4. combustor · 2017-12-21 · combustion is the process whereby the hydrogen and carbon in fuels...

Gas Turbines for Power Plants 4. Combustor 1 / 118 Combined Cycle Power Plants

4. Combustor


Fuel and Combustion Theory 2 1

Factors Affecting Combustor Design 40 3

Combustor Type 50 4

NOx Formation and Its Control 63 5

Diffusion Combustor 75 6

Dry Low NOx Combustor 84 7

Catalytic Combustor 109 8

Major Components 22 2


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

accelerating the gases into the turbine section. The resulting expanded and accelerated high

temperature exhaust gas is used to turn the turbine and produce power to drive a generator.

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, the gas is accelerated.

Air Inlet Compressor Combustors Turbine Exhaust

Cold Section Hot Section

Combustor


The combustor is the heart of the

engine, as it is where the fuel is

burnt. Without that, the engine

would produce no power.

The combustor consists of an outer

casing, an inner perforated liner, a

fuel injection system, and a starting

ignition system.

Combustors are made from

materials that can withstand the

high temperatures of combustion.

The casing must withstand the cycle

pressures and may be a part of the

structure of the gas turbine.

Fundamentals for Combustor [1/2]

The combustion chamber has the difficult task of burning large quantities of fuel with extensive volumes of air,

and releasing the heat.

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

the limited space available.

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

combustor.

In the gas turbine engine, the maximum temperature is limited to 1700°C by the materials from which the

turbine blades and nozzles are made.

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 liner.

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.

Fundamentals for Combustor [2/2]

Gas Turbines for Power Plants 4. Combustor 6 / 118

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 [1/5]


Combustion [2/5]

Combustion 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 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 kerosene) 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 kerosene were burned in air, which contains 21% oxygen and 78% nitrogen, the exhaust could also

include nitrous oxides (NOx, nitrogen + oxygen).


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. (Kerosene 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 [3/5]


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 [4/5]


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) 등유 (C12H26)의 공연비 = 15.02 : 1

Fuel-Air Ratio

Combustion [5/5]

=


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


Natural gas will be the choice of most combined cycle power plants if natural gas is available since its effects

on pollution are minimal and maintenance cost will also be the lowest and gives longer machine life.

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.

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

high temperature gas turbines.

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.

Natural Gas [1/3]


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]


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.

The Wobbe index is used to compare the combustion energy output of different composition fuel gases in the

combustor. If two fuels have identical Wobbe indices then for given pressure and valve settings the energy

output will also be identical.

The natural gases have Wobbe index of 1220 10%.

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.

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]


DLN Combustor Range


About 40% of the turbine power installed operates on liquid fuels. Natural gas is the preferred fuel for

industrial engines. If natural gas is not available, however, 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]


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.

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.

Maximum allowable limits are normally set by gas turbine manufacturers for trace metals because of their

corrosive effects in the hot gas path parts of the gas turbine.

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

Vanadium pentoxide (V2O5) is 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.

Liquid Fuel [2/4]


Sodium and potassium levels can be reduced by providing a system for washing the oil with water.

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.

Crude is attractive as a fuel, because it is burned in pumping station directly from the pipiline.

Liquid Fuel [3/4]


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.

Sulfur oxides in combination with excess air raise the dew point temperature very significantly, producing

liquid sulfuric acid at well above the dew point temperature.

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 salts, 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]


Fuel Requirements

Fuel requirements are important in the design of a combustion system and any necessary fuel treatment

equipment.

The heating value of a fuel affects the overall size of the fuel system. Fuel heating value is more important

concern in connection with gaseous fuels because they have various types and have wide range of heating

value from natural gas to process gas. The less heating value, the larger fuel systems.

Cleanliness of the fuel must be monitored if the fuel is naturally dirty or can pick up contaminants during

transportation.

• Contaminants can cause damage or fouling in the fuel system and result in poor combustion.

• Cleanliness is a measure of the water and sediment (ash) and the particulate content. Water and sediment are found in

liquid fuels, while particulates are found in gaseous fuels.

• Particulates and sediments cause clogging of the fuel filters, erosion and deposition on hot section parts. Water leads to

oxidation in the fuel system and poor combustion.

Corrosivity is important because corrosion by usually occurs in the hot section of the engine.

Deposition and fouling can occur in the fuel system and in the hot section of the gas turbine. Deposition rates

depend on the amount of certain compounds contained in the fuel.

• Carbon residue, pour point, and viscosity are important properties in relation to deposition and fouling.

• The carbon residue is a measure of the carbon compounds left in a fuel after the volatile compounds have vaporized.

• The carbon residue property shows the tendency of a fuel to carbon deposit on the fuel nozzles and combustion liner.

• Both pour point and viscosity measure the tendency of a fuel to foul the fuel system.

Fuel availability should be considered. If the fuel reserves are unknown, or seasonal variations are expected,

dual fuel capability must be considered.

The ash content of liquid fuels is important in relation to cleanliness, corrosion, and deposition.


