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Revista mexicana de ciencias forestales
versión impresa ISSN 2007-1132
Rev. mex. de cienc. forestales vol.7 no.38 México nov./dic. 2016
Technical Essay
Chemistry of plant biomass upon yield during torrefaction: a review
1Doctorado Institucional en Ciencias Agropecuarias y Forestales, Universidad Juárez del Estado de Durango. México.
2Instituto de Silvicultura e Industria de la Madera, Universidad Juárez del Estado de Durango. México.
3Facultad de Ciencias Forestales, Universidad Juárez del Estado de Durango. México.
Biomass is a source of energy that has great anatomical and chemical structural heterogeneity, low energy density, high moisture content and hygroscopicity, hydrophilic and with low calorific value. For their use, the application of thermal treatments, known as roasting, promote greater efficiency in the conversion to biofuels. In the present work the composition of the vegetal biomass, its thermal degradation, mass-energy yield and the chemical properties of the final product are analyzed. During the roasting process, an inert atmosphere and temperature intervals of 200 to 300 ° C are required. This can improve the energy characteristics of the biomass. The solids, liquids and gases obtained are used to generate heat energy. Solids have lower oxygen and moisture content; Which increases the calorific value, produces little smoke, is not fermented, is hydrophobic and resists rotting. The liquids are made up of water, acetic acid, furfural acid, formic acid, methanol, lactic acid, phenol, aldehydes and ketones. The gases generated are mainly CO, CO2, H2 and CH4. Finally, to characterize the properties and quality of biofuels, techniques of proximal and elemental analysis are used. The roasting process improves the chemical properties of vegetable biomass by increasing its quality as a fuel.
Key words: Biofuels; bioenergy; energy density; pyrolysis; calorific value; heat treatment
La biomasa es una fuente de energía que presenta gran heterogeneidad estructural anatómica y química, con baja densidad energética, alto contenido de humedad e higroscopicidad, es hidrófila y con bajo poder calorífico. Para su aprovechamiento, la aplicación de tratamientos térmicos, conocidos como torrefacción promueven mayor eficiencia en la conversión a biocombustibles. En el presente trabajo se analizan la composición de la biomasa vegetal, su degradación térmica, rendimiento másico-energético y las propiedades químicas del producto final. Durante el proceso de torrefacción, se requiere de una atmósfera inerte e intervalos de temperatura de 200 a 300 °C. Este puede mejorar las características energéticas de la biomasa. Los productos sólidos, líquidos y gases obtenidos se emplean para generar energía calorífica. Los sólidos tienen menor contenido de oxígeno y humedad; lo que aumenta el poder calorífico, produce poco humo, no se fermenta, es hidrofóbico y resiste la pudrición. Los líquidos están conformados por agua, ácido acético, ácido furfural, ácido fórmico, metanol, ácido láctico, fenol, aldehídos y cetonas. Los gases generados son principalmente CO, CO2, H2 y CH4. Finalmente, para caracterizar las propiedades y la calidad de los biocombustibles, se emplean técnicas de análisis proximales y elementales. El proceso de torrefacción mejora las propiedades químicas de la biomasa vegetal, al aumentar su calidad como combustible.
Palabras clave: Biocombustibles; bioenergía; densidad energética; pirólisis; poder calorífico; tratamiento térmico
Introduction
In recent years, the greatest challenge has been to achieve a balance between economic development and the quality of the environment (REN21, 2016). The main cause of the environmental deterioration is the increasing demand for energy, caused by the prevailing economic models and the significant increase in lighting, heating, cooling and transportation requirements (Demirbas, 2005). After the 1973 oil crisis, the importance of diversifying sources of fuel and environmentally friendly technologies became evident (Maffeo, 2003).
Current progress in the use of renewable energy and energy efficiency plays a crucial role in reducing the consumption of fossil fuels, greenhouse gases and environmental pollution. In 2014, the production of renewable energies represented about 58.5 % of the global capacity of energy generation. In 2016, China was the leader in the installed capacity on the matter, with 83.3 billion dollars (REN21, 2016). Renewable energies that are most consumed and traded globally, according to their level of importance are: wind (51 %), geothermal (17 %), bioenergy (12 %), hydroelectric (10 %) and Solar (1.1 %) (Alemán-Nava et al., 2014).
