Reduction Denitration Research Articles

Modulating NH3 oxidation and inhibiting sulfate deposition to improve NH3-SCR denitration performance by controlling Mn/Nb ratio over MnaNbTi2Ox (a = 0.6-0.9) catalysts

The MnaNbTi2Ox (a = 0.6–0.9) catalysts for NH3 selective catalytic reduction denitration were prepared using the co-precipitation method. Among them, Mn0.7NbTi2Ox exhibits well low-temperature catalytic performance, wide activity temperature range (180–480 ℃), and worthy resistance to SO2 even with H2O. XRD was used to investigate the structure of MnaNbTi2Ox, in which Mn and Nb oxides highly dispersed in the MnaNbTi2Ox catalysts and Nb can dope into the crystal lattice of TiO2. XRF and XPS results show Nb can affect the transfer of electrons to Mn4+, and changing the Mn/Nb ratio can regulate the Mn4+ content on the MnaNbTi2Ox catalysts. H2-TPR, NH3 and NO oxidation results verify that Nb inhibits the oxidation capacity of MnaNbTi2Ox, and altering the Mn/Nb ratio can get appropriate oxidation property, which facilitates low-temperature NH3 activation and limit non-selective oxidation for NH3. In-situ DRIFTS results show Nb-OH bonds can provide new Brønsted acid sites, and both Lewis and Brønsted acid sites are active. Furthermore, Nb addition prevents sulphate deposition on the catalyst. The effect of Mn/Nb on catalytic performance, N2O formation and inhibition, SO2 poisoning, SO2 effect on NH3-SCR, and the enhancement of SO2 tolerance are also analyzed.

Insights into the mechanism of enhancing resistance of MnAlOx-Str NH3-SCR catalyst derived from streptomycin waste residue and LDHs

Streptomycin waste residue composited with Layered double hydroxides (LDHs) derived Mn1Al1Ox-Str-2 catalyst was proposed for NH3 selective catalytic reduction (NH3-SCR) denitration technology, which improved the overall catalytic performance. In the wide temperature range of 150-300℃, the preferred Mn1Al1Ox-Str-2 catalyst had Nitrogen oxides (NOx) conversion of exceeded 96%, which was higher than that of the control Mn1Al1Ox and Mn/Al2O3 catalysts. In addition, Mn1Al1Ox-Str-2 catalyst had better resistance to SO2 and/or H2O, as well as good regeneration ability. Various analytical techniques had confirmed that the Mn1Al1Ox-Str-2 catalyst derived from streptomycin residue composite LDHs precursor had better surface properties with more active species, abundant surface oxygen, higher reduction capacity and better adsorption of NOx and NH3. Analytical techniques revealed that the Lewis acid site on the surface of Mn1Al1Ox-Str-2 catalyst was involved in SCR reaction and both the Eley-Rideal mechanism and Langmuir-Hinshelwood mechanism existed simultaneously in the NH3-SCR reaction, and SO2 had a weak effect on the adsorption of reactants with fewer sulfate species formation. This work provided feasible strategies for efficient and stable SCR technology and efficient utilization of streptomycin residue waste resources, which was significant for the design and application of SCR technology in the future.

Application of Selective Non Catalytic Reduction (SNCR) Denitration Technology in W-flame Boiler

Application of Selective Non Catalytic Reduction (SNCR) Denitration Technology in W-flame Boiler

Tailoring tunnel-type potassium-free manganese oxide catalyst via cerium substitution for catalytic NO reduction with NH3 at ultra-low temperatures

Manganese oxide (KMn8O16, K-MO) was widely used for selective catalytic reduction (SCR) denitration due to its superior ability for activating and converting NO to N2 with NH3. However, the harmful K+ in its tunnel structure tends to consume NH3, leading to limited low-temperature activity and stability. Herein, a Ce substitution strategy was developed for the preparation of K-free Ce-MO for NH3-SCR. As-prepared Ce-MO exhibits a high NOx conversion (95%) at an ultra-low temperature of 75°C, with negligible activity decay within 60 h. Systematic characterizations elucidate the important role of Ce substitution in improvement of catalytic efficiency and stability for NO removal. The introduction of Ce not only eliminates harmful K+ and improves the stability of the catalyst, but also enhances the removal efficiency of NOx by the generation of oxygen vacancies. In addition, the formation of intermediate NH4NO3 is critical to improving the catalytic efficiency and stability over Ce-MO base on in-situ DRIFTS. This work provides a favorable strategy for the preparation of free-K tunnel-type manganese oxide catalyst for denitration with high activity and superior stability at ultra-low temperature.

