ANALYSIS OF AN INNOVATIVE INLET
AIR COOLING SYSTEM BASED ON
LIQUID NITROGEN EVAPORATION
FOR IGCC POWER AUGMENTATION
Mirko Morini
MechLav – Università di Ferrara
Michele Pinelli – Pier Ruggero Spina – Anna Vaccari – Mauro Venturini
Engineering Department – Università di Ferrara
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Summary
IGCCs are energy systems mainly composed of a gasifier and a combined cycle power plant. Since the gasification process usually requires oxygen as the oxidant, the plant also has an Air Separation Unit. Since these plants are based on gas‐steam combined cycle power plants they suffer from a reduction in performance when ambient temperature increases.
In this paper, an innovative system for power augmentation in IGCC plants has been presented. The system is based on gas turbine inlet air cooling by means of liquid nitrogen spray. In fact, nitrogen is a product of the ASU, but is not always exploited. In the proposed plant, the nitrogen is first chilled and liquefied and then it can be used for inlet air cooling or stored for a postponed use. This system is not characterized by the limits of water evaporative cooling (where the lower temperature is limited by air saturation) and refrigeration cooling (where the effectiveness is limited by pressure drop in the heat exchanger).
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Inlet air cooling technologies
Many strategies for inlet air cooling have been proposed in recent years. These technologies can be grouped into two main categories:
•Evaporative cooling is based on the adiabatic evaporation of water in air which causes an increase in air humidity and a reduction in air temperature to wet bulb temperature. •Refrigeration is based on the use of a cold stream (e.g. water) which exchanges heat with the compressor inlet air by means of a cooling coil. The profitability of inlet air cooling systems is strongly dependent on the location of the plant.
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Liquid nitrogen
Liquid nitrogen latent heat of vaporization in currently used in food freezing and chilling applications.
The innovative system for power augmentation in IGCC plants imagined in this work consists of the liquefaction of the nitrogen stream which is usually discharged by the ASU by using an electric chiller. Then, the liquefied nitrogen can be directly sprayed into the inlet duct of the gas turbine as done for industrial fast freezing and chilling.
Moreover, liquid nitrogen can operate as thermal energy storage: the nitrogen is produced during nightime when electricity is cheap and used in daytime when electricity is expensive.
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The model
air
oxygen
nitrogen
syngas
flue gas
66.9 kg/s
In order to implement the power augmentation system some modifications to the model of a traditional IGCC are performed:
27.4 kg/s
ASU
55.8 kg/s
G
GC
649.9 kg/s
GT
SS
68.1 kg/s
•ASU is designed in order to allow the production of a stream of 100 kg/s of high pressure (10 bar) nitrogen (this does not significantly increase the size of the ASU since this stream is mostly recuperated from the gases usually discharged by the ASU);
100.0 kg/s
EC
641.0 kg/s
ST
2.3 kg/s
66.9 kg/s
AMBIENT
Gross power
[MW]
409.3
Total auxiliaries
[MW]
70.8
•an electric chiller (COP = 3) is added in ASU
[MW]
39.8
order to liquefy the high pressure nitrogen.
Electric chiller
[MW]
17.1
Net power
[MW]
333.8
Net electric [%]
39.6
efficiency
with respect to 377.9 MW
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Plant operating mode
66.9 kg/s
27.4 kg/s
ASU
air
oxygen
nitrogen
syngas
flue gas
55.8 kg/s
G
GC
649.9 kg/s
GT
SS
air
oxygen
nitrogen
syngas
flue gas
ASU
G
GC
GT
SS
68.1 kg/s
100.0 kg/s
EC
641.0 kg/s
< 100.0 kg/s
ST
IACS
ST
2.3 kg/s
66.9 kg/s
AMBIENT
AMBIENT
Off peak
Peak
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Performance maps
400
In order to simulate the behavior of the plant in the four selected locations, plant performance maps are built
350
250
200
-20
nitrogen production - RH = 100 %
nitrogen production - RH = 10 %
inlet air cooling - RH = 100 %
inlet air cooling - RH = 10 %
IGCC normal operation - RH = 100%
IGCC normal operation - RH = 10 %
-10
0
10
20
30
40
50
Ambient temperature [°C]
100
inlet air cooling - RH = 100 %
inlet air cooling - RH = 10 %
75
50
2
300
N [kg/s]
Net Power [MW]
450
25
0
5
10
15
20
25
30
35
40
45
Ambient temperature [°C]
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Effect of nitrogen injection
The injection of a large amount of liquid nitrogen at gas turbine inlet can seen disputable.
