*2.1. Precombustion Carbon Capture*

This method implies an alternative of combusting fuel directly in a combustor. At first, fuel is converted to a combustible gas. This gas is used for power generation [8]. CO2 is separated and sequestered from this gas generated from fossil fuel before combustion [9]. A schematic diagram of the process is illustrated in Figure 1.

**Figure 1.** Schematic diagram of the pre combustion carbon capture process [10].

At first, synthesis gas (syngas), which is a mixture of mainly H2 and CO with a trace of CO2, is produced from a fossil fuel. It can be done by adding steam to the fossil fuel. This process is known as steam reforming [11]. Another way to produce syngas is by supplying pure oxygen after separating it from air to fossil fuel which is being used to produce power. This process is known as partial

oxidation when it is applied to liquid or gaseous fuels. When it is applied for solid fuels, it is known as gasification. The reactions for this process are given below [11]:

Steam reforming: *CxHy* + *x*H2O → *x*CO + *x* + *y*2 H2 Partial oxidation: *CxHy* + *x*2O2 → *x*CO + - *y*2H2 .

The syngas produced in this way is then converted to CO2 from CO by water-gas shift reaction.

.

Water gas shift reaction: CO + H2O - CO2 + H2 .

The products of the water shift gas reaction remain under high pressure which facilitates the removal of CO2. It is removed at ambient temperature. The remaining gas is mainly hydrogen with little impurities. This gas is used to generate power in a combined cycle power plant. High pressure (typically 2–7 MPa) and a high concentration of CO2 (15–60% by volume) before the separation of CO2/H2 demand less energy for CO2 separation and compression than post combustion carbon capture [6]. However, the energy requirement becomes high in this process due to the air separation and reforming or gasification processes. One way to reduce this energy penalty is to use Sorption Enhanced Water Gas Shift (SEWGS) technology. Water gas shift reaction and CO2 separation can be integrated through this technology [12]. SEWGS increases the conversion rate of CO by removing CO2 from the product of the water gas shift reaction. This results in an additional reduction of CO2 emission [13]. The process is almost the same for any fossil fuel, but if any fuel other than the natural gas is used, then more refining stages should be included since more contaminants are produced [11].

Currently, the main research focus of precombustion carbon capture is to use this method in Integrated Gasification Combined Cycle (IGCC) power plants. A layout of the IGCC is shown in Figure 2.

**Figure 2.** A schematic layout of an IGCC power plant using pre combustion carbon capture [14].

Here, oxygen is separated from air in a cryogenic air separation plant [14]. This oxygen is passed to a gasifier where coal is gasified at high pressure to produce syngas at high temperature. After cooling and preliminary cleaning, syngas is shifted through a water gas shift reaction in a water gas reactor and converted to H2S, H2, and CO2. After several cleaning steps to remove sulphur, mercury, water, and other impurities, the syngas only consists of CO2 and H2. This gas mixture is passed through the CO2 removal process where CO2 is captured. Hydrogen is then used to produce power. Most commercially developed technologies employ physical solvents to separate CO2 from syngas. A lot of work has been carried out for best performance of CO2 separation from the syngas. Some of the important works using different separation technologies are summarized in Table 1.


### **Table 1.** Summary of some important studies on precombustion carbon capture.

### *2.2. Post Combustion Carbon Capture*

This technique is used in the existing power plants without a major modification of the plant. For this reason, it has the advantage of easier retrofitting compared to the other CCS processes [28–30]. It is the simplest technique to capture CO2. In this method, CO2 is removed from the exhaust flue gases of the power plants. Normally the flue gases exit at atmospheric pressure. The concentration of CO2 in these gases is very low. A typical concentration of CO2 in the flue gas is shown in Table 2.


**Table 2.** Amount of CO2 in flue gases of power plants [31].

Due to the low concentration of CO2, the driving force is too low to capture it from the flue gas [31]. Large sized equipment and high capital cost are required to handle a huge volume of flue gases. Therefore, a cost-effective way to capture CO2 from the flue gas needs to be identified. Also, the flue gas contains various types of contaminants such as SOx, NOx, fly ash, etc. They cause the separation process to become more costly with existing technologies [32].

Separation process for CO2 from flue gas is challenging for some reasons. Equipment design is required to withstand the high temperature of the flue gas. The gas must be cleaned up before separating CO2. Merkel et al. [32] has proposed a flow process to clean up the gas as shown in Figure 3. The hot exhaust gas leaving the boiler is passed through an electrostatic precipitator (ESP) that removes all the large particulates. After that, the sulphur products are removed through a flue gas desulphurization unit (FGD). Post combustion carbon capture technology is designed to treat the outlet gas of FGD. In this state, the gas mixture contains around 10–14% CO2 mainly in a mixture of nitrogen. A schematic of a power plant using coal as fuel with solvent-based absorption post combustion carbon capture is shown in Figure 4.

**Figure 3.** Schematic diagram of a simplified flue gas cleanup process for post combustion carbon capture [32].

Here, coal is pulverized and combusted with air to generate heat. This heat is used to produce steam which in turn produces power through three different steam turbines of various pressures. Low-quality exhaust steam is condensed in a condenser and sent back to the boiler. The exhaust flue gas from the boiler is passed through the cleaning process to remove sulphur, ash, NOx and other impurities. After the final stage of cleaning, the gas is sent to the CO2 capture process.

The complexity is much reduced when natural gas is used as fuel. A typical layout of a post combustion carbon capture combined cycle power plant using natural gas as fuel is shown in Figure 5. Natural gas is combusted with compressed air and the product is expanded through a gas turbine to produce power. The exhaust of the gas turbine remains at high temperature. This high-temperature flue gas is used to make steam. It produces additional power through a steam turbine. The cooled flue gas is then passed to the CO2 capture process. Figure 5 shows a solvent-based CO2 capture system using MEA. MEA scrubs CO2 from the flue gas in the absorber column leaving clean gas to the exhaust. Later, the MEA is purified in the stripper column to use again in the absorber column. CO2 is captured

from the stripper column and compressed for storage. Using MEA is the most common method to separate CO2 from flue gas. Other technologies are also used to separate CO2 from the gas mixture.

**Figure 4.** Layout of a post combustion carbon capture coal-fired power plant [33].

**Figure 5.** Layout of a post combustion carbon capture power plant operating with natural gas as the fuel [33].

Some of the important work on post combustion carbon capture using different separation technology is summarized in Table 3.


### **Table 3.** Some of the important studies on post combustion carbon capture.

A careful literature survey on post combustion carbon capture reveals the research is being directed lately to membrane absorption separation technology. This method combines the advantages of both absorption and membrane separation in a single technology [47].
