NOx Concentrations Research Articles

THE IMPROVEMENT OF THE ECONOMIC AND ENVIRONMENTAL CHARACTERISTICS OF COGENERATION PLANTS BASED ON HEAT ENGINES WITH THE THERMOCHEMICAL METHOD OF CLEANING THEIR EXHAUST GASES

The article presents the results of research aimed at improving the economic and environmental characteristics of cogeneration plants based on heat engines (ICE, GPE, GTU), with a thermochemical device for the neutralization of harmful components of its waste gases, and a boiler-utilizer (BU). The neutralization process takes place in a layer of red-hot carbon material and a gaseous medium, which includes: methane, hydrogen and carbon monoxide. The effectiveness of the proposed process was investigated both analytically, using specialized computer programs for energy management, and experimentally, by a special laboratory device, which made it possible to simulate in a wide range of temperatures (500...1000°C) how the products of natural gas combustion (the base fuel for the GPA and GTU), with a defined coefficient of excess air (oxygen), as well as the concentration of CO and NOx in their composition. The primary task of the experimental study was to determine the effect of temperature on the neutralization of nitrogen oxides and carbon monoxide in natural gas combustion products. The results of the studies (in the considered temperature range of 750...950℃) showed the possibility of reducing the concentration of nitrogen oxides by 2.5...3 times, and the concentration of carbon monoxide by almost 5 times. At the same time, it was established that the efficiency of NOx neutralization at the considered temperature range does not change significantly (within 10%), which makes it possible to carry out the neutralization process at a temperature of 750...780℃ and reduce the energy cost. The final neutralization of the exhaust gases of the engine and the utilization of their thermal potential is carried out in the boiler-utilizer. The placement of the thermochemical device in front of the boiler-utilizer, on the one hand, changes the thermal characteristics of the cogeneration plant to direction of increasing its heat loads, and on the other hand, increases the maneuverability of the plant, providing possibility of independent change of electrical and thermal load. In addition, the use of solid fuel with its gasification in the device allows not only to neutralize the harmful components of the engine's exhaust gases, but also to provide a partial replacement of natural gas in the engine and the boiler - utilizer with a corresponding reduction of thermal and harmful emissions in to the environment.

A novel hybrid forecasting approach for NOx emission of coal‐fired boiler combined with CEEMDAN and self‐attention improved by LSTM

AbstractThe precise prediction of NOx generation concentration in coal‐fired boilers serves as the foundational cornerstone for the judicious optimization and control of selective catalytic reduction denitrification (SCR) systems. Owing to the intricate nature of the denitrification process within SCR, there exists a temporal delay in regulating the ammonia injection rate based on the monitored data of NOx concentration at the SCR inlet. Such delays can give rise to ammonia leakage and subsequent obstruction of the air preheater. In light of this, a predictive model, CEEMDAN‐LSTM‐SA, is proposed as an amalgamation of data decomposition and the LSTM (long short‐term memory) fusion self‐attention mechanism within a deep learning network, which is introduced to forecast the NOx emission concentration at the SCR inlet of coal‐fired units. To mitigate the impact of data outliers on the training effectiveness of the model, a clustering method coupled with a statistical testing strategy is initially applied to refine the dataset first. CEEMDAN data decomposition technology is leveraged to facilitate the breakdown of data, alleviating its non‐stationary and intricate characteristics. Subsequently, through spectral analysis, the decomposed components are grouped and aggregated to form novel data elements, which are then subjected to prediction by the constructed LSTM‐SA deep learning network. The ultimate NOx emission concentration prediction value is derived through a process of fusion. Upon scrutinizing and comparing the predictions derived from various models using coal‐fired power plant data, it is evident that the performance metrics of CEEMDAN‐LSTM‐SA predictions exhibit a mean absolute error of 7.425, mean absolute percentage error of 2.415%, root mean square error of 9.715, R‐squared (R2) value of .789, mean absolute relative error of 2.109%, and a Theil's information criterion of .016. In contrast to other models, including traditional self‐attention networks, LSTM, and LSTM‐SA combination networks, CEEMDAN‐LSTM‐SA proposed in this study demonstrates superior prediction accuracy and enhanced generalization capabilities. Consequently, this predictive model stands poised to furnish an efficacious framework for the SCR ammonia injection strategy within thermal power units.

