air8

°“√»÷°…“°“√¥—°®—∫‰Œ‚¥√‡®π —≈‰ø¥å‚¥¬„™âÀ‘πªŸπRemoval of H2S using Raw Limestone and Spray-Calcined Limestonein a Fixed Bed Reactor at High Temperatures

*»Ÿπ¬å«‘®—¬·≈–Ωñ°Õ∫√¡¥â“π ‘Ëß·«¥≈âÕ¡ °√¡ àß‡ √‘¡§ÿ≥¿“æ ‘Ëß·«¥≈âÕ¡ ‡∑§‚π∏“π’ µ.§≈ÕßÀâ“ Õ.§≈ÕßÀ≈«ß ®.ª∑ÿ¡∏“π’ 12120 ‚∑√. 0-2577-1136 ‚∑√ “√. 0-2577-1138Environmental Research and Training Center, Department of Environmental Quality Promotion. Technopolis. Klong 5 Klong Luang, Pathumthani 12120 e-mail: [email protected]**Department of Fuel and Energy, University of Leeds, UK.

N. Milne* and W. Nimmo**

∫∑§—¥¬àÕß“π«‘®—¬π’È ‰¥â¡’°“√∑¥ Õ∫ª√– ‘∑∏‘¿“æ°“√

¥—°®—∫°ä“´‰Œ‚¥√‡®π´—≈‰ø¥å (H2S) ‚¥¬„™âÀ‘πªŸπ

(CaCO3) ·≈–·§≈‡´’¬¡ÕÕ°‰´¥å (CaO) ÷Ëßº≈‘µ®“°

À‘πªŸπ„π drop tube reactor (DTR) ∑’ËÕÿ≥À¿Ÿ¡‘

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H2S „π fixed-bed reactor (FBR) ∑’ËÕÿ≥À¿Ÿ¡‘

√–À«à“ß 873 ∂÷ß 1173 K æ∫«à“„π¢∫«π°“√

calcination ‡¡◊ËÕÕÿ≥À¿Ÿ¡‘ Ÿß¢÷Èπ¡’°“√·ª√√Ÿª¢Õß

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¢Õß√Ÿæ√ÿπ ∑’ËÕÿ≥À¿Ÿ¡‘ 1323 K ª√– ‘∑∏‘¿“æ°“√

·ª√√Ÿª‡ªìπ CaO Ÿß∂÷ß 95% CaO “¡“√∂¥Ÿ¥´—∫

Drop Tube Reactor (DTR)

Fixed - Bed Reactor (FBR)

H2S ‰¥â¥’°«à“ CaCO3 º≈®“°°“√»÷°…“π’È™’È„Àâ‡ÀÁπ«à“

CaO ÷Ëßº≈‘µ‰¥â®“°¢∫«π°“√·ª√√ŸªÀ‘πªŸπ„π

DTR “¡“√∂π”‰ªª√–¬ÿ°µå„™â„π°“√¥Ÿ¥ —∫°ä“´ H2S

„π‚√ßß“πº≈‘µ‰øøÑ“®“°∂à“πÀ‘π‰¥â Õ¬à“ß‰√°Á¥’

ª√– ‘∑∏‘¿“æ°“√¥Ÿ¥´—∫¢÷ÈπÕ¬Ÿà°—∫§ÿ≥ ¡∫—µ‘‡©æ“–

¢Õß«—µ∂ÿ¥‘∫∑’Ë„™â °≈‰°°“√∑”ªØ‘°‘√‘¬“ √«¡∑—Èß‡ß◊ËÕπ‰¢

¢Õß¢∫«π°“√¥Ÿ¥´—∫π—ÈπÊ

ABSTRACTThe effectiveness of raw and spray-

calcined limestone for flue gas desulphu-

risation, in particular, H2S removal from coal

gasif icat ion, has been invest igated by

experimental studies using drop tube reactor

(DTR) and fixed-bed flow reactor (FBR). The

limestone was ground and sprayed into a

DTR for calcination at 1073 and 1323 K.

