Energy & Fuels 2009, 23, 1545–1550
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Characteristics of HCN Removal Using CaO at High Temperatures Houzhang Tan, Xuebin Wang,* Congling Wang, and Tongmo Xu State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong UniVersity, Shaanxi 710049, China ReceiVed October 29, 2008. ReVised Manuscript ReceiVed January 4, 2009
Experimental investigation on the removal of hydrogen cyanide (HCN) using calcium oxide (CaO) was carried out in a fixed bed reactor at temperature ranging from 300 to 1173 K, and the original HCN was produced during the pyrolysis of pyridine. Effects of temperature, volume space velocity, and initial HCN concentration on HCN removal were discussed. The results of temperature-programmed experiments show that temperature is the main factor affecting HCN removal. With the formation of CO, HCN starts to decrease from 473 K, and remains unchanged from 673 to 873 K. At 873 K, there is a further decrease in HCN without CO formation, and when temperature is higher than 1023 K, HCN is removed completely. In the isothermal experiments, CaCN2 was detected at 723 K, but at higher temperatures of 923 and 1123 K, there was no CaCN2 in the solid residues, and the nitrogen in the removed HCN was equal to that in the formed N2. This indicates that at a lower temperature CaO is consumed to remove HCN, CaO + 2HCN f CaCN2 + CO + H2; but at a higher temperature, CaO acts as a catalyst for HCN removal, 2CiHj + 2HCN f N2 + (j + 1 k)H2 + 2Ci+1Hk. The investigation on the removal efficiency shows that there is a critical temperature and a critical volume space velocity at which the HCN removal efficiency is able to reach up to 100%.
Introduction Emission of nitrogen oxides and sulfur oxides from the coal combustion system continues to be a significant threat to the environment.1 Calcium-based materials are commonly proposed and applied to realize the high efficiency desulphurization in fluidized-bed combustion as well as in pulverized coal combustion.2 Meanwhile, considerable investigations have indicated that calcium also plays some role in forming nitrogen oxides.3-8 Experimental research has found that calcium causes a significant N2O decrease in fluidized-bed combustion,9,10 while others argue that the influence is little.3,11 Even under the pulverized furnace condition, the injection of calcium materials is also proposed to affect NO emission.5-8,12,13 Despite considerable investigations, the effect of calcium on the emission of nitrogen oxides from coal combustion is still confusing and is generally * To whom correspondence should be addressed. E-mail:
[email protected]. (1) Beer, J. M. Prog. Energy Combust. Sci. 2000, 26, 301–327. (2) Hu, G.; Dam-Johansen, K.; Wedel, S.; Peter Hansen, J. Prog. Energy Combust. Sci. 2006, 32, 386–407. (3) Hayhurst, A. N.; Lawrence, A. D. Combust. Flame 1996, 105, 511– 527. (4) Li, Y.; Loh, B. C.; Matsushima, N.; Nishioka, M.; Sadakata, M. Energy Fuels 2002, 16, 155–160. (5) Patsias, A. A.; Nimmo, W.; Gibbs, B. M.; Williams, P. T. Fuel 2005, 84, 1864–1873. (6) Zhong, B. J.; Tang, H. Combust. Flame 2007, 149, 234–243. (7) Nimmo, W.; Patsias, A. A.; Hall, W. J.; Williams, P. T. Ind. Eng. Chem. Res. 2005, 44, 4484–4494. (8) Guo, F.; Hecker, W. C. Symp. (Int.) Combust. 1996, 26, 2251–2257. (9) Sasaoka, E.; Sada, N.; Hara, K. I.; Uddin, M. A.; Sakata, Y. Ind. Eng. Chem. Res. 1999, 38, 1335–1340. (10) Shen, B. X.; Mi, T.; Liu, D. C.; Feng, B.; Yao, Q.; Winter, F. Fuel Process. Technol. 2003, 84, 13–21. (11) Wo´jtowicz, M. A.; Pels, J. R.; Moulijn, J. A. Fuel Process. Technol. 1993, 34, 1–71. (12) Nimmo, W.; Patsias, A. A.; Hampartsoumian, E.; Gibbs, B. M.; Williams, P. T. Fuel 2004, 83, 149–155. (13) Verdone, N.; De Filippis, P. Chemosphere 2004, 54, 975–985.
