Solid Fuels

1
Solid Fuels

• Properties and Testing of Coal

2
Testing of Coal

• Proximate analysis of coal
• Ultimate Analysis
• Determination of Calorific Value
• Swelling Index

3
Testing of Coal

• Ultimate Analysis
• Determination of Carbon and Hydrogen
• A known amount of coal is burnt in dry oxygen
• C and H are converted into CO2 and H2O
  respectively
• The products of combustion are passed over
  weighed tubes of anhydrous CaCl2 and KOH
• The increase in the weight of CaCl2 tube
  represents the weight of water formed
• The increase in the weight of KOH tube represents
  the weight of CO2 formed

4
Testing of Coal

• Ultimate Analysis
• Determination of Carbon and Hydrogen
• Xweight of coal sample
• Yincrease in the weight of CaCl2 tube
• Zincrease in the weight of KOH tube
• carbon in coal
• Hydrogen in Coal

5
Testing of Coal

• Ultimate Analysis
• Determination of Sulphur
• A known quantity of coal is burnt in bomb
  calorimeter in oxygen
• The residue ash is treated with dilute
  hydrochloric acid
• Acid extract is treated with barium chloride
  solution to precipitate the sulphate as barium
  sulphate
• The precipitate is filtered washed dried and
  weighed
• The age of sulphur is computed from the weight
  of BaSO4

6
Testing of Coal

• Ultimate Analysis
• Determination of Nitrogen
• The KjeldahlGunning macro method is the one most
  widely used for determining nitrogen (ASTM
  D-3179)
• By this method, any nitrogen present in the
  sample is converted into ammonium salts by a
  hot mixture of concentrated sulfuric acid and
  potassium sulfate
• sodium or potassium hydroxide is added to
  alkaline the mixture
• ammonia is expelled which is absorbed into a
  sulfuric acid solution
• The excess acid is titrated with sodium
  hydroxide solution

7
Example

• The analysis of the coal in boiler is
• C 81 , H2 4.5 , O2 8 and remainder is
  incombustible
• The dry flue gas analysis is CO2 8.3 , CO 1.4
  O2 10 N2 80.3
• Determine
• The weight of air supplied per kg of coal
• The percentage of excess air

8
Solution
Volume Mol. weight Proportional weight Analysis by weight Carbon /kg of constituent Weight of Carbon/kg dry flue gas
CO2 CO O2 N2 8.3 1.4 10.0 80.3 44 28 32 28 365.2 39.2 320.2 2249 0.1228 0.0132 0.1076 0.756 12/44 12/28 0.0335 0.00566
Total 100 2973 0.03916
9
Solution
Weight of dry flue gas per kg of coal
0.81/0.03916
20.68 kg Water formed
0.045 x 9 0.405 kg per kg of coal Incombustible
s 1 -0.81 -0.045 -0.08 0.065 kg / kg of
coal Air supplied per kg of coal ? Air Coal
Dry Flue gas Water Incombustibles Air
20.15 kg per kg of coal
Dry Flue Gas 20.68 kg H2O 0.405
kg Incombustibles0.065 kg
Air ? Coal 1 kg
Combustion
Theoretical air 11.6 C 34.8(H2
O2/8)10.6175 age excess air (20.15
10.6175)/10.6175 ?
10
Determination of Calorific Value

• Determination of Calorific Value by Bomb
  Calorimeter
• Already discussed
• If a coal does not have a measured calorific
  value, it is possible to make a
• close estimation of the calorific value (CV) by
  means of various formulas, one of the most
  popular formula is
• Modified Dulong Formula for G.C.V.
• G.C.V.

C, H, O, N and S are the percentages of carbon,
hydrogen, oxygen, nitrogen and sulphur
11
Swelling Index/Number of Coal

• It denotes the caking capacity of Coal. Caking
  power is the ability to form a fused coke when
  coal is heated out of contact with air.
• The crucible swelling index is determined by
  heating 1 gm of coal in a special crucible to 820
  C under standardized conditions for 2.5 minutes
  or until no observable volatile matter is evolved
  
• The profile of the coke produced is compared with
  series of standards

12
Swelling Index/Number of Coal

• No. Less than 2.5 very weak caking properties or
  non-caking. Coal is suitable for stem raising but
  unsuitable for carbonisation
• No. 3 - 3.5 weak to moderate caking power.
  Suitable for all combustion purposes. Marginally
  suitable for carbonisation
• No. 4 - 6.5 coals of moderate caking power.
  These are suitable for combustion but may be
  strongly caking for some forms of mechanical
  stokers. They are suitable for gas-work and
  second grade metallurgical coke

13
Swelling Index/Number of Coal

• No. 7- 9 Strongly caking coals. These are too
  strongly caking to be suitable for combustion.
  They are best for metallurgical cokes