FOSSIL (Also see Coal Power Plant Flash animation)

There are three classes of fossil fuels:

  1. coal
  2. oil
  3. natural gas

The general chemical formula for the hydrocarbon compounds is Hx(H2C)y although they are many times composed with nitrogen, oxygen, and sulfur derivatives. Some important hydrocarbons are methane (CH4), ethane (C2H6), propane (C3H8), and butane (C4H10). Burning of sulfur creates SO2 which reacts with atmospheric water vapor to form acid rain.

Natural gas consists mostly of methane and ethane; whereas liquified petroleum gas (LPG) is primarily composed of propane and butane. Coal is the most abundant fossil fuel in the U.S. Coal found in the eastern U.S. typically has high-sulfur and is found in deep deposits (underground shaft mined); conversely, western coal is of low-sulfur content and found near the surface (strip or open-pit mined).

One of the important properties of a fuel is its heating value (the negative of the heat of combustion). In most instances, the higher heating value (HHV) of the fuel is quoted. However, the lower heating value (LHV) may be a more appropriate quantity since the heat to vaporize any moisture in the fuel is accounted for (i.e., LHV=HHV - hfg·masswater where hfg is the latent heat of vaporization).

Combustion

In any discussion of fossil fuels it is necessary to examine the combustion process. There are three important exothermic chemical reactions:

  1. C + O2 ® CO2 carbon dioxide
  2. 2H2 + O2 ® 2H2O water
  3. S + O2 ® SO2 sulfur dioxide

The latter reaction is a consequence of the presence of sulfur with the true fuel materials of carbon and hydrogen. Sulfur dioxide (SO2) is a major pollutant. Some important characteristics of these three fuels are listed below:

Characteristic Carbon Hydrogen Sulfur
Ignition Temperature 407°C (765°F) 582°C (1080°F)

Highest of the three
243°C (470°F)

Lowest of the three
Heating Value (HHV and LHV) QH=QL=32,778 kJ/kg QH=142,097 kJ/kg
QL=120,067 kJ/kg
QH=QL=9257 kJ/kg
Reaction Kinetics Oxidation of C is slower and more difficult than H2 or S H2 is a gas, so it burns rapidly S burns after H2 but before C
Reaction(s) 2C + O2 ® 2CO

2CO + O2 ® 2CO2

note C to CO, then CO to CO2
2H2 + O2 ® 2H2O S + O2 ® SO2

In addition to specifications on the fuel materials, constituents of air are given below for reference

Constituent Approximate Concentration of Air
By Volume By Weight
Nitrogen 79% 76.8%
Oxygen 21% 23.2%
Others Small Small

If the combustion temperature is too high, then the following disassociation reactions occur

N2 + O2 ® 2 NO {needs at least 2900°F}
N2 + 2 O2 ® 2 NO2 {NO2 is a yellow-brown gas}
2 CO2 ® 2 CO + O2  

All of these reactions are endothermic, and result in pollutants of NOx (nitrous oxides) and CO (carbon monoxide). NOx emissions are reduced through control of the combustion process, that is, either reducing the combustion temperature or lowering the air-fuel ratio. CO production is reduced by introducing excess air.

There are different types of combustion systems (burners) for each of the three fossil fuels. Natural gas burners are the easiest gas to burn, since one needs only to proportion the gas-air mixture, mix with air, and ignite the mixture. Oil burners prepare the oil by vaporization or gasification by heating it in the burner, or by atomization in the combustion-air stream. Atomization can be accomplished with high-pressure air or steam, and is best suited for variable loads. Atomization can also be done by mechanical means (centrifugal force) which is better suited for steady loads and high capacities.

Petroleum

Commercial petroleum fuels are divided into ASTM grades, which are based on the fuel viscosity (high viscosity means that the fuel is more resistant to flow). Grade No. 6, otherwise known as Bunker "C" Fuel Oil, is the most widely used in power plants. Crude oil can be used directly, but this is not very desirable. Oil is also used for non-energy products such as plastics, asphalt, and waxes.

Some of the advantages of oil is that it is cleaner; easier to handle, store and transport; easier to burn than coal and produces little ash; and the oil can be atomized for a good mix with combustion air (for gas-like performance). The disadvantages of oil include that the ash is very adhesive and difficult to remove; some oils are high in sulfur and sulfur is difficult to remove; and the presence of vanadium, once oxidized, causes corrosion of ferrous materials found in most boilers.

Shale oil is a future petroleum source. The U.S. has the world's largest oil shale reserves (in Colorado and Wyoming), and shale oil is the U.S.'s most abundant fossil fuel next to coal. The main problem with shale oil is the development of an economical and environmentally acceptable recovery process. The petroleum yield is around 25 gallons per ton of shale oil.

combustion_tubine

Combustion Turbine Power Plant Schematic; this power plant is capable of burning either natural gas or fuel oil [Source: Tennessee Valley Authority (TVA)].

Natural Gas

Natural gas is the easiest of the fossil fuels to burn as it mixes well with air and burns cleanly with little ash. Natural gas has the highest gravimetric heating value of all fossil fuels, but has the lowest known fuel reserves. The gas is piped directly to the plant, otherwise, it is difficult to store unless cryogenic temperature tanks are available. For instance, overseas gas is converted to liquified natural gas (LNG) for transport (at -197°F). Natural gas high in hydrogen sulfide (H2S) is known as sour gas (versus sweet gas).

