NOx removal by selective noncatalytic reduction with urea solution in a fluidized bed reactor

A fluidized bed reactor has been developed to overcome the plugging problem of urea injection by employing a sparger rather than nozzles in the SNCR process for simultaneous removal of SO₂ and NO_x. In a developed fluidized bed reactor, the optimum temperature to remove NO_x is shifted to lower values, the reaction temperature window is widened with the presence of CO in flue gas, and NO conversion is higher than that in a flow reactor. The optimum amount of urea injection in the reactor is found to be above 1.2 based on the normalized stoichiometric molar ratio (NSR) with respect to NO conversion. In the simultaneous removal of SO₂/NO, conversions of SO₂ and NO reach 80–90%, nearly the same values for the individual removal of SO₂ and NO above 850 ‡C.     

CO含量对烟气选择性非催化还原反应的影响

分别以氨水和尿素溶液作为还原剂,在管式反应器上详细研究了CO含量对烟气SNCR（选择性非催化还原）脱硝性能的影响。结果表明,CO含量增加导致：（1）SNCR脱硝温度窗口和最佳脱硝温度向低温方向移动,同时脱硝温度窗口宽度变窄、最大脱硝效率降低,尤其是以氨水作为还原剂时更为突出;（2）N₂O排放的峰值升高;当以尿素溶液作还原剂时,N₂O排放温度窗口主要向低温方向扩展;当以氨水作还原剂时,N₂O排放温度窗口向低温和高温两个方向扩展;（3）NH₃及HNCO残留量曲线向低温方向移动。维持相同的φ（CO）/φ（urea）,氨氮比增加将导致最佳脱硝温度略微降低。氨氮比或氧含量的变化对NO_x、N₂O、NH₃及HNCO的排放趋势没有影响。     

Mechanism of NO decomposition over Cu-ZSM-5

In the preceding Letter Shelef [1] has proposed a mechanism for NO decomposition involving coordinatively unsaturated Cu²⁺ sites on which NO molecules are chemisorbed in the gem-dinitrosyl form. At reaction temperature this complex is supposed to decompose into N₂ and O₂ without involving a redox process. That such a process is feasible has been pointed out by Moser [2]. Shelef cited several reasons in support of this view and others that have led him to think that a cyclic redox mechanism is not operative. These arguments are countered herein and some new data are presented showing the infrared spectra of surface species recorded under in situ reaction conditions.On leave from Central Research Institute for Chemistry, Hungarian Academy of Sciences, H-1525 Budapest, Hungary.     

The role of NO2 in the reduction of NO by hydrocarbon over Cu-ZrO2 and Cu-ZSM-5 catalysts

The reduction of NO _x with propene or propane in the presence of 1 or 4% O₂ was studied at low conversions over a 7.4 wt% Cu-ZrO₂ and a 3.2 wt% Cu-ZSM-5 catalyst. The rates of N₂ production were compared in experiments using only NO or a mixture of NO and NO₂ in the feed. They were also compared with the rates of NO₂ reduction to NO under the same conditions, and of NO oxidation to NO₂ in the absence of hydrocarbon. It was found that the reduction of NO₂ to NO was very fast, consistent with literature data. The data were best explained by a reaction scheme in which the hydrocarbon was activated primarily by reaction with adsorbed NO₂ to form an adsorbed oxidized N-containing hydrocarbon intermediate, the reaction of which with NO was the principal route to produce N₂ under lean NO _x conditions.On leave from State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, PR China.     

Investigations of the reduction of NO to N2 by reaction with Fe

The paper presents the results of the investigation of nitric oxide reduction by its reaction with metallic iron, within the temperature range of 750-1200 °C. Experiments were carried out in a one-dimensional tubular flow reactor externally heated in an electrical furnace. The spherical and cylindrical iron samples of well-defined and fixed surface area were placed in the center of the reactor. Mixtures NO/N₂, NO/O₂/N₂, NO/CO₂/N₂ of different molar compounds fractions were continuously fed into the reactor. The surfaces and cross-sections of partly oxidized iron samples were observed using a scanning microscope. The rate of nitric oxide reduction in the first stage of iron oxidization was measured. The obtained values lie between those presented in the literature and measured in the fluidized bed reactor and thermogravimetrically. A very strong influence of oxygen but a very weak one of carbon dioxide on the efficiency of NO reduction was observed within the investigated range of temperature.     

