CCUS energy requirements vary widely as a function of the removal process. Precombustion CO2 capture from natural gas processing is essentially cost-free as the gas separation is a required step to ensure that the natural gas sent through the pipes meets the required energy density and chemical composition. Most of the energy capture cost for these operations is associated with CO2 transport and injection into a well, which involves compression, pumping and injection. Similarly, the CO2 generated by the fertilizer production process or by methane reforming is quite pure. In these situations, the cost associated with separating CO2 from other gases is low and allows for the recovery of up to some 85%-95% in most industrial processes (Leung et al 2014).
Energy costs rapidly increase with the complexity of CO2 removal and the efforts required to separate these molecules from others. Notably, CO2 capture from post-combustion thermal plants can significantly reduce the efficiency of the plants measured, for example, by the net electricity produced by GJ of fossil burned. For natural gas, the energy penalty for capturing CO2 is on the order of 15% and can exceed 30% for coal plants, as shown in Table 12.1, not including the extra energy costs associated with CO2 storage.
These energy costs increase with the proportion of CO2 removed. Thus, a natural gas or a coal thermal plant is typically set to remove between 85% and 90% of the CO2 emitted at combustion even though, technically, it is possible to remove 99% of emission, but with additional effort.
From a life cycle standpoint, it is necessary to expand GHG emission accounting to include upstream emissions associated with the extraction, processing and transport of the fuel to the combustion site. These emissions depend closely on local regulations, the nature of the extraction and the distance to the power plant. They are also likely to decrease over the coming years as tighter regulation is implemented. At present, the IEA estimates that from extraction to combustion, more than 80% of natural gas produces 0.53 kgCO2e/kWh of electricity produced, and coal, 1 kgCO2e/kWh with current technologies (IEA 2020). This means that about 0.17 kg.eq.CO2 per kWh of electricity produced is emitted upstream from the thermal plant for both coal and natural gas, without CCUS.
When accounting for the reduced efficiency of CCUS-based gas and coal thermal plants, the overall net emissions per kWh of electricity generated from CCUS-equipped facilities therefore remains at 0.35 kgCO2e for coal and 0.24 kgCO2e for gas. If these remaining GHGs were to be captured using direct air technologies in a net-zero pathway, the overall primary energy cost for the same kWh would be almost double (196%) for coal power plants and 170% for gas power plants, as compared with current thermal plants. Adding the cost of CO2 storage explains why these technologies are unlikely to become significant, except for peak or other very specific uses, given the existence of alternative low-cost and low-carbon sources of electricity.