Scholarly article on topic 'Vacuum Regeneration of Amine Solvent for Post-Combustion Carbon Capture with Compression Train Integration'

Vacuum Regeneration of Amine Solvent for Post-Combustion Carbon Capture with Compression Train Integration Academic research paper on "Chemical engineering"

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Abstract of research paper on Chemical engineering, author of scientific article — Yann Le Moullec

Abstract The main aim of this study is the assessment of vacuum operating pressure for amine based solvent regeneration with respect to plant efficiency and economic in order to conclude about its industrial potential. The capture process considered is the conventional two columns configuration with MEA solvent. A regeneration pressure range from 0.06 to 2.5bar have been investigated. The thermal integration with the power plant has been performed on a new built, advanced supercritical power plant adapted for CO2 capture. Flue gases condensation heat and CO2 compression heat have been fully integrated in the steam cycle. Influence of stripper pressure on optimal lean loading ratio and columns basic design (height and diameter) have been investigated, with a focus on the influence of the CO2 compression heat integration strategy. Calculations of plant efficiency have been completed by simplified economical calculations for the levelized cost of electricity (LCOE) and avoided CO2 (LCCO2) in order to assess the industrial interest of stripper vacuum operation. Regarding plant efficiency the optimal pressure is at the minimum value: i.e. 0.06bar with a loss of efficiency of 7.6%pt. In the pressure range from 0.5bar (medium vacuum) to 2.5bar (standard stripper pressure), plant efficiency is quite stable with a minimum around atmospheric pressure with 9.4%pt loss of efficiency. Regarding plant economics the main impact of vacuum regeneration is not the cost of the larger stripper but the cost of the very large compressor needed to maintain vacuum condition. At very low pressure, absorber and stripper have the same operating temperature therefore the economizer is no longer needed. Coupled with the improved plant efficiency, the effect of pressure on cost of electricity and cost of avoided CO2 is very small. The expected gain for deep vacuum stripper is not large enough to justify pilot demonstration of such operating parameters.

Academic research paper on topic "Vacuum Regeneration of Amine Solvent for Post-Combustion Carbon Capture with Compression Train Integration"

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Energy Procedia 37 (2013) 1814- 1820

GHGT-11

Vacuum Regeneration of Amine Solvent for Post-Combustion Carbon Capture with Compression Train Integration

Yann Le Moullec*

_R&D, Fluid Dynamics Power Generation and Environment Department, 6 quai Watier, 78401 Chatou, France_

Abstract

The main aim of this study is the assessment of vacuum operating pressure for amine based solvent regeneration with respect to plant efficiency and economic in order to conclude about its industrial potential. The capture process considered is the conventional two columns configuration with MEA solvent. A regeneration pressure range from 0.06 to 2.5 bar have been investigated. The thermal integration with the power plant has been performed on a new built, advanced supercritical power plant adapted for CO2 capture. Flue gases condensation heat and CO2 compression heat have been fully integrated in the steam cycle. Influence of stripper pressure on optimal lean loading ratio and columns basic design (height and diameter) have been investigated, with a focus on the influence of the CO2 compression heat integration strategy. Calculations of plant efficiency have been completed by simplified economical calculations for the levelized cost of electricity (LCOE) and avoided CO2 (LCCO2) in order to assess the industrial interest of stripper vacuum operation.

Regarding plant efficiency the optimal pressure is at the minimum value: i.e. 0.06 bar with a loss of efficiency of 7.6 %pt. In the pressure range from 0.5 bar (medium vacuum) to 2.5 bar (standard stripper pressure), plant efficiency is quite stable with a minimum around atmospheric pressure with 9.4 %pt loss of efficiency. Regarding plant economics the main impact of vacuum regeneration is not the cost of the larger stripper but the cost of the very large compressor needed to maintain vacuum condition. At very low pressure, absorber and stripper have the same operating temperature therefore the economizer is no longer needed. Coupled with the improved plant efficiency, the effect of pressure on cost of electricity and cost of avoided CO2 is very small. The expected gain for deep vacuum stripper is not large enough to justify pilot demonstration of such operating parameters.

© 2013 The Authors. Published by Elsevier Ltd. Selection and/or peer-review under responsibility of GHGT

Keywords: Post-combustion, Amine absorption, vacuum regeneration, plant integration, economics evaluation

1. Introduction

In the past few years, many studies have assessed capture process optimized flow schemes in order to reduce the heat needed for solvent regeneration. However, few have tried to reduce the quality of needed

* Corresponding author. Tel.: +33-1-30877731; fax: +33-1-30877108. E-mail address: yann.le-moullec@edf.fr.

