Quality Assessment of Quercus scytophylla Liebm Charcoal Produced in a Metal Kiln in the Cordón Grande Ejido, Guerrero, Mexico

Abstract

The present study evaluates the quality of charcoal produced from Quercus scytophylla Liebm. in Guerrero, Mexico, using a portable metal oven, namely, the Guadiana Valley Experimental Field (CEVAG) type. A $2\times 3$ factorial design was employed to analyse the influence of wood heterogeneity (sapwood vs. heartwood) and position within the oven (low, medium, high) on the yield and physicochemical properties of the charcoal. The mean yield of the process was found to be 20.0–26.7%. The characteristics of six properties were determined: moisture content, volatile matter, ash content, fixed carbon, basic density, and calorific value. The charcoal exhibited a low moisture content (1.49–3.56%) and ash content (2.18–2.52%), meeting international standards. Volatile matter was higher in heartwood (22%). Fixed carbon (73.73–74.05%) was close to the optimal parameters of international standards. The calorific value exhibited marked variations in accordance with the position during the process of carbonisation, with elevated values observed in the lower section (6751–7508 cal g⁻¹). The basic density of the wood was higher in the sapwood, with a maximum value of 0.57 g cm⁻³ observed in the upper section. A positive linear relationship was identified between the basic density and calorific value, although the coefficient of determination was small ($R^2=0.67$) and therefore inconclusive. The analysis showed the type of relationship that can be established between these two variables. The upper part of the kiln exhibited the optimal physicochemical properties, with the levels deemed acceptable. The utilisation of this oak for charcoal production fosters sustainable forest management and engenders direct economic benefits for rural communities. In conclusion, the research provides a viable technical model for sustainable wood energy production in forestry regions and underscores the need to evaluate other timber species with this potential.

1. Introduction

Despite the fact that Mexico’s energy supply is predominantly reliant on fossil fuels, including oil, charcoal, and natural gas, a decline in global demand is anticipated in the medium term [1]. It is therefore imperative to diversify non-fossil and renewable energy sources. In this regard, forest biomass fuel is a viable solution that complies with these principles and mitigates the adverse environmental effects caused by the use of traditional fossil fuels [2,3].

Charcoal is a forest by-product obtained by pyrolysis or carbonisation [4]. It is utilised for various purposes, including heating, cooking, and recreational activities, due to its high calorific value [5]. Charcoal is characterised by a high percentage of fixed carbon [6], a low ash content [7,8], a high density, and low moisture content [9,10,11]. Additionally, it is used in industrial applications, such as iron sintering, when it exhibits high quality [12].

Globally, 1966.2 million m³ of wood are utilised as an energy source, with the production of 54.9 million tons of charcoal being particularly significant. The countries of Africa (5.1%), the Americas (22.1%), and Asia (17.5%) are the predominant producers and consumers of this product, with Brazil, Nigeria, Ethiopia, India, and the Democratic Republic of Congo being the primary producers of charcoal [13].

The wood carbonisation process is typically conducted in earthen, brick, or metal kilns, with the specific type chosen based on the prevailing socioeconomic conditions of the site. In 2018, charcoal constituted 5.9% of Mexico’s total timber production. That same year, 64.0% of the authorised timber volume in the Mexican states of Guanajuato, Sonora, Tamaulipas, and Yucatán was allocated for charcoal production [14].

Between 2017 and 2018, Mexico recorded a 15.9% increase in the volume of wood used for charcoal production. In 2018, the volume of timber authorised for charcoal production in Mexico was 620,195 $m^{3}r$, of which 271,647 $m^{3}r$ was Q. sp. During this period, 3971 $m^{3}r$ was authorised for this purpose in Guerrero [14]. In 2022, charcoal production in Mexico was 38.4 million $m^3$, equivalent to 2.0% of global production, but national timber production between 2020 (538,719 $m^{3}r$) and 2021 (418,330 $m^{3}r$) fell by 22.3%, and by 2022, Guerrero had allocated 2728.7 $m^{3}r$ for charcoal production [15].

Charcoal production is linked to the social economy of small producers [16]. More than 60.0% of production is carried out on a family scale [17], using traditional methods with different species and in rudimentary earthen kilns with low-yield techniques [18,19,20]. Furthermore, there are no standardised processes to homogenise characteristics and evaluate product quality.

