“Combustion of biomass can be considered as taking place in four stages. All the stages can, however, be taking place at the same time. For example, a log of wood does not burn in one go. Similarly, a stove that is recharged during burning will be experiencing different phases of combustion simultaneously – see picture.

Stage 1. In lighting the fire, supplying the initial heat source, the water content of the fuel is evaporated off at about 100C.

Stage 2. When the fuel temperatures reach between about 200C and 350C the volatile gases (compounds of carbon, hydrogen and oxygen) are released.

Stage 3. The volatile gases mix with oxygen and ignite at temperatures in excess of about 450C and burn with a yellowish flame radiating heat. Some of this heat is reabsorbed by the fuel releasing more volatiles. This process should become self-sustaining until all the volatiles have been released. The volatiles need sufficient heat, oxygen, space and time to ignite. If any of these are lacking the volatiles may leave the combustion chamber without igniting; the combustion will then be incomplete and inefficient; the fire will be smoky and may die altogether. When all the volatiles have been released, charcoal remains. (Charcoal is mostly fixed carbon).

Stage 4. The charcoal burns (oxidises), provided there is sufficient oxygen at the fire-bed, at temperatures around 800C. The carbon monoxide produced reacts with oxygen (again provided oxygen is available) just above the fire-bed, to give carbon dioxide. The charcoal will usually continue to burn long after the volatiles have been used up.

A charcoal fire requires oxygen both at the fire-bed (primary air) and just above the coals (secondary air). If there is insufficient air, the fire will give off carbon monoxide, which can be dangerous to the stove user especially in enclosed spaces.”
(Improved wood, waste and charcoal burning stoves – B.Stewart, 1987)



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:

Species Choice
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 difference between the CV of different species when tested at the same moisture content. The main difference between species is the moisture content when the timber is green, and the rate of drying.

Wood Density
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 tend to burn for a longer period of time than softwood meaning fewer ‘top ups’ are required to keep a log stove burning for a given length of time. If you measure wood by volume you will receive more kilowatt hours (kWh) of heat from a cubic metre (m3) of hardwood than softwood, though this will tend to be offset to some extent by the higher calorific value of many softwoods.

Moisture Content
The moisture content of wood has the greatest effect on CV of any of the variables. Any water in the timber has to evaporate away before the wood will burn, and this will reduce the net energy released as useful heat (as opposed to steam up the chimney).” (Wood as fuel - technical supplement – Biomass Energy Centre - www.biomassenergycentre.org.uk)

From the extracts above, you should be able to deduce the following: The drier the wood being burned, the less energy is wasted driving off water from firewood. PAR-CHAR™ will have less than 10% moisture content. If the firewood is consistent in its size and dryness then reloading does not cause massive fluctuations (inefficiencies) in the combustion process. PAR-CHAR is consistent in size and dryness making the combustion process stable and more efficient. The myth that Ash is the best firewood goes up in smoke – all species, (including softwoods) when dried properly, provide similar levels of heat per unit of fuel. PAR-CHAR is the best firewood (and someone needs to rewrite that poem!).

Thus far we have touched-upon the inefficiencies involved in harvesting and processing BIG trees for firewood, we have explained the waste-issues of water in woodfuel and we have described, the combustion process according to inefficient principles which are based around poorly designed woodstoves and poorly prepared firewood.