EFFECT OF WC1 ON SULFIDATION OF CALCIUM OXIDE

J, ABBASIAN, J. R. WANGEROW and A. H. HJLL Institute of Gas Technology, 3424 S. Stale Street, LYhicago, IL af6, U.SA.

Abmw-The sulf&tion 01 calcined limestone and dolomite was studied in an ambient pressure quartz thermogravimetric analyzer (TGA) reactor unit at 982°C Comparative sulRdation experiments were pet-lurmed with and without the prcxence olHCI using txlth calcined limestone and dolomite in two particle siaes. The reactant gas contained H,S, H,, H,D, and nitrogen. The H,S and HCI content ofthe reactant gas was chosen to represent oxygen-blown and air-blown coal gasification environments. The muIts of this study indicate that the addition of both steam and HCI enhances the overdl rate ofsulfidation reaction and that the rate is linearly depndent un the KC1 cxmterst of the reactant gas up 1~ 100 ppm for limestone and 150 ppm for dolomite. The sulfidation rexlion rate in insensitive to HCI concentrations above thm levels fat thcsz sorhnts. The rcsdts obtained in this study sugge~ that the enhancemmt of the sulfidaridn reaction rate in the presence ol HCI is eaused by surface phenomena that are related to rhc prssm~ ol steam.

INTRODUCTION

Some coals, in addition to having high sulfur contents, also have high chlorine contents. During gasification of coal, the sulfur compounds present in coal are converted to hydrogen sulfide (H,S) while the chlorine in coal is converted to hydrogen chloride. To comply with environmental regulations, a high frw tion of sulfur and chlorine compounds must k re- moved from the gas stream.

One approach for removal of H,S from fuel gas is in situ desulfurization using calcium-based sorbents such as limestone and dolomite. The available experirnentaf data of the reaction between hydrogen sulfide and calcium-based sorbents (Abbasian et al., !99Oa,b; Rehmat et al., 1987) indicate that calcined limestone and dolomite can remove up to 90% OF the hydrogen sulfide from the fuel gas within the gasi6er:

CaO + H,S e CaS + H,O. (A)

Magnesium oxide in the calcined dolomite does not react with H,S at the operating conditions prevailing in the gtiifiet; therefore, it is treated as inert in the solid sorbcnt. The high chlorine content has no ad- verse effect on coal conversion in the gasifier. hs a matter of fact, higher chlorine content has been pre ven to have svme catalytic effect on coal conversion [Hill er al., 1987).

The effect of chlorine compounds on reactions in- volving calcium-bad sorbents and H,S has not been addressed in the literature; however, tests in coal combustion and MSW incineration have shown that sulfur removal effitiency is improved in the presence of HCl (Chang et al., 1989; Cooper and Johnston, 1989; Blythe and Rhudy, 1989; Lange PI al., 1987). The results of the study by Blythe and Rhudy indicate that, by spiking calcium chloride to simulate a coal with an approximate O.054.25 wt% chloride level, SO1 removal eficiency was increased by l&20%. In another study (Lange et ul., 1987) the SO, removal efficiency in 3 dry sorbent injection system more than

doubled when about 250 ppm of HCl was added to the system. The exprimeatal data repvr&d in the above studies are from large-scale equipment.

It has been postulated that the beneficial effect of HQ on SO2 removal involves the formation of C&I, through the reaction

ZHCI -t CaO # CaCI, + H,O. (B)

Figure 1 shows the equilibrium HCl concentration as a function of the temprature and steam content of the gas mixture. The chlorine content of a &as stream in a coal gasifier vr coal cvmbustor is usually well below 1000 ppm. From Fig. 1 it is clear that at temperature above WOOC, a sieam content as low as 2% will move reaction (B) towards the Ieft, making C&l, unstable in the gasifier or combustor environ- ment; however, the short-lived formation of CaCI, on the surface of the lime [Ca(OH)2] can improve the surfa= absorption of gas moisture, which has been shown to improve lime reactivity.

