Sulphur retention in circulating fluidized bed coal combustion. Modelling and simulation

Coal Science

J.A. Pajares and J.M.D. Tasc6n (Editors) 9 1995 Elsevier Science B.V. All rights reserved.

1839

Sulphur retention in circulating fluidized bed coal combustion. Modelling and simulation. J. Ad~inez, L.F. de Diego, F. Garcfa-Labiano and P. G a y s . Instituto de Carboqufmica (C.S.I.C.), P.O. Box 589, 50080 Zaragoza, Spain 1. INTRODUCTION Circulating fluidized bed coal combustion (CFBC) with sorbent addition allows the combustion of coals of different rank even with high sulphur and ash contents in a clean way. High combustion efficiencies together with high sulphur retentions are the principal objectives to be reached. To optimise the process and make predictions about the combustor behaviour in a broad range of operating conditions it is necessary to make a mathematical model of the process. This model was developed integrating hydrodynamic and sulfation kinetic submodels.The model was validated previously [1] with experimental results obtained in pilot plant and good fitting was found. 2. MODEL DESCRIPTION Riser hydrodynamic was modelled taking into account previous work [2,3]. In this work the bed was divided in two regions. A dense region with constant voidage of 0.82 in which the solids present were perfectly mixed, and the dilute region, was considered with a core-annulus structure with a net dispersion of solids from the core to the annulus. The axial voidage profile in the dilute region was analysed with an exponential decay model. To calculate the hydrodynamic properties of the bed, mean particle size and density of solids in the bed (partially sulphated limestone, coal and ashes) were used. The size distribution of ashes and limestone in the bed is calculated considering that the ashes had the same size distribution as the coal particles fed into the boiler, and then a fraction of them, Pr, had a breakage giving fines with a function of size distribution Pf(r)lash. With this initial assumption the mean particle size and density of the solids present in the bed were calculated and the hydrodynamic model was solved. Then, taking into account the collection efficiency of solids by the cyclone, the composition and size distribution of solids in the recirculation stream were calculated. This process was repeated iteratively until the convergence on the ash fraction in the bed was reached The axial solid distribution in dilute region of CFB was determined with the exponential decay model of Kunii and Levenspiel [4] modified by Ad~inez et al. [2] by the following equation: (1- ~;) = [ ( 1 - ~*) + (g*- eRo) exp

(-a hf)]- K

(1)

The K constant was calculated [2] as a function of the solid circulation flux, the solid flux above TDH and the solid downward velocity. The main parameter in this model is the decay constant "a", which was calculated with the following equation [2]: a

(Uo -

ut) 2 D 0 " 6 = 0 . 8 8 - 4 2 0 dp

(2)

1840 In the dilute region the solids rise through the core with a voidage ee and descend through the annulus at a velocity of lm/s in a denser suspension. A modified Rhodes model [5] was used in which the flowrate of solids transferred from the core to the annulus is proportional to the solid concentration present in the core and to the interface surface. The solid dispersion constant was calculated as a function of the operating conditions [3].Plug flow of gas was considered in the bed and perfect mixing to the solids. The solution of the hydrodynamic model indicates, at each bed height, the following aspects: mean voidage, wall and core voidage, core radius, upward solids flow in the core, downward solids flow in the wall and external circulation solid flux. Then, to calculate the mean residence times of particles it is necessary to know all the solid fluxes in the system. The solid fluxes are dependent on the recovery efficiency of solids by the cyclone. The mean residence time is defined as a function of the volume of solids in the bed (Ve), the volume of solids of size ri (Ve(ri)), the volumetric flow rate of solids leaving the fast column (v4) , and the volumetric flowrate of solids recirculated to the fast column (v 1), by the following expression: r(ri) =[v4/Ve- vl(ri)/Ve(ri)] (3) Different SO2 generation rates depending on the height in the bed were considered. These differences were due to differences in the char combustion rate because of the existence of axial oxygen concentration profiles due to the plug flow in the bed. Moreover, the place of coal devolatilization must be taken into account because the combustion of volatiles consumes 02. The model of coal combustion utilised corresponds to that proposed by Ad~ez et al. [6]. The SO2 disappearance rate will depend on the SO2 concentration in the differential element considered in each region. Moreover, the sulfation reactivity depends on the SO2 concentration and the reaction time for each size and it is expressed by the equation: dXs/dt = Kc ~ Csup exp (- Kc0 Csup t/Xs,max)

(4)

In a differential element of the reactor for a SO2 concentration and a limestone particle size, the mean reactivity for each size of limestone particle will depend on the age distribution of the limestone particles: for perfect mixing of solids in the bed E(t) depends on the mean residence time using the following equation: oo

(-~-t) ( d X = f) d p"~- dp E(t) dt

where

E(t)=et/r/"r

(5)

U

For model solving the mean conversion in the bed achieved for each limestone cut size was assumed. With these conversions the hydrodynamic submodel was solved iteratively. The axial SO2 concentration profiles were obtained by solving the coal combustion model in CFB previously developed [6]. When the SO2 generation rates were determined, the SO2 retention was calculated dividing the bed into compartments (400 in dense region and 100 in dilute region). In each of them the SO2 in the outlet was determined iteratively calculating Cs02,f and the mean conversion of each limestone cut size in the compartment. The mean conversions in the bed for each limestone size were compared with those previously assumed and the process was repeated iteratively using as convergence criterion the mean conversion of each limestone cut size. 3. RESULTS AND DISCUSSION The simulation was made considering a reactor with 6 m high and 20 cm of diameter. The

1841 excess air was 15% with a 20% introduced as secondary air at 2.5 m high. The pressure drop in the bed was 8000 N/m 2. A lignite with a particle size distribution less than 2 mm and a limestone with particle size distribution between 0 and 0.8 mm were used. Moreover the efficiency of solids recovery is the resultant of the standard curve of a high gas throughput cyclone. Pressure drop: The effect of different pressure drop in the combustor have been simulated in the Figure 1, which represents the sulphur retentions (SR) as a function of the air velocity. The pressure drop is important in the retention prediction because increases the retention when increases the pressure drop, due to an increase of mean residence time of solids in the bed. Bed height: Figure 2 shows the effect of different bed heights between 5 and 8 m on sulphur retention as a function of air velocity using the same pressure drop to combustor height ratio. It can be seen, that an increase in the combustor height gives a slight increase in sulphur retention. Secondary air: Figure 3 shows, the SR obtained as a function of Ca/S molar ratio using different percentages of secondary air. As can be seen, an increase in percentage of secondary air shown an increase in the retention. Moreover, the effect of the introduction height of secondary air was analysed, it was found a little effect on the retention, being negligible at high velocities. From the point of view of sulphur retention, the percentage must be high inside the range studied, while heights of introduction of 2.5 m are enough in the operating conditions analysed. Sulphur in coal: The sulphur coal content is distributed in different way between char and volatiles. For this reason the effect of the form of sulphur distribution was analysed and a poor effect on sulphur retention was found. Besides, it was studied the effect of sulphur on SR. Figure 4 shows the SR as a function of Ca/S molar ratio using a lignite with three different sulphur contents from 3 to 9 %. It was observed that for a fixed Ca/S molar ratio, it can be reached more retentions increasing sulphur contents due to the bigger SO2 concentrations present in the bed. Particle size distribution: The particle size distribution of the limestone fed can be very different, therefore the combustor behaviour was simulated using several Rosin-Ramler limestone feed distributions between 0 and 0.8 mm. It has been found a great effect of the form of the particle size distributions on the sulphur retention predictions. Moreover, there is a range of values of n (2-3) which shows the best sulphur retentions in the system, because these distributions have particle sizes so large that can be recovered by the cyclones, but enough fine that can reach high particle conversions, as can be seen in Figure 5. With the bigger values of n, in which the distribution has a great proportion of coarse particles, high retentions are asured because high sulphation particle conversion are reached. Cyclone: The effect of the recovery efficiency curve of cyclone on SR was simulated. Figure 6 shows the predicted sulphur retentions (SR) obtained when working with the different curves of the cyclone, increasing the retentions when the efficiency of the solids recoverd by the cyclone increases. From these results, it can be said that the knowledge of the curve of recovery of solids by the cyclone is an important parameter in the modelling and operation of circulating fluidized bed combustors. REFERENCES 1- Ad~ez J., De Diego L.F., Gay~in P., Armesto L., Cabanillas A." Eurotherm Seminar n ~ 38. Marseille (1994). 2- Ad~inez J., Gay~in P., Garcfa-Labiano F., De Diego L.: Powder Tech. 81(3), 25 (1994). 3- De Diego L.F., G a y ~ P., Ad~ez J.: Powder Technol. (accepted). 4- Kunii D., Levenspiel O.: Powder Technol. 61, 193 (1990). 5- Rhodes M.J.: Powder Technol. 60, 27 (1990). 6- Ad~ez J., De Diego L.F., G a y ~ P., Armesto L., Cabanillas A.: Fuel (in press).

1842

100

10090~ ~ / S = 2

.Ca/S=2

80

A P = 10000N/m 2

70

80

~0 70

50

5, a

40 5,0

I

9

I

5,5

,

I

6,0

9

I

6,5

9

I

7,0

m

9

7,5

8,0

5,0

Figure 1. Effect of air velocity on sulphur retention using different pressure drops.

9O

100 '

u=6m/s

2

~

.

5,5

i

.

|

.

i

.

I

.

6,5 7,0 7,5 8,0 U(m/s) Figure 2. Effect of air velocity on sulphur retention using different bed heights.

u (m/s)

100

i

6,0

u = 6m/s

9% S / . f ~

9O

8O

80

70 60 50 50

4O

40

30

~

I

1,0

,

I

1,5

9

2,0

'

9

'

2,5

9

3,0

3,5

Ca/S Figure 3. Effect of percentage introduced as secondary air on sulphur retention.

.

i

.

i

2.0

.

i

.

1,5

2,5

i

.

3,5 Ca/S Figure 4. Effect of Ca/S molar ratio on sulphur retention using different sulphur contents. 1.0

3,0

100

100 Ca/S=2

90 t Ca/S=2 8O

n=2

g80

.

60

70

40

9

5,0

I

5,5

9

I

6,0

,

I

6,5

9

l

7,0

.

l

7,5

9

8,0

u (m/s)

Figure 5. Effect of air velocity on sulphur retention using different particle size distributions.

60 5,0

5,5

6,0

6,5 U(m/s)

7,0

7,5

8,0

Figure 6. Effect of air velocity on sulphur retention with different curves of cyclone.