Source: Gas Turbine Engineering Handbook, M.P. Boyce

Effect of Fuel on Operation and Maintenance


Fuel and Combustion Theory 1

Factors Affecting Combustor Design 3

Combustor Type 4

NOx Formation and Its Control 5

Diffusion Combustor 6

Dry Low NOx Combustor 7

Catalytic Combustor 8

Major Components 2


Primary air: combustion (연료와 반응하여 고온가스 생성)

Secondary air: cooling (연소기 및 고온가스 냉각)

연소기 구성 및 공기흐름

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Diffuser

디퓨저는 압축기를 빠져나온 고속의 압축공기 속도를 줄여서 연소기로 보내줌

일반적으로 압축기를 빠져나오는 압축공기는 마하수 0.5 정도의 속도를 가짐. 그런데 압축공기가 이렇게 빠른 속도로 연소기에 공급되면 화염이 안정적으로 형성되기 어려움

따라서 압축기를 빠져나온 압축공기를 디퓨저를 통과시켜 마하수 0.35 정도로 감속시켜 줌

연소기에서 화염이 안정적으로 유지되기 위해서는 연소기 입구에서 공기속도가 느려야 함

디퓨저 출구에서 일반적으로 나타나는 마하수 0.35의 속도는 매우 빠른 편임. 그러나 디퓨저에서 마하수 0.35 이하의 속도로 낮추기 어려움

이는 디퓨저에서 공기속도를 더 낮추기 위해서는 디퓨저의 확산각도를 증가시켜야 하는데, 이 경우 디퓨저에서 유동박리가 일어나기 쉬우며, 유동박리가 일어나면 디퓨저 기능을 상실할 뿐만 아니라 난류강도가 증가하여 압력손실이 증가하는 문제가 발생하기 때문임

따라서 디퓨저에서 감속시킬 수 있는 한계는 마하수 0.35 정도이며, 압축공기 속도는 연소기 입구에서 추가적으로 낮추어 줌

디퓨저 [1/2]


디퓨저에서는 압축공기 속도가 줄어들기 때문에 압축공기의 정압(static pressure) 상승

따라서 디퓨저 출구에서 가스터빈에서 가장 높은 정압이 나타남

그러나 디퓨저에서 정체압력은 일정하게 유지

그러나 실제적으로 디퓨저 벽면에서 마찰손실이 발생하기 때문에 디퓨저를 통과하는 동안 압축공기의 정체압력은 약간 낮아짐

디퓨저 [2/2]


연료분사기 [1/2]

가스터빈 연료는 최종적으로 연료분사기를 통해 연소영역으로 분사

가스터빈에 사용하는 액체연료는 연소시키기 위해 먼저 기화시켜야 함

연료분사기(fuel injector)는 액체연료가 쉽게 기화될 수 있도록 액체연료를 무화시켜 작은 액적으로 만들어 표면적을 증가시킴

액적 상태의 연료는 가스터빈 연소기에서와 같이 높은 압력의 공기 속에서 기화가 촉진되어 쉽게 연소

연료분사기 종류: 압력무화식, 공기충돌식, 증발식, 예혼합식

1) 압력무화식(pressure-atomizing) 연료분사기

• 연료에 500 psi의 높은 압력을 가하여 연료를 무화시킴

• 압력무화식 연료분사기는 매우 간단하다는 장점을 가지고 있지만 몇 가지 단점도 가지고 있다.

• 첫째, 연료계통이 고압에 견딜 수 있도록 견고하게 제작해야 하는데, 이는 연료계통 무게 증가의 원인이 된다.

• 둘째, 연료 무화가 고르게 일어나지 않는 경향을 가지는데, 이는 더 많은 공해물질과 연기(smoke)를 발생시키는 불완전연소 또는 고르지 못한 연소의 원인이 된다.

2) 공기충돌식(air blast) 연료분사기

• 얇은 막 형태로 분사되는 연료(sheet of fuel)에 고속으로 공기를 충돌(blast)시켜 균일한 액적으로 연료를 무화시킴

• 연료를 무화시키기 위하여 단지 1차공기의 일부분 사용

• 공기충돌식 연료분사기를 적용하여 최초로 연소기에서 연기가 발생하는 문제 해결

• 공기충돌식 연료분사기는 압력무화식에 비해 낮은 연료압력 사용


연료분사기 [2/2]

3) 증발식 연료분사기

• 공기충돌식과 유사하며, 연료가 연소영역으로 분사되면서 1차공기와 혼합

• 그러나 연료-공기 혼합기는 연소영역 내부에 있는 튜브를 따라 흘러감

• 이 과정에서 연소영역의 열이 연료-공기 혼합기로 전달되어 연소되기 전에 연료의 일부가 증발하며, 이로 인해 연료와 공기의 혼합이 촉진

• 증발식 연료분사기를 이용하면 복사열이 줄어든 상태에서 연소가 일어나기 때문에 연소기 라이너를 보호할 수 있음

• 그러나 증발튜브는 그 내부를 흘러가는 연료에 의해 냉각되기 때문에 연료유량이 적은 경우에 수명이 짧아지는 심각한 문제를 가지고 있음

4) 예혼합(premixing) 연료분사기

• 연료가 연소영역에 도달하기 전에 공기와 미리 혼합하여 연소영역에 공급

• 연소가 일어나기 전에 연료와 공기를 미리 혼합시키면 연료와 공기를 매우 균일하게 혼합시킬 수 있으며, 이를 통해 공해물질 발생을 줄일 수 있음

