J i
THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS
345 E. 47th St., New York, N.Y. 10017
The Society shall not be responsibte for statements or opinions advanced in
papers or discussion at meetings of the Society or of its Divisions or Sections,
or printed in its publications; Discussion is printed only if the paper is published in an ASME Journal. Papers are available from ASME for 15 months
after the meeting.
.
Printed in U.S.A.
94-GT-255
Copyright © 1994 by ASME
MARS SoLoNOx - LEAN PREMIX COMBUSTION
TECHNOLOGY IN PRODUCTION
C. J. Etheridge
Solar Turbines Incorporated
San Diego, California
ABSTRACT
Solar Turbines has applied it's SoLoNOx lean premix
combustion technology to the Mars 100S (103 MW, 14,100 hp
ISO) and Mars 90S (9.4 MW, 12600 hp ISO) gas turbine engines
and now offers engines with guaranteed NOx and CO emissions
less than 42 and 50 ppmv respectively and with near teen
expectations of offering NO. emissions below 25 ppmv when
operating on natural gas fuels. The development has been ongoing
since January 1990 and includes a field demonstrator engine which
is mnning on a gas pipeline in the North West of USA.
This paper introduces the basic design of the Mars SoLoNOx
combustion system and presents some of the unique characteristics
found with this lean premix technology during the program.
✓
A
1
INTRODUCTION
Solar Turbines now has low emissions versions of it's natural
gas fired Mars 100S and 90S gas turbines in production (Figure 1).
The development program that commenced in January 1990 was
-centered around replacing the complete combustion system of the
existing engine with a new lean bum/premixed version.
The basic design requirements for the system were as
follows:
•
Guarantee the following emissions levels corrected to 15
percent oxygen between 50 to 100 percent load and 0 to
100°F (-18 to 38°C) on natural gas:
- 42 ppmv NO.
- 50 ppmv CO
- 25 ppmv unburned hydrocarbons
•
Develop design parameters to meet 25 ppmv NO,
guarantee levels over the same operating range for
production engines in 1995.
`°`^
FIGURE 1. MARS 100S SoLoNOx GAS TURBINE ISO
RATED AT 10.5 MW (14,100 HP)
•
Be able to retrofit existing packages by engine exchange
and with minimum fuel system modifications.
•
Incorporate an Annular Combustion Chamber
•
Develop dual fuel low emissions capability in the future
Solar had been researching lean premix injector technologies
in the mid 1980's (Roberts, P.B., et al, 1981; Smith, KO., et al.,
Presented at the International Gas Turbine and Aeroengine Congress and Exposition
The Hague, Netherlands — June 13-16, 1994
Downloaded From: http://proceedings.asmedigitalcollection.asme.org/pdfaccess.ashx?url=/data/conferences/asmep/82312/ on 06/18/2017 Terms of Use: http://www.asme.org/abo
air/fuel ratios which enabled the design requirements to be met
(Figure 3 presents the results of this testing).
The results show that the injector premix duct is required to
operate at an air/fuel ratio (AFR) of 31.2 at the design point, with
the ability to turn down to 32.4 AFR before the CO exceeeds the
target level. These AFR's correspond to a theoretical combustion
temperature range from 2700 to 2800°F (1775 to 1810 K), which
corresponds to the widely accepted maximum operating
temperature for minimum NO,, emissions. Calculations also carried
out, showed that the engine would only be capable of operating
over a range between 96 and 100 percent load, which showed that
some other additional control would be required to extend the
operating range down to the 50 percent load design goal.
1986; and Smith K.O., et al, 1987) and the decision to apply the
basic injector concept to the Mars engine was taken in early 1989
in response to the increasing need to reduce emissions being
emitted into the atmosphere.
The Mars development program commenced in January 1990
with an initial combustion test verification program on Natural gas,
followed by a complete system design, an engine development
program and an engine durability test program in conjunction the
Pacific Gas Transmission Company in Washington State, USA. The
program was a success and both the Mars 100S and Mars 90S gas
turbine engines are now in production under the trade name of
"SoLoNOx".
This paper presents some of the unique findings of
developing and operating a gas fired lean premix combustion
system in the Mars Gas Turbine engine.
70
•.
".
60
BASIC INJECTOR DESIGN
The basic research injector concept developed in the mid
1980's is presented in Figure 2. It is comprised of a swirler and
parallel mixing duct into which, natural gas is injected through fuel
injection spokes. The fuel mixes with the swirling inlet air to
produce a homogeneous gas/air mixture which is then injected into
N
a combustion chamber. The injector also includes a pilot fuel
circuit which enables a portion of the fuel to be burned in the
combustor with a diffusion flame. This pilot circuit is used for start
up and low power operation and it provides stable combustion in
a region where lean premix combustion is not sustainable.