Biomass, a source of renewable energy, is all kinds of organic matter that originates in a biological process (Kumar et al., 2015). In plants, during photosynthesis (reduction oxide process) solar energy is absorbed in chlorophyll and carbon dioxide (CO2) is transformed from air and soil water into carbohydrates that store chemical energy, through the photoelectrochemistry conversion process of photosynthesis (Bustamante-García et al., 2013). Carbohydrates are composed mainly of carbohydrates, lipids and proteins, which are in a variable proportion, depending on the nature of the biomass.
Unfortunately, the use of biomass as fuel has disadvantages because of its heterogeneity in anatomical, physical and chemical structure. From the physical point of view, it is characterized by high moisture content, low density, high volume, high hygroscopicity, storage difficulty and high toughness (Zheng et al., 2013). The chemical composition includes larger fractions of hydrogen, oxygen and volatiles, as well as lower amounts of carbon and calorific value compared to fossil fuels.
The disadvantages of biomass as an energy source can be improved with thermochemical pre-treatments (Shankar-Tumuluru et al., 2012; Chen y Kuo, 2011; Arteaga-Pérez et al., 2015). According to Rousset et al. (2013) the main methods used to transform biomass into energy are biological (aerobic digestion, fermentation), chemical (esterification), mechanical (densified and gravimetric reduction) and thermochemical (combustion, liquefaction, gasification, pyrolysis and torrefaction processes). Most of these help to break the amorphous and crystalline regions of the biomass (Shankar-Tumuluru et al., 2012).
Roasting is a slow pyrolysis that improves the properties of lignocellulosic biomass to convert it into good quality fuel. It consists of a slow heating of the biomass in an inert atmosphere (absence of oxygen), at a temperature ranging from 200 to 300 °C (Chen et al., 2015), during a residence time from a few minutes to one hour (Rousset et al., 2013). The improvements observed after torrefaction are reducing the moisture content, increasing the energy density, increasing the C / O ratio, improving hydrophobicity, flammability, reactivity and grinding; Storage and transport are facilitated (Couhert et al., 2009).
Based on the above, the objective of this document is to carry out a detailed bibliographic review on the roasting of the biomass, which includes; a) chemical composition of the biomass, b) thermal degradation, c) roasting products, d) mass efficiency, e) energy efficiency, and f) chemical properties of the fuel. The purpose is to provide recent information about this process, which is considered in the future will be one of the main technologies to obtain fuels with high energy value from vegetable biomass.
Chemical composition of plant biomass
The structural unit of every plant organism is the cell, which is composed of a cell wall which is a resistant but generally flexible layer, although it is sometimes rigid and provides structural support to plants. In addition, it gives protection to mechanical and thermal stresses. Its main constituents are hemicellulose, cellulose, lignin and extractable components (Montoya-Arbeláez et al., 2014).
Hemicellulose. It is a complex polymer composed of heterogeneous polysaccharide groups such as pentoses (D-xylose and L-arabinose) and hexoses (D-glucose, D-mannose and D-galactose) forming branched chain chains; And 4-O-methylglucuronic, D-galacturonic and D-glucuronic acids with β-1,3 links (Sun and Tomkinson, 2003; Scheller and Ulvskov, 2010). It structures in chains of 500-3 000 units of sugar is responsible for approximately 15 to 35 % of the dry weight of broadleaf wood and 20 to 32 % of conifers (Table 1).
Table 1 Chemical composition of different types of plant biomass.
Biomass | Hemicellulose (%) | Cellulose (%) | Lignin (%) | Extractives (%) |
---|---|---|---|---|
Rice shell1 | 18.47 | 42.20 | 19.40 | - |
Bambusa sinospinosa McClure2 | 14.44 | 62.33 | 13.01 | 2.52 |
Fagus ssp. wood 3 | 34.40 | 40.90 | 24.80 | - |
Straw4 | 27.20 | 34.00 | 14.20 | - |
Betula pendula Roth wood4 | 25.70 | 40.00 | 15.70 | - |
Salix spp.5 | 14.10 | 49.30 | 20.00 | - |
Cocconut shell6 | 28.40 | 52.20 | 36.00 | - |
Betula pendula Roth7 | 27.70 | 40.00 | 15.70 | - |
Broadleaves8 | 15-35 | 40-44 | 18-25 | - |
Conifers8 | 20-32 | 40-44 | 25-35 | - |
Cryptomeria japonica (Thunb. ex L. f.) D. Don9 | 7.65 | 46.86 | 42.11 | - |
Abies alba Mill.10 | 25.00 | 37.10 | 35.00 | 2.80 |
Picea abies (L.) H. Karst10 | 20.00 | 42.40 | 35.10 | 2.40 |
Source: 1 = Abdullah et al. (2010); 2 = Chen et al. (2015a); 3 = Septien et al. (2012); 4 = Mohan et al. (2006); 5 = Bridgeman et al. (2008); 6 = Montoya-Arbeláez et al. (2014); 7 = Shankar-Tumuluru et al. (2011); 8 = Shah and Gardner (2012); 9 = Lu et al. (2013); 10 = Peng et al. (2013).