• 연료와 공기를 미리 혼합하면 연료-공기 혼합기를 더 낮은 온도에서 연소시킬 수 있기 때문에 NOx 발생을 크게 줄일 수 있음

• 예혼합 연료분사기의 단점은 미리 혼합된 연료-공기 혼합기가 연소영역에 도달하기 전에 자동점화(auto-ignition)될 수 있다는 점

• 아울러 미리 혼합된 연료-공기 혼합기의 속도가 화염 전파속도보다 작은 경우 연료-공기 혼합기가 연소영역에 도달하기 전에 화염이 역방향으로 전파되어 연소. 이를 플래시백(flashback)이라 함

• 자동점화와 플래시백이 일어나면 연소기는 심각한 손상을 입게 됨


선회기 [Swirler]

① 연료와 공기의 혼합 촉진

② 화염 길이를 짧게 유지시켜 연소실 길이 축소. 이를 통해 가스터빈 무게 절감

③ 연료노즐 하류에 재순환영역을 만들어 화염정착을 도모함으로써 연소기 내부에 화염이 안정적으로 유지되게 해줌


점화기 [Igniter]

대부분의 가스터빈은 자동차에 사용하는 점화기와 유사한 전기불꽃점화기(electrical spark igniter) 사용

점화기는 연료와 공기가 혼합되어 있는 연소영역에 위치해야 하지만 연소에 의해 손상 받지 않도록 연소기 상류에 위치해야 함

점화기에 의해 일단 연소가 일어나면 연소기 내부에 형성되는 재순환영역에 의해 화염이 유지되기 때문에 점화기는 더 이상 필요하지 않음

따라서 점화기는 점화가 완료된 후에 집어넣을 수 있는 형태(retractable type)로 되어 있는 것도 있음


케이싱은 연소기 외부를 구성하는 얇은 판으로서 매우 단순한 구성품

케이싱은 내부를 흐르는 공기에 의해 연소기에서 발생하는 열로부터 보호되기 때문에 열적 성능은 전혀 문제되지 않음

그러나 케이싱 내부는 높은 압력이 작용하는 데 반해 외부는 낮은 압력이 작용하기 때문에 연소기 케이싱은 일종의 압력용기

따라서 연소기는 구조적으로 안정돼야 함

아울러 연소기 케이싱은 가스터빈 외관을 결정하는 요소이기 때문에 압축기 및 터빈 케이싱과 조화를 이루어야 함

케이싱


Liner

라이너 [1/7]

라이너는 연소기에 형성되는 화염을 둘러싸고 있는 일종의 튜브 (Liner is a wall to contain the flame)

따라서 라이너는 화염관(flame tube)이라고도 불림

라이너에는 연소기 내부로 1차공기(primary air)와 2차공기(secondary air)를 안정적으로 공급하기 위하여 다공판 형태로 많은 구멍이 뚫려 있음

https://www.google.co.kr/url?sa=i&rct=j&q=&esrc=s&frm=1&source=images&cd=&cad=rja&uact=8&ved=0ahUKEwj5lYPJ9qXKAhWVBI4KHWATA5oQjRwIBw&url=https://en.wikipedia.org/wiki/Combustor&psig=AFQjCNFE-sPy_-FCJFu-UYtlphDXjK2GUw&ust=1452744652659027


A representative flow pattern in an annular combustor with double air swirler

(from Tanica, 2000)

Primary zone

Intermediate zone

Dilution zone

Mixing and combustion

Completion of combustion (2CO+O2 2CO2)

Cooling

연소기 내부 공기흐름 및 연소영역

라이너 [2/7]

Primary, secondary and tertiary

injector holes through the liner,

these are often plunged (rounded)

to improve cd (discharge coefficient)

and jets positional stability. Mach

number through the holes is of the

order of 0.3 to provide sufficient

penetration of the jets into the

combustor.

A slow moving recirculating primary

zone to enable the fuel injected to

be mixed sufficiently with the air to

facilitate combustion and flame

stabilization.

A secondary zone where further air

is injected and combustion is

completed.

A tertiary zone where the remaining

air is injected to quench the mean

exit temperature to that required for

entry to the turbine, and to control

the radial and circumferential

temperature traverse.


