Technical-economic and [628927]

Absorber design for the improvement of the ef ficiency of post-
combustion CO 2capture
Cristian Dincaa,*, Adrian Badeaa,b, Laurentiu Stoicaa, Adrian Pascua
aThe Power Plant Department, Politehnica University of Bucharest, Bucharest 060042, Romania
bThe Romanian Academy of Scientists, Bucharest 050094, Romania
Keywords:
CO2chemical absorption
Absorber unit designPost-combustion CO
2capture
CFBC pilot installationabstract
The objective of this paper is to design the absorber unit in order to separate the CO 2of the flue gases
resulted from coal combustion. The design model was made for various types of packages using plastic,
ceramic or metal rings. It was also considered that the capture process ef ficiency was 85%, and the
monoethanolamine concentration in the solution varied between 10 and 30%. We analyzed the oldestand the newest types of package rings (Raschig rings, ceramic Berl saddle rings), as well as various types
of metal and plastic Pall rings. In the model developed in this paper, the physical properties of package
materials were taken into account (density, surface area, void fraction, etc). By applying the proposedmodel, we obtained results concerning the in fluence of the L/Gratio on the CO
2capture ef ficiency, the
rich and the lean loading solvent, the solvent flow pumped through the absorber unit, the type and the
size of package (height, diameter, etc). The objective of the paper is to compare the experimental resultsobtained in the CFBC (Circulating Fluidized Bed) pilot installation with those obtained in the mathe-
matical model.
©2014 Energy Institute. Published by Elsevier Ltd. All rights reserved.
1. Introduction
The aim of the present paper is to analyze the CO 2capture process by chemical absorption in order to reduce the amount of thermal
energy (heat) required by the regeneration of chemical solvents (amines, blended amines, piperazine injection, etc). In the present paper we
analyzed only monoethanolamine.
The chemical absorption process is nowadays one of the most promising and viable technologies, taking into account its integration in
the new power plants, but especially in the existing coal power plants [18]. Many research studies focus on the chemical absorption process
in order to minimize its integration in energy or non-energy processes [14,16] . However, the CO 2capture process by chemical absorption has
a major drawback which consists in the thermal energy required by the chemical solvents regeneration. In the power plants with CO 2
capture by chemical absorption, in order to regenerate solvents, a part of the steam flow is extracted from a fixed outlet fitted on the low
pressure steam turbine. Taking into account the extracted steam pressure as well as the extracted steam flow, the thermal power plant
efficiency is reduced from 45% (for a thermal power plant with supercritical parameters using hard coal) to about 30% [5]. Therefore, many
papers in scienti fic literature have analyzed different solutions for the reduction of the quantity of heat required by the chemical solvents
regeneration such as: two or three stages for solvent regeneration; the use of solar systems to partially ensure the quantity of the required
heat; or the use of new chemical solvents. The chemical absorption process of CO 2aims to increase the process ef ficiency and to reduce the
operating costs.
The reduction of the greenhouse gas emissions generated during many energy processes represents one of the main objectives set in the
worldwide energy strategies [11,15,17] . The concerned measures and methods can be divided into two categories: those aiming at the energy
sector development through the integration of renewable energy technologies, including those based on nuclear resources, and those
aiming at the development of a fossil fuel-based energy industry. According to IPCC, the main greenhouse gases are: CO 2,C H 4and N 2O.
*Corresponding author.
E-mail address: crisflor75@yahoo.com (C. Dinca).
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Journal of the Energy Institute
journal homepage: http://www.journals.elsevier .com/journal-of-the-energy-
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http://dx.doi.org/10.1016/j.joei.2014.08.003
1743-9671/ ©2014 Energy Institute. Published by Elsevier Ltd. All rights reserved.Journal of the Energy Institute xxx (2014) 1 e10
Please cite this article in press as: C. Dinca, et al., Absorber design for the improvement of the ef ficiency of post-combustion CO 2capture, Journal
of the Energy Institute (2014), http://dx.doi.org/10.1016/j.joei.2014.08.003

However, the environmental tests performed on different energy installations which use fossil fuel revealed the fact that CO 2emissions have
the most important contribution to the greenhouse effect [8]. Consequently, Fig. 1 shows the main CO 2emission reduction technologies,
these being categorized according to their place of integration into the thermal power plant [6].
