Photosynthetic biogas upgrading: technological advancements and integration into a circular economy

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Bose, Archishman
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University College Cork
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Biomethane is a viable alternative to natural gas and diesel and its use has significant applications in decarbonising the hard to abate sectors of industry, heavy transport, and agriculture. Yet, commercial technologies for upgrading biogas from anaerobic digestion (AD) to biomethane (such as amine and water scrubbing) bring high costs and significant energy demand. In this research, photosynthetic biogas upgrading was studied as an alternative to conventional biogas upgrading. The two-step bubble column photobioreactor setup was established as the suitable technological configuration. Five criteria were developed to select favourable microalgae species for photosynthetic biogas upgrading; Spirulina platensis was evaluated as optimal amongst common microalgae species. Following a systems approach, the bubble column (the least optimised component of photosynthetic biogas upgrading) was interrogated in detail through design of experiments to advance the understanding of the overall technology. Findings concluded that the liquid inlet pH, the liquid to gas flow (L/G) ratio and their interactions are major factors for bubble column operations. A lower algal concentration would ensure both a higher CO2 removal efficiency and a lower O2 concentration in biomethane. Regression equations, developed from experiments, were found to adequately predict CO2 removal efficiency, overall CO2 mass transfer coefficient, O2 concentration in biomethane and pH of the outlet algal medium for bubble column operations at liquid inlet pHs between 9.4 and 10.2, alkalinities between 1.3 g-inorganic carbon (IC)/L and 2.1 gIC/L, superficial gas velocities (uG) between 0.3 cm/s and 0.6 cm/s and L/G ratios between 0.3 and 0.8. A minimum pH of 10, alkalinity of 1.7 gIC/L with L/G ratios between 0.6 and 0.7 and uG between 0.5 and 0.6 cm/s were found sufficient to achieve grid quality biomethane (CO2 and O2 less than 2.5%vol and 0.8%vol respectively) at a CO2 mass transfer coefficient above 150 hr-1. These optimal conditions were also favourable for the growth of Spirulina platensis in a connected photobioreactor. Following technological optimisation, several strategies were developed to integrate photosynthetic biogas upgrading into a circular economy system using a Cascading Algal Biomethane Biorefinery System (CABBS). A microalgae composition-based decision tree was proposed to facilitate the maximisation of the profitability of these integrated biorefineries. A detailed model of a fuel (biomethane), food (Spirulina powder), biofertiliser (digestate) polygeneration process was developed. Through a multi-criteria performance assessment, the economic and the environmental benefits of the process were shown. Contrary to the energy allocation approach of the Recast EU Renewable Energy Directive (RED-II) methodology, the energy, emissions, land, and water footprints of each co-product from the polygeneration system were found to be best represented by economic allocation while allowing cost-competitiveness with market available alternatives for food and fertiliser. Using the carbon intensity of electricity grids and the individual market price of each co-product, an economic allocation was found to facilitate the production of sustainable biomethane (as defined by carbon intensity of less than 14.4 gCO2-eq/MJ for biomethane for use in heat as per RED-II). Spirulina powder and biofertiliser were also assessed to have lower emissions than meat protein and synthetic nitrogenous fertilisers respectively. The conditions for the profitability of the polygeneration plant was shown at industrial, medium and small scales while selling biomethane at the price of natural gas (3 c€/kWh). A negative marginal abatement cost established the industrial-scale process as a profitable technology comparing very favourably to conventional biomethane production processes. Future lower carbon intensity of electricity grids and trading of biomethane in the EU emission trading scheme should facilitate financial viability of smaller-scale processes by 2030.
Biomethane , Biorefinery , Biogas upgrading , Anaerobic digestion , Microalgae , Circular bioeconomy , Technoeconomic analysis , Design of experiments , Renewable gas , CO2 capture and reuse , Polygeneration system , Multicriteria performance assessment , Low-carbon technology , Greenhouse gas assessment , Bubble column , Process modelling and simulation , Marginal abatement cost
Bose, A. 2021. Photosynthetic biogas upgrading: technological advancements and integration into a circular economy. PhD Thesis, University College Cork.
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