Much ongoing research shows how it is extremely difficult to achieve the net-zero emission target without including carbon dioxide removal (CDR) from the atmosphere. Ocean liming is a new and promising marine CDR strategy, aimed at removing significant amounts of atmospheric CO2 by discharging alkaline materials into the ocean. Terrestrial-enhanced weathering is a further CDR option based on the distribution of mafic rocks on land. Both rely on the use of raw materials. The first part of this work clarifies whether and to which extent, the energy and cost requirements to extract and process massive amounts of raw materials may affect the engineering feasibility of such technologies at the global scale. Hence, firstly, an in-depth literature review on two main stages of the raw material process chain (i.e., surface mining and comminution) and long-haul transportation has been made to elucidate each activity's technical challenges, cost, energy demand, and energy-saving strategies. Afterward, the related energy-saving potential has been assessed. The overall conclusion of this part is that there are remarkable energy-saving opportunities, especially in the most energy-intensive activities (i.e., fine and ultrafine grinding and road transport), if investment in cutting-edge solutions, energy efficiency improvements, application of best practices and policies to upscaling them, could take place at a fast-enough pace and within a wide system approach. The second part of the work addresses the environmental impact caused by discharging alkaline materials into the ocean from a cost and energy point of view. The results show that the total cost of spreading can be kept low (around 10 $/tCO2) only if appropriate conditions of discharge rate and water requirement to create the alkaline mix are chosen. The estimations suggest a combined use of at least 50 kg/s and a slurry instead of an alkaline solution requiring high water amounts. The final part of the work discusses the cost and energy requirement of a novel carbon capture storage option called Buffered Accelerated Weathering of Limestone (BAWL). The capture and storage of industrial CO2 is achieved in two steps: the first one involves a chemical reaction between CO2, seawater, and calcium carbonate within a long tubular reactor whereas the second one consists of buffering the unreacted amount of CO2 exiting the reactor with an alkaline solution made of calcium hydroxide and seawater. The chemical background of BAWL has already been presented by Caserini et al (2021b). The present work carried out a sensitivity analysis to examine how cost and energy consumption can change with the variation of three important design parameters: depth of the discharge (100-3,000 m), length of the dissolution reactor pipeline (10-100 km) and grain size of calcium carbonate (10-1 μm). The results show that the total electricity consumption and cost to store 1 tonne of Steam-Methane Reforming CO2 vary between 1.3-2.2 MWh and 142-189 € depending on the configuration of the plant. The recent evolution of BAWL consists of using only one pipeline to allow calcium hydroxide and seawater to react with CO2. The maximum length of the pipeline is 2 km, the buffered solution is discharged at a depth of 20 m, and the grain size of calcium hydroxide is not lower than 74 μm. In such a case, the energy consumption and cost to store 1 tonne of Steam-Methane Reforming CO2 are 214 GWh and 159 € respectively.
Recenti ricerche mostrano come sia ormai estremamente difficile raggiungere l'obiettivo della neutralità climatica (net-zero target) senza ricorrere a strategie di rimozione della CO2 dall'atmosfera. L'alcalinizzazione artificiale degli oceani rappresenta uno dei metodi più promettenti, dato l'alto potenziale di mitigazione (1-100 GtCO2 all'anno) e il contrasto all’acidificazione degli oceani che garantirebbe. Essa consiste nello spargimento di sostanze alcaline sulla superficie marina. Basandosi sull'uso di materie prime come il calcare o rocce ad alto contenuto di olivina, valutazioni riguardanti la disponibilità della materia prima, la domanda energetica e di costo richiesta dall’estrazione e da trattamenti successivi sono requisiti primari per stabilire l’effettiva fattibilità ingegneristica di questa tecnica. Questo studio si è occupato di valutare in maniera mirata la domanda energetica e di costo dovuti all'estrazione, frantumazione, polverizzazione e trasporto della materia prima (calcare e roccia mafica), e di quantificare, inoltre, il potenziale risparmio energetico derivante dall'applicazione di opportune strategie. I risultati hanno mostrato come la macinazione fine, ultrafine e il trasporto di lungo raggio rappresentino le fasi più critiche della catena di approvvigionamento, ma offrano, al contempo, significativi margini di risparmio energetico se si decide di investire in nuove tecnologie e migliori pratiche. In una prospettiva di futura implementazione dell’alcalinizzazione artificiale degli oceani, le riduzioni di energia elettrica dovuta alla macinazione fine del calcare (diametro di 5 micron) potrebbero arrivare fino al 65% se l'efficienza energetica del processo di macinazione raggiungesse il 10%. Sono stati inoltre effettuati calcoli sul consumo energetico e sul costo dovuto allo sversamento di idrossido di calcio in mare tramite navi. Lo studio termina con un'analisi tecnica, energetica e di costo di un’innovativa tecnologia di cattura e stoccaggio dell'anidride carbonica emessa da processi industriali, chiamata Buffered Accelerated Weathering of Limestone.
