Thanks Kurt, Orvi, stoneware and Yggdrasil. I’m sourcing materials now. Considering a 200L drum instead of 50L; is that too large, or could the larger one handle, say, 5-10 kg of chips at a time? I assume you weld the upper stage to the lower one, or will high temp mortar work?
How far up in the upper chamber should the air be injected?
Several posters mention using 650C heat guns instead of propane fired burners. Any opinions on this? Wouldn’t a heat gun incinerate right away (because of the air) rather than pyrolyse in the lower stage, which is what we’re trying to achieve down there—pyrolysis?
Yeah, it will pyrolyse and incinerate from the start. And there is nothing bad about it, you just need to know it and adjust for it. It depends greatly on what is your end goal. With pyrolysis, you need to burn the outgas anyway... So in my opinion, direct incineration with plenty of air present is beneficial, as you use the combustion heat to get to right temperature. From my experience, you use full throttle on the heatgun just from the start, then you switch to low heat, and then just air for the last stage. Very little external heat required.
And if the carbon isn´t a problem for your further operation, just stop at the point where material stops emmiting gaseous pyrolysis products and just carbonaceous material is left. When you switch down the airflow and heat, it will slowly cool itself and burning of carbon dies down.
With larger drum/vessel, you need bigger heating capacity to onset the pyrolysis and incineration. So 2-3kW heatgun would be probably less efficient. But as the material start to burn (firstly in the place where is the input of the hot air, then it spreads to the whole layer) it will self-suffice the heat required. Unfortunately, for this stage, you need that afterburner going on full throttle from the start of operation, as temperature in main chamber isn´t yet high enough to support clean burning, and lots of unburnt gasses are emitted and need to be burned in upper stage.
As to the air injected to the afterburner, it should be injected as early as possible, because you need longest time possible to achieve full combustion of volatiles to CO2 and water. Air pump for inflating matresses for example is very good for this task, but only if it is paired with some PWM regulator or regulated power supply, so you does not need to run it on full power. Or optionally, you can make some "valve", by sticking the piece of sheet metal inside the tube, where cut is made for it´s insertion. And by adjusting the slit, you can regulate the air volume passed in. These "valves" I also recommend to use in the main drum on the input of hot air from heatgun - much better regulation than just two stages which typical heatgun offers. You observe the whole operation, and if you see smoke from the chimney, you crank the afterburner torch and air to max, and heat from the bottom slightly down to maintain it, and so on and so on
Main chamber and afterburner - i personally taken the lid from the main drum, cut a proper sized hole in it, and insulated the lid from the inner side with wool. To the bottom part of afterburner is cut hole of same diameter, and after insulating it with wool, these two stages are just put one on another, no need for sealing it in my opinion. Hot air want to move upwards and that "chimney effect" will effectively drag the air in from places where sealing isn´t perfect.