A highly effective N2 fixation method based on reverse vortex flow gliding arc plasma under water

A gliding arc plasma has the advantages of strong reactivity, high energy density, and easy scale-up, so it is very competitive in the field of plasma-based N2 fixation. In this work, we designed an underwater transfer arc discharge plasma N2 fixation system based on a reverse vortex flow gliding arc. In full contact with air and water, the plasma could realize the efficient conversion of N2, O2, and H2O, obtaining gaseous NO, NO2, and activated water rich in NOx*. The experiment showed that an underwater transfer arc discharge was formed when the current was greater than 0.6 A. Under this operating condition, the conversion rate of N2 reached 1.63%, the concentration of NOx reached 2.4%, and the production rate of nitrogen-containing products was about 3.41 mol/h. In addition, the energy consumption was as low as 2.21 MJ/mol, the lowest reported value for atmospheric-pressure rotary gliding arc plasma-based N2 fixation. The generated activated water had abundant NO3− and NO2− ions, which could be directly applied as N2 fertilizer in agriculture, promoting plant germination and growth. This paper describes an environmentally friendly and efficient technical route toward developing industrial N2 fixation/green agriculture, strongly supporting the carbon neutrality concept.

Significant reductions of urban daytime ozone by extremely high concentration NOX from ship’s emissions: A case study

In this paper, a four nested Weather Research and Forecasting-Community Multiscale Air Quality (WRF-CMAQ) model was applied to analyze ozone (O3) concentration reductions under the influence of extremely high NOx concentrations emitted from ships at the Hankou Jiangtan (HJ) station adjacent to the Yangtze River in Wuhan, China. By adding NO emissions along the waterway for the sensitivity experiments, the model successfully reproduced two cases of high NOx concentrations on April 9 and April 28, 2020, and one case of persistent and extremely high NOx concentrations on the night of May 3, 2020. During daytime, strong photochemical reactions generated new O3, while the high concentration NOx from ship emissions continuously reduced O3 through catalytic cycling. This led to the O3 concentrations at the HJ station being consistently lower than the surrounding stations in the afternoon, with the high NOx plume dropping O3 concentrations by 51 ppb in 1 h, and NO2 concentrations up to 110 ppb. The presence of large amounts of fresh NO (up to 265 ppb) in the ship emission plumes rapidly reduces O3 concentrations through titration reactions during night, resulting in a wide area of low O3 concentrations (2–4 ppb) over a long period of time. This study demonstrates that the pollution and impact of emissions from ships traveling in waterways on the atmosphere of the surrounding area cannot be ignored.

Impact of COVID-19 lockdowns on NOx emissions: A sector-specific analysis of transportation and power generation

To limit the transmission of COVID-19, a series of rigorous lockdown measures were implemented worldwide. These measures have caused a massive impact on the socio-economic conditions, especially on traffic flow as well as the demand for electricity. This study designs an integrated approach that incorporating an atmospheric dispersion model into a state-of-the-art traffic flow and power consumptions models to simulate the dispersion of the resultant emissions of NOx, considering the changes during the full and partial lockdown periods in comparison to the corresponding pre-pandemic period. This study is geographically focused on Kuwait, examining the impact of COVID-19 lockdown measures on NOx emissions within its transportation and power generation sectors. The results show that hourly averaged electricity generating decreased by 5.6% and 4.5% as a result of the partial lockdown and full lockdown respectively. Crude oil consumption increased from 12% to 27% during the partial lockdown, and rose to represent 20% as a result of the full lockdown. This is because power plants had to act quickly to secure supply chains and manage fuel inventory. More natural gas was consumed as a result of the partial lockdown, raising its share of total energy consumption to 57%. This resulted in a decrease of 14% and 36% in NOx emissions. Hotspots for NOx concentrations were observed over densely populated residential areas during the full lockdown period, since transportation-related emissions accounted for the bulk of NOx emissions during the full lockdown. In contrast, maximum NOx concentration hotspot for the partial lockdown period was observed in the vicinity of power plants. This sector-specific analysis provides insights into sustaining air quality gains by targeting interventions for transportation and energy use.

