Wood Stove Catalytic Converter Retrofit

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In Wood Stove Efficiency and Emissions: 1984, we reviewed the progress that’s being made toward developing clean-burning, efficient wood stoves. The latest high-tech wood burners use 30% less wood and emit up to 90% fewer pollutants than did the best heaters built only five years ago. However, a lot of us simply don’t have roughly $1,000 to lay out for the latest catalytic wood stoves.

Luckily, there is a course of action available that’s a great deal more affordable than purchasing a complete new wood stove and that still provides much of the efficiency improvement and emissions reduction made possible by these state-of-the-art combustors. There are, you see, over a half-dozen retrofittable wood stove catalytic converter devices on the market today, and test results show that they offer significant performance gains over a plain old airtight heater. If you’re in the market for such a device, here are some of the things to look for.

Any retrofit catalyst is a compromise because the stove that it’s installed in was not designed to insure an adequate supply of oxygen and good mixing of the smoke and oxygen at the combustor. Also, add-on catalysts are often positioned farther from active flames than are designed-in units, so the temperature of gases entering them is generally lower. This means that the add-on (and particularly the external) catalyst may be reluctant to “light off” at the beginning of a burn or after refueling.

Catalyst retrofits that are positioned inside the firebox offer some obvious advantages. (We presented plans for a build-it-yourself model in A Wood Stove Catalytic Converter You Can Build.) Their close proximity to the burn zone allows them to get into action early and keep working until there’s little left but coals. Having the unit too close to the fire can, however, be a liability. If adequate precautions aren’t taken to prevent flames from reaching the ceramic monolith, thermal shock can quickly destroy the catalyst: There have been reports of combustors that disintegrated after only a few months’ use in stoves designed without adequate flame impingement protection. Internal combustors are probably best suited to stoves that are used continuously at low to medium heat outputs.

Many designers, including Robert Vincel — who developed the Riteway internal and the Uni-Com external catalyst retrofits — have concluded that the stress to the combustor inside a stove isn’t generally worth the gains provided by that position. Vincel’s Uni-Com uses elaborate mixing geometry to insure good distribution of gases, and is fitted with cast refractory to give the unit thermal mass. The latter measure helps the combustor get through transient cool conditions — such as occur during reloading — without dropping below ignition temperature.

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