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Biofuels and Biomass
Pelletization of agroindustrial biomasses from tropic as energy resource: implications of pellet quality Carlos F. Valdés, Gloria Marrugo, Farid Chejne, Kevin Andrés Cogollo, and Diego Fernando Vallejos Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b01673 • Publication Date (Web): 01 Oct 2018 Downloaded from http://pubs.acs.org on October 1, 2018
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Energy & Fuels
Pelletization of agroindustrial biomasses from tropic as energy resource: implications of pellet quality
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Abstract
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The effect of the binder amount and particle size on the quality and agglomeration
Carlos F. Valdés, Gloria Marrugo, Farid Chejne1, Kevin Cogollo and Diego Vallejos Universidad Nacional de Colombia, Facultad de Minas, Escuela de Procesos y Energía, TAYEA Group, Carrera 80 No. 65-223, Medellín (Colombia).
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mechanical of pellets produced from three biomasses of agroindustrial crops (Rice Husk
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“RH”, Coffee Husk “CH” and Palm Rachis “PR”) were studied. Pellets quality parameters
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such as production yield, bulk density, pellet sizes distribution (length), water resistance,
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impact resistance, ultimate and proximate analysis were analyzed, according to the
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European Standards and were related to a possible energy exploitation. In addition, pellets
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morphology was evaluated by Scanning Electron Microscope and Stereoscope
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visualization. The connectivity between the biomass particles is explained by mean of
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mechanical interlock for PR; whereas to CH and RH were a combination of mechanisms
18
like adhesion and interparticle attraction forces. The results showed that for the smallest
19
particle size analyzed (0.6 mm), pellets from laminar shaped biomasses (RH and CH)
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showed a high impact resistance and water resistance. Likewise, pellets from the fibrous
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biomass (PR) showed a high impact resistance; but a low water resistance.
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1
Corresponding author at: Universidad Nacional de Colombia, Facultad de Minas, Escuela de Procesos y Energía, Medellín, Colombia. Email address:
[email protected] ACS Paragon Plus Environment
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Keywords: Biomasses; Pelletization; Agglomeration mechanisms; Pellets quality; Energy
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exploitation
25 26 27
1.
Introduction
The energy exploitation of solid waste has become a recommended practice in the
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agroindustrial, especially in tropical countries like Colombia, due to the availability of a
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wide range of agricultural waste to produce clean energy; e.g. in Colombia according to the
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latest inventory of renewable energy potential carried out, annually around 72 million tons
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of agricultural residual biomass are produced 1,2. Much of this biomass could be used to
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satisfy the country’s energetic demand; nevertheless, there are some barriers for the
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adequate use of biomass as: its heterogeneity (size and shape) and its low bulk density that
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cause difficulties in the transport and the storage 3; in addition, the biomass has a low
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calorific value compared to fossil fuels4. Therefore, it is necessary to implement
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pretreatments to enhance these characteristics5.
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To increase the biomass bulk density, mechanical processes like briquetting and
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pelletization are used 6. The main benefits obtained through densification are: greater
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energy density, homogeneity through uniformity in shape and size, lower humidity
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contents, dust and ashes control, reduction of transport and storage costs, compositions, and
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sizes distribution standardized that facilitating the feeding in domestic and industrial
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equipment 7–10.
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Abundant information about the pelleting process has been published 11–15. The research
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focuses on parametric evaluations of the process and the intensification of industrial
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processes through the use of waste pellets. The study of the effect of the variables in the
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pelletization process leads to obtaining the optimal operating conditions of each biomass
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for the production of high quality pellets 12,15–18; nonetheless, research that describe the way
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how particles agglomerate and the effects of the agglomeration mechanisms on the pellets
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properties are not common; e.g. Tumuluru et al.11 studied the effect of the biomass
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physicochemical characteristics, the morphology and process variables on the densification.
