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Ind. Eng. Chem. Res. 2001, 40, 5640-5648
Flue-Gas Desulfurization in Circulating Fluidized Beds: An Empirical Model from an Experimental Pilot-Plant Study P. Ollero,†,‡ F. J. Gutie´ rrez Ortiz,*,† A. Cabanillas,§ and J. Otero§ Departamento de Ingenierı´a Quı´mica y Ambiental, Universidad de Sevilla, Camino de los Descubrimientos s/n, 41092 Sevilla, Spain, and Centro de Investigaciones Energe´ ticas y Medioambientales (CIEMAT), Avenida de la Complutense 22, 28040 Madrid, Spain
Flue-gas desulfurization in a circulating fluidized-bed absorber (CFBA) is quite a novel technology that has significant potential advantages, in comparison with other semidry desulfurization processes. A high desulfurization yield, a high sorbent utilization, low investment costs, and the fact that relatively little extra space is required are the most outstanding features of this technology. This paper shows the main results obtained in a 3-MWe equivalent pilot plant specifically designed and constructed to assess the performance of a flue gas cleaning system composed of a CFBA unit and an electrostatic precipitator. The results of an experimental program were used to determine the parameters of a semiempirical model that correlates the SO2 removal efficiency to the main operating variables. The model, based on the number of overall mass-transfer units, shows a coefficient of determination (R2) close to 0.9, indicating a good correlation between the measured and calculated efficiency values. Background The circulating fluidized-bed absorber (CFBA) concept involves the injection of a sorbent, typically slaked lime, in conjunction with flue-gas humidification by spraying water into a circulating fluidized-bed unit located downstream from the air preheater but upstream from the particulate collection equipment which is usually a fabric filter (FF) or an electrostatic precipitator (ESP). However, a medium-efficiency mechanical dust collector should also be placed between the CFBA unit and the final particulate collection equipment in order to reduce the dust load of the gas entering this last device. As much as 98% of the material collected in both collectors is recycled to the fast fluidized bed, where the solid concentration may build up to 2000 g/Nm3 or even more. The major operating parameters of these dry desulfurization processes are the fresh Ca/S ratio (CASf), the approach to the adiabatic saturation temperature (AST), and the solid recirculation ratio (RR). Increasing the Ca/S ratio leads to higher SO2 removal yields but at the expense of lower sorbent utilization. On the other hand, decreasing the AST has a strong positive effect on the SO2 removal efficiency and sorbent utilization. However, there is a minimum practical temperature approach due to the growing risk of solid deposition on the absorber walls and on the internal parts of the process equipment. Increasing the RR has a positive effect on the sulfur removal yield but at the expense of a greater gas pressure drop and a greater impact on the final particulate collector. Besides these operating parameters, there are two additional variables that may affect the gas cleaning system: the flue gas SO2 inlet concentration and the flue gas flow rate, which depends on the power plant load. * To whom correspondence should be addressed. Phone: 00 34 95 448 72 60. Fax: 00 34 95 446 17 75. E-mail:
[email protected]. † Universidad de Sevilla. ‡ E-mail:
[email protected]. § CIEMAT.
