Personally, wet scrubbing the gas isn’t worth the effort. The sawdust filter Matt referenced will handle anything a charcoal gasifier will spit out. Same for the filter that Kristijan talked about. On my Mazda B2000 I ran an updraft with an upholstery bag filter with no issues. I used a 55 gallon drum for the gasifier body and I would get about 60/70 miles before the gas started to get a little hot.
With an updraft make sure your charcoal stays as dry as possible, sealed bags and containers for storage. Also add water drip at the nozzle area. I’ll take some photos of what remains of my Mazda’s gasifier(I was hit by a stolen car and it crushed the barrel). Stay tuned I’ll edit them into this post.
Here’s the gasifier, I had 5 1/2" holes in a very thick walled 2.625" DOM tube, wall thickness was 5/8" for the heat. I think I could have gotten away with just using 3 of those holes, the sides got pretty hot closest to the outer holes.
I kept both ends open for an even flow of air.
For water drip I had a 5 gallon square tank with a rubber hose and a valve, and on the end of the hose was about a foot long piece of stainless steel drinking straw. I set it to 1 drip per second.
And my gas exit was that pipe at the top. I had a rolled up tube up diamond expanded sheet metal to keep the biggest pieces from getting sucked into the filter box. I used about 6 feet of flexible exhaust pipe to route to the filter.
Here’s my filter, a tall ammo can with two pipes. Dirty gas entered at the bottom pipe, and would get caught by the outside of the bag. I did it this way so excess soot could fall off of the bag to prevent too much clogging. The bag is just hose clamped to the pipe coming out of the lid.
Inside the bag I did put in some heavy door screen, steel not aluminum, that’s mostly there to keep the bag held open, and act as an emergency last stand filter if the bag gets any holes or tears that I haven’t noticed.
I was able to go country highway speeds, 55mph, no problem. Top speed I managed to get to was 60mph but I didn’t have an opportunity to go faster than that. That poor engine only has 80 horsepower when it was new, so acceleration was like a VW Beetle when it was running on charcoal gas.
The gasifier that I built is by no means the best or easiest or something I came up with. The flute nozzle design is something Kristijan discovered with his Mercedes.