Domestic biomass combustion overview
Woodsmoke arises due to the fact that wood is difficult to burn completely, especially under the conditions present in a relatively simple stove. Part of the problem is that there is no such thing as ‘standard wood’: one species of tree burns differently from another and the moisture content and size of the logs also plays a part, meaning that wood burning stoves have to be designed so that they are not too fussy about what goes into them.
The result is that flue gases of wood burning stoves contain some substances which can be categorised as pollutants. These include gases, liquids and even solids. Some of the gases are responsible for the characteristic smell of burning wood, which is not always welcome, especially in congested cities. Liquid pollutants comprise tars and creosote which may condense on the internal walls of the chimney where they will gradually build up and constitute a fire hazard. Finally, solid particles may be carried some distance by the hot air currents in the flue gas but will eventually come to Earth as dust and soot.
Domestic Biomass, Chemistry
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:
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.
Domestic Biomass, Catalysis
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.
Domestic Biomass, Regulations
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.
The Ecodesign directive applies to solid fuel local space heaters with output up to 50 kW and solid fuel boilers with output up to 1 mW. It sets limits for oxides of nitrogen, particulate matter, carbon monoxide and organic gaseous compounds. The Ecodesign directive for solid fuel boilers comes into force on 1st January 2020 and applies to solid fuel local space heaters with effect from 1st January 2022.
Domestic Biomass, Testing
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.
Another consideration is that domestic biomass stoves don’t burn in a constant manner (especially if they’re burning logs) so the emissions fluctuate significantly. However, getting a rough measurement of carbon monoxide can be done using a standard flue-gas analyser, and solid particulate matter can also be quantified using a smoke or dust meter. In Germany the emissions from biomass stoves are checked every year by chimney sweeps!
Products for domestic biomass combustion
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 is preferable. In this respect, ceramic honeycombs are better than metal ones.
More sophisticated domestic biomass heating appliances incorporating fans to drive air through the catalytic converter may utilise reticulated foam substrates. Metal substrates can also be suitable but when using any substrates which ash cannot pass through it is necessary to include some means to prevent the ash from reaching them.