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THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS
345 E. 47th St., New York, N.Y. 10017
97-GT-125
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All Rights Reserved
Copyright 0 1997 by ASME
. Printed in U.S.A
DESIGN OF A SEMICLOSED CYCLE GAS TURBINE WITH
CARBON DIOXIDE- ARGON AS WORKING FLUID
lnaki Ulizar
Industrie de Turbopropulsores - Nalvir
Torrejon de Ardoz
Madrid, Spain
111111111111,111 111111111
Pericles Pilidis
School of Mechanical Engineering
Cranfield University
Cranfield, Bedford, U.K.
ABSTRACT
INTRODUCTION
The main performance features of a
semiclosed cycle gas turbine with carbon dioxide-argon
working fluid are described here. This machine is
designed to employ coal synthetic gas fuel and to
produce no emissions.
The present paper outlines three tasks carried
out. Firstly the selection of main engine variables,
mainly pressure and temperature ratios. Then a sizing
exercise is carried out where many details of its
physical appearance are outlined. Finally the offdesign performance of the engine is predicted.
This two spool gas turbine is purpose built for
the working fluid, so its physical characteristics reflect
this requirement. The cycle is designed with a turbine
entry temperature of 1650 K and the optimum pressure
ratio is found to be around 60. Two major alternatives
are examined, the simple and the precooled cycle.
A large amount of nitrogen is produced by the
air separation plant associated with this gas turbine and
the coal gasifier. An investigation has been made on
how to use this nitrogen to improve the performance of
the engine by precooling the compressor, cooling the
turbine nozzle guide vanes and using it to cool the
delivery of the low pressure compressor.
The efficiencies of the whole plant have been
computed, taking into account the energy requirements
of the gasifier and the need to dispose of the excess
carbon dioxide. Hence the overall efficiencies indicated
here are of the order of 40 percent. This is a low
efficiency by current standards, but the fuel employed is
coal and no emissions are produced.
The continuing concern over the emission of
greenhouse gases coupled with the ever increasing
demand for electrical energy poses a very difficult
challenge to power engineers. One possible solution to
these conflicting requirements, if solid fossil fuels are
to be employed, is to collect and dispose of the
emissions in a controlled way.
An internal combustion semi-closed power
cycle, where the working fluid is carbon dioxide, with
oxygen and fuel injected in the combustion chamber
and excess carbon dioxide collected at the outlet seems
to be a very attractive environmental proposition. Such
a cycle has a dual advantage. By collecting and storing
safely the excess carbon dioxide, the emission of
greenhouse gases is controlled. By having a clean fuel
and no air in the working fluid, there are no emissions
of oxides of sulphur and nitrogen.
This proposition becomes particularly attractive
when coal is considered. This fuel is in plentiful supply
and when gasified it can be a very clean source of
energy. The major drawbacks of such a cycle are the
complexity of the equipment and the significant
reduction in efficiency caused by the need to produce
pure oxygen.
This paper describes the physical aspects of
such a machine and how to capitalise on the integration
of the machine and the gasifier. Of particular interest is
the use of the large amount of cold nitrogen produced
by the air separation plant required to produce pure
oxygen for the gas turbine and the gasifier.
Presented at the International Gas Turbine & Aeroengine Congress & Exhibition
Orlando, Florida — June 2-June 5, 1997
This paper has been accepted for publication in the Transactions of the ASME
Discussion of it will be accepted at ASME Headquarters until September 30, 1997
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THE ENGINE
requirements have been included and the machine has
been optimised to account for them. The predictions
outlined here have been produced with a developed
version of GTSI (Ref. 5). It works on the main
principles of TURBOMATCH (Ref. 4), the Cranfield gas
turbine simulation program. The gas properties are
calculated based on what is published in Reference 2
and 6.
This paper describes an internal combustion
turbine where the working fluid is carbon dioxide, the
main mass of which is recirculated in the engine. The
choice of working fluid is dictated by the fuel, based on
coal and the requirement to make carbon dioxide
collection an easy process.
