Chiang Mai Journal of Science

Enhancing Solid Fuel Properties of Sawdust Torrefaction using a Rotary Drum Reactor

1. INTRODUCTION

Biomass is a significant element of energy and synthetic fuels production. However, its intrinsic toughness makes it difficult to grind effectively for use in boilers or high temperature gasifiers in power and heating plants. Torrefaction is an emerging key technique to improve biomass properties and make it an environmentally friendly energy source with coal-like properties. This method is characterised by a moderate type of pyrolysis in which the biomass is heated to temperatures between 200 °C and 300 °C under air pressure in an inert atmosphere and this improves its physical, chemical and biological properties for better use in co-firing and gasification. In Thailand, sawdust is a common waste of biomass, particularly in the north, where lignite coal is the main solid fossil fuel. Both materials are promising candidates for optimal primary energy sources to gradually replace depleting fossil fuels to achieve the targets of sustainable and renewable syngas production [1-2]. Rubberwood sawdust is an abundant by-product of the wood-processing industry. Although it has strong potential as a renewable solid fuel, it remains largely underutilized. Therefore, using this sawdust as a feedstock for torrefaction can effectively transform an industrial residue into a higher-value biofuel. Torrefied biomass is water-resistant, less prone to decay, and easier to store. It gains in uniformity and energy value but loses in hardness, which makes it easier to grind and to use in industry. This suggests that torrefied biomass can sometimes substitute fossil fuels. The changes in the biomass characteristics are dependent on the torrefaction severity, which is determined by the temperature and time of the process. Severity is often expressed as the mass loss occurring during torrefaction. More heat or a longer time will result in more mass loss [3].

A thorough review of recent studies on the torrefaction of sawdust for biofuel production underscores the significance of this process in enhancing biomass fuel characteristics, offering valuable information on optimization methods and the possibility of sustainable energy alternatives. There is a consensus in the literature review that torrefaction has advantages in enhancing the calorific value, density and overall fuel quality of sawdust from different species of wood. Biomass fuel properties have been optimized in several studies. The aim of this study was to investigate the thermal treatment of torrefaction to improve the calorific value and density of sawdust biomass using four different wood species. The study revealed that the torrefaction of the sawdust at different time intervals and high temperature also significantly enhanced the fuel properties of the sawdust which can be a competitive alternative to the conventional fossil fuels [4]. The authors concluded that torrefied sawdust pellets are an environmentally friendly, cost-effective and efficient source of energy with potential market advantages over untreated biomass and traditional fossil fuels. The effect of torrefaction and pelletising on the design parameters of a fixed-bed gasifier was considered by Luo et al. (2020) in an experimental study of raw and torrefied wood gasification. They provide detailed experimental results on the pyrolysis and CO₂ gasification of raw and torrefied wood pellets. A numerical model was developed to predict the devolatilization times of thermally thick particles, and the predictions were compared with experimental results to improve the design and efficiency of biomass gasification processes[5]. Pio et al. (2026) investigated the integration of torrefaction and briquetting processes to convert eucalyptus sawdust into high-quality solid biofuel. The feedstock used was eucalyptus sawdust, which was torrefied in a fixed-bed reactor under an inert nitrogen atmosphere at temperatures ranging from 200 to 300 °C for 30 minutes, followed by mechanical densification into briquettes at 18 MPa and 120 °C [6] Michal Šafářet al. (2025) explored the relationship between thermal energy and the torrefaction severity index (TSI) of spruce wood sawdust. The feedstock used was finely ground spruce sawdust (<0.125 mm), and torrefaction was conducted using a thermogravimetric analyzer under an inert nitrogen atmosphere. The experiments were carried out at temperatures of 200, 250, and 300 °C with residence times ranging from 10 to 60 minutes. Advanced analytical techniques, including thermogravimetric analysis (TGA) and TD-GC/MS, were employed, along with machine learning models (ANN, C&RT, SVM, KNN) to analyze and predict process behavior[7].

