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CERNE

Print version ISSN 0104-7760On-line version ISSN 2317-6342

CERNE vol.25 no.1 Lavras Jan./Mar. 2019  Epub May 20, 2019

http://dx.doi.org/10.1590/01047760201925012602 

Articles

THERMAL PROFILE OF WOOD SPECIES FROM THE BRAZILIAN SEMI-ARID REGION SUBMITTED TO PYROLYSIS

Ananias Francisco Dias Júnior1  + 
http://orcid.org/0000-0001-9974-0567

Carlos Rogério Andrade2 
http://orcid.org/0000-0001-7243-3073

Thiago de Paula Protásio3 
http://orcid.org/0000-0002-5560-8350

José Otávio Brito5 
http://orcid.org/0000-0002-0873-0994

Paulo Fernando Trugilho4 
http://orcid.org/0000-0002-6230-5462

Michel Picanço Oliveira1 
http://orcid.org/0000-0001-9241-0194

Graziela Baptista Vidaurre Dambroz1 
http://orcid.org/0000-0001-9285-7105

1 Federal University of Espírito Santo, Jerônimo Monteiro , Espirito Santo, Brazil

2 Federal University of Goias, Jataí, Goías, Brazil

3 Federal Rural University of Amazonia

4 Federal University of Lavras, Lavras, MInas Gerais, Brazil

5 University of Sao Paulo, Piracicaba, São Paulo, Brazil -


ABSTRACT

The objective of this study was to evaluate the thermal decomposition profile of 10 wood species from the semi-arid region of Brazil using thermogravimetric analysis (TGA) to investigate their potential as biomass energy sources. First, flash carbonization was carried out in a muffle furnace, in which wood samples were heated to a maximum temperature of 500°C, and the product yields were determined. The chemical analysis of the wood of each species for the determination of extractive, lignin and ash contents was performed. TGA was performed using sawdust samples heated at 10°C.min−1 up to 625°C under a nitrogen atmosphere at a flow rate of 50 mL.min−1. The thermal decomposition profile was used to evaluate which wood species was more thermally stable. Mimosa tenuiflora and Poincianella pyramidalis woods were the most suitable as biomass energy sources for charcoal production because of their thermal stability and satisfactory carbonization yields. The thermal stability of the 10 wood samples was confirmed by the analysis of the carbonization yields.

Keywords: Thermal effect; Thermal degradation of biomass; Carbonization; Heat action on wood

INTRODUCTION

The semi-arid biome known as Caatinga covers most of the Northeastern region of Brazil. This biome is dominated by one of the few types of vegetation unique to Brazil, harboring several endemic plant and animal species (Silva and Oren, 1997). According to Oliveira et al. (2006) and Paes et al. (2012), the demand for new biomass energy products has increased in the Brazilian semi-arid region, enhancing the interest in native vegetation as a source for firewood and charcoal production. Despite the importance of Caatinga vegetation as an energy source, there is a lack of information about its native species and their energy potential, specifically regarding charcoal production for industrial and domestic uses (Dias Júnior et al., 2018). Some Caatinga species have already been studied for energy, such as Mimosa tenuiflora, Piptadenia stipulacea, Caesalpinia pyramidalis, Aspidosperma pyrifolium among others (Carneiro, et al., 2013; Dias Júnior et al., 2018). However, the knowledge of a greater number of species can facilitate the management practices in the region in order to obtain raw material for various industrial activities. Thus, knowledge of the chemical composition and thermal behavior of wood species sourced from forest management areas is essential for sustainable energy generation in regional enterprises. This effort is driven by the need to restore this biome, threatened with extinction, using vegetation restoration approaches and, at the same time, to meet the demands of wood industries (Kelty, 2006; Amazonas et al., 2018).

Pyrolysis of wood consists in the heating of wood to temperatures of up to 500°C in a non-oxidizing atmosphere and has as one of its products a solid material rich in carbon, called charcoal. Its volatile fraction consists of non-condensable and condensable gases, which compose the pyroligneous liquid (Jesus et al., 2018; Demirbas, 2003; Guillén et al., 2001). Pyrolysis is a complex process that consists of a series of chemical reactions, accompanied by heat and mass transfer (Yang et al., 2007). The pyrolysis of any biomass, including wood, can be considered as the thermal decomposition of the three main biomass components (hemicellulose, cellulose, and lignin) and the loss of water.