Sulphidation experiments were performed in

a FBR at temperature range from 873 to 1173

K. For calcination, the higher the temperature,

the greater conversion at shorter residence

time. The surface area, pore size and structure

have influence on the rate of decomposition

of CaCO3 to form CaO. 95% conversion was

obtained at 1323 K. Sulphidation of the raw

limestone (CaCO3), gave less performance in

sulphur conversion than the calcined form

(CaO). The results from this study indicated

that calcined forms of limestone can be

applied for flue gas desulphurisation from

coal-fired power plants. However, the efficiency

depends very much on the characteristics of

the materials used, the mechanisms of

conversion, and the process conditions.

1. IntroductionCoal is the most abundant, safe,

secure and economical fossil fuel. However,

environmental impact from coal-fired power

plants has been very much concerned.

Sulphur Oxides (SOX) emission from fossil

fuel combustion systems has a significant

impact to the environment as it causes acid

rain, which could damage the ecosystem and

human health. Therefore, the implementation

of measures to control the emission of SO2 is

an essential part in limiting such impact.

At present, the retrofitting of large coal-fired

power stations with flue gas desulphurisation

(FGD) units has been the focus of the strategy

in the U.K. for the reduction of sulphur

emissions1. Current technologies involve the

use of calcined limestone as an agent for

sulphur absorption, which is injected into

the flue gas at the appropriated conditions of

temperature and Ca/S ratios. It is proposed

that similar technologies using limestone

may be applied to the removal of sulphur from

the product gas2-7 as part of the development

of the next generation of clean coal gasification

systems, which is so called the integrated

gasification-combined cycle (IGCC). In IGCC,

the coal is gasified in a restricted air (or more

commonly oxygen) supply in the reaction

vessel and the resultant fuel gas is then burnt

with air in the combustion chamber of a gas

turbine. During gasification, impurities arising

from sulphur, nitrogen and chlorine in the coal

are present in their reduced forms (H2S, COS,

NH3 and HCl), which can be removed to

a very low level using conventional technology.

Desulphurisation at high temperatures (up

to 1200 ÌC) would make a substant ia l

contribution in improving the thermal efficiency

of electric power generation in IGCC systems

which can be as much as 2-3%2. Amongst

the possible sorbents for H2S removal at high

temperatures, calcium-based materials have

the advantage of being cheap, abundantly

available, and commonly used as bulk

chemical. Limestone, dolomite, and calcium

hydroxide are the most common calcium-

based sorbents used in this application.

Recently, the use of other materials has been

suggested as alternatives to l imestone

injection, namely the carboxylic acetate salts

of Mg and Ca (CMA), but the studies have been

mainly concern with SO2 capture.

»Ÿπ¬å«‘®—¬·≈–Ωñ°Õ∫√¡¥â“π ‘Ëß·«¥≈âÕ¡ °√¡ àß‡ √‘¡§ÿ≥¿“æ ‘Ëß·«¥≈âÕ¡ §-93

propably due to the onset of the sintering of

CaS crystals forming a new, less prenetrable

coating around the limestone particles. Under

these conditions the rate of sulphidation is

limited by the rate of diffusion of H2S through

the sintered product layer. This is probably

the main reason for the poor performance

of limestone as an absorption medium for

H2S removal.

At temperatures greater than about

900 ÌC - at the conditions mentioned above,

the limestone will calcine at rates faster than

sulphidation to form CaO which reacts relatively

quickly to give CaS. There is evidence to

suggest that at temperatures just above the

calcination temperature the CaO is consumed

almost as rapidly as it is formed (Fenouil and

Lynn, 1995, Part 2)4. At temperature about

50 ÌC above the calcination temperature the

calcination rate is fast enough not to limit the

sulphidation process. Experiments have

shown that at these temperatures (about

950 ÌC), the rate of sulphidation is not affected

by the initial form of the sorbent, whether it

be raw limestone or pre-calcined limestone.