연료가 분사되는 연료노즐 바로 하류에 1차영역(primary zone) 형성

1차영역에서는 연료와 공기의 혼합이 일어나면서 연소가 일어남

연료와 혼합되어 연소에 사용되는 공기를 1차공기라 함

1차공기의 일부는 선회기를 통해 연소기 중앙부로 공급되며, 일부는 라이너 상류에 있는 작은 구멍을 통해 반경방향으로 공급. 1차공기 양은 압축기를 빠져 나온 전체 공기의 약 1/3이하 정도

2차영역(secondary zone, or intermediate zone)에서는 계속해서 연소가 진행

가스터빈 연료는 탄소와 수소가 결합되어 있는 탄화수소. 탄소는 연소과정에서 일차적으로 산화되어 CO 형성. 이렇게 생성된 CO는 2차영역에서 다시 산소와 결합하여 CO2가 되면서 탄소의 연소 완료

따라서 탄소는 폭발적으로 연소하는 수소에 비해 연소가 느리게 진행. 그러므로 연료의 연소는 2차영역에서 종료되며, 1차영역과 2차영역을 합친 길이는 연소기 전체 길이의 약 75% 차지

연소기에서 생성된 배기가스 온도는 터빈으로 직접 보내기에는 너무 높음

따라서 2차영역 하류에 있는 희석영역(dilution zone, or tertiary zone)에 추가적으로 압축공기를 공급하여 배기가스와 혼합시켜 배기가스 온도를 터빈에 적합한 온도로 낮추어 줌

희석공기는 라이너 하류에 있는 구멍을 통해 연소기로 유입

최근에 개발된 가스터빈은 성능을 향상시키기 위하여 증가된 TIT를 적용함에 따라 더 적은 양의 희석공기를 사용. 이렇게 줄어든 희석공기는 다시 연료의 연소에 사용할 수 있기 때문에 가스터빈 성능이 더욱 향상되며, 출력도 증가

라이너 [3/7]


라이너는 연소기에서 생성된 화염을 감싸고 있기 때문에 냉각 필요

라이너는 냉각공기를 이용하여 냉각시킴.연소기에서 희석공기와 냉각공기를 합쳐서 2차공기라 함

라이너는 장기간 높은 화염온도에 노출되기 때문에 초합금(superalloy)으로 제작

그러나 초합금을 이용하여 제작하더라도 라이너는 냉각이 필요하며, 이를 위해 연소기를 빠져나온 압축공기를 이용하여 충돌냉각과 막냉각 또는 침출냉각을 실시하며, 라이너 내부에 열차폐코팅 실시

막냉각은 라이너 외부에 있는 냉각공기를 라이너에 있는 작은 구멍을 통해 라이너 바로 안쪽으로 주입하는 방법. 이렇게 주입된 냉각공기는 라이너를 높은 열로부터 보호하기 위하여 라이너 안쪽 표면에 얇은 냉각공기 막을 형성

침출냉각은 라이너를 다공성 물질로 제작하여 라이너를 냉각시키는 방법

침출냉각을 이용하면 적은 양의 냉각공기를 이용하여 막냉각과 비슷한 냉각효과를 얻을 수 있음

침출냉각과 막냉각의 큰 차이점 두 가지는 냉각 결과 나타나는 라이너의 온도분포와 냉각에 사용되는 냉각공기 양임

• 침출냉각은 다공성 물질로부터 무수히 많은 작은 구멍을 통해 냉각공기가 균일하게 주입되기 때문에 막냉각에 비해 훨씬 균일한 라이너 온도분포를 얻을 수 있음

• 막냉각의 경우 냉각공기는 구멍이나 루버를 통해 라이너 내부로 유입되기 때문에 구멍과 구멍 사이는 상대적으로 온도가 높고, 구멍 주위는 온도가 낮아서 라이너 온도분포가 균일하지 못함

• 더욱 중요한 것은 침출냉각은 막냉각에 비해 훨씬 적은 양의 냉각공기를 사용한다는 점임

• 막냉각이 전체 공기의 20~50%를 냉각에 사용하는 데 반해 침출냉각은 10% 정도만을 사용

• 냉각공기 양을 줄이면 더 많은 공기를 연소에 사용할 수 있기 때문에 가스터빈 성능이 향상되고 엔진 출력도 증가

라이너 [4/7]


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

can cause hot spots or block cooling air passages.

막냉각 [Film Cooling]

라이너 [5/7]


막냉각과 충돌냉각

충돌냉각(impingement cooling)은 냉각공기를 라이너에 고속으로 충돌시켜 냉각시키는 방법

공기를 고속으로 물체 표면에 충돌시키면 대류열전달계수가 증가하여 물체를 더욱 효과적으로 냉각시킬 수 있음

충돌냉각에 사용된 냉각공기는 라이너에 연속적으로 형성되어 있는 슬롯을 따라 흐르면서 냉각공기 막을 형성하여 이차적으로 막냉각이 일어나게 함

라이너 [6/7]


