# High temperature fast pyrolysis of waste biomass in a solar-assisted quartz drop-tube reactor

Vignesvar Krish Subramani, Sylvain Rodat, Elmeri Pienihäkkinen, Pouya Sirous-Rezaei, Stéphane Abanades

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Vignesvar Krish Subramani, Sylvain Rodat, Elmeri Pienihäkkinen, Pouya Sirous-Rezaei, Stéphane Abanades. High temperature fast pyrolysis of waste biomass in a solar-assisted quartz drop-tube reactor. Biomass and Bioenergy, 2026, 210, pp.109071. 10.1016/j.biombioe.2026.109071. hal-05512228

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# High temperature fast pyrolysis of waste biomass in a solar-assisted quartz drop-tube reactor

Vignesvar Krish Subramani<sup>a, *, c</sup>, Sylvain Rodat<sup>a, c</sup>, Elmeri Pienihäkkinen<sup>b, c</sup>,
Pouya Sirous-Rezaei<sup>b</sup>, Stéphane Abanades<sup>a, c</sup>

## ARTICLE INFO

### Keywords:

- Waste biomass

- Fast pyrolysis

- Concentrated solar power

- Syngas

- Sustainable aviation fuel

- Drop-tube reactor

## ABSTRACT

Solar-assisted pyrolysis is a sustainable process for converting biomass into syngas, bio-oil, and biochar using renewable solar thermal energy, with a potentially zero carbon footprint. It generates both high-value gaseous and liquid fuels while transforming the atmospheric CO₂ captured in biomass in the form of solid carbon that can be long-term sequestrated or valorized. The EU's target of reducing net greenhouse gas emissions to at least 55% by 2030 sets the stage for effective measures to limit carbon emissions and achieve a sustainable future. This study presents the development of an innovative fast pyrolysis quartz drop-tube reactor using concentrated solar power and its performance for bio-waste valorization. Extensive raw material characterization was carried out, which provides valuable insights into demolition wood and rye straw feedstocks properties. Solar pyrolysis runs revealed key dependencies of product yields on operational parameters such as feedstock type, nitrogen gas flow rate (0.7-1.4 NL/min), and heating profile in the temperature range 800-900 °C. Operation at such high temperatures promoted gas production (>50% gas yield in mass) over liquid and solid products. In similar conditions, rye straw showed higher gas yield as compared to demolition wood. In addition, preheating or increasing the gas residence time favored gas production with negligible impact on gas composition. The solar drop tube pyrolysis reactor appears as a sustainable option to upgrade waste feedstocks into valuable products using concentrated solar energy with net zero CO₂ emission.

## 1. Introduction

The rapid rise in energy-related CO₂ emissions is having a lasting impact on the environment and the global ecosystem. Average temperatures have reached record highs. In 2023, annual energy-related emissions grew by 2.1%, surpassing 40 gigatons of carbon dioxide equivalent for the first time. China, the United States, and India are the top three emitters, collectively contributing to more than half of global emissions [1]. As stated in the synthesis report from the Intergovernmental Panel on Climate Change (IPCC, 2023), the average annual greenhouse gas emissions between 2010 and 2019 were higher than in any previous decade. However, the growth rate during this period (1.3% per year) was lower than the 2.1% per year observed from 2000 to 2009 [2].

According to the European Environment Agency's (EEA) report, the transportation sector is a significant source of emissions, accounting for 28% of global CO₂ emissions [3,4]. Within this sector, aviation alone

contributes approximately 12% of transport-related emissions. To curb these emissions and achieve climate neutrality in the EU, the 'Fit for 55' package introduced a series of proposals to expand the use of Sustainable Aviation Fuels (SAF) in aviation. The plan mandates that at least 2% of SAF be available at EU airports by 2025, with a target of 70% by 2050. This goal can be partially met through the production of advanced biofuels and green hydrogen.

The development of carbon-neutral and low-emission fuels derived from renewable resources, such as biomass, is becoming increasingly important in the gradual transition away from conventional fossil fuels. Biomass refers to biological material from living or recently living organisms, produced directly or indirectly through photosynthesis, primarily consisting of plants or plant-derived substances [5-7]. According to the International Energy Agency (IEA), the share of biofuels in total liquid fuel transport demand is projected to rise from 5.6% in 2023 to 6.4% by 2030 (on a volume basis), reaching 215 billion liters per year (5.7 EJ), with the aviation and shipping industries accounting for up to

<footnote><sup>a</sup> CNRS Processes, Materials, and Solar Energy Laboratory, PROMES-CNRS, 66120 Font-Romeu Odeillo, France</footnote>

<footnote><sup>b</sup> VTT Technical Research Centre of Finland Ltd., P.O. Box 1000, FI-02044 Espoo, Finland</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: vignesvar.krish-subramani@cnrs.fr (V. Krish Subramani).</footnote>
75% of total biofuel demand [8].

Biomass can be transformed into fuels and chemicals using either biochemical methods, such as anaerobic/aerobic digestion and fermentation [9,10], or thermochemical techniques, including pyrolysis, gasification, hydrothermal liquefaction, and hydrothermal carbonization [11,12]. Among these thermochemical approaches, pyrolysis, a process that thermally decomposes organic materials without oxygen, has been extensively explored as a viable method for generating fuels and chemicals from diverse biomass sources [13].

## 2. Pyrolysis

The term “pyrolysis” is derived from Greek, where “pyro” means “fire” and “lysis” means “separation.” Unlike complete combustion, which occurs in the presence of oxygen and results in the production of carbon dioxide and water, pyrolysis takes place without oxygen. The earliest records of pyrolysis traces back to the Egyptian era, where liquids from pyrolysis process were used in the sealing of watercrafts [14, 15]. Biomass pyrolysis produces three phases of matter. The gaseous products include carbon monoxide and hydrogen (syngas), methane, short-chain hydrocarbons, and carbon dioxide. The liquid phase, known as bio-oil and tar, consists of aliphatic and aromatic compounds, phenols, aldehydes, levoglucosan, hydroxy acetaldehyde, hydrocarbon chains, and water. Finally, the solid phase recovered is carbon-rich material called char.

The pyrolysis process can be classified into four main industrial types based on heating rate (Fig. 1): slow, medium, flash, and fast pyrolysis. The operating temperature can be divided into two ranges: low temperature (500 °C) and high temperature (1000 °C). Slow pyrolysis is a batch process that takes place over an extended duration, sometimes lasting several days, with a slow heating rate (<1 °C/s). Due to its long residence time (>10 min), it mainly produces char and tar through recombination reactions. Medium pyrolysis features an intermediate heating rate (10 °C/s) and residence time (~1 min). Fast pyrolysis operates at higher heating rates (>100 °C/s) and short gas residence times (1 s). Bio-gas yields reach 50 - 70 wt% (on a dry biomass basis) for the high temperature range (1000 °C). Flash pyrolysis, involves extremely high heating rates (>1000 °C/s) and very short residence times (<0.1 s), yielding primarily bio-oil for the low temperature range and gas for the high temperature range [14,15,16–18].

Pyrolysis reactors require heat energy to break down biomass. Traditional external heating methods, such as biomass combustors or internal electric heaters, are commonly used but either consume raw materials or rely on high-grade electricity for heating [20]. Integrating solar thermal energy into biomass conversion presents an innovative alternative, allowing long-term energy storage in the form of high-density fuels while enhancing efficiency and minimizing emissions by eliminating fossil fuel dependence and contamination [21]. This

approach represents a sustainable solution, utilizing an abundant renewable energy source with the potential for a zero-carbon footprint.

