Desulfurization of hot coal gas in fluidized bed with regenerable zinc titanate sorbents

Fuel Processing Technology, 37 (1994) 53~5 Elsevier Science B.V., Amsterdam

53

Desulfurization of hot coal gas in fluidized bed with regenerable zinc titanate sorbents W a h a b M o j t a h e d i a, K a r l Salo.~ a n d J a v a d A b b a s i a n b'*

a Enviropower Inc., Tekniikantie 12, SF-02150 Espoo (Finland) b Institute of Gas Technology, 3424 South State Street, Chicago, IL 60616-3986 (USA) (Received October 19, 1993; accepted in revised form May 25, 1993)

Abstract

Integrated gasification combined cycle (IGCC) power generation processes are considered to be among the most attractive technologies for the 21st century. In such processes, solid fuels such as coal are gasified at pressure and the fuel gas is cleaned and combusted in the gas turbine. The gas cleanup is necessary not only for the protection of the gas turbine hardware, but also to comply with environmental regulations. In the so-called "simplified" IGCC process, the fuel gas is cleaned at high temperature and pressure to improve the overall cycle efficiency. The hot gas cleanup system being developed by Enviropower Inc., a joint venture of Tampella Power, Inc. (a leading Finnish boiler manufacturer) and Vattenfall AB (a major Swedish utility), includes a hightemperature, high-pressure desulfurization unit and a particulate removal system. The former comprises two fiuidized bed reactors utilizing regenerable zinc titanate sorbents capable of removing the sulfur gases (primarily H2S) to below 50 ppmv. The latter employs rigid ceramic filter elements operating at up to 700 °C and 20 bar and is capable of reducing the "fines" concentration to an acceptable level for a gas turbine. Novel regenerable zinc titanate sorbents suitable for fluidized-bed application have been tested. The sulfur capture and attrition characteristics of these sorbents have been evaluated in extensive testing in a bench-scale fluidized-bed reactor operating at high pressure and temperature conditions expected in IGCC operation. Two different gas mixtures representing air-blown gasifier exit gas with and without in-situ desulfurization with Ca-based sorbents have been used. H 2S removal efficiencies of higher than 99% at acceptable levels of sorbent conversion have been achieved in all these experiments with minimal sorbent deterioration.

1. INTRODUCTION O n e o f t h e m o s t a t t r a c t i v e t e c h n o l o g i e s for p o w e r g e n e r a t i o n f r o m c o a l i n t h e 21st c e n t u r y is I n t e g r a t e d G a s i f i c a t i o n C o m b i n e d C y c l e ( I G C C ) p r o c e s s . E n v i r o p o w e r I n c . , a j o i n t v e n t u r e of T a m p e l l a P o w e r , I n c . (a l e a d i n g F i n n i s h b o i l e r m a n u f a c t u r e r ) a n d V a t t e n f a l l A B (a m a j o r S w e d i s h u t i l i t y ) , is d e v e l o p ing a so-called "simplified" IGCC process which incorporates the pressurized f l u i d i z e d - b e d g a s i f i c a t i o n o f s o l i d fossil f u e l s ( t h e U - G a s process), a p p l y i n g a i r - b l o w n gasification a n d hot gas clean-up. These are i n t e g r a t e d i n t o a power a n d h e a t g e n e r a t i n g c o m b i n e d cycle. T h i s t y p e of I G C C - s y s t e m h a s t h e

* To whom correspondence should be addressed. 0378-3820/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved S S D I 0378-3820(93) E0041-C

54

W. Mojtahedi et al./Fuel Processing Technol. 37 (1994) 53-65

advantages of higher power generation efficiency, high power-to-heat ratio for cogeneration, excellent environmental performance, simple plant configuration and modularity. Several IGCC processes are being developed which will be demonstrated in the U.S. Department of Energy Clean Coal Technology Program, including Enviropower's IGCC process, to be demonstrated in the United States, in the Clean Coal Technology - Round IV. The "simplified" IGCC process includes three subsystems (Fig. 1): A - the gasification plant, including fuel preparation and feeding, gasifier, gas cooling to about 550°C and hot gas clean-up, B - the gas turbine plant including gas turbine and the booster compressor heat exchanger system for the gasifier air supply, and C - the steam cycle including heat recovery steam generator, steam turbine and conventional parts of a steam cycle. The "simplified" IGCC process has been described in detail elsewhere [1, 2]. This paper focuses on the hightemperature, high-pressure desulfurization tests carried out at the Institute of Gas Technology (IGT), in support of Enviropower's IGCC process development.

