Catalytic Converter for Wood Stove vs Filter – What is the Difference?

A catalytic converter for wood stove applications is often referred to as a filter. This is understandable because of the small channels that pass through it, but it is incorrect.

A filter is designed to do one of the following:

• Remove solids from liquids or gases
  
• Remove liquids from gases

The function of a catalytic converter, on the other hand, is to convert toxic gases into less harmful substances. Filters work by physically separating solids or liquids from a fluid, whereas catalytic converters for wood stoves are designed to promote chemical reactions that transform harmful gases into less harmful substances.

Filtering Particulate Matter from Wood Smoke

Many emissions from wood stoves are volatile, meaning that they change from gas to liquid and then from liquid to solid as the flue gas cools down in the chimney and after leaving it. However, particulate matter that remains in solid form at the high temperatures found in the wood stove, flue, and chimney requires a different approach.

Particulate matter that remains in solid form at these temperatures can be filtered out, but doing so requires either a filter with extremely small pores to trap the particles, much smaller than the channels in a catalytic converter, or a device known as an electrostatic precipitator (ESP).

catalytic converter for wood stove

There are several types of filter media with suitable pore sizes and the ability to withstand high temperatures, but there is one major problem with this approach: the filter needs to be cleaned or replaced frequently to prevent an unacceptable flow restriction. In practice, this could mean maintenance every few hours rather than every few days. Some of the solids that would be trapped by a filter exposed to hot flue gas would consist of carbon, which could in theory be oxidised to carbon dioxide gas. However, there is also ash suspended in the flue gas, and this cannot be converted into a gas.

Electrostatic Precipitators and Foam Filters

An electrostatic precipitator, also known as an ESP, is an alternative method of removing particles from hot flue gas that dispenses with the need for filter media. An ESP creates a very high voltage differential that attracts particles to one of its electrodes. Unlike filter media, it therefore cannot become blocked in the same way.

When the electrical supply is switched off, the trapped particles simply fall back into the combustion chamber. ESPs are very effective at removing particles that are in the solid or liquid phase at high temperatures, but they are expensive to buy and install and are unsuitable for off-grid situations because they require an electrical power supply.

Filtering out particles from wood smoke using filter media creates unacceptable maintenance requirements, while ESPs are too expensive for many applications. Meanwhile, conventional catalytic converters for wood stoves of the type where the flue gas flows through in a straight line are deliberately designed with channels that are large enough not to trap particles. Allowing the particles to pass through prevents them from causing a blockage.

Foam filters, also known as sponge filters, work differently from a conventional catalytic converter for wood stoves. Instead of passing straight through the structure, the flue gas is required to twist and turn on its way from one side to the other. This process causes some of the particles to become trapped, at least on a temporary basis. If the trapped particles contain carbon, retaining them for several minutes may give them enough time to oxidise into carbon dioxide, with the process promoted by the catalytic coating applied to the foam filter.

The main problem is ash, which cannot be converted into a gas and may have difficulty flowing all the way through the filter, thereby causing an unacceptable flow restriction. This problem is reduced in stoves and boilers equipped with a fan in the flue because the additional pressure helps the ash pass through the foam filter, while the fan keeps the flue gas moving even when the filter is partially blocked.

Why a Catalytic Converter Can Reduce Particulate Emissions

Volatile missions from wood stoves change from gases to liquids and then solids as they cool while travelling up the chimney and into the outside air. These emissions, also known as condensable particulate matter, make up about 80% of the total particulate matter emitted from wood stoves. While the flue gas is still hot inside the wood stove and chimney, these substances remain in gaseous form and cannot be filtered out by filter media, an ESP, or a foam filter. In this gaseous form, however, they can be oxidised by a catalytic converter into carbon dioxide and water.

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Filters vs Catalytic Converters for Wood Stoves

The most effective and reliable way to reduce particulate matter emissions from wood stoves is to focus on the volatile compounds that account for approximately 80% of total particulate emissions. A conventional catalytic converter allows solid particles to pass through without causing a blockage while oxidising volatile gases before they can leave the chimney and cool down into particulate matter.

A catalytic converter for wood stove applications is not a filter, but it can provide a highly effective solution for reducing particulate matter emissions from wood stoves and boilers.