An electrochemically-mediated gas separation process for carbon abatement

Transcription

1 Avilble online t Energy Procedi 37 (2013 ) GHGT-11 An electrochemiclly-medited gs seprtion process for crbon btement Michel C. Stern, Fritz Simeon, Howrd Herzog b, T. Aln Htton *, Deprtment of Chemicl Engineering, Msschusetts Institute of Technology, 77 Msschusetts Ave., Cmbridge, MA 02139, USA b MIT Energy Inititive, Msschusetts Institute of Technology, 77 Msschusetts Ave., Cmbridge, MA 02139, USA Abstrct This work describes promising lterntive to conventionl therml processes for bsorber/desorber processing of for removl of CO 2 from flue gs strems t fossil fuel fired power plnts. Our electrochemiclly-medited mine regenertion (EMAR) process offers the dvntges of n electricl system coupled with the desirble high output purities typicl of mine sorbents tht re difficult to chieve with most electric systems such s pressure-swing sorption, membrne seprtion, nd oxy-fuel combustion. Preliminry experimentl results re presented tht demonstrte the fesibility of using ethylenedimine s the CO 2 sorbent nd copper electro-cycling to isothermlly modulte the mine ffinity for CO 2. Cupric ions entirely dectivte ethylenedimine t rtio of 2:1 dimine to copper. Open-circuit potentil mesurements t 50 o C indicte the required energy to desorb CO 2 nd regenerte the ethylenedimine is 18 kj/mole CO 2 under open-circuit conditions. Kinetic over-potentils re sufficiently low to ensure cceptble energy losses. Lower energies cn be chieved by incresing the temperture or by chnging the mine The Authors. Published by by Elsevier Ltd. Ltd. Selection nd/or peer-review under responsibility of GHGT of GHGT Keywords: Crbon cpture; mine scrubbing; electrochemistry; ethylenedimine 1. Introduction The compelling evidence for the role of nthropogenic crbon dioxide (CO 2 tmosphere s primry cuse of globl climte chnge tht is mjor thret to globl public helth nd welfre highlights the need for the development nd deployment of crbon btement technologies nd the legisltion to enforce their implementtion [1]. Crbon cpture nd sequestrtion (CCS) from lrge point-source emitters, such s fossil fuel burning power plnts, is potentilly effective strtegy for * Corresponding uthor. Tel.: ; fx: E-mil ddress: thtton@mit.edu The Authors. Published by Elsevier Ltd. Selection nd/or peer-review under responsibility of GHGT doi: /j.egypro

2 Michel C. Stern et l. / Energy Procedi 37 ( 2013 ) crbon mitigtion [2], lthough the high cost of implementtion, which hs been estimted to nerly double the cost of electricity, hs deterred both politicl nd industril entities from supporting CCS [3]. Therml mine scrubbing is the current benchmrk technology for seprting CO 2 from combustion flue gses in CCS systems [4]. CO 2 is bsorbed by mine solutions, usully monoethnolmine in wter, t flue gs tempertures between 40 o C nd 60 o C nd then desorbed in the stripper by low-pressure stem t between 100 o C nd 120 o C [5]. The need for low-pressure stem generlly requires significnt frction of the stem be routed from the stem turbines to the mine stripper. For n existing power plnt, retrofitting requires such high cpitl cost to reconfigure the internls of the plnt tht rebuild is often more economiclly fesible [3]. Alterntive strtegies tht do not rely on low-pressure stem hve been shown to be economiclly superior when considering retrofit to n existing plnt [6]. These processes, which include oxy-fuel combustion nd membrne seprtion, use electric compressors to perform the work of seprtion, thereby voiding the inherently difficult demnds of therml scrubbing, but t the expense of lower output purities of CO 2 [6,7]. This could be significnt drwbck bsed on the downstrem storge processes, since even smll quntities of impurities in the CO 2 outlet cn significntly lter the supercriticl properties of the strem [8]. We hve proposed new strtegy tht combines the dvntges of both electricl nd trditionl scrubbing systems using the process shown schemticlly in Figure 1. The Electrochemiclly-Medited Amine Regenertion (EMAR) process opertes in mnner kin to trditionl mine scrubbing opertions, but elimintes the cross-flow het exchnger nd stripper required in the usul configurtions by replcing them with n electrochemicl cell, where desorption nd regenertion is chieved through the electrochemicl cycling of Lewis cid. The trnsition between copper nd cupric ions is currently the most promising cid cycle for n EMAR process. This process hs both logisticl nd operting dvntges reltive to therml processes for desorption of the CO 2 nd regenertion of the mine sorbents. Much lower len lodings of CO 2 cn be chieved due to the drmticlly fster kinetics of the competitive desorption pproch compred to those possible with therml desorption opertions; thus the hydrulic lod on the scrubber cn be reduced, leding to smller scrubbers. Moreover, since the desorption does not require sweep gs, the entire electrochemicl cell cn be pressurized llowing for elevted desorption pressures tht led to significntly lower compression requirements downstrem. It should be possible to eliminte t lest one compressor in the intercooled compressor trin required to produce CO 2 t the 100 to 150 brs necessry for storge. In this pper, we evlute the potentil utility of the proposed pproch by reporting experimentl results obtined using ethylenedimine (EDA) to demonstrte the bility to desorb CO 2 nd then regenerte the mine isothermlly by electro-cycling between copper metl nd cupric ions. Becuse EDA is strong chelting gent towrds cupric ions [9], CO 2 cn be relesed through the ddition of cupric ions into CO 2 sturted EDA solution. EDA is vible CO 2 sorbent nd hs been investigted previously for therml scrubbing processes [10].

3 1174 Michel C. Stern et l. / Energy Procedi 37 ( 2013 ) Len Outlet Flsh T nk Rich Outlet Absorber Gs Cthode Anode Pump Fig. 1. Process digrm for n EMAR system for CO 2 scrubbing H 2 N H 2 N 2CO 2 Absorber NH 2 NH 2 Len Amines + H 3 N + H 3 N Loded Amines NHCO NHCO 2 Cu 2+ Anode Complexed Amines H 2 N Cu 2CO 2 NH 2 H 2 N NH 2 2+ Cu 2+ Cthode 2. Experimentl methods 2.1. Absorption experiments Fig. 2. Chemicl trnsitions in n EMAR system bsed on EDA nd copper Severl 40 ml solutions contining 1M EDA, 1M potssium nitrte, nd vrying mounts of copper nitrte from 0 to 0.5 molr were prepred in 50 ml round bottom flsks. During the experiments, the solutions were heted to 50 o C nd vigorously stirred. Simulted flue gs (15% CO 2, 85% N 2 ) ws bubbled through the flsk t 66 ml/min for severl hours. The outlet ws smpled every 2 minutes using gs chromtogrph to determine the CO 2 concentrtion. The totl quntity bsorbed ws clculted by subtrcting the mount of CO 2 tht exited the solution from the mount of CO 2 tht ws bubbled into the solution. All reported mesurements were djusted by subtrcting out the quntity of CO 2 bsorbed by 1M potssium nitrte solution without ny mine Open-circuit potentil mesurements Experiments were performed by mesuring the open-circuit potentil between copper wire electrode nd seled Ag/AgCl reference electrode. All solutions contined 1M EDA nd 1M NNO 3 nd vrying mounts of copper nitrte from 0 to 0.5M. The difference in the open-circuit potentil t given copper

4 Michel C. Stern et l. / Energy Procedi 37 ( 2013 ) loding ws clculted by subtrcting the open-circuit potentil of solution tht ws under inert tmosphere from the potentil of solution with the sme copper loding but sturted with CO 2. CO 2 sturtion ws chieved by bubbling pure CO 2 through the solution until the open-circuit potentil reched stedy vlue. All experiments were done t 50 o C Kinetic over-potentil mesurements Experiments using Norml Pulse Voltmmetry (NPV) were performed on model solution (1M EDA, 0.25M Cu(NO 3 ), 0.5M KNO 3, 0.5M NCl). Non-Frdic current ws eliminted through mesurement of the resistnce nd cpcitnce of the system using millisecond-long chronopotentiometic mesurements followed by estimtion of the chrging current s n RC circuit. Copper electrodes were creted through electroplting of cupric ions onto smooth vitreous crbon electrode (dimeter = 3mm) from 1M H 2 SO 4 nd 0.5M CuSO 4 solution t negtive of overpotentil of 100mV nd gentle stirring for 15 minutes. The electrode ws ctivted by ppliction of negtive 200mV overpotentil for 1 minute before n experiment. 3. Results nd Discussion In the process schemtic for the EMAR system shown in Figure 1, crbon dioxide is bsorbed in trditionl bsorption column where it rects with EDA to form the corresponding crbmte; in principle, one mole of EDA will be required per mole of CO 2, in contrst to bsorption with MEA, where two moles of the mine re required per mole of CO 2. The CO 2 -loded mine solution leving the bsorber is fed to the node side of the electrochemicl cell, where cupric ions relesed on oxidtion of copper electrodes compete fvorbly with the CO 2 for the EDA. The displced CO 2 is relesed s gs, nd compressed for subsequent sequestrtion or utiliztion. The Cu 2+ -complexed mine solution is then pssed through the cthode side of the electrochemicl cell, where the copper is reduced nd pltes out on the electrodes, nd the mine is regenerted before being returned to the bsorber. The optiml node desorption pressure depends on the need to reduce the len lodings of the bsorbent to cceptble levels, nd on the voided work of compression. Elsewhere in these conference proceedings, we hve shown tht to include the pressure in the nlysis, the efficiency of the process with respect to reversible system must be used insted of the work of seprtion. Eq. (1) shows the efficiency of n EMAR process s function of the CO 2 binding constnt K CO2 nd the prtil pressures in the flue gs nd the node. EMAR ln ln 1 1 ln 1 P CO2 K CO2 1 K CO2 K CO2 ln (1) The efficiency clculted in Eq. (1) is n incresing function of node prtil pressure, indicting tht n efficient EMAR process should be operted t s high pressure s possible. The mximum chievble efficiency of n EMAR process is shown s function of desorption pressure in Fig. 3 for CO2 flue gs prtil pressure of 0.1 br. This result shows tht, potentilly, thermodynmic efficiencies of over 75% cn be chieved. Kinetic nd Ohmic overpotentils nd other minor losses will, however,

