Academic literature on the topic 'Wood chips – Drying'

Solid biofuels such as woodchips have always been a significant source of fuel in the field of renewable energy. However, the drying of wood chips has been a challenge in preparing biofuels and other applications. The moisture content of below 25% in the wood chips of grade EN 14961 is considered the premium wood chip material. Solar drying emerged as a leading solution for the drying of wood chips. The paper investigates the comparison of moisture removal rate using a natural convective solar dryer compared to open sun drying developed at Miskolc, Hungary (48°06'15.0"N 20°47'30.0"E).

Pine, birch, and cotoneaster wood chips were segregated and exposed to microwave radiation. Moisture content was measured before and after microwave treatment, and the surface temperature of wood chip samples was recorded. The results showed that due to the selective nature of the process, the duration of microwave radiation should be adjusted taking into account the size fraction of the examined material. Wood chips exposed to microwaves for more than 30 s heated up to over 100 °C. Finer wood chips were found to lose moisture more slowly.

Abstract Sustainable biomass resources are limited and their utilization therefore needs to be more efficient. In addition, there is an urgent need for low-cost energy storage, particularly for solar energy. Drying considerably increases the calorific value of woody biomass, and the resulting dried biomass provides easy seasonal energy storage. The drying both improves the quality of the biomass and extends its storage life. To investigate the technology and feasibility of solar-enhanced drying, several drying experiments were conducted on wood chips in VTT’s 12 kWpeak convective dryer in Jyväskylä, Finland. Drying times varied from 3.5 to 27 h and the final moistures from 12 to 32 w-%. VTT’s experiments show that solar heat can be successfully applied to thermal drying of biomass. The moderate drying temperatures used (typically 20–50 °C) are advantageous for ensuring homogenous drying of wood particles and for preventing changes to the physical structure of the biomass and loss of volatiles. Due to the low efficiency of the system, still in its prototype phase, the calculated payback times were not highly attractive to the entrepreneur. In the experiments, best profitability was achieved by drying seasoned wood, for which a payback time of 12–17 years was estimated for a large scale system such as a biomass terminal. Based on the assumptions that technical improvements are made, the best drying efficiency reached in our experiments is achieved, and some investment subsidy is secured; scale-up is feasible. With these criteria met, the payback time of such a dryer could be brought below 10 years. This requires, however, that drying increases the selling price of wood chips.

Short-rotation plantations are harvested in cycles of three to twenty years, depending on the production aim. In contrast to the establishment and recultivation costs, the costs for harvesting and processing of the wood occur regularly. The harvesting technology should be chosen with respect to the desired outcome – logs or chips. This is crucial for the process costs as well as the possible performance. A combination of forestry harvesters and forwarders is recommended when logs will be harvested. If the wood will be used energetically, an agricultural combined harvester and chipper fitted with suitable harvesting aggregates is the best technology with respect to productivity and performance, as well as for economic reasons. For drying the wood chips, an air ventilation process based on the self-heating of freshly harvested wood chips is an effective method for drying the wood without external energy input. With this method, it is possible to reduce the water content to 30% within three months.

Processes occurring during storage of wood chips for energetic or furniture industry purposes were presented. As a result of carried out investigations, dependences of temperature and relative humidity changes of surrounding air were shown. Modified Henderson equation can be utilized for computer simulation of storing and drying processes concerning wood chips for energetic and furniture industry purposes. It reflects also obtained results from experiments carried out with above mentioned material. Using computer simulation program we can examine different wood chips storing conditions to avoid overheating and loss problems.

AbstractBased on the exploitation research, the authors evaluated the possibilities of using the heat conducted through the side surface of the waste gases exhaust pipe of SB 1.5 drum drier for drying wood chips. According to the estimated calculations within one hour approximately 173 thousand of kJ of heat may be obtained from the external surface of the waste gases exhaust pipe with the height of 7 m and temperature of approximately 78°C which constitutes an equivalent of approximately 4 kilo of heating oil. In case the above mentioned heat source for drying wood chips in SPA 20 silo with a volume of 6800 kilo within 50% to 20% humidity is used, one may expect that the drying time will be approximately 100 hours.

