Mobile Carbon Dioxide Removal: A Baseline Cost Estimate and Testing Protocol

There has been an extensive focus in recent years on compounds ideal for post-combustion carbon capture at power plants; an assortment of metal organic framework materials have been synthesized that can selectively adsorb CO₂. These compounds are designed for large-scale pressure swing adsorption at stationary power generation facilities, where the footprint, mass, and operational pressure are not a concern. This traditional carbon capture reduces greenhouse gas emissions from the electricity generation sector, but overlooks the ~30% of emissions from the transportation sector. The Organization for Economic Cooperation and Development projects that transport emissions will double by 2050[1], while the World Bank estimates that transportation will be the single largest source of global GHG emissions by 2035 and 80% of total emissions by 2050[2].

Federal air quality standards have addressed particulate matter and nitrous oxides, but regulations are not yet in place for carbon dioxide. There are two general approaches to reduce atmospheric concentrations of carbon dioxide: either capture CO₂ from vehicle exhaust at the source or from ambient air through direct air capture. Both methods rely on physical adsorption, but at a volumetric CO₂ concentration of 0.04% in air versus 13% for vehicle exhaust.

Using the thermodynamic minimum work for separation and compression of CO₂, the cost of mobile carbon dioxide removal (MCDR) can be estimated. In this study, an on-board system which captures carbon dioxide from the average daily commute of a passenger vehicle and then regenerates daily is shown to reduce emissions from the U.S. light-duty sector by nearly 80%. Furthermore, off-board adsorbent regeneration and CO₂ compression using low carbon electricity can reduce operational power costs by 50% over systems that use on-board energy from the waste heat of the internal combustion engine, while also avoiding additional CO₂ emissions.

The total carbon abatement cost of MCDR is comparable to stationary carbon capture and significantly less than direct air capture; MCDR is likely to cost 2-3 times less than direct air capture based on cost per carbon emission avoided. When comparing against stationary capture, most of the cost premium for MCDR can be attributed to CO₂ collection from distributed sources and the fuel economy penalty associated with increased vehicle mass. The mass penalty can be minimized by increasing the frequency of sorbent regeneration and CO₂ compression to every 30 miles, equivalent to the average daily vehicle commute, rather than at every refueling (~300 miles). Thus, MCDR is a novel strategy to reduce CO₂ emissions from the transportation sector with high potential for positive environmental benefits and costs comparable to stationary carbon capture at power plants.

The development of on-board carbon capture is strongly warranted; preliminary screening of potential adsorbents is currently underway using a novel exhaust gas testing regime at the US EPA’s National Vehicle and Fuel Emissions Laboratory. For vehicles, the mass and volume requirements for capture of all carbon emissions would be prohibitive based on cost, target fuel economy, and consumer support. These requirements are minimized by using an off-loading and regeneration frequency of 30 miles and a low-carbon home electricity energy source. Adsorbent materials that could work for MCDR in the transportation sector would address CO₂ capacity and selectivity, hydrothermal stability, trace contaminant tolerance, high flow rates, elevated temperatures, and ambient operating pressure.

Testing for all of these parameters required an approach not yet explored; to reach a pilot scale demonstration of MCDR, several apparatuses were designed that progress through a laboratory to pilot scale-up. The laboratory-scale apparatus uses gram quantities of solid compounds and controls flow rate, temperature, and relative humidity. Starting at the bench scale, kilogram quantities of materials are tested using engine exhaust, while still controlling flow rate and humidity. The proof-of-concept scale is an off-board system that operates with vehicle exhaust under full flow at ~1:4 mass scale for CO₂ uptake. The pilot scale design operates at full scale on-board a vehicle, targeted at >90% capture of CO₂ from vehicle exhaust during a typical drive cycle. The testing protocol that accompanies the apparatuses will illuminate what candidate materials, unit configurations, and operating conditions would be ideal for MCDR, and will hopefully serve as a framework for the evaluation of CO₂ removal from complex exhaust gas for the transportation sector.