1.1. High-temperature sulfur removal The principal sulfur compound formed during gasification of coal is hydrogen sulfide (H2S) with lesser amounts of COS (carbonyl sulfide) and CS2 (carbon disulfide).

GASIFICATION PLANT COAL ~

~

DOLOMITE ASH

TAIL GAS

SORBENT AIR STEAM

SORRENT

ASH

I

t I, Roo .R,

T

~ ISTEAMCYCLE

/

II II,

.WATER

I

i l FLOE GAS

L GAS TURBINE PLANT

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L . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Fig. 1. "Simplified" IGCC power plant.

w. Mojtahedi et al./Fuel Processing Technol. 37 (1994) 53-65

55

Sulfur removal to meet the European Environmental Standards and the United States Clean Air Act and National Source Performance Standards (NSPS) will be mandatory, requiring at least 90% sulfur removal for all new coal-based power plants. However, IGCC is being looked upon as a clean power plant of the future with a promise of achieving 99% sulfur removal. A two-stage desulfurization of the fuel gas is employed in the IGCC process: 1.1.1. In-situ desulfurization The bulk of fuel-bound sulfur is removed in the gasifier by means of a desulfurizing agent such as limestone or dolomite. Hydrogen sulfide reacts with these and forms calcium sulfide (CaS). Up to 90% of the sulfur can be removed by this method. The sulfided sorbent is discharged with ash from the gasification zone. This compound is not environmentally stable and, therefore, is not suitable for direct disposal. A second reaction zone, also of the fluidized-bed type, is provided in the ash discharge path where the residue is reacted with air. The CaS is converted to CaSO4 which is stable and an environmentally acceptable compound. Samples of the stabilized solid waste material were subjected to Environmental Protection Agency (EPA) proposed sulfur leaching tests. The results show that the sulfur released from all stabilized sulfided calcium-based samples were below the United States EPA limits and could be regarded as non-hazardous [3]. 1.1.2. Post-bed desulfurization (polishing) Post-gasifier desulfurization is undertaken to polish the product gas, i.e. to reduce the remaining H2S concentration to below 50 ppm. This is accomplished in a two-fluidized-bed reactor system. HES is contacted with a regenerable sorbent in the first reactor (sulfider) at around 550°C. Zinc titanate-based (ZT) sorbents appear to be the most promising regenerable sorbents among those that have been investigated. They are capable of selectively removing HES from a gasifier product gas and can withstand many sulfidation/regeneration cycles at high temperature and pressure. A two-fluidized-bed-reactor system was chosen for the post-bed desulfurization as it offers many advantages including smaller and lower cost equipment over fixed- and moving-bed systems [4]. 1.2. Sulfidation

The basic reaction occurring in the sulfider is the reaction of H2S with zinc oxide (ZnO) ZnO + H2S --* ZnS + H20

(1)

Titanium oxide does not participate in the reaction but is used to stabilize ZnO and to prevent ZnO reduction to zinc followed by the loss of zinc by vaporization. Extensive studies have been carried out in laboratory-scale packed- and

56

W. Mojtahedi et al./Fuel Processing Technol. 37 (1994) 53-65

fluidized-bed tests using zinc titanate sorbent particles in different size ranges. H2S concentrations of below 50 ppm were achieved in all the tests with no zinc vaporization observed [1]. Excellent attrition resistance, durability and mechanical integrity of advanced zinc titanate sorbents, developed for the HTHP desulfurization unit, promise to achieve a very high degree of H2S removal at a reasonable cost. 1.3. Regeneration

Sulfided sorbent is regenerated in the second reactor (regenerator) for further use in subsequent cycles. Regeneration of the sorbent is a highly exothermic process requiring large amounts of diluent (typically 3-8% 02 in the diluent) when heat removal is not incorporated in the reactor. The diluent may be nitrogen or steam depending on the system. The regeneration temperature must also be tightly controlled in a narrow range to prevent zinc sulfate formation at low temperature and sintering/degradation at high temperature. By incorporating heat removal in the fluidized bed, it is possible to use less dilution and it may be feasible to even use undiluted air depending on the rate of heat removal. Based on laboratory experiments a temperature window between 700 and 750°C is needed for the regeneration reaction: ZnS + ~O2 --* ZnO + SO2

(2)

The feed to the regenerator consists of a mixture of air and diluent. If steam is used as a diluent, the steam/air ratio will have to be adjusted to provide a temperature of about 750 °C in the regenerator.

