# Carbon capture through solar-driven CO2 gasification of oil palm empty fruit bunch to produce syngas and biochar

Saqr A.A. Al-Muraisy, Srirat Chuayboon, Lais Americo Soares, J.G.

Buijnsters, Shahrul Bin Ismail, Stéphane Abanades, Jules B van Lier, Ralph

E.F. Lindeboom

► To cite this version:

Saqr A.A. Al-Muraisy, Srirat Chuayboon, Lais Americo Soares, J.G. Buijnsters, Shahrul Bin Ismail, et al.. Carbon capture through solar-driven CO2 gasification of oil palm empty fruit bunch to produce syngas and biochar. Energy, 2025, 323, pp.135805. 10.1016/j.energy.2025.135805. hal-05026403

HAL Id: hal-05026403

https://hal.science/hal-05026403v1

Submitted on 9 Apr 2025

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
# Carbon capture through solar-driven CO₂ gasification of oil palm empty fruit bunch to produce syngas and biochar

Saqr A.A. Al-Muraisy<sup>a, *, ID</sup>, Srirat Chuayboon<sup>b, e</sup>, Lais Americo Soares<sup>a</sup>, J.G. Buijnsters<sup>c, ID</sup>, Shahrul bin Ismail<sup>d</sup>, Stéphane Abanades<sup>b, ID</sup>, Jules B. van Lier<sup>a</sup>, Ralph E.F. Lindeboom<sup>a, ID</sup>

## ARTICLE INFO

### Keywords:

Oil palm empty fruit bunch

Biomass

Concentrated solar power

Solar driven gasification

Carbon capture

Biochar

## ABSTRACT

Oil palm empty fruit bunch (OPEFB) is an abundant organic waste in Malaysia that is often disposed of through field burning. A previous study has shown that solar-driven steam gasification of OPEFB can produce hydrogen-rich syngas with an energy upgrade factor of 1.2 and a carbon conversion efficiency of 95.1 %. Beyond its potential as a biofuel, OPEFB can also act as a carbon sink, capturing photosynthetically stored carbon. This study explores the potential of amplifying OPEFB's negative carbon emissions through solar-driven gasification, using CO₂ as the gasifying agent. In this work, a Central Composite Design (CCD) approach was employed to assess the influence of temperature (1100–1300 °C) and CO₂/OPEFB molar ratio (1.6–3.0) on H₂/CO molar ratio and energy upgrade factor, with a constant OPEFB flow rate of 1.8 g/min. The results demonstrated that at an energy upgrade factor of 1.4, 94.9 % of the total carbon was converted into syngas with a H₂/CO molar ratio of 0.3. The maximum observed net carbon capture yield of 0.4 g C/g OPEFB was achieved at 1300 °C and a CO₂/OPEFB molar ratio of 3.0. The remaining carbon (94.4–95.7 wt %) was converted into biochar with low heavy metal content, which has potential as a soil enhancer.

## 1. Introduction

The production of crude palm oil (CPO) from oil palm fresh fruit bunch (OPFFB) generates various types of solid and liquid residues [1–3]. Notably, oil palm empty fruit bunch (OPEFB), mesocarp fibre (MF), and palm kernel shell (PKS) are the solid residues, whereas palm oil mill effluent (POME) is the liquid residue [1]. Disposal of these types of economically undervalued residues is not environmentally favourable [4]. Therefore, it is important to convert harmful oil palm residues into value-added products.

OPEFB is the most abundant solid residue in Malaysia's palm oil industry, with an estimated 21.8 million tonnes generated in 2020 [5–7]. When utilized for energy production, OPEFB can mitigate approximately 218.6 kg CO₂-equivalent per ton, potentially reducing annual greenhouse gas emissions of 4.8 million metric tonnes in Malaysia [8]. Generally, OPFFB residues are incinerated to generate

steam for CPO production, which contributes to the energy demands of palm oil mills [1]. However, the incineration of OPEFB is problematic due to its lignocellulosic structure and the high moisture content of about 60 wt % [9]. The field-burned OPEFB further produces white smoke, which is harmful to the environment [10]. Moreover, OPEFB is left in dump sides, forming anaerobic conditions that cause emissions of greenhouse gases (GHG), i.e., CH₄ and CO₂ [2]. Therefore, adequate treatment and utilisation of OPEFB are urgently needed to reduce the environmental burden. Some amounts of OPEFB are used as a bio-fertiliser in palm oil plantations and for mushroom cultivation [9]; however, this practice is not sufficient to utilise the large amounts of produced OPEFB [1].

Carbon sequestration or capture, along with energy recovery, offers a more sustainable alternative to field burning [2]. Gasification plants, focusing on bioenergy production with carbon capture, have gained attention as a key component of global mitigation strategies [4,11–14].

<footnote><sup>a</sup> Delft University of Technology, Faculty of Civil Engineering and Geosciences, Department of Water Management, Sanitary Engineering Section, 2628 CN, Delft, the Netherlands</footnote>

<footnote><sup>b</sup> Processes, Materials and Solar Energy Laboratory, PROMES-CNRS, 7 Rue Du Four Solaire, Font-Romeu, 66120, France</footnote>

<footnote><sup>c</sup> Department of Precision and Microsystems Engineering, Faculty of Mechanical Engineering, Delft University of Technology, 2628 CD, Delft, the Netherlands</footnote>

<footnote><sup>d</sup> Universiti Malaysia Terengganu, School of Ocean Engineering, Easter Corridor Renewable Energy (ECRE), 21030, Terengganu, Malaysia</footnote>

<footnote><sup>e</sup> Department of Mechanical Engineering, King Mongkut's Institute of Technology Ladkrabang, Prince of Chumphon Campus, Chumphon, 86160, Thailand</footnote>

<footnote>* Corresponding author.
E-mail address: s.a.a.al-muraisy@tudelft.nl (S.A.A. Al-Muraisy).</footnote>
Utilising OPEFB as feedstock in these plants could minimise landfill use and generate value-added products. In addition, converting OPEFB into syngas and biochar via solar-driven gasification could be an energetically more attractive solution [2]. Malaysia's abundant solar energy potential of 4–6 kWh/(m².day) and 4–8 daily hours of sunshine [2,15], make it conducive for solar-driven gasification.

Even though it is still at an early stage of development, CO₂ gasification emerges as a promising alternative to the conventional process of syngas production from natural gas [16]. In this regard, solar-driven gasification using CO₂ as the gasifying agent is one of the promising negative emission technologies (NETs) and carbon capture methods [16]. Solar-driven CO₂ gasification utilises highly concentrated solar power to locally generate temperatures above 1000 °C [17]. Under these conditions, OPEFB reacts with CO₂ to generate CO-rich syngas, as expressed in Equation (1). The method to develop the molecular formula of OPEFB was reported in a previous study [2].

$$C_{7.4}H_{12.1}O_{5.1} + 2.3 CO_2 \rightarrow 9.7 CO + 6.05 H_2 \Delta H^{\circ 1} = +2765.8 \text{ kJ/mol} \quad (1)$$

Syngas (CO and H₂), the main product of solar-driven CO₂ gasification of OPEFB, is an important precursor and intermediate gas for the synthesis of various commodity chemicals. For instance, CO is used in the manufacturing of acrylic acid, formic acid, dimethyl carbonate, propionic acid, acetic anhydride, acetic acid, and formamides, which possess multiple industrial downstream applications [16]. CO produced from bio-waste can also be used as fuel for solid oxide fuel cells (SOFCs) to generate electricity at high efficiency [18]. In addition, CO is simultaneously produced with H₂, forming syngas, which is an energy-rich gaseous fuel mixture. Syngas serves as a carbon and energy source for various industrial processes, such as the Fischer-Tropsch (FT) process, ammonia production, methanol production, dimethyl ether production, power and heat generation, and a substitute for natural gas [16]. Different applications require specific H₂/CO molar ratios in syngas. To expand the application potential of CO-rich syngas, it is necessary to mix it with H₂ to achieve the desired molar ratio for a variety of downstream applications [16]. For instance, the synthesis of methanol, dimethyl ether, and formaldehyde requires a H₂/CO molar ratio of 2, ethanol requires a H₂/CO molar ratio of ≤1, FT fuels require H₂/CO molar ratio of ≥2, and power generation requires a H₂/CO molar ratio of ≥50 [19].

Carbon sequestration through biochar is an attractive option among negative emission technologies (NETs) with relatively high technical readiness, good carbon abatement potential, and moderate costs [20]. Biochar from gasification is a carbon-rich material with potential applications in soil amendment and carbon sequestration, acting as a stable carbon sink which contributes to negative carbon emissions [20–22]. Additionally, advancements in biochar modification methods open up diverse applications beyond soil amendment [23]. These developments highlight the potential of utilising solar-driven CO₂ gasification of OPEFB to produce biochar.

