Removal of SO2 with particles of dolomite limestone powder in a binary fluidized bed reactor with bubbling fluidization

Removal of SO₂ with particles of dolomite limestone powder in a binary fluidized bed reactor with bubbling fluidization

R.Pisani Jr.I, ^*; D.Moraes Jr.^II

^IUniversidade Federal de São Carlos, Universidade de Ribeirão Preto, Departamento de Engenharia Química, Rua José Bonifácio 799, CEP 13560-610, Phone [phone] and (16) 9111-0776, São Carlos - SP, Brazil. E-mail: [e-mail] ^IIUniversidade Santa Cecília, Universidade Federal de São Carlos, Rua Oswaldo Cruz, 266 CEP 11045-907, Phone: [phone], ext. 144, Fax [phone], Boqueirão, Santos - SP, Brazil. E-mail: [e-mail]

ABSTRACT

In this work, SO₂ was treated by reaction with dolomite limestone (24 µm) in a fluidized bed reactor composed of 500-590 µm sand particles. The influence of operating temperature (500, 600, 700 and 800ºC), superficial gas velocity (0.8, 1.0 and 1.2 m/s) and Ca/S molar ratio (1, 2 and 3) on SO₂ removal efficiency for an inlet concentration of 1000 ppm was examined. Removal of the pollutant was found to be dependent on temperature and Ca/S molar ratio, particularly at 700 and 800ºC. A maximum removal of 76% was achieved at a velocity of 0.8 m/s, a temperature of 800°C and a Ca/S of 3. The main residence time of the powder particles was determined by integrating normalized gas concentration curves as a function of time; the values found ranged from 4.1 to 14.4 min. It was concluded that the reactor operated in bubbling fluidization under every operational condition.

Keywords: powder-particle fluidized bed, sulfur dioxide, limestone, bubbling fluidization, gas-solid reaction.

INTRODUCTION

The SO₂ treatment processes are classified as wet, wet-dry and dry medium, depending on the amount of liquid phase present (Pisani Jr. & Moraes Jr., 1999). The pollutant treatment process in noncatalytic dry medium is a gas reaction at around 800ºC with porous CaO particles from the calcination of calcite and dolomite limestone and hydrated lime. The product of this reaction – for temperatures at which the process is efficient (from 700 to 900ºC) – is calcium sulfate, especially in surplus O₂ (Borgwardt, 1970; Marsh & Urichson, 1985; Al-Shawabkeh et al., 1995; Lyngfelt et al., 1995).

The physical properties of calcined particles, such as specific surface area, porosity and pore diameter distribution, are strongly influenced by the conditions under which calcination occurs: temperature, time and type of raw material used, which indicate the experimental determination of their values (Borgwadt & Harvey, 1972; Hartman et al., 1978; Hayashi, 1996). Comparative studies carried out in differential reactors have demonstrated that hydrated lime yields higher conversions to sulfate than to calcite and dolomite limestone (Bruce et al., 1989; Al-Shawabkeh et al., 1994 and 1995). However, Borgwadt and Harvey (1972), Silcox et al. (1984) and Hayashi (1996) determined that dolomite limestone produces better retention of the CaO present in the calcined particle than does calcite limestone.

The velocity constant in the sulfation reaction is proportional to the square of the specific surface area (Borgwardt & Bruce, 1986). Since particles with smaller diameters have larger surface areas and lower resistance to the diffusion of gases through the porous structure of the solid, their use as a source of CaO is recommended for SO₂ treatment processes. However, limestone particles with diameters smaller than 50 µm, which are classified as pertaining to Geldart's group C (Geldart, 1986), tend to clog during fluidization and may be easily elutriated in a bed due to their low terminal velocity.

