## 1 Introduction  
Sawdust and other by-products generated in forest industries are often considered a problem in work areas and woodlands, since disposal can create wildfires during periods of intense heat, generating dust in the air, and block areas in production facilities [1, 2]. Regarding biomass availability, the genus _Pinus_ in Mexico represents the most important forest resource, reaching 5.0 million m\({}^{3}\) year\({}^{-1}\) of rolled wood in the last decade [3]. Much of the waste produced by the forest industry comes from the harvesting of coniferous forests and pine species [1]. [PERSON] et al. [4] estimated that around 31,000 Mg year\({}^{-1}\) of forest residues from the genus _Pinus_ are generated in the region of El Salto, Mexico; this volume could generate energy of approximately 65.6 GW h year\({}^{-1}\). For comparison, [PERSON] [5] and [PERSON] et al. [6] calculated that 43.7 \(\pm\) 5% of residual biomass in the state of Durango comes from a forest industry, of which 7.95% corresponded to sawdust from conifers. These aforementioned by-products may appear in the form of sawdust, bark, or chips, and are considered biomass with ideal potential for thermal energy production through briquette densification with applications in generators of steam, boilers, and turbines [7]. Briquettes are solid biofuels made with or without additives from biomass densification, and with specific properties derived from the proportion of material/additive, pressure, temperature, and time utilized in their manufacture as well as the typical physicochemical properties of biomass [8]. The production of briquettes promotes the cleanliness and care of the environment because they are made from waste, and expanded production promotes local economic development, reducing energy dependence on fossil fuels [9]. The particle size of sawdust from which briquettes are manufactured significantly affects their physical and mechanical properties as well as combustion characteristics [10]. Additionally, processing parameters such as moisture content of the sawdust, pressing temperature, and compacting pressure are factors that have a significant impact on briquette quality and strength. Generally, briquettes with lower moisture content are of lower quality. However, it has been determined in some studies that excessively low moisture content degrades the mechanical properties of briquettes [11]. According to [PERSON] et al. [12] briquettes manufactured at lower pressures (30 to 60 MPa) fall to pieces easily, but at higher pressures (150 to 250 MPa), are consistent and compact. Additionally, increasing the compaction temperature causes plasticizing of the particles and activation of natural binders in the material, increasing the strength of the briquettes [13]. For this reason, making briquettes with the right proportion of different particle sizes is important, because briquettes obtained with irregular particle sizes and shapes suffer some disadvantages [14]. Consequently, it is recommended to use sawdust mixtures with different proportions of particle sizes and evaluate their biomass characteristics in terms of physical, mechanical, and energy properties to produce briquettes with consistent physical properties (size, shape, and density) that facilitate the feeding of furnaces and boilers [15]. The response surface methodology (RSM) is one of the methodologies used to thoroughly assess the effect of particle size on other physical-mechanical and energetic properties of solid biofuels. This methodology consists of a set of mathematical and statistical procedures that analyze the phenomena or experiments in which one or more variables are influenced by others [16]. Thus, it is known as the response surface to the relationship between variable response and factor levels in an experiment. The difference between RSM and a factorial experimental design is that an experimental design aims to locate the \"winner\" treatment among all those that have been tested.