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Ind. Eng. Chem. Res. 1999, 38, 1384-1390
Microstructural Changes in the Desulfurization Reaction at Low Temperature M. J. Renedo,* J. Ferna´ ndez, A. Garea, A. Ayerbe, and J. A. Irabien Departamento de Quı´mica, E.T.S.I.I.y T., Universidad de Cantabria, Avda, Los Castros s/n, 39005-Santander, Spain
Sorbents for SO2 removal from flue gas were prepared on the basis of fly ash and calcium hydroxide at different slurring times; they were tested in the desulfurization reaction at low temperature and characterized before and after the reaction, by determining the particle size, specific surface area, and pore size distribution in order to study the influence of the slurry time in the microestructure of the sorbent and to establish the main changes in the desulfurization reaction at low temperature. Small macro- and mesopore volumes were the structural properties of the sorbent related to the conversion values, with a maximum in conversion and porosity for the sorbent at a slurring time of 15 h despite the continuous increase in the specific surface area (microporosity). These results confirm that the pore size distribution and volume are the main variables which can be related to the maximum utilization of the sorbent at low temperatures. The increase in the specific surface area of the sulfated sorbents at high relative humidity (80%), due to the microporosity increase, may be explained by taking into account that the partial condensation of water can happen in the pore structure and a gas-liquid-solid reaction, with a fast nucleation step, would produce the observed microporosity increase. Introduction Dry desulfurization by direct injection of sorbents into the flue gas duct offers an attractive alternative to semidry or wet methods for controlling SO2 emission at low temperature, with a simple technology1-3 as a retrofit option for existing coal-fired power plants. In-duct injection systems work with a dry powdered sorbent, typically calcium hydroxide which reacts with the humidified flue gas. Because of the short residence time of the solids in the ductwork, a reactive sorbent must be used to achieve acceptable levels of SO2 removal. Sorbents obtained by mixing lime or hydrated lime with different sources of silica led to significantly higher conversion of calcium (mol of SO2/mol of Ca in the sorbent) compared to the conversion obtained using hydrated lime.4-6 When fly ash is used as the silica source, the pozzolanic reaction of silica with Ca(OH)2 was considered to be responsible for the improvement of solid utilization.4,7-11 Reactivation and reuse of different sorbents as fly ash or spent sorbents by hydration12-17 have been evaluated in order to increase the use of calcium in the sorbents. In all cases where fly ash and CaO or Ca(OH)2 are introduced in the hydration process, the pozzolanic reaction takes place. When a spent desulfurization solid is used, CaSO4 takes part of the pozzolanic system. Products of the pozzolanic reaction are different, depending on the ratio of components, fly ash composition, and so forth, but highly hydrated compounds with higher specific surface areas than calcium hydroxide are formed. These solids are very reactive toward SO2. The influence of the structural properties of the sorbent, mainly the specific surface area, on the SO2 * To whom correspondence should be addressed. E-mail:
[email protected]. Fax: 34-42-201591.
capture ability, was widely mentioned.18-20 An influence of the specific surface area on the reaction rate has been reported working with Ca(OH)2;21 for sorbents prepared by slurring lime or Ca(OH)2 with fly ash, results of the correlation between specific surface area and conversion are divergent.5,6,11,15,22,23 The influence of the specific surface area on the desulfurization yield was correlated with the ability of the sorbent for carrying moisture on the solid surface.18,24 Deliquescent salts as additives were proved to increase the reactivity of the Ca(OH)2 toward SO2.25 For desulfurization sorbents prepared by slurring Ca(OH)2 and fly ash, several additives were also tested to increase the reaction rate between fly ash and Ca(OH)2 and hence to reduce the slurring time.8,13,22,26-28 Peterson and Rochelle’s studies8 that focused on the influence of the dissolved calcium concentration in the slurry show that solids with high surface areas formed in slurries containing between 10 and 100 ppm of calcium ion were more reactive toward SO2 than the solids formed in solutions containing less or greater amounts of dissolved calcium. They also correlated gassolid reactivity with the moisture content of the sorbents (dependent on surface area, the presence of deliquescent salts on the surface of the solids, and the relative humidity). Slurring processes under pressure were also performed to increase the reaction rate between fly ash and Ca(OH)2 by increasing the dissolution rate of fly ash; this step was assumed to be the limiting step of the hydration reaction rate. The effect of the pressure on the hydration was to allow operation at higher slurring temperatures.6,29 Jozewicz et al.6 indicated that there were two necessary factors for a sorbent to react with SO2: a large solid surface area and amorphous surface structure. They found a good correlation between specific surface area and conversion at low temperatures.