Consequently, the main objective of this paper is to model the CO 2chemical absorption by using various types of rings (Raschig, Pall, etc)
in order to dimension the absorber unit.
The usage of both programming languages and of specialized software in process modeling makes possible a detailed analysis of
chemical processes in both the industrial and the energy sector. Whenever chemical processes are to be designed and optimized, these
instruments become indispensable because they offer the necessary information about how to develop these processes. In the present study
we made experimental tests on several types of rings with the purpose of identifying the latter's behavior within CO 2absorption processes
which use amines. In addition, we present the simpli fied mathematical model for a package column used in the CO 2absorption processes
which use monoethanolamine (MEA); the results were compared with those obtained experimentally. This research is a part of an ambitious
project which envisages to design and to optimize a thermo-electrical plant with post-combustion CO 2capture.
2. Mathematical model for the design of the absorber unit
In order to dimension the absorber unit, it is necessary to calculate the carbon dioxide solubility in the absorbent used (amine and
piperazyne, etc) at a certain temperature of the chemical absorption process corresponding to a stated equilibrium. The determination of the
ratio between the molar content of the carbon dioxide in gaseous and liquid phases allows one to establish the operating line of the absorber
unit and the minimum value of ( L/G)minratio. Fig. 2 shows the fraction between the molar ratio of CO 2in gaseous and liquid phases as well as
the expulsive force given by the difference between the concentrations of CO 2in the two stages at the inlet and the outlet of the absorber
unit, respectively.
Fig. 1. The main ways to reduce the CO 2emissions generated in the combustion process.
Fig. 2. The operating and the equilibrium line in an absorber unit.C. Dinca et al. / Journal of the Energy Institute xxx (2014) 1 e10 2
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The mathematical model is based on the rate-based model [1,2,3,7,9] . Knowing the value of the mass flow rate (or the volume flow rate)
of the flue gases, based on the ratios de fining the lignite combustion process, the required solvent flow was determined by formula
(L/G)operating . In order to determine the flow parameter ( FLG) the formula below shall be used:
FLG¼L
G/C18rG
rL/C190:5
(1)
In the Renewable Energy Laboratory of the Faculty of Power Engineering e“Politehnica ”University of Bucharest, a CFBC (circulating
fluidized bed combustion) pilot installation was conceived and designed; it was equipped with post-combustion CO 2capture by chemical
absorption. The lean and the rich loading solvents of the MEA solution were measured at the inlet and at the outlet of the absorber unit,
respectively. Both were determined experimentally and through a mathematical model under various concentrations of the MEA in the
solvent. The analyzed CO 2absorption process is shown in Fig. 3 .
The chemical absorption process of CO 2from the flue gas is based on the use of a solvent which separates CO 2from the other components
of the flue gas following some chemical reactions which take place in the absorber unit. At the outlet of the absorber unit, the chemical
solvent carries a high CO 2burden and, in order for it to be reused, its regeneration is necessary. The solvent regeneration takes place in the
stripper unit by using a high amount of heat which, in the energy processes, comes from the fixed outlet of low pressure steam turbine (the
pressure of the fixed outlet is approximately 2 bar). Fig. 3 presents a solution to reduce thermal energy consumption by the gradual
regeneration of the solvent used. This involves recovering and using a part of the flue gas heat ( Fig. 3 efirst regeneration stage) in order to
heat the solvent until it reaches a temperature which is as close as possible to the one required for the CO 2separation.
Table 1 shows the dry gas composition starting from the elementary composition of the used lignite.
