2412
Ind. Eng. Chem. Res. 2002, 41, 2412-2417
Preparation, Characterization, and Calcium Utilization of Fly Ash/Ca(OH)2 Sorbents for Dry Desulfurization at Low Temperature M. J. Renedo* and J. Ferna´ ndez Departamento de Ingenierı´a Quı´mica y Quı´mica Inorga´ nica, ETSIIT, Universidad de Cantabria, Avenida Los Castros s/n, 39005 Santander, Spain
Sorbents for SO2 removal were prepared by hydration at 90 °C of calcium hydroxide and coal fly ash at different fly ash/Ca(OH)2 initial ratios, from 1/3 to 15/3, and slurring times, from 3 to 37 h. Properties characterized in the sorbents included the particle size, the Brunauer-EmmettTeller surface area, and the porosity distribution. The Ca(OH)2 conversion as a measure of the pozzolanic reaction extent was also determined; all of these sorbent properties were related to the preparation variables. The obtained sorbents were tested in a flue gas desulfurization reaction at 57 °C and 52% relative humidity. The calcium utilization in this reaction was quantified as moles of SO2 captured per moles of calcium × 100. The Ca(OH)2 conversion in the hydration reaction and the raw material ratios have been revealed as the most efficient parameters to predict the behavior of the sorbent in the desulfurization process. The hydration reaction between fly ash and Ca(OH)2 should be performed just until the completion of the pozzolanic reaction and with a fly ash/Ca(OH)2 initial weight ratio of 5/3 or 9/3 to have an optimum sorbent in the reaction with SO2. Introduction The control of SO2 emissions by injection of calcium sorbents is being widely used at low (below 150 °C) and high temperature (furnace injection, 700-1200 °C) and less developed at medium temperature (economizer injection, 300-600 °C).1,2 Dry desulfurization by direct injection of sorbents into the flue gas duct is an attractive alternative to semidry or wet methods at low temperature. Sorbents obtained by hydration of calcium hydroxide and different sources of silica have led to significantly higher conversion of calcium compared to the conversion obtained using hydrated lime.3-8 The use of fly ash as the silica source presents advantages both economically and environmentally because fly ash is the most voluminous byproduct from all coal-fired power plants.9,10 In all cases where fly ash or other silica sources and CaO or Ca(OH)2 are present in the hydration process, the pozzolanic reaction takes place and hydrated calcium silicates with a general composition of (CaO)x(SiO2)y(H2O)z are formed. Kind and Rochelle11 proposed a mechanism for the reaction between the fly ash and Ca(OH)2 in which the rate-limiting step is the dissolution of the silica from the fly ash when there is enough calcium hydroxide. On the basis of this mechanism, the kinetic study of this reaction has been carried out.12 The influence of the Si/Ca initial ratio on the composition of the product has been investigated by several authors.13-15 They conclude that when working with pure reactive and during the first hours of hydration, the Si/Ca ratio and water content in the final product depend on the Si/Ca initial ratio and temperature. Because of the influence of the Si/Ca initial ratio, silica source, hydration time, and temperature on the composition and specific surface area of the final sor* To whom correspondence should be sent. E-mail: renedomj @unican.es. Telephone: 34-42-201580. Fax: 34-42-201591.
bent, this last property has been related to the referred preparation conditions of the sorbent and to the reactivity toward SO2. Table 1 summarizes the bibliographic review of this subject, including the experimental conditions, in order to compare the results. Taking into account data in Table 1, at low values of the fly ash/Ca(OH)2 initial ratio, hydration time, and temperature, the surface area of the sorbent increases as these variables do. However, a tendency to stop or decrease in the Brunauer-Emmett-Teller (BET) surface generation is observed, depending on the maximum value of the surface area on the hydration conditions as well as on the reactivity of the silica source. The correlation between specific surface area and conversion values in the desulfurization process is not found in all of the works. The summarized results do not show a clear relationship between sorbent preparation variables and the surface area or between the surface area and calcium utilization in a desulfurization reaction. The sorbent porosity distribution8 and the particle size of the raw materials7,14 and of the sorbents7,8,19 have also been related to the desulfurization ability. With this state of knowledge, this work tries to clarify the relationship between the preparation variables and the properties of the obtained sorbents and between these properties and the desulfurization behavior. Following the preceding works, a set of experiments have been performed to prepare sorbents from fly ash and hydrated lime, varying the raw material ratio and the hydration time. Properties to be characterized in the sorbents include the particle size distribution, BET surface area, and pore size distribution. The Ca(OH)2 conversion (Ca(OH)2 reacted with the fly ash) will also be determined as a measure of the pozzolanic reaction extent. All sorbents will be tested in a desulfurization reaction at low temperature to obtain the calcium utilization expressed as moles of SO2 per moles of Ca.
