Circular Economy in Charcoal Production: Valorization of Residues for Increased Efficiency and Sustainability

Abstract

This study explores a circular economy approach in charcoal production, utilizing combustion gases from the process itself to optimize efficiency and quality, minimizing waste and reducing emissions. The research investigates the pre-drying of Eucalyptus sp. wood with these gases before carbonization, through an innovative system that directs gases from the carbonizing furnace to a separate drying furnace. Wood samples were dried at 120 °C and 150 °C for 15, 22.5, and 30 h before carbonization. The analysis included the gravimetric yield of charcoal, semi-carbonized wood, and fines, in addition to evaluating key charcoal properties. Results demonstrated that drying with combustion gases at 150 °C increased the charcoal yield by 7%, regardless of drying time. Furthermore, this pre-drying improved charcoal quality, raising fixed carbon content from 74.68% to over 81% and reducing volatile matter from 24.40% to below 18%. These findings highlight that the utilization of combustion gases for wood drying not only significantly enhances the efficiency and quality of charcoal production but also contributes to the reduction in greenhouse gas emissions, promoting a more sustainable and environmentally friendly alternative compared to conventional methods.

1. Introduction

Energy is fundamental to human development, underpinning technological, economic, and social progress. Fossil fuels play a critical role in driving the industrial revolution and fostering the growth of modern economies. However, reliance on these finite resources has contributed to global warming and pollution [1]. Consequently, shifting the energy matrix towards renewable sources, such as solar, wind, hydro, and biomass, is essential [2,3]. Investing in clean and renewable technologies is therefore crucial to ensuring a prosperous and sustainable future for the coming generations, promoting development that respects planetary boundaries.

Brazil produces five million tons of charcoal annually, with 71% consumed by the steel and metallurgical sector, positioning the country as a leader in sustainability within this sector, which accounts for 7% of global greenhouse gas emissions [4]. Demand for charcoal is expected to increase due to its high quality and its potential as a renewable energy source to replace coal [5]. However, its competitiveness depends on a robust production chain that employs advanced technology and utilizes all byproducts of the process [6].

Wood moisture content is a major challenge in charcoal production. Carbonizing wood with high moisture content increases carbonization time, pollutant emissions, fine particle generation, and reduces charcoal gravimetric yield [7,8]. Freshly harvested wood has a high moisture content, often with the water mass exceeding the wood mass [9,10]. Controlling drying is difficult because it is influenced by both wood characteristics and environmental parameters. High-density wood with a larger diameter and the presence of bark, when stored in environments with low temperature and high relative humidity, tends to dry more slowly [11,12]. Furthermore, water interacts with wood in different ways, forming weak capillary bonds in void spaces and strong hydrogen bonds with hydroxyl groups in the cell wall, resulting in varying drying rates over time [13].

Air drying is the most common method for reducing the moisture content of wood intended for charcoal production due to its low cost and the possibility of implementation near harvesting sites, thus reducing forest transport costs [14]. However, it is a lengthy process, typically taking between six months and a year, and often fails to achieve the 35% moisture content recommended for wood carbonization [15]. Artificial drying presents itself as an efficient alternative to complement traditional field drying, allowing for the carbonization of wood with a lower moisture content. This innovative approach significantly enhances the gravimetric yield of the process, yielding positive economic and environmental impacts. By boosting charcoal production, artificial drying supports sector development, while reducing the land required for eucalyptus plantations mitigates the environmental footprint of the activity. Furthermore, optimizing the production process by eliminating the transportation stage between the dryer and the carbonization furnace leads to a substantial reduction in production costs.

Therefore, there is a need for methods to improve the charcoal production process. This study aimed to evaluate the effect of channeling combustion gases to dry wood within the furnace, varying the time and temperature employed, and analyzing the resulting impacts on charcoal yield and quality.

2. Materials and Methods

2.1. Acquisition and Characterization of the Raw Material

Seven-year-old Eucalyptus sp. trees were felled and sectioned into 1 m logs with diameters ranging from 13.5 to 16.5 cm; the species and age were chosen because they are those used in the production of charcoal in Brazil. This size was chosen to fit the dimensions of the carbonization furnace. Wood properties were determined by selecting 12 logs per diameter class and extracting 5 cm thick disks at 25%, 50%, and 75% of each log’s total length. Heartwood and sapwood areas on the disks were visually analyzed to determine the heartwood/sapwood ratio. These same samples were used to determine basic density using the ratio of dry mass to saturated volume.

