Use of Invasive Acacia Biomass to Produce Biochar and Solid Biofuels

Abstract

This study focuses on the production of high-density solid biofuels and high-fixed-carbon biochar from two invasive acacia species harvested in a protected area in Northwestern Spain, thereby contributing to its silviculture management challenge. It is possible to transform the initial biomass into briquettes and pellets reaching high densified values above 1300 kg/m³ and 500 kg/m³, respectively. Using thermochemical conversion processes in a simple double chamber oven, a biochar was obtained with a fixed carbon of 66%. Several parameters were analyzed and compared between the original biomass and the resulting products. The results show that an added value could be achieved without applying a complex system to transform, starting from residual biomass from invasive species.

1. Introduction

The utilization of waste and invasive plant biomass represents a crucial possibility for reducing CO₂ emissions, mitigating the greenhouse effect, and addressing global warming. This approach not only creates additional value in waste and/or invasive plant biomasses, but also increases economic activities in rural areas, where these materials are mainly located, promoting population growth and sustaining local economies, particularly through the production of value-added products like biofuels or biochar. Acacia spp. was introduced in Spain and Portugal over one hundred years ago, principally to control coastal erosion, and is linked to the production of sticks and posts for vineyards and fruit trees [1]. However, this species has spread and is now threatening the native flora, becoming an environmental issue. Nowadays, it is considered an invasive species in Spain [2,3,4]. Its distribution area is unclear, but some studies indicate a substantial increase in recent decades when comparing the last two Spanish National Forest Inventories [5,6], suggesting an increase in its dominance. Of particular significance are Acacia dealbata (with altitudes between 1100 m and 280 m) and Acacia melanoxylon (with altitudes between 710 m and 250 m) and, to a lesser extent, Acacia longifolia. Specific locations have also been found in the Iberian Peninsula with forest cover of A. farnesiana, A. salicina, A. mearnsii, A. decurrens, and A. retinodes [6,7]. Managing these invasive species is a complex challenge, as they tend to dominate certain areas due to their invasive capacity and resilient ability to resprout, particularly following fires and clear cutting without adequate management [8,9]. Harvesting these species to obtain added-value products could generate returns to offset the costs associated with their management. Therefore, it is crucial to propose strategies for the sustainable utilization of biomass from invasive acacias, transforming them into a renewable raw material that provides societal benefits with the potential to replace other materials, especially when seeking to avoid petroleum-derived products. Furthermore, the use of invasive plants helps deal with their management or eradication. This is of significant interest since added value could be achieved by starting with residual biomass from invasive plants, contributing to the enhancement of sustainable management practices in protected areas. Studies of the potential of using Acacia spp. as a raw material for bioenergy sources through biofuels, like briquettes or pellets, and for biochar shows great potential but its suitability needs to be checked [10,11]. Among the currently available technologies, mechanical densification and thermal transformation constitute important alternatives for the recovery of this biomass and also allows for adequate adjustments to obtain different products and different properties. As a result of these processes, bioproducts (biochar) and solid biofuels like briquettes and pellets can be produced.

Pyrolysis is the thermal decomposition of the biomass under inert atmosphere conditions (absence of oxygen), with temperatures usually over 300 °C, and is the most common method to produce biochar. This thermal process also converts the initial waste biomass into other sources, such as oil and gas [12]. The preparation methods for biochar are mainly divided into different classifications: torrefaction [13,14], pyrolysis [13,15], hydrothermal carbonization-HTC [16], and microwave carbonization [17]. Within the pyrolysis process, slow, fast, and very fast systems can be distinguished [18,19,20], where the times and/or temperatures reached in the pyrolization process are employed as variables. As a result of this, there are several parameters in the process that have physicochemical influences on the final properties of biochar, with the most relevant being the characteristics of the original raw material, reaction temperature, heating rate, residence time, and reaction atmosphere [21,22]. Different waste materials were processed with adequate results using slow pyrolysis under low-moderate temperatures (400–500 °C) with long residence time (2–3 h) [23,24]. By using different agricultural and forest wastes, as well as pine chip and cork, biochar with good properties can be obtained [20,25,26]. Utilizing acacia wood in a slow pyrolysis process (temperatures mainly in the range of 300–500 °C), the authors of [24] obtained satisfactory results, with an optimum temperature of 435 °C for 1 h using a laboratory electrical furnace.