Cellulose, a polysaccharide formed exclusively by β-D-anhydroglucopyranose units bound by glycosidic bonds, is the main chemical constituent of wood, accounting for 42% of its dry matter (Sjöström, 1993; Rowell et al., 2005). This polymer decomposes at temperatures between 315 and 400°C (Yang et al., 2007). Hemicelluloses represent, on average, 20 to 30% of wood dry mass (Sjöström, 1993). They are generally amorphous, low molecular weight polymers, consisting of a central chain of repeating units from which side chains branch out (Sjöström, 1993; Rowell et al., 2005). Hemicellulose thermal degradation occurs between 190 and 360°C (Shen et al., 2010). Finally, lignins are three-dimensional, amorphous, branched macromolecules that present phenylpropane as the basic unit bound by ether (C-O-C) and carbon-carbon (C-C) bonds (Rowell et al., 2005). Its thermal decomposition starts at approximately 100°C and continues until about 900°C (Müller-Hagedorn et al., 2003; Yang et al., 2007).

Due to the complexity of the conversion process from wood to energy, it is important to assess of its thermal decomposition, mainly considering that wood is heterogeneous material. For this, it is necessary to use techniques that facilitate the understanding of the process. The thermogravimetric analysis (TGA) accompanies the of the sample as a function of time, it being possible to determine the temperature when the thermal degradation starts and when it is most intense. Also provide the amount of residue is produced at a certain temperature, being of this important tool when you want to select species for energy purposes (Carneiro et al., 2013; Araújo et al., 2016; Azizi et al., 2017; Martinez et al., 2018; Fernandez et al., 2019).

In 2006, the Plants of the Northeast Association (APNE) started field, technological, and social studies in rural settlements of the semi-arid region of Pernambuco, Brazil, to promote the sustainable forest management of the Caatinga. The set of actions, supported by several partnerships, has provided favorable socioeconomic results and the consolidation of methodologies. It is within this context that the economic activities of charcoal production and extraction of pyroligneous liquid take place. An important part of the actions includes laboratory studies to obtain basic results and support field-scale charcoal and pyroligneous liquid production (Dias Júnior et al., 2018).

Information on materials that are potential sources of biomass energy (for combustion and carbonization) is fundamental for the selection of more suitable species, aiming at the optimization and increased efficiency of processes for input generation. Considering the great challenges of managing overexploited biomes, such as the Caatinga, studies on wood properties are relevant for the successful selection of wood species and the elaboration of management plans.

With the aim of obtaining more information on the Caatinga, a biome that is characteristic of Brazil but about which there is a lack of technical and scientific information, we evaluated the thermal decomposition of ten wood species from the semi-arid region of Brazil by thermogravimetric analysis (TGA) in order to obtain scientifically accurate information on their thermal profile.

MATERIAL AND METHODS

Wood samples were supplied by the Plants of the Northeast Association (APNE) from forest management areas located in rural settlements in the state of Pernambuco, Brazil. Ten tree species of the Caatinga were selected, as described in Table 1.

TABLE 1 Caatinga wood species analyzed in the study. 

Species Popular name Family
Anadenanthera colubrina var. cebil (Griseb.) Altschul Angico-de-caroço Fabaceae
Poincianella pyramidalis (Tul.) L.P. Queiroz Catingueira Fabaceae
Cnidoscolus quercifolius Pohl Faveleira Fabaceae
Piptadenia stipulacea (Benth.) Ducke Jurema branca Fabaceae
Mimosa tenuiflora (Willd.) Poir. Jurema preta Fabaceae
Manihot carthaginensis subsp. glaziovii (Müll.Arg.) Maniçoba Euphorbiaceae
Aspidosperma pyrifolium Mart. Pereiro Apocynaceae
Platycyamus regnellii Benth. Pereira brava Fabaceae
Jatropha grossidentata Pax & K.Hoffm Quebra-faca Euphorbiaceae
Commiphora leptophloeos (Mart.) J.B.Gillett Umburana-de-cambão Burseraceae

Non-destructive samples (bark-to-bark) were collected from three specimens of each species at the diameter at breast height. Samples of the tree wood were removed with the help of a tread in five distinct points of the trunk.

