Torrefaction effects on composition and quality of biomass wastes pellets

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1 Torrefaction effects on composition and quality of biomass wastes pellets C. Nobre, M. Gonçalves & B. Mendes Mechanical Engineering and Resources Sustainability Center, Department of Sciences and Technology of Biomass, Faculty of Sciences and Technology, New University of Lisbon, Caparica, Portugal C. Vilarinho & J. Teixeira Mechanical Engineering and Resources Sustainability Center, Mechanical Engineering Department, University of Minho, Guimarães, Portugal ABSTRACT: The torrefaction of pellets produced from different biomass wastes was performed at different temperatures and residence times to evaluate the influence of those parameters on the pellet quality. The fixed carbon, ash content and high heating value increased with the torrefaction temperature and time while the volatile matter content, apparent density and mechanical durability decreased. Using torrefaction temperatures lower than 250 ºC or adding a binder may allow to obtain pellets with good fuel properties and still preserve adequate mechanical properties. 1 INTRODUCTION The conversion of biomass into solid biofuels such as pellets is a strategy developed to overcome questions such as low energy density, high moisture content and decentralized location of the raw biomass resources. The consistent growth of the global market for wood pellets and the increasing awareness of the benefits brought by the use of biofuels, have triggered the need to diversify the raw materials used as feedstock. Pelletization is a flexible way to process biomass, compatible with the use of dedicated wood materials or biomass wastes such as fruit tree prunings (Arranz et al., 2015), food industry wastes (Ruiz Celma et al., 2012), wood industry wastes (Rabaçal et al., 2013), agricultural wastes (Miranda et al., 2011, Niedziółka et al., 2015) or forestry wastes (Acda M., 2014). Diversification of raw materials for pellet production could enable the energetic valorisation of various biomass wastes available in significant amounts and with good fuel characteristics that are currently landfilled or burned in open air. The material and energetic densification that results from pelletization, can be further improved by torrefaction, a thermochemical treatment also so known as mild pyrolysis, that uses low temperatures (between 200 C and 300 C), in an inert or oxygen-deficient atmosphere. Torrefaction converts raw biomass in a solid homogeneous fuel, with low moisture content and a higher calorific value (Medic et al., 2012, Grigiante and Antolini, 2015, Wang et al., 2013). During this process biomass loses water and part of its volatile matter, becoming dark, dry, friable and with a heating value closer to that of coal (Ghiasi et al., 2014). The increased hydrophobicity after torrefaction is justified by the elimination of superficial hydroxyl groups that causes a decrease in the ability to establish hydrogen bonds and therefore interact with water molecules (Bergman and Kiel, 2005). Also, this process promotes the thermochemical formation of non-polar unsaturated structures, a property that can also contribute to a reduction of biodegradability (Patel et al., 2011). Usually torrefaction is carried out as a pre-treatment directly on raw material and then the raw material is subjected to a densification process, such as pelletizing. The energy density of torrefied biomass pellets reaches 18 GJ/m 3, a value 20% higher than commercial pellets, although lower than the energy density of coal, typically higher than 20 GJ/m 3 (Uslu et al., 2008).

