Energy & Fuels 2009, 23, 2025–2030
2025
Effect of Sulfated CaO on NO Reduction by NH3 in the Presence of Excess Oxygen Tianjin Li, Yuqun Zhuo,* Yufeng Zhao, Changhe Chen, and Xuchang Xu Key Laboratory for Thermal Science and Power Engineering of Ministry of Education, Department of Thermal Engineering, Tsinghua UniVersity, Beijing 100084, People’s Republic of China ReceiVed September 5, 2008. ReVised Manuscript ReceiVed January 13, 2009
The effect of sulfated CaO on NO reduction by NH3 in the presence of excess oxygen was investigated to evaluate the potential of simultaneous SO2 and NO removal at the temperature range of 700-850 °C. The physical and chemical properties of the CaO sulfation products were analyzed to investigate the NO reduction mechanism. Experimental results showed that sulfated CaO had a catalytic effect on NO reduction by NH3 in the presence of excess O2 after the sulfation reaction entered the transition control stage. With the increase of CaO sulfation extent in this stage, the activity for NO reduction first increased and then decreased, and the selectivity of NH3 for NO reduction to N2 increased. The byproduct (NO2 and N2O) formation during NO reduction experiments was negligible. X-ray photoelectron spectroscopy (XPS) analysis showed that neither CaSO3 nor CaS was detected, indicating that the catalytic activity of NO reduction by NH3 in the presence of excess O2 over sulfated CaO was originated from the CaSO4 product. These results revealed that simultaneous SO2 and NOx control by injecting NH3 into the dry flue gas desulfurization process for NO reduction might be achieved.
1. Introduction The removal of SO2 and NOx from flue gas has received much attention, because of the many environmental problems that are associated with them, such as acid rain and the impact on human health.1 The wet scrubbing technique is commercially welcomed for flue gas desulfurization (FGD), because of its high desulfurization efficiency and wide adaptability. However, it consumes a large amount of water and requires water treatment. In many arid regions, water shortage is a significant problem. These drawbacks limit the application of wet SO2 removal techniques in the arid regions and the developing countries. For such cases, the low cost calcium-based dry FGD technique, based on the rapid hydration preparation of sorbent from lime and fly ash at ambient temperatures, has become an alternative approach.2-4 The bench-scale circulating fluidized bed (CFB) reactor results5 showed that 83% SO2 removal efficiency was observed at 350 °C with a Ca/S molar ratio of 2.2. Similar results were obtained in pilot-scale CFB reactor experiments,6 although the SO2 removal efficiency was only 40%-80% for Ca/S ratios of 1.5-3.0 at temperatures of 250-400 °C, which was lower than that of the bench-scale experiments, because of the competition between CO2 and SO2 for the active sites over Ca(OH)2. Determining how to eliminate * Author to whom correspondence should be addressed. E-mail:
[email protected]. (1) Xu, X. C.; Chen, C. H.; Qi, H. Y.; He, R.; You, C. F.; Xiang, G. M. Fuel Process. Technol. 2000, 62, 153–160. (2) Li, Y.; Nishioka, M.; Sadakata, M. Energy Fuels 1999, 13, 1015– 1020. (3) Li, Y.; Loh, B. C.; Matsushima, N.; Nishioka, M.; Sadakata, M. Energy Fuels 2002, 16, 155–160. (4) Zhang, J.; Zhao, S. W.; You, C. F.; Qi, H. Y.; Chen, C. H. Ind. Eng. Chem. Res. 2007, 46, 5340–5345. (5) Matsushima, N.; Li, Y.; Nishioka, M.; Sadakata, M.; Qi, H. Y.; Xu, X. C. EnViron. Sci. Technol. 2004, 38, 6867–6874. (6) Hou, B.; Qi, H. Y.; You, C. F.; Xu, X. C. Energy Fuels 2005, 19, 73–78.