Heat addition is achieved by injecting oxygen
and coal gas fuel in the combustion chamber. To keep
a constant mass flow through the device water is
collected at the outlet of the cooler and some carbon
dioxide is extracted at the HPC delivery. This gas is
compressed to 60 atmospheres for efficient collection.
All calculations include this compression work The
background of such a device was reported in
Reference 6.
ADVANCED ENGINE CYCLE
From the outset it was clear that a special
turbine would need to be designed for this duty. Using
existing equipment would result in high losses. A study
was carried out and it was found that the use of
existing, airbreathing, equipment would incur a thermal
efficiency penalty of 10 percent (Ref. 2). The design of
the engine studied here was made with a Turbine Inlet
Temperature of 1650 K as a benchmark. This high
temperature presupposes clean gas fuel.
A cycle such as that reported in Reference 6
without accounting for the coal gasifier energy
requirements would yield thermal efficiencies of 26 and
48 percent for the gas turbine and the combined cycle
respectively.
Similar cycles are shown in Figures 2, 3 and 4
for a pressure ratio of 48, labeled Simple Cycle-1450K,
Each value of overall pressure ratio is represented by a
clusters of points representing the various splits of LPC
and HPC pressure ratios to give one given value of
overall pressure ratio. Figure 2 and 3 show the thermal
efficiencies of the simple cycle and the combined cycle,
respectively. Figure 4 shows the specific work of the
gas turbine. If the coal gasifier energy requirements are
included, the simple cycle efficiency would be about 17
percent (instead of 26, without allowing for the gasifier
energy inputs, as reported in Reference 6) and
combined cycle efficiency would be about 36 (instead of
46 reported previously). These figures are shown for
the purposes of comparison.
Figure 1. Diagram of the Semi-Closed Cycle Engine
Hirr
Boiler
Cooler
4-- 0O2+Ar
In
CO2+Ar H20
Figure 1 shows the engine selected. It is a
two-spool gas generator where the low pressure turbine
drives the LPC and the load. The coal gasifier energy
I
M
r
FIGURE 2. GAS TURBINE THERMAL EFFICIENCY
0.25
1111 1111111•1111111
Thermal Efficiency
0.2
0.15
0.1
0.05
SRBB
Me
0
la Simple Cyne (IMO ig
isi Simple Cyne din Preceding (1650 K)
manse cyrn) we, pleading (PGVs Cooling 1E60 K)
0
darned Cscie (145D K)
,
0
20
so
so
100
120
Gas Turbine Overall Pressure Ratio
2
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140
160
FIGURE 3. COMBINED CYCLE THERMAL EFFICIENCY
029
0.38
0.37
•
a
0.35
0.33
0.32
• 5002 0001850 If)
O ample 0.02 Mb Pre:Ding (1650 oq
• Simple Csie
Pre:oth• (NGVs Ceding !
o Singe Cyle ('CO K)
0.31
0.3
K)
a
20
40
80
60
100
120
140
160
Gas Turbine Overall Pressure Ratio
FIGURE 4. COMBINED CYCLE SPECIFIC POWER
5E0
5C0
450
0
"6
ce 400
•
U
3E0
3D3
• •
• Simple Oide (1650K)
• Simple Oide MIK Precat•(1C60 K)
• stm0a cycle KIM Prna (NGWOr• 100 K)
• Simple awl. (140 K)
1 250
<0
Oa
203
0
20
40
80
60
top
120
140
163
Gas Ttrbine Overall Pressure Ratio
It is interesting to observe that the simple cycle
efficiency is not much better with the higher turbine
entry temperature of 1650 (Figure 2). This is because
of the additional compressor work required to elevate
the pressure of the gas fuel to that required for entry
into the combustor. In such a machines the fuel and
oxidant flow is predicted to be in excess of one tenth of
the working fluid. The combined cycle efficiency is
better, however, by about one percent because of the
higher exhaust mass flow of the gas turbine.