New applications of biochar, in addition to the known use as energy, have acquired special relevance, not only in the agriculture and livestock sector, where it is used to improve the physical and chemical properties of the soil [27], but also as domestic and industrial products (filters, animal feed, cosmetics, etc.). The use of biochar in soil can be beneficial as it can increase the amount of organic matter, it is a fertility enhancer, it contributes to carbon sequestration, it is a nutrient leaching preventer, and it is an acidity modifier (pH). Due to its porous structure, biochar is also used as an additive in low rainfall areas, where it has been proved to stabilize the level of moisture in the soil [28], or as a structuring agent and base for nutrients during the composting process, where it has optimized the degradation of organic matter while reducing emissions of ammonia and greenhouse gasses [29]. It has been shown to be effective in reducing the absorption of heavy metals and helps to sequester solid carbon in agricultural fields for hundreds and even thousands of years [30]. The market viability of certain types of biochar and solid biofuels has been demonstrated, primarily due to their low production costs—attributable to the use of low-cost feedstocks, often derived from waste—and their scalability for large-scale production [31].

This study developed densified solid biofuel and biochar from two invasive plants biomass material and compared their parameters with those of original biomass. The production of biochar in this environment aligns with various sustainability goals, principles of circular economy, and the achievement of the Sustainable Development Goals (SDGs). Through the application of thermochemical conversion processes, slow pyrolysis, using a mobile straightforward double-grade reactor [32], the initial biomass could be transformed into a biochar with high-fixed-carbon. These principles fit into the guidelines for sustainable biochar production issued by the European for Biochar Foundation (EBC) [33].

With fast pyrolysis, high-fixed-carbon contents are usually not achieved, and with gasification, generally biochar with higher carbon content is produced, although its quality may not be sufficient for different applications, mainly due to the fact that it still contains harmful substances such as polycyclic aromatic hydrocarbons [34]. In these two processes, biochar is a low-yield byproduct, whereas higher yields are achieved in oil and gas production. In this work, slow pyrolysis was used, as it is a methodology with a foreseeable commercial relevance, without an excessively advanced technique, suitable for simpler use, and able to achieve an adequate release of volatile components and a relatively high-fixed-carbon content of the solid [35].

2. Materials and Methods

Solid biofuels and biochar were elaborated from biomass of invasive A. dealbata and A. melanoxylon. Materials of these exotic species were collected in a natural area named Cañones del Sil (Northwestern Spain, WGS84 coordinate: N 42.411024749820186, E −7.632084153695484), within the Natura 2000 protected area. This preserved space gains significance when addressing the management of invasive species, particularly the acacias, whose proliferation is fueled by climate change and anthropogenic pressure. The material was obtained from trees of both species with normal diameters below 10 cm and heights of 4 m.

To adjust this study, four clearly distinguishable types of biomasses were separated: small branches and leaves (phyllodes) called “branch/leaf” (Figure 1a); woody and trunk biomass fraction of higher volume called “wood” (Figure 1b); and these original types of biomasses were ground (Figure 1c) to subsequently elaborate briquettes, pellets, and biochar (Figure 1d–f) that were then analyzed.

For the characterization of the original biomass, solid biofuels, and biochar, different methodologies were employed: UNE-EN ISO 18134-3 for moisture content (%) [36], UNE-EN ISO 17828 for density (kg/m³) [37], UNE-EN ISO 18123 for volatile matter (%) [38], UNE-EN ISO 18122 for ash (%) [39], and UNE-EN 13037 for pH (%) [40].

Pellets were elaborated using a Euro-Tolds device, with a flat matrix system of 300 mm and 6 mm pellet diameter, by traditional pelletization [41]. To elaborate the briquettes, a TI device with a double-head and bi-directional hydraulic system was used with 14.71 MPa pressure and 80 mm briquette diameter. The size of the evaluated briquettes and pellets was 80 × 100 mm and 6 × 3 mm, respectively, and all characterization tests were carried out in triplicates.

