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.     

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.     

Reburning and burnout simulations of natural gas for heavy oil combustion

Reburning and burnout simulations were carried out through PLUG code of CHEMKIN-III using a reduced mechanism, in order to determine preliminary experimental parameters for achieving maximum NO_x reduction to implement the reburning technology for heavy oil combustion in pilot scale equipments in Brazil. Gas compositions at the entrance of the reburning zone were estimated by the AComb program. Simulations were performed for eight conditions in the usual range of operational parameters for natural gas reburning. The maximum NO reduction (ca. 50%) was reached with 10 and 17.5% of power via natural gas and 1.5 and 3.0% O₂ excess, respectively, at 1273 K. The model predicts 250 ppm of NO, 50 ppm of CO and air mass flows in the range of about 50-130 kg/h for burnout.     

Detailed measurements in a laboratory furnace with reburning

This article presents a detailed experimental characterization of the reburning process in a large-scale laboratory furnace. Natural gas, pine sawdust and pulverized coal were used as reburn fuels. Initially, the study involved the collection of in-flame combustion data, without reburning, in order to define appropriate locations for the injection of the reburn fuels. Next, flue-gas data were obtained for a wide range of experimental conditions using the three reburn fuels and, subsequently, detailed measurements of local mean O₂, CO, CO₂, HC and NO_x concentrations, and gas temperatures have been obtained in the reburn zone for three representative furnace operating conditions, one for each reburn fuel studied. The flue-gas data revealed that the sawdust reburning leads to NO_x reductions comparable or even higher than those attained with natural gas reburning, while coal reburning yields much lower NO_x reductions. The detailed data obtained in the reburn zone indicates that the reburning process remains active throughout all the reburn zone in the cases of natural gas and sawdust reburning, while in the case of coal reburning its relatively low volatile matter content is insufficient to establish an effective reburn zone. In the cases of the sawdust and coal reburning the burnout levels remain approximately constant, regardless of the NO_x emissions reduction, with the sawdust reburning leading to higher particle burnout performance than the coal reburning.     

An experimental parametric study of gas reburning under conditions of interest for oxy-fuel combustion

An experimental parametric study on the NO reduction efficiency by reburning under oxy-fuel conditions has been performed through the gas-phase interactions between different gas mixtures and NO in a CO₂ atmosphere, under flow reactor conditions in the 800-1800 K temperature range. The study provides a wide amount of experimental data on reburning under oxy-fuel conditions to be further used in modeling studies. A higher NO reduction is attained in a N₂ atmosphere compared to CO₂ under fuel-rich and stoichiometric conditions, although the efficiency is similar in both atmospheres under fuel-lean conditions. The formation of HCN in a fuel-rich environment is higher in N₂ than in CO₂ but comparable for other stoichiometries. The influence of the main parameters of the process under oxy-fuel conditions has been found to present similar trends to those observed in literature for reburning in air. A significant NO reduction can be obtained at moderately high temperatures, fuel-rich conditions, high values of the reburn fuel/NO ratio, sufficiently high residence times and low water vapor contents. C₂H₆ has been detected to act as a better reburn fuel as CH₄, whereas CO itself is unable to reduce NO.     

Evaluation of the use of coal volatiles as reburning fuel for NOx reduction

In this study, computational fluid dynamic (CFD) and kinetic models were used to investigate the relative performances of coal volatiles and natural gas reburning. This modeling approach considers fluid dynamic and non-isothermal effects, which were not considered in past laboratory flow reactor studies. The commercial CFD code FLUENT 6.1 was used to predict the residence times and temperatures for reburning tests in the down-fired combustor (DFC), a 0.5 MMBTU/h research combustor at The Pennsylvania State University. To predict NO_x concentrations within the combustor, this data was then applied to an advanced reburning kinetic model used in past studies. For equal firing rates and stoichiometric ratios, reburning using methane yielded lower concentrations of NO_x (and, therefore, better NO_x reduction performance) than reburning using coal volatiles. The coal volatiles give increased flame temperature over natural gas, which apparently offsets the increased reburn zone hydrocarbon radical yield of coal volatiles over natural gas.     

