9. fuel and combustion · 2018-01-13 · thermodynamics 9. fuel and combustion 5 / 127 combustion...

Thermodynamics 9. Fuel and Combustion 1 / 127Thermodynamics

9. Fuel and Combustion


Fuel and Combustion Theory 21

NOx Formation and Its Control 252

Combustor Types 443

Important Parameters in Combustor Design 604

Diffusion and Premixed Combustion 755

Dry Low NOx Combustors 846

Catalytic Combustors 1077

Combustor Cooling 1218


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


Diffuser

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.

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


Combustion of fuel is irreversible process, and entropy is produced during the combustion of the fuel.

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

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

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.

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.

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.

Generals for Combustor [1/2]


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

within the limited space available.

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

required maximum temperature 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.

Generals for Combustor [1/2]


비점: 1기압 하에서 액화되는 온도

탄소계 분류 제품 비점(C) 밀도(kg/l)발열량

MJ/kg (kcal/kg)

C2 이하 천연가스메탄(CH4)

에탄(C2H6)

-162

-89

0.3

0.37

52 (12425)

48 (11470)

C3~C4 LPG프로판(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

Fuel


C + O2 = CO2 + 33.9 MJ/kg

H2 + 1/2O2 = H2O(water) + 143.0 MJ/kg (HHV)

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

S + O2 = SO2 + 9.28 MJ/kg

Combustion


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 elevated 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 [1/2]

Fuel


The composition on a molar basis of natural gases is as follows:

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.

Composition, mol% A B C D E F

Methane

Ethane

Propane

Isobutane

Normal butane

Isopentane

Normal pentane

Hexane

Nitrogen

Carbon dioxide

Hydrogen sulphide

Heating value, Btu/ft3

95.0

1.9

0.5

0.5

0.1

0.1

0.1

0.1

1.5

0.2

0.0

?

94.3

2.1

0.4

0.0

0.2

0.0

0.0

0.0

0.0

0.0

2.8

1010

72.3

5.9

2.7

0.2

0.3

0.0

0.2

0.0

0.0

17.8

0.1

934

88.9

6.3

1.8

0.1

0.2

0.0

0.0

0.0

0.0

2.2

0.1

1071

75.4

6.4

3.6

0.6

1.0

0.2

0.1

0.0

0.0

12.0

0.1

1044

85.6

7.8

1.4

0.1

0.0

0.1

0.0

0.0

0.0

4.7

0.2

1051

Natural Gas [2/2]

Fuel


Liquid fuels are used in gas turbines are distillates and ash-bearing fuels.

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 [1/3]

Fuel


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 [2/3]

Fuel


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.

The compounds that result during combustion (sodium sulfates, sodium vanadates, and vanadium pentoxide)

are semimolten and corrosive at metal temperatures normally associated with gas turbine operation.

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 [3/3]

Fuel


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.

The sulfur contained in the fuels produce ammonium bisulfate (NH4HSO4) and ammonium sulfate

((NH4)SO4).

Ammonium bisulfate causes rapid corrosion of boiler tubes; and both ammonium compounds cause fouling

and plugging of the boiler and increase of PM-10 (particulate matter smaller in diameter than 10 microns)

emissions.

Ammonium sulfate is not corrosive, but its formation contributes to fouling, plugging, and higher particulate

emissions.

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), and the temperature.

Use of Sulfur-Bearing Fuels

Fuel


The heat rate will be different by the type of heating value.

In the US, the standard is HHV, whereas in Europe the practice is to use LHV.

The fuel HHV is obtained by laboratory analysis in an oxygen bomb calorimeter.

The LHV of the fuel is computed by subtracting the latent heat of vaporization for water produced by fuel

hydrogen combustion and fuel moisture content.

LHV = HHV – Hfg (M + 8.94H2)/100

where, M is fuel moisture % by weight, Hfg is water latent heat at reference temperature 25C, H2 is fuel

hydrogen % by weight.

