3.2. Comparison of Extraction Procedures
Given the lab-scale size of the experimental apparatus, the recovery of the HTL products is a challenging task, as some can be retained in the reactor wall after the experiments. In order to collect the largest amount of biocrude from these small reactors, a solvent extraction procedure was developed; it is technically not possible to separate the biocrude and the aqueous phase gravimetrically. This would instead be the preferred solution in case of large scale continuous processes and the same approach should be considered also in lab-scale experiments, as reported also by Castello, Pedersen and Rosendahl [9
reports the effects of the collection procedure on the composition of the light biocrude fraction (biocrude 1) and on the aqueous phase obtained from an experiment performed at 350 °C, 10 min, 10%. It is clearly visible that by using Procedure 1, the light biocrude has a higher amount of organics and, in particular, catechol, creosol, acetic acid, benzoic acid and 4-ethylguaiacol are under the detection limit in the case of Procedure 2. Accordingly, in the aqueous phase, the situation is reversed: a greater concentration of organics is obtained in the sample collected through the Procedure 2; this is true for all the calibrated compounds, except for lactic acid, glycerol and glycolic acid, whose concentrations are comparable. In addition, Table 4
shows the difference in products yield between the two collection procedures: a higher amount of BC1 and a lower amount of WSO are recovered by means of Procedure 1. This behavior is explained by the fact that in Procedure 1 water is not removed prior to DEE extraction of BC1 and therefore water-soluble organics are in part recovered in the light biocrude. From now on, the results showed in this study were based on this latter collection procedure, which was adopted because it allowed for a larger recovery of organics in the biocrude. However, it should be kept in mind that Procedure 2 would be more suitable for a direct comparison with a scaled-up/continuous process, where the biocrude would be gravimetrically separated from the water.
3.3. Yields and Influence of Operating Parameters
shows the yield of the HTL products, which were obtained at the operating conditions selected according to the experimental plan. The unidentified WSO were evaluated by difference and take into account also the losses due to the collection procedure.
A high yield of total biocrude was obtained, ranging from 44.1% to 65.7% w
, with the amount of light and heavy fraction changing with reaction conditions. In general, by increasing the reaction temperature, an increase in the yield of BC1 and a decrease in that of BC2 are observed, while the solid residue is approximately constant throughout all operating conditions, being char yields between 11.4% and 19.6% w
. The maximum total biocrude yield was achieved at 300 °C, 10 min, 20% but nearly 74% of it was composed by BC2. At 350 °C, 10 min, 10%, the total biocrude yield was 51.8% w
and the maximum BC1 yield was obtained (29.3% w
). The yields of the detected WSO and of the gas products were lower and the latter experienced an increase at 350 °C, as a higher temperature is known to enhance gasification reactions [27
]. It is known from the literature [18
] that the hydrothermal liquefaction of lignin is more likely to produce a rather high amount of solid product and therefore the use of alkali catalysts, such as KOH, K2
or NaOH [27
], and capping agents as phenol or boric acid [15
] have been suggested to limit the char formation hampering polymerization, as well as different reaction medium than just water, as ethanol, methanol or water-mixture thereof [21
]. For instance, Arturi et al. [30
] investigated the effect of phenol in the HTL of Kraft pine lignin with K2
and, in the temperature range of 280–350 °C, at a concentration of 3.2%–3.6% w
of phenol obtained comparable solid yields to the present study, where no additives were adopted and with the use of a similar solvent extraction procedure.
A statistical analysis was also performed in order to assess the influence of process parameters (temperature, time, B/W) and their interaction on BC1, BC2 and total biocrude yield. The significance level for this model was chosen to be 0.05 (95% confidence level). A Pareto plot [33
] is reported in Figure 5
, Figure 6
and Figure 7
to visually highlight the absolute values of the main factors and the effect of interaction between the three parameters. The reference line in the chart indicates the limit between significance. The Pareto plot is useful to discriminate which process parameters can be neglected and which ones have an importance in the hydrothermal conversion process. However, to have a deeper understating of the positive and negative effects, the main effects plot and the interactions plot are reported in Appendix B
, Figure A2
and Figure A3
), showing how positively or negatively each parameter or combination thereof affects the biocrude yield.
