Advantages of Clean Burn Wood Appliances

Robert Burke, Technical Director of HETAS advises about the advantages of Clean Burn Advantages of Clean Burn Wood AppliancesWood Appliances

Using modern clean burning and efficient wood burning appliances gives us a huge opportunity to save money on fuel and to protect the environment. In the April (#4) Technical Bulletin from HETAS we wrote an article “Improving Air Quality”. This article builds on the subject by taking a look at the science of combustion and how the latest clean burn technology can massively reduce emissions and at the same time cut down on the quantity of fuel being used.

Wood as a Fuel
To understand how wood burns we must first look at the composition of wood, which is made up of approximately 50% carbon 6% hydrogen and 44% oxygen, none of which is harmful to health. Unfortunately, no fossil fuel burning appliances can claim a 100% full combustion reaction and therefore all gas, oil and solid fuel, wood, biomass appliances have to deal with the fumes they generate effectively.

Advantages of Clean Burn Wood Appliances 2When wood is heated to around 280ºC it undergoes an exothermic reaction called pyrolysis which releases gases from the wood and leaves behind carbon or charcoal and ash. Up to 60% of the heat energy contained in wood is contained in the organic gases which are initially released as smoke. Within the products of combustion there are many different chemical compounds, all of which in the ideal situation can be burned away leaving only carbon dioxide and water. Though as stated above there is never 100% combustion in a real world situation. For any fossil fuel, we are left with varying volumes of gases including carbon monoxide and hydrocarbons in the exhaust gas dependent on how good the combustion reaction is. The very best modern wood burning appliances produce much cleaner exhaust gases and fewer particulates, and at the same time burn less fuel for the same heat output.

The Combustion Reaction
Knowing that wood burns as both a gas (the hydrocarbons Advantages of Clean Burn Wood Appliances 3produced during pyrolysis) and as a solid (the carbon element left as charcoal), setting up both an efficient and clean combustion environment can be quite a challenge. But it is a challenge being taken on with excellent results by today's designers of clean burn appliances. To get as close to 100% combustion of wood as is technically possible a number of elements have to be incorporated into a firebox design. The ideal design should:

  • supply sufficient oxygen to satisfy the combustion reaction, but not too much to cool or carry the hydrocarbon gases away from the firebox before they can burn,

  • allow the hydrocarbon gases sufficient time in the right conditions so they can burn,

  • generate a high enough temperature so the hydrocarbon gases can ignite, and

  • provide sufficient turbulence so the hydrocarbon gases and oxygen can mix together.

The above conditions are known within the combustion industry as the three T's, Time, Temperature, and Turbulence.

The above conditions can only be provided in the well-engineered and controlled environment of a clean burn closed stove. The advantages of a designer aiming to get as close to the 100% complete reaction are more heat output from each kilogram of fuel used, and greatly reduced emissions. 

Influences of Fuel Quality
 Whilst the appliance designer can spend a great deal of effort ensuring the firebox provides for the best possible arrangement of the three T's, all his efforts can be undermined by poor quality fuel. The biggest influence on fuel quality is the moisture content of the wood which can significantly alter the combustion reaction for the worst.

Advantages of Clean Burn Wood Appliances 4Wet Wood – When a tree is felled, even one that has been dead for several years the wood will contain a substantial amount of free water. Free water is water that is not chemically combined as part of the woods structure; fifty to sixty per cent of the weight of the wood can be water. A two kilogram freshly cut log can therefore contain in excess of one litre of water which needs to be dried out of the log before the log can be safely used as firewood. Green wood is usually seasoned or kiln dried to bring the moisture content down to between fifteen and twenty five per cent dry weight of the wood. If green wood is used the one litre of water in each two litre log will need to be boiled off before the wood will burn effectively. This has the effect of suppressing the Temperature (one of the three T's) in the firebox and stopping the hydrocarbons burning, greatly increasing emissions and significantly reducing the available heat.

Overly Dry Wood – This presents different problems to the combustion reaction, in as much as the wood will burn quickly and very hot. This rapid pyrolysis of the hydrocarbons can overwhelm the available oxygen being supplied to the firebox preventing total combustion of the gases. Again if the hydrocarbons aren't burned they will form smoke and pollution degrading the local air quality.

Reductions in Emissions Being Achieved
Fortunately as an industry our designers have progressed in constantly improving their product, with greater efficiency and substantially reduced emissions. New clean burn appliances offer a significant improvement in their environmental credentials compared to older stoves or open fires. It is reported in America that clean burn appliances have reduced emissions by more than 90% on a similar heat output from an older open appliance.

Combating Emissions – A Call to Action
Vast number of people across UK and Europe like to sit in front of an open fire for the ambience of the flickering wood flames. Some are dedicated to replacing other fossil fuel use with wood and biomass products.  However they are unlikely to know how much they can help themselves and the general environment by getting rid of old open fires and very old appliances in favour of modern clean burn and efficient stoves and boilers.

Advantages of Clean Burn Wood Appliances 5As solid fuel installers, chimney sweeps and service engineers, carrying out your normal business, you are in the ideal position to educate the customers. If you see them burning wood inefficiently and polluting the local area, or you see an open fire being used for wood burning, please let the user know the downside of what they are doing. Have a conversation about how they can improve their own lives and benefit the environment by changing to a modern clean burn and efficient appliance. With your help, as the people on the front line, we can over the coming years clean up the name of wood burning in the UK and replace open wood fires and old inefficient appliances with clean burn modern appliances; and at the same time keep our industry buoyant and progressive.”


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