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.     

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.     

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.     

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.     

Experiment and modelling of parameters influencing natural wind drying of willow chunks

The objective of this study was to investigate the parameters that govern the drying process of willow chunks. Indicative chunk drying trials were conducted to assess the potential of natural wind drying. Supportive model simulations were conducted to gain insight into the influence of different process parameters (particle size, pile depth) on drying. Natural wind drying experiments showed that willow chunks could be dried from 50% (wet basis) to around 10% within 5 months. Internal heating in the pile did not occur and dry matter losses were reasonably low (3.5–5%). The drying time of willow chunks depended on drying air conditions, particle size and pile dimensions (depth). The particle size of chunks should be large enough to create a low airflow resistance in the pile, but small enough to avoid that internal diffusion of moisture limits the drying process.     

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.     

Biomass granular screw feeding: An experimental investigation

Successful feeding is critical to biomass utilization processes, but difficult due to the heterogeneity, physical properties and moisture content of the particles. The objectives of the present study were to find the mechanisms of blockage in screw feeding and to determine the effects of particle mean size (0.5-15 mm), size distribution, shape, moisture content (10-60%), density and compressibility on biomass particle feeding at room temperature. Wood pellets, sawdust, hog fuel and wood shavings were tested in a screw feeder/lock hopper system previously employed to feed sawdust into a pilot-scale circulating fluidized-bed gasifier. Experimental results showed that large particles, wide size distributions, large bulk densities and high moisture contents generally led to larger torque requirements for screw feeding. The “choke section” and seal plug play important roles in determining the torque requirements.     

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.     

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.     

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.     

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.     

The influence of drum chipper configuration on the quality of wood chips

The efficiency of adequately chosen and monitored technology during the production of wood chips is shown by the quantity of energy stored within the wood fuel, and in the manufacture and preparation costs per product unit. Several influential material, construction, and technological parameters have to be encountered during the production of wood chips when using a mobile drum chipper. A set of different experiments regarding the production of wood chips using a drum chipper were made in order to find the more suitable combinations of production parameters (technological and constructional) for different types of inputted materials (hard round wood, soft round wood, and branches).By monitoring the energy contents within different kinds of wood chips, it was determined that soft wood biomass having rougher structures and hard wood biomass having finer structures are more suitable for larger systems. Dust particles within the wood fractions are undesirable; however constant forms and size–structures regarding wood chips are welcome.The aim of our research was to build a decision model that would help a user to choose the right combination of parameters on a drum chipper, thus ensuring the production of wood fuel with characteristics close to the desired optimum, yet at minimum production costs.     

Methods for size classification of wood chips

Methods for size classification of wood chips were analysed in an international round robin using 13 conventional wood chip samples and two specially prepared standard samples, one from wood chips and one from hog fuel. The true size distribution of these two samples (according to length, width and height) had been determined stereometrically (reference method) using a digital calliper gauge and by weighing each of the about 7000 wood particles per sample. Five different horizontal and three rotary screening devices were tested using five different screen hole diameters (3.15, 8, 16, 45, 63 mm, round holes). These systems are compared to a commercially available continuously measuring image analysis equipment.The results show that among the devices of a measuring principle—horizontal and rotary screening—the results are quite comparable, while there is a severe incompatibility when distributions are determined by different measuring principles. Highest conformity with the reference values is given for measurements with an image analysis system, whereas for all machines with horizontal screens the median value of the size distribution only reached between one-third to half of the reference median value for the particle length distribution. These deviations can be attributed to a higher particle misplacement, which is particularly found in the larger fractions. Such differences decrease when the particle's shape is more roundish (i.e. sphericity closer to one). The median values of length distributions from screenings with a rotary classifier are between the measurements from an image analysis and horizontal screening devices.     

Pelletizing properties of torrefied spruce

Torrefaction is a thermo-chemical conversion process improving the handling, storage and combustion properties of wood. To save storage space and transportation costs, it can be compressed into fuel pellets of high physical and energetic density. The resulting pellets are relatively resistant to moisture uptake, microbiological decay and easy to comminute into small particles. The present study focused on the pelletizing properties of spruce torrefied at 250, 275 and 300 °C. The changes in composition were characterized by infrared spectroscopy and chemical analysis. The pelletizing properties were determined using a single pellet press and pellet stability was determined by compression testing. The bonding mechanism in the pellets was studied by fracture surface analysis using scanning electron microscopy. The composition of the wood changed drastically under torrefaction, with hemicelluloses being most sensitive to thermal degradation. The chemical changes had a negative impact, both on the pelletizing process and the pellet properties. Torrefaction resulted in higher friction in the press channel of the pellet press and low compression strength of the pellets. Fracture surface analysis revealed a cohesive failure mechanism due to strong inter-particle bonding in spruce pellets as a resulting from a plastic flow of the amorphous wood polymers, forming solid polymer bridges between adjacent particles. Fracture surfaces of pellets made from torrefied spruce possessed gaps and voids between adjacent particles due to a spring back effect after pelletization. They showed no signs of inter-particle polymer bridges indicating that bonding is likely limited to Van der Waals forces and mechanical fiber interlocking.     

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.     

Numerical modelling of a fixed bed updraft long stick wood gasifier

A computational model to evaluate the anticipated performance characteristics of an updraft fixed bed gasifier utilizing long stick wood as the source of fuel is presented. This type of gasifier obtains high-energy release rates due to the lower inlet air velocity and extended reaction zone. The numerical model couples the performance of the individual particles and the external gas phase reactions to describe the gasifier performance. By calculating the performance of individual particles, the computational model is able to determine the interaction between the combustion of charcoal particles in the lower portion of the bed and the pyrolysis of wood sticks in the upper region. Most importantly, the model can describe the effect on overall system performance when conditions within the gasifier are changed, such as changes in the wood or char particle size, moisture content, gasifier height, inlet velocity, etc. The results of the model are compared with the gasifier performance for a gasifier using sticks with a 6 cm diameter, a length of 68 cm and a dry density of 600 kg m⁻³. The model results are used to investigate the performance of the gasifier under a variety of load conditions, fuel sizes, and moisture conditions.     

Pressure resistance to air flow during ventilation of different types of wood fuel chip

Of two types of willow chip (28 and 50 mm cutting length), two types of willow chunk (100 and 200 mm cutting length) and one type of forest chip (28 mm cutting length), the pressure resistance to air flow during ventilation was measured. The influence of air velocity and particle size has been determined and the results are described by a mathematical equation.The measured pressure resistance correlated very well with the size distribution of the different chip and chunk types. The lowest pressure resistance was found in 200 mm long willow chunk at only 2 Pa/m at an air flow rate of 0.1 m/s, while the highest was found in the 28 mm chip of willow and forest material at 39 Pa/m.The pressure resistance was shown to be a factor 10 to 100 lower for these wood fuels than for agricultural crops like rape seed and grain.