Valorization of Forest Biomass Through Biochar for Static Floating Applications in Agricultural Uses

Abstract

The feasibility of utilizing biochar as a static floating material for agricultural applications was researched to prevent evaporation from open water static storage systems or as a floating barrier in slurry pits, for instance. Five types of biochar were created from chips, bark, and pellets of pine and residues from two acacia species using a pyrolysis time between 60 and 120 min and mean temperatures between 380 and 690 °C in a simple double-chamber reactor. Biomass and biochar were characterized for their main properties: bulk density, moisture content, volatile matter, ash content, fixed carbon, and pH. Biochar was also evaluated through a basic floatability test over 27 days (648 h) in distilled water. The highest fixed carbon content was observed in pine bark biochar (69.5%), followed by the pine pellets (67.4%) and pine chips (63.4%). Despite their high carbon content, the pellets exhibited a low floatability level, whereas pine bark biochar showed superior static floatage times, together with chip and ground chip biochar. These results suggest that biochar produced from bark and wood chips may be suitable for application as floatability material in water or slurry management systems. These results warrant further research into the static floating of biochar.

1. Introduction

Thermochemical conversion technologies used to transform waste biomass into biofuels, and also in other high-added-value products, have garnered growing attention in recent years from researchers, governmental administrations, and industrial stakeholders [1].

The pyrolysis of lignocellulosic biomass offers several advantages, such as a reuse strategy, including the valorization of agricultural and forestry residues into high-value products such as biochar, bio-oil, and syngas [2]. This thermochemical conversion process contributes to waste reduction, promotes carbon sequestration through stable biochar production, and supports energy recovery without reliance on fossil resources. Additionally, it aligns with circular economic principles by transforming low-value biomass into renewable energy carriers and soil amendments while mitigating the environmental impacts associated with conventional waste disposal. Furthermore, waste biomass comes from the photosynthetic assimilation of atmospheric carbon dioxide, so the use of biofuels/biochar has the potential to be effectively carbon-neutral in terms of net greenhouse gas emissions.

Among the various thermochemical conversion techniques, pyrolysis (moderate temperatures between approximately 300 and 700 °C) and gasification (high temperatures over 750 °C) are widely acknowledged as two of the most advanced and promising thermochemical approaches for converting biomass into cleaner fuel alternatives and are regarded as particularly promising pathways for transforming biomass into environmentally sustainable fuel sources [1]. Within these two techniques, pyrolysis is considered to be the most suitable route for converting low-moisture-content biomass into biofuels and biobased products, owing to its relative simplicity, operational flexibility, and high efficiency [3]. Based on specific operational parameters, pyrolysis can be classified into three processes: slow pyrolysis, fast pyrolysis, and flash pyrolysis [4]. Slow pyrolysis represents a traditional form of the pyrolysis process, characterized by a low heating rate [5]. This gradual thermal input is typically auspicious to the production of solid char over liquid and gaseous fractions [6]. Moreover, the extended residence time within the reactor allows for the gas-phase intermediates to undergo further secondary reactions, promoting additional char formation. The solid product resulting from a thermal conversion process using pyrolysis and gasification is a carbon-rich material commonly referred to as biochar [4]. Temperature, time, biomass characteristic, and the pressure process, among other variables, affect the physicochemical properties of biochar, such as pH, surface area, porosity, surface functional groups, impurities, and floating properties [7].

To achieve biomass heating, a reactor is required, which increases temperature in the absence of very-low-level oxygen. There are various classifications in the literature [1] depending on the capacity and purpose (analytical with small capacity, or industrial/pilot-scale with greater capacity), continuous or discontinuous reactors, with fixed or fluidized beds, rotary kiln systems, ablative or auger reactors, microwave heating systems, etc. [6]. The use of a reactor heated by the biomass itself, without excessive technical requirements, could allow for the optimization of the energy balance and simpler use.

Biochar from waste biomass has shown great potential as an effective and low-cost material not only as a soil amendment, contaminant neutralizer, but also as a filtering of fluids or a floating element in water or slurry ponds, which must subsequently be used without other material that includes some contaminants [8]. In this sense, it is a material that does not prevent the subsequent use of water; it even improves the application with slurries [9] or sorbent for the removal of water contaminants [10].

