Fuel Processing Technology 91 (2010) 635–640

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Fuel Processing Technology

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Landfill gas (LFG) processing via adsorption and alkanolamine absorption

Ankur Gaur a, Jin-Won Park a,⁎, Sanjeev Maken b, Ho-Jun Song a, Jong-Jin Park a

a Department of Chemical and Biomolecular Engineering, Yonsei University,134 Shinchon-dong, Seodaemun-gu, Seoul 120-749, Republic of Koreab Department of Chemistry, Deenbandhu Chhotu Ram University of Science and Technology, Murthal-131 039, Haryana, India

Abbreviations: mm, millimeter; ppmv, parts per mil⁎ Corresponding author. Department of Chemical a

Yonsei University,134 Shinchon-dong, Seodaemun-guKorea. Tel.: +82 2 364 1807; fax: +82 2 312 6401.

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

0378-3820/$ – see front matter © 2010 Elsevier B.V. Aldoi:10.1016/j.fuproc.2010.01.010

a b s t r a c t

a r t i c l e i n f o

Article history:Received 17 September 2009Received in revised form 30 December 2009Accepted 6 January 2010

Keywords:Landfill gasCarbon dioxideMethaneAbsorptionActivated carbonAdsorption

Landfill gas (LFG) was upgraded to pure methane using the adsorption and absorption processes. Differenttoxic compounds like aromatics and chlorinated compounds were removed using granular activated carbon.The activated carbon adsorbed toxic trace components in the following order: carbon tetrachlorideNtolueneNchloroformNxyleneNethylbenzeneNbenzeneN trichloroethylene≈tetrachloroethylene. After removingall trace components, the gas was fed to absorption apparatus for the removal of carbon dioxide (CO2). Twoalkanolamines,monoethanol amine (MEA) anddiethanol amine (DEA)were used for the removal of CO2 fromLFG.ThemaximumCO2 loading is obtained for 30 wt.%MEAwhich is around2.9 mol L−1 of absorbent solutionwhereasfor sameconcentrationofDEA it is around1.66 mol L−1 of solution. 30 wt%MEAdisplayedahigherabsorption rateof around 6.64×10−5 mol L−1 min−1. DEA displayed a higher desorption rate and a better cyclic capacity ascompared to MEA. Methane obtained from this process can be further used in the natural gas network for city.

lion by volume.nd Biomolecular Engineering,, Seoul 120-749, Republic of

l rights reserved.

© 2010 Elsevier B.V. All rights reserved.

1. Introduction

In the present energy crisis scenario, new recovery methods forlandfill gas (LFG) are garnering huge interest. LFG has a very high-energy potential due to the presence of methane (CH4) and thus is agreat source for energy production. Many countries have showninterest in collection and subsequent use of landfill gas to reduceGreen house gas (GHG) emissions from the landfill and to replace thefossil fuels [1]. The waste management policies in Korea are quiteeffective as compared to other countries. Korea has one of the biggestlandfill sites in the world. There are 238 landfill sites operatingcurrently in Korea [2] that offer a great opportunity to use CH4 as anenergy resource which is produced in landfill by the process ofanaerobic digestion of the organicmaterial contained in themunicipalsolid waste [3]. LFG is a mixture of CH4 (35 to 60%) and CO2 (35 to55%) along with other numerous trace components like aromatics,chlorinated organic hydrocarbons, siloxanes and sulfur compounds[4]. Therefore, LFG offers a very attractive option to oil in this energystarved country [5]. Upgrading the LFG to high purity CH4 enhancesthe energy density of LFG [6] that could be used in transportationsector or as a city gas [7]. The best way to remove the tracecomponents is via adsorption because the process is economical andsafe [8,9]. Also high gas quality is obtained. It is important to removethese trace components because of their harmful effects on human

health and also due to corrosion caused by impurities like hydrocar-bon, chlorinated organic compounds, and sulfur compounds whichreduces the operating life of boilers and combustion engines [10–12].