The study demonstrated that increasing torrefaction temperature and residence time significantly enhances the torrefaction severity index (TSI), while reducing mass yield and improving energy density. Strong correlations were identified between TSI, higher heating value (HHV), and elemental composition (C, H, O). Machine learning models, particularly artificial neural networks (ANN), showed excellent predictive accuracy (R² ≈ 0.9998), enabling precise estimation of TSI under varying conditions. These findings highlight the importance of combining experimental analysis with machine learning to optimize biomass torrefaction processes, improve fuel quality, and support scalable, efficient bioenergy production[8]. Biomass torrefaction technology has been pointed out as a possibility of producing solid fuels with better quality and less environmental impact. This technology is especially good for bioenergy production and for environmental sustainability [9-11]. The physiological and chemical properties of White Spruce Sawdust (WSS) for bio-fuel production was studied and it was observed that there was a significant loss of mass due to the decomposition of hemicellulose but improvements were seen in the heating value, hydrophobicity, bulk particle density and pellet density. In the same way, the eucalyptus sawdust was studied to improve its energetic properties for combustion and biomass valorization[12]. Their study evaluated the effects of torrefaction at different temperature and residence time conditions based on the improvements of hygroscopicity, volatile matter, fixed carbon contents and the heating value of the biomass. The research showed that torrefaction greatly reduces the moisture affinity of the biomass and increases its energy density, thus it can be a potential intermediate between raw biomass and charcoal for energy applications [13-14]. The effect of different moisture contents and binder ratios on the physical and mechanical properties of raw and torrefied pellets was investigated. The study showed that torrefaction can significantly improve the higher heating values and ash contents of the pellets, thus enhancing their potential as biofuels.

Moreover, Mohamed et al. (2023) investigated the kinetics of isothermal torrefaction of sawdust. The sawdust was torrefied in a fixed bed reactor with nitrogen flow and the Coats–Redfern method was applied. The changes of functional groups were analysed by FTIR spectroscopy. The feedstock used was raw sawdust. Torrefaction was carried out in a fixed-bed reactor under inert nitrogen atmosphere at a temperature between 200-300 °C for a holding time of 20-60 minutes under isothermal conditions. The study showed that the mass yield decreased with the increase in the torrefaction temperature and residence time by the decomposition of hemicellulose and partial cellulose fractions. The kinetic analysis revealed that the activation energy decreased from 48.21 kJ/mol to 41.77 kJ/mol with the increase in holding time, indicating the enhanced reactivity at longer torrefaction durations. FTIR analysis showed the decrease in the functional groups like O–H, C–H, and C=O with an increase in severity. The results show that torrefaction enhances the reactivity of biomass and modifies its chemical structure, confirming its potential as an upgraded solid fuel, and providing important kinetic parameters for the optimization of the process [15]. Their experimental study was to optimize torrefaction parameters for improving fuel properties of biomass. They found that optimal torrefaction conditions are different for wood and sewage sludge, with large enhancements in energy content for wood at particular temperatures and times, while torrefaction of sewage sludge was considered to be energetically unjustified. Previous studies have demonstrated that torrefaction can improve the fuel properties of various biomass resources. However, most torrefaction experiments have been performed using electrically heated laboratory-scale systems, such as fixed-bed reactors, tubular reactors, or electrical furnaces [16-17]. Although well controlled, these systems do not fully represent industrial operation, where heat is typically supplied by fuel combustion or process heat. A rotary drum reactor has the potential to improve product uniformity compared with electrically heated stationary reactors. This study may provide preliminary data for future pilot-scale development of torrefaction systems using practical heat sources, such as biomass residue fuels or hot flue gas from combustion processes.

This research investigates the physicochemical mechanisms of sawdust torrefaction using a rotary drum powered by LPG. This research clarified how torrefaction parameters influence the efficiency of biomass conversion into a superior bioenergy, thereby reinforcing its potential as a high-quality solid biofuel in sustainable environments and energy systems.

