Energy & Fuels 2002, 16, 155-160
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Chain Reaction Mechanism by NOx in SO2 Removal Process Yan Li,† Boon Chye Loh, Norihiko Matsushima, Masateru Nishioka, and Masayoshi Sadakata* Department of Chemical System Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan Received June 15, 2001. Revised Manuscript Received September 28, 2001
The effects of NOx, CO2, and reaction temperature on the SO2 removal process have been investigated in a fluidized bed reactor by using CaO/fly ash sorbent in order to demonstrate any available method to promote the calcium utilization rate and form valuable gypsum as a byproduct for the dry desulfurization process. It was found that NOx increased the selectivity of CaSO4 as the final product in the sorbent during the desulfurization process. The effect of NOx on SO2 removal rate was also investigated and results showed that the presence of NOx enhanced the SO2 removal rate, even when mole ratio of NO/SO2 was less than 1.0. It was deduced that the reaction mechanism involved NOx-related chain reactions. On the basis of FTIR analysis of reactions product, the chain reaction mechanism could be outlined that Ca(NO3)2 was formed from the reaction of Ca(OH)2 with NO2, and then Ca(NO3)2 reacts with SO2 to form CaSO4 + NO. On the other hand, it was found that the CO2 negative effect on SO2 removal was reduced with NOx presence.
Introduction SOx and NOx emissions from coal combustion facilities have given significant environmental and human health effects. SO2/NOx can react with water vapor and other chemicals in the air to form very fine particles. These airborne particles form a key element of urban smog and are a significant health hazard. The wet desulfurization technique of flue gas has been developed commercially. However, the system of wet process is complicated and costly, and it requires a considerable amount of water consumption and water re-treatment. This makes the technique inapplicable where the water reserves are rather limited. For such cases, the dry flue gas desulfurization process has become an alternative approach. However, the SO2 removal rate in the dry process generally is quite low. It is important to find a novel method to make the dry desulfurization process more efficient. As the most widely used absorbent for the dry desulfurization process is CaO or Ca(OH)2, the SO2 removal reaction mechanism by calcium sorbent has been studied widely. The calcium utilization rate is significantly influenced by solid particle characteristics including its surface area, pore size and volume for SO2 diffusion into calcium sorbent particles and the reaction in the solid to form CaSO4/CaSO3. However, the commercial Ca(OH)2/CaO powder is produced mainly from CaCO3 decomposition with a small surface area (generally less than 10 m2/g). As a result, it is difficult to realize a high calcium utilization rate, especially for the desulfurization process of flue gas under low tempera* Author to whom correspondence should be addressed. Tel: 81-35841-7344. Fax: 81-3-5841-7489. E-mail:
[email protected]. † E-mail:
[email protected].
ture. Therefore, it is necessary to improve the reactivity of sorbent particles for SO2 absorption by physical and chemical processes. Sanders et al.1 found that the sorbent prepared by mixing fly ash and hydrated lime slurries could enhance the SO2 removal rate. It was considered that the enhancement resulted from the presence of calcium silica hydrate material formed by the hydration reaction between calcium and the alumina silicate in the fly ash, and the difference in reactivity was caused by the structure of calcium silicate hydrate material formed. On the other hand, experimental results of Svoboda et al.2 showed that the products from SO2 capture were strongly related to the general reactivity of the sorbent(γ-alumina-CaO). Most of reaction product was found to be CaSO4 when the absorbent had large specific surface, high CaO content, and wide pore size distribution. It was therefore concluded that oxidation of CaSO3 in the temperature range 120-240 °C was related to the sorbent reactivity. Studies by Hiroaki et al.3,4 indicated that the aging of the slurry containing calcium hydroxide, calcium sulfate, and fly ash at about 100 °C and successive drying were essential for producing active sorbent. It was considered that the high activity was realized by (1) Sanders, J. F.; Keener, T. C.; Wang, J. Heated Fly Ash/Hydrated Lime Slurries for SO2 removal in Spray Dryer Absorbent. Ind. Eng. Chem. Res. 1995, 34, 302-307. (2) Svoboda, K.; Lin, W.; Hannes, J.; Korbee, R.; Bleek, C. M. Lowtemperature flue gas desulfurization by Alumina-CaO regenerable sorbents. Fuel 1994, 73 (7), 1144-1150. (3) Hiroaki, T.; Tomohiro, I.; Hideki, N.; Tsutomu, U.; Hideshi, H. Removal of Sulfur Dioxide from Flue Gas by the Absorbent Prepared from Coal Ash: Effects of Nitrogen Oxide and Water. Ind. Eng. Chem. Res. 1996, 35, 851-855. (4) Hiroaki, T.; Tomohiro, I.; Tsutomu, U.; Hideshi, H.; Hideaki, K. High Active Absorbent for SO2 Removal Prepared from Coal Fly Ash. Ind. Eng. Chem. Res. 1995, 34, 1404-1411.
