*5.1. Coal-Based Gasification Power Plants*

For decarbonization of gasification-based power plants, two technical options are available. The pre-combustion route when the shifted syngas is decarbonized before combustion in a combined cycle. The post-combustion route when the syngas is used for power production in a combined cycle as in any conventional IGCC design without carbon capture, and then the combustion gases are treated for CO2 capture (see Figure 3). For gasification systems, the general opinion is that the pre-combustion configurations are more efficient than the post-combustion ones considering the partial pressure of CO2 in the gas to be decarbonized [16]. This work is considering both pre- and post-combustion capture options to illustrate, in a quantitative manner, the advantages of the pre-combustion capture option. For post-combustion capture, only the calcium looping option was considered. This consideration was based on its higher energy efficiency compared to the chemical scrubbing option. The following gasification-based power plant concepts were evaluated:

Case 1.1—Conventional gasification-based power plant without decarbonization;

Case 1.2—Decarbonized power plant based on the pre-combustion concept using reactive gas-liquid absorption (MDEA);

Case 1.3—Decarbonized power plant based on the pre-combustion concept using reactive gas-solid system (CaL);

Case 1.4—Decarbonized power plant based on the post-combustion concept using reactive gas-liquid absorption (MDEA).

The most important techno-economic and environmental performance indicators of evaluated coal-based gasification power plants are summarized in Table 2.

As shown in Table 2, the pre-combustion capture configurations have a lower decarbonization energy penalty than the post-combustion option (9.3–9.7 vs. 11.7 net energy efficiency percentage points). This energy efficiency difference of about two net percentage points between pre-combustion and post-combustion capture cases can be explained by the significantly higher partial pressure of CO2 in the gas subject to decarbonization (12–14 bar for pre-combustion cases vs. 0.13–0.16 bar for post-combustion cases). For the pre-combustion options, the MDEA concept (Case 1.2) shows higher net efficiency than the CaL concept (Case 1.3).

The specific consumption of primary energy for CO2 avoided (SPECCA indicator) shows slightly better performances for the MDEA system compared to the CaL one (either prior or after combustion). In addition, CaL design (which uses a Circulated Fluidization Bed—CFB system) is more complicated to be adjusted for operation at high pressures (about 30–40 bar) as required in the pre-combustion

option [33]. Specific CO2 emissions (carbon footprint) for all decarbonized plants are significantly reduced compared to the benchmark case without carbon capture. A full Life Cycle Analysis (LCA) reveals (as illustrated below in case of super-critical combustion-based power plants) that other environmental impact indicators increase by plant decarbonization [30]. This negative element of process decarbonization is explained by increasing the raw materials consumptions, reducing energy efficiency, and introducing new plant sub-units (CO2 capture and conditioning units).


**Table 2.** Gasification power plants techno-economic and environmental performance indexes.

The decarbonization process of gasification-based power generation brings significant economic penalty (23–75% increase in the specific capital investment, 35–50% increase in the electricity cost). The economic indicators show that pre-combustion capture (either gas-liquid absorption or calcium looping) is definitely better than post-combustion capture in term of specific capital investment (reduced by 20–30%), levelized cost of electricity (reduced by 6–10%) and CO2 avoided cost (reduced by 19–30%). The MDEA-based decarbonization option has slightly better electricity cost, and CO2 avoided cost than the calcium looping option (for the technical reasons mentioned above).

One relevant element to be mentioned here in connection with gasification systems represents the ability of this partial oxidation technology to generate, in a flexible manner, various energy carriers. For instance, after syngas decarbonization in a pre-combustion capture configuration, the hydrogen-rich gaseous stream could be employed for the generation of power, hydrogen, or other synthetic carbon-based fuels (methanol, substitute natural gas, synthetic hydrocarbons via Fischer–Tropsch process). One key advantage of these systems represents high cumulative energy efficiency. This specific design characteristic of gasification-based energy conversion systems represents an important element for the future low carbon higher efficiency systems [34].
