Debarking and drying of downy birch (Betula pubescens) and Scots pine (Pinus sylvestris) fuelwood in conjunction with multi-tree harvesting

Fuelwood moisture content is an important parameter for small and medium-sized heating plants. Optimal storage location and good fuelwood storage maintenance promotes drying efficiency and ensures good fuelwood quality. The effective drying period is limited to spring and summer. It is known that partial debarking of wood will enhance drying and improve fuelwood quality.The aim of the study was to test strip debarking as a mean to speed up the drying rate and to lower moisture and bark contents. The prerequisite was that debarking had to be integrated into the processing phase of a single grip harvester. The changes made in the harvester head had to be simple, easily installed and inexpensive.The mechanical objective was solved by drilling holes into the delimbing knives and placing bolts through the knives. In this manner the bolt head would strip bark off the stems when fed through the delimbing knives. The qualitative goal was to see if debarking improved drying rate and if harvesting season and species played a role.The data was collected on non-frozen and frozen pine wood in spring 2005 and in winter 2006, and on non-frozen birch in spring 2007. The debarking results of the non-frozen pine were 3-6% of the total surface area for the normal harvester head and 7-9% for the modified harvester head. The results for frozen pine were correspondingly 1% and 2-2.5%. The debarking results of non-frozen birch were 5-6% and 5-8%. Both studies of pine show that the bolts in delimbing knives doubled the amount of bark removed. There was no significant difference in birch.The delimbed Scots pine fuelwood storages reached 30% moisture content on fresh weight basis during the first drying season at landing. This result was obtained independently of the rate of debarking or the harvesting season. Seasoning over the second summer lowered the moisture content of pine fuelwood by another 5% units to 25%. Birch reached 20% during the first season.     

Validation of prediction models for estimating the moisture content of logging residues during storage

Increased use of forest biomass for energy and rising transportation costs are forcing biomass suppliers towards better moisture content management in the supply chain. Natural drying is used to decrease moisture content of energy wood. Drying is dependent on wood characteristics and weather conditions. Weather-dependent drying models for estimating the optimal storage time based on average moisture changes in fuel wood stacks stored outdoors have been developed for different stem wood and logging residues. Models are an easy option for estimating the moisture content of energy wood piles compared to sampling and measuring the moisture of samples. In this study, stand and roadside storage models for logging residues were validated against data from field studies and forest companies. Over 200 reference piles for the stand model, 23 piles for the roadside model and 10 piles for the combined model were studied. Results of the validation are promising. The difference between measured and modelled moisture was on average only 0.35%. The presented models can be implemented anywhere in Finland, because the Finnish Meteorological Institute has a weather observation service offering weather history data for every location in Finland. For international use, parameters need to be estimated on a case by case basis, but it should be possible to implement the approach also elsewhere.     

Development and test of a simplified method to calculate dry matter loss during open-air storage of poplar wood chips by analysing ash contents

Short rotation coppice (SRC) in agriculture can be established successfully, only if the entire process chain is economically competitive. Despite the substantial dry matter loss (up to 25%) occurring during open-air storage of wood chips, it is the most applied storage technique. Since the particle size of wood chips plays an important role in storage and drying processes, two storage piles (>500 m³) with fine and coarse wood chips were investigated comprehensively under the weather conditions of North-East-Germany over a period of 9 months.The objective of this experiment was to develop a simplified method to calculate dry matter loss by determining the ash content and to compare the results with the conventional sample bag method. The new method delivered statistically sound results, particularly when ash contents were calculated from separated bark and wood samples instead of from wood chip samples. Furthermore, the significant and consistent increase in ash content of the bark samples during storage suggests, that the development of a model solely based on ash content of separated bark could be advantageous in terms of simplified yet reliable determination of dry matter losses. Since the ash content of wood chips is largely governed by the bark content, exact data for the proportion of bark is required, which varies between 18 and 35% depending on tree size represented by the stem diameter at cutting height (SDCH). Moreover, it was found that the fuel quality depends more on the SDCH of the SRC-harvest than on the produced wood chip format.     