2. MATERIALS AND METHODS

2.1 Materials
The raw material used in this study was rubberwood sawdust obtained from a rubberwood processing factory in Thailand. All feedstock samples were firstly air dried and then ground to 1-3 mm. Then, the power samples were dried in an oven at 105 °C for 24 hrs and finally stored in jars at room temperature. The raw biomass samples were dried to minimize their initial moisture content and achieve a uniform level. Excess moisture in untreated biomass decreases thermal conversion efficiency [31]. Using biomass with a uniform moisture level standardizes the physical and chemical properties of all samples before the torrefaction process. The biomass was characterized by calorific value, proximate analysis, and elemental analysis. The elemental composition analysis of raw sawdust was performed using ASTM D5373 (for C, H, and N) and ASTM D3176 (for O, using the difference method) standards. The results showed carbon, hydrogen, nitrogen, and oxygen content as follows: 41.86, 6.90, 0.00, and 48.61, respectively.

2.2 Methods
2.2.1 Torrefaction process
Torrefaction reactors can generally be categorized according to their design and operational method. The selection of a suitable reactor depends on factors such as the intended production scale, the biomass type, and the required quality of the final product. The torrefaction reactor in this work is a rotary drum reactor, which utilizes a rotating drum to carry out the torrefaction process. These reactors are known for their durability and are well-suited for processing mixed or varied biomass feedstocks. A rotary drum reactor features a cylindrical drum that rotates slowly, causing the biomass to tumble and move gradually through the system. The drum is heated from the outside, and the residence time is adjusted by modifying the drum’s rotation speed and length. Horizontal rotating torrefaction reactor using standard seamless stainless steel pipe SUS304 sch10S nominal size DN150 (6 in.), reactor capacity 16 L (length 80 cm) and installed internal sweep blades along the length of the reactor, using nitrogen as a carrier gas throughout the torrefaction process, and nitrogen flow rate into the reactor 1 L/min. The torrefaction reactor rotation set uses an electric motor with a gear reducer set and a VSD set to adjust the rotation speed of the reactor at 3-5 rpm. The LPG fuel combustion system for heating the reactor is positioned at the bottom along the length of the torrefaction reactor to transfer heat from the exhaust gas of combustion to the torrefaction reactor while it rotates.

2.2.2 Characteristics of raw material and torrefied biomass
TG measurements were performed for thermal degradation in an N₂ environment. Biomass degradation procedures for TG measurements were carried out under atmospheric pressure and at temperatures ranging from 25 to 900°C at a rate of 15°C/min. The proximate analysis was performed following ASTM to classify the sample in terms of moisture (ASTM E871), volatile matter (ASTM E872), fixed carbon (ASTM E872), and ash (ASTM D1102/D3174-04)[19]. The fixed carbon content was calculated by difference. The surface characteristics and microstructure of the torrefied sawdust were analyzed using scanning electron microscopy (SEM; JEOL models JSM-5410 and JSM-6301F). SEM images were taken under 15 kV acceleration with magnifications ranging from 1000x to 5000x to examine the structure of the torrefied product. Fourier transform infrared (FTIR) spectra of the torrefied sawdust samples were obtained by a Perkin Elmer Spectrum100 spectrometer in transmittance mode, with scanning frequencies from 4000 to 600 cm^-1 wave number range to determine their functional groups. A bomb calorimeter (IKA-C6000) was used to determine the calorific value of the sawdust samples and torrefied sawdust. The quantity of heat released upon full combustion is used to calculate the calorific value. The torrefied product exhibits a high calorific value (HHV) (MJ/kg), which is the benchmark for lignite A (14.7–19.3 MJ/kg) as defined by ASTM D388 [20].

3. RESULTS AND DISCUSSION

3.1 Characteristics of Sawdust
The sawdust sample used in this study was from the industry in Thailand. Properties of sawdust feedstock compared with other works are summarized in Table 1. The ash content was lower compared to the values in the literature (15.48–23.55 %). Furthermore, the volatile matter (VM) to fixed carbon (FC) ratio was higher in this sample, which can lead to larger torrefied yields.

Moreover, the thermal behavior and the thermal properties of the sawdust were studied by a thermogravimetric/differential thermal gravimetric analysis (TG/DTG). Figure 2 shows the thermal gravimetric of the sawdust curve, and the sawdust thermal decomposition was in the range of 100–450 °C. The relatively wide degradation range, followed by a slight drop at 110°C, revealed that the sawdust contained a small water content. The relative degradation range shows a rapid drop at 250-450 °C, which represents the amount of volatile organic compounds.