10.1021/ie010938g CCC: $22.00 © 2002 American Chemical Society Published on Web 04/11/2002
Ind. Eng. Chem. Res., Vol. 41, No. 10, 2002 2413 Table 1. Bibliographic Revision types of reactives fly ash (Class F); Ca(OH)2 fly ash (Class F); Ca(OH)2 quartz SiO2; Ca(OH)2
SiO2/Ca(OH)2 or fly ash/Ca(OH)2 weight ratio
hydration temp (°C)
hydration time (h)
max BET surface area (m2/g)
3 3
90 170
1-50 1-8
65 (50 h) 64 (4 h)
1.66
90
3-30
32 (30 h)
1.2
150 90
1-50 1-90
0.81/4.9
90
8 48
172 (20 h) 172 (60 h) maintained 35 (4.9 ratio) 148 (1.2 ratio) decreasing after
0/4.9
90
1-86
fly ash; Ca(OH)2
0.5/5 2.3
fly ash (Class F); Ca(OH)2 fly ash (Class C); Ca(OH)2 fly ash (Class F); Ca(OH)2 fly ash (Class F); Ca(OH)2
0.5/20
150 230 180 65
2 0.5-8 0.5-8 4
16
25-92
2-24
2.5/14
77
8
35 (2.5 ratio)
1/25 12
60 40-90
4 4; 6
26 (90 °C, 6 h)
a
desulf conditions
max desulf results: (mol of SO2/ mol of initial Ca)
lit. ref Ferna´ndez et al.16
57 °C; 50% RH
Jung et al.15
65 °C; 60% RH 20 (5 ratio)
Renedo et al.8
0.59 (15 h)
64 °C; 60% RH 66 °C; 54% RH
Jung et al.17
0.6 (1.2 ratio, 48 h) 0.8 (4.9 ratio, 20 h) 0.6 (5 ratio)a 0.32 (1 h) 0.34 (4 h) 0.78 (20 ratio)a
Jozewicz et al.4 Jozewicz and Rochelle3
0.8 (92 °C, 5 h) 0.4 (25 °C, 16 h) Al-Shawabkeh et al.18 80 °C; 50% RH
Davini6
0.8 (25 ratio) 0.8 (90 °C, 6 h)
Conversion values are calculated considering only the Ca from the Ca(OH)2.
Experimental Section Preparation of Sorbents. The sorbents were prepared using commercial calcium hydroxide supplied by Calcinor S.A., ASTM Class F coal fly ash from a pulverized coal boiler and collected in an electrostatic precipitator of Pasajes (Guipuzcoa, Spain), a bituminous coal-fired power plant. Ca(OH)2 and fly ash chemical composition and physical properties are shown in Table 2. Solids were prepared by slurring fly ash and calcium hydroxide mixtures in a 2 L stirred tank reactor at a constant temperature of 90 °C, provided of three mouths with a reflux refrigerator to maintain a constant liquid/ solid ratio of 10/1 and at a stirring rate of 1400 rpm. The amount of liquid was 1.5 L and the amount of total solid 150 g, varying the amounts of fly ash and calcium hydroxide, depending on the fly ash/Ca(OH)2 ratio. For a ratio of 5/3, the amounts were 93.75 g of fly ash + 56.25 g of Ca(OH)2 ) 150 g. Samples of the reaction mixture were taken out at different reaction times, from 3 to 37 h. The fly ash/Ca(OH)2 weight ratios tested were 1/3, 5/3, 9/3, and 15/3. Samples were filtered with a 0.45 µm mesh filter, dried in an oven at 105 °C for 1 h, and ground. An experiment using the same experimental conditions as those to study the self-hydration of the fly ash was performed, and samples of the mixture water/fly ash were withdrawn at different hydration times to obtain the values of the BET surface area. Sulfation Test. The reaction between the solid sorbents and SO2 was performed in a glass-made jacketed fixed-bed reactor, under isothermal conditions at 57 °C and a relative humidity of 52%. Approximately 0.5 g of the sorbent was weighed and dispersed manually in 30 g of an inert silica sand bed (particle size range
Table 2. Physical Properties and Chemical Composition of Ca(OH)2 and Fly Ash
Physical Properties adsorption-desorption of N2 BET specific surface area (m2/g) intrusion-extrusion of Hg pore volume (cm3/g) macropore volume (>50 nm) (cm3/g) mesopore volume (6.7-50 nm) (cm3/g) skeletal density (g/cm3) laser diffraction mean volume diameter (D ) 4.3)
hydrated lime
fly ash
16.20
2.98
1.395 1.319 0.076 1.830 6.59
0.680 0.658 0.022 1.889 27.64
Chemical Composition (wt %)a SiO2 Al2O3 Fe2O3 MgO CaO Ca(OH)2 CaCO3 Na2O K2O insoluble loss by heat impurities
49.26 30.04 6.92 0.85 1.91 85.6 9.6 0.64 2.44 0.17 6.31 4.8
a
Composition obtained by TG. This composition is slightly modified as CaCO3 is formed from Ca(OH)2.
