Lefebvre A.H., Ballal D.R. Gas Turbine Combustion: Alternative Fuels and Emissions

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360 Gas Turbine Combustion: Alternative Fuels and Emissions, Third Edition

9.2 Concerns

The exhaust from an aircraft gas turbine is composed of CO, carbon dioxide

(CO

), water vapor (H

O), unburned hydrocarbons (UHC), particulate matter

(mainly carbon), NO

, and excess atmospheric oxygen and nitrogen. CO

and

O have not always been regarded as pollutants because they are the natu-

ral consequence of complete combustion of a hydrocarbon fuel. However,

they both contribute to global warming and can only be reduced by burning

less fuel. Thus, improvements in engine thermal efciency not only reduce

direct operating costs, but also reduce pollution.

The principal pollutants are listed in Table 9.1. CO reduces the capacity of

the blood to absorb oxygen and, in high concentrations, can cause asphyxia-

tion and even death. UHC are not only toxic, but they also combine with NO

to form photochemical smog. Particulate matter (generally called soot or

smoke) creates problems of exhaust visibility and soiling of the atmosphere.

It is not normally considered to be toxic at the levels emitted, but recent

studies by Seaton et al. [1] indicate a strong association between asthma and

other respiratory diseases and atmospheric pollution by concentrations of

small particles in the microgram range. Moreover, some smoke suppressants

contain heavy metals, such as barium, which add another pollutant to the

exhaust gases. NO

(NO + NO

), of which the predominant compound at

high emission levels is NO, not only contribute to the production of photo-

chemical smog at ground level, but also cause damage to plant life and add

to the problem of acid rain.

Relative to other sources, aircraft engines are only minor contributors to

the overall NO

burden. For example, in the United States, NO

emissions

from aircraft engines account for only about 2% of the total emissions

nationwide from all sources [2]. On a global basis, NO

emissions from

aircraft engines constitute less than 3% of all man-made NO

emissions.

However, of special concern is that these emissions lead to the formation

of ozone in the troposphere—the region that extends from ground level

to approximately 12 km above the earth’s surface. This is the region in

TABLe 9.1

Principal Pollutants Emitted by Gas Turbines

Pollutant Effect

Carbon monoxide (CO) Toxic

Unburned hydrocarbons (UHC) Toxic

Particulate matter (C) Visible

Oxides of nitrogen (NO

) Toxic, precursor of chemical smog,

depletion of ozone in stratosphere

Oxides of sulfur (SO

) Toxic, corrosive

Emissions 361

which stationary gas turbines and subsonic aircraft operate. The relevant

reaction mechanisms are

= NO + O,

O + O

= O

Measurements taken over a long period of time at altitudes from 1 to 3 km

indicate that the level of ozone over western Europe is now approach-

ing 50 ppb (parts per billion). Prolonged exposure to ozone concentrations

around 100 ppb is associated with respiratory illnesses, impaired vision,

headaches, and allergies. Ground-level ozone is especially important in

regions where the topographical features prevent the local weather system

from removing the ozone formed in combustion, and where strong sunshine

can promote the photochemical reactions that lead to smog. Los Angeles is

the classic example of such a region. It is hardly surprising, therefore, that

the drive toward very stringent emissions legislation on NO

rst emanated

from this city.

Similar studies indicate that NO

emissions emitted at the extreme altitudes

at which supersonic aircraft are required to operate can deplete the strato-

spheric ozone layer via the reactions

NO + O

= NO

+ O

+ O = NO + O

Note that at the end of these reactions, the NO is liberated to produce more

ozone.

Depletion of the ozone layer allows increased penetration of solar ultra-

violet radiation, which produces a corresponding increase in the incidence

of skin cancer.

With stationary engines burning residual fuels, an additional pollutant of

concern is oxides of sulfur (SO

), mainly SO

and SO

, which are formed when

sulfur-containing compounds in the fuel react with oxygen in the combus-

tion air. They are toxic and corrosive and lead to the formation of sulfuric

acid in the atmosphere. Since virtually all the sulfur in fuel is oxidized to

, the only viable limitation strategy is to remove sulfur from fuel before

combustion.

