Novel technologies

The first large-scale CO2 capture plant will probably be a post combustion process based on amine absorption, or an oxyfuel process. But if we look ahead to 2030 and onwards, new and improved concepts for CO2 capture that are not available today will probably have been developed.

Looking ahead to 2030

The introduction of new technologies always starts with promising prototypes that are heavily improved as time goes by. That will probably be the case with CO₂ capture technology as well.

Consider the mobile phone. The first prototypes twenty years ago were big, heavy and expensive. Few people could see that this was the start of the development of the modern mobile phone that nearly every one of us cannot live without today.

A similar development of CO₂ capture technology can also be expected. The first large-scale CO₂ capture plants will probably be based on post-combuston amine absorption. Soon after oxyfuel and pre-combustion plants will follow. All these first-of-its-kind CO₂ capture plants will probably turn out to be big and expensive and consume lots of energy.

But if we look ahead to 2030 and onwards there will probably be new alternatives that are efficient and cost-effective. Some of the future alternatives are membranes, adsorption (not absorption!) and chemical looping.

Membrane

Membranes can be used for separating CO₂ from other gas components. The technology is available today but it will take many years and lots of research before we will see a large-scale unit for CO₂ capture by membranes.

The concept is simple. Membranes are materials that some materials can penetrate while others can not.

Membrane for CO2 separation. Two gas components enter the inside of membrane fibres. Only one of the components is able to penetrate the membrane, and the result is one stream of purified CO2 and another stream nearly free of CO2.

A coal power plant emits large volumes of CO₂ and will consequently require large areas of membrane. The membrane unit must be small and compact to keep the cost down. In the figure above this is solved by manufacturing the membrane as thin hollow fibres. Thousands of such membrane fibres are then put together into a membrane module in order to obtain a high membrane area within a small volume.

There biggest challenges is related to the membrane material. In a good membrane most of one component can be transported through the membrane while most of the other components are not let through. But many membranes only let through some of the first component and only stops some of the other components. Most research activities on membranes for CO₂ capture focus on developing improved membrane materials.

Furthermore, energy is required to push material through the membrane, and the thicker membrane, the more energy is needed. It is a challenge to produce membranes as thin as possible to reduce the energy requirements.

Chemical looping

Chemical looping is a new technology for combustion with inherent CO₂ capture. It combines two reactors, one air reactor and one fuel reactor.

In the air reactor oxygen from the air reacts with a metal-based material to form a metal-oxide. This metal-oxide is transfered to the fuel reactor where it reacts with fuel, for example natural gas. The reaction produces CO₂, water and pure metal during the release of energy. The metal is then recycled to the air reactor.

The beauty of this process is that it has the same advantages as oxyfuel, but it solves the challenges related to oxyfuel. Oxyfuel has the advantage of having a simple and cheap CO₂ separation process, but the challenge of producing cheap and pure oxygen.

In chemical looping the reaction of oxygen from air with a metal does not require large quantities of energy like in the oxyfuel process. Furthermore, the gas produced in chemical looping is CO₂ and water vapour which can easily be separated by a traditional condensing process.

But there is one big challenge: existing metal-based materials for reacting with oxygen and the fuel are far from being efficient. The challenge is to design new material that reacts easily with oxygen and fuel and releases large amounts of energy.

Integrated fuel cells

Integrated fuel cells enable production of the clean energy carriers electricity and hydrogen from fossil fuel or bio-fuel with ultra-high efficiency and integrated CO₂ capture.

As an example a process developed by ZEG Power is described in the following sub-section.

The electricity is produced from a Solid Oxide Fuel Cell (SOFC) module. The SOFC module converts fuel to electricity electrochemically without any conventional combustion. The efficiency of the SOFC module is in the 50% to 70% range, depending on the operating conditions (ZEG Power). The energy that is not converted into electricity is released at a high temperature (appr. 1000°C). Even the ohmic losses and other losses in the SOFC are released as high temperature heat, and can be utilised in the process.

Hydrogen is produced from natural gas or other hydrocarbons in a reforming process that requires significant heat input. In the process the required heat for the reforming process comes from the excess heat from the SOFC module, and the need for heat generation by combustion is eliminated. In order to use all the excess heat from the fuel cell, more hydrogen is produced than is consumed in the fuel cell.

Combining a SOFC with a hydrogen reformer for co-production of electricity and hydrogen makes a system with a theoretical efficiency of 100%. CO₂ capture is enabled without theoretical energy losses, but some energy losses due to the operation is however inevitable in real processes. CO₂ capture processes require energy, usually in the form of heat. In an amine process as well as in the ZEG process, heat is required to release CO₂. The same amount of heat is released while the CO₂ is absorbed, but at a lower temperature. When this heat is taken from a heat powered system, the CO₂ capture will reduce the overall efficiency of the system. In the ZEG process the heat is released at a temperature that is high enough for the hydrogen reformer, and the waste heat from the CO₂ capture process can be fully utilised in the reformer.

This particular process also eliminates the need for an afterburner, usually required in SOFC systems. It further improves the operating conditions for the SOFC, resulting in higher efficiency and/or lower costs for the SOFC module. The process completely eliminates the need for combustion, and NOx emissions are virtually zero. The challenge is to prove the design for larger (>200Mwh) plants.

Adsorption

As previously mentioned, the first large-scale CO₂ capture plants will probably be based on post-combustion absorption. But a future alternative to absorption is adsorption.

During CO₂ capture by absorption a liquid chemical, also called solvent, will react with the CO₂, while in adsorption the CO₂ will be attached to the surface of a solid component, also called sorbent.

The challenges for adsorption are similar to those related to absorption; finding new sorbents that react more efficiently and faster with CO₂ and that require less energy to be regenerated.

CO₂ capture by adsorption is not a mature technology, but the technology is avalable on laboratory scale. A lot of research programes are underway and hopefully there will be a technological breakthrough that can pave the way for adsorption as a future solution for CO₂ capture.

See also

• CO₂ capture
• Post cimbustion CO₂ capture
• Pre combustion CO₂ capture
• Oxyfuel with CO₂ capture
• CCS challenges

External links

More information about membranes:
- CO2 Capture Project

More information about chemical looping:
- Instituto de Carboquimica
- Western Kentucky University
- ENCAP R&D project
- Chemical looping animation

More information about integrated fuel cells:
- Zero Emission Gas Power

More information about research activities on adsorption:
- IEA GHG RD&D project database
- Reseach paper on Pressure Swing Adsorption