I will also add that the hi-efficiency stoves are all water jacketed on the primary chamber to help keep the wood from burning up there. So I’m envisioning a large OWB type unit with the firetubes running up from the secondary burn chamber, but instead of going through a small water jacket, have them go through a 150 gallon or so water tank that surrounds the upper firebox (and lower firebox to catch heat that makes it through the refractory)..... Gasifier with a decent amount of thermal storage built right in!!
This isn't quite making sense to me. Surrounding the primary firebox with water insures it stays "cool," which will result in additional creosote condensation on the walls of the firebox -- not what you want. All that creosote otherwise would be wood gas material to be gasified below. I think you want the primary firebox walls to be hot to maximize wood gas production and minimize creosote condensation. You also want the wood to burn in the primary firebox with controlled oxgen input to enhance pyrolysis. A cool firebox is contrary to the theory of gasification. The primary firebox should be insulated from the water (the more the better) to maximize heat and pyrolysis in the firebox. Controlled oxygen input controls the fire, not the temperature around the firebox.
The lower gasification burn chamber is not a secondary burn chamber as secondary burn is understood. Secondary burn operates at roughly 500-1100F. The refractory in this chamber is super-hot (2000F) to combust nearly 100% of the various chemicals given off during the primary burn. A water jacket around the back, sides and bottom of the gasification chamber likely is not problematical, as the gasification burn takes places in a refractory isolated from the walls of the chamber, and all that leaves the refractory is nearly 100% combusted gases (CO2 and water vapor).
Firetube design, at least in the Tarm, is nearly what you describe, if you omit surrounding the primary firebox with a water jacket. Instead, gases during primary burn are driven by a downdraft into the gasification refractory, and CO2 and water vapor remaining (virtually all other gases are now reduced simply to CO2 and water), which are now very hot, are exhausted through firetubes surrounded by water in the rear portion of the boiler. I'm assuming in the Tarm that the back wall of the primary firebox, for purposes of economy in construction, is the front wall of the water/firetube chamber, and that the back and bottom of the gasification chamber also are surrounded by water. Water also could surround the sides of the gasification chamber, but this would require a more complicated and expensive design.
The water chamber could be large (about 50 gal in the Tarm) like the Garn, but that makes a large, heavy, and expensive boiler. So, the alternative is a separate water storage tank, smaller, lighter, and less expensive boiler. Same result though.
Haha..........
