Search

The strong oxidation capabilities of Mn sites render Mn-based catalysts showing high activities of selective catalytic reduction (SCR) at temperatures < 250℃, but simultaneously lead to the generation of by-product N 2 O with strong greenhouse effect. The activity-N 2 -selectivity trade-off was a prevalent observation in the development of Mn-based SCR catalysts. Here, a polyoxometallic acid enhancement strategy was devised to simultaneously promote the N 2 selectivity and low-temperature SCR (LT-SCR) activity of Mn-based metal oxide catalysts, where the introduction of uniformly dispersed phosphomolybdic acid (HPMo) tuned the oxidation capabilities and enhanced NH 3 adsorption-storage capability of catalysts. The combination of transient reaction, in-situ DRIFTS analysis and DFT calculations revealed that the main origin of N 2 O on the Mn-based catalysts was the deep oxidation of NH 3 , which was inhibited by the interaction between HPMo and Mn sites. Additional NH 3 molecules adsorbed by the abundant acid sites supplied by HPMo further facilitated the SCR reaction. This strategy could guide the design of high-activity and high-selectivity LT-SCR catalysts for industrial denitration. [Display omitted] • The introduction of HPMo tuned the oxidation abilities and enhanced the acidity. • The loading of HPMo can inhibit the oxidation of NO to NO 2 and deep oxidation of NH 3. • Abundant acid sites supplied by HPMo can accelerate the SCR reaction. • The activity-selectivity trade-off can be broken by the modification of HPMo. [ABSTRACT FROM AUTHOR]

Investigation of the low-temperature (LT) selective catalytic reduction (SCR) of NOx has adsorbed extensive efforts, and is now a hot topic in environmental catalysis. In this work, we cautiously explore the mechanism beneath LT-SCR via a combination of chemical-trapping methods, using BaO/Al2O3 as a NOx-intermediate trap, temperature-programmed desorption (TPD) and in situ FTIR spectroscopy over a series of metal-oxide catalysts. Our results show a linear correlation between LT-SCR rates and amounts of nitrites generated from NO oxidative activation and sequentially stored on BaO/Al2O3, implying key roles of nitrites/nitrite-precursors in LT-SCR. [ABSTRACT FROM AUTHOR]

Under the extensive implementation of ultra-low emission (ULE) facilities in coal-fired power plants of China, sulfur trioxide (SO3) has received increasing attention due to its impact on human health and operation safety of power plants. However, systematic research and evaluation for controlling SO3 emission in various ULE facilities are still lacking. Here, a systematic study was conducted based on 378 in situ performance evaluation tests carried out in 148 coal-fired power plants. The results illustrate that the SO2/SO3 conversion rate of the selective catalytic reduction devices can be controlled within 1% before and after ULE retrofit. Also, the synergistic removal efficiency of SO3 in the low-low-temperature electrostatic precipitator and the wet electrostatic precipitator can be higher than 70%. The removal efficiency of SO3 in the wet limestone-gypsum flue gas desulfurization scrubber is 33–64% before ULE and 31–81% after, and the average efficiency of the double scrubbers is 8.7% higher than that of the single scrubber. Due to the different SO3 removing abilities of various technologies, the overall efficiency of SO3 removal is in the range between 27 and 95% adopting different ULE technical routes. Average concentration of SO3 emission can be decreased by 51.8% after ULE application. [ABSTRACT FROM AUTHOR]

Highlights • The SCR reaction occurred within a thin layer (ca. 0.2 mm) in the wall near the catalyst channel. • The SO 2 oxidation occurred in the entire catalyst wall. • The effects of the operating parameters were investigated synthetically. • A strategy for catalyst design, i.e., applying thin-walled design was proposed. Abstract A three-dimensional model that combined the selective catalytic reduction (SCR) of NO with ammonia and SO 2 oxidation reactions over monolith honeycomb catalyst was established to study the effects of catalyst structure and operating parameters on NO reduction and SO 2 oxidation. The model was to proved to be valid by experimental data. Simulation results showed that the SCR reaction only took place within a thin layer (ca. 0.2 mm) of the catalyst wall surface, whereas SO 3 generated via SO 2 oxidation accumulated throughout the entire wall. The effects of typical operating parameters, i.e., space velocity, temperature, feed concentrations of NO, NH 3 and SO 2 were studied. Temperature and gas velocity were found to be the major influencing factors on SO 2 oxidation. A strategy for monolith catalyst struction optimization, i.e., using thin-walled catalyst was proposed on the basis of these results. [ABSTRACT FROM AUTHOR]

