Fuel Processing Technology 91 (2010) 1500–1504

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Fuel Processing Technology

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Metal-carbonate formation from ammonia solution by addition of metal salts—Aneffective method for CO2 capture from landfill gas (LFG)

Ankur Gaur, Jin-Won Park ⁎, Jung-Hwa JangDepartment of Chemical and Biomolecular Engineering, Yonsei University, 262 Seongsanno Seodaemun-gu, Seoul 120-749, Republic of Korea

Abbreviations: Å, Angstrom;mm,millimeters; kPa, killiter per minute; mol L−1, mole per liter.⁎ Corresponding author. Tel.: +82 2 364 1807; fax: +

E-mail address: jwpark@yonsei.ac.kr (J.-W. Park).

0378-3820/$ – see front matter © 2010 Elsevier B.V. Adoi:10.1016/j.fuproc.2010.05.027

a b s t r a c t

a r t i c l e i n f o

Article history:Received 28 January 2010Received in revised form 18 May 2010Accepted 25 May 2010

Keywords:AlkalinityAmmoniaCarbon dioxideNeutralization

The absorption of CO2 from LFG in different weight concentration ammonia solution and metal salts(Zinc and Barium) is investigated in this study. Addition of metal salts results in useful metal carbonateswhen LFG is passed through the solution. Barium salts show a better potential of removing CO2 as comparedto Zinc salts. Addition of Barium salts to ammonia solution results in a new absorbent as no study has beenfocused on it till date. Also metal salts are added to alkaline wastewater which not only decreases the pH ofthe wastewater but also useful metal carbonates are obtained from wastewater when LFG is passed throughit. Different parameters like CO2 loading, reaction rate and change in pH are investigated. Formation ofcarbonates is proved by using SEM and XRD analysis. Raman spectroscopy was performed on the discardedliquid after removal of carbonates to understand the formation of bicarbonates, carbonates and carbamates.

opascal; kW, kilowatt; L min−1,

82 2 312 6401.

ll rights reserved.

© 2010 Elsevier B.V. All rights reserved.

1. Introduction

Landfill gas (LFG) is a high methane content fuel consisting of someimpurities such CO2 and poisonous compounds [1–4]. Numerous wayscan be applied to remove these impurities from LFG gas and obtainpurified methane [5]. There are mainly four methods for CO2 removalfrom LFG. Physical absorption [6], chemical absorption [7], adsorptionandmembrane separation are thesemainmethods. Eachmethod has itsown advantages and disadvantages. The choice of particular methoddepends on economics and quantity of gas being purified. In this study acompletely new method is applied for removal of CO2 from LFG. Onlyone previous publication is reported for this method [8]. The idea is touse ammonia (NH3) solution consisting of metal salts to remove CO2.Reaction of CO2 with NH3 is of great industrial interest. A lot of studieshave been focused on removal of CO2 using ammonia absorption as itoffers numerous advantages like high CO2 loading capacity and lessdegradation of absorbing solution, thereby keeping material costs lowbut there are small disadvantages likehigh regeneration costs comparedto amine technology [9]. Tomake the processmore economical additionofmetals salts to the ammonia solution is proposed. Themetal salts usedin this study are Zinc sulfate heptahydrate (ZnSO4·7H2O), Zinc chloride(ZnCl2) and Barium chloride dehydrate (BaCl2·2H2O). A study donepreviously focuses only on Zinc salts but in this study a new CO2