Experimental investigation on dynamic characteristics of methane/air combustion in double‐layer pellets porous media

AbstractThe double‐layer porous media burner is considered as an effective way to realize stable lean‐burn. In order to quickly achieve a stable combustion state in a double‐layer porous media burner, this work investigated the dynamic characteristics of methane/air premixed combustion in a bench‐scale double‐layer porous media burnspanning from ignition to stable combustion and ultimately flameout. The experimental results indicate that regulating the equivalence ratio and the inlet velocity enables the establishment of a stable flame front and the φ = 0.75 and the Vin = 0.20 m/s are the appropriate start‐up conditions. The average propagation velocity of the combustion wave variation along the axial direction and ranged approximately from −0.022 to −0.078 mm/s. Moreover, the transition time to a stable combustion state is reduced by nearly 47.14% as the equivalence ratio increases from 0.60 to 0.70. During start‐up stage, there are significant fluctuations in CO and NOx concentrations, but both emissions remain low during steady combustion state, with the maximum concentrations of 37.5 and 40.2 mg/m3, respectively. Furthermore, the porous media combustion exhibits a pronounced re‐ignition capacity. At higher equivalence ratios, longer interruptions of premixed gas are allowed.

Study on Exhaust Gas Characteristics of SI Engine Using Fuel (Gasoline) & HHO Gas

Experiments have been carried out to investigate the effect of HHO gas on the emission and performance of SI engines. Water was converted into hydrogen and oxygen consisting of a 2:1 volume ratio by electrolysis process, which is called HHO (oxyhydrogen gas). An HHO fuel cell had been fabricated to generate the HHO gas required to feed the engine. This HHO is then mixed with filtered air, which is after the air filter system and before the inlet manifold system of the vehicle. In this investigation, a 125-cc single-cylinder 4-stroke air-cooled engine was used. The exhaust gas characteristics were sampled and measured using a gas analyzer. It was observed that when HHO gas had been introduced into the air/fuel mixture, it consequently reduced the concentration of NO, NOx, CO, engine temperature, and HC by about 50%, 50%, 38%, 5% & and 45% respectively. Also, a reduction in fuel consumption is noticed for the same engine RPM and load conditions when HHO gas was imported into the A/F mixture. Also, when HHO is introduced into the system, the concentration of O2 CO2 increases by about 50% and 23% respectively.

Effects of thermal disposal of oil-water residues on operation heating plant boiler

Industrial zones are often the source of specifically polluted waters, depending on type of industry. These waters are usually not in the limits for outtake into public sewers and it is necessary to dispose them externally or build-up technologies for their pre-treatment. The article is about the real case of an industrial. A separate sewage network is built for oily water. Solid impurities are separated from the oily water and oily water is treated by evaporation technology. The evaporator produces two types of products - condensate and concentrate. Condensate is a water with high organic content, and it is necessary to purified it by a chemical treatment plant in next step. Evaporator technology remove from water the most of metal content and significantly decrease organic and water content in concentrate. Concentrate is oily stream with lower water content. Water content in concentrate decreased in next technology steps to ca. 40-50 %. The final product is certificated technological fuel, and its final elimination is in the heating plant fluidized bed boiler. The article is detailed focused on the influence of concentrate combustion. A local temperature increases around 10-15 °C extending across the combustion chamber was confirmed. There was also an increase in HF concentration in flue gas. CO concentrations were higher in the first moments of combustion, but the control system responded by increasing the combustion air supply, and CO concentrations were finally lower at the level of 3 mg·m-3. During combustion, there was no provable increase in concentrations of NOX or other substances, and the co-combustion did not have a significant effect on the operation of the boiler, so this way was evaluated like technically and economically effective.