The CFB principle for absorbing acid gases was developed in Germany in the 1970s.1 In the mid-1980s, this process was used for flue gas desulfurization in coalfired power stations. Several papers describe the fundamentals and the applications of this semidry desulfurization process.2-4 Since 1984, only a few installations have been built in Germany and some neighboring countries. Besides these plants, two North American units have been operational since 1995. Data offered by the CFB system suppliers5 show a removal efficiency between 90 and 95% with Ca/S ratios between 1.2 and 1.5. The Tennessee Valley Authority has tested a very similar process, the gas sorbent absorption (GSA) concept, at a 10-MWe equivalent plant.6 In the first phase of the demonstration program, this type of absorber, combined with an ESP, yielded an SO2 removal efficiency of 80-90% at a Ca/S ratio of 1.3 for 3% sulfur coal when the AST was 4.4 °C. The Pilot Plant The pilot plant (Figures 1 and 2) is located in southern Spain at the Los Barrios Power Plant. The power plant has one 550-MWe unit and burns Colombian and South African coal with a sulfur content of less than 0.7%. The pilot plant is made up of three integrated main units: a spray dryer, a CFB, and an ESP. There is a long gas duct upstream from the precipitator to provide enough residence time for in-duct desulfurization. This installation is probably unique in Europe because it is possible to emulate several semidry desulfurization processes by means of a set of butterfly valves, which allows us to assemble these main units in different ways. This multipurpose pilot plant can process up to 12 000 Nm3/h of flue gases, which are withdrawn upstream and/or downstream from the power-plant precipitator at 130-150 °C. This allows us to mix flue gases with and without fly ash in any proportion, thus simulating the performance of an ash precollector located upstream from the desulfurization unit. There is also an SO2
10.1021/ie010152i CCC: $20.00 © 2001 American Chemical Society Published on Web 10/13/2001
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Figure 1. View of the pilot plant.
injection unit to increase the SO2 gas concentration from 350 up to 3000 ppm. The pilot plant is fully instrumented with wet and dry temperature sensors, flow rate and pressure measurement devices, and an SO2 analyzer with three measuring probes located at the pilot-plant inlet, after the desulfurization units, and downstream from the ESP at the pilot-plant outlet. The signals from these sensors are processed in a data acquisition system provided with SCADA software. The CFB desulfurization unit is made up of three main parts: the reactor, the lime feeding system, and the product recirculation system. Inside the absorber, just above the diverging section of the venturi gas distributor, there is a pneumatic injection nozzle to humidify the flue gas. If desired, the flue gas may instead be humidified upstream from the CFB in a
Figure 2. Pilot-plant flowsheet.
humidification chamber. Figure 3 shows a flow diagram of the whole unit, while Table 17 gives its main dimensions and technical characteristics. This engineering design agrees with widely accepted design rules for this type of desulfurization unit, namely, a flue gas superficial velocity of around 6 m/s at full load and an average gas residence time of around 3 s at the same conditions. This guarantees that, if the operating conditions of this CFB unit, i.e., the Ca/s ratio, the product RR, and the AST, are close to the common values in practice (are inside the common range in practice), its behavior will be similar to that of a commercial one. In the humidification chamber, the flue gas at about 150 °C is humidified with finely atomized water (mean drop diameter, 60 µm), dropping its temperature to about 75 °C and reducing its fly ash content somewhat as well. The flue gas enters the absorber through a tangential inlet located at the plenum under the gas distributor. Thanks to the cyclonic effect due to the tangential inlet, some of the fly ash is collected in the plenum and discharged through its bottom. In the divergent section