Parabolic Dish Collector as a New Approach for Biochar Production: An Evaluation Study

Abstract

The main factors influencing biochar properties are feedstock biomass and pyrolysis operational conditions. A solar parabolic dish collector was proposed as a new green approach to the pyrolysis process. The technique of this reactor was designed to produce biochar from sesame feedstock (SF) by concentrating solar radiation. This research aims to compare the main physical and chemical properties of biochar produced by the solar reactor to those of the conventional reactor (muffle furnace, SB-3). Biochar produced by the parabolic dish collector was a heterogeneous brown color. Depending on color intensity, biochar was divided into the biochar formed around the inner sidewalls of the internal chamber (SB-1) and the biochar formed in the upper part of the internal chamber (SB-2). Generally, the physiochemical properties of the SB-2 biochar were similar to the SB-3 biochar, while SB-1 biochar was similar to SF. This was because the temperature distribution was not uniform in the solar reactor. The proposed solar parabolic dish collector needs some modifications to upgrade the biochar production to be close to that produced by the electric instrument. SB-2 is preferred as a soil amendment depending on its pH, cation exchange capacity (CEC), elemental composition, ion molar ratio (H/C, O/C, and (O+N)/C), and acidic functional groups.

1. Introduction

Biochar is a black, solid carbonaceous material obtained through the pyrolysis process. Pyrolysis is the thermochemical degradation of organic waste (feedstock) in the absence or minimal condition of oxygen conducted at a temperature higher than 300 °C [1,2,3]. The biochar expression is not a new term; it is derived from the ancient Greek word ‘bio’, which means life, and the word ‘char’ is a short English word for charcoal [4]. Historically, biochar application is not a new concept; for instance, Cro-Magnon people, early European modern humans, used a woody charcoal material to document their events on cave walls [5]. The Pharaohs used the tar produced from pyrolyzed wood for embalming their dead, while the wood char was used for medicinal purposes [6]. In addition, the formation of highly fertile soil due to the storage of undegradable carbon in the Amazon Basin soil is called “Terra Preta”, which has received considerable quantities in the residues from incomplete biomass burning [7,8]. Recently, biochar has gained significant attention due to its unique physical and chemical characteristics that could be used as a soil amendment and adsorbent material in wastewater treatment to remove multiple pollutants. Such physicochemical characteristics are high resistance to biological decomposition, high carbon percentage, enormous surface functional groups, high cation exchange capacity (CEC), and alkaline material. Biochar also has high sorption capacity and porosity, a large specific surface area (SSA), and valuable amounts of nutrients [1,9]. However, the physiochemical properties of biochar vary and are largely influenced by the type and structure of feedstock, pyrolysis environment (maximum temperature, heating rate, and pyrolysis duration), and modification process [10,11,12]. The organic compounds present in the biomass decompose at a specific temperature in an oxygen-limited environment, yielding several gases (carbon dioxide, carbon monoxide, hydrogen, methane, and ethane), liquid (tar), and a black solid material as a byproduct (char) [2,13].

