Keywords

1 Introduction

Using wood wastes from SFMP as fuel is a sustainable alternative for diversifying the national and even global energy matrix. In Amazonia, these lignocellulosic wastes are formed mainly by branches, buttresses, and stumps of felled trees, usually of large dimensions. Such wastes can represent up to two-thirds of the dry mass of the tree [29]. Therefore, they are highly available and economically viable. In addition, waste consumption is heading towards maximum efficiency in using natural resources.

Some studies have discussed interesting aspects of the environmental impact of extracting wastes from SFMP worldwide [1, 25] and found no negative effect on tree growth. On the contrary, they indicated positive effects of stump extraction on forest regeneration [25, 28]. Based on published data worldwide, the literature indicates that waste extraction increases nutrient export [1]. The export effect is complex because it is affected by the harvest cycle and specific characteristics of the logged forest ecosystem, such as the input of nutrients by weathering and output by leaching [32, 44]. Compared to fossil fuels, burning wood and charcoal emits low amounts of greenhouse gases [11]. The carbon emitted will be stored again during forest regeneration [9] promoted by techniques associated with SFMP [28].

Carbonization is an important conversion route for using by-products from forest management. In addition to its wide domestic use, charcoal is used in blast furnaces of steel mills as a biothermoreducer for pig iron, in the production of the called “green steel.” The Carajás region, located in eastern Amazonia, has the largest pig iron production in northern Brazil [41]. Forest wastes partially supply the region's firewood demand for charcoal production [24].

The great challenge for the energy use of wastes from SFMP refers to the high variability of their characteristics. Lima et al. [19] found variation in the wood basic density from 0.525 to 0.895 g/cm3. In addition, the wood presents high moisture, representing up to 45% of the wet mass without bark [24]. Basic density and moisture influence the productivity of the kilns [4]. This variability is increased by the lack of carbonization control [39] commonly carried out in brick kilns of low technologic level known as “hot tail.” The carbonization in these handmade kilns is controlled empirically, without temperature control or waste separation into quality classes.

Studies have been carried out to understand the variation in the quality of charcoal derived from waste [19, 23]. In general, the separation of wastes into groups of species according to the different wood characteristics significantly improves the productivity of the kilns, increases the yield, and reduces the variation in the charcoal quality compared to the conventional method [5]. The next steps involve conducting research that will contribute to more effective control of carbonization, increase its yield, improve the quality of charcoal derived from forest wastes in Amazonia, and mitigate gas emissions during carbonization. This chapter aims to describe the advances obtained so far related to the energy use of forest wastes arising from SFMP in Amazonia.

2 Background of Studies with Wastes from SFMP

The project entitled “Valuation of wood wastes from sustainable forest management for bioenergy in Legal Amazonia” (Public Selection Notice for Scientific and Technological Research—Edition 2018) was developed by the Federal Rural University of Amazonia (website: https://novo.ufra.edu.br/) and financed directly by the Banco da Amazônia. Several advances were achieved in understanding the characteristics of primary wastes from SFMP in Amazonia. It was possible to understand the variability of the main physical, chemical, and energy properties of the forest biomass used for bioenergy in Amazonia [19, 23]. Consequently, the data collected will allow decision-making in industrial units, such as steel mills and ceramics.

The various studies analyzed the wood characteristics of species from 22 genera. The waste characteristics that varied the most were basic density (0.525–0.895 g/cm3), ash content (0.3–2.5%), and total extractives (1.7–17.9%). However, the elemental carbon content (49.18–52.16%), total lignin (30.2–38.1%), fixed carbon (16.5–22.0%), volatile matter (76.7–82. 8%), and higher heating value (19.4–20.4 MJ/kg) were characteristics that showed the smallest variations between the studied species [19]. Within the same species, wood properties have smaller ranges of variation. In the study of Lima et al. [19], Manilkara elata and Dinizia excelsa had a basic density of 0.900 and 0.890 g/cm3, respectively. On the other hand, Lima et al. [24] reported values of 0.872 and 0.927 g/cm3 of basic density for the same species. Between the two studies, there was a difference of about 3%. This variation can be considered low since this residual biomass has no genetic, age, or site control. These findings indicate the reliability and possibility of data extrapolation to represent the overall quality of waste from these species.

Based on the physical, chemical, and energy characterization of the wastes, it was demonstrated that the wood of the species has the quality to generate electric energy in boilers [19] and in charcoal production for domestic and industrial use. Lima et al. [19] verified a species effect on the technological properties of residual woods, making it necessary to qualify them for energy production. The use of wood wastes with up to 30% moisture (wet basis) has proven to be viable in modern cogeneration systems.

