Industrial Processes for Biomass Drying and Their Effects on the Quality Properties of Wood Pelle

Biomass and Bioenergy 27 (2004) 621–628 Industrial processes for biomass drying and their effects on the quality properties of wood pellets M. St( ahl à , K. Granstro¨ m, J. Berghel, R. Renstro¨ m Division for Engineering Sciences, Physics and Mathematics, Department of Environmental and Energy Systems, Karlstad University, SE-651 88, Karlstad, Sweden Received 4 December 2002; received in revised form 7 July 2003; accepted 14 August 2003 Abstract T
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  Biomass and Bioenergy 27 (2004) 621–628 Industrial processes for biomass drying and their effects on thequality properties of wood pellets M. St ( ahl  , K. Granstro ¨m, J. Berghel, R. Renstro ¨m Division for Engineering Sciences, Physics and Mathematics, Department of Environmental and Energy Systems, Karlstad University,SE-651 88, Karlstad, Sweden Received 4 December 2002; received in revised form 7 July 2003; accepted 14 August 2003 Abstract This paper contributes to the discussion of how different kinds of industrial scale dryers for biomass influence thequality properties of wood pellets. It also discusses how the drying technique can affect the environment. The mostcommon biomass drying processes in use, i.e., convection dryers are discussed. The discussion of drying techniques isbased on advantages and disadvantages with a focus on the drying medium, temperature and residence time. The choiceof drying technique is particularly important if the end-user’s choice of pellets is made due to the specific requirementsfor the heating system used. Some specific parameters were tested in order to investigate how the choice of dryingtechnique affects the pellet quality. The parameters tested were moisture content and the emissions of volatilehydrocarbons. Pellets available on the market were chosen for the tests. The amount of volatile hydrocarbons left insawdust after drying vary with drying technique, as emissions of terpenes are larger in dryers with long residence times.Low emissions of volatile hydrocarbons would improve the energy content of the sawdust, and by decreasing airpollution improve the work environment and the environment in the surroundings of the dryers. r 2004 Elsevier Ltd All rights reserved. Keywords:  Wood fuel; Biomass drying; Pellet quality; Sawdust; VOC-emissions 1. Introduction The high taxes on emissions of carbon dioxidefrom fossil fuels and the high price of fuel oil hasmade the residential heating market demand forpellets in Sweden grow very rapidly the last fewyears, and last year especially. In March this year(2002) the environmental ambassadors in theEuropean Union accepted the commission’s pro-posal to approve the Kyoto Protocol. Thatdecision implies that the European Union pledgesto decrease the emissions of greenhouse gases by8% until 2010 in comparison with the level of  ARTICLE IN PRESS$-see front matter r 2004 Elsevier Ltd All rights reserved.doi:10.1016/j.biombioe.2003.08.019  Corresponding author. Tel.: +46-54-700-1000; fax: +46-54-700-1165. E-mail address: (M. St ( ahl). URL:  emissions in 1990. This could increase the pelletmarket in Sweden even further [1]. From the year2000 to 2001 the pellets burner market for theSwedish domestic market has gone from 5000burners sold to almost 15000 burners sold [2]. Thesmall-scale customers used about 250000 tonnesof pellets in 2001 and that market is still growing[2]. Last year 22 pellet factories in Swedenproduced about 715000 tonnes of pellets althoughthe production capacity is more than 1 milliontonnes a year [3]. Sweden also imported about180000 tonnes of pellets and exported about60000 tonnes of pellets [4].Cutter shavings and dry sawdust are preferredmaterials for production of wood pellets. Todaymost pellets are made from sawdust or shavingsand those materials are used to the maximumextent. As the demand for pellets increases thesupply of dry sawdust will be insufficient andother raw materials will be used. At first morewet sawdust will be dried and used and if theuse of pellets continues to grow the raw materialsbark, branches and crowns (grot in Swedish),stem wood and recycled fuel should come intouse in that specific order [5]. The drying of bio- mass has increased during the last decades andwith a growing pellet market it will increasefurther. The wet raw material used today forpellet production contains about 50–55% water.For production of pellets the Swedish manu-facturers normally dry the raw material towater content 8–12% before the pelleting processbegins [6].Drying of wood-based biofuels is important,since wet wood results in low combustion tem-peratures, low