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

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

9.5.3.2 Selective Catalytic Reduction

This is a method for converting NO

in a gas turbine exhaust stream into

molecular nitrogen and H

O by injecting ammonia into the stream in the

presence of a catalyst. Exhaust gases rst pass through an oxidation catalyst

and are then mixed with ammonia before entering the SCR catalyst. The

oxidation catalyst removes the CO and UHC emissions by oxidizing them

to CO

and H

O. To reduce NO

emissions, ammonia is injected in a manner

designed to achieve intimate mixing with the engine exhaust stream. After

mixing, the exhaust gases pass over a catalyst (usually vanadium pentoxide),

which results in the selective reduction of NO

to form N

and H

O. The

principal reactions are:

6NO + 4NH

→ 5N

+ 6H

6NO

+ 8NH

→ 7N

+ 12H

Water or steam injection is used rst to reduce the NO

level down to around

40 ppmv, leaving the SCR process to achieve a further reduction down to less

than 10 ppmv [40]. SCR works best with natural gas fuel and is fairly intolerant

to sulfur-bearing liquid fuels. It requires that the temperature of the exhaust

stream be within a fairly narrow range from 560 to 670 K, and so is restricted

to systems in which the exhaust gas ows into a heat recovery device, usu-

ally a steam generator [33]. A major problem with this method is the require-

ment for a control system that feeds the requisite amount of ammonia, and

the need for a continuous monitoring system that can give the feedback to

the ammonia supply mechanism under differing load conditions. Another

problem is the size and weight of the equipment. According to Davis and

Washam [33], for an 83 MW MS7000 gas turbine, an SCR designed to remove

90% of the NO

from the exhaust stream has a volume of 175 m

and weighs

111 tons. Despite these drawbacks, the method is quite widely used.

9.5.3.3 Exhaust Gas Recirculation

The underlying principle of this approach is the reduction of ame temper-

ature by recirculating cooled combustion products back into the primary

zone, as illustrated in Figure 9.16. The practical feasibility of this method

of NO

reduction has been investigated by Wilkes and Gerhold [42], who

found that signicant reductions (50%) could be achieved with recirculation

rates of 20% or less at base load conditions. The major thermal effect stems

from the reduced concentration of oxygen in the inlet air, but there is also a

secondary effect due to the higher heat capacity of this air with an increased

O and CO

content.

The main advantage of the method is that little or no combustor devel-

opment is required and standard production combustors can be used. Its

main drawback lies in the need for an intercooler between the exhaust and

Emissions 391

inlet. This virtually rules it out for simple gas turbines, but application to

combined cycle plants offers more promise due to the substantially lower

exhaust gas temperatures. Another drawback is that only very clean fuels

can be used to avoid problems of fouling and contamination.

9.6 Pollutants Reduction by Control of Flame Temperature

Of all the factors inuencing pollutant emissions from gas turbine combus-

tors, the most important by far is the temperature of the combustion zone.

With conventional combustors, this can range from 1000 K at low-power

operation to 2500 K at high-power operation, as indicated in Figure 9.17. This

gure also shows that too much CO is formed at temperatures below around

1670 K, whereas excessive amounts of NO

are produced at temperatures

higher than around 1900 K. Only in the fairly narrow band of temperatures

between 1670 and 1900 K are the levels of CO and NO

below 25 and 15 ppmv,

respectively. The basic objective of all the various approaches toward low-

emissions combustors described below is to maintain the combustion zone

(or zones) within a fairly narrow band of temperatures over the entire power

range of the engine.

9.6.1 Variable geometry

An ideal variable-geometry system would be one in which large quantities

of air are admitted at the upstream end of the combustion liner at maxi-

mum power conditions to lower the primary-zone temperature and provide

adequate lm-cooling air. With a reduction in engine power, an increasing

Fuel

Heat

exchanger

Pump

Figure 9.16

Schematic diagram to illustrate the principle of exhaust gas recirculation.

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

proportion of this air is diverted to the dilution zone to maintain the primary-

zone temperature within the low-emissions “window” shown in Figure 9.17.

Practical ways of achieving some variation in airow distribution include

the use of variable-area swirlers to control the amount of air owing into the

combustion zone [43,44], variable air openings into the dilution zone [45,46],

or a combination of these.

