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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Calcium looping | 2/6 | https://en.wikipedia.org/wiki/Calcium_looping | reference | science, encyclopedia | 2026-05-05T10:46:20.922266+00:00 | kb-cron |
Combustion of fuel in an external chamber and conduction of energy in to the vessel Combustion of fuel in an external chamber and use of a heat transfer medium. Indirect methods are generally less efficient but do not require the provision of oxygen for combustion within the calciner to prevent dilution. The flue gas from the combustion of fuel in the indirect method could be mixed with the flue gas from the process that the CaL plant is attached to and passed through the carbonator to capture the CO2. One efficient way of transferring heat into the calciner is by means of heat pipes. The indirectly heated calcium looping (IHCaL) using heat pipes has high potential to decarbonize the lime and cement industry. The deployment of this technology with refuse-derived fuels would allow to achieve net negative CO2 emissions.
== Recovery of energy from the carbonator == Although the heat from the carbonator is not at a high enough temperature to be used in the calciner, the high temperatures involved (>600 °C) mean that a relatively efficient Rankine cycle for generating electricity can be operated. Note that the waste heat from the market-leading amine scrubbing CO2 capture process is emitted at a maximum of 150 °C. The low temperature of this heat means that it contains much less exergy and can generate much less electricity through a Rankine or organic Rankine cycle. This electricity generation is one of the main benefits of CaL over lower-temperature post-combustion capture processes as the electricity is an extra revenue stream (or reduces costs).
== Sorbent degradation == It has been shown that the activity of the sorbent reduces quite markedly in laboratory, bench-scale and pilot plant tests. This degradation has been attributed to three main mechanisms, as shown below.
=== Attrition === Calcium oxide is friable, that is, quite brittle. In fluidised beds, the calcium oxide particles can break apart upon collision with the other particles in the fluidised bed or the vessel containing it. The problem seems to be greater in pilot plant tests than at a bench scale.
=== Sulfation === Sulfation is a relatively slow reaction (several hours) compared with carbonation (<10 minutes); thus it is more likely that SO2 will come into contact with CaCO3 than CaO. However, both reactions are possible, and are shown below.
Indirect sulfation: CaO + SO2 + 1/2 O2 → CaSO4 Direct sulfation: CaCO3 + SO2 + 1/2 O2 → CaSO4 + CO2 Because calcium sulfate has a greater molar volume than either CaO or CaCO3 a sulfated layer will form on the outside of the particle, which can prevent the uptake of CO2 by the CaO further inside the particle. Furthermore, the temperature at which calcium sulfate dissociates to CaO and SO2 is relatively high, precluding sulfation's reversibility at the conditions present in CaL.
== Technical implications == Calcium looping technology offers several technical advantages over amine scrubbing for carbon capture. Firstly, both carbonator and calciner can use fluidized bed technology, due to the good gas-solid contacting and uniform bed temperature. Fluidized bed technology has already been demonstrated at large scale: large (460MWe) atmospheric and pressurized systems exist, and there is not a need for intensive scaling up as there is for the solvent scrubbing towers used in amine scrubbing. Also, the calcium looping process is energy efficient. The heat required for the endothermic calcination of CaCO3 and the heat required to raise the temperature of fresh limestone from ambient temperature, can be provided by in-situ oxy-fired combustion of fuel in the calciner. Although additional energy is required to separate O2 from N2, the majority of the energy input can be recovered because the carbonator reaction is exothermic and CO2 from the calciner can be used to power a steam cycle. A solid purge heat exchanger can also be utilized to recover energy from the deactivated CaO and coal ashes from the calciner. As a result, a relatively small efficiency penalty is imposed on the power process, where the efficiency penalty refers to the power losses for CO2 compression, air separation and steam generation. It is estimated at 6-8 % points, compared to 9.5-12.5 % from post combustion amine capture. The main shortcoming of Ca-looping technology is the decreased reactivity of CaO through multiple calcination-carbonation cycles. This can be attributed to sintering and the permanent closure of small pores during carbonation.
=== Closure of small pores === The carbonation step is characterized by a fast initial reaction rate abruptly followed by a slow reaction rate (Figure 2). The carrying capacity of the sorbent is defined as the number of moles of CO2 reacted in the period of fast reaction rate with respect to that of the reaction stoichiometry for complete conversion of CaO to CaCO3. As seen in Figure 2, while mass after calcination remains constant, the mass change upon carbonation- the carrying capacity- reduces with a large number of cycles. In calcination, porous CaO (molar volume = 16.9 cm3/g) is formed in place of CaCO3 (36.9 cm3/g.). On the other hand, in carbonation, the CaCO3 formed on the surface of a CaO particle occupies a larger molar volume. As a result, once a layer of carbonate has formed on the surface (including on the large internal surface of porous CaO), it impedes further CO2 capture. This product layer grows over the pores and seals them off, forcing carbonation to follow a slower, diffusion dependent mechanism.