Introduction:
China faces a great challenge to secure the electricity supply for a steadily growing economy and at the same time use environmentally friendly technologies. One way to improve the environmental efficiency of power generation is the simultaneous production of electric power and heat (Combined Heat and Power = CHP, synonymous with cogeneration). It is a prerequisite for CHP plants that there are users for the waste heat, as heat cannot be stored or transported over distances. In China there are more than 300 cities with a district heating infrastructure and a daily growing number of industrial facilities with power and heat demand.
Therefore China can benefit substantially from the advantages of CHP plants: fuel saving, fewer emissions of carbon dioxide and other pollutants.
CHP Technology
CHP plants can be based on different technologies. Each technology has different properties in terms of fuel, power to heat ratio, electrical efficiency and heat quality.
In the small power range (less than 10 MW) piston engines are the widest spread technology.
In the higher power range (more than 10 MW) steam and combustion turbines are the dominant technology. Generally the boundary conditions like the availability of fuel (such as coal, natural gas, waste gases and oil), fuel prices, power and heat demand curve define the power generation technology.
The standard solution for CHP in China is coal-fired power plants, which use the waste heat from the steam process. This process is well proven and profits from advanced combustion and turbine technology.
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If natural gas is available for power generation, the most efficient CHP solution in terms of environmental efficiency is a combined cycle power plant. This type of power plant is very flexible in terms of the ratio of power and heat output.
In terms of total efficiency (the sum of electricity and useful heat divided by fuel input) all cogeneration technologies can reach very high efficiencies of up to 90% and more, provided the boundary conditions like heat level, return condensate temperature and heat load curve are appropriate for the respective technology. The following table shows a range of CHP technologies and their properties.
PRIME
MOVER
EXTRACTION
STEAM
TURBINE
FUEL USED SIZE
RANGE
(MWe)
ANY FUEL
HEAT:
POWER
RATIO
1 to 300+ 3:1 to
8:1+
ELECTRICAL
GENERATING
EFFICIENCY
20 – 35%
TYPICAL
OVERALL
EFFICIENCY
UP TO 90%
BACK
PRESSURE
STEAM
TURBINE
COMBINED
CYCLE GAS
TURBINE
OPEN CYCLE
GAS
TURBINE
COMPRESS.
IGNITION
ENGINE
ANY FUEL
GAS
BIOGAS
GASOIL
LFO
LPG
NAPHTHA
GAS
BIOGAS
GASOIL
HFO
LFO
LPG
NAPHTHA
GAS
BIOGAS
GASOIL
HFO
LHO
NAPHTHA
0.5 to
500
3 to 300+
0.25 to
50+
0.2 to 20
3:1 to
10:1+
1:1 to
3:1*
1.5:1 to
5:1*
0.5:1 to
3:1*
Alfa value
0.9-2
20 - 35%
35 – 55%
25 – 42%
35 – 45%
UP TO 90%
70 - 90%
65 – 90%
65 - 90%
SPARK
IGNITION
ENGINE
GAS
BIOGAS
LHO
NAPHTHA
0.003 to
6
1:1 to 3:1
Alfa value
0.9-2
25 - 43% 70 – 90%
* Highest heat to power ratios for these systems are achieved with supplementary firing.
HEAT QUALITY
STEAM AT
MULTIPLE
PRESSURE
LEVELS
STEAM AT
MULTIPLE
PRESSURE
LEVELS
MEDIUM GRADE
STEAM
HIGH
TEMPERATURE
HOT WATER
HIGH GRADE
STEAM
HIGH
TEMPERATURE
HOT WATER
LOW
PRESSURE
STEAM
LOW AND
MEDIUM
TEMPERATURE
HOT WATER
LOW AND
MEDIUM
TEMPERATURE
HOT WATER
From this chart it can easily be seen that there is no “right” or “wrong” CHP technology, all technologies cover a market segment which is defined by fuel, size or heat level. One of the main differentiators for environmental efficiency of a plant is the heat to power ratio. This ratio describes how much electrical power a plant produces in relation to the heat output. The
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following chart shows in an example, what kind of output can be produced with three typical
CHP plants of different technologies. For the example a fuel input of 100 MW for each plant was assumed.
Typical Power and Heat Output of
CHP Power Plant Concepts
Industrial Applications
With the same heat input, GT and CC power plants yield more electric power
120
100
80
60
40
20
0
Losses
Heat
El. Power
Steam
Turbine
All fuels
GT Cogen CC Cogen
Gas/Oil Gas/Oil
06.06.2005
PG I5 Presentation Power Generation 12
PG I5BD
The appropriate heat to power ratio for a specific application is defined by the demand profile for heat and power. Demand profiles are characterized by a full load demand and daily or seasonal load variations.
Therefore it is necessary to choose carefully the right plant type and specifications with regard to the load profile.
