NOx Emission Trends Research Articles

Comparative experimental study on the co-firing characteristics of water electrolysis gas (HHO) and lean coal/lignite with different injection modes in a one-dimensional furnace

Zero-carbon fuels (H2/NH3) replacing coal co-firing is a promising way to address carbon emissions. Hydrogen co-firing helps improve combustion stability at low loads and control pollutant emissions. This study used a safer water electrolysis gas (HHO) for the multi-coal co-firing adaptability test. The effects of HHO injection mode and flow rate on combustion intensity, flue gas emissions, and fly ash characteristics were investigated. The results showed that HHO and lean coal/lignite co-firing could significantly increase the combustion temperature. The maximum temperature increases after co-firing were 108 °C and 95 °C. As the HHO increased, the CO2 in lignite co-firing increased, and CO and unburnt carbon decreased, while the opposite results were observed for lean coal (LC). At the 2200L/h HHO co-firing, CO2 decreases by 14.46 % and 27.22 % for LC premixed co-firing (PC) and staged co-firing (SC). Compared to SC, PC is more conducive to promoting the complete combustion of coke. The NOX emissions of HHO/LC co-firing are lower than those of pure coal, reaching the lowest value of ∼ 9 ppm under SC. As the HHO increases, the NOX of Lignite-PC first increases and then slowly decreases, while Lignite-SC has the opposite effect. Although various NOX emission trends are observed under different conditions, the results are dominated by a common mechanism − the competition between NOX reduction caused by HHO gas and the increase in co-firing temperature on NOX generation. The NOX of SC is all lower than that of PC. The SO2 emission from the LC co-firing decreases with the increase of HHO, while the SO2 from lignite increases. The SO2 of LC-SC can be reduced from ∼ 142 ppm to 0 ppm. HHO gas injection inhibits the release and oxidation of sulfur in lean coal. The differences in coal types lead to different changes in SO2 of co-firing. HHO gas reduced the SO3 content of lignite fly ash by 38.01 %. The high-temperature reduction atmosphere promotes the decomposition of sulfates and increases SO2 emissions. HHO co-firing refines the particle size of fly ash. The ultra-fine and micron particle matter (PM) increases, and the content of PM>100μm in Lignite-SC decreased by 10.32 %.

Mesoporous SO42− / kit-6-catalyzed hydrocracking of waste chicken oil

In this study, we studied the hydrocracking of waste chicken oil (WCO) catalyzed by mesoporous SO42−/KIT-6. The study included WCO extraction, SO42−/KIT-6 catalyst synthesis, hydrocracking, and catalytic characterization. XRD patterns revealed intense peaks in the low-angle region, with shoulder peaks showing an increase in sulphate loading from 10% to 30%. The BET-specific surface area for the pure KIT-6 supports measured at 1003 m2/g, indicative of a well-defined mesoporous structure. Thermogravimetric analysis (TGA) showed a two-stage weight loss, attributed to the elimination of hydrated water (about 200 °C) and decomposition of sulphate ions (400–450 °C). SEM analysis highlighted the surface morphology of the active SK-2 catalyst. Hydrocatalytic and catalytic cracking reactions were performed, and about 99.8% conversion was achieved with 20 mL/H H2 flow, whereas higher production of bioliquids was observed at a flow of 15 mL/h. The hydrocracking mechanism was also studied to understand the formation of lower hydrocarbons. GC analyses of simulated distilled gasoline, kerosene, and diesel showed diverse hydrocarbon compositions. For engine testing, non-hydrocracked fuel rose to 28 kW at 3000 rpm and declined to 21 kW at 3500 rpm. Emission analysis revealed decreasing trends in NOX emissions of hydrogen-rich blends, with values of 65 ppm, 54 ppm, and 48 ppm for petrol, NHBL, and HBL, respectively. Similarly, SO2 emissions reduced from petrol to NHBL and HBL at 910 ppm, 800 ppm, and 600 ppm, respectively, suggesting reduced environmental impact. CO emissions exhibited a substantial reduction in NHBL (0.90%) and HBL (0.54%) compared to petrol (2.70%), emphasizing the cleaner combustion characteristics. Our results provide a comprehensive exploration of waste chicken oil hydrocracking, emphasizing catalyst synthesis, fuel characterization, engine performance, and environmental impact, thereby contributing valuable insights to the field of sustainable bioenergy.

