Burning Wood

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 dependant on its size and the amount of foliage it has.

Understanding the burning process helps to make better use of the wood

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.
   Please see the tips below on seasoning and storing wood, which can help to prevent this waste.

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.

3. Charcoal - When most of the tar and gasses have burned the remaining substance is charcoal. A hot bed of coals can enhance the combustion process when burning larger pieces of wood.  Start with a small fire to develop a bed of hot coals. As the coal bed develops and the stove heats up, slowly add larger and larger pieces of wood. It takes time to build a good coal-bed, but its worth the effort

What is the best wood to buy?
 Some notes about soft wood and hard wood.

The difference in so-called 'hard' woods and 'soft' is the density of their cells or fibres. So, the harder a wood, the greater the density and quantity of fibres in any square inch of that wood. 
As a general rule, the deciduous trees (those that loose their leaves in the autumn) are usually thought of as hardwoods and the evergreen trees (such as pines, firs and larches) as the softwoods. But generalisations are of course always subject to many  exceptions. Some evergreens may well be harder than some deciduous trees. Birch, for example, is not very hard at all. So we should understand that there is a whole range of densities amongst our tree species, including medium dense woods, which cannot be satisfactorily classed as hard or soft.
For construction purposes and different wood working projects, the shape and arrangement of the individual cells and its growth patterns are also important considerations, but when judging the value of a bit of wood for its use as a firewood, we look especially at the density of fibres.
One of the reason for this is that firewood tends to be sold by volume rather than weight. Assuming that the wood is reasonably dry, the weight of a square foot of good hardwood may be double of that of a square foot of softwood. This means that the same volume of hardwood will provide you with more fuel to burn than an equal amount of softwood, simply because it contains more substance.
Yet the price of that hardwood will normally not be double that of softwood, because it took the same amount of labour to prepare. So, if a trailer full of softwood costs £100 and the same size trailer full of hardwood costs £120, the more expensive option may well be the most economical.
The other advantage of good hard firewoods are that the stove does not need to be fed as often and the coal-beds made by the glowing wood may burn more easily overnight.
However, the ideal situation would be to have a store of both hard and soft woods, because the softer woods also have distinct advantages. They light more easily than the slower burning hardwoods and if the softwoods are dry, they create a hotter, more intense fire. The draft created by the hotter fire moves the air up the chimney faster.
After reading the notes above about the burning process, you will understand that means less pollution in the form of smoke and less creosote condensation in your chimney.
The denser hardwoods tend to smoulder more easily when the fire is lit first, so their flue gas temperature will be much cooler.
Because softwoods like pine and larch contain a lot of resins and pitch, people think that they will fur up the chimney with creosote more easily than a hardwood like oak. This is not necessarily true at all. It's not the pitch that is the problem, it's the water IN the pitch. Once the water in the wood has evaporated, that pitch becomes high octane fuel! When dry, softwoods burn extremely hot!
There is also the matter of seasoning (drying out of the wood - please see below) to be considered. When you buy wood, it will usually have been cut in the winter of the year you buy it. Hard woods tend to take longer than softwoods to fully dry out.
Softwoods cut in the previous winter should, with proper storage, be ready to burn the next autumn, whereas many hardwoods may take a bit longer than that. Oak, for example, is very slow to dry out and ideally left for two years.
Summarising we can say that it is always sensible to buy this year for next years fuel supply and that it is very handy to have both soft and hardwoods. You can use the softwoods to start a good fire and you will have additional control over the fire (besides regulating it by opening and closing the vents and so on) by adding slower burning wood to fast ones if you want to leave the fire without looking after it for a bit. Alternatively you can add some fast burning softwood logs to the slower hardwood logs to instantly produce a cheery flame or rev up a fire, which has been neglected.

Tips on seasoning and storing wood

Wood, which has recently been cut and is still full of sap and water is known as "green" wood.
Greenwood will generally burn poorly and inefficiently, because it can have from 50% water (for example: Ash) to as much as 140% water (for example: Elm) in its cells. It may be hard to light, smoulder, not put out any heat and cause more than the usual amount of creosote to build up in your chimney. One fresh-cut cord of oak may contain enough water to nearly fill six, 55 gallon drums.
So our aim will be to dry the wood out to below 25% moisture content and this process is called seasoning. As the name implies, we basically store the wood for a season or so, while it dries, but there are things we can do to speed up seasoning by cutting the wood now rather than just before we use it.
Wood is composed of bundles of microscopic tubes that were used to transport water from the roots of the tree to the leaves. These tubes will stay full of water for years even after a tree is dead. This is why it is so important to have your firewood cut to length for 6 months or more before you burn it, it gives this water a chance to evaporate since the tube ends are finally open and the water only has to migrate a foot or two to escape. Splitting the wood helps too by exposing more surface area to the sun and wind, but cutting the wood to shorter lengths is of primary importance.
Here is a tips to decide whether your wood is ready or not: Well seasoned firewood generally has darkened ends with cracks or splits visible, it is relatively lightweight, and makes a clear "clunk" when two pieces are beat together. Green wood on the other hand is very heavy, the ends look fresher, and it tends to make a dull "thud" when struck.
Another thing we can do to help is store our wood properly. It is all just plain boring common sense of course: Store if off the ground by building the pile on some longer logs (or whatever method you can devise). A shed with an open side makes an ideal storage place, as the air can circulate around the logs and help to dry it out. Unventilated spaces or plastic tarps, which never get taken off will prevent the drying and evaporation process and cause moulds and rot. So, if a tarp is your only option, take it off frequently to air the wood on fine days. And remember to put it back on again in time: your seasoned firewood will reabsorb large amounts of water if exposed to rain, snow and excessive dew, which is liable to make it rot and be unfit for making a good fire.
When you build up a store of firewood, remember that the wood may start to deteriorate after 4 to 5 years, although this is of course variable and depending on storage conditions and species involved.

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