10.1021/ef0101309 CCC: $22.00 © 2002 American Chemical Society Published on Web 12/15/2001
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the formation of microporous structure in the sorbent. It was also reported that the activity of sorbent was increased with the hydration time and reached a maximum activity in 12 h. It was considered that the hydration resulted in the formation of ettringite and calcium silicate.5,6 Research by D. Paolo7 showed that the hydration process between hydrated lime slurries and fly ash needed temperatures higher than 80 °C and a long reaction time with several hours to obtain a high reactivity of the sorbent. Although it was verified that the reactivity of the prepared sorbent was enhanced in the hydration process by mixing hydrated lime slurries with fly ash, the technique of application in actual process was difficult because the hydration process generally needed high temperature and long reaction time. Studies by Li, Sadakata, et al.8,9 gave a new method to prepare the highly active sorbent with low-cost by mixing the CaO and fly ash in water at ambient temperature with a short preparation time. The calcium utilization rate in sorbent was found to be 2-3 times higher than that of the original CaO particles under identical conditions. It was also found that the mixing time of CaO and fly ash had not a significant effect on the calcium utilization rate. SEM and EDX results showed that the CaO particle was separated into several small particles of Ca(OH)2 due to the significant heat release of the reaction between CaO and water, and the tiny particles of Ca(OH)2 covered the surface of fly ash particles with drying process. These results indicated that the significant increase of calcium utilization rate was mainly attributed to the Ca(OH)2 covering the surface of fly ash particles, and the hydration reactions could not play an important role under the sorbent preparation condition at ambient temperature. On the other hand, it was reported that NOx has effects on both conversion rate and reaction product at low temperature. Hiroaki et al. found that reaction products consisted exclusively of CaSO4 with NOx presence at reaction temperature between 100 °C and 130 °C. It was supposed that SO2 was oxidized to SO3, and then reacted with Ca(OH)2 to form CaSO4. Up to now, it is not clear for the mechanism of NOx effects on CaSO4 formation. In this paper, the experiment of the desulfurization reaction was conducted in a fluidized bed reactor. The chemical compositions of byproducts were analyzed by FTIR. Separate tests to investigate possible reaction routes were carried out in a TGA system. Experimental Method The experiments have been carried out in a fluidized bed reactor depicted schematically in Figure 1. The inside diameter of reactor is 42.6 mm with 500 mm in height. The temperature in the fluidized bed of the reactor was measured by a thermocouple and controlled automatically. (5) Moore, A. E.; Taylor, H. F. W. Crystal Structure of Ettringite. Nature(London) 1968, 218, 1048-1049. (6) Moore, A. E.; Taylor, H. F. W. Crystal Structure of Ettringite. Acta Crystallogr. 1970, B26, 386-393. (7) Paolo D. Investigation of the SO2 adsorption properties of Ca(OH)2-Fly ash system. Fuel 1996, 75 (6), 713-716. (8) Yan, Li; Sadakata, M. Study of Gypsum Formation for Appropriate Dry Desulfurization Process of Flue Gas. Fuel 1999, 78, 10891095. (9) Yan, Li; Nishoka, M.; Sadakata, M. High Calcium Utilization and Gypsum Formation for Dry Desulfurization Process. Energy Fuels 1999, 13, 1015-1020.
