The last filtration step in a wood gas system before an engine should be some sort of fine filter.  I have had very good luck with spin on pool filters.  I like them because:

• They do not allow any flow to get through unfiltered.
• They are made to be wet.
• They have a nice plastic supporting structure inside to handle the pressure across the media.They coalesce mist into droplets nicely.
• They are not subject to variations based on how tightly they are packed.
• They can be back blown easily.
• They hang so they do not trap moisture.
• They can handle significant pressure across the media.

Below is the model that I offer for sale and also use on the Gas Station:

 

THIS FILTER WILL NOT ELIMINATE TAR.  It is meant for fine particles. Low temperatures and fast turns are the best way I have found to remove tar (condense and splat!). 

A Type R shop vacuum filter sleeve fits nicely over this pleated filter and can be used as two stage filtration.  That is the way I am currently running mine.

The housing has a 1.5 inch female NPT inlet and outlet.  I have tested it to 17cfm and it has performed very well, only allowing a small amount of soot through.  This makes it suitable for engines under roughly 25hp.

These units are available for $225 plus shipping.

Integrated Gas Cooler Option:

The final filter can also be ordered with an integrated gas cooler.

 This cooler uses a 25W fan to lower the gas temperature.  When tested on the Mini Gas Station it lowered the temperature of a 12cfm flow from 350F to 130F.  Please note, it is important that the heat exchanger be kept clean for best results.  This unit will perform badly if it has a coating of tar inside the housing acting as insulation.

This option is available for an extra $150 or a total of $375 with the filter.