The flue gas generated from glass furnaces contains particles with high concentrations of alkali metals and hazardous heavy metals, which could reduce the lifespan of selective catalytic reduction (SCR) catalysts and lead to catalyst deactivation. Removing the particles before SCR could improve system operation reliability and reduce nitric oxides (NOx) emission. In this paper, the characteristics of fly ash from glass furnaces and the particle removal properties in the electrostatic precipitator (ESP) from 363 K to 623 K were studied. The particle collection efficiency was found to decrease with increasing temperature. A method of enhancing particle removal was proposed and tested through particle conditioning in a lab-scale experiment. In order to change the characteristics of the fly ash, calcium carbonate powders were used as conditioning particles and the particle collection efficiency increased from 65.8% to 87.6% at 623 K. This method was further applied in an industrial high-temperature ESP for a glass furnace. When the ratio in mass between the particles from the glass furnace and from the conditioning proportion was 1:1.5, the particle collection efficiency of the ESP was improved significantly and the outlet particle concentrations decreased from 103.23 mg/Nm 3 to 39.25 mg/Nm 3 . [ABSTRACT FROM AUTHOR]

Four typical coal-fired power plants were employed to explore the performance evolution within the whole life cycle of catalysts. The results revealed that the amount of active ingredients in the catalysts decreased over time. The content of V 2 O 5 and WO 3 in four units had decreased by 3.32%–32.8% and 12.9%–22.9% respectively. However, alkali metals and alkaline earth metals in flue gas were gradually deposited on the catalyst and led to catalyst poisoning, especially in two units due to arsenic poisoning. The specific surface area of catalysts decreased significantly in the first year of operation, accounting for 87.3%, 47.8%, 55.1% and 86.7% of the decline in the lifetime, which may be the main reason for the decline of catalyst activity during this period. The wall thickness of catalysts decreased linearly with the operation time in the lifetime. The relative activity of catalysts at the end of designed service lifetime was between 0.90 and 0.93, whereas the decrease range of SO 2 /SO 3 conversion rate was within 15.4%–35.6%. Influenced by the catalyst design and the variations of practical operation conditions, the difference between the actual and designed service lifetimes of catalysts in the four units was 2.08%, 44.0%, 4.17%, and − 25.0%, respectively. • SCR catalyst of four typical coal-fired power plant was investigated. • Catalyst performance evolution in three consecutive years of operation were analyzed. • A method for checking actual service lifetime of SCR catalyst was proposed. • Difference between the actual and designed catalyst lifetimes was obtained. • K/K 0 at the end of catalyst service lifetime was between 0.91 and 0.93. [ABSTRACT FROM AUTHOR]