Storage of whole-tree chips from high-density energy plantations of Eucalyptus in Brazil

In this paper, the drying of whole-tree chip (WTC) storage from young Eucalyptus plantation managed at short-rotation coppice in Brazil was studies. The biomass was converted from high-density energy plantations of Eucalyptus grandis at 2 years old into four piles. Wood chip particles had 5, 15, and 30 mm length were disposed on a paved surface to evaluate the effect on the chip drying. An additional covered pile (30-mm wood chip) was installed to evaluate the effect of coverage condition. The non-ventilated and uncovered piles were not affected by WTC length, and the final moisture content (MC) was 48.4–53.5% and temperature inside the piles (storage temperature) was approximately 36 °C. However, the coverage showed beneficial effect on drying wood chip process, collaborating to keep the MC lower than 35%, conventionally recommended for energy purposes. Among storage systems studied, the higher daily moisture content was assigned to covered pile, about 0.197% day⁻¹ during the first 30 days. This paper can be used as a reference for further studies with wood chip pile storage at tropical conditions.     

Storage dynamics and fuel quality of poplar chips

Poplar cultivation for wood/timber production has a growth production cycle of about 10–15 years. Usually the stem is separated from the crown and used to produce material of different kind such as veneer, pallets, panels, etc. For wood industries, crowns generally represent waste material to be disposed of, causing economic and time losses. It is generally believed that the costs of managing crown biomass are higher than the potential incomes obtainable. Nonetheless, it is worthwhile investigating the possibility of using these byproducts as energy source and evaluating their value as a fuel. However, storing such residues presents several problems connected with spontaneous microbial degradation.The aim of this work was to evaluate the storage effects on chipped biomass deriving from the crown and stem wood of poplar and how they affect fuel quality and dry matter losses.A storage trial was carried out with three piles of stem wood chips and three of crown chips coming from a 15 year old poplar plantation. The piles were stored outdoors for six months under the same climatic conditions.The effect of storage on fuel quality was evaluated with respect to moisture content, gross and net calorific values, chemical composition, ash content, and bulk density.The variation of temperatures inside each pile due to heat development was continuously monitored and showed different trends between piles depending on source material. Results showed that chips from crown material had better storage properties and exhibited lower decay than chips from stem wood.     

Open-air storage of fine and coarse wood chips of poplar from short rotation coppice in covered piles

The cultivation of short rotation coppices (SRC) on agricultural land represents an economically and environmentally promising option for sustainable provision of bioenergy. Not only the further development of efficient harvesting machinery, but also the development of harvest-optimised storage systems are necessary to implement cost-efficient cultivation and use strategies for SRC in practice. The storage of fine wood chips from poplar harvest with a forage harvester results in high dry matter losses of up to 25%. Tractor-mounted mower-chippers can harvest coarse wood chips that might possess more favourable storage and drying properties. The main objective of the current research project was to develop and perform a storage experiment in which the storage behaviour of fine and coarse wood chips could be examined and compared in detail over a period of nine months. In this experiment two covered storage piles (height 3.5 m), with over 500 m³ fine and coarse wood chips respectively, were examined under practice scale conditions in Germany. After nine months of storage the fine chips in the core of the storage pile had dried to a moisture content of 34% with dry matter losses of 22%. Coarse chips, on the other hand, achieved a moisture content of 29% and dry matter losses of 21% in the same period. The maximum moisture content of 40% required by heating plants in practice is achieved by fine chips after 6.5 months and by the coarse chips already after 3.5 months.     

Effects of particle size and pile height on storage and fuel quality of comminuted Salix viminalis

Biomass from short rotation forestry can provide a valuable energy source but maintaining the quality of the material during handling and storage is challenging. The relatively young plant material is highly prone to microbial degradation and can consequently suffer changes in fuel quality. The aim of this work is to evaluate the role of particle size and pile height on the processes which occur during storage and how it affects fuel quality. Fresh willow shoots (Salix viminalis) were comminuted into chips and chunk wood (nominal size 22–45 mm), stored in 3 and 6 m high piles for 2 and 3 months respectively. Results showed that the four piles exhibited different behaviour. Temperature development was rapid and prominent in the chipped willow particularly in the 6 m high pile. In the chunk woodpiles, temperature rise was very slow and became notably higher than ambient temperature only after 2 months of storage in the 6 m high pile. The effect of storage on fuel quality was evaluated with respect to moisture content, heating value, particle size distribution, and ash content. In general, salix chips had a relatively lower moisture content and energy value than chunk wood by the end of storage. Evaluation of fungal activity during storage showed that marked increase in spore counts was closely associated with heat development.     