1876-6102 © 2013 The Authors. Published by Elsevier Ltd. Selection and/or peer-review under responsibility of GHGT doi:10.1016/j.egypro.2013.06.059

steam for solvent boiling. Some patents show the interest of vacuum regeneration for hot carbonate process or ionic liquid but vacuum regeneration for amine-based solvent allows using a lower quality of extracted steam and less thermal stress on solvent, thus limiting its degradation. Main drawbacks are an important increase in compression power consumption needed to produce supercritical CO2 and a larger regeneration column. The main aim of this study is the assessment of vacuum operating pressure for amine based solvent regeneration with respect to plant efficiency and economic in order to conclude about its industrial potential. Simulations of capture process at different stripper pressure have been performed. The capture process has been, then, integrated in a reference power plant. Finally, calculations of plant efficiency have been completed by simplified economical calculations for the levelized cost of electricity (LCOE) and avoided CO2 (LCCO2) in order to assess the industrial interest of stripper vacuum operation.

2. Methodology

This work is based on simulations performed with ASPEN Plus software. Main hypotheses of the capture process model are briefly described and the overall process description and simulation parameters are detailed.

2.1. Model description

The thermodynamic behaviour of the mixture of H2O-MEA-CO2 is represented by a NRTL model adapted for electrolyte solutions and already implemented in ASPEN Plus®. This thermodynamic model is corrected by Henry's law for gaseous species with low molar masses (O2, N2, CO2). In this type of mixture, the chemical and thermodynamic equilibriums are highly interdependent. The equilibrium coefficients used are based on the work of Augsten et al. [1] and Jou et al. [2,3]. Among the five equilibriums, the two that do not involve only transfers of protons are considered to be kinetically slower than the three involving only transfer of protons, which are considered to be instantaneous. The kinetic coefficients of these two reactions are taken from the work of Hikita et al. [4].

The transfer model used is the module RateSep® available in the ASPEN Plus® software. This software also takes into account the binary interactions between the compounds using the Krishna and Standard theory of transfer of multi-component. This approach implies the discretization of the thickness of liquid on the packing and solving all equations describing the chemistry of the system in each of these volumes. This provides a strict representation of the contribution of the chemical reactions, whether rapid or not, to the transfer. This approach requires a great deal of time and its use has recently spread thanks to the increase in computing capacity available [5-7]. Non-linear, non-equidistant discretization gives a better compromise between the computing time and accuracy of the calculations. Six non-equidistant segments are considered to be the optimum [6,7]. This type of model has been successfully validated on Esbjerg's capture pilot plant data from the European project CASTOR, the data have been taken from Dugas et al. [8].

2.2. Process description and modelling assumption

The capture process considered is the conventional two columns configuration using aqueous 30 % mass monoethanolamine (MEA) solution. A regeneration pressure range from 0.06 to 2.5 bar has been investigated. Columns sizes have been adapted for each stripper operating pressure. The temperature pinches in the economizer and the reboiler are taken equals to 5 K. All other heat exchangers have temperature pinches of 10 K. For very low pressure stripper, the economizer is removed because the stripper operates at almost the same temperature as the absorber. Isentropic efficiency of fans and pumps are taken equals to 75 %.

The compression train is considered as a succession of stage coupling a compressor, with 85 % isentropic efficiency, and a condenser. The compression ratio of each stage is limited to 2 in order to limit the overall energy consumption of the compression train.

Three different integration patterns have been investigated:

1) Standard integration: each condenser cools the flow down to 30 °C, heat recovered above 40 °C is used to preheat the boiler feedwater.

2) Medium temperature integration: each condenser cools the flow down to 40 °C, all the recovered heat is used to preheat the boiler feedwater.

3) High temperature integration: same as above but the compression ratio of each stage is raised to 10.

The reference power plant used for integration calculation is based on a supercritical power cycle at 290 bar/600 °C/620 °C with a net LHV efficiency of 45.5 %. Figure 1 shows the power plant flow scheme and the main flue gas and steam operating parameters. The thermal integration on the power plant has been performed on a new built, advanced supercritical power plant adapted for CO2 capture. Flue gases condensation heat and CO2 compression heat have been fully integrated in the steam cycle.

Figure 1: Reference power plant flow scheme

3. Results

3.1. Absorber and stripper heights and operating conditions

The pressure of the flue gas at the absorber inlet is considered to be constant. The cost of increasing of pressure of the flue gas is prohibitive because of the very high volumetric flow rate. Therefore, the absorber always operates at almost atmospheric pressure.