For all types of stationary gas turbines, the problems posed by exhaust gas

emissions are no less challenging than for aircraft engines. World energy

demand is forecast to grow over the next 30 years at around 1.8% per annum.

This demand will be met predominantly by the combustion of fossil fuels

[3]. Thus, the manufacturers and users of gas turbines for utility power gen-

eration now nd themselves at the forefront in regard to responsibility for

emissions issues.

362 Gas Turbine Combustion: Alternative Fuels and Emissions, Third Edition

9.3 Regulations

9.3.1 Aircraft engines

The International Civil Aviation Organization (ICAO) has promulgated reg-

ulations for civil subsonic turbojet/turbofan engines with rated thrust levels

above 26.7 kN (6000 pounds) for a dened landing-takeoff cycle (LTO), which

is based on an operational cycle around airports. This LTO cycle is intended

to be representative of operations performed by an aircraft as it descends

from an altitude of 914 m (3000 ft) on its approach path, to the time it subse-

quently attains the same altitude after takeoff.

ICAO standards for gaseous emissions are presented in Table 9.2, in which

is the engine pressure ratio at takeoff. They are expressed in terms of

a parameter that consists of the total mass in grams of any given gaseous

pollutant emitted during the LTO cycle per kilonewton of rated thrust at sea

level. We have

Emission Emission IndexEng

g/kN g/kg fuel

()

=×iineSFC Time in mode

kg fuel/hrkN

()

.×

(9.1)

This equation shows that two methods are available to the engine manu-

facturer for reducing NO

. One is to make improvements to the combustor

that reduce its emissions index (EI), and the other is to choose an engine cycle

that yields a lower SFC. Since the CO and UHC levels of modern engines

have been signicantly reduced at all low-power conditions, and only NO

emitted in appreciable amounts at altitude cruise, in practice the emissions

generated by aircraft engines consist primarily of NO

. A typical example of

the emissions mass distribution associated with the ight of a modern sub-

sonic aircraft has been provided by Bahr [4] and is shown in Table 9.3. This

table represents a ight of 500 nautical miles and shows that NO

emissions

predominate both in the vicinity of the airport and during altitude cruise.

TABLe 9.2

ICAO Gaseous Emissions Standards

Emission (g/kN)

Subsonic Turbojet/Turbofan

Engines

Supersonic Turbojet/Turbofan

Engines

HC 19.6

140 (0.92)

CO 118.0

4550 (π

)

−1.03

32 + 1.6π

36 + 2.4π

−1.04 + 2π

(2007 + engines)

Source: www.icao.org.

Newly manufactured engines with rated takeoff thrust greater than 26.7 kN.

Emissions 363

For a longer ight, NO

emissions would account for an even larger fraction

of the total emissions mass.

The ICAO standard for smoke measurement (engine year of manufacture

2000+) is expressed in terms of a smoke number (SN), which is related to the

engine takeoff thrust (F

) by the expression

SN = 83.6(F

)

−0.274

. (9.2)

This expression is shown graphically in Figure 9.1. The intention of this

standard is to eliminate any visible smoke from the engine exhaust. As smoke

ICAO smoke emission standard

Regulatory level = 83.6 (F

)

–0.274

Smoke number

050 100

Rated thrust (F

), kN

150

200

250

300

Figure 9.1

ICAO smoke emissions standards.

TABLe 9.3

Typical Distribution of Total Emission Mass Quantities Generated During a Flight

of An Aircraft Equipped with Modern Engines

Percent of Total Emission Mass

Category

During ICAO Landing-Takeoff

Cycle

During Climbout Cruise/

Descent Overall

Smoke − 0.1 0.1

HC 0.6 1.0 1.6

CO 5.4 7.0 12.4

7.8 78.1 85.9

Total 13.8 86.2 100.0

(56.5% NO

) (90.6% NO

)

Source: Bahr, D.W., ASME Paper 92-GT-415, 1992. With permission.

Note: Aircraft: twin-engine transport; range: 500 nautical miles.

364 Gas Turbine Combustion: Alternative Fuels and Emissions, Third Edition

visibility depends on both the smoke concentration, as indicated by the value

of SN, and on the viewing path length, the allowable SN of a high thrust

engine is lower than for a low thrust engine because of its larger exhaust

diameter.