The thermal degradation of hemicellulose in the torrefaction occurs in the temperature range of 130 to 260 °C; the highest weight loss occurs above 180 °C (Mohan et al., 2006), due to its chemical composition and its relationship with lignin (Demirbas, 2009). Table 1 shows the percentage of hemicellulose in different types of biomass, whose values after roasting vary from 7.0 to 34.4 %. Agricultural biomass is different from woody biomass, since crops have a high hemicellulose content, which decomposes easily when heated (Wang et al., 2011). Rowell (2012) indicates that hemicellulose originates from acetyl and methyl groups, which are responsible for the release of light volatile gases (CO and CO2) during a temperature thermal treatment between 200 and 300 °C. Scheller and Ulvskov (2010), indicate that the proportion of tar and coal is lower than those obtained from the torrefaction of cellulose.
Cellulose. It is the main component of the cellulosic biomass (starches) and lignocellulosic (bagasse, leaves, fruits, etc.), constitutes between 40 and 60 % of the cell wall content of broadleaved and coniferous wood (Table 1). It is a high molecular weight polymer that forms the fibers of the biomass; it has a structure of 7 000 to 15 000 glucose molecules (Basu et al., 2013). The D-glucose polymer is linked by β- (1→4) glycosidic bonds to form cellobiose molecules (Nhuchhen et al., 2014). It is formed by a structure of long linear chains (microfibrils) joined by hydrogen bridges with oxygen and intramolecular van der Waals forces, which develops a crystalline and amorphous fibrillar structure (Cuervo et al., 2009).
The crystalline region is difficult to penetrate by solvents and reagents; moreover, it is more resistant to thermal depolymerization (Sjöström, 1993). The amorphous one contains water of hydration and maintains free water inside the plant, it is more accessible and susceptible to all reactions; when it is rapidly heated, it becomes steam that can break down the structure of cellulose (Shankar-Tumuluru et al., 2011). Therefore, according to the type of biomass its cellulose content after torrefaction varied from 34.00 to 62.33 % (Table 1).
Cellulose is important because it favors stiffness in plant cells, due to the union of the microfibrils in a carbohydrate matrix. Hydroxyl groups increase the ability to form hydrogen bonds and are responsible for their hygroscopic behavior and influence the swelling and contraction of the biomass by absorbing or losing moisture; (Nhuchhen et al., 2014). In order to obtain a thermal treatment, there are contraction phenomena that cause dimensional variations (Nhuchhen et al., 2014). The thermal degradation of the cellulose begins between 240 and 350 °C, which transforms it into anhydrous cellulose and levoglucosan (Mohan et al., 2006).
Lignin. It is a non-crystalline, amorphous, three-dimensional and branched polymer with a structural base of phenylpropane units (C10H15O3)n, with a large number of aromatic rings joined together by cycles of furan or ether bonds (β-O-4-aryl) (Vanholme et al., 2010). It is formed from the union of various acids, phenylpropyl alcohols and multiple sugars; it originates in the cell wall of many plant cells, its content ranges from 18 to 25 % in hardwood and 25 to 35 % in coniferous wood (Table 1).
Lignin is mainly located in the middle sheet, where it is deposited during the lignification of the plant tissue; it covers the spaces in the cell wall between the components of cellulose, hemicellulose and pectin (Brebu and Vasile, 2010). By its nature it is hydrophobic and aromatic; it allows water, nutrients and metabolites transport; in turn, provides hardness, stiffness and resistance to the cell wall and acts as a bridge between the cells of the wood, and is considered a glue, since it unites by joining adjacent cells (Bergman et al., 2005a).
