Combustion Air

Primary and Secondary Combustion Air and Combustion Gases

Combustion Air is the air drawn through the firebox by the draughting system which allows combustion to take place. Only the oxygen content of the air (approx 18%) is used in the combustion process, the remainder (mostly nitrogen) being inert and serving no function other than wasting energy and cooling the fire.

Combustion Air comes in two forms:

  • Primary Combustion Air which is drawn upwards through the ashpan, grate and firebed, and
  • Secondary Combustion Air which is drawn in over the top of the fire – e.g. through the firehole door.

Essentially, the primary air releases heat from the fuel and the secondary combustion air releases heat from volatile gases released from the hot coal in the firebed. The chemical reactions that release heat from the fuel are sometimes complex, but the end-product is a combination of carbon dioxide gas and water vapour mixed with small quantities of carbon monoxide and oxides of other impurities (plus large volumes of nitrogen). These reaction products are termed Combustion Gases.

The quantity of combustion gases released from the firebox can be much larger than the volume of steam exhausted from the cylinders. For instance at the 5AT’s maximum designed drawbar power (1800 kW at 113 km/h), Wardale calculated that the mass of combustion gas ejected through the chimney would be 2.2 times the mass of steam passing through the blast nozzles.

In normal steam locomotive operation, the bulk of combustion air is drawn through the ashpan and grate, thus being classified as “primary air”. Secondary air is normally only available when the firehole door is opened, usually for firing and occasionally for more extended periods to burn off volatile material and thus reduce smoke emissions.

The Gas Producer Combustion System (GPCS), as described elsewhere on this website, involves a continuous flow of secondary air, usually through ducts passing through the outer and inner firebox sides and crown, and corresponding reduction in primary air.

To reduce temperature drop in the firebox, it is possible to preheat the air before it is mixed with the fuel, however the first time this advance was ever implemented in practice was by David Wardale in the development of a modernized design of the QJ 2-10-2 freight locomotive. Wardale never complete the final design for this machine, but one locomotive – QJ No 8001 – was modified by being fitted with an experimental heat exchanger for the heating combustion air. The photo below of the modified machine has been copied from Hugh Odom’s Ultimate Steam website.

To be continued ….