FLUE GAS DESULPHURIZATION IN A CIRCULATING FLUIDIZED BED: INVESTIGATION AFTER 10 YEARS OF SUCCESSFUL

COMMERCIAL OPERATION AT THE FACILITY OF PILSEN/CZ

Leuschke, F.*; Bleckwehl, S.

AE&E Lentjes GmbH D-40880 Ratingen, Germany * now with Imtech Deutschland GmbH & Co.

KG, D-82152 Planegg, Germany

Ratschow, L.**; Werther, J.

Hamburg University of Technology D-21071 Hamburg, Germany ** now with Pöyry Energy GmbH,

Hamburg,Germany

Abstract – At the cogeneration plant in Pilsen, Czech Republic, a circulating fluidized bed (CFB) technology for flue gas desulphurization (FGD) is operating successfully for already more than 10 years (Hansen, et al. (1994), Sauer and Porter (1994), Sauer and Baege (1998)). The semi-dry process is based on the conversion of calcium hydroxide with sulphur to calcium sulphite and calcium sulphate. To investigate the current plant operation and the chemical-physical processes directly in the CFB absorber, a measurement campaign was performed by AE&E Lentjes in conjunction with Hamburg University of Technology, utilizing a system of double-channel capacitance probes as presented by Wiesendorf and Werther (2000) with integrated thermocouples. This system has been used for the first time in a CFB FGD reactor. In the atmosphere of very cohesive particles, the system was still suitable for local measurements of solids concentrations, as well as strand velocities and temperature distributions. Commonly accepted characteristics of CFB systems, such as core-annulus flow structure with strands of particles falling down in the proximity of the wall and a concentration gradient with higher solids volume concentrations in the proximity of the wall, could be validated for this very lean FGD system. The results of the measurement campaign presented contain solids concentration distributions, as well as solids velocity and temperature distributions to describe the processes directly in the absorber. Furthermore, the influences of different operating parameters such as CFB absorber temperature, pressure drop across the CFB absorber, SO2 emission, as well as the sorbent material on the desulphurisation efficiency are described.

INTRODUCTION Circulating fluidized bed (CFB) technology was commercially introduced by Lurgi in the early eighties for the scrubbing of acid gases, especially for flue gas desulphurisation (FGD). The semi-dry process is based on the conversion of calcium hydroxide with sulphur to calcium sulphite and calcium sulphate. Worldwide, more than 60 plants based on this CFB FGD technology are in commercial operation (Yi et al. (2005)).

After 10 years of successful operation, a check of the facility at Pilsen provided the opportunity to use up- to-date measurement equipment in order to verify basic assumptions of the CFB FGD technology. At the Pilsen facility, a single reactor with an inner diameter of 8.4 m performs the flue gas desulphurization of a cogeneration plant, consisting of 5 older boilers.

Several necessary parameters for the design of commercial scale plants have never been measured and validated before due to the complexity of the CFB process and its chemical reactions. Therefore, a measurement campaign at the CFB FGD plant of the cogeneration plant in Pilsen was performed by AE&E Lentjes in conjunction with Hamburg University of Technology, to investigate experimentally the process depending on the main operating parameters, as well as, the chemical-physical processes directly in the CFB absorber.

PRINCIPLE OF THE CFB FGD TECHNOLOGY The dry desulphurisation following the principle of the expanded circulating fluid bed (CFB) operates at a temperature of approximately 75 °C, typically using hydrated lime for the combined absorption of SO2, SO3, HF and HCl. The process produces a dry product which is collected at a filter and is easy to handle. The absorber is arranged downstream the boiler(s). The process scheme of the dry CFB FGD and its control is shown in Fig. 1. The CFB absorber is an empty vertical flue gas duct with venturi shaped nozzles at the bottom. The solids matter with mean particle sizes around 5 µm consists of lime, recirculated

