Catalytic Converters: What’s the Best Way to Reduce Particulate Emissions from a Woodstove or Boiler?

Wood is a sustainable fuel for generating heat and is carbon neutral, unfortunately though, burning it releases some toxic emissions which are detrimental to local air quality. Catalytic converters are increasingly used to reduce particulate emissions from wood-burning appliances by oxidising pollutants before they leave the chimney.

Understanding Particulate Emissions from Wood Burning

The biggest concern is about particulate emissions, which may also be referred to as fine particles, PM10 or PM2.5, which are references to their maximum size measured in microns. Particles can be inhaled deep into the lungs where they may cause irritation and breathing difficulties. They also incorporate some carcinogenic chemicals, and the extremely small ones can even cross over from the lungs into the bloodstream, meaning that inhalation of fumes from woodburning can be responsible for heart disease too.

There are three components to particles from woodburning, as follows:

• Soot. This is just carbon.
  
• Ash. This consists of various minerals and metals.
  
• Wood tar. Sometimes referred to as creosote, this is a black, sticky substance.

In reality, one particle can contain all of the components above.

Before we go any further, we need to be aware that there are different definitions of what constitutes particles, and the distinction is based on the temperature at which they are measured. This matters because wood tars are volatile, meaning that at the typical temperature range found in chimneys, they are actually gases but they condense into particles when they cool down after leaving the chimney. Therefore, if you measure particles inside the chimney where the flue gases are hot then you only detect soot and ash, whereas if you wait until the flue gases have cooled down to ambient temperature then you will also detect wood tars. This might seem like a technical point, but including wood tars in the particle measurement will increase the mass by roughly six times.

Environmental scientists always measure particles at ambient temperature. Woodstove engineers and regulatory bodies in the USA, Canada, Australia and New Zealand also measure at ambient temperature, but in the EU they measure inside the flue at a temperature of 180 OC. For this reason, it is very important for someone wishing to reduce particulate emissions from wood stoves or boilers to decide whether their goal is to comply with a regulation, and if so, which one.

When selecting a particulate reduction strategy, many appliance manufacturers evaluate whether a catalytic converter for wood stove applications can achieve the desired emissions performance while maintaining efficient operation.

Catalytic Converters vs Electrostatic Precipitators

Both soot and wood tars can be reduced either by careful optimisation of the combustion chamber itself or by aftertreatment in the flue. Factors which a woodstove engineer will need to consider in the design of the combustion chamber include temperature, air supply, mixing of the air with the combustion products, and residence time. However, even with the best internal design, some soot and wood tars will still exit the stove and enter the flue. This is where aftertreatment can take place.

There are two types of aftertreatment technology which are used on wood burning appliances – electrostatic precipitators (ESPs) and catalytic converters, with the catalytic converter offering a different approach to particulate reduction.

Electrostatic precipitators create a very high voltage to attract particles to an electrode. They are very effective at capturing soot and ash (more than 90%), but don’t achieve anything against wood tars because these are gases at the temperature prevailing in the chimney where the ESP is installed, and unfortunately about 80% of particulate matter emitted by woodstoves (after it has cooled down) consists of wood tars. ESPs also require an electrical power supply.

The other alternative is the catalytic converter. A catalytic converter for wood stove systems promotes chemical reactions that oxidise pollutants before they are emitted to the atmosphere. They can be effective at oxidising both soot (in solid form) into carbon dioxide and wood tars (in gaseous form) into carbon dioxide and water. Compared to ESPs catalytic converters are less effective against soot, offering a reduction of approximately 25%, and they are not effective against ash, however they can oxidise 80% or more of the gaseous wood tars, thereby preventing them from condensing into particulates when they leave the chimney.

For applications where wood tar emissions represent the largest share of particulate matter, a catalytic converter can provide a highly effective solution. A properly designed catalytic converter for wood stove installations can substantially reduce emissions while operating without an external power supply. For this reason, catalytic converters are increasingly being considered alongside ESP technology in modern wood-burning systems.