Pulverized coal-fired boiler
Encyclopedia
A pulverized coal-fired boiler is an industrial or utility boiler
that generates thermal energy
by burning pulverized coal (also known as powdered coal or coal dust
since it is as fine as face powder in cosmetic makeup) that is blown into the firebox.
The basic idea of a firing system using pulverised fuel is to use the whole volume of the furnace
for the combustion of solid fuels. Coal is ground to the size of a fine grain, mixed with air and burned in the flue gas flow. Biomass and other materials can also be added to the mixture. Coal contains mineral matter which is converted to ash during combustion. The ash is removed as bottom ash and fly ash. The bottom ash is removed at the furnace bottom.
This type of boiler dominates the electric power industry, providing steam to drive large turbines. Pulverized coal provides the thermal energy which produces about 50% of the world's electric supply.
where the fuel was mechanically distributed onto a moving grate at the bottom of the firebox in a partially crushed gravel like form. Air for combustion was blown upward through the grate carrying the lighter ash and smaller particles of unburned coal up with it, some of which would adhere to the sides of the firebox. In 1918 the Milwaukee Electric Railway and Light Company, later Wisconsin Electric, conducted tests in the use of pulverized coal at its Oneida Street power plant
. These experiments helped Fred L. Dornbrook to develop methods of controlling the pulverized coal's tarry ash residues with boiler feed water tube jackets that served to reduce the surface temperature of the firebox walls and allowed the ash deposits be easily removed. This plant became the first central power station in the United States to use pulverized fuel.
The Oneida Street power plant near Milwaukee's City Hall was subsequently decommissioned and renovated in 1987. It is now the site of the Milwaukee Repertory Theatre.
The fly ash is carried away with the flue gas
and is separated in various hoppers in the path and finally in an ESP or a bag filter.
s. For example, see Prussian G 12
.
evaluated a pulverized coal-boiler on the Mercer, a 9,500 ton merchant ship. According to their report the Mercer with its pulverized coal-boiler ran at 95% efficiency and was cheaper to operate than ship's boilers using oil as fuel.
Boiler
A boiler is a closed vessel in which water or other fluid is heated. The heated or vaporized fluid exits the boiler for use in various processes or heating applications.-Materials:...
that generates thermal energy
Thermal energy
Thermal energy is the part of the total internal energy of a thermodynamic system or sample of matter that results in the system's temperature....
by burning pulverized coal (also known as powdered coal or coal dust
Coal dust
Coal dust is a fine powdered form of coal, which is created by the crushing, grinding, or pulverizing of coal. Because of the brittle nature of coal, coal dust can be created during mining, transportation, or by mechanically handling coal.-Explosions:...
since it is as fine as face powder in cosmetic makeup) that is blown into the firebox.
The basic idea of a firing system using pulverised fuel is to use the whole volume of the furnace
Furnace
A furnace is a device used for heating. The name derives from Latin fornax, oven.In American English and Canadian English, the term furnace on its own is generally used to describe household heating systems based on a central furnace , and sometimes as a synonym for kiln, a device used in the...
for the combustion of solid fuels. Coal is ground to the size of a fine grain, mixed with air and burned in the flue gas flow. Biomass and other materials can also be added to the mixture. Coal contains mineral matter which is converted to ash during combustion. The ash is removed as bottom ash and fly ash. The bottom ash is removed at the furnace bottom.
This type of boiler dominates the electric power industry, providing steam to drive large turbines. Pulverized coal provides the thermal energy which produces about 50% of the world's electric supply.
History
Prior to the developments leading to the use of pulverized coal most boilers utilized grate firingGrate firing
Grate firing was the first combustion system used for solid fuels. It now is used mainly for burning waste and biomass, but also for smaller coal furnaces.* Capacities 0.3 to 175 MWth in industry and CHP...
where the fuel was mechanically distributed onto a moving grate at the bottom of the firebox in a partially crushed gravel like form. Air for combustion was blown upward through the grate carrying the lighter ash and smaller particles of unburned coal up with it, some of which would adhere to the sides of the firebox. In 1918 the Milwaukee Electric Railway and Light Company, later Wisconsin Electric, conducted tests in the use of pulverized coal at its Oneida Street power plant
Oneida Street Station
Oneida Street Station, also known as the East Wells Power Plant, was a power plant operated by The Milwaukee Electric Railway & Light Company. Constructed from 1898 to 1900, it was designed by architect Herman Esser in neoclassical revival style. The building is located in downtown Milwaukee,...