To develop the biochar, a simple mobile double-chamber reactor (of internal chamber of 50 L) on a laboratory scale was employed. The system was discontinuous and autonomous. The previous residual temperature within a slow pyrolysis system was used, concatenating successive processes. Additionally, the syn-gas produced was redirected to the heating process, thereby avoiding loss of energy and emission into the atmosphere. This also prevented the oven from cooling down between processes; thus residual heat was used. The aim of this method is to achieve simple manufacturing using low-cost-effective materials and easy operation, with a mobile format, in order to employ it near the biomass source. The heating process does not require energy from electricity or fossil sources, since it is heated by the temperature generated by the biomass itself [32]. This system increases the temperature using waste biomass in the pyrolysis chamber up to between 300 and 600 °C. The temperature is generated through a co-fuel process involving biomass waste. Temperature profiles were constant with a FT-data logger and measured with 4 K thermocouples. In the reactor, as soon as the pyrolysis chamber reached a temperature of ≥300 °C, the power of the biomass chamber was sustained, and the combustion of the pyrolysis gas was redirected inside the biomass chamber. Figure 2 represents the temperatures recorded in each of the chambers and at the gas outlet of the pyrolization chamber for a time process of 120 min. The process used for wood fraction biomass began with the cold reactor. For the branch/leaf biomass fraction, the thermal process started with the residual heat of the previous process, making use of certain thermal inertia and initial heating conditions. In the upper part of the combustion chamber, low-density co-fuel of waste wood pine from used palettes was employed to increase the initial temperature, due to its high flammability, and high-density products of pellets were used in the lower part of the chamber as co-fuel to maintain the temperature. Both types of materials enabled the co-combustion process and provided pyrolysis temperature to the pyrolysis chamber during the process time.

2.1. Biochar Mass Yield

Biochar yield was determined as the ratio of biochar weight in relation to the initial weight of the biomass [35], according to Equation (1):

$$Biocharmassyield=\left({\displaystyle \frac{w2}{w1}}\right)\times 100,$$

(1)

where w₁ is the weight of the acacia biomass at the moisture content on wet base before pyrolysis (

Table 1. Yields, losses, maximal temperatures, and times in the thermal process.

Materials	Mass Yield (%)	Losses (%)	Pyrolysis Means Temperature (°C)	Pyrolysis Time (>300 °C) (min)	Fixed Carbon (%)
D-biochar-wood	52.0	48.0	401.4 ± [44.93]	60	66.0 ± [2.86]
D-biochar-branch/leaf	59.0	51.0	321.5 ± [49.06]	367	51.0 ± [3.84]
M-biochar-wood	46.0	54.0	460.0 ± [79.70]	60	66.8 ± [3.80]
M-biochar-branch/leaf	25.0	75.0	450.9 ± [82.04]	110	61.0 ± [4.36]

) and w₂ is the biochar weight at the end of the thermal transformation process. These values were calculated from dry biomass at room humidity, with a mean value of 14.7 ± [1.88]%. Gravimetric measurements were used in this study, since the initial humidity of the biomass was low and the value obtained was easily interpreted.

2.2. Moisture Content

Moisture content was measured for both the initial material and the solid biofuel produced. Biochar is an anhydrous product, since all the moisture content present beforehand evaporates in the thermal process. A gravimetric system according to UNE-EN ISO 18134-3 standard [36] was used with a balance, a drying oven, and a desiccator. The values related to the wet mass were calculated with the following Equation (2):

$$Moisturecontent={\displaystyle \frac{(m2-m3)}{(m2-m1)}}\times 100,$$

(2)

where m₁ is the mass in grams of the empty tray, m₂ is the mass in grams of the tray with the material before drying, and m₃ is the mass in grams of the tray with the material after drying.

2.3. Density

Bulk density was measured, according to the UNE-EN ISO 17828 standard [37], as the mass to volume ratio, using a container of 1 L (1 dm³) for biomass, and a container of 0.5 L (0.5 dm³) for biochar, due to the greater compaction of the latter. The density of pellets and briquettes was also determined by their mass to volume ratio. The values were calculated with the following Equation (3):

$$D={\displaystyle \frac{m2-m1}{v}},$$

(3)

where D is the bulk density in kg/m³, m₁ is the mass of the empty container in kg, m₂ is the mass of the filled container in kg, and v is the volume of the measuring container in m³.

2.4. Fixed Carbon, Volatile Matter, Ash, and pH

Since a smaller sample size (≤1 mm) of the original material and biochar was required, the latter were ground using a laboratory mill Retsch SC-100 in two steps: 1 cm and then 1 mm mesh, in order to facilitate the transformation of the material and avoid possible obstructions of the mesh.