Solar-thermal conversion reactors can be heated through either direct or indirect irradiation. In indirect heating, concentrated solar rays heat the opaque walls of the reactor, transferring energy via conduction and convection. In contrast, direct heating involves a transparent wall, which lets solar radiation directly heat the reactants more efficiently by minimizing heat losses. The reactor's design, including the configuration of the receiver and choice of materials, plays a key role in optimizing solar collection and conversion efficiency. In both cases, the materials used must be able to withstand thermal shocks and resist corrosion [22–27]. Michael J. Antal was one of the pioneers in biomass pyrolysis (1976–1989), experimenting with the conversion of biomass into gases and liquid bio-oils using concentrated solar radiation, laying the groundwork for future research on solar-powered pyrolysis and reactor design [28,29]. Notably, the use of a quartz tube solar reactor was evaluated [30]. Biomass was fed from the top while steam was introduced from the bottom, leading to gasification reactions.

Solar-thermal conversion reactors can be categorized into several types based on design and heating mechanisms [25].

* Fixed-bed reactors have a simple structure and low operating costs, making them suitable for laboratory studies. However, they suffer from poor heat and mass transfer [31,32].

* Mobile/moving bed reactors function similarly but incorporate periodic motion to improve mixing. Rotary reactors utilize rotation to enhance heat and mass transfer, ensuring uniform temperature distribution, but require higher operational energy [33].

* Fluidized bed reactors suspend biomass particles in a gas stream, ensuring efficient heating and uniform reaction conditions [34–36].

* Drop tube reactors (DTR) are continuous flow reactors with low operational difficulty where a carrier gas flow and the biomass particles are fed from the top, which, by free fall, travel inside the reactor tube of high-temperature fields with a shorter residence time [31,37–40].

* Vortex-flow/cyclone reactors use centrifugal force for rapid product separation, making them suitable for high-temperature applications [41–43].

For fast and flash pyrolysis, fluidized bed and drop-tube reactors are among the most effective technologies due to their high heating rates, efficient heat transfer, and short residence times (<1 s). Fluidized bed reactors maintain uniform temperature distribution, preventing hot-spots, while drop-tube reactors rapidly volatilize biomass by suspending it in a high-temperature gas stream. Additionally, drop-tube reactors require significantly less gas flow compared to fluidized bed reactors, thereby reducing heat losses and inert gas consumption, a factor particularly relevant for solar applications where gas heating in solar receivers is challenging, and particle suspension is preferred.

The Circular Fuels project [44], funded by the European Union (Horizon Europe) in partnership with nine organizations, seeks to produce Sustainable Aviation Fuels (SAF) from the pyrolysis of biowaste feedstocks by utilizing concentrated solar energy. Through advancements in renewable energy harvesting and technological innovation, the project aims to convert low-cost and abundant waste wood and agricultural residues into renewable bio-oil using solar-assisted fast pyrolysis. This approach eliminates combustion while simultaneously valorizing by-products, enhancing sustainability in biofuel production. Fast pyrolysis (non-solar) of biomass was demonstrated at a pilot scale by VTT Finland in a circulating fluidized bed, which led to collaborative development projects such as Valmet, Fortum, and UPM based on the patented integrated pyrolysis technology developed by VTT [45]. An industrial-scale setup was demonstrated at the Valmet facilities, and the largest pre-commercial biomass fast pyrolysis plant to date (30 MWth, 50,000 tons per year) was established by Fortum in Joensuu, Finland. The plant is now owned by Savon Voima [46].

<figure><img src="image_2.png" /><figcaption>Fig. 1. Summarizing different pyrolysis conditions and the effect on product distribution [19].</figcaption></figure>
The developed solar-assisted drop-tube pyrolysis reactor at CNRS-PROMES (France) was operated at high reaction temperatures of around 800-900 °C. The initial sizing and design of the reactor were executed through numerical CFD thermal studies carried out using the ANSYS FLUENT 2020 commercial solver [47]. This paper presents a brief introduction to the reactor experiment set-up, instrumentation, followed by the experimental results from the first successful campaign of 2025.

### 3. Analytical methods and experimental setup

#### 3.1. Feedstock characterization

The feed materials consist of rye straw and demolition wood harvested and collected from Finland. Feedstocks were ground to a particle size smaller than 2 mm prior to characterization and solar-assisted pyrolysis experiments. Particle size distribution of the feedstocks was measured by sieving, and the distributions are presented in Fig. 2. Distributions were similar with both feeds. The majority of the particles were in the range of 0.18 – 1.25 mm (85.1 - 85.7%), but some smaller (7.0 - 7.2%) as well as larger (7.2 - 7.9%) particles were included in the feedstocks.

Moisture content of the feeds was analyzed according to the method SFS-EN ISO 18134. Ash and volatile matter were analyzed with Leco TGA701, according to the methods SFS-EN ISO 18122 and SFS-EN ISO 18123, respectively. Elemental composition was analyzed with Leco CNH828 according to the method SFS-EN ISO 16948. Heating values were measured with IKA Werke C 6000 bomb calorimeter, according to the method SFS-EN 18125. Chlorine and sulfur contents were analyzed by an automated Combustion Ion Chromatography (CIC) system consisting of Nittoseiko AQF-2100H combustion unit and Dionex Integron HPIC system. In the CIC, the sample is first combusted at 1000 °C, and the resultant combustion gases are absorbed in an aqueous absorption solution. The combustion converts the halogen-containing substances to halide ions and sulfur to sulphate, which are quantitatively analyzed by injecting the absorption solution into the IC.

For the carbohydrate and lignin composition, the samples were hydrolyzed with sulfuric acid at two stages, adapted from the NREL/TP-510-42618 method issued by the US National Renewable Energy Laboratory [48]. The resulting monosaccharides were determined by high-performance anion-exchange chromatography (HPAEC) with pulse amperometric detection (Dionex ICS 3000 A equipped with a CarboPac PA1 column) [49]. The polysaccharide content in the samples was calculated from the corresponding monosaccharides using an anhydro correction of 0.88 for pentoses and 0.90 for hexoses. The Klason lignin content, that is, the insoluble residue from the hydrolysis, was determined gravimetrically. Acid-soluble lignin was determined from the hydrolysate based on UV spectroscopy at 215 and 280 nm using an equation described by Goldschmid [50]. Extractives were determined via heptane extraction.

For the metal content analysis, approximately 0.3 g of the sample was first microwave-digested using the Milestone microwave system

<figure><img src="image_3.png" /><figcaption>Fig. 2. Particle size distribution of the feedstocks.</figcaption></figure>

Ethos UP. Digestion was carried out in an acid solution containing HNO₃, H₂O₂, HCl, and HF. After digestion, metals were analyzed with an Agilent 5110 SVDV ICP-OES equipped with a nebulizer and spray Chamber (Agilent). For calibration purposes, ICP multi-element standard solution IV (1000 mg/L) from Merck, CCS 5 (100 mg/L), CCS 6 (100 mg/L), and Labdig-29 were purchased from Inorganic Ventures. Standards were diluted with 1% HNO₃ (analytical grade) in the range of 0.1 ppm to 100 ppm for calibration.

#### 3.2. Reactor design

From the CFD simulation [47], a design has been set for the pyrolysis reactor (Fig. 3). The proposed solar pyrolysis reactor is of a tubular type that was operated as a drop tube, and it can alternatively be operated either as a fixed bed or fluidized bed. The vertically fed millimetric biomass particles are suspended in an inert nitrogen gas stream, which creates a dynamic environment that enhances heat/mass transfer and mixing. Nitrogen flowrate was monitored using a BROOKS SLA5850 mass flow controller with an accuracy of ±0.6% of reading. A 1 m long external quartz tube of 30 mm diameter and 2.5 mm thickness was set at the focus of the 2 m diameter solar concentrator available in the PROMES lab. The reactor tube is covered by insulation blocks that prevent heat losses to the surroundings. This insulation block consists of an open-air receiver cavity of 20 cm height with a circular front orifice of 15 mm diameter. Electric heaters of 10 cm and 20 cm lengths were added on top of it to permit biomass preheating. A set of 8 type-K thermocouples was positioned around and inside the test rig, as displayed in Fig. 3, where four of the thermocouples are introduced inside the drop tube to measure local temperatures at the four levels: electric heater 1 (R1, TCK1), electric heater 2 (R2, TCK2), solar cavity (TCK5), and gas outlet (TCK8). The other four additional thermocouples (TCK3, TCK4, TCK6, and TCK7) are positioned to measure the temperature at the tube wall along the solar cavity to check for temperature distribution. The solar reactor can operate at a maximum entering power of 1.5 kW, yielding a mean irradiation flux of about 8 MW/m² at the aperture. The irradiation directly heats the reactor tube and the waste biomass particles, thereby undergoing fast pyrolysis.

