The following paper was presented at Bioenergy 84 held on 15-21 June 1984 at the Swedish Trade Fair Center 
in Goteborg, Sweden.

TORREFIED WOOD FROM TEMPERATE AND TROPICAL SPECIES. ADVANTAGES AND PROSPECTS

J.P. BOURGEOIS* and JACQUELINE DOAT**

*    ARMINES -ECOLE DES MINES DE PARIS, 60, Boulevard Saint-Michel, 
     75272 PARIS CEDEX (France)

**  CENTRE TECHNIQUE FORESTIER TROPICAL, 45Bis, Avenue de la Belle Gabrielle,
     94130 NOGENT SUR MARNE (France)

ABSTRACT

This paper gives the results obtained by the thermo-condensation of a few tropical and temperate woods with special reference to energetic yields and potential uses of this new product, called "torrefied wood".

PRELIMINARY  

Firewood represents 5.4% of world energy. However, the use of wood is different from one part of the world to the other. Wood represents 60% of the energy consumption in Africa, 17% in Asia, 21% in South America and only 0.7% in developed countries. In the developing countries (having more than 30% of the global population), wood and charcoal the main sources of energy.

The judicious use of forestry resources has already been the subject of many system studies.

Our researches are guided towards:

To day, charcoal is the main product obtained from the thermal processing of wood. Therefore, according to the technology, charcoal contains from 30% to 60% of the initial energy of wood. The energy loss, which occurs during pyrolysis, is located, firstly, in the incondensable components and secondly, in the pyroligneous acids and the phenolic tars.

The ARMINES' technical experiments have led to the isolation of products that mainly correspond to the destruction of hemi-celluloses (intermediate matter between dry wood and charcoal, containing 90% of the initial energy).

We give hereafter the results of our work on a few temperate and tropical species.  

WOOD CHARACTERISTICS 

Four different samples were selected for the study: two temperate woods and two tropical species. The characteristics of the samples are given in Table 1.

Table 1. Characteristics of the Samples 

The samples were converted into chips and air-dried. First of all, the woods were analyzed on a chemical point of view.
We measured also their calorific value and elementary composition.

The results are given in the Table 2.  

Table 2. Chemical Composition of the Raw Materials (on bone-dry woods) 

The chemical composition of the woods is different depending on the species. The temperate pine contains a lot of solvent extractives while the temperate hardwoods have a lot of water extractives (tannins). The tropical pine has the highest quantity of lignin and the temperate hardwoods the highest % of peritosans, softwoods contains mannans.

PRODUCTION OF TORREFIED WOOD

The aim of the process is to concentrate the wood energy into a product formed during a short and low temperature pyrolysis.  What happens during the pyrolysis of a ligno-material?  

The main steps are summarized hereafter.  

First of all, the wood loses water corresponding to its moisture, then some more water coming from the chemical reactions of thermo-condensation and also carbon dioxide gas. Acetic acid and light compounds appear when the temperature reaches a level of 180 degrees C. The endothermic character of the reaction failed then, while the degradation of hemi-celluloses occurs. At about 280 C. the reaction becomes quite exothermic, the gas production increases with a change in the chemical composition (CO and hydrocarbons are formed). At the same time, phenols, cresols and other heavier products are produced and can be found into the pyrolignous phase. The temperature of the reaction increases and reaches 400 C. or more. For the production of torrefied wood, the process shall be stopped just before the exothermic phase (as shown in Figure A). At this step, almost all the acetic acid is removed and the wood becomes a little more crushable.  

Figure A.  Changes in Friability & Production of Acetic Acid

TECHNOLOGY

Different routes and technologies had been studied by ARMINES. For the production of torrefied wood, it is necessary to assure a large heat transfer in a narrow scale of temperature.

First, a rotative kiln had been experimented on a laboratory scale. The wood was dried and torrefied in a controlled atmosphere with external heating. Then, to improve the energetic yield, a three fixed-beds apparatus was performed, with three sections: pre-drying, drying, & roasting.

Nowadays, a simpler process with a sole kiln was set up. Two pilot plants have been built; a batch one and a continuous one. They give good and interesting results.

CHARACTERISTICS OF THE TORREFIED PRODUCTS

The four samples were treated without problems and gave "torrefied woods" that have been analyzed.
The results of the tests are given in Table 3.  

Table 3. Torrefaction Results 

The results show that it is possible to obtain, from each species, a torrefied product that can be used in several purposes.
The yield in weight is good (74 to 77%,) .The torrefied woods have a low ashes content and contain 28 to 31% of fixed carbon.
The high calorific value, depending on the volatiles and ashes, varies from 23,000 to 24,000 kJ/kg (or 5,400 to 5,700 kcal/kg).
On the diagram O/C and H/C, torrefied wood is located between lignins and coals. (see Figure B).

Figure B. Characteristics of Different Fuels

ENERGETIC YIELDS AND CARBON YIELDS

The energetic yields and the carbon yields are quite good. They are given in Table 4.

Table 4.  Energetic Yields and Carbon Yields 

ADVANTAGES OF TORREFIED WOOD

The isolated products through the partial thermic degradation of the biomass that have been classified together under the term of "torrefied wood", possess the following characteristics:

POSSIBLE USES OF TORREFIED WOOD

CONCLUSION

Through the partial thermo-condensation of wood, it is possible to obtain stable ligneous and cellulosic compounds that have retained most of their initial energy potential. These compounds can be used for different purposes (domestic cooking, combustion, gasification, metallurgy, etc.).

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