Evaluation of a charcoal production process from forest residues of Quercus sideroxyla Humb., & Bonpl. in a Brazilian beehive kiln

Industrial Crops and Products 42 (2013) 169–174

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Evaluation of a charcoal production process from forest residues of Quercus sideroxyla Humb., & Bonpl. in a Brazilian beehive kiln Veronica Bustamante-García a,∗ , Artemio Carrillo-Parra a , Humberto González-Rodríguez a , ˜ a Roque Gonzalo Ramírez-Lozano b , José Javier Corral-Rivas c , Fortunato Garza-Ocanas a

Universidad Autónoma de Nuevo León, Facultad de Ciencias Forestales, Carr. Nac. No. 85, km 145, Linares, Nuevo León, , C.P. 67700, Mexico Universidad Autónoma de Nuevo León, Facultad de Ciencias Biológicas, San Nicolás de los Garza, Nuevo León, , Mexico c Universidad Juárez del Estado de Durango, Facultad de Ciencias Forestales, Río Papaloapan y Blvd. Durango S/N, Col Valle del Sur, 34120, Durango, , Mexico b

a r t i c l e

i n f o

Article history: Received 13 February 2012 Received in revised form 18 April 2012 Accepted 19 April 2012 Keywords: Charcoal production Quercus sideroxyla Brazilian beehive oven Elemental analysis Proximate analysis

a b s t r a c t Carbonization process from Brazilian beehive ovens and quality of charcoal from residues of branches and cracked firewood from Quercus sideroxyla were evaluated. Oven temperature and time, charcoal yields, quality and calorific value were also assessed. In addition, charcoal quality was determined using proximate and elemental analysis. Moreover, charcoal was classified according to its size. Since values for immediate and elemental analysis were expressed as percentage, data were transformed using the arcsine square root function for each studied variable. The relationship between temperature and time process for cracked firewood and branches were R2 = 0.99; p < 0.0001 and R2 = 0.98; p < 0.0001, respectively. The carbonization of cracked firewood was slower (131.6 h), oven temperature reached 975 ◦ C and had higher yield (5.4 m3 t−1 ), compared to branches (86.7 h, 1007 ◦ C and 9.2 m3 t−1 , respectively). The best charcoal quality was obtained from the middle section of the oven when using cracked firewood; with a mean calorific value of 32,000 J g−1 , moisture content 3.3%, volatile materials 19.0%, ash 5.2%, fixed carbon 72.2%, and carbon 89.41%. Elemental analysis had the following mean values: H = 2.95%, O = 2.93%, N = 0.2%, and S = 0.01%. The quality size of the charcoal of branches was acceptable according to France and Belgium standards. The quality of charcoal produced from branches can be improved by controlling air intakes to prevent increments in temperature. © 2012 Elsevier B.V. All rights reserved.

1. Introduction New initiatives have been developed for energy models with the aim of alleviating poverty, energy crisis, as well as to reduce the damage of environment for excessive use of fossil fuels that may contribute to greenhouse effect and global warming (Flotats, ˜ 2008; Banos et al., 2011; Nakata et al., 2011). Dendroenergy is a sustainable locally available alternative in saw mills. It is a clean energy source that fixes CO2 from the atmosphere during the photosynthetic processes (Demirbas, 2004; Martínez, 2009; Conesa and Domene, 2011). The use of energy from biomass increases the effectiveness of forest management when obtaining bioheat, bioelectricity and biofuels throughout direct combustion, thermochemical or biochemical processes, respectively. Charcoal quality depends mainly

∗ Corresponding author. E-mail addresses: veronica [email protected] (V. Bustamante-García), arte [email protected] (A. Carrillo-Parra), [email protected] (H. González-Rodríguez), [email protected] (R.G. Ramírez-Lozano), ˜ [email protected] (J.J. Corral-Rivas), [email protected] (F. Garza-Ocanas). 0926-6690/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.indcrop.2012.04.034

