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Production and characterization of fuel pellets from rice husk and wheat straw
Renewable Energy 145 (2020) 500e507
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Renewable Energy journal homepage: www.elsevier.com/locate/renene
Production and characterization of fuel pellets from rice husk and wheat straw s M. Ríos-Badra n a, Iva n Luzardo-Ocampo a, Juan Fernando García-Trejo b, Ine rrez-Antonio a, * Santos-Cruz a, Claudia Gutie Jose a b
noma de Quer Universidad Auto etaro, Facultad de Química, Cerro de las Campanas s/n Col. Las Campanas, Quer etaro, Quer etaro, C.P. 76010, Mexico noma de Quer Universidad Auto etaro, Facultad de Ingeniería, Cerro de las Campanas s/n Col. Las Campanas, Quer etaro, Quer etaro, C.P. 76010, Mexico
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 December 2018 Received in revised form 16 April 2019 Accepted 10 June 2019 Available online 14 June 2019
In this work, the production and characterization of pellets made from rice husks and wheat straws is presented. For this, physicochemical and energetic characterization of pellets produced with rice husks and mixtures of rice husks with wheat straws were carried out, followed by a comparison with those properties stablished by the ISO 17225-6 standard. The rice husk pellets exhibited the lowest calorific value (3090.64e4049.05 kcal/g) and highest ash content (12.81e17.51%), while the pellets made from mixtures obtained the highest calorific power (4301.10e4573.50 kcal/g) and a reduced amount of ashes (11.43e13.06%). Moreover, the pellets produced with both biomasses exceed the moisture, ashes and nitrogen content, but complied with diameter, length and durability components. These results suggest that these biomasses blends improve the quality and combustion characteristics of the pellets, aiming to enhance the research for the manufacturing of these sustainable biofuels. © 2019 Elsevier Ltd. All rights reserved.
Keywords: Rice husks Wheat straw Fuel pellets Waste biomass Calorific power Physicochemical characterization
1. Introduction According with the International Energy Agency, in 2015 the world energy consumption was 9383.60 Mtoe [1], being transport along with heat and power generation the sectors that consumed more energy. The forecasts of the International Energy Agency indicate that global energy needs rise more slowly than in the past; however, there is still a growing by 30% between today and 2040 [2]. In order to generate all this huge amount of energy, 32,294 Mt de CO2 were released to the atmosphere, which contributes to the aggravation of the climate change problem. Therefore, it is necessary the search of new alternative energy sources, that allows the development of the different economic sectors; moreover, these
Abbreviations: ASTM, American Society for Testing and Materials; ISO, International Standards Organization; EN, European Norm; Mtoe, Million tons of oil equivalent; Mt, Million tons; PJ, Peta Joules; MJ, Mega Joules; C, Complied; NC, Not complied; RH, Rice husks; RWP, Rice husks and wheat straws pellets; LOI, Loss on ignition; TGA, Thermogravimetric analysis; TKN, Total nitrogen content (total Kjeldahl nitrogen); TOC, Total organic carbon; WS, Wheat straws; SD, standard deviation. * Corresponding author. rrezE-mail addresses: [email protected], [email protected] (C. Gutie Antonio). https://doi.org/10.1016/j.renene.2019.06.048 0960-1481/© 2019 Elsevier Ltd. All rights reserved.
alternative energy sources must to be renewables, and its conversion processes must to be of reduced both energy consumption and environmental impact. In particular, the heat and power generation sector is one of the most dynamics. According to the International Energy Agency the global electricity demand doubled between 1990 and 2016, and was set to grow at twice the pace of energy demand as a whole in the next 25 years [3]. There are several renewable options for electricity production such as wind, solar, tidal, geothermal, and hydropower. The application of these energetic alternatives includes the installation of new facilities and equipment for the electricity both production and transmission. Nevertheless, another alternative is the development of solid biofuels that can replace the use of coal in traditional power plants; in this case, the actual infrastructure for both production and transmission can be used. Solid biofuels are defined as any plant matter used directly as fuel or converted into other forms, usually physical processes, before its combustion [4]. Among solid biofuels, we can find different types as briquettes, pales, cubes, wood chips and pellets; the last ones are the most popular. The pellets are a densified biofuel made from milled biomass, with or without additives; usually with a cylindrical form, with typical length between 5 and 40 mm [5]. The pellets are produced from wood and other
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biomasses as agro-industrial and forest residues. In particular, the use of agro-industrial residues has been explored recently for the production of biofuel pellets, since they are available in great amounts, which represent a contamination problem; moreover, in most of the cases, these residues are not used to generate addedvalue products. Due to this, the production of biofuel pellets from agro-industrial residues has gained attention in the scientific community. One of the most important characteristics of pellets is its durability, which can be affected by the feedstock characteristics, the moisture content or size reduction during pre-processing, and by pelletization conditions, including the use of binders, feedstock mixes, temperatures or die pressures [6]. Thus, the works reported in the literature have been focused in the effect of these variables on the properties of fuel pellets; different biomasses have been considered, such as oil palm and para-rubber tree residues [7], mixtures of bark (eucalyptus), fruit shell (mangosteen) and fruit peel (papaya) [8], olive mills [9], mixtures of pine sawdust, wheat straw and rapeseed straw [10], Norway spruce (Picea abies), European beech (Fagus sylvatica), and wheat straw (Triticum aestivum) [11], bamboo sawdust, eucalyptus sawdust, corn cob, rubber tree branches, palm fibre and lippia grass [12], oil palm empty fruit bunch, oil palm frond, oil palm shell and oil palm mesocarp [13], waste from palm plantation and palm industry [14], waste canola meal biomass [15], rice straw [16], grape vine and sunflower husk residues [17], corn stover [18], barley straw [19], wastes from Scenedesmus microalgae [20], downed coconut [21], canola hull [22], wheat straw and peat [23], among others. In Mexico, the use of biomass as primary energy source reached 5.3% in 2005, a considerably reduction from 1965, when used to cover 15.3% of this energy supply [24]. Nonetheless, there is a high potential for the production of biomass-derived energy in Mexico, since it has been estimated that between 54% and 81% of the total brut energy production could come from biomass, which ranges between 3035 and 4550 PJ/year, meaning 10 times the actual use [25]. Moreover, the production of biomass in Mexico is considerable. In 2006, 75.73 Mt of dry matter were produced from 20 different food crops, among primary and secondary agroindustrial wastes (60.13 and 15.60 Mt, respectively) derived from postharvest processing such as sugarcane and agave bagasse, corncobs, coffee pulp, among others [26]. Amidst the production of these wastes, those derived from corn, sorghum, barley, bean and wheat are important because of the high production of these crops in Mexico [27]. One of the crops that recently has increased its production is rice; at 2015, a production of 236,017.92 ton of rice was reported [28]. In addition, it is estimated that for every kilogram of harvested paddy, between 0.41 and 3.96 kg of rice straw will be produced, while rice husk accounts for between 20% and 33% of paddy weight [29]. Rice husk is a type of biomass that constitutes the most important byproduct from the rice milling industry. Considering that China produced in 2017 the 22.01% of the world's rice production [30], their total amount of rice husks generation was about 42.89e70.76 Mt. This reality has stimulated the research focused in the exploitation of rice husks for the manufacturing of materials such as zeolite, ceramics or catalysts, while their use as fuel has gained interest due to their potential energetic performance [27]. In particular, the uses of rice residues for the production of fuel pellets have been explored in the literature. In 2012, Lim et al. [31] presented an interesting review about the applications of the biomass of the rice industry. In that work, the direct combustion of rice straw and rice husk was included in the energetic applications. In addition, gasification and pyrolysis of these residues are considered. Especially for the rice husk, most of this residue is burned in fields, causing an important energy waste
