NOx Abatement Research Articles

Numerical analysis of ammonia homogenization for selective catalytic reduction application

Selective catalytic reduction based on urea water solution as ammonia precursor is a promising method for the NOx abatement form exhaust gasses of mobile diesel engine units. It consists of injecting the urea-water solution in the hot flue gas stream and reaction of its products with the NOx over the catalyst surface. During this process flue gas enthalpy is used for the urea-water droplet heating and for the evaporation of water content. After water evaporates, thermolysis of urea occurs, during which ammonia, a known NOx reductant, and isocyanic acid are generated. The uniformity of the ammonia before the catalyst as well as ammonia slip to the environment are important counteracting design requirements, optimization of which is crucial for development of efficient deNOx systems.The aim of this paper is to show capabilities of the developed mathematical framework implemented in the commercial CFD code AVL FIRE®, to simulate physical processes of all relevant phenomena occurring during the SCR process including chemical reactions taking part in the catalyst. First, mathematical models for description of SCR process are presented and afterwards, models are used on the 3D geometry of a real SCR reactor in order to predict ammonia generation, NOx reduction and resulting ammonia slip. Influence of the injection direction and droplet sizes was also investigated on the same geometry. The performed study indicates importance of droplet sizes on the SCR process and shows that counterflow injection is beneficial, especially in terms of minimizing harmful ammonia slip to environment.

High NOx Reduction Activity of an Ultrathin Zirconia Film Covering a Cu Surface: A DFT Study

NO reduction by CO over a c-ZrO2(110) ultrathin film covering a Cu(110) surface (ZrO2/Cu) has been examined by means of density-functional theory calculations. Spontaneous transfer of electronic charge from Cu to zirconia gives the latter an ability to reduce oxidants. Gaining this excess charge, NO adsorbs and activates on the cationic Zr site of the oxide surface. Thus activated, two NO on adjacent Zr sites can react to form ONNO with an activation energy of 0.39 eV (8.9 kcal/mol), which readily decomposes into N2O and an O adatom. N2O also decomposes into N2 and an O adatom with negligible activation energy of 0.06 eV (1.4 kcal/mol). CO reacts off the O adatoms left on the surface as CO2 with an activation energy of 0.36 eV (8.2 kcal/mol), completing a redox cycle. These results indicate high potentials of oxide-covered metal catalysts (metal@oxide) for NOx abatement.

Numerical Study on Reduction of Ambient NOx Concentration by a Flow-through ACF (Activated Carbon Fibers) Fences

A method to use fences filled inside with activated carbon fibers (ACF) is presented for reduction of high NO2 concentration in urban atmosphere without excess energy use. Polluted air mass in road space moves with natural- and car-induced- winds to contact with- and to flow through- ACF fences, and the ACF layer removes these pollutants. Two parameters determining the performance of the ACF fences in terms of NOx abatement were identified as (1) resistance coefficient of the ACF layer to air flow, and (2) chemical reactivity of ACF layer for NOx removal. By varying these two parameter values in their ranges obtained by laboratory experiments, the reduction of NOx concentration at roadside by the ACF fences was numerically evaluated under an idealized situation in which two cases were considered; one is the double-fences placed at both sides of road and the other is single fence set at downwind side of the road. The results showed that in both cases these “energy-free” equipment can decrease NOx concentration by 15~55% at roadside. Furthermore, it was indicated that in the double-fence case the ACF fence with lower permeability (larger resistance coefficient) shows higher reduction of NOx, while in the single fence case higher permeability reduces NOx more.

