Hot Coal-Gas Desulfurization with Calcium-Based Sorbents in a

Energy & Fuels 2004, 18, 1543-1554

1543

Hot Coal-Gas Desulfurization with Calcium-Based Sorbents in a Pressurized Moving-Bed Reactor A. Abad, J. Adańez,* F. Garcıá-Labiano, L. F. de Diego, and P. Gayań Instituto de Carboquı´mica (CSIC), Department of Energy and Environment, Miguel Luesma Casta´ n 4, 50015 Zaragoza, Spain Received March 30, 2004. Revised Manuscript Received June 16, 2004

The performance of a pressurized moving bed on hot coal-gas desulfurization was analyzed for the countercurrent and cocurrent configurations. The behavior of several calcium-based sorbents, two limestones, and a dolomite was determined at temperatures in the range of 1073-1273 K and a pressure of 1 MPa. The limestones were used only under calcining conditions, whereas the dolomite was used under both calcining and noncalcining conditions. The desulfurization level and sorbent conversion, as well as the longitudinal H2S concentration profiles in the transference zone, were determined as a function of the main design and operating variables, including the Ca/S molar ratio, the bed height, the type of flow configuration (countercurrent or cocurrent), the temperature, the H2S concentration, and the particle size. The length of the transference zone, and then the reactor length necessary for full desulfurization, was highly dependent on the particle size and the solids velocity. In both countercurrent and cocurrent configurations, it was possible to reach high desulfurization levels with a H2S concentration at the gas outlet that were similar to the thermodynamic equilibrium. However, the countercurrent configuration was always more effective for desulfurization than the cocurrent configuration, and smaller reactors were necessary in the first case. The mathematical model developed to predict the experimental results obtained at small scale was used for design considerations of desulfurization reactors that use moving beds of calcium-based sorbents.

1. Introduction In gasification processes, such as integrated gasification with combined cycle (IGCC), the sulfur contained in coal reacts in the reducing atmosphere of a gasifier, forming gaseous compounds. More than 90% of the sulfur in the gas phase has been reported to be H2S. This gaseous contaminant must be removed prior to combustion of the coal gas, to prevent damage to turbine equipment and to comply with the emissions legislations. The use of calcium-based sorbents for coal-gas desulfurization at high temperature and high pressure offers several advantages over other gas-cleaning methods, primarily higher thermal efficiency, reduced volume of processing vessels, and low price of the sorbents. A current industrial practice is to add limestone to the fluidized bed gasifier. This achieves moderately successful removal of H2S but requires a significant excess over the stoichiometric quantity of limestone theoretically necessary to attain close-to-equilibrium removal of H2S and produces large amounts of mixed-limestones/ slag solid waste.1,2 Sulfurous gases are also removed after the coal conversion process by sorbent injection into the gas * Author to whom correspondence should be addressed. E-mail address: [email protected]. (1) Thambimuthu, K. V. Gas Cleaning for Advanced Coal-Based Power Generation; IEA Coal Research: London, 1993. (2) Fenouil, L. A.; Lynn, S. Ind. Eng. Chem. Res. 1995, 34, 23242333.

stream and/or by gas percolation through an external fixed or moving bed of sorbent particles.1 Sulfur retentions up to 70% with Ca/S molar ratios of 3 were achieved in the injection of calcium-based sorbents in an entrained bed reactor.3 However, if the gases obtained in the IGCC process are to be used in a gas turbine, sulfur removal near to the thermodynamic equilibrium would be desirable. The H2S removal in moving-bed reactors allows the almost-complete utilization of the sorbent, reaching H2S concentrations at the outlet near equilibrium.4 Moreover, to avoid efficiency loss, H2S removal in moving-bed reactors from coal gas in IGCC systems should be conducted at medium pressures. The H2S concentration at the equilibrium is dependent on the operating conditions. Under noncalcining conditions, as the existing in a fluidized bed gasifier,5 the direct sulfidation of the limestone or half-calcined dolomite occurs:

CaCO3 + H2S h CaS + CO2 + H2O

(1)

CaCO3‚MgO + H2S h CaS‚MgO + CO2 + H2O (2) (3) Adańez, J.; Garcıá-Labiano, F.; de Diego, L. F.; Fierro, V. Energy Fuels 1998, 12, 726-733. (4) Fenouil, L. A.; Lynn, S. Ind. Eng. Chem. Res. 1996, 35, 10241043. (5) Kristiansen, A. Understanding Coal Gasification; IEA Coal Research: London, 1996.

10.1021/ef040039f CCC: $27.50 © 2004 American Chemical Society Published on Web 08/03/2004

1544 Energy & Fuels, Vol. 18, No. 5, 2004

Abad et al.

The thermodynamic equilibrium of these reactions is given by the following equation:6

Kds )

PCO2PH2O PH2S

(

) 4.66 × 1012 exp -

13212 T

)

(3)

Under calcining conditions, as the existing at the gas outlet of an entrained bed gasifier,5 the sulfidation of the calcined sorbents occurs:

CaO + H2S h CaS + H2O

(4)

CaO‚MgO + H2S h CaS‚MgO + H2O

(5)

Table 1. Chemical Analysis and Physical Characteristics of Sorbents

composition (wt %) CaCO3 MgCO3 other surface area, Sg (m2/g) 0

Sa´stago limestone

Omyacarb limestone

Sierra de Arcos dolomite

95.7 0.9 3.4 1.2a (6.8)b 0.30a (0.66)b

97.1 0.2 2.7 0.6a (19)b 0.03a (0.56)b

52.5 40.5 7.0 9.6c (30.4)b 0.35c (0.57)b

a Raw sorbent. b Data for calcined sorbent are given in parentheses. c Half-calcined dolomite.