BIBLIOGRAPHICAL REVIEW

To overcome the inconveniences of fluidization caused by small-diameter particles, Kato et al. (1994a) proposed the use of SO₂ treatment in a binary fluidized bed known as a powder-particle fluidized bed. In this system, the powder solid reagent, usually with diameters of less than 50 µm, was continually fed into a fluidized bed of coarse particles so that only the fine particles were elutriated continuously. This configuration allowed operation at higher superficial velocities because the conditions of fluidization were determined almost exclusively by the properties of the coarse particles. These authors carried out the SO₂ treatment in a 20 mm internal diameter, 430 mm high stainless steel column, feeding calcite limestone into a fluidized bed with 495-991 µm sand particles. They achieved 100% removal at 800ºC, with a 5 µm limestone particle diameter, a 2.5 Ca/S molar ratio, a 10.0 cm static sand bed height, a 1.0 m/s superficial velocity and a 1000 ppm inlet gas concentration.

Later, for a 29 µm dolomite limestone with a (Ca+Mg)/S molar ratio of 2, Tashimo et al. (1998) achieved a removal efficiency of approximately 50% at 800ºC, a superficial velocity of 1.0 m/s, a static bed height of 10.0 cm and a gas concentration of 1000 ppm compared to the 65% achieved by Kato et al. (1994a) under the same conditions. Tashimo et al. concluded that the CO₂ concentration had little effect on SO₂ removal efficiency, particularly under conditions of solid reagent surplus (Ca/S ratios of 2 and 3 and a (Ca+Mg)/S ratio of 2). They also found that the diameter of the solid reagent exerted a strong influence, i.e., SO₂ removal efficiency dropped from 85% using 2.9 µm dolomite limestone to 40% using the same substance with a 53 m m diameter under the same experimental conditions of 850ºC, a (Ca+Mg)/S molar ratio of 2, a superficial velocity of 1.0 m/s, a static bed height of 10.0 cm and a SO₂ concentration of 1000 ppm.

Tashimo et al. (1999) studied the process of calcination of calcite limestone particles with 2.0, 5.0, 9.9, 23.3, 30.9 and 64.0 µm diameters in a binary fluidized bed composed of sand particles with diameters ranging from 420 to 840 µm. The study revealed the influence of the temperature (800 to 950ºC), the superficial velocity (0.25, 0.45 and 1.0 m/s), the static bed height (10.0 and 20.0 cm) and the mass flow rate of fine solids (5 to 15 g/h) on the conversion from calcium carbonate to calcium oxide. They concluded that the effect of particle diameter (between 2.0 and 23.3 µm) played a minor role in carbonate conversion for velocities of 0.25 and 0.45 m/s, but a significant one for 1.0 m/s, a fact that was attributed to the shorter residence time of fine particles inside the reactor under these conditions. Their results show that CaCO₃ conversion to CaO reached 88% at 800ºC, 0.45 m/s velocity and 9.9 µm diameter, and 72% for a particle diameter of 23.3 µ m under the same experimental conditions.

The conditions under which the contact between the gaseous and solid phases occurred were found to directly interfere with the performance of the processes described herein, evidencing the need to characterize the fluid dynamic operational regime as a function of operational conditions.

MATERIALS AND METHODS

Characterization of the Flow Regime

The use of an empirical correlation is a function to estimate the critical velocity for the transition from bubbling to turbulent fluidization (u_c). Cai et al. (1989) proposed Equation 1 to estimate u_c based on several assays carried out between 0.1 and 0.8 MPa, from room temperature to 500ºC, on eight different types of particles with diameters ranging from 53 to 1057 µm and densities of 706 to 2580 kg/m³. Bai et al. (1996) adapted this equation and applied it to binary beds, confirming its validity by means of the pressure drop oscillation method.

This set of equations, however, requires knowledge of the mass fraction of powder solids retained in the bed in order to calculate x_c and u_c. This can be done by knowing the elutriation constant (K) of the bed or the main residence time (q) of the powder solids inside the binary bed. Ma and Kato (1998) proposed a correlation to calculate K in a binary fluidized bed consisting of coarse particles of Geldart's group B and fine particles of types A and C, which took into account the effect of the adhesion force between the particles and the existence of a critical diameter at which the inversion of the behavior of K values occurs as a function of the diameter of the powder solids at room temperature.

where

The main residence time of powder solids (q) can be estimated by dividing the sand mass in the bed (1029 g) by the product of K with the area of the cross section of the column (6.67* 10^-3 m²) in the steady state, since this represents approximately the mass of particles that forms on the bed divided by the particle mass outflow exiting the bed (Kunii & Levenspiel, 1991).

