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SEPARATIONS TGA Study on Humidified Activation and Desulfurization Performance of High-CaO Coal Ash at Low Temperature Juan Yu,* Mingchuan Zhang, Mengli Chen, Yubao Song, Weidong Fan, and Yuegui Zhou Department of Energy Engineering, Shanghai Jiaotong University, 800 Dong Chuan Road, Minhang, Shanghai 200240, China
A novel SO2-removal reaction system, based on the use of a thermogravimetric analyzer (TGA), has been developed and tested with humidified high-CaO coal ash as the sorbent. The system operates satisfactorily and proves to be a valuable tool in determining the SO2-removal ability of sorbents. The experimental results also show that the presence of unbound water is an important factor in the desulfurization reaction of high-CaO coal ash at low temperature (30130 °C). Once the water in the ash is completely evaporated, the reaction will stop. The effects of the H2O/CaO ratio, temperature, and Ca/S ratio on the Ca utilization of coal ash have been experimentally and theoretically analyzed. As a result, with increasing H2O/CaO ratio, the Ca utilization increases, whereas with increasing temperature and Ca/S ratio, the Ca utilization decreases. Introduction Both the emission of sulfur dioxide in flue gases and the disposal of ash have been considered to be severe and important problems in coal-fired power plants. Various flue gas desulfurization (FGD) processes, including dry, semidry, and wet FGD processes, have been applied to reduce SO2 emissions. In the dry FGD process, a reactive dry, calcium-based compound is used as the SO2 sorbent. However, the expensive sorbents must be used repeatedly, which makes the dry FGD process complex and costly. Wet FGD processes are widely used in thermal power plants. The sorbent, e.g., lime or limestone-gypsum, is added in the form of a solution or slurry in which the chemical reaction takes place. However, the consumption of large amounts of water and treatment of the resulting wastewater remain as disadvantages of the wet FGD process. Semidry FGD processes have been developed and adopted commercially since the 1980s, including the spray-drying process,1 the injection of sorbent as a slurry into the duct,2,3 and the injection of dry sorbent with flue gas conditioned by spraying water into the duct.4-6 Lime, hydrated lime, and limestone are usually used as sorbents, and the SO2-removal efficiency reaches 90%. Although the reaction between the droplets and SO2 occurs in the liquid phase, the sorbent consumed can be removed in its solid state from the flue gas. Thus, no wastewater is produced, and sludge-handling equipment is not required.7-8 Therefore, the semidry FGD process is attractive to both engineers and researchers. It is expected that continued progress will lead to lower costs, higher efficiencies, and maximal savings of material and energy. * To whom correspondence should be addressed. Tel.: 8621-5474-2847. Fax: 86-21-5474-2996. E-mail: yujuanjuan@ hotmail.com.
Fly ash is a waste product from coal-fired power plants. It is well-known that coal fly ash slurried with CaO can enhance the reactivity and utilization of sorbent.9-11 However, fly ash is hardly used alone as a sorbent because of its low content of alkaline oxide. In some regions of China, the coals are rich in CaO. The CaO content of coal ash is commonly up to about 25%. This enables the fly ash to be an alternative sorbent with the benefit of reducing the preparation expenses of sorbent and the treatment expenses of ashes. The reactivity of humidified high-CaO fly ash toward SO2 is experimentally investigated in this paper. Subscale scrubbers and tube reactors are typically used to develop an understanding of sorbent capacity for SO2 removal.12-16 Whether these units are benchscale or full-scale, the initial construction costs and operation and maintenance expenses are inevitably high. In