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Current Opinion in Structural Biology

Volume 59, December 2019, Pages 91-97

Structure and mechanisms of ROS generation by NADPH oxidases

Author links open overlay panelFrancescaMagnaniAndreaMattevi

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https://doi.org/10.1016/j.sbi.2019.03.001Get rights and content

Highlights

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ROS are deliberately produced by the NADPH oxidases in response to specific physiological stimuli.

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The first three-dimensional structure of the NOX catalytic core has been recently solved.

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ROS are generated by a heme cofactor through an uncommon outer sphere mechanism.

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Prolonged NOXs activity can ultimately lead to fibrosis-related diseases, such as cancer and muscular dystrophy.

NADPH oxidases (NOXs) are integral membrane enzymes that produce reactive oxygen species. Humans have seven NOX enzymes that feature a very similar catalytic core but distinct regulatory mechanisms. The recent structural elucidation of the NOX catalytic domains has been a step forward in the field. NADPH, FAD, and two hemes form a linear array of redox cofactors that transfer electrons across to the two sides of the membrane. Oxygen is reduced through an unusual outer sphere mechanism that does not involve any covalent intermediate with the heme iron. Several recent studies have expanded the roles of NOXs in cell signaling, innate immune response, and cell proliferation including oncogenic transformation. This work reinforces NOX-generated ROS as powerful signaling molecules. A challenging question is to understand the specific mechanisms of enzyme regulation and to harness the growing insight on NOXs' structure and biochemistry to generate more powerful small-molecule modulators of NOX activities.

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Special issue articlesRecommended articlesCiting articles (15)

The rox1 and rox2 RNAs have been suggested to be components of the dosage compensation machinery in Drosophila. We show that both rox RNAs colocalize with the male-specific lethal proteins at hundreds of specific bands along the male X chromosome. The rox RNAs and MSL proteins also colocalize with the X chromosome in all somatic cells and are expressed in the same temporal pattern throughout development. Genetic analysis shows that the functions of the rox genes are redundant and required for the association of the MSL proteins with the male X chromosome. These data provide compelling evidence for a direct function of the rox RNAs in dosage compensation

NOx Emission

NOx emissions averaged 0.245 lb/MM Btu, SO2 removal efficiencies in excess of 90 percent were achieved with typical emissions of 0.038 lb/MM Btu, particulate matter emissions were 0.0047 lb/MM Btu, and CO emissions were less than 130 ppm at 3.0 percent O2 [1].

From: Clean Coal Engineering Technology, 2011

Related terms:

Biodiesel

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Carbon Monoxide

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Assessment of performance, combustion, and emission behavior of novel annona biodiesel-operated diesel engine

Senthil Ramalingam, Silambarasan Rajendran, in Advances in Eco-Fuels for a Sustainable Environment, 2019

14.3.4.3 NOx emission

NOx emission is found in an engine due to higher in-cylinder temperature. Though the diesel fuel or the ester-based biodiesel does not contain nitrogen, the NOx are formed by taking the nitrogen from the air because 78% of nitrogen is present in the air.

Fig. 14.7 shows the variation of NOx emission with brake power for different proportions of AME diesel blends and diesel. It is noted that the NOx increases with increasing brake power for all AME-diesel blends at all loads. Among the different AME-diesel blends, A20 has a lower NOx emission than that of other blends. The NOx for A20 is 620 ppm and for diesel is 552 ppm at maximum load. The diesel fuel has less NOx emission because the diesel fuel contains no oxygen content. When the biodiesel is present in the combustion chamber, the temperature becomes very high due to the presence of higher oxygen content, which enhances combustion and results in higher NOx emission.

Fig. 14.7. Variation of NOx with BP for diesel and different proportions of AME diesel blends.

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Integrated Gasification Combined-Cycle Power Plants

Neville A.H. Holt, Seymour B. Alpert, in Encyclopedia of Physical Science and Technology (Third Edition), 2003

I.G NOx Emissions Control

NOx emissions from the combustion turbine can result from two sources: (a) fuel-bound nitrogen in such species as ammonia and (b) thermally produced NOx. In most of the commercial-size IGCC plants there is a water wash step in the cold gas cleanup that removes ammonia and HCN from the syngas before entering the AGR process, thereby removing the components of fuel-bound nitrogen.

