In the wood-energy chain, storage assumes particolar importance in the enhancement of a product. Storage allows users to have fuel in time for planting operations (continuously throughout the year for power plants of medium/ large size and seasonally for the plants). In addition, storage permits to dry the material to moisture levels consistent with the specifications of the designer and manufacturer of the boiler. During storage, however, microbiological phenomena inevitably occur, leading to a loss of dry matter. These phenomena depend on several factors (microclimate, size, method of storage), which are not all controllable. In other words, for a fuel that guarantees satisfactory operation, you must endure the loss of product. Panacea group was started in 2007 to study the storage of SRF poplar and the work is still ongoing. The following sections summarize the objectives and results obtained during experiments in order to elucidate the effects of storage on energy content.

The quality of combustion is the only variable that can ensure that the emission limits for CO (carbon monoxide), TOC (total organic carbon) and NOx (oxides of nitrogen) are not exceeded, even during possible transitional periods. The emissions of these compounds depend on the performance of combustion and on the design of the combustion chamber. As the best performance of combustion is obtained through the best possible combination of fuel and oxidizing (oxygen in the air), it is extremely important to use a fuel in accordance with the configuration of the

combustion system, as specified by the designer, for the efficiency of combustion and ultimately planting.

Woody biomass can be classified in two ways:

• not prepared;
• already prepared for the power conversion system.

The unprepared fuel is to be pre-treated to reduce its size. Some users prefer to make the chips during planting, because it allows them to directly control the size and to control the physical characteristics of the product for optimum combustion technology.

However, there are many situations in which the biomass, already reduced in size, is attributed to users, in order to facilitate the harvesting and handling of the plants in the field as well as to contain costs. The operation can be performed with organ hammers (mallets), or machines (with) knives.

In the first case, the machine tolerates inerts better, and the maintenance costs are lower. However, there are risks with the use of machines with mallets. At the time of conversion, you may  experience the following drawbacks:

• The material may be too exhausted, leading to the flooding of power systems;

• The presence of inert material, because the product comes from a mixed soil (near plant roots) with elevated ash production.

In the second case, the quality of chips most often corresponds to the characteristics required by the

designer of the plant. In this case, the maintenance costs of the chopping system are higher but the cost of energy conversion decreases. Hence the importance of the type of car: whatever the technology used, the product must meet the specifications of the boiler as closely as possible.

The use of an unsuitable fuel causes a reduction in planting yield: the stop time increases and the energy production, consequently, fails, compromising its environmental performance.

Among fuel characteristics, the following are particularly important:

• Net calorific value (NCV);
• Humidity (%);
• Ash (%)

The NCV and the ash content may be related to the product or to dry matter. NCV expresses the net energy content of the latent heat of water vapour formed during combustion (C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy), which does not depend on the moisture content of the fuel. The increase in moisture content causes a loss of useful energy, which is used to vaporize the water in the fuel, to reach the point of zero self-combustion, and also compromises the environmental performance of the system (increase in CO). A fuel with a high NCV, but with a water content which is too high, adversely affects efficiency and emissions into the atmosphere. Larger plantings are more subsidized than those of small size, because they are able to mix biomasses of different quality in order to obtain an optimal mix for planting. The ashes are the minerals present in plant tissues and those found in pollutants (dirt, usually inert). For the woody biomass, a high ash content indicates a high proportion of bark compared to the heartwood, and in particular shows pollution from inert substances. This translates into more maintenance and larger quantities of ash to manage.

It is possible to identify the following phenomena related to degradation into a pile: from 10-15 ° C to 60-65° C we find biochemical reactions; above 60-65° C only chemical reactions (oxidation) take place. Both reactions lead to decomposition of organic matter with consequent loss of dry matter. The process can be slowed, reduced, and even avoided, but with high expenditure of energy resources (e.g., artificially dried material). Referring to specific texts for a study in depth, it must be said that the main microorganisms responsible for this phenomenon are aerobic fungi, including the Basidiomycota that are the most active. The degradation process is influenced by oxygen and water present in the biomass. The fungi attack cell wall through enzymatic pathways, invading the cell lumens through existing pores or piercing the tissues, first impairing cellulose and hemicellulose, and only then the lignin. It is evident that too fine of a size increases the area for microorganism attack, as well as leading to problems during combustion. The flow of oxidizing air transports

unburned particles of wood that are too light and at a risk for subsequent burning on the heat exchanger (with progressive loss of performance) or on systems of flue gas cleaning (with gradual increases in the cost of maintenance).