The commercial rotary dryers used to dry wood wafers (of approximate dimensions 0.63 mm thick, 50 mm wide and 76+ mm long) for the production of panelboard are modified versions of agricultural dryers and have not been designed for the optimal drying of wood wafers. The lack of available information on wafer drying necessitated that the first goal of this research was the characterization of wafer drying behaviour. After the important parameters of wafer drying were identified, the applicability of fluidized bed technology to wafer drying was assessed and an industrial size dryer was designed. The proposed fluidized bed wafer dryer was then compared to a commercial rotary dryer in terms of energy efficiency. Wafer drying behaviour was investigated in two factorial experiments. Three lengths of wafers (25 mm, 44 mm and 63 mm) were individually dried in a 0.15 m draft tube at temperatures of 90°C, 120°C and 150°C. The statistical analysis of the resultant drying rate curves showed that the drying behaviour of aspen wafers was influenced by the effect of wafer length on the external heat and mass transfer rates to the wafer surface, and on the length of internal pathways for bulk flow and diffusion of water. The external drying conditions had a decreasing effect on drying rate until about 10% moisture content at which time drying became limited by internal heat and mass transport. The initial assessment of fluidized bed technology for wafer drying used a 0.15 m semi-cylindrical column for the determination of wafer drying rate curves and wafer behaviour in a fluidized bed of inert particulate solids at excess superficial velocities of 0.25 to 1.0 m/s. Wafer drying times in a bed of 0.5 mm sand at 150°C were about 40% of the drying times for wafers dried by forced convection of air at the same temperature and twice the superficial velocity (~ 1 m/s). Wafer movement in the fluidized bed followed the circulation patterns of the emulsion phase and was thus dependent on the bubbling behaviour of the bed. A minimum excess superficial velocity of 0.25 m/s (depending on distributor design) was required to prevent permanent settling of the wafers to the distributor. Preliminary experimentation on a 2-compartment bed showed that wafers could be circulated through the two compartments in near plug flow. However, the application of this technique to a 4-compartment continuous fluidized bed wafer dryer was unsuccessful because of the separation of sand and wafers caused by slugging beds in two of the compartments. A preliminary design was prepared for an industrial size, 5-compartment fluidized bed wafer dryer to approximate plug flow of wafers by a series of well-mixed fluidized beds in series. The design calculations showed that this dryer was more efficient in terms of energy and plant space than a conventional triple pass rotary dryer.
Applied Science, Faculty of
Chemical and Biological Engineering, Department of
Graduate

The kiln drying of wood produces huge amounts of vapour. The vapour is released to the environment when the process purges some of the saturated hot air. The main environmental issue regarding the use of kiln drying process are the release of the water vapour which contains organic contaminants. Some of them are hazardous to human health. In addition, there are some wood particles which may released with the water vapour purging process. In this research, the vapour is condensed and analysed for its organic contaminants and their biodegradability. The result showed that the dominant contaminants present in the condensate were ethanol and methanol with the concentration of approximately 65 mg/L and 25 mg/L respectively. The average COD concentration of the condensate was 159 ± 40 mg/L. The analysis also showed that the contaminants were biodegradable. In order to treat the wastewater, a trickling filter process using bark chips as a support medium was used to treat an artificial wastewater. The artificial wastewater contained the dominant contaminant present in the wood drying condensate. In the experiment, different sizes of bark chips were used. In addition, the loading rate of the treatment system was varied by changing the flow rate and contaminant concentration. The 30 cm long trickling filter using bark chips varying between of 2.8 – 4 mm diameter as the support medium gave a maximum removal of 36.4 % with removal capacity of 8.34 kg COD/m³bed•day at a flow rate of 2.8 cm/min and average inlet COD load of 20.4 kg COD/m3bed•day. The trickling filter with bark chips varying between 5.6 – 8 mm diameter as the support medium was operated using variations in contaminant concentration and flow rate. The operation using different inlet concentration gave the highest removal rate of 13.5 kg COD/m3bed•day at average initial load of 84.9 kg COD/m³bed•day, flow rate of 2.8 cm/min and theoretical initial concentration of 680 mg/L. The trickling filter operated with flow rate variation showed the highest removal rate of 10 kg COD/m³bed•day at an average inlet load of 53.3 kg COD/m³bed•day and flow rate of 7.1 cm/min. The removal rate of the contaminants in treatment was limited. There is a number of possible explanations. First is the active surface area, which indicating the area where the contact between the biofilm surface and feed happened. The active surface area increased as the flow rate increased. Second is the residence time of the feed in the bed. The residence time of the feed varied with the flow rate. It decreased as the flow rate increased. Third is the influence of the contaminants in the feed. The presence of methanol and methanol in the feed inhibited each other’s degradation. The dimention of a full-scale biotrickling filter to be used in actual kiln was also estimated. The estimation was made based on the maximum removal rate and optimum flow rate obtained in the experiments. The result of the estimation showed to obtain significant removal, the required bed would have to be 2.35 m in diameter and 160 in height.

