3.1 Scanning electron microscopy (SEM)
The samples were submitted to microscopic analysis to evaluate the topography of the four lignins extracted. The micrographs can be seen in Fig. 2, in which they were generated with a resolution of 1000 times for better visualization of each one. It was possible to visualize the morphological characteristics of lignins as a typical porous material, demonstrating uniformity and homogeneity on their surfaces, which facilitates the process of slow release of organic compounds incorporated into the lignin molecule [5].
3.2 Ultraviolet Visible Spectroscopy (UV-Vis)
UV-Vis spectroscopy is very convenient for lignin analysis due to its aromatic nature, as can be seen in Fig. 3. It shows the spectrograms of each lignin sample diluted in dioxane-water. Even at different absorbance intensities, all of them have small events in the region of 250–350 nm. The absorption at 280 nm is attributed to the presence of aromatic rings substituted with oxygenated groups, while around 250 nm there is a maximum region, that can be attributed to the presence of aromatic rings with substituted oxygen groups, like phenols appears clearly in the LigBg and LigEu lignins. In the other two lignins, these shoulders also appear, but less accentuated, mainly in LigMg. In the samples of LigBg and LigP more evident shoulders occur around 330 nm, characteristic also of phenolic groups or more oxidized phenols such as hydroquinones, or to the presence of ferrulic and p-coumaric acid in that samples [8, 9].
3.2 Fourier-transform infrared spectroscopy (FT-IR)
The Fig. 4 demonstrates the spectra in the infrared region for all lignin samples. The spectra show characteristic signs of lignin, containing small changes, mainly in intensity and molecular structure. In this analysis, the main functional groups and the preservation of the aromatic structure can be observed. Table 1 presents a correlation between the frequency of the elementary bands and their attributions.
Table 1
Infrared frequency assignments for lignins.
Wavenumber (cm3)
|
Assignments
|
LigBg
|
LigEu
|
LigP
|
LigMg
|
3450
|
3453
|
3423
|
3400
|
O-H Stretch on Hydrogen Bonds
|
2935
|
2938
|
2924
|
2914
|
Aliphatic C-H Asymmetrical Stretch
|
2841
|
2840
|
2850
|
2841
|
Symmetrical C-H Aliphatic Stretch
|
1700
|
1720
|
1710
|
1705
|
C = O stretch of carboxylic acids, ketones and aldehydes
|
1602
|
1600
|
1586
|
1596
|
C = C aromatic ring
|
1518
|
1514
|
1514
|
1513
|
C = C aromatic ring
|
1460
|
1454
|
1464
|
1460
|
Aliphatic C-H asymmetric strain
|
1410
|
1410
|
1420
|
1425
|
C = C aromatic ring
|
1205
|
1210
|
1210
|
1214
|
C-O Stretch, C-C, with C-O Stretch Sensitive Aromatic Ring Replacement
|
1106
|
1106
|
1102
|
1111
|
C-H deformation (typical of syringyl ring) and C-O deformation of secondary alcohol and aliphatic ether
|
1048
|
1018
|
1024
|
1018
|
Deformation aromatic C-H in plane and C-O deformation of the primary alcohol,
|
832
|
828
|
828
|
840
|
Deformation aromatic C-H out of plane
|
In this characterization, the stretching at 2935 to 3450 cm− 1 is observed, characterized by the asymmetric stretching of aliphatic C-H and O-H in hydrogen bonds, respectively; the mean peak at 1705 cm− 1 representing the stretching of C = O in unconjugated ketones, carbonyls and ester group; in 1600, 1518 and 1410 cm− 1, the stretching of C = C occurred, attributed to ring vibrations; between 1210 cm− 1 type stretches C-C and C-O; 1102 at 1111 cm− 1 C-H deformation (typical of syringyl ring) and C-O deformation of secondary alcohol and aliphatic ether; 1018–1048 cm− 1 was characterized by in-plane deformation of aromatic CH and CO deformation in primary alcohols, and in the ranges from 828 cm− 1 characteristic of out-of-plane CH-type deformation [4, 7].