In this work, residence time, q, was calculated by integrating the normalized SO₂ concentration curves as a function of time in the transient regime (Equation 7). Because the main residence time of the gas phase in the reactor (within a range of 0.83 to 1.25 s) can be considered negligible compared to the characteristic time of the fluid dynamics of the bed, the reduction of the SO₂ concentration at the outlet can be "instantaneously" attributed to the increase in solid reagent mass retained in the bed.

The mass fraction of coarse particles in the mixture comprising the bed (x_c) and shown in Equations 3 and 4 was calculated by

SO₂ removal efficiency (X) was defined by

The mass flow rates of the dolomite limestone were obtained from the equation of the ideal gas state and the stoichiometric balance, as will be shown later.

Equipment and Accessories

The equipment utilized to desulfurize the gas flow is shown in Figure 1. The reactor column consisted of a 316 stainless steel tube with an internal diameter of 85 mm and a height of 1.0 m. The sand bed, composed of particles ranging in diameter from 500 to 590 µm and initially static, was 10.0 cm high, although measured from the coupling point of the solid reagent feeder, the total height was 12.0 cm. The screw feeder of the powdered material, powered by an electric motor with variable rotation, was connected above the gas current distributor (a perforated plate with 1% free area for flow). The calibration equation for the dolomite mass flow rate was

with 0.8 s < p < 75.2 s

The 1000 ppm SO₂ concentration at the reactor inlet, which was produced by mixing the fluidization air with pure gas (99.9% in mass) from a cylinder, was kept constant during the experiment by a rotameter installed at the SO₂ line. The analyzer employed was a Horiba PG 250 model and the analytical method was infrared radiation absorption with 1% precision of the scale bottom and the smallest division of 1 ppm. A data acquisition system was used to register the value of SO₂ concentration every 10 s.

A granulometric analysis of the dolomite limestone (of the Minercal brand for agricultural use) using a Malvern Mastersizer device showed an average diameter of 24.2 µm based on the equivalent sphere volume. The dolomite and sand densities were measured with an Accupyc model 1330 Micrometrics Helium pycnometer, and the values obtained were 2805 kg/m³ and 2638 kg/m³, respectively.

The limestone was chemically characterized using an AtonScan 25 atomic emission spectrometer (Thermo Jarrel Ash) and differential thermal analysis. Table 1 shows the composition estimated from these analyses.

The temperature was measured at nine points distributed in three radial positions (0, R/2 and 3R/4) and five axial positions (13.5, 27.0, 50.0, 73.5 and 96.0 cm), employing type k thermocouples with iconel lining.

Experimental Procedure

Initially, 1029 g of presieved sand with diameters ranging from 500 to 590 mm was placed in the

column to form a 10.0 cm high static bed. The mass flow rate of the air was adjusted to obtain the desired superficial velocity (0.8, 1.0 and 1.2 m/s) in the reactor at the chosen temperature (500, 600, 700 and 800ºC). The rotation period of the screw feeder axis was adjusted to the necessary dolomite mass flow rate according to the superficial velocity, temperature and Ca/S molar ratio of the test. After the motor of the particle material feeder was turned off, the SO₂ was fed into the system up to a concentration of approximately 1000 ppm at the reactor inlet. A point on the SO₂ rotameter scale was chosen as a reference to keep the concentration constant at the inlet. This concentration was measured at the system outlet, but without feeding the solid reagent into the system. The adjusted operational conditions were checked for 30 minutes.

Once the equipment was set up, the test began by simultaneously turning on the variable rotation motor and starting the acquisition of the gas concentration data at the reactor outlet for 60 min, which sufficed for the system to reach the steady state. After 60 min of operation, the feeding of dolomite was stopped and the SO₂ concentration was recorded for another 30 min in order to verify the tendency of the concentration to return to its initial value.

RESULTS AND DISCUSSION

Figure 2 illustrates the typical behavior found in this study, above all at 700 and 800°C, at which the strongest drop in the concentration of SO₂ occurred as a function of time. This behavior was found to begin at 600°C, albeit less intensively, probably due to the low velocities of limestone calcination and sulfation.