The performance of the cycle is heavily
penalised by the requirement of emissions control. Its
efficiency is 15 to 20 percent lower than state of the art,
currently available, combined cycle equipment. The
need to operate with carbon dioxide as working fluid
and to use pure oxygen for combustion results in three
major energy drains:
The steam cycles employed in the present
analysis are current state of the art back pressure
cycles, with a steam inlet temperature and pressure of
813 K and 52.6 bar. The back pressure is 0.06 bar.
INTEGRATION WITH THE COAL GASIFIER
The 1650 K cycle described above was taken
as a baseline. Three modifications were investigated to
improve its performance. These entailed the use of the
nitrogen produced by the cryogenic air separation plant
that produces pure oxygen for the coal gasifier and the
gas turbine combustor.
For each mass unit of oxygen produced for the
engine, one half unit is required by the gasifier. The byproduct of this air separation plant is liquid nitrogen, in
very large quantities. It can be employed to boost
engine performance, as follows:
Power for the air separation plant
Compression of the fuel and oxidant gases
Compression of the gas for disposal
Cooling the compressor inlet gas
Cooling the turbine nozzles
Cooling the LPC exit gas
3
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FIGURE 7. HPT COOUNG BLEED
(YVcoolIVV2, %) SIMPLE CYCLE, 1650 K
E'8•
Percent
0
HPCPressure Ratio
10 20-24
"681•••••••iii:
' 4561•B••••••
c4INIMSEMBEIS
—411691••••
"a41••••
016-20
012-16
• 8-12
0 4-8
00-4
,
3
5 6 7 8
10
11
12
LPC Pressure Ratio
FIGURES. HPT COOLING BLEED (WcoollW2, %)
SIMPLE CYCLE WITH PRECOOLING, 1650 K
7.U0NE0Va,.
.E:TIMESPAVA6:
W.feaMESWAS
..
'.g101151
Percent
• 12-14
210-12
08-10
"411/1112111111181161SQ.
0 6-8
• 4-6
"qqaMEIVINEUESINI
-1"‘EIBINIEFIIM
"MIEN
02-4
-
00-2
2 3
4 5
6
7
12
8 9 10 11
LPC Pressure Ratio
FIGURE 9. HPT COOLING BLEED SIMPLE CYCLE
WITH PRECOOUNG & NGVs COOLING, 1650 K
cv
(WcooUW2, %)
. •
• 6-7
05-6
04-5
03-4
."4110121MINEMENEtia
41311MINEEIN
"MEIEMBIE
'MOE
"4 ;1
• 2-3
-
01-2
e
07
CM
2 3
4 5 6 7 8 9 10 11 12
1-12C Pressure Ratio
6
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00-1
vanes of the turbines. The results were marginal, there
was no efficiency gain compared to the cycle with
precooling and NGV cooling, even when a higher
compressor pressure ratio was selected. Again the
gains in efficiency were counterbalanced by the fuel
compression work.
When the performance of the combined cycle
is examined, it can be observed that the efficiency is
nearly double that of the simple cycle. The power
increase accrued from an unfired steam cycle is larger
than what would be expected from a conventional
combined cycle. This is because this engine
experiences large energy drains of the gasifier and
associated equipment This deduction is much larger
than in a conventional gas turbine. In the present case
it accounts for approximately one third of the gas
turbine output. It can also be observed that the
combined cycle efficiency is better in the precooled
case, but the difference is now much smaller, of the
order of two percent. This is because the precooled gas
turbine cycle has a larger specific power, therefore a
lower steam to gas flow ratio.
ENGINE DESIGN
An overall pressure ratio of 60 appears to be
the optimum, or close to the optimum in every case.
This figure is selected, because in addition to yielding
very good efficiencies the selection of this pressure ratio
will also simplify the equipment required. 60 bar is the
disposal pressure of the CO2 in the cycle. If the gas
'turbine pressure ratio selected is also 60, the CO2 can
be stored directly from a bleed at the compressor
delivery without the requirement for an additional
compressor.
Cooling The Turbine Nozzles
The nitrogen, once it emerges from the
compressor precooter is still very cold. This low
temperature gas can be put to further use. It could be
recycled through the nozzle guide vanes of the turbine
to cool the aerofoils. In the design proposed here this is
achieved by means of a closed loop circuit so that no
nitrogen is injected into the main gas stream. The rotors
are .cooled in a conventional way (Figures 7 to 9).
