To meet global goals for decarbonizing our energy consumption, it is readily apparent that the capture of CO2 from flue gases (and to a lesser extent process/syngas from hydrogen generation) will play a key role. One area of significant debate amongst the authors of the dozens of studies that this author has reviewed, is the amount of energy that will be lost to carbon capture through solvent or sorbent regeneration, and ultimately CO2 compression and transport to a forever-home via sequestration. Energy loss is also commonly referred to as parasitic load – but perhaps that is too limiting to the power generation world since CO2 capture will be often times from industry as well.
The good news is that CO2 can be readily captured as a simple acidic gas by more than a dozen capture technologies, including solvents like aqueous amine solutions, potassium carbonates (and other carbonates), physical sorbents, membranes, and even refrigerated solvents like methanol. Some methods are quick to capture CO2 (in more technical terms they have high kinetic rates) but are difficult to get to release their CO2 once they have either adsorbed or absorbed it. Other methods are slower to capture CO2 but are more easily triggered with energy to release it. Sadly there is no free lunch – and one must pick his poison in the design of a carbon capture system – slow reacting but easily regenerated systems require fairly large capture devices to make up for their slow kinetic reaction rate…whereas more active solvents require more energy to regenerate them – and hence have a higher lifetime operating cost.
In terms of operating energy losses for carbon capture, most published reports from real-world projects (Petra-Nova in Wharton Texas, Air Products at Valero’s Port Arthur Refinery, and several others) indicate a total energy loss of 22-30% depending on how one calculates all of the externalities. In addition, those projects that strip CO2 from a high-pressure and higher-concentrated stream such as the effluent syngas from a steam methane reformer’s low-temp-shift reactor outlet (10-20% CO2 concentration @ 500 psi-ish) are able to operate more efficiently than a coal plant that is capturing flue gas from a slipstream at nearly atmospheric pressure with perhaps 10% CO2 concentration.
Within these values, I am including the electricity (or steam in some instances) required to power the very large compressor needed to bring CO2 up to an adequate density for transmission and sub-surface sequestration (more to come on this topic in the separate blog).
In simpler terms, a 1000 MW power station capturing its flue gas for CO2 sequestration will be spending 30% of its generated electrical power (and steam) on its own carbon capture. 300 MW of power going into the self-support of carbon is a big number, no matter how it is calculated.
The ramifications of the power consumed by CO2 capture systems are that 1) it is expensive to burn hydrocarbons with a net parasitic load of 30%…it will take about 43% more power to deliver the same MWs to end users. 2) alternative energy sources like hydro and wind should expect to benefit from the structural shift in the cost curve with much higher clearing prices for energy 3) carbon capture credits values must be high enough to incentivize a portion of the power sector to capture their CO2 4) heat rates for power plants will be more important in the future as the CO2 capture parasitic load works to amplify differences in the dispatch curve 5) other emitters of CO2 who can make their process more efficient or have high pressure CO2 streams are more likely to come in around the 22% energy loss for CO2 capture and could offer alternatives to CO2 capture versus power plants.
Author: Jon Paul Ruggles is a partner at Inddevco and an energy expert