Wood moisture content (dry basis) during artificial drying was determined using three logs per diameter class, totaling nine logs per kiln load. Three 5 cm thick disks were cut from each log at 25%, 50%, and 75% of its total length. Moisture content was calculated using the following equation: UBs = (Um − Ms)/Ms, where UBs = moisture content (dry basis), Ms = dry mass, and Um = wet mass. The average moisture content of the disks was taken as the moisture content of the respective treated logs.

2.2. Description of the Kiln-Furnace System and the Gas Transport System

The furnace system consisted of four circular furnaces connected to a central gas burner via ducts (Figure 1). Each furnace had a volumetric capacity of approximately 1 m³ (1.4 st) of wood and was constructed of solid ceramic bricks laid with clay mortar. The internal dimensions of each furnace were 1.20 m in diameter, with 1.00 m high walls and a 0.40 m dome. Four 5 × 5 cm air intake openings (air controllers) were evenly distributed around the furnace perimeter at ground and floor level.

The furnace for burning the carbonization gases was divided into two components: the combustion chamber and the chimney. The combustion chamber was built with double 20 cm walls of solid ceramic bricks and a clay, sugar, and cement mortar mixture, forming a cylinder with internal dimensions of 75 cm in height and 65 cm in diameter. It was also internally lined with ceramic fiber blanket for improved thermal insulation.

A 40 × 40 cm hinged window was installed on the side of the combustion chamber for auxiliary fuel feeding, ignition, and maintenance. A 15 × 15 cm metal grate was installed in the center of the furnace base to provide air intake to the combustion chamber via a 15 × 15 × 75 cm (height × width × length) underground duct.

After constructing the combustion chamber, a constriction was created, reducing the internal diameter to 35 cm to increase gas residence time. Above this constriction, the chimney, with an internal diameter of 50 cm, was built. The chimney was constructed of double-walled masonry, using the same materials as the combustion chamber, and extended two meters above the combustion chamber. It was also internally lined with ceramic fiber blanket.

The carbonization gas transport ducts were built with solid ceramic bricks and laid with clay mortar. Four ducts, each with internal dimensions of 15 cm high, 15 cm wide, and 85 cm long, connected the furnaces to the furnace. A manually operated valve to control the flow rate of carbonization gases to the furnace was installed in the center of each duct, 60 cm from the furnace wall

In this system, two furnaces were used for carbonization and gas generation, while the other two were used for drying. The gas transport system was designed to utilize combustion gases from the furnace by injecting them into the drying furnaces (Figure 1).

The gases generated during carbonization were burned in the furnace’s combustion chamber and released as thermal energy through the chimney. A centrifugal fan and metal piping drew the gases into the chimney and directed them to the drying furnace via masonry ducts.

Combustion gas extraction from the chimney occurred 2.10 m above the furnace base through 2 mm thick metal tubing with a 180 mm internal diameter. At this point of the chimney, the metal tubing was installed at a 90° angle to the furnace floor to extract the gases. Immediately below this tubing, a section of 2 mm thick galvanized sheets with a 180 mm internal diameter was connected to transport the combustion gases to the location of the centrifugal fan. A safety valve was installed in this tubing, 1.80 m from the floor, to interrupt the combustion gas flow if necessary. Below this valve, 1.5 m from the floor, a manually operated damper (12 × 20 cm opening) was also installed to control the combustion gas temperature in the drying furnace.

The 0.5 hp, 1800 rpm centrifugal fan drew the carbonization gases and conveyed them to the drying furnace. A Y-shaped duct, constructed of solid ceramic bricks and clay mortar with a 15 × 15 cm square cross-section, was connected to the fan outlet to direct the combustion gases to the drying furnaces.

Combustion gases were injected into the drying furnace through a 15 × 15 cm opening located on the furnace’s side wall. After passing through the wood load in the drying furnace, the combustion gases were released into the atmosphere through the chimney due to the positive pressure generated by the centrifugal fan.

