Revue6-EN.pdf

ENVIRONMENT ASHES

The recycling of wood ash in forests France and Sweden

Emil POPOVICI, Enrico BENETTO, Patrick ROUSSEAUX, INSA Lyon

The dispersal of wood ash on the ground of the forest of origin : a means of developing in a sustainable way the use of wood energy, which is still underexploited in France.

THE FRENCH FORESTS, A RESOURCE FOR ENERGY

ITEBE

Figure 1 : Letting the branches dry some weeks in the forest avoids the losses of fertility.

France provides more than 20 % of the European production of primary energy from wood, which corresponds to about 10 million ton equivalent of petrol (TOE) a year. The advantages of the use of wood energy are very specific : it is a renewable resource, the balance of CO2 on the cycle of the life of wood is neutral, the emissions per MWh are generally lower than those of fossil fuels and the solid by-products of combustion (wood ash) can be utilized.

Using wood for producing energy does not endanger the immense French forest heritage (28 % of the total surface of France), which keeps increasing 30 000 hectares a year in a natural way and a big part of which is still underexploited. Also significant quantities of by-products of the current forest industry, such as bark and sawdust, have yet to be utilized.

The supplementary energy that can be reclaimed from these sources is thus estimated at more than 3 millions of TOE a year (that is 12 million tons of wood).

FOREST WOOD ASH : WASTE OR PRODUCT ?

The mineral substances that remain after the combustion form the ash. In the case of wood, the ash content v ar y wid e l y f ro m 0 , 2 % t o 2 % according to the species and part of the tree. For example, it is four times higher in bark than in limbs.

The proportion of mineral substances in wood ash varies according to the wood type, the woodland soil and the part of the boiler where they are gathered. On average, the composition of ashes is as follows : Ca (17 %), Si (7,6 %), K (4,2 %), Mg (1,9 %), Al (1,4 %), Mn (1,2 %), Fe (1 %), P (0,9 %), Na (0,7 %), S (0,5 %) and traces of heavy metals (0,5 %). The heavy metals have a concentration from 1,5 to 3 times higher in fly ash (smoke) compared to firebox ash.

Most of the nutrients accumulated by the tree during its growth, mainly the calcium, the potassium, the magnesium and the phosphor and in lesser measure other micro-nutriments, can be found in important concentrations in wood ashes. This is why, in France, most of the ashes stemming from boiler rooms which burn only forest wood are used by farmers who have contractual agreements with the producers of energy. These ashes, which otherwise would be disposed of, are given free to the farmers who use them as natural fertilizers and take care of the costs of transport and spreading.

IMPACT OF ASHES ON ECOSYSTEMS The impact of harvesting stem wood on the forest ecosystem is unimportant, even though it interrupts the cycle of decomposition of the wood that would transfer the nutrients accumulated by the trees back into the ground. When also branches and leaves are removed, the impact becomes more important because about 60 % of the phosphor, nitrogen and potassium are in these parts.

WOOD ENERGY N°6 < JULY 2002 > 46

HOW AND WHEN IS WHAT RECYCLED ?

Mainly it comes to removing branches and other residues from the forests, where recycling ashes stemming from combustion, or contributing compensating nutrients, are possible. To support the growth of a new generation of trees optimally, the dispersal of nutrients contained in the ashes has to begin five years before the final felling. The recycling cycle of bioashes stemming from combustion includes the collection, the treatment, the transport and the dispersal in the forest (figure 2). It can also be necessary to store ashes in intermediate warehouses between these main stages.

In Sweden, for example, all these operations are strictly regulated to prevent the forming of ash leachate, by which most of the nutrients would be removed by rainwater and percolate to other land than that of the forest of origin. The recycled quantity is limited according to the quality of soil, the species of the wood extracted and the quantity of wood that will remain in the forest. The maximum quantity that can be recycled in the case of removal of the wood and all its residues is 3 tons per hectare during the time of growth of a tree (50 or 60 years). This quantity is limited to 2 tons in the case when trunks

and bark are removed.

The recycled ashes must not be too solidified and agglomerated to facilitate the spreading and the slow dissolution over 5 to 25 years, and the concentration of nutrients and heavy metals is strictly regulated.

The recycled ashes must be stabilized by adding water and then solidified.

That is done by : - Auto-solidification, which includes mechanical grinding to the desired size, uniform dispersal on the ground, addition of water and compression ; - Granulation : in fixed installations of compressing or rolling and in the presence of water, this technique is the best adapted to small deposits of ashes and gives a product that presents a rather high dissolving time.

To maximize the economic profitability of recycling, it is necessary to try to use equipment already existing on the spot, which require minimal adaptation, for example lorries with exchangeable bodies or tractors with spreading devices (figure 3). In isolated sites or sites having reduced accessibility, it is possible to use helicopters but costs are three times as high.

The cost of disposing of waste has increased due to environmental pressures and to the tax on waste disposal. At the moment, the cost varies from 68 to 114 euros per ton, taxes included. On average, in Sweden the costs of recycling ashes in forests amount to 150 euros / ha, or from 50 to 75 euros for a ton.

In conclusion, considering the costs of stocking and the advantages for the forest and boiler site owners, this option seems more and more promising. ᔤ

Figure 3 : For an economic dispersal of ashes, existing equipment is used.

CONTACT : Enrico Benetto, Emil Popovici EcoInnova sas Environment Park Via Livorno 60 10144 Torino Italy ebenetto@ecoinnova.it epopovici@ecoinnova.it www.ecoinnova.it

The ash recycling is a good example of industrial ecology because it is a closed circuit.

FROM " WOOD FUELS BASIC INFORMATION PACK ", JYVÄSKYLÄ2000

Nutrition losses become evident in the long term and the fertility of the forest ground can decrease noticeably.

To avoid all this, during the harvest of conifer wood, the branches can be left to dry in the forest for a few weeks (figure 1). Needles will then fall and release most of the nutrients into the ground. However, in that case, the rate of biomass harvest decreases and the total cost can increase.

Another solution to re-stabilize the balance of nutrients in the ground is the recycling of ashes arising from combustion of wood. In that case, all the nutrients taken from the forests during the harvest of the biomass (except for nitrogen and some other constituents that are lost in smoke) return to the ground and the cations Ca2+, Mg2+, K2+, with their alkaline character, help to decrease the acidity of the soil.

Recycling can be considered a very good example of industrial ecology of wood energy because no substance external to the ecosystem or manmade contaminants are introduced.

ITEBE

ENVIRONMENT ASHES

WOOD ENERGY N°6 < JULY 2002 > 47