this paper, a simple, novel system based on the technique of thermogravimetric analysis has been established to study the desulfurization performance of sorbents. The whole system simplifies operation and maintenance, decreases costs, and is characterized by high precision and robust data processing. The major objectives of the present investigation were to explore the process of gas desulfurization at low temperatures in the new type of reaction system, as well as to study the effects of operating parameters such as the H2O/CaO molar ratio, temperature, and Ca/S ratio on the Ca utilization of humidified high-CaO coal ash. Experimental Section Preparation of Humidified Coal Ash. The coal fly ash used for this study was obtained from a 300 MW pulverized coal boiler of Wujing Power Plant (Shanghai, China). The chemical composition of the coal ash is
10.1021/ie000789m CCC: $20.00 © 2001 American Chemical Society Published on Web 07/17/2001
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Figure 1. Experimental system schematic diagram: 1, SO2 steel cylinder; 2, nitrogen steel cylinder; 3, valve; 4, rotameter; 5, pressure tank; 6, infrared SO2 gas analyzer; 7, thermogravimetric analyzer. Table 1. Composition of Coal Ash (wt %) CaO
Fe2O3
Al2O3
MgO
SiO2
SO3
25.75
9.38
10.38
4.60
33.92
13.26
Table 2. Experimental Conditions and the Total Weight Loss of Samples sample
M′ (mg)
M′′ (mg)
T (°C)
Ca/S
1 2 3 4 5 6 7 8 9 10 11 12 13 14
3.78 3.79 3.81 3.83 3.75 3.79 3.80 3.79 3.79 3.79 3.83 3.77 3.78 3.82
3.78 5.82 7.37 8.65 10.41 3.79 7.58 8.2 9.32 10.93 8.65 8.76 8.80 8.82
50 50 50 50 50 70 70 70 70 70 50 50 50 50
0.81 0.81 0.81 0.81 0.81 0.81 0.81 0.81 0.81 0.81 0.81 1.04 1.52 1.89
H2O/CaO desired practical 0 5 10 15 20 0 5 10 15 20 15 15 15 15
0 5.13 9.23 13.30 19.40 0 6.60 8.90 13.00 17.80 12.91 13.16 13.21 13.04
X (%) 0.00 34.01 45.74 52.32 60.34 0.00 49.62 51.96 56.98 63.30 52.32 54.28 54.78 54.79
shown in Table 1, from which we can see that the CaO content is 25.75 wt %. Most of the dry coal ash samples had an initial mass of 3.75-3.85 mg (Table 2), which was measured by an analytical balance. The humidified coal ash sample was held in an aluminum crucible and stored for about 30 min. Precautions should be taken to keep the sample from contacting air to reduce carbonate formation and water loss. The ash sample in conjunction with the aluminum crucible was put into the reactor of a thermogravimetric analyzer (TGA). Experimental Process. Figure 1 depicts the experimental system, which includes the simulated flue gas feed system and the thermogravimetric analyzer. The simulated flue gas feed system consists of nitrogen and sulfur dioxide storage and metering systems and a pressurized mixing tank. The SO2 concentration of the flue gas was measured with the Shimazu infrared SO2 gas analyzer (SOA-7000). Low-temperature sulfur-removal experiments were carried out using the thermogravimetric analyzer. The SO2-laden flue gas entered the reactor from the top under ambient pressure conditions. The wet ash sample undergoes evaporation as well as reaction with SO2 in the flue gas. Thus, a variation in the mass of the ash sample corresponds to the quantity of evaporated water minus adsorbed SO2. The total mass variation, X, is
Figure 2. Typical thermogravimetric curve graph.
Figure 3. Experimental thermogravimetric curve graph.
displayed in a thermogravimetric curve graph supplied by the TGA (see Figures 2 and 3). A set of experiments was carried out to study the effect of the presence of water in ash on the SO2-removal process. The samples included two wet coal ashes and a dry coal ash. The weight loss processes of these samples were measured by heating them from 30 to 130 °C in the TGA with a heating rate of 10 °C/min (see Figure 3).
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Figure 4. Effects of the H2O/CaO ratio and temperature on the Ca utilization.