The characteristics of whole-tree fuel stocks from silvicultural cleanings and thinnings

The fuel properties of small-sized whole-tree fuel stocks were studied in roadside and in-stand storages. The significance of pile cover, season and storage site on moisture content and heating value of Scots pine (Pinus sylvestris) and pubescent birch (Betula pubescence) fuel stocks were observed.When the fuel stocks are placed in a well-ventilated location moisture content may be lowered below the 40% mark during one summer period. Covering the piles will give up to 6% units lower moisture content in comparison with non-covered piles. The most benefit is gained from covering during snow melting in spring and the least in midwinter when the snow is dry.Multi-tree harvested stemwood with no limbs seasoned equally well as the whole trees both in roadside and in-stand trials. This is because the processing of multi-tree bunches caused some debarking to take place. As a result, the transpiration drying capability of whole trees was equalized by the evaporation of moisture via the open wood surface. Both assortments reached moisture contents below 30% in in-stand conditions during one summer's seasoning.The effective heating values of neither birch nor pine showed significant changes. However, heating value deviated with the composition of the fuel stock. The presence or absence of crown material was a factor.The microbial counts of mesophilic fungi in whole-tree fuel stocks were only 1% of that found in logging residues. The counts of thermo-tolerant fungi were still much smaller. Leaving the Scots pine piles uncovered will increase the number of mesophilic fungal spores and bacteria.     

Simulation of temperature and moisture changes during storage of woody biomass owing to weather variability

The objective of this study is to describe moisture content and temperature profile in the woody biomass pile by a two dimensional mathematical model. Woody biomass in the form of wood chips and bundles was stored for a period of one year. Heat and moisture transfer model for drying processes was solved by finite element method using MATLAB programming. The simulation was performed using the recorded climate conditions during the experiment and constant drying air conditions. The temperature change inside the bundles shows the same trend and effect with ambient air temperature, however, in case of wood chips shows lesser effect at various ambient air temperature. Uniformly declined moisture content was observed inside the covered wood chips pile during the storage period. The proposed two dimensional model is in close agreement with experimental data to describe the moisture and temperature profile of the pile wood chips and bundles. However, as the wood chips pile height increases more than 3 m temperature development inside the pile could be rapid and the effect of chemical reaction in the wood chips pile has to be included for better accuracy of prediction.     

Poplar wood chip storage: Effect of particle size and breathable covering on drying dynamics and biofuel quality

Used as alternative energy sources, solid biofuels have the advantage of simultaneously reduce fossil fuel dependence and mitigate climate changes by reducing greenhouse gas emissions. Biomass storage and handling play a key role for wood fuel quality assessment: in particular, the presence of covering systems influences biomass drying process and factors leading to bulk compaction (e.g. reduced particle size) negatively affect woody biomass preservation till final use. To study the effect that particle size and covering have on biofuel quality, Short Rotation Forestry (SRF) poplar trees were harvested and chipped by means of a Claas Jaguar 890 operating machine equipped with a “GBE-1” header for SRF. The chipping procedure was carried out in March 2009 using on the same header both the standard chipping drum and a special on purpose drum designed by CRA-ING to obtain coarser particles. The chipped biomass was subsequently stored in stacks on a paved surface for a 120 days period to evaluate: i) effect of particle size on uncovered stacks drying dynamic and fuel quality, ii) effect of the presence of non-woven tarp on drying and fuel quality of biomasses standardly chipped with the conventional drum. Coarser stack was found to have a less prompt dehydration and, at the end of the storage period, have less energy and dry matter loss. In case of biomass chipped with the conventional drum, at the end of the storage period the effect of covering ended up in wood fuel with significantly higher heating value.     

Dewatering of high-moisture wood chips by roller compression method

In seeking an efficient method for drying very wet chips, we developed and tested an original continuous dewatering system using mechanical compression (10–30 MPa). The moisture content of cedar chips decreased from 100–270% to 85–130% (dry basis), and the energy required to remove the water was 9% (power consumption basis) and 25% (primary energy basis) for heat energy to evaporate the water. In the case of combined drying, in which roller compression was applied first, followed by thermal drying, the energy required to dry wet cedar chips to a moisture content as low as 20% was 49% (power consumption basis) and 58% (primary energy basis) of that required for thermal heating alone. These results indicate that the combined method uses less energy to dry very wet wood chips.     

