Synthetic coal chars for the elucidation of NO heterogeneous reduction mechanisms

In the present work, the mechanisms involved in NO–char heterogeneous reduction have been studied using a synthetic coal char (SC char) as carbon source. Another synthetic char (SN char) without nitrogen in its composition has also been employed in these studies. Isothermal reduction tests at different temperatures have been carried out. Two temperature regimes were considered: low temperature (T < 250 °C) where NO chemisorption takes place and high temperature (T > 250 °C) where NO–C reaction occurs. Step response experiments combining consecutive reaction stages with NO and ¹⁵NO were performed in order to determine the role of nitrogen surface complexes, C(N), in the reduction process. The results revealed N₂ and CO₂ to be the main reduction products under the experimental conditions employed in this work. NO chemisorption at lower temperatures results in N₂ emission and surface complexes (mainly oxygenated) formation, while char gasification by NO involves a direct NO attack on the char surface to form surface complexes. As a consequence of desorption of these complexes new sites of reaction are created.     

Kinetic relationship between NO/N2O reduction and O2 consumption during flue-gas recycling coal combustion in a bubbling fluidized-bed

Flue-gas recycling combustion of a sub-bituminous coal and its rapid pyrolysis char at 1120 K has been simulated experimentally in a bubbling fluidized-bed. O₂, CO₂ and H₂O, and NO or N₂O were pre-mixed and fed into the bed together with coal/char particles with the O₂ concentration in the exit gas maintained at 3.5 vol%. Increasing the inlet O₂ concentration, thus increasing the O₂ consumption rate and decreasing the flue-gas recycling ratio, caused the once-through conversion of fuel-bound nitrogen into N₂O to decrease while the conversion to NO to remain unchanged. The in-bed reductions of NO and N₂O were both first order with respect to the respective nitrogen oxide, with the rate constants to increase linearly with the rate of O₂ consumption in the bed and thus also with that of char/volatiles consumption. This finding, which indicated linear increase in the concentrations of reactive species involved in NO/N₂O reduction with the rate of O₂ consumption, enabled consideration that the homogeneous and heterogeneous reduction rates of NO and N₂O were proportional to the consumption rates of O₂ by the volatiles and char, respectively. The rate analysis of the kinetic data revealed the relative importance of burning volatiles and char as the agents for the reduction of NO and N₂O. While the reduction in the gas phase was fully responsible for the NO-to-N₂O conversion, the reactions over the char surface governed the NO-to-N₂ reduction. The volatiles and char had comparable contributions to the reduction of N₂O to N₂. The NO-to-N₂ and N₂O-to-N₂ reductions over the char surface were, respectively, accelerated and decelerated by increasing the H₂O concentration.     

Low cost coal-based carbons for combined SO2 and NO removal from exhaust gas

The aim of this paper is to show how a cheap carbonaceous material such as low rank coal-based carbon (or char) can be used in the combined SO₂/NO removal from exhaust gas at the linear gas velocity used in commercial systems (0.12 m s⁻¹). Char is produced from carbonization and optionally activated with steam. This char is used in a first step to abate the SO₂ concentration at the following conditions: 100 °C, space velocity of 3600 h⁻¹, 6% O₂, 10% H₂O, 1000 ppmv SO₂, 1000 ppmv NO and N₂ as remainder. In a second step, when the SO₂ concentration in the flue gas is low, NO is reduced to N₂ and steam at the following experimental conditions: 150 °C, space velocity of 900 h⁻¹, 6% O₂, 10% H₂O, 0-500 ppmv SO₂, 1000 ppmv NO, 1000 ppmv NH₃ and N₂ as remainder.It has been shown that the presence of NO has no effect on SO₂ abatement during the first step of combined SO₂/NO removal system and that low SO₂ inlet concentration has a negligible effect on NO reduction in the second step. Moreover, this char can be thermally regenerated after use for various cycles without loss of activity. On the other hand, this regenerated char shows the highest NO removal activity (compared to parent chars, either carbonized or steam activated) which can be attributed to the activating effect of the sulfuric acid formed during the first step of the combined SO₂/NO removal system.     