A set of experiments was carried out to investigate the effect of the H2O/CaO molar ratio on the Ca utilization of coal ash at constant temperature. Before the desired temperature was achieved, only pure nitrogen was fed into the TGA. To avoid excess evaporation, a fast heating rate of 25 °C/min was chosen. Sulfurremoval experiments started when the desired temperature was reached and the gas flow was switched from nitrogen to simulated flue gas. Thus, the practical H2O/ CaO ratio differed from the desired H2O/CaO ratio, which was changed from 5 to 20. The practical H2O/ CaO ratio can be calculated from the thermogravimetric curve. For example, suppose that the SO2-removal temperature of B is scheduled (see Figure 2). From Figure 2, we can see that AB is the heating-up period of the sample and, correspondingly, that CD is the total weight loss caused by water evaporation before the desulfurization reaction took place. Subtracting this fraction of water, we can obtain the new H2O/CaO ratio. The simulated flue gas in the experiments passed through the reactor for 5 min, and the SO2 concentration remained unchanged at around 2600 ppm (see Table 2, experiments 1-10, and Figure 4). A set of experiments was conducted to elucidate the effect of temperature on the Ca utilization. Only two temperatures of 50 and 70 °C were selected because of the restrictions on the experimental conditions. The H2O/CaO ratio ranged from 0 to 20, and the SO2 concentration was still constant at 2600 ppm (see Table 2, experiments 1-10, and Figure 4). Another set of experiments was performed to study the effect of the Ca/S molar ratio on the Ca utilization at an isothermal temperature of 50 °C. The Ca/S molar ratio was changed within the range of 0.5-2.0 by adjusting the SO2 concentration from 1110 to 2600 ppm. The desired H2O/CaO ratio was 15 (see Table 2, experiments 11-14, and Figure 5). Ca Utilization. The Ca utilization η is defined herein as the percentage of calcium oxide in the coal ash that is chemically bound to sulfur dioxide. It can be expressed as
η)
m′′ × 100% m′
where m′ (mg) is the amount of calcium oxide in the fresh dry ash and m′′ (mg) is the amount of calcium oxide bound to sulfur dioxide.
Figure 5. Effects of the Ca/S ratio on the Ca utilization.
On the basis of the measurements, m′ and m′′ were determined from
[
m′ ) CaO × M′
]
(100 - X) 56 - M′ × 100 64
m′′ ) M′′
where M′ (mg) and M′′ (mg) are the amounts of fresh dry coal ash sample and wet coal ash sample, respectively, and X (%) is the total weight loss of sample, which is supplied by the TGA with good accuracy (see Table 2). CaO denotes the CaO content in the dry ash. In this study, CaO is around 25.75 wt %. Ca utilization is an important index in evaluating the desulfurizing capacity of humidified coal ash. In this paper, the effects of a number of operating parameters, including the H2O/CaO ratio, temperature, and Ca/S ratio, on the Ca utilization of wet ash were experimentally investigated under isothermal conditions. Results and Discussion Effects of Water. The CaO in the coal ash used in this study differs from the fresh CaO in that the former is often encased by a partially sulfated shell17 that impedes the chemical reaction between SO2 and CaO from taking place further. Curve 1 in Figure 3 also illustrates this point. No changes of mass were observed in curve 1 when the dry ash was heated from 30 to 130 °C. However, in the presence of water, the sample began to lose weight until a total weight loss of 41.91% was reached (curve 2 in Figure 3). It cannot be proved that the SO2-removal reaction occurred. Therefore, another sample, about the same as the first wet sample, was heated with pure nitrogen as the protective gas to obtain the water evaporation curve (curve 3 in Figure 3). A 43.35% weight loss resulted. A comparison of the two total weight loss rates indicates that a certain amount of SO2 was adsorbed by the first sample so that the total weight loss of the first sample was decreased. Compared with the dry ash, water in the wet ash plays a significant role in SO2 removal. This is because water can penetrate the partially sulfated shell and then react with CaO to form calcium hydroxide. As the molar volume of calcium hydroxide (33.1 cm3/mol) is larger than that of CaO (16.9 cm3/mol), calcium hydroxide expands and disrupts the partially sulfated shell. Consequently, calcium hydroxide can make contact with SO2 and capture it. In addition, water can form a liquid film
