2.3.5. Calcium Looping Technology

The calcium looping carbon capture system utilizes a different technique to capture CO2 from a gas stream. In this method, a direct reaction takes place between CO2 and CaO. This reaction produces solid calcium carbonate which is easily separable from the other gases. The main reversible reaction for this process is as follows [80].

$$\text{CaO} + \text{CO}\_2 \leftrightharpoons \text{CaCO}\_3\tag{1}$$

The forward reaction, known as carbonation reaction, is exothermic. The reverse reaction is called calcination reaction, which is endothermic. The initial rate of a carbonation reaction is very fast but it comes to an abrupt slow rate after some time [81]. Due to the endothermic reaction at the calcination reactor, it needs a large amount of heat to be supplied at a high temperature. Often this

heat is supplied by oxy combustion of coal or natural gas inside the calcination reactor [33]. After the recovery of CO2 from the calcination reactor, it is compressed and stored. This process can be used for both precombustion and post combustion carbon capture. The following reaction is the desired key reaction in the gasifier of precombustion carbon capture [80]:

$$\rm CaCO + H\_2O + CaCO \leftrightharpoons CaCO\_3 + H\_2 \tag{2}$$

Precombustion carbon capture has some advantages using calcium looping process. CaCO3 and CaO increase the destruction rate of tar which is complex when hydrogen is used as fuel. The removal of CO2 from the gas mixture also increases the rate of conversion from CH4 and CO to H2 [80].

The main use of this process is in post combustion carbon capture [82]. A schematic of the process is shown in Figure 10. Here, the limestone captures CO2 from the exhaust flue gases of a power plant with the help of a circulating fluidized bed carbonator. The sorbent is then passed to a calciner which operates at a higher temperature. After regeneration, the sorbent is again passed to the carbonator. Coal or natural gas is burnt in an oxy fuel environment of the calciner to produce necessary heat.

**Figure 10.** A schematic diagram of post combustion carbon capture using Calcium looping [82].

The overall reaction to form solid carbonate is exothermic. The high-grade heat produced at the carbonator can be supplied for a steam cycle in order to produce more power. This lowers the energy penalty from conventional post combustion capture [83]. The limestone is available in huge quantities and it is a non-hazardous substance. The price of the limestone is also much lower than the amines used for scrubbing in post combustion carbon capture. Used or spent sorbents can be further utilized for other purposes.

The sorbent is recycled and used repeatedly for CO2 capture. The reversibility of the main reaction decreases with the increase of the number of cycles [84]. Therefore, the sorbent loses its carrying capacity with repeated use in the cycle. After the first cycle, the capacity of the sorbent is reduced by 1535% depending on favorable and unfavorable conditions. This loss of capacity decreases in each cycle [81]. A huge amount of makeup sorbent is required for this process.

### **3. Carbon Capture by Water Condensation**

This method is comparatively novel in power generation. Here, instead of supplying air to the combustion chamber, pure oxygen is supplied for combustion. As a result, the combustion products consist of mainly CO2 and steam. The CO2 content of the mixture is captured by condensing steam. Thus, there is no need to apply any of the CO2 separation technologies described in the previous

section. Therefore, this method is economically more viable. If the oxygen is produced using an Air Separation Unit (ASU), the process is known as oxy-combustion carbon capture. Another way of supplying oxygen is to use a metal oxide with the help of a chemical looping known as chemical looping combustion [85,86].