Water reduction in the wood is due to a physical phenomenon involving the slow balance of its moisture with that of the surrounding environment. Natural drying proceeds in less humid areas until the wood has achieved the lowest possible degree of moisture in the given pressure and temperature conditions. Drying proceeds more quickly with a greater difference between the water content in the wood and the air inside the stored product. This difference is related to its exchange (wind or forced ventilation), humidity and the air temperature outside the pile. It is also important to note that:

1.water loss does not occur uniformly and synchronously in the wood, but gradually proceeds from the outside inwards; not prepared;

2. the supply of water proceeds much more quickly with the grain, and the speed of the removal of water imbibition is greater than the scavenging of water saturation;

3. light woods dry before heavy woods, even if the porosity does not indicate the permeability;

4. shade, poor ventilation, contact with the soil and contact with the weeds not only slow down the natural drying, but also encourage the development of bacterial and fungal infections leading to woody substance loss;

5. the presence of bark (because of its histological structure) slows the escape of steam.

In addition to the passage of water from the wood to the external environment, we must consider the reverse transition, i.e. the partial entry into the wood of meteoric water (rain, snow, fog) if the pile is not sheltered from events such as meteorites. In this case it is also important to note that:

1. the presence of the bark is essential to slow the absorption of water by the wood;

2. more porous and lighter wood more easily absorbs the water with which it is in contact;

3. in chips, the greater the surface of the material not protected by the bark, the greater is the percentage of exposed heartwood and thus the greater is the potential for absorption of meteoricwater.

We can therefore say that the stacked outdoors and uncovered ligno-cellulosic biomass faces two processes: the release of water vapour from the wood to air in the gaps of the pile and meteoric water absorption by the wood, related to the size of the harvested product (whether bark is covering the wood or not). It follows, therefore, that the size of the chips can affect the potential of the biomass can to be naturally dried in uncovered piles. It is therefore important to develop a logistics system based on the harvest of whole plants in order to operate in the best possible conditions for natural drying. However, as previously mentioned, the choice of a specific harvesting solution affects the subsequent stages of the logistic/handling chain, and the handling and transport costs. Hence it is important to identify the storage method of the product size that allows the containment of degradation phenomena.

Almost all the experimental evaluations related to wood chip storage, have been conducted in northern Europe, and in climatic conditions completely different from those of our country. In order to identify the storage system best able to reduce the fermentative phenomena, in 2007, Panacea started gathering experimental proof related to the evaluation of the temperatures inside the piles, moisture content changes, the variation in NCV and dry substance losses in different operating conditions. First and foremost, experimental methodology and  necessary scientific equipment were developed. The equipment consists of a data acquisition system: a central unit and a series of sealed boxes, one for each pile. The boxes, consisting of 24 channels each, are devoted to the temperature detected by a temperature sensor PT100 (electrical resistance) placed inside the piles. The temperature sensors are connected to the box by a cable covered with silicone rubber that can withstand both acid ph values and high temperatures. An internal clock, for each box, can capture the data from each channel every 10 seconds. The box is connected to the central unit, consisting of

a computer and sheets for the preparation of related signals. A weather unit is connected to the central unit and records the evolution of the local microclimate throughout the entire period of experimentation: rain, wind speed, solar radiation, temperature and humidity of the outside air.

