiency and the reduction of visible

smoke, both were solved by the early 1970s.

Typical values for combustion efficiency is 99%.

Combustion efficiency at off-design conditions, such as idle, must exceed 98.5% to satisfy regulations on

exhaust carbon monoxide and UHC.

The combustion efficiency can be determined by the chemical analysis of the combustion products.

Knowing the air/fuel ratio used and the proportion incompletely burnt constituents, it is possible to calculate

combustion efficiency.

Combustion efficiency = Actually released energy

Theoretically available energy

Combustion efficiency = Fuel burnt in the combustor

Total fuel input

Combustion Efficiency

LHVf

afa

theretical

actualcomb

m

hmhmm

h

h

23

h2 = enthalpy leaving the compressor

h3 = enthalpy entering the turbine

ma = mass flow rate of air

mf = mass flow rate of fuel

LHV = lower heating value of fuel


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1

1,

2,

21

o

o

T

TKKPLF

The pressure loss in a combustor is a major problem

because it affects gas turbine efficiency and power

output.

The pressure loss occurred in the combustor is a very

important parameter, because the efficiency of the gas

turbine will be reduced by an equal percentage.

Typical values for pressure loss in combustor is usually

in the range of 2~4% of static pressure.

Pressure loss in a combustor is caused by friction,

turbulence and the temperature rise due to

combustion.

The pressure loss due to friction is called as friction

loss and is found to be much higher.

The fundamental loss is caused by temperature rise

due to combustion.

The overall stagnation pressure loss is the sum of the

fundamental loss and friction loss.

PLF = Cold loss + Hot loss

PLF = Pressure loss factor, K1 = cold loss, K2 =

fundamental loss

1 2 3 0

40

30

20

10

Temperature ratio, To,2/To,1

Pre

ssu

re lo

ss fa

cto

r

Cold loss, K1

Fundamental

pressure loss

Pressure Loss in a Combustor

V1 V2

1 2

1

2/ 1,

2,

2

11

2,1,

o

ooo

T

T

V

pp


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Uniformity of the Combustor Outlet Temperature Profile [1/2]

The uniformity of the combustor outlet temperature

profile can be investigated along two directions,

tangential and radial.

The uniformity of the combustor outlet temperature

profile affects the useful level of TIT, because the

average gas temperature is limited by the peak

gas temperature.

The figure shown in the right hand side is a

temperature profile measured at the exit of the gas

turbine at various loads.

The profile factor is the ratio between the

maximum exit temperature and the average exit

temperature.

This is a very important parameter for determining

the health of the gas turbine caused by thermal

fatigue.

The settings for shutdown of gas turbines using

natural gas as a fuel are set at about 100F

between the maximum and minimum temperatures

at any given time at the exit.

Temperature difference between adjacent probes

should not exceed 40~50F for turbines using

natural gas as a fuel.

[ An example of exit temperature profile of a gas

turbine for various loads, 16 probes were used ]

Profile Factor


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The turbine downstream of the combustor has to withstand very

high temperatures and stresses, due to the centrifugal loads.

These stresses are highest towards the hub of the blade, so the

radial temperature profile in the combustor is controlled, with the

peak temperatures around two thirds of the way up the blade.

Temperature factor, also known as traverse factor, is defined as:

1) The peak gas temperature minus mean gas temperature

divided by mean temperature rise in the nozzle design. The

traverse number must have a lower value between 0.05 and

0.15 in the turbine nozzle vanes.

2) The difference between the highest and the average radial

temperature.

Temperature Factor

Uniformity of the Combustor Outlet Temperature Profile [2/2]

Tip

Hub

Temperature


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Combustor Type 3





Combustor Cooling 8


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Function of a diffuser = Velocity energy Pressure energy

The velocity of the air leaving the compressor is decreased before it enters the combustor in order to

reduce the combustor pressure loss and the air velocity in the combustor.

Reduced air velocity in the combustor contributes to flame stability

The velocity of compressor discharged air is so high – 150 m/s – and this velocity is reduced to about 60

m/s by the diffuser.

Combustion must maintained in a stream of air moving with high velocity of 30-60 m/s.

Diffuser

http://upload.wikimedia.org/wikipedia/commons/4/4c/Jet_engine.svg


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Can-Type Combustor [1/8]

Arrangement

GE 9FB.05


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Arrangement



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Can type combustor, which is also called as multiple chamber, or can-annular combustor, is commonly used

for industrial gas turbines.

The major advantage is that development can be carried out on a single can using only a fraction of the

overall airflow and fuel flow.

It is relatively easy to maintain them. That is, each can be removed easily and worked on independently.

The individual combustors are interconnected with small cross-fire tubes (interconnector tubes ) so that, as

combustion occurs in the two combustors with igniter plugs, the flame can move to all of the remaining cans.

Another mission of the cross-fire tubes is that this allows each can to operate at the same pressure, which

make the engine vibration free.

Flow at the exit of the combustor is not uniform, which make the engine vibration higher.

The combustion ignition system uses two spark plugs and two flame detectors, along with cross-fire tubes.

Ignition in one of the chambers produces a pressure rise which forces hot gases through the cross-fire tubes,

propagating ignition to other cans within one second.

Flame detectors, located diametrically opposite the spark plugs, signal the control system when ignition has

been completed.



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Can type combustors can be of the straight-through design or reverse-flow design and have single fuel

nozzles in the diffusion combustors, while each combustor have three to eight nozzles and one pilot nozzle

in the center in the DLN combustor.

In theory, a large number of fuel nozzles provide better distribution of the fuel gas (or greater atomization of

the liquid fuel droplets) and more rapid and uniform burning and heat release. But the problems of equally

distributing fuel to each fuel nozzle significantly limit the number of fuel nozzles employed.

The straight-through design is used on aircraft engines, while a reverse-flow design is used on heavy-duty

gas turbines.


http://www.power-technology.com/projects/kwangyang/


HIoPE


Transition Piece 1st Stage Nozzle

1st Stage

Blades

3rd Stage

Blades

Combustion

Can

2nd Stage

Nozzle

2nd Stage

Blade

3rd Stage

Nozzle

Transition Piece



HIoPE


[GEAE ] [ Siemens ]

The development of a can type combustor requires experiments with only one can, while the annular

combustor must be treated as a unit and requires much more hardware and a large amount of compressor

flow.



HIoPE


① 선회유동 형성 (수백 fps의 축방향 공기속도를 5~6 fps로 감속). 만약, 축방향 공기속도가 너무 빠르면,

• 연소정지( flame-out) 초래

• 연소기 압력강하 초래

• 연소기 효율저하 초래

② 연료-공기 혼합 촉진

③ 화염길이 짧게 유지

• 연소실 길이 축소

• 터빈으로 화염전파 방지

Swirler



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A free vortex increases tangential velocity at the

center.

The higher velocity at the center produces a lower

static pressure and thus a radial pressure gradient.

This is the reason that a recirculation zone is

formed just downstream of swirl vanes.

The recirculation zone acts as flame holders during

continuous operation of gas turbine.

Recirculation Zone



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It consists of an outer casing, a liner

completely annular in form, and an

inner casing.

The annular combustor is commonly

used today in all sizes of aero engines.

Annular Combustor [1/5]

Configuration


HIoPE


V94.3 gas turbine consists of 16-stage axial flow

compressor followed by an annular combustor and a

four-stage reaction type axial-flow turbine.