The situation in regard to compliance with ICAO regulations is generally

satisfactory for subsonic aircraft engines, due mainly to the efforts of the

engine manufacturers during the past 20 years in improving combustor

design and in reducing engine-specic fuel consumption. However,

there is continuing pressure to reduce NO

emissions from all sources.

In Sweden, a tax is now imposed on NO

LTO emissions generated dur-

ing domestic ights, whereas in Switzerland the charge bands are based

on both NO

and UHC LTO emissions [5]. Current ICAO regulations

are restricted to operations at low altitudes in and around airports, but

growing concerns regarding ozone depletion at high altitudes could lead

to them being extended to other ight regimes, such as altitude cruise,

where the bulk of NO

emissions occur. The feasibility of introducing cer-

tication standards covering these ight regimes is being considered by

ICAO [6].

The emissions standards shown in Table 9.2 for supersonic turbojet engines

were set to ensure that the Olympus engine, which powers the Concorde,

would be in compliance. For future supersonic transport (SST) engines,

NASA has proposed a cruise NO

EI of 5 g/kg fuel [7]. This target does not

seem to be too challenging when examined alongside the cruise NO

EI lev-

els of 8–12 produced by modern subsonic aircraft. However, due to the large

pressure rise across the sonic wave generated by SST aircraft, combustor inlet

temperatures will be exceptionally high and the application of current com-

bustor technology would yield EI NO

levels of around 45. Thus, the future

of second generation supersonic aircraft depends crucially on compliance

with goals that can only be met by the use of yet-to-be-developed ultralow

combustor designs.

9.3.2 Stationary gas Turbines

Regulations governing emissions from stationary gas turbines tend to be

highly complex because the legislation varies from one country to another

and is supplemented by local or site-specic regulations and ordinances

governing the size and usage of the plant under consideration and the type

of fuel to be used. Detailed information on environmental legislation and

regulations for stationary engines may be found in Schorr [8]. For the large

number of engines burning natural gas, the emissions of UHC, particulate

matter, and SO

are negligibly small, and most of the drive toward more

stringent regulations for stationary gas turbines has been directed at NO

. In

the United States, the Environmental Protection Agency (EPA) has promul-

gated emissions standards (Federal Register 71 FR 38482 dated July 6, 2006

Emissions 365

Standards 40 CFR Part 60), which depend on the engine’s input energy and

intended use (utility or industrial).

For new, electricity-producing, turbine-ring natural gas, the NO•

limits are: 42 ppmv below 3 MW (4000 HP); 25 ppmv (3–110 MW);

and 15 ppmv (above 110 MW)

For new electricity-producing turbines ring fuels other than natural •

gas, the NOx limits are: 96 ppmv below 3 MW (4000 HP); 74 ppmv

(3–110 MW); and 42 ppmv (above 110 MW)

For new mechanical drive turbines (below 3.5 MW), the NOx limits •

are: 100 ppmv for natural gas ring and 150 ppmv for fuels other

than natural gas

SO•

emissions are limited to 110 ng/J gross energy output for turbines

that are located in continental areas, and 780 ng/J gross energy out-

put for turbines located in noncontinental areas.

Note that the above limits are expressed in parts per million by volume

(ppmv), referenced to 15% oxygen on a dry basis. The purpose is partly to

remove ambiguity when comparing different sets of experimental data, but

also to indicate that combustors burning less fuel are expected to produce

less NO

The correction formula is

(NO

)

ref15%oxygen

= (5.9)(NO

xmeas

) / (20.9 − O

2meas

), (9.3)

where NO

concentrations are expressed in ppmv (dry) and O

content is

expressed in percentage by volume.

European and Japanese regulations are broadly in line with EPA standards.