The quality of charcoal depends on the species, section of the tree, physical and chemical properties of the wood, types of kiln, and carbonisation process [21,22,23]. The calorific value indicates the amount of thermal energy produced by a fuel when burned and is the most important property of charcoal, in addition to the friability, moisture content, volatiles, ash, and fixed carbon, as expressed in percentage terms [10,19,24]; these properties that can be altered by transport, the storage time, and the charcoal size [25]. In this study, the calorific values of wood and charcoal were evaluated using the international standard ASTM E711-87’s criteria [26].

Some authors have shown [27,28] that wood characteristics, including basic density, have a greater influence on charcoal yield and properties than the pyrolysis process. In this regard, heartwood is expected to produce charcoal with a higher basic density and calorific value than sapwood in the species Q. scytophylla.

The objective of this study was to evaluate the quality of charcoal produced from sapwood and heartwood of Q. scytophylla in portable metal kilns installed at logging sites in the Cordón Grande forest community in the municipality of Técpan de Galeana, state of Guerrero, Mexico.

2. Materials and Methods

2.1. Study Area

The study was conducted in stands 118 and 119 (Figure 1) of cutting area 8 for the year 2022, located at the geographic coordinates 17° 37′ 27′′ LN, 100° 37′ 12′′ LW; altitude 2046 m above sea level; physiographic province XII Sierra Madre del Sur; subprovince 66 Cordillera Costa del Sur; and topographic system 100 Sierra, according to the physiographic map scale 1:1,000,000 [29]. The soil consists of extrusive igneous rock and lithological unit intermediate andesite-tuff (geological map 1:250,000). Climate C (W2) temperate subhumid with rainfall in summer and higher humidity (climate map 1:1,000,000). Vegetation: pine forest and pine–oak forest. Hydrological region: 19 A Tecpan River Sub-basin [29].

Figure 1. Logging plan for the Cordón Grande communal land, Técpan de Galeana, Guerrero.

2.2. Characteristics of the Kiln, Oak, and Carbonisation Process

The charcoal was produced in a portable octagonal metal furnace of the Guadiana Valley Experimental Field (CEVAG) type, consisting of three octagonal sections that can be assembled one on top of the other (upper, middle, and low sections). It has sixteen vents with covers, two on each side of the octagon in section 1, as well as a top cover with ventilation for moisture vaporisation. It also has four tubes or chimneys (Figure 2). The oven has a capacity of 3000 to 3200 kg per load, which yields 500 to 600 kg of charcoal. The use of this oven represents an accessible technological improvement, greater efficiency in the pyrolysis process, better product quality, and portability for areas with limited access in the mountains of Guerrero, allowing carbonisation to be carried out on site near the biomass source, thereby reducing transportation costs. The process was in accordance with [30]. During the pyrolysis process, gases are released, and the air intake is controlled by manually operated dampers in the chimneys located at the bottom of the furnace. Total airtightness is achieved by closing the air intakes at the bottom, and the process is carried out by managing the oxygen intake to achieve controlled combustion [31]. It should be noted that, during the carbonisation process, it was not possible to record temperature readings that would allow for a thermal profile to be generated.

Figure 2. CEVAG-type metal oven. Figure used with permission from INIFAP. Brochure for Producers No. 69. December 2020 [30].

The white oak species (Q. scytophylla) is native to western and central Mexico, from Sonora and Chihuahua to Chiapas [32], with a widespread distribution in the study area. The trees grow up to 20 m tall and 50 cm in diameter. The species is deciduous, with thick, leathery, long (20 cm) leaves that have sharp, pointed teeth along the edges.

The raw material for carbonisation came from cutting area 8, based on the 2022 Forest Management Program of the Cordón Grande ejido, with 4353.60 m³ v.t.a., authorised on 299.74 ha. The weight of the charcoal was obtained using a Rhino brand portable digital scale with a capacity of 500 kg and an accuracy of 100 g. The moisture content of the green wood was determined based on Mexican standard NOM-EE-117-1981 [33]. The average moisture contents in the sapwood and heartwood were measured in the laboratory as 43% and 26%, respectively.

2.3. Selection of Charcoal and Wood Samples

The raw material for charcoal production consisted of sections of the trunk and branches, both split and whole, and took approximately one week to produce.

The samples for analysis came from a single harvest; due to the distance, the land in the cutting area was not easily accessible.