. These experiments helped Fred L. Dornbrook to develop methods of controlling the pulverized coal's tarry ash residues with boiler feed water tube jackets that served to reduce the surface temperature of the firebox walls and allowed the ash deposits be easily removed. This plant became the first central power station in the United States to use pulverized fuel.
The Oneida Street power plant near Milwaukee's City Hall was subsequently decommissioned and renovated in 1987. It is now the site of the Milwaukee Repertory Theatre.
How it works
The concept of burning coal that has been pulverized into a fine powder stems from the belief that if the coal is made fine enough, it will burn almost as easily and efficiently as a gas. The feeding rate of coal according to the boiler demand and the amount of air available for drying and transporting the pulverized coal fuel is controlled by computers. Pieces of coal are crushed between balls or cylindrical rollers that move between two tracks or "races." The raw coal is then fed into the pulverizer along with air heated to about 650 degrees F from the boiler. As the coal gets crushed by the rolling action, the hot air dries it and blows the usable fine coal powder out to be used as fuel. The powdered coal from the pulverizer is directly blown to a burner in the boiler. The burner mixes the powdered coal in the air suspension with additional pre-heated combustion air and forces it out of a nozzle similar in action to fuel being atomized by a fuel injector in modern cars. Under operating conditions, there is enough heat in the combustion zone to ignite all the incoming fuel.Ash removal
There are two methods of ash removal at furnace bottom:- Dry bottom boiler
- Wet bottom boilerWet bottom furnaceA wet-bottom furnace or wet-bottom boiler is boiler that contains a wet bottom furnace. It is a kind of boiler used for pulverised fuel firing.In wet bottom boiler the bottom ash is kept in a molten state and tapped off as a liquid...
, also called Slag tapWet bottom furnaceA wet-bottom furnace or wet-bottom boiler is boiler that contains a wet bottom furnace. It is a kind of boiler used for pulverised fuel firing.In wet bottom boiler the bottom ash is kept in a molten state and tapped off as a liquid...
The fly ash is carried away with the flue gas
Flue gas
Flue gas is the gas exiting to the atmosphere via a flue, which is a pipe or channel for conveying exhaust gases from a fireplace, oven, furnace, boiler or steam generator. Quite often, the flue gas refers to the combustion exhaust gas produced at power plants...
and is separated in various hoppers in the path and finally in an ESP or a bag filter.
Current Technologies
Pulverized coal power plants are broken down into three categories; subcritical pulverized coal (SubCPC) plants, supercritical pulverized coal (SCPC) plants, and ultra-supercritical pulverized coal (USCPC) plants. The primary difference between the three types of pulverized coal boilers are the operating temperatures and pressures. Subcritical plants operate below the critical point of water (647.096 K and 22.064 MPa). Supercritical and ultra-supercritical plants operate above the critical point. As the pressures and temperatures increase, so does the operating efficiency. Subcritical plants are at about 37%, supercriticals at about 40% and ultra-supercriticals in the 42-45% range.Steam locomotives
Pulverized coal firing has been used, to a limited extent, in steam locomotiveSteam locomotive
A steam locomotive is a railway locomotive that produces its power through a steam engine. These locomotives are fueled by burning some combustible material, usually coal, wood or oil, to produce steam in a boiler, which drives the steam engine...
s. For example, see Prussian G 12
Prussian G 12
The Prussian G 12 was a 2-10-0 goods train locomotive with the Prussian state railways .It was built because it had been shown during the First World War that it was a great disadvantage, from a servicing and maintenance point of view, for each state railway to have its own locomotive classes with...
.
Merchant Ships
In 1929 the United States Shipping BoardUnited States Shipping Board
The United States Shipping Board was established as an emergency agency by the Shipping Act , 7 September 1916. It was formally organized 30 January 1917. It was sometimes referred to as the War Shipping Board.http://www.gwpda.org/wwi-www/Hurley/bridgeTC.htm | The Bridge To France by Edward N....
evaluated a pulverized coal-boiler on the Mercer, a 9,500 ton merchant ship. According to their report the Mercer with its pulverized coal-boiler ran at 95% efficiency and was cheaper to operate than ship's boilers using oil as fuel.
External links
- Article on pulverized coal power at the World Resources InstituteWorld Resources InstituteThe World Resources Institute is an environmental think tank founded in 1982 based in Washington, D.C. in the United States.WRI is an independent, non-partisan and nonprofit organization with a staff of more than 100 scientists, economists, policy experts, business analysts, statistical analysts,...
- University of Stuttgart: Cylcone Furnace