Common feedstock used to synthetize biochar includes residues from forestry and agriculture, as well as organic waste materials like animal manure and sewage sludge [11]. Pine residues are a lignocellulosic material generated in substantial quantities worldwide as by-products of agroforestry industries and have demonstrated considerable potential as feedstocks for biochar production.

Various thermochemical conversion processes have yielded promising outcomes in transforming pine waste into biochar, with notable efficiency and performance metrics [12]. Moreover, invasive species such as acacias exhibit significant potential for conversion into biochar with promising functional properties [13].

The physicochemical characteristics of biochar are highly influenced by several factors, mainly feedstock composition, processing temperature, and residence time during the thermal treatment [14]. Therefore, the effectiveness of biochar applications varies in part due to the influence of feedstock type and thermochemical processing parameters on its physicochemical properties. Mainly for this reason, the development of standardized characterization protocols and certification schemes is recommended to ensure its suitability for targeted environmental management practices [15]. The key parameters used to characterize biochar mainly include fixed carbon, ash content, volatiles, and pH, among others, and for more specific uses, a relevant parameter is also floatability, through the formation of a crust (physical barrier for fluids) [16]. When used for these purposes, after storage, water or manure is applied to the soil as fertilizer, especially the solid fraction after slurry separation [17]. Furthermore, the solid fraction enriched with biochar, which contains high amounts of stable carbon, could be beneficial for long-term carbon sequestration [18].

In this study, biochar was synthesized from five types of waste forest biomass, and its characteristics were analyzed in comparison to the original feedstocks. The biochar production process implemented in this context supports key sustainability objectives, aligns with circular economy principles, and contributes to the realization of the Sustainable Development Goals (SDGs).

Utilizing a mobile, double-chamber reactor system with slow pyrolysis [19], the biomass was thermochemically converted into high fixed-carbon biochar. This approach is consistent with the sustainability guidelines for biochar production established by the European Biochar Foundation (EBC) [20].

2. Materials and Methods

2.1. Biomass Materials

A total of five biomass materials has been tested (Figure 1). The biomass of diverse waste materials was obtained from a conventional Pinus pinaster sawmill in northern Spain in the form of slabs and bark (Figure 1a). The slabs were chipped to create pine chips (Figure 1b) with a P31 (G50) classification. For comparison, commercial pellets class Aplus A1 of pinus wood (Figure 1c) were purchased from a European company (Coruña, Spain). Also, biochar was created from biomass of invasive acacias (A. dealbata and A. melanoxylon) obtained in the northwest of Spain from trees with a 4 m height and 10 cm normal diameter (Figure 1d).

2.2. Biochar Synthesis and Characterization

Figure 1 shows the biochar of pine bark (Figure 1e), pine chips (Figure 1f), pine pellets (Figure 1g), and acacia (Figure 1h).

Biochar was created using a mobile double-chamber reactor (Figure 2), equipped with a 50 L internal capacity and designed for simplified operation, developed in previous works [19]. The system of the reactor was discontinuous and autonomous. One of the main aims of this technology is to achieve simple manufacturing using low-cost and effective materials and easy operation, with a mobile format, in order to employ a local biomass source.

The heating mechanism operates independently of electrical or fossil energy inputs as it utilizes the thermal energy released from the combustion of biomass itself [19]. The temperatures in this system within the pyrolysis chamber are elevated to a range of 300–690 °C using waste biomass as a primary heat source.

The temperature profile inside the pyrolysis chamber, outside in the combustion chamber, and in the pyrolization gas outlet were measured with four thermocouples (type K sensor) and registered with an FT-data logger RS-PRO 1384 (accuracy of ±0.05%) for pine chips (Figure 3a), pine bark (Figure 3b), pine pellet (Figure 3c), and acacia (Figure 3d).

Once the pyrolysis chamber reaches a temperature of ≥300 °C, the system maintained the thermal input and the pyrolysis gas produced was redirected to the heating process, thereby avoiding loss of energy and emission into the atmosphere.