Wood samples without the presence of bark were characterized by chemical composition analysis, in which the total extractive content was determined according to the method T-12 05-75 of the Technical Association of the Pulp and Paper Industry (Tappi, 1975) and the Klason lignin content was determined according to TAPPI method 222 05-74 (Tappi, 1974). The ash content was determined by immediate analysis according to the method NBR 8112 of the Brazilian Association of Technical Standards (Abnt 1986), and the higher calorific value was determined in accordance with NBR 8633 (ABNT, 1984).

For pyrolysis, 5 ± 1 g of wood samples were chipped and milled in a Wiley-type mill using a 40-mesh sieve. Analysis was conducted on a Gray-King pyrolysis apparatus consisting of a glass retort inserted in an electric heating muffle furnace under inert atmosphere saturated with nitrogen gas at 500°C. Thermocouples were placed in the reaction zone of the glass retort that contained the wood sample for temperature control. A vertical U-tube immersed in crushed ice was used to collect the condensable gases. The procedures followed the recommendations of Dias Júnior et al. (2018).

After pyrolysis, the mass of charcoal contained in the glass tube and the mass of pyroligneous liquid deposited in the U-tube were measured. The carbonization yields of charcoal, pyroligneous liquid, and non-condensable gases were then determined. Details of the apparatus are shown in Figure 1.

FIGURE 1 Gray-King pyrolysis apparatus. A = muffle furnace; B = thermocouple; C = opening for nitrogen gas insertion; D = ice container; E = U-tube for collection of pyroligneous liquid. Source: Dias Júnior et al. (2018). 

TGA was performed on a Shimadzu TGA-60 under a nitrogen gas atmosphere at a constant flow of 50 mL min−1 and a heating rate of 10°C min−1. For this analysis, approximately 4 mg of wood sawdust with a particle size of 200-270 mesh was used. The thermograms were obtained over a temperature range of 25°C (room temperature) to 625°C. This temperature range was defined based on what is practiced in Brazil the majority of systems of production of charcoal, temperatures between 400 and 600°C (Carneiro et al., 2013; Pereira et al., 2013). The loss of mass for the main temperature ranges was obtained along the decomposition in relation to the initial mass analyzed.

Data were submitted to Shapiro-Wilk test for normality and Levene’s test of homogeneity of variances. For analysis of variance (ANAVA), a completely randomized design with five replications per wood species was used, and the Scott Knott test at 95% probability was employed for multiple comparison of means.

RESULTS

Table 2 shows the characteristics and properties of the studied wood species. Anadenanthera colubrina, Mimosa tenuiflora, and Comminphora leptophloeos presented the highest total extractive content​.

TABLE 2 Basic density and chemical composition of Eucalyptus wood. 

Species TEX (wt.%) LIG (wt.%) ASH (wt.%) HHV (MJ.kg −1 ) CHY (wt.%) PAY (wt.%) NCGY (wt.%)
Anadenanthera colubrine 20.68a 24.89c 1.78c 18.70c 26.81c 26.76d 46.42b
Standard mean error (±) 0.52 1.06 0.11 0.11 2.38 1.66 0.98
Poincianella pyramidalis 15.15c 25.19c 2.82a 17.96d 30.01b 36.39a 33.60e
Standard mean error (±) 0.71 0.68 0.29 0.08 2.20 1.65 1.01
Cnidoscolus quercifolius 17.25b 27.53b 0.80d 20.46a 23.56d 34.77b 41.67c
Standard mean error (±) 0.95 0.18 0.02 0.06 1.98 1.95 1.55
Piptadenia stipulacea 13.60c 24.91c 0.45e 19.28c 28.98b 37.94a 33.08e
Standard mean error (±) 0.49 0.70 0.01 0.06 3.33 2.22 1.49
Mimosa tenuiflora 23.31a 32.80a 0.92d 20.56a 32.72a 30.77c 36.52d
Standard mean error (±) 0.33 0.60 0.05 0.08 3.42 2.35 1.78
Manihot carthaginensis 14.64c 23.68c 1.79c 18.35d 31.88a 32.44c 35.68d
Standard mean error (±) 2.04 0.81 0.11 0.11 1.91 1.28 2.01
Aspidosperma pyrifolium 17.69b 25.31c 0.92d 19.62b 23.27d 24.22d 52.52a
Standard mean error (±) 0.37 0.28 0.02 0.05 5.33 3.32 0.99
Platycyamus regnellii 12.59c 24.86c 0.32b 19.31c 32.52a 34.87b 32.61e
Standard mean error (±) 0.55 0.26 0.02 0.05 2.37 1.10 1.36
Jatropha grossidentata 16.16b 27.09b 1.05d 19.89b 29.39b 34.34b 36.27d
Standard mean error (±) 0.71 1.49 0.16 0.05 4.21 3.20 1.47
Commiphora leptophloeos 21.25a 27.04b 0.64e 19.20c 28.81b 33.09b 38.10c
Standard mean error (±) 0.51 0.24 0.01 0.06 4.20 3.49 1.36