I en värld med ökande koldioxidhalter i atmosfären och höjd medeltemperatur, råder det inga tvivel om att vi står inför en rad utmaningar för att minska användandet av bland annat fossila bränslen som generar skadliga utsläpp. Det finns många alternativ till oljebaserade bränslen, och ett som har ökat markant de senaste åren är bränslepellets. Bränslepellets är ett träbaserat biobränsle som i sitt kompakta tillstånd erbjuder ett bra värmevärde och är klimatneutralt. För att tillverka pellets måste råmaterialet först termiskt torkas, från en fukthalt på ca 55 % till ca 10 %, vilket i dagsläget kan motsvara upp till en fjärdedel av hela energiåtgången i pelleteringsprocessen. Med den ökade efterfrågan av bränslepellets finns också ökade förutsättningar för energieffektiviseringar i pelletsproduceringen, speciellt i torkningssteget.  Drinor AB har tagit fram en avvattningsmaskin av biomaterial som heter CDP, och med den är det möjligt att avvattna biomaterial till ca 30 %, vilket skulle reducera både tiden och energin det tar att termiskt torka materialet. Avvattningen sker under tryck på minst 40 ton, där vattnet mekaniskt pressas ut ur råmaterialet. Hur avvattningen påverkar råmaterialet, speciellt i en pelletframställning, är oklart och syftet med detta arbete var att ta reda på hur pelleteringsegenskaperna påverkas efter pressning med CDP, och om det finns möjligheter att spara energi i malningsdelen i pelleteringsprocessen. Målet med arbetet var att ta reda på hur CDP påverkar pelletskvaliteter i form av hårdhet och densitet, samt om friktionsutvecklingen i pelleteringsmatrisen förändras, genom att framställa pellets ur 3 scenarion. I ett scenario ska en traditionell pelletsframställning liknas, med endast termisk torkning och i de resterande två scenarion implementeras CDP som försteg till den termiska torkningen. I ett av dessa två scenarion mals inte träflisen innan pelletering, för att se om energi kan sparas utan att offra pelletskvalitet. Ett annat mål var att, per scenario, ta reda på vid vilken fukthalts- och temperaturkombination de bästa pelletsen tillverkades med avseende på hög densitet och hårdhet samt låg friktionsutveckling. Resultaten visade att scenariot med CDP som komplement till termisk torkning och utan malningprocess, producerade pellets med högst hårdhet, högst densitet och lägst friktionsutveckling under båda fukthalterna på pelleteringsmaterialet och nästintill samtliga matristemperaturer. Det scenario som hade endast termisk torkning producerade pellets med lägst densitet och hårdhet samt högst friktion under nästan alla temperaturer och fukthalter. När den bästa fukthalts- och temperaturkombinationen togs fram per scenario, så var scenariot med CDP och utan malning bäst. Pellets producerade där hade ökad densitet, nästan tredubblad hårdhet samt mer än halverad friktionsutveckling i pelleteringsmatrisen, jämfört med scenariot som imiterade traditionell pelletsframställning med endast termisk torkning. Skulle det scenariot med CDP och utan malningsprocess implementeras i en verklig industriell skala skulle det innebära stora förutsättningar för ökad produktion av pellets med bättre kvalitet, samt ett minskat energianvändande i form av reducerad termisk torkning och minskat användande av malningsprocessen.
In a world with growing carbon dioxide contents in the atmosphere and elevated average temperature, there is no doubt that we are faced with a number of challenges to reduce the use of, among other things, fossil fuels that generate harmful emissions. There are many alternatives to oil-based fuels, and one that has increased markedly in recent years is fuel pellets. Fuel pellets are a wood-based biofuel that, in its compact state, offers a good thermal value and is climate neutral. In order to produce pellets, the raw material must first be thermally dried, from a moisture content of about 55 % to about 10 %, which can currently account for up to at least a quarter of the total energy consumption in the pelleting process. With the increased demand for fuel pellets, there are also increased possibilities for energy efficiency in the pellet production, especially in the drying stage. Drinor AB has developed a biomaterial dewatering machine called CDP, with which it is possible to drain the biomaterial to a moisture content of about 30%, which would reduce both the time and the energy it takes to thermally dry the material. The dewatering pressure is at least 40 tonnes, where the water is mechanically squeezed out of the raw material. How the dewatering affects the raw material, especially in a pellet production, is unclear and the purpose of this work was to find out how the pelleting properties are affected after pressing with CDP and if there is potential for saving energy in the grinding process in the pelleting process. The aim of the work was to find out how CDP affects pellet qualities in terms of hardness and density, and if the friction development in the pelleting dye changes, by making pellets out of 3 scenarios. In one scenario, traditional pellets production should be resembled, with only thermal drying, and in the remaining two scenarios, CDP is implemented as a complement to thermal drying. In one of these two scenarios, the wood chips were not milled before pelleting, to see if energy can be saved without sacrificing pellet quality. Another goal was to determine, by each scenario, what moisture and temperature combination the best pellets were produced with respect to high density and hardness and low friction development. The results showed that the CDP scenario, as a complement to thermal drying and without grinding process, produced the hardest pellets, highest density and lowest friction development during both moisture levels of the pelleting material and almost all die temperatures. The scenario that only had thermal drying produced pellets with the lowest density and hardness, as well as maximum friction under almost all temperatures and moisture levels. When the best moisture and temperature combination was achieved by each scenario, the scenario with CDP and without grinding was the best. Pellets produced there had increased density, almost triple the hardness, and more than half the friction development in the pelleting die, compared to the scenario that imitated traditional pellets production with only thermal drying. Should the scenario with CDP and without grinding process be implemented on a real industrial scale, it would provide great conditions for increased production of better quality pellets, as well as reduced energy use in the form of reduced thermal drying and reduced use of the grinding process.

