World Resource Review Vol. 16 No.2 CAPTURING CARBON DIOXIDE DIRECTLY FROM THE ATMOSPHERE Frank S. Zemanand Klaus S. Lackner Departmentof Earth and EnvironmentalEngineering Columbia University 918 Mudd, MC 4711 500 West 120dtStreet New York, NY, 10027 USA Email: [email protected] Keywords: Air extraction,COb sequestration,climate change,kraft process SUMMARY The increasingconcern over rising CO2levels in the atmosphereand their potentialclimate effectsis fuelling researchaimed at carbonmanagement. One areaof researchfocuses on capturingthe CO2after combustionand sequesteringit underground.Capture schemes would operateat the site of generationtaking advantageof the elevatedconcentrations of CO2in the effluent. Here, however,we proposean indirectmethod of carboncapture that removes CO2from the atmosphere.This processcombines existing technologies with recenttechnological innovations into a novel carboncapture concept termed air extraction. The processuses dissolved sodium hydroxide to removethe CO2from ambientair. The resultantsodium carbonate solution is causticizedusing calcium hydroxideto regeneratethe sodiumhydroxide solutionand precipitate calcite. The calcite is then thermallydecomposed to producelime and CO2.The lime is hydratedto completethe process.These latter stages are usedroutinely in the paperindustry and the calcinationof limestoneis centralto the cementindustry. This paperreviews the processand highlights pertinentresearch to developa likely cost-effectiveprocess. It is shownthat the proposedprocess is well defmed and technicallyfeasible. The wide parameterspace and potentialfor improvementssuggest that further efficiency improvementsare attainable. The most significantimprovements will be derived from efficient heatmanagement. () 2004 World ResourceReview. All rightsreserved. 157 World Resource Review Vol. 16 No.2 1 INTRODUCTION Recentyears have seenattention drawn to issuessun-ounding changes in the global climate inducedby humanactivities. Concernover these potential changeshas led to the formation of internationalagencies, such as the IntergovernmentalPanel on ClimateChange (IPCC), whose mission is to consider this problem. The IPCC hasstated that carbondioxide (COJ is the greenhouse gascontributing the largestportion of the anthropogenicincrease in global radiative forcing, which the IPCC defmesas an externallyimposed perturbation in the radiative energybudget of the Earth's climate system. AnthropogenicCO2 producesapproximately 62% of the increaseor 1.46 W1m2 out of a total increase of 2.43 W/m2(Houghton and Ding, 2001). The increasein atmosphericCO2 levels can be largely attributedto the combustionof fossil fuels, with a small contributionfrom cementproduction and land usechanges. The total global emissionsreached 6,611 million metric tons of carbonin 2000, which representsa 1.8%increase over 1999(Marland, Bodenand Andres,2003). Giventhe world's economicdependence on fossil fuels andthe expectedincrease in consumption, methodsfor mitigating the atmosphericrelease of CO2need to be developed. The primary targetsfor mitigationare power plantsas theyproduce large concentratedstreams of CO2(Herzog and Drake,1996). Thesesources account for aboutone third of the worldwide CO2emissions. Even aftereliminating all emissionsfrom power plants,the remainingtwo thirds would still be releasedto the atmosphere.Roughly half of all emissionsarise from smalldistributed and oftenmobile sources. Mitigating their CO2impact on the atmosphereis more difficult. One approachis to provide fuels that are carbonfree, which is the reasonmuch emphasisis being placedon hydrogenas a transportationfuel. Here,we proposean alternativemethod for mitigating CO2emissions from sourcesother than powerplants. We proposethat CO2 is removeddirectly from the atmospherein a costeffective, industrialprocess hereafter referred to as air extraction. Air extractioncan be viewed as a variationof flue gasscrubbing where the fluid is at atmospherictemperature and pressure with a CO2concentration of 0.037%. The implementationof the processdiscussed here uses a sodium hydroxide (NaOH)based, alkaline liquid sorbentto removethe CO2from the ambientair by producingdissolved carbonate ions. In orderto recoverthe sodiumhydroxide, the resultantsodium carbonate (NazCO3) solution is mixed with calciumhydroxide (Ca(OH)J to producesodium hydroxide and calcium 0 2004 World ResourceReview. All rightsreserved. 