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).

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}).

^{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.

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.

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).

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.

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 %.

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 %.