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Flue gas desulfurization in a circulating fluidized bed
Coal Science J.A. Pajares and J.M.D. Tasc6n (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
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Flue gas desulfurization in a circulating fluidized bed R. Ortiz de Salazar ", P. Ollero b, A. Cabanillas a, J. Otero-Ruiz a, L. Salvador b. aCIEMAT - Tecnologia de Combustibn y Gasificacibn. Avda. Complutense 22, 28040 Madrid. bAICIA. Avda. Reina Mercedes s/n, 41080 Sevilla. KEYWORDS: FGD, CFB, SO 2 abatement.
1. I N T R O D U C T I O N
In the last years major concern has been drawn upon SO 2 emissions from combustion power plants. SO 2 emission standards for new and existing coal-fired plants were introduced in the EC on 24 Nov. 19881. Spain, as EC member country is legally bound to meet these targets. There is a moratorium for Spain, though. It should not fully comply with the directive until 2003. Therefore, SO 2 removal from the flue gases is and will become necessary to meet environmental regulations in the future, as fuel switching, cleaning and/or blending may not be sufficient as regulations are made stricter. Nowadays there are many ways of reducing SO 2 emissions from coal utilization: wet or dry scrubbers or slurry sorbent injection, high or low temperature processes... 2' 3 This project develops a Flue Gas Desulfurization (FGD) system using a Circulating Fluidized Bed (CFB) as a dry scrubber, taking advantage of the special gas-solid contact characteristics of this technology"' s. In a first phase a CFB pilot plant for FGD has been built up at CIEMAT, Madrid. This desulfurization system cleans the flue gases (200 Nm3/h) produced in a CFB combustor. These exhaust gases are conditioned to simulate those produced in a pulverized coal power plant. The conditioning implies adjusting gas temperature and dust load. This first phase has a duration of two years. In this paper we present the objectives and design of the operational tests. Soon the process will be scaled to a plant of 15000 Nm3/h to be built up in a commercial power plant. The design, built-up and operational tests of this plant will be carried out during the second phase of this project.
2. O B J E C T I V E S
The main objective of this project is to evaluate the technology of desulfurization in a CFB at low temperature using calcium hydroxide as sorbent. This means establishing the maximum SO 2 abatement to be achieved in the plant and its
1844
approximation to maximum solid conversion 6, and measuring the consumption of sorbent, power and water. The second objective, closely related to the previous, is the design of an industrial plant and the establishment of working parameters ( Ca/S ratio, approximation to adiabatic saturation temperature, solid recirculation...).
3. PROCESS DESCRIPTION
Acidic gases in the combustion efluent can be removed in a purposed designed vessel after the furnace. Post furnace sorbent injection offers great residence time and better control of gas temperature, thus enhancing sulphur capture. There is also the option of injecting water or stream to promote the reactions. The main disadvantage of this technique is that sorbents need to be more reactive and hence more expensive than those used for in-furnace desuiphuration. In this project Ca(OH) 2 will be used as sorbent in a circulating fluidized bed reactor. The flue gases leaving the combustion chamber and energy recovery circuit enter the desulphuration reactor at a relatively low temperature. Acidic gases are then removed by dry reactive particles (Ca(OH)2) injected into the fluidised bed. The reactions that take place are:
802 § H20 Ca(OH)2+H2S03
9 H2SOs CaSO3.H O
If HCI is present it is removed by Ca(OH)2 at a temprerature > 120 C. The reaction product is very hygroscopic and cannot be handled at lower temperature. SO= is also removed at this temperature by the following reactions: 2 HC! + Ca(OH) 2
CaGI2,2H20 + Ca(OH)a + S02
CaCi2.2H20 CaCI2.2H2 0. CaSOs + H20
These reactions are temperature dependent. Generally they are more effective as the flue gas temperature approaches the adiabatic water saturation temperature. The desulphuration resultant residues are calcium sulphite, unreacted sorbent and flyash. These residues are recirculated to improve sorbent utilisation.
