| dc.description.abstract | The acceleration of climate change and the depletion of oil reserves forced the society to look for alternative fuel sources. Among the available alternatives, second generation biofuel is gaining attention due to several advantages. Second generation biofuels (mainly bioethanol) are produced from plant material which is not used for food purpose. The challenging step in biofuel production is the pretreatment of biomass to reduce the recalcitrance by removing most of the lignin and part of the hemicellulose. Organosolv pretreatment is gaining popularity as a suitable pretreatment method. Several parameters affect the pretreatment efficiency and the subsequent hydrolysis process. The optimization of this process is necessary for an economically feasible production.
In this study, a model was developed to predict the product composition after pretreatment using the H-factor concept used in pulping. The total solid yield fits well with the H-factor. The concept is extended to apply to the individual components of the biomass. A new factor called O-factor was introduced which uses individual activation energies for the species such as lignin and hemicelluloses. A good fit was obtained for lignin with a parameter value of 96 kJ/mol. A decent fit was observed for other hemicelluloses, such as xylan and mannan. This study showed that the O-factor concept is a useful tool in designing the pretreatment process.
The redeposition of lignin to the fiber surface after pretreatment was studied in this project. The redeposition was studied using two types of reactors. The surface analysis of the samples pretreated in conventional autoclave reactor using SEM showed that the redeposition occur during cooling stage of the reaction. Further, the redeposition is believed to depend on the effluent lignin concentration. The redeposited lignin particle size and population density are dictated by the effluent concentration, which is consistent with the supersaturation theory. The use of a displacement reactor proved that the displacement of effluent before the cooling stage reduces the redeposition significantly. The redeposition of lignin on regions which are less accessible to washing liquid, such as the fiber lumen, was still observed, suggesting that a complete prevention of redeposition was not achieved.
Finally, the structural properties and characteristics of the isolated lignin from two type of reactors were studied. The lignin is a valuable starting material for several platform chemicals. Hence, the valorization of lignin is gaining attention. The knowledge of lignin structure, purity and properties is important for the valorization process. The molecular mass analysis of lignin samples isolated using an autoclave reactor showed that the molecular mass and polydispersity reduced as the temperature/severity of the reaction increased. The polydispersity index value of around 2 for samples treated at higher temperatures suggest that the depolymerization was random at these temperatures and more lignin bonds are broken. High polydispersity at lower temperature suggests that the redeposition of lignin to the fiber surface might have affected the polydispersity. The displacement reactor did not show a low molecular mass at high temperature. This substantiates the hypothesis of lignin redeposition influencing the polydispersity. Further, the FTIR analysis of lignin samples showed that samples having lower molecular mass have more free phenolic hydroxyls, which agrees with the high bond breakage at higher temperatures. The carbohydrate and Py-GC-MS analysis showed that the organosolv lignin is of high purity. The TGA analysis showed that the highest lignin degradation happened around 400 °C. This study shoes that the organosolv lignin is of high purity and low molecular mass, which makes organosolv lignin favorable for valorization.
The enzymatic hydrolysis study of pretreated samples showed that the glucose yield increased as the lignin content of the pretreated sample decreased. Further, the samples from the displacement reactor showed lower yield of glucose for similar lignin content. Considering the displacement reactor samples have lower amount of redeposited lignin on the surface, it can be assumed that the redeposited lignin has little effect on the hydrolysis. The data is not sufficient to arrive at a solid conclusion, and there is room for further investigation. Overall, this study suggests that ethanol organosolv pretreatment is a suitable pretreatment technique and the organosolv lignin can play a key role in a future biorefinery sector as a lignin first biorefinery concept. | en_US |