The thermodynamic equilibrium of these reactions is given by the following equation:6

Ksc )

PH2O PH2S

(7262 T )

) 1.127 exp

(6)

H2S removal in moving beds has several advantages, with respect to the fixed-bed operation. Moving beds operate in a continuous way with a high sorbent utilization and do not need heating and cooling steps. Moreover, the gas flow and sorbent renewal in moving beds avoid the plugging problems that exist in the fixed beds by the fines entrained from the gasifier. The disadvantages of the moving beds are mainly based on a more-complex control and operation, and the existence of higher solids attrition. In a moving bed, the solids fall down by gravity. The moving-bed configuration can be countercurrent or cocurrent, when the gas flow is upward or downward, respectively. Because the sulfidation reaction is not instantaneous, there is a transference zone where the sorbent reacts with the H2S. In this zone, there are longitudinal profiles of H2S concentration and solid conversion, which are constant with time at steady-state operation. There are few papers in the open literature about H2S desulfurization in moving-bed reactors. In 1996, Fenouil and Lynn4 modeled the behavior of moving beds in both cocurrent and countercurrent configurations at 0.1 and 3 MPa. They showed that H2S concentration near thermodynamic equilibrium and high sorbent utilization can be reached with a required length of the bed between several centimeters to a few meters, depending on the particle size of the sorbent and the superficial velocity of the gas. However, the validity of the model was not tested with experimental systems. The objective of this work was to analyze the hot coalgas desulfurization with different calcium-based sorbents in a pressurized moving-bed reactor. The main operating and design variables were varied including the system configuration (countercurrent and cocurrent), bed height, Ca/S molar ratio, particle size, temperature, and H2S concentration, as well as the type of sorbent (limestone and dolomite) and type of reaction (direct sulfidation or sulfidation of calcined sorbents). Moreover, a mathematical model that describes the moving-bed operation was developed, in which the (6) Barin, I. Thermochemical Data of Pure Substances; VCH: Weinheim, Germany, 1989.

Figure 1. Schematic diagram of the pressurized moving-bed reactor.

kinetic parameters determined in previous works on desulfurization were used.7-10 2. Experimental Section 2.1. Materials. The materials used for this work were two limestones (“Omyacarb” and “Sa´stago”) and a dolomite (“Sierra de Arcos”). Table 1 shows the chemical analyses and physical characteristics of these sorbents. Sa´stago limestone is a Spanish sorbent that is characterized by high porosity and reactivity. Particle sizes with narrow size intervals between 1.0 mm and 3.0 mm (ranges of +1.0-1.25 mm, +1.6-2.0 mm, and +2.5-3.0 mm) were used. Silica sand (>99% SiO2) with the same particle size intervals was used as an inert and diluting material in the moving bed. 2.2. Experimental System. A pressurized moving bed was built to analyze the hot coal-gas desulfurization with calciumbased sorbents at temperatures in the range of 1073-1273 K and a total pressure of 1 MPa. Figure 1 shows the experimental setup, which was composed of a continuous system for (7) Adańez, J.; de Diego, L. F.; Garcıá-Labiano, F.; Abad, A. Energy Fuels 1998, 13, 617-625. (8) Abad, A. H2S Retention from Coal-Gas into Pressurized MovingBed Reactors. Ph.D. Dissertation, University of Zaragoza, Zaragoza, Spain, 2003. (9) Adańez, J.; Abad, A.; de Diego, L. F.; Garcıá-Labiano, F.; Gayań, P. Ind. Eng. Chem. Res. 2004, 43, 4132-4139. (10) Garcıá-Labiano, F.; Adańez, J.; Abad, A.; de Diego, L. F.; Gayań, P. Energy Fuels 2004, 18, 761-769.