Software internal to the central unit sets all control parameters of the various sensors, saving the data to be sent to the Panacea offices, in Rome, via the Internet using a modem UMTS. A uninterruptible power supply (UPS) permits autonomy of the system for about 2 hours. If the system does not work correctly, messages are sent to those responsible for controlling the system so that they can act remotely via VNC software. In the pile, usually built in the months of January-February, bags containing a known quantity of the same material as that forming the pile are placed in two longitudinal sections. The first section is opened in July to pick up the bags and evaluate the parameters listed above (intermediate time), whereas the second one is opened in November, at the end of the experiment (final time). The first tests were conducted close to Franco Alasia Vivai in Savigliano (CN) by comparing piles of chips built in different ways:

• with natural ventilation (induced): The goal was to assess possible improvement of drying protocol. The pile was built around a metal structure, consisting of a passing horizontal duct and two vertical chimneys;
• discovered: this pile was carried out to compare, with the other, in order to determine the solution that was easiest to implement;

• covered: the mass was covered with a waterproof geotextile fabric TopTex ®, which at the same time allowed the escape of water vapour out of the pile. The TopTex ® is a nonwoven polypropylene that is not attacked by environmental agents; it is able to resist degradation and can be used for at least five years due to UV stabilization of polymers. The advantages to fuel quality have recently been evaluated;

• compacted and discovered: the pile was pressed by wheeled tractor in order to reduce the amount of oxygen inside the pile.

The final opening of the piles revealed the best way to dry the product, in uncovered and covered piles (L’Informatore Agrario No. 39/2008 p. 52). The uncovered pile has a wet outer layer, whose thickness is not in relation to the pile, as the tests performed in the following year at the Bando D'Argenta power plant have shown. Consequently, the wet share decreases if the height of the pile increases. In the ventilated pile, the moisture reduction was localized only in the immediate vicinity of the ventilation ducts, and the rest of the mass was not suitable for energy conversion, as the moisture values were too high at the end of storage (60%). The compaction of the material leads to a product for the most part unusable, indeed, there is an increase in moisture (from 50% to > 65%). From a practical point of view, the building of the compacted pile by the tractor passage involves considerably long times and risks for the operator (of the industry).

During the experimental proof cited above, two other piles were also monitored: a size-discovered pile and a pile of whole plants (L’Informatre Agrario No 10/2009 p.29). The discovered size, an intermediate solution between chips and plants, retains a wet outer layer, and had reached a considered moisture of 40% after eight months. The whole plants, with the associated harvesting difficulties and spaces for storage, instead reached a humidity of 20%.

In 2008, we wanted to test the influence of size on the final quality of the chips, once we had identified the two most promising storage systems. For organizational and technical reasons, two

uncovered piles had been built: the first pile, largest in size and obtained by Spapperi size chipper, was built at Alasia Franco Vivai in Savigliano (CN). The second one, a smaller size obtained with Claas Jaguar, was built at the power plant of Bando d’ Argenta (FE) in San Marco Spa Bioenergie. The results, reported in the specific work published here, have shown a direct relation between drying speed and size: materials of larger size dry faster. The dry matter losses are directly proportional to the time of storage in both sites. These results show that for the storage of the largest-size product, dry matter losses can cause, especially in the early months, faster drying, yielding a product readily placed on the market.

In order to validate the results obtained in previous years, in 2009 the trials were conducted on the same site in collaboration with Enervision of Dosolo, MN, building piles to heights of 7 m in order to evaluate the following:

• large-size uncovered pile, obtained by Claas Jaguar, equipped with the modified “Panacea” rotor;
• thin/fine-size uncovered pile obtained by Claas Jaguar, equipped with current rotor;
• thin/fine-size covered pile with geotextile fabric obtained by Claas Jaguar, equipped with current rotor.

The parameters currently being evaluated are: evolution of temperatures inside the piles, changes in NCV, dry substance losses and the variation of moisture in relation to size, the use of geotextile fabrics with “fine” products. The place on which the piles were built is asphalted in order to exclude the possible contamination of the chips while inert during handling, as well as to limit the possible contribution of moisture from the soil.

In addition, three more piles were monitored, in order to compare the storage of SRF poplar with other lignocellulose biomasses:

• pile of exhausted bark;
• pile from forest essence;
• pile of scraps of peeled poplar.

