DeNOx Process Research Articles

Promotional effect of ceria on the catalytic behaviour of new V2O5–WO3–TiO2 aerogel solids for the DeNOx process

New cerium and/or tungsten modified V2O5–TiO2 aerogel catalysts (V2O5–TiO2,V2O5–WO3–TiO2, V2O5–CeO2–TiO2 and V2O5–WO3–CeO2–TiO2) were successfully developed, by associating the sol gel procedure and supercritical drying approach. A combination of various analytical techniques, including XRD, N2-Physisorption at 77 ​K, DRUV-Vis spectroscopy, NH3-TPD and H2-TPR, was used to characterize the obtained solids. Their catalytic performances were evaluated in the low temperature NO-SCR by NH3. The results reveal that all the materials exhibit high cristallinity of TiO2 anatase phase and developed mesoporous texture. The addition of tungsten (W) increases the surface acidity of V2O5–TiO2 and thereby enhances its reactivity at high temperatures (>420 ​°C). However, the addition of cerium (Ce) improves the redox properties of the latter catalyst and increases significantly its NO-SCR reactivity, especially at low temperatures. The NO-SCR activity increases over the aerogel systems, in the 200–400 ​°C temperature range, following this order: V2O5–WO3–TiO2 ​< ​V2O5–TiO2 ​< ​V2O5–CeO2–TiO2 ​< ​V2O5–WO3–CeO2–TiO2. The new V2O5–WO3–CeO2–TiO2 system was found to be the most active catalyst for converting NO into N2 (with 60% and 90% NO conversions at 250 ​°C and between 320 and 420 ​°C, respectively). The highest SCR activity of this new aerogel solid if compared to the other samples was essentially correlated with a good reactivity of its acidic and redox sites which in turn was related to the existence of strong interactions between cerium and the other active elements (mostly Ce↔ V and Ce ↔ W).

Modified Zeolite Catalyst for a NOx Selective Catalytic Reduction Process in Nitric Acid Plants

Natural zeolite of the heulandite-type framework was modified with iron and tested as a catalyst for the selective catalytic reduction of nitrogen oxides with ammonia (NH3-SCR) in the temperature range of 150–450 °C. The catalyst was prepared at a laboratory scale in a powder form and then the series of experiments of its shaping into tablets was conducted. Physicochemical studies of the catalyst (N2 sorption at −196 °C, FT-IR, XRD, UV-vis) were performed to determine the textural and structural properties and identify the surface functional groups, the crystalline structure of the catalysts and the form and aggregation of the active phase. The activity tests over the shaped catalyst were performed industry-reflecting conditions, using tail gases from the pilot nitric acid plant. The influence of a temperature, catalyst load, and the amount of reducing agent (ammonia) on the NOx reduction process were investigated. The results of catalytic tests that were performed on model gas mixture showed that non-modified clinoptilolite exhibited around 58% conversion of NO at 450 °C. The temperature window of the shaped catalyst shifted to a higher temperature range in comparison to the powder sample. The catalytic performance of the shaped Fe-clinoptilolite in the industry-reflecting conditions was satisfactory, especially at 450 °C. Additionally, it was observed that the ratio of N2O concentration downstream and upstream of the catalytic bed was below 1, which indicated that the catalyst exhibited activity in both DeNOx and DeN2O process.

Remarkable low temperature catalytic activity for SCR of NO with propylene under oxygen-rich conditions over Mn0.2La0.07Ce0.05Ox/ZSM-5 catalyst

Unlike NH3-SCR systems, even in oxygen-poor conditions, SCR of NO with propylene traditionally exhibited good catalytic activity only at high temperature (≥300 °C) due to the reduction properties of propylene. Most of the researches of low temperature HC-SCR are still focused on the development of noble metal (Ag, Pt, Pd, Rh etc.) catalysts. Herein, a series of rare earth metals LayCez co-doped Mn-based microporous zeolite ZSM-5 (MnxLayCez/ZSM-5) were prepared by co-impregnation method. Under the simulated oxygen-rich condition of actual diesel engines, we first reported a non-noble metal catalyst that exhibited high activity (≥90%) and selectivity (≥90%) for low temperature (180–270 °C) C3H6-SCR. The excellent low temperatures catalytic performance could be attributed to the more Brønsted acid sites, the higher superficial dispersion of manganese species, the more oxygen vacancy and the larger pore size (4.42 nm) characterized by XRD, SEM, HR-TEM, HAADF-STEM, NH3-TPD, H2-TPR and FT-IR. Moreover, the XPS results showed that the introduction of La could effectively enhance the oxygen vacancies of manganese oxide and cerium oxide, which significantly increase the chemical adsorption of NO, reducing the reaction activation energy, and promoting the conversion of NO under low-temperature. These advantages suggest the material has huge application potential in the deNOX process.

