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Laboratory testing of different SO2 sorbents for dry sorbent injection
Coal Science J.A. Pajares and J.M.D. Tasc6n (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
1851
Laboratory testing of different SO2 sorbents for dry sorbent injection A. B. Fuertes and M. J. Fernandez Instituto Nacional del Carb6n, C.S.I.C., Apartado 73, 33080-Oviedo
1. INTRODUCTION Dry sorbent injection technology (DSI) is a relatively recent alternative to wet scrubbing and spray drying. In general three DSI approaches have been extensively investigated [1]: i) Furnace injection of calcium-based sorbents (limestone or hydrated lime at a temperature region of around 1200~ ii) Low temperature (70-100 ~ duct injection of hydrated lime at high relative humidity and iii) Low temperature (120-175~ duct injection of sodium-based sorbents (i.e. sodium carbonate and sodium bicarbonate). Recently, especial attention has been given to injection of different kinds of sorbents in the flue gases at temperatures around 400-600~ [2, 3]. The sorbent injection in this temperature window (existing in the economizer) avoids the problems derived from the impact of sorbent particles on tube banks and allows the utilization degree of some sorbents to be improved. This paper reports results of different tests for several candidate sorbents to be used in the 300-600~ temperature window. Especial enphasis is placed in the analysis of the sulfation of sodium carbonate at different temperatures and SO2 concentrations.
2. EXPERIMENTAL The experiments were performed in a fixed bed reactor using a simulated combustion gas mixture. The fixed bed reactor is a vertical packed bed formed by sorbent particles dispersed in silica sand. The entire bed is supported on a 2 cm diameter fritted quartz plate contained in the quartz cylinder. The main characteristics of the fixed bed reactor and experimental conditions are given in Table 1. For these experimental conditions, the Peclet number values for this fixed bed reactor are in the range of 70-100. This suggests that the effect of gas dispersion can be neglected so that a plug flow model is a reasonable assumption. Eight classes of sorbent have been tested: Calcium hydroxide (HCa), calcium acetate (ACa), dolomite (D), magnesium hydroxide (HMg), magnesium acetate (AMg), sodium acetate (ANa), sodium bicarbonate (BNa) and sodium carbonate (CNa). The dolomite is the only natural sorbent, being the rest analytical reagent grade. All the particles were below 53 tzm. Prior to testing, the reactor was by-passed, and the test gas SO2 concentration was established. During the experiment the gas passed from the fixed bed reactor via the water trap to SO2 analyzer (IR Beckman 880). SO2 absorption in the fixed bed reactor was calculated by integration of SO2 concentration/time curve.
1852 Table 1. Fixed-bed characteristics and experimental conditions.
Fixed-bed characteristics Silica sand size range (/~m) Bed height (cm) Sorbent/inert ratio (g/g) Bed porosity Sorbent size (/~m) Experimental conditions Gas composition (%vol) O2 CO2 H20 SO2 (ppm) Temperature range (~ Gas flow rate (STP 1/min)
180-355 3 1/173, 1/100 0.44 20-53
5 15 10 900-3200 300-600 --- 0.5
3. RESULTS AND DISCUSSION.
3.1. Sorbent selection In Figure 1 the sulfation capacity (mol SO2/mol sorbent) of the tested sorbents is indicated. Calcium and magnesium sorbents show very low sulfur capture under the studied conditions. From these results, only Na-based sorbents (mainly sodium carbonate and sodium
Figure 1. Sulfur capacity of the different type of sorbents.
Figure 2. Influence of gas composition on sulfur capture.