Previous studies have explored conventional CO₂ gasification of various residues to produce CO-rich syngas, including wood biomass [24,25], plastics [26], and coal [27]. However, there is limited research on solar-driven CO₂ gasification [17,28–30]. Bellouard et al. [30] found that solar-driven CO₂ gasification of wood biomass achieved a H₂/CO molar ratio of 0.4–0.7 and a carbon conversion rate of 97.1 wt %, for a temperature range of 1100–1400 °C and CO₂ flowrate range of 0.2–0.6 NL/min. These findings illustrate the efficient process of solar-driven CO₂ gasification to produce CO-rich syngas from biomass. Moreover, Chew et al. [4] investigated the CO₂ gasification of torrefied oil palm biomass but found that torrefaction negatively affected energy yield and limited the range of attainable H₂/CO molar ratios, which subsequently hindered the possible downstream applications. Furthermore, Chan et al. [16] extensively investigated the recent progress of CO₂ gasification of various types of feedstocks and its promising potential to produce CO-rich syngas. However, thus far, solar-driven CO₂ gasification of raw OPEFB to produce CO-rich syngas and biochar was never reported

before. Therefore, the main objectives of the present study are: a) to assess the multifactor parametric effects of operating temperature and CO₂/OPEFB molar ratio on the response variables, i.e., H₂/CO molar ratio and energy upgrade factor, using CO₂ as the gasifying agent, b) to determine the optimum operating conditions within the range of the operating variables, and c) to characterise the produced biochar and propose its potential application. In this study, central composite design (CCD) method was applied. CCD has proven to be a useful statistical technique for optimising multifactor parameters in the gasification of various types of biomasses [2,31,32]. CCD provides a comprehensive analysis of the independent and interaction effects of operating variables, offering a deeper understanding compared to conventional analysis methods [2].

This study contributes to the development of NETs by harnessing the abundance of solar thermal energy, CO₂, and OPEFB to generate value-added syngas and biochar. The findings of this study could be a stepping-stone towards decarbonisation efforts within the palm oil industry. It is also in line with the global commitment towards achieving the UN Sustainable Development Goals (SDGs) 7, 12, and 13, as well as the 2015 Paris Agreement on Climate Control [20].

The reduction of GHG emissions through NETs can generate carbon credits—tradable certificates that represent the avoidance of one metric ton of CO₂ emissions [33]. As global demand increases and regulatory frameworks strengthen, the value of carbon credits is projected to rise significantly, potentially reaching $238 per ton by 2050 [34]. These credits can be traded in carbon markets, providing financial incentives for industries to adopt cleaner technologies [35]. Additionally, the future cost of Concentrated Solar Power (CSP) is expected to decline due to advancements in power blocks, receivers, thermal storage, and solar fields [36]. According to NREL projections, CSP Levelised Cost of Energy (LCOE) could drop substantially, improving economic viability, from $88.7/MWh in 2025 to an estimated $43.4 - $59.5/MWh by 2050 [36].

## 2. Materials and methods

### 2.1. Biomass feedstock

OPEFB was obtained from a palm oil mill in Malaysia. Prior to shipment, OPEFB fibres were washed, dried, shredded, grinded, and then stored at room temperature until use. More details are available in a previous work [2].

The proximate analysis, i.e., moisture content, total and volatile solids, and ash contents of OPEFB was performed according to the standard method reported by APHA [37]. The ultimate analysis, i.e., carbon, hydrogen, nitrogen, and sulfur contents of OPEFB was performed via CHNS elemental analysis (Flash 2000, Thermo Scientific, USA). The oxygen content (wt. %) was calculated by the difference, i.e., 100 - (C + H + N + S). The lower heating value (LHV) of OPEFB was taken from the literature [38]. Table 1 summarises OPEFB characteristics and comparison with the literature.

### 2.2. Experimental setup

The complete experimental system is illustrated in Fig. 1. Its main components consist of a sun-tracking heliostat, solar concentrating parabola dish, conical spouted bed solar reactor connected with a biomass feeding hopper, a filtering unit, and a gas analysis system. More details on the solar reactor concept, design, and experimental protocol have been previously reported [30,40,41].

The experimental setup, located at CNRS-PROMES solar laboratory in Odeillo, France, uses a 2-m parabolic concentrator to reflect and concentrate solar radiation. A sun-tracking heliostat, located 30 m beneath the setup trap door, reflects the solar radiation vertically to the concentrator. The motorised trapdoor partially closes or opens to regulate the reflected incident solar power and thus the reactor temperature. The reactor, positioned under the concentrator, receives the
<table><caption>**Table 1**
Oil palm empty fruit bunch (OPEFB) characteristics.</caption><thead><tr><th rowspan="2">OPEFB characterisation</th><th colspan="3">Composition (wt. %)</th></tr><tr><th>This study</th><th>Farid et al. [39]</th><th>Chang et al. [10]</th></tr></thead><tbody><tr><td>Moisture content</td><td>11.3 ± 0.1</td><td>7.9 ± 0.4</td><td>2.4–14.3</td></tr><tr><td><strong>Proximate analysis (%<sup>a</sup>)</strong></td><td></td><td></td><td></td></tr><tr><td>Volatile matter</td><td>87.7 ± 0.1</td><td>71.0 ± 0.7</td><td>70.0–83.9</td></tr><tr><td>Fixed carbon</td><td>1.0 ± 0.1</td><td>14.6 ± 0.7</td><td>9.0–18.3</td></tr><tr><td>Ash</td><td>1.0</td><td>6.4 ± 0.5</td><td>1.3–13.7</td></tr><tr><td><strong>Ultimate analysis (%<sup>b</sup>)</strong></td><td></td><td></td><td></td></tr><tr><td>C</td><td>48.4 ± 0.1</td><td>44.9 ± 0.6</td><td>43.8–54.8</td></tr><tr><td>H</td><td>6.7 ± 0.1</td><td>6.6 ± 0.1</td><td>4.4–7.4</td></tr><tr><td>N</td><td>0.6 ± 0.1</td><td>0.9 ± 0.1</td><td>0.3–1.2</td></tr><tr><td>S</td><td>0.1</td><td>–</td><td>0.04–1.1</td></tr><tr><td>O<sup>b</sup></td><td>44.2 ± 0.4</td><td>47.6 ± 0.1</td><td>38.3–47.8</td></tr><tr><td>Lower Heating Value (MJ/kg)</td><td>16.1<sup>c</sup></td><td>16.9 ± 0.4</td><td>16.8–19.2</td></tr></tbody></table>

a Weight percent on a dry basis.

b By difference (100-(C + H + N + S)).

c Value taken from literature [38].

concentrated solar flux through a 2-cm aperture. The flux density at the focal point can go up to 10 MW/m² with direct normal irradiation (DNI) of 1 kW/m², following a Gaussian distribution.

Gasification reactions occurred in the reactor's cone-like cavity, which was insulated at the bottom and sides. The gasification of OPEFB is a multifaceted thermochemical process involving several stages: drying, devolatilization (pyrolysis), heterogeneous char reactions, and homogeneous gas-phase reactions [42,43]. When OPEFB particles enter the high-temperature cavity, they rapidly heat up, leading to moisture evaporation [44]. As the temperature continues to rise, devolatilization takes place, during which volatile compounds decompose and are released from the biomass [44]. OPEFB gasification using CO₂ as the gasifying agent primarily involves heterogeneous and homogeneous reactions that drive the conversion of solid carbon into syngas. The key reaction could likely be the Boudouard reaction (C + CO₂ ⇌ 2CO), which is endothermic and enhances CO production at high temperatures [42–44]. Additionally, biochar gasification (C + CO₂ → CO) plays a crucial role in carbon conversion, while secondary reactions such as the reverse water-gas shift (CO₂ + H₂ ⇌ CO + H₂O) influence the H₂/CO ratio [42]. The reaction kinetics are largely controlled by temperature, biochar reactivity, and the availability of CO₂ at the reaction sites, with higher temperatures favouring increased conversion rates and syngas yields [43]. Further details on the reaction kinetics and simulation of biomass gasification are reported in the literature [42–44].

Biomass was injected into the cavity through a motorised feeding hopper connected via a screw feeding mechanism. Temperature was measured using a B-type thermocouple and verified with a solar-blind optical pyrometer operating at 4.8–5.2 µm (Impac, LumaSense Technologies, Inc., USA). Pressure of the reactor was atmospheric in all the experiments.

A gas flow was injected into the reactor cavity consisting of Ar carrier gas, at a constant flowrate of 2.5 NL/min, and the gasifying agent (CO₂). The carrier gas was introduced to create an anaerobic atmosphere, and transport biomass particles to the top of the cavity for better solar power absorption [30]. It also ensures proper mixing of the biomass with CO₂ [30].

The syngas products were cooled in a gas bubbler and filtered to remove the carried ashes, char, and tars before entering the gas analyser. Syngas composition was measured via a syngas analyser (X-Stream XEGP, Emerson, USA, precision ±1 % of full scale) which utilises Non-Dispersive Infrared (NDIR) and Thermal Conductivity Detection (TCD) technologies for the measurement of main gases in syngas. The concentrations of CO, CO₂, CH₄, and C_nH_m were measured using NDIR cells, while H₂ concentration was measured using a thermal conductivity detector. Moreover, the produced syngas composition was periodically measured using a gas chromatograph (GC, Varian CP4900, Agilent, USA) for further verification of the results [30] which uses chromatographic techniques to separate and quantify components in complex mixtures like syngas. The GC was equipped with two columns (MolSieve 5A PLOT and PoraPLOT U for light hydrocarbons) and the carrier gas was argon. More details of the analysis methods are available in the literature [40,45]. Gas flowrates were controlled via mass flow controllers (MFC, Brooks Instruments model 5850 S). After each experiment, the outlet components were weighted for mass balance. The mass and energy balances of the experiments are discussed in the Supplementary Material (sections 2 and 3, respectively).