The structure of lignin has various thermal stabilities; therefore, it breaks at different temperatures (Brebu and Vasile, 2010). According to Yang et al. (2007), it decomposes to a wide range of temperatures, ranging from 280 to 900 °C, due to the different functional groups that compose it and complete devolatilization of the wood. Shah and Gardner (2012) report that, at temperatures of 250-500 °C, lignin is the most difficult structural element to decompose thermally. Phenols are produced by the separation of ether bonds and carbon-carbon bonds (Demirbas, 2009; Mohan et al., 2006).
Extractable components. Devolatilization depends on the amount and location of the extractables in the biomass. These compounds are divided into organic and inorganic (mineral) components. Organic matter is responsible for the biomass characteristics, such as color, odor, taste, density, hygroscopicity and flammability (Mohan et al., 2006). They are soluble compounds in different solvents, like benzene alcohol that solubilizes waxes, fats and resins; In cold water the tannins, gums, sugars and coloring materials are solubilized, while in warm water the starches. They are classified as volatile acids, essential oils, resin acids and polyphenols (Lima, 2013).
The inorganic (mineral) components present some metallic ions, essential for the optimum development of the tree, its content varies from 2.5 to 12.0 % of the weight of the biomass. High mineral contents are found in leaves, branches, bark, roots; Their quantity is influenced by soil and age conditions (Mohan et al., 2006). The most abundant components are calcium, potassium and magnesium, to a lesser extent phosphorus, sodium, iron, silicon, manganese, copper and zinc (Kim et al., 2012).
Torrefaction Products
In the torrefaction process different products are obtained: uniform solids, condensable organic compounds (liquids) and non-condensable gases (van der Stelt et al., 2011). Bergman et al. (2005b) state that the quantity and type of products generated depends on operating conditions such as heating rate, temperature and residence time, as well as the physical, anatomical and chemical properties of the biomass.
Solid products. The production of solids or also known as mass yield is defined as the dry weight ratio of the untreated biomass and the weight of the ash-free dry solid product (Basu et al., 2013). The thermal decomposition of hemicellulose, cellulose and lignin has a large influence on the loss of mass of the lignocellulosic materials. Chew and Doshi (2011), based on a mass spectrometric analysis, document that the weight loss of biomass is due to the reduction of hemicellulose and lignin (Na et al., 2013).
Wang et al. (2011) observed that increasing the temperature in the torrefaction of stems of Gossypium herbaceum L. and Triticum aestivum L. decreases the mass or solids yield, but increases the volatile fractions. Table 2 shows that the solids yield varies by the type of biomass and the maximum temperature reached by the torrefaction process, as the temperature increases yield decreases. In general, the post-process mass yield has a high carbon content and high energy density. Shankar-Tumuluru et al. (2011) indicate that the solid phase varies from brown to black and can be used for bioenergy, since it is composed of a structure of sugars, products originating from the reaction, coal and ash (Bergman et al., 2005b).
Table 2 Yield of products obtained from torrefaction as a function of temperature and biomass.
Biomass | Temperature (%) | Mass yield (dry weight %) | Liquid yield (dry weight %) | Gas yield (dry weight %) |
---|---|---|---|---|
Stem of Gossypium herbaceum L.1 | 200 | 63.89 | 4.14 | 31.97 |
Stem of Gossypium herbaceum L.1 | 250 | 33.80 | 13.80 | 52.40 |
Stem of Gossypium herbaceum L.1 | 300 | 30.04 | 17.28 | 52.68 |
Stem of Triticum aestivum L.2 | 200 | 47.56 | 3.93 | 48.51 |
Stem of Triticum aestivum L.2 | 250 | 42.24 | 9.69 | 49.07 |
Stem of Triticum aestivum L.2 | 300 | 31.61 | 11.57 | 56.82 |
Salix sp.3 | 280 | 87.50 | 1.40 | 1.40 |
Salix babylonica L.4 | 230 | 91.00 | 8.00 | 1.00 |
Salix babylonica L.4 | 250 | 85.50 | 13.00 | 1.50 |
Salix babylonica L.4 | 280 | 79.00 | 18.00 | 3.00 |
Betula pendula Roth4 | 230 | 93.20 | 6.00 | 0.80 |
Betula pendula Roth4 | 250 | 88.00 | 10.80 | 1.20 |
Betula pendula Roth 4 | 280 | 79.00 | 19.00 | 2.00 |
Miscanthus spp.4 | 230 | 89.00 | 10.00 | 1.00 |
Miscanthus spp.4 | 250 | 83.00 | 15.00 | 3.00 |
Miscanthus spp. 4 | 280 | 69.00 | 24.00 | 7.00 |
Bambusa sinospinosa McClure 5 | 250 | 74.00 | 8.00 | 18.00 |
Bambusa sinospinosa McClure 5 | 300 | 46.00 | 25.00 | 30.00 |
Bambusa sinospinosa McClure5 | 350 | 36.00 | 27.00 | 37.00 |
Source: 1 = Wang et al. (2011); 2 = Wang et al. (2011); 3 = Shankar-Tumuluru et al. (2011); 4 = Zanzi et al. (1989); 5 = Chen et al. (2015).