Fixed carbon is the solid carbon in the biomass and biochar that remains in the pyrolysis process after drying and the devolatilization of materials and substances [35]. It was determined from the following Equation (4):

$$FixedCarbon=1-Vd-Ad,$$

(4)

where Vd and Ad are the volatile matter content and the percentage of ashes on a dry basis.

Volatile matter was determined according to the UNE-EN ISO 18123 standard [38] by calculating the loss of mass without moisture content. Heating rate and time along test were controlled to obtain a comparable total mass loss. The values were calculated with the following Equation (5):

$$Vd=\left({\displaystyle \frac{100\ast \left(m2-m3\right)}{m2-m1}}-Mad\right)\times \left({\displaystyle \frac{100}{100-Mad}}\right),$$

(5)

where Vd is the volatile matter content, expressed on a dry basis, m₁ is the mass in grams of the bottle, m₂ is the mass in grams of the bottle and the material before the thermal process, m₃ is the mass in grams of the bottle and the material after the thermal process, and Mad is the humidity as a percentage.

Ash content is an important parameter for biofuels, as it is a by-product of combustion, which ends up as bottom or fly ash and needs to be disposed of. Furthermore, the chemical composition of ash contributes to slagging and corrosion in combustion equipment [34]. Nevertheless, depending on the legal aspects, ash can be deposited or used to produce other products. The values were calculated according to UNE-EN ISO 18122 standard [39] with Equation (6):

$$Ad={\displaystyle \frac{(m3-m1)}{(m2-m1)}}\times 100\times {\displaystyle \frac{100}{100-Mad}}$$

(6)

where Ad is the percentage of ashes on a dry basis, m₁ is the mass in grams of the empty tray, m₂ is the mass in grams of the tray and material before the thermal process, and m₃ is the mass in grams of the tray and material after the thermal process.

The pH values were determined according to UNE-EN 13037 standard [40], using the methodology of soils and substrates, with a solution of water and calcium chloride (CaCl₂) in a 5:1 ratio.

2.5. Statistical Analysis

Statistical analyses were conducted using Statgraphics Centurion XV 15.2.14, and the graphs were prepared using SigmaPlot for Windows version 11.0 (Build 11.0.0.77) and Microsoft Excel 2023. Shapiro–Wilk method was used for the normality tests of the density groups. Levene’s homogeneity of variance tests were employed to test significant differences in the groups, and one-way ANOVA was used to determine significant differences in the fixed carbon obtained in the biochar, as a main result.

3. Results and Discussion

According to previous references, with thermochemical biomass conversion [35,42,43,44,45,46], large hydrocarbon molecules found in biomass present a decomposition into smaller molecules and, to achieve high values of fixed carbon without pressure, it is necessary to use relatively high temperatures (commonly over 300 °C), with a heating rate between 1 and 30 °C/min. Fast pyrolysis mainly produces liquid fuels, known as bio-oil and gas, while slow pyrolysis namely results in solid charcoal [35], and depends on the process conditions, e.g., biomass, temperature, and residence time [42,43]. According to the results of this work (Table 1), processes with the highest temperatures, and with the shortest times, provide high-fixed-carbon values for wood and branch/leaf fraction. Therefore, it can be considered an optimal balance, considering this parameter as biochar quality and its net energy cost of production.

The yields obtained in the processes are between 46 and 59%, except in the case of the branch/leaf material of A. melanoxylon, with a yield of 25%. All biomass with larger lignin contents usually achieve a high mass yield [35], with wood biomass showing slight increases with respect to non-wood waste for each species. Considering that the temperature and residence time of the process, as well as the characteristics of the original biomass, have the most relevant effects on mass yield and fixed carbon [43], according to these results, it can be inferred that the wood biomass of both species, for a similar temperature and residence time, obtains similar mass yield and fixed-carbon values. By contrast, the small fraction shows a clear difference, increasing the fixed-carbon and reducing mass yield with high temperatures and short times, while reducing fixed carbon and increasing mass yield for lower temperatures and longer times.

Table 2. Mean and standard deviation values of the different tests carried out on the elaborated products.