LO
---t*._ -------o-
50
GAS
GAS INJECTION
CO Development Goal
'•,
40
U
30
••^•'•.
NOx Development Goal
20
s.•
CO
O 10
z
24
AXIAL
Single Injector Test "Centaur Size"
Mars Engine Conditions
IN - 198 psia, TIN - 770°F
26
28
30
32
34
SWIRLER AIR/FUEL RATIO
PREMIX
RE93084M
FIGURE 3. SINGLE INJECTOR EMISSIONS DATA
AT MARS CONDITIONS
ENGINE PART LOAD OPERATION
The results of the basic injector design program clearly
demonstrated that the limited operating range of the injectors would
not allow a Gas Turbine engine to meet the emissions goals across
the 50 to 100 percent load range. A concept was therefore required
which would enable the injectors to tun at their optimum AFR at
any point in the operating load range.
Several options were considered which included fuel staging
concepts and air control concepts, both by mechanical means and
aerodynamic means. The chosen concept was required to be as
reliable as possible and be easily maintained, since this is a prime
consideration for operators. The design also had to meet fairly
rigorous unit delta cost targets, and all these considerations drove
a need for the design to be as simple as possible.
Fuel staging was considered, but in order for this concept to
meet requirements, a plurality of injectors would be required, which
could not be accomodated with the developed basic injector
concept, which calls for only fourteen injectors in the Mars engine.
Fuel staging would therefore involve turning off individual injectors
as necessary to divert the fuel to the other injectors. This method
would cause potentially life limiting radial temperature distortions
around the turbine and also isolate injector burning zones between
zones of cold air, which would increase the possibilities of engine
Same out and increased CO emissions.
H,wr
FIGURE 2. BASIC RESEARCH INJECTOR DESIGN
The basic injector concept was originally developed using
Solai's Centaur full load engine combustor inlet conditions of 100
psia and 600°F (690 kPa and 590 K) and the design therefore had
to be extrapolated to meet the more arduous conditions found in the
Mars engine combustor which are typically in the 220 psia, 800°F
(1520 kPa, 700 K) region at full load.
Firstly, one Centaur sized injector was tested close to Mars
Design Point conditions in a tubular combustor to determine
whether it was feasible to meet the development goals of 25 ppmv
NO= and 50 ppmv CO corrected to 15 percent 0 2. This was
achieved and the tests also determined the range of swider premix
2
Downloaded From: http://proceedings.asmedigitalcollection.asme.org/pdfaccess.ashx?url=/data/conferences/asmep/82312/ on 06/18/2017 Terms of Use: http://www.asme.org/abo
As the design study progressed, internal aerodynamic air
control became the favoured option because it became clear that
control of air into the injector premix ducts by mechanical means
would require a more complex and possibly a less reliable, design
and controls strategy to ensure that emissions would remain low.
In view of this, it was decided to opt for a relatively simple control
method of using overboard injector bleed which demonstrated
excellent control characteristics on a modified basic injector (Figure
4). The rig results, when converted to engine conditions, indicated
that the low emissions would easily meet the 50 percent load
requirements.
h
F
BLEED FLOW
RE93104M
FIGURE 4. AIR BLEED INJECTOR
INJECTOR BLEED DESIGN CONCEPT
The advantage of the injector bleed design concept lies in its
simplicity of operation. Figure 5 presents a side view of the engine
and it shows a single butterfly control valve located on the side of
the engine. This valve is attached to an air bleed manifold which
is connected in turn to each of the fourteen injectors.
When the injector premix duct leans out to a point where the
CO approaches the 50 ppmv limit during engine turn down, the
bleed valve opens, and draws air away from the swider, which has
the effect of restoring the correct AFR to keep the CO emissions
below guarantee levels.
Control of the valve is relatively straightforward. The Mars
engine already has an array of 17 thermocouples measuring the
power turbine inlet temperature (T s), and these are used to control
the engine's maximum operating condition. The control system has
easily made use of these to control the position of the valve, and
this is achieved by establishing and following a T S set point curve,
which is set up during an emissions test to maintain both NO, and
CO within design goals.
A typical set of engine test data generated using this method
is presented in Figure 6. The graphs show how the NO„ CO, T S
and engine bleed as a percentage of compressor mass flow vary
with reducing engine speed. The curves show a number of
characteristics:
a) The lower the gas producer (GP) speed - the higher the
bleed, as a direct result of the leaner operation of the
engine.
