NO Emission Levels Research Articles

A simplified model of nitric oxide emission from a circulating fluidized bed combustor

AbstractA simplified mathematical model leading to a closed form of solution is developed for estimation of nitric oxide emission from a coal fired circulating fluidized bed (CFB) furnace. The furnace is divided into two sections: a lower section below and an upper section above the secondary air injection level. Reactions in the cyclone and the return leg are neglected. Furnace dimensions, coal feed rate, coal composition and furnace temperature are inputs to the model which was validated against several pilot scale and commercial units. Experimental results from two pilot plants and two commercial power plants agree with model predictions. A sensitivity analysis was carried out using the model to examine the effect of different operating parameters and coal properties on the overall NO emission from the furnace. It was found that excess air and furnace temperature are most important factors influencing the NO emission level. The primary to secondary air ratio influences the NO emission level reasonably. Properties of coal are other factors which affect the NO emission to a large extent. The model, though it invovles some simplification, predicts the overall emission of NO with a level of accuracy accepted in commercial operation.

流動層燃焼装置からのＮ２ＯとＮＯｘの発生と燃料中窒素の化学結合形態との関連についての実験的検討

To elucidate the effect of nitrogen functionality of coals on N2O and NOx emissions, five kinds of model compounds, such as pyridine, pyrrole and aniline, were burnt with kerosene or coal in an experimental fluidized bed having 0.16m inner diameter and 3m height.No remarkable difference in N2O and NOx emission levels was found among the mod- el compounds in spite of the different nitrogen functionality. Furthermore, simulteneous formation of HCN and NH3, which would be of precursors respectively for N2O and NOx formation reactions, in the combustor was recognized in all of the combustion experi- ments. These results suggest the existence of the conversion reactions such as NH3 formation from HCN in the fluidized bed. The ratio of fuel-nitrogen conversion to N2O was lower in model compounds/kerosene combustion in comparison with model compounds/coal combustion, while the conversion ratio to NOx was higher in the former case.

On the aero-thermal characteristics of gasifier-fuelled gas turbine combustors with complex geometries

This paper describes the application of a mathematical model to predict the aerodynamics, combustion,and NO emission performances of a gas turbine combustor. The numerical basis of the model is a specially constructed code that uses a generalised nonorthogonal and boundary conforming coordinate system. The gas turbine is a component of the British Coal “Topping Cycle,” a coal-fired fluidised-bed-based combined-cycle technology. A distinctive feature of this study is therefore that the fuel for the gas turbine is produced from the partial gasification of coal. Since this fuel contains nitrogenous species derived from the fuel-bound nitrogen of the parent coal, there is concern that these species might give rise to high NO emissions from the combustor. In addition to the fuel NO, the prompt and thermal NO are modelled. The aerodynamic and combustion performances of the combustor, a Frame-9 unit, for this rather unusual fuel are found to be generally good, with no indication of flame stabilisation difficulties and only a relatively small temperature nonuniformity over the exit plane. This nonuniformity can be traced to products that manage to “skirt round” the dilution jet flows. The predicted level of NO emission corresponds remarkably well with that measured. Most importantly, a significant reduction of NO to N 2 by NH 3 occurs within the primary zone, with the result that the NO emission from the gas turbine component of the Topping Cycle is considerably diminished.

気泡流動層石炭燃焼装置からのＮ２ＯとＮＯｘの発生特性および環境流動層燃焼との比較

The effects of operating conditions, air staging, limestone feed and coal types on N2O and NOx emissions were examined using an experimental bubbling fluidized-bed combustor. N2O emission level increased with decreasing in bed temperature and increasing in residual oxygen concentration and superficial gas velocity. Both of air staging and limestone feed into the bed were effective for reduction of N2O emission. In general, N2O emission increased with nitrogen content in coal, though no reasonable correlation has been found between NOx emissions and the nitrogen content.The emission levels of N2O and NOx from the bubbling combustion were compared with those from circulating combustion, which was reported previously. Although total emissions of N2O and NOx from both types of combustion were almost same level, N2O emission from the bubbling combustion showed lower level than that from the circulating combustion. The reason for the difference in the emission levels of N2O and NOx from both types of combution was qualitatively discussed to take into account both differences in flow dynamics of bubbling and circulating beds and in chemical reactions under heterogeneous and homogeneous systems.