10.1021/ie980016m CCC: $18.00 © 1999 American Chemical Society Published on Web 03/17/1999
Ind. Eng. Chem. Res., Vol. 38, No. 4, 1999 1385
An increase on the specific surface area was related to the morphological characteristics of the products, differentiated gel-like amorphous surface structures from needle-shaped crystals with much less water of hydration than needed for the sorbent to be reactive with SO2. Diffenbach et al.29 did not find such a good correlation and they observed an optimum slurring time for obtaining reactive sorbents. Garea et al.30 and Ferna´ndez et al.31 investigated the appropriate experimental conditions to form sorbents with high surface areas in a set of experiments performed in a Parr reactor, preparing sorbents according to a factorial design of experiments. The influence of temperature, pressure, water/solid ratio, fly ash/Ca(OH)2 ratio, and time were studied and sorbents with different specific surface area were prepared. An influence of the slurring time in the specific surface area generation was found, but despite their different areas, a constant maximum desulfurization yield of 56% was shown, tested with a pure humidified SO2 stream. Other structural properties, such as particle size or pore size distribution, have not been studied in fly ash/ Ca(OH)2 sorbents. But working in desulfurization processes with hydrated lime at low temperatures, Ortiz et al.32 identified the region of pore sizes between 9 and 200 nm as the effective zone in this reaction. For sorbents prepared by the hydration of Ca(OH)2 and fly ash, constituted by different compounds (hydrated calcium silicates and silicoaluminates, unreacted Ca(OH)2, etc.) used for low-temperature desulfurization processes, it was pointed out from the literature that the specific surface area was not the key factor in the desulfurization yield and the calcium concentration does not show a clear influence on the desulfurization yield. Tsuchiai et al.,15 with sorbents prepared by the hydration of fly ash, hydrated lime, and CaSO4, observed a pore volume generation during the hydration time, finding a maximum mean pore diameter and desulfurization activity at 130 °C for 15 h of hydration; the specific surface area did not reach the maximum at that time. In a previous work, Garea et al.,30 for sorbents prepared under pressure by the hydration of fly ash and Ca(OH)2, found a great increase of pore volume in the mesopore and small macropore region when a sorbent with high specific surface area was formed. Taking into account these literature data, it is possible to point out that the time of slurry (in fly ash/Ca(OH)2 sorbents) strongly modifies the calcium concentration in the solution and the specific surface area; for similar sorbents (with CaSO4 in the slurry) medium pore diameter has a maximum with slurry time; dissolved calcium concentration, specific surface area, pore size distribution, and particle size are related to the desulfurization ability; the mesopores and small macropores seem to be the changing pore zone in the desulfurization processes. According to the literature in this work, the evolution of dissolved calcium, specific surface area, particle size, and pore size distribution in sorbents prepared by slurring Ca(OH)2 and fly ash at 90 °C with the time of hydration will be studied, and the changes of the main microestructural parameters (specific surface area and pore size distribution) during the sulfation reaction will be established in order to relate them to the maximum conversion values obtained and to clarify the influence of the sorbent microestructure in the low-temperature desulfurization processes.
