This set of Energy Engineering Multiple Choice Questions & Answers (MCQs) focuses on “Pulverised Fuel Burning”.
1. Proper regulation of Pulverized fuel results in _________
a) better fuel economy
b) high output work
c) efficient regulation of heat
d) proper combustion
Explanation: The rate of pulverized coal feed can be regulated properly resulting in fuel economy. Since coal is in the powdered form, coal of any grade can be used. Wide grade of low grade fuel can be burnt easily. Practically the coal used does not form any slagging or clinkers.
2. Why is combustion rate faster when the usage of pulverized coal is done in pulverized fuel handling?
a) Due to greater surface area
b) For using fuel in powdered form
c) Decrease in excess air
d) The speed at which the coal is fed
Explanation: The combustion rate is faster due to greater surface area of coal per unit mass of coal. It means more coal surface is exposed to heat and oxygen. This decreases excess air required for complete combustion and also decreases fan power. There is an increased rate of evaporation and higher boiler efficiency due to the complete combustion of fuel.
3. What is the advantage of breaking coal into smaller pieces in pulverized fuel handling?
a) Easy coal feeding
b) No clogging coal since they are in powder form
c) Exposes more surface of the coal
d) Increase in combustion rate
Explanation: By breaking, a given mass of coal into smaller pieces exposes more surface area for combustion. The increase of surface area exposed per unit volume with the decreasing diameter of coal particle. As shown in graph below, this increase varies rapidly after 0.01 inch diameter of the coal particle and there is no substantial increase after 0.001 inch diameter.
4. What is the main aim of Automatic control combustion in pulverized fuel firing?
a) To regulate the powder coal
b) To draw in air required for combustion
c) To maintain constant steam pressure
d) To draw out the ash formed
Explanation: Automatic control combustion applied to pulverized fuel fired boilers is effective in maintaining an almost constant steam pressure under wide load variations. It also gives fast response to load changes as rate of combustion can be controlled easily and immediately.
5. How is that using the pulverized system causes less trouble?
a) Rate of combustion is high
b) The operation is very quick and easy
c) No moving part in furnace
d) By using pulverized fuel system is totally free from slagging or clinker formation
Explanation: There are no moving parts in the furnace, subjected to high temperature therefore the life of the system is more and the operation is trouble free. The boilers can be started from cold very rapidly and effectively. The external heating surface is free from corrosion and fouling as smokeless combustion is possible.
6. Usage of highly preheated secondary air in pulverized fuel firing helps in ______
a) Heats fuel at pace
b) Takes less time to finish the process
c) Rapid flame propagation
d) Reduces the troubles and problems caused in the system
Explanation: Using highly preheated secondary air helps in raid flame propagation. This also increases the ability of system to use high combustion air temperature. The furnace makes it possible to complete combustion with minimum travel of flame travel since it is pre-heated.
7. In which type of machine is the coal reduced to fine powders?
Explanation: Coal is reduced to fine powder in grinding mill. Coal is pulverized to increase its surface exposure. The pulverization of coal means of exposing a large surface area of the coal to action of oxygen and consequently accelerates combustion.
8. Where are pulverized fuel firers used?
a) Large capacity plants
b) Small capacity plants
c) Medium capacity Plants
d) Domestic plants
Explanation: Pulverized fuel firers are used in large capacity plants. The conventional or stoker firing methods were unable to meet the variable loads on the plants and were unsuitable for large capacity plants. Using pulverized fuel firer gives higher thermal efficiency, better control as per load on the plant and uses low grade coal.
9. With what is pulverized fuel mixed to burn like gas or oil?
b) Crude oil
Explanation: In the pulverized fuel firing system the coal is reduced to fine powder state by using a grinding mill and then projected in the combustion chamber with the help of hot air current. The coal in powder state may be mixed with crude oil and may burn like gas or oil.
10. Which air is supplied separately to ensure complete combustion in pulverized fuel firing system?
a) Primary air
b) Turbulence air
c) Secondary air
d) Blower air
Explanation: The amount of secondary air required to complete the combustion is blown separately and resulting turbulence in the high pressure combustion chamber helps in through mixing of fuel and air. For better combustion result this type of mixing is done.
11. What decides the efficiency of pulverized fuel firing system?
a) Turbulence of air
b) Size of coal powder
c) Primary air
Explanation: The size coal powder decides the efficiency of the pulverized fuel firing system. The fineness of the coal should be such as 70%. Of it would pass through a 200 mesh sieve and 90% through 50 mesh sieve.
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