Li et al. (2012) state that in torrefaction the mass yield is greater than in carbonization, by obtaining a large part of biochar (70-85 % by weight), which is feasible to use as fertilizer, fuel in the thermoelectric and metallurgical industries and for the manufacture of chemicals.
Bergman and Kiel (2005) indicate that after the torrefaction of the wood of Salix spp. at 280 °C, the mass yield is 87.5 %. Bergman et al. (2005a) made a calculation of 70 %, while the remainder (30 %) of the mass is converted into gases. Almeida et al. (2010) showed that this product can be used as a quantitative indicator to measure the degree of torrefaction.
Condensable compounds (liquids)
As the temperature increases in the torrefaction process, the higher the release of condensable gases (liquid compounds) and non-condensable (gases), due to competition between carbonization and devolatilization reactions (Table 3). According to Van der Stelt et al. (2011), the liquid products are divided into four subgroups: 1) reaction water produced from the thermal decomposition; 2) free water obtained from evaporation; 3) organic compounds and lipids produced during devolatilization and carbonization; and 4) compounds such as waxes and fatty acids. In contrast, Shankar-Tumuluru et al. (2011) cite that liquid products comprise water, acetic acid, furfural acid, formic acid, methanol, lactic acid, phenol, aldehydes and ketones.
Table 3 Yield of the obtained gas from torrefaction of the biomass compounds.
Compund | Yield of the produced gas (m M g-1 biomass) | |||||
---|---|---|---|---|---|---|
H2 | CO | CH4 | CO2 | C2H4 | C2H6 | |
Hemicellulose | 8.75 | 5.37 | 1.57 | 9.72 | 0.05 | 0.37 |
Cellulose | 5.48 | 9.91 | 1.84 | 6.58 | 0.08 | 0.17 |
Lignin | 20.84 | 8.46 | 3.98 | 7.81 | 0.03 | 0.42 |
Source: Yang et al. (2007).
Water is the main condensable product of torrefaction, and it is released during drying when moisture evaporates and in the dehydration reactions between organic molecules. Acetic acid originates mainly from the methoxy groups present as side chains in units of xylose (part of the hemicellulose fraction) and acetoxy (Shah and Gardner, 2012).
Non-condensable compounds (gases)
The type and amount of gas released during torrefaction depend on the type of raw material, anatomical, molecular and chemical composition. Yang et al. (2007) record that the main gases that are formed during torrefaction are H2, CO and CO2 (Table 3). Hemicellulose with higher carboxyl functional groups showed higher CO2 yield. Cellulose has a high yield of CO, due to the thermal degradation of carbonyl and carboxyl. Lignin has the highest yield of H2 and CH4, which is attributable to the high content of aromatic rings and methoxyl groups (O-CH3), responds to cracking and deformation of this compound, resulting in a large release of H2 and CH4.
The release of C2H4 and C2H6, in general, is very low compared to other gases. Dehydroxylation of the carboxyl group (-COOH) plays a crucial role in the production of the oxygen-containing gas (Table 3). H2 is comes from the cracking and deformation of the alkene groups (C = C) and carbon-oxygen (C-H) bonds, while CH4 is mainly due to the cracking of the methoxy group (-O-CH3).
Shah and Gardner (2012) cite that CO formation is due to dehydration or decarboxylation reactions; The increase in CO production is caused by the reaction of CO2 and water steam. The release of CO2 is presented by the decarboxylation of the acid groups of the biomass.
The non-condensable gases formed in the torrefaction of stems of G. herbaceum and straw of T. aestivum were mainly: CO and CO2. Due to the increase in temperature and to the fact that the volatile content of G. herbaceum is higher (76.92%) than in the T. aestivum straw (71.59 %); small amounts of CH4 and H2 are observed, which are released at 400 °C (Wang et al., 2011). Shah and Gardner (2012), when comparing the composition of wood gas and agricultural residues observed that the residues are characterized by a higher production of CO2. The ratio of CO and CO2 increases with temperature, because cellulose and lignin decompose at higher temperatures.