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with consequently environmental problems; thus, some countries with high production of rice have developed electric generators using rice husks burning [31]. In these cases, the pretreatment of the raw material such as pelletization is essential for meeting handling, storage and transportation optimal conditions, as well as complete combustion in furnaces [32]. Later, Liu et al. [29] presented the production of fuel pellets from bamboo and rice straw. They proposed the use of mixed biomass, in order to decrease the ash contents resulting from the combustion of rice straw. The resulting fuel pellets have ash content lower than 8.0% and a calorific power higher than 17,500 J/g. In 2014, the influence of moisture content, particle size and forming temperature on the calorific power and durability of rice straw fuel pellets were reported [16]. Their results show that it is possible to obtain pellets with 12 MJ/kg for the lower heating value, and durability major than 95%. One year later, a similar study for the production of fuel pellets from rice straw was presented [33]. They analyzed the moisture content, the use of starch as additive, the temperature formation, and three flat dies on the mechanical properties of the fuel pellets. The results show that a durability of 99.31% was obtained in the pellet of the highest quality. The calorific power was not reported. Recently, the fabrication of fuel pellets with rice straw and rice husk was presented [34]. The results showed that it is possible to obtain higher heating values, which are slightly lower than those of commercial wood pellets, when the residual biomass is dried. From the revision of the literature, we can observe that the use of agroindustrial residues have been reported for the production of fuel pellets. In particular, the use of waste biomass for the rice industry has been less explored, specially the use of rice straw. In counterpart, the use of rice husk is just consider in one work [34], where the authors claim that it is possible to obtain high heating values. However, the main reported problem is the high content of ash. In order to reduce the content of total ashes, rice husks can be blended with other agroindustrial wastes. In particular, wheat straws are a suitable biomass that have been studied for the manufacturing of pellets; due to their high production as the main consumed cereal in the world, and also because they are a fuel alternative for heat and power production to woody biomass [35]. Wheat straw pellets are commonly produced in pellet mills whereas the high temperature and pressure produce a softening, due to the generation of polymeric interpenetration that add an additional bond strength for the macromolecular entanglement. Hence, the specific chemical composition of the straws (proteins, lignin, cellulose and hemicellulose) can act as binders of the resulting pellet, improving their heat exposition [19]. Nonetheless, pellets must fit into quality parameters such as homogeneity in particle density and size, moisture and ashes content, and net calorific power, which are important variables that are critical for a proper functioning and complete combustion in the low-scale furnaces [16]. Another important parameters are C, H, N, S, Cl, and K concentrations as well as heavy metals (Cd, Pb, Zn, Cr, Cu, and As) levels, on which pellets may comply with the maximum allowed values of the ISO 17225-6 standard in order to be certified and commercialized [36]. As observed in previous reports, the use of blends of rice husks and wheat straws as raw materials for the manufacturing of pellets has not been explored yet. Thus, in this work we present the production of fuel pellets from rice husk and its mixtures with wheat straw. For this, the effect of moisture content and the wheat straw amount were studied on the calorific power and mechanical properties of the pellets, in comparison with the values specified in the standard ISO 17225-6. The article is organized as follows. In the second section the materials and methods are described, while the discussion of the results is presented in section 3. Finally, the
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concluding remarks of the work are given in section 4. 2. Materials and methods 2.1. Biomass collection
was changed from 2 to 5 kVA; also, an iron-isolate plaque was added to the flat die in order to reach a higher temperature (102e110 C) inside the pelletizer, allowing the manufacturing of complete pellets. Fig. 1bee shown the biomasses and pellets from the selected agroindustrial residues, respectively.
Rice (Oryza sativa) husks were collected from Arrocera del Bajío, xico. The wheat (Triticum aeslocated at Cortazar, Guanajuato, Me tivum) straws were donated by Todo Pellet, located in Agropark at xico. The damaged husks and straws were Irapuato, Guanajuato, Me removed, and samples were homogenized using a 250 mm mesh prior to physicochemical characterization.
2.3. Biomasses and pellets physicochemical characterization
2.2. Pellets manufacturing
2.3.2. Elemental analysis Total organic carbon (TOC) was determined by the method of loss on ignition (LOI) [18]. Briefly, the sample was calcined at 375 C by 16 h. Considering that at least the 50% of the organic material was carbon, TOC was calculated using a factor of 1.724 [18] with the equation:
Before the pelletizing procedure, there were assayed different biomasses concentrations and moisture levels. At first, three different moisture levels were assayed for rice husks (1.00, 8.27 and 10.00%) and wheat straws (1.00 and 8.20%). The selection of these moisture values was accordingly to the initial quantification of the original raw materials, and they are representative of the reported moisture values that typically can be found for rice husks (0.32e12%) [34e36] and wheat straws (4.1e12.1%) [37e42]. The samples were dried using an oven (110 ± 2 C), and their moisture was measured each 30 min during 5 h using a hygrometer (Benetech GM640). However, with these moisture values it was not possible to densify the residues in the pelletization machine; thus, in order to produce the rice husks pellets the moisture was raised up to 21 and 23%, with which it was possible to densify the biomass. Thus, the biomass material was mixed using different proportions of rice husks and wheat straws: 100:0, 50:50 and 75:25, with two different initial moistures of both biomasses (21 and 23%), measured using a hygrometer. All combinations were assayed by triplicate. The initial moisture of biomasses blends was recorded, and biomasses mixtures were placed into a modified pellet machine (KL120/BC, Handan City Electrical and Mechanical Company Limited) (Fig. 1a). The modification is described briefly next. The pelletizer holes from the rotative flat die (2) were enlarged, from 4 mm diameter and 18 mm length to 6 mm diameter and 22 mm length. Additionally, pellets were classified using an adapted waste drainage (3), located at the hopper, (1) and the original engine (4)
2.3.1. Proximate analysis Moisture and volatiles compounds were quantified using the reported Hach methods [15,16]. Ashes were calculated using the ASTM D 1102-84 method [17].
% TOC ¼ [((A-B)*100)/A]/1.724
(1)
where A is the mass of dry material (g) and B is the mass of the incinerated material (g). The total nitrogen content (TKN) was analyzed by the modified Kjeldahl method from Hach (Method 8075) [16], quantifying just the nitrogen compounds organically bounded. The generated nitrogen compounds were measured using a spectrophotometer (HACH DR6000; 190e1100 ± 1 nm of wavelength accuracy) at 460 nm wavelength. 2.3.3. Thermal analysis The calorific powder was calculated on a calorimeter (Parr Instruments 6200; 0.05e0.1% precision; 0.0001 C Temperature resolution). The thermal characterization of biomasses and pellets was conducted by a thermogravimetric analysis, TGA, (TA Instruments Q500; Temperature accuracy: þ/- 0.1 C; Temperature precision: þ/- 0.01 C), which is one of the most common technique for the comparison of thermal processes, as well the combustion and pyrolysis kinetics of raw materials such as biomass and the resulting products [19].
Fig. 1. a) Modified pelletizer machine; b) Rice (Oryza sativa) husks; c) Wheat (Triticum aestivum L.) straws; d) Rice husks pellets; e) Rice husks/wheat straws pellets. In Figures b, c, and e the superior rule scale is in centimeters, while the inferior in inches; in Figure c the scale is in inches.