## 2.3. Experimental design

CCD, a response surface methodology, was used in this study to optimize the operating conditions for the solar-driven gasification of OPEFB [46–48]. More details on the design of circumscribed CCD can be found in the literature [49]. An array of designed experiments was generated using Statistica Software (StatSoft v. 8.0). The generalized equation for CCD with two factors, based on the quadratic Taylor series polynomial approximation, is presented in Equation (2).

$$Y_i = \beta_0 + \sum_{j=1}^{k} \beta_j x_{ij} + \sum_{j=1}^{k} \beta_j x_{ij}^2 + \sum_{j=1}^{k-1} \sum_{f>j}^{k} \beta_{jj} x_{ij} x_{fj} + \varepsilon_i \quad (2)$$

<figure><img src="image_2.png" /><figcaption>**Fig. 1.** Schematic diagram of the experimental setup.</figcaption></figure>
The experimental design included temperature and CO₂/OPEFB molar ratio as the operating variables, with a constant OPEFB flowrate of 1.8 g/min which is reported to be suitable for such experiments [50]. The operating variables were coded at three levels: -1 (low), 0 (medium), and +1 (high). To obtain a rotatable CCD model, the extreme levels of the star design (-α, +α) were added to the model whereby |α| is equal to 1.41 [51]. The selected values and levels of the temperature and CO₂/OPEFB molar ratio were determined based on prior studies on the solar gasification of OPEFB and wood biomass [2,41,50]. For two factors, a CCD requires 9 experiments. In this study, the central-level experiment was conducted in duplicate. Since the results of the duplicate were consistent, only one replicate is presented and discussed. Table 2 provides further details on the investigated levels of the independent operating variables.

### 2.3.1. Selection of response variables

To assess the produced syngas quality for possible applications, H₂/CO molar ratio is the suitable response variable [31]. Other gases like CO₂, CH₄, and C₂H₃ are generated as by-products in smaller quantities. The energy upgrade factor is the other response variable used to assess the efficiency of solar-driven gasification by measuring the storage of solar energy in the syngas products. Syngas yield was also investigated to identify the quantity of syngas produced for each operating condition.

H₂/CO molar ratio, H₂/CH₄ molar ratio, and energy upgrade factor were calculated from the molar syngas yields, the LHV of OPEFB and syngas, and the solar energy input. Additionally, carbon conversion efficiency was determined based on the carbon content in OPEFB and syngas components, providing insight into how effectively carbon was utilized in the gasification process. Carbon consumption rate was also analysed to quantify the rate at which carbon was converted into gaseous products under varying conditions. Furthermore, the net carbon capture yield was evaluated to assess the overall effectiveness of the process in capturing and retaining carbon within useful syngas components. Further explanations of the methods of data processing and evaluation are reported in an earlier study [2] and provided in the Supplementary Material (section 1).

A total number of 9 experiments were performed under the designed operating conditions. Table 2 shows the CCD experimental design consisting of the selected operating variables versus response variables.

### 2.3.2. Statistical analysis

Principal component analysis (PCA) was conducted using RStudio software (version 1.1.456) to assess the correlation between the operating and response variables [52,53]. Standardization of variable values was performed to reduce dataset dimensionality. Each eigenvector in a PCA biplot serves as a single operating or response variable. Eigenvectors pointing in identical directions are positively correlated, whereas eigenvectors with opposite directions are inversely correlated.

Perpendicular eigenvectors indicate an independent correlation. The weight of a variable eigenvector, determined by its length, indicates the relative importance of each principal component.

Two principal components (PC1 and PC2) were selected as they captured over 95% of the data variability for the H₂/CO molar ratio and energy upgrade factor. PC1 and PC2 in our dataset correspond to the primary directions of variation within the data. PC1 accounts for the largest variance, while PC2 represents the next highest variance, orthogonal to PC1.

The obtained results were visualised and statistically analysed using Design-Expert Software (version 13) and RStudio Software (version 1.1.456). Significant interactions were identified through PCA and ANOVA to quantify the impact of each operating variable on the selected response variables.

## 2.4. Biochar characterisation

Biochar samples were collected for analysis from two locations: (i) the solar reactor alumina tube at the exit of the conical cavity and (ii) the gas filtering unit, after each solar-driven gasification experiment. A comparison was made between biochar produced from solar-driven gasification using steam (as presented in our previous work [2]) and CO₂ as the gasifying agents. The elemental composition of the biochar, including carbon, hydrogen, and nitrogen content, was determined using an elemental analyser (Flash 2000 Elemental Analyser, Thermo-Fisher, USA). The surface area, pore volume, and pore size of the biochar samples were measured using a surface analyser (Gemini VII 2390p, Micromeritics Instrument Co., USA) with N₂ at 77 K. Prior to analysis, the samples were heated at 60 °C for 24 h to remove adsorbed gases. The specific surface area was determined using the Brunauer-Emmett-Teller (BET) method, while the pore size distribution was obtained through Barret-Joyner-Halenda (BJH) adsorption analysis.

The metal composition was analysed using inductively coupled plasma - optical emission spectrometry (ICP-OES) (Optima 5300DV, PerkinElmer Co., USA). Biochar and positive control samples (raw OPEFB with particle size < 1 mm) were subjected to pretreatment using the microwave digestion method (Multiwave PRO, Anton Paar GmbH, Austria). Approximately 25 mg of each sample was destructed in a solution consisting of 4.5 mL of 30 % HCl, 1.5 mL of 65 % HNO₃, and 0.2 mL of 40 % HF. Then, solutions containing the biochar samples were further digested with an extra 6 mL of 65 % HNO₃. Biochar samples, which proved to be more recalcitrant to destruction; were further mixed with 2 mL of 30 % H₂O₂. The destruction time in the microwave was 60 min at maximum power and temperature (13 kW and 210 °C, respectively). After the destruction, all samples were diluted with Milli-Q water to 50 mL.

The metals analysed included phosphorus (P), potassium (K), sodium (Na), magnesium (Mg), manganese (Mn), calcium (Ca), iron (Fe),

<table><caption>**Table 2**
Central Composite Design (CCD) experimental matrix and results of solar-driven CO₂ gasification of OPEFB (at OPEFB flowrate = 1.8 g/min).</caption><thead><tr><th rowspan="2">Run</th><th colspan="3">Operating Variables</th><th colspan="2">CCD response variables</th><th colspan="3">Carbon capture results</th><th colspan="3">Related results</th></tr><tr><th>Temp. (°C)</th><th>CO₂/OPEFB molar ratio</th><th>CO₂ flowrate<sup>a</sup> (NmL/min)</th><th>H₂/CO molar ratio</th><th>Energy upgrade factor</th><th>Carbon conversion (wt. %)</th><th>Net carbon consumption rate (g/min)</th><th>Net carbon capture yield (gC/gOPEFB)</th><th>H₂/CH₄ molar ratio</th><th>Syngas yield (mmol/g OPEFB)</th><th>η<sub>solar-to-fuel</sub> (%)</th></tr></thead><tbody><tr><td>1</td><td>1059 (-1.41)</td><td>2.3 (0)</td><td>506.0</td><td>0.4</td><td>1.2</td><td>57.0</td><td>0.3</td><td>0.2</td><td>3.6</td><td>59.0</td><td>10.2</td></tr><tr><td>2</td><td>1100 (-1)</td><td>1.6 (-1)</td><td>352.0</td><td>0.4</td><td>1.0</td><td>65.3</td><td>0.3</td><td>0.2</td><td>4.7</td><td>54.3</td><td>7.3</td></tr><tr><td>3</td><td>1100 (-1)</td><td>3.0 (+1)</td><td>662.2</td><td>0.4</td><td>1.3</td><td>77.1</td><td>0.3</td><td>0.2</td><td>4.5</td><td>69.8</td><td>11.7</td></tr><tr><td>4</td><td>1200 (0)</td><td>1.3 (-1.41)</td><td>286.0</td><td>0.5</td><td>1.2</td><td>84.1</td><td>0.5</td><td>0.3</td><td>8.7</td><td>68.1</td><td>13.9</td></tr><tr><td>5</td><td>1200 (0)</td><td>3.3 (+1.41)</td><td>726.0</td><td>0.3</td><td>1.3</td><td>68.0</td><td>0.6</td><td>0.3</td><td>6.5</td><td>69.7</td><td>14.4</td></tr><tr><td>6</td><td>1200 (0)</td><td>2.3 (0)</td><td>506.0</td><td>0.4</td><td>1.3</td><td>85.4</td><td>0.6</td><td>0.3</td><td>6.5</td><td>71.6</td><td>12.9</td></tr><tr><td>7</td><td>1300 (+1)</td><td>1.6 (-1)</td><td>352.0</td><td>0.4</td><td>1.3</td><td>86.1</td><td>0.6</td><td>0.3</td><td>10.2</td><td>73.4</td><td>12.6</td></tr><tr><td>8</td><td>1300 (+1)</td><td>3.0 (+1)</td><td>662.2</td><td>0.3</td><td>1.4</td><td>94.9</td><td>0.7</td><td>0.4</td><td>9.9</td><td>79.5</td><td>13.7</td></tr><tr><td>9</td><td>1341 (+1.41)</td><td>2.3 (0)</td><td>506.0</td><td>0.4</td><td>1.4</td><td>87.7</td><td>0.7</td><td>0.4</td><td>12.1</td><td>77.4</td><td>8.8</td></tr></tbody></table>

<footnote><sup>a</sup> CO₂ flowrate (g/min) is calculated from multiplying CO₂/OPEFB molar ratio with OPEFB flowrate.</footnote>
cadmium (Cd), chromium (Cr), copper (Cu), nickel (Ni), lead (Pb), and zinc (Zn). All values were reported as the average of three determinations per sample and expressed on a dry matter basis. This analysis aimed to demonstrate the potential application of biochar as a biofertiliser [54,55]. The particle structure and surface topography of raw OPEFB and biochar were examined using scanning electron microscopy (SEM) (XL30ESEM, Philips, NL).

