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Description

The biggest thing we have to contribute to the current Wikipedia entry on Calcium Looping is specificity to carbon capture and storage. While the entry does contain a section titled “Benefits of CaL compared with other post-combustion capture processes”, it is empty. We have several resources specifically describing the efficacy of Calcium Looping in relation to capture methods like solid sorbent capture and membrane scrubbers present in the bibliography. The entry offers a very basic technical explanation of the process which we will make more specific and comprehensive. While it does explain the basic technology, it fails to cover the political implications; something that is necessary to understand in order to evaluate the true viability of the process. It is politically divisive, seen as financially extravagant and an unnecessary expense. While the entry describes in brief the process of calcium looping, it does not describe how the process would be implemented; nor does it describe in depth the drawbacks to calcium looping as a process. While calcium looping is a viable carbon capture strategy, some resources we have compiled raise caveats to its efficacy, notably the high amounts of waste solvent that make the process expensive to run for extended periods of time. Providing the conflicting viewpoints presented in these sources would make the entry more comprehensive and less opinionated. In order to surmount these obstacles, technological advances in decreasing the waste product would have to be made; advances which are described in portions of our bibliography, but not at all in the entry. Finally, the entry is poorly referenced. Similar carbon capture articles contain an excess of 100 references, whereas the calcium looping entry only contains 16. Just in preliminary research we have amassed a bibliography of 21 sources, dwarfing the number of the current entry. Overall, there is a great deal we can add to the current Wikipedia entry on Calcium looping, and we are looking forward to getting started.

Background of Ca-Looping

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 2 vessels.

ΔH= +178kJ/mol --- (1)

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.

File:Figure 1 wiki.png

Figure 1: A potential post combustion process using the calcium looping cycle (Blamey et al., 2010)

Flue gas containing CO₂ is fed to the first vessel (the carbonator), where carbonation occurs. 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 ^oC 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 ^oC 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.

Technical Implications

Calcium looping technology offers several technical advantages over amine scrubbing. 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.

Figure 2 Repeated calcination/carbonation cycles of limestone in a TGA (Blamey et al, 2010)

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, whilst 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 =16.9cm3/g) is formed in place of CaCO₃ (36.9cm3/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.

Sintering

CaO is also prone to sintering, or change in pore shape, shrinkage and grain growth during heating. Ionic compounds such as CaO mostly sinter because of volume diffusion or lattice diffusion mechanics. As described by sintering theory , vacancies generated by temperature sensitive defects direct void sites from smaller to larger ones, explaining the observed growth of large pores and the shrinkage of small pores in cycled limestone.  It was found that sintering of CaO increases at higher temperatures and longer calcination durations, whereas carbonation time has minimal effect on particle sintering. A sharp increase in sintering of particles is observed at temperatures above 1173 K , causing a reduction in reactive surface area and a corresponding decrease in reactivity.

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.

Benefits of CaL Compared with Other Post-Combustion Capture Processes

Calcium Looping compares favorably with several post-combustion capture technologies. Amine scrubbing is the capture technology closest to being market-ready, and CaL has several marked benefits over it. When modeled on a 580 MW coal-fired power plant, Calcium looping experienced not only a smaller efficiency penalty (6.7-7.9% points compared to 9.5% for Monoethanolamine and 9% for chilled ammonia) but also a less complex retrofitting process. Both technologies would require the plant to be retrofitted for adoption, but the CaL retrofitting process would result in twice the net power output of the scrubbing technology . Furthermore, this advantage can be compounded by introducing technology such as cryogenic O2 storage systems. This ups the efficiency of the CaL technology by increasing the energy density by 57.4%, making the already low energy penalties even less of an issue .

CaL 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.

Economic Implications

Calcium looping has several economic advantages.

Cost/metric ton for CO₂ captured

Firstly, Ca-looping offers greater cost advantage compared to conventional amine-scrubbing technologies. The cost/metric ton for CO₂ captured through Ca-looping is ~$23.70 whereas that for CO₂ captured through amine scrubbing is about $35-$96 .  This can be attributed to the high availability and low cost of the CaO sorbent (derived from limestone) as compared to MEA. Also, as discussed in section 2, Ca-looping imposes a lower energy penalty than amine scrubbing, resulting in lower energy costs. The amine scrubbing process is energy intensive, with approximately 67% of the operating costs going into steam requirements for solvent regeneration. A more detailed comparison of Ca-looping and amine scrubbing is shown below.

Table I: Comparison of Ca-looping and Amine Scrubbing

	Amine Scrubbing	Ca-Looping
Cost/CO2 avoided	~ $35-96/ton	~$23.70/ ton
Raw material cost (Rao and Rubin,2002)	$1,250/ton MEA	$25/ton CaCO3
Efficiency Penalty	6-12%	6-8%

Cost of CO₂ emissions avoided through Ca-looping

In addition, the cost of CO₂ emissions avoided through Ca-looping is lower than the cost of emissions avoided via an oxyfuel combustion process (~$23.8USD/t). This can be explained by the fact that, despite the capital costs incurred in constructing the carbonator for Ca-looping, CO₂ will not only be captured from the oxy-fired combustion, but also from the main combustor (before the carbonator). The oxygen required in the calciners is only 1/3 that required for an oxyfuel process, lowering air separation unit capital costs and operating costs.

File:Figure 3 wiki.png

Figure 3: Project Sensitivity Analysis (Mackenzie, 2007)

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.

