Article pubs.acs.org/IECR
Experimental and Kinetic Study on the Influence of Iron Oxide on the Selective Noncatalytic Reduction DeNOx Process Shi-Long Fu, Qiang Song,* and Qiang Yao Key Laboratory of Thermal Science and Power Engineering of Ministry of Education, Department of Thermal Engineering, Tsinghua University, 100084 Beijing, China S Supporting Information *
ABSTRACT: The influence of Fe2O3 on the SNCR deNOx process was investigated in a fixed-bed reactor. The experimental results demonstrated that Fe2O3 has an inhibitory effect on the SNCR deNOx process that is more notable at low temperatures. Interactions between Fe2O3 and NH3, Fe2O3-catalyzed NH3 oxidation, and NO reduction by NH3 were studied experimentally. In the absence of O2, Fe2O3 was reduced by NH3 to form Fe, which then catalyzed NH3 decomposition to N2 and H2. Fe2O3 also catalyzed NO reduction by NH3. In the presence of O2, Fe2O3 mainly catalyzed NH3 oxidation. All reactions showed first-order characteristics with respect to NH3, which indicates that the adsorption of NH3 and its dissociation into −NH2 on the surface of Fe2O3 is the rate-controlling step in these processes. The presence of O2 and NO affected the selectivity of further −NH2 reaction. A kinetic model was developed, and the simulation results agreed well with the experimental data.
1. INTRODUCTION Nitrogen oxides (NOx) are well-known hazardous environmental pollutants. In China, the cement and metallurgical industries are significant sources of NOx emissions, making up over 15% of the total NOx emissions of the country.1,2 Stricter emission standards will be enacted by the Chinese government in the near future; hence, all cement kilns and blast furnaces in the country must apply appropriate deNOx technologies. European and American cement kilns have widely installed built-in selective noncatalytic reduction (SNCR) deNOx systems, and several cement kilns have installed selective catalytic reduction (SCR) deNOx systems.3 However, only a few blast furnaces feature deNOx devices, and these devices mainly use SCR systems. Considering cost and performance, SNCR is expected to be the most suitable deNOx technology for cement kilns that can also be applied to blast furnaces with some modification. The SNCR deNOx process has been well studied, and this technology has been relatively well developed for boilers.4 Temperature, residence time, flue-gas composition, reducing agent, and mixing degree are all important factors influencing the SNCR process.5 Based on the SNCR deNOx mechanism and flow model, the SNCR process for a designated boiler can be well simulated and designed to achieve optimal performance.6 However, SNCR processes applied in cement kilns show diverse deNOx performance and deNOx efficiencies ranging from 15% to 80%.7 The high concentration of active particulates in flue gas differentiates the SNCR process in cement kilns from that in boilers. A good understanding of the influence of active particulates on the SNCR process will thus help improve and optimize SNCR designs for cement kilns and blast furnaces. About 60% of the particles in cement kilns are Ca-based particles. Ca-based particles catalyze the oxidation of the reducing agent and inhibit the SNCR deNOx process.8,9 Fe2O3 makes up about 5% of the particulates in cement kilns and a higher proportion in blast furnaces. As Fe2O3 is more active than Ca-based particulates, the influence of Fe2O3 on the SNCR © 2014 American Chemical Society
deNOx process must be clarified to support the application of the SNCR deNOx process in the cement and metallurgical industries. The SNCR deNOx process involves reactions among NH3, NO, and O2. The influence of Fe2O3 on the NH3 + NO + O2 reaction has been mainly studied for SCR at temperatures below 773 K. Long and Yang10 found that the deNOx efficiency of Fe/ ZSM-5 could be as high as 100% at temperatures between 673 and 773 K and a gas hourly space velocity of 460000 h−1. Frey et al.11 and Klukowski et al.12 found that Fe/HBEA catalysts have higher deNOx efficiencies than Fe/ZSM catalysts because of the larger pore size of the former compared with that of the latter. Yang et al.13 and Long and Yang10,14 studied the conversion of NH3 on the surface of Fe2O3 and found that NH3 is first adsorbed and then dissociated to −NH2. NO reacts with adsorbed oxygen to form NO2, and −NH2 reacts with NO2 to form N2. Lietti et al.15 used Fourier transform infrared (FTIR) spectroscopy and found that −NH2 production is the rate-controlling step in the conversion of NH3 on the surface of Fe2O3. Angeles Larrubia and Ramis16 and Ramis et al.17 studied the influence of Fe2O3 on the NO reduction of NH3 using FTIR spectroscopy and confirmed a fast reaction, shown in eq 1, based on the sudden disappearance of −NH2 in the presence of NO NH 2 + NO → NH 2NO → N2 + H 2O
(1)
Klukowski et al.12 confirmed that the mechanism of Fe/HBEA catalysts involves the adsorption and reaction of NH3 and NO on neighboring Fe3+ sites. Willey et al.18 pointed out that NH3 dissociates H atoms after adsorption on Fe2O3 and that Fe2O3 is reduced by H atoms. O2 then oxidizes iron oxides of low valence to Fe2O3 to recover activity. Received: Revised: Accepted: Published: 5801
January 9, 2014 March 15, 2014 March 19, 2014 March 19, 2014 dx.doi.org/10.1021/ie500109r | Ind. Eng. Chem. Res. 2014, 53, 5801−5809
Industrial & Engineering Chemistry Research
Article
Figure 1. Sketch of the fixed-bed reactor system.