the interference from CO2 and enhance the Ca effective utilization efficiency for SO2 capture was a serious challenge. Hou et al.7 investigated the effect of CO2 on SO2 removal using a rapid hydration sorbent at 300-800 °C by themogravimetric analysis (TGA). The results showed that the negative effect from CO2 could be neglected at temperatures above 700 °C. In our previous work, the pilot-scale CFB-FGD results8 confirmed that SO2 removal efficiency could be as high as 85%-95% at a Ca/S molar ratio of 2 in the temperature range of 700-800 °C. The elevated desulfurization temperature has enabled a new possibility of the simultaneous removal of multipollutants in flue gas. Previous studies have reported that trace elements such as selenium9 and arsenic10,11 can also be captured by CaO in the same temperature window. For NOx reduction from the flue gas of coal-fired power plant, NH3 is the widely used agent for both selective noncatalytic reduction (SNCR) and selective catalytic reduction (SCR).12 It has been well-noticed that, with the addition of a small amount of combustible gases such as CO or CH4,13,14 homogeneous SNCR reaction could be fulfilled at ∼800 °C. That is to say, the simultaneous removal of SOx, NOx, and trace elements (e.g., (7) Hou, B.; Qi, H. Y.; You, C. F.; Xu, X. C. J. Tsinghua UniV. (Sci. Technol.) 2004, 44, 1571–1574. (8) Zhang, J.; You, C. F.; Qi, H. Y.; Hou, B.; Chen, C. H.; Xu, X. C. EnViron. Sci. Technol. 2006, 40, 4300–4305. (9) Li, Y. Z.; Tong, H. L.; Zhuo, Y. Q.; Wang, S. J.; Xu, X. C. EnViron. Sci. Technol. 2006, 40, 7919–7924. (10) Jadhav, R. A.; Fan, L.-S. EnViron. Sci. Technol. 2001, 35, 794– 799. (11) Li, Y. Z.; Tong, H. L.; Zhuo, Y. Q.; Li, Y.; Xu, X. C. EnViron. Sci. Technol. 2007, 41, 2894–2900. (12) Muzio, L. J.; Quartucy, G. C.; Cichanowicz, J. E. Int. J. EnViron. Pollut. 2002, 17, 4–30. (13) Bae, S. W.; Roh, S. A.; Kim, S. D. Chemosphere 2006, 65, 170– 175. (14) Javed, M. T.; Irfan, N.; Gibbs, B. M. J. EnViron. Manage. 2007, 83, 251–289.
10.1021/ef800745v CCC: $40.75 2009 American Chemical Society Published on Web 02/19/2009
2026 Energy & Fuels, Vol. 23, 2009
Figure 1. Schematic diagram of the experimental apparatus.
selenium and arsenic) might be achieved in a temperature window of 700-850 °C in a single reactor. Although CaO, as a SO2 sorbent, has a catalytic effect on NH3 oxidation to NO,15-18 NH3 oxidation reactivity could be greatly suppressed by H2O19,20 and SO2.21,22 Lee et al.21 investigated the effect of sulfated limestone on NO reduction by NH3 in the presence of excess O2 at 827 °C. The results showed that sulfated limestone had a catalytic effect on NO reduction. A 52% NO reduction was observed with a reaction time of 60 ms in their experiments. However, similar fixed-bed reactor experiments that were performed by Iisa et al.23 at 750 °C showed that sulfated limestone had no obvious effect on NO reduction. The reason of this difference was unclear. Also, the NH3 selectivity for NO reduction to N2 and the formation of other byproducts (NO2 or N2O) was not reported in the study of Lee et al.21 The objective of this study is to further investigate the effect of sulfated CaO on NO reduction by NH3 in the temperature window of 700-850 °C. 2. Experimental Section 2.1. Apparatus. The experiments were conducted in a fixedbed quartz reactor. A schematic diagram of the experimental apparatus is shown in Figure 1. The flow rate of each cylinder gas was controlled by a mass flow controller (MFC) to obtain the desired inlet gas concentrations. The typical reactant gas concentration used in this study was as follows: NO, 500 ppm; NH3, 500 ppm; O2, 3%; and N2, balance. The total flow rate of the inlet gas mixture was 1000 mL/min (STP). (15) Shimizu, T.; Tachiyama, Y; Fujita, D.; Kumazawa, K.; Wakayama, O.; Ishizu, K.; Kobayashi, S.; Shikada, S.; Inagaki, M. Energy Fuels 1992, 6, 753–757. (16) Hayhurst, A. N.; Lawrence, A. D. Combust. Flame 1996, 105, 511– 527. (17) Zijlma, G. J.; Jensen, A. D.; Johnsson, J. E.; van den Bleek, C. M. Fuel 2002, 81, 1871–1881. (18) Li, T. J.; Zhuo, Y. Q.; Chen, C. H.; Xu, X. C. In Proceedings of the 6th International Symposium on Coal Combustion, Wuhan, China, 2007; pp 533-537. (19) Zijlma, G. J.; Jensen, A.; Johnsson, J. E.; van den Bleek, C. M. Fuel 2000, 79, 1449–1454. (20) Shimizu, T.; Hasegawa, M.; Inagaki, M. Energy Fuels 2000, 14, 104–111. (21) Lee, Y. Y.; Soares, S. M. S.; Sekthira, A. In Proceedings of the 9th International Conference on Fluidized-Bed Combustion, New York, 1987; pp 1184-1187. (22) Zijlma, G. J.; Jensen, A. D.; Johnsson, J. E.; van den Bleek, C. M. Fuel 2004, 83, 237–251. (23) Iisa, K.; Salokoski, P.; Hupa, M. In Proceedings of the 11th International Conference on Fluidized Bed Combustion, New York, 1991; pp 1027-1033.