Experimental study on the effects of ammonia cofiring ratio and injection mode on the NOx emission characteristics of ammonia-coal cofiring

Ammonia (NH3) cofiring is a promising approach to reduce the CO2 emissions from coal-fired power plants. However, the potential drastic increase of NOx emissions may inhibit the wide implementation of NH3 cofiring. To explore the potential NOx control strategies, the effects of NH3 cofiring ratio (RNH3), NH3 injection mode and overfire air (OFA) rates on the NOx emission characteristics of NH3/coal cofiring are studied in a test furnace that allow for flexible control of the injection positions and combustion environment of NH3. The results show that when NH3 is injected with coal stream (top mixing mode), the NOx emissions increase and then decrease with the increase of RNH3 under all OFA rates. However, when NH3 is fed through the side port of furnace separately (side mixing mode), the NOx emissions exhibit distinct trends under different OFA rates. Under lower OFA rates, the NOx emissions show monotonical decrease with the increase of RNH3, while under higher OFA rates the NOx emissions first decrease and then increase with the increase of RNH3. Consequently, higher OFA rates could produce higher NOx emissions than lower OFA rates. This indicates that air-staging is not always effective in the NOx reduction of NH3 cofiring. Thus, although the side mixing mode of NH3 cofiring exhibits overall better performance in NOx control, caution is needed to accommodate the OFA rate with NH3 cofiring ratio. Although a variety of trends of NOx emissions were observed in this study, the results demonstrate that all these trends are in fact governed by the same mechanism – the competition between the NO-formation and NO-reduction reactions of NH3 in the varying O2 environment in the furnace. By creating appropriate O2 environment in the furnace, the NOx emissions of NH3/coal cofiring could be substantially lower than that of pure coal combustion. The findings of these paper are instrumental in the design and operation of NH3 cofiring system in full scale coal-fired boilers to achieve effective NOx control.

Exploring Deposition Observations of Oxidized Sulfur and Nitrogen as a Constraint on Emissions in the United States

AbstractEmissions of anthropogenic sulfur and nitrogen oxides form secondary pollutants that impact human health, ecosystems, and climate. Accurate estimates of emissions trends are needed to verify the effectiveness of the regulation of these species. We explore the utility of deposition measurements of SOx (=SO2 + SO42−) and TNO3 (=HNO3 + NO3−) as constraints on emissions trends of SOx and NOx in the conterminous United States (CONUS) from 1990 to 2021. The GEOS‐Chem model captures observed annual SOx and TNO3 wet deposition at NADP‐NTN sites in 2011 with a −15% and +15% normalized mean bias (NMB), respectively. The model overestimates the dry deposition of SOx and TNO3 estimated at CASTNET sites in 2011 (NMB >100%), however, this bias is substantially reduced when using an alternate derived dry deposition data set at the same sites, highlighting the uncertainty in dry deposition velocities. Despite this, we find that the model (driven by scaled NEI emissions) captures the relative trend in dry deposition of SOx (−93% observed and −94% simulated) and TNO3 (−66% observed and −68% simulated) from 1990 to 2021 and that these decreases closely reflect the trends in anthropogenic SO2 emissions (−93%) and anthropogenic NOx emissions (−71%), respectively. SOx and TNO3 wet deposition observations are dominated by soluble secondary products and are more influenced by natural and transboundary sources, and therefore have decreased more modestly over the same period (−78% and −52%). Natural sources of NOx are relatively constant during this time and therefore moderate the reduction in total NOx emissions (−55%).

Modelling nitrogen oxide emission trends from the municipal solid waste incineration process using an adaptive bi‐directional long and short‐term memory network

