In the controlled-combustion (or airtight) stove, control of heat output is gained at the expense of combustion efficiency, especially during low burn rates. Low burn rate is a relative value. For this discussion, low burn rate means rates that result in excessive smoke and poor combustion efficiencies (50% to 70%). To overcome this problem, draft regulators, secondary combustion chambers, and baffling have been used to improve combustion efficiencies. But these methods typically fall short during medium and low burn rates because of difficulty in maintaining temperature and air/fuel ratio.

Temperature Requirements. Wood gases normally require temperatures in excess of 1,000°F (500°C) to ignite and burn. The Süd-Chemie Catalytic Combustor causes wood gas to burn (oxidize) at temperatures as low as 500°F (260°C). Even at the lowest usable burn rates these temperatures are common, so it is now possible to burn wood gases at low burn rates.

Air/Fuel Ratio Requirements. For every conventional combustion process, there is an optimum air/fuel ratio in which exactly enough air is present to ensure that 100% of the fuel is burned.

Wood combustion is a complex and variable process. Many wood-gas components are produced at the same time, each requiring a different amount of air for the same combustion. The rate of wood-gas released varies constantly. In addition, regulatory air supply to the fire is difficult because air-inlet devices are imprecise. The proper air/fuel ration is elusive.

To complicate matters, there are flammability limits-ranges of air/fuel ratios that the mixture of fuel and air will support combustion if exposed to a spark or a flame. When concentrations are above or below these limits "flame-out" can occur.

Since catalytic combustion is a flameless burning process, air/fuel ratios are much less important, especially at the excess-air end of the range. High fuel concentrations can be burned until oxygen is depleted. Thus, use of a catalytic combustor extends the range of wood-combustion conditions.

Higher Heat-Transfer Efficiencies.

The catalytic combustor allows higher heat-transfer efficiency at low burn rates. This is possible for two reasons. First, a useful by-product of catalytic oxidation is heat. Elevated exhaust-gas temperatures will enhance the amount of heat transferred by an appliance to the surroundings. Second, the combustor is the most effective at low burn rates. When low flow conditions exist, there is more time for the transfer of heat from hot exhaust gases through the stove.

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Technical Advantages

What Do You Gain By Using a Stove With a Catalytic Combustor

· Added efficiency

· Reduced creosote

· Less pollution

High Combustion Efficiency at Low Burn Rates.

Temperature Requirements.

Air/Fuel Ratio Requirements.

Higher Heat-Transfer Efficiencies.