The species studied, such as D. excelsa, M. elata, P. altissium, and G. glabra, showed better energy properties than species and clones traditionally used in energy forests. D. excelsa had the highest mass of CO2eq fixed in 1 m3 of wood wastes (1687 kg), meaning that the use of 1 m3 of wood wastes of this species would mitigate the emission of 1687 kg of CO2eq. Finally, the wastes of this species showed the best properties for bioenergy.

The second step of the project proposed waste segregation for charcoal production. Principal component analysis (PCA), a multivariate statistical technique, enabled forming species groups with similar properties [19, 34].

Correlations of the physical and energetic properties of the wood with colorimetry parameters were found [20]. Near-infrared (NIR) spectroscopy was also evaluated with the same objective [21]. Both methods were effective in separating waste.

Lima et al. [20] reported that woods with greater red pigmentation (M. elata and D. excelsa had higher energy density. All groups formed by PCA can be recommended for bioenergy; however, the group formed by the species M. elata and D. excelsa, with a purplish-brown color, is the most promising.

In general, waste segregation increases carbonization efficiency, mainly due to the effect of wood basic density. High-density waste has less moisture, allowing to add a greater amount of wood mass in the kiln, increasing production and productivity per carbonization cycle. Figure 1 shows the productivity parameters of segregated and non-segregated woods in the different studies carried out during the execution of the project.

Fig. 1
A chart of valuation of wood wastes from sustainable forest management for bioenergy in the legal Amazonia. It has tables for non-segregated wood traditional carbonization, segregated wood high-medium basic density, and segregated wood low basic density.

(Source The authors 2023)

Summary of results related to kiln productivity during project execution

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Lima et al. [22] reported a species effect on the physical, chemical, and energy properties of charcoal derived from SFMP wastes. The results showed that the content of extractives in the wood positively influenced the gravimetric yields of charcoal, the carbonization mass balance, the heating value, and the energy performance index of charcoal. Apparent relative density, gravimetric yields of charcoal, gravimetric yields of non-condensable gases, ash content, fixed carbon yield, energy density, energy yield, and retained carbon were the properties with a wide variation between species. The PCA separated charcoal samples from SFMP wastes of 48 species into five distinct groups. These groups can be used in carbonization in brick kilns to supply the Carajás Steel Pole and cogeneration systems in remote Amazonian communities. Finally, the D. excelsa species wastes showed the best charcoal properties and processes.

Table 1 presents the project outputs to date. Seven articles were published aiming to understand the waste properties and improve charcoal production through its segregation.

Table 1 Products of the project financed directly by the Banco da Amazônia “Valuation of wood wastes from sustainable forest management for bioenergy in Legal Amazonia”
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Currently, the project “Generation of bioenergy from wastes from sustainable forest management: decentralization of the energy matrix and socio-environmental impacts in Amazonia” (CNPq/MCTI/FNDCT No. 59/2022) is in effect. The project aims to clarify the combustibility processes of burning co-products from SFMP in Amazonia and mitigate the emission of greenhouse gases during the wood carbonization by implementing a furnace-kiln system, which allows the burning of gases formed during the carbonization.

3 Traditional Production of Charcoal from SFMP Wastes in Amazonia

The model of charcoal production in Brazilian Amazonia mainly includes rudimentary brick kilns of the hot tail type, which are semi-spherical kilns built by combining clay and sandy textured soil to avoid cracking. These relatively simple, handmade kilns present low conversion efficiency and are empirically controlled by experienced workers based on the color and amount of smoke expelled, in addition to touch. Consequently, these brick kilns have low productivity and gravimetric yields of charcoal between 15 and 25% on a wet basis [33, 34]. Typically, brick kilns do not include gas cleaning systems and may release unburned by-products into the environment [42]. A typical hot tail kiln is presented in Fig. 2.

Fig. 2
A photograph of a dome shaped brick kiln. It is covered in mud on the outside. There is a tag on it that reads 62.

