since WW2 there is a discussion about EGR and steam. As you said, EGR is quite simple and self regulating, thus it was used much more often. Other advantages: no freezing water tank in winter, a bit saving on char and easier to develop for the manufacturer.
Adding water/steam was often regarded as too delicate, especially in cold climate.
With us, it is different. We are much more dedicated to our systems than the average driver in WW2 and want to develop them further.
Both CO2 to CO and H2O to H2 consumes heat, so the reduction of one is stealing the heat of each other. If I remember right, reduction of CO2 needs a bit lower temps, so if you have already quite low temps most of the steam will just pass the reaction zone but also carries the heat away.
Adding water creates more powerful gas. Thus, if you want maximum power just add steam, no EGR. As evaporating consumes also a lot of heat, try to feed the water already as hot steam into the reaction.
As Andres mentioned, a kind of “water carburetor” can be also self regulating according to the flow. Just add as much water as you can convert to hydrogen, as otherwise the superheated steam would just carry away the heat from the reaction. So it’s best to start on the “lean” side.
Edit: I have seen the drawing of a setup in the old book from Switzerland:
A little expalantion of the german legend: This is a heat exchanger, preheating the air (Luft) and the water with the hot gases taken from the gasifier. The amount of water is adjustet to the temperature in the heat exchanger by means of a thermostat, which actuates a valve (Ventil). There is also a manual actuated valve (Hahn) for the possibilty of additional water ins some situations like start-up.
This “Calori element” was actually built and used in a charcoal gasifier in Switzerland.
It is stated that for a good increase of the heat value, the gasifier should have a high thermal efficiency. Otherwise there is not a remarkable increase in the quality of gas.
This means: Using the heat of the gas for preheating air and steam. Also updraft system are good suitable, as the char is preheated by the gases.
It is further written in the book: The variable need for water due to different qualities of charcoal makes the optimal dosage of water very complicated, so there are just a few systems with added water in Switzerland today (1944). The majority of designers favours the simple construction just using air more then the possibility of higher power or saving fuel. In most cases, the reason was to get soon into business, no one wanted to waste valuable time with long development processes. But it is hoped that now, with better rules about quality of charcoal, adding water gets more and more common. Up to the end of 1943, there were only 4 systems using water.