**2. Carbon Capture Technologies for E**ffi**cient Decarbonization of Industrial Applications**

As decarbonization technologies, two carbon capture methods were assessed in view of integration into fossil-intensive industrial applications. The first option considers a mature technology based on chemical scrubbing employing alkanolamines [10]. This technology represents the conventional option when acid gas removal (e.g., CO2, sulfur compounds, etc.) is required in various chemical industrial processes (e.g., natural gas reforming for ammonia synthesis, sulfur removal from oil refinery, etc.). For this main reason, the reactive gas-liquid absorption technology was considered as potential decarbonization process for the evaluated energy-intensive industrial applications. To illustrate this decarbonization technology, Methyl-DiEthanol-Amine (MDEA) was selected as a chemical solvent. The selection of the MDEA solvent was based on the following key advantages over more conventional Mono-Ethanol-Amine (MEA): higher CO2 loadings (1 mole CO2/mole MDEA vs. up to 0.5 mole CO2/mole MEA), higher solution concentration (50% vs. 30%), lower degradation, corrosion and toxicity,

better thermal stability, etc. [11]. The overall chemical reaction for MDEA-based decarbonization is presented below:

$$2\text{ CO}\_2 + \text{H}\_2\text{O} + \text{MDEA} \leftrightarrow \text{MDEAH}^+ + \text{HCO}\_3^- \tag{1}$$

For this decarbonization technology, the above global chemical reaction is used in a cycle, as presented in Figure 1. The gas to be decarbonized is treated in the absorption column being put in contact with an MDEA aqueous solution (50% wt.). The loaded (rich) solvent is pumped in a separate column where using heat, the CO2 is desorbed, and thus, the solvent regenerated. The regenerated (lean) solution is pumped back in the absorption column (some make-up being necessary to cover the solvent losses). The CO2 is treated for moisture removal and compressed to the final delivery pressure (120 bar) prior to storage/utilization. A key element of this decarbonization technology represents the heat consumption (at the bottom of the desorption column) for CO2 desorption and solvent regeneration. Currently, for the post-combustion CO2 capture configurations applied to fossil-based power generation plants (10–15 volumetric percentages of CO2 content in the gas to be treated), this heat duty is about 3 GJ/t CO2 [12]. For pre-combustion capture configurations, the heat consumption for solvent regeneration is significantly reduced to about 0.6–0.8 GJ/t due to pressure reduction [13].

**Figure 1.** CO2 capture by chemical scrubbing via an absorption-desorption cycle.

The second evaluated decarbonization option makes use of an innovative reactive system based on calcium adsorption (Calcium Looping—CaL). Similar with chemical scrubbing option presented above, the chemical looping cycle can also be used for pre- and post-combustion decarbonization configurations. This technique uses two separate reactors for decarbonization as follow [9]:

The carbonation reactor where the gas to be decarbonized is put in contact with the calcium sorbent for CO2 capture. The reactions for pre- and post-combustion decarbonization are exhibited below:

$$\text{Post}-\text{combustion capture} \colon \text{ CO}\_2 \, + \text{ CaO} \, \leftrightarrow \text{ CaCO}\_3 \tag{2}$$

$$\text{Pre}-\text{combustion capture} : \text{CO} + \text{H}\_2\text{O} + \text{CaO} \leftrightarrow \text{CaCOs} + \text{H}\_2\tag{3}$$

Calcination reactor when CaCO3 is thermally disintegrated to CaO and CO2 according to the next chemical reaction:

$$\text{CaCO}\_3 \leftrightarrow \text{CaO} + \text{CO}\_2 \tag{4}$$

The innovative calcium looping method was selected as decarbonization technology for the fossil-intensive industrial applications based on the following reasons: it represents a promising technology in reducing the CO2 capture energy and cost penalties, possibility to use the spent sorbent (deactivated calcium sorbent) within the main process, sorbent lower cost and large availability, etc. [14]. For this decarbonization technology, the conceptual design is presented in Figure 2. As for gas-liquid absorption decarbonization technology, the calcium looping cycle requires an additional energy input (for the calcination reactor). In the calcination reactor, some sort of fuel (e.g., natural gas, syngas,

coal, etc.) is to be oxy-combusted for providing the required energy input for the calcium carbonate decomposition. Oxygen must be used for combustion in order not to dilute the CO2 captured stream with nitrogen. Different for gas-liquid decarbonization technology, the CaL cycle is operating at significantly higher temperatures: carbonation (CO2 fixation) reactor to 500–650 ◦C and calcination reactor to about 850–1000 ◦C [15]. These operating conditions enable high-temperature heat recovery with positive consequences on the overall energy conversion yield [16].

**Figure 2.** CO2 capture by the calcium-based sorbent looping cycle.