Ocean liming as a carbon dioxide removal technology: potential, limitations and environmental impacts
De Marco, Serena
2023/2024
Abstract
Much ongoing research shows how it is extremely difficult to achieve the net-zero emission target without including carbon dioxide removal (CDR) from the atmosphere. Ocean liming is a new and promising marine CDR strategy, aimed at removing significant amounts of atmospheric CO2 by discharging alkaline materials into the ocean. Terrestrial-enhanced weathering is a further CDR option based on the distribution of mafic rocks on land. Both rely on the use of raw materials. The first part of this work clarifies whether and to which extent, the energy and cost requirements to extract and process massive amounts of raw materials may affect the engineering feasibility of such technologies at the global scale. Hence, firstly, an in-depth literature review on two main stages of the raw material process chain (i.e., surface mining and comminution) and long-haul transportation has been made to elucidate each activity's technical challenges, cost, energy demand, and energy-saving strategies. Afterward, the related energy-saving potential has been assessed. The overall conclusion of this part is that there are remarkable energy-saving opportunities, especially in the most energy-intensive activities (i.e., fine and ultrafine grinding and road transport), if investment in cutting-edge solutions, energy efficiency improvements, application of best practices and policies to upscaling them, could take place at a fast-enough pace and within a wide system approach. The second part of the work addresses the environmental impact caused by discharging alkaline materials into the ocean from a cost and energy point of view. The results show that the total cost of spreading can be kept low (around 10 $/tCO2) only if appropriate conditions of discharge rate and water requirement to create the alkaline mix are chosen. The estimations suggest a combined use of at least 50 kg/s and a slurry instead of an alkaline solution requiring high water amounts. The final part of the work discusses the cost and energy requirement of a novel carbon capture storage option called Buffered Accelerated Weathering of Limestone (BAWL). The capture and storage of industrial CO2 is achieved in two steps: the first one involves a chemical reaction between CO2, seawater, and calcium carbonate within a long tubular reactor whereas the second one consists of buffering the unreacted amount of CO2 exiting the reactor with an alkaline solution made of calcium hydroxide and seawater. The chemical background of BAWL has already been presented by Caserini et al (2021b). The present work carried out a sensitivity analysis to examine how cost and energy consumption can change with the variation of three important design parameters: depth of the discharge (100-3,000 m), length of the dissolution reactor pipeline (10-100 km) and grain size of calcium carbonate (10-1 μm). The results show that the total electricity consumption and cost to store 1 tonne of Steam-Methane Reforming CO2 vary between 1.3-2.2 MWh and 142-189 € depending on the configuration of the plant. The recent evolution of BAWL consists of using only one pipeline to allow calcium hydroxide and seawater to react with CO2. The maximum length of the pipeline is 2 km, the buffered solution is discharged at a depth of 20 m, and the grain size of calcium hydroxide is not lower than 74 μm. In such a case, the energy consumption and cost to store 1 tonne of Steam-Methane Reforming CO2 are 214 GWh and 159 € respectively.| File | Dimensione | Formato | |
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https://hdl.handle.net/10589/220192