DOE/MC/23 170-96/C05 12

Performance of Low-Btu Fuel Gas Turbine Combustors

Authors:

Stephen Bevan John H. Bowen Alan S. Feitelberg

Stephen L. Hung Michael A. Lacey Kenneth S. Manning

Contractor:

GE Environmental Services, Inc. 200 N. Seventh Street Lebanon, Pennsylvania 17046

Contract Number:

DE-AC2 1 -89MC23 170

Conference Title:

Advanced Coal-Fired Power Systems '95 Review Meeting

Conference Location:

Morgantown, West Virginia

Conference Dates:

June 27-29, 1995

Conference Sponsor:

U.S. Department of Energy, Morgantown Energy Technology Center ( M E W

DISCLAIMER

This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.

This report has been reproduced directly from the best available copy.

Available to DOE and DOE contractors fiom the Office of Scientific and Technical Information, 175 Oak Ridge Turnpike, Oak Ridge, TN 37831; prices available at (615) 576-8401.

Available to the public from the National Technical Information Service, U.S. Department of Commerce, 5285 Port Royal Road, Springfield, VA 22 16 1 ; phone orders accepted at (703) 487-4650.

5.2 Performance of Low Btu Fuel Gas Turbine Combustors

CONTRACT INFORMATION

Contract Number

Contractor

DE-AC21-87MC23 170

GE Environmental Services, 'Inc. 200 N. Szventh St. Lebanon, PA 17042 (7 17) 274-7000

Contractor Project Manager Stephen Bevan

Principal Investigators

METC Project Manager

Period of Performance

Schedule and Milestones

John H. Bowen, GE Corporate Research and Development Alan S. Feitelberg Stephen L. Hung Michael A. Lacey Kenneth S. Manning

Justin L. Beeson

September 30, 1987 to September 30,1995

Program Schedule

1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 1.10 2.1 2.2

1987 1988 1989 1990 1991 1992 1993 1994 1995 Project Plan - - HGCU/Gasifier Site Prep - HGCU System Construction - Gasifier Refurbishment - Preliminary System Test - Gas Turbine Combustor Design - - - Turbine Simulator Site Prep I Combustor/Simulator ProcurementfConstruction - - Advanced Gas Turbine System Studies Advanced HGCU Processes Integrated System Testing Data Evaluation

- -250-

OBJECTIVES

General Electric Company is developing and testing low Btu fuel gas turbine combustors for use in integrated gasification combined cycle (IGCC) power generation systems. Conducting low Btu fuel combustion tests in the turbine simu- lator located at GE Corporate Research and Development is a significant portion of this effort. The primary objective of recent turbine simulator tests has been to compare and contrast the performance of several different low Btu fuel nozzle designs. Parameters related to fuel nozzle performance include: (1) emissions of CO, NO,, and unburned hydrocarbons; (2) flame stability; and (3) combustor liner and fuel nozzle tempera- tures. A total of five fuel nozzles, based on three distinct nozzle concepts, have been evaluated over a wide range of operating conditions.

GE is also developing and testing a rich- quench-lean (RQL) low Btu fuel gas turbine com- bustor. The overall objective of this work is to develop an RQL combustor with lower conver- sion of fuel bound nitrogen (FBN) to NO, than a conventional diffusion flame gas turbine combus- tor. Initial tests were conducted in RQL1, a reduced scale (0.75 lb/s total flow) combustor. Insights gathered from the successful RQLl tests have been incorporated into the design of RQL2, a full scale rich-quench-lean gas turbine combus- tor now under construction.

BACKGROUND INFORMATION

Although the low Btu gas fuel nozzle devel- oped several years ago [I] performed reasonably well, this fuel nozzle was too large for use in large gas turbine combustor cans which typically use multiple fuel nozzles. For this reason, GE Power Generation began development of smaller low Btu gas fuel nozzles. The long term objec- tive of this effort is to develop a single fuel nozzle concept that is suitable for use with low

Btu fuel produced from any type of gasifier (air or oxygen blown) and any type of cleanup system (low or high temperature). In addition, the nozzle should be suitable for use in both relatively small combustors that use a single fuel nozzle and large combustors that use multiple fuel nozzles. The single fuel nozzle tests described in this paper complement multi-nozzle tests conducted by GE Power Generation at the Gas Turbine Develop- ment Lab in Schenectady, NY.

Depending upon the feedstock and the pro- cess conditions, the low Btu fuel produced by a gasifier and high temperature desulfurization system will contain hundreds to thousands of parts per million of FBN (primarily NH,). In a conventional combustor, a significant fraction of the FBN will be converted into NO,. RQL com- bustion can reduce the conversion of FBN to NO,, because a large fraction of the FBN is con- verted into non-reactive N, in the fuel rich stage. Additional air is introduced in the quench stage, and the lean stage provides sufficient residence time to complete combustion.

The subscale RQL combustor described in this paper (RQL1) is a significant improvement over the reduced scale RQL combustor discussed in previous work [2,3]. The most important design enhancement is the incorporation of a con- verging rich stage combustor liner, which sepa- rates the rich stage recirculation zones from the quench stage and lean stage air. Although pre- mixing the low Btu fuel and the rich stage air would probably improve the performance of RQL1, premixing is no longer considered to be a requirement for low conversion of FBN to NO,.

PROJECT DESCRIPTION

Turbine Simulator The low Btu gas turbine simulator is shown

in Figure 1. High temperature low Btu fuel is supplied by the pilot scale coal gasification and

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hot gas cleanup (HGCU) facility at GE Corporate Research and Development in Schenectady, NY. The turbine simulator is designed to operate at full gasifier capacity, 8000 l b h of fuel gas pro- duced by gasifying 1800 lbhr of coal. The gasi- fier produces low Btu fuel at a pressure of 20 atm and a nominal temperature of 1 0 0 0 O F . A typical fuel composition (after H,S removal by the hot gas cleanup system) is given in Table 1. Details of the gasifier and HGCU system can be found in Gal et al. [4].

The turbine simulator includes: a low Btu gas fuel nozzle; a modified GE MS6000 combus- tor liner; a film cooled, first stage LM6000 nozzle assembly; and an impingement cooled transition piece (see Figure 1). A recent addition to the tur- bine simulator test stand is the optional ability to blend natural gas into the low Btu fuel. The added natural gas allows the turbine simulator to

maintain 'F' conditions (combustor exit tempera- ture = 255OOF) even when the gasifier is operated at 80% of capacity. Comparisons of combustor liner temperatures, fuel nozzle temperatures, and CO emissions typically show that the addition of natural gas has little effect on turbine simulator performance.

The most recent turbine simulator tests (des- ignated as Tests 5,6,7A, 4A, and 7B) have focused on evaluating the performance of differ- ent low Btu fuel nozzle designs (see Table 2). The three basic nozzle concepts are shown in Fig- ures 2,3, and 4. Nozzle N5-40, designed for low Btu fuel combustion several years ago [l], con- sists of concentric slot-type axial fuel and air swirlers. Nozzle N5-15 is identical to N5-40, except the N5-15 fuel swirl angle is 1 5 O , not 40". The results of tests with both of these fuel nozzles were discussed by Abuaf et al. [5 ] .

High Temperature Fuel Control Valve Cooling Air Cooling Air Low Btu Fuel Combustion

From HGCU Svstem

I iiece - er .MS6000

Combus.or Reverse Transition Liner Flow

Sleeve Gas

out

lsokinetic Particulate Sample Probe

Exhaust Figure 1. The Turbine Simulator Test Stand

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Both the N7A nozzle type (Figure 3) and the N7B nozzle (Figure 4) have a smaller overall diameter than the N5 nozzle type. The N7A fuel nozzle type is best described as an axial fuel swirler and a radial air swirler. Nozzles N7A-20 and N7A-30 are identical, except for the location and angle of the fuel holes. The N7A-20 fuel swirl angle is 20°, and the fuel hole centers are located on a 2.5" diameter circle. The N7A-30 fuel swirl angle is 30°, and the the fuel hole cen- ters are located on a 3.25" diameter circle. These changes were designed to increase the fuel/air mixing and flame stability of the N7A nozzle type.

