If you have enough sufficiently dry wood, downdraft gasification models burn extremely cleanly (they're effectively rocket stoves), but those models require a fair bit of maintenance, especially if your wood is slightly wet or green, because soot can quickly build up on the water envelope.
https://en.wikipedia.org/wiki/Outdoor_wood-fired_boiler
If you have enough sufficiently dry wood, downdraft gasification models burn extremely cleanly (they're effectively rocket stoves), but those models require a fair bit of maintenance, especially if your wood is slightly wet or green, because soot can quickly build up on the water envelope.