AIR BLEED
HOSE
BLEED
E
INJECTOR
M
FIGURE 5. MARS SoLoNOx AIR BLEED SYSTEM LAYOUT
Downloaded From: http://proceedings.asmedigitalcollection.asme.org/pdfaccess.ashx?url=/data/conferences/asmep/82312/ on 06/18/2017 Terms of Use: http://www.asme.org/abo
1370
u;
1360
u_
Turbine Inlet
Temperature, T5
1350
1340
1
1330
8
Bleed
Control
6
z
4
I
Engine Bleed, WiN
4
50
60
70
90
80
REM
201—
C
Control
NOx
n 15
e10
40
Mars SoLoNOx Engine
0
20
40
60
LOAD, %
80
100
120
M
FIGURE 7. TYPICAL THERMAL EFFICIENCY
CURVES FOR MARS ENGINES
MARS SoLoNOx COMBUSTION SYSTEM DESIGN
The Mars SoLoNOx system includes a full annular
Bl
0
i
w
Injected Engine
an engine with water injection to meet 42 ppmv. It can be seen that
up to 4 percentage points are lost with bleed at lower loads
compared to a standard engine.
301-
QC 20
42 ppmv Water
} 20
U
15
U
LL
W 10
100
40
U ® 10
25
0
40
LOAD, %
0
Standard Engine
30
5
20
35
50
60
70
LOAD, %
80
100
90
W
FIGURE 6. MARS SoLoNOx I00S ENGINE TEST DATA
b) The Ts set point varies with GP speed to accomodate a
variable CO characteristic.
combustor and fourteen injectors. Approximately sixty percent of
the combustion air is fed through the injectors and the remaining
forty percent is used for cooling of the combustor walls. No
dilution zone is included in the design as the pattern factor is low
due to the premixed nature of the system. Acceptable radial profile
has been provided by setting up the required airflow in the last
cooling ring of the inner and outer combustor barrels. Figure 8
presents a cross section of the combustor and injectors. The two
gas fuel circuits can be seen in the injector and the bleed annulus
at the swider inlet is also shown. When bleed is required to richen
up the injector, it is taken evenly from an annular space located
adjacent to the injector swider plenum. The bleed system is setup
in such a way that all injectors are equally reduced in air flow.
The combustor primary zone volume was determined using
the formula for loading Q , (Smith, K.O., 1987):
9
c) NO1 starts to rise as the premix duct AFR richens to
compensate for the reduced combustor inlet air
temperature.
d) NO1 emissions remain constant with reducing combustor inlet pressure, thereby indicating that NO ; formation in
lean bum combustors is fairly independant of pressure.
The primary disadvantage of using bleed to control emissions
in this way, is the loss of engine thermal efficiency at part loads
when the bleed is operating. This loss is due to the fact that work
is done compressing air which is then expanded into the exhaust
duct. Any loss of engine performance is undesirable, but to put this
loss into perspective, Figure 7 provides typical thermal efficiency
vs load curves for a standard engine, a Mars SoLoNOx engine and
_
wf
V,c Pc
where
Wr = Fuel flow rate in Btu/hr
V R = Combustor primary zone volume in ft'
P, = Combustor pressure in atmospheres
Smith, 1987, defines a loading requirement 1.5 x 10 6 Btu/hr1t3-aim for the Mars engine for adequate CO burnout which
produces a required 4.32 ft 3 (0.1223 m3)volume.
From this the geometry was selected based upon the given
length and inside diameter constraints outlined in the specification
Downloaded From: http://proceedings.asmedigitalcollection.asme.org/pdfaccess.ashx?url=/data/conferences/asmep/82312/ on 06/18/2017 Terms of Use: http://www.asme.org/abo
3OM
FIGURE 8. MARS SoLoNOx COMBUSTION SYSTEM LAYOUT
of requirements. The net result was to produce a combustor which
defined the requirement for a new larger combustion section outer
casing. Finally the interface with the turbine section was required
to be the same as the existing engine and this was included in the
design shown in Figure 8.
BASIC ENGINE OPERATION
The complete control strategy for engine operation is
illustrated in Figure 9 with the engine starting up initially in a high
pilot fuel transient mode. Here, the bleed valve is nominally closed
and 30 percent of the fuel flows through the pilot circuit to provide
high combustion stability. The engine will stay in this mode of
operation until it passes through a threshold where it can accelerate
in the "low emissions" or normal operating mode, which is
typically above 86 percent of gas producer speed (Ngp).
At 88 percent Ngp, the controls trigger the system to change
to the "low emissions" mode. First of all, the bleed valve ramps
open under T S control to raise the primary zone AFR to the
required conditions for low emissions. After this is achieved the
pilot fuel is reduced to low levels, and the emissions then fall in to
the required limits. This change will typically take about 15
seconds. From this point on, the engine will normally operate in
"low emissions" mode at any point above 88 percent Ngp.
