[Stoves] Excess air and combustion efficiency

[Crispin at newdawn.sz]

Dear Friends

I have been crunching some numbers from the testing of a new stove in Senegal and a very interesting fact comes to light.

The stove is basically a Rocket stove in that it has a combustion chamber with wood fed in from the side, air from underneath and no grate.

Without going into details about the other features of the design, I found the following:

When the excess air ratio (the air that is not needed to burn the wood) is reduced, the CO/CO2 ratio drops.  That is, the combustion efficiency improves and the emisson of unburned CO drops in concert with a drop in excess air.  

Clearly there is a limit to this because if I kept choking off the air, the fire would starve and the CO would rise.  During all the readings the EA was 200% or more and that limit was not reached.  As the EA increased, the CO/CO2 ratio increased continuously, i.e. more and more of the CO was left unburned.

This indicates something fundamental about how to plan the air supply in a Rocket stove.  In this case, the air is preheated and that may make a difference, in that the preheating might increase when the excess air was reduced, which again would show something of fundamental importance concerning combustion in a Rocket stove.  

At the moment I have no figures on the temperature of the preheated air, however it does remind me that I had CO/CO2 ratios of less than 2% in a large Lion Stove when it was running at full power.  The number of data points was very small.

I have selected those emission readings where the stove was boiling (not simmering) and where the EA was less than 1000%, which gave me 23 readings.  The correlation is strong and apparently linear.  Maybe someone who is good at math can tell us how good the correlation really is.

I will post the chart with the data sorted for EA in a new article that will appear momentarily on the stove.  

Best regards
Crispin