3.3 Properties of lignins samples
This technique determines the content of the elements carbon, nitrogen, hydrogen and sulfur. The approximate values of oxygen present in each sample were calculated by an estimate of subtraction of the percentage of the other elements obtained in the elemental analysis (Table 2). It is known that softwood lignins are characterized by carbon content in the range of 60–64%, while the mass fraction of carbon for hardwood lignins is slightly lower – 58–60%. In accordance with this, all lignins are close, as expected. The highest element content present was carbon, followed by oxygen. Emphasizing that lignin molecules are also rich in oxygenated groups, such as hydroxyls and ethers [5, 10]
Table 2
Elemental analysis, ash and moisture content*.
Sample
|
C (%) m/m
|
H (%) m/m
|
N (%) m/m
|
S (%)
m/m
|
O (%)
m/m
|
MC%
|
AC %
|
References
|
C (%)
|
H (%)
|
N (%)
|
Authors
|
LigBg
|
54.12
|
5.04
|
0.58
|
1.33
|
38.93
|
3.95
|
10.03
|
60.02
|
7.13
|
2.24
|
[11]
|
LigEu
|
60.53
|
6.00
|
0.36
|
2.87
|
30.24
|
5.80
|
1.11
|
64.76
|
5.78
|
0.03
|
[12]
|
LigP
|
58.15
|
6.71
|
1.51
|
2.31
|
31.32
|
6.03
|
4.68
|
40.8
|
6.07
|
0.83
|
[13]
|
LigMg
|
60.75
|
8.02
|
1.79
|
1.70
|
27.74
|
5.59
|
0.38
|
45.9
|
6.02
|
0.50
|
[14]
|
* Oxygen values were calculated by subtracting the values obtained from the other elements in the analysis, where the sum totals 100%. |
According to Protassio [15], for the use and production of bioenergy it is necessary that the biomass has high levels of carbon and hydrogen and low levels of oxygen, because the elements that make up a biomass are correlated with the calorific value. Even though these elements are not the only ones present in plant biomass, they are the most relevant to be studied. In our analysis, only the levels of oxygen were not obtained directly, due to the lack of equipment.
The moisture content shows the percentage of water present in each material, related to its dry weight. Lignin, due to the great presence of aromatics in the structure, has a more hydrophobic character. In turn, the ash content provides the amount of inorganic residue that remains after burning. It is noted that all samples showed few significant differences in relation to the moisture content, with values close to those found in the literature references. Ash is an impurity that will not burn, so biomass with low ash content is better suited for pyrolysis than biomasses with higher ash content. In this case, lignin sugarcane bagasse had the highest ash content, because their composition has an abundance of mineral elements. According to Joshua et al. (2016), the combustion volume and efficacy of a fuel reduces with increasing ash content. It is worth noting that for all samples, results were obtained similar to those that were used as reference by other researches [16].
3.4 Differential scanning calorimetry (DSC) and Thermogravimetric analysis (TGA)
The DSC curves of isolated lignins are demonstrate in the endothermec thermograms in Fig. 5–6. The first scan was from the temperature of 20 ºC to 220 ºC (Fig. 5) and you can see the output of the volatiles present in the samples, like water. We can be seen in this result is that lignin has different capacities to retain water in their structure. While sugarcane bagasse lignin has less retention capacity, pineapple lignin has greater hydration capacity. This can be seen in the intensities of the lignin endothermic peaks around 100 ºC. Aside the fact that different structures can influence the hydrophobic lignin character, the presence of a greater number of polysaccharides, increases the sample's hydrophilicity. These results agree with the FT-IR spectral data for the hydroxyl bands present around 3400 cm− 1. The lignin that differs from the others in this region is that of pineapple, which appears to have other volatiles in its structure, or to have more or less strongly connected waters in it. This can be seen by the three peaks around 84 ºC, 101 ºC and 148 ºC. The first at 84 ºC, is very close to the same peak, in eucalyptus lignin, around 92 ºC. It is interesting to note that the maximum of the most intense peaks to the water elimination of the samples occurs at different temperatures. In the case of eucalyptus lignin around 92 ºC, bagasse occurs around 97 ºC, pineapple around 101 ºC and mango around 107 ºC [7, 17].
Figure 6 shows the exotherms peaks related to the degradation lignin, starting at 370 ºC and going up to 440 ºC. The maximum of two samples (mango and bagasse) is around 415 ºC, eucalyptus lignin has two degradation maximums, 399 ºC and 432 ºC. Pineapple lignin also peaks at 432°C. According to the tests carried out by Hollfman et al. (2021), the difference in the endothermic peaks presented by the lignin DSC is explained by the fact that they are from different species and, therefore, demonstrate differences in the structural components [17].
