This section presents the gas quantification and evaluation of results from the slow pyrolysis experiments.
FTIR-TGA Studies
Under slow pyrolysis and combustion conditions, the gases (CO2, CO, CH4, and H2O) were measured. The amount of tars formed are considered to be the discrepancy in the original dry sample mass and the total amount of char and pyrolytic gases detected. Figure 1 below shows the quantified gases for each experiment. On the other hand, Fig. 2 shows the tar, char and gaseous that were produced in pyrolysis.
The lignin samples, especially almond lignin, had the highest gas yield of all the samples. Additionally, as would be expected, the major species during combustion was CO2. On the other hand, for samples of lignin and cellulose during the pyrolysis stage, CO is the predominant species, whereas for samples of biomasses, CO2 is emitted in greatest amounts, up to about 12 wt% on a dry basis.
Methane has the lowest yield of any gas; on average, it is 1 weight percent on dry basis. The samples that include lignin are those that yield more than 2 weight percent on dry basis, with almond lignin having the greatest yield at 2.6 weight percent on dry basis. The lowest methane yield, however, was seen in virgin biomass samples, with hazelnut shells producing only 0.1 wt% on dry basis.
The main process that produces CO is pyrolysis, and the average amount is about 6.3 wt% on d.b. The samples with the highest lignin concentrations have the largest CO yields, whereas samples of virgin biomass have the lowest yields. The difference is not significant. Additionally, CO is produced when cellulose sample burning is taking place.
In addition to combustion, slow pyrolysis also results in significant amounts of CO2 production. Regarding the pyrolysis and combustion stages, respectively, its typical production is around 6.6 and 77 wt% on d.b. Virgin biomass samples, particularly sunflower shell, yield the majority of the pyrolytic CO2 that is formed. The char generation in the pyrolysis, on the other hand, is what causes the CO2 to be formed upon burning. Hazelnut shell with the largest lignin structure and other isolated lignin fractions from biomass samples gave the highest CO2 yield in char combustion. The isolated lignin samples from virgin biomass such as AL, HL and SL yielded the most CO2 gas, about 143, 135 and 134 wt% on the basis of the initial sample in combustion of char, respectively. Here, it is important to indicate that the reason of larger than 100 wt.% values is due to the adding oxygen to the mass.
Pyrolysis and combustion both produce water; the latter depends on the hydrogen concentration of the char that is produced. The pyrolytic water yield is 3.5 wt%% on d.b. on average, with sunflower samples yielding the largest amounts at about 6.3 wt% on d.b. However, only 1.6 wt% on average of the d.b. of water is created during char burning. Almond lignin and virgin samples are the main sources of production. As lignin also has the largest char outputs, it typically produces the most water during combustion.
The tars fraction evolved during pyrolysis is the species that FTIR was unable to identify, and it was considered to be the basis of the discrepancy between the dry sample's initial mass and the total amount of char and pyrolytic gases obtained. Lignin samples, particularly almond lignin, showed the lowest tar generation of any biomass samples. However, cellulose samples had the highest tar yield, with the exception of almond biomass, which also produced large amounts of tar. According to published data, cellulose samples produced the highest tar output whereas lignin samples produced the lowest 19.
FTIR-DTG
The mass loss and the formation of carbon-containing gases during pyrolysis are shown in Figs. 3–11 below.
The findings of the FTIR study revealed that methane had the lowest yield of all the gas species from all of the biomass samples. Methoxy groups breaking down at temperatures below 500 oC is what causes methane to develop 11. The biggest mass yields are seen in CO and CO2, which also release energy more quickly. At low temperatures, ether bridges raptured to create CO; at high temperatures, diaryl ether dissociates to become the dominant source of CO 6. The main source of the CO2 release was the breaking and reformation of the carboxyl, carbonyl, and ether functional groups 25. The synthesis of complex organics, primarily phenols, aldehydes, alcohols, and acids, is shown by the FTIR spectra for bands between 800 and 2000 cm− 1 at the mass loss peak for all biomass samples. The most productive samples within this band region are indicated to be cellulose samples, but because the FTIR equipment is not calibrated, there is no way to quantify these gaseous chemicals.