Although [27] argued that the carbonisation process does not influence the product quality, Refs. [23,34] have shown that the position (low, medium, high) of the charcoal in the oven can influence its quality parameters. Based on this, charcoal samples were taken from the lower (0.00–0.70 m), middle (0.70–1.40 m), and upper (1.40–2.00 m) parts of the kiln for sapwood and heartwood charcoal. These were labeled and placed in brown paper and plastic bags to prevent moisture absorption during transport to the laboratory. Sapwood and heartwood samples were also collected and labeled to determine their basic density and calorific value.

2.4. Charcoal Performance in the Furnace

The charcoal yield per bake was determined using Equation (1) [35]:

$$\%\text{Performance}=\left({\displaystyle \frac{\text{Anhydrous}\text{weight}\text{of}\text{charcoal}}{\text{Anhydrous}\text{weight}\text{of}\text{wood}}}\right)\times 100$$

(1)

2.5. Moisture Content and Basic Density of Wood and Charcoal

Sapwood and heartwood samples were collected, from which $2\times 2\times 2$ cm cubes were made to obtain the moisture content and basic density using Equations (2) and (3) [36]:

$$\%\text{Moisture}\text{content}=\left({\displaystyle \frac{\text{Wet}\text{weight}-\text{Anhydrous}\text{weight}}{\text{Anhydrous}\text{weight}}}\right)\times 100$$

(2)

$$\text{Basic}\text{density}={\displaystyle \frac{\text{Anhydrous}\text{weight}}{\text{aturated}\text{volume}}}$$

(3)

The charcoal samples were obtained from the bottom, middle, and top of the kiln for sapwood and heartwood. The samples (wood and charcoal) were then placed in a Memmert VN100 drying oven at a temperature of 103 ± 2 °C until a constant weight was obtained to determine the anhydrous weights of the wood and charcoal, as specified in Mexican standard NOM-EE-117-1981 [33].

2.6. Charcoal Quality Assessment

The granulometry was determined using screens with mesh openings of 20 × 20, 50 × 50, and 100 × 100 mm, stacked from largest to smallest, using the methodology of [19]. Each screen, including the residues, was separated and weighed individually. The calculations were based on the ratio between the initial weight and the mass retained in each sieve and residue, as specified in [19]. With this data, the friability was determined using Equation (4):

$$\%\mathit{\text{Friability}}=\left({\displaystyle \frac{\text{Starting}\text{weight}-\text{Final}\text{weight}}{\text{Starting}\text{weight}}}\right)\times 100$$

(4)

The calorific value of wood and charcoal was determined using a Parr® 6200 calorimeter equipped with a Parr® 6510 manual water heating system and a Star® printer, using wood and charcoal test specimens of $1\pm 0.1$ g, in accordance with the international standard ASTM E711-87’s criteria [37].

The percentages of moisture content, volatiles, ash, and fixed carbon in the charcoal were obtained by incinerating the samples in a Felisa muffle furnace using the criteria of ASTM D 1762-84 [37]. All tests were performed in the Wood Anatomy and Technology Laboratory of the Forest Sciences Division at the Autonomous University of Chapingo and calculated using Equations (5)–(7):

$$MV=\left({\displaystyle \frac{m{m}_{105}-m{m}_{950}}{m{m}_{105}}}\right)\times 100$$

(5)

$$CC=\left({\displaystyle \frac{m{m}_{950}}{m{m}_{105}}}\right)\times 100$$

(6)

$$CF=100-\%CH-MV-CC$$

(7)

where

• $MV$: Volatile material;
• $CC$: Ash content;
• $CF$: Fixed carbon;
• $m{m}_{105}$: Sample mass after drying at 105 °C (g);
• $m{m}_{950}$: Residual mass of the sample after incineration at 950 °C (g);
• $\%CH$: Percentage of moisture content.

The samples (chips and charcoal) were ground in a Cole–Parmer model 4301-00 mill at 20,000 rpm for ten seconds to avoid the generation of very small particles (<0.25 mm) that can cause errors in obtaining volatiles; the ground material (wood or charcoal) was sieved to use the material retained in the 40- and 60-mesh screens. The samples were dried in a Memmert VN100 digital oven for 24 h. To obtain the moisture, volatile, and ash contents, samples of 1 ± 0.1 g of ground material were weighed on an OHAUS Scout Pro electronic scale with an accuracy of 0.01 g.

