ca ion-exchanged coal char as h2s sorbent

Ca ion-exchanged coal char as H2S sorbent

Benjamin Garcia* , Takayuki Takarada

Department of Biological and Chemical Engineering, Gunma University, 1-5-1 Tenjin-cho, Kiryu 376, Gunma, Japan

Accepted 29 September 1998

Abstract

H2S removal using Ca ion-exchanged coal char was studied in a fixed bed reactor at a temperature of 9008C. Yallourn, an Australian browncoal was used for ion exchange. The CaO obtained from the pyrolysis of Ca ion-exchanged coal was finely dispersed in the char and muchmore reactive toward H2S than that from limestone. Ca ion-exchanged coal char showed high H2S capture and almost no emission of H2S wasobserved in the earlier reaction stage. At a reaction time of 100 min, the S/Ca atomic ratio in Ca ion-exchanged coal char increased to 1.2.This was due to H2S capture capacity of both the coal char itself and the CaO distributed in the char. The sorption capacity of char wasdetermined as 1.5 mmol-H2S/g-char. The desulfurization characteristics of calcined limestone strongly depended on the particle size,whereas those of Ca ion-exchanged coal char was almost independent of the char particle size.q 1999 Elsevier Science Ltd. All rightsreserved.

Keywords:Ca ion-exchange; Ultra-fine CaO particles; H2S removal

1. Introduction

Coal gasification is a versatile method of coal utilizationwhere the coal gas generated can subsequently be used toproduce energy. Clean coal technology has become a stan-dard phrase in recent days. It has generally been taken asreferring to methods of producing power and heat in whichlow emissions of sulfur and nitrogen oxides are includedinto the design [1]. The topping cycle (or advanced-PFBC,as it is named in Japan) is one of the advanced coal-firingtechnologies for power generation which combines featuresof systems based on gasification and combustion. The mainadvantage of the topping cycles over the other systems isthat topping cycles can exploit the potential of high inlettemperature gas turbines, while minimizing the energylosses incurred in the production of fuel gas. It is wellknown that H2S is the most abundant sulfur-containingcompound emitted from coal gasification. Therefore, it isnecessary to remove the H2S as much as possible from thecoal gasification stream to protect the turbines and relatedequipment from the corrosive action of this compound. Hotcoal desulfurization, which uses solid sorbents such asoxides of metals (e.g. of iron, manganese, zinc, and vana-dium) and limestones, possesses several technical andeconomic advantages over low temperature desulfurizationprocesses.

Limestones and dolomites are the most common sourcesof calcium oxide used as sulfur sorbent. However, its lowconversion makes it necessary to oversupply the amount ofsorbent to obtain an acceptable sulfur capture [2]. The ther-mal decomposition of calcium carbonate and the sulfidationof the calcium oxide produced can be described by the over-all reactions:

CaCO3 ! CaO1 CO2 �1�

CaO1 H2S! CaS1 H2O �2�The reaction of CaO with H2S is of technical interest in

both gasification [3, 4] and fuel rich combustion systems [4,5] when CaO is used as a sulfur sorbent. Experimentsperformed in TGA showed a weak influence of temperatureon the initial sulfidation rates, and low activation energieswere obtained (5.16 kcal/mol [6] and 3.6 kcal/mol [7]). Thesulfidation experiments realized by Westmoreland et al. [6]gave a linear relation between the initial reaction rate ofreagent grade CaO and the H2S concentration in the reactivemixture at temperatures in the range of 3008C–9008C.Evangelos and Stratis [8] found that highly porous sorbentparticles are produced from limestone or dolomite calcina-tion and, as a result, the overall rate of sulfidation reaction isin general influenced by both the transport in the intraparti-cle space and surface reaction. Moreover, since the solidproduct (CaS) in the reaction Eq. (2) has larger molarvolume than the solid reactant (CaO), various phenomenaassociated with pore structure (such as pore plugging and

Fuel 78 (1999) 573–581

0016-2361/99/$ - see front matterq 1999 Elsevier Science Ltd. All rights reserved.PII: S0016-2361(98)00183-5

* Corresponding author. Tel.:181-277-30-1450; fax:181-277-30-1454.

formation of inaccessible pore space) may play an importantrole in determining the reactivity of the solid with theprogress of its sulfidation. Therefore, the sorbent utilizationstrongly depends on its particle size [9], and high utilizationwill be expected by using sorbents with small particle size.

