The utilisation of heat produced In combination withelectricity is related to the impossibility of converting thermalenergy Into mechanical or electric energy without considerablelosses. For modern coal-fired power stations of 200-600 MW usingseawater as coolant, the maximum conversion efficiencies obtainablewill be no better than about 45%. And these levels could be realisedonly at steam pressures as high as 240 bar, temperatures of 560°C.and a single reheat of steam. In this paper efficiency factorsare given in relation to the lower heat value of the fuel andcooling seawater temperatures of approx. 10°C.
The use of natural gas in a combined cycle powerplant makes it possible to increase the maximum efficiency toapprox. 50% due to the higher temperature of the working medium.In a combined cycle plant a gas turbine is placed before a boilerproducing steam to operate a steam turbine.
The higher the temperature of the working medium(gas or steam) and the lower the temperature of the coolant inproducing mechanical energy in a turbine or combustion engine,the higher the output of mechanical energy or electricity. Theseare Important factors when considering combined heat and power(CHP) systems In which case the coolant is the district heatingwater. In the following the district heat (DH) temperatures areconsidered to be 100°C (supply) and 50°C (return).
The advantages of CHP production are illustratedIn figures 1, 2, and 3 by comparing three different types of powerstations:
-a condensing power plant using seawater as coolant (fig. 1)In the back-pressure CHP plant all steam is condensed at a relatively high coolant temperature, thus reducing the electric output to approx. 36% compared with a condensing plant of efficiency 45%, however, an increase In the overall efficiency (up to 92%) is achieved by a heat output of 56%.
The back-pressure CHP plant has a fixed ratio.Cm of power to heat output as shown in the example below:
Cm = 36%/56% = 0.64
The advantage of CHP production Is illustrated bycomparing two large plants without and with CHP: a condensingplant (45%) and a back-pressure plant (36% and 56%):
Reduced power production at the back- pressure plant(calculated as units of energy produced per 100 units of energy in the fuel):
45 - 36 = 9 units
Achieved heat production: 56 units
The change from a condensing plant to a back-pressure plant with the same fuel input (100 units) thus means that 9 units of electricity have to be produced elsewhere. e.g. at anothercondensing plant. If this is the case an "energy efficiency"for the heat production can be calculated comparing the heat achievedat the back-pressure plant with the additional fuel used at thecondensing plant:
Additional fuel used for producing 9 units of powerat a condensing plant:
9 units/0.45 = 20 units
"Energy efficiency" of the achieved heatproduction:
56 x 100/20= 280%
This means that in order to produce 1 unit of heating, only fuel corresponding to about one-third of the unit is needed. This calculation is relevant only in full back-pressure operation with an overall efficiency of more than 90%. Modern extraction CHP plants operate between the two regimes depending
on the heat consumption relative to the electricity production. In such a case the annual efficiency of a CHP plant Is typically 70-75% and the "energy efficiency" of the heat production achieved is typically 200%.
It follows from the calculations that CHP is important to make rational use of energy. Comparing with a condensing power station a CHP plant approximately doubles the utilisation of the energy content of the fuel (figures 1 to 3).
Another rule of thumb Is the following: CHP saves
about 30% fuel as compared with separate production of electricity
and heat.
a) Condensing Power PlantsIn the turbine, the thermal energy of the steam isconverted into mechanical energy to rotate a generator shaft.Approximate steam conditions following the energy transfer are0.04 bar and 40°C. Steam condensation takes place in a condenser,which is cooled with large quantities of water. In Denmark mainlyseawater is used for this purpose, heated 8-10°C In the condenser.The condensed steam (condensate) is then pumped back into theboiler.
b) Back-pressure CHP Plants
As indicated in figure 1 about 47% of the fired energy
is rejected with the cooling water in a condensing power plant.
Some of this heat can be utilised In a CHP generation process
by raising the temperature of the supply water of a DH system.
In a conventional power plant the steam Is condensed at about
400 C a temperature too low for supplying DH. In a
CHP plant It Is therefore necessary to condense the steam In the
condenser under pressure and temperature conditions that enable
DH water to reach 85-120°C.
In the back-pressure plant the ratio of electricity
to heat production Is fixed. This Is a disadvantage that can be
somewhat alleviated by using heat accumulators.
c) Extraction CHP Plants
This type of plant is characterized by a variable
ratio of power to heat generation (fig. 3).
The DH water Is Indirectly heated by steam extracted from the turbine. The extraction arrangement at the turbine enables the DH water to be heated to l00-120°C. The greater the steam extraction for hot water production, the smaller the power generation. This means that when more hot DH water is produced, less heatenergy is lost at the con denser.
In the case of large CHP units (125 to 500 MWe). the production of hot DH water takes place almost exclusively at extraction power plants because of their great flexibility.
d) Combined cycle CHP Plants
The hot flue gas from the gas turbine is utilised
in the boiler (see fig. 4) to produce steam for the steam turbine.
It is necessary to use either natural gas or fuel oil for the
gas turbine, whereas it Is possible to use either gas or coal
as supplementary fuels for the steam boner.
The steam turbine of a small-scale combined cycle plant is normally a back-pressure unit, whereas a large-scale plant normally has an extraction turbine.
