fossil fuel power plant

Mohave Generating Station, a 1,580 MW coal power plant near Laughlin, Nevada

A fossil fuel power plant is an energy conversion center that burns fossil fuels to produce electricity, designed on a large scale for continuous operation.

Basic concepts

In a fossil fuel power plant the chemical energy stored in fossil fuels such as coal, fuel oil, natural gas or oil shale 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 that may run on natural gas or diesel.

The burning of fossil fuel is summarized in the following chemical reaction:

and the simple word equation for this chemical reaction is:

All fossil fuels generate carbon dioxide, when combusted. Chemical side reactions also take place, generating, among others, sulfur dioxide (predominantly in coal) and oxides of nitrogen. Each fossil fuel power plant is a highly complex, custom-designed system. Present construction costs, as of 2004, run to US$1,300 per kilowatt, or $650 million for a 500 MWe unit. Multiple generating units may be built at a single site for more efficient use of land, natural resources and labor.

Coal Power Station in Tampa FL

Fuel transport and delivery

Coal is delivered by mass transport systems, truck, rail, barge or collier. A large coal train called a "unit train" may be two kilometers long, containing 100 cars with 100 tons of coal in each one, for a total load of 10,000 tons. A large plant under full load requires at least one coal delivery this size every day. Plants may get as many as three to five trains a day, especially in "peak season", during the summer months when power consumption is high. A large thermal power plant such as the one in Nanticoke, Ontario stores several million tons of coal for winter use when the lakes are frozen.

Modern unloaders use rotary dump devices, which eliminate problems with coal freezing in bottom dump cars. The unloader includes a train positioner arm that pulls the entire train to position each car over a coal hopper. The dumper clamps an individual car against a platform that 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 unit train takes about three hours.

Shorter trains may use railcars with an "air-dump", which relies on air pressure from the engine plus a "hot shoe" on each car. This "hot shoe" when it comes into contact with a "hot rail" at the unloading trestle, shoots an electric charge through the air dump apparatus and causes the doors on the bottom of the car to open, dumping the coal through the opening in the trestle. Unloading one of these trains takes anywhere from an hour to an hour and a half. Older unloaders may still use manually operated bottom-dump rail cars and a "shaker" attached to dump the coal. Generating stations adjacent to a mine may receive coal by conveyor belt or massive diesel-electric-drive trucks.

A collier (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. Colliers are large, seaworthy, self-powered ships. For transporting coal in calmer waters, such as rivers and lakes, flat-bottomed vessels called barges are often used. Barges are usually unpowered and must be moved by tugboats or towboats.

For startup or auxiliary purposes, the plant may use fuel oil as well. Fuel oil can be delivered to plants by pipeline, tanker, tank car or truck. Oil is stored in vertical cylindrical steel tanks as large as 90,000 barrels (14,000 m³). The heavier no. 5 "bunker" and no. 6 fuels are 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) in size. The coal is transported from the storage yard to in-plant storage silos by rubberized conveyer belts at rates up to 4,000 tons/hour. A 400 ton silo may feed each coal pulverizer (coal mill) at a rate of up to 60 tons per hour. Coal fed into the top of the pulverizer and ground to a powder, the consistency of face powder, is blown, 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.

Feedwater heating and deaeration

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 (1.25 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 (0.38 m³/s).

Diagram of boiler feed water deaerator (with vertical, domed aeration section and horizontal water storage section

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 through a deaerator^[1][2] that removes dissolved air from the water, further purifying and reducing its corrosivity. The water may be dosed following this point with hydrazine, a chemical that removes the 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 air-blown into the furnace from fuel nozzles at the four corners and it rapidly burns, forming a large fireball at the center. The thermal radiation of the fireball heats the water that circulates through the boiler tubes near the boiler perimeter. 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 3,200 psi (22.1 MPa). It is separated from the water inside a drum at the top of the furnace. The saturated steam is introduced into superheat pendant tubes that hang in the hottest part of the combustion gases as they exit the furnace. Here the steam is superheated to 1,000 °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, and North Dakota. 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 the furnace-exit level and mix it with the incoming coal in fan-type mills that exhaust the pulverised coal and hot gas mixture into the boiler.

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

Steam turbine generator

Rotor of a modern steam turbine, used in a power station

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 loses pressure and temperature energy it expands in volume, requiring increasing diameter and longer blades at each succeeding stage 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 (at 3 rpm) so that the shaft will not bow even slightly and become unbalanced. This is so important that it is one of only four functions of blackout emergency power batteries on site. They are emergency lighting, communication, station alarms and turbogenerator lube oil.