The benefits of such an approach would be the
reduction of cooling flow extracted from the HPC
delivery to cool the turbine. This would result in a
higher average gas temperature in the turbine,
improving the cycle characteristics.
The cooling flow can be reduced quite
significantly. In the simple cycle case, turbine cooling
flows can exceed 20 percent of the compressor delivery
gas. When precooling is employed, this is reduced by
about one third, due to the cooler compressor delivery
temperature. If Nitrogen is used, to cool the turbine
nozzle guide vanes, the requirement for cooling air is
halved, when compared to the previous case.
It can be noticed that this will have a very
minor effect in the efficiency of the simple cycle, but
the effect on the combined cycle is of the order of one
quarter to one third of a percent (Figures 2 and 3
respectively). The effect is smaller than expected than
in a conventional combined cycle because part of the
efficiency gain is counteracted by the need to compress
more fuel gas.
Using nitrogen to cool the turbine nozzle guide
vanes has small effect on efficiency, similar to using a
higher turbine inlet temperature. The effect on the
specific power of the cycle is quite marked, an increase
of nearly 10 percent for the gas turbine alone and
slightly more in the combined cycle (Figure 4).
This effect does not include the improvement
of turbine efficiency due to the reduction of the cooling
flows. It is anticipated this would be between one and
two percent, which would translate into a similar
improvement in gas turbine thermal efficiency.
Item
Simple
Cycle
Precooled
Cycle
LPC Press. Ratio
HPC Press. Ratio
Overall Press. Ratio
HPT Press. Ratio
LPT Press. Ratio
LPC Stages
HPC stages
HPT stages
LPT Stages
LPC inlet Temp (K)
HPC exit Temp (K)
HPT inlet Temp (K)
LPT exit Temp (K)
Gas Turbine Power(MW)
Steam Turbine Power (MW)
Nett Output (MW)
Mass Flow (kg/s)
Fuel Flow (kg/s)
7
8
56
3.2
15.3
9
12
2
6
300
708
1650
960
161
103
193
500
52
5
12
60
2.6
20.0
8
14
2
7
216
563
1650
921
230
112
253
500
64
Tlgt
0.172
0.216
Tlec
LPC inlet diameter (m)
LPT outlet diameter (m)
Overall gas turbine length (m)
RPM HPC
RPM LPC
0.367
1.75
2.14
5.4
5000
3000
0.388
1.56
2.14
5.5
5000
3000
Table I: Engine features for two engine models
Having selected the overall pressure ratio of
60, the authors then examined the split of LPC and
I-PC pressure ratios. The only constraint introduced
was that no compressor would have a pressure ratio
greater than 12. This is a pressure ratio, which for
carbon dioxide is quite high, equivalent to the state of
the art in current air breathing industrial gas turbine
compressors.
Efficiency changes for different pressure ratio
splits were examined and it can be seen in Figure 10
that there is little effect on efficiency as the pressure
ratio split between the two compressors changes. The
only exception is that of the intercooled cycle where the
lowest LPC pressure ratios give higher efficiencies.
Cooling the LPC exit gas
Using the Nitrogen to cool the LPC exit gas in
an intercooled cycle, is another scheme that was
investigated by the authors.
In this case the nitrogen left the compressor
precooler, was then ducted to the intercooler between
the two compressors and then to the Nozzle guide
5
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FIGURE 7. HPT COOUNG BLEED
(YVcoolIVV2, %) SIMPLE CYCLE, 1650 K
E'8•
Percent
0
HPCPressure Ratio
10 20-24
"681•••••••iii:
' 4561•B••••••
c4INIMSEMBEIS
—411691••••
"a41••••
016-20
012-16
• 8-12
0 4-8
00-4
,
3
5 6 7 8
10
11
12
LPC Pressure Ratio
FIGURES. HPT COOLING BLEED (WcoollW2, %)
SIMPLE CYCLE WITH PRECOOLING, 1650 K
7.U0NE0Va,.
.E:TIMESPAVA6:
W.feaMESWAS
..
'.g101151
Percent
• 12-14
210-12
08-10
"411/1112111111181161SQ.