The lower heating value of the gas is one in which the H2O in the products has not condensed. The lower

heating value is equal to the higher heating value minus the latent heat of the condensed water vapor.

Because in general the latent heat in the water vapor in the exhaust is not recovered, Brayton cycle

efficiencies are calculated using LHV, rather than HHV.

For most hydrocarbon fuels, the difference between LHV and HHV is about 4 percent.

Heating Value [1/3]

Fuel


The LHV of the fuel is important because it defines the mass flow of fuel supplied to the gas turbine.

The lower the LHV, the higher the mass flow of fuel required to provide a certain chemical heat input,

normally resulting in a higher power output and efficiency. However, there is no clear relationship between

fuel lower heating value and output.

This is why low BTU gases can result in high power outputs if they are supplied at the pressure required by

the gas turbine.

This effect is noted even though the mass flow of methane is lower than the mass flow of distillate fuel.

Here the effects of specific heat were greater than that of mass flow.

Heating Value [2/3]

Fuel

Thermodynamics 9. Fuel and Combustion 17 / 127

[Exercise 9.1]

어떤 발전소 열효율이 HHV를 기준으로 45%이다. 이 발전소의 열효율을 LHV를 기준으로 계산하시오. 이 발전소에 사용하는 석탄의 HHV는 12540 Btu/lb이며, 석탄은 5.2%의 수분과 4.83%의 수소를 포함하고 있다.

[Solution]

Heat rate를구하면다음과같다.

HHV = 3412.14/HRHHV = 0.45 HRHHV = 7,582.5 Btu/kWh

LHV를계산한다.

LHV = HHV – Hfg (M + 8.94H2)/100 = 12540 – 1049.7 (5.2 + 8.94 4.83)/100

= 12032.15 Btu/lb

LHV/HHV를계산한다.

LHV/HHV = 12032.15/12540 = 0.9595

따라서 LHV를기준했을때 heat rate는다음과같다.

HRLHV = HRHHV 0.9595 = 7275.41 Btu/kWh

LHV = 3412.14/HRLHV = 3412.14/7275.41 = 46.9%

Heating Value

Heating Value [3/3]


Combustion

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).


Exhaust usually occurs as a gas; the temperature of the exhaust is high because of the heat released.

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

generated.

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

present in your 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.

As a result of combustion, exhausts are created and heat is released.

Combustion


All turbine and engine manufacturers quote heat rates in terms of the LHV of the fuel.

However, the usable energy content of fuels is typically measured on a HHV basis.

In addition, electric utilities measure power plant heat rates in terms of HHV.

Combustion

연소의정의

• 연료중의가연성분(C,H,S)이공기중의산소와결합하여산화되는발열반응

• 연소반응의세가지요소: 가연성연료, 산소, 점화원(불씨)

완전연소

• 산소가충분한상태에서가연분이완전히산화되는반응

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

• 탄소는연소하면서일차적으로 CO가된후에이차적으로 CO2 가됨. 따라서탄소는수소에비해연소에더많은시간이소요됨

불완전연소

• 산소가불충분한상태에서가연분이불완전하게산화되는반응

C + O = CO + 10.3 MJ/kg

[CO + 1/2O2 = CO2 + 10.1 MJ/kg (CO는다시산소와반응하여완전연소될수있음)]


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.Compressor

Fuel

Turbine

Air

Power

Exhaust NOx ~

25 ppm350C

Bypass Air

1800C1300C

Use of Air in a Combustor


GasChemical

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

[표] 공기조성

공기조성


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 것을나타낸다. 공연비는 질량비이므로 원자량을 탄소 12, 수소 1, 산소 16으로 하면, 연료와 산소의 질량은

연료: 12n + 1m산소: 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 : 12) 옥탄(C8H18)의 공연비 = 15.13 : 13) 경유 (C17H36)의 공연비 = 14.95 : 1

Fuel-Air Ratio


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

The combustion process must be stable over the wide range of fuel flows required for ignition, start-up, and

full power.