Recyclable Iron Reduction Denitration: A Strategy for Removal of Nitrogen Oxides

Recyclable Iron Reduction Denitration: A Strategy for Removal of Nitrogen Oxides

Carbon monoxide+ammonia coupling denitration at low temperatures over manganese–cerium/activated carbon catalysts

To identify the mechanism of the low-temperature CO + NH3 coupling selective catalytic reduction (SCR) denitration process, a series of Mn–Ce/activated carbon catalysts differing in the Ce content were prepared and characterized via SEM, EDS, XRD, XPS, FTIR and evaluated in the CO + NH3 coupling SCR denitration. The results show that the denitrification rates of Mn-Ce/AC catalysts are in the order of 7Mn-3Ce/AC > 7Mn-5Ce/AC > 7Mn-1Ce/AC at a temperature range of 150 °C–300 °C and an oxygen content of 9%. A moderate Ce content provides a smooth catalyst surface with a homogeneous distribution of metal particles and pores, resulting in the optimization of the physical and chemical performance of the Mn-based catalyst. As a result, more active oxygen species and lattice defects are formed, which promoted the dispersion of active components on the catalyst surface, increased the adsorption sites of CO and NH3, and provided more active sites for denitration reaction. Mn4+ and Ce3+ both increased to varying degrees, and oxygen-containing functional groups carboxyl and lactone groups increased, which promoted the synergistic reaction between MnCe, enhancing the NO adsorption and conversion rate. At 150 °C–250 °C, the CO + NH3 coupling denitration is mainly governed by physical adsorption, whereas chemical adsorption is the predominant mechanism at 250 °C–300 °C. The denitration process includes four stages: reaction gas adsorption, adsorption molecular dissociation, catalytic denitration of metal active components and product desorption.

A New Regularized Spatiotemporal Attention-Based LSTM with Application to Nitrogen Oxides Emission Prediction.

The data collected from complex process industries are usually time series with considerable nonlinearities and dynamics, as well as excessive redundancy. Moreover, there are temporal and spatial correlations between input variables and key performance variables. These characteristics bring great difficulties to data-driven modeling of the key performance variables. To overcome the problems, a new regularized spatiotemporal attention (STA)-based long short-term memory (LSTM) was developed. First, a standard LSTM network with an STA module was trained to capture the dynamic relationship between input and target variables. Second, the least absolute shrinkage and selection operator was introduced to optimize the STA module. Third, the hyperparameter representing the regularization strength of the algorithm was determined using a moving window cross-validation strategy. Finally, the proposed algorithm was compared to other state-of-the-art algorithms using artificial data, and then it was used to predict the nitrogen oxide emissions of a selective catalytic reduction denitration system. Simulation results showed that the proposed algorithm achieved more accurate predictions than the other algorithms. Furthermore, the statistics and analysis of the importance of the variables are consistent with known chemical-reaction mechanisms and observations of field experts. Thus, the proposed method can provide technical support for the predictive control and optimization of such systems.

Investigation on low energy-consumed embedded selective catalytic reduction technology for pelletizing flue gas and the CO2 emission reduction assessment.

