Electrofuels in a circular economy

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Rusmanis, Davis
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University College Cork
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With the global shift away from traditional dispatchable fossil hydrocarbon fuels, the requirement for energy storage is of increasing importance. Renewable electricity generation is predominately in the form of variable renewable electricity, produced by wind and solar technologies. Intermittent production of electricity leads to an inevitable mismatch in supply and demand between the grid and the consumers. This can lead to periods of surplus power generation – and subsequent dispatch down – during low demand; conversely it can also lead to energy shortages during periods when there is insufficient power generation to match high demand which can lead to grid blackouts. Due to the difficulty in storing significant quantities of electricity via the grid, or batteries, sustainable alternative methods of energy storage must be devised. In recent years, electrofuels have become a centre-stage topic due to the opportunity to store electricity as low-carbon energy vectors which can be utilised where electrification may not be ideal, such as the hard-to-abate sectors of haulage, shipping, agriculture, and industry (iron, steel and chemicals). Further, the generation of these electrofuels could be carried out using electricity which would otherwise be lost, during times of excess production, and low grid demand. Combining technologies such as electrolysers (capable of producing hydrogen and oxygen) and biomethanation (which can combine carbon dioxide with hydrogen to form biomethane), offer a green alternative to fossil natural gas, and can use the existing gas grid as both a distribution system and a sustainable energy storage method. Based on the literature review and previous research, the initial thesis work focused on the design and commission of a prototype three-phase cascading biomethanation system. The fabricated prototype used diffusers as the agitation method and was deemed to be at a technology readiness level (TRL) of 4. The system was designed based on the results of a previously simulated system published by this research group (Voelklein et al., 2019); the results from this study were of similar performance to the previous model. The commissioning process produced carbon conversion rates between 72% and 97% across 3 reactors which can operate in parallel or series. A methane evolution rate of 2.9 L CH4/LVR/d was achieved at medium flow rates. Increasing the flow rates resulted in substantial drops in the conversion efficiency of the system. This limitation was mooted as likely due to the low bubble column height of the system. The design of the system was limited by the dimensions of the system, which were associated with health and safety concerns of the University. Assessing the integration of biomethanation technology into the wider energy, and environment sector, an initial small-scale case study of a circular economy system was carried out, based on local industry. Using a dataset from two local wind turbines to assess available surplus electricity, a small 122 kW electrolyser was proposed to be of a suitable size for the local site. This electrolyser would generate enough by-product oxygen to supply 8.9% of the oxygen demand of the aeration process for the local wastewater treatment plant of approximately 65,000 population equivalent. The product hydrogen could be used directly as a transport fuel, or convert 40% of the CO2 generated by the anaerobic digesters at the wastewater treatment plant into biomethane. This system could reduce the wastewater treatment plant emissions by 3.6% due to the reduced aeration requirement (pumping oxygen instead of air would reduce electricity usage). Should an appropriately sized electrolyser be used, up to a 40% reduction in emissions and energy use could be achieved. However, the small scale of the system was identified as a major barrier to the application of the technology, with the levelised cost of hydrogen evaluated at €8.92/kg H2 (or 27c/kWh). This resulting high cost cannot justify the implementation of a small-scale system to capture intermittent curtailed electricity as initially proposed. Expanding the circular economy system is possible with a carbon-negative emission pathway integrating pyrolysis technology to generate biochar. Anaerobic digesters and pyrolysis systems could potentially reduce greenhouse gas emissions by 42.7 kt CO2 through biomethane production from substrates within a 10km radius and through biochar production from digestate. When considering the sustainability considerations set by the latest version of the EU Renewable Energy Directive, electrofuels may only be certified as a renewable energy supply by affecting a 70% emission saving when compared to the current fossil hydrocarbon fuels. Additionally, the Directive does not allow for emission saving associated with carbon capture and reuse within the biological methanation system despite the added capture and use of otherwise curtailed electricity. Assessing a large-scale circular economy system, once again co-locating electrolysers with anaerobic digesters and biomethanation systems on new or existing large wastewater treatment plants can offer significant benefits. To supply a 10 MW electrolyser operating at an 80% operating capacity, a wind farm of 144MW size (18 x 8 MW turbines) operating at 57 % capacity would be necessary, with an assumed 10% curtailment scenario, reflective of the Irish grid dispatch down of the recent years. The 10 MW electrolyser could supply oxygen for a wastewater treatment plant of 426,000 population equivalent, reducing the plant emissions (and power demand) by 40% when compared to traditional gaseous air aeration. The produced wastewater sludge can be digested to produce biogas. The CO2 component of biogas can be converted to methane using a biomethanation system, requiring 22% of the electrolyser-produced hydrogen. This would result in the capture of 16 ktCO2e per annum while producing enough electrofuels to offset 390 diesel trucks with 94 compressed biomethane trucks and 296 compressed hydrogen trucks.
Electrolyser , Wastewater treatment , Biological methanation , Circular economy , Decarbonisation , Heavy-goods vehicles , Green Hydrogen , Design and commission , Hydrogen , Carbon capture
Rusmanis, D. 2023. Electrofuels in a circular economy. PhD Thesis, University College Cork.
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