• A new corrosion-resistant experimental system was designed to study SOx absorption reaction with alkaline sorbents. • The SO 3 absorption performance of the three alkaline sorbents was Na 2 CO 3 > Ca(OH) 2 ≈ Mg(OH) 2. • The influence of operating parameters on SO 3 removal had been deeply studied. • The SO 3 absorption variation above 300 °C was similar to the condition without vapor. • The external diffusion control model was established for SO 3 absorption rate. Selective Catalytic Reduction (SCR) system can efficiently remove NOx in coal-fired power plant, while raise SO 3 concentration in the flue gas. Alkaline sorbent injection is a major approach to remove SO 3. Experiments were conducted to study the SO 3 absorption performance of alkaline sorbent. The SO 3 absorption performance of the three alkaline sorbents was Na 2 CO 3 > Ca(OH) 2 ≈ Mg(OH) 2. The ratio of SO 3 absorption rate to SO 2 absorption rate for Na 2 CO 3 increased with the increasing temperature. The key parameters on the SO 3 absorption performance of Na 2 CO 3 were studied. The SO 3 absorption by Na 2 CO 3 was increased by 72.7% with increasing temperature from 150 °C to 300 °C, and the temperature had little influence on SO 3 absorption above 300 °C. The increasing SO 3 concentration and decreasing particle size of Na 2 CO 3 enhanced SO 3 absorption. However, when the average particle diameter was less than 50 μm, the SO 3 absorption was no longer significantly increased. The increasing CO 2 concentration slightly reduced the SO 3 absorption. After adding vapor into the flue gas, the SO 3 absorption was obviously increased below 300 °C. The SO 3 absorption variation above 300 °C was similar to the condition without vapor. When reaction temperature was 300 °C or above and reaction time was less than 20 min, external diffusion became the control step for SO 3 absorption. On the basis of external diffusion control model, the mass transfer coefficient slightly increased from 1.68 × 10−3 m/s to 1.75 × 10−3 m/s when the reaction temperature increased from 300 °C to 400 °C. [ABSTRACT FROM AUTHOR]

Considering the practical requirements for continuous operation under part load condition, the commercial honeycomb selective catalytic reduction (SCR) catalyst was modified with Sb addition. Experiments were performed to investigate the effect of modification on long-time SCR performance under part load condition. Characterizations for the original and modified catalysts were also conducted to analyze the changes of the catalysts. The results indicated that the activity of modified catalyst was obviously enhanced in the temperature range of 275–325 °C and it achieved about 64.5% removal efficiency during the 30 h stability test at 275 °C. The characterization results indicated that the ammonium sulfate was chemically adsorbed on the catalyst surface at low temperatures, which led to the decrease of the specific surface area, pore volume, and V4+/V5+ ratio of the catalysts. These are the reasons for the decrease of the catalyst activity at low temperatures, while the deposition amount of ammonium sulfate was relatively small over the modified catalyst. In addition, the decomposition temperature of the ammonium sulfate was reduced in the modified catalyst compared with the original one. NH4+ ions decomposed at 275 °C by reacting with the NOx in the flue gas, and the dynamic equilibrium of this reaction was achieved on the modified catalyst after a short period of time. Therefore the modified catalyst can be continuously and stably operated at this temperature, and the part load operation of the SCR system in the coal-fired power plant can be realized. [ABSTRACT FROM AUTHOR]

• Oxidative activation of NO to a mobile nitrite-precursor over VOx/CeO 2 is proposed. • A mechanism for the reduction half-cycle of low-temperature SCR is established. • Synergy of VO x and CeO 2 surface is revealed in electronic and kinetic manners. • A combined experimental and theoretical method is reported for SCR mechanism study. Elucidation of redox mechanism is vital to develop highly active catalysts for selective catalytic reduction (SCR) of NOx. Here, we apply an integrated experimental and theoretical approach to investigate low-temperature SCR (LT-SCR) mechanism over VOx/CeO 2 , a model system containing industrially-relevant vanadia active-species and an eye-catching redox support, i.e. ceria, frequently documented in SCR studies. We show that NO oxidative activation to a gaseous nitrite-precursor intermediate, which was trapped by BaO/Al 2 O 3 and further spectroscopically and computationally validated, serves as a key step in LT-SCR. This scheme involves paired contributions from vanadia and ceria, in which vanadium stabilizes at +5 while Ce3+/Ce4+ varies in the redox cascade, and coupling of V5+−OH and proximal Ce4+−O reduces the NO oxidative activation barrier. These findings progress the understanding of LT-SCR mechanism and deliver a specific perspective on the synergy of surface active-sites and supports, which are essential for the design of further improved SCR catalysts. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • The SCR activity and alkali resistance was tested over a series of vanadia-titania catalysts. • V 4 /Ti catalyst displayed the best performance after doping with potassium. • V4+ species were sensitive to be affected by alkali metals. • Acidity played a critical role in the poisoned catalysts. • Competition of NH 3 oxidation mainly caused the deactivation with V > 4%. Abstract A series of vanadia-titania catalysts with different vanadia loadings for the selective catalytic reduction (SCR) of NO by ammonia was prepared via incipient impregnation method. The alkali resistance and SCR activity at low temperature has been investigated on the prepared catalysts. Increasing the vanadia loading from 1% to 10%, the NO removal efficiency of the catalysts gained a 70% growth at 200 °C. With the increase of the vanadia loading, the high loading vanadia-titania catalysts exhibited higher ratio of V5+/(V4++V5+), which enhanced the oxidation of NH 3 at high temperatures and also improved the standard SCR performance at low temperature. After doping with potassium, V 4 /Ti catalyst (V 2 O 5 = 4 wt%) displayed the best performance over which 80% initial activity was retained from 300 °C to 400 °C. More acid sites and active vanadium species were retained in the high loading vanadia-titania catalysts, which made them exhibit better low temperature alkali resistance. However, when vanadia content was higher than 4%, the competition of NH 3 oxidation severely deactivated the catalytic activity above 250 °C. Additionally, in the presence of SO 2 and H 2 O the SCR performance of the high loading vanadia-titania catalysts were also studied in order to investigate the influence of the actual working conditions. [ABSTRACT FROM AUTHOR]