Li et al. The reactor was similar to that used by Dam-Johansen et al.24 for heterogeneous reaction. Both ends of the outer quartz tube were sealed by silica-gel plugs. The bed material could be placed on the sintered porous quartz disk, and the inner tube was then put back in place without significantly changing the temperature profile. The reactor was placed in an electrically heated furnace capable of maintaining a zone (∼20 cm long) of constant reaction temperature. NH3 and NO were separated from O2 and were directly injected into the bed to reduce the early SNCR reaction, because the SNCR effect might have an important role when temperature was >750 °C.25 The quartz disk was located near the bottom section of the constant temperature zone. The main stream and NH3 injection stream could be preheated to the desired temperatures. Below the porous disk, the reactor tube size was decreased and the temperature dropped sharply to reduce the homogeneous reactions. Detailed information of the reactor assembly was shown in Figure 2. 2.2. Gas Analysis. The concentrations of NO, NO2, N2O, NH3, and SO2 could be continuously monitored by a Fourier transform infrared (FTIR) spectrometer (Thermo Nicolet Corporation, Model NEXUS670) aided by a MCT detector with liquid N2 cooling and a 2-m gas cell. The gas-cell infrared (IR)-transparent windows were made of ZnSe (spectral range of 4000-650 cm-1). The resolution chosen was 0.5 cm-1.The FTIR spectrometer had been carefully calibrated beforehand and had been verified and used in our previous work18,26 for multicomponent gas measurement. The gas-cell temperature was controlled to be constant at 150 °C. The pipeline between the outlet of the quartz reactor and the inlet of the FTIR gas cell was heated to 90 °C by heating tape to prevent the potential reaction among the components of the gas mixtures. The measurement accuracy for a single gas component is estimated to be (2%. However, when applied to a simultaneous monitoring of multiple gas species, the measurement accuracies are believed to be (2% (NO), (3% (NH3), and (3% (SO2), respectively. NO conversion (XNO), NH3 conversion (XNH3), and the apparent selectivity of NH3 for NO reduction to N2 (SNO) were defined as follows:
(
XNO (%) ) 1 -
out CNO in CNO
)
( )
XNH3 (%) ) 1 -
SNO (%) )
out CNH 3
in CNH 3
in out CNO - CNO in out CNH - CNH 3 3
× 100
(1)
× 100
(2)
× 100
(3)
in and Cout are the respective NO concentrations (given where CNO NO in parts per million (ppm)) at the inlet and outlet of the quartz in out reactor, and CNH 3 and CNH3 are the respective NH3 concentrations (given in ppm) at the inlet and outlet of the quartz reactor. 2.3. Experimental Procedures. The empty tube blank experiment was first conducted. In blank tests, the NO and NH3 conversions were 90% in this temperature window. It can be concluded that, with increasing extent of CaO sulfation, although NH3 conversion was strongly inhibited, the NH3 selectivity toward NO reduction to N2 was actually enhanced. Figure 5d shows the effect of sulfated CaO on the byproduct (NO2 and N2O) formation during NO reduction experiments. In blank tests, the N2O concentration was below the detection limit of the FTIR spectrometer (1.5 ppm). A trace of N2O (
