# Impact of different agricultural biomass residues on the performance of continuous solar-steam gasification

Srirat Chuayboon, Stéphane Abanades

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Srirat Chuayboon, Stéphane Abanades. Impact of different agricultural biomass residues on the performance of continuous solar-steam gasification. Industrial Crops and Products, 2025, 236, pp.122048. 10.1016/j.indcrop.2025.122048. hal-05307960

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# Impact of different agricultural biomass residues on the performance of continuous solar-steam gasification

Srirat Chuayboon<sup>a, D</sup>, Stéphane Abanades<sup>b, *</sup>

## ARTICLE INFO

**Keywords:**

Renewable energy

Concentrated solar power

Solar reactor

Thermochemical gasification

Waste biomass valorization

Crop residue

Biofuel

## ABSTRACT

Solar-driven biomass gasification represents a promising avenue for sustainable carbon-neutral fuel production. Nevertheless, the types of raw biomass materials play a vital role in continuous solar gasification performance. In this study, continuous solar-steam gasification with various agricultural crop residues was experimentally carried out in a 1.5 kW<sub>th</sub> solar gasifier to investigate the influence of biomass types on performance and efficiency under different operating temperatures up to 1400 °C. Seven agricultural residues were used as feedstocks, including oil palm wastes (palm mesocarp fiber, palm empty fruit bunch, and palm kernel shell), bagasse, betel nut, coconut fiber, and rice husks. Results demonstrated that the system can effectively perform with all biomass types with high-quantity and high-quality syngas production. The process revealed exceptional performance and efficiency, including the total maximum syngas yield range of 67.9–81.5 mmol/g<sub>dry biomass</sub>, reaching 81.4–95.2 % of the theoretical total syngas yields, maximum energy upgrade factor (1.05–1.35), reaching 94.2–97.3 % of the theoretical values, and maximum carbon conversion (87.2–96.9 %). The feedstock types showed a significant influence on gasification outcomes. Palm oil empty fruit bunch, betel nut, and palm mesocarp fiber were promising candidates for solar gasification with their high volatile content and significant decomposition potential, followed by coconut fiber and bagasse. Nevertheless, palm kernel shell and rice husk were found to be unsuitable biomasses for continuous solar-steam gasification because of the issues of reduced gasification activity and reaction rate limitations, due to a high density for palm kernel shell and a high ash content for rice husk. A temperature of 1300 °C was recommended to carry out continuous solar-steam gasification, leading to the maximum solar-to-fuel energy conversion efficiency in the range 13.7–18.9 %. This study provides insights into the influence of agricultural residue types on the solar-steam gasification process, while assisting in the proper biomass residue selection for efficient continuous solar gasification.

## 1. Introduction

Fossil fuel consumption and CO₂ emissions have been rising in many parts of the world, particularly in developing countries where energy demand is overgrowing (Prasertsan and Sajjakulnukit, 2006). One of the solutions for reducing the country's dependency on imported fossil energy has been to use crops in energy production (Wang et al., 2021). Thailand holds a variety of waste biomass sources with substantial potential for energy production and environmental sustainability (Jusakulvijit et al., 2021). Among these resources, agricultural waste biomass emerges as a promising candidate. Rice husks, sugar cane waste (bagasse), oil palm wastes (such as palm fiber, palm kernel shell, palm empty fruit bunch), coconut waste (coconut fiber), betel waste, and rice

waste (rice husk waste) become key players by increasing their share in biofuel production as substitutes for imported crude oil (Basha et al., 2020; Jusakulvijit et al., 2021; Prasertsan and Prasertsan, 1996). The Ministry of Energy in Thailand has set a substantial goal in the form of a 20-year development plan, namely the Alternative Energy Development Plan (AEDP), to promote energy production within the full potential of domestic renewable energy resources (Misila et al., 2020).

Gasification is one of the well-known processes in valorizing waste and biomass (Molino et al., 2016; Taqvi et al., 2024). The concept of partial combustion of biomass in an autothermal gasifier is generally used to produce synthesis gas (syngas) (Nipattummakul et al., 2011). Syngas primarily consists of hydrogen (H₂) and carbon monoxide (CO). It can be used as a versatile intermediate for production of hydrogen (Shen, 2024), ammonia (Dziva et al., 2024), methanol (Rajaee et al.,

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

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

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: stephane.abanades@promes.cnrs.fr (S. Abanades).</footnote>
<table><thead><tr><th>Nomenclature</th><th>η<sub>solar-to-fuel</sub></th><th>Solar-to-fuel energy conversion efficiency (%)</th></tr></thead><tbody><tr><td>T</td><td>Temperature (°C)</td><td></td></tr><tr><td>LHV<sub>syngas</sub></td><td>Lower heating value of syngas (J/kg)</td><td></td></tr><tr><td>m</td><td>Mass (kg)</td><td></td></tr><tr><td>Yield<sub>ex</sub></td><td>Experimental syngas yield (mmol/g<sub>dry biomass</sub>)</td><td></td></tr><tr><td>Yield<sub>th</sub></td><td>Theoretical syngas yield (mmol/g<sub>dry biomass</sub>)</td><td></td></tr><tr><td>F<sub>i</sub></td><td>molar flow-rate of species i (mol/s)</td><td></td></tr><tr><td>Q<sub>solar</sub></td><td>Solar power input (W)</td><td></td></tr><tr><td>m<sub>x</sub></td><td>Global mass balance (-)</td><td></td></tr><tr><td>X<sub>c</sub></td><td>Carbon conversion (%)</td><td></td></tr><tr><td>U<sub>ex</sub></td><td>Experimental energy upgrade factor (-)</td><td></td></tr><tr><td>U<sub>th</sub></td><td>Theoretical energy upgrade factor (-)</td><td></td></tr></tbody></table><table><thead><tr><th colspan="2">Abbreviations</th></tr></thead><tbody><tr><td>DNI</td><td>Direct Normal Irradiance</td></tr><tr><td>CSP</td><td>Concentrated solar power</td></tr><tr><td>NL</td><td>Normal liter (at 0°C, 1 atm)</td></tr><tr><td>PKS</td><td>Palm Kernel Shell</td></tr><tr><td>BN</td><td>Betel Nut</td></tr><tr><td>BG</td><td>Bagasse</td></tr><tr><td>PEFB</td><td>Palm Empty Fruit Bunch</td></tr><tr><td>CCF</td><td>Coconut Fiber</td></tr><tr><td>RH</td><td>Rice Husk</td></tr><tr><td>PMF</td><td>Palm Mesocarp Fiber</td></tr></tbody></table>

2024), or a precursor for Fischer-Tropsch synthesis (producing synthetic fuels like diesel and gasoline) (Rogachuk and Okolie, 2024), and electricity generation (Li et al., 2024). In the process, carbonaceous solid materials are reacted with a restricted amount of air/steam/O₂/CO₂ to get desired gaseous products with usable heating value (Xu et al., 2024). The partial combustion in the autothermal gasification process becomes a significant disadvantage of this technology since 30–40 % of biomass feedstock is lost from combustion (Zhu et al., 2024), and producer gases are not clean as containing N₂ from air or CO₂ from combustion (Chen et al., 2024; Yang et al., 2024).

Solar gasification is proposed to overcome limitations of traditional gasification (Gregg et al., 1980). It refers to using solar energy as an external heat source to drive the gasification process without combustion (Chuayboon and Abanades, 2020; Piatkowski et al., 2011). Therefore, biomass can be entirely converted to syngas, and no contamination by N₂ from air or CO₂ from combustion occurs (Taylor et al., 1983). This technology outstandingly allows the integration of two renewable energies (solar and biomass) in a single process, offering an innovative pathway toward achieving sustainable and green biofuel production (Boujjar et al., 2019b, 2020; Chuayboon and Abanades, 2023). Solar energy is stored into a high-quality syngas and can easily be transported as a convertible and dispatchable chemical form. The cleaner syngas from solar steam gasification is advantageous for applications in chemical synthesis (e.g., liquid biofuel), which require high purity in H₂ and CO. However, solar gasification technology comes at the expense of the dependence of solar irradiation on intermittence and flux stability, which can be alleviated by using auxiliary heating such as hybrid solar gasification combining autothermal and allothermal gasification (Boujjar et al., 2019a; Rodat et al., 2020). In addition, dynamic control of solar power input and biomass feed rate are essential for advanced real-time control systems adjusting biomass feed rate and steam injection under variable solar irradiance (Curcio et al., 2022, 2025).