含氮焦炭异相还原NO反应机理的密度泛函理论研究

选用合理简化的含氮焦炭模型,使用密度泛函理论对含氮焦炭异相还原NO释放出N2的反应机理在分子水平上进行了研究.使用B3LYP/6-31G(d)方法优化得到了含氮焦炭模型、产物、中间体和过渡态的几何构型.在相同水平上计算得到了优化所得结构的单点能并进行了零点能校正,从而得到整个反应的势能面及各中间反应的活化能和反应热.N...     

Catalytic Reduction of Nitric Oxide by Hydrogen Sulfide Over γ-alumina

The ability of H₂S to reduce NO in a fixed bed reactor using a γ-alumina catalyst was studied with the objective of generating new methods for conversion of NO to N₂. Compared to the homogenous reaction of NO with H₂S, the catalyzed reaction showed improved conversions of NO to N₂. Using a gas space velocity of 1000 h⁻¹ and a feed of 1% NO and 1% H₂S in argon, it was found that the conversion of NO to N₂ was complete at 800 °C. This result compared to a 38% conversion of NO to N₂ for the homogeneous gas phase reaction at 800 °C. At temperatures below 800 °C, a short fall in the nitrogen balance was discovered when the γ-alumina was employed as a catalyst. This discrepancy was explained by conversion of NO to NH₃ and subsequent reaction of the NH₃ with any SO₂ in the system to form ammonium sulfur oxy-anion salts. This suggestion is supported by the finding that when larger amounts of H₂S were used relative to NO, more NH₃ was formed together in tandem with lower N₂ mass balances. Several reaction pathways have been proposed for the catalytic reduction of NO by H₂S.     

Reduction of NO in the exhaust gas by reaction with N radicals

N atoms can serve as a reducing agent for NO in the exhaust gases from fossil fuel power plants. In order to investigate the reduction of NO, synthetic exhaust gas was plasma treated in a dielectric barrier discharge (DBD) (direct treatment) as well as treated by adding nitrogen atoms generated in a pure nitrogen DBD (remote treatment). A DBD with a coaxial electrode geometry was used. Oxygen could be added to the synthetic exhaust. A complete NO reduction was achieved in dry, oxygen free exhaust gas with direct treatment, but oxygen and humidity in the exhaust gas promotes formation of N_xO_y and reduces the efficiency of NO reduction. A 90% NO reduction was achieved with remote treatment. Remote treatment is insensitive to oxygen in the exhaust, but requires large amounts of nitrogen and energy. Fourier transform infrared spectroscopy and ultraviolet absorption spectroscopy were employed in order to detect NO, NO₂, and N₂O in the treated exhaust gas stream.     

Modelling of NO and N2O emissions from biomass-fired circulating fluidized bed combustors

A model for NO and N₂O emissions from biomass-fired circulating fluidized bed (CFB) combustors has been developed and evaluated in this study. All the model parameters were chosen for a typical woody biomass-pinewood chips. Both drying and devolatilization of biomass particles were modelled with limited rates, which were selected from the literature based on woody biomass fuels. The partition of fuel-nitrogen between volatiles and char was also specifically chosen for pinewood based on available experimental data from the literature. Volatile nitrogen was assumed to consist of NH₃, HCN and N₂ with the distribution between three species as input parameters to the model. Twenty-five homogenous and heterogeneous global chemical reactions were included in the model, of which 20 reactions represents the global fuel-nitrogen reactions. Both gaseous and solid phase were assumed to be in plug flow. The model has been applied to the modelling of a 12 MW_th CFB boiler. The predicted N₂O emissions were always less than 5 ppmv for pinewood combustion, which was consistent with the experimental results. The predicted NO emissions increased with the total excess air of the riser and the fuel-N content while the predicted percentage conversion of fuel-N to NO decreased with increasing fuel-N content. The NO emissions were also predicted to decrease with increasing primary zone stoichiometry. These predictions agree with the experimental results. The predicted NO emissions decreased slightly with increasing bed temperature, whereas experiments showed that NO emissions slightly increased with bed temperature for birch chips combustion and did not change with bed temperature for fir chips combustion. Sensitivity analyses reveal that the reaction between NO and char is the key reaction to determine the NO emissions.     