AbstractAccurately predicting trends in NOx emission is essential for effectively controlling pollution in municipal solid waste incineration (MSWI) power plants. However, the MSWI process exhibits notable dynamic nonlinearity, time series characteristics, and fluctuations that are distinct from those present in fossil fuel combustion processes. Therefore, the model must possess excellent capabilities in handling time series and nonlinear features while achieving adaptive updates to account for complex working conditions. To address these issues, we have developed a robust prediction model for NOx emission trends using the bi‐directional long short‐term memory (Bi‐LSTM) deep learning algorithm. This model encompasses maximum information coefficient and expert experience for input variables selection, parameter optimization using the linear inertial weight particle swarm algorithm (LDWPSO), and an adaptive update strategy based on probabilistic statistics. The prediction performance of this model was compared to that of the traditional and widely used backpropagation neural network (BPNN), extreme learning machine (ELM), and LSTM. Furthermore, we verified the adaptive update effect of the proposed model using additional data. The results demonstrate that the proposed model exhibits robust prediction and adaptive capabilities. This study's originality is presenting a satisfactory trend prediction for NOx emission from the MSWI process using an adaptive LDWPSO‐(Bi‐LSTM) model. It will be essential for the optimization and control of NOx emissions from the MSWI process.

A pilot-scale test facility of 500 kWth for industrial CFB boilers on low nitrogen combustion-discussion of design, experiment, and economic analysis

The design and construction of a pilot-scale test facility of 500 kWth for a novel CFB boiler with three separate sub-furnaces and two steps for material circulation are introduced in detail. A thorough test was conducted with the facility to verify the applicability of fuel staging on NOx abatement in the novel CFB. The varied trends of temperature distribution and NOx emissions during fuel staging are presented in the study. It was observed that fuel staging can effectively and immediately reduce the initial emission concentration of NOx in the flue gas. Under the tested conditions, the NOx removal efficiency can reach up to 40% at a fuel-staged proportion of 5%. The initial NOx emission concentration can be controlled to below 80 mg/m3, even. The main reason for the NOx reduction is the heterogeneous reduction of NO on the surface of coal char formed after the devolatilization of staged fuel. In addition, economic analysis was made by comparing fuel staging and SNCR. The estimated operating costs showed that fuel staging can be applied as an alternative technology for SNCR when the initial concentration of NOx in the flue gas ranges from 300 to 360 mg/m3.

Unexpected deterioration of O3 pollution in the South Coast Air Basin of California: The role of meteorology and emissions

Tropospheric ozone (O3) pollution has long been a prominent environmental threat due to its adverse impacts on vulnerable populations and ecosystems. In recent years, an unexpected increase in O3 levels over the South Coast Air Basin (SoCAB) of California has been observed despite reduced precursor emissions and the driving factors behind this abnormal condition remain unclear. In this work, we combine ambient measurements, satellite data, and air quality modeling to investigate O3 and precursor emission trends and explore the impacts of meteorological variability and emission changes on O3 over the SoCAB from 2012 to 2020. Changes in O3 trends were characterized by declining O3 in 2012–2015, and increasing O3 afterwards with the most extreme O3 exceedances in 2020. Basin-wide increases of MDA8 O3 concentrations over warm season were depicted between 2012 and 2020, with the most significant enhancements (5–10 ppb) observed in San Bernardino County. Persistent heatwaves and weak ventilation on consecutive days were closely correlated with O3 exceedances (r2 above 0.6) over inland SoCAB. While decreasing trends in NOx (−4.1%/yr) and VOC emissions (−1.8%/yr) inferred from emission inventory and satellites during 2012–2020 resulted in a slow transition for O3 sensitivity from VOCs-limited to NOx-limited, model simulations performed with fixed meteorology indicate that unfavorable meteorological conditions could largely offset regulation benefits, with meteorology anomaly-induced monthly O3 changes reaching 20 ppb (May 2020) and the deterioration of O3 pollution in 2016, 2017, and 2020 was largely attributed to unfavorable meteorological conditions. Nevertheless, anthropogenic emission changes may act as the dominant factor in governing O3 variations across the SoCAB when net effects of meteorology are neutral (typically 2018). This work provides a comprehensive assessment of O3 pollution and contributes valuable insights into understanding the long-term changes of O3 and precursors in guiding future regulation efforts in the SoCAB.

Numerical simulation of the influence of different oxygen concentration on the combustion characteristics of double tangential boiler at low load

In order to cope with the demand for deep peak regulation of high-parameter coal-fired units, this paper uses numerical simulation methods to systematically explore the influence of different secondary wind oxygen enrichment concentrations on the combustion performance and characteristics of singlefurnace double-cut round boilers during low-load operation, so as to provide theoretical support for the actual operation of power plants. The results are as follows: (1) The use of oxygen enriched secondary air combustion during low-load operation of the boiler can improve the overall temperature of the main combustion zone of the furnace and the local temperature of the burner nozzle section, and enhance the combustion stability of pulverized coal in the furnace. (2) When the secondary air oxygen concentration is 35% and 40%, the overall temperature of the main combustion area of the boiler increases the most obviously, and the combustion is the most stable. (3) Oxygen-enriched combustion effectively reduced the NOx emission concentration in the furnace, and the overall trend of NOx emission decreases first and then increases with the increasing trend of secondary air oxygen concentration.

