Selective Non-catalytic Reduction Process Research Articles

CFD-based investigation of NO x removal from industrial waste gas by selective catalytic reduction (SCR) and selective non-catalytic reduction (SNCR) process using NH3

CFD-based investigation of NO_<i>x</i> removal from industrial waste gas by selective catalytic reduction (SCR) and selective non-catalytic reduction (SNCR) process using NH₃

Experimental investigation of enhanced de-NOx efficiency by Na/K combined additives on the urea-based SNCR process

The urea-based selective non-catalytic reduction (SNCR) process is widely favored for its significant economic advantages over selective catalytic reduction. However, in most real-world applications, it often falls short of reaching the required high-efficiency temperature range. Alkali metals are recognized as potential promoting additives for higher de-NOx efficiency and wider temperature window. The effects of individual additives have been widely studied, but the combined use of Na/K additives has not been extensively investigated, particularly with regard to their underlying mechanisms. This paper explores the promoting mechanism of Na/K on the urea-based SNCR process, and propose a synergistic enhancement mechanism for the combined use of Na/K additives. Results showed that the Na/K additives into urea significantly improve the de-NOx efficiency, especially in a low temperature range of 600–1000 °C, which indicating a much wider effective temperature window. Na/K combined additives (Na2CO3 + K2CO3 and Na2CO3 + CH3COOK) exhibit a strong and synergistic promotion of urea-based SNCR efficiency. These combined additives effectively broaden the lower limit of effective temperature window from 950 °C and 804–892 °C to 774 °C compared to the none-additive and the single-Na or K additive conditions. Notably, the de-NOx efficiency of combined additives reaches 3–4 times of that non-additive conditions at relatively low temperatures, and the ammonia escape and N2O emission is respectively reduced by ∼ 1/3 at 600 °C and ∼ 1/2 at 800 °C. This study can provide a deep insight into the enhanced urea-based SNCR efficiency by Na/K combined additives and enhance the feasibility of urea-based SNCR technology under lower temperatures.

The reaction characteristics and mechanism of polymer non-catalytic reduction (PNCR) for NOx removal

To overcome the defects of the traditional selective non-catalytic reduction (SNCR) process (e.g., low efficiency, narrow temperature range), a new modified SNCR technology based on the solid complex polymer reducing agents, also called polymer non-catalytic reduction (PNCR), was investigated both in the laboratory and pilot scale to reveal its reaction characteristics and mechanism. The PNCR process demonstrates excellent removal efficiency (about 90%) of NO in furnace in the wide temperature range (850–1150 °C), and possesses promising application feasibility with an average NOx emission concentration of 68.72 mg·m−3 even on unstable industrial operating conditions. The NO removal behaviors influenced by O2, temperature, or water steam illuminate the unique O2-independent and H2O-promoted reaction characteristics of PNCR in the wide temperature range. The thermogravimetric infrared spectra/mass spectrometry (TG-IR/MS) results further reveal a pyrolysis-assisted formation mechanism of active NH2/NH free radicals without the requirement of O2 and high temperature, which avoids the overoxidation of active radicals and accounts for the wide denitrification temperature window, low oxygen compliance and high denitrification efficiency of PNCR process. The excellent NO removal performance as well as the unique reaction characteristics/mechanism of PNCR forebode its broad industrial application prospect in the field of flue gas cleaning.

Numerical Simulation of Selective Non-Catalytic Reduction Denitrification Process in Precalciner and the Effect of Natural Gas Injection on Denitrification

Cement production is the third largest source of nitrogen oxides (NOx), an air pollutant that poses a serious threat to the natural environment and human health. Reducing NOx emissions from cement production has become an urgent issue. This paper aims to explore and investigate more efficient denitrification processes to be applied in NOx reduction from precalciner. In this study, firstly, the flow field, temperature field, and component fraction in the precalciner are studied and analyzed using numerical simulation methods. Based on this, the influence of the reductant injection height and amount on the SNCR was studied by simulating the selective non-catalytic reduction (SNCR) process in the precalciner. The effect of natural gas on the NOx emissions from the precalciner was also investigated. The simulation results showed that, with the increase in height, the NOx concentration in the precalciner decreased, then increased, then decreased, and then increased again. The final NOx concentration at the exit position was 531.33 ppm. In the SNCR denitrification process, the reductant should be injected in the area where the precalciner height is 26–30 m so that the reductant can fully react with NOx and avoid the increase of ammonia escape. The NSR represents the ratio of reductant to NOx, and the results show that the larger the NSR is, the higher the denitrification rate is. However, as the NSR approaches 2, the denitrification rate slows down and the ammonia escape starts to increase. Therefore, according to the simulation results, the NSR should be kept between 1 and 1.6. The denitrification rate reached the maximum value of 42.62% at the optimal condition of 26 m of reductant injection height and 1.6 of NSR. Co-firing of natural gas with pulverized coal can effectively reduce the NOx generation in the furnace. The denitrification rate reached the maximum value of 32.15% when the natural gas injection amount was 10%. The simulation results of natural gas co-combustion and SNCR combined denitrification showed that combined denitrification was better than natural gas co-combustion or SNCR denitrification. Under the condition of NSR of 1 and natural gas injection of 10%, the denitrification rate increased by 29.83% and 31.64% compared to SNCR-only or co-combustion-only denitrification, reaching 61.98%, respectively. Moreover, less reductant is used in co-denitrification, so the problem of excessive ammonia emissions can be avoided. The results of this study provide useful guidance for denitrification process development and NOx reduction in cement production.

Influence and mechanism of the adsorption and reactions of residual NH3, NO, and O2 on coal ash after the selective noncatalytic reduction process

NH3 as reducing agent in selective noncatalytic reduction (SNCR) is widely used for the deNOx of coal-fired boiler flue gas, where fly ash significantly influences the conversion of residual NH3 that does not participate in deNOx. However, few studies have been conducted on the exact nature of this influence. The adsorption and reaction of NH3, NO, and O2 on the fly ash at 50–850 °C were investigated in this study. At 50–350 °C, NH3, NO, and O2 co-adsorbed on the coal ash surface, and the presence of NO and O2 reduced the physical adsorption of NH3 on the ash and promoted its chemical adsorption on the ash. At 450–850 °C, coal ash catalyzed NO reduction by NH3 and NH3 oxidation by O2. In the presence of O2, the oxidation reaction became dominant and the reduction reaction was inhibited. The mechanism of adsorption and reaction of coal ash showed that NH3 was adsorbed on the Brönsted and Lewis sites on coal ash and NH3 at the Lewis site could be dissociated into *NH2 and *NH. The NO ad-species in coal ash were mainly nitrate and nitrite. *NH2 reacted with NO to form NH2NO, which desorbed as N2 at 450–850 °C. The main oxidation intermediates for NH3 ad-species were monodentate nitrate and bridged nitrate, which desorbed as NO at 450–850 °C. The results provide a theoretical basis for adjusting the migration and conversion of residual NH3 and reducing ammonia slip from power plants.

Nitric oxide (NO) and nitrous oxide (N2O) emissions during selective non-catalytic reduction and selective catalytic reduction processes in a pulverized coal/Ammonia Co-fired boiler

The global warming and climate change are accelerated by the emissions of CH4, chlorofluorocarbon, and N2O gases along with CO2 resulting from the use of fossil fuels. Recently, a new combustion technology that combines carbon-free ammonia (NH3) fuel and coal has been proposed to reduce CO2 emissions from thermal power plants. In this study, we examined the emission levels of NO and greenhouse gas N2O from the existing coal-fired and future carbon-neutral coal/ammonia co-fired thermal power plants. As a result, a correlation between the amounts of NO and N2O gases generated during combustion in the furnace of a thermal power plant was established, and possible relationships between NO and N2O emissions during the selective non-catalytic reduction (SNCR) and selective catalytic reduction (SCR) of NOx were experimentally analyzed. In addition, a linear correlation between the CO and N2O concentrations was observed. The air-staged combustion during SCR (300 °C, urea) produced the lowest total emission concentration of NO and N2O (9 ppm) under the utilized conditions. The findings of this study can help control the emission levels of air pollutant NO and greenhouse gas N2O throughout the entire process of thermal power plants.

Adsorption and catalytic oxidation of residual NH3 on coal ash after selective non-catalytic reduction in coal-fired boilers

Selective non-catalytic reduction (SNCR) with NH3 as the reducing agent is widely used for the denitrification of flue gas in coal-fired boilers, where fly ash significantly influences the conversion of the residual NH3 that does not participate in denitrification. However, there have been few studies on the exact nature of this influence, particularly the adsorption and reaction mechanisms of NH3 on fly ash. In this study, temperature-programmed desorption (TPD) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) were used to study the mechanisms of NH3 adsorption and reactions over coal ash. In the absence of oxygen, in the temperature range of 50–450 °C, NH3 was adsorbed on the surface of the coal ash. The adsorption capacity of lignite ash was higher than that of anthracite ash. This difference was attributed to the large specific surface area and surface acidity of the lignite ash. However, between 450–850 °C, coal ash had a catalytic effect on NH3 decomposition and oxidation. Due to the high surface lattice oxygen content of lignite ash, its catalytic oxidative ability was superior to anthracite ash. Moreover, NH3 was first adsorbed over Lewis and Brønsted acid sites on the surface of coal ash and later underwent hydrogen abstraction to produce either the NH2 or the NH intermediate. The intermediates further reacted with the surface lattice oxygen of coal ash to produce NO and N2O. These results might be helpful for the management of NH3 residues from SNCR processes and the utilization of amino reducing agents in coal-fired boilers.

The Technology of Processing of Bed Combustion Fly Ash for Cement Composite Production

Insufficient offer of high-temperature fly ash meeting the requirements of standard EN 4501 is becoming an increasingly significant problem for concrete technology. This trend is generally caused by relentless effort to green electricity generation and reduce the carbon footprint, leading to the decommissioning of coal-fired power plants and the greening of existing. The most often ways are combustion processes of fluidized-bed and selective non-catalytic nitrogen oxide reduction (SNCR) processes. Thanks to these processes, the demand for this raw material has significantly exceeded its supply in recent years. It is, therefore, more than current to look for ways to use the large amount of ash produced from coal combustion, which is not high-temperature fly ash, but can be classified as a bed combustion fly ash or cinder. The paper aims to show the possible processing and subsequent use of these waste materials for the production of cement composites. The possibilities of grinding these coarse-grained fly ashes and the subsequent impact of their use on the cement composite in comparison with conventional fly ash according to the standard EN 450 from the same sources were verified. The impact of their use on the properties of composites in the fresh and hardened state was analysed, and these impacts were related to, for example, their chemistry, grain morphology, loss on ignition, or fineness.

An experimental and modelling study of the Selective Non-Catalytic Reduction (SNCR) of NOx and NH3 in a cyclone reactor

The Selective Non-Catalytic Reduction process was investigated through experiments and modelling in a bench scale cyclone reactor. Outlet and internal profiles of NO and NH3 concentrations were measured at varying operating conditions. Cyclone inlet temperatures between 866 and 1023 °C were used, while the inlet NO concentration was kept at ~500 ppm. Gaseous ammonia was injected in the cyclone inlet, with nitrogen carrier gas. 60–69% NO reduction was achieved at inlet temperatures of 975–982 °C, and stoichiometric ratio NH3/NO β≈1.6, with no ammonia slip. The main reaction zone was the inlet and upper cyclone chamber. The experiments was modelled using simple and detailed kinetics combined with simple reactor models and a compartment model suggested in this work. The qualitative trends of NO reduction and ammonia slip were predicted by the models. However, the ammonia slip was significantly over-predicted in most cases. The outlet concentrations were insensitive to choice of reactor model, while sensitive to temperature and kinetics. The compartment model quantitatively reproduced the cyclones NO concentration profiles, through its simplified representation of the cyclone flow and temperature gradients, while the NH3 profiles were only qualitatively predicted. The choice of bypass fractions showed a large influence on the ammonia profiles.

