low-nox combustion technology - tytlabs.com · we employed multistage lean premixed (lp) combustion...

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R&D Review of Toyota CRDL Vol. 41 No. 1

Low-NOx Combustion Technology

Yoichiro Ohkubo

ResearchReport

Abstract

Simple-cycle and recuperated-cycle micro gasturbines (MGT) were developed for use incogeneration systems. A simple-cycle MGT isbetter suited to applications that require steadyheat energy (steam) more than electric power. Arecuperated-cycle MGT, however, can be used forthose applications that need both electricity andheat energy.

Firstly, lean premixed combustion with amultistage fuel supply was investigated for a 300-kWe class simple-cycle MGT. An NOx emissionlevel of less than 15 ppm (O2 = 16 %) with town

gas as the fuel was demonstrated when theequivalence ratio of the primary lean pre-mixturewas held at a constant value of less than 0.6 andthe pilot fuel constituted about 10 % of the totalfuel flow rate. Secondly, to investigate low-NOxcombustion for a 50-kWe class recuperated-cycleMGT, we examined lean premixed combustionthat produces a NOx level of less than 10 ppm(O2 = 16 %) with town gas and also leanpremixed, pre-vaporized combustion withkerosene that produces a NOx level of less than20 ppm (O2 = 16 %).

KeywordsLow-NOx, Lean premixed combustion, Lean premixed pre-vaporized combustion,Combustor, Gaseous fuel, Liquid fuel, Micro gas turbine, Recuperated-cycle GT,Cogeneration system

Special Issue Core Technology of Micro Gas Turbine for Cogeneration System


1. Introduction

The continuous combustion of a gas turbinecombustor simultaneously offers both low NOx andlow CO/HC emissions, and the combustiontechnology does not require an exhaust after-treatment device. This, in fact, is one of the mainadvantages of continuous combustion over areciprocating engine. Furthermore, cleancombustion can be realized with many differenttypes of fuel.

One type of conventional continuous combustionis diffusion combustion, in which the fuel is injecteddirectly into the combustion chamber where it mixeswith air. Because the flame stability is very good,diffusion combustion has been adopted for use in anautomotive gas turbine that can operate over a widerange of equivalence ratios. The exhaust emissioncharacteristics of diffusion combustion areinfluenced by both the operating conditions and thefuel properties. For example, the 300-kWe classsimple-cycle micro gas turbine (MGT)commercialized by TOYOTA Turbine and Systems(TT&S) has a maximum NOx emission level ofaround 100 ppm. Unfortunately, reducing the NOxemissions leads to large amounts of unburntcomponents (such as CO and HC) being exhausted,as shown in Fig. 1. In general, it is difficult toreduce NOx emissions while maintaining a highcombustion efficiency, because there is a tradeoffbetween the amount of unburned hydrocarbons (CO,HC) and the NOx emissions.

When a gas turbine is incorporated into a

cogeneration system, the NOx emissions must beless than 70 ppm (O2 = 16 %), as required by Japan'sAir Pollution Control Act. Furthermore, a lower andmore demanding NOx emission figure is applied inlarge cities, as shown in Fig. 2. For example, Aichiimposes a limit on NOx emissions of 35 ppm (O2 =16 %) with gaseous fuel, and 50 ppm (O2 = 16 %)with liquid fuel.

There are several technologies, such as water orsteam injection, that can be applied to satisfy thesesevere NOx regulations. These particulartechnologies are collectively known as "wetdiffusion combustion" and work by lowering thecombustion temperature and thus limiting theformation of thermal NOx. Figure 3 shows oneexample of the design used to inject steam into thecombustion chamber. A nozzle through whichsteam is injected is provided in the vicinity of thefuel injector. The NOx emission levels arecontrolled by varying the amount of steam, and are

13

0

100

200

300

400

500

0 20 40 60 80 100

2

Heavy oil (type A)Kerosene

Town gas (type 13A)

CO

exh

aust

em

issi

on ;

ppm

( O

=

16

% )

NOx exhaust emission ; ppm ( O = 16 % ) 2

Fig. 1 NOx and CO emissions exhausted from adiffusion combustor.

0 50 100 150 200

Gas turbine

Gas engine

Diesel engine

NOx exhaust emission ; ppm(O2 = 16 %)

594

TargetLess than 15ppm

Exhaust emission Regulation

Guidance standardTokyo

Yokohama-shi, Kawasaki-shiSaitama, Chiba-shi

OsakaAichi

70

20

35

3524

594

TargetLess than 15ppm

70

20

35

3524

143

19

Fig. 2 NOx emission regulations in Japan.