2.3 H2S decomposition and the water-gas equilibrium

The principal components of the gas

mixture which comprise the products of coal

gasification, namely CO2, CO, H2O, H2 exist

in equilibrium

CO + H2O → CO2 + H2 (4)

With a temperature dependent

equilibrium constant,

K = [CO2] [H2] , equilibrium constant. [CO][H2O]

H2S from coal sulphur may decompose

if the temperature is high enough,

H2S → H2 + S (5)

Additionally, H2S may react with CO2

This study is aimed to investigate the

sulphidat ion per formance of calcined

limestone and raw limestone as an agent

for H2S removal from the flue gas from coal

combustion. Spray pyrolysis/calcination was

performed using a drop tube reactor (DTR)

for calcination of limestone to produce CaO,

and the CaO was then sulphided under

controlled conditions of temperatures and

gas composition in a fixed bed reactor (FBR).

The sulphidation efficiency was compared with

that of limestone and the calcined form.

2. Background

2.1 CalcinationLimestone consists of predominantly

CaCO3 and undergoes calcination at a

temperature of which is determined principally

by the CO2 partial pressure in the gas mixture

surrounding the particles.

CaCO3 → CaO + CO2 (1)

Limestone will calcine under an inert

atmosphere at about 700 ÌC (Borgwardt,

1985)23 and at about 900 ÌC (Fenouil and

Lynn, 1995, Part 3)5 under pure CO2, at 1 bar.

The calcination process opens up pores in

the lime particles enhancing their reactivity

towards H2S absorption due to greater surface

availability.

2.2 SulphidationH2S can be removed from coal gas by

CaCO3 or CaO to form CaS.

CaCO3 + H2S → CaS + H2O + CO2 (2)

CaO + H2S → CaS + H2O (3)

Under an atmosphere of 2 mol% H2S,

5 mol% H2O and 88 mol% CO2 (Fenouil and

Lynn, 1995, Part 1)3 the lowest temperature

at which sulphidation of CaCO2 takes place, at

1 bar, was found to be less than 10% under

these conditions. At temperatures greater than

660 ÌC the rate of sulphidation decreased;

§-94 »Ÿπ¬å«‘®—¬·≈–Ωñ°Õ∫√¡¥â“π ‘Ëß·«¥≈âÕ¡ °√¡ àß‡ √‘¡§ÿ≥¿“æ ‘Ëß·«¥≈âÕ¡

CO2 + H2S → COS + H2O (6)

It may be necessary to be aware of

the possible affect of these equilibria on the

actual gas composition surrounding the

sorbent samples under test, since the initial

feed composition will not remain unchanged

in the reactor. The actual composition may be

computed and/or measured by extracting gas

samples for analysis by gas chromatography.

The kinetic data obtained will therefore

pertain to the sorbent gas environment.

2.4 The effect of O2

Observed increases in the rate of

limestone conversion in the presence of O2

(Nimmo and Agnew, 1999)1 may be explained

by the formation of SO42- ions which can break

the metastable CaS crust and enhance the

rate of conversion of CaCO3 (Fenouil and Lynn,

1995 Part 1)3. The presence of small amounts

of oxygen has been reported as apparently

enhancing sulphidation. Enhanced sulphidation

rates may be explained by the misinter-

pretation of sulphidation measurements due

to the formation of CaSO4 via,

CaS + 2O2 → CaSO4 (7)

The enhancement is likely to be due

to the higher molecular weight of CaSO4 or to

some catalytic effect exercised by CaSO4

(Heesink and Swaaij, 1995)7. Therefore, under

sulphidation conditions, it is important that

sweep gases should be oxygen-free to avoid

errors in kinetic studies. Oxygen may be

removed from carrier/reactant gases by, for

example, beds of copper-based catalyst

pellets.

The presence of CO (1%) in the

sweep gas has been found to prevent CaSO4

formation from the oxidation of CaS by CO2

under atmosphere of CO2/N2 (Fenouil and

Lynn, 1995)4.

2.5 CaO sinteringLimestone-derived CaO will begin to

reduce in surface area due to sintering at

temperatures greater than 700 ÌC under inert

atmosphere (Borgwardt, 1985)23. The onset

of sintering of CaO produced from ultra-pure

CaCO3 occurs at a temperature of about

900 ÌC. The reason for this may lie in the

presence of latt ice defects caused by

impurities in the limestone-derived CaO.