In some locations, notably Southern California and parts of Japan, growing

public awareness of the contribution of NO

to the production of smog has

created pressures for increasingly stringent NO

standards, and some local

regulations now call for NO

limits as low as 9 ppmv. In all countries, the

published standards are considered to be minimum requirements and there

is often a requirement to use the “best available control technology” (BACT)

or the “lowest available emission rate” (LAER). This has led to concerns such

as those expressed by Angello and Lowe [9], that if a new technology is devel-

oped that signicantly improves the ability to reduce NO

emissions, it effec-

tively sets the emission standard that all subsequent plants must meet. Thus,

as new technologies are developed to meet the ever increasingly restrictive

emission limits, they become the standard by which the next round of emis-

sion regulations is guided.

Until the late 1980s, the BACT for achieving NO

levels down to 25 ppmv

was by water or steam injection into the combustion zone. The technology cur-

rently used to reduce NO

concentrations to below 10 ppmv is a combination of

366 Gas Turbine Combustion: Alternative Fuels and Emissions, Third Edition

diluent injection (water or steam), or lean, premixed combustion, supplemented

by exhaust gas cleanup using selective catalytic reduction (SCR), as described

below. This process is not only very expensive, but it also requires the use of

additional control systems. Furthermore, it can exacerbate the overall pollution

problem by releasing ammonia gas and increased CO concentrations into the

atmosphere. This illustrates the difculties involved in determining how all the

various technical, economic, and environmental tradeoffs should be assessed.

At the present time, there are no EPA standards for CO and UHC emissions,

but many local standards exist that are usually established on a site-specic

basis. Typical CO limits range from 10 to 40 ppm.

The attainment of low CO levels has not usually presented any major dif-

culties in the past, due largely to the user’s insistence on high combustion

efciencies to minimize fuel consumption. However, the continuing pressure

to reduce NO

emissions has resulted in copious amounts of water or steam

being injected into the combustion zone and, more recently, to the adoption of

lean, premix combustion. The success of these techniques relies on the lower-

ing of ame temperature, which tends to promote the formation of CO. Thus,

the control of CO levels is now posing a more difcult problem than when the

emissions regulations for stationary engines were rst formulated.

9.4 Mechanisms of Pollutant Formation

The concentration levels of pollutants in gas turbine exhausts can be related

directly to the temperature, time, and concentration histories of the combus-

tion process. These vary from one combustor to another and, for any given

combustor, with changes in operating conditions. The nature of pollutant

formation is such that the concentrations of CO and UHC are highest at low-

power conditions and diminish with an increase in power. By contrast, NO

and smoke are fairly insignicant at low-power settings and attain maximum

values at the highest power condition. These characteristic trends are illus-

trated in Figure 9.2.

9.4.1 Carbon Monoxide

When a combustion zone is operating fuel-rich, large amounts of CO are

formed owing to the lack of sufcient oxygen to complete the reaction to CO

If, however, the combustion zone mixture strength is stoichiometric or mod-

erately fuel-lean, signicant amounts of CO will also be present due to the

dissociation of CO

(see Chapter 2). In practice, CO emissions are found to be

much higher than predicted from equilibrium calculations and to be highest

at low-power conditions, where burning rates and peak temperatures are

relatively low. This is in conict with the predictions of equilibrium theory,

Emissions 367

and it suggests that much of the CO arises from incomplete combustion of

the fuel, caused by one or more of the following:

Inadequate burning rates in the primary zone, due to a fuel/air ratio •

that is too low and/or insufcient residence time

Inadequate mixing of fuel and air, which produces some regions in •

which the mixture strength is too weak to support combustion, and

others in which over-rich combustion yields high local concentra-

tions of CO

Quenching of the postame products by entrainment into the liner •

wall-cooling air, especially in the primary zone

In principle, it should be possible to reduce the CO formed in primary com-

bustion to a very low level by the staged admission of additional air down-

stream to achieve a gradual reduction in burned gas temperature. However,

once formed, CO is relatively resistant to oxidation, and in many practical

systems its oxidation is rate determining with respect to the attainment of com-

plete combustion. At high temperatures, the major reaction removing CO is

CO + OH = CO

+ H.

This is a fast reaction over a broad temperature range. At lower temperatures,

the reaction

CO + H

O = CO

+ H

is important as a means of removing CO.

02040

Percent of takeoff power

60 80 100

Smoke

UHC

Pounds per 1000 pounds of fuel

Smoke number

Figure 9.2

Emissions characteristics of gas turbine engines.