2 On the other hand, recent studies indicate that it is harder to compress torrefied biomass into pellets using regular densification conditions because the friction in the pellet mill increases, leading to higher temperatures in the die (Wang et al., 2013, Li et al., 2012, Peng et al., 2013). This situation can be controlled by pre-conditioning the torrefied biomass to increase its moisture and by increasing the mechanical force applied (Peng et al., 2015, Shang et al., 2012). Therefore the use of torrefied biomass for pellet production can increase the energy requirements of the process and reduce the lifetime of the pelletizing equipment while yielding pellets with poor mechanical properties that can be extensively degraded during transportation and handling. An alternative to this methodology is to produce pellets and then subject them to the torrefaction process. Although a negative correlation was still observed between the torrefaction conditions and the pellet mechanical properties (Peng et al., 2015, Shang et al., 2012) the process of torrefaction after pelletization requires less energy than the pelletization of torrefied biomass and provides a similar final product (Ghiasi et al., 2014). The aim of this work was to study the torrefaction of pellets produced from different biomass wastes and to determine the conditions that produce an improvement in the pellet fuel quality without causing a significant degradation of their mechanical properties. 2 MATERIAL AND METHODS 2.1 Pellet formulation and production The industrial wood wastes pellets were produced with a mixture of end-of-life wood materials, furniture wastes and lignocellulosic materials from waste management units. Pellets produced from orchard wastes contained prunings from Malus domestica, Pyrus communis, Prunus persica and Prunus domestica, provided by a fruit producer (COOPERFRUTAS). Urban lignocellulosic wastes included aerial parts and branches from fifteen different species: Arundo donax L., Sophora japonica, Tipuana speciosa, Prunus cerasifera var pissardii, Melia azedarach, Celtis australis, Pinus pinea, Fraxinus angustifolia, Grevillia robusta, Betula celtiberica, Platanus x hybrida, Olea europaea, Eucalyptus globulus, Causarina equisetifolia, Acer negundo. These wastes were collected in cleaning operations of urban green spaces in Almada, Setúbal, Portugal. The three types of pellet were produced in an industrial pellet production unit (Casal e Carreira Biomassa Lda, Alcobaça, Portugal) with a production capacity of 2.5 t/h. The raw materials were milled to a diameter lower than 4 mm and admitted to the press for densification. During pelletization the biomass was heated up to 90 C and extruded through die orifices with a diameter of 6 mm for the industrial wood wastes pellets and a diameter of 8 mm for the orchard and urban lignocellulosic wastes pellets. 2.2 Torrefaction experiments The biomass pellets were subject to torrefaction at temperatures from 200 C to 250 C and residence times of 30, 60 and 120 minutes, using a muffle furnace (Nabertherm). The torrefied samples were placed in a desiccator and allowed to cool to room temperature. 2.3 Proximate analysis and higher heating value Total moisture content and ash content were determined gravimetrically according to standards BS EN :2009 and BS EN 14775:2009, respectively. The volatile matter content of each sample was determined according to the standard BS EN 15148: 2009 and the fixed carbon was calculated by difference. The higher heating value was determined according to the equation established by Parikh et al. (2005) (Equation 1). HHV (MJ/kg)=0.3536FC VM A (1) Where FC, VM and A correspond to fixed carbon, volatile matter and ash contents, respectively.

3 2.4 Quality parameters The same pellets were subjected to torrefaction at 250 C for 60 minutes, in a larger scale (3.2 kg to 8.7 kg), using an industrial rotary pyrolysis furnace (MJ Amaral, model FR 100). The mechanical durability and content of fines of the torrefied pellets were determined following the standard CEN/TS :2009 and their bulk density was determined through an adaptation of standard EN 15103: RESULTS AND DISCUSSION The torrefaction conditions (temperature and time) had a clear effect in the pellet appearance, conferring a darker colour, a more uniform appearance and slightly lower dimensions (Figure 1). It was observed that the pellet becomes darker for increasing residence time this process is faster with increasing temperatures, as stated by other authors (Unsal and Ayrilmis, 2005). These colour changes are related with the vaporization of the lighter biomass components and the decomposition and rearrangement of the heavier ones, namely cellulose and lignin (González-Peña and Hale, 2009). Figure 1. Orchard wastes pellets: A- Reference; B- 200 ºC, 120 min; C- 250 ºC, 120 min. The pellets also developed a rougher surface and became more brittle with increasing temperature and residence time. These features were more noticeable for torrefaction tests with longer residence times (120 min). Volatile matter, fixed carbon and ash content and high heating value of the reference pellets and the corresponding torrefied pellets are presented in Figure 2. The reference pellets contained 5-8% of residual moisture that was eliminated during torrefaction; since the torrefied pellets were always kept in a desiccator their moisture content was considered to be approximately zero. Even after re-equilibration with the atmospheric humidity, torrefied pellets typically regain only 1-6% of the initial moisture content (Bergman et al., 2005). The pellets produced from urban biomass wastes presented a higher content in volatile matter and lower contents of ash and fixed carbon than the other two pellet types, but a similar high heating value. Regardless of the raw materials composing each pellet, the torrefaction process resulted in a decrease of the volatile matter and increase of the fixed carbon and ash contents. These changes were proportional to the temperature and residence time and contributed to an increase in the pellet high heating value, as evaluated from the proximate analysis. The increase in the fixed carbon and the ash contents correspond to a concentration of those components in the torrefied pellets as more volatile components are eliminated and have opposite effects in the pellet heating value.

