Analysis of the use of waste heat in the turbine regeneration system

Analysis of the use of waste heat in the turbine regeneration system

Piśmiennictwo
[1]http://www.ncbir.pl/programy-strategiczne/zaawansowane-technologie-pozyskiwania-energii
[2] Ruszel M., Polska perspektywa pakietu energetyczno-klimatycznego, Nowa Energia 2009, nr 4, s. 5-8.
[3]http://www.is.pcz.czest.pl/strategiczny
[4] Chen L., Yong S.Z., Ghoniem A.F., Oxy-fuel combustion of
pulverized coal: Characterization, fundamentals, stabilization and CFD modeling, Progress in Energy and Combustion
Science 2012, Vol. 38, pp. 156-214.
[5] Nowak W., Czakiert T. (red.), Spalanie tlenowe dla kotłów
pyłowych i fluidalnych zintegrowanych z wychwytem CO2,
Wydawnictwo Politechniki Częstochowskiej, Częstochowa
2012.
[6] Nowak W., Rybak W., Czakiert T. (red.), Spalanie tlenowe dla
kotłów pyłowych i fluidalnych zintegrowanych z wychwytem
CO2. Kinetyka i mechanizm spalania tlenowego oraz wychwytu CO2, Wydawnictwo Politechniki Częstochowskiej,
Częstochowa 2013.
Henryk Łukowicz 1), Andrzej Kochaniewicz 2)
Silesian University of Technology in Gliwice,
Department of Flow Machinery and Power Engineering
at the Institute of Power Engineering and Turbomachinery
Analysis of the use of waste heat
in the turbine regeneration system of a 900 MW
supercritical coal-fired power unit
Analiza wykorzystania ciepła ze spalin
w regeneracji turbiny nadkrytycznego bloku węglowego
o mocy elektrycznej 900 MW
Brown and hard coal will remain the principal source of thermal energy in the process of heat and power generation in Poland.
It is a consequence of the fuel availability and of the forecasts concerning its prices. However, the use of coal will also depend on
whether or not numerous criteria are satisfied. And the most essential will be those dictated by ecology. Primarily, they concern the
need to reduce the emissions of carbon dioxide, which is the main
greenhouse gas. The process of carbon dioxide capture, transport
and storage (CCS) is energy-consuming and has a significantly
adverse impact on the efficiency of electricity generation [3-5,8],
which in consequence leads to a higher fuel consumption. One of
the ways to reduce CO2 emissions is to improve electricity generation efficiency. For currently designed and built power plants this
is achieved by raising the live and reheated steam parameters.
Therefore, attempts are made to achieve supercritical and ultrasupercritical steam parameters [1,7]. Apart from raising steam
Henryk Łukowicz, (Ph.D., D.Sc., Eng., Associate professor),
e-mail: [email protected]
2)
Andrzej Kochaniewicz, Ph.D., e-mail: [email protected]
1)
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parameters, the technological structure of the power plant is upgraded, too. The power unit efficiency may be greatly improved by
drying brown coal fed into the boiler. In addition to these methods
of improving the power unit efficiency, there is another way which
is applied more and more often: to use the heat recovered from the
boiler flue gases from which sulphur has been removed in the wet
flue gas desulphurization (FGD) system. In terms of heat recovery
in the turbine regeneration system, the use of waste heat from the
gas turbine seems to be an interesting solution [9].
Reference cycle
The diagram of the thermal cycle of a power unit with
a gross power capacity of 900 MWe, for which this analysis was
performed, is shown in Fig. 1. It corresponds to the structure of
supercritical power units with a single steam reheat which are
now designed and constructed. It is assumed that the feed pump
is driven by electric motors. Table 1 presents the basic parameters of the reference cycle.
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·
Qout – heat flux carried away from the cycle into the surroundings.
