A Boiler is a closed vessel in which steam is generated (from water to steam) with the help of coal-burning or other fuel substances. It is used in thermal power plants for steam generation and further, the steam is used for rotating the turbine for the generation of electricity.
Pulverized Coal-fired Boiler
A pulverized coal-fired boiler is an industrial boiler that generates thermal energy by burning pulverized coal.
How does a steam boiler work?
In a pulverized coal-fired boiler, The coal is pulverized to a fine powder, so that less than 2 % is +300 micro-meter and 70 – 75 % is below 75 microns. The pulverized coal is blown with part of the combustion air into the boiler plant through a series of burner nozzles. Combustion takes place at temperatures from 1300 – 1700 °C, depending largely on coal grade.
The flue gas formed in the furnace passes over the superheater, economizer, air-preheater, and ESP, and finally is evacuated by the induced draft fan into the chimney.
The ash from the combustion chamber, which falls is collected in the bottom ash hoppers. The ash which is fine & flies with the flue gas is separated by ESP and collected in ESP hoppers.
The Feed water enters the boiler through the economizer tubes provided in the path of the flue gas. The feed water is heated in the economizer and then enters the boiler drum situated outside the furnace at the top of the Boiler.
The water is circulated in the tubes and converted into steam by gaining heat inside the furnace. The dry and saturated steam from the boiler drum then passes through the superheater section and is finally available at the boiler outlet header.
Advantages of Pulverized Coal-Fired Boiler
- Ability to burn all ranks of coal.
- It permits combination firing (i.e. can use coal, oil, and gas in the same burner)
Disadvantage of Pulverized Coal-Fired Boiler
- The high power demand for pulverizing.
- Require more maintenance.
- Fly ash erosion
- Pollution complicates unit operation
Coming to classification there are multiple types of boilers but mainly it has been classified into two types:
- Fire Tube Boiler
- Water Tube Boiler
Fire tube boiler: A fire-tube boiler is a boiler that has fire (high-temperature exhaust gas) inside the tubes and is surrounded by water.
The type of boiler is called fire tube boiler” because the hot flue gas flows through the tubes and the water surrounds the tubes.
There are various types of fire tube boilers and here is the following list:
- Cochran Boiler
- Cornish Boiler
- Locomotive Boiler
- Velcon Boiler
- Lancashire Boiler
- Simple Vertical and
- Scotch Marine Boiler.
Water-tube boiler: A water-tube boiler is a boiler in which water is contained in the tubes and fire (high-temperature exhaust gas) surrounds the water.
The type of boiler is called water tube boiler because the water flows through the tubes and the hot exhaust gas surrounds the tubes.
- There are various types of water tube boilers. Some of them are the following:
- Benson Boiler
- Lamont Boiler
- Babcock and Wilcox Boiler
- Loeffler boiler and
- Yarrow boiler.
Differences between Fire Tube and Water Tube Boilers:
| Sr No. | Fire Tube Boiler | Water Tube Boiler |
| 1 | The Hot flue gases flow inside the tube and water has surrounded the tube. | Whereas In the water tube boiler, The water flow inside the tube, and the hot flue gases have surrounded the tube. |
| 2 | This is heavy in weight. | This is light in weight. |
| 3 | This is also called an internal fire tube boiler and | This one is called an Externally Fire-tube boiler. |
| 4 | Here the pressure is limited to only 20 bar but | In the water tube boiler, the pressure is around 100 bar and more. |
| 5 | The rate of Steam generation is Lower. Why? because the pressure is limited to only 20 bar. | The rate of Steam generation is High. Why? because the pressure is limited to 100 bar and more. |
| 6 | The explosion chances are less in a fire tube boiler because of low pressure. | Here explosion chances are more because of high pressure. |
| 7 | The overall efficiency is around 75 percent. | In the water tube, the overall efficiency is around 90 percent. |
| 8 | Efficiency is less compared to the water tube. | Efficiency is more. |
| 9 | Load fluctuations cannot be handled. | Load fluctuations can be handled easily. |
| 10 | The direction of water circulation is not well defined in this boiler. | The direction of water circulation in a water tube boiler is well-defined. |
| A definite path is provided for the circulation of water. | ||
| 11 | This is not suitable for a large Power Plant. It is used in the process industry. | But It is suitable for Large Power plants. |
| 12 | It is simple in design. | It is complex in design. |
| 13 | A less skilled operator can work on this boiler. | But here it requires a more skilled operator. |
| 14 | This is having a low maintenance cost. | But the Water tube boiler has High maintenance costs. |
| 15 | Fire Tube Boiler example: | Water Tube boiler example: |
| Cochran Boiler | Benson Boiler | |
| Cornish Boiler | Lamont Boiler | |
| Locomotive Boiler | Babcock and Wilcox Boiler | |
| Velcon Boiler | Lamont Boiler | |
| Simple Vertical and | Loeffler boiler and | |
| Scotch Marine Boiler. | Yarrow boiler. | |
| 16 | A fire-tube boiler requires more floor area for a given output. | The water-tube boiler requires less floor area for a given output. |
| 17 | The fire-tube boiler is bulky and difficult to transport. But | These (Water Tube Boilers) are light in weight, hence transportation is not a problem. |