Combustion of Fuels – Questions and Answers

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1. Explain the term combustion with example.

Ans. Combustion is an exothermal chemical reaction which is accompanied by development of heat and light at a rapid rate so that temperature rises considerably.

Eg. Combustion of carbon and oxygen.

 C(s) + O2(g)  →  CO2(g) + 97KCal.

Combustion of Fuels - Questions and Answers


2. Distinguish between gross and net calorific values.

S. No Gross Calorific value Net Calorific value
(i) It is also called higher calorific value. It is known as lower calorific values.
(ii) It is the total amount of heat produced, when one unit of the fuel has been burnt completely and the products of combustion have been cooled to room temperature. It is the net heat produced when unit mass/volume of the fuel is burnt completely and the products are permitted to escape.
(iii) The latent heat of condensation of stream also included in the measured heat. It is the measure of the difference between high calorific value and latent heat of water vapour formed.

3. What is meant by spontaneous ignition temperature ?

Ans. It is defined as the minimum temperature at which the fuel catches fire (ignites) spontaneously without external heating.

4. What are the factors, influence the rate of combustion?

Ans. The rate of combustion depends on the following factors,
(i) Nature of the combustible substance (fuel).
(ii) Temperature.
(iii) Concentration of the fuel and air.

5. Define calorific intensity.

Ans. It is the maximum temperature reached when the fuel is completely burnt in the theoretical amount of air.

6. What are the uses of flue gas Analysis ?

Ans. Flue gas analysis gives an idea about the complete or incomplete combustion process,

(i) If the flue gases contain considerable amount of CO, it indicates that incomplete combustion.

(ii) If the flue gases contain considerable amount of O2 , it indicates that complete combustion.


7. Name the important units of calorific values.

Ans.
(i) Calorie
(ii) Kilocalories
(iii) British Thermal Unit (B.T.U)
(iv) Centigrade Heat Unit (C.H.U)

8. What is meant by explosive range of a fuel?

Ans. For every combustible gaseous mixture there exists an upper limit and lower limit corresponding to largest and smallest proportion of combustible gas which mixed with air will undergo combustion. This range is called explosive range of fuel.


9. The lower and upper explosive limits of ethane are 3 and 15 respectively. What does this mean ?

Ans. This is called explosive range or the limits of inflammability of a gaseous fuel. For continuous burning, the amount of gas present in  the fuel air mixture should not go below the lower limit or above the upper limit.


10. What are the functions of three bulbs kept in orsat’s gas apparatus ?

Ans. The first bulb contains KOH solution to absorb CO2 only. The second bulb contains a solution of alkaline pyrogallol to absorb  CO2 and  O2. The third bulb contains ammoniacal cuprous chloride to absorb CO2, O2 and CO.

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