The raw agricultural bio-residue type plays a crucial role in gasification performance (Ferreiro et al., 2024). It significantly impacts the gasification reaction in terms of gasification activity (Li et al., 2019), reaction rate (Mohammed et al., 2012), syngas composition (Ajorloo et al., 2025), and char characteristics (Strandberg et al., 2017), highlighting the importance of selecting appropriate agricultural residues for efficient gasification processes. Biomasses rich in cellulose, like pine sawdust, produce more tar and less hydrogen than lignin-rich biomasses, such as used ground coffee and wheat straw (Ajorloo et al., 2025). Specific elements and compounds in the biomass affect its reactivity during gasification (Fürsatz et al., 2021). For example, the ash composition of the biomass, particularly the content of alkali metals (such as potassium, sodium, calcium, and magnesium), can act as either catalysts or inhibitors, which enhance or downgrade the gasification process (Li et al., 2019; Strandberg et al., 2017; Zou et al., 2018). NiO/CaO catalysts facilitate the cracking of tar components generated during the gasification of palm kernel shells (Dechapanya et al., 2020). The type of

biomass also influences the composition of the produced syngas. For example, the H₂/CO ratio in the syngas varied between 2 and 4 depending on the different biomass types considered regarding rice husk, rice straw, maize straw, cotton straw, wood chip, coconut fiber, and chicken manure (Li et al., 2019).

Furthermore, a higher volatile matter content leads to a higher yield of combustible gases, while fixed carbon contributes to char formation during pyrolysis/gasification process. For instance, palm oil empty fruit bunch has a high content of volatiles, which significantly increases its reactivity during gasification (Mohammed et al., 2012). Moreover, smaller waste biomass particle sizes resulted in higher gas yields due to a larger surface area, leading to faster heating rates and more efficient gasification reactions (Li et al., 2009). Conversely, larger particles show slower heating rates and less efficient gasification. Smaller particle sizes enhance heat and mass transfer, improving syngas yields; however, the type of biomass (i.e., composition) has a more pronounced effect on the overall gasification performance than the particle size alone (Chuayboon et al., 2018a). In addition, an economic analysis related to solar-driven steam gasification of biomass was conducted, and the hydrogen production costs for different gasification processes (solar-only, hybrid, and autothermal) under various economic scenarios were assessed. Key factors influencing cost competitiveness were identified, including heliostat and solar tower costs, as well as biomass feedstock prices (Boujjar et al., 2021). The biomass price was varied in the 0–1 €/kg interval to cover a large range of biomass types (such as food waste, agricultural and crop waste, and solid recovered fuels) and also a potential increase in the resource price (for instance, due to the increasing pressure on the resource or to extra-cost associated with additional feedstock pre-treatment). An increase in the feedstock price favored the solar process economic viability over the conventional autothermal process, because of a more efficient conversion of the biomass feedstock, thereby reducing fuel production costs.

In summary, gasification reaction and performance significantly depend on the type of biomass residues. Notably, the above studies have been mostly carried out with the conventional gasification process in a lab-scale reactor driven by partial combustion, electric heating, or artificial light simulators. Besides, a few research was carried out on solar gasification using coal (Piatkowski and Steinfeld, 2008) or beech wood biomass (Chuayboon et al., 2019). For this reason, the effect of the type of raw agricultural residues on continuous solar gasification must be experimentally investigated in a prototype solar gasifier, entirely driven by natural solar radiation energy.

In this study, the continuous solar-steam gasification of different agricultural residues was experimentally investigated to gain understanding on the impact of the type of raw agricultural residues on solar gasification reaction, performance, and efficiency. Seven agricultural residues were employed as feedstocks, including oil palm wastes (palm mesocarp fiber (PMF), palm empty fruit bunch (PEFB), palm kernel shell (PKS)), bagasse (BG), betel nut (BN), coconut fiber (CCF), and rice husks
(RH), due to their high availability and favorable characteristics according to their different physical and chemical properties (Dechapanya et al., 2020; Jusakulvijit et al., 2021; Wang et al., 2021). A comprehensive feedstock screening was thus proposed, providing a detailed comparison of their performance. The main objective was to demonstrate that the reactor can accommodate to various feedstocks types, thus showing solar process flexibility and reliability for the conversion of wastes to valuable fuels.

The global stoichiometric reaction for solar steam gasification of such agricultural residues is given according to Eq. 1:

$$C_x H_y O_z + (x-z) H_2 O \rightarrow \left(\frac{\gamma}{2} + x - z\right) H_2 + xCO \quad (1)$$

The high-temperature solar gasification of biomass wastes from various agricultural residues for renewable syngas production was considered. The steam gasification reaction was entirely driven by highly concentrated sunlight to demonstrate the feasibility of the reactor operation under real solar heating conditions. The process thus integrates solar energy utilization for the conversion and upgrading of waste feedstocks to high-value solar fuels. High-temperature concentrated solar heat is used to provide the reaction enthalpy, and therefore, bio-waste feedstock is entirely dedicated producing syngas. Although numerous studies have investigated the gasification of biomass residues, very few have specifically focused on the use of concentrated solar energy as the process heat source, enabling bioresources saving and emissions mitigation. This represents a notable research gap and an original feature, particularly in the context of developing sustainable and carbon-neutral processes for solar fuels production. This study focuses on the experimental study of solar-aided bio-waste gasification in a high-temperature solar reactor, with emphasis on the effect of the biomass waste type, composition, and temperature on syngas yields and reactor efficiencies. It unravels the impact of different types of agricultural biomass residues on the performance of continuous solar steam gasification. All the feedstocks were shaped as pressed pellets for their continuous injection, as opposed to particles in prior work (Chuayboon and Abanades, 2022, 2024; Chuayboon et al., 2019). The high-temperature solar reactor was successfully operated continuously with the different biomass wastes to determine and compare relevant performance results. Key performance metrics encompassing syngas production rate, syngas yield, syngas composition, lower heating value, carbon conversion, energy upgrade factor, and solar-to-fuel energy

conversion efficiency were systematically analyzed under different temperatures ranging from 1100 °C to 1400 °C, showcasing the high performance of solar concentrating technology and reactor facilities. Experiments also showed that the solar reactor can accommodate to various waste feedstocks types, shapes, and properties, confirming process flexibility and robustness. This work further provided guidelines in selecting appropriate agricultural residues for continuous solar-steam gasification to produce sustainable and green biofuel. Agricultural bio-wastes from different resources were proved to be compatible with continuous solar gasification for their ultimate conversion to clean and dispatchable synthetic fuels.

## 2. Experimental setup and methods

### 2.1. Agricultural residues

Seven agricultural residues, including oil palm wastes (palm mesocarp fiber (PMF), palm empty fruit bunch (PEFB), and palm kernel shell (PKS)), bagasse (BG), betel nut (BN), coconut fiber (CCF), and rice husks (RH), were used as solid reactants for conducting solar-steam gasification reactions. Some were obtained from the Asia Biomass Public Company Limited, Thailand, and others were obtained from oil palm industries in Chumphon province, Thailand, with different size distributions. Table 1 shows the physical and chemical properties of such agricultural residues, and a difference in these properties was observed. The highest carbon content was found in PKS (52.25 %), followed by BN (54.97 %), PEFB (48.36 %), BG (47.92 %), PMF (47.31 %), CCF (46.56 %), and RH (42.09 %). Moreover, PEFB has the highest volatile matter content at 83.68 %, making it a promising candidate for solar gasification. PMF and BN also exhibit high volatile content (78.21 % and 73.78 %, respectively). PKS has the highest fixed carbon content at 24.60 %. The highest ash contents were found in RH (16.46 %), followed by CCF (13.09 %). The other feedstocks featured ash content between 1.01 % and 5.06 %. Physically, the palm shell is strong, exhibiting high carbon content and hardness. Therefore, the effect of biomass type on their physical and chemical properties can be experimentally investigated through solar gasification.