The flowsheet of the experimental installation is presented in Fig. 4. It starts with a custom-made gravimetric feeder system that enables precise control of the biomass mass flow rate, and a mass flow meter to control nitrogen flow through the hopper. The hopper is manufactured

<figure><img src="image_4.png" /><figcaption>Fig. 3. 2D cut of the prototype and scheme of the tubular reaction zone.</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>Fig. 4. Flowsheet of the solar reactor installation.</figcaption></figure>

using 3D Fused Deposition Modeling, ensuring a reasonably lighter structure for the function. The material used is carbon fiber reinforced high temperature nylon (HTN + CF) or polylactic acid, depending on the mechanical requirements of the various pieces. The feeder system uses a stepper motor screw feed drive along with a high-precision balance (0.001 g) that enables providing feedback on the injected mass flow rate. Special attention was paid for the biomass flowrate control, which is the main source of uncertainty in this kind of set-up. To ensure reproducibility, the screw feeder was calibrated to deliver a constant mass flow rate of 1 g/min. The actual flow rate was subsequently verified by weighing the hopper before and after each experiment. The maximum uncertainty on the biomass flowrate was ±4%. The injected biomass and nitrogen gas then flow through the drop-tube reactor (DTR) system. The injected biomass particles undergo pyrolysis upon traversing the heated reactor tube length, where the main products of pyrolysis, gases, bio-oil, and char, are evolved. The evolved gases pass through a cartridge filter and then through a condenser, where the bio-oil condensates are recuperated. The pyrolysis gas products then flow into a gas pre-treatment system (bubbler, filter, dryer) before being analyzed by a multi-gas (H₂(0-50%), N₂(0-100%), O₂(0-10%), CO(0-50%), CO₂(0-50%), CH₄(0-50%), C₂H₂(0-10%), C₂H₄(0-10%), C₂H₆(0-10%), C₃H₆(0-10%), C₃H₈ (0-10%), and n-C₄H₁₀ (0-10%)) Laser Raman Spectrometer continuous monitoring system (reference LASER RAMAN LRGA-3100 from POLLUTEK). Precision provided by the manufacturer is ±2% of the full scale. For security reasons (in case of reactor blocking by char/ tars), a pressure relief valve is set upstream. A custom LabVIEW-based data acquisition and control system was developed to record DNI (Direct Normal Irradiance), control gas flow rates, adjust thermal resistance heating power, and operate the shutter for regulating solar flux. Set points and measurements are logged as. csv files with a 1s time step for further analysis.

The entire experimental installation is positioned at the focal point of a 1.5 kW horizontal-axis parabolic solar concentrating system, which serves as the primary source of thermal energy for the reactor. In this configuration, solar radiation is continuously redirected toward the

parabolic reflector by means of a two-axis-tracking heliostat, ensuring that the incoming rays are always optimally aligned with the reflector surface throughout the day. The concentrated sunlight is then focused precisely onto the solar cavity orifice of the reactor, as illustrated in Fig. 5, where the highest flux density is achieved. A shutter located between the heliostat and the parabola enables heating control by providing smooth regulation of the incoming power from 0 to 100% of the nominal power using movable blades, while two power supplies were adjusted to control the temperature of the electrical heated zones. To ensure stability and repeatability, experimental runs were carried out during sunny periods (no clouds) with stable heating conditions to avoid any strong disturbance.

Following the physical assembly of the installation, a comprehensive commissioning phase was undertaken to confirm that all systems and subsystems were functioning as intended. This validation process included mechanical checks, optical alignment verification, and operational tests of the heating, control, and measurement subsystems. Once the commissioning was completed and the reliability of the integrated system was established, the facility proceeded with the execution of the initial experimental campaign. The results obtained from this first round of solar-assisted pyrolysis experiments are presented in detail in the following sections and are subsequently analyzed and discussed to provide insight into the performance of the reactor under real operating conditions.

### 3.3. Experimental conditions

The first campaign study consists of four runs with varying operating conditions, including different kinds of waste biomasses, different gas residence times, and the utilization of pre-heaters. Table 1 provides a summary of the experimental runs, consisting of the flow rates of the carrier gas and biomass, the type of biomass used, along with the time average temperatures of the thermocouple sensors T1 to T8 (as positioned in Fig. 3).

The influence of 3 parameters was analyzed. The effect of 2 biomass
<figure><img src="image_6.png" /><figcaption>Fig. 5. Drop-tube reactor at the focus of the solar concentrator and solar concentrating system.</figcaption></figure>

<table><caption>**Table 1**
Experimental conditions summary.</caption><thead><tr><th rowspan="3">S. No</th><th rowspan="3">Carrier gas</th><th rowspan="3">N<sub>2</sub> Gas flow rate (NL/min)</th><th rowspan="3">N<sub>2</sub> Gas residence time (s)</th><th rowspan="3">Biomass type</th><th rowspan="3">Biomass feed rate (g/min)</th><th colspan="2">Resistance 1 (10 cm)</th><th colspan="2">Resistance 2 (20 cm)</th><th colspan="5">Solar cavity (20 cm)</th><th rowspan="3">Outlet</th></tr><tr><th colspan="8">Temperatures (°C)</th></tr><tr><th>T1</th><th>T2</th><th>T3</th><th>T4</th><th>T5</th><th>T6</th><th>T7</th><th>T8</th></tr></thead><tbody><tr><td>1</td><td>Nitrogen</td><td>0.7</td><td>5.2</td><td>Demolition wood</td><td>1</td><td>647.8</td><td>714.6</td><td>808.1</td><td>886.8</td><td>875.1</td><td>862.3</td><td>770.7</td><td>129.3</td></tr><tr><td>2</td><td>Nitrogen</td><td>1.4</td><td>2.7</td><td>Demolition wood</td><td>1</td><td>642.4</td><td>721.5</td><td>848.1</td><td>923.4</td><td>891.7</td><td>902.8</td><td>801.3</td><td>136.0</td></tr><tr><td>3</td><td>Nitrogen</td><td>1.4</td><td>1</td><td>Demolition wood</td><td>1</td><td>34.5</td><td>217.9</td><td>833.8</td><td>909.0</td><td>901.1</td><td>903.8</td><td>767.5</td><td>121.1</td></tr><tr><td>4</td><td>Nitrogen</td><td>0.7</td><td>5.5</td><td>Rye straw</td><td>1</td><td>641.3</td><td>714.8</td><td>853.6</td><td>905.6</td><td>917.1</td><td>903.5</td><td>801.6</td><td>135.8</td></tr></tbody></table>

types on the pyrolysis yields (Table 1) was compared between Run-1 with demolition wood and Run-4 with rye straw, both injected at 1 g/min (0.06 kg/h), and the carrier gas N₂ flow rate was maintained at 0.7 NL/min. The effect of the gas residence time on the pyrolysis yields (Table 1) was carried out between Run-1 and Run-2 with demolition wood injection at 1 g/min (0.06 kg/h), while the carrier gas N₂ flow rate was maintained at 0.7 NL/min and 1.4 NL/min. The influence of the preheating (Table 1) was studied between Run-2 and Run-3 with demolition wood injection at 1 g/min (0.06 kg/h) and the carrier gas N₂ flow rate at 1.4 NL/min. The preheaters R1 and R2 were maintained at 640 °C and 700 °C in Run-2, and then were deactivated for Run-3, therefore attaining an average of 34.5 °C and 218 °C at R1 and R2, respectively. The solar heating at the reactor was maintained at similar temperatures (900 °C) across all the experimental runs.