There are many methods of removing the CO2 from the landfill gaslike chemical absorption, adsorption and membrane separation [13–15]. Each process has its own advantages and disadvantages. Here thefocus is on chemical absorption, which is very old and most suitablemethod for separating CO2 from other gases. Aqueous alkanolaminesare widely used absorbents for removal of CO2 and among themaqueous monoethanol amine (MEA) solution is the industrially mostimportant absorbent because of its rapid reaction rate, low cost,thermal stability and low solubility of hydrocarbons. There are somemajor disadvantages which MEA suffers from like degradationthrough oxidation of amine, high enthalpy of reaction and somecorrosion problem. Therefore, in this study we used MEA and also asecondary amine, diethanol amine (DEA). MEA and DEA arecommercial absorbents and widely used in CO2 removal from fluegases [16,17]. The reaction mechanism of primary and secondaryamine is almost similar. CO2 reacts with aqueous solutions of primaryand secondary amines, reaching equilibrium of carbamate, bicarbon-ate and carbonate. Maximum absorption of CO2 occurs when all of theabsorbed gas exists as bicarbonate. The carbamate formation reactionof primary and secondary amines takes place through the zwitterionsmechanism. The carbamate formation is very unstable due to the alkylgroup attached to the amine and this results in fast hydrolysis reaction.Alkanolamines remove CO2 from gas stream by the exothermicreaction of CO2 with the amine functionality of the alkanolamine.Both amines have been extensively studied in the previous researchworks [18] but most of the study limited to low concentration CO2

absorption from flue gas and hydrocarbon effect is not taken into

Table 1Concentration of main and toxic trace components of landfill gas.

Constituents Concentration

This paper Brousseauand Heitz[20]

Christensenet al. [10]

Main components(vol.%)

Methane 47–55% 53.3 50–60Carbon dioxide 45–52% 25.6 40–50

Toxic tracecomponents(ppmv)

AromaticsBenzene 6.7–11 2.1 0.1–4.3Toluene 21.8–44.9 34.9 0.1–163.2Ethyl benzene 4.5–12.5 7.3 0.12–54.4Xylene 4.6–14.4 2.6 0.1–88.2

Chlorinated compoundsChloroform 78.6–183.9 2.45 0–10.2Carbon tetrachloride 41.5–124.3 No data No dataTrichloroethylene 0.9–2.6 2.1 0–58.1Tetrachloroethylene 0.9–6.9 5.2 0–36.9

SulfursHydrogen sulfide 15.1–427.5 0.002 vol.% 0–50.2Methyl mercaptan 12.1–84.9 0–61

636 A. Gaur et al. / Fuel Processing Technology 91 (2010) 635–640

consideration. Moreover, the amount of CO2 was around 5 to 15 vol.%in previous studies. Mostly nitrogen was used along with CO2 insteadof CH4 [19]. In this study, a high concentration of CO2 of around 48%was used, in order to understand how CH4 (present in LFG) affects theabsorption of CO2 in the amine. The main objective of this study is tointegrate two technologies of gas separation one related to theremoval of trace components via adsorption and the other to theremoval of CO2 via absorption from LFG.

2. Materials and experimental methods

2.1. The raw landfill gas

The LFG was collected, stored and transported to a laboratory fromthe Sudokwon landfill site located in the west coast of Inchon city,which is the largest waste treatment site in Korea. Experimentaldiagram of the LFG extraction and collection into gasholder isrepresented in Fig. 1. The LFG was compressed to gasholder from anextraction well. The static pressure of the extraction well is around0.2–0.3kgf cm−2. The moisture content of raw LFG was removed bypassing through cyclone type dehydrator and compressor. Thecompressor had 3 HP and compressed up to 50kgf cm−2 in thegasholder. The dehumidified gas was then collected in the gasholder.The composition of raw landfill gas from extraction well is shown inTable 1 and found to have similar pattern with other reported data by[20]. The concentrations of themain components CH4 and CO2 are 47–55 vol.% and 45–52 vol.% respectively. There are three groups of toxiccomponents such as aromatics, chlorinated compounds and sulfurcompounds. The moisture content of raw landfill gas was removedusing the cyclone type dehydrator and compressor.

