Boilers for steam generation in power plants and process industries use coal as fuel. The percentage of boilers operating with coal as fuel outnumbers the boilers using all other fuels combined. Coal is pulverized before firing for achieving a stable and efficient combustion. Many types of pulverizers are used in boilers by different designers.
History of pulverization
The history of pulverization dates back as early as 1824 and was envisioned by Carnot in a coal fired engine. In 1890 Diesel made use of pulverized coal in his diesel engine. Pulverized coal firing was first developed in the cement industry and then migrated to the power and process industries. Actually Thomas Alva Edison and the Niepce brothers of France were pioneers in pulverized coal firing. This technology gained momentum after World War I in the power generating industry. It was John Anderson, chief engineer of power plants at the Wisconsin Electric Power Company who introduced pulverized coal firing in power stations.
Pulverized coal is the most efficient way of using coal in a steam generator. The coal is ground so that about 70 % will pass through 200 mesh (0.075 mm) and 99 % will pass through 50 mesh (0.300 mm). A pulverized coal boiler can be easily adapted for other fuels like gas if required later without much difficulty. However, during the design stage it is possible to make boilers firing multiple fuels. With pulverization technology, large size boilers could be designed, manufactured, erected, and run much more efficiently.
Types of pulverizers
Mainly there are three types of pulverizer used in industry: the slow speed mills like ball tube mills, the medium speed mills like bowl, ball and race, roller mills fall in this category, and the third type is the high speed impact mill. The slow speed and medium speed mills are selected for coals ranging from sub-bituminous to anthracite. The high speed mills are used mainly for lignite.
The purpose of a pulverizer in a coal fired boiler
- To supply pulverized coal to the boiler as per requirement of steam generation
- Transport the pulverized coal from pulverizer to the burners in the boiler
- To remove moisture in coal to an acceptable level for firing in boiler
- To remove high density inorganics from coal during pulverization
- To classify coal particles to the required level of fineness, normally 70 % through 200 mesh and less than 2% on 50 mesh
Coal parameters affecting pulverizer output
While selecting a pulverizer, the coal characteristics play an important role. The Hardgrove index, total moisture, input coal size, output fineness, and mill wear have direct impact on the mill output.
- The Hardgrove index of coal tells us about the ease with which it can be pulverized. A higher Hardgrove index indicates the coal is easier to grind. 50 HGI normally is taken for calculating the base capacity of the mill. When coal with HGI higher than 50 is fed to the pulverizer, the output will be higher than base capacity, and below 50 HGI, the output will be lower.
- The total moisture in coal has a high effect on mill output. The higher the moisture, the lower the output.
- Higher pulverized coal fineness increases the recirculation in the mill and the output reduces.
- The inlet size of the coal also affects the mill output directly.
- Mill air flow variations result in changes in mill outlet temperature and fineness as well as capacity.