The peer-reviewed paper, titled Integrated techno-economic and life cycle assessment of a novel algae-based coating for direct air carbon capture and sequestration, is a Life Cycle Analysis (LCA) and Techno-Economic Analysis (TEA) for the technology, called Carbon Capture Coatings, in a solar-based configuration (the technology can be configured to use waste or solar energy). Within the 3rd party independent study, the LCA accounts for all Scope I and II carbon emissions for the described Carbon Capture Coatings facility. The authors conclude that the technology is scalable, has a 50% efficiency in the solar configuration (meaning that to account for system emissions, two tons of CO2 must be captured and sequestered to net one ton of sequestered CO2), and the cost is in the $700-$1,500 range per ton of CO2 captured and sequestered. Jason Quinn, PhD, CEO of the independent third party that evaluated the technology for the LCA and TEA, concluded that ‘the capture costs estimated with this study are on par with existing Direct Air Carbon Capture and Storage (DACCS) technologies, including Climeworks’ low-temperature sorbent system. Furthermore, the proposed technology offers rapid scale-up potential, which is important if we are to be able to address the removal of billions of tons of CO2 in time to make a difference in climate change. Moreover, Carbon Capture Coatings technology does not rely on injecting billions of tons of CO2 deep into the earth like many of the other sequestration techniques relied upon by other carbon removal systems.’
“Integrated techno-economic and life cycle assessment of a novel algae-based coating for direct air carbon capture and sequestration”
The carbon embodied in the cellulose material is converted to biochar through pyrolysis to ensure durable carbon sequestration without the need for underground storage. The proposed system offers many advantages including modularity and scalability, the potential for high water retention rates, and long periods of operation with minimal maintenance and management. Three scenarios were evaluated using conservative, baseline, and optimistic assumptions to capture the true range in performance of the system. Results from the modeling work show a carbon removal efficiency ranging from 51% to 73% and carbon capture and sequestration costs of $702–$1585 per tonne CO2 sequestered. Furthermore, the modular design of the coated substrate system and utilization of solar energy supports the rapid upscaling necessary to meet mid-century carbon removal goals. Discussion focuses on the key performance drivers of the system and the challenges and feasibility of meeting target metrics to support economic and environmental sustainability.
Anthropogenic greenhouse gas (GHG) emissions have caused the global temperature to rise approximately 1.18 ◦C since the 19th century resulting in a multitude of devastating environmental impacts. Some of these impacts include warming oceans and shrinking ice sheets, sea level rise, loss of biodiversity, and increased severity and frequency of natural disasters and wild fires . Outlined in the 6th Assessment Report from the International Panel on Climate Change, the best path to limiting warming under 1.5 ◦C by 2100 is to achieve a 45% reduction in human-caused CO2 emissions by 2030 and to reach net-zero emissions by 2050 . While the majority of GHG emissions reductions are expected to come from the implementation of clean renewable energy and changes to major carbon-emitting industries, a number of essential sectors lack a clear pathway towards net-zero emissions. These hard-to-abate sectors make up nearly 30% of global GHG emissions and include the manufacturing of essential chemicals and materials like cement, steel, aluminum and fertilizers, as well as heavy duty transportation industries including shipping, trucking and aviation . The National Academy of Sciences released a report in 2019 describing a number of existing technologies that need to be adopted to meet short-term climate goals. Through a combination of soil conservation practices, increased forest management efforts, biomass capture, processing, and distribution and other negative emissions technologies, it may be possible to achieve 10 billion tonnes of CO2 removal per year in the first half of the century . In order to reach large-scale and long-term removal goals of 20 billion tonnes of CO2 per year in the second half of the century, costlier and less developed technologies like Direct Air Carbon Capture and Storage (DACCS) will need to be adopted at unprecedented levels .
