We managed to start a small fire here about a month ago; one of my helpers was smoking, and a spark got into the sawdust pile. It was a very slow burn; I had to track down the fire more by smell than anything else. But the burn pattern was interesting; there was a tiny dot on the surface, and a fairly big fire going on down out of sight.
That got me to wondering about burning sawdust for heat, and I found that there’s really only one basic pattern for a sawdust burner, and they’re quite efficient. Essentially, take a cylinder, pack it full of sawdust, and hollow out the centre. This makes a chimney which houses the flame. The longer the chimney, the hotter the burn. This is a 36″ long piece of 8″ snap-together ducting. The bottom cap is an end cap fastened with a couple of sheet metal screws, and it has a 2 1/4″ hole bored in it. I packed the sawdust around a section of PVC pipe, and rammed it down with a 2×4. The top is covered in sand, to prevent an inefficient burn from starting from the top down. I carefully removed the pipe, propped the burner up over two bricks, and dropped tightly furled paper down the chimney, and lit from below. It took several pieces of paper to get it going (feeding the paper up from the bottom was basically impossible).
I made a smaller version of this out of a paint can, but the burn was so slow that there was no flame; it just glowed for hours. These images were taken two hours into the burn. I don’t know how long it’ll last (it’s still going!) but I suspect another four hours, minimum. I was hoping to make an outdoor heater this way, but the sawdust is an excellent insulator, so almost all the heat goes straight up. It’s decorative; I think I’ll make a bunch of these and scatter them around the yard for the next party.