# Generating Electrical Power: THERMAL GENERATING STATIONS

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The hydraulic resources of most modern countries are already fully developed. Consequently, we have to rely on thermal and nuclear stations to supply the growing need for electrical energy.

Thermal generating stations produce electricity from the heat released by the combustion of coal, oil, or natural gas. Most stations have ratings between 200 MW and 1500 MW so as to attain the high efficiency and economy of a large installation.

Such a station has to be seen to appreciate its enormous complexity and size.

Thermal stations are usually located near a river or lake because large quantities of cooling water are needed to condense the steam as it exhausts from the turbines.

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The efficiency of thermal generating stations is always low because of the inherent low efficiency of the turbines. The maximum efficiency of any ma chine that converts heat energy into mechanical energy is given by the equation where 1') efficiency of the machine [%] T1 temperature of the gas entering the turbine l K J T2 = temperature of the gas leaving the turbine I KJ In most thermal generating stations the gas is steam. In order to obtain a high efficiency, the quotient T2/T1 should be as small as possible. However, temperature T2 cannot be lower than the ambient temperature, which is usually about :W°c. As a result, T2 cannot be less than :wo + 2730 293 K This means that to obtain high efficiency, T should be as high as possible. The problem is that we cannot use temperatures above those that steel and other metals can safely withstand, bearing in mind the corresponding high steam pressures. It turns out that the highest feasible temperature T, is about 550°C. As a result, It follows that the maximum possible efficiency of a turbine driven by steam that enters at 823 K and exists at 293 K is 1') (I 293/823) I 00 64.4ll Due to other losses, some of the most efficient steam turbines have efficiencies of 45%. This means that 65% of the thermal energy is lost during the thermal-to-mechanical conversion process. The enormous loss of heat and how to dispose of it represents one of the major aspects of a thermal generating station.

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Makeup of a thermal generating station

--- 17 Principal components of a thermal power plant.

---.18 Low-pressure section of a 375 MW, 3600 RPM steam turbine generator set, showing the radial blades.

---.19 This forced-draft fan provides 455 m^3/s of air at a pres sure difference of 5.8 kPa for a thermal power station. It’s driven by a 3-phase induction motor rated 12000 hp (8955 kW), 60 Hz, 890 rim in.

The basic structure and principal components of a thermal generating station are shown.

They are itemized and described below.

• A huge boiler (1) acts as a furnace, transferring heat from the burning fuel to row upon row of water tubes S I' which entirely surround the flames. Water is kept circulating through the tubes by a pump PI'

• A drum (2) containing water and steam under high pressure produces the steam required by the turbines. It also receives the water delivered by boiler-feed pump P:;. Steam races toward the high pressure turbine HP after having passed through superheater. The superheater, composed of a series of tubes surrounding the flames, raises the steam temperature by about 200°C. This increase in temperature ensures that the steam is absolutely dry and raises the overall efficiency of the station.

• A high-pressure (HP) turbine (3) converts thermal energy into mechanical energy by letting the steam expand as it moves through the turbine blades. The temperature and pressure at the out put of the turbine are, therefore, less than at the input. In order to raise the thermal efficiency and to prevent premature condensation, the steam passes through a re-heater S" composed of a third set of heated tubes.

• The medium-pressure (MP) turbine (4) is similar to the high-pressure turbine, except that it’s bigger so that the steam may expand still more.

• The low-pressure (LP) turbine (5) is composed of two identical left-hand and right-hand sections.

• The turbine sections remove the remaining avail able energy from the steam. The steam flowing out of LP expands into an almost perfect vacuum created by the condenser (6).

• Condenser (6) causes the steam to condense by letting it flow over cooling pipes S4' Cold water from an outside source, such as a river or lake, flows through the pipes, thus carrying away the heat. It’s the condensing steam that creates the vacuum.

• A condensate pump P2 removes the lukewarm condensed steam and drives it through a re-heater (7) toward a feedwater pump (8).

• The re-heater (7) is a heat exchanger. It receives hot steam, bled off from high-pressure turbine HP, to raise the temperature of the feed water. Thermodynamic studies show that the overall thermal efficiency is improved when some steam is bled off this way, rather than letting it follow its normal course through all three turbines.