Combustor must perform within desirable range of emissions, exit temperature, and fuel properties.

The parasitic pressure drop between compressor and turbine should be minimized.

Combustor hardware should be simple, rugged, and small enough to be properly cooled by the available air.

Combustor hardware must have acceptable life and be accessible, maintainable, and repairable.

Requirements for a Combustor

OperationEnvironmental

regulationsPerformance Durability

• Reliable ignition

• High combustion

stability

• Low smoke

• Satisfactory emission

levels

• High combustion

efficiency

• Minimum pressure loss

• Satisfactory combustor

exit temperature

distribution

• Life


Fuel and Combustion Theory1


Combustor Types 3



Dry Low NOx Combustors6


Combustor Cooling 8


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 농도0.12 ppm/h 이상

(오존주의보)0.3 ppm/h 이상

(오존경보)0.5 ppm/h 이상(오존중대경보)

인체에 나타나는현상

• 눈과 코 자극

• 불안감과 두통 유발

• 호흡수 증가

• 호흡기 자극

• 가슴압박

• 시력감소

• 폐기능 저하

• 기관지 자극

• 패혈증 유발

Ozone


대류권에 시간당 오존농도가 0.12ppm 이상일 때 내려지는 주의보.

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

그러나 오존농도가 일정기준 이상 높아지면 호흡기나 눈이 자극을 받아 기침이 나고 눈이 따끔거리거나 심할 경우 폐기능 저하를 가져오는 등 인체에 피해를 주기도 하며, 농작물 수확량 감소를 가져오는 유독물질이된다.

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 사용자제가 요청되고 해당지역의 유치원 학교는 실외학습을 자제해야 한다.

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

중대경보일 때에는 소각시설 사용과 자동차 통행이 금지되며, 주민의 실외활동 금지가 요청된다.

오존 주의보


Thermal NOx

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.


Compressor

Fuel

Turbine

Air

Power

Exhaust NOx

~ 25 ppm350C

Bypass Air

1800C

1300C

3200F

1700

2200 2400 2600 2800 3000

1200 1300 1400 1500 1600

140

120

100

80

60

40

20

0

Reaction Temperature

NO

x, ppm

Required TIT

Temperature

required for

flame

1800C

160

The threshold for thermal NOx formation

is reached at 2800F(1538C)

Temperature and NOx Formation

Thermal NOx


Emission in Diffusion Combustor [1/4]

1000

0.5

4000

3000

2000

1.51.0

Equivalence Ratio

Fla

me

Te

mp

.,

F

100

300

200

NO

xR

ate

, p

pm

v

High Smoke

EmissionsHigh CO

Emissions

Sto

ich

iom

etr

ic C

on

ditio

nFuel RichFuel 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


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

01500 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

xE

mis

sio

n, p

pm

Optimum Band



Combustion in gas turbines with a diffusion combustor, the fuel is sprayed into the center of an air stream.

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.

The hot combustion products are cooled by dilution with excess air to temperatures acceptable to

combustor walls and turbine blades.

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.



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.



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 파이럿 화염을 적용하면 화염이꺼지는 염려가 없이 넓은 잉여공기비에 걸쳐서운전이 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 NOx

와 CO 발생 교차지점으로 제한한다.

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

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

Emission in DLN Combustor


200

250

100

150

50

0

25

Diffusion

CombustorDLN

Combustor

Catalytic

Combustor

NO

xE

mis

sio

n, p

pm

vd

Fuel: Natural Gas

Fla

me

Te

mp

era

ture

Fuel/Air Ratio

Fuel RichFuel Lean

Diffusion Combustor

Premixed

Combustor

Extinction

of Lean

Premix

Flame

NOx Emission Level


NOx Control Methods for Gas Turbines

1. Diluent addition

• Steam, CO2, N2 or other diluent is added 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


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.

