6  Boiler

Adapted from (Paoli and Cullen 2020)

Burners are conversion devices that transform the chemical energy of fuel into useful heat. Hot combustion products are used to increase the temperature of water or to generate steam. A boiler is defined as the combination of the burner and heat exchanger producing hot water or steam as output.

6.1 Characterisation

Boilers are used in many different applications, for heating systems, and to supply hot water. Two main categories are:

  • Residential Boilers are normally below 35 kW rated capacity, and they can be fired with all types of fossil fuels.

  • Commercial Boilers are used in commercial buildings such as office or retail space. Their nominal capacity is usually between 35 kW and 500 kW. They can be bought off the shelf from boiler manufacturers or be custom built.

6.2 Key issues that affect efficiency

  • The amount of excess air determines the combustion efficiency: a certain amount of air is needed for complete combustion of the fuel, but too much air takes energy to heat up which is wasted.

  • The heat exchanger effectiveness determines the temperature loss between the combustion gasses and the water flow temperature. There is no technical limit to how small this loss can be made, by increasing the size of the heat exchanger, but there are practical cost and space limitations.

  • The water inlet temperature determines how much heat can be extracted from the combustion gasses. Lower temperatures allow greater efficiency.

  • Heat loss from the boiler (i.e. heat from combustion which does not end up in the output water) reduces the overall efficiency.

6.3 Efficiency limits

According to Paoli and Cullen (2020), the current efficiency and efficiency limits for boilers are:

Buildings Industrial
Current \(\eta_D\) 80–93% 70–90%
BAT (Best Available Technology) 93% 90%
TEL (Technological Efficiency Limit) 93–101% 90–101%

There are different efficiencies for boilers used in buildings (with lower water flow temperatures) and industrial boilers used to produce higher-temperature steam.

6.4 Final Energy used

Boiler use is broken down into 4 categories in the final energy data. To match with the two types of boilers discussed above, each of which has a different potential efficiency improvement, the 4 energy categories have been combined as follows in the data below:

  • Use in buildings is the sum of Hot Water and Space Heat energy
  • Use in industrial processes is Process Heat-Boiler
  • Mechanical use has been neglected, as has direct use of primary heat energy (these are both small categories)

This table shows the quantity of final energy \(F\) used in the UK in one year:

    Energy [PJ/year]
End use type Fuel  
Buildings Coal 50.4
Gas 1472.4
Liquid fuel 171.7
Solid biomass & Waste 71.3
Industrial Coal 32.4
Gas 201.6
Liquid fuel 82.3
Solid biomass & Waste 10.5