TEX = total extractive content; LIG = lignin content; ASH = ash content; HHV = higher heating value; CHY=charcoal yield; PAY=pyroligneous liquid yield; NCGY = non-condensable gases yield. Means followed by the same letter in a column do not differ from each other by the Scott-Knott test at 95% probability. Source: Dias Júnior et al. (2018).

It is observed values higher than 20% of extractive content in the wood of the species studied. In contrast, for the lignin content, characteristic associated with the yield in charcoal, it is observed the greatest value for Mimosa tenuiflora. Woods of the Cnidoscolus quercifolius, Jatropha grossidentata and Comminphora leptophloeos species presented values close to 27% and the other species had mean values close to 25% for this variable. Ash content, the Piptadenia stipulacea and Comminphora leptophloeos species presented the lowest values, which the higher mean value detected was for the Poincianella pyramidalis wood (Figure 2). The species from the Brazilian semi-arid region (Poincianella pyramidalis and Manihot carthaginensis) with higher ash content and lower lignin content showed lower higher heating value. There was a highly significant correlation between higher heating value, Klason lignin and ash contents. The proportion explained of the HHV by Klason lignin and ash content was 81%.

FIGURE 2 Calorific values of different wood species from the Brazilian semi-arid region. Where: the circles indicate the groups formed by the Scott-Knott test for the higher heating value (HHV). 

It is observed in Table 2 that woods from the Mimosa tenuiflora, Manihot carthaginensis and Platycyamus regnelli species obtained the highest charcoal yields. For yield in pyroligneous liquid, Poincianella pyramidalis and Piptadenia stipulacea presented themselves as potential species for obtaining this product. In consequence of these results, these species, together with Platycyamus regnelli, presented the lowest mean yield values in non-condensing gases.

Thermogravimetric analysis

Figure 3 shows the thermograms of the 10 analyzed wood species, in which TG curves are shown as blue lines that represent mass loss as a function of temperature and DTG curves are shown as red lines.

FIGURE 3 TG and DTG curves of Caatinga wood species. 

Table 3 presents the mass loss by thermal decomposition of each wood species according to temperature ranges. An average mass loss of 8.57 wt.% was observed in the first temperature range. This phase corresponds to the drying of wood, in which the loss of water molecules linked to the cell wall occurs through the absorption of heat, characterizing an endothermic process (Pereira et al., 2013; Martinez et al., 2018). During the pyrolysis of the wood occur two distinct stages of mass loss (Carneiro et al., 2013; Pereira et al., 2013; Protásio et al., 2014a), that is, evaporation of the water (drying), and in second stage, thermal degradation of the carbohydrates (between 200°C to 400°C).

TABLE 3 Basic density and chemical composition of Eucalyptus wood. 

Wood species Mass loss (wt.%) Residual mass (wt.%)
25-100 (°C) 100-200 (°C) 200-300 (°C) 300-400 (°C) 400-500 (°C) 500-600 (°C)
Anadenanthera colubrina 9.23 1.07 18.62 40.46 5.63 3.56 21.43
Poincianella pyramidalis 8.82 1.85 18.85 37.17 6.57 4.61 22.13
Cnidoscolus quercifolius 9.26 1.08 21.71 32.38 10.25 7.92 17.40
Piptadenia stipulacea 6.56 0.72 20.72 42.39 6.89 6.23 16.49
Mimosa tenuiflora 9.55 1.21 17.32 35.13 6.27 3.62 26.90
Manihot carthaginensis 9.46 1.43 16.48 35.63 5.62 2.93 28.45
Aspidosperma pyrifolium 8.51 0.86 19.87 38.07 4.98 2.66 25.05
Platycyamus regnellii 7.63 0.49 19.95 41.68 5.91 5.54 18.80
Jatropha grossidentata 8.05 2.01 20.85 36.47 6.84 4.86 20.92
Commiphora leptophloeos 8.60 0.11 20.09 40.03 3.93 2.12 25.12
Average 8.57 1.08 19.45 37.94 6.29 4.41 22.27

DISCUSSION

It is possible to observe that the Anadenanthera colubrina, Mimosa tenuiflora and Comminphora leptophloeos species presented the highest values for total extractive content in the wood (>20%). These values can be considered quite high when compared, for example, with the average value of 5% for woods from four Eucalyptus clones analyzed by Santos et al. (2011).