This study has focused on the determination of the effect of hydrogen chloride on the rate of reaction between hydrogen sulfide and two calcined calcium- based sorbents (a limestone and a dolomite). The result ol this work can be used in the optimization of the calcium-to-sulfur ratio based on the chlorine con- tent of the coal.

EXPEBIMEKFAL Two calcium-based sorbents (a limestone and a

dolomite) were sefeckd and used in this investigation. The sorbents were crushed and screened into narrow particle size ranges. Two cuts from each sorbent were selected for testing in this program. The selected cuts included coarse particles (- 20 + 30 mesh, (7, = 0.07 cm) and fine particles (- 643 + SO mesh, & = 0.02 cm).

The chemical composition of both cuts of the dolomite and the limestone sorbents are presented in Table 1. The chemical compositions of different cuts

2690 J. ARRAWN et ~1.

Ca0+2HCI=CaCl2+H20

Fig. 1. Equilibrium concentmtion of HCL.

Limstone Dolornile

Analyses @t*/$) Coarse Fine Coarse Fine

Calcium 38.7 39.6 22.2 22.3 Magnesium 0.59 0.52 13.2 14.1 Potaksium 0.5 0.5 0.5 0.5 Iron 0.084 0.10 0.11 0.16 AIuminum 0.05 0.05 0.069 0.071 Siliwn 0.11 0.10 0.25 Slonrium 0.015 0.017

:&I5 0.005

Carbon dioxide 44,7 44.5 4x10 4X.2 Oxygen (by diff.) t 5.25 1 14.613 15.616 14.414 Total 100.00 100.00 100.00 1clcl.Qo

of the same sorbents are very similar. The average calcium content of the limestone is 39.2%. which corresponds to 98% calcium carbonate. The molar ratio of calcium carbonate to magnesium carbonate in the dolomite is 0.98, indicating that both the lime- stone and the dolomite are reasonably pure.

The rates of reaction between calcined sorbents and H2S were determined in a thermogravimetric analyzer (TGA) reactor system. The schematic diagram of the TGA unit is shown in Fig. 2. All wetted parts of the unit that come in contact with the HCI were changed to either quartz or Teflon@ to prevent corrosion.

The procedure for these tests included heating the sorbent in nitrogen atmosphere (upward flow) to 982°C to calcite the sorbent. At this point, steam is introduced into the bottom of the reactor by bubbling a metered flow of nitrogen through the saturator, that is maiatained at a predetermined temperature. The gas mixture pontaining H,S, H,, and N, with the predetermined composition is then introduced into the reactor. The ratio of H,/H,S in the gas mixture

was tintained at 4 to avoid HIS decomposition at high temperatunzs. The mixture of reactant gas and steam flows upward past the sortit while Ihe changes in the weight of the sample exposed to the reactant gas mixture is monitored and recorded until the sample reaches its maximum weight.

The test parameters included the effects of steam, HCl content of the reactant gas, H2S concentration, sorbent type, and sorbent particle size on the overall sulfidation reaction rate. The H,S and HCI concen- *ration corresponded to those of air-blown as well as oxygen-blown gasifiers using coals containing 24 wt% sulfur, with limestone as the in-bed de- sulfurization sorbent.

The sfkxt of the steam content of the fuel gas on the sulfidation of akined limestone in oxygen-blown gasification is presented in Fig. 3, indicating that the rate of suliidation reaction increases with the addition of steam to the gas. The rate of sulIidation reaction,

L__.J WATER mMxm_rr WT

FLOWMETERS

U U

Fig. 2. Process flow diagram of the ambient pressure quartz TGA unit.

