Selective Reduction Process Research Articles

Metallurgical Behavior of Adjusted Converter Slag in Reduction Process

The metallurgical behavior of adjusted converter slag components during the selective reduction process were investigated by thermodynamic calculating with different modified slag composition, addition of reducing agent and reduction temperature. The activities of main slag components were drawn from the calculated values. The results showed that the activity of SiO2increased with increment of its mass fraction in slag. The solubility of SiO2increased with increment of temperature. The selective reduction was promoted by selecting the appropriate amount of modifier. Reduction order was elucidated in this paper, Fe was reduced from the slag followed by P, Mn and Si and the reduction rate of Si could reach about 51%. The metal phase was rich in Fe, Si, Mn and P as a result of the selective reduction.

Cost-Efficient Nitrogen Oxides Control by a Hybrid Selective Noncatalytic Reduction and Selective Catalytic Reduction System on a Utility Boiler

Abstract The hybrid process of selective noncatalytic reduction (SNCR) followed by selective catalytic reduction (SCR) is an up-to-date nitrogen oxides control technique and can achieve a high efficiency (60%–95%) with moderate cost. The SNCR/SCR hybridization was investigated through experimentation on a 100 MW, coal-fired utility boiler in China. The baseline nitrogen oxides emission of the hybrid urea-based SNCR/SCR system was 170 ppm at 6% O2 controlled by coal reburning technology. The hybrid SNCR/SCR system including only 89 m3 catalysts could control NO emission under 25 ppm with ammonia slip <4 ppm at 300–430 tons/h of boiler load. Total NO reduction efficiency was generally about 85% and could be beyond 90%, in which SCR's efficiency was 50%–70% individually and SNCRs was about 60%–75%. NO emission decreased from about 65 to 15 ppm when the flow rate of 35% urea solution increased from 0.15 to 0.47 tons/h. At higher boiler load, more urea solution had to be injected and higher layer of the agent ...

N&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;O Formation in Selective Non-catalytic NO&amp;lt;sub&amp;gt;x&amp;lt;/sub&amp;gt; Reduction Processes

Nitrous oxide is not an environmentally regulated species in the U.S., but it does participate in the stratospheric ozone chemistry and contributes to the greenhouse effect. Nitrous oxide has been found to be a by-product of the selective non-catalytic reduction process. Chemical kinetic calculations demonstrated that the formation of nitrous oxide in the urea-based selective non-catalytic reduction process is linked to the conversion of NO by cyano species released from the process parent compounds. This conversion occurs within in temperature window between 850 and 1050℃. With urea injection, nitrous oxide emissions represent up to 20 percent conversion of the NOx reduced. The amount of nitrous oxide formed depends primarily on the process temperature, the amount of chemical injected, the initial NOx level, and the carbon monoxide level in the gas stream. These observations, which were based on the chemical kinetics of the process, should be considered in designing selective non-catalytic reduction systems to minimize nitrous oxide by- product formation.

Full-Scale Numerical Investigation of a Selective Noncatalytic Reduction (SNCR) System in a 100 MW Utility Boiler with Complex Chemistry and Decoupling Approach

To efficiently model the selective noncatalytic reduction (SNCR) process in a larger utility boiler, a decoupling numerical simulation approach, to separately treat coal combustion and SNCR NOx red...

Experimental and modeling study of the effects of multicomponent gas additives on selective non-catalytic reduction process

The influence of multicomponent additives on NO reduction by selective non-catalytic reduction process has been investigated experimentally in an electricity-heated tube reactor. The multicomponent additives are composed of two species of CO, CH 4 and H 2, and the molar ratio of their two components varies from 1/3 to 3/1. The results show that all the investigated additives could decrease the optimal temperature for NO reduction effectively, but the contributions of their components are different. The performance of multicomponent additive composed of CO and CH 4 depends mainly on CH 4 component. The function of CO component is shifting the temperature window for NO reduction to lower temperature slightly and narrowing the temperature window a little. The temperature window with multicomponent additive composed of H 2 and CH 4 is distinct from that with its each component, so both H 2 and CH 4 component make important contributions. While the fraction of CO is no more than that of H 2 in multicomponent additives composed of them, the performance of multicomponent additives is dominated by H 2 component; while the fraction of CO becomes larger, the influence of CO component becomes notable. Qualitatively the modeling results using a detailed chemical kinetic mechanism exhibit the same characteristics of the temperature window shift as observed experimentally. By reaction mechanism analysis, the distinct influences of CO, CH 4 or H 2 component on the property of multicomponent additive are mainly caused by the different production rates of OH radical in their own oxidation process.