Fig. 3 Schematic of a diffusion combustor with steaminjection.

less than 40 ppm (O2 = 16 %) for a fuel to steamweight ratio of around 1. Figure 4 compares NOxemissions both with and without steam injectionwhen kerosene is being used as the fuel. The NOxemissions fall as the flow rate of the steam isincreased, but there is a limit on the extent to whichthe NOx emissions can be reduced because there is acorresponding increase in the amount of COemissions.

Further reductions in the NOx levels to less than20 ppm (O2 = 16 %) are required in giant cities suchas Tokyo and Osaka. This level cannot be achievedby wet diffusion combustion. Lean premixedcombustion, however, offers a means of achievingboth low NOx and low CO emissions.

On the other hand, demand to be able to use towngas as a fuel is increasing because it produces lowerlevels of CO2 emissions. Unlike liquid fuel, gaseousfuel allows us to achieve low NOx levels when it isapplied to lean premixed combustion.

This paper describes the concept of the dry low-NOx combustors and the test results for a 300-kWeclass simple-cycle and a 50-kWe class recuperated-cycle MGT.

2. Concept of lean premixed combustion

Conventional combustion, namely, diffusioncombustion or spray combustion, has a particularadvantage in that the ideal air-fuel mixture willalways be formed at some point within the diffusionflame even if the fuel flow rate changes, such thatthe fuel-air mixture burns at the optimum ratio ofaround 1. It is observed that a bell shaped flame is

formed in the center of the combustion chamber, asshown in Fig. 5 (a). The bell-shaped flame becomesslender as the fuel flow rate falls, and widens as thefuel flow rate increases. As a result, the diffusionflame burns stably over a wide range of equivalenceratios.

The main technology that sustains the low-emission combustion1-4) is lean premixedcombustion. This involves forming a pre-mixture ofair and fuel beforehand, which is then burned in thecombustion chamber. The lean premixed flame hasa uniform spread and forms a blue flame in thecombustion chamber, as shown in Fig. 5 (b). Theflammable limit of lean premixed combustionclosely depends on the equivalence ratio. Forexample, CO emissions increase at an equivalenceratio of less than 0.45. On the other hand, NOx isproduced exponentially with temperature when theequivalence ratio is higher than 0.7. Given thesepreliminary considerations, the mixture temperature

14


0

100

200

300

400

500

0 20 40 60 80 100

w/o Steam injection

Steam injection( Ps=0.6 MPa )

NOx exhaust emission ; ppm ( O = 16 % )2CO

exh

aust

em

issi

on ;

ppm

( O

= 1

6 %

)2

Fuel ; kerosene

Fig. 4 Comparison of NOx/CO emissioncharacteristics with and without steam injection.

Fig. 5 (a) Spray combustion flame.(Using kerosene as the fuel)

Fig. 5 (b) LPP combustion flame.(Using kerosene as the fuel)

needs to be higher than 1250 OC to oxidize methanewithin 1 ms, which is the residence time of themixture in the combustion chamber, but lower than1550 OC to reduce the NOx emissions when a perfectmixture is formed. Actually, the upper temperatureshould be less than 1450 OC because the fuel and airare partially mixed. To attain this narrowtemperature window between 1250 OC and 1450 OC,a specially designed combustion device is necessary.Several methods for stabilizing the lean premixedflame have been proposed. One is combinedcombustion in which a small diffusion flame (calledthe pilot flame) is formed in the central region of thelean premixed flame. Another proposal involvesmultistage combustion in which the combustionchamber is divided into many smaller chambers witheach zone of lean premixed combustion beingcontrolled separately and optimally.

When developing a combustor, it is important totake the operating conditions of the gas turbine intoconsideration. The operating conditions for arecuperated-cycle MGT are different from those of asimple-cycle MGT. The combustor inlet airtemperature (CIT) is about 280 OC in a 300-kWeclass simple-cycle MGT (compression ratio 6.5),while it is about 600 OC in a 50-kWe classrecuperated-cycle MGT (compression ratio 3.5).The rotational speed of the turbine axis does notvary in the 300-kWe class MGT that is typicallyused to power a conventional generator. It is varied,however, in the 50-kWe class MGT as this type iswell suited to directly powering high-speedgenerators. For the 50-kWe MGT, the most suitableequivalence ratio depends on the fuel flow rate,which is obtained by changing the air flow rate inproportion to the rotational speed. The combustorfor the 300-kWe class MGT features multistagecombustion, that is a simple load operating systemthat controls only the fuel flow rate, and which doesnot rely on any variable geometry for the airflowcontrol.

