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

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

Combustion tests showed that NO

emissions were lowest with either total

inner fueling or combined inner and outer fueling. The combustor yielded

around 12 ppmv NO

at 0.6 MPa and 20 ppmv at 0.9 MPa. CO was always

below 50 ppmv. In common with most well-mixed systems, low concentrations

of both CO and NO

were attainable only over a fairly narrow range of operat-

ing conditions. Potential improvements for this concept include increasing the

number of fuel-injection holes used to deliver fuel to the lming surface to aid

in the formation of a more uniform lm, and lengthening the injector center-

body to allow a longer time for fuel evaporation and mixing [54].

9.7.2 Siemens Hybrid Burner

This burner was originally developed to operate on natural gas in either

diffusion or premix modes. It has two separate air passages—an inner one,

which features an axial swirler near its exit, and a concentric outer passage,

which is designated in Figure 9.24 as “diagonal swirler” because it is tilted

Fuel oil return

Fuel gas flow to the

pilot burner nozzle

Fuel gas flow to the

diffusion burner nozzle

Fuel oil

diffusion burner

Fuel gas

diffusion burner

Fuel gas

pilot burner

Axial swirler

Main air flow

Fuel gas flow to the

premixed burner nozzle

Fuel oil flow to the

premixed burner nozzle

Water injection

Main air flow

Steam or water

Fuel oil inlet

Diagonal swirler

Fuel/air mixture for the

premixed operating mode

Steam injection

Figure 9.24

Siemens hybrid burner. (From Angello, L. and Lowe, P., ASME Paper 89-GT-254, 1989; Bonzani,

F., Di Meglio, A., Pollarolo, G., Prade, B., Lauer, G., and Hoffmann, S., Presented at Power-Gen

Europe ‘97, Madrid, June 1997. With permission.)

Emissions 401

with respect to the burner axis. The inner passage, which carries about 10%

of the total air, contains the gas-diffusion and pilot burners. During startup

and low load operation, all the fuel is conned to this passage. As the load

increases, a fuel/air ratio is eventually reached at which the burner switches

from a diffusion to a premix mode. The gas is then injected into the outer

air passage through small premixer tubes (one per diagonal swirler vane

channel), each containing ten holes. This arrangement ensures a high level

of radial and circumferential uniformity in the fuel–air mixture entering the

premix combustion zone. Good stability is achieved by the presence of the

pilot burner situated in the inner passage. Essentially, the system functions

as a diffusion burner at low engine loads, and then operates as a premix

burner in the upper load range.

For liquid fuels, the burner is equipped with a central oil burner lance. A

further nozzle system for water or steam injection into the diffusion ame

ensures that NO

emissions can be reduced to low values over the entire

engine operating range [56].

The Siemens hybrid burner is now fully established as a low-emissions

system for engines in the 150 MW class, and has consistently achieved

single-gure NO

emissions levels (9 ppmv) when burning natural gas. Its

exibility has been demonstrated by its application by MAN GHH to its

THM 1304 engine. This 9 MW class gas turbine is a two-shaft, heavy frame

machine that features two tubular combustion chambers mounted on top

of the engine casing. NO

emissions are less than 5 ppmv between 75% and

100% load when operating on natural gas.

In the early Siemens silo combustors, all the hybrid burners were of the

same design and size. Adaptation to different combustor sizes was accom-

plished by changing the number of burners. However, in the new hybrid

burner ring (HBR) annular combustors, the number of burners is kept con-

stant at 24 in order to achieve a satisfactory temperature pattern factor (see

Chapter 4). This means that the size of the burner must be varied to suit

the size of the combustor. However, the basic hybrid burner design remains

unchanged [57].

The new annular combustor developed jointly by Siemens AG and Ansaldo

Energia in Italy for the V64.3A engine features premixed operation for both

natural gas and fuel oil. The gaseous fuel is injected into the diagonal swirl-

ers in the manner described above; the liquid fuel is injected through plain-

orice atomizers into the crossowing airstream issuing from these swirlers.