Combined cycle plants have become a popular generation scheme in recent years. A combined cycle uses a (top) gas turbine (Brayton) cycle with the excess heat going to a (bottom) steam turbine (Rankine) cycle. The combustion gases are first used to drive the gas turbine, then the exiting gases are sent to a heat recovery steam generator (HRSG), and lastly to the stack. The overall thermal efficiency of combined cycle plants being built today is a remarkable 60%. Combined cycle plants are designed for intermediate load since they are relatively quick to start. Advantages of these plants are that they can be built in a relatively short time period (about 2 years), and their use of natural gas, which is an environmentally good choice (except for greenhouse gas emission), and a reasonably priced fuel.

combined_cycle

Combined Cycle Plant Diagram [Source: Texas Utilities (TU)].

Coal

Coals are classified in order to identify end-use, and also to provide data useful in specifying and selecting burning and handling equipment, and in the design and arrangement of heat transfer surfaces. The coal grades institutes by ASTM are primarily based upon age. Arizona coal is subbituminous.

Because of the cost of transportation coal-fired plants are sometimes built next to the coal mine (i.e., a mine mouth plant).

Fuel Ignition Temperature Characteristics HHV/LHV (kJ/kg)
Hydrogen 1080°F As gas, burns rapidly 142,097
120,067
Sulfur 470°F Lowest ignition temperature 9257
Carbon 765°F Slowest of the three in burning 32,778

Coal furnaces

There are several types of coal furnaces including

The stoker furnace is of limited capacity and does not lend itself to power plants but rather it is used in industrial processes. Coal is introduced on a grate, and it is finally burned on a stationary bed. The primary air enters below the burning bed and initiates the combustion process, and also cools the grate. Secondary air is introduced over the burning bed to complete the combustion process.

The cyclone furnace employs several (as many as 16) independent combustion chambers. The main combustion chamber operates at a temperature of 3200°F. These were popular in the 1950s and 1960s but are no longer being built since they have difficulty burning low-sulfur coals and the high temperature results in significant NOx formation.

The pulverized coal furnace attempts to burn finely powdered coal and air in a gaseous torch. This is accomplished through pulverizing the coal by crushing, impact and attrition (rubbing) of the coal to a size finer than face powder (diameter < 0.3 mm). The primary air dries and transports the coal. The advantages of a pulverized coal furnace include its ability to burn all ranks of coal from anthracitic to lignitic, and it permits combination firing (i.e., can use coal, oil and gas in same burner). Because of these advantages, there is widespread use of pulverized coal furnaces. The disadvantages of the pulverized coal furnace are that the coal pulverizer has a significant power demand of its own and requires more maintenance, flyash erosion and pollution complicate unit operation and increase exhaust system maintenance requirements, and pulverized systems have higher initial cost and require larger furnace volumes for the combustion process.

For a fluidized-bed furnace, the velocity of combustion gas (air) entering the bottom of the furnace is maintained such that the coal and limestone or dolomite particles are suspended (resembling a boiling liquid). The boiler tubes are immersed in the fluidized bed. Fluidized-bed combustion systems are categorized as pressurized vs. atmospheric bed systems, and circulating vs. stationary bed systems. Advantages of fluidized-bed systems include higher rates of heat transfer between combustion process and boiler tubes (thus reduced furnace area and size required), and combustion temperature (1500-1600°F) is lower than in a conventional furnace. The primary advantage is reduced pollutants, for example, the lower furnace temperatures means reduced NOx production. In addition, the limestone (CaCO3) and dolomite (MgCO3) react with SO2 to form calcium and magnesium sulfides, respectively, which are solids that do not escape up the stack; however, it does require about 50% more limestone/dolomite as compared to a wet-scrubber system. This means the plant can easily use high sulfur coal. Disadvantages of fluidized-bed systems include 1) erosion of tubes by the particles rubbing the tubes, 2) requires more fan power to suspend the particles, and 3) system appears better suited for low-power applications.

coalart

Coal-Fired Power Plant Schematic [Source: Tennessee Valley Authority (TVA)].

Boilers

Types of steam generation systems include

Early fire tube boilers consisted of a water-filled pressure vessel with hot gases traveling inside tubes through the vessel. This system is probably the simplest and least expensive, but such a configuration limits the steam to low pressures because of pressure-vessel code limitations. Also, a single tube failure is catastrophic and could cause a steam explosion in the furnace.

The water tube boiler is best suited for high-pressure, high-capacity steam generators. High-pressure water and steam flows from tube headers or drums through tubes in the furnace walls or in tube bundles mounted in the exhaust gas duct. The water tube boilers are classified as either natural-circulation (older units) or forced circulation. Pressures greater than 2400 psia require forced circulation through the evaporators.

The water-cooled integral furnace/boiler design places the water tubes on furnace periphery.

Once-through boilers (universal pressure or Benson boilers) are the most widely used forced-circulation boiler system in the U.S. The water is feedpumped in at 5000 psia and enters turbine at 3500 psia. These systems require extremely pure water to avoid deposition on boiler tubing.


Last updated: June 13, 2006

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