2.3. Drying Inside the Kiln and Subsequent Wood Carbonization

Wood loading was performed manually, arranging the wood vertically up to one meter in height. In the remaining space, in the furnace dome, the wood was positioned horizontally. Larger diameter logs (15 ± 3 cm) were placed near the air controllers, while smaller diameter logs (9 ± 3 cm) were placed in the center of the furnace. Logs with a diameter of 12 ± 3 cm were placed in the remaining areas.

After loading, the doors were closed with solid ceramic bricks and clay mortar, leaving two centered openings, one at the top and one at the bottom, for furnace ignition. Ignition was performed as follows: first, the gas burner’s combustion chamber was ignited, followed by the carbonization furnace. Ignition in the carbonization furnace occurred at the top of the door. After this stage was established, the top opening was closed to begin carbonization. Lignocellulosic residues were used as biomass fuel for ignition in both procedures. Carbonization was conducted while monitoring the temperature in the furnace dome according to the theoretical curve, with slight variations depending on the wood’s initial moisture content (Figure 2).

Furnace temperature was monitored using a Minipa MT-350 infrared pyrometer (Minipa Indústria e Comércio Ltd., São Paulo, Brazil) with a measurement range of 30 °C to 550 °C and controlled by adjusting the air controllers. The process was considered complete when the carbonization front reached the last air controllers near the gas outlet to the duct connecting the furnace to the gas burner.

The procedure concluded with the closing of all air controllers and the gas conduction duct. Clay was applied to seal any cracks or fissures in the furnace to aid cooling.

The gases generated during the carbonization process were burned in the furnace’s combustion chamber and transported to the drying furnace for artificial wood drying within the furnace. The total mass of wood used was measured to calculate moisture loss and yields.

In the drying furnace, wood loading and furnace closing were performed similarly to the carbonization furnace. Ignition was then started in the drying furnace, repeating the operational procedure used for the carbonization furnace (Figure 3).

After cooling and opening the furnaces, material smaller than 10.5 mm (fines), partially carbonized wood (embers), and charcoal were separated and weighed. Charcoal samples were then collected for a qualitative analysis.

2.4. Thermal Profile of the Carbonization Kiln and the Drying Kiln

In both the carbonization and drying furnaces, temperature monitoring points were installed: one at the top and two on the furnace wall, 50 cm from the ground. Temperature data were collected using a Minipa MT-350 infrared temperature sensor with a measurement range of 30 °C to 550 °C. In the drying system, temperature was also monitored at the gas inlet to the drying furnace using type K thermocouples (Thermo Sensors Corporation, Garland, TX, USA) and an ICEL TD-880 datalogger (Hall Technologies, Coppell, TX, USA).

The temperatures of the carbonization furnaces, drying furnaces, and combustion gases were monitored and recorded at regular 1 h intervals from the start of each treatment. These data were used to create a thermal profile of the wood drying system using the burner’s combustion gases.

To evaluate the effect of wood drying time inside the furnace on the charcoal production cycle, the total carbonization time was measured. This involved recording the time spent on each drying treatment and the carbonization time itself, with the latter considered to be the time until the furnaces were completely sealed.

2.5. Calculation of Carbonization Yields

The gravimetric yield in charcoal, semi-carbonized wood and fines were calculated according to Equations (1)–(3).

CGY = 100 × DMS/Mms

(1)

where CGY = charcoal gravimetric yield (%); DMS = dry mass of charcoal (kg); and DWM = dry wood mass (kg).

GYF = 100 × Dmf/Mms

(2)

where GYF = gravimetric yield in fines (%); Dmf = dry mass of fines (kg); and Mms = dry wood mass (kg).

Sw = 100 × Sm/Dm

(3)

where Sw = semi-carbonized wood Yield (%); Sm = semi-carbonized wood dry mass (kg); e Dm = dry wood mass (kg).

2.6. Charcoal Properties

Charcoal samples were ground and sieved, and the fraction passing through a 60-mesh sieve was collected. These samples were then dried in an oven at 103 ± 2 °C until a constant mass was achieved, according to NBR 6923 [16]. The higher heating value (HHV) of the charcoal was determined using an adiabatic bomb calorimeter, in accordance with NBR 8633 [17]. The determination of volatile matter (VM), ash content (AC), and fixed carbon (FC) followed the procedures outlined in NBR 8112 [18].

