Long time lurked here, first post ever. If I might chime in on the subject as I have studied and experimented myself with syngas. The proper place to "tap into" the burner would actually be in the secondary chamber. But the process would require that absolutely NO secondary air is introduced, and that the producer gas must be cooled immediately after it leaves the coals to prevent it from igniting. So the heavily insulated secondary chamber would hinder the process. The gas cannot be pulled from the primary chamber, as only pyrolosis has happened at this point and not the reduction phase that is necessary. The heat from the bed of coals releases the wood gas in its raw state. This is the pyrolosis phase. Then the raw wood gas must be passes through the bed of burning coals with just the right amount of oxygen at just the right velocity to induce "carbon cracking" and to initiate the exothermic process of "reduction" where all the chemical compounds are reduced to basic elements of flammable gases of hydrogen and carbon monoxide. The inert atmospheric gases pass through to help carry the heat and the carbon settles out in the form of fine ash. It is possible to do such a thing with a boiler, but the conditions would have to be absolutely perfect.