The first attempts to reduce NOx introduced a heat sink in the flame by injecting water to reduce combustion

temperature, which is the primary parameter affecting NOx formation.

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, which may be viewed as a measure of the inefficiency of the combustion process, also

increases as the injection rate of steam or water increases.

The lowest practical levels achieved with injection are generally 25 ppm when firing natural gas and 42 ppm

when firing oil.

1. Water or Steam Injection [1/3]



1. Water or Steam Injection [2/3]

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

Re

lative

NO

xP

rod

uctio

n R

ate

Steam injection

Water injection

Mixture of natural gas and steam



Base Load Operation (MS7001E Gas Turbine)

Peak Load Operation (MS7001E Gas Turbine)

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

1. Water Injection – Experimental Results [3/3]

TR-108057 (EPRI)



2. Selective Catalytic Reduction [1/4]

SCR is a exhaust gas NOx reduction system that uses ammonia to react with NOx over

a catalyst that convert NOx into molecular nitrogen and water.

This system increases plant installation and operating cost.

Typically, the SCR catalyst operates under temperature range of 570F/300C and

750F/400C.



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

catalytic reduction (SCR) systems.

SCR is the most effective and proven technology to reduce NOx emissions, greater than 90%.

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.

Conventional SCR technology operates in a narrow temperature range (288C-399C).

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 cost of the system, the efficiency penalty due to the pressure drop introduced by the catalyst, and the

potential for NH3 slip are the major disadvantages of this system.

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 (수산화암모늄) 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.

This systems are relatively expensive to install and maintain.

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.




SCONOX is a post-combustion catalytic system that removes both NOx and CO from the gas turbine

exhaust, but without ammonia injection.

The catalyst is platinum and the active NOx removal reagent is potassium carbonate.

Currently, SCONOX is used for only LM2500.

SCONOX is very sensitive to sulfur, even the small amount in pipeline natural gas.

The initial capital cost is three times the cost of SCR.

It has moving parts, reliability and performance degradation due to leakage may be significant issues.

Use of any exhaust gas treatment technology (SCR or SCONOX) results in pressure drop that reduces gas

turbine efficiency.






Combustor Types 3





Combustor Cooling 8


Can-Type Combustor [1/8]

Arrangement

GE 9FB


Arrangement



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.



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.

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/


Transition Piece 1st Stage Nozzle

1st Stage

Blades

3rd Stage

Blades

Combustion

Can

2nd Stage

Nozzle

2nd Stage

Blade

3rd Stage

Nozzle

Transition Piece



[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.



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 촉진③ 화염길이 짧게 유지

• 연소실 길이 축소• 터빈으로 화염전파 방지

Swirler



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 [1/5]

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]



GT26 & GT24 [Alstom]



DLE Combustor for Aeroderivative Gas Turbines (GE)

Premixer

Combustion Liner

Heat Shield

Three rings of

fuel nozzles



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.

• Although a large number of fuel jets can be

employed, it is more difficult to obtain an even fuel-

air distribution and an even 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.

• Most of the development work should be carried out

on the complete combustor, requiring 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.



Industrial gas turbines, relieved of weight reductions, initially employed large external (silo) combustors in

order to ensure efficient combustion, which is caused by lower gas velocities.

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

The initial use of diffusion burners increased NOx formation.

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

affected burn-out.

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 V94.3GE 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 Types 3





Combustor Cooling 8


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.

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

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

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.

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.

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


p

2

1

Qin

3

4Qout

Win

Wout

h

s

1

2

3

4

3

4

2

Pressure Loss in a Combustor

The pressure loss in a combustor is a major problem because it affects gas turbine efficiency and power

output.

Total pressure loss is usually in the range of 2~4% of static pressure.

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.


1

2,

3,

21

o

o

T

TKKPLF

Pressure loss in a combustor is caused by friction, turbulence and the temperature rise due to combustion.

The overall stagnation pressure loss is the sum of the fundamental loss and friction loss.