열차폐코팅

Hot gases

Thermal Barrier Coating

Bond Coat

Base

Material

Cooling gases

1400C 1200 1000

열차폐코팅(TBC; Thermal Barrier Coating)이란 고온가스로부터 라이너 모재로 전달되는 열을 줄여주기 위해 열전도도가 매우 낮은 세라믹을 이용하여 라이너 내부 표면에 얇은 두께로 코팅함으로써 절연층을 만들어 주는 것

이렇게 라이너 내부 표면에 절연층이 형성되어 있으면 라이너 표면을 고온의 배기가스로부터 절연시켜 라이너 표면온도를 낮게 유지시켜줄 수 있음

이 경우 라이너 온도를 낮추기 위한 냉각공기 양을 줄일 수 있으며, 이로 인해 가스터빈 성능이 향상되고 엔진 출력도 증가

열차폐코팅은 두 개의 코팅층, 즉 본드코팅과 탑코팅으로 이루어짐

본드코팅은 라이너의 산화저항성과 부식저항성을 향상시키기 위해 실시

고온의 배기가스 열이 라이너로 전달되는 것을 차단하기 위해 실시하는 탑코팅 재료는 세라믹

라이너 [7/7]

http://www.google.co.kr/url?sa=i&rct=j&q=&esrc=s&frm=1&source=images&cd=&cad=rja&uact=8&ved=0ahUKEwi73ZDut6XKAhWDJpQKHYQdBVMQjRwIBw&url=http://www.arci.res.in/programmes-clpm&psig=AFQjCNGi7KLmJriY9KdymsOtllmxfVyj9A&ust=1452728038940930


Steam Cooling

(Supply)

Steam

(Return)

(Return) Bypass valve

Premixing nozzle

Pilot nozzle


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 is increased.

Use of Air in a Combustor

Compressor

Fuel

Turbine

Air

Power

Exhaust NOx

~ 25 ppm 350C

Bypass Air

1800C 1300C




Combustor Type 4





Major Components 2


1) High combustion efficiency at all operating conditions.

2) Minimized pollutants and emissions: Low levels of unburned hydrocarbons (unburned fuel), smoke (carbon

particles), carbon monoxide, oxides of nitrogen.

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 contributes to form of fouling on turbine blades and HRSG tube banks.

Design Requirement for Combustors


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, but 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.

= 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


The combustion intensity is a measure of the rate of heat release per unit volume.

The heat released by a combustion chamber is dependent on the volume of the combustion area. Thus, to

obtain the required high power output, a comparatively small and compact gas turbine combustion chamber

must release heat at exceptionally high rates.

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.

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.

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.

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


Combustion Stability

The ability of the combustion process to sustain itself

in a continuous manner is called combustion stability.

Combustion stability means smooth burning and the

ability of the flame to remain alight over a wide

operating range.

Stable and efficient combustion can be upset by too

lean or too rich mixture.

This situation causes blowout of the combustion

process.

The effect of mass flow rate, combustion volume and

pressure on the stability of the combustion process are

combined into the Combustor Loading Parameter

(CLP), defined as

VolumeCombustion p

mCLP

n

mixture


Local Mach Number

Low annulus Mach number is essential to maintain a level of Mach number for the injector ports of around 0.3,

since a ratio of injector port to annulus Mach number of greater than 2.5 is required for good coefficient of

discharge.

The injector port Mach number of 0.3 is a compromise between minimizing pressure loss while achieving

good penetration. Unless the ports are angled it is reasonable to assume that half of the air entering the

primary ports joints the upstream primary zone, and half the downstream secondary zone.

The flow regime in the primary zone is complex with a recirculation. This is essential to mix the fuel and air

properly, and to provide a region of slow velocity in which the flame may be stabilized.

The mean axial Mach number leaving the primary zone must be of the order 0.02~0.05.

After the secondary zone air flow has been introduced, the Mach number within the liner may rise to around

0.075~0.1.

Finally, the tertiary air is introduced and the flow is accelerated along the turbine inlet duct to about 0.2 at the

nozzle guide vane leading edge.

The contents are written based on the diffusion combustor for aviation using liquid fuel.

[ Combustor Mach numbers ]


Pressure Loss in a Combustor [1/2]

The pressure loss occurred in a combustor is very important parameter because it affects gas turbine

efficiency and power output.

The gas turbine efficiency will be reduced by an equal percentage of pressure loss.

Compressor exit Mach number will be of the order of 0.2~0.35. This must be reduced in the combustor entry

diffuser to between 0.05 and 0.1 around the can, otherwise can wall pressure loss will be unacceptably high.

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 combustor cold loss is due to the dump of air being injected through the wall.

The fundamental loss or hot loss is caused by temperature rise due to combustion.

Flow in a duct with heat transfer is called Rayleigh flow and the fundamental thermodynamics dictate that

there is a pressure loss associated with the heat release; reduced density increases velocity, requiring a

pressure drop for the momentum change.

With the typical combustor Mach number of 0.025 design point hot loss is around 0.05% and 0.15% for

temperature ratios of 2 and 4 respectively.


