A Novel Method of CO2 Capture from

High Temperature GasesK. Nakagawat and 1. Ohashi

Toshiba Research and Development Center Saiwai-ku, Kawasaki 210, Japan

ABSTRACT

Lithium zirconate powder reacts immediately with ambient CO2 in the range of 450 to 550°C. Moreover, the productsreact and return reversibly to lithium zirconate at temperatures above 650°C. Utilizing this reaction, the possibility of aCO2 separation system which operates at around 550°C is suggested. A CO2 separation process operable at temperaturesbeyond 500°C may have a special benefit, because the separation can be achieved directly during the fuel reformingprocess, where high CO2 concentration is expected.

IntroductionEnvironmental concerns have stirred up much interest in

CO2 exhaust from energy power plants. A large part of CO2emitted to the atmosphere originates from the combustionof fossil fuels, especially coal and petroleum. To reduce theCO2 exhaust from flue gases, several methods of CO2 sepa-ration have been proposed."2 However, they are still toocostly for commercial use. Here we report the application ofthe lithium zirconate-zirconia reversible reaction for CO2removal.

Lithium zirconate has been investigated as a candidate ofthe additive material for molten carbonate fuel cell (MCFC)electrolyte plates.3 A durability test of this material re-vealed that the lithium zirconate which had been synthe-sized by the heat-treatment of zirconia and lithium carbon-ate returned to the starting materials after 1000 h in thepresence of CO2. This reaction is represented by

Li2ZrO3 + CO2 = Li2CO3 + Zr02 [1]

Although the reaction between zirconia and lithium car-bonate is well known, there are no published studies on theinverse reaction. In this study, we examined the reaction be-tween lithium zirconate and CO2 using the thermogravimet-nc method. On the basis of experimental results, the possi-bility of a CO2 separation system utilizing the reaction isdiscussed.

ExperimentalLithium zirconate powder having particle diameters of

0.5 to 1.0 p.m was synthesized by the heat-treatment of zir-conia and lithium carbonate. The reaction between the lith-ium zirconate powder and CO2 was studied in controlled gasflowing environments using a thennogravimetric instru-ment (TGA-50, Shimadzu). Atmospheric gases used in theexperiment were mixtures of C02, air, and H2 having fourdifferent compositions (gas A: CO2 0%; gas B: CO2 20%; gasC: CO2 70%, balanced by air; and gas D: CO2 20%, balancedby H2). We used two types of heating procedures, i.e., con-stant rate heating (5°C/mm) up to 800°C to examine the re-action temperature, and constant temperature heating withplateaus at 500 and 680°C, to examine the reaction rate.

Results and DiscussionFrom the thermodynamic data in Ref. 5, the Gibbs free

energy change of Eq. 1 was calculated on the basis of Eq2. Figure 1 shows the values as the function of temperatureand CO2 partial pressure. Negative values indicate right-ward progress of Eq. 1. It is obvious that the reaction canbe considered to be reversible depending on the tempera-ture and CO2 partial pressure

AG = All— TAS — RTln(p02) [2]

(where All: enthalpy change, AS: entropy change, T: absolutetemperature, R: gas constant, pco2: CO2 partial pressure).

* Electrochemical Society Active Member.

It has been also confirmed that the reaction is independentof gas species other than CO2 which usually exists in theexhaust gas of energy power plants. These features suggestthe possibility of CO2 absorption by the use of this reaction.

Figure 2 shows the weight change of the lithium zir-conate powder during the temperature increase to 800°C atvarious CO2 partial pressures. As Eq. 1 shows, the weightincrease and decrease are indicators of the absorption andemission of C02, respectively. There are clear differencesdepending on the presence of C02; we can see an obviousweight increase in C02-containing gases B and C, whereasno weight change was observed in gas A, which is free fromCO2. Judging from the weight increase curves in Fig. 2, thepreferable temperature range for the absorption of CO2

C)600

400 1

200

Fig. 2. Weight change of lithium zirconate powder during temper-ature sweep. Gas A: CO2 0%, Gas B: CO2 20%, Gas C: CO2 70%,balanced by air, atmospheric pressure. Initial sample weight: 24mg. Gas flaw rate: 200 mI/mm.

