There's no real need for a grill at the bottom unless you intend to burn out the char in bottom lit updraught mode, in which case you will up the temperature everything has to withstand from the 600C of the tlud to >1100C (steel burns out quickly above 700C)
This depends entirely on the amount of primary air you let in and the mass of material yo stuff in the tube. Basically you "liberate" about
2.5kWhrs of heat and produce some char for every 1kg of dry wood.With the lazy flame I showed most of the heat is lost in convection and you only benefit slightly from the radiant heat from the tube above the pyrolysis front and flame. You'd need to add a "bluff body" of some sort to get heated up and radiate this down. At the same time premixing the secondary air will stabilise the flame.
I service a pellet boiler that has been installed in a solar thermal system who's control system has needed a lot of snagging. Each time something goes wrong I have to clean out the boiler. As the silo hatch is 3 floors up I give them the opportunity of carrying the pellets I empty out of the boiler back up or I offer to dispose of them ;-).
There are half a dozen places making them, 3 major ones (Bridgend, Chester and Durham) and the firm I sub to imports them from France and Latvia.
Wow two non PC thoughts in one sentence!
It's simply a tin can, pierced at the bottom, with a wooden dowel stood vertically in the middle. The section around this packed and pounded with dry sawdust. Then withdraw the dowel and light the top.
These tlud things are very sensitive to moisture. Above 25% and they simply refuse to burn because the heat feedback to the fuel below is low compared with a conventional updraught fire. The interesting thing is that at the moisture content rises from 0-25% the char decreases from 25% of the dry mass to zero.
The other main criteria is that bits of burning debris should not be able to fall to the bottom of the fuel bed, else it simply turns into an updraught fire. I just started a simple one using some wood from a broken softwood wardrobe, pictures posted at
one for Martyn (cork stuck so for demo purposes only), and this is exactly what happed as I had not packed it well enough. You can see how rudimentary primary air control can be. Also note the secondary air path.
AJH