Production of biofuels in a circular cascading bio-based energy system

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Wu, Benteng
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University College Cork
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Anaerobic digestion (AD) systems which incorporate carbon capture, utilization and sequestration technologies may offer a solution to significantly reduce greenhouse gas (GHG) emissions in the heavy duty sectors of transport and industry. However, serious challenges faced by AD hinder its development, and new strategies are needed to achieve sustainable biofuel production. In this thesis bespoke circular cascading bio-based systems, with AD as the core element, integrating electro-fuel production via CO2 biomethanation and value-added pyrochar via pyrolysis of solid digestate, were proposed to produce sustainable biofuels, namely biogas and biomethane. The preliminary energy analysis revealed that the proposed system could increase final net energy output by 70% as compared with a conventional biomethane system if the electricity used to produce hydrogen is assumed to be otherwise curtailed. Pyrochar derived from solid digestate was proposed to be beneficial for both the AD and CO2 biomethanation processes. It is hypothesised that carbonaceous materials could significantly promote direct interspecies electron transfer (DIET) and accordingly enhance and stabilize AD performance. The first experimental work added carbonaceous materials (nanomaterial graphene and more cost-effective pyrochar) to digestion of thin stillage which was under stress due to an acidic shock, to investigate the potential role in improving the stability of the AD process. Results showed that the addition of graphene could help stabilize AD of thin stillage after acidic shock, presumably due to DIET. Graphene amendment (of 1.0 g/L) improved biomethane yield by 11.0% compared with the control group (without material addition) and accelerated the degradation of propionic acid. In comparison, pyrochar addition (both at 1.0 g/L and 10 g/L) shortened lag time but failed to enhance biomethane yield. Modelling of microbial electron transfer estimated that when 50% of the electrons produced from propionate oxidation are transferred through DIET, approximately 85 kJ/mol more energy can be obtained as compared to that of indirect hydrogen transfer. There is a gap in the state of the art in the potential role of carbonaceous materials in ex-situ biomethanation systems (CO2 + 4H2 → CH4 + 2H2O) facing flexible operation conditions due to intermittent gas injection. The variability in gas supply would be caused by green hydrogen production associated with variable renewable electricity. In the second experimental work CO2 biomethanation was examined with graphene amendment (1.0 g/L); the work demonstrated how such amendment could contribute to stabilizing ex-situ biomethanation after repeated periods of intermittent gas supply. The rationale for the stabilizing effect of graphene amendment is suggested as due to the high electrical conductivity and large specific surface of graphene. With the shock of intermittent gas injection, graphene addition enhanced the gas conversion efficiency by 18.2% and production rate by 267% as compared with the control group. However, pyrochar amendment (1.0 g/L) did not lead to promotional effects on the upgrading performance. Extending applications of carbonaceous materials in biofuels to biochemicals (in the third experimental work) the promotional effects of pyrochar amendment were observed in microbial chain elongation for medium chain fatty acid (MCFA, containing 6-12 carbon atoms) production. Optimal pyrochar addition could achieve 115% more MCFA yield than no pyrochar addition; this was demonstrated to be attributed to the high electrical conductivity and surface redox groups of pyrochar. Moreover, the optimal electron donor (ethanol) to electron acceptor (acetate) molar ratio was 2 mol/mol in a pyrochar mediated chain elongation system. Thermodynamic calculations modelled an energy benefit of 93.50 kJ/mol reaction for pyrochar mediated n-caproate production. To assess the economic and environmental benefits of circular cascading bio-based energy systems, process designs for production of gaseous biofuel (biomethane), liquid biofuel (biomethanol), and biofertilizer (digestate) were developed. The minimum marginal abatement cost for the pyrolysis incorporated case was –111.1 €/t CO2-eq when biomethane was sold at 1.03 €/Nm3. The significance of the hydrogen price may be noted as the marginal abatement cost rose to –58.2 €/t CO2-eq when H2 was purchased at €3.40/kg. When methanol was sold at 425 €/t (global weighted average value), the marginal abatement cost for the pyrolysis incorporated case (with H2 at 1€/kg) was 136.5 €/t CO2-eq, which is higher than current carbon credits at 33.5 €/t CO2. Integration of AD, CO2 biomethanation and pyrolysis technologies could be economically and environmentally compelling to produce biomethane, while for biomethanol production, minimisation of methane loss and use of low carbon electricity were necessary to lower the abatement cost.
Circular cascading bio-based system , Anaerobic digestion , Biomethane , Pyrochar , Graphene , Power to gas , Chain elongation
Wu, B. 2022. Production of biofuels in a circular cascading bio-based energy system. PhD Thesis, University College Cork.
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