Due to its high flame temperature, the clean syngas can lead to high NOx emissions in the combustion turbine unless controlled by other means. Two main techniques are used to lower the flame temperature for NOx control in IGCC systems. One is to saturate the syngas with hot water derived from low-temperature heat recovery elsewhere in the process. The other is to use nitrogen from the ASU. In both cases, mass is also added to the syngas and additional power is thereby generated in the gas turbine and steam cycle. At Wabash NOx control is by saturation and some steam injection. At Tampa the NOx is controlled by nitrogen injection, while at Buggenum and Puertollano a combination of saturation and nitrogen is used.

At the Sierra Pacific project the nitrogen in the low-heating-value syngas from the air-blown gasifier should reduce the flame temperature sufficiently to meet the NOx limits. However, this has not yet been demonstrated and there may also be some NOx production from ammonia at this plant since there is no water wash step in the gas cleanup. If extremely low NOx levels, such as 1–2 ppmv, are required, a selective catalytic reduction (SCR) bed can be placed within the HRSG at a suitable temperature.

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Air Pollution from Small Two-Stroke Engines and Technologies to Control It

Yuji Ikeda, ... Eran Sher, in Handbook of Air Pollution From Internal Combustion Engines, 1998

13.1.1 NOx Emissions

NOx emissions are mainly composed of NO molecules and a small amount of NO2, in a typical ratio of 1 to 100. NO is one of the intermediate products of chemical reactions involving nitrogen and oxygen atoms and molecules, which occur at the high-temperature burned gases behind the flame. Rapid cooling (below 1800 K), due to the expansion stroke, does not allow the products to attain chemical equilibrium and some intermediate products (including NO) "freeze" and leave NO concentrations far in excess of levels corresponding to equilibrium at exhaust conditions. The high residual fraction, which is typical to two-stroke-cycle engine, results in lower maximum temperatures and therefore lower NO production.

The strong effect of the temperature is evident; a reduction of the maximum temperature from 2500 K to 2300 K results in a cutback of 90 percent in the initial rate of NO production. Increasing the residuals mass fraction is an effective and practical method of reducing the maximum temperature and thus the NO emission level. Due to the peculiar gas exchange process in two-stroke-cycle engines the recycled exhaust gas is an inherent property, and the typical low NO emission level of a two-stroke-cycle engine is anticipated. Owing to the exceptionally high value of water-latent heat, introducing water spray into the induction manifold during the cylinder (or crankcase) charging process is another practical method to depress the adiabatic flame temperature and, hence, the NO emission level.

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Clean Coal Technologies for Advanced Power Generation

Bruce G. Miller, in Clean Coal Engineering Technology, 2011

Limestone Effects

The effect of limestone addition on NOx emissions is complex. In a BFBC boiler, the limestone is primarily in the dense phase of the bed where CO concentration is high and O2 concentration is low, and the catalyzed reduction of NO by CO may dominate over oxidation of NH3, thereby leading to lower NO emissions [19]. In a CFBC boiler, however, the limestone is distributed through the entire combustor, and oxidation of volatiles to NO may occur in the upper zone where CO concentrations are lower, thereby increasing NO emissions. The addition of limestone is usually found to decrease N2O emissions, although the effect is minor [19, 47]. The decrease is attributed to the limestone catalyzing the decomposition of N2O.

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Reduction of nitrogen oxide emissions

Marek Pronobis, in Environmentally Oriented Modernization of Power Boilers, 2020

4.3.5 Flue gas recirculation

The effect of recirculation, which decreases the NOx emissions, consists in a reduction of the content of O2 in the area of chemical reactions producing NOx, as well as lowering both average values and local maxima of temperature in the combustion zone. The easiest way to achieve this is to feed re-circulated cool flue gas directly to the burners or, alternatively, as a mixture of flue gas with combustion air.

In gas and oil fired boilers, FGR is an effective NOx reduction method, but its effectiveness may vary considerably. The values shown in Fig. 4.21 (Gas & oil A [45], Gas B [46], Gas C [47]) differ even three times for the same percentage of FGR.

Figure 4.21. The impact of flue gas recirculation on NOx reduction.

It is generally believed that the effectiveness of flue gas recirculation regarding NOx reduction in PF boilers is not very high, and the reduction in emissions does not exceed several per cent (Fig. 4.21) – coal [45]. However, the reduction in NOx emissions is sometimes bigger of the order of 10%. Moreover, properly designed recirculation diminishes the non-uniformity of the distribution of temperatures in the furnace and superheaters.

