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°“√»÷°…“°“√¥—°®—∫‰Œ‚¥√‡®π´—≈‰ø¥å‚¥¬„™âÀ‘πªŸπ Removal of H2S using Raw Limestone and Spray-Calcined Limestone in a Fixed Bed Reactor at High Temperatures N. Milne* and W. Nimmo**

Fixed - Bed Reactor (FBR)

∫∑§—¥¬àÕ ß“π«‘®—¬π’È ‰¥â¡’°“√∑¥ Õ∫ª√– ‘∑∏‘¿“æ°“√ ¥—°®—∫°ä“´‰Œ‚¥√‡®π´—≈‰ø¥å (H2S) ‚¥¬„™âÀ‘πªŸπ (CaCO3) ·≈–·§≈‡´’¬¡ÕÕ°‰´¥å (CaO) ´÷Ëߺ≈‘µ®“° À‘πªŸπ„π drop tube reactor (DTR) ∑’ËÕÿ≥À¿Ÿ¡‘ 1073 ·≈– 1323 K ·≈–π”¡“∑¥ Õ∫°“√¥—°®—∫°ä“´ H2S „π fixed-bed reactor (FBR) ∑’ËÕÿ≥À¿Ÿ¡‘ √–À«à “ ß 873 ∂÷ ß 1173 K æ∫«à “ „π¢∫«π°“√ calcination ‡¡◊Ë Õ Õÿ ≥ À¿Ÿ ¡‘ Ÿ ß ¢÷È π ¡’ ° “√·ª√√Ÿ ª ¢Õß À‘πªŸπ‡ªìπ·§≈‡´’¬¡ÕÕ°‰´¥å¡“°¢÷Èπ ·≈–„™â‡«≈“ „π°“√∑”ªØ‘°‘√‘¬“πâÕ¬≈ß Õߧåª√–°Õ∫∑’Ë ”§—≠´÷Ëß ¡’º≈µàÕ°“√·ª√√Ÿª§◊Õ æ◊Èπ∑’˺‘« ¢π“¥·≈–‚§√ß √â“ß ¢Õß√Ÿæ√ÿπ ∑’ËÕÿ≥À¿Ÿ¡‘ 1323 K ª√– ‘∑∏‘¿“æ°“√ ·ª√√Ÿª‡ªìπ CaO Ÿß∂÷ß 95% CaO “¡“√∂¥Ÿ¥´—∫ Drop Tube Reactor (DTR) *»Ÿπ¬å«‘®—¬·≈–Ωñ°Õ∫√¡¥â“π ‘Ëß·«¥≈âÕ¡ °√¡ à߇ √‘¡§ÿ≥¿“æ ‘Ëß·«¥≈âÕ¡ ‡∑§‚π∏“π’ µ.§≈ÕßÀâ“ Õ.§≈ÕßÀ≈«ß ®.ª∑ÿ¡∏“π’ 12120 ‚∑√. 0-2577-1136 ‚∑√ “√. 0-2577-1138 Environmental Research and Training Center, Department of Environmental Quality Promotion. Technopolis. Klong 5 Klong Luang, Pathumthani 12120 e-mail: nittaya@deqp.go.th **Department of Fuel and Energy, University of Leeds, UK.

H2S ‰¥â¥’°«à“ CaCO3 º≈®“°°“√»÷°…“π’È™’È „Àâ‡ÀÁπ«à“ CaO ´÷Ë ß º≈‘ µ ‰¥â ® “°¢∫«π°“√·ª√√Ÿ ª À‘ π ªŸ π „π DTR “¡“√∂𔉪ª√–¬ÿ°µå„™â „π°“√¥Ÿ¥´—∫°ä“´ H2S „π‚√ßß“πº≈‘ µ ‰øøÑ “ ®“°∂à “ πÀ‘ 𠉥â Õ¬à “ ߉√°Á ¥’ ª√– ‘ ∑ ∏‘ ¿ “æ°“√¥Ÿ ¥ ´— ∫ ¢÷È π Õ¬Ÿà °— ∫ §ÿ ≥ ¡∫— µ‘ ‡ ©æ“– ¢Õß«—µ∂ÿ¥‘∫∑’Ë„™â °≈‰°°“√∑”ªØ‘°‘√‘¬“ √«¡∑—È߇ß◊ËÕπ‰¢ ¢Õߢ∫«π°“√¥Ÿ¥´—∫π—ÈπÊ

ABSTRACT The effectiveness of raw and spraycalcined limestone for flue gas desulphurisation, in particular, H2S removal from coal gasification, has been investigated 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. Introduction Coal 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 »Ÿπ¬å«‘®—¬·≈–Ωñ°Õ∫√¡¥â“π ‘Ëß·«¥≈âÕ¡ °√¡ à߇ √‘¡§ÿ≥¿“æ ‘Ëß·«¥≈âÕ¡

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 Ì C ) would make a substantial 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 calciumbased sorbents used in this application.