Fundamental understanding of the Di-Air system (an alternative NOx abatement technology). I: The difference in reductant pre-treatment of ceria

Toyota's Di-Air DeNOx system is a promising DeNOx system to meet NOx emission requirement during the real driving, yet, a fundamental understanding largely lacks, e.g. the benefit of fast frequency fuel injection. Ceria is the main ingredient in Di-Air catalyst composition. Hence, we investigated the reduction of ceria by reductants, e.g. CO, H2, and hydrocarbons (C3H6 and C3H8), with Temporal Analysis of Product (TAP) technique. The results show that the reduction by CO yielded a faster catalyst reduction rate than that of H2. However, they reached the same final degree of ceria reduction. Hydrocarbons generated almost three times deeper degree of ceria reduction than that with CO and H2. In addition, hydrocarbons resulted in carbonaceous deposits on the ceria surface. The total amount of converted NO over the C3H6 reduced sample is around ten times more than that of CO. The deeper degree of reduction and the deposition of carbon by hydrocarbon explain why hydrocarbons are the most powerful reductants in Toyota's Di-Air NOx abatement system.

CoOx and FeOx supported on ZrO2 for the simultaneous abatement of NOx and N2O with C3H6 in the presence of O2

MeOx/ZrO2 (Me=Co and Fe) catalysts were studied for the simultaneous selective catalytic reduction of NO and N2O in the presence of O2 using C3H6 as reducing agent (SCRsim). To give a better insight in the simultaneous process we investigated the reactions related to SCRsim (SCRN2O, SCRNO, N2O decomposition and C3H6 combustion) as well as the abatements in the absence of O2 (CRsim, CRN2O, CRNO).Catalytic results showed that, in the presence of O2 excess, CoOx/ZrO2 and FeOx/ZrO2 catalysts were scarcely active and unselective for the separate NO and N2O abatements with C3H6 and are ineffective for their simultaneous abatement. Because C3H6 preferentially reacted with O2, NO was poorly reduced and N2O was abated, at a temperature above that of complete C3H6 conversion, via both SCRN2O and decomposition. Conversely, in the absence of O2 in the feed, on both catalysts NO and N2O were efficiently reduced by C3H6, but undesired by-products formed.The activity for SCRsim strongly depended on the C3H6/O2 feeding ratio. With suitable feeding mixture O2 was completely consumed and the residual propene efficiently and simultaneously reduced NO and N2O, with negligible formation of by-products. In hydrothermal conditions both catalysts were slightly and reversibly deactivated.Characterization by XRD, UV–vis DRS and FTIR after catalytic experiments showed that dispersed Co2+ and Fe3+ species were stable on zirconia surface and that no significant segregation phenomena occurred in hydrothermal conditions.

Promotion of transition metal oxides on the NH3-SCR performance of ZrO2-CeO2 catalyst

Chromium oxide and manganese oxide promoted ZrO2-CeO2 catalysts were prepared by a homogeneous precipitation method for the selective catalytic reduction of NOx with NH3. A series of characterization including X-ray diffraction (XRD), high-resolution transmission electron microscope (HR-TEM), Brunauer–Emmett–Teller (BET) surface area analysis, H2 temperatureprogrammed reduction (H2-TPR), and X-ray photoelectron spectroscopy (XPS) were used to evaluate the influence of the physicochemical properties on NH3-SCR activity. Cr-Zr-Ce and Mn-Zr-Ce catalysts are much more active than ZrO2-CeO2 binary oxide for the low temperature NH3-SCR, mainly because of the high specific surface area, more surface oxygen species, improved reducibility derived from synergistic effect among different elements. Mn-Zr-Ce catalyst exhibited high tolerance to SO2 and H2O. Cr-Zr-Ce mixed oxide exhibited>80% NOx conversion at a wide temperature window of 100°C–300°C. In situ DRIFT studies showed that the addition of Cr is beneficial to the formation of Bronsted acid sites and prevents the formation of stable nitrate species because of the presence of Cr6 +. The present mixed oxide can be a candidate for the low temperature abatement of NOx. Open image in new window

