How to use catalytic converters to reduce fumes from wood stoves and boilers
Burning wood is generally considered to be an environmentally-friendly way of producing heat, either for heating rooms (e.g. wood stoves) or water (e.g. wood boilers). Its main advantage over competing heat sources is that wood is a renewable fuel. This means that the carbon which is released when it is burned is cancelled out by the carbon which was absorbed by the trees which were grown to produce the timber.
However, burning wood also emits some toxic compounds such as carbon monoxide and fine particles, which reduce air quality, especially in urban areas. In many advanced countries, regulations are in place to control the emissions of these pollutants, and the regulations are getting stricter.
Installing a catalytic converter in a wood stove or boiler is an easy way to reduce pollutants so that they comply with the new regulations.
What is a catalytic converter ?
Most people think of a catalytic converter as something in the exhaust system of their car. It is basically a chamber which enables chemical reactions to take place.
A catalytic converter contains a very large surface area coated with precious metals such as platinum. As hot gases pass through the catalytic converter, reactions occur which are similar to burning, but without any flames. For this reason, catalytic converters are also commonly known as catalytic combustors.
The aim is to convert as much as possible of the pollutants into either carbon dioxide or water.
In the case of a wood stove or boiler, the catalytic converter also releases heat energy as it oxidises the smoke, so it improves efficiency and reduces pollution at the same time !
How to design a wood stove incorporating a catalytic converter
A wood stove or boiler with a catalytic converter inside looks very similar to the traditional type. The catalytic converter needs to be installed in a location where the flue-gas temperature is hot enough but not too hot (somewhere between 300 and 500 C is ideal).
It is a good idea to have a by-pass damper so that the flue gas does not go through the catalytic converter until the fire has got going nicely. It also makes sense to choose a position so that most of the heat released by the catalytic converter does something useful, rather than disappearing up the chimney ! This illustration shows an example of a good design for a wood stove with a catalytic converter:
The reactions which the catalytic converter promotes are temperature-dependant. If the flue gas temperature is above a certain threshold then the catalytic converter will start working, and it will get hotter than the flue gas entering it.
This is known as ‘light-off’ and usually occurs between 250–300 C. When the flue gas is below ‘light-off’ temperature the catalytic converter will not do anything. In fact, it is worse than that because some of the oils and tars contained in the flue gas (e.g. creosote) will get deposited on the catalytic converter, and if this happens for a long time it will need to be cleaned.
To avoid this problem, many woodstoves with catalytic converters fitted also incorporate a thermometer so that the user only closes the bypass damper when the temperature is high enough.
Although it sounds complicated, owners tend to get used to the procedure very quickly, and many obtain a form of satisfaction from being involved in using their appliance efficiently.
To summarise, installing a catalytic converter is often the simplest and least expensive way to reduce the pollution form a wood stove or boiler, whilst at the same time extracting more heat from the wood which is burned.
It enables the benefits of burning the oldest source of heat known to man to be compatible with the latest standards for air-quality.
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