Hot Coal-Gas Desulfurization with Ca-Based Sorbents solids feeding and extraction, a pressurized reactor, and a gas analysis system by gas chromatography. To prevent corrosion problems, the sulfidation was performed inside a Kanthal reactor tube (0.027 m inner diameter, 1.4 m in length). The reactor was placed inside a furnace with three independent heating zones, to avoid longitudinal temperature profiles. A thermocouple, located inside a Kanthal tube, could be moved longitudinally to obtain the temperature along the reactor. The solids were fed to the reactor using a screw feeder from a lock hopper located in the upper portion of the system. The solids passed downward via gravity and were extracted by means of another screw feeder. The entire system was pressurized, although the reacting gas only passed through the reactor. The velocity of the screw feeders was regulated to control the solids flow through the reactor. The gas and solid flow could be countercurrent or cocurrent. The syngas was obtained by blending N2, CO2, CO, and H2, which were controlled using specific mass flow controllers. The H2O content was obtained by complete evaporation of a constant flow rate of water proceeding from a pressurized (1.2 MPa) deposit. The H2S was added in a zone near the gas inlet of the reactor. The H2S concentration at the inlet and at the exit gas line was measured using a Varian Star 3400cx gas chromatograph with a flame photometric detector (FPD) in a semicontinuous way. A pressure valve located at the gas exit line controlled the reactor pressure. The bed height was determined with the Ergun equation11 and the pressure-drop measurements were obtained with a differential pressure sensor. 2.3. Procedure. The transference zone for pure sorbent was only of few centimeters. To reduce the errors in the H2S longitudinal profiles determined under the different operating conditions, the sorbent was diluted with inert sand and the mixture was charged in the upper deposit. The operating procedure was dependent on the type of gas-solid contact that was desired. 2.3.1. Countercurrent Configuration. The reaction temperature and pressure were reached with a nitrogen flow through a homogeneous bed of solids (mb ) 0.43) with the desired height. The reaction gases then were introduced at the bottom of the reactor, and the solids feeding and extraction started with the same velocity. The steady state was reached within ∼1 h, and it was maintained constant for 2 h. The analysis of the H2S concentration at the gas outlet gave us the desulfurization level that was reached under the selected operating conditions. Finally, the solids feeding was stopped and the H2S concentration at the gas outlet increased as the bed height decreased, and the H2S concentration corresponding to each height was obtained. This gave the longitudinal profile of H2S concentration inside the reactor during the steady-state operation. Some experiments were repeated to obtain the solid conversion profiles along the reactor. At steady-state operation, the feeding and extraction of solids was stopped and the reacting gases were replaced by nitrogen. Several solids fractions were extracted at different bed heights after reactor depressurization and solids cooling. The solids fractions were completely sulfided in a fixed bed to obtain, by difference, the solid conversion in each zone of the moving bed during the steadystate operation. 2.3.2. Cocurrent Configuration. For these experiments, the bed was filled with sand at the desired height. After the temperature and pressure were stable, the reaction gases were introduced from the upper portion of the reactor. The H2S concentration at the gas outlet was the same to the inlet, because the bed was filled with inert material. At this moment, (11) McCabe, W. L.; Smith, J. C.; Harriott, P. Unit Operation of Chemical Engineering; McGraw-Hill: New York, 1985.

Energy & Fuels, Vol. 18, No. 5, 2004 1545 the feeding and extraction of solids (sorbent + sand) began with the same velocity and the desulfurization process started. The H2S concentration data that were obtained during this process corresponded to the interphase between the inert sand and the reacting solids (sorbent + sand). These data gave the longitudinal profile of H2S concentration during the steadystate operation of a reactor working at cocurrent configuration. After the reactor was filled with the mixture of sorbent and sand, the H2S concentration was maintained constant, and the steady-state operation continued for 2 h. Finally, the solids feeding and extraction were stopped and the reacting gases were replaced by nitrogen to obtain the solid conversion profiles in a similar way to that described previously for the countercurrent operation. 2.4. Experiments. The H2S removal in a moving bed, under both countercurrent (CT) and cocurrent (CO) configurations, has been analyzed in this work. The experiments were performed at a total pressure of 1 MPa, and the composition of the reacting gas was as follows: 6 vol % CO2, 3 vol % CO, 5 vol % H2, 0.5 vol % H2S, the amount of H2O necessary to reach the water gas shift reaction equilibrium, and the balance was N2. The calcination temperature under these conditions was 1135 K. The dolomite was used under both noncalcining and calcining conditions. However, the use of limestone was restricted to calcined conditions, because the direct sulfidation of limestones is a process of low practical interest.12 Table 2 shows the operating conditions used in the experiments. For the sorbent residence times encountered in these experiments, the degree of solid conversion is not expected to be above 85%, because, after that point, the sulfidation reaction rate was extremely slow.8,10,13,14 In this work, it was considered that only this fraction (Xe ) 0.85) was effective for desulfurization in a moving bed. An effective Ca/S|e molar ratio was defined considering only the calcium that could be converted in the reactor and the sulfur that can be removed from the gas stream:

Ca/S|e )

NCa(Xe - Xin) ug(CH2S,in - CH2S,eq)

(7)

A Ca/S|e molar ratio of 1 corresponds to the minimum sorbent flow rate practically needed, taking the aforementioned constraints into account, to obtain the H2S equilibrium concentration at the reactor outlet. The gas analysis system was checked prior to detect the effect of the dispersion on the experimental system. The introduction of a step function of H2S (up to 5000 vppm) into the reactor was detected as the same function at the gas analysis system. Moreover, the calculation of the axial dispersion coefficient (D/uL) gave values of ∼10-6 for the experimental system. According to Levenspiel (1986),15 the flow in the system can be considered as plug flow with values of this parameter of 0) (16) ∂CH2S ∂R

)0

(at R ) 0 and t > 0)

(17)

The external mass-transfer coefficient (kg,H2S) was calculated with the following equations:17

Sh ) 1.17Pe0.49Sc-0.16

(if Re < 95)

(18)

-0.26

(if Re > 95)

(19)

0.59

Sh ) 0.74Pe

Sc

(2) A differential mass balance equation for the gas diffusion in the product layer, which gives a relation between the concentration in the pores of the particle and the concentration at the reaction interface. The analytical solution of this differential mass balance allowed us to express the local reaction rate (-rS) in terms of the gas concentration in the pores, using the equation

(-rS) )

ks(CH2S - CH2S,eq)S0(r2/r0)2 1 + (ksr2/Ds)[1 - (r2/r1)]

(20)

The average reaction rate per volume unit was calculated by integration of the local conversions:

