Showing papers on "NOx published in 2019"

Anthropogenic drivers of 2013–2017 trends in summer surface ozone in China

Abstract: Observations of surface ozone available from ∼1,000 sites across China for the past 5 years (2013–2017) show severe summertime pollution and regionally variable trends. We resolve the effect of meteorological variability on the ozone trends by using a multiple linear regression model. The residual of this regression shows increasing ozone trends of 1–3 ppbv a−1 in megacity clusters of eastern China that we attribute to changes in anthropogenic emissions. By contrast, ozone decreased in some areas of southern China. Anthropogenic NOx emissions in China are estimated to have decreased by 21% during 2013–2017, whereas volatile organic compounds (VOCs) emissions changed little. Decreasing NOx would increase ozone under the VOC-limited conditions thought to prevail in urban China while decreasing ozone under rural NOx-limited conditions. However, simulations with the Goddard Earth Observing System Chemical Transport Model (GEOS-Chem) indicate that a more important factor for ozone trends in the North China Plain is the ∼40% decrease of fine particulate matter (PM2.5) over the 2013–2017 period, slowing down the aerosol sink of hydroperoxy (HO2) radicals and thus stimulating ozone production.

Science and technology of ammonia combustion

Abstract: This paper focuses on the potential use of ammonia as a carbon-free fuel, and covers recent advances in the development of ammonia combustion technology and its underlying chemistry. Fulfilling the COP21 Paris Agreement requires the de-carbonization of energy generation, through utilization of carbon-neutral and overall carbon-free fuels produced from renewable sources. Hydrogen is one of such fuels, which is a potential energy carrier for reducing greenhouse-gas emissions. However, its shipment for long distances and storage for long times present challenges. Ammonia on the other hand, comprises 17.8% of hydrogen by mass and can be produced from renewable hydrogen and nitrogen separated from air. Furthermore, thermal properties of ammonia are similar to those of propane in terms of boiling temperature and condensation pressure, making it attractive as a hydrogen and energy carrier. Ammonia has been produced and utilized for the past 100 years as a fertilizer, chemical raw material, and refrigerant. Ammonia can be used as a fuel but there are several challenges in ammonia combustion, such as low flammability, high NOx emission, and low radiation intensity. Overcoming these challenges requires further research into ammonia flame dynamics and chemistry. This paper discusses recent successful applications of ammonia fuel, in gas turbines, co-fired with pulverize coal, and in industrial furnaces. These applications have been implemented under the Japanese ‘Cross-ministerial Strategic Innovation Promotion Program (SIP): Energy Carriers’. In addition, fundamental aspects of ammonia combustion are discussed including characteristics of laminar premixed flames, counterflow twin-flames, and turbulent premixed flames stabilized by a nozzle burner at high pressure. Furthermore, this paper discusses details of the chemistry of ammonia combustion related to NOx production, processes for reducing NOx, and validation of several ammonia oxidation kinetics models. Finally, LES results for a gas-turbine-like swirl-burner are presented, for the purpose of developing low-NOx single-fuelled ammonia gas turbine combustors.

A two-pollutant strategy for improving ozone and particulate air quality in China

Abstract: Fine particulate matter (PM2.5) decreased by 30–40% across China during 2013–2017 in response to the governmental Clean Air Action. However, surface ozone pollution worsened over the same period. Model simulations have suggested that the increase in ozone could be driven by the decrease in PM2.5, because PM2.5 scavenges hydroperoxy (HO2) and NOx radicals that would otherwise produce ozone. Here we show observational evidence for this effect with 2013–2018 summer data of hourly ozone and PM2.5 concentrations from 106 sites in the North China Plain. The observations show suppression of ozone pollution at high PM2.5 concentrations, consistent with a model simulation in which PM2.5 scavenging of HO2 and NOx depresses ozone concentrations by 25 ppb relative to PM2.5-free conditions. PM2.5 chemistry makes ozone pollution less sensitive to NOx emission controls, emphasizing the need for controlling emissions of volatile organic compounds (VOCs), which so far have not decreased in China. The new 2018–2020 Clean Air Action plan calls for a 10% decrease in VOC emissions that should begin to reverse the long-term ozone increase even as PM2.5 continues to decrease. Aggressive reduction of NOx and aromatic VOC emissions should be particularly effective for decreasing both PM2.5 and ozone. Observations confirm that cleaning up fine particulate matter in the North China Plain has exacerbated ozone pollution, suggesting that both NOx and VOC emissions need to be reduced to improve air quality.

Ozone Pollution: A Major Health Hazard Worldwide

Abstract: Oxides of nitrogen (NOx) and volatile organic compounds (VOCs) released into the atmosphere can react in the presence of solar irradiation, leading to ozone formation in the troposphere. Historically, before clean air regulations were implemented to control NOx and VOCs, ozone concentrations were high enough to exert acute effects such as eye and nose irritation, respiratory disease emergencies, and lung function impairment. At or above current regulatory standards, day-to-day variations in ozone concentrations have been positively associated with asthma incidence and daily non-accidental mortality rate. Emerging evidence has shown that both short-term and long-term exposures to ozone, at concentrations below the current regulatory standards, were associated with increased mortality due to respiratory and cardiovascular diseases. The pathophysiology to support the epidemiologic associations between mortality and morbidity and ozone centers at the chemical and toxicological property of ozone as a strong oxidant, being able to induce oxidative damages to cells and the lining fluids of the airways, and immune-inflammatory responses within and beyond the lung. These new findings add substantially to the existing challenges in controlling ozone pollution. For example, in the United States in 2016, 90% of non-compliance to the national ambient air quality standards was due to ozone whereas only 10% was due to particulate matter and other regulated pollutants. Climate change, through creating atmospheric conditions favoring ozone formation, has been and will continue to increase ozone concentrations in many parts of world. Worldwide, ozone is responsible for several hundreds of thousands of premature deaths and tens of millions of asthma-related emergency room visits annually. To combat ozone pollution globally, more aggressive reductions in fossil fuel consumption are needed to cut NOx and VOCs as well as greenhouse gas emissions. Meanwhile, preventive and therapeutic strategies are needed to alleviate the detrimental effects of ozone especially in more susceptible individuals. Interventional trials in humans are needed to evaluate the efficacy of antioxidants and ozone-scavenging compounds that have shown promising results in animal studies.

Exploring 2016–2017 surface ozone pollution over China: source contributions and meteorological influences

Abstract: . Severe surface ozone pollution over major Chinese cities has become