Evaluation of the use of different hydrocarbon fuels for gas reburning

Gas reburning is a NO_x reduction technique that has been demonstrated to be efficient in different combustion systems. An experimental study of gas reburning performance in the low temperature range (at and under 1100°C) has been carried out. An evaluation of the use of different hydrocarbon fuels, such as natural gas, methane, ethane, ethylene and acetylene was performed and the influence of the temperature and stoichiometry is considered. The results show that the reburning process is effective under appropriate conditions at the low temperatures used in this work. However, as the temperature diminishes, the influence of the reburn fuel becomes more marked and the use of acetylene or ethane and ethylene leads to better performance than natural gas or methane, the classical reburn fuels for high temperature applications.     

Effect of gas-liquid phase compositions on NO2 and NO absorption into ammonium-sulfite and bisulfite solutions

The effect of gas-liquid phase compositions on NO and NO₂ absorption into ammonium-sulfite and bisulfite solutions is investigated. Preliminary experiment results indicate that the concentrations of (NH₄)₂SO₃ or NH₄HSO₃ solution and the molar ratio for HSO₃⁻ to the total solution concentrations all have significant impact on NO₂ and NO absorption rates. While the solution concentration is constant, the absorption of NO_x mixture is strongly related to the ratio NO₂/NO_x. The absorption rate of NO is primarily affected by NO₂ inlet concentration, and the NO absorption rate reaches the maximum value in (NH₄)₂SO₃ solution with the increase of NO₂ inlet concentration, which is determined by the reaction of NO and NO₂ with SO₃²⁻ as well as NO formation. Moreover, when the solution is NH₄HSO₃ the best ratio of NO₂/NO for the maximum value of the NO absorption rate becomes less or smaller. Meanwhile, the presence of NO in the gas phase is also favorable to the absorption rate of NO₂ in ammonium-sulfite or bisulfite solutions. The total results suggest that the coexistence of NO and NO₂ in the flue gas could enhance the absorption of each other to some extent.     

Optimisation of NOx reduction in advanced coal reburning systems and the effect of coal type

The advanced reburning process for NO_x emission control was studied in a down-fired 20 kW combustor by evaluating the performance of 15 pulverised coals as reburning fuels. The proximate volatile matter contents of the coals selected ranged from around 4 to 40 wt% (as received) with elemental nitrogen contents from around 0.6 to 2.0 wt%. The effects of reburn fuel fraction, reburning zone residence time, ammonia agent injection delay time (relative to the reburn fuel and burnout air injection points) and the nitrogen stoichiometric ratio are reported in detail and the optimum configurations for advanced reburning, established as a function of operating condition and coal type. The experimental results show that advanced reburning can reduce NO_x emissions up to 85%. The maximum benefits of advanced reburning over conventional reburning were observed at the lower reburn fuel fractions (around 10%). The results demonstrate that under advanced reburning conditions equivalent or higher levels of NO_x reduction can be achieved while operating the reburn zone closer to stoichiometric conditions compared with conventional reburning operating at high reburn fuel fractions (20-25%). Thus the practical problems associated with fuel-rich staged operation can be reduced. The effect of coal properties on the advanced reburning performance was also investigated. As with conventional reburning, the fuel nitrogen content of the coal used was found to have little influence on the NO_x reduction efficiency except at the highest reburn fuel fractions. There was, however, a strong correlation between the effectiveness of advanced reburning and the volatile content of the reburning fuels, which not only depended on the reburn fuel fraction, but also the mode (rich or lean) of advanced reburning operation. These parameters are mapped out experimentally to enable the best operating mode to be selected for advanced reburning as a function of the reburning fuel fraction and volatile content.     