∫0R 4πR2(-rS) dR 0

(-rjS) )

4 3 πR 3 0

(21)

The model was solved using the finite differences method for the transference zone, assuming perfect mixing of solids in each differential element. The model (17) Froment, G. F.; Bischoff, K. B. Chemical Reactor Analysis and Design, 2nd ed.; Wiley: New York, 1990.

predicted the longitudinal profiles of gas concentration and solid conversion through the bed and at the reactor outlet. 4. Results and Discussion The desulfurization level reached in a pressurized moving bed is dependent on several factors. In this work, the effect of the main variables was analyzed through the knowledge of the gas and solid conversions at the reactor outlet, as well as the longitudinal profiles developed inside the bed and, more specifically, in the transference zone. This zone is the region of the reactor where the sorbent reacts with the H2S. The concentration profile in this zone varied, for a given Ca/S molar ratio, from the maximum concentration at the reactor inlet and the minimum H2S concentration given by the mass balance. In any case, the maximum H2S removal was limited by the thermodynamic equilibrium. The experimental data will be also used to validate the mathematical model developed in this work. 4.1. Effect of the Ca/S|e Molar Ratio. To analyze the effect of the Ca/S|e molar ratio on the desulfurization process, several experiments (E1, E2, and E3) were performed with the dolomite under calcining and noncalcining conditions and the Sa´stago limestone under calcining conditions. The solids velocity was varied in the calcining experiments, and the gas flow was the variable in the noncalcining experiments. Figure 3 shows an example of the longitudinal profiles of H2S concentration obtained both at countercurrent and cocurrent flow for the dolomite under calcining conditions. The location of the transference zone in the countercurrent configuration was different, depending on the Ca/S|e molar ratio, although in all cases, the length of this zone was smaller than the bed length (1.2 m). For Ca/S|e < 1, the transference zone was located in the upper portion of the bed. This was because the H2S concentration at the gas outlet was higher than that given by the thermodynamic equilibrium and, at the bottom of the bed, there was no reaction, because the sorbent had reached its effective conversion. For Ca/S|e ) 1, the transference zone was located at an intermediate zone of the bed. In this case, the solid conversion at the reactor outlet was maximum and the H2S concentration corresponded to that given by the thermodynamic equilibrium. For Ca/S|e > 1, the outlet sorbent was partially reacted and in the upper portion, there was no reaction, because the H2S had reached the equilibrium concentration. Because of these facts, the transference zone was located at the bottom of the reactor. For a cocurrent flow, the transference zone was always located in the upper portion of the bed. With a Ca/S|e molar ratio of 0.75, the sorbent reached the effective conversion at lengths smaller than the bed height used in this work (1.2 m). In this case, the transference zone (0.8 m) was lower than the bed length, and the H2S concentration at the outlet of the bed was limited by the Ca/S|e molar ratio used. For higher values of Ca/S|e (1.0 and 1.25), increasing desulfurization levels were obtained for a same bed height. However, the H2S equilibrium concentration at the reactor outlet was not reached in any of these experiments, because the length of the transference zone was greater (3.15 and 2.25 m,

Hot Coal-Gas Desulfurization with Ca-Based Sorbents

Energy & Fuels, Vol. 18, No. 5, 2004 1549

Figure 3. Longitudinal concentration profiles obtained with different Ca/S|e ratios for dolomite (particle size of +1.6-2.0 mm) at 1180 K. Calcining conditions: 1 MPa, 0.5 vol % of H2S, and L0 ) 1.2 m ((0) experiment E1, Ca/S|e ) 1; (4) experiment E2, Ca/S|e ) 1.25; and (]) experiment E3, Ca/S|e ) 0.75). Solid lines represent model predictions.

Figure 4. Effect of the Ca/S|e ratios on the length of the transference zone at 1 MPa, 0.5 vol % H2S, particle size of +1.6-2.0 mm, and 1180 K (calcining conditions) ((0) dolomite, and (4) Sa´stago limestone) or 1120 K (noncalcining conditions) ((]) dolomite). Solid lines represent model predictions.

respectively) than the bed length (1.2 m). Figure 3 also shows the model predictions as a continuous line for both reactor configurations. It can be observed that the model predicted the longitudinal profiles of gas concentration well, showing the validity of the assumptions used in the mathematical model. As previously mentioned, the transference zone differed greatly, depending on the type of gas-solid contact. The length of this zone determines the minimum length of the reactor necessary to obtain the maximum H2S removal, that is, the H2S equilibrium concentration at the gas outlet. Figure 4 shows the length of the transference zone (Lt) as a function of the Ca/S|e molar ratio for the two possible bed configurations. The values of Lt for the cocurrent configuration with Ca/S|e molar ratios of 1 (experiment E1CO) and

