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Fuel Pellets from Wheat Straw: the Effect of Lignin Glass Transition and Surface Waxes on Pelletizing Properties

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Fuel Pellets from Wheat Straw: the Effect of Lignin Glass Transition and Surface Waxes on Pelletizing Properties Bioenerg. Res. (2012) 5:450-458 DOI 10.1007/s12155-011-9169-8 Fuel Pellets from Wheat Straw: The Effect of Lignin Glass Transition and Surface Waxes on Pelletizing Properties Wolfgang Stelte • Craig Clemons • Jens K. Holm • Jesper Ahrenfeldt • Ulrik B. Henriksen • Anand R. Sanadi Published online: 13 December 2011 CO Springer Science+Business Media, LLC. 2011 Abstract The utilization of wheat straw as a renewable 63°C, respectively. Pellets are pressed from wheat straw and energy resource is limited due to its low bulk density. straw where the waxes have been extracted from. Two pellet- Pelletizing wheat straw into fuel pellets of high density izing temperatures were chosen—one below and one above increases its handling properties but is more challenging the glass transition temperature of lignin. The pellets compres- compared to pelletizing woody biomass. Straw has a lower sion strength, density, and fracture surface were compared to lignin content and a high concentration of hydrophobic each other. Pellets pressed at 30°C have a lower density and waxes on its outer surface that may limit the pellet strength. compression strength and a tendency to expand in length after The present work studies the impact of the lignin glass transi- the pelletizing process compared to pellets pressed at 100°C. tion on the pelletizing properties of wheat straw. Furthermore, At low temperatures, surface extractives have a lubricating the effect of surface waxes on the pelletizing process and pellet effect and reduce the friction in the press channel of a pellet strength are investigated by comparing wheat straw before and mill while no such effect is observed at elevated temperatures. after organic solvent extraction. The lignin glass transition Fuel pellets made from extracted wheat straw have a slightly temperature for wheat straw and extracted wheat straw is higher compression strength which might be explained by a determined by dynamic mechanical thermal analysis. At a better interparticle adhesion in the absence of hydrophobic moisture content of 8%, transitions are identified at 53°C and surface waxes. Keywords Wheat straw • Pellets • Glass transition • W. Stelte () • J. Ahrenfeldt U. B. Henriksen Biosystems Department, Risø National Laboratory Lignin • Wax for Sustainable Energy, Technical University of Denmark, Frederiksborgvej 399, DK-4000 Roskilde, Denmark Introduction e-mail: [email protected] C. Clemons Wheat is one of the most common grown crop species Forest Products Laboratory, worldwide and its annual production has been estimated to United States Department of Agriculture, be about 685 million tons for the year 2009 [1]. Wheat straw One Gifford Pinchot Dr, Madison, WI 53726-2398, USA is therefore a large potential resource for heat and power production and a possible alternative to woody biomass in J. K. Holm countries with little forest resources. The bulk density of Chemical Engineering, DONG Energy Power A/S, loose straw is only about 40 kg/m3 [2]. Therefore, it is Nesa Alle 1, 3 DK-2820 Gentofte, Denmark usually compacted into bales of about 120 kg/m directly after the harvest [3]. Wheat straw's low bulk density makes A. R. Sanadi handling expensive since volume limitations of carriers are Biomass and Ecosystem Science, Faculty of Life Sciences, reached before the load limit, and large spaces are required University of Copenhagen, Rolighedsvej 23, for its storage. One way to overcome this limitation is to DK-1958 Frederiksberg C, Denmark compact wheat straw into fuel pellets or briquettes, which Springer Bioenerg. Res. (2012) 5:450-458 451 have a much higher density and allow long distance trans- The present work investigates the lignin Tg of both untreated portation for a fraction of the cost [4]. Fuel pellets made from and organic solvent-extracted wheat straw, where the hydro- agricultural residues are of increasing interest to researchers phobic waxes from the surface have been removed. The [5-10] and are also successfully being used by energy pro- removal of surface waxes was monitored by means of attenu- viders in large scale combined heat and power plants [11]. ated total reflectance—Fourier transform-infrared spectroscopy Recently, the pellets have also been successfully tested in (ATR-FTIR) that allows surface analysis of a very low depth. different household-size pellet boilers demonstrating their The Tg was determined by means of dynamic mechanical suitability for small-scale applications [12]. Nevertheless, thermal analysis (DMTA). Pellets were produced well below there are still significant drawbacks to overcome such as ash and above the Tg from untreated and organic solvent-extracted deposit formation and corrosion. wheat straw. The impact of the lignin Tg and the surface Wheat straw pellets are produced in pellet mills in which the extractives on the