How accelerating the electrification of the van sector in Great Britain can deliver faster CO2 and NOx reductions

As a major emission contributor with significant growth potential, the light goods vehicles (vans) play an important part in achieving net-zero. In 2020 the UK government committed to phasing out sales of new internal combustion engine (ICE) vans by 2030, but the impact of the policy and how far are we to decarbonize the entire van fleet by 2050 is unclear. This paper investigates the CO2 and NOx emission trends in the van sector in Great Britain under the 2030 ICE phase-out. ECCo model11http://www.element-energy.co.uk/sectors/low-carbon-transport/project-case-studies/ is used to forecast the future van population by powertrain. The annual van mileage is estimated based on the van activity survey22https://www.gov.uk/government/statistics/van-statistics-2019-to-2020. The instantaneous emission model PHEM, NAEI emissions inventory and remote sensing measurements are used to parameterize real-world driving emission factors of CO2 and NOx. Scenarios have been set out to assess the impact of enablers and barriers affecting the pace of emission reductions. Results suggest vans are on track to reach the tailpipe net-zero target by 2050 under all scenarios, and the speed of NOx reduction is even faster. A rapid transition to battery electric vans in the early to mid-2020s will significantly lower CO2, with associated estimated monetary benefits of £1.3 billion.

Impact of ammonia addition on knock resistance and combustion performance in a gasoline engine with high compression ratio

Increasing the compression ratio of gasoline engines is a promising method to increase the engine's fuel economy. However, engine knock caused by auto-ignition is still a large obstacle to improving thermal efficiency and engine load for high compression ratio hybrid engines. Spark induced compression ignition (SICI) is an effective way to utilize auto-ignition to solve the aforementioned problems. Meanwhile, ammonia, a carbon-free fuel, with an outstanding antiknock property, has the great potential to be used in SICI mode. In this study, the effects of ammonia addition on knock suppression, combustion characteristics, thermal efficiency, and emission performance were investigated in a high compression ratio (15.5), four-valve, single-cylinder gasoline engine under SICI combustion mode. In experiments, gasoline was directly injected into the cylinder while ammonia was injected into the intake port. The results show that blending ammonia could resist engine knock and improve thermal efficiency. Within the knock limitation, the duration of flame propagation under ammonia blending conditions could be shortened and meanwhile, the auto-ignition becomes weakened compared with pure gasoline. Benefiting from combustion phase optimization, the thermal efficiency and engine load could be increased or maintained at optimal ammonia blending ratio. The maximum increase of thermal efficiency and engine load is 2.46% and 0.2 MPa respectively. Moreover, the increased engine load can extend the limit of the ammonia blending ratio. For nitrogen emissions, blending ammonia results in NOx emission deterioration due to the formation of fuel-type NOx. NOx emission has a weak dependence on the ammonia blending ratio, and the trend of NOx emission varied with spark timing is opposite to pure gasoline conditions, which is closely related to the pressure sensitivity of fuel-type NOx. Ammonia slip was also detected in the engine exhaust because of the incomplete combustion.

Effects of second hydrogen direct injection proportion and injection timing on combustion and emission characteristics of hydrogen/n-butanol combined injection engine