Effect of oxygenated liquid additives on the urea based SNCR process

An experimental investigation was performed to study the effect of oxygenated liquid additives, H 2O 2, C 2H 5OH, C 2H 4(OH) 2 and C 3H 5(OH) 3 on NO x removal from flue gases by the selective non-catalytic reduction (SNCR) process using urea as a reducing agent. Experiments were performed with a 150 kW pilot scale reactor in which a simulated flue gas was generated by the combustion of methane operating with 6% excess oxygen in flue gases. The desired levels of initial NO x (500 ppm) were achieved by doping the fuel gas with ammonia. Experiments were performed throughout the temperature range of interest, i.e. from 800 to 1200 °C for the investigation of the effects of the process additives on the performance of aqueous urea DeNO x. With H 2O 2 addition a downward shift of 150 °C in the peak reduction temperature from 1130 to 980 °C was observed during the experimentation, however, the peak reduction efficiency was reduced from 81 to 63% when no additive was used. The gradual addition of C 2H 5OH up to a molar ratio of 2.0 further impairs the peak NO x reduction efficiency by reducing it to 50% but this is accompanied by a downward shift of 180 °C in the peak reduction temperature. Further exploration using C 2H 4(OH) 2 suggested that a 50% reduction could be attained for all the temperatures higher than 940 °C. The use of C 3H 5(OH) 3 as a secondary additive has a significant effect on the peak reduction efficiency that decreased to 40% the reductions were achievable at a much lower temperature of 800 °C showing a downward shift of 330 °C.

Materials for thermohydrolysis of urea in a fluidized bed

The production of ammonia from urea was investigated in a fluidized bed reactor in order to avoid the formation of N 2O in the selective non-catalytic reduction (SNCR) process, which occurs when urea is directly injected into an SNCR reactor. Among about 20 tested materials, γ-Al 2O 3 proved to be the best suited as a fluidized bed material due to its high catalytic activity as well as relatively good stability. Even better catalytic results were obtained with titanium oxide, which was, however, much less attrition resistant than the aluminum oxides. Silicon dioxide and silicates showed only a weak catalytic effect. The tested molecular sieves and zeolites showed catalytic results between those of silicates and alumina. The results of the catalytic tests are in accordance with a two-step mechanism: urea rapidly decomposes to ammonia and isocyanic acid initiated by simple heat transfer from the fluidized material, followed by the rate-determining catalytic hydrolysis of isocyanic acid. The screening showed that a compromise had to be found for the catalytically active fluidized bed material, since good catalytic properties require high surface areas, which is in conflict with the desired high abrasion resistance of the material. Scale-up experiments in the pilot plant with γ-Al 2O 3 “CAT 250” as the best material among the screening experiments demonstrated that it is possible and straightforward to thermohydrolyze 10.6 kg/h of urea to 6 kg/h ammonia. Gas analyses of the product gas showed that urea was almost completely converted and that only minuscule amounts of isocyanic acid were formed.

Effect of hydrogen generated by dielectric barrier discharge of NH 3 on selective non-catalytic reduction process

Plasma-assisted selective non-catalytic reduction (SNCR) has been investigated to clarify which species generated by the plasma play a crucial role in NO reduction. We find that the presence of O 2 is indispensable and only H 2 is observed to be a stable product by dielectric barrier discharge (DBD) of NH 3. As the extent of NH 3 decomposition by DBD increases, the commencement temperature of SNCR processes is lowered and the working temperature window is widened. This propensity may be attributed to the chemical reaction of H 2 with O 2 to generate OH and H radicals which make it possible to yield NH 2 radicals even at low temperature.

Removal of ammonium chloride generated by ammonia slip from the SNCR process in municipal solid waste incinerators

The selective non-catalytic reduction (SNCR) process is one of the methods used to reduce NOx to N2 and H2O by injecting NH3 or urea solution into a high-temperature furnace. Merits of this method include simple handling, low cost, and energy savings. However, a critical problem of the SNCR process is the generation of ammonia slip; in reactions with HCl in flue gas, ammonium chloride is generated and forms detached white plumes near the stack. Using a laboratory-scale experimental apparatus, we examined the possibility of NH4Cl collection and removal by a bag filter (BF). The molar NH3/HCl ratio of the compound collected at the filter was nearly one, regardless of gas temperature, retention time, and concentration, confirming the formation of NH4Cl. The NH4Cl generation ratio increased as reaction temperature decreased, indicating that the collection efficiency of NH4Cl should increase if the BF is operated at the lowest possible temperature while avoiding the critical point causing low-temperature corrosion (e.g., 150°C). In addition, the use of activated carbon injection in the front of the BF and the dust layer on the BF are expected to capture slipped ammonia at the BF and reduce NH4Cl fume generation in the stack.