The synergistic effect of NOx and SO2 on the formation and light absorption of secondary organic aerosols from ο-xylene photooxidation

Both NOx and SO2 are important anthropogenic air pollutants that significantly affect secondary organic aerosols (SOA) formation. However, our understanding of the combined effects of NOx and SO2 on SOA formation is still insufficient. The concentrations, optical properties, and chemical compositions of SOA formed through ο-xylene photooxidation under various NOx concentrations were investigated with or without SO2 in the chamber. It was found that SOA mass concentration increased from 60 to 129 μg m−3 when the initial NOx concentration was increased from 0 to 900 ppb. The presence of 50 ppb SO2 not only enhanced ο-xylene SOA formation by 1.6–2.9 times, but also changed the evolution of SOA with NOx. The enhancement of ο-xylene SOA formation was primarily attributed to acid-catalyzed heterogeneous reactions initiated by nitric acid and sulfuric acid. However, SOA concentration decreased under 900 ppb NOx when SO2 was present, likely due to the decrease in sulfate production because of the competition between NOx and SO2 for ·OH. In addition, nitrogen-containing organic compounds (NOCs) concentrations and mass absorption coefficients (MACλ=365 nm) were promoted by both NOx and SO2. Our research indicates that the promotion of both NOCs formation by NOx, and oligomer formation by SO2 were the main reasons for the enhanced SOA light absorption. The present research provides a scientific basis for understanding the combined effects of both NOx and SO2 on ο-xylene SOA formation, and illustrates that the imbalanced reduction of anthropogenic air pollutants would lead to new characteristics in SOA pollution.

NOx emissions during oxygen-enriched combustion of sewage sludge and corresponding hydrochar

This study proposed an oxygen-enriched combustion technology for the resource utilization of hydrochar prepared from sewage sludge (SS) through hydrothermal carbonization (HTC). However, its application potential relies heavily on NOx emissions. To investigate NOx emissions during oxygen-enriched combustion of hydrochars prepared at different temperatures (200 °C, 240 °C, and 280 °C), real-time NOx concentrations were recorded under varying conditions, including different carrier gases (N2 and CO2), temperatures (600 °C, 800 °C, and 1000 °C), and oxygen concentrations (20 %, 30 %, and 40 %). In comparison to air combustion processes, the NOx emissions during the devolatilization stage were considerably depressed in oxygen-enriched combustion. A high concentration of CO2 inhibited NH3 production and HCN oxidation, and generated increased amounts of CO to facilitate NOx reduction. Elevated temperatures also reduced the NOx release during the oxygen-enriched combustion of SS and hydrochar, as the accelerated oxygen consumption weakened the oxidation reactions of HCN and NH3; the enhanced reduction reaction rate and increased CO production also promoted NOx reduction. Elevated oxygen concentrations, however, promoted NOx release owing to the increased NH3 and HCN oxidation by an increased number of oxidizing groups. Additionally, the reduction of reducing gases and rapid char combustion hindered NOx reduction. The NOx-N conversion of the hydrochars was much lower than that of SS under different conditions. HTC effectively reduced the nitrogen content of SS, with nitrogen in HC-280 existing as stable heterocyclic -N. HC-280 showed significantly less nitrogen conversion at low temperatures (9.03 % reduction at 600 °C) and high oxygen concentrations (2.01 % reduction at 40 % O2) compared to SS, highlighting the potential of combining HTC and oxygen-enriched combustion.

Simultaneous absorption of SO2 and NOx from simulated flue-gas exploiting the enhancing oxidative ability of aqueous euchlorine as scrubbing solution

This paper reports the performances of a pilot-scale Wet Oxidative Scrubber (WOS) in the treatment of a simulated flue-gas containing 500 ppmv SO2 and 1030 ppmv NOx and operated at 60 °C and 1 atm. The scrubber is fed with an oxidizing solution containing NaClO2 at either 0.75 % or 1 % w/w initially at pH > 9 and then acidified to pH down to 3 with HCl, to obtain the additional oxidative contribution given by euchlorine species during the simultaneous absorption. Further experiments of single-component NOx absorption are performed to better understand the role exerted by SO2 in the simultaneous absorption. These experimental tests are carried out to investigate the effect of the acidity of the chlorite-based scrubbing solution on the simultaneous absorption performance of SO2 and NOx, highlighting the intensification with respect to the bare de-NOx system. Experiments also allow determining the optimal dosages and solution pH for a complete and optimized removal of NOx and SOx also being less energy-intensive. For a scrubbing solution under acid-induced conditions (pH = 3), the retrieved results show that the NaClO2 molar dosage needed for a complete simultaneous removal results 3.6 times greater than the total NOx concentration in the flue-gas. This allows an interesting reduction of 83 % in oxidant consumption and a water-saving of 70 % with respect to the same process applied to the simultaneous NOx absorption but without altering the solution pH. Overall, the results confirm the enhancing role exerted by euchlorine deriving from chlorite acidification and sulfur dioxide presence in the flue-gas.