4 Volatile matter (%wt, bs) A Ash (% wt, bs) B 60 2 Fixed carbon (% wt, bs) C High heating value (MJ/kg) D Figure 2. Volatile matter (A), ash (B), fixed carbon (C) and high heating value (D) of pellets produced with industrial wood wastes (IW), orchard wastes (OW) and urban lignocellulosic wastes (UW), after torrefaction at 200 C and 250 C and different residence times. The volatile matter content decreased less than 5% (wt.) for torrefaction at 200 ºC, and around 25 % (wt.), for torrefaction at 250 ºC (Fig. 2-A), indicating that at this condition, not only moisture and volatile organic compounds are lost but also an important fraction of structurally relevant components such as hemicellulose (Medic et al., 2012). The enhancement in fixed carbon is also more pronounced at 250 ºC, reaching values that are 15% (wt,) higher than the ones of non torrefied pellets (Fig 2-C). The ash content of the torrefied pellets suffered almost no change at 200 ºC and increased of 1% 2.5% (wt.) at 250 ºC (Fig 2-B). In order to evaluate the effect of the different torrefaction conditions on the high heating value (HHV) of the studied pellets, the correlation proposed by Parikh et al. (2005) was used. The HHV of the reference pellets was improved at every torrefaction condition and the increase was directly proportional to the torrefaction temperature and residence time. Urban lignocellulosic wastes pellets were the most affected by the torrefaction process. These pellets show a greater decrease in volatile matter (from 82.0% to 63.1%) and a more significant increase in fixed carbon (from 14.9% to 31.0%) and ash content (from 3.1% to 6.0%) at the most severe torrefaction conditions (Fig. 2). The effect of the torrefaction process on the pellet mechanical properties was then evaluated at 250 ºC and 60 min, a condition in which the three types of pellets presented different proximate compositions. This test was also performed in a larger scale in order to obtain representative samples for the subsequent measurements (Table 2).

5 Table 2. Quality parameters of the reference pellets and respective torrefied pellets. Torrefaction Industrial wood Orchard Urban lignocellulosic wastes pellets Quality parameters conditions wastes pellets wastes pellets No torrefaction (reference) mass, kg Bulk density, kg/m Mechanical durability, % Fine content, % ºC, 60 min mass, kg Bulk density, kg/m Mechanical durability, % Fine content, % The registered mass loss of the pellets after torrefaction is between 7-14 %. This can correspond to the loss of residual moisture and volatiles and the larger loss corresponds to the urban lignocellulosic pellets which presented the higher volatile content. At higher temperatures, the mass loss during torrefaction can be positively correlated with a loss of energy content (Shang et al., 2012). The bulk density also decrease after torrefaction, leaving the three pellet types below the normative value ( 600 kg/m3) and therefore these pellets will have a lower energy density, influencing costs for transportation and storage (Obernberger and Thek, 2004). The torrefaction process also had a negative impact in the mechanical durability and fines content of the pellets, especially for the urban lignocellulosic wastes pellets that reached a fines content of 21.9%. The mechanical durability of the three types of torrefied pellets was bellow values the ENplus normative value for mechanical durability for domestic or industrial pellets ( 97.5 % and 96.5 %, respectively). This indicates that some of the lost mass had a binding effect essential for the pellet integrity and milder torrefaction conditions should be used to preserve it. Another option to avoid degradation of the pellet mechanical properties is to add an appropriate binder before or after the torrefaction process. 4 CONCLUSIONS The use of the torrefaction process after pelletization promotes changes in the chemical and quality properties of the pellets namely a uniform appearance, a significant increase in fixed carbon content and a consequent increase in their calorific value. These changes were more extensive at higher torrefaction temperatures and longer residence times. The variations in proximate composition and quality parameters between the different types of pellets are also influenced by the feedstock from which they were produced. However, this treatment is negatively correlated with the mechanical properties of the pellets, decreasing their mechanical durability and their bulk density which can affect transport, storage and performance of the pellets in combustion systems. The use of torrefaction temperatures lower than 250 ºC and the addition of binders should be further investigated in order to retain the positive aspects of this thermal treatment regarding proximate composition and minimize the negative impact in the mechanical properties. ACKNOWLEDGEMETNS The authors gratefully acknowledge the support from project PROPELLET (Vale Inovação, Projeto nº37769) and the project promotor CMC Biomassas, Lda. REFERENCES ACDA M., D. E Physico-chemical properties of wood pellets from forest residues. Journal of Tropical Forest Science, 26,

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