In the case of the PF boilers, with relatively large furnaces (volumetric heat release qv ≈ 100–120 kW/m3) the introduction of flue gas recirculation into the combustion area (directly, via suitable nozzles, via pulverizers or mixing secondary air with flue gas) complicates the technical solutions substantially and fails to produce significant effects of reducing NOx emission. This is because the reducing effect of recirculation consists mainly of the lowering of temperatures in the furnace so that the secondary formation of nitrogen oxides in its top part does not happen. In small furnaces, with a high qv, the furnace outlet temperatures are above 1400oC, and the effect of recirculation is more visible.

However, recirculation of flue gas may make sense as an additional element of steam temperature regulation when the boiler has to work at a wide range of capacity. Recirculation then helps to keep the required temperatures of superheated steam without the necessity to increase the excess air number in the furnace at a low load, which would disturb the process of NOx reduction. In addition to the described primary low NOx methods, there are also other technologies that could be used in modernized PF boilers. One of them is Flameless Oxidation - FLOX used in natural gas furnaces. Its advantage is significant suppression of thermal NO formation. It is also a promising technology for burning coal [48,49].

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Introduction

Franz F. Pischinger, in Handbook of Air Pollution From Internal Combustion Engines, 1998

9.3.1 Chemical Thermodynamics

The crucial task for lowering diesel emissions is to improve particulate and NOx emissions simultaneously—a difficult task, as there is usually a trade-off between these two emissions. To determine if this task is at all possible, basic fundamentals of chemical thermodynamics must be explored.

Figure 9.9 shows a typical mixture distribution in a diesel injection spray just prior to ignition, ranging from rich in the spray core to lean in the spray edge. The diagram on the left shows the temperature history of the zones with different relative air-fuel ratios (λ), proceeding from the curve of mixture temperature before combustion up to the curve of burned gases after combustion. The diagram also shows the zones of soot and NOx formation, the darker shadowed zones indicating higher formation rates. It can be seen that in an indicated target λ-range, combustion is possible without soot formation and only very low NOx formation. Thus, in principle, smokeless, low-NOx diesel combustion should be possible, if the mixture formation could be narrowed to the required mixture ratio. This may not be possible with the combustion of typical sprays. However, even if soot formation cannot be completely avoided, there is a possibility of burning the soot during the high-temperature combustion process, as is shown in the diagram on the right side of the figure. This, of course, requires intensive mixing at the beginning of the expansion process while the temperatures are still on a high level, although not too high because of NOx formation. In this way the soot generated can be burned nearly completely during the process.

Fig. 9.9. Soot formation and nitrogen oxide formation in a diesel fuel spray and its dependence on stoichiometric air-fuel ratio λ.

It can be concluded that a careful mixing strategy is the key to low-emission diesel combustion. Since mixing is controlled by fuel injection as well as by air motion in the combustion chamber, both processes must be developed concurrently. Future diesel combustion development approaches follow these guidelines. Typical design features addressing this fact are discussed next.

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Combustion-Related Emissions in CI Engines

J. Gary Hawley, ... Roy W. Horrocks, in Handbook of Air Pollution From Internal Combustion Engines, 1998

10.6.2.1 Background

Exhaust gas recirculation (EGR) is a well-proven technique for the effective suppression of NOx emissions. The reductions in emissions due to the influence of EGR are shown in Figure 10.15 for a Ford prototype 16-valve, 1.8-liter TCI DI engine. For the automotive diesel engine to meet future European NOx emissions standards, EGR will be the primary emission control technique. Most gasoline engines can recirculate up to a maximum of 20 percent of the exhaust gases without affecting combustion stability, while diesel engines are capable of recirculating up to 50 percent of the exhaust gases. Amounts greater than 50 percent EGR can be achieved by throttling the diesel intake. Since EGR reduces the oxygen content of the intake charge, it can only be used at part load, or when the fuel-air ratio is sufficiently high so as not to cause a dramatic increase in smoke, HCs, and PM. The EGR rate contours over the operating range for the eight-valve version of the engine considered in Figure 10.15 shown in Figure 10.16.

Fig. 10.15. General trend on emissions due to the influence of EGR (taken from a Ford prototype 1.8-liter DI engine).

Fig. 10.16. Contours of percentage EGR rate for a Ford prototype 1.8-liter DI diesel engine.

EGR has an effect on the flame structure, temperature, and species profiles following ignition. The reductions in NOx emissions with EGR can be explained on the basis of a predicted decrease in peak flame temperature in a diffusion flame coupled with a reduced oxygen mass fraction. (The Zeldovich mechanism gives the NOx formation rate rising exponentially with temperature.)