The total masses of pyrolysis gases are calculated (in mmol and g) from the gas analyzer-recorded molar fractions with N₂ as the tracer gas species, according to the following calculation procedure:

$$ \dot{V}_{tot} = \frac{\dot{V}_{N_2}}{X_{N_2}} \qquad (1) $$

$$ V_i = \int_{0}^{t} \dot{V}_i dt = \int_{0}^{t} [\dot{V}_{tot} * X_i] dt \qquad (2) $$

$$ n_i = \int_{0}^{t} \dot{n}_i dt = \int_{0}^{t} \left[ \frac{\dot{V}_i * P_0}{R * T_0} \right] dt \qquad (3) $$

The gas residence time of the carrier gas N₂ is calculated from the normalized volumetric flow rate and the reactor volume as follows,

$$ t_{gas\ residence} = \frac{V_{reactor}}{\dot{V}_{N_2} * \left(\frac{T_1}{P_1}\right) * \left(\frac{P_0}{T_0}\right)} \qquad (4) $$

The yield rates of the pyrolysis gases (in mmol/g<sub>biomass</sub>), evolved during the pyrolysis reaction, are calculated using the following equation,

$$ yield_i = \frac{\int_{0}^{t} \dot{n}_i dt}{\int_{0}^{t} \dot{m}_{bio} dt} \qquad (5) $$

# 4. Results

## 4.1. Feedstock characterization

Feedstock analytics on a dry mass basis for demolition wood and rye

<table><caption>**Table 2**
Feedstock analyses on a dry mass basis.</caption><thead><tr><th>Analysis</th><th>Rye straw</th><th>Demolition wood</th></tr></thead><tbody><tr><td>Moisture, wt%</td><td>9.7</td><td>6.6</td></tr><tr><td>Ash 550 °C, wt% dry</td><td>5.7</td><td>1.5</td></tr><tr><td>Volatiles, wt% dry</td><td>74.9</td><td>82.1</td></tr><tr><td>C, wt% dry</td><td>48.9</td><td>51.9</td></tr><tr><td>H, wt% dry</td><td>5.5</td><td>5.9</td></tr><tr><td>N, wt% dry</td><td>0.7</td><td>0.2</td></tr><tr><td>O, wt% dry</td><td>39.0</td><td>41.0</td></tr><tr><td>S, wt% dry</td><td>0.1</td><td>0.1</td></tr><tr><td>Cl, wt% dry</td><td>0.1</td><td>0.0</td></tr><tr><td>HHV, MJ/kg dry</td><td>18.8</td><td>20.3</td></tr><tr><td>LHV, MJ/kg dry</td><td>17.4</td><td>19.0</td></tr></tbody></table>
<table><caption>**Table 3**
Lignin, extractive, and polysaccharide composition of the feedstocks.</caption><thead><tr><th></th><th>Rye straw</th><th>Demolition wood</th></tr></thead><tbody><tr><td>Lignin total, wt%</td><td>24.8</td><td>31.7</td></tr><tr><td>- Acid soluble lignin, wt%</td><td>0.3</td><td>0.0</td></tr><tr><td>- Klason lignin, wt%</td><td>24.5</td><td>31.7</td></tr><tr><td>Extractives, wt%</td><td>1.4</td><td>2.0</td></tr><tr><td>Total polysaccharides, wt%</td><td>65.8</td><td>56.8</td></tr><tr><td>Monosaccharide composition</td><td></td><td></td></tr><tr><td>- Ramnose, wt%</td><td>0.2</td><td>0.2</td></tr><tr><td>- Arabinose, wt%</td><td>2.6</td><td>1.3</td></tr><tr><td>- Galactose, wt%</td><td>0.8</td><td>2.1</td></tr><tr><td>- Glucose, wt%</td><td>41.0</td><td>43.0</td></tr><tr><td>- Xylose, wt%</td><td>30.0</td><td>7.0</td></tr><tr><td>- Mannose, wt%</td><td>&lt;0.1</td><td>10.0</td></tr><tr><td>- Fructose, wt%</td><td>&lt;0.1</td><td>&lt;0.1</td></tr></tbody></table>

straw are presented in Table 2. Rye straw had a moisture content of 9.7% and contained 5.7% ash and 74.9% volatile matter on a dry basis. Its elemental composition included 48.9% carbon, 5.5% hydrogen, 0.7% nitrogen, and 39% oxygen. On a dry basis, it provided a higher heating value (HHV) of 18.8 MJ/kg and a lower heating value (LHV) of 17.4 MJ/kg. Demolition wood had a moisture content of 6.6%, with 1.5% ash and 82.1% volatile matter on a dry basis. Its composition included 51.9% carbon, 5.9% hydrogen, 0.2% nitrogen, and 41% oxygen. The corresponding HHV and LHV of demolition wood are a bit higher, 20.3 MJ/kg and 19.0 MJ/kg, respectively, likely due to the lower ash content.

More detailed chemical composition of the feeds, including lignin, extractive, and polysaccharides content, is presented in Table 3. Demolition wood contained a bit more lignin compared to straw, while straw had higher polysaccharide content. Especially, xylose content was higher in straw, which is expected as xylans are abundant hemicellulose components in straws [51]. On the other hand, mannose was higher in demolition wood, indicating the presence of softwood where manannans are more abundant hemicellulose components compared to hardwoods [52]. Extractive content was in a similar range in both feeds.

Inductively Coupled Plasma (ICP) analysis revealed distinct elemental compositions between the two investigated feedstocks, as shown in Fig. 6. Rye straw contained high levels of potassium (10894 ppm), phosphorus (1678 ppm), calcium (2616 ppm), sulfur (1197 ppm), magnesium (554 ppm), and silicon (614 ppm). In contrast, demolition wood exhibited elevated concentrations of calcium (2182 ppm), iron (290 ppm), zinc (185 ppm), copper (79 ppm), chromium (34 ppm), and sodium (177 ppm), while potassium (597 ppm) and phosphorus (78 ppm) were comparatively low.

## 4.2. Solar-assisted pyrolysis

### 4.2.1. Typical result

First of all, a typical result is detailed (Run-4) to get insights into the collected data and associated calculations. Rye straw was injected for 1.5 h. Fig. 7 presents the time evolution of solar DNI, solar power, reactor temperatures (positions described in Fig. 3), and resistance powers of R1 and R2, respectively. It is observed that a stable solar DNI and, consequently, a stable solar power and solar tube heating are achieved over the period of biomass injection. The solar power onto the experiment is controlled using the shutter opening control system (Fig. 5).

The reactor temperatures plotted in Fig. 7 indicate that a maximum temperature of 640 °C and 720 °C is attained at the T1 and T2 sensors, positioned at the preheaters R1 and R2 at 108 W and 248 W,

<figure><img src="image_7.png" /><figcaption>**Fig. 6.** Inorganic composition of the feedstocks by ICP.</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>**Fig. 7.** Evolution of solar DNI, solar power, and reactor temperatures of Run-4.</figcaption></figure>

respectively. The thermocouple T5, positioned inside the tube at the solar aperture level, attained a stable temperature of 917 °C during the biomass injection by the regulation of the solar power between 600 and 640 W thanks to the solar power adjustment with the shutters (dotted red line in Fig. 7). The temperature average of T4 and T6, positioned on the reactor tube wall close to the solar heating aperture (Fig. 3), stabilized at 904 °C, while the T3 and T7 sensors at the cavity ends attained a temperature of 850 °C and 800 °C, respectively, due to the thermal gradient inside the solar cavity. The temperature at the reactor outlet (T8) is maintained at a temperature range of ≈80 °C-130 °C in order to avoid premature condensation of the pyrolysis gases at the reactor's cartridge filter.