# 3. Results and discussion

## 3.1. Global performance analysis

Syngas components produced from solar-driven CO₂ gasification of OPEFB proportionally increased with increasing temperature and CO₂/OPEFB molar ratio. OPEFB flowrate was kept constant at 1.8 g/min. At the applied flowrate, the solar reactor showed effective experimental performance as indicated by stable syngas production and syngas composition, and no pyrolytic smoke was observed through visual observation of the reactor window. Thus, it was assumed that the OPEFB feed flowrate was suitable for the kinetic rate of the gasification process. This finding comes in agreement with reported literature [50].

Fig. 2 illustrates the syngas yields at various operating conditions. In Fig. 2, CO₂ yield was excluded from the total syngas yield from an energy perspective because it is the most oxidised form of carbon. However, for syngas volumetric composition, black box analysis was used to differentiate the proportion of CO₂ generated from the solar-driven gasification of OPEFB and the unreacted CO₂ injected into the reactor. In this regard, it was observed that the CO₂ consumption rate was positively proportional to the operating temperature. For instance, carbon dioxide was consumed and converted at a rate of up to 0.3, 0.6, and 0.7 g/min at 1100 °C, 1200 °C, and 1300 °C, respectively. This observation comes in agreement with reported literature values [50]. Net carbon capture yield followed a similar trend reaching a maximum of 0.4 gC/gOPEFB at 1300 °C and a CO₂/OPEFB molar ratio of 3.0.

At 1300 °C, total syngas yield reached 77.4 mmol/g OPEFB with a composition of 28.0 % and 57.3 % v/v for H₂ and CO, respectively. The generated syngas yield was 89.5 % of the theoretical value of 86.5 mmol/g OPEFB, with a stoichiometric composition of 38.4 % and 61.6 % for H₂ and CO, respectively, using Equation (1). The syngas yield obtained in our present study was higher than that of a reported study reaching 61.0 mmol/g biomass from the solar-driven gasification of wood

biomass at 1300 °C [45]. Results obtained at 1300 °C showed that the H₂ fraction was lower and the CO fraction higher than the results of Bellouard et al. [45], who found 31 % v/v for H₂ and 42 % v/v for CO. This observation indicates that in our present work, more efficient gasification of OPEFB and conversion of CO₂ feed took place. This finding is supported by the calculated carbon conversion efficiency of 94.9 %, which was also higher than the 82.4 % of the cited work [45]. These results could identify that the type and structure of the biomass have an impact of the gasification process efficiency and CO₂ conversion.

At 1200 °C, the syngas yield reached a maximum of 71.6 mmol/g OPEFB, with a composition of 22.6 % and 54.4 % v/v for H₂ and CO, respectively. The syngas yield achieved 82.8 % of the theoretical value, according to Equation (1). At 1200 °C, the obtained syngas showed a lower H₂ fraction and a higher CO fraction than the results of Bellouard et al. [40] who reported 24.0 % and 44.0 % v/v for H₂ and CO, respectively. A carbon conversion efficiency of 85.4 % was achieved, which was comparable to the 87.7 % in the cited work [40] in which wood was gasified at 1200 °C using CO₂ as the gasifying agent.

At 1100 °C, the highest syngas yield was 69.8 mmol/g OPEFB, consisting of 14.4 % and 37.2 % v/v for H₂ and CO, respectively. The achieved syngas yield represented 80.7 % of the theoretical value. The generated syngas showed slightly lower concentrations of H₂ and CO than Bellouard et al. [40], who found 16.0 % and 44.0 % v/v for H₂ and CO, respectively. A carbon conversion efficiency of 77.1 % was achieved, which was higher than the 70.4 % in the cited study [40].

Results clearly demonstrated that an efficient syngas yield could be generated from the solar-driven gasification of OPEFB using CO₂ as the gasifying agent. Experiment #8 showed the highest syngas yield and carbon conversion efficiency of 79.4 mmol/g OPEFB and 94.9 %, respectively. This experiment also achieved a H₂/CO molar ratio of 0.3. Based on stoichiometric values, a H₂/CO molar ratio of 0.6 was expected. The lower experimental H₂/CO molar ratio indicated that extra CO was obtained from the gasifying agent (CO₂) by the Boudouard reaction [40].

The experimental ηsolar-to-fuel achieved a range of 7.3–14.4 % in this study. The effect of changing the operating variables was statistically not significant (p > 0.05) on ηsolar-to-fuel. However, it showed a slightly proportional increase with X₂ (CO₂/OPEFB molar ratio). To illustrate, at 1100 °C, increasing X₂ from 1.6 to 3.0 increased ηsolar-to-fuel from 7.3 to 11.7 %. Furthermore, at 1200 °C, increasing X₂ from 1.3 to 3.3 increased ηsolar-to-fuel from 13.9 to 14.4 %. Similarly, at 1300 °C, increasing X₂ from

<figure><img src="image_3.png" /><figcaption>**Fig. 2.** Syngas yield and product composition for solar-driven gasification of OPEFB under 2 varying operating conditions: temperature (1100–1300 °C) and CO₂/OPEFB molar ratio (1.6–3.0) at constant OPEFB flowrate (1.8 g/min).</figcaption></figure>
1.6 to 3.0 increased η<sub>solar-to-fuel</sub> from 12.6 to 13.7 %. Temperatures above 1300 °C reduced η<sub>solar to fuel</sub> due to the increase in heat losses (radiative and conductive) [2], which explained why a η<sub>solar to fuel</sub> of 8.8 % at 1341 °C was obtained.

The η<sub>solar to fuel</sub> value using CO<sub>2</sub> as the gasifying agent was lower compared to when steam was used, which could be due to the slower reaction kinetics [56], resulting in higher reaction duration, in turn increasing solar energy consumption and heat losses. For instance, the highest η<sub>solar to fuel</sub> using solar-driven steam gasification of OPEFB was 19.6 % [2] compared to 14.4 % in our present study. These findings are supported by similar observations in literature in which carbonaceous materials were gasified [56]. According to Numazawa et al. [56], the frequency factor, i.e., the reaction kinetics, of steam gasification of metallurgical coke was approximately 10 times higher than that of CO<sub>2</sub> gasification.

## 3.2. Analysis of the operating and response variables

The influence on the response variables (H₂/CO molar ratio and η<sub>solar to fuel</sub>) resulting from changing the operating variables was experimentally investigated and statistically analysed by CCD. Principle component analysis (PCA) was also used to identify the statistical significance of the effect of each operating variable on the response variables. PCA was performed by considering the scaled values of the operating variables with respect to the scaled values of the response variables.

### 3.2.1. Influence on H₂/CO molar ratio

Fig. 3A shows that CO₂/OPEFB molar ratio (X₂) had a strong inverse correlation with H₂/CO molar ratio (Y₁), whereas temperature (X₁) showed a nearly independent effect on Y₁, indicated by their nearly perpendicular correlation. These observations are further supported by ANOVA analysis reporting a p-value of 0.0326 which indicates its statistical significance. Moreover, ANOVA analysis of the parameters showed p-values of 0.4013 and 0.0135 for X₁ and X₂, respectively, indicating that only X₂ is statistically significant. Further details on ANOVA results are available in the Supplementary Material (section 4.1, Table A.2). To validate the results of PCA and ANOVA analyses, the effects of changing process variables (X₁ and X₂) on Y₁ were experimentally conducted and statistically investigated by CCD. As shown in Fig. 3B, at constant X₂, Y₁ did not change significantly which is in line

with the findings of PCA and ANOVA analyses. To illustrate, at a constant X₂ of 2.5, changing X₁ between 1000 and 1400 °C resulted in a constant Y₁ at 0.4. On the other hand, at a constant X₁ of 1200 °C, changing X₂ between 1.3 and 3.3 resulted in significant changes of Y₁ between 0.3 and 0.5. Further analysis of the interaction effects between the temperature and CO₂/OPEFB molar ratio, and their impact on the H₂/CO molar ratio, can be found in the Supplementary Material (section 5, Figure A.3(A)).

Injecting CO₂ at a higher flowrate resulted in enhancing the produced CO quantity, thus decreasing Y₁, which was expected from its theoretical gasification reaction (Equation (1)). The experimentally obtained values of Y₁ were between 0.3 and 0.5. As observed in Fig. 3B, there was no optimum condition achieved in this study for the H₂/CO molar ratio, which could be due to the maximum allowable process conditions, i.e., temperature ≤1400 °C, and CO₂ flowrate ≤700 NmL/min. Hence, the optimal conditions of the H₂/CO molar ratio, represented by a CCD plateau curve, are likely to occur at higher operating conditions that surpass the upper limit of the solar gasification reactor. Consequently, establishing a quadratic correlation was not possible.