Efficient and cost effective reburning using common wastes as fuel and additives

Potential substitutes of natural gas and lignite fly ash as NO and HCN reducing agents, respectively, for heterogeneous reburning were examined in a bench-scale apparatus equipped with a simulated reburning and a burnout furnace. Selection of NO reducing agent is based on fuel volatility and nitrogen functionality. HCN reducing agent selection is based on literature data. A wide range of waste materials and industrial by-products show overall NO reduction efficiency up to 88% at reburning stoichiometric ratio 0.90 or 0.95. Mixed fuel containing scrap tire and Fe₂O₃ is particularly effective. Though its cost is constrained by the energy-intensive operation of grinding the tire, the estimated raw-material cost is better than that of natural gas reburning and highly competitive against SCR. A first-level approximation study of the selectivities of nitrogen species to form NO in burnout zone reveals the importance of HCN and char nitrogen reaction mechanisms.     

NO reduction capacity of four major solid fuels in reburning conditions - Experiments and modeling

The combustion of solid fuels in the rotary kiln and in the calciner of a cement plant generates fuel and thermal NO. This NO can be reduced inside the reducing zone of the calciner. This occurs in two different ways: homogeneous reduction by hydrocarbons and heterogeneous reduction by char. The purpose of this paper is to identify the relative contribution of volatile matters or char on the NO reduction process, which largely depends on the nature of the solid fuel used for reburning.Experiments were undertaken in an Entrained Flow Reactor (EFR), at three temperatures: 800, 900 and 1000 °C. Four major fuels used in the cement production process were studied: a lignite, a coal, an anthracite and a petcoke. Specific experiments were undertaken to determine (i) their devolatilisation kinetics and the gas species released. A wide range of species influencing the NO chemistry was carefully analyzed. Then, (ii) the char oxidation and (iii) the char NO reduction kinetics were characterized. Finally, (iv), the “global” NO reduction capability of each fuel was quantified through experiments during which all phenomena could occur together. This corresponds to the situation of an industrial reactor in reducing conditions. Anthracite and petcoke reduce only very small quantities of NO whereas lignite and coal reduce, respectively, 90% and 80% of the initially present 880 ppm of NO (at 1000 °C after 2 s).The four types of experiments described above were then modeled using a single particle thermo-chemical model that includes heterogeneous reactions and detailed chemistry in the gas phase. This model reveals that both NO reduction on char and NO reduction by volatiles mechanisms contribute significantly to the global NO reduction. After short residence times (several tenth of a second), gas phase reactions reduce NO efficiently; after long residence times (several seconds) the char reduces larger quantities of NO.     

The destruction of N2O in a pulsed corona discharge reactor

The removal of N₂O by a pulsed corona reactor (PCR) was investigated. Gas mixtures containing N₂O were allowed to flow in the reactor at various levels of energy input, and for different background gases, flow rates, and initial pollutant concentrations. The reactor effluent gas stream was analyzed for N₂O, NO, NO₂, by means of an FTIR spectrometer. It was found that destruction of N₂O was facilitated with argon as the background gas; the conversion dropped and power requirements increased when nitrogen was used as the background gas.Reaction mechanisms are proposed for the destruction of N₂O in dry argon and nitrogen. Application of the pseudo-steady state hypothesis permits development of expressions for the overall reaction rate in these systems. These reaction rates are integrated into a simple reactor model for the pulsed corona discharge reactor. The reactor model brings forth the coupling between reaction rates, electrical discharge parameters, and fluid flow within the reactor. Comparison with experiment is encouraging, though the needs for additional research are clearly identified.     