1

1,

2,

21

o

o

T

TKKPLF

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

V1 V2

1 2

1

2/ 1,

2,

2

11

2,1,

o

ooo

T

T

V

pp

Pressure Loss in a Combustor [2/2]


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

The failure mechanism of a turbine blade is related primarily

to creep and corrosion and secondarily to thermal fatigue.

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 buckets 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


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




Combustor Type 4





Major Components 2


Can-Type Combustor [1/7]

Arrangement

HA Gas Turbine, GE


Arrangement



Transition

Piece

1st Stage

Nozzle

1st Stage

Blades

3rd Stage

Blades

Combustion

Can

2nd Stage

Nozzle

2nd Stage

Blade

3rd Stage

Nozzle

Transition Piece


Transition piece guides the flow of the hot gas from the combustor to the inlet of the turbine.


Can type combustor, which is also called as multiple chamber, 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.



Can type combustors can be of the through-flow 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.

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

turbines.

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.



Swirler


① 선회유동(소용돌이) 형성 (수백 fps의 축방향 공기속도를 5~6 fps로 감속)

• 축방향 공기속도를 줄여 연소정지( flame-out) 방지

• 재순환영역을 만들어 화염 정착 도모

② 연료-공기 혼합 촉진

③ 화염길이 짧게 유지

• 연소실 길이 축소

• 터빈으로의 화염전파 방지


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



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 [1/3]

Configuration


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]



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 in 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.

• A major disadvantage is that it is hard to repair it.


[GEAE ] [ Siemens ]


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


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




Combustor Type 4





Major Components 2


The primary pollutants produced by gas turbines are NOx, CO, smoke, and UHC.

Smoke is produced due to the production of finely divided soot particle in fuel rich regions of the flame and

can be produced anywhere in the combustion zone where mixing is inadequate.

Most of the soot produced in the primary zone is consumed in the high temperature regions downstream.

Sulfur dioxide, particulate matter (PM) and trace amounts of hazardous air pollutants may also be present

when liquid fuels are used.

Gas turbine engineers are continually challenged to enhance thermal efficiency while maintaining or reducing

emissions.

However, this is difficult because these are conflicting goals.

In order to improve thermal efficiency, higher TIT is required. However, higher TITs promote NOx formation

because the thermal NOx formation increases exponentially at higher 1538C, the threshold for themal NOx

formation.

Furthermore, reducing available oxygen to reduce NOx can result in higher CO and UHC emissions due to

incomplete combustion.

OEMs have developed processes that use air as diluent to reduce combustion flame temperatures and

reduce NOx by premixing fuel and air before they enter the combustor.

This lean premixed combustion process is referred to various trade names, GE and Siemens use DLN (Dry

Low NOx), Rolls-Royce uses DLE (Dry Low Emission), and Solar Turbines uses SoLoNOx.

Emission


NOx Formation [1/2]

There are two mechanisms of oxides of nitrogen (NO and NO2, collectively called 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(1538C). 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.

Fuel NOx are also produced by the conversion of a fraction of any nitrogen chemically bound in the fuel.

Emission of fuel NOx is insignificant when use natural gas as a fuel, but must be considered when use lower

quality distillates and syngas.


[ General Emission Performance of a Lean Premix Combustor ]

1700 1200 1300 1400 1500 1600

140

120

100

80

60

40

20

0

Reaction temperature, C

NO

x, C

O,

vp

pm

@ 1

5%

O2

CO emissions

1800

160

Pulsation

NOx emissions

Operating

range

Pre

ssu

re p

uls

atio

ns

NOx Formation [2/2]


The main reactions of thermal NOx in diffusion combustors are

O2 O + O

N2 + O NO + N

N + O2 NO + O

The rate of formation of NO has been determined theoretically and is described by

[NO] = K1exp(-K2/T)[N2][O2]0.5t

The quantity of NO formed is an exponential function of temperature and is proportional to the concentration

of N2, the square root of O2 concentration and the residence time, t, at the high temperature.

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

turbines.

The other NOx producing mechanisms are fuel NOx and prompt NOx.

The fuel NOx results from reactions with nitrogen present in the fuel – a significant component in coal and, to

a small extent, in liquid fuel but no present in natural gas.

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.

NOx Formation in Gas Turbines


Emissions Control Methods

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.

4. Post combustion treatment

• Catalytic clean-up of NOx and CO from the gas turbine exhaust gas (usually used in conjunction with the

other two methods)


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.

Currently, water/steam injection rarely applies due to water/steam consumption and the penalty of reduced

efficiency. The amount of water required to reduce NOx emission levels is approximately one-half of the fuel

flow. However, there is a 1.8% efficiency penalty associated with water injection for oil-fired simple-cycle gas

turbines. Output increase of approximately 3% can be obtained by water/steam injection.

Water/Steam Injection [1/3]


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 combustor

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.


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

@ 1

5%

O2

50

Experimental Results TR-108057 (EPRI)



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


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 300C and 400C because the

reaction rate maximizes at around 350C.

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.



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, which is extremely corrosive, 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.