[Display omitted] • CeO 2 -Fe 2 O 3 -Nb 2 O 5 catalyst presents similar activity with CeO 2 -Nb 2 O 5 catalyst. • CeO 2 -Fe 2 O 3 -Nb 2 O 5 catalyst owns an enhanced SO 2 -tolerant ability. • SO 4 2− on the CeO 2 -Fe 2 O 3 -Nb 2 O 5 catalyst is easy to be reduced. • Fe 2 O 3 in the CeO 2 -Fe 2 O 3 -Nb 2 O 5 catalyst protects CeO 2 from being poisoned by SO 2. In this study, CeO 2 -Nb 2 O 5 and CeO 2 -Fe 2 O 3 -Nb 2 O 5 mixed oxides used for the selective catalytic reduction of NO with NH 3 (SCR) were prepared and the performance of these two samples was investigated. Given the modification of Fe 2 O 3 , catalyst sulfur resistance was significantly improved with the SCR activity remaining almost unchanged. For CeO 2 -Nb 2 O 5 sample, SO 2 reacted with the main redox sites, CeO 2 , to form inactive species, Ce 2 (SO 4) 3. By contrast, SO 2 preferentially interacted with Fe 2 O 3 , which left surface cerium oxygen species preserved for CeO 2 -Fe 2 O 3 -Nb 2 O 5 catalyst and enabled this sample with an enhanced SO 2 -resistant ability. At the same time, SO 4 2− on the Fe 2 O 3 -added catalyst exhibited a lower stability. It seemed that CeO 2 -Fe 2 O 3 -Nb 2 O 5 sample might be easily recycled via H 2 regeneration at a relatively low temperature point and be a potential catalyst for the applications in the SCR systems. [ABSTRACT FROM AUTHOR]

NH 3 oxidation over V-0.5Ce(SO 4 ) 2 /Ti catalyst used for NH 3 -SCR was investigated and compared to that occurred over a commercial V-W/Ti catalyst. It was found that much more significant NH 3 oxidation into N 2 and N 2 O occurs over V-0.5Ce(SO 4 ) 2 /Ti than commercial V-W/Ti catalyst at high temperatures (>300 °C). According to the kinetics and characterization results obtained, stronger surface redox properties and a relatively higher concentration of polymerized vanadates present on V-0.5Ce(SO 4 ) 2 /Ti should contribute to its higher NH 3 conversion values. In-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) studies suggested the formation of -NH imido species, based on which a successive NH 3 dehydrogenation-like mechanism is proposed, which consistently explains well the experimental observations. [ABSTRACT FROM AUTHOR]