Low rank coals such as brown coal and lignite have alarge amount of oxygen-containing functional groups andthe proton on carboxyl groups is easily exchanged withmetal cations. When Ca-exchanged low rank coal is pyro-lyzed, the carboxylate salt dissociates, releasing carbondioxide and leaving highly dispersed calcium compoundssuch as CaO and CaCO3. The chemical form of the calciumcompound depends on the pyrolysis conditions. It is wellknown that finely dispersed calcium compounds have aquite large catalytic effect for char gasification in O2, CO2

and steam.In the present study, a new desulfurization technique

using Ca ion-exchanged coal as an H2S sorbent in thetopping cycle system was proposed. That is to say, the Caion-exchanged coal with raw coal is fed into the gasifier,where the coal gasification and the desulfurization takeplace. A highly effective desulfurization is expected byusing ultrafine CaO particles produced from the Ca ion-exchanged char. The remaining chars and the producedCaS are transferred into the fluidized bed combustor,where the chars are burnt and the CaS is oxidized toCaSO4 for safe disposal. The scope of this study does notcover the aspect of the CaS oxidation. Fine limestone parti-cles of sub-micron meter size are able to be prepared bymechanical grinding, but an enormous energy for grindingwill be needed. Moreover, since fine particles easily makeaggregates, it is quite difficult to handle fine particles in thedispersion state. In this technique, there is no problem inhandling fine CaO particles, because fine CaO particles areincorporated into large coal char particles. In the toppingcycle system, the chemical form of calcium compounds in agasifier depends on the gasification conditions. Both CaO

and CaCO3 can react with H2S according to reaction Eq. (2)and the following reaction, respectively.

CaCO3 1 H2S! CaS1 H2O 1 CO2 �3�Equilibrium calculations revealed that CaCO3 was

calcined to CaO under conditions where there was a gasifi-cation temperature of over 9008C and 0.12 CO2 partial pres-sure [10]. In this paper, the reactivities of CaO preparedfrom Ca-exchanged brown coal and limestone toward H2Swere investigated.

2. Experimental

2.1. Material

As the ion-exchange capacity of coal depends on theamount of oxygen-containing functional groups present inthe brown coal matrix [11], low rank coals are the mostappropriate coals to be used as ion-carriers. Yallourn coal,an Australian brown coal, was selected in this study as anion-exchanged coal. Carboxyl and hydroxyl groups in thecoal were determined as 1.7 and 6.0 mequiv/g (daf) of coal[12]. The limestone used was supplied by Yoshizawa LimeIndustry Co. Ltd. It was from Kuzu, Tochigi prefecture inJapan. The analyses of coal and limestone are shown inTables 1 and 2, respectively.

2.2. Ca ion exchange

The calcium addition to the coal matrix was achieved byion-exchange with calcium hydroxide. Yallourn coal (20 g)was soaked in a milk of lime which was prepared by mixing10 g of Ca(OH)2 with 1000 ml of deionized water. The Ca-ion exchange was carried out by stirring the slurry with amagnetic stirrer at room temperature for 24 h. The pH of themixture was kept in the range of 11.8–12.0. The coal wasseparated from the solution by filtration, then washed withdeionized water to remove the excess of Ca(OH)2 and driedunder vacuum at 1078C. The calcium from Ca ion-exchanged coal (YL–Ca coal) was extracted using a dilutedsolution of hydrochloric acid (HCl: 10%). The amount ofcalcium incorporated in the Yallourn coal was determinedby atomic absorption analysis (Shimadzu AA-6400F atomicabsorption flame emission spectrometer). The calciumcontent in the sample was 9.6 wt.%.

2.3. Reaction with H2S

The pyrolysis of YL–Ca coal, as well as the limestonecalcination, were carried out in a fixed bed reactor at 9008Cfor 10 min to decompose the carboxylic sites, and provideCaO crystallites in the char. Complete pyrolysis of YL–Cacoal or limestone calcination was achieved in 10 min. Thepyrolyzed YL–Ca coal (YL–Ca char) was stored in a sealedflask and kept in a desiccator until it was used. Limestonecalcination was done just before each experiment.

B. Garcia, T. Takarada / Fuel 78 (1999) 573–581574

Table 1Analysis of coal. The particle size of the sample used was 0.25–0.5 mm,unless otherwise stated

Sample Ultimate analysis (wt.%, daf)

C H N S O

YL a coal 65.1 4.6 0.6 0.3 29.4YL(Ca)b char 87.4 1.5 0.8 0.2 10.1

a YL � Yallourn.b YL(Ca) char� Calcium ion-exchanged coal char, 15.6% calcium.

