Fossil fuel power plant

A fossil fuel power plant is an energy conversion center that combusts fossil fuels to produce electricity, designed on a large scale for continuous operation. The principle of chemical to energy conversion as exploited in a battery produces relatively small amounts of electricity for temporary or intermittent use. In contrast, in a fossil fuel power plant, the chemical energy stored in fossil fuels such as coal, fuel oil, or natural gas is converted successively into thermal energy, mechanical energy, and finally electrical energy for continuous use and distribution across a wide geographic area. Almost all large fossil fuel power plants are steam-electric power plants, except for gas turbines and utility-sized reciprocating engines, which may run on natural gas or diesel. Each fossil fuel power plant is a highly complex, custom-designed system. Present construction costs, as of 2004, run to $1,300 per kilowatt, or US$650 million for a 500 MWe unit. Multiple generating units may be built at a single site for more efficient use of land, resources, and labor.

Fuel transport and delivery

Coal is delivered by mass transport systems, either truck, railroad, barge or collier. A large coal train may be nearly a mile long, containing 100 cars, each with 100 tons of coal, for a total load of 10,000 tons. A large plant requires a coal delivery this size every day. Modern unloaders use rotary dump devices. The unloader includes a train positioner arm that moves the entire train to position a car over a coal hopper. The dumper clamps an individual car against a platform, which swivels the car upside down to dump the coal. Swiveling couplers enable the entire operation to occur while the cars are still coupled together. Unloading a train takes about three hours. Older unloaders may still use bottom dump rail cars. Generating stations adjacent to a mine sometimes haul coal with massive diesel-electric drive trucks with 140 ton capacity. Trucks this large typically have 8 ft (2.5 m) diameter tires, too large and heavy to be licensed for highway use.

A collier, or cargo ship carrying coal, may hold 40,000 tons of coal and takes several days to unload. Some colliers carry their own conveying equipment to unload their own bunkers; others depend on equipment at the plant's coal yard. While a collier is a large, seaworthy, self-powered ship, a barge is a much smaller, flat-bottomed vessel designed for calmer waters such as rivers and lakes.

For startup or auxiliary purposes, the plant may use no. 2, no. 5 or no. 6 fuel oil as well. Fuel oil may be delivered by tanker truck or train car. It is stored in vertical cylindrical steel tanks as large as 90,000 barrels (14,000 m³). The heavier no. 5 "bunker" and no. 6 fuel must be steam-heated before pumping in cold climates.

Plants fueled by natural gas are usually built adjacent to gas transport pipelines or have dedicated gas pipelines extended to them.

Fuel processing

Coal is prepared for use by crushing the rough coal to pieces less than 2 inches (50 mm) size. The coal is transported from the storage yard to in-plant storage silos by rubberized conveyer belts at rates up to 4000 tons per hour. A 400 ton silo may feed a coal pulverizer (coal mill) at a rate of up to 60 tons per hour. It is introduced into the top of the pulverizer which grinds the coal to a powder the consistency of face powder and blows powder mixed with air into the furnace. A 500 MWe plant will have six such pulverizers, five of which can supply coal to the furnace at 250 tons per hour under full load.

Natural Gas is delivered to sites, usually by pipeline. For the sites burning light oil, the delivery to the specific sites are either by pipeline, tanker, or trucks.

Feedwater heating

The feedwater used in the steam boiler is a means of transferring heat energy from the burning fuel to the mechanical energy of the spinning steam turbine. The total feedwater consists of recirculated condensed steam, referred to as condensate, from the steam turbines plus purified makeup water. Because the metallic materials it contacts are subject to corrosion at high temperatures and pressures, the makeup water is highly purified before use. A system of water softeners and ion exchange demineralizers produces water so pure that it coincidentally becomes an electrical insulator, with conductivity in the range of 0.3–1.0 microsiemens per centimeter. The makeup water in a 500 MWe plant amounts to perhaps 20 US gallons per minute (1 L/s) to offset the small losses from steam leaks in the system.

The feedwater cycle begins with condensate water being pumped out of the condenser after travelling through the steam turbines. The condensate flow rate at full load in a 500 MWe plant is about 6,000 US gallons per minute (0.4 m³/s). The water flows through a series of six or seven intermediate feedwater heaters, heated up at each point with steam extracted from an appropriate duct on the turbines and gaining temperature at each stage. Typically, the condensate plus the makeup water then flows though a deaerator, which removes dissolved air from the water, further purifying and reducing its corrosivity. The water may be dosed following this point with hydrazine, a chemical which removes the last remaining oxygen in the water to below 5 parts per billion (ppb). It is also dosed with pH control agents such as ammonia or morpholine to keep the residual acidity low and thus non-corrosive.