2. EXPERIMENTAL 2.1. Sorbents

Two zinc titanate (ZT) sorbents were prepared by United Catalyst Inc. for this study. These sorbents include L-3014 sorbent with a zinc to titanium molar ratio of 0.8 (0.8 ZT), and L-3140 with a zinc to titanium ratio of 1.5 (1.5 ZT). The average particle diameter of these sorbents is about 270 gm. The minimum fluidization velocity of the sorbents is about 6 cm/s, and the complete fluidization velocity is about 24 cm/s. 2.2. A p p a r a t u s

A unique state-of-the-art high-pressure/high-temperature reactor (HPTR) system was designed and constructed by IGT to evaluate candidate sorbents in cyclic sulfidation/regeneration tests. The test unit includes simulated hot coal-derived gas feed systems, an absorption/regeneration reactor, and associated process instrumentation and control devices. A detailed diagram of the high-pressure reactor is shown in Fig. 2.

W. Mojtahedi et al./Fuel Processing Technol. 37 (1994) 53-65 I

CERAMIC THERMOWELL

57

EXIT GAS TO CONDENSER TO GAS CHROMATOGRAPH

N2 PURGE T O BALANCE SHELL PRESSURE

QUARTZ

I

FOUR ZONE

POROUS QUARTZ DISK

PROCESS GAS

CORROSIVE GAS

Fig. 2. Detailed diagram of the high pressure batch fluidized-bed reactor.

The design of the r e a c t o r is based on a double-shell balanced pressure system. All the H2S wetted parts of the r e a c t or are constructed of quartz or ceramic material to prevent corrosion and loss of sulfur. The react or systems used by other investigators for hydrogen sulfide absorption at elevated pressures and temperatures, consist of reactors t hat are made of stainless steel and/or Alonized (Allon processed) stainless steel material. Earlier efforts at IGT indicated t h a t these materials are reactive toward H2S, even after long time exposure to a gas mixture containing high concent rat i on of hydrogen sulfide (saturation test), which significantly affects the accuracy of H2S measurements for determination of the performance of the sorbent. Some important design aspects of this system are as follows: • The HP TR vessel is a pressure-balanced system. A nitrogen purge prevents the H2S and other corrosive gases from contacting metal surfaces such as the pressure-retaining vessel wall, r e a c t o r he a ter assembly, and insulation. • Reactor materials t hat contact H2S containing gases are constructed of quartz or ceramic material to prevent corrosion and loss of H2S. This is crucial to accu rat e measurement of the sorbent performance.

58

W. Mojtahedi et al./Fuel Processing Technol. 37 (1994) 53 65

• The exit gas is sampled in the disengaging zone of the fluidized-bed reactor using a ceramic probe. • The HPTR unit is internally heated using a diffusion-type heater divided into four temperature-control zones. • Feed gases (dry gas and the steam feed) are preheated within the reaction vessel. H2S and other corrosive gases are fed directly to the bottom of the quartz HPTR where it mixes with the wet hot gases before entering the reactor. The HPTR unit is capable of operation at the following conditions: Maximum temperature: 1000 °C Maximum pressure (at maximum temperature): 30 bar 1000 L/min, STP Maximum flowrate: Feed gas composition: Sulfidation CO, C02, CH4, H2, H20, N2, H2S Regeneration N2, HzO, H2 Maximum fluidized-bed height: 36 cm The pressure-retaining vessel houses the 5- or 7.5-inch O.D. quartz reactor insert, the r eactor heater assembly, metal liners, ceramic tubes and thermowells. Dry feed gases enter at the bottom of the reactor vessel. Deionized water is pumped to the bottom of the reactor vessel and into a stainless steel coil wrapped around the lower section of the outer liner. The water is vaporized in the coil and superheated. The steam produced mixes with the dry feed gases at the exit of the coil in the bottom of the reactor vessel. The annul ar space between the two metal liners is used as an a n n u l a r heat exchanger to preheat the feed gases and super heat the steam. The hydrogen sulfide and other corrosive gases enter at the bottom of the react or vessel and flow up t h ro u g h a small quartz tube. The bulk of the exit gas leaves the reactor top through a ceramic tube. A separate gas stream that is used for gas sampling leaves t hrough a second, smaller ceramic tube. A ceramic thermowell, extending down from the top of the r eacto r vessel, contains five thermocouples to monitor the bed temperatures at various levels in the reactor and the exit gas temperature. The exit gas is sampled in the disengaging zone of the quartz react or insert using a ceramic probe. The sampling gas stream is cooled immediately after leaving the reactor. The gas is sent directly to a dual-column gas chromatograph for analysis. The composition of the r e act or exit gas is measured with a dual column gas chromatograph t hat is equipped with a flame photometric detector (FPD), a thermal conductivity detector (TCD) and auto-samplers. The FPD is used to detect low levels (0-100 ppm) of H2S and COS while TCD can be used to detect the sulfur dioxide (SO2). The react or pressure, reactor temperature (eight locations), fluidized bed differential pressure and flow rates of various gases are controlled and monitored by a PC-based data acquisition system. The fluidized-bed temperature was measured at four different locations inside the bed. The results indicated uniform temperature distribution throughout the bed in all the tests. Several blank tests were conducted in which a gas

