Anna University Long Questions and Answers

Unit - IV

Chapter 6

Combustion of Fuels

Anna University Long Questions & Answers

 Part - B

1. Explain: Gross and Net calorific value. (A.U. Dec 06)

(a) Higher (or) Gross calorific value (GCV)

It is defined as the total amount of heat produced, when a unit quantity of the fuel is completely burnt and the products of combustion are cooled to room temperature.

When a fuel containing hydrogen is burnt, the hydrogen is converted into steam. If the combustion products are cooled to room temperature, the steam gets condensed into water and latent heat is evolved. Thus, the latent heat of condensation of steam is also included in gross calorific value.

(b) Lower (or) Net Calorific Value (NCV)

It is defined as the net heat produced, when a unit quantity of the fuel is completely burnt and the products of combustion are allowed to escape.

ஃ NCV = GCV - Latent heat of condensation of water vapour produced.

= GCV - Mass of hydrogen × 9 × Latent heat of condensation of water vapour.

1 part by weight of H₂ produces 9 parts by weight of H₂O as follows. The latent heat of steam is 587 cal/gm.

Thus,

NCV = GCV – (9/100) H × 587

= GCV - 0.09H × 587

where

H = % of H₂ in the fuel.

 

2. Define gross and net calorific values. Calculate gross and net calorific values of a coal sample containing 84% carbon, 1.5% sulphur, 6% nitrogen, 5.5% hydrogen and 8.4% oxygen. (A.U May 2015)

(i) GCV & NCV

Refer Annexure Q.No.1; Page 61

(ii) Problem

Solution

(i) Gross calorific value  (GCV)

(ü) Net Calorific Value (NCV)

 

3. Calculate the gross and net calorific value of coal having the following compositions.

Carbon - 85%, hydrogen - 8%, sulphur - 1%, nitrogen - 2% and ash - 4% (A.U. May 2007)

Solution

 (i) Gross calorific value (GCV)

= 1 / 100 [8080 × % C + 34500 (% H - %O / 8) + 2240 ×  % S] kcal/kg

= 1 / 100 [8080 × 85 + 34500 (8 – 0/8 ) + 2240 × 1] kcal/kg

= 1/ 100  [6,86,800 + 2,76,000 + 2240 ] kcal/kg

= 1 / 100 [ 9,65,040 ] kcal/kg

= 9,650.4 kcal/kg.

(ii) Net Calorific Value (NCV)

=  GCV – (9 / 100) H × 587 kcal/kg

= 9650.4 – (9 / 100)  × 8 × 587 kcal/kg

= 9650.4 – 422.64

= 9227.76 kcal/kg.

 

4. With a neat diagram explain the analysis of flue gas by Orsat apparatus and mention the precautions to be followed during the analysis. (Chen A.U. Dec 2009 July 2016)

(or)

Explain flue gas analysis by ORSAT method. Give suitable diagram.  (A.U. June 2014, Dec 2014, May 2017)

Description of orsat's apparatus

It consists of a horizontal tube. At one end of this tube, U-tube containing fused CaCl₂ is connected through 3-way stop cock. The other end of this tube is connected with a graduated burette. The burette is surrounded by a water-jacket to keep the temperature of gas constant. The lower end of the burette is connected to a water reservoir by means of a rubber tube. The level of water in the burette can be raised or lowered by raising or lowering the reservoir (fig ) The horizontal tube is also connected with three different absorption bulbs 1, 2, and 3 for absorbing CO₂, O₂ and CO.

Working

The 3-way stop-cock is opened to the atmosphere and the reservoir is raised, till the burette is completely filled with water and air is excluded from the burette. The 3-way stop-cock is now connected to the flue gas supply and the flue gas is sucked into the burette and the volume of flue gas is adjused to 100 cc by raising and lowering the reservoir. Then the 3-way stop cock is closed.

(a) Absorption of CO₂

The stopper of the absorption bulb-1, containing KOH solution, is opened and all the gas is passed into the bulb-1 by raising the level of water in the burette. The gas enters into the bulb-1, where CO₂ present in the flue gas is absorbed by KOH.

Fig. Orsat's apparatus

The gas is again sent to the burette. This process is repeated several times to ensure complete absorption of CO₂, The decrease in volume of the flue gas in the burette indicates the volume of CO₂ in 100 cc of the flue gas.

(b) Absorption of O₂

Stop-cock of bulb-1 is closed and stop cock of bulb-2 is opened. The gas is again sent into the absorption bulb-2, where O₂ present in the flue gas is absorbed by alkaline pyrogallol. The decrease in volume of the flue gas in the burette indicates the volume of O₂.