Residual heat from preceding cycles within the slow pyrolysis system could be used for concatenating successive processes. The overpressure developed within the pyrolysis chamber, maintained through a single pyrolysis gas outlet, effectively inhibited the ingress of stoichiometric oxygen.

Biomass and biochar were characterized for their main properties. The bulk density or apparent density of biomass and biochar was calculated according to the standard UNE-EN ISO 17828 [21] with a 2000 mL ± 10 mL container and a balance of 1000 and accuracy of ±0.1 g.

The moisture content of the original biomasses was calculated in the laboratory using the oven drying method (gravimetric method) according to standard ISO 18134-3 [22], in order to use this as a reference for the apparent density.

The ash and volatile matter content were calculated according to the standards UNE-EN ISO 18122 [23] and UNE-EN ISO 18123 [24], respectively, using a specific particle size distribution of 1 mm or less in anhydrous state. This ensures that the biomass and biochar test material for the termination of ash and volatile matter does not contain moisture in the test. With a laboratory mill, the samples are ground first through a 6 mm sieve and second through a 1 mm sieve. This procedure is performed to facilitate the final grinding. Before and after each grinding process, the mill is thoroughly cleaned to avoid possible contamination with different materials.

The pH was measured according to the standard UNE-ISO 10390 [25] using a mixture with each material in deionized water with CaCl₂ (1 L of distilled water with 1.47 g of CaCl₂) for 1 h (±10 min), and then it was allowed to stand for at least 1 more hour and no more than 3 h.

The sample was mixed with a solution of one part of material to five parts of solution (5 g of material to 25 g of solution). The dilutions had a temperature of 22.7 ± [0.75] °C when the pH was measured.

The biochar yield was calculated as the proportion of the biochar mass relative to the initial dry weight of the biomass feedstock [26], as expressed in Equation (1):

$$Biochar mass yield= \left({\displaystyle \frac{w_2}{w_1}}\right) \times 100$$

(1)

where w₁ is the weight of biomass before pyrolysis and w₂ is the weight of biochar at the end of the transformation process.

2.3. Floatability Analysis

The floatability test was carried out with biochar material applied directly in distiller water and, in parallel, with ground pine chips biochar produced from pine chip biochar applying mechanical friction grinding with a steel ball mill for 1 h. The initial granulometry of the pine chips biochar became a powder format.

Biochar was deposited slowly on top of a graduated cylinder (measuring cylinder or mixing cylinder) of 1000 ± 10 mL to cover a 30 mm surface layer. The cylinders are transparent in order to measure the size of the floating layer and the settle layer (if settling occurs). Figure 4c shows the schematic of the test setup. The thickness of the floating biochar is measured, which is initially 30 mm. In the case of pine pellet biochar, a large proportion of it settles instantly to the bottom of the cylinder.

The floatability behavior was analyzed with five periodic measurements over 27 days. The water was in the laboratory temperature at around 20 °C. The measurements sought to determine both the size of the layer that remained afloat (floating layer) and the layer that formed at the bottom of the container due to the settling of the material (settled layer).

The initial moment of the test is shown in Figure 4a, and the final moment of the test is shown in Figure 4b. The graduated cylinder on the left shows the initial moment of the pine pellets, which settle instantly, while the cylinder on the right with pine chips initially has no biochar deposited at the bottom and, once the test period has elapsed, shows a slight settling.

3. Results and Discussion

3.1. Results and Discussion of the Biomass Materials

The characterization results of the original biomass are summarized in

Table 1. Mean and standard deviation values of the original biomass.