The thermal behaviors of biomass and its derivatives, providing valuable insights for optimizing biofuel production processes at various temperatures affect the physicochemical properties, demonstrating that torrefaction significantly improves the higher heating value, grindability, and combustion properties of the material [23].

3.2 Physicochemical of Torrefied Sawdust
Scanning electron microscopy (SEM) is technique that provides the morphology of samples. Microstructure was characterized by scanning electron microscope (SEM; JEOL, JSM-5410 and JSM-6301F). The SEM images of the torrefied sawdust are shown in Figure 3. It can be seen that all samples present aggregates of variable morphology and size. The main particles appeared as an agglomeration of particles to the size of less than 2.5 µm.

The percentage weight (wt.%) of volatile matter (VM) and ash were measured using an ASTM standard with the unit on a dry basis (db) for both raw material and torrefied samples. At the same time, fixed carbon (FC) content was determined by difference. A proximate analysis of sawdust and torrefied sawdust is displayed in Table 2.

Characterization of the biochar composites using FTIR spectrophotometry gave spectra as reported in Figure 3. In the infrared spectra of sawdust, two important regions were observed in which bands corresponding to organic functional groups above ~1100 cm^-1 are found, as well as bands corresponding to inorganic functional groups below 1100 cm^-1.  Guzmán et al. (2015) noted that the position and the type of band suggest that the inorganic functional groups are found below 1100 cm^-1, it related to the SiO₂. Meanwhile, the band at 3200-3500 cm^-1 is observed in the sawdust and is attributed to the vibrations of the hydroxyl groups O-H which are related to the function of H₂O. As the torrefaction temperatures are increased, the bands associated with the organic part begin to appear (T > 250 °C).

3.3 Energy Yield of Torrefied Sawdust
Temperature and residence time significantly affect the characteristics of torrefied biomass. As the temperature increases, the mass yield fraction decreases. Similarly, a longer retention time also leads to a reduction in mass yield. However, temperature has a greater impact on mass yield compared to retention time [27]. The mass yield of biomass samples obtained from torrefaction processes was measured by weighing the samples before and after the torrefaction process [28]. Mass yield of torrefied sawdust was in the range of 65.20 - 82.35%. In torrefaction, the primary structural elements in torrefaction that cause weight loss are cellulose and hemicellulose. Under very mild circumstances for these processes, lignin, the most thermally stable component, hardly participates in conversions[10]. The heating value of torrefied sawdust was in the range of 17.33 -21.13 MJ/kg as shown in Figure 6(a). It was found that the torrefied process time did not have a significant effect because the heating value of all three temperatures at 30 min and 45 min increased by less than 5 percent. Therefore, it can be concluded that when the torrefied time was increased from 30 min to 45 min, it did not have a significant effect on the heating value.

One of the key advantages of torrefaction is the increased energy density and improved quality of the resulting material. The energy yield from torrefaction depends on both the process temperature and duration, with higher temperatures and longer durations generally leading to greater energy yields and improved material quality. Two critical indicators for assessing torrefaction efficiency are mass yield and energy yield [12]. As defined by Escribano-Uriarte, B., et al. (2024) mass yield refers to the ratio of dry torrefied biomass to dry untreated biomass (see Eq. 1), while energy density is calculated as the ratio of the higher heating value (HHV) of the torrefied product to that of the raw biomass (see Eq. 2)[3].

Mass yield (MY, %) = $\frac{{\mathrm{m}}_{\mathrm{tor}}}{{\mathrm{m}}_{\mathrm{raw}}}$ x 100 (1)

Energy density ratio (ED) = $\frac{{\mathrm{HHV}}_{\mathrm{tor}}}{{\mathrm{HHV}}_{\mathrm{raw}}}$ (2)

Energy yield (EY, %) = $\frac{{\mathrm{HHV}}_{\mathrm{tor}}}{{\mathrm{HHV}}_{\mathrm{raw}}}\mathrm{\text{× MY}}$ (3)

Where; mass yield (MY) represents the weight loss; m_raw is the biomass before torrefaction and m_tor is the amount of torrefied biomass at the time (g). EY represents the energy yield; the higher heating value HHV_tor of the sawdust after torrefaction, and HHV_raw represents the HHV of the untreated sawdust sample.

