This set of Energy Engineering Multiple Choice Questions & Answers (MCQs) focuses on “Principle of Stoker”.
1. How is the coal fed to the grate in over feed system?
a) Above the point of air admission
b) Between the beds of incandescent coke and green coal
c) Above the ash bed
d) Between the point of air admission
Explanation: The coal is fed on the grate above the point of air admission. The pressurized air coming from forced draught fan enters under the bottom of the grate. The air passing through the grate is heated by absorbing the heat from the ash and grates itself. Due to this the ash and grate are cooled.
2. The amount of CO2 formed in overfeed system entirely depends on ___________
a) Amount of ash produced
b) Carbon content in fuel
c) Rate of air supply
d) Intensity of process
Explanation: The rate of carbon-dioxide entirely depends on the rate of air supply. When the hot air after cooling the ash passes to incandescent coke bed, here the O2 reacts with C to form CO2. Generally for fuel bed of 800mm depth all the O2 in the air is consumed in the incandescent region.
3. Which is the zone where the water vapors carried in air reacts with carbon in over-feed system?
a) Green coal bed zone
b) Incandescent zone
c) Combustion zone
d) At the ash pit
Explanation: The water vapor carried with air reacts with Carbon in incandescent zone and forms CO, CO2, and H2. Part of CO2 formed reacts with air C passing through incandescent zone and converts into CO. The gases leaving the incandescent region of fuel bed consist of N2, CO2, CO, H2 and H2O.
4. How is volatile matter of raw coal is lost in the overfeed system?
Explanation: Raw coal that is continuously supplied on the surface of the bed loses it volatile matter by distillation in this zone. The required for the distillation of coal is given by incandescent coke below the fresh fuel, hot gases diffusing through the surface of the bed and hot gases and flame in the furnace above.
5. Where does the ignition zone lie in overfeed system?
a) Below the raw fuel undergoing distillation
b) Below the combustion zone
c) In between coke bed and flame zone
d) Between coke bed and green coal bed
Explanation: The ignition zone lies directly below the raw fuel undergoing distillation. The gases leaving the upper surface of the fuel bed contain combustible volatile matter formed from the raw fuel N2, CO2, O2, H2O and CO if the burning is incomplete.
6. Why is secondary air supplied in overfeed system?
a) To burn the unburnt gases
b) To maintain constant heat intensity
c) To completely remove water vapors formed due to primary air supply
d) To create distillation process for the raw coal supplied
Explanation: Additional secondary air is supplied at the top of the bed to burn the remaining combustible gases (volatile matter+CO+H2). The secondary air is supplied at very high speed to create turbulence for complete combustion of unburnt gases.
7. Why does burnt gases with content of N2, CO2, H2, O2 and H2O re-enter the boiler in overfeed system?
a) To dismiss the ash content in the gases formed
b) Due to incomplete burning
c) For complete removal of moisture
d) To complete the burn the coal
Explanation: The burnt gases re-enter the boiler containing N2, CO2, O2, H2O and CO if the burning is incomplete. During incandescence, the fuel is continuously loses its carbon by oxidation until only the ash remains.
8. In which direction does fuel, coke and ash in fuel bed move with respect to air and gases in overfeed system?
a) Upward direction
b) Same direction
c) Opposite direction
d) Downward direction
Explanation: Fuel, coke and ash in the fuel bed move in the direction opposite to that of air and gases. The primary air entering from the bottom cools the ash until it finally rests in a plane immediately adjacent to the grate.
9. In which direction does fuel and air in under-feed system move?
c) Same direction
d) Counter direction
Explanation: In under feed type of stokers, the fuel and air move in the same direction. In this stoker air after passing through the holes in the grate comes in contact with the raw coal. As air diffuses through the bed of raw coal, it mixes with the volatile matter generated by the raw coal.
10. Which is the process from which heat for distillation comes, in under feed system?
c) Synthesis reaction
d) Circulation of hot air gases
Explanation: The heat for the distillation comes by conduction from the mass of incandescent fuel bed existing above the raw coal. The air mixes with the volatile matter and passes through the ignition zone and then enters into the region of incandescent coke.
11. Which reaction differs over feed and under feed system?
a) Changes occurring in physical properties of the system
b) Breaking of molecular structure
c) Changes in chemical composition
d) Existence of volatile matter
Explanation: The reactions which take place in the incandescent zone of under-feed stoker is exactly same as in the incandescent zone of over feed stoker except some breaking of molecular structure of volatile matter. Part of the broken volatile matter reacts with the oxygen of air.
12. What is the purpose of secondary fan’s air in under feed system?
a) To support gases coming out of fuel bed
b) To clear the ash from the upper beds
c) To maintain constant pressure in system
d) To support combustion of system
Explanation: The gases coming out of raw fuel bed pass through a region of incandescent ash on the surface of the fuel and finally discharged to the furnace. As the gases coming out of fuel bed also contain combustible matter the supply of secondary air is required in this case. The ash left at the bottom of the stoker is at a higher temperature than the over feed stoker.
13. What is the significance of rate at which air is supplied in the under feed system?
a) Increase depth of incandescent zone
b) Pushes up ignition zone
c) Burns fuel for long time
d) Supports combustion
Explanation: The rate of air supply has marked effect on the speed of ignition in case of under feed burning as the rising air cools the raw fuel prior to ignition. At low rate of air fuel, the heat flow by conduction from the incandescent fuel bed downward is faster than the removal of heat by air. This pushes the ignition plane down below the bed surface and creates a maximum depth of incandescent zone.
14. Which type of coal is used in the under feed system?
a) Anthracite coal
b) Bituminous coal
c) Briquette coal
d) Pulverized coal
Explanation: The under feed method uses bituminous coals which have high volatile matter. The volatile matter gets heated to a high temperature as it passes through incandescent region of coal. The volatile matter being at a high temperature before entering the furnace burns quickly when mixed with secondary air.
15. Why does over feed burning system takes longer time for complete burning?
a) The volatile matter will be cooler at initial
b) Due to the presence of dust and ash
c) The effect of design of system
d) Effects of air fuels on the system
Explanation: In case of over-feed burning, the volatile matter will be considerably cooler than the furnace gases and therefore it requires longer time for complete burning and also the air passes though incandescent zone and then enters ignition zone. This may create a tendency to form smoke.
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