Energy efficiency
The energy yield reflects the magnitude of the energy conversion of the biomass during the torrefaction process (Wang et al., 2011). Bergman and Kiel (2005) recorded an energy yield of 94.9 % (Table 4), whereas Bergman et al. (2005a) reported an energy yield of 90 % of biomass energy and 10 % of energy was converted to heat. Bates and Ghoniem (2013) point out that the energy yield (in terms of calorific value) of the solid product obtained is an important parameter of torrefaction.
Table 4 Energy yield of torrefacted biomass at different temperatures of torrefaction.
Biomass | Temperature (°C) | Energy yield (%) |
---|---|---|
Salix spp. 1 | 280 | 94.90 |
Woody2 | 240 | 99.90 |
Woody2 | 260 | 91.40 |
Woody2 | 280 | 80.40 |
Leucaena spp.3 | 200 | 94.10 |
Leucaena spp.3 | 250 | 76.20 |
Wood briquettes4 | 220 | 95.90 |
Wood briquettes4 | 250 | 78.40 |
Pinus spp. splinters5 | 250 | 90.00 |
Pinus spp. splinters5 | 275 | 87.00 |
Pinus spp. splinters5 | 300 | 71.00 |
Forest residues5 | 250 | 92.00 |
Forest residues5 | 275 | 82.00 |
Forest residues5 | 300 | 72.00 |
Liriodendron tulipifera L.5 | 240 | 99.90 |
Liriodendron tulipifera L.5 | 260 | 91.40 |
Liriodendron tulipifera L.5 | 280 | 80.40 |
Source: 1 = Bergman and Kiel (2005); 2 = Kim et al. (2012); 3 = Wannapeer et al. (2011); 4 = Chew and Doshi (2011); 5 = Phanphanich and Mani (2011).
Chemical properties of the solid products obtained in torrefaction
Torrefaction influences significantly the chemical properties of the biomass. The characterization of the proximal and elemental composition of the biomass allows to determine, if a product is acceptable as fuel. The proximal analyzes are to determine the percentage of moisture, fixed carbon, volatile material and ash (Na et al., 2015). Elemental analyzes are a tool to characterize product properties (Sadaka and Negi, 2009).
Proximal analyzes
When the temperature of the torrefaction process is high, there are significant changes in the proximal composition due to the decrease in moisture content, volatile material and ashes, but the percentage of fixed carbon increases due to the devolatilization of the biomass. Fixed carbon is the mass of remaining organic matter remaining after the volatile material and moisture are released. It is considered the most important energy component, which generates a crystalline structure, in which the chemical bond breaks down into carbon atoms (Bustamante-García et al., 2015). It is formed by carbonaceous structures that at the end of the process form the solid carbonaceous mass of the treated biomass until the combustion. In several investigations it is documented that in the torrefaction of different types of biomass the fixed carbon content increases as the temperature of the process increases (Table 5).
Table 5 Results of proximal analysis of different types of biomass subjected to different torrefaction temperatures (values on dry basis).
Biomass | Temperature (°C) | Fixed Carbon (%) | Volatile material (%) | Ashes (%) |
---|---|---|---|---|
Stem of Gossypium herbaceum L.1 | 250 | 44.48 | 36.13 | 12.69 |
Stem of Gossypium herbaceum L.1 | 300 | 48.00 | 31.16 | 14.74 |
Stem of Triticum aestivum L.1 | 250 | 43.66 | 28.21 | 23.30 |
Stem of Triticum aestivum L.1 | 300 | 55.43 | 14.84 | 24.95 |
Torrefacted wood2 | 250 | 28.00 | 55.00 | 1.50 |
Torrefacted wood2 | 300 | 30.00 | 65.00 | - |
Bambusa sinospinosa McClure3 | 250 | 19.00 | 66.00 | 4.00 |
Bambusa sinospinosa McClure3 | 300 | 30.00 | 42.00 | 5.00 |
Bambusa sinospinosa McClure3 | 350 | 53.00 | 32.00 | 6.00 |
Eucalyptus sp. 4 | 250 | 24.70 | 75.00 | 0.30 |
Eucalyptus sp. 4 | 300 | 44.10 | 55.60 | 0.30 |
Picea spp.5 | 200 | 15.95 | 83.92 | 0.12 |
Picea sp.5 | 225 | 25.12 | 74.40 | 0.14 |
Betula pendula Roth5 | 200 | 14.76 | 85.15 | 0.09 |
Betula pendula Roth5 | 225 | 26.09 | 73.71 | 0.13 |
Eucalyptus grandis W.Hill6 | 240 | 20.37 | 79.56 | 0.07 |
Eucalyptus grandis W.Hill6 | 280 | 24.13 | 75.60 | 0.27 |
Pinus sp.7 | 225 | 22.75 | 76.37 | 0.87 |
Pinus sp.7 | 250 | 29.02 | 70.03 | 0.94 |
Pinus sp.7 | 275 | 42.49 | 56.53 | 0.98 |
Source: 1 = Wang et al. (2011); 2 = Shankar-Tumuluru (2011); 3 = Chen et al. (2015a); 4 = Lu et al. (2012); 5 = Bach et al. (2014); 6 = Rousset et al. (2012); 7 = Lee-Carter (2012).