Water or steam is not required to meet the emissions regulations.

9.7.3 general electric DLN Combustor

The GE DLN combustor, shown schematically in Figure 9.25, is a two-stage

concept designed for application to natural gas-red, heavy-duty gas turbines,

but capable of liquid fuel operation with diluent injection to control NO

. It

has been described in some detail in a number of publications, including

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

those of Maughan et al. [27], Davis and Washam [33], Hilt and Waslo [34],

Schorr [40], Davis [51], and Washam [58]. The essence of this concept is the

use of two-stage combustion to achieve low emissions and high operabil-

ity over the entire load range. The combustion system consists of four main

components: primary fuel nozzles, liner, venturi, and cap/centerbody assem-

bly. These components are arranged to provide three main zones:

1. A primary zone that extends from the six primary nozzles mounted

on the cap face to the end of the centerbody

2. A secondary zone that includes the volume from the centerbody exit

to the plane of the dilution holes

3. A dilution zone that occupies the space from the dilution holes to the

end of the liner

The combustor operates in four distinct modes, which are designated as

primary, lean-lean, secondary, and premix as described below.

9.7.3.1 Primary

As illustrated in Figure 9.25a, lightoff is accomplished with fuel owing

through the primary nozzles located in the head end of the liner. Primary

combustion air enters through swirlers surrounding each nozzle and through

the primary air holes. This mode of operation is used to ignite, accelerate,

and operate the machine at low-power settings up to around 40% full load.

Primary

fuel and

swirler

Primary

Venturl cooling

Liner venturl

Liner

Ignitor

Primary

flame zone

Secondary

fuel and

centerbody

Mixing

fuel and air

Premixed

primary

Premixed

secondary

Premixed

secondary

flame zone

(a) (b)

(c)

Jets

Figure 9.25

GE DLN combustor in various operational modes: (a) primary; (b) lean-lean; and (c) premix.

(From Maughan, J.R., Luts, A., and Bautista, P.J., ASME Paper 94-GT-252, 1994. With

permission.)

Emissions 403

9.7.3.2 Lean-Lean

As the engine load increases, fuel is supplied to the secondary zone from

four radial stub pipes located in the centerbody, as shown in Figure 9.25b.

This fuel mixes with air and then ows through a swirler at the centerbody

exit to create a swirl-stabilized secondary combustion zone. The primary and

secondary zones both operate at low equivalence ratios, hence the term lean-

lean for this operating mode, which can raise turbine output to base load.

9.7.3.3 Secondary

This mode represents a transition between lean-lean and premix modes.

The fuel supply to the primary zone is gradually reduced while increasing

the fuel ow to the secondary zone. Eventually, the primary ame is extin-

guished, leaving ame only in the secondary zone.

9.7.3.4 Premix

Fuel is reintroduced through the primary nozzles and the primary com-

bustion zone (now premixed) is shifted to a region downstream of the liner

venturi, where it is ignited by the secondary ame zone (Figure 9.25c). The

venturi consists of a converging–diverging section that accelerates the ow

from the rst stage to prevent ashback. It also creates a toroidal recirculation

zone over its downstream conical surface to stabilize the primary combustion

zone in this premix mode, which is attained at or near the engine design

point. This mode corresponds to minimum pollutant emissions.

If required, both the primary and secondary fuel injectors can be dual-fuel

nozzles, to permit automatic transfer from gas to oil throughout the load range.

The system can achieve NO

and CO levels of 9 and 25 ppmv, respectively, at

base load when operating on natural gas [51]. With liquid fuels, the NO

and

CO emissions from the DLN combustor at loads less than 20% of the base load

are similar to those obtained with the standard combustor. This result is hardly

surprising because both systems feature diffusion ames in this range. The

combustor operates in the lean-lean mode between 20 and 50% load and in the

premix mode from 50 to 100% load. NO

emissions are appreciably lower than

for a standard combustor due to the premixing, but are considerably higher

than the low levels achievable with gas in the fully premixed mode. With

water injection, the combustor achieves NO

and CO levels of 42 and 20 ppmv,

respectively, at base load when operating on distillate oil fuel [51].

