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.     

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.     

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.     

Development of bio-physical properties during storage of poplar chips from 15 ha test fields

A novel tractor-mounted mower-chipper for harvesting of short rotation coppice has been developed and tested in the field. The cutting length of the chipping module is adjustable in the range of 20–100 mm. Measurements during storage of the poplar chips in either a bunker silo or a covered pile (500 m³ each), over a period of about eight months, include the pile or bunker temperature, the loss of dry matter, the change in moisture content, and the mould formation. The effect of particle size has been examined as well. It appeared that fine chips, having an average chip length of 25 mm, show slightly better results than the coarse chips (75 mm). During storage of the fine chips in a pile, the moisture content decreased from around 60 to 33 weight percent, while the loss of dry matter was approximately 24 weight percent. The differences with the results obtained for coarse chips were however marginal. As a matter of fact, the favorable storage conditions expected for the coarser chips could not be confirmed at practice scale, despite significant differences in the development of the pile temperature and mould contamination.     

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.     

Natural drying treatments during seasonal storage of wood for bioenergy in different European locations

Research into the methods of producing high quality wood chips for a rapidly growing energy sector is becoming increasingly important. For example, small wood chip heating plants require high quality wood chips to ensure efficient operation, thereby minimizing maintenance costs. Moisture content is considered to be an important quality parameter regarding wood based fuels. The objective of this study is to investigate methods to promote the natural drying of wood for bioenergy purposes. The effects on the drying process through covering the wood piles and partial debarking of stems were tested in order to identify methods to reduce the moisture content of the woody material in the storage. Drying trials were established in Finland, Italy and Scotland, utilizing tree species typically used for energy purposes in each area. The results show that natural drying is a viable and effective method to enhance the energy efficiency of wood based fuel products in all the regions studied. Furthermore, by adapting current harvesting methods and storage procedures even better results can be achieved. In addition, the results also indicate that broadleaved trees dry more effectively, if some partial debarking is carried out and that covering of piles is of utmost importance in Scotland and Finland.     

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.     

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.     

Drying of firewood - the effect of harvesting time, tree species and shelter of stacked wood

Firewood represents a renewable source of energy and is the main source of energy for about half the World’s population. When burning firewood in domestic stoves, combustion and thus energy efficiency is dependent on the moisture content of the wood. In Denmark, it is generally recommended that moisture content should be no more than 180 g kg⁻¹ total weight. This study aims to assess the effect of species, harvesting time and shelter on the drying of stacked firewood. After felling, the moisture content declined to a relative stable level for all species. The rate of drying depended on the felling time, tree species, and the presence of shelter. The lower asymptotic moisture content depended mainly on the presence of shelter and averaged 188 g kg⁻¹ total weight for frames left in the open and 154 g kg⁻¹ total weight for frames covered by a shelter. It is concluded that Norway spruce felled during the early summer may obtain an acceptable moisture content at the onset of the heating season. Deciduous trees should be felled during the winter or early spring and stored under shelter to be suitable for burning before the heating season. Shelter was found to be of great importance to maintain an acceptable moisture content of firewood in storage during winter.     

The release of terpenes during storage of biomass

During outdoor storage of large quantities of wood fuels, hydrocarbons are emitted into air in gas phase and leached out into the ground by precipitation. To investigate to what extent these emissions have environmental or health effects, sampling has been done on wood chip piles in an existing terminal storage situated in the south of Sweden, north east of Växjö. Sampling was done by solid phase microextraction (SPME) and analyzed on a gas chromatograph equipped with a mass spectrometer. The storage period, during which sampling was performed, stretches from June through January, a total of 200 days. Sampling was performed at 7 occasions. The results show that emissions into air increases when the temperature directly above the pile increases. The temperature above the pile decrease with the ambient temperature, up until September for the forest residue and October for the bark and root reducing wood chips, when the temperature above the pile suddenly increases. This could be due to the ambient temperature, the storage time or microbial activity.The PAH content in the leachate, 27.27 μg/l, does not seem to be in the range where it might give large effects on the environment.     

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.     

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.     

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.     

Fuel quality changes during seasonal storage of compacted logging residues and young trees

Forest fuel procurement creates logistical problems, as large stocks are accumulated along the supply chain. The purpose of this study was to examine fuel quality (moisture content, ash content and calorific value) of compacted young trees (mainly downy birch) and both uncompacted and compacted logging residues (LR) (mainly Norway spruce). The materials were examined before and after storage, with and without cover, and effects of handling were considered. Dry matter losses from compacted LR during storage and handling were determined. Fuel quality and mass were determined before and after storage and handling. The moisture content of LR dropped to 28.6% when stored in small piles after fuel adapted logging at the clear felling site for 3 weeks in May. Drying continued after compaction into cylindrical bales (length 3.4 m, diameter 0.7 m) and during storage in windrows (9 and 12 months), the moisture content falling to 18.2–20.7% for the covered and 18.8–24.9% for the uncovered material. The windrow of loose LR remoistened to 40.8% (by snow contamination) resulted in a 6% lower net calorific value as received, compared to cylindrical bales. Ash contents were in the range 1.6–2.2% for LR and 1.0–1.2% for young trees. Dry matter losses ranged from 8.4% to 18.1% on compacted LR. Remoistening during the winter is higher for loose than for compacted LR. Early summer in northern Sweden provides favourable conditions for drying forest fuels.     

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.     

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.     

Sampling procedure in a willow plantation for estimation of moisture content

Heating value and fuel quality of wood is closely connected to moisture content. In this work the variation of moisture content (MC) of short rotation coppice (SRC) willow shoots is described for five clones during one harvesting season. Subsequently an appropriate sampling procedure minimising labour costs and sampling uncertainty is proposed, where the MC of a single stem section with the length of 10–50 cm corresponds to the mean shoot moisture content (MSMC) with a bias of maximum 11 g kg⁻¹. This bias can be reduced by selecting the stem section according to the particular clone. The average difference in MSMC between the largest and smallest shoot in a stump was 31 g kg⁻¹. This variation is only marginally smaller than the variation found in MC between stumps. The MC of individual stem sections may vary as much as 190 g kg⁻¹ in one shoot. Variation in whole shoot moisture content was primarily influenced by the shoot diameter, but in addition significant effects of clone and shoot age were found.     

Supply chain cost analysis of long-distance transportation of energy wood in Finland

The increasing use of bioenergy has resulted in a growing demand for long-distance transportation of energy wood. For both biofuels and traditional forest products, the importance of energy efficiency and rail use is growing. A GIS-based model for energy wood supply chains was created and used to simulate the costs for several supply chains in a study area in eastern Finland. Cost curves of ten supply chains for logging residues and full trees based on roadside, terminal and end-facility chipping were analyzed. The average procurement costs from forest to roadside storage were included. Railway transportation was compared to the most commonly used truck transportation options in long-distance transport. The potential for the development of supply chains was analyzed using a sensitivity analysis of 11 modified supply chain scenarios.For distances shorter than 60 km, truck transportation of loose residues and end-facility comminution was the most cost-competitive chain. Over longer distances, roadside chipping with chip truck transportation was the most cost-efficient option. When the transportation distance went from 135 to 165 km, depending on the fuel source, train-based transportation offered the lowest costs. The most cost-competitive alternative for long-distance transport included a combination of roadside chipping, truck transportation to the terminal and train transportation to the plant. Due to the low payload, the energy wood bundle chain with train transportation was not cost-competitive. Reduction of maximum truck weight increased the relative competitiveness of loose residue chains and train-based transportation, while reduction of fuel moisture increased competitiveness, especially of chip trucks.     

Von Rittinger theory adapted to wood chip and pellet milling,in a laboratory scale hammermill

The study draws upon the milling theories developed for the ore processing industry (Von Rittinger, Kick and Bond theories) in order to define a method for characterising wood chip and pellet energy consumption during milling.Energy consumption during wood milling depends on three main factors: the material moisture content, the particle size difference between the feed and the milled product, and the material itself. The latter may be characterised by a single parameter based on an adaptation of Von Rittinger's constant.A relation characterising wood pellet energy consumption as a function of the particle size distribution of the pellet ingredients and the milled pellets is proposed. This is characteristic of each type of pellet for each moisture content value considered.     

Numerical and experimental assessment of thermal performance of vertical energy piles: An application

A district space heating and cooling system using geothermal energy from bearing piles was designed in Shanghai and will be installed in two years before 2010. This paper describes the pile-foundation heat exchangers applied in an energy pile system for an actual architectural complex in Shanghai, 30% of whose cooling/heating load was designed to be provided by a ground-source heat pump (GSHP) system using the energy piles. In situ performance tests of heat transfer are carried out to figure out the most efficient type of energy pile and to specify the design of energy pile system. Numerical investigation is also performed to confirm the test results and to demonstrate the medium temperature variations along the pipes. The averaged heat resistance and heat injection rate of different types of energy piles are calculated from the test and numerical results. The effect of pile type, medium flow rate and inlet temperature on thermal performance is separately discussed. From the viewpoint of energy efficiency and adjustability, the W-shaped underground heat exchanger with moderate medium flow rate is finally adopted for the energy pile system.