Volatile organic compounds are removed from the biomass; As the temperature increases, volatile content decreases and becomes more hydrophobic, difficult to ignite, and changes from brown to dark (Ratte et al., 2011; Kim et al., 2012); however, the high volatile content may be beneficial in the co-combustion with mineral coal, because the mixture allows to increase the ignition temperature and, consequently, the calorific value (Demirbas, 2004). Demirbas (2003) indicates that the product with high percentage of volatile material and low fixed carbon content reduces the friability and fragility; It also increases its resistance to compression and cohesion.
Ashes are solid inorganic wastes generated in the complete oxidation of biomass at high temperatures, are composed of silicates, carbonates, sulfates and other minerals (Lee-Carter, 2012). The ash indicates the amount of minerals the solid product has that will be subjected to complete oxidation; as its value decreases ensures that the biomass will be converted to fuel efficiently. Thyrel et al. (2013) report that ash is considered an industrial by-product and environmental pollutant because it has a high alkali metal content; therefore, they are highly reactive.
Concentrations of inorganic elements in biomass such as silica, sulfur and alkali metals are a disadvantage because they form alkali silicates or sulfates that melt or soften at temperatures of 700 °C (Shankar-Tumuluru et al., 2012). Ahmaruzzaman (2009) found that, during the combustion of the solid product, the ash can react with sulfur and chlorine, which causes the formation of slag, dirt and corrosion problems.
Inorganic compounds can absorb heat and moisture, which lowers process efficiency; Therefore, it is a parameter that directly affects the calorific value. In wood fuels, the bark content influences the ash content; mainly because the crust has higher levels of mineral impurities, such as sand and earth.
Kim et al., (2012) observed that the content of inorganic compounds in the raw and roasted biomass of Liriodendron tulipifera L., changes slightly, except for Fe and Zn, which were probably released on the stainless steel surface of the reactor, where torrefaction was made (Table 6). Bridgeman et al. (2008) document that inorganic compounds absorb heat in the same way as moisture, thereby decreasing the efficiency of torrefaction and stimulating the formation of carbonaceous residues (char). Therefore, the content of the inorganic influences, directly, the calorific value.
Table 6 Inorganic compounds obtained from the roasted biomass of Liriodendron tulipifera L. at different conditions.
Source: Kim et al. (2012).
Elementary analysis
The elemental analyzes detect important features of the biomass, since they show, exactly, the atomic elements that it contains. The composition may vary according to the type of biomass and the region where it is collected. Their determination in the organic fraction of the roasted wood allows to establish the balance of the mass and energy of the combustion, in order to efficiently use the fuel (Bustamante-García et al., 2015). In addition, it is important to determine its content to balance chemical equations that help predict products that result from a chemical reaction (Friedl et al., 2005; Sheng and Azevedo, 2005). Elemental analyzes are performed with oxidation, decomposition and reduction methods. Table 7 shows that in the elemental composition of various types of biomass, carbon varies from 48.05 to 69.56 %, hydrogen 1.58 to 3.63 %, oxygen from 17.63 to 27.60 %, nitrogen to a maximum of 0.70 % and Sulfur less than 0.02 %.
Table 7 Elemental analysis results of different types of biomass subjected to roasting (dry basis).
