For use in thermal power
plants, coal is ground into dust using a device called a powdered coal
mill. The resulting product, calledpowdered coal or pulverized
coal, is then generally used in a fossil fuel power plant for electricity
generation. Pulverised coal is a significant dust explosion hazard, as large
quantities are suspended in air for transfer from the mill to the power plant.
Explosions have occurred when the flow drops and flames in the burning chamber
pass back along the ductwork delivering fuel.
Pulverized coal power plants
first appeared in the 1920s and serve as the backbone of the power sector in
the U.S. Pulverized coal remains a relatively simple technology, converting a
little more than one-third of the fuel’s energy potential into useful
electricity.
Pulverized coal power generation starts by crushing coal into
a fine powder that is fed into a boiler where it is burned to create heat. The
heat produces steam that is used to spin one or more turbines to generate
electricity. Subcritical plants make up the bulk of the U.S. pulverized coal
system, with efficiencies for new plants usually around 37 percent. Supercritical
plants use higher pressure and temperatures to boost efficiency to 40 percent
or more. Ultra-supercritical, using still higher pressures, achieves 42-45
percent efficiency.
Pulverized coal power plants have gained renewed interest
this decade due to surging natural gas prices. The levelized cost
of electricity generated at most pulverized coal plants in the U.S. is
currently less than that of natural gas combined-cycle plants. While the
capital costs of gas-fired combined-cycle units are only half that of
pulverized coal, the fuel costs are much higher and often unstable. Utilities
pay special attention to predictability of prices and currently shy away from
high and unstable natural gas prices. Moreover, as domestic production of
natural gas is limited, further additions to gas-fired power generation would
require more imported liquefied natural gas, and exacerbate energy security
concerns.
Coal dust suspended in air is explosive -- coal dust has far
more surface area per unit weight than chunks of coal, and is more susceptible
to spontaneous combustion. As a result, a nearly empty coal store is a greater
explosion risk than a full one. The worst mining accidents in history have been
caused by coal dust explosions, such as the disaster at Senghenydd in South Wales in 1913 in which 439 miners
died, the Courrières mine disaster in
Northern France which killed 1099 miners, the Luisenthal Mine
disaster in Germany, which claimed 299 lives in 1962, and the worst: the
explosion at Benxihu Colliery, China, which
killed 1549 in 1942. Such accidents were usually initiated by firedamp
ignitions, the shock wave of which raised dust from the floor of the mine
galleries to make an explosive mixture. The problem was investigated by Michael
Faraday and Charles Lyell in the explosion at the colliery at Haswell County
Durham of 1844, but their conclusions were ignored at the time.
The main attempts at prevention include using safety lamps,
adding stone dust coffers to mine galleries, watering workings and ensuring
efficient ventilation of all the workings.
Coal workers' pneumoconiosis, or black lung disease, is
caused by inhaling coal dust, typically dust produced in coal mining.
According to George Orwell, coal miners could develop
characteristic tattoos owing to coal dust getting into wounds. This can also
occur with substances like gunpowder.
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). 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.
The Milwaukee Electric Railway and Light Company, later
Wisconsin Electric, conducted tests in the use of pulverized coal at its Oneida
Street power plant in 1918. These experiments helped Fred L. Dornbrook to develop methods of controlling the
pulverized coal's tarry ash residues with boiler fed 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.
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 with
subcritical plants operating at the lowest temp and pressure ranges,
ultra-supercritical plants are at the highest ranges, and supercritical plants
in the middle. 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.
Pulverised fuel firing is a solid fuel burning technique
in which the fuel is pulverised before being ignited. It is the most common
method of burning coal and oil shale for power generation. 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 There are
two methods of ash removal at furnace bottom:
● Dry bottom boiler
● Wet bottom boiler, also called Slag tap
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.