Sustainable Drying and Torrefaction Processes of Miscanthus for Use As a Pelletized Solid Biofuel and Biocarbon-Carrier for Fertilizers

Home , Miscanthus, Thermal depolymerization, Torrefaction

molecules

Article Sustainable Drying and Torrefaction Processes of Miscanthus for Use as a Pelletized Solid Biofuel and Biocarbon-Carrier for Fertilizers

Szymon Szufa 1,* , Piotr Piersa 1, Łukasz Adrian 1, Justyna Czerwi ´nska 1, Artur Lewandowski 1, Wiktoria Lewandowska 1, Jan Sielski 2, Maria Dziku´c 3 , Marek Wróbel 4 , Marcin Jewiarz 4 and Adrian Knapczyk 4

1 Faculty of Process and Environmental Engineering, Lodz University of Technology, Wolczanska 213, 90-924 Lodz, Poland; [email protected] (P.P.); [email protected] (Ł.A.); [email protected] (J.C.); [email protected] (A.L.); [email protected] (W.L.) 2 Department of Molecular Engineering, Lodz University of Technology, Wolczanska 213, 90-924 Lodz, Poland; [email protected] 3 Faculty of Economics and Management, University of Zielona Góra, ul. Licealna 9, 65-246 Zielona Góra, Poland; [email protected] 4 Department of Mechanical Engineering and Agrophysics, University of Agriculture in Kraków, Balicka 120, 30-149 Kraków, Poland; [email protected] (M.W.); [email protected] (M.J.); [email protected] (A.K.) * Correspondence: [email protected]; Tel.: +48-606-134-239

Citation: Szufa, S.; Piersa, P.; Abstract: Miscanthus is resistant to dry, frosty winters in Poland and most European Union countries. Adrian, Ł.; Czerwińska,J.; Miscanthus gives higher yields compared to native species. Farmers can produce Miscanthus Lewandowski, A.; Lewandowska, W.; pellets after drying it for their own heating purposes. From the third year, the most efficient plant Sielski, J.; Dzikuć,M.; Wróbel, M.; development begins, resulting in a yield of 25–30 tons of dry matter from an area of 1 hectare. Jewiarz, M.; et al. Sustainable Drying Laboratory scale tests were carried out on the processes of drying, compacting, and torrefaction of and Torrefaction Processes of this biomass type. The analysis of the drying process was conducted at three temperature levels Miscanthus for Use as a Pelletized of the drying agent (60, 100, and 140 ◦C). Compaction on a hydraulic press was carried out in the Solid Biofuel and Biocarbon-Carrier pressure range characteristic of a pressure agglomeration (130.8–457.8 MPa) at different moisture for Fertilizers. Molecules 2021, 26, contents of the raw material (0.5% and 10%). The main interest in this part was to assess the influence 1014. https://doi.org/10.3390/ of drying temperature, moisture content, and compaction pressure on the specific densities (DE) and molecules26041014 the mechanical durability of the pellets (DU). In the next step, laboratory analyses of the torrefaction

Academic Editor: Farid Chemat process were carried out, initially using the Thermogravimetric Analysis TGA and Differential Scaning Calorimeter DSC techniques (to assess activation energy (EA)), followed by a flow reactor ◦ Received: 18 January 2021 operating at five temperature levels (225, 250, 275, 300, and 525 C). A SEM analysis of Miscanthus Accepted: 10 February 2021 after torrefaction processes at three different temperatures was performed. Both the parameters of Published: 14 February 2021 biochar (proximate and ultimate analysis) and the quality of the torgas (volatile organic content (VOC)) were analyzed. The results show that both drying temperature and moisture level will affect Publisher’s Note: MDPI stays neutral the quality of the pellets. Analysis of the torrefaction process shows clearly that the optimum process with regard to jurisdictional claims in temperature would be around 300–340 ◦C from a mass loss ratio and economical perspective. published maps and institutional affil- iations. Keywords: torrefaction; drying; miscanthus; biocarbon; fertilizers; kinetics

Copyright: © 2021 by the authors. 1. Introduction Licensee MDPI, Basel, Switzerland. Poland as a member of the European Union is active participator in new European This article is an open access article Green Deal which will be a revolution in terms of polish energy sector. New regulations distributed under the terms and in terms of climate and environment are new challenges in upcoming years. The Polish conditions of the Creative Commons energy sector is currently facing serious challenges such as high energy demand, an inade- Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ quate level of infrastructure development production and transport of fuels and energy, 4.0/). extraction of external energy natural gas, receiving crude oil from an external supplier,

Molecules 2021, 26, 1014. https://doi.org/10.3390/molecules26041014 https://www.mdpi.com/journal/molecules Molecules 2021, 26, 1014 2 of 29

and, in the long term, environmental protection, given the current climate issues, of which early reactions are strongly related to potential response to the fuel and energy situation. In between the renewable energy sources, thermodynamic and kinetic analysis aspects of Miscanthus torrefaction are characterized by unflagging interest. This paper shows clearly that Miscanthus is a very interesting material, both in pellet production and in further processing for biochar production, used not only as an energy carrier, but also as a new type of carbon source in fertilizer mixtures where it is shown to be a carrier for organic fertilizers. Torrefied Miscanthus are used as an additive for organic fertilizer and pelletized solid biofuel. The aim of this research was to use kinetics data from TGA analysis of the Miscanthus torrefaction process for designing counter-flow reactor for continuous torrefaction process. The presented technique of Miscanthus torrefaction process is based on the assumption that the authors will find out the optimal torrefaction process conditions for Miscanthus (kinetics, residential time of torrefaction process, and torrefaction temperatures in isothermal conditions) as a feedstock in new design installation for conducting continuous torrefaction processing of different energy crops in a semi-pilot scale (steps: Drying Miscanthus with 40% to 5%, torrefaction of Miscanthus with humidity content 5% to 2%). Of all the renewable energy sources, biomass is a CO2 neutral biofuel that is used for big-scale heat and electrical energy production, but it can be also used for distributed energy systems and as other bio-based products. Biomass comes from many sources and occurs in the form of shoots of woody plants [1–5] and herbaceous plants [6,7]. As a result of the research on Miscanthus torrefaction process, the author intends to design and build a small installation with a superheated steam counter-current reactor. Products in the form of torrefied Miscanthus will also be able to serve the farmers who produce them, as biochar in the form of an additive (carrier of the carbon element) to organic fertilizer, suitable for agriculture and greenhouses, improving soil properties and increasing productivity. Po- mades, fruit stones, kernel shells, and other residuals are also used, such as waste cooking oil [8–13]. Recently, so-called water biomass, such as hyacinths or algae, has also become popular [14–16]. The classification of biomass according to source and origin is found in ISO 17225-1:2014 [17]. Extensive research is constantly expanding the spectrum to include new types of biomass [18–20]. Biomass has relatively uniform energy parameters due to its composition—lignin, cellulose, and hemicellulose [21–23], although it is physically very variable [9,24–29]. Therefore, it often requires combustion systems dedicated to a given type and form [30–33]. Another way to use this diversified raw material effectively is to process it into a standardized fuel form such as pellets or briquettes [10,34–36]. However, the production processes of compacted biofuels depend heavily on the raw material’s physical properties already mentioned [28,37]. For the quality of the final product (briquette or pellets) to be at the required level, the raw material must first have adequate moisture content. A range of 6–17% is often stated [38], but mostly it is about 10–12% [20,39–43]. The correct fragmentation of raw material is just as important [34,44–48]. Proper preparation of the raw material facilitates high solid density (DE) and mechanical durability (DU), two quality parameters depending on the properties of the raw material, and properly adjusted parameters of the compaction process [48–51]. Torrefaction is one such technology which, depending on the process parameters, makes it possible to unify the properties of a diversified raw material [2,28,52,53]. Such a standardized product is characterized, for example, by its better grindability [54–57], and it can be more widely used, not only as a fuel with similar properties to those of coal. Miscanthus is a plant that is often proposed as a promising species for use in energy and not only as a fuel but also for agricultural applications, such as a carrier for organic fertilizers [6,58,59]. Miscanthus has a yield of at least several tons per ha, it requires low expenditure in cultivation, enjoys a simple harvesting technology and it is easy to store. The annual amount of precipitation should be between 400 and 600 mm. This plant tolerates a temporary water deficit well, which is not without significance in the periods of drought occurring in Poland [6]. One of the bio-based products which can be generated from biomass in a true torrefaction process is biocarbon, with a role as a carrier rich in C for the production of bio-fertilizers. Biomass Molecules 2021, 26, 1014 3 of 29

has the biggest potential from all kind of renewable energy sources in Poland. Increas- ing the biomass share in conventional coal-power plants is a very effective and fast in implementation way of reducing a carbon dioxide and other pollutant gases emission. To increases the biomass share up to 30% or even 50% thermal, the biomass particles must be milled down to sizes where caloric values can be expected. When comparing coal with biomass, of which both are still the dominant solid fuels for heat and electricity production in Poland, often reveal inferior properties of biomass. When we look closely at wooden biomass fuel properties we see that biomass has, in most cases, a high moisture content, resulting in storage complications such as self-heating and biological degradation, lower energy densities, is a bulkier fuel (with poorer transportation and handling characteristics), and more tenacious (the fibrous nature of biomass makes it difficult to reduce to small homogeneous particles). Biomass properties mentioned above have negative impacts during energy thermal conversion such as gasification system design limitations and lower combustion and co-firing efficiencies. As a torrefaction product we get a hydrophobic solid fuel with greatly increased grindability and energy density (on a mass basis). What is more important is that we lower the required energy to process the torrefied biomass and determine that it no longer requires additional separate handling facilities, like when we co-combusted new fuel with coal in operating power plants. It is suggested that torrified biomass can be compacted into high grade pellets with substantial superior fuel properties compared with standard wood pellets from untreated biomass. The carbonization process can be combined together with drying and palletization process, with both energy and economy benefits [60]. In this paper, the torrefaction process of the most promising of energy crops plus low-cost effective feedstock was performed using an electrical furnace in the presence of CO2 gas and a thermogravimetric analyzer with two other inert gases, argon and nitrogen. In addition, Miscanthus, with its sorptive and fertile characteristics of torrefied biocarbons, was evaluated with respect to its potential as a carbon sequesterer and soil fertility enhancer [61–65]. Biocarbon as a carrier for natural biofertilizer has a huge potential in farming [66,67] as an additive for bio-fertilizers for greenhouse cultivation, agriculture, and horticulture. One of the main objectives of this research was to establish proper Miscanthus torrefaction process conditions (torrefaction temperature, heating ratio and resident time, kinetics, and energy activation) to design installation with capacity 50 kg/h of wet biomass in a semi-pilot scale with counter-flow torrefaction reactor for continues torrefaction process to produce: Carbonized solid biofuel and biochar for fertilizers. Demand for biocarbon, not only as a bio-product but primarily for the products manufactured in its use, will stimulate an increased interest in biocarbon produced by thermo-chemical conversion in the Polish market: Fuels for biogas plants, sugar production, bioethanol production plant, saw mills, and pulp and paper factories [68,69].

2. Results 2.1. Analysis of Miscanthus Drying Process and Pellets Properties of Miscanthus Figure1 shows the particle composition analysis of the samples dried in the temperature range examined. Both the bar graphs, showing the mass shares of sieve class as well as the cumulative graph, indicate that there are no differences in particle size distribution (PSD) between the samples. This shows that the calculated value of d50 for all samples is 0.2 mm. These results help us to conclude that the particle composition did not inﬂuence the DE and DU quality parameters of the pellets. The degree of sample fragmentation is at the level recommended by Mani et al. [48]. MoleculesMolecules 20212021,, 2626,, x 1014 44 of of 31 29

FigureFigure 1. 1. AnalysisAnalysis of of samples’ samples’ particle particle size size distribution. distribution.

FigureFigure 2 shows the pellets’pellets’ specificspecific densitydensity DEDE changeschanges duedue toto temperaturetemperature dryingdrying andand compaction compaction pressure. pressure. For For the the samples samples of of dry dry material, material, the the DE DE values values increased increased from from −3 811811 to to 1008 1008 kg·m kg·m−3 (Figure(Figure 2a).2a). In Inthis this case, case, thethe lowest lowest pressure pressure at which at which the threshold the threshold was −3 ◦ reachedwas reached (1000 (1000 kg·m kg−3) ·wasm )determined was determined for a sample for a sample dried dried at 100 at °C 100 andC was and wasabout about 350 ◦ MPa.350 MPa Samples. Samples dried dried at 60 at and 60 and 140 140°C reachedC reached the the assumed assumed DE DE value value at at about about 390 390 MPa. MPa. ForFor samples samples with with 5% 5% moisture moisture content, content, the the DE DE surface surface was was shifted shifted up up and and reaches reaches values values −3 inin a a range range of of 890–1075 890–1075 kg·m kg·m−3 (Figure(Figure 2b).2b). The The DE DE threshold threshold was was reached at 230 MPa for allall samples, samples, suggesting suggesting that that the the drying drying temper temperature,ature, in in this this case, case, was was not not important, important, as as opposedopposed to to the the moisture moisture content, content, the the growth growth of of which which results results in in an an increase increase in in DE. DE. When When · −3 itit increased increased to to 10%, 10%, the the DE DE surface surface again again shifts shifts up up (DE (DE range range is is 990–1090990–1090 kg·m kg m−3) with) with a thresholda threshold at ata pressure a pressure level level of of 140 140 MPa MPa regardless regardless of of drying drying temperature temperature (Figure(Figure 2c).2c) . FigureFigure 3 shows pellet changes in DU. Threshold values were set at 96 and 97.5% (quality classclass B B and and A A for for non-woody non-woody pellets pellets according according to to ISO ISO 17225-6:2014 17225-6:2014 st standard).andard). DU DU thresh- thresh- olds for pellets made from material in a dry state (Figure3a) were not reached and no olds for pellets made from material in a dry state (Figure 3a) were not reached and no effect on the drying temperature could be observed. When the moisture content of the effect on the drying temperature could be observed. When the moisture content of the material rose to 5%, the first DU threshold (96%) was reached at pressures of 300 MPa material rose to 5%, the first DU threshold (96%) was reached at pressures of 300 MPa (material dried at 140 ◦C) and of about 340 MPa (material dried at 100 ◦C and 60 ◦C). The (material dried at 140 °C) and of about 340 MPa (material dried at 100 °C and 60 °C). The second threshold was not reached in this variant except for a small peak at a pressure second threshold was not reached in this variant except for a small peak at a pressure of of 400 MPa for material dried at 140 ◦C (Figure3b). A further increase in the moisture 400 MPa for material dried at 140 °C (Figure 3b). A further increase in the moisture content content to 10% was significant for an increased pellet DU. The first threshold was reached to 10% was significant for an increased pellet DU. The first threshold was reached at 180 at 180 MPa (dried at 140 ◦C) and above 240 MPa for the rest of the samples (Figure3c). MPa (dried at 140 °C) and above 240 MPa for the rest of the samples (Figure 3c). The The second threshold was reached only for material dried at 140 ◦C and at about 230 MPa. second threshold was reached only for material dried at 140 °C and at about 230 MPa. The The results presented indicate that water (moisture content 5% and 10%) is a binder and results presented indicate that water (moisture content 5% and 10%) is a binder and leads leads to improved connections between particles [39,40]. This results in an increase in DE toand improved DU values. connections Slightly higherbetween DE particles values for[39,40]. material This driedresults at in 140 an◦ increaseC can be in caused DE and by DUthe sugarsvalues. containedSlightly higher in the DE raw values material for cells material and released dried at due 140 to °C this can temperature. be caused by These the sugarssugars contained in combination in the with raw moisture material act cells as aan binder.d released However, due to this this needs temperature. to be confirmed These sugarsin further in combination tests. with moisture act as a binder. However, this needs to be confirmed in furtherThe statisticaltests. analyses carried out allowed the identification of groups between which there are significant statistical differences. Results for DE are presented in Table1, and for DU in Table2, with significance level α = 0.05.