The highest value of lignin content (Table 2), commonly associated with charcoal yield, was observed for Mimosa tenuiflora, and Cnidoscolus quercifolius, Jatropha grossidentata, and Commiphora leptophloeos had the second highest lignin content. However, lignin content alone cannot determine the potentiality of a tree species for charcoal production (Wang et al., 2017); it is also necessary to analyze the density, dry mass yield, and chemical characteristics of the wood.

High lignin contents tend to result in high charcoal yields (Protásio et al., 2012; Wang et al., 2017). Vale et al. (2010), in an experiment with Brazilian Cerrado species, found lignin contents ​​varying from 25.16 to 32.31wt.%. Costa et al. (2014), in a study evaluating five Cerrado wood species, reported lignin contents of 19.88 to 26.87wt.%. Protásio et al. (2013) found lignin values varying ​​between 28.01 and 35.12wt.% for eucalyptus specimens that had similar characteristics to the species analyzed in the present study. On the basis of literature results, we highlight the potential of Mimosa tenuiflora for charcoal production.

The species Piptadenia stipulacea and Commiphora leptophloeos presented the lowest ash contents, whereas Poincianella pyramidalis had the highest percentage of ash (Table 2 and Figure 2). Paes et al. (2013) evaluated three wood species from the Caatinga and reported ash contents of less than 2.10wt.%. A high ash content can result in equipment damage and increases the cleaning frequency required for systems that use wood as an energy source. The ash contents observed in the present study (Table 2) were low and did not compromise the energy potential of the wood. Comparison of the ash content of the studied wood species with other fuels used in Brazil for power generation evidences the advantages of Caatinga tree species. Ash contents of 5wt.% to 35wt.% have been reported for coal, a non-renewable fuel widely used for electricity generation and reduction of iron ore (Ward et al., 2008; Wang et al., 2011; Wang et al., 2012; Magdziarz and Wilk, 2013). Sugarcane bagasse, an important energy source for Brazilian industries, can have a mineral content of up to 21wt.% (Scatolino et al., 2018), which is considerably higher than the values observed in this study for Caatinga wood.

According to Santos et al. (2011) and Telmo and Lousada (2011a; 2011b), the calorific value is an important property of wood fuels, as it expresses the amount of energy released as heat when the material undergoes complete combustion. The calorific value of wood species is influenced mainly by their chemical composition, particularly in relation to extractives, lignin and ash contents (Demirbas, 2001; Telmo and Lousada, 2011a; Telmo and Lousada, 2011b). This fact was in satisfactory agreement with the high higher calorific value, high lignin content and lower ash content of Mimosa tenuiflora and Cnidoscolus quercifolius woods.

The chemical composition of extractives, as well as their thermal stability, can affect charcoal yield (Protásio et al., 2012). A high ash content is not desirable, as it results in charcoal of high mineral content and may compromise the calorific value of this bio-reducer (Soares et al., 2014). Santos et al. (2012) obtained values ​​of 18.59 MJ kg−1 for catingueira (Poincianella pyramidalis) wood, a result similar to that found in this study. Considering that eucalyptus wood, which is widely used for power generation, has an average higher heating value of 19 MJ kg−1 (Couto et al., 2013; Protásio et al., 2013), the results of this study demonstrate the suitability of native species of the Brazilian semi-arid region of the state of Pernambuco as fuels for heat generation.