Experiment and Computational Fluid Dynamics (CFD) Simulation of Urea-Based Selective Noncatalytic Reduction (SNCR) in a Pilot-Scale Flow Reactor

A turbulent reacting flow computational fluid dynamics (CFD) model involving a droplet size distribution function in the discrete droplet phase is first built for selective noncatalytic reduction (SNCR) processes using urea solution as a NOx removal reagent. The model is validated with the experimental data obtained from a pilot-scale urea-based SNCR reactor installed with a 150 kW gas burner. New kinetic parameters of seven chemical reactions for the urea-based NOx reduction are identified and incorporated into the three-dimensional turbulent flow CFD model. The two-phase droplet model with the non-uniform droplet size is also combined with the CFD model to predict the trajectory of the droplets and to examine the mixing between the flue gas and reagents. The maximum NO reduction efficiency of about 80%, experimentally measured at the reactor outlet, is obtained at 940 °C and a normalized stoichiometric ratio (NSR) = 2.0 under the conditions of 11% excess air and low CO concentration (10−15 ppm). At the reaction temperature of 940 °C, the difference of a maximum of 10% between experiments and simulations of the NO reduction percentage is observed for NSR = 1.0, 1.5, and 2.0. The ammonia slip is overestimated in CFD simulation at low temperatures, especially lower than 900 °C. However, the CFD simulation results above 900 °C show a reasonable agreement with the experimental data of NOx reduction and ammonia slip as a function of the NSR.

NO&lt;SUB&gt;x&lt;/SUB&gt; control by ammonium carbonate and ammonia with hydrocarbons as additives

AbstractAn experimental study was performed to analyse the effect of hydrocarbon additives, CH4 and C3H8, on NOx reduction by the selective non-catalytic reduction process using ammonia as a reducing agent as well as the use of ammonium carbonate as an independent reducing agent. The experiments were performed in an isothermal flow reactor in which the flue gas contains 500 ppm NOx and 3%O2 in the temperature range 800–1100°C. A down ward shift of 200°C is observed with the addition of C3H8/NH3 at a molar ratio of 2˙0 but with some impaired NOx reduction efficiency. The addition of methane has also exhibited a similar trend; lowering the peak reduction temperature by up to 60°C. The use of ammonium carbonate has shown that up to 78% reduction can be achieved at temperatures ~1025°C with a molar ratio of 2˙0 thereby demonstrating the potential of ammonium carbonate as a reducing agent for the control of NOx in the flue gases of the combustion sources.

Computational fluid dynamics and interactive multiobjective optimization in the development of low-emission industrial boilers

A CFD-based model is applied to study emission formation in a bubbling fluidized bed boiler burning biomass. After the model is validated to a certain extent, it is used for optimization. There are nine design variables (nine distinct NH3 injections in the selective non-catalytic reduction process) and two objective functions (which minimize NO and NH3 emissions in flue gas). The multiobjective optimization problem is solved using the reference-point method involving an achievement scalarizing function. The interactive reference-point method is applied to generate Pareto optimal solutions. Two inherently different optimization algorithms, viz. a genetic algorithm and Powell's conjugate-direction method, are applied in the solution of the resulting optimization problem. It is shown that optimization connected with CFD is a promising design tool for combustion optimization. The strengths and weaknesses of the proposed approach and of the methods applied are discussed from the point of view of a complex real-world optimization problem.