(Source The authors 2023)

Rudimentary brick kiln commonly used in charcoal production with SFMP wastes in Brazilian Amazonia

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The study conducted by Barros et al. [5] describes the carbonization model with wastes in Amazonia. The authors reported kilns with a base diameter of 3.20 m and a height of 2.5 m, with six openings at the base, six at the top, and one chimney, with a height of 70 cm for gas exhaustion. The kilns have a capacity of 10 m3 of wood. The carbonization cycle normally ranges from 12 to 13 days, including filling, ignition, carbonization, cooling, and kiln discharge steps. The cooling stage refers to the application of water mixed with clay on the kiln wall (Fig. 3a), with the aid of a tractor (Fig. 3b), intending to seal the kilns and reduce their internal temperatures to about 50 ℃, which reduces the chances of charcoal reignition after opening the kiln.

Fig. 3
Two photos of cooling the kilns, a and b. A contains a man spraying water on the surface of the kiln. B has a tractor for the process.

(Source The authors 2023)

Cooling of the kilns through the application of water and clay (a) with the aid of a tractor, Massey Ferguson 275 model (b)

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Allied to the low technological apparatus used in the charcoal production model, the raw material used is very heterogeneous (Fig. 4). Typical wood wastes present low energy density, high moisture content, and variable diameters, hindering their use as energy sources, mainly for charcoal production with suitable quality for domestic and industrial applications. The empiricism of the activity is also related to raw material control, as it is carbonized without diametric standardization and moisture control. It is known that the higher the water content in wood, the lower its heating value, and the combustion will occur in an inadequate regime [12]. In addition, the charcoal yield reduces since the energy expenditure for the process’s first stage, drying, is likely to be high. Canal et al. [7] demonstrated that the emissions of condensable gases (pyroligneous liquid) and non-condensable gases (CO2, CO, CH4, and H2) increased, and the gravimetric yield of charcoal decreased with the increase in wood moisture during the carbonization processes.

Fig. 4
A photograph of S F M P wastes in a storage yard. The logging waste is kept on the left side.

(Source The authors 2023)

SFMP wastes in the storage yard of a charcoal production unit in Brazilian Amazonia

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Currently, there is no control over the moisture content of forest wastes for charcoal production in Legal Amazonia and, consequently, there is the excessive expenditure of energy, the appearance of cracks and internal fissures in the charcoal due to the sudden release of water in the form of steam and, consequent decrease in the mechanical properties of charcoal. Moreover, countless forest species are carbonized concomitantly; therefore, the wood's qualitative and quantitative aspects are not under control. Finally, it is necessary to improve carbonization by, for example, adjusting the process control ranges as a function of time and temperature, considering the thermal degradation of wood to maximize charcoal yield [8, 10, 30].

The effect of raw material heterogeneity and empirically controlled carbonization on charcoal production can be evidenced by data from a Brazilian Amazonia production unit (Table 2). SFMP wastes are carbonized in this production unit, especially branches. Table 2 shows the mass and volume balances obtained in twelve brick kilns in 2017, as well as the descriptive statistics associated with the analyzed variables.

Table 2 Mass and volume balances per carbonization cycle of SFMP wastes in Brazilian Amazonia
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The average values of the wet mass of wastes in the kiln (MW), estimated dry mass of wastes in the kiln (MSM), and estimated volume of wastes in the kiln (VW) obtained by the traditional carbonization model were 8.456 ± 0.371 t, 6.577 ± 0.295 t, and 17.16 ± 0.86 st, respectively. The evaluated kilns produce, on average, 1.762 ± 0.126 t of charcoal per cycle. The average volume of charcoal in the hot tail kilns was 6.01 ± 0.36 m3.

The gravimetric and volumetric surveys performed were based on the operational conditions of the charcoal production unit and not necessarily on experimental conditions, which, in turn, are controlled. In this way, the factors inherent to the raw material, labor, and the control of carbonization can considerably affect the production and productivity of hot tail kilns. This effect can be verified by analyzing the mass of semi-carbonized pieces (MSP) produced per cycle, in which kiln 4 did not produce these by-products. In contrast, kiln 6 produced a high amount of semi-carbonized pieces (1.240 t) and, consequently, a low gravimetric yield of charcoal. Therefore, they cannot be considered wood or charcoal but a by-product of carbonization.

The following variations were reported for the gravimetric yield of charcoal—GYC (%, on a wet and dry mass basis) and gravimetric yield of semi-carbonized pieces—GYS (%, on a wet and dry mass basis): 16.13–24.20% (GYC, wet mass basis), 21.02–31.12% (GYC, dry mass basis), 0.00–13.70% (GYS, wet mass basis), and 0.00–17.85% (GYS, dry mass basis) (Table 3).

