Wednesday, February 22 was another mild day, but not as warm, with significant clouds early in the day. As we started our burn, clouds cleared and blue sky dominated.
We learned from earlier burns how critical air supply is to a successful, clean burn. Multiple signs, beginning with excess smoke, indicated our burner was choking for air. We had a 4-inch air intake and an 8-inch exhaust chimney with a thermal booster. Do the math.
We need to increase the air entering the bottom of the barrel to feed the biomass fire. We also need more air in the chimney to burn the woodgas in the chimney, rather than in and outside the cap. We also need to control and vary the air intake, because demand for air changes in the course of a burn.
|VIDEO: Last Gasp
1 min 32 sec; 3.4 Mbyte, .mov
|Burn #8 Begins
1 min 32 sec; 3.4 Mbyte, .mov
|Full Flame at Peak
x min y sec; z.z Mbyte, .mov
So, one important upgrade to our equipment was to build a solid, level base of cinderblocks and brick for the barrel to sit on. The next TLUD will have a metal ring to sit in and clamp onto. All air vents, gas pipes and other accessories are built into this fixed docking ring. Barrels of biomass are rolled on and off this permanent base.
But the major upgrade to our burner was to install three 4-inch vent pipes and elbows to guide air into the bottom of the barrel. The tubes are buried in soil under the barrel, facing downwind and downhill for protection, and soil is pulled up all around the barrel bottom (see photo below).
|New Triple Air Intakes
4-inch vent pipes
This simple device effectively triples the air volume entering the barrel, and should curtail smoke, boost temperature, and improve a burn's even symmetry. Most crucial, these three tubes give us near complete control over air supply, allowing us to quickly suffocate a burn, or alter air flow in increments. Or we can install small electric blowers to push air into the barrel and accelerate a burn.
The extra air vents sharply improve performance of the primary biomass fire in the barrel. Fire ignited faster, burned more vigorously and updraft was stronger. Smoke was noticably less. The brief wisps of smoke I saw are due to wind, or to inadequate air-gas mixing in the chimney.
We have further equipment upgrades underway to address two problems: wind, and air-gas mixing. The essential solution is to turbo-charge the chimney air intakes by inducing a vortex motion of the air-gas mix. But we already release enough energy in the chimney. We must design a second firebox to contain, control and harness the gas flare.
Burning biomass is easy, simple and physical. But a 55-gallon burn releases a powerful thermal genie from the barrel. While the biomass smolders downward through the barrel, a huge volume of BTUs spiral up the chimney as dense "smoke" of combustible hydrocarbons. Most of the energy is rushing up the chimney. Taming this flaming gas dragon is the true challenge to TLUD combustion engineering.
|Red-hot Charcoal Fire
glows in the barrel bottom
Clearly, we must design another combustion chamber to insert in the chimney. Too little air is entering the chimney to burn more than a tiny fraction of the gas in the stovepipe. Most of the gas is burning when it exits the chimney inside the cap, because then it has enough air to truly, fully burn. We need a second burn box to contain the immense energy of the gas flare. Then we can harness that energy usefully.
|The Vanishing Brush Pile
going, going, nearly half gone......