Before experiments, the agricultural residues of PEFB, PMF, BG, and BN were cut into small particles using a grinder machine. PKS was crushed into small pieces with a mortar, and CCF was cut with scissors. They were then screened with a sieving method for size reduction and

<table><caption>**Table 1**

Physical and chemical properties of agricultural residues.</caption><thead><tr><th rowspan="2">No</th><th rowspan="2">Biomass specie</th><th rowspan="2">Lower heating value (MJ/kg)</th><th rowspan="2">particle size distribution (mm)</th><th rowspan="2">Apparent pellet density (g/cm³)</th><th colspan="4">Proximate analysis (wt%)*</th><th colspan="5">Ultimate analysis** (wt%) as dry basis</th></tr><tr><th>Moisture (arb)</th><th>Ash (adb)</th><th>Volatiles matter (adb)</th><th>Fixed carbon (adb)</th><th>C</th><th>H</th><th>O</th><th>S</th><th>N</th></tr></thead><tbody><tr><td>1</td><td>Palm kernel shell, PKS (C<sub>9.7</sub>H<sub>12.3</sub>O<sub>5.2</sub>)</td><td>19.09</td><td>1–5</td><td>0.95</td><td>13</td><td>4.33</td><td>71.07</td><td>24.60</td><td>52.25</td><td>5.54</td><td>37.38</td><td>0.03</td><td>0.46</td></tr><tr><td>2</td><td>Betel nut, BN (C<sub>8.9</sub>H<sub>12.9</sub>O<sub>5.2</sub>)</td><td>20.10</td><td>≤ 1</td><td>0.87</td><td>9.03</td><td>2.68</td><td>73.78</td><td>20.02</td><td>54.97</td><td>6.72</td><td>42.88</td><td>0.25</td><td>1.13</td></tr><tr><td>3</td><td>Palm empty fruit bunch, PEFB (C<sub>7.1</sub>H<sub>11.6</sub>O<sub>4.8</sub>)</td><td>16.20</td><td>1–3</td><td>0.86</td><td>11.31</td><td>1.01</td><td>83.68</td><td>10.97</td><td>48.36</td><td>6.67</td><td>44.24</td><td>0.13</td><td>0.60</td></tr><tr><td>4</td><td>Bagasse, BG (C<sub>6.2</sub>H<sub>9.6</sub>O<sub>4.5</sub>)</td><td>15.71</td><td>1–15</td><td>0.67</td><td>9.21</td><td>5.06</td><td>73.53</td><td>19.75</td><td>47.92</td><td>6.18</td><td>46.20</td><td>0.05</td><td>0.37</td></tr><tr><td>5</td><td>Palm Mesocarp fiber, PMF (C<sub>6.0</sub>H<sub>9.8</sub>O<sub>4.5</sub>)</td><td>15.68</td><td>-</td><td>0.69</td><td>8.98</td><td>3.69</td><td>78.21</td><td>17.37</td><td>47.31</td><td>6.53</td><td>47.65</td><td>0.04</td><td>0.48</td></tr><tr><td>6</td><td>Coconut fiber, CCF (C<sub>6.5</sub>H<sub>9.2</sub>O<sub>4.5</sub>)</td><td>15.05</td><td>1–5</td><td>0.74</td><td>12.17</td><td>13.09</td><td>62.18</td><td>24.54</td><td>46.56</td><td>5.50</td><td>42.65</td><td>0.05</td><td>0.36</td></tr><tr><td>7</td><td>Rice husk, RH (C<sub>6.5</sub>H<sub>10.5</sub>O<sub>4.6</sub>)</td><td>14.23</td><td>1–6</td><td>0.69</td><td>9.06</td><td>16.46</td><td>66.29</td><td>16.70</td><td>42.09</td><td>5.71</td><td>40.12</td><td>0.04</td><td>0.61</td></tr></tbody></table>

arb = as received basis, adb = as dry basis

*Method: In-house method based on ASTM D7582, **Method: CHNS/O Analyzer
biomass particle size in a range of 1–2 mm was utilized. In addition, the moisture contents of biomass were reduced by sun drying methods, in which biomass was spread in thin layers to expose it to sunlight, and the moisture contents were monitored by using a moisture meter. Subsequently, they were made in pellets (with 10 mm diameter and 10–15 mm height) using a pellet-making machine consisting of a hydraulic press at a pressure of ~80 kg/cm² for 3 min/pellet (biomass mass range: 0.42–1.05 g/pellet and volume: 2.51–4.39 cm³/pellet, resulting in pellet density: 0.67–0.95 g/cm³).

The goal of preparing biomass shaped in pellets was to inject different biomass types and efficiently handle the biomass into the reactor cavity receiver with continuous injection (each run consisted in injecting 5–7 biomass pellets). The photographs of agricultural residue pellets are shown in Fig. 1, and for each type, the mass and density are different.

## 2.2. Experimental setup

Fig. 2 displays the experimental setup of continuous pellet-fed solar-steam gasification of agricultural waste biomass. As shown in Fig. 2a, the system comprises a continuous solar reactor, an automatic sun-tracking heliostat, and a 2-meter diameter parabolic solar concentrator. A heliostat is a system of mirrors that precisely tracks the sun and reflects sunlight onto the parabolic concentrator. The solar concentrator can provide heat, with up to 1.5 kWth of thermal power at its focal point, achieving a solar flux density of up to 12,000 kW/m² for a direct normal irradiation (DNI) of 1 kW/m², thereby reaching the high temperatures (above 1300°C) required for efficient steam gasification. The concentrated solar radiation enters the reactor cavity after passing through a transparent glass window in the downward direction. This window allows concentrated sunlight to be absorbed by the reactor cavity via the upper aperture, while minimizing radiation heat losses and maintaining a sealed environment. The continuous reactor is symmetrically cylindrical and directly irradiated. It mainly consists of an alumina cavity

<figure><img src="image_4.png" /><figcaption>Fig. 1. Photograph of agricultural residue pellets: (a) palm kernel shell (PKS), (b) betel nut (BN), (c) bagasse (BG), (d) palm empty fruit bunch (PEFB), (e) coconut fiber (CCF), (f) rice husk (RH), and (g) palm mesocarp fiber (PMF).</figcaption></figure>

receiver (121.4 cm³ volume), a water-cooled reactor shell, a hemispherical transparent window, and a pellet biomass-feeding delivery system. More details of the solar reactor can be found in previous works (Chuayboon and Abanades, 2022, 2024). The reactor temperature ($T_1$) is measured inside the cavity receiver using B-type thermocouples and compared with that measured by a pyrometer ($T_{pyrometer}$). The reactor pressure is monitored with a pressure transducer ($P_1$). Gas ($N_2$) and liquid ($H_2O$) flow rates are controlled by thermal mass flow controllers (MFC, Brooks Instruments model SLA5850S, range 0–2 NL/min ±0.2 % of full scale).

The biomass feeding system was designed for the continuous injection of pressed pellets, as opposed to particles in previous works (Chuayboon and Abanades, 2022, 2024; Chuayboon et al., 2019). Prior to the experiment, biomass pellets were first loaded into the alumina feeding tube. Subsequently, the feeding tube equipped with biomass inside was installed for direct pellet injection in the reactor cavity receiver. A protective $N_2$ flow of 0.2 NL/min was introduced through the biomass feeding tube. The solar reactor was sucked by a venturi pump and concomitantly flushed by a $N_2$ flow to purge the residual air from the system. As illustrated in Fig. 2b, the solar reactor was then heated by highly concentrated solar power under $N_2$ flow. An automatic shutter was positioned between the heliostat and the parabolic dish to control the solar power input. A protective $N_2$ flow of 1.5 NL/min was injected through two stainless steel tubes toward the transparent window, protecting the window from pyrolytic gases depositing on the surface.

Once the targeted operating temperature was reached, steam was introduced into the cavity receiver from the bottom along with an $N_2$ carrier gas flow of 0.2 NL/min. Biomass feedstock pellets (total weights of 0.97–5.66 g per run depending on biomass type), consisting of 5–7 pellets, were continuously fed one by one with a manual push rod through the inlet reactant port. The steam/biomass molar ratio was controlled at a fixed value slightly over the stoichiometric steam/biomass molar ratio (10 % excess of steam) to promote biomass-to-syngas conversion in gasification reaction (Chuayboon et al., 2018a). Note that the stoichiometric steam/biomass molar ratio (Eq. 1) was 4.5 for PKS, followed by 2.4 for BN, 2.2 for PEFB, 2.0 for CCF, 1.9 for RH, 1.7 for BG, and 1.5 for PMF. The biomass gasification with steam was performed continuously during the feeding. According to thermodynamics of steam gasification (Chuayboon and Abanades, 2023), a temperature in the range 1100–1400 °C was found to promote syngas yield and quality.

The produced syngas exited through the gas outlet, and flowed through a cleaning unit equipped with a water scrubber and microfilter (0.1 µm pore diameter), which removed entrained char particles and unconverted water prior to gas analysis. The biomass was entirely gasified in the reactor while only the ash fraction remained in the reactor cavity receiver and was removed after the experiments. The syngas composition was continuously monitored by an online gas analyzer (GAS 3100 P), and an automatic data collector recorded all data every 1 s.

The global mass balance ($m_x$) is defined as the ratio of the total output (syngas, solid products, and unconverted water) to the total input (agricultural residues and steam). This balance indicates the principle of mass conservation, as illustrated in Eq. 2.

$$m_x = \frac{m_{\text{syngas}} + m_{\text{solid products+water}}}{m_{\text{biomass}} + m_{\text{steam}}} \quad (2)$$

where $m_i$ represents the mass of species i (kg).

