Wood gas is produced by applying very high heat to wood in a reduced oxygen environment. In the Wood Gun, gasification is driven by the hot refractory lining the bottom of the fuel chamber. This causes the release of gases through a process known as pyrolysis. In the Wood Gun, the wood gas is drawn through the bottom of the firebox and into our uniquely designed refractory by an induction fan. The gases are combined with oxygen and combusted in the refractory. The super heated gases then pass through the interconnected refractory tunnels and into our uniquely designed swirl chamber where the bulk of heat exchange takes place. The superheated refractory then continues to gasify the wood remaining in the firebox. After combustion the residual ash is combined with high velocity air flow to scrub the heat exchanger helping to prevent build up. This process allows us to maintain unexcelled consistency in heat exchange. Most ash exits the unit and is collected in the cyclone ash separator for fast and easy removal.
Wood gasification has been used to power lighting, run vehicles (including tractors) and as a method of improving efficiency in wood and biomass boilers. It is the latter cause that is served by using it in the Wood Gun. Organic matter in the fuel changes its physical state and chemical composition during this process. The resulting gases are more readily mixed with air, for increased combustion efficiency. In the Wood Gun, the low oxygen zone would correspond to space just above the refractory surface. Incoming air is rapidly mixed with the gases produced in this low oxygen zone as they pass through the slotted center brick. Rapid combustion continues to occur as the gases pass through the refractory tunnels inside the side brick. This combustion drives more heat upward through the refractory, which in turn produces more pyrolytic gases from the fuel just above.
The Wood Gun cycles on and off much like an oil or gas fired boiler. During the off cycle, fuel is not consumed because oxygen is not present in the combustion chamber. Due to the heat retained in the large, super-heated refractory mass, re-ignition of the fuel occurs automatically when a drop in water temperature signals the need for heat, and air is reintroduced. Re-ignition can take place successfully 4-6 hours (or more) after the last firing cycle. Boiler size and the vigor of the last burning cycle impact this. During the off cycle, wood gas inside the firebox condenses on the inner walls forming a solid that will eventually fall off. This combines with the remaining fuel thus adding energy that in many other boilers goes up the chimney. By utilizing this ability, the Wood Gun can operate efficiently without thermal storage. By virtue of its stainless steel firebox, the Wood Gun is immune to corrosion by the pyrolytic acids that form during the off cycle.
When burning conventional firewood, or medium to high moisture particle fuel, gasification is simply a must for attaining high combustion efficiency. Nevertheless, high combustion efficiency alone is insufficient to achieve overall high performance. The Wood Gun also features high thermal capture efficiency, through use of the swirl tube heat exchanger design.
The Wood Gun heat exchanger consists of an array of inner firetubes (or single inner firetube) inside a large outside tube. In all cases, the large outside tube features a swirl chamber. The swirl chamber funnels exhaust, and causes it to swirl around the tube. This swirling motion is accompanied by an increase in velocity. Because the gases are traveling in a corkscrew motion around the chamber, contact time with the heat exchange surfaces is improved versus a straight through design. Higher velocity contributes to a self-cleaning effect, a key feature of the design. The ash contained in the exhaust stream serves as a scouring agent.
This heat exchanger design pairs well with the high combustion efficiency that comes from utilizing gasification by combining it with great thermal capture. This thermal capture efficiency shows up in very low stack temperatures. The Alternate Heating Systems Wood Gun™ boiler will often have stack temperatures that are under 300° Fahrenheit. With such low stack temperatures, it is common to see exhaust gases condense to visible water vapor shortly after exiting the chimney (see below). Note the "condensation gap"; as the gases exit the chimney, the water vapor is at first invisible. After it condenses, it can then be seen. The smaller the gap between exiting the chimney and becoming visible, the cooler the vapor (or surrounding air). If there is no visible steam, then the gases contain enough heat energy to disperse before becoming visible. A chimney venting into warmer, drier air will be less likely to show steam, because the vapor disperses before it can condense and be seen.