Elaborated Products
Material	Moisture Content (%)	Bulk Density (kg/m3)	pH (%)	Volatile Matter (%)	Ash (%)	Fixed Carbon (%)
D-biochar-wood	anhydrous	162.4 ± [11.46]	8.0 ± [0.14]	31.6 ± [1.52]	2.39 ± [1.55]	66.0 ± [2.86]
D-biochar-branch/leaf		192.0 ± [0.57]	7.5 ± [0.00]	46.3 ± [1.94]	3.07 ± [1.88]	51.0 ± [3.84]
M-biochar-wood	anhydrous	159.9 ± [1.27]	9.5 ± [0.16]	21.5 ± [3.92]	11.75 ± [9.90]	66.8 ± [3.80]
M-biochar-branch/leaf		150.9 ± [2.12]	8.1 ± [0.04]	34.0 ± [0.76]	4.96 ± [3.60]	61.0 ± [4.36]

and Figure 3 show the results of the different tests carried out with biomass and the elaborated products. The treatment temperature had an influence on the pH values of biochar, since the highest and lowest temperature values correspond to the highest and lowest acidity values, with no relevant influence of residence time [47]. A 43% increase in pH over the average was achieved as a consequence of a decrease in functional groups during pyrolysis [35].

The ash content of biochar is largely dependent on the ash content of the biomass, but also on the process temperature. In the case of M-biochar-wood, the highest ash value was reached, and it corresponds to the highest pyrolysis temperature. With regard to fixed carbon, a 72% increase in biochar was obtained with respect to the original, with higher values in the wood material compared to the branch/leaf material, and a percentage above 50% in all results. For the pyrolysis conditions with slow heating rates and residence times of many hours for dealbata with branch/leaf material, the volatile matter content seems to be relevant, obtaining over 45%, although the pH values are the highest. Similar values were observed with an alkaline pH of 11.2 at a pyrolytic temperature of 450 °C in ragweed and horseweed biochar [48]. For volatile matter, the reduction in biochar with respect to the original biomass was 30–80% (Figure 3).

The usual densities obtained with briquettes elaborated with industrial hydraulic systems vary usually between 600 and 1000 kg m⁻³ [49]. “Impact” and “extrusion” briquetting devices usually generate densities of 800–1100 kg m⁻³ and 1100–1200 kg m⁻³ [50,51], respectively, at room temperature. The physicochemical properties of briquetted biofuels are significantly influenced by biomass type—classified as herbaceous or woody, and further as hardwood or softwood—as well as by densification parameters, including temperature and applied pressure during compaction [41]. Previous works [51,52] report that briquette bulk density is a key physical characteristic that varies substantially among different biomass categories. Herbaceous materials such as grasses generally display low bulk densities, typically between 40 and 150 kg m⁻³, while agricultural residues tend to exhibit intermediate values, ranging from 80 to 100 kg m⁻³. In contrast, woody biomass—owing to its more compact structure—demonstrates considerably higher bulk densities, commonly between 150 and 200 kg m⁻³. According to these references, the values of densification obtained both in the briquettes and in the pellets (Table 2 and Figure 4) would allow considering the possibility of producing biofuels with the analyzed raw material. In the case of the briquettes, that is, high-quality briquette fuel without binders, a moderate density was obtained, due to the system used at room temperature (low-pressure hydraulic briquetting). In the production of pellets, high density and quality were obtained, with a density close to 1375 kg/m³.

Biochar bulk density values were slightly lower than those of the original biomass, as expected (Figure 5). These values are in line with those obtained for biomass-derived biochar, although they are slightly lower than the values typically reported for coal and coke [53].

The Shapiro–Wilk method was employed, using a comparison of the quartiles to determine the normality of the original biomass bulk density groups. As the p-value was 0.146842 (≥0.05), the idea that the groups come from a normal distribution cannot be rejected with 95% confidence interval.

The fixed carbon value of the original material ranges between 14.8% and 17.7%, compared to 50.7% to 66.8% of the biochar (Figure 3). Previous works [43] with charcoal produced from the trunks of different acacia species, obtained a fixed carbon of around 20% in the biomass, and their biochar obtained a fixed carbon of 70%, elaborated with 500 °C (rate of about 25 °C/min) in an electric furnace. The one-way ANOVA for fixed carbon based on the normality of the groups showed significant differences. Furthermore, LSD and Levene’s test were used to test the homogeneity of means and variance at 95% confidence level. Since all p-values in Levene’s test were ≥0.05, there was no statistically significant difference between variances.