Biomass | Temperature (°C) | C (%) | H (%) | O (%) | N (%) | S (%) | Ashes (%) |
---|---|---|---|---|---|---|---|
Eucalyptus spp. 1 | 250 | 57.80 | 4.90 | 37.00 | 0.00 | 0.00 | 0.30 |
Eucalyptus spp. 1 | 300 | 68.20 | 4.20 | 27.30 | 0.00 | 0.00 | 0.30 |
Bambusa sinospinosa McClure2 | 250 | 55.81 | 1.50 | 38.45 | 0.52 | 0.00 | 3.83 |
Bambusa sinospinosa McClure2 | 300 | 69.56 | 1.58 | 23.25 | 0.12 | 0.00 | 5.38 |
Oryza sativa L. shell3 | 250 | 48.05 | 4.63 | 33.40 | 0.70 | 0.00 | 13.18 |
Oryza sativa L. shell3 | 300 | 69.56 | 4.77 | 17.63 | 0.28 | 0.00 | 23.11 |
Torrefacted wood4 | 250 | 51.30 | 5.90 | 40.90 | 0.40 | - | 1.50 |
Liriodendron tulipifera L.5 | 260 | 52.42 | 5.81 | 29.22 | 0.15 | - | - |
Liriodendron tulipifera L.5 | 280 | 54.42 | 5.65 | 27.21 | 0.15 | - | - |
Abies alba6 | 225 | 56.92 | 5.87 | 37.07 | 0.07 | <0.02 | - |
Betula pendula Roth6 | 225 | 56.92 | 5.86 | 37.13 | 0.09 | <0.02 | - |
Coffee residues7 | 503K | 55.10 | 6.36 | 36.29 | 2.25 | Nulo | - |
Coffee residues7 | 563K | 58.82 | 6.01 | 32.53 | 2.64 | Nulo | - |
Coffee residues7 | 623K | 67.03 | 4.95 | 25.28 | 2.74 | Nulo | - |
Pinus sp.8 | 225 | 53.67 | 5.70 | 39.02 | 0.78 | - | 0.87 |
Pinus sp.8 | 250 | 57.14 | 5.27 | 35.83 | 0.84 | - | 0.96 |
Pinus sp.8 | 275 | 64.17 | 4.81 | 29.27 | 0.81 | - | 0.98 |
Source: 1 = Lu et al. (2012); 2 = Chen et al. (2015a), 3 = Chen et al. (2012); 4 = Shankar-Tumuluru (2011); 5 = Kim et al. (2012); 6 = Bach et al. (2014); 7 = Tsai and Liu (2013); 8 = Lee-Carter (2012).
When the process is carried out at high temperatures, the carbon content in the mass increases; meanwhile, hydrogen and oxygen are reduced; in contrast, at low temperatures and reduced residence time, changes are limited. According to Di Blasi (2008), biomass treated at low temperatures retains high percentages of nitrogen, and at high temperatures it is released as a gas with HCN. Some authors assert that the amount of nitrogen in the solid product does not produce negative effects on the environment compared to fossil fuels with values ranging from 1.0 to 1.5 % (Jenkins et al., 1998; Demirbas, 2005).
If the product has a high content of sulfur, it can have negative effects, since it reduces the temperature in the process, it limits the expulsion of gases and when it oxidizes it forms SO3, that causes problems in boilers, to finally transform into sulfuric acid (H2SO4), which is released into the atmosphere. Shankar-Tumuluru et al. (2012) state that, when the biomass is roasted at 300 °C, the sulfur content in the biomass decreases by approximately 50 %.
Conclusions
The torrefaction process improves the chemical properties of plant biomass, as it increases its quality as fuel. The chemical composition is transformed as hemicellulose, cellulose and lignin are thermically degraded, mainly, the first one. The hydroxilous groups are fragmented, which allows to obtain an hydrophobic product. Solid, liquid and gas products are obtained in torrerfaction. The quality of the solid product depends on biomass and the conditions of the process, as temperatura rises and the time of residence, the percentage of fixed carbon and calorific power do as well. The mass and energy yield dependo n the condicions in which torrefaction is made, as both yields decrease when temperature gets higher; conversely, it increases the liquie and gas yield. Therefore, mass and energy yield is an indicator of the efficiency of the torrefaction process.
Acknowledgements
The first author wishes to extend her gratitude to Dr. José Rodolfo Goche Telles and advisers for the support he provided throughout the doctoral studies and to the Consejo Nacional de Ciencia y Tecnología (National Council of Science and Technology) (Conacyt) for the financing granted for the same purpose.
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Received: November 06, 2016; Accepted: December 23, 2016