Effect of Mixing on Natural Gas Reburning

The purpose of this work is to understand the influence of the mixing process on reburning performance. Modeling is done in a one-dimensional approximation by utilizing a detailed chemical mechanism, staged addition of reactants, and mixture stratification. Natural gas is used as both the main fuel and the reburning fuel. The most important parameters that affect the efficiency of the reburning process are: the amount of the reburning fuel, the initial NO level, the initial temperatures of the injected reburning fuel and overfire air (OFA), temperature of flue gas at the points where reburning fuel and OFA are injected, and intensity of mixing of the reburning fuel and OFA with the stream of flue gases. It is shown that fuel stratification in the mixing zone improves reburning efficiency for small heat inputs of the reburning fuel and degrades reburning efficiency for large heat inputs. Initial temperatures of the reburning fuel and OFA affect NO reduction and can be optimized for deeper NO control. Reactions of N-containing species in the burnout zone play an important role in NO reduction for large heat inputs of the reburning fuel.     

热解燃烧链条炉低NOx排放特性的数值模拟

利用Fluent软件,对功率为1.4 MW的新型热解燃烧链条炉的NOx排放特性进行了数值模拟,其中,煤热解产生的还原性可燃气简化为CH4,采用添加元素N的乙烯-空气混合物模拟链条炉排半焦层燃烧及其生成的NO. 数值计算结果表明,在过量空气系数为1.2、再燃比为30%的燃烧条件下,热解燃烧比传统燃烧可降低NO排放14.6%. 热解燃烧链条炉由于热解气的再燃作用,在炉膛中形成一局部还原区,可较有效地降低NOx排放,证明了热解燃烧技术的可行性. 增大再燃比和减小炉排前段风室配风量可提高出口NO还原率,减小炉膛前拱长度和前后拱间距会使NO还原作用增强.     

The formation of N2O during the reduction of NO by NH3

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 N₂O by-product. The NO + NH₃ 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 NH₃, in nitrogen as the carrier gas, with O2 added in some experiments. The product gases were analyzed for NO, N₂O and NH₃. The effects the following parameters were studied: temperature inside the reactor between 850 and 1250 K, height of the sand bed, NH₃/NO molar ratio over the range 0.54–2.0 and the addition of 1 or 2% of O₂ 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 N₂O formation also appeared to be strongly enhanced, except at the highest temperatures. Higher molar NH₃/NO ratios favored NO reduction and N₂O production, both with and without sand. The reduction of NO did not appear to require the presence of O₂, but the introduction of 1% or 2% of O₂ 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.     

Mechanism of the Reduction by Ammonia of Nitrates Stored onto a Pt–Ba/Al2O3 LNT Catalyst

The mechanism involved in the formation of N₂ and of N₂O during the reduction of nitrates stored onto a Pt–Ba/Al₂O₃ LNT catalyst is investigated using labeled NO and unlabeled ammonia, in the presence and in the absence of NO in the gas phase. The reduction of the stored NO _x species (labeled nitrates) with NH₃ leads to the selective formation of N₂. Based on the isotopic distribution, it appears that N₂ formation occurs primarily through the statistical coupling of N-atoms formed by dissociation of NO and NH₃ at metal Pt sites. When the reduction of the stored nitrates is carried out in the presence of NO in the gas phase, NO is preferentially reduced. This implies that the rate determining step of the reduction of nitrates by ammonia is likely associated with the release of stored NO _x . Negligible amounts of nitrous oxide have been observed during the NH₃-TPSR with adsorbed nitrates, whereas relevant quantities of N₂O have been detected at low temperatures (below 180 °C) in the runs performed in the presence of NO in the gas phase. The data converge to indicate that N₂O formation involves the presence of gaseous NO and this suggests that the formation of nitrous oxide occurs either through the coupling of two adsorbed NO molecules or the recombination of an adsorbed NO molecule with an adsorbed NH _x species.     