As seen above, the storage of lignocellulose leads to the degradation of certain components (mainly cellulose and hemicellulose) with product loss. The different storage techniques can reduce these losses, even if not completely cancelling them. Conversely, a dried product can be kept for a long

time (if you avoid the intake of other water) and the efficiency of the boiler increases. In other words, moisture reduction through storage can be avoided if the product is to be part of a blend made according to the specifications of the designer/manufacturer of the boiler. It is evident that this solution is only feasible in large-size power plants outfitted for electricity production, where it is possible to mix a product of very low moisture content with poplar chips. This does not occur for medium-size power plants or business/homes users: obtaining a biomass of good quality is essential to optimize planting performance. Within the curve of thermal efficiency of a small boiler (50 kW) shown in Figure 2, using the material as received (55% humidity) results in a 55% yield.

Planting performance was optimized by feeding with material with humidity between 25-30%. With regard to experimental data surveyed in experiments conducted in 2008 on piles of thin chips, we alculated the energy obtainable from 1 ha SRF arable land, supposing that SRF is used to feed the planting. Plant yield is shown in Fig.2 for the months of October, July and December. Crop yield depends on several factors including climatic conditions and the level of crop input, so production can vary from 8 to 18 t of dry matter per hectare per year, corresponding to 55% water content and 18-40 t of biomass per hectare. For a production of 40 t/ha/year of material in its natural state (55% moisture on wet basis and without ashes), in reference to the experimental evaluations of 2008, the results obtained are reported in te following table.

		Initial data	1th section data	2nd section data
		October 2007	July 2008	Dec. 2008
Moisture	(%)	55	22	34
Boiler yielding	(%)	55	78	76
DM losses	(%)	--	15,0	18,9
NCV ashes free	(MJ/kg)	5,7	12,5	10,8
Prod. ashes free	(t/ha/year)	40,0	19,6	22,1
Energy theory	(MJ/ha)	226.583	244.487	239.774
Effective energy	(MJ/ha/year)	124.620	190.700	182.228
Effective energy	(kWh/ha/year)	34.617	52.972	50.619

 

After four months of storage, we observed dry matter losses of 15%, with a reduction of water content from 55% to 22%. Since there was snow at the end of the storage period, in December the average moisture of the product had increased to 34%. As expected, dry matter losses increased to 19%. The dry matter losses were considerable; however, rather than the slight increase of theoretically convertible energy, the increase in energy actually obtained derived from the increased efficiency in the boiler due to the improvement of the quality of the chips.

After four months, compared with dry matter losses of 15%, the quantity of biomass energy provided by a single hectare increased from 34 MWh/ha/year to nearly 53 MWh/ha/year. This means that each hectare is able to provide 35% more energy each year if moisture levels are reduced, even in the face of dry matter losses. Even when the chips again appear partly humidified (Remoisten) as a result of meteorite events, the gain in energy is still interesting. This suggests some considerations: storage aimed at drying the product in the face of inevitable dry matter losses, however, can result in a better product in energy terms, if properly conducted. This is essential if there is not an opportunity to mix different biomasses with moisture in order to obtain a fuel with an optimum water content for the characteristics of the plant. Storage can possibly be avoided if the material will be part of a blend made in order to approximate the characteristics of the same to the specifications of the designer / manufacturer of the boiler, or if there is a source of unused thermal energy (electrical equipment not co-generative). It is clear therefore that such a result is basically feasible only in larger-size power plants for the production of electricity.

Further issues to be considered are dust emissions and the increase of inert material during storage.

The dust, also related to moisture of the product, can cause leakage into the environment. This can occur following storage in power plants of large size, where it is sometimes necessary to sprinkle piles to comply with the authorization requirements. This problem usually does not occur for small/medium-size plants, given the small quantities stored. Storage generally involves an increase in the specific content of ashes (inert). This is due in part to dry matter losses (carbon and hydrogen, and the specific share increases), but particularly to the handling, which, if not conducted carefully, inevitably increases the level of pollution in biomass due to inert compounds.

The experiments conducted by Panacea group have certainly helped to identify the dynamics of SRF poplar chip storage in northern Italy. The surveyed data can be useful in strategic assessments in each operating facility. However, it is clear that each situation requires specific investigations on the matter. The experiments under way in 2009 are a key point of this process. The effort is expected to validate the results obtained so far and to provide additional detail about the storage of poplar chips raised in SRF.