pellet stability are investigated by means of chopped straw particles are forced through cylindrical press compression testing, density and expansion measurements. channels. The high pressure in combination with an elevated Scanning electron microscopy (SEM) was used for fracture temperature of about 80-100°C, results in a softening and flow surface analysis and determination of the bonding type. of the wheat straws amorphous polymers. Polymer inter- penetration between the straw particles during pelletization Materials and Methods increases the bond strength due to macromolecular entangle- ment [13, 14]. Much work has already been done to determine Wheat (Triticum aestivum L.) was harvested in late summer of the optimal processing conditions for pellet production from 2007 on the island of Sjælland, Denmark and straw collected agricultural residues [5-10]. Nevertheless, the mechanical after being air dried in the field. The wheat straw was milled to properties of wheat straw pellets are lower as compared to particles less than 5 mm in size using a hammer mill (Model conventional fuel pellets made from wood [14]. This is likely 55, Jensen and Sommer Aps, Denmark). The material was due to the different chemical composition and structure of packed in paper bags permeable to air and moisture and stored wheat straw. In general, the lignin content in grasses is lower in a dry and dark storage room. and its structure is different compared to wood, i.e., many monolignols in grasses are substituted with acetyl or ferulic Material Pretreatment acid residues [15]. Another important difference is that wheat straw contains a cuticula, which is present on the outer surface After about 2 years of storage, samples were taken from the of all grasses and is basically a hydrophobic layer composed of bags and particles less than 1 mm and greater than 2.8 mm cutin and hydrophobic waxes. Cutin is a polymeric network of in size were removed using analytical sieves and a sieve polyhydroxylated C16 and C18 fatty acids cross-linked by ester shaker (Retsch, Germany). Extractives were removed from bonds [16]. It is amorphous and insoluble in water and closely the wheat straw by soxlet extraction in n-hexane for 8 h. associated with semicrystalline wax particles. The plant cuticula Prior to the experiments, the materials were conditioned acts as a barrier allowing the plant to regulate its water balance in a climate chamber at 65% RH and 27°C for at least and protecting it against physical, chemical, and biological 1 week. The equilibrium moisture content was 8.0% for aggression (i.e., fungi and insects) [16]. This layer is likely to straw and 8.1% for extracted straw. have a negative effect on the bonding between the biomass particles within a pellet since weak boundary layers between Determination of Extractives Content adjacent fibers and particles can be formed during biomass densification processes resulting in poor mechanical stability About 8 g of wheat straw particles were dried at 105°C of the densified products [14, 17, 18]. overnight and extracted as described above. The extracted The softening, flow, and subsequent hardening of lignin is sample was dried again and the extract content was calculated likely the major bonding factor and, as such, is worth studying based on the weight difference. in greater detail. The temperature at which a polymer softens and passes from a glassy into a plastic state is called the glass Attenuated Total Reflectance Infrared Spectroscopy transition temperature (Tg) and is an important characteristic of a polymer. To verify the extractives removal, ATR-FTIR was used to study Many studies have investigated the Tg of wood lignins [19– the surface of the biomass particles. Spectra were recorded at 23] and have shown that they depend on species, moisture 30°C using a Fourier transform infrared spectrometer (Nicolet content, and sample preparation. In general, the Tg decreases 6700 FT-IR, Thermo Electron Corporation, USA), equipped with an increasing moisture content (water acts as plasticizer) with a temperature-adjustable ATR accessory (Smart Golden and degree of substitution. For example, the presence of many Gate diamond ATR accessory, Thermo Electron Corporation, acetyl groups in hardwood lignins lowers its Tg [22, 23]. USA). Samples were dried at 105°C for 4 h and stored in 411 Springer 452 Bioenerg. Res. (2012) 5:450-458 airtight containers until used for testing. A minimum of five of a cylindrical die 7.8 mm in diameter, made of hardened measurements per sample was performed. To ensure good steel, lagged with heating elements and thermal insulation. contact, all samples were pressed against the diamond surface The end of the die was plugged using a removable backstop. using a metal rod with consistent pressure. All spectra were Pressure was applied to the biomass using a piston made out obtained with 200 scans for the background (air) and 100 of hardened steel connected to a pneumatic laboratory press scans for the sample with a resolution of 4 cm-1 from 500 to (P25, Compac, Denmark) and a load cell (C2S, NTT-Nordic 4,000 cm'.
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