absorbent in form of ammonia solution+Barium chloride dihydrate is

proposed. Addition of BaCl2·2H2O to ammonia solution increases theCO2 loading capacity to great extent as compared to the Zinc salts. Thestudy also uses alkaline wastewater obtained from nearby catalyticindustry. Addition of metal salts not only decreases the pH ofwastewater but we are also able to obtain useful metal carbonatesfrom wastewater when we pass the LFG through wastewater solution.The study highlights the fast and efficient absorption of CO2 by aqueousammonia along with high quality Zinc and Barium carbonate produc-tion. In theprocessmost of the capturedCO2 is carbonated as solidmetalcarbonates. The process offers a great advantage to evade the drawbacksof energy-consuming steps associated with both regeneration andseparation of NH3 from CO2 and also reduces the amount of pure CO2 tobe deposited deep underground or under oceans. The effects ofconcentration of NH3 and metal salts are investigated and discussed.The experiments are performed at room temperature. The carbonatesobtained from these procedures are profitable products. Zinc carbonatecan be used as used as an astringent and excipient in shampoo. It canalso is also beusedasafireproofingfiller for rubber andplastics, as a feedadditive, as a pigment, in cosmetics and lotions, and in the manufac-turingof porcelain, pottery, and rubber. Bariumcarbonate iswidely usedin the ceramic industry as an ingredient in glazes. It acts as a flux, amatting and crystallizing agent and combines with certain coloringoxides to produce unique colors not easily attainable by othermeans. Inthe brick, tile, earthenware and pottery industries Barium carbonate isadded to clays to precipitate soluble salts (calcium and magnesiumsulfates) that cause efflorescence. Thus converting CO2 from LFG intouseful carbonatesmakes theprocess veryattractiveandeconomical. Thepurified LFG gas which consists mainly of methane can be used in city'snatural gas network.

Fig. 1. Schematic diagram of adsorption and absorption experimental apparatus.

Fig. 3. Breakthrough curve for 5 wt.% ammonia solution and metal salts (D) 50 gZnSO4·7H2O (E) 50 g ZnCl2 and (F) 50 g BaCl2·2H2O.

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2. Material and experimental methods

2.1. The raw landfill gas

The LFGwas collected, stored and transported to laboratory from theSudokwon landfill site which is located in the west cost of Inchon Cityand is the largestwaste treatment site inKorea. Asmain components theconcentration of CH4 and CO2 are 47–55 vol.% and 45–52 vol.%respectively and some poisonous compounds are present. LFG wascompressed to the gasholder fromextractionwell. The static pressure ofthe extraction well is around 19.61–29.4 kPa. The raw LFG was passedthrough the compressor which was 2.20 kW and compressed up to4903.33 kPa in the gas holder. The moisture content of raw landfill gaswas removed using the cyclone type dehydrator.

2.2. Chemical absorbents

Granular activated carbon is used in this study whose apparentdensity is around 0.40 to 0.43g cm−3 and the particle size is around1.00 mmto1.41 mm. The averageporediameter is between14and18 Åand BET surface area ranges from 1100 m2 g−1 to 1200 m2 g−1.Activated carbon was obtained from Samchonli limited. Alkalinewastewater displayed a very high alkalinity and a great bufferingcapacity [10]. Different concentration ammonia solutions wereprepared using 25 wt.% standard solution using simple stoichiometriccalculations. The standard ammonia solutionwas obtained fromDuksanpure chemical company limited. Zinc sulfate heptahydrate, Zinc chlorideand Barium chloride dihydrate were obtained from Duksan pure

Fig. 2. Breakthrough curve for 2 wt.% ammonia solution and metal salts (A) 50 gZnSO4·7H2O (B) 50 g ZnCl2 and (C) 50 g BaCl2·2H2O.

chemical company limited. There were a total of nine solutions usedin this study—2 wt.% ammonia+50 g ZnSO4·7H2O (solution A), 2 wt.%ammonia+50 g ZnCl2 (solution B), 2 wt.% ammonia+50 g BaCl2·2H2O(solution C), 5 wt.% ammonia+50 g ZnSO4·7H2O (solution D), 5 wt.%ammonia+50 g ZnCl2 (solution E), 5 wt.% ammonia+50 g BaCl2·2H2O(solution F), wastewater+50 g ZnSO4·7H2O (solution G),wastewater+50 g ZnCl2 (solution H), and wastewater+50 g BaCl2·2H2O (solution I).

2.3. Analytical method

Raman spectroscopy measurements were done in backscatteringgeometry with a JY LabRam fitted with a liquid nitrogen cooleddetector. The spectra were collected under ambient conditions usingthe 514.50 nm line of argon-ion laser to compare the reactionsimilarity between ammonia and wastewater.

Alkalinity of liquid samples was measured by using titration usingstandardized sulfuric acid (H2SO4) solution using end point indicators(methyl green) and/or pH meter.

The characterization of solid products was done by using a scanningelectronic microscope (JEOL, J-5600) and an X-ray diffractometer(Mac Science, MXP-3TXJ-7266).

The amount of carbonates was calculated by using standard methodnumber 2540 B mentioned in reference [11].