desulphurisation products and fly ash. It is suspended in the flue gas prior to entering the FGD absorber through the nozzles. Within the circulating fluid bed the solids are distributed over the entire height of the absorber. The average solids velocity is much lower than the average gas velocity, the difference is called “slip velocity”. This slip velocity between flue gas and solids is the characterising criterion for the optimal heat and mass transfer behaviour within the circulating fluid bed absorber. This allows the injection of water directly into the fluid bed and the control of the absorption temperature independently from the amount of lime feed. The absorption product which mainly consists of calcium sulphite (CaSO3 x ½ H2O), calcium sulphate (CaSO4 x ½ H2O), limestone (CaCO3) and fly ash, is separated from the clean gas in the downstream filter. The product is recirculated back to the absorber via air-slides to prolong the solids retention time of the sorbent. This aims at reducing the Ca/S molar ratio of the process, which is approximately between 1.1 to 1.5 depending on the SO2 raw gas and clean gas concentrations and the amount/composition of the fly ash. The clean gas passes through the booster fan and is vented to the atmosphere via the stack. The amount of product which corresponds to the amount of hydrated lime fed to the absorber, the inlet fly ash quantity and the collected acid gases, is discharged and conveyed to the product silo. The CFB FGD process requires three main control circuits to regulate, i.e. the reagent feed rate as a function of the flue gas flow rate and the SO2 content of the raw gas and the clean gas, the water injection rate into the fluid bed as a function of the temperature at the outlet of the absorber and the rate of discharge from the solids circuit as a function of the pressure drop across the CFB absorber (Fig. 1). Additionally, the flue gas recirculation controls the minimum gas volume flow to the absorber to establish the fluidized bed independent from boiler load.

EXPERIMENTAL INVESTIGATIONS Plant Design

The experimental investigations of the CFB FGD process were carried out at the CFB absorber integrated in the flue gas cleaning system of the cogeneration plant in Pilsen (Fig. 2). The Pilsen combined heat and power plant consists of 5 different boilers, burning local lignite. All units are equipped with individual dust filter and ID fans and feed their gases to a common duct upstream of the CFB FGD plant. The desulphurisation plant consists of a single CFB absorber with an electrostatic precipitator (ESP) for dedusting and an additional booster fan to cover the pressure drop of the new equipment. The flue gas – after passing the ESP – is discharged to the stack without reheating. The CFB FGD plant cleans a gas quantity of maximal 688.000 Nm3/h from the 5 boilers and the raw gas contains a SO2 concentration of up to 5.200 mg/Nm3. The clean gas parameters which have to be guaranteed downstream of the CFB FGD are ≤ 50 mg/Nm3 for dust and ≤ 1.490 mg/Nm3 for SO2. This equals a desulphurisation efficiency of 71% thus, meeting the statutory requirements. Due to the good part load performance of the

Flue Gas

Clean Gas

Flue Gas Recirculation

Dry Product Discharge

Water

Absorbent

CFBAbsorberAb-

sorbentSilo

De-Dusting

Sta

ck

Booster Fan

T

dP

SO2

Storage inHoppers

V

Flue Gas

Clean Gas

Flue Gas Recirculation

Dry Product Discharge

Water

Absorbent

CFBAbsorberAb-

sorbentSilo

De-Dusting

Sta

ck

Booster Fan

TT

dPdP

SO2SO2

Storage inHoppers

VV

Fig. 1: Dry CFB flue gas desulphurisation process scheme

Fig. 2: Flow diagram of the cogeneration plant in Pilsen

CFB FGD process, the desulphurisation plant is capable of covering a load range of operation from 1 to 5 boilers. Hydrated lime serving as sorbent is produced on site in a “dry lime hydration plant”. Due to the controlled water injection into the hydrator, lime (CaO) is hydrated to hydrated lime (Ca(OH)2) in a dry mode. The solid product of the CFB FGD plant is used as stabilising material which is produced in a plant installed below the storage silos for the CFB FGD product.

Measuring Program

The experimental studies of the CFB FGD process comprise two main topics, i.e. the analysis of the current plant operation and the investigation of the chemical-physical processes directly in the CFB absorber. The investigation of the existing plant operation contains the experimental examination of the operation measurements such as the SO2 raw gas concentration at the inlet of the CFB absorber, the SO2 clean gas concentration at the inlet of the stack, the volume flow rate and the weighing of the sorbent. Furthermore, the desul-phurisation efficiency depending on the CFB absorber temperature, the SO2 emission, the pressure drop across the CFB absorber as well as the type of sorbent was investigated (Tab. 1). To analyse the chemical-physical processes in the CFB absorber, which has a total height of nearly 42 m and an inner diameter of approximately 8.4 m, local measurements of solids concentrations, as well as strand velocities and temperature distributions were performed with a system of double-channel capacitance probes with integrated thermocouples in different vertical and horizontal measuring levels (Fig.3). A major problem during the measurements was the stickiness of the particles. Minutes only after the insertion of the probes into the reactor the particles formed a thick coating on the capacitance needles which made reliable measurements impossible. However, attaching a mechanical vibrator to the probe tube (in fact, a drill with jack hammer function was used) prevented the particles from attaching to the probe tip and thus enabled the measurements.

