## 1 Introduction  
Due to fossil fuel reserves being depleted, the world is shifting toward the production of renewable energy. Biomass materials are one of the most intriguing alternatives due to their widespread availability. Because of their low bulk density, biomass materials are difficult to handle, store, and transport. Pelletization can densify the materials into pellets by using pressure and heat [1]. The use of raw materials, such as agricultural, forestry, and food waste, and surpluses from the agri-food industry has recently increased dramatically [2]. Wood pellets are a cost-effective, environmentally friendly alternative to traditional fossil fuels. In terms of calorific value, softwoods had a higher range between 19.66 and 20.36 MJ/kg, while hardwoods ranged between 17.63 and 20.81 MJ/kg [3]. Sawdust, planer shavings, and dry chips have been the primary source materials used in the production of wood pellets in Sweden since 2001. Because of their widespread availability, alternative forms of biomass, such as bark and logging residues, have sparked interest as raw materials [4]. A previous study [5] investigated the physical, mechanical, and energy properties of wood pellets derived from three common tropical species: _Acacia wrigthiti_, _Ehenopsis ehano_, and _Havardia pallens_. According to the findings, the pellets obtained from the species have a high energy density, making them ideal for commercial and industrial heating applications. Aside from wood residues and uncommon species, a variety of biomasses, including almond shells, olive stones, coffee dregs, coffee husks, grape pomace, hazelnut shells, miscanthus, pine kernel shells, and switchgrass, were blended to produce high-quality pellets for industrial use [1]. Similarly, [PERSON] and [PERSON] [6] found that willow, miscanthus, wheat, barley, and rape straws were also used. Additionally, [PERSON] et al. [7] showed the potential of empty fruit bunches as a biomass source for pellet production. Thailand can grow plants all year, ensuring a steady supply of biomass feedstock. Thailand exported 172,441 tonnes of wood pellets to South Korea and Japan in 2019. Thai entrepreneurs want to increase wood pellet exports to Japan for power generation. According to the Kasikorn Research Center, Thailand will export 80,000-100,000 tonnes of wood pellets to Japan in 2020 [8]. The Thai government restricts the use of wood pellets to industrial heat production rather than electricity generation. In Thailand, biomass power plants use less expensive substitutes, such as palm shells, paddy, and bagasse, instead of wood pellets [9]. Rubberwood has the potential to produce a substantial amount of energy. It outperforms fruit fibers, rice husks, coconut husks, bagasse, and logging residues, and it even slightly outperforms wood residues. It does, however, have less potential than empty fruit bunches and palm shells. Rubberwood has lower ash and nitrogen content compared to oil palm biomass. There are numerous opportunities in Thailand for wood pellet production using para-rubber waste wood as a feedstock. Consumption of wood pellets can help reduce greenhouse gas emissions while also supporting the Thai government's efforts to use wood pellets for domestic purposes. Furthermore, there is a growing demand for wood pellets for export [10]. The global market for pellets is expanding. The output in 2008 was 9.8 \(T_{g}\), with a global increase to 14.3 \(T_{g}\) in 2010 and an excess of 26 \(T_{g}\) in 2015 [11]. Because of the high demand for rubberwood in the wood product manufacturing industry, the quantity of rubberwood biomass available for energy production is relatively low compared to the abundance of oil palm biomass. Rubberwood biomass utilization as a fuel source is severely limited due to (1) systematic replanting, (2) extraction of all above-ground biomass up to 10 cm in diameter from the field, and (3) utilization of waste and residue from secondary milling activities in the panel products sector [12].