MINIMISATION OF NOx EMISSION BY FLAME TEMPERATURE CONTROL

Emission control technology has advanced to an extent whereby pollutant levels are now controlled by legislation. Techniques for controlling emissions vary and include improved fuel mixture preparation, multi stage combustion, exhaust gas recirculation, inhibitors such as ammonia and other techniques which reduce the intensity of combustion and hence flame temperature.

This paper examines ways in which ultra low NO_x, emission combustors can be produced by applying technology developed for burning poor quality fuels.     

PERFORMANCE CHARACTERISTICS OF NOx REMOVAL BY AQUEOUS EMULSIONS OF YELLOW PHOSPHORUS

The absorption oflean NO_x (200?1000 ppm) gases in aqueous /% emulsions was investigated in a mechanically stirred vessel. The chemical reaction rate between NO, and P₄ in an alkaline solution was shown zero order with respect to NO_x concentration and three-seconds order to P₄ concentration. In the presence of S0_x=2, the chemical reaction rate between NO_x, and P₄ in alkaline solution was again zero order with respect to NO_x concentration, but changed to second order to P₄ concentration.

In addition, a robust design method was also used to evaluate the relative importance of various processing variables on the performance of this deNO_x process by P₄$ solution. S/N analysis of the results indicated that the P₄ concentration, liquor temperature and NO_x, concentration were the major factors in affecting NO_x absorption efficiencies.     

CHEMICAL AND BIOCHEMICAL PROCESSES FOR NOx CONTROL FROM COMBUSTION OFF-GASES

Nitrogen oxides (NO_x) are major air pollutants as they have adverse effects on health and the environment. Various types of combustion equipment, including biosolids incinerators, are the sources of NO_x emissions in the atmosphere. Various source control measures in the form of the use of low NO_x burners and burner staging techniques, for example, are commonly practiced by the generators. Treatment processes such as selective catalytic reduction (SCR) and selective non-catalytic reduction (SNCR) are also widely used, especially by power plants. Newer processes involving chemical scrubbing of the flue gas have been under development for the past two decades and are being gradually introduced in full-scale commercial operation. This article provides a comprehensive review of recently developed chemical and biochemical processes for NO_x control. Equations for the reaction rates, scrubber sizes, and chemical requirements are also presented.     

SEPARATION OF ETHYLBENZENE-XYLENES MIXTURES BY DISTILLATION: A POSSIBLE APPLICATION OF VAPOUR RECOMPRESSION CONCEPT

The application of direct vapour recompression to an existing plant, fractionating ethylbenzene-xylenes mixtures, has been studied.

The behaviour of the saturated liquid and vapour curves for ethylbenzene in the Temperature-Entropy diagram has been evaluated theoretically by means of well known equations of state. The most important operating parameters of the process (as functions of ΔT) in the reboiler-condenser have been obtained. The economic analysis performed indicates that for the case considered, the best value for ΔTis about 6°C, but only a small energy saving can be achieved.     

Model Studies of NOx Storage and Sulphur Deactivation of NOx Storage Catalysts

The storage of NO _x under lean conditions in model NO _x storage catalysts as well as the deactivation by sulphur have been studied. We find that NO₂ plays an important role in the storage mechanism as an oxidising agent. Two different mechanisms for this are discussed: the formation of surface peroxides and the oxidation of nitrites to nitrates. FTIR studies show that NO _x is stored as surface nitrates. The sulphur deactivation is found to be more severe when SO₂ is added during the rich phase than when SO₂ is added during the lean period. FTIR shows the formation of bulk sulphates both under lean and rich conditions.     

NOx Storage-Reduction Three-Way Catalyst with Improved Sulfur Tolerance

The NO _x storage-reduction catalyst (NSR catalyst) is poisoned by SO₂ caused by fuel sulfur, thus its activity is reduced. In order to improve the NSR catalyst, the sulfur poisoning phenomenon has been analyzed. Based on this result, we developed TiO₂ and Rh/ZrO₂ to promote the sulfur desorption. The developed catalyst has made remarkable progress in its sulfur tolerance, about 50% improvement in NO _x purification performance compared with the conventional one.     