2.1. Mass balances on the CO 2absorber unit
The mass balance was made per absorber unit, supposing that the process takes place at a constant temperature and pressure. Firstly, in
order to determine the molar flow of the flue gas at the inlet and at the outlet of the absorber unit, respectively, the ideal gas law: pV¼nRT
Fig. 3. The chemical absorption process for CO 2capture.C. Dinca et al. / Journal of the Energy Institute xxx (2014) 1 e10 3
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was used. Also, the amine molar flow rate has been determined taking into account the fact that in the absorption process, mono-
ethanolamine was used in a mass concentration in the solvent which varied between 15 and 35%. Starting from a known value of the ef-
ficiency of the CO 2capture process, the CO 2burden of the rich/lean solution was determined in mol CO 2/mol MEA.
Equation (2)was used in order to determine the molar flow rate of the flue gas ( nfg):
nfg¼pfg*Vfg
Rfg$Tfg(2)
where pfgetheflue gas pressure was considered as atmospheric pressure 1.013 /C2105N/m2;
Vfgerepresents the flue gas volume resulted from the combustion process, m3/s;
Rfgerepresents the perfect gas constant, 0.08205 atm/(mol K);
Tfgethe absolute temperature of the flue gas, K.
Formula (3) was used in order to determine the molar flow rate of CO 2in the flue gas before the flue gas flow enters the absorber unit (see
Table 1 ):
nbottom
CO2¼CCO2$nfg (3)
where: CCO2erepresents the content in the flue gases at the inlet in the absorber unit, in %.
The molar flow rate of the flue gas containing other gases besides CO 2,ðnogȚwas determined by calculating the difference between nfg
and nCO2Knowing the ef ficiency of the CO 2absorption process ( ε), the CO 2concentration in flue gas at the outlet of the absorber unit was
determined by using the following equation:
Cout
CO2¼CCO2$/C16
1/C0ε/C17
(4)
The Equation (5)was used to determine the number of CO 2moles, nabs
CO2, absorbed by the solvent used (monoethanolamine in different
concentrations):
nabs
CO2¼nbottom
CO2/C0/C16
nfg/C0nog/C17
(5)
where: nog¼nfg/C0nbottom
CO2.
The number of moles in the flue gases at the inlet of the absorber unit was determined by using Equation (6):
ntop
CO2¼nog
1/C0Cout
CO2(6)
Taking into account that the solvent used was monoethanolamine (MEA), the loading was determined as being the difference between
the rich loading at the bottom of the absorber unit ðxbottom Țand the lean loading ðxtopȚat the top of the absorber unit as in Equation (7):
xabs¼xbottom /C0xtop (7)
The rich and the lean loading were measured in five points indicated in Fig. 3 : the absorption unit output (point A), inside the tanker
“MEA rich ”(point B), the heat exchanger output (point C), after the electrical re-boiler (point D) and inside the tanker “MEA lean ”(point E).
Table 2 shows the values of CO 2loading in the MEA solution at the points indicated in Fig. 3 .
In this paper, the carbon dioxide loading was measured with a total organic carbon (Shimadzu TOC-5050A TOC) analyzer. The contained
inorganic carbon was determined by using the TOC-5050 analyzer. The method used to determine the contained inorganic carbon consists
in injecting a diluted solution (8 e10ml) in a phosphoric acid of 1% H 3PO4.concentration.
The CFBC pilot installation is equipped with an emissions monitoring system at different points of the installation. In consequence,
different components of the flue gases could be measured (NO x,S O x, CO, etc) in order to determine the in fluence that some solvents could
have on them.Table 1
Analysis of the lignite combustion process.
Elementary composition of lignite
Ci, [%] Hi, [%] Si, [%] Oi, [%] Ni, [%] Wi, [%] Ai, [%] LHVa, [kJ/kg]
21.55 1.25 1 2.55 0.65 36 37 7542.59
Flue gases composition
Dry condition Wet condition
CO2, [%] 11.626 9.94
SO2, [%] 0.202 0.173
N2, [%] 80.179 68.55
O2, [%] 7.993 6.833
H2O, [%] e 14.504
aLHVelow heating value of lignite.C. Dinca et al. / Journal of the Energy Institute xxx (2014) 1 e10 4
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In the proposed model, it was considered that at the inlet of the absorber unit the solvent is always pure, xtop¼0 kmol =kmol.
The CO 2concentration in the solvent at the top of the absorber unit is determined according to the balance equation knowing the values
for the CO 2concentration in the flue gases at the inlet and outlet of the absorber unit, respectively, gbottom , and gtop:
gbottom ¼nbottom
CO2
nfg(8)
gtop¼nbottom
CO2/C0nabs
CO2
nfg(9)
The mass balance equation per absorption unit can be written as follows:
Lm$/C0xbottom /C0xtop/C1¼Gm$/C0gbottom /C0gtop/C1(10)
For the absorber/stripper units belonging to CO 2capture processes, ratio ðLm=GmȚminis considered to be 0.7. Ratio ðLm=GmȚoperating fall
onto the range (1.1 e1.5). In the present research, it was considered that ðLm=GmȚoperating ¼1:1$ðLm=GmȚmin[13].
In order to determine the diameter of absorption unit ( Dabs) the following equation was used:
Dabs¼/C18
Aabs$4
p/C190:5
(11)
where: Aabserepresents the absorber section in m2.
The absorber section was determined by using Equation (12), being dependent on the speci ficflow of flue gases passing through the
packing ( Gsp):
Aabs¼Gb
Gsp(12)
Gsp¼/C18Fd$rfg$g
m0:2$Fr$j/C190:5
(13)
where: Fdethe dimensional factor is determined by using nomograms (see Fig. 4 ) in accordance with the pressure drop in Dppacking and
the density of the amine-based solution;
rfgerepresents the flue gas density in kg/m3;gegravitational acceleration in m/s2;
methe liquid's viscosity in cP; Frepacking speci fic factor in m2/m3;
jethe ratio between the density of the water and that of the monoethanolamine at the process pressure and temperature.
The package height ðHabsȚwas determined by using Equation (14):
Habs¼Htg$NtG (14)Table 2
Lean and rich loadings at the five points for the 30% MEA concentration in the solvent.
Sample Rich loading MEA solvent ðmol CO 2=mol MEA Ț Lean loading MEA solvent ðmol CO 2=mol MEA Ț
ABC D E
1 0.512 0.597 0.52 0.351 0.413
2 0.514 0.59 0.526 0.348 0.4163 0.521 0.593 0.527 0.344 0.4184 0.513 0.59 0.531 0.341 0.4085 0.524 0.585 0.524 0.347 0.4126 0.506 0.58 0.529 0.353 0.4217 0.498 0.592 0.519 0.342 0.4258 0.514 0.593 0.524 0.339 0.421
9 0.52 0.586 0.526 0.337 0.418
10 0.519 0.597 0.531 0.331 0.41911 0.532 0.588 0.53 0.327 0.42312 0.524 0.591 0.527 0.329 0.42213 0.527 0.584 0.531 0.325 0.4214 0.522 0.586 0.534 0.326 0.42115 0.517 0.591 0.533 0.327 0.427
16 0.513 0.594 0.527 0.331 0.422
17 0.511 0.592 0.532 0.333 0.42618 0.508 0.596 0.536 0.33 0.42519 0.504 0.587 0.537 0.317 0.42820 0.514 0.591 0.527 0.322 0.421Average 0.516 0.59 0.529 0.335 0.42C. Dinca et al. / Journal of the Energy Institute xxx (2014) 1 e10 5
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Htg¼GMy
FG$a(15)
NtG¼Zyn
y0interp ðvs;y;fa;QȚdQ (16)
where: GMyespeci fic molar flow rate of solution taking into account the package area, in mol/(m2s);vs¼cspline ðy;faȚecubic interpolation
function where fa¼½1=y/C0yint/C138/C131/C131/C131/C131/C131/C131/C131 !. At the beginning of the iteration the value for yintwas 0.