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2.3.4. Physical analysis The bulk density was determined using the EN 15103 [20]. The measurement of pellet diameter and length was done using a Vernier Caliper (Mitutoyo Corp; Instrument uncertainty: 0.008 mm). The mechanical durability was performed using the ASTM D3038-93 standard [21] with slight modifications. Briefly, prior to determination, the pellet mass was quantified using a precision scale. Subsequently, pellet was dropped from a 1.85 m height over a plastic tile, and the mass of the major piece was recorded, selecting it as the final pellet mass. This procedure was performed three times. The impact resistance was calculated using the equation: MD ¼ (A/B)*100
(2)
where A is the initial mass (g) and B is the final mass (g). Fines quantity (%) was calculated following the ISO 17827-2 procedure [43] for solid biofuels using a vibrating sieve method with a screen aperture equal than 1 mm. 2.4. Statistical analysis The results were expressed as the means ± SD from at least three experiments. The data was analyzed using the JMP v.8.0 software, and the statistical analysis was done following one-way ANOVA and Tukey-Kramer's Test for the identification of significant differences among samples. The level of significance was established at p ¼ 0.05. 3. Results and discussion 3.1. Proximal, elemental and energetic characterization of the biomasses Table 1 shows the proximal, elemental and energetic characterization of the biomasses used for the manufacturing of pellets. Rice (Oryza sativa) husks (RH) exhibited a significant (p < 0.05) high of volatile and ashes content from all samples, whereas wheat (Triticum aestivum L.) straws (WS) showed the highest TOC and TKN contents. Regarding the energetic characterization, wheat straws presented the highest values. The proximal, elemental and energetic values of biomass coincide with the typical parameters for location of these products into the biomasses area of a VanKrevelen diagram [22]. The rice husks are among the reported ranges for nitrogen, carbon and ashes (47.8, 0.1%, and 14.7%, respectively) [23,24]. Certain differences found among samples might be explained by several factors such as growing soil conditions and the genetic environment of the plant, and consistent with the high silica content of rice husks [25]. The higher TOC values for wheat straws might be explained by its high content of
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carbohydrates, represented mainly by hemicelluloses, cellulose and lignin (39.4, 28.8, and 18.6%, respectively); which is a desirable composition for better binding properties of the resulting pellet, but undesirable regarding NOx emissions, corrosion and ash deposition during biomass combustion [23,26]. Nonetheless, the carbon content from these biomasses make them as important candidates as potential source for the production of biofuel pellets, beyond their traditional consideration as limited materials due to their low nutritional value, together with their tough nature and composition [25,27]. In general, biomasses can be considered more reactive than traditional solid fuels, such as coal, but the wide range of variability among their chemical properties considerably affect the energetic performance. Such limitations can be studied through a TGA analysis, aiming to a better biomass combustion understanding; along with the possibility to apply thermal and kinetics models for biomass characterization, and functional properties in the development of successful solid biofuels [28]. Fig. 2 shows the thermogravimetric analysis of rice (Oryza sativa) husks pellets at two different initial moistures: 1% and 8.27%. The temperature increases from room temperature to 950 C resulted in a three steps of weight losses. Fig. 2a shows the first two steps at room temperature-50 C and 203e345 C, with a loss weight of 8.116 and 47.5%, respectively; and a continuous third step of continuous light weight loss of 345e685 C with a 9.152% of loss. The first step could be associated to water vaporization, while the second step it is attribute to the degradation of hemicellulose and cellulose. The third degradation region could be associate to the cellulose and lignin degradation; while in the second derivative of the weight loss (blue color in Fig. 2a) the maximums correspond to the principal temperature of the weight loss, in the first step to 35.54 C and 321.83 C, respectively. Thus, the initial decomposition of rice husks started at 203e218 C, reaching a final temperature in the first reaction zone at 320e350 C, showing a rapid degradation of sample due to the volatile products. In this area, the observed degradation rates were 16.17 and 17.59 min1, respectively for 1% and 8.21% rice husks (Fig. 2a and b). The cellulose pyrolysis was observed between 320 and 400 C, with a complete decomposition at a maximum temperature of 328.94 C and 321.83 C, respectively, for 1% and 8.27% moisture. Above 400 C there was a sudden mass loss, representing the release of the remaining volatiles and their ignition. The latter small mass loses corresponded to the slow burning of the partly carbonized residue.
3.2. Proximal, elemental, physical and energetic characterization of fuel pellets Table 2a shows the proximal, elemental and energetic characterization of both rice (Oryza sativa) husks pellets and rice (Oryza sativa) husks/wheat (Triticum aestivum L.) straws pellets; also,
Table 1 Proximal, elemental and energetic characterization of rice (Oryza sativa) husks and wheat (Triticum aestivum L.) straws. Initial moisture1
Rice Husks (RH) 1.00%
Wheat Straws (WS) 8.27%
10.00%
1.00%
8.20%
83.29 ± 0.15b 14.61 ± 0.01b 37.70 ± 0.03c 0.21 ± 0.01
85.37 ± 0.10a 17.51 ± 0.01a 40.54 ± 0.05b 0.43 ± 0.01
70.09 ± 1.26d 11.43 ± 0.18e 49.10 ± 0.76a 0.22 ± 0.01
67.75 ± 0.82e 13.06 ± 0.22c 49.96 ± 0.93a 0.63 ± 0.02
3262.08 ± 54.26d
3090.54 ± 16.72e
4573.50 ± 9.20a
4301.10 ± 55.30b
1
Proximal and elemental characterization Volatile 82.12 ± 0.12c Ashes 12.81 ± 0.01d TOC 31.13 ± 0.04c TKN 0.19 ± 0.01 Energetic characterization2 Calorific Power 4049.05 ± 69.22c
The values are expressed as means ± SD of two independent experiments. Different letters express significant differences (p < 0.05) by row using Tukey-Kramer's Test. Expressed in percentage (%). 2 Expressed in kcal/kg.
1
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Fig. 2. Thermogravimetric analysis (TGA) of rice (Oryza sativa) husk with a) 1% and b) 8.27% of initial moisture.
Table 2b includes their comparison with the ISO 17225-6 standard [36] for cereal straws, and the percentage of fitting of each parameter within the standard, when applicable. The rice husks pellets exhibited a higher volatile matter and ashes values than reported ones for this kind of pellets (65.1 and 9.3%), but lower TOC and TKN (46.6% and 0.7%) [27]. The rice husks/wheat straws pellets exhibited the highest moisture, volatile, TOC, TKN and calorific power values. These chemical characteristics derived from rice husks pellets allow a generation of higher H2 and lower CO2 content in the gasification of these pellets, giving a higher stability of the generated gas due to a higher energy density, when compared with
traditional gas derived from coal and petroleum-derived products [27]. Mixing several biomasses may optimize energetic properties of biomass solid fuel [29]. As wheat straws have an important amount of cellulose and lignin, the depolymerization reactions that occurs when these chemical components are submitted to high temperature conditions, reducing the polymer length from 1000 to 200 monomers units, together with the metal composition of the biomass, allow a high calorific power [40]. Nevertheless, a number of inorganic and organic products might be released during combustion by wheat straws; mixtures of wheat straws with rice husk
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Table 2 a) Proximal, elemental and energetic characterization of rice (Oryza sativa) husks pellets and rice (Oryza sativa) husks/wheat (Triticum aestivum L.) straws pellets; b) ISO 172256 parameters for cereal straws and percentage of fit of each parameter for each pellet. A
Rice (Oryza sativa) husks pellets (RHP)
Rice (Oryza sativa) Husks: Wheat (Triticum aestivum L.) pellets (RWP)
Initial moisture1/mixture
21%
23%
50:50 (21%)
75:25 (21%)
11.06 ± 0.20d 83.68 ± 0.93c 16.57 ± 0.10a 33.69 ± 0.01c 0.50 ± 0.01c
12.22 ± 0.23c 83.62 ± 0.68c 17.22 ± 0.66a 33.31 ± 0.02d 0.30 ± 0.02d
15.95 ± 0.22a 89.54 ± 1.26a 11.50 ± 0.15c 35.81 ± 0.38a 1.58 ± 0.02a
14.16 ± 0.23b 86.08 ± 0.01b 14.09 ± 0.17b 34.79 ± 0.52b 1.04 ± 0.01b
3512.72 ± 67.48c
3312.77 ± 66.60c
3688.26 ± 0.93a
3519.25 ± 1.30bc
Proximal and elemental characterization1 Moisture Volatile Ashes TOC TKN Energetic characterization2 Calorific Power B
Rice (Oryza sativa) husks pellets (RHP)
Rice (Oryza sativa) Husks: Wheat (Triticum aestivum L.) pellets (RWP)
Initial moisture1/mixture
21%
23%
50:50 (21%)
75:25 (21%)
22.15 ± 2.33 e 146.00 ± 0.49 e 57.71 ± 0.01
59.54 ± 2.21 e 64.29 ± 2.22 e 125.00 ± 3.03
41.55 ± 2.33 e 101.29 ± 2.42 e 47.86 ± 1.01
10 e 7 e 0.7
NC
C
NC
3511e4538
Proximal and elemental characterization1 Moisture 10.55 ± 2.05 Volatile e Ashes 136.71 ± 1.41 TOC e TKN 29.05 ± 0.78 Energetic characterization2 Calorific Power C
ISO 17225-6
The values are expressed as means ± SD of two independent experiments. Different letters express significant differences (p < 0.05) by row using Tukey-Kramer's Test. 1 Expressed in percentage (%). 2 Expressed in kcal/kg. 3 The percentage of fit (%) was calculated using the maximum value for each parameter according to ISO 17225-6 standard for cereal straws, except for calorific power. C: Complied; NC: Not complied.
