Anna University 2 Marks Questions and Answers

5. Fuels

 

1. Write the characteristics of a good fuel. (A.U.T Coim July 2010)

1. It should be cheap and readily available.

2. It should be safe and economical for storage and transport.

3. It should not undergo spontaneous combustion.

4. It should have higher calorific value.

5. It should have moderate ignition temperature.

6. The combustion should be easily controllable.

 

2. What is coalification (or) metamorphism of coal.

The process of conversion (or alteration) of vegetable matter to anthracite is called coalification or metamorphism of coal.

 

3. What are the different varieties of coal? How do you rank them in the order of increasing calorific value? (A.U. (Chem-II) Dec 2006)

(or) How coals are classified. (A.U. Jan. 2018)

Coal is classified on the basis of its rank. The rank of coal indicates its degree of maturity. Various types of coal are:

Calorific values increases from left to right.

 

4. Name the various determinations of proximate analysis.

It involves the determination of percentage of

(i) Moisture content.

(ii) Volatile matter.

(iii) Ash content.

(iv) Fixed carbon in coal.

 

5. What is the significance of volatile matter in coal.

High percentage of volatile matter is undesirable because

(i) it reduces the calorific value of coal,

(ii) large proportion of fuel on heating will distill over as vapour, which escapes out unburnt,

(iii) coal with high percentage of volatile matter burns with a long flame with high smoke,

(iv) the coal containing high percentage of volatile matter do not coke well.

 

6. What is the importance of kjeldahl's method.

The determination of nitrogen content is carried out by Kjeldahl's method.

 

7. Mention the significance of nitrogen content in coal.

(i) Nitrogen does not have any calorific value, and its presence in coal is undesirable.

(ii) Good quality coal should have very little nitrogen content.

 

8. What is meant by the term “Fixed carbon”. (B.D.U. - Nov. 95)

It is the pure non-volatile, carbon content present in the coal. Higher the percentage of fixed carbon greater is its calorific value.

 

9. Distinguish between proximate and ultimate analysis. (Coim A.U. July 2009, A.U May 2017)

Proximate analysis

 (i) It involves the It involves the determinations of physical constituents like moisture, volatile, ash and fixed carbon contents in coal.

(ii) It gives the approximate composition of the main constituents of coal.

Ultimate analysis

(i) It involves the determination of chemical constituents like carbon, hydrogen, nitrogen and sulphur and oxygen contents in coal.

(ii) It gives the exact composition of the elementary constituents of coal.

 

10. What is carbonisation of coal.

(or)

What is metallurgical coke?

When bituminous coal is heated strongly in the absence of air, the volatile matter escapes out and the mass becomes hard, strong, porous and coherent which is called Metallurgical Coke. This process is called carbonisation.

 

11. What are the desirable characteristics of metallurgical coke. (A.U. June 2013)(Coim A.U.T July 2010)

(i) Purity: The moisture, ash, sulphur contents in metallurgical coke should be low.

(ii) Porosity: Coke should be highly porous.

(iii) Strength: It should have high mechanical strength.

(iv) Calorific Value: The calorific value of coke should be high.

 

12. How is coke superior to coal? (or) Distinguish between coal and coke. (Chen A.U.T. June 2010)(Coim A.U.T May 2011)

(i) Percentage of fixed carbon and hence the calorific value of coke is high.

(ii) Percentage of moisture, volatile and ash contents are higher in coal, where as they are low in coke.

 

13. What is the difference between caking coals and coking coals. (A.U. Dec 2012)

When coals are heated strongly, the mass becomes soft, plastic and fuses to give a coherent mass. Such type of coals are called Caking Coals. But if the mass so produced is hard, porous and strong then the coals are called Coking Coals. Coking coals possess lower volatile matter and are used for the manufacture of metallurgical coke.

 

14. What is the drawback of presence of sulphur in the coal? (Chen A.U. June 2009, May 2015)

(i) The combustion products of sulphur, i.e., SO₂ and SO₃ are harmful and have corrosion effects on equipments.

(ii) The coal containing sulphur is not suitable for the preparation of metallurgical coke as it affects the properties of the metal.

 

15. What are the advantages of Otto-Hoffman's process.

1. Valuable by products like ammonia, coal gas, naphthalene etc., are recovered.

2. The carbonisation time is less.

3. Heating is done externally by producer gas.

 

16. Why is coke used in metallurgical process than coal? (A.U. Dec 2007)

(or)

Why coke is prefered to coal for metallurgical purpose? (TCY A.U. July, Dec 2009)(A.U.T Madurai)

(i) Percentage of fixed carbon in coke is more.

(ii) The moisture, ash, sulphur contents are very low.