Investigation of the Current Plant Operation

The experimental examination of the operation measurements i.e. the SO2 raw gas and clean gas concentration, the volume flow rate and the weighing of sorbent showed that these measurements were resilient, plausible and correct. The variation of the CFB absorber temperature showed that with increasing temperature from 77 to 82 °C due to the degradation of the kinetic reactions conditions the desulphurisation efficiency decreased, in case of constant SO2 emission limit resulting in increased sorbent consumption. The decrease of the reactor temperature caused the opposite influence. Compared to the absorber temperature which had a significant influence on the desulphurisation efficiency, the pressure drop across the CFB absorber, being proportional to the total solids content within the absorber, had no noticeable influence on the desulphurisation efficiency. The reduction of the SO2 stack emission down to 50 mg/Nm3 besides constant operating conditions generates, in fact, an increase in the sorbent consumption, but shows that desulphurisation efficiency > 99 % is achieved and that tightened emission limits could be maintained without any problems. Furthermore, the direct application of lime (CaO) as absorbent was tested, showing significant increase of the sorbent consumption, but the prescriptive limits were also maintained with direct use of CaO, ensuring a high availability of the CFB FDG plant as a quite reasonable

5

21m Level 9,3m Level

21m Level

9,3m Level

14,8m Level

30°60°30°

60° 30°

30°

60°60°

7

8

21

6

4

3

1413121110

CFB-Absorber

Water-Injection

1,5m

Top of Venturi Nozzles = 0m

5

21m Level 9,3m Level

21m Level

9,3m Level

14,8m Level

30°60°30°

60° 30°

30°

60°60°

7

8

21

6

4

3

1413121110

CFB-Absorber

Water-Injection

1,5m

Top of Venturi Nozzles = 0m Fig. 3: Arrangement of the different measuring levels in the CFB absorber

Tab. 1: Parameters for Investigation of the Current Plant Operation

Operating Value

Pressure Drop across Absorber [mbar] 8,5 6,0

Sorbent Ca(OH)2

12,0

SO2 Emission [mg/Nm 3] 1490,0 400,0 50,0

CFB Absorber Temperature [°C] 77,0 72,0 82,0

ParameterVaried Values

CaO

Investigation of the Plant Operation

alternative in case of mechanical problems with the hydrator.

Investigation of the Chemical-Physical Processes in the CFB Absorber The experimental studies of the chemical-physical processes in the absorber were performed by using a system of double-channel capacitance probes with integrated thermocouples as presented by Wiesendorf and Werther (2000). This system was used in the present work for the first time in a CFB FGD reactor. In the presence of very cohesive particles, this system was still suitable for local measurements of solids concentrations, as well as strand velocities and temperature distributions. The results of the measurements of vertical and horizontal distributions in the reactor are detailed in the following sections.

Time-averaged horizontal profiles of the solids concentration

Time plots of the solids concentration can be drawn, aided by the measurements with the capacitance probes in the CFB FGD. These time dependent plots were averaged for a time period of 30 seconds each. Measurements were carried out at various penetration depths, giving a radial concentration profile at each measuring port. These radial profiles have been interpolated for two horizontal layers in the CFB FGD with the help of the commercial software Matlab.

At the horizontal layer 21 m above the Venturi nozzles, only the region opposite the outlet to the electro-static precipitator can be drawn out, the rest of the horizontal layer had not been accessible for the probes (Fig. 4 left). Therefore, only half of these horizontal layers are plotted. Due to good accessibility of the measurement ports 1 - 8 at 9.3 m above the Venturi nozzles, it was possible to draw an entire concen-tration profile for this height, as can be seen in Fig. 4 on the right side. In Fig. 4, as well as in all following plots of horizontal profiles, the location of the measurement ports has been indicated together with the day of the measurement. Due to the complex measurement procedure, it has not been possible to perform the measurements on each port on the same day.

The solids volume concentrations in the plots range from 0 to 1.5%. In the lower part of the CFB FGD, there was a local maximum when measuring port 2 at a penetration depth of 1 m. Several repetitions of the measurement at this port have confirmed the maximum. Other concentration maxima were measured at ports 5, 6, and 8. There is an obvious subsidence in the solids volume concentration.