Furthermore, it was observed that extending the residence time beyond 30 min had no significant effect because the heating values of all three temperatures at 30 min and 45 min were less than 5% compared to the untreated sample. Therefore, it can be concluded that in an operational torrefaction oven, the torrefaction time increase from 30 min to 45 min did not significantly affect the heating value under typical operating conditions. The torrefaction performance is influenced by the residence time. While not as important as the torrefaction temperature, it does have an impact on the characteristics and composition of the final solid torrefied biofuels [31]. From the study of Escribano-Uriarte et al. (2024) who studied the torrefaction of cockwood powder, it was found that the torrefaction biomass had a mass yield and energy yield at 246 °C for 60 min of 85.47 and 88.98%, respectively, which were lower than those of torrefaction sawdust due to the lower heating value than torrefaction sawdust [32].

4. CONCLUSIONS

This study investigated the enhancement of fuel properties of rubberwood sawdust from industry by using a rotary drum torrefaction process at different temperatures (220, 250, and 280 °C) for various times (15 30, and 45 min). The product yield and in-depth characteristics of torrefied sawdust were then determined. The torrefied mass yields in the range of 65.20 - 82.35% by weight. The thermal and physiochemical properties of the torrefied sawdust were analyzed. FTIR analysis revealed significant chemical structural changes, particularly a reduction in O-H and C-O functional groups, indicating degradation of hemicellulose and cellulose and improved hydrophobic properties. Furthermore, SEM analysis revealed clear morphological changes, showing increased porosity and the formation of small amorphous flakes with a particle size reduction to less than 2.5 μm. These physical and chemical changes directly contribute to increased energy density and improved crushability of the material. Therefore, the research demonstrates that torrefaction at 250°C can efficiently transform sawdust into a lignite-like solid fuel, resulting in a high-quality product suitable for sustainable alternative energy applications. The result indicated that higher temperatures in the torrefaction process cause increased decomposition of chemical components of biomass, resulting in more mass loss. In the chemical energies, solid fuels are stored in fixed carbon and volatile matter, which has a higher reaction of carbonization and increase to 25.47% and 88.06%, respectively. The torrefaction at 250 °C for 30 min yielded a biofuel with more than 80% energy yield, which allowed classification of this biofuel as lignite, with the optimum at 250°C and a duration of 30 min. Optimizing conditions can improve torrefied biomass production by enhancing the higher heating value and energy density, producing solid fuels with good quality and lower environmental impact, and greater sustainability.

ACKNOWLEDGEMENTS

This work was supported by the research funding from the National Research Council of Thailand (NRCT), Science, Research and Innovation Fund (SRI): Bioeconomy and Circular Economy Research Group (Grant No. N23H6600048). We would also like to thank the support from the National Research Council of Thailand (NRCT), the basic research project of Rajamangala University of Technology Rattanakosin, contract number FRB660025/0170.

AUTHOR CONTRIBUTIONS

Prangtip Rittichote Kaewpengkrow: Conceptualization, Methodology, Resources, Project administration, Funding acquisition, Writing- Reviewing and Editing. Yutthana Sripha: Visualization, Methodology, Resources, Data curation, Writing - Original draft preparation. Chinaruk Thianpong: Visualization, Investigation, Supervision. Tinnapob phengpom: Validation, Formal analysis, Writing.

CONFLICT OF INTEREST STATEMENT

The authors declare that they hold no competing interests.

DECLARATION OF USE OF GENERATIVE AI

During the preparation of this work, the author(s) used “Grammarly and Qillbot” to improve the readability and language of the manuscript. After using this tool, the author(s) reviewed and edited the content as necessary and take full responsibility for the publication's content.

FUNDING

This research was financially supported by the National Research Council of Thailand (Grant No. N23H6600048).

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