Solid Fuels Properties and Testing of Coal

Testing of Coal Proximate analysis of coal Ultimate Analysis Determination of Calorific Value Swelling Index

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 CO 2 and H 2 O respectively The products of combustion are passed over weighed tubes of anhydrous CaCl 2 and KOH The increase in the weight of CaCl 2 tube represents the weight of water formed The increase in the weight of KOH tube represents the weight of CO2 formed

Testing of Coal Ultimate Analysis Determination of Carbon and Hydrogen X=weight of coal sample Y=increase in the weight of CaCl2 tube Z=increase in the weight of KOH tube % carbon in coal= %Hydrogen in Coal =

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

Testing of Coal Ultimate Analysis Determination of Nitrogen The Kjeldahl–Gunning 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

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 (a)The weight of air supplied per kg of coal (b)The percentage of excess air

Solution Volume%Mol. weightProportional weight Analysis by weight Carbon /kg of constituent Weight of Carbon/kg dry flue gas CO2 CO O2 N /44 12/ Total

Solution Weight of dry flue gas per kg of coal= 0.81/ =20.68 kg Water formed = x 9 = kg per kg of coal Incombustibles = = kg / kg of coal Air supplied per kg of coal = ? Air + Coal = Dry Flue gas + Water + Incombustibles Air = kg per kg of coal Combustion Air ? + Coal 1 kg Dry Flue Gas =20.68 kg H2O = kg Incombustibles=0.065 kg Theoretical air = 11.6 C (H2 – O2/8)= %age excess air = (20.15 – )/ =?

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

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

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 : weak to moderate caking power. Suitable for all combustion purposes. Marginally suitable for carbonisation No : 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

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