The first gas species to be released in relation to the cellulose samples are CO and CO2. Their development begins at temperatures about 200 oC and ends at lower temperatures than 700 oC. This is consistent with literature because cellulose depolymerizes to yield active cellulose, which will then react to produce CO and CO2 gases through decarboxylation and decarbonylation processes 14,26,27.
The weight of sunflower cellulose begins to decrease among the samples at a temperature just about 200 oC. In contrast, the other two samples begin to lose weight about 210 oC. CO2 is released first and more quickly, and then CO yield. On the other hand, methane has the lowest yield and is formed at a higher temperature, around 400 oC. These results were in agreement with the literature 19.
The CO2 yield from sunflower reveals two peaks, one at 260 oC and the other at 335 oC, and it was not observed in the other cellulose samples. CO corresponding peak is also much more distinctive in this sample. Regarding with other samples as almond, hazelnut, and sunflower celluloses, the CO and CO2 peaks are, at 345, 350, and 325 oC, respectively. Figures 3 and 5 shows that the devolatilization occurred in a rather narrow peak, as has been seen in previous studies 27,28. It was not observed in sunflower stalk because of the small amounts of hemicellulose in the structure.
The higher amount of carbonyl group may be responsible for the sunflower cellulose sample's two different peaks in CO2 release. The maximal rates of devolatilization and gas peaks both revealed the same temperature. But, they are lower than the literature's devolatilization rate peak for pure cellulose 18.
At about 200 oC, the weight of every sample of lignin begins to decline. While lignin samples completed devolatilization at a temperature of about 890 oC. Because of lignin fraction reacts at low temperatures, it is believed that a major amount of it will turn into char 7. According to the literature, the lignin pyrolysis process produced the maximum char yield (48% wt d.b.); the samples produced the same quantity of char 18.
At lower temperatures, CO2 is the main gas emitted, however at higher temperatures, additional CO and methane are created. This is consistent with literature 14,19. This is particularly true of the lignins from sunflower and hazelnut. However, in the case of almond lignin, CO formation is also considerable at lower temperatures. Regarding almond, hazelnut, and sunflower lignins, the measured CO peaks are at 545, 495, and 530 oC, respectively. Regarding almond, hazelnut, and sunflower lignins, the CO2 peaks are at 400, 360, and 350 oC, respectively.
The only samples showing a double peak at methane yield are lignin samples Faix et al. and Jensen et al.,who investigated wood-originated lignin pyrolysis, both observed the same finding. They proposed that the first peak, which appears at around 450 oC, is caused by the degradation of methyl functional groups, while the second peak is caused by the degradation of the hydrocarbon skeleton following deoxygenation 18,29. Due to the substantially higher ash concentration that can catalyze the process, the first methane peak was detected in this study at a lower temperature.
The weight loss in virgin biomass samples started, on average, at around 185 oC, with sunflower stalk starting the process first and then hazelnut shell. According to the TGA data, the almond biomass sample includes the most reactive hemicellulose, which is the biopolymer that reacts most rapidly. Likewise, according to the research currently available, CO2 and CO are the primary gases [19]. In an almond shell sample, CO2 exhibits two noticeable shoulders at 288 and 355 oC, with a peak at 415 oC.
The peak of CO2 is observed in virgin samples of hazelnut and sunflower at 372 and 322 oC, respectively. However, for almond, hazelnut, and sunflower, the CO peaks are found at 409, 375, and 325 oC, respectively.
Holocellulose at low temperatures (500°C) and lignin at high temperatures (> 500°C) are considered to have produced the most to the release of CO2 from biomass pyrolysis, according to the results of research on virgin biomass and lignin. This finding is exactly clear in the hazelnut shell sample because the lignin sample was always releasing CO2 at high temperatures (> 500°C). Similar to this, the devolatilization of lignin should be the main reason of the formation of CO and CH4 at high temperatures (> 500°C).
Whether mass loss rate information from extracted biomass is sufficient to predict the mass loss behavior of the entire sample was also investigated in this research. The results for this are provided in Figs. 12–14. Almond matches are superior, hazelnut matches are average, and sunflower matches are the worst of the three biomass types. Among the samples, almond shell has the highest concentration of hemicellulose, but it also gives the best results due to its low levels of ash and extractive matter and high levels of cellulose and lignin. Although the hemicellulose fraction was not separated in this investigation, it appears that there are synergies among the biomass, particularly at high temperatures.