It can be noticed from Figure 5
that temperature (A) had the greatest effect on BC1 yield, but also the residence time (B) is statistically significant at 95% confidence level, being both above the mentioned reference line. On the contrary, the ratio B/W (C), as well as the combination of factors, can be considered as not significant for the yield of BC1. This validates the fact that the temperature and partially the residence time drive the reactions pathways that lead to the formation of lighter intermediates that forms the BC1, as already shown in other works [30
depicts the Pareto chart reporting the absolute standardized effect of the factors for the BC2 yield. In this case, beyond temperature and time that remains important in the generation of BC2 heavy branched molecules, the combined interaction between temperature and B/W becomes significant. This means that a relative variation in the solid material introduced in the slurry, combined with a variation of the reaction temperature, has more influence in the reaction mechanisms that produce BC2 rather than varying the B/W alone.
Interestingly, concerning the total biocrude yield (Figure 7
), the most significant factor is the interaction between temperature and B/W, followed by temperature. In this case, time is not significant, suggesting that, in order to detect a statistically significant effect, longer residence time should be investigated. This effect demonstrates that increasing (or decreasing) the biomass content together with a variation in the reaction temperature drive the degradation reactions that form the biocrude (e.g., phenols, methoxyphenols and longer oxygenated aromatics chains). In general, this means that, if the objective is to optimize the process in terms of total biocrude yield without considering its quality, both of these factors have to be jointly taken into account.
In addition to the experiments of the DOE, four other reaction conditions were tested in duplicates, by increasing temperature to 370 °C and time to 15 and 20 min, collecting the products with Procedure 1. Figure 8
reports the solid residue and biocrudes yields from the experiments carried out at a B/W of 10% w
. At 370 °C, 5 min, 10% an increase in the yield of the light biocrude and a decrease in that of the heavy one is achieved; BC2 yield decreases with residence time, while BC1 yield reaches the maximum value of 41.7% w
at 15 min.
Although Castello, Pedersen and Rosendahl [9
] recently reported that favorable HTL conditions can be obtained also at supercritical condition, it is known that the HTL temperature range where the biocrude is maximized lies between 300 and 350 °C [8
]. At lower temperatures, partial conversion occurs, whereas at higher values the production shifts towards gases and char. In the present study, the maximum total biocrude yield was obtained at 350 °C, but the peak of its light fraction was achieved at 370 °C, indicating that higher temperatures are needed in order to optimize the conversion of this particular lignin-rich material.
3.4. Elemental Analysis and Higher Heating Value
With respect to the elemental analysis of the LRS (Table 2
), both light and heavy biocrude reported an increase in the C and H content and a decrease in O and ash concentration, confirming the energy densification effect of the process. The as-received elemental analysis of biocrude 1 and 2 is reported in Appendix D
and Table A4
). In general, a lower C content and a higher H and O content characterize the light biocrude fraction. These values are in line with literature: Arturi et al. [30
] performed batch HTL of Kraft lignin at 300 °C, 15 min, 6% lignin concentration with the addition of 1.6% of K2
and obtained a biocrude with 69.9% and 23.6% w
(d.b.) of carbon and oxygen content, respectively. The feedstock, the two biocrudes and the solid residues CHO compositions are given in the van Krevelen diagram of Figure 9
. The light biocrudes have a wider range of H/C and O/C molar ratios with changing reaction conditions, while the heavy biocrudes are less dispersed, having an H/C comprised between 1.10 and 1.25 and an O/C between 0.25 and 0.30. The solid residues are the products that mostly differ from the LRS, having O/C ratios similar to BC2 but lower H/C. The H/C and O/C values of the BC2 obtained in this study are in line with those reported in the review of Ramirez, Brown and Rainey [35
], concerning HTL of lignocellulosic biomass. The decrease in the O/C and the increase in the H/C ratio of BC1 with respect to the feedstock suggest that the production of light biocrude was mainly due to decarboxylation rather than dehydration, which, on the contrary, was more evident for the production of BC2 and the solid residues.
A significant energy densification effect was achieved through the HTL treatment: the higher heating values of the biocrudes ranged between 24.9 and 29.5 MJ kg−1
(see Table A6
in Appendix D
). Although rather similar values were observed, HHVs of heavy biocrudes were generally higher than those of light ones. The HHV of the total biocrude was determined as a yield-based weight-average from that of BC1 and BC2. The maximum increase with respect to the feedstock (27%) was achieved at 350 °C, 5 min, 10%, the same operating condition, which produced the maximum amount of total biocrude at a B/W of 10%. When BC1 and BC2 are considered separately, their yields and energy densifications, in terms of calorific value, have contrasting trends. Indeed, the yield of BC1 increases with severity, while that of BC2 decreases. The opposite is shown for the HHV: that of BC1 nearly decreases, while that of BC2 increases with severity (Figure 10
). However, considering the total biocrude as the sum of BC1 and BC2, both yield and HHV reach a maximum at the same condition, i.e., 350 °C, 5 min, 10%, as the HHV of total biocrude is evaluated as a yield-based weight-average from that of BC1 and BC2.
