2. Wood Energy and Environment

A Printable PDF-version of section 2.1

Contents of this section:

2. Wood energy and environment

2.1 Forest energy and greenhouse effect

2.1.1 Forest and carbon cycle

2.1.2 Old growth forests

2.1.3 Conclusions

2.2 Emissions from Wood Fuels

2.2.1 Sulphur oxides

2.2.2 Nitrogen oxides

2.2.3 Production chain emissions

2.3 Dust and organic compounds emissions

2.3.1 Organic compounds

2.3.2 Dust

2.3.3 Heavy metals

2.3.4 Conclusions

2.4 Nutrient loss from forests

2.4.1 Nutrient balance of forests

2.4.2 Ash recycling

Page by Markus Huhtinen 1/2006

2.1 Forest energy and greenhouse effect

2.1.1 Forest and carbon cycle

The green plants take in carbon dioxide (CO₂₎ from the atmosphere and store carbon to their biomass as long as the photosynthesis process produces more biomass than the cell respiration process relases. All combustion and decaying processes of biomass release carbon dioxide to the atmosphere. Simply put: A plant stores carbon as long as it keeps getting bigger.

Forests behave the same way. As long as the carbon assimilation of green plants is stronger than the respiration and decomposition of vegetation, the ecosystem functions as a carbon sink. Again, this means  that the total biomass of the forests keeps getting bigger.

The net accumulation of CO₂ by a forest stand equals to its net biomass volume growth. There are always dying and regeneration processes going on within a forest stand. Majority of the trees removed in low thinnings would eventually die and decay before the forest reaches a mature stage.

One hectare of Middle-European forest will take an average of 7.5 tonnes of carbon out of the air each year during the first 20-60 years depending on the species and site productivity.

Thinning has little effect to the total production and to the CO₂ intake of the forest stand. The carbon assimilation of the remaining trees reaches very fast higher level. This means that the remaining trees simply grow faster. 

Recent evidence suggests that expanding forest cover worldwide could absorb a considerable amount of carbon dioxide from the atmosphere (Pearce 1992).

If, in general, the wood is harvested at a sustainable rate, using it for energy purpose does not result in any net increase in atmospheric carbon dioxide. In most European countries the net volume growth of forests is positive. 

Theoretically, the long-lifecycle wood products such as wood used for construction would fix carbon and keep it away from the atmosphere for a long time.  In general the energy wood is mainly low-value timber with no alternative use. If it is not burned, the carbon it contains will be released to the atmosphere through normal decaying process.

2.1.2 Old Growth Forests

There has never been any pressure to utilise mature forests for energy production purposes. However, some  aspects are worth mentioning here. 

In a climax-type ecosystem, such as mature rainforest the carbon balance is  close to zero, and most of the carbon is stored as the stemwood biomass. The assimilation rate may be high, but the decomposition and respiration are also rapid.

Some very humid types of rainforest actually form layers of peat transforming slowly into brown coal. This kind of areas naturally keep acting as CO₂ sinks as long as the forest cover remains uncut.

On drier regions the forest fires eventually release most of the carbon assimilated by trees back to the atmosphere. The risk of forest fires gets naturally higher as the forest grows older and the amout of dead, dry wood and litter becomes bigger.

In some northern boreal forests the forest cover creates a cool and humid microclimate which favors the accumulation of the peat. If the forest cover is removed the decomposition of peat may follow  releasing CO₂ to the atmosphere.

Some studies also indicate that old-growth forest continues to remove carbon even when fully mature, and that old and wild forests are better than plantations at removing carbon dioxide from the atmosphere. The short-rotation plantations only store carbon during their rotation period.

Soils in undisturbed tropical rain forests and temperate woodlands contain carbon derived from fallen leaves, twigs and buried roots that can bind to soil particles. When such forests are cut, the trees' roots decay and soil is disrupted, releasing more carbon dioxide than the amount which is in the stemwood.

2.1.3 Conclusions

Any energy production using wood biomass is likely to replace the use of fossile fuels thus reducing the total emissions of greenhouse gases.

Reforestation processes create areas which can act as carbon sinks for decades. The removals from thinnings can be burned without large-scale effect to the carbon balance.

A final cut of the forest stand releases all the carbon that the the forest has accumulated back to the atmosphere.

Clearcuts of old forests can lead to large CO₂ emissions from forest soils. Especially areas which accumulating peat layers are efficient CO₂ sinks.

As in all burning processes, the most effective way to reduce the amount of greenhouse gas emissions of energy production is to use as efficient methods as possible. The co-production of electricity and heat  at so-called CHP plants can have an efficiency rate of over 90 %, whereas the electricity production efficiency rate with wood burners (condensing powerplants) usually falls below 45%.  :

2.2.Emissions

All burning processes release CO₂ and water vapor to the atmosphere

Page and images by Markus Huhtinen 25.10.2006  

Carbon Cycle. The figures indicate the amount of carbon in gigatonnes (GT) (Image by Wheeling Jesuit University/NASA)

The extensive disturbance may lead to increased CO₂ emissions from soil (image by Markus Huhtinen).