technological modifications to commercial pelletizers, which presents limitations in the workable moisture values for pellets manufacturing. Table 3 shows the physical characterization of both rice (Oryza sativa) husks pellets and rice (Oryza sativa) husks/wheat (Triticum aestivum L.) straws pellets, as well as their comparison with the ISO 17225-6 standard [36] for cereal straws and the percentage of fitting of each parameter within the standard. All the elaborated pellets complied with the ISO 17225-6 standard [36] in the diameter, length, apparent density and durability, except for the 50:50 rice husks:wheat straws pellets, which exhibited a significant lower durability than the other pellets. Furthermore, none of the pellets complied with the apparent density and fines percentage. The geometric dimensions of the pellets, considering both diameter and
might reduce the potential generation of these products, resulting in a significant lower amount of ashes when the pellets made from rice husks/wheat straws blends are compared with rice husks or wheat straws alone. Additionally, the pelletization process permitted a better handling, transport, storage and use of the biomass, elements that come together with the densification process of biomasses [29]. However, the manufactured pellets must fit into quality standards such as the ISO 17225-6 [36]. Regarding this, the pellets only complied with the energetic specifications of the standard, while the 50:50 rice husk:wheat straw pellet exhibited the highest calorific power among all samples. However, the lack of fitting of pellets represents an opportunity area to improve the composition of pellets by assaying different blends of biomasses and making
Table 3 a) Physical characterization of rice (Oryza sativa) husks pellets and rice (Oryza sativa) husks/wheat (Triticum aestivum L.) straws pellets. b) ISO 17225-6 parameters (cereal straws) and fitting of each parameter within the standard. A
Rice (Oryza sativa) husks pellets (RHP)
Rice (Oryza sativa) Husks: Wheat (Triticum aestivum L.) pellets (RWP)
Initial Moisture1/Mixture
21%
23%
50:50 (21%)
75:25 (21%)
Diameter2 Length2 Apparent Density2 Fines1 Durability1
6.01 ± 0.01b 25.10 ± 0.03a 101.94c ± 0.09 2.51 ± 0.01c 95.50 ± 0.70b
6.02 ± 0.01b 24.72 ± 0.74a 100.26 ± 0.36d 2.05 ± 0.07d 97.00 ± 0.01a
6.30 ± 0.12a 25.75 ± 0.36a 104.00 ± 1.41b 3.03 ± 0.03a 89.50 ± 0.70d
6.38 ± 0.06a 25.75 ± 0.53a 109.00 ± 1.41a 2.83 ± 0.04b 92.0 ± 1.41c
B
Rice (Oryza sativa) husks pellets (RHP)
Rice (Oryza sativa) Husks: Wheat (Triticum aestivum L.) pellets (RWP)
Initial Moisture1/Mixture
21%
23%
50:50 (21%)
75:25 (21%)
Diameter2 Length2 Apparent Density3 Fines1 Durability1,4
C C NC NC C
C C NC NC C
C C NC NC NC
C C NC NC C
ISO 17225-6
6e8 15e40 650 0e1 92e100
The values are expressed as means ± SD of two independent experiments. Different letters express significant differences (p < 0.05) by row using Tukey-Kramer's Test. 1 Expressed in percentage (%). 2 Expressed in mm. 3 Expressed in kg/m3. 4 Assayed by triplicate. The percentage of fit (%) was calculated using the maximum value for each parameter according to ISO 17225-6 standard for cereal straws, except for diameter, length, fines and durability. C: Complied; NC: Not complied.
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length, are crucial factors that might affect combustion [29]. Hence, thinner pellets allow a uniform combustion, especially for those pellets oriented for small furnaces. In addition, pellets dimension can alter fuel feeding properties as the shorter the pellets, the easier can a continuous flow be reached [33]. There are no reports about pellets manufactured from mixtures of rice husks and wheat straws pellets; however, Liu et al. [29] reported bigger apparent density values for pellets than those found in this research, reaching amount between 540 and 640 kg/ m3, for different mixtures of rice and bamboo straws, and similar values for rice straw pellets (135 kg/m3). It has been reported that low density of materials or products might imply transportation and storage problems, thus the granulation procedure could improve the apparent density of the material; as the particle size of the pellets is coarser than other materials, their apparent density becomes smaller, affecting the way that material behave when submitting to pressure conditions [34]. The complete formation of pellets depending of the applied pressure and the binding properties of biomass, lead to the formation of fines, affecting the yielding of pellets. The formation of fines and the low apparent density are conditions that could be improved in the development of pellets by modifying initial moisture and working conditions; thus, machine properties need an improve to be adjusted in order to produce more consistent and densified pellets. However, a high durability was exhibited in the resulting pellets, which match very well with the ISO 17225-6 standard, except for the 50:50 rice husks:wheat straw pellets, with a durability below than 92%. The values reported for durability can be considered high, particularly those for the rice husks pellets, guaranteeing low mass losses of the pellets during storage and transportation [14]. 4. Conclusions In this research, it was proposed a mixture of rice husk/wheat straw pellet (RWP) which exhibited high calorific power, fitting with several quality parameters of the ISO 17225-6 standard such as diameter, length and durability. Moreover, the TGA analysis showed a total decomposition of organic matter, indicating the complete transformation of the raw materials into energy. Despite the non-compliance with proximal parameters such as density, fines, ashes and TKN, there is an enormous potential for the densification of these agroindustrial wastes into sustainable pellets with improved combustion characteristics. Additionally, this research might contribute to the improvement of operating conditions of the pelletizer machines, and the use of novel biomass sources with the desired quality characteristics. Conflicts of interest The authors have declared that there is not a conflict of interest. Acknowledgements Financial support provided by CONACyT, grant 279753, for the s development of this project is gratefully acknowledged. Also, Ine n was supported by a scholarship from CONACYTM. Ríos-Badra SENER. References [1] U. Energy Information Administration, International Energy Outlook 2018 Overview, 2018, pp. 1e76. https://www.eia.gov/outlooks/ieo/pdf/0484(2017). pdf. [2] OECD, Economic, Environmental and Social Statistics, OECD Factb, 2016, 20152016, https://doi.org/10.1787/factbook-2015-39-en.