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on the surface of the calcium hydroxide particles, which greatly accelerates the reaction at low temperature because the desulfurization reaction converts from a gas-solid reaction to an ionic reaction. It is also shown in Figure 3 that curve 3 changes into a straight line and essentially no longer descends when the temperature is higher than 90 °C, whereas curve 2 no longer descends when the temperature is higher than 91 °C. These results demonstrate that the evaporation of water is completed at 90 °C, whereas the chemical reaction ends at 91 °C. This implies that, at low temperature, the evaporation process and the SO2removal process occur simultaneously and that, once the water is entirely evaporated, the desulfurization reaction stops. Therefore, the presence of water is a prerequisite to the SO2-removal reaction of high-CaO ash at low temperature. Effects of H2O/CaO Ratio. The H2O/CaO ratio is defined as the molar ratio of H2O to CaO in ash. The experimental conditions are shown in Table 2. The effects of the H2O/CaO ratio within the range 0-20 on the Ca utilization at constant temperature are shown in Figure 4. As shown in Figure 4, the Ca utilization increases with increasing H2O/CaO ratio under isothermal conditions. This is because the higher the water content is in the ash, the longer the chemical reaction will be maintained, and hence, the more the SO2 will be adsorbed by the ash. However, different H2O/CaO ratios have different effects on the Ca utilization. When the H2O/CaO ratio is less than 5, the Ca utilization increases gradually with increasing H2O/CaO ratio. When the H2O/CaO ratio is between 5 and 13, the Ca utilization increases rapidly, and when the H2O/CaO ratio is more than 13, the curve tends to vary slowly again. Therefore, it seems that the H2O/CaO ratio of 13 is the most economical for the experimental conditions because of the higher Ca utilization and lower amount of water consumed. Effects of Temperature. Two sets of experiments were conducted at the temperatures of 50 and 70 °C with H2O/CaO ratios ranging from 0 to 20. The experimental results are shown in Figure 4. When the H2O/ CaO ratio is constant, the Ca utilization decreases with increasing temperature. This is because the evaporation rate of water in ash is faster at higher temperature and, hence, the SO2 capture potential of the ash is decreased. On the other hand, it is commonly considered that, in the presence of water, the chemical reaction between calcium hydroxide and SO2 undergoes six steps as follows:18,19
(1) Hydration of CaO CaO + H2O ) Ca(OH)2 (2) Diffusion of SO2 from the flue gas to the liquid film (3) Absorption of SO2 in the liquid film (4) Dissolution of SO2 to form HSO3- and SO32(5) Dissolution of Ca(OH)2 in the liquid film Ca(OH)2 ) Ca
2+
+ 2OH
-
(6) Liquid-phase reaction of calcium and sulfur species 1 1 Ca2+ + SO32- + H2O ) CaSO3‚ H2O 2 2 In these steps, the diffusion process and the dissolution of reactants in the liquid film are the keys to controlling the whole rate, whereas the other reactions can be considered to instantaneously go to completion.20 The higher the reaction temperature, the greater the diffusion coefficient, and the lower the solubility. The result that the Ca utilization at 70 °C is lower than that at 50 °C illustrates that, under the experimental conditions, the dissolution of reactants is the dominant influencing factor and the reaction rate decreases with increasing temperature. Effects of Ca/S Ratio. In this study, the Ca/S ratio is defined as the number of moles of calcium in the ash supplied per mole of sulfur in the flue gas. This ratio is used because it clearly shows the consumption of ash, which is a significant economic factor in the process. The effects of the Ca/S ratio within the range 0.5-2.0 at the temperature of 50 °C on the Ca utilization are shown in Figure 5. From Figure 5, we can see that, as the Ca/S ratio increases, the Ca utilization decreases. The same trend was also obtained by Ma and Kaneko et al.21 in their experimental study on SO2 removal from flue gas. The shape of this curve, which varies from an initial dramatic reduction to a gradual reduction, demonstrates that a suitable Ca/S ratio can be found within the lower range of values. This is important in decreasing the sorbent costs and achieving an optimal process. Conclusion This is the first study of the SO2-removal capacity of wet high-CaO coal ash by means of a reaction system based on the use of a thermogravimetric analyzer (TGA). The experimental results show that this system is a valuable tool in quantitatively determining the SO2 absorption and Ca utilization of sorbents. The effects of water in the ash were experimentally investigated. It was found that, at low temperature (30-130 °C), unbound water is the prerequisite to the desulfurization reaction of high-CaO coal ash. Once the water is completely evaporated, the reaction ends. In this study, the Ca utilization was used to evaluate the SO2-removal potential of wet coal ash, and a number of experimental parameters such as the H2O/CaO molar ratio, temperature, and Ca/S ratio were chosen in ranges common for power-plant flue gas. Experimental data show that, with increasing H2O/CaO ratio, the Ca utilization increases, whereas with increasing temperature and Ca/S ratio, the Ca utilization decreases. Acknowledgment The work reported in this paper was funded by the Shanghai Electric Power Corporation and the Special Fund for National Key Fundamental Research (G1999022209). The authors acknowledge their financial support. Discussions with Chen Kaichao, Liao Yongbo, and Qiu Xin of the Energy Engineering Department at Shanghai Jiaotong University are also greatly appreciated. Literature Cited (1) Ollero, P.; Salvador, L.; Canadas, L. An Experimental Study of Flue Gas Desulfurization in a Pilot Spray Dryer. Environ. Prog. 1997, 16, 20.
