*3.3. Thermal Analysis Results*

Figure 5a shows the TG/DTG results. The PLA mass was almost constant up to around 290 ◦C, where thermal decomposition started. The PLA decomposed totally in one step at temperatures of ~300–400 ◦C, with the maximum peak at 367 ◦C. Backes et al. [48] show that PLA composition (additive presence) affects thermal degradation, and some components reduce the activation energy of initiation of thermo-degradation reactions. As a result, the decomposition onset temperature and maximum peak can differ up to 40 ◦C depending on the processed PLA [48]. Additionally, maximum decomposition peaks occur at 353–385 ◦C [48]. The DSC analysis results are shown in Figure 5b. The analysis shows that during PLA pyrolysis several reactions related to polymer phase transition occurred. The first phase transition at 64 ◦C is the glass transition of PLA. At 149 ◦C, the endothermal melting transformation was observed and finally, at 372 ◦C, the main endothermal decomposition peak was found. These findings agreed well with the result of Sousa et al. [49]. The results show that, for some reason, the DSC decomposition peak was shifted in comparison to DTG at about 5 ◦C (Figure 5a,b). Nevertheless, these findings explain that torrefaction could not significantly change the properties of PLA, as the temperature was too low for efficient devolatilization.

For PAP, three peaks were observed by DTG. First at 80 ◦C, second at 326 ◦C, and third at 550 ◦C with 1.3%, 74.6%, and 5.4% mass change (Figure 5a, grey curve), respectively. The first and third peaks are almost not visible in Figure 5a. The first peak is related to residual water evaporation, while the second peak is probably related to cellulose decomposition. This is due to the fact that white paper is made mainly from cellulose, (85–99%) with the addition of lignin of 0–15% [50]. Nevertheless, reprocessed paper (e.g., newspaper) has less cellulose (40–55%), more lignin (18–30%), and comparable content hemicellulose (25–40%) in comparison to white paper [50]. Additionally, the previously mentioned substances could affect the PAP sample decomposition. Typically, the hemicellulose, cellulose, and lignin decompose at 225–325 ◦C, 305–375 ◦C, and 250–500 ◦C, respectively [51]. According to Porshnov et al. [52], the temperature range of 250–300 ◦C is a characteristic interval for hemicellulose decomposition, 300–350 ◦C for cellulose decomposition, while above 400 ◦C the residue of lignocellulosic substances decomposed at a very slow rate. Lignin decomposition reactions were reported to occur at up to 900 ◦C [52]. Therefore, it is highly probable that PAP's third peak is related to lignin decomposition. The DSC results showed that, during PAP pyrolysis, four endothermal transformations occurred. The first transformation at 91.4 ◦C was probably related to residual moisture removal [53], and the following transformations were related to the decomposition of elements of the PAP sample. Similar results were obtained by Yang et al. [53], who tested clean cellulose and found the main endothermal peak related to decomposition at 355 ◦C. In this study, this peak was found at 329.6 ◦C (Figure 5b) and, similarly to the PLA, the DSC peak of PAP was shifted in comparison to DTG at about 3.6 ◦C.

The kinetic parameters were determined at <sup>β</sup> = 10 ◦C·min−<sup>1</sup> using the Coats–Redfern method. The kinetic triplets were determined for the whole process (30–800 ◦C) and the main peaks observed at TG/DTG plots (Figure 5a). The whole decomposition process for PAP and PLA were described by a reaction order of 1.56 and 2.02, respectively, and relative low activation energy of 33.11 kJ·(mol·K)−1, and 46.24 kJ·(mol·K)−1, respectively (Table 2). Here, it is worth noting that, for PLA, the determination coefficient was low, at 0.66, which was a result of the one-stage decomposition process, which occurred at 290–400 ◦C. Additionally, other kinetic triplets were determined with high determination coefficients (Table 2). The main PLA decomposition reaction was described by a reaction order of 0.42 and 160.05 kJ·(mol·K)−<sup>1</sup> activation energy, while PAP exhibited an order of 2.12, and 122.55 kJ·(mol·K)−<sup>1</sup> (Table 2). The first peak for PAP was omitted, as it was only residual water evaporation. It is worth noting that the suspected lignin decomposition at the third peak of the PAP sample had the highest activation energy of 173.05 kJ·(mol·K<sup>−</sup>1), which was about 51 kJ·(mol·K)−<sup>1</sup> larger than the main decomposition of cellulose. This finding is contrary to Noszczyk et al. [54] who studied several types of biomass materials and noticed that the cellulose content had a significant impact on the Ea, and the highest Ea was observed at the second stage of reaction, which was related to the cellulose decomposition [54].