Boiler operation

The boiler is a rectangular furnace about 50 ft (15 m) on a side and 130 ft (40 m) tall. Its walls are made of a web of high pressure steel tubes about 2.3 inches (60 mm) in diameter.

Pulverized coal is blown into the furnace from fuel nozzles at the four corners and it rapidly combusts, forming a large fireball at the center. This heats the water that circulates through the boiler tubes. The water circulation rate in the boiler is three to four times the throughput, and is typically driven by pumps. As the water in the boiler circulates, it absorbs heat and changes into steam at 700 °F (370 °C) and 3200 psi (22.1 MPa), which is separated from the water inside a drum at the top of the furnace. The saturated steam is introduced into superheat pendant tubes hanging in the hottest part of the combustion gasses as they exit the furnace. Here the steam is superheated to 1000 °F (540 °C) to prepare it for the turbine.

Plants designed for lignite (brown coal) are increasingly used in locations as varied as Germany, Victoria, Australia, and the midwestern US. Lignite is a much younger form of coal than black coal. It has a lower energy density than black coal and requires a much larger furnace for equivalent heat output. Such coals may contain up to 70% water and ash, yielding lower furnace temperatures and requiring larger induced draft fans. The firing systems also differ from black coal, and typically draw hot gas from furnace exit level and mix it with the coal in fan-type mills that exhaust the pulverised coal/hot gas mix into the boiler.

Plants that use gas turbines as a source of heating the water for conversion to steam, use boilers known as HRSGs, Heat Recovery Steam Generators. The waste (exhaust) heat from the gas turbines are used to make superheated steam that is then used in a conventional water-steam cycle.

Steam turbine generator

The turbine generator consists of a series of steam turbines interconnected to each other and a generator on a common shaft. There is a high pressure turbine at one end, followed by an intermediate pressure turbine, two low pressure turbines, and the generator. As steam moves through the system and drops in pressure, it expands in volume, requiring larger diameter and longer blades in each succeeding turbine to extract the remaining energy. The entire rotating mass may be over 200 tons and 100 ft (30 m) long. It is so heavy that it must be kept turning slowly even when shut down (3 rpm) so that the shaft will not sag even slightly and become unbalanced. This is so important that it is one of just four functions of blackout emergency power batteries on site: emergency lighting, communication, station alarms, and turbine generator turning gear.

In operation, the superheated steam from the boiler passes through 14–16 in (350–400 mm) diameter piping down to the high pressure turbine, where it falls in pressure to 600 psi (4 MPa) and 600 °F (315 °C), exits through 24–26 in (600–650 mm) diameter cold reheat lines and passes back up into the boiler where the steam is reheated in special reheat pendant tubes back to 1000 °F (540 °C). The hot reheat steam is conducted down to the intermediate pressure turbine where it falls again in both temperature and pressure, and exits directly to the large bladed low pressure turbines, and finally enters the condenser.

The generator, 30 ft (9 m) long and 12 ft (3.7 m) diameter contains a stationary stator and a spinning rotor, each containing miles of heavy copper conductor— no permanent magnets here. In operation it generates up to 21,000 amps at 24,000 volts AC (504 MWe), as it spins at 3600 rpm, synchronized to the 60 Hz power grid. The power grid frequency is 60 Hz across North America and 50 Hz in Europe, Oceania, Asia (Korea and parts of Japan notable exceptions) and parts of Africa. This electricity flows to a distribution yard, where transformers step the voltage up to 115, 230, 500, or 765 kV AC as needed for transmission to its destination. The rotor spins in a sealed chamber cooled with hydrogen gas, selected because it has the highest known heat transfer coefficient of any gas, and for its low viscosity which reduces windage losses. This system requires special handling during startup, with air in the chamber displaced by carbon dioxide first, before filling with hydrogen. This ensures that the highly flammable hydrogen will not mix with oxygen in the air.

Steam condensing

The lower the pressure of the exhaust steam leaving the low pressure turbine, the more efficient is the overall series of turbines. The exhaust steam from the low pressure turbines contacts condenser tube bundles which have cooling water circulating through them. Typically, the cooling water causes the steam to condense at a temperature of about 90–100 °F (32–38 °C) and that creates an absolute pressure in the condenser of about 1.5–2.0 in Hg (5–7 kPa), which is a vacuum of about 28 in Hg (95 kPa) below atmospheric pressure. The condenser in effect creates the low pressure required to increase the efficiency of the turbines. The limiting factor is the temperature of the cooling water and that, in turn, is limited by the prevailing average climatic conditions at the power plant's location. From the bottom of the condenser, powerful pumps recycle the condensed steam back to the feedwater heaters for reuse. The heat absorbed by the circulating cooling water in the condenser tubes must be removed to maintain the ability of the water to cool as it circulates. This is done by pumping the warm water from the condenser through either natural draft, forced draft or induced draft cooling towers (as seen in the image to the right), which reduce the temperature of the water by 20–30 °F (11–17 °C) by evaporation. The circulation flow rate of the cooling water in a 500 MWe unit is about 225,000 US gallons per minute (14.2 m³/s) at full load. ^*