Annular combustors are superior to can combustors

in terms of overall temperature distribution factor

(OTDF). Can combustors have a relative higher

OTDF that may result in thermo-mechanical fatigue

problems.

Annular combustor popularity increases with higher

temperatures or low-BTU gases, because the

amount of cooling air required is much less than in

can type designs due to a much smaller surface

area.

The amount of cooling air required becomes an

important consideration in low-BTU gas applications,

because most of air is used up in the primary zone

and little is left for film cooling.

V94.3 & V84.3 [Siemens]



HIoPE


GT26 & GT24 [Alstom]



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DLE Combustor for Aeroderivative Gas Turbines (GE)

Premixer

Combustion Liner

Heat Shield

Three rings of

fuel nozzles



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Advantages Disadvantages

• The main advantage of the annular combustion

chamber is that for the same power output, the

length of the chamber is only 75 per cent of that of a

can-annular system of the same diameter, resulting

in a considerable saving in weight and cost.

• Its minimal surface area requires less cooling air.

• Another advantage is the elimination of combustion

propagation problems from chamber to chamber.

• Flow at the exit of the combustor is uniform, which

make the engine vibration free.

• It has a reduced surface exposed to the gas, which

should result in less pressure loss across the

chamber.

• It is difficult to obtain an uniform fuel-air distribution

and an uniform outlet temperature distribution, in

spite of employing a large number of fuel jets.

• The structure of annular combustors is inevitably

weak. Therefore, it has big possibility of buckling of

the hot flame tube walls.

• The development of an new annular chamber

should be carried out with a test facility capable of

supplying the full engine air mass flow. This requires

a huge layout and involves enormous cost.

• However, a major disadvantage is that it is hard to

repair it.



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Industrial gas turbines initially employed large external (silo) combustors in order to ensure efficient

combustion, which is caused by lower gas velocities.

Typically emission levels of CO 10 ppm were achieved and UHC emission was undetectable.

Silo combustors require too much cooling air and at part load the large areas of cooling surface adversely

affected combustion efficiency.

The initial use of diffusion burners increased NOx formation.

Long residence times also increased NOx formation.

Because of high level of NOx formation and other structural reasons silo combustors were replaced by small

multiple chambers.

Siemens V94.3 GE 10 Siemens

Silo Combustor


HIoPE


Type Advantages Disadvantages

Can • Better maintenance

• Easier development

• Flow is not uniform at the combustor outlet

• Higher vibration

• Heavier

• Longer

Annular

• Flow is uniform at the combustor outlet

• Vibration free

• Not heavier

• Higher thermal efficiency because of

smaller cooling area

• No components for flame propagation

• Maintenance is not easy

• Development is difficult

Silo • Complete combustion

• Minimization of CO emission

• Higher NOx emission

• No good for part load operation

Comparison of Combustors


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Combustor Type 3





Combustor Cooling 8


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Thermal NOx [1/4]

There are two mechanisms of NOx formation in gas turbine combustors.

1) The oxidation of atmospheric nitrogen found in the combustion air (thermal NOx and prompt NOx), and

2) The conversion of nitrogen chemically bound in the fuel (fuel NOx).

Thermal NOx is formed by a series of chemical reactions in which oxygen and nitrogen present in the

combustion air dissociate and subsequently react to form NOx.

Prompt NOx, a form of thermal NOx, is formed in the proximity of the flame front as intermediate combustion

products such as HCN, N and NH that are oxidized to form NOx.

Prompt NOx is formed in both fuel-rich flame zones and DLN combustion zones.

The contribution of prompt NOx to overall NOx emissions is relatively small in conventional diffusion

combustors, but this contribution is a significant percentage of overall thermal NOx in DLN combustors.

For this reason, prompt NOx becomes an important consideration for DLN combustor designs, establishing

a minimum NOx level attainable in lean mixtures.

The thermal route is a primary mechanism for NOx when flame temperatures are above approximately 1800

K(1523C). Below this temperature, the thermal reactions are relatively slow.

Thus, a common approach to NOx control is to reduce the combustion temperature so that very little thermal

NOx can form.


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General Emission Performance of a Lean Premix Combustor

Thermal NOx [2/4]

1700 1200 1300 1400 1500 1600

140

120

100

80

60

40

20

0

Reaction temperature, C

NO

x,

CO

, vp

pm

@ 1

5%

O2

CO emissions

1800

160

Pulsation

NOx emissions

Operating

range

Pre

ssu

re p

uls

atio

ns

Compressor

Fuel

Turbine

Air

Power

Exhaust NOx

~ 25 ppm 350C

Bypass Air

1800C

1300C


HIoPE


The mechanisms of NOx formation are as follows.

• Extended Zeldovich mechanism:

(1) O + N2 NO + N

(2) N + O2 NO + O

(3) N + OH NO + H

• Nitrous oxide:

(4) N2 + O + M N2O + M (여기서 M은 N2, O2, CO2 같은 제 3의 물질)

(5) N2O + O NO + NO

(6) N2O + H NO + NH

• Prompt:

(7) N2 + CH HCN + N

In the absence of thermal NOx, the other mechanisms become significant. Non-equilibrium concentration of

O or H atoms in the flame region can produce NOx via reactions (1) to (3), and this is known as Zeldovich

NOx.

The prompt mechanism is followed by a sequence of reactions converting HCN to NO; reaction (7) is just

the initiation.

Prompt NOx is formed very briefly during the combustion process by the interaction of CH radicals on N2,

the quantity of NO produced in this manner being also relatively small.

Thermal NOx [3/4]


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NOx product is approximately 95% of NO, with NO2 making up the balance plus a small amount of N2O.

Emission limits are defined as the quantity of nitrogen dioxide equivalent, as the NO is ultimately converted

into NO2 in the atmosphere.

The rate of NO formation has been determined theoretically by

The reduction in temperature becomes the main strategy for controlling thermal NOx emission from gas

turbines.

t = residence time at high temperature (sec)

T = absolute temperature (K)

[N2], [O2] = concentration of nitrogen, oxygen (mol/cm3)

d[NOx]/dt = rate of NOx formation (mol/cm3/s)

The Rate of Thermal NOx Formation

5.0

225.0

16

x ON69090

exp100.6]NO[

TTdt

d

Thermal NOx [4/4]


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Fuel NOx

Fuel NOx is the conversion of fuel-bound nitrogen (FBN) to NOx during combustion.

During combustion, the nitrogen bound in the fuel is released as a free radical and ultimately forms free N2

or NO.

Fuel NOx can contribute as much as 50% of total emissions from oil and 80% from coal. This is because the

amount of FBN is large in coal, small in oil, but little in natural gas.

Natural gas typically contains little or no FBN. As a result, when compared to thermal NOx, fuel NOx is not a

major contributor to overall NOx emissions from stationary gas turbine firing natural gas. Molecular nitrogen,

present as N2 in natural gas, does not contribute significantly to fuel NOx formation.

Some low-Btu synthetic fuel contain nitrogen in the form of ammonia (NH3).

Other low-Btu fuels such as sewage and process waste stream gases also contain nitrogen.

When these fuels are burned, the nitrogen bonds break and some of the resulting free nitrogen oxidizes to

form NOx.

With excess air, the degree of fuel NOx formation is primarily a function of the nitrogen content in the fuel.

The fraction of FBN converted to fuel NOx decreases as the nitrogen content increases, although the

absolute magnitude of fuel NOx increases. For example, a fuel with 0.01 percent nitrogen may have 100

percent of its FBN converted to fuel NOx, whereas a fuel with a 1.0 percent nitrogen may have only a 40

percent conversion rate.