As the engine speed increases, the bleed valve starts to close
in response to the swirlier premix flow richening up. This is
achieved by using the controls to modulate the bleed valve to a set
Ts value, which has been calibrated to maintain the emissions
below requirements. As the engine speed increases, the bleed valve
will eventually close as the normal T s value for engine operation
exceeds the T S set point Once this point has passed the engine then
operates as a standard engine right up to maximum load.
If the engine is to be shut down the reverse sequence occurs.
As the engine Ts falls below the set point for bleed valve control,
the controls command the valve to open to maintain the set point
value, and as the speed is reduced the valve continues to open.
When the engine speed goes below 86 percent Ngp, the controls
command the engine to go back into the high pilot transient mode.
Firstly, the pilot flow is increased to 30 percent and then the bleed
valve closes shut. The engine continues to reduce speed to idle and
then shuts down after a cooldown period.
One ramification of this sequence of operations with a high
pilot transient mode and a "low emissions" mode is that users of
the gas turbines will have to ensure that their operating permits
adequately address the two step emissions signature that results
(Figure 10), and factor this into their overall operation.
Downloaded From: http://proceedings.asmedigitalcollection.asme.org/pdfaccess.ashx?url=/data/conferences/asmep/82312/ on 06/18/2017 Terms of Use: http://www.asme.org/abo
Main
is reached when the CO starts to increase as the primary
zone leans out.
fl
F_ 1.0SoLoNox
JMode
w
u-
Non SoLoNOx
Mode
-
• CO reduces with reducing ambient temperature at part
loads for a given Ngp as the controls act to maintain a
constant value of T s.
0
LEAN PREMIX COMBUSTION PRESSURE OSCILLATIONS
z SoLoNOx
¢ O Open_
Mode
C l-
trN
-J°-
w
2
Non SoLoNOx
Mode
Lean Premix Annular combustors have been found to
experience pressure pulsations under certain operating conditions.
The pulsations occur at a unique frequency which coincides with
the standing wave natural frequency of the circumferential length
of the combustor operating at the given gas temperature. These
oscillations can become quite severe, and up to 2 psi (14 kPa) nns
has been measured using a probe located in the torch igniter tube.
k$
< closed
70
80
NGP, %
20
90
40
60
POWER, %
100
80 100
It soon became apparent from the engine field demonstration
program, that the Combustor oscillations were producing a new
vibration environment in the engine which was causing high
Flu
FIGURE 9. BASIC ENGINE FUEL AND AIR CONTROL
70
NOflSO LONOx
Mode
60
_'1
CO>
2OOpmv
z° 50
30
SoLONOX Mode
p 25
Fuel Switch Point
9
E
a
a 40
O 30
;20
CO
a
ze
NO x
'c
x 20
O
z 10
10
-20
0
0
25
50
75
LOAD, %
100
0
RES31
FIGURE 10. TYPICAL INDICATIVE MARS SoLoNOx
EMISSIONS CHARACTERISTICS
40
o
FIELD ENGINE EMISSIONS PERFORMANCE
As part of the SoLoNOx program, a field demonstration
engine has been pinning at a Pacific Gas Transmission site in
North West USA since the Summer of 1992 (Stitt, 1993). The
objectives of the program are to gain field operating experience,
identify any design deficiencies, provide emissions data over a
range of ambient conditions, and demonstrate the robustness of the
control strategy for keeping CO emissions within specifications.