The results from the multi-gas Laser Raman Spectrometer continuous monitoring system are plotted in Fig. 8. The plot exhibits that the product pyrolysis gas consists of higher amounts of carbon monoxide (CO) (≈19.29 %vol), hydrogen (H₂) (≈12 %vol), and carbon dioxide (CO₂) (≈6.91 %vol). There were also relatively smaller amounts of methane (CH₄) (≈3.22 %vol), acetylene (C₂H₂) (≈0.13 %vol), ethylene (C₂H₄) (≈2.04 %vol), ethane (C₂H₆) (≈0.16 %vol), propylene (C₃H₈) (≈0.08 %vol), propane (C₃H₈) (≈0.06 %vol), and n-butane (n-C₄H₁₀) (≈0.15 %vol) present in the product gas.

The integrals over gas production time (Equation (5)) of product yield and mass composition are plotted in Fig. 9, which exhibits that out of 58.2 wt% of pyrolysis product gas (non-condensable gases) measured, CO (11.20 mmol/gbiomass), H₂ (7.09 mmol/gbiomass), and CO₂ (4.04 mmol/gbiomass) were in larger amounts (in mmol per gram of biomass
<figure><img src="image_9.png" /><figcaption>Fig. 8. Molar concentrations of the pyrolysis gases in the carrier gas (N₂) of Run-4.</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>Fig. 9. Yields of the product gases (a) and product mass composition pie-chart (b).</figcaption></figure>

injected). In addition to the gas product, about 20.4 wt% of char and 15.4 wt% of liquid condensate (condensable gases) were obtained with a 6 wt% mass loss during the recuperation process. This mass loss by recuperation is due to unrecovered mass deposits observed at the condenser, beaker, bubbler, and secondary cartridge filter. The liquid recovered had a water content beyond 70% (Karl Fischer titration) and was not analyzed further. As expected from the literature (Fig. 1), high temperature pyrolysis with short residence time is expected to favor gas production, which was effectively observed in all runs with syngas being the major product. Consequently, the subsequent parametric study mainly focuses on gas production and its composition.

### 4.2.2. Influence of waste biomass type

The influence of 2 different waste biomasses on pyrolysis product yields was studied. This section compares the Run-1 with demolition wood to the Run-4 with rye straw as waste biomass injected, whose operating conditions are listed in Table 1. For both runs, a N₂ gas flow of 0.7 NL/min was maintained, therefore yielding a gas residence time of 5.2 s and 5.5 s attained from their volumetric flow rate and the reactor volume and temperature. The controlled waste biomass feed rate was maintained at 1 g/min (0.06 kg/h) using the gravimetric feeder system. The preheaters R1 and R2 were maintained at 648 °C and 715 °C on Run-1, while R1 and R2 attained 641 °C and 715 °C on Run-4, respectively. The mole fraction of the product pyrolysis gases and their yields in mmol per gram of biomass injected are presented in Fig. 10a. Relatively higher yields were observed for the rye straw than for the demolition wood: CO

(11.20 mmol/grye straw vs 7.44 mmol/gdemolition wood), H₂ (7.09 mmol/grye straw vs 2.54 mmol/gdemolition wood), CO₂ (4.04 mmol/grye straw vs 1.18 mmol/gdemolition wood), CH₄ (1.87 mmol/grye straw vs 1.32 mmol/gdemolition wood), and CₙHₘ (1.53 mmol/grye straw vs 0.87 mmol/gdemolition wood). The mole fraction plot in Fig. 10b exhibits that the demolition wood waste biomass yielded a higher fraction of CO (0.56 in demolition wood vs 0.44 in rye straw) and CH₄ (0.10 in demolition wood vs 0.07 in rye straw). In contrast, the rye straw waste biomass contained higher fractions of H₂ (0.28 in rye straw vs 0.19 in demolition wood) and CO₂ (0.16 in rye straw vs 0.09 in demolition wood).

There are different reasons explaining a better gas production from rye straw. First, rye straw has a much higher ash content than demolition wood (5.7% vs 1.5% respectively), which can catalyze gas production, especially potassium (10894 ppm vs 597 ppm) (Fig. 6) [53]. Indeed, potassium has been widely studied in literature for its catalytic effect on pyrolysis. Trendewicz et al. [54] confirmed the specific role of potassium in increasing the yield of char and gases produced via a catalyzed dehydration reaction. Fan et al. [55] also analyzed the role of potassium and showed that potassium could favor the cleavage and reforming of carboxyl and carbonyl groups, which results in an increase in CO₂ production. This increase of CO₂ is indeed observed with rye straw as compared to demolition wood (4.04 vs 1.18 mmol/gbiomass, respectively (Fig. 10a)). Also, rye straw presents a higher hemicellulose content (mainly from xylose) while demolition wood is more lignin-rich. Hemicellulose is known to decompose earlier than cellulose and lignin, and the former is harder to decompose [56].
<figure><img src="image_11.png" /><figcaption>Fig. 10. Influence of waste biomass type on yield rate (a) and mole fraction of the product gases (b).</figcaption></figure>

### 4.2.3. Influence of gas residence time

The influence of different gas residence times on pyrolysis yields was studied. This section compares Run-1, with a 0.7 NL/min N₂ gas flow rate, to Run-2, with a 1.4 NL/min N₂ gas flow rate, whose operating conditions are listed in Table 1. For both runs, the same demolition wood at 1 g/min (0.06 kg/h) was injected while maintaining a similar tube thermal heating using the preheaters and the solar heating control system. The gas residence times of nitrogen gas at each discrete zone of the reactor are presented in Fig. 11. Upon doubling the nitrogen gas flow (1.4 NL/min) in Run-2, the gas residence time drops to a near 50% (2.7

s) of that of Run-1 with a 0.7 NL/min N₂ gas flow rate (5.2 s). The gas residence time at the preheater R1 and R2 zones was 0.6 s and 1.1 s for the Run-2 with a 1.4 NL/min nitrogen flow rate, while for the Run-1, the gas residence time at the R1 and R2 zones was 1.2 s and 2.2 s.

The mole fraction of the product pyrolysis gases and their yields in mmol per gram of biomass injected are presented in Fig. 12a. Relatively higher yields of CO (10.04 mmol/g<sub>biomass</sub> in Run-2 vs 7.44 mmol/g<sub>biomass</sub> in Run-1), H₂ (4.80 mmol/g<sub>biomass</sub> in Run-2 vs 2.54 mmol/g<sub>biomass</sub> in Run-1), CO₂ (2.57 mmol/g<sub>biomass</sub> in Run-2 vs 1.18 mmol/g<sub>biomass</sub> in Run-1), CH₄ (1.57 mmol/g<sub>biomass</sub> in Run-2 vs 1.32 mmol/g<sub>biomass</sub> in Run-1), and

<figure><img src="image_12.png" /><figcaption>Fig. 11. Gas residence time in the different zones of the tubular reactor.</figcaption></figure>
<figure><img src="image_13.png" /><figcaption>Fig. 12. Influence of gas residence time on yield (a) and mole fraction of the product gases (b).</figcaption></figure>

$C_nH_m$ (1.37 mmol/$g_{biomass}$ in Run-2 vs 0.87 mmol/$g_{biomass}$ in Run-1) were observed in the Run-2 with a shorter gas residence time. The mole fraction plot of Fig. 12b exhibits that the molar fraction compositions of the product pyrolysis gas were nearly similar in both experimental runs 1 and 2, yielding higher fractions of CO (0.56 in Run-1 vs 0.49 in Run-2), $H_2$ (0.19 in Run-1 vs 0.24 in Run-2), and $CO_2$ (0.09 in Run-1 vs 0.13 in Run-2). Hence, this confirms that a shorter gas residence time favors higher pyrolysis gas production (mainly CO, $H_2$, $C_nH_m$, $CO_2$) for a given biomass at 900 °C in accordance with trends plotted in Fig. 1. As the flow remains laminar for the tested $N_2$ flow range, the increased convective heat transfer is not expected to have a strong impact on pyrolysis kinetics. The higher gas production is more related to the fact that shorter residence times avoid any secondary reaction that could consume the primary gas production.

