Soft woods burn most readily, first because of their lower density, and secondly because the resins within them add to the calorific (heat) value of the wood. The heat given off is increased by as much as ten or twenty percent in soft woods such as pine, because of these resins. Oils and gums in hardwoods also contribute to the calorific values of those woods.
The heavier the wood when dry, the greater will be its calorific value. Green wood has a much lower heating capacity than dry wood because some of the heat from the wood is wasted in driving off the moisture. A twenty percent loss of heat here, in burning wet, or ‘green’ wood, represents one extra load for every five loads burned.
The wood should be kept as dry as possible. Wood, although ‘dry’, will nevertheless absorb moisture from the atmosphere and raise its moisture content. Wet wood only creates a lot of smoke and provides less useful heat.
Bark too burns, but it has a much higher ash content on combustion than hardwoods – up to ten percent compared to under one percent for both hardwoods and softwoods.
While the density of woods can vary markedly from species to species, the calorific value per unit weight is about the same, but slightly higher for softwoods. Potential heat output of many hardwoods is in the vicinity of 19-20,000 kJ/kg; that of softwood is around 21-22,000 kJ/kg.
Because the density of softwoods is about half that of hardwoods, one would need almost twice the volume of wood to contribute about the same amount of heat.
With the burning of wood, it is not sufficient merely to get combustion – there is more to burning wood in a fuel stove or a fireplace than applying a match. It is important to derive energy efficiently from the wood.
As wood burns, numerous changes occur. Wood is first dried, then, at higher temperatures, volatilisation of some of the components occurs; these burn to yield carbon dioxide and water, and produce most of the heat from a fire. A little ash will form. If combustion is incomplete, carbon monoxide may be produced as well. Charcoal will be formed, and this is also burned, and provides the remainder of the heat produced by combustion. The gums, oils and resins all burn readily, and help to maintain the fire.
The gases driven off have high ignition temperatures, usually well above 500 degrees C, so the temperature of the fire must be kept high. It must also be high enough for combustion of the charcoal – somewhere around 1100 degrees C or more in the fire chamber. Some fuel stoves are much more efficient than others in achieving these high temperatures.
All combustibles must be mixed with oxygen from the air. The air, on entering the combustion area, will immediately lower the temperature inside the fire, but with combustion, the temperature will be raised again. In effect, this lowering and raising of temperature is an ongoing procedure.
If the wood is too closely packed together, insufficient air will be available for combustion, it may not mix thoroughly with the combustible materials; if spaced too far apart, sufficiently high temperatures are difficult to obtain and maintain. Too much air, on the other hand, will cool the fire and reduce its efficiency. Any excess air will be heated nevertheless, without contributing to the heat of the fire. This warmed air will be lost up the chimney.
With incomplete combustion – either because of restricted oxygen or too low a temperature – smoke will be lost (or created!) with removal of potential combustibles and loss of recoverable energy.
This simply means that, for an effective, efficient wood heater or stove, a high temperature must be maintained, and sufficient oxygen added and mixed well with the combustibles.
So what sort of wood heaters are efficient and contribute to the ideal burning of wood? Or more importantly, what sorts of heaters are inefficient?
While open fires are a delight to sit in front of with a good book or magazine on a cold night, most of the heat from this type of heater is lost up the chimney; little convected heat enters the room. Most of the heat from these fireplaces comes from radiated heat. A person feels warm sitting in front of the coals or the flames, but the fire seldom heats the air in the room to a comfortable temperature. Too much air is drawn into the fire and that air needs to be replaced at a very fast rate from within the room. The air in turn is replaced by cold air from outside – achieving the opposite of what a cosy fire is intended for. The large quantity of air lost up the chimney or flue means that the fire will seldom be hot enough for complete combustion. These fires may have an efficiency as low as ten percent.
A second type of popular fire is the hood-type. Although this type is open, it is more efficient than the built-in open fire. It will warm the room, but not as efficiently as an enclosed heater.
Efficiencies of twenty to thirty percent are attainable with enclosed heaters, such as pot-belly heaters.
Controlled combustion (slow combustion) heaters have better efficiencies – up to around seventy or nearly eighty percent. Baffles are incorporated in many of these to increase efficiency by maintaining a high temperature of combustion, while further efficiencies can be achieved by increasing temperatures in the combustion zone with fire bricks or other insulating material, and by pre-heating the air prior to combustion. But with this type of heater, when the air intake is reduced to slow the burning, incomplete combustion may result, with a corresponding lowering of efficiency.
But whatever type of heater is used, wood will continue to provide idyllic indoor conditions on those cold winter nights.
The following information is from the Forestry Commission’s technical supplement: ‘Wood as Fuel, A Guide to Choosing and Drying Logs’ .
Wood Calorific Value
When choosing wood for burning there are three factors which have an effect on the calorific value (CV) or the amount of available heat per unit of fuel:
1. Species Choice
2. Wood Density
3. Moisture Content
The general differences are that hardwoods (deciduous, broadleaved tree species) tend to be denser, and softwoods (evergreen, coniferous species) tend to contain more resins. There is little variation of CV between species when tested at the same moisture content. The main differences between species are moisture content when the timber is green at the time of felling, and the rate at which this moisture is lost during seasoning.
As hardwood species are generally denser than softwood species, a tonne of hardwood logs will occupy a smaller space than a tonne of softwood logs. Dense woods will burn for longer than less dense woods, this means you will need fewer ‘top ups’ to keep a log stove burning. If you measure wood by volume you will generally receive more kilowatt hours (kWh) of heat from a cubic metre (m3) of hardwood than softwood. However, softwoods are often cheaper and easier to source.
The moisture content of wood has the greatest effect on CV of any of the variables. Not only does any water in the timber represent less fuel when buying by weight, but it also has to evaporate away before the wood will burn, using some of the fuel energy reducing the net energy released as useful heat.
How much firewood generates a Kilowatt hour of heat?
Assuming a regular hardwood used typically for firewood has a net calorific value of 4.06 kwh/kg at a moisture level of 20%, if this is burnt in a stove with an efficiency level of 70% then it would take a 0.352kg piece of firewood to produce 1kWh of heat