The minimum H₂/CO molar ratio of 0.3 was experimentally reached. It should be noted that widening the resulting H₂/CO molar ratios could diversify CO applications, such as in fuel cells [57], catalyst production, electronics, semiconductor applications, and the manufacture of metal carbonyls [58].

Optimisation of Y₁ depended on the desired outcome; if more CO is needed, then X₂ between 2 and 3.5 and temperature between 1100 and 1400 °C could be chosen as optimised conditions to attain a Y₁ of 0.3–0.35. In addition, it was evidenced that the gasifying agent played a crucial role in the syngas composition, which was also reported in the literature [59]. Therefore, choosing a suitable gasifying agent is essential to control and optimize the desired product.

### 3.2.2. Carbon sequestration potential of OPEFB solar-driven syngas

From a carbon sequestration perspective, it is interesting to analyse the composition gap between the OPEFB solar-driven syngas and the gas requirements of selected carbon-based materials. Examples include advanced carbon materials synthesis through chemical vapour deposition (CVD) and other carbon-based chemicals through the FT process. Advanced carbon materials are considered to be the backbone of engineering and scientific innovation due to their outstanding chemical,

<figure><img src="image_4.png" /><figcaption>**Fig. 3.** (A): PCA of scaled operating variables (X₁, X₂) vs. scaled H₂/CO molar ratio (Y₁), (B) 3D response surface plot analysing the effects of operating variables (X₁, X₂) on H₂/CO molar ratio (Y₁) at constant OPEFB flowrate of 1.8 g/min (X₁ = temperature, X₂ = CO₂/OPEFB molar ratio).</figcaption></figure>

<figure><img src="image_5.png" /></figure>
physical, and electrical properties, which would result in long-term carbon sequestration potential [60]. The synthesis of FT chemicals from OPEFB solar-driven syngas can represent a process with long-term carbon sequestration potential [61]. The products of FT synthesis can include long-lived chemicals, waxes, or even construction materials [62]. Even when FT products are used as fuels, the carbon released can be part of a circular carbon economy if the CO₂ is captured and reused, contributing to reducing the reliance on fossil fuels and lowering the overall carbon footprint [62].

Solar-driven gasification of OPEFB using either steam [2] or CO₂ as the gasifying agents could potentially produce large amounts of syngas with varying H₂/CO molar ratios ranging from 0.3 in the present study, to 1.6 in the previous work [2]. Table 3 compares the target carbon sequestration methods, experimentally achieved OPEFB solar-driven syngas composition, and the upgrade gap analysis.

The economic viability of carbon sequestration and carbon-based material synthesis depends on the evolving carbon markets [34]. The projected rising values of carbon credit could support OPEFB-derived syngas in sustainable industries. Additionally, the advancements in CSP infrastructure and the projected drop in its LCOE [36] could lower energy costs, improving the scalability of solar-driven gasification and its role in carbon sequestration. Further details on the scalability prospects of the solar reactor are provided in Supplementary Material (Section 7).

### 3.2.3. Influence on energy upgrade factor

The influence on the energy upgrade factor due to changing operating variables was experimentally assessed and statistically evaluated, as shown in Fig. 4(A and B). Further details on ANOVA results are available in the Supplementary Material (section 4.2, Table A.3). Most experiments showed an energy upgrade factor higher than 1.0, demonstrating that the calorific value of the generated syngas was higher than the original feedstock [2]. The directions of the PCA eigenvectors in Fig. 4A illustrated that temperature (X₁) and CO₂/OPEFB molar ratio (X₂) were positively correlated with the energy upgrade factor (Y₂). It is understandable that increasing temperature and CO₂/OPEFB molar ratio added more heat and carbon sources into the reactor, which contributed to the calorific value of the syngas output, thus enhancing the energy upgrade factor [50].

Fig. 4B illustrates the 3D response surface of the energy upgrade factor as a function of temperature and CO₂/OPEFB molar ratio at a constant OPEFB flowrate of 1.8 g/min. A linear relationship was observed between the operating variables and the energy upgrade factor. Further investigation of the interaction effects between the

temperature and CO₂/OPEFB molar ratio, and their impact on the energy upgrade factor, can be found in the Supplementary Material (section 5, Figure A.3(B)).

The highest energy upgrade factor of 1.4 was achieved at 1300 °C and a CO₂/OPEFB molar ratio of 3.0 (experiment #8), indicating that approximately 40 % of the generated calorific value originated from the storage of solar energy in the form of syngas [2]. The lowest energy upgrade factor of 1.0 was obtained at 1100 °C and a CO₂/OPEFB molar ratio of 1.6 (experiment #2). This low value could be due to the insufficient amount of heat source and/or CO₂ injected into the reactor cavity, thus resulting in low syngas yield. However, results still illustrated that the feedstock calorific value was fully recovered.

The observed trends indicated that the energy upgrade factor is directly proportional to the investigated operating variables. These findings agreed with the reported literature [50]. Moreover, the energy upgrade factor of syngas generated in this study was approximately 2-fold higher than those obtained from conventional autothermal gasification of OPEFB of 0.7 [66,67]. This improvement is likely due to the advantages of solar energy providing higher, more consistent temperatures (1100–1300 °C), promoting efficient carbon conversion and syngas production [2]. This leads to an enhanced yield of high-energy syngas components like CO and H₂. Unlike conventional autothermal gasification, which relies on biomass combustion for heat supply and can suffer from heat losses and incomplete reactions, solar-driven gasification allows for better temperature control and reaction kinetics, resulting in higher-quality syngas [2]. It is worth noting that 25–35 wt % of biomass is used for heating in conventional gasification [68].

As observed in Fig. 4B, no optimum condition was achieved in this study for the energy upgrade factor, which could be due to the maximum allowable processing conditions (temperature ≤1400 °C, and CO₂ flowrate ≤700 NmL/min). Therefore, the optimal conditions of the energy upgrade factor, illustrated by the CCD plateau curve, likely occur at higher operating conditions exceeding the upper limit of the solar gasification reactor. As a result, a quadratic function could not be formulated.

Although a maximum energy upgrade factor of 1.4 was experimentally reached, CCD predicts that it could exceed 1.5 at temperatures above 1400 °C and a CO₂/OPEFB molar ratio higher than 3.0. It is worth noting that the theoretical energy upgrade factor of Equation (1) is 1.43. Therefore, the CCD predicted values higher than 1.43 may be attributed to gasification reactions with the 11.3 % humidity in OPEFB (Table 1). Additionally, excess CO₂ can react with carbon to form 2CO (Boudouard reaction), and with H₂ to form CO and H₂O (reverse water gas shift reaction) [40]. However, it was not possible to validate the CCD model

<caption>**Table 3**</caption>

Comparison of target carbon sequestration methods (potential product, synthesis technology, and gas requirements) with experimentally achieved OPEFB solar-driven syngas composition (using steam [2] and CO₂) and upgrade gap analysis.

<table><thead><tr><th colspan="4">Target carbon sequestration methods</th><th colspan="2">Experimentally achieved OPEFB solar syngas composition and upgrade gap analysis</th></tr></thead><tbody><tr><td>Potential products</td><td>Ref.</td><td>Synthesis technology</td><td>Gas composition requirement</td><td>OPEFB solar-driven syngas composition</td><td>Upgrade gap analysis<sup>a</sup></td></tr><tr><td>Graphene/ graphite</td><td>[60]</td><td>CVD</td><td>H<sub>2</sub>/CH<sub>4</sub> = 49</td><td>H<sub>2</sub>/CH<sub>4</sub> = 24.8 (experiment #15 [2])</td><td>1) 100 % CO removal, 2) 98 % H<sub>2</sub> addition or 49 % CH<sub>4</sub> removal, and 3) C<sub>n</sub>H<sub>m</sub> trace pollutant removal</td></tr><tr><td>Carbon nanotubes</td><td>[63]</td><td>CVD</td><td>H<sub>2</sub>/CH<sub>4</sub> = 4.5</td><td>H<sub>2</sub>/CH<sub>4</sub> = 4.6 (experiment #2 [2])</td><td>1) 100 % CO removal, 2) 2.2 % H<sub>2</sub> removal or 2.2 % CH<sub>4</sub> addition, and 3) C<sub>n</sub>H<sub>m</sub> trace pollutant removal</td></tr><tr><td>Synthetic diamond</td><td>[64]</td><td>CVD</td><td>H<sub>2</sub>/CH<sub>4</sub> = 49</td><td>H<sub>2</sub>/CH<sub>4</sub> = 24.8 (experiment #15 [2])</td><td>1) 100 % CO removal, 2) 98 % H<sub>2</sub> addition or 49 % CH<sub>4</sub> removal, and 3) C<sub>n</sub>H<sub>m</sub> trace pollutant removal</td></tr><tr><td>Methanol</td><td>[19]</td><td>FT</td><td>H<sub>2</sub>/CO = 2</td><td>H<sub>2</sub>/CO = 0.5 (experiment #4 (this study))</td><td>1) 100 % CH<sub>4</sub> removal, 2) 300 % H<sub>2</sub> addition or 75 % CO removal, and 3) C<sub>n</sub>H<sub>m</sub> trace pollutant removal</td></tr><tr><td>Dimethyl ether</td><td>[19]</td><td>FT</td><td>H<sub>2</sub>/CO = 2</td><td>H<sub>2</sub>/CO = 0.5 (experiment #4 (this study))</td><td>1) 100 % CH<sub>4</sub> removal, 2) 300 % H<sub>2</sub> addition or 75 % CO removal, and 3) C<sub>n</sub>H<sub>m</sub> trace pollutant removal</td></tr><tr><td>Formaldehyde</td><td>[19]</td><td>FT</td><td>H<sub>2</sub>/CO = 2</td><td>H<sub>2</sub>/CO = 0.5 (experiment #4 (this study))</td><td>1) 100 % CH<sub>4</sub> removal, 2) 300 % H<sub>2</sub> addition or 75 % CO removal, and 3) C<sub>n</sub>H<sub>m</sub> trace pollutant removal</td></tr><tr><td>FT fuels</td><td>[19, 65]</td><td>FT</td><td>H<sub>2</sub>/CO ≥ 2</td><td>H<sub>2</sub>/CO = 0.5 (experiment #4 (this study))</td><td>1) 100 % CH<sub>4</sub> removal, 2) ≥300 % H<sub>2</sub> addition or ≥75 % CO removal, 3) C<sub>n</sub>H<sub>m</sub> trace pollutant removal</td></tr><tr><td>Ethanol</td><td>[19]</td><td>FT</td><td>H<sub>2</sub>/CO ≤ 1</td><td>H<sub>2</sub>/CO = 0.3–0.5 (all experiments of this study)</td><td>1) C<sub>n</sub>H<sub>m</sub> trace pollutant removal</td></tr></tbody></table>

