*3.1. Results of Proximate and Ultimate Analyses*

Table 1 presents the results of the determination of the physicochemical characteristics of the AR biomass samples. Proximate and ultimate analysis results indicate the potential use of the studied samples in thermochemical conversion processes. In the structure of all parts of the plant, a high content of volatile components was found, 68.3 to 77.9 wt%. This indicates a high reactivity of the material, as well as the fact that they can be converted into pyrolysis products with a large amount of gaseous and liquid components. The samples had a moisture content of about 7 wt%, which did not exceed the permissible limit of 10 wt%. A higher moisture content can lead to an increase in drying costs due to the need for additional thermal energy and a decrease in the efficiency of the thermal conversion of biomass [37].


**Table 1.** Results of the proximate and ultimate analyses of the AR biomass samples.

The elemental composition of all samples and their ash content are within the limits characteristic of lignocellulosic materials [38–40]. High ash content is considered a problem in thermochemical conversion processes because it can cause fouling or aggregation, as well as lead to some disposal problems, lower energy conversion rates, and ultimately higher recycling costs. AR leaves are characterized by the highest ash content. Given that this plant was grown on unprepared soil and without fertilizing, the increased ash content in the leaves can be explained by the biological characteristics of the plant. The HHV of the samples was determined from the results of the elemental analysis. For leaves, it was the minimum value equal to 21 MJ/kg; the maximum value was 25.6 MJ/kg for AR stems. The high HHV values in this study are comparable to those for various municipal solid wastes [41–43].

#### *3.2. Thermal Degradation Analysis*

The thermogravimetric (TG) and differential thermogravimetric (DTG) curves of the AR samples are shown in Figure 3. Mass loss during thermal decomposition was recorded in an inert medium at a temperature of 10 ◦C/min.

**Figure 3.** TG (**a**) and DTG (**b**) curves of the AR biomass at a heating rate of 10 ◦C/min.

The thermal decomposition of the three biomass samples studied is similar to the process of pyrolysis of lignocellulosic materials and proceeds in three main stages, which are characterized by the processes of dehydration, devolatilization, and carbonation (Table 2). The dehydration stage starts from 38 ◦C and proceeds on average up to 200 ◦C for the studied samples. Mass loss at this stage is considered to be the removal of free moisture, accompanied by evaporation from the sample surface upon heating, as well as chemically bound moisture. In addition to evaporation of the moisture, a slight release of volatile components is possible at this stage [44–46]. The average mass loss at this stage is 7.7%. At this stage, small peaks are found on the DTG curves. The first peak on the DTG curve is associated with the removal of free moisture at a temperature of about 100 ◦ C.


**Table 2.** Main stages of thermal decomposition.

The main stage of thermal decomposition for all samples starts at about 200 ◦C and ends in the temperature range of 502–520 ◦C. The processes that occur at this stage are associated with the release of volatile components. Significant mass loss is reflected as a peak in the DTG curve, which is due to complex thermochemical reactions during the conversion of biomass organic matter. In addition, the peak shown on the DTG curve characterizes the maximum decomposition rate, which is due to the breakdown of hemicellulose and cellulose [47]. As is known, the thermal decomposition of hemicellulose occurs in the temperature range of 180 to 300 ◦C, and cellulose from 300 to 480 ◦C [48,49]. Hemicellulose is composed of short-chain heteropolysaccharides and has an amorphous and branched structure. Monosaccharides are the main functional groups of hemicellulose with a small amount of uronic acids and acetyl groups [50]. The behavior of hemicellulose during pyrolysis is largely reflected by the characteristics of these building blocks during the thermal conversion process. Cellulose is a linear macromolecular polysaccharide consisting of a long chain of glucose units linked by β-1,4-glycosidic bonds [50]. The decomposition of cellulose is carried out by the depolymerization of various chemical bonds with the formation of carbon monoxide and dioxide, and carbon residues, as well as by the formation of bonds at high temperature to obtain liquid pyrolysis components.

Peaks in the DTG curves at the second stage indicated the decomposition of hemicellulose, which decomposes in the temperature range of 200–340 ◦C. Peaks at temperatures of 320–450 ◦C confirmed the thermal decomposition of cellulose. However, the temperature range changed as a result of the different biochemical compositions of the samples studied. For example, during the pyrolysis of AR stems, the maxima on the DTG curves shift, probably because of the high content of hemicellulose, which decomposes before cellulose.

At the carbonation stage, the processes of thermal cracking and dehydrogenation, as well as the decomposition of the solid carbonaceous residue and inorganic substances in its composition, take place [51]. The average mass loss in the carbonation stage is 8.7% for the studied samples. The main process at this stage is the thermal conversion of the complex structure of lignin, which decomposes almost throughout the entire temperature range from 190 to 900 ◦C. Unlike the carbohydrate structure of cellulose and hemicellulose, lignin has an aromatic matrix that gives strength and rigidity to the cell walls. Cellulose and hemicellulose have been shown to rapidly decompose over a short temperature range, while lignin slowly decomposes over a wider temperature range up to the end temperature of the experiment with the maximum formation of solid carbonaceous residues [52–54]. At temperatures above 680 ◦C, apparent shoulder peaks appeared in the DTG curves, which is due to the decomposition of inorganic components with low thermal stability [55,56]. At temperatures above 750 ◦C, practically no mass loss was observed, and the average residual weight as a result of heat treatment was 27.5% (Table 3).


**Table 3.** TG-data on changes in mass.

The content of hemicellulose, cellulose, and lignin in biomass affects the yield of pyrolysis products [57]. A high content of cellulose and hemicellulose contributes to the production of bio-oil, while a higher concentration of lignin results in more biochar produced. The structural complexity and stability of lignin make it difficult to destroy it during pyrolysis, leading to a higher yield of biochar [58].