Tungsten and tungsten-copper for coal-fired MHD power generation

Magnetohydrodynamics (MHO) can improve the thermal efficiency and reduce levels of SOx and NO emissions of existing coal-fired power generation plants. Although the thermal and electrochemical environments for a coal-fired MHO channel challenge the materials used, platinum, tungsten, and tungsten-copper have been found to be suitable choices. Evaluations indicate these materials perform adequately as electrodes and other gas-side surfaces in the coal-fired MHO channel. Analysis of test elements has resulted in the identification of wear mechanisms. Testing of a prototypical coal-fired MHO channel incorporating these materials is under way and will be completed in 1993.

Formation and measurement of N 2O in combustion systems

Direct N2O emissions from fossil fuel combustion have previously been reported to be equivalent to 25–40% of the NOx levels. At these levels, fossil fuels have been suggested to be a major anthropogenic source of N2O. Recent tests have shown these measurements to be in error, most of the N2O having been formed by reaction between NOx, SO2, and H2O in the sample containers. Time resolved measurements of gas samples stored in Tedlar bags, supported by chemical kinetic calculations, indicate that the majority of N2O forms over a time period of 6 hours. The conversion of NOx to N2O in the sample containers is shown to depend on the amount of SO2 present. This sampling artifact raises questions about the validity of the existing data base, collected by grab sampling methods. As a result, a continuous infrared analyzer, developed primarily for characterization of N2O emissions from full scale combustion sources, was used to perform on line N2O measurements at several full scale utility combustion systems. A variety of conventional and advanced utility combustion systems (firing pulverized coal, oil, and gas) were tested. The measurements from conventional systems (natural gas, oil, and pulverized coalfired) indicate that the direct N2O emission levels are generally less than 5 ppm and are not related to the NOx levels in the flue gas. However circulating fluidized bed units produced elevated N2O emissions. At one circulating fluidized bed combustor firing a bituminous coal, N2O levels ranged from 84 to 126 ppm as the load was varied from 100% to 55%, respectively. The N2O emissions from the circulating fluidized bed appeared to be inversely related to the bed temperature. However, temperature is not the only parameter affecting N2O emissions from fluidized beds; all three of the units studied operated at similar temperatures during full load operation, but the N2O emissions ranged between 25 and 84 ppm. N2O emissions were also elevated at a full-scale boiler using selective non-catalytic NOx reduction with urea; 11–13% of the reduced NOx was converted to N2O.

Nitrous oxide emissions from fossil fuel combustion

The role of coal combustion as a significant global source of nitrous oxide (N2O) emissions was reexamined through on‐line emission measurements from six pulverized‐coal‐fired utility boilers and from laboratory and pilot‐scale combustors. The full‐scale utility boilers yielded direct N2O emission levels of less than 5 ppm. The sub‐scale combustor test data were consistent with full‐scale data, and also showed N2O emission levels not exceeding 5 ppm, although these levels increased slightly when various combustion modifications to lower NO emissions were employed. These on‐line emission measurements are very different from previously published data. The discrepancy is shown to be due to a sampling artifact by which significant quantities of N2O can be produced in sample containers which have been used in establishing the previously employed N2O data base. Consequently, we conclude that N2O emissions bear no direct relationship to NO emissions from these combustion sources, and that this direct source of N2O is negligible. Other indirect routes for the conversion of NO into N2O outside the combustor and other combustion sources not examined by this study, however, cannot be ruled out.

石炭の循環流動層燃焼におけるＮ２Ｏ生成

Parametric study was made on emission of nitric oxide (NO) and nitrous oxide (N2O) from a bench scale circulating fluidized bed combustor (CFBC) of coal. It was found that significant amount of N2O was emitted from the CFBC. N2O emission levels were dependent on the bed temperature and coal types, and were parti cularly sensitive to former. Under the condition of lower bed temperature and/or low volatile coals, the N2O emission increased. The dependency of N2O emission on the bed temperature and the coal types was opposite to that of NO.The total amount of NO and N2O was rather independent of the operating conditions except for the coal types and became large for low volatile coals.

Low NOx, High Efficiency Multistaged Burner: Fuel Oil Results

This paper discusses the fuel-oil portion of an evaluation, utilizing a multistaged combustion burner designed for in-furnace NOx control and high combustion efficiency, for high nitrogen-content fuel and waste-incineration application in a 0.6-MW package boiler simulator. A low-NOx precombustion chamber burner was reduced in size by about a factor of two (from 600 to 250 ms first-stage residence time) and coupled with (1) air staging, resulting in a three-stage configuration, and (2) natural gas fuel staging, yielding up to four stoichiometric zones. Natural gas, doped with ammonia to yield a 5.8% fuel nitrogen content, and distillate fuel oil, doped with pyridine to yield a 2% fuel nitrogen content, were used to simulate high nitrogen content fuel/waste mixtures. The multistaged burner reduced NO emissions by 85% from emission levels from a conventional unstaged burner mounted on a commercial package boiler. A minimum NO emission level of 110 ppm was achieved in the fuel-oil tests, from a level of 765 ppm for conventional firing.