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Contents lists available at ScienceDirect
Renewable Energy journal homepage: www.elsevier.com/locate/renene
Production and characterization of fuel pellets from rice husk and wheat straw s M. Ríos-Badra n a, Iva n Luzardo-Ocampo a, Juan Fernando García-Trejo b, Ine rrez-Antonio a, * Santos-Cruz a, Claudia Gutie Jose a b
noma de Quer Universidad Auto etaro, Facultad de Química, Cerro de las Campanas s/n Col. Las Campanas, Quer etaro, Quer etaro, C.P. 76010, Mexico noma de Quer Universidad Auto etaro, Facultad de Ingeniería, Cerro de las Campanas s/n Col. Las Campanas, Quer etaro, Quer etaro, C.P. 76010, Mexico
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 December 2018 Received in revised form 16 April 2019 Accepted 10 June 2019 Available online 14 June 2019
In this work, the production and characterization of pellets made from rice husks and wheat straws is presented. For this, physicochemical and energetic characterization of pellets produced with rice husks and mixtures of rice husks with wheat straws were carried out, followed by a comparison with those properties stablished by the ISO 17225-6 standard. The rice husk pellets exhibited the lowest calorific value (3090.64e4049.05 kcal/g) and highest ash content (12.81e17.51%), while the pellets made from mixtures obtained the highest calorific power (4301.10e4573.50 kcal/g) and a reduced amount of ashes (11.43e13.06%). Moreover, the pellets produced with both biomasses exceed the moisture, ashes and nitrogen content, but complied with diameter, length and durability components. These results suggest that these biomasses blends improve the quality and combustion characteristics of the pellets, aiming to enhance the research for the manufacturing of these sustainable biofuels. © 2019 Elsevier Ltd. All rights reserved.
Keywords: Rice husks Wheat straw Fuel pellets Waste biomass Calorific power Physicochemical characterization
1. Introduction According with the International Energy Agency, in 2015 the world energy consumption was 9383.60 Mtoe [1], being transport along with heat and power generation the sectors that consumed more energy. The forecasts of the International Energy Agency indicate that global energy needs rise more slowly than in the past; however, there is still a growing by 30% between today and 2040 [2]. In order to generate all this huge amount of energy, 32,294 Mt de CO2 were released to the atmosphere, which contributes to the aggravation of the climate change problem. Therefore, it is necessary the search of new alternative energy sources, that allows the development of the different economic sectors; moreover, these
Abbreviations: ASTM, American Society for Testing and Materials; ISO, International Standards Organization; EN, European Norm; Mtoe, Million tons of oil equivalent; Mt, Million tons; PJ, Peta Joules; MJ, Mega Joules; C, Complied; NC, Not complied; RH, Rice husks; RWP, Rice husks and wheat straws pellets; LOI, Loss on ignition; TGA, Thermogravimetric analysis; TKN, Total nitrogen content (total Kjeldahl nitrogen); TOC, Total organic carbon; WS, Wheat straws; SD, standard deviation. * Corresponding author. rrezE-mail addresses: [email protected], [email protected] (C. Gutie Antonio). https://doi.org/10.1016/j.renene.2019.06.048 0960-1481/© 2019 Elsevier Ltd. All rights reserved.
alternative energy sources must to be renewables, and its conversion processes must to be of reduced both energy consumption and environmental impact. In particular, the heat and power generation sector is one of the most dynamics. According to the International Energy Agency the global electricity demand doubled between 1990 and 2016, and was set to grow at twice the pace of energy demand as a whole in the next 25 years [3]. There are several renewable options for electricity production such as wind, solar, tidal, geothermal, and hydropower. The application of these energetic alternatives includes the installation of new facilities and equipment for the electricity both production and transmission. Nevertheless, another alternative is the development of solid biofuels that can replace the use of coal in traditional power plants; in this case, the actual infrastructure for both production and transmission can be used. Solid biofuels are defined as any plant matter used directly as fuel or converted into other forms, usually physical processes, before its combustion [4]. Among solid biofuels, we can find different types as briquettes, pales, cubes, wood chips and pellets; the last ones are the most popular. The pellets are a densified biofuel made from milled biomass, with or without additives; usually with a cylindrical form, with typical length between 5 and 40 mm [5]. The pellets are produced from wood and other
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biomasses as agro-industrial and forest residues. In particular, the use of agro-industrial residues has been explored recently for the production of biofuel pellets, since they are available in great amounts, which represent a contamination problem; moreover, in most of the cases, these residues are not used to generate addedvalue products. Due to this, the production of biofuel pellets from agro-industrial residues has gained attention in the scientific community. One of the most important characteristics of pellets is its durability, which can be affected by the feedstock characteristics, the moisture content or size reduction during pre-processing, and by pelletization conditions, including the use of binders, feedstock mixes, temperatures or die pressures [6]. Thus, the works reported in the literature have been focused in the effect of these variables on the properties of fuel pellets; different biomasses have been considered, such as oil palm and para-rubber tree residues [7], mixtures of bark (eucalyptus), fruit shell (mangosteen) and fruit peel (papaya) [8], olive mills [9], mixtures of pine sawdust, wheat straw and rapeseed straw [10], Norway spruce (Picea abies), European beech (Fagus sylvatica), and wheat straw (Triticum aestivum) [11], bamboo sawdust, eucalyptus sawdust, corn cob, rubber tree branches, palm fibre and lippia grass [12], oil palm empty fruit bunch, oil palm frond, oil palm shell and oil palm mesocarp [13], waste from palm plantation and palm industry [14], waste canola meal biomass [15], rice straw [16], grape vine and sunflower husk residues [17], corn stover [18], barley straw [19], wastes from Scenedesmus microalgae [20], downed coconut [21], canola hull [22], wheat straw and peat [23], among others. In Mexico, the use of biomass as primary energy source reached 5.3% in 2005, a considerably reduction from 1965, when used to cover 15.3% of this energy supply [24]. Nonetheless, there is a high potential for the production of biomass-derived energy in Mexico, since it has been estimated that between 54% and 81% of the total brut energy production could come from biomass, which ranges between 3035 and 4550 PJ/year, meaning 10 times the actual use [25]. Moreover, the production of biomass in Mexico is considerable. In 2006, 75.73 Mt of dry matter were produced from 20 different food crops, among primary and secondary agroindustrial wastes (60.13 and 15.60 Mt, respectively) derived from postharvest processing such as sugarcane and agave bagasse, corncobs, coffee pulp, among others [26]. Amidst the production of these wastes, those derived from corn, sorghum, barley, bean and wheat are important because of the high production of these crops in Mexico [27]. One of the crops that recently has increased its production is rice; at 2015, a production of 236,017.92 ton of rice was reported [28]. In addition, it is estimated that for every kilogram of harvested paddy, between 0.41 and 3.96 kg of rice straw will be produced, while rice husk accounts for between 20% and 33% of paddy weight [29]. Rice husk is a type of biomass that constitutes the most important byproduct from the rice milling industry. Considering that China produced in 2017 the 22.01% of the world's rice production [30], their total amount of rice husks generation was about 42.89e70.76 Mt. This reality has stimulated the research focused in the exploitation of rice husks for the manufacturing of materials such as zeolite, ceramics or catalysts, while their use as fuel has gained interest due to their potential energetic performance [27]. In particular, the uses of rice residues for the production of fuel pellets have been explored in the literature. In 2012, Lim et al. [31] presented an interesting review about the applications of the biomass of the rice industry. In that work, the direct combustion of rice straw and rice husk was included in the energetic applications. In addition, gasification and pyrolysis of these residues are considered. Especially for the rice husk, most of this residue is burned in fields, causing an important energy waste