The same behavior did not occur at 500°C, indicating the nonexistence of any chemical reaction at this temperature, as shown in Table 2, since practically zero SO₂ removal fractions were obtained under the conditions corresponding to this temperature. It was found that, in general, the system took about 30 min to enter the steady state in the tests in which the Ca/S molar ratios were 2 and 3, while in those where the Ca/S molar ratio was 1, this time dropped to about 10 min. This finding confirmed that the 60 min adopted in the study methodology was appropriate.

These 60 min were interrupted to feed the system with dolomitic limestone, after which the concentration of SO₂ was monitored for another 30 min to check whether the concentration of gas tended to return to its initial value.

It was found that stopping the feeding of dolomite caused an almost immediate response from the equipment, which began to increase the gas outflow concentration, tending toward values close to the initial ones at the end of the period, similarly to what was obtained when the feeding of the reagent on the bed began. This indicated the fast loosening of the solid reagent from the sand bed. The mean residence time was determined in this second phase of the experiment by means of the following equation:

After the 30 min initially stipulated, the concentration of gas had practically returned to its initial values, which was expected since it required approximately 30 min for the equipment to reach the steady state.

The dispersion of points shown in Figure 2 for the Ca/S molar ratio of 1 indicates that the limestone recirculated in the dense phase of the bed (Levenspiel, 1999). Hence, there were alternating periods of greater and lesser presence of solid reagent on the bed, which was reflected by the oscillation of the SO₂ outflow concentration. The increase in mass outflow of the solid reagent diminished the importance of the phenomenon of fine solid recirculation, as can be seen in the same figure for the Cs/S molar ratios of 2 and 3. The increase in the amount of retained limestone likely favored a more uniform distribution of the solid reagent, reducing its effect on the SO₂ outflow concentration.

Table 2 shows the SO₂ removal efficiency achieved with the continuous operation of the system in the steady state.

Table 2 shows that pollutant removal efficiency increased significantly as a function of temperature under every condition studied. Very low values were determined for temperatures of 500 and 600ºC, probably due to the low calcination rate of the limestone. Kinetic studies on the sulfation reaction of CaO, precalcined limestone and hydrated lime particles showed that this reaction occurred under similar conditions (Borgwardt, 1970; Borgwardt & Harvey, 1972; Allal et al., 1992 and Pisani Jr., 2002). As a source of CaO, Pisani Jr. (2002) used hydrated lime, whose dehydroxylation reaction occurs at 450°C, in the reaction with SO₂ under the same conditions (Ca/S ratio, gas velocity and height of the static sand bed). He obtained removal fractions ranging from 23.6 to 78.3% at 500°C, which indicated the existence of the sulfation reaction under those conditions. Therefore, when using dolomite limestone under such conditions, the limiting stage of the process is assumed to be the calcination reaction of the limestone, which does not make CaO available for sulfation.

The increase in SO₂ removal efficiency as a function of the Ca/S molar ratio was confirmed. A maximum efficiency of 76.0% was obtained for a gas flow temperature of 800ºC, Ca/S ratio of 3 and superficial velocity of 0.8 m/s. The results obtained in this work were generally inferior to those of Kato et al. (1994a). This may be attributed to the effect of particle diameter on main residence time, q, and sulfation reaction rate. In other words, for dolomite particles with diameters of less than 35 µm, the smaller the diameter the greater the interaction between fine and coarse particles, implying longer retention in the bed. The specific surface area of the solid reagent increased as the diameter decreased; hence, the sulfation reaction rates were higher for small particles at the same operational temperatures because they are directly related to the kinetic constants as well as to the specific surface areas.

Figure 3 depicts a typical example of the SO₂ concentration curves normalized as a function of time for the experimental conditions of this study. These curves were integrated based on Equation 7 using the Microcal Origin 4.0 software program, and the results of the mean residence time of the solid reagent are given in Table 3.