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(2) Yoon, H.; Stouffer, R.; Rosenhoover, W. A.; Withum, J. A.; Burke, F. R. Pilot Process Variable Study of Coolside Desulfurization. Environ. Prog. 1988, 7, 104. (3) Merrick, D.; Vernon, J. Review of Flue Gas Desulfurization System. Chem. Ind. 1989, 6, 55. (4) Stouffer, R.; Yoon, H.; Burke, F. P. An Investigation of the Mechanisms of Flue Gas Desulfurization by In-Duct Dry Sorbent Injection. Ind. Eng. Chem. Res. 1989, 28, 20. (5) Jocewicz, W.; Chang, S. C. J.; Sedman, B. C.; Brna, B. T. Silica-Enhanced Sorbents for Dry Injection Removal of SO2 from Flue Gas. J. Air Pollut. Control Assoc. 1988, 38, 1027. (6) Hall, W. B.; Singer, C.; Jocewicz, W.; Maxwell, A. M. Current Status of the ADVACATE Process for Flue Gas Desulfurization. J. Air Waste Manage. Assoc. 1992, 42, 103. (7) Rubin, E. S.; Cushey, M. A.; Marnicio, R. J. Controlling Acid Deposition: The Role of FGD. Environ. Sci. Technol. 1986, 20, 960. (8) Lane, W. R.; Khan, S. Spray Dryer or Wet Limestone FGD Costs and Selection for Retrofit versus New Plants. Proc. Am. Power Conf. 1993, 52, 95. (9) Peterson, J. R.; Rochelle, G. T. Liqueous Reaction of Fly Ash and Ca(OH)2 to Produce Calcium Silicate Absorbent for Flue Gas Desulfurization. Environ. Sci. Technol. 1988, 22, 1299. (10) Jocewicz, W.; Rochelle, G. T. Fly Ash Recycle in Dry Scrubbing. Environ. Prog. 1986, 5, 219. (11) Sanders, J. F.; Keener, T. C.; Wang, J. Heated Fly Ash/ Hydrated Lime Slurries for SO2 Removal in Spray Dryer Absorbers. Ind. Eng. Chem. Res. 1995, 34, 302. (12) Wieckowska, I. Catalytic and adsorptive desulfurization of gases. Catal. Today 1995, 24, 405. (13) Kyte, W. S. Some Chemical and Chemical Engineering
Aspects of Flue Gas Desulfurization. Trans. Chem. Eng. 1981, 59, 219. (14) Harriott, P.; Smith, K.; Benson, L. B. Simultaneous Removal of NO and SO2 in Packed Scrubbers or Spray Towers. Environ. Prog. 1993, 12, 110. (15) Strock, T. W.; Gohara, W. F. Experimental Approach and Techniques for the Evaluation of Wet Flue Gas Desulfurization Scrubber Fluid Mechanics. Chem. Eng. Sci. 1994, 49, 4667. (16) Sahar, A.; Kehat, E. SO2 Removal From Hot Flue Gas by Lime Suspension Spray in a Tube Reactor. Ind. Eng. Res. 1991, 30, 435. (17) Schmal, D. Improving the Action of Sulfur Sorbents in the Fluidized-Bed Combustion of Coal. Ind. Eng. Chem. Process Des. Dev. 1985, 24, 72. (18) Newton, G. H.; Kramlich, J. Modelling the SO2-Slurry Droplet Reaction. AIChE J. 1990, 36, 1865. (19) Neathery, J. K. Model for Flue-Gas Desulfurization in a Circulating Dry Scrubber. AIChE J. 1996, 42, 259. (20) Babu, D. R.; Narsimhan, D.; Phillips, C. R. Absorption of Sulfur Dioxide in Calcium Hydroxide Solutions. Ind. Eng. Chem. Fundam. 1984, 23, 373. (21) Xiaoxun, M.; Kaneko, T.; Guo, Q.; Xu, G.; Kato, K. Removal of SO2 from Flue Gas Using a New Semidry Flue Gas Desulfurization Process with a Powder-Particle Spouted Bed. Can. J. Chem. Eng. 1999, 77, 356.
Received for review August 30, 2000 Revised manuscript received May 14, 2001 Accepted May 15, 2001 IE000789M