There are numerous techniques for biochar formation from organic residues by thermochemical degradation, which are mainly classified into traditional and modern approaches. According to Gabhane et al. (2020), traditional reactors are manufactured from different materials such as clay burners, firebrick pits, brick kilns, iron retorts, and steel oven reactors [2]. Depending on the temperature range and the heating rate, pyrolysis may be slow or fast. Slow pyrolysis takes place at a temperature range of 300–600 °C and a heating rate of 5 to 7 °C per minute, whereas fast pyrolysis reactors use temperatures above 500 °C and a heating rate of 300 °C per minute. Modern technologies include flash pyrolysis (temperature above 1000 °C for one minute and a heating rate of 1000 °C per second), vacuum pyrolysis (residues are subjected to pressures between 0.5 and 2 bar and a temperature of 450–600 °C), gasification (temperature more than 700 °C), and microwave pyrolysis [2]. Regardless of the type of pyrolysis, the energy source used greatly influences the environment and economic sectors. Lately, solar radiation as a renewable source of energy for the pyrolysis process has received more attention due to its potential to reduce pollutants and gas emissions. Through the use of solar energy directly, it is possible to reduce the negative impact caused by the combustion of fuels and/or the generation and transportation of electricity [13]. In that sense, all available amounts of agricultural residues can be transformed into biochar, hence providing a higher proportion compared with conventional systems. Many solar collectors have been proposed to concentrate the radiation beam directly to the reactor for agricultural residue pyrolysis and management, for instance, elliptical reflectors, mirrors, deep-dish parabolic mirrors, and lamp reflector windows. Depending on the solar concentrator, the temperature ranged from 300 °C to 900 °C [14]. The average temperature obtained by the parabolic dish reactor that utilizes a low-temperature reactor was 360–600 °C [15]. Chintala et al. (2017) produced bio-oil (20%) from Jatropha biomass by a parabolic dish reflector to concentrate the solar radiation onto a pyrolysis reactor, which ranged from 250 to 320 °C [16].

However, the biochar properties produced by parabolic dish reactors have not been thoroughly studied in the literature. The present study proposes a cavity chamber sealed from air and fixed on a solar parabolic dish collector to directly heat the agricultural residues. Samples of sesame stalk were prepared and filled in the reactor, aiming to evaluate their physicochemical properties after being pyrolyzed in the chamber, including ash content and elemental analysis, cation exchange capacity (CEC), zeta potential, infrared spectroscopy (FTIR), and scanning electron microscopy. The characteristics of the biochar produced by the solar reactor will be compared to those produced by the muffle furnace as one of the conventional methods for biochar production.

2. Materials and Methods

2.1. Solar Pyrolysis Chamber Design

The technique of the pyrolysis chamber reactor was designed to concentrate solar radiation as a source of energy to produce biochar from agricultural waste. It consists of a double-jacket chamber made from a stainless-steel sheet (3 mm thickness) rounded and welded together to form the shape as shown in Figure 1. The internal chamber consists of an inlet hole with a diameter of 15 cm and a height of 4 cm. This hole allows the solar radiation to enter the cavity coming from the solar parabolic dish’s reflected surfaces. The inner cavity dimensions are 22 cm in diameter and 10 cm in height. This cavity is designed to receive all reflected rays from the dish surface and minimize the heat losses to the outside environment. The agricultural residues will fill the space surrounding this heated cavity. The diameter and height of the outer chamber are 38 cm and 27 cm, respectively, with a movable cover on the top to save the inert environment during the carbonization process. Moreover, the fine holes near the top of the outside chamber were made to allow the gases to escape during pyrolysis, as shown in Figure 1a. The chamber after fabrication and installation is shown in Figure 1b.

2.2. Feedstock and Biochar Preparation

Sesame (Sesamum indicum) stalk residues were selected to be used as a feedstock (SF) source for biochar production (SB). The collected sesame stalks were dried in the air and cut into smaller particles of the desired size and shape to give fractions of less than 5 cm using a rotary cutting mill. The feedstock material was put inside the outer chamber, and the movable cover was well-sealed to avoid oxygen and burn the material in an inert environment. The pyrolysis chamber reactor was fixed on a stand facing the solar parabolic dish collector. During the pyrolysis process, the temperature was measured, and the maximum temperature of the reactor was kept at 300 °C; this process extended to 3 h. The focus was to create solid biochar; therefore, there was no interest to collect the constable. The present reactor was designed to produce biochar rather than condensable gasses; additionally, the temperatures recorded during the solar pyrolysis were not sufficient to produce biogas or bio-oil, and their amounts can be neglected. Despite that, small holes were made on top of the chamber (Figure 1a) to facilitate gas escape from the reactor during the carbonization process. After cooling to room temperature (≃25 °C), it was observed that the color of the formed biochar was brown. However, the produced biochar was divided into two types due to the heterogeneous distribution of solar radiation in the reactor field. The biochar formed around the inner sidewalls of the chamber was denoted by SB-1, whereas the biochar formed on the upper part of the internal chamber was denoted by SB-2. The collected biochar types were ground in a mortar and passed through a 0.5 mm sieve for further investigation. The conventional biochar was produced using a muffle furnace (HYSC, Hanyang Science Lab Co., Ltd., Seoul, Korea) at 300 °C for 3 h (denoted here by SB-3), which is used to compare the physicochemical parameters of the solar pyrolyzed biochar.