Storage of biomass is often associated with problems such as heat development, drymatter losses and reduction of fuel quality. The rise in temperature can potentiallycause a risk of self-ignition in the fuel storage. Moreover, emissions from storage pilescan cause health problems in the surrounding. The dry matter losses and reduction offuel quality can have economical effects. The aim of this thesis project is to developguidelines on how to store large amount of biomass at Vattenfalls heat and powerplants in an optimal way. Storage trials at Idbäckens CHP were done in order to studythe effect of storage on fuel quality, dry matter losses and temperature development.Two storage trials were performed over six weeks with waste wood chips and stemwood chips stored in about 4.5 m high outdoor piles. A trial over four days in whichwaste wood chips was placed on a heated surface was evaluated. A study to test thepossibility of using waste heat to dry waste wood chips was performed.Small but not negligible dry matter losses were observed in both of the piles of storedmaterial. The largest weekly losses were found during the first week of storage and adeclining behavior could thereafter be seen. The accumulated losses during six weeksof storage were 2.0 % and 1.7 % respectively, for waste wood and stem wood. Storageduring six weeks of waste wood and newly chipped stem wood did not cause anymajor deterioration of the fuel quality as such, beside the substance losses.No drying effect could be seen in the heated surface trial. The surface became warm,about 50°C, but it was not sufficient to dry the chips. The conclusion is that it is notpossible to dry large amount of chips on a heated surface with the design used hereand during four days.The overall conclusion is that in order to minimize the dry matter losses the materialshould be handled according to the LIFO (last in first out) principle. Wheneverpossible, try to purchase fuel that has been stored for a while since the more easilydegraded compounds has already been degraded through microbial activity. There is apossibility that the largest losses has already occurred. Furthermore, try also tocomminute the material (reduce the particle size) at the plant and as close in time tocombustion as possible.