4. EXPERIMENTAL PLANT
The flow diagram of the experimental plant built in CIEMAT, Madrid, is shown in
1845
figure 1. Flue gas entering the CFB scrubber is humidified and cooled to within 20-30-~ of the adiabatic saturation temperature with a water spray. Then it flows vertically upward throgh the bell plate and through the bed of fresh reagent and recirculated material, where the SO 2 retention takes place. The gases leaving the reactor are cleaned of particles in the bag filter. The solids collected in the filter are recirculated to the CFB absorber by means of a screw feeder. By this way unused sorbent remains in the process for a long residence time and conversion increases. An electrical heater and an ash feeder are used to condition the exhaust gases produced in a CFB combustor, so that they simulate those produced in a pulverized coal power plant.
5. FUTURE A C T I O N S
The cleaning performance of the CFB absorber will be evaluated at two ranges of SO 2 concentration in the flue gases: 500-1000 ppm and 2500-3500 ppm. In both cases the coals used to generate the gas are actually being used in Spanish pulverized coal power plants. Different Ca/S ratios, approximations to adiabatic saturation temperature, solid recirculations and fluidization velocities will be tried in a factorial design. Experimental tests will begin in May 1995.
REFERENCES
1.88/609/EC 2. Takeshita, M., Soud, H. "FGD performance and experience on coal-fired plants', IEACR/58, lEA Coal Research, July 93. 3. Pan,Y.S. "Recent advances in flue gas desulfurization technologies', DOE/PETC/TR-91/4. 4. Porter, D. "Dry removal of gaseous pollutants from flue gases with the circulating fluid bed scrubber'. Low Cost Emission Control Systems Seminar, IMeChE, London, 20 th Oct. 1994. 5. Keene, T.C. et al. "The use of a circulating fluidized bed absorber for control of sulfur dioxide', AIChE Annual Meeting, Washington D.C., Nov.27- Dec. 2, 1988. 6. Irabien, A. et al. "Kinetics of flue gas desulfurization at low temperatures: nonideal surface adsorption model', Chem. Eng. Sc., Vo1.47, N-~ pp 1533-1543, 1992.
This work is supported by the ECSC and OCIDE- PIE.
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1843
Flue gas desulfurization in a circulating fluidized bed R. Ortiz de Salazar ", P. Ollero b, A. Cabanillas a, J. Otero-Ruiz a, L. Salvador b. aCIEMAT - Tecnologia de Combustibn y Gasificacibn. Avda. Complutense 22, 28040 Madrid. bAICIA. Avda. Reina Mercedes s/n, 41080 Sevilla. KEYWORDS: FGD, CFB, SO 2 abatement.
1. I N T R O D U C T I O N
In the last years major concern has been drawn upon SO 2 emissions from combustion power plants. SO 2 emission standards for new and existing coal-fired plants were introduced in the EC on 24 Nov. 19881. Spain, as EC member country is legally bound to meet these targets. There is a moratorium for Spain, though. It should not fully comply with the directive until 2003. Therefore, SO 2 removal from the flue gases is and will become necessary to meet environmental regulations in the future, as fuel switching, cleaning and/or blending may not be sufficient as regulations are made stricter. Nowadays there are many ways of reducing SO 2 emissions from coal utilization: wet or dry scrubbers or slurry sorbent injection, high or low temperature processes... 2' 3 This project develops a Flue Gas Desulfurization (FGD) system using a Circulating Fluidized Bed (CFB) as a dry scrubber, taking advantage of the special gas-solid contact characteristics of this technology"' s. In a first phase a CFB pilot plant for FGD has been built up at CIEMAT, Madrid. This desulfurization system cleans the flue gases (200 Nm3/h) produced in a CFB combustor. These exhaust gases are conditioned to simulate those produced in a pulverized coal power plant. The conditioning implies adjusting gas temperature and dust load. This first phase has a duration of two years. In this paper we present the objectives and design of the operational tests. Soon the process will be scaled to a plant of 15000 Nm3/h to be built up in a commercial power plant. The design, built-up and operational tests of this plant will be carried out during the second phase of this project.