Numerical simulation on coking process of coke oven with external flue gas recirculation

The coking behaviour and NOx emission characteristics of the coke oven are of great significance for coke production. Based on the realisation k-epsilon turbulence model, the P-1 radiation model, the species transport model with eddy dissipation, and the thermal NOx model. A comprehensive model for traditional regenerative coke oven was established with reversing mode and external flue gas recirculation model considering temperature-dependent thermophysical properties. The convective term of model equations was the discretised using the second-order upwind scheme, while the others were applied by the central difference scheme. The coking behaviour of the coke oven with external flue gas recirculation was numerically solved through the SIMPLE algorithm, and the effects of reversing period and external flue gas recirculation ratio on the coking behaviour were explored. When increasing the reversing period, the mean temperature of the vertical flue increases, and the coking time decreases, but the temperature uniformity of the carbonisation chamber becomes worse. As the external flue gas recirculation ratio increases, the average temperature of the vertical flue decreases, and the NOx concentration at the outlet decreases, yet the coking time increases. These results would provide useful information for guiding the improvement of coke oven operation and reducing NOx emissions.

Prediction of Air Pollution with Machine Learning Algorithms

Air pollution has become an important problem due to its threats. Air pollutants are in complex interaction with atmosphere and environment. For this reason, it is important to study air pollution issues. In recent years, studies on prediction of air pollutants with machine learning methods have gained momentum. In this study, some air pollutants are predicted with various machine learning algorithms considering meteorological factors. In machine learning phase, a separate study is conducted with various machine learning algorithms (multilayer perceptron neural network, stochastic gradient descent, ridge regression, cross decomposition) considering temperature, relative humidity, wind, pressure and air pollutant measurements of previous hour. Consistencies of these algorithms in estimating pollutant concentrations are compared. Various statistical metrics are used to analyze the consistencies. As a result, the coefficient of determination of all algorithms are found above 0.67, considering the test section. It is found that the coefficient of determination of the multilayer perceptron neural network algorithm provides better results than other algorithms.

Zone-specific longshore sampling as a strategy to reduce uncertainties of SGD-driven solute fluxes from high-energy beaches

Submarine groundwater discharge (SGD) from intertidal beach systems forms a source of fresh and saline, often nutrient-enriched water to the near-shore across the world. The chemical composition of solute flux via SGD depends on several factors, including solid phase composition, organic matter supply, hydraulic gradients, as well as pore water residence time and origin, which are partly related to the beach topography. High-energy sandy beaches are especially prone to frequent sediment relocation and a variable topography, often characterized by a ridge and runnel structure. Topography may not only change on a temporal scale, but also in the longshore direction, which may alter SGD-driven solute fluxes. However, sampling high-energy beach sites is challenging and often does not allow for high resolution cross- and longshore sampling. We tested the approach of zone-specific pore water sampling, whereby samples were taken at the high water line, in the runnel, on top of the ridge, and close to the low water line of eight cross-shore transects. This was done in order to assess the longshore variability of pore water composition and flux over a 2.5 km wide beach section. In this study we relied on relatively few data points per transect and still found a characteristic cross-shore distribution of pore water parameters, related to redox conditions of each zone. Recharge zones (high water line and ridge) exhibited higher O2 and NOx concentrations and discharge zones (runnel and low water line) exhibited higher nutrient, DOC, Mn, and Fe concentrations. Furthermore, DOC concentrations in pore waters were lower than in coastal seawater, supporting previous assumptions of rapid organic matter turnover. Nevertheless, we observed large concentration ranges among samples assigned to the same zone, which could partly be related to longshore topography differences. The measured pore water concentrations and calculated SGD-driven solute fluxes indicated that sampling of single cross-shore transects may be representative for a beach section when assessing a redox-independent parameter like Si, while it is less representative for parameters affected by more complex biogeochemical processes like P, NH4, Mn, and Fe. For these, we suggest that longshore distributed data coverage combined with higher resolution sampling in discharge zones is a good strategy to reduce the uncertainty associated with estimating SGD-driven solute fluxes in a practicable way.

