Of all the things I have gotten wrong while experimenting with gasification, fuel preparation was probably the worst and gave me the most trouble. I initially wanted to create a machine that was indifferent to the fuel that it was fed. Was I ever naïve!!! I intentionally ran huge nozzle circle diameters that I thought would accept just about anything especially small branches. What I ended making were machines that had their combustion zones end too soon and the tars could go right down the center of the machine and out. They also had huge pyrolysis zones that made more tar than I could combust. Some times it was so bad that it looked like I was trying to repave my driveway. I have actually seen flaming tar balls with streamers shooting out of my flare! If you are reading this hopefully you will not have to make the same mistakes. Here are some basic findings to keep in mind:
1.) Remove the biggest twigs. Twigs act like rebar in concrete. They keep the pile together and prevent good fuel flow. Try to remove the big stuff.
This is the smallest fuel I would consider running. It was created with a Wallenstein BX-40 3 point PTO tractor chipper. I would also try to remove whatever twigs I could get easily.
2.) Remove the small pieces. A gasifier is an unintentional filter. Unless the ash is free to blow through the fuel bed it will be retained, melt, and resolidify in the cooler reduction zone. That will leave a hard object that looks like a barbeque lava rock preventing the fuel from flowing through the machine and the whole unit will stop functioning.
Below is a picture of a fuel sample that I made by screening chips from my experimental converted chipper. The screen was 1/2″ “hardware cloth” screen.
The good fuel is on the left and the “fines” on the right will be used around the farm for plant mulching and weed suppression. It was made with this machine:
I use chips because the processing equipment is cheap and available, but it must be a large commercial size chipper. Regular home owner size chippers make chips that are too fine and the fuel bed will plug with ash and clinker.
3.) Make sure your fuel is dry. Preheat gives you some protection against wet wood but it only goes so far. It takes me five days to dry the fuel shown above:
if I lay them out on a table in a thin pile and stir them every day.
Northern Self Reliance 
i really like your site and what you are doing ..do you recommend using the MTD approach to chip/chunk the fuel if that is all you have ..or to get a larger chipper…is the MTD is ok… do you think the 5 horsepower version is ok or just the 8 HP version?
Hi Murray. Bunch of issues here…That MTD I am running has a special custom rotor that I built in my shop. It works fine. I will release it for sale once I can afford the product liability for that class of product. My recommendation would be to get all your feedstock ready and rent a 6″ chipper from Home Depot for $200/day. The 8hp may have a heavier crankshaft to take more of a beating but I don’t know that for sure. Both the 5 and 8hp will chip through the same size branch because it is the rotor inertia not the engine power that does the cutting. The 5hp will just take longer to come back up to speed after it bogs down.
I recently read got some information from another gasifier developer that roughly 50cm (2 inch) x 50cm wood chunks ran better than chips. Do you have an opinion about this assertion?
Hi Glenn. No question. Blocks are more forgiving on tar. All the space between the fuel chucks gives lots of opportunity for the tar gas to be consumed rather than the O2 consuming char. I have seen it and Jim Mason has reported it. If chunkers were as readily available as chippers I would have built block machines. Gasifier internal geometry has to be somewhat fuel specific. I would hate to put chips in a machine built to the WW2 Imbert tables. It would be a tarry mess.
So you would not recommend your gasifiers unless they are fueled by chips?
Correct. You would have all sorts of fuel hang ups if you tried to run blocks in Victoria. I have plans for a big wood “Elizabeth” but I haven’t gotten around to building it. I have no problem saying my machines are chip and pellet only. I would much rather lose a sale than have an unhappy customer.