2. O B J E C T I V E S
The main objective of this project is to evaluate the technology of desulfurization in a CFB at low temperature using calcium hydroxide as sorbent. This means establishing the maximum SO 2 abatement to be achieved in the plant and its
1844
approximation to maximum solid conversion 6, and measuring the consumption of sorbent, power and water. The second objective, closely related to the previous, is the design of an industrial plant and the establishment of working parameters ( Ca/S ratio, approximation to adiabatic saturation temperature, solid recirculation...).
3. PROCESS DESCRIPTION
Acidic gases in the combustion efluent can be removed in a purposed designed vessel after the furnace. Post furnace sorbent injection offers great residence time and better control of gas temperature, thus enhancing sulphur capture. There is also the option of injecting water or stream to promote the reactions. The main disadvantage of this technique is that sorbents need to be more reactive and hence more expensive than those used for in-furnace desuiphuration. In this project Ca(OH) 2 will be used as sorbent in a circulating fluidized bed reactor. The flue gases leaving the combustion chamber and energy recovery circuit enter the desulphuration reactor at a relatively low temperature. Acidic gases are then removed by dry reactive particles (Ca(OH)2) injected into the fluidised bed. The reactions that take place are:
802 § H20 Ca(OH)2+H2S03
9 H2SOs CaSO3.H O
If HCI is present it is removed by Ca(OH)2 at a temprerature > 120 C. The reaction product is very hygroscopic and cannot be handled at lower temperature. SO= is also removed at this temperature by the following reactions: 2 HC! + Ca(OH) 2
CaGI2,2H20 + Ca(OH)a + S02
CaCi2.2H20 CaCI2.2H2 0. CaSOs + H20
These reactions are temperature dependent. Generally they are more effective as the flue gas temperature approaches the adiabatic water saturation temperature. The desulphuration resultant residues are calcium sulphite, unreacted sorbent and flyash. These residues are recirculated to improve sorbent utilisation.
4. EXPERIMENTAL PLANT
The flow diagram of the experimental plant built in CIEMAT, Madrid, is shown in
1845
figure 1. Flue gas entering the CFB scrubber is humidified and cooled to within 20-30-~ of the adiabatic saturation temperature with a water spray. Then it flows vertically upward throgh the bell plate and through the bed of fresh reagent and recirculated material, where the SO 2 retention takes place. The gases leaving the reactor are cleaned of particles in the bag filter. The solids collected in the filter are recirculated to the CFB absorber by means of a screw feeder. By this way unused sorbent remains in the process for a long residence time and conversion increases. An electrical heater and an ash feeder are used to condition the exhaust gases produced in a CFB combustor, so that they simulate those produced in a pulverized coal power plant.
5. FUTURE A C T I O N S
The cleaning performance of the CFB absorber will be evaluated at two ranges of SO 2 concentration in the flue gases: 500-1000 ppm and 2500-3500 ppm. In both cases the coals used to generate the gas are actually being used in Spanish pulverized coal power plants. Different Ca/S ratios, approximations to adiabatic saturation temperature, solid recirculations and fluidization velocities will be tried in a factorial design. Experimental tests will begin in May 1995.
REFERENCES
1.88/609/EC 2. Takeshita, M., Soud, H. "FGD performance and experience on coal-fired plants', IEACR/58, lEA Coal Research, July 93. 3. Pan,Y.S. "Recent advances in flue gas desulfurization technologies', DOE/PETC/TR-91/4. 4. Porter, D. "Dry removal of gaseous pollutants from flue gases with the circulating fluid bed scrubber'. Low Cost Emission Control Systems Seminar, IMeChE, London, 20 th Oct. 1994. 5. Keene, T.C. et al. "The use of a circulating fluidized bed absorber for control of sulfur dioxide', AIChE Annual Meeting, Washington D.C., Nov.27- Dec. 2, 1988. 6. Irabien, A. et al. "Kinetics of flue gas desulfurization at low temperatures: nonideal surface adsorption model', Chem. Eng. Sc., Vo1.47, N-~ pp 1533-1543, 1992.
This work is supported by the ECSC and OCIDE- PIE.
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