Carbon conversion ($X_c$) is the ratio of the carbon content in the syngas to the carbon content in the biomass feedstock, as presented in Eq. 3.
<figure><img src="image_5.png" /><figcaption>Fig. 2. (a) Schematic diagram of the solar reactor set up with solar concentrating facilities and (b) photograph of on-sun continuous steam gasification of agricultural residues.</figcaption></figure>

$$X_c = \frac{\int_{0}^{t} F_{CO}(t)dt + \int_{0}^{t} F_{CO_2}(t)dt + \int_{0}^{t} F_{CH_4}(t)dt + \int_{0}^{t} 2F_{C_2H_m}(t)dt}{\int_{0}^{t} xF_{C_xH_yO_z}(t)dt} \quad (3)$$

where $F_i$ represents the molar flow rate of species i (mol/s).

The energy upgrade factor ($U_{ex}$) is defined as the ratio of the syngas energy content to the biomass feedstock energy content, which represents the fraction of solar energy stored in the syngas, according to Eq. 4.

$$U_{ex} = \frac{(LHV_{syngas} \bullet m_{syngas})}{(LHV_{feedstock} \bullet m_{feedstock})} \quad (4)$$

Where $LHV_{syngas}$ and $LHV_{feedstock}$ are the lower heating values of syngas products and biomass feedstock (J/kg), $m_{syngas}$ and $m_{feedstock}$ are the mass of syngas and feedstock (kg), respectively.

The solar-to-fuel energy conversion efficiency ($\eta_{solar-to-fuel}$) is defined as the ratio of the energy content of the syngas to the total energy input including the solar energy input and the energy content of the agricultural residues, as described in Eq. 5, which quantifies how well solar energy is utilized for the process:

$$\eta_{solar-to-fuel} = \frac{(LHV_{syngas} \bullet m_{syngas})}{Q_{solar} + (LHV_{feedstock} \bullet m_{feedstock})} \quad (5)$$

where $Q_{solar}$ is the solar energy input (J).

## 3. Results and discussion

### 3.1. Overview of performance assessment

Table 2 summarizes the operating parameters and experimental results of solar gasification of seven agricultural residues representing 28 on-sun runs. The process was performed under the following parameters: initial agricultural residue mass range ($m_{biomass}$): 0.97–5.6 g, steam feeding rate ($\dot{m}_{H_2O}$): 253–500 mg/min, reaction temperature: 1100–1400 °C, solar power input ($\dot{Q}$): 0.79–1.29 kWth, operating pressure: 0.88–0.90 bar (slightly above atmospheric pressure of 0.85 bar at the lab site), and nitrogen flow rate: 1.9 NL/min. Using the known total inert flow rate of $N_2$ and the measured species mole fractions ($y_i$), the gas production rate of each species ($F_i$) was determined by: $F_i = F_{N_2} \bullet$

$y_i/y_{N_2}$ (expressed in NL/min). The syngas yield (producer gas) was then calculated by integrating the gas production rates over the entire operating time. The syngas yield is presented in the unit of mmol/gdry biomass (millimole per gram of dry biomass). Within seven biomass types, syngas yields were obtained in the range as follows: $H_2$ (28.0–42.3 mmol/gdry biomass), CO (16.6–32.3 mmol/gdry biomass), $CO_2$ (3.6–7.4 mmol/gdry biomass), $CH_4$ (1.8–6.0 mmol/gdry biomass), and $C_2H_m$ (0.0–1.1 mmol/gdry biomass), resulting in the total syngas yield (Yield$_{ex}$) range of 56.9–81.5 mmol/gdry biomass compared to the theoretical total syngas yield (Yield$_{th}$) range of 80.2–95.8 mmol/gdry biomass, according to Eq. 1. $m_x$ was obtained in the range 91.9–99.6 % within the considered biomass types, implying excellent mass conservation. As seen in Table 2, $X_c$ in the range of 83.1–96.9 % was achieved, revealing efficient carbon-to-syngas conversion. Within seven biomass types, experimental syngas heating value (LHV$_{syngas}$) was obtained in the range 16.6–21.9 kJ/gdry biomass, approaching the theoretical syngas heating value (LHV$_{th}$) range of 20.9–25.0 kJ/gdry biomass (calculated from Eq. 1). The experimental energy upgrade factor ($U_{ex}$) was achieved in the range 0.96–1.35, compared to the theoretical energy upgrade factor ($U_{th}$) in the range of 1.11–1.53 (calculated from Eq. 1), indicating high-performance biomass-to-syngas conversion and successful solar energy storage in the syngas.

Within the considered biomass types, corresponding values of $\eta_{solar-to-fuel}$ were obtained in the range of 10.3–18.9 %, which was comparable with previous related studies (Chuayboon and Abanades, 2022, 2023), due to higher operating temperatures up to 1400 °C, thus indicating efficient conversion of solar energy and biomass to syngas.

### 3.2. Syngas evolution

Syngas evolution of continuous steam gasification of seven agricultural biomass residues with pellet injection is represented in Fig. 3. The evolution of $H_2$, CO, $CO_2$, $CH_4$, and $C_2H_m$ production rate plotted along with the representative reactor temperature ($T_1$) of 1400 °C over 30 min of an entire assessment period is shown for PKS (Fig. 3a), BN (Fig. 3b), BG (Fig. 3c), PEFB (Fig. 3d), CCF (Fig. 3e), RH (Fig. 3f), and PMF (Fig. 3g), respectively.

Overall, continuous solar-steam gasification was successfully carried out on seven biomass types. Specifically, syngas was continuously produced with high rates of $H_2$ and CO and much lower $CO_2$, $CH_4$, and $C_2H_m$ rates, implying high-quantity and high-quality syngas produced. This points out the production of a high-value syngas composed of mainly $H_2$ and CO, with low quantities of by-products. For any biomass type, a
<table><caption>**Table 2**

