A pre-combustion process separates air into its two major components of nitrogen and oxygen , with highly purified oxygen entering the power station boiler with the fuel to be burnt . The relative ease in separation of CO 2 and water vapour ( by condensing out the water vapour ) leaves 95 %– 99 % of CO 2 to be piped or transported to a storage facility .
Although this is an efficient process for capturing CO 2 , separating large volumes of air into its constituent gases can use a significant percentage 9 of the power produced at a power plant , resulting in increased energy consumption . A power station ’ s conventional base design also needs to be adapted by adding equipment and processes at the front end , prior to combustion taking place in the boiler . The boiler design must additionally change to accommodate the air separation process and input of oxygen . Burning fuel in pure oxygen ( in the absence of nitrogen to dilute the flames and gases ) results in extremely high temperatures that the combustion chamber may not be able to withstand . Some of the combustion gases therefore need to be diverted back into the combustion chamber to provide the dilution effect that limits temperature rises to acceptable levels .
These design requirements make oxyfuel technology more suited for incorporation into the design of new-build power plants from the outset , rather than being retrofitted to a conventional power plant .
• Pre-combustion capture – Pre-combustion carbon capture requires conversion of the fuel precombustion to separate out the carbon for capture , in a process known as gasification . Air is channelled through an air separation unit to generate a high level of very pure oxygen , which is used with steam to convert the fuel into a synthesis gas ( or “ syngas ”) of carbon monoxide and hydrogen – this differs slightly from the dominant method of producing hydrogen , known as steam methane reforming , which involves the use of steam alone .
A water shift reaction process follows , in which the carbon monoxide in the syngas reacts with water to produce CO 2 and more hydrogen . CO 2 can then be captured via a chemical solvent process , while the hydrogen can go on to be burnt as a carbon-free fuel .
Pre-combustion capture benefits from a long industrial history with decades of cumulative expertise and know-how for the gasification of fuel into syngas . Some power stations in the USA and Europe , for example , already use gasification to produce syngas that is sent directly to gas turbines to generate electricity , albeit without the carbon capture elements .
The energy input required for the gasification and water shift reaction processes , however , results in a less efficient power station . Particularly for natural gas power stations , where all the gaseous fuel needs to react with steam and oxygen to produce CO 2 and hydrogen , the economic advantage of pre-combustion carbon capture over postcombustion carbon capture has yet to be established .
The electricity generation process , where hydrogen is produced from the fuel to generate electricity in a gas turbine , also requires a significantly different design from that of conventional combustion processes . 10 This limits the application of pre-combustion technology to new-build power stations , and excludes the ability to retrofit older coal power plants , which currently comprise much of the world ’ s installed base of fossil fuel power .
• Transport – Captured CO 2 needs to be safely and efficiently transported , either for onward industrial use or to a permanent underground storage site in a suitable geological formation , often depleted oil and gas reservoirs . It is typically compressed under high pressure into a liquid , as dense liquid is easier to transport than gas and allows transportation of greater volumes .
Compared with other transport options , pipelines are often seen as the most cost-efficient and viable long-term option for transporting large quantities of CO 2 to be captured from industrial sources such as power stations and hydrocarbon production , despite the cost associated with pipeline construction . CO 2 is already widely transported today via pipelines , in accordance with established industrial safety standards and regulations . For example , the US has seen pipeline transportation of liquid CO 2 for oil recovery for almost four decades . 11
However , depending on the location of the CO 2 capture and the geological formation used for storage as well as availability of land and pipeline construction and operation regulatory regimes , other forms of transportation ( such as shipping or trucking ) may also be appropriate . For example , in the absence of a UK-wide network of CO 2 transportation pipelines , it may be more economical to transport any CO 2 captured in the south of the UK and destined for storage in geological formations in the North Sea by ship than by building a dedicated pipeline .
Moreover , the use of pipelines for mass transportation of CO 2 would require a dramatic expansion of existing pipeline networks . An estimated 40 million tonnes of CO 2 is captured and stored annually today 12 , compared with projections by the International Energy Agency that climate change abatement scenarios would require up to 1.6bn tonnes ( Gt ) of CO 2 annually to be safely transported and stored underground from 2030 , rising to 7.6 to 10Gt of CO 2 annually from 2050 . 13 Such vast volumes of CO 2 would require , in the case of the high-end estimate of
The use of pipelines for mass transportation of CO 2 would require a dramatic expansion of existing pipeline networks .
74 Project Finance International April 6 2022