Selective non-catalytic reduction of nitrogen oxides in the production of iron ore pellets

Purpose. Using mathematical modeling, to assess the feasibility of introducing a Selective Non-Catalytic Reduction (SNCR) system as a measure to reduce nitrogen oxide emissions from the production of iron ore pellets. To determine the peculiarities of using ammonia solution and urea solution as reagents for the SNCR process, the influence of the injection of these reagents on the temperature regime during iron pellet production, as well as assess the expected efficiency of the SNCR method for purification of exhaust gases from nitrogen oxides. Methodology. The research results have been obtained using CFD-modeling in the ANSYS Fluent software package. To model this process, a computational domain is constructed, which corresponds in size to the preheating zone (PRE zone) of the actual iron pellet production plant. Two series of calculations are performed for this domain: the first, without adding a reagent, and the second, with a urea solution as a reagent for the SNCR system. Findings. For the first series of calculations, the temperature field and the pressure field in the computational domain is obtained. Experimental research makes it possible to assert that the physical conditions of the mathematical model are close to those at a real plant for the production of pellets. In the second series of calculations, the temperature field in the computational domain is obtained and the influence of the reagent injection of the SNCR system is determined, namely, the temperature decrease in the PRE zone of the pellet production plant by 1025 . The expected efficiency of reduction of nitrogen oxides using a 50% urea solution is about 60%. Originality. It has been revealed that the process of urea solution evaporation is intense, which accelerates the beginning of urea decomposition and, accordingly, the reduction reaction of nitrogen oxides. The temperature drop in the zone of moisture evaporation does not exceed 1025 C. The reagent injection (50% urea solution) with a consumption of 219 kg/h does not significantly affect the temperature regime in the PRE zone. Modeling the chemical reactions of the SNCR process with the injection of 50% urea solution droplets through lances into the PRE zone chamber indicates a 60% reduction in nitrogen oxide emissions. Practical value. The introduction of the SNCR system at pellet production plant can reduce nitrogen oxide emissions, which will have a positive impact on the environmental situation in metallurgical regions.

Reduction of NOx Emission from the Cement Industry in South Korea: A Review

As climates change around the world, concern regarding environmental pollutants emitted into the atmosphere is increasing. The cement industry consistently produces more than 4000 million metric tons of cement per year. However, the problem of air pollutants being emitted from the calcination process is becoming more critical because their amount increases proportionally with cement production. Each country has established regulatory standards for pollutant emission. Accordingly, the cement industry is equipped with facilities to reduce air pollutants, one of which is the NOx removal process. NOx reduction processes under combustion conditions are modified to minimize NOx generation, and the generated NOx is removed through post-treatment. In terms of NOx removal efficiency, the post-treatment process effectively changes the combustion conditions during calcination. Selective non-catalytic reduction (SNCR) and selective catalytic reduction (SCR) processes are post-treatment environmental facilities for NOx reduction. Accordingly, considering the stringent NOx emission standards in the cement industry, SNCR is essential, and SCR is selectively applied. Therefore, this paper introduces nitrogen oxide among air pollutants emitted from the South Korean cement industry and summarizes the technologies adapted to mitigate the emission of NOx by cement companies in South Korea.