It is relatively easy to realize lean premixedcombustion with gaseous fuels such as town gas orLPG. But, prevaporization is necessary to achievelean combustion with liquid fuels such as keroseneor gas oil. Given this fact, the combustor for a liquidfuels will have a complex structure.

3. Lean premixed combustion for simple-cyclemicro gas turbine

3. 1 PDLP combustorWe employed multistage lean premixed (LP)

combustion to achieve low NOx emissions with ahigh combustion efficiency over a wide operatingrange. To this end, we developed a pilot flameassisted double-swirler lean premixed (PDLP)combustor5-6) to provide low-emission combustionfor the 300-kWe class simple-cycle MGT. A cross-section of the 300-kWe class MGT with the PDLPcombustor is shown in Fig. 6.

The PDLP combustor, shown in Fig. 7, has acentral diffusion combustion zone, with the primaryand secondary lean combustion zones in series. Thediffusion flame formed by the pilot nozzle isintended to stabilize the lean premixed flames

15


Height

1,381 mm

Intake Air

Exhaust Gas

Height

1,381 mm

Intake air

Exhaust gas

Fig. 6 Cross section of 300-kWe class simple-cycle &two-shaft MGT.

Pilot nozzle

Secondary nozzle Primary nozzle

Fig. 7 PDLP combustor for a 300-kWe class simple-cycle MGT.

generated by the primary or secondary annularnozzles. The amount of pilot fuel must not exceed10 % of the total fuel flow rate to suppress theformation of thermal NOx as much as possiblewithout the risking flame instability.

3. 1. 1 Flame holding and combustionA spark ignitor is set in the center of the pilot

burner to ensure stable ignition. The pilot fuel isinjected in both the axial and radial directions. Inthe first case, it is axially injected into the pilotburner region to form a flame kernel. In the secondcase, it is radially injected so as to spread a leandiffusion flame. These diffusion flames anchor theprimary and secondary lean premixture flame, asfollows.

The pilot burner has an axial vane swirler forwhich the swirl number is 0.8. The primary andsecondary nozzles have a radial vane swirler at theinlet to each nozzle. These three swirlers form a co-swirling flow that rotates in the same direction. Theswirl number is 1.2 for the primary and 1.0 for thesecondary flow. The injectors for the primary andsecondary fuel are spaced circumferentially at theinlet of each swirler vane. The fuel and air aremixed sufficiently while the mixture is introducedthrough the swirler into the combustion chamber.The swirling flow of the mixture creates an internalrecirculation zone, which stabilizes the flame.

3. 1. 2 Heat-resistant measuresA unique characteristic of this combustor is that

part of the primary lean premixed combustion ductis made of Si3N4 ceramic, and the combustion lineris cooled by turbulent air convection that ispromoted by small ribs that protrude from the linersurface. No cooling air is introduced. High-velocityexternal cooling air is required to keep the walltemperature low enough. The shoulder wall of thecombustion liner has many cooling holes but theseare configured in such a way that the secondarypremixture is not excessively diluted with air.

The Si3N4 ceramic duct can endure a temperatureof 1400 OC. The use of the ceramic duct ensures thatthe unit is durable at primary equivalence ratios inexcess of 0.7, even though the equivalence ratio isdesigned to be less than 0.6. The ceramic duct andsmall ribs are employed to withstand the thermalload and ensure a long service life.

3. 1. 3 Fuel scheduleFigure 8 shows the fuel schedule for a range of

engine speeds. When starting the engine, the pilotfuel is injected to form a diffusion flame. After theformation of the pilot diffusion flame, as judgedfrom the rotational speed and exhaust temperature,the primary fuel is supplied to accelerate the engine.

The fuel injection schedule is divided into tworanges. In the low-load mode, both the pilot andprimary fuel are supplied up to half of the maximumpower. While the pilot fuel flow is held constantthroughout the operation range, the primary fuelflow, as controlled by a metering valve, is increasedaccording to the engine load. In the high-load mode(half to full load), the secondary fuel is added to thepilot and primary fuel and controlled according tothe load. When the equivalence ratio of the primarylean premixture is held below 0.6, very little thermalNOx is generated.