of the gas distributor, the flue gas comes into contact with finely atomized water and with a mixture of fresh lime and dry recirculated solid product (fly ash, unconverted lime, and reaction products, mainly sulfite and calcium sulfate). This mixture is fed into the CFB absorber by means of an air slide. The gas, which leaves the absorber at about 60 °C laden with fly ash, reaction products, and unconverted dry lime, comes first into an inertial dust collector and afterward into the pilot ESP so that those solids can be separated from the flue gas. The fresh lime hopper, just above the air slide feeder, is filled every day by means of an intermittent pneumatic device using a programmed sequence of air pulses. The lime is discharged into the air slide through a variable-speed screw conveyor in order to adjust the appropriate input flow corresponding to each test. The lime hopper rests on three load cells that continuously measure the weight of the remaining lime, allowing us to check the discharge flow. Table 2 shows the particle size distribution of the fresh lime. The inertial dust collector is continuously discharged by gravity into the product hopper through a constantspeed rotary valve. The dry dust collected in the ESP is carried by means of a pneumatic conveyor to the product hopper. The solid product for recirculation is discharged at the bottom of this hopper by means of a
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Figure 3. Flowsheet of the CFB unit. Table 1. Main Dimensions and Technical Characteristics of the Desulfurization Unit plenum gas distributor
reaction zone
maximum flow rates and loads
water atomization system inside the absorber
diameter height gas inlet type throat diameter throat height convergent angle divergent angle total height type diameter height thickness gas outlet angle diameter flue gas flow rate calcium hydroxide solid recirculation flow rate water flow rate SO2 injection fly ash load no. of nozzles geometrical arrangement type: air pressure air flow water pressure water flow
950 mm 2000 mm tangential venturi 300 mm 1000 mm 70° 79° 3600 mm cylindrical 950 mm 18 000 mm 5 mm 90° 437 mm 12 000 Nm3/h 140 kg/h 13 700 kg/h 700 L/h 58 kg/h 7000 mg/Nm3 7 hexagonal (6 + 1)
Table 2. Cumulative Mass Size Distribution of the Sorbent size (µm)
under (%)
size (µm)
under (%)
300 224 166 102 62.5 34.3 17.2
0 99.9 99 95.5 90.7 80.6 70.2
10.5 7.08 5.27 3.92 2.64 1.78 1.2
61.0 51.1 42.0 32.3 20.3 10.9 3.0
product is continuously discharged by means of a screw located in the hopper at a medium height that acts as a weir. The temperature downstream from the humidification chamber is controlled by adjusting the water flow rate to the atomizers by means of a variable-speed pump. On the other hand, the temperature at the outlet of the CFB absorber is controlled by adjusting the water flow rate to the atomizer, located inside the reactor, by means of a control valve in the return line of the water injection system. Because the wet bulb temperature of the flue gas is measured by a wet bulb temperature sensor, a specified AST can be obtained. Experimental Results
ultrasonic biphase mix 3-6 bar 20.8-21.1 Nm3/h 0.5-3 bar 0.07-1.03 L/min
variable-speed rotary valve into the air slide. This valve allows us to manipulate the RR. The surplus solid
Based on the theory of design and analysis of experiments,8 an experimental program was designed to evaluate the behavior of the CFB desulfurization unit with respect to the main operating variables: (i) the CASf, (ii) the AST, (iii) the solid RR, (iv) the SO2 flue gas concentration (SO), and (v) the superficial gas velocity (U0) through the absorber.
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Figure 4. Time record of a desulfurization test. Table 3. Design Matrix for the Fractional Factorial Designa
a
CASf
AST
RR
SO
U0
1.2 1.5 1.2 1.5 1.2 1.5 1.2 1.5 1.2 1.5 1.2 1.5 1.2 1.5 1.2 1.5
10 10 15 15 10 10 15 15 10 10 15 15 10 10 15 15
95 95 95 95 98 98 98 98 95 95 95 95 98 98 98 98
400 400 400 400 400 400 400 400 2000 2000 2000 2000 2000 2000 2000 2000
6 4 4 6 4 6 6 4 4 6 6 4 6 4 4 6
The U0 is the generator of this particular fraction.