explained to stem from the catalysis of calcium, while the mechanism is not fully demonstrated. As compared to the abundant research on the direct effect of calcium on nitrogen oxides, less attention has been directed to the mechanisms related to the effect of calcium on hydrogen cyanide (HCN), which is recognized to be a predominant intermediate precursor of nitrogen oxides in the transformation of nitrogen in coal.14-16 The general scheme for the role of HCN in coal nitrogen transformation is given in Figure 1. Clearly, the demonstration on the reaction mechanism between calcium and HCN is helpful in comprehending how calcium affects the emission of nitrogen oxides. Experimental research has indicated that, in the process of coal pyrolysis or gasification, the addition of calcium oxide (CaO) inhibits HCN formation but promotes the nitrogen in coal conversion to N2 efficiently.17-25 Meanwhile, as massive HCN from the coal chemical industry results in heavy toxicity, further study on the characterization of HCN (14) Dagaut, P.; Glarborg, P.; Alzueta, M. U. Prog. Energy Combust. Sci. 2008, 34, 1–46. (15) Glarborg, P.; Jensen, A. D.; Johnsson, J. E. Prog. Energy Combust. Sci. 2003, 29, 89–113. (16) Hill, S. C.; Douglas Smoot, L. Prog. Energy Combust. Sci. 2000, 26, 417–458. (17) Jensen, A.; Johnsson, J. E.; Johansen, K. D. AIChE J. 1997, 43, 3070–3084. (18) Ohtsuka, Y.; Watanabe, T.; Asami, K.; Mori, H. Energy Fuels 1998, 12, 1356–1362. (19) Ohtsuka, Y.; Zhiheng, W.; Furimsky, E. Fuel 1997, 76, 1361–1367. (20) Liu, H.; Liu, Y.; Liu, Y.; Che, D. J. Fuel Chem. Technol. 2008, 36, 134–138. (21) Scha¨fer, S.; Bonn, B. Fuel 2002, 81, 1641–1646. (22) Hayashi, J.-I.; Kusakabe, K.; Morooka, S.; Nielsen, M.; Furimsky, E. Energy Fuels 1995, 9, 1028–1034. (23) Mori, H.; Asami, K.; Ohtsuka, Y. Energy Fuels 1996, 10, 1022– 1027. (24) Ohtsuka, Y.; Mori, H.; Nonaka, K.; Watanabe, T.; Asami, K. Energy Fuels 1993, 7, 1095–1096. (25) Tsubouchi, N.; Ohshima, Y.; Xu, C.; Ohtsuka, Y. Energy Fuels 2001, 15, 158–162.
10.1021/ef800935u CCC: $40.75 2009 American Chemical Society Published on Web 02/05/2009
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Tan et al.
Figure 1. General scheme for the role of HCN in the process of coal nitrogen transformation.
Figure 2. Schematic diagram of experimental system for HCN formation and removal.
reduction by CaO may also provide a new approach to solving the problem of HCN pollution. The present study focuses on the characterization of HCN removal using CaO, and the effects of temperature, volume space velocity (SV), and initial HCN concentration are discussed. The original HCN was generated during the pyrolysis of pyridine, gaseous species (CO, HCN, H2, CH4, and N2) were monitored quantitatively by using a gas analyzer coupled with gas chromatography (GC), and the solid residues were detected by Fourier transform infrared spectroscopy (FT-IR). Hopefully, the results will be helpful in elucidating the reaction mechanism between HCN and CaO at high temperatures.
The experiment of HCN removal was carried out in a quartz reactor B, which was 20 mm in inner diameter and 1000 mm in length. An electric furnace with a SiC tube as electrothermal element supplied heat to the reactor and could continuously work at a temperature as high as 1473 K, and the constant-temperature heating zone was longer than 600 mm. A Ni-Cr/Ni-Si thermocouple was used to measure the reaction zone temperature controlled by a SHIMADEN FP93 PID regulator. The precision of temperature control was (2 K. Analytically pure CaO powder, which was squeezed in the mold at 5 MPa for 5 min, then crushed and sieved to 1-1.7 mm, was placed onto an orifice in the middle of the reactor. To characterize the effective time of HCN passing through the CaO layer, the volume space velocity (h-1) is calculated as
Experimental Section HCN Generation and HCN Reduction. In this Article, pyridine was chosen as the source of HCN generation, and pyrolysis of pyridine was performed in a quartz reactor A at 1273 K. As shown in Figure 2, volatile pyridine in the vessel was carried out by the blowing of high-purity argon (>99.999%). To guarantee a constant sample quantity, the flow rate of argon was kept stable at 0.036 m3 h-1 by using a mass flow meter. Before entering reactor B, the gaseous products from the pyridine pyrolysis passed through a filter filled with CaCl2 to remove water steam and residual pyridine from reactor A.