1853 acetate) are reliable candidates to be used under the economizer conditions. The differences in sulfur capture can be explained as a result of possible equilibrium constrains and competing reactions such as carbonation and hydration. This fact is illustrated in Figure 2 where the breakthrough curves corresponding to Ca(OH)2 sulfation under different gas compositions are displayed. Thus, it is observed that the presence of CO2 in the gas composition strongly inhibits the sulfur capture. It can be considered that under these reaction conditions, the dehydration of Ca(OH)2 occurs and simultaneously the recarbonation of CaO rapidly takes place, inhibiting the sulfation reaction. On the other hand, it is observed that the presence of H20 hardly have influence on sulfur capture under these conditions. Surprisingly, sodium bicarbonate shows a lower degree of sulfur capture than sodium carbonate. This fact is contrary to the results obtained during sulfation at lower temperatures (around 150~ [4]. During these experiments sodium bicarbonate rapidly decomposes into sodium carbonate. The sodium carbonate formed presents a high surface area and consequently high reactivity. However, at temperatures above 300~ a loss in surface area due to sintering is observed. The effect of sintering on reactivity and sulfur capture could therefore explain the low sulfur loading observed for sodium bicarbonate. 3.2. S o d i u m carbonate-SOz reaction under economizer conditions
Different studies indicate that sodium sorbents, mainly sodium bicarbonate, have great activity in sulfur capture at temperatures around 120-175~ [5]. However, in this study we demostrate that sodium sorbents (i.e. sodium carbonate) can be also efficiently used at temperatures around 550~ The overall reaction between SO2 and sodium carbonate can be
0.8
4
Cso~ = 1500 ppm Reaction time = 1 h
O
Cso2, ppm
Capacity
3300
0.64
e,i
Z
0.6
c5
0
3
x o
0.4
o
9U-
2
o
o
r r
ora~ ,.,
0.2
rj
1
O 0.0
i
300
400
500
600
Temperature, ~ Figure 3. Variation of Na2CO3 sulfur capacity with temperature.
|
i
i
|
I
10 20 30 40 50 60 70 Time, min Figure 4.
I n f 1u e n c e
o f
S O
2
concentration on the NazCO 3 sulfur capture (T=550~
1854 written as NazCO 3 + SO 2 + 8 9 z ~ Na2SO 4 + CO 2 The sulfur capture of sodium carbonate is strongly dependent on reaction temperature. In Figure 3 the variation of sorbent sulfur capacity with reaction temperature is represented. The sulfur capture increases from 0.11 mol SO2/mol sorbent at 300~ to 0.66 mol SO2/mol sorbent at 550~ A maximum in sulfur capture is found at this temperature. At higher temperatures the sulfur capture decreases. The modification of SO2 concentration does not have any influence on sulfur capture, as observed in Figure 4 where the breakthrough curves for different SO2 concentration levels between 900 ppm and 3300 ppm are showed.
4. CONCLUSIONS Our observations indicate that several competing processes contribute to the sulfur capture of the tested sorbents. This fact suggests that simulation of boiler conditions is required for meaningful evaluations. From all tested sorbents, sodium carbonate presents the higher sulfur loading under the experimental conditions utilized (temperatures around 550~ and gas composition typical of combustion flue gases). The sodium carbonate presents a maximum in sulfur capture of 0.66 mol SO2/mol sorbent at a temperature of 550~ This fact suggests that this sorbent is a good candidate to be utilized under economizer conditions. Additionally, it has been detected that the change in SO2 concentration between 900 and 3300 ppm does not have any influence on sulfur capture.
ACKNOWLEDGEMENTS The financial acknowledged.
assistance
of
the
DGCICYT
(PB91-0101)
is
gratefully
REFERENCES .
.
.
.
5.
M. Takeshita and H. Sound, FGD performance and experience on coal-fired plants, lEA Coal Research, IEACR/58, lEA, London, 1993. P. V. Smith, in Processing of High Sulfur Coals IV, Ed. by P. R. Dougan, D. R. Quigley and Y. A. Attia, Elsevier, Amsterdam, 791, 1991. S. M. Dalton and B. C. Syrett, in Desulphurization in Coal Combustion Systems, IChemE SYMPOSIUM SERIES No. 106, The Institution of Chemical Engineers, London, 161, 1989. T. C. Keener and S. J. Khang, Chem. Engng. Sci., 48 (1993) 2859. M. Hartman, Int. Chem. Eng., 18 (1978) 712.
