Publications

We describe a process for capturing CO₂ from the atmosphere in an industrial plant. The design captures ~1 Mt-CO₂/year in a continuous process using an aqueous KOH sorbent coupled to a calcium caustic recovery loop. We describe the design rationale, summarize performance of the major unit operations, and provide a capital cost breakdown developed with an independent consulting engineering firm. We report results from a pilot plant that provides data on performance of the major unit operations. We summarize the energy and material balance computed using an Aspen process simulation. When CO₂ is delivered at 15 MPa, the design requires either 8.81 GJ of natural gas, or 5.25 GJ of gas and 366 kWhr of electricity, per ton of CO₂ captured. Depending on financial assumptions, energy costs, and the specific choice of inputs and outputs, the levelized cost per ton CO₂ captured from the atmosphere ranges from 94 to 232 $/t-CO₂.

We present a conceptually simple method for optimizing the design of a gas–liquid contactor for capture of carbon dioxide from ambient air, or ‘air capture’. We apply the method to a slab geometry contactor that uses components, design and fabrication methods derived from cooling towers. We use mass transfer data appropriate for capture using a strong NaOH solution, combined with engineering and cost data derived from engineering studies performed by Carbon Engineering Ltd, and find that the total costs for air contacting alone—no regeneration—can be of the order of $60 per tonne CO₂. We analyse the reasons why our cost estimate diverges from that of other recent reports and conclude that the divergence arises from fundamental design choices rather than from differences in costing methodology. Finally, we review the technology risks and conclude that they can be readily addressed by prototype testing.

In this chapter, our aim is to provide an overview of air capture technologies that focuses on three broad topics. First, we provide an economic and technical rationale for considering direct capture of CO₂ from ambient air. Second, we describe some of the more important constraints and trade-offs involved in the design of air capture systems from the standpoint of chemical engineering and physics. Third, we provide an overview of a particular air capture technology that we are developing which combines a contactor using a high molarity sodium hydroxide solution with a titanate-cycle caustic recovery process.

Air capture is an industrial process for capturing CO₂ from ambient air; it is one of an emerging set of technologies for CO₂ removal that includes geological storage of biotic carbon and the acceleration of geochemical weathering. Although air capture will cost more than capture from power plants when both are operated under the same economic conditions, air capture allows one to apply industrial economies of scale to small and mobile emission sources and enables a partial decoupling of carbon capture from the energy infrastructure, advantages that may compensate for the intrinsic difficulty of capturing carbon from the air.

We used a 6.5 m tall packed tower prototype to study the capturing rate of CO₂ from air. The tower was operated at a pressure drop of less than 27 pa in the packing at 0.7m/sec air speed with a counter current flow mode and with NaOH or KOH solution as the absorbent. The tower consumed an average of ~30 kJe per mole CO₂. We found that via an intermittent operation with a 5% duty cycle, the fluid pumping work reduced by 90%. A novel process for removing carbonates ions from alkaline solutions based on titanate compounds is compared to the traditional lime cycle for the caustic recovery. The titanate process reduces the high-grade heat requirement by ~50%. The results from experimental data of leaching and precipitation test support process design of the titanate cycle. In this paper, we also present the chemical process design.

To reduce the risks of climate change, atmospheric concentrations of greenhouse gases must be lowered. Direct capture of CO₂ from ambient air, ‘‘air capture’’, might be one of the few methods capable of systematically managing dispersed emissions. The most commonly proposed method for air capture is a wet scrubbing technique which absorbs CO₂ in an alkaline absorbent, i.e. sodium hydroxide producing an aqueous solution of sodium hydroxide and sodium carbonate. In most of the previous works it was assumed that the absorbent would be regenerated and CO₂ liberated from the alkaline carbonate solution using a lime and calcium carbonate causticization cycle. We describe a novel technique for recovering sodium hydroxide from an aqueous alkaline solution of sodium carbonate and present an end-to-end energy and exergy analysis. In the first step of the recovery process, anhydrous sodium carbonate is separated from the concentrated sodium hydroxide solution using a two-step precipitation and crystallization process. The anhydrous sodium carbonate is then causticized using sodium tri-titanate. The titanate direct causticization process has been of interest for the pulp and paper industry and has been tested at lab- and pilot-scale. In the causticization process, sodium hydroxide is regenerated and carbon dioxide is liberated as a pure stream, which is compressed for use or disposal. The technique requires 50% less high-grade heat than conventional causticization and the maximum temperature required is reduced by at least 50 ℃. This titanate cycle may allow a substantial reduction in the overall cost of direct air capture.

