State-of-the-art process flow scheme

The Process Flow Diagram (PFD) considered as benchmark state-of-the-art configuration for NH3-based capture processes and used in this work as basic scheme for the implementation of advanced configurations is shown in Fig. 1. The description of the process and the purposes of each process section and unit operation have been detailed elsewhere [14]. Such process flow scheme is the result of implementing the patents related to the CAP [10][48][49][50][51], including process sections that allow for the control of NH3 emissions and for the recuperation of NH3, as well as simple process modifications broadly applied in solvent-based capture processes that allow to decrease the overall energy consumption with minor impact on the process complexity and operability, e.g. the RSS. More specifically and most relevant to this work, the benchmark PFD considers:

Fig. 1. Benchmark state-of-the-art flow scheme of the CAP used as reference configuration of NH3-based capture processes. Boxes with dashed lines are introduced for the identification of the different sections of the process, i.e. A to G. Labels provide the name of the process section or, if within a unit operation, the equipment or equipment section identification. The operating variables whose value govern the performance of the process are indicated in the flow scheme and have been colour-coded according to the process section to which they belong, i.e.

for CO2 absorber operating variables,

for CO2 desorber operating variables and

 for FG-WW column operating variables. (For interpretation of the references to colour in this figure caption, the reader is referred to the web version of this article.)

The absorber pumparound (within the CO2 absorption section (F) in Fig. 1), a slip stream of the cold CO2-rich stream that is cooled, chilled and sent to the top of the CO2 absorber in order to minimize the NH3 slip to the gas.

The FG post-conditioning section (A), which is able to further decrease the NH3 concentration in the CO2-depleted FG to values that meet the regulatory limits at the stack. It consists of a Flue Gas Water-Wash (FG-WW) column that is an NH3 absorber, followed by an acid-wash scrubber using an aqueous solution of H2SO4.

The CO2 Water-Wash (CO2-WW) section (B), which is used to minimize gaseous NH3 losses from the CO2 desorber, consists of a Direct Contact Cooler (DCC) that uses water to remove NH3 from the CO2 gas stream produced in the CO2 desorber.

The solvent recovery section (C), whose goal is to minimize the NH3 make-up requirements, is composed of two different distillation columns, namely: (i) the NH3 desorber, whose purpose is the recuperation of NH3 (and CO2) from the NH3-rich solution exiting the NH3 absorber; and, (ii) the appendix stripper, which recovers NH3 (and CO2) from the solvent stream purged from the CO2 absorber/CO2 desorber loop and from the liquid stream purged from the CO2-WW section. Both the NH3 desorber and the appendix stripper operate at atmospheric pressure and generate an almost pure water stream at the bottom of the column, requiring ca. 100 °C at the reboiler.

The RSS (within the CO2 desorber section (E)), a split of the cold CO2-rich stream that bypasses the rich/lean heat exchanger and that is directly sent to the top of the CO2 desorber instead. In addition to control the temperature in the CO2 desorber thus the NH3 slip to the CO2 stream, its optimization, together with the pressure in the CO2 desorber, allows minimizing the energy demand while avoiding solid formation downstream in the process [14].

The solvent is regenerated and CO2 is stripped-off in the CO2 desorber (E), which operates at pressures between 7.5 and 30.0 bar and that reaches temperatures in the reboiler between 115 and 150 °C.

The operating variables that govern the overall performance of the benchmark state-of-the-art CAP are indicated in Fig. 1 [14]. They are enumerated below, and arranged according to the capture process section to which they belong:

CO2 absorber (F) decision variables, i.e. the apparent NH3 concentration in the CO2-lean stream, cˆNH3 [molNH3kgH2O−1], the CO2 loading of the CO2-lean stream considering apparent species, llean [molCO2molNH3−1], the liquid-to-gas flowrate ratio for the CO2-lean liquid and inlet gas streams entering the CO2 absorber, Llean∕Gin [kg kg−1], the temperature of the pumparound stream, Tpa [°C], and the pumparound split fraction, fs [-].

CO2 desorber (E) decision variables, i.e. the pressure in the CO2 desorber, PCO2des [bar], and the cold-rich bypass split fraction, fcr [-].

FG-WW column (A) decision variables, i.e. the apparent NH3 concentration in the NH3-lean stream, cˆNH3FG−WW [molNH3kgH2O−1], the liquid-to-gas flowrate ratio for the NH3-lean liquid and the CO2-depleted FG entering the NH3 absorber, (L∕G)FG−WW [kg kg−1], the temperature of the NH3-lean stream, TleanFG−WW [°C], the temperature of the pumparound stream, TpaFG−WW [°C], and the pumparound split fraction, fsFG−WW [-].

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