Typical emission results are presented in Figure 11, where curves
of NO. and CO against ambient temperature for a variety of engine
Ngp speeds are shown. The main conclusions of these results are
as follows:
•
15
30
20
40
60
AMBIENT TEMPERATURE, °F 80
100
a^oaw^
Mars T-14,000 SoLoNOx
Average:
All Part-Load Conditions
0 20
E
a
Average:
0 10
Maximum Load
0 1
-20
0
1
1
1
20
60
40
AMBIENT TEMPERATURE, °F
1
1
80
100
FIGURE 11. AVERAGE EMISSIONS MEASURED ON
FIELD ENGINE AS A FUNCTION OF
AMBIENT TEMPERATURE
• NO, increases and CO decreases at full load with
reducing ambient temperature as the premix duct AFR
increases to compensate for reducing combustor inlet
temperature to maintain T S topping until maximum Ngp
G
Downloaded From: http://proceedings.asmedigitalcollection.asme.org/pdfaccess.ashx?url=/data/conferences/asmep/82312/ on 06/18/2017 Terms of Use: http://www.asme.org/abo
no
o^
z O
40
50
92%
40
30
0
---- ---------------
9
NO x Development Target
20
40
- 30
-------------------CO Margin
'O
n'o 20
20
V 10
92%
U0
2
4
8
6
10
PILOT FLOW, %
12
14
OL
90
92
94
96
98
NGP, % 100
RES3100M
50
0.4
E
z
0.3
N 40
0
92%
30
--------
¢ 0.2
w
0
cc
0
Z 0.1
0
-----------20
z
0
W
10
0
y 0
2
4
6
8
10
12
90
14
92
96
94
NGP, %
PILOT FLOW, % R
98
100
102M
FIGURE 13. TYPICAL MARS SoLoNOx ENGINE
EMISSIONS AND VIBRATION OPERATING
FIGURE 12. EFFECTS OF PILOT FLOW ON VIBRATION
AND EMISSIONS FOR VARIOUS ENGINE
PARAMETERS
SPEEDS
the extra pilot can be turned off, NO,, values of 20 ppmv are shown
up to full load with CO falling from the mid 20's down to around
10 ppmv. The figure also shows a hysteresis loop for emissions to
ensure controls stability when operating in the 93 to 94 percent
Ngp range.
frettage and fatigue in certain internal sheet metal components and
that this problem would have to be addressed.
EFFECT OF VARIABLE PILOT FLOW ON COMBUSTOR
OSCILLATIONS
It was found during testing that increased pilot flows act to
damp down the oscillations, by providing additional stability as the
core of the primary recirculation zone becomes hotter.
Using pilot however, increases both the NO,, and CO
emissions and this effect is illustrated in Figure 12, where vibration
amplitude, NOx and CO are plotted for variable pilot flow for a
given Ngp.
The current position on emissions is shown Figure 13 where
a typical NO,, and CO emissions signature is presented for a
production engine. In the 90 to 94 percent Ngp range
(approximately 50 to 70 percent load), NO, emissions are shown
between 30 and 40 ppmv with CO levels between 35 and 50 ppmv
because of the need to use higher pilot. At the higher end, where
CONCLUSIONS
The Mars SoLoNOx development program has successfully
produced an advanced, durable and reliable gas turbine that meets
the latest emissions targets of 42 ppmv NO,, and 50 ppmv CO and
low unburned hydrocarbons between 50 and 100% load over wide
ambient temperature conditions when conning on Natural gas. The
transition to full scale production has been achieved and engines
are now running in the field in both Europe and the USA. During
the program, engines have also demonstrated that a 25 ppmv NO,
version is achievable, and further development is being
aggressively pursued to achieve this as a standard with 15 ppmv
and ultimately single digit NO = emissions to follow.
7
Downloaded From: http://proceedings.asmedigitalcollection.asme.org/pdfaccess.ashx?url=/data/conferences/asmep/82312/ on 06/18/2017 Terms of Use: http://www.asme.org/abo
REFERENCES
Roberts, P.B., Kubasco, A.J., and Sekas, N.T. - Solar
Turbines Inc 1981, "Development of a Low NOx Lean Premixed
Annular Combustor", ASME 81-GT-40
Smith, K.O., Angello, L.C., and Kurzynske, FR., - Solar
Turbines Inc 1986, "Design and Testing of an Ultra-Low NOx Gas
Turbine Combustor", ASME 86-GT-263
Smith, K.O., Kurzynske, F.R., and Angello, L.C., - Solar
Turbines Inc 1987, "Experimental Evaluation of Fuel Injection
Configurations for a Lean- Premixed Low NO, Gas Turbine
Combustor", ASME 87-GT-141
Solar Turbines Inc - Principal Investigator K.O. Smith 1987,
"NO. Reduction for Small Gas Turbine Powerplants", EPRI AP5347
Stitt, D.H. - Pacific Gas Transmission Co, San Francisco,
CA, "PGT's Experience with Low NOx Combustors on Pipeline
Gas Turbines", Pacific Coast Gas Association Transmission
Conference, Vancouver, British Columbia, Canada - Apr 21 to 23
1993
ACKNOWLEDGEMENTS
To Jim Piper, Layout group, Graham Ogbome, Controls
group, Ed Shranko and Joe Smith in Experimental Fabrication at
Solar Turbines for their unwavering support in the early days of the
project. Also to Pacific Gas Transmission Co. for their excellent
support with the Durability program.
Downloaded From: http://proceedings.asmedigitalcollection.asme.org/pdfaccess.ashx?url=/data/conferences/asmep/82312/ on 06/18/2017 Terms of Use: http://www.asme.org/abo