Waste or forest selection wood must be treated to achieve a good energy supply. Let's see how

The total Italian forest area has tripled in the last 80 years and, if we consider more restricted time frames, for example , from 2005 to 2015, it would be easy to say that the increase was 53,000 hectares per year, to the detriment of pastures, uncultivated land in mountainous areas and hilly terraces.

If on the one hand the increase in forested area can be seen as a positive fact, on the other the disposition in mountainous areas of most of the forest resources, combined with the progressive depopulation of the same areas, leads to a problem of management and maintenance of the woods.

Selective cutting and cleaning of wood residues in forest areas remains an important and necessary activity for plant life and for the reuse of waste, which can be used in many sectors, such as the furniture sector, in which Italy is a major player.

Furthermore, the supply chain of wood products intended for the production of calorific energy would require better management of national forests, being able to reduce imports of timber intended for these purposes from abroad.

As regards the production of elements suitable for combustion, we can investigate which processes are necessary to transform a wood waste into a suitable element to develop thermal energy.

Davide Pettenella helps us in this story, who studied the Italian forest supply chain, with the aim of analyzing the production of woody biomass for the production of thermal energy.

To release and use the energy contained in plant materials, various conversion technologies are available: some of these are now reliable, economical, comfortable and simple applications in use, likely to be acquired both individually and collectively and industrially.

Others, on the other hand, are still in the development phase, require high-tech structures and dimensions industrial to be economic, even if the development of the latter depends on an extensive use of the biomass resource as an energy function.

In the case of forest biomass, characterized, among other things, by relatively low water contents >, the thermo-chemical processes of energy conversion ("dry process") are mainly used, while the other processes particularly concern agricultural biomass, with a higher moisture content ("wet process").

The first part of this chapter will present the treatments necessary to increase and standardize the energy quality of wood materials, while the second part will describe the actual conversion technologies.

Conditioning techniques

Wood products potentially usable for energy purposes are characterized by an extreme heterogeneity in terms of composition, size and shape: in fact, we pass from wood dust to sawdust, from shavings to twigs and logs.

When using them for energy conversion, they require treatments that vary according to the physico-chemical characteristics of the material has and the type of system that uses them.

The aim of these operations is to obtain a fuel with higher energy quality and greater ease of use, which makes dendro-energy as comparable as possible to other conventional fuels.

Drying and storage

After the cutting, preparation and logging phases, the wood cannot be «conveniently» used as it is due to the high percentage humidity; this can assume very different values and, on average, in a freshly cut forest plant it is around 50%.

In these conditions, a large part of the energy contained in the wood would be used to release the water contained, with obvious considerable losses in energy conversion efficiency.

A simple and economical solution to reduce the water content of wood to burn transpiration (or biological) drying.

It consists of a seasoning of the wood, the duration of which can vary from a few months to more than a year, in relation to the seasonal climatic trend and the type of wood; this, after cutting, is usually stored in the forest, at the edge of the road or near the places of use, in the open or sheltered.

Obviously the loss of humidity leads to an increase in the calorific value of the wood, which reaches its highest value with humidity between 12 and 15%. In fact, an excessively low humidity content in the wood would lead, in most of the combustion plants available today, to rapid burning and the fumes would be quickly evacuated before releasing the heat.

These humidity values also correspond to a better stability of the material. In reality, the gain is not so macroscopic, if we consider that the weight of the wood decreases during drying; in fact, if, as it should be, we considered the variation of calorific power on the same volume, the gain in energy terms would be more limited.

On the other hand, it must also be considered that drying the wood leads to a better quality of combustion, less production of smoke and tar, a less wear of the equipment and higher thermodynamic yields.

Chipping

To make the composition of wood materials homogeneous chipping is used, a mechanical operation which reduces woody assortments of different sizes in small flakes (Chips, hence the name).

In this way, the handling of the material and the supply of the systems are considerably facilitated.

The geometry of the chips varies with the cutting techniques, in fact the dimensions required depend on the type of system and, above all, of its power system. They have a length that varies from 15 to 50 mm, a width equal to half and a thickness equal to 1/5-1/10 of the length (generally the dimensions are 40x20x3mm).

The geometry, the size, the density are important characteristics if the chips are intended for the wood industry; homogeneity, on the other hand, is the most important parameter for chips intended for combustion.

The presence of chips of uneven dimensions often causes annoying blockages in the feeding systems of automatic systems.

The homogeneity of the material can be obtained by sizing through sieves. A moisture content of the wood higher than 40% can cause problems in the functioning of the chipper: after cutting, therefore, it is necessary to park the material on site at the roadside or in the square.

In any case, the humidity of the wood must not fall below 25%.

The storage of chips poses problems for drying, since deterioration and loss of material can occur due to respiration processes and microbiological fermentation, the more intense the deeper the storage pile, the humidity of the fuel and the external temperature (the same processes of respiration and fermentation, on the other hand, by increasing the temperature and favoring the evaporation of the water contained in the chips).

On the Italian market there are chippers of various powers up to 15 MW, capable of processing timber of various sizes (with work capacities ranging from a few up to a few tens of tons per hour), both self-propelled and carried by agricultural tractors.

The former are able to process timber of all types of species, up to a diameter of 30 cm. and have high investment and operating costs (the rent costs about 150 Euros for an hour of operation), but they have the advantage of considerable working capacity and low use of labour.

Chippers mounted at the three points of the tractor have low investment costs and relatively low operating costs, but require manual feeding and offer low work capacities.

They are distinguished by the cutting system: this can be disc or drum. The first, generally used in small-powered chippers, has the drawback of producing chipping tails which block the worm of the boiler supply system, but on the other hand has reasonable investment costs, ease of use relatively easy maintenance, lower power requirements.

The second cutting system is more common in self-propelled machines of higher power.

Densification

At the different industrial levels of the wood supply chain, significant quantities of small particle size wood residues are produced (shavings, sawdust, wood, etc.) which are not easily used and sometimes pose disposal problems.

The densification of these materials allows to obtain dense fuel, in the form of cubes, pellets (small cylinders of 8-10 mm in diameter and 20-30 mm in length) and briquettes (in the shape of a bar of soap or a cylinder with a length between 50 and 300 mm), which can be used within the production process or which can be sold to other users.

Densified products are characterized by high energy density, stability and uniformity of dimensions, low percentage of humidity. Consequently, they have lower transport costs and greater ease of storage and use, better control and greater combustion efficiency.

The density of densified products varies from 0.9 to 1.4 g/cm3, while their humidity can vary within very broad ranges (generally from 5% at 10%).

The production process is divided into three phases: storage and preparation of the material, drying, densification.

Initially the wood is separated from impurities, reduced to smaller and more uniform dimensions and stored on platforms; from here it is transported inside drying ovens, where the humidity is reduced to values around 10%.

Then the material is densified by compression (screw or piston press) or extrusion.

The machines that work by extrusion heat the material, causing, after cooling, the formation of a protective film of lignin which opposes a return of humidity.

The quality of the product and the energy cost required (on average 20% of the energy contained in the material to be densified) depend on the chemical-physical characteristics of the raw material and the type of process used.

Other conditioning techniques

Among the conditioning products, roasted wood should be mentioned, (an intermediate form of energy between wood and coal, with a calorific value higher than 5000 kcal/h, obtained by treating wood at temperatures of about 300 degrees, more stable and more homogeneous than the starting material and which creates fewer storage and transport problems) and the sawdust-fuel mixture .

The use of these products responds, however, rather to the need for the disposal of processing residues than to the need for energy recovery.

Machine translation. We apologize for any inaccuracies. Original article in Italian.