For a 30% MEA concentration in the solution and for a 40/C14C temperature, the balance curve of the CO 2concentration in the flue gases was
determined between the inlet and the outlet of the absorber unit ( Fig. 4 ).
In the proposed mathematical model, the initial value of the CO 2concentration in the solution at the inlet of the absorption unit was
chosen to be zero because when the balance is reached, the value of this parameter will reach 0.18 mol CO 2/mol MEA.
2.2. Package material data
In this paper various types of package materials were analyzed in order to determine their in fluence on the ef ficiency of the CO 2capture
process, as well as that on the absorber unit dimensions. The types of packages and their geometric characteristics (dimensions, package
area and void factor, etc) are shown in Table 3 .
The study also considered the modi fication of the monoethanolamine concentration in the solvent, and, to this purpose, the following
variants were analyzed: 15%; 20%; 25%; 30% and 35% wt. The indicators CL,CVvary with the package material as reported in specialized
literature [12].
Fig. 4. The operating line of the absorber unit at a 30% wt. MEA in the solution and 40/C14C temperature.
Table 3
Package material data used for testing the absorption process.
Package type Size (mm) Area (m2/m3) Fp (ft2/ft3) CL CV Reference
Pall rings ceramic 25 58 179 1.361 0.412 [4]
Pall rings ceramic 15 111 580 1.276 0.401 [10]
Pall rings plastic 25 63 55 0.905 0.446 [12]
Pall rings metal 25 63 56 1.44 0.336 [19]
Berl saddle ceramic 13 142 240 1.364 0.232 [12]
Berl saddle ceramic 25 76 110 1.246 0.387 [12]C. Dinca et al. / Journal of the Energy Institute xxx (2014) 1 e10 6
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2.3. Mathematical model parametrical study
The proposed mathematical model allowed the evaluation of the absorber unit in case of a monoethanolamine concentration in the
solution of 30% for the process temperature of 40/C14C and 60/C14C, respectively. The results are reported in Fig. 5 .
The operational line of the absorber unit was also determined for various MEA mass concentrations in the solution: 15%, 20%, 25%, 30%
and 35% ( Fig. 6 ). The operational line was determined considering the absorption process temperature of 40/C14C.
3. Discussions
In this paper we analyzed various types of package materials presented in Table 3 . The diameter and the height were determined for each
type of package ( Fig. 7 ).
When calculating the column diameter, the porosity of the materials was considered to be 0.93. However, the in fluence of the porosity on
the column diameter is insigni ficant. The parameters which in fluenced the value of the column diameter are the package void fraction (Fp)
and the speci fic package area. The pressure drop values in the package material were considered to be 0.5 bar for all types of the analyzed
packages. The concentration of MEA in the solution was 30% wt. However, the modi fication of the MEA concentration in the solvent did not
lead to an obvious modi fication of the column diameter ( Fig. 8 ). This situation was pointed out in the case of Raschig rings which have a
13 mm diameter. The modi fication of the MEA concentration in the solution has a powerful impact on the CO 2capture process ef ficiency; the
temperature of the absorption process is considered to be 40/C14C(Fig. 9 ).Fig. 5. The operational and equilibrium line for 30% MEA and absorber temperature of 40/C14C and 60/C14C.
Fig. 6. The operational and equilibrium line for 30% MEA and absorber temperature of 40/C14C and 60/C14C.C. Dinca et al. / Journal of the Energy Institute xxx (2014) 1 e10 7
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of the Energy Institute (2014), http://dx.doi.org/10.1016/j.joei.2014.08.003

Considering the Raschig ceramic rings, it was observed that the modi fication of the MEA concentration in the solvent leads to the
modi fication of the ef ficiency of CO 2chemical absorption, the variation being linear.
The package height for the cases presented in Table 2 was determined for a MEA concentration of 30% in the solvent, for which the
efficiency of the CO 2absorption process was approximately 85% ( Fig. 10 ).