of plant species, size of materials, type of kiln and carbonization process (Pascal, 2005). Moreover, procedures, techniques and raw materials used for charcoal production are very important for performance and quality. In many countries, the charcoal production is carried out by traditional methods such as earth kilns or stacks; however, industrial procedures involve brick kilns, transportable metal kilns, and retorts (Martín, 1989; Stassen, 2002). Despite of the fact that charcoal is an important energy source in Mexico, there is a lack of national standards practices and norms that regulate the quality and performance ratio and a classification system. European and Asian markets regulate the charcoal quality based on standards of physicochemical characteristics such as colour, sound, ignition velocity and particle size (Ayón, 2003). In the United States of America, the charcoal quality is based upon fixed carbon according to DIN EN 1860-2:2005, the size of charcoal pieces, homogeneity, non-sparking, amount of dust and impurities ´ 2011). The main problem (Stassen, 2002; Petrovic´ and Glavonjic, of the charcoal production chain in Mexico is the low carbonization yield with high production costs and waste. Additionally, there are no regulations governing the production process and charcoal quality from different systems. By controlling the production process may allow to producers an increasing in the competitiveness

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Fig. 1. Right: Brazilian beehive kiln prototype that shows: (a) steel door, (b) chimney, (c) air inlet, (d) ignition chamber. Left: diagram of Brazilian beehive kiln prototype.

on international markets. Therefore, the objectives of the study were to evaluate a charcoal production process, charcoal quality and yield from branches or cracked firewood of Quercus sideroxyla.

2. Materials and methods Two types of raw materials from logging areas were evaluated: (a) branches and (b) cracked firewood. The size of branches varied from 3 to10 cm in diameter and from 30 to 35 cm in length. The sizes of cracked firewood, of an advanced decay stage, were wood pieces that ranged from 15 to 20 cm from each side and 30 to 35 cm long. Twenty-one samples were randomly selected from each type of residue with three replicates. The materials were carbonized in Brazilian beehive ovens. This type of oven was selected because is more efficient than the traditional ones (Flores and Quinteros, 2008; Arias et al., 2010). The Brazilian beehive ovens are characterized by a circular shape, built of bricks, the roof dome diameter of 7.06 m, height 3.8 m, average capacity of 81 m3 of wood, with 36 holes distributed around the oven for air exchange (Fig. 1). The oven was divided in three levels (top, middle and bottom) according to its height. In order to compare the variables among levels, seven samples, identified by different nail sizes, were placed in each level. The holes were used to control the gas produced during the carbonization. The temperature (◦ C) of each oven was monitored at 4 h intervals using Omega thermocouples. The carbonization time (h) was also determined. The charcoal quality produced by each type of residue was characterized by two physical–chemical tests: immediate analysis and elemental analysis. The International Standard ASTM D 1762-84 (ASTM, 2001) was used to determine the immediate analysis variables such as moisture content, volatile materials, ash and fixed carbon. A LECO CHN628 analyzer (LECO Corporation, Michigan, USA) was used to estimate carbon, hydrogen, and nitrogen content according to the International Standard ASTM D 5373 (ASTM, 2002) (Romero et al., 2007; Rojas and Barraza, 2009). Oxygen was calculated as the difference of the total ash-free mass and the sum of carbon, nitrogen, and hydrogen content (Baldock and Smernik, 2007; Conesa and Domene, 2011). Sulphur content was determined by means of using an elemental analyzer module TruSpec® S (LECO Corporation, Michigan, USA). The charcoal size grading was determined randomly from a selection of 300 bags containing 2.5 kg of charcoal. The classification was made according to the following charcoal dimensions: class 1 (<2 cm), class 2 (from 2 cm to 5 cm), class 3 (from 5 cm to 10 cm) and class 4 (from 10 cm to 15 cm). The charcoal was weighed with a digital electronic balance Tor-Rey (0.001 kg accuracy). Charcoal yield was determined from the total volume of wood used for the production process, multiplying by a factor of 0.6 and divided by the

weight of the charcoal produced (Whiteman et al., 2002; Picardo et al., 2008). Since data corresponding to immediate analysis, elemental analysis and charcoal size fragments (quality) were expressed on a percentage basis, but they did not have the assumption of normality, thus were transformed to the arcsine square root function using the Kolmogorov–Smirnov test (Steel and Torrie, 1980). The significance of results was analysed using the PROC MODEL and PROC TTEST according to the SAS/ETS® statistics software package (SAS Institute Inc., 2004). Experimental data were statistically analysed using one-way analysis of variance with a factorial arrangement being type of residue (branches and cracked firewood) and position in the oven (top, middle and bottom) the factors (Steel and Torrie, 1980). Where the F-test was significant (p < 0.05), the differences were validated using the Tukey’s honestly significant difference (HSD) test and were considered statistically significant at p = 0.05 for all pair-wise comparisons (Steel and Torrie, 1980). To compare charcoal yield (m3 t−1 ), temperature (◦ C) and carbonization time (h) the t-test was performed. The relationship between temperature and carbonization time was adjusted to the model (Eq. (1)) proposed by Challice and Clarke (1965):



y(T ) = a × e

 t − x 2 

−0.5

0

ˇ0

(1)