Fuel injection in the engine control map isscheduled depending on the electric power, as shownin Fig. 9. Secondary fuel feed begins at half of thefull power and is controlled according to theincrease in the electric power. In the high-loadmode, the primary fuel is controlled so as tomaintain a constant equivalence ratio of around 0.5because little thermal NOx forms at low combustiontemperatures. In practice, we modified the primaryfuel in the high-load mode according to the engineinlet air temperature (EIT) and the engine speedbecause the inlet air density varies.

16


100

40

15

0

0 300

Start 38,000 45,000 49,000

Primay

Secondary

Pilot

Output power ; kW

Fuel

flo

w r

ate

; %

90

High mode

Total

Range of operation

Low mode

Engine speed ; rpm

Fig. 8 Fuel schedule for a range of engine speed in the300-kWe class simple-cycle & two-shaft MGT.

3. 2 Test results3. 2. 1 Combustion characteristicsA gas turbine test was conducted on an engine

bench. The exhaust gas was sampled at the turbineoutlet duct and the NOx, CO, HC and O2 emissionlevels were measured. The NOx emissions andcombustion efficiencies of the PDLP combustor areplotted in Fig. 10, relative to those of a diffusioncombustor with steam injection.

In the high-load mode range from 150 kWe to 300kWe, we achieved a low NOx output of less than 10ppm (O2 = 16 %). No acoustic noise or vibrationoccurred across the entire operation range of thePDLP combustor. At a power output of 150 kWe, aNOx level of more than 17 ppm (O2 = 16 %) wasproduced since the primary equivalence ratio

reached the maximum of around 0.7 with the shutoffof the secondary fuel. In the low-load mode below150 kWe, the NOx emissions increased in almostlinear proportion to the primary equivalence ratio orthe output power. Combustion efficiencies in excessof 99.5 % were maintained over a wide power rangeabove 90 kWe.

3. 2. 2 Field testsWe carried out a field test of the 300-kWe MGT

during one year, and the total run time was 6,200hours. The gas temperature at the inlet to the gasgenerator turbine (TIT) increases during the ratedoperation, without control, with an increase in theEIT. Therefore, it is necessary for the output powerto be controlled in order to keep the TIT constant.For an EIT above 15 OC, the output power felllinearly by 2.8 kWe for every 1 OC increase in theEIT. The thermal efficiency and total efficiencydecreased by 1.4 % and 1.7 %, respectively, for anEIT increase of 10 OC.

As shown in Fig. 11, NOx emissions relative to theEIT fall by 0.7 ppm for every 1 OC when the EIT isabove 15 OC, but the output power falls to maintain aconstant TIT. For this reason, the operatedequivalence ratio of the primary and secondary fuelbecomes lean as the EIT increases, in spite of theconstant TIT. For an EIT below 15 OC, the outputpower is controlled to maintain a constant maximumpower of 295 kWe. NOx emissions increase by 1.5ppm per 1 OC, because the equivalence ratio of eachof the three fuel lines increases as the thermalefficiency and air density decrease. As a result, the

17


0

10

20

30

40

50

0 50 100 150 200 250 300Electric power ; kWe

2=

16

%)

95

96

97

98

99

100

Com

bust

ion

effi

ciei

ncy

; %

LP Comb. ; Comb.Effic.

Steam/Diffus.Comb. ; Comb.Effic.

Steam/Diffus.Comb. ; NOx

LP Comb. ; NOx

NO

x ; p

pm (

O

Fig. 10 NOx emissions and combustion efficiencies ofthe PDPA combustor, compared with those of adiffusion combustor with steam injection.

0

5

10

15

0 5 10 15 20 25 30 35

2=

16%

)

0

100

200

300

CO

Electric power

NOx

Engine inlet air temperature ;

CO

; ppm

, el

ectr

ic p

ower

; kW

e

NO

x ; p

pm (

O

Fig. 11 NOx emission characteristics of a field test whenfuel schedule is controlled by the engine inlet airtemperature (EIT).

0

0.2

0.4

0.6

0.8

0 50 100 150 200 250 300

total

primary

secondary

pilot

Electric power ; kWe

Equ

ival

ence

rat

io

Fig. 9 Equivalence ratio controlled by fuel injection inthe 300-kWe class simple-cycle & two-shaftMGT.

