Since we started looking at fire and stoves, the question of what combination of the “Three T’s” results in ‘complete combustion’ has remained unanswered. Looking into the literature is always rewarding (We don’t go there enough!) and has recently shed some light on the situation. In the past, test results from Tom Reed’s forced draft TLUD seemed to indicate that thorough mixing at high temperatures without much volume for mixing can achieve really low emissions of CO and PM 2.5. The combustion zone is thin!

Bomana (2005)* finds the same order of importance in a study of pellet stove combustion. When woodgas and flame were well mixed, temperatures were above 850C, for at least 0.5 seconds, products of incomplete combustion can disappear. No more smoke or carbon monoxide.

“The results in the present study have shown…a total depletion of products of incomplete combustion by high temperature, intensive, air rich and well mixed conditions in the bed section. During such conditions the importance of residence time at high temperature was shown to be limited, although isothermal conditions during 0.5-1.0 s would be preferable for optimized results in practical situations.”

Unfortunately, the needed amount of mixing caused by turbulence for complete combustion is not yet quantified. ARC is working to remedy this situation and we are reading the literature looking for math! Dr. Nordica MacCarty, at Oregon State University, is directing progress on CFD modelling that should eventually get us closer to predicting the functions of the “Three T’s”.

*Effects of Temperature and Residence Time on Emission Characteristics during Fixed Bed Combustion of Conifer Stem-wood Pellets Christoffer Bomana*, Esbjörn Petterssona,b, Fredrik Lindmarka , Marcus Öhmana , Anders Nordina , Roger Westerholmc  2005