Post-combustion CO2 capture (PCC) is the methodology of capturing CO2 from the flue gas after the combustion of fossil fuels. The capturing of CO2 can be performed by using different mass transfer operations such as absorption and desorption, adsorption and membrane separation. Each technique has its own advantages and disadvantages. The process optimization has to be done considering both energy demand, capital and operation cost of the process. A conceptual diagram of the PCC is shown in Figure 1.4.
Figure 1.4: Schematic of post-combustion capture [5]
Post-combustion CO2 capture using a solvent process is regarded as one of the most mature carbon capture technologies [2]. Figure 1.5 illustrates an overview of PCC including other aspects like adsorption and membrane technologies.
Figure 1.5: Main carbon separation/capture methods in the post-combustion CO2 capture: Aghaie, et al. [11]
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1.2.1 Absorption and Desorption Process
The technology is included with both chemical and physical absorption of CO2. Alkanolamines, amino acid salts, aqueous chilled ammonia and ionic liquids react and capture CO2 from the flue gas stream. The amine-based technology is already in use for CO2 capture from natural gas. Probably, it will be the dominant technology for removing CO2 from flue gas of coal-fired power plants in 2030 [12]. Aqueous alkanolamines are widely used and investigated for the CO2 removal from flue gas streams.
Amino acid salts contain an amino group (-NH2) as in the amine to absorb CO2. The volatility of the solution is reduced by converting the carboxylic group into a salt [13].
The high CO2 absorption rate, high thermal stability, high biodegradability, low ecological toxicity, low volatility and resistance to O2 degradation have made amino acid salts favourable for PCC [14, 15].
Aqueous ammonia shows several advantages over conventional amines such as low cost, less corrosiveness and it does not degrade due to the presence of O2 and other species in the flue gas [16]. The escape of ammonia with the CO2 product stream at the stripper gas outlet is a loss and requires ammonia makeup, which is a disadvantage of using aqueous ammonia in PCC.
Ionic liquids can capture CO2 through either chemical absorption or physical absorption [17]. They are organic salts, which form a stable liquid at room temperature [18].
Generally, the CO2 solubility is more influenced by the anion than cation in physical absorption. Other factors that affect the CO2 solubility are free volume and size of the ionic liquid. For chemical absorption, ionic liquids with an amino-functional group that can react with CO2 can be used [17]. A systematic review of the use of ionic liquids in PCC is presented by Aghaie, et al. [11].
1.2.2 Adsorption Process
Cyclical removal of CO2 from flue gas using adsorption is an alternative to challenge disadvantages engaged with aqueous amine processes like low contact area between gas and liquid, low CO2 loading and corrosion effects [17]. The rate-limiting factor for the process is the diffusion of CO2 from flue gas to the pores of the adsorbent. The review performed by Choi, et al. [19] listed details of potential physisorbents and chemisorbents for CO2 removal. The use of zeolites and activated carbon as physisorbents have been reported through isotherm and kinetic studies. Due to the acid nature of CO2, alkaline metal oxides, especially with low charge/radius ratio like (Na2O/K2O) and (CaO/MgO) are applicable as chemisorbents to capture CO2. There are possibilities to improve adsorption and selectivity via chemical modifications on the surface of the solid materials to acquire high surface area. This is achieved by using amine-impregnated and amine-grafted materials. A critical analysis of adsorbents in the literature is performed by Sayari, et al. [20] who describe the different characteristics of different materials.
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1.2.3 Membrane Separation Process
The potential of membrane separation has been recognized as an energy efficient process for the CO2 capture from flue gas [21, 22]. Brinkmann, et al. [23] reported studies using different polymer and ceramic membranes for the coal-fired power plants. Possible types of membrane modules for the gas separation applications are envelope-type, spiral wound and hollow fiber modules. Merkel, et al. [24] outlined some general design issues, which affect the selection of the optimum membrane and module for PCC. The performance of the membrane system is restricted by the pressure ratio of the membrane and cannot achieve 90% capture of CO2 from a single-stage membrane process. Thus, a multi-stage treatment process is required to enhance the CO2 recovery and purity. Figure 1.6 illustrates the flow diagram of a two-step vacuum membrane process for CO2 capture from flue gas.
Figure 1.6: Simplified flow diagram of a two-step vacuum membrane process to capture and sequester CO2 in flue gas: Merkel, et al. [24]
Leung, et al. [25] provide a comparison of different separation technologies. The advantages of the absorption process are it is the most matured process for CO2 separation, which gives a high absorption efficiency (> 90%)[5]. The high energy requirement (with MEA: 3 MJ/kg CO2) for the CO2 desorption and inadequate understanding of environmental impacts related to solvent degradation are considered as disadvantages [25-27]. For the adsorption process, advantages are the high absorption efficiency and availability of low cost physical adsorbents. High energy demand for regeneration is a drawback for this technology [2, 20, 27]. The advantage of using membranes is low energy requirement (0.5-6 MJ/kg CO2) compared to other available technologies. The associated disadvantages are the low purity of CO2 removal and low removal efficiencies[28].
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