HIoPE


Tropospheric ozone has been a significant air pollution problem because it is the primary constituent of

smog.

Many countries do not meet ozone standard and thereby expose large segments of the population to

unhealthy levels of ozone in the air.

NO2 reacts in the presence of air and ultraviolet light (UV) in sunlight to form ozone and nitric oxide (NO).

The NO then reacts with free radicals in the atmosphere, which are also created by the UV acting volatile

organic compounds (VOC).

The free radicals then recycle NO to NO2. In this way, each molecule of NO can produce ozone multiple

times.

In addition to the ozone concerns, NOx and SOx in the atmosphere are captured by moisture to form acid

rain.

Acid rain impacts certain ecosystems and some segments of our economy.

All of these facts indicate the need to reduce NOx emissions, and understanding of its generation and control

mechanisms.

오존 농도 0.12 ppm/h 이상

(오존주의보) 0.3 ppm/h 이상

(오존경보) 0.5 ppm/h 이상 (오존중대경보)

인체에 나타나는 증상

• 눈과 코 자극

• 불안감과 두통 유발

• 호흡수 증가

• 호흡기 자극

• 가슴압박

• 시력감소

• 폐기능 저하

• 기관지 자극

• 패혈증 유발

Ozone


HIoPE


대류권에 시간당 오존농도가 0.12ppm 이상일 때 내려지는 주의보.

성층권의 오존은 지구상의 생명을 보호하는 보호막 역할을 한다. 그리고 대류권에 오존(ozone: O3, 강력한 발암물질)이 적당량 존재할 경우 강력한 산화력으로 살균, 탈취작용을 한다.

그러나 오존농도가 일정기준 이상 높아지면 호흡기나 눈이 자극을 받아 기침이 나고 눈이 따끔거리며, 심한 경우 폐 기능저하를 가져오는 등 인체에 피해를 주기도 하며, 농작물 수확량 감소를 가져오는 유독물질이 된다.

오존 경보제에 의해 각 자치단체장이 권역별로 시간당 오존농도가 0.12 ppm에 달하면 주의보, 0.3 ppm으로 오르면 경보, 0.5 ppm 이상 치솟으면 중대경보를 내리게 된다.

농도가 '주의보' 발령 수준일 때 1시간 이상 노출되면 호흡기와 눈에 자극을 느끼고, 기침을 유발한다. 따라서 주의보가 발령되면 호흡기 환자나 노약자, 5세 이하의 어린이는 외출을 삼가고 운전자도 차량 이용을 자제해야 한다.

'경보'가 발령되면 소각시설과 자동차의 사용 자제가 요청되고 해당지역의 유치원과 학교는 실외학습을 자제해야 한다.

'중대경보'가 발령되는 0.5 ppm에 6시간 노출되면 숨을 들이마시는 기도가 수축되면서 마른기침이 나오고 가슴이 답답해지고 통증을 느끼게 된다. 특히 물에 잘 녹지 않는 오존이 장시간 폐 깊은 곳까지 들어가면 염증과 폐수종을 일으키며, 심하면 호흡곤란을 일으켜 실신하는 수도 있다.

중대경보가 발령되면 소각시설 사용과 자동차 통행이 금지되며, 주민의 실외활동 금지가 요청된다.

오존 주의보


HIoPE


NOx Control Methods in Gas Turbines

1. Diluent injection

• Steam, CO2, N2 or other diluent is injected to the combustion zone of diffusion combustor.

• Since NOx formation is a function of flame temperature, the addition of diluent lowers the flame

temperature to reduce NOx formation.

2. Premixed fuel lean combustion

• Typical premixed combustion mixes the fuel and oxidant upstream of the burner.

• Premixed combustion allows use of leaner fuel mixtures that reduce the flame temperature, and

therefore thermal NOx formation.

• This is the basis for DLN combustor operation.

3. Catalytic combustion

• Lean premixed combustion is also the basis for achieving low emissions from catalytic combustors.

These systems use a catalytic reactor bed mounted within the combustor to burn a very lean fuel air

mixture.

• The catalyst material stability and its long term performance are the major challenges in the development

of an operational catalytic combustor.

• Catalytic combustion is also an unlikely solution for retrofitting existing turbines.

4. Post combustion treatment


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Since September 1979, when regulations required that NOx emission be limited to 75 ppmvd, many gas

turbines have accumulated millions of operating hours using either steam or water injection to meet

required NOx levels. Most gas turbines control NOx emission with diluent injection into the combustor until

1990.

The injected diluent used as a heat sink that lowers the combustion zone temperature, which is the primary

parameter affecting NOx formation. As the combustion zone temperature decreases, NOx production

decreases exponentially.

The increased diluent injection lowers the thermal efficiency because some of the energy of combustion

gases is used to heat the water or steam.

Water (or steam) injection for power augmentation economically attractive in some circumstance, such as

peaking applications. However, the process required large quantities of clean water – to at least boiler feed

water standard – to avoid corrosion of blade or fouling and blocking of cooling air holes by impurities.

In case of water injection, however, there was an increase in levels of pressure fluctuations associated with

combustion. Such dynamic pressures can excite acoustic resonance which may shorten combustor life.

Steam injection, while lacking the cooling effect of water evaporation, can nevertheless give better mixing

and lower dynamic pressure levels than water injection.

Carbon monoxide, representing the measure of the inefficiency of the combustion process, also increases

as the diluent injection increases.

The lowest practical levels achieved with diluent injection are generally 25 ppm when firing natural gas and

42 ppm when firing oil.

Water/Steam Injection [1/3]


HIoPE


Water to fuel ratio

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0

1.0

0.8

0.6

0.4

0.2

0.0

Rela

tive

NO

x p

rod

uctio

n r

ate

Steam injection

Water injection

Mixture of natural

gas and steam


Water injection is extremely effective means for

reducing NOx formation.

To maximize the effectiveness of the water

injection, fuel nozzles have been designed with

additional passage to inject water into the

combustor head end.

The water is thus effectively mixed with the

incoming combustion air and reaches the flame

zone at its hottest point.

Steam injection for NOx reduction follows

essentially the same path into the bombustor

head end as water.

However, steam is not as effective as water in

reducing thermal NOx.

The high latent heat of water acts as a strong

heat sink in reducing the flame temperature.

In general, for a given NOx reduction,

approximately 1.6 times as much steam as

water on a mass basis is required for control.

There are practical limits to the amount of water or steam that can be injected into the combustor. This is

because the increased quantities of water/steam were proved detrimental to cycle efficiency and part lives,

and the emission rates for other pollutants also began to rise significantly.


HIoPE


Base load operation (MS7001E)

Peak load operation (MS7001E)

100exp(-1.58W/F ratio)

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

100

10

Water to fuel ratio

NO

x, p

pm

vd @

15%

O2

50

Experimental Results TR-108057 (EPRI)



HIoPE


Selective Catalytic Reduction [1/3]

SCR is the most effective and proven technology to reduce NOx emissions, greater

than 90%.

NOx level less than 9 ppmvd can be obtained at 15% oxygen for all combined

cycle plants with selective catalytic reduction (SCR) systems.

760F

O2=17%

895F

O2=16.4%

NOx=30 ppmv

CO=15 ppmv

680F

SC

R c

ata

lyst

CO

ca

taly

st

395F

O2=16.4%

NOx=6 ppmv

CO=3 ppmv

Duct burner


HIoPE


NOx contained in the gas turbine exhaust gas is converted into harmless molecular nitrogen and water on

the catalyst bed by the reaction with ammonia.