Low cost catalytic sorbents for NOx reduction. 2. Tests with no reduction reactives

Coal, char and activated char doped with model vanadium compounds (V₂O₅ and NH₄VO₃) and petroleum coke ash, PCA, (main metal components: V, Fe and Ni) have been tested as catalytic sorbents for NO reduction without the addition of a reduction reactive. The sorbents prepared have shown to be active for NO reduction at temperatures higher than 350 °C. The most efficient sorbents are those obtained from unactivated chars and doping with model vanadium compounds or PCA does not upgrade significantly their behaviour. On the other hand, for the samples prepared from activated char, an improvement of the reduction efficiency is observed after impregnation with model vanadium compounds or PCA.SSA of the samples does not play a relevant role on the NO reduction efficiency while surface chemistry significantly affect the samples’ behaviour: higher the CO₂/CO ratio determined by TPD, higher the NO reduction efficiency of the sample.Slightly higher NO conversions are observed for the samples loaded with pure compounds but PCA is perspective for producing catalyst doped activated carbons.     

A USM turbulence-chemistry model for simulating NOx formation in turbulent combustion

A unified second-order moment (USM) turbulence-chemistry model for simulating NO_x formation in turbulent combustion is proposed. All the correlations, including the correlation of the reaction-rate coefficient fluctuation with the concentration fluctuation, are closed by the transport equations in the same form. This model abandons the series expansion approximation of the exponential term or the approximation of using a product of two single-variable PDFs instead of a joint PDF. The proposed model is used to simulate methane-air jet diffusion combustion and NO_x formation. The combustion prediction results are compared with those using the EBU-Arrhenius model and other two versions of the second-order moment model. The NO_x prediction results are compared with those using the pure presumed PDF model. Validation of predictions using the experimental data given by the Sandia National Laboratory, USA indicates that the proposed model gives better results than other models, and it is much economical than other refined models.     

Low cost catalytic sorbents for NOx reduction. 1. Preparation and characterization of coal char impregnated with model vanadium components and petroleum coke ash

Spanish coal, char and activated char doped with model vanadium components (V₂O₅ and NH₄VO₃) and petroleum coke ash (enriched in V, Fe, and Ni) were prepared and characterized as potential catalytic sorbents for NO_x reduction. The phase-mineral and chemical composition, content and behavior (capture, retention, distribution, and redistribution) of transition metals, as well as morphogenesis, surface area, acid-base properties, surface active sites and oxidation-reduction transformations of the catalytic sorbents were characterized. It was found that minerals and phases such as anhydrite, calcite, clay minerals, pyrite, pyrrhotite, magnetite and fusinoid-type ingredients have a leading role for the behavior of loaded transition metals. Some original (pyrite, jarosite, shcherbinaite, coulsonite, trevorite, Ni oxide) and newly formed (pyrrhotite, magnetite, wuestite, hematite, paramontroseite, karelianite) Fe, V and Ni minerals in the catalytic sorbents are perspective redox indicators for the physicochemical conditions in such complex system. The data indicate that the V-Fe-Ni containing minerals dispersed onto and into the carbon support may be the most active catalytic sites. The preparation procedure that could provide the most favorable conditions for the production of effective and low cost catalytic sorbents for NO_x reduction is also described.     

Steam effect on NO x reduction over lean NO x trap Pt–BaO/Al2O3 model catalyst

The effect of steam on NO _x reduction over lean NO _x trap (LNT) Pt–Ba/Al₂O₃ and Pt/Al₂O₃ model catalysts was investigated with reaction protocols of rich steady-state followed by lean–rich cyclic operations using CO and C₃H₈ as reductants, respectively. Compared to dry atmosphere, steam promoted NO _x reduction; however, under rich conditions the primary reduction product was NH₃. The results of NO _x reduction and NH₃ selectivity versus temperature, combined with temperature programmed reduction of stored NO _x over Pt–BaO/Al₂O₃ suggest that steam causes NH₃ formation over Pt sites via reduction of NO _x by hydrogen that is generated via water gas shift for CO/steam, or via steam reforming for C₃H₈/steam. During the rich mode of lean–rich cyclic operation with lean–rich duration ratio of 60 /20 s, not only the feed NO, but also the stored NO _x contributed to NH₃ formation. The NH₃ formed under these conditions could be effectively trapped by a downstream bed of Co²⁺ exchanged Beta zeolite. When the cyclic operation was switched into lean mode at T < 450 °C, the trapped ammonia in turn participated in additional NO _x reduction, leading to improved NO _x storage efficiency.     