### 4.2.4. Influence of pre-heating

The influence of pre-heating on pyrolysis yields was finally studied. This section compares the Run-2 with pre-heating activated to the Run-3 without the pre-heating, whose operating conditions are listed in Table 1. For both runs, the same demolition wood biomass feed at 1 g/min (0.06 kg/h) and a 1.4 NL/min $N_2$ gas flow rate was injected while maintaining a similar solar cavity heating. The probe temperatures of the reactor along its length, as per their positions (Fig. 3), are presented in Fig. 13, where a sharp difference in the temperatures at T1 and T2 is observed. This is due to the temperature difference between the activation of pre-heaters R1 (at 641 °C) and R2 (at 710 °C) in Run-2 and the deactivation of the pre-heaters R1 (at 35 °C) and R2 (at 218 °C) in Run-3, respectively.

The mole fraction of the product pyrolysis gases and their yields in mmol per gram of biomass injected are presented in Fig. 14a. An overall lower yield of product gases (in mmol/$g_{biomass}$) was observed in Run-3 without pre-heating than in Run-2 with pre-heating activated. The

yields of CO (7.62 mmol/$g_{biomass}$ for Run-2 vs 4.19 mmol/$g_{biomass}$ for Run-3), $C_nH_m$ (1.02 mmol/$g_{biomass}$ for Run-2 vs 0.65 mmol/$g_{biomass}$ for Run-3), and $H_2$ (2.36 mmol/$g_{biomass}$ for Run-2 vs 1.05 mmol/$g_{biomass}$ for Run-3) were drastically reduced among the pyrolysis product gases of Run-3 with the deactivation of preheating. The mole fraction plot of Fig. 14b exhibits the evolution of gas compositions for both experimental runs 2 and 3, with CO (0.49 in Run-2 and 0.57 in Run-3) and $H_2$ (0.24 in Run-2 vs 0.18 in Run-3) being the main gases produced with the highest mole fractions. Hence, the activation of preheating in Run-2 favored higher gas production (mainly $H_2$, CO, and $C_nH_m$) due to its larger heated reactor length (50 cm in Run-2 vs. 20 cm in Run-3). This highlights that heating only 20 cm height is not sufficient to run pyrolysis because biomass is not sufficiently heated during the particle drop, and thus the pyrolysis reaction is limited.

<figure><img src="image_14.png" /><figcaption>Fig. 13. Influence of preheating on reactor temperature profile.</figcaption></figure>
<figure><img src="image_15.png" /><figcaption>Fig. 14. Influence of preheating on yield rate (a) and mole fraction of the product gases (b).</figcaption></figure>

**5. Conclusion**

The main innovative assets of the developed solar assisted quartz drop tube reactor are: (i) the continuous feeding of the biomass feedstock particles (ii) the avoidance of a quartz window, favoring reactor scale up and reducing costs, (iii) the simultaneous online monitoring of a wide variety of gaseous species with a dedicated gas analysis system based on laser Raman spectroscopy, (iv) the combined solar/electrical heating for reactor hybridization. It was tested for two biomass wastes: rye straw and demolition wood. Both biomasses were extensively characterized to share valuable data about their composition. Solar pyrolysis experiments were carried out in the high temperature range (800-900 °C), and syngas production was promoted. In particular, direct solar heating experiments highlighted the critical roles played by operational parameters such as the specific type of waste biomass used as feedstock, the carrier gas flow rate (impacting the gas residence time), and the thermal heating profile provided along the pyrolysis tube. By systematically varying these parameters, the study was able to shed light on how each factor, individually, influences the conversion pathways and final products distributions. The use of rye straw was associated with enhanced gas production as compared to demolition wood (CO: 11.20 mmol/g<sub>rye straw</sub> vs 7.44 mmol/g<sub>demolition wood</sub>, H<sub>2</sub>: 7.09 mmol/g<sub>rye straw</sub> vs 2.54 mmol/g<sub>demolition wood</sub>), underscoring the importance of biomass composition, in particular ash potassium content (10894 ppm vs 597 ppm), in dictating pyrolysis behavior. Similarly, the gas yield was favored by shorter gas residence times (CO: 10.04 mmol/g<sub>biomass</sub> vs 7.44 mmol/g<sub>biomass</sub>, H<sub>2</sub>: 4.80 mmol/g<sub>biomass</sub> vs 2.54 mmol/g<sub>biomass</sub> for residence times of 2.7 s and 5.2 s, respectively). Another significant observation was the marked improvement in gas yields when the reactor was operated with pre-heaters (CO: 7.62 mmol/g<sub>biomass</sub> vs 4.19 mmol/g<sub>biomass</sub> and H<sub>2</sub>: 2.36 mmol/g<sub>biomass</sub> vs 1.05 mmol/g<sub>biomass</sub> with and without preheating, respectively). Indeed, biomass preheating proved to be necessary, so that biomass particles can reach sufficiently high temperatures for the pyrolysis reaction. In further work, indirect solar heating could be explored by replacing the quartz tube with an opaque

tube. This would indicate whether the heating mode (direct vs indirect) could bring different chemical behaviors. Pyrolysis is a versatile process that can be tuned for various applications by adjusting the operating parameters, particularly the heating rate and residence time. Consequently, different products can be targeted, ranging from bio-oil to high-value syngas. The present solar process is especially well suited for operation at high heating rates, favoring bio-oil production at moderate temperatures (~500 °C) and syngas production at higher temperatures (~1000 °C). Syngas can be further upgraded into other fuels (e.g., hydrogen or methanol) through processes such as the water-gas shift reaction or Fischer-Tropsch synthesis, or it can be used directly in gas turbines. Bio-oil can also be up-graded into liquid drop-in biofuels, including Sustainable Aviation Fuels (SAF).

**CRediT authorship contribution statement**

**Vignesvar Krish Subramani:** Writing – review & editing, Writing – original draft, Visualization, Software, Resources, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. **Sylvain Rodat:** Writing – review & editing, Writing – original draft, Visualization, Validation, Supervision, Resources, Project administration, Methodology, Investigation, Funding acquisition, Formal analysis, Data curation, Conceptualization. **Elmeri Pienihäkkinen:** Writing – review & editing, Writing – original draft, Investigation, Data curation. **Pouya Sirous-Rezaei:** Writing – review & editing, Writing – original draft, Investigation, Data curation. **Stéphane Abanades:** Writing – review & editing, Writing – original draft, Visualization, Validation, Supervision, Project administration, Methodology, Investigation, Formal analysis.

**Funding sources**

This work was supported by the European Union through the Horizon project Circular Fuels (ID: 101118239).
Declaration of competing interests

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

The authors express their sincere gratitude to Régis Rodriguez,

Emmanuel Guillot, Roger Garcia, Ilya Heintz, Jean-Jacques Huc, Emmanuel Hernandez, and Michaël Tessonneaud for their support in the conception, instrumentation, and operation of the solar reactor and solar facility. The solar drop tube reactor could be open to collaborative experiments involving other types of biomass worldwide.

Nomenclature

$\dot{m}_{bio}$ Mass flow rate of biomass (g/s)

$n_i$ Number of moles of the gas species (i) (mol)

$\dot{n}_i$ Molar flow rate of the gas species (i) (mol/s)

$P_1$ Volume-averaged static pressure of the gas at the reactor (Pa)

$P_0$ Normal pressure of the gas in the reactor (101325 Pa)

$R$ Universal gas constant (8.314 J/mol.K)

$T_0$ Normal temperature of the gas at the reactor (273 K)

$T_1$ Volume-averaged static temperature of the gas at the reactor (K)

$t$ Time of injection of biomass

$\dot{V}_i$ Volumetric flow rate of pyrolysis gas species (i) (Nm³/s)

$\dot{V}_{N_2}$ Volumetric flow rate of the carrier gas N₂ (Nm³/s)

$V_{reactor}$ Volume of the heated reactor zone (cylindrical) (m³)

$\dot{V}_{tot}$ Total volumetric flow rate of the N₂ injected (Nm³/s)

$X_i$ Molar fraction of the gas species (i) as measured by the gas analyser

$X_{N_2}$ Molar fraction of the N₂ gas as measured by the gas analyser

Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.biombioe.2026.109071.