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The most relevant result of this study was the identification of the plastic and elastic
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deformation of the particles, like key stages for an adequate compaction, with an energy
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consumption of 40% of the total energy. However, the analysis was focused on wood
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waste; which are chemically and morphologically different of agroindustrial residual
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biomass that abounds in the countries tropic-located.
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According to the above, palm oil industry waste has been subjected to palletization in
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Thailand. Waste as palm rachis “PR” or oil palm empty fruit bunch “OPEFB”, oil palm
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frond “OPF”, palm kernel shell “PKS” and oil palm mesocarp fiber “OPMF” and other
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waste 19,20, were pelletized using high temperatures on pelletizer die (150, 200 and 250°C).
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The best pellets quality was obtained using PKS19; additionally, it was shown that the
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shape, the particle size and the chemical composition of the raw biomass are relevant on the
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process performance. However, in these investigations the pellets were formed by a single
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unit press under condition laboratory with temperature and pressure controlled. These are
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difficult variables to scale and control in an industrial equipment.
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Likewise, evaluations of pellets production from agroindustrial waste were carried out in
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Costa Rica (Central America), (coffee pulp 6,12, pineapple leaves, giant cane, PKS, OPMF,
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sugarcane, herbaceous species 12 and wood tropical species). They used an industrial type
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pelletizer and the pellets were characterized from the energy content, its physical and
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mechanical characteristics, using standard procedures. Among the most important findings,
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highlights the compliance with most of the quality standards currently required for these
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solid fuels (EN ISO 17225-2 2014); however, the physicochemical composition of
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biomasses from tropical origin, prevents that some of the standard values required for
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properties such as ash content, calorific value and mechanical durability be reached, which
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are normally met for wood pellets.
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Regarding the parameters of the process, the moisture content of the biomass is a source of
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continuous research, its effects on the process and quality of the pellets, as well as the water
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added as a binder for pelleting, represents one of the research challenges. It is known that
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the biomass moisture content because it increases the Van der Waals forces during contact
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between particles. Therefore, it have effects in the formation of solid bridges and the
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decrease of glass transition temperature, which is reflected in the duration and effectiveness
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of the plastic phase during the agglomeration of the particles 11,13,14,21,22.
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In the research developed by Said et al.14 and Ishii Furuichi13, the influence of feeding and
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operating conditions on the pellets quality produced from rice straw was determined. They
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found that the properties most affected by the biomass moisture content are durability and
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bulk density. The optimum values of biomass moisture content were found between 13-20
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wt.%. Other important aspect is the optimal biomass-binder relation to ensure the resistance
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and durability of the pellets. It has been found root shaped bifurcations in the pellets
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obtained of softwoods with moisture content between 0 and 10 wt.% 6. Meanwhile,
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biomass moisture content above 20 to 25 wt.%, has the tendency to form piston-shaped
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cracks sectioned along the pellet22–24.
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In accordance with the above, Stelte et al.22, found that when water is used as binder, the
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quantity suitable for stability or integrity of the pellets was between 5 - 15 wt.%. Likewise,
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it was determined that the difference in the binder amount needed to produce good quality
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pellets is directly related to the physicochemical characteristics of the raw materials; this
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because, the moisture content and the lignocellulosic components, under high pressure may
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be expelled from the particles, facilitating the binding and stabilization of particles 11,22,25.
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Also in important highlight that, given the heterogeneous nature in size and shape of raw
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biomasses, during pelletizing different agglomeration mechanisms may be present. These
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are reason of research and poorly understood 26–28; e.g. in the process of compressing
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fibrillar biomasses, flat shaped and voluminous particles can be interlaced, resulting that the
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dominating mechanism is mechanical interlock. In addition, the presence moisture between
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the particles can generates cohesive forces during agglomeration through hydrogen bonds,
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affecting directly the durability and pellet density13,14,21. Then, the understanding of the
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interaction different physical forces that interact (forces of attraction between particles,
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interfacial forces and capillary pressure, forces of adhesion and cohesion, solid bridges
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(stable) and links mechanical interlocking), the shape and size of the particles are essentials
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to understand the pelletization process11, and these are the principal reason that justifies the
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continuous research on pelletizing.