2.2. Activated carbon and chemical absorbents

Among various adsorbent materials, activated carbon was selected,as it is a very versatile adsorbent. They have a very large surface area,good micro porous structure, high adsorption capacity and high degreeof surface reactivity and it is cheaply available. The physical properties ofgranular activated carbon used in this study are shown in Table 2.Granular activated carbon is used in this study whose apparent densityis around 0.4 to 0.43 g cm−3 and the particle size is around 1.0 mm to1.41 mm. The average pore diameter is between 1.4 and 1.8 nmand BETsurface area range from 1100 m2 g−1 to 1200 m2 g−1. Activated carbonwas obtained from Samchonli limited. MEA and DEA used in this studywere obtained from Sigma-Aldrich with a mass purity N99%. Theaqueous solutions were prepared from doubly distilled water. Allsolutionswere prepared bymass with a balance precision of 1×10−4 g.

2.3. Analytical procedures

Gas samples were taken from raw gas and product gases. Maingases CH4 and CO2 were quantitatively analyzed using a Non-Dispersive Infra Red gas analyzer (NDIR) obtained from KinscoTechnology Company of Korea. Aromatics (benzene, toluene, xylene)

Fig. 1. Schematic diagram of LFG collection from the extraction well.

and chlorinated compounds (chloroform, carbon tetrachloride, tri-chloroethylene, tetrachloroethylene) were analyzed according to theNational Institute for Occupational Safety and Health (NIOSH)manualof analytical method. HP 5890 GC with a flame ionization detector(FID) using a OV-1WHP100/120 column (HP) and a 1% SP-1000 60/80carbopackB column (Supelco) were used to analyze aromatics andchlorinated compounds, respectively. Sulfur compoundswere directlyanalyzed by using a HP 5890 series II GC with sulfur chemilumines-cence detector (GCSCD, Sievers Model 355) using 30 m long, 0.32 mmID, 4 mm film thickness SPB-1 capillary column (Supelco). Theexperiment was repeated successfully for a couple of times to checkuncertainty which was found to be ±1%.

2.4. Experimental setup and procedure

The equipment used in this experiment consists of two parts(shown in Fig. 2), one is for adsorption for the removal of toxic tracesand the other is for absorption of CO2 from the purified LFG in order toseparate CH4. The gasholder was connected to the mass flowcontroller at the inlet section of adsorption. Further moisture wasremoved from the gas using a calcium chloride impinger. In the nextstep gas was fed to adsorption tower and regularly monitored usingGas chromatograph. The adsorption tower used in this study has a bedlength of 10 cm having the inside diameter 1 cm, the outside diameter1.3 cm, the packing length 3.6 cm and the amount of packing usedwas1.2 g. The remaining section in both ends of the column was filledwith bead type ceramic ball with 2 mm outside diameter to maintainthe uniform gas distribution and prevent the carryover of adsorbentparticles. The landfill gas free from the trace components was fed tothe absorption apparatus. The apparatus consisted of a saturator and areactor. The gas obtained from the adsorption column was fed to thesaturator and afterwards passed to the reactor. Around 300 g of aminesolution was put into the reactor. The reactor was maintained at thetemperature of 303 K using water thermostat during absorption. Thesystem was purged with nitrogen to remove any air from the system.A condenser was used to cool down the vapors of the solution. An

Table 2Physical properties of granular activated carbon.

Property Granular

Apparent density (g cm−3) 0.4–0.43Particle size (mm) 1.0–1.41Average pore diameter (nm) 1.4–1.8BET surface area (m2 gm−1) 1100–1200

Table 4

Fig. 2. Schematic diagram of adsorption and absorption experimental apparatus.

637A. Gaur et al. / Fuel Processing Technology 91 (2010) 635–640

NDIR analyzer was used to continuously monitor the concentration ofCH4 and CO2 in the outlet gas. The flow rate of incoming gas was keptconstant at 1.6 L min−1 using a flow meter. Desorption experimentwas carried out after the absorption was complete. The reactor washeated to 343 K to remove CO2 from the saturated absorbent. Theexperiment for absorption and desorption was done in three cycleswithout changing the absorbent for calculating the cyclic capacity andfinding the best absorbent. Cyclic capacity is the real effective CO2

loading capacity which is calculated from the difference between CO2

rich loading of absorption and CO2 lean loading of desorption.

