CALORIFIC VALUES

Calorific Values The calorific value of a combustible substance is the heat generated when 1 kg of that substance is completely burned (also known as the heat of combustion or heat value for the fuel).

Calorific Values The NET (or lower) calorific value for a fuel is calculated when the water, H 2 O, in the combustion products is in its vapour form. The GROSS (or higher) calorific value for a fuel is calculated when the water in the combustion products is in its liquid form. The latent heat of vapourisation of water is 2.5 MJ/kg.

Measurement of Calorific Values The calorific value of a fuel is measured in calorimetric tests where the products where the products of combustion are cooled to normal atmospheric conditions. The total heat released in the process of cooling is measured.

Measurement of Calorific Values During this process, water vapour present in the combustion products condenses, releasing its latent heat, and resulting in more heat being liberated than would be available had no condensation occurred.

Measurement of Calorific Values The calorific value indicated by such tests is thus the gross calorific value. The net calorific value is then obtained by subtracting the latent heat of the water present from the gross calorific value.

Atomic Weights and Molecular Masses The atomic weights for the constituent atoms of the fossil fuels and the molecular masses of the elements in fossil fuels are: ATOMIC WEIGHTS H1 C12 S32 O16 N14 MOLECULAR MASSES H 2 2H 2 O18 C12CO 28CO 2 44 S32SO 2 64 O 2 32 N 2 O44 A mole is the molecular mass of a substance expressed in grams.

Calorific Values of Various Common Fuels (MJ/kg) FUEL Formula Molecular Higher Calorific Lower Calorific Weight Value (MJ/kg) Value (MJ/kg) Hydrogen Gas H Methane CH Propane C 3 H Butane C 4 H Fuel Oil Petrol Kerosene CH Residual (heavy) fuel Oil Orimulsion Diesel Oil Carbon C Town Gas 32.0 Coke Anthracite 29.9 Bituminous Coal 26-32

Calorific Values of Other Fuels (MJ/kg) FUEL Formula Molecular Higher Calorific Lower Calorific Weight Value (MJ/kg) Value (MJ/kg) Ethanol (ethyl alcohol, liquid) C 2 H 6 O Municipal refuse22.0 Wood (dry) Wood (20% moisture)16.2 Cattle manure (dry) Cellulose17.1 Carbohydrates16.7 Starch, Cellulose C 6 H 10 O Methanol (Wood Alcohol) CH 3 OH Peat Wood (50% moisture)10.2 Carbon Monoxide CO Sulphur S Wood (80%) moisture) 4.1

Gross and Net Calorific Values The net calorific value is obtained by subtracting the latent heat of the water present from the gross calorific value. E.g. Methane, from combustion calculations later, CH 4 + O 2 = CO 2 + 2H 2 O 16kg32kg44kg2 x 18kg 1kg2kg2.75kg 2.25kg The Latent Heat of Vapourisation of Water is 2.5 MJ/kg So, 2.25 kg have MJ of Latent Heat From the Measured Gross Calorific Value of 55.8 MJ/kg, the Net Calorific Value is = 50.2 MJ/kg

Approximations of of Calorific Values The gross calorific value depends mainly upon the combinations of H, C and O in the fuel. The higher the ratio of H (with its high calorific value) to C (with its lower calorific value), the higher the calorific value of the fuel. The presence of O indicates that some of the H and C have already formed chemical compounds with oxygen and so the H and C tied up in these compounds are not available for combustion, resulting in a lower calorific value.

Approximations of of Calorific Values Element Atomic Weight Higher Calorific Value (MJ/kg) Carbon Hydrogen 1144 To a first approximation, E.g. Methane.Molecular WeightHigher Calorific Value (MJ/kg) CH x x 144 = 57 (56 16 measured) E.g. Kerosene.Molecular WeightHigher Calorific Value (MJ/kg) CH x x 144 = 44 (43 14 measured)