Scholarly article on topic 'Kinetic study on promoted potassium carbonate solutions for CO2 capture from flue gas'

Kinetic study on promoted potassium carbonate solutions for CO2 capture from flue gas Academic research paper on "Chemical sciences"

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{"Post-combustion CO2 capture" / MEA / "Potassium carbonate" / Piperazine / Absorption / Desorption / Rates / Capacity}

Abstract of research paper on Chemical sciences, author of scientific article — P. Behr, A. Maun, K. Deutgen, A. Tunnat, G. Oeljeklaus, et al.

Abstract Two different reactors were used to gather information about the kinetics of promoted potassium carbonate solutions for CO2 capture from flue gas. A bubble reactor was used to enable the fast evaluation of a wide rang of different solutions under a defined variation of experimental parameters as temperature and gas flow rate. Apart from the documentation of the CO2 absorption rate the bubble reactor also provided the possibility of a simultaneous documentation of the pH-value of the evaluated solution during the absorption process. These experiments underlined that the pH-value can be used as an indicator for the loading of a known solution during a absorption process. Secondly a wetted wall column reactor was used to obtain kinetic data of gas molecules on a clearly defined liquid surface. A variation of the experimental parameters as pressure, temperature and composition of the phases enables a process -steering in and out of chemical equilibrium.

Academic research paper on topic "Kinetic study on promoted potassium carbonate solutions for CO2 capture from flue gas"

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Energy Procedía 4 (2011) 85-92

Energy Procedía

www.elsevier.com/locate/procedia

GHGT-10

Kinetic study on promoted potassium carbonate solutions for CO2 capture from flue gas

P. Behr1*, A. Maun, K. Deutgen, A. Tunnat, G. Oeljeklaus, K. Görner

Chair of Environmental Process Engineering and Plant Design Universität Duisburg-Essen, Leimkugelstraße 10, 45141 Essen

Abstract

Two different reactors were used to gather information about the kinetics of promoted potassium carbonate solutions for CO2 capture from flue gas. A bubble reactor was used to enable the fast evaluation of a wide rang of different solutions under a defined variation of experimental parameters as temperature and gas flow rate. Apart from the documentation of the CO2 absorption rate the bubble reactor also provided the possibility of a simultaneous documentation of the pH-value of the evaluated solution during the absorption process. These experiments underlined that the pH-value can be used as an indicator for the loading of a known solution during a absorption process. Secondly a wetted wall column reactor was used to obtain kinetic data of gas molecules on a clearly defined liquid surface. A variation of the experimental parameters as pressure, temperature and composition of the phases enables a process-steering in and out of chemical equilibrium.

© 2010 Published by Elsevier Ltd.

Keywords: Post-combustion CO2 capture; MEA; Potassium carbonate; Piperazine; Absorption; Desorption; Rates; Capacity

1. Introduction

Despite the growing recognition of the need to reduce CO2 emissions to mitigate the risks associated with climate change, Carbon dioxide emissions continue to rise due to the worldwide demand for the use of fossil-fuelled power plants. Because our current state of technology does not yet provide any alternative source of such amounts of sustainable energy, the adoption of carbon dioxide capture and storage (CCS) technologies is increasingly considered to be a potentially significant contributor to the energy infrastructure.

* Corresponding author. Tel.: +49-201-1837512; fax: +49-201-1837513. E-mail address: peter.behr@uni-due.de

doi:10.1016/j.egypro.2011.01.027

We have to face that there are changes required to stabilize atmospheric carbon dioxide concentration. Therefore, the efficient treatment of CO2 containing waste gases is becoming a significantly important work step.

Regarding large point sources, like fossil fuel fired power plants, capture by absorption methods provides the most economical solution to separate CO2 from bulk gas streams. The chemical or physical absorption of CO2 into solvents is a well developed technology. It has been applied to numerous commercial processes, including gas treating and ammonia production [1]. A lot of solvents have been applied to gas treating, but the most effective are generally considered to be aqueous amines or hot potassium carbonate (hotpot) solvents. Amines have an advantage over the hotpot process. The absorption rate of CO2 by amines is fast, however, the heat of absorption is also high [2]. In contrast, absorption into potassium carbonate solutions has a relatively low heat of absorption, similar to physical solvents, but is limited by slow absorption rates [3].

To achieve high CO2 absorption kinetics and to maintain the low energy demand required by potassium carbonate solutions simultaneously, the LUAT is particularly investigating an array of various additives. The lab-scale experimental research was carried out within a national funded research project supported by the German Government and german power companies within the COORETEC-project as well as additional suppliers.

A bubble reactor and a wetted wall column reactor were used for gathering equilibrium and rate data on various promoted potassium carbonate solutions. Addressing the rate of absorption completes the understanding of overall solvent performance. Research on the vapour-liquid equilibrium of CO2 in the solvent defines capacity and heats of absorption. In promoted K2CO3 systems, the impact of high ionic strength on equilibriums is largely unknown.