3. Results and discussion

3.1. Effect of activated carbon

In order to determine the adsorption pattern of each tracecomponents model gases were used and experimental trials wereinitially performed at two concentrations (400 and 600 ppmv) foreach model adsorbate. The adsorption capacity of the three mainmodels adsorbate benzene, toluene and ethyl benzene is tabulated inTable 3. The amount of benzene adsorbed at 400 ppmv concentrationis 0.301 g/g AC whereas at 600 ppmv concentration it is 0.343 g/g AC.The breakthrough time for benzene at 400 ppmv concentration is100 min and at 600 ppmv concentration is 75 min. As compared tobenzene the amount of toluene adsorbed at 400 and 600 ppmv ismore. At 400 ppmv amount of toluene absorbed is 0.402 g/g AC and at600 ppmv it is 0.497 g/g AC. The breakthrough time for toluene at400 ppmv was similar to benzene at 100 min but the breakthroughtime at 600 ppmv was slightly more than benzene. The third purecompound was ethyl benzene whose adsorbed amounts at 400 ppmvand 600 ppmv were 0.627 g/g AC and 0.72 g/g AC, respectively. Thebreakthrough times for ethyl benzene at 400 ppmv and 600 ppmvconcentrations were 105 min and 100 min, respectively. The break-through time is the time needed to detect 1% of inlet concentration.These results show that when pure compound adsorption was done,the amount of ethyl benzene adsorbed was more than toluene thatwas more than benzene. From these results it was found that first thetoxic trace components have different adsorption capacity on theactivated carbon, second the order of the adsorbed amount was

Table 3Adsorption characteristics of pure benzene, toluene and ethyl benzene by using modelgases.

Components Concentration(ppmv)

Pure compoundadsorbed amount(mg/g AC)

Breakthrough time(min)

Benzene 400 301 100600 343 75

Toluene 400 402 100600 497 83

Ethyl benzene 400 627 105600 720 100

tolueneNethyl benzeneNbenzene when LFG free from moisture wasused. At the simplest qualitative level, the observed order ofadsorption capacity may be attributed to the adsorbate–adsorbentinteractions between π-electron of benzene and adsorbate. When anelectron donating methyl group was introduced in benzene (as intoluene), the electron density of π-electron cloud increases [21] andthese interactions become stronger and this should lead to highervalue of adsorption capacity than benzene. The introduction of ethylgroup in benzene (as in ethyl benzene), should cause furtherenhancement of these attractive interactions and it should furtherincrease the adsorption capacity. This is indeed true in our case(Table 3).

The LFG from the gasholder was used in adsorption experiment.There was a small deviation from the concentration of the feed gas atthe sampling time of the LFG. The different adsorption characteristicswere determined by detecting the influent and effluent concentra-tions at the same time because it was quite difficult to maintain theconstant feed concentration. In the process of compressing LFG, allsulfur compounds were eliminated in the gasholder so adsorptioncharacteristics of sulfurs were not determined during the course ofthis experiment. The adsorption characteristics of benzene, toluene,ethyl benzene and xylene (BTEX) in LFG are tabulated in Table 4. Thebreakthrough times of benzene, toluene, ethyl benzeneandxylenewere330, 520, 700, and 820 min, respectively. The adsorption capacities pergram of activated carbon of benzene, toluene, ethyl benzene and xylenewere 12, 83, 31, and 42 mg, respectively. Similar trends were alsoreported by [22] for the adsorption of BTEX vapor on activated charcoal.During the course of the competing adsorption in multicomponentsystem, adsorbates with strong interaction forces displace less firmlybounded substances with relatively short breakthrough time. Size andstructure of themolecule also affects adsorption. In the case of aromaticsbenzene, toluene, ethyl benzene and xylene, multi-components wereadsorbed completely onto the activated carbon surface at the beginningof the adsorption process. Later, weakly bounded benzene was foundincreasing its concentration in the effluent gas showing the benzenebeing replaced by other aromatics. Among toluene, ethyl benzene andxylene, toluene adsorbed maximum in spite of the higher π-electrondensity of xylene. This might be due to the steric hindrance offered byincreased molecular size of xylene and ethyl benzene due to thepresence of bulky two-methyl and ethyl group, respectively. Theadsorption characteristics of chlorinated compounds are also shownin Table 4. The breakthrough times of chloroform, carbon tetrachloride,trichloroethylene, and tetrachloroethylene were 120, 240, 360, and360 min, respectively. The adsorption capacities per gram of activatedcarbon of chloroform, carbon tetrachloride, trichloroethylene, andtetrachloroethylenewere 73, 104, 5, and 4 mg, respectively. The smalleradsorption capacity in case of trichloroethylene, and tetrachloroethy-lene might be due to the large size and non polar nature of thesemolecules which make their adsorption into the pore of AC moredifficult [23,24]. The AC adsorbed toxic trace components in the

