The first large-scale CO capture plant will probably be a post combustion process based on amine absorption, or an oxyfuel process.
But as we look ahead to 2030 and onwards, new and improved concepts for CO2 capture will become available. Technologies listed below are currently in various stages of development and expected to become commercially available in the coming decades.
Looking ahead to 2030
The introduction of new technologies always starts with promising prototypes that are heavily improved as time goes by. This evolutionary improvement is also expected to be the case with CO2 capture technology.
Consider the mobile phone. The first prototypes twenty years ago were large, bulky and expensive. Few people could imagine that this was the beginning of a development that would alter how people across the globe communicate and conduct commerce. Today there are more than 5 billion mobile phones, many affordable to people in the most disadvantaged nations.
A similar development of CO2 capture technology can also be expected. The first large-scale CO2 capture plants will probably be based on post-combustion amine absorption. Soon after oxyfuel and pre-combustion plants will follow.
All these first-of-its-kind CO2 capture plants will be more capital intensive and less efficient then subsequent generations. As greater numbers of CO2 capture plants are installed cost will reduce due to greater operational knowledge and a refinement of the technologies.
But if we look ahead to 2030 and onwards there are likely be new alternatives that become cost-effective offering greater efficiency gains. Some of these future alternatives include membranes, chemical looping and adsorption (not absorption!).
Membranes can be used for separating CO2 from other gas components. The technology is available today but will take some years and further research before it may be available for large-scale CO2 capture at generation plant.
The concept is simple. Similar to a filter, components which diffuse more readily through a material under pressure may be separated out. Generally CO2 capture membranes are designed to be selective for smaller gasses, for example allowing Nitrogen to pass through while leaving a pure stream of CO2 behind. Existing membrane technology currently available does not provide the CO2 purity necessary for transport. It is possible to increase purity with multi stage membranes but this also substantially increases the energy penalty required for repeated compression.
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 combined into a membrane module in order to obtain a high membrane area within a small volume.
In an ideal membrane, a high selectivity for the chosen gas will be achieved at low pressures and low energy costs. With regard to CO2 capture, current research focus on overcoming challenges related to the material design and improving membrane performance. Furthermore, energy is required to force gases 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 is a new technology for combustion with inherent CO2 capture. It is a significant departure from traditional combustion methods. It is flameless combustion technology combining 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 and heat. This metal-oxide becomes an oxygen carrier and is transferred to the fuel reactor where it reacts with a fossil fuel. The reaction consumes some heat while producing CO2, water and regenerating the metal to a pure state. The metal is then recycled to the air reactor and the cycle repeated.
The beauty of this process is that it has many of the 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 a challenge is to produce cheap and pure oxygen. Similar to oxyfuel chemical looping produces a relatively pure stream of CO2 and water vapour which can be readily separated by a traditional condensing process.
A major challenge is the development of robust and efficient oxygen carrier particles. The reactants must be highly reactive, thermodynamically stable while resistant to deactivation and degradation. Metal-based materials for reacting with oxygen and the fuel are currently under development. At present the development of chemical looping technology is in the early stages and significant development is needed to commercialise the technology.
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).
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.
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, called solvent, will react with the CO2, while in adsorption the CO2 will be attached to the surface of a solid component, called sorbent.
Pressure swing adsorbtion is a process that utilises the different solubility’s of gaseous components in a solid. At high pressure, CO2 is adsorbed onto a porous materials, when the pressure is decreased the gas is desorbed from the porous sorbent producing a pure CO2 stream. The sorbent can be reused for subsequent adsorption as the chambers continue to oscillate between high and low pressure selectively to adsorb and desorb CO2.
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. The development of regenerable sorbents that have high selectivity for CO2 is critical for the adoption of the pressure swing adsorption process.
CO2 capture by adsorption is not yet a mature commercial technology, but is currently being tested at a laboratory scale. Research programs are underway with advances in the technological expected, paving the way for adsorption as a future solution for CO2 capture.