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.

Cement Production

Finally, favorable economics can be achieved by using the purged material from the Ca-looping cycle in cement production. The raw feed for cement production includes ~ 85wt% limestone with the remaining material consisting of clay and additives (e.g. SiO2, Al2O3 etc.) . The first step in the process involves calcinating limestone to produce CaO, which is then mixed with other materials in a kiln to produce clinker.

File:Figure 4 Wiki.png

Figure 4: Process flow diagram of a cement production (Alsop et al., 2007)

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. For example, adding Ca-looping to all coal-fired plants in the UK would generate enough waste for 33Mtonnes/yr of cement production, whereas current cement production in the UK is only 12.5 Mtonnes/yr. Hence, if the technology is applied on a large scale, the purge rate of CaO should be optimized to minimize waste.

Sample Evaluation

Assumptions:

1.)  For a Ca-looping cycle installed on a 500 MW power plant, the purge rate is 12.6 kg CaO/s.  (Blamey, et al. 2010)

2.)  For the cement production process, 0.65 kg CaO is required/ kg cement produced (Blamey, et al.)

3.)  U.S. electric generation capacity (only fossil fuels): Natural gas = 415 GW, Coal= 318 GW & Petroleum = 51 GW  (EIA, 2010)

4.)  Cement consumption in U.S. = 110,470 x 103 metric tons = 1.10470 x 108 metric tons = 1.10470 x 1011 kg (USGS, 2008)

Calculations:

For a single Ca-looping cycle installed on a 500MW power plant:

Amount of CaO from purge annually = 12.6 kg CaO/s x 365 days/year x 24 hr/day x 3600s / hour = 3.97 x 108 kg CaO/ year

Cement that can be obtained from purge annually= 3.97 x 108 kg CaO/ year x 1 kg cement/ 0.65 kg CaO = 6.11 x 108 kg cement/ year

Net electricity generation in US:  (415 + 318 + 51) GW = 784 GW = 7.84 x 1011 W

Number of 500 MW power plants: 7.84 x 1011 W / 5.00 x 108 W = 1568 power plants

Amount of cement that can be produced from Ca-looping waste: 1568 x 6.11 x 108 kg cement/ year = 9.58 x 1011 kg cement/ year

Production from Ca-looping waste as percent of total annual cement consumption = [(9.58 x 1011 kg)/ (1.10470 x 1011 kg)] x 100 = 870 %

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.

Political/ Environmental Implications

To fully gauge the viability of calcium looping as a capture process, it is necessary to consider the political, environmental, and health effects of the process as well.

Political Implications

Though many recent scientific reports (e.g.: the seven-wedge stabilization plan by Pacala and Socolow) convey an urgent need to deploy CCS, this urgency has not spread to the political establishment , mainly due to the high costs and energy penalty of CCS The economics of calcium looping are integral to its political viability.  One economic and political advantage is the ability for Ca-looping to be retrofitted onto existing power plants, rather than requiring new plants to be built. The IEA sees power plants as an important target for carbon capture, and has set the goal to have all fossil fuel based power plants deploy CCS systems by 2040. However, power plants are expensive to build, and long lived. Retrofitting of post-combustion capture systems, such as Ca-looping, seems to be the only politically and economically viable way to achieve the IEA’s goal.

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 . 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.

Environmental Implications

The starting material for calcium looping is limestone, which is environmentally benign and widely available, accounting for over 10 % (by volume) of all sedimentary rock. Limestone is already mined and cheaply obtainable. The mining process has no major known adverse environmental effects, beyond the unavoidable intrusiveness of any mining operation. However, as the following calculation shows, despite integration with cement industry, waste from Ca-looping can still be a problem.

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 . 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 .  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.

Advantages and drawbacks

Advantages of the Process

Calcium looping is considered as potential promising solutions to reduce C02 capture energy penalty . There are many advantages from the calcium looping methods. Firstly, the method has been proved to yield a low efficiency penalties (5-8% points) while other mature C02  capture systems yield a higher efficiency penalties (8-12.5%). Moreover, the method is well suited for a wide range of flue gases. Calcium looping is applicable for new builds and retrofits to existing power stations or other stationary industrial C02 sources because the method can be implemented using large-scale circulating fluidized beds while other methods such as amine scrubbing is required a vastly upscale solvent scrubbing towers.  In addition, crushed limestone used in calcium looping as the sorbent is a natural product, which well distributed all over the world, non-hazardous and really cheap. Many cement manufacturers or any power plants, which located close to limestone sources would consider integrating Calcium looping for C02 capture. The waste sorbent can be used in cement manufacture.

Drawbacks

Apart from these advantages, there are several disadvantages needed to take into considerations.  The plant integrating Ca-Looping might require a high construction investment because of the high thermal power of the post-combustion calcium loop. The sorbent capacity decreases significantly with the number of cycles for every carbonation-calcination cycle so the calcium-looping unit will require a constant flow of limestone. In order to increase the long-term reactivity of the sorbent or to reactivate the sorbent, there are some methods are investigated such as thermal pretreatment, chemical doping and the production of artificial sorbents . The method applying the concept of fluidized bed reactor, but there are some problems causing the uncertainty for the process. Attrition of the limestone can be a problem during repeated cycling.

Articles

Calcium looping

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Carbon capture and storage

References

Category:Carbon dioxide Category:Chemical engineering Category:Chemical processes