2.2.1. Torrefaction Process—CSF Production

The CSF was produced at different temperatures of 200–300 ◦C in intervals of 20 ◦C and kept for 20, 40, and 60 minutes each. For the torrefaction procedure, 10 g of dry samples were placed in ceramic crucibles. These crucibles were placed into the chamber of the muffle furnace (Snol 8.1/1100, Utena, Lithuania), which was purged with CO2 gas to create an inert atmosphere before the samples were heated to the setpoint temperature. During the torrefaction process, CO2 gas was continuously supplied to the chamber to prevent sample ignition. The CO2 flow was shut off after the treatment period and when the temperature of the chamber declined to <150 ◦C. The mass of samples before and after the process was used to calculate mass yield following Equation (1). Then, using the results of HHV, the energy densification ratio was calculated (Equation (2)), and then the energy yield of CSF was determined according to Equation (3).

$$MY = \frac{m\_b}{m\_r} \cdot 100\tag{1}$$

where *MY*. is the mass yield, %; *mb* is the mass of material after torrefaction, g (CSF); and *mr* is the mass of material before torrefaction, g.

$$EDr = \frac{HHV\_b}{HHV\_r} \tag{2}$$

Where *EDr*. is the energy densification ratio; *HHVb*. is the high heating value of material after torrefaction (J·g<sup>−</sup>1) (CSF); and *mr* is the high heating value of material before torrefaction (J·g<sup>−</sup>1).

$$EY = MY \cdot EDr \tag{3}$$

where *EY* is the energy yield, %; *MY*. is the mass yield, %; and *EDr* is the energy densification ratio.

#### 2.2.2. Proximate Analysis and HHV Determination

For all samples, the proximate analysis was performed. The moisture content (MC) was determined by the drying method at 105 ◦C using a laboratory dryer (WAMED, KBC-65W, Warsaw, Poland) according to PN-EN 14346:2011 standard [28]. The volatile matter (VM) was measured by a thermogravimetric method using a tubular furnace (Czylok, RST 40 × 200/100, Jastrz ˛ebie-Zdrój, Poland), according to [29]. The ash content (AC) was measured by sample incineration in a muffle furnace (Snol 8.1/1100, Utena, Lithuania) according to PN-Z-15008-04:1993 standard [30], and fixed carbon was measured by difference. Additionally, samples were tested for volatile solids content (VS) and combustible part content (CP) using the muffle furnace (Snol 8.1/1100, Utena, Lithuania) according to PN-EN 15169:2011 [31] and PN-Z-15008-04:1993 [30] standards, respectively. All samples were tested for high heating value using a calorimeter (IKA, C200, Staufen, Germany), according to PN EN ISO 18125:2017-07 [32]. To ensure repeatability, each experiment was triplicated.