1.25 (experiment E2CO) were obtained with experiments E4CO and E9CO through the use of eq 8 to represent the data with the same sorbent fraction. As expected, the required length was always larger with gas and solid cocurrent flow than in the countercurrent configuration. Moreover, the value of Lt was maximum for a Ca/S|e molar ratio of 1. However, this was the only case where it was possible to reach the effective sorbent conversion, together with the maximum gas desulfurization, with the subsequent savings in sorbent and waste generation. When the Ca/S|e molar ratio was different than 1, the length of the transference zone was smaller, as a consequence of the higher reaction rates produced by a higher mean H2S concentration (Ca/S|e < 1) or a lower mean solid conversion (Ca/S|e > 1). 4.2. Longitudinal Profiles of Solid Conversion. Although the longitudinal profiles of the sorbent conversion can be calculated from the gas concentration profiles by means of a mass balance in the bed (see eq 13 or 14), in this work, they were also experimentally measured in the E1 experiments. The method used has been described in the Experimental Section. Figure 5 shows the average solid conversions experimentally determined, both at countercurrent and cocurrent configurations, for the dolomite under noncalcining conditions. This figure also shows the solid conversions obtained from the gas concentration profiles (see eq 13 or 14) and the model predictions. It can be observed that the sorbent conversion profiles obtained both by experimental techniques and by the model predictions are in fair agreement for both reactor configurations. Similar results were obtained for other sorbents and operating conditions. 4.3. Effect of the Bed Height. Figure 6 shows a comparison of the longitudinal profiles of H2S concentration in the transference zone for the Sa´stago limestone under calcining conditions, when the length of the


Figure 5. Conversion profiles in the moving-bed reactor obtained during desulfurization with dolomite (particle size of +1.6-2.0 mm) at 1120 K. Noncalcining conditions: 1 MPa, 0.5 vol % H2S, Ca/S|e ) 1, and L0 ) 1.2 m, obtained from direct measure (([) countercurrent and (1) cocurrent) or from concentration profiles ((0) countercurrent and (4) cocurrent). Solid lines represent model predictions.

bed (L0) was longer or smaller than the transference zone (Lt). Similar results were obtained with the dolomite, both under calcining and noncalcining conditions. When L0 > Lt, there was a region of the bed without reaction, because the gas concentration was that given by the thermodynamic equilibrium or because the sorbent had reached the effective conversion. Under these conditions, the gas and solid concentration profiles in the transference zone were the same for different bed heights above Lt, and are shown in Figure 6. For

Abad et al.

experiments E1CT (L0 ) 1.20 m, Lt ) 0.78 m) and E4*CO (L0 ) 3.6 m, Lt ) 2.58 m), the H2S concentration obtained at the outlet was almost the same as that corresponding to the thermodynamic equilibrium. The concentration profile named E4*CO was obtained; it transformed that obtained in experiment E4CO with eq 8 to the conditions (FS and fW,sorb) of experiment E1CO. When L0 < Lt, the residence time of the sorbent in the reactor in experiments E4CT (L0 ) 0.4 m, Lt ) 0.78 m) and E1CO (L0 ) 1.2 cm, Lt ) 2.58 m) was lower than that necessary to obtain the complete conversion of the sorbent and the gas. The effect of the bed height on the concentration profiles was dependent on the reactor configuration. The concentration profiles in the cocurrent configuration were the same, independent of the length. An increase in the reactor length only produced a continuous decrease in the H2S concentration, as it can be seen in experiments E1CO and E4*CO. However, for the countercurrent configuration, the concentration profiles were different, depending on the L0/Lt ratio. The height needed to obtain a fixed H2S concentration was lower when L0 < Lt (point Q) than when L0 > Lt (point P), because of a higher average reaction rate of the sorbent in the bed. However, the desulfurization level attained at the outlet reactor was lower in the first case. 4.4. Effect of the Type of Gas-Solid Contact. To analyze the effect of the gas and solid flow, the E1 desulfurization experiments were compared under the same operating conditions for the countercurrent and cocurrent reactor configurations. Figure 7 shows the longitudinal gas profiles of the transference zone in both cases for the Sa´stago limestone and the model predictions. The gas inlet point to this zone was considered as the reference point in both cases, which corresponded to z ) 0 in Figure 7; i.e., the cocurrent profiles were inverted. The first zone of the cocurrent bed was more effective for desulfurization than the countercurrent bed, because a low converted solid was reacting with a gas with high

Figure 6. Effect of the bed height (L0) on the concentration profiles in the moving-bed reactor with Sa´stago limestone (particle size of +1.6-2.0 mm) at 1180 K (calcining conditions), 1 MPa, 0.5 vol % H2S, Ca/S|e ) 1 and countercurrent (Lt ) 0.78 m) ((0) L0 ) 1.2 m and (4) L0 ) 0.4 m) or cocurrent (Lt ) 2.58 m) ((0) L0 ) 1.2 m, and (4) L0 ) 3.6 m). Solid lines represent model predictions.



Figure 9. Effect of reaction temperature on the length of the transference zone in countercurrent configuration under the E1CT operational conditions (1 MPa, 0.5 vol % H2S, and a particle size of +1.6-2.0 mm: (0) dolomite under calcining conditions, (4) Sa´stago limestone under calcining conditions, and (]) dolomite under noncalcining conditions. Solid lines represent model predictions. Figure 7. H2S concentration profiles obtained in countercurrent and cocurrent configuration with Sa´stago limestone (particle size of +1.6-2.0 mm) at 1180 K (calcining conditions), 1 MPa, 0.5 vol % H2S, Ca/S|e ) 1, and L0 ) 1.2 m ((0) countercurrent and (]) cocurrent). Solid lines represent model predictions.

Figure 8. Effect of particle size on the length of the transference zone in countercurrent configuration under the E8CT operational conditions, 1 MPa, 0.5 vol % H2S, and 1180 K (calcining conditions) ((0) dolomite, and (4) Sa´stago limestone) or 1120 K (noncalcining conditions) ((]) dolomite). Solid lines represent model predictions.