Superheated steam from the boiler is delivered through 14–16 inch (350–400 mm) diameter piping to the high pressure turbine where it falls in pressure to 600 psi (4 MPa) and to 600 °F (315 °C) through the stage. It exits via 24–26 inch (600–650 mm) diameter cold reheat lines and passes back into the boiler where the steam is reheated in special reheat pendant tubes back to 1,000 °F (540 °C). The hot reheat steam is conducted to the intermediate pressure turbine where it falls in both temperature and pressure and exits directly to the long-bladed low pressure turbines and finally exits to 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 either 3,000 or 3,600 RPM, synchronized to the power grid. 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 first displaced by carbon dioxide before filling with hydrogen. This ensures that the highly flammable hydrogen does not mix with oxygen in the air.

The power grid frequency is 60 Hz across North America and 50 Hz in Europe, Oceania, Asia (Korea and parts of Japan are notable exceptions) and parts of Africa.

The 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.

Steam condensing

Diagram of a typical water-cooled surface condenser

The condenser condenses the steam from the exhaust of the turbine into liquid to allow it to be pumped. If the condenser can be made cooler, the pressure of the exhaust steam is reduced and efficiency of the cycle increases. The condenser is usually a shell and tube heat exchanger commonly referred to as a surface condenser. Cooling water circulates through the tubes in the condenser's shell and the low pressure exhaust steam is condensed by flowing over the tubes as shown in the adjacent diagram. The tubing is designed to reduce the exhaust pressure, avoid subcooling the condensate and provide adequate air extraction. Typically the cooling water causes the steam to condense at a temperature of about 32–38 °C (90–100 °F) and that creates an absolute pressure in the condenser of about 5–7 kPa (1.5–2.0 in Hg), a vacuum of about 95 kPa (28 mmHg) relative to atmospheric pressure. The condenser, in effect, creates the low pressure required to drag steam through and 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 (it may be possible to lower the temperature beyond the turbine limits during winter, causing excessive condensation in the turbine).

From the bottom of the condenser, powerful extraction pumps recycle the condensed steam (water) back to the water/steam cycle. The heat absorbed by the circulating cooling water in the condenser tubes must also be removed to maintain the ability of the water to cool as it circulates.

A Marley mechanical induced draft cooling tower

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) that reduce the temperature of the water by evaporation, about 11–17°C (20–30 °F) - expelling waste heat to the atmosphere. The circulation flow rate of the cooling water in a 500 MWe unit is about 14.2 m³/s (225,000 US gal/minute) at full load.^[3]

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 plants include an automatic cleaning system that circulates sponge rubber balls through the tubes to scrub them clean without the need to take the system off-line.

The cooling water used to condense the steam in the condenser returns to its source without having been changed other than having been warmed. If the water returns to a local water body (rather than a circulating cooling tower), it is tempered with cool 'raw' water to prevent thermal shock when discharged into that body of water.

Another form of condensing system is the air-cooled condenser. The process is similar to that of a radiator and fan. Exhaust heat from the low pressure section of a steam turbine runs through the condensing tubes, the tubes are usually finned and 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. Air-cooled condensers typically operate at a higher temperature than water cooled versions. Whilst saving water, the efficiency of the cycle is reduced (resulting in more carbon dioxide per MW of electricity).

Diagram

Simplified thermal power station

1. Cooling tower	10. Steam control valve	19. Superheater
2. Cooling water pump	11. High pressure steam turbine	20. Forced draught (draft) fan
3. Three-phase transmission line	12. Deaerator	21. Reheater
4. Step-up Transformer	13. Feedwater heater	22. Combustion air intake
5. Electrical generator	14. Coal conveyor	23. Economiser
6. Low pressure steam turbine	15. Coal hopper	24. Air preheater
7. Boiler feedwater pump	16. Coal pulverizer	25. Precipitator
8. Surface condenser	17. Boiler steam drum	26. Induced draught (draft) fan
9. Intermediate pressure steam turbine	18. Bottom ash hopper	27. Flue gas stack

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 the manufacturing of concrete. This cleaning up of flue gases, however, only occurs in plants that are fitted with the appropriate technology. Still, the majority of coal fired power plants in the world do not have these facilties.^{[citation needed]} Legislation in Europe has been efficient to reduce flue gas pollution. Japan has been using flue gas cleaning technology for over 30 years and the US has been doing the same for over 25 years. China is now beginning to grapple with the pollution caused by coal fired power plants.

Flue gas stack at GRES-2 Power Station in Ekibastus, Kazakhstan

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 50 °C (120 °F). A typical flue gas stack may be 150–180 m (500–600 ft) tall to disperse the remaining flue gas components in the atmosphere. The tallest flue gas stack in the world is 420 m (1,375 ft) tall at the GRES-2 power plant in Ekibastusz, Kazakhstan.

In the United States and a number of other countries, atmospheric dispersion modeling^[4] studies are required to determine the flue gas stack height needed to comply with the local air pollution regulations. The United State also limits the maximum height of a flue gas stack to what is known as the "Good Engineering Practice (GEP)" stack height.^[5][6] In the case of existing flue gas stacks that exceed the GEP stack height, any air pollution dispersion modeling studies for such stacks must use the GEP stack height rather than the actual stack height.