The reduction in NOx due to EGR was for a long time thought to be a result of the increase in the heat capacity of the charge caused by the presence of recirculated CO2. However, recent work has shown that the reduction of oxygen in the inlet charge (dilution effect) is the dominant factor [57]. The work investigated the way that EGR influences diesel engine combustion and emissions. Findings from the investigations included the effect of CO2 dissociation (chemical effect) on exhaust emissions is small; the high heat absorbing capacity of CO2 (thermal effect) had only a small effect on exhaust emissions including NOx. The reduction in the inlet charge oxygen (dilution effect) is the dominant effect on emissions, resulting in very large reductions in exhaust NOx at the expense of higher particulate and unburned hydrocarbon emissions and lower engine output and fuel economy.

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Renewable Energy Policies and Barriers

Fredric Beck, Eric Martinot, in Encyclopedia of Energy, 2004

4.1 Renewable Energy Set-Asides

To meet National Ambient Air Quality Standards, the U.S. Environmental Protection Agency requires 22 U.S. states and the District of Columbia to reduce NOx emissions significantly by 2007. States can meet emission reduction targets through actual emission reductions or purchase of emission reduction credits from other states participating in a region-wide NOx trading program. States can allocate, or set-aside, a percentage of the total state NOx allowances to energy efficiency and renewable energy. Eligible renewable energy producers receive these set-aside allowances and can sell them to fossil-fuel-based electricity generators to enable those generators to stay within their NOx allocation. The additional revenue from sales of these set-aside allowances can potentially provide stimulus for renewable energy development, although to date few states have implemented renewable energy set-asides.

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Nitrogen Cycle, Atmospheric

Dan Jaffe, in Encyclopedia of Physical Science and Technology (Third Edition), 2003

II.C.2 Acid Rain

Acid precipitation, or acid rain, can causes significant impacts on freshwater, coastal, and forested ecosystems. Both NO3− (from NOx emissions) and SO42− (from SO2 emissions) contribute significantly to acid rain. The relative ratio of SO42− / NO3− in precipitation will be substantially determined by the regional emissions of SO2 and NOx. In regions that get most of their energy from coal and other high-sulfur fuels, there will be significant emissions of SO2 unless scrubber technology is employed. Due to declining emissions of SO2 in developed regions of the world and increasing NOx emissions, from automobiles, the relative contribution of NO3− is changing, with NO3− contributing an increasing fraction of the acidity in acid rain. In ice cores collected in remote regions of the northern hemisphere, SO42− and NO3− concentrations have increased significantly in the past 50 years, reflecting the large increase in source emissions due to anthropogenic sources.

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Modernization of fuel grinding systems

Marek Pronobis, in Environmentally Oriented Modernization of Power Boilers, 2020

5.3.1 Improvement of pulverized coal quality

The majority of hard coals combusted in the Polish power sector are coals of high Vdaf content. According to Polish standard PN-82/G97002, they are coals of types 31 and 32. For their grinding in power industry are used almost exclusively medium-speed compression mills, the majority of which is equipped with static centrifugal classifiers.

Before the introduction of low-NOx combustion systems relatively coarse pulverized coal was used (R0.09 = 30%–50%, R0.20 = 8%–20%). In the course of numerous retrofits, aimed at introducing low-NOx combustion technologies, burner suppliers as a necessary condition for the proper operation of the system recognized to improve the quality of coal milling. For dry-bottom boilers, depending on the coal properties, dimensions of the furnace, types of burners, and demanded losses due to unburned combustibles as well as earlier experiences with the combustion of Polish coal, the demanded qualities were changed. From rather easy to fulfil R0.09 ≤ 20% and R0.20 ≤ 2%, to the difficult to reach R0.09 < 15% and R0.20 < 0.2.

To achieve the first of these ranges in compression mills proved sufficient the new generation of static classifiers redesigned and of increased volume, which is produced in Poland for all types and sizes of mills [40]. The new classifiers are characterized by advantageous uniformity number n = 1.4–1.8. It should be added that the pair R0.09 ≤ 20% and R0.20 ≤ 2% corresponds to the relatively low (at least for compression mills) polydispersity number of dust n = 1.08. At higher values of n, the relations between R0.09 and R0.20 are different. For example, at n = 1.4 the R0.09 = 20% corresponds to R0.20 = 0.62%, while R0.20 = 2% corresponds to R0.09 = 28.95%. The need to improve the quality of milling arises mainly from a condition not exceeded certain combustible matter content in fly ash and slag, which is determined by the share of coarse fractions in the pulverized coal (R0.20). As an overriding requirement, the sufficiently low value of R0.20, for example 2%, should, therefore, be considered. When the value of n is high, the R0.09 resulting from the dust particle size distribution can be relatively large without deteriorating the combustion efficiency and while avoiding excessive costs of milling (Table 5.1). To obtain the better pulverizing quality, which corresponds to the range R0.09 < 15% and R0.20 < 0.2, the mills should be equipped with dynamic classifiers.