PRIs3JRE: 1 awn TEMPEFiATuRE: aant SDRBENT CALCINED LIMESTONE PARTICLE OIAMETER: 0 M cm HCI CONCENTRATDN: 0 ppn &S CONCENTRATION: 1% , I

2 4 TIM!, min

Fig. 3. Effect of steam on the ~Ifidration reaction rate of fine limestone particks in oxygen-blown gasification.

however, does not appear to be sensitive to steam Similar results were obtained during the sulfidation concentrations above 15 K. The effect of HCI addition of fine calcined dolomite partiel= in simulated oxy- to the reactant gas on the limestone sulfidation reac- gen-blown as well as air-blown gasikation conditions tion rate in air-blown gasification mode with 10% (Fig. 5). In the case of dolomite, however, the rate of steam is shawn in Fig. 4, indicating that the suffid- sutidation reaction appears to be dependent on HCI ation reaction rate increases with an increasing HC1 concentration in the range &150 ppm. The effects of concentration in the range. CL 100 ppm, and is inscns- stt~m and HCl concentration on tbe stidation itive to HCI conoantrations above 100ppm. reaction rate involving eoarae so&em par-ticks

Fig. 4. E&ct of HCI concentration on the sulfidation reaction rate of fine limestone particles in air-blown gasification.

Fig. 5. Effects of steam and HCI on the sulfidarion reaction rate of fine d&mite particles in wxygen-blown gasification.

(2, = 0.07 cm) were similar to those observed with hne particles (dP = 0.02 cm).

Analyses of the reacted samples indicate that CaM, did not form during the course of the exJxximents and this is consistent with the thermodynamic equilibrium data shown in Fig. 1.

The results obtained in this study suggest that the enhancement of the sulftdation reaction rate in the presence of HCI is caused by some surface phenomena requiring certain levels of ACI in the gas stream to activate the reaction sites, ;ind that the mechanism of rate euhanoement is associated with the presence of steam. Unfortunately, bxause of the CaCl, form&ion in the absence of sleam, this cannot be verified inde- pendently.

To obtain further information regarding the reac- tion mechanism, petrographic samples of the partially sulfided sorbents were mounted. sectioned and polished aceorcbng to the method (ASTM D2797) used in coal petrography (ASTM, i981) and analyzed by scanning electron microscope @EM). The partially sulfi&d samples of the sorbents were prepared by mounting the samples in a l-inch-&am&er briquette mold with a mixture of BUEHLER epoxy that con- tained 10% TiCI? pigment. While the epoxy was still fluid, the samples were subjected to a vacuum to force the epoxy and pigment into cracks and pores. This was done to ensure that the sample would not bz pulled out of the epoxy binder during the grinding and polishing steps, The epoxy was allowed to harden

Effect of HCI un sulfidarion of calcium oxide 2693

overnight. The hardened briquettes were removed this sample was 0.07 cm (25 mesh). This sample was from the mold and were polished without water MI 50% sulfided in the ambient, pressure quartz thermo- prevent decomposition of the calcium sulfide. balance. The sulfur map of this particle is shown in

Figure 6 shows a close-up picture of a partially Fig. 7. From this sulfur map it appears that the sulfidti lime particle. The average particle diameter of shrinking-core model is ati appropriate model to

Fig. 6. C%sc-up picture of a partially sulfided limestone particle.

Fig. 7. S~lhur map or a panially, sulf~dd limestone particle.

2694

0.10 ’ L ._ ,... A.. 1

0 100 zoo 300 4vo 600 E[K) 700 BOO 800 1000 HCI iN -REACTANT GAS, ppmv

.Fig. 8. Dependence al the sulfidatiun reaction rats on the I-El content Al the reactant gsw.

describe the sulfidatioa reaction because a partially sulfided particle consists of calcium oxide in the center and calcium sulfide in the outer layer.