Based on the unique technique property of grate-kiln pellet production process and its demand of ultra-low emission of NOx, a low energy-consumed embedded selective catalytic reduction (SCR) denitration technology was proposed. The temperature of the NOx-containing tributary flue gas was 350-500 °C, which basically accorded with the temperature range of SCR by V2O5-based catalyst. Considering thepotential inhibition effect of high SO2 concentration (8000-10000 mg/m3) and metal-containing dust in the pelletizing flue gas, the catalyst compositions were optimized to V2O5 (0.5%) and WO3 (5%), giving NO conversion over 90% with low yield of N2O by-product. Compared with the low-medium temperature SCR technology, it was no longer necessary to reheat the flue gas, showinga remarkable CO2 emission reduction effect. The CO2 emission reduction ratios were 94% and 66% contributed by the decrease of fuel and electricity consumption for the embedded SCR technology, respectively. The operating cost was also greatly reduced from 11.4 CNY/t-pellet to 3.1 CNY/t-pellet (Chinese Yuan).

Integrated technology for dust removal and denitration of high-temperature flue gas in coal-fired power plants

A new integrated technology of high-temperature dust removal and denitration, which solves the catalyst problems of wear, blockage, and poisoning caused by the operation of selective catalytic reduction (SCR) denitration systems in coal-fired power plants under high concentrations of particulate matter, is developed. A pilot plant was established for the integrated technical performance test research based on the actual engineering flue gas. The test research results agree well with the design indicator values. The maximum denitration efficiency and potential of the high-temperature dust removal and denitration integrated pilot plant were no <88.445 % and 2.97, respectively. The removal efficiency of sulfur trioxide was > 60 %, and the efficiency improved with the addition of alkaline substances. The removal effect of NaHCO3 on sulfur dioxide was higher than that of calcium hydroxide. The mercury concentration in the flue gas at the outlet increased with an increase in the filtration velocity. The test results provide a reference for the optimization and engineering application of the developed integrated technology and equipment.

Performance Analysis and Optimization for Static Mixer of SCR Denitration System under Different Arrangements

In order to solve the poor flow performance issues of selective catalytic reduction (SCR) denitration systems, the effect of the static mixer on the flow field was studied using computational fluid dynamics (CFD) numerical simulations. Based on the analysis of the original SCR denitration system, two static mixers were selected to explore their influence on system performance. The results show that both static mixers can effectively improve the denitration performance under different conditions. The static mixer with a rotating arrangement showed a better performance in the uniformity of concentration. The pressure loss without a static mixer is 834 Pa, and the pressure loss increases by 94 Pa and 73 Pa for rotating and X-arranged static mixers, respectively. Meanwhile, a static mixer will increase energy loss. Therefore, power plants can choose the layout of their static mixers according to the actual situation to achieve the optimal performance.

CeO2-doped WO3 composite catalyst based on acid site enhancement for diesel exhaust gas denitration

Based on the complicated preparation of current diesel vehicle exhaust gas denitration catalysts, an in-situ deposited composite oxide catalyst, CeO2-WO3 mixed oxide catalyst, was prepared by electrodeposition and hydrothermal methods, which was loaded on the titanium mesh and applied to the selective catalytic reduction denitration of diesel vehicle exhaust. Denitration performance of the catalysts was tested by a fixed bed reactor, and the influence of different electrodeposition time of CeO2 was investigated. The results demonstrate that 20 min is the best electrodeposition time of CeO2 (100% NOx conversion at 250–350 °C). The high dispersion of active CeO2 on WO3 promotes the synergistic effects among different components. The as-prepared catalysts were characterized by SEM, XRD, XPS, H2-TPR, NH3-TPD and in-situ DRIFTS. It is evident that Ce3+ is successfully introduced by loading CeO2, which enhances the chemisorption of oxygen. Meanwhile, the increased acidity including both weak acid and medium-strong acid sites of CeO2-WO3 composite catalyst is observed, which improves the co-adsorption of NH3 on Lewis acid and Brønsted acid simultaneously and facilitates the denitration process. Through the characterization by in-situ DRIFTS, it is elucidated that the NH3-SCR reactions are mainly carried out following the Eley-Rideal (E-R) pathway in a medium-temperature range (250–350 °C).