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Given the above conditions, to carry out this study, three representative residual biomasses
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of agroindustrial activities in Colombia were selected and pelletized: palm rachis (RP), rice
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husk (RH) and coffee husk (CH). From the physicochemical characteristic of the
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biomasses, it was possible to establish that the pellets produced did not achieve the
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requirements by European standards (EN ISO 17225-2 2014) because the ash content of
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raw material is in some cases higher than the established limit by standards. However, as
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these pellets represent an opportunity for energy exploitation environmentally sustainable;
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therefore, were characterized under various quality tests and the implications of these
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properties on the energy exploitation were discussed. Then, the research goals were
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directed to: 1). Establish the influence of the shape and size of the biomass on process
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behavior and pellets quality. 2). Provide an explanation of the determination of
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agglomeration mechanisms in relation to the shape and size of the biomass particles. As
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novelty, in this investigation the effect of the parameters of interest on process and in the
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quality properties of the pellets were evaluated using a pilot-scale pelletizer, with
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characteristics similar to industrial equipment, unlike the devices normally used for this
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type of research; since most of the studies are performed on single-pellet laboratory presses.
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2.
Experimental section
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2.1. Pelletization equipment
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A horizontal die pelletizer with fixed rollers and a feed capacity until 300 kg/h was used.
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The die has 210 mm of diameter and 41mm of thickness, with 102 holes of 8 mm diameter.
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The pressure rollers are located over the die; the axis allows adjust them against the die by
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a shell that has two screws which the extrusion pressure of the process is fixed. More
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details about pelletization equipment can be found in our previous research29.
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2.2. Biomass and pellets characterization
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2.2.1. Biomasses
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Three typical and available Colombian agroindustrial waste were selected: Rice Husk (RH),
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Palm Rachis (PR)and Coffee husk (CH) (see Figure 1) from the departments of Tolima
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(4°26′00″N 75°14′00″O), Antioquia (6°13′00″N 75°34′00″O) and Meta
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(4°09′00″N 73°38′00″O), respectively.
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Palm Rachis (PR) was exposed to solar drying because of its high moisture content (42.02
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wt.%) until reaching its equilibrium moisture (∼10wt.%). The biomasses were ground to
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reduce their particle size using a hammer mill with tilting knives, powered at 6500 rpm, it is
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having a mesh of 2 mm.
Figure 1. Physical aspects of biomasses. a) RH; b) PR; c) CH 158 159
The physicochemical characterization of representative biomass samples was tested
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through different techniques. The results and normative procedure used are described in
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Table 1.
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Table 1. Physicochemical characterization of selected biomasses Criterion Unit Norm RH CH PR Form Relative NA Sheetlike Sheetlike Fibrilar Hardness Relative Mohs scale 1.50 2.00 3.00 Density kg/m3 ASTM C127-04 160 245 65 Ultimate Analysis High heating value kJ/kg ASTM D5865-13 14089 18003 17740 C wt.% ASTM D5373 33.87 43.50 43.40 H wt.% ASTM D5373 4.57 5.80 5.70 N wt.% ASTM D5373 0.84 1.40 0.14 S wt.% ASTM D4239-14 0.12 0.18 0.15 O wt.% ASTM D5373 41.27 45.48 45.87 Proximate analysis Residual moisture wt.% ASTM D3173-11 6.92 6.86 6.05 Fixed Carbon wt.% ASTM D3172-1 17.90 16.92 16.98 Volatile material wt.% ISO 562-10 55.85 72.58 72.23 Ash wt.% ASTM D3174-12 19.33 3.64 4.74 Lignocellulosical composition (DAF basis) Hemicellulose wt.% NREL TP-510-42618 24.09 24.60 28.95 Cellulose wt.% NREL TP-510-42618 38.99 27.63 31.89 Lignin wt.% NREL TP-510-42618 35.23 36.69 29.59 Extractives wt.% NREL TP-510-42618 1.69 11.08 9.57
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2.2.2. Pellets
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The characterization of pellets was carried out following parameters specified on the
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standard pellet production (EN ISO 17225-2) regarding its physical and chemical
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properties. The study of Atuesta and Sierra30 was taken as a starting point. The analyses
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were realized by triplicate according to standardized procedures.