Adsorption characteristics for compounds present in Landfill gas as feed gas.

Compound in LFG Adsorbed compound(mg/g AC)

Breakthrough time(min)

Aromatic compoundsBenzene 12 330Toluene 83 520Ethyl benzene 31 700Xylene 42 820

Chlorinated compoundChloroform 73 120Carbon tetrachloride 104 240Trichloroethylene 5 360Tetrachloroethylene 4 360

Fig. 3. Equilibrium breakthrough curve between CO2 and absorbent through absorption and desorption steps.

638 A. Gaur et al. / Fuel Processing Technology 91 (2010) 635–640

following order; carbon tetrachlorideN tolueneNchloroformNxyleneNethyl benzeneNbenzeneN trichloroethylene≈tetrachloroethylene.

3.2. Chemical absorption of CO2

From the adsorption results, purified LFG without trace compo-nents was obtained. The concentrations of CH4 and CO2 were 52 vol.%and 48 vol.% respectively. This mixture was fed to an apparatus ofabsorption to find the ability of CO2 separation with MEA and DEA,respectively, from the landfill gas. In Fig. 3 the variation in measuredCO2 equilibrium curve as a function of time is displayed. From theabsorption results, it can be noticed that the maximum amount of CO2

was removed by the 30 wt.% MEA. When the concentration of MEA

Fig. 4. Reaction rate throughout ab

was reduced to 20 wt.% the amount of CO2 absorbed was lesser butstill MEA absorbed large amount of CO2 as compared to 30 wt.% and20 wt.% DEA. In the case of desorption step, the secondary amine DEAis more effective in desorbing CO2 than primary amine MEA. Fig. 4shows the absorption rate and desorption rate as function of time forMEA and DEA. The absorption rate for CO2 capture from mix gas wasdefinedby loading capacity [CO2mol L−1 of absorbent solution] per unitof reaction time [min]. The rate of desorption was also defined by usingthe same unit for CO2 released. Among the two absorbentsMEA showedhigher absorption reaction rate than DEA and 30 wt.% MEA displayedthe highest average absorption rate of 6.64×10−5 mol L−1 min−1. Incase of desorption, DEA displayed a much better desorption rate ascompared to MEA. The reason for higher absorption rate of MEA lies in

sorption and desorption steps.

Fig. 5. Carbon dioxide loading for the three cycles of the experiment.

639A. Gaur et al. / Fuel Processing Technology 91 (2010) 635–640

the structure and size of MEA which is smaller than DEA. The large sizeof DEA offers more stearic hindrance during the interaction of CO2 withamino group and hence reduces the extent of absorption in DEAmolecule as compared to MEA. This might be responsible for theimproved desorption rate and less regeneration energy of DEAmentioned by [25,26]. The characteristics of desorption are also animportant point to select the absorbent because high desorptioncapacity decreases the reboiling heat and saves the regenerationenergy. The rich amine CO2 loading in absorption step and lean amineCO2 loading in desorption step can be determined by the area of curveformed by CO2 outlet concentration and reaction time for theseabsorbents. TheCO2 loading data through three cycleswithout changingthe absorbents are displayed in Fig. 5. The cycles consist of absorptionfollowed by desorption and same cycle again. It is interesting to notethat CO2 loading capacity increased linearly with an increase inabsorption time and reverse happened during desorption. The CO2

Table 5Cyclic capacity of different absorbents through the regeneration process.

