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.     

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.     

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.     

Poplar woodchip storage in small and medium piles with different forms,densities and volumes

Wood biomass is one of the main sources of biofuel for bioenergy production worldwide. Generally, the exclusive use of comminuted biomass in automated boilers is preferred because these woodchips consist of homogeneous particles with a specified size. Wood biomass is harvested mainly in autumn and winter, whereas the demand for biomass-fired power stations is continuous throughout the year. Nevertheless, large amounts of woodchips are also produced in the spring and summer from residual materials obtained from the utilisation of conventional poplar plantations.This study focused on uncovered small and medium woodchip piles. In particular, the influence of form, density, and the size of piles on the biofuel quality during woodchip storage was analysed. The woodchip moisture contents and dry matter losses were considered when evaluating the storage dynamics.The results suggest that a storage system can be selected to service only the needs of thermal stations because any difference between the form (trapezoidal and cone), volume (35 and 70 m³), and density of the piles was observed on woodchip quality analysis. In fact, a mean moisture content of 18% and average dry matter losses of 10% were recorded at the end of storage period for all treatments. Notably, the climate conditions and storage periods affected the results of this experiment.     

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.     

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.     

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.     

An optimisation framework for drying of heat-sensitive products

An equipment model for a two-stage evaporator system has been presented and validated. A material model incorporating liquid and vapour migration as well as the heat transfer in a potato slice, as a model heat-sensitive product, was formulated and validated. Combining the two models, optimisation of the heat pump drying process was conducted for intermittent air temperature drying cycles. The independent parameters considered were the mean starting temperature, T_m, and the intermittent cycle ratio of the drying air, τ_on/τ_cycle. The constraints imposed were the desired moisture content, drying time, ascorbic acid (AA) content and non-enzymatic browning (NEB). The objective functions were minimising the loss of AA content and NEB degradation, and maximising the heat recovery of the two-stage evaporator system. The implications of relaxing implicit and explicit constraints on heat recovery and product quality parameters (AA and NEB) were also studied.     

The influence of feedstock drying on the performance and economics of a biomass gasifier–engine CHP system

The need to dry biomass feedstocks before they can be gasified can place a large energy and capital cost burden on small-to-medium scale biomass gasification plants for the production of heat and power. Drying may not always be unavoidable, but as biomass moisture content to the gasifier increases, the quality of the product gas deteriorates along with the overall performance of the whole system. This system modelling study addresses the influence of feedstock moisture content both before and after drying on the performance and cost of a biomass gasifier–engine system for combined heat and power at a given scale and feedstock cost. The scale range considered 0.5–$\text{3.0}\text{MW}{}_{\mathrm{<mtext>e</mtext>}}$. The system comprises an updraft gasifier with external thermal and catalytic tar cracking reactors, gas clean-up and a spark-ignition gas engine. A spreadsheet-based system model is constructed, with individual worksheets corresponding to sub-models of system components, and a number of drying technology options and modes of operation are examined. Wherever possible, data supplied by manufacturers or taken from real systems is used in the construction of the sub-models, particularly in the derivation of cost functions.     

Performance study of a heat pump dryer system for specialty crops—Part 2: Model verification

The experimental and predicted performance data of a heat pump dryer system is reported. Chopped alfalfa was dried in a cabinet dryer in batches and also by emulating continuous bed drying using two heat pumps operating in parallel. Results showed that alfalfa was dried from an initial moisture content of 70% (wb) to a final moisture content of 10% (wb). The batch drying took about 4.5 h while continuous bed drying took 4 h to dry the same amount of material. The average air velocity inside the dryer was 0.36 m s^?1. Low temperatures (30–45°C) for safe drying of specialty crops were achieved experimentally. The heat pump drying system used in this study was about 50% more efficient in recovering the latent heat from the dryer exhaust compared to the conventional dryers. Specific moisture extraction rate (SMER) was maximum when relative humidity stayed above 40%. The dryer was shown to be capable of SMER of between 0.5 and 1.02 kg kW^?1 h^?1. It was concluded that continuous bed drying is potentially a better option than batch drying because high process air humidity ratios at the entrance of the evaporator and constant moisture extraction rate and specific moisture extraction rate values can be maintained. An uncertainty analysis confirmed the accuracy of the model. Copyright © 2002 John Wiley & Sons, Ltd.     