The power unit electricity generation efficiency (gross)
B
HP
IP
G
LP
DSH
C
HPH 3
DEA
HPH 2
LPH 4
HPH 1
LPH 3
LPH 2
where:
·
We – electric power
m· fuel – fuel mass flow
LHV – lower heating value
LPH 1
Unit heat consumption
Fig. 1. Technological structure of the reference cycle
Unit consumption of the fuel chemical energy
Table 1
Parameters of the analyzed reference cycle
Parameter
Live steam temperature
Live steam pressure
Reheated steam pressure
Reheated steam temperature
Pressure in the condenser
Feed water temperature
Value
650.0
30.0
6.0
670.0
0.005
310.0
Unit
°C
MPa
MPa
°C
MPa
°C
Average temperature of the heat supplied and carried away
where:
∑ m· i (hout – hin )i – heat flux supplied to the cycle,
i
∑ m· i (sout – sin )i – entropy flux supplied to the cycle.
i
Fuel and the boiler flue gases
Boiler efficiency
The analysis of the use of waste heat was performed for
two kinds of fuel fed into the boiler. The elemental composition
of the two kinds of coal and the waste heat flux are presented in
Table 2.
Table 2
Characteristics of fuel in working state and the waste heat flux
Item
1
2
3
4
5
6
7
8
9
Parameter
Lower heating value, MJ/kg
Moisture content
Ash content
c content
h content
o content
n content
s content
Flue gas mass flow m· , kg/s
EG
m· fuel , kg/s
·
Waste heat flux QEG , MW
10
11
Hard coal
23.00
0.0900
0.200
0.600
0.0380
0.0500
0.0120
0.0100
Brown coal
7.75
0.514
0.114
0.232
0.0192
0.1050
0.0032
0.0126
833.44
1104.52
79.78
250.74
64.1
30.6
Indices of the cycle operation
The following indices were defined in the analysis of the
impact of individual parameters on the power unit efficiency [2]:
The cycle thermal efficiency
where:
·
Qin – heat flux supplied to the medium in the boiler and in the
flue gas heat exchangers,
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The boiler efficiency was determined with the following assumptions:
• complete and perfect combustion occurs in the boiler,
• the ash contraction coefficient equals one,
• the relative radiation heat loss is assumed as 0.005.
The boiler waste heat
The temperature of flue gases from hard coal-fired power
units is approximately 120°C, whereas for brown coal-fired plants it
is about 170°C. The flue gas temperature is limited by the dew point
for the flue gas component pressure. In the combustion process,
the sulphur contained in coal is oxidized to SO2. During the flue gas
cooling process, a part of sulphur dioxide, due to the excess of oxygen relative to the stoichiometric amount, is further oxidized to SO3.
The produced sulphur trioxide reacts with water vapour contained in
flue gases and produces vapours of sulphuric acid.
In cycles with no heat recovery, the flue gases after the
boiler are directed to electrostatic precipitators and then to a,
usually wet, flue gas desulphurization installation. Before sulphur
is removed from flue gases, they are cooled, which is necessary
due to the consumption of process water used for desulphurization. After sulphur is removed from flue gases, they are directed
to the stack. However, before they are emitted to the environment, they need to be heated. For this reason, they are supplied
with waste heat recovered before the flue gas desulphurization
system (Fig. 2).
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Stack
Flue Gas Heater
Flue Gas Cooler
LUVO
B
FGD
ESP
Fig. 2. Diagram of a cycle with no waste heat recovery [6]
The following values of temperatures at the flue gas desulphurization (FGD) system inlet can be found in publications:
• 85°C for a hard coal-fired power unit,
• 120°C for a brown coal-fired power unit.
Waste heat recovered before the FGD installation can be
used in the power unit system, e.g. to heat the main cycle condensate. In this case, it is necessary to carry flue gases away
through a cooling tower (Fig. 3).
For a hard coal-fired power unit, the flue gas temperature
makes it possible to heat a part of the condensate to a value
corresponding to the parameters after the LP2 exchanger,
where the temperature is about 94°C. In the case of a brown
coal-fired power unit, the flue gas temperature makes it possible to heat the condensate to a value after the LP4 exchanger,
where the temperature is about 154°C. It follows that brown
coal flue gases can heat the cycle medium to a temperature
much higher than that obtained using hard coal flue gases.