To quantify .the effect of. HCI on the overall sulf- idation rate, the shrinking-m model for spherical solid particles of. unchanging size. (Levenspiel 1967) was applied to the expwimental data obtained from sulfidation reaction tests conducted in this study. Based on this model, if the overall reaction rate if totally controlled by the chemical reaction. then

F,(x) = 1 - (1 - x,&3 = ; (1)

where xcao ix the extent of the calcium oxide conver- sion, t is the reaction time, atid ‘E is the time required for complete conversion, which is expresd as

In cq, (2), pm is the molar density of the calcium oxide in the sorbent, R is the radius of the solid particlq k, is the intrinsic reaction rate WrIstant, [HzS J is the concentration of hydrogen sulfide, and n is the order of reaction with respect to hydrogen sulfide concent@ion. If the overall, rate of reaction is

totally controlled by the.intraparticIe diffusion of the reactant gas through the product layer, then

F,(x) = 1 - 3(1 - x&l3 + 2(1 - x&= ; (3)

where

In eq. (4), D, is t,he effective diffusivity OC the re- actant gas through the solid product layer of the solid particle.

Both F,(r) and F2(x) were plotti against reaction time in the CaO conversion range of 0 d x C 0.66 representing the range of sorhnt conversion during

the in situ desulfurization with calcium-to-sulfur ratio of greater than 1.5.

F,(x) was not linear with respect to reaction time while F>(X) was nearly linear. This indicates that although tht enhancement of the overall reaction rate is believed to be related to the chemical reactivity of the sorbent, the interparticle diffusion. appears to be the major factor in controlling the overall reaction rate to the extent that it can be used alone to approx- imate the ouerall reaction rate to quantify the benefi- cial effect df HCl on the sulfidation reaction rate. The slope of the plot (i.e. I/z) represents the ovwall reaction rate constant. A plot of 1,‘~ vs the HCl content of the reactant gas is presented in Fig. 8, indicating that the overall sulfidation rate is linearly dependmt on the HCI mntent of the reactant gas in the range &tOO ppmv for limestone and &i50 ppmv for do- lomite. The overall sulfidation rate is increased by 2550% in the presence of sufficient amount of HC’I in the reactant gas.

CONCLUSIONS AND RECOMMENDATIONS The following conclusiotis are made from the work

performed in this study:

Addition of steam to the reactant gas enhances the sulfidation reaction rate. Addition of HCl to the reactant gas (containing stem) enhances the sulfidatiofl reaction rate further. The sulfidation reaction .rate is insensitive to the steam content of the reactant gas.above 10%. The reaction ofHsS with Ca-based sorbents can he described by a shrinkingcore model. The overall-&ion rate is increwd by 2550% ia the vnce of sufficient amoin! of HCl in the reactant gas.

AcknowI&gernent+Thc aurhors thank Dr Frank Honea ol the Center for RcJtcarch on Sulfur in Coal For his valuable contributions during the course of’ this study. This work was

Effect of I-ICI on sdfidation of calcium oxide 2695

prepared with the support, in part, by grants made Fsible and SO, from MSW incineratw flue gas. Paper presented bv the Illinois Denartment of Enernv and Natural Resources at CRSO’EPRI First Inwrnational Merence on Chlor- &ough its Coa~Development 3&d and Center for Re- ine in Coal, October 1989, Chicago. search on Sulfur in CoaZ and the US Dqxzrtment of Enerpgr Coowr, J. R. and Johnston. I. A.. 1989. The ini%tence of coal [Grant Numbers DE-FG21-90MC27ti (METC) and DE- -

_ _

FG22,90PC90176 (PETC!)]. Any opinions, findings, conclu- chlo&re content upon the FGD prwess system con6gura- tion lor DRAX (UK) power station. Paper presented at

sions, or recommendations expressed herein, however, are CRSC/EPRT First lnteruational Conference on Chlorine those of the authors and do not necessarily refleot the views in Coal, October 1939. Chicago. of IDENR, CRSC and the DOE. Hill, A. H., Anderson, G. L., Feldkirchner, H. L. and

Punwani. D. V.. 1987. Testina of novel catalvtic coal

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