Well it's far too long since I wrote here, but I / we have finally cracked the 'free fuel' with sawdust burner. We have been pondering this for over a year and this winter with thought and experimenting we have enough good results to share this knowledge.
If you have a top loading log burner or wood fired cooker you can use dry sawdust (not shavings) and get a good hot fire that lasts between 2 - 24 hours depending on the size of container, packing, and dryness of the dust.
Basically you line the bottom of your burner with tinfoil (aluminium foil) and make a single hole in the centre, about 1.5 inches diameter.
Then place a wick from the burn chamber down to the ash bin underneath where you will light the sawdust. We have found that a strip of cotton cloth about 4 inches long x 1 inch wide works well.
Then place a tube vertically over the hole with the wick up the bottom of the tube. The tube should be about 1.5 inches wide - you can use wider - it increases the burn rate. The tube can be plastic, metal or cardboard - you take it out after packing.
Then start putting in the sawdust - about 3 to 4 inches, then pack it down firmly with something sturdy - a log in our case.
Add more dust and pack again, continue like this until your burner is about 2/3 full. The more you pack the sawdust the longer it burns.
Then gently and slowly pull the centre tube out of the dust. This leaves a central chimney which is where the burn will start.
Light the wick from the ash box and that's it.
The burner gets very hot and will smoulder for a long time.
We did this burn for 3 hours, then added logs to the middle to get our oven hot enough to bake in.
After 5 hours from lighting there is still sawdust around the edges and corners of the burner and in 3 hours it raised the temperature in the room by 3 degrees.
Shaving logs to come next and outdoor camping or BBQ burners.