• The burners (9) supply and control the amount of gas. oil, or coal injected into the boiler. Coal is pulverized before it’s injected. Similarly, heavy bunker oil is preheated and injected as an atomized jet to improve surface contact (and combustion) with the surrounding air.

• A forced-draft fan (10) furnishes the enormous quantities of air needed for combustion.

• An induced-draft fan (11) carries the gases and other products of combustion toward cleansing apparatus and from there to the stack and the outside air.

• Generator G, directly coupled to all three turbines, converts the mechanical energy into electrical energy.

In practice, a steam station has hundreds of other components and accessories to ensure high efficiency, safety, and economy. For example, control valves regulate the amount of steam flowing to the turbines; complex water purifiers maintain the required cleanliness and chemical composition of the feedwater; oil pumps keep the bearings properly lubricated. However, the basic components we have just described enable us to understand the operation and some of the basic problems of a thermal station.

Turbines

The low-, medium-, and high-pressure turbines possess a series of blades mounted on the drive shaft: The steam is deflected by the blades, producing a powerful torque. The blades are made of special steel to withstand the high temperature and intense centrifugal forces.

The HP, MP, and LP turbines are coupled together to drive a common generator. However, in some large installations the HP turbine drives one generator while the MP and LP turbines drive another one having the same rating.

Condenser

---Condenser rated at 220 MW. Note the large pipes feeding cooling water into and out of the condenser.

The condenser is as important as the boiler in thermal and nuclear power stations.

We have seen that about one-half the energy produced in the boiler has to be removed from the steam when it exhausts into the condenser.

Consequently, enormous quantities of cooling water are needed to carry away the heat. The temperature of the cooling water increases typically by 5°C to 10°C as it flows through the condenser tubes. The condensed steam (condensate) usually has a temperature between 27°C and 33°C and the corresponding absolute pressure is a near-vacuum of about 5 kPa. The cooling water temperature is only a few degrees below the condensate temperature.

Cooling towers

If the thermal station is located in a dry region, or far away from a river or lake, we still have to cool the condenser, one way or another. We often use evaporation to produce the cooling effect. To understand the principle, consider a lake that exposes a large surface to the surrounding air. A lake evaporates continually, even at low temperatures, and it’s known that for every kilogram of water that evaporates, the lake loses 2.4 MJ of heat. Consequently, evaporation causes the lake to cool down.

Consider now a tub containing 100 kg of water at a certain temperature. If we can somehow cause 1 kg of water to evaporate, the temperature of the remaining 99 kg will inevitably drop by 5.8°C. We conclude that whenever 1% of a body of water evaporates, the temperature of the remaining water drops by 5.8°C. Evaporation is, therefore, a very effective cooling process.

But how can we produce evaporation? Surprisingly, all that is needed is to expose a large surface of water to the surrounding air. The simplest way to do this is to break up the water into small droplets, and blow air through this artificial rain.

In the case of a thermal station, the warm cooling water flowing out of the condenser is piped to the top of a cooling tower, where it’s broken up into small droplets. As the droplets fall toward the open reservoir below, evaporation takes place and the droplets are chilled. The cool water is pumped from the reservoir and recirculated through the condenser, where it again removes heat from the condensing steam. The cycle then repeats. Approximately 2% of the cooling water that flows through the con denser is lost by evaporation. This loss can be made up by a stream, or small lake.

--- Cooling tower installed in a nuclear power station in Oregon. The generator output is a 1280 MVA at a power factor of 0.88. Tower characteristics: height: 152 m; diameter at the base: 117 m; diameter at the top: 76 m; cooling water: 27 m^3/s; \N6.ter loss by evaporation: 0.7 m^3/s. The temperature of the-cooling water drops from 44.5° to 24° as it passes through the tower.

---This 540 MW steam-turbine generator set runs at 3600 r/min, generating a frequency of 60 Hz. The low-pressure turbine and alternator are in the background.

Boiler-feed pump

The boiler-feed pump drives the feedwater into the high-pressure drum. The high back pressure together with the large volume of water flowing through the pump requires a very powerful motor to drive it. In modern steam stations the pumping power represents about 1% of the generator output. Although this appears to be a significant loss, we must remember that the energy expended in the pump is later recovered when the high-pressure steam flows through the turbines. Consequently, the energy supplied to the feed pump motor is not really lost, except for the small portion consumed by the losses in the motor and pump.