Typically, the SCR catalyst operates under a narrow temperature range of 570F/300C and 750F/400C.

The equipment is comprised of segments stacked in the exhaust duct. Each segment has a honeycomb

pattern with passages aligned to the direction of the flow.

A catalyst such a vanadium pentoxide is deposited on the surface of the honeycomb.

For a GE turbine MS7001EA an SCR designed to remove 90% of the NOx has a volume of 175 m3 and

weights 111 tons.

The major disadvantages of this system are the cost of installation and maintenance of the system, the

efficiency penalty due to the pressure drop introduced by the catalyst, and the potential for NH3 slip.

HRSG flue gas draft losses: approximately 25 mbar, 35 mbar if catalysts are required.

A certain amount of ammonia, that is excess ammonia, may pass through the catalyst unreacted and

emitted into the atmosphere as “ammonia slip”.

Both NOx and ammonia are acutely toxic, and they contribute to fine particle formation, acidifying deposition.

In most cases, ammonia slip is currently limited by permit condition to either 5 or 10 ppm at 15% O2,

because ammonia is a hazardous material.



HIoPE


Ammonium hydroxide (수산화 암모늄) solution sprayed over a mesh containing titanium and vanadium

oxide catalysts reacts with the NOx to form nitrogen and water.

The reaction rate shows peak level at around 350C, and this temperature is appeared between the

evaporator and economizer sections of HRSG.

However, when the NOx emission should be controlled less than 10 ppm, this system can be used with the

combination of water injection.

Anhydrous ammonia (NH3) is the most cheap reagent.

Aqueous ammonia (NH4OH) is a safer to transport, handle and store than anhydrous ammonia. For these

reasons, many end-users and operators use it.

SCR systems are sensitive to fuels containing more than 1000 ppm sulfur.

Ammonia can lead to fouling of HRSG tubes downstream of the SCR if moderate quantities of sulfur are

present in the flue gas.

Ammonia and sulfur react to form ammonium bisulfate, a sticky substance that forms in the low temperature

section of HRSG (usually the economizer).

The deposited ammonium bisulfate is difficult to remove and can lead to a marked increase in pressure

drop across the HRSG.



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Combustor Type 3





Combustor Cooling 8


HIoPE


Diffusion Combustion

Both fuel and oxidizer are supplied to the reaction zone

in an unmixed state in a diffusion combustor.

Fuel mixes with the air by turbulent diffusion and the

flame front can be considered the locus of the

stoichiometric mixture where temperatures reach

approximately 2000C.

Stoichiometric mixture will lead to both the highest flame

temperature and the fastest reaction rates.

Optimal conditions for combustion are restricted to the

vicinity of the surface of stoichiometric mixture. This is

the surface where fuel and air are locally mixed in a

proportion that allows both to be entirely consumed.

Since combustion is much faster than diffusion in most

cases, the latter governs the rate of entire combustion

process. This is the reason why those flames are called

diffusion flames.

Diffusion combustion has been used extensively

because there is no backfire.

Fuel

Post flame

radiation

Surface of

stoichiometric

mixture

Air Air


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Combustion Zones

The fuel injected into the combustor is evaporated and burnt in the primary zone, where air-fuel ratio is

about 60:1.

The fuel is burnt almost stoichiometrically with one-third or less of the compressor discharge air.

The combustion process consists of three phases, the endothermic dissociation of the fuel molecules,

followed by a fast, exothermic formation of CO and H2O, and finally the slower, exothermic oxidation of CO

to CO2.

About 80% of the energy is released in the second phase during the formation of CO. The slower burn-out to

CO2 can require 75% of the combustion zone length.

The hot combustion products are cooled by dilution in the dilution zone with excess air to temperatures

acceptable to combustor walls and turbine blades.


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Emission in a Diffusion Combustor [1/2]

1000

0.5

4000

3000

2000

1.5 1.0

Equivalence ratio

Fla

me

te

mp

era

ture

, F

100

300

200

NO

x r

ate

, p

pm

v

High smoke

emissions High CO

emissions

Sto

ich

iom

etr

ic c

on

ditio

n

Fuel rich Fuel lean

Op

tim

um

ba

nd

Stoichiometric condition means that

the proportions of the reactants are

such that there are exactly enough

oxidizer molecules to bring about a

complete reaction to stable molecular

forms in the products.

Equivalence ratio ( ) is the ratio of

the oxygen content at stoichiometric

condition and actual condition

[ Emission for diffusion combustors using No. 2 oil as a fuel ]

With precisely enough air to

theoretically consume all of the fuel,

combustion is referred to as a

“stoichiometric” f/a ratio.

Adding more air produces combustion

that is fuel-lean, and adding less air

produces combustion that is fuel-rich.

= (f/a) stoichiometric

(f/a) actual


HIoPE


The overall air/fuel ratio is in the region of 100:1,

while the stoichiometric ratio is approximately 15:1

The air/fuel ratio might vary from about 60:1 to 120:1

for simple cycle gas turbines and from 100:1 to 200:1

if a heat exchange is used.

1400

NOx limit

CO

em

issio

n, p

pm

Primary zone temperature, K

120

100

80

60

40

20

0 1500 1600 1700 1800 1900 2000

0

5

10

30

25

20

15

CO limit

Permissible temperature

range to meet both CO and

NOx limits (optimum band)

NO

x e

mis

sio

n, p

pm

Emission in a Diffusion Combustor [2/2]

Carbon monoxide is produced when incomplete

combustion occurs.

Increasing combustion temperatures improves

burning and thus reduces carbon monoxide

emissions.

Nitrogen is the dominant element in the

atmosphere. Raising the temperature of air

causes it to react with oxygen, producing nitrogen

oxides (NOx).

Normal combustion temperatures rage from

1871C to 1927C. At this temperature, the

volume of NOx in the combustion gas is about

0.01%.

The higher the air temperature and exposure time

to these temperatures, the greater the formation

of NOx.

There is an optimum band, where both CO and

NOx emissions are low.

The ideal combustor would therefore always burn

fuel within this band, independent of the engine

operating condition.


HIoPE


[ Stability loop ]

It is necessary to maintain an optimum air-fuel ratio to

ensure ignition and sufficiently fast combustion.

Rich mixture will result in cracking(열분해) of the fuel

with the formation of amorphous carbon which is

difficult to burn. Although insufficient air is a cause of

carbon formation, the problem is intimately associated

with improper mixing.

Lean mixture (or poor atomization) and poor mixing will

lead to failure of combustion.

Operation outside the region of stable burning results

in unstable combustion causing vibration and

combustion failure.

The stability range and air-fuel ratio range decreases

as the air velocity is increased.

Stability Limits

Normally, combustors are designed with an inlet air velocity not exceeding 80 m/s at design load.

In order to cool the products of combustion to a temperature acceptable to turbine blades, it is necessary to

use a total air-fuel ratio far in excess of those permitting stable combustion.

This difficulty is usually avoided by admitting a satisfactory amount of primary air so as to maintain stable

combustion.

The products of combustion are then cooled by introducing additional air called secondary air.

The air-fuel ratio calculated with respect to the sum of primary and secondary air is known as the total air-

fuel ratio.


