This work compares three postcombustion CO2 capture processes based on mature technologies for CO2 separation, namely, (i) absorption using an aqueous piperazine solution, (ii) adsorption using Zeolite 13X in conventional fixed beds (either vacuum swing adsorption or temperature swing adsorption), and (iii) multistage membrane separation using a polymeric material (with CO2/N2 selectivity of 50 and permeability for CO2 of 1700 GPU). All three capture plants are assumed to be retrofitted to a generic industrial CO2-emitting source with 12% CO2 v/v (with 95% relative humidity at the inlet temperature and pressure of 30 °C and 1.3 bar, respectively) to deliver CO2 at 96% purity. In the cases of adsorption and membranes, the flue gas is dried before feeding it to the CO2 capture unit. In a first step, the capture processes (i.e., components and design parameters) are optimized based on their technical performance, defined through process exergy requirement and plant productivity; exergy–productivity Pareto fronts are computed for varying CO2 recovery rates. Second, the economic performance of the processes is assessed through a cost analysis. Estimates of CO2 capture costs are provided for each process as a function of the plant size and CO2 recovery rate. The comparative assessment shows that, although the adsorption- and membrane-based processes analyzed may become cost competitive at the small scale (i.e., below sizes of 100 tons of flue gas processed per day) and low recovery rates (i.e., below ca. 40%), the absorption-based process considered is the most cost-effective option at most plant sizes and recovery rates.

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This article addresses the application of the indirect mineral carbonation process to recycled concrete aggregates (RCA). In such a process, calcium is first leached from the RCA feedstock into an aqueous ammonium nitrate solution and then it is carbonated to precipitate calcium carbonate. The calcium leaching step is modeled in this contribution, whereas it was investigated experimentally in part 1 of this series, where the focus was more on thermodynamic aspects than on kinetics. In this part 2 paper, kinetics aspects are also included by modeling them using two different approaches. In the first approach, the reaction–diffusion model is implemented, accounting for the congruent dissolution of the calcium hydroxide phase and the incongruent dissolution of the calcium silicate hydrate components of concrete fines. In the second approach, a double-shrinking-core model is used, which for the sake of simplicity assumes congruent dissolution of calcium silicate hydrates as well, infinitely fast reaction/dissolution kinetics, and pseudo-steady-state diffusion. The model parameters have been either determined through targeted experiments or estimated by fitting measurements obtained in experiments with two different materials, both in the size range of up to a diameter of 4 mm. Both models exhibit satisfactory accuracy in describing the experimental data; particularly for particles larger than about 0.1 mm in diameter, the two models calculate a nearly identical evolution of the calcium concentration in solution. The simplifications of the double-shrinking-core model lead to a loss of resolution in characterizing the evolution of leaching inside the particles, whereas they enable faster computations. The latter feature makes such a model a convenient tool for studying process performance, namely, productivity, calcium recovery, and solvent efficiency, through parametric analysis; this has been demonstrated as reported in the last part of the article.