2 Experimental set up

2.1 Bubble column reactor for absorption measurements

The test equipment allows a fast evaluation of different sorbents. Fig. 1 shows an outline of the experimental set

Figure 1: Experimental set up of the bubble column reactor

The thermostattable absorber / desorber is equivalent to a half continuous bubble column reactor. In this reactor the gas mixture flows through the examined sorbents. At the end of the reactor a non-dispersive IR gas analyzer is used to determine the CO2-concentration after the absorption- or desorption-process. An upstream gas cooler avoids cross-axis sensitivities and disturbances by water vapour.

In addition to that the experimental setup contains a continuous pH value recording. CO2-concentrations and pH-values are recorded simultaneously, this provides the possibility to evaluate the correlation between these two values.

2.2 Wetted wall column

A wetted wall column reactor is used to study the interaction of gas molecules on a clearly defined liquid surface. A scheme of the overall wetted wall column is shown in Figure 2a. Figure 2b shows a more detailed picture of the reaction chamber.

Figure 2a: Experimental set up of the wetted wall column reactor

Figure 2b: Wetted wall column reaction chamber

Carbon dioxide is mixed with N2, O2 and different trace gases using mass flow controllers to create a simulated flue gas of defined concentration. The gas is heated up to the preset experimental temperature before entering the wetted wall column reaction chamber. In the chamber the gas countercurrently contacts the falling liquid film on the surface of the stainless steel rod. CO2 is, depending on the experimental parameters, either absorbed or desorbed into the gaseous phase.

A continuous and simultaneous documentation of the different gas components (CO2, O2, H2O, NH3, SO2, NO2, NO, HCl and HF) at the entry and exit of the reactor is realized with a FTIR-spectrometer. A variation of the experimental parameters as pressure, temperature and composition of the phases enables a process-steering in and out of chemical equilibrium.

At chemical equilibrium parameters as gas-solubility, absorption- and desorption-enthalpy and equilibrium-constants are evaluable. Out of chemical equilibrium we are mostly able to examine mass- and heat-transfer processes and reaction-kinetics in the phase-boundary and in the condensed phase.

3. Data measured using the bubble column reactor

This table shows an overview of the different evaluated solutions, additives and used parameters over the different experiments

Chemicals Additives Parameters

Bicarbonate Organic polymers Temperature

Sodium -carbonate Monoamines 30 -90 °C

Potassium -carbonate Diamines CO2-concentration

Calcium -carbonate Triamines 2 - 14 vol.-%

MEA Piperazine Sorbent-concentration

DEA Cyclene 0,1 -4,9 mol/l

DGA Polyols Gas flow rate

Sodium hydroxide 40 - 80 l/h

Potassium hydroxide Volume of reactionchamber

Potassium acetate 20 -100 ml

Ammoniumacetate Gas inlet

Natriumphosphate Vitreous frit, Impinger

Ionic liquids

Alanine

Glycine

Arginine

Among all gathered data, the combined use of amines with organic polymers to promote the potassium carbonate solutions showed the most promising results.

Figure 3: Separation rates of 1M potassium carbonate solutions with different additive combinations plotted against the time

Figure 3 summarizes some important experimental results. Displayed are the time dependent separation rates of CO2 after passing the sorbent. The integrated areas under the plotted lines are equal to the total amount of absorbed CO2. Within the evaluated amines piperazine showed the highest initial separation rates. It is assumed that this occurs due to a fast parallel reaction with the amine in combination with the mutual base catalysis of the potassium carbonate and amine.

The use of an organic polymer as a single additive showed slightly enhanced initial separation rates, but no increase of the saturation. Therefore we assume that the organic polymer only enhances the mass transfer into the liquid phase without being directly involved in a chemical absorption reaction.

The most promising results were gained by using a combination of piperazine and organic polymers to promote the potassium carbonate solutions. Due to the different mechanisms of enhancement of the separation rate -chemical for the piperazine and physical for the organic polymer - very high separation rates can be achieved with the combination of both additives. This also underlines the fact that the different mechanisms do not effect each other and that the combination of different promoting additives can lead to an even better enhancement of the solution's performance than the use of a single additive.

The following diagrams sum up some experimental data gathered by the simultaneous recording of pH-value and CO2 concentration with the bubble reactor. Displayed are the recorded pH-Values depending on the absorbed amount of CO2 expressed by mole CO2 per mol sorbent.

Mole Fraction [mole/mole]

Figure 4: pH-values of potassium carbonate solutions with different amounts of piperazine plotted against the mole fraction (mole CO2 / mole solvent)

The slightly steeper decrease of the pH-value of the potassium carbonate solution with only 7 weight percent of piperazine shows that the absorption process of CO2 is dominated by the reaction of the gas with the hydroxide ions in the solution. Whereas the higher amounts of piperazine cause a flatter decrease of the pH-value. This flatter decrease indicates the fast carbamate parallel reaction of the gas with the piperazine in the solution. Figure 9 also underlines that higher amounts of piperazine lead to a smaller pH-range with a similar amount of absorbed gas than lower amounts of the additive.