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with consequently environmental problems; thus, some countries with high production of rice have developed electric generators using rice husks burning [31]. In these cases, the pretreatment of the raw material such as pelletization is essential for meeting handling, storage and transportation optimal conditions, as well as complete combustion in furnaces [32]. Later, Liu et al. [29] presented the production of fuel pellets from bamboo and rice straw. They proposed the use of mixed biomass, in order to decrease the ash contents resulting from the combustion of rice straw. The resulting fuel pellets have ash content lower than 8.0% and a calorific power higher than 17,500 J/g. In 2014, the influence of moisture content, particle size and forming temperature on the calorific power and durability of rice straw fuel pellets were reported [16]. Their results show that it is possible to obtain pellets with 12 MJ/kg for the lower heating value, and durability major than 95%. One year later, a similar study for the production of fuel pellets from rice straw was presented [33]. They analyzed the moisture content, the use of starch as additive, the temperature formation, and three flat dies on the mechanical properties of the fuel pellets. The results show that a durability of 99.31% was obtained in the pellet of the highest quality. The calorific power was not reported. Recently, the fabrication of fuel pellets with rice straw and rice husk was presented [34]. The results showed that it is possible to obtain higher heating values, which are slightly lower than those of commercial wood pellets, when the residual biomass is dried. From the revision of the literature, we can observe that the use of agroindustrial residues have been reported for the production of fuel pellets. In particular, the use of waste biomass for the rice industry has been less explored, specially the use of rice straw. In counterpart, the use of rice husk is just consider in one work [34], where the authors claim that it is possible to obtain high heating values. However, the main reported problem is the high content of ash. In order to reduce the content of total ashes, rice husks can be blended with other agroindustrial wastes. In particular, wheat straws are a suitable biomass that have been studied for the manufacturing of pellets; due to their high production as the main consumed cereal in the world, and also because they are a fuel alternative for heat and power production to woody biomass [35]. Wheat straw pellets are commonly produced in pellet mills whereas the high temperature and pressure produce a softening, due to the generation of polymeric interpenetration that add an additional bond strength for the macromolecular entanglement. Hence, the specific chemical composition of the straws (proteins, lignin, cellulose and hemicellulose) can act as binders of the resulting pellet, improving their heat exposition [19]. Nonetheless, pellets must fit into quality parameters such as homogeneity in particle density and size, moisture and ashes content, and net calorific power, which are important variables that are critical for a proper functioning and complete combustion in the low-scale furnaces [16]. Another important parameters are C, H, N, S, Cl, and K concentrations as well as heavy metals (Cd, Pb, Zn, Cr, Cu, and As) levels, on which pellets may comply with the maximum allowed values of the ISO 17225-6 standard in order to be certified and commercialized [36]. As observed in previous reports, the use of blends of rice husks and wheat straws as raw materials for the manufacturing of pellets has not been explored yet. Thus, in this work we present the production of fuel pellets from rice husk and its mixtures with wheat straw. For this, the effect of moisture content and the wheat straw amount were studied on the calorific power and mechanical properties of the pellets, in comparison with the values specified in the standard ISO 17225-6. The article is organized as follows. In the second section the materials and methods are described, while the discussion of the results is presented in section 3. Finally, the
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concluding remarks of the work are given in section 4. 2. Materials and methods 2.1. Biomass collection
was changed from 2 to 5 kVA; also, an iron-isolate plaque was added to the flat die in order to reach a higher temperature (102e110 C) inside the pelletizer, allowing the manufacturing of complete pellets. Fig. 1bee shown the biomasses and pellets from the selected agroindustrial residues, respectively.
Rice (Oryza sativa) husks were collected from Arrocera del Bajío, xico. The wheat (Triticum aeslocated at Cortazar, Guanajuato, Me tivum) straws were donated by Todo Pellet, located in Agropark at xico. The damaged husks and straws were Irapuato, Guanajuato, Me removed, and samples were homogenized using a 250 mm mesh prior to physicochemical characterization.
2.3. Biomasses and pellets physicochemical characterization
2.2. Pellets manufacturing
2.3.2. Elemental analysis Total organic carbon (TOC) was determined by the method of loss on ignition (LOI) [18]. Briefly, the sample was calcined at 375 C by 16 h. Considering that at least the 50% of the organic material was carbon, TOC was calculated using a factor of 1.724 [18] with the equation:
Before the pelletizing procedure, there were assayed different biomasses concentrations and moisture levels. At first, three different moisture levels were assayed for rice husks (1.00, 8.27 and 10.00%) and wheat straws (1.00 and 8.20%). The selection of these moisture values was accordingly to the initial quantification of the original raw materials, and they are representative of the reported moisture values that typically can be found for rice husks (0.32e12%) [34e36] and wheat straws (4.1e12.1%) [37e42]. The samples were dried using an oven (110 ± 2 C), and their moisture was measured each 30 min during 5 h using a hygrometer (Benetech GM640). However, with these moisture values it was not possible to densify the residues in the pelletization machine; thus, in order to produce the rice husks pellets the moisture was raised up to 21 and 23%, with which it was possible to densify the biomass. Thus, the biomass material was mixed using different proportions of rice husks and wheat straws: 100:0, 50:50 and 75:25, with two different initial moistures of both biomasses (21 and 23%), measured using a hygrometer. All combinations were assayed by triplicate. The initial moisture of biomasses blends was recorded, and biomasses mixtures were placed into a modified pellet machine (KL120/BC, Handan City Electrical and Mechanical Company Limited) (Fig. 1a). The modification is described briefly next. The pelletizer holes from the rotative flat die (2) were enlarged, from 4 mm diameter and 18 mm length to 6 mm diameter and 22 mm length. Additionally, pellets were classified using an adapted waste drainage (3), located at the hopper, (1) and the original engine (4)
2.3.1. Proximate analysis Moisture and volatiles compounds were quantified using the reported Hach methods [15,16]. Ashes were calculated using the ASTM D 1102-84 method [17].
% TOC ¼ [((A-B)*100)/A]/1.724
(1)
where A is the mass of dry material (g) and B is the mass of the incinerated material (g). The total nitrogen content (TKN) was analyzed by the modified Kjeldahl method from Hach (Method 8075) [16], quantifying just the nitrogen compounds organically bounded. The generated nitrogen compounds were measured using a spectrophotometer (HACH DR6000; 190e1100 ± 1 nm of wavelength accuracy) at 460 nm wavelength. 2.3.3. Thermal analysis The calorific powder was calculated on a calorimeter (Parr Instruments 6200; 0.05e0.1% precision; 0.0001 C Temperature resolution). The thermal characterization of biomasses and pellets was conducted by a thermogravimetric analysis, TGA, (TA Instruments Q500; Temperature accuracy: þ/- 0.1 C; Temperature precision: þ/- 0.01 C), which is one of the most common technique for the comparison of thermal processes, as well the combustion and pyrolysis kinetics of raw materials such as biomass and the resulting products [19].
Fig. 1. a) Modified pelletizer machine; b) Rice (Oryza sativa) husks; c) Wheat (Triticum aestivum L.) straws; d) Rice husks pellets; e) Rice husks/wheat straws pellets. In Figures b, c, and e the superior rule scale is in centimeters, while the inferior in inches; in Figure c the scale is in inches.