Reduction in porosity and internal surface

area will influence the accessibility of H2O

and CO2 to accelarate the rate of CaO

sintering due to catalytic effects (Borwardt,

1989; Mai and Edgar, 1989). The effect of

H2 and CO on sintering may be indirect

through the water gas shift reaction (reaction

(4)), therefore, increasing H2 will increase

the H2O content and increasing CO content

will increase the CO2 content. Reduced

sulphidation rates in the presence of H2 and

CO have been attributed to competitive co-

adsorption on the CaO surface (Heesink and

van Swaaij, 1995)7.

2.6 CaS sinteringCaS sintering is dependent on the

composition of the atmosphere surrounding the

particles (Fenouil, 1995)3. Under atmosphere

containing CO2, CaS was observed to sinter at

temperatures above 850 ÌC, but under N2

structural difference was observed under

examination by scanning electron microscope

(SEM). This observation was confirmed by

surface area measurement. Sintering of CaS

is characterised by a smoothing surface of

individual grains and even a merging of grain

boundaries can be observed. Carbon dioxide

appears to catalyse the sintering of CaS even

when the CO2 concentration is 5% by volume.


3. Experimental Equipment andAnalytical Methods

3.1 Drop Tube Reactor (DTR).A schematic diagram of the drop

tube furnace (2000 mm x 40 mm i.d.) is

shown in Figure 1. The furnace consisted of

six heated sections rated at 0.5 kW each

of which were l inked to provide three

independently controllable temperature zones.

The limestone was ground in a sealed

limestone hopper (Figure 2). Then the particles

were fed into the reactor through a water

cooled spray injection system, utilising an

internally mixing, twin fluid atomiser and a

constant-pressure feed system, to ensure

a steady flow of solution to the nozzle of

about 10 ml/min. Atomising air was fed at

a rate of about 3 ml/min, and carrier air was

fed at up to 40 l/min. The flow of liquid was

varied with the carrier gas flow to produce

difference residence times at each port in the

reactor. Quenching, due to the evaporation of

water from the spray at the top of the DTR,

meant that calcination conditions (1073 and

1323 K) prevailed only in the bottom two-thirds

of the tube, which was controlled to give the

desired range of reaction temperatures. The

time the particles spent in this section was

taken as the residence time, up to 0.8 s.

The low temperature at the top also meant

that the reactor could not be operated with

a uniform temperature throughout its length;

therefore, a fixed profile (Figure 3) was

used, with the temperature varied in the last

section only.

Solid particle sampling from the DTR

was performed using a sampling probe, which

was inserted into the gas flow, and a portion

of the flow was directed through a two-stage

sample recovery system. Whilst the reaction

temperatures were still at over 400 K, particles

greater than 3-4 mm were removed by a

cyclone separator with a heated catchpot

(353 K). Then the remaining fine materials

were trapped just downstream by a poly

(tetrafluoroethylene) filter.Figure 1 : Schematic diagram of the drop

tube reactor (DTR).

Figure 2 : Limestone feeder

Figure 3 : Temperature profiles in the DTRwith a common profile in evaporation zone up

to 1073 K but differing profiles in the calcination tofinal temperatures of 1073 and 1323 K.


3.2 Fixed Bed Reactor (FBR).Sulphidat ion experiments were

performed in a fixed bed reactor operating

under dif ferent conditions. Calcination,

sulphidation, and sintering experiments have

been reported using bench-scale laboratory

reactor3, 23, where small batches of sample,

<50 mg, were reacted under controlled

conditions of temperature and gas composition.

The reactor used in this study was based

on these proven designs (Figure 4) and

operated at atmospheric pressure. Gas flows

were accurately metered using mass flow

controllers. The concentration of H2S was

maintained at about 2% throughout the

temperature range of concern (873-1173 K)

by the inclusion of H2 to prevent H2S decom-

position. The gas mixtures used in the

sulphidation studies in the FBR are shown

in Table 1 at the two relevant reactor

temperatures obtained from equilibrium

calculations. Checking of gas composition

was performed by extracting gas samples

for analysis by gas chromatography. An

important feature of the design permitted

the solid sample to be withdrawn from the

hot zone of the furnace and cooled under

nitrogen.