HIoPE




Combustor Type 3





Combustor Cooling 8


HIoPE


Catalytic combustor

Design Change of Combustors

Diffusion combustor Wet combustor DLN combustor

Steam or water injection Inclusion of catalyst

Single fuel nozzle Multiple fuel nozzle

Reduced NOx emission Low NOx emission Near zero NOx emission

Premix fuel and air

before combustion

Fuel injector Spark

plug D

iese

l e

ng

ine

Sp

ark

ig

nitio

n e

ng

ine


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Diffusion Flame Premixed Flame

• Fuel and air mix and burn at the same time

• An example for diffusion combustion is a Diesel

engine, where a liquid fuel spray is injected into the

compressed hot air within the cylinder. It rapidly

evaporates and mixes with the air and then auto-

ignition under partly premixed conditions

• Flame color is bright yellow

• NOx formation in post-flame regions

• Fuel and air mixed and then burn

• In a spark ignition engine, a premixed turbulent

flame front propagates from the spark through the

combustion chamber until the entire mixture is

burnt.

• Flame color is blue to bluish-green

• Low NOx burners

Different Modes of Laminar Combustion

Post flame

oxidation and

radiation

Premixed

flame front

Fuel + air Fuel

Post flame

radiation Surface of

stoichiometric

mixture

Air Air


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The high costs of both water injection and SCR systems give opportunities to develop advanced

combustors, so-called dry low NOx (DLN) combustors.

Moreover, the introduction of steam or water to the gas turbine combustor is a thermodynamic loss, due to

taking some of the energy from combustion gases to heat water or steam. However, DLN combustor has no

impact on the cycle efficiency. Therefore, DLN combustor is more desirable than steam/water injection.

DLN combustor premixes air and fuel, and makes a fuel lean mixture that significantly reduces peak flame

temperature and thermal NOx formation.

Another important advantage of the DLN combustor is that the amount of NOx formed does not increase

with residence time.

Since long residence times are required to minimize CO and unburned hydrocarbon (UHC) emissions, DLN

systems can achieve low CO and UHC emissions while maintaining low NOx levels.

To minimize flame temperature and hence NOx formation the fuel/air mixture is weakened to as near the

extinction point as can safely be realized. The main problems associated with lean premix flames are

stability, inflexibility and the limited turn-down range.

To stabilize the flame, hybrid system having two fuel injectors of main fuel and pilot fuel is used commonly.

In the hybrid system, the bulk of the fuel (more than 75%) is burned in a premixed burner, the remainder

being supplied to a small pilot diffusion flame embedded in the flow.

The main fuel is injected into the air stream immediately downstream of the swirler at the inlet to the

premixing chamber. The pilot fuel is injected directly into the combustion chamber with little if any premixing.

A small portion of the fuel is always burned richer to provide a stable “piloting” zone, while the remainder is

burned lean.

In both cases, a swirler is used to create the required flow conditions in the combustor to stabilize the flame.

DLN Combustor [1/2]


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With the flame temperature being much closer to the lean limit than in a diffusion combustor, some action

has to be taken when the engine load is reduced to prevent flame out.

If no action was taken, flame out would occur since the mixture strength would become too lean to burn.

Due to flame instability limitations of the DLN combustor below approximately 50% of rated load, the

combustor is typically operated in a conventional diffusion flame mode, resulting in higher NOx levels.

DLN fuel injector is much larger because it contains the fuel/air premixing chamber and the quantity of air

being mixed is large, approximately 50-60% of the combustion air flow.

The operation is limited to a narrow range of fuel/air ratio between the production of excessive NOx and

excessive CO.

Some manufacturers are now offering dual-fuel DLN combustors.

However, DLN operation on liquid fuels has been problematic due to issues involving liquid evaporation and

auto-ignition.

This consideration becomes more important as power producers consider converting from natural gas only

to dual-fuel operation as natural gas price rise.

DLN Combustor [2/2]


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1) 순수한 예혼합연소는 잉여공기비(excess air ratio)가 증가함에 따라 NOx 발생이 급격히 줄어들며, 잉여공기비가 2 이하인 경우 CO 발생은 매우 적다.

2) 순수한 예혼합연소는 운전영역이 매우 좁은데, 이는 잉여공기비가 2에 가까워지면 화염이 꺼지기 때문이다.

3) 예혼합연소에 파일럿 화염을 적용하면 화염이 꺼지는 염려가 없이 넓은 잉여공기비에 걸쳐서 운전이 가능하다. 이와 같은 이유 때문에 GE사와 Siemens사 모두 파일럿 화염을 적용한 예혼합연소기를 개발하여 사용하고 있다.

4) 파일럿 화염을 적용한 예혼합연소는 NOx와 CO의 발생을 최소화하기 위해서 운전영역을 NOx와 CO 발생 교차지점으로 제한한다.

5) 확산연소는 잉여공기비 전 영역에 걸쳐서 예혼합연소에 비해 NOx 발생량이 많다.

6) 확산연소의 운전영역은 예혼합연소에 비해 더 큰 잉여공기비를 가지는 부분에서 형성되며, NOx와 CO의 배출량도 훨씬 많다.

Emissions in a DLN Combustion [1/2]

CCPP includes gas turbines with DLN combustors that can

operate with stack gas NOx emission concentration as low

as 25 ppmvd at 15% oxygen without steam or water

injection, when the natural gas is used as a fuel.

NOx can be reduced to less than 9 ppmvd by the installation

of SCR in the HRSG.

Excess air ratio

NO

x a

nd

CO

em

issio

ns, p

pm


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Fla

me

te

mp

era

ture

Fuel to air ratio

Fuel rich Fuel lean

Diffusion combustor

Premixed

combustor

Extinction

of lean

premix

flame

200

250

100

150

50

0

25

Diffusion

combustor DLN

combustor

Catalytic

combustor

NO

x e

mis

sio

n, p

pm

vd

Fuel: natural gas

Emissions in a DLN Combustion [2/2]

Reduction of emissions in the premix combustor

NOx are reduced by

• Lowering flame temperature by lean combustion

• Elimination of local hot spots

CO and UHC are reduced by

• Increasing combustion residence time (volume)

• Combustor design to prevent local quenching


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Diffusion vs. Premix

Discuss the advantages of a lean premix combustor

1) Lower NOx emission Low flame temperature

2) Larger power output Less cooling air is required

3) Lower CO and UHC emission Increased residence time

4) Extended hot gas parts No water/steam injection

Diffusion

combustor

Lean premix

combustor


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1) The fuel-air equivalence ratio and residence time in the flame zone

to be low enough to achieve low NOx.

2) Acceptable levels of combustion noise (dynamics).

3) Stability at part-load operation.

4) Sufficient residence time for CO burn-out.

GE DLN Combustor [1/5]

DLN-1 Combustor


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Operating Modes of DLN-1 Combustor



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A small portion of the fuel is always burned richer to provide a stable ‘piloting’ zone, while the remainder is

burned lean.

Primary

Flame is in the primary stage only. This mode is used to ignite, accelerate and operate the machine over

low- to mid-loads, up to pre-selected combustion reference temperature.

Lean-Lean

Flame is in both the primary and secondary stages. This mode is used for intermediate loads between two

pre-selected combustion reference temperature.

Secondary

Flame is in the secondary stage only. This mode is a transition state between lean-lean and premix modes.

This mode is necessary to extinguish the flame in the primary zone, before fuel is reintroduced into the

primary zone.