(c) Absorption of Co

Now stop-cock of bulb-2 is closed and stop-cock of bulb-3 is opened. The remaining gas is sent into the absorption bulb-3, where CO present in the flue gas is absorbed by ammoniacal cuprous chloride. The decrease in volume of the flue gas in the burette indicates the volume of Co. The remaining gas in the burette after the absorption of CO₂,  O₂,& CO is taken as nitrogen.

Precautions

1. Care must be taken in such a way that, the reagents in the absorption bulb 1, 2 and 3 should be brought to the etched marked level one by one by raising and lowering reservoir bottle.

2. All the air from the reservoir bottle is expelled to atmosphere by lifting the reservoir bottle.

3. It is essential that CO₂, O₂ and CO are absorbed in that order only

4. As the CO content in flue gas is very small, it should be measured quite carefully.

 

5. Write notes on (i) IT & SIT (ii) Explosive range (i)

(a) Ignition Temperature (IT)

It is defined as, "the lowest temperature to which the fuel must be heated, so that it starts burning smoothly".

Ignition temperature of coal is about 300°C. In the case of liquid fuels, the ignition temperature is called the flash point, which ranges from 200 – 450°C. For gaseous fuels, the ignition temperature is in the order of 800°C.

(b) Spontaneous Ignition Temperature (SIT)

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

If the ignition temperature of a fuel is low it can catch fire very quickly. On the other hand if the ignition temperature is high it is difficult to ignite the fuel. If the heat evolved in a system is unable to escape, temperature of the system goes on increasing and when SIT is reached, the system burns on its own.

(ii) Explosive Range (or) Limits of Inflammability

All gaseous fuels have two limits called upper limit and lower limit. These limits represents percentage by volume of fuel present in fuel-air mixture.

1. Lower limit represents the smallest proportion of combustible gas (fuel).

2. Upper limit represents the largest proportion of combustible gas.

The range covered by these limits is termed as explosive range of the fuels. For continuous burning the amount of fuel present in the fuel-air mixture should not go below the lower limit or above the upper limit.

Example

The explosive range of petrol is 2-4.5. This means that when the concentration of petrol vapour in petrol-air mixture is between 2 and 4.5 by volume, the mixture will burn on ignition. When the concentration of petrol vapour in petrol-air mixture is below 2% (lower limit) or above 4.5% (upper limit) by volume, the mixture will not burn on ignition.

Thus, explosive range (or) explosive limit is the limiting composition of a gas-air mixture beyond which the mixture will not ignite and continue to burn is called explosive range (or) explosive limit.

 

6. What is carbon footprint? Mention any 5 important sources? How to lower carbon footprint?

Definition

It is the total amount of green house gases (including CO₂ and CH₄) that are generated (emitted) by our direct and indirect activities.

Individual carbon footprint

It is the sum total of their direct and indirect carbon emissions over the course of a year.

i.e., Smaller your carbon footprint : better for the future

Bigger your carbon footprint : Have bigger negative impact in environment

The average carbon footprint for a person in united state is 16 tons. Globally, the average is closer to 4 tones. To avoid 2°C rise in global temperatures, the average global carbon footprint per year needs to drop under 2 tons by 2050.

Fig. Carbon Footprint

1. Sources of carbon footprint

(i) Climate change.

(ii) Natural process like volcanos.

(iii) Green house gases emitted from human activities.

(iv) Pollution released by human beings doing human things.

(v) Transportation accounted for about 28% of total country.

(vi) Electricity generation accounted for about 28%

(vii) Industrial activities 22%

(viii) Heating and cooling in homes and businesses contribute 11%

How to lower carbon footprint

Lowering individual carbon footprint from 16 tons to 2 tons does not happen over night. But, by making small changes in our action we can reduce carbon footprint.

Once you understand where your emission comes from, you can take steps to reduce your impact.

1. Calculate your carbon footprint.

2. Drive less.

3. Switch to an electric (or) hybrid car.

4. Travel smart. 5. Switch to renewable energy.

6. Consider solar panels.

7. Make your home more efficient.

8. Turn your thermostat just 2 degrees cooler in winter and 2 degrees warmer in summer.

9. Get energy efficient appliances.

10. Unplug electrical devices when not in use.

11. Buy locally - sourced food.

12. Start a home garden.

13. Eat less meat.

14. Don't waste water.

15. Reduce, reuse and recycle.