Data availability

Data will be made available on request.

References

[1] W.J. Ripple, et al., The 2024 state of the climate report: perilous times on planet Earth, Bioscience 74 (12) (Dec. 2024) 812–824, https://doi.org/10.1093/biosci/biae087.

[2] K. Calvin, et al., IPCC, 2023: climate change 2023: synthesis Report, in: H. Lee, J. Romero (Eds.), Contribution of Working Groups I, II and III to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change [Core Writing Team, IPCC, Intergovernmental Panel on Climate Change (IPCC), Geneva, Switzerland, Jul. 2023, https://doi.org/10.59327/IPCC/AR6-9789291691647.

[3] European Environment Agency, Decarbonising Road Transport: the Role of Vehicles, Fuels and Transport Demand, Publications Office, 2022, LU, https://data.europa.eu/doi/10.2800/68902. (Accessed 13 March 2025).

[4] European Union Aviation Safety Agency, European environment agency. European Aviation Environmental Report 2022, LU: Publications Office, 2023. https://data.europa.eu/doi/10.2822/04357. (Accessed 13 March 2025).

[5] H. Long, X. Li, H. Wang, J. Jia, Biomass resources and their bioenergy potential estimation: a review, Renew. Sustain. Energy Rev. 26 (Oct. 2013) 344–352, https://doi.org/10.1016/j.rser.2013.05.035.

[6] E.M. Rubin, Genomics of cellulosic biofuels, Nature 454 (7206) (Aug. 2008) 841–845, https://doi.org/10.1038/nature07190.

[7] M.J. Realff, Biorenewable resources: engineering new products from agriculture, by Robert C. Brown, J. Ind. Ecol. 7 (3–4) (Jul. 2003) 227–228, https://doi.org/10.1162/108819803323059541.

[8] I. International Energy Agency, Renewables 2024, IEA, Paris, 2024. Licence: CC BY 4.0, https://www.iea.org/reports/renewables-2024.

[9] E. Kwietniewska, J. Tys, Process characteristics, inhibition factors and methane yields of anaerobic digestion process, with particular focus on microalgal biomass fermentation, Renew. Sustain. Energy Rev. 34 (Jun. 2014) 491–500, https://doi.org/10.1016/j.rser.2014.03.041.

[10] A. Limayem, S.C. Ricke, Lignocellulosic biomass for bioethanol production: current perspectives, potential issues and future prospects, Prog. Energy Combust. Sci. 38 (4) (Aug. 2012) 449–467, https://doi.org/10.1016/j.pecs.2012.03.002.

[11] H.B. Goyal, D. Seal, R.C. Saxena, Bio-fuels from thermochemical conversion of renewable resources: a review, Renew. Sustain. Energy Rev. 12 (2) (Feb. 2008) 504–517, https://doi.org/10.1016/j.rser.2006.07.014.

[12] D.C. Elliott, Hydrothermal processing, in: R.C. Brown (Ed.), Thermochemical Processing of Biomass, first ed., Wiley, 2011, pp. 200–231, https://doi.org/10.1002/9781119990840.ch7.

[13] S. Wang, G. Dai, H. Yang, Z. Luo, Lignocellulosic biomass pyrolysis mechanism: a state-of-the-art review, Prog. Energy Combust. Sci. 62 (Sep. 2017) 33–86, https://doi.org/10.1016/j.pecs.2017.05.004.

[14] B.J. Alvarez-Chavez, S. Godbout, J.H. Palacios-Rios, É. Le Roux, V. Raghavan, Physical, chemical, thermal and biological pre-treatment technologies in fast pyrolysis to maximize bio-oil quality: a critical review, Biomass Bioenergy 128 (Sep. 2019) 105333, https://doi.org/10.1016/j.biombioe.2019.105333.

[15] T.Y.A. Fahmy, Y. Fahmy, F. Mobarak, M. El-Sakhawy, R.E. Abou-Zeid, Biomass pyrolysis: past, present, and future, Environ. Dev. Sustain. 22 (1) (Jan. 2020) 17–32, https://doi.org/10.1007/s10668-018-0200-5.

[16] M.S.D. Jesus, C.L.M. Martinez, L.J. Costa, E.G. Pereira, A.C.O.D. Carneiro, Thermal conversion of biomass: a comparative review of different pyrolysis processes, RCM (Rapid Commun. Mass Spectrom.) 11 (1) (Mar. 2020) 12–22, https://doi.org/10.12953/2177-6830/rcm.v11n1p12-22.

[17] A.K. Vuppaladadiyam, et al., Biomass pyrolysis: a review on recent advancements and green hydrogen production, Biore sour. Technol. 364 (Nov. 2022) 128087, https://doi.org/10.1016/j.biortech.2022.128087.

[18] W. Jerzak, A. Mlonka-Medrala, N. Gao, A. Magdziarz, Potential of products from high-temperature pyrolysis of biomass and refuse-derived fuel pellets, Biomass Bioenergy 183 (Apr. 2024) 107159, https://doi.org/10.1016/j.biombioe.2024.107159.

[19] X. Deglise, A. Donnot, Bois énergie. La Construction Responsable, Jul. 2004, https://doi.org/10.51257/a-v1-be8535.

[20] M.U.H. Joardder, P.K. Halder, A. Rahim, N. Paul, Solar assisted fast pyrolysis: a novel approach of renewable energy production, J. Eng. 2014 (2014) 1–9, https://doi.org/10.1155/2014/252848.

[21] K. Zeng, D. Gauthier, J. Soria, G. Mazza, G. Flamant, Solar pyrolysis of carbonaceous feedstocks: a review, Sol. Energy 156 (Nov. 2017) 73–92, https://doi.org/10.1016/j.solener.2017.05.033.

[22] Y. Makkawi, M. Ibrahim, N. Yasir, O. Moussa, Solar-thermal conversion of biomass: principles of solar concentrators/reactors, reported studies, and prospects for large-scale implementation, Fuel Process. Technol. 264 (Nov. 2024) 108139, https://doi.org/10.1016/j.fuproc.2024.108139.
[23] M. Ringer, V. Putsche, J. Scahill, Large-Scale Pyrolysis Oil Production: a Technology Assessment and Economic Analysis, National Renewable Energy Laboratory (NREL), Golden, CO, Nov 2006, https://doi.org/10.2172/894989. US, NREL/TP-510-37779.

[24] E. Alonso, M. Romero, Review of experimental investigation on directly irradiated particles solar reactors, Renew. Sustain. Energy Rev. 41 (Jan. 2015) 53–67, https://doi.org/10.1016/j.rser.2014.08.027.

[25] G. Zsembinszki, A. Solé, C. Barreneche, C. Prieto, A.I. Fernández, L.F. Cabeza, Review of reactors with potential use in thermochemical energy storage in concentrated solar power plants, Energies 11 (9) (Sep. 2018) 2358, https://doi.org/10.3390/en11092358.

[26] C. Wieckert, R. Palumbo, U. Frommherz, A two-cavity reactor for solar chemical processes: heat transfer model and application to carbothermic reduction of ZnO, Energy 29 (5–6) (Apr. 2004) 771–787, https://doi.org/10.1016/S0360-5442(03)00183-X.

[27] M. Kruesi, Z.R. Jovanovic, E.C. Dos Santos, H.C. Yoon, A. Steinfeld, Solar-driven steam-based gasification of sugarcane bagasse in a combined drop-tube and fixed-bed reactor – thermodynamic, kinetic, and experimental analyses, Biomass Bioenergy 52 (May 2013) 173–183, https://doi.org/10.1016/j.biombioe.2013.03.003.