2.3. Sesame Feedstock and Produced Biochar Characterization

2.3.1. Ash Content and Elemental Analysis

The ash content of feedstock (SF) and different produced biochar types (SB-1, SB-2, SB-3) were determined by dry combustion of the different biomaterials in the air using a muffle furnace at 600 °C for 12 h. After combustion and reaching room temperature, the weight of ash was determined to calculate the ash content percentage according to El-Gamal et al. (2017) as follows [11]:

Ash Content (%) = W_a/W_m × 100,

(1)

where W_m and W_a are the weight of materials before and after combustion (g), respectively.

The total inorganic elementals in both feedstock and different biochar types were extracted from the ash using AquaRegia extraction (70 % HNO₃: 30 % HCl, v/v). The total concentration of Na, K, Mg, Ca, Cu, Fe, Mn, and Zn ions were measured in the acid extract by atomic absorption spectroscopy (AAS—ZEEnit 700—Analytik Jena). The yellow color intensity formed by ammonium paramolybdate-vanadate reagent was measured at the 420 nm wavelength to determine the total P concentration in the same extract using T80 UV/VIS Spectrophotometer, PG Instruments Ltd. [17].

Carbon, hydrogen, nitrogen, and sulfur contents of the feedstock and different biochar types were conducted by CHNS Elemental Analyzer (Vario MACRO cube, Elementar, Germany). The oxygen content was obtained according to Ferraro et al. (2021) by the following equation [4]:

O (%) = 100 − (C + N + H + S + Ash)

(2)

2.3.2. Cation Exchange Capacity (CEC)

The modified ammonium-acetate compulsory displacement method (Sumner and Miller, 1996) was followed to determine the cation exchange capacity (CEC-NH₄) of the biochar, as described by [18]. Before starting the CEC extraction, a pre-leaching step was applied to reduce interference from soluble salts using de-ionized water by shaking the suspension (1: 20, biochar: water) at 180 rpm for 5 min (five times). Then, the biochar sample was saturated with sodium acetate solution (1M Na-OAc, pH 8.2) by adding 15 mL to the sample, and the mixture was shaken for 10 min (repeated three times). After that, the samples were washed three times with ethanol to remove excess Na from the biochar surface. Finally, NH₄-acetate (1M NH₄-OAc, pH 7) was used three times to displace sodium ions, and the released Na⁺ ion was measured by atomic absorption spectroscopy.

2.3.3. Zeta Potential

The water suspension of feedstock and different biochar powders (0.1 g: 200 mL) was prepared to measure the zeta potential shaken at 150 rpm for 12 h [19], and a Zetasizer (ZP—Malvern, UK) was used to measure the zeta potential of different biomaterial samples in the suspension. Negative values indicate that biochar particles carry a net negative surface charge.

2.3.4. Infrared Spectroscopy (FTIR)

The Fourier transform infrared (FTIR) spectral method was used to identify the functional groups of the feedstock and different biochar types. The spectral range investigated was from 400–4000 cm⁻¹ using (Shimadzu FT/IR-5300, Tokyo, Japan).