Table 3 shows the design matrix. Figure 4 shows a typical but short time record for the SO2 content, and we can observe the dynamic response of the desulfurization process with respect to the sorbent injection and the onset of the recirculation system. The response is quite quick, and the SO2 concentration reaches a quasi-steady state a few minutes (1-3 min) after the process change. Beside this, the effect of the start-up of the recirculation system on the gas flow rate
and on the gas opacity downstream from the precipitator can also be seen. Under these conditions, the dust load entering the precipitator is so high that a low dust emission level cannot be maintained. Tables 4 and 5 contain the experimental results of the main set of tests, which were obtained by taking time-averaged values from similar but longer SO2 concentration records for each test. All of these tests were carried out without ash precollection but with humidification upstream from the CFB unit and inside it. Figures 5-7 illustrate these results in a more understandable way. However, they cannot accurately show the individual effect of each independent variable on the SO2 removal efficiency. This is because the applied fractional factorial design did not include changing one variable at a time while maintaining the others at constant values. Moreover, it was often impossible to operate at the nominal values established by the factorial design during the whole test. Nevertheless, the set of experimental results is sufficient to develop an empirical model by applying a complete statistical analysis, as will be seen later. With such a model, it is easy to assess the effect of each individual variable. Desulfurization Yield vs the Major Operating Variables Figure 5 shows the SO2 removal efficiency at 400 ppm vs AST for the two RRs and the two U0s but without
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Table 4. Experimental Tests with 400 ppm actual CASf (mol/mol)
U0 (m/s)
AST (°C)
RR (%)a
gas flow rate (Nm3/h)
recirculated solids (kg/h)
1.59 1.04 1.28 1.39 1.48 1.69 1.54 1.49 1.8 1.8 1.77b 1.63 1.34 1.41 1.65 1.65 1.65 1.2 1.2 1.2 1.37 1.27 1.25 1.85 1.07 1.8 1.85 1.59 1.22 1.29 1.54 1.03 1.37 1.41 1.43
6 4 4 4 4 4 4 4 4 4 4 4 6 6 6 6 6 4 4 4 4 4 4 4 4 4 4 4 6 6 6 6 6 6 6
10 13 12 10 10 13 13 9 8 14 8 8 13 10 12 8 13 9 10 10 10 10 10 11 10 10 10 10 8 13 13 10 9 10 8
98 0 95 medium medium 0 low medium 98 0 99.3 98 low medium-high 95 95 0 98 0 low very low low low low low 95 low low 98 low low low 95 95 medium
12590 8318 8393 8318 8318 8318 8393 8318 8318 8318 8318 8318 12477 12477 12477 12477 12477 8393 8393 8393 8393 8393 8393 8393 8393 8393 8393 8393 12477 12477 12477 12590 12590 12590 12590
6200 0 1518
92.3 43.0 73.2 64.7
0
45.0 69.8
4096 0 4096 4096
92.2 30.9
CFB
58.3 2382 2382 0 3952 0
1603 5875
2299 2299
84.7 93.8 45.7 90.5 49.2 57.9 60.2 66.8 66.8 72.3 89.6 72.3 58.5 94.7 61.2 60.0
efficiency (%) CFB + ESP 94.8 52.4 75.4 72.3 48.7 66.6 78.5 96.4 40.0 96.6 94.8 66.7 81.5 84.7 96.4 42.5 93.7 56.5 64.4 52.0 61.5 69.2 79.0 90.3 79.0 66.8 96.2 64.2 62.6 70.7
87.1 89.4 75.5
a Ranges of values for RR are as follows: low, 90%. b This test was conducted simulating fly ash precollection.
Table 5. Experimental Tests with 2000 ppma gas actual recirculated efficiency (%) CASf U0 AST RR flow rate solids CFB + (mol/mol) (m/s) (°C) (%)a (Nm3/h) (kg/h) CFB ESP 1.14 1.04 1.57 1.24 1.35 1.36 1.05 1.42 1.1
4 4 4 4 4 6 6 6 6
13 13 7 7 13 10 7 8 10
low 95 98 98 95 95 98 98 95
8318 8318 8318 8393 8393 12477 12477 12590 12590
0 3168 9067 8244 3548 5274 12522 13725 4795
60.0 75.1 95.6 90.6 82.0 96.3 91.0 82.3
67.0 76.0 97.9 96.4 85.9 89.5 96.9 95.3
a Ranges of values for RR are as follows: low, 90%.