Where T is the temperature (◦ C), ˛ is the parameter representing the peak of temperature (◦ C), t is the time (h), xo is the position of the temperature peak, ˇo is the parameter describing the peak wide at the middle of minimum temperature. 3. Results and discussion The coefficients of determination (R2 ) to relate the temperature of carbonization as a function of time for cracked firewood and branches were 0.9955 and 0.9734, respectively (Fig. 2). The carbonization process for branches was 86.7 h and the maximum temperature was 1007 ◦ C (Fig. 2). Conversely, 150 h were required for the cracked firewood process with a maximum temperature of 1000 ◦ C (Fig. 2). During the carbonization process, the three main wood components i.e., lignin, cellulose and hemicellulose, were degraded at different levels, being the hemicellulose the first component to breakdown at 100 ◦ C. In a closed system, the burning intensifies, and consequently, crystalline cellulose breakdown range from 300 to 400 ◦ C (Rothermel, 1972; Rentería et al., 2005). However, Aragón (2009) noted a release of energy from 220 to 350 ◦ C as a result of the breakdown of glycosidic linkages and occurs a partial depolymerization of the cellulosic component. The lowest charcoal yield was obtained when using branches (9.2 m3 t−1 ; Table 1). Branches are smaller than cracked firewood

V. Bustamante-García et al. / Industrial Crops and Products 42 (2013) 169–174 1400

Branches Cracked firewood Branches Cracked firewood

T=1795.55*exp(-0.5*((t-121.40)/33.28)^2) 2 1200 R =0.9734 p=0.0001

Temperature (°C)

1000 800 600 400 200

T=1022.66*exp(-0.5*((t-122.11)/32.88)^2) R2=0.9955 p=0.0001

0

0

20

40

60

80

100

120

140

160

Time (h) Fig. 2. Temperature pattern as a function of time during the carbonization process from branches and cracked firewood of Quercus sideroxyla in Brazilian beehive kiln. Experimental plotted values are denoted in filled (branches) and open (cracked firewood) circles. The curves correspond to the fitted model (Eq. (1); see Section 2).

and consequently more material is needed to reach temperatures above 1000 ◦ C. Neves et al. (2011) related charcoal yields with material size and they found that these reactions induce the release of tars. The higher wood volume without conversion to charcoal was obtained from the oven containing branches due to the heterogeneous carbonization process. Free areas between materials during the carbonization process may facilitate air circulation, and leads to a sharp temperature increase, modifying the carbonization process (García, 2010). Cracked firewood produced more charcoal yields per unit of raw material (5.4 m3 t−1 ). Font et al. (1993) and Bhat and Agarwal (1996) mentioned that low heating rates and moderate final temperatures are required to achieve acceptable charcoal oxidation levels. In this study, significant differences between residues were detected for charcoal humidity, volatile materials and fixed carbon; while for oven positions, statistical differences were observed in volatile materials and fixed carbon (Table 2). Similarly, significant differences between residues, positions and the interaction residues × positions were found for calorific value, carbon, hydrogen, oxygen, nitrogen and sulphur content data (Table 2). The charcoal humidity data (3.2–3.9%; Table 3) were lower than the 7% that is the maximum value accepted by the European quality standards (NBN M11-001 and NF N 846 E). Total volatile materials ranged from 12.5% to 27.9% being higher in branches than in cracked firewood (Table 3). It seems that volatile materials content in charcoal increases the compressive strength, friability and fragility (Ayón, 2003; Demirbas¸, 2003). Volatile concentration materials from 12% to 16% reduce smoke release (Di Blasi, 2008). According to Ayón (2003) Asian and European markets allow a maximum rate of 12% of volatile materials. Table 1 Cubic meters of residues required to produce a ton of charcoal, and non-carbonized wood from the carbonization process of cracked firewood and branches from Quercus sideroxyla in a Brazilian beehive kiln. Values are means (n = 3) ± standard errors. Variable

Residues (m3 ) Non-carbonized wood (m3 )

Residue Cracked firewood

Branches

5.4 ± 0.1 b 1.4 ± 0.3 b

9.2 ± 0.2 a 4.2 ± 0.6 a

t value

p value

424.06 16.75

0.0005 0.0149

Means followed by different letters (a, b) indicate significant differences between residues at p = 0.05 according to the t-test for each comparison.

