Understanding the Impacts of Pyrolysis Temperature on the Energy Performance of Eucalyptus spp. Charcoal ^†

Abstract

This study aimed to investigate the influence of two pyrolysis temperatures (300 °C and 450 °C) on the energy quality of charcoal using a mix of commercial eucalypt woods. In this study, pyrolysis was carried out at a heating rate of 3.33 °C.min⁻¹ for a duration of 3 h. The apparent density, bulk density, immediate analysis, high heating value, energy density, and combustibility index of the charcoal were measured. Under the conditions analyzed, pyrolysis performed at a final temperature of 450 °C resulted in charcoal with better energy performance than that produced at 300 °C.

1. Introduction

Primary energy resources, such as wood energy, are sources of energy and supplies obtained from nature [1,2]. Humanity has been using energy derived from biomass since the dawn of history. This fuel type has been widely used for heat production, converting wood energy for cooking and/or heating [1,2,3]. Currently, biomass is the only source of fuel for domestic use in many households in developing countries. It is the most important renewable energy source, accounting for about 6% of the total primary energy supply [1]. The search for energy efficiency has led to charcoal becoming an important wood product used in several applications over time.

There are several commercial processes available for pyrolyzing biomass and turning it into charcoal. Historically, kilns have been used with intensive labor and require a high degree of control to produce good quality and high charcoal yields [4]. These charcoal production methods are considered rudimentary, even though they have been practiced for centuries [5,6]. Charcoal’s specific properties are dictated by the control over the process variables and the homogenization of the raw material, constituting significant challenges for obtaining a high-quality product. Thus, understanding the influence of different pyrolysis temperatures becomes important since the temperature is an essential variable in determining the energy quality of charcoal [7,8].

Although much research has been carried out with a focus on the wood quality of Eucalyptus species and the pyrolysis variables [8,9,10], it is necessary that research continues to be developed to directly assist in decision making on the charcoal production process and contribute important technical information for the global charcoal sector. This study investigated the effect of two pyrolysis temperatures of a eucalypt wood mix on the energetic properties of charcoal.

2. Materials and Methods

2.1. Wood Preparation

To perform this study, a commercial Eucalyptus plantation area was selected in the southwest region of Bahia State, Brazil. The location is characterized by flat to slightly undulating relief, and the climate is classified as Cwb (tropical altitude), according to the Köppen classification, with an average temperature of 21 °C and annual precipitation of 700 mm [11]. Discs were obtained at different trunk positions from the collected trees (at 0%, 50%, and 100% of the commercial height; minimum diameter = 8 cm), homogenized, and transformed into smaller samples for the charcoal process.

Previous analyzes showed that the wood used had an average basic density of 500 kg.m⁻³, 29% of lignin, and 5% of extractives.

2.2. Charcoal Production

Wood samples measuring approximately 3 cm × 3 cm × 6 cm were dried in an oven at 103 ± 2 °C and placed in a metallic reactor whose volume was 1.34 dm³. Approximately 420 g of wood was used in each pyrolysis test. Five pyrolysis tests were carried out at a heating rate of 3.33 °C.min⁻¹, with a duration of 3 h and two final temperatures (300 °C and 450 °C) to make a total of 10 tests. The tests were performed in an electric muffle furnace with a water-cooled condenser and condensable gases collector (Figure 1).

2.3. Characterization of Charcoal

To evaluate the charcoal samples, the following analyzes were performed:

• Immediate analysis (volatile materials, ash, fixed carbon, %)—D-1762-84 [12];
• Apparent density (kg.m⁻³)—NBR 11,941 [13];
• Bulk density (kg.m⁻³)—NBR 6922 [14];
• High heating value, Useful calorific value (MJ.kg⁻¹) [15];
• Combustion test—ICOM [16].

The energy density (GJ.m⁻³) of the charcoal was defined as the maximum amount of energy per unit volume of charcoal, determined by the product of the bulk density and the useful calorific value. To determine the combustibility index, 149 ± 1.22 g of dry charcoal with a homogeneous particle size of 16 mm was used [17]. Ignition was carried out using 4.5 g of anhydrous alcohol 96° INPM. The temperature reached, and the mass consumed throughout the test were recorded every 3 min. The beginning of the test was marked by when the alcohol volatilization occurred in the system, and the end was determined as when the combustion of the material was complete, i.e., after the system does not show mass variation for five consecutive readings.

2.4. Data Analysis

The data obtained were subjected to Student’s t test after checking the normality and homoscedasticity of the residuals at 5% significance, using the Shapiro–Wilk and Bartlett tests, respectively.

3. Results

The apparent density and ash were the same for both pyrolysis temperatures. The charcoal pyrolyzed at 450 °C provided the higher value of fixed carbon content (82.98%), lower volatile matter content (15.92%), and higher heating value (32 MJ/kg) (

Table 1. Results obtained from the charcoal characterization.