— —-

.O1

C)o,4o —%

C..

72*ti..(o.i ...,...,.#.,.,

Fig. 1. Values of the Gibbs free energy change for Eq. 1.

800

I,0)C(U.CC4-0)(U

50 100 150 200Duration (mm)

0

1344 J. Electrochem. Soc., Vol. 145, No. 4, April 1998 The Electrochemical Society, Inc.

J. Electrochem. Soc., Vol. 145, No. 4, April 1998 The Electrochemical Society, Inc. 1345

seems to be 400 to 590°C. The emission of CO2 proceedsmainly at temperatures above 600°C. Since CO2 separationfrom gas with a high CO2 concentration is generally advan-tageous, these temperatures are convenient for efficient CO2absorption in the fuel reforming process, where high CO2concentration is expected at temperatures of 400 to 600°C. 6Further, it is also clear from Fig. 2 that CO2 is remitted evenin gas C, which contains as much CO2 as 70%. The fact indi-cates a possibility of CO2 gas enrichment from 20 to 70%.

As the next step, the rate of the reaction was tested in aC02/hydrogen gas mixture, which is considered to be anactual gas environment for the fuel reforming process. Fig-ure 3 shows the weight change of the lithium zirconatepowder when it was kept at 500°C, and then at 680°C. Sim-ilar to Fig. 2, a clear weight increase and decrease were ob-served during heating at these temperatures. The value ofthe weight increase was 21.9% after 180 mm heating, whichcorresponds to 76.3% of the theoretical maximum. Thehighest CO2 absorption rate was 290 cm2/h for 1 cm3 of lithi-um zirconate. An X-ray diffraction analysis showed thatlithium carbonate and zirconia were present after keepingthe sample at 500°C, but they disappeared after the heatingat 680°C. These experimental results clearly indicate thatthe reaction represented by Eq. 1 is reversible with practi-cal reaction rates and is applicable to the CO2 capture fromhigh-temperature gases.

Besides the reaction in which lithium zirconate is in-volved, there are some known reactions of alkaline or alka-line earth metal oxides in regard to CO2 absorption. Amongthese substances, magnesia has been reported as a candi-date of the CO2 absorbent (Eq. 3) because it has a moderatereaction temperature (300°C). However, there are twomajor differences between magnesia and lithium zirconate.The first one is the reactivity with steam which is expectedto coexist with CO2 in the fuel reforming process. General-ly, alkaline or alkaline earth metal oxides have a higherreactivity with steam. Among others, magnesia reacts withsteam as shown in Eq. 4, up to ca. 260°C. Therefore, the CO2absorption by the process of Eq. 3 and the steam absorptionby the process of Eq. 4 proceed simultaneously. This fact re-sults in the waste of magnesia from the viewpoint of effi-cient CO2 absorption. However, the reaction between lithi-um zirconate and steam in Eq. 5 does not proceed attemperatures over 200°C. Since the preferable temperaturerange for CO7 absorption is 400 to 590°C, the lithium zir-conate exclusively absorbs CO2 within this temperature

MgO + CO2 = MgCO3

MgO + H20 = Mg (OH)2

Li2ZrO2 + H2O = 2Li (OH) + Zr02

I 21.9%

800

Fig. 3. Weight change of lithium zirconate powder during heatingat two temperatures. Gas D: CO2 20%, balanced by H2, atmospher-ic pressure. Initial sample weight: 20 mg. Gas flow rate:100 mt/mm.

Fig. 4. Models of the volume change of absorbent particles dur-ing CO2 absorption. (a) The case of magnesia and (b) the case oflithium zirconate.