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Figure 1. Fluidized bed reactor. The reactant gas was fed into the reactor with the balance steam of N2 under the demanded concentration and humidity. A filter was set up to collect the entrained particles in the exhaust gas. The gas analyzer (Horiba PG250) was used to measure the gas concentrations of SO2, O2, CO2, and NO/NO2 in the entrance and exit flows. The system was operated batchwise with a single charge of particles. In a typical operation, the fluidized velocity in the bed was set at 5 cm/s. The gas composition was as follows: SO2[1480 ppm], O2[8 vol %], CO2[12 vol %], NOx[1000-300 ppm], and N2 as balance. The bed was loaded with 48.5 g of sorbent. After the demanded reaction temperature was reached, the gas sample was fed to the fluidized bed reactor and the the exhaust gas concentration was continuously monitored. The product after desulfurization was analyzed by FTIR to check its composition. The sorbent was prepared from CaO and coal fly ash with the average diameter of particles 60 µm. In a typical preparation process of the sorbent, 20% CaO and 80% fly ash were mixed in water at ambient temperature, and then the mixture was dried in a constant temperature oven. Sorbent particle characteristics including its BET surface area and pore volume were investigated. Sorbent activity for SO2 removal was measured in the TGA system. It was found that the calcium utilization rate in the sorbent was achieved 2-3 times higher than that of the original CaO particles under identical reaction conditions. The results in detail can be found in reference 9.
Results Sorbent Activity Testing. The desulfurization activity of the sorbent was examined at 250-350 °C with the gas composition of SO2[1480 ppm], O2[8 vol %], CO2[12 vol %] with N2 balance. A control test run using a mixture of 20 wt % untreated Ca(OH)2 and 80 wt % flyash was also conducted under identical conditions. The results in Figure 2 confirmed that the sorbent possessed better desulfurization activity. FTIR analysis of the reacted sorbent revealed that the main product obtained was CaSO3. According to Li and Sadakata,9 the improvement in activity of the sorbent could be attributed to the chemical and physical changes in the sorbent during the preparation process. When the CaO was added to water with fly ash, it reacted with water to form Ca(OH)2. This reaction is exothermic with an enthalpy change of 65 kJ/mol. Due to rapid heat release, the particles disintegrated into finer particles, and with the existence of fly ash, the fine particles were prevented from re-coagulation. In this manner, a larger relative surface area of the sorbent was induced and this improved its activity. Effects of Temperature and CO2. The effect of a higher temperature on the desulfurization activity of the
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Figure 2. The activity comparison of sorbent and Ca(OH)2 [SO2: 1480 ppm, O2: 8 vol %, CO2: 12 vol %, reaction temperature: 350 °C]. Figure 4. The effects of NO (1000 ppm) on SO2 removal rate [SO2: 1480 ppm, O2: 8 vol %, CO2: 12 vol %, reaction temperature: 350 °C].
the dominant product formed at this temperature was CaSO3. It could be deduced that the desulfurization process occurred according to the following steps: k1
Ca(OH)2 + SO2 98 CaSO3 + H2O k2
Ca(OH)2 + CO2 98 CaCO3 + H2O k3
CaCO3 + SO2 98 CaSO3 + CO2
Figure 3. The effects of temperature and CO2 on SO2 removal rate [SO2: 1480 ppm, O2: 8 vol %, CO2: 12 vol %].