As an example, a 350 MW extraction plant will have
the following main data:
Power, max. 350 MW
Efficiency, condensing operation 55%
Power back-pressure operation 315 MW
Heat, back-pressure operation 250 MJ/s
Cm 1.26
Efficiency, back-pressure operation 88%
a) Combustion-type Engines
Combustion-type engines are now available in sizes
up to about 47 MW and down to a few kW Electrical efficiencies
range from about 30% for the smallest and 35-40% for ignition
lean-burn gas engines. For large diesel engines the electrical
efficiencies can be as high as 50%. An example of a combustion
engine with turbo-charger for natural gas equipped with a waste
heat boiler is shown in figure 5.
Due to the utilisation of heat it is possible to
attain a total efficiency above 85% and CM values ranging from
0.85 to 1.0.
b) Gas Turbines
A CHP plant based on a gas-fired gas turbine is shown
in fig. 6. Gas turbines range in size from 200 kW to 220 MW. Theirelectrical efficiencies are about 20% for the smallest units to40% for the largest.
Figure 6 shows an example of a medium- size localCHP plant equipped with a waste heat recovery boiler. Total efficienciesabove 85% are possible and CM values are In the range of approx.0.5 - 0.6.
c) Combined-cycle CHP Plants
The principle of a combined-cycle CHP plant hasbeen described earlier (fig. 4). The minimum size will be approx.10 MW power.
The small-scale plants are normally back- pressureunits with a CM value of approx. 1.0.
d) Biomass or Coal-fired CHP Plants
These plants use wood chips, straw, waste or coal
to fuel a steam boiler supplying a back- pressure steam turbine(fig. 2). Due to low steam data and no reheating of steam, theCM values are rather low for this type of CHP plant, viz. 0.3- 0.5. However, biofuels have the environmental advantage of beingclose to CO2 neutral.
In the field of biomass extensive research and developmentefforts are being made to develop methods for gasifying the biomassin order to use combined cycle plants or combustion engines andattain a higher CM value.
When planning the implementation of a CHP system,whether It be a renovation or the establishment of an entirelynew system, it is important to assess and optimise the systemdesign.
It is particularly important to choose the correctlevels of supply and return temperatures. For the previously mentionedCHP plants it is assumed that the supply temperature will be 100°Cand the return temperature 50°C.
Lowering the level of district heating temperatureswill raise the CM value and vice versa.
The optimisation of a CHP plant should include thetransport costs for heat. Likewise. the possibility of reducingthe costs of production and transport of heat by means of heataccumulators should be included in the optimisation.
Future progress in savings and heat insulation ofbuildings as well as new connections to customers and new supplyareas will also influence total system design. Operating the systemat a lower temperature level will typically result in increasedtransport charges but overall decreased production costs.
Increasing the CM value of a system will usuallyprove to be economically advantageous. It is possible to improvethis CM value by changing either the production technology orthe pipeline network. Sometimes improvements may be made at aconsiderably lower cost by altering the production technologyrather than the pipeline network.
Typical guidelines of optimal supply-pipe temperaturesare shown in figure 7.
a) Combustion Engines
In recent years, gas engines based on natural gashave improved vigorously towards higher electric efficienciesand thereby higher CM values. For lean-burn engines of sizes 1-2MW electric efficiencies are approaching 40% The trend of thedevelopment has been to optimise the control of the combustionprocess by using a combustion prechamber and increasing the excessair factor. *, to 2.0 for lean-burn engines. By so doing, themean effective pressure in the combustion process is increasedand the NOx formation kept at a low level.
b) Back-pressure CHP Plants
High-temperature-resistant
ferritic steels used in such places as boiler
superheaters andlive steam pipes offer high flexibility at elevated steam parametersin contrast to austenitic steels. They play a key role in improvingthe ability of thermal power stations to attain higher efficiencies,lower pollution levels, lower fuel consumption, and lower manufacturingcosts. Based on present development it can be assumed that withthe improved ferritic steels containing 9-10% CrMoVNb. the presentlimit of the steam admission temperature of 560°C can beraised to approximately 600°C. The modified 9% Cr- Steel(P91/T91) constitutes a good example of this steel category.
By raising the steam temperature from 560 to 600°C,the electric efficiency of a condensing plant can be increased2 percentage points.
For a back-pressure plant, the electric efficiencywill increase by approx. 3 percentage points corresponding toan improvement of about 5 percentage points in the CM value.
As can be seen, the elevated steam data may be evenmore relevant for a back-pressure plant than for a condensingunit.
c) Gas Turbines
The electric efficiency, and thus the CM value, isusually increased by raising the flue gas temperature at the inletfor the first row of blades as much as possible. e.g. to 11000C This requires improved materials in the combustion chamber andturbine blades as well as an advanced cooling technique for thecomponents. Besides, certain parts of the blades may be coated.The combustion process itself is controlled in order to obtainthe lowest possible NO, formation. Today this is done by meansof a special design of the combustion chamber called dry low-NO,type, while the design used up till now has been water or steaminjection. Today the air volume can be regulated down to 75-80%by means of stationary blade shifting. This enables the electricefficiency to be increased at part load.