In order to ensure higher efficiency of ball-ring mills and ensure their stable operation at low power unit loads, it is necessary to increase the speed of the transporting-drying medium on the path between the milling unit and the gravitational separation zone of the mill. The aforementioned effect can be obtained by implementation of new nozzle rings, which enable more efficient transport of the pulverized material from under the grinding unit to the gravitational separation zone. In the elutriation zone thicker particles are returned under the balls to improve the milling effect. The system of adjusting the flow resistance of the rotating nozzle ring, proposed by FPM SA, allows for limitation of excessive material overflow to pyrites chamber.

Another solution in this area for HP, RP, and RS/RPS Mills is described in Ref. [41].

In comparison to the compression mills, fan mills for hard coal, especially those with higher performance, produce PF with worse parameters: milling is more coarse, and the number of polydispersity is lower (n = 0.7–0.9). Therefore, obtaining the fineness required for low-NOx-combustion is more difficult. The improvement of the quality of product can, in this case, be achieved by means of changing the cross-section of the housing, the volume and design of the classifier as well as by changing the number, dimensions and location of control elements.

Figure 5.13 shows modernized version of the fan mill MWK-16 type classifier. In standard construction (left), the guide vanes were positioned in the same vertical row, applying the highest load on the upper blade. The uneven load distribution is highly noticeable during classifier maintenance, the upper blade is characterized by the highest wear. Shifting the mutual position of the guide vanes, which was implemented in modernized fan mill classifier construction (right), allows for more uniform material-gas mixture distribution.

Figure 5.13. Modernization of classifier of the fan mill [12,26].

PF gas mixture is directed through pulverized fuel inlet to the classifier. The coarse particles are separated from the main flow due to decrease in mixture velocity, caused by extension of the flow cross section area. Additionally material classification occurs also due to change in flow direction on adjustable guide vanes. The position of the blades allows for milling quality control. Rejected particles fall down to reject return duct, which leads back to the milling chamber.

In order to avoid negative influence of excessive mill ventilation on boiler operation, for high mill capacities, the internal recirculation system is introduced. Part of the transportation-drying medium is returned to the milling chamber through reject return/internal recirculation duct. The amount of recirculated gas is adjusted by recirculation damper position, which narrows or widens the flow cross-section. Similar design is also used in the mill MWk-9 (Fig. 5.5).

The measurements showed the possibility to achieve R0.09 = 20% and R0.20 = 2%, which is a significant improvement concerning the state before modernization. Similar results were obtained introducing the centrifugal classifiers [42], but the high velocity of fuel/air mixture at the inlet to the classifier caused serious wear problems.

In Ref. [43] it is proposed to introduce a so-called "barriershoop" (Fig. 5.14), which is placed inside a spiral housing, along with the circumference of the beater wheel. The study shows that increased grinding quality allows grinding of lignite in a mill without a classifier.

Figure 5.14. Reconstruction of the fan mill by setting up the obstacles along the spiral housing.

1, Beater wheel; 2, Spiral housing; 3, Obstacles

In fan mills generally exists the possibility of using the mixture of air and flue gas as a drying medium. It is an advantageous tool to reduce NOx-emissions by means of decreasing the oxygen concentration in the combustion zone. To further reduce the O2 concentration in the drying medium, the following actions can be undertaken:

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partly recirculation of the relatively cold mixture of coal dust and gas from the outlet of the pulverizer on its inlet,

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replacing (at least partially) the hot air through relatively cold flue gas from the outlet of the boiler,

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control of the rotational speed of the wheel to reduce the ventilation at low load,

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injection of water to the mill, and

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adaptation of the milling system to the operation with low ventilation and increased temperature of the drying medium.

Other interesting information about modernization of classifiers in MPS and Atrita pulverizers is presented in Ref. [44].