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Ash content ranged from 3.7% (cracked fire wood and bottom position) to 6.7% (branches and top position; Table 3). Lower values (2.7%) obtained from oak charcoal, were reported by Syred (2006). The ash content is an industrial pollutant product. In fact, it affects the management of the charcoal and increases the costs of the process (Ahmaruzzaman, 2009; Serrano, 2009). According to Blanco et al. (2007) the ash content can reduce flammability by covering the fuel from oxygen. Belgium and France norms do not consider the ash content during the evaluation of charcoal quality; however, for the German norms the maximum acceptable level is 6%. Higher fixed carbon amounts (78.8%) where found at the top position in the oven when using cracked firewood while values of about 63.2% were observed in branches in the bottom and middle positions (Table 3). Higher fixed carbon percentage is related to lower values of volatile materials. In Europe, a charcoal for domestic use must contain percentages of fixed charcoal of 75%, 78% and 82% according to NBN M11-001, DIN 51749 (1989) and NF N 846 E standards, respectively. The highest calorific value (33,900 J g−1 ) was obtained from cracked firewood in the middle position of the oven, whereas the lowest (25,200 J g−1 ) corresponds to branches in the middle position (Table 4). Márquez et al. (2001) reported that charcoal obtained from Pinus caribaea, Eucalyptus saligna and Pinus tropicalis under laboratory conditions at 700 ◦ C, showed calorific values of 32,200 J g−1 , 32,700 J g−1 and 32,200 J g−1 , respectively. Ordaz (2003) in a similar study found the highest calorific value (8,29,300 J g−1 ) from the middle of the oven. Syred (2006) reported calorific figures of 32,500 J g−1 from oak charcoal produced on metal oven. In this study, charcoal produced from cracked firewood obtained from oak wood showed the highest values. In this study, the largest percentage of fixed carbon (89.41%) corresponded from the middle of the oven with cracked firewood, and the lowest value (75.78%) was obtained from branches in the middle of the oven (Table 4). Higher values (93.1%, 93.6% and 92.7% respectively) produced under laboratory conditions were reported by Márquez et al. (2001) in P. caribaea, E. saligna and P. tropicalis. It seems that the higher percentage values resulted from a uniform, high carbonization temperature and speed. However, Demirbas¸ (2003) analysed the charcoal used in an industrial boiler and found 81.5% of fixed carbon. The same author reported that the charcoal produced from pruning residues had a fixed carbon value that varied from 65 to 85%. In this study, the highest percentage of hydrogen in charcoal was obtained from branches at the top and bottom of the oven with values of 3.35% and 3.32%, respectively (Table 4). A higher value (4.0%) was obtained by Demirbas¸ (2003) in charcoal samples, which may indicate that hydrogen and oxygen molecules are released during the steaming process from the reactions that form mainly H2 , CO and CO2 . Then, throughout the synthesis of gas they are converted into ammonia, methanol and other products. In this study, the highest percentage of oxygen was obtained from branches (9.1%) at the bottom level of the oven and the lowest content (2.9%) occurred in the middle of the oven for cracked firewood (Table 4). The lowest percentage of oxygen and calorific value occurred in branches at the bottom of the oven. These findings agree with Calventus et al. (2009) and Márquez et al. (2001) who reported that low oxygen levels might decrease the fuel calorific value. In this study, the greatest nitrogen content (0.42%) was observed in branches placed in the middle of the oven (Table 4). The amount of nitrogen obtained from charcoal has not the same negative effect to the environment as compared to fossil fuels (Jenkins et al., 1998; Demirbas¸, 2003; Robinson et al., 2003; Serrano, 2009). In addition, Di Blasi (2008) suggested that the charcoal produced at low temperatures retains more nitrogen; however, at higher temperatures hydrogen, carbon, and nitrogen are released. Sulphur levels were highest (0.04%) for charcoal obtained from branches in the middle of the oven (Table 4). High content of

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Table 2 Calculated means squares (MS), F and p values of the analysis of variance to detect significant differences for humidity, volatile materials, ash, fixed carbon, calorific value, carbon, hydrogen, oxygen, nitrogen and sulphur of charcoal produced from cracked firewood and branches at three oven positions. Source of variation Variable