Using flue gases collected before the air heater, it is possible
to achieve a higher temperature of the condensate. Their temperature makes it possible to heat a part of the condensate
flowing in the high-pressure recovery line (Fig. 5) or in the deaerator bypass.
The flue gas temperature before the air heater is [6]:
• 330°C for a hard coal-fired power unit,
• 380°C for a brown coal-fired power unit.
HP heater
2
3
4
1
3
LP heater
2
1
DEA
Flue Gas Cooler
BOILER
B
LUVO
To cooling tower
FGD
ESP
Cooling Tower
Waste Heat
Recovery
Flue gas
cooler
LUVO
ESP
Fig. 3. Diagram of a cycle with waste heat recovery [6]
FDG
Water air heater
For a 900 MW hard coal-fired power unit, the flue gas mass
flow amounts to approximately 830 kg/s, which makes it possible
to recover heat of about 30 MW. In the case of a brown coalfired power unit, the boiler flue gas mass flow is much bigger – it
amounts to approximately 1100 kg/s. For these flue gases, it is
possible to recover about 64 MW of heat.
Using the boiler waste heat to heat
the main cycle condensate
Due to the temperature of the flue gases before and after
the Ljüngström air heater (LUVO), the recovery exchanger may
be installed at two places. The flue gas temperature after the
LUVO makes it possible to heat only a part of the condensate
flowing in the low-pressure regeneration (Fig. 4).
DEA
4
1
Cooling Tower
Table 3 and Table 4 present the calculation results of the
power unit operation indices for cycles with the boiler waste
heat recovery system. Many variants of the possible application
of waste heat in the regeneration system are considered in this
analysis: to heat the condensate in the low-pressure regeneration, to heat the water in the deaerator bypass or to raise the temperature of feed water in the high-pressure regeneration. Heating the condensate with heat recovered from flue gases leads to
a reduction in the mass flow of steam directed from the turbine
bleeds to regenerative heaters. This results either in a rise in the
electric power of the turbine set at the same boiler efficiency or
allows a reduction in the amount of steam generated in the boiler
keeping the same power capacity of the turbine set.
ESP
Flue gas cooler
FGD
Option for brown coal
Option for hard coal
Fig. 4. The concept of waste heat recovery in a low-pressure
recovery system depending on fired coal
strona Calculation results
Conclusions
LUVO
B
LP heater
3
2
Fig. 5. A concept of waste heat recovery
in high-pressure regeneration
792
The increment in the power unit efficiency depends on how
heat is supplied to the cycle medium. For a hard coal-fired power
unit it may reach from 0.15 percentage points for heat recovery
in low-pressure regeneration to 0.60 percentage points for heat
recovery in high-pressure regeneration (Fig. 6).
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Table 3
Power plant operation indices under different conditions of load and exchanger configuration for the use of waste heat recovered
from brown coal flue gases
W·e
Working conditions
1
2
Diagram of heat
exchangers
Without recovery
–
W·e = const
m· 0 = const
W· = const
m· 0
ηth
ηB
ηe
q
Q· EG
m· fuel
qfuel
Tav
10
12
14
C
kg/s
MWt
3
4
5
6
7
8
9
MWe
kg/s
%
%
%
kJ/kWh
kJ/kWh
o
900.0
618.8
50.92
89.14
42.85
6927.3
7771.5
444.1
250.7
0.0
900.0
613.2
49.74
92.44
43.24
7093.6
7700.8
420.9
248.4
63.5
908.3
618.8
49.74
92.44
43.24
7093.5
7700.7
420.9
250.7
64.1
900.0
63.4
612.1
49.84
92.44
43.32
7081.1
7687.2
422.9
248.0
m· 0 = const
W· = const
909.9
618.8
49.84
92.44
43.32
7081.0