Thermal process and NO emission reduction characteristics of a new-type coke oven regenerator coupled with SNCR process

Based on the cascade utilization of high-temperature waste heat and selective non-catalytic reduction (SNCR) technology, a new-type coke oven regenerator with ammonia injection space was proposed. By changing the structure and operating parameters, the high-temperature heat storage and NO emission reduction are achieved synergistically. A three-dimensional model was used to study the flow, heat transfer, and SNCR process inside the regenerator. The validity of the model was verified. Results showed that the new-type regenerator had a significant cascade utilization characteristics of waste heat. There was a competitive reaction between NH3 + NO → N2 and NH3 → NO during the coupled SNCR process. The optimal reaction temperature window was 911 ~ 984°C, and the residence time was 0.21 ~ 0.38 s. The mixing of the reductant with the flue gas and the complete contact of NH3 with NO determined high NO reduction efficiency. Furthermore, combining multiple operating parameters, a strategy was proposed to ensure the outlet NO/NH3 concentration meets the emission standard requirements.

An Experimental Study on the Characteristics of NOx Distributions at the SNCR Inlets of a Large-Scale CFB Boiler

The unknown NOx distributions inside large-scale CFB (circulating fluidized bed) boilers have always hindered the economy of the SNCR (selective non-catalytic reduction) process. In this study, field tests were carried out on a typical 300 MW CFB boiler, where multi-level 316 L-made probe and Ecom-J2KN/Testo 350 analyzers were used to perform detailed two-dimensional distributions of flue gas composition at SNCR inlets for the first time. The penetration depth inside the horizontal flue pass was up to 7 m. The NOx distributions were analyzed in detail combining with the auxiliary test in the dilute phase zone. Key results show that the average O2 concentrations in #A and #C regions were 6.52% and 0.95%, respectively. The vertical NOx distributions of #A and #C SNCR inlets were similar, showing a trend of first increasing and then decreasing with peak value all appeared at 5 m depth, while the NOx distribution of #B SNCR inlet was basically increasing. Some local areas with extremely high NOx concentration (over 2000 mg/m3) were observed near the inclined edge of SNCR inlets, which has never been reported before. Based on this, the optimization of urea injections was conducted, which could save 15.7% of the urea solution consumption while ensuring ultra-low emission of NOx.

Effects of hydrogen peroxide, sodium carbonate, and ethanol additives on the urea-based SNCR process

The issue of secondary pollutants represents a classic problem in the urea-based selective non-catalytic reduction (SNCR) process. Therefore, this study took to investigate the evolutions of secondary pollutants when including additives during the urea-based SNCR process. Results indicated that additives, namely hydrogen peroxide, sodium carbonate, and ethanol, clear improved denitration efficiency between 750 °C - 925 °C due to the increase of OH groups. Compared to rates without any additives, the “temperature window” width of hydrogen peroxide, sodium carbonate, and ethanol increased by 30%, 30%, and 52%, respectively. The temperature at the maximum denitration efficiency was decreased by 25 °C with the addition of ethanol. The addition of hydrogen peroxide, sodium carbonate, and ethanol decreased the amount of ammonia leakage at 725 °C - 900 °C. Besides, the emissions of HNCO and N2O were decreased by adding sodium carbonate during 725 °C - 900 °C. Therefore, the addition of sodium carbonate proved to be beneficial for reducing the secondary pollutant emissions in SNCR. This study aims to provide a deeper understanding of the urea-based SNCR process in combustion.

