Pulverized Coal Power Plant
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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.
Explosions
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.
Lung and skin problems
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.
Pulverized coal-fired boiler
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
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.