Whether using a liquid or a solid substrate, the governing principles are the same; CO2 from ambient air is fixed in the chemical liquid solvent or chemical solid sorbent with a chemical bond and then released as a high-purity CO2 stream through the application of heat. The purified CO2 stream must then be compressed, cooled, transported and injected into an underground CO2 storage reservoir to achieve permanent sequestration. While chemical liquid solvent systems often require high-quality heat at 900 ◦C to separate the CO2 from the solvent, chemical solid sorbent systems only require heat at 100 ◦C for the separation process, thereby enabling co-location with a number of industrial processes producing waste heat . Synergistic placement of solid sorbent DACCS systems with geothermal or nuclear power plants would utilize low-carbon waste heat or use an industrial slip stream to provide 80% of the total energy demand for the system. The remaining 20% of the total energy demand is primarily for fans to move air through the system and is typically supplied by the local grid or from on-site renewable energy generation. Several recent studies have evaluated the economic and environmental performance of chemical sorbent DACCS systems currently operating at commercial scales. Several common metrics are used to quantify the economic performance of DACCS systems: $ per tCO2 captured, $ per tCO2 net-delivered, and $ per tCO2 net-sequestered. The first metric, $ per tCO2 captured, quantifies the cost to operate the system long enough to remove or capture 1 tonne of CO2 from ambient air. The second metric, $ per tCO2 net-delivered, considers the direct GHG emissions from the system during the capture process and represents the cost to operate the system long enough to achieve a net removal of 1 tonne of CO2 from the atmosphere. The third metric, $ per tCO2 net-sequestered, incorporates the GHG emissions and costs of compressing, cooling, transporting, injecting, and storing the CO2 stream and represents the cost to achieve a net removal and durable sequestration of 1 tonne of CO2 from the atmosphere. For all three metrics, various economic assumptions and cash flow models can be used to incorporate the time value of money, depreciation, loan payments and taxes into final capture and sequestration metrics. A recent study from Deutz and Bardow  provides a comprehensive life cycle assessment (LCA) of low-temperature sorbent systems from Climeworks currently operating in Switzerland and Iceland and found that these plants are achieving carbon removal efficiencies of 85% and 93%, respectively. While the results from Deutz and Bardow  provide valuable insight on the environmental performance of the evaluated DACCS system and associated supply chains, their results exclude environmental impacts from the compression, injection and storage stages necessary to achieve durable long-term sequestration. To the authors knowledge, the only comprehensive LCA of low-temperature sorbent DACCS systems including the injection and storage stage is from Terlouw et al. . This recent study quantifies the impacts of utilizing various sources of heat (i.e., waste heat, electricity, and solar heat) and electricity (i.e., solar PV and grid energy) with Climeworks’ low-temperature sorbent technology across a total of eight different locations. Of all the evaluated scenarios, carbon removal efficiencies range from 9% (high temperature heat pump and grid energy in Greece) to 97% (waste heat and grid energy in Norway) indicating a considerable dependence on low-carbon heat and energy to achieve desirable carbon removal efficiencies . Another recent study from McQueen et al.  offers a robust techno-economic analysis (TEA) of Climeworks’ low-temperature sorbent system when using natural gas, geothermal brine and a nuclear power plant slip stream as heat sources. For each of these sources respectively, McQueen et al.  report carbon removal efficiencies of 35%, 71%, and 71% and capture costs of $223, $205 and $233 per tCO2 (and therefore capture costs accounting for removal efficiencies are $637, $288, and $328 per tCO2 net-delivered) . It should be noted, however, that the system boundary used by McQueen et al.  stops at compressed CO2 delivery for use or storage and excludes actual injection into underground reservoirs. Regardless of the source of heat, the capture costs estimated by McQueen et al.  when using low-temperature sorbents are approximately $200 per tCO2 captured. To understand the impact of including injection and storage in the system boundary, we can combine a baseline capture cost of $200 per tCO2 with the range of carbon removal efficiencies reported by Terlouw et al. . This suggests a potential range of $206 to $2222 per tCO2 net-sequestered for solid sorbent technologies, highlighting the necessity to obtain low-carbon sources of heat and electricity for the capture and separation processes. Additionally, the need for low-carbon heat and electricity limits the scalability of low-temperature sorbent technologies by necessitating co-location with waste heat. Additionally, concerns exist regarding land use change when considering electricity generation using solar PV. Terlouw et al.  estimated land requirements as high as 4.7 km2 for a solar PV system capable of supplying energy for DACCS plant with a capacity of 100 ktCO2 per yr.
