Renewable energy from biogas with reduced carbon dioxide footprint: Implications of applying different plant configurations and operating pressures

Abstract

Renewable energy from biogas has the potential to decarbonise energy systems. For example, biomethane derived from raw biogas may partially displace fossil fuels in the transportation sector. The implemented renewable energy actually decarbonises energy systems only if its life cycle CO2 footprint is lower than that of displaced conventional technologies, which is sometimes uncertain. Therefore, this study has been undertaken to review and synthesise knowledge available in the academic literature on the CO2 footprint of renewable energy from biogas. The typical life cycle CO2 footprint of biogas reported in literature is between 50 and 450kgCO2/MWhel. The review analyses three phases associated with biogas: (i) biomass production, (ii) biomass-to-biogas conversion, and (iii) biogas end use. It is found that remarkable CO2 footprint reduction can be achieved by innovating the biomass-to-biogas phase through limiting the amount of CO2 liberated to biogas. The mechanism for reducing CO2 footprint is proposed and suitable solutions are discussed and evaluated. The literature review is followed by a case study that improves the practical understanding of CO2 footprint reduction potentials. In the case study anaerobic digestion (AD) and pressurised anaerobic digestion (PAD) are compared in terms of their biomethane, power and heat generations, and CO2 emissions. Six plant configurations involving AD, biogas upgrading and combined heat and power (CHP) generation are modelled and simulated. The results show that due to the methane enrichment in biogas (94% CH4 at the self-sustained digester pressure of 5MPa) CO2 footprint is reduced. It is revealed that PAD based biogas plants may generate high purity biomethane with the extremely low direct CO2 footprint of about 13kgCO2/MWhf which contrasts with conventional CHP systems achieving about direct CO2 footprint of 700kgCO2/MWhel. The study also explores the fundamentals of PAD which is one of emerging biogas technologies.*Corresponding author.