Charcoal friability was assessed using the “pan test”. This test involves crushing approximately 500 g of quartered charcoal and sieving it. The fraction retained on a 25.4 mm sieve was used for the test. This material was placed in a covered steel pan and subjected to a pressure of 1 metric ton in a laboratory press for 1 min. The pressure was then released, and the charcoal was sieved again, with all particles smaller than 10.5 mm being discarded. Friability was calculated based on the final mass after sieving.

2.7. Statistical Analysis

The experiment was conducted using a completely randomized design (CRD) in a factorial arrangement with two temperatures (120 and 150 °C), three drying times (15, 22.5, and 30 h), and four replications; this temperature was selected to facilitate the drying of the wood while minimizing structural damage. The yields and properties of the charcoal were assessed for normality using the Shapiro–Wilk test and for homogeneity of variances using Bartlett’s test. Two analyses of variance (ANOVAs) were performed, one including the control values. When a statistically significant difference was observed using the F-test, at a 5% significance level, Dunnett’s test was used for post hoc comparisons.

3. Results and Discussion

3.1. Wood Properties

The wood exhibited an average basic density of 485.8 kg/m³ and a mean heartwood-to-sapwood ratio (H/S) of 0.88 (

Table 1. Carbonization process yields.

	Drying Time (Hour)	Charcoal Gravimetric Yield (%)	Semi-Carbonized Wood Yield (%)	Fine Yield (%)
Control	-	31.2	6.50	3.98
120 °C	15	35.21	10.87	4.18
	22.5	36.20	6.91	3.08
	30	35.58	5.54	3.64
150 °C	15	36.55	3.14	2.88
	22.5	38.84	3.61	2.31
	30	39.03	6.09	3.00

). These values are consistent with those reported in the literature for Eucalyptus sp. clones cultivated for charcoal production [19].

The heartwood-to-sapwood ratio (H/S) exhibits a negative correlation with the charcoal yield, as a greater proportion of heartwood impedes fluid transport due to vessel occlusion by tyloses [20]. Moreover, in Eucalyptus sp. clones, regardless of age, heartwood demonstrates low permeability, ranging from 0 to 3.9 cm³/cm·atm·s, whereas sapwood permeability ranged from 270 to 400 cm³/cm·atm·s [21]. Consequently, heartwood impermeability causes water vaporization within the wood during carbonization, leading to cracking in the charcoal and increased production of fines.

3.2. Parameters of Wood Drying in the Kiln and Carbonization

Carbonization performed without the use of combustion gases for wood drying closely approximated the theoretical carbonization curve (Figure 4). Following 35 h of kiln carbonization, the average wall temperature exceeded that of the bottom region, indicating the advancement of the carbonization front.

The introduction of combustion gases from carbonization in other furnaces facilitated the wood drying and influenced the material’s carbonization curve (Figure 5 and Figure 6).

For a drying temperature of 120 °C, temperature peaks of 61.6 °C, 65 °C, and 65.45 °C were observed after 15, 22.5, and 30 h of drying, respectively. At a drying temperature of 150 °C, the observed temperature peaks were 83.8 °C, 86.55 °C, and 87 °C for drying times of 15, 22.5, and 30 h, respectively.

The utilization of high-temperature carbonization gases for artificial drying followed by carbonization facilitated the artificial drying and initial heating of the wood load and kiln structure, resulting in process efficiency gains. The gas cooling process proved effective, as these gases exited the carbonization kiln at temperatures exceeding 300 °C [22]. Without this cooling, there would be a risk of self-ignition of the loaded wood and degradation due to high-temperature exposure [23].

Regardless of the evaluated time and temperature, at the end of the artificial drying process, when the kilns were ignited, there was a sudden increase in internal temperature, surpassing the theoretical curve. This occurred due to the ignition process of the kiln to initiate carbonization, resulting from the combustion of lignocellulosic residues and part of the loaded wood. In kilns with artificial wood drying, this increase was more pronounced, as minimal energy was required to remove the remaining moisture. This efficiency gain implies a reduction in carbonization time and an increase in gravimetric yield [8,24]. After the ignition, the temperatures observed at the top of the kilns after the artificial drying followed by carbonization were within the range stipulated by the theoretical curve, demonstrating that artificial drying inside the kiln does not interfere with process control.

In the third and fourth phases of carbonization, higher temperatures were observed on the kiln wall compared to the control, especially in the back region. The insertion of carbonization gases occurred on only one side of the kiln, and the preferred path taken by these gases caused a greater reduction in moisture content in this region and, consequently, a greater heat exchange with the kiln structure, resulting in higher temperatures in these areas during carbonization.

