Air
Table of contents:
Fires cannot burn by wood alone - they also need air!
What is the Function of Air?
To many the sole purpose of air is to bring in oxygen, to enable burning. Yes, this is essential, but it also helps distribute heat in the kiln through convection and also by radiation. Whilst the main constituents of air are transparent to radiant heat, both water vapour and carbon dioxide will emit radiant heat - the water vapour comes from the air supply and from water evaporating from the wood, and the carbon dioxide comes from the air supply and the burning of the fuel.
How much air is needed?
In steady state laboratory conditions, we would just need enough air to burn the fuel, and no more. But wood fired kilns are far from steady state. When we add fuel the temperature drops as it heats up to vapourise components. then they catch fire and reduce the oxygen levels, with the heat rising, and then as the wood turns to charcoal and burns in the ember bed oxygen levels increase again. And, of course, each load will vary in weight and possibly composition of the wood.
Looking at industry data for furnaces, gas requires about 15% extra air, and liquids like heating oil require 15-25% excess. They have an oxygen sensor in the firebox to monitor the oxygen level, or measure the oxygen level in the flue gase (where 10 - 13% oxygen is about right for a wood fired kiln) and adjust the air supply to suit. For wood firing, my guess is that the excess is 50% or more.
Inadequate air
When the kiln has inadequate air, the wood burns inefficiently, creating less heat and emitting more carbon monoxide, carbon particles and other pollutants - exacerbated by the low burning temperature.
Excess air
When there is excess air the fire doesn't burn hotter or faster. What happens is there is a temperature drop because the excess air needs to be heated up to kiln temperature, which results in less efficient burning and more pollutants created. Also, the concentrations of H2O and CO2 are reduced, so efficiency drops because less radiant heat is output, especially at higher temperatures where radiant heat is more important. As an example, in a gas fired furnace if the excess air is increased from 15 to 25% the oxygen level increases from 3.0 to 4.6%, but the emissivity (heat output) of the air leaving the firebox drops by 3%, and the flame temperature drops by 93 °C. Overall, this drops the heat output by 5%.