Experimental and kinetic modeling study of the reduction of NO by hydrocarbons and interactions with SO2 in a JSR at 1 atm

The reduction of nitric oxide (NO) by a mixture of methane, ethylene and acetylene with and without addition of SO₂ has been studied in a fused silica jet-stirred reactor operating at 1 atm in simulated conditions of the reburning zone. The temperatures were ranging from 800 to 1400 K. In these experiments, the initial mole fractions of NO and SO₂ were 0 or 1000 ppm, that of methane, ethylene and acetylene were, respectively, 2400, 1200 and 600 ppm. The equivalence ratio has been varied from 0.5 to 2.5. It was demonstrated that the reduction of NO varies as the temperature and that for a given temperature, a maximum NO reduction occurs slightly above stoichiometric conditions. The addition of SO₂ inhibited the process of reduction of NO under the present conditions. The present results generally follow those obtained in previous studies involving simple hydrocarbons or natural gas as reburn fuel. A detailed chemical kinetic modeling of the present experiments was performed using an updated and improved kinetic scheme (1006 reversible reactions and 145 species). An overall reasonable agreement between the present data and the modeling was obtained. Also, the proposed kinetic mechanism can be successfully used to model the reduction of NO by ethane, ethylene, a natural gas blend (methane-ethane 10:1). The kinetic modeling indicates that the reduction of NO proceeds via the following sequence of reactions: HCCO+NO=HCNO+CO; HCCO+NO=HCN+CO₂; HCN+O=NCO+H; HCN+O=NH+CO; HCN+H=CN+H₂; HCNO+H=HCN+OH; CN+O₂=NCO+O; NCO+H=NH+CO; NCO+NO=N₂O+CO; NCO+NO=CO₂+N₂; NH+NO=N₂O+H; NH+NO=N₂+OH. The inhibition of this process by SO₂ is explained by the sequence of reactions H+SO₂+M=HOSO+M and HOSO+H=SO₂+H₂ that acts as a termination process: H+H+M=H₂+M.     

煤燃烧过程中NOx的生成和还原

燃煤过程中ＮＯｘ 的排放是一个复杂的过程, 既包括ＮＯｘ 的生成过程又包括生成的ＮＯｘ 进行均相和多相还原反应． 简述了煤燃烧过程中ＮＯｘ 生成和还原的机理． 认为燃料氮是ＮＯｘ的主要来源,ＮＯｘ 的排放与煤阶、煤中氮含量以及温度等因素有关;ＮＯｘ 与半焦的多相反应是ＮＯｘ还原的主要原因, 其中包括ＮＯｘ 在半焦表面的化学吸附、表面络合物的生成以及产物的生成和脱附等过程     

Low cost catalytic sorbents for NOx reduction. 2. Tests with no reduction reactives

Coal, char and activated char doped with model vanadium compounds (V₂O₅ and NH₄VO₃) and petroleum coke ash, PCA, (main metal components: V, Fe and Ni) have been tested as catalytic sorbents for NO reduction without the addition of a reduction reactive. The sorbents prepared have shown to be active for NO reduction at temperatures higher than 350 °C. The most efficient sorbents are those obtained from unactivated chars and doping with model vanadium compounds or PCA does not upgrade significantly their behaviour. On the other hand, for the samples prepared from activated char, an improvement of the reduction efficiency is observed after impregnation with model vanadium compounds or PCA.SSA of the samples does not play a relevant role on the NO reduction efficiency while surface chemistry significantly affect the samples’ behaviour: higher the CO₂/CO ratio determined by TPD, higher the NO reduction efficiency of the sample.Slightly higher NO conversions are observed for the samples loaded with pure compounds but PCA is perspective for producing catalyst doped activated carbons.     