Molecules 2021, 26, x 5 of 31 MoleculesMolecules 2021 2021,,26 26,, 1014 x 55 ofof 2931

Figure 2. Changes of test pellet specific density DE due to the temperature of drying and compac- FigureFigure 2.2. ChangesChanges ofof testtest pelletpellet specific specific density density DE DE due due to to the the temperature temperature of of drying drying and and compaction compaction pressure: (a) dry sample, (b) moisture content at a level of 5%, and (c) moisture content at a pressure:tion pressure: (a) dry (a sample,) dry sample, (b) moisture (b) moisture content content at a level at of a 5%,level and of 5%, (c) moisture and (c) moisture content at content a level ofat 10%. a level of 10%. level of 10%.

FigureFigure 3.3. Changes of of test test pellet pellet mechanical mechanical durability durability DU DU due due to tothe the temperature temperature of ofdrying drying and and Figure 3. Changes of test pellet mechanical durability DU due to the temperature of drying and compaction pressure: (a) dry sample, (b) moisture content at a level of 5%, and (c) moisture con- compactioncompaction pressure:pressure: ((aa)) drydry sample,sample, ((bb)) moisturemoisture contentcontent atat aa levellevel of of 5%, 5%, and and ( c(c)) moisture moisture content content at a level of 10%. attent a levelat a level of 10%. of 10%.

Molecules 2021, 26, x 6 of 31

The statistical analyses carried out allowed the identification of groups between which there are significant statistical differences. Results for DE are presented in Table 1, Molecules 2021, 26, 1014and for DU in Table 2, with significance level α = 0.05. 6 of 29

Table 1. Three-way ANOVA results for specific density.

Table 1. Three-way ANOVASS resultsdf for speciﬁcMS density. F Value p-Value Intercept 216951606 1 216951606 2465092 0.00 SS df MS F Value p-Value Temperature 65 2 33 0 0.69 Moisture Intercept589762 2 216,951,606294881 1 3351216,951,606 0.002,465,092 0.00 Pressure Temperature666172 5 133234 65 21514 330.00 0 0.69 Moisture 589,762 2 294,881 3351 0.00 Temperature x Moisture 4045 4 1011 11 0.00 Pressure 666,172 5 133,234 1514 0.00 Temperature x Pressure 1682 10 168 2 0.046 Temperature x Moisture 4045 4 1011 11 0.00 Moisture x PressureTemperature 91084 x Pressure 10 1682 9108 10 103 168 0.00 2 0.046 Temperature x Moisture x Moisture x4169 Pressure 20 91,084 208 10 2 9108 0.0016 103 0.00 PressureTemperature x Moisture x Pressure 4169 20 208 2 0.0016 Error Error14258 162 14,25888 162 88

Table 2. Three-way analysis of variance ANOVA—DU. Table 2. Three-way analysis of variance ANOVA—DU. SS df MS F Value p-Value Intercept 1523952 1 SS1523952df 7800727 MS 0.00 F Value p-Value Temperature Intercept687 2 1,523,952343 11758 1,523,952 7,800,7270.00 0.00 Moisture Temperature29810 2 68714905 276294 343 0.00 1758 0.00 Pressure Moisture47372 5 29,8109474 248497 14,905 0.00 76,294 0.00 Temperature x Moisture Pressure 136 4 47,372 34 5 174 9474 0.00 48,497 0.00 Temperature x PressureTemperature x Moisture 211 10 136 21 4 108 34 0.00 174 0.00 Moisture x PressureTemperature x Pressure29103 10 211 2910 10 14897 21 0.00 108 0.00 Temperature x MoistureMoisture x x Pressure 29,103 10 2910 14,897 0.00 Temperature x Moisture233 x Pressure20 233 12 20 60 120.00 60 0.00 Pressure Error 32 162 0 Error 32 162 0

It was shownIt wasthat shownthe drying that temperature the drying temperature has no influence has no on inﬂuence DE and has on DEa slight and has a slight effect on DU.effect Therefore, on DU. a Therefore,layered graph a layered was made graph to was show made how to the show changes how thein DE changes and in DE and DU progressDU in progressaccordance in accordancewith the moistu withre the content moisture and content compaction and compactionunder pressure under pressure (Figure 4). (Figure4).

Figure 4. ChangesFigure of 4. test Changes pellet of quality test pellet due toquality moisture due to content moisture and content compaction and compaction pressure:( apressure:) speciﬁc (a density) spe- and (b) mechanical durability.cific density and (b) mechanical durability.

2.2. Kinetic Analysis of Miscanthus Torrefaction Process 2.2. Kinetic Analysis of Miscanthus Torrefaction Process Thermogravimetric analysis of the Miscanthus torrefaction process was performed using three different heating ratios 5, 10, and 20 K/min. The results of the TGA analysis for Miscanthus are shown in Figure5. Molecules 2021, 26, x 7 of 31

Molecules 2021, 26, x 7 of 31

Thermogravimetric analysis of the Miscanthus torrefaction process was performed Molecules 2021, 26, 1014usingThermogravimetric three different heating analysis ratios of 5, the 10, Miscanthus and 20 K/min. torrefaction The results process of the was TGA performed analysis 7 of 29 forusing Miscanthus three different are shown heating in Figure ratios 5,5. 10, and 20 K/min. The results of the TGA analysis for Miscanthus are shown in Figure 5.

Figure 5. Thermogravimetric analysis results for heating speeds of 5, 10, and 20 K/min determined Figure 5. ThermogravimetricforFigure Miscanthus. 5. Thermogravimetric analysis results analysis for heating results speedsfor heatin ofg 5, speeds 10, and of 20 5, K/min 10, and determined 20 K/min determined for Miscanthus. for Miscanthus. Making assumptionsMaking assumptions for the stability for thef(x) stabilityat a given f(x) level at a of given conversion, level of this conversion, technique this technique enablesMaking the enableslogarithm assumptions the dx/dt logarithm for to the be stability dx/dtplotted tof(x) as be aat plotted functiona given aslevel of a temperature function of conversion, of temperature inverse, this technique and inverse, the and the activationenables the energy activationlogarithm (Figure energydx/dt 6) and to (Figure bepre-exponential plotted6) and as pre-exponential a function coefficients of temperature coefﬁcientsto be estimated to inverse, be for estimated each and level the for each level ofactivation conversion energyof while conversion (Figure not knowing 6) while and pre-exponential not the knowing reaction themechanism coefficients reaction mechanism(Figure to be estimated7). Table (Figure 3 for contains7). each Table level the3 contains the activationof conversion energyactivation while and not energypre-exponential knowing and pre-exponentialthe reaction coefficien mechanismt coefﬁcientas a function (Figure as aof function the7). Tablelevel of of3 the contains conversion level of the conversion by byactivation the Ozawa–Flynn–Walla energythe Ozawa–Flynn–Walla and pre-exponential method. method. coefficient as a function of the level of conversion by the Ozawa–Flynn–Walla method.

Figure 6. Presentation of the Ozawa–Flynn–Wall logarithm dx/dt function for Miscanthus. Figure 6. Presentation of the Ozawa–Flynn–Wall logarithm dx/dt function for Miscan- thus.Figure 6. Presentation of the Ozawa–Flynn–Wall logarithm dx/dt function for Miscan- Table 3. Pre-exponential coefﬁcient and activation energy as a function of the degree of conversion thus. by the Ozawa–Flynn–Walla method for Miscanthus.

Fract. Mass Loss Activation Energy (kJ/mol) Log (A/s−1) 0.02 115.33 ± 34.46 8.05 0.05 105.80 ± 23.54 6.87 0.10 109.45 ± 18.57 7.27 0.20 99.07 ± 18.21 6.27 0.30 100.06 ± 21.66 6.30 0.40 104.41 ± 25.94 6.63 0.50 119.38 ± 28.20 7.93 ± 0.60 145.97 27.97 10.23 0.70 186.50 ± 22.91 13.67

Molecules 2021Molecules, 26, x 2021, 26, 1014 8 of 31 8 of 29

Figure 7.FigurePre-exponential 7. Pre-exponential coefficient coefficient and activation and activation energy energy of Ozawa–Flynn–Walla of Ozawa–Flynn–Walla conversion conversion for Miscanthus. for Miscanthus. Using the Netzsch Kinetics 3 program, based on thermogravimetric TG curves estimated as a function of temperature, a mathematical adjustment of one thermal depoly- Table 3. Pre-exponential coefficient and activation energy as a function of the degree of conversion by the Ozawa–Flynn–Wallamerization reactionmethod for model Miscanthus. was made. By using the estimated reaction model, kinetic parameters were delivered, that is to say, the velocity constant (pre-exponential factor) k Fract. Mass Loss Activation Energy (kJ/mol) 2 Log (A/s^−1) and activation energy Ea, in addition to the fitting factor R in Table4. The most optimal fit 0.02for the experimental data for115.33 the ± Miscanthus 34.46 sample was found 8.05 for the n-order reaction 0.05model shown in AppendixA105.80, Table ± A1 23.54 for n = 2.9. The correlation 6.87 coefficient R2 was 0.9932, 0.10 109.45 ± 18.57 7.27 the pre-exponential coefficient 10.42, and the activation energy 142.52 kJ/mol. Figure8 0.20 99.07 ± 18.21 6.27 represents the fit of the equation with estimated coefficients fitted to the experimental data. 0.30 100.06 ± 21.66 6.30 0.40 104.41 ± 25.94 6.63 Table 4. Proximate0.50 and ultimate analysis of Miscanthus119.38 ± 28.20 before and after the torrefaction 7.93 process. 0.60 145.97 ± 27.97 10.23 HHV Mass Loss Energy Crop M (%) C (%) N (%) H (%) S (%) O (%) Cl (%) V (%) A (%) d ad d 0.70 d d d 186.50 ±d 22.91 d d 13.67d (MJ/kg) (%) Miscanthus 5.3 48.5 0.27 6.20 0.05 42.56 0.115 91.29 2.3 15.82 - Torrefied Using the Netzsch Kinetics 3 program, based on thermogravimetric TG curves esti- Miscanthus: mated as a function of temperature, a mathematical adjustment of one thermal depoly- ◦ (257 C, 9 min)merization 2.8 54.37 reaction 0.19model was 5.37 made. By 0.05 using 36.06the estimated 0.014 reaction 73.37 model, 3.94 kinetic pa- 21.70 30.69 (300 ◦C, 6 min) 1.5 57.04 0.16 4.93 0.05 32.36 0.013 61.11 5.44 26.72 43.56 (525 ◦C, 5 min)rameters 1.1 were 59.29 delivered, 0.14 that 3.69is to say, the 0.04 velocity 27.61 constant 0.012 (pre-exponential 44.27 factor) 9.21 k and 27.69 76.24 activation energy Ea, in addition to the fitting factor R2 in Table 4. The most optimal fit for Molecules 2021, 26M—moisture, x content, C—carbon content, N—nitrogen content, H—hydrogen content, S—sulfur content, O—oxygen content,9 of 31 Cl—chlorine the experimental data for the Miscanthus sample was found for the n-order reaction content, V—volatile matter content, A—ash content, HHV—higher heating value, ad—air dried state, and d—dry state. model shown in Appendix A, Table 7 for n = 2.9. The correlation coefficient R2 was 0.9932, the pre-exponential coefficient 10.42, and the activation energy 142.52 kJ/mol. Figure 8 represents the fit of the equation with estimated coefficients fitted to the experimental data.

Table 4. Proximate and ultimate analysis of Miscanthus before and after the torrefaction process.

Mad Cd Nd Hd Sd Od Cld Vd Ad HHVd Mass Loss Energy Crop (%) (%) (%) (%) (%) (%) (%) (%) (%) (MJ/kg) (%) Miscanthus 5.3 48.5 0.27 6.20 0.05 42.56 0.115 91.29 2.3 15.82 - Torrefied Miscanthus: (257 °C, 9 min) 2.8 54.37 0.19 5.37 0.05 36.06 0.014 73.37 3.94 21.70 30.69 (300 °C, 6 min) 1.5 57.04 0.16 4.93 0.05 32.36 0.013 61.11 5.44 26.72 43.56 (525 °C, 5 min) 1.1 59.29 0.14 3.69 0.04 27.61 0.012 44.27 9.21 27.69 76.24 M—moisture content, C—carbon content, N—nitrogen content, H—hydrogen content, S—sulfur content, O—oxygen content, Cl—chlorine content, V—volatile matter content, A—ash content, HHV—higher heating value, ad—air dried state, and d—dry state.

Figure 8. Fitting the calculated kinetic model to the experimental data for the Miscanthus. Figure 8. Fitting the calculated kinetic model to the experimental data for the Miscanthus. One important fact that has to be remembered is that the estimated kinetic trio is only the optimal mathematical fit of the equation to the experimental data, and in this case there is no exact physical significance (reaction order). By using the above fitting procedure, the activation energy, thus delivered, is visible: It is only used to correlate the model with the experimental data and does not matter with reference to the definition. The conclusions are that kinetic analysis methods employing the results of different heating ratios were more suitable than a single heating ratio method.