<footnote><sup>a</sup> All percentages are expressed in mol %.</footnote>
A

PC1 and PC2: 96.0%

<figure><img src="image_6.png" /></figure>

B

<figure><img src="image_7.png" /><figcaption>**Fig. 4.** (A): PCA of scaled operating variables ($X_1$, $X_2$) vs. scaled energy upgrade factor ($Y_2$), (B) 3D response surface plot analysing the effects of the operating variables ($X_1$, $X_2$) on energy upgrade factor ($Y_2$) at constant OPEFB flowrate of 1.8 g/min ($X_1$ = temperature, $X_2$ = $CO_2$/OPEFB molar ratio).</figcaption></figure>

predicted value as the operating conditions were beyond the operational limits of the used solar reactor.

### 3.3. Analysis of the generated biochar

Biochar is produced as a by-product of the solar-driven gasification of OPEFB, although in small amounts. However, due to the large annual quantities of generated OPEFB of approximately 21.8 million tonnes generated per year [5-7], biochar-generated mass will become a residue of interest that requires proper treatment and/or direct utilisation methods. It is worth noting that biochar generation represents a long-term method of carbon sequestration [69]. Therefore, biochar produced from the solar-driven gasification, using steam [2] and $CO_2$ (this study), was characterised and a prospective application was proposed.

During gasification, the carbon content in biochar increases. The generated biochar from solar-driven gasification of OPEFB using steam and $CO_2$ as the gasifying agents generated a carbon content of up to 94.4 [2] and 95.7 dry wt. % (present research), respectively. Further details and data discussions are reported in the Supplementary Material (section 6).

The surface area ($S_{BET}$) of biochar characterises its reactivity and combustion behaviour [54]. Biochar produced at 1100 °C showed the lowest $S_{BET}$ due to its high ash content and poor amorphous structure [70]. The surface area and pore volume increased with the operating temperature reaching 53.4 m²/g and 0.15 cm³/g for solar-driven steam gasification and 60.9 m²/g and 0.19 cm³/g for solar-driven $CO_2$ gasification at 1300 °C. The small surface areas indicated that the generated biochar had low adsorptive capacities, likely due to the high operating temperatures used during solar-driven gasification. Therefore, further activation processes are needed to utilise the produced biochar as activated carbon with high adsorptive capacity. Biochar can have a low specific surface area at lower temperatures [71,72]. For example, biochar from de-alcoholised grape marc via entrained flow technology at 1200 °C had an $S_{BET}$ of 60 and 35 m²/g with air and steam as gasifying agents, respectively [71].

Metal content analysis was performed to determine the feasibility of using biochar as a nutrient source in soil applications. Most of the main elements (P, K, Na, Mg, Mn, Ca, Fe, S) are important plant nutrients in

agriculture [73]. The content of P, K, Na, and S in biochar was proportional to the operating temperature (1100-1300 °C). At 1300 °C, results showed that the P, K, and Na content in biochar reaching 1.1, 5.9, and 0.2 wt %, respectively, were 10-20-fold higher than in raw OPEFB. Moreover, the S content in biochar at 1300 °C of 0.2 wt % was around 2-fold higher than in raw OPEFB. However, the amount of remaining elements (Mg, Mn, Ca, and Fe) decreased in the biochar compared to the raw feedstock. This quantity decrease indicates that they were solubilised during the process of solar-driven gasification, which was visually observed as melted metals deposited as ash on the alumina particles at the bottom of the solar reactor cavity. Nonetheless, the concentrations of this group of elements were still in the acceptable range for soil fertilisers [73], which also agreed with other studies on the use of biochar pyrolysed at a temperature of 700 °C [55,74]. The most abundant metals in the biochar samples were P, K, Fe, and S, which are essential soil micronutrients for plants and human health [74].

Heavy metals such as Cd, Cr, Cu, Ni, and Pb were found to be extremely low or negligible in raw OPEFB and the biochar. At 1300 °C, Zn content of 0.2 wt % in the biochar was found to be 17-40-fold higher than in raw OPEFB using steam [2] and $CO_2$ (present study). Except for Pb, Cd, and Cr, the analysed heavy metals were considered micronutrients for plants [73]. Cd was below the detection limit (<0.01 wt %) in raw OPEFB and the biochar, whereas Cr was found in a very small amount, i.e., 0.01 wt %, in raw OPEFB, but less than the detection limit in all biochar samples. Even though essential and non-essential elements can be potentially harmful elements once they are found in high concentrations [73], the concentration ranges found in the biochar of our present study is comparable with other biochar characterisation analyses [73,74]. However, threshold concentrations for heavy metals are not determined yet and are required for biochar certification [73].

SEM analysis was used to study the morphological features of raw OPEFB and biochar at two magnifications, i.e., 125x and 500x (Figure A.5 in the Supplementary Material). A predominance of small pore sizes was shown in all samples. Both raw OPEFB and the produced biochar presented a compact structure. The fibrous texture shown in raw OPEFB was lost in the biochar, likely due to the applied high-temperature range of 1100-1300 °C during the solar-driven gasification. The loss of fibrous texture in the biochar is in line with the findings of $S_{BET}$ analysis.
Considering the biomass origin and high content of the main ele-
ments (P, K, Na, Mg, Mn, Ca, Fe, S), which are beneficial for soils and
plants [74], and low levels of heavy metals (Cd, Cr, Cu, Ni, Pb), biochar
is most suitable for use as a biofertiliser. However, if the intention is to
use them as an adsorbent, a biochar activation process is recommended,
as suggested in other studies using activating agents such as K₂CO₃ [54,
75].

4. Conclusion

Carbon capture through solar-driven gasification of OPEFB was achieved by utilising CO₂ as the gasifying agent to generate syngas. This process reached a net carbon consumption rate of 0.7 g/min and a carbon conversion efficiency of 94.9 %. The highest net carbon capture yield of 0.4 g C/g OPEFB was achieved at the highest temperature of 1300 °C and a CO₂/OPEFB molar ratio of 3.0. An effective syngas yield of 77.4 mmol/g OPEFB was achieved, representing 89.5 % of the maximum theoretical yield. The constant OPEFB flowrate of 1.8 g/min achieved stable and efficient experimental performance. The increase in temperature (1100–1300 °C) and CO₂/OPEFB molar ratio (1.6–3.0) at the OPEFB flowrate of 1.8 g/min showed a linear relationship with both H₂/CO molar ratio and energy upgrade factor. By using central composite design (CCD), the CO₂/OPEFB molar ratio, process temperature, and gasifying agent can be optimised, which is important for determining the H₂/CO molar ratio in the produced syngas.

A temperature of 1300 °C and CO₂/OPEFB molar ratio of 3.0 were found to be the optimum conditions, in this study, to achieve the lowest H₂/CO molar ratio of 0.3 (corresponding to H₂ and CO yields of 19.2 mmol/g OPEFB, and 58.5 mmol/g OPEFB, respectively) and the highest energy upgrade factor of 1.4. The favourable energy upgrade factor of 1.4 indicates that 40 % of the generated calorific value was efficiently stored from solar energy in the form of syngas. However, CCD predicted that energy upgrade factors exceeding 1.5 may be reached at temperatures above 1400 °C and a CO₂/OPEFB molar ratio higher than 3.0. However, these operating conditions are beyond the operational limits of the used solar reactor.

The produced biochar was characterised by a high carbon content of 94.4–95.7 wt %. The surface area and pore volume of the biochar reached 60.9 m²/g and 0.19 cm³/g, respectively, whereas the pore diameter ranged between 11.4 and 14.1 nm. Elemental composition analysis showed a high content of the main elements (P, K, Na, Mg, Mn, Ca, Fe, S) of interest for soil conditioning and fertilisation purposes. Heavy metal concentrations (Cd, Cr, Cu, Ni, Pb) were found to be extremely low or negligible.