Effect of primary amine hydrocarbon chain length for the selective catalytic reduction of NOx from diesel engine exhaust

The effect of increasing primary amine hydrocarbon chain length on the SCR of NO_x from diesel engine exhaust was investigated and compared to ammonia. Methylamine (CH₃NH₂), ethylamine (CH₃CH₂NH₂), propylamine (CH₃CH₂CH₂NH₂) and butylamine (CH₃CH₂CH₂CH₂NH₂) were tested using a 12 cell mini core NH₃ - SCR catalyst cut from a 400 cpsi block. There is a steady decrease in NO_x conversion as the length of the hydrocarbon chain increases (from 50% for methylamine to 26% for butylamine). For the same number of carbons in the amine, primary amines are more active reductants than methyl substituted secondary or tertiary amines. For example, ethylamine (NO_x conversion of 45%) is more active than dimethylamine (NO_x conversion of 34%).Since the amines are reactive in the gas phase in the temperature range of diesel engine exhaust, gas phase conversions were estimated by replacing the mini core SCR catalyst with an equivalent length of quartz beads. There was no smooth transition in gas phase NO and NO_x conversions with increasing hydrocarbon chain length. The results suggest a different mechanism for gas phase reactions depending on the nature of the amine.     

NO2 formation and its effect on the selective catalytic reduction of NO over Co/ZSM-5

Catalytic performance of Co/ZSM-5 with different metal loadings and of HZSM-5 was compared in the NO + O₂, C₃H₈ + O₂, and NO + C₃H₈ + O₂ reactions. It was found that Co/ZSM-5 catalysts containing only isolated cobalt ions in cationic positions are inactive in NO₂ formation. To achieve appreciable NO conversion in the SCR process over these catalysts higher reaction temperatures are required. These results make it possible to suggest that NO₂ formation is not a prerequisite for the SCR of NO with hydrocarbons over Co/ZSM-5. With increasing Co loading, however, Co/ZSM-5 begins to exhibit activity in NO₂ formation. This is explained by the formation of cobalt oxide particles on the zeolite carrier, which are active in the NO₂ formation. Increase in NO₂ formation strongly enhances catalytic activity in SCR of NO at lower reaction temperatures. Comparison of the C₃H₈ conversion in the C₃H₈ + O₂ and C₃H₈ + O₂ + NO reactions provides evidence that NO₂ activates hydrocarbon molecules resulting in the formation of the reaction intermediates of the SCR process.On leave from N.D. Zelinskii Institute of Organic Chemistry, Leninskii Pr. 47, Moscow, Russia.     

A reduced NOx reaction model for pulverized coal combustion under fuel-rich conditions

A reduced NO_x reaction model was developed for analysis of industrial pulverized coal firing boilers. The model was developed from experiments of laminar premixed combustion under a variety of stoichiometric ratios, burning temperatures, coal ranks (from sub-bituminous coal to anthracite) and particle diameters. Calculations agreed with experimental results for NO_x and nitrogen species (NH₃ and HCN), if the model assumed that the hydrocarbon radicals were formed not only from pyrolysis of volatile matter, but also from char oxidation and gasification. The presence of hydrogen in char at the final burnout stage supported this assumption. NO_x reduction by hydrocarbon radicals was the most important reaction in high temperature (>1500 K), fuel-rich, char combustion regions. NO_x reduction from nitrogen species was sensitive to peak NO_x concentration in volatile combustion regions, but NO_x emission downstream had little influence from the peak NO_x concentration. The heterogeneous reaction between char and NO_x was important for fuel-lean or low-temperature conditions.     

An acid catalyzed step in the catalytic reduction of NO x to N2

Contact of adsorbed ammonium nitrite, NH₄NO₂, with HCl vapor or a solid acid such as the zeolite HY, significantly lowers the temperature of its decomposition to N₂ + H₂O. Protonated NH₄NO₂ decomposes at room temperature. The decomposition of ammonium nitrite is one of the steps in the catalytic reduction of NO _x with ammonia or other reductants.     

Application of the thermal DeNOx process to diesel engine DeNOx: an experimental and kinetic modelling study

Diesel DeNO_x experiments have been conducted using the selective noncatalytic ‘thermal DeNO_x’ process in a diesel fuelled combustion-driven flow reactor which simulated a single cylinder (966 cm³) and head equipped with a water-cooling jacket and an exhaust pipe. NH₃ was directly injected into the cylinder to reduce NO_x emissions. A wide range of air/fuel ratios (A/F=20-40) was selected for NO_x reduction where an initial NO_x of 530 ppm was usually maintained with a molar ratio (β=NH₃/NO_x) of 1.5.The results indicate that a 34% NO_x reduction can be achieved from the cylinder injection in the temperature range, 1100-1350 K. Most of the NO_x reduction occurs within the cylinder and head section (residence time<40 ms), since temperatures in the exhaust are too low for additional NO_x reduction. Under large gas quenching rates, increasing β values (e.g. 4.0) substantially increase the NO_x reduction up to 60%, which is comparable with those achieved under isothermal conditions. Experimental findings are analysed by chemical kinetics using the Miller and Bowman mechanism including both N/H/O species and CO/hydrocarbon reactions to account for CO/UHC oxidation effects, based on practical nonisothermal conditions. Comparisons of the kinetic calculations with the experimental data are given as regards temperature characteristics, residence time and molar ratio. In addition, the effects of CO/UHC and branching ratio (α=k₁/(k₁+k₂)) for the reaction NH₂+NO=products are discussed in terms of NO reduction features, together with practical implications.     