Hydrogen and n-butanol are superior alternative fuels for SI engines, which show high potential in improving the combustion and emission characteristics of internal combustion engines. However, both still have disadvantages when applied individually. N-butanol fuel has poor evaporative atomization properties and high latent heat of vaporization. Burning n-butanol fuel alone can lead to incomplete combustion and lower temperature in the cylinder. Hydrogen is not easily stored and transported, and the engine is prone to backfire or detonation only using hydrogen. Therefore, this paper investigates the effects of hydrogen direct injection strategies on the combustion and emission characteristics of n-butanol/hydrogen dual-fuel engines based on n-butanol port injection/split hydrogen direct injection mode and the synergistic optimization of their characteristics. The energy of hydrogen is 20% of the total energy of the fuel in the cylinder. The experimental results show that a balance between dynamics and emission characteristics can be found using split hydrogen direct injection. Compared with the second hydrogen injection proportion (IP2) = 0, the split hydrogen direct injection can promote the formation of a stable flame kernel, shorten the flame development period and rapid combustion period, and reduce the cyclic variation. When the IP2 is 25%, 50% and 75%, the engine torque increases by 0.14%, 1.50% and 3.00% and the maximum in-cylinder pressure increases by 1.9%, 2.3% and 0.6% respectively. Compared with IP2 = 100%, HC emissions are reduced by 7.8%, 15.4% and 24.7% and NOx emissions are reduced by 16.4%, 13.8% and 7.9% respectively, when the IP2 is 25%, 50% and 75%. As second hydrogen injection timing (IT2) is advanced, CA0-10 and CA10-90 show a decreasing and then increasing trend. The maximum in-cylinder pressure rises and falls, and the engine torque gradually decreases. The CO emissions show a trend of decreasing and remaining constant. However, the trends of HC emissions and NOx emissions with IT2 are not consistent at different IP2. Considering the engine's dynamics and emission characteristics, the first hydrogen injection proportion (IP1) = 25% plus first hydrogen injection timing (IT1) = 240°CA BTDC combined with IP2 = 75% plus IT2 = 105°CA BTDC is the superior split hydrogen direct injection strategy.

Understanding the Simulated Ammonia Increasing Trend from 2008 to 2015 over Europe with CHIMERE and Comparison with IASI Observations

The objective of this study is to assess and understand the NH3 recent trends and to identify the key components driving its concentrations. We have simulated the seasonal cycle, the interannual variability, and the trends in NH3 vertical column densities (VCD) from 2008 to 2015 over Europe, with the CHIMERE regional chemistry–transport model. We have also confronted the simulations against the Infrared Atmospheric Sounding Interferometer (IASI) satellite observations. IASI often shows a strong maximum in summer in addition to the spring peak, whereas CHIMERE only shows a slight peak in summer some years. This result could point to a misrepresentation of the temporal profile of the NH3 emissions, i.e., to missing emission sources during summertime either due to more than expected fertilizer use or to increased volatilization under warmer conditions. The simulated NH3 VCDs present an increasing trend over continental Europe (+2.7 ± 1.0 %/yr) but also at the national scale for Spain, Germany, UK, France, and Poland. Sensitivity tests indicate that these simulated positive trends are mainly due to (i) the trends in NH3 emissions, found heterogeneous in the EMEP NH3 emissions with strong disparities depending on the country, and (ii) the negative trends in NOx and SOx emissions. The impact of reductions in NO2 and SO2 emissions on NH3 concentrations should therefore be taken into account in future policies. This simulated NH3 VCD increase at the European scale is confirmed by IASI-v3R satellite observations in spring and summer, when ammonia emissions strongly contribute to the annual budget in accordance with crop requirements. Nevertheless, there are remaining differences about the significance and magnitude between the simulated and observed trends at the national scale, and it warrants further investigation.

Long-term trends of ozone and precursors from 2013 to 2020 in a megacity (Chengdu), China: Evidence of changing emissions and chemistry

Elevated ozone (O3) pollution in the warm season is an emerging environmental concern affecting global highly urbanized megacities. In southwestern China, full characterization of causes for O3 pollution has been stymied by limited observations and the dominant factors that influence O3 variability on a long-term basis still lack understanding. Herein, we identified O3 variations and inferred trends in precursor emissions in Chengdu over 2013–2020 based on extensive ambient measurements, emission inventory, and satellite data. Numerical models were used to investigate the changes in meteorological variability and biogenic emissions. Trends of O3 in urban areas show deterioration (+14.0% yr−1) between 2013 and 2016 followed by a slight decrease over 2017–2020, while O3 levels in rural areas generally show a downward trend (−2.9% yr−1) during 2014–2020. Both emission inventory (−3.7% yr−1) and OMI satellite columns (−4.5% yr−1) depict strong decline trends in NOx emissions, while satellite HCHO columns exhibit a flattened downward trend of VOC emissions (−1.8% yr−1), which caused rural areas shifted from VOCs-limited to transitional or NOx-limited regime since 2016. Considering metropolitan Chengdu remains VOCs-limited regime over time, the existing regulatory framework involving simultaneous NOx and VOCs control would result in evident O3 improvements in the near future. Despite benefits from anthropogenic emission reductions, we demonstrate that meteorological conditions and enhanced biogenic emissions over the warm season could partially or even fully offset effects attributed to emission changes, making the net effects obscure. This finding provides robust evidence of reductions in NOx and VOCs emission and informs effective O3 mitigation policies for megacities which undergo similar emission pathways in Chengdu.