The N7B nozzle consists of axial air and fuel swirlers and an air-cooled mixing cup. Unlike the N5 and N7A nozzle types, only one version of the N7B nozzle has been tested in the turbine simula- tor.

RQL Combustor

The RQLl combustor has a modular design, which allows for rapid evaluation of different

The RQLl test stand is shown in Figure 5.

Table 1: Typical Pilot Plant Low Btu Fuel Composition

Species Mole Percent Q L v.u

17.3 2.7

30.1 12.6 28.0 0.3 0.4

100.0

hardware configurations. The rich and lean stages are separate components, and the air flow to each stage can be varied independently. Although both stages are impingement cooled, only the rich stage has a separate cooling air supply that is not used for combustion. The RQLl low Btu fuel nozzle is similar in design to the N7A nozzle type, although much smaller.

Both the turbine simulator and the RQLl combustor were fired with pilot plant fuel during Test 4A (completed November 1994). RQLl was fired for 37 hours, during which time all of the RQL test objectives were completed. Because of the low fuel flow rate needed for the subscale RQLl combustor, the gasifier was fueled with anthracite during the RQL1 portion of Test 4A to minimize tar deposition in the fuel line. The use of anthracite resulted in a lower than usual fuel ammonia content (see Table 2.) Following com- pletion of the RQLl portion of the test, the fuel

b- 4.25" -1

Figure 2. Nozzle N5-15 (concentric air swirler not shown)

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line and some shared instrumentation lines were disconnected from RQLl and connected to the turbine simulator, which was fired for a total of 25 hours.

RQLl has been fired with both pilot plant low Btu fuel and natural gas/N, blends. The nat- ural gas/N2 tests were completed before begin- ning Test 4A. The main gods of the natural gas/N2 tests were to: (1) check the instrumenta-

Table 2. Recent Long Duration Tests

Test Date Firing

Time (hours) Turbine Simulator

Fuel Nozzle NH, Concentration in Low Btu Fuel (ppmv)

5 11/93 41 6 5/94 67

7A 8/94 77 4A 11/94 25 (simulator)

37 (RQL1)

N5-40 N5- 15

N7A-20 N7A-30

4100 4400 4250

2200 (simulator) 1650 (RQL1)

7B 3/95 111 N7B 4600

20".

I diameter

w 0.312' 16 fue diameter

equally spaced

!I holes

Nozzle Base

SECT, A-A

I 16 air slots

0.44" x 0.55"

Nozzle Tip Nozzle Assembly (side view)

Figure 3. Nozzle N7A-20

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tion and data acquisition software; (2) verify the effectiveness of the cooling scheme; (3) observe flame stability, dynamic behavior, and ignition characteristics; and (4) provide an opportunity for operator training. Given the limited time avail-

able during a pilot plant test, these tasks had to be completed before beginning pilot plant Test 4A. Only the results of the pilot plant test with low Btu fuel will be presented in this paper.

16 air holes

equally spaced

SECTION B-B

44 air cooling hoios 0.125" diameter equally spaced

4.50'

I Nozzle Base Air Cooled Mixing Cup

Figure 4. Nozzle N7B

Lean Stage Lean Stage Gas

Sample I I , Liner probe Backpressure

Valve

Rich Stage Rich Stage Combustion Air Cooling Air

T

U I / Y

Impingement h e p for Lean Stage Flame Stabilization

Converging Rich Stage Liner Cooling Sleeves

Figure 5. RQLl Test Stand

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400 0" 8

s 350

-0 5 300 !& a Y

g 250

E zoo 0 ul v )

W

.-

.-

RESULTS

Turbine Simulator NO, emissions measured during the last 4

turbine simulator tests are shown in Figure 6 (only emissions measurements without natural gas added to the low Btu fuel are shown). Most of the NO, formed in the turbine simulator comes from NH, in the fuel. NO, emissions decrease as combustor exit temperature increases because the head end of the combustor becomes increasingly fuel rich as exit temperature increases.