Tests were performed for different

residence times so the rates of sulphidation

could be obtained from conversion data using

TGA analysis. Experiments were performed

under differential conditions using high gas

flow rates and small solid sample weights

to ensure that the inlet and outlet gas

concentrations were as close as possible.

These conditions ensured that the particle

reactions were not affected by changes in

the ambient gas concentrations due to, for

example, CO2 evolution under calcination

conditions. The particles were dispersed in

a substrate of quartz wool, so that interparticle

effects were minimised.

The sulphidation per formance of

sprayed-calcined limestone was compared

with that of raw limestone (Omyacarb) in the

FBR at the same conditions of temperature

and residence times.

3.3 CaO Sulphidation and TGA Analysis.A method was developed to analyze

the material from sulphidation experiments

using the fixed-bed reactor by TGA, in a

two-stage programme that enabled the

determination of carbonate and sulphide in the

sample, and oxygen by difference, as follows:

Figure 4 : Schematic diagram of fixed bed reactor used in sulphidation studies.


(1) Heating of solid to 750 ÌC under

N2 and holding for 15 min.

CaCO3 + CaO + CaS → 2CaO + CaS + CO2

(2) Heating from 750 to 900 ÌC in air

and holding for 25 min.

CaO + CaS + air → CaO + CaSO4

From a known initial mass of sample,the degree of sulphidation can be calculated.From procedure 1, the mass of carbonatepresent can be calculated in the initial samplefrom the mass loss, and from procedure 2,the mass of sulphide present can be calculatedfrom the mass gain. Hence the mass of CaOin the original sample can be calculated bydifference:

mCaO = mtotal - (mCaS + mCaCO3) (1)

From this, the percent conversion to

sulphide can be calculated:

X = (MCaS/Mtotal)100 (2)

Where m = mass of compound n, M

= molar quantity of compound n, and X = %

conversion.

3.4 Particle Characterisation.Malvern Mastersiser instruments were

used to set up the atomiser nozzles andmeasure the size of the spray-formed CaOpar ticles. Samples from the latter wereprepared by suspending the CaO powder indry ethanol, and particle separation wasensured by mild vibrational agitation beforeparticle sizing was performed. Surface areameasurements were made using a TechmationQuantasorb instrument using the three-pointBrunaur, Emmett, and Teller (BET) theory, inwhich N2 is absorbed at three different partialpressures to give three different coveringvolumes and fitted according to the BETisoterm.

Scanning electron microscope images

were produced using a Camscan 4 instrument

linked to a PC for storage and electronic

manipulation.

3.5 Limestone Samples.The limestone used in the experiments

presented here were sourced in Spain as part

of a larger study on the performance of

different sorbents for H2S removal at high

temperature in the next generation of clean

coal gasification systems. The particle size

range used in the calcination and sulphidation

studies was 75-106 µm.

4. Results and Discussion

4.1 Particle Structure.Evidence for macropore formation

during calcination can be seen when the poresize distribution is examined. The BET surfaceareas of the calcined CaO from limestoneobtained at 1323 K was 23 m2g-1, comparedwith uncalcined values of 0.3 and 4 m2g-1.The results indicate that the calcined formof limestone has greater proportion of thesurface area of pores greater than about100 Å. However, these data alone cannotaccount for the greater capacity of dolomitefor sulphur capture. Examination of sulphidedand raw samples was per formed usingscanning electron microscopy (SEM) toexamine the macrostructure of the particles.An SEM images of the raw limestone, spray-calcined form, and sulphided spray-calcinedlimestone are presented in Figure 5, 6 and 7,respectively.

Figure 5 : Sub-10 µm diameter particles of rawlimestone showing crystal structure


4.2 Calcination.Batches of calcined limestone were

prepared in the drop tube reactor at a

temperature of 1023 and 1323 K. The result

showed that the higher the temperature,

the greater the rate of calcination, indicated

by greater conversion at shorter residence

times. The final degree of conversion was in

the region of 95%. These results led to the

assumption that the rate of calcination is

proportional to the initial surface area of the

uncalcined material and treated in a manner

similar to that given by Borgwardt, 198523 (21)

where the rate of calcination of small particles

can be described by

ln(1-x) = -ksSgt (4)

where x is the fraction converted to

CaO, ks is the rate constant of the surface

reaction (mol cm-2 s-1), Sg is the BET surface

area (cm2 mol-1), and t is the residence time

in the reactor.