2.7. Statistical Analysis

The charcoal samples were evaluated under a $2\times 3$ factorial arrangement where the first factor was the type of wood (sapwood and heartwood) and the second factor was the position of the charcoal samples in the kiln: low, medium, and high. The study variables are shown in Table 1. For the analysis of variance, it was verified that the assumptions of normality (Shapiro–Wilk test [38]) and homogeneity of variances between treatments (Bartlett’s test [39]) were met. The Box–Cox transformation [40] was used when the variable did not meet the assumption of normality and/or homogeneity of variances. When the Box–Cox transformation did not work in the cases of the calorific value of wood and the basic density of charcoal, the nonparametric Mann–Whitney U test was used (Table 1).

Table 1. Study variables for wood and charcoal from Q. scytophylla.

Variable	Transformation	Shapiro–Wilk Test	Bartlett Test	Test for Comparing Means
Heat capacity: Wood	Box–Cox	0.07801 *	0.02195 NS	Mann–Whitney U
Heat capacity: Charcoal	None	0.4205 *	0.2681 *	Tukey
Basic density: Wood	Box–Cox	0.8524 *	0.09368 *	Tukey
Basic density: Charcoal	Box–Cox	NS	NS	Mann–Whitney U
Granulometry $20\times 20$	None	0.7263 *	0.0565 *	Tukey
Granulometry $50\times 50$	None	0.1924 *	0.2457 *	Tukey
Granulometry $100\times 100$	Box–Cox	0.1560 *	0.07577 *	Tukey
Moisture content (%)	None	0.4168 *	0.8441 *	Tukey
Volatile (%)	Box–Cox	0.1809 *	0.3193	Tukey
Ashes (%)	Box–Cox	0.2560 *	0.2160 *	Tukey
Fixed carbon (%)	None	0.6634 *	0.0780 *	Tukey
Friability	None	0.445 *	0.464	Tukey

* Significant at $\alpha =0.05$; NS—not significant.

According to [34], there was variability in the charcoal quality.

The Box–Cox transformation was performed using the powerTransform function in the car package [41]. The normality test was performed using the shapiro.test function from the stats package [42]. The homogeneity of the variances test was performed using the bartlett.test function from the stats package. Statistical analyses were performed using R statistical software [43].

3. Results

3.1. Oven Performance

The initial weight of the green wood averaged 3600 kg, and the final weight of the charcoal was 720 kg, corresponding to a yield of 20%. However, based on the ratio between the dry weights, the yield was calculated at 26.7%. Even so, with both calculations, the yield was comparable with that specified by the manufacturer, who states that the firewood capacity per load of 3000 to 3200 kg yields a charcoal production per batch of 500 to 600 kg [30].

3.2. Heat Capacity

There are no statistically significant differences in the calorific values of sapwood and heartwood (Table 2). In charcoal, there are significant differences in calorific value (F = 6.30, p-value < 0.001). The calorific value in the lower layer is higher than in the rest, while the middle and upper layers have a lower calorific value (Table 2).

Table 2. Calorific value: Comparison of averages for wood and charcoal Q. scytophylla.

Material	Type	n	Mean ± Standard Error (cal g−1)	Group
Wood	Sapwood	5	4623.55 ± 9.96	a
	Heartwood	5	4561.82 ± 39.15	a
Charcoal	Lower sapwood	5	7508.26± 113.33	a
	Lower heartwood	5	7296.28 ±113.33	ab
	Middle sapwood	5	7016.38 ±113.33	abc
	Upper sapwood	5	6952.37 ±113.33	bc
	Upper heartwood	5	6862.99 ±113.33	bc
	Middle heartwood	5	6751.14 ±113.33	c

All averages with the same letters are statistically equal.

3.3. Basic Density

There are significant differences in the wood basic density (F = 103.72, p-value < 0.001), where it is higher in heartwood (Table 3). Sapwood charcoal in the upper layer had a higher basic density; in fact, sapwood charcoal in any of the layers has a higher density than heartwood charcoal (Table 3).

Table 3. Basic density: comparison of averages in wood and charcoal Q. scytophylla.

Material	Type	n	Mean ± Standard Error (g cm−3)	Group
Wood	Heartwood	32	0.80±0.01	a
	Sapwood	37	0.70±0.01	b
Charcoal	Upper sapwood	50	0.57 ± 0.01	a
	Middle sapwood	50	0.53 ± 0.01	b
	Lower sapwood	32	0.53 ± 0.01	b
	Lower heartwood	30	0.52 ± 0.02	bc
	Middle heartwood	50	0.50 ± 0.01	cd
	Upper heartwood	50	0.50 ± 0.01	d

All averages with the same letters are statistically equal.