NOx emission peaks at around 10 ppm (O2 = 16 %)at an EIT of 15 OC.

By the way, it is known that the NOx emissiondecreases accroding to the amount of the humidity inthe atomosphere. When the NOx emissions inFig. 11 were corrected by the reference equation,7)

NOx emissions increase by 0.2 ppm per 1 OC above15 OC of the EIT, too.

4. Lean premixed combustion for aregenerative cycle micro gas turbine

4. 1 TLP combustorThis chapter explains LP combustion for a 50-kWe

class recuperated-cycle MGT (Fig. 12) usinggaseous fuel. The engine control for the MGT'slow-emission combustor must be as small andsimple as possible. The developed combustoremploys a simple load operating system that controlsthe fuel flow rate and engine rotational speed, usingneither a variable geometry (to control the air flowrate) nor multistage lean premixed combustion.

Figure 13 shows a cross section of a tandem-typelean premixed (TLP) combustor for gaseous fuelssuch as town gas or LPG. This combustor consistsof a pilot nozzle with a bluffbody at its center and acoaxial annular nozzle for premixing the fuel, with aradial vane swirler at the inlet to the nozzle. Thediffusion flame formed by the pilot fuel is intendedto stabilize the lean premixed flame supported by theannular nozzle. The internal recirculation zonecreated by the swirling flow of the mixturedownstream from the bluffbody is also indispensableto stabilizing the lean combustion flame.

The amount of pilot fuel must be held at less than10 % of the total fuel flow rate to keep the amountof thermal NOx as small as possible without anyflame instability. The gas fuel for premixing isinjected through nozzles into the combustion airstream just upstream of the swirler vane. The fueland combustion air are mixed and then introducedinto the combustion chamber through the coaxialannular nozzle. The combustion liner is cooled bythe turbulent air convection that is promoted bysmall ribs without introducing cooling air, which isthe same technique as that explained in the previouschapter.

4. 2 Combustion characteristicsThe engine control schedule is divided into three

modes. Only the pilot fuel is supplied from start-upto the low electric power range (low diffusioncombustion mode), both the pilot fuel and thepremixing fuel are supplied in the middle range(medium mode of LP combustion, assisted by a pilotflame), and only the premixing fuel is supplied inthe high load mode until full load (high LPcombustion mode) is reached, as shown in Fig. 14.

In the high LP combustion mode, both thepremixing fuel and air flow rate are controlled bymeans of a feedback signal to maintain a constanttemperature at the turbine outlet. Actually, the airflow rate is varied according to the rotational speedof the compressor because the fuel flow rate iscontrolled according to the electric power demand.This means that the equivalence ratio of the lean pre-mixture is held almost constant at the combustorinlet. This variable speed operation is advantageous

18


High speed generator

TLP combustor

Recuperator

Fig. 12 50-kWe class recuperated-cycle MGT.

Premixing-fuel injection

Liner with small ribs

Bluffbody

Pilot-fuel iniection

Fig. 13 TLP combustor for a 50-kWe class recuperated-cycle MGT.(TLP: Tandem-type Lean Premixed Combustor)

in that it prevents the deterioration of thecombustion efficiency under partial load conditions.The gas turbine is operated so as to maintain acertain exhaust gas temperature under high loads, soadequately low levels of emissions can bemaintained over a wide range of output power.

The low load diffusion combustion mode isapplied below 20 kWe. The medium LP combustionmode, assisted by the pilot flame, is suitable forloads between 20 kWe and 40 kWe, while the highLP combustion mode is optimized for loads between40 kWe and full load. The NOx emissions are about10 ppm (O2 = 16 %) when LP combustion is assistedby a pilot flame and when the pilot fuel flow rate isset to around 10 % of the total fuel flow rate. Verylow NOx levels of less than 10 ppm (O2 = 16 %) areformed as a result of LP combustion. A combustionefficiency in excess of 99.5 % can be maintained

throughout all the modes. Variable speed operationis effective for improving not only the combustionefficiency but also the thermal efficiency underpartial loads.

The characteristics of the pilot nozzle wereselected to provide the rated power with only thepilot flame in order to ensure independent operationwithout a connection to the electric power grid. Andalso, the characteristics of the pilot nozzle affectboth the ignition and the stability of leancombustion.