Table 3 Gravimetric yields of charcoal and semi-carbonized pieces per brick kiln in a charcoal production unit in Brazilian Amazonia
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The production of semi-carbonized pieces is undesirable, as it decreases the kilns’ production and productivity (Fig. 5a) and the gravimetric conversion coefficient (Fig. 5b). Kiln 6 presented GYC, wet mass basis, of 16.13%. Kilns 4 and 12, on the other hand, with lower production of semi-carbonized pieces, stood out in converting wood into charcoal. As already mentioned, the generation of semi-carbonized pieces is related to the raw material and the control of carbonization.

Fig. 5
A set of 2 graphs, a and b. A plots G Y C versus G Y S in percentage. B plots V c h versus G Y S. They represent data of 12 kilns. For a, G Y C is the highest for kiln 12 and G Y S for 6. For b, V c h is the highest for kiln 10 and G Y S for 6.

(Source The authors 2017)

Relationship between the gravimetric yield (GYC) (a) and volume of charcoal (b) with the gravimetric yield of semi-carbonized pieces (GYS) in the brick kilns of a charcoal production unit in Brazilian Amazonia

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The kilns evaluated showed, on average, a GYC of 20.87 ± 1.35% (wet mass basis) and 26.84 ± 1.73% (dry mass basis). The GYS was 5.00 (based on wet mass) and 6.44% (based on dry mass), with high variation between kilns. These results indicate that the carbonization was performed differently in the kilns, probably due to the empirical character associated with charcoal production (lack of control based on specific temperature ranges and time).

Figure 6 shows the gravimetric yields of products and by-products generated in a charcoal production unit concerning the initial total wet mass (110.74 t). 67.51% of forest wastes are transformed into gases during carbonization, 19.09% result in charcoal, 4.59% in semi-carbonized pieces, and 8.81% are by-products generated in the sectioning wastes (sawdust, wood fragments, and bark).

Fig. 6
An exploded pie diagram of gravimetric yields of products and by-products. The data is as follows. Gases, 67.51%. Charcoal, 19.09%. Sawdust, 7.15%. Semi-carbonized wood, 4.59%. Wood fragments, 0.84%. Bark, 0.82%.

(Source The authors 2017)

Gravimetric yields of products and by-products generated in charcoal production, considering the initial total mass with 22.22% of moisture

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The charcoal bulk density per kiln (CBD) evaluated in 2017 and the average value obtained for this initial prospecting can be seen in Fig. 7. On average, the CBD of waste was 0.293 t/m3. The variation between the kilns was low; consequently, the minimum (0.270 t/m3) and maximum (0.317 t/m3) values were similar.

Fig. 7
A bar graph plots C B D versus kiln. The data is as follows. Kiln 1, 0.276. 2, 0.270, 3, 0.309. 4, 0.293. 5, 0.296. 6, 0.280. 7, 0.293. 8, 0.284. 9, 0.298. 10, 0.286. 11, 0.317. 12, 0.312. Average, 0.293.

(Source The authors 2017)

Charcoal bulk density (CBD) per kiln evaluated in the production unit in Brazilian Amazonia

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In summary, using different species with different physicochemical properties without prior separation for carbonization, combined with the diametric differences of the waste logs and empirical control of carbonization, help explain the previously reported results. Thus, alternatives are needed to maximize kiln productivity based on raw material and process control.

4 Characteristics of Wood Wastes from SFMP

According to previously published studies, the physical, chemical, and energy properties of SMFP wood wastes corroborate wide heterogeneity [19]. In addition, it is a heterogeneous biomass concerning diameter classes, formats, and origins (root, stem, and branches). A diametric variation of 0.123–0.760 m was reported by Barros et al. [5] for carbonized wastes in brick kilns in Amazonia. The wide variation in wood properties negatively influences charcoal production [23]. In addition, the need to classify woods based on diameter is highlighted since this property strongly influences the thermal profile, heat transfer, and carbonization rate [14, 17].

Regarding the physical properties of wastes, the basic density and moisture have already been studied. Pereira et al. [34] reported a variation of 0.221 (Sterculia pruriens) at 0.867 g/cm3 (Pseudopiptadenia psilostachya), evaluating wood wastes of 18 tropical species from Brazilian Amazonia. Values ranging from 0.525 (Couratari guianensis) to 0.895 g/cm3 (M. elata) were described by Lima et al. [19], evaluating wood wastes of 20 native species to the Amazonia. More recently, evaluating wood wastes with the near-infrared (NIR) spectroscopy technique, Lima et al. [24] reported basic density values between 0.354 (Simara guianensis) and 0.927 g/cm3(D. excelsa).

