Calorific value

The term ‘calorific value’ (CV) is a measure of heating power, and refers to the amount of energy released when a fuel is completely combusted under specific conditions. For solid and liquid fuels this is measured at constant volume and for gaseous fuels it is measured at constant pressure.

Calorific vales for a range of fuels are published by the UK government.

Typically, calorific vales are expressed in Megajoules per cubic metre (MJ/m3) for solid and liquid fuels or Megajoules per kilogram (MJ/kg) for gaseous fuels.

Calorific value can be expressed as a Net Calorific Value (NCV, or Lower Heating Value), or a Gross Calorific Value (GCV, a Higher Heating Value):

• The Net Calorific Value considers that the combustion products contain water of combustion to the vapour state, and so the heat energy in the water is not recovered.
• The Gross Calorific Value considers that the water of combustion is condensed, and so the heat energy in the water is recovered.

NB Embodied Carbon, The Inventory of Carbon and Energy (ICE), By Prof. Geoffrey Hammond and Craig Jones, Ed. Fiona Lowrie and Peter Tse, published by BSRIA in 2011, defines the Calorific Value (CV) of energy as: ‘The energy content of a fuel (as may be released through combustion). It may be expressed as a gross calorific value (GCV) or net calorific value (NCV). The former is always larger than (or equal to) the latter. The difference is due to latent heat (energy) remaining in condensation (water vapour) after combustion. The difference is typically 5-10 per cent (e.g. 10 per cent for natural gas, 5 per cent for coal).’

Combined heat and power quality assurance (CHPQA) guidance notes, published by the Department for Business, Energy & Industrial Strategy in 2014, suggests that the: ‘Gross Calorific Value (GCV) of a fuel is the total energy available from that fuel (solid, liquid or gas) when it is completely burnt. It is expressed as heat per unit weight or volume of fuel. ‘Gross’ signifies that the water formed or liberated during combustion is condensed to the liquid phase. The GCV of a solid or liquid fuel is determined at constant volume and the GCV of a gaseous fuel is determined at constant pressure.’

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