When considering biomass heat in pence per kilowatt hour (p/kWh), one is talking about the price of the useful energy that is coming OUT of the biomass boiler. When a gas price is quoted, it is the price of the gas going INTO the boiler.

When reviewing a wood heating proposal one needs to have a clear understanding of the relationship between the price paid for fuel and the price paid for the useful energy. This difference is often misinterpreted when comparing biomass with gas as one has to consider the fuels calorific value and the boiler’s efficiency. 

Calorific value of fuel

Gas is bought and sold based on its Gross Calorific Value (GCV), not Net Calorific Value (NCV).  This is ­­explained below.

Natural gas contains hydrogen atoms, and when it is burnt, one of the products of combustion is water (H2O).  In a normal combustion process, this water is produced in the form of vapour (or steam). When working out how much energy the fuel will provide, one has to take account the energy needed to evaporate the water that is formed in the combustion process. Therefore, a fuel’s NCV is the energy released when the fuel is burnt minus the energy needed to evaporate the water that is formed in the combustion process.

A fuel’s GCV is the total energy released when the fuel is burnt, including the energy needed to evaporate the water.  In the case of natural gas, its GCV is about 10% higher than its NCV (i.e. when gas is burnt, about 10% of the total energy it contains just goes into evaporating the water that is a product of its combustion). 

This means that if you measure the fuel going into a boiler in terms of its GCV, you have to take account of the difference between GCV and NCV when working out how much energy will come out of the boiler.

Boiler efficiency

When any fuel is burnt, some of its energy is lost (mostly up the chimney), so not all of its energy content is put to good use.

A worked example:
Converting natural gas to useful energy
1 kg of natural gas (with a GCV of, say, 15 kWh/kg) is burnt in an 85% efficiency boiler.

• You buy the gas at a price of 3 p/kWh (on the basis of its GCV) and have therefore paid 45p for a kg of fuel.
  
  
• The NCV of natural gas is about 10% less than its GCV, therefore GCV of 15 kWh/kg = NCV of 13.5 kWh/kg
  
  
• NCV x 0.85 (85% boiler efficiency on NCV-basis) = 11.475 kWh/kg of useful energy.
  
  
• 45p/kg of fuel ÷ 11.475 kWh/kg = 3.92p/kWh of useful energy.

So, whilst the price for the fuel is 3 p/kWh, the actual price of the useful energy is 3.92 p/kWh.