With regard to carbonization yields, Mimosa tenuiflora, Manihot carthaginensis, and Platycyamus regnellii woods achieved the best results (Table 2). Poincianella pyramidalis and Piptadenia stipulacea had the best yields in pyroligneous liquid and were thus considered potential species for extraction of this product. Overall, these results show that Poincianella pyramidalis, Piptadenia stipulacea, Mimosa tenuiflora, Manihot carthaginensis, and Platycyamus regnellii had the lowest mean yield ​​in non-condensable gases. These results are in agreement with those obtained by Oliveira et al. (2006) for Mimosa tenuiflora, Poincianella pyramidalis, Manihot carthaginensis, and Platycyamus regnellii but are lower than the yields reported by the authors for Mimosa tenuiflora wood, which ranged from 37.82 to 41.06wt.% for charcoal and from 30.56 to 34.31wt.% for pyroligneous liquid.

Pereira et al. (2013) stated that the lignin syringyl/guaiacyl (S/G) ratio should be low for a high charcoal yield, as it implies a more condensed lignin structure. This may explain why woods of high lignin content do not always result in high charcoal yields. A high concentration of extractives of low thermal resistance can also lead to low charcoal yields (Poletto et al., 2012; Poletto, 2016).

In carbonization processes aimed at charcoal production, companies expect to obtain the highest charcoal yields, because, in most cases, gases are not recovered for other uses and account as losses. The yields in charcoal, pyroligneous liquid, and non-condensable gases are affected by carbonization conditions and chemical and physical characteristics of the wood (Oliveira et al., 2006; Vale et al., 2010; Protásio et al., 2014b).

The wood species had a similar mass loss profile in the TG curves (Figure 3). Two main thermal events can be observed: a drying phase (elimination of moisture) and a thermal degradation phase. In stage I, occurs evaporation of the water (drying), and in the second stage, between 200°C to 400°C, the mass decreases rapidly due mainly to the thermal degradation of the carbohydrates (holocellulose). From 400°C the mass decreases less intensely due to the decomposition of the lignin macromolecule and carbonization products (Yang et al., 2007; Protásio et al., 2014a). According to Yang et al. (2007) and Pereira et al. (2013), no specific lignin degradation range is detected in TGA analysis, due the wide temperature ranges of the thermal decomposition of lignin. Thus, there is loss of mass of the lignin macromolecule in the other stages of thermal decomposition.

These observations are consistent with the results obtained by Elyounssi et al. (2012), Carneiro et al. (2013) and Protásio et al. (2014a). With respect to DTG curves, the thermograms show peaks at approximately 80°C, representing the drying phase. However, in spite of similarities, differences in the temperatures of maximum degradation are evident, mainly related to the degradation of hemicellulose and cellulose. With the exception of the DTG curves for Cnidoscolus quercifolius and Manihot carthaginensis, it is possible to observe an attenuation in degradation rate between 250°C. The maximum rate of decomposition was observed in 350°C and correspond the thermal degradation of the cellulose. The peak of the DTG curve indicates when the cellulose degradation rate starts to decrease. At 270-300°C, the degradation rate starts to increase again, which is indicated in the graphs by an arrow. The first temperature range corresponds to the final phase of hemicellulose degradation, and the second temperature range is attributed to cellulose degradation. For all species, stabilization of the degradation rate occurs at approximately 400°C, which corresponds to the end of cellulose degradation, according to Yang et al. (2007) and Azizi et al. (2017).

The studied wood species had an average mass loss of 1.08% between 100 and 200°C, a very small mass variation. This temperature interval is known as the range of thermal stability of wood, being limited by the initial temperature of thermal decomposition of its main components (Raad et al., 2006; Yang et al., 2007). Carneiro et al. (2013) reported similar values in this temperature range to those of the present study for the average mass loss of wood species from the Caatinga of Rio Grande do Norte, Brazil. From 200 to 300°C, the average mass loss was 19.4wt.%. This phase probably comprises the thermal decomposition of hemicelluloses. Santos et al. (2012) observed mass losses of 16 to 19wt.%, between 200 and 300°C, which is in agreement with the values obtained in the present study. The highest average mass losses occurred in the ranges of 300-400°C and 400-500°C, corresponding to a variation of 37.94 and 6.29% in wood weight, respectively, totaling 44.23%. This finding can be explained by the fact that the peak mass loss for cellulose occurs at a higher temperature than that of hemicelluloses, as cellulose requires a great amount of energy for chain depolymerization and degradation of monomers (Liao, 2003; Fernandez et al., 2019).