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

1 2 30

40

30

20

10

Temperature Ratio, To,3/To,2

Pre

ssu

re L

oss F

acto

rCold Loss, K1

Fundamental

Pressure Loss


The fundamental loss is caused by temperature

rise due to combustion.

The pressure loss due to friction is found to be

much higher.


Force balance:

(1)

Continuity equation:

Stagnation pressure:

(2)

Eqn. (1) (2);

The stagnation as a fraction of the inlet dynamic head then becomes

(3)

Equation (3) finally becomes, since

2211 VmApVmAp

25. Vppo

VAm

2

11

2

22121,2, 5. VVpppp oo

2

11

2

221212 VVVVmpp

2

11

2

221,2, 5. VVpp oo

11

2/ 2

1

2

11

2

22

2

11

2,1,

V

V

V

pp oo

1

2/ 1

2

2

11

2,1,

T

T

V

pp oo

T/1

2211 VV

V1 V2

1 2

Pressure Loss in 1-D Duct



Uniformity of the Combustor Outlet Profile

The uniformity of the combustor outlet profile affects the useful level of TIT, because the average gas

temperature is limited by the peak gas temperature.

The profile factor is the ratio between the maximum exit temperature and the average exit temperature.

The figure shown in the right hand side is a

temperature profile measured at the exit of the

gas turbine at various loads.

This is a very important parameter for determining

the health of the gas turbine.

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 ]


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.

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.

Traverse Factor

Tip

Hub

Temperature


Satisfactory Operation and Life of Combustor

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.

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.

Minimum carbon deposits and smoke emission are essential for satisfactory operation.

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.


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.

50550686.0

000,1

..

amb

bTGrSp

LHVW

Wobbe Index


[ 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.

Stability Limits [1/2]


The stability range and air-fuel ratio range decreases as the air velocity is increased.

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.

Stability Limits [2/2]


The size of combustion chamber is determined by the rate of heat release required.

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

It is in the region of 2 ~ 5x104 kW.m3-atm for aircraft gas turbines, while industrial gas turbines have much

lower value for this because of larger volume of combustion space available.

combustion intensity = heat release rate

combustor vol. * pressure[kW / m3-atm]

Combustion Intensity


Flame Temperature

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.


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.

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

exhaust carbon monoxide and UHC.

Typical values for combustion efficiency and pressure loss in combustor are 99% and 2~8%, respectively.

A typical subsonic airliner cruises at a altitude of 11000m, where the ambient pressure and temperature are

0.227bar and 217K, respectively.

Altitude effects are still significant for industrial gas turbines because some engines are operated at altitudes

of 4000m in regions of South America.

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.

Operational Requirements




Combustor Types 3





Combustor Cooling 8


Different modes of laminar combustion

Fuel

InjectorSpark

Plug

Die

se

l E

ng

ine

Sp

ark

Ig

nitio

n E

ng

ine

Premixed

Flame

Diffusion

Flame

Premixed

Flame

Diffusion

Flame


Types of Flame

Diffusion Premixed

• 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


Flame Temperature and NOx Emissions

Reduction of emissions in the premix combustor

NOx are reduced by

• Lowering flame temperature by lean combustion

• Elimination of hot spot

CO and UHC are reduced by

• Increasing combustion residence time (volume)

• Combustor design to prevent local quenching

Fla

me T

em

pera

ture

Fuel/Air Ratio

Fuel RichFuel Lean

Diffusion Combustor

Premixed

Combustor

Extinction

of Lean

Premix

Flame


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 life of hot gas parts No water/steam injection

Diffusion

combustor

Lean premix

combustor


Diffusion Flame

In diffusion flame combustion, both fuel and oxidizer are supplied to the reaction zone in an unmixed state.

Fuel is mixed with the surrounding air by convection and diffusion during combustion.

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.

This will lead to both the highest flame temperature and the fastest reaction rates.

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.

The rate of combustion process can be increased by turbulent flow.