Characteristics and Formation Mechanism of Ozone Pollution in Demonstration Zone of the Yangtze River Delta, China

Emerging research indicates that ground-level ozone (O3) has become a leading contributor to air quality concerns in many Chinese cities, with the Yangtze River Delta (YRD) region facing particular challenges. This study investigated the characterization of air pollutants in Wujiang, which is located within the YRD demonstration zone, during the warm season (April–September) of 2022. The contributions of emission and meteorology to O3 were identified, the O3-NOX-VOC sensitivities were discussed, and the VOC sources and their contributions to O3 formation were analyzed. A random forest model revealed that the high O3 concentration was mainly caused by a combination of increased emission intensity due to the resumption of work and production after the COVID-19 pandemic, along with adverse meteorological conditions. The results revealed more than 92% of the pollution days were related to O3 during the warm season, and the impact of O3 precursor emissions was slightly greater than that of the meteorological conditions. O3 formation was in the VOC-limited regime, and emission reduction strategies targeting VOCs, particularly aromatics such as toluene and xylene, have been identified as the most effective approach for mitigating O3 pollution. Changes in O3-NOX-VOC sensitivity were also observed from the VOC-limited regime to the transitional regime, which was primarily driven by variations in the NOX concentrations. The VOC source analysis results showed that the contributions of gasoline vehicle exhaust and diesel engine exhaust (mobile source emissions) were significantly greater than those of the other sources, accounting for 20.8% and 16.5% of the total VOC emissions, respectively. This study highlights the crucial role of mobile source emission control in mitigating O3 pollution. Furthermore, prioritizing the control of VOC emission sources with minimal NOX contributions is highly recommended within the VOC-limited regime.

Performance and emission characteristics of a diesel engine fuelled with Mesua ferrea biodiesel with chromium oxide (Cr2O3) nanoparticles: Experimental approach and response surface methodology

This study investigates the effects of using nano-fuelled diesel engines, including Mesua ferrea biodiesel and chromium oxide (Cr2O3) nanoparticles. The RSM model was created to make accurate predictions of the operating parameters. The amount of Cr2O3 nanoparticles was set at 60, 80, and 100 mg/L and an equal amount of surfactant and dispersant was added to the nanoparticles. The test was carried out under different compression ratios and load conditions. The parameters evaluated were the engine loads (3, 6, 9, and 12 kgf), fuel samples, and compression ratios (16.5:1, 17.5:1, and 18.5:1) as inputs and the parameters BTE, BSFC, CO, UHC, NOx, and smoke opacity as outputs. The BD20 + Cr2O3 80 mg/L + DSP 80 mg/L fuel sample resulted in a significant improvement in BTE by 16.58 % and a reduction in BSFC by 0.58 % compared to the other samples, while CO concentration decreased by 31.85 %, UHC concentration by 22.23 %, NOx concentration by 6.16 % and smoke opacity by 62.61 % at CR 18.5:1 and full load. The correlation coefficient (R2), which was determined using response surface methods and artificial neural networks, was between 0.96 and 0.98 for all output parameters. The observed range of values demonstrates a strong correlation between the experimental data and the predicted results.

NOx degradation by cement mortar containing B-TiO2/MgAl-CLDH under visible light: Experimental and modeling

In this paper, an advanced photocatalyst, B-TiO2/MgAl-CLDH, was synthesized and applied in cement mortar. The photocatalytic NOx degradation ability of cement mortar containing B-TiO2/MgAl-CLDH under different initial NOx concentrations and flow rates was investigated. The results show that B-TiO2/MgAl-CLDH has stronger visible light absorption, lower recombination rate of electron-hole pairs and narrower optical band gap than commercial nano-TiO2 (P25), resulting in enhanced NOx removal efficiency. Specifically, the maximum NOx degradation ratio of B-TiO2/MgAl-CLDH is 23.4% within 30 min, which is about 5 times that of P25. For photocatalytic mortar, the initial NOx concentration (from 1.0 ppm to 2.0 ppm) and flow rate (from 1 L/min to 3 L/min) were positively correlated with NOx removal amount, while negatively correlated with NOx removal ratio. Based on the internal molecular diffusion properties of NOx, Langmuir-Hinshelwood and the power law kinetic models were used to predict the NOx degradation ability of photocatalytic mortar. The modeling of NOx degradation process presents a reliable prediction of the NOx removal ability of photocatalytic mortar, serving as a valuable tool for assessing the mortar’s NOx removal effectiveness for policymakers and engineers.

