The replacement of fossil fuels with a sustainable alternative like biomass from organic wastes represents a key issue to bring CO2 emission below zero in a short time. Most of waste have a high lignin content and non-hydrolysable biopolymers that cannot be transformed efficiently into biofuels with existing technology.
The conversion of biomass to biofuels can be achieved via biochemical and thermochemical processes. Hydrothermal liquefaction (HTL) is an interesting way to generate renewable energy from biomass and provides several advantages over other techniques since hot pressurized water is used as a reaction medium and reactant. As such, other chemicals are unnecessary and the whole process is versatile and environmentally friendly. Typical HTL conditions range from 250 to 350 °C and 5–20 MPa for residence times of 10–60 min. HTL allows the production of different fractions: an organic bio-crude fraction, a solid residue, a small fraction of gaseous product and an aqueous phase as main by-product.
This aqueous phase is rich in organic fraction that need treatments. The upgrading/valorisation into biomethane by anaerobic digestion (AD) could be an interesting option which is challenging for recalcitrant substrates, like lignin. The peculiar lignocellulosic structural assemblage makes its deconstruction into simple products accessible to microorganisms extremely difficult. In particular, lignin cannot be converted into biofuels with conventional biological processes, precluding the conversion of a consistent fraction of biomass, as it can constitute over the 30 % of dry mass. The deconstruction of biomass can be accomplished by heat which breaks biopolymers into lower molecular mass fragments allowing the biodegradation from anaerobic bacteria.
The project is aimed at investigating the potential of AD to the treatment of HTL products obtained at KIT from lignin-rich substrates through batch reactors experiment. During my stay at the Institute of Catalysis Research and Technology IKFT of Karlsruhe, I had the opportunity to perform HTL experiments in batch systems to obtained aqueous phase products to perform AD tests later at University of Bologna. In particular 6 experiments were conducted with micro batch reactors and one experiment with the PAUL reactor.
Micro batch experiments: fir sawdust (100 g/L) and an alkaline lignin solution (100 g/L) were processed in HTL process at different temperatures (250°C and 350°C) and different reaction time (30 and 60 minutes) to obtain an aqueous phase product rich in deconstructed lignin. The experiments were performed in batch reactors (25 ml) and pressure range 4 – 16.5 MPa. Samples are put in stainless steel reactors under standard pressure and sealed. The reaction time starts after the target temperature is reached. After the reaction, the reactors are put under cold water to cool down and stop the reaction.
PAUL reactor experiment: fir sawdust (100 g/L) were processed at 350°C for 60 minutes. The experiment was performed in 2L stirred batch reactor at 17.4 MPa. The reaction time starts after the target temperature is reached. A sampling system allow to collect the liquid after the desired retention time. The solid residue is taken when the whole reactor cool down. At the end of each experiments, samples were filtered under vacuum to separate the aqueous phase from the solid residue. The solid residue was dry overnight at 105°C to calculate the net amount. The gas, where present, was calculated through a GC-TCD.
All HTL products will be analytically characterized at Bologna University and used in AD experiments. A scientific paper is expected as final outcome.