7687.1
422.9
250.7
64.1
900.0
612.2
49.83
92.44
43.31
7081.9
7688.1
422.2
248.0
63.4
m· 0 = const
W· = const
909.8
618.8
49.83
92.44
43.31
7081.8
7688.0
422.2
250.7
64.1
900.0
611.1
49.92
92.44
43.39
7068.8
7673.9
424.1
247.5
63.3
m· 0 = const
W· = const
911.5
618.8
49.92
92.44
43.39
7068.7
7673.8
424.1
250.7
64.1
900.0
609.5
49.92
92.44
43.39
7021.9
7592.4
433.9
244.9
49.8
m· 0 = const
913.8
618.8
50.26
92.49
43.86
7021.8
7592.3
433.9
248.7
50.6
W·e = const
900.0
597.3
50.57
92.49
44.10
6983.6
7550.9
435.1
243.6
59.3
m· 0 = const
932.4
618.8
50.57
92.49
44.10
6983.6
7550.7
435.1
252.3
61.5
e
e
e
e
Table 4
Power plant operation indices under different conditions of load and exchanger configuration for the use of waste heat recovered
from hard coal flue gases
W·e
1
m· 0
ηth
ηB
ηe
q
Q· EG
m· fuel
qfuel
Tav
10
12
14
C
kg/s
MWt
2
Diagram of heat
exchangers
3
4
5
6
7
8
9
Working conditions
MWe
kg/s
%
%
%
kJ/kWh
kJ/kWh
Without recovery
–
o
900.0
618.8
50.92
94.38
45.37
6927.3
7339.7
444.1
79.8
0
W·e = const
m· = const
900.0
616.8
50.30
96.05
45.52
7014.5
7315.5
432.0
79.5
30.5
903.0
618.8
50.30
96.05
45.52
7014.5
7315.4
432.0
79.8
30.6
W·e = const
m· = const
900.0
616.5
50.32
96.05
45.54
7010.7
7311.5
433.1
79.5
30.4
903.5
618.8
50.32
96.05
45.54
7010.7
7311.5
433.1
79.8
30.6
W·e = const
900.0
611.7
50.42
96.10
45.72
6999.0
7283.1
436.3
79.2
37.7
m· 0 = const
910.4
618.8
50.42
96.10
45.72
6998.9
7283.0
436.3
80.1
38.2
W·e = const
900.0
610.0
50.70
96.10
45.97
6961.1
7243.6
440.3
78.7
27.1
m· 0 = const
913.1
618.8
50.70
96.10
45.97
6961.0
7243.5
440.3
79.9
27.4
0
0
44,10
45,97
43,86
45,72
43,39
43,31
45,54
43,32
45,52
43,24
Initial power unit
Initial power unit
45,37
42,85
42,20 42,40 42,60 42,80 43,00 43,20 43,40 43,60 43,80 44,00 44,20
45,00 45,10 45,20 45,30 45,40 45,50 45,60 45,70 45,80 45,90 46,00 46,10
Power unit electricity generation efficiency, %
Power unit electricity generation efficiency, %
Fig. 6. Change in electricity generation efficiency for a hard
coal-fired power unit for the analyzed variants of waste heat recovery
in the turbine regeneration system
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Fig. 7. Change in electricity generation efficiency for a brown
coal-fired power unit for the analyzed variants of waste heat recovery
in the turbine regeneration system
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For a brown coal-fired power unit, the increment in electricity generation efficiency ranges from 0.39 percentage points in
the turbine low-pressure regeneration system to 1.25 percentage
points in high-pressure regeneration (Fig. 7).
Using the boiler waste heat to raise the temperature of the
cycle medium in the turbine regeneration system results in a reduction in the cycle efficiency compared to the reference cycle.
This concerns both hard and brown coal-fired power plants. The
drop in efficiency is caused by lower average temperature of the
heat supplied to the cycle. Despite this, however, the power plant
efficiency rises due to a smaller stack loss. The calculations were
performed using an in-house code.
The results presented in this paper were obtained from
research work co-financed by the Polish National Centre for
Research and Development within the framework of Contract
SP/E/1/67484/10 – Strategic Research Programme – Advanced
technologies for obtaining energy: Development of a technology
for highly efficient zero-emission coal-fired power units integrated with CO2 capture.
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