Adsorbate-assisted NO decomposition in NO reduction by C3H6 over Pt/Al2O3 catalysts under lean-burn conditions

The rates and product selectivities of the C₃H₆-NO-O₂ and NO-H₂ reactions over a Pt/Al₂O₃ catalyst, and of the straight, NO decomposition reaction over the reduced catalyst have been compared at 240C. The rate of NO decomposition over the reduced catalyst is seven times greater than the rate of NO decomposition in the C₃H₆-NO-O₂ reaction. This is consistent with a mechanism in which NO decomposition occurs on Pt sites reduced by the hydrocarbon, provided only that at steady state in the lean NO _x reaction about 14% of the Pt sites are in the reduced form. However, the (extrapolated) rate of the NO-H₂ reaction at 240C is about 10⁴ times faster than the rate of the NO decomposition reaction thus raising the possibility that NO decomposition in the former reaction is assisted by H_ads. It is suggested that adsorbate-assisted NO decomposition in the C₃H₆-NO-O₂ reaction could be very important. This would mean that the proportion of reduced Pt sites required in the steady state would be extremely small. The NO decomposition and the NO-H₂ reactions produce no N₂O, unlike the C₃H₆-NO-O₂ reaction, suggesting that adsorbed NO is completely dissociated in the first two cases, but only partially dissociated in the latter case. It is possible that some of the associatively adsorbed NO present during the C₃H₆-NO-O₂ reaction may be adsorbed on oxidised Pt sites.     

Kinetic modeling of storage and reduction with different reducing agents (CO, H2, and C2 H4) on a Pt–Ba/-Al2O3 catalyst in the presence of CO2 and H2O

In this paper a global reaction kinetic model is used to understand and describe the NO_x storage/reduction process in the presence of CO₂ and H₂O. Experiments have been performed in a packed bed reactor with a Pt–Ba/γ-Al₂O₃ powder catalyst (1 wt% Pt and 30 wt% Ba) with different lean/rich cycle timings at different temperatures (200, 250, and ) and using different reductants (H₂, CO, and C₂H₄). Model simulations and experimental results are compared. H₂O inhibits the NO oxidation capability of the catalyst and no NO₂ formation is observed. The rate of NO storage increases with temperature. The reduction of stored NO with H₂ is complete for all investigated temperatures. At temperatures above , the water gas shift (WGS) reaction takes place and H₂ acts as reductant instead of CO. At , CO and C₂H₄ are not able to completely regenerate the catalyst. At the higher temperatures, C₂H₄ is capable of reducing all the stored NO, although C₂H₄ poisons the Pt sites by carbon decomposition at . The model adequately describes the NO breakthrough profile during 100 min lean exposure as well as the subsequent release and reduction of the stored NO. Further, the model is capable of simulating transient reactor experiments with 240 s lean and 60 s rich cycle timings.     

Influence of oxygenated additives on the NOxOUT process efficiency

The reduction of nitric oxide by urea has been investigated experimentally in the presence of various additives using a well-stirred laboratory reactor operated from 900 to 1450 K at atmospheric pressure. The influence of injecting various oxygenated organic compounds on the maximum NO reduction has been discussed, as well as the possibility of widening the range of temperature where the NO_xOUT process is effective. All the species investigated have been able to enlarge the width of the temperature window where NO abatement occurs without compromising significantly the maximum efficiency of the process.     

Low-temperature selective catalytic reduction of NO with NH3 based on MnOx-CeOx/ACFN

MnO _x -CeO _x /ACFN were prepared by the impregnation method and used as catalyst for selective catalytic reduction of NO with NH3 at 80°C-150°C. The catalyst was characterized by N₂-BET, scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FT-IR). The fraction of the mesopore and the oxygen functional groups on the surface of activated carbon fiber (ACF) increased after the treatment with nitric acid, which was favorable to improve the catalytic activities of MnO _x -CeO _x /ACFN. The experimental results show that the conversion of NO is nearly 100% in the range 100°C-150°C under the optimal preparation conditions of MnO _x -CeO _x /ACFN. In addition, the effects of a series of performance parameters, including initial NH₃ concentration, NO concentration and O₂ concentration, on the conversion of NO were studied. __________ Translated from Chemical Industry and Engineering Progress, 2007, 27(1): 87–91 [译自: 化工进展]