Absorbent Conc.(wt.%)

Maximum rich loading(mol [sorbent L]−1)

MEAa 30 2.920 2.10

DEAa 30 1.6620 1.20

MEAb 30 0.9120 0.83

DEAb 30 0.9520 0.91

MEAc 30 0.8920 0.78

DEAc 30 0.8520 0.83

a First cycle.b Second cycle.c Third cycle.

loading capacity increased rapidly during the first few minutes ofabsorption and after some time it displayed little capacity. For everyabsorption cycle the temperature was kept constant (303 K), themaximumof CO2 rich loading of 30 and 20 wt.% ofMEAwas 2.9 mol L−1

and 2.1 mol L−1, respectively, during the first cycle of absorption(Table 5). However, for DEA this loading was 1.66 mol L−1 and1.2 mol L−1 for 30 and 20 wt.%, respectively, for the first cycle. In caseof desorption step, the minimum amount of CO2 during lean loading of30 and 20 wt.% MEA at 343.15 K was 2.19 mol L−1 and 1.43 mol L−1

respectively, while for 30 and 20 wt.% DEA, it was 0.7 mol L−1 and0.5 mol L−1, respectively. Cyclic capacity is an important parameter foraccurately estimating the size and energy requirements for pilot or fullscale plant for CO2 capture from landfill gas. The cyclic capacity wascalculated for every cycle from the difference between maximum CO2

rich loading and minimum CO2 lean loading for respective cycle andshown in Table 5. The cyclic capacity decreased, in case of 30 wt.% MEA,

Minimum lean loading(mol [sorbent L]−1)

Cyclic capacity(mol [sorbent L]−1)

2.19 0.711.43 0.670.70 0.860.50 0.700.33 0.580.40 0.430.18 0.770.26 0.650.36 0.530.39 0.390.13 0.720.22 0.61

640 A. Gaur et al. / Fuel Processing Technology 91 (2010) 635–640

by almost 18% during the second cycle and 9% during the third cyclewhereas this was around 10% and 6% in case of 30 wt.% of DEA. Undersame temperature and gas flow condition DEA displayed a higher cycliccapacity in the first cycle. After the first cycle, MEA was not able tomaintain its high cyclic capacity. There is a large decrease in absorptionrate as well as in absorption capacity after the first cycle. The averageabsorption and desorption rates also display some decrease for everyadditional cycle. Thus,MEAwasnot an effective absorbent for absorbingCO2 over a longer period of time because cyclic carbamate or urea iscreated by heat as the residual products of reaction. Formation of cycliccarbamates acts as a hindrance in absorption of CO2 and reduces theeffective concentration of absorbent, thus resulting in the decrease ingas solubility/absorption which lowers the CO2 cyclic capacity after the1st cycle. Similar effects were also observed in DEA but to the lesserextent when compared to MEA. Another interesting observation madeduring this study was the effect of CH4 on the absorbent. CH4 did notaffect the absorption of CO2 much. Since the start of the experimentthere was no notable change in the concentration of CH4 for both MEAand DEA. Aminesmight be absorbing very small amount of CH4 that canbe neglected in comparison to the large amount (54%) of CH4 in the LFG.Thus, we can say that there was no noticeable change in theperformance of amine by the presence of CH4 in the feed gas.

4. Conclusions

The poisonous compounds present in the LFG were removedvia adsorption using the activated carbon. The activated carbonadsorbed harmful trace components in the following order: carbontetrachlorideN tolueneNchloroformNxyleneNethyl benzeneNbenzeneNtrichloroethylene≈ tetrachloroethylene. Two absorbents, DEA andMEA were used for separating CO2 from landfill gas free from tracecompounds. In the absorption–desorption step though MEA showedhigher absorption rate than DEA but DEA displayed better cycliccapacity as compared to MEA. The cyclic capacity decreased, in caseof 30 wt.%MEA, by almost 18% during the second cycle and 9% duringthe third cycle whereas this was around 10% and 6% in case of 30 wt.%of DEA. Thus, MEA was quite effective in the first absorption cyclebut it was unable to effectively remove CO2 in the second and thirdcycles.

Nomenclature

HP horse powerDEA diethanol amineMEA monoethanol amineAC activated carbon

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