2.3.5. Scanning Electron Microscopy (SEM)

SEM was conducted to characterize the surface morphology of the feedstock and biochar using a SEM, Model JSM—IT200, JEOL Ltd., Tokyo, Japan. Before the investigation, the samples were coated with gold to avoid the buildup of local electrical charges using a sputtering coater (Model JFC-1100E Ion Sputtering Device JEOL Co., Tokyo, Japan).

3. Results

3.1. Effect of Solar Pyrolysis Temperature on Biochar Production

Figure 2 shows the sesame stalk feedstock and the different biochar types obtained from the solar parabolic dish collector chamber and conventional reactor. The biochar produced by the solar pyrolysis chamber was brown in color and lightweight because of moisture loss. Depending on the color intensity, the biochar was divided into two types because the uniform distribution of solar radiation was not homogenous in the reactor field. The first type (SB-1, Figure 2b) was collected from carbonized material formed around the inner sidewalls of the internal chamber, while the second type (SB-2, Figure 2c) was collected from the upper part of the internal chamber. The FS (Figure 2a) had a light color, whereas the produced biochar by the solar parabolic dish collector had a heterogeneous color (SB-1 and SB-2). In comparison, the biochar produced under controlled conditions by the conventional method (muffle furnace) had a homogeneous black color (Figure 2d).

3.2. The Physicochemical Properties of Produced Biochar

The physicochemical parameters of raw sesame stalk (SF) and the corresponding biochar samples obtained by different pyrolysis conditions are presented in

Table 1. The main physicochemical parameters of raw sesame stalk and its biochar produced by different conditions.

Parameter	Unit	SF 1	SB-1	SB-2	SB-3
Ash	%	4.79	5.73	8.13	11.07
Volatile Gases	%	95.21	94.27	91.87	88.93
pH		5.36	5.60	6.88	7.49
EC	dS m−1	0.62	0.65	0.77	0.91
CEC	cmol kg−1	100.48	59.59	57.61	51.42
Total Elements
N	%	0.23	0.29	0.45	0.49
C	%	43.26	49.39	53.20	62.61
H	%	5.59	4.86	3.74	3.00
S	%	1.47	3.32	6.08	7.49
O	%	44.66	36.41	28.40	15.34
P	%	0.31	0.47	0.78	0.72
Ca	%	0.65	0.70	0.76	0.86
Mg	%	0.27	0.67	0.41	0.40
K	%	0.70	1.55	1.78	2.66
Na	%	0.41	0.36	0.48	0.55
Cu	mg kg−1	7.78	9.37	9.52	12.59
Fe	mg kg−1	146.20	193.30	209.30	341.00
Mn	mg kg−1	7.09	10.32	11.94	16.93
Zn	mg kg−1	33.68	83.27	105.60	97.17

¹ Sesame feedstock: SF, biochar produced around the inner sidewalls of the internal chamber: SB-1, biochar formed on the upper of the internal chamber: SB-2, and conventional biochar: SB-3.

. Ash percentages in different sesame biochar types increased from 4.79% in SF to 11.07% in the conventional method (SB-3). At the same time, the volatile percentages decreased with increasing ash content. Compared to feedstock, the ash percentages of different biochar types were increased by about 19.62%, 69.73%, and 131.11% for SB-1, SB-2, and SB-3, respectively. Generally, the ash content and volatile gas percentages of SB-1 are approximately near those of SF. At the same time, the biochar collected from the upper part of the internal chamber (SB-2) was relatively close to that produced by the conventional method (SF-3). In general, from the volatile matter, ash content, and elemental constituents, the biochar that formed around the inner sidewalls of the internal chamber (SB-1) was more related to raw biomass (SF) due to the low conversion occurred (semi-biomass), while the biochar obtained from the upper part of the internal chamber (SB-2) was much more similar to that produced by the conventional method (SB-3) (Table 1). The biochar produced by the conventional process was more homogeneous compared to that produced by the parabolic dish collector (Figure 2).

