Catalytic converters for woodstoves and boilers

People have been burning wood to create heat for tens of thousands of years, and even in the 21st Century it still has a role to play. Wood is a renewable fuel and as such, it complies with the Net Zero agenda of reducing emissions of carbon dioxide (CO2), because the CO2 released into the atmosphere when burning wood exactly matches the CO2 absorbed by the tree when it was growing. Another point often overlooked is that trees don’t live for ever. They die, get blown over in storms, or need to be cut down because they are in the wrong place. Generating heat is a good use of wood that otherwise would be left to rot, and far better than using fossil fuels.

So, if burning wood is carbon neutral, why are so many people opposed to it? The problem is that, as well as releasing CO2, wood burning also emits some substances which are harmful to human health, and the most serious of these is fine particles. Fine particles (also known as PM2.5 because they are less than 2.5 microns in diameter) from wood combustion consist mainly of carbon and hydrogen atoms. Because they are so small, they can be inhaled deep into the lungs where they can cause diseases such as cancer. They also irritate the breathing passages, especially for people who already suffer from asthma or bronchitis.

In order to reduce harmful emissions from wood heating, in wealthy countries there are regulations which limit the emissions from stoves and boilers. These cover new appliances and, in some countries, for example Italy and Germany, older appliances with higher emissions are banned.

The need to reduce emissions from wood burning appliances has forced manufacturers of stoves and boilers to redesign their products so that they burn more cleanly and efficiently. One technology which can help with this is the catalytic converter for wood stove. Catalytic converters first came into the public consciousness when they were first installed on cars to reduce the harmful exhaust emissions, so it is not surprising that tinkerers tried them out on wood stoves, beginning in the USA in the 1980s.

The principle of catalytic converters for woodstoves and boilers is that it promotes reactions between flue gases without actually changing its own chemical composition. In an ideal world, the only components in the flue gas from a wood stove would be carbon dioxide and water. But in reality, combustion is never perfect and therefore some chemicals are also present due to incomplete combustion. These include carbon monoxide, hydrocarbons and soot, all of which are harmful to health.

Catalytic converters for woodstoves and boilers facilitate a second combustion process in which these pollutants react with oxygen. As well as cleaning up the flue gas, this process also releases additional heat, thereby improving the efficiency of the stove or boiler. There’s even another benefit too. Many of the hydrocarbon gases created when burning wood condense into tars as they cool down, and these can stick to the walls of flue pipes, where they obstruct the flow and may even cause a chimney fire. Because catalytic converters for wood stoves oxidise the hydrocarbon gases before they condense into tars, the problem is reduced and the chimney sweep’s job is made easier.

The main purpose of a catalytic converter for wood stove is to promote reactions between gases, but wood stoves and boilers also emit particles of carbon, which are in the solid form even at the high temperatures inside the appliance. There are three ways that catalytic converters for woodstoves and boilers can help to oxidise soot, as follows:

By increasing the flue gas temperature
Catalytic converters for woodstoves and boilers oxidise gases such as carbon monoxide and hydrocarbons, and because these reactions are exothermic, this increases the flue gas temperature. If the concentrations of carbon monoxide and hydrocarbons are very high (as they were on stoves and boilers designed before emissions controls were introduced), then the temperature of the flue gas can be increased to over 600 OC, at which point the carbon simply combusts. For this reason, the first generation of catalytic converters for woodstoves and boilers were known as ‘catalytic combustors’.

By direct catalytic action
Some types of catalytic converter materials (e.g. ceria) can promote the oxidation of carbon at temperatures as low as 400 OC, which are often found inside woodstoves and boilers.

By creating an oxidising agent
An oxidising agent is a molecule incorporating oxygen atoms which is unstable, meaning that it is easy for it to transfer an oxygen atom to carbon. Nitrogen dioxide (NO2) is an example of an oxidising agent and this reacts with carbon at temperatures as low as 250 OC. The flue gas of woodstoves contains nitrogen monoxide (NO) and the right type of catalytic converter materials can oxidise this into NO2.

How a catalytic converter for wood stove is made

A catalytic converter for wood stove consists of a ‘substrate’ which is a structure with a large surface area. The substrate normally has channels which pass straight through it, in which case it is referred to as a ‘honeycomb’ substrate. Honeycomb substrates can be made from either a ceramic material called cordierite, or very thin metal foil. An alternative type of substrate is known as a ‘foam filter’. This is made from a ceramic material and resembles a sponge.

With a foam filter, the channels do not pass straight through, meaning that the gas flow is more turbulent and there is a greater chance of solid particles getting trapped. This can cause problems though because wood smoke also contains ash, and if ash build up in the catalytic converter it will cause a blockage.

The substrate is coated with materials which have a very high surface area (at the microscopic levels, and on top of these, catalytic converter materials. Traditionally the catalytic converter materials are platinum and/or palladium but as these are very expensive, research is underway to find cheaper alternatives.

Design considerations

When designing a catalytic converter for wood stove or other wood burning appliance, the main factors to be considered are as follows: efficiency, pressure drop, maintenance and cost. To deal with these individually:

Efficiency:
Efficiency derives from the quality of the chemistry used in the coating and also from the surface area of the substrate. The surface are of the substrate is referred to as ‘geometric surface area’ and this derives from the volume of the substrate and also the size of the cells. Another phrase that people will come across when first getting involved with catalytic converters is ‘space velocity’. This compares the volume of the substrate with the volume of air flowing through it in a given period of time, which determines the amount of time that polluted gas spends in the substrate. Most suppliers of catalytic converters will specify a maximum space velocity for their products.

Pressure drop:
Many woodstoves rely on convection to extract flue gases from the appliance, and therefore the catalytic converter cannot cause too much obstruction to the gas flow. This necessitates keeping the flue gas velocity through the catalytic converter below a certain threshold, which in turn requires a cross- sectional area above a certain threshold. It is often a good idea to incorporate a by-pass into a woodstove equipped with a catalytic converter, so that flue gas can be diverted around it when lighting the stove, before a healthy draught has been established in the chimney.

Maintenance:
Catalytic converters need periodic maintenance to remove ash, so they must be installed in a position where they can be easily removed for this purpose. They will also need to be removed when the chimney is swept. The size of the cells in the substrate must be chosen so that the majority of ash passes straight through, otherwise it will quickly become blocked. It is also beneficial if the flue gas flows vertically up through the substrate, which means that some ash which is unable to pass through will fall back down into the fire.

Cost:
Ceramic substrates are less expensive than metal ones, but require special tooling so cannot be made in specific sizes unless the customer commits to buying a large number. It is therefore sensible for appliance manufacturers to consider using standard sized ceramic substrates or bulk buying. Most catalytic converters on the market at the moment are based on precious metals, which are obviously expensive, however there is very little correlation between the amount of precious metals used in a catalytic converter and its efficiency.

It is therefore worthwhile looking for a catalytic converter supplier which utilises an advanced coating process that requires less precious metals to achieve the desired result. It is also worth trying out (and maybe funding the development of) catalytic converters which use alternatives to precious metals. Examples of alternative catalytic converter materials are Hopcalite and Perovskites.