Supercritical steam plants

Above the critical point for water of 705 °F (374 °C) and 3,212 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. Subcritical fossil fuel power plants can achieve 36–38% efficiency. Supercritical designs have efficiencies in the low to mid 40% range, with new "ultra critical" designs using pressures of 4,400 psia (30 MPa) and dual stage reheat reaching about 48% efficiency.

Nuclear power plants must operate far below the temperatures and pressures than coal fired plants do. This limits their thermodynamic efficiency to the order of 34–37%. One proposed plant design, the Supercritical water reactor, operates at temperatures and pressures similar to current coal plants, producing comparable efficiency.

Gas turbine combined-cycle plants

480 megawatt GE H series power generation gas turbine

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 fueled with natural gas or diesel. While highly efficient and very quick to construct (a 1,000 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.

Environmental impacts

Fossil fuel power contributes to acid rain, global warming, and air pollution (electricity generation is responsible for 39 percent of USA carbon dioxide emissions).^[7] Acid rain is caused by the emission of nitrogen oxides and sulfur dioxide into the air. These themselves may be only mildly acidic, yet when it reacts with the atmosphere, it creates acidic compounds such as sulfurous acid, nitric acid and sulfuric acid that fall as rain, hence the term acid rain. In Europe and the USA, stricter emission laws have reduced the environmental hazards associated with this problem.

Another danger related to coal combustion is the emission of fly ash, tiny solid particles that are dangerous for public health. (Natural gas plants emit virtually no fly ash) These can be filtered out of the stack gas, although this does not happen everywhere. The most modern plants that burn coal use a different process, in which synthesis gas is made out of a reaction between coal and water. This is purified of most pollutants and then used initially to power gas turbines, then the residual heat is used for a steam turbine. The pollution levels of such plants are drastically lower than those of "classical" coal power plants. However, all coal burning power plants emit carbon dioxide. Research has shown that increased concentration of carbon dioxide in the atmosphere is positively correlated with a rise in mean global temperature, also known as climate change.

Coal also contains low levels of uranium, thorium, and other naturally-occurring radioactive isotopes whose release into the environment leads to radioactive contamination. While these substances are present as very small trace impurities, enough coal is burned that significant amounts of these substances are released. A 1,000 MW coal-burning power plant could release as much as 5.2 tons/year of uranium (containing 74 pounds of uranium-235) and 12.8 tons/year of thorium. The radioactive emission from this coal power plant is 100 times greater than a comparable nuclear power plant with the same electrical output; including processing output, the coal power plant's radiation output is over 3 times greater.^[8]

Trace amounts of mercury can exist in coal and other fossil fuels.^[9] When these fuels burn, mercury vapor can be released and the mercury is a neurotoxic heavy metal which bioaccumulates in food chains and is especially harmful to aquatic ecosystems. According to the United States Department of Energy, the worldwide emission of mercury from both natural and human sources was 5,500 tons in 1995.^[9] and coal-fired plants in the USA release an estimated 48 tons annually, which is less than 1 percent of the worldwide emissions.^[9] The Environmental Working Group (a privately funded environmental advocacy organization) alleges that coal-fired power plants are the largest emitters of mercury in the USA.^[10]

Alternatives to fossil fuel power plants include solar power and other renewable energies (see non-carbon economy).

Clean coal

Main article: clean coal

Recent developments in the application of fossil fuels include the utilisation of clean coal. Here carbon dioxide produced from the combustion process can be stored in geological formations. This is particularly applicable to empty oil and gas deposits. A number of these carbon sequestration schemes are being planned, most notably for the North Sea.

See also

• Boiler
• Combined heat and power
• Cooling tower system
• Flue gas stacks
• Geothermal power
• Global warming
• Greenhouse gas
• Petroleum dependence
• Power station
• Thermal power station
• Water-tube boiler
• Mercury vapour turbine

References

Bibliography

• Steam: Its Generation and Use (2005). 41st edition, Babcock & Wilcox Company, ISBN 0-9634570-0-4
• Steam Plant Operation (2005). 8th edition, Everett B. Woodruff, Herbert B. Lammers, Thomas F. Lammers (coauthors), McGraw-Hill Professional, ISBN 0-07-141846-6
• Power Generation Handbook: Selection, Applications, Operation, Maintenance (2003). Philip Kiameh, McGraw-Hill Professional, ISBN 0-07-139604-7
• Standard Handbook of Powerplant Engineering (1997). 2nd edition, Thomas C. Elliott, Kao Chen, Robert Swanekamp (coauthors), McGraw-Hill Professional, ISBN 0-07-019435-1

External links

• Power plant diagram
• Power Plant Reference Books
• Large industrial cooling towers

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