3. Discussion 3.1. TGA Analysis

A TGA analysis of Miscanthus was performed in an inert atmosphere N2 using TGA Netzsch Tarsus 209 FC (Netzsch, SelbBavaria, Germany). In Figure 9 we see the derivative thermogravimetric Differential Thermogravimetric DTG and thermogravimetric TG analysis results of the Miscanthus sample torrefied in an N2 atmosphere.

Figure 9. Thermogravimetric analysis of the Miscanthus torrefaction process for the production of biocarbon at a temperature of 525 °C (heating ratio 20 K/min) in an N2 atmosphere.

Molecules 2021, 26, x 9 of 31

Molecules 2021, 26, 1014 9 of 29

Figure 8. Fitting the calculated kinetic model to the experimental data for the Miscanthus.

One importantOne fact important that has factto be that remembered has to be rememberedis that the estimated is that the kinetic estimated trio is kineticonly trio is only the optimalthe mathematical optimal mathematical fit of the equation fit of the to equation the experimental to the experimental data, and data, in this and case in this case there there is no exactis no exactphysical physical significance significance (reaction (reaction order). order). By using By usingthe above the above fitting fitting proce- procedure, the dure, the activationactivation energy, energy, thus thus delivered, delivered, is visible: is visible: It is It on isly only used used to correlate to correlate the model the model with the with the experimentalexperimental data data and and does does not notmatte matterr with with reference reference to the to definition. the definition. The con- The conclusions clusions areare that that kinetic kinetic analysis analysis methods methods empl employingoying the results the results of different of different heating heating ratios ratios were were more suitablemore suitable than a thansingle a heating single heating ratio method. ratio method.

3. Discussion3. Discussion 3.1. TGA Analysis 3.1. TGA Analysis A TGA analysis of Miscanthus was performed in an inert atmosphere N2 using A TGA analysis of Miscanthus was performed in an inert atmosphere N2 using TGA TGA Netzsch Tarsus 209 FC (Netzsch, SelbBavaria, Germany). In Figure9 we see the Netzsch Tarsus 209 FC (Netzsch, SelbBavaria, Germany). In Figure 9 we see the derivative derivative thermogravimetric Differential Thermogravimetric DTG and thermogravimetric thermogravimetric Differential Thermogravimetric DTG and thermogravimetric TG anal- TG analysis results of the Miscanthus sample torreﬁed in an N2 atmosphere. ysis results of the Miscanthus sample torrefied in an N2 atmosphere.

Figure 9. ThermogravimetricFigure 9. Thermogravimetric analysis of the Miscanthusanalysis of the torrefaction Miscanthus process torrefaction for the production process for of the biocarbon production at a temperatureof ◦ 2 of 525 C (heatingbiocarbon ratio 20 at K/min) a temperature in an N of2 atmosphere. 525 °C (heating ratio 20 K/min) in an N atmosphere.

The ﬁrst phase of the Miscanthus roasting process can be detected at temperatures of approximately 257 ◦C because of the thermal depolymerization and destruction of the organic compounds which contain the Miscanthus. The thermal depolymerization of the Miscanthus was stopped at 525 ◦C. A mass reduction resulted in a lower energy yield. In this research, the focus was on the determination of apparent kinetic parameters, and it is of great use for the engineering design stage of the installation of both a biomass regimental dryer and a reactor. The main focus of this work was not on conducting research to get to know the fundamental chemical mechanisms of the torrefaction of Miscanthus.

3.2. DSC Analysis The torrefaction process initiation temperatures along with the heat liberated at different heating ratios were calculated from the DSC curves. The initiation temperature rises together with increasing heating ratios, and this is in agreement with the data presented in Figure 10. The initiation temperature increased by nearly 20 ◦C when increasing the heating ratio from 10 to 20 ◦C/min. The heat liberated during the torrefied Miscanthus (torrefaction temperature; 257 ◦C) combustion increased by 995 W/g when increasing the heating ratio from 10 to 20 K/min. In Figure 10 we can see the DSC pyrolysis curves of torrefied Miscanthus at different heating ratios of 10, and 20 K/min in an N2 atmosphere in order to investigate the pyrolysis of torrefied Miscanthus in different temperatures. Molecules 2021, 26, 1014 10 of 29

Decomposition of the first component (hemicellulose) of the torrefied Miscanthus, followed by a broad exothermic pyrolysis peak of the mixture of three principal constituents that shifted to a higher temperature with an increasing heating ratio from 10 to 20 K/min. The torrefied Miscanthus pyrolysis decomposition peaks appeared at 320.3, 320.6, 336.4, and 345.1 ◦C with heating ratios of 10 and 20 K/min, respectively (torrefaction temperature; 257 ◦C). Le Brech et al. [48,50] have studied the combustion of nearly 30 common biomass types, including Miscanthus, and they reported that Miscanthus showed two temperature ranges of combustion of 240–340 and 450−550 ◦C using the DTG data. Furthermore, they found that the combustion characteristics of the lignocellulosic biomass are about the same, and thus they proposed that the two steps of the kinetic reaction are:

X(solid)→Y(solid) + G1(gas) Step 1

Molecules 2021, 26, x 11 of 31 Y(solid)→A(ash) + G2(gas) Step 2

◦ ◦ Figure 10. DSCFigure analysis 10. DSC of analysis torreﬁed of Miscanthus torrefied Miscanthus 257 C and 257 300 °CC and (heating 300 °C ratios: (heating 10, 20ratios: K/min) 10, 20 in anK/min) N2 atmosphere. in an N2 atmosphere. 3.3. C, H, N, O, S Analysis 3.3. C, H, N, O, S Analysis In Table4 the results of a proximate analysis of Miscanthus before and after the In Table 4 the results of a proximate analysis of Miscanthus before and after the ther- thermochemical conversion process are presented. It is quite clear that the C% (weight) in mochemical conversion process are presented. It is quite clear that the C% (weight) in the the biomass thermochemical process bio-products increases in tandem with an enhanced biomass thermochemical process bio-products increases in tandem with an enhanced Mis- Miscanthus torrefaction process temperature. This is contrary to the weight percentages of canthus torrefaction process temperature. This is contrary to the weight percentages of H H and O which showed a decreasing trend. We can conclude from the above mechanism and O which showed a decreasing trend. We can conclude from the above mechanism that dehydration takes place as well as decarbonization during the Miscanthus torrefaction that dehydration takes place as well as decarbonization during the Miscanthus torrefac- process. This clearly shows that a decrease in the H and O contents of torreﬁed biomass tion process. This clearly shows that a decrease in the H and O contents of torrefied bio- will result from the emission of CO2, CO, or H2O[22]. A rising percentage of the C content mass will result from the emission of CO2, CO, or H2O [22]. A rising percentage of the C is due solely to a decrease in the O content [25,26,36,37]. content is due solely to a decrease in the O content [25,26,36,37].

Molecules 2021, 26, 1014 11 of 29

3.4. Energy Yield, Mass Yield, Volatile Matter, Ash Content, High Heating Value In Figure 10, TGA Miscanthus carbonization process curves are shown with green points in the drawing. The Miscanthus torrefaction process was performed up to 525 ◦C for 9 min where the samples were kept under isothermal conditions. As is evident, the percentage of Miscanthus mass reduction was 78.80%. The mass decreased by 0.13% while the samples were held under isothermal conditions at 525 ◦C. As seen in Figure 10, it was at about a temperature of 80 ◦C where the initial evaporation of moisture from the sample occurs. In the case of carbonization up to 257 ◦C, the maximum heating ratio was 2.1%/min at 266 ◦C (the maximum instantaneous temperature of this process during stabilization to 257.5 ◦C). The maximum carbonization process speed for Miscanthus up to 300 ◦C was 5.16%/min at 300 ◦C, and in the case of thermochemical conversion up to 525 ◦C, the Molecules 2021, 26, x 12 of 31 maximum process speed was 10.42%/min at a temperature of about 337 ◦C. Figure 11 shows the trajectory of torrefaction curves. In the drawing, the blue points show all carbonization process curves representing the end of the heating process and firstthe stage ﬁrst of stage maintaining of maintaining the roasting the roasting sample sampleat a specific at a speciﬁctemperature. temperature. Miscanthus Miscanthus thermochemicalthermochemical conversion conversion was conducted was conducted up to up 257 to °C 257 and◦C andwith witha residential a residential time time under under isothermalisothermal conditions conditions equal equal to to 9 9min, min, and and it it was was evident evident that that the the Miscanthus Miscanthus mass mass reduc- reduction tionstood stood at 10.62%.at 10.62%. However, However, the the mass mass reduction reduction was was 11.68% 11.68% after after a period a period of 6 of min 6 min at 300 at ◦C, 300and °C, when and when it was it held was underheld under isothermal isothermal conditions conditions at 525 at◦C 525 the °C mass the reducedmass reduced by 0.13%. by A 0.13%.set of A maximum set of maximum process process speeds speeds (minima (minima on DTG on curves)DTG curves) were were given given alongside alongside a set of a setTG-DTG. of TG-DTG. As is evident As is evident in Figure in Figure11, the initial11, the evaporation initial evaporation of moisture of moisture from the from Miscanthus the Miscanthusstraw sample straw occurs sample at theoccurs quickest at the speedquickest at aroundspeed at 70 around◦C where 70 °C torrefaction where torrefaction takes place takesup toplace 257 ◦upC. Theto 257 maximum °C. The Miscanthus maximum torrefactionMiscanthus process torrefaction speed process was 8.01%/min speed was where 8.01%/minthe temperature where the was temperature around 300 was◦C and around for torrefaction 300 °C and upfor totorrefaction 525 ◦C the up maximum to 525 °C process the maximumspeed was process 8.81%/min speed wherewas 8.81%/min the temperature where the was temperature 305 ◦C. was 305 °C.

Figure 11. Analyses of Miscanthus mass loss with DTG curves during the torrefaction process. Figure 11. Analyses of Miscanthus mass loss with DTG curves during the torrefaction process. As can be seen, the Miscanthus weight decrease was 2.10%/min when the holding time As can be seen, the Miscanthus weight decrease was 2.10%/min when the holding under isothermal conditions was 6 min at 300 ◦C in the Miscanthus torrefaction process time under isothermal conditions was 6 min at 300 °C in the Miscanthus torrefaction pro- carried out up to 257 ◦C while keeping the sample 9 min under isothermal conditions. In cess carried out up to 257 °C while keeping the sample 9 min under isothermal conditions. addition, the weight decrease was 5.16%/min while when holding the sample at 525 ◦C, In addition, the weight decrease was 5.16%/min while when holding the sample at 525 °C, and the mass reduction was 10.42%/min. and the mass reduction was 10.42%/min. The torrefaction process was performed on Miscanthus to establish the best torrefac- The torrefaction process was performed on Miscanthus to establish the best torrefaction temperature for a variety of industrial applications such as carbonized solid biofuel, tion temperature for a variety of industrial applications such as carbonized solid biofuel, a carrier for bio-fertilizers, and activated carbon. Pyrolysis is the term used where the a carrier for bio-fertilizers, and activated carbon. Pyrolysis is the term used where the thermal treatment is above 300 ◦C. Visible rises in the high heating value (HHV) up to thermal treatment is above 300 °C. Visible rises in the high heating value (HHV) up to 400 °C are connected to the removal of oxygen and hydrogen. There is a reduction in the HHV when the temperature rises above 500 °C. In Table 5 there is presented content of mineral elements in the torrefied Miscanthus and untreated Miscanthus at three different temperatures.

Molecules 2021, 26, 1014 12 of 29

400 ◦C are connected to the removal of oxygen and hydrogen. There is a reduction in the HHV when the temperature rises above 500 ◦C. In Table5 there is presented content of mineral elements in the torreﬁed Miscanthus and untreated Miscanthus at three different temperatures.

Table 5. Content of mineral elements in the torreﬁed Miscanthus and untreated Miscanthus at three different temperatures.

Mineral Composition of Torrefied Mineral Composition of Torrefied Mineral Composition of Torrefied Miscanthus (257.5 ◦C) Miscanthus (300 ◦C) Miscanthus (525 ◦C) Arithmetic Standard Aritmetic Standard Arithmetic Standard Element Average Deviation Average Deviation Average Deviation C 68.33 1.175 73.98 1.742 83.75 2.195 O 29.78 1.535 24.11 2.552 14.23 2.556 Al 0.09 0.017 0.87 0.749 0.07 0.016 Si 1.49 0.443 1.05 0.260 0.54 2.508 K 0.08 0.064 0.08 0.015 0.82 0.356 Ca 0.07 0.017 0.19 0.038 0.13 0.040 Mg 0.06 0.10 0.07 0.000 0.11 0.028 S 0.06 0.000 0.23 0.052 0.15 0.008 P 0.11 0.033 0.12 0.040 0.17 0.049

The torrefied Miscanthus produced as a result of ecological fertilization is characterized by low cultivation costs and the heat energy produced in this carbonized solid biofuel requires about 25% less expenditure than during fertilization with chemical fertilizers. Figure 12 SEM microscopic images of torrefied Miscanthus in three different temperatures. We can see how the –OH groups are degraded with torrefaction process temperature increase. This article presents results showing that the energy yield drops slowly from 90% to 33% as the temperature of the Miscanthus carbonization process increases. During the experiment, reducing the weight of Miscanthus during the roasting process at 300 ◦C shows the nearest weight reduction to 50%. Miscanthus was then held for 6 min, and the weight reduction corresponding to this residence time was 40.72%. These data are used to calculate the profitability of using Miscanthus as a carrier for bio-fertilizers. A high carbonization rate and low energy input in combination have the most significant impact on the economics of the torrefaction process. Therefore, it was assumed that the use of Miscanthus as a carrier for bio-fertilizers after roasting should not exceed 50% in weight reduction. So that this can be achieved, the holding time needs to be as low as possible. In order to produce activated carbon, the weight loss should not exceed 75%, while during thermochemical conversion of Miscanthus at 525 ◦C it was noticed that the residence time with the shortest period was 5 min, and the weight loss was 78.80%. Figure 13 shows that the mass residua of Miscanthus torrefaction products proceed at three temperatures, 257.5 ◦C, 300 ◦C, and 525 ◦C, in residential times from 5 to 10 min (under isothermal conditions) using three different heating ratios, 5, 10, and 20 K/min. Three heating ratios were deployed, and data were checked on the elemental composition and thermal stability of the isotherm research material. It shows a different mass loss ratio in correlation with different residential times. More detail analysis of the results presented at Figure 13 was described in Section 3.5 Biomass Torrefaction Residence Time and its effect on the process. At 300 ◦C, an alteration may be observed, which indicates a substantial variance between the values before and after the biomass carbonization process, a maximum mass loss of 44.10%, and minimum mass loss of 32.38%. In spite of that, the residual masses at 257.5 ◦C are the highest of all recorded in the thermogravimetrical analysis. This is an excellent result that demonstrates mass reduction with an increase of temperature: The mass percent of the biomass at 525 ◦C is the lowest of all measurements. When the isotherm is extended at 300 ◦C, the Miscanthus mass decreases. Dynamic and isothermal measurements at a 10 K/min heating ratio were made on samples of Miscanthus. This yielded data on the thermal stability and the elemental composition of the isotherm research material. This was divided into five samples, from 5 to 10 min, according to the Molecules 2021, 26, 1014 13 of 29

measurements found at the three temperatures of 525, 300, and 257 ◦C[63]. As a result of these studies, it was found that the most optimal torrefaction temperatures for Miscanthus for the production of a biocarbon as a carrier for fertilizers lie between 300 and 340 ◦C looking at it from a mass loss ratio and economical perspective. However, the temperature may differ in practice based on the initial state of the raw material, the heating ratio, and the chemical composition of the raw biomass [2]. Researchers from Norway who have been working on biochar use as an additive for bio fertilizers have found that persistent biochar C with a half-life 60 times higher than the parent feedstock can be achieved at pyrolysis temperatures as low as 370 ◦C, with no further gains to be made at higher temperatures. Biochar produced from Miscanthus and corn cob re-applied to soil previously incubated at the highest temperatures was mineralized faster than when applied to non-incubated soil. Their results from a 1-year incubation study show that biochars with long residence time in soils can be produced from Miscanthus and corncob feedstocks when a threshold temperature requirement of 370 ◦C is reached, and there is no gain in resistance beyond this value. It is also important to mention that they state that the mechanisms behind these Molecules 2021, 26, x effects need to be better understood if we want to predict the long-term stability14 of 31 of biochar

in soil.

(a) (b) (c)

(d) (e) (f)

(g) (h) (i) FigureFigure 12. SEM 12. SEM microscopic microscopic images images of of torrefied torrefied Miscanthus:Miscanthus: 257 257 °C◦ Cmagnification magnification (a) 5000×, (a) 5000 (b)× 1000×,,(b) 1000and ×(c), 500× and (andc) 500 × and 300 °C magnification (d) 5000×, (e) 1000×, (f) 500×, 525 °C (g) 5 000×, (h) 1 000×, and (i) 500×. 300 ◦C magnification (d) 5000×, (e) 1000×,(f) 500×, 525 ◦C(g) 5000×,(h) 1000×, and (i) 500×. Figure 13 shows that the mass residua of Miscanthus torrefaction products proceed at three temperatures, 257.5 °C, 300 °C, and 525 °C, in residential times from 5 to 10 min (under isothermal conditions) using three different heating ratios, 5, 10, and 20 K/min. Three heating ratios were deployed, and data were checked on the elemental composition and thermal stability of the isotherm research material. It shows a different mass loss ratio in correlation with different residential times. More detail analysis of the results presented at Figure 13 was described in section: 3.5 Biomass Torrefaction Residence Time and its effect on the process.

Molecules 2021, 26, 1014 14 of 29 Molecules 2021, 26, x 15 of 31

(a)

(b)

(c)

Figure 13. TorrefactionFigure 13. process Torrefaction of Miscanthus: process of Miscanthus: Thermogravimetric Thermogravimetric analysis results analysis and results optimal and processoptimal conditions for the productionprocess of carbonized conditions solid for biofuel, the production carrier forof carbonized natural fertilizer solid biofuel, and activated carrier for carbon natural (Figure fertilizer 13 heating and ratios: (a) activated carbon (Figures 13 heating ratios: (a) 5 K/min, (b) 10 K/min, and (c) 20 K/min). 5 K/min, (b) 10 K/min, and (c) 20 K/min).

Molecules 2021, 26, 1014 15 of 29

3.5. Biomass Torrefaction Residence Time and its Effect on the Process A thermogravimetric analysis of the Miscanthus torrefaction process contributed to the observation that biomass thermochemical conversion temperatures above 300 ◦C resulted in decreasing energy and mass yield. In Figure 13a correlation can be seen between the effects of the Miscanthus torrefaction residual time and the characteristics of the Miscanthus carbonization process. It was observed that the result of thermochemical conversion residual time on volatile compounds in terms of ash content was not as large as that of the Miscanthus carbonization process conditions such as temperature. In Figure 13, we observed that the mass yield reduces when holding time increases. The decrease in the water content and volatile matter of the Miscanthus provides an explanation for this. In spite of that, there was a very high mass loss at the start of the biomass torrefaction process while the change of mass yield was not so meaningful with regard to a longer torrefaction residence time. The explanation for this lies in the degradation of more reactive biomass elements at the ﬁrst stage of the torrefaction process. From Figure 13 it is possible to infer that the HHV increases with Miscanthus torrefaction time. Nevertheless, in research performed thus far, the Miscanthus thermochemical conversion temperature is more signiﬁcant than residual mass.

3.6. Volatile Organic Content (VOC) Analysis When Miscanthus carbonization was conducted at temperatures up to 300 ◦C it was found that mass loss approximated to that of the untreated Miscanthus before the thermochemical conversion process indicated by the mass yield of Miscanthus. This clearly demonstrated that the extent of torrefaction for Miscanthus up to 300 ◦C was small when equated to values above 300 ◦C. The first visible weight loss is closely related to the moisture loss in the first stage of the process (the fastest at 64–74 ◦C to 110 ◦C), and the second loss is thermal decomposition, which creates volatile organic compounds such as water vapor, carbon monoxide, carbon dioxide, acetic acid, and other organic substances (Figure 14). This effect can be observed in moisture loss and then in the depolymerization of secondary cell wall components, hemicellulose, cellulose, and lignin, as can be seen in Figure 13. Weight reduction during thermochemical conversion at lower temperatures can mainly be explained by H2O loss. In the process of torrefaction of Miscanthus at a temperature over 300 ◦C, the reduction is associated with the thermal decomposition of this energy crop. The key effect that was observed during this process is the increase in volatile matter content, while ash content was reduced where temperatures were over 525 ◦C. The HHV increases constantly with a rise of volatile compounds and the decrease of ash content. Figures 14–16 show the results of volatile organic measurements for Miscanthus. The research results found that a Miscanthus carbonization process carried out below 300 ◦C is more suited to producing solid biofuels. Hydrocarbon emissions were observed above 370 ◦C for the torrefaction process, with a direct and negative impact on torrefaction energy efficiency. As can be seen in Figure 11, between 260 and 280 ◦C the most active thermal- chemical conversion temperatures for Miscanthus are disturbed in order to obtain a size difference of 45–50%. At temperatures exceeding 300 ◦C, HHV remained at 24 MJ/kg in this range, and the increase in energy was maintained above 70%. Energy efficiency in relation to HHV remains at the most optimal level at a higher range. The area of carbonization changes in cases of new physico-chemical properties of the primary material and heating speed [64]. Molecules 2021, 26, x 17 of 31

The first visible weight loss is closely related to the moisture loss in the first stage of the process (the fastest at 64–74 °C to 110 °C), and the second loss is thermal decomposition, which creates volatile organic compounds such as water vapor, carbon monoxide, carbon dioxide, acetic acid, and other organic substances (Figure 14). This effect can be observed in moisture loss and then in the depolymerization of secondary cell wall components, hemicellulose, cellulose, and lignin, as can be seen in Figure 13. Weight reduction during thermochemical conversion at lower temperatures can mainly be explained by H2O loss. In the process of torrefaction of Miscanthus at a temperature over 300 °C, the reduction is associated with the thermal decomposition of this energy crop. The key effect that was observed during this process is the increase in volatile matter content, while ash Molecules 2021, 26, 1014 content was reduced where temperatures were over 525 °C. The HHV increases constantly 16 of 29 with a rise of volatile compounds and the decrease of ash content.

Molecules 2021, 26, x FigureFigure 14. 14.ThermogravimetricThermogravimetric curve curve and ion and current ion current curves curvesfor Miscanthus for Miscanthus thermochemical thermochemical con- conver-18 of 31 ◦ Molecules 2021, 26, x versionsion performed at at 525 525 °C.C. 18 of 31

Figures 14–16 show the results of volatile organic measurements for Miscanthus. The research results found that a Miscanthus carbonization process carried out below 300 °C is more suited to producing solid biofuels. Hydrocarbon emissions were observed above 370 °C for the torrefaction process, with a direct and negative impact on torrefaction energy efficiency. As can be seen in Figure 11, between 260 and 280 °C the most active thermal-chemical conversion temperatures for Miscanthus are disturbed in order to obtain a size difference of 45%–50%. At temperatures exceeding 300 °C, HHV remained at 24 MJ/kg in this range, and the increase in energy was maintained above 70%. Energy efficiency in relation to HHV remains at the most optimal level at a higher range. The area of carbonization changes in cases of new physico-chemical properties of the primary material and heating speed [64].

FigureFigure 15. 15. MassMass loss lossof Miscanthus of Miscanthus after a after torrefaction a torrefaction process processin a CO2 inatmosphere a CO2 atmosphere using an elec- using an Figure 15. Mass loss of Miscanthus after a torrefaction process in a CO2 atmosphere using an elec- tricalelectrical furnace. furnace. trical furnace.

Figure 16. Volatile organic content (VOC) emission in ppm of the Miscanthus torrefaction process in Figure 16. Volatile organic content (VOC) emission in ppm of the Miscanthus torrefaction process an electric oven in a CO2 atmosphere. in an electric oven in a CO2 atmosphere. Figure 16. Volatile organic content (VOC) emission in ppm of the Miscanthus torrefaction process in an Onelectric Figure oven 17 inemission a CO2 atmosphere. index for the torrefied Miscanthus in electric oven is presented which is defined as: The amount of pollutants emitted in relation to the amount of fuel subjectedOn Figure to a specific 17 emission process. index The for emission the torrefied factor Miscanthusis given in mg in electricof a given oven pollutant is presented whichreferred is to defined one gram. as: The amount of pollutants emitted in relation to the amount of fuel subjected to a specific process. The emission factor is given in mg of a given pollutant referred to one gram.

Molecules 2021, 26, 1014 17 of 29

On Figure 17 emission index for the torrefied Miscanthus in electric oven is presented which is defined as: The amount of pollutants emitted in relation to the amount of fuel Molecules 2021, 26, x 19 of 31 subjected to a specific process. The emission factor is given in mg of a given pollutant referred to one gram.

4 Emission index (mg/g) index Emission

0 200 250 300 350 400 450 500 550 Temperature (°C)

Figure 17. Emission index for the Miscanthus torrefaction process using an electric oven. Figure 17. Emission index for the Miscanthus torrefaction process using an electric oven. 4. Materials and Methods 4.4.1. Materials Drying andand PelletizingMethods Methods 4.1. DryingThis researchand Pelletizing concentrated Methods on the drying and torrefaction process of one energy crop, Miscanthus.This research Stems concentrated were harvested on the in thedrying winter and of torrefaction 2019/20, chipped, process and of one then energy sieved. crop, The Miscanthus.study was conducted Stems were on harvested chips that in went the winter through of a 2019/20, 16 mm sieve chipped, and remainedand then sieved. on an 8 The mm studysieve. was Separation conducted was on carried chips outthat on went a shaker through (LPzE-4e, a 16 mm Morek sieve Multiserw, and remained Marcypor˛eba, on an 8 Poland). Specimens were then split into three samples and then dried (SLW 115 dryer, Pol- mm sieve. Separation was carried out on a shaker (LPzE-4e, Morek Multiserw, Mar- Eko, Wodzisław Sl´ ˛aski,Poland) at temperatures of 60, 100, and 140 ◦C until they reached a cyporęba, Poland). Specimens were then split into three samples and then dried (SLW 115 dry state. After that, all of them were milled using a hammer mill with a 1 mm hole size bot- dryer, Pol-Eko, Wodzisław Śląski, Poland) at temperatures of 60, 100, and 140 °C until tom screen (PX-MFC 90D Polymix, Kinematika, Luzern, The Switzerland). For each sample, they reached a dry state. After that, all of them were milled using a hammer mill with a 1 PSD was determined in line with the ISO 17827-2 standard [70]. Samples were divided into mm hole size bottom screen (PX-MFC 90D Polymix, Kinematika, Luzern, The Switzer- 5 sieve classes, C : 0.1—diameter of particle d ≤ 0.1 mm, C : 0.25—diameter of particle in land). For each sample,1 PSD was determined in line with the2 ISO 17827-2 standard [70]. the span 0.1 < d ≤ 0.25 mm, C3: 0.5–0.25 < d ≤ 0.5 mm, C4: 0.71–0.5 < d ≤ 0.71 mm, and Samples were divided into 5 sieve classes, C1: 0.1—diameter of particle d ≤ 0.1 mm, C2: C5: 1–0.71 < d ≤ 1 mm. Based on the data received, the cumulative PSD and median value 0.25—diameter of particle in the span 0.1 < d ≤ 0.25 mm, C3: 0.5–0.25 < d ≤ 0.5 mm, C4: 0.71– of particle size d50 were determined. The sample of dry and ground raw material was 0.5 < d ≤ 0.71 mm, and C5: 1–0.71 < d ≤ 1 mm. Based on the data received, the cumulative split into nine sub-samples. Three of them were left in a dry state (moisture content 0%) PSDand and six were median placed value in aof climatic particle chamber size d50 (KBF-S were 115,determined. Binder, Tuttlingen, The sample Germany) of dry and and groundmoisturized raw material to 5% (3 was subsamples) split into and nine to sub-samples. 10% (3 subsamples). Three of Test them pellets were were left madein a dry in a statehydraulic (moisture press content (P400, Sirio,0%) and Meldola, six were Italy) placed at a pressurein a climatic range chamber of 130.8–457.8 (KBF-S MPa.115, Binder, After a Tuttlingen,stabilization Germany) period (24 and h), moisturized the pellets were to 5% measured (3 subsamples) (diameter and and to 10% height) (3 subsamples). and weighed. TestBase pellets on this, were the made specific in a density hydraulic of each press pellet (P400, was Sirio, calculated. Meldola, The Italy) mechanical at a pressure durability range ofwas 130.8–457.8 determined MPa. using After modified a stabilization standards period (ISO (24 17831-1:2016-02) h), the pellets were [71]. measured The 500 g (diame- mass of terthe and sample, height) required and weighed. by the standard, Base on wasthis, metthe byspecific ballast density material of witheach whichpellet thewas samples calculated.were tested.The mechanical All measurement durability methods was determ of thisined part using of the modified research are standards described (ISO in detail17831- in 1:2016-02)previous [71]. research The work500 g mass [41,43 of,60 the]. Tosample, confirm required the significance by the standard, of the was changes met by in specificballast materialdensity with and mechanicalwhich the samples durability were a tested. statistical All measurement analysis was run.methods A three-way of this part analysis of the research(ANOVA) are was described conducted, in detail and in for previous each variant, research the work normality [41,43,60]. of the To distributionconfirm the sig- was nificancechecked of (by the a Shapiro–Wilkchanges in specific test) withdensity an assumptionand mechanical of equal durability variance a statistical (Brown–Forsythe analysis wastest) run. [72, 73A ].three-way For all variants, analysis this (ANOVA) equality was conducted, met. Finally, and a one-way for each analysis variant, (ANOVA) the normality of the distribution was checked (by a Shapiro–Wilk test) with an assumption of equal variance (Brown–Forsythe test) [72,73]. For all variants, this equality was met. Fi- nally, a one-way analysis (ANOVA) and post-hoc analysis (Scheffé’s test) were carried out, which gave an indication as to which groups showed statistically significant differences. In the final stage, a one-way analysis (ANOVA) and post-hoc analysis (Scheffé’s test) were conducted.