[28] J. Lédé, Pyrolysis and gasification of biomass in solar and simulated solar environments: the pioneering works of Michael J. Antal in the period of 1976–1989, Energy Fuels 30 (10) (Oct. 2016) 7896–7903, https://doi.org/10.1021/acs.energyfuels.6b00887.

[29] M.J. Antal Jr., M. Rodot, C. Royere, A. Vialaron, Solar Flash Pyrolysis of Biomass, vol. 1, 1979, pp. 159–163. https://ui.adsabs.harvard.edu/abs/1979sun2.conf..159A. (Accessed 13 March 2025).

[30] M.J. Antal, L. Hofmann, JoséR. Moreira, C.T. Brown, R. Steenblik, Design and operation of a solar fired biomass flash pyrolysis reactor, Sol. Energy 30 (4) (1983) 299–312, https://doi.org/10.1016/0038-092X(83)90185-8.

[31] Q. Bellouard, S. Abanades, S. Rodat, N. Dupassieux, A High Temperature drop-tube and packed-bed Solar Reactor for Continuous Biomass Gasification, presented at the SOLARPACES 2016: International Conference on Concentrating Solar Power and Chemical Energy Systems, Abu Dhabi, United Arab Emirates, 2017 100001, https://doi.org/10.1063/1.4984458.

[32] M. Millhé, L. Van De Steene, M. Haube, J.-M. Commandré, W.-F. Fassinou, G. Flamant, Autothermal and allothermal pyrolysis in a continuous fixed bed reactor, J. Anal. Appl. Pyrolysis 103 (Sep. 2013) 102–111, https://doi.org/10.1016/j.jaap.2013.03.011.

[33] I. Ermanoski, Moving Bed Reactor for Solar Thermochemical Fuel Production (No. 8,420,032), 2013. US8420032B1.

[34] T. Kodama, N. Gokon, S. Enomoto, S. Itoh, T. Hatamachi, Coal coke gasification in a windowed solar chemical reactor for beam-down optics, J. Sol. Energy Eng. 132 (4) (Nov. 2010) 041004, https://doi.org/10.1115/1.4002081.

[35] T. Kodama, Y. Kondoh, T. Tamagawa, A. Funatoh, K.-I. Shimizu, Y. Kitayama, Fluidized bed coal gasification with CO₂ under direct irradiation with concentrated visible light, Energy Fuels 16 (5) (Sep. 2002) 1264–1270, https://doi.org/10.1021/ef020053x.

[36] N. Gokon, R. Ono, T. Hatamachi, L. Liuyun, H.-J. Kim, T. Kodama, CO₂ gasification of coal cokes using internally circulating fluidized bed reactor by concentrated Xe-light irradiation for solar gasification, Int. J. Hydrogen Energy 37 (17) (Sep. 2012) 12128–12137, https://doi.org/10.1016/j.ijhydene.2012.05.133.

[37] C. Guizani, S. Valin, J. Billaud, M. Peyrot, S. Salvador, Biomass fast pyrolysis in a drop tube reactor for bio oil production: experiments and modeling, Fuel 207 (Nov. 2017) 71–84, https://doi.org/10.1016/j.fuel.2017.06.068.

[38] A. Bieniek, W. Jerzak, M. Gajek, A. Magdziarz, Numerical investigations of biomass pyrolysis with partial oxidation in a drop tube reactor, J. Clean. Prod. 401 (May 2023) 136774, https://doi.org/10.1016/j.jclepro.2023.136774.

[39] S. Septien, S. Valin, M. Peyrot, C. Dupont, S. Salvador, Characterization of char and soot from millimetric wood particles pyrolysis in a drop tube reactor between 800°C and 1400°C, Fuel 121 (Apr. 2014) 216–224, https://doi.org/10.1016/j.fuel.2013.12.026.

[40] C. Ndibe, J. Maier, G. Scheffknecht, Combustion, cofiring and emissions characteristics of torrefied biomass in a drop tube reactor, Biomass Bioenergy 79 (Aug. 2015) 105–115, https://doi.org/10.1016/j.biombioe.2015.05.010.

[41] D. Hirsch, Solar hydrogen production by thermal decomposition of natural gas using a vortex-flow reactor, Int. J. Hydrogen Energy 29 (1) (Jan. 2004) 47–55, https://doi.org/10.1016/S0360-3199(03)00048-X.

[42] A. Zgraggen, P. Haueter, D. Trommer, M. Romero, J. Dejesus, A. Steinfeld, Hydrogen production by steam-gasification of petroleum coke using concentrated solar power—Ii Reactor design, testing, and modeling, Int. J. Hydrogen Energy 31 (6) (May 2006) 797–811, https://doi.org/10.1016/j.ijhydene.2005.06.011.

[43] F. Müller, et al., A pressurized high-flux solar reactor for the efficient thermochemical gasification of carbonaceous feedstock, Fuel 193 (Apr. 2017) 432–443, https://doi.org/10.1016/j.fuel.2016.12.036.

[44] Circular fuels. https://circularfuels.eu.

[45] Y. Solantausta, et al., Bio-oil production from biomass: steps toward demonstration, Energy Fuels 26 (1) (Jan. 2012) 233–240, https://doi.org/10.1021/ef201109t.

[46] A. Oasmaa, J. Lehto, Y. Solantausta, S. Kallio, Historical review on VTT fast pyrolysis bio-oil production and upgrading, Energy Fuels 35 (7) (Apr. 2021) 5683–5695, https://doi.org/10.1021/acs.energyfuels.1c00177.

[47] V. Krish Subramani, S. Rodat, S. Abanades, Waste biomass fast pyrolysis in a drop-tube reactor using concentrated solar power: circular FUELS - EU PROJECT, SolarPACES Conf Proc 3 (Nov) (2025), https://doi.org/10.5282/solarpaces.v3i2359.

[48] A. Sluiter, et al., Determination of structural carbohydrates and lignin in biomass, in: Laboratory Analytical Procedure (LAP), National Renewable Energy Laboratory, Jan. 2008.

[49] S. Willför, et al., Carbohydrate analysis of plant materials with uronic acid-containing polysaccharides—A comparison between different hydrolysis and subsequent chromatographic analytical techniques, Ind. Crop. Prod. 29 (2–3) (Mar. 2009) 571–580, https://doi.org/10.1016/j.indcrop.2008.11.003.

[50] C. Ludwig, K. Sarkanen, O. Goldschimid, “Ultraviolet Spectra,” *Lignins: Occurrence, Formation, Structure and Reactions*, 1971, pp. 241–298.

[51] E. Pienihäkkinen, C. Lindfors, T. Ohra-aho, A. Oasmaa, Improving fast pyrolysis bio-oil yield and quality by Alkali removal from feedstock, Energy Fuels 36 (7) (Apr. 2022) 3654–3664, https://doi.org/10.1021/acs.energyfuels.1c04331.

[52] D. Fengel, G. Wegener, Wood: Chemistry, Ultrastructure, Reactions, Walter de Gruyter, 2011.

[53] A.V. Bridgwater, Review of fast pyrolysis of biomass and product upgrading, Biomass Bioenergy 38 (Mar. 2012) 68–94, https://doi.org/10.1016/j.biombioe.2011.01.048.

[54] A. Trendewicz, R. Evans, A. Dutta, R. Sykes, D. Carpenter, R. Braun, Evaluating the effect of potassium on cellulose pyrolysis reaction kinetics, Biomass and bioenergy 74 (2015) 15–25.

[55] H. Fan, J. Gu, Y. Wang, H. Yuan, Y. Chen, B. Luo, Effect of potassium on the pyrolysis of biomass components: pyrolysis behaviors, product distribution and kinetic characteristics, Waste Management 121 (2021) 255–264.

[56] H. Yang, R. Yan, H. Chen, D.H. Lee, C. Zheng, Characteristics of hemicellulose, cellulose and lignin pyrolysis, Fuel 86 (12–13) (Aug. 2007) 1781–1788, https://doi.org/10.1016/j.fuel.2006.12.013.