The second difference is the morphologic change of thepowder. It is considered that the volume change of the par-ticle is harmful for fabricating a stable porous body of theabsorbent. Figure 4 shows models of the volume change ofthe absorbent particles during the CO2 absorption. In thecase of magnesia, the particle size changes dramaticallyafter the absorption of C02; by calculations based on theformula weight and the density, the value of the magnesiaparticle increases up to 249% (see Fig. 4a). On the contrary,the volume of lithium zirconate is estimated to be no morethan 134% (see Fig. 4b). Therefore, we can expect that a sta-ble porous body can be fabricated using lithium zirconate.

[3] ConclusionIn order to build a system for continuous CO2 separation,

[4] at least two reaction vessels which contain lithium zir-1 1

conate are required. Figure 5 shows an example of the sys-15J tem. The operation of each vessel is periodically altemated

from the absorption to the emission of CO2. In the firstphase, a high-temperature gas from a fuel reformer is fed to

CO2 lean gas (exhaust)

Fig. 5. An example of the application of the CO2 capture systemto an energy power plant having a fuel reforming process.

Volume1.00

Volume2.49

Absorption . .

Magnesium carbonate

4Emission

MgCO3

Volume1.34

MgO + CO2

Volume1.00

Absorption

4Emission

• t, cc, 1 a. —— — a 6 S — S

LI2ZrO3 + CO2

Layer of lithium carbonateLa a a, . —— —

Zr02 + LI2CO3

+20

atCCCa

C)— +10C0

0

450'-'550'C

600 90

Is0'lull a.E

200

Reformer

0

Vessel A

100

CO2 separator

Vessel B

200 300 400Duration (mm)

Fuel

Air

Combustor

To boiler

1346 J. Electrochem. Soc., Vol. 145, No. 4, April 1998 The Electrochemical Society, Inc.

vessel A. During this process, the temperature of the gasand the absorbent in this vessel should be kept in the rangeof 450 to 550°C to ensure complete CO2 absorption. The gaswhich does not react with the absorbent is led to the com-bustor. The second phase starts when vessel A is saturatedby CO2. In this phase, the gas flow is switched to vessel B,while vessel A is heated to temperatures above 650°C. Theabsorbed CO2 is separated from the absorbent at these tem-peratures and led to another plant where CO2 fixation isperformed. After the second phase, the operation returns tothe first phase. We believe that there are two major advan-tages in the system concerning the thermal efficiency. Thefirst one is the temperature of extracted CO2. Since the fix-ation process, viz, hydrogenation, requires high tempera-tures, feeding CO2 at high temperatures has a certain bene-fit. The second point is the fact that CO2 absorption is anexothermic reaction at around 500°C. The residual gas isheated during the absorption, which results in the preheat-ing of the fuel gas before the combustor.

The reaction between the lithium zirconate powder andCO2 is reversible and therefore applicable to CO2 capturein the presence of hydrogen and steam. Furthermore, itshould be noted that the CO2 absorption beyond 500°Cwith heat generation has special value, since this temper-ature is high enough to let the absorption proceed directlyduring fuel reforming process.

AcknowledgmentsWe thank Akiko Watanabe and Noriko Nakazawa for per-

forming TG experiments.

Manuscript submitted March 20, 1997; revised manu-script received November 26, 1991.

Toshiba Corporation assisted in meeting the publicationcosts of this article.

REFERENCES1. A. M. Wolsky, E. J. Daniels, and B. J. Jodh U.S. Depart-

ment of Energy, Report ANL/ES/CP/80346 (1993).2. J. Tsubaki, Membrane, 19, 146 (1994).3. H. Ohzu, K. Shinozaki, Y. Akasaka, and A. Tsuge, in

Proceedings of the Annual Meeting of the CeramicSociety of Japan, Tokyo, 2A33 (1986).