energy efficiency and high emissionsof hydrocarbons and particles compared to forexample pellets. If biofuels are dried and com-pressed to pellets or briquettes, the fuel will havecontrolled moisture content (MC), have a higherenergy density and be easier to transport. It willalso take up less room and be less susceptible tomould and insect attacks during storage. Theshape of the pellet, diameter 6–8mm and a lengthof 4 or 5 times the diameter according to theSwedish standard [7] also make it easier to use infully automatic operation. Pellets have an addi-tional advantage in being a standardised fuel,simplifying construction and operation of burners.It is important to the pellet producers to havequality standardisation. It increases the customer’sconfidence since the customer wants to know whatproduct he or she buys [8]. This is particularlyimportant when dealing with home heating custo-mers, as they are not as tolerant as large-scaleusers for differences in pellet quality. Small-scaleusers now demand a high and even quality of pellets. Therefore, it is very important to havegood control over drying parameters like tempera-ture and residence time.Dryers can be classified according to themedium used in the drying process. In Swedenflue gas dryers and superheated steam dryers areused for commercial drying of sawdust. Rotary-drum dryers using combustion gases as the heatingmedium are the most common technique fordrying sawdust in Sweden. Dryers investigated inthis paper work as convection dryers where a gasboth supplies the necessary energy and transportsaway the emitted steam. According to Mujumdar[9] the superheated steam dryers have some keyadvantages compared to air dryers. No oxidationor combustion reactions are possible. Steam dryershave higher drying rates than air and gas dryers.Steam drying also avoids the danger of fire orexplosions and allows toxic or valuable liquids tobe separated in condensers. However, the systemsare more complex and even a small steam leakageis devastating to the energy efficiency of the steamdryer [10].In a steam drying process, Johansson et al. [11]investigated the temperature in a small wood piecedried in superheated steam. They found that thetemperature of the material stays at the boilingtemperature until the MC decreases to the criticallevel at 10% (water wet basis, wb). Below this thematerial will gradually approach steam tempera-ture. In flue gas dryers the dried material is heatedto the wet bulb temperature until the MC reachesthe critical MC, below this the material tempera-ture will approach the gas temperature. Flue gasrotary dryers combine high inlet temperature withlong residence time. According to Wimmerstedt etal. [12] this can result in pyrolysis and partialgasification. In lumber drying a kiln uses heatedair as the drying medium. According to Rosen [13] ARTICLE IN PRESS M. St ( ahl et al. / Biomass and Bioenergy 27 (2004) 621–628 622  the conventional inlet temperature is 60–82 1 C.The temperature of the wood is near the wettemperature.In the presence of nitrogen oxides and sunlightthe volatile organic compounds (VOC) emitted toair during drying contribute to the formation of harmful photo-oxidants. Elevated levels of photo-oxidants are an important cause of widespreadforest and crop damage in Europe. Photo-oxidantsare also harmful to humans, as they causeirritation in the respiratory tract and in sensitiveparts of the lungs. Monoterpenes (C 5 H 8 ) 2  are themost volatile group of the components present inwood, and is of particular interest in studies of emissions during drying. Due to the short atmo-spheric lifespan of monoterpenes, the highestphoto-oxidant concentrations can be expectedwithin 5h after the emission takes place, andwithin a distance of 50km [14].When emissions during drying of wood fuel areconsidered in relation to the type of dryer used, theparameters residence time, temperature and dryingmedium are most significant. The release of VOC(g/s kg oven dried weight (odw)) is rapid early inthe drying process, with a small second emissionpeak at about 10% MC [15,16]. The total amountof emitted VOC (g/kg odw) from wood willincrease rapidly at the beginning of the dryingprocess, and then level out [16]. Increaseddrying medium temperature increases the totalamount of emitted VOC (g/kg odw) [16–18]. TheMC of the drying medium does not affect thereduction of terpene content in sawdust duringdrying [16].In the Swedish Standard Institutes test of pelletquality the dimensions, MC, density, bulk density,ash content, heating value, durability, sulphur andchlorine content and additives are specified. InLehtikangas [19] paper all the pellet qualityparameters according to the Swedish standard [7]are tested. The purpose of this paper is to discussthe drying media, the residence time and thetemperature used in the drying processes. Further-more, the aim is to discuss how those para-meters influence the MC and the heatingvalue with a special focus on the emissions of VOC from the dryers but also from the pelletproduction. 