3.4. Granulometry and Friability

In the $20\times 20$ granulometry, there are significant differences in the means (F = 21.08, p-value < 0.001); the granulometry is greater in the lower layer screen, while in the upper and intermediate screens, there are no significant differences (Table 4).

Table 4. Granulometry: Comparison of means by screening.

Screen Size (mm)	Layer	n	Mean ± Standard Error (g*)	Group
$20\times 20$	Lower	10	423.40 ± 24.64	a
	Upper	10	250.90 ± 24.64	b
	Middle	10	210.30 ± 24.64	b
$50\times 50$	Upper	10	310.80 ± 32.28	a
	Lower	10	259.80 ± 32.28	ab
	Middle	10	188.40 ± 32.28	b
$100\times 100$	Middle	10	497.70 ± 56.63	a
	Upper	10	270.20 ± 56.63	b
	Lower	10	127.500;± 56.63	b

* The averages represent the grams of charcoal recovered per thousand grams of sample for each screen size. All averages with the same letters are statistically equal.

In the $50\times 50$ granulometry, there are significant differences in the means (F = 3.63, p-value < 0.05); the granulometry in the upper and lower layers are statistically equal (Table 4). In the $100\times 100$ granulometry, there are significant differences in the means (F = 12.75, p-value < 0.001); the granulometry is greater in the middle layer of the sieve (Table 4).

Regarding friability, this is greater in the lower layer (F = 4.49, p-value < 0.05), followed by the middle and then upper layers (Table 5).

Table 5. Friability: Comparison of means by layer.

Capa	n	Mean ± Standard Error (%)	Group
Lower	10	18.93 ± 2.104	a
Middle	10	10.36 ± 2.104	b
Upper	10	16.81 ± 2.104	ab

All averages with the same letters are statistically equal.

There are significant differences in the averages of the moisture content (F = 134.97, p-value < 0.001), where the moisture content is higher in sapwood charcoal (Table 6). There are significant differences in the average volatile content (F = 5.47, p-value < 0.05), where the percentage of volatiles is higher in heartwood charcoal (Table 6). There are significant differences in the percentage of ash (F = 51.23, p-value < 0.001), where the percentage of ash is higher in heartwood charcoal (Table 6).

Table 6. Percentages of moisture, volatiles, ash, and fixed carbon: Comparison of averages for sapwood and heartwood.

Variable	Material	n	Mean ± Standard Error	Group
% Humidity	Charcoal sapwood	10	3.56±0.12	a
	Charcoal heartwood	10	1.49 ± 0.12	b
% Volatile	Charcoal heartwood	10	22.26 ± 0.58	a
	Charcoal sapwood	10	20.21 ± 0.58	b
% Ashes	Heartwood charcoal	10	2.52 ± 0.04	a
	Charcoal sapwood	10	2.18 ± 0.04	b
% Fixed carbon	Charcoal sapwood	10	74.05 ± 0.7	a
	Heartwood charcoal	10	73.73 ± 0.7	a

All averages with the same letters are statistically equal.

There are no significant differences in the averages of the fixed carbon percentage (F = 0.11, p-valor = 0.74) (Table 6).

3.5. Statistical Model

An analysis was conducted regarding the relationship between the calorific value of charcoal and the basic density of charcoal, moisture content, volatiles, ash, and fixed carbon. The results show a linear relationship between the calorific value of charcoal and its basic density (Table 7 and Figure 3a. From the adjusted simple linear regression model, we obtain an $R^2=0.67$; although the model is not conclusive, explaining only 67% of the total sample variance of the calorific value of charcoal, it does give us indications of the existence of a linear relationship between the calorific value and the basic density of charcoal.

Table 7. Estimated simple linear model.

Parameter	Estimate	Standard Error	t Value	p-Value
Intercept	1229.85	964.78	1.27	0.22
Basic density	11,207.05	1837.71	6.10	0

Figure 3. Estimated model for the calorific value of charcoal with basic density as an explanatory variable. (a) Fitted model. (b) Q-Q plot of residuals.

