This set of Energy Engineering Interview Questions and Answers for Experienced people focuses on “Pulverised Mills – 2”.
1. In which mill is the pulverization of coal is made by the impact?
a) Hammer mill
b) Bowl mill
c) Ball mill
d) Ball and race mill
Explanation: In the hammer mill the pulverization of coal is made by impact and this is also called as impact mill. The coal in pulverizer remains in suspension during the entire pulverizing process. All the grinding elements and the primary air fan are mounted on a single shaft.
2. What is done in the primary stage of the grinding in impact mill?
a) Coal is reduced to fine granular state
b) Coal is broken down in medium pieces
c) Coal is dried up to remove the moisture content
d) Coal is mixed up with the other state of fuel to improve the combustion efficiency
Explanation: The primary air fan induces flow of air through the pulverizer carrying the coal to the primary stage of grinding. In the primary stage of grinding, the coal is reduced to fine granular state by impact with a series of hammers and then into the final stage where pulverization is completed by attrition.
3. What is the purpose of rotary scoop shaped rejecter arms in impact mill?
a) To send back the coal back into grinder for second run
b) To throw the large particles back into the grinder
c) To eliminate ash from the pulverizer
d) To eliminate rocks and solid particles that has mixed up with the coal
Explanation: The finely pulverized coal in the final stage of grinding is carried with the air to the burner through the rotating scoop rejecter arms which throw the large particles back into the grinding section. The final stage grinding consists of pegs carried on a rotating disk and travelling between stationary pegs.
4. How is pulverization done in bowl mill?
a) By bowl and steel balls
b) By bowl and roller
c) By bowl and impact hammer
d) By bowl and batan
Explanation: The bowl mill consists of rollers and power driven bowl in which pulverization takes place as the coal passes in between the sides of the bowl and the rollers. The automatic control charges the supply of coal to the bowl.
5. What does the primary air that is induced by draught does in a bowl mill?
a) Blows away ash
b) Control the flow of coal
c) Provides air for the combustion
d) Draws heated air
Explanation: The primary air by induced draught fan draws a stream of heated air through the mill, carrying the pulverized coal into a stationary classifier located at the top of pulveriser. The classifier returns the coarse particles of coal to the desired fineness is carried to the burner through the fan.
6. Which force is used to throw away the heavy particles in the bowl mill?
a) Gravity force
b) Centrifugal force
c) Centripetal force
d) Spring force
Explanation: The coal pulverized is carried to the burner through the fan. The impurities in the coal containing heavy particles are thrown over side by centrifugal force as these enter the rotating bowl/ the heavy particles thrown out fall into the space below the bowl.
7. On what basis is selection of bowl mill is not done?
a) Total moisture content of coal
b) Volatile matter
c) Ash producing capacity
Explanation: The selection of bowl mill is done on the basis of total moisture content of coal, sulfur and mineral content (used to determine pyrite removal sizing requirement), Grindability(used to select mill size), volatile matter (used to determine the required coal fineness).
8. What is not the cause of mill fire?
a) Hot startup with normal load
b) Pyrite building from pyrite plows failure
c) Accumulation of coal dust in mill devices
d) Localized hot spots from poor primary air flow
Explanation: Mill fires are more often caused due to pyrite building, hot startup on heavy load, accumulation of coal dust etc. the cause of fire is also dependent upon coal properties that exceed pulverizing rating.
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