Table 2Analysis of limestone (wt.%). The particle size of the sample used was0.25–0.5 mm, unless otherwise stated

Ig. loss SiO2 Fe2O3 Al2O3 CaO MgO

42.11 2.16 0.10 0.06 54.24 0.23

The desulfurization was carried out in a reactor made ofquartz, with an inner diameter of 2.5 cm and length of75 cm. A plate of sintered quartz filter in the reactor wasused as a gas distributor and sample support. The schematicdiagram is shown in Fig. 1. The sample of YL–Ca char(0.75 g) or limestone (0.3 g) put into the reactor was heated

electrically under nitrogen atmosphere at 208C/min to thedesired temperature. The temperature was kept for 10 minbefore introducing the reactant gas. The reactant gasconsisted of a mixture of 1500 ppm hydrogen sulfide andnitrogen. It was fed into the upper part of the reactor. Theexperiment was allowed to continue for 100 min. Since theSO2 analyzer cannot realize direct measurements of hydro-gen sulfide, oxygen was fed just under the plate of sinteredquartz filter with the purpose of converting the unreactedH2S into SO2. The exhaust gas from the reactor was continu-ously analyzed with an SO2 analyzer (Yanako, EIR-500S),and with an infrared gas analyzer (Shimadzu CGT-7000).After 100 min, the flows of H2S, as well as oxygen, weresuspended, and then the reactor was cooled down undernitrogen stream to prevent undesired reactions of theproduct with the atmosphere.

Calcium conversion was defined as the moles of H2Scaptured, divided by the moles of calcium present in thesample.

X � �Cin 2 Cout�CCa

�4�

whereX is the apparent calcium conversion,Cin is the molesof H2S at the inlet of the reactor,Cout is the moles of H2S atthe outlet of the reactor, andCCa is the content of calcium(moles) in the sample.

The characterization of the sample was carried out by X-ray diffraction (XRD) (Rigaku Geigerflex), scanning elec-tron microscopy (SEM) (JEOL JSM-5300LV) and nitrogenadsorption (Quantachrome Autosorb 1).

3. Results and discussion

3.1. Characterization of Ca ion-exchanged coal char

3.1.1. Crystallinity of Ca compoundsYL–Ca coal was pyrolyzed in the temperature range of

7008C–10008C and the crystallinity of the calciumcompounds in the char was determined by XRD analysis.Fig. 2 shows the XRD patterns for YL–Ca chars prepared atdifferent temperatures in a N2 atmosphere. No diffractionpeaks were observed for YL–Ca char prepared at 7008C,in spite of a high calcium content of about 16 wt.%. Thisresult suggests that the calcium compounds in this stagewere finely dispersed in the char. The dispersion mode ofcalcium compounds observed in this study was quite similarto those reported by some researchers [13, 14]. Radovic etal. [15] carried out XRD measurements of calcium-exchanged North Dakota lignite char prepared at 7028Cand reported that no diffraction peaks attributable to calciumcompounds were observed. Ohtsuka and Tomita [13]observed a similar dispersion mode for calcium-loadedcoal chars prepared using Ca(CH3OO)2, Ca(NO3)2,Ca(OH)2 and CaCl2. They also reported that, in a nitrogen

B. Garcia, T. Takarada / Fuel 78 (1999) 573–581 575

Fig. 1. Schematic diagram of fixed bed reactor.

Fig. 2. XRD patterns of YL–Ca char at different pyrolysis temperatures inan N2 atmosphere.


Fig. 3. Pore distribution of Ca ion-exchanged Yallourn coal after heat treatment.

Fig. 4. SEM images of Yallourn coal char (a,b) and Ca ion-exchanged Yallourn coal char (c,d). Pyrolysis: 9008C, 10 min, in N2.

atmosphere, CaCO3 is decomposed to CaO in the temperaturerange 6508C–7008C and the decomposition is promoted bycarbon. When YL–Ca coal was pyrolyzed at highertemperatures, the broad diffraction peaks assigned to CaOappeared as shown in Fig. 2. The average crystallite size ofCaO prepared at 8008C and 9008C was evaluated by theDebye–Scherrer method as 20 and 28 nm, respectively.These results mean that ultra-fine CaO particles are easilyprepared by the pyrolysis of calcium ion-exchanged browncoal. The intensity of the CaO peak was increased with anincrease in the temperature, due to the sintering of the CaOcrystallites [16].

3.2. Specific surface area

Physical properties of samples were determined by nitro-gen adsorption. The BET surface area of YL–Ca char andcalcined limestone prepared at 9008C was 214 and 19 m2/g,respectively. The pore distribution profile of YL–Ca char isshown in Fig. 3. It was found that the micro and mesopores were developed in the char. The large specificsurface area of YL–Ca char observed is mainly due to thesurface area attributed to micro pores. From these results, itcan be said that YL–Ca char was a highly porous materialand well suited to uniformly disperse CaO in the charmatrix.