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Recommended publications,NOx formation was measured during combustion of pulverized coals and pulverized coal char in N2 and CO2 environments under isothermal and nearly constant oxygen conditions (i.e. using dilute coal loading). Three different oxygen concentrations (12% O2, 24% O2, and 36% O2) and two representative US coals were investigated, at a gas temperature of 1050 °C. To investigate the importance of NO reburn reactions, experiments were also performed with an elevated concentration (550 ppm) of NO in the gases into which the coal was introduced. For low levels of background NO, the fractional fuel-nitrogen conversion to NOx increases dramatically with increasing bath gas oxygen content, for both N2 and CO2 environments, though the fuel conversion is generally lower in CO2 environments. Char N conversion is lower than volatile N conversion, especially for elevated O2 concentrations. These results highlight the importance of the volatile flame and char combustion temperatures on NOx formation. For the high background NOx condition, net NOx production is only observed in the 36% O2 environment. Under these dilute loading conditions, NO reburn is found to be between 20% and 40%, depending on the type of coal, the use of N2 or CO2 diluent, the bulk O2 concentration, and whether or not one considers reburn of volatile-NOx. This dataset provides a unique opportunity to understand and differentiate the different sources and sinks of NOx under oxy-fuel combustion conditions.The importance of NOx production by lightning in the tropics has been assessed by using satellite lightning measurements from the Lightning Image Sensor (LIS). The lightning data from LIS over the period of 1998–2000 are analyzed and partitioned based on the latitude to obtain the numbers of cloud-to-ground (CG) and cloud-to-cloud (intracloud or IC) flashes. The average annual lightning counts over the 3-yr period are 3.0×108 CG flashes and 1.1×109 IC flashes between 35°N and 35°S. The resulting lightning distributions are employed to calculate the production of NOx. We obtain a lightning NOx production of 6.3 Tg N yr−1 over this latitudinal region, using representative production values of 6.7×1026 and 6.7×1025 NO molecules for each CG and IC flash, respectively. NOx production by lightning varies slightly on a seasonal basis in accordance with the lightning distribution, with the maximum production occurring in the months of September, October, and November. The geographical and seasonal production of NOx by lightning is compared to NOx emissions from other sources (i.e., from anthropogenic activity, biomass burning, and soil emissions). The results indicate that production of NOx by tropical lightning is significant throughout the year. Lightning accounts for almost all of the NOx emitted over the oceans and 50–90% of NOx emitted over some continental areas on a seasonal basis. On the annual basis, the contribution of lightning to total NOx production is 23% in the tropics, and globally the lightning NOx production occurs predominately in this region. The uncertainty in estimating NOx production by lightning over the tropics is discussed.Atmospheric Environment

Volume 39, Issue 26, August 2005, Pages 4793-4802

Evidence of an increasing NO2/NOX emissions ratio from road traffic emissions

Author links open overlay panelDavid C.Carslaw

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https://doi.org/10.1016/j.atmosenv.2005.06.023Get rights and content

Abstract

A statistical analysis of roadside concentrations of nitrogen oxides (NOX) and nitrogen dioxide (NO2) in London shows that from 1997 to 2003 there has been a statistically significant downward trend (at the p=0.004 level) in NOX averaged across a network of 36 sites. Conversely, there has been no statistically significant trend in the concentrations of NO2 over the same period. Hourly modelling using a simple constrained chemical model shows that the NO2/NOX emissions ratio from road traffic has increased markedly from a mean of about 5–6 vol% in 1997 to about 17 vol% in 2003. Calculations show that if the NO2/NOX emissions ratio had remained the same as that towards the beginning of each time series, 14 out of the 36 sites would have shown a statistically significant downward trend in NO2 at the p=0.10 level compared with only five that did. The increase in the NO2/NOX emissions ratio from road traffic in recent years has therefore had a significant effect on recent trends in roadside NO2 concentrations. It is shown that the increased use of certain types of diesel particulate filters fitted to buses is likely to have made an important contribution to the increasing trends in the NO2/NOX emissions ratio. However, it is unlikely that these filters account for all of the observed increase and other effects could be important, such as the increased penetration of diesel cars in the passenger car fleet and new light- and heavy-duty engine technologies and management approaches.In this paper, the annual data of 50 China Ⅳ public transit buses fueled with diesel in five cities have been analyzed, and the effects of vehicle speed and acceleration on real-road NOx emission and fuel consumption characteristics have been discussed. Results showed that the annual percentage of low speed range (0-30.0 km/h) was over 70%, the acceleration range of -0.5-0.5 m/s2 accounted for nearly 75%, and distribution of each month remained roughly stable. The highest NOx emission was concentrated in region with low vehicle speed (10.0-30.0 km/h) and high acceleration (1.0-2.0 m/s2), and the highest fuel consumption was concentrated in region with high vehicle speed (40.0-50.0 km/h) and high acceleration (1.0-2.0 m/s2). Both the NOx emission and fuel consumption characteristics deteriorated in summer and winter, which was mainly attributed to the using of air conditioning.This work presents the results of an experimental investigation on NOx emissions from coal combustion in a pilot scale test facility. Three oxidiser atmospheres have been compared, namely air, CO2/O2, and O2 enriched recirculated flue gas. NOx emissions from two different combustion modes have been studied, swirl flame and flameless combustion. The influence of the burner oxygen ratio and the oxidiser O2 concentration on NOx formation and reduction have been analysed. With increasing burner oxygen ratio, an increase of NOx emissions has been obtained for air and CO2/O2 in both, swirl flame and flameless combustion. In case of the swirl flame, flue gas recirculation leads to a reduction of NOx emissions up to 50%, whereas in case of flameless combustion this reduction is around 40% compared to CO2/O2. No significant impact of the oxidiser O2 concentration in the CO2/O2 mixture on NOx emissions is observed in the range between 18 and 27 vol.% in swirl flames. An analysis of NOx formation and reduction mechanisms showed, that the observed reduction of NOx emissions by flue gas recirculation cannot be attributed to the reduction of recirculated NOx alone, but also to a reduced conversion of fuel-N to NO.Fuel Processing Technology

