Energy & Fuels 2005, 19, 73-78
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Dry Desulfurization in a Circulating Fluidized Bed (CFB) with Chain Reactions at Moderate Temperatures Bo Hou,* Haiying Qi, Changfu You, and Xuchang Xu Institute of Thermal Engineering, Tsinghua University, Beijing 100084, People’s Republic of China Received January 17, 2004. Revised Manuscript Received September 21, 2004
A series of experiments in a circulating fluidized bed (CFB) pilot plant has explored a new dry desulfurization process, using the NOx in the flue gas and a new sorbent that has been prepared from fly ash and lime. Various desulfurization operating parameters were tested for the thermodynamic, chemical, and dynamic states for temperatures of 523-673 K. The NOx increased the calcium conversion ratio in the desulfurization process. In addition, with the NOx, the desulfurization byproduct was determined to be mostly CaSO4, instead of CaSO3, as a result of the chain reaction caused by the NOx. Therefore, the NOx in the flue gas can improve the efficiency of the dry desulfurization process.
1. Introduction The emission of SO2 from coal combustion facilities has resulted in significant environmental and human health effects. The SO2 emissions come mainly from combustion processes that use low rank coal with higher sulfur content. Desulfurization equipment must be used to remove SO2 and produce a valuable byproduct. Because of the lack of water resources in most areas, an economical dry desulfurization technology is needed, especially for developing countries and areas. Therefore, the circulating fluidized bed (CFB) flue gas desulfurization (FGD) process at moderate temperatures (523-673 K) has been undergoing development. In this temperature range, the dry desulfurization process with a CFB does not perform well, because of the low calcium conversion ratio (ηCa) of lime sorbent.1 In this temperature range, the reaction rate between the original calcium-based sorbent (such as CaO/Ca(OH)2) and the SO2 is too low to give satisfactory desulfurization efficiency. Therefore, a new method is needed to improve the ηCa value of the sorbent. The effect of NOx on the SO2 removal process may also provide some additional positive effect on the desulfurization process. This paper investigates the activity of a new sorbent that has been prepared from coal fly ash and lime, the chain reaction caused by the NOx in the desulfurization process, and other factors that affect the flow gas desulfurization process at moderate temperatures in a pilot plant. * Author to whom correspondence should be addressed. E-mail address:
[email protected]. (1) Changfu, Y.; Haiying, Q.; Aijun, W.; Xuchang, X. Experimental Study on the Flue Gas Desulfurization Technology by Circulating Fluidized Bed and the Reactivation of Desulfurizer by Steam in 400800 °C. In Proceedings of National Symposium on Combustion; Chinese Society of Engineering Thermophysics: Beijing, 1999; pp v-51-v-57.
With a calcium-based sorbent, the ηCa value is significantly influenced by the particle characteristics, including the particle surface area, pore size, and pore distribution, as well as the pore volume.2 With the commercial lime usually used in the SO2 removal process, the microcosmic particle characteristics are not good enough to provide high removal rates, especially at lower temperatures. Therefore, the activation of the sorbent must be improved by other physical and chemical processes. The conventional method uses inexpensive coal fly ash and lime to prepare the sorbent using the hydration process.3 Ueno found that a sorbent that was prepared as a mixture of CaO, CaSO4, and fly ash provided better SO2 removal,4 and he thought that the high removal rate was due to the improved microcharacteristics of the hydration and drying procedures in the sorbent preparation process.4 The surface area and particle pore volume were 8 times larger than that of the original material, using hydration at 350 K for 8 h and drying at 360 K for 1 day.5 The activity of the prepared sorbent was known to be enhanced in the hydration process by mixing hydrated lime slurries with coal fly ash; however, the actual process was not practical, because of the difficult preparation process with high temperatures and long hydration reaction times. Therefore, a new, low-cost, on-line sorbent preparation process was developed by hydrating lime and fly ash at ambient temperature for a short time. This paper investigates the activity of such a sorbent at moderate temperatures. (2) Li, Y.; Sadakata, M. Study of Gypsum Formation for Appropriate Dry Desulfurization Process of Flue Gas. Fuel 1999, 78, 1089-1095. (3) Ho, C.-S.; Shih, S.-M. Ca(OH)2/Fly Ash Sorbents for SO2 Removal. Ind. Eng. Chem. Res. 1992, 31 (4), 1130-1135. (4) Ueno, T. Process for Preparing Desulfurizing and Denitrating Agents, U.S. Patent No. 4,629,721, December 16, 1986. (5) Al-Shawabkeh, A.; Matsuda, H. Comparative Reactivity of Treated FBC- and PCC-Fly Ash for SO2 Removal. Can. J. Chem. Eng. 1995, 73 (October), 678-685.
10.1021/ef049975l CCC: $30.25 © 2005 American Chemical Society Published on Web 12/10/2004
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Energy & Fuels, Vol. 19, No. 1, 2005
Hou et al.
Figure 1. Pilot-scale circulating fluidized bed (CFB) experimental system. The pilot-plant system consisted of 10 subsystems: air supply; flue gas generation (oil burner and boiler); NO conversion; cooling; sorbent preparation; bed material feeder and drain; CFB reactor riser with a diameter of 0.305 m and a height of 6 m and cyclones; dust removal and back-feed (bag filter); control and measurement; and compressed air source.
