Technical and Economic Evaluation

The NH₃ capture process was analyzed for different mass concentrations of 2, 5, 7, and 15% respectively, comparing the results with the CO₂ capture reference process based on MEA in 30% mass concentration. All results presented were for 90% CO₂ capture efficiency. The annual number of operating hours of the plant was estimated at 7500 h/year. The generator efficiency was 98.7% and the mechanical efficiency was 99.6%. Generator losses were between 2.6 and 3.1 MW and mechanical losses between 0.8 and 0.9 MW. The quantity of heat per 1 kg of steam was approximately 2820 kJ/kg for all cases studied. The solvent temperature at the regeneration inlet was 363.15 K and at the regeneration outlet, it was between 398 and 433 K. The specific electrical consumption includes the electrical consumption of pumps, compressors, and the cooling of ammonia solutions. The results for the technical and economic indicators are shown in Table 6.

Before the CO₂ capture process, the overall efficiency of the steam power plant was 42.45%. After integration, the overall efficiency fell to about 27.58% for NH₃ = 2% wt. and 30.83% for MEA = 30% wt.%. In the case of using NH₃ in 2 and 5% wt., the energy penalty was higher due to the electricity consumption to cool the solution at the inlet of the absorption column. Although better results have been obtained using mass concentrations greater than NH₃ in solution, it is not preferable to select a mass concentration greater than 7% due to the volatilization of ammonia in the absorption process. The overall efficiency of the power plant decreased by 6.17 to 14.87 percentage points in the case of NH₃ use, depending on the mass concentration. In the case of MEA, the overall efficiency decreased by 11.62 percentage points. In the study conducted by Molina and Bouallou (2017), a decrease in the overall efficiency for a 450 MW coal-fired power plant of 7.2 percentage points was obtained when using NH₃ in a mass concentration of 3%. In the case of using MEA in a mass concentration of 30%, they obtained a decrease of 11.82 percentage points [43].

The investment costs for a steam power plant without carbon capture process were 2497 €/kW, whereas after integration of the capture process, they amounted to 4333 €/kW. The price of the fuel used was estimated at 55 €/t [44], a discounted rate of around 8%, and a lifespan of 30 years. An important advantage of using ammonia in the chemical absorption capture process is that it has a lower purchase price of 0.672 €/kg, compared to the MEA purchase price of 2 €/kg per liter [27]. Once the CO₂ capture process was integrated, the LCOE increased by 53.8−104.7% compared to the LCOE for the steam power plant without CO₂ capture. Only in the case of NH₃ = 2% wt. was a higher LCOE value obtained than in the case of MEA = 30% wt. due to the higher specific electricity consumption and specific heat, which led to a lower amount of electricity produced. In terms of the CO₂ capture cost, according to the analysis made by the IEA in 2021, for the energy sector, it is between 45–90 €/tCO₂, depending on the type of CO₂ capture technology used [3]. In this study, the CO₂ cost capture ranged from 59.07 to 76.21 €/tCO₂.

An analysis of the costs of electricity is presented below, approximated by considering the cost of a ton CO₂ certificate. Currently, energy regulations on reducing greenhouse gases have led to a tax on carbon dioxide emitted into the atmosphere by steam power plants. The CO₂ emissions tax varies depending on the certificate market. The CO₂ emissions tax was 31.62 €/tCO₂ in 2021, and it reached 96.93 €/tCO₂ in 2022 [45]. Thus, this analysis considered the price of a certificate for a ton of CO₂ emitted into the atmosphere to vary between 15 €/tCO₂ and 100 €/tCO₂ to observe when the integration of the chemical absorption capture process was better in terms of LCOE compared to when a capture process was not integrated, and the CO₂ emissions tax was paid.

For a CO₂ emissions tax of around 70 €/MWh, ammonia solutions with a mass concentration of 7 and 15% were better in terms of LCOE. Where MEA = 30% by weight, a lower LCOE was only obtained if the CO₂ the tax was greater than 80 €/MWh (Figure 4). In this case, if the cost of a ton-CO₂ certificate continues to grow (this has increased over the last decade to reach the average last year of 60 €/MWh), capture technologies can become a reliable alternative to reduce CO₂ emissions from steam power plants.

Regarding the economic criterion of the NPV (Figure 5), we note that without the integration of a carbon capture technology, the project is profitable even if a CO₂ emissions tax of 100 €/tCO₂ is considered, but the NPV was decreased by 65%. In the case of solutions with a capture process, it was found that the capture process using a mass concentration ammonia solution at 2% is not economically efficient if the CO₂ emissions tax is 100 €/tCO₂, the NPV value being less than 0. For 5%, 7%, and 15% solvent mass concentration of NH₃, the NPV had positive values regardless of the CO₂ emissions tax. Regarding the MEA solution for a CO₂ emissions tax of 60 €/tCO₂, the NPV was 163 M€. NH₃ capture solutions (7% and 15% by mass concentration) were economically better than the MEA solution, because the amount needed to regenerate the solvent is lower, and therefore a smaller additional amount of fuel is used, resulting in lower fuel costs. Another reason why the NH₃ based solution was best is that the cost of purchasing ammonia is much lower than the cost of purchasing MEA. Regarding the other economic indicators (Figure 6, Figure 7 and Figure 8), the same conclusions can be drawn as for the NPV indicator. The PI indicator shows, for example, that the case based on NH₃ in a mass concentration of 2% is not a cost-effective solution as the PI is less than 1 for a CO₂ emissions tax of 100 €/tCO₂. For all other cases studied, the PI indicator had values greater than 1, which means that they are profitable (Figure 8).