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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.
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.
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.Blamey, J.; Anthony, E.J.; Wang, J.; Fennell, P.S. 2010, The Calcium Looping Cycle for large-scale capture. Progress in Energy and Combustion Science vol. 36 (2) p. 260-279 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.
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.
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:
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.
Note that the waste heat from the market-leading Amine scrubbing 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).
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.
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.Romeo LM, Abanades JC, Escosa JM, Pano J, Giminez A, Sanchez-Biezma A, et al. Oxyfuel carbonation/calcination cycle for low cost CO2 capture in existing power plants. Energy Conversion and Management 2008;49(10):2809–14 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.Blamey J., Anthony EJ, Wang J, Fennell PS, The calcium looping cycle for large-scale capture, Progress in Energy and Combustion Science 2010
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.
Solutions: Several options to reduce sorbent deactivation are currently being researched. An ideal sorbent would be mechanically strong, maintain its reactive surface through repeated cycles, and be reasonably inexpensive. Using thermally pre-activated particles or reactivating spent sorbents through hydration are two promising options. Thermally pre-activated particles have been found to retain activity for up to a thousand cycles. Similarly, particles reactivated by hydration show improved long term (after~20 cycles) conversions.
The lifecycle CO2 emissions for power generation with CaL and the first three disposal techniques have been calculated. Before disposal of the CaO coal power with CaL has a similar level of lifecycle emissions as amine scrubbing but with the CO2-absorbing properties of CaO CaL becomes significantly less polluting. Ocean disposal was found to be the best, but current laws relating to dumping waste at sea prevent this. Next best was use in cement manufacture, reducing emissions over an unabated coal plant by 93%.
This calcium oxide could be sourced from other point sources of CO2 such as power stations, but most effort has been focussed on integrating calcium looping with Portland cement manufacture. By replacing the calciner in the cement plant with a calcium looping plant, it should be possible to capture 90% or more of the CO2 relatively inexpensively. There are alternative set-ups such as placing the calcium looping plant in the preheater section so as to make the plant as efficient as possible or to indirectly heat the calciner for increased energy efficiency.
Some work has been undertaken into whether calcium looping affects the quality of the Portland cement produced, but results so far seem to suggest that the production of strength-giving phases such as alite are similar for calcium looped and non-calcium looped cement.
This technology has been successfully piloted in Europe by a cooperative industry group with support from the European Union as the Low Emission Intensity Lime And Cement (LEILAC1) reactor project. The study report concluded that the technology could capture C02 from full scale lime and cement kilns at €14 to €24/t. Transport and storage costs are not included in this estimate and will be dependent upon infrastructure available near the cement or lime plant
A FEED study is underway for a larger commercial demonstration kiln proposed for the Heidelberg Cement plant in Hannover (LEILAC2). This commercial demonstration kiln is designed to capture 100ktpa . Leilac-2 passed its Financial Investment Decision (FID) in March 2022, and its Front End Engineering Design (FEED) Study Summary was completed and published on 13 October 2023, leading to a new and improved design and revised timeline. The next milestone is procurement of long lead items, currently underway (2023).
This type of kiln is also being studied as a potential method to decarbonise shipping through both looping and single use processes. The single use process would involve using CaCO3 to be sown over the ocean, thereby permanently capturing addition carbon from the ocean as the CaCO3 reacts to form Ca(HCO3)2 and reversing ocean acidification.
| + Comparison of Ca-looping and Amine Scrubbing | !Amine Scrubbing !Ca-Looping |
Sensitivity Analysis: Figure 3 shows how varying 8 separate parameters affects the cost/metric ton of CO2 captured through Ca-looping. It is evident that the dominant variables that affect cost are related to sorbent use, the Ca/C ratio and the CaO deactivation ratio. This is because the large sorbent quantities required dominate the economics of the capture process. Low costs of CO2 avoided for the indirectly heated Ca-looping process have been reported for integrated concepts in the lime production.
These variables should therefore be taken into account to achieve further cost reductions in the Ca-looping process. The cost of limestone is largely driven by market forces, and is outside the control of the plant. Currently, carbonators require a Ca/C ratio of 4 for effective CO2 capture. However, if the Ca/C ratio or CaO deactivation is reduced (i.e. the sorbent can be made to work more efficiently), the reduction in material consumption and waste can lower feedstock demand and operating costs.
Using purged material from a Ca-looping system would reduce the raw material costs for cement production. Waste CaO and ash can be used in place of CaCO3 (the main constituent cement feed). The ash could also fulfill the aluminosilicate requirements otherwise supplied by additives. Since over 60% of the energy used in cement production goes into heat input for the precalciner, this integration with Ca-looping and the consequent reduced need for a calcination step, could lead to substantial energy savings (EU, 2001). However, there are problems with using the waste CaO in cement manufacture. If the technology is applied on a large scale, the purge rate of CaO should be optimized to minimize waste.
A further political advantage is the potential synergy between calcium looping and cement production. An IEA report concludes that to meet emission reduction goals, there should be 450 CCS projects in India and China by 2050. However, this could be politically difficult, especially with these nations' numerous other development goals. After all, for a politician to commit money to CCS might be less advantageous than to commit it to job schemes or agricultural subsidies. Here, the integration of calcium looping with the prosperous and (particularly with infrastructure expansion in the developing world) vital cement industry might prove compelling to the political establishment.
This potential synergy with the cement industry also provides environmental benefits by simultaneously reducing the waste output of the looping process and decarbonizing cement production. Cement manufacture is energy and resource intensive, consuming 1.5 tonnes of material per tonne of cement produced.Jankovic, Alex. Walter Valery, Eugene Davis, "Cement grinding optimisation, Minerals Engineering, Volume 17, Issues 11–12, November–December 2004, Pages 1075-1081 In the developing world, economic growth will drive infrastructure growth, increasing cement demand. Deploying a waste product for cement production could therefore have a large, positive environmental impact.
From the environmental and health standpoint, Ca-looping compares favorably with amine scrubbing. Amine scrubbing is known to generate air pollutants, including amines and ammonia, which can react to form carcinogenic nitrosamines.Lag M., Lindeman B., Instanes C., Brunborg B., & Schwarze P. Health effects of amines and derivatives associated with CO2 capture. Norwegian Institute of Public Health: 2011 Calcium looping, on the other hand, does not produce harmful pollutants. In addition, not only does it capture CO2, but it also removes the pollutant SO2 from the flue gas.Coppola A., Montagnaro F., Salatino P., Scala F. Fluidized bed calcium looping: The effect of on sorbent attrition and capture capacity, Chemical Engineering Journal, Volumes 207–208, 1 October 2012, 445-449 This is both an advantage and disadvantage, as the air quality improves, but the captured SO2 has a detrimental effect on the cement that is generated from the calcium looping wastes.
Calcium looping already has an energy advantage over amine scrubbing, but the main problem is that amine scrubbing is the more market-ready technology. However, the accompanying infrastructure for amine scrubbing include large solvent scrubbing towers, the likes of which have never been used on an industrial scale. The accompanying infrastructure for calcium looping capture technologies are circulating fluidized beds, which have already been implemented on an industrial scale. Although the individual technologies differ in terms of current technological viability, the fact that the infrastructure needed to properly implement an amine scrubbing system has yet to be developed keeps calcium looping competitive from a viability standpoint.
Therefore, amount of cement production from Ca-looping waste of all fossil fuel based electric power plants in US will be far greater than net consumption. To make Ca-looping more viable, waste must be minimized (i.e. sorbent degradation reduced) to ideally about 1/10th of current levels.
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