Molecules 2021, 26, 1014 18 of 29

Molecules 2021, 26, x and post-hoc analysis (Scheffé’s test) were carried out, which gave an indication as to20 which of 31

groups showed statistically significant differences. In the final stage, a one-way analysis (ANOVA) and post-hoc analysis (Scheffé’s test) were conducted. 4.2. TGA Method 4.2. TGA Method Before the conceptual work, a thermogravimetric analysis (TGA) was performed to Before the conceptual work, a thermogravimetric analysis (TGA) was performed findto find out outthe mass the mass reduction reduction and ki andnetics kinetics of Miscanthus of Miscanthus in a semi-scale in a semi-scale installation installation with a regimentalwith a regimental dryer and dryer regimental and regimental torrefaction torrefaction reactor. reactor.In this research, In this research, a TG 209 a TGTarsus 209 NetzschTarsus NetzschTGA was TGA used was for the used energy for the crop energy thermochemical crop thermochemical process. A set-up process. with A a set-up con- tinuouswith acontinuous working dryer working and reactor dryer and for reactora torrefaction for a torrefaction process using process superheated using superheated steam is plannedsteam is so planned as to identify so as to the identify proper the mass proper loss mass to energy loss to loss energy ratio loss for ratiothe highest for the energy highest densityenergy [74]. density The [ 74Miscanthus]. The Miscanthus carbonization carbonization process processwas performed was performed using specific using specificequip- mentequipment with a with thermogravimetric a thermogravimetric analyzer analyzer in an in inert an inert atmosphere, atmosphere, nitrogen, nitrogen, and and argon— argon— FigureFigure 18. 18 .

Figure 18. Set-up for a MiscanthusFigure 18. torrefaction Set-up for processa Miscanthus for the torrefaction production process of carbonized for the solidproduction material of carbonized and biocarbon solid as ma- carriers for bio-fertilizers. terial and biocarbon as carriers for bio-fertilizers.

TheThe energy energy crop crop Miscanthus Miscanthus was was positioned positioned for for analysis analysis in in accordance accordance with with ISO ISO standardsstandards [75,76]. [75,76]. An An elemental elemental and and an an ultimate ultimate analysis analysis were were also also carried carried out out following following carbonization.carbonization. When When a a torrefaction torrefaction process process was was performed, performed, the the HHVs HHVs of of the the Miscanthus Miscanthus werewere estimated estimated in in accordance accordance with with ISO ISO standards standards [76]. [76]. The The HHVs HHVs were were estimated estimated using using a a calorimetercalorimeter bomb bomb (Parr (Parr Instrument Co.,Co., ModelModel 1672,1672, Moline,Moline, IL, IL, USA). USA). C, C, H, H, N, N, S, S, and and an an O Odetermination determination analysis analysis of of the the Miscanthus Miscanthus were were conducted conducted before before and and after after the the torrefaction torrefac- tionprocess, process, and samplesand samples were analyzedwere analyzed using proximate using proximate analysis. analysis. Parameters Parameters of the equipment of the equipmentused in experimental used in experimental research are research found inare Table found6. in Table 6.

TableTable 6. 6. EquipmentEquipment used used in in experimental experimental research research on on the the Miscanthus Miscanthus torrefaction torrefaction process. process.

DeviceDevice Model Model Producer ProducerDevice Paramet Devicer Parametr Heating ratio: 0.001 K/minHeating to 200 ratio: K/min 0.001 TGA K/min to ThermogravimeterThermogravimeter TG 209TG F3 209 Tarsus F3 Tarsus Netzsch (Germany) Netzsch (Germany) resolution:200 K/min0.1 µg TGA resolution: 0.1 µg Heat Flux sensor, Temperature range: Heat Flux sensor, Temperature−170 ◦ Crange: to 600 ◦C DifferentialDifferential Scanning Scanning Netzsch −170 °C toHeating 600 °C ratios: 0.01 K/min to DSC 3500 Sirius Netzsch CalorimeterCalorimeter DSC AnalyzerDSC Ana- DSC 3500 Sirius (Germany)Heating ratios: 0.01 K/min to 100 K/minK/min (Germany) Temperature accuracy: 0.1 K lyzer Temperature accuracy: 0.1 K Enthalpy accuracy: Generally < 2% Enthalpy accuracy: Generally < 2% Maximum temperature of measurement: 1350 Przemysłowy Insty- Laboratory electrical °C PR-45/1350M tut Elektroniki PIE tube furnace Accuracy of temperature control: +/− 5 °C (Poland) Furnace heating speed: 100 °C/8 min

Molecules 2021, 26, 1014 19 of 29

Table 6. Cont.

Device Model Producer Device Parametr Maximum temperature of measurement: 1350 ◦C Laboratory electrical Przemysłowy Instytut Elektroniki PR-45/1350M Accuracy of temperature control: tube furnace PIE (Poland) +/− 5 ◦C Furnace heating speed: 100 ◦C/8 min Measurement method: Continuous ﬂame ionization detection (FID) Measurement range: 0–100 000 ppm Stationary analyzer VOC FID 3-500 J.U.M. Engineering (Germany) Tenderness: Maximum 1 ppm CH4 for full scale Response time: 1.2 s Linearity: 1% Precision class instrument: 0.1% Temperature Resolution: 0.0001 ◦C Calorimeter PARR 6400 Parr Instrument Company (USA) Calorie sample range: 5000–8000 Linearity across operating range: 0.05% EDX elemental map of a gallium arsenide single crystal sample with <011> EDX Oxford X-Max Oxford orientation, obtained using Energy Dispersive spectrometer-SEM FEI Instruments HD-2700-spherical aberration X-ray Spectroscopy Quanta 200FEG microscope (USA) corrector, dual SDDs. The acceleration voltage is 200 kV, the pixel number is 128 by 96, the acquisition time is 12 min.

4.3. TGA, DSC, Kinetic Analysis, VOC, and SEM Analysis A TGA of the Miscanthus torrefaction process was carried out using samples weighing 10 mg. Nitrogen was injected at a flow rate of 20 mL/min. To create an inert atmosphere. The Miscanthus sample was positioned in the oven of the TGA analyzer. The laboratory was configured with an electric oven and analyzer for determining the VOC and was calibrated using a precision electronic balance (with a resolution of 0.01 g) to find out the weight loss during the torrefaction process. The torrefaction temperature and residential time process conditions were registered using a computer. For thermogravimetric- spectrophotothermal TG-MS analysis, a Luxx 409 PG Netzsch thermogravimetric analyzer (Netzsch, Selb/Bavaria, Germany) was used together with an Aeolos Netzsch QMS 403D mass spectrometer (Netzsch, Selb/Bavaria, Germany). Tests were conducted under an argon atmosphere at a flow of about 25 mL/min. Since it was necessary to stabilize the weight by means of a water jacket, an initial temperature of 40 ◦C was determined. The mass of the biomass samples used in the biomass torrefaction process using TG-MS technique was 5.0 mg. Samples were located in Al2O3 crucibles with a diameter of 6 mm. A Differential Scanning Calorimetric analysis was performed using a DSC 3500 Sirius apparatus (Netzsch, Selb/Bavaria, Germany). The measurement methodology was as follows. A sample was prepared by weighing a high-pressure steel crucible (maximum 500 ◦C, maximum 100 bar) with a volume of 27 microliters without the tested biomass sample. The crucible was then weighed again with the mass of the tested material. The weighing process was based on the Radwag scale, XA 52.3 Y model working with the accuracy (52 g/0.01 mg). The high-pressure crucible was closed with a sealing press for high pressure crucibles in an oxygen atmosphere. The reference crucible was weighed empty, inserting the sample into the DSC measuring chamber. After inserting the sample and the reference crucible into the measuring chamber, the DSC 3500 Sirius with 1-414/6 software (Netzsch, Selb/Bavaria, Germany) was started and the process parameters were set. The specifications were: Initial temperature 30 ◦C, final temperature 500 ◦C; heating ratio 5, 10, and 20 K/min; gas flow; and ample weight 1.4–1.5 mg. Analysis of the results: DSC measurement data analysis was performed using Proteus version 6.1.0 (Netzsch, Selb/Bavaria, Germany). A kinetic analysis of the Miscanthus carbonization process was carried out using a thermogravimetric Molecules 2021, 26, 1014 20 of 29

procedure with heating ratios of 5, 10, and 20 K/min. The Miscanthus sample masses were 10.0 ± 2 mg. A kinetic analysis was carried out in temperature ranges of 150 to 500 ◦C. The data were analyzed by means of Netzsch Kinetics 3 software. First, an iso-conversion analysis was conducted, which permitted a calculation of the activation energy without any knowledge of the reaction model [77]. The kinetics of the process were determined using special NETZSCH Kinetics Neo software (Netzsch, Selb/Bavaria, Germany) [78,79] by conducting a TGA analysis of the Miscanthus torrefaction process with different heating ratios of 5, 10, and 20 K/min. First, a Kissinger analysis according to ASTM E698 was performed based on the method assuming that the maximum reaction can be obtained from one step with the same degree of conversion regardless of the degree of heating. Note that this assumption is only partially correct and the resulting errors are minor. Based on Equation (1), the Kissinger method makes it possible to calculate the activation energy of the thermal decomposition reaction of a solid by plotting the logarithm of the sample heating ratio as a function of the inverse of temperature at the moment of the highest mass loss rate [80]. − Ee f (T) = Ae RT (1) 1 where A—pre-exponential factor (rate constant) ( /s), Ea—activation energy (J/mol), R—gas constant (J/mol·K), and T—temperature (K).

4.4. Volatile Organic Compounds

The value of the VOC emission indicator wz in pollutants (in mg of pollutant analyzed per 1 g of combusted biofuel) was determined on the basis of:

Q·cavr·τ wz = (2) mp

3 where: Q —air ﬂow rate (m /s), mp—sample mass (mg), τ—sample torrefaction time (s), 3 cavr—average concentration of pollutants (mg/m ), calculated from the dependencies:

1 Z t cavr = c(τ)dt (3) τ 0 The samples were investigated by scanning electron microscopy (SEM) and energy- dispersive X-ray spectroscopy (EDX) using an SEM FEI Quanta 200FEG microscope equipped with an EDX Oxford X-Max spectrometer (Oxford Instrument, High Wycombe, UK). Measurements using the EDX technique (Oxford Instrument, High Wycombe, UK) were performed in at least 10 different spots for a given sample, and then an average atom concentration and its standard deviation were calculated for each of the identiﬁed elements. The electron energy of 20 keV was used in investigations.

4.5. Experimental Procedure and Device The samples were dried in the oven for 4 h at 110 ◦C to obtain homogeneous experimental conditions. In this research, a Miscanthus sample was processed using 2 g of substrate at atmospheric pressure. In this experiment, a horizontal tubular reactor was used with a length of 600 mm and an internal diameter of 150 mm to carry out the process of biomass carbonization, as shown in Figure 19. CO2 washing was carried out until the oxygen concentration in the furnace reactor fell below 1%. After ﬂushing CO2 inside the reactor (2 L/min), the temperature in the furnace chamber was increased to various levels between 250 and 350 ◦C at a constant heating ratio of 10 K/min. When the torrefaction process attained a set temperature, the heating process was halted, and the carbon dioxide was turned off. The torreﬁed Miscanthus sample was immediately removed and weighed. Biomass holding time tests were carried out at different residential times starting from 0–50 min for 250, 300, 350, and 525 ◦C. The Miscanthus thermochemical process was carried out in an electrical oven PR-45/1350M (Przemysłowy Instytut Elektroniki, Warszawa, Poland) under a CO2 atmosphere. At the beginning, dried biomass with a mass of 2–2.5 g Molecules 2021, 26, 1014 21 of 29

MoleculesMolecules 2021 2021, 26, ,26 x , x 23 23of of31 31 was located on a quartz glass boat, which was in turn positioned in an electrical furnace heated to the appropriate temperature for a period of one hour. The analysis was conducted ◦ at 225, 250, 275, 300, and 525 C. The CO2 ﬂow was 1/min. Simultaneously, an emission of emissiontheemission sum ofof of VOCthe the sum recordedsum of ofVOC VOC in recorded the recorded stationary in inth the JUM stationarye stationary FID 3-500 JUM JUM analyzer FID FID 3-500 3-500 was analyzer performed analyzer was was (every per- per- formed10formed s). Three (every (every replicates 10 10 s). s). Three wereThree replicates carried replicates out were forwere eachcarried carried temperature out out for for each each value. temperature temperature Secondly, value. during value. Sec- theSec- ondly,Miscanthusondly, during during carbonization the the Miscanthus Miscanthus process, carbonization carbonization the sample process, wasprocess, weighed the the sample sample again was to was estimate weighed weighed the again weightagain to to estimateloss.estimate Torreﬁed the the weight weight material loss. loss. can Torrefied Torrefied be seen material in material the photo can can be in be seen Figure seen in in20the the. photo photo in inFigure Figure 20. 20.

Figure 19. Installation for VOC analysis of “torgas” during the Miscanthus torrefaction process using FigureanFigure electric 19. 19. Installation furnace. Installation for for VOC VOC analysis analysis of of“torgas” “torgas” during during the the Miscanthus Miscanthus torrefaction torrefaction process process usingusing an an electric electric furnace. furnace.