Recently, the use of other materials has been suggested as alternatives to limestone injection, namely the carboxylic acetate salts of Mg and Ca (CMA), but the studies have been mainly concern with SO2 capture. §-93

This study is aimed to investigate the sulphidation performance 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 Calcination Limestone consists of predominantly CaCO 3 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 Ì C (Borgwardt, 1985) 23 and at about 900 Ì C (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 Sulphidation H2S can be removed from coal gas by CaCO3 or CaO to form CaS. CaCO3 + H2S → CaS + H2O + CO2 CaO + H2S → CaS + H2O

(2) (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

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 »Ÿπ¬å«‘®—¬·≈–Ωñ°Õ∫√¡¥â“π ‘Ëß·«¥≈âÕ¡ °√¡ à߇ √‘¡§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 misinterpretation 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.

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2.5 CaO sintering Limestone-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 lattice defects caused by impurities in the limestone-derived CaO. Reduction in porosity and internal surface area will influence the accessibility of H2O and CO 2 to accelarate the rate of CaO sintering due to catalytic effects (Borwardt, 1989; Mai and Edgar, 1989). The effect of H 2 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 CO 2 content. Reduced sulphidation rates in the presence of H2 and CO have been attributed to competitive coadsorption on the CaO surface (Heesink and van Swaaij, 1995)7.

2.6 CaS sintering CaS 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 Ì C , 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.

§-95

3. Experimental Equipment and Analytical 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 linked 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

Figure 2 : Limestone feeder

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.

Figure 3 : Temperature profiles in the DTR with a common profile in evaporation zone up to 1073 K but differing profiles in the calcination to final temperatures of 1073 and 1323 K.

Figure 1 : Schematic diagram of the drop tube reactor (DTR).

§-96

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.

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3.2 Fixed Bed Reactor (FBR).

Sulphidation experiments were performed in a fixed bed reactor operating under different 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 decomposition. 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, CO 2 evolution under calcination conditions. The particles were dispersed in a substrate of quartz wool, so that interparticle effects were minimised.

The sulphidation performance 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.

»Ÿπ¬å«‘®—¬·≈–Ωñ°Õ∫√¡¥â“π ‘Ëß·«¥≈âÕ¡ °√¡ à߇ √‘¡§ÿ≥¿“æ ‘Ëß·«¥≈âÕ¡

§-97

(1) Heating of solid to 750 ÌC under N2 and holding for 15 min.

linked to a PC for storage and electronic manipulation.

CaCO3 + CaO + CaS → 2CaO + CaS + CO2

3.5 Limestone Samples.

(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 carbonate present can be calculated in the initial sample from the mass loss, and from procedure 2, the mass of sulphide present can be calculated from the mass gain. Hence the mass of CaO in the original sample can be calculated by difference: 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 and measure the size of the spray-formed CaO particles. Samples from the latter were prepared by suspending the CaO powder in dry ethanol, and particle separation was ensured by mild vibrational agitation before particle sizing was performed. Surface area measurements were made using a Techmation Quantasorb instrument using the three-point Brunaur, Emmett, and Teller (BET) theory, in which N2 is absorbed at three different partial pressures to give three different covering volumes and fitted according to the BET isoterm.

Scanning electron microscope images were produced using a Camscan 4 instrument §-98

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 pore size distribution is examined. The BET surface areas of the calcined CaO from limestone obtained at 1323 K was 23 m2g-1, compared with uncalcined values of 0.3 and 4 m2g-1.

The results indicate that the calcined form of limestone has greater proportion of the surface area of pores greater than about 100 Å. However, these data alone cannot account for the greater capacity of dolomite for sulphur capture. Examination of sulphided and raw samples was performed using scanning electron microscopy (SEM) to examine the macrostructure of the particles.

An SEM images of the raw limestone, spraycalcined form, and sulphided spray-calcined limestone are presented in Figure 5, 6 and 7, respectively.

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

»Ÿπ¬å«‘®—¬·≈–Ωñ°Õ∫√¡¥â“π ‘Ëß·«¥≈âÕ¡ °√¡ à߇ √‘¡§ÿ≥¿“æ ‘Ëß·«¥≈âÕ¡

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

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 performance of calcination and sulphidation between two different sizes

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

(<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 reproducibility of the sulphidation results was ±5% of the quoted values. It is evidenced that the percentage conversion depended on temperature, partial pressure of the sulphurcontaining gases, surface 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

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

Temperature (K) 1073 1323 »Ÿπ¬å«‘®—¬·≈–Ωñ°Õ∫√¡¥â“π ‘Ëß·«¥≈âÕ¡ °√¡ à߇ √‘¡§ÿ≥¿“æ ‘Ëß·«¥≈âÕ¡

Limestone (Omyacarb) 1.75 x 10-6 7.07 x 10-6 §-99

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).

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

CaO + H2S → CaS + H2O (CaCO3 + H2S → CaS + H2O + CO2)

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 CaSO 4 formed. Sulphide content was calculated, with total sulphur being obtained from the dissociation weight loss. Bases on TGA experiment, CaSO4 assumed to decompose via:

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.

CaSO4 → CaO + SO3

5. Conclusions Calcination: 5.1 The higher the temperature, the higher percentage of decomposition of the limestone carbonate.

CaCO3 → CaO + CO2 §-100

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 efficiency 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. Acknowledgement Thanks 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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