Reducing greenhouse gas emissions from the transportation sector may be the most difficult aspect of climate change mitigation. We suggest that carbon neutral hydrocarbons (CNHCs) offer an alternative pathway for deep emission cuts that complement the use of decarbonized energy carriers. Such fuels are synthesized from atmospheric carbon dioxide (CO₂) and carbon neutral hydrogen. The result is a liquid fuel compatible with the existing transportation infrastructure and therefore capable of a gradual deployment with minimum supply disruption. Capturing the atmospheric CO₂ can be accomplished using biomass or industrial methods referred to as air capture. The viability of biomass fuels is strongly dependent on the environmental impacts of biomass production. Strong constraints on land use may favour the use of air capture. We conclude that CNHCs may be a viable alternative to hydrogen or conventional biofuels and warrant a comparable level of research effort and support.

In contrast to conventional carbon capture systems for power plants and other large point sources, the system described in this paper captures CO₂ directly from ambient air. This has the advantages that emissions from diffuse sources and past emissions may be captured. The objective of this research is to determine the feasibility of a NaOH spray-based contactor for use in an air capture system by estimating the cost and energy requirements per unit CO₂ captured. A prototype system is constructed and tested to measure CO₂ absorption, energy use, and evaporative water loss and compared with theoretical predictions. A numerical model of drop collision and coalescence is used to estimate operating parameters for a full scale system, and the cost of operating the system per unit CO₂ captured is estimated. The analysis indicates that CO₂ capture from air for climate change mitigation is technically feasible using off-the-shelf technology. Drop coalescence significantly decreases the CO₂ absorption efficiency; however, fan and pump energy requirements are manageable. Water loss is significant (20 mol H₂O/mol CO₂ at 15 °C and 65% RH) but can be lowered by appropriately designing and operating the system. The cost of CO₂ capture using NaOH spray (excluding solution recovery and CO₂ sequestration, which may be comparable) in the full-scale system is 96 $/ton-CO₂ in the base case, and ranges from 53 to 127 $/ton-CO₂ under alternate operating parameters and assumptions regarding capital costs and mass transfer rate. The low end of the cost range is reached by a spray with 50 µm mean drop diameter, which is achievable with commercially available spray nozzles.

It is physically possible to capture CO₂ directly from the air and immobilize it in geological structures. Air capture differs from conventional mitigation in three key aspects. First, it removes emissions from any part of the economy with equal ease or difficulty, so its cost provides an absolute cap on the cost of mitigation. Second, it permits reduction in concentrations faster than the natural carbon cycle: the effects of irreversibility are thus partly alleviated. Third, because it is weakly coupled to existing energy infrastructure, air capture may offer stronger economies of scale and smaller adjustment costs than the more conventional mitigation technologies. We assess the ultimate physical limits on the amount of energy and land required for air capture and describe two systems that might achieve air capture at prices under 200 and 500 $/tC using current technology. Like geoengineering, air capture limits the cost of a worst-case climate scenario. In an optimal sequential decision framework with uncertainty, existence of air capture decreases the need for near-term precautionary abatement. The long-term effect is the opposite; assuming that marginal costs of mitigation decrease with time while marginal climate change damages increase, then air capture increases long-run abatement. Air capture produces an environmental Kuznets curve, in which concentrations are returned to preindustrial levels.