Numerical Simulation of Urea Based SNCR Process in a Trinal-Sprayed Precalciner

In order to study the combustion characteristics, NOx emission and NH3 slip in a new trinal-sprayed precalciner, the simulations of combustion and aqueous urea solution based selective non-catalytic reduction (SNCR) process were conducted by computational fluid dynamics in this precalciner, the effects of different injection heights, different injection flow rates and stratified injection under different flow rates on SNCR process were studied. The results showed that the flow field was symmetrically distributed in the precalciner, and the flue gas from the rotary kiln formed the recirculation region on both sides of the cone body, which increased the residence time of the solid particles. The temperature was mainly between 1100 K and 1250 K in the middle and upper column of the precalciner, which met the demand of the pulverized coal combustion and raw material decomposition. The concentration of NO at the outlet of the precalciner was 559 ppm, moreover, different injection heights and different injection flow rates had a strong influence on NOX removal efficiency and NH3 slip. The aqueous urea solution should be injected at SNCR-1 to prolong the residence time of NH3, and injection flow rate had an optimal flow rate but not the higher the better. When the injection flow rate under stratified injection was 0.019 kg/s, which could play a better optimization role on NO removal efficiency on the basic of the injection flow rate. In consideration of cost effective, a stratified injection with an injection flow rate of 0.019 kg/s and an injection height of 20 m, 25 m and 30 m was suggested as a compromise of a satisfactory NOx reduction rates and reasonable NH3 slip. Under this condition, numerical simulation result showed that NOx concentration at the outlet of precalciner was 297.27 mg/Nm3 and NH3 slip was 4.67 mg/Nm3, meeting emission standard.

Experimental kinetic studies on the effects of organic additives on ammonia-based selective non-catalytic reduction process.

The effect of organic components including ethanol, acetic acid, and benzene on ammonia-based selective non-catalytic reduction (SNCR) process has been investigated. Experiments were performed through a quartz tube reactor with simulated flue gas and conducted from 973 to 1373 K. The combustion and pyrolysis properties of organic components were carried out first, it was found that all the combustion processes were completed ahead of the pyrolysis, and the combustion and pyrolysis temperatures for benzene are the highest among the three organic components. Ethanol addition promoted the removal of NO in the temperature range of 973 to 1073 K, and the NO reaction temperature window was broadened, while NO removal was greatly inhibited under a higher temperature over 1100 K. With regard to the effect of addition of multiple organic components on NO removal, the combination of C2H6O and C2H4O2 significantly promoted this process, while the combination of C2H6O/C2H4O2 and C6H6 showed a negative effect on NO removal. The mechanism based on the radicals' reaction has been illustrated, showing the competition of these reactions under different situations.

Chemical Kinetics Modeling and Analysis of Monomethylamine for Power Plants Selective Non-Catalytic Reduction (SNCR) Systems

A study was performed to investigate monomethylamine (MMA) as an alternative or supplementary reagent for selective non-catalytic reduction (SNCR) of nitrogen oxides (NOx) for power plants especially the coal-fired type. A detailed chemical kinetics mechanism for MMA, related to the SNCR NOx reduction process, was modeled using a simplified plug flow reactor (PFR) approach using the CHEMKIN software. Model validation results were in line with reported experimental results. A model sensitivity analysis showed that the normalized MMA/NO stoichiometric ratio (NSR) and the initial NOx concentration are the most critical parameters that significantly affect MMA-based process performance. Additionally, a reduced reaction mechanism and its mechanism tree for this particular process are proposed, consisting of only 18 reactions and 21 species. Simulation comparisons with three other reagents (ammonia, urea, and cyanuric acid), under similar process conditions, indicate that MMA-based SNCR NOx reduction process has a superior performance at much lower temperatures (under 800 K) than conventional SNCR processes, with the added benefit that it does not suffer from ammonia slip problems. The MMA-based SNCR NOx removal system could be applied in cascade mode with other post-combustion NOx removal approaches to extend NOx control to lower temperatures and eliminate the temperature window limitation and potential ammonia slip of conventional SNCR systems. The cascade design is a perfect solution for coal power plants especially at partial load modes of operation which provides more flexibility in performance for such plants. The results of the simulations in this study show that the potential of MMA for SNCR NOx reduction systems at full-scale power plants has significant merit.

