*3.2. Thermal Degradation Characteristics of GG at Different Heating Rates*

Figure 2a,b present the thermograms of GG in the N2 and air atmospheres, respectively, at different heating rates.

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**Figure 2.** Thermograms of mass loss and mass-loss rate (DTG) for GG in (**a**) N2 and (**b**) air atmospheres.

The thermograms for the thermal degradation of GG in N2 and air reveal that the degradation trend consists mainly of three regions, namely dehydration (I), devolatilization (II), and solid burnout or char formation (III). The first region is often ascribed to moisture loss and occurs below 100 ◦C for all heating rates. Shortly after, around 220 ◦C, is a characteristic "shoulder" that is indicative of hemicellulose degradation. Of the polymeric constituents, it is the least stable and thus the most reactive. Prior to the "shoulder", between 140 and 200 ◦C, a mass loss of less than 1% is noted. This may be due to the breakdown of some low-molecular organics with very weak bonds. At around 290 and 370 ◦C, a sharp and conspicuous DTG peak emerges. This is typically assigned to cellulose degradation because, as it has been noted, at temperatures above 250 ◦C cellulose is totally decomposed [22]. The devolatilization region is where substantial parts of the bonds in hemicellulose and cellulose and partly in lignin are deconstructed, leading to a release of large amounts of volatiles. This notion is supported by other researchers who have also observed that this region is suggestive of a simultaneous decomposition of the hemicellulose, cellulose, and lignin components [8]. Lignin decomposes over a wide range of temperatures, and thus, there is the trailing effect that extends to much higher temperatures. At about 600 ◦C, a barely visible peak appears—this may be indicative of the deconstruction of lignin's strongest bonds and/or the thermal cracking of some of the condensed lignin structures formed from the previous primary reactions [50].

Figure 3 depicts the thermograms of the isolated polymeric constituents (xylan, cellulose, and lignin (softwood and hardwood)), and the GG decomposition under N2 conditions at 20 ◦C/min. This provides insight into the sequence of the decomposition of the lignocellulosic biomass constituents. It is shown that hemicellulose (xylan) is the most reactive with a DTG peak at about 270 ◦C. Then, cellulose appears, with a prominent DTG peak at about 400 ◦C, which is closely followed by lignin. There are quantitative variations in terms of the DTG peaks (height and temp) between the different constituents, and these affirm the discussion on the trend observed for GG thermal decomposition. The apparent discrepancy may be due to the fact that the isolated constituents are devoid of the complexity associated with the lignocellulosic macromolecular structure in their natural matrix.

**Figure 3.** Thermograms of mass-loss rate (DTG) of cellulose, xylan, lignin, and GG in N2 atmosphere at 20 ◦C/min.

Table 5 presents the thermal characteristics data from the decomposition of the GG in both atmospheres. A marginal difference exists in the residual mass as the heating rate rises for the decomposition in both the N2 and the air atmosphere—this underscores the independence of the mass-loss trend on the heating rate. Similar trends have been shown in previous findings for peanut shells [1] and other biomass residues [3] in N2 and air atmospheres. It is also demonstrated that the DTG peaks increase in height and shift to higher temperatures with an increase in the heating rate. For instance, the maximum DTG peaks in absolute terms for N2 at 5, 10, and 20 ◦C/min read 3.21%/min (327 ◦C), 6.45%/min (339 ◦C), and 12.3%/min (358 ◦C), respectively. This is consistent with findings from the published literature for the pyrolysis and combustion of tobacco wastes [51]. This could be due to the heat transfer limitations that arise from the successive rise in the thermal gradient. This phenomenon usually occurs when there is rapid heating at the exterior relative to the inner parts of the sample, thus pushing the decomposition temperature to higher values [3].

**Table 5.** Heating rate, maximum peak temperature, residual mass, and mass-loss rate of GG decomposition under N2 and air atmosphere.


*β* = heating rate, *Tmax* = maximum peak temperature, *Rw* = residual mass.

Specifically, the residual mass after thermal degradation in the air is far less than (above 100%) in N2. This agrees with the findings from the published literature [3]. Thermal degradation in the air is an oxidative reaction (combustion), and it is expected to possess a comparatively higher decomposition rate. This is further attested to by the mass-loss rate peak (DTG), which is shown to be much higher in the air atmosphere for the corresponding stages of degradation. For example, at this 20 ◦C/min the DTG peaks for the air and N2 environments, respectively, read −23.1 and −12.3%/min. It is important to note that under air, a prominent peak appears at about 450 ◦C, as opposed to a very small peak under N2. This conforms with the published data [8,52,53]. These observations suggest a thermal degradation phenomenon that is related to the combustion of char as char formation is predominant in the latter stage of oxidative degradation processes [8,52].