Fundamental studies of NOx storage at low temperatures

A commercial NO_x storage catalyst (Pt, BaO and alumina containing) was investigated by temperature programmed desorption (TPD) experiments in the temperature range 100–400 °C. The catalyst stored a substantial amount of NO_x at 100 °C using NO + O₂. Nitrites or loosely bound NO species are suggested for this storage, since no NO was oxidised at this low temperature. In addition, the released NO_x during the temperature ramp consisted of mainly NO and at lower temperatures the NO₂ dissociation is limited. Water and CO₂ was found to decrease the storage substantially, 92% for the NO + O₂ adsorption at 100 °C. The total storage for 60 min using NO₂ + O₂ at 200 °C was similar when introducing CO₂ and H₂O. However, the initial total uptake of NO_x was decreased. Initially we probably formed loosely bound NO_x species, which likely are strongly influenced by water and CO₂. After longer time periods are barium nitrates probably formed and they can remove the carbonates by forming stable nitrates, thus resulting in the same total uptake of NO_x.     

Effects of Lean/Rich Timing and Nature of Reductant on the Performance of a NOx Trap Catalyst

A NO _x trap catalyst was studied in a laboratory reactor under simulated diesel passenger car conditions. The effects of lean/rich duration and the nature of reductant are investigated. At 300°C, the average NO _x conversion decreases with increasing lean duration; conversely the NO _x conversion increases with increasing rich duration. The NO _x conversion at this temperature was found to be a direct function of reaction stoichiometry. That is, the quantity of trapped NO _x under lean conditions must be balanced by the quantity of reductant during the rich trap regeneration step. At extreme temperatures, other factors, reaction kinetics (at lower temperatures) and NO _x storage capacity (at higher temperatures), dominate the NO _x conversion process. Overall, carbon monoxide was found to be the most effective reductant. Hydrocarbon, e.g., C₃H₆, is effective at higher temperatures (T>350°C), while H₂ is more efficient than other reductants at low temperatures (T<200°C). The individual steps of the NO _x conversion process are discussed.     

The mechanism for NOx storage

The mechanisms for storing of NO_x in platinum–barium–alumina catalysts during lean–rich transients are investigated. Oxidation of NO to NO₂ is found to be an important step. NO₂ is found to be important for oxidation of the catalyst or of nitrites to form nitrates. NO_x is then stored in the form of surface nitrates. FTIR studies show no formation of bulk nitrates in these experiments. This revised version was published online in July 2006 with corrections to the Cover Date.     

Simultaneous Removal of NOx and SOx from Flue Gas of a Glass Melting Furnace using a Combined Ozone Injection and Semi-dry Chemical Process

In this study, we propose a plasma-chemical hybrid NO_x removal process using nonthermal plasma for the treatment of flue gases emitted from glass melting furnaces; the process is demonstrated through a laboratory-scale model experiment conducted using a semi-dry desulfurization apparatus. The performance of the system for simultaneous removal of SO₂ and NO_x is investigated. As a result, NO is effectively oxidized to NO₂ by injecting ozone into the spray region and the removal efficiencies of 90% and 50% were obtained for NO and NO_x, respectively. In addition, the SO₂ removal efficiency of 84% was achieved.     

Modeling bio-atmospheric coupling of the nitrogen cycle through NOx emissions and NOy deposition

The tropospheric and terrestrial nitrogen cycles are connected to one another through the emissions of NO_x and NH_x from soils and vegetation and the subsequent redeposition of these compounds and their products elsewhere. These connections play an important role in the Earth system influencing tropospheric concentrations of NO_x, O₃, and CO₂. Estimates of the biogenic sources of NO_x, soil emissions and biomass burning, are amongst the most variable terms in the global budget of NO_x and are eclipsed only by lightning. A 3-D chemistry transport model, IMAGES, was used to examine how soil emissions and biomass burning influence tropospheric concentrations of NO_x and O₃ as well as NO_y deposition. Soil and biomass burning emissions of NO_x contributed the most to atmospheric NO_x concentrations closest to the surface and south of 30°N. The influence of these emissions on tropospheric O₃ and NO_x concentrations dissipated with height suggesting that these surface emissions are most important to surface ozone concentrations. The removal of either the soil or biomass burning source resulted in a 5-20% difference in tropospheric O₃ concentrations over large regions of the atmosphere. Both sources are also important contributors to N deposition, particularly south of 30°N which, in turn, can generate significant carbon storage. These exercises demonstrate both the importance and complexity of the connections between atmospheric chemistry and the terrestrial biosphere.     