Catalytic decomposition of NO2 over a copper-decorated metal–organic framework by non-thermal plasma

Efficient catalytic conversion of NO2 to non-harmful species remains an important target for research. State-of-the-art deNOx processes are based upon ammonia (NH3)-assisted selective catalytic reduction (NH3-SCR) over Cu-exchanged zeolites at elevated temperatures. Here, we describe a highly efficient non-thermal plasma (NTP) deNOx process catalyzed by a Cu-embedded metal-organic framework, Cu/MFM-300(Al), at room temperature. Under NTP activation at 25°C, Cu/MFM-300(Al) enables direct decomposition of NO2 into N2, NO, N2O, and O2 without the use of NH3 or other reducing agents. NO2 conversion of 96% with a N2 selectivity of 82% at a turnover frequency of 2.9 h−1 is achieved, comparable to leading NH3-SCR catalysts that use NH3 operating at 250°C–550°C. The mechanism for the rate-determining step (NO→N2) is elucidated by in operando diffuse reflectance infrared Fourier transform spectroscopy, and electron paramagnetic resonance spectroscopy confirms the formation of Cu2+⋯NO nitrosylic adducts on Cu/MFM-300(Al), which facilitates NO dissociation and results in the notable N2 selectivity.

Activity enhancement of acetate precursor prepared on MnOx-CeO2 catalyst for low-temperature NH3-SCR: Effect of gaseous acetone addition

MnOx-CeO2 catalysts are developed by hydrolysis driving redox method using acetate precursor (3Mn1Ce-Ac) and nitrate precursor (3Mn1Ce-N) for the selective catalytic reduction (SCR) of NOx by NH3. A counterpart sample (Cop-3Mn1Ce) was prepared by the NH3·H2O co-precipitation method for comparison purpose. Combining the results of physicochemical properties characterization and performance test, we find that the 3Mn1Ce-Ac catalyst with some nanorod structures is highly active for the deNOx process. The SCR activity of the 3Mn1Ce-Ac catalyst is more admirable than the 3Mn1Ce-N and the Cop-3Mn1Ce catalysts due to plentiful Lewis acid sites, excellent low-temperature reducibility, and superior surface area resulted from O2 generation during the preparation procedure. The 3Mn1Ce-Ac still exhibits the greatest performance for the deNOx process when gaseous acetone is in the SCR feed gas. The NOx conversion and N2 selectivity over the 3Mn1Ce-Ac are both improved by gaseous acetone above 150 °C due to the inhibition of SCR undesired side reactions (NSCR & C-O reactions) and “slow-SCR” process.

A Dynamic Model Incorporated with Delay Estimation and Variable Selection for Control of SCR De-NOx Process

In order to control NH3 injection for the selective catalytic reduction of nitrogen oxide (NOx) denitration (SCR de-NOx) process, a model that can accurately and quickly predict outlet NOx emissions is required. This paper presents a dynamic kernel partial least squares (KPLS) model incorporated with delay estimation and variable selection for outlet NOx emission and investigated control strategy for NH3 injection. First, k-nearest neighbor mutual information (KNN_MI) was used for delay estimation, and the effect of historical data lengths on KNN_MI was taken into account. Bidirectional search based on the change rate of KNN_MI (KNN_MI_CR) was used for variable selection. Delay–time difference update algorithm and feedback correction strategy were proposed. Second, the NH3 injection compensator (NIC) and the outlet NOx emission model constituted a correction controller. Then, its output and the output of the existing controller are added up to suitable NH3 injection. Finally, the KNN_MI_CR method was compared with different algorithms by benchmark dataset. The field data results showed that the KNN_MI_CR method could improve model accuracy for reconstructed samples. The final model can predict outlet NOx emissions in different operating states accurately. The control result not only meets the NOx emissions standard (50 mg/m3) but also keeps high de-NOx efficiency (80%). NH3 injection and NH3 escape are reduced by 11% and 39%.