158 World Resource Review Vol. 16 No.2 with calcium hydroxide (Ca(OH)2)to producesodium hydroxide and calcium carbonate(CaCO3) in a reactionknown as causticizing. This reactiontransfers the carbonateanion from the sodiumto the calcium cation. The calcium carbonateprecipitates, leaving behind a regeneratedsodium hydroxide sorbent. The precipitate is dried, washedand thermally decomposedto producelime (CaO). This step. known as calcination, is followed by hydratingthe lime. known as slaking, which completesthe process.The processof recycling of sodiumhydroxide using calcium hydroxide hasbeen in usein the pulp and paper industry since 1884,where it is known as the Kraft Process(Miner and Upton, 2002). This processalso involves the calcinationof limestone,which is at the heart of the cementmanufacturing industry. In short. eachunit processrequired is known to be technicallyfeasible and the only remaining questionssurround their efficienciesand costs. This paperwill presenta re'tiew of the indi~idual components comprisingthe air extractionprocess with a view to highlighting the potential benefitsand concerns. The issuessurrounding their integrationinto a functional air captureS}.stem will also bediscussed. The layout of the paperwill follow the path of the C~ moleculethrough the system. A simplified schematicof the processis shownin Figure 1. Figure 1 Overviewof Air ExtractionProcess. Note that two reactionsare not shown; !hying (d), and hydrating(h). 0 Z~ WorldResourre Review AD rights reserved 159 World ResourccRcyicw Vol. 16 No.2 2 ALKALINE SODIUM SORBENTS The removal of a gaseouscomponent through contactwith a liquid is known as wet scrubbihg. Wet scrubbingcan be divided into processeswhere there is a chemicalreaction between the sorbateand the sorbentand wherethe sorbateis physically dissolvedinto the sorbentsolution. For the air extraction processwe proposean alkaline sodiumsolvent which reactschemically with the entrainedCO2. The chemicalreaction for this processis shownbelow as reaction (1). 2NaOH (aq) + CO2(g) -+ Na2CO3(aq)+ H2O; (1: The aqueouscarbonate reaction can be simplified by omitting the cation, resulting in the following ionic reaction. 20H-(aq) + CO2 (g)-+ CO32-(aq)+ H2O 0) (2) 6.GO= -56.1 kJ/mol (Lllio= -109.4 kJ/mol) Note that the enthalpyand free energyof the reactionare for a nominal I molar solution. The thermodynamicdata, given at 298K and a pressureof 1 bar, was obtainedfrom the availableliterature (Lide, 2000). As a comparison,the free energyof mixing CO2with nitrogenand oxygento form ambientair is given by ~G = RT In (P atrnlPCoJ-20 kJ/mol. (3) Clearly, sodiumhydroxide provides a sufficientdriving force to effectively collect CO2from ambientair. Eventhough a lower binding energy might be desirable,the high binding energyof chemicalsorbents proves useful in absorbingCO2 from streamswith low partialpressures of CO2(White, et al.,). As an alternativewith a weakerbinding energy,one may considersodium or potassiumcarbonate buffer solutionsas a sorbent. In this casethe absorptioncan be describedby: CO2(g)+ CO]2- + H2OQ) -+ 2 HCO]' (4) IlGO = -14.3 kJ/mol (ilHO=-27.6 klimat) Even in this caseit is possibleto mustera sufficientthermodynamic driving force to removeCO2 from the air. For a two molar solutionof bicarbonate 2004World Resource Review. All rightsreserved. 1(;0 World Resource Review Vol. 16 No.2 ions, the free energyof the reactionfrom ambientair is negativeif the bicarbonate concentrationstays below 0.15 molar. A similar resultcan be obtainedby calculatingthe massaction equilibriumusing empiricalvalues for the equilibrium constants. Reaction(4) is effectivelytrimolecular and is the result of a sequence of reactionswhich have fastkinetics at high temperaturesor very high carbonate to bi-carbonateratios. Otherwisethe processoccurs in the diffusion regime, which is much slower(Astarita, 1967),making this reactionimpractical for air extractionas it is kinetically limited. In a recentreview focusedon flue gas,White et al. (2003)do not specificallydiscuss absorption on sodiumhydroxide (NaOH), ratherthey consider aqueoussolutions of sodiumand potassiumcarbonate. These sorbents were comparedto mono-ethanolamine(MEA), the industrystandard, and it was concludedthat MEA providesa substantiallymore cost-effectivesolution (Leci and Goldthorpe,1997). At thesehigher CO2 concentrations there would be no advantagein the higherbinding energyof the hydroxideand the heatof this reactionis in any caseirretrievably lost. An additionalcomplication arises becausethe productcarbonate cannot be decomposedwith eitherheat or pressure. The resultantNazCa] mustbe efficiently decomposedchemically. We proposeto accomplishthe chemicaldecomposition using calciumhydroxide as an intermediary. A sodiumhydroxide solutionprovides a liquid sorbentthat is more easily cycled through a piping systemthan a calciumhydroxide suspension.Its binding energyis strongenough and its reactionkinetics fast enoughto obviatethe need for heating,cooling or pressurizingthe air. BecauseCO2 is so dilute any such actionwould result in an excessiveenergy penalty. The hydroxide solutionavoids all