Fractionation of biomass from Eucalyptus globulus using subcritical water

Figure 1. SEM image of eucalyptus wood before and after the extraction with water at T=285ºC and flowrate=5 mL/min.
Figure 1. SEM image of eucalyptus wood before and after the extraction with water at T=285ºC and flowrate=5 mL/min.

A green way to obtain renewable biofuels starting from woody biomass is studied, according with the biorefinery concept. Eucalyptus globulus wood is used as raw material.

Eucalyptus is a tree of considerable importance, due to its wide expansion and its spread use in industrial applications. High biomass yield and low water consume make it very attractive from an industrial point of view, not only for paper production, but also as sustainable and carbon-neutral source for liquid fuels. Cellulose and hemicellulose contained in woody biomass can be hydrolysed to monomeric sugars, which can be fermented to ethanol, or can be converted to higher value products. A promising, clean and cheap way to depolymerize cellulose and hemicellulose into monosaccharides is the process called autohydrolysis, which simply consists on treating biomass with hot water/steam.

During the reaction, most of the hemicellulose is extracted and hydrolysed to monomers, with a consequent release of acetic acid originated from the acetyl groups bonded to the oligosaccharides; a lower amount of cellulose is released, due to the crystalline structure of the polymer, which makes it more difficult to dissolve.

The experiments were carried out in a laboratory-scale fixed bed reactor, i.e. biomass wood chips in batch and fresh water pumped in continuous; the aim of the experiments was to investigate different conditions of temperature and liquid flowrate in order to find the best conditions to extract glucans and hemicellulose hydrolysis products from Eucalyptus globulus wood, and to analyze the influence of these parameters onto byproducts formation.

Results in figure 2 represent the fraction of the total amount of soluble compounds (sugars and degradation products) in the liquid leaving the reactor during the whole reaction (tsol=90 min), respect to the total amount of soluble compounds in the raw material, detected by HPLC.

Figure 2. (a) Fraction of C6 and C5 sugars in liquid, in function of the reaction time at constant temperature 185°C and different liquid flowrates. (b) Fraction of C6 and C5 sugars in liquid, in function of the temperature at constant liquid flowrate 5mL/min.
Figure 2. (a) Fraction of C6 and C5 sugars in liquid, in function of the reaction time at constant temperature 185°C and different liquid flowrates. (b) Fraction of C6 and C5 sugars in liquid, in function of the temperature at constant liquid flowrate 5mL/min.

The amount of degradation products in figure 3 was calculated as total mass of degradation products at the end of the reaction (tsol =90min), divided by the total amount of sugars (C5+C6) produced at the same time. The amount of acetic acid is also evidenced.

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Figure 3. (a) Fraction of total degradation products, total acetic acid, acetic acid coming from hemicellulose rupture and acetic acid produced as degradation product in liquid, respect to the total mass of sugars, in function of the reaction time at constant temperature 185°C and different liquid flowrates. (b) Fraction of total degradation products, total acetic acid, acetic acid coming from hemicellulose rupture and acetic acid produced as degradation product in liquid, respect to the total mass of sugars, in function of the temperature at constant liquid flowrates 5mL/min.

Results can be summarised saying that:

  •  At constant temperatures, high liquid residence times led to an increase of the concentration of acetic acid in the solution, that catalyses the depolymerisation of hemicellulose, and thus to the production of larger amounts of C5 sugars. Degradation products increased with increasing residence time, because sugars, once formed, were not removed quickly from the reactor and were degraded to other components.
  •  At constant flowrates, high temperatures led to an increase of the final amount of C6, C5 and degradation products. High temperatures are in fact necessary to break the crystalline bonds of cellulose. High temperatures also favour the breakage of hemicellulose bonds, increasing the kinetic constant of the depolymerisation reaction. On the other hand, the kinetics of reactions leading to by-products formation are also improved.

Gianluca Gallina – Project FracBioFuel ENE2012-33613

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