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

At the beginning of gas turbine development, the primary design goal was to optimize performance.

Therefore, emphasis was placed on maximizing combustion efficiency by minimizing the emission of UHC

and CO.

In the early 1970’s, the primary concern for emission was shifted to NOx, and the injection of water or steam

into the combustion zone was introduced to reduce for NOx reduction.

As the greater NOx reduction requirements imposed during 1980’s, increased quantities of water/steam were

introduced. These attempts proved detrimental to cycle efficiency and part lives, and the emission rates for

other pollutants also began to rise significantly.


The fuel injected into the combustor is evaporated and burnt in the primary zone.

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

Ideally, all fuel should be burnt at the end of the primary zone.

The hot gases produced in the primary zone are cooled in the dilution zone.

In the primary zone, fuel-air ratio is about 60:1.

Fuel-Air Ratio

Diffusion Flame


Primary zone

Intermediate zone

Dilution zone

Mixing and combustion

Completion of combustion

Cooling

Primary zone Secondary zone

Mixing zone

Intermediate zone

Tertiary zone

Dilution zone

Combustion Zones in a Typical Diffusion Combustor

Diffusion Flame


Premixed Flame

Lean premixed stationary gas turbine means any stationary gas turbine designed to operate at base load

with the air and fuel thoroughly mixed to form a lean mixture before delivery to the combustor.

Premixed flames are used whenever intense combustion is required within a small volume.

Gas turbine OEMs have significantly reduced CO emissions from gas turbines by developing lean premix

technology.

Lean premix combustion design not only produces lower NOx than diffusion flame technology, but also

lowers CO and volatile organic compounds (VOC), due to increased combustion efficiency. Lower NOx level

is because premixing prevents local hot spot within the combustor that can lead to significant NOx formation.

Atmospheric nitrogen acts as a diluent in premixed combustor, as fuel is mixed with air upstream of the

combustor at deliberately fuel-lean conditions. The fuel/air ratio typically approaches one-half of the ideal

stoichiometric level, meaning that approximately twice as much air is supplied as is actually needed to burn

the fuel. This excess air is key to limiting NOx formation, as very lean conditions cannot produce the high

temperatures that create thermal NOx.

Characteristics

• Reacts rapidly

• Constant pressure

• Propagates as thin zone




Combustor Types 3





Combustor Cooling 8


Catalytic combustor

Design Change of Combustors

Diffusion-type 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

A typical reverse-flow can-type

diffusion combustor


Premix Flame

Diffusion Flame

Fuel Rich

NO

xF

orm

atio

n R

ate

Excess AirStoichiometric

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.

If fuel and air are mixed before combustion in

a so-called premix flame, the combustion

temperature, and therefore the NOx formed, is

a strong function of the fuel-air ratio.

DLN Combustor [1/3]

By using a lean fuel/air mixture the rate of NOx formation can be significantly reduced.

The main problems associated with lean premix flames are stability, inflexibility and the limited turn-down

rage.

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.

Thorough mixing is also essential to avoid unsteady combustion and even flashback with subsequent

combustor damage.


To stabilize the flame, hybrid system is used commonly. In the hybrid system, the bulk of the fuel is burned

in a premixed burner, the remainder being supplied to a small pilot diffusion flame embedded in the flow.

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

excessive CO.

DLN combustors burn most (at least 75%) of the fuel at cool and fuel-lean conditions to avoid any

significant production of NOx.

DLN combustor premixes air and fuel, and makes a lean fuel 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.

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 combustors also tend to create harmonics in the combustor that may result in vibration and acoustic

noise.

In addition, 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.

DLN Combustor [2/3]


DLN combustors can also experience “flash-back” in which fuel upstream of the burner ignites prematurely

damaging combustor components.

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 combustors have two fuel injectors: main fuel and pilot fuel.

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.

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.

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 flue 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.