A casual inspection of Figure 6 suggests that the conversion of NH3 to NO, differs between fuel nozzles. However, when the variation in fuel ammonia content between tests is taken into account (see Table 2), the conversion of NH3 to NO, is seen to follow a uniform trend for all of the fuel nozzles (Figure 7). Although the consis- tency between the nozzles is somewhat surpris- ing, keeping the axial distribution of fuel/air ratio the same for all of the nozzles probably con-

OX 150 I 1000 1300 1600 1900 2200 2500

Combustor Exit Temperature ( O F )

Figure 6. Turbine Simulator NO Emissions x

tributes to the observed uniformity. The overall conversion in the turbine simulator is somewhat lower than expected from a commercial installa- tion, because the simulator operates at reduced flows, and the gas residence time in the combus- tor is greater than at full flow.

Although the fuel nozzles cannot be ranked based on their conversion of NH3 to NO,, flame stability and CO emissions vary significantly between nozzles (see Figure 8). The CO emis- sions shown in Figure 8 are a combination of measurements made both with and without natu- ral gas added to the low Btu fuel (natural gas addition has almost no measurable influence on CO emissions). CO emissions were typically below 10 ppmv (on a dry, 15% 0, basis) at com- bustor exit temperatures greater than 2100°F. CO emissions increase rapidly as the combustor exit temperature decreases and the combustor approaches blowout. For most of the fuel nozzles, blowout occurred at combustor exit tem- peratures below 1400°F.

80 l ' ~ l " l " l '

- h 0 Nozzle N5-15 - v 8 - 4 A Nozzle N7A-20 -

A Nozzle N7A-30 $ 0 Nozzle N7B 0 eo 1 c

z 0 S

.c

.P 40 E al iz s

20 1000 1300 1600 1900 2200 2500

Combustor Exit Temperature ( O F )

Figure 7. Conversion of NH3 to NO in the Turbine Simulator

X

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Although not shown in Figure 8, nozzle N5-40 produced lower CO emissions than any of the other nozzles. This nozzle also resulted in unacceptably high capkowl temperatures, and as a result the capkowl was damaged during Test 5. The decreased fuel swirl angle of nozzle N5-15 reduced cap/cowl temperatures to acceptable levels, but resulted in unacceptably high CO emissions.

Nozzle N7A-20 improved the CO emissions over nozzle N5-15, but yielded unacceptably poor flame stability. When using Nozzle N7A-20, the combustor tended to blow out at low combustor exit temperatures (T 2 175OOF) when the fuel heating value was low. (Due to the motion of the stirrer in the fixed bed gasifier, fuel heating value fluctuates sinusoidally with an amplitude of +15% and a period of about 20 minutes.) Of the five fuel nozzles tested, only Nozzle N7A-20 exhibited this phenomenon. Nozzle N7A-30, with an increased fuel swirl angle and fuel holes relocated closer to the air swirler, had improved

" I " I " I " I Nozzle

1200 1500 1800 2100 2400 Combustor Exit Temperature ("F)

Figure 8. Turbine Simulator CO Emissions

flame stability. However, these changes also sig- nificantly increased combustor liner temperatures (see Figure 9).

Nozzle N7B improved upon N7A-30 and had good flame stability, reasonable CO emissions (Figure 8) and very low liner temperatures (Figure 9). Nozzle N7B also performed well in multi-nozzle tests conducted at the GE Power Generation Gas Turbine Development Lab. For this reason, nozzle N7B has been selected for use in the GE MS7001FA gas turbine being supplied for the Tampa Electric Co. Polk County IGCC power plant.

