An ordinary wood gasifier turns wood (or other solid hydrocarbon fuel) into gaseous fuel, for burning in an engine. However, the gas produced has a very low energy density, mostly due to having a large amount of nitrogen in it. (The nitrogen comes from the air that's fed into the gasifier). By using
a nitrogen-free oxidizer, we can produce a fuel with higher energy density. If its temperature is high enough, steam act as an oxidizer... therefore, that's what I propose:
Start with a "regular" gasifier, with all of the normal accessory components.
Add the following four components: Two high efficiency counter current heat exchangers (HX1 & HX2), a combined fuel mixer / combustion chamber (C), and a combined boiler / superheater (B).
All of these components (wherever it's reasonable to do so) should be constructed from, or surrounded by, high quality, high temperature, insulating materials (fire clay comes to mind).
In normal operation, there two main streams of fluid and one minor stream of fluid: Fresh air from the atmosphere moves through HX1, C, HX2, HX1 and back to the atmosphere. Water moves through B, HX2, the gasifier, back through B, and onward to the engine (as fuel). Some of the fuel coming out of the gasifier is fed into C.
The air is heated on it's first pass through HX1, and as it goes through C, and cooled going through HX2 and through HX1 the second time.
Similarly, water is heated, boiled, then superheated as it goes through B, superheated some more as goes through HX2, cooled as it passes through G and chemically reacts with the carbon there to transform into "water gas", and cooled further as it passes (uncombusted) through the superheater, then the boiler, of part B.
Of the original gasifier's other parts, the cyclone is placed immediately downstream of the gasifier, before the fuel splits into two streams (to C and B), and the radiator and fine filter are placed downstream of B, after the fuel gas has been cooled by moving it's heat into the feed water.
Wood chips (or whatever) can be added, and ashes removed, via airlocks [link].
After the gaseous fuel has been used, the water that was added at the beginning of the process will have been re-created. Cooling that exhaust will produce liquid water. If the engine and gasifier are reasonably close to one another, that water can be re-used, reducing the total amount of water needed.