Volume 96, April 2012, Pages 237-249

Review

Review of the effects of biodiesel on NOx emissions

Author links open overlay panelS. KentHoekmanCurtisRobbins

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https://doi.org/10.1016/j.fuproc.2011.12.036Get rights and content

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Compared to conventional diesel fuel, use of biodiesel is generally found to reduce emissions of hydrocarbons (HC), carbon monoxide (CO), and particulate matter (PM); but to increase oxides of nitrogen (NOx) emissions. This paper reviews and summarizes relevant literature regarding the so-called "biodiesel NOx effect, and presents theories" to explain this effect. In modern diesel engines, several factors related to fuel composition and engine control strategies are important, though no single theory provides an adequate explanation of the biodiesel NOx effect under all conditions. There is evidence to suggest that effects on injection timing, ignition delay, adiabatic flame temperature, radiative heat loss, and other combustion phenomena all play some role. The biodiesel NOx effect can be mitigated by modifying engine control settings — particularly by retarding injection timing and increasing exhaust gas recirculation (EGR). The absolute magnitude of the biodiesel NOx effect appears to be reduced with modern engines, although there are cases where the percentage change is still substantial. Sophisticated after-treatment systems required to achieve the 2010 diesel engine emissions standards do not appear to be significantly affected by use of biodiesel. However, longer term study is warranted, as such systems have only been in commercial use for a short time.

Highlights

► Use of biodiesel usually, but not always, increases NOx exhaust emissions. ► Fuel composition, engine technology and operating conditions all affect NOx. ► The biodiesel NOx effect can be mitigated by changes in engine operation parameters.Atmospheric Environment. Part A. General Topics

Volume 25, Issue 9, 1991, Pages 1961-1969

Measurements of reactive nitrogen compounds

The production of atmospheric NOx and N2O from a fertilized agricultural soil

Author links open overlay panelM.F.ShepherdD.R.Hastie

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https://doi.org/10.1016/0960-1686(91)90277-EGet rights and content

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The source strength of atmospheric trace gases from rural or remote locations must be quantified in order to assess the effect of such inputs on the background tropospheric chemistry. To assess the importance of biological production of NOx and N2O from fertilized agricultural soil, enclosure techniques have been used to determine the emission fluxes of NOx and N2O at a site in Southern Ontario, Canada. NOx fluxes on the unfertilized soil range from 1.5 to 41.6 μg(NO) m−2 h−1. The corresponding N2O fluxes are 0–61.8 μg(N2O)m−2h−1. For the most fertilized soil NOx fluxes range from 3.1 to 583 μg(NO) m−2h−1 and the N2O fluxes from 0 to 446 μg(N2O) m−2h−1. The fluxes increase linearly with fertilizer application, with 11% of the nitrogen in the fertilizer converted to NOx and 5% to N2O. The emission rates were studied as functions of the soil parameters temperature, moisture, ammonium, nitrate and pH, to attempt to understand better the production mechanisms, although a model for the process could not be developed. In rural areas away from transportation corridors the increased NOx emission from fertilized soil may dominate local oxidant production but is not significant on the Province-wide scale

Applied Thermal Engineering

Volume 137, 5 June 2018, Pages 584-593

Research Paper

NOx emission of biodiesel compared to diesel: Higher or lower?