SCR systems are relatively expensive to install and maintain them.





Combustor Type 4





Major Components 2


Diffusion vs. Premixed Combustor

Diffusion Combustor Premixed Combustor

• In diffusion combustors, the fuel and air are injected

separately into the reaction zone, the reactants

combine by a diffusion process and as a result, the

flame speed is limited by the rate of diffusion.

• The geometry is relatively simple and the flame is

stable over the wide range of operation.

• The main drawback of diffusion combustors is the

difficulty in controlling NOx emissions, this is

because the reaction stabilizes in stoichiometric

regions where the temperature and consequently

NOx formation are high.

• In this system, NOx reduction is possible by the

injection of diluent, such as water or steam.

• In DLN systems, the fuel and air are mixed together

before they enter the reaction zone.

• Thus, the fuel/air ratio can be controlled and the

flame temperature reduced by lean combustion to

get a low level of NOx generation.


A flame burns best when there is just enough fuel to react with the available oxygen.

Equivalence ratio ( ) is the local fuel to air ratio divided by the corresponding stoichiometric value.

Different fuels have different stoichiometric f/a ratios, it is convenient to normalize the f/a ratio by the

stoichiometric value, producing the term equivalence ratio.

With this stoichiometric mixture (equivalence ratio of1.0) the flame temperature is the highest and the

chemical reactions are the fastest, compared to cases where there is either more oxygen (“fuel lean,” < 1.0)

or less oxygen (“fuel rich,” > 1.0) for the amount of fuel present.

In a gas turbine, the maximum temperature of the hot gases exiting the combustor is limited by the tolerance

of the turbine nozzles and buckets.

This temperature corresponds to an equivalence ratio of 0.4 to 0.5 (40% to 50% of the stoichiometricfuel flow).

In the combustors used on modern gas turbines, this fuel-air mixture would be too lean for stable and efficient

burning.

Therefore, only a portion of the compressor discharge air is introduced directly into the combustor reaction

zone (flame zone) to be mixed with the fuel and burned.

The balance of the airflow either quenches the flame prior to the combustor discharge entering the turbine or

cools the wall of the combustor.

= (f/a) stoichiometric

(f/a) actual

Equivalence Ratio


Another important combustion parameter is the flame temperature.

Flame temperature is determined by a balance of energy between reactants and products.

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

would be consumed.

Practically, however, the effects of species dissociation and heat capacity shift the peak temperature to

slightly above stoichiometric ( 1.05).

Fuel type is important in determining the flame temperature. The list below compares calculated adiabatic

flame temperatures of two hydrocarbons, CO and H2 during stoichiometric combustion in ambient air.

The methane flame temperature is approximately 150C lower than hydrogen and CO.

This distinction makes it somewhat easier to produce low-emissions from natural gas, compared to syngases

containing undiluted H2 and CO.

Compared to natural gas, the stoichiometry of syngas requires a smaller volume of air for complete

combustion, producing higher temperatures.

Flame Temperature

Species Adiabatic Flame temperature (C)

Methane (CH4) 1,950

Propane (C3H8) 1,988

Carbon Monoxide (CO) 2,108

Hydrogen (H2) 2,097


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. Fuel

Post flame

radiation

Surface of

stoichiometric

mixture

Air Air

Diffusion combustion has been used extensively because there is no backfire.

However, the main disadvantage of diffusion combustor is the emission as high temperature of the primary

zone produced larger than 70 ppm NOx in burning natural gas and more than 100 ppm for liquid fuels. This is

because there will be always stoichiometric regions regardless of overall stoichiometry in a diffusion flame.


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.


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

ichio

me

tric

co

nd

itio

n

Fuel rich Fuel lean

Op

tim

um

ba

nd

For a given fuel, since the flame temperature is a

unique function of the equivalence ratio, the rate of

NOx generation can be cast as a function of the

equivalence ratio.

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.

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.

The rate of NOx generation dramatically decreases

as flame temperature decreases (i.e., the flame

becomes fuel lean). This is because of the

exponential effect of the temperature in the

Zeldovich Mechanism and is the reason why

diluent injection (usually water or steam) into a

combustor flame zone reduces NOx emissions.

[ Emission for diffusion combustors

using No. 2 oil as a fuel ]


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]

Lean, dry control is desirable for reaching the lower

NOx levels, and also to avoid the turbine efficiency

penalty associated with diluent injection.

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.


[ 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.




Combustor Type 4





Major Components 2


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


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


The term dry relates to the fact that no water or steam is injected into the combustor to lower flame

temperature and hence NOx.

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.

DLN Combustor [1/2]


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.

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]


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 교차지점 주위로 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. 이로 인해 확산연소는 연소에 더 많은 공기가 필요하게 된다.