Iron-based denitration catalyst derived from Fenton sludge: Optimization analysis of selective dealkalization and influence mechanism of calcination temperature

Fenton sludge from sewage treatment plants is a kind of hazardous waste that causes serious harm to the ecological environment. In this study, the potential of preparing a novel selective catalytic reduction (SCR) denitration (deNOx) catalyst was evaluated through selective dealkalization followed by calcination using Fenton sludge. The effects of water leaching, carbon dioxide leaching, and acid leaching methods (organic and inorganic acid) on Fenton sludge dealkalization were studied, taking the total dealkalization and active component retention rates and deNOx efficiency as comprehensive considerations. Results showed that acid leaching method has the best leaching effect on free alkali and chemically combined alkali, the order of dealkalization rates was FSH2SO4 (94.23%)>FSHCl (92.18%)>FSHNO3 (89.93%)>FSCH3COOH (81.07%). Through comprehensive comparison, FSHCl exhibited the best catalytic active and excellent selective dealkalization performance, achieving relatively high total dealkalization rate while retaining the active components to the greatest extent (86.73%). On this basis, the effects of calcination temperature on catalytic activity, as well as physical and chemical properties of the catalyst were studied. The MFS-450 catalyst exhibited the largest surface area (76.53 m2/g), pore volume (0.23 cm3/g), abundant oxygen-containing functional groups, excellent surface acidity (0.779 μmol/m2), and redox properties (5.07 mmol/g). Besides, it has the best low-temperature SCR activity and the widest reaction temperature window, with nitrogen oxide (NOx) conversion exceeding 90% at temperatures between 293 and 471 °C. This research contributes a representative example of converting Fenton sludge into valuable materials and may open a new avenue to synthesizing high-activity SCR catalyst from low-cost feedstocks.

One-pot synthesis of rare earth modified Cu/SAPO-34 for enhanced selective catalytic reduction denitration performance

Four rare earth elements (gadolinium (Gd), dysprosium (Dy), neodymium (Nd), cerium (Ce)) modified Cu/SAPO-34 are synthesized by one-step hydrothermal method, and the effects of rare earth on the selectivity catalytic reduction (SCR) denitration (de-NOx) performance of Cu/SAPO-34 are systematically studied. Among them, Dy-modified Cu/SAPO-34 (Dy-Cu) exhibits the best de-NOx efficiency (100% NO conversion between 210–500 °C, and 100 % N2 selectivity between 210–390 °C) under a gas hourly space velocity (GHSV) of 36,000 h−1. It is revealed that rare earth modification effectively reduces the formation of CuO, enhances the Cu2+ content, increases the number of acid sites, and improves the NO oxidation performance, which are beneficial to improve the de-NOx performance of Cu/SAPO-34. Compared with Gd, Nd and Ce, Dy is more favorable for Cu2+ to move toward the eight-membered ring of Cu/SAPO-34, and Cu2+ in the eight-membered ring is more beneficial to improve the NH3-SCR activity. The in-situ diffuse reflectance infrared Fourier-transform spectra show that the NH3-SCR reaction on Dy-Cu and Cu/SAPO-34 follows the Langmuir-Hinshelwood mechanism.

Analysis uneven selective catalytic reduction flow field of coal-fired units under the situation of double carbon

With the implementation of China’s carbon peaking and carbon neutralization strategy and the accelerating transformation of energy structure, higher requirements are put forward for the carbon reduction and operation and maintenance capacity of coal-fired units. Based on the analysis of the uneven Selective catalytic reduction flow field of a coal-fired unit in a power plant, the field tests under different conditions are used to study the inlet flow field distribution and outlet NOx concentration distribution of the denitration device of the unit, and the targeted optimization, adjustment of ammonia injection branch pipe is carried out, which effectively solves the uneven Selective catalytic reduction denitration flow field and can provide reference for similar units.

Progress of selective catalytic reduction denitrification catalysts at wide temperature in carbon neutralization.