In addition to low-temperature sorbent systems, existing literature studies have evaluated the economic performance of high temperature chemical solvent DACCS technologies. In a study from 2018, Keith et a.  provide a robust TEA evaluating the technology used by Carbon Engineering and found that the baseline system with Nth plant economic assumptions was capable of capturing CO2 from the atmosphere in the range of $126 to $170 per tCO2 captured depending on the assumed capital recovery factor (7.5–12%). While the analysis lacks a comprehensive LCA, the preliminary LCA performed in the study estimates a carbon removal efficiency of 90% when using natural gas combustion with emissions recovery as the primary source of energy. When combined with the cost of capture, this yields a range of $140 to $189 per tCO2 net-delivered for the chemical solvent DAC technology from Carbon Engineering. While the costs reported by Keith et al.  show favorable economic performance over low-temperature sorbent systems, high levels of uncertainty exist in economic results without a comprehensive LCA examining the system and quantifying all sources of re-emission. Additionally, while chemical solvent systems allow for centralized re-generation units and can achieve economies of scale, highly complex and costly infrastructure and the potential for a large water footprint provide potential obstacles in up-scaling to meet climate goals.
While decades of research and development have focused on the advancement of chemical solvent and sorbent technologies for DACCS systems, this study aims to assess a novel and emerging technology which exploits the biological process of photosynthesis to perform direct air capture on a large scale. The cultivation of microalgae for food and renewable fuels has been a research focus for biologists and engineers alike. Impressive photosynthetic rates drive high biomass yields per area and remove carbon from the atmosphere while producing a high value feedstock containing carbohydrates, lipids and proteins . The composition of microalgae allows for a variety of high value co-products and can be made into renewable transportation fuels through a number of conversion processes including transesterification and hydrothermal G.M. Cole et al. Journal of CO2 Utilization 69 (2023) 102421 3 liquefaction . This work exploits the fixation of carbon from the atmosphere in bacterial cellulose to perform direct air capture. When natural, carbon-fixing and polymerization genes from Gluconacetobacter hansenii (ATCC 53582, formerly Gluconacetobacter xylinus) are transfected into algal cells, the resulting recombinant algal cells can generate extracellular bacterial cellulose [14,15]. Similar engineered strains, when placed under growth-arrested conditions, have been shown to divert > 80% of biomass to photobiological production of compounds such as sucrose . This work assumes the potential that algae under growth-arrested conditions over long terms effectively divert all biomass to the production of cellulose. Though incidental, significant algae cell replication outside growth-arrested conditions may increase the required nutrient content of the coating. Accumulation of cellulose in the coatings is approximated by experimental data for the growth of Chlorella on coated sheets. This approximation is corroborated by previous work showing engineered algal cells under arresting-growth condition to have increased carbon pulldown rates compared to wild-type cells .
In this study, we evaluate the carbon capture potential of a coated surface made from hydrogel, microalgae, nutrients and water. The coated surface, housed in a modular enclosure, has been designed to mimic the biological processes of lichens, by converting atmospheric CO2 into bacterial cellulose which builds up on the surface over time. The coating is then harvested, and the constituents are separated with a chelator, allowing the generated cellulose to be converted to biochar through pyrolysis. The biochar is then land applied, providing an environmental service while durably sequestering the embodied carbon. While still in the early stages of development, there are many predicted advantages of this novel technology including the ability to quickly reach large-scale carbon removal using non-arable land. Additionally, once the initial surface has been seeded and deployed, the system is designed to remove carbon from the atmosphere and accumulate cellulose for months to years with minimal system inputs or required maintenance. The potential long operational life of this biomimetic surface would allow for rapid large-scale deployment across the globe. While the specific longevities of the algal cultures contained in the coating are currently being evaluated through lab-scale experiments, the economic and environmental impacts of this parameter are captured by modeling coating lifetimes of 1 year, 2.5 year, and 5 years in conservative, baseline and optimistic scenarios, respectively. Using these scenarios, this study aims to quantify the near-term environmental and economic performance of this novel carbon capture coating using a robust systems-level modeling approach, which serves as the foundation for integrated TEA and LCA with sensitivity and scenario analyses.