250-300 µm), and the whole bed was supported on a 3.6 cm diameter fritted glass plate contained in the glass cylinder. The volume composition of the gas stream was 5000 ppm SO2, 12% CO2, 2% O2, and balance N2 at a rate of 1000 cm3/min. This gas passed through the humidification system where the gas was in contact with water in an absorber flask of 250 cm3, that contained glass spheres used to improve the contact between gas and
2414
Ind. Eng. Chem. Res., Vol. 41, No. 10, 2002
Table 3. Physicochemical Properties and Calcium Utilization of Sorbents Prepared by Slurrying Fly Ash and Hydrated Lime at Various Slurrying Times (h) fly ash/Ca(OH)2 ratio 1/3 Ca(OH)2 conv (%) diameter 4.3 (µm) diameter 3.2 (µm) BET surface area (m2/g) macropore vol. (cm3/g) mesopore vol. (cm3/g) micropore vol. × 10-3 (cm3/g) (mol of SO2/mol of Ca) (%)
5/3
9/3
15/3
3
7
13
37
3
7
15
37
3
7
13
37
3
7
13
37
15.1
17.2 9 4 22.7 0.05 0.12 1.56 29
25.7
44.8
82.7 13 7.4 34.3 0.04 0.18 3.31 51
39.3 16.1 5.3 11.4 0.04 0.06 0.64 39
78.6 12.8 6.5 21.9 0.06 0.1 2.34 57
98.1
98.5 12.2 6 64.6 0.05 0.30 5.60 58
61.7
94.3 16.0 7 20.9 0.03 0.1 1.2 55
96.7
98.1
38.5 0.06 0.17 1.84 37
51.5 12.5 6.9 15.6 0.04 0.07 0.80 35
96.6
29 0.04 0.14 1.80 33
22.8 16.9 6.3 9.8 0.03 0.05 0.24 26
34.6 0.04 0.15 3.0 52
47.4 0.02 0.23 3.7 57
12.4 0.04 0.06 0.23 22
liquid phases. The absorber flask was submerged into a water bath at constant temperature of 50 °C. After humidification, the gas mixture flowed to the reactor. The water content in the gas stream was obtained by cooling the stream with cool water in a reflux refrigerator for 3 h and measuring the condensed water. From these data the experimental vapor pressure at the reaction temperature (57 °C) was obtained. The quotient between the experimental pressure vapor and the saturation pressure vapor at the reaction temperature gave the relative humidity. When the reaction time (1 h) was over, the reactor content was sieved to separate the sorbent from the sand and the sorbent was analyzed by the thermogravimetric technique. Physical and Chemical Analysis. The particle size distributions and the mean diameters of the sorbents with different fly ash/Ca(OH)2 ratios and slurrying times were measured by laser diffraction in a Malvern Instruments Mastersizer X. The amount of Ca(OH)2 in the solid was measured by dissolving a small sample of the sorbent in a sugar solution at ambient temperature and titrating the solved Ca(OH)2 with HCl following the procedure given by Peterson and Rochelle.20 This amount is a measure of the hydration extent. The Ca(OH)2 conversion, obtained from this value, is a quantitative expression of the pozzolanic reaction:
Ca(OH)2 used in the preparation of solid Ca(OH)2 after hydration Ca(OH)2 used in the preparation of solid
× 100
The Ca(OH)2 used in the preparation of the solid was calculated by considering the composition of commercial Ca(OH)2 (Table 2) and by subtracting its solubility in water at the reaction temperature. Three replicates of each sample were done, with the relative standard deviation being less than 1%. The pore size distribution and specific surface area were measured in the sorbents using a Micromeritics ASAP-2000 apparatus. The specific surface area was determined following the BET standard method. The experimental error was evaluated using three replicates of some data, and a relative standard deviation of less than 10% was found. The pore size distribution was calculated by applying the Barret-Joyner-Halenda (BJH) method from nitrogen adsorption data. The porosity ranges studied were microporosity (
