Decarbonizing Freight Transport: Mobile Carbon Capture from Heavy-Duty Vehicles

Abstract

Predictions for future carbon dioxide emission reductions largely rely on power generation shifts to renewable energy sources and passenger vehicle electrification, while emissions from on-road freight shipping using heavy-duty vehicles (HDV) are expected to increase significantly over the coming decades. Mobile carbon capture (MCC) using porous solid adsorbents is a yet unexplored decarbonization strategy, the evaluation of which requires a study of the ideal materials and conditions for capture as well as the environmental, economic, and social implications of a global mobile carbon capture program for heavy-duty vehicles (HDVCC). While many porous materials are researched as carbon capture adsorbents, their carbon dioxide storage capacity at higher temperatures, in the range of 40°C to 75ºC and representative of vehicle exhaust streams, is critical to assess performance under realistic conditions. To quantify the impact on uptake capacity of elevated temperatures characteristic of vehicle exhaust, pressure swing isotherms were conducted on eight commercially available porous adsorbents at temperatures from 25°C to 100ºC. The materials tested included two activated carbons, two zeolite molecular sieves, and four metal-organic framework (MOF) adsorbents. An average decrease of 25% in the CO2 adsorption capacity was observed for zeolites, activated carbons, and MOFs at 101 kPa pressure for each 15ºC stepwise increase in the measured isotherm. Isosteric heats of adsorption are obtained for each material using the Clausius-Clapeyron equation and are in good agreement with adsorption enthalpies reported for these materials at similar temperatures. Among the materials considered, the reduction in CO2 adsorption capacity with increasing temperature is least pronounced for zeolites 5A and 13X, which correspondingly have the largest heats of adsorption for carbon dioxide. Candidate materials for HDVCC were then examined through a series of adsorption tests using dynamic flow of representative exhaust gas blends containing CO2, CO, NO, and H2O at temperatures and pressures characteristic of tailpipe exhaust. Of the materials tested, Zeolite 5A is a prime candidate for MCC, capturing approximately 11 weight % from representative wet diesel exhaust. Uptake can be further enhanced by cooling or removing water vapor from the exhaust gas; adding a high surface area heat exchanger prior to the adsorption bed accomplishes both, increasing capture to 15 weight %. After establishing the technical feasibility of capturing carbon from HDV, we then explore if HDVCC is a viable and sustainable decarbonization strategy for the transportation sector. Publications addressing MCC claim it is cost-prohibitive because of high mass requirements, often offering direct air capture as a better means of indirectly reducing vehicle emissions. In the economic evaluation, we show that the hypothetical carbon abatement cost of HDVCC is competitive with both stationary carbon capture and battery electric vehicles at ~$100/tCO2 avoided. The environmental impact of HDVCC was explored using an open-source simple climate model, the primary result of which is a range of warming (0.12°C – 0.15°C) that could be avoided if HDVCC is implemented between 2025 and 2040. Finally, a framework for the design of emerging technology was adapted to build an evaluation tool for consumers to compare HDVCC against traditional and electric HDV (using overhead catenary lines). The science and sustainability components encompass a comprehensive assessment of HDVCC, which is found to be a practical, cost-effective, and sustainable approach to mitigating carbon emissions from on-road sources and would ideally be implemented and integrated alongside stationary carbon capture.