H2S concentration. However, the final step was less effective for reaction, because a high converted solid was reacting with a diluted gas. Therefore, to obtain the same desulfurization level, higher residence times and taller reactors were necessary in the cocurrent configuration than in the countercurrent configuration. This was valid for any Ca/S ratio used. 4.5. Effect of Particle Size. The effect of the particle size on the desulfurization process was tested in the countercurrent moving bed under the operating conditions given in experiment E8CT. Figure 8 shows the length of the transference zone for the three particle sizes used (ranges of +1.0-1.25 mm, +1.6-2.0 mm, and +2.5-3.0 mm) and the model predictions. The lower reaction rates as the particle size increased9,10 produced

an important increase in the Lt values for all the sorbents. Therefore, this parameter is of great importance in the design of the moving-bed desulfurization reactors. Small particle sizes would be desirable, because smaller beds are necessary to obtain the same desulfurization level. However, the particle size to be used in a countercurrent moving bed will be limited by the minimum fluidization velocity of the sorbent and other problems derived from the flow of small particles. 4.6. Effect of Temperature. Several experiments were performed with the dolomite under calcining and noncalcining conditions, and with the Sa´stago limestone under calcining conditions under countercurrent configuration. Figure 9 shows the length of the transference zone as a function of the temperature for the experimental conditions given in the E1CT experiment. This corresponded to the minimum reactor length necessary to obtain a H2S concentration at the gas outlet equal to that given by the thermodynamic equilibrium. An increase in the temperature produced a decrease in Lt, because of the increase in the reaction rate. Figure 9 also shows the model predictions, using the corresponding sulfidation model in each case, direct sulfidation, or sulfidation of the calcined sorbents. Under the operating conditions normally used, there was a temperature interval of ∼30 K where the calcination could affect the sulfidation.18 This fact could be important if there is not a zone in the moving-bed reactor for the sorbent calcination prior to the sulfidation. In this case, a simultaneous model for the calcination and sulfidation reactions must be used.18 On the other hand, under noncalcining conditions, only dolomite can be used as a desulfurization sorbent, because of the low conversions attained by the limestone under these conditions.8,12 4.7. Effect of H2S Concentration. Three different H2S concentrations (2500, 5000, and 10000 vppm) were used to analyze the effect of this parameter on the moving-bed desulfurizer operation under the conditions given in experiment E1CT. The solids flow was varied to maintain a constant Ca/S|e molar ratio of 1. Figure 10 shows the experimental results obtained and the model predictions. Surprisingly, the length of the trans-


Abad et al.

Figure 10. Effect of the inlet H2S concentration on the length of the transference zone in countercurrent configuration. E1CT operational conditions, 1 MPa, particle size of +1.6-2.0 mm, and 1180 K (calcining conditions) ((0) dolomite, and (4) Sa´stago limestone) or 1120 K (noncalcining conditions) ((]) dolomite). Solid lines represent model predictions.

ference zone was not affected by the H2S concentration. This behavior was consequence of two opposite effects. On one hand, a higher concentration produced a higher reaction rate and a lower length of the transference zone. On the other hand, a higher solids flow, which is necessary to reach a constant Ca/S|e molar ratio, produced a higher downward solids velocity and an increase in the length of the transference zone. It must be considered that Lt was calculated as the product of the residence time in the transference zone (tr) and the downward solids velocity (usol). The combination of eqs 7 and 11 yields the following equation:

MCaCO3ug(Ca/S|e) Lt ) trusol ) tr × FjsolfW,sorbfCaCO3(1 - mb)(Xe - Xin) (CH2S,in - CH2S,eq) (22) Therefore, Figure 10 demonstrated that Lt was constant when there was a variation both in the H2S concentration at the inlet and in the solids velocity but maintaining a constant Ca/S|e molar ratio at the same total pressure, temperature, and particle size.

k1tr(CH2S,in - CH2S,eq) ) constant

(23)

with

k1 )

MCaCO3ug(Ca/S|e) FjsolfW,sorbfCaCO3(1 - mb)(Xe - Xin)

(24)

4.8. Effect of the Type of Sorbent. For a better comparison of the type of sorbent, the longitudinal profiles of gas concentration were transformed to a bed composed by 100% sorbent. Equations 8 and 11 were used to calculate the concentration profiles and the downward solids velocity, respectively. Figure 11 shows the longitudinal profiles obtained for the dolomite and the Sa´stago and Omyacarb limestones under calcining conditions and for the dolomite under noncalcining (18) de Diego, L. F.; Abad, A.; Garcıá-Labiano, F.; Adańez, J.; Gayań, P. Ind. Eng. Chem. Res. 2004, 43, 3261-3269.