differentiating with respect to the Ca/S ratio. Here we can see the positive effect of operating closely to the saturation temperature. The favorable effect of increasing the RR is also clear because the desulfurization efficiency rises roughly 20 points. However, the effect of the superficial velocity of the gas is not conclusive, especially at low temperatures. At high temperatures, the full load (6 m/s) yield is greater than the partial load (4 m/s) yield. Figure 6 again shows the desulfurization yield vs AST but now differentiating only with respect to the two Ca/S ratios. The positive effect of the CASf is quite a bit weaker than expected; in fact, it is almost imperceptible at low operating temperatures. However, this conclusion must be seen in light of the fact that the experimental points reflect the effect of more operating parameters
Figure 5. Effect of the RR and U0.
because they correspond to tests with different RRs and gas velocities. Finally, Figure 7 shows the desulfurization efficiency of the CFB absorber and of the global system (i.e., including the ESP) versus the mass flow rate of recirculated solids without differentiating with respect to any other variable. Here we can see that the average contribution of the ESP is around 5 yield points. At the same time, it seems clear that higher RRs than those used in the tests (95 and 98%), which would lead to higher solid flow rates, may not be justified because the yield gain is very small and the pressure drop increases linearly.
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Figure 6. Effect of the Ca/S ratio.
Figure 8. Measured versus calculated removal efficiencies. Table 6. Adjustable Parameters of the Empirical Model CFB CFB + ESP
R2
k
a
b
no. of tests
0.896 0.912
14.984 16.184
0.299 0.299
-1.259 -1.212
18 19
model assumes the general form
Figure 7. Effect of the solid flow rate.
4
xiR ∏ i)1
X)k
Empirical Model The physics-based empirical model used in this work has the following basic form:
E ) 1 - exp(-X)
(1)
Here, E is the SO2 removal efficiency and X represents the number of overall mass-transfer units. Obviously, the number of mass-transfer units of a specific CFB unit depends on the operating conditions, namely, the CASf, the AST, the product RR, the SO2 inlet concentration (SO), and the U0. This last variable has been taken into account to assess the performance of the unit with respect to the flue gas flow rate, which is subject to changes in the power plant load. However, in this empirical model the CASf and the RR have been considered together using the "internal Ca/S ratio" (CASint) concept, defined as the total moles of fresh and recirculated calcium hydroxide fed along with the solid stream into the fluidized-bed absorber per mole of SO2 entering the unit with the flue gas. In a CFB, the internal Ca/S ratio is much greater than the CASf because of the high product recirculation that contains unconverted lime and can best explain the unit behavior. Using a simple mass balance and the definition of RR as the mass of recirculated solids divided by the mass of solids leaving the fluidized-bed reactor, it can be easily shown that the internal Ca/S ratio may be expressed as
[
CASint ) CASf 1 +
(
RR E 11 - RR CASf
)]
i
where the xi values represent the four remaining independent variables (CASint, AST, SO, and U0) and the Ri values are exponential coefficients that take into account the different influences of each independent factor on the efficiency. To apply a linearized least-squares method to obtain the k and Ri values from the experimental data, the basic model equation should be transformed by taking logs two times for both sides
[ ( )]
ln ln
1
1-E
4
) ln k +
Ri ln xi ∑ i)1
The experimental data used to determine the model parameters are those included in Tables 4 and 5, except the ones corresponding to the “low” and “medium” RRs because it was not possible to assign to them a reliable RR value. Using a statistical software package (SPSS 7.5),9 a preliminary linear multivariable regression analysis has shown that both the SO and U0 have an extremely low statistical significance and cannot be considered to be important factors for the desulfurization process. This agrees with the previous analysis of the empirical data and with the conclusions of a pure statistical analysis made based on a previous polynomial model.10 Thus, with rejection of these variables, the number of overall mass-transfer units can be formulated as follows:
(2)
For the number of overall mass-transfer units, the
CASinta X)k ASTb
(3)
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Figure 9. (a) Effect of the CASf and of the AST on the SO2 removal efficiency and on the SU. (b) Effect of the CASf and of the RR on the SO2 removal efficiency and on the SU. (c) Effect of the RR and of the AST on the SO2 removal efficiency and on the SU.