In order to improve the stability of the pilot flameand the deposition of soot on the surface of thebluffbody, the combustion test rig was used toobserve combustion flameout. Flame out orunstable combustion at cold start occurredoccasionally in the diffusion flame mode. As aresult of our observations, we modified the shapes ofthe pilot nozzle and the bluffbody from those of thepreliminary design. Photographs of the flame asviewed from downstream after the modifications inthe two modes are shown in Fig. 15.

Moreover, the introduction of fresh air into arecirculation zone downstream of the bluffbodyprovides a very effective means of stabilizing theflame. Part of the compressed air from thecompressor outlet is introduced through the pipe intothe combustion chamber of the engine.

4. 3 Ignition characteristicsThe diameter of the pilot injection hole has an

influence on the ignition of the pilot flame and thestability of the lean premixed flame. By measuring

19


<Premixed flame assisted by pilot-flame>

Fig. 15 Photographs of the diffusion flame (Left) and the premixed flame with pilot-flame (Right), using town gas as the fuel.

<Diffusion flame>

0

20

40

60

80

100

0 10 20 30 40 50

Electric power ; kWe

2

95

96

97

98

99

100

Com

bust

ion

effi

cien

cy ;

%

Diffusion combustion

LP combustionassisted by pilot-flame

NO

x ; p

pm (

O

= 1

6 %

)

LP combustion

Fig. 14 NOx emission and combustion efficiency in the50-kWe class recuperated-cycle MGT.

the concentration in the vicinity of the spark pluggap, we can ensure stable ignition provided thevelocity of the fuel injected from the pilot injector ishigh enough to reach the spark gap. So, the numberand diameter of the pilot injector holes should bemodified.

The relationship between the gaseous fuelconcentration and ignition characteristics wasinvestigated using an instrument that is based on theinfrared absorption method for the time-resolvedmeasurement8) of the equivalence ratio in thevicinity of the spark gap. Figure 16 shows a timehistory of the measured equivalence ratio in thevicinity of the spark plug gap at engine start-up. Theequivalence ratio increases steeply approximately0.3 seconds after the solenoid valves for total fuelcutoff are opened. And, it reaches its maximumlevel in 1 second, which is adjusted to the overall

average equivalence ratio in the combustionchamber. Then, the equivalence ratio falls becausethe air flow rate increases with the rotational speed.The ignition test at start-up indicated an ignitiontiming of between 0.3 and 0.5 seconds after thevalve was openedon 3 sec. in Fig. 16.

5. Lean premixed prevaporized combustion fora recuperative-cycle micro gas turbine

5. 1 Design of TLPP combustorTo use liquid fuel such as kerosene or gas oil, the

application of lean premixed prevaporized (LPP)combustion allows us to attain low levels of NOx.We developed a tandem-type lean premixed pre-vaporized (TLPP) combustor for the 50-kWe classrecuperated-cycle MGT.9) Figure 17 shows theTLPP combustor. The TLPP combustor consists ofa pilot spray injector with a bluffbody at its centerand a coaxial annular premixing nozzle with a radialvane swirler at its the inlet, in the same way as forthe TLP combustor for gaseous fuel. Although theTLPP combustor has approximately the samestructure as the TLP combustor for gaseous fuel, itdiffers in the following aspects.

A louver vane is placed in the premixed, pre-vaporized passage to prevent the formation of highfuel concentrations. The prevaporizing passage isdivided into two sections by the louver vane. Liquidfuel is injected from the swirl-type injectors towardsthe louver vane surface. This structure promotesspray evaporation and premixing as anycomparatively large liquid droplets collide with thevane surface, and then evaporate completely within

20


Pilot iniector

Air

Louver vane

LPP injector

(b) Photograph of TLPP combustor.(a) Cross section of TLPP combustor.

Fig. 17 Tandem-type lean premixed pre-vaporized (TLPP) combustor.

6000

12000

0

0.5

1

1.5

2

2.5

3

3.5

4

0 1 2 3 4 5 6 7 8

Time; sec

Equ

ival

ence

rat

io

0

2000

4000

8000

10000

14000

16000

Eng

ine

spee

d N

e ; r

pm

Averaged equivalence ratio m

Ne

Valve open

Range of ignition

Range of ignition

Fig. 16 Measurement of the equivalence ratio in thevicinity of the spark gap at the engine start-up.

about 1 millisecond after fuel injection. There is a problem of deposits forming in the fuel

passage and at the tip of the injector when the fuelinjector is always exposed to an atmospherictemperature of around 600 OC. Compressor exit airat a comparatively low temperature (around 200 OC)is introduced into the injector tip to preventoverheating of the injector itself. A little pilot fuel isalso injected to significantly improve the durabilityof the TLPP combustor but too much for cooling theinjector itself, although the NOx emissions increase.