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The morphological analysis was made on SEM, similarly as the analysis made for raw
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biomasses. In addition, transversal sections of pellets were observed on Stereoscope Leica
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brand. Pellet size distribution was determined by the geometry measuring its diameter and
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length with a Vernier caliper with 0.05mm precision, according to ASABE
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standard31. Bulk density was calculated measuring the volume occupied by a determined
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mass in a cylindrical container of known volume. The mass was measured using a balance
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with 0.1g precision. Impact resistance was evaluated according to the free fall method
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described by Odeen and Noren32 and Mina-Boac et al33. The test was developed by freely
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dropping a sample of 100 g of pellets five times from a height of 1.8 m. Then, the mass of
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particles larger than 0.5 mm was quantified and the impact resistance index was calculated
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according to Eq.1.
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M − M . % IR = ∙ 100 M
Eq. 1
184 185
Water resistance index was measured by immersing a pellets sample of 10 g in a water
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volume during 30s, according to Lindley and Vossoughi34. The water resistance index is the
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relation between initial mass and the mass of pellets after 30s under water and it was
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calculated by using Eq.2.
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% WR = 1 −
M − M ∙ 100 M
Eq. 2
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For proximate and ultimate analysis of pellets samples, it was required grind up to reach
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250 µm particle size and subsequently the analysis was performed according to the same
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protocols used for the raw biomasses. The energy density is the energy content into the
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pellets per volume unit (MJ/m3) and was determined like the product among both its bulk
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density and high heating value (HHV).
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2.3. Experimental procedure
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Taking as a starting point a background review11,21,35, and preliminary pelletization tests,
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the mixtures biomass-binder were determined (Table 2).
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With the same preliminary tests, the feeding regime was related with the electric current
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consumed (Amperages). Additionally, the die pressure was close-fitting with the turn
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number of screws located on two ends of the housing. The turn number or their fraction
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(1/4 to 1 turn) was counted to adjust the screws from a similar reference point for all the
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tests. Holding a current of 10A in the equipment and a pressure over die of ½ turn, the
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continuous production was guaranteed; by this reason these parameters were fixed. At these
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conditions, die temperature vary between 50°C and 70°C.
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Table 2. Mixtures biomass - binder Binder (wt. %) Biomass Dp,mm 1.63 10 15
210 211
In Figure 3 a general scheme of pelletized process is presented. To each biomass, first the
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particle size was selected (Table 2). Then, the homogeneous mixture was prepared with 1
213
kg of material and the binder proportion was selected (10 and 15 wt.%). In this case the
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binder added was water.
215 216
The mixture is feeding to the equipment at the conditions of feed rate and pressure fixed
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and the pellets produced in each test are characterized (Section 2.1.2). This process was
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repeated to each biomass particle size and each amount of binder selected.
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To analyze the process, the yield of production (Yp) was determined. This yield was defined
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as the ratio of the mass of good quality pellets produced (Mgood pellets) and biomass fed into
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pelletizer (Mmaterial fed) as shown in Eq 3.
223
=
! "##"$% × 100 &'$"()'# *"!
Eq. 3
224 225
According to European standards EN ISO 17225-236, many characteristics can define a
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good quality pellet. The main criteria are the pellet dimensions, where length (L) has to be
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in the range of 3.15 - 40mm and the diameter (D) is determined by the diameter of the holes
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in the die. In this research, the diameter of the holes in the die was 8mm, but the range of L
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established by the standard operation of device is quite broad; therefore, an ideal length
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distribution of pellets was defined, according with the performance of the pelletizing
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equipment, which showed like ideal distribution of length pellets a 70 to 80 wt.% between
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20-35 mm, 5 to 10 wt.% between 10-20 mm and less than 15% between 3.5 and of 10mm.
