## 1 Introduction
Pretreatment and blending diverse raw materials for co-pelletizing is a promising approach for improving biomass pellet quality, and co-pelletizing requires cost-effective, eco-friendly, and sustainable raw materials. Furthermore, in the coming years, co-pelletizing will be a viable option for increasing the competitiveness of large-scale biomass pellet fuel production [13]. Co-pelleting of woody and herbaceous crops; co-pelleting of crops containing high amounts of starch and oil, such as microalgae and peanut shells, with woody or herbaceous crops; mixed pelleting of different parts of the same crop, such as the bark and leaves of similar quality; and co-pelleting of biomass with solid waste, such as municipal sludge and paper waste, are examples of common co-pelleting combinations. The co-pelletization of non-biomass-based materials with biomass, such as household waste and inorganic additives, simplifies pelletizing and improves pellet fuel quality. The addition of 50% rubberwood sawdust and 50% lignite reduced the ash content by 50% [14]. Refuse-derived fuel (RDF) is a fuel substitute derived from waste management facilities. The use of RDF for heat and power production adheres to the European Union's waste hierarchy [15]. Materials such as refuse-derived fuel (RDF) fractions are used as fuel in cement or combined heat and power (CHP) plants. However, the low bulk density creates a number of transportation and storage issues [16]. In the case of biomass, these issues result in a decrease in pelletization. Because of its low bulk density, raw RDF has a high potential for densification operations. Furthermore, the negligible content of natural binders necessitates die redesign. Furthermore, studies have shown that temperature has a significant impact on pellet quality. For all variations of compaction pressure and channel diameter, the temperature condition of 120 \({}^{\circ}\)C produces the most durable pellets. This is due to plastics melting. Using appropriate operation combinations, RDF from municipal solid waste, packaging, wood, paper, and plastics could be more manageable and storable, with more predictable characteristics and specifications such as higher heating value and proximate and ultimate analyses of pellets [17]. RDF has gained value as an alternative fuel due to the vast quantity of non-recyclable combustible waste materials, which can replace traditional generation burning fuels [18; 19]. The impact of fossil fuels (coal, oil, and gas) on global warming is highly unsustainable for developing economies worldwide, and they are the most significant contributor to global climate change [20]. The use of biomass as a biofuel can communicate the need for low-carbon energy while also reducing pollution, paving the way to carbon neutrality. As a result, the critical decision of using municipal solid waste and biomass as sustainable energy resources to manage energy supply and demand issues, as well as climate change issues, must be made [21; 22]. Previous literary works have revealed a potential ability to co-pelletize. To the best of the authors' knowledge, there have been few studies on the co-pelletization of rubberwood and RDF. As a result, there is a limited understanding of the effects of co-pelletization on the performance of the produced pellets. Our previous study produced mixed pellets from rubberwood and RDF and tested their mechanical durability and calorific value (ultimate and proximate) [23]. Meanwhile, the goal of this research is to determine the physical properties, higher heating value (HHV), and elemental analysis of pellets made from a rubberwood and RDF mixture. The findings of this research could support the viability of mixed pellet fuel as a feedstock in thermal waste-to-energy technologies.