The pH value of biochar was higher than that of feedstock. It was observed that SB-3 became more alkaline (pH = 7.49) than SB-1 and SB-2. However, SB-2 and SB-3 were near neutral, but SB-1 appeared to be more acidic, and its value was near that of SF. Figure 3 showed a good exponential correlation (R² = 0.89) between the pH values and the percentages of oxygen content.

Electric conductivity (EC) of the water extraction is a factor used to assess the biochar salinity degree by measuring the total dissolved salts. The EC was higher in SB-2 and SB-3 than in SF and SF-1. All produced biochar types are generally characterized as non-saline materials, whose values are less than the unity.

Table 1 shows that feedstock has a cation exchange capacity (CEC) higher than that of biochar. Moreover, there is a gradient decrease in the CEC values from SB-1 to SB-3. The lowest value was found in the case of SB-3 (23.76 cmol kg⁻¹). The temperature of the pyrolysis process for both methods markedly affected the value of the CEC, which decreased in biochar produced by the conventional method compared to that produced by the parabolic dish collected method.

The elemental analysis of carbon, nitrogen, and sulfur showed higher contents in different biochar types than in feedstock (Table 1). The prime elemental constituent in biochar is carbon; SB-3 contained a more considerable percentage of carbon, which was higher by about 13.22% and 9.41% compared to SB-1 and SB-2, respectively. The concentrations of H and O in the SF were much higher than in the different biochar types. In addition, the molar ratios of H/C, O/C, and (O+N)/C were higher in the SF than in the produced biochar. These molar ratios decreased with increases in the temperature effect, indicating that SF has much higher polar group content (Figure 4). Generally, the biochar produced by different conditions showed higher chemical compositions relative to SF (Table 1). SB-2 and SB-3 contained approximately close P, Mg, Na, and Zn percentages, while the percentages of Ca, K, Cu, Fe, and Mn in SB-1 were approximately close to SB-2. However, SB-3 contained higher levels of Ca, K, Na, Cu, Fe, and Mn than those in biochar produced by the solar parabolic dish reactor.

3.3. Zeta Potential

Pyrolysis temperature significantly influences the zeta potential attributed to the colloidal biochar surface charge. The increased negative charges may indicate that biochar particles form stable suspensions to be more mobile in subsurface environments. As seen in Figure 5, the zeta potential of SF had the lowest value (−14.3) compared to the three produced biochar types. However, there was no significant change between the different biochar types, which ranged from −23.1 (SB-1) to −25.4 (SB-3), indicating that all biochar particle samples carried negative charges on their surfaces.

3.4. Fourier-Transform Infrared (FTIR)

FTIR spectra and band assignments of sesame stalk feedstock (SF) and its different biochar types produced by the solar parabolic dish collector method (SB-1 and SB-2) and conventional method (SB-3, muffle production) are presented in Figure 6. It was observed that the FTIR spectra of feedstock were less dissimilar from those of sesame biochar types regardless of the peak intensity due to the low temperature of the pyrolysis process. Moreover, there was a slight change in peak intensity and width between SB-2 and SB-3, while SB-1 was more related to feedstock (SF), especially in the fingerprint region. This remark indicates that the pyrolysis condition of SB-2 was near to that pyrolyzed under control conditions (SB-3). The alcohol and hydroxyl compounds in SF and SB-1 were detected by the broad peak of the H-bond at 3338 cm⁻¹, indicating O-H stretch vibration and affirming the presence of water and hydroxyl ascribed to cellulose and hemicellulose [11,20].