Residue (R) Value

MS

Humidity Volatile materials Ash Fixed carbon Calorific value Carbon Hydrogen Oxygen Nitrogen Sulphur

2.3 304.2 39.2 308.6 86,15,400.5 80.7 0.1 21.0 0.9 0.3

R×P

Position (P) MS

F

p

6.0 16.8 2.9 20.8 616.0 3482.4 119.0 110.5 3325.8 345.8

0.02 0.01 0.09 0.01 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001

Value

0.2 89.8 3.0 53.6 13,59,838.9 5.0 0.7 33.8 0.1 0.1

Error

MS

F

p

0.6 4.9 0.2 3.6 97.2 216.3 786.0 177.7 380.7 50.7

0.53 0.01 0.80 0.03 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001

Value

1.1 16.2 10.8 9.4 62,29,926.0 44.0 0.2 3.3 0.5 0.1

F

p

2.8 0.9 0.8 0.6 445.5 1899.8 177.3 17.2 1700.1 146.2

0.07 0.42 0.46 0.53 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001

0.4 18.1 13.6 14.8 13,985.4 0.02 0.01 0.19 0.01 0.01

Table 3 Humidity, volatile materials, ash and fixed carbon percentages from charcoal of cracked firewood and branches at three oven positions in a Brazilian beehive kiln. Values are means (n = 18) ± standard errors. Residue

Oven position

Humidity (%)

Cracked firewood

Bottom Middle Top Bottom Middle Top

3.9 3.3 3.8 3.2 3.4 3.3

Branches

± ± ± ± ± ±

Volatile materials (%)

0.1 a 0.1 a 0.1 a 0.1 a 0.1 a 0.1 a

22.1 19.0 12.5 27.1 27.9 23.0

± ± ± ± ± ±

Ash (%)

2.5 ba 1.7 ba 2.0 b 1.8 a 3.6 a 2.9 a

3.7 5.2 4.8 6.3 5.2 6.7

± ± ± ± ± ±

Fixed carbon (%)

1.0 a 1.5 a 1.0 a 0.4 a 1.5 a 1.1 a

70.1 72.2 78.8 63.2 63.2 66.8

± ± ± ± ± ±

2.4 ba 2.5 ba 1.7 a 1.7 b 3.5 b 2.8 b

Means followed by different letters (a, b) indicate significant differences for the combination residue × oven position at p = 0.05 according to Tukey’s honestly significant difference (HSD) test for each comparison.

Table 4 Calorific value (J g−1 ) and contents (%) of carbon, hydrogen, oxygen, nitrogen and sulphur of cracked firewood and branches at three oven positions in a Brazilian beehive kiln. Values are means (n = 18) ± standard errors. Residue

Oven position

Cracked firewood

Bottom Middle Top Bottom Middle Top

Branches

Calorific value 30,800 33,900 32,000 30,500 25,200 31,400

± ± ± ± ± ±

Carbon 5.3 cd 4.4 a 7.5 b 5.3 d 6.8 e 1.7 cb

82.02 89.41 84.28 80.53 75.78 82.83

± ± ± ± ± ±

Hydrogen 0.23 d 0.05 a 0.06 b 0.04 e 0.13 f 0.02 c

3.06 2.95 3.25 3.35 2.86 3.32

± ± ± ± ± ±

0.02 c 0.01 d 0.02 b 0.03 a 0.01 e 0.01 a

Oxygen 6.63 2.93 5.80 9.12 5.21 6.22

± ± ± ± ± ±

Nitrogen 0.29 b 0.06 d 0.10cb 0.01 a 0.38 c 0.01 b

0.28 0.21 0.30 0.28 0.42 0.36

± ± ± ± ± ±

0.01 d 0.01 e 0.02 c 0.01 d 0.01 a 0.01 b

Sulphur 0.02 0.01 0.01 0.02 0.04 0.02

± ± ± ± ± ±

0.01 b 0.01 c 0.02 c 0.01 b 0.02 a 0.01 b

Means followed by different letters (a–f) indicate significant differences for the combination residue × oven position at p = 0.05 according to Tukey’s honestly significant difference (HSD) test for each comparison.