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


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


Diffusion vs. Premix

Discuss the advantages of a lean premix combustor

1) Lower NOx emission Lower flame temperature

2) Larger power output Less cooling air is required and higher combustion air flow

3) Lower CO and UHC emission Increased residence time

4) Extended life of hot gas parts No water/steam injection

Diffusion

combustor

Lean premix

combustor


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 [1/6]

DLN-1 Combustor

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Operating Modes of DLN-1 Combustor



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



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)



[ DLN-2.6 Fuel Nozzle Arrangement ]

DLN-2 Combustor


[ DLN-2.6 Fuel Nozzle ]

[ DLN-2.6 Combustor ]

[ DLN-2 Combustor ]

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DLN-2 Combustor


New type of combustor would be need as the firing temperature increases, such as FA machines.

DLN-2 eliminated the venturi and centerbody assemblies requiring cooling air in order to maintain an optimal

air fuel ratio in the combustion zone.

DLN-2 can operate on both gaseous and liquid fuel. On gas, DLN-2 operates in a diffusion mode at low loads

(< 50% load), and a premixed mode at high loads ( > 50% load). While the combustor can operate in the

diffusion mode across the load range, diluent injection would be requires for NOx control. Oil operation on this

combustor is in the diffusion mode across the entire load range, with diluent injection for NOx control.

DLN-2.6 eliminated diffusion mode at startup and low load. The result was a premixed-only combustor with 4

manifolds: 3 premixed manifolds staging fuel to the six burners (PM1, PM2 and PM3), and a fourth premixed

manifold for injecting quaternary fuel for dynamics abatement.


Siemens DLN Combustor [1/3]

Hybrid Burner


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


(Supply)

Steam

(Return)

(Return) Bypass valve

M501G steam cooled

liner in fabrication

Premixing nozzle

Pilot nozzle

MHI : G Series Combustor


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.

Hydrogen (H2) has a very high flame speed and very short ignition delay. Thus, it is very difficult to avoid auto-

ignition and/or flashback in a premixed combustor.

Syngas contains significant quantities of H2 and CO. Thus, the standard approach to premixing is unlikely to

work for syngas fuel.

Problems in DLN Combustors

1. Auto-Ignition Source: Gas Turbine Engineering Handbook, M.P. Boyce


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 occurs during unexpected engine transients, such as compressor surge.

The resultant change of air velocity would almost certainly result in flashback.

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 [1/3]

In the event of a flashback, the metal temperatures increase to

unacceptable levels and hardware damage occurs.

In some cases, these events have caused forced outages and

adversely impacted availability.

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.

[Damage to fuel nozzles due to flashback]

Source: Gas Turbine Engineering Handbook, M.P. Boyce

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Natural gas should be clean

dry gas because DLN

combustors are very

sensitive to any liquid carry-

over into the combustor,

which leads to flash back.

DLN combustors have no

tolerance for liquids in the

fuel gas.

Typically, coalescing filters

will remove all droplets and

solids larger than about 0.3

microns.

[Natural gas fuel handling system]


2. Flashback [2/3] Source: Gas Turbine Engineering Handbook, M.P. Boyce


2. Flashback [3/3]


Source: The design of high-efficiency turbomachinery

and gas turbines, D.G. Wilson, 1998.

Fairing

Fused Tip

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

[GE DLN-2 un-faired fuel nozzle]


Flame area and reaction

rate oscillations

Combustion

products

Vortex/flame

interactions Unsteady mixing,

vaporization, atomization

Flow rate

oscillations

Fuel flow rate

oscillations

Fuel/air ratio

oscillations

Flame Reactants

Feedback

Acoustic

oscillations

Combustion

products

3. Combustion Instability [1/3]



The disadvantage of DLN combustors is combustion instability, especially at low equivalence ratios.

Combustion instabilities are characterized by large-amplitude oscillations of one or more natural acoustic

modes of the combustor.

Such instabilities have been encountered during the development and operation of propulsion(rockets,

ramjets, and afterburners), power generation, boiler and heating systems, and industrial furnaces.

These instabilities are spontaneously excited by a feedback loop between an oscillatory combustion process

and, in general, one of the natural acoustic modes of the combustor.

In general, the occurrence of instabilities is problematic because they produce large-amplitude pressure and

velocity oscillations that result in thrust oscillations, severe vibrations that interfere with control system

operation, enhanced heat transfer and thermal stresses to combustor walls, oscillatory mechanical loads that

result in low- or high-cycle fatigue of system components, and flame blowoff or flashback.

These phenomena may result in premature component wear that could lead to costly shutdown or

catastrophic component and/or mission failure.

Consequently, considerable research and development efforts have been invested to elucidate the processes

responsible for the excitation of these instabilities and the development of approaches for their prevention.

3. Combustion Instability [2/3]



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 [3/3] Source: Gas Turbine Engineering Handbook, M.P. Boyce


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.




Combustor Type 4





Major Components 2


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


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

Washcoat layered on mechanical support (substrate)


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.


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 ]


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.


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.


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


Catalytic combustor in the 501D5

Catalytic combustor in the SGT6-5000F

Catalytic Combustor


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4. combustor · 2017-12-21 · combustion is the process whereby the hydrogen and carbon in fuels...

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