Figure 20. Torrefied Miscanthus samples after process at selected temperatures. FigureFigure 20. 20.Torreﬁed Torrefied Miscanthus Miscanthus samples samples after after process process at at selected selected temperatures. temperatures. 4.6. Kinetics Models 4.6.4.6. Kinetics Kinetics Models Models A AFriedman Friedman analysis analysis consists consists of ofdetermining determining the the logarithm logarithm of ofthe the degree degree of ofconver- conver- sion andA Friedman the function analysis of temperature consists of determininginverse according the logarithm to Equation of the (4) degree [81] of conversion andsion the and function the function of temperature of temperature inverse inverse according according to Equation to Equation (4) [81 ](4) [81] dx E dx E lnln =lnA−=lnA− +ln+lnfxfx (4) dxdt RTEa (4) ln dt= lnA − RT+ lnf(x) (4) dt RTmax TheThe Ozawa–Flynn–Walla Ozawa–Flynn–Walla method method is isbased based on on the the integral integral form form of ofthe the equation equation [81] [81] The Ozawa–Flynn–Walla method is based on the integral form of the equation [81] dx A −x dx A −x G Gxx= = = = expTexp dt dt (5) Z fx β Z RT (5) dxfx A β −RTxa G(x) = = exp dt (5) f(x) β RT ForFor the the Miscanthus Miscanthus samples samples analyzed, analyzed, the the assumptionTo assumption was was made made that that the the Miscanthus Miscanthus torrefactiontorrefaction process process proceeds proceeds in inone one step step according according to tothe the scheme: scheme: A−1→BA−1→B wherewhere B—is B—is the the torrefied torrefied Miscanthus. Miscanthus. InIn linear linear form, form, the the thermokinetic thermokinetic equation equation takes takes the the form form [81]. [81].

Molecules 2021, 26, 1014 22 of 29

For the Miscanthus samples analyzed, the assumption was made that the Miscanthus torrefaction process proceeds in one step according to the scheme:

A − 1 → B

where B—is the torreﬁed Miscanthus. In linear form, the thermokinetic equation takes the form [81].

g(X) A E ln = ln − (6) T β RT

5. Conclusions This article investigates the conditions of the Miscanthus drying and torrefaction processes used to obtain two types of products. In this work, we primarily examined the influence of the material drying temperature, moisture content, and compaction pressure on test pellets’ DE and DU. Furthermore, this varies depending on the material tested. The drying temperature has no effect on the course of DE changes but has a slight effect on DU changes (drying at 140 ◦C allows DU 97.5% at 240MPa and 10% moisture content to be obtained). DE and DU depend on moisture content and compression pressure. Higher pressure levels do not cause significant differences. It has been shown that moisture content has a major influence on the course of DE and DU changes. Although the drying temperature changes the properties of the material, these changes do not significantly affect the compaction process (there is no clear dependence of DE and DU on the drying temperature). This is a result which means that the material dried over a wide temperature range can be used for the compaction stage without the risk of losing the quality parameters of the granulates. This allows the right drying temperature to be selected for the material preparation processes, thus reducing the effort required for these processes. For example, the drying temperature significantly influences the grinding process. Jewiarz and others have shown that regardless of the tested material (Scots pine, European beech, Giant Miscanthus, and Cup plant) Sokolowski’s grinding criterion G0.25 shows that the energy inputs in the grinding process decrease as the raw material drying temperature increased. Secondly, the authors quantified various parameters of the biomass carbonization process like it was done on different biomass in [82–84]. These included kinetics, optimal temperature of the biomass roasting process, and residence time for specific mass loss ratios [85–92]. Additionally, high VOC concentrations in the tracks show that torgas can be used as a heat source for the roasting process. It was found that (257 ◦C, 9 min under this isothermal conditions) temperature at which during Miscanthus torrefaction process we are in the closest temperature at which mass loss of torrefied material have lost 30% of it original mass which according to the previous studies on many different biomass torrefaction process the most optimal mass loss to have the highest increase in caloric value (with a 30% mass loss we are losing only approximately 10% of energy of the torrefied biomass) [93–97]. At (300 ◦C, 6 min) there was observed closest to 50% mass loss which according to the literature is one of the most optimal mass loss for the biochar production as additive/carrier of C for fertilizers. At (525 ◦C, 5 min) the mass loss was closest to 75% mass loss which from the process and it economic point of view is reasonable temperature for the production of biosorbents/activated carbons [98,99]. It can be also added that this can impact the bio economy by the fact that an optimal mass loss ratio to energy loss for different energy crops during the torrefaction process has a significant impact on both carbonized solid biofuel and biochar as carriers for fertilizer prices (too much or too little heat for the process has a significant impact on the final product prices) [100]. In the next stage of the future research on the Miscanthus torrefaction process, there will be a focus on building an installation for a continuous torrefaction process using superheated steam and using kinetics data from this article to design a biomass rotary dryer run on hot air and counter-flow reactor for torrefaction process run on dry steam (superheated steam). In the spring of 2021, a new set-up will be built and experimental research will be carried out on Molecules 2021, 26, 1014 23 of 29

the conditions of the torrefaction process of Miscanthus and other woody and agri-biomass for the production of carbonized solid biofuels, biocarbon as a carrier for organic fertilizers, and activated carbon. Molecules 2021, 26, x 25 of 31 Author Contributions: S.S., M.J., M.W., A.K., P.P., Ł.A., W.L., J.C.; methodology, J.S., M.D., P.P., S.S., M.J., M.W., A.K., P.P., Ł.A., W.L., J.C.; software S.S., M.J., M.W., A.K., P.P., Ł.A., A.L., W.L., J.C.; validation, S.S., M.J., M.W., A.K., P.P., Ł.A., A.L., W.L., J.C. formal analysis, S.S. and Ł.A.; investigation, J.C.; validation,J.S., S.S., M.D., M.J. M.W., P.P., S.S., A.K., M.J., P.P., M.W., Ł.A., A.K., A.L., P.P., W.L., Ł.A., J.C. W.L., formal J.C.; analysis, resources, S.S., S.S., Ł.A. M.J., and M.W., K.P.; A.K., P.P., Ł.A., investigation, J.S.,A.L., M.D., W.L., P.P., J.C; S.S., data M.J. curation, M.W., S.S., A.K., M.J., P.P., M.W., Ł.A., A.K., W.L., P.P., J.C.; Ł.A., resources, A.L., W.L., S.S., J.C.; M.J. writing—original M.W., draft A.K., P.P., Ł.A.,preparation, A.L., W.L., J.C; S.S., data M.J., curation, M.W., A.K., S.S., P.P.,M.J.Ł.A., M.W., A.L., A.K., W.L., P.P., J.C.; Ł.A., writing—review A.L., W.L., J.C.; and writ- editing, S.S., P.P., ing—original draftW.L. preparation, and Ł.A.; visualization, S.S., M.J. M.W., S.S., A.K., M.J., P.P., M.W., Ł.A., A.K., A.L., P.P., W.L., Ł.A., J.C.; A.L., writing—review W.L., J.C; supervision, S.S., M.J., and editing, S.S.,M.W., P.P., A.K.;W.L. projectand Ł.A.; administration, visualization, S.S.,S.S., M.J.,M.J. M.W., A.K., P.P., Ł.A.,Ł.A., W.L.,A.L., J.C.;W.L., funding J.C; acquisition, supervision, S.S.,S.S., M.J. M.J., M.W., M.W., A.K.; A.K., project P.P.,Ł.A., administration, W.L., J.C. All S.S., authors M.J. M.W., have readA.K., and P.P., agreed Ł.A., toW.L., the published version J.C.; funding acquisition,of the manuscript. S.S., M.J. M.W., A.K., P.P., Ł.A., W.L., J.C. All authors have read and agreed to the published version of the manuscript. Funding: The research results in this article were financed by (NCBR) from Warsaw Poland–LIDER Funding: The research(grant no. results 0155/L-9/2017). in this article Thiswere study financed was by also (NCBR) conducted from andWarsaw financed Poland–LIDER by NCN Krakow Poland (grant no. 0155/L-9/2017).(grant no. 2018/31/B/HS4/00485), This study was also conducted program and OPUS. financed This researchby NCN wasKrakow supported Poland by the Ministry (grant no. 2018/31/B/HS4/00485),of Science and Higher program Education OPUS. ofThis the research Republic was of supported Poland (Faculty by the ofMinistry Production of and Power Science and Higher Education of the Republic of Poland (Faculty of Production and Power Engi- Engineering, University of Agriculture in Krakow). neering, University of Agriculture in Krakow). Institutional Review Board Statement: Not applicable. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: The data presented in this study are available on request from the Data Availability Statement: The data presented in this study are available on request from the corresponding author. corresponding author Conflicts of Interest: The authors declare no conflict of interest. Conflicts of Interest: The authors declare no conflict of interest. Sample Availability: Samples of the compounds of dry and torrefied Miscanthus are available from Sample Availability: Samples of the compounds of dry and torrefied Miscanthus are available the authors. from the authors. Appendix A Appendix A The activationThe energy activation value energyobtained value for Mi obtainedscanthus for is Miscanthus27 kJ/mol. A is lower 27 kJ/mol. error in A lower error this method wasin this 13 kJ/mol. method The was level 13 kJ/mol. of conver Thesion level and of speed conversion of reaction and speedwere both of reaction de- were both rived from thederived formulas. from Due the to formulas. the fact Duethat tof(x) the is factconstant that f(x) for isa constantgiven conversion, for a given the conversion, the log with dx/dtlog as witha function dx/dt of as temperature a function of inverse temperature gives straight inverse giveslines with straight a slope lines equal with a slope equal to Ea/R. Figureto 21 Ea shows/R. Figure logs A1of theshows dx/dt logs loga ofrithm the dx/dt as a logarithmfunction of as temperature a function of inverse temperature inverse determined bydetermined Friedman’s by method Friedman’s for Miscanthus. method for Miscanthus.

FigureFigure A1. Log 21. dx/dt Log dx/dt graph graph as a function as a function of temperature of temperature reverse reverse using using the Friedman the Friedman method method for Miscanthus. for Miscanthus.

Using the Friedman method, it is possible to show the value of pre-exponential coefficient and activation energy as a function of the level of conversion. Figure 22 shows these graphs for a sample of Miscanthus.

Molecules 2021, 26, 1014 24 of 29

Using the Friedman method, it is possible to show the value of pre-exponential Molecules 2021, 26, x coefﬁcient and activation energy as a function of the level of conversion.26 of Figure31 A2 shows

these graphs for a sample of Miscanthus.

Figure A2. FigureActivation 22. Activation energy and energy pre-exponential and pre-exponential coefﬁcient coeffici as a functionent as a offunction the degree of the of degree conversion of conver- by the Friedman method forsion Miscanthus. by the Friedman method for Miscanthus.

Analysis of FigureAnalysis 22 gives of Figure the activation A2 gives energies the activation of Miscanthus energies ofwhich Miscanthus differ de- which differ depending on thepending extent of on conversion. the extent ofIn conversion.Table 7 (Appendix In Table A),A1 we(Appendix can findA summarized), we can ﬁnd summarized the theoretical theequations theoretical of the equations solid-state of the distribution solid-state distributionmodels of g(x), models which of g(x), we which have we have taken taken to determineto determine the best fit. the best ﬁt.

Table 7. Pre-exponentialTable A1. coefficientPre-exponential and activation coefﬁcient ener andgy activationas a function energy of the as degree a function of conversion of the degree of conversion by the Friedman bymethod the Friedman for Miscanthus. method for Miscanthus.

Fract. Mass Loss Fract. Mass Loss Activation En Activationergy (kJ/mol) Energy (kJ/mol) Log (A/s^-1)Log (A/s−1) 0.02 108.12 ± 26.14 7.17 0.02 108.12 ± 26.14 7.17 ± 0.05 0.05103.12 18.65 103.12 ± 18.656.65 6.65 0.10 0.1091.07 ± 20.76 91.07 ± 20.76 5.55 5.55 0.20 0.20101.12 ± 20.73 101.12 ± 20.73 6.40 6.40 0.30 0.30102.77 ± 32.18 102.77 ± 32.18 6.39 6.39 0.40 0.40126.43 ± 31.52 126.43 ± 31.52 8.42 8.42 0.50 0.50170.58 ± 23.37 170.58 ± 23.37 12.21 12.21 0.60 0.60216.56 ± 12.43 216.56 ± 12.43 16.07 16.07 ± 0.70 0.70301.28 ± 16.43 301.28 16.43 23.02 23.02

The activation Theenergy activation and pre-exponential energy and pre-exponential coefficient as a coefﬁcient function of as the a function level of of the level of conversion by theconversion Ozawa–Flynn–Walla by the Ozawa–Flynn–Walla method for Miscanthus method forcan Miscanthus be found in can Table be found3. in Table3 . On Figures 23 andOn 24 Figures thermogravimetric A3 and A4 thermogravimetric and ion current curves and for ion the current Miscanthus curves torre- for the Miscanthus faction processtorrefaction up to 257.5 °C process and up up to to 300 257.5 °C◦ areC and presented. up to 300 ◦C are presented.

Molecules 2021, 26, x 27 of 31 Molecules 2021, 26, x 27 of 31 Molecules 2021, 26, 1014 25 of 29

FigureFigure 23. A3. ThermogravimetricThermogravimetric and and ion ioncurrent current curves curves for forthe the Miscanthus Miscanthus torrefaction torrefaction process process up up to to 257.5Figure257.5 °C. 23.◦ C. Thermogravimetric and ion current curves for the Miscanthus torrefaction process up to 257.5 °C.