Figure 10 shows unburned hydrocarbon (UHC) emissions measured during the last four turbine simulator tests. Because adding natural gas to the low Btu fuel had almost no measurable effect on UHC emissions, the data in Figure 9 are a combination of measurements made with and without natural gas added. Note that unlike CO and NO, UHC emissions have not been corrected

Nozzle m A A N7A-30 .- cn

N7B

- - - - - - -

P - -

G

4 600 - - -

0 5 10 15 20 25 I I ' I I I I I I ' I I I I I I -

Axial Distance from Combustor Entrance (inches)

Figure 9. Turbine Simulator Combustor Liner Temperatures at a Combustor Exit

Temperature of 17S0°F

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to a dry, 15% O2 basis. UHC emissions, like CO emissions, tend to increase as the combustor exit temperature decreases and the combustor approaches blowout. UHC emissions were fre- quently at or near the detection limit of about 2 ppmv. For this reason, identifying clear differ- ences in UHC emissions from the different fuel nozzles is difficult.

RQLl Test conditions for the RQLl portion of

Test 4A are listed in Table 3. The relatively low flow rates of fuel and air resulted in lower than expected fuel and air temperatures. The low fuel NH, concentration (1650 ppmv) was not unex- pected, and was a result of gasifying anthracite during the RQLl portion of Test 4A (usually, Illi- nois #6 is gasified). To find the optimal operating conditions, three sets of measurements were con- ducted with the RQLl combustor. In the first set of measurements, 40% of the total combustion air was sent to the rich stage, and the remaining 60% was sent to the lean stage. The air split was held constant as the fuel flow rate was varied. The resulting NO, emissions are shown as a function of combustor exit temperature in Figure 11.

h

0 N5-15 A N7A-20

.- A N7A-30 v)

W

1000 1300 1600 1900 2200 2500 Combustor Exit Temperature (“F)

Figure 10. Turbine Simulator Unburned Hydrocarbon Emissions

The most striking feature of Figure 11 is the minimum in NO, emissions. This type of trend is expected in a rich-lean combustor and has been widely reported in other experimental [e.g., 61 and theoretical [e.g., 71 studies. For a fixed air split between the rich and lean stages, there is an optimum rich stage fueVair ratio which minimizes the conversion of NH, to NO,. Factors which reportedly influence the optimum fuel/air ratio include the rich stage residence time and the fuel composition.

The overall conversion of NH3 to NO, is also shown in Figure 11. Since the contributions of non-FBN NO, (eg , thermal NO, and prompt NO,) cannot be separated from the total measured NO, emissions, the conversion has been calcu- lated assuming that there is no thermal NO, for-

Table 3. RQLl Test Conditions

Pressure: Rich Stage Residence Time: Rich Stage Air Temperature: Lean Stage Air Temperature: Low Btu Fuel Temperature: Low Btu Fuel Composition:

10 atm

27-51 ms

460°F

620°F

520°F

34% N2 28% H2O

14% H2

12% co 10% co, 1.4% CH,

1650 ppmv NH,

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mation. In other words, all of the measured NO, has been assumed to come from NH, in the fuel. Using this approach, the conversion of NH3 to NO, is overestimated slightly.

The second set of measurements was con- ducted with 60% of the total combustion air sent to the rich stage and 40% sent to the lean stage (see Figure 12). Comparing Figures 11 and 12 shows that the minimum in NO, emissions shifted to a higher combustor exit temperature. The third set of measurements was conducted with 67% of the total combustion air sent to the rich stage and 33% sent to the lean stage (see Figure 13). The minimum in NO, emissions shifted to an even higher combustor exit temperature, about 2350'F. For a given RQL combustor configuration, the minimum in NO, emissions can be made to occur at any desired combustor exit temperature by carefully selecting the air split between the rich and lean stages.

h s - 42.5 - - - 9

0 c - I" - z

- 27.5 6

- 2 - z - s

K 0

a,

- .-

I I I I I I I I I I I l l 1 ' Combustor Exit Temperature (OF)

12.5 1500 1750 2000 2250 2500

Figure 11. RQLl NO Emissions at a 40% Rich / 60% Lean

Combustion Air Split

X

Figure 12 shows that at the optimal rich stage fuel/air ratio, the conversion of NH, to NO., is about 15%. This conversion is a substantial improvement over a standard, diffusion flame based gas turbine combustor. At these condi- tions, the conversion of NH, to NO, in a diffu- sion flame based gas turbine combustor is expected to be about 30%.

