## 2 Technology Selection
### Amines-Based Absorption  
Amines are organic ammonia (NH\({}_{3}\)) derived compounds, whether it is classified as a primary, secondary, or tertiary amine depends on the number of substituents replacing the hydrogen atoms. The most common base solvent for carbon capture is monoethanolamine (MEA), which acts as a weak base to neutralize the acidic CO\({}_{2}\) molecule. The reaction forms a carbamate compound as described in Equation (1):  
\[2\text{R}-\text{NH}_{2}+\text{ CO}_{2}\rightarrow\text{R}-\text{NH}_{3}^{+}+ \text{R}-\text{NHCOO}^{-}+\text{heat} \tag{1}\]  
Two other reactions occur simultaneously with the carbamate formation: base-catalyzed CO\({}_{2}\) hydration and carbonic acid formation. However, the contribution of both reactions is minimal as compared to Equation (1) in the overall absorption rate. If heat is applied (typically \(\sim\)100-200 \({}^{\circ}\)C) the carbamate ion, having a nitrogen-carbon bond, dissociates easily, reversing the absorption process. Despite the fact that MEA reacts strongly with CO\({}_{2}\) there is a constraint for the loading that can be achieved, a maximum of 0.5 moles CO\({}_{2}\) can be loaded per mole MEA. When choosing a member of the amine family, three key requirements are considered: (i) fast reaction rate, (ii) low regeneration energy, and (iii) high loading capacity. As for the reaction rate, primary amines offer the best option, allowing for a smaller equipment size, thus reducing the capital cost of the plant. As for the regeneration energy and loading capacity, the tertiary amine offers the best option. Caution must be taken when SO\({}_{2}\) and NO\({}_{2}\) are present in flue gases as they react irreversibly with the amines forming heat-stable sulfates and nitrates. This leads to loss in the solvent absorption capacity that must be compensated with fresh make-up amine and waste disposal stage [9]. Thus, scrubbing of NO\({}_{x}\) and SO\({}_{x}\) is of great importance to prevent the formation of salts that are not reclaimable. NO\({}_{2}\), in particular, is the problematic species, and less than 20 ppmv is recommended in the flue gas stream. As for the SO\({}_{x}\) component, there has to be a tradeoff between the desulfurization cost and the cost of the required make-up solvent replacing the degraded amount. When using MEA as a solvent, a level of 10 ppmv is desired according to the Fluor Daniel Econamine(tm) process [11]. Another operational drawback in using amines is their corrosive nature. Either corrosion-resistant alloys have to be deployed in the plant or corrosion inhibitors are added, especially in stages undergoing elevated temperature [9], inhibitors also prevent MEA degradation [11]. Amine degradation is another concern when using amine solvents and can occur through either carbamate polymerization or oxidative degradation. Degradation through carbamate polymerization occurs in the presence of elevated temperatures and carbon dioxide (more prone at the rich end of the stripper containing high CO\({}_{2}\) loading and the reboiler sump). A temperature lower than 100 \({}^{\circ}\)C poses an insignificant risk in terms of carbamate polymerization. Using lower amine concentration would lower the degradation rate and reduce the risk of the carbamate polymerization mechanism. Oxidative degradation occurs due to the presence of oxygen in the flue gas stream. The reaction is independent of carbon dioxide concentration and temperature. Aldehydes and organic acids are some of the products of oxidative degradation, having a significant environmental impact if released into the atmosphere. Commercial vendors indicate that the solvent is suitable for capturing CO\({}_{2}\) with oxygen-containing flue gases [12]. Nevertheless, published data indicate the importance of taking precautions to prevent the entrainment of oxygen into the amine solvent. The first strategy is to prevent the O\({}_{2}\) from being absorbed during the capture process by using molecular sieves or fuel-rich combustions upstream of the absorber.