Figure A4. Thermogravimetric and ion current curves for the Miscanthus torrefaction process up to Figure 24. Thermogravimetric and ion current curves for the Miscanthus torrefaction process up to 300 ◦C. 300Figure °C. 24. Thermogravimetric and ion current curves for the Miscanthus torrefaction process up to 300 °C. ReferencesReferences References 1. 1. Karbowniczak,Karbowniczak, A.; A.; Hamerska, Hamerska, J.; J.;Wróbel, Wróbel, M.; M.; Jewiarz, Jewiarz, M.; M.; N N˛ecka,K.ęcka, K. Evaluation Evaluation of of Selected Selected Species Species of of Woody Woody Plants in Terms of 1. Karbowniczak, A.; Hamerska, J.; Wróbel, M.; Jewiarz, M.; Nęcka, K. Evaluation of Selected Species of Woody Plants in Terms of SuitabilitySuitability forfor Energy Energy Production. Production. In InRenewable Renewable Energy Energy Sources: Sources: Engineering, Engineering, Technology, Technology, Innovation. Innovation. Springer Springer Proceedings Proceedings in Energy in ; of Suitability for Energy Production. In Renewable Energy Sources: Engineering, Technology, Innovation. Springer Proceedings in EnergyMudryk,; Mudryk, K., Werle, K., Werle, S., Eds.; S., SpringerEds.; Springer Proceedings Proceedings in Energy: in Energy: Berlin/Heidelberg, Berlin/Heidelberg, Germany, Germany, 2018; 2018; pp. 735–742. pp. 735–742. Energy; Mudryk, K., Werle, S., Eds.; Springer Proceedings in Energy: Berlin/Heidelberg, Germany, 2018; pp. 735–742. 2. 2. Robbins,Robbins, M.P.; M.P.; Evans, Evans, G.; G.; Valentine, Valentine, J.; J.;Donnison, Donnison, I.S.; I.S.; Allison, Allison, G.G. G.G. New New opportunities opportunities for for the the exploitation exploitation of ofenergy energy crops crops byby 2. Robbins, M.P.; Evans, G.; Valentine, J.; Donnison, I.S.; Allison, G.G. New opportunities for the exploitation of energy crops by thermochemicalthermochemical conversion conversion in innorthern northern Europe Europe and and the the UK. UK. Prog.Prog. Energy Energy Combust. Combust. Sci. Sci. 20122012, 38, ,38 138–155., 138–155. [CrossRef] thermochemical conversion in northern Europe and the UK. Prog. Energy Combust. Sci. 2012, 38, 138–155. 3. 3. Telmo,Telmo, C.; C.; Lousada, Lousada, J. Heating J. Heating values values of ofwood wood pellets pellets from from different different species. species. BiomassBiomass Bioenergy Bioenergy 20112011, 35, 35, 2634–2639., 2634–2639. [CrossRef] 3. Telmo, C.; Lousada, J. Heating values of wood pellets from different species. Biomass Bioenergy 2011, 35, 2634–2639. 4. 4. Filbakk,Filbakk, T.; T.; Jirjis, Jirjis, R.; R.; Nurmi, Nurmi, J.; J.; Høibø, Høibø, O. TheThe effecteffectof of bark bark content content on on quality quality parameters parameters of Scotsof Scots pine pine (Pinus (Pinus sylvestris sylvestrisL.) pellets. L.) 4. Filbakk, T.; Jirjis, R.; Nurmi, J.; Høibø, O. The effect of bark content on quality parameters of Scots pine (Pinus sylvestris L.) pellets.Biomass Biomass Bioenergy Bioenergy2011, 201135, 3342–3349., 35, 3342–3349. [CrossRef ] pellets. Biomass Bioenergy 2011, 35, 3342–3349. 5. 5. Križan,Križan, P.; P.;Matúš, Matú M.;š, M.; Šooš, Šooš, L.; L.; Beniak, Beniak, J. Behavior J. Behavior of ofbeech beech sawdust sawdust during during densification densiﬁcation into into a solid a solid biofuel. biofuel. EnergiesEnergies 20152015, 8,, 8, 5. Križan, P.; Matúš, M.; Šooš, L.; Beniak, J. Behavior of beech sawdust during densification into a solid biofuel. Energies 2015, 8, 6382–6398.6382–6398. [CrossRef] 6382–6398. 6. Lewandowski, I.; Clifton-Brown, J.C.; Scurlock, J.M.O.; Huisman, W. Miscanthus: European experience with a novel energy crop. Biomass Bioenergy 2000, 19, 209–227. [CrossRef]

Molecules 2021, 26, 1014 26 of 29

7. Lewandowski, I.; Scurlock, J.M.O.; Lindvall, E.; Christou, M. The development and current status of perennial rhizomatous grasses as energy crops in the US and Europe. Biomass Bioenergy 2003, 25, 335–361. [CrossRef] 8. Khomina, V.; Trach, I.; Semenyshyna, I.; Koberniuk, O.; Mudryk, K.; Jewiarz, M.; Wróbel, M.; Styks, J. Potential of Soybean Straw in Ukraine and Solid Biofuel Production. In Renewable Energy Sources: Engineering, Technology, Innovation. Springer Proceedings in Energy ICORES 2018; Wróbel, M., Jewiarz, M., Szl˛ek,A., Eds.; Springer Proceedings in Energy: Berlin/Heidelberg, Germany, 2020; pp. 163–170. [CrossRef] 9. Hebda, T.; Brzychczyk, B.; Francik, S.; Pedryc, N. Evaluation of suitability of hazelnut shell energy for production of biofuels. Proc. Eng. Rural Dev. 2018, 17, 1860–1865. 10. García, R.; Gil, M.V.; Rubiera, F.; Pevida, C. Pelletization of wood and alternative residual biomass blends for producing industrial quality pellets. Fuel 2019, 251, 739–753. [CrossRef] 11. Zamorano, M.; Popov, V.; Rodríguez, M.L.; García-Maraver, A. A comparative study of quality properties of pelletized agricultural and forestry lopping residues. Renew. Energy 2011, 36, 3133–3140. [CrossRef] 12. Bry´s,A.; Zielińska,J.; Głowacki, S.; Tulej, W.; Bry´s,J. Analysis of possibilities of using biomass from cherry and morello cherry stones for energy purposes. E3s Web Conf. 2020, 154, 01005. [CrossRef] 13. Zawi´slak,K.; Sobczak, P.; Kraszkiewicz, A.; Niedziółka, I.; Parafiniuk, S.; Kuna-Broniowska, I.; Tana´s,W.; Zukiewicz-Sobczak,˙ W.; Obidziński,S. The use of lignocellulosic waste in the production of pellets for energy purposes. Renew. Energy 2020, 145, 997–1003. [CrossRef] 14. Rezania, S.; Ponraj, M.; Din, M.F.M.; Songip, A.R.; Sairan, F.M.; Chelliapan, S. The diverse applications of water hyacinth with main focus on sustainable energy and production for new era: An overview. Renew. Sustain. Energy Rev. 2015, 41, 943–954. [CrossRef] 15. Davies, R. Ignition and Burning Rate of Water Hyacinth Briquettes. J. Sci. Res. Rep. 2013, 2, 111–120. [CrossRef] 16. Benemann, J. Microalgae for Biofuels and Animal Feeds. Energies 2013, 6, 5869–5886. [CrossRef] 17. ISO 17225-1:2014. Solid Biofuels—Fuel Specifications and Classes—Part. 1: General Requirements; International Organization for Standardization: Geneva, Switzerland, 2014. 18. Knapczyk, A.; Francik, S.; Wójcik, A.; Bednarz, G. Influence of Storing Miscanthus x gigantheus on Its Mechanical and Energetic Properties. In Renewable Energy Sources: Engineering, Technology, Innovation. Springer Proceedings in Energy ICORES 2017; Mudryk, K., Werle, S., Eds.; Springer Proceedings in Energy: Berlin/Heidelberg, Germany, 2018; pp. 651–660. [CrossRef] 19. Manouchehrinejad, M.; Yue, Y.; de Morais, R.A.L.; Souza, L.M.O.; Singh, H.; Mani, S. Densification of Thermally Treated Energy Cane and Napier Grass. Bioenergy Res. 2018, 11, 538–550. [CrossRef] 20. Theerarattananoon, K.; Xu, F.; Wilson, J.; Ballard, R.; Mckinney, L.; Staggenborg, S.; Vadlani, P.; Pei, Z.J.; Wang, D. Physical properties of pellets made from sorghum stalk, corn stover, wheat straw, and big bluestem. Ind. Crop. Prod. 2011, 33, 325–332. [CrossRef] 21. White, R.H. Effect of lignin content and extractives on the higher heating value of wood. Wood Fiber Sci. 1987, 19, 446–452. 22. Demirba¸s,A. Relationships between heating value and lignin, fixed carbon, and volatile material contents of shells from biomass products. Energy Sources 2003, 25, 629–635. [CrossRef] 23. Demirbas, A. Higher heating values of lignin types from wood and non-wood lignocellulosic biomasses. Energy Sources Part A Recover. Util. Env. Eff. 2017, 39, 592–598. [CrossRef] 24. Wróbel, M.; Mudryk, K.; Jewiarz, M.; Głowacki, S.; Tulej, W. Characterization of Selected Plant Species in Terms of Energetic Use. In Renewable Energy Sources: Engineering, Technology, Innovation. Springer Proceedings in Energy ICORES 2017; Mudryk, K., Werle, S., Eds.; Springer Proceedings in Energy: Berlin/Heidelberg, Germany, 2018; pp. 671–681. 25. Prosiński,S. Chemia Drewna; PWRiL: Warszawa, Poland, 1984; ISBN 83-09-00674-8. 26. Bry´s,A.; Bry´s,J.; Głowacki, S.; Tulej, W.; Zajkowski, P.; Sojak, M. Analysis of Potential Related to Grass-Derived Biomass for Energetic Purposes. In Renewable Energy Sources: Engineering, Technology, Innovation. Springer Proceedings in Energy ICORES 2017; Mudryk, K., Werle, S., Eds.; Springer Proceedings in Energy: Berlin/Heidelberg, Germany, 2018; pp. 443–450. [CrossRef] 27. Nielsen, H.K.; Lærke, P.E.; Liu, N.; Jørgensen, U. Sampling procedure in a willow plantation for estimation of moisture content. Biomass Bioenergy 2015, 78, 62–70. [CrossRef] 28. Stelte, W.; Sanadi, A.R.; Shang, L.; Holm, J.K.; Ahrenfeldt, J.; Henriksen, U.B. Recent developments in biomass pelletization—A review. BioResources 2012, 7, 4451–4490. [CrossRef] 29. Larsson, S.H.; Thyrel, M.; Geladi, P.; Lestander, T.A. High quality biofuel pellet production from pre-compacted low density raw materials. Bioresour. Technol. 2008, 99, 7176–7182. [CrossRef] 30. Kubica, K.; Jewiarz, M.; Kubica, R.; Szl˛ek,A. Straw Combustion: Pilot and Laboratory Studies on a Straw-Fired Grate Boiler. Energy Fuels 2016, 30, 4405–4410. [CrossRef] 31. Dong, L.; Gao, S.; Xu, G. NO Reduction over Biomass Char in the Combustion Process. Energy Fuels 2010, 24, 446–450. [CrossRef] 32. González, J.F.; González-García, C.M.; Ramiro, A.; González, J.; Sabio, E.; Gañán, J.; Rodríguez, M.A. Combustion optimisation of biomass residue pellets for domestic heating with a mural boiler. Biomass Bioenergy 2004, 27, 145–154. [CrossRef] 33. Ryu, C.; Yang, Y.; Bin, K.A.; Yates, N.E.; Sharifi, V.N.; Swithenbank, J. Effect of fuel properties on biomass combustion: Part I. Experiments—Fuel type, equivalence ratio and particle size. Fuel 2006, 85, 1039–1046. [CrossRef] 34. Shaw, M.D.; Karunakaran, C.; Tabil, L.G. Physicochemical characteristics of densified untreated and steam exploded poplar wood and wheat straw grinds. Biosyst. Eng. 2009, 103, 198–207. [CrossRef] Molecules 2021, 26, 1014 27 of 29

35. Tumuluru, J. Effect of Moisture Content and Hammer Mill Screen Size on the Briquetting Characteristics of Woody and Herbaceous Biomass. Kona Powder Part. 2019, 241–251. [CrossRef] 36. Francik, S.; Knapczyk, A.; Knapczyk, A.; Francik, R. Decision Support System for the Production of Miscanthus and Willow Briquettes. Energies 2020, 13, 1364. [CrossRef] 37. Kaliyan, N.; Vance Morey, R. Factors affecting strength and durability of densified biomass products. Biomass Bioenergy 2009, 33, 337–359. [CrossRef] 38. Dyjakon, A.; Noszczyk, T. The Influence of Freezing Temperature Storage on the Mechanical Durability of Commercial Pellets from Biomass. Energies 2019, 12, 2627. [CrossRef] 39. Serrano, C.; Monedero, E.; Lapuerta, M.; Portero, H. Effect of moisture content, particle size and pine addition on quality parameters of barley straw pellets. Fuel Process. Technol. 2011, 92, 699–706. [CrossRef] 40. Pietsch, W. Agglomeration Processes. Phenomena, Technologies, Equipment; Wiley-VCH Verlag GmBH.: Weinheim, Germany, 2002; ISBN 3-527-30369-3. 41. Styks, J.; Wróbel, M.; Fr ˛aczek,J.; Knapczyk, A. Effect of Compaction Pressure and Moisture Content on Quality Parameters of Perennial Biomass Pellets. Energies 2020, 13, 1859. [CrossRef] 42. Puig-Arnavat, M.; Shang, L.; Sárossy, Z.; Ahrenfeldt, J.; Henriksen, U.B. From a single pellet press to a bench scale pellet mill—Pelletizing six different biomass feedstocks. Fuel Process. Technol. 2016, 142, 27–33. [CrossRef] 43. Wróbel, M.; Jewiarz, M.; Mudryk, K.; Knapczyk, A. Influence of Raw Material Drying Temperature on the Scots Pine (Pinus sylvestris L.) Biomass Agglomeration Process—A Preliminary Study. Energies 2020, 13, 1809. 44. Zafari, A.; Kianmehr, M.H. Factors affecting mechanical properties of biomass pellet from compost. Env. Technol. 2014, 35, 478–486. [CrossRef] 45. Peng, J.H.; Bi, H.T.; Sokhansanj, S.; Lim, J.C. A study of particle size effect on biomass torrefaction and densification. Proc. Energy Fuels 2012, 26, 3826–3839. [CrossRef] 46. Matúš, M.; Križan, P.; Kováˇcová, M.; Beniak, J. The influence of size fraction on the compressibility of pine sawdust and the effectiveness criterion for densification. Acta Polytech. 2014, 54, 52–58. [CrossRef] 47. Franke, M.; Rey, A. Improving Pellet Quality and Efficiency. Feed Technol. 2006, 10, 12–15. 48. Mani, S.; Tabil, L.G.; Sokhansanj, S. Effects of compressive force, particle size and moisture content on mechanical properties of biomass pellets from grasses. Biomass Bioenergy 2006, 30, 648–654. [CrossRef] 49. Muramatsu, K.; Massuquetto, A.; Dahlke, F.; Maiorka, A. Factors that Affect Pellet Quality: A Review. J. Agric. Sci. Technol. A 2015, 5.[CrossRef] 50. Tumuluru, J.S.; Wright, C.T.; Hess, J.R.; Kenney, K.L. A review of biomass densification systems to develop uniform feedstock commodities for bioenergy application. Biofuels Bioprod. Biorefin. 2011, 5, 683–707. [CrossRef] 51. Mostafa, M.E.; Hu, S.; Wang, Y.; Su, S.; Hu, X.; Elsayed, S.A.; Xiang, J. The significance of pelletization operating conditions: An analysis of physical and mechanical characteristics as well as energy consumption of biomass pellets. Renew. Sustain. Energy Rev. 2019, 105, 332–348. [CrossRef] 52. Wróbel, M.; Hamerska, J.; Jewiarz, M.; Mudryk, K.; Niemczyk, M. Influence of Parameters of the Torrefaction Process on the Selected Parameters of Torrefied Woody Biomass. In Renewable Energy Sources: Engineering, Technology, Innovation. Springer Proceedings in Energy; Mudryk, K., Werle, S., Eds.; Springer Proceedings in Energy: Berlin/Heidelberg, Germany, 2018; pp. 691–700. 53. Mierzwa-Hersztek, M.; Gondek, K.; Jewiarz, M.; Dziedzic, K. Assessment of energy parameters of biomass and biochars, leachability of heavy metals and phytotoxicity of their ashes. J. Mater. Cycles Waste Manag. 2019, 21, 786–800. [CrossRef] 54. Phanphanich, M.; Mani, S. Impact of torrefaction on the grindability and fuel characteristics of forest biomass. Bioresour. Technol. 2011, 102, 1246–1253. [CrossRef] 55. Bridgeman, T.G.; Jones, J.M.; Williams, A.; Waldron, D.J. An investigation of the grindability of two torrefied energy crops. Fuel 2010, 89, 3911–3918. [CrossRef] 56. Saleh, S.B.; Hansen, B.B.; Jensen, P.A.; Dam-Johansen, K. Influence of Biomass Chemical Properties on Torrefaction Characteristics. Energy Fuels 2013, 27, 7541–7548. [CrossRef] 57. Kanwal, S.; Munir, S.; Chaudhry, N.; Sana, H. Physicochemical characterization of Thar coal and torrefied corn cob. Energy Explor. Exploit. 2019, 37, 1286–1305. [CrossRef] 58. Fagernäs, L.; Brammer, J.; Wilén, C.; Lauer, M.; Verhoeff, F. Drying of biomass for second generation synfuel production. Biomass Bioenergy 2010, 34, 1267–1277. [CrossRef] 59. Frühwirth, P.; Graf, A.; Humer, M.; Hunger, F.; Köppl, H.; Liebhard, P.; Tumfart, K. Miscanthus sinensis “Giganteus”. Chinaschilf als nachwachsender Rohstoff. Anbau, Inhaltsstoffe, Kosten, Heiztechnik; Landwirtschaftskammer Österreich: Wien, Austria, 2006; ISBN 3-902325-02-X. 60. Nunes, L.J.R. A Case Study about Biomass Torrefaction on an Industrial Scale: Solutions to Problems Related to Self-Heating, Difficulties in Pelletizing, and Excessive Wear of Production Equipment. Appl. Sci. 2020, 10, 2546. [CrossRef] 61. Wilk, M.; Magdziarz, A.; Gajek, M.; Zajemska, M.; Jayaraman, K.; Gokalp, I. Combustion and kinetic parameters estimation of torrefied pine, acacia and Miscanthus giganteus using experimental and modelling techniques. Bioresource Technol. 2017, 243, 304–314. [CrossRef] Molecules 2021, 26, 1014 28 of 29