368 Gas Turbine Combustion: Alternative Fuels and Emissions, Third Edition

The main factors inuencing combustion efciency, and hence CO emis-

sions, are engine and combustor inlet temperatures, combustion pressure,

primary-zone equivalence ratio and, with liquid fuels, the mean drop size of

the spray. All these aspects have been investigated by many workers, includ-

ing Rink and Lefebvre [10], who used a continuous ow tubular combus-

tor, 150 mm in diameter, in conjunction with an array of 36 equally spaced

“microscopic” airblast atomizers, to achieve a uniform distribution of liquid

fuel in the mixture entering the combustion zone. This method of fuel injec-

tion had another useful advantage in that it allowed the mean drop size in

the fuel spray to be varied in a controlled manner while maintaining all

other ow conditions constant. All measurements of pollutant emissions

were carried out at a distance of 170 mm from the fuel injectors.

9.4.1.1 Inﬂuence of Equivalence Ratio

Some of the results obtained by Rink and Lefebvre [10] for a light diesel oil (DF

2) are presented in Figure 9.3, which shows the variation of CO emissions with

an equivalence ratio for three values of inlet air pressure. All three curves

exhibit the same general characteristics. They show that CO emissions

diminish with an increase in the equivalence ratio, reaching minimum val-

ues at an equivalence ratio of around 0.8, above which any further increase

200

100

CO, g/kg fuel

SMD = 70 µm

Fuel = DF 2

= 573 K

, Mpa

0.76

1.01

1.27

0.6 0.8 1.0

Equivalence ratio, φ

1.2 1.4

Figure 9.3

Inuence of pressure and equivalence ratio on CO. (From Rink, K.K. and Lefebvre, A.H.,

International Journal of Turbo and Jet Engines, 6(2), 113–22, 1989. With permission.)

Emissions 369

in the equivalence ratio causes CO emissions to rise. These trends are typical

of those observed for other types of combustion systems. The high levels of

CO at low equivalence ratios are due to the slow rates of oxidation associated

with low combustion temperatures. An increase in the equivalence ratio

raises the ame temperature, which accelerates the rate of oxidation so that

CO emissions decline. However, at temperatures higher than around 1800 Κ,

the production of CO by chemical dissociation of CO

starts to become

signicant. Thus, only in a fairly narrow range of equivalence ratios around

0.8 can low levels of CO be achieved.

9.4.1.2 Inﬂuence of Pressure

Figure 9.3 also demonstrates the benecial effect of an increase in combus-

tion pressure in reducing CO emissions. In recent high-pressure industrial

combustor tests, performed at 9 and 14 bars, it was found that at low equiva-

lence ratios, a 50% increase in pressure decreases CO by a factor of two and at

high equivalence ratios, the same increase in combustion pressure virtually

eliminated CO emissions by suppressing chemical dissociation.

9.4.1.3 Inﬂuence of Ambient Air Temperature

Hung and Agan [11] have examined the inuence of ambient air temperature

on the CO emissions from a 7 MW industrial engine supplied with natural

gas fuel. A strong air temperature effect on measured CO was observed. CO

emissions for an air temperature of 287 K were three to four times higher

than the corresponding values at 298 K. A correlation of these data carried

out by Hung [12] yielded the following expression for calculating the effect of

ambient air temperature on CO. It is considered to be valid for temperatures

up to 303 K.

/ CO

288

= 1 − 0.0634(T − 288), (9.4)

where CO

is the emissions of CO in ppmv for 15% oxygen at ambient temper-

ature T, and CO

288

is the emissions of CO in ppmv for 15% oxygen at 288 K.

This equation should be used with caution because it is likely to be very

engine specic. Nevertheless, it serves to highlight the strong dependence of

CO emissions on ambient air temperature, and helps to explain some of the

anomalies that are sometimes encountered when analyzing CO measure-

ments obtained from repeat tests carried out over a period of time.

9.4.1.4 Inﬂuence of Wall-Cooling Air

An important factor inuencing CO emissions is the amount of liner wall-

cooling air employed in the primary combustion zone. CO formed in primary

combustion can migrate toward the liner walls and become entrained in the