Hayhurst and co-workers19,20 and Apostolescu et al.21 studied the reaction between Fe and NO at temperatures ranging from 973 to 1173 K. They found that Fe could be oxidized by NO and that this reaction was first-order with respect to NO. Gradon and Lasek22 studied NO reduction by Fe and found that 4.5% O2 can rapidly reduce the reaction rate to zero. Koebel et al.23 and Singoredjo et al.24 found that the oxidizing ability of NO is weak and that the reaction rate between NH3 and NO is low in the absence of O2. Two mechanisms have been presented to describe NH3 conversion during the SCR reaction on the surface of Fe2O3: the Langmuir−Hinshelwood (L−H) mechanism and the redox mechanism. Yang et al.13 and Long and Yang10,14 found that the reaction between NH3 and NO catalyzed by Fe2O3 follows the L−H mechanism. Lei25 studied the effects of Fe2O3 on NO reduction by NH3 in the presence of O2 and found that the overall reaction follows the redox mechanism. In the absence of O2, the catalytic oxidation of NH3 occurs. Lissianski and co-workers26,27 found that injecting Fe, Fe2O3, and Fe3O4 into the reburning zone increases deNOx efficiency. For the same amount of Fe, Fe2O3 was found to exhibit the best effects. Their study indicated that Fe2O3 has a high activity at high temperatures. Hence, Fe2O3 is very likely to have a catalytic effect on the SNCR deNOx process. The existing research has mainly focused on the activity of Febased catalysts in the SCR process, and the studied temperatures have always been below 773 K. The influence of Fe2O3 on the SNCR deNOx process in the temperature range of 1123−1323 K has yet to be reported. In the present work, the influence of Fe2O3 on the SNCR deNOx process was studied using a fixedbed reactor in the temperature range of 923−1373 K. The reaction mechanism of Fe2O3 was analyzed, and a kinetic model
was developed. The results obtained will be helpful for the improved application of SNCR in the cement and metallurgical industries.
2. EXPERIMENTAL SECTION 2.1. Samples. Analytically pure Fe2O3 and Fe samples were used in this work to avoid interferences presented by impurities. The samples were dried at 393 K for 5 h before use. The particle size of Fe2O3 was 63.7 μm. 2.2. Setup and Gas Analysis. The fixed-bed reaction system is shown in Figure 1. A quartz reactor consisting of external and internal sections was used in the experiments. The internal section was 25 mm in length and 10 mm in diameter. Fe2O3 particles were loaded on quartz wool at the bottom of the internal section. The quartz reactor was inserted into an electric furnace equipped with a digital temperature controller. The internal section was within the constant-temperature zone. The model gas used in the experiments consisted of NH3, NO, and O2, with N2 as the balance, and the total gas flow rate was 500 N mL/min. The gas pipe was made of polytetrafluoroethylene (PTFE) to reduce the influence of NH3 adsorption on the reaction. Reactant gases were separately fed into the constant-temperature zone of the reactor. NH3 and NO were fed through the two top inlets, whereas O2 was fed through the bottom inlet. The inlet gases were radially injected to minimize the effects of diffusion. The exhaust gas was analyzed using an FTIR instrument (Nicolet 6700) calibrated by the method presented by Li28 to avoid the interference of H2O and CO2 in the optical path. The measurement error was