The Shapiro–Wilk test shows that the residuals meet the normality assumption (W = 0.93, p-value = 0.18), which is also observed in Figure 3b.

4. Discussion

The production of charcoal in Mexico using CEVAG-type metal kilns is considered a technological innovation for charcoal production that complies with regulatory standards. These kilns were developed by INIFAP and offer significant advantages over traditional methods, including a capacity of 3000-3200 kg of firewood per load, production of 500–600 kg of charcoal, and a process lasting 28 to 36 h. Portable steel kilns, such as the RMV steel kiln [44] and Mark V [45], can be a benchmark for CEVAG, reducing the percentage of pollution from various greenhouse gases compared with traditional systems, and thus, mitigating the environmental impact.

In this study on Q. scytophylla, it was observed that depending on the position within the kiln, there are statistically significant differences in the basic density of the charcoal, with the maximum being 0.57 g cm⁻³ for sapwood in the upper part. This demonstrates that temperature gradients and air flow affect the quality of the charcoal produced in this kiln type [46]. On the other hand, the best quality charcoal from Quercus sideroxila Humb. & Bonpl. was obtained from the middle section of the Brazilian beehive-type kiln, with an average calorific value of 8096.88 cal g⁻¹, a moisture content of 3.3%, 19.0% volatile material, 5.2% ash, and 72.2% fixed carbon. This quality is acceptable according to international standards (from France and Belgium) [5].

The calorific values obtained for Q. scytophylla charcoal (6751.14–7508.26 cal g⁻¹) in this study have already been reported for this and other species in Mexico, for example, Quercus laurina (7332.57 cal g⁻¹) and other species of oak trees (6926.534 and 8359.61 cal g⁻¹). The variations between the different positions within the oven (lower, middle, and upper) were similar to previous research, demonstrating that temperature and air flow within the oven influence the energy properties of the resulting charcoal [47].

This study did not generate the thermal profile [48,49], which would allow the carbonisation process to be controlled and optimised, as well as provide correlations with the quality parameters (moisture content, ash, volatile material, and fixed carbon). In the context of the CEVAG system, this would constitute a complementary tool for advanced standardisation, but it is not a requirement for evaluating the performance of this technology.

The quality of the charcoal (fixed carbon) of 73.73-74.05% is close to the optimal parameters (≥75%) of international standards DIN 51749 and EN 1860-2:2023 [50,51]; therefore, we can consider the values obtained in this study to be acceptable. On the other hand, considering the moisture content (1.49–3.56%) and ash content (2.18–2.52%) results observed in this study, it is considered that national and international standards are met. However, the moisture content is below the recommended maximum limit of 8.0%, which is an indicator of the efficiency of the carbonisation process, but is comparable with that of species such as Eucalyptus (5.0–6.1%) [52].

The moisture content of the wood utilised during the carbonisation process exerts negligible influence on the majority of the variables that ensure the quality of the charcoal, with the exception of the friability value. In this particular instance, a positive correlation is observed between the moisture content of the wood and friability. Consequently, it can be concluded that the moisture content of the wood employed for carbonisation must be less than 20% [53].

The charcoal yield obtained in the portable metal furnace in this study is low (26.7% anhydrous basis), although its basic density is high (0.80 g cm⁻³ heartwood and 0.70 g cm⁻³ sapwood). This is attributed to the high moisture content (25.8%) of the Q. scytophylla wood before the carbonisation process. This situation was also found with woods of Q. laurina and Q. crassifolia [23], which, despite their high basic densities, had low yields. However, the yields observed in our research on Q. scytophylla wood are within the parameters for the carbonisation of Mexican forest species; for example, species such as Q. spp. yield 16 to 30% of the raw material weight. The results are similar when using brick kilns at 4.4 m³ ton⁻¹ [23].

The preferred wood for wood energy in Oaxaca, Mexico, is oak (Q. spp.), which has a calorific value of 7332.57 cal g⁻¹, a moisture content of 3.14%, 21.93% volatile matter, 3.16% ash content, and 74.91% fixed carbon. Regarding charcoal quality, the best indicators were found at the top of the kiln: lower moisture content, lower percentages of volatile matter and ash, and a higher percentage of fixed carbon. Overall, the charcoal produced in Ixtlán de Juárez, Oaxaca, complies with international standards for moisture and ash content, and the charcoal from the upper part of the kiln also meets the requirements for fixed carbon [23]. On the other hand, the dendroenergetic properties of wood from four Brazilian juvenile tropical tree species (Eucalyptus grandis Hill, Acacia mearnsii De Willd, Mimosa scarabella Benth., and Atelia glazioviana Baill) determined by [54] fluctuated between 4467 and 4545 cal g⁻¹ of calorific value, 75.33% and 82.90% of volatile material, 1.14% and 1.75% of ash content, and 15.96% and 23.21% of fixed carbon content. In our work, higher calorific values were found in the inner layer for sapwood (7508.26 cal g⁻¹) and heartwood (7296.28 cal g⁻¹).