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2.3.4. Physical analysis The bulk density was determined using the EN 15103 [20]. The measurement of pellet diameter and length was done using a Vernier Caliper (Mitutoyo Corp; Instrument uncertainty: 0.008 mm). The mechanical durability was performed using the ASTM D3038-93 standard [21] with slight modifications. Briefly, prior to determination, the pellet mass was quantified using a precision scale. Subsequently, pellet was dropped from a 1.85 m height over a plastic tile, and the mass of the major piece was recorded, selecting it as the final pellet mass. This procedure was performed three times. The impact resistance was calculated using the equation: MD ¼ (A/B)*100
(2)
where A is the initial mass (g) and B is the final mass (g). Fines quantity (%) was calculated following the ISO 17827-2 procedure [43] for solid biofuels using a vibrating sieve method with a screen aperture equal than 1 mm. 2.4. Statistical analysis The results were expressed as the means ± SD from at least three experiments. The data was analyzed using the JMP v.8.0 software, and the statistical analysis was done following one-way ANOVA and Tukey-Kramer's Test for the identification of significant differences among samples. The level of significance was established at p ¼ 0.05. 3. Results and discussion 3.1. Proximal, elemental and energetic characterization of the biomasses Table 1 shows the proximal, elemental and energetic characterization of the biomasses used for the manufacturing of pellets. Rice (Oryza sativa) husks (RH) exhibited a significant (p < 0.05) high of volatile and ashes content from all samples, whereas wheat (Triticum aestivum L.) straws (WS) showed the highest TOC and TKN contents. Regarding the energetic characterization, wheat straws presented the highest values. The proximal, elemental and energetic values of biomass coincide with the typical parameters for location of these products into the biomasses area of a VanKrevelen diagram [22]. The rice husks are among the reported ranges for nitrogen, carbon and ashes (47.8, 0.1%, and 14.7%, respectively) [23,24]. Certain differences found among samples might be explained by several factors such as growing soil conditions and the genetic environment of the plant, and consistent with the high silica content of rice husks [25]. The higher TOC values for wheat straws might be explained by its high content of
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carbohydrates, represented mainly by hemicelluloses, cellulose and lignin (39.4, 28.8, and 18.6%, respectively); which is a desirable composition for better binding properties of the resulting pellet, but undesirable regarding NOx emissions, corrosion and ash deposition during biomass combustion [23,26]. Nonetheless, the carbon content from these biomasses make them as important candidates as potential source for the production of biofuel pellets, beyond their traditional consideration as limited materials due to their low nutritional value, together with their tough nature and composition [25,27]. In general, biomasses can be considered more reactive than traditional solid fuels, such as coal, but the wide range of variability among their chemical properties considerably affect the energetic performance. Such limitations can be studied through a TGA analysis, aiming to a better biomass combustion understanding; along with the possibility to apply thermal and kinetics models for biomass characterization, and functional properties in the development of successful solid biofuels [28]. Fig. 2 shows the thermogravimetric analysis of rice (Oryza sativa) husks pellets at two different initial moistures: 1% and 8.27%. The temperature increases from room temperature to 950 C resulted in a three steps of weight losses. Fig. 2a shows the first two steps at room temperature-50 C and 203e345 C, with a loss weight of 8.116 and 47.5%, respectively; and a continuous third step of continuous light weight loss of 345e685 C with a 9.152% of loss. The first step could be associated to water vaporization, while the second step it is attribute to the degradation of hemicellulose and cellulose. The third degradation region could be associate to the cellulose and lignin degradation; while in the second derivative of the weight loss (blue color in Fig. 2a) the maximums correspond to the principal temperature of the weight loss, in the first step to 35.54 C and 321.83 C, respectively. Thus, the initial decomposition of rice husks started at 203e218 C, reaching a final temperature in the first reaction zone at 320e350 C, showing a rapid degradation of sample due to the volatile products. In this area, the observed degradation rates were 16.17 and 17.59 min1, respectively for 1% and 8.21% rice husks (Fig. 2a and b). The cellulose pyrolysis was observed between 320 and 400 C, with a complete decomposition at a maximum temperature of 328.94 C and 321.83 C, respectively, for 1% and 8.27% moisture. Above 400 C there was a sudden mass loss, representing the release of the remaining volatiles and their ignition. The latter small mass loses corresponded to the slow burning of the partly carbonized residue.
3.2. Proximal, elemental, physical and energetic characterization of fuel pellets Table 2a shows the proximal, elemental and energetic characterization of both rice (Oryza sativa) husks pellets and rice (Oryza sativa) husks/wheat (Triticum aestivum L.) straws pellets; also,
Table 1 Proximal, elemental and energetic characterization of rice (Oryza sativa) husks and wheat (Triticum aestivum L.) straws. Initial moisture1
Rice Husks (RH) 1.00%
Wheat Straws (WS) 8.27%
10.00%
1.00%
8.20%
83.29 ± 0.15b 14.61 ± 0.01b 37.70 ± 0.03c 0.21 ± 0.01
85.37 ± 0.10a 17.51 ± 0.01a 40.54 ± 0.05b 0.43 ± 0.01
70.09 ± 1.26d 11.43 ± 0.18e 49.10 ± 0.76a 0.22 ± 0.01
67.75 ± 0.82e 13.06 ± 0.22c 49.96 ± 0.93a 0.63 ± 0.02
3262.08 ± 54.26d
3090.54 ± 16.72e
4573.50 ± 9.20a
4301.10 ± 55.30b
1
Proximal and elemental characterization Volatile 82.12 ± 0.12c Ashes 12.81 ± 0.01d TOC 31.13 ± 0.04c TKN 0.19 ± 0.01 Energetic characterization2 Calorific Power 4049.05 ± 69.22c
The values are expressed as means ± SD of two independent experiments. Different letters express significant differences (p < 0.05) by row using Tukey-Kramer's Test. Expressed in percentage (%). 2 Expressed in kcal/kg.
1
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Fig. 2. Thermogravimetric analysis (TGA) of rice (Oryza sativa) husk with a) 1% and b) 8.27% of initial moisture.
Table 2b includes their comparison with the ISO 17225-6 standard [36] for cereal straws, and the percentage of fitting of each parameter within the standard, when applicable. The rice husks pellets exhibited a higher volatile matter and ashes values than reported ones for this kind of pellets (65.1 and 9.3%), but lower TOC and TKN (46.6% and 0.7%) [27]. The rice husks/wheat straws pellets exhibited the highest moisture, volatile, TOC, TKN and calorific power values. These chemical characteristics derived from rice husks pellets allow a generation of higher H2 and lower CO2 content in the gasification of these pellets, giving a higher stability of the generated gas due to a higher energy density, when compared with
traditional gas derived from coal and petroleum-derived products [27]. Mixing several biomasses may optimize energetic properties of biomass solid fuel [29]. As wheat straws have an important amount of cellulose and lignin, the depolymerization reactions that occurs when these chemical components are submitted to high temperature conditions, reducing the polymer length from 1000 to 200 monomers units, together with the metal composition of the biomass, allow a high calorific power [40]. Nevertheless, a number of inorganic and organic products might be released during combustion by wheat straws; mixtures of wheat straws with rice husk
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Table 2 a) Proximal, elemental and energetic characterization of rice (Oryza sativa) husks pellets and rice (Oryza sativa) husks/wheat (Triticum aestivum L.) straws pellets; b) ISO 172256 parameters for cereal straws and percentage of fit of each parameter for each pellet. A
Rice (Oryza sativa) husks pellets (RHP)
Rice (Oryza sativa) Husks: Wheat (Triticum aestivum L.) pellets (RWP)
Initial moisture1/mixture
21%
23%
50:50 (21%)
75:25 (21%)
11.06 ± 0.20d 83.68 ± 0.93c 16.57 ± 0.10a 33.69 ± 0.01c 0.50 ± 0.01c
12.22 ± 0.23c 83.62 ± 0.68c 17.22 ± 0.66a 33.31 ± 0.02d 0.30 ± 0.02d
15.95 ± 0.22a 89.54 ± 1.26a 11.50 ± 0.15c 35.81 ± 0.38a 1.58 ± 0.02a
14.16 ± 0.23b 86.08 ± 0.01b 14.09 ± 0.17b 34.79 ± 0.52b 1.04 ± 0.01b
3512.72 ± 67.48c
3312.77 ± 66.60c
3688.26 ± 0.93a
3519.25 ± 1.30bc
Proximal and elemental characterization1 Moisture Volatile Ashes TOC TKN Energetic characterization2 Calorific Power B
Rice (Oryza sativa) husks pellets (RHP)
Rice (Oryza sativa) Husks: Wheat (Triticum aestivum L.) pellets (RWP)
Initial moisture1/mixture
21%
23%
50:50 (21%)
75:25 (21%)
22.15 ± 2.33 e 146.00 ± 0.49 e 57.71 ± 0.01
59.54 ± 2.21 e 64.29 ± 2.22 e 125.00 ± 3.03
41.55 ± 2.33 e 101.29 ± 2.42 e 47.86 ± 1.01
10 e 7 e 0.7
NC
C
NC
3511e4538
Proximal and elemental characterization1 Moisture 10.55 ± 2.05 Volatile e Ashes 136.71 ± 1.41 TOC e TKN 29.05 ± 0.78 Energetic characterization2 Calorific Power C
ISO 17225-6
The values are expressed as means ± SD of two independent experiments. Different letters express significant differences (p < 0.05) by row using Tukey-Kramer's Test. 1 Expressed in percentage (%). 2 Expressed in kcal/kg. 3 The percentage of fit (%) was calculated using the maximum value for each parameter according to ISO 17225-6 standard for cereal straws, except for calorific power. C: Complied; NC: Not complied.