Figure 11. H2S concentration profiles in a moving-bed reactor in countercurrent configuration for the different sorbents used (particle size of +1.6-2.0 mm) at 1 MPa, 0.5 vol % H2S, Ca/ S|e ) 1, fW,sorb ) 1, and 1180 K (calcining conditions) ((0) dolomite, (4) Sa´stago limestone, and (O) Omyacarb limestone) or 1120 K (noncalcining conditions) ((]) dolomite).

conditions, with the operating conditions given in experiment E1CT. Under these conditions, the length of the transference zone was a few centimeters. This was the reason to work with sorbents diluted in sand to obtain beds of longer length and increase the exactness of the H2S concentration data. The differences in the concentration profiles and in the Lt values for the different sorbents could be explained through their composition and sulfidation reactivity.8,10 The length of the moving bed necessary to obtain complete gas desulfurization with the Omyacarb limestone was ∼4 times that necessary for the other sorbents, because of its low reactivity. However, Sa´stago limestone gave Lt values similar to that of the dolomite. The higher calcium content of the limestone compensated for the lower reactivity of this sorbent, with respect to the dolomite, and a lower solids flow was necessary for a given Ca/S|e molar ratio. On the other hand, the reactivity of the dolomite under calcining and noncalcining conditions was similar, which produced similar values of Lt. 4.9. Design Considerations. The moving-bed model developed can be used to design a moving bed for hightemperature and high-pressure desulfurization with limestone or dolomite. The model gives the length of the transference zone and the conversion and concentration profiles obtained under different operating conditions. A basic design for countercurrent configuration was performed under the following operational conditions: Ca/S|e molar ratio of 1, 1123 K under noncalcining conditions and 1223 K under calcining conditions, and total pressure of 2 MPa. The desulfurization process was performed with pure sorbent (fW,sorb ) 1), with a particle size in the range of 1-5 mm, and for the treatment of a coal-gas stream similar to that obtained in a entrained bed gasifier with dry coal feeding (27 vol % H2, 61 vol



higher Lt value was obtained for the less-reactive Omyacarb limestone (2.10 m). Other important factors, besides the bed height needed to reach the maximum desulfurization in a pressurized moving-bed reactor, are involved in selecting the best calcium-based sorbent. Limestones can be only used under calcining conditions, whereas dolomite can be used both under calcining and noncalcining conditions. The waste produced in the process is another important factor to consider. Dolomites produce higher amounts of residues, because the MgO acts as an inert material; however, it is easier to stabilize than limestones. 5. Conclusions Figure 12. Design factor (FD) obtained with the theoretical model for countercurrent configuration at 2 MPa, Ca/S|e ) 1, and 1223 K (calcining conditions) ((-) dolomite, (- -) Sa´stago limestone, and (-‚‚-) Omyacarb limestone) or 1123 K (noncalcining conditions) ((‚‚‚) dolomite) as a function of particle size.

% CO, 4 vol % CO2, 2 vol % H2O, 5 vol % N2, 0.5-1.5 vol % H2S).5 The H2S equilibrium concentration under these conditions was 84 vppm at 1123 K and 73 vppm at 1223 K. Under these conditions, the length of the transference zone was calculated as a function of the gas and solid velocities, and for different H2S concentrations. The results obtained could be grouped into a design factor (FD), defined as

FD )

CH2S,in - CH2S,eq Lt usol

(25)

the value of which was constant for a fixed Ca/S|e molar ratio, temperature, pressure, and particle size. This fact is a consequence of the results obtained from the analysis of the effect of the H2S concentration (eq 23) and from the identity defined in eq 8. Equation 25 can be used for the design of desulfurization reactors working with different H2S concentrations, solid velocities, and/or gas flows, but maintaining a fixed Ca/S|e molar ratio. When the H2S concentration and solid velocity are varied, the (CH2S,in - CH2S,eq)/usol ratio is almost constant. In this case, Lt is also constant, which is in agreement with the results shown in Figure 10. Figure 12 shows the variation of FD, obtained for the conditions previously mentioned, as a function of the particle size for different sorbents both under calcining and noncalcining conditions. Graphs similar to this could be obtained for other Ca/S|e molar ratios, temperatures, or pressures. Therefore, the length of the transference zone in a moving-bed desulfurizer can be calculated as a function of the solid velocity (usol) and the H2S concentration at the inlet (CH2S,in) using Figure 12 and eq 25. As an example, for a sorbent particle size of 3 mm and a gas velocity of 0.85 m/s (near the minimum fluidization velocity), the downward velocity for pure sorbent was 1.05 m/h for the dolomite, 0.70 m/h for the Sa´stago limestone, and 0.51 m/h for the Omyacarb limestone. The length of the transference zone calculated from the FD values obtained from Figure 12 and eq 25 was similar for the dolomite, both under calcining (0.66 m) and noncalcining (0.83 m) conditions, and for the Sa´stago limestone (0.64 m). However, a

The performance of a pressurized moving bed on the hot coal-gas desulfurization was analyzed for the countercurrent and cocurrent configurations, and using different calcium-based sorbents. The bed height and the Ca/S molar ratio were two more-important design and operating variables in the performance of this type of desulfurization reactors. Both were used to determine the H2S concentration at the gas outlet and the sorbent utilization, and both affected the longitudinal profiles of concentration and solid conversion in the bed. The length of the transference zone, and then the reactor length for maximum desulfurization, was highly dependent on the particle size and the solids flow. However, this length did not vary with the inlet H2S concentration when the solid velocity was also varied, to maintain a constant Ca/S molar ratio. It was found that it was possible to reach a full coalgas desulfurization with calcium-based sorbents, limited only by the thermodynamic equilibrium, in a pressurized moving bed (1 MPa) working at high temperatures (in the range of 1073-1273 K) both in countercurrent and cocurrent configuration. The limestones were only effective for desulfurization under calcining conditions, whereas the dolomites could be used both under calcining and noncalcining conditions. Under these conditions, the effective conversion of the sorbents used was ∼85%. The sulfidation at higher conversions was extremely slow, and it was not effective during the operation in a moving-bed reactor. The countercurrent configuration was more effective in the H2S removal, and smaller beds were necessary to reach a given desulfurization level. Using a Ca/S|e molar ratio of 1, the optimum sorbent utilization was obtained. Under these conditions, the height of the transference zone was maximum, although, in an industrial reactor, the height would not be excessively high and could vary from several centimeters (for dolomite and reactive limestone) to a few meters (for nonreactive limestone). The length of the transference zone can be calculated, for a given set of operating conditions (temperature, pressure, particle size, and Ca/S molar ratio), as a function of the solids velocity and the H2S concentration at the gas inlet stream by means of a design factor. Acknowledgment. This research was performed with financial support from the Comisioń Interministerial de Ciencia y Tecnologıá (Project No. AMB980883).