The fixed bed drying of western hemlock and Douglas-fir biomass particles at temperatures ranging from 50��C to 200��C and air velocities from 0.3 to 0.9 m/s was investigated. The objectives were to describe the drying characteristics of the particles, fit a model for thin-layer drying, and develop and test a deep bed drying model based on the thin-layer model. The effects of temperature and air velocity were determined in a bed approximately 1.3 cm in depth and a model for the drying curve was developed. The thin-layer model was then used to predict what would happen in a deeper bed. Model results were compared to drying curves measured in a 23-cm-deep bed. The deep bed model predicted both the experimental drying times and the moisture and temperature profiles in the bed.
Graduation date: 2013

This paper deals with energy savings by the heat recovery of waste vapour from moist biomass drying in energy systems. Drying is an energy-intensive process. Energy consumption can be reduced by using indirect drying by recuperating the heat of waste vapour generated in the process; however the vapour is polluted by air and small mechanical particles. Experiments with green wood chips were realized on an indirect dryer with a condensing heat exchanger to experimentally verify the grade and conditions of heat recovery from waste vapour. On the basis of the experimental results, the potential of the heat recovery from waste vapour was evaluated. Keywords: Indirect drying; Biomass; Waste vapour; Heat recovery

The present work reports a computer simulation and optimization study of heat transfer in a rotary kiln used for drying and preheating of wood chips with superheated steam at 1 bar. A rotary kiln employed for drying and preheating of wet solids consists of a refractory lined cylindrical shell mounted at a slight incline from the horizontal plane. The kiln is very slowly rotated about its longitudinal axis. Wet solids are fed into the upper end of the cylinder and during the process, are dried and heated by the countercurrent flow of the hot gas. Finally, it is transferred to the lower end where it reaches the desired temperature and discharged. The heat transfer model includes radiation exchange among hot gas, refractory wall and the solid surface, transient conduction in the refractory wall, and mass and energy balances of the hot gas and the solids. A finite-difference based computational heat transfer approach is used. A Univariate Search method has been used to obtain minimum kiln length with respect to various kiln operating parameters subject to a constraint on the inlet gas temperature. The parametric study lent a good insight into the physics of the drying process in a rotary kiln. The optimization study reveals that for an economical design of a rotary kiln low rotational speed, small inclination angle and medium gas flow rate is required.

Sludge from wastewater treatment plants in five Swedish pulp and paper mills has been burned together with wood in a circulating fluidised bed (CFB) boiler. The sludge was either mechanically dewatered or pre-dried. The mechanically dewatered sludge had to be fed with a pump, but the pre-dried sludge could be fed by the fuel feed system normally used for coal, wood chips or wood pellets. In parallel to the combustion tests in the CFB boiler the sludges were also investigated as single fuels in a small laboratory FB. The Swedish pulp and paper industry produces three major fractions of sludge: pure fibre sludge, sludge produced by employing a precipitation species like ironaluminiumsulphate, and finally, sludge subjected to biological cleaning. The way of production of the sludge influences its content of, for example, nitrogen, sulphur and chlorine, but the composition of the sludge is also influenced by the pulp and paper process. The present measurements show that the concentrations of nitrogen, sulphur and chlorine in the sludge have a great impact on the corresponding gaseous emissions from combustion. Actions to prevent these emissions could be necessary, depending on the origin of the sludge and treatment process used. In the present project all sludges were burned with wood-pellets as the main fuel under identical operating conditions, typical for a CFB boiler. Wood pellets were chosen as a well defined, low-polluting fuel that makes comparison of emissions from the sludges clear. Co-combustion with wood-pellets has the advantage of enabling operation also with wet sludges that cannot be used as single fuels without pre-drying. No actions were taken to improve sulphur and chlorine retention, by for example adding limestone. From a combustion point of view the co-combustion works well with low levels of carbon monoxide present in the flue gas and no light hydrocarbons.

