Wood is stored solar power!

From all terrestrial organisms only the green plants are in the position to take up and chemically store solar power. This procedure is called photosynthesis.

Carbon dioxide and water are converted into sugar connections by supply of solar power. From these the components of the wood are formed. The chlorophyll of the leaves and needles makes CO2 and water react with one another.

The solar power needed for photosynthesis is measurable. For the formation of glucose (C6H12O6 ) 2,872 kJ (0.8 KW/H) is used and thus stored.

As by-product of this procedure oxygen results.

With the burning of wood the process of photosynthesis is turned around. From cellulose water and CO2 develop again. A closed cycle!

The complete cycle in nature lets biomass develop and at the same time divides it again into the individual parts. The fossil energy-sources oil, coal and gas were created millions of years ago by the bond of CO2. At that time the CO2-content of the air did not permit human life. By the storage of carbon the CO2 content in the atmosphere went down. Since 100 years we burn these carbon supplies, thereby produce additional CO2, which cannot be stored in the same measure again. This rise of the CO2 content in the atmosphere causes the so-called greenhouse effect - increased absorbing of the solar power in the atmosphere and thus its heating up. Climatic changes and natural catastrophes are the results.

Wood consists to nearly 50% of cellulose, which lends structure to the wood and which together with the Hemizellulose provides the support frame. The lignin as bonding agent is embedded into this structure. . Wood can be compared to reinforced concrete: the cellulose represents steel and the lignin is the cement.

The heat value of the wood

The amount of heat a fuel is actually able to produce is called heat value (Hu). The heat value of the wood depends essentially on two variables:

•  Water content

•  Weight per each volume unit (density)

The more water is contained in the wood the smaller its heat value, since the water evaporation in the process of burning consumes warmth too. The water content (w) of the wood is the amount of water, specified in per cent, related to the total mass. Example: "Forest-fresh wood", which consists in equal measures of pure wood mass and water, has thus a water content of 50% (100% wood moisture).

From the diagram the heat value (Hu) in dependence of the water content can be extracted. The usable energy content considerably decreases with increasing water content.

Example: With a water content of w= 20% the heat value comes to 4,0 KW/H per kg wood

The following diagram shows the dependence of the lower heat value on the moisture content of wood with resinous wood and hardwood:

Related to the mass (weight) of the wood the difference of the individual wood is negligibly small concerning the heat value.

. The following bar chart represents the heat value of different tree species with 25% water content compared with beech. The basis of the comparison is the weight! Wood briquettes or pellets are traded according to weight. Wood briquettes or pellets are traded according to weight. It becomes clear that resinous woods have a higher heat value than hardwood due to the higher resin portion.

Relative heat value comparison of different wood species concerning weight

In the case of a water content of 25% the comparison of unit volume shows another picture. In respect to volume, poplar has only 60% of the calorific value compared to beech, however almost the same heat value in respect to weight.

Stages of the wood burn

The burn of wood is a complex procedure, which runs off in several stages. The burn begins with the drying and degassing phase. Firstly water evaporates. Since energy is needed for that processs, the heat value is the more reduced the higher the water content. In this phase the temperature of the wood hardly rises over 100°C. When the water has evaporated the temperature rises.

The thermal decomposition takes place at a temperature of 160 – 180°C (Pyrolyse und Degassing phase). With rising temperature the decomposition increases. 250°C onwards it is on its peak. Now the decomposition generates more warmth than it uses up. In this phase the pyrolyse-reactions are strongly exotherm. The wood has lost about 85% of its mass (water, carbon and combustible gases). Charcoal is left.

The burning itself is subdivided in four stages. Often these stages take place at the same time.

Wood burn needs air:

• about 80% primary air for decomposition (Pyrolyse),
  gas production and charcoal-burning;
• approx. 20% hot secondary air for complete outburn of wood gases.

During the degassing phase about 70% of the calorific value are released. The gas produced during degassing contains the inflammable constituents carbon monoxide, hydrogen and organic compounds. Under air supply it reacts, producing CO2, H2O and releasing heat. If the burning process is interrupted during this phase, e.g. by withdrawal of heat, a high emission and odourful gas will be the result.

It is necessary to provide enough oxygen during the reactions. In the course of burning wood gases and charcoal are produced. Ca. 2/3 of the energy is provided by gas, whereas ca. 1/3 is produced by charcoal.