9.7.4 ABB eV Burner

The ABB company has developed a conical premix burner module, called

the EV-burner, which has demonstrated good performance in a wide range

of DLN combustion applications [59,60]. A cross-sectional view to illustrate

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

the operating principles of the burner is given in Figure 9.26. Each burner is

formed by two offset half cones, which are shifted to form two diametrically

opposed air inlet slots of constant width. Gaseous fuels are injected into

the combustion air owing into the slots by means of two fuel distribution

tubes containing rows of small holes that inject the fuel across the airstream.

Fairly complete mixing of fuel and air is obtained shortly after injection and

the swirling mixture ows out of the cone and into the ame zone. A unique

feature of this burner is that ame stabilization is achieved in free space near

the burner outlet because of the sudden breakdown of a swirling ow.

The device can operate satisfactorily on both gaseous and liquid fuels. The

latter are injected at the apex of the cone using a pressure or air-assist type of

atomizer. The fuel is not fully evaporated by the time it reaches the stabiliza-

tion zone and a diffusion-type ame penetrates a short distance upstream

into the burner, which explains why no pilot stage is needed to achieve ade-

quate ame stability. Also, because the ame never touches the walls, the

cone body remains clean and metal temperatures are relatively low.

In February 1991, an ABB GT11N gas turbine was retrotted with a new silo

combustor of the type shown in Figure 9.27. This silo combustor is equipped

with 37 EV burners, all of which operate in a pure premix mode [59]. For part

load operation, fuel is supplied to only a fraction of the total number of burn-

ers. NO

values of 13 ppmv have been reported by Aigner and Muller [60] for

the base load conditions of 1.25 MPa and T

= 643 K.

When burning MBtu syngases, a different fuel-injection strategy is called

for than for natural gas because of the very high ame speeds and fast reac-

tion times of MBtu fuels [61]. Injection of the fuel along the inlet air slots is

no longer appropriate. Instead, fuel injection is delayed until the burner exit,

where it enters the swirling airstream through a number of plain holes that

direct the fuel radially inward.

Combustion air

Gas fuel stage 2

Atomization

nozzle

Gas injection ports

Gas fuel stage 1

Liquid fuel

Figure 9.26

Operating principle of ABB EV conical premix burner. (From Sattelmayer, T., Felchin, M.P.,

Haumann, J., Hellat, J., and Styner, D., Journal of Engineering for Gas Turbines and Power, 114(1),

118–25, 1992. With permission.)

Emissions 405

The EV-burner technology has also been used in the design of annular

combustors. The ABB GT10 (23 MW) combustor features a single row of

18 EV burners, whereas the heavy-duty ABB GT13E2 gas turbine (>150 MW)

has 72 EV-burners that are arranged in two staggered circumferential rows

within the annular combustor [62].

In 1994, the 17 MW GT35 7-can combustor was equipped with 3 EV burn-

ers per can. It operates on gas only and startup is achieved using one burner

per can. Between idle and half load, a second burner is also used. At higher

loads, all three burners are in operation. Burner staging has a surprisingly

small effect on combustor pattern factor [63].

From the different sizes of engines and from the different types of combus-

tor (can, annular, and silo) and the number of burners employed, it is clear

that the burner modules cannot have the same size for all engines. Thus, a

major asset of the EV burner is that it can be scaled with only minor modi-

cations to suit a wide range of engine applications.