Usually, flue gas from coal-fired boilers contains 500∼600 ppm NOx (mainly in the form of NO). The following reactions occur between the NO and the SO2:6
1 NO + O2 f NO2 2
(1)
NO2 + SO2 f SO3 + NO
(2)
SO3 has a much higher reactivity than SO2 in the desulfurization process, which will accelerate the sulfate reaction and produce a higher desulfurization efficiency. NOx affects both the ηCa value and the reaction products in the desulfurization process, which implies that the byproducts probably contain more CaSO4, instead of CaSO3.7 However, the first reaction is very limited for temperatures of 573-673 K; therefore, a method was sought to intensify this reaction to improve the desulfurization process. These experiments investigated the effect of NO on the desulfurization process. 2. Pilot-Scale CFB Reactor System The pilot-scale CFB reactor system is shown in Figure 1. An oil-fired burner and a boiler were used to generate the flue gas, which contained the desired O2, CO2, NOx, and SO2. According to the data given by Li et al.,7 the gas temperature should reach 823 K for NO conversion and then be reduced to 473-673 K by an air cooler for the SO2 removal reaction in the CFB riser. Because of a higher excess-air coefficient, the flue gas from the oil burner was diluted; the CO2 content then was lower than the level in real flue gas from a coal-fired boiler. Therefore, as an alternative, an oil boiler was designed to give the necessary CO2 content (>13%). (6) Tsuchiai, H.; Ishizuka, T.; Ueno, T.; Hattori, H.; Kita, H. Highly Active Absorbent for SO2 Removal Prepared from Coal Fly Ash. Ind. Eng. Chem. Res. 1995, 34 (4), 1404-1411. (7) Li, Y.; Loh, B. C.; Matsushima, N.; Nishioka, M.; Sadakata, M. Chain Reaction Mechanism by NOx in SO2 Removal Process. Energy Fuels 2002, 16, 155-160.
Figure 2. Example of a real-time desulfurization efficiency curve. Parameters were as follows: bed temperature (Tbed), 633 K; gas velocity (U0), 2 m/s; bed density (Fbed), 48 kg/m3; SO2 concentration ([SO2]), 1500 ppm; NO concentration ([NO]), 700 ppm; CO2 concentration ([CO2]), 3.5%; and sorbent residence time (τbed), 3.4-7 h. NO was added to the flue gas before the NO converter and SO2 was added before the CFB riser. The NO was converted to NO2 by injecting methanol. Solid particles were collected in the cyclones and bag filter from the flue gas and fed back into the riser for further circulation. The flue gas temperatures and static pressures at different positions along the height of the riser were measured automatically with thermocouples and pressure sensors. The CO, CO2, O2, NO, NOx, and SO2 concentration in the flue gas were measured on-line at the inlet and outlet of the CFB reactor, using a gas analyzer. Figure 2 shows typical results for the real-time treatment of the measured data. As shown in Figure 3, solid particles at four positionssat the distributor, J-valve, secondary cyclone, and bag filterswere sampled for further analyses using X-ray diffractometry (XRD), scanning electron microscopy (SEM), Fourier transform infrared (FTIR) spectroscopy, and thermogravimetric analysis (TGA). The particle size distribution of the bed material, taken from different positions, is shown in Figure 4. Both the particle size
Dry Desulfurization in a CFB
Energy & Fuels, Vol. 19, No. 1, 2005 75 Table 2. Operation Conditions for the Pilot-Scale Circulating Fluidized Bed (CFB) Reactor parameter
value
bed temperature superficial velocity in the riser bed density two-stage cyclone separation efficiency
523-673 K 1.2-2.6 m/s 33.3-66.7 kg/m3 99.9%
The operation conditions for the pilot-scale CFB reactor was shown in Table 2. The desulfurization efficiency is defined as
( )
ηdesox (%) ) 1 Figure 3. Diagram showing the bed material sampling positions (distributor, J-valve, secondary cyclone, and bag filter).
SiO2 Al2O3 Fe2O3 CaO MgO K2O Na2O MnO TiO2 P2O5 lime A 8.37 1.50 0.76 75.73 4.33 0.45 0.01 0.08 0.13 lime B 3.64 0.34 0.44 86.45 3.51 0.048 0.01 0.013 0.11 fly ash 48.64 38.10 4.30 2.93 0.64 0.76 0.43 0.30 1.39 0.39
× 100
(3)
3. Results and Discussion 3.1. Activity of New Sorbent. With a dry desulfurization process in an open system such as a CFB reactor, the calcium conversion ratio (ηCa) of the sorbent is the most important parameter that determines the desulfurization efficiency (ηdesox); the desulfurization efficiency is related to the calcium conversion ratio by the expression
and the size distribution range decrease as the flue gas flowed through the gas-solid separators. The new sorbent was prepared using commercial lime (in the form of 20-mm lumps) with the coal fly ash collected from the electrostatic precipitator (ESP) of a coal-fired power plant. The average diameter of the fly ash was 83 µm. The mineral analysis of the lime and fly ash is listed in Table 1. The sum of SiO2 and Al2O3 content in the fly ash was >80%, with some Fe2O3 and CaO. The lime was mainly CaO, with some SiO2 and MgO. Lime A was milled to a fine powder, which was determined to contain ∼20% Ca(OH)2 and CaCO3, because of the exposure in the air. Lime B was simply incinerated in the oven without milling. A typical preparation procedure was to mix the lime, fly ash, and water in the desired proportions for