sulphur in charcoal has negative effects since it could reduce the temperature for expulsion of gases and the oxidizing process, allowing the synthesis of SO3. These alterations could produce technical troubles in boilers and eventually induce sulphuric acid (H2 SO4 ), which is then released to the atmosphere (Jenkins et al., 1998; Robinson et al., 2003). Demirbas¸ (2003) determined a content of sulphur of about 3.0% from charcoal used in a boiler. However, this value does not represent an environmental concern since the content are lower than fossil fuels. Strahler (1992) reported that sulphur content in oil varies from 0.1% to 5.5%. The results from charcoal size from bags had significant differences between type of residues (Table 5). The highest production of

Table 5 Analysis of variance of charcoal size classification from two forest residues from Quercus sideroxyla. Variable

Means squares

t value

p value

Class 1 Class 2 Class 3 Class 4

3034.79 3414.68 1027.19 1837.08

52.77 95.67 48.65 20.97

<0.001 <0.001 <0.001 <0.001

charcoal from Class 4 (10–15 cm) was obtained from cracked firewood with 51.5% (Fig. 3). Charcoal Class 3 (5–10 cm), from branches had the highest percentage (30.5%) and charcoal from cracked firewood was 23.8%. The highest value of charcoal from Class 2 (2–5 cm) corresponded to branches (29.4%), while a value of 12.5% was observed in cracked firewood. Cracked firewood was the type of residues that had the highest percentage of charcoal for Class 1 (<2 cm) with 12.2%. Charcoal obtained from branches had the lowest percentage of fines (4.0%). The size of packed charcoal from the two types of studied residues exceeded the maximum allowable by DIN EN 1860-2:2005 for US markets. Less than 10% of charcoal should exceed 8 cm. However, charcoal from Class 3 and Class 2 produced from both raw materials are acceptable by this standard, considering at least 80% greater than 2 cm and not more than 7% of the charcoal should measure 1 cm. From the two raw materials, only cracked firewood exceeded the maximum rates. According to the specifications sizes of produced charcoal, it is accepted for Belgium and France markets, except Class 1 from cracked firewood, which exceeded the maximum acceptable limits. Rojas and Barraza (2009) stated that the main criterions used for the market are moisture content, ash, volatile material and calorific value. These properties are related to the initial temperature and

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References

80 70

Percent (%)

60

173

Class 1 (<2 cm) Class 2 (2 to 5 cm) Class 3 (5 to 10 cm) Class 4 (10 to 15 cm)

50 40 30 20 10 0

Branches

Cracked firewood Residues

Fig. 3. Charcoal size classification from two types of residues of Quercus sideroxyla. Values are means (n = 100) ± standard deviation.

time when maximum energy is released, and the total time for the carbonization process. It is also important to avoid losses of energy by heating the ash. However, the quality of charcoal from branches was low due to the heterogeneity of the carbonization process, even though it was exposed to high temperatures, it showed a high percentage of volatile materials and low yields. Another factor that may affect is the energy loss during the process, since part of the material has been consumed during the process. Moreover, there were spaces in the oven with cold spots due to the gaps, resulting in a poor heat transfer inside the oven (García et al., 2009). 4. Conclusions The production process and quality of the charcoal obtained in Brazilian beehive kiln when using cracked firewood was acceptable. The carbonization process was slow but stable since the materials were controlling the temperature inside the oven. The homogeneity of the process is reflected in the maximum performance, high calorific value, and acceptable values for carbon, nitrogen, sulphur and hydrogen content. The highest quality was obtained at the top of the oven with cracked firewood. The percentage of fixed carbon, volatile materials, ash and moisture content were within the range of international standards. In addition, charcoal particle size distribution satisfied international standards, except charcoal Class 1 (<2.0 cm) from cracked firewood. In order to produce and assure charcoal of good quality following international standards, it is highly recommended to crack the charcoal before packing. The carbonization process when using branches produced a heterogeneous charcoal. The lowest quality was observed in the middle of the oven with the lowest values of fixed carbon, heating, elemental carbon, as well as the highest percentage of hydrogen, nitrogen and sulphur. It was observed that the temperature increased rapidly, leading to an increase in the consumption of raw material, conducing to low yields and low charcoal quality. Acknowledgements The authors wish to thank the support from the company Noram de Mexico, SA de CV, especially to M.Sc. Jose Guadalupe Garcia Molina and M.S. Allison L. Ludvik Vanderhop for their technical assistance. Useful suggestions from two anonymous reviewers helped to improve the manuscript.

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