The latest engine from ABB—the GTX100—is a single shaft 43 MW

machine that features an annular combustor containing 30 AEV burners,

which represent the most recent development in EV burner technology. The

Fuel lance

Burner

Flame tube

To turbine From compressor

Figure 9.27

Silo burner tted with ABB EV burners. (From Aigner, M. and Muller, G., Journal of Engineering

for Gas Turbines and Power, 115(3), 533–6, 1993. With permission.)

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

AEV burner features an increase in the number of air inlet slots from two

to four. Gas feed pipes located along the slots inject gaseous fuel through

“tuned” holes with “tuned” spacing into the combustion air owing into

the slots. When operating at part load, gas is also injected through six equi-

spaced holes around each mixing tube exit to produce a ring of diffusion

ames with good weak extinction performance. To meet the requirement

for operation on liquid fuel, a pressure atomizer is located in the apex of

the burner cone. This atomizer provides four separate fuel sprays—one for

each slot in the burner cone. At its center is a small pilot atomizer, which

produces a narrow-angle spray of low penetration. The purpose of this pilot

nozzle is to create a small fuel-rich zone in the center of the mixing tube

exit, thereby extending the lean blowout limit. Downstream of the four cone

segments is a short transition piece whose function is to convert the four

individual fuel-airstreams into a single coherent ow. Further downstream

is a cylindrical mixing tube in which fuel evaporation and fuel–air mixing

proceed to completion. The efux from the mixing tube is then discharged

into an annular liner, where the ame is anchored in free space as in the

EV burner. The emissions performance of the AEV burner on the GTX100

engine between 50 and 100% full load is 15 ppmv NO

and CO on gaseous

fuel and 25 ppmv NO

and CO on gas oil [63].

9.7.5 rolls royce rB211 industrial Burner

Most of the operating experience gained in DLE technology has been with

heavy-duty gas turbines whose applications call for extended periods of base

load running. However, there are many other applications, such as mechani-

cal drive for pipeline compressors, where considerable operational exibility

is required. Aeroderivative engines have much to offer in this regard, and

there are a number of stationary engines in the small-to-medium-size cate-

gory that have been derived from successful high-performance aero engines.

One notable example is the Rolls Royce RB211 engine in which the annular

aero-combustor has been replaced by nine radially positioned reverse ow

combustors, as shown in Figure 9.28 from Willis et al. [31]. This arrangement

results in an 80% increase in combustion volume. The primary zone is fed by

two counter-rotating air swirlers, with several gaseous fuel-injection points

located in each swirl passageway. The secondary mixing duct is wrapped

around the primary combustor, but is separated from it by another annu-

lar duct that provides the wall-cooling air. Gaseous fuel is injected into the

secondary duct from 36 equispaced axial spray bars, each containing six

injection holes. This fuel bar arrangement was determined by trajectory cal-

culations and an air velocity prole predicted by a CFD code. Fuel sampling

and combustion tests showed uniformity of fuel–air mixing to within 4%.

Combustion testing, carried out over a range of pressures from 0.1 to 2.0 MPa,

demonstrated the ability of this axially staged, DLE combustor to achieve

simultaneously low NO

and low CO over wide ranges of power and ambient

Emissions 407

temperature without resorting to either variable geometry or air bleeds. It

was also demonstrated that a uniform fuel distribution before combustion is

essential for achieving low emissions, especially at high pressures. Based on

the test data obtained so far, at full base load conditions the predicted engine

emissions are 17.4 ppmv NO

, 5 ppmv CO, and zero UHC.

9.7.6 egT DLN Combustor

The European Gas Turbine company (EGT) has adopted a simple xed-

geometry, partially premixed system for its G30 DLN combustor. The NO

emission goal of 25 ppmv is achieved by partially premixing the fuel with half

of the total combustor airow. Also, the incorporation of an impingement-

cooled, thermal barrier coated liner greatly reduces the wall-quenching effects

associated with conventional lm-cooled liners and limits CO and UHC emis-

sions to below 50 and 20 ppmv, respectively.

