Calcium looping is being developed as it is a more efficient, less toxic alternative to current post-combustion capture processes such as amine scrubbing.
Whilst the process can be theoretically performed an infinite number of times, the calcium oxide sorbent degrades as it is cycled.
[7] 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.
Calcination occurs at this stage, and the regenerated CaO is quickly passed back to the carbonator, leaving a pure CO2 stream behind.
Oxy-combustion of coal: Pure oxygen rather than air is used for combustion, eliminating the large amount of nitrogen in the flue-gas stream.
CaL is usually designed using a dual fluidised bed system to ensure sufficient contact between the gas streams and the sorbent.
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.
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.
A solid purge heat exchanger can also be utilized to recover energy from the deactivated CaO and coal ashes from the calciner.
This product layer grows over the pores and seals them off, forcing carbonation to follow a slower, diffusion dependent mechanism.
[31] After cycling several times and being removed from the calcium loop, the waste sorbent will have attrited, sulfated and become mixed with the ash from any fuel used.
The reactor tubes are heated from the outside using electricity or fuel ensuring the CO2 stream is pure and not contaminated with air or combustion products.
[35] 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 amine scrubbing process is energy intensive, with approximately 67% of the operating costs going into steam requirements for solvent regeneration.
[23] Sensitivity Analysis: Figure 3 shows how varying 8 separate parameters affects the cost/metric ton of CO2 captured through Ca-looping.
Low costs of CO2 avoided for the indirectly heated Ca-looping process have been reported for integrated concepts in the lime production.
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.
Finally, favorable economics can be achieved by using the purged material from the calcium looping cycle in cement production.
The raw feed for cement production includes ~ 85 wt% limestone with the remaining material consisting of clay and additives (e.g. SiO2, Al2O3 etc.).
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,[44] mainly due to the high costs and energy penalty of CCS [45] The economics of calcium looping are integral to its political viability.
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.
An IEA report concludes that to meet emission reduction goals, there should be 450 CCS projects in India and China by 2050.
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.
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.
[49] 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.
[52] In addition, crushed limestone used in calcium looping as the sorbent is a natural product, which is well distributed all over the world, non-hazardous and inexpensive.
Many cement manufacturers or power plants located close to limestone sources could conceivably employ Calcium looping for CO2 capture.
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.
[26] The accompanying infrastructure for calcium looping capture technologies are circulating fluidized beds, which have already been implemented on an industrial scale.
To make Ca-looping more viable, waste must be minimized (i.e. sorbent degradation reduced) to ideally about 1/10th of current levels.