**3. Conceptual Designs, Main Design Assumptions, and Process Integration Elements**

To illustrate the influence of plant decarbonization over the most relevant techno-economic and environmental indexes, some key fossil-intensive industrial applications were considered as follow:

Cases 1: Coal-based gasification power plants;


The evaluated gasification plants consider both pre- and post-combustion decarbonization scenarios is based on the Integrated Gasification Combined Cycle (IGCC) design [17]. The conventional gasification-based power plant design without carbon capture involves a partial oxidation process (with oxygen and steam) of the solid fuel to syngas (mainly a mixture of hydrogen and carbon monoxide). Further, the syngas is treated for sulfur removal in an acid gas removal unit, and the clean gas is used for power generation in a combined cycle gas turbine unit [18].

For the pre-combustion capture, the syngas is decarbonized either by gas-liquid absorption or calcium looping, and the hydrogen-rich stream is then used for power generation (in a combined cycle gas turbine unit) or hydrogen and power co-generation. For the post-combustion capture, the flue gases from the syngas-fueled gas turbine are treated for decarbonization with the same two carbon capture technologies (MDEA-based gas-liquid absorption and calcium looping). The conceptual plant layouts of decarbonized IGCC power plants are shown in Figure 3.

The evaluated coal-based super-critical power plants are based on the conventional state of the art design [19]. The combustion-based power plants involve total oxidation of solid fuel with air. The hot flue gases are then used for steam generation. The steam cycle parameters were selected in line with industrial standards: live steam at 290 bar/582 ◦C also having two steam reheats at 75 bar/580 ◦C, and 20 bar/580 ◦C. The cooled flue gases are treated for particulate matter, NOx, and SOx removal prior to decarbonization. The two decarbonization technologies analyzed in this paper (MDEA-based chemical scrubbing by gas-liquid absorption and calcium-based gas-solid looping cycle) were evaluated in a post-combustion capture configuration. The conceptual plant layout of the decarbonized super-critical power plant, is presented in Figure 4.

**Figure 3.** Decarbonized Integrated Gasification Combined Cycle (IGCC) power plant options.

**Figure 4.** Decarbonized super-critical power plant.

The evaluated decarbonization scenario for iron and steel production considers an integrated steel mill in accordance with the current state of the art [20]. The iron and steel production involve sinter production, iron production (a blast furnace), desulphurization plant, steel production (basic oxygen furnace), and various metallurgical steps. Within an integrated steel mill, there are numerous CO2 emission sources; this analysis considers the carbon capture for the main ones: captive power and heat (steam) plant, blast furnace and hot stoves, lime and coke production systems [21]. The two decarbonization technologies (MDEA-based chemical scrubbing by gas-liquid absorption and calcium-based gas-solid looping cycle) were evaluated in a post-combustion configuration. The conceptual plant layout of decarbonized iron and steel production system is presented in Figure 5.

The evaluated decarbonization scenario for cement production considers the current conventional design [22]. The cement production involves raw meal production, preheating, calcination (clinker production), and grinder (cement production) steps. The generated CO2 within the cement production process has two main sources—one from the fuel to be combusted in the calcination step and one from the calcium carbonate decomposition [23]. The two decarbonization technologies (MDEA-based chemical scrubbing and calcium-based gas-solid looping cycle) were evaluated in a post-combustion capture configuration. The conceptual plant layout of the decarbonized cement plant is presented in Figure 6.

**Figure 5.** Decarbonized integrated steel mill.

**Figure 6.** Decarbonized cement plant.

Table 1 shows the most important design assumptions of evaluated fossil-intensive industrial applications (power generation, iron, steel, and cement production) to be decarbonized as well as the two CO2 capture technologies (reactive gas-liquid absorption and calcium looping cycle). More detailed specifications are provided in the reference sources indicated in Table 1. Assumptions were furthermore used for modeling and simulation of assessed case studies. In this respect, ChemCAD software was used as a process flow modeling tool. Then the simulation results were employed to evaluate the most important plant performance indexes (e.g., fuel consumption, ancillary ene.g., and raw materials consumption, plant decarbonization rate, carbon footprint, etc.).


**Table 1.** Main design elements of evaluated decarbonized industrial processes.

The evaluated carbon capture designs were assessed in view of heat and power integration analysis for optimization of overall energy conversion yield [24]. In this respect, Pinch Analysis was used for Heat Integration of hot and cold streams within the plant. The main focus of Heat Integration analysis of the two carbon capture technologies was to enhance heat recovery potential by process-to-process heat exchange and to reduce external hot and cold utility consumptions [25]. To show the fundamental advantage in terms of high-temperature heat recovery of calcium-based gas-solid looping cycle over the chemical scrubbing option, Figure 7 presents the Hot and Cold Composite Curves for CaL cycle used for super-critical power plant decarbonization [16].

**Figure 7.** Calcium looping cycle Heat Integration analysis.

In contrast with the calcium looping cycle, the reactive gas-liquid absorption cycle has low-temperature hot streams (40–60 ◦C); therefore, the available heat cannot be used in an energy-efficient manner (e.g., for steam generation) but only to be taken by cooling water (external cooling utility) [11]. It can be observed that the high-temperature heat recovery capacity of the CaL unit significantly improves the overall energy conversion yield. In fact, the CaL unit can be seen not only as a carbon capture system but also as an energy conversion system since additional fuel (coal) is oxy-combusted with the goal to provide the heat input for CaCO3 decomposition.