technological modifications to commercial pelletizers, which presents limitations in the workable moisture values for pellets manufacturing. Table 3 shows the physical characterization of both rice (Oryza sativa) husks pellets and rice (Oryza sativa) husks/wheat (Triticum aestivum L.) straws pellets, as well as their comparison with the ISO 17225-6 standard [36] for cereal straws and the percentage of fitting of each parameter within the standard. All the elaborated pellets complied with the ISO 17225-6 standard [36] in the diameter, length, apparent density and durability, except for the 50:50 rice husks:wheat straws pellets, which exhibited a significant lower durability than the other pellets. Furthermore, none of the pellets complied with the apparent density and fines percentage. The geometric dimensions of the pellets, considering both diameter and
might reduce the potential generation of these products, resulting in a significant lower amount of ashes when the pellets made from rice husks/wheat straws blends are compared with rice husks or wheat straws alone. Additionally, the pelletization process permitted a better handling, transport, storage and use of the biomass, elements that come together with the densification process of biomasses [29]. However, the manufactured pellets must fit into quality standards such as the ISO 17225-6 [36]. Regarding this, the pellets only complied with the energetic specifications of the standard, while the 50:50 rice husk:wheat straw pellet exhibited the highest calorific power among all samples. However, the lack of fitting of pellets represents an opportunity area to improve the composition of pellets by assaying different blends of biomasses and making
Table 3 a) Physical characterization of rice (Oryza sativa) husks pellets and rice (Oryza sativa) husks/wheat (Triticum aestivum L.) straws pellets. b) ISO 17225-6 parameters (cereal straws) and fitting of each parameter within the standard. A
Rice (Oryza sativa) husks pellets (RHP)
Rice (Oryza sativa) Husks: Wheat (Triticum aestivum L.) pellets (RWP)
Initial Moisture1/Mixture
21%
23%
50:50 (21%)
75:25 (21%)
Diameter2 Length2 Apparent Density2 Fines1 Durability1
6.01 ± 0.01b 25.10 ± 0.03a 101.94c ± 0.09 2.51 ± 0.01c 95.50 ± 0.70b
6.02 ± 0.01b 24.72 ± 0.74a 100.26 ± 0.36d 2.05 ± 0.07d 97.00 ± 0.01a
6.30 ± 0.12a 25.75 ± 0.36a 104.00 ± 1.41b 3.03 ± 0.03a 89.50 ± 0.70d
6.38 ± 0.06a 25.75 ± 0.53a 109.00 ± 1.41a 2.83 ± 0.04b 92.0 ± 1.41c
B
Rice (Oryza sativa) husks pellets (RHP)
Rice (Oryza sativa) Husks: Wheat (Triticum aestivum L.) pellets (RWP)
Initial Moisture1/Mixture
21%
23%
50:50 (21%)
75:25 (21%)
Diameter2 Length2 Apparent Density3 Fines1 Durability1,4
C C NC NC C
C C NC NC C
C C NC NC NC
C C NC NC C
ISO 17225-6
6e8 15e40 650 0e1 92e100
The values are expressed as means ± SD of two independent experiments. Different letters express significant differences (p < 0.05) by row using Tukey-Kramer's Test. 1 Expressed in percentage (%). 2 Expressed in mm. 3 Expressed in kg/m3. 4 Assayed by triplicate. The percentage of fit (%) was calculated using the maximum value for each parameter according to ISO 17225-6 standard for cereal straws, except for diameter, length, fines and durability. C: Complied; NC: Not complied.
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length, are crucial factors that might affect combustion [29]. Hence, thinner pellets allow a uniform combustion, especially for those pellets oriented for small furnaces. In addition, pellets dimension can alter fuel feeding properties as the shorter the pellets, the easier can a continuous flow be reached [33]. There are no reports about pellets manufactured from mixtures of rice husks and wheat straws pellets; however, Liu et al. [29] reported bigger apparent density values for pellets than those found in this research, reaching amount between 540 and 640 kg/ m3, for different mixtures of rice and bamboo straws, and similar values for rice straw pellets (135 kg/m3). It has been reported that low density of materials or products might imply transportation and storage problems, thus the granulation procedure could improve the apparent density of the material; as the particle size of the pellets is coarser than other materials, their apparent density becomes smaller, affecting the way that material behave when submitting to pressure conditions [34]. The complete formation of pellets depending of the applied pressure and the binding properties of biomass, lead to the formation of fines, affecting the yielding of pellets. The formation of fines and the low apparent density are conditions that could be improved in the development of pellets by modifying initial moisture and working conditions; thus, machine properties need an improve to be adjusted in order to produce more consistent and densified pellets. However, a high durability was exhibited in the resulting pellets, which match very well with the ISO 17225-6 standard, except for the 50:50 rice husks:wheat straw pellets, with a durability below than 92%. The values reported for durability can be considered high, particularly those for the rice husks pellets, guaranteeing low mass losses of the pellets during storage and transportation [14]. 4. Conclusions In this research, it was proposed a mixture of rice husk/wheat straw pellet (RWP) which exhibited high calorific power, fitting with several quality parameters of the ISO 17225-6 standard such as diameter, length and durability. Moreover, the TGA analysis showed a total decomposition of organic matter, indicating the complete transformation of the raw materials into energy. Despite the non-compliance with proximal parameters such as density, fines, ashes and TKN, there is an enormous potential for the densification of these agroindustrial wastes into sustainable pellets with improved combustion characteristics. Additionally, this research might contribute to the improvement of operating conditions of the pelletizer machines, and the use of novel biomass sources with the desired quality characteristics. Conflicts of interest The authors have declared that there is not a conflict of interest. Acknowledgements Financial support provided by CONACyT, grant 279753, for the s development of this project is gratefully acknowledged. Also, Ine n was supported by a scholarship from CONACYTM. Ríos-Badra SENER. References [1] U. Energy Information Administration, International Energy Outlook 2018 Overview, 2018, pp. 1e76. https://www.eia.gov/outlooks/ieo/pdf/0484(2017). pdf. [2] OECD, Economic, Environmental and Social Statistics, OECD Factb, 2016, 20152016, https://doi.org/10.1787/factbook-2015-39-en.
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