**3. Process simulation with the equilibrium approach**

In general, the computer simulation of a chemical process can be conducted in two different manners: the equilibrium- and the rate-based approaches, depending on whether the kinetics is not taken or it is taken into consideration, respectively. The results at the equilibrium represent the performance theoretically achievable, while the rate-based the realistic one.

Evaluating the integration of a power plant with the ammonia-based capture using the equilibrium approach, as both chilled and cooled processes, Bonalumi et al. [36] focus on the flue gas from a coal-fired plant, as opposed to a gas-fired. The main difference is the CO<sup>2</sup> concentration, which is in the neighborhood of 15%, on a volume and dry basis, for coal- and of 4% for gas-fired. The reference power plant is the one defined by the European Benchmark Task Force [39] with the scope of establishing a framework for the consistent comparison of capture technologies. The plant has a nominal net electric power output and efficiency of 754 MW<sup>e</sup> and 45.5%. The carbon dioxide flow is 160.7 kgCO2/s at a concentration of 15.2%.

In their evaluation, Bonalumi et al. [36] adopt values of the design parameters differentiated between the chilled and the cooled process, as indicated by **Table 2**. Moreover, in the chilled process, the temperature of the streams entering the absorber is 7°C, leading to a maximum temperature in the absorber of around 18°C, despite the reaction of absorption being exothermic, and promoting the salt precipitation in a wide range of concentrations of the reactants. In the cooled process, instead, the temperature of those streams is 20°C, leading to a maximum temperature in the absorber of around 27°C and preventing the solid formation.

Different indexes can be defined to assess the carbon capture performance. First, the carbon capture efficiency is defined as the ratio of the flow rates [kmol/s or kg/s] of the carbon dioxide exiting the compression island and of that entering the exhaust chilling island. As a second


**Table 2.** Design parameters for the chilled and the cooled aqueous ammonia capture proposed by Bonalumi et al. [36].

common performance index, the specific heat duty [MJth/kgCO2] is defined as the ratio of the reboiler heat duty [MWth] and the mass flow rate [kgCO2/s] of effectively captured carbon dioxide. However, this second index does not include the information on the capture efficiency (first index) nor on the temperature at which the heat duty is required (or, in equivalent terms, the loss of electric power generation from the steam turbine due to the steam bled for the regenerator).

for the water wash on top of the absorber. Water wash is required indeed to minimize the

The ammonia-based capture is proposed typically for existing coal- and natural gas-fired power plants. Nonetheless, Bonalumi and Giuffrida [37] consider it for an air-blown integrated gasification combined cycle (IGCC) fired with high-sulfur coal, while Pérez-Calvo

In general, the computer simulation of a chemical process can be conducted in two different manners: the equilibrium- and the rate-based approaches, depending on whether the kinetics is not taken or it is taken into consideration, respectively. The results at the equilibrium represent the performance theoretically achievable, while the rate-based the realistic one.

Evaluating the integration of a power plant with the ammonia-based capture using the equilibrium approach, as both chilled and cooled processes, Bonalumi et al. [36] focus on the flue

tration, which is in the neighborhood of 15%, on a volume and dry basis, for coal- and of 4% for gas-fired. The reference power plant is the one defined by the European Benchmark Task Force [39] with the scope of establishing a framework for the consistent comparison of capture technologies. The plant has a nominal net electric power output and efficiency of 754 MW<sup>e</sup>

In their evaluation, Bonalumi et al. [36] adopt values of the design parameters differentiated between the chilled and the cooled process, as indicated by **Table 2**. Moreover, in the chilled process, the temperature of the streams entering the absorber is 7°C, leading to a maximum temperature in the absorber of around 18°C, despite the reaction of absorption being exothermic, and promoting the salt precipitation in a wide range of concentrations of the reactants. In the cooled process, instead, the temperature of those streams is 20°C, leading to a maximum

Different indexes can be defined to assess the carbon capture performance. First, the carbon capture efficiency is defined as the ratio of the flow rates [kmol/s or kg/s] of the carbon dioxide exiting the compression island and of that entering the exhaust chilling island. As a second

**Table 2.** Design parameters for the chilled and the cooled aqueous ammonia capture proposed by Bonalumi et al. [36].

concen-

gas from a coal-fired plant, as opposed to a gas-fired. The main difference is the CO<sup>2</sup>

and 45.5%. The carbon dioxide flow is 160.7 kgCO2/s at a concentration of 15.2%.

temperature in the absorber of around 27°C and preventing the solid formation.

**Parameter Unit Chilled Cooled** Ammonia initial concentration %(mass) 20 7.5 Ammonia-to-carbon dioxide ratio kmol/kmol 3.2 5 Recycle fraction — 0.8 0.2 Regeneration pressure bar 20 5 Regeneration temperature °C 95.4 105.6

tendency of ammonia to escape from the absorber, which is called ammonia slip.

et al. [38] for cement plants, both achieving promising indications.