W. Mojtahedi et al./Fuel Processing Technol. 37 (1994) 53 65

59

mixture containing a few ppmv of H2S was introduced into the empty react or under typical reaction conditions and the exit gas was analyzed. No sulfur loss was observed, indicating t ha t the H2S in the gas is not absorbed by the empty reactor. 2.3. Cyclic sulfidation/regeneration tests

A series of sulfidation/regeneration tests was conducted in the high pressure fluidized-bed r eact or using both sorbents. In a typical test, about 900 grams of sorbent are loaded into the r eact or and the unit is pressurized to the desired pressure using nitrogen and the r e a c t or is brought up to the desired temperature. The r e a c t a n t gas mixture containing a known concent rat i on of HES is then passed through the fluidized bed at a predetermined flow rate to achieve the desired fluidization velocity. The bulk of the react or exit gas is cooled to condense the steam which is collected in a k n o c k o u t pot, and the gas is then burned in a combustor. The r e a c t or exit gas is sampled in the disengaging zone of the quartz r e a c t o r using a ceramic probe. The sampling gas stream is cooled immediately after leaving the reactor, depressurized and is sent to ~he gas c h r o mato g r ap h for analysis. The flow of the r e a c t a n t gas is switched to nitrogen when the HES content of the r e a c t or exit gas reaches about 300 ppmv, and the sulfidation test is terminated. Regeneration of sulfided sorbent is carried out with a gas containing 3 to 5% oxygen and 0-30% steam, balanced with nitrogen. The SO2 cont ent of the r eact or exit gas during regeneration is measured by the gas chromatograph. The regeneration test is terminated by switching the gas to nitrogen when the SO2 content of the reactor exit gas falls below 10 ppmv. The cyclic sulfidation/regeneration tests were conducted with both sorbents over 2-5 cycles at a variety of operating conditions with a simulated fuel gas containing 1500 to 5000 ppmv H2S. The chemical composition of the sorbents after both sulfidation and r e ge ne r at i on tests were determined. The particle size distribution of the sorbents undergoing cyclic tests was also determined. 2.4. A t t r i t i o n tests

The attrition resistance properties of the sorbents in the fluidized-bed were evaluated by determining the particle size distribution of the sorbents in the sulfidation/regeneration tests after a predetermined number of cycles. The sorbents' attr itio n in the conveying line was determined using an attrition tester at P a r t i c u l a t e Solid Research Inc. (PSRI). The unit consists of a loop with several bends in which the solids are circulated at the desired loading for a predetermined number of cycles and the particle size distribution of the sorbents are determined. A proprietary theoretical model was used by PSRI to predict the rate of attrition. This experimental technique coupled with the theoretical model has been shown to be very reliable for prediction of the attrition resistance properties of many catalysts (such as FCC catalyst), polymers, and other particulate materials used in chemical and petrochemical industries where fluidized beds are employed.

60

W. Mojtahedi et al./ Fuel Processing Technol. 37 (1994) 53-65

The particle size distribution of the sorbents was obtained after 12, 40 and 100 passes through the bends in the loop. These tests were conducted with both sorbents at gas velocities of 18 and 30 m/s with a solid flow rate of 0.17 kg/s. The mass flux of the solid in the coveying line was 88 kg/m z s.