Table 3. Results of LSD and Levene’s homogeneity of variance tests.

Groups	SD	Diff.	+/− Limits	F-Ratio	LSD Anova p-Value	Levene p-Value
D-wood and M-wood		0.20	0.253454	4.80	0.0936	0.274577
D-branch/leaf and M-branch/leaf		−0.55	0.646184	5.58	0.0774	0.157484
D-wood and D-branch/leaf	*	−2.25	0.646184	93.46	0.0006	0.157484
M-wood and M-branch/leaf	*	−3.00	0.253454	1080.00	0.0000	0.274577
D-Biochar-wood and M-Biochar-wood		−0.86	22.60290	0.01	0.9203	0.454108
D-Biochar-branch/leaf and M-Biochar-branch/leaf	*	−10.31	9.35910	9.36	0.0377	0.941239
D-Biochar-wood and D-Biochar-branch/leaf	*	15.23	7.69322	30.22	0.0053	0.807901
M-Biochar-wood and M-Biochar-branch/leaf		5.78	23.2228	0.48	0.5271	0.526589

SD and * = significant difference, D = A. dealbata, M = A. melanoxylon for each fraction biomass size.

shows the significant differences obtained after determining the contrast between the interest groups.

According to the results, there are no statistically significant differences between the fixed carbon values obtained in the “wood” fractions of both species. These results could be due to the fact that the heat generated system facilitates the evaporation of volatile matter, generating poor gasses. Moreover, in this process, the pyrolysis gasses were led to the process as ignite gasses to promote the heating necessary for pyrolysis, which prevented them from being lost into the atmosphere, with the consequent pollution and loss of heat and efficiency of the overall process.

For ash content, a linear increase was found depending on the mass lost during the pyrolysis process. These results could be explained by the fact that the amount of inorganic matter is the same in both green biomass and biochar; therefore, it is logical that, with less dry weight, the ash increases its value. In relation to the pH, the values obtained in the biochar were higher than those obtained in the original material, i.e., 8–9 for most of the biochar samples, compared to 4–5 for the original material. Values slightly higher than 9 were found in A. melanoxylon wood biochar. This increase could be attributed to the loss of its acidic organic functional groups and the accumulation of alkaline inorganic substances with the increase in the pyrolytic temperature [48,54]. In any case, it could be stated, according to these results, that the use of biochar with respect to the original biomass enhances its fertilization capacity in soils, various substrates, and compost.

4. Conclusions

The yields obtained in the processes are around 50%, except in the case of the branch/leaf material of A. melanoxylon, with a yield of 25%. An optimal balance is achieved, considering this parameter as biochar quality and the net energy cost of production.

According to the obtained results, using low pyrolysis temperatures and short times in a simple oven with double chamber, moderately high values are achieved compared to the references and the original biomass material in all samples. Wood material of A. melanoxylon and A. dealbata obtained the highest fixed carbon value, with 66%, very similar to the fixed carbon value of A. melanoxylon leaf/branch. No significant difference was detected between the values obtained for the two species in wood material, although a significant difference was obtained for the leaf/branch material, which reached a higher value. In any case, the results show that it is possible to obtain biochar with fixed carbon above 50%, which can be used for various applications, such as soil recovery or amendments, reducing the carbon emissions and ecological footprint, allowing for the manufacturing of all types of added-value products, such as activated biochar.

Ash values were coherent and predictable in view of the initial moisture content and fixed carbon obtained. In relation to pH, the values obtained for biochar samples enable the use of this type of substrate to improve acidic soils that are very typical in Northwestern Spain.

Pellets biofuels showed high density, close to 1400 kg m⁻³. It can be concluded that, with acacia waste, it would be possible to obtain the best pellets potentially manufactured in theory and with better physical and energetic densities than the products usually sold at the conventional market. This study highlights the effectiveness of this approach in the sustainable management of protected areas, contributing to the management of invasive species, addressing environmental issues, and promoting the circularity of residual biomass. Ultimately, this study underscores the innovation and promising utility of Acacia biomass-derived biochar in enhancing the sustainability and management of protected spaces.