The solar-driven gasification of OPEFB could help reduce around 4.8 million metric tons of GHG emissions per year in Malaysia. This process is largely independent of grid electricity and does not rely on critical raw materials required for PV or wind energy systems, positioning it as a viable and sustainable energy production technology. In the broader context, the solar-driven gasification of OPEFB has the potential to play a key role in the global energy transition by providing an eco-friendly solution for both carbon management and energy generation.

CRediT authorship contribution statement

**Saqr A.A. Al-Muraisy:** Writing – review & editing, Writing – original draft, Visualization, Validation, Software, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. **Srirat Chuayboon:** Writing – review & editing, Validation, Resources, Investigation. **Lais Americo Soares:** Software, Methodology. **J.G. Buijnsters:** Writing – review & editing, Validation, Conceptualization. **Shahrul bin Ismail:** Supervision, Resources. **Stéphane Abanades:** Writing – review & editing, Validation, Resources, Investigation, Funding acquisition, Data curation. **Jules B. van Lier:** Writing – review & editing, Supervision, Resources, Project administration, Funding acquisition. **Ralph E.F. Lindeboom:** Writing – review & editing, Supervision, Resources, Project

administration, Funding acquisition.

Funding

This work was supported by the TU Delft | Global Initiative (project No. 15DGF220), a program of the Delft University of Technology to boost science and technology for global development. Solar experiments were performed through an academic joint collaboration between TU Delft (the Netherlands) and PROMES-CNRS (France).

Collaboration: The visualisation of CCD results was executed with the support of Fauzul Imron, affiliated at Delft University of Technology, Faculty of Civil Engineering and Geosciences, Department of Water Management, Sanitary Engineering Section, 2628 CN, Delft, the Netherlands.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

**Appendix A. Supplementary data**

Supplementary data to this article can be found online at https://doi.org/10.1016/j.energy.2025.135805.

Data availability

Data will be made available on request.

**References**

[1] Anyaoha KE, Sakrabani R, Patchigolla K, Mouazen AM. Critical evaluation of oil palm fresh fruit bunch solid wastes as soil amendments: prospects and challenges. Resour Conserv Recycl 2018;136:399-409.

[2] Al-Muraisy SA, Soares LA, Chuayboon S, Ismail SB, Abanades S, van Lier JB, et al. Solar-driven steam gasification of oil palm empty fruit bunch to produce syngas: parametric optimization via central composite design. Fuel Process Technol 2022; 227:107118.

[3] Chuayboon S, Abanades S. Continuous solar-driven gasification of oil palm agricultural bio waste for high-quality syngas production. Waste Manag 2022;154: 303-11.

[4] Chew JJ, Soh M, Sunarso J, Yong S-T, Doshi V, Bhattacharya S. Isothermal kinetic study of CO₂ gasification of torrefied oil palm biomass. Biomass Bioenergy 2020; 134:105487.

[5] Supian MAF, Amin KNM, Jamari SS, Mohamad S. Production of cellulose nanofiber (CNF) from empty fruit bunch (EFB) via mechanical method. J Environ Chem Eng 2020;8(1):103024.

[6] Derman E, Abdulla R, Marbawi H, Sabullah MK. Oil palm empty fruit bunches as a promising feedstock for bioethanol production in Malaysia. Renew Energy 2018; 129:285-98.

[7] Cheah WY, Siti-Dina RP, Leng STK, Er A, Show PL. Circular bioeconomy in palm oil industry: current practices and future perspectives. Environ Technol Innovat 2023; 30:103050.

[8] Dolah R, Karnik R, Hamdan H. A comprehensive review on biofuels from oil palm empty bunch (EFB): current status, potential, barriers and way forward. Sustainability 2021;13(18):10210.

[9] Liew ZK, Chan YJ, Ho ZT, Yip YH, Teng MC, Chong S, et al. Biogas production enhancement by co-digestion of empty fruit bunch (EFB) with palm oil mill effluent (POME): performance and kinetic evaluation. Renew Energy 2021;179:766-77.

[10] Chang SH. An overview of empty fruit bunch from oil palm as feedstock for bio-oil production. Biomass Bioenergy 2014;62:174-81.

[11] Shahbaz M, AlNouss A, Ghiat I, Mckay G, Mackey H, Elkhalifa S, et al. A comprehensive review of biomass based thermochemical conversion technologies integrated with CO2 capture and utilisation within BECCS networks. Resour Conserv Recycl 2021;173:105734.

[12] Al-Ansari T, Korre A, Nie Z, Shah N. Integration of biomass gasification and CO2 capture in the LCA model for the energy, water and food nexus. Computer aided chemical engineering. Elsevier; 2016. p. 2085-90.

[13] Wang Y, Yang Y. Research on greenhouse gas emissions and economic assessment of biomass gasification power generation technology in China based on LCA method. Sustainability 2022;14(24):16729.

[14] Fernández-Lobato L, Aguado R, Jurado F, Vera D. Biomass gasification as a key technology to reduce the environmental impact of virgin olive oil production: a Life Cycle Assessment approach. Biomass Bioenergy 2022;165:106585.
[15] NASA. NASA Prediction of Worldwide Energy Resource (POWER). Climatology resource for SSE-renewable energy. 2020.

[16] Chan YH, Rahman SNFSA, Lahuri HM, Khalid A. Recent progress on CO-rich syngas production via CO2 gasification of various wastes: a critical review on efficiency, challenges and outlook. Environ Pollut 2021;278:116843.

[17] Dai S, Chang Z, Chang C, Akhatov JS, Li X. Numerical study on the directly-irradiated vortex reactor for solar CO2 coal gasification. Sol Energy 2019;188:573-85.

[18] Abdallah M, Ni D, Simson A. Evaluation of biochars derived from food waste for synthesis gas production via pyrolysis and CO2 gasification. Biomass Energy 2020;143:105883.

[19] Chae HJ, Kim J-H, Lee SC, Kim H-S, Jo SB, Ryu J-H, et al. Catalytic technologies for CO hydrogenation for the production of light hydrocarbons and middle distillates. Catalysts 2020;10(1):99.

[20] Li L, Yao Z, You S, Wang C-H, Chong C, Wang X. Optimal design of negative emission hybrid renewable energy systems with biochar production. Appl Energy 2019;243:233-49.

[21] Lee M, Lin Y-L, Chiueh P-T, Den W. Environmental and energy assessment of biomass residues to biochar as fuel: a brief review with recommendations for future bioenergy systems. J Clean Prod 2020;251:119714.

[22] Mosa A, Mansour MM, Soliman E, El-Ghamry A, El Alfy M, El Kenawy AM. Biochar as a soil amendment for restraining greenhouse gases emission and improving soil carbon sink: current situation and ways forward. Sustainability 2023;15(2):1206.

[23] You S, Ok YS, Chen SS, Tsang DC, Kwon EE, Lee J, et al. A critical review on sustainable biochar system through gasification: energy and environmental applications. Bioresour Technol 2017;246:242-53.

[24] Lahijani P, Zainal ZA, Mohamed AR, Mohammadi M. CO2 gasification reactivity of biomass char: catalytic influence of alkali, alkaline earth and transition metal salts. Bioresour Technol 2013;144:288-95.

[25] Shen Y, Li X, Yao Z, Cui X, Wang C-H. CO2 gasification of woody biomass: experimental study from a lab-scale reactor to a small-scale autothermal gasifier. Energy 2019;170:497-506.

[26] Singh P, Déparrois N, Burra K, Bhattacharya S, Gupta A. Energy recovery from cross-linked polyethylene wastes using pyrolysis and CO2 assisted gasification. Appl Energy 2019;254:113722.

[27] Kim YT, Seo DK, Hwang J. Study of the effect of coal type and particle size on char-CO2 gasification via gas analysis. Energy Fuel 2011;25(11):5044-54.

[28] Parvez AM, Mujtaba IM, Pang C, Lester E, Wu T. Effect of the addition of different waste carbonaceous materials on coal gasification in CO2 atmosphere. Fuel Process Technol 2016;149:231-8.

[29] Gokon N, Ono R, Hatamachi T, Liuyun L, Kim H-J, Kodama T. CO2 gasification of coal cokes using internally circulating fluidized bed reactor by concentrated Xe-light irradiation for solar gasification. Int J Hydrogen Energy 2012;37(17):12128-37.

[30] Bellouard Q, Abanades S, Rodat S. Biomass gasification in an innovative spouted-bed solar reactor: experimental proof of concept and parametric study. Energy Fuels 2017;31(10):10933-45.

[31] Raheem A, Ji G, Memon A, Sivasangar S, Wang W, Zhao M, et al. Catalytic gasification of algal biomass for hydrogen-rich gas production: parametric optimization via central composite design. Energy Convers Manag 2018;158:235-45.

[32] Fermoso J, Gil MV, Arias B, Plaza MG, Pevida C, Pis J, et al. Application of response surface methodology to assess the combined effect of operating variables on high-pressure coal gasification for H2-rich gas production. Int J Hydrogen Energy 2010;35(3):1191-204.

[33] University CS. Carbon credits 101 2025. https://warnercn source.colostate.edu/ast-k-the-experts-carbon-credits-101/

[34] BloombergNEF. Carbon credits. https://about.bnef.com/blog/carbon-credits-face-biggest-test-yet-could-reach-238-ton-in-2050-according-to-bloombergnef-port/?utm_source=chatgpt.com; 2025.