An augmentation, from the reference, of the packing height in the absorber has a non-significant impact on the rich solvent loading because of the thermodynamic pinch in the bottom of the absorber. A diminution of packing height induce a reduction of rich solvent loading which increase the solvent flow

rate needed and therefore the boiler duty (figure 2). It can be highlighted that a taller absorber induces a smaller optimal lean solvent loading (figure 2).

0,05 0,1 0,15 0,2 0,25 0,3 0,35 0,4

Lean loadingfaclor

Figure 2: specific reboiler duty with respect to column packing height and lean loading

The height of the stripper and the reboiler pressure influence strongly the reboiler duty. A pinch analysis shows that the minimum lean solvent loading is comprise between 0.2 and 0.25. Figure 2 shows that the energy savings between a packing height of 3 m and 10 m is approximately 2.5% whereas the gain between a height of 10 m and one of 15 is approximately 1.2%. These results, combined with those of precedent paragraph, show that it is more important to give priority to the absorber in terms of height of packing rather than to the stripper, which is confirmed by the calculation of the efficiency of the power plant in the optimum case. It should be noted that a large packing bed in the absorber ensures less boiler duty at high lean loading and an increase in the size of the packing bed in the stripper provides less boiler duty at low lean loading. From there, it is probable that strict technical-economic optimisation should not consider the size of packing beds as a constant but should, on the contrary, adapt it to each type of process architecture.

0,05 0,1 0,15 0,2 0,25 0,3 0,35 0,4 0,45 0,5 Lean loading factor

Figure 3: specific reboiler duty with respect to stripper pressure and lean loading

The temperature of the stripper is strongly linked with the pressure at its bottom. Some authors have already explored the effect of stripper pressure on boiler duty such as Freguia and Rochelle [9]. Figure 3 shows the evolution of boiler duty for different lean loadings and boiler pressures. In order, to regenerate the solvent at lean load loading greater than 0.3 all packing heights are equivalent. For smaller lean

loading, the height of packing becomes a crucial parameter. The optimal lean loading seems to be around 0.225 for a high pressure stripper (2.5 bar) and 0.3 for an atmospheric pressure stripper, consistent with the simulation of Abu-Zahra et al. [10].

The stripper cross over area increases as the pressure decreases (figure 4), the variation is exponential in the low pressure cases. As the pressure decreases a larger amount of steam is needed to strip the CO2 from the solvent. Both effects: lower gas density and higher H2O/CO2 ratio, explain the trends of the curve shown in figure 4.

2 0 0 0,5 1 1,5 2 2,5 3

Stripper absolute pressure(bar)

Figure 4: optimal specific reboiler duty and specific stripper crossover area with respect of stripper pressure 3.2. Plant integration results

For each pressure and for each compression heat integration pattern, the minimal efficiency loss has been calculated with respect to the lean loading ratio. Results of this parametric study are shown in figure 5. The standard and medium temperature compression heat integration option does not present a clear optimum in the pressure range 0.125 to 2.5 bar. All pressure lead to approximately 9.4 %pt for the standard and 9.7 %pt for the medium temperature integration.

£ 10,0 -8

O typical compression rest integration

* medium temperature compression heat irtegratio

• high temperature compression heat integration

Stripper pressure {bar]

Figure 5: plant efficiency for different regeneration pressure and compression heat integration options

The difference between this work and Romeo et al. [11], which concludes the opposite, is explained by the integration of flue gas condensation heat which competes with the low temperature compression heat. The high temperature heat integration decreases significantly the overall performance of the power plant

(figure 5). Very low regeneration pressure (< 60 mbar) leads to very low energy penalty for the three integration patterns, at around 7.5 %pt. Heat integration options do not significantly affect this result.

3.3. Techno-economical results

Literature correlations [12-14] have been used for economical evaluation. Overall coal power plant cost has been kept constant for the different studied cases, only the capture process and the compression train have been recalculated. Table 1 summarizes the main characteristics of the investigated processes and table 2 summarizes the technical-economic results. It is shown that the low pressure stripper does not provide better CO2 capture economics despite the better plant efficiency. This is mainly due to the increase of the capture process cost and the sharp increase of the compression train cost. Finally, the best regeneration pressure seems to be around atmospheric pressure but the difference between 2.5 bar stripper and atmospheric stripper is very small: less than 2 %.