Operating parameters and experimental results of continuous solar-steam gasification of seven agricultural residues.</caption><thead><tr><th>Run No.</th><th>Type<br>(-)</th><th>m<sub>biomass</sub><br>(g)</th><th>ṁ<sub>H₂O</sub><br>(mg/min)</th><th>T₁<br>(°C)</th><th>Q̇<sub>solar</sub><br>(kW)</th><th>H₂ yield</th><th>CO yield</th><th>CO₂ yield<br>(mmol/g<sub>dry biomass</sub>)</th><th>CH₄ yield</th><th>C₂H₆ yield</th><th>Yield<sub>ex</sub></th><th>Yield<sub>th</sub></th><th>m<sub>x</sub><br>(%)</th><th>X<sub>c</sub><br>(%)</th><th>η<sub>solar-to-fuel</sub><br>(%)</th><th>U<sub>ex</sub><br>(-)</th><th>U<sub>th</sub><br>(-)</th><th>LHV<sub>syngas</sub><br>(kJ/g<sub>dry biomass</sub>)</th><th>LHV<sub>th</sub></th></tr></thead><tbody><tr><td>1</td><td>Palm Kernel Shell<br>(PKS)</td><td>4.28</td><td>500</td><td>1100</td><td>0.81</td><td>31.9</td><td>21.4</td><td>7.4</td><td>6.0</td><td>1.0</td><td>67.7</td><td>95.8</td><td>93.8</td><td>83.1</td><td>10.3</td><td>1.04</td><td>1.31</td><td>19.8</td><td>25.0</td></tr><tr><td>2</td><td></td><td>4.15</td><td>500</td><td>1200</td><td>0.97</td><td>34.5</td><td>24.7</td><td>6.7</td><td>4.4</td><td>0.8</td><td>71.2</td><td>95.8</td><td>94.3</td><td>84.7</td><td>10.7</td><td>1.04</td><td>1.31</td><td>19.9</td><td>25.0</td></tr><tr><td>3</td><td></td><td>4.58</td><td>500</td><td>1300</td><td>1.18</td><td>35.4</td><td>25.9</td><td>6.8</td><td>4.9</td><td>0.7</td><td>73.6</td><td>95.8</td><td>95.6</td><td>88.6</td><td>13.7</td><td>1.08</td><td>1.31</td><td>20.7</td><td>25.0</td></tr><tr><td>4</td><td></td><td>4.85</td><td>500</td><td>1400</td><td>1.27</td><td>38.9</td><td>30.9</td><td>5.5</td><td>2.8</td><td>0.1</td><td>78.3</td><td>95.8</td><td>96.6</td><td>90.6</td><td>12.1</td><td>1.07</td><td>1.31</td><td>20.5</td><td>25.0</td></tr><tr><td>5</td><td>Betel Nut<br>(BN)</td><td>4.30</td><td>500</td><td>1100</td><td>0.92</td><td>33.0</td><td>23.1</td><td>7.0</td><td>4.7</td><td>0.8</td><td>68.7</td><td>85.6</td><td>95.8</td><td>87.5</td><td>13.8</td><td>0.96</td><td>1.11</td><td>19.4</td><td>22.4</td></tr><tr><td>6</td><td></td><td>4.44</td><td>500</td><td>1200</td><td>1.05</td><td>37.5</td><td>26.5</td><td>5.9</td><td>4.2</td><td>0.8</td><td>75.0</td><td>85.6</td><td>96.5</td><td>92.0</td><td>16.4</td><td>1.04</td><td>1.11</td><td>21.0</td><td>22.4</td></tr><tr><td>7</td><td></td><td>5.62</td><td>500</td><td>1300</td><td>1.15</td><td>38.2</td><td>29.2</td><td>5.6</td><td>3.8</td><td>0.4</td><td>77.2</td><td>85.6</td><td>97.3</td><td>95.5</td><td>18.9</td><td>1.05</td><td>1.11</td><td>21.1</td><td>22.4</td></tr><tr><td>8</td><td></td><td>5.66</td><td>500</td><td>1400</td><td>1.17</td><td>42.1</td><td>32.3</td><td>4.8</td><td>2.2</td><td>0.1</td><td>81.5</td><td>85.6</td><td>99.2</td><td>96.3</td><td>16.3</td><td>1.05</td><td>1.11</td><td>21.2</td><td>22.4</td></tr><tr><td>9</td><td>Palm Empty Fruit Bunch<br>(PEFB)</td><td>3.51</td><td>386</td><td>1100</td><td>0.79</td><td>35.1</td><td>22.9</td><td>6.3</td><td>5.4</td><td>1.1</td><td>70.9</td><td>86.6</td><td>96.9</td><td>89.4</td><td>12.3</td><td>1.28</td><td>1.39</td><td>20.7</td><td>22.5</td></tr><tr><td>10</td><td></td><td>3.32</td><td>386</td><td>1200</td><td>1.03</td><td>38.7</td><td>25.4</td><td>5.4</td><td>4.4</td><td>0.9</td><td>74.9</td><td>86.6</td><td>98.6</td><td>90.2</td><td>14.6</td><td>1.31</td><td>1.39</td><td>21.2</td><td>22.5</td></tr><tr><td>11</td><td></td><td>3.35</td><td>386</td><td>1300</td><td>1.19</td><td>39.6</td><td>28.9</td><td>4.8</td><td>4.3</td><td>0.5</td><td>78.2</td><td>86.6</td><td>97.5</td><td>96.4</td><td>18.7</td><td>1.35</td><td>1.39</td><td>21.9</td><td>22.5</td></tr><tr><td>12</td><td></td><td>3.45</td><td>386</td><td>1400</td><td>1.26</td><td>42.3</td><td>31.0</td><td>4.7</td><td>3.0</td><td>0.2</td><td>81.2</td><td>86.6</td><td>99.1</td><td>96.9</td><td>15.7</td><td>1.33</td><td>1.39</td><td>21.6</td><td>22.5</td></tr><tr><td>13</td><td>Bagasse<br>(BG)</td><td>4.4</td><td>300</td><td>1100</td><td>0.81</td><td>33.5</td><td>21.0</td><td>6.5</td><td>4.8</td><td>0.8</td><td>66.6</td><td>81.4</td><td>95.9</td><td>83.6</td><td>12.6</td><td>1.21</td><td>1.35</td><td>19.0</td><td>21.3</td></tr><tr><td>14</td><td></td><td>5.03</td><td>300</td><td>1200</td><td>0.95</td><td>34.0</td><td>22.8</td><td>5.7</td><td>4.1</td><td>0.8</td><td>67.4</td><td>81.4</td><td>96.8</td><td>83.9</td><td>12.3</td><td>1.21</td><td>1.35</td><td>18.9</td><td>21.3</td></tr><tr><td>15</td><td></td><td>4.91</td><td>300</td><td>1300</td><td>1.03</td><td>36.4</td><td>27.0</td><td>4.5</td><td>3.3</td><td>0.6</td><td>71.8</td><td>81.4</td><td>96.5</td><td>89.3</td><td>16.4</td><td>1.26</td><td>1.35</td><td>19.8</td><td>21.3</td></tr><tr><td>16</td><td></td><td>5.16</td><td>300</td><td>1400</td><td>1.24</td><td>37.5</td><td>29.3</td><td>4.1</td><td>2.4</td><td>0.2</td><td>73.4</td><td>81.4</td><td>99.6</td><td>90.3</td><td>14.5</td><td>1.24</td><td>1.35</td><td>19.5</td><td>21.2</td></tr><tr><td>17</td><td>Palm Mesocarp Fiber<br>(PMF)</td><td>0.97</td><td>253</td><td>1100</td><td>0.85</td><td>34.1</td><td>23.1</td><td>6.1</td><td>4.2</td><td>0.7</td><td>68.3</td><td>80.2</td><td>95.5</td><td>87.6</td><td>11.0</td><td>1.22</td><td>1.34</td><td>19.1</td><td>20.9</td></tr><tr><td>18</td><td></td><td>3.27</td><td>253</td><td>1200</td><td>0.87</td><td>35.5</td><td>25.3</td><td>5.1</td><td>4.1</td><td>0.7</td><td>70.7</td><td>80.2</td><td>96.4</td><td>90.0</td><td>13.7</td><td>1.27</td><td>1.34</td><td>19.9</td><td>20.9</td></tr><tr><td>19</td><td></td><td>3.6</td><td>253</td><td>1300</td><td>1.02</td><td>37.8</td><td>27.4</td><td>4.7</td><td>3.4</td><td>0.6</td><td>74.0</td><td>80.2</td><td>98.6</td><td>92.3</td><td>17.2</td><td>1.30</td><td>1.34</td><td>20.4</td><td>20.9</td></tr><tr><td>20</td><td></td><td>2.81</td><td>253</td><td>1400</td><td>1.16</td><td>39.8</td><td>31.5</td><td>3.6</td><td>1.8</td><td>0.7</td><td>76.9</td><td>80.2</td><td>99.1</td><td>94.3</td><td>13.7</td><td>1.28</td><td>1.34</td><td>20.1</td><td>20.9</td></tr><tr><td>21</td><td>Coconut Fiber<br>(CCF)</td><td>3.2</td><td>357</td><td>1100</td><td>0.83</td><td>32.8</td><td>20.3</td><td>6.7</td><td>5.0</td><td>0.7</td><td>65.5</td><td>82.5</td><td>94.4</td><td>84.9</td><td>10.8</td><td>1.23</td><td>1.43</td><td>18.5</td><td>21.6</td></tr><tr><td>22</td><td></td><td>2.87</td><td>357</td><td>1200</td><td>1.04</td><td>33.3</td><td>24.7</td><td>5.2</td><td>3.6</td><td>0.5</td><td>67.2</td><td>82.5</td><td>94.6</td><td>88.0</td><td>13.4</td><td>1.23</td><td>1.43</td><td>18.5</td><td>21.6</td></tr><tr><td>23</td><td></td><td>2.94</td><td>357</td><td>1300</td><td>1.16</td><td>34.2</td><td>26.8</td><td>4.8</td><td>3.6</td><td>0.2</td><td>69.7</td><td>82.5</td><td>96.4</td><td>91.8</td><td>15.4</td><td>1.26</td><td>1.43</td><td>18.9</td><td>21.6</td></tr><tr><td>24</td><td></td><td>3.02</td><td>357</td><td>1400</td><td>1.29</td><td>36.7</td><td>30.7</td><td>3.8</td><td>2.2</td><td>0.0</td><td>73.4</td><td>82.5</td><td>98.9</td><td>94.9</td><td>14.0</td><td>1.28</td><td>1.43</td><td>19.3</td><td>21.6</td></tr><tr><td>25</td><td>Rice Husk<br>(RH)</td><td>3.29</td><td>323</td><td>1100</td><td>0.81</td><td>28.0</td><td>16.6</td><td>6.3</td><td>5.3</td><td>0.7</td><td>56.9</td><td>83.4</td><td>91.9</td><td>83.2</td><td>10.4</td><td>1.17</td><td>1.53</td><td>16.6</td><td>21.7</td></tr><tr><td>26</td><td></td><td>3.33</td><td>323</td><td>1200</td><td>1.05</td><td>32.9</td><td>19.7</td><td>5.0</td><td>4.6</td><td>0.5</td><td>62.7</td><td>83.4</td><td>94.5</td><td>85.6</td><td>12.8</td><td>1.25</td><td>1.53</td><td>17.8</td><td>21.7</td></tr><tr><td>27</td><td></td><td>3.41</td><td>323</td><td>1300</td><td>1.15</td><td>34.5</td><td>22.4</td><td>4.2</td><td>3.6</td><td>0.3</td><td>64.9</td><td>83.4</td><td>94.5</td><td>87.3</td><td>15.4</td><td>1.26</td><td>1.53</td><td>17.9</td><td>21.7</td></tr><tr><td>28</td><td></td><td>3.23</td><td>323</td><td>1400</td><td>1.20</td><td>37.3</td><td>24.5</td><td>3.8</td><td>2.3</td><td>0.0</td><td>67.9</td><td>83.4</td><td>96.6</td><td>87.2</td><td>14.2</td><td>1.25</td><td>1.53</td><td>17.7</td><td>21.7</td></tr><tr><td></td><td>Max value</td><td>5.66</td><td>500</td><td>1400</td><td>1.29</td><td>42.3</td><td>32.3</td><td>7.4</td><td>6.0</td><td>1.1</td><td>81.5</td><td>95.8</td><td>99.6</td><td>96.9</td><td>18.9</td><td>1.35</td><td>1.53</td><td>21.9</td><td>25.0</td></tr><tr><td></td><td>Min value</td><td>0.97</td><td>253</td><td>1100</td><td>0.79</td><td>28.0</td><td>16.6</td><td>3.6</td><td>1.8</td><td>0.0</td><td>56.9</td><td>80.2</td><td>91.9</td><td>83.1</td><td>10.3</td><td>0.96</td><td>1.11</td><td>16.6</td><td>20.9</td></tr></tbody></table>
<figure><img src="image_6.png" /><figcaption>Fig. 3. Typical syngas production rate evolution during continuous solar-driven pellet-fed steam gasification of seven agricultural residues at 1400 °C.</figcaption></figure>
similar pattern in the syngas production rate was observed with a sharp
peak of the syngas production rate over a given period. Such a pattern of
syngas was due to the biomass pellet injection method, in which the
pellet was injected piece by piece into the reactor. The total reaction
duration for each run was chiefly dependent on the total weight of
biomass injected (as listed in Table 2). Thus, the higher the amount of
biomass injected (as in the case of PKS or BN), the higher the reaction
duration. In addition, the biomass was injected once the steady-state
temperature was achieved, which explains why the start of the reaction
differed depending on the gasification experiments. The