1851
Laboratory testing of different SO2 sorbents for dry sorbent injection A. B. Fuertes and M. J. Fernandez Instituto Nacional del Carb6n, C.S.I.C., Apartado 73, 33080-Oviedo
1. INTRODUCTION Dry sorbent injection technology (DSI) is a relatively recent alternative to wet scrubbing and spray drying. In general three DSI approaches have been extensively investigated [1]: i) Furnace injection of calcium-based sorbents (limestone or hydrated lime at a temperature region of around 1200~ ii) Low temperature (70-100 ~ duct injection of hydrated lime at high relative humidity and iii) Low temperature (120-175~ duct injection of sodium-based sorbents (i.e. sodium carbonate and sodium bicarbonate). Recently, especial attention has been given to injection of different kinds of sorbents in the flue gases at temperatures around 400-600~ [2, 3]. The sorbent injection in this temperature window (existing in the economizer) avoids the problems derived from the impact of sorbent particles on tube banks and allows the utilization degree of some sorbents to be improved. This paper reports results of different tests for several candidate sorbents to be used in the 300-600~ temperature window. Especial enphasis is placed in the analysis of the sulfation of sodium carbonate at different temperatures and SO2 concentrations.
2. EXPERIMENTAL The experiments were performed in a fixed bed reactor using a simulated combustion gas mixture. The fixed bed reactor is a vertical packed bed formed by sorbent particles dispersed in silica sand. The entire bed is supported on a 2 cm diameter fritted quartz plate contained in the quartz cylinder. The main characteristics of the fixed bed reactor and experimental conditions are given in Table 1. For these experimental conditions, the Peclet number values for this fixed bed reactor are in the range of 70-100. This suggests that the effect of gas dispersion can be neglected so that a plug flow model is a reasonable assumption. Eight classes of sorbent have been tested: Calcium hydroxide (HCa), calcium acetate (ACa), dolomite (D), magnesium hydroxide (HMg), magnesium acetate (AMg), sodium acetate (ANa), sodium bicarbonate (BNa) and sodium carbonate (CNa). The dolomite is the only natural sorbent, being the rest analytical reagent grade. All the particles were below 53 tzm. Prior to testing, the reactor was by-passed, and the test gas SO2 concentration was established. During the experiment the gas passed from the fixed bed reactor via the water trap to SO2 analyzer (IR Beckman 880). SO2 absorption in the fixed bed reactor was calculated by integration of SO2 concentration/time curve.
1852 Table 1. Fixed-bed characteristics and experimental conditions.
Fixed-bed characteristics Silica sand size range (/~m) Bed height (cm) Sorbent/inert ratio (g/g) Bed porosity Sorbent size (/~m) Experimental conditions Gas composition (%vol) O2 CO2 H20 SO2 (ppm) Temperature range (~ Gas flow rate (STP 1/min)
180-355 3 1/173, 1/100 0.44 20-53
5 15 10 900-3200 300-600 --- 0.5
3. RESULTS AND DISCUSSION.
3.1. Sorbent selection In Figure 1 the sulfation capacity (mol SO2/mol sorbent) of the tested sorbents is indicated. Calcium and magnesium sorbents show very low sulfur capture under the studied conditions. From these results, only Na-based sorbents (mainly sodium carbonate and sodium
Figure 1. Sulfur capacity of the different type of sorbents.
Figure 2. Influence of gas composition on sulfur capture.