Review of Recent After-Treatment Technologies for De-NOx Process in Diesel Engines

The removal of nitrogen oxides (NOx), which are major atmospheric pollutants, in exhaust gas after-treatment systems, has been studied extensively. The catalytic reduction of NOx in lean-burn environments is challenging in diesel after-treatment technology. The most relevant catalytic NOx reduction technologies for diesel engines are summarized in this review paper, focusing on current catalytic after-treatment systems for compliance with emission standards. Four promising after-treatment technologies for the abatement of tail-pipe NOx emissions are discussed: (i) lean NOx trap (LNT) and LNT combined with selective catalytic reduction (SCR) (LNT + SCR), (ii) Urea/NH3-SCR, (iii) Hydrocarbon-SCR (HC-SCR), and (iv) CO/H2-SCR. The relevant reaction mechanisms and current major challenges are reviewed in detail. The effects of the active phases and support modifications of the respective catalysts are examined to evaluate their contribution to the after-treatment systems. Major issues in practical applications, such as hydrothermal aging, chemical poison, and metallic additives for improving de-NOx performance, are extensively reviewed in this paper.

Fabrication of Cu-CHA composites with enhanced NH3-SCR catalytic performances and hydrothermal stabilities

Cu-SSZ-13 and Cu-SAPO-34, both with CHA topology but different compositions, are highly active NH3-SCR catalysts for the abatement of harmful nitrogen oxides from the exhausts of lean-burn engines. However, the former suffers from high preparation cost and worse high-temperature hydrothermal stability, and the latter is limited by its poor low-temperature hydrothermal stability. Herein, a new Cu-CHA composite (named Cu-CHADNL) containing both microenvironments of SSZ-13 and SAPO-34 was synthesized by employing one-pot synthesized Cu-SSZ-13 as Cu source, seeds and part of Si/Al source. The resultant materials have cubic-like morphology, high Si contents, controllable Cu loadings and uniform Si/P/Al/Cu elemental distributions. Solid state MAS NMR, NH3-DRIFTS and H2-TPR demonstrate the coexistence of aluminosilicate and SAPO zones in the Cu-CHADNL crystals. Notably, Cu-CHADNL exhibits high NH3-SCR activity in wide temperature window and robust resistance against both steaming at 800 °C and soaking in 80 °C water. These advantages are owing to the unique composite microstructures of Cu-CHADNL and suggest the material has huge application potential in deNOx process.

A MnOx@Eu-CeOx nanorod catalyst with multiple protective effects: Strong SO2-tolerance for low temperature DeNOx processes

A novel MnOx@Eu-CeOx catalyst with multiple protective attributes was designed and fabricated using a chemical precipitation method and tested for its low temperature SCR activity. The subject MnOx@Eu-CeOx nanorod catalyst exhibited superior SCR performance and strong SO2-tolerance. The formation of the composite-shell structure enhanced the catalysts' surface acidity and redox performance, which resulted in excellent SCR performance. Moreover, the TG results suggested that the protective effect of the EuOx-CeOx composite-shell effectively reduced the deposition of the surface sulphates. The XPS, XRD analysis results of the subject catalyst together with theoretical calculations provided strong evidence that there was a strong interaction between Mn and Ce in the MnOx@Eu-CeOx. This significant interaction could provide maximum protection to the core from the effect of SO2, which also contributed to the high SO2 resistance of the catalyst. In situ FT-IR results also indicated that the chemisorbed species on MnOx@Eu-CeOx were much more stable in the presence of SO2 compared to Eu-CeOx/MnOx, which resulted in the deposition of significantly less sulphates. This low temperature SCR catalyst with multiple protective attributes, including composite shell, strong interaction and core-shell structure, is the key to long-term resistance to SO2.