TPR	VM	AS	FC	AD	BD	ED	HHV
(°C)	(%)	(%)	(%)	(kg·m−3)	(kg·m−3)	(GJ·m−3)	(MJ·kg−1)
300	33.23 * (1.26)	1.07 (0.01)	65.70 (1.25)	299.74 (21.48)	133.86 * (2.33)	3.58 * (0.01)	28.25 (0.06)
450	15.92 (1.65)	1.11 (0.03)	82.98 * (1.68)	320.90 (15.27)	105.36 (0.02)	3.22 (0.02)	32.00 * (0.21)

Results are mean values followed by standard error. TPR = temperature; VM = volatile matter; AS = ash; FC = fixed carbon; AD = apparent density; BD = bulk density; ED = energy density; HHV = high heating value. * Significant at the 5% using the t test.

).

In contrast, the charcoal produced at 300 °C showed a higher energy density (3.58 GJ.m⁻³) and higher bulk density (133.86 kg.m⁻³). The charcoal produced at a temperature of 450 °C had a combustion index of 0.135 and the charcoal produced at 300 °C had combustion index of 0.058.

Figure 2 illustrates the variation in temperature (A) and mass consumption (B) during the pyrolysis process.

4. Discussion

Increasing the temperature causes the degradation of cellulose, hemicellulose, and lignin, concentrating the carbon and increasing the calorific value of charcoal [18,19]. However, degradation of the wood components leads to a marked loss of mass with a low loss of volume, making the material less dense [20]. As the peaks of wood degradation during the pyrolysis process occur at different temperatures, depending on its chemical and elemental composition, this loss of mass and reduction in the charcoal density occurred differently for the charcoal produced at 300 and 400 °C. While the degradation of hemicellulose and cellulose occurs at lower temperature ranges (220–315 °C and 315–400 °C, respectively), lignin has greater thermal stability, and despite initiating degradation at lower temperatures, its mass loss is slow and occurs even at higher temperatures (160–900 °C) [21]. Despite losing some of the components due to thermal degradation, the samples carbonized at 300 °C still had a higher concentration of lignin and cellulose in their composition compared with the samples carbonized at 450 °C. This higher concentration directly reflects the high content of volatile matter, and consequently, the lower fixed carbon content of charcoal, resulting in products with different physical and chemical characteristics [20,22,23].

Charcoal that has a high fixed carbon content (>73%) accompanied by a low content of volatile matter (<25%) and a low ash content (<1.5%) is suitable for application both in the steel industry and as charcoal for barbecues [9,20,24]. According to the data collected in this work, only charcoal produced at 450 °C reached the levels required for these two uses. In addition to these properties, another considered essential to defining the energy quality of charcoal is its calorific value. Although the calorific value is intrinsic to the material being used, the pyrolysis parameters can influence the content of other properties such as fixed carbon. High fixed carbon contents are related to high calorific values of charcoal. Confirming this trend, charcoal with a higher fixed carbon content (produced at 450 °C) also had a higher heating value (32 MJ.kg⁻¹) compared with the other analyzed charcoal (28.25 MJ.kg⁻¹).

Regarding the inorganic portion of charcoal, the ash content must be considered when evaluating the energy efficiency of solid fuel. However, the ash content of the material is not changed by the pyrolysis process as the mineral composition is intrinsic to the wood. High contents of ash, which are related to the mineral fraction of charcoal, can compromise the use of solid fuel in some types of boilers [20]. Therefore, it is interesting that strategies are devised to reduce the ash content in waste reused for energy generation. As Eucalyptus has a low ash content, eucalypt wood blends can be the solution to reduce ash and improve the energy properties of other biomasses used as fuel [25].

In addition, the energy properties determined in this study depend on the combustion efficiency of the material. The Combustion Index (ICOM) evaluates the amount of energy released in relation to the amount of mass consumed during the combustion process [25]. That is, the larger the ICOM, the greater the heat generation for the same amount of mass [25]. The final carbonization temperature influenced the ICOM of the studied materials, providing an increase of about 42.3% in the ICOM for the charcoal produced at a temperature of 450 °C. Investigating the influence of carbonization temperature on the energy quality of charcoal is essential to increasing knowledge about the thermal behavior of biomass when exposed to heat. However, other pyrolysis parameters can influence the physical, mechanical, chemical, and energetic properties of charcoal, and they must be studied in a complementary way.

5. Conclusions

Under the conditions analyzed, pyrolysis performed at a final temperature of 450 °C resulted in better charcoal energy performance than charcoal produced at 300 °C. This study can serve as a basis for new research assessing the influence of other pyrolysis parameters on the energy quality of different charcoal produced from diverse eucalypt wood mixes. Future studies should assess the practical production of charcoal from eucalypt wood mixes and improve its applications and operational efficiency for the steel industry or for barbecues.