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So, the mass of good quality pellets was evaluated by Eq.4.
234 235
236 237 238
! "##"
%$= × ,
% -. && / %'& #"
Where: = 011 23 4566571 4829:;59 % -. && = 011 23 456657 ≥ 10 @@ %'& #" = 011 23 4566571 10@465 1565;759 328 7ℎ5 7517
3.
Results and discussion
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Eq. 4
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3.1. Physical characterization of biomasses
240 241
The Figure 3 shows a photograph of the three biomasses used in this research. It can be
242
observed that despite of the particle size reduction, the shape of the biomass is conserved.
Figure 3. Biomass particle size distribution 243
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Both RH and CH are sheetlike, but with different textures; RH presents roughness and is
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brittle to the touch, while CH is softer and slightly malleable making it less brittle than the
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RH. On the other hand, PR fibers are hard and resist torsion.
247 248
In addition, the biomass morphological structure with sizes of 1.27 mm was studied
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through a SEM (JEOL JSM 5910 LV with a SEI detector). The SEM images in Figure 3,
250
show the exocarp of the RH, which it is a symmetric structure conformed by convex cells
251
called simple papillae. These simple papillae are separated by grooves and grains of silica
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compounds scattered over the surface 2. The CH has non-uniform but softened areas;
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therefore, this material is smoother to the touch than RH. Although both are sheetlike and
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the PR fibers are smooth and have rigidity, it can be observed in its surface small grooves
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that appear to be mineral material 2.
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The results of physicochemical characterization (Table 1), indicate that the RH has the
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lowest calorific value, which is due to its lower volatile material content and higher ashes
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content 37. The high ash content of raw material has a significant impact on the combustion
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process 38; nevertheless, the pelletizing improves the emissions control of particulate
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material and facilitates the handling of the ashes of combustion processes, because the
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pellets combustion allow the ash to agglomerate in the form of pellets and pellets
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agglomerated in the bottom of the device 29,39,40. This depending on the ash content may
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imply a greater cleaning frequency of thermochemical device and can be decisive in the
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choice of the most suitable material as solid fuel.
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The Table 1 show that the average carbon content for the samples is over 45 wt.% (daf) and
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oxygen is greater than 51 wt.% (daf). These values are characteristic for lignocellulosic
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biomasses due to the presence of highly oxygenated groups that give it higher reactivity
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compared to fossil fuels. As it was expected, sulfur content is low, this represents an
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environmental advantage. All these results physicochemical agree with values reported in
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the literature 2.
273 274
In the Table 1, the lignocellulosic material characterization of biomasses is presented in dry
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ash free basis (daf). In comparison, RH has the highest cellulose content and CH has the
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highest lignin content. As for the PR its components are distributed more evenly.
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Extractives content are higher in CH and PR, and a lower content is presented in RH. To
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know the content of lignocellulosic material is important for the pelletizing process due to
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cellulose, lignin and some extractives can be act as own biomass binders under pressure
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conditions 41,42. In this context, it can be observed that the contents of the different
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substances (hemicellulose, cellulose, lignin and extractives) vary in each biomass,
282
highlighting the differences in the extractive contents and the homogeneous distribution of
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lignocellulosic components in the PR.
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3.2. Physical characterization of pellets
286 287
In order to identify physical mechanisms of agglomeration, visualizations of the pellets
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produced through a SEM and a stereoscope were realized. In Figure 4, a difference in
289
agglomeration mechanisms of particles from different biomasses can be observed. These
290
differences were determined by the morphological characteristics presented on each
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biomass. In this order, the RH and CH are sheetlike, whereas the PR is fibrillar shaped with
292
an acceptable percentage of amorphous particles.