Preparation of activated sewage sludge char for low temperature De-NOx and its CO emission inhibition

Sewage sludge (SS) char can be potentially applied to De-NOx processes but it should be active enough and the relevant CO emissions should be controlled. In this paper activated SS chars have been prepared by using a simple KOH impregnation-carbonisation method and the activated chars are applied to remove NOx from flue gases within temperature range of 100–250 °C, acting as both reductants and catalysts. Special attention is paid to inhibiting CO emission in the process. Four reductive agents are adopted to check the catalytic effect of the activated SS chars. The results show that the activated SS chars (i.e. SC-KOHs) present a higher adsorption-reduction ability with much lower CO emission than that of non-activated SS chars (i.e. SC-Raws). SC-KOH produced at 800 °C (SC–KOH-800) shows obvious chemical adsorption behaviors, its adsorption capacity for NO is higher than 8.06 mg.L−1, superior to the coal-based activated carbon. Hydrazine hydrate is found to enhance De-NOx performance with SC-KOHs acting as catalysts, especially at higher temperatures. However, the De-NOx efficiency decreased when ammonia, urea and urea involved reductants were used. The physicochemical structure of the SS chars was characterized to show that the SC-KOHs are more porous with higher BET areas and pore volumes. Simultaneously, the SC-KOHs are crystallized to much lower extent, less graphitised, but have richer O-containing functional groups and zeolite structure on the surface when compared to the SC-Raws, which contributed to their high activities. Moreover, SC-KOH-800 can be recommended as the suitable adsorbent/catalyst for De-NOx within 100–250 °C.

Taming NO oxidation efficiency by γ-MnO2 morphology regulation

Nitric oxide (NO) emitted from the combustion of fossil fuels has drawn global concern, and the oxidation of NO contributes greatly to the DeNOx process.

In situ electron-induced reduction of NOx via CNTs activated by DBD at low temperature

In this study, a new in situ electron-induced process is presented with carbon nanotubes (CNTs) as a reduction agent activated by dielectric barrier discharge (DBD) for nitrogen oxide (NOx) abatement at low temperature (< 407 K). Compared with a single DBD system and a DBD system with activated carbon (DBD-AC), a DBD system with carbon nanotubes (DBD-CNT) showed a significant promotion of NOx removal efficiency and N2 selectivity. Although the O2 content was 10%, the NOx conversion and N2 selectivity in the DBD-CNT system still reached 64.9% and 81.9% at a specific input energy (SIE) of 1424 J/L, and these values decreased to 16.8%, 31.9% and 43.2%, 62.3% in the single DBD system and the DBD-AC system, respectively. X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) were utilized to investigate surface changes in the CNTs after activation by DBD to explore the NOx reduction abatement mechanism of this new process. Furthermore, the outlet gas components were also observed via Fourier transform infrared spectroscopy (FTIR) to help reveal the NOx reduction mechanism. Experimental results verified that carbon atoms excited by DBD and the structure of CNTs contributed to the synergistic activity of the DBD-CNT system. The new deNOx process was accomplished through in situ heterogenetic reduction reactions between the NOx and carbon atoms activated by the plasma on the CNTs. In addition, further results indicated that the new deNOx process exhibited acceptable SO2 tolerance and water resistance.

Cr3+ substituted Zn-Al layered double hydroxides as UV–Vis light photocatalysts for NO gas removal from the urban environment

The ZnAl-CO3, ZnAlCr-CO3 and ZnCr-CO3 LDH samples were studied as De-NOx photocatalysts in this work. Samples without Cr and increasing the presence of Cr3+ in the LDH framework in the 0.06, 0.15 and 0.3 Cr/Zn ratio were prepared by co-precipitation method, all of them constituted by pure LDH phase. The increase of chromium content in the LDH framework leads to lower crystallinity and higher specific surface area in the samples. Moreover, the CrO6 octahedron centres expand the photo-activity from UV to Visible light and assist to decrease the recombination rate of the electrons and holes. The favourable textural, optical and electronic properties of Cr-containing LDH samples explain the good NO removal efficiency (55%) and outstanding selectivity (90%) found for the analysed De-NOx process.

In situ, portable and robust laser sensor for simultaneous measurement of ammonia, water vapor and temperature in denitrification processes of coal fired power plants

Simultaneous measurement of the ammonia, flue gas temperature and water vapor concentration in denitrification (deNOx) processes is of great importance for improving the deNOx efficiency of coal-fired power plants (CFPPs) and maximizing energy conservation and emission reduction. A portable, robust, low-cost and low-power laser sensor was developed based on wavelength modulation spectroscopy (WMS) with a single near infrared diode laser, which covers the optimized NH3 spectral line at 6612.73 cm−1 and H2O line pair at 6612.02 and 6609.85 cm−1. A calibration-free for WMS with varied modulation amplitude (WMS-VMA) was proposed to improve the performance of the tri-spectral line analysis. A probe, based on Herriott cell structure, coupled with deformation resistance, dust-proof, air-tight and anti-seismic, was design to adapt to the harsh environment of CFPPs. Lab-based sensor evaluation showed that the measurement accuracies for the temperature, H2O and NH3 concentration are 2.9 %, 4.8 % and 5.1 %, respectively. Allan variance analysis shows that the sensor has good stability, and the measurement sensitivity at five-second resolution are 0.1 ppm and 0.03 % for NH3 and H2O concentration, respectively. Furthermore, the feasibility of the sensor system was also verified by field tests at both deNOx inlet and outlet in a practical CFPP, denoting great potential of ammonia injection optimization in deNOx process.

Investigation of in situ high temperature sensor based on the direct absorption spectroscopy signal of ammonia gas for coal-fired power plant

The selective catalytic reduction and non-selective catalytic reduction (SCR/SNCR) are the most popular de-NOx processes for coal-fired power plant, where the real-time temperature is a significant parameter. A new developed tunable diode laser spectroscopy used for measuring the temperature of ammonia (NH3) in the process of SCR/SNCR influenced by strong exhaust gas matrices CO2 and H2O was presented in this paper. Two lines near 6605 cm−1 were selected, with rather weak temperature dependency and minimal interface from CO2 and H2O according to HITRAN12 database. A homemade heating equipment for high temperature laser spectroscopy measurement was developed based on an ARM microcontroller. The temperature calibration experiments of absorption lines were carried out at variable concentrations under different temperatures. The spectral signals of 2.5%, 3.5% and 4.5% NH3 were observed for validation of the generalization and robustness of the sensor. The calibration and validation experimental results show that the maximum measurement error and global average error of the temperature sensor with the temperature ranging from 453 to 653 K are less than 2% and 1.38%, respectively. It demonstrates that our proposed sensor with high accuracy and linearity, and can be utilized to measure the temperature in SNCR/SCR process.

The application of modified cenospheres in DeNOx process

Cenospheres were modified with iron, manganese and/or copper ions by the hydrotalcite method. The obtained catalysts were characterized by FTIR, XRD and low-temperature nitrogen sorption. The best catalyst at low temperature (200 °C) was CBFe-Mn while at the highest measured temperature of 500 °C both CBFe-Mn and CBMn-Cu showed similar performance.

Simultaneous catalytic reduction of N2O and NOx for tertiary N2O abatement technology: A field study in a nitric acid production plant

To investigate the feasibility of the simultaneous abatement of N2O and NOx in a single reactor, this field study was performed in a nitric acid production plant. An Fe/BEA powder catalyst prepared by Fe ion‐exchange with steam‐treated BEA zeolite was extruded in pellets and honeycombs. In the simultaneous reduction of N2O and NOx, the pellet‐ and honeycomb‐type catalysts showed more than a 90% and 95% conversion of N2O and NOx, respectively, at a reaction temperature between 350 °C and 380 °C which is typical for the DeNOx process. The catalytic performance of the pellets and honeycombs was also stable during the reactor operation period. With higher N2O and NOx conversions of over 90%, NH3 concentration in exhaust gas streams can be reduced to satisfy domestic regulations for air pollutant emissions. Accordingly, it is suggested that the simultaneous reduction of N2O and NOx in a single reactor with the Fe/BEA catalyst may be a reasonable option for a tertiary abatement technology for nitric acid production plants.Novelty or Significance: The simultaneous catalytic reduction of N2O and NOx in a tail gas from a nitric acid production plant was investigated with Fe/BEA catalysts prepared by Fe ion‐exchange with steam‐treated BEA zeolite. Pellet‐ and honeycomb‐type catalysts were applied to abatement using NH3 as the reducing agent. Over 90% of N2O and NOx from the nitric acid production process was simultaneously removed from a single reactor at a reaction temperature between 350 °C and 380 °C. © 2018 American Institute of Chemical Engineers Environ Prog, 38: 451–456, 2019

Simulation of the Sour-Compression Unit (SCU) process for CO2 purification applied to flue gases coming from oxy-combustion cement industries

The purpose of the present work was to investigate, for a gas issued from a full oxy-fuel combustion in the cement industry, a CO2 de-SOx and de-NOx process, called “Sour-Compression Unit” (SCU), thanks to simulations with Aspen PlusTM. An important stage necessary for the SCU modeling has been the construction of an accurate chemical mechanism. Two-column and single-column configurations have been evaluated and compared. A parametric study and a Design Of Experiments have been conducted on a single-column process to study the influence of the operating parameters on the SOx and NOx abatement ratios. As a demonstration of the effectiveness of the model, three SOx and NOx purity specifications (depending on the further applications of the CO2) were applied to the purified gas. The feature of the investigated model lies on optimizing the way to reach the purity target in order to decrease installation costs (CAPEX) and energy requirements (OPEX).

Case Studies for Clean Technology Development in the Chemical Industry Using Zeolite Based Catalysts

This paper deals with the practical implementation of cleaner technologies in the chemical industry, using two case studies as an illustration. The first case study deals with the removal of NOx and N2O gases over an iron-doped ZSM-5 catalyst developed for tail gas treatment in nitric acid manufacturing. The aim for this case study was to investigate the efficiency of the zeolitic catalyst in the DeNOx process and to compare its catalytic activity with the conventional vanadia-titania. By the experiments carried out, it can be concluded that the new technological developments could significantly contribute to a decrease in environmental pollution. The second case study focuses on zeolite-based catalysts prepared from zeolitic tuff by the impregnation method, for biodiesel production from waste sunflower vegetable oil. The effects of operating and processing variables such as reaction temperature and time were investigated. The results showed that the highest biodiesel yield was 96.7% at an 11.5 MeOH/oil molar ratio, in the presence of 6.4 wt % catalyst at a 50 °C reaction temperature and reaction time of 2 h. The properties of the biodiesel that was produced, such as the viscosity, meet the required specifications of standard JUS EN14214. The common feature of the two different case studies is that both technologies use zeolite catalysts, namely naturally-occurring zeolitic tuff and synthetic ZSM type zeolite catalyst. The examples shown emphasize the importance of the zeolites in clean chemical technologies, which contribute to the protection of the environment.

In-situ electrochemical DeNOx under mild conditions depending on perovskite-modified gas diffusion electrode

NOx is a significant atmospheric pollutant triggering haze weather. The existing DeNOx technology has drawbacks of needing additional reducing agent and high working temperature. In order to overcome those defects, a novel La0.6Sr0.4Fe0.8Mn0.2O3 (LSFM)-modified gas diffusion electrode (GDE) was prepared to establish an in-situ electrochemical DeNOx process. The DeNOx efficiency was enhanced by increasing Nafion and polytetrafluoroethylene (PTFE) content in the GDE within an appropriate range. Nafion created the proton transport pathway of electrolyte-Nafion-three phase interfaces (TPIs) and PTFE improved the hydrophobic gas channels. The highest DeNOx efficiency of 71.1% was obtained with inlet NOx of 1000 ppm under ambient temperature (25 °C) when the mass ratio of LSFM:Nafion:PTFE was 1:0.117:0.23. In the electrochemical DeNOx process, i) NOx was absorbed by the gas diffusion layer and then transferred to the catalyst layer through the hydrophobic channels; ii) NOx was electrochemically decomposed to N2 and O2− under the catalysis of LSFM; iii) O2− was transformed to OH− on the TPIs by combining with H+ and oxidized to O2 in the anode. Simultaneously N2 was released. Its advantages of no reducing agent, mild operating condition and high energy-efficiency indicate great potential for practical application.