Phenomenology of high-ozone episodes in NE Spain

Abstract. Ground-level and vertical measurements (performed using tethered and non-tethered balloons), coupled with modelling, of ozone (O3), other gaseous pollutants (NO, NO2, CO, SO2) and aerosols were carried out in the plains (Vic Plain) and valleys of the northern region of the Barcelona metropolitan area (BMA) in July 2015, an area typically recording the highest O3 episodes in Spain. Our results suggest that these very high O3 episodes were originated by three main contributions: (i) the surface fumigation from high O3 reservoir layers located at 1500–3000 m a.g.l. (according to modelling and non-tethered balloon measurements), and originated during the previous day(s) injections of polluted air masses at high altitude; (ii) local/regional photochemical production and transport (at lower heights) from the BMA and the surrounding coastal settlements, into the inland valleys; and (iii) external (to the study area) contributions of both O3 and precursors. These processes gave rise to maximal O3 levels in the inland plains and valleys northwards from the BMA when compared to the higher mountain sites. Thus, a maximum O3 concentration was observed within the lower tropospheric layer, characterised by an upward increase of O3 and black carbon (BC) up to around 100–200 m a.g.l. (reaching up to 300 µg m−3 of O3 as a 10 s average), followed by a decrease of both pollutants at higher altitudes, where BC and O3 concentrations alternate in layers with parallel variations, probably as a consequence of the atmospheric transport from the BMA and the return flows (to the sea) of strata injected at certain heights the previous day(s). At the highest altitudes reached in this study with the tethered balloons (900–1000 m a.g.l.) during the campaign, BC and O3 were often anti-correlated or unrelated, possibly due to a prevailing regional or even hemispheric contribution of O3 at those altitudes. In the central hours of the days a homogeneous O3 distribution was evidenced for the lowest 1 km of the atmosphere, although probably important variations could be expected at higher levels, where the high O3 return strata are injected according to the modelling results and non-tethered balloon data. Relatively low concentrations of ultrafine particles (UFPs) were found during the study, and nucleation episodes were only detected in the boundary layer. Two types of O3 episodes were identified: type A with major exceedances of the O3 information threshold (180 µg m−3 on an hourly basis) caused by a clear daily concatenation of local/regional production with accumulation (at upper levels), fumigation and direct transport from the BMA (closed circulation); and type B with regional O3 production without major recirculation (or fumigation) of the polluted BMA/regional air masses (open circulation), and relatively lower O3 levels, but still exceeding the 8 h averaged health target. To implement potential O3 control and abatement strategies two major key tasks are proposed: (i) meteorological forecasting, from June to August, to predict recirculation episodes so that NOx and VOC abatement measures can be applied before these episodes start; (ii) sensitivity analysis with high-resolution modelling to evaluate the effectiveness of these potential abatement measures of precursors for O3 reduction.

Highly Active Sb–V–CeO2/TiO2 Catalyst Under Low Sulfur for NH3-SCR at Low Temperature

The Cu based zeolite NH3-SCR catalysts are well-known for after treatment of exhaust gases from light-and heavy-duty diesel engines. However, these catalysts are easily deactivated under the co-presence of H2O and SO2. In the present study, Sb–V–CeO2/TiO2 catalyst showed superior conversion for the abatement of NOx with NH3 under H2O and SO2 condition compared to commercial Cu–SAPO-34 and CuFe–ZSM-5 zeolite catalysts. The physico-chemical properties of fresh and used catalysts were examined by BET surface area, TEM (Transition electron microscopy) with EDS/EDX (Energy dispersive spectrometer/X-ray), XPS (X-ray photoelectron spectroscopy), DRIFTS (in situ diffuse reflectance infrared Fourier transformed spectroscopy) and H2-TPR (temperature programmed reduction). Higher than 95% of initial NOx conversion was obtained over Sb–V–CeO2/TiO2 catalyst at 250 °C under 14% H2O and 50 ppm SO2 conditions. The SO2 on/off test of the CuFe–ZSM-5 and Cu–SAPO-34 catalysts indicated a drastic decrease of NOx conversion when SO2 was turned on. In contrast, Sb–V–CeO2/TiO2 catalyst maintained a high NOx conversion and initial NOx conversion was completely recovered when SO2 was turned off.