Numerical simulation of combustion characteristics and emission predictions of methane-air and hydrogen-air mixtures in a constant volume combustion chamber using multi-point laser-induced spark ignition

ABSTRACT Lean-burn combustion using higher spark energy can enhance the combustion characteristics and emission performance. A laser is electrodeless with a high energy ignition source and can achieve multi-point ignition easily. This paper presents the computational fluid dynamics (CFD) modeling of combustion of methane-air and hydrogen-air mixture in a constant volume combustion chamber (CVCC) by using multi-point laser-induced spark ignition (LISI). The numerical simulation is carried out using ANSYS Fluent software, and a standard turbulence model (k – ε) is used. The pressure-time history and reaction progress was plotted for various equivalence ratios (ϕ). Numerical result shows that multi-point ignition can completely burn methane-air and hydrogen-air mixture in the range of 0.77 to 1.68 and 0.36 to 4.1, respectively. The maximum peak pressure (Pmax) and shortest combustion duration were observed at ϕ of 1.29 for the methane-air mixture and ϕ of 1.0 for the hydrogen-air mixture. The relative error between the numerical and the experimental results from the literature are within 10%. The faster reaction progress was observed for the hydrogen-air mixture compared to the methane-air mixture for all equivalence ratios. Finally, the validated numerical model was used to predict the CO, CO2, and NOx emission trends. For the methane-air mixture, CO emission increases as the equivalence ratio increases. However, the maximum CO2 emissions were observed at stoichiometric conditions. The highest NOx emissions were observed for the hydrogen-air mixture compared to the methane-air mixture at a stoichiometric equivalence ratio.

Reductions in nitrogen oxides over the Netherlands between 2005 and 2018 observed from space and on the ground: Decreasing emissions and increasing O3 indicate changing NOx chemistry

Satellite measurements of tropospheric NO2 columns are valuable for monitoring long-term changes in air quality. However, direct linkage of satellite-derived NO2 trends with changes in underlying NOx emissions and NOx surface concentrations is complicated by the contribution of background NO2 to the column, by changes in the chemical regime wherein emissions take place, and by data sampling differences. Here we study the 2005–2018 changes in nitrogen oxides concentrations over the polluted Netherlands. We use the QA4ECV OMI NO2 retrievals, RIVM surface measurements of NO, NO2, and O3, wet deposition fluxes of nitrate, and NOx emissions reported in two European inventories (EMEP and TNO-MACC-III). We interpret the observed changes in concentrations with simulations by CLASS, a box model accounting for boundary layer dynamics and chemistry. Nationally averaged, OMI column NO2 and RIVM surface NO2 concentrations are reduced by 30% and 32% respectively between 2005 and 2018. This is in line with Dutch national NOx emissions from the TNO-MACC-III and EMEP inventories showing decreases of 32%–39%, respectively, between 2005 and 2018. There is no indication that the decrease in NO2 concentrations slows down after 2010. The observed reductions in nitrogen oxides differ between winter and summer and turn out to be modulated by ozone chemistry. The RIVM surface measurements show a stronger reduction in NOx than in NO2, accompanied by an increase in O3 of 4–6 ppbv, which is due to diminished NO-titration following the lower NOx emissions, especially in winter. CLASS simulations confirm that daytime O3 increases have shifted the NO–NO2 equilibrium more towards NO2, explaining the weaker reductions in NO2 (−30%) than in NOx (−40%) concentrations over time. The O3 increases occur both during day and during night, and have likely shortened the NOx lifetimes both in summer (via faster OH + NO2+M during daytime) and in winter (via faster nighttime N2O5 formation and subsequent hydrolysis). Our findings suggest a reduction in anthropogenic NOx emissions of approximately 30% in the southern part of the Netherlands, and propose that interpreting satellite NO2 trends as a proxy for trends in NOx emissions is well-possible over a high-NOx region, but requires careful analysis in terms of changes in the chemical regimes and NOx lifetime.