A compilation of average waste basic density values from the previous studies and the basic density classes proposed by the International Association of Wood Anatomists [16] is shown in Table 4. Three classes were used, namely: low density (Db ≤ 0.500 g/cm3); medium density (0.500 < Db ≤ 0.720 g/cm3); and high density (Db > 0.720 g/cm3). These are 55 tropical species, ranging from 0.221 to 0.935 g/cm3.

Table 4 Basic density of wood wastes of tropical species from Brazilian Amazonia
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The amplitudes reported for Amazonian wood wastes are within the variation range (0.140–1.210 g/cm3) published by Fearnside [13], evaluating the wood properties of 268 tropical species from the same biome. The literature demonstrates that low-density woods (< 0.500 g/cm3) are not suitable for energy purposes. Thus, they should not be carbonized with the high basic density woods to produce bioreducers.

In the reality of Amazonia, wood in all density ranges is carbonized, negatively influencing the gravimetric yield and charcoal productivity of brick kilns. Protásio et al. [37] reported a positive relationship between wood basic density and charcoal apparent relative density of Eucalyptus sp., indicating that denser woods result in dense charcoals.

Pereira et al. [34] also reported moisture values under operational conditions ranging from 24.99 (Cordia goeldiana) to 159.26% (Sterculia sp.) on a dry basis. Moisture is negatively correlated with the wood basic density [24], indicating that high-density woods present lower moisture due to the smaller volume of empty spaces in the wood [31]. For energy purposes, woods with moisture below the fiber saturation point (< 30%, dry basis) are recommended. Above that, the negative effects are significant in the gravimetric yield of charcoal, as a considerable part of the firewood is burned to release energy to meet the drying stage, as well as in the carbonization cycle, making it longer. In this sense, greater control of the moisture of wood wastes under operational conditions is necessary.

Average values of the chemical properties of wood wastes, such as total lignin (LigT), total extractives (EXT), and elemental carbon (C), can be seen in Table 5. These are data on wood from branches of 20 logged tropical species by the reduced impact logging method in an SFMP certified by the Forest Stewardship Council (FSC) in Brazilian Amazonia, previously published by Lima et al. [19].

Table 5 Total extractives, total lignin, and elemental carbon from SFMP wastes in Brazilian Amazonia
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EXT (1.8–17.9%, dry mass basis), LigT (30.2–38.1, dry mass basis free of extractives), and C (49.2–52.4%, basis mass dry) demonstrated high variability in native tropical woods of Amazonia. Wood species with high levels of EXT, LigT, and C are promising for energy purposes, especially to supply the charcoal-producing complex in the Carajás region, located between Maranhão and Pará states, in Brazil. Lima et al. [23] demonstrated that the EXT had a positive relationship with the GYC, indicating that the species D. excelsa, P. altissimum, M. elata, and G. glabra showed the best carbonization mass balances.

The C value (42.82%) described by Haqiqi et al. [15] for Eucalyptus pellita and the variation range (47.23–48.80%) reported by Santos et al. [40] of four Eucalyptus hybrid clones (three Eucalyptus urophylla  × E. grandis and one Eucalyptus camaldulensis × E. grandis) at seven years of age, were lower than the values found for wood wastes, which ranged from 49.2 to 52.4%. This range of C indicates that the wastes are very promising since C is the main energetic element of biomass [2]. Woods of the Eucalyptus genus are the most used to compose energy forests, Brazil's main source of forest biomass. Thus, alternative renewable sources are important for diversifying the energy sector’s raw materials.

Pereira et al. [35] discussed that woods with LigT above 28% are desired for charcoal production. All species shown in Table 5 showed values above the reference published by the authors, indicating that the species are suitable for this purpose. Wood species with high LigT values are more thermally stable and contribute positively to the gravimetric yield of charcoal of the production unit [27]. In addition, they have a high heating value, which indicates more energy is generated during combustion [43].

Although wastes from SFMP have suitable quality for charcoal production, the carbonization of wood with different physical and chemical characteristics negatively affects charcoal production unity, productivity in brick kilns, and the quality of the charcoal produced. Barros et al. [5] demonstrated that the carbonization of different woods together, which is the traditional model of carbonization in Amazonia, results in a reduction in the charcoal quality, negatively affecting friability, apparent relative density, ash content, volatile matter, fixed carbon, higher heating value, and energy density. The authors highlighted the need for better control of the raw material factor under operational conditions, especially with the wood segregation before carbonization, aiming to reduce the effect of heterogeneity on the production, productivity, and quality of the charcoal produced in traditional kilns.