With the looming goal of carbon neutrality and increasingly stringent environmental protection policies, gas purification in coal-fired power plants is becoming more and more intense. To achieve the NOx emission standard when coal-fired power plants are operating at full load, wide-temperature denitrification catalysts that can operate for a long time in the range of 260–420°C are worthy of study. This review focuses on the research progress and deactivation mechanism of selective catalytic reduction (SCR) denitration catalysts applied to a wide temperature range. With the increasing application of SCR catalysts, it also means that a large amount of spent catalysts is generated every year due to deactivation. Therefore, it is necessary to recycle the wide temperature SCR denitration catalyst. The challenges faced by wide-temperature SCR denitration catalysts are summarized by comparing their regeneration processes. Finally, its future development is prospected.

A novel online modeling for NOx generation prediction in coal-fired boiler

The selective catalytic reduction (SCR) denitration technology is widely used in coal-fired generating units. The NOx concentration of boiler outlet is an important parameter in the feedforward control of SCR denitration. However, its measurement lag leads to a large range fluctuation of NOx emission, which affects the safe and economic operation of the unit. In order to solve the problem of boiler outlet NOx concentration measurement lag in denitration control, and improve the timeliness of fluctuation response for denitration control. Many studies have reported on NOx concentration prediction models based on the long short-term memory (LSTM) algorithm, support vector machines (SVM) algorithm, et al. However, there are no reports on online modeling, particularly none on predictive values of boiler outlet NOx concentration ahead of the measured values. Thus, in this study, a 1000 MW ultra-supercritical coal-fired boiler was selected, and 2404 sets of measured samples were collected to predict NOx concentration. A novel online modeling method for NOx concentration of boiler outlet was proposed. For the first time, a high-precision online real-time prediction model of boiler outlet NOx concentration was innovatively established based on improved long short-term memory network (ILSTM). A feature quantity weight analysis method based on the RRelieff algorithm is adopted, and the change rates of feature quantities were used as input in the model. The results showed that the root mean square error (δR) and computation time of ILSTMN reduced relatively by 17.97 % and 1.97 s, respectively. The online model with satisfied accuracy is trained in 1 s, which uses the latest recent data from decentralized control system (DCS). The NOx concentration of boiler outlet predicted by the online model is 22 s ahead of the measurement NOx concentration, and the prediction accuracy is still as high as 96 % without the intervention after two years of operation. As a feedforward of SCR denitration control system, NOx concentration predicted by the model can significantly improve the timeliness of control response. The online model provides theoretical support for suppressing large fluctuations of NOx emissions.

Experimental and Mechanistic Study of Synergistic Removal of Hg by Evaporation from Desulfurization Wastewater

The flue evaporation of desulfurization wastewater can solve the problem that it is difficult to remove some heavy metal ions and chloride ions by conventional methods. A large amount of chloride ions in desulfurization wastewater can also promote the catalytic oxidation removal of Hg in the flue gas. The migration character of chloride ions in the flue evaporation process of desulfurization wastewater was studied by using the coal-fired thermal state experimental platform. The concentrations of Hg0 and Hg2+ in the flue gas at the inlet and outlet of selective catalytic reduction denitration (SCR), electrostatic precipitator (ESP), and wet desulfurization (WFGD) devices were tested, and the synergistic removal of traditional pollutant removal equipment by flue evaporation of desulfurization wastewater was analyzed. The influence of Hg and the effect of the evaporation of desulfurization wastewater at different positions on the removal of Hg in the flue gas were compared and analyzed, and the catalytic mechanism of Hg on the SCR surface was further revealed. The results show that 10% chloride ions enter the flue gas after the desulfurization wastewater evaporates. The content of chlorine elements and evaporation temperature influence the evaporation of desulfurization wastewater. The mechanism of SCR catalytic oxidation of Hg0 was explored; oxygen atoms have catalytic oxidation effects on Hg0 at different positions in the V2O5 molecule in SCR; and chloride ions can enhance the catalytic oxidation of Hg0 by V2O5. The intermediate product HgCl is generated, which is finally converted into HgCl2. The oxidation efficiency of Hg0 in electrostatic precipitation (ESP) is increased from 3% to 18%, and the removal efficiency of Hg is increased from 5% to 10%. The removal efficiency of Hg2+ in WFGD is basically maintained at approximately 85%. In addition, a small amount of Hg2+ was restored to Hg0 in WFGD. The removal efficiency of Hg0 in the flue gas of evaporative desulfurization wastewater before SCR is 65%, and the removal efficiency of gaseous Hg is 62%. When the evaporative desulfurization wastewater before ESP, the synergistic removal efficiency of Hg0 is 39%, and the gaseous Hg removal efficiency is 39%, and the removal efficiency of Hg is 40%. Evaporation of the desulfurization wastewater before SCR was more conducive to the coordinated removal of Hg by the device.