The basic densities of wood from five common tropical forest species (Alnus acuminata subsp. arguta (Schltdl.) Furlow, Arbutus xalapensis Kunth, Myrsine juergensenii (Mez) Ricketson & Pipoly, Persea longipes (Schltdl.) Meisn., and Prunus serotina Ehrh.) used for charcoal production in Oaxaca, Mexico, range from 0.37 to 0.50 g cm⁻³, containing 75.41% to 83.66% volatile material and 0.56% to 1.50% ash, and reach 4657.50 to 5968.76 cal g⁻¹ of calorific value; meanwhile, the charcoal produced from these woods contains between 28.4% and 34.3% volatile material and 1.13% and 4.83% ash, with a calorific value of 7017.30 to 7669.34 cal g⁻¹. These tests were carried out under laboratory conditions and the carbon yields ranged from 26.2% to 34.1% [35]. These researchers demonstrated that wood converted to charcoal becomes a more efficient fuel. Charcoal produced from woods with a very high basic density, such as Ebenopsis ebano (Berland.) Barneby & J.W. Grimes and Prosopis laevigata (Humb. & Bonpl. ex Willd.) M.C. Johnst. but produced in a pit-type oven [5] presented the following charcoal quality values: moisture contents of 3.6% and 3.5%; volatile matter contents of 22.8% and 24.9%; ash percentages of 2.8% and 3.2%; fixed carbon contents of 70.8% and 68.6%; and calorific values of 7222.94 and 7099.70 cal g⁻¹, respectively. These are ranges within which the values obtained for Q. scytophylla in this research fall.

On the other hand, the estimated linear regression model suggests that the calorific value of charcoal can be explained by its basic density (R² = 0.67); although the model is not conclusive, it allows us to identify a type of correlation between the two variables, which is essential for predicting the energy quality. This coincides with other research demonstrating the importance of basic density as an energy predictor. For example, the basic density of Q. scytophylla charcoal (0.50–0.57 g cm⁻³) is comparable with other species in Mexico [47,55].

The study provides a robust experimental design (factorial $2\times 3$) that allows for the simultaneous evaluation of multiple factors affecting charcoal quality. This methodological approach is valuable for future studies on the energy characterisation of Mexican forest species. This research validates the effectiveness of CEVAG metal kilns in producing high-quality charcoal from Mexican species. This is particularly relevant considering that Mexico allocated 620,195 m³ of wood for charcoal production in 2018, with Q. spp. accounting for 271,647 m³ [47].

The present study also demonstrates that regarding calorific value, basic density, and quality variables in wood and charcoal, sapwood yields superior results in comparison with heartwood.

5. Conclusions

Regarding the sustainable utilisation of forest resources in Mexico, charcoal emerges as a renewable alternative, underscoring the economic and environmental significance of the ecosystem services provided by this wood resource.

The present study offers results that identify opportunities for improvement in fixed carbon content to achieve optimal international standards. In particular, it is suggested that Q. scytophylla is a viable species for the production of charcoal of acceptable quality. This is relevant for sustainable forest management, especially in forest ejidos such as Cordón Grande, where charcoal production can be integrated as part of comprehensive forest utilisation strategies. The carbon yield (20.0%) in this study corresponds to the index estimated by the manufacturer of the CEVAG furnace.

This research study serves to reinforce the scientific underpinnings that support the advancement of wood energy in Mexico. In addition, it contributes to the global corpus of knowledge pertaining to the carbonisation of tropical and subtropical species. Consequently, it establishes itself as a significant point of reference for future studies in the forestry and energy sectors.

Future research arising from the analysis of the data presented includes the following: optimisation of carbonisation parameters (temperature and time); full life cycle analysis of charcoal production; and characterisation of other Mexican species with potential, such as the genus Q. spp.