Why is pulverized coal system used in power plants?
What are advantages and disadvantages of pulverized coal system.
Explain two stages of coal pulverization system.
Compare/Difference between pulverized coal firing and stocker firing.
Pulverized Coal System
The first commercial application of pulverized coal firing for steam generation was made in the early 1920s. Since then it has become almost universal in central utility stations using coal as fuel. Coal is first ground to dust like size and powdered coal is then carried in a stream of air to be fed through burners into the furnace. As the entering coal particles get heated in high temperature flames in the furnace, the volatile matter is distilled off and this reduces the coal particles to minute sponge like masses of fixed carbon and ash. The volatile gases mix with the oxygen of the air, get ignited and burn quickly. Oxygen of the hot air reacts with the carbon surface to release energy. The combustion products form a blanket on carbon particles, which is stripped off by turbulent mixing of these particles and air. Proper burning of fuel needs the supply of correct proportion of air, mixing of fuel and air, high temperature, and adequate time to complete combustion reactions.The ash resulting from combustion (i) partly falls to the furnace bottom and (ii) the rest is carried in gas stream as fly ash to flue-gas outlet, or (iii) is deposited on the boiler heating surfaces.
Modern central station boiler furnaces have water-cooled walls that form part of the heat-absorbing surfaces in steam generation.
Primary air: the amount of air which is used to dry the coal and convey the powdered fuel to furnace.
Secondary air: the amount of air which is blown separately to complete the combustion.
To burn pulverized coal successfully, the following two conditions must be satisfied:
1. Large quantities of very fine particles of coal, usually those that would pass a 200-mesh sieve must exist to ensure ready ignition because of their large surface-to-volume ratio.
2. Minimum quantity of coarser particles should be present since these coarser particles cause slagging and reduce combustion efficiency.
Process of coal pulverization in two stages
Stage 1: Raw and lump coal is crushed to a particle size not more than 15-25mm in the crusher.
Stage 2: Crushed coal is delivered into raw coal bunkers and from here it is transferred to grinding mills that grind the feed into final particles of 200-300 mesh size. During grinding hot air is blown through the fuel to dry it to impart good fluidity of the coat dust.
Advantages of pulverized coal firing
(1) Greater surface area per unit mass of coal allows faster combustion reactions because more carbon becomes exposed to heat and oxygen. This reduces the excess air needed to complete combustion.
(2) Less excess air is required for complete combustion because of greater surface area of fuel exposed.
(3) Any type of coal right from anthracite to peat can be successfully burnt.
(4) Better response to variation in load due to better combustion control.
(5) Large amount of heat release makes it very suitable for super thermal power stations where the rate of steam generation is as high as 2000 t/h.
(6) Clinkering and slagging problems are low,
(7) Carry over of unburnt fuel to ash is practically nil.
(8) It has low ash handling problem.
(9) It can operate successfully in combination with gas and oil fired system.
(10) Cold start-up of boilers is very rapid and efficient.
(11) Less furnace volume is required.
(12) It has smokeless operation.
(13) It can use highly preheated air for combustion which helps for rapid flame propagation.
(14) There are no moving parts in the furnace subjected to high temperature, therefore the life of system is more and the operation is trouble less.
Disadvantages of pulverized coal firing
(1) The capital cost is high as it requires many additional equipment compared to stoker firing.
(2) Its operation cost is high compared to stoker firing.
(3) Lot of fly ash in the exhaust which makes the removing of fine dust uneconomical.
(4) Since coal will burn like a gas, there will always be danger of explosions.
(5) Special equipment is required for starting
(6) The maintenance of furnace brick work is costly because of higher working temperature.
(7) The skilled operators are required.
(8) High air pollution due to the emission of very fine particles of grit and dirt
Comparison/difference between pulverized coal firing and stoker firing
| Pulverized coal firing | Stoker firing |
| 1. Coal is to be supplied in powder form to furnace. 2. Powder form of coal burns like a gas 3. It can be used for large capacity more than 100 MW) power plant. 4. Less amount of excess air is required for complete combustion 5. More building space is required 6. The maintenance and operating costs are more. 7. The possibilities of explosion are more. 8. More efficient combustion | 1. Coal is to be supplied in solid form to furnace. 2. Coal burns on grate. 3. It can be used for low to medium capacity power plant. 4. More excess air is required for complete combustion. 5. Less building space is required. 6. The maintenance and operating costs are less. 7. It is free from danger of explosion. 8. Less efficient combustion. |