On the Influence of Kinetic Uncertainties on the Accuracy of Numerical Modeling of an Industrial Flameless Furnace Fired With NH3/H2 Blends: A Numerical and Experimental Study

Ammonia/hydrogen-fueled combustion represents a very promising solution for the future energy scenario. This study aims to shed light and understand the behavior of ammonia/hydrogen blends under flameless conditions. A first-of-its-kind experimental campaign was conducted to test fuel flexibility for different ammonia/hydrogen blends in a flameless burner, varying the air injector and the equivalence ratio. NO emissions increased drastically after injecting a small amount of NH3 in pure hydrogen (10% by volume). An optimum trade-off between NOx emission and ammonia slip was found when working sufficiently close to stoichiometric conditions (ϕ = 0.95). In general, a larger air injector (ID25) reduces the emissions, especially at ϕ = 0.8. A well-stirred reactor network with exhaust recirculation was developed exchanging information with computational fluid dynamics (CFD) simulations, to model chemistry in diluted conditions. Such a simplified system was then used in two ways: 1) to explain the experimental trends of NOx emissions varying the ammonia molar fraction within the fuel blend and 2) to perform an uncertainty quantification study. A sensitivity study coupled with latin hypercube sampling (LHS) was used to evaluate the impact of kinetic uncertainties on NOx prediction in a well-stirred reactor network model. The influence of the identified uncertainties was then tested in more complex numerical models, such as Reynolds-averaged Navier–Stokes (RANS) simulations of the furnace. The major over-predictions of existing kinetic scheme was then alleviated significantly, confirming the crucial role of detailed kinetic mechanisms for accurate predictive simulations of NH3/H2 mixtures in flameless regime.

Stomatal conductance influences interannual variability and long-term changes in regional cumulative plant uptake of ozone

Ambient ozone uptake by plant stomata degrades ecosystem and crop health and alters local-to-global carbon and water cycling. Metrics for ozone plant damage are often based solely on ambient ozone concentrations, overlooking the role of variations in stomatal activity. A better metric is the cumulative stomatal uptake of ozone (CUO), which indicates the amount of ozone entering the leaf over time available to cause physiological damage. Here we apply the NOAA GFDL global earth system model to assess the importance of capturing interannual variations and 21st century changes in surface ozone versus stomatal conductance for regional mean CUO using 20-year time-slice simulations at the 2010s and 2090s for a high-warming climate and emissions scenario. The GFDL model includes chemistry-climate interactions and couples atmospheric and land components through not only carbon, water, and energy exchanges, but also reactive trace gases—in particular, ozone dry deposition simulated by the land influences surface ozone concentrations. Our 20-year time slice simulations hold anthropogenic precursor emissions, well-mixed greenhouse gases, and land use distributions fixed at either 2010 or 2090 values. We find that CUO responds much more strongly to interannual and daily variability in stomatal conductance than in ozone. On the other hand, long-term changes in ozone explain 44%–90% of the annual CUO change in regions with decreases, largely driven by the impact of 21st century anthropogenic NOx emission trends on summer surface ozone. In some regions, increases in stomatal conductance from the 2010s to 2090s counteract the influence of lower ozone on CUO. We also find that summertime stomatal closure under high carbon dioxide levels can offset the impacts of higher springtime leaf area (e.g. earlier leaf out) and associated stomatal conductance on CUO. Our findings underscore the importance of considering plant physiology in assessing ozone vegetation damage, particularly in quantifying year-to-year changes.