temperature dropped consistently where the peak syngas production
rate occurred due to the fall of the biomass pellet, coursing endothermic
gasification reaction. A stable pattern in the peak of the syngas pro-
duction rate was mainly obtained. These results indicated that such
selected biomasses could be employed for the continuous solar-steam
gasification process.

For any biomass type, the peak production rates of H₂ and CO were
found to be roughly 0.4 NL/min, while those of CO₂, CH₄, and C₂H₈
were found to be approximately 0.03 NL/min. Note that a sudden shoot
in the syngas production rate of BG above 0.8 NL/min and PMF above

<figure><img src="image_7.png" /><figcaption>**Fig. 4.** (a) Comparison of mass balance of seven biomass types between input reactants and output products, and (b) global mass balance ($m_x$) as a function of temperatures (1100–1400 °C).</figcaption></figure>
0.6 NL/min at the beginning of the reaction (Fig. 3) is caused by double pellets of biomass falling into the reactor simultaneously, thus reducing the total reaction duration. As previously described, the continuous gasification reaction was entirely driven by concentrated sunlight as high-temperature process heat. A weak drop in the temperature during biomass injection was observed at any peaks of the syngas production rate. In addition, a rapid decline in the temperature at the end of the reaction is due to a stop in the solar energy input by closing the shutter after reaction completion. These representative experiments revealed the practical operation of on-sun solar gasification with continuous pellet injection. The feasibility and reliability of continuous solar gasification with various agricultural biomass residues were thus demonstrated.

### 3.3. Global mass balance

Global mass balance ($m_x$) approaches offer a structured way of tracking the total quantity of a specific material between input reactant and output product flows. In this study, the ratio of the mass leaving the system to the mass entering the system of solar steam gasification of seven agricultural residues was reported through the $m_x$ indicator, as shown in Fig. 4. The steam input and syngas product quantities were determined by integrating their flow rates over time. The mass of unreacted H₂O and remaining char (mixed together) were quantified using an analytical balance by weighting the outlet components, including the bubbler, alumina outlet tube, and stainless-steel tube, before and after experiments.

Fig. 4a displays the $m_x$ values of seven biomass types at a representative temperature of 1400 °C. Overall, a difference in the water mass quantity of each biomass was observed. The required water of each biomass type was different (3.88–18.66 g), depending on the initial amount of biomass (in the range of 2.81–5.66 g) and the chemical composition of each biomass type, according to Table 1. The excellent conservation of mass was obtained at any biomass type, as the mass of output was almost equal to the mass of input, as evidenced by the value of $m_x$ of 96.6 % for PKS, 99.2 % for BN, 99.5 % for BG, 99.0 % for PEFB, 98.9 % for CCF, 96.5 % for RH, and 99.1 % for PMF, respectively. The values of $m_x$ of less than 100 % were due to the tiny residual amount of ash and unconverted char formation deposited in the reactor cavity receiver and outlet components. According to Fig. 4b, a positive impact of the temperature (1100–1400 °C) on $m_x$ was observed, as $m_x$ increased with rising temperatures at any biomass type, reaching its maximum $m_x$ value at 1400 °C. This indicated that an increase in the temperature favored the solid conversion to syngas in the reactor, thus leading to better mass balance at higher temperatures.

Moreover, the influence of the biomass types on $m_x$ was identified. RH and PKS exhibited lower mass balance than the others. This is because RH showed relatively high remaining ashes (Li et al., 2019) while PKS exhibited high remaining char due to high carbon content. Such remaining solids were found in the reactor cavity receiver, as

<figure><img src="image_8.png" /><figcaption>**Fig. 5.** Ash remaining in the cavity receiver after experiments: (a) bagasse ash in Run#13–16 and (b) rice husk ash in Run#25–28.</figcaption></figure>

shown in Fig. 5, which was hard to collect entirely. Significantly higher ash content from rice husk than from bagasse was revealed (Fig. 5). Ash sintering at high temperature must also be addressed, which may counteract the beneficial effect of increasing temperature. The presence of remaining ash directly depends on the type of agricultural residues (Table 1). This may cause reduced heat transfer to the biomass, and clogging due to accumulation. This issue can be mitigated by blending with other biomass types such as PEFB (Al-Muraisy et al., 2025), BN (Chuayboon and Abanades, 2024), PMF, or PKS, or further designing reactors with features enabling ash removal (Abanades et al., 2021). The following section discusses the effects of biomass type and temperature on syngas yield in more detail.

### 3.4. Syngas yield

The influence of biomass types and temperature on syngas yields (mmol/g<sub>dry biomass</sub>) is represented in Fig. 6. The temperature ($T_1$) was adjusted at 1100 °C, 1200 °C, 1300 °C, and 1400 °C. Overall, a significant impact of the biomass type and temperature on syngas yield was emphasized. As seen in Fig. 6a, a remarkable H₂ yield was found from PEFB, followed by BN and PMF, respectively, due to high volatile matter (Mohammed et al., 2012). For instance, the H₂ yield from PEFB rose from 35.1 mmol/g<sub>dry biomass</sub> at 1100 °C to 42.3 mmol/g<sub>dry biomass</sub> at 1400 °C, approaching its theoretical value of 46.0 mmol/g<sub>dry biomass</sub>. The H₂ yields from BG, CCF, and PKS were slightly lower than that from PEFB due to lower volatile matter (Mohammed et al., 2012), and their tendency was found to be similar as they overlapped each other. However, RH exhibited the lowest H₂ yield in the 28.0–37.3 mmol/g<sub>dry biomass</sub> range compared to its theoretical yield of 43.5 mmol/g<sub>dry biomass</sub>, possibly due to high ash formation. At any biomass type, H₂ increased with temperature and reached the maximum at 1400 °C.

Like the H₂ yield trend, a significant increase in the CO yield (Fig. 6b) with rising temperature was observed at any biomass type. Within the considered temperature range, BN exhibited the highest CO yield in the range of 23.1–32.3 mmol/g<sub>dry biomass</sub> due to high fixed carbon, followed closely by PEFB in the range of 22.9–31.0 mmol/g<sub>dry biomass</sub>, and PMF in the range of 23.1–31.5 mmol/g<sub>dry biomass</sub>. The CO yields associated with CCF, PKS, and BG were found to be similar in the range of 20.3–30.7 mmol/g<sub>dry biomass</sub>, 21.4–30.9 mmol/g<sub>dry biomass</sub>, and 21.0–29.3 mmol/g<sub>dry biomass</sub>. As expected, RH revealed the lowest CO yield in the range of 16.6–24.5 mmol/g<sub>dry biomass</sub>, indicating the limitation of this biomass for continuous solar gasification, possibly due to the impact of high ash formation, which can accumulate in the reactor cavity receiver and eventually lead to blockages during long-term operation (Surjosatyo et al., 2017). In comparison, at 1400 °C, H₂ and CO yields for all the biomass types were approximately five times higher than CO₂, demonstrating high-quantity and high-quality syngas produced.

