Biomass waste has been widely used for producing biofuel as an alternative source to replace depleted fossil fuels, especially in European Union countries. However, it is rarely used in developing countries like Thailand, Indonesia, and Malaysia (Malek et al. 2020). In fact, Malaysia has abundant amounts of biomass waste, especially from the palm oil industry. Palm kernel shell (PKS) and empty fruit bunch (EFB) are examples of biomass waste generated from palm oil mills. Current practices show that EFB is commonly utilized as fertilizer since it is abundant in minerals such as phosphorus, magnesium, potassium, nitrogen, and carbon. These minerals are beneficial for soil pH increments, soil erosion reduction, and soil moisture conservation (Sukiran et al. 2017). Hence, EFB is transformed into chemical fertilizers through the incineration process. However, this method is not environmentally friendly because it produces high amounts of white smoke and fly ash throughout the process, resulting in air pollution (Loh 2017). Meanwhile, PKS has been commonly utilized as solid fuel for a steam boiler in the milling plant since PKS has a comparatively higher heating value compared to other lignocellulosic biomass. However, current practice has limited the potential of biomass usage and the commercial value of EFB and PKS, and most practices contribute to the degradation of environmental conditions. The limited application of both EFB and PKS is due to their inferior characteristics such as low bulk density, hygroscopic nature, and low energy density, which ultimately contributes to inconveniences during handling, transportation, and storage processes, as well as reduced combustion efficiency as a solid fuel (Kumar et al. 2017). In fact, both EFB and PKS have high potential to be utilized as an energy commodity by upgrading their characteristics through suitable pre-treatment methods.
Torrefaction and pelletization are widely used pre-treatment methods for improving the fuel quality of biomass, which eventually enhances their commercialized value as solid biofuel. The main reasons for using pre-treatment methods are to enhance the heating value, inhibit rotting behavior, as well as improve the bulk and energy density of biomass waste (Sukiran et al. 2017; Pradhan et al. 2018; Mostafa et al. 2019). Theoretically, the torrefaction process is a thermochemical pre-treatment method that occurs in a mild temperature range (200°C to 300°C) under atmospheric conditions without oxygen supply (Mostafa et al. 2019; Siyal et al. 2020). The torrefaction process transforms biomass waste with a hygroscopic nature into hydrophobic by destroying hydroxyl groups in biomass (Manouchehrinejad et al. 2021). Furthermore, the torrefaction process improves the heating value of biomass waste, reducing moisture content while increasing the fixed carbon content of biomass waste (Kanwal et al. 2019). However, the main challenge in employing torrefied biomass as solid biofuel is related to the delicate and brittle properties of torrefied biomass, which are unfavorable for handling, transportation, and storage processes (Larsson et al. 2013). Hence, biomass needs to be transformed into a uniform particle size with enhanced mechanical strength to reduce losses due to fracture and breakage during handling, transportation, and storage processes. This limitation can be overcome by using pelletization, where this physical pre-treatment method can compact biomass waste into uniform-sized particles with increased bulk density (Sukiran et al. 2017; Picchio et al. 2020; Sambeth et al. 2022). In addition, pelletization is an effective method that enables easier and more efficient handling, transportation, and storage processes. However, the limitation of pelletization is that this method does not improve the combustion properties of raw biomass waste. As a solution, torrefaction and pelletization processes must be combined to enhance both the combustion and physical characteristics of biomass waste.
The combination of torrefaction and pelletization approaches is able to produce biomass pellets with better high heating value, hydrophobicity, a compact structure, and uniform size (Bach and Skreiberg 2016). However, suitable operating conditions for torrefaction and pelletization must be considered to produce the desired quality of biomass pellets. For example, residence time and torrefaction temperature are two significant operating conditions in the torrefaction process. Torrefaction temperature is a more significant operating condition compared to residence time because residence time shows a less substantial effect on enhancing the characteristics of biomass waste. Additionally, torrefaction temperature is the most significant variable that affects the higher heating value (HHV) of the pellet formed and the binding mechanisms of biomass waste during the pelletization process. This is due to the fact that a higher torrefaction temperature results in more severe degradation of lignocellulosic components, which naturally act as a natural binder. Usually, lignocellulosic components form solid bridges between biomass particles and improve their cohesive strength. Several research works have been conducted to investigate the impacts of torrefaction temperature on the binding mechanisms of biomass waste during the pelletization process, and the effect varies depending on the properties of the biomass employed. For example, biomass processed at high torrefaction temperatures produces more fragile and brittle pellets, and more energy is consumed during the co-pelletization process (Bach and Skreiberg 2016; Rudolfsson et al. 2017). Although a high HHV of torrefied biomass can be obtained at higher torrefaction temperatures, a low compression strength of the pellet is expected, suggesting that biomass undergoing severe torrefactions (275°C to 300°C) tends to have low quality in terms of mechanical properties (Stelte et al. 2013).
Thus, torrefaction and pelletization present an ideal approach to transform EFB and PKS wastes into pellet products for energy-related applications. Based on the availability of oil palm solid wastes in Malaysia, around 18.88 million tonnes per year of EFB has been generated compared to only 4.72 million tonnes per year of PKS waste (Sukiran et al. 2017). This indicates that EFB has abundant resources compared to PKS. However, EFB has a calorific value of 18.88 MJ/kg, which is lower than the calorific value of PKS, which is around 20.09 MJ/kg (Loh 2017). In addition, PKS contains a high lignin content, around 50.7 wt%, which is preferable to produce stronger and more compact pellets (Siyal et al. 2020). High lignin content can strengthen solid bridges between particles, preventing pellet fracture and breakage during handling, transport, and storage processes. This theoretically shows that PKS will produce better pellets in terms of energy content compared to EFB. The main drawback is that EFB is the main priority for sustainable energy production in Malaysia due to a significant amount of waste. Therefore, blending EFB with PKS through co-pelletization provides another approach to produce high-quality pellets. Previous studies show that a better quality pellet can be obtained using a combination of different biomass materials. For example, oil palm trunk bark was blended with corncob at different mixing ratios, and the blended pellet exhibited better quality in terms of heating values, ash content, and compression strength (Kpalo et al. 2020). Torrefied canola meal and canola hull were blended using a mixer at five different ratios (100:0, 80:20, 60:40, 40:60, 100:0), and the blended pellet with more than 40 wt% of torrefied canola hull possessed better heating value and mechanical strength than the pure torrefied pellet (Faizal et al. 2018).
Currently, research found in literature involving EFB and PKS is focusing only on the torrefaction process, and in some cases, working on pelleting raw EFB and PKS. Based on our knowledge, no work has been conducted on the influence of torrefaction and the blending ratio of raw and torrefied EFB and PKS on the co-pelletization process. Hence, the aim of this study is to investigate the effect of torrefaction temperature (210°C, 240°C, 270°C) and the blending ratio of PKS to EFB (0:100, 25:75, 50:50, 75:25, 100:0) on the physical and combustion characteristics of biomass pellets. The combustion properties such as HHV, moisture and ash content, as well as physical properties such as compression strength, pellet density, density changes, and dimensional stability, were measured to evaluate the quality of co-pellets. Moreover, univariate analysis of variance (UANOVA) was conducted to investigate the impact of the studied parameters (torrefaction temperature and blending ratio) themselves, as well as the interaction effects of the studied parameters on the combustion and physical characteristics of co-pellets produced.