0 6-8
• 4-6
"qqaMEIVINEUESINI
-1"‘EIBINIEFIIM
"MIEN
02-4
-
00-2
2 3
4 5
6
7
12
8 9 10 11
LPC Pressure Ratio
FIGURE 9. HPT COOLING BLEED SIMPLE CYCLE
WITH PRECOOUNG & NGVs COOLING, 1650 K
cv
(WcooUW2, %)
. •
• 6-7
05-6
04-5
03-4
."4110121MINEMENEtia
41311MINEEIN
"MEIEMBIE
'MOE
"4 ;1
• 2-3
-
01-2
e
07
CM
2 3
4 5 6 7 8 9 10 11 12
1-12C Pressure Ratio
6
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00-1
,
FIGURE 10. COMBINED CYCLE EFFICIENCY VARIATION WITH LPC
PRESSURE RATIO (PR=60)
0.39 0.385 -
Therm alEfficiency
-is-Simple Cycle (1650 K)
0.38 - a-Simple Cycle with Retooling (1650 K)
0.375 -6-Simple Cycle with Preceding (NGVs
Cooling, 1650K)
0.37 -
- 0-Interceded Cycle with Promo
' ling
(NGVs Cooling, 1650 K)
-a-Simple Cycle (1450 K)
0.365 0.36 0.355
4
5
6
9
a
7
10
II
12
LPC Pressure Ratio
TABLE III: Advanced Semi-Closed Cycle Performance
(Simple & Combined Cycle- With Precooling)
Taking this advanced cycle as the baseline, the
next step was to make an approximate estimate of
component sizing. Techniques developed from those
described in Reference 1 were employed to produce the
results shown in Table I. The two engines illustrated
are the Simple Cycle and the Precooled Cycle with
nitrogen cooled turbine nozzle guide vanes.
The large effect, approximately 90 MW, of the
energy drains of the gasifier on the nett output of the
plant can be observed in Table I. The largest drains are
the power for the fuel gas compression and for the air
separation plant.
Having selected the engine cycle and having
performed the preliminary design, the off design
performance of the engine was predicted for the gas
turbine and the combined cycle.
HPC Exit T(K)
100
300
7.0
8.0
709
LPC RPM (%)
100
I-IPC RPM (%)
103
LPC P.Ratio
HPC P.Ratio
100
99
89
76
64
54
300
300
300
300
300
300
7.5
82
8.1
7.4
6.8
6.2
7.3
6.4
5.6
5.0
4.5
4.0
697
692
631
664
646
629
100
100
100
100
100
100
97
94
92
89
86
83
TET (K)
1650 1550 1450 1350 1250 1150 1050
Fuel (Kg Is)
10.2
9.21
8.05
CT Power MW
161
144
121
ST Power MW
103
85
72
6.32
4.64
3.32
2.32
86
53
28
11
54
38
26
17
•
94
78
61
48
37
29
216
225
233
241
248
255
262
LPC P.Ratio
5.0
5.2
5.0
4.4
3.8
3.5
3.1
HPC P.Ratio
12.0
10.5
8.9
7.6
6.5
5.5
4.5
HPC Exit T(K)
566
556
553
542
531
519
506
LPC RPM (%)
100
100
100
100
100
100
100
100
97
94
91
88
85
81
TET (19
1650 1550 1450 1350 1250 1150 1050
Fuel (Kg Is)
12.68 10.76
8.00
5.57
3.81
2.58
1.70
CT Power MW
230
194
134
83
48
25
10
ST Power MW
112
94
71
51
37
26
17
The results shown above are quite interesting.
In both cases the turbine power includes the work
required for the gasifier. These results are preliminary
an optimisation of the control laws is required
In the cycle without precooling (Table II), the
LPC pressure ratio rises and then falls. Also the speed
of the LPC remains constant because it is on the power
shaft connected to the electricity generator. The
changes in mass flow are a result of closing the stators.
In the present exercise the surge margin of the two
compressors is adequate so the HPC does not require
any variable stators. However, these may be necessary
if further investigations reveal that the surge margin of
the LPC is not satisfactory.