(iii) The mechanical strength, calorific value is high.

 

17. What is meant by refining of petroleum?

The process of removing impurities and separating the crude oil into various fractions having different boiling points is called refining of petroleum.

 

18. What is Cottrell's process in crude oil refining.

(TCY A.U. Dec, July 2009) The crude oil from oil well is an extremely stable emulsion of oil and salt water. The crude oil is allowed to flow between two highly charged electrodes, where colloidal water droplets combine to form large drops, which is then separated out from the oil.

 

19. What do you mean by synthetic petrol? Give the suitable reaction. (A.U.T Coim Dec 2011)

The conversion of solid coal into liquid fuel by heating it with hydrogen to higher temperature under high pressure is called synthetic petrol.

 

20. What are the important by-products recovered from Otto - Hoffman's by product oven.

(i) Tar (ii) Ammonia (iii) Naphthalene (iv) Benzene (v) H₂S.

 

21. Explain the chemical structure and knocking

The knocking tendency of fuel hydrocarbons mainly depends on their chemical structures. The knocking tendency decreases in the following order.

Straight chain paraffins > Branched chain paraffins > Cycloparaffins > Olefins > Aromatics.

Thus olefins of the same carbon-chain length possess better anti-knock properties than the corresponding paraffins.

 

22. How to improve the antiknock properties of gasoline (or) octane number.

The octane number of fuel can be improved by

(i) blending petrol of high octane number with petrol of low octane number, so that the octane number of the latter can be improved.

(ii) the addition of anti-knock agents like Tetra-Ethyl Lead (TEL).

(iii) now a days aromatic phosphates are used as antiknock agent because it avoids lead pollution.

 

23. Select the compound which possesses highest octanenumber and highest cetane number out of n-heptane, n-hexadecane, n-octane and iso octane.

(i) Highest octane number = Iso octane

(ii) Highest cetane number = n-hexadecane.

 

24. What is the cause of knocking in internal combustion engine? Name a commonly used antiknock compound. (A.U. June 2006)

(or) Explain the term “knocking' of a petrol engine. (Chen A.U. Dec 2009)(A.U. Dec 2005, June 2013)

Knocking is a kind of explosion due to rapid pressure rise occuring in petrol engine.

This can be reduced by adding TEL.

Example: Tetra ethyl lead.

 

25. Define octane number (or) Octane rating of a petrol? How can it be improved? (Chen A.U. May 2008, Coim. A.U.T. 2010, May 2011)

Octane number is defined as “the percentage of iso-octane present in a mixture of iso-octane and n-heptane."

Octane number can be improved by adding anti-knock agents like Tetra Ethyl Lead (TEL).

 

26. Why should leaded petrol not to be used? (Mds - Apr. 2001)

(i) Lead deposits on the spark plug and on cylinder walls, which is harmful to engine life.

(ii) This creates atmospheric pollution.

 

27. Define cetane number (or) cetane rating of a diesel oil? How can it be improved? (Chen A.U. July 2009)(TCY A.U.T. June 2010)

Cetane number is defined as, “the percentage of cetane present in a mixture of cetane and a-methyl naphthalene.”

The cetane number of a diesel oil can be improved by adding dopes like Ethyl nitrate.

 

28. Define cetane number. Mention the cetane number of two hydrocarbons. (A.U.T Chen June 2011)

Thus the cetane number is defined as the percentage of hexa decane present in a mixture of hexa decane and ɑ-methylnaphthalene, which has the same ignition lag as the fuel under test.

 

29. What is leaded petrol? Give its significance in automobiles. (A.U. (chem - II) Dec 2006)

When the petrol is mixed with Tetra - Ethyl Lead (TEL), it is called leaded petrol.

Significance: Knocking in petrol engine is minimised.

 

30. How will you improve the anti-knocking characteristics of diesel. (A.U. (chem - II) Dec 2006)

Antiknocking characteristics of diesel can be improved by increasing n-cetane (hexa decane) value of the fuel.

Cetane value can be increased by adding dopes like ethyl nitrate, iso-amyl nitrate.

 

31. What is meant by hydrogenation of coal? (Bharathiar - Apr. 97)

If coal is heated with hydrogen to high temperature under high pressure, it is converted to gasoline. The preparation of liquid fuels from solid coal is called hydrogenation of coal.

 

32. What is power alcohol? Mention the advantage. (AU. June 2014)

When ethyl alcohol is blended with petrol at a concentration of 5-10%, it is called power alcohol.

1. It is cheaper than petrol.

2. If any moisture is present, power alcohol absorbs it.

3. As ethyl alcohol contains oxygen atoms, complete combustion occurs, so emission of CO, hydrocarbon, particulates are reduced.