Cnidoscolus quercifolius and Mimosa tenuiflora suffered a less pronounced mass loss in this temperature range. This interesting finding may be related to their chemical composition, as these two species had the highest lignin contents among the studied wood species. On the basis of these results, Mimosa tenuiflora can be regarded as a species of great thermal resistance and high charcoal yield (Table 2). These properties can be related to the resistance of lignin to thermal effects, which enhances the transformation of wood into charcoal.

Residual mass values varied from 16.49wt.% for Piptadenia stipulacea to 28.45wt.% for Manihot carthaginensis, indicating that the studied species had different resistances to thermal degradation. Some of the species evaluated in this study were more thermally resistant than the 7-year old eucalyptus specimens evaluated by Santos et al. (2012) by TGA. The authors reported an average residual mass of 14.75wt.%. Interestingly, the species Piptadenia stipulacea, which had the lowest residual mass, had the lowest ash content.

Regarding the potential for charcoal production, the results of the present study demonstrate the low contribution of cellulose to charcoal yield. Moreover, other studies have shown that the residual mass of cellulose ranges from 5 to 10wt.% at 450°C (Yang et al., 2007; Shen and Bridgwater, 2010; Fernandez et al., 2019). Lignin does not have a specific degradation peak, because its thermal decomposition occurs over a wide range of temperatures, that is, its degradation occurs at a slow and steady rate. When considering the use of these species for charcoal production, attention must be paid to the chemical composition of wood (lignin content) as well as to variables of the production process. It is important to highlight that 450°C is the maximum temperature recommended for charcoal production, as it provides higher yields, according to Pereira et al. (2013). The authors explained that lignin degradation is more intense at temperatures above 450°C.

According to the results presented in Figure 3 and Table 3, the greatest mass loss for all species occurred between 300 and 400°C. Yang et al. (2007) reported that this temperature range provides the highest thermal degradation of cellulose. The energy absorbed in the first phases of the process is related to evaporation of water from the material. The reaction becomes exothermic at approximately 300°C (Pereira et al., (2013), coinciding with the maximum mass loss peak (Table 3), when the greatest loss of volatile compounds occurs.

The biomass components directly influence its pyrolysis behavior, which was elucidated by TGA. This fact was demonstrated by Raad et al. (2006) and Yang et al. (2007), two studies that report the behavior of the main components of wood separately subjected to pyrolysis. Only in this manner can overlapped similarities of different constituents be distinguished at the same time (Haykiri-Acma et al., 2010).

The second thermal event is attributed to the pyrolysis of the main wood constituents (cellulose, hemicelluloses, and lignins). Each component has a different behavior when exposed to heat. According to Yang et al. (2007), hemicelluloses begin to decompose at 220-315°C. According to these authors, cellulose undergoes pyrolysis at a higher temperature range, between 315 and 400°C. At temperatures above 400°C, most cellulose has already been degraded. Lignin is the most difficult component to decompose, as its degradation occurs slowly from the beginning of carbonization of other constituents up to 900°C (Yang et al., 2007). Above 400°C, the thermal degradation of wood decreases considerably, corresponding to lignin degradation. At this temperature, cellulose and hemicelluloses have already been thermally degraded.

CONCLUSIONS

Mimosa tenuiflora and Poincianella pyramidalis were the most suitable wood species for sustainable charcoal production.

The thermal decomposition profiles of wood samples obtained by TGA can aid in the selection of species of greater energy potential for charcoal production. TGA allows the control of carbonization processes on an experimental scale and the determination of the relationship between temperature and degradation of wood components. The thermal stability behavior of 10 wood species was confirmed by analysis of charcoal yield.

It is recommended specific studies that can relate the thermal decomposition in different temperature ranges and the respective gases emitted (TGA-GC/MS/PY) to conclusions about environmental sustainability.

ACKNOWLEDGEMENTS

The authors would like to thank the Plants of the Northeast Association (APNE) for providing the wood species used in this study. This research was financially supported by CNPq (National Council for Scientific and Technological Development, Brazil) and CAPES (Higher Education Personnel Improvement Coordination, Brazil).

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HIGHLIGHTS

We analyzed native and endangered species of the Brazilian semi-arid region.

The thermal profile of the species was questioned for energy generation.

Mimosa tenuiflora and Poincianella pyramidalis were the most suitable wood species for sustainable charcoal production.

The energy characteristics will allow the efficient management of the species in mixed plantations for this purpose.

Received: October 07, 2018; Accepted: March 12, 2019

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