293 294
According to the Figures 4a and b, the particle orientation agglomerated respect to its fiber
295
plane can be described like transverse orientation for RH grain and longitudinal for CH
296
grain. The longitudinal orientation tend to retain their structure more than the transverse
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shape as found by Nielsen et al.43. It is observed in Figures 4b and 5b that to CH grains
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there are more laminar structures preserved compared to RH; therefore, the adhesion of
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these particles by short-range forces was favored, which occur through binder substances
300
between the particles and the solid-solid bonds that are generated during the plastic
301
deformation.
302
Figure 4. SEM visualization of the pellets. Above the smallest particle size 0.6mm and down the largest one, 1.63mm. a) RH; b) CH; c) PR 303 304
On the other side, the PR fibers does not show a particular orientation; its shape and its
305
heterogeneous distribution of particles sizes, seem to favor bonding mechanisms like the
306
formation of solid bridges and mechanical interlocks. These orientations have an effect on
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the energy consumption for compression and extrusion through the pelletization die43–45.
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This can be explained from the greatest or lowest extent of intraparticle frictional forces
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with the walls of the extrusion holes, making a difference in the energy consumptions
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during the production of pellets. The PR fibers have a high Young’s module which implies
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greater energy consumption during palletization.
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a)
b)
Figure 5. Stereoscope images of pellets cross section and scheme of union mechanisms predominant. Particle size 1.6 mm. a) RH; b) CH; c) PR 313 314
The RH is covered by a cuticle that is waxes rich, with the physiological function of
315
improving the protection of the grain, but it gives less resistance to bending compared to
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the sheetlike of the CH; therefore, can promote lower energy consumption during the
317
extrusion of the grain through the pelletization die. Above in Figure 4a, it can be seen that
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the superficial morphology of the RH has undulations own of biomass, which in the case of
319
adjacent accommodation of the sheets during the compression and plastic deformation,
320
favors binding mechanisms such as the mechanical interlock27,46,47; however, the
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occurrence of these mechanisms for RH particles needs to a particular accommodation of
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each particle, which makes the pelleting process lack randomness.
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For all the above, it is cannot be determined through SEM visualization that the dominant
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mechanism for RH particles is mechanical interlock. In general, it is possible to make
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conclusions about the presence of interlocks along with other types of forces such as
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chemical forces of short range, Van der Waals forces and solid bridges that lead to the
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adhesion (Figure 5a), by the presence of binder between the particles.
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As for CH, it has a sheetlike irregular shape (Figure 4b above), i.e., the elongated shape of
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the structure is not well defined as in RH. This favors the greater presence of agglomeration
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mechanisms such as mechanical interlocks (Figures 4b below and 5b), compared with RH
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(Figure 4a below and 5a) where the physical mechanisms of adhesion and cohesion may be
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more predominant. Mechanical interlocks agglomeration forms are frequently present in PR
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fibers (Figure 4c), since its flexible morphology promotes the presence of such
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agglomeration forms (Figure 5c) through the fiber interlocking, rather than the short-range
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ones which it is promoted by flat particles.
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It is also important to note that another factor responsible for the binding process is
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lignocellulosic composition of raw biomass. This assertion is supported by several
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studies21,27,47,48, which generally indicate that constituents such as lignin, protein, starch, fat,
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waxes and water soluble in carbohydrate, are natural binders that can be activated
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(softening or melting locally) by high humidity, high temperature of the die or by
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superheated steam pretreatment when it is used21.
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In general, SEM and stereoscope visualization (Figure 5) revealed that the most important
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binding mechanisms for pellet production from the selected biomass are mechanical
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Energy & Fuels
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interlocking, followed by solid bridge or interparticle attraction and finally physical bonds
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as short-range forces favored by the binder49,50. Furthermore, depending on the
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lignocellulosic composition, these constituents variously affect compaction.
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3.3. Effects of the binder added and the particle size on performance of the
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pelletizing process
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From the results from each pelletizing test for all biomasses, the evaluation of the effect of
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selected particle size and binder content (10 and 15 wt.%) on the yield of good pellets
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produced (Figure 6) was carried out.
358 a) Yield , wt. %
60% 50% 40% 30% 20% 10% 0%