The drawbacks to all forms of variable-geometry systems include complex

control and feedback mechanisms that tend to increase cost and weight and

reduce reliability. Problems of achieving the desired temperature pattern in

the combustor efux gases could also be encountered, especially if the liner

pressure drop is allowed to vary too much. The incentive for surmounting

these practical problems is that variable geometry has the potential for simul-

taneously reducing all the main pollutant species without sacricing other

aspects of combustion performance. It also has several other advantages; for

example, as the combustion temperature never falls below a certain mini-

mum value of around 1670 K, chemical reaction rates are always relatively

high. This enables the combustion zone to be made smaller, with consequent

advantages in terms of reductions in combustor size and weight. For aircraft

applications, variable geometry also has the potential for wide stability lim-

its and improved altitude relight performance.

Ideally, variable geometry combustors should be used in conjunction with

premix-prevaporize fuel-injection systems. Only in this way is it possible to

120

100

Temperature

range for

low

emissions

1500 1700 1800 1900

2500 K

2000 21001600

Carbon monoxide, ppmv

Oxides of nitrogen, ppmv

Primary temperature, K

1000 K

Temperature range of

conventional combustors

Figure 9.17

Inuence of primary-zone temperature on CO and NO

emissions.

Emissions 393

avoid the local high-temperature, high NO

-forming regions, created by the

presence of fuel droplets in the combustion zone.

Although variable geometry has been used in some large industrial

engines, there have been few successful applications of this technique in

small-to-medium-size gas turbines due to size and cost limitations and also

because of concerns regarding operational reliability [47].

9.6.2 Staged Combustion

With variable-geometry systems, the combustion temperature is controlled

to within fairly narrow limits by switching air from one zone to another with

changes in engine power setting. By contrast, the airow distribution within

staged combustors remains constant; the fuel ow is switched from one

zone to another in order to maintain a fairly constant combustion tempera-

ture. One simple method of fuel staging is by “selective fuel injection,” as

described by Bahr [48]. With this technique, fuel is supplied only to selected

combinations of fuel injectors at lightoff, relight, and engine idle conditions,

as illustrated in Figure 9.18. Only at power settings above idle is the full

Unfueled

Fueled

Figure 9.18

Illustration of the use of selective fuel injection.

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

complement of fuel injectors employed. The objective of this modulation

technique is to raise the equivalence ratio and hence also the temperature

of the localized combustion zones at low-power operation. This approach,

which is now in common use, not only reduces CO and UHC emissions, but

also has the added advantage of extending the lean blowout limit to lower

equivalence ratios.

A major drawback of selective fuel injection is the “chilling” of chemical

reactions that occurs at the outer edges of the individual combustion zones.

This chilling lowers combustion efciency, as discussed above, and increases

the formation of CO and UHC. Furthermore, the circumferentially nonuni-

form exit temperature distribution results in loss of turbine efciency. These

limitations have led to the development of “staged” combustors in which no

attempt is made to achieve all the performance objectives in a single combus-

tion zone. Instead, two or more zones are employed, each of which is designed

specically to optimize certain aspects of combustion performance.

A typical staged combustor has a lightly loaded primary zone, which

provides all the temperature rise needed to drive the engine at low-power

conditions. It operates at an equivalence ratio of around 0.8 to achieve high

combustion efciency and low emissions of CO and UHC. At higher power

settings, its main role is to act as a pilot source of heat for the main combus-

tion zone, which is supplied with a fully premixed fuel–air mixture. When

operating at maximum power conditions, the equivalence ratio in both zones

is kept low at around 0.6 to minimize NO

and smoke.

An important choice for the designer is whether the staged combustion

should take place in “series” or in “parallel.” The latter approach, often called

“radial staging,” features the use of a dual-annular combustor, as illus-

trated in Figure 9.19. One of these combustors is designed to operate lightly

loaded and provide all the temperature rise needed at startup, altitude relight,

and engine idle conditions. At idle, the equivalence ratio of the combustion

zone is selected to minimize the emissions of CO and UHC. The other annu-

lar combustor is specically designed to optimize the combustion process at

high-power settings. It features a small, highly loaded combustion zone of

short residence time and low equivalence ratio to minimize the formation of

and smoke.

The main advantage of radial staging is that it allows all the combustion

performance goals to be achieved, including low emissions, within roughly

the same overall length as a conventional combustor. This short-length fea-

ture is attractive from the standpoints of low engine weight and reduced

rotor dynamics problems [48].