The conversion of NH, to NO, in the RQLl combustor has been modeled with a series net- work of perfectly stirred reactors (PSRs) and plug flow reactors (PFRs). The rich stage was mod- eled as an equivolume PSR and PFR in series. Model predictions were insensitive to the relative sizes of the rich stage PSR and PFR. The flow exiting the rich stage PFR was combined with the lean stage air in a second PSR, which fed a second PFR. Inputs to the model were the known rich stage and lean stage combustor volumes, as well as measured pressures, fuel and air flow flow rates, fuel composition, and temperatures. Since

200 0" 8 s '2 42.5 -

z 0

-$ 150 3

0"

I*

v) 27.5 .- 0 100 .- v) 2 E z s

c

z - a v

0

a,

c

v) .- w

gx 50 12.5 1500 1750 2000 2250 2500

Combustor Exit Temperature (OF)

Figure 12. RQLl NO Emissions at a 60% Rich / 40% Lean

Combustion Air Split

X

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\

heat loss from the RQLl combustor to the sur- roundings was significant, a heat removal term was included in the first PSR. The three other model reactors were assumed to be adiabatic. For each data point, the rate of heat loss from the first PSR was varied until the model predicted com- bustor exit temperature matched the measured exit temperature. The chemical kinetic mecha- nism included more than 50 species and 250 ele- mentary reaction steps [8]. Calculations were performed using the CHEMKIN I1 package of subroutines and associated programs [9].

The model predictions are shown as the solid line in Figure 13 (modeling of the NO, emissions for the other air splits has not been completed). The model reproduces both the magnitude of NO, emissions and the location of the minimum fairly well. Using these types of models, key reaction pathways responsible for NH, oxidation and destruction can be identified. These type of models can also be used to perform 'what-if cal- culations, and to investigate the influence of oper- ating conditions (e.g., fuel composition, total rich

200 0" s 51 i. TI 150 s E .- s 100

R

In v

In In

W

.- E ' 50

1500 1750 2000 2250 2500

Combustor Exit Temperature ("F)

Figure 13. RQLl Emissions and Model Predictions at an Air Split of

67% Rich / 33% Lean

stage residence time) on the performance of an RQL combustor.

RQLl CO emissions were low under all test conditions (see Figure 14). The air split between the rich and lean stages had relatively little influ- ence on CO emissions.

FUTURE WORK

Fabrication and assembly of RQL2, a full scale rich-quench-lean combustor, is now under- way (see Figure 15). Like RQL1, the RQL2 design includes a converging rich stage liner and a backward facing step at the entrance to the lean stage to enhance lean stage flame stabilization. The RQL2 design also allows the air flow rates to the rich and lean stages to be varied indepen- dently. RQL2 has been designed to take the place of the turbine simulator and to connect to the existing turbine simulator transition piece and LM6000 cascade. The first pilot plant test of RQL2 is tentatively scheduled for the third quar- ter of 1995.

, [ I f I ( ( , 1 1 1 1 , [ [ 1 1 , h - 0" 0 40% rich 160% lean - .O 7

0 60% rich 140% lean A 67% rich f 33% lean

0'

-0

1500 1750 2000 2250 2500

Combustor Exit Temperature ( O F )

Figure 14. RQLl CO Emissions at Three Combustion Air Splits

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Figure 15. RQL2 Test Stand

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Springfield, VA: National Technical Informa- tion Service.

DOE/METC-93/6 13 1. NTIS/DE93000289.

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