The fluid dynamics in a circulating fluidized bed are commonly described by a core-annulus flow structure, consisting of a solids-rich annulus where the solids fall down and a lean core region where the solids are entrained upwards by the fluid. In the lower part of the CFB FGD, the concentration profile has

cV [%]

H = 21 m above nozzles H = 9.3 m0.0 0.5 1.0 1.5

port 14; Oct 20

port 13; Oct 20

port 12; Oct 20

port 10; Oct 20

H2O

outflowto ESP

inflow direction of flue gas

H2O

inflow direction of flue gas

Fig. 4: Time-averaged solids volumetric concentration in the CFB FGD. Left: Interpolation from measurements in a horizontal layer 21 m above the Venturi-nozzles. Right: Interpolation from the measurements in a horizontal layer 9.3 m above the Venturi nozzles.

v [m/s]

H = 21 m above nozzles H = 9.3 m-4 -2 0 2 4 6 8

port 14; Oct 20

port 13; Oct 20

port 12; Oct 20

port 11; Oct 20

port 10; Oct 20

outflowto ESP

H2O

inflow direction of flue gas

H2O

inflow direction of flue gas

Fig. 5: Time-averaged strand velocity in the CFB FGD. Left: Interpolation from measurements in a horizontal layer 21 m above the Venturi-nozzles. Right: Interpolation from the measurements in a horizontal layer 9.3 m above the Venturi nozzles.

such a typical pattern. While the solids concentration varies in the core region between 0.5 vol.-% and 1 vol.-%, it increases to values of roughly 3 vol.-% in the proximity of the wall. The annulus region has a thickness of 0.05 m to 0.1 m only which is smaller than in CFB combustors (Werther (2005)).

In the upper part of the CFB FGD, the measurement at port 11 was discarded due to sticky solids on the tip of the measurement probe which led to aberrant values. A core-annulus flow pattern cannot be clearly identified at this height. There may be several reasons for this phenomenon. One explanation lies in the location of the ports opposite the exit to the ESP. The whole flue gas stream drifts into the direction of the outlets which leads to the destruction of a typical concentration profile. The darker color indicates an overall lower solids concentration in this region of the CFB FGD.

Time-averaged horizontal profiles of the vertical strand velocity

By means of the capacitance probes, it is also possible to determine solids velocities. In order to do so, time plots of the signals obtained from measuring with the two tips of one probe, are compared. By means of a cross correlation the average time shift between the two signals can be found. The vertical distance between the tips of the probe divided by the time shift of the signals can be interpreted as the strand velocity. Assuming that the solids strands move in average with the same velocity as the particles they consist of, solids velocity distributions can be shown. In Fig. 5, the solids velocity distribution is shown for two horizontal planes. At the lower level, the core-annulus flow structure can be clearly identified. The vertical solids velocities range from -4 m/s in the annulus region to +6 m/s in the core region. In analogy to the concentration profiles, regions with lower solids velocity can be identified in the core region. They are at the locations of high solids concentration (compare to Fig 4).

The core-annulus intersection is commonly identified at the radius of net zero vertical solids velocity. Applied on the solids velocity profile of the lower part of the CFB FGD, the width of the annulus region would be roughly 0.25 m and deviates from the observation of a very shallow annulus region from the concentration measurements. This observation is probably system-specific: in many known CFB systems, the fluid is fed through a nozzle floor. In this case of a Venturi CFB, it is possible that at the height of 9.3 m above the Venturi nozzles, flue gas stream has not yet spread out to the full width of the reactor diameter.

In the upper part of the CFB FGD, the solids velocities are in the range of the theoretical mean superficial gas velocity of circa 4 m/s. The terminal settling velocity of the solids is very low, due to a very small mean particle diameter of 4 – 6 µm. Therefore, it is likely that the solids and the flue gas have approximately the same velocity in the very lean regions. Contrary to the observation of the solids concentrations, a core-annulus structure can also be found in the upper part of the riser.

Time-averaged horizontal temperature profiles

Some of the probes were equipped with an additional thermocouple at the probe tip. Fig. 6 shows a temperature distribution from measurements that were carried out on the 23rd October. During this measuring period, the location of water injection for the desulphurization process was at a port below measuring port 5. Fig. 6 (right) shows a temperature distribution at the 9.3 m level. The flue gas temperature at the outlet of the CFB FGD was set to 72°C. It is clearly visible that the flue gas temperature is lower at the side of water injection. Furthermore, the temperature in the proximity of the wall is lower than the temperature in the core. The temperature profile is not symmetric to the point of water injection. The temperature is higher at the side of the outflow to the ESP. A calculation of the solids mass flux profile from the solids concentration and solids velocities has shown that the mass flux on the side of the outflow, is twice the mass flux on

outflowto ESP

H2O

60 65 70 75 80 85 90

Tclean gas = 72 °C

T [°C]

H = 21 m above nozzles H = 9.3 m

∆T ≈ 5 °C due to increased water injection

port 14; Oct 23

port 13; Oct 23

port 12; Oct 23

port 11; Oct 23

port 10; Oct 23

inflow direction of flue gas

H2Oinflow direction of flue gas

Fig. 6: Time Time-averaged solids Temperature profile in the CFB FGD. Outlet temperature = 72°C. Left: Interpolation from measurements in a horizontal layer 21 m above the Venturi-nozzles. Right: Interpolation from the measurements in a horizontal layer 9.3 m above the Venturi nozzles.

the opposite side. This indicates that the temperature on the side of the outflow to the ESP is higher, because more water can be evaporated, due to higher solids flux. In Fig. 6, on the left, the temperature profile is shown for the upper part of the CFB FGD. The overall temperature is about 10°C lower, due to complete evaporation of the injected water at this height. At this height, the temperature is still lower at the side of water injection. In the proximity of the walls, the temperature dropped below 60°C.