Experimental and modeling study of the effect of CH 4 and pulverized coal on selective non-catalytic reduction process

The reduction of nitric oxide using ammonia combined with methane and pulverized coal additives has been studied in a drop tube furnace reactor. Simulated flue gas with 1000 ppm NO x and 3.4% excess oxygen was generated by cylinder gas. Experiments were performed in the temperature range of 700–1200 °C to investigate the effects of additives on the DeNO x performance. Subsequently, a kinetic mechanism was modified and validated based on experimental results, and a computational kinetic modeling with CHEMKIN was conducted to analyze the secondary pollutants. For both methane and pulverized coal additives, the temperature window is shifted towards lower temperatures. The appropriate reaction temperature is shifted to about 900 and 800 °C, respectively with 1000 ppm methane and 0.051 g min −1 pulverized lignite coal. The addition of methane and pulverized coal widens the temperature window towards lower temperature suggesting a low temperature application of the process. Furthermore, selective non-catalytic reduction (SNCR) reaction rate is accelerated evidently with additives and the residence time to complete the reaction is shortened distinctly. NO x reduction efficiency with 80% is achieved in about 0.3 s without additive at 1000 °C. However, it is achieved in only about 0.2 s with 100 ppm methane as additive, and only 0.07 and 0.05 s are needed respectively for the cases of 500 and 1000 ppm methane. The modified kinetic modeling agrees well with the experimental results and reveals additional information about the process. Investigation on the byproducts where NO 2 and N 2O were analyzed by modeling and the others were investigated by experimental means indicates that emissions would not increase with methane and pulverized coal additions in SNCR process and the efficacious temperature range of SNCR reaction is widened approximately with 100 °C.

The influence of mixing between NH 3 and NO for a De-NOx reaction in the SNCR process

The objective of this study was to determine the mixing effect on De-NOx in the selective non-catalytic reduction (SNCR) process using NH 3 as a reductant. A bench scale reactor set-up was used to perform experimental investigations on the mixing in the SNCR process, using an injection of NH 3, with or without an air injection, into the bulk gas. Mixing between NH 3 and NO is a very important parameter, as are kinetic parameters, such as the NSR and reaction temperature. The type of NH 3 with NO mixing method caused different De-NOx results, in which for the case of NH 3 injected with air was more effective for NO reduction than under NH 3 premixing conditions. For the case of air injection conditions, the NO reduction efficiency increased with increasing NSR, initial NO concentration, and retention time. The temperature for the maximum NO removal efficiency was 900 °C with ammonia premixing conditions, but was around 800 °C when ammonia was premixed and air was injected at the same time. That is, the optimization of the reductant mixing and injection conditions are very useful for improving the NO removal efficiency, as well as lowering the reaction temperature to that were optimal efficiency is observed. The optimum momentum ratio ( J), which refers to the mixing characteristics, was found to be about 100 in this study.

The formation of N 2O during the reduction of NO by NH 3

The process of selective non-catalytic reduction of NO, SNCR, is important for limiting emissions of nitrogen oxides from coal-fired power plants. Such a process has been studied for many years, both in the laboratory and under practical conditions. This work was an attempt at elucidating some of the problems associated with the method when used under circulating fluidized bed (CFB) conditions and in particular, the formation of the N 2O by-product. The NO + NH 3 reaction has been studied in the laboratory, over quartz sand in a heated fixed bed flow reactor. In comparison with a combustion environment, the composition of the gas phase was drastically simplified and limited to NO and NH 3, in nitrogen as the carrier gas, with O2 added in some experiments. The product gases were analyzed for NO, N 2O and NH 3. The effects the following parameters were studied: temperature inside the reactor between 850 and 1250 K, height of the sand bed, NH 3/NO molar ratio over the range 0.54–2.0 and the addition of 1 or 2% of O 2 in volume. Baseline tests with an empty reactor were also made. With no sand in the reactor, the results were both qualitatively and quantitatively different. The sand helped to increase the efficiency of NO reduction, particularly at lower temperatures, but N 2O formation also appeared to be strongly enhanced, except at the highest temperatures. Higher molar NH 3/NO ratios favored NO reduction and N 2O production, both with and without sand. The reduction of NO did not appear to require the presence of O 2, but the introduction of 1% or 2% of O 2 gave some benefit. The results confirmed that under practical conditions more attention should be paid to the role of the bed solids in the SNCR process.