Tuning the Synthetic Parameters to Obtain Smart C‐N Co‐Doped Titania Photocatalysts for NOx Abatement

AbstractGlobal society is constantly seeking for sustainable and effective processes to fight environmental pollution. The abatement of NOx, a class of hazardous contaminants, can be performed by means of photocatalytic oxidation using titania‐based photocatalysts. Non‐metal doping is highly desirable for an efficient exploitation of solar light. The co‐presence of nitrogen and carbon represents an appealing possibility though their introduction generally requires post‐synthesis treatments. Otherwise, in this work carbon and nitrogen have been introduced in titania photocatalysts simultaneously by a one‐pot synthetic approach using a natural and sustainable source of dopants, namely chitosan. Several synthetic parameters have been changed in order to evaluate their effect on the physicochemical properties of the final materials and to establish structure‐activity relationship as for the photocatalytic performances in NOx oxidation under visible light irradiation.

Ceria modified FeMnOx—Enhanced performance and sulphur resistance for low-temperature SCR of NOx

Low-temperature NH3-SCR is an environmentally important reaction for the abatement of NOx from stationary sources. In recent years FeMnOx has attracted significant attention as a potential catalyst for this process, however its catalytic activity and SO2 resistance require further improvement. In this contribution FeMnOx has been modified to examine the effect of ceria on catalytic activity and SO2 resistance in the low temperature region. Preparation of catalysts via the citric acid method generate modified materials that exhibit enhanced NO turnover compared to FeMnOx. A reduction in NH3 adsorption (NH3-TPD) and a suitable ratio between NO sites (NO-TPD) and chemisorbed surface oxygen (XPS) are beneficial for the promotion of fast-SCR. A comparative SO2 resistance study of FeMnOx and Ce(12.5) showed that the latter exhibited improved stability in the presence of SO2, as indicated by the retention of pore volume (N2 adsorption) and surface composition (XPS). In-situ DRIFTS demonstrated that chemisorbed N-containing species on Ce(12.5) were much more stable in the presence of SO2 compared to FeMnOx, which resulted in the formation of significantly less metal sulphates and NH4HSO4.

Effect of samarium and vanadium co-doping on structure, ferroelectric and photocatalytic properties of bismuth titanate

A systematic analysis of the structure, ferroelectricity and photocatalytic activity (PCA) of Sm and V modified Bi4Ti3O12was performed. Highly reproducible gas–solid phase PCA of NOxabatement and improved ferroelectricity were achieved.

Experimental investigation of N2O formation in selective non-catalytic NOx reduction processes performed in stoker boiler

Abstract Stoker fired boiler plants are common throughout Eastern Europe. Increasingly strict emission standards will require application of secondary NOx abatement systems on such boilers. Yet operation of such systems, in addition to reducing NOx emissions, may also lead to emission of undesirable substances, for example N2O. This paper presents results of experimental tests concerning N2O formation in the selective non-catalytic NOx emission reduction process (SNCR) in a stoker boiler (WR 25 type). Obtained results lead to an unambiguous conclusion that there is a dependency between the NOx and N2O concentrations in the exhaust gas when SNCR process is carried out in a coal-fired stoker boiler. Fulfilling new emission standards in the analysed equipment will require 40–50% reduction of NOx concentration. It should be expected that in such a case the N2O emission will be approximately 55–60 mg/m3, with the NOx to N2O conversion factor of about 40%.