DLN Combustor [3/3]


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


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

00 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

% O

2(p

pm

vd

)

NOx

CO

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



DLN-2.6 Fuel Nozzle Arrangement



Siemens DLN Combustor [1/4]

Hybrid Burner


NOx Emission - Siemens V84.3A Engine with 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



Gas only support housing

Dual Fuel Support Housing

Dual Fuel Pilot Nozzle

Combustor Basket

ULN Burner



EV Cone Burner

Alstom DLN Combustor [1/2]


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]


(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.

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.

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 reducing auto-ignition delay time, 3) fuel residence time incorrectly calculated,

4) auto-ignition triggered early by ingestion of combustible particles.

Problems in DLN Combustors

1. Auto-Ignition


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.

Fairing

Fused Tip

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



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.

3. Combustion Instability



The main advantage of combined cycle power plants is its high specific work; power per unit mass flow.

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

has increased; increasing wear and necessitating more frequent maintenance.

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

many gas turbine manufacturers.

4. Noise


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


The flame stability is inherently greater in diffusion combustors over a wider range of fuel-air ratio.

On the other hand, the NOx emissions are much greater than DLN combustors.

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.

In DLN combustors, the interaction of turbulent flow and chemistry is essential.

5. Flame Stability



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

liner and nozzle.

In addition, there are corrosion and distortion problems.

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.

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

vibrations, leading to major failures.

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.

Damage to fuel nozzles due

to flash back

6. Durability





Combustor Types 3





Combustor Cooling 8


Development History of Combustors

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


Schematic of a Monolithic Catalyst

Catalytic Coating

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

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.


Concept of Catalyst System


Traditional ceramic coating

Siemens metal ceramic coating

Catalytic Coating


Low Temperature Catalytic Combustors

For uncooled turbines such as micro-turbines,

TITs are generally within catalyst material

temperature limits (less than 1000C (1830F)).

This means that the catalyst and substrate can

tolerate the maximum required flame

temperature, and if complete combustion can be

sustained, there is no need to separate the gas-

phase combustion zone from the catalyst.

Therefore, a simple catalytic combustor for this application can comprise only a premixer and a catalyst bed.

This simple system is called as a single-stage system.

The reaction should be completed (99.9% combustion efficiency) within the catalyst bed, this single-stage

system is also known as a total conversion catalytic reactor.

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.

Premixer Catalytic Reactor

[Total Conversion]

Fuel

Air


Moderate Temperature Catalytic Combustors

For modern, cooled turbines having TITs well above 1100C (2000F), two-stage catalytic combustor is used,

and metal substrates have generally been adopted for their robustness in the gas turbine environment.

Therefore, catalyst-stage temperature must remain below 950C (1750F), while gas-phase combustion

temperature may reach 1525C (2780F) before NOx emissions increase beyond acceptable levels.

Ultra-low NOx emissions have been successfully demonstrated in recent engine tests of two different two-

stage systems.

[Partial Conversion]

Fuel

Air

PremixerCatalytic

Reactor

Gas-

Phase

Burnout

950C

1100C


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.


Catalytic combustor in the 501D5

Catalytic combustor in the SGT6-5000F

Catalytic Combustor


Catalytic combustor in the SGT6-5000F

Catalytic Combustor


LCL

RCL

Type of Catalytic Combustor


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.


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.

Rich Catalytic Lean Burn (RCL) Design


Rich Catalytic Module

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 *.

* Lightoff is defined as the temperature at which

the catalyst surface initially becomes active.

Rich Catalytic Lean Burn (RCL) Design




Combustor Types 3





Combustor Cooling 8


Film Cooling

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.


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


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 bond coat is to insulate oxide.


Steam Cooling

(Supply)

Steam

(Return)

(Return)Bypass valve

Premixing nozzle

Pilot nozzle


Convective and Film Cooling


질의 및 응답

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

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

9. fuel and combustion · 2018-01-13 · thermodynamics 9. fuel and combustion 5 / 127 combustion...

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