SV )
Qa V
(1)
where Qa is the carrier gas flow rate (m3 h-1) under the actual condition, and V is the stacking volume (m3).
Qa ) Qs ×
T 273
(2)
and
V)
M F
(3)
Characteristics of HCN RemoVal Using CaO
Energy & Fuels, Vol. 23, 2009 1547
Figure 3. Chemical structures of pyridinic-N and pyrrolic-N.
Figure 5. Profile of gaseous species at different temperatures. Table 1. Profiles of Gaseous Species at Three Typical Temperatures concentration of outlet-components/ppm temperature/K HCN-Cin/ppm HCN 723 923 1123
Figure 4. Fluctuation of gaseous species from pyridine pyrolysis.
where Qs is the carrier gas flow rate 0.036 m3 h-1 under the standard condition, M is the stacking mass (kg), and F is the stacking density to be kept constant (680 kg/m3). We can then calculate SV as:
QsF T × SV ) M 273
(4)
The SV is mainly controlled by changing the stacking mass of the CaO layer. The removal rate of HCN is defined as ηHCN, which is the difference between the initial HCN concentration (Cin) and outlet HCN concentration (Cout) of reactor B.
ηHCN )
Cin - Cout × 100% Cin
(5)
Gaseous Products Analysis and Solid Residues Detection. The concentration of HCN, CO, and CH4 in flue gas was detected online by a Helsinki made DX-4000 FT-IR gas analyzer, which has a 1.07 L gas analysis cell with a path of 5 m, resolution of 8 cm-1, response time 1123 K in the process of coal pyrolysis. Furthermore, it is also indicated that Ca may only exist as CaO in the process of N2 formation. Consequently, the presumed reaction eqs 1 and 5 have been verified by quantitative analysis of gas species and FT-IR detection. Conclusion The reaction mechanisms of HCN removal by CaO were studied in a fixed bed, detected by the gas analyzer, and GC coupled with FT-IR. Original HCN was generated in the pyrolysis of pyridine. The effects of temperature, SV, and initial HCN concentration were discussed. The conclusions are summarized as follows: (1) CaO is efficient at removing HCN from the flue gas. Temperature is the most important factor affecting the HCN removal rate, and the HCN removal rate increases with the increase in temperature. Corresponding to a constant initial HCN concentration, there is a critical temperature and a critical SV at which the HCN removal rate can reach up to 100%. (2) In temperature-programmed experiments, with the formation of CO at 473 K, HCN starts to decrease, and remains unchanged from 673 to 873 K. At 873 K, there is a further decrease in HCN, and CH4 also begins to decrease, but there is no CO formation again. When the temperature is higher than 1023 K, HCN has been removed completely. In the isothermal experiments, CaCN2 is detected at 723 K, but at higher temperatures of 923 and 1123 K, there is no CaCN2 formed in the solid residues, and the nitrogen in the removed HCN is equal to that in the formed N2. Results of FT-IR and gas analysis indicate that, at a lower temperature CaO reacts with HCN to form CaCN2 by the route CaO + 2HCN f CaCN2 + CO + H2, transforming the nitrogen in HCN into CaCN2, but at a higher temperature CaO transforms all of the nitrogen in HCN to N2 totally by another catalysis route: 2CiHj + 2HCN f {CaO}N2 + (j + 1 - k)H2 + 2Ci+1Hk. Acknowledgment. The present work was supported by the State Basic Research Development Program (No. 2005CB 221206) and National Key Technology R&D Program in the 11th Five-Year Plan of China (No. 2006BAK02B03). EF800935U