Perovskite LaFeO3-SrTiO3 composite for synergistically enhanced NO removal under visible light excitation

Perovskite oxides (ABO3) are widely studied as excellent sorbing and catalytic materials for NOx abatement in automobile or stationary depollution processes, and recently they have attracted significant interest in solar conversion reactions due to the flexible composition, facile optical and electronic tuning properties. In this work, perovskite LaFeO3 microspheres were synthesized and employed as photocatalysts to remove parts-per-billion level NO, and it is found that the photocatalytic efficiency was dramatically improved by coupling with SrTiO3 nanocubes. The LaFeO3-SrTiO3 composite with proper mass ratio (0.3–1) displayed 3.1 and 4.5 fold enhancement in NO removal rate as compared to the pristine LaFeO3 and SrTiO3, respectively. Moreover, the LaFeO3-SrTiO3 composite exhibited decreased NO2 yield possibly due to the basic surface property of strontium sites. The synergistically improved activity was due to broad visible light harvest, enlarged surface area, and most importantly, the depressed surface charge recombination originating from the perfectly matched LaFeO3-SrTiO3 interface and facile charge transfer along the staggered band alignment. The temperature programmed desorption (TPD) analysis revealed that the composite had efficient chemisorption for NO. Further, the electron spin resonance (ESR) combined with the radical scavenger tests and density functional theory (DFT) calculations suggested that the photocatalytic NO oxidation via superoxide radicals (O2−) from SrTiO3 and direct hole (h+) transfer from LaFeO3 might be the predominant routes. We believe that this study provides some new insights into perovskite nanomaterials as photocatalyst for NOx abatement under ambient conditions.

Durability of Cementitious Photocatalytic Building Materials

Photocatalytic building materials have been shown to be a promising remediation technique for air pollutants such as NOx and VOC’s. However, there are still doubts and questions about the durability of the air cleaning characteristic in time. In this respect, a pre-normative research is being undertaken at BRRC as a first step towards evaluation of the life time of these photo-active air purifying products. This is done by combining and adapting existing durability tests for cement-based materials with standardized test methods to determine the photocatalytic activity. First results of this research are presented here, including a state-of-the-art on accelerated ageing testing as well as the validation of applicable ageing procedures in combination with a normalized evaluation test for photocatalytic NOx abatement. Finally, recommendations for future work regarding durability testing on photocatalytic building materials are also given.

Cost and potential of energy conservation and collaborative pollutant reduction in the iron and steel industry in China

This study investigated the cost and potential of energy conservation and collaborative pollutant emission reduction in the iron and steel industry in China. A total of 24 technologies were selected, and their investments, operation costs, energy saving benefits, abatement benefits, abatement coefficients and popularising rates were collected and calculated using the 2013 data as the baseline. A new parameter, APeq, reflecting the characteristics of energy conservation and multi-pollutant collaborative emission reduction was set up. The conservation and emission reduction supply curves (CERSCs) were analysed. These 24 technologies resulted in the abatement of SO2, NOx, PM10, CO2 and APeq to 621.6kt, 398.9kt, 390.2kt, 291.1Mt and 699.5kt, respectively. The influences of discount rates and weight factors on the abatement supply curves were also discussed. Results show that, with the increase in the discount rate, the abatement costs increase and the amount of that abatement cost below zero decreases. Finally, the pollutant reductions in three key areas in China were analysed. The reduction potentials of the Beijing–Tianjin–Hebei region, Yangtze River Delta and Pearl River Delta are 107, 79 and 13kt, respectively, and the corresponding abatement costs are 7.52, −51.54 and −1.76 billion Yuan. Policymakers can choose suitable and economic technologies according to the calculation of the CERSCs.

NOx abatement from stationary emission sources by photo-assisted SCR: Lab-scale to pilot-scale studies

In developing a higher activity for NOx abatement from stationary emission sources, photo-assisted selective catalytic reduction (photo-SCR) was systematically investigated over TiO2 supported on a spherical α-Al2O3/γ-Al2O3. The formation of NO2 was virtually suppressed in the reaction temperature range of 110–200°C. The NOx removal efficiencies of TiO2/γ-Al2O3 photocatalyst at 120°C for real flue gas (in pilot-scale photoreactor) and simulated flue gas (in lab-scale photoreactor) were 68–75% and 43.5%, respectively, in the presence of reducing reagent (C4H10). SEM and XPS techniques revealed that sulfur temporarily occupied the active sites of the photocatalyst, causing deactivation. Spent TiO2/γ-Al2O3 photocatalyst was regenerated via desulfurization by either air or hydrogen at 400–500°C. The effects of regeneration temperature and regeneration time on the photocatalytic activity are methodically investigated, demonstrating that using hydrogen at 500°C in 1h could completely regenerate the spent photocatalyst.