Torch igniter

Injector

Combustor

Air transfer

casing

HP turbine

Discharge

nozzle

Diffuser

Figure 9.28

Rolls Royce Industrial RB211 DLE combustor. (From Willis, J.D., Toon, I.J., Schweiger, T., and

Owen, D.A., ASME Paper 93-GT-391, 1993. With permission.)

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

The essential features of the G30 combustor have been described by Norster

and DePietro [64] and are shown schematically in Figure 9.29. The basic

design philosophy is to achieve good mixing at high ring temperatures to

limit NO

production, and relatively poor mixing at lower temperatures to

give a good stability margin and low CO/UHC emissions.

The tubular combustor incorporates a simple radial inow swirler, a

swirler slot fuel-injection system, and a premixing chamber, all of which

are attached to the upstream end of the main barrel of the combustor. These

components provide major control of combustion air, fuel injection, and

mixing. Ignition and ame stabilization occurs within the vortex core of

the prechamber. Gaseous fuel is injected at the entrance of each swirler slot

through a metering jet that is sheltered below a step on the upstream side

of the slot. At low fuel ows, corresponding to low ring temperatures, the

injected fuel remains close to the rear wall of the slot and delivers a poorly

mixed fuel–air mixture to the prechamber. With increasing fuel ow, the

fuel penetrates further across the swirler slot and mixes more effectively

with the swirler air, thereby providing a more uniform mixture to the burn-

ing zone.

During starting and engine acceleration, there is a need for a piloting ame

of high stability and efciency. This is provided by a pilot/igniter burner,

which is centrally located in the head of the prechamber (see Figure 9.29). The

amount of fuel supplied to the pilot under starting and acceleration conditions

is adjusted automatically to achieve smooth and consistent starting. At full

engine speed, with no load or low ring temperature, the pilot fuel proportion

Main

swirler

Impingement

liner

Piolt

burner

Premix

chamber

Softwall

Plunged dilution

hole

Figure 9.29

EGT dry low-emissions combustor. (From Norster, E.R. and DePietro, S.M., Institution of Diesel

and Gas Turbine Engineers, DEUA Publication 495, 1996; Gallimore, S., Vickers, R.M, and Boyns,

M.B., ASME Paper 97-GT-159, 1997. With permission.)

Emissions 409

is fairly high (around 50%) to assist ame stability at these fuel-lean condi-

tions. This proportion is gradually reduced with an increase in ring tempera-

ture and reaches a minimum value at full load.

The G30 combustor was designed and developed initially for the 4.9 MW

Typhoon gas turbine. It is intended to be retrotable across EGT’s Typhoon/

Tornado/Tempest range of small engines (<10 MW). Base load emissions

of below 15 ppmv NO

and 10 ppmv CO, along with zero UHC, have been

achieved on the Tornado single-shaft industrial engine [65].

9.7.7 general electric LM6000 Combustor

Another important aeroderivative gas turbine is General Electric’s LM6000.

Figure 9.30 gives a cross-sectional view of the DLN combustor designed for

this engine, as described by Leonard and Stegmaier [19] and Joshi et al. [66].

This premix combustor employs about twice the volume of the conventional

annular combustor it replaces in order to maintain low levels of CO and UHC

while greatly reducing the emissions of NO

. Part of the air used in combus-

tion, which at maximum power is around 80% of the total combustor airow,

ows into the combustion zone through three annular rings of premixers, as

shown in Figure 9.30. The two outer rings each have 30 fuel–air premixers,

whereas the inner ring has 15. This arrangement of premixers facilitates fuel

staging at part-load operation. The total of 75 fuel nozzles is formed by hav-

ing 15 stems with three premixers on each stem, as shown in Figure 9.31, plus

Figure 9.30

General Electric LM6000 dry low-NO

combustor. (From Leonard, G. and Stegmaier, J., Journal

of Engineering for Gas Turbines and Power, 116, 542–6, 1993. With permission.)