The condenser tubes are made of brass or stainless steel to resist corrosion from either side. Nevertheless they may become internally fouled during operation by bacteria or algae in the cooling water, or by mineral scaling, all of which inhibit heat transfer and reduce thermodynamic efficiency. Many systems include an automatic cleaning system which circulates sponge rubber balls through the tubes to scrub them clean without needing to take the system off-line.

Another form of condensing systems is the air-cooled condenser. While these systems are similar in operation to mechanical cooling towers, they typically are more environmentally acceptable forms of condensing steam. The process is similar to that of a radiator and fan...exhaust heat form the low pressure section of a steam turbine runs through the condensing tubes. The tubes are usually finned and when ambient air is pushed through the fins with the help of a large fan, the steam condenses to water to be reused in the water-steam cycle.

Diagram

Stack gas path and cleanup

see Flue gas emissions from fossil fuel combustion and Flue gas desulfurization for more details

As the combustion flue gas exits the boiler it is routed through a rotating flat basket of metal mesh which picks up heat and returns it to incoming fresh air as the basket rotates, This is called the air preheater. The gas exiting the boiler is laden with fly ash, which are tiny spherical ash particles. The flue gas contains nitrogen along with combustion products carbon dioxide, sulfur dioxide, and nitrogen oxides. The fly ash is removed by fabric bag filters or electrostatic precipitators. Once removed, the fly ash byproduct can sometimes be used in manufacture of concrete. Where required by law, the sulfur and nitrogen oxide pollutants are removed by stack gas scrubbers which use a pulverized limestone or other alkaline wet slurry to remove those pollutants from the exit stack gas. The gas travelling up the flue gas stack may by this time only have a temperature of about 120 °F (50 °C). The flue gas stack may be 500–600 ft (150–180 m) tall to disperse the remaining flue gas components in the atmosphere. In fact, the flue gas stack in this photograph] of the GRES-2 power plant in Ekibastusz, Kazachstan is 1375 ft (420 m) tall.

Super critical steam plants

Above the critical point for water of 705 °F (374 °C) and 3212 psia (22.1 MPa), there is no phase transition from water to steam, but only a gradual decrease in density. Boiling does not occur and it is not possible to remove impurities via steam separation. In this case a new type of design is required for plants wishing to take advantage of increased thermodynamic efficiency available at the higher temperatures. These plants (also called once-through plants because boiler water does not circulate multiple times) require additional water purification steps to ensure that any impurities picked up during the cycle will be removed. This takes the form of high pressure ion exchange units called condensate polishers between the steam condenser and the feedwater heaters.

Nuclear power plants generally cannot reheat process steam due to safety requirements for isolation from the reactor core. This limits their thermodynamic efficiency to the order of 34–36%. Subcritical fossil fuel power plants can achieve 36–38% efficiency. Super critical designs have efficiencies in the low to mid 40% range, with new "ultra critical" designs using pressures of 30 MPa and dual stage reheat reaching about 48% efficiency.

Gas turbine combined cycle plants

An important class of fossil power plant uses a gas turbine, sometimes in conjunction with a steam boiler "bottoming" cycle. The efficiency of a combined cycle plant can approach 60% in large (500+ MWe) units. Such turbines are usually fuelled with natural gas or light fuel oil. While highly efficient and very quick to construct (a 1000 MW plant may be completed in as little as two years from start of construction), the economics of such plants is heavily influenced by the volatile cost of natural gas. The combined cycle plants are designed in a variety of configurations composed of the number of gas turbines followed by the steam turbine. For example, a 3-1 combined cycle facility has three gas turbines tied to one steam turbine. The configurations range from (1-1),(2-1),(3-1),(4-1), (5-1), to (6-1).

Simple-cycle gas turbine plants, without a steam cycle, are sometimes installed as emergency or peaking capacity; their thermal efficiency is much lower. The high running cost per hour is offset by the low capital cost and the intention to run such units only a few hundred hours per year.

External links

Power plant diagram
Power Plant Reference Books
Coal Combustion: Nuclear Resource or Danger - Article discussing the presence of trace amounts of radioactive materials in coal used for fuel, and the radiation released from combustion of such fuel.
Large industrial cooling towers

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