5 Carbonization of Similar Wood Wastes

Several proposals were presented in the literature to reduce the heterogeneity of the residual raw material to produce charcoal based on the properties of the wood [19, 20, 34] and charcoal [23]. Carbonization should prioritize woods with similar properties to homogenize the process phases and the bioreducer quality. The wood basic density can be a criterion to separate wastes, as well as several combined properties, through multivariate statistical analyses of grouping.

Pereira et al. [34] proposed carbonization considering basic density classes. In this study, the authors separate wood wastes into classes of low (Sterculia pruriens, Sterculia sp., and Cordia goeldiana), medium (Tetragastris altissima, Pouteria sp., Ocotea sp., Peltogyne sp., Sclerolobium sp., Protium sp., Lecythis sp., and Caryocar villosum), and high (Eschweilera amazonia, Lecythis lurida, Eschweilera sp., Manilkara sp., Hymenaea sp., Terminalea sp., and Pseudopiptadenia psilostachya) density. They recommended wood of medium and high basic density for steel charcoal production, as it will result in a bioreducer with adequate apparent relative density.

Lima et al. [19] verified four groups of similar residual woods through the PCA technique, using physical (basic density, moisture, and maximum moisture content); chemical (proximate analysis: fixed carbon, volatile matter, and ash; and molecular analysis: total extractives, soluble, insoluble, and total lignin); and energy (higher heating value and energy density) properties; in addition to the specific consumption of firewood in charcoal production. This proposal promoted improvements of (+) 22, (−) 9.4, (+) 2.0, (−) 2.3, (+) 1.0, and (+) 23.6% in apparent relative density, ash content, fixed carbon, friability, higher heating value, and energy density of charcoal produced in brick kilns in Amazonia [5]. Table 6 presents the wood groups evaluated by the authors and the basic density variation ranges.

Table 6 Species groups segregated by PCA technique for carbonization in a charcoal production unit in Brazilian Amazonia
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Lima et al. [23] used the same grouping analysis to separate species in charcoal production units in Amazonia based on physical, chemical, and energy properties. Furthermore, the authors considered the yields of the carbonization process under laboratory conditions. Colorimetric characteristics (lightness, green/red axis, blue/yellow axis, color saturation, and hue angle), physical (moisture and basic density), chemical (total extractives and total lignin), and energy density of these residual woods contributed to the formation of six species groups using the PCA technique [20]. In the study, the basic and energy densities were negatively correlated with lightness, blue/yellow axis, color saturation, and hue angle, demonstrating that darker woods, such as D. excelsa and M. elata, present greater energy potential.

Two methods were tested to segregate residual wood on an operational scale. The first was based on the basic density [34], and the second was based on several characteristics of the wood [19]. In this sense, positive effects are evident in the productivity of brick kilns with raw material control. Figure 8 shows the effects of separating wood into density classes (medium–high and medium–low) on charcoal production at an operational scale.

Fig. 8
Four graphs of effects of waste segregation into density classes. A plots B D versus M C, and has a declining trend. B plots M c h versus M C. The trend is declining. C is a box-plot. It plots G Y C versus densities. Medium high is the highest. D plots M c h versus G Y C. The trend is declining.

(Source The authors 2023)

Effects of waste segregation into density classes (medium–high and medium–low) on charcoal productivity at operational scale in Brazilian Amazonia. Relationship between basic density—BD (g/cm3) and moisture content—MC (%) (a), mass of charcoal—Mch (t) and MC (%) (b), basic density—BD (g/cm3) and gravimetric yield of charcoal—GYC (%) (c), and GYC (%) and Mch (t) (d)

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Medium–high basic density woods (0.739–0.993 g/cm3) had lower average MC (41.1%, dry mass basis), while medium–low basic density woods (0.517–0.630 g/cm3) had higher MC (67.5%, based on dry mass). The trend of water reduction in woods with high BD can be seen in Fig. 9a. Traditional carbonization brings together woods with a wide variation in BD, which is not interesting due to the different MCs of the woods, which makes it difficult to control the carbonization process, reducing Mch (Figs. 9b and d) and GYC (Fig. 9c). Traditional carbonization showed a GYC of 30.3% (based on dry mass), lower than the average values reported for the medium–high (33.4%, based on dry mass) and medium–low wood groups (31.6%, based on dry mass) density. It is important to note that traditional carbonization negatively affects bioreducer production, the unit's operational efficiency, and revenues. Segregation promotes greater production of charcoal with the same quantity of wood mass inserted in the kiln. From an operational and charcoal productivity point of view, BD as a criterion to separate residual wood is the most appropriate and simple method to be carried out by the employees.