62. Doumer, M.E.; Arízaga, G.G.C.; da Silva, D.A.; Yamamoto, C.I.; Novotny, E.H.; Santos, J.M.; dos Santos, L.O.; Wisniewski, A.; de Andrade, J.B.; Mangrich, A.S. Slow pyrolysis of different Brazilian waste biomasses as sources of soil conditioners and energy, and for environmental protection. J. Anal. Appl. Pyrolysis 2015, 113, 434–443. [CrossRef] 63. Eibisch, N.; Schroll, R.; Fuß, R. Effect of pyrochar and hydrochar amendments on the mineralization of the herbicide isoproturon in an agricultural soil. Chemosphere 2015, 134, 528–535. [CrossRef][PubMed] 64. Schimmelpfenniga, S.; Müller, C.; Grünhage, L.; Koch, C.; Kammannaa, C. Biochar, hydrochar and uncarbonized feedstock application to permanent grassland—effects on greenhouse gas emission sand plant growth. Agric. Ecosyst. Environ. 2015, 191, 39–52. [CrossRef] 65. Trazzi, P.A.; Leahy, J.J.; Hayes, M.H.B.; Kwapinski, W. Adsorption and desorption of phosphate on biochars. J. Environ. Chem. Eng. 2016, 4, 37–46. [CrossRef] 66. Cruz, D.C. Production of Bio-Coal and Activated Carbon from Biomass. Master’s Thesis, The University of Western Ontario, London, ON, Canada, 2012. 67. Gładki, J. Biow˛egielSzans ˛adla Zrównowazonego˙ Rozwoju [BIOCHAR as a Chance for Sustainable Development], 2nd ed.; Oficyna Poligrafi czna Apla Sp.J.: S˛edziszów, Poland, 2017; pp. 60–85. ISBN 978-83-65487-05-06. 68. Xue, G.; Kwapinska, M.; Kwapinski, W.; Czajka, K.M.; Kennedy, J.; Leahy, J.J. Impact of torrefaction on properties of Miscanthus×giganteus relevant to gasification. Fuel 2014, 121, 189–197. [CrossRef] 69. Gucho, E.M.; Shahzad, K.; Bramer, E.A.; Akhtar, N.A.; Brem, G. Experimental Study on Dry Torrefaction of Beech Wood and Miscanthus. Energies 2015, 8, 3903–3923. [CrossRef] 70. ISO 17827-2:2016. Solid Biofuels—Determination of Particle Size Distribution for Uncompressed Fuels—Part. 2: Vibrating Screen Method Using Sieves with Aperture of 3.15 mm and Below; International Organization for Standardization: Geneva, Switzerland, 2016. 71. ISO 17831-1:2015. Solid Biofuels—Determination of Mechanical Durability of Pellets and Briquettes—Part. 1: Pellets; International Organization for Standardization: Geneva, Switzerland, 2015. 72. ISO 16994:2016. Solid Biofuels—Determination of Total Content of Sulfur and Chlorine; International Organization for Standardization: Geneva, Switzerland, 2016. 73. ISO 16948:2015. Solid Biofuels—Determination of Total Content of Carbon, Hydrogen and Nitrogen; International Organization for Standardization: Geneva, Switzerland, 2015. 74. ISO 18122:2015. Solid Biofuels—Determination of Ash Content; International Organization for Standardization: Geneva, Switzerland, 2015. 75. ISO 18123:2015. Solid Biofuels—Determination of the Content of Volatile Matter; International Organization for Standardization: Geneva, Switzerland, 2015. 76. ISO 18125:2015. Solid Biofuels—Determination of Calorific Value; International Organization for Standardization: Geneva, Switzer- land, 2017. 77. Gramsa, J.; Kwapińska,M.; J˛edrzejczyk,M.; Rze´znicka,I.; Leahy, J.J.; Ruppert, A.M. Surface characterization of Miscanthus × giganteus and Willow subjected to torrefaction. J. Anal. Appl. Pyrolysis 2019, 138, 231–241. [CrossRef] 78. Stelmach, J.; Kuncewicz, C.; Adrian, Ł.; Jirout, T.; Rieger, F. Change in Mixing Power of a Two-PBT Impeller When Emptying a Tank. Processes 2021, 9, 341. [CrossRef] 79. Magdziarz, A.; Wilk, M.; W ˛adrzyk,M. Pyrolysis of hydrochar derived from biomass—Experimental investigation. Fuel 2020, 267, 117246. [CrossRef] 80. Standard Test Method for Kinetic Parameters for Thermally Unstable Materials Using Differential Scanning Calorimetry and the Flynn/Wall/Ozawa Method Active Standard ASTM E698|Developed by Subcommittee: E37.01, Book of Standards Volume: 14.05. Available online: https://td.chem.msu.ru/uploads/files/courses/special/specpracta/lit/ASTM%20E%20698%20-%2016.pdf (accessed on 15 April 2020). 81. ISO 17225-6:2014. Solid Biofuels—Fuel Specifications and Classes—Part. 6: Graded Non-Woody Pellets; International Organization for Standardization: Geneva, Switzerland, 2014. 82. Siuda, R.; Kwiatek, J.; Szufa, S.; Obraniak, A.; Piersa, P.; Adrian, Ł.; Modrzewski, R.; Ławińska,K.; Siczek, K.; Olejnik, T.P. Industrial Verification and Research Development of Lime–Gypsum Fertilizer Granulation Method. Minerals 2021, 11, 119. [CrossRef] 83. Szufa, S.; Adrian, Ł.; Piersa, P.; Romanowska-Duda, Z.; Ratajczyk-Szufa, J. Torrefaction process of millet and cane using batch reactor. In Renewable Energy Sources: Engineering, Technology, Innovation. Springer Proceedings in Energy; Wróbel, M., Jewiarz, M., Szl˛ek,A., Eds.; Springer Proceedings in Energy: Berlin/Heidelberg, Germany, 2020; pp. 371–379. [CrossRef] 84. Szufa, S.; Wielgosiński,G.; Piersa, P.; Czerwińska,J.; Adrian, Ł.; Dzikuć,M.; Lewandowska, W.; Marczak, M. Torrefaction of straw from Oats and Maize for use as a fuel and additive to organic fertilizers—TGA analysis, kinetics as products for agricultural purposes. Energies 2020, 13, 2064. [CrossRef] 85. Dzikuć,M.; Kuryło, P.; Dudziak, R.; Szufa, S.; Dzikuć,M.; Godzisz, K. Selected aspects of combustion optimization of coal in power plants. Energies 2020, 13, 2208. [CrossRef] 86. Wilk, M.; Magdziarz, A. Hydrothermal carbonization, torrefaction and slow pyrolysis of Miscanthus giganteus. Energy 2017, 140, 1292–1304. [CrossRef] Molecules 2021, 26, 1014 29 of 29

87. Szufa, S.; Dzikuć,M.; Adrian, Ł.; Piersa, P.; Romanowska-Duda, Z.; Marczak, M.; Błaszczuk, A.; Piwowar, A.; Lewandowska, W. Torrefaction of oat straw to use as solid biofuel, an additive to organic fertilizers for agriculture purposes and activated carbon— TGA analysis, kinetics. E3s Web Conf. 2020, 154, 02004. [CrossRef] 88. Romanowska-Duda, Z.; Piotrowski, K.; Wolska, B.; D˛ebowski,M.; Zieliński,M.; Dziugan, P.; Szufa, S. Stimulating effect of ash from Sorghum on the growth of Lemnaceae—A new source of energy biomass. In Renewable Energy Sources: Engineering, Technology, Innovation. Springer Proceedings in Energy; Wróbel, M., Jewiarz, M., Szl˛ek,A., Eds.; Springer Proceedings in Energy: Berlin/Heidelberg, Germany, 2020; pp. 341–349. [CrossRef] 89. Wnukowski, M.; Owczarek, P.; Nied´zwiecki,Ł. Wet torrefaction of miscanthus–Characterization of hydrochars in view of handling, storage and combustion properties. J. Ecol. Eng. 2015, 16, 161–167. [CrossRef] 90. Szufa, S.; Adrian, Ł.; Piersa, P.; Romanowska-Duda, Z.; Grzesik, M.; Cebula, A.; Kowalczyk, S. Experimental studies on energy crops torrefaction process using batch reactor to estimate torrefaction temperature and residence time. In Renewable Energy Sources: Engineering, Technology, Innovation. Springer Proceedings in Energy; Mudryk, K., Werle, S., Eds.; Springer Proceedings in Energy: Berlin/Heidelberg, Germany, 2018; pp. 365–373. [CrossRef] 91. Adrian, Ł.; Szufa, S.; Piersa, P.; Romanowska-Duda, Z.; Grzesik, M.; Cebula, A.; Kowalczyk, S. Experimental research and thermographic analysis of heat transfer processes in a heat pipe heat exchanger utilizing as a working fluid R134A. In Renewable Energy Sources: Engineering, Technology, Innovation. Springer Proceedings in Energy ICORES 2017; Mudryk, K., Werle, S., Eds.; Springer Proceedings in Energy: Berlin/Heidelberg, Germany, 2018; pp. 413–421. [CrossRef] 92. Szufa, S.; Romanowska-Duda, B.Z.; Grzesik, M. Torrefaction process of the Phragmites Communis growing in soil contaminated with cadmium. In Proceedings of the 20th European Biomass Conference and Exhibition, Milan, Italy, 18–22 June 2014; pp. 628–634, ISBN 978-88-89407-54-7. [CrossRef] 93. Szufa, S. Use of superheated steam in the process of biomass torrefaction. Przemysł Chem. 2020, 99, 1797–1801. (In Polish) [CrossRef] 94. Marczak, M.; Karczewski, M.; Makowska, D.; Burmistrz, P. Impact of the temperature of waste biomass pyrolysis on the quality of the obtained biochar. Agric. Eng. 2016, 20, 115–124. 95. Junga, R.; Pospolita, J.; Niemiec, P. Combustion and grindability characteristics of palm kernel shells torrefied in a pilot-scale installation. Renew. Energy 2020, 147, 1239–1250. [CrossRef] 96. Ławińska,K.; Szufa, S.; Obraniak, A.; Olejnik, T.; Siuda, R.; Kwiatek, J.; Ogrodowczyk, D. Disc Granulation Process of Carbonation Lime Mud as a Method of Post-Production Waste Management. Energies 2020, 13, 3419. [CrossRef] 97. Jewiarz, M.; Wróbel, M.; Mudryk, K.; Szufa, S. Impact of the Drying Temperature and Grinding Technique on Biomass Grindability. Energies 2020, 13, 3392. [CrossRef] 98. Ławińska,K.; Szufa, S.; Modrzewski, R.; Obraniak, A.; W˛ezyk,˙ T.; Rostocki, A.; Olejnik, T.P. Obtaining Granules from Waste Tannery Shavings and Mineral Additives by Wet Pulp Granulation. Molecules 2020, 25, 5419. [CrossRef][PubMed] 99. Szufa, S.; Piersa, P.; Adrian, Ł.; Sielski, J.; Grzesik, M.; Romanowska-Duda, Z.; Piotrowski, K.; Lewandowska, W. Acquisition of Torrefied Biomass from Jerusalem Artichoke Grown in a Closed Circular System Using Biogas Plant Waste. Molecules 2020, 25, 3862. [CrossRef][PubMed] 100. Lapczynska-Kordon, B.; Lis, S.; Tomasik, M. Control of the microwave drying process of selected fruits and vegetables. Przegl ˛ad Elektrotechniczny 2019, 9, 74–77. [CrossRef]