4. K. Nakagawa and T. Ohashi, Abstract 3F11, 64th Annu-al Meeting of the Electrochemical Society of Japan,Yokohama, 1997.

5. The NBS table of chemical thermodynamic properties,J. Phys. Chem. Ref. Data, 11 (1982).

6. J. fi. Johnson, Chemistry of Coal Utilization, 2nd Suppl.Volume, p 1497, Wiley-International, New York (1981).

7. G. V Colombo, P. Rey, and E. S. Mills, U.S. Pat.3,557,011 (1971).

Low Pressure Chemically Vapor Deposited W03 Thin Films forIntegrated Gas Sensor Applications

D. Davazoglou* and K. Georgouleas

NCSR "Demokritos," Institnte of Microelectronics, 153 10 Agia Paraskevi, Attihi, Greece

ABSTRACT

The gas sensing properties of substoichiometric WO2 films have been investigated when activated with some goldmonolayers sputtered on their surface and in the as-deposited form. These films were deposited on silicon substrates, intwo steps: (i) chemical vapor deposition of tungsten films using W(CO)1, vapors in a horizontal cold wall reactor at a pres-sure of 13.3 Pa and temperatures between 773 and 823 K, and (ii) oxidation of the tungsten films inair at 873 K for 10 mm.For the tests, the substoichiometric W02 films were packed in classic dual in-line packages. The resistance variationsofthese configurations caused by changes in their environment were monitored. Reversible changes, of the order of severalkilohms, were observed in the presence, or upon removal of H2, at concentrations down to 100 ppm, the magnitude ofwhich depended on the concentration and the temperature measured. The response times measured were also dependenton temperature and H2 concentration, and were of the order of some minutes.

IntroductionThe operation of chemical sensors is based on the re-

versible change of a certain physical property exhibited bysome specific materials, induced by changes in their chem-ical environment. This change of physical properties istransformed to an electrical signal, the measurement ofwhich permits the detection of the nature and the magni-tude of the change in the chemical environment. The suit-ability of a material to be used in the fabrication of chem-ical sensors depends on its ability to return to its initialstate upon removal of the cause that induced the change ofits properties. Many materials have been used up to now inthe formation of chemical sensors. Among them is palladi-um for the detection of hydrogen,1 5n02 for the detectionof flammable gases,2 W02 for the detection of nitrogenoxides3 and H2S, etc.

The goal of our investigation was to examine the suit-ability of the tungsten substoichiometric oxide films (inthese films, the O/W atomic ratio is less than 3, but arereferred to in this paper as WO3 films) to be used in thefabrication of silicon-based integrated chemical sensors.These films can be easily deposited,2'6'8 are easily oxidized

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and reduced reversibly, and the variation of their oxida-tion state induces significant changes in their electrical(resistivity)2 and optical (dielectric)12'" properties. In aprevious paper,8 we have shown the compatibility of theprocessing of tungsten oxide films deposited by low pres-sure chemical vapor deposition (LPCVD) from W(CO)6vapors at pressures of 13.3 Pa and temperatures rangingbetween 723 and 873 K, with that of integrated circuitsmade on silicon. These films adhere well on silicon oxideand nitride and polycrystalline silicon films, and can beeasily reactive ion etched (RIE) using SF chemistry. Fur-thermore, these films were characterized with respect totheir morphology (grain size and surface roughness) usingatomic force microscopy (AFM) measurements, theirstructure by taking X-ray diffraction (XRD) spectra, andtheir resistivity was measured at temperatures rangingbetween 77 and 300 K. Also, films deposited at tempera-tures around 773 K suffer considerable stresses, and theabsorption and desorption of oxygen induces reversiblechanges in their resistivity. For this reason, this paperfocuses on the gas sensing characteristics of WO3 filmsdeposited by LPCVD at temperatures around 773 K.

This paper is organized as follows: the details of thepreparation of the samples are described in the second see-