The Effect of Carbon Monoxide and Hydrocarbons on NOx Storage at Low Temperature

One possible way to reduce NO _x in lean exhausts is by using NO _x trap catalysts. This paper addresses storage of NO _x on such catalysts at temperatures below the catalyst light-off. Experiments carried out on commercial samples in synthetic exhausts revealed a large capacity for storage of NO _x when NO₂ was added at temperatures below 150°C. In contrast, when NO was added instead, no storage took place. CO was found to decrease the storage by reacting with NO₂ and forming NO and CO₂. Propene inhibited the reaction between NO₂ and CO and therefore gave rise to larger NO _x storage when CO was present. The paper concludes with a discussion of a possible mechanism for the storage of NO _x at low temperatures.     

Statistical Behavior of O3, OX,NO, NO2, and NOx in Urban Environment

ABSTRACT

This paper presents the results of the statistical modeling of the ozone concentration in Campo Grande, Brazil in 2016. Five sets of data, summer (January–March), autumn (April–June), winter (July–September), spring (October–December), and all year round were used. The results show that the maximum concentrations of oxidants occur at 3:00 p.m., the diurnal NO variation, the concentrations show a cycle with two peaks at 7:00 and the other at 11:00 p.m. It has been found that the best distribution for the five datasets is the lognormal distribution of three parameters. The seasonality of the datasets shows greater asymmetry during the summer, due to the greater tail distribution, mainly due to the greater photochemical activity.     

Spectroscopic Evidence for a Nitrite Intermediate in the Catalytic Reduction of NOx with Ammonia on Fe/MFI

The mechanism of selective catalytic reduction (SCR) of NO_x with NH₃ over Fe/MFI was studied using in situ FTIR spectroscopy. Exposing Fe/MFI first to NH₃ then to flowing NO + O₂ or using the reversed sequence, invariably leads to the formation of ammonium nitrite, NH₄NO₂. In situ FTIR results in flowing NO + NH₃ + O₂ at different temperatures show that NH₃ is strongly adsorbed and reacts with impinging NO_x. The intensity of the NH₄NO₂ bands initially increases with temperature, but passes through a maximum at 120 °C because the nitrite decomposes to N₂ + H₂O. The mechanistic model rationalizes that the consumption ratio of NO and NH₃ is close to unity and that the effect of water vapor depends on the reaction temperature. At high temperature H_2O enhances the rate because it is needed to form NH₄NO₂. At low temperature, when adsorbed H₂O is abundant it lowers the rate because it competes with NO_x for adsorption sites.     

Numerical Study of NOx Abatement Using Ozone Injection Integrated with Wet Absorption

Nitrogen oxides emitted from power plants and the chemical industry are poisonous to humans and animals, contribute to ozone depletion, and cause acid rain. More than 90% of nitrogen oxides (NO_x) consist of nitric oxide (NO), which is insoluble in water. Among the various available techniques of NO_x abatement, ozone injection is a promising method in which NO is oxidized to higher-order nitrogen oxides (NO₃, N₂O₃, N₂O₄, and N₂O₅), which can easily be absorbed in a wet scrubber. In this article, the ozone injection process integrated with an absorber column is numerically modeled and simulated at various operating conditions. The predicted results of NO_x oxidation with ozone injection and absorption in water agree with the published experimental results. The ozone injection process is modeled using a plug flow reactor, while the wet absorption is based on a rigorous rate-based RateFrac model. Detailed kinetic mechanisms of O₃-NO_x oxidation and absorption of nitrogen oxides in water are incorporated in the model to simultaneously predict the performance efficiency of the ozone reactor and absorber column. Thermodynamic properties of the components are estimated using an Electrolyte NRTL model. The influence of performance parameters (such as feed gas flow rate, inlet gas temperature, reactor configurations, ozone concentration, and NO/NO₂ molar ratio) on the oxidation efficiency of NO_x in the reactor and absorber column is investigated to predict the optimal operating conditions.     

Coarse-scale soil–atmosphere NOx exchange modeling: status and limitations

Gaseous NO and NO₂ (collectively termed NO_x) are trace atmospheric constituents with important functions in various atmospheric and ecosystem processes. Because nitrification and denitrification in soil are included among major sources of the gases, simulation models for predicting soil-atmosphere NO_x exchange should incorporate the strong dependence of these two microbial processes on soil temperature, soil water content, and substrate N availability. We briefly review current understanding of these controls, then describe how various authors have incorporated that knowledge into model parameterization schemes, and finally present some general thoughts regarding how well those schemes work and what might be missing. Existing coarse-scale models have evolved to the point that they are beginning to explicitly address the influences, interactions, and dynamics of all three microscale controllers in formulations suitable for studies at regional to global and seasonal to interannual scales. Perhaps the greatest limitation of the models, as well as their predictor variables, is that they often fail to account for the large pulse of gaseous N oxide emissions commonly observed following wetting of very dry soil. This failure is exacerbated by mounting evidence that similar pulses may occur following sudden removal of other environmental limitations on microbial growth and metabolism. Such pulses ostensibly make a large contribution to soil- atmosphere NO_x exchange, especially in semi- arid, subhumid, and seasonally dry tropical regions of the globe where the exchange has been poorly characterized despite being subject to intense anthropogenic disturbance.     