Computation of NO Levels for Various Operations in SI Engines

The method of computation is proposed to estimate NO emission levels of spark ignition engines operating with not only homogeneous but also nonhomogeneous mixtures, because mixture states other than a homogeneous one may deliberately be used to improve pollutant emissions and engine performance. First proposed is a model valid for homogeneous mixtures, and then it is so modified as to be applicable to various nonhomogeneous mixtures such as a stratified charge. These models allow us to survey the relation between NO emissions and the variations of mixture states or operating conditions in SI engines.

小型ガスタービンにおける動特性と汚染物質NO<SUB>x</SUB>のシミュレーション

It is attractive to investigate the gas turbine engine by simulating technique, since it gives an improved understanding of engine dynamics and control problems without any dangers. And it will save both cost and time of engine development.As a few examples of transient operations, the sudden changes of fuel flow rate during acceleration and deceleration are examined. It is shown clearly that the complicated dynamic behaviour of a small gas turbine can be analyzed, and these calculated results are compared with experimental data during the transient operations.Pollutant emissions (CO, THC, NOx and NO) from the gas turbine are measured and analyzed during acceleration and deceleration. Especially, the NO emission levels are calculated by the Zeldovich mechanism during accelerations, and these data are compared with those obtained experimentally. As can be seen, the data from both tests agreed with each other fairly well.

Urban kinetic chemical calculations with altered source conditions

Chemical kinetic computations representing upwind, urban and downwind areas in northern New Jersey under conditions of westerly wind flow have been performed. A number of points in the NO-HC source emissions plane, covering a range of 1 4 to 3 times current emissions inventories, are included. All results demonstrate the frequently-observed features of an ozone decrease as the urban area is approached from upwind followed by a subsequent increase downwind of the urban area. In the urban area, ozone peak values increase by < 3% for a 3-fold increase in hydrocarbon emissions, but decrease by ~ 14% as NO emission increases 3-fold. Hydrocarbon emission changes do not markedly perturb the NO 2/NO ratio, because their influence in NO → NO 2 oxidation is offset by a concomitant influence on organic nitrate formation. The effect of additional NO emissions is to lower that ratio locally, thus lowering the ozone concentration, but to enhance it downwind, thus raising the ozone level downwind of urban areas as a result of precursor transport. The hydroxyl radical concentration in these urban calculations turns out to be proportional to the NO 2 concentration. As a result, the formation of the photochemical products acrolein, PAN, nitric acid, gaseous sulfate, and total particulate matter is strongly influenced by NO emission levels. Thus, a reduction in the emission rate of NO appears likely to result in reductions in a variety of undesirable atmospheric species in geographical areas with conditions approximating those utilized for these computations.

Predicted effects of fluid dynamic parameters on nitric oxide formation in turbulent jet diffusion flames

The effects of various fluid dynamic parameters on the flowfield and, consequently, nitric oxide formation in turbulent jet diffusion flames are analyzed in the present study. The diffusion flame is considered to form in the mixing zone between a finite jet of fuel, such as methane, and a parallel semi-infinite stream of oxidizer, such as air. The theoretical results show that, as the air velocity is increased from stagnant conditions, the NO emission level first decreases and then increases after reaching a minimum at an air-fuel velocity ratio of about 0.5. For air-fuel velocity ratios greater than unity, the emission level decreases monotonically as the air velocity is increased. Flue gas recirculation in fuel stream is found to be more effective than recirculation in air stream by several-fold in reducing NO emission. These results are shown to be the consequences of turbulent diffusion. The effects of turbulence level, air preheat, turbulent Prandtl-Schmidt number, and jet orifice size on NO emission are also investigated and discussed.

The Role of Kinetics in Engine Emission of Nitric Oxide

Abstract The use of chemical kinetic calculations to predict the formation of nitric oxide in spark ignition engines and gas turbine combustors is reviewed. Although the basic chemical kinetic mechanism is the same in both cases, differences in thermodynamics and fluid mechanics account for different NO emission levels. In spark ignition engines it is necessary to account for temperature gradients in the engine. In gas turbines the NO kinetics are frozen before the NO is formed in equilibrium amounts.