Premix

Fuel to both primary and secondary zones. Flame is in the secondary stage only. Optimum emissions are

generated in this mode by premixed flow. In the premix mode, the first stage thoroughly mixes the fuel and

air and delivers a uniform, lean, and unburned fuel/air mixture to the second stage.

A pilot nozzle produces a stable diffusion flame that can maintain high flammability in the premixed flame.

Operating Modes of DLN-1 Combustor



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CO

(p

pm

vd

)

350

300

250

200

150

100

50

0 0 10 20 30 40 50 60 70 80 90 100

0

10

20

30

40

50

60

70

80

90

100

ISO ambient conditions

Gas turbine load, %

NO

x @

15%

O2 (

pp

mvd

)

NOx

CO

Emission Level - GE DLN-1 Combustor (Fuel: NG)



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DLN-2.6 Fuel Nozzle Arrangement



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Siemens DLN Combustor [1/3]

Hybrid Burner


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Most of the fuel is injected through eight main

fuel nozzles in the support housing, which is

divided into two fuel stages of four main nozzles

each.

The remainder of the fuel is divided between the

C-stage and the pilot.

The pilot nozzle includes a diffusion stage and a

premix pilot stage.

By injecting fuel through multiple injection holes

in the swirler vanes, enhanced fuel/air mixing is

achieved, thus reducing the peak temperature of

local hot spots that contribute NOx formation.

[ ULN (Ultra-Low NOx) combustor cross-section ]

ULN Burner

Siemens DLN Combustor [2/3]

Gas only support housing Combustor basket Dual fuel pilot nozzle Dual fuel support housing


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(Supply)

Steam

(Return)

(Return) Bypass valve

M501G steam cooled

liner in fabrication

Premixing nozzle

Pilot nozzle

MHI : G Series Combustor


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EV Cone Burner

Alstom DLN Combustor [1/2]


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Alstom EV burner consists of an axially split diffusing cone, the two halves offset to form tangential slots.

Combustion air is fed through the slots forming a powerful vortex in the cone.

Gas is injected into the vortex via small holes at the edge of the slots.

Pre-mixing in the cone is followed by combustion at exit where the vortex breaks down, allowing

recirculation and flame stabilization.

As the flame is outside the burner, the burner structure remains relatively cool.

The burner is guaranteed by ABB to give less than 25 ppm NOx with natural gas and 42 ppm with oil firing

and water injection.

EV Cone Burner

Alstom DLN Combustor [2/2]


HIoPE


Auto-ignition is the spontaneous self-ignition of a combustible mixture.

For a given fuel mixture at a particular temperature and pressure, there is a finite time before self-ignition

will occur.

DLN combustors have premix ducts on the head of the combustor to mix the fuel uniformly with air.

To avoid auto-ignition, the residence time of the fuel in the premix duct must be less than the auto-ignition

delay time of the fuel.

Auto-ignition delay times for fuels do exist, but a literature survey will reveal that there is considerable

variability for a given fuel.

Reasons for auto-ignition could be classified as follows: 1) long fuel auto-ignition delay time assumed, 2)

variations in fuel composition, 3) fuel residence time incorrectly calculated, 4) auto-ignition triggered early

by ingestion of combustible particles.

If auto-ignition does occur in the premix duct, then it is probable that the resulting damage will require repair

and/or replacement of parts before the engine is run again at full load.

Problems in DLN Combustors

1. Auto-Ignition


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Flashback into a premix duct occurs when the local flame

speed is faster than the velocity of the fuel-air mixture leaving

the premix duct.

Flashback usually happens during unexpected engine

transients, such as compressor surge.

The resultant change of air velocity would almost certainly

result in flashback.

Unfortunately, as soon as the flame-front pressure drop will

cause a reduction in velocity of the mixture through the duct.

This amplifies the effect of the original disturbance, thus

prolonging the occurrence of the flashback.

2. Flashback

Advanced cooling techniques could be offered to

provide some degree of protection during a flashback

event cause by engine surge.

Flame detection systems coupled with fast-acting fuel

control valves could also be designed to minimize the

impact of a flashback.

Thorough mixing is also essential to avoid unsteady

combustion and flashback.

Fairing

Fused Tip

[GE DLN-2 fully faired (flashback resistant) fuel nozzle]


[Damage to fuel nozzles due to flashback]

http://www.power-technology.com/projects/kwangyang/


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Combustion instability, called as “rumble”, only used to be a problem with conventional combustors at very

low engine powers.

It was associated with the fuel-lean zones of a combustor where the conditions of burning are less attractive,

and this is a main cause of oscillatory burning.

In a conventional combustor, the heat release from these oscillatory burning was only a significant

percentage of the total combustor heat release at low power conditions.

In DLN combustors, most of the fuel is burned very lean to reduce flame temperature.

Therefore, these lean zones that are prone to oscillatory burning are now present from idle to full load. This

is the reason why resonance usually occur within the combustor.

The pressure amplitude at any given resonant frequency can rapidly buildup and cause failure of the

combustor.

The use of dynamic pressure transducer in the combustor ensures that each combustor can is burning

evenly. This is achieved by controlling the flow in each combustor can until the spectrums obtained from

each combustor can match.

This technique has been used and found to be very effective and ensures combustor stability.

Fundamentally, stable combustion in DLN combustors requires more accurate control of fuel-air ratio in

combustors at all loads.

Many factors affect the combustor flame stability such as changes in fuel composition, heating value, grid

frequency, ambient conditions, operating load transients, and even operator-influenced conditions during

transient operations.

3. Combustion Instability



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DLN combustors tend to create harmonics in the combustor that may result in vibration and acoustic noise.

As the firing temperature getting higher, dynamic pressure oscillation activity within the combustor, noise,

has increased; increasing wear and necessitating more frequent maintenance.

In DLN combustors, especially in the lean premix chambers, pressure fluctuations can set up very high

vibrations, leading to major failures.

Multi-fuel-nozzle combustion system has been adopted popularly to reduce the noise from combustor by

many gas turbine manufacturers.

The heat from combustion, pressure fluctuation, and vibration in the compressor may cause cracks in the

liner and nozzle.

The edges of the holes in the liner are of great concern because the holes act as stress concentrators for

any mechanical vibrations and, on rapid temperature fluctuations, high-temperature gradients are formed in

the region of the hole edge, giving rise to a corresponding thermal fatigue.

O&M costs for turbines equipped with DLN combustor can be higher because of a variety of factors,

including replacement of blades and vane due to damage resulting from dynamic pressure pulsation, and

combustor sensitivity to changes in fuel composition.

4. Noise


Damage to a GE 7FA fuel nozzle caused by

combustion dynamic instabilities. The

damage from combustion dynamic

instabilities can easily extend to other high-

temperature components including liners and

transition pieces.


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Combustor Type 3





Combustor Cooling 8


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Catalytic Combustor

• The fuel and air are injected

separately into the combustion

zone where they mix and react.

• It tend to have flame temperatures

that are typical of stoichiometric

combustion and therefore produce

high NOx emissions.

• Obtaining reasonable emissions

from a diffusion flame combustor,

generally requires the injection of

diluents into the combustion

section to lower the flame

temperature, typically either steam

or water.

• F-class firing temperatures

produce 25 ppm of NOx.

• The fuel and air are premixed

upstream of flame zone.

• This results in significantly lower

flame temperature than the

diffusion flame combustor resulting

in lower NOx emissions without

diluent injection.

• The limitation on low emissions

from the lean premixed

combustion system is the

combustion instabilities which

occur as the lean flammability limit

of the mixture is approached.