Author links open overlay panelHaoChenYisongChen

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https://doi.org/10.1016/j.applthermaleng.2018.04.022Get rights and content

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An overall experiment on NOx emission of biodiesel has been carried out.

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Biodiesel emits lower NOx than diesel at low loads under low and medium speeds.

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A systematic influence path for NOx of biodiesel is proposed.

Abstract

A little decrease in NOx emission of biodiesel compared to diesel is observed at low loads under low and medium engine speeds. An overall experimental investigation on NOx emission based on combustion characteristics of biodiesel has been carried out. Negative effects of high viscosity and distillation temperature on the spray quality and homogeneity of air fuel mixture are clear at low loads under low and medium speeds. Brake thermal efficiency (BTE) and combustion temperature of biodiesel are lower than those of diesel, resulting in a lower level of NOx emission. With the increase of load, in-cylinder thermal condition and the injection pressure are improved, and with the increase of speed, air swirl movement is strengthened. Oxygenated biodiesel produces more reactive radicals than diesel, and these radicals will surely accelerate the combustion speed, improves the intensity of diffusion combustion, shortens the combustion duration and increase the BTE. Consequently, the diffusion combustion temperature, especially the peak combustion temperature of biodiesel is higher than that of diesel. On the whole, NOx emission of biodiesel is higher than that of diesel in most cases, except in the condition of low loads under low and medium speeds.Due to new emission regulations, coverage of monitoring requirements extends to new boiler classes. In Europe, Industrial Emission Directive brings new 15-50 MW boilers under the directive in special cases, so inexpensive NOx emission monitoring tools are appreciated. This paper presents models to estimate NOx emission in two similar natural gas fired municipal hot water boilers. The models utilize online process measurements also available in relatively simple set-ups. In the context, the performance of linear regression model and nonlinear neural network model are introduced. Despite the similarity, the results state that the examined boilers behave differently in terms of linearity and emission levels. Therefore, every boiler should be identified separately to exploit the simplest models in practical installations.Atmospheric Environment

Volume 45, Issue 39, December 2011, Pages 7053-7063

Recent evidence concerning higher NOx emissions from passenger cars and light duty vehicles

Author links open overlay panelDavid C.CarslawaMartin L.Williamsa

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https://doi.org/10.1016/j.atmosenv.2011.09.063Get rights and content

Abstract

Ambient trends in nitrogen oxides (NOx) and nitrogen dioxide (NO2) for many air pollution monitoring sites in European cities have stabilised in recent years. The lack of a decrease in the concentration of NOx and in particular NO2 is of concern given European air quality standards are set in law. The lack of decrease in the concentration of NOx and NO2 is also in clear disagreement with emission inventory estimates and projections. This work undertakes a comprehensive analysis of recent vehicle emissions remote sensing data from seven urban locations across the UK. The large sample size of 84,269 vehicles was carefully cross-referenced to a detailed and comprehensive database of vehicle information. We find that there are significant discrepancies between current UK/European estimates of NOx emissions and those derived from the remote sensing data for several important classes of vehicle. In the case of light duty diesel vehicles it is found that NOx emissions have changed little over 20 years or so over a period when the proportion of directly emitted NO2 has increased substantially. For diesel cars it is found that absolute emissions of NOx are higher across all legislative classes than suggested by UK and other European emission inventories. Moreover, the analysis shows that more recent technology diesel cars (Euro 3–5) have clear increasing NOx emissions as a function of Vehicle Specific Power, which is absent for older technology vehicles. Under higher engine loads, these newer model diesel cars have a NOx/CO2 ratio twice that of older model cars, which may be related to the increased use of turbo-charging. Current emissions of NOx from early technology catalyst-equipped petrol cars (Euro 1/2) were also found to be higher than emission inventory estimates – and comparable with NOx emissions from diesel cars. For heavy duty vehicles, it is found that NOx emissions were relatively stable until the introduction of Euro IV technology when emissions decreased by about 30%. The more limited data available for urban buses shows that there has been little change in NOx emissions from Euro I to Euro IV. There is general much better consistency across the different estimates of heavy duty vehicle NOx emissions than for light duty vehicles.

Highlights

► Trends in ambient NOx concentrations have tended to stabilise in recent years. ► Little change in light duty vehicle NOx emissions over 20 years or so. ► Increased diesel car power demand leads to increased NOx for newer vehicles. ► Older catalyst-equipped vehicles emit more NOx than previously thought. ► Important implications at a European level for meeting NO2 limits.

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