3.6. FTIR Analysis
FTIR spectra of the LRS, BC1 and BC2 were qualitatively analyzed in order to evaluate functional moieties modifications after the HTL treatment (Figure 12
). If compared with the feedstock, biocrudes exhibit a decrease in intensity at 1000–1070 cm−1
, along with the loss of a peak at 1056 cm−1
. This is probably due to breaking the β–O–4 or/and α–O–4 ether bonds of lignin, as also confirmed by other studies [20
], suggesting that the feedstock underwent to hydrolysis depolymerization [10
]. The macromolecular lignin backbone is in fact preferentially fragmented by ether bonds, which can be more easily broken than the C-C linkages through hydrolysis reactions [10
]. The two biocrudes show only slight differences. The presence of a relevant peak in the BC2 spectrum around 1700 cm−1
, typical of νC=O [20
], is presumably related to the presence of acetone residues, confirmed by the peaks around 1360 and 1419 cm−1
. The peaks in the region of aromatics, typical of the lignin structure, around 1600, 1515 and 1460 cm−1
], are always present in the three samples, suggesting that the lignin aromatic rings were, in general, preserved during HTL. An enhancement of the intensity around 1250–1200 cm−1
is probably related to guaiacols and mainly syringols [40
], first products of lignin depolymerization [10
3.7. Molecular Weight Analysis
In order to gain further insight on differences between the light and the heavy biocrude, their molecular weight (or molar mass) was evaluated by gel permeation chromatography (GPC). The weight-average molecular weight (Mw
), the number-average molecular weight (Mn
) and the polydispersity index (PDI = Mw
) are reported in Appendix E
). In addition, it was attempted to determine the average molecular weight of the lignin-rich stream, but only ~10% of this was soluble in THF (ambient temperature) and therefore this value was not estimated. Differently, the biocrude samples were completely THF-soluble and, as expected, the molar masses of the light biocrudes were far lower than the ones of the BC2. The former is comprised between 390 and 490 g mol−1
, while the latter range between 1030 and 1400 g mol−1
. The values of Mw
at 10% and 20% B/W for BC1, BC2 and total biocrude are shown in Figure 13
. The Mw
of the total biocrude was determined as a yield-based weight-average from that of BC1 and BC2. Concerning BC1, a higher B/W, in general, produces a higher molar mass. An increase with residence time is shown at 300 °C, while an opposite behavior is reported at 350 °C. A maximum is reached at 350 °C, 5 min, both at 10% and 20% w
of B/W, though the latter is subjected to high standard deviation, and the minimum values are reached at 370 °C (400 and 391 g mol−1
, respectively). Despite the complex trend of the molecular weight of BC2, the Mw
of the total biocrude clearly decreases with temperature and time, changing from 1146 to 565 g mol−1
, indicating that a higher extent of depolymerization occurred at harsher reaction conditions.
Furthermore, it is interesting to look at the effect of reaction conditions on the molar mass distributions (Figure 14
). The shape of the distribution is only slightly altered by changing residence time at fixed temperature and B/W but greatly changes with temperature, suggesting the latter to be a more influencing parameter, at least at the investigated reaction conditions. Concerning the light biocrude, at 300 °C and 10%, there is a contribution of low weight compounds (between 40 and 100 g mol−1
), which disappears at 350 °C and reappears, even with a slightly different shape, at 370 °C, confirming the presence of a maximum. On the contrary, the molar masses of the heavy biocrudes are more homogeneous, as confirmed by the lower PDI. Considering the BC1 samples obtained at a B/W of 20%, at 300 °C they still exhibit a low-molecular-weight peak, but, in this case, it is narrower and shifted towards higher values, precisely between 100 and 200 g mol−1
. The shapes of the distributions of samples at 350 and 370 °C are more similar to the ones of the 10% case. The distributions of BC2 are comparable to those obtained at 10%, only a more marked hump between 500 and 600 g mol−1
is present in the samples obtained at 300 °C.
These findings, along with the FTIR results, show that LRS was effectively depolymerized during the HTL treatment, even if without any alkali catalyst or capping agent and with far lower residence time than commonly reported in lignin depolymerization experiments, where the reaction time is generally extended up to several tens of minutes or hours [20