• These instabilities can lead to

large pressure fluctuation in the

combustor.

• F-class firing temperature produce

7-9 ppm of NOx.

• The goal of the ATS program was

the development of a high

efficiency, high firing temperature

engine (>1700 K) with NOx

emissions less than 10 ppm for

lean premixed systems and 5 ppm

for the catalytic system.

• It shows promise to achieve lower

emissions because the

combustion instabilities at the lean

flammability limit are no longer a

limiting factor.

• Although catalytic combustion

systems have not yet been

employed in large industrial gas

turbines, results from current

development are encouraging and

emissions in the range of 2-3 ppm

are achievable.

Diffusion Flame Combustor DLN Combustor

Development History of Combustors


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Catalysts influence a chemical reaction by changing its mechanism:

Reaction without catalyst: A + B = AB (final product)

Reaction with catalyst: A + K = AK (transient product )

AK + B = AB + K

K – catalyst

Catalyst K is preserved in the chemical reaction.

Catalytic Reactions

Siemens metal ceramic coating


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NOx Reduction in a Catalytic Combustor

Definition of catalytic agent: “A substance by its mere presence alters the velocity of a reaction, and may be

recovered unaltered in nature or amount at the end of the reaction.” or “Catalysts is a chemical compound

which influence the rate of chemical reaction by lowering its activation energy, i.e. the initial energy

necessary to initiate the chemical reaction.”

A catalyst promotes a chemical reaction, such as fuel with oxygen, but is itself neither consumed nor

produced by the reaction.

There are three basic classes of reactions that one may desire to promote combustion in gas turbines: fuel

preparation such as reforming prior to combustion, fuel oxidation with heat release, and pollutant destruction.

Catalytic combustion normally refers to fuel oxidation with heat release, particularly when the catalyst is

placed within combustor casing.

The primary motivation of the catalytic combustion is to make combustion temperature lower for reduced

NOx emissions.

The presence of a combustion catalyst enables complete combustion at lower temperatures than otherwise

possible. That is, catalytic combustor can operate stably with flame temperatures far below 1525C.

In addition, catalytic combustor offers lower dynamic pressure oscillations.

Most non-catalytic combustors operate with peak flame temperatures higher than 1525C (2780F) to ensure

adequate flame stability and margin flow blowout.

NOx emissions even for perfectly premixed flames at 1525C can exceed 3 ppm.

Although catalytic combustion systems have not yet been employed in large industrial gas turbines, results

from current development are encouraging and emissions in the range of 2-3 ppm are achievable.


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Fundamentals of Catalytic Combustors

The requirements for the application of catalytic combustor in a gas turbine are as follows:

• Ignition of the fuel/air mixture at typical compressor outlet temperatures. (if a pilot flame has to be used to

reach the ignition temperature, this can produce a significant amount of thermal NOx.)

• High catalyst activity to maintain complete conversion of fuel into thermal energy.

• Low pressure drop over the catalyst.

• Thermal shock resistance.

• Retention of high specific surface area and catalytic activity under high temperatures of operation.

A single substance cannot fulfill all of these. Therefore, a typical catalytic combustor for gas turbines usually

consists of three main components: the support, the washcoat , and catalyst.

[ Schematic of a monolithic catalyst ]


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Main Components for Catalytic Combustion

1. Support

A mechanical support is the base for a high efficiency catalytic combustion system.

In order to minimize the pressure drop over the system, normally a honeycomb structure is used.

Honeycombs give large area to volume ratios, allowing high mass flow with lower pressure drops.

Other requirements of the support are a high thermal shock resistance, low thermal expansion, and

chemically inert to combustion gases.

The monolith material should have a porosity of 30-40% with large pores of a diameter of 5-15 m to obtain

the proper surface for adhesion of a washcoat.

Oxidation is a concern because the support thickness of only 0.01 in.


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Main Components for Catalytic Combustion

2. Washcoat and Catalyst

The washcoat is applied onto the surface of the mechanical support to provide a large specific surface area

that has to be maintained at high temperatures.

Thermal expansion of the washcoat must not differ much from the support material to avoid separation from

support during combustion.

The active metal catalyst can be platinum (Pt), palladium (Pd), rhodium (Rh) or mixture of these compounds.

The activity of catalysts for gas turbine combustors should also be stable and last for at least one year of

operation without problems.

Pd is the most commonly used catalytic combustion due to its enhanced thermal stability compared to Pt

and its high reactivity for CH4 combustion.

At temperatures below about 970 K, Pd is present as PdO. As the temperature increases, a reversible

reduction to metallic Pd takes place, resulting in a decrease of activity. As the temperature increases further,

the activity of metallic Pd exceeds that of PdO.

Pt has a higher activity for CO and saturated hydrocarbons.

However, Pd and Pt have problems in terms of sintering (loss of active surface area) and evaporation at

high temperatures resulting in a deactivation of the catalyst. This is a main obstacle in the development of

catalytic combustor. Additionally, these two materials are expensive noble metals.


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Air

Air

Fuel Gas-Phase

Burnout

Fuel

Fuel

Gas-Phase

Burnout

Air

Air

Fuel

Gas-Phase

Burnout

Air

Air

Fuel

Secondary fuel/air

Secondary fuel/air

Air

Air

Fuel

Premixer Catalytic Reactor

Type of Catalytic Combustors

(e) Rich catalytic lean

burn Gas-Phase

Burnout

Air

Air

Fuel + Air Premixer Catalytic Reactor

Air cooled catalyst

(d) Hybrid

combustion with

secondary fuel/air

(c) Hybrid combustion,

full burn-out

downstream of the

catalysts

(b) Hybrid combustion

with secondary fuel

(a) Fully catalytic

combustion by

multiple segments


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1. Fully catalytic combustion by multiple segments

The total premixed fuel/air stream is oxidized by a set of catalyst segments.

The reaction should be completed (99.9% combustion efficiency) within the catalyst bed. Therefore, there is

no need to separate the gas-phase combustion zone from the catalyst.

Thus, the temperature just downstream of the last segment is same as the turbine inlet temperature.

The advantage of this type of combustor is its simplicity as no secondary fuel or air has to be mixed in and

no flame has to be anchored.

However, the very high temperatures in the final segments as well as the difficulty of controlling the heat

release in the first segments will probably make this design difficult for most gas turbine applications.

Normally, highly active noble metal catalysts have been used in the first segments in order to initiate the

oxidation at a low inlet temperature. Therefore, the implementation of a pilot flame for start-up procedure is

not necessary.

The difficulty of finding suitable materials maintaining high catalytic activity and thermal stability under

prolonged exposure time above 1300C has yet to be solved.

Metal substrates are more suitable than ceramic ones for gas turbine applications because they are able to

withstand gas turbine demands such as thermal stress and thermal shock.

However, metal temperatures must normally by limited to less than about 950C (1750F) for long durability.

Air

Air

Fuel


(a) Fully catalytic combustion by multiple segments


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2. Hybrid combustion with secondary fuel

Most of the fuel passes through the catalyst segments but some of the fuel is mixed downstream of the last

catalyst segment and is burnt in a homogeneous reaction zone downstream catalysts.

This design is supposed to keep the temperature of the catalyst segments below 1000C.

The catalyst will work as a preheater increasing the temperature to a level where ultra-lean combustion may

take place.

Hence, the flame limits could be extended to fuel-air mixtures yielding temperatures well below the ones

needed for the formation of thermal NOx.