The fuel for premixing is injected, using four sprayinjectors, into the passage carrying the combustionair. The fuel flow rate to the pilot spray injector iscontrolled so as not to exceed around 12 % of thetotal fuel flow rate during LPP combustion assistedby the pilot flame. Compressed air is introducedfrom the circumference of the spray injector topromote atomization and to cool the injectors. Weobserved the spray impingement on the surface ofthe air passage wall at the air temperature of 600 OC,

as shown in Fig. 18.Only pilot fuel is supplied from start-up to a load

of 20 kWe (low spray combustion mode), and boththe pilot fuel and premixed fuel are supplied at loadsover 20 kWe (high LPP combustion mode assistedby pilot flame).

5. 2 Combustion characteristicsObservations of the combustion flame are shown

in Fig. 19. LPP combustion forms a blue flame inthe annular region of the combustion chamber. LPPcombustion assisted by a pilot flame forms abrilliant yellow flame that anchors the LPP flame.

Figure 20 shows the behavior of the NOxemission and the combustion efficiency forkerosene. With a pilot fuel flow rate of less than 12% of total fuel flow rate, the NOx emission is lessthan 20 ppm (O2 = 16 %) and the combustionefficiency is higher than 99.5 % over a power rangeof 20 kWe to 50 kWe. The lower pilot fuel flow rateis an effective means of reducing the NOx

21


0

20

40

60

80

100

0 10 20 30 40 50Electric power ; kWe

2

95

96

97

98

99

100

Com

bust

ion

effi

cien

cy ;

%

Gpilot /Gtotal =12%

Gpilot /Gtotal

Gpilot /Gtotal

LPP combustion assisted by pilot-flame

NO

x ; p

pm (

O =20%

=16%

Spray combustion

= 1

6 %

)

Fig. 20 NOx emission and combustion efficiencyaffected by pilot fuel flow rate in the range of thehigh LPP combustion mode (more than 15 kWe).

Fig. 19 Photographs of LPP combustion assisted by pilot-flame (Left) and LPP combustion flame without pilot-flame(Right), using kerosene as the fuel.

Fig. 18 Photograph of kerosene spray injected into thepremixed prevaporized passage of TLPPcombustor.(Air temperature: 600 OC, Pressure: 0.3 MPa)

emissions. However, the combustion efficiency at apower output of around 15 kWe deteriorates becauseof the bad characteristics of the liquid atomizationand prevaporization.

The pilot fuel is controlled by a feedback signal toadjust the engine load in the low spray combustionmode. But, the pilot fuel flow rate is held at around12 % of the total fuel flow rate to stabilize the pilotflame in the middle and in the high mode of pilotflame assisted LPP combustion.

Figure 21 shows a comparison of the NOxemissions for kerosene and gas oil. Although theNOx emissions for kerosene are higher than thosefor gas oil in the low mode below 15 kWe of spraycombustion, both NOx emission levels are almostthe same in the medium and high mode of LPPcombustion that is assisted by the pilot flame.

6. Summary

Lean premixed combustion was investigated as ameans of simultaneously realizing low NOxemissions and high combustion efficiencies forgaseous or liquid fuels. To overcome the inherentcharacteristic in that lean premixed combustion canbe stabilized only in a narrow range of equivalenceratios, the uniformity of the fuel-air premixture andthe controllability of the lean premixed combustionwere improved by using three types combustors, asfollows.

(1) Multistage lean premixed combustion wasinvestigated for a conventional simple-cycle microgas turbine (MGT), using gaseous fuel such as town

gas or LPG. The developed pilot flame assisteddouble swirler type lean premixed (PDLP)combustor has a diffusion combustion zone at itscenter, with a primary and secondary leancombustion zone arranged in series. It is possiblethat the diffusion flame formed by the pilot nozzlestabilizes the lean premixed flames generated by theprimary or secondary annular nozzle. Low NOxlevels of less than 15 ppm (O2 = 16 %) andcombustion efficiencies in excess of 99.5 % areachieved over a wide range from 50 % to full loadfor a 300-kWe class simple-cycle MGT.