1853 acetate) are reliable candidates to be used under the economizer conditions. The differences in sulfur capture can be explained as a result of possible equilibrium constrains and competing reactions such as carbonation and hydration. This fact is illustrated in Figure 2 where the breakthrough curves corresponding to Ca(OH)2 sulfation under different gas compositions are displayed. Thus, it is observed that the presence of CO2 in the gas composition strongly inhibits the sulfur capture. It can be considered that under these reaction conditions, the dehydration of Ca(OH)2 occurs and simultaneously the recarbonation of CaO rapidly takes place, inhibiting the sulfation reaction. On the other hand, it is observed that the presence of H20 hardly have influence on sulfur capture under these conditions. Surprisingly, sodium bicarbonate shows a lower degree of sulfur capture than sodium carbonate. This fact is contrary to the results obtained during sulfation at lower temperatures (around 150~ [4]. During these experiments sodium bicarbonate rapidly decomposes into sodium carbonate. The sodium carbonate formed presents a high surface area and consequently high reactivity. However, at temperatures above 300~ a loss in surface area due to sintering is observed. The effect of sintering on reactivity and sulfur capture could therefore explain the low sulfur loading observed for sodium bicarbonate. 3.2. S o d i u m carbonate-SOz reaction under economizer conditions
Different studies indicate that sodium sorbents, mainly sodium bicarbonate, have great activity in sulfur capture at temperatures around 120-175~ [5]. However, in this study we demostrate that sodium sorbents (i.e. sodium carbonate) can be also efficiently used at temperatures around 550~ The overall reaction between SO2 and sodium carbonate can be
0.8
4
Cso~ = 1500 ppm Reaction time = 1 h
O
Cso2, ppm
Capacity
3300
0.64
e,i
Z
0.6
c5
0
3
x o
0.4
o
9U-
2
o
o
r r
ora~ ,.,
0.2
rj
1
O 0.0
i
300
400
500
600
Temperature, ~ Figure 3. Variation of Na2CO3 sulfur capacity with temperature.
|
i
i
|
I
10 20 30 40 50 60 70 Time, min Figure 4.
I n f 1u e n c e
o f
S O
2
concentration on the NazCO 3 sulfur capture (T=550~
1854 written as NazCO 3 + SO 2 + 8 9 z ~ Na2SO 4 + CO 2 The sulfur capture of sodium carbonate is strongly dependent on reaction temperature. In Figure 3 the variation of sorbent sulfur capacity with reaction temperature is represented. The sulfur capture increases from 0.11 mol SO2/mol sorbent at 300~ to 0.66 mol SO2/mol sorbent at 550~ A maximum in sulfur capture is found at this temperature. At higher temperatures the sulfur capture decreases. The modification of SO2 concentration does not have any influence on sulfur capture, as observed in Figure 4 where the breakthrough curves for different SO2 concentration levels between 900 ppm and 3300 ppm are showed.
4. CONCLUSIONS Our observations indicate that several competing processes contribute to the sulfur capture of the tested sorbents. This fact suggests that simulation of boiler conditions is required for meaningful evaluations. From all tested sorbents, sodium carbonate presents the higher sulfur loading under the experimental conditions utilized (temperatures around 550~ and gas composition typical of combustion flue gases). The sodium carbonate presents a maximum in sulfur capture of 0.66 mol SO2/mol sorbent at a temperature of 550~ This fact suggests that this sorbent is a good candidate to be utilized under economizer conditions. Additionally, it has been detected that the change in SO2 concentration between 900 and 3300 ppm does not have any influence on sulfur capture.
ACKNOWLEDGEMENTS The financial acknowledged.
assistance
of
the
DGCICYT
(PB91-0101)
is
gratefully
REFERENCES .
.
.
.
5.
M. Takeshita and H. Sound, FGD performance and experience on coal-fired plants, lEA Coal Research, IEACR/58, lEA, London, 1993. P. V. Smith, in Processing of High Sulfur Coals IV, Ed. by P. R. Dougan, D. R. Quigley and Y. A. Attia, Elsevier, Amsterdam, 791, 1991. S. M. Dalton and B. C. Syrett, in Desulphurization in Coal Combustion Systems, IChemE SYMPOSIUM SERIES No. 106, The Institution of Chemical Engineers, London, 161, 1989. T. C. Keener and S. J. Khang, Chem. Engng. Sci., 48 (1993) 2859. M. Hartman, Int. Chem. Eng., 18 (1978) 712.