Air

Air

Fuel Gas-Phase

Burnout

Fuel

Fuel Premixer Catalytic Reactor

(b) Hybrid combustion with secondary fuel


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3. Hybrid combustion using inactive channels

All of the premixed fuel/air enter the first segment.

Selected number of channels of the monolith support have not been coated with catalysts.

The unburnt fuel passing through the inactive channels is oxidized downstream of the last segment in a

homogenous reaction zone.

This design has two advantages. One is that there is no need for a secondary mixing zone after the last

catalyst segment. The other is that the unreacted gas in the inactive channels will cool the active channels

and thereby limit the temperature of the catalysts.

In this case, combustor outlet temperatures of up to 1400C and at the same time keeping the catalyst

temperature below 1000C were demonstrated by GE.

Clearly, this design has the advantage of reducing the operating temperature of the catalyst, thus increasing

the lifetime of the catalyst.

Gas-Phase

Burnout

Air

Air

Fuel


(c) Hybrid combustion, full burn-out downstream of the catalysts


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4. Hybrid combustion using secondary fuel/air

Secondary fuel/air is injected downstream of the last catalyst segment.

The disadvantage of such a design is the difficulty of achieving a homogeneous mixture of fuel and air in

the post-catalytic reaction zone.

Advantage is the catalyst temperature can keep low enough to avoid deterioration.

It has been demonstrated that the catalyst bed temperature could be kept between 700 and 800C with the

ignition temperature being 360C. Combustor outlet temperature as high as 1390C at 13 bar pressure

realized.

A Pt/Pd/Rh-based cordierite honeycomb catalyst has been used in the experimental work and NOx level

below 5 ppm were measured.

Gas-Phase

Burnout

Air

Air

Fuel

Secondary fuel/air

Secondary fuel/air


(d) Hybrid combustion with secondary fuel/air


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5. Rich catalytic lean burn [1/2]

All the above-mentioned designs are based on lean premixed combustor design.

The inlet air flow to the catalyst is separated into two streams, and a portion of the air is mixed with the fuel

and reacts on the surface of the catalyst under fuel rich conditions.

The remaining air is used to cool the backside of the catalyst.

The two streams mix at the catalyst exit and then react and burnout in the homogeneous reaction zone.

By operating the catalyst in the fuel rich region, the reaction rate is limited by the rate of diffusion of oxygen

to the catalyst surface.

Therefore, RCL design is able to tolerate wider variations in fuel-air ratio within the catalyst region than the

LCL design.

RLC does not require the preburner, because the fuel and air react at compressor discharge temperature of

typical gas turbines.

The choice of catalyst is critical for RCL in order to insure proper catalyst lightoff *.

(e) Rich catalytic lean burn

Gas-Phase

Burnout

Air

Air

Fuel + Air Premixer Catalytic Reactor

Air cooled catalyst

* Lightoff is defined as the temperature at which the catalyst surface initially becomes active.


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Rich catalytic module


5. Rich catalytic lean burn [2/2]

[ Rich catalytic lean burn ] [ Lean catalytic lean burn ]


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High Temperature Catalytic Combustors

In the high firing temperature gas turbines, such as FB-class and H-class GTs, gas-phase combustion

temperatures may need to exceed 1525C (2780F), before addition of cooling air, in order to meet required

TIT.

At the current level of firing temperature and catalysts development, NOx emissions will exceed 3 ppm at

15% O2.

However, it has been reported that catalytic combustors have low combustion dynamics, because gas-

phase energy release in the combustor is the driving force for combustion-induced pressure oscillations

(combustion dynamics) and these oscillations are reduced when a portion of the fuel is catalytically reacted

prior to gas-phase combustion.

Regardless of pollutant emission levels, however, catalytic combustion may prove useful even when

temperatures must be well in excess of 1525C (2780F).

In fact, combustion dynamics often become most problematic at high flame temperatures and a solution is

most needed.

In addition, non-catalytic premixed combustors employ piloting or fuel staging to improve combustion

dynamics.

Thus, a catalytic combustor with low combustion noise without piloting or fuel staging may offer reduced

NOx emissions as compared to an equivalent non-catalytic system.


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Catalytic combustor in the 501D5

Catalytic combustor in the SGT6-5000F

Catalytic Combustor


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Lean Catalytic Lean Burn (LCL) Design

All of the fuel and air are premixed and

enter catalyst section under fuel lean

conditions.

At the end of the catalyst section any fuel

not reacted is burned out in a reaction zone.

To insure proper catalyst activity, the inlet

temperature of fuel-air mixture to the

catalyst of approximately 500C. Since this

temperature is higher than the compressor

discharge temperature of a typical gas

turbine, a preburner will be necessary.

Operation of the catalyst in the lean region

requires very close control of the fuel-air

ratio in the vicinity of the catalyst to avoid

high reaction rates and excessive catalyst

temperatures.

This technology has been commercially

operated on a small gas turbine, Kawasaki

1.5 MW, and has been studied by GE and

Siemens on large gas turbines.


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Combustor Type 3





Combustor Cooling 8


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In early gas turbine engines

25~30% : used for combustion

70~75% : used for cooling

In the modern engines

45% : used for combustion

35% : used for cooling the

combustor

20% : used for cooling the turbine

By using more air to support combustion, the

thermal efficiency of the engine is improved

and the size of the engine for a given output is

reduced.

Use of Air in a Combustor

Compressor

Fuel

Turbine

Air

Power

Exhaust NOx ~

25 ppm 350C

Bypass Air

1800C 1300C


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Film Cooling [1/2]

The liner is exposed to a high temperature because of heat radiated by the flame and combustion.

To extend the life of the liner, it is necessary to lower the temperature of the liner and use a material having a

high resistance to thermal stress and fatigue.

The air film cooling method reduces the temperature of the liner.

This reduction is accomplished by fastening a metal ring inside the liner to leave a definite annular clearance.

Air is admitted into this clearance space through rows of small holes in the liner and impingement cooling is

done at this stage, and the air is directed by the metal rings as a film of cooling air along the liner inside.


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Air film flow prevent carbon from forming on the inside of the liner. Carbon deposits

can cause hot spots or block cooling air passages.

Liner Types

Film Cooling [2/2]


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Thermal Barrier Coating

Modern combustors also make extensive use of thermal barrier

coatings to further insulate the metal from the extreme gas

temperatures.

TBC is now standard on most high-performance gas turbines.

The thickness of coating layer is 0.4~0.6 mm and can reduce

metal temperatures by 50~150C on the basis coating material of

ZrO2-Y2O3. (cooling temperature is about 4~9C per mil of coating

layer thickness)

TBC consists of two layers. The first layer is a bond coat of

NICrAlY and the second is a top coat of YTTRIA-stabilized

zirconia.

The purpose of the TBC is to increase the thermal efficiency of the

gas turbine through reduced cooling air and maintaining higher

gas temperature.

The purpose of the bond coat is to insulate oxide.

Hot gases

Thermal Barrier Coating

Bond Coat

Base

Material

Cooling gases

1400C 1200 1000


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Steam Cooling

(Supply)

Steam

(Return)

(Return) Bypass valve

Premixing nozzle

Pilot nozzle


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질의 및 응답

작성자: 이 병 은 (공학박사) 작성일: 2015.02.11 (Ver.5) 연락처: [email protected]

Mobile: 010-3122-2262 저서: 실무 발전설비 열역학/증기터빈 열유체기술

Documents

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