Carbon capture, utilisation and storage
A critical tool in the climate energy toolbox
Carbon, capture utilisation and storage (CCUS) is one of the only technology solutions that can significantly reduce emissions from coal and gas power generation and deliver the deep emissions reductions needed across key industrial processes such as steel, cement and chemicals manufacturing, all of which will remain vital building blocks of modern society.
CO2 capture involves the separation of CO2 from industrial processes and energy-related point sources such as power plants. Separating the CO2 requires energy and often involves modifications to existing processes by adding extra process steps. After separation, the CO2 stream can be further purified and compressed to make it ready for transport. CO2 capture is typically divided into four main categories. In certain cases, these categories can be combined to create hybrid routes to capture.
CO2 is separated from a mixture of gases at the end of the industrial or energy process, for example from combustion flue gases. This route is referred to as post-combustion capture in power generation applications. Most post-process capture technologies used in large-scale CCS projects today are amine-based absorption systems. Examples are the 115 megawatt (MW) unit at the coal-fired Boundary Dam power plant in Canada and the 640 MW unit at the coal-fired Petra Nova power plant in Texas, United States, together capturing well over 2 million tonnes of CO2 per year.
Instead of air, nearly pure oxygen can be used to combust fuel - this is oxy-fuel combustion. This way, virtually all the flue gas will be composed of CO2 and water vapour. Part of the flue gas is recycled to the combustion chamber to control the combustion temperature, while the remainder is dehydrated to obtain a high-purity CO2 stream. Oxygen is commonly produced by separating oxygen from air, often produced in an air separation unit.
Research efforts focus mainly on the development of membranes as an alternative technology for air separation and novel oxy-fuel technologies, such as chemical looping and pressurised oxy-fuel combustion. Both show large energy reduction potentials, but are still in their infancy. Another promising new concept is supercritical CO2 power cycles as an option for capturing CO2 from natural gas or coal-based syngas. In these cycles, fuel gas is combusted at high pressure using oxygen, moderated by recycled CO2 and/or H2O, and the resulting hot high-pressure gas is expanded in a turbine to generate electricity.
Supercritical CO2 cycles offer efficiencies comparable with conventional natural gas cycles (without CO2 capture) at similar capital cost, and they can be net producers, not consumers, of water. In May 2018, NETPower, one of the developers of these supercritical CO2 cycles, started operation of a 50 MW plant in La Porte, Texas that should demonstrate the key aspects of the technology.
In a process called syngas/hydrogen capture, fossil fuels or bioenergy can be processed with steam and/or oxygen to produce a gaseous mixture called syngas, consisting of carbon monoxide and hydrogen (a process referred to as reforming or gasification). The former is reacted with more steam (water-gas shift reaction) to yield additional hydrogen and convert the carbon monoxide to CO2. The CO2 can be separated from the high-pressure gas mixture, yielding a raw syngas for chemical production or energy feedstock for the generation of heat (in a boiler or furnace) or electricity (in a combined-cycle gas turbine, CCGT, or fuel cell).
The capture process has been applied at scale at the Great Plains in North Dakota, which converts coal into synthetic natural gas, and captures 3 million tonnes of CO2 on a yearly basis. Pre-combustion CO2 capture can be applied in the short term to high CO2-emitting industries including the chemical and iron and steel industries.
Certain processes in industry and fuel production generate high-purity CO2 streams as an intrinsic part of the process (e.g. gas processing, ethanol production). Without CO2 capture, the produced CO2 is vented to the atmosphere. This is inherent separation.
The cost of capture varies greatly per point source, ranging from around 20 USD per tonne of CO2 for high-purity sources (e.g. natural gas processing) to well over 100 USD per tonne of CO2 for small dilute point sources (e.g. small industrial boilers).