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Ecodesign and wood burning technology

It’s been nearly 2 years since the EU Energy Related Products directive Ecodesign requirements for solid fuel stoves were put in place,but the majority of UK marketed stoves are still not ready for this change coming in 2022. The Stove Industry Alliance has just launched the Ecodesign Ready scheme to promote the latest generation Ecodesign compliant stoves but what is it about?

The main point of the Ecodesign rules for wood burning stoves is to significantly reduce unwanted emissions from burning wood in domestic stove fireboxes, whilst maintaning a good thermal efficiency. By unwanted emissions we mean unburned fuel (Gaseous Hydrocarbons, OGC), Carbon monoxide and microscopic particles of soot and dust, and these emissions lead to poor air quality.

This kind of ruling to improve air quality is not going to go away, BREXIT or not, as the UK government has already been found wanting when taken to court about poor air quality in UK cities. In fact, the real downside risk for our industry is that it will happen sooner rather than later.


What can we do about it?

For many years it was thought to be very difficult to get wood burning stoves through the UK Defra clean air act exemption process, but today this has become something of a formality for modern clean burning stoves. Ecodesign is now a big step up from Defra exemption and is definitely a challenge even for the best R&D departments in the UK. But it is far from impossible and several EU manufacturers have been doing it for years.

Charlton & Jenrick have had their first Ecodesign compliant fireboxes independantly tested and certified during 2013 and have completed several more since then. True enough it means that some firebox configurations and layouts will become extremely difficult to get to the new mandatory combustion or efficiency levels but there are still plenty of options to work on.


What about costs?

It will definitely increase investment needed in new product design, development and approval plus added cost for features and design solutions incorporated in the production of the stoves themselves. Of course it is dependant on the actual technical solutions utilised in the end but in general, prices will rise for the consumer for manufacturers to implement the necessary improvements in performance.

So we already know it is difficult for the stove development engineer to achieve these new clean burning levels, but what exactly is needed within a firebox to produce cleaner emissions? Elements to be considered in every new design are as follows:

1. Gasification: A solid log does not just burn by itself, it needs to be broken down into constitutent parts by thermally degrading it into gaseous and solid matter. The solid elements act almost like a wick in a candle and the gases produced burn as bright attractive flames above the fuel within the firebox. To achieve this thermal breakdown it is important to get the right burning conditions, especially the retention of heat within the fuel bed. Contrary to popular belief this does not always have to be in a bed of ash, the char on a correctly designed grate will work just fine, but the conditions must always be right.

2. Secondary Air: This is invariably present in modern wood burners primarily as an air wash for the objective of maintaining clean glass, but secondary air is an important part of burning off the gases produced by logs within the combustion chamber. Sometimes a portion of the secondary air can be used to act as primary air as well in promoting the gassing off from the solid logs during the burn cycle and burning off the solid carbon char at the end.

3. Tertiary air: This is probably the development that has most reduced smoke emissions in recent years and is in fact extra secondary combustion air by a different route. A popular term used in the commercial combustion industry is “air staging” and tertiary air is doing just the same job in combination with primary and secondary air in a stove firebox. The entry points are usually above the fuel in the rear or side of the firebox and the air continues combustion flames once the majority of primary and secondary air have done their bit.

4. Mixing: This is very often overlooked but it is critical to mix the fuel and air well inside the firebox. Air that plays no part in the combustion process just cools the firebox and carries away heat into the flue - hence the poor thermal efficiency of an open fire. Fuel gases that don’t burn because they are not mixing with air will exit as emissions that will fail to meet Ecodesign. The entries for the various air injection stages plus the shape of the firebox itself contribute to the mixing and therefore completeness of combustion.

5. Baffling: Stove fireboxes originally had no baffle and so heat was lost into the flue way almost immediately. By elongating the flame path with a baffle of some kind, more heat is exchanged to the room plus a longer dwell time then exists for the gases within the burn chamber to complete the combustion process.

6. Conditions: The conditions inside the firebox are critical to the effects of all the foregoing stages. For example if the heat is lost too quickly then the flames will stop burning before combustion is completed. Also, no matter how hot they are, pockets of unburned gases with no oxygen present cannot burn any further. Air staging, the right firebox shape and good mixing help to create the right conditions but there is no sure-fire recipe for success. This is where the good development engineer earns his living.


In conclusion

The only way to succeed at achieving Ecodesign levels of combustion performance is to get the various elements of the whole system to work as one to maximise the full combination of fuel gases and air under the correct conditions. The new aspect of thermal efficiency is also not to be forgotten because in providing lots of secondary and tertiary air it can suffer due to dilution by excess air in the system. It is possible to develop and use innovations in this process such as Charlton and Jenrick’s own patented tertiary air profiling system used within Purevision stoves which optimises the mixing of tertiary air into the gases above the fuelbed.

Development of a high performance Ecodesign ready solid fuel stove is not something that can be done from a textbook or in theory, it is a highly practical and iterative process. In fact it can be compared very closely to the elite sports theory and practice of “marginal gains”. A product design tends to progress in small steps or iterations towards the final outcome which is full compliance with the new requirements.

There are many uncertainties in the solid fuel stove industry but what’s certain is there will be continued pressure to improve the combustion process to reduce unwanted emissions of particulates and gaseous carbon compounds.


Information supplied from fires and fireplaces 2017.


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