[Display omitted] • Catalyst high-temperature activity was greatly enhanced through adding HSiW. • Doping HSiW led to the formation of abundant acid sites on the catalyst surfaces. • Adding HSiW lowered the amount and reactivity of catalyst surface oxygen species. • The balance of the acidity and oxidability played a key role in the SCR activity. A series of H 4 SiW 12 O 40 -modified Fe 2 O 3 catalysts were synthesized using the precipitation method. Over 80% NO x conversion was obtained in the temperature region of 250–450 °C for the catalysts with the H 4 SiW 12 O 40 content of 5% and 10%. The relationship between the physicochemical properties and performances of the series catalysts was established. Abundant acid sites were introduced with doping H 4 SiW 12 O 40 , which promoted the adsorption of reductant NH 3 on the catalysts at high-temperature regions. Moreover, adding H 4 SiW 12 O 40 lowered the amount and reducibility of catalyst surface reactive oxygen species, which inhibited the oxidation of NH 3 to NO x at higher temperatures to some extent. Thus, most ammonia species were prone to participating in the reaction with NO x instead of being directly oxidized to NO x in the higher temperature region over the H 4 SiW 12 O 40 -decorated catalysts. And it could be concluded that the balance of the acid and redox properties constituted the main reason for the superior activity of the high H 4 SiW 12 O 40 content catalysts. Additionally, SO 2 exerted a slight and reversible inhibition effect on the catalyst activity at 300 °C, suggesting that HSiW-modified Fe 2 O 3 oxide might be a promising catalyst in practical applications. [ABSTRACT FROM AUTHOR]

A series of V-Ce(SO 4 ) 2 /Ti catalysts for selective catalytic reduction (SCR) of NO with ammonia are prepared by impregnation method. Low temperature SCR activity and alkali resistance of the optimal V-0.5Ce(SO 4 ) 2 /Ti sample are found to be better than on the commercial V-W/Ti catalyst. Also, V-0.5Ce(SO 4 ) 2 /Ti shows an excellent durability in the presence of SO 2 and H 2 O, indicating to have prospects for the industrial application. Based on catalysts characterization and in-situ DRIFTS studies, a higher proportion of surface active oxygen generated by the introduction of Ce and a much faster H 2 reduction point out the improved redox properties of V-0.5Ce(SO 4 ) 2 /Ti, which results in a stronger NO oxidative activation and is confirmed by a more abundant formation of surface NO + and NO 3 − species. When exposed to SCR conditions where both NH 3 and NO are present, this enhanced NO activation can produce more reactive nitrite and/or NO + intermediates which then readily react with adsorbed NH 3 and decompose to N 2 and H 2 O, accounting for the improved SCR activity of V-0.5Ce(SO 4 ) 2 /Ti at low temperatures. The addition of Ce(SO 4 ) 2 also provides abundant reactive acid sites and adsorbed NH 3 species thus increase. Even after Na poisoning, adequate surface acidity and redox properties still remain. Furthermore, relatively higher contents of V-OH are preserved owing to the interaction between Na and O S O, acting as a protection for the active sites. These promotional effects contribute to the better alkali resistance of V-0.5Ce(SO 4 ) 2 /Ti. Therefore, all the results suggest that V-0.5Ce(SO 4 ) 2 /Ti is a promising candidate as a catalyst for NH 3 -SCR in coal-fired power plants, especially under high Na-content conditions. [ABSTRACT FROM AUTHOR]

The life cycle environmental impacts of widely used flue-gas cleaning processes, including limestone-gypsum wet flue-gas desulfurization (FGD), selective catalytic reduction denitration (SCR) and electrostatic precipitators (ESP), are compared. The abbreviations FGD, SCR and ESP in this article refer in particular to the above listed processes. Energy consumption, resource consumption and emission inventories of each life cycle phase are established. The potentials of five environmental impact categories, namely, global warming, acidification, nutrient enrichment, photochemical ozone formation, soot and ashes are finally obtained. The current paper reveals that the total energy consumption and resource consumption are dominated by the process operation. The three processes are compared and it is found that to achieve equal environmental burden reductions, the SCR process consumes more energy and approximately the same amount of air and inert rock as the FGD. But the water consumption of the FGD is much higher than the SCR. The ESP process consumes both the least energy and the least resource in general. The economic assessment indicates that the performance of the ESP process tends to be the best and the FGD follows when equally invested. [ABSTRACT FROM AUTHOR]