The TGA analysis presented in Fig. 7 expresses the thermal behavior in terms of mass losses of lignin samples as a function of temperature. Thus, it is observed that bagasse lignin has at least two thermal events (Tonset 230°C; Tendset 831°C), while eucalyptus lignin has at least four thermal events (276 ºC, 426 ºC,
513.5 ºC, 676°C). The other two samples show three thermal events, being pineapple lignin at 276°C, 521°C, 626°C and mango lignin at 201°C, 350°C, 823°C. It should be noted that between 200–300 ºC, the degradation of aliphatic alcohols, acids and esters occurs; around 500 ºC we have the decomposition of the aromatic rings [5].
3.5 Superior Calorific Power (SCP)
Determination of the calorific value in a calorimetric pump must take into account all parameters that cause deviations. The energy of burning of the sample is done considering the exclusion of heat energy related to the burning of the metallic wire used (whose nominal value is 96.25 J). The results obtained for all studied lignins are shown in Table 3.
Table 3
Calorific power results for the bagasse (LigBg), eucalyptus (LigEu), pineapple (LigP) and mango (LigMg) samples.
Samples
|
Calorific Power (j g− 1°C− 1)
|
Calorific Power (kcal kg− 1)
|
LigBg
|
18465 ± 520
|
4413.24
|
LigEu
|
19193 ± 480
|
4587.24
|
LigP
|
21890 ± 620
|
5231.83
|
LigMg
|
11850 ± 280
|
2832.22
|
Heat capacity measures the variation of the internal energy (U) of a system with temperature, which in a system with constant volume as in a calorimetric pump, dU = qv = Cv dT. it is the amount of heat necessary to raise the temperature of the body, that is, the greater the heat capacity of the body the greater the amount of heat to increase the temperature [11]. Table 4 shows a decreasing order in relation to the specific heat of each sample, with LigP > LigEu > LigBg > LigMg. It is worth noting that the greater the specific heat of a substance, the greater the amount of heat that must be supplied or removed from it for temperature variations to occur [18].
Table 4
Correlation between calorific value found in each sample, with the respective biomass, whose data was referenced from other research.
|
Calorific Power Lignin (kcal kg− 1)
|
Calorific Power Biomass (kcal kg− 1)
|
References
(Authors)
|
Sugarcane bagasse
|
4413.24
|
4274.48
|
[13]
|
Eucalyptus
|
4587.24
|
4650.00
|
[12]
|
Pineapple Peel
|
5231.83
|
4247.13
|
[13]
|
Mango Core
|
2832.22
|
4440.73
|
[14]
|
Correlating the values obtained in the analysis with the values found in the literature (Table 4), it was observed that, according to Dourado et al. (2017) in a study with Eucalyptus grandis, under different fertilizations, it was found an average SCP of 4,650 kcal kg− 1, a value close to the calorific value found for LigEu of 4,587.24 kcal kg− 1. For sugarcane bagasse lignin, values similar to the studies carried out by Paula et al. (2011) whose SCP result was 4274.48 kcal kg− 1, while our result was 4413.24 kcal kg− 1. Interesting to note in these two cases is the inversion of SCP values between biomass and lignin from that biomass. For eucalyptus, biomass has a SCP higher than the SCP of its lignin, while for bagasse, lignin has a SCP value higher than that of biomass. Despite this, the values are very close, with an error of 1% and 3% respectively, situated within the margin of error of the measures [19, 20].
In the cases, pineapple peel and mango seed, there is also an inversion between the calorific value of lignin and biomass from literature. But in this case the values were more distant from each other. For Santos’s study [13], pineapple peel biomass obtained values of 17.77 MJ kg− 1, which is 81.2% of the SCP value (21.89 MJ kg− 1), for pineapple peel lignin, LigP obtained in this work. This was the sample with the greatest potential to assess the feasibility of its use in power generation, although there is this discrepancy in the SCP values. The lowest calorific value obtained was for the sample LigMg, mango seed lignin, with a value of 11.85 MJ kg− 1. It is observed that this result was 63.8% of the reported in the research by Andrade [14], whose SCP of the Uba mango seed was 18.58 MJ kg− 1.