For the cycle with precooling (Table III) some
interesting features can be observed. The large mass
flow reduction is achieved by variable stators and by the
higher compressor inlet temperature resulting from the
lower mass flow of nitrogen through the precooler.
Also, the HPC needs only a small modulation of
variable stators, such that variable inlet guide vanes
would suffice. Most of the surge margin control is
achieved using the variable stators of the LPC.
TABLE II: Advanced Semi-Closed Cycle Performance
(Simple & Combined Cycle- No Preceding)
LPC Inlet T (K)
100
LPC Inlet I (C)
HPC RPM (%)
OFF-DESIGN PERFORMANCE
Mass Flow (96)
Mass Flow (%)
7
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CONCLUSION
REFERENCES
1
This paper describes the preliminary analysis
of a semi closed cycle gas turbine. The working fluid is
carbon dioxide and the fuel is low heating value gas
synthesised from coal. The selected configuration is a
two spool simple cycle gas generator with an integral
power turbine.
The benefits of integration of the cycle with the
gasifier are shown here. In particular, the use of the
liquid nitrogen produced by the air separation plant to
precool the compressor inlet. This scheme increases
the overall efficiency of the gas turbine by about 5
percent and of the combined cycle by approximately 2
percent.
The analysis was extended to examine the
physical features of the engine and to assess its offdesign performance. Rotational speeds and size do not
present any extraordinary features. It is expected that
the mechanical design will present no more problems
that of a conventional one, although careful attention
will need to be paid to the aerodynamic design.
Some control issues are noteworthy, such as
the careful variable stator control to achieve surge
control and mass flow modulation.
The prize offered by this type of engine is clean
electricity. There are many difficulties envisaged in the
development and construction of such a machine.
Furthermore its efficiency will be lower than a
conventional open cycle combined cycle gas turbine.
Its development depends on the willingness to pay a
premium for clean electricity.
The main issue will be an economic one. It is
very difficult to make a cost estimate of such a power
plant. Uncertainties in the development, the number of
machines to be built, etc., contribute to this difficulty. It
is clear that such a machine will be much more
expensive than a conventional one. In the balance will
be its higher cost and lower efficiency against its ability
to use coal and its attractive environment features.
A large amount of work needs to be done to
understand the fundamental principles of the machine.
The current knowledge base rests on experimental and
computational techniques based on air as a working
fluid. This needs to be replicated for carbon dioxide.
The combustion of a low calorific value fuel in an
oxygen stream needs to be assessed. It may be
necessary to burn with some excess oxygen to ensure
all the fuel is consumed. This will require a slightly
larger energy drain for the air separation plant These
and other issues need to be explored prior to any
development work.
Hunter, I.H.
Design of Turbomachinery for Closed
and Semiclosed Gas Turbine Cycles.
M.Sc. Thesis, Cranfield University, 1994
2
Keenan, J. H., Kaye, J. and Chao, J.
Gas Tables, International Version,
2nd Edition, (SI Units) 1983
3
Navaratnam, M.
The investigation of an Aeroderivative Gas
Turbine Using Alternative Working Fluids in
Closed and Semiclosed Cycles.
M.Sc. Thesis, Cranfield University, 1994
4
Palmer, JR.;
The TURBOMATCH scheme for Gas Turbine
Performance Calculations; Users' Guide
Cranfield Institute of Technology, 1983
5)
I. Ulizar, P. Pilidis
GTSI, Simulation of Gas Turbines with
Unorthodox Working Fluids and Fuels.
Turbomacchine 96, Italian Thermotechnical
Conference, Genova, July, 1996
6)
I. Ulizar, P. Pilidis
A Semiclosed Cycle Turbine With Carbon
Dioxide- Argon As Working Fluid
ASME Paper 96-GT-345, Presented at the
Turbo Expo, Birmingham, U.K, June, 1996
ACKNOWLEDGMENT
The authors are grateful to their colleagues in I.T.P.
and Cranfield for their support and encouragement and to
professor W.D. Morris for giving the inspiration, in an
indirect way, to the idea of nitrogen precooling.
8
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