sorbent was studied by carrying out an experiment run at 350 °C. Another experiment in which CO2 was not included in the inlet gas was also conducted at 350 °C to study the effect of CO2 presence at that temperature. The result of these two experiments, combined with the result to be performed at 250 °C, is given in Figure 3. It was observed that the SO2 removal efficiency was better at the higher temperature or when CO2 was absent. FTIR analysis of reaction product revealed that
(1) (2) (3)
In the absence of CO2, desulfurization took place according to step (1). However, when CO2 was present, step (2) occurred simultaneously. This reaction was confirmed from a TG experiment which was carried out at 350 °C between Ca(OH)2, CO2, and H2O. It was observed that k2 increased rapidly with increasing temperature. The sulfation rate of CaCO3 was reported to be smaller than that of Ca(OH)2. A TG experiment whereby pure CaCO3 was reacted with 3000 ppm SO2 at 350 °C was conducted and no conversion was observed. As a result, it could be concluded that the presence of CO2 resulted in a partial conversion of the Ca(OH)2 into less
Figure 5. NO concentration effect on SO2 removal SO2: 1480 ppm; O2: 8 vol %; CO2: 12 vol %; H2O: 10 vol %; N2 balance, temp. ) 350 °C.
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Figure 6. FTIR analysis of product in fluidized bed after 90 min desulfurization by sorbent at temp. 350 °C [SO2: 1480 ppm, O2: 8 vol %, CO2: 12 vol %, NO: 1000 ppm; or NO2: 1000 ppm with N2 balance].
reactive CaCO3 and therefore, caused a decrease in the overall desulfurization activity of the sorbent. Reaction (2) increased tremendously above 350 °C. Although the kinetics for the above-mentioned series of reaction was unknown, it could be deduced that there might exist an optimum temperature for the desulfurization process to take place. Further studies might be necessary in this area. NOx Effects. The effect of NO on the desulfurization activity and gypsum conversion of the sorbent at 350 °C was investigated by including 1000 ppm/380 ppm NO in the inlet gas. The results derived from this experiment are illustrated in Figure 4 and Figure 5. It was observed that a high desulfurization activity of the sorbent could be maintained even when CO2 was present. The NO effect with 1000 ppm was similar with 380 ppm NO effect on SO2 removal as mole ratio of NO/ SO2 less than 1.0. These results revealed that the chain reaction by NOx was proceeding. It was noted that the present sorbent also showed an activity in the removal of NO, although it was not as high as the one for SO2. FTIR analysis of the product in Figure 6 indicated that CaSO4 was the main product when NOx existed. NOx effect on the reaction activity of calcium sorbent in the absence of fly ash was also investigated in TGA. The result in Figure 7 showed that NO could provide a similar enhancement in reactivity on calcium hydroxide. It was also observed that the NOx effect on the desulfurization reactivity of sorbent with fly ash was more apparent than that on calcium hydroxide, which could be attributed to the improvement of reaction activity in the CaO/fly ash sorbent preparation.9 According to these results, it was concluded that the activity for SO2 removal and gypsum conversion of the sorbent were strongly enhanced by the presence of NOx, and the CO2 negative effect on SO2 removal was reduced with the presence of NOx. Discussion of NOx Effect Mechanism on de-SO2 Process There are many possible reactions between NOx and SO2 with O2 and H2O. These reactions were investigated
Figure 7. Calcium utilization rate reacted with SO2 [1000 ppm]; O2[8%]; NO2 [1000 ppm]; with N2 balance, reaction temperature: 400 °C.
Figure 8. Reaction temperature effect on NO2 conversion rate for reaction NO + 1/2O2 f NO2 ([O2] 10%; [NO]0 1000 ppm).
in the sulfur acid production process. The main available reactions were listed as below:
2NO + SO2 f N2O + SO3 NO + 2SO3 f (SO3)2NO
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Figure 9. Effect of reaction temperature on the conversion rate of SO2 to SO3 by SO2 + NO2 f SO3 + NO ([NO2]o; [SO2]o 1000 ppm). Figure 11. FTIR analysis of reaction product: Ca(NO3)2 + SO2 [3314 ppm]; temperature: 300 °C.
Figure 10. FTIR analysis of reaction product: Ca(OH)2 + NO2 [2900 ppm]; temperature: 300 °C.
Figure 12. FTIR analysis of reaction product: CaCO3 + NO2 [2900 ppm]; temperature: 300 °C.
NO2 + 2SO2 f (SO3)2NO (3NO2 + 2SO2 f S2N2O9 + NO; S2N2O9 f S2NO7 + NO2)
N2O4 + 2NO f 2N2O3