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Figure 10. Effect of the ESP on the SO2 removal efficiency and on the SU.
and the complete empirical model is made up of eqs 1-3. When a linear multivariable regression analysis was applied, two sets of model parameters (Table 6) were obtained, one for the CFB unit and another for the global system CFB + ESP. The high coefficient of determination for both models shows that the applied method works well. Figure 8 shows the measured versus the calculated removal efficiency for both the CFB and global system (CFB + ESP). It can be seen that almost all of the points lie in a narrow (5% band. Figures 9 and 10 summarize a technical analysis of the CFB process carried out with the empirical model. The effect of the three independent variables on the SO2 removal and on the sorbent utilization defined as
SU ) E/CAS is shown. In a commercial application, the CFB desulfurization technology may be combined with an ESP, as in the Los Barrios Pilot Plant, or with a fabric filter. Thus, it is very useful to distinguish between the CFB performance and the yield of the global system (CFB + ESP). Parts a-c of Figure 9 show the model results related to the CFB unit, and Figure 10 shows the contribution of the ESP to the global efficiency. Figure 9a shows the minor effect of the CASf on the removal efficiency and on the sorbent utilization. Thus, an increase of 60% in the consumption of lime elevates the removal efficiency only 10 percentage points. On the other hand, the removal efficiency is very sensitive to the AST. The consumption can be minimized by operating at the lowest AST compatible with a trouble-free operation, i.e., thus avoiding solid deposits. Typically, the AST should be higher than 8-10 °C. Figure 9b shows that the removal efficiency is also very sensitive to the RR. Increasing the recirculation leads to a higher removal efficiency or, alternatively, to a savings in lime consumption. Although this increases the required power, it is still a good practice because the pressure drop in the absorber is quite low in any case. Figure 9c shows that the removal efficiency is particularly sensitive to the RR at high values of this operating parameter. This is because the solids flow rate through the absorber increases greatly in this range of RRs (>95%) and so do the CASint ratios.
Although not included in the set of figures, the global efficiency versus the three independent variables plots show similar trends but are displaced upward to higher efficiencies and sorbent utilizations. Indeed, Figure 10 shows that the contribution of the ESP to the global efficiency is around 5 percentage points. As can be seen, it is quite significant, and it should not be disregarded in the design of an FGD plant. Despite the fact that the model was developed based on the results from our pilot plant, it can be used with confidence in industrial plants, given the size of the equipment and the fact that it was designed and operated in accordance with current standards. For the same reason, the qualitative results (i.e., “U0 and inlet SO2 concentration do not affect the yield”) can also be accepted with confidence. Conclusions An extensive experimental program was carried out at a 3-MWe equivalent scale pilot plant to assess the CFB desulfurization technology and to develop a semiempirical efficiency model. The pilot CFB absorber processes real flue gases withdrawn from a 550-MWe power station doped with SO2 to increase the sulfur concentration level up to the value required by the experimental design. The pilot-plant study demonstrated that high SO2 removal efficiencies (up to 95-97%) may be reached at reasonable values of the operating parameters (CASf ) 1.2, AST ) 10 °C, RR ) 98%), for up to a 3% sulfur content coal. This means a high sorbent utilization (SU ) 75%) and thereby implies moderate lime consumption and operating costs. These results were obtained without ash precollection at low (400 ppm) and at high (2000 ppm) SO2 concentrations in the inlet flue gas. According to our experimental results, the CFB unit performs equally well at low and at high power plant loads. The fractional factorial design of experiments, comprised of more than 40 tests, allowed us to collect enough data to develop an empirical model based on the number of overall mass-transfer units concept. Despite the fact that the model was developed based on the results from our pilot plant, it can be used with confidence in industrial plants because our pilot CFB was designed and operated according to widely accepted engineering rules in this field.
