Engineering Chemistry Multiple Choice Questions on “Efficiency of Combustion”.
1. Which of the following is the correct condition for the efficient combustion of coal?
a) Coal should be of high calorific value
b) Intimate mixing of air with the combustible matter
c) The ignition temperature should be high
d) The volatile matter should be low
Answer: b
Clarification: For efficient combustion, it is essential that the coal and its distillation products are brought into intimate contact with sufficient quantity of air to burn all combustible material. Also, sufficient time should be allowed to complete the combustion process.
2. Which of the following furnace is best suitable for the efficient combustion of coal?
a) Blast furnace
b) Pudding furnace
c) Open hearth furnace
d) Boiler furnace
Answer: d
Clarification: This type of furnaces are used for efficient combustion of coal since these convert the heat into thermal energy which are then used for generation of power. Pudding furnace and open hearth furnaces are types of steelmaking furnaces.
3. Which of the following is calculated by flue gas analysis?
a) Amount of air used in combustion
b) Rate of combustion
c) Time required for combustion
d) Amount of air present after combustion
Answer: a
Clarification: The only practical way for determining the amount of air used in furnace is to analyse the flue gas for the constituents CO2, CO an O2. The control of air is by means of dampers.
4. What happens when carbon monoxide is obtained in flue gas analysis?
a) Combustion is complete
b) Combustion does not take place
c) Combustion is incomplete
d) It increases the rate of combustion.
Answer: c
Clarification: If the analysis shows the presence of free CO, it means that the combustion is incomplete which calls for an intermediate increase in the amount of air used for combustion. Also, a diminution in the rate of supply of coal is required.
5. Which of the following compound other than CO2, CO an O2 can be determined by the flue gas analysis?
a) C2H6
b) C6H6
c) C6H11O22
d) Ethylene
Answer: a
Clarification: Sometimes N2, H2 and hydrocarbons like CH4 and C2H6 are obtained by flue gas analysis of coal. If N2 is obtained after the analysis, the combustion will be incomplete.
6. What happens when too much excess air is supplied to the furnace in the process of combustion of coal?
a) There is a gain in heat in the furnace
b) No change occurs in the furnace
c) There is loss in heat in the furnace
d) The excess air escapes out
Answer: c
Clarification: In ordinary furnaces, 50 to 100 % excess air is generally supplied. If the excess air is supplied above this amount, there is a loss of heat in the furnace. In such cases, the supply of air to the hearth is cut short or the rate of fuel supply is increased.
7. If both CO and O2, both are indicated in the appreciable amount in the flue gas analysis, which type of combustion is seen?
a) Regular and non-uniform combustion
b) Irregular and non-uniform combustion
c) Regular and uniform combustion
d) Irregular and uniform combustion
Answer: b
Clarification: If both CO and O2 are present in an appreciable amount, then the combustion is irregular and non-uniform. That means, in some parts of the furnace, there is an excess of air and in some other parts, the supply of air is insufficient.
8. Short orsat apparatus gives more precise results than long orsat in the analysis of flue gas.
a) True
b) False
Answer: b
Clarification: Long orsat or precision model orsat is used for analysis of quite complicated mixtures and which gives very accurate results in competent hands. The precision orsat assemblies are mounted on substantial stands.
9. At what temperature does the CO is converted to CO2 in orsat apparatus for analysis of flue gas?
a) 280-295 0C
b) 250-265 0C
c) 350-365 0C
d) Less than 100 0C
Answer: a
Clarification: At the temperature range of 280-295 0C, the CO is converted into CO2. Also the hydrogen present is oxidised to water. The diminution in the volume of gas sample indicates the volume of hydrogen.
10. Which of the following apparatus cannot be used for flue gas analysis?
a) Ambler’s apparatus
b) Hemple’s apparatus
c) Able’s apparatus
d) Bunte’s apparatus
Answer: c
Clarification: Able’s apparatus is used for calculation of flash point and fire point of an oil. Flash point is the minimum temperature of oil, at which the vapours of oil produces flash in a burning match stick when brought close to the vapours of oil.