Ceria has absorbed extensive interests in functioning as catalysts for low-temperature (LT) selective catalytic reduction (SCR) of NOx due to its abundant surface oxygen species and salient redox property. The mechanistic interpretations of how such redox property contributes to its LT-SCR activity, however, still remain debated. Here, we use a model tungsten-promoted ceria catalyst (WO 3 /CeO 2), known as preeminent in LT-SCR reactions, and combine steady-state kinetic, structural characteristic and in situ spectroscopic experiments with theoretical treatments to reveal the mechanistic connections between NO oxidative activation and LT-SCR turnovers. We show that, compared with NO oxidation to NO 2 , NO oxidative activation to nitrite intermediates is both kinetically and thermodynamically more favorable; surface nitrate species are not detected in the present case, indicating their more difficult formation than nitrites. All these results thus suggest the prevalence of NO activation to nitrites over NO to nitrates or oxidation to NO 2 and subsequent occurrence of fast SCR in the WO 3 /CeO 2 -catalyzed LT-SCR reaction. These findings progress the understanding of LT-SCR reaction mechanism on CeO 2 -based catalysts and convey a detailed perspective of different reaction intermediates and their consequences for LT-SCR turnovers, which may contribute to the rational design of catalysts towards further improved LT-SCR activity. [ABSTRACT FROM AUTHOR]

Arsenic (As) is found to be poisonous to the commercial V 2 O 5 WO 3 /TiO 2 catalysts for selective catalytic reduction of NO x with NH 3 . The NO x conversion of catalysts declines and N 2 O formation dominates at high temperature (above 300 °C) after As poisoning. A series of activity and characterization experiments are applied to reveal the deactivation mechanism caused by As. Results indicate that doping of As on the catalysts, which exists as H 2 AsO 4 − and HAsO 4 2− , doesn’t seriously change surface area of catalysts or TiO 2 phase, but greatly decreases both the Lewis and Brønsted acid sites. It is found that V OH is destroyed and less reactive As OH is newly formed. V OH site is crucial to the NH 3 adsorption and its destruction by As contributes to catalyst deactivation. Besides, stronger oxidizability of catalysts that resulted from more surface-active oxygen aroused by As leads to substantial non-selective catalytic reduction reaction and NH 3 oxidation at high temperature. Both of these two aspects result in lower NO x conversion and higher N 2 O formation. However, SO 4 2− can provide remarkable surface acidities and the sites that destroyed by As can thus be supplied. Benefited from these new reactive acid sites, catalysts with prominent SO 4 2− loading show superior arsenic resistance even with a high As content, which indicate to be a promising anti-poisoning formula. Finally, mechanism of arsenic poisoning and resistance effect of SO 4 2− is proposed based on the above analysis. [ABSTRACT FROM AUTHOR]

• Established a dual-layer monolith model coupling reaction and mass transfer. • Gave suggestions on design of active layer thickness by analyzing over 175 cases. • Analyzed the influences of average pore size of catalyst on SCR and SO 2 Oxidation. Nitrogen oxides (NOx) and sulfur dioxide (SO 2) are common gaseous pollutants in the flue gas of stationary resources, such as thermal power plants. The undesired oxidation of SO 2 on the catalytic monolith for Selective Catalytic Reduction (SCR) with ammonia will cause deposition of ammonium bisulfate, corrosion and extra pressure drop to downstream devices. A dual-layer model of catalytic monolith was established in this work to thoroughly investigate the effects of active layer thickness on SCR reaction and SO 2 oxidation under various operating parameters. Besides, the influences of different sizes of nanopore in catalyst on both reactions are discussed. Thus, optimal parameters and suggestions were given for the design of SCR catalytic monolith with low SO 2 oxidation. [ABSTRACT FROM AUTHOR]