Pre combustion

Pre-combustion CO2 capture is a process where the carbon in the fuel is separated, or removed, before the combustion process. Instead of burning coal or natural gas in a combustion plant, the fuel can be converted to hydrogen and CO2 prior to combustion. The CO2 can then be captured and stored, while the hydrogen is combusted to produce power. Pre-combustion capture technology is only applicable to new fossil fuel power plants because the capture process requires strong integration with the combustion process.

CO₂ can be separated from fossil fuel before combustion, which is the so-called pre-combustion CO₂ capture method.

The principle of this process is to first convert the fossil fuel into CO₂ and Hydrogen gas (H₂). The H₂ and the CO₂ is then separated in the same way as in the post-combustion process, although a smaller installation can be used. This results in a hydrogen-rich gas which can be used in power plants or as fuel in vehicles. The combustion of hydrogen does not lead to any production of CO₂.

Oxyfuel combustion with CO2 capture. Illustration: Prosjektlab

When using coal or natural gas for power production, the fossil fuel and steam is converted into synthesis gas (or syngas) in a traditional steam reformer. Synthesis gas is a common industrial gas consisting of carbon mono-oxide (CO) and hydrogen gas. The CO subsequently reacts with steam to form CO₂.

WIth pre-combustion CO₂ capture about 90 percent of the CO₂ from a power plant can be removed. As the technology requires significant modifications of the power plant, it is only viable for new power plants, not for existing plants.

Pre-combustion CO2 capture.

Using today's technologies, the investment costs for a gas power plant with pre-combustion CO₂ capture will be much higher than for a similar plant using post-combustion capture of flue gas. The separation of CO₂ from fossil fuel prior to combustion will become increasingly interesting as technological development brings down investment and operating costs.

Plans for new coal power plants often focus on the IGCC technology (Integrated coal Gasification Combined Cycle), where coal is converted into CO₂ and H₂ before combustion.

IGCC is a new technology that is under development. IGCC represents a large difference from traditional power plants. Traditional plants combust coal directly, while hydrogen will be combusted in an future IGCC plant.

Impurities are removed prior to combustion. This results in lower emissions of sulfur dioxide, particulates and mercury and it also gives improved efficiency compared to conventional coal power plants.

The plant is called "integrated" because its syngas is produced in an unit that is an integrated part of the combustion process.

The "combined cycle" indicates that two power production units are combined in the process. The first power production unit is a gas turbine fuelled by hydrogen that generates electricity and waste heat. The waste heat is used to make steam to generate additional electricity in a steam generator unit. 

Read more at Wikipedia

The pre-combustion CO₂ capture technologies are not as mature as the post-combustion capture technologies. But in 15 to 20 years it is expected that already ongoing research activities will improve the technology and reduce the cost related to pre-combustion technology.

Within a decade or two scientists expect that pre-combustion capture will have lower energy loss and lower costs than post-combustion CO₂ capture. 

Pre-combustion CO₂ capture by the IGCC process could be the preferred technology in the future for CO₂ capture. However, it is not possible to pick a technology winner now. Several other options could turn out to be just as applicable. Oxyfuel and novel technologies like membranes, adsorption and chemical looping could become equally good alternatives.

There are three main challenges related to pre-combustion CO₂ capture:

1. Pre-combustion is only applicable for new power plants because the capture process has to be an integrated part of the combustion process. Existing plants that will be retrofitted with CO₂ capture will mainly chose post-combustion capture.
2. Pre-combustion technologies like IGCC are not as mature as post-combustion capture technologies.
3. Gas turbines running on hydrogen are a huge challenge. A lot of research and development remains before a good functioning hydrogen gas turbine becomes available.

• Post combustion CO2 capture
• Oxyfuel
• Novel technologies

• R&D projects focusing on pre combustion CO₂ capture and IGCC: Dynamis and ENCAP
• Read about pre combustion and IGCC at the NETL home page and the RWE home page