The method employed to calculate q from the experimental data could not be applied to the limestone under the conditions of 500°C, since the tracer (the SO₂ itself) was insensitive to alterations in the bed. The values for the final concentrations of SO₂ (C_f) were obtained by calculating the arithmetical mean of the concentration values between 55 and 60 min of operation, i.e., in the last 5 min of the operation in the steady state.

Temperature increases generally caused a reduction in the mean residence time of the limestone inside the bed (Table 3), indicating the influence of the viscous effect on the drag of the fine solids. Superficial velocity was not found to influence time, q, under the conditions studied. No reports of investigations under similar experimental conditions, which would allow for comparisons with estimated q results, were found in the literature. Kato et al. (1994b) studied the main residence times of powdered solids in a binary bed, but his studies were carried out at room temperature with superficial velocities close to that of minimum fluidization.

The q values listed in Table 3 are compatible with the SO₂ concentration curves as a function of time (Figure 2), since the time required for the system to reach the steady state was less than 40 min.

Determination of time, q, allowed us to estimate the values of x_c using Equation 8 and then to calculate the critical transition velocities for the operational conditions based on Equations 1 and 4. Table 3 shows the final results of these calculations.

It was found that the values of u_c lay in the range of 1.54 to 1.81 m/s, which allowed for the fluidization regime to be classified as bubbly under all the experimental conditions applied in this study.

The q results calculated from the elutriation constant K (Equations 5 and 6) ranged from 17.6 to 297.3 min, indicating that Equation 5 overestimated the values of q. Nevertheless, they supplied values of u_c ranging from 1.17 to 1.54 m/s. This corroborates the characterization of the fluidization of the bed in this study as the bubbling regime for the operational conditions studied here.

In the results of x_c – the mass fraction of coarse solids in the bed – it was found that the mass of solid reagent retained in the fluidized sand bed increased as a function of the superficial velocity (reduction of x_c), which should have contributed toward increasing the SO₂ removal fraction (X). However, this was not observed, since X decreased as a function of this variable, as can be seen in Table 2. Therefore, the reduction of the mean residence time of the gas should be a determining factor in the decreasing the SO₂ conversion as a function of superficial velocity.

CONCLUSIONS

This work allowed us to reach the following conclusions for the experimental conditions studied:

a) The equipment and the methodology employed were appropriate for analysis of the SO₂ treatment process in a binary fluidized bed;

b) The method developed to determine the main residence time of the dolomite particles (in the range of 4.1 to 14.4 min) produced results compatible with the experiment because the time required for the equipment to reach the steady state was approximately 30 min;

c) The pollutant treatment was ineffective at 500ºC because no CaO was available for the sulfation reaction deriving from the calcination reaction;

d) The pollutant removal efficiency was highly dependent on the temperature of the reaction medium, for example 0.0, 9.4, 39.0 and 76.0% for temperatures of 500, 600, 700 and 800ºC, respectively, with a velocity of 0.8 m/s and a Ca/S molar ratio of 3. The same behavior was observed under the other experimental conditions;

e) The Ca/S molar ratio influenced removal efficiency. For a superficial velocity of 1.2 m/s and a temperature of 800ºC, efficiencies of 17.2, 40.6 and 66.4% were obtained with molar ratios of 1, 2 and 3.

f) The SO₂ treatment process employing dolomite limestone had an efficiency of up to 76.0%;

g) The increase in mean residence time of the gas flow (0.83, 1.00 and 1.25 s) explained the behavior of increasing SO₂ removal efficiency as a function of the decrease in superficial velocity of the gas (1.2, 1.0 and 0.8 m/s);

h) The fluidization for all experimental conditions was that of the bubbling regime, since the superficial velocities used in the experiment (0.8, 1.0 and 1.2 m/s) were lower than the critical velocities (in the range of 1.54 to 1.81 m/s);

NOMECLATURE

Greek Letters

ACKNOWLEDGEMENTS

This work was supported by the Brazilian research funding agencies Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) and Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES).

Received: October 6, 2001

Accepted: September 13, 2002

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To whom correspondence should be addressed

Publication Dates

• Publication in this collection
  25 June 2003
• Date of issue
  June 2003

History

• Accepted
  13 Sept 2002
• Received
  06 Oct 2001