Table 1: main characteristics of the capture and compression process for different stripper pressure

Stripper Solvent Absorber size Stripper size Heat integration # comp.

pressure flow (# x h x d) (# x h x d) stage

2.5 bar 17.2 t/tco2 2 x 30m x 20m 2 x 25m x 10m Typical 5

1.0 bar 27.4 t/tco2 2 x 30m x 20m 2 x 25m x 13m Typical 6

500 mbar 40.3 t/tco2 2 x 40m x 20m 2 x 20m x 15m Typical 7

125 mbar 60.7 t/tco2 2 x 40m x 20m 3 x 20m x 17m No economizer 9

60 mbar 121 t/tco2 2 x 40m x 20m 4 x 15m x 20m No economizer 10

Table 2: summary of the techno-economical results

Stripper pressure CCS plant efficiency Rel. capture CAPEX Rel. comp. cAPEX Rel. electricity price Rel. avoided Co2 price

2.5 bar 35.5 % 1.00 1.00 1.00 1.00

1.0 bar 36.1 % 1.10 1.08 0.99 0.98

500 mbar 36.1 % 1.32 1.20 1.01 1.04

125 mbar 35.8 % 1.26 1.64 1.04 1.15

60 mbar 37.9 % 1.37 1.81 1.00 1.00

4. Conclusion

The influence of the stripper pressure on plant efficiency and plant economics has been studied. Plant efficiency, with or without compression heat integration, the optimal pressure is at the minimum value tested : i.e. 0.06 bar with an efficiency penalty of 7.6 %pt. Integration of compression waste heat needs a complete modification of the feed water preheating train especially for very low pressure stripper. In the best configuration tested it allows an improvement of 0.5 %pt efficiency. In the pressure range from 0.5 bar (medium vacuum) to 2.5 bar (standard stripper pressure), plant efficiency is quite stable with a minimum around atmospheric pressure with 9.4 %pt loss of efficiency.

Regarding plant economics the main impact of vacuum regeneration is not the cost of the larger stripper but the cost of the very large compressor needed to maintain vacuum conditions. At very low pressure, absorber and stripper have the same operating temperature therefore the economizer is no longer needed. Moreover, for low pressure cases, the part of CO2 recovered through pressure swing become significant and the reboiler must be adapted. Coupled with the improved plant efficiency, the effect of pressure on

cost of electricity and cost of avoided CO2 is very small. The expected gain for deep vacuum stripper is not large enough to justify pilot demonstration of such operating parameters. But, it must be kept in mind that there are almost no experiment of deep vacuum CO2 stripping with amine based solvent. Moreover it could be emphasized that Aspen Plus e-NRTL model does not produce the same results as the e-UNIQUAC model, the latter being significantly higher in terms of reboiler duty than the former. More definitive conclusion could be made after some deep vacuum stripping experiment at laboratory scale.

References

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[2] Jou, F., Y., Mather, A., E., Otto, F., D., Solubility of Hydrogen Sulfide and Carbon Dioxide in Aqueous Methyldiethanolamine Solutions. Ind. & Eng. Chem. Process Des. & Dev. 1982, 21:539

[3] Jou, F., Y., Carroll, J., J., Mather, A., E., Otto F. D., Solubility of Mixtures of Hydrogen Sulfide and Carbon Dioxide in Aqueous N-Methyldiethanolamine Solutions. J. of Chem. Eng. Data 1993, 38:75

[4] Hikita, H., Asai, S., Ishikawa, H., Honda, M., The kinetics of reactions of carbon dioxide with monoethanolamine, diethanolamine and triethanolamine by rapide mixing method. Chem. Eng. J. 1977, 13:7.

[5] Kucka, L., Muleer, I., Kenig, E.Y., Gorak, A., On the modelling and simulation of sour gas absorption by aqueous amine solutions. Chem. Eng. Sci. 2033, 58:3571-3578.

[6] Lawal, A., Wang, M., Stephenson, P., Yeung, H., Dynamic modeling of CO2 absorption for post combustion in coal-fired power plants. Fuel 2008, 88(12):2455-2462.

[7] Chen, E., Carbon dioxide absorption into piperazine promoted potassium carbonate using structured packing. Ph. D. thesis, Austin Univesity of Texas 2007.

[8] Dugas, R., Alix, P., Lemaire, E., Broutin, P., Rochelle, G. Absorber model for CO2 by monoethanolamine - application to CASTOR pilot results. Energy Procedia 2009, 1:103-107.

[9] Freguia, S., Rochelle, G.T., Modeling of CO2 capture by aqueous monoethanolamine. AIChE J.2003, 49(7):1676-1686

[10] Abu-Zahra, M.R.M., Niederer, J.P.M., Feron, P.H.M., Versteeq, G.F., CO2 capture from power plants : Part II. A parametric study of the economical performance based on mono-ethanolamine. Int. J. ofGreen h. Gas Cont. 2007, 1:135-142.

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