If the combustor domes of the inner and outer stages are arranged to be

radially in-line, the fuel injector tips for both stages can be mounted on a

common feed arm, as shown in Figure 9.19. An important advantage of this

arrangement is that the main stage fuel injectors are cooled by the continu-

ously owing pilot fuel, as illustrated in Figure 9.20. This prevents coking of

Emissions 395

Shingled liner

construction

Main dome

Centerbody

dilution

Pilot dome

Split duct

diffuser

Pressure atomizing

fuel nozzle assembly

Counterrotating

swirlers

Figure 9.19

General Electric dual-annular combustor. (From Bahr, D.W., Journal of Propulsion and Power, 3(2),

179–86, 1987. Reprinted with permission from AIAA.)

Main inlet

Pilot inlet

Cooling flow

Pilot

nozzle

Main

nozzle

Figure 9.20

Fuel nozzle for GE dual-annular combustor. (Courtesy of Parker Hannin Corporation.)

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

the main stage nozzles when they are unfueled but still exposed to the hot

engine environment.

There are a number of drawbacks to radial staging. One basic drawback

is that all zones are supplied with air at the compressor outlet temperature,

which means that all zones have the same relatively poor lean blowout

limit. It is also clear that pollutants reduction is achieved at the expense of

increased design complexity and a marked increase in the number of fuel

injectors. The larger liner wall surface area demands additional cooling air,

which has an adverse effect on pattern factor. Furthermore, the peaks of the

radial temperature prole could shift in radial position as a result of fuel

staging, with potential adverse effects on the hot sections downstream of the

combustor. Another basic problem with radial staging is that of achieving

the desired performance goals at intermediate power settings where both

zones are operating well away from their optimum design points.

The radially staged combustor shown in Figure 9.19 was designed by the

General Electric company. It achieved around 35% reductions in CO and UHC,

and 45% reduction in NO

, in comparison with the corresponding single-

annular combustor. The GE CFM56-5B engine, tted with this dual-annular

combustor, is now in service on Airbus Industrie A320 and A321 aircraft. The

GE90 dual-annular combustor has also received ight certication.

With “series” or “axial” fuel staging, a portion of the fuel is injected into a

fairly conventional primary combustion zone. Additional fuel, usually pre-

mixed with air, is injected downstream into a “secondary” or “main” combus-

tion zone, which operates at low equivalence ratios to minimize the formation

of NO

and smoke. The primary combustion zone is used on engine startup

and generates the temperature rise needed to raise the rotational speed up

to engine idle conditions. At higher power settings, fuel is supplied to the

secondary combustion zone and, as the engine power rises toward its maxi-

mum value, the function of the primary zone becomes increasingly one of

providing the heat needed to initiate rapid combustion of the fuel supplied

to the second stage.

Axial staging does have certain advantages over radial staging. Since

the main stage is downstream of the pilot stage, ignition of the main stage

directly from the pilot is both rapid and reliable. Also, the hot gas ow from

the pilot into the main combustion zone ensures high combustion efciency

from the main stage, even at low equivalence ratios. According to Segalman

et al. [49], the radial temperature prole at the combustor exit can be devel-

oped to a satisfactory level using conventional dilution hole trimming and,

once developed, does not change signicantly as a result of fuel staging.

The main drawback to axial staging is that the in-line arrangement of

stages tends to create additional length, which makes the problem of retrot

difcult for some engines. In comparison with conventional combustors, the

liner surface area that needs to be cooled is higher. The fuel injectors for

the two combustion stages require separate feed arms, which involve two

separate penetrations of the combustor casings. Furthermore, the pilot fuel

Emissions 397

cannot be used to cool the main stage fuel as can be done quite conveniently

with radial staging.

Figure 9.21 shows a cross-sectional view of an axially staged combustor

developed by the Pratt and Whitney company [50]. For clarity, the main stage

fuel injectors are shown rotated half an injector pitch to be in line with the

pilot stage injectors. The engine centerline is at the bottom of the gure. This

combustor has the benets of the axially in-line stage arrangement without

any length penalty and is designed to t into the existing P&W V2500-AS

engine. The pilot combustion zone is specically designed to provide wide

stability limits and high combustion efciency (low CO and UHC). With

an increase in engine power above idle, fuel is admitted to the main com-

bustion zone, where combustion is initiated and sustained by the hot gas

emanating from the pilot zone. The relative amounts of fuel supplied to the

pilot and main zones is such that no thrust lag is created when fuel is rst

introduced into the main zone. In combination, the pilot and main zones

maintain a low equivalence ratio that ensures low NO

emissions at higher

power settings.