Fig. 9
Four scatterplots of weighted moisture content of the wastes. A plots M C w versus groups 1 to 5 with 3 highest. B plots B D W w versus groups with 2 highest. C plots M W t versus groups with 1 highest. D plots V W versus groups with 3 highest.

(Source The authors 2023)

Weighted moisture content of the wastes (a), weighted basic density of wastes (b), wet mass of wood (c), and volume of wood (d) by waste groups in brick kilns in Brazilian Amazonia. Error bars refer to standard deviations

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Additional wood characteristics can be used as a separation criterion to improve the production and quality of the steel bioreducer. In this sense, the carbonization of the groups proposed by Lima et al. [19] was tested under operational conditions in brick kilns. Figure 9 presents weighted average data for moisture—MCw (Fig. 9a) and basic density—BDWw (Fig. 9b), wet mass—MW (Fig. 9c), and volume—VW (Fig. 9d) of wood per hot-tail kiln. Groups 2 (9.117 m3) and 3 (9.500 m3) filled the kilns with the highest average volumes of waste. Groups 1, 2, and 4 showed average MW, MCw, and BDWw above group 5 (conventional model).

Waste groups 1, 2, and 4, which combine medium and high-density species, had the highest MW. Consequently, they allowed better use of the kiln's internal space. In addition, such combined wood species had lower MCw and higher BDWw. The woods in group 3 presented a lower MW inside the kiln due to the lower BDWw. In summary, grouping similar tropical woods provided encouraging results related to the wood amount inserted in the kilns. It is known that the greater the dry wood mass in the kiln, the greater the mass of charcoal produced. The traditional model of carbonization promotes greater variation in the MW, and VW used in the process, which is not interesting, as it makes the monthly and annual planning of wood in the charcoal plant difficult.

Compared to the traditional model without wood separation, the charcoal productivity dataset from brick kilns demonstrates the positive effects of waste wood segregation in Amazonia (Fig. 10). In order, groups 1 (7.84 m3), 2 (7.33 m3), 4 (6.61 m3), and 3 (6.43 m3) showed better average values of charcoal volume (Vch), compared to the group without segregation, which produced 6.02 m3. Regarding mass (Mch), gravimetric yield (GYC), and bulk density (CBD) of charcoal, the highest average values were reported for groups 1, 2, and 4.

Fig. 10
Four scatterplots. A plots V c h versus groups 1 to 5 with 1 highest and 5 lowest. B plots M c h versus groups with 1 highest and 3 lowest. C plots G Y C versus groups with 1 highest and 5 lowest. D plots C B D versus groups with 1 highest and 3 lowest.

(Source The authors 2023)

Volume (a), mass (b), gravimetric yield (c), and bulk density (d) of charcoal produced in brick kilns at a production plant in Brazilian Amazonia using segregated waste groups

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The highest Mch and GYC results reported for group 1 (D. excelsa) are associated with the high content of extractives and lignin in the wood (see Table 5). The H/C ratio and the high thermal stability of the extractives, mainly the phenolics, contribute to the increase in Mch and GYC [36]. In addition, this group will present reduced emissions of condensable and non-condensable gases into the atmosphere. Woods with higher densities explain the high CBD mean values of groups 1 (0.372 t/m3), 2 (0.339 t/m3), and 4 (0.304 t/m3). In this sense, segregation will promote a greater mass of charcoal transported to the steel industries compared to group 5. On the other hand, the charcoal of group 3 is less dense and voluminous and can be considered as a basis for commercialization.

The production of charcoal must be better controlled, especially concerning raw materials and factors associated with the process. Clearly, the separation of wood promotes encouraging results related to the production and quality of charcoal, which can influence the operational and financial planning of the plant. Thus, the methodology to adequately control the quality of the consumed raw material must be adjusted. As charcoal is commercialized based on its mass, the basic density can be used as a qualitative index to discriminate the material by its quality. Furthermore, it is an easily determined property. The literature corroborates that dense woods generate charcoals with high apparent relative density [23]. On the other hand, groups with similar physical, chemical, and energy properties can clearly improve charcoal quality.

