CO2 Capture technologies should be implemented within acceptable cost levels in terms of energy and value. Because the costs of CCS technologies are high, of which the energy required is a main cost driver, there is a need to further develop existing technologies and to create novel ones.
Post-combustion technology captures CO2 from the flue gases produced by combustion of fossil fuels and biomass. Post-combustion capture typically uses a solvent to capture the CO2 from the flue gases after the combustion process. The main line of research in this area is on the development of reactive solvent systems with a lower need for energy and improved environmental performance. Reactive solvents are the preferred choice due to the low partial pressure of CO2 (between 3-15 kPa) in the flue gases. Nevertheless, other techniques, such as membranes and adsorption systems are attractive alternatives that are under development and some have approached the marked.
Pre-combustion capture is applicable to syngas production with gasifiers and reformers. For the power sector, the Integrated Gasification Combined Cycle (IGCC) power plant is an attractive option for the application of CCS with pre-combustion technology. The synthesis gas (syngas) from the gasifier can be used for the production of different type of products, because of its use as an intermediate, mainly in the production of petrochemicals and hydro-treatment of heavy feed in refinery. Syngas can also be used to produce liquid synthetic fuels using Fisher-Tropsch process. Syngas is mainly composed of the colourless, odourless, highly flammable gases carbon monoxide and hydrogen.
Gasification is an advanced technical process that uses heat, pressure, the reactants air, oxygen and steam to convert any carbon based raw material into synthesis gas. Gasification differs from more traditional energy-generating schemes, because it is not a combustion process but rather a conversion process. Inside of the gasifier it is starved of oxygen to prevent or limit combustion and the result is a partial oxidation of the fuel which results in production of syngas.
To enable pre-combustion capture, the syngas is further processed in a Water Gas Shift (WGS) or so called CO-shift reactor, which converts CO into CO2 while producing additional H2, thus increasing the CO2 and H2 concentrations. Separation of the produced CO2 from the shifted syngas renders a fuel stream very low in carbon also called “Hydrogen rich syngas”.
In practice, there are two distinct ways to carry out CO-shifting:
An acid gas removal system, such as Selexol™, could be used to separate the CO2 from the H2. After CO2 removal, the H2 rich syngas is used as a fuel in a gas turbine combined cycle power plant to generate electricity.
The current commercial available pre-combustion CO2 capture technologies that could be applied to IGCC systems—the glycol-based Selexol™ process and the methanol-based Rectisol® process— employ physical solvents that preferentially absorb CO2 from the syngas mixture. There are several Selexol™ and Rectisol® systems in use at commercial scale, although not fully integrated at IGCC power plants.
Membrane- and sorbent-based (Reactor) technologies are promising technologies for pre-combustion capture at various level of development:
In oxy-fuel combustion, almost pure oxygen mixed with recycled flue gas is used for the combustion process instead of air. This will result in a flue gas that is mainly CO2 saturated with water vapour. The flue gas is composed with approximately 83% CO2, 15% H2O, which makes it possible to purify and store CO2 with less downstream large treatment equipment. However, the combustion temperature with pure oxygen is excessively high. Therefore, dilution or moderation is required, via recycling the CO2 rich flue gas, to make the combustion temperature similar to that of a normal air-blown combustor. Oxygen is produced by a cryogenic Air Separation Unit so called “cold box”. Novel techniques with lower energy consumption and cost are currently the key focus topics. Oxygen transport membrane is one such technology for separation of O2 from air.
An alternative type of oxy-fuel combustion is the chemical looping combustion (CLC) process. CLC is an advanced coal or gas oxy-combustion technology that involves the use of a metal oxide or other compound as an oxygen carrier to transfer oxygen from the combustion air to the fuel, avoiding direct contact between fuel and combustion air.