Biomass	Moisture Content (%)	Bulk Density (kg/m3)	Volatile * (%)	Ash Content * (%)	Fixed Carbon (%)	pH
Pine chips	11.2 ± [0.08]	298.5 ± [2.69]	85.0 ± [0.04]	0.47 ± [0.10]	14.5 ± [0.10]	4.1 ± [0.02]
Pine bark	17.8 ± [0.49]	193.7 ± [2.40]	71.4 ± [1.30]	0.92 ± [0.30]	27.9 ± [1.22]	3.3 ± [0.02]
Pellet	8.4 ± [1.66]	647.6 ± [14.85]	77.4 ± [0.64]	0.69 ± [0.01]	22.0 ± [0.64]	4.5 ± [0.30]
A. dealbata	13.6 ± [0.53]	212.8 ± [36.53]	82.3 ± [2.14]	1.65 ± [0.82]	16.1 ± [1.34]	4.7 ± [0.20]
A. melanoxylon	17.7 ± [2.93]	230.6 ± [10.00]	81.3 ± [2.75]	2.50 ± [1.05]	16.3 ± [1.74]	4.9 ± [0.13]

* values determined whit anhydrous biomass.

. The bulk density of the original material from the arboreal resources was very analogous for a similar moisture content. The pine pellets were of a high value due to the densification of the material, and pine bark showed a low value due to its large amount of internal air. Previous works have found similar density values for the materials analyzed for pine chips [27], pine pellets [28,29], for pine bark [30], and for acacia [13].

In relation to the values obtained from the ash content and volatile matter, a previous work [31] has found similar values for pine chips, 0.65 ± [0.02]% and 84.44 ± [0.18]%, respectively, and for pine pellets [32] of 0.60% and 2.50% and 77.80% and 76.62%, respectively.

For fixed carbon values, the same studies have found a value of 14.91 ± [0.16]% and values between 14.23 and 14.70%, and [33] found a value of around 16%. Sharman et al. [12] include ash content of 0.53–2.61% and volatile content of 74.85% for pine wood and cone. These works conclude that pine waste has an adequate value for the low cost of the material and the necessary process.

3.2. Results and Discussion of Biochar Synthesis and Characterization

Table 2. Yields, losses, maximal temperatures, and times in the thermal process.

Biochar	Mass Yield (%)	Losses (%)	Mean Temperature of Pyrolysis (°C)	Pyrolysis Time (>300 °C) (min)
Pine chips	54	46	499.4 ± [160.3]	100
Pine bark	75	25	386.5 ± [97.4]	70
Pine pellet	70	30	504.5 ± [181.3]	120
A. dealbata	52	48	401.4 ± [44.93]	60
A. melanoxylon	46	54	460.0 ± [79.70]	60

summarizes the most relevant parameters of the pyrolysis processes for the different types of biochar. The yields obtained in the process are between 46% and 75%. Considering that all biomass with a higher lignin content usually achieves a high mass yield [26], the acacia biomass consisted of wood fractions and fractions of branches and leaves, which would justify lower yields, although similar to those of pine chips.

Given that temperature, residence time, and the intrinsic properties of the feedstock are key factors influencing the mass yield and fixed carbon content [34], the results suggest that the biomass from both acacia species produced comparable mass yields and fixed carbon levels under equivalent thermal conditions and residence times.

In contrast, pine pellet and pine bark show a difference, increasing mass yield and fixed carbon at different temperatures and times. The pellets required long processing temperatures and times. Pine chips reach a yield of 54% with 500 °C, while in previous similar works [3] with different pine species and the same temperatures, the yield has not exceeded 33%.

Figure 5 shows the temperature and time values reached in the production of the different types of biochar. According to previous references, in the thermochemical conversion of biomass, the macromolecular hydrocarbons break down into smaller molecular fragments and reach an elevated fixed carbon content under atmospheric pressure conditions that require the application of relatively high temperatures, typically with a heating rate ranging from 1 to 30 °C/min [26,35]. In Figure 5, a heating rate is estimated considering temperatures and times above 100 °C, obtaining values between 3 and 5 °C/min, within the range of slow pyrolysis.

Under the same conditions, the fast pyrolysis process produces primarily liquid fuels, known as biofuels, and gases, while a slow pyrolysis process produces a greater amount of solid carbon [26]. In this study, mean temperatures close to 500 °C and slow pyrolysis are used.

Table 3. Mean and standard deviation of the tests carried out on the biochar. Pyrolysis conditions are summarized in Figure 3 and Table 2.

