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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Calcium looping | 1/6 | https://en.wikipedia.org/wiki/Calcium_looping | reference | science, encyclopedia | 2026-05-05T10:46:20.922266+00:00 | kb-cron |
Calcium looping (CaL), or the regenerative calcium cycle (RCC), is a second-generation carbon capture technology. It is the most developed form of carbonate looping, where a metal (M) is reversibly reacted between its carbonate form (MCO3) and its oxide form (MO) to separate carbon dioxide from other gases coming from either power generation or an industrial plant. For this reason, calcium looping is also known as carbonate looping. In the calcium looping process, the two species are calcium carbonate (CaCO3) and calcium oxide (CaO). The captured carbon dioxide can then be transported to a storage site, used in enhanced oil recovery or used as a chemical feedstock. Calcium oxide is often referred to as the sorbent. Calcium looping is being developed as it is a more efficient, less toxic alternative to current post-combustion capture processes such as amine scrubbing. It also has interesting potential for integration with the cement industry, as well as waste incineration and waste-to-energy plants.
== Basic concept ==
There are two main steps in CaL:
Calcination: Solid calcium carbonate is fed into a calciner, where it is heated to 850-950 °C to cause it to thermally decompose into gaseous carbon dioxide and solid calcium oxide (CaO). The almost-pure stream of CO2 is then removed and purified so that it is suitable for storage or use. This is the 'forward' reaction in the equation above. Carbonation: The solid CaO is removed from the calciner and fed into the carbonator. It is cooled to approximately 650 °C and is brought into contact with a flue gas containing a low to medium concentration of CO2. The CaO and CO2 react to form CaCO3, thus reducing the CO2 concentration in the flue gas to a level suitable for emission to the atmosphere. This is the 'backward' reaction in the equation above. Note that carbonation is calcination in reverse. Whilst the process can be theoretically performed an infinite number of times, the calcium oxide sorbent degrades as it is cycled. For this reason, it is necessary to remove (purge) some of the sorbent from the system and replace it with fresh sorbent (often in the carbonate form). The size of the purge stream compared with the amount of sorbent going round the cycle affects the process considerably.
== Background == In the Ca-looping process, a CaO-based sorbent, typically derived from limestone, reacts via the reversible reaction described in Equation (1) and is repeatedly cycled between two vessels. The forward, endothermic step is called calcination while the backward, exothermic step is carbonation. A typical Ca-looping process for post-combustion CO2 capture is shown in Figure 1, followed by a more detailed description. Flue gas containing CO2 is fed to the first vessel (the carbonator), where carbonation occurs. The CaCO3 formed is passed to another vessel (the calciner). Calcination occurs at this stage, and the regenerated CaO is quickly passed back to the carbonator, leaving a pure CO2 stream behind. As this cycle continues, CaO sorbent is constantly replaced by fresh (reactive) sorbent. The highly concentrated CO2 from the calciner is suitable for sequestration, and the spent CaO has potential uses elsewhere, most notably in the cement industry. The heat necessary for calcination can be provided by oxy-combustion of coal below. Oxy-combustion of coal: Pure oxygen rather than air is used for combustion, eliminating the large amount of nitrogen in the flue-gas stream. After particulate matter is removed, flue gas consists only of water vapor and CO2, plus smaller amounts of other pollutants. After compression of the flue gas to remove water vapor and additional removal of air pollutants, a nearly pure CO2 stream suitable for storage is produced. The carbonator's operating temperature of 650-700 °C is chosen as a compromise between higher equilibrium (maximum) capture at lower temperatures due to the exothermic nature of the carbonation step, and a decreased reaction rate. Similarly, the temperature of >850 °C in the calcinator strikes a balance between increased rate of calcination at higher temperatures and reduced rate of degradation of CaO sorbent at lower temperatures.
== Process description == CaL is usually designed using a dual fluidised bed system to ensure sufficient contact between the gas streams and the sorbent. The calciner and carbonator are fluidised beds with associated process equipment for separating the gases and solids attached (such as cyclones). Calcination is an endothermic process and as such requires the application of heat to the calciner. The opposite reaction, carbonation, is exothermic and heat must be removed. Since the exothermic reaction happens at about 650 °C and the endothermic reaction at 850-950 °C, the heat from the carbonator cannot be directly used to heat the calciner. The fluidisation of the solid bed in the carbonator is achieved by passing the flue gas through the bed. In the calciner, some of the recovered CO2 is recycled through the system. Some oxygen may also be passed through the reactor if fuel is being burned in the calciner to provide energy.
=== Provision of energy to the calciner === Heat can be provided for the endothermic calcination step either directly or indirectly. Direct provision of heat involves the combustion of fuel in the calciner itself (fluidised bed combustion). This is generally assumed to be done under oxy-fuel conditions; i.e. oxygen rather than air is used to burn the fuel to prevent dilution of the CO2 with nitrogen. The provision of oxygen for the combustion uses much electricity and is associated with high investment costs. Other air separation processes are being developed. The penalties of calcium looping may be reduced by providing the heat for the calcination indirectly. This can be done in one of the following ways: