Maximum NO Reduction Research Articles

Kinetic Simulation of the Promoted Effect of Methanol on the NO<sub>x</sub>OUT Process

Subscript textThe kinetic model for simulating the mechanism of the promoted effect of methanol on the NOxOUT process has been established and it mainly includes the optimal sub-mechanisms respectively for the NOxOUT process and the chemical reaction of methanol. The oxygen concentration does not obviously influence the maximum NO reduction efficiency in the range of 1-6 %, but the temperature window is overall shifted to lower temperature zone with oxygen concentration increased. Meanwhile, the mole ratio of urea to nitric oxide by a factor of 2 should be maintained between 1.5 and 2 from both the efficiency and running cost view. Also, ample residence time of 300/T-400/T s must be guaranteed for the reduction occurring thoroughly. Methanol does not compromise the maximum NO reduction efficiency and broadens the temperature window towards low temperature zone. The promoted mechanism of methanol on the NOxOUT process is the abundant OH formation through the methanol regenerative reaction of CH2OH/CH3O+H2O=CH3OH+OH and methanol should be maintained at 50-100 ppm for an obvious promoted effect. During the co-injection of methanol and urea, the “ammonia slip” is depressed, especially at 1173 K where the promoted effect on NO reduction is obvious, but emission of nitrous oxide is also markedly increased at this temperature.

Effect of operating conditions on NO reduction by acetylene–ethanol mixtures

An experimental and theoretical study of the influence of different operating conditions on NO reduction by acetylene–ethanol mixtures has been carried out. The present investigation includes the evaluation of the impact of adding different amounts of ethanol for NO reduction by an acetylene–ethanol mixture, and the evaluation of the capacity of acetylene–ethanol mixtures to reduce different amounts of NO. The experiments were conducted in a quartz flow reactor at atmospheric pressure in the 775–1375 K temperature range operating under different oxygen concentrations, from fuel-rich to fuel-lean conditions, considering the influence of these operating variables. The experimental results have been simulated and interpreted in terms of a literature detailed gas-phase kinetic mechanism. The present results show that ethanol addition to acetylene shifts the onset for NO consumption to higher temperatures; however, the maximum NO reduction levels reached by pure acetylene oxidation are not significantly modified by its addition. The initial concentrations of NO and oxygen are important parameters affecting NO reduction by acetylene–ethanol mixtures oxidation, since they are closely coupled to the fate of hydrocarbon radicals responsible of NO consumption.

Experimental and Modeling Study of the Effects of Gas Additives on the Thermal DeNO x Process

An experimental study of thermal DeNO x process with different additives was performed in an electricity-heated tubular flow reactor, showing that CO is less effective to lower the optimum temperature than H 2 and CH 4. The maximum NO reduction is lowered with H 2 added, while it is hardly affected by CO or CH 4. The temperature window is widened appreciably with CH 4 added, while it is narrowed slightly by H 2 or CO. The disadvantage of CH 4 is that it causes CO emission due to its incomplete oxidation, and the maximum conversion of CH 4 to CO is more than 50%. In general, the calculation using a detailed chemical kinetic model predicts most of the process features reasonably well. The analysis on reaction mechanism shows that the effects of these additives on NO reduction are achieved principally by promoting the production of ·OH radical.

NO REDUCTION BY SELECTIVE NONCATALYTIC REDUCTION USING AMMONIA-EFFECTS OF ADDITIVES

An experimental study of nitric oxide removal by selective noncatalytic reduction (SNCR) using ammonia as reducing agent has been performed in a semi-industrial reactor over the temperature range 973−1200 K. Several chemical compounds, such as CH4, C2H4, C2H6, CH3OH, C2H5OH, and CO, which are usually used in the literature as additives for the SNCR process, have been evaluated. As a result, very high efficiencies (up to 82%) have been achieved in our optimal experimental conditions with the classical SNCR process. The additives seem to make the NO reduction process more effective at lower temperatures. These additives induce a downward shift (up to more than 100 K) of the optimal temperature window for the reduction process, and a slight decrease of the maximum NO reduction level depending on the species and on the additive concentration. The additive utilization effectively decreases residual NH3 emissions.

Experiment and Computational Fluid Dynamics (CFD) Simulation of Urea-Based Selective Noncatalytic Reduction (SNCR) in a Pilot-Scale Flow Reactor

A turbulent reacting flow computational fluid dynamics (CFD) model involving a droplet size distribution function in the discrete droplet phase is first built for selective noncatalytic reduction (SNCR) processes using urea solution as a NOx removal reagent. The model is validated with the experimental data obtained from a pilot-scale urea-based SNCR reactor installed with a 150 kW gas burner. New kinetic parameters of seven chemical reactions for the urea-based NOx reduction are identified and incorporated into the three-dimensional turbulent flow CFD model. The two-phase droplet model with the non-uniform droplet size is also combined with the CFD model to predict the trajectory of the droplets and to examine the mixing between the flue gas and reagents. The maximum NO reduction efficiency of about 80%, experimentally measured at the reactor outlet, is obtained at 940 °C and a normalized stoichiometric ratio (NSR) = 2.0 under the conditions of 11% excess air and low CO concentration (10−15 ppm). At the reaction temperature of 940 °C, the difference of a maximum of 10% between experiments and simulations of the NO reduction percentage is observed for NSR = 1.0, 1.5, and 2.0. The ammonia slip is overestimated in CFD simulation at low temperatures, especially lower than 900 °C. However, the CFD simulation results above 900 °C show a reasonable agreement with the experimental data of NOx reduction and ammonia slip as a function of the NSR.

The performance of CNT as catalyst support on CO oxidation at low temperature

A carbon nanotube (CNT) was used as catalyst support impregnated with transition metal cobalt for CO oxidation at low temperature. Catalyst properties were analyzed by X-ray powder diffractometer (XRD), X-ray photoelectron spectrometer (XPS), and transmission electron microscope (TEM). Analytical results for TEM and XRD demonstrated that cobalt particles were highly dispersed on the carbon nanotube (20–30 nm) with nanosized cobalt particles (10–15 nm). These investigations indicated that Co/CNT generates about 99% of the high activity for CO conversion at 250 °C and thermally stability that is superior to Co/activated carbon (AC). The optimum reaction conditions for CO conversion were O 2 concentration 3%, operation temperature 250 °C, CO concentration 5000 ppm, and space velocity 156,000 h −1. At 250 °C, CO may act as a reductant for NO reduction over Co/CNT in the presence of oxygen, whereas CO/NO = 2.5 showed that maximum NO reduction was 30%. Under H 2 rich conditions, the optimum reaction temperature for CO conversion was under 300 °C, and performance of CO 2 selectivity was better at 200 °C than 250 °C as the oxygen concentration increased.

Promotion of surface SO x on the selective catalytic reduction of NO by hydrocarbons over Ag/Al 2O 3

The promotion of sulfur oxides on the selective catalytic reduction (SCR) of NO by hydrocarbons in the presence of a low concentration of sulfur oxides over Ag/Al 2O 3 has been investigated by a flow reaction test and in situ infrared spectroscopy. When the C 3H 6 (or C 10H 22) + NO + O 2 feed-flow reaction was tested, maximum NO reduction was below 30% over fresh Ag/Al 2O 3. After the addition of SO 2 to the feed flow, conversion increased slightly. Conversion increased further after SO 2 was cut-off from the feed flow. This demonstrated that the increase in NO reduction activity of the catalyst was related to SO x adsorbed on the catalyst. SO x adsorbed on the catalytic surface (1375 cm −1) was detected by IR spectroscopy and was stable within the temperature range. NCO species, as an intermediate in NO reduction, on SO x -adsorbed Ag/Al 2O 3 in a C 3H 6 + NO + O 2 feed flow was observed in in situ IR spectra during the elevation of the reaction temperature from 473 to 673 K, while it was only observed at 673 K on fresh Ag/Al 2O 3 under the same experimental conditions. We suggest that SO x in low concentrations depressed the combustion of reductants by contaminating hydrocarbon combustion active sites on the catalyst, resulting in an increase in NO reduction efficiency of the reductants.

Bi2O3/Al2O3 catalysts for the selective reduction of NO with hydrocarbons in lean conditions

Bi2O3/Al2O3 catalysts were prepared by sol–gel and impregnation methods, and their properties in the selective catalytic reduction of NOx with propene and propane investigated. Under simulated diesel exhaust gas conditions (500ppm NO, 10% O2, 6% H2O, GHSV of ca. 30,000h−1) with 525ppm propene as reductant, maximum conversion levels to N2 reached 62% for NO and 47% for NO2, while maximum NOx reduction was observed at 550°C. Increasing the propene concentration to 1315ppm (corresponding to a [C1]/[NO] ratio of 8) resulted in a maximum NO reduction level of 96%, while the temperature corresponding to the maximum shifted to 525°C. Propane was also found to function as an efficient reductant; maximum NO conversion levels were only slightly lower than for propene, although the temperature window for NO conversion was shifted by ∼25°C to higher temperature relative to propene. While the presence of oxygen was found to be essential for SCR activity, water significantly inhibited NOx reduction by competing for adsorption sites with the propene and/or NOx reactants. Optimum SCR activity was observed for sol–gel catalysts at bismuth loadings of 3–7wt%, XRD and XPS measurements showing that the bismuth oxide in these catalysts was present as a highly dispersed phase. The results of steady-state and transient DRIFTS measurements suggested a reaction scheme in which surface nitrate and acetate groups react to afford N-containing intermediates, including organo-nitrite and/or nitrate species, as well as isocyanate, which react further to form ultimately N2.

Reburning and burnout simulations of natural gas for heavy oil combustion

Reburning and burnout simulations were carried out through PLUG code of CHEMKIN-III using a reduced mechanism, in order to determine preliminary experimental parameters for achieving maximum NO x reduction to implement the reburning technology for heavy oil combustion in pilot scale equipments in Brazil. Gas compositions at the entrance of the reburning zone were estimated by the AComb program. Simulations were performed for eight conditions in the usual range of operational parameters for natural gas reburning. The maximum NO reduction (ca. 50%) was reached with 10 and 17.5% of power via natural gas and 1.5 and 3.0% O 2 excess, respectively, at 1273 K. The model predicts 250 ppm of NO, 50 ppm of CO and air mass flows in the range of about 50–130 kg/h for burnout.

Influence of oxygenated additives on the NO xOUT 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 x OUT 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.

Bimetallic PtAu cluster-derived catalysts for the selective catalytic reduction of NO by propylene

The selective catalytic reduction of nitric oxide by propylene in the presence of excess oxygen has been investigated over a series of silica supported Pt and PtAu catalysts. Cluster-derived catalysts were prepared by wet impregnation using a Pt 2Au 4(CC tBu) 8 organometallic cluster precursor, and compared to catalysts obtained by incipient wetness impregnation using individual Pt and Au salts. The addition of Au by co-impregnation resulted only to a small shift (20–25 °C) in the light-off of propylene towards higher temperatures. A markedly different catalytic behavior was observed in the case of cluster-derived catalysts, where a 150 °C delay was observed both in the temperature of maximum NO reduction and the light-off of propylene in the presence and in the absence of NO. Most importantly, the selectivity towards N 2 increased by 50% compared to the monometallic Pt catalysts. The results of characterization studies indicate that the monometallic Pt/SiO 2 and the co-impregnated PtAu/SiO 2 samples have similar metal particle sizes. A more narrow particle size distribution was observed with the cluster-derived bimetallic sample.

A Study of Furan as a Model Oxygenated Reburn Fuel for Nitric Oxide Reduction

Reduction of NO with furan under pyrolytic and oxidative conditions is investigated. Experiments in a single pulse shock tube are performed under the conditions covering the temperature range of 1210−1950 K, pressures from 12.4 to 15.3 atm, residence times of 570−1140 μs, initial NO concentrations of 300−380 ppm, and initial furan concentration of 1.44 mol % for the pyrolysis series and 0.36 mol % for the two oxidation series with equivalence ratio, φ, of 5.0 and 2.4. Furan removed NO at progressively lower temperature as the equivalence ratio decreased. The maximum NO reduction achieved in the temperature range of this study is 50% for pyrolysis and 80% for the oxidation series. The only N-containing products observed are N2 and low yields of HCN. A kinetic reaction model which reproduces the experimental data substantially is presented and this is used to compare the efficiency of NO reduction by furan and other hydrocarbons under shock tube and stirred flow reactor conditions. The efficiency of NO removal by furan is comparable to that by propene. Acetylene is more effective than either fuel. Low-temperature conversion of NO by C2H2, however, is predicted to lead to the formation of NO2. The ability of furan, a major product of thermal decomposition of biomass, to reduce NO in the absence of O2 suggests that biomass may be an efficient and inexpensive alternative to other reburning fuels.

Reduction of NO by propane in a JSR at 1 atm: experimental and kinetic modeling

The reduction of nitric oxide (NO) by propane in simulated conditions of the reburning zone has been studied in a fused silica jet-stirred reactor operating at 1 atm. The temperatures were in the range from 1150 to 1400 K. In the present experiments, the initial mole fraction of NO was 1000 ppm, that of propane was 2490–2930 ppm. The equivalence ratio has been varied from 0.6 to 2. It was demonstrated that the reduction of NO varies with the temperature and that for a given temperature, a maximum NO-reduction occurs slightly above stoichiometric conditions. The present results generally follow those obtained in previous studies involving simple hydrocarbons or natural gas as reburn fuel. The neat oxidation of propane was also studied in the same conditions of temperature, pressure and residence time. A detailed chemical kinetic modeling of the present experiments was performed using an updated and improved kinetic scheme (892 reversible reactions and 113 species). An overall reasonable agreement between the present data and the modeling was obtained. Also, the proposed kinetic mechanism can be successfully used to model the reduction of NO by ethane, ethylene, a natural gas blend (methane–ethane 10:1). According to this study, the main route to NO-reduction by propane involves ketenyl radical. The kinetic model indicates that the reduction of NO proceeds via: C 3H 8→C 2H 4→C 2H 2→ HCCO, CH; HCCO+NO→ HCNO+CO and HCN+CO 2; CH+NO→HCN; HCNO+H→ HCN+OH; HCN+O→ NCO→ NH; NH+H→N; N+NO→ N 2 ; NH+NO→ N 2 O followed by N 2O+H→ N 2 .

On the promotional effect of Sn in Co-Sn/Al2O3 catalyst for NO selective reduction

NO reduction with propylene over Co/Al2O3 and Co–Sn/Al2O3 catalysts has been investigated. For the Co/Al2O3 catalyst, a calcination temperature exceeding 800 °C led to a decrease of NO conversion. Calcination of the Co/Al2O3 catalyst at 1000 °C resulted in the formation of α-Al2O3 and Co3O4. The presence of 20% water vapor showed a significant shift for the maximum NO reduction temperature from 450 to 600 °C over Co/Al2O3. It has been found that modification of 6 wt% Co/Al2O3 with 2 wt% Sn significantly enhanced the catalyst thermal stability and improved the inhibitory effect of water on NO conversion and reaction temperature. The promotional effect of Sn on the catalyst thermal stability was attributed to the suppression of the phase transformation from highly dispersed Co2+ species on γ-Al2O3 to α-Al2O3 and Co3O4. The smaller influence of water vapor on NO reduction conversion and temperature over Co–Sn/Al2O3, compared to Co/Al2O3, was attributed to the dispersion effect of Sn species on Co2+ species as well as the involvement of Sn species in NO reduction at a relatively lower temperature. The synergetic effect between the octahedral Co2+ species and γ-alumina plays a significant role in the catalysis of NO selective reduction by C3H6.

Catalytic selective reduction of NO with propylene over Cu-Al 2O 3 catalysts: influence of catalyst preparation method

NO reduction to N 2 by C 3H 6 was investigated and compared over Cu-Al 2O 3 catalysts prepared by four different methods, namely, the conventional impregnation, co-precipitation, evaporation of a mixed aqueous solution, and xerogel methods. It was found that the catalyst preparation method as well as the Cu content exerts a significant influence on catalyst activity. For the catalysts prepared by the first three preparation methods, with the increase of Cu content from 5 to 15 wt%, the maximum NO reduction conversion decreased slightly, but the temperature for the maximum NO reduction also decreased. For the xerogel Cu-Al 2O 3, there was a significant decrease in NO reduction conversion with the increase of Cu content from 5 to 10 wt%. In the absence of water vapour, the Cu-Al 2O 3 catalyst prepared by the impregnation method exhibited the highest activity toward NO reduction. The purity of alumina support was found to be a crucial factor to the activity of the Cu-Al 2O 3 catalyst prepared by impregnation. In the presence of water vapour, a substantial decrease in NO conversion was observed for the Cu-Al 2O 3 catalysts prepared by the first three methods, especially for the impregnated Cu-Al 2O 3 catalyst. In contrast, the presence of water vapour showed only a minor influence on the xerogel 5 wt% Cu-Al 2O 3 and it showed the highest activity for NO reduction in the presence of 20% water vapour. The xerogel 5 wt% Cu-Al 2O 3 catalyst was also found to be less affected by a 5 wt% sulfate deposition than the Cu-Al 2O 3 catalysts prepared by other methods.

Detailed measurements in a pulverized coal flame with natural gas reburning

Gas composition and temperature profiles were measured inside a 200 kW, entrained flow, pulverized coal Controlled Profile Reactor, using reburning for NO reduction. NO, CO, CO 2, NO x , O 2, and NH 3 samples were collected with a water-cooled and water-quenched stainless steel probe and analyzed on a dry basis with on-line gas analyzers and an ion-sensitive electrode (NH 3) over a grid of 36 locations within the reactor. Temperature data were obtained with a suction pyrometer using an S-type shielded thermocouple. NO reduction with reburning was investigated over a range of residence times and reburning zone stoichiometric ratios to optimize for maximum NO reduction in the flue gases. A decrease in stoichiometric ratio of the reburning zone resulted in an increase in NO reduction up to a maximum of 70% at a stoichiometric ratio of 0.78. The residence time in the reburning zone was varied by moving the tertiary air injector up and down axially. Increasing residence time in the reburning zone was initially beneficial but became unimportant when residence time became longer than 700 ms. The location of the reburning zone was found to be optimal when reburning fuel was directed at the location of highest NO concentration (i.e., immediately following NO formation). In comparison to a baseline case without reburning, the rate of carbon burnout was found to be higher with 20 cm of primary fuel injection, but proceeded very slowly through the reburning zone. At the end of the reactor, burnout was more complete in the baseline case. The species, temperature, and solid sample elemental concentrations appeared to be self-consistent and should provide accurate data for comparison with modeling results.

Zeolite supported Pt catalysts for reduction of NO under oxygen excess: a comparison of C 3H 6, HNCO and NH 3 as reducing agents

The effect of the zeolite SiO 2/Al 2O 3 ratio of ZSM-5 supported Pt catalysts on the reduction of NO under oxygen excess has been investigated. The activity and selectivity of three reducing agents were compared, i.e. C 3H 6, HNCO and NH 3. The maximum NO reduction with both C 3H 6 and HNCO as the reducing agent was affected by the SiO 2/Al 2O 3 ratio of the support. Although no effect was observed on the maximum NO reduction for NH 3, the yields of N 2 and N 2O were affected. The results are put in perspective of reaction mechanisms proposed in the literature.

Catalytic reduction of NO by hydrocarbons over a mechanical mixture of spinel Ni–Ga oxide and manganese oxide

NO reduction with propylene over Mn2O3, spinel Ni–Ga oxide and their mechanical mixtures has been investigated. Mn2O3 has no activity to NO reduction, but has a high activity for NO oxidation to NO2. Spinel Ni–Ga oxide showed an apparent activity to NO reduction only at temperatures above 400°C. Mixing of Mn2O3 to the Ni–Ga oxide resulted in a significant enhancement of NO reduction in the temperature range of 250–450°C. The optimal Mn2O3 content in the mixture catalyst was about 10–20 wt%. It is suggested that the synergetic effect of Mn2O3 and Ni–Ga oxide plays an important role in the catalysis of NO reduction. The Ni–Ga oxide and Mn2O3 mixture catalyst is superior to Pt/Al2O3 and Cu-ZSM-5 by showing a higher NO reduction conversion, resistance to water and negligible harmful by-product formation. Other lower hydrocarbons C2H4, C2H6 and C3H8 also give a maximum NO reduction conversion as high as 50%. The difference from using C3H6 is that the temperature at the maximum NO reduction is higher than it is with C3H6.

Control of NO x and N 2O in pressurized fluidized-bed combustion

Experiments with a test rig on the formation of NO x and N 2O and their reduction in pressurized fluidized-bed combustion of bituminous coal are reported. The emission of NO x is controlled by two NO reduction methods—ammonia injection and staging of combustion air—and the emission of N 2O by increasing the combustion temperature. Both ammonia injection and staging of combustion air are effective in NO reduction. The emission of NO x decreases with increasing ammonia flow, and ∼75–85% NO x reduction can be achieved at an NH 3 NO x molar ratio of 4–5. However, ammonia injection increases N 2O emission and ammonia slip occurs. In the staging of combustion air a maximum NO reduction of 75% can be achieved with primary air ratio ≈ 1. A further decrease in primary air ratio to sub-stoichiometric conditions leads to an increase in NO x concentration, and has other negative effects on sulfur capture and combustion performance. No great change in N 2O emission is observed when air staging is changed. Increasing combustion temperature reduces N 2O emission significantly. Extremely low N 2O concentrations are obtained at high bed temperature and pressure. Freeboard temperature profile also plays an important role in N 2O emission.