Feasibility of the Nordic forest energy harvesting technology in Poland and Scotland

Finland and Sweden have been forerunners in the development of wood harvesting machinery and methods. In both countries, small- and large-scale supply systems for wood chips have been in operation for several decades. More recently, the production and use of forest chips from logging residues and small diameter trees has been growing rapidly.The European Union (EU) has set ambitious targets for the use of renewable energy to mitigate climate change and to increase domestic energy security and self-sufficiency. The largest unutilised source for renewable energy in the EU is forest biomass. European forests could fulfill one third of the goal set for biomass-based energy production in the EU’s Biomass Action Plan. In addition, member countries have started national programmes to promote the use of biomass for energy.As a result, interest in Nordic forest energy technology has been increasing rapidly in other parts of the EU. The Finnish Forest Research Institute and its collaborators have been running a technology transfer project in ten European countries, with the goal of tailoring and adapting Nordic forest technology to local conditions through analysing the applicability, costs and overall competitiveness of selected feedstock supply technologies.This paper summarizes the findings of feasibility studies carried out in Poland and Scotland and gives an overview of the current situation and development trends of forest energy in the European Union.     

Evaluation of greenhouse gas emission risks from storage of wood residue

The use of renewable energy sources instead of fossil fuels is one of the most important means of limiting greenhouse gas emissions in the near future. In Finland, wood energy is considered to be a very important potential energy source in this sense. There might, however, still be some elements of uncertainty when evaluating biofuel production chains. By combining data from a stack of composting biodegradable materials and forest residue storage research there was an indication that rather great amounts of greenhouse gases maybe released during storage of wood chip, especially if there is rapid decomposition. Unfortunately, there have not been many evaluations of greenhouse gas emissions of biomass handling and storage heaps. The greenhouse gas emissions are probably methane, when the temperature in the fuel stack is above the ambient temperature, and nitrous oxide, when the temperature is falling and the decaying process is slowing down. Nowadays it is still rather unusual to store logging residue as chips, because the production is small, but in Finland storage of bark and other by-products from the forest industry is a normal process. The evaluations made indicate that greenhouse gas emissions from storage can, in some cases, be much greater than emissions from the rest of the biofuel production and transportation chain.     

Industrial supply chains and production machinery of forest chips in Finland

Metsäteho Oy surveyed the industrial supply chains used in the production of forest chips in 2006 in Finland. The Metsäteho study also conducted a survey of the production machinery of forest chips used by energy plants in 2007, and provided an estimate of industrial supply chains and future machinery requirements for forest chip production in Finland.The majority of the logging residue chips and chips from small-sized thinning wood were produced using the roadside chipping supply chain in 2006. The chipping at plant supply chain was also significant in the production of logging residue chips. The majority of all stump wood chips consumed were comminuted at the plant, and with only around one fifth comminuted at terminals. The role of the terminal chipping supply chain was also significant in the production of chips from logging residues and small-sized wood chips. It was predicted that the roles of both terminal chipping of logging residues and chipping at the plant will increase by the year 2010. Regarding the production of chips from small-diameter wood, it was estimated that the role of chipping at the plant will also increase in coming years. The proportion of roadside chipping in the production of small-sized wood chips and logging residue chips is expected to decrease.The study estimated that a total of 1100 machine and truck units were employed in the production of forest chips for energy plants in 2007. Increasing forest chip consumption will create considerable demand for additional forest chip production resources in the future.     

Influence of particle size and moisture content on tendency to bridge in biofuels made from willow shoots

Different systems for harvesting and storage of wood fuel from willow shoots give fuels with different particle size distribution, particle shape and moisture content. These factors influence the tendency for wood fuel particles to form a stable bridge over openings, which prevents the feeding of the fuel. The influence of cutting and storage method on the tendency to bridge was studied for chips and chunks made from 3 to 5 year old willow shoots harvested in January and December. Shoots were cut with four different machines to produce five fuel assortments with nominal particle length from 28 to 200 mm, and stored outdoors, in central Jutland, Denmark, in 160 m³ loose volume piles. Some piles were uncovered, some covered with plastic and two were sealed in an airtight silage plastic film enclosure. The bridging tendency was measured at the end of May and September by determination of how wide a “bridge” of fuel over a slot opening could be before it collapsed. With a 500 mm thick layer of fuel above the slot opening, the bridge width varied between 58 mm for the small chips and 977 mm for the large chunks. Most of the variation was due to two fuel properties, the proportion of particles longer than 100 mm and the moisture content of the fuel.     