Heat and mass transfer during drying of a bed of shrinking particles – Simulation for carrot cubes dried in a spout-fluidized-bed drier

The purpose of the present study was to develop a model to describe the heat and mass transfer during the drying of carrot cubes in a spout-fluidized-bed drier. The model took into account the non-homogeneous shrinkage of the material. The Arbitrary Lagrange–Eulerian (ALE) formulation was applied to enter the problem with moving boundaries. Three phases of drying were distinguished according to the behavior of changes in percent local error of estimation: an initial phase of warming up the material – characterized by a low level of error of moisture content prediction, a second phase – characterized by an increase in the error of moisture content prediction and a phase of decreasing error. A simple test of the sensitivity of the model to the changes in heat transfer coefficient was performed in order to improve the ability of the model to predict the changes in moisture content and temperature of dried carrots. The predicted changes in both the moisture content and the temperature of carrot cubes during drying in a spout-fluidized-bed drier indicate that the model can be successfully applied to describe moisture content, temperature and deformation of dried particles in cases when the very high accuracy of moisture content and temperature prediction is not a crucial element of investigation of the drying process.     

Thermal analysis in fluidized bed drying of moist particles

This paper deals with thermal modeling of the fluidized bed drying of wet particles to study heat and mass transfer aspects and drying thermal efficiencies. The model is then validated with the literature experimental data obtained for corn. A parametric investigation is undertaken to study the effects of the inlet air temperature, the air velocity and the initial moisture content of the material (i.e. corn) on the process thermal efficiency. The results show that the thermal efficiencies of the fluidized bed drying decrease sharply with decreasing moisture content of corn and hence increasing drying time, and apparently become the lowest at the end of the drying process. This clearly indicates that the moisture transfer from the material depends strongly on the air temperature, air velocity and the moisture content of material. A good agreement is obtained between the model predictions and the available experimental results.     

入炉污泥含水率对污泥干化焚烧工艺影响研究

以上海某污水处理厂为例,通过理论计算和工程设计分析,研究了入炉污泥含水率(质量分数)对污泥干化焚烧工艺的影响.结果表明,随着干化程度的提高,对干燥机处理能力的要求提高,对干燥机型式的选择余地缩小,对焚烧炉、余热锅炉等设备的要求也将提高,对设备材质、系统安装、运行管理的要求也将相应提高.随着污泥干化程度的降低,进料量和烟气量增大,导致焚烧和烟气处理设备体积庞大.由于污泥泥质特性随时间变化大,在污泥热值整体偏低的地区,采用60%入炉污泥含水率存在一定的风险.污泥入炉含水率对污泥焚烧处理工程中的工艺选择及布置影响较大,工程设计中不应简单照搬国内外类似工程,而应根据当地污泥泥质特性、热值、辅助热源等实际情况,合理选择入炉污泥含水率.     

Solar drying in sludge management in Turkey

Two main wastewater treatment plants in Bursa city in Turkey will start to operate and produce at least 27,000 tons of dry solids per year by the end of 2006. The purpose of this study was to investigate an economical solution to the sludge management problem that Bursa city would encounter. The general trend in Turkey is mechanical dewatering to obtain a dry solid (DS) content of 20%, and liming the mechanically dewatered sludge to reach the legal landfilling requirement, 35% DS content. This study recommends limited liming & solar drying as an alternative to only-liming the mechanically dewatered sludge. Open and covered solar sludge drying plants were constructed in pilot scale for experimental purposes. Dry solids and climatic conditions were constantly measured. Faecal coliform reduction was also monitored. The specially designed covered solar drying plant proved to be more efficient than the open plant in terms of drying and faecal coliform reduction. It was found that, if the limited liming & solar drying method was applied after mechanical dewatering instead of only-liming method, the total amount of the sludge to be disposed would be reduced by approximately 40%. This would lead to a reduction in the transportation, handling, and landfilling costs. The covered drying system would amortize itself in 4 years.     