3.3. SEM observation

The SEM images of Yallourn coal char (YL char) andYL–Ca char prepared at 9008C are shown in Fig. 4. Theouter surfaces for both samples were relatively rough, andno definite difference in the morphology between two charswas observed [Fig. 4(a) and (c)]. A careful observation,however, revealed that small particles of several micro-meters in diameter were often observed on the YL–Cachar [Fig. 4(d)]. The particles may be attributable to calcium

compounds. In this study, calcium was loaded from a milkof lime and the calcium loading was 9.1 wt.%. It is possiblethat a part of the calcium adheres on the char surface and thepore surface in the char and, on heating, the unexchangedcalcium compound forms small particles, as observed withSEM. However, judging from the results of the XRDmeasurements shown in Fig. 2, it seems that the contentof unexchanged calcium compounds in YL–Ca char isquite low.

3.4. Desulfurization

3.4.1. Effect of sorbent typeFig. 5 shows typical profiles of H2S removal for both of

the sorbents studied. Limestone showed a considerablereduction of H2S emissions in the first 10 min, althoughthe calcium conversion achieved was only 12 wt.%. Afterthis period, the emissions gradually increased. The high H2Scapture observed for limestone at the early reaction stagecan be due to the nascent CaO particles formed by thethermal decomposition of CaCO3. Evangelos and Stratis[8] showed that the CaO produced by the thermal decom-position of CaCO3 is highly porous and gives a large surfacearea for the reaction. As the reaction progresses, it can besupposed that the CaS produced was plugging the poreslimiting the gas diffusion and, as an outcome, the H2Scapture decreased. On the other hand, YL–Ca char showeda high capacity for H2S capture, and almost no H2S emissionwas detected for about 45 min. The desulfurization effi-ciency of YL–Ca char was higher than that of limestone.The H2S removals achieved at 100 min of the experimentwere 47% and 80% for limestone and YL–Ca char, respec-tively. These results agree with those obtained from theexperiments of fuel-rich combustion of coals containingcalcium realized by Freund and Lyon [5]. The CaOproduced by the pyrolysis of Ca ion-exchanged coal was


Fig. 5. H2S removal profile for limestone and YL–Ca char at 9008C.

finely dispersed on the char and the char was a highlyporous material, as mentioned previously. The ultrafineCaO particles may considerably reduce the influence ofproduct layer diffusion resistance and the reactant gasmay easily diffuse through the pores in the char. Theseproperties of YL–Ca char are the main reasons for thehigh performance of YL–Ca char as a desulfurizingagent.

3.5. XRD patterns of calcium species

The XRD analyses of reacted limestone and YL–Ca charat different reaction times give information on the progressof CaS formation. Fig. 6 shows the XRD patterns ofcalcined limestone during the reaction with H2S. Thediffraction peaks assigned to CaO and Ca(OH)2 wereobserved in the sample before the reaction. It is consideredthat Ca(OH)2 is formed by the reaction between CaO andwater vapor in an air atmosphere. At a reaction time of30 min, strong peaks attributable to CaS were clearly

seen. After that, the peak intensity from CaS graduallyincreased and that from CaO gradually decreased with anincrease in the reaction time. The XRD pattern at 100 minwas very similar to that at 60 min. From these results, it isconsidered that, in the early stage of the reaction immedi-ately after the calcination of limestone, the CaO produced ishighly porous and the capture of H2S can be carried outefficiently before pore plugging occurs and, in the latterreaction stage, the CaO from limestone is deactivated dueto the product layer formed on the surface of the CaOparticle.

The XRD patterns of YL–Ca char under the same condi-tions exhibited different behaviors during the reaction ofCaO with H2S, as shown in Fig. 7. The gradual increase inthe CaS peak intensity and the decrease in the CaO peakintensity with an increase in the reaction time, indicate thatthe reaction was uninterrupted during the experiment. Theultra-fine CaO particles produced from YL–Ca char seemsto diminish the reaction limitations observed for the CaOfrom limestone.


Fig. 6. XRD patterns for limestone during the H2S removal at 9008C. Fig. 7. XRD patterns for YL–Ca char during the H2S removal at 9008C.