5 1176 Michel C. Stern et l. / Energy Procedi 37 ( 2013 ) reduce the ctul operting efficiency of rel process. For competitive opertion, then, desorption pressures should be bove 10 brs nd the CO 2 binding equilibrium coefficient should not exceed 500. Mximum Achievble Efficiency K = 500 CO P (Br) CO2 Fig. 3. Thermodynmiclly chievble efficiency of n EMAR process bsed on node desorption pressure nd CO 2 binding constnt, K CO2. We evluted the EMAR process through (i) bsorption experiments to show the effect on the CO 2 loding of the ddition of cupric ions to the solution, (ii) mesurements of the open-circuit potentils to enble determintion of the electricl work required to seprte the CO 2 nd to determine the binding constnts, nd (iii) study of the kinetic over-potentils under vrious operting conditions to enble ssessment of potentils needed to sustin desirble current density. Figure 4 shows the CO 2 cpcity per mine functionl group s function of the cupric ion loding per mine functionl group, from which it is evident tht s cupric ions re dded, the CO 2 cpcity of n EDA solution drops by 2 moles CO 2 per mole of Cu 2+. In the bsence of cupric ions, the EDA binds slightly over 0.5 moles CO 2 per mole of mine functionl group, which indictes tht crbmte formtion domintes over bicrbonte formtion. When the cupric ion concentrtion reches hlf the ethylenedimine concentrtion, the CO 2 cpcity of the solution becomes negligible. Thus, we conclude tht the chnge in CO 2 cpcity upon oxidtion of copper electrode is 1 mole CO 2 per mole of electrons [CO 2 ]/[Am] [Cu]/[Am] Fig. 4. Thermodynmiclly chievble efficiency of n EMAR process bsed on node desorption pressure nd CO 2 binding constnt, K CO2.

6 Michel C. Stern et l. / Energy Procedi 37 ( 2013 ) The open-circuit potentil difference between nodic (CO 2 sturted) nd cthodic (inert tmosphere) conditions bsed on n node pressure of 1 tmosphere is given in Figure 5. The potentil is nerly constnt over the entire rnge of copper lodings nd verges 0.19 volts t 50 C. A constnt potentil is expected since the EDA/copper complex does not chnge under the different copper lodings tested or due to the presence of CO 2. UV-visible spectr (dt not shown) confirm tht the complex is consistent under ll tested conditions. The slight drop in potentil is likely due to concentrted solution effects. Open-Circuit Potentil Difference (V) Copper Loding [Cu]/[Am] Fig. 5. Open-circuit potentil difference of EDA/Cu 2+ solution between nodic nd cthodic conditions t 40, 50 nd 60 o C. The work required is the product of the chrge nd voltge nd equls 18.3 kj/mole CO 2 seprted t 50 C. This number is slightly more thn double the reversible vlue required for complete seprtion of 10% CO 2 in nitrogen strem t 50 o C. By operting t higher tempertures, the open-circuit potentil is reduced nd leds to lower energies of seprtion. If the temperture is incresed beyond 75 C, however, the loding of the CO 2 -sturted EDA solution will drop below 0.5 mole CO 2 per mole of mine functionl group nd begin to increse the chrge required for seprtion. The work of seprting one mole of CO 2 from flue gs conditions to pure CO 2 strem t other node pressures cn be obtined from the eqution W CO2 RT ln 1 K CO2 1 K CO2 K CO2 (2) Vlues of the CO 2 binding constnt were determined s function of temperture using mesured opencircuit potentils in the presence of 1 br CO 2 nd in ir using the eqution K CO2 exp F RT V CO 2 V ir (3) where V CO2 nd V ir refer to the open-circuit potentils under CO 2 nd ir tmospheres, respectively. Figure 6, which shows the vrition of K CO2 with temperture, indictes the optiml temperture rnge over which the vlue of K CO2 is between 250 nd 500 runs from bout 63 to 70 degrees Celsius.

