Expanding Biogas Processes into Broader Biorefinery and Bioeconomy Systems
To optimise environmental and economic sustainability of biogas processes, we need to scavenge all products and by-products of anaerobic digestion. There are numerous potential configurations and pathways we can design to maximise sustainability (both environmental and economic), typically involving circular bioeconomy concepts utilising a series of cascading technologies.
We can consider three main products of anaerobic digestion; biomethane (CH4), biogenic carbon dioxide (CO2), and digestate. A stable continuous production of energy in the form of biogas may not be optimal. We can produce energy on demand, we can ramp up production to suit a particular time schedule, and in so doing, increase the economic return by availing of the day ahead electricity market. If the desired end-product is biomethane (termed renewable natural gas), then CO2 within the biogas can be converted to micro-algae. Such a system would involve biogas upgrading via a carbonate bicarbonate tower whereby CO2 in the biogas supports growth of micro-algae, a valuable product with uses including as a super food. Alternatively, we may not produce energy but rather produce PHAs (bioplastics) from volatile fatty acids produced within the digester.
Solid digestate can be converted to biochar (via pyrolysis). We can tune biochar: at high temperatures (700oC) we can optimise porosity and achieve surface areas of 400m2/g; at low temperatures (300oC) we can optimise conductivity. Conductivity is shown to enhance bacterial activity when biochar is added to a fermentation process; it also has a beneficial role in direct interspecies electron transfer between bacteria and archaea. Porosity is preferred in filtering out contaminants in wastewaters (as required in recast EU Water Directive) and in 3D printing of electrodes. Addition of electrodes with a voltage differential of 1 volt within a digester can convert CO2 to CH4 in-situ and in essence increase energy output of a biogas facility by over 80%, whilst negating the need for traditional physio-chemical biogas upgrading.
We can go beyond CH4 as an energy vector. For example, we can use bio-electrochemistry in enhancing microbial elongation in production of high value medium chain carboxylic acids (such as C6 caproic acid) from CO2 in biogas. We can make liquid fuels, such as biomethanol from steam reforming of biomethane, or we can use renewable hydrogen to react with biogenic CO2 (by-product of biomethane) to make e-methanol. Methanol (e- or bio-) is likely to be a shipping fuel of the future. Methanol is also a precursor for sustainable aviation fuel via an alcohol to jet process.
For net zero we must expand biogas processes into broader biorefinery and bioeconomy systems.
Bio: Jerry d. Murphy served as director of the Research Ireland MaREI centre for energy, climate and marine from 2015 to 2026. In that time the centre graduated 220 Ph.D. researchers and engaged in research co-design with 110 industry partners. He set up the Circular Economy, Energy, and Environmental Systems research Group in UCC in 2007; he has supervised to graduation c. 30 Ph.D. researchers in this theme and has published c. 230 peer review journal papers. Scholar GPS ranks his work in the top 12 academics worldwide researching “Biofuels.”
He was awarded the Engineers Ireland Excellence Award (2015); Excellence in Marine Research (2017); adjunct professor of the University of Southern Queensland (2018); International Advisor to DBFZ (The German Bioenergy Research centre) (2019); the Mary B Upton Visiting Professorship in Cornel University (2024); a visiting Professor at The International Energy Agency (2024) and a Visiting Professor at the Robert Frederick Smith School of Chemical and Biomolecular Engineering, Cornell (2026). He led the International Energy Agency (IEA) technology collaboration programme (TCP) in Bioenergy, Task “Energy from Biogas” from 2016 – 2021 and grew this Task to 19 member countries with the addition of Brasil, Canada, China, Estonia, India, and Italy and in this role authored/edited 18 IEA Bioenergy reports, 23 case stories and chaired 12 IEA Bioenergy Symposia.
Jerry presently currently serves as Professor Chair of Civil Engineering in University College Cork, Ireland.