The calcination rate constant for each

temperature was extracted from a plot of ln

(1-x) versus t. Nimmo et al.1 has shown that

the per formance of calcinat ion and

sulphidation between two different sizes

(<38 µm and 75-125 µm) are comparable.

This is due to the fact that the difference

between the size fraction is not great. The rate

constants for limestone calcination are shown

in Table 1. 95% conversion was obtained in

this study at 1323 K (1050 ÌC).

4.3 Sulfidation.Samples previously calcined in the

DTR to 70-95% conversion (at 1023 and 1323

K) were sulphided at 873, 1073 and 1173 K

in the FBR. The performance of the calcined

materials is shown in Figures 8. The reproduc-

ibility of the sulphidation results was ±5%

of the quoted values. It is evidenced that

the percentage conversion depended on

temperature, partial pressure of the sulphur-

containing gases, sur face area of the

materials (particle size and morphology), and

pore size. Nimmo et al.1 also observed that

not only the surface area of the material has

an influence on the sulphidation, the pore

size of the calcined form is also to be

considered. The larger pore sizes will be less

prone to blockage by buildup of a sulphide layer,

thereby permitting deeper penetration of H2S

into the particle and resulting, ultimately, in

Figure 6 : Sub-10 µm diameter particles of spray calcinedlimestone at 1323 K showing conversion from CaCO3 to CaO.

Figure 7 : Sub 10 µm diameter particles of sulphidedspray-calcine limestone at 873 K

Table 1 Calcination surface rate constants, ks, for limestone (mol cm-2 s-1).

Temperature (K) Limestone (Omyacarb)

1073 1.75 x 10-6

1323 7.07 x 10-6


greater degrees of particle conversion. As

residence time increased in the fixed bed

reactor (H2S), sulphidation increased rapidly

in the first 10 minutes. After 10 minutes, 17%

conversion was obtained. After 20 minutes,

20% conversion was found.

The apparent loss in sulphide

conversion observed at 1173 K has been

attributed to partial oxidation of the CaS.

This proposal was tested with Derbyshire

limestone at 1073 and 1173 K, with samples

sulphided for 10 min. (This apparent loss in

conversion is only observed at temperature

above 1073 K.) Samples obtained from the

FBR were analysed with the TGA method

described previously for sulphide conversion.

However, on completion of the oxidation, the

sample was heated further under nitrogen to

decompose the CaSO4 formed. Sulphide

content was calculated, with total sulphur

being obtained from the dissociation weight

loss. Bases on TGA experiment, CaSO4 assumed

to decompose via:

CaSO4 → CaO + SO3

5. Conclusions

Calcination:5.1 The higher the temperature, the

higher percentage of decomposition of the

limestone carbonate.

CaCO3 → CaO + CO2

5.2 The surface area, and pore size

and structure have influence on the rate of

decomposition.

Sulphidation:5.3 Sulphidation of the raw limestone

(CaCO3) gave less performance in sulphur

conversion than the calcined form (CaO).

CaO + H2S → CaS + H2O

(CaCO3 + H2S → CaS + H2O + CO2)

5.4 Rapid conversion during the first

10 minutes in the FBR was observed.

5.5 Current research is now using

calcium acetate, Ca (CH3COO)2, solution to

produce small CaO particles.

In general:5.6 The results from this study

indicated that calcined forms of limestone

can be applied for flue gas desulphurisation

from coal-fired power plants. However, the

ef f ic iency depends very much on the

characteristics of the materials used, the

mechanisms of the conversion, and the

process conditions, which further research

should be investigated.

6. AcknowledgementThanks to Dr E. Hampartsoumian

and staff at the Department of Fuel and

Energy, and the Department of Materials,

University of Leeds, for their technical help.

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Figure 8 : Sulphidation of precalcined limestone. FBRtemperature = 873 K. DTR calcinations temperature = 1323 K.


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