6 Future Prospects for Charcoal Production from Wood Wastes

The energy use of forest wastes contributes to reducing the environmental impact caused by improper disposal and the exacerbated use of “dirty” energy sources and, consequently, to the mitigation of greenhouse gas emissions. Forest biomass stands out among the various renewable materials used for energy purposes due to the numerous solid, liquid, and gaseous fuels obtained through thermochemical, biochemical, and mechanical routes and their conciliation. It is worth noting that Normative Instruction No. 5, of December 11, 2006, published by the Ministry of the Environment, mentions in Article 2, item XIV, that forest logging wastes (branches, buttresses, and remains of trunks, for example) can be used as secondary products of SFMP for wood and energy production [6]. However, there is still a lack of scientific research and technology transfer for the sustainable valorization of these resources for bioenergy generation on industrial and domestic scales.

Many advances have been obtained with the understanding of the quality of SFMP wastes destined for energy generation and steel charcoal production. However, there is a need to continue research to reach technological maturity for a real understanding of the energy potential of this biomass. Furthermore, converting this raw material into energy products is not sufficiently mastered. In this sense, thermochemical conversion routes must be sufficiently analyzed with integrated methodologies (energetic-environmental-economic). For example, the life cycle inventory will provide a technical–economic-environmental assessment of strategies for the sustainable use of residual biomass in SFMP associated with the carbon balance of conversion processes.

The studies carried out so far have promoted important advances in clarifying the quality parameters of forest wastes destined for energy production. Despite this, some aspects of its energy use need to be elucidated. As pointed out by Lima et al. [19], the thermal behavior and combustion kinetics of this fuel remains unclear. It is also necessary to know the energy performance of the wastes at different final carbonization temperatures. These studies will allow more efficient use of wood and subsidize new conversion routes, such as densification, torrefaction, gasification, and liquefaction. Biomass has been widely studied for producing electricity, bio-oil, and chemical compounds [26]. Although there is research on wood from Amazonian species [38] and other biomass types common in the region [3], there is no large-scale research on woody Amazonian wastes for this purpose.

From a strategic point of view, biomass energy conversion processes are economically viable techniques to generate energy for local communities and industrial production systems, even in remote locals, as in several regions of the Amazonia. In Brazil, direct combustion and carbonization are the two most widespread ways of using wood wastes from SFMP, the main legal tool for the rational use of timber and non-timber forest products in Brazilian Amazonia.

Previously reported studies were limited to a single charcoal production unit from waste wood. Future research needs to include more wooden species and Amazonian forest management regions in their studies. It is also necessary to know the amount of waste stored in the forests. Thus, efficient harvesting and storage strategies for these biomasses must be developed.

The sustainability of the entire production chain for the use of waste must become one of the main objects of future research, generating technological impacts for the full use of waste and improving sustainable technologies for bioenergy use of tropical forest resources; with new, creative, and viable solutions that contribute to technological, economic, and social developments, through research. Thus, the following questions must be answered: (i) How much carbon is emitted from the collection, transport, carbonization, and combustion of waste?; (ii) Does the carbon stored in the forest soil and the regeneration of trees after harvesting neutralize the carbon emitted by the production chain?; (iii) Are the strategies effective in minimizing environmental impacts?

The answers to these questions will support the development of new solutions in Amazonia for increasing the efficiency of waste use and minimizing polluting gas emissions during wood carbonization. The technologies used for planted forests burn polluting gases formed during carbonization [8] and can be improved, adapted, and applied to the carbonization of SFMP wood wastes.

7 Conclusions

SFMP wastes have suitable properties for charcoal production for domestic and industrial use. However, there are great challenges to producing sustainable energy with this Amazonian biomass, mainly due to the low technological level of the carbonization kilns and the high variation in the waste properties concerning their dimensions and physical, chemical, and energy properties.

For example, waste diameter and basic density show high variability among species, studies were carried out to characterize and classify these wood wastes into different groups to solve these problems. The results demonstrate an increase in the gravimetric yield of charcoal, the productivity of the kilns, and an improvement in the bioreducer quality.

Therefore, the importance of technological research is emphasized to enable the development of carbonization, direct heat generation, and other energy routes to adapt the industry in Brazil to the current requirements of the national and international markets. Future research should highlight technological improvements in energy production using SFMP wastes to increase efficiency, quality, and sustainability.