Biochar	Bulk Density (kg/m3)	Volatile * (%)	Ash Content * (%)	Fixed Carbon (%)	pH
Pine chips	215.1 ± [9.63]	35.3 ± [3.03]	1.28 ± [0.42]	63.4 ± [3.24]	5.3 ± [0.09]
Pine bark	139.4 ± [4.58]	29.3 ± [0.05]	1.19 ± [0.07]	69.5 ± [0.03]	7.6 ± [0.34]
Pellet	384.9 ± [20.00]	30.1 ± [17.76]	2.52 ± [0.28]	67.4 ± [17.49]	6.5 ± [2.05]
A. dealbata	177.2 ± [18.33]	39.9 ± [8.58]	3.70 ± [0.54]	56.4 ± [9.13]	6.9 ± [1.28]
A. melanoxylon	155.4 ± [5.39]	29.1 ± [6.26]	12.28 ± [6.03]	58.6 ± [0.47]	8.8 ± [0.76]

* values determined with anhydrous biochar.

summarizes the most relevant parameters of the produced biochar. All of the biochar materials synthesized and studied exceed 50% of the fixed carbon value.

The highest value was achieved with pine bark, reaching 69.5%, followed by the value of 63.4% for pine chips, which although it did not exceed pine pellets, it required higher temperatures and times, which would limit their production costs. Regarding the pH value of the biochar, compared with the original biomass, it shows in all cases increases between 23% and 57%, with the bark biochar increasing this value the most.

According to previous work [32], devolatilization occurs at a very slow rate at low temperatures, and mass losses increase for any specific temperature (650 and 734 °C); the higher the temperature, the faster the pellets release volatile matter. In this case, pine pellets have reached the highest value of fixed carbon, but their pyrolysis process has required higher temperatures and times.

3.3. Results and Discussion of Floatability Analysis

Figure 6 shows the values obtained after the floatability test. When applying the different biochar to the top of the test, some particles (small fractions) immediately precipitate, except in the case of pine bark and ground pine chips (with a bulk density of 266.0 ± [8.49] kg/m³). This is the reason why the initial values of the surface layer in the graph already start below 30 mm.

As the test progresses, the floating biochar (solid lines in Figure 6) begins to settle, producing total settling in the pine pellets, compared to the pine bark and ground pine chips with minimal settling (dashed lines in Figure 6).

It is noteworthy that ground pine chips biochar behaved similarly to pine bark, not only in terms of the static position situation but also in the movement of distilled water. It floats even with fluid movement.

Figure 7 shows an example of the initial and final test measurements. The initial floating layer of acacia (Figure 7a) and the final settled layer of pine chips (Figure 7b) and pine bark (Figure 7c) can be observed.

The highest floatability value was achieved with pine bark biochar, which also has the highest fixed carbon value. In a similar way to other previous works [36], during the test, it was verified that before reaching the decantation phenomenon, and as floatability decreases with time, a certain amount remains floating, part undergoes decantation to the bottom, and the suspension phenomenon also occurs. When comparing the results of the pine chips biochar with the same material but ground to powder size, it is evident that the ground biochar is unsinkable, even when the water stops being static and becomes dynamic through agitation.

4. Conclusions

The highest fixed carbon content in biochar was observed in the pine bark (69.5%), followed by pine pellets (67.4%), and pine chips (63.4%). The use of relatively high carbonization temperatures and times did not result in significant weight losses during the process for these biochars.

The highest floatability result was achieved with a pine bark biochar, which corresponds to the highest fixed carbon value. However, the ground pine chip biochar achieved the same result, which was higher than the original pine chip biochar. Despite their high carbon content, the pellets exhibited the lowest floatability level. According to the results obtained, the use of low-density original biomass, such as pine bark and chips, showed superior floatability.

The floatability of biochar in water can be affected by several factors. This work demonstrates how bulk density and particle size, among others, play a significant role. Lower-density biochar tends to float more easily (pine bark biochar), and particle size reduction and milling have improved floatability (ground pine chip biochar), both in terms of static floatability states and in dynamic states with agitation.

This work lays the groundwork for further detailed research on floatability, focusing primarily on low relative densities, particle size reduction, and the improved properties resulting from milling, which can increase hydrophobicity and its interaction with water.