Table 1. Physical Properties and Chemical Composition of Ca(OH)2 and Fly Ash physical properties adsorption-desorption of N2: specific surface BET (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) (µm) chemical composition (wt %) SiO2 Al2O3 Fe2O3 MgO CaO Ca(OH)2 CaCO3 Na2O K2O insolubles loss by heat impurities
hydrated lime
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 fly ash 49.26 30.04 6.92 0.85 1.91
84.32 11.98 0.64 2.44 0.17 6.31 3.70
Experimental Section Preparation of Sorbents. The sorbents were prepared using commercial Ca(OH)2 supplied by Calcinor S. A. and ASTM Class F coal fly ash obtained from a pulverized coal boiler and collected in an electrostatic precipitator of Pasajes (Guipuzcoa, Spain), a bituminous coal-fired power plant. The chemical composition and physical properties of these materials are shown in Table 1. Solids were prepared by slurring 15 g of fly ash/Ca(OH)2 mixtures at a constant ratio of 5/3 with 300 cm3 of water in a Pyrex glass beaker placed in a water bath at 90 °C and stirring the mixture at a rate of 650 rpm for different times of 3, 7, 15, and 30 h. The ratio between fly ash and Ca(OH)2 was taken from the central point of the factorial design of experiments which has been shown in previous works30,31 that afford enough Ca(OH)2 to obtain reactive calcium silicates without a great deal of inert fly ash. After the hydration reaction, the reactor and its content were cooled at 35 °C and the slurry was filtered with a 0.45 µm mesh filter; the filter cake was dried in an oven at 105 °C until constant weight, ground, and sieved through a 60 µm mesh. The filtrate was analyzed for dissolved calcium. 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 different relative humidity. The sorbent was dispersed in an inert silica sand bed and the entire bed supported on a 3.6 cm diameter fritted glass plate contained in the glass cylinder. Two gas streams were used: (i) 7000 ppm of SO2 and balance N2 and (ii) 5000 ppm of SO2, 12% CO2, 2% O2, and balance N2, passed through the humidification system where the gas was in contact with water in one or two absorber flasks of 250 cm3, that contained glass spheres in order to improve the contact between gas and liquid phases and submerged in a water bath at constant temperature. After water saturation, the humidified gas mixture (50% relative humidity (RH) when
1386 Ind. Eng. Chem. Res., Vol. 38, No. 4, 1999 Table 2. Physicochemical Properties and Conversion Values of Sorbents Prepared by Slurrying Fly Ash and Hydrated Lime slurrying time (h)
3
Ca2+ (ppm) unreacted Ca(OH)2 (g in 15 g of initial solid) particle size (µm) d(0.1) d(0.5) d(0.9) D(4,3) D(3,2) BET surface area (m2/g) macropore volume (cm3/g) mesopore volume (cm3/g) micropore volume (cm3/g) conversion (%) 50% RH (mol of SO2/mol of Ca) 80% RH 80% RH (3 h)
the gas stream was as in (i) and 80% RH when the gas stream was as in (ii) flowed to the reactor. The difference between the reactor temperature and the absorbers temperature was used to establish the relative humidity, which was measured by two procedures: by using a Delta OHM, HD8901 thermohygrometer under test conditions (without SO2 in the gas stream) and by cooling the stream at the end of the system for 2 h and collecting the water condensed. When the reaction time (1 h normally) concluded, the reaction product was sieved using a Retsch Vibro type sieving unit to separate the sorbent from the sand. In order to check if the maximum solid conversion was achieved in the standard period of 1 h of reaction, a reaction was performed for 3 h, with the sorbent prepared for 7 h of slurry time. Physical and Chemical Analysis. The sorbent was analyzed by the thermogravimetric technique in a Perkin-Elmer TGA-7 unit, with a temperature furnace program between 50 and 1250 °C, a PE 7500 microprocessor, and a TAC-7 thermal analysis controller. Synthetic air was used as the carrier gas (30 mL/min). The sample weight was always between 10 and 20 mg. The temperature program was (20 °C min-1)
(20 °C min-1)
50 °C 98 600 °C (20 min) 98 1200 °C (1 °C min-1)
1200 °C 98 1250 °C (15 min) TG curves of the reacted calcium hydroxide indicated a mass loss occurring between 850 and 1300 °C that was attributed to the sulfate decomposition in CaO (s) and SO3 (g). This analysis allowed the calculation of the amount of reacted SO2 as calcium sulfate. The thermogravimetric analysis of sulfation products, as well as the complete characterization of the mass loss, were described elsewhere.33 Dried solids before and after reaction with SO2 at the two experimental conditions were tested for BET nitrogen specific surface area and pore size distribution using a Micromeritics ASAP-2000 apparatus with 0.5 g of sample weight approximately. Specific surface area was determined following the BET standard method, and pore size distribution was calculated applying the BJH method from nitrogen adsorption data using the adsorption branch of the hysteresis loop. The experimental error was evaluated using three replicates of each data, and a relative standard deviation of 15% was experimentally found. The regions obtained were microporosity (