Fig. 4. Breakthrough curve for alkaline waste water and metal salts (G) 50 g ZnSO4·7H2O(H) 50 g ZnCl2 and (I) 50 g BaCl2·2H2O.

Fig. 5. Reaction rate for 2 wt.% ammonia solution andmetal salts (A) 50 g ZnSO4·7H2O (B)50 g ZnCl2 and (C) 50 g BaCl2·2H2O.

Fig. 7. Reaction rate for alkalinewastewater andmetal salts (G) 50 g ZnSO4·7H2O (H) 50 gZnCl2 and (I) 50 g BaCl2·2H2O.

Table 1Change in pH for different solutions before and after passing LFG.

Solutions Initial pH Final pH

2 wt.% ammonia+50 g ZnSO4·7H2O (A) 9.03 7.722 wt.% ammonia+50 g ZnCl2 (B) 8.16 6.712 wt.% ammonia+50 g BaCl2·2H2O (C) 10.60 7.165 wt.% ammonia+50 g ZnSO4·7H2O (D) 9.81 7.375 wt.% ammonia+50 g ZnCl2 (E) 9.71 7.305 wt.% ammonia+50 g BaCl2·2H2O (F) 10.82 7.31Wastewater+50 g ZnSO4·7H2O (G) 8.64 7.09Wastewater+50 g ZnCl2 (H) 7.40 6.59Wastewater+50 g BaCl2·2H2O (I) 9.27 7.16

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2.4. Apparatus and procedure

The landfill gas was passed through the activated carbon to removeany gaseous contaminants and then first through saturator and thenthrough reactor. The experimental setup is shown in Fig. 1. The reactorcontained 1 liter solution of different absorbents. A porous ceramicbubble spargerwasused in the reactor to distribute the gas evenly in thesolution.Gasflow rate of LFGwas controlled bygasflow-meter obtainedfrom MKP industries. During the experiment the reactor and saturatorwere placed in the water bath to maintain room temperature at around293 K. The CO2 concentration in the outlet gas mixture was constantlyanalyzed by an IR analyzer (KINSCO). A silica gel trap was used in orderto remove the humidity from gas phase and to protect the instrument.The pH of the liquid solvent was continuously measured using a pHmeter (Orion 420 A+). The flow rate of LFG is kept at 1.5 L min−1

controlled by mass flow controller obtained from MKP.In a typical experiment 1 liter (1 L) of the liquid solvent was placed in

the reservoir vessel and LFG mixture consisting of certain CO2

concentration was fed to the system. During the initial start of theexperiment the liquid absorbent completely absorbed the CO2 from thefed mixture. As the absorption process progressed the CO2 wascontinuously accumulated in the liquid absorbent and after some timeCO2 started to evolve in the outlet stream. At the end of each run thesolution became completely saturated with CO2 and the concentration of

Fig. 6. Reaction rate for 5 wt.% ammonia solution and metal salts (D) 50 g ZnSO4·7H2O(E) 50 g ZnCl2 and (F) 50 g BaCl2·2H2O.

CO2 in the outlet was equal to inlet. One cycle of experiment took almost50 min.

3. Results and discussion

The study deals with different absorption characteristics of numer-ous mixtures. The solutions are divided into three main categories. Thesolutions are A, B, C, D, E, F, G, H and I. Solutions A, D and G consist of2 wt.% ammonia. Solutions B, E and H consist of 5 wt.% ammonia.Solutions C, F and I consist of alkalinewastewater. The reason for taking50 g of metal salt instead of using molar equality is due to economicalconsideration for future study at pilot plant scale. Fig. 1 describes thediagram of the experimental procedure. Figs. 2, 3 and 4 deals with theCO2 outlet concentration curve or breakthrough curve. Figs. 5, 6 and 7shows the reaction rate for all 9 solutions. Among the 9 solutions thesolutions consisting of BaCl2·2H2O show the best CO2 breakthroughcurve and reaction rate graph proving that among three metal saltswhich we added to the different concentration ammonia solutions andwastewater BaCl2·2H2O is the best pick. In Table 1 the change in pH is

Table 2Maximum CO2 loading for different solutions.