3.3. Charcoal Production Yields

The use of carbonization gases increased the gravimetric yield in all treatments and reduced the content of sticks and fines at a temperature of 150 °C (

Table 2. Effect of combustion gas wood drying on charcoal properties: volatile matter (VM), ash (A), fixed carbon (FC), and higher heating value (HHV).

Properties	Control	120 °C			150 °C
		15 h	22.5 h	30 h	15 h	22.5 h	30 h
VM (%)	24.40	17.30	16.49	17.31	16.45	17.96	17.05
FC (%)	74.68	82.09	82.92	82.40	82.78	81.31	82.36
A (%)	0.92	0.60	0.59	0.68	0.67	0.60	0.65
HHV (MJ/kg)	30.53	32.61	33.65	32.78	33.62	32.94	33.01

).

Gravimetric yield increased between 12.8% and 25.1% when the wood was dried using carbonization gases. The smallest increases were observed at 120 °C for 15 h, while the largest occurred at 150 °C for 30 h. These gains were attributed to the removal of water from the wood by the combustion gases, thereby reducing the amount of woody material required to be fully combusted to provide the energy needed for residual moisture removal. Additionally, the pre-heating of the wood load and the furnace during the drying process contributed to a reduction in the wood consumption during the ignition and endothermic phases of carbonization.

The increased gravimetric yield represents important environmental and economic benefits, as a smaller area of planted forests will be required to meet the demand. This translates to reduced consumption of fertilizers, pesticides, fossil fuels, and other inputs used in the eucalyptus wood production chain.

The use of 150 °C combustion gases reduced ember formation during wood carbonization. The increased energy input for artificial drying decreased the water mass within the loaded wood. Consequently, given the self-thermal nature of the process—where the material itself provides energy—less energy was consumed for water removal, enhancing wood-to-charcoal conversion. This also resulted in a reduced quantity of semi-carbonized wood. The observed values align with the previous studies [25] and were considered low, indicating a near-complete wood conversion to charcoal and demonstrating the suitability and effectiveness of the theoretical carbonization curve employed.

Employing combustion gases for artificial drying also reduced the generation of fines at 150 °C. During carbonization, the rapid temperature increases cause water within the wood to vaporize. This volume expansion can lead to the collapse or rupture of anatomical components, reducing charcoal’s mechanical strength and increasing fine production [26]. Thus, lower moisture content during carbonization results in fewer fines.

3.4. Charcoal Properties

The application of combustion gases in charcoal production led to an improvement in product quality, as evidenced by an increase in the higher heating value and fixed carbon content, coupled with a reduction in volatile matter content.

The fixed carbon content was 74.68%. All treatments using carbonization gases for wood drying showed an increase of at least 9%. Fixed carbon corresponds to the material that burns in solid form during combustion, being desirable for use in steel mills (ref.). The increase in fixed carbon content reduced the volatile matter content, with a decrease of up to 31% in its content with wood drying. The ash content remained constant among treatments. The changes in the proximate composition of the charcoal were reflected in the higher heating value of the produced charcoals, where the treatments with artificial drying presented values higher than 32.22 MJ/kg, and the control presented 30.53 MJ/kg. Therefore, artificial wood drying using carbonization gases increased the carbonization yield and the quality of the produced charcoal, generating quantitative and qualitative gains.

4. Conclusions

The results of this study clearly indicate the positive impact of employing carbonization gases for artificial wood drying prior to carbonization. The pre-drying process effectively reduced wood moisture content, leading to a more efficient carbonization process and several key improvements in charcoal production. Notably, gravimetric yields increased significantly (up to 25.1%), minimizing the formation of undesirable byproducts such as semi-carbonized wood and fines. Furthermore, the resulting charcoal exhibited superior quality characteristics, including a substantial increase in fixed carbon content (at least 9%), a marked reduction in volatile matter content (up to 31%), and consequently, a higher heating value, exceeding 32.22 MJ/kg. These improvements are attributed to the enhanced control over the carbonization process facilitated by the pre-drying stage. This technique offers a promising approach to enhance the sustainability and economic viability of charcoal production by reducing raw material consumption and producing a higher quality product, suitable for demanding applications such as steelmaking.