Domestic biomass combustion overview
Wood has been used as a heat source for thousands of years but it is only fairly recently that the air quality problems that this leads to have been fully appreciated.
In an ideal world the only emissions from biomass combustion would be carbon dioxide and water, but in practice some substances which are harmful to human health are also emitted, including gases such as carbon monoxide and oxides of nitrogen as well as fine particles.
As a result of the growing awareness of the health problems that wood smoke can cause or exacerbate, regulations covering woodstoves and boilers are getting more stringent and therefore the latest appliances have to be designed to burn much more cleanly than in the past.
One of the technologies which helps to reduce emissions from woodstoves is the catalytic converter which promotes chemical reactions between the flue gases, rather like a second combustion process.
The chemistry of emsisions from domestic biomass combustions
Burning wood sounds simple, but it’s actually a surprisingly complicated process involving six discrete stages, during which the solid wood is gradually converted into flammable gases, which are then oxidised to produce heat. In an ideal combustion process the only emissions would be carbon dioxide and water, but in reality some pollutants are also produced.
These pollutants can be divided into three categories:
- Those which arise from incomplete combustion,
- Those which arise from reactions between the components of the air,
- Inorganic components of the fuel.
In the first category the principle pollutants are carbon monoxide, soot (i.e. carbon), hydrocarbons and wood tar compounds (creosote etc).
The second category comprises mainly oxides of nitrogen (NOx), which are generated both from the combustion air as well as the nitrogen content of the wood itself. If the NOx group is broken down further, the majority will be found to be nitric oxide (NO) with most of the remainder consisting of the toxic nitrogen dioxide (NO2).
The third category refers to ash.
Cutting emissions from domestic biomass combustion
Discover how catalytic converters can minimise fumes from wood burning stoves.
Abatement of fumes from wood burning
The fumes resulting from the incomplete combustion of biomass can be oxidised into carbon dioxide and water, as long as the temperature is right.
The purpose of a catalytic converter is to lower this temperature so that the appropriate conditions can more easily be obtained.
However, a range of different organic pollutants are present in biomass fumes, and each requires a different temperature threshold before it will oxidise.
In addition, these pollutants may be in the gas, liquid or even solid phases, and most catalysts used for this application only promote the oxidation of pollutants in the gas phase.
Fortunately, the oxidation of organic pollutants releases heat (i.e. it is exothermic) so the catalytic converter gets going (‘lights off’ in the jargon) with those pollutants that have the lowest light-off temperature at about 225oC and then gets hotter.
As the temperature increases it reaches the light-off temperature for additional pollutants, which may have switched to the gas phase, and these release more heat and so on in a virtuous circle.
For this reason, a catalytic converter which is working well will often glow red-hot.
Once the temperature reaches about 600oC, any organic matter (no matter which phase it is in) which has resisted catalytic oxidation so far will simply burn. This is why catalytic converters used in biomass stoves are often referred to as ‘catalytic combustors’.
Designing an efficient catalytic biomass stove is not simply a matter of starting with a standard model and inserting a catalytic converter in the flue.
A well designed catalytic biomass stove uses the heat emitted from the catalytic converter as part of its output, rather than wasting it up the chimney, and also ensures that the catalytic converter runs at a high enough temperature so that it burns off any tar which lands on it.
The subject is quite complicated but Whitebeam offers design guidelines and advice to assist stove manufacturers which are unfamiliar with this technology.
Testing emissions from domestic biomass stoves
Quantifying the fumes from woodstoves is surprisingly complicated.
Measuring pollutants from wood fuelled heaters
Measuring the emissions from a biomass stove is actually a discipline in its own right, and for this reason even quite large stove manufacturers tend to use external laboratories which are equipped for this purpose.
One of the complications is that for repeatable emissions measurements it is necessary to know the thermal output of the stove during testing, and quantifying this is quite difficult.
Also, measuring the organic gaseous compounds (OGC) requires the use of an instrument called a flame-ionisation detector, which is expensive to buy or hire and requires a trained person to operate.
Different testing procedures are used around the World, meaning that it is difficult to compare the emissions figures of stoves from one regulatory block with those from another.
The main difference arises from the way that particulates are defined. In the EU, particulates are quantified by the hot filter method meaning that they only include substances which are in the solid or liquid phase at the elevated temperature of the filter (usually 180oC).
Wood tars which are in the gas phase at this temperature are defined as ‘OGC’ (organic gaseous compounds), even though when they cool down to ambient temperature they will condense and become particulates. In North America by contrast, particulates are quantified using the dilution tunnel method, which involves cooling the flue gas down to ambient temperature before the measurement is taken.
This results in a definition of particulates which correlates more closely with that used by scientists in the air quality sector.
At present most regulations still include limits for particulate emissions on a gravimetric basis i.e. the mass contained in a cubic metre of flue gas.
However, since the health effects of particulates depend on their size, with the smallest being the most dangerous, attempts are being made to introduce regulations containing limits of the number of particulates permitted.
Regulations relating to emissions from domestic biomass combustion
In an attempt to improve air quality, the rules covering fumes from wood stoves are becoming increasingly stringent.
The limits for fumes from woodstoves and boilers
The most common reason that biomass is burned domestically is to heat spaces, for example using wood stoves, or to heat water in a biomass boiler.
Emissions regulations tend to be different depending on whether it is space or water which is being heated, although there are overlaps, for example in the case of stoves with back boilers.
n Europe, the minimum standard applying to solid fuel local space heaters with output up to 50 kW and solid fuel boilers with output up to 1 mW is the Ecodesign Directive (EN16510-1:2022). It sets limits for oxides of nitrogen, particulate matter, carbon monoxide and organic gaseous compounds.
However, the limits set in the Ecodesign Directive are widely regarded as being very unambitious and therefore some individual EU member states have decided to implement their own standards which are more demanding. For example, in Germany a regulation referred to as BlmSchV now applies.
As an alternative to regulations, some countries have introduced accreditation schemes to make consumers aware of the environmental performance of woodstoves. Examples of this include the Flammevert scheme in France and the Nordic Swan in Scandinavia.
There are increasing calls from air quality campaigners for wood stoves to be banned completely in cities so wood stove producers have realised that, regardless of regulations, unless they can produce much cleaner stoves, their future is in jeopardy.
Products to minimise fumes from domestic biomass stoves
We offer a range of catalytic converters to reduce emissions from woodstoves.
Catalytic technology for domestic wood burning
In simple wood stoves, there is normally no fan, so the catalytic converter must not cause a significant pressure drop.
Another consideration is that ash will be present, which could block the channels in the catalytic converter.
For these two reasons, it is best to specify one based on a honeycomb substrate with a low cell-density, for example 18 or 25 cells per square inch.
The flue gas temperature in appliances which burn logs can vary significantly, which means that a substrate which can act as a heat store may be preferable.
In this respect, ceramic honeycombs are better than metal ones.
Metal substrates offer the advantage that they can be welded directly to other components without the need for gaskets, and they are also more robust which is advantageous when they are handled by the customer, for example during routine cleaning.