Concerning unwanted products, CO₂ yields (Fig. 6c) for any biomass type were found to be similar except for PKS. PKS exhibited the highest CO₂ (5.5–7.4 mmol/g<sub>dry biomass</sub>) likely due to its high fixed carbon content with high density from its lignocellulosic structure, which involve the conversion of fixed carbon to CO₂ (Lapuerta et al., 2004; Mohammed et al., 2012). The CO₂ yield from BN was also high (4.8–7.0 mmol/g<sub>dry biomass</sub>), also because of high fixed carbon content according to Table 1, thus showing the impact of biomass type. The CO₂ yields of other biomass types remained in the range of 4.1–6.5 mmol/g<sub>dry biomass</sub> (BG), 4.7–6.3 mmol/g<sub>dry biomass</sub> (PEFB), 3.8–6.7 mmol/g<sub>dry biomass</sub> (CCF), 3.8–6.3 mmol/g<sub>dry biomass</sub> (RH), and 3.6–6.1 mmol/g<sub>dry biomass</sub> (PMF), because they have a lower fixed carbon content than PKS and BN. In addition, the CO₂ yield can be significantly reduced by increasing the gasification temperature, due to favorable reactions including the Boudouard reaction (C+CO₂ ↔ 2CO) and carbon gasification reaction (C+H₂O ↔ CO+H₂), in agreement with previous studies (Chuayboon and Abanades, 2021; Chuayboon et al., 2018a, b). For example, CO₂ yield from BN decreased from
<figure><img src="image_9.png" /><figcaption>**Fig. 6.** Effect of biomass type on syngas yields at 1100–1400°C: (a) H₂, (b) CO, (c) CO₂, (d) CH₄, (e) C₂H₆, and (f) total syngas yield (Yield<sub>ex</sub>).</figcaption></figure>

7.0 mmol/g<sub>dry biomass</sub> at 1100 °C to 4.8 mmol/g<sub>dry biomass</sub> at 1400 °C. CH₄ yields (Fig. 6d) were found to be in the range of 3.0–6.0 mmol/g<sub>dry biomass</sub> at any biomass type, even if CH₄ from PKS appeared to be slightly higher than other biomass types due to high fixed carbon (Li et al., 2009). In addition, they can be diminished with increasing temperature, due to the shift of chemical reactions equilibrium, the favored production of lighter gases like CO and H₂, and the kinetics of the reactions involved (Chuayboon and Abanades, 2021; Chuayboon et al., 2018a). Byproducts related to C₂H₆ (Fig. 6e) were relatively low (0.2–1.1 mmol/g<sub>dry biomass</sub>), and they can be reduced by raising the temperature, enabling high-quality syngas. The effect of biomass type on C₂H₆ was not clear; however, RH and CCF appeared to yield lower amounts than other biomass types.

As expected, PEFB and BN were found to exhibit the highest total syngas yield (Yield<sub>ex</sub>, Fig. 6f) (70.9–81.2 mmol/g<sub>dry biomass</sub> for PEFB and 68.7–81.5 mmol/g<sub>dry biomass</sub> for BN), due to the promoted H₂ and CO yields. Similar total syngas yields were reached between PKS and PMF (in the range of 67.7–78.3 mmol/g<sub>dry biomass</sub> for PKS and 68.3–76.9 mmol/g<sub>dry biomass</sub> for PMF) and between BG and CCF (in the range of 66.6–73.4 mmol/g<sub>dry biomass</sub> for BG and 65.5–73.4 mmol/g<sub>dry biomass</sub> for CCF).

The higher the volatile matter, the higher the syngas yield as observed for PEFB, BN, and PMF. As expected, RH produced the lowest total syngas yield (56.9–67.9 mmol/g<sub>dry biomass</sub>) due to the low H₂ and CO production. An increase in the H₂ and CO with temperature led to an increase in the total syngas yield, reaching the maximum value at 1400 °C (e.g., 81.2 mmol/g<sub>dry biomass</sub> for PEFB at 1400 °C, close to the theoretical total yield (Yield<sub>th</sub>) of 86.6 mmol/g<sub>dry biomass</sub>). At 1400 °C, the maximum total syngas yields of the considered biomasses approached their total theoretical yields (Yield<sub>th</sub>) by 95.9 % for PMF, followed by 95.2 % for BN, 93.8 % for PEFB, 90.2 % for BG, 89.0 % for CCF, 81.8 % for PKS, and 81.4 % for RH, respectively. Thus, PMF, PEFB and BN were identified to be excellent candidates for continuous solar steam gasification, followed by BG, CCF, and PKS. However, due to the lowest syngas yield achieved, RH was optional for continuous solar steam gasification.

In conclusion, the type of agricultural residues was found to impact the continuous solar-steam gasification process considerably. Temperature significantly promoted gasification performance, leading to upgraded syngas yield (high H₂ and CO) and quality (low CO₂, CH₄, and C₂H₆) at any biomass type. PMF, PEFB, and BN were suitable biomass
types in the gasification reaction, followed by BG, CCF, PKS, and RH, as confirmed by high syngas yields within the considered temperature range. However, it is necessary to pay attention on using PKS due to its high density and RH due to its high ash formation, which revealed a negative effect on the continuous solar-steam gasification performance, as evidenced by the lower syngas yield produced. Co-gasification of biomass mixture is a promising strategy to improve the gasification performance of RH. However, a well-designed experimental program is required to determine the optimal biomass blend ratio and evaluate the overall efficiency compared to gasifying each biomass separately.

### 3.5. Solar reactor performance and efficiency

The solar reactor performance and efficiency of continuous solar-steam gasification of different biomass types are presented in Fig. 7. Syngas heating value ($LHV_{syngas}$, Fig. 7a), solid carbon conversion ($X_c$, Fig. 7b), energy upgrade factor ($U_{ex}$, Fig. 7c), and solar-to-fuel energy conversion efficiency ($\eta_{solar-to-fuel}$, Fig. 7d) of each biomass type are plotted as a function of temperatures and compared thoroughly with the theoretical syngas heating value ($LHV_{th}$), and theoretical energy upgrade factor ($U_{th}$). Overall, the influence of the biomass type and temperature on solar reactor performance was unraveled.

In agreement with syngas tendency, PEFB exhibited the highest $LHV_{syngas}$ in the range of 20.7–21.9 kJ/g<sub>dry</sub> biomass ($LHV_{th}=22.5$ kJ/g<sub>dry</sub> biomass), followed by BN: 19.4–21.2 kJ/g<sub>dry</sub> biomass ($LHV_{th}=22.4$ kJ/g<sub>dry</sub> biomass), PKS: 19.8–20.7 kJ/g<sub>dry</sub> biomass ($LHV_{th}=25.0$ kJ/g<sub>dry</sub> biomass), PMF: 19.1–20.4 kJ/g<sub>dry</sub> biomass ($LHV_{th}=20.9$ kJ/g<sub>dry</sub> biomass), BG: 18.9–19.8 kJ/g<sub>dry</sub> biomass ($LHV_{th}=21.3$ kJ/g<sub>dry</sub> biomass), CCF: 18.5–19.3 kJ/g<sub>dry</sub> biomass ($LHV_{th}=21.6$ kJ/g<sub>dry</sub> biomass), and RH: 16.6–17.9 kJ/g<sub>dry</sub> biomass ($LHV_{th}=21.7$ kJ/g<sub>dry</sub> biomass), respectively. At any specified biomass type, $LHV_{syngas}$ increased with temperature, peaked at 1300 °C, and then leveled off at 1400 °C due to a reduction in the unwanted gas yields ($CH_4$ and $C_2H_m$), which have high calorific

heating value.

As expected, $U_{ex}$ values (Fig. 7c) generally exceeded one, demonstrating that the energy content of the produced syngas is higher than reactant biomass, thus indicating solar energy storage into syngas. $U_{ex}$ values differed depending on the biomass types. Therefore, a comparison of the $U_{ex}$ values with their theoretical energy upgrade factors ($U_{th}$) was addressed.

