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For some time now the fuel market is radically changing. This has been verified by the Shell-study: the survey predicts that the market share of alternative recources will increase to develop 40% until 2050. At the moment the share amounts to about 3%.
Apart from water, wind and solar energy, wood is the regenerative energy source of the future. However, an efficient and reliable supply has to be provided. In contrast to water, wind and solar energy, wood-energy can be produced according to the actual consumption. 1. Wood-sources Which parts of wood can be used?
Using
bark for the supply of energy requires a high temperature
in the combustion chamber. As most of the bark is quite humid,
only heating systems in large heating systems achieve the
necessary burning-temperatures. The resulting residue of wood in saw mills (kindling, little pieces) has to be chopped before burning in the heating system. But because of its in-homogeneity in quality it is quite cheap and can be suitable for heating. The actual wood chips occur in every saw mill that uses a common cutting technique. Yet there are considerable differences in quality. Principal buyers are the cellulose-industry, the sanding-industry and chipboard-manufacturers. The so-called TMP-quality is suitable for the production of cellulose and fetches a good price. These relatively high prices make this sort of wood of no interest to heating-purposes. Wood chips as bought by chipboard manufacturers on the other hand are suitable for burning – in quality as well as in price. Sawdust is useful for heating systems only when these have been specially designed for sawdust / shavings. To combine sawdust with wood chips is costly. But there is the possibility to press sawdust and shavings into pellets and wood briquettes. Forest
wood contains an enormous potential of application. Nowadays
only fractions of the low-quality wood that could be used,
is actually used for heating purposes. Merely ¾ of
the increase of our forests are utilized (the average in Germany:
increase 6 cubic metres per hectare, utilization 4 cubic meter
/ha). In addition 10-20 % wood of every felled tree in the
forest.
So far, forest-industrial wood that has been harvested goes to the cellulose- and the chipboard industry. The market is not ready to assimilate the whole amount of material that could be utilised. The problem concerning energetically utilization of wood chips consists in the high grade of humidity and the price. Wood that results from the streets and landscape architecture is quite cheap, yet it is not available all over the year (mainly autumn and early spring). Additionally the degree of humidity comes to about 50% because green wood is chopped. The illustration shows the changes in the buying-behaviour of the chipboard industry during the last 30 years. The shaves of raw materials shows the tendencies at the market. The share of forest wood falls because it is the most expensive raw material of the chipboard industry. In contrast to this the recycling industry sells more and more old and used wood to the timber industry.
This tendency may very well be applied to the procuring of wood for energy generation. Criteria for quality and price of wood chips are first and foremost humidity as well as shares of bark and shavings. Among experts the chopped material with bark is called black – it is cheaper – and that without bark is called white. Therefore mainly the black product is used for energy purposes. Wood chips from saw mills are available in large amounts and – in relation to round timber – are underpaid. The reason for that fact is the connection with the globally unstable price of cellulose. The following illustration shows the prices for individual fuels:
This table shows different fuels and their market price per ton, the second table the different humidities and consequently their heating value. To calculate the prices of fuel per KWh, the basis is formed by 4,5 / 4 kWh/kg with dry wood and 2,2 / 2 kWh/kg with humid material. Especially relatively dry – although more expensive – materials come off better than humid wood. It can be perceived that technological drying of wood chips may be useful, too. 3. Supply of wood according to the customer’s need Which one of all these wood-fuels should be used? There is no general answer. The choice depends on : - the size of the installation - possiblities of storage - benefit from subsidy programmes - conditions for putting the system into operation During cold phases in winter when need of warmth is at its highest, it is possible to achieve an increase in power by the energy inherent in the wood.
Especially
small heating systems depend on dry material to reach the
temperature that is necessary for complete burning.
The big saw mills endeavour to saw their round timber as fresh as possible and the by-products are correspondingly humid. The
number of smaller saw mills that produce structural timber
and have the trees barked and let them dry for 6 months in
the forest is on the decrease. For energetical purposes, however,
this procedure is quite advantageous, because then the refuse
wood, too, contains only 30% humidity. Yet the main part of
the supply is fresh from the forest, and it might be worthwhile
to dry some of the residual-wood from saw mills artificially. 4. Drying Here the anatomy of the wood has to be taken into consideration. Water is not only bound in the cells but also in little pipes that serve to absorb water from the ground. It is relatively easy to remove the free water from the pipes; the difficulty lies in the removal of water from the cells: Cohesive forces have to be overcome. The grade of humidity, when all free water has been eliminated, is called fibre saturation point (about 20-25%). Consequently it doesn’t make sense to unbind the water in the cells with a high input of energy; 20% humidity is the limit of the drying process. There are two procedures for drying wood chips:
The fresh wood chips are stored under a roof. Depending on the outside-temperature the microbiological conversion starts after about 11 days. Even if the outside-temperature comes to only 10°C the temperature inside the heap reaches 35°C. After 2 months the content of humidity has sunk from 40% to 20%. During this period 2-8% of the dry substance has been decomposed by fungi. In the outer regions of the heap a condensation layer has developed; in it the growth of fungus is quite marked, and intense flight of spores is a concomitant phenomenon. This may have physical effects on man.
How should an optimal conception of "wood for energy" look like? The head office. in this case the HEZ, has integrated several supplier of wood on contract in a radius of 150 km. It is the head office’s task to coordinate the individual amounts of the providers with the respective demands of the heating systems. For this customer-related providing of fuel it is necessary to have a great variety of wood with different degrees of humidity and quality. The „tailor-made suit“ of the respective heating system guarantees for example that the supplied wood has an average of 35% humidity, is in accordance with the Austrian ÖNORM and contains a forest wood-share of 20%. It may have become comprehensible that such a flexibility concerning supply is only achieved with the help of several reliable suppliers. One single saw mill or forest owner couldn’t answer these demands. The supply interval and the transport itself have to be coordinated with the needs of the customer. The vehicles are either container-lorries or trailer lorries; for short distances it may be efficient to use agricultural vehicles.
There are different methods for the providing of forest wood:
The
latter system requires an expensive special-purpose vehicle
and will be only worthwhile when there is high demand of wood
chips from the forest. Additionally the share of leaves and
needles reduces the heating value.
All-terrain choppers are partly employed for forest protection. Material that has been infested by insects is chopped to get rid of the insects. In nature reserves certain unwanted tree species are felled, chopped, and then the material is removed. This material is quite cheap although it isn’t chopped primarily for fuel purposes. Shouldn’t mechanical solutions to the forest protection-problem be preferable to chemical controlling in our days? The
regular saw material is exposed to strong competetive pressure,
as already described. Therefore it should be considered carefully
where the wood chips come from and how they are produced.
Cellulose- and chipboard industry have marked preferences for particular tree species. So beech and spruce are preferred to tree species with a high share of resin a low density or heart wood. For energetic purposes these disadvantages are not of such consequence as to the chipboard industry. The differences in market price however are considerable. The heating values on the other hand don’t differ too much. 7. Summary Useful wood fuels for medium and large heating systems come from the sawing industry, the forest, the cultivation of the countryside and in limited number from the recycling industry. The most important wood fuel for these systems consists in wood chips from saw mills. Humidity and price are the most significant criteria for the utilization of this fuel. Further factors that matter are shares of shavings and contents such as leaves and needles. Because most of all small heating systems need dry wood chips, wood can be dried to a certain grade of humidity. There are different methods for the production of wood chips from the forest. The fuel has to be geared to the respective heating system. This is realized by a multitude of suppliers and in a relatively large catchments area. |