#### *3.4. Theoretical Mass and Energy Balance of the Torrefaction Process*

Table 3 summarizes the theoretical mass and energy balance to produce 1 g CSF s given. The table compares the temperature and time. The third and fourth headings present the input mass needed to produce 1 g of CSF, and the chemical energy contained in this material. The fifth heading presents external heat provided to the torrefaction process. The sixth heading shows energy contained in 1 g of CSF. The seventh heading present a mass of gas released from the substrate during torrefaction, and the last heading show energy contained in this gas. The energy in gas was calculated as a sum of external energy provided to conduct a process and energy of substrate that was not converted into CSF.


**Table 3.** Torrefaction mass and energy balance for production of 1 g of CSF from PLA and Paper wastes.

\* value determined using DSC analysis result. \*\* value determined using calorimetric analysis result (HHV). \*\*\* value is the sum of chemical energy contained in gas and heat from external energy, assuming that no external energy stays in CSF.

> The result shows that more PAP than PLA substrate is needed to produce 1 g of CSF. In the case of 300 ◦C at 60 min, the double mass of PAP is needed compared to PLA (Table 3). The reason for this large input substrate demand originates from the low mass yield of PAP torrefaction (Figure 2b). As a result, much more chemical energy contained in PAP is put into the process to produce 1 g of CSF (23,833 J for PLA vs. 43,551 J for PAP). Additionally, the DSC results showed that more energy was needed to heat PAP than PLA to 300 ◦C, (458 J·g−<sup>1</sup> CSF vs. 940 J·g−<sup>1</sup> CSF) (Table 3). This is caused probably by the mostly higher specific heat value (Sp) of PAP in comparison to PLA. Depending on chemical composition, Sp of PAP varies from 1150 to 1650 J·(g·K)−<sup>1</sup> [55] while, for PLA, the value varies from 1180 to 1210 J·(g·K)−<sup>1</sup> [56]. On the other hand, PLA has a higher thermal conductivity, 0.12–0.15 W·(m·K)−<sup>1</sup> than PAP 0.08–0.11 W·(m·K)−<sup>1</sup> [55,56].

> During torrefaction, torrgas are produced. The analysis showed that a small mass of torrgas is produced from PLA and, depending on process conditions, these vary from 0.004 g·g−<sup>1</sup> CSF to 0.227 g·g−<sup>1</sup> CSF. As the production of 1 g of CSF from PAP needs far more substrate, much more torrgas is produced and varies from 0.054 g·g−<sup>1</sup> CSF to 1.485 g·g−<sup>1</sup> CSF (Table 3). As a result, during torrefaction at 300 ◦C, for each gram of produced CSF, around 1.5 g of torrgas is generated, and these torrgas contain more energy than produced CSF, while for PLA it is only 0.23 g of torrgas with around four times less energy than produced CSF (Table 3).