3. RESULTS AND DISCUSSION Figure 3 shows the H2S br eakt hr ough curve for the L-3014 sorbent (0.8 ZT) at 550°C an~ 20 atm over five cycles. The sorbent conversion is based on conversion of Z1tO in the sorbent to ZnS. The HES c oncent rat i on in the reactor exit gas reaches 100 ppmv at the sorbent conversion of 10 to 15% over five cycles. The results of similar tests conducted with the L-3140 sorbents (Fig. 4) indicate a higher range of conversion (17 to 22%) when HES concentration in the reactor exit gas reaches 100 ppmv. The H2S breakt hrough curves for the L-3140 (1.5 ZT) sorbent in the polishing mode (HES = 1500 ppmv) is presented in Fig. 4. The results of the tests indicate t hat the L-3140 (1.5 ZT) sorbent has better pore structure making zinc oxide readily available for reaction with hydrogen sulfide. In general, high capacity combined with higher conversion indicates that L-3140 is the superior sorbent. The effects of H2S concentration on the H2S b r e a k t h r o u g h curve appears to be minimal, suggesting t hat the initial sulfidation reaction rate is very fast and the b r e a k t h r o u g h concentration is mainly controlled by mass transfer between the bubble phase and cloud (or emulsion) phase. The results of parametric study with both sorbents for the effects of react or pressure

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Fig. 3. HES Breakthrough curve for L-3014 (0.8 ZT) sorbent at 550°C.

30

61

W. Mojtahedi et al./Fuel Processing Technol. 37 (1994) 53-65

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(10-20 atm), steam content (5-10%), fluidized-bed height (22-33 cm), and fluidization velocity (20-30 cm/s) indicate t hat none of these operating variables has a significant effect on the H2S removal efficiency of the sorbents. The effect of t e m p e r a t u r e on H2S removal efficiency appears to be quite significant. Figure 5 shows the H2S b r e a k t h r o u g h curve for the L-3140 sorbent at 650°C. The H2S co n ten t of the exit gas reaches 100 ppmv at a sorbent conversion of greater t h a n 30%. The results of these tests indicate t hat these sorbents are very reactive toward hydrogen sulfide, and can remove more t h a n 99% of H2S from the hot coal-gas at a very high space velocity (i.e. ~ 2 5 0 0 0 h r - 1 ) . Similar sulfur removal efficiencies can be obtained in larger reactors if the gas/solid flow p a t ter n can be duplicated. The fluidized-bed distributor used in these experiments was a porous quartz plate capable of generating very small bubbles with a very short jet penetration. Because the mass transfer between the bubble and emulsion or cloud phases appears to be the controlling step in desulfurization, especially at such a high level of gas conversion (99%), the design of the distributor may become crucial in achieving the desired level of desulfurization. The r e a c t o r bed height can be increased to allow a longer gas and solids residence time to overcome the effect of the distributor. For example, in a pilot-scale desulfurization operation assuming a fuel gas flow rate of 7 x 104 L/min, STP with a H2S c ont e nt of about 1000 ppmv to achieve a superficial gas velocity of 20 cm/s, a r eact or bed diameter of about one meter is required. If the fluidized-bed height in the desulfurization react or is maintained at 1 m, the gas residence time will be about 5 s. To achieve a sorbent conversion of about 5% with L-3140 sorbent (sulfur loading of 10 g/kg sorbent)

62

W. Mojtahedi et al./Fuel Processing Technol. 37 (1994) 53-65 300

8orbent

L-S140 (t 6ZT) 650 "C

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Fig. 5. H2S Breakthrough curve for L-3140 (1.5 ZT) sorbent at 650°C.

in this pilot-scale reactor, a sorbent circulation (feed) rate of about 166 g/s is required, resulting in a solid residence time of about 83 min. The H2S exit concentration in the above pilot reactor can be expected to be similar to those observed in the 3-inch reactor at 5% sorbent conversion (<50 ppmv), if the longer gas residence time in the pilot unit can make up for the effect of larger bubbles generated by a different distributor design. The results of bench-scale tests also indicate that the overall reactivities of the sorbents are generally decreasing over the three to five cycles tested. This is mainly due to changes in the physical properties of the sorbent such as porosity and specific surface area which are caused by the cyclic chemical changes that the sorbents undergo in the course of sulfation/regeneration cycles. However, the physical properties of the sorbents are expected to stabilize after a finite number of cycles in an actual process. Although long-term "life cycle" testing of the candidate sorbents is necessary to determine suitability of the sorbents for a cyclic hot coal-gas desulfurization process, the high reactivities of the sorbents coupled with moderate initial reduction in the sorbents reactivities over three to five cycles suggest that these sorbents are suitable candidates for a cyclic hot gas desulfurization process. The results of chemical analysis of the fresh, sulfided, and regenerated sorbents (from different cycles) indicate excellent sulfur balance and regeneration efficiency (> 99%) during cyclic operation. Figure 6 shows the particle size distribution of the fresh and regenerated L-3140 sorbent after the second and fifth cycles indicating that the sorbent attrition rate in the fluidized bed is very small.