With the same total efficiency, the plant with the lower heat to power ratio has the better environmental efficiency due to the higher exergy value of electricity compared to heat. The benefit of CHP plants can be seen if the CO2 emission of a CHP plant is compared to the separate generation of electricity and heat by means of the best available technology. If natural gas is available, the best available technology for power generation is a large-scale combined cycle power plant with a net efficiency of 57.5%. The best available technology for heat generation is a gas fired boiler with 90% efficiency. In order to produce the same amount of electricity and heat, a typical modern Combined Cycle CHP plant (50% electrical efficiency and 85% total efficiency) uses 20% less fuel and consequently emits 20% less CO2
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than separate power and heat generation plants. In this example it is assumed that the plant operates at full power and heat load for 8000 hours per year. el. Power MW
Heat MW
Fuel Input MW
GHG emissions t/a
CCPP
70
122
194783
Boiler
50
56
88889
Total Separate
Generation
70
50
177
283671
CCPP CHP
70
50
140
224000
It has to be mentioned that these favorable results can only be reached if the heat load is constant enough to reach a high number of annual full load hours for heat delivery. If the heat load is not as constant as in the previous example, the advantage will be smaller. But with a heat load of more than 3000 full load hours per year, a CHP plant provides lower fuel consumption and CO2 emissions than separate generation.
The standard solution for CHP in China is coal fired power plants, which use the waste heat from the steam process. This process is well proven and profits from advanced combustion and turbine technology. The heat to power ratio of this plant type is relatively high, i.e. most of the fuel input is converted into heat. With urban and industrial development, the power demand rises much faster than the heat demand, which means the heat to power ratio of the
CHP plants should be lower to guarantee a constant heat load for the plant.
One possibility to modify coal fired plants for higher efficiency and more electricity production is a so-called “repowering”, the addition of a gas turbine with heat recovery boiler and the use of the heat in the existing steam process. Of course this requires the availability of natural gas on site.
In the following, this paper will focus on Combined Cycle Power Plants, which are, in terms of environmental efficiency, the favorable solution in the larger (> 20 MW) power scale.
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Fig. 4, Combined Heat and Power Process with a Combined Cycle Power Plant
Siemens offers turnkey solutions for combined cycle power plants in all power ranges. If used as a CHP plant, combined cycle power plants can generate heat levels from 50 up to 500 deg
C, with a high flexibility of power and heat generation.
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Fig. 4, Siemens Industrial Combined Cycle Plant Portfolio
Application examples
In October 2003, Göteborg Energi and Siemens Power Generation Industrial Applications signed a contract regarding the construction of a new cogeneration power plant of the combined-cycle SCC-800 at Rya, Gothenburg. The assignment pertains to a cogeneration power plant with three SGT-800 lines, plus the capability to add another line in the future.
According to the project schedule, the power plant will be ready for test operations during
November 2006. Behind the decision to build this plant is an increasing demand for both electricity and district heating in Gothenburg.
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Fig.5, Combined Cycle Power Plant at Rya, Gothenburg
In 2003, Siemens Power Generation Industrial Applications received the order for a turnkey modernization of a combined heat and power station for the Erlangen municipality
(Stadtwerke Erlangen) in the south of Germany. The SCC-400 (20 MW el
) combined cycle power plant will be used as an extension of an existing coal fired district heating plant. The power and heat demand profile requires high plant operational flexibility. During the summer period only a minimum heat output is required (<10MW), whereas the peak heat demand reaches more than 25 MW. In order to increase the flexibility of power and heat output, an additionally fired heat recovery steam generator was used. The electrical efficiency of the plant is about 48%, which is very high for a plant of this size. Even more importantly, up to
90% of the fuel will be converted into power and useful heat, resulting in a considerable saving of fuel and reduction of CO2 compared to the existing plant. More than 70,000 tonnes per year CO
2
will be saved compared to the old coal fired CHP plant.
Financial Benefit of Greenhouse Gas Emission Reductions
The reduction of greenhouse gas (GHG) emissions like CO2 can be converted into a considerable economic value in the new GHG trading schemes following the Kyoto Protocol.
Predictions for the future price level of GHG emission certificates vary widely in a range of 5 to 20 US$ per ton CO2 equivalent.
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In China the savings of CO2 emissions can be converted into financial benefit if the project complies to the rules of the CDM (Clean Development Mechanism), one of the instruments of the Kyoto Protocol. A certified CDM project produces so called CERs (Certified Emission
Reductions) which can be traded on a global basis, for example in the European Emissions
Trading System. CHP projects are usually very suitable for this mechanism. In any case it is very important that every project needs to be evaluated individually for applicability for
CDM. If certified, a considerable income stream for a CHP plant can come from reduced emissions, amounting to as much as 10% of the electricity sales.
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