3.2.1. Elemental Analysis
The results of ultimate analysis of different bio-oil samples obtained from fast pyrolysis of PM and Sida with and without catalysts are tabulated in Table 3. The non-catalytic bio-oil from PM contained 42.34% of C which was lower than that reported by Mohan et al. (2006) [38] for typical pyrolysis oil as 54–58%. Moreover, O content (49.94%) of this bio-oil sample was higher than the typical O content of bio-oil which varied between 35 and 40%. However, H content (7.21%) of the sample agreed with the literature [39]. In general, it can be seen that O content of bio-oil samples increased during CFP except for Na2CO3, while H content decreased. In O content point of view, the results of this study were opposite to the literature [38], where the application of catalyst leaded to decrease in O content of bio-oil. This difference can be due to the fact that in our study bio-oil samples were analyzed directly without any fractionation. The C content increased or decreased depending on catalysts type, where it increased when Na2CO3 was applied. The N and S contents of bio-oil samples highly depend on the raw biomass content. The HHV of bio-oil samples were calculated by using the ultimate analysis results. Since the HHV is highly influenced by O content, sample with high O content showed low HHV. The bio-oil sample obtained with ZnO had the highest O (62.71%) and lowest C and H contents, thus showed the lowest HHV of 12.01 MJ kg− 1 among the six samples. When ZSM-5 was applied the corresponding bio-oil sample with O content of 51.32% and the highest H content (8.26%) had the highest HHV compared to the other samples. The HHV of non-catalytic bio-oil sample (18.11 MJ kg− 1) was higher than that obtained by using ZnO, CeO2, and ZrO2, and lower than that obtained with Na2CO3 and ZSM-5.
Table 3
Elemental analysis of bio-oil produced from non-catalytic and catalytic fast pyrolysis.
Biomass | Bio-oil Sample | % | |
C | H | O | N | S | HHV (MJ kg− 1) |
Pearl Millet | Non-catalytic | 42.34 | 7.21 | 49.94 | 0.47 | 0.04 | 18.11 |
CeO2 | 35.80 | 7.03 | 56.78 | 0.35 | 0.04 | 14.91 |
ZnO | 30.25 | 6.73 | 62.71 | 0.27 | 0.04 | 12.01 |
ZrO2 | 41.57 | 7.06 | 50.87 | 0.44 | 0.07 | 17.57 |
Na2CO3 | 43.36 | 7.03 | 49.03 | 0.49 | 0.09 | 18.35 |
ZSM-5 | 39.96 | 8.26 | 51.32 | 0.34 | 0.13 | 18.38 |
Sida cordifolia | Non-catalytic | 37.70 | 7.39 | 53.47 | 1.29 | 0.15 | 16.34 |
CeO2 | 27.79 | 7.78 | 63.41 | 0.97 | 0.05 | 12.29 |
ZnO | 32.90 | 7.77 | 58.10 | 1.16 | 0.06 | 14.60 |
ZrO2 | 32.47 | 7.59 | 58.82 | 1.07 | 0.06 | 14.18 |
Na2CO3 | 21.84 | 7.71 | 69.09 | 1.27 | 0.09 | 9.54 |
ZSM-S | 26.08 | 7.83 | 65.16 | 0.86 | 0.08 | 11.55 |
Ultimate analysis of bio-oil samples obtained from CFP and NCFP of Sida presented different trends compared to PM, probably due to the initial composition of each biomass. The C content of bio-oil sample from Sida was lower than that of PM, while H content showed the opposite except for ZSM-5. Also, O content was higher in bio-oil sample from Sida which leaded to the low HHV observed for these samples. During fast pyrolysis of Sida, C and S contents of bio-oil samples decreased when catalysts were applied, whereas O and H contents increased. Bio-oil sample obtained by using Na2CO3 had the lowest C but highest O content, and thus the lowest HHV. According to HHV, samples can be ordered as Na2CO3 < ZSM-5 < CeO2 < ZrO2 < ZnO < no catalyst. As for PM, N contents of all bio-oil samples from Sida were almost zero while S content was around 1%. For both biomasses CFP with ZSM-5 produced bio-oil with the highest H content.