63

W. Mojtahedi et al./Fuel Processing Technol. 37 (1994) 53-65

The particle size d i s t r i b u t i o n s of the L-3140 s o r b e n t in the c o n v e y i n g line a t t r i t i o n tests are given in Fig. 7 for a different n u m b e r of passes t h r o u g h the bends in the solid c i r c u l a t i o n loop. A c o m p a r i s o n of the a t t r i t i o n r a t e c o n s t a n t s (calculated using the P S R I p r o p r i e t a r y model) for different solid materials tested u n d e r similar o p e r a t i n g c o n d i t i o n s are s h o w n in Table 1 i n d i c a t i n g t h a t b o t h

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64

W. Mojtahedi et al./Fuel Processing Technol. 37 (1994) 53 65

TABLE 1 Comparison of attrition rate constants for several materials Material

Attrition rate constant, K¢f× 105

L-3140 L-3014 FCC Limestone Coke breeze Alumina

1.6 1.7 16 19 20 130

sorbents have lower attrition rate constants (higher attrition resistance) compared to FCC and other solid material tested and therefore are suitable for fluidized bed application. In the actual process, the small particles leaving the react or with the react or effluent enter a series of cyclones where a high fraction of them are collected and returned to the reactor. The results of these tests indicate t hat at the conditions tested, after going through 100 bends, about 1% of the particles will have diameters less t hat 44 #m. Assuming a constant rate of attrition per cycle and particles smaller than 44 #m as elutriable particles, and 90% cyclone collection efficiency, the fresh sorbent make-up for the process will be 1% per 1000 cycles indicating t hat the attrition resistance properties of these sorbents are quite acceptable for hot gas desulfurization process.

4. CONCLUSIONS The following conclusions can be drawn from this study: • Both sorbents are capable of reducing the H2S content of feed gas to below 50 ppmv in high pressure fluidized-bed operation. • Both sorbents have acceptable attrition resistance properties for fluidizedbed and lift line application. • Sorbent regeneration of greater than 99% can be achieved with both sorbents. • Steam content, r e a c t or pressure, and H2S inlet concentration do not appear to significantly affect the sulfur removal efficiency of the sorbents. • Reaction temperature strongly affects the H : S exit gas concentration.

ACKNOWLEDGEMENTS The work presented in this paper was jointly financed by Enviropower Inc. and the Finnish Ministry of Trade and Industry through the National Combustion Research Program "Liekki". The authors would like to t h a n k A. H orvat h of Enviropower Inc., J.R. Wangerow and A.H. Hill of IGT, and T.M. Knowlton of IGT/PSRI for their valuable contributions.

W. Mojtahedi et al./Fuel Processing Technol. 37 (1994) 53-65

65

REFERENCES 1 Mojtahedi, W., Horvath, A., Salo, K. and Gangwal, S.K., 1991. Development of Tampella IGCC Process. Paper presented at the 10th EPRI Conf. on Coal Gasification Power Plants, San Francisco, CA, EPRI, Palo Alto, CA. 2 Horvath, A., Mojtahedi, W., Salo, K., Patel, J. and Silvonen, R., 1991. Tampella IGCC Process: Cleaner and more efficient power from solid fuels. Paper presented at PowerGen. '91 Conf., Tampa, FL. 3 Abbasian, J., Rehmat, A. and Banerjee, D.D., 1991. Sulfation of partially sulfided calciumbased sorbents, Ind. Eng. Chem. Res., 30(8): xxx. 4 Gangwal, S.K., 1990. Enhanced durability of disulfurization sorbents for fluidized-bed application. Proc. of the 10th Annual Gasification and Gas Stream Cleanup System Contractors Review Meeting, Vol. 1. DOE/METC-90/6115. US DOE, Morgantown, WV.