[35] IEA. The role of carbon credits in scaling up innovative clean energy technologies. https://www.iea.org/reports/the-role-of-carbon-credits-in-scaling-up-innovative-clean-energy-technologies; 2025.

[36] NREL. Concentrating solar power. https://atb.nrel.gov/electricity/2024/concentrating_solar_power; 2025.

[37] Carranzo IV. Standard Methods for examination of water and wastewater. Conference Standard Methods for examination of water and wastewater, vol. vol. 5. Universidad Complutense de Madrid, p. 185.

[38] Olisa Y, Kotingo K. Utilization of palm empty fruit bunch (PEFB) as solid fuel for steam boiler. European Journal of Engineering and Technology 2014;2(2).

[39] Farid MAA, Hassan MA, Roslan AM, Samsudin MH, Mohamad ZJJ, Othman MR, et al. Carbon monoxide reduction in the flue gas during biochar production from oil palm empty fruit bunch. J Clean Prod 2020;258:120580.

[40] Bellouard Q, Abanades S, Rodat S, Dupassieux N. Solar thermochemical gasification of wood biomass for syngas production in a high-temperature continuously-fed tubular reactor. Int J Hydrogen Energy 2017;42(19):13486-97.

[41] Chuayboon S, Abanades S, Rodat S. Insights into the influence of biomass feedstock type, particle size and feeding rate on thermochemical performances of a continuous solar gasification reactor. Renew Energy 2019;130:360-70.

[42] Bai Z, Hu W, Zhu X, Wang S, Gu Y. Solar-driven biomass steam gasification by new concept of solar particles heat carrier with CPFD simulation. Energy Convers Manag 2023;293:117500.

[43] Chuayboon S, Abanades S. Carbon-neutral synfuel production via continuous solar H2O and CO2 gasification of oil palm empty fruit bunch. Energy 2023;281:128212.

[44] Safarian S, Unnbörsson R, Richter C. A review of biomass gasification modelling. Renew Sustain Energy Rev 2019;110:378-91.

[45] Bellouard Q, Rodat S, Abanades S, Ravel S, Frayssines P-È. Design, simulation and experimental study of a directly-irradiated solar chemical reactor for hydrogen and syngas production from continuous solar-driven wood biomass gasification. Int J Hydrogen Energy 2019;44(35):19193-205.

[46] Inayat A, Inayat M, Shahbaz M, Sulaiman SA, Raza M, Yusup S. Parametric analysis and optimization for the catalytic air gasification of palm kernel shell using coal bottom ash as catalyst. Renew Energy 2020;145:671-81.

[47] Mansur FZ, Faizal CKM, Monir MU, Samad NAFA, Atnaw SM, Sulaiman SA. Co-gasification between coal/sawdust and coal/wood pellet: a parametric study using response surface methodology. Int J Hydrogen Energy 2019;44(31):16234-47.

[48] Raheem A, Zhao M, Dastyar W, Channa AQ, Ji G, Zhang Y. Parametric gasification process of sugarcane bagasse for syngas production. Int J Hydrogen Energy 2019;44(31):16234-47.

[49] Ait-Amir B, Pougnet P, El Hami A. Meta-model development, vol. 2. Elsevier; 2020. p. 157-87. Embedded mechatronic systems.

[50] Chuayboon S, Abanades S, Rodat S. Comprehensive performance assessment of a continuous solar-driven biomass gasifier. Fuel Process Technol 2018;182:1-14.

[51] Habchi S, Lahboubi N, Asbik M, El Bari H. Enhancing biomethane production from food waste using olive pomace hydrochar: an optimization study. Environmental Advances 2024;15:100477.

[52] Martínez MG, Floquet P, Dupont C, da Silva Perez D, Meyer X-M. Assessing the impact of woody and agricultural biomass variability on its behaviour in torrefaction through Principal Component Analysis. Biomass Energy 2020;134:105474.

[53] Xin X, Pang S, de Miguel Mercader F, Torr KM. The effect of biomass pretreatment on catalytic pyrolysis products of pine wood by Py-GC/MS and principal component analysis. J Anal Appl Pyrolysis 2019;138:145-53.

[54] Onorevoli B, da Silva Maciel GP, Machado ME, Corbelini V, Caramao EB, Jacques RA. Characterization of feedstock and biochar from energetic tobacco seed waste pyrolysis and potential application of biochar as an adsorbent. J Environ Chem Eng 2018;6(1):1279-87.

[55] Chen T, Zhang Y, Wang H, Lu W, Zhou Z, Zhang Y, et al. Influence of pyrolysis temperature on characteristics and heavy metal adsorptive performance of biochar derived from municipal sewage sludge. Bioresour Technol 2014;164:47-54.

[56] Numazawa Y, Hara Y, Matsukawa Y, Matsushita Y, Aoki H, Shishido T, et al. Kinetic modeling of CO2 and H2O gasification reactions for metallurgical coke using a distributed activation energy model. ACS Omega 2021;6(17):11436-46.

[57] Saadabadi SA, Thattai AT, Fan L, Lindeboom RE, Spanjers H, Aravind P. Solid oxide fuel cells fuelled with biogas: potential and constraints. Renew Energy 2019;134:194-214.

[58] Linde. Carbon monoxide: an industrial gas. 2020.

[59] Sadeghi M, Jafari M, Yari M, Mahmoudi S. Exergoeconomic assessment and optimization of a syngas production system with a desired H2/C0 ratio based on methane tri-reforming process. J CO2 Util 2018;25:283-301.

[60] Bashir A, Mehvish A, Khalil M. Advanced carbon materials for sustainable and emerging applications. 21st century advanced carbon materials for engineering applications A comprehensive handbook. 2021.

[61] dos Santos RG, Alencar AC. Biomass-derived syngas production via gasification process and its catalytic conversion into fuels by Fischer Tropsch synthesis: a review. Int J Hydrogen Energy 2020;45(36):18114-32.

[62] Newman AJ, Dowson GR, Platt EG, Handford-Styring HJ, Styring P. Custodians of carbon: creating a circular carbon economy. Front Energy Res 2023;11:1124072.

[63] Coquay P, Peigney A, De Grave E, Vandenberghie RE, Laurent C. Carbon nanotubes by a CVD method. Part II: formation of nanotubes from (Mg, Fe) O catalysts. J Phys Chem B 2002;106(51):13199-210.

[64] Sartori AF, Overes BH, Fanzio P, Tsigkourakos M, Sasso L, Buijsters JG. Template-assisted bottom-up growth of nanocrystalline diamond micropillar arrays. Diam Relat Mater 2019;95:20-7.

[65] Placido A, Challman D, Liu K, Andrews R, Jacobs G, Davis B, et al. Small-scale pilot plant for gasification of coal and coal/biomass blends and conversion of derived syngas to liquid fuels via Fischer-Tropsch synthesis. Univ. Of Kentucky, Lexington, KY (United States). Reasearch Foundation; 2018. https://doi.org/10.2172/1458389.

[66] Lahijani P, Zainal ZA. Gasification of palm empty fruit bunch in a bubbling fluidized bed: a performance and agglomeration study. Bioresour Technol 2011;102(2):2068-76.

[67] Monir MU, Abd Aziz A, Kristanti RA, Yousuf A. Co-gasification of empty fruit bunch in a downdraft reactor: a pilot scale approach. Bioresour Technol Rep 2018;1:39-49.

[68] Basu P. Biomass gasification, pyrolysis and torrefaction: practical design and theory. Academic press; 2018.

[69] Dziejarski B, Krzyżyńska R, Andersson K. Current status of carbon capture, utilization, and storage technologies in the global economy: a survey of technical assessment. Fuel 2023;342:127776.

[70] Benedetti V, Patuzzi F, Baratieri M. Characterization of char from biomass gasification and its similarities with activated carbon in adsorption applications. Appl Energy 2018;227:92-9.

[71] Hernández JJ, Lapuerta M, Monedero E. Characterisation of residual char from biomass gasification: effect of the gasifier operating conditions. J Clean Prod 2016;138:83-93.

[72] Li H, Dong X, da Silva EB, de Oliveira LM, Chen Y, Ma LQ. Mechanisms of metal sorption by biochars: biochar characteristics and modifications. Chemosphere 2017;178:466-78.
[73] Bachmann HJr, Bucheli TD, Dieguez-Alonso A, Fabbri D, Knicker H, Schmidt H-P, et al. Toward the standardization of biochar analysis: the COST action TD1107 interlaboratory comparison. J Agric Food Chem 2016;64(2):513-27.

[74] Onorevoli B, da Silva Maciel GP, Machado ME, Corbelini V, Caramão EB, Jacques RA. Characterization of feedstock and biochar from energetic tobacco seed waste pyrolysis and potential application of biochar as an adsorbent. J Environ Chem Eng 2018;6(1):1279-87.

[75] Maciel GPdS, Lazzari E, Bjerk TR, Ahmad SM, Carvalho APBd, Nogueira JMF, et al. Trace analysis of carbazole in commercial diesel by using adsorption on activated biochar from rice husk pyrolysis. International Journal of Engineering Research & Science (IJOER)[recurso eletrónico] Bikaner, India Aug 2017;3(8):46-57. 2017.