Formation and reduction of nitric oxide in fixed-bed combustion of straw

Mechanisms of nitric oxide (NO) formation and reduction in fixed-bed combustion of straw have been modeled mathematically and verified experimentally. The model for the straw combustion and nitrogen chemistry consists of sub-models for evaporation, pyrolysis, tar and char combustion, nitrogen conversion, and energy and mass conservation. Twenty chemical reactions are included, of which 12 belong to the fuel nitrogen reaction network. Volatile nitrogen is assumed to be NO, NH₃, HCN and HNCO, and char nitrogen is converted to NO during char oxidation. The model predictions are in qualitative agreement with the measurements during the ignition phase, i.e. when the combustion front passes through the un-burnt fuel. The yield of NO can be reduced considerably by using a low primary air flow due to the longer gas residence in the fixed-bed, while the NO exhaust concentration is insensitive to the bed temperature. The NO exhaust concentration initially reaches a maximum and then decreases towards a stable value after the straw bed is ignited. Variations of NO, NH₃, HCN, and HNCO concentrations in the ignition flame front indicate that a large quantity of NO can be reduced in the thin flame front zone. The developed model is further validated by separate experiments in which NO or NH₃ was added at the middle through tubes or at the bottom of the bed with the primary air flow. Both the simulations and measurements showed that the variation of the NO exhaust concentration is small as compared with the injected NO or NH₃ concentration. According to the simulations and experiments, it is proposed that flue gas recirculation may be a very effective method of reducing NO emissions from flue gas in the fixed-bed combustion of straw. Calculations indicated that about 20% of the flue gas may be recirculated without significantly affecting the combustion behavior.     

Investigation of flue-gas treatment with O3 injection using NO and NO2 planar laser-induced fluorescence

Direct ozone (O₃) injection is a promising flue-gas treatment technology based on oxidation of NO and Hg into soluble species like NO₂, NO₃, N₂O₅, oxidized mercury, etc. These product gases are then effectively removed from the flue gases with the wet flue gas desulfurization system for SO₂. The kinetics and mixing behaviors of the oxidation process are important phenomena in development of practical applications. In this work, planar laser-induced fluorescence (PLIF) of NO and NO₂ was utilized to investigate the reaction structures between a turbulent O₃ jet (dry air with 2000 ppm O₃) and a laminar co-flow of simulated flue gas (containing 200 ppm NO), prepared in co-axial tubes. The shape of the reaction zone and the NO conversion rate along with the downstream length were determined from the NO-PLIF measurements. About 62% of NO was oxidized at 15d (d, jet orifice diameter) by a 30 m/s O₃ jet with an influence width of about 6d in radius. The NO₂ PLIF results support the conclusions deduced from the NO-PLIF measurements.     

NO reduction with H2 in the presence of excess O2 over Pd/MFI catalyst

The NO SCR (selective catalytic reduction) activity with H₂ in the presence of excess O₂ was investigated over Pd/MFI catalyst prepared by sublimation method. With GHSV=90?000 h⁻¹, a very high steady-state conversion of NO to N₂ (70%) is achieved at 100 °C. Significant reorganizations take place inside the catalyst upon its first contact with all reactants and products at the reaction temperature. Pd⁰, which has a significant role in the NO-H₂-O₂ reaction, is possibly the active site for NO reduction. The formation of Pd-β hydride deactivates the catalyst for NO reduction. Throughout the entire NO-H₂-O₂ reaction, no N₂O or NO₂ is formed; N₂ is the only N-containing product. The presence of O₂ inhibits the formation of undesirable NH₃. The rate of the NO+H₂ reaction is fast or comparable to that of the H₂+O₂ reaction. The oxidation of Pd⁰ and subsequent agglomeration of PdO are responsible for the decreased NO reduction activity at high temperature.     

Transformations and destruction of nitrogen oxides—NO, NO2 and N2O—in a pulsed corona discharge reactor

There has been an increasing recent research interest in the removal of NO_x from combustion gases using electrical discharges, especially pulsed corona discharge reactors. The major issues in development of this technology are (a) the energy consumption required to achieve the desired pollutant reduction; and (b) the formation of undesirable byproducts. In this study, the transformations and destruction of nitrogen oxides—NO, NO₂ and N₂O—were investigated in a pulsed corona discharge reactor. Gas mixtures—NO in N₂, N₂O in N₂, NO₂ in N₂ and NO-N₂O-NO₂ in N₂—were allowed to flow through the reactor with initial concentrations, flow rates and energy input as operating variables. The reactor effluent gas stream was analyzed for N₂O, NO, NO₂, by means of an FTIR spectrometer. In some experiments, oxygen was measured using a gas chromatograph.Reaction mechanisms were proposed for the transformations and destruction of the different nitrogen oxides within a unified model structure. The corresponding reaction rates were integrated into a simple reactor model for the pulsed corona discharge reactor. The reactor model brings forth the coupling between reaction rates, electrical discharge parameters, and fluid flow within the reactor. It was recognized that the electron-impact dissociation of the background gas N₂ leads to both ionic and radical product species. In fact, ionic reactions were found responsible for N₂O destruction. Radical reactions were dominant in the transformation and destruction of NO and NO₂. However, decomposition of N₂⁺ ions also leads to indirect production of N radicals; this appears to be a less-power intensive route for NO destruction though longer residence times may be necessary. In addition, the decomposition of N₂⁺ ions limits the N₂O destruction that can be achieved. Comparison with our experimental data, as well as data in the literature, was very encouraging.     