太阳能辅助热泵干燥粮食的数值模拟研究

粮食的干燥过程实质上是多孔介质热湿耦合传递的过程。基于多孔介质热质传递理论,通过数值模拟的方法,针对利用太阳能辅助热泵干燥粮食时热风随时间变化的情况,采用综合温度和空气绝对湿度作为瞬态边界条件来对干燥过程中粮食内部温度和水分的变化进行模拟研究。模拟结果显示小麦水分在干燥150h后达到安全水分13.6%(干基),而实验结果显示小麦水分在干燥135h后达到安全水分13.6%(干基),二者对比相差不大,并且模拟温度与试验温度吻合较好。     

Spatial and temporal effects in drying biomass for energy

This study evaluates the impact of the moisture content of biomass on thermal efficiency and relative boiler size which directly represent the economic merits of biomass drying. A model for predicting the moisture content of bundled Leucaena (Leucocephala) trees under open environment was validated for tropical Hawaii. Cumulative precipitation and evapotranspiration (ET) are the major factors affecting the biomass moisture content change. ET was computed using Hargreave's model, which requires only temperature and solar radiation data. Integration of these models made it possible to calculate the thermal efficiency and relative boiler size when using bundled trees as a fuel under a given drying regime and for a specific geographical location. A geographic information system provided the temperature and precipitation data required for evaluating the spatial variation in boilder efficiency and size for the 1440 km² island of Kauai. Depending on the time of harvest, the Leucaena moisture content varied from 35 to 69% (on wet basis) following a period of 6 months of in-field drying. Boiler efficiency using fuelwood with this range of moisture content varied from 49 to 73%. Boiler relative size varied from 1.2 to 2.2 times the size required when Leucaena with 0% moisture content is used as a fuel. The spatial and temporal effects on the value of biomass were thus found to be important factors for various sites in the study area. The methods for quantifying the merit of biomass moisture management proposed in this paper demonstrate how GIS modeling can lead to appropriate decision-making capability in bioenergy.     

Mathematical Modeling of Heat and Mass Transfer during Convective Dehydration of an Anisotropic Cylindrical Foodstuff

Two‐dimensional, unsteady‐state mass transfer was studied for air drying of an anisotropic finite cylindrical body. A mathematical model was developed for predicting the temperature of the drying sample at any time and moisture in any position in the drying sample at any time. The anisotropic nature of the drying material was considered in the mathematical model by taking into account the different moisture diffusivities in the axial and radial directions. A cut fresh green bean was used as an anisotropic material and a pilot‐scaled dryer was set up to investigate the drying behavior of this foodstuff. Several length‐to‐diameter ratios of fresh green beans were considered and the mathematical model was validated by comparison of the predicted values of average moisture content with the experimental data. The model was used to predict the moisture distributions inside the drying samples. Different moisture content distributions in the axial and radial directions during drying confirms the anisotropic nature of cut green bean samples. © 2013 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.21119     

Moisture transfer models for slabs drying

This paper deals with an experimental and theoretical investigation of drying of moist slabs. Experimental part includes the measurement of the moisture content distributions of eggplant slices with 5 mm thickness and 35 mm diameter during drying at the temperatures of 55 °C, 65 °C, and 75 °C and the velocities of 1.0 and 1.5 m/s, respectively. Four drying models are used to determine drying process parameters (e.g., drying coefficient, lag factor, and half-drying time) and moisture transfer parameters (e.g., moisture diffusivity and moisture transfer coefficient), and to calculate moisture content distributions. The calculated values are then compared with the experimental moisture data during the drying of eggplant slices at different drying air temperatures and flow velocities. An excellent agreement is obtained between the calculations and experimental measurements for the cases considered. Also, the experimental drying times are determined and compared with the ones obtained through four different drying models. The results show that all four models are capable of estimating the drying parameters and moisture content distributions. The experimental drying data and model findings are expected to be useful to drying industry.