(2) Because modern MGTs directly power high-speed generators, the air flow rate is changed basedon the rotational speed of the engine. A simple loadoperating system that controls both the fuel and airflow rates is easily applied to a tandem-type leanpremixed (TLP) combustor using gaseous fuel. LowNOx levels of less than 10 ppm (O2 = 16 %) andcombustion efficiencies in excess of 99.5 % areachieved over a wide range from 40 % to full loadfor a 50-kWe class recuperated-cycle MGT.

(3) Lean premixed prevaporized (LPP) combustionwas investigated in order to enable the use of liquidfuel such as kerosene or gas oil. The structuredevised for the TLPP combustor promotes sprayevaporation and premixing within about 1millisecond after fuel injection at the air temperatureof 600 OC. Low NOx levels of 20 ppm (O2 = 16 %)and higher combustion efficiencies are also achievedover a wide load range from 40 % to full load for the50-kWe class recuperated-cycle MGT.

References

1) Mori, M., Ishizuka, A., Miyahara, M. and Kuwabara, S. : "Development of Double Swirler LowNOx Combustor for Gas Turbine", 20th CIMACG03, (1993), London

2) Hosoi, J., Watanabe, T., Toh, H., Mori, M., Sato, H.and Ishizuka, A. : "Development of a Dry Low NOxCombustor for 2MW Class Gas Turbine", ASMEPap., No. 96-GT-53(1996)

3) Ichikawa, H., Kumakura, H., Sasaki, M. and Ohkubo, Y. : "Development of a Low EmissionCombustor for a 100-kW Automotive Ceramic GasTurbine (IV)", ASME Pap., No. 97-GT-462(1997)

4) Bhargava, A., Kendrick, D. W., Casleton, K. H. andMaloney, D. J. : "Pressure Effect on NOx and COEmissions in Industrial Gas Turbines", ASME Pap.,No. 2000-GT-97(2000)

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0

10

20

30

40

50

60

70

0 10 20 30 40 50Electric power ; kWe

Gas Oil: Gpilot /Gtotal = 12%

Kerosene: G pilot total = 12%

Spray combustion LPP combustion assisted by pilot-flame

2N

Ox

; ppm

( O

=

16

%)

Kerosene: G /G = 0

/G

pilot total

Fig. 21 NOx emissions using kerosene as the fuelcompared with those of gas oil.

5) Sato, H., Hase, K., Tokumoto, T., Ohkubo, Y.,Azegami, O., Idota, Y. and Higuchi, S. :"Development of a Dry Low Emission Combustor for300 kW Class Gas Turbine", 23rd CIMAC, Hamburg,(2001)

6) Ohkubo, Y., Azegami, O., Idota, Y., Sato, H. andHiguchi, S. : "Development of Dry Low-NOxCombustor for 300kW Class Gas Turbine Applied toCo-generation Systems", ASME Pap.,No. 2001-GT-83(2001)

7) Hung, W. S. Y. and Agan, D. D. : "The Control ofNOx CO Emissions from 7-MW Gas Turbines withWater Injection as Influenced by AmbientConditions", ASME Pap., No. 85-GT-50(1985)

8) Ohkubo, Y., Idota, Y. and Azegami, O. :"Measurement of Equivalence Ratio in Spark PlugGap for Low Emission Combustor", 30th GTSJAnnual Conf. Pap., Oct.9-10, (2002), 115-120

9) Azegami, O., Ohkubo, Y., Idota, Y,. Higuchi S. andOkabayashi, K. : "Development of Dry LowEmission Combustor for 50kW Class Gas Turbine",Asian Congr. on Gas Turbines 2005, Nov.16-18,(2005)

(Report received on Dec. 16, 2005)

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Yoichiro OhkuboResearch fields : Gas Turbine and

reciprocating engine, Combustion,Fuel injection and atomization

Academic degree : Dr. Eng.Academic society : Jpn. Soc. Mech. Eng.,

Gas Turbine Soc. Jpn., Soc. Autom.Eng. Jpn., Combust. Soc. Jpn., Int.Inst. Liquid Atomization and SpraySystems-Jpn.

Award : Paper Award of Gas TurbineSoc. Jpn. (1991)Technical Award of Combust. Soc.Jpn. (1996)

low-nox combustion technology - tytlabs.com · we employed multistage lean premixed (lp) combustion...

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