A reduced NOx reaction model for pulverized coal combustion under fuel-rich conditions

A reduced NO_x reaction model was developed for analysis of industrial pulverized coal firing boilers. The model was developed from experiments of laminar premixed combustion under a variety of stoichiometric ratios, burning temperatures, coal ranks (from sub-bituminous coal to anthracite) and particle diameters. Calculations agreed with experimental results for NO_x and nitrogen species (NH₃ and HCN), if the model assumed that the hydrocarbon radicals were formed not only from pyrolysis of volatile matter, but also from char oxidation and gasification. The presence of hydrogen in char at the final burnout stage supported this assumption. NO_x reduction by hydrocarbon radicals was the most important reaction in high temperature (>1500 K), fuel-rich, char combustion regions. NO_x reduction from nitrogen species was sensitive to peak NO_x concentration in volatile combustion regions, but NO_x emission downstream had little influence from the peak NO_x concentration. The heterogeneous reaction between char and NO_x was important for fuel-lean or low-temperature conditions.     

Effects of gas temperature fluctuation on the NO release from pulverized coal particle during char combustion

The effects of gas temperature fluctuation on the NO release from pulverized coal particle during char combustion are investigated. The computed results show that the NO formation through the heterogeneous oxidation and reduction reactions is influenced by the gas temperature fluctuation for the particles with initial diameters of 10-50 μm. The gas temperature fluctuation leads to faster NO release from the particle. The heterogeneous NO formation during the char combustion is further enhanced by the increase in the fluctuation amplitude of gas temperature.     

Denitrification in aqueous FeEDTA solutions

The biological reduction of nitric oxide (NO) in aqueous solutions of FeEDTA is an important key reaction within the BioDeNOx process, a combined physico‐chemical and biological technique for the removal of NO_x from industrial flue gasses. To explore the reduction of nitrogen oxide analogues, this study investigated the full denitrification pathway in aqueous FeEDTA solutions, ie the reduction of NO₃^?, NO₂^?, NO via N₂O to N₂ in this unusual medium. This was done in batch experiments at 30 °C with 25 mmol dm^?3 FeEDTA solutions (pH 7.2 ± 0.2). Also Ca²⁺ (2 and 10 mmol dm^?3) and Mg²⁺ (2 mmol dm^?3) were added in excess to prevent free, uncomplexed EDTA. Nitrate reduction in aqueous solutions of Fe(III)EDTA is accompanied by the biological reduction of Fe(III) to Fe(II), for which ethanol, methanol and also acetate are suitable electron donors. Fe(II)EDTA can serve as electron donor for the biological reduction of nitrate to nitrite, with the concomitant oxidation of Fe(II)EDTA to Fe(III)EDTA. Moreover, Fe(II)EDTA can also serve as electron donor for the chemical reduction of nitrite to NO, with the concomitant formation of the nitrosyl‐complex Fe(II)EDTA–NO. The reduction of NO in Fe(II)EDTA was found to be catalysed biologically and occurred about three times faster at 55 °C than NO reduction at 30 °C. This study showed that the nitrogen and iron cycles are strongly coupled and that FeEDTA has an electron‐mediating role during the subsequent reduction of nitrate, nitrite, nitric oxide and nitrous oxide to dinitrogen gas. Copyright © 2004 Society of Chemical Industry