Control of NOx and other emissions in micro gas turbine combustors fuelled with mixtures of methane and ammonia

Methane–ammonia mixtures have potentials as low-carbon fuels for gas turbines, however significantly high fuel NOx production in their flames present challenges to their application. This study aims to provide deep insight into the physical and chemical processes involved in the formation and control of emissions from the combustion of CH4NH3–air with up to 30% ammonia by heat fraction in gas turbine combustors. Hence, laser diagnostics techniques such as Particle Image Velocimetry (PIV), and Planar Laser Induced Fluorescence (PLIF) imaging, in addition to Fourier Transform Infrared (FTIR) gas analysis were employed to study the flow field, flame structure and emissions characteristics of a micro gas turbine swirl combustor fuelled with CH4NH3–air mixtures. The control of emissions from the flames was further studied using Large Eddy Simulation (LES) of a model swirl combustor. The results show that NOx emissions from premixed CH4NH3–air in single-stage combustion were more than 5000 ppmv at equivalence ratios, Φ = 0.8–1.1, which is about twice more than the values already reported for NH3–air. Trends in NOx emissions correspond with the trends in OH radicals concentration in the combustor owing to the relevance of OH radicals in fuel NOx production. Emissions control leading to significantly low emissions such as 49 ppmv of NOx, 2 ppmv of CO and approximately zero N2O, HCN and NH3 emissions with a 99.8% combustion efficiency was achieved using rich-lean combustion. An optimum Φ of the primary combustion zone for low NOx emission was identified, which varied from 1.30 to 1.35 depending on the ammonia fraction. For Φ richer (leaner) than the optimum Φ, NOx emission increased due to an increase in NOx production in the secondary (primary) combustion zone. Rich-lean combustion of CH4NH3–air emitted less NOx than that of NH3-air because the higher flame speed of CH4NH3–air mixtures ensured lower NOx production in the secondary combustion zone.

Impact of Air Pollution Controls on Radiation Fog Frequency in the Central Valley of California

AbstractIn California's Central Valley, tule fog frequency increased 85% from 1930 to 1970, then declined 76% in the last 36 winters. Throughout these changes, fog frequency exhibited a consistent north‐south trend, with maxima in southern latitudes. We analyzed seven decades of meteorological data and five decades of air pollution data to determine the most likely drivers changing fog, including temperature, dew point depression, precipitation, wind speed, and NOx (oxides of nitrogen) concentration. Climate variables, most critically dew point depression, strongly influence the short‐term (annual) variability in fog frequency; however, the frequency of optimal conditions for fog formation show no observable trend from 1980 to 2016. NOx concentration, which has a decreasing north‐south concentration gradient, declined continuously over this period, consistent with the long‐term temporal and spatial trends in fog. As development in the Central Valley increased direct particle and other pollutant emissions from 1930 to 1970, fog frequency increased. Following the Clean Air Act, particle emissions quickly declined, and NOx emissions declined steadily, reducing the cloud condensation nuclei (CCN) available for fog formation. As a precursor of ammonium nitrate aerosols, which are efficient CCN, we used NOx measurements and emission trends as a proxy for the CCN trend. We conclude that while the short‐term fog variability is dominantly driven by climate fluctuations, the longer‐term temporal and spatial changes in fog have been driven by changes in air pollution. For conditions close to the dew point, a decrease in fog of 5 days/year per 10 parts per billion NOx decrease occurred across the Central Valley.

Evaluation of Heavy- and Medium-Duty On-Road Vehicle Emissions in California's South Coast Air Basin.

Emission measurements were collected from heavy-duty (HDVs) and medium-duty vehicles (MDVs) at the Peralta weigh station long-term measurement site near Anaheim, CA, in 2017. Two Fuel Efficiency Automobile Test units sampled elevated and ground-level exhaust vehicles totaling 2 315 measurements. HDVs (1844 measurements) exhibited historical reductions in fuel specific oxides of nitrogen (NOx) from the 2008 measurements (55%) with increased use of exhaust gas recirculation and selective catalytic reduction systems. However, as these technologies have aged, the in-use benefits have declined. Infrared % opacity measurements of tailpipe soot decreased 14% since 2012 with increased diesel particulate filter (DPF) use, DPF longevity, and fleet turnover. Sixty-three percent of the HDV fleet in 2017 was chassis model year 2011+ compared to only 12% in 2012. The observed MDV fleet (471 measurements) was 1.4 years older than the HDV fleet with average NOx 14% higher. A significant reduction in MDV NOx occurred ∼2 model years prior to similar HDV reductions (2014 versus 2016 chassis model year). MDV chassis model years 2014+ were able to meet their corresponding NOx laboratory certification standards in-use, whereas HDVs remain slightly above this threshold. Similar MDV NOx emission trends were also observed in data previously collected in Chicago, IL.