Additionally, the narrow band at 2915 cm⁻¹ shows aliphatic compounds assigned to saturated C–H stretching vibrations of the methylene group that are related to hemicellulose and cellulose. These peaks could have disappeared in the SB-2 and SB-3 cases due to the dehydration of cellulosic components and methylene groups and the increase in the aromatic structure [4]. A similar observation was observed at the peak around 1036 cm⁻¹ of alcohol, C-O stretch, and aliphatic amines C-N that decreased in different biochar types. The peak around 1240 cm⁻¹ is related to the phenol and C-O stretch group. The band around 1730 cm⁻¹ demonstrates the presence of hemicelluloses, specified by carbonyl compounds (C=O stretching vibration) of the carboxylic acid, ketones, and ester groups. The NH bend, N-H bend, C=C stretching vibrations, and C=C-C aromatic ring stretch around 1600 cm⁻¹ were attributed to primary amines, secondary amines, alkenes, and aromatic functional groups.

Moreover, C–H, N–O, and O–H vibrations dominated the fingerprint region of the spectrum from 1508 to 610 cm⁻¹. However, the strong peaks around 1510 and 1450 cm⁻¹ are attributed to the aromatic ring (C=C-C). The peaks between 1500 cm⁻¹ and 1030 cm⁻¹ were higher in the feedstocks than those in the three biochar types, and that group became weaker in SB-2 and SB-3 [21].

3.5. Scanning Electron Microscopy (SEM)

Figure 7 shows the surface structures and morphological characteristics of sesame stalk feedstock (SF) corresponding to the biochar (SB-1, SB-2, and SB-3) that were identified by SEM images. SEM magnified at 500X represents the morphological appearances of sesame biochar produced by different pyrolysis conditions. The raw sesame stalk (Figure 7a) had a smaller pore size with relatively lower porosity and blocked tubular structure compared to its biochar. It was clear that an evident porous tubular structure and channels occurred with different pyrolysis conditions, and the grid structure was detected at high magnification, which was 2000X for feedstock and 1000X for biochar. The parabolic dish collector method increased the porous numbers and diameters of the different biochar types (SB-1 and SB-2), which in turn developed the fibrous channels with a highly complex network of pores. The pores of SB-3 were relatively different in size and shape compared to those of SB-1 and SB-2. On the other hand, the SEM images of SB-2 and SB-3 (1000X) were relatively close and presented a variety of shapes and sizes of microspores and macrospores compared to SB-1.

4. Discussion

4.1. Effect of Solar Pyrolysis Temperature on Biochar Production

The main factors influencing the biochar properties are feedstock types and pyrolysis operational conditions. A solar parabolic dish collector was proposed as a new approach to pyrolysis. The physicochemical parameters of raw sesame stalk (SF) and the corresponding biochar samples obtained by different pyrolysis conditions showed different variations compared to biochar produced under controlled conditions (SB-3) because of the nonuniform temperature during the pyrolysis. That means that the carbonization process by solar radiation of the used reactor was heterogeneous; at the same time, the used chamber design needs some modification to enhance the uniformity of radiation distribution. Further studies are needed to increase the pyrolysis process’s residence time during exposure to solar energy to permit the uniform distribution of heat inside the chamber. However, the data revealed a gradient in ash and volatile contents from SB-1 to SB-3 due to the irregular temperature distribution in the solar pyrolysis reactor. However, the temperature around the inner sidewalls of the internal chamber was lower than in the upper part. Gogoi et al. (2020) observed in their study of the thermochemical properties of sesame stalk pyrolyzed at different temperatures that the volatile gas content decreased from 33.45 to 27.12 with the rising temperature from 350–650 °C, while the ash percentage increased from 6.57% to 8.20% in the biochar produced at the same temperature [22]. Generally, many researchers have reported that the ash percentage was lower in feedstock compared to biochar and vice versa in the case of volatile percentage due to an increase in inorganic constituents’ concentration [11,22,23].

