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Sunday, 2 May 2010

Wood Fuel Facts

When you have a basic understanding of the processes involved in buying, preparing, storing and burning wood, we can make wood a more environmentally friendly fuel and get more value for our money.

Burning Wood

Apart from the Sun itself, trees have supplied the warmth to enable us to live in cooler climates for hundreds and hundreds of years and for many people all over the world have provided the means to cook foods, which would otherwise be impossible or unpalatable to eat. Firewood was of course also a major source of light in the long dark winters before the event of electricity.

Even today a huge part of the growing world population, probably between 2 and 3 billion people, depend in some way or another on trees for heat and cooking. That puts a great strain on our disappearing forests, as well as on the people, who have to walk further and further to get some wood. In many parts of the world it is not unusual to spend a large part of the day, traveling many miles, to keep a family in wood and water.

Burning wood in a stove is more efficient than burning wood on an open fire and can easily cut down the amount needed by half.

In Great Britain wood fires have evolved from a basic necessity to almost a luxury. Of course there are still many rural people who use local resources to heat their homes, but in urban areas, there is often a smoke control zone.

The technology of wood burning stoves did not change much for hundreds of years until the 1980's, when a new generation of highly efficient wood burners were developed to comply with the increasing need to prevent air pollution. These newer models increase the heat efficiency of the stove, as well as dealing with the 'exhaust' products of the fire so that they are able to be used in smoke control areas.

Clean burn stoves, Cleanheat Stoves

Cleanburn or Cleanheat stoves are incorporated with a sophisticated system that allows warm air to be introduced just above the normal height of the fire. The effect is to allow the combustion of unburned hydrocarbons in the smoke stream. This, in turn, provides not only a ‘cleaner burn’ (i.e. less soot particles going up the chimney/flue and into the atmosphere) but also generates up to twice the heat output from the same amount of fuel. Furthermore, you will enjoy the sight of even more flames. For more Information check out our article on Cleanburn and cleanheat stoves.

Is wood an environment-friendly fuel?

Many people think that wood is a truly green source of fuel, because, unlike fossil fuels, it does not release any extra carbon dioxide (CO2) into the atmosphere. This is true only if we the trees we use up are continually replaced. Preferably with a generous safety margin. What follows is an explanation why this is so for those of you not familiar with the subject.

Carbon dioxide is one of the famous greenhouse gasses, which in large quantities has the effect of creating an invisible blanket around the Earth This 'blanket' traps the heat of the sun and so contributes to climate warming.

Back in the dawn of pre-history, our Earth was a hot place with an atmosphere full of CO2 and other gasses, which no human being could have lived in. Over the course of millions of years many generations of plants and trees have created an atmosphere, where animals and people can breathe. Plants do this because they take CO2 (a molecule consisting of 1 part carbon and two parts oxygen) from the air and use the carbon in this gas as food to build their tissues and release excess oxygen back into the air. Animals and people do the opposite: we breathe in oxygen and breathe out CO2 as a waste product.

I have read somewhere that the fossil fuels (coal, oil, natural gas, etc.) we use up in just one single year account for about 1 million years of patient work by plants and trees to convert the air into the carbon compounds, we now so 'conveniently' use to heat our homes, provide our lights and fuel our cars. That is a truly staggering statistic!

When we burn the wood of a tree, this will release no more CO2 than this particular tree took out of the air when it grew. So as long as we make sure that we grow enough trees to continually replenish the CO2 absorbing capacity of what we burn up, there is a balance. It is like living on the income of the interest of a bank account. As long as you spend no more than the interest generated that year, your income will remain stable. As soon as you use up more, you are on a downward spiral. Your capital disappears and the interest you receive will be less and less.

Therefore wood is only an environment-friendly fuel if it comes from well-managed woodlands and forests, where more trees are grown each year than are harvested. The reason why many more trees need to be planted than we cut down, is because the new young trees will of course be much smaller than the larger ones we harvest. The CO2 absorbing capacity of a tree is of course very much dependent on its size and the amount of foliage it has.

Understanding the burning process

It helps a lot to have a basic insight into what actually happens when wood is burned for at least two reasons. First of all you will be able to burn your wood with the least impact possible on the environment. Secondly you will also benefit by having less maintenance to do on your chimney, gain the knowledge to enable you to get more out of your wood and have warmer fires.
There are 3 stages in the wood-burning process:

1) Evaporation - When you light your fire a lot of energy will be needed at first to boil away any moisture, which is left in the wood. Using energy to drive off excess water in firewood robs the stove of energy needed for an efficient and clean burn. Also, much of the energy wasted in evaporating water is energy that could have heated your home. This is both wood, money and effort wasted.

Using unseasoned or damp wood is therefore not a good thing. To sum it up: The effective available heat is MUCH less because there are less wood fibers in each pound of wood put in the woodburner; a good percentage of that heat must be used to evaporate all that water before those wood fibers can burn and the presence of all that moisture tends to keep "putting out" the fire, and therefore making it burn very poorly, which tends to produce a lot of creosote and pollution.

2) Emissions - As the heat of your fire intensifies, waste-gases (smoke) are released from the wood. Unburned smoke is emitted into the air as either as pollution, or condensed in the chimney causing creosote build-up. It takes time for the air in your chimney to heat up. When it is still cold you get an effect similar to the condensation of hot breath on a colder window or mirror. So when the by-products of combustion (smoke in the form of gasses) exit the fireplace or wood stove, and flow up into the relatively cooler chimney, condensation occurs.

The resulting residue that sticks to the inner walls of the chimney is called creosote. Creosote is formed by unburned, flammable particulates present in the smoke. It is black or brown in appearance. It can be crusty and flaky, tar-like, drippy and sticky or shiny and hardened. Quite often, all forms will occur in one chimney system.
If the wood you are using is rain logged, or green, the fire will tend to smolder and not warm the chimney sufficiently. Wet wood causes the whole system to be cool, and inefficient. In contrast: dry wood means a hot fire, which results in a hot flue, and a hot flue means much less creosote clogging up your chimney. If your fire is hot enough to burn up the gasses and particals released from the wood, there will of course also be less air-pollution. Waste gases from wood need oxygen in order to burn. This is why starving a fire for air, or “banking down a fire” is the worst way to burn. Always give a fire a generous supply of combustion air.

You can improve the situation by insulating your chimney to make it easier to heat up, as well as starting the fire with a good supply of lovely dry kindling, which will also help to heat the air in the chimney.

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