Energy flow diagram for a steam plant

Modern thermal generating stations are very similar throughout the world because all designers strive for high efficiency at lowest cost. This means that materials are strained to the limits of safety as far as temperature, pressure, and centrifugal forces are concerned. Because the same materials are available to all, the resulting steam plants are necessarily similar. --- shows a typical 540 MW turbine-generator set, and is a view of the control room.

Most modern boilers furnish steam at a temperature of 550°C and a pressure of 16.5 MPa. The overall efficiency (electrical output/thermal input) is then about 40%. The relative amounts of energy, steam flow. losses, and so forth, don’t change very much. provided the temperature and pressure have the approximate values indicated above. This enables us to draw a diagram showing the energy flow, steam flow, water flow, and so on, in a reduced-scale model of a typical thermal generating station. ++ ++ such a model producing 12 MW of electrical power.

Using this model, we can estimate the characteristics of any thermal power station. For example, a 480 MW station (40 times more powerful than the model) has the following approximate characteristics:

Electric power output 40 X 12 MW 480 MW

Coal consumption 40 x I kg/s 40 kg/s

====

Air intake; Boiler thermal power; Steam output; Cooling water; Heat carried away by the cooling water

-- 40 X 10 kg/s 40 X 30MW 40 X 8 kg/s 40 X 360 kg/s 40X 15MW

--- 400 kg/s 1200MW 320 kg/s 14400 kg/s 600MW

====

If a large river or lake is not available and a cooling tower is required, it would have to evaporate q 2% X 14400 288 kg/s of cooling water. This loss by evaporation has to be made up by a local source of water.

---Scale model of a typical thermal generating station.

Thermal stations and the environment

The products of combustion of thermal generating stations are an increasing subject of concern, due to their impact on the environment.

Carbon dioxide (CO2), sulfur dioxide (SO2), and water are the main products of combustion when oil, coal, or gas are burned. Carbon dioxide and water produce no immediate environmental effects, but sulfur dioxide creates substances that give rise to acid rain. Dust and fly ash are other pollutants that may reach the atmosphere. Natural gas produces only water and CO2 , This explains why gas is used (rather than coal or oil), when atmospheric pollution must be reduced to a minimum.

A good example of pollution control is the large generating station. It’s equipped with a special fabric filter flue gas cleaning system. The fabric filters act like huge vacuum cleaners to remove particles from the boiler-gas flue stream. The fabric filter for each boiler is composed of 48000 filter bags, each 5 m long and 16 cm in diameter. When a boiler operates at full capacity, they capture dust particles at the rate of 28 kg/so A substantial proportion of this material is later mixed with concrete for road-building projects. The following technical specifications enable us to appreciate the size of this station.

Electrical data:

• number of generators: 4
• power per generator: 660 MW
• speed: 3000 r/min
• voltage: 23000 V
• frequency: 50 Hz, phases: 3

Thermal and mechanical data:

• number of steam turbines: 4
• number of condensers: 4
• number of boilers: 4
• steam flow per turbine: 560 kg/s
• steam temperature: 540°C
• steam pressure: 16.55 MPa
• cooling water per condenser: 21 000 kg/s
• coal consumption per boiler: 51.5 kg/s
• dust captured by cleaning system: 28 kg/s
• length of one turbine-generator unit: 50 m
• weight of one turbine-generator unit: 1342 tons
• number of emission stacks: 2
• height of emission stack: 200 m
• outside diameter at bottom: 20 m
• outside diameter at top: 11.6 m

Another interesting feature is that coal for the station is brought in by conveyor belts from two mines that are only 1.5 km and 4.5 km away. Thus, the station is ideally located near its source of fuel and near its source of cooling water, on the shore of Lake Macquarie.

--- View of the Power Station. The large building on the left is the turbine-generator hall: 27 m wide x 38 m high x 418 m long. To the right can be seen the four structures that house the steam boilers. A portion of the flue gas cleaning system can be seen between the emission stack in the foreground and the boiler structures.

--- General construction of the flue gas cleaning system showing the filter bags that capture the dust, which then falls into the hoppers below.

--- Installation of the fabric filter bags. Each bag is 15 m long and 16 cm in diameter.

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Monday, October 1, 2012 13:21