Calcium looping

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. In the calcium looping process, the two species are calcium carbonate (CaCO₃) and calcium oxide (CaO). The carbon dioxide can then be transported to a storage site, used in Enhanced Oil Recovery (EOR) or used as a chemical feedstock. Calcium oxide is often referred to as the sorbent.

CaL 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.

Basic concept

$CaCO_{3}\leftrightharpoons CaO+CO_{2}\qquad \qquad \Delta H=+178kJ/mol$

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 a carbonator. It is cooled to approximately 650 °C and is brought into contact with a flue gas containing aa 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 amount 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.

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 lots of electricity; other air separation processes are being developed.

Indirect provision of heat to the calciner involves either:

• Combustion of fuel outside the vessel and conduction of energy in to the vessel
• Combustion of fuel in another vessel 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.

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. [REF] The problem seems to be greater in pilot plant tests than at a bench scale.

Sulphation

Sulphation 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 sulphation: $CaO+SO_{2}+1/2O_{2}\Rightarrow CaSO_{4}$

Direct sulphation: $CaCO_{2}+SO_{2}+1/2O_{2}\Rightarrow CaSO_{4}+CO_{2}$

Because calcium sulphate has a greater molar volume than either CaO or CaCO₃ a sulphated 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 sulphate dissociates to CaO and SO₂ is relatively high, precluding sulphation's reversibility at the conditions present in CaL.

Disposal of waste sorbent

Properties of waste sorbent

After cycling several times and being removed from the calcium loop, the waste sorbent will have attrited, sulphated and become mixed with the ash from any fuel used. The amount of ash in the waste sorbent will depend on the fraction of the sorbent being removed and the ash and calorific content of the fuel. The size fraction of the sorbent is dependent on the original size fraction but also the number of cycles used and the type of limestone used.

Disposal routes

Proposed disposal routes of waste sorbent include:

• Landfill;
• Disposal at sea;
• Use in cement manufacture;
• Use in flue gas desulphurisation (FGD).

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%.