All biochar types have higher pH than SF due to the thermal decomposition of easily degradable compounds during the pyrolysis process (carboxylic functional groups) and increasing of basic functional groups [23]. This means that rising pH occurs due to the decomposition of oxygen-rich functional groups [9]. Additionally, rising biochar pH is evidence of the higher ash content and the concentration of nonvolatile inorganic elements, which are affected by the pyrolysis temperature uniformity. El-Gamal et al. (2017) in their study showed that sugar cane bagasse and rice husk biochar produced at 450 °C had lower pH values (8.40 and 8.33, respectively) than those obtained at 550 °C (9.00 and 9.44, respectively). This difference could be due to the concentration of non-pyrolyzed inorganic elements [11]. This observation was confirmed by the chemical compositions of different biochar (Table 1), especially basic cations (Ca, Mg, K, and Na), which were higher in different biochar types than in sesame stalk feedstock [9]. All biochar types are generally characterized as non-saline material, whose values are less than the unity. These results agree with other researchers [9,11,20,21].

The dissimilarity in CEC values between SB-1 and SB-2 could be ascribed to the nonuniform pyrolysis temperature distribution in the solar reactor. However, Murtaza et al., 2022, reported that the difference trend in the CEC values of biochar depends on the biomass type, the ash content in feedstock, the cellulose and lignin contents, and the pyrolysis temperature [9]. For instance, Cely et al., 2015, found that the huge variation in CEC values of biochar derived from diverse manure wastes was due to the difference in ash content, which was higher in chicken manure than pig manure [24]. It was reported that lignin-rich biomass produces a higher CEC than biomass with higher cellulosic content [11,24].

In addition, the reduction of H and O values is due to the decomposition of the simple organic compound during the pyrolysis process and the release of hydrogen compounds such as light hydrocarbons and small-chain polymers [25]. Decreasing the O/C ratio increases the aromaticity and decreases the degradation rate [22]. The molar ratio of H/C can be used as an indicator of the carbonization of different biochar. Feedstock has the highest H/C because H is principally essential to the organic carbon of plants. The H/C ratio was less than one in the case of SB-2 and SB-3, meaning that SB-2 and SB-3 were carbonized significantly better than SB-1. Both biochar types showed a great formation of aromatic compounds. In addition, different decrements in this ratio of various biochar types compared to feedstock indicate that the pyrolysis condition is different [26].

During the pyrolysis process, the deprotonation of functional groups creates a negative charge on the surfaces of the biochar particles, which can form an electrical double layer in the solution phase near the surface [27]. Generally, many researchers have reported that the zeta potentials of the biochar increase with increased pyrolysis temperatures; therefore, the higher pyrolysis temperature of biochar production obtains higher negative surface charges consistent with reduced density of carboxylic and phenolic functional groups than those produced at the lower temperatures, thus lowering the electronegativity [19,28,29].

Generally, the FTIR bands and peaks of SB-1 differed from those of SB-2 and SB-3. This is because the pyrolysis temperature uniformity was not the same as in the cases of SB-1 and SB-2. There was a slight change in peak intensity and width between SB-2 and SB-3, while SB-1 was more related to feedstock. This mention indicates that the pyrolyzed biochar formed on the upper internal chamber (SB-2) was similar to the pyrolyzed biochar formed under control conditions (SB-3, conventional method). This is due to the thermal decomposition of the lignocellulosic structure (three main components) of plant residue [4,30].

Moreover, the changes in the morphological structures were due to the elimination of easily degradable organic compounds in the form of volatile gases [12,22]. This indicates that the thermal decomposition of cellulose and hemicellulose was higher for the conventional method (SB-3) than those produced by the solar parabolic dish collectors. Batista et al. (2018) perceived the same result in the study of the effect of surface morphology of different biochar types produced at 350 °C on water holding capacity. They observed that biochar types have heterogeneous surface morphologies with excessive pores and channels [12].