To investigate the effect of this procedure simulations are carried out by varying the liquid nitrogen mass flow rate for a given ambient condition.
25
Air composition changes, thought not considerably, after nitrogen injection. Ambient temperature = 45 °C
Ambient relative humidity = 60 %
oxygen molar fraction [%]
20
The oxygen content seems always high enough to allow a regular combustion. 15
10
Ambient air
GT inlet air
Exhaust gas
5
0
20
40
60
80
100
Anyway local effects will be further investigated.
liquid nitrogen mass flow rate [kg/s]
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Nitrogen enrichment
The main effect of adding nitrogen to the gas turbine inlet air is the reduction of the oxygen molar fraction in the oxidant stream. In literature, the combustion in gas turbines with low oxygen concentration has been investigated mainly for two different technologies: (i) exhaust gas recirculation (EGR) and (ii) humidified gas turbines. According to literature, the EGR is limited to the minimum oxygen content in the combustion air, which is evaluated in 16 % ‐ 18 %.
In this paper, a small scale combustion chamber geometry (330 kWth) is taken into consideration since its model has been widely validated by the Authors.
•Analyzed cases (@ constant volume flow rate)
S1 ‐ a standard dry air composition (79 % N2, 21 % O2)
S2 ‐ an N2‐enriched air (83 % N2, 17 % O2)
S3 ‐ a standard dry air composition (79 % N2, 21 % O2) + 2 % water overspray
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Test Combustor Geometry
o Reverse flow tubular combustor which is part of a 100 kW Micro Gas Turbine for which full validated model and experimental data are available o Two fuel supply lines: pilot line (diffusive combustion), main line (premixed combustion)
o Reference fuel distribution: 15 % pilot line, 85 % main line
OUTER
FLAME TUBE
SPARKPLUG
PILOT
FUEL
LINE
SECONDARY
SWIRLER
LINER
Air inlet
Main fuel line
Pilot fuel line
MAIN
FUEL
LINE
PRIMARY
SWIRLER
DILUITION
HOLES
INNER
FLAME TUBE
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Results: velocity distributions
Syngas feeding
No significant variation of the velocity field can be highlighted.
A variation in the velocity field is noticeable only downstream of the dilution holes.
S1
S2
S3
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Results: velocity distributions
S1
S2
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Results: temperature distributions
Syngas feeding
Reduction of the temperature in the primary combustion zone: in the case of both nitrogen (S2) and water vapor (S3) air vitiation.
This is due to the fact that nitrogen and water vapor are characterized by a higher specific heat and, therefore, for a given heat release the increase of exhaust gas temperature is lower.
The effect of nitrogen vitiation is even lower when syngas feeding is considered: the higher combustion reactivity of the hydrogen than methane mitigates the effect of the oxygen dilution.
S1
Non dimensional temperature
S2
S3
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Effect of nitrogen injection
The trend is not linear in the whole investigated range of nitrogen mass flow rate. 50
Ambient temperature = 45 °C
Ambient relative humidity = 60 %
inlet air temperature [°C]
45
40
When the temperature decreases under the dew point temperature part of the liquid nitrogen is used for water condensation. 35
30
25
20
0
20
40
60
80
100
liquid nitrogen mass flow rate [kg/s]
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For this reason, the rate of temperature reduction decreases after this temperature.
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Effect of liquid water at GT inlet
When warm humid air is cooled by spraying liquid nitrogen, the air can saturate by forming liquid water. This fine dispersed fog enters into the gas turbine and contributes to a further gas turbine power augmentation as wet compression or overspray.
The curves at different humidity diverge. When the target inlet air temperature cannot be achieved due to high water content in the air the gas turbine power decreases by increasing the ambient temperature as the gas turbine inlet air increases. MechLav
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Locations
Since the effectiveness of power augmentation technologies proved to be strictly related to climatic conditions four locations around the world have been chosen.
