Calorific value measurement in engineering ppt

Fuel chemistry

1. FUEL Definition, Classification Calorific value of fuels (gross and net), Dulong’s formula ( Numericals ) , Determination of calorific value of fuels using bomb’s calorimeter ( Numericals ) , Determination of calorific value of fuels using Boy’s Gas Calorimeter ( Numericals ),

2. Cracking – Thermal & catalytic cracking, Octane & Cetane numbers with their significance. High & Low temperature carbonization, Manufacture of coke (Otto – Hoffmann oven) Proximate and ultimate analysis of Coal (Numerical) Some solid liquid and gaseous fuels: CNG, LPG, Natural gas, Oil gas, water gas, coal gas , Producer gas Combustion of fuels (Numerical) .

3. Fuel Any combustible substance, chemical or reactant containing carbon as the main constituent which on proper burning produces heat that can be used economically for domestic and industrial purposes and in generation of power. Fuel + O 2 Products + heat

4. Classification : - Primary Fuels :- These include the naturally occurring fuels found freely on earth’s crust. Solid Fuels ( Wood ) Liquid Fuels(Petroleum ) Gaseous Fuels (Natural Gas)

5. Secondary or Derived Fuels:- These are artificially manufactured or are derived from primary fuels. Solid Fuels(Coke, Charcoal ) Liquid Fuels(Gasoline, Diesel) Ga seous Fuels (coal gas, biogas)

6. Characteristics of a good fuel- High calorific value Low moisture content Low noncombustible matter Moderate ignition temperature Easy to handle ,transport and store. Readily available in bulk and at low cost. Environment friendly

7. Calorific Value of Fuel- The amount of heat obtained by the complete combustion of a unit mass of the fuel. Calorific values are of two types – 1. Gross Calorific Value ( GCV ) 2. Net Calorific Value (NCV )

8. GCV or Higher Calorific Value :- It is the total amount of heat generated when a unit mass of fuel is completely burnt and the products of combustion are cooled down to 60 o F, 288 o K or 15 o C. NCV or Lower Calorific Value :- It is the net heat produced when one unit mass of fuel is completely burnt and the products are allowed to escape.

9. NCV= GCV – Latent heat of condensation of water vapor produced H 2 + ½ O 2 H 2 O 2g 18g 1g 9g One Part by weight of Hydrogen gives nine parts by weight of water. The latent heat of steam is 587 cal/g of water vapor produced. NCV = GCV – 9 x H/100 x 587 Or LCV = ( HCV – .09 H x 587 ) cal/gm

10. Units of Calorific Value:- Solid & Liquid Fuels - Calorie – The amount of heat required to increase the temperature of 1 gm of water through 1 C. Calories/gm or Kcal/gm British thermal unit/pound Gaseous Fuel - Kcal/m 3 BThU /Ft 3

11. Determination of Calorific Value:- Bomb Calorimeter - Principle – A known unit mass of the fuel is burnt and the quantity of the heat produced is absorbed in water & measured. Then the quantity of heat produced by burning a unit mass of the fuel is calculated. This apparatus is used to find the calorific value of solid/liquid fuels.

12. Construction -

13. It consist of the following parts - A strong cylindrical Bomb made up of stainless steel (chromium nickel-molybdenum) steel, resistant to corrosion and capable of withstanding high pressure up to 50 atm. It is provided with a lid which can be screwed firmly on the bomb. The lid in turn is provided with two electrodes & an oxygen inlet valve.

14. 2 . A copper calorimeter vessel with a known weight of water in which the bomb stands. 3 . The calorimeter is surrounded by an air jacket and water jacket to prevent the loss of heat due to radiation. 4 . The calorimeter is provided with an electrical stirrer for stirring water & a Beckmann thermometer which can read accurately temperature difference up to .01 of a degree. 5 . The crucible used is made up of Ni, Stainless steel or fused silica.

15. Working - A weight amount of the fuel is placed in the silica crucible supported over the ring. A fine magnesium wire touching the sample of the fuel is stretched across the electrodes. About 10 ml of distilled water is introduced into the bomb to absorb the vapors of sulphuric acid & nitric acid formed during the combustion. Oxygen supply is forced into the bomb to a pressure of 25-30 atm.

16. The bomb is then carefully placed in the calorimetric vessel containing a known amount of water. The stirrer is driven and the initial temperature of water is noted. The electrodes are then connected to a battery circuit is completed. The combustion of the fuel take place with the evolution of heat. The heat produced by burning is transferred to the water which is stirred is recorded. The calorific value of the fuel is calculated as -

17. Calculations - Let x = mass of the fuel sample in the crucible(gm) W = mass of the water in calorimeter (gm) w = water eq. of calorimeter, stirrer, bomb & thermometer T 1 = initial temp. of water in calorimeter T 2 = final temp. of water in calorimeter L = Higher calorific value (GCV) of fuel (cal/gm)

18. Heat liberated by the burning of fuel = x L cal Heat absorbed by water = W x (T 2 -T 1 ) Heat absorbed by apparatus (calorimeter) = w x (T 2 -T 1 ) Hence total heat absorbed/gained by water, apparatus etc = W x (T 2 -T 1 ) + w x (T 2 -T 1 ) cal = [(W + w) (T 2 -T 1 )] cal . Heat liberated by the fuel = Heat gained by the water & calorimeter

19. Therefore X L = [(W + w) (T 2 -T 1 )] L = (W + w) (T 2 -T 1 ) cal/gm X Corrections – Fuse Wire Correction ( t f )– The heat liberated, also include the heat given out by the ignition of fuse wire used. Hence it must be subtracted from the total value.

20. Acid Correction ( t a )- Fuels containing S and N are oxidized, under high pressure and temp. of ignition, to H2SO4 and HNO3 respectively. S + H2 + 2O2 H2SO4 + Heat 2N + H2 + 3O2 2HNO3 + Heat Formation of these acids are exothermic reactions. So, the measured heat also includes the heat given out during the acid formation. The amount of these acids may be measured by the titration.

21. Cooling Correction ( t c ) - Time taken to cool the water in calorimeter from maximum temp to room temperature is noted. From the rate of cooling and the actual time taken for cooling, The cooling correction is added to the rise in temp. Cotton Thread Correction ( t t )– As the burning of cotton thread also generates heat.

22. Thus, Higher/Gross Calorific Value, GCV = (W + w) (T 2 -T 1 + tc ) – ( ta + t f + t t ) cal/gm X Numerical Examples – A sample of coal contains C = 93%, H = 6% & ash = 1%. Wt of coal burnt = 0.92 gm Wt of water taken = 2200 gm Water eq of bomb calorimeter = 550 g Rise in temp. = 2.42 o C Fuse wire correction = 10.0 cal Acid correction = 50 cal GCV & NCV = ?