Health benefits of reducing NOx emissions in the presence of epidemiological and atmospheric nonlinearities

Recent epidemiological evidence suggests that the logarithm of concentration is a better predictor of mortality risk from long-term exposure to ambient PM2.5 and NO2 than concentration itself. A log-concentration-response function (CRF) predicts a heightened excess risk per unit concentration at low levels of exposure that further increases as the air becomes less polluted. Using an adjoint air quality model, we estimate the public health benefits of reducing NOx emissions, on a per-ton and source-by-source basis. Our estimates of benefits-per-ton assume linear in concentration and log-concentration CRFs for NO2 and a CRF that is linear in concentration for O3. We apply risk coefficients estimated using the Canadian Census Health and Environment Cohort. We find that a log-concentration CRF for NO2 leads almost consistently to larger benefits-per-ton than a linear in concentration CRF (e.g., $500 000 ton−1 compared to $270 000 ton−1 for Ottawa). We observe that concentrations gradually decline due to widespread, progressive emissions abatement, entailing increasing health benefits as a result of (1) a log-concentration CRF for NO2 and (2) the nonlinear response of O3 to NOx emissions. Our results indicate that NOx abatement has the potential to incur substantial and increasing health benefits, by up to five times with 85% emission reductions, for Canada into the future.

Modelling Inhibition Effects of Short-Chain Hydrocarbons on a Small-Pore Cu-Zeolite NH3-SCR Catalyst

NH3-SCR currently represents the leading technology for NOx abatement from lean exhaust gases, e.g. from diesel engines. Hydrocarbon (HC) slip onto the SCR catalyst can have a negative impact on its DeNOx performance. In order to better understand hydrocarbon poisoning, the influence of different light HCs on the catalytic activity of a small-pore Cu-zeolite has been investigated in a synthetic gas test rig. Based on these data and on further indications from the literature, an existing SCR kinetic model has been extended to represent major effects of light HCs on the behavior of zeolite SCR catalysts. The developed model is able to describe such effects in a simple but effective way.

Effect of H2O and SO2 on the Selective Catalytic Reduction of NO with NH3 Over Ce/TiO2 Catalyst: Mechanism and Kinetic Study

Ce-based catalysts now were regarded as a promising alternative to substitute V2O5/WO3–TiO2 catalyst for the abatement of NOx in the flue gas. However, H2O and SO2 showed a notable effect on NO reduction over Ce based catalysts and the mechanism was not very clear. In this work, the mechanism of H2O and SO2 effect on NO reduction over Ce/TiO2 was studied using the steady state kinetic study and the reaction kinetic parameters were compared. Although H2O and SO2 both showed a promotion on NO reduction over Ce/TiO2 at higher temperatures, their mechanisms were quite different. The effect of SO2 on NO reduction over Ce/TiO2 was mainly attributed to the sulfation. The selective catalytic reduction (SCR) reaction over Ce/TiO2 at higher temperatures was promoted after the sulfation due to the promotion of NH3 adsorption. Meanwhile, the catalytic oxidation of NH3 to NO (i.e., the C–O reaction) over Ce/TiO2 at higher temperatures was suppressed after the sulfation. Therefore, the promotion of NO reduction over Ce...

The byproduct generation analysis of the NOx conversion process in dielectric barrier discharge plasma

Abatement of NOx through non-thermal plasma (NTP) processes has been developed over the past several years.