Solutions MaximumCO2 loading (mol[sorbent L]−1)

2 wt.% ammonia+50 g ZnSO4·7H2O (A) 0.292 wt.% ammonia+50 g ZnCl2 (B) 0.342 wt.% ammonia+50 g BaCl2·2H2O (C) 0.725 wt.% ammonia+50 g ZnSO4·7H2O (D) 1.355 wt.% ammonia+50 g ZnCl2 (E) 1.135 wt.% ammonia+50 g BaCl2·2H2O (F) 1.86Wastewater+50 g ZnSO4·7H2O (G) 0.32Wastewater+50 g ZnCl2 (H) 0.26Wastewater+50 g BaCl2·2H2O (I) 0.50

Table 3Amount of carbonates formed in 1 liter of solution.

Solutions Amount of carbonates formed (g[sorbent L]-1)

2 wt.% ammonia+50 gZnSO4·7H2O(A) 6.982 wt.% ammonia+50 g ZnCl2 (B) 32.722 wt.% ammonia+50 g BaCl2·2H2O (C) 37.185 wt.%ammonia+50 gZnSO4·7H2O(D) 0.005 wt.% ammonia+50 g ZnCl2 (E) 0.005 wt.% ammonia+50 g BaCl2·2H2O (F) 37.14Wastewater+50 g ZnSO4·7H2O (G) 1.46Wastewater+50 g ZnCl2 (H) 49.00Wastewater+50 g BaCl2·2H2O (I) 32.97

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mentioned. Addition of Zncl2 and ZnSO4·7H2O decreases the pH toconsiderable extent but BaCl2·2H2Oadditiondoesn't affect the pHmuchaswe can see inTable 1. The pHof pure 2 wt.% ammonia andpure 5 wt.%ammonia is around11but the addition of ZnSO4·7H2Odecreases thepHto around 9.03 for a 2 wt.% solution and to 9.81 for a 5 wt.% solution andto 8.64 in case of alkalinewastewater. The addition of ZnCl2 affected thepH more as compared to ZnSO4·7H2O. The pH of 2 wt.% ammonia and5 wt.% ammonia decreased to 8.16 and 9.71. The pH of alkalinewastewater decreased to 7.40 after addition of ZnCl2. Addition ofBaCl2·2H2O didn't affect the pH much. The pH dropped only by 0.40 to10.60 in case of 2 wt.% ammonia and to 10.82 in 5 wt.% ammonia case.

Fig. 8. XRD analysis of (a)ZnCO3 and (b) BaCO3 after completion of the experiment.

The alkalinewastewaterpHdecreased to9.27while addingBaCl2·7H2O.NowwhenLFG ispassed throughall the9 solutionsmost of the solutionswere neutralized as seen in Table 1. The final pH of all solutions hoversaround 7. Some solutions were neutralized quickly whereas other tooksomemore time to neutralize. From the breakthrough curvesmaximumCO2 loadings for all 9 solutions are calculated and displayed in Table 2.The CO2 loading is arranged in increasing order for all 9 solutionsFbAbCbDbIbGbEbBbH. ThemaximumCO2 loading is displayedby 5 wt.%ammonia solution+BaCl2·2H2Owhich is around 1.86 mol L−1. Table 3mentions about amount of carbonates formed in 1 liter solution. Theamount of carbonates formed is also displayed in increasing orderB=EbCbAbDbIbHbGbF. Carbonate formation is a complex mechanism.

The reaction mechanism for ammonia and CO2 is:

NH3 þ CO2 þ H2O↔NH4þ þ HCO

−3 ð1Þ

NH3 þ HCO−3 ↔NH2CO

−2 þ H2O ð2Þ

NH3 þ HCO−3 ↔CO

2−3 þ 2NH

4þ: ð3Þ

The reaction mechanism for Zn salts are

Zn2þ þ 2HCO

3−↔CO2 þ ZnCO3 þ H2O ð4Þ

Zn2þ þ CO

2−3 ↔ZnCO3 ð5Þ

Zn2þ þ NH2CO

2− þ H2O↔ZnCO3 þ NH4þ: ð6Þ

The formation of Barium carbonate follows the mechanism similarto Zinc carbonate

Ba2þ þ 2HCO

3−↔CO2 þ BaCO3 þ H2O ð7Þ

Ba2þ þ CO

2−3 ↔BaCO3 ð8Þ

Ba2þ þ NH2CO

2− þ H2O↔BaCO3 þ NH4þ: ð9Þ

The reaction of CO2with ammonia solution results in the formation ofdifferent products like bicarbonate, carbonate and carbamate as repre-sented in Eqs. (1), (2) and (3). These species further react withmetal ionspresent in the solution forming metal carbonates as represented in Eqs.(4) to (9).Whenusing5 wt.%ammonia solutionno carbonates are formedwhen 50 g each ZnCl2 and ZnSO4·7H2O is added to solution after passingthe LFG but addition of 50 g BaCl2·2H2O results in around 37.14g L−1