> When energy contained in torrgas is higher than the external energy needed to produce CSF, it theoretically can be assumed that the process is self-sufficient. This is true when torrgas are incinerated to provide heat for a substrate. With that assumption can be stated that PLA torrefaction can be self-sufficient at process temperatures higher than 300 ◦C and 40 min, while PAP is similar from 200 ◦C and 20 min (Table 3). Nevertheless, these results do not include heat losses, process efficiency, and energy needed for water evaporation that is in the real feedstock. Due to many different approaches to reactors design, it is hard to assume any heat losses and process efficiency. However, the contribution of water can be calculated and added to the results obtained in this study. To remove 1% of the water from solid fuel, at least 22.57 J (2257 J·g−<sup>1</sup> H2O is the latent heat of water evaporation at 100 ◦C) is needed, as well as the energy needed to heat this water to 100 ◦C [57]. For this

reason, the herein presented calculations serve as a starting point that has to be adapted for a particular reactor system and different feedstocks.

#### **4. Summary**

The results of this study showed that PLA's fuel properties cannot be improved by torrefaction, as no calorific values increase were observed with increasing process temperature and time. The reason is that PLA hardly decomposes, with negligible charring effects at torrefaction temperatures. On the other hand, PAP's fuel properties can be improved up to 10% by applying temperatures higher than 280 ◦C, which is probably caused by a partial cellulose decomposition. Additionally, the kinetic analysis revealed that PLA is decomposed in a one-stage process, that takes place at ~290–400 ◦C, with Ea of 160.05 kJ·(mol·K)−1, while PAP is decomposed in a two-stage process, at ~240–400 ◦C, and ~668–760 ◦C, with Ea of 122.55 kJ·(mol·K)−<sup>1</sup> and 173.05 kJ·(mol·K)−1, respectively. Moreover, the calculations showed that PLA torrefaction cannot be self-sufficient for CSF production and external energy is required, while CSF production from PAP proves to be self-sufficient under assumptions of no heat loss.

These results provide the first step towards an understanding of the PLA torrefaction process, but further research is needed to investigate higher temperatures of thermal PLA processing embracing gaseous and liquid products rather than solids, as PLA decomposes entirely into volatile components. Moreover, future studies should focus on PLA copyrolysis with conventional plastic, as a separation in waste management facilities is currently not possible from the MSW stream. Such a separation may be possible for separately collected and clean plastic wastes, but will fail in the case of plastics with organic adhesions, which are typical for plastic in MSW.

Regarding waste management scenarios, our study showed that the thermal properties of PLA qualify this material neither as a fuel surrogate in waste incinerators nor for an improvement by torrefaction process when we compare PLA with conventional high energy plastics. Therefore, a successive substitution of high caloric plastics by PLA may be reasonable when the end-of-life-scenario for the material is composting, but will raise the demand of conventional fuel when its thermally treated.

**Supplementary Materials:** The following are available online at https://www.mdpi.com/article/10.3 390/ma14227051/s1, Table S1: CSF Production, Table S2: Proximate Analysis, Table S3: TG-DTG-DSC.

**Author Contributions:** Conceptualization, K.S; methodology, K. ´ S.; software, K. ´ S.; validation, K. ´ S. ´ and A.B, formal analysis, K.S.; investigation, K. ´ S.; resources, K. ´ S and A.B.; data curation, K. ´ S.; ´ writing—original draft preparation, K.S; writing—review and editing, K. ´ S., C.Z., and A.B; visualiza- ´ tion, K.S.; supervision, C.Z. and A.B.; project administration, K. ´ S.; and funding acquisition, A.B. All ´ authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** All data derived during the experiments are given in the paper or the Supplementary Materials.

**Acknowledgments:** The presented article results were obtained as part of the activity of the leading research team—Waste and Biomass Valorization Group (WBVG). Paper has been prepared during research scholarship of Kacper Swiechowski at BOKU financed under the Leading Research Groups ´ support project from the subsidy increased for the period 2020–2025 in the amount of 2% of the subsidy referred to Art. 387 (3) of the Law of 20 July 2018 on Higher Education and Science, obtained in 2019—project number N040/0012/20.

**Conflicts of Interest:** The authors declare no conflict of interest.