Of special interest in Figure 9.21 is the inboard location of the pilot combus-

tion zone. This greatly reduces the susceptibility to ame blowout in heavy

rain because the compressor centrifuges the water to the outer portion of the

airow path. Another advantage of having the main zone outside the pilot

is that the radial temperature prole at the combustor outlet peaks toward

the outer radius of the turbine owpath, a situation that is conducive to long

turbine blade life.

In the longer term, it is possible that staging of the airow by variable geom-

etry in conjunction with fuel staging may become more of a design option.

Pilot fuel

injectors

Main stage

Pilot stage

Main fuel

injectors

Figure 9.21

Pratt and Whitney axially staged combustor.

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

9.7 Dry Low-Oxides of Nitrogen Combustors

In the design of DLN combustors for stationary gas turbines, there are two

major performance criteria to be met. As pointed out by Davis [51], one obvi-

ous requirement is that of meeting the emissions goals at base load on both

gas and liquid fuels and controlling the variation of emissions levels across

the load range of the engine. Another, equally important, requirement is for

high system operability to achieve stable combustion at all operating condi-

tions, good system response to rapid load changes, acceptable levels of com-

bustion noise and, if required, capability for switching smoothly from gas to

liquid fuel, and vice versa.

This chapter reviews some of the approaches that various manufacturers

are following in their endeavors to achieve low pollutant emissions, in par-

ticular low NO

, without having to resort to the injection of water or steam.

Combustors of this type are known as “dry low-NO

” (DLN) or “dry low-

emissions” (DLE) combustors.

9.7.1 Solar Dry Low-emissions Concepts

Solar Turbines in San Diego has been among the pioneers in the develop-

ment of DLE combustors for industrial gas turbines. The results of this com-

pany’s efforts have appeared in a number of publications, e.g., White et al.

[41], Roberts et al. [45], Smith et al. [52–54], and Etheridge [55]. Figure 9.22

Swirl

vanes (18)

Pilot fuel

Channel

injection fuel

Combustor

dome

Primary air flow

Swirler injection

fuel spoke (18)

Pilot fuel

injection

Channel fuel

injection orifice

Figure 9.22

Solar low-NO

fuel injector for natural gas. (From Smith, K.O., Angello, L.C., and Kurzynske,

F.R., ASME Paper 86-GT-263, 1986. With permission.)

Emissions 399

shows a cross-sectional view of a fuel injector designed for installation in

multiple-can combustion systems for the Mars and Centaur engines. An

18-vane radial ow swirler is used to impart a high degree of rotation to

the combustor primary air, which serves both to promote fuel–air mixing

and to induce a recirculatory ow in the primary zone. The fuel injector/

air swirler assembly permits three different modes of fuel injection, as indi-

cated in Figure 9.22. Best mixing is achieved by injecting the gaseous fuel

through 18 spokes, each spoke being located between a pair of swirl vanes.

As each spoke contains six holes of 0.89 mm diameter, the total number of

injection points is 108. Combustion tests carried out with this fuel injector

assembly attached to a cylindrical combustion liner showed that the con-

cept is capable of achieving NO

emissions below 10 ppmv when burning

natural gas at pressures up to 1.1 MPa, along with low values of CO and

UHC [52].

The manner in which the fuel-injection system described above was

adapted for liquid fuels by Smith and Cowell [54] is shown in Figure 9.23.

The system employs two different modes of liquid fuel injection. The “inner

lming” mode involves lming of the fuel on the cylindrical swirler center-

body. Fuel is delivered to the outer surface of the centerbody through eight

holes located around the centerbody circumference. This fuel forms a lm

that is carried downstream by the swirling primary airow. It vaporizes and

mixes with air as the lm progresses along the centerbody and into the pri-

mary zone. The “outer lming” mode operates in the same manner, but the

lm formation now takes place on the outer cylindrical surface of the air

swirler channel.

Filming

injector

orifices

Gas pilot

orifices

Combustor

liner

Swirler

centerbody

Main fuel

Axial

swirler

Air

flow

Methanol

supply

Translating

swirler

inlet plate

Air

flow

Pressure

atomizing

injectors

Premixing

gas

injectors

Figure 9.23

Solar low-NO

fuel injector for liquid fuels. (From Smith, K.O. and Cowell, L.H., ASME Paper

89-GT-264, 1989. With permission.)