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Brown and hard coal will remain the principal source of ther-mal energy in the process of heat and power generation in Poland. It is a consequence of the fuel availability and of the forecasts con-cerning its prices. However, the use of coal will also depend on whether or not numerous criteria are satisfied. And the most es-sential 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 genera-tion 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 ultra-supercritical steam parameters [1,7]. Apart from raising steam
1) Henryk Łukowicz, (Ph.D., D.Sc., Eng., Associate professor), e-mail: henryk.lukowicz@polsl.pl
2) Andrzej Kochaniewicz, Ph.D., e-mail: andrzej.kochaniewicz@polsl.pl
parameters, the technological structure of the power plant is up-graded, 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 param-eters of the reference cycle.
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
Fuel and the boiler flue gases
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.
Q·out – heat flux carried away from the cycle into the
surround-ings.
The power unit electricity generation efficiency (gross)
where:
W·e – electric power m·fuel – fuel mass flow LHV – lower heating value
Unit heat consumption
Unit consumption of the fuel chemical energy
Average temperature of the heat supplied and carried away
where:
∑ i
m·i (hout – hin )i – heat flux supplied to the cycle, ∑
i
m·i (sout – sin )i – entropy flux supplied to the cycle.
Boiler efficiency
The boiler efficiency was determined with the following as-sumptions:
• 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 oxy-gen 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 desulphuriza-tion. After sulphur is removed from flue gases, they are directed to the stack. However, before they are emitted to the environ-ment, they need to be heated. For this reason, they are supplied with waste heat recovered before the flue gas desulphurization system (Fig. 2).
Fig. 1. Technological structure of the reference cycle
Table 1
Parameters of the analyzed reference cycle
Parameter Value Unit
Live steam temperature 650.0 °C
Live steam pressure 30.0 MPa
Reheated steam pressure 6.0 MPa
Reheated steam temperature 670.0 °C
Pressure in the condenser 0.005 MPa
Feed water temperature 310.0 °C
DEA HP IP LP G C B DSH HPH 3 HPH 2 HPH 1 LPH 4 LPH 3 LPH 2 LPH 1 Table 2
Characteristics of fuel in working state and the waste heat flux
Item Parameter Hard coal Brown coal
1 Lower heating value, MJ/kg 23.00 7.75
2 Moisture content 0.0900 0.514 3 Ash content 0.200 0.114 4 c content 0.600 0.232 5 h content 0.0380 0.0192 6 o content 0.0500 0.1050 7 n content 0.0120 0.0032 8 s content 0.0100 0.0126
9 Flue gas mass flow m·EG , kg/s 833.44 1104.52
10 m·fuel , kg/s 79.78 250.74
11 Waste heat flux Q·EG , MW 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:
Q·in – heat flux supplied to the medium in the boiler and in the
The following values of temperatures at the flue gas desul-phurization (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 con-densate. 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 possi-ble 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 tem-perature makes it possible to heat a part of the condensate flowing in the high-pressure recovery line (Fig. 5) or in the de-aerator 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. Fig. 2. Diagram of a cycle with no waste heat recovery [6]
ESP FGD
Stack
Flue Gas Cooler Flue Gas Heater
LUVO
B
Fig. 3. Diagram of a cycle with waste heat recovery [6]
ESP FGD
Flue Gas Cooler
Cooling Tower
LUVO
B
Waste Heat Recovery
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 coal-fired 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).
Fig. 4. The concept of waste heat recovery in a low-pressure recovery system depending on fired coal
ESP
FGD Flue gas cooler
Cooling Tower LUVO B LP heater DEA 1 2 3 4
Option for brown coal Option for hard coal
Fig. 5. A concept of waste heat recovery in high-pressure regeneration ESP BOILER Flue gas cooler To cooling tower LUVO FDG LP heater DEA 1 2 3 4
Water air heater HP heater
1 2 3
calculation results
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 regenera-tion, to heat the water in the deaerator bypass or to raise the tem-perature of feed water in the high-pressure regeneration. Heat-ing 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.
conclusions
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).
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 m·0 ηth ηB ηe q qfuel Tav m·fuel Q
·
EG
1 2 3 4 5 6 7 8 9 10 12 14
Working conditions Diagram of heat exchangers MWe kg/s % % % kJ/kWh kJ/kWh oC kg/s MWt
Without recovery – 900.0 618.8 50.92 89.14 42.85 6927.3 7771.5 444.1 250.7 0.0 W·e = const 900.0 613.2 49.74 92.44 43.24 7093.6 7700.8 420.9 248.4 63.5 m·0 = const 908.3 618.8 49.74 92.44 43.24 7093.5 7700.7 420.9 250.7 64.1 W·e = const 900.0 612.1 49.84 92.44 43.32 7081.1 7687.2 422.9 248.0 63.4 m·0 = const 909.9 618.8 49.84 92.44 43.32 7081.0 7687.1 422.9 250.7 64.1 W·e = const 900.0 612.2 49.83 92.44 43.31 7081.9 7688.1 422.2 248.0 63.4 m·0 = const 909.8 618.8 49.83 92.44 43.31 7081.8 7688.0 422.2 250.7 64.1 W·e = const 900.0 611.1 49.92 92.44 43.39 7068.8 7673.9 424.1 247.5 63.3 m·0 = const 911.5 618.8 49.92 92.44 43.39 7068.7 7673.8 424.1 250.7 64.1 W·e = const 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 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 m·0 ηth ηB ηe q qfuel Tav m·fuel Q
·
EG
1 2 3 4 5 6 7 8 9 10 12 14
Working conditions Diagram of heat exchangers MWe kg/s % % % kJ/kWh kJ/kWh oC kg/s MWt
Without recovery – 900.0 618.8 50.92 94.38 45.37 6927.3 7339.7 444.1 79.8 0 W·e = const 900.0 616.8 50.30 96.05 45.52 7014.5 7315.5 432.0 79.5 30.5 m·0 = const 903.0 618.8 50.30 96.05 45.52 7014.5 7315.4 432.0 79.8 30.6 W·e = const 900.0 616.5 50.32 96.05 45.54 7010.7 7311.5 433.1 79.5 30.4 m·0 = const 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
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 45,97 45,72 45,54 45,52 45,37 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, % Initial power unit
44,10 43,86 43,39 43,31 43,32 43,24 42,85
Power unit electricity generation efficiency, % 42,20 42,40 42,60 42,80 43,00 43,20 43,40 43,60 43,80 44,00 44,20 Initial power unit
Fig. 7. Change in electricity generation efficiency for a brown coal-fired power unit for the analyzed variants of waste heat recovery
For a brown coal-fired power unit, the increment in electric-ity 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 re-duction 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 integrat-ed with CO2 capture.
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