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A careful statistical analysis based on a preliminary model that included all of the operating variables showed that neither the SO2 inlet concentration (4002000 ppm) nor the superficial gas velocity (4-6 m/s) significantly influences the removal efficiency. Thus, although the final proposed model is very simple and has only two independent variables (the internal Ca/S ratio, which in turn depends on the product RR, and the approach to the saturation temperature), it shows a high coefficient of determination (R2 ) 0.9). Finally, another important conclusion derived from the experimental work and reproduced by the model is the significance of the sulfur removal that takes place in the ESP, around 5 percentage points. Acknowledgment This study was supported by the European Coal and Steel Community (ECSC) and the Commission of Science and Technology of Spain (CICYT). The authors acknowledge the staff of the Los Barrios Power Plant for its technical support during the construction and operation of the pilot plant. Nomenclature Variables CASf ) fresh calcium to sulfur molar ratio (mol/mol) AST ) approach to the adiabatic saturation temperature (°C) (i.e., the difference between the gas temperature at the outlet of the absorber and the adiabatic saturation temperature of the gas) RR ) solid recirculation ratio (%), defined as (mass flow rate of recirculated solids/mass flow rate of solids at the absorber outlet) × 100 U0 ) superficial gas velocity (m/s) SO ) SO2 flue gas concentration (ppm) E ) SO2 removal efficiency X ) number of overall mass-transfer units k ) constant xi ) independent variables Ri ) exponential coefficients SU ) sorbent utilization (%), defined as desulfurization efficiency/CASf
Abbreviations FGD ) flue gas desulfurization CFBA ) circulating fluidized-bed absorber SD ) spray dryer ESP ) electrostatic precipitator GSA ) gas sorbent absorption FF ) fabric filter
Literature Cited (1) Graf, R. Circulating Fluidized Beds in the Flue Gas ScrubbingsDevelopments, Applications and Operating Experiences in the years 1970 to 2000. 6th International Conference on Circulating Fluidized Beds, Wu¨rzburg, Germany, 1999; pp 601607. (2) Neathery, J.; Schaefer, J.; Stencel, J.; Burnett, T.; Norwood, T. Circulating bed absorption for flue gas desulphurisation: a fundamental study. Pittsburgh Coal Conference, Pittsburgh, PA, 1994. (3) Porter, D. Dry removal of gaseous pollutants from flue gases with the circulating fluid bed scrubber. Low cost SO2 emission control systems for power and process plants. Seminar, London, 1994. (4) Sage, P. W.; Ford, N. Review of sorbent injection processes for low cost SO2 control. Low cost SO2 emission control systems for power and process plants. Seminar, London, 1994. (5) Makansi, J. CFB technology injects life into dry scrubbing. Power 1993, Oct. (6) Burnett, T. A.; Puschaver, E. J.; Little, T. M.; Lepovitz, L. R.; Altman, R. F. Results from the Phase II of the Gas Suspension Absorption Flue Gas Desulfurization Technology at the Center for Emissions Research. 1995 SO2 Control Symposium, Miami, 1995. (7) Ollero, P.; Gutie´rrez Ortiz, F. J.; Otero, J.; Cabanillas, A. Flue Gas Desulfirization by means of a Circulating Fluidized Bed Absorber. Final Report on the DELEF Project (ECSC Agreement No. 7220-ED/067); European Community of Steel and Coal: Brussels, Belgium, 1999. (8) Box, G.; Hunter, W.; Hunter, J. S. Statistics for Experimenters; Wiley: New York, 1978. (9) SPSS. SPSS Base 7.5 for Windows, User’s Guide; SPSS Inc.: Chicago, 1997. (10) Gutie´rrez, F. J.; Ollero, P.; Cabanillas, A.; Otero, J. A Technical Pilot Plant Assessment of Flue Gas Desulphurisation in a Circulating Fluidised Bed. Adv. Environ. Res. 2001, in press.
Received for review February 13, 2001 Revised manuscript received August 2, 2001 Accepted August 15, 2001 IE010152I