Model and simulation of a solar kiln with energy storage

A solar kiln with energy storage can be used for continuous drying. This kiln consisted of several units which were modeled to simulate it in operation. A model was proposed for each unit, and another based on laboratory tests for drying a wooden board by passing air across. These models were combined to produce a global model. Simulation results were then analyzed and showed that the use of storage was justified to reduce drying time. Moreover, with the judicious use of storage and air renewal, drying schedules could be produced for a better quality of dried wood.     

Changes in feedstock quality in willow chip piles created in winter from a commercial scale harvest

Storage and handling are important facets of biomass logistics because there are associated costs and biomass properties can change significantly as material proceeds through the supply chain. Thus, this aspect of biomass supply systems requires continued study. Shrub willow chips were harvested, and used to create six piles that each contained between 10 and 22 Mg (fresh biomass). Material was monitored for several months in temporary storage to assess changes in biomass quality (moisture, ash and energy content). Internal pile temperatures increased due to biological activity and conditions within a pile quickly differentiated based on location (shell, core, top, and side). Mean moisture content increased from 42 to 47% (mass fraction) between harvest and delivery of the chips, but ranged between 37 and over 60% over the next three months depending on pile location with the shell generally drier than the core. Mean ash content increased 1 to 2% points (mass fraction) between harvesting, reloading and delivery to the trial location, but became more variable during storage. Higher heating values (HHV) were stable between 18.6 and 19.0 MJ kg⁻¹ over the six months, but lower heating values (LHV) ranged between 8.6 and 11.7 MJ kg⁻¹ and mirrored changes in moisture content. There was minimal change in chip quality over two months, but quality became more variable over longer time periods. This period could be extended, and negative effects on chip quality could be mitigated, by improving storage methods, blending different types of chips, or employing pretreatments.     

Heat storage for wood drying using solar heat

储热技术有利于提高太阳能的利用率,降低传统木材干燥的能耗,对木材工业的可持续发展具有重要意义。本文系统地阐述了储热技术的基本原理,简要介绍了其应用概况;结合国内外研究现状,分析了储热技术在木材太阳能干燥中的应用,并指出了储热技术在木材太阳能干燥中的发展趋势。     

Transport control of forest fuels by fleet manager,mobile terminals and GPS

Chip transportation is undertaken by private transportation enterprises that are contracted by a larger wood fuel trader. Compared with industrial roundwood supply the volumes and value of wood fuel supply are markedly lower. As a result, the possibilities to invest in information systems for transport management are limited. New portable, wireless communication techniques and internet-based systems for fleet management enable more cost-efficient control systems with low investment costs in hard and software and also low operating costs. Mobile handsets with GPS, digital road map display and global positioning systems (GPS) or general purpose radio service data transfer protocol can be used as mobile terminals in chippers and trucks. In addition, transport management personnel can use them to locate in-forest wood fuel storage piles into the map database as well as point suitable routing to the storage piles. In a pilot study, conducted in Central Finland, an internet based management tool, Arbonaut Fleet Manager™, was tailored for forest fuel supply chain management and trailed for three months. It was found that use of mobile handsets with GPS and map display assisted especially in exact location of in-forest wood fuel storage piles by managers. They assisted also trucks and chippers in navigation to storages and landings, but the screen was too small for reading during driving. The management system was found helpful in stock accounting and GPS-based vehicle tracking gave transportation distances directly for invoicing.     

Modelling natural drying efficiency in covered and uncovered piles of whole broadleaf trees for energy use

Small dimensions regenerated forests are considered a useful fuel resource for small local heat plants in Norway, since it is not relevant for the timber industry. Most small heat plants built so far are constructed for moisture contents of about 35% on wet basis. Therefore, the material must be dried. Because artificial drying induces additional costs, storing the material in piles roadside as whole trees until desired moisture content is obtained is considered beneficial. Traditionally, leaf seasoning has been considered an efficient method. To increase the understanding of these processes, a study on drying whole trees in piles has been accomplished at three different locations with different climatic conditions. The study focuses on the following explanatory variables: harvesting season, location, climatic conditions, position in the pile, tree species, and relative crown length. The effect of covering the piles in order to reduce the moisture uptake during winter was also studied. Models, estimating the moisture content with time profiles, were developed.During spring and summer the moisture content was reduced to approximately 35% also when the material was harvested in the autumn the year before. The climatic conditions were important for the drying result, but drying was effective also in the moist climate in western Norway. Covering the dry piles before the winter was important in order to maintain the requested moisture content. The effect of covering the material harvested in autumn was limited.