3.2.2. GC-MS Analysis of Bio-oil
GC-MS analysis was performed in order to determine different chemical compounds contained in bio-oil samples. Bio-oil from fast pyrolysis of biomass is known as a very complex liquid containing up to 400 chemical compounds [9]. Bio-oil samples obtained from fast pyrolysis of PM and Sida also contained a multitude of compounds which can be grouped as acids, ketones, hydrocarbon, aromatics (phenol, benzene), and others (amine, alcohol, aldehyde ether, ester). The proportions of these chemical groups in bio-oil varied with the initial biomass structural composition and pyrolysis operating parameters. It is well known that feedstock with high lignin content produces bio-oil with high aromatic contents [40]. The use of catalysts having the aim of enhancing the quality of bio-oil, the proportions of chemical groups in bio-oil are also affected by the presence or absence of catalyst. Five different catalysts used in this study influenced bio-oil composition. Due to the large number of chemical compounds in bio-oil, only chemicals with the peak areas ≥ 1% were examined in this study. Acids, ketones, alkanes, aromatics (phenol, toluene, benzene), and others (alcohols, amines, aldehydes) contents of bio-oil samples were examined to determine the best catalyst for the production of each compound from fast pyrolysis of PM and Sida, and the results are presented in Figs. 4 and 5.
Effects of catalysts on acids production
Acids contents of bio-oil samples were examined in order to determine the most suitable catalyst for acids production from fast pyrolysis of PM and Sida. Due to its high corrosivity, acids are mainly considered as undesired products in bio-oil [35]. Acetic, propanoic, and butanoic acids were found as the main carboxylic acids in bio-oil samples. Acetic acid resulted from the removal of acetyl groups contained in xylose unit [41]. Rasrendra et al. [42] produced acetic acid through alcohol carbonylation. Carboxylic acids contents of bio-oil samples from fast pyrolysis of PM increased with catalyst application except for Na2CO3. When ZnO, ZSM-5, and ZrO2 were used, acids contents initially 9.47% in non-catalytic bio-oil sample rised to 13.60, 14.05, and 14.45%, respectively. The highest acids contents were reached with CeO2, while the lowest was observed with Na2CO3. Among carboxylic acids, acetic acid was the most represented in all bio-oil samples. For example, by using ZSM-5, the proportion of acetic, propanoic, and butanoic acids were 6.59, 5.11, and 2.35%, respectively. Bio-oil with the lowest acid content (Na2CO3) and the highest acid content (CeO2) did not contain butanoic acid. In bio-oil from CeO2, pentanoic acid (2.26%) and octadecenoic acid (3.50%) were also detected. In general, CeO2 was found to be the most suitable catalyst for carboxylic acid production from fast pyrolysis of PM. However, for specific production of acetic acid non-catalytic bio-oil sample with 7.04% was the best.
Acids contents of bio-oil samples from fast pyrolysis of Sida showed variable trends compared to PM. In fact, acids contents decreased with the application of catalyst except for CeO2 which produced the highest acids contents. From the point of view of acids contents, bio-oil samples from Sida can be ordered as Na2CO3 < ZrO2 < ZnO < ZSM-5 < no catalyst < CeO2. Bio-oil samples from Sida contained more carboxylic acids types than that from PM. Indeed, it contained acetic, octadecenoic, n-hexadecanoic, propanoic, butanoic, benzoic, and valeric acid which proportions changed with catalysts. For example, acetic acid from the application of Na2CO3 decreased from 7.73% in non-catalytic bio-oil to 2.20% and then increased to 8.13% by using ZSM-5. This change can be due to the nature of catalyst and its chemical composition [16, 43]. The highest acid content of 16.28 and 15.39% in bio-oil samples from PM and Sida respectively were observed with CeO2 application. Thus, it can be said that CeO2 was the best catalyst to produce carboxylic acids from fast pyrolysis of PM and Sida.
Effects of catalysts on ketones production
A ketone is a compound containing a carbonyl functional group bridging two groups of atoms [44]. Ketones can be obtained by careful oxidation of a secondary alcohol using a strong oxidant. In industry, ketones are mainly synthesized by oxidation of hydrocarbons by oxygen in the air. This is how cyclohexanone, a precursor to nylon, is made from cyclohexane. Acetone or propanone is the simplest component of ketones. Thus, ketones are very important in the plastics production industries [45].
Ketones contents of bio-oil samples from NCFP of PM was found as 17.38%. After the application of CeO2 it was still substantially equal (17.35%) and then decreased with ZrO2 application (16.44%). The ketones contents of 22.71, 25.07, and 28.46% were reached with ZSM-5, ZnO and Na2CO3, respectively. Among ketones compounds, butanone and pentanone were the most represented in all bio-oil samples. For bio-oil samples from Sida, ketones contents decreased from 29.30% in non-catalytic bio-oil to 19.61, 23.67, 24.51, and 28.34% during CFP with Na2CO3, ZrO2, ZnO, and ZSM-5, respectively but increased with CeO2 (31.33%). It can be seen that like acids, ketones also showed varying trends from PM to Sida. For a high production of ketones, Sida seemed to be the most promising than PM, even without catalyst bio-oil sample from Sida contained more ketones than that from PM.
