Direct Air Capture and Methanation: Process design and development

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https://research.tue.nl/en/publications/direct-air-capture-and-methanation-process-design-and-development

Direct Air Capture and Methanation: Process design and development

Francesco Sabatino

Summary

As a consequence of COVID-19, global CO2 emissions fell by 6.4% in 2020, when many countries underwent a lockdown. While this decrease is remarkable, it is still well below the needed cut of 7.6% per year in the next decade to prevent global warming above 1.5 °C. Moreover, the emissions level quickly bounced back to pre-pandemic levels the following year. Within this context, it is becoming ever more likely that, to avoid overshooting the 1.5 °C target, net removal of CO2 from the atmosphere will be required. Several negative emissions technologies have been proposed in the literature, with direct air capture (DAC) being one of the most promising. DAC is a process in which CO2 is extracted from ambient air and concentrated for sequestration or use as feedstock. Although sequestration is the only option that ensures negative emissions, utilization is of interest as a way to produce more sustainable fuels or to store renewable energy. The complete decarbonization of our society will not take place overnight, with some sectors (e.g. aviation) requiring a crucial technological leap to become fully independent from carbon sources. The growth of renewable energy, moreover, also calls for the development of measures to balance the inherent fluctuations associated with solar and wind energy supply. In this regard, power-to-X (PtX) is often brought up when discussing the future renewable energy system. PtX refers to technologies that convert electricity into a variety of fuels or energy carriers through a two-step process. Hydrogen is initially produced via water electrolysis and converted to a gaseous or liquid energy carrier by synthesis with CO or CO2. The utilization of dense energy carriers produced from atmospheric carbon and renewable hydrogen would reduce or even eliminate the emissions of distributed sources such as the one associated with the transport sector. Moreover, dense energy carriers provide an efficient way of storing and transporting renewable energy using the existing infrastructure. 

DAC has experienced an incredible advancement in the past decade, both in terms of number of scientific publications and technological readiness. PtX has also been investigated in detail, with different demonstration plants currently in operation. However, the possible integration of DAC with the conversion of CO2 has yet to be fully explored. Different synergies might arise from the optimal combination of CO2 capture and hydrogenation, the most obvious being reduction of the energy demand through efficient heat integration. Moreover, novel and intensified processes based on a more interconnected integration could provide additional advantages. The aim of this work is to quantify these possible synergies and identify an optimal process design for the production of an energy carrier, specifically synthetic natural gas, from CO2 captured from air and renewable hydrogen. DAC is still a very expensive technology with a high potential for improvement. The high cost of current DAC processes would place a considerable weight on the economics of any CO2 capture and utilization (CCU) system. Therefore, this thesis begins with a thorough assessment and optimization of the most prominent DAC processes, starting from absorption-based technologies. Two processes are considered, namely liquid scrubbing with aqueous solutions of potassium hydroxide and alkanolamines. The former has been developed by Carbon Engineering Ltd. and is a benchmark in DAC, while the latter is the most widely adopted technology for carbon capture and storage (CCS) applications. Using process simulations and mathematical optimization, energy consumption and productivity are computed and peak performances are identified. The results show that both technologies can efficiently separate CO2 from air and provide it at high purity. Solvent regeneration dominates the energy demand of these technologies. As a matter of fact, in the Carbon Engineering process CO2 is recovered at 900 °C, a temperature that is reached through oxy-combustion of natural gas. Although the solvent regeneration is carried out at only 120 − 130 °C, the amine scrubbing alternative actually has a higher energy demand, as considerable amounts of water need to be heated up. Thus, the efficient CO2 capture of liquid scrubbing DAC technologies is hindered by solvent regeneration processes that are either very energy intensive or unsustainable. 

Electrochemical alternatives for the recovery of CO2 from spent absorbents have also been proposed. These technologies can be easily integrated with renewable energy sources, as the only energy input is electricity. In this work, the focus is on the application of bipolar membrane electrodialysis (BPMED) in the potassium hydroxide scrubbing process. Two options based on different electrodialysis cell designs have been modelled and optimized. The technical assessment is complemented by a detailed economic analysis, underlying the advantages but also the current shortcomings of this technology and pathways for advancement. With current membrane performance, it is estimated that the energy demand would be almost four times higher than the one required by the thermochemical cycle adopted by Carbon Engineering. Several solutions to further abate power consumption are reviewed, with the most promising providing a 29% reduction. Membrane performance, particularly the electrical conductivity and ion selectivity, are currently limiting factors. Moreover, the high cost of membranes weighs heavily on the process economics. In a scenario where cheaper and better membranes would become available, total costs below 250 $ ton−1 CO2 would be feasible, making BPMED a viable fully electrified alternative to other technologies requiring natural gas. 

Subsequently, the adsorption-based approach to DAC is explored. As several CO2 adsorbents have been proposed for DAC, a screening on the basis of the working capacity is carried out to identify the best sorbent candidates. Moreover, the influence of water co-adsorption and mass and heat transfer rates is investigated. A temperature-vacuum swing adsorption (TVSA) process has been adopted in this work, and the design for the adsorption unit is based on data published by DAC company Climeworks. The TVSA cycle is modelled and optimized to minimize energy demand and maximize productivity. The results show that the adsorption cycle can be engineered to obtain high purity CO2. The solid-based process has the potential to offer the best performance, however fast adsorption kinetics and an optimal H2O co-adsorption mechanism are required to outperform liquid scrubbing processes. Translating productivity and energy performance into cost of CO2 capture via a simple model, it is found that the capital cost is the main cost driver. All technologies have the potential to operate below 200 $ ton−1 CO2 under favorable, yet realistic, energy and reactor costs. The solid-sorbent process achieves this under broader conditions, and becomes less dependent on the installation cost when high mass transfer rates are achieved. 

The final part of this thesis is devoted to the integration of adsorption-based DAC with CO2 methanation. The synergies that might arise from the optimal combination of DAC with methanation could greatly benefit the overall process efficiency and, therefore, the economic feasibility. Three different processes are developed and assessed, exploring different integration strategies, viz. i) only heat integration; ii) DAC sorbent regeneration with high pressure H2 to produce a H2/CO2 gas mixture; iii) complete integration of DAC and methanation in a single unit. As these concepts differ considerably, different modelling methodologies are adopted for their evaluation. Sensitivity analyses are carried out to identify optimal operating conditions with regards to the productivity and energy demand. It is shown that novel multi-functional process concepts can provide important benefits, particularly related to a more efficient heat integration and avoidance of CO2 compression. Integrated direct air capture and methanation (DACM) processes can achieve overall autothermal operation, effectively exploiting the heat liberated during methanation for sorbent regeneration.