Kinetic study and optimization on SNCR process in pressurized oxy-combustion

Pressurized Oxy-combustion is one of the most promising technologies for carbon capture, utilization, and storage (CCUS) of coal utilization. In pressurized oxy-combustion, the removal of acid gas pollutants and the latent heat of moisture condensation are mainly carried out by an integrated direct-contact wash column (DCC). Recent studies have shown that the inlet concentration of NOx in the column is important for the combined removal of NOx and SOx, therefore, it is essential to control the NOx emissions before the DCC. Selective non-catalytic reduction (SNCR) is a promising and economical technology of NOx reduction; however, no study has been conducted on SNCR at elevated pressure, especially in pressurized oxy-combustion. In this paper, kinetic modeling is carried out to explore and optimize the NOx reduction by SNCR in pressurized oxy-combustion. First, the detailed mechanism used in this study is validated by the experimental results in literature. Based on the validation of the detailed mechanism, the effects of different parameters including temperature, oxygen concentration, moisture content, NH3 concentration, and SO2 concentrations on the SNCR performance are analyzed at higher pressures in oxy-combustion atmosphere. The modeling results show that by increasing the pressure from 1 atm to 15 atm, the SNCR de-NOx efficiency increased by 2–3%. The width of the optimum temperature window for maximum De-NOx is broadened from 1250K to 1450K with increasing the pressure from 1 atm to 10 atm. The increase of pressure has no significant effect on thermal de-NOx efficiency with increasing the NH3/NO ratio. At higher pressure (10 atm) the increase in O2 concentration from 1 to 20% the de-NOx efficiency decreases almost twice as much as at 1 atm. The increase of SO2 and moisture contents at high pressure have no significant effect on the de-NOx efficiency. Rate of production (ROP) and sensitivity analysis are performed to determine the dominant reaction paths of de-NOx in pressurized-oxy SNCR process. This study can provide guidance for the process optimization of de-NOx by SNCR in pressurized oxy-combustion.

Effect of Combustible Gas Components on SNCR Reaction with Reducing Agent in Ceramic Kiln

Selective non-catalytic reduction (SNCR) is a denitration method used in NOx process-control of ceramic kiln. In this study, the effect of combustion gas components (CO, H2, CH4) on NOx removal in the SNCR process was investigated, using ammonia and urea as reducing agent in a lab-scale experimental platform. The results showed that the addition of CO slightly promoted the NOx removal efficiency when ammonia was used as reducing agent, while when urea was used as reducing agent, the addition of CO inhibited the denitrification. The increase of H2 and CH4 addition ratio in the simulated flue gas reduced the de-NOx efficiency. With the same addition ratio, the effect of CH4 on the reduction of denitration efficiency was much more obvious than that of H2. When ammonia was used as reducing agent, the ammonia escape was much greater than that of urea.

Catalytic oxidation of NH3 over circulating ash in the selective non-catalytic reduction process during circulating fluidized bed combustion

Selective non-catalytic reduction (SNCR) with NH3 as the reducing agent is widely used for the denitrification of flue gas in circulating fluidized bed combustion (CFB) process, where the circulating ash can cause the oxidation of NH3. In this study, the behaviors and mechanisms of NH3 catalytic oxidation over circulating ash were systematically investigated. The results revealed that NH3 could be catalytically oxidized to form NO efficiently by circulating ash. Along with the temperature increasing from 550 °C to 950 °C, the formation of NO increased at first and then decreased. Oxygen was crucial to NH3 catalytic oxidation, while its effects on NO formation were slight at high concentrations. Meanwhile, as the initial NH3 concentration increased, the concentration of NO in the flue gas increased while the selectivity of NO decreased because of the enhanced homogeneous reduction between the residual NH3 and NO. Fe2O3 was confirmed to be one of the principal mineral components in circulating ash to catalyze NH3 oxidation. In addition, the mechanisms of NH3 oxidation over circulating ash were also investigated. It was validated that NH2 on Lewis acid sites were the main NH3 adsorbed species (ad-species), and the main oxidation intermediates of NH3 ad-species were bidentate and bridged nitrates in the presence of O2.