3.6. Calcium conversion

The conversion of CaO to CaS was calculated by meansof gas analysis, as mentioned previously. The apparentcalcium conversion achieved for calcined limestone andYL–Ca char after 100 min was 70% and 120%, respec-tively. The conversion of over 100% observed for theYL–Ca char is due to the fact that the char can capture apart of the H2S. In order to determine the sulfur retention forthe char, the Yallourn coal (0.75 g) was pyrolyzed under anitrogen atmosphere at 9008C for 10 min, and the H2Scapture was done under the same experimental conditionsused for the other sorbents. Fig. 8 shows the profilesobtained for YL char and YL–Ca char. It is clear that thechar is able to retain considerable amounts of H2S. Theamount of sulfur retained in the YL char and YL–Ca char

at a reaction time of 100 min were 1.5× 1023 and 4.7×1023 mol/g-char, respectively.

Assuming that the H2S capture by YL char and CaOcontained in YL–Ca char independently progresses in paral-lel, the calcium conversion at 100 min was estimated as88%. However, in order to obtain precise calcium conver-sions in YL–Ca char, further investigations on the carriereffects of carbonaceous material of char on the reactionbetween CaO and H2S will be needed.

3.7. Effect of particle size

In the case of limestone, it is well known that the sorbentutilization strongly depends on the particle size. The use ofsorbent with a small particle size gives a large surface areaand small resistance to the product layer diffusion. To


Fig. 8. YL char and YL–Ca char profiles for the capture of H2S at 9008C.

Fig. 9. Effect of particle size of limestone during the capture of H2S.

observe the effect of particle size of YL–Ca char on thedesulfurization behavior, YL–Ca chars with mean particlesizes of 0.38, 0.20 and 0.11 mm were prepared. Limestonewas also used for comparison. The profiles of the H2Scapture for different particle sizes of calcined limestoneare shown in Fig. 9. The reaction profile of CaO with H2Swas strongly influenced by particle size. The H2S removalincreased with the decrease in the particle size. This obser-vation agrees with that reported by Evangelos and Stratis[3]. High H2S removal was observed for all the samples usedat the beginning of the reaction. The smallest particle(0.11 mm) showed high activity during 40 min, keepingthe emissions at about 100 ppm. However, comparingwith YL–Ca char, it is noteworthy that in the early stage

of the reaction, the H2S concentration at the outlet gas wasmuch higher than that observed from YL–Ca char. Thesmallest particles (0.11 mm) maintained a high reactionrate until they achieved almost 100% conversion. Thelarge particles showed a high initial reaction rate, but thereaction rate rapidly declined, as is shown in Fig. 9. Thishigh initial rate can be interpreted as evidence that the reac-tion is carried out on the nascent CaO surface and large H2Scapture can be done before the product layer forms. Thisinterpretation is consistent with the behavior of the reactionbetween CaO and SO2, which has been shown to occurthroughout the pore structure [17]. The calcium conversionachieved after 100 min of reaction for the largest particlesize used (0.75 mm) was 43%. The product layer seems to


Fig. 10. Effect of particle size of YL–Ca char during the capture of H2S at 9008C.

Fig. 11. Dispersion of CaO in YL–Ca char at different particle sizes.

be an important resistance to the gas diffusion through thepore when sorbent with large particle size is used.

As is illustrated by Fig. 10, YL–Ca char is much moreeffective for retaining H2S by all the particle sizes used thancalcined limestone. All samples with different sizesachieved similar high H2S capture. In order to investigatethe relation between the particle size of YL–Ca char and thedistribution of CaO particles in the char, measurement of theCaO dispersion were realized. The analysis of CaO disper-sion was carried out according to the method developed byLinares-Solano et al. [18], where CO2 is reacted with CaO at3008C. At this temperature, CO2 reacts only on the surfaceof CaO, therefore, knowing the amount of reacted CO2, it ispossible to calculate the dispersion of CaO. Fig. 11 showsthe CaO dispersion of three different particle sizes of YL–Ca char. Almost no variation in the results is observed,indicating that the particle size of YL–Ca char does notaffect the dispersion of CaO. This result agrees with thatof Fig. 9, corroborating that the particle size and dispersionof CaO is independent of the particle size of the YL–Cachar.

4. Conclusions

The CaO from the calcium ion-exchanged coal wasshown to be a highly efficient H2S removal agent. The reac-tivity of CaO prepared from Ca ion-exchanged coal is muchhigher than that prepared from limestone. The use of Ca ion-exchanged coal char provides high sorbent utilization. The

H2S removal is not a function of the particle size of the Caion-exchanged coal char, because ultra-fine, highly reactiveCaO particles are produced from Ca ion-exchanged coalindependently of its particle size. This characteristic canmake it a useful sorbent to be used in topping cyclessystems.

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ca ion-exchanged coal char as h2s sorbent

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