Taking into account that the higher the package is, the higher the investment costs are, we chose to use plastic Pall rings with a 25 mm
diameter and with the lowest height, namely 3.11 m.
Table 4 shows a comparative analysis of the data obtained in the experimental analysis and from the mathematical model.
We could notice that the maximal ef ficiency was obtained for the 30% MEA wt. concentration in the solution. The data obtained from the
mathematical model for the lean and rich loading solvent is not so different from the ones obtained experimentally on the pilot installation,
the values being relatively similar (temperature and pressure of the absorption process, the flow of the solvent through the absorption unit,
the temperature of the cooling water, etc). The obtained results are useful for modeling the absorption unit-desorption unit ensemble.Fig. 7. The column diameter for different package materials used.
Fig. 8. The in fluence of the MEA concentration on the column diameter.
Fig. 9. The in fluence of the MEA concentration on the CO2 capture ef ficiency.C. Dinca et al. / Journal of the Energy Institute xxx (2014) 1 e10 8
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4. Conclusions
The aim of the article was to test various types of materials in the CO 2absorption column in view of reducing the thermal energy
consumption needed for the amine regeneration. Within the experimental study we tested materials such as Raschig rings, metal and plastic
Pall rings and ceramic Berl saddle rings. The present study focused only on the analysis of the monoethanolamine solvent with the following
concentrations: 15, 20 and 30% wt. The study of the chemical absorption process modeling was carried out only for the variant of 30% MEA in
the solvent, and thus the highest value of the capture process was obtained, namely 85%.
It was noticed that when Raschig ceramic rings were used, the quantity of required thermal energy was minimal, with a value of 3.1 GJ/
ton CO 2. This is due to a higher height of the absorption column in comparison with other types of packages. On the other hand, the MEA
consumption used for approximately the same ef ficiency of the CO 2capture process was lower in the case of Raschig ceramic rings, 3.5 mol
liquid/mol MEA.
In the mathematical model, both the ef ficiency of the CO 2capture process (85%) and the MEA concentration in the solvent (30%) were
kept constant. In this case, it was found that, in order to reduce the investment costs, the use of plastic Pall rings is recommended.
The value of the absorption column is approximately 90 mm, while the value of the column height is approximately 3.1 m. On the other
hand, there are the ceramic Raschig rings for which the diameter of the absorption column is 100 mm and the height is 4.6 m.
Acknowledgments
The study has been funded by the UEFISCDI within the National Project number 38/2012 with the title: “Technical-economic and
environmental optimization of CCS technologies integration in power plants based on solid fossil fuel and renewable energy sources
(biomass) ”eCARBOTECH. The work has also been funded by the Sectorial Operational Program Human Resources Development 2007 e2013
of the Ministry of European Funds through the Financial Agreement POSDRU/159/1.5/S/134398.
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Parameter Experimental analysis Mathematical model
MEA concentration in solvent, [%] 15 20 30 15 20 30
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CO2capture process ef ficiency, [%] 65 79 85 68 81 86
L/Gratio, [mol liquid/mol gas] 1.7 2.4 3.5 1.89 2.25 3.37
Structure of the absorber package Metal Pall ring, 25 mmEnergy requirement, [GJ/tonCO
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CO2capture process ef ficiency, [%] 61 74 85 60 77 83
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CO2capture process ef ficiency, [%] 62 76 83 61 79 82
L/Gratio, [mol liquid/mol gas] 1.9 2.5 3.7 1.92 2.31 3.4
aThe stripper unit is not analyzed in this paper.Fig. 10. The column height according to the package type.C. Dinca et al. / Journal of the Energy Institute xxx (2014) 1 e10 9
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of the Energy Institute (2014), http://dx.doi.org/10.1016/j.joei.2014.08.003

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Please cite this article in press as: C. Dinca, et al., Absorber design for the improvement of the ef ficiency of post-combustion CO 2capture, Journal
of the Energy Institute (2014), http://dx.doi.org/10.1016/j.joei.2014.08.003