A comparative kinetic study of SNCR process using ammonia

The paper presents comparative kinetic modelling of nitrogen oxides (NOx) removal from flue gases by selective non-catalytic reduction process using ammonia as reducing agent. The computer code SENKIN is used in this study with the three published chemical kinetic mechanisms; Zanoelo, Kilpinen and Skreiberg. Kinetic modeling was performed for an isothermal plug flow reactor at atmospheric pressure so as to compare it with the experimental results. A 500 ppm NOx background in the flue gas is considered and kept constant throughout the investigation. The ammonia performance was modeled in the range of 750 to 1250 ºC using the molar ratios NH3/NOx from 0.25 to 3.0 and residence times up to 1.5 seconds. The modeling using all the mechanisms exhibits and confirms a temperature window of NOx reduction with ammonia. It was observed that 80% of NOx reduction efficiency could be achieved if the flue gas is given 300 msec to react with ammonia, while it is passing through a section within a temperature range of 910 to 1060 ºC (Kilpinen mechanism) or within a temperature range of 925 to 1030 ºC (Zanoelo mechanism) or within a temperature range of 890 to 1090 ºC (Skreiberg mechanism).

Comparison of Global Ammonia Chemistry Mechanisms in Biomass Combustion and Selective Noncatalytic Reduction Process Conditions

The success of computational fluid dynamics (CFD)-based combustion modeling is strongly dependent on several submodels which are required to close the Favre-averaged conservation equations for mass, momentum, and energy and transport equations for scalar quantities. Due to the requirement of high computer capacity, global mechanisms are frequently applied to model chemical reactions in industrial-scale CFD. The present study compares the performance of five global ammonia chemistry mechanisms in the conditions typical of the biomass combustion in fluidized beds. A special emphasis is given to the modeling of the selective noncatalytic reduction (SNCR) process. A modification of the standard k–e model is used to model turbulence, the eddy dissipation combustion model and the eddy dissipation concept are used to model turbulence–chemistry interaction, and the finite-volume method (discrete ordinates) together with the weighted sum of gray gases model are used to model radiative heat transfer. A simplified a...

Experimental and modeling study of the effect of CO and H 2 on the urea DeNO x process in a 150 kW laboratory reactor

An experimental and modeling investigation has been performed to study the effect of process additives, H 2 and CO on NO x removal from flue gases by a selective non-catalytic reduction process using urea as a reducing agent. Experiments were performed with a flow reactor in which flue gas was generated by the combustion of propane in air at 3% excess oxygen and the desired levels of initial NO x (500 ppm) were achieved by doping the flame with ammonia. Experiments were performed throughout the temperature range of interest, i.e. from 850 to 1200 °C for investigation of the effects of the process additives on the performance of aqueous urea DeNO x . Subsequently, computational kinetic modeling with SENKIN code was performed to analyze the performance of urea providing a direct comparison of modeling prediction with experimental measurements. With CO addition, a downwards shift of 215 °C in the peak reduction temperature from 1125 to 910 °C was observed during the experimentation while the kinetic modeling suggests it to be 150 °C, i.e. from 1020 to 870 °C. The addition of H 2 impairs the peak NO x reduction but suggests a low temperature application of the process. A downward shift of 250 °C in the peak reduction temperature, from 1020 to 770 °C, was observed during kinetic modeling studies. The kinetic modeling shows a good qualitative agreement with the experimental observations and reveals additional information about the process.

Nitrous oxide formation and emission in selective non-catalytic reduction process

Pulverized coal-fired boilers are not nitrous oxide sources because of high temperature combustion. But selective non-catalytic reduction may produce N2O by NO reduction reactions. Chemical kinetics calculation and experimental research were used to find out the mechanism between N2O and N-agent species, N-agent/NO nitrogen stoichiometric ratio (NSR), reaction temperature, reaction time, etc. The results show that N2O emission decreases with increasing reaction temperature and NSR decreases when reaction time is enough. N2O concentration first increases then decreases as SNCR reactions keep on occuring. Ammonia SNCR tests indicated that N2O emission was 0–7 μmol/mol. About 8.7% of NO was transformed to N2O, and N2O emission was 27.8 μmol/mol at urea-SNCR test. Urea-SNCR is likely to bring N2O emission problem.

The Synthesis and Characteristics of Homogenously Dispersed CNT-Al&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt; Nanocomposites by the Thermal CVD Method and Pulsed Electric Current Sintering Process

An optimum route to synthesize Al2O3-based composite powders with a homogeneous dispersion of carbon nanotubes (CNTs) was investigated. CNT/Metal/Al2O3 nanocomposite powders were fabricated by thermal chemical vapor deposition (CVD) over a metal catalyst homogeneously dispersed into an Al2O3 matrix by the means of chemical and selective reduction processes. The nanocomposite powders were densified by Pulse Electric Current Sintering (PECS). The experimental results show that the CNT/Metal/Al2O3 nanocomposites have unique electrical properties.