116 Carbon Capture, Utilization and Sequestration

**3. Process simulation with the equilibrium approach**

A third index is adopted to solve this issue about the specific heat duty. Consequently, the new index allows to compare consistently plants characterized by different capture efficiencies, regeneration temperatures, and electric efficiency penalties. The Specific Primary Energy Consumption for Carbon Avoided (*SPECCA*) [MJth/kgCO2] is defined as

$$\text{SPECCA} \stackrel{\text{def}}{=} \frac{\text{HR} - \text{HR}\_{\text{pEI}}}{\text{E}\_{\text{nEI}} - \text{E}} \equiv \frac{3600 \left( \frac{1}{\text{\%}} - \frac{1}{\text{\%}\_{\text{aux}}} \right)}{\text{E}\_{\text{nEI}} - \text{E}} \tag{5}$$

where all parameters refer to either the power plant equipped with the carbon capture or the reference plant without it: *HR* is the heat rate [MJth/MWh<sup>e</sup> ], *E* the specific CO<sup>2</sup> emission [kgCO2/MWh<sup>e</sup> ], *<sup>η</sup><sup>e</sup>* [−] the net electric efficiency, and *REF* stays for reference.


**Table 3.** Predicted electric consumption of the capture island for the chilled and the cooled aqueous ammonia capture computed by Bonalumi et al. [36].


**Table 4.** Overall performances of the chilled and the cooled processes compared against a reference power plant (without carbon capture) and a plant integrated with MEA aqueous solution computed by Bonalumi et al. [36].

**5. Economic and environmental assessments**

Specific CO<sup>2</sup>

et al. [34, 36].

ultimately, the resulting cost of avoided CO2

abiotic depletion elements, are better in the case of calcium looping.

The integration of the chilled process and an ultra supercritical power plant is analyzed by Valenti et al. [41] via a parametric analysis from the energy and the economic perspectives. The capture island is simulated with an equilibrium approach. In the parametric investigation, five parameters are varied singularly: (1) ammonia initial concentration in the aqueous solution, (2) ammonia-to-carbon dioxide ratio in the absorber, (3) regeneration pressure, (4) regeneration temperature, and (5) absorber chiller evaporation temperature. The economic analysis, with respect to a reference power plant rated at the net electric production of over 750 MW<sup>e</sup>

**Table 5.** Performances of the cooled process computed with the equilibrium and the rate-based approaches by Bonalumi

**Parameter Unit Cooled equilibrium Cooled rate based**

emission kgCO2/MWh<sup>e</sup> 138.9 141.2

Electric power loss MW<sup>e</sup> 129.3 136.4 Net electric power MW<sup>e</sup> 624.7 617.6 Net electric efficiency % 37.70 37.27 Specific heat duty MJ/kgCO2 2.98 3.02

*SPECCA* MJ/kgCO2 2.58 2.77

shows that the capital investment of the capture island is estimated to be a relatively small portion of that of the power island. However, due to other costs and due to the performance penalties, the cost of electricity increases significantly by 37.5%, from 59.90 to 82.38 €/MWh<sup>e</sup>

A detailed environmental life cycle analysis for an ultra supercritical power plant with and without carbon capture is proposed by Petrescu et al. [42]. Three capture islands are considered: (1) gas-liquid absorption with MDEA (monodiethanolamine), (2) gas-liquid absorption with aqueous ammonia, and (3) gas-solid absorption with calcium oxide. The environmental evaluation is performed using the "cradle-to-grave" methodology considering several upstream and downstream processes. Eleven environmental impact categories, according to the method CML 2001, are compared using GaBi software. The study highlights that carbon capture technologies decrease the global warming potential indicator, but they may increase other indicators. The amine technology achieves a good performance from the perspective of global warming, but not satisfactory from that of all others. Aqueous ammonia adsorption and calcium looping prove to be better. Some indicators, such as acidification potential, eutrophication potential, or those related to lethal concentrations (e.g., human toxicity potential, freshwater aquatic ecotoxicity potential, and marine aquatic ecotoxicity potential), are better in the case of aqueous ammonia. By contrast, some others, such as abiotic depletion fossil and

is approximately 38.64 €/tCO2.

Chemical Absorption by Aqueous Solution of Ammonia http://dx.doi.org/10.5772/intechopen.78545

,

119

;

Regarding the results for the chilled and the cooled process, **Table 3** compares the predicted electric consumptions. The exhaust cooling and the absorption-regeneration sections are more penalizing for the chilled process due to the major consumption of the chillers. By contrast, the power island is more penalizing for the cooled process, on one side, because of a large contribution due to the higher amount of NH3 that must be recovered by the water wash section. On the other, because of another major contribution due to the higher specific heat duty and the higher regeneration temperature that require more steam bleeding at a higher value of pressure and enthalpy from the turbine. In addition, the compression stage is more penalizing for the cooled process since the regeneration pressure is lower. Hence, from the electric consumption, the chilled process is less penalizing than the cooled one.

In its turn, **Table 4** summarizes the performances for the chilled and the cooled processes and it compares them against those of the reference power plant (without any carbon capture) and a plant integrated with carbon capture in MEA aqueous solution. From the index *SPECCA*, which is as seen the most consistent perspective for evaluating a capture technology, the cooled process is less penalizing than the chilled one, by far, than MEA.
