Pulverized Coal System
- The pulverized coal system is used now a days for big power plants of capacity 100 MW or more.
- The coal is pulverized in order to increase its surface area, thus, promotes the rapid combustion (like burning of gas) without the need of supplying much of excess air, consequently, it results into high thermal efficiency.
- For burning the pulverized coal successfully in a furnace, it requires that the fineness of coal should be such that 70% of it would pass through 200 mesh (0.074 mm) sieve and 98% of it would pass through 50 mesh (10.297 mm) sieve in order to promote ignition and obtain high combustion efficiency.
- The disadvantage of pulverized coal system is that it is costly compared to stoker firing. Also, it needs special equipment for removal of fine dust like electrostatic precipitator, ir requires more space and it adds to air pollution.
- The pulverized coal is obtained by grinding the raw coal in pulverisers called pulverising mills.
- The various types of mills used are ball mill, hammer mill, ball and race mill, bowl mill etc.
- Every type of mill has the means of reducing the size of coal e.g. in case of ball mill the coal is crushed in a drum having steel balls.
- As the drum rotates driven by a motor, the coal gets Pulverized due to combined impact of coal and balls.
- The system has a classifier to regulate fineness of coal and finally, the coal is supplied to burner by exhaust fan.
- The elements of a pulverized coal system are shown in Figure used for each boiler unit.
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| Figure A |
- The working of the system is as follows :
- Large crushed coal is supplied to the main hooper. After weighing, it drops into the service hooper from where it is supplied to pulverizing mill by the feeder. Hot air is supplied into the mill. Due to supply of hot air, the milling of coal into fine particles and its drying takes place in the pulverizing mill.
- The pulverized air-fuel mixture is transferred to separator where large dust particles are separated and returned back to the mill. Fine dust particles from separator passes into cyclone where the dust particles are separated from air.
- The fine dust particles from cyclone falls into dust bin from where it is supplied to burner along with the hot air drawn by fan.
- Secondary hot air is also supplied to the burner through air ducts as shown in Figure. The fuel burns in presence of air in the furnace at a very high combustion efficiency.