As a result, $U_{ex}$ of PEFB was in the range of 1.28–1.35 ($U_{th}=1.39$), followed by PMF: 1.22–1.30 ($U_{th}=1.34$), CCF: 1.23–1.28 ($U_{th}=1.43$), RH: 1.17–1.26 ($U_{th}=1.53$), BG: 1.21–1.26 ($U_{th}=1.35$), BN: 0.96–1.05 ($U_{th}=1.11$), and PKS: 1.04–1.08 ($U_{th}=1.31$). $U_{ex}$ from PEFB and PMF was excellent, reaching 97.3 % of $U_{th}$, followed by BN (94.2 %), BG (93.3 %), CCF (87.7 %), PKS (83.0 %), and RH (82.4 %), respectively. In addition, $U_{ex}$ increased with increasing temperature, reached the maximum value at 1300 °C, and slightly decreased at 1400 °C. For example, at 1300 °C, the maximum $U_{ex}$ value was found for PEFB (1.35), PMF (1.30), RH (1.26), and BG (1.26), indicating an optimal temperature. The decrease in $U_{ex}$ at 1400 °C was consistent with a reduction in the $LHV_{syngas}$, which resulted from a decline of $CH_4$ and $C_2H_m$.

$X_c$ values of PEFB (89.4–96.9 %) and BN (87.5–96.3 %) were found to be considerably greater than other biomass types, as compared with PMF (87.6–94.3 %), CCF (84.9–94.9 %), BG (83.6–90.3 %), PKS (83.1–90.6 %), and RH (83.2–87.2 %). From these results, PEFB exhibited the highest $X_c$ value, while RH exhibited the lowest value. The effect of biomass type on carbon conversion was thus evidenced. High carbon conversion of PEFB, BN, and PMF can be attributed to their high volatile matter contents. For any specified biomass type, a sharp increase in $X_c$ in the temperature range of 1000–1300 °C was noticed, followed by a slight increase in $X_c$ slope at 1400 °C, where the maximum $X_c$ values exited in the range of 87.2–96.9 %, in agreement with $LHV_{syngas}$ trends.

As expected, $\eta_{solar-to-fuel}$ increased with temperature, reached a peak at 1300 °C, and then decreased meaningfully with increasing

<figure><img src="image_10.png" /><figcaption>Fig. 7. (a) syngas lower heating value ($LHV_{syngas}$), (b) solid carbon conversion ($X_c$), energy upgrade factor ($U_{ex}$), and solar-to-fuel energy conversion efficiency ($\eta_{solar-to-fuel}$) as a function of temperature for different biomass types.</figcaption></figure>
temperature to 1400 °C (Fig. 7d). Therefore, the optimal temperature exited at 1300 °C. The maximum values of $\eta_{solar-to-fuel}$ were 18.9 % for BN, 18.7 % for PEFB, 17.2 % for PMF, 15.4 % for CCF and RH, and 13.7 % for PKS. $\eta_{solar-to-fuel}$ of BN was in the range of 13.8–18.9 %, closely followed by PEFB: 12.3–18.7 %, PMF: 11.0–17.2 %, BG: 12.3–16.4 %, RH: 10.4–15.4 %, and PKS: 10.3–13.7 %.

In summary, the types of agricultural biomass residues showed a significant effect on the continuous solar steam gasification. PEFB, BN, PMF, CCF, and BG were recommended for solar gasification, while RH was less suitable because of high ash formation, which can cause ash accumulation and would require extraction for continuous processing. PKS appeared to be challenging to be gasified due to its high density and hardness. High density and ash content in feedstocks negatively impact gasification activity. On the one hand, high density feedstock leads to a lower specific surface, which means less surface area available for contact with steam to react with biomass, thus hindering reaction kinetics. On the other hand, high ash content leads to ash accumulation in the reactor cavity receiver, decreasing the heat transfer and steam/biomass reaction contact, thus adversely impacting the gasification activity and decreasing the syngas yield. Reactor performance and efficiency can be enhanced by increasing the temperature to its optimal point, which substantially improves $LHV_{syngas}$, $X_c$, $U_{ex}$, and $\eta_{solar-to-fuel}$. A temperature of 1300 °C was discovered to be optimal in maximizing $\eta_{solar-to-fuel}$ to 18.9 %, providing guidance for scaling up such systems. These key findings highlighted successful continuous solar steam gasification of agricultural biomass residues, achieving stable syngas production with high carbon conversion rates and energy efficiency. The screening of a broad range of crop waste feedstocks, all processed into pressed pellets for continuous injection, is a key contribution to demonstrating the process flexibility and reliability, further confirming the system's performance under real, variable solar conditions. The adaptability of the solar reactor to different types of biomass wastes was demonstrated, showing that it can be a viable technology for valorizing various waste streams into high-quality syngas.

## 4. Conclusions

Continuous solar-steam gasification of different agricultural residues has been successfully performed in a continuous solar gasifier, driven by real high-flux concentrated solar heat. This study showcases an innovative approach in both valorizing agricultural residues and converting sunlight into high-quality and green syngas fuel. Seven crop residues were successfully utilized for continuous solar-steam gasification, confirming that the reactor can accommodate a variety of biomass types, with suitable flexibility and reliability. Solar steam gasification was conducted continuously with a pellet biomass injection method, which could further be applied at an industrial scale. An important effect of agricultural residue types on the key performance indicators, including syngas production rate, lower syngas heating value ($LHV_{syngas}$), carbon conversion efficiency ($X_c$), energy upgrade factor ($U_{ex}$), and solar-to-fuel efficiency ($\eta_{solar-to-fuel}$), was pointed out.

As a result, syngas was generated with high yields and quality. For any biomass type, a temperature increase upgraded the syngas yield (high H₂ and CO) and its quality (low CO₂, CH₄, and C₂H₆). This, in turn, improved reactor performance ($LHV_{syngas}$, $X_c$, $U_{ex}$, and $\eta_{solar-to-fuel}$), at the expense of increased solar energy consumption, which can eventually downgrade $\eta_{solar-to-fuel}$. The total syngas yield was maximum at 1400 °C, confirming the significant impact of temperature on solar gasification. Within seven biomass types and a temperature range of 1100–1400 °C for 28 on-sun runs, the total syngas yield was in the range of 56.9–81.5 mmol/g<sub>dry</sub> biomass, thus approaching the theoretical total yield by 68.2–95.2 %. In addition, the values of $m_x$, $LHV_{syngas}$, $X_c$, $U_{ex}$, and $\eta_{solar-to-fuel}$ were in the range of 91.9–99.6 %, 16.6–21.9 kJ/g<sub>dry</sub> biomass, 83.1–96.9 %, 0.96–1.35, and 10.3–18.9 %, respectively, and as such, the experimental $LHV_{syngas}$ and $U_{ex}$ values also approached the

theoretical ones (20.9–25.0 kJ/g<sub>dry biomass</sub> and 1.11–1.53, respectively). The maximum values of $LHV_{syngas}$ (17.9–21.9 kJ/g<sub>dry biomass</sub>), $U_{ex}$ (1.05–1.35), and $\eta_{solar-to-fuel}$ (13.7–18.9 %) obtained at 1300 °C confirmed the optimal temperature. Thus, this temperature was suggested for operating the continuous solar-steam gasification of agricultural residues.

Due to their outstanding syngas yields and reactor performance, PEFB, BN, and PMF were proven to be appropriate biomass feedstocks in the gasification reaction, followed by CCF, BG, PKS, and RH. However, PKS and RH were less suitable for gasification due to high hardness for PKS and ash formation for RH. This study demonstrated the significant impact of agricultural residue types on the gasification performance, and further aided in assessing process efficiencies for suitable biomass selection. Minimizing the ash accumulation in the reactor cavity receiver is essential for the long-term performance stability of the continuous solar gasifier. Future research will be focused on solar hybrid gasification of agricultural residues to demonstrate controlled operation under variable solar irradiation.

## CRediT authorship contribution statement

**Stéphane Abanades:** Writing – review & editing, Visualization, Validation, Supervision, Software, Resources, Project administration, Methodology, Investigation, Funding acquisition, Formal analysis, Data curation, Conceptualization, Writing – original draft. **Srirat Chuayboon:** Writing – original draft, Visualization, Validation, Software, Resources, Project administration, Methodology, Investigation, Funding acquisition, Formal analysis, Data curation, Conceptualization.

## 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.

## Acknowledgements

This research was performed in a collaboration between King Mongkut's Institute of Technology Ladkrabang (KMITL), Thailand and Processes, Materials and Solar Energy Laboratory (PROMES), France. It received funding from National Science Research and Innovation Fund (NSRF), Thailand under grant number: RE-KRIS/FF67/004. This work was also funded by Region Occitanie in France (Défi clé Hydrogène Vert - RHyO – HYDROMET project). The authors thank Asia Biomass Public Company Limited, Thailand, for supplying agricultural residues, as well as The Office of Scientific Instruments and Testing Center by KMITL for characterizing the physicochemical properties of agricultural residues.

## Data availability

Data will be made available on request.

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