Temporal Evolution of Vehicle Exhaust Plumes in a Congested Street Canyon Environment

Air pollutants from traffic make an important contribution to human exposure, with pedestrians likely to experience rapid fluctuation and high concentrations on the pavements of busy streets. This monitoring campaign was on Hennessy Road in Hong Kong, a densely populated city with deep canyons, crowded footpaths and low wind speeds. Kerbside NOx concentrations were measured using electrochemical sensors with baseline correction and subsequently deconvoluted to determine concentrations at 1-s resolution to study the dispersion of exhaust gases within the first few metres of their on-road source. The pulses of NOx from passing vehicles were treated as segments of a Gaussian plume originating at the tailpipe. The concentration profiles in segments were fit to a simple analytical equation assuming a continuous line source with R2 > 0.92. Least squares fitting parameters could be attributed to vehicle speed and source strength, dispersion, and sensor position. The width of the plume was proportional to the inverse of vehicle speed. The source strength of NOx from passing vehicles could be interpreted in terms of individual emissions, with a median value of approximately 0.18 g/s, but this was sensitive to vehicle speed and exhaust pipe position. The current study improves understanding of rapid changes in pollutant concentration in the kerbside environment and suggests opportunities to establish the contribution from traffic flow to pedestrian exposure in a dynamic heavily occupied urban microenvironment.

Numerical Simulation Study on Combustion of Low Calorific Value Waste Blended with Biomass.

Numerical simulations of a 600 t/day waste incinerator was carried out using the fluid dynamic incinerator code and Fluent to evaluate the effect of biomass blending on furnace temperature, pollutant generation, and selective noncatalytic-reduction (SNCR) denitrification when treating low calorific-value waste. The results show that as the biomass blending ratio increases, the water content gradually decreases, the calorific value increases, and the maximum temperature of the incinerator gradually increases from 1227 to 1408 K, while the content of exported NOx increases from 579 to 793 mg/Nm3; during the combustion of low-quality waste, the residence time of the flue gas in the high-temperature region (above 1123 K) is 1.62 s. When the biomass blending ratio exceeds 20%, the residence time of the flue gas in the high-temperature region is more than 2 s, which can effectively curb the generation of dioxin. When the biomass blending ratio is 20%, and the normalized stoichiometric ratio (2nurea/nNO) of urea injected into the SNCR is 1.1, the NOx concentration at the outlet is 230.08 mg/Nm3, which satisfies the NOx emission standard of less than 250 mg/Nm3.

Urban–Rural Comparisons of Biogenic Volatile Organic Compounds and Ground-Level Ozone in Beijing

Ground-level ozone (O3) pollution has been a severe environmental and health problem for decades. The importance of biogenic volatile organic compounds (BVOCs) in the formation of tropospheric photochemistry O3 has been highlighted, especially in areas of rapid urbanization. We conducted simultaneous measurements of trace gases, including NO, NOX, O3, and BVOCs (i.e., isoprene and α-pinene), in the urban and rural forest areas of Beijing to determine the relationships between them. The results highlight the differences between the urban and rural forest areas of Beijing in terms of ambient air concentrations of BVOCs and O3, and the interrelationships between BVOCs, NOX, and ozone were quantified. Moreover, the isoprene concentration was found to be higher in the atmosphere of the urban site than of the rural site, which had higher α-pinene concentrations and higher O3 concentrations. The NOX concentration was higher at the urban site than at the rural site, and there was a significant exponential relationship between NOX and O3 at the urban site, indicating that the impact of NOx on O3 at the urban site was greater than that at the rural site. The O3 concentration increased with rising isoprene and α-pinene in both sites. In the case of substantially increased BVOC concentrations, declining NOX concentrations strongly promote the formation of O3. Consideration should be given to planting tree species with low-BVOC emissions, as they are crucial for mitigating O3 pollution in urban areas. Additionally, the relationships between BVOCs, NOX, and O3 should be considered in policymaking related to O3 control.