The role of acid sites in cobalt zeolite catalysts for selective catalytic reduction of NOx

The role of the acidic support in ion-exchanged cobalt-zeolite, lean NO_x catalysts has been determined by studying the individual steps in the selective reduction pathway. At a GHSV of 10,000 and reaction temperatures below 400°C, NO oxidation is not sufficiently rapid to obtain equilibrium over, for example, 1–4 wt% Co-mordenite catalysts. The NO oxidation rate increases in the order H⁺Co²⁺ Co oxide, and neither the number, nor the strength of the acid sites affects the specific rate of the Co²⁺ ions. For reduction of NO₂ by propylene at 300°C and methane at 400°C, the formation of N₂ is suggested to occur at support protons sites. In addition, the rate of N₂ formation increases linearly with an increase in the number of acid sites, and the specific activity increases with an increase in acid strength. Cobalt (2+) ions do not contribute significantly to the formation of N₂, but do non-selectively reduce NO₂ to NO. It is proposed that the formation of N₂ occurs by protonation of the reducing agent followed by attack of the carbocation by gas phase NO₂. Thus, the selective reduction of NO requires two catalytic functions, metal and acid sites. This revised version was published online in July 2006 with corrections to the Cover Date.     

Influence of vent air valve opening on combustion characteristics of a down-fired pulverized-coal 300 MWe utility boiler

Industrial experiments have been performed on a down-fired pulverized-coal 300 MWe utility boiler with vent air valve opening of 100% and 40%. The gas temperature distribution along the primary air and coal mixture flow, gas temperature distribution in the furnace, and gas components such as O₂, CO, CO₂ and NO_x in the near-wall region were measured for the first time. The influence of vent air valve opening on coal combustion in the furnace was determined. The results indicate that ignition of the primary air and pulverized-coal mixture is delayed. The position of the gas temperature peak is above the arches. Emission of NO_x is up to 2101 mg/m³ (at 6% O₂ dry) with vent valve opening of 40%.     

Theoretical Study of the Ozone Production and NOx Molecules Processing in N2/O2 Pulsed Discharge

Environmental decontamination is a major challenge due to the rapid growth of industrial and technological development, requiring an important consumption of fossil energies. Nowadays, a new way to treat polluting molecules based on the use of nonequilibrium reactive plasmas, is under development. These plasmas are generated by electrical discharges at atmospheric pressure, for neutralizing or transforming the toxic oxides. In this respect, the goal of the present work is to analyze the possibilities of treatment of NO_x molecules and ozone generation by pulsed discharge. The study was carried out by using a one-dimensional model based on the parallel resistor network concept, consisting in dividing the discharge volume into plasma elements which are connected in no way but through their contribution to the total resistance of the plasma. The model calculations indicate the effect of gas heating and inhomogeneous preionization on the ozone production and NO_x destruction.     

燃煤过程中CaO及钙基固氟剂对氟析出的控制

燃煤过程中CaO与HF反应生成CaF₂.在燃烧温度900 ℃时,CaO对煤中氟析出的抑制范围为12.2%～61.0%,平均为39.5%.CaO固氟最佳条件是：燃烧温度800～1000 ℃,停留时间5～10 min, Ca/S（摩尔比）为2.5～3.0.钙基固氟剂由钙基吸收剂和添加剂组成,主要经历钙基吸收剂的分解反应、CaO固氟反应及硅铝钙等化合物的复合反应而形成高温稳定的固氟产物.工业链条炉燃烧试验表明:钙基固氟剂在全预混添加方式下固氟率为54.0%～64.8%,平均为59.3%;在半预混半喷射两段添加方式下固氟率为72.5%～80.5%,平均为75.5%,明显高于全预混添加方式. CaO和钙基固氟剂的固氟效果与燃烧条件、燃烧方式及固氟剂的种类和成分有关.