The selective non-catalytic reduction of nitric oxide using ammonia at up to 15% oxygen

Selective non-catalytic reduction of nitric oxide (NO) using ammonia was studied with up to 15% (by volume) oxygen at 102 kPa. The experiments were conducted in an electrically heated laminar-flow, quartz reactor using mixtures of N₂, O₂, NO, and CO to simulate exhaust gas. The base case condition included 330 ppmv of NO, 495 ppmv of NH₃, and 15% O₂. At a reactor temperature of 1050 K, 77% of the NO was removed. For a lower oxygen concentration of 1%, the NO removal was as high as 98% at 1100 K. The degraded performance at high oxygen concentrations is attributed to increases in the oxidation reactions. A major result of this work was the quantification of the amount of N₂O in the treated gases. For the base case conditions, 21 ppmv of N₂O was measured for a reactor temperature of 1075 K. Increasing the ratio of NH₃ to NO (by increasing the NH₃ concentration) increased the maximum NO removal and decreased the temperature at which this level of NO removal was achieved. For the higher NH₃ concentrations, however, the N₂O concentration increased to as high as 54 ppmv. The oxidation products of ammonia (in the absence of NO) for these conditions were found to include first N₂O beginning at 900 K and then NO beginning at 1050 K. Comparisons between these experimental results and predictions from the Miller and Bowman (1989) model indicate that further enhancements of the model may be necessary to incorporate the features of high oxygen conditions.     

富氧燃烧系统中NO的还原及其排放

富氧燃烧方式具有排烟中的CO2浓度高、CO2捕捉回收处理容易等优点,是很有前途的新型清洁煤燃烧方式.通过实验得出了富氧燃烧过程中燃料氮生成NO的转化率随CO2浓度的变化、循环NO在挥发分燃烧火焰中的还原率、循环NO对燃料氮生成NO的转化率的影响,并得出了相应的定量关系式.结合实验结果和系统物质平衡理论分析,得出了整个系...     

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.     

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.     

Mathematical modelling of NO emissions from high-temperature air combustion with nitrous oxide mechanism

A study of the mathematical modelling of NO formation and emissions in a gas-fired regenerative furnace with high-preheated air was performed. The model of NO formation via N₂O-intermediate mechanism was proposed because of the lower flame temperature in this case. The reaction rates of this new model were calculated basing on the eddy-dissipation-concept. This model accompanied with thermal-NO, prompt-NO and NO reburning models were used to predict NO emissions and formations. The sensitivity of the furnace temperature and the oxygen availability on NO generation rate has been investigated. The predicted results were compared with experimental values.The results show that NO emission formed by N₂O-intermediate mechanism is of outstanding importance during the high-temperature air combustion (HiTAC) condition. Furthermore, it shows that NO models with N₂O-route model can give more reasonable profile of NO formation. Additionally, increasing excess air ratio leads to increasing of NO emission in the regenerative furnace.     

Regeneration of initial activity of a pitch-based ACF for NO–NH3 reaction at ambient temperature

The reactivity of adsorbed NO (including NO₂) and NH₃ in the presence of 4.0% oxygen in He was examined over a pitch-based ACF calcined at 800°C. Regeneration at 30°C by 4% O₂ in He without NH₃ was found to be optimum for the recovery of the initial activity with complete removal of NO within 3 h, with minimum leaks of adsorbed NO and NH₃. A higher temperature of 40°C for regeneration increased the liberation of adsorbed NO, and NH₃ over ACF was rather slow at a lower temperature of 25°C, slow regeneration being achieved. Oxygen appears necessary to regenerate the ACF through enhancing the reaction of adsorbed NO and NH₃ for the initial activity, which was ascribed to the catalytic activity for NO–NH₃ and adsorption of both NO and NH₃. NH₃ in the gas phase appears to inhibit the regeneration reaction of adsorbed species, by using the leaking amount during the regeneration.     

Ammonia decomposition in coal gasification atmospheres

Ammonia present in the product gas from coal gasification may increase NO_x emissions from IGCC systems. A fixed bed reactor was used to study the effect of calcined limestone (CaO) on NH₃ decomposition and reaction of NH₃ and NO. Reactions at temperatures to 900°C in helium and in gas compositions typical of air-blown gasifiers were studied. Although CaO enhanced ammonia decomposition in helium, reaction in the gasification atmosphere resulted in the loss of this catalytic activity. Increasing the total pressure further reduced the rate of NH₃ decomposition. CaO enhanced conversion of NO to NH₃ in gasification atmospheres.