Effects of catalysts on hydrocarbons production
Hydrocarbons are highly desirable for fuels and valuable chemicals productions from fast pyrolysis oil [46]. Bio-oil from fast pyrolysis of biomass contained mainly carboxylic acids and oxygenated compounds which limit its direct use as fuel [16]. In this study, alkanes contents of bio-oil samples from fast pyrolysis of PM and Sida were examined. Alkanes contents of bio-oil samples from fast pyrolysis of PM increased during CFP. The highest alkanes content of 12.92% was reached with CeO2, while the lowest was observed with ZnO (4.54%). Zeolite is known as the most suitable catalyst to produce high hydrocarbons yields at a low cost. Bio-oil obtained by using ZSM-5 had 5.15% of alkanes. It is important to note that the selectivity of zeolite depends on its pore structure and acidic sites. Acidic site concentration in zeolite maximizes the level of aromatic compounds in bio-oil because Brønsted acidic sites produce aromatics, while Lewis acidic sites produce alkanes [47]. Alkanes proportion of bio-oil samples from fast pyrolysis of Sida increased with catalyst application except ZnO, where it decreased from 8.01 (non catalytic) to 6.43%. Unlike bio-oil sample from PM, here the highest alkanes content of 14.15% was observed in bio-oil from Na2CO3 catalyst. In point of view of alkanes contents, catalysts can be ordered as Na2CO3 > ZSM-5 > ZrO2 > CeO2 > ZnO. When comparing two biomasses alkanes production was found to be more suitable from fast pyrolysis of Sida than PM.
Effects of catalysts on aromatics production
Aromatics such as phenols, benzenes, and toluenes in pyrolysis oil have already been reported in the literature [43]. Among all bio-oil chemicals phenolic compounds are important industrial ones and could be used to produce solvents or phenolic-based adhesives i.e. novolac and resole resins. Aromatics are important in chemical industry for plastics production and value-added chemicals [35]. It was also reported that bio-oil could substitute for fossil phenol (up to 25 wt%) in the synthesis of phenolic resins. Bio-oil samples from NCFP of PM contained 26.27% of aromatics which increased during CFP. Jeong et al. [40] reported that phenols contents of bio-oil increased with catalyst application. In fact, it rised from 26.27 to 47.30% when ZrO2 was applied. Nair and Vinu [48] reported that ZrO2 application during fast pyrolysis improved the guaiacol content of bio-oil sample. The lowest aromatics content of 26.39% was observed with Na2CO3. The CeO2, ZSM-5, and ZnO catalysts enhanced the proportions of aromatics in bio-oil with the yields of 29.77, 30.68, and 39.10%, respectively. The aromatics contents of bio-oil samples from fast pyrolysis of Sida also increased with catalyst application except ZnO, where the lowest percentage was observed. The CeO2 and ZSM-5 with 25.11 and 23.56% respectively represented the most important proportions of aromatics. Compared to bio-oil from PM, bio-oil from Sida contained less aromatics. Thus, PM was the most suitable biomass for aromatics production through fast pyrolysis specially by using ZrO2.
Effects of catalysts on the other compounds
Alcohols, amines/amides, aldehydes, ethers, and esters were considered as the other compounds due to their low percentages in bio-oil samples. Alcohols are highly desirable for valuable chemicals and fuels productions from fast pyrolysis of biomass [49]. Amides may be used to form resilient structural materials (e.g., nylon, Kevlar). Among amides dimethylformamide is one of the most important organic solvents usually used in drug production [44]. An ether is an organic compound that contains two alkyl or aryl groups by an oxygen atom and used in medicine [50]. Alcohols such as methanol and ethanol from bio-oil of PM were improved by using Na2CO3, ZnO, and CeO2. Amines contents of bio-oil samples increased significantly with Na2CO3 and ZSM-5 and decreased with the other catalysts. Aldehydes contents increased only when CeO2 was applied. During fast pyrolysis of Sida, alcohols contents of bio-oil samples increased with Na2CO3, ZnO, and ZrO2 and decreased with ZSM-5 and CeO2. Amines contents increased during CFP regardless of catalyst type. The highest amines content was observed in bio-oil sample from Na2CO3.