Nomenclature CH2S ) H2S concentration (mol/m ) CH2S,∞ ) bulk H2S concentration in the gas (mol/m3) CH2S,eq ) H2S concentration at the thermodynamic equilibrium (mol/m3) CH2S,ext ) H2S concentration in the external particle surface (mol/m3) CH2S,i ) H2S concentration at the axial coordinate zi (mol/m3) CH2S,in ) inlet H2S concentration (mol/m3) CH2S,out ) outlet H2S concentration (mol/m3) Ca/S ) calcium to sulfur molar ratio Ca/S|e ) effective Ca:S molar ratio, calculated using eq 7 dp ) particle diameter (m) De ) effective diffusivity of H2S within the sorbent particles (m2/s) De,P ) effective diffusivity of H2S within the sorbent particles at pressure PT (m2/s) DH2S ) molecular diffusion coefficient of H2S (m2/s) DK ) Knudsen diffusion coefficient (m2/s) Ds ) product layer diffusion coefficient (m2/s) Ds,P ) product layer diffusion coefficient at pressure PT (m2/s) Ds,P0 ) product layer diffusion coefficient at PT ) 0.1 MPa (m2/ s) Dds,0 ) pre-exponential factor of the product layer diffusion coefficient in the direct sulfidation (m2/s) Dsc,0 ) pre-exponential factor of the product layer diffusion coefficient in the sulfidation of calcined sorbents (m2/s) ED,ds ) activation energy of the product layer diffusion coefficient in the direct sulfidation (J/mol) ED,sc ) activation energy of the product layer diffusion coefficient in the sulfidation of calcined sorbents (J/mol) Ek,ds ) activation energy of the chemical reaction rate constant in the direct sulfidation (J/mol) Ek,sc ) activation energy of the chemical reaction rate constant in the sulfidation of calcined sorbents (J/mol) fV,sorb ) volume fraction of sorbent in the bed fW,sorb ) weight fraction of sorbent in the bed fCaCO3 ) weight fraction of CaCO3 in the sorbent FD ) design factor defined in eq 25 (mol s m-3) Fsol ) solid flux in the bed (kg m-2 s-1) h ) axial coordinate in the bed (m) kg,H2S ) mass-transfer coefficient in the gas film around the solid particle (m/s) ks ) chemical reaction rate constant (m/s) kds,0 ) pre-exponential factor of the chemical reaction rate constant in the direct sulfidation (m/s) ksc,0 ) pre-exponential factor of the chemical reaction rate constant in the sulfidation of calcined sorbents (m/s) Kds ) equilibrium constant for the direct sulfidation reaction (Pa) 3

Abad et al. Ksc ) equilibrium constant for the sulfidation reaction of calcined sorbents L ) bed length (m) L0 ) total bed length (m) Lt ) length of the transference zone (m) m ) total pressure exponent in the pore diffusivity NCa ) molar flux of calcium (mol m-2 s-1) Pi ) partial pressure of component i (Pa) P0 ) reference pressure (MPa) PT ) total pressure (MPa) Pe ) Peclet number, ugR0/DH2S r0 ) initial grain radius (m) r1 ) grain radius after some reaction (m) r2 ) radius of unreacted grain core (m) (-rS) ) local reaction rate in the particle (mol m-3 s-1) (-rjS) ) mean reaction rate in the particle (mol m-3 s-1) R ) radial coordinate within the particle (m) R0 ) particle radius (m) Re ) Reynolds number, ugFgR0/µg s ) total pressure exponent in the product layer diffusivity S0 ) initial reaction surface (m2/m3) Sg ) specific surface area (m2/g) Sc ) Schmidt number, µg/(FgDH2S) Sh ) Sherwood number, kgR0/DH2S t ) time (s) tr ) residence time in the transference zone (s) T ) temperature (K) ug ) gas velocity (m/s) usol ) solid velocity (m/s) Xin ) inlet sulfidation conversion Xe ) effective sulfidation conversion X ) sulfidation conversion Xout ) outlet sulfidation conversion Xz ) sulfidation conversion at a position z in the transference zone z ) axial coordinate in the transference zone (m) Greek Letters 0 ) initial particle porosity mb ) moving-bed porosity µg ) gas viscosity (kg m-1 s-1) Fg ) gas density (kg/m3) Fjsol ) mean solid density in the bed (kg/m3) Acronyms CT ) countercurrent configuration CO ) cocurrent configuration EF040039F

Hot Coal-Gas Desulfurization with Calcium-Based Sorbents in a

Recommend Documents