2. Materials and methods  2.1. Test materials Samples from six different producers (A–F)were tested. Three pellet producers suppliedsamples of wet and dry sawdust as well as pellets.All the pellet assortments had 8mm diameter.Samples of sawdust before and after drying in alaboratory steam dryer were also obtained. Fromthe pellets producers using raw material fromlumber dried wood (E and F), dried material andpellets were examined from E and pellets from F(Table 1).Four producers uses sawdust as raw material,one uses dried sawdust and cutter shavings, andone wet sawdust, cutter shavings and groundcutter shavings (Table 2). The sawdust and cuttershavings used for pellet production are fromNorway spruce ( Picea abies ), Scots pine ( Pinussylvestris ) or both.  2.2. Industries for pellet manufacturing Two of the producers use the drum-dryingtechnique; one uses the steam-drying techniqueand two use sawdust and cutter shavings fromwood dried in a lumber kiln. A laboratory scalespouted bed steam dryer was also included in thecomparison (Table 2). As the factories usedifferent kinds of dryers, the drying temperatureand the residence time varies between the dryers.  2.3. Laboratory procedures Collection of the samples and their preparationwere conducted using standard procedures [20,21].The sawdust and pellets were stored in polyethy-lene bags in a freezer before analysis. The MC was ARTICLE IN PRESS Table 1Samples from producers (x=received sample)Producer A B C D E FWet raw material x x x xDry raw material x x x x xPellets x x x x x M. St ( ahl et al. / Biomass and Bioenergy 27 (2004) 621–628  623  tested with the SS 187170 [22] as a guideline, andthe oven set to 103 7 2 1 C.For the analysis of terpene content thesawdust samples (1.85–2.64g) and the crushedpellet samples (3.81–6.1g) were extracted with50ml acetone in a Soxhlet extractor for 8h.The acetone extract was diluted with 100mlwater, followed by dual extraction with 10mlisooctane. The isooctane was analysed with a GC-MS (GC Perkin Elmer AutoSystem with anunpolar capillary column, injector temperature200 1 C, temperature program 45 1 C for 2min,40 1 C/min to 80 1 C held for 8min, 15 1 C/min to250 1 C; MS Perkin-Elmer Qmass 910, scan35–300 m = e ). The amounts were calculated froman external standard series of the three mostabundant monoterpenes. This method is simi-lar to the one used by Englund and Nussbaum[17], though they used a Soxtec s extractorwhich shortens the extraction time to 3h, andextracted the acetone phase three times withpentane. 3. Results 3.1. Moisture content The raw material had MC between 41.2 (D) and53.5% (B) (Table 3). After drying the material MCbetween 5.9 (B) and 11.7% (A) are reached and thepellet has MC between 6.0 (E) and 7.8% (D).According to the Swedish standard SS 187120, theMC of pellets should be less then 10% or less than12% depending on which standard group itbelongs to [7]. 3.2. Terpene content Material A contains much more terpenes thanthe other materials (Table 4). The terpene contentin pellets is more than 100mg/kg odw for A and F,somewhat less than 100mg/kg odw for E and D,and as low as 14mg/kg for material C.The species of tree in the sawdust samples isknown only for material E. In Sweden, the sawmill ARTICLE IN PRESS Table 2Production technologies used in the comparison and their key parametersProducer A B C D E FDryingtechniqueFlash dryer Spouted bed Rotary drumdryerRotary drumdryerLumber kiln Lumber kilnRotary drumdryer a Drying media SuperheatedsteamSuperheatedsteamFlue gas fromforest residuesFlue gas fromsawdust(powder)Air AirRaw material Wet sawdust Wet sawdust Wet sawdust Wet sawdust Dry sawdustand cuttershavingsWet sawdustand cuttershavingsTemperature >140 1 C Inlet temp240 1 C, outlettemp.130 1 COutlet temp90–100 1 COutlet temp115 1 C60–82 1 C b 60–82 1 C b Residence time Short Short 2.5min c Medium Medium Long pine 110hspruce 80hLong pine 110hspruce 80hAdditives No No Lignine d No No Wafolin S d,ea Lumber kiln dried cutter shavings are mixed with wet sawdust and dried in a rotary drum dryer. b Rosen [13]. c Experiments conducted by Renstro ¨m (unpublished) have shown dryer B to have a mean residence time of 2.5min for sawdust withfinal MC of 15% (wb). d Olsson [6]. e Lignosulphonate made of pine oil. M. St ( ahl et al. / Biomass and Bioenergy 27 (2004) 621–628 624
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