Energy and cost comparison per carbon avoided for CO2 removal from air compared to low-carbon electricity [electronic resource]
Keywords: Mechanical Engineering.

Our inability to curb CO2 emissions has increased interest in geoengineering: "the deliberate large-scale manipulation of the planetary environment to counteract anthropogenic climate change." One of the safest types of geoengineering is carbon dioxide removal from the air, which is generally very expensive and resource intensive. On the other hand, the electricity generation sector has a number of options to lower carbon emissions, and the emissions from electricity also influence the efficacy of atmospheric CO2 removal technologies. Because removing carbon from the atmosphere and preventing emissions from electricity generation are equivalent from a climate perspective, it is desirable to compare them on a quantitative basis. However, because technologies to remove carbon dioxide from the air and low-carbon electricity are designed for different purposes and services, it is very difficult to find the appropriate quantitative metrics for comparison. In order to compare disparate systems fairly we use a broader definition of carbon avoided, one which includes life-cycle emissions and uses a common reference electricity mix for all technologies considered. Using this metric we compare two currently-deployable carbon-negative options (one biological and one chemical) against several electricity generation technologies both with and without carbon capture and sequestration. The electricity generation technologies with carbon capture include current state-of-the-art coal and natural gas electricity, and the low-carbon electricity generation technologies without carbon capture include nuclear and wind electricity. We evaluate the systems on the basis of: carbon avoided per unit of physically captured carbon; energy consumption per unit of carbon avoided; and cost per unit of carbon avoided. We use the present day average U.S. electricity mix as the initial reference electricity, and then vary the carbon intensity and cost in order to understand the sensitivity of the results. We show that on an energy and cost per unit avoided basis, the biological atmospheric CO2 removal system is strongly favored over the chemical system of atmospheric CO2 evaluated. We also show that when considered relative to a common reference, the biological atmospheric CO2 removal system (a bioelectricity system with carbon capture and sequestration) is less expensive and more efficient for avoiding carbon than either a natural gas or coal electricity generation system with carbon capture and sequestration. However, overall we show that the least expensive option for avoiding carbon in the electricity system of the U.S. today is by generating wind electricity. The next lowest cost options for avoiding carbon are by generating nuclear electricity or bioelectricity with carbon capture and sequestration. The system for bioelectricity with carbon capture and sequestration investigated here only becomes less expensive than wind electricity for avoiding emissions once the electricity emissions of the overall U.S. electricity mix are reduced by about 80%.