BaCO3 formation. When using 2 wt.% ammonia solution addition ofZnSO4·7H2O results around 6.98 g L−1 ZnCO3 formation and addition ofZnCl2 results in around 32.72 gL−1 ZnCO3 formation and BaCl2·2H2Oaddition forms around 37.18 gL−1 BaCO3. In case of alkaline wastewatermaximumamount of carbonateswas obtained in case of addition of Zncl2around 49 gL−1 followed by BaCl2·2H2O which resulted in 32.97 gL−1

BaCO3 formation and least carbonates were obtained in case of ZnSO4·7-H2O around 1.46 gL−1 ZnCO3. Zncl2 and Bacl2·2H2O are bettermetal saltsfor obtaining metal carbonates compared to ZnSO4·7H2O. In Fig. 8 XRDanalyses of the carbonates obtained were done using the standardtechniques. XRD was used to study crystal morphology of carbonatesobtained from the process. As far as morphology is concerned only twotypes of X-ray diffraction pattern were obtained in this study as shown inFig. 8 which matched the one numbered 41-0373 in JCPDS fororthorhombic BaCO3 and number 38-154 for ZnCO3. In Fig. 9 SEMpictures of carbonate samples are displayed. ZnCO3 obtained fromZnSO4·7H2O had rod like structures and that obtained from ZnCl2 was amore powdered form. As seen in Fig. 9 the BaCO3 obtained wasorthorhombic in nature. Fig. 10 shows the Raman spectroscopy imagesfor all 9 solutions after completion of the CO2 absorption. Ramanspectroscopyhelps us understand the formation of solidmetal carbonatesand reactionmechanisms. Aswe can see in Fig. 10when 2 wt.% ammoniasolution andmetal salts aswell as alkalinewastewater andmetal salts areused there are no distinct peaks for carbonate or bicarbonate suggesting

Fig. 9. SEM picture of (a) ZnCO3 and (b) BaCO3 obtained after completion of the experiment.

1504 A. Gaur et al. / Fuel Processing Technology 91 (2010) 1500–1504

thatmost of the ions are converted into solid carbonates butwhenwe areusing a high concentration ammonia solution the peaks for bicarbonateand carbonate can be distinctly seen suggesting that ammonia played abigger role in CO2 absorption as compared to metal salts and no metalcarbonates are obtained in 5 wt.% case except for Barium carbonate. Theexperimentswere repeated numerous times. A new CO2 absorbent in theform of ammonia solution+Barium salt is obtained. Future study willfocusmoredeeply on reactionmechanismbetweenammonia andBariumsalts. The study is a breakthrough and new approach in the field of CO2

absorption. The quality of methane obtained from this process isextremely high quality and it can be used in numerous processes likenatural gas network or in fuel cells. The process requires a deepeconomical study which is also a focus for future study. The process canbe easily scaled up to industry level. Korea is an advanced country and thisprocess can help obtain pure methane and cheap metal carbonates. Thiscan help Korea achieve self sufficiency in fuel as it lacks natural resources.

4. Conclusions

CO2 is removed from LFG using a new technique. Metal salts areadded in the ammonia solution and alkaline wastewater. When LFG ispassed through these solutions puremetal carbonates are obtained. The

Fig. 10. Raman spectroscopy result after completion of the experiments.

method is successful when low ammonia concentration is used. In caseof alkalinewastewater addition ofmetal salts not only reduces the pHofwastewater but metal carbonates are also obtained. The process isextremely economical and simple to scale up. Among the three metalsalts used Barium salts offer the best alternative asmaximum carbonateformation and CO2 loading occurs when Barium salt is used.

Nomenclature

HP horse powerLFG landfill gasAC activated carbon

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