7 1178 Michel C. Stern et l. / Energy Procedi 37 ( 2013 ) CO 2 Binding Constnt (K CO2 ) Temperture C) Figure 6: Clculted CO 2 binding constnts bsed on open-circuit potentil mesurements t different tempertures. For the EDA solution to demonstrte sufficiently fcile kinetics, the totl over-potentil should be less thn 50 mv for the node nd cthode combined in the thermodynmiclly desirble temperture rnge. Mesurement of the cthode over-potentils ws done in n ir tmosphere (since the CO 2 should lredy hve been removed before the solution enters the cthode) while node mesurements were done in CO 2 sturted solutions. If the process of copper oxidtion is concerted (the CO 2 leves the mine nd the mine ttcks the copper surfce simultneously), then we would not expect significnt differences in the kinetic over-potentils when operting t CO 2 pressures bove 1 tmosphere. Figure 7 shows the nodic nd cthodic overpotentils required t given temperture to chieve current density of 50 A/m 2. In the thermodynmiclly desirble window of 63 o C to 70 o C, the kinetic over-potentils re below the 50mV gol, which is strongly encourging result. Note tht these results re for solution tht is more dilute (four fold so) thn the hypotheticl working solution, nd thus overestimtes the needed overpotentil. Conversely, the trnsport limittion encountered during these short pulses (0.1 to 0.2 seconds) is less thn wht might be encountered in flow cell. Overpotentil (mv) Cthode Totl Anode Te mperture C) Figure 7: Kinetic overpotentils of the node, cthode, nd combined s function of temperture.

8 Michel C. Stern et l. / Energy Procedi 37 ( 2013 ) Conclusion Electrochemiclly-medited mine regenertion is new strtegy with the potentil to drsticlly reduce the implementtion costs of post-combustion CO 2 bsorption seprtions, without the need for intrusive retrofitting of n existing power plnt to extrct low-pressure stem for the stripper. The process hs the dded dvntge tht the stripper will provide considerbly lener solvent feed to n bsorber, reducing the required bsorber size. It hs been shown tht mines such s ethylenedimine (EDA) relese cptured CO 2 when they form soluble complexes with cupric ions in solution, nd cn be regenerted on reduction of the copper ions. Preliminry experimentl results show the bility to desorb CO 2 nd regenerte n ethylenedimine solution t only double the theoreticlly-reversible work. Optimiztion of the chemistry nd operting conditions should led to significntly higher ttinble efficiencies, nd re the subject of ongoing studies. Acknowledgements We grtefully cknowledge finncil support from Siemens CKI reserch fund nd ARPA-E reserch grnt, DE-AR References [1] Prry, M.L.; Cnzini, O.F.; Plutikof, J.P.; vn der Linden, P.J.; nd Hnson, C.E., ed., Climte Chnge 2007: Impcts, Adpttion nd Vulnerbility, Contribution of Working Group II to the Fourth Assessment Report of the Intergovernmentl Pnel on Climte Chnge, Cmbridge University Press,ISBN (pb: ). [2] Metz, B.; Dvidson, O.R.; Bosch, P.R.; Dve, R.; nd Meyer, L.A., ed., Climte Chnge 2007: Mitigtion of Climte Chnge, Contribution of Working Group III to the Fourth Assessment Report of the Intergovernmentl Pnel on Climte Chnge, Cmbridge University Press, ISBN (pb: ). [3] Ktzer J, Moniz E, Deutch J, Ansolbehere, S. Beer, J. et l. The future of col: n interdisciplinry MIT study. Cmbridge, MA, Msschusetts Institute of Technology Avilble t: [4] Rochelle GT. Amine scrubbing for CO2 cpture. Science. 2009;325: [5] Kothndrmn A., Nord L., Bollnd O., Herzog H.J., McRe G.J. Comprison of solvents for post-combustion cpture of CO 2 by chemicl bsorption. Energy Procedi. 2009;1: [6] Singh D, Croiset E, Dougls PL, Dougls MA. Techno-economic study of CO2 cpture from n existing col-fired power plnt: MEA scrubbing vs. O2/CO2 recycle combustion. Energy Conversion nd Mngement. 2003;44(19): [7] Angewndte Chemie (Interntionl ed. in English). 2010;49(35): [8] Seevm PN, Rce JM, Downie MJ, Hopkins P. Trnsporting the Next Genertion of CO2 for Crbon, Cpture nd Storge : The Impct of Impurities on Supercriticl CO 2 Pipelines. Proceedings of IPC :1 13. [9] McIntyre GH, Block BP, Fernelius WC. A Thermodynmic Study of Some Metl-Amine Coördintion Compounds. Journl of the Americn Chemicl Society. 1959;81(3): [10] Zhou S, Chen X, Nguyen T, Voice AK, Rochelle GT. Aqueous ethylenedimine for CO 2 cpture. ChemSusChem. 2010;3(8):913 8.