CONCLUSIONS A measurement campaign was carried out at the CFB FGD cogeneration plant in Pilsen and performed by Lentjes in conjunction with the Hamburg University of Technology in order to investigate the desulphurisation efficiency depending on the main operating parameters like CFB absorber temperature, SO2 emission and pressure drop across the CFB absorber as well as the chemical-physical processes directly in the CFB reactor. A system of double-channel capacitance probes with integrated thermocouples was used to measure the local solids concentrations, as well as strand velocities and temperature distributions in different vertical and horizontal measuring levels of the CFB reactor. The variation of the CFB absorber temperature showed that with increasing temperature, due to the degradation of the kinetic reactions conditions the desulphurisation efficiency decreased whereby in case of constant SO2 emission limit the sorbent consumption increased. In case of decreasing the reactor temperature the opposite influence is noticed. Compared to the absorber temperature which had a significant influence on the desulphurisation efficiency, the pressure drop across the CFB absorber had no influence on the efficiency. The reduction of the SO2 emission limit to 50 mg/Nm3 besides constant operating conditions generates, in fact, an increase in the sorbent consumption, but also showed that desulphurisation efficiencies > 99 % are achieved and that tightened EU emission limits could be maintained without any problems.

In the lower part of the CFB FGD, the fluid dynamical measurements confirmed a core-annulus flow structure, consisting of a solids-rich annulus where the solids fall down and a lean core region where the solids are entrained upwards by the fluid. While the solids concentration varies in the core region between 0.5 vol.-% and 1 vol.-%, it increases to values of roughly 3 vol.-% in the proximity of the wall. The concentration profile in a horizontal layer is not homogeneous; a subsidence slope concerning the solids concentration and solids velocity was detected. At the height of the exit to the ESP, a core-annulus flow structure was not detected with regard to solids concentration. The whole flue gas stream drifts into the direction of the outlets which leads to the destruction of a typical concentration profile.

The flue gas temperature is lower at the side of water injection. In the proximity of the wall, it is lower than in the core. Furthermore, the horizontal temperature profile at 9.3 m above the Venturi nozzles is not symmetric to the location of water injection. The temperature is higher at the side of the outflow to the ESP. A calculation of the solids mass flux profile from the solids concentration and solids velocities has shown that the mass flux on the side of the outflow is twice the mass flux on the opposite side. This indicates that the temperature on the side of the outflow to the ESP is higher because more water can be evaporated, due to higher solids flux. In the upper part of the CFB FGD, the overall temperature is about 10°C lower, due to complete evaporation of the injected water at this height.

REFERENCES Hansen, S.K., Toher, J., Lanois, G., Sauer, H.: High efficiency, dry flue gas SOx and combined SOx/NOx -

removal experience with the Lurgi circulating fluid bed dry scrubber - a new economical retrofit option for U.S. utilities for acid rain remediation, International Power Generation Conference, San Diego, 1991.

Sauer, H.; Porter, D. E.: Dry Removal of Gaseous Pollutants from Flue Gases with the Circulating Fluid Bed Scrubber, C479/022 IMechE (1994).

Sauer, H.; Baege, R.: Recent Developments on CFB FGD Technology, Power Gen Europe, Milano, 1998. Werther, J.: Fluid dynamics, temperature, and concentration fields in large-scale CFB combustors, Proc. 8th

Int. Conf. Circulating Fluidized Beds (K.Cen, (Ed.)), Hangzhou, China 2005, pp. 1-25. Wiesendorf, V., Werther, J.: Capacitance Probes for Solids Volume Concentration and Velocity

Measurements in Industrial Fluidized Bed Reactors, Powder Technology 110 (2000), pp. 143-157. Yi, J.; Sauer, H.; Leuschke, F; Baege, R.: What is possible to achieve on flue gas cleaning using the CFB

technology, 8th International Conference on Circulating Fluidized Beds, Hangzhou, 2005.

ACKNOWLEDGMENT We would like to thank the operator of the cogeneration plant in Pilsen for the excellent collaboration and the possibility to perform the experimental investigations during the commercial operation.