The conventional fuel firing method (stoke firing) were found to be unable to take the fluctuations in the loads on the plant due to limited capacity of combustion. Further, these conventional methods employed were found to be unsuitable for large capacity (generating plant capacity more than 100MW) plants and coal containing high percentage of ash due to the difficulties experienced in removing large quantities of ash and interference of the formed ash in the combustion process. The pulverized fuel system are nowadays are universally used for large capacity plants and using low cost (low grade) fuel as it gives high thermal efficiency and better control as per the load demand.
In the pulverized fuel firing system, the coal is reduced to a fine powder with the help of grinding mills and projected into combustion chamber with the help of hot air current. The amount of air required to complete the combustion is supplied separately to the combustion chamber. The resulting turbulence in the combustion chamber helps in the uniform mixing of fuel (coal) and air and through combustion. The amount of air which is used to carry the coal and to dry it before entering into combustion chamber is known as “primary air” and the amount of air which is supplied separately for complete the combustion is known as “secondary air”. The amount of primary air may vary from 10% to almost the entire combustion air requirement as per the type of pulverizer and load on it.
The efficiency of the pulverized fuel firing system mostly depends on the size of the powder. The fineness of the coal should be such as 70% of it should pass through 200 mesh sieve and 98% through 50 mesh sieve.
Many modern thermal power plants use pulverized fuel system when the availability of the coal is cheap and is not suitable for stoke firing. The advantages and disadvantages of pulverized fuel over stoke firing system is given below:
Advantages of Pulverized Coal Firing:
- The main advantage of pulverized firing system lies in the fact that by breaking a given mass of coal into smaller pieces exposes more surface area for combustion.
- Greater surface area of coal per unit mass of the coal allows faster combustion as more coal surface is exposed to heat and oxygen. This reduces the excess air required to ensure complete combustion and the required fan power also
- Wide variety and low grade coal can be burnt more easily when the coal is pulverized
- Pulverized coal gives faster response to load changes as the rate of combustion can be controlled easily and immediately. Automatic control applied to pulverized coal fired boilers is effective in maintaining an almost constant steam pressure under wide load variations
- This system is free from clinker and slagging troubles
- This system works successfully with or in combination with the gas and oil
- It is possible to use highly pre-heated secondary air (350oC) which helps in rapid flame propagation
- The pulverized system can be repaired easily without cooling the system as the pulverizing equipment is located outside the furnace
- Large amount of heat release is possible in this system compared to stoke firing system
- The banking losses are low compared to stoke firing system
- The boiler can be started from cold very rapidly and efficiently. This is highly important when grid stability is of the important concern
- The external heating surface is free from corrosion and fouling as smokeless combustion is possible
- There are no moving parts in the furnace or boiler subjected to high temperature. Therefore the life of the pulverized fuel firing system is more and operation is trouble-less
- Practically no ash handling problem in this type of firing system
- The furnace volume required is considerably less as the use of the burners which produce turbulence in the furnace makes it possible to complete combustion with minimum travel
Disadvantages of Pulverized Coal firing:
- The capital cost of the pulverized coal firing system is considerably high as it requires many additional auxiliary equipment. Its operation cost is also high compared to stoke firing system
- The system produces fly ash (fine dust) which requires special and costly fly-ash removal equipments as electrostatic precipitators
- The flame temperatures are high and the conventional types of refractory lined furnaces are not inadequate. It is always necessary to provide water cooled walls for the safety of the furnace. The maintenance cost is also high as working temperature is high which causes rapid deterioration of the refractory surface of the furnace
- The possibility of the explosion is more as coal burn like gas
- The storage of powdered coal requires special attention and high protection from the fire hazards