Mole Fraction [mole/mole]

Figure 5: pH-values of potassium carbonate solutions with different amines plotted against the mole fraction (mole CO2 / mole solvent)

Comparing MEA and piperazine with the tertiary amine MDEA, it becomes obvious that its pH-value decrease is even steeper. This occurs due to the fact that the tertiary amine does not directly react with the absorbed gas. A similar gradient was also recorded with the organic polymer with a molar mass of 400 g/mole.

Figure 10 also shows that the potassium carbonate solution with piperazine, in contrast to MEA and MDEA, absorbs a high amount of CO2 within a smaller pH-range than the other two solutions.

Further experiments, using different combinations of MEA and the organic polymer 400, also showed promising results. An even higher absorption rate than with the addition of MEA only could be achieved. A similar effect, but with a slightly lower absorption rate, was also achieved by using MDEA in combination with MEA. This illustrates that the different additives enhance different reactive steps within the absorption process. This leads to the point that a better understanding and a directed promotion of these steps may lead to even better performances of absorption solutions.

4. Data measured using the wetted wall column reactor

In the next section the most promising results concerning piperazine and some other non-cyclic di- and triamines are compared to some different MEA-solutions. Within these data we will focus here only on experiments using the following parameters.

T emperature 30 °C

CO2 -concentration 14 %

gas-flowrate 250 l/h

sorbent-flowrate 40 l/h

reactionchamber 130 ml

o cL c o

= = 25 o .2

ï £ <u <U o m

<5 n o R

1 M MEA

-S:—

3000 4000

Time [s]

Figure 6: Results MEA 1mole/l

The ordinate of figure 6 contains: volume percent of CO2, O2, H2O, and mg/m3 of NH3. The absorbed amount of CO2 is equivalent to the CO2-difference between raw gas and clean gas. The CO2-absorption in a 1mole/l MEA-solution can be described with two different kinetics; the fairly fast carbamat formation up to 1400s and the rather slow formation of bicarbonate. Due to the increasing saturation of the solvent the absorption-rate of CO2 decreases whereas the desorption-rate increases along the absorption-process. The steam saturation of about 2,5 volume percent H2O occurs within the first few seconds. Furthermore the release of NH3, due to the degradation of MEA, at the beginning of the absorption process was recorded.

cn —

c= 25 o o

■18 !

<v <U U tn

o J 15 ° 8 £ 10 z

1 M K2C O3 + 30% P z (0,48 M)

IWiA .

rfl/vVr1!

3000 4000

Time [s]

Figure 7: Results of a 1M K2CO3 solution with 30 weight percent piperazine

The direct comparison between the MEA- and the piperazine promoted carbonate-solution provides some qualitative information about the different solutions. The piperazine promoted carbonate-solution shows a higher initial absorptionrate than the MEA-solution. The saturation of the carbonate-solution shows, just as expected due to

the reaction order, an exponential trend. The integral ofthe separation rate of CO2 shows that this solution provides a higher loading of CO2. The release of NH3, due to a degradation of piperazine, was not observed at 30°C.

Furthermore the adoption of the characteristic mass transfer rate enables a direct quantitative comparison of the different sorbents. It is defined as the absorbed amount of gas per temperature [K], pressure [bar] and area unit[m2]. This characteristic also is a typical number for the dimensioning of absorption columns.

Figure 8: Mass transfer rates of different carbonate + additive combinations in comparison to two MEA solutions. Recorded values refer to the unloaded solutions at the beginning of the absorption process. Percent values show weight percent referring to the amount of potassium carbonate.

The comparison of the different solution shows that the promoting amines significantly increase the mass transfer rate in respect o f the amount o f added amine. Within the added amines piperazine shows the most promising enhancement of the mass transfer rate; a solution of 1 M K2CO3 with 30 mass percent of piperazine shows a similar performance as a 4.9 M MEA-solution.

4. Conclusions

To achieve high CO2 absorption kinetics using potassium carbonate the most promising results were obtained by using a combination of piperazine and organic polymers. Assuming different mechanisms of enhancement the experiments indicate that the different mechanisms do not significantly effect each other and that the combination of different additives lead to an even better enhancement of the solution's performance than the use of a single additive

References:

1. Kirk-Othmer Encyclopedia of Chemical Technology, 27 Volume Set, 5th Edition

2. S. Bishnoi and G. T. Rochelle, "Absorption of Carbon Dioxide into Aqueous Piperazine: Reaction Kinetics, Mass Transfer and Solubility." Chem. Engr. Sci 55: 5531-5543, 2000

3. J.T. Cullinane, "Thermodynamics and Kinetics of aqueous piperazine with potassium carbonate for carbon dioxide absorption. Chemical Engineering, Austin, TX, The University of Texas at Austin: 295, 2005