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 coal-fired power stations, providing steam to drive large turbines.
History Prior to the developments leading to the use of pulverized coal, most boilers utilized grate firing 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. Those 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. That 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 decommissioned and renovated in 1987. It is now the site of the Milwaukee Repertory Theatre.
Pulverization of coal Coal pulverizers may be classified by speed, as follows:
Low Speed Medium Speed High Speed
Low speed A ball mill is a low speed pulverizer that consists of a horizontal rotating cylinder, up to three diameters in length, containing a charge of tumbling or cascading steel balls, pebbles, or rods. A tube mill is a revolving cylinder of up to five diameters in length used for fine pulverization of ore, rock, and other such materials; the material, mixed with water, is fed into the chamber from one end, and passes out the other end as a slurry. Both types of mill include liners that protect the cylindrical structure of the mill from wear. Thus the main wear parts in these mills are the balls themselves, and the liners. The balls are simply "consumed" by the wear process and must be re-stocked, whereas the liners must be periodically replaced. The ball and tube crushers are low-speed machines that grind the coal with steel balls in a rotating horizontal cylinder. Due to its shape, it is called a tube mill and due to use of grinding balls for crushing, it is called a ball mill, or both terms as a ball tube mill. These mills are also designated as an example size, BBD-4772:
B – Broyer (Name of inventor). B – Boulet (French word for balls). D – Direct firing. 47 – Diameter of shell (in decimeters) i.e. 4.7 m diameter. 72 – Length of shell (in decimeters) i.e. 7.2 m length. The grinding in the ball and tube mill is produced by the rotating quantity of steel balls by their fall and lift due to tube rotation. The ball charge may occupy one third to half of the total internal volume of the shell. The significant feature incorporated in the BBD mills is its double end operation, each end catering to one elevation of a boiler. The system facilitated entry of raw coal and exit of pulverized fuel from same end simultaneously. This helps in reducing the number of installations per unit.
Mill construction details A ball tube mill may be described as a cylinder made of steel plates having separate heads or trunions attached to the ends with each trunion resting on suitable bearings for supporting the machine. The trunions are hollow to allow for the introduction of discharge of the materials undergoing reduction in size. The mill shell is lined with chilled iron, carbon steel, manganese steel, or high chrome liners attached to shell body with counter sunk bolts. These liners are made in different shapes so that the counter inside surface of the mill is suited for requirement of a particular application. The shells are of three pieces. The intermediate shell connects to the end shells by flange joints and the total length of shell is 7.2 m. The liners are fastened to the inner side of mill shell (cylindrical part) to protect the shell from the impact of the steel balls. There are 600 liners of ten variants in each shell weighing 60.26 tonnes. The original lift value of the liners is 55 mm. and the minimum lift allowed is 20 mm.
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