In my opinion gas cleaning is one of the most overlooked parts of gasification when really, it is probably the most important. Your gas cleaning assembly will determine how much dirt ingestion, oil contamination, moisture ingestion, and tar fouling you will have in your engine. None of those things are good.
The three things you need to remove from your gas are dirt, namely fly ash and charcoal, steam, and tars. The dirt is the easiest. Generally systems use a cyclone filter which will get +90% of the particulates followed by some sort of fine filter. I like spin on pool filters for my fine filtration. They filter to the micron level, they are made to be wet, they can be back blown, and they dry quickly. Other people use hay or sawdust for their fine filters. My problem with these methods is that the size of the filter media can be variable and when it gets wet it can become a soggy non-breathing mass. It can also retain moisture and lead to accelerated corrosion. If sawdust gets excessively hot it can start to break down and release pyrolysis(tar) gasses directly to the engine which can lead to intake valve sticking.
Steam is the next easiest. Just cool the gas with a heat exchanger. Remember that tars will also condense at some point so whatever you build or use, be sure that it is able to be taken apart and cleaned. There is no good solvent for tar. The only way I know of to clean tar is to burn it off.
Since steam is always present in a wood gas system corrosion is always a problem. I have a general rule that ANYTHING that I am likely to want to take apart in a year should be assembled with a bolted flange joint and gasket. Screwed together NPT threads will rust tightly and be very difficult or impossible to get apart.
Tars are the last and hardest thing to remove. Wood gasser Dutch John once said that trying to filter out tars is like trying to filter butane out of air and I completely agree. You will not be able to filter it out with charcoal or hay.
I do not try to filter them out but rather I try to condense them out. I follow the example of my woodstove chimney. If you look in the entry thimble you will see that it is always clean to the ceramic and the red clay liner is plainly visible. At the top of the chimney where the ceramic is exposed to temperatures as low as -20F there is a black glass like glaze where the tars have condensed. Generally the thickest build up of tars is right at the roof line where the temperature suddenly drops.
Applying this logic to a wood gas system, I come out of my gasifier and immediately cool the gasses as low as possible. On my big machine the outlet temperature is about 240F(116C) due to preheating so almost immediately water and any tars begin to form droplets in the gas stream. Next I go into the cyclone. This makes any droplets impact the side walls and stick. I would rather have my tars sticking to the inside of my cyclone than the intake manifold of my engine. Many people like to keep their cyclones hot to avoid condensation and corrosion, but I advocate removing all the caps and covers at shut down so the system can dry and controlling corrosion that way.
My final step is filtration in the pool filter and on to the engine. Some people like to preheat their combustion air to be sure that the gasses entering the engine are dry or above the dew point.