4.2. Application of Biochar Produced by Parabolic Dish Collector

Biochar is widely used for wastewater treatment and soil amendment strategies by recycling organic waste. The ability of biochar to eliminate pollutants (organic and inorganic) from wastewater depends on its adsorption capacity, which is linked to biochar stability and surface physicochemical properties due to the increased hydrophobic nature with well-organized carbon layers [31]. It is reported that higher pyrolysis temperatures produce biochar with higher aromaticity, hydrophobicity, surface area, pore volume, ash content, and pH. In contrast, biochar produced at lower temperatures are rich in oxygen-containing functional groups (acidic), have higher molar ratios of H/C and O/C (low aromaticity), and lower pH [32]. The biochar with low pH is suitable for soil due to the increased availability of soil nutrients at low pH, i.e., acidic conditions [33]. Applying biochar as a soil amendment has variable effects on soil corresponding to the biochar physicochemical properties and soil type. Depending on the physicochemical parameters of the biochar presented here, biochar produced by the parabolic dish pyrolysis reactor may predict the efficiency of biochar amendment to modify soil properties, especially in alkaline soil.

The pH levels of the obtained biochar were 5.60 and 6.88 for SB-1 and SB-2, respectively, which could decrease soil alkalinity. Applied low-pH biochar can increase N retention in soil by enhancing NH₄⁺ formation and reducing NH₃ emissions [3]. Biochar application could increase the soil organic matter percentage and subsequently, nitrogen and phosphorus availability [34]. Salem et al. (2019) applied eggplant shoots biochar (BC, pH = 7.6) and low-pH composted biochar with farmyard manure (BC-FYM, pH = 7.4) and sulfur (BC-S, pH = 6.9) in alkaline soil planted with potatoes. They found that the treatment of BC-FYM increased the potato yields by 11.7% and 25.13% under both inorganic and organic fertilizers, respectively, and by 10.53% in the case of BC-S using organic fertilization [35].

In contrast, original BC reduced the yield in the presence of mineral and organic fertilization by more than 6% and 10%, respectively. This result confirmed that the lower biochar pH reduces the soil pH, which might improve nutrient solubility and availability for plant growth in alkaline soil. The CEC values of 59.59 and 57.61 cmol kg⁻¹ for SB-1 and SB-2, respectively, were higher than SB-3 due to the presence of carboxyl functional groups primarily induced in acidic groups, confirmed by FTIR data [9]. It was demonstrated that the surface functional group could influence the capability of the biochar water-holding capacity [4]. In addition, it has been reported that biochar as a soil amendment could improve some soil physical properties, especially in coarse-textured and fine-textured soils, by increasing soil porosity, decreasing soil bulk density, and improving soil aggregation [36]. The complicated structure and pore system of the produced biochar could provide a protection arm for the soil microbes, such as fungi, bacteria, actinomycetes, and mycorrhizae [26].

5. Conclusions

The present study proposed a direct solar pyrolysis reactor to produce biochar as a promising alternative source of biochar. Its scaling up is promising, depending on the feedstock biomass type and size reactor. To evaluate the biochar production by solar parabolic dish collector, the present study provides an overview of the impacts of this method on the physiochemical properties of the obtained biochar compared to the conventional condition. The uniformity of the solar distribution significantly influences the physiochemical attributes of biochar. It was observed that the biochar formed on the upper part of the internal chamber was similar to the conventional method at the same pyrolysis conditions. The proposed solar parabolic dish collector needs some modifications to upgrade the biochar production to be close to that produced by the electric instrument. However, the produced biochar through this reactor is preferred for use as a soil amendment depending on its pH, CEC, elemental composition, ion molar ratios (H/C, O/C, and (O+N)/C), and presence of acidic functional groups (carboxylic groups). Developing a pyrolysis process using solar radiation as a source of energy for biochar production gives several environmental and economic benefits. In the conventional pyrolysis method, nonrenewable energy resources are used to obtain the desired temperature for biochar production. In addition, the huge amount of biomass in rarer areas is used as a fuel in low-efficiency traditional furnaces or direct combustion in the agricultural field, which causes air pollution and is extremely energy inefficient. Therefore, biochar production using solar radiation can help to mitigate some environmental problems and provide a green economic concept by the reduction of greenhouse gas emissions, air pollution, and energy consumption.