Dubai
Ferrara
Darwin
Johannesburg
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Climatic data
65%
55%
50%
relative frequency
45%
40%
35%
30%
25%
20%
65%
> 95 %
85 to 95 %
75 to 85 %
65 to 75 %
55 to 65 %
45 to 55 %
35 to 45 %
25 to 35 %
15 to 25 %
RH < 15 %
> 95 %
85 to 95 %
75 to 85 %
65 to 75 %
55 to 65 %
45 to 55 %
35 to 45 %
25 to 35 %
15 to 25 %
RH < 15 %
60%
55%
50%
45%
relative frequency
60%
15%
10%
5%
40%
35%
30%
25%
20%
15%
10%
5%
0%
< -5 °C
-5 to 5 °C
5 to 15 °C 15 to 25 °C 25 to 35 °C 35 to 45 °C
0%
> 45 °C
< -5 °C
-5 to 5 °C
Ambient temperature
Ferrara
65%
65%
50%
relative frequency
45%
40%
35%
30%
25%
20%
> 95 %
85 to 95 %
75 to 85 %
65 to 75 %
55 to 65 %
45 to 55 %
35 to 45 %
25 to 35 %
15 to 25 %
RH < 15 %
> 95 %
85 to 95 %
75 to 85 %
65 to 75 %
55 to 65 %
45 to 55 %
35 to 45 %
25 to 35 %
15 to 25 %
RH < 15 %
60%
55%
50%
45%
relative frequency
55%
> 45 °C
Ambient temperature
Darwin
60%
5 to 15 °C 15 to 25 °C 25 to 35 °C 35 to 45 °C
15%
10%
40%
35%
30%
25%
20%
15%
10%
5%
5%
0%
0%
< -5 °C
-5 to 5 °C
5 to 15 °C 15 to 25 °C 25 to 35 °C 35 to 45 °C
< -5 °C
> 45 °C
-5 to 5 °C
5 to 15 °C 15 to 25 °C 25 to 35 °C 35 to 45 °C
> 45 °C
Ambient temperature
Ambient temperature
Johannesburg
Dubai
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Dew point temperature
Dew point temperature can be considered as an index of the limit temperature for evaporative cooling. For some localities (Darwin, Dubai), the dew point temperature in summer time is always higher than 15 °C (black dotted line), and, therefore, evaporative cooling cannot provide suitable power augmentation.
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Model application
Electrical energy [TWh/yr]
Peak
Off Peak
Tot.
Peak
IGCC
Off Peak
Tot.
NC
Darwin
0.99
2.08
3.07
1.08
2.01
3.10
Dubai
0.97
2.05
3.02
1.07
1.98
3.06
Ferrara
1.07
2.25
3.31
1.10
2.22
3.32
Johannesburg
1.05
2.24
3.29
1.08
2.22
3.30
The power augmentation strategies proposed lead to an increase in the total electrical energy produced.
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Model application
Coal
[Mt/yr]
Efficiency [%]
Coal
Off Peak
[Mt/yr]
Peak
Tot.
Efficiency [%]
Peak
IGCC
Off Peak
Tot.
NC
Darwin
1.19
41.3
41.5
41.5
1.23
41.3
40.4
40.7
Dubai
1.17
41.4
41.5
41.5
1.21
41.4
40.5
40.8
Ferrara
1.27
42.1
42.2
42.2
1.28
42.0
41.8
41.9
Johannesburg
1.26
42.1
42.3
42.2
1.27
42.1
41.9
42.0
The increase in electrical energy production is obtained through higher coal consumption, with the annual average efficiency decreasing.
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Economic results
In order to present some basic economic considerations about the profitability of the system it is assumed that:
•peak hours run from 8:00 a.m. to 7 p.m. Monday to Friday;
•the value of electrical energy during peak hours is 100 €/MWh, while during off peak hours it is 50 €/MWh;
•the price of coal is equal to 100 €/t;
•extra costs for maintenance are not considered.
IGCC
NC
TEC
HPF
OS
CCC
Cash Flow
Diff.
Diff.
Diff.
Diff.
Diff.
[M€/yr]
[M€/yr]
[M€/yr]
[M€/yr]
[M€/yr]
[M€/yr]
Darwin
83.3
3.0
2.8
3.2
9.9
3.3
Dubai
82.1
3.7
1.6
1.9
2.7
3.1
Ferrara
92.4
0.9
0.5
0.7
4.8
0.6
Johannesburg
91.6
0.9
0.8
1.1
7.0
0.5
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Conclusions
In this paper, an innovative system for power augmentation in IGCC has been presented. The system consists of the liquefaction by means of an electric chiller of the nitrogen stream which is usually discharged by the ASU when the oxygen for the gasification process is produced. The system is studied by means of a thermodynamic model of an IGCC developed in a commercial code. The model is then applied to a whole year on an hourly basis for four different locations with different temperature and humidity profiles.
The thermal energy storage strategy proved to be profitable, since, in practice, the system uses low remunerated energy during the night to produce high remunerated energy during the day. The profitability of the system is strictly related to climatic conditions.
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Questions?
Mirko Morini, PhD
mirko.morini@unife.it
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