On reading the latest edition of the HETAS technical bulletin I notice a strong focus not only on compliance for fitting stoves and the regulations involved with this, but also covering such topics as ventilation strategy, combustible wall construction and ‘air tightness’. All seems pretty complex stuff for fitting a stove but all pretty crucial to ensure your home safety.
Even more topical now however is the importance of efficient stoves and the drive from the industry towards new cleaner technologies in solid fuel and the drive is nearly all based around wood burning. The buzz word around government now has changed from ‘low emissions’ to ‘clean air’ and so the industry, with strong support from The Stove Industry Alliance (SIA) is working hard at being ahead of the game in terms of clean air.
Interestingly the bulletin looks in detail at how these more efficient stoves improve combustion, lower emissions and in turn reduce the amount of fuel required.
It looks at the composition of wood which is made up of 50% carbon, 6% hydrogen and 44% oxygen. In the ideal situation the wood could be completely burned giving 50% of its volume to carbon and 50% to water, but 100% combustion does not happen with any fuel burned, even gas or oil.
When wood is heated to around 280oC it undergoes a reaction called pyrolysis which liberates combustible gases from the wood and leaves behind carbon as a solid charcoal. Here is the important bit – Up to 60% of the heat energy contained in wood is contained within these organic gases which are initially released as visible smoke.
Whilst the appliance designer goes to great effort to ensure the firebox provides the best possible combustion, all his efforts can be undermined by poor quality fuel.
Most people now know that the biggest influence on fuel quality is the moisture content of the wood which can significantly affect the combustion reaction. Freshly cut wood can contain up to 60% of ‘free water’ i.e. water that is not chemically combined as part of the wood’s structure. A 2kg log can therefore contain over 1kg of water and if this log is burnt, this water will need to be driven off before the wood will burn effectively. This not only suppresses the temperature within the firebox but inhibits the combustion process as energy is used to evaporate the moisture before it can achieve pyrolysis, increasing emissions and significantly reducing efficiency.
So, as an industry, not only is there a significant drive for more efficient stoves and the encouragement to replace open fires with clean burning stoves but also a concerted effort to improve fuel quality. As a combination this will help to reduce emissions, promote clean air and conserve valuable resources.