Ryszard Parosa, Andrzej Brożyński, Piotr Grześkowiak, Krzysztof Kowalczyk, Marek Natoński, Piotr Ziętek and Janusz Żytkiewicz PROMIS-TECH Poland Keywords: microwave treatment, low pressure heating, microwave drying Uniquely favourable characteristics of biological product can be obtained through the use of the microwave method in vacuum heating process. Microwave-vacuum drying is superior to other methods in terms of dried products' structure, flavour, colour and biological active compounds contents. But applications of such a methods seems to be much wider: drying of fruits and vegetables for consumption, drying of herbs for extraction of valuable biological compounds, for modification of seed structure (sunflowers seeds, pumpkin seeds), for pasteurisation etc. A universal system was designed for testing such processes in laboratory scale and several industrial scale system have been developed. Process of thermal treatment can be carried out with plastic drum installed inside of multi-mode microwave cavity and cavity which is connected by microwave line with reflectometer and circulator - to microwave generator. In laboratory unit generator 2.45 GHz with controlled power (from 50W to 800W) was applied. System was equipped with vacuum pump with pressure control and is controlled by computer. Most important technical parameters, like: microwave power, time of treatment, pressure inside of drum, temperature of steam – are controlled and recorded. Laboratory scale unit is shown below. Basing on laboratory scale test several technologies in industrial scale was developed. Industrial scale unit equipped with 8 generators of 3 kW (2.45 GHz) was constructed and for last 8 years has been successfully used for “production” of crispy chips which are now popular in Polish marked. Exemplary industrial scale installation is shown in photo below. Multi – drum microwave drier. Two cavity microwave industrial drier Another system for modification od seed is now constructed – ordered by big industrial producer of batons and sweet snacks. Process will be carried our inside of dielectric drum in low pressure and reactor will be equipped with 8 generators of 3 kW (2.45 GHz). Treatment time will be reduced to 3-4 minutes and next material (seeds) will be cooled down. Last project which now realized is connected with drying of wood flour applied in composite material production. System will work continuously with two airlocks and with dielectric drum and will be connected with 4 microwave generators (3 kW, 2.45 GHz). In next step planed installation will be equipped with microwave high power generator ca. 60 kW with frequency 915 MHz.

Biomass has received wide attention as a substitute for fossil fuel in the generation of energy because of its renewability and carbon neutrality. However, raw biomass combustion is hindered by physical properties such as low energy density and high moisture content. Biomass torrefaction is a mild pyrolysis thermal treatment process carried out at temperature of 200 to 300°C under inert conditions to improve the fuel properties of parent biomass. This yields a higher energy per unit mass product but releases non-condensable and condensable gases which results in energy and mass losses. The condensable gases (volatiles), can result in tar formation on condensing hence, system blockage. Fortunately, the hydrocarbon composition of volatiles also represents a possible auxiliary energy source for torrefaction. The present study investigated energy recovery from volatiles through clean co-combustion with NG for feedstock drying and/or the thermal treatment process of pine wood chips. The research also studied the effect of torrefaction pretreatment temperatures on the amount of energy recovered for various combustion air flow rates. For all test conditions, blue visual flames and low CO and NOx emissions at the combustor exit consistently signified clean and complete premixed combustion. Torrefaction temperature at 283–292 °C had relatively low energy recovered from volatiles, mainly attributed to higher moisture content evolution and low molecular weight of volatiles evolved. At lowest torrefaction pretreatment temperature, smaller amount of volatiles was generated with most energy recovered from the volatiles. Energy conservation evaluation on the torrefaction reactor indicated that about 40% of total energy carried by the exiting volatiles and gases has been recovered by the co-fire of NG and volatiles at the lowest temperature while 20% and 22% of the total energy were recovered at the intermediate and highest torrefaction temperature respectively.