Mechanism of CO selective catalytic reduction denitration on Fe–Mn/AC catalysts at medium and low temperatures under oxygen atmosphere

To explore the modification mechanism of Fe and Mn on activated carbon and the denitration mechanism by selective catalytic reduction of carbon monoxide (CO-SCR) on Fe–and Mn-loaded activated carbon (Fe–Mn/AC) catalysts at medium and low temperatures under 8% oxygen, we selected a coconut shell activated carbon as a catalyst carrier and prepared Fe–Mn/AC catalysts. The physical adsorption of CO mainly occurs at low temperatures, and an increase in temperature is beneficial to the CO denitration activity of the metal active components. High Fe2+ content increases the main active sites of the CO + NO reaction, and high Mn3+ content inhibits the conversion of γ-Fe2O3 to α-Fe2O3 and increases the denitration activity. Surface oxygen species (Oα) with high reactivity and mobility are also formed during the denitration process. After denitration, Oα is mainly converted to the lattice oxygen (Oβ), and Mn2+ and Mn3+ are oxidized to Mn4+ in a small amount. The iron oxides and manganese oxides penetrate the internal structure of the catalyst and destroys its oxide crystal structure and the surface functional groups of the activated carbon, resulting in the formation of more oxygen vacancies and lattice defects. Consequently, reduction characteristics and absorption capacities increased and the spectrum peaks of H2-TPR, CO-TPR, and NO-TPR shifted to low temperatures. Moreover, the number of Lewis and Brønsted acid sites increased, resulting in sites that are more reactive. The denitration process involves the following four steps: (1) adsorption of reactant molecules, (2) dissociation of adsorbed molecules, (3) catalytic denitration by metal active components and recombination of active substances for denitration, and (4) product molecule desorption and diffusion.

Alkali Metal Poisoning and Regeneration of Selective Catalytic Reduction Denitration Catalysts: Recent Advances and Future Perspectives

Alkali Metal Poisoning and Regeneration of Selective Catalytic Reduction Denitration Catalysts: Recent Advances and Future Perspectives

Distribution and emission characteristics of filterable and condensable particulate matter in the flue gas emitted from an ultra-low emission coal-fired power plant

Total particulate matter emitted from coal-fired power plants can be classified into filterable particulate matter (FPM) and condensable particulate matter (CPM). In this study, the FPM and CPM in the flue gas were sampled from a 300 MW ultra-low emission coal-fired power plant by the simultaneous sampling system, which was conducted according to ISO 23210-2009 and U.S. EPA method 202. The results show that the emission concentration of CPM rose from 5.15 mg/Nm3 to 7.19 mg/Nm3 at the stack when coal mixed with sludge. Almost all of the air pollutant control devices have a positive effect on the removal of CPM and FPM in the flue gas, except the influence of the selective catalytic reduction (SCR) denitration device on CPM. And the SCR denitration equipment increased the concentration of inorganic components of CPM in flue gas. The low-low temperature electrostatic precipitator had the most obvious removal effect on CPM and FPM, and the removal efficiency for FPM and CPM was more than 90% and 75% respectively. The organic fraction in CPM was mainly composed of hydrocarbons, esters, organosilicon, and other organics. In particular, the proportion of hydrocarbons and organosilicon was relatively high. In the case of co-combustion of sludge and coal, the concentration of CPM and FPM in flue gas increased as a whole, but the distribution trend of CPM and FPM was consistent with that of the non-combustion sludge and the distribution of organic components in CPM was almost the same.