*3.2. Proximate Analysis and HHV Results*

The samples of materials used to produce CSF were also analyzed for volatile matter (VM), ash content (AC), fixed carbon (FC), volatile solids (VS), combustibles parts (CP), and high heating value (HHV). The PLA materials had 100%, 0%, 0%, 100%, and 100% of VM, AC, FC, VS, and CP, respectively, while the PAP material had 88.2%, 3.6%, 8.2%, 96.3%, and 96.4% of VM, AC, FC, VS, and CP, respectively. The HHV of PLA and PAP were 19,420 and 17,525 J·g<sup>−</sup>1, respectively (Table 1).



For PLA samples, an unexpected result was found for FC and AC, 0%, while VM, VS, and CP were 100%. The same results were obtained for all CSF made from PLA (Table 1). Moreover, the Tukey test shows that there were no significant changes between the HHV of CSF made from PLA. Therefore, it can be stated that torrefaction does not affect PLA fuel properties. These unexpected results can be explained in two ways: (I) The amount of ash (minerals) in PLA was too small to be detected by equipment that was used, or (II) there were no minerals in the PLA material at all. In case of a lack of minerals (case II), the results would be correct, as all organic matter was incinerated/devoltalized during experiments. In the other case (I), a correction for the undetected mass would have to be

performed. However, the error of the undetected mass is ±0.1 mg at an input of 1 g and therefore negligible.

In the literature, both cases can be found for PLA. In favor of assumption (II) were results from Camacho-Muñoz et al. [37] that showed 100% of vs. in a PLA sample. However, Jing et al. [38] showed that PLA is a type of thermally degradable material that burns at a relatively rapid heat release rate with negligible chars, suggesting that at least some FC should remain.

For CSF made of PAP, a decrease in VM with increasing temperature and time was observed. With increasing process temperature and time from 200 ◦C and 20 min to 300 ◦C and 60 min, the VM decreases from 86.6% to 55.7%, while FC and AC increase from 9.9% to 34.6%, and from 3.4% to 9.7%, respectively (Table 1). The observed decrease in VM is related to the devoltalization of materials. On a molecular level, large cellulose molecules in PAP are broken into smaller ones until they are small enough to be removed by convection [39]. Depending on the chemical composition, more or fewer of such small molecules are released and, as a result, different values of VM can be observed. Unlike VM, the AC and FC content increase mainly as a result of the loss in VM. Unlike AC, which is related to the mineral present in the sample, additional FC can be produced during secondary reactions [40]. Nevertheless, for biomass, the presence of components such as hemicellulose and cellulose is the main contributor of VM production, while lignin is the same for FC production [41].

Both tested materials were characterized by a relatively high level of VM, and low and zero content of FC (PAP and PLA, respectively). For comparison, wood biomass has 86% of VM, 15% of FC, and 0.4% of AC [42], torrefied wood at 300 ◦C in 30 min has 71% of VM, 29% of FC, and 0.4% of AC [43], while high-rank bituminous (coal) has 27.6% of VM, 65% of FC, and 7.4% of AC [44]. It is clear that fuel properties of torrefied paper and biodegradable plastic are not close to conventional solid fuels. Nevertheless, the positive aspect of PLA material is its zero-ash content, which decreases the costs for managing the ash.

The high heating value of 19.4 MJ·kg−<sup>1</sup> for PLA is more than twice lower than that of conventional plastics such as polyethylene [45]. Moreover, torrefaction does not increase the HHV of PLA (Table 1). On the other hand, torrefaction was found to be suitable for PAP. The HHV of PAP increased from 17.5 MJ·kg−<sup>1</sup> to 19.5 MJ·kg−<sup>1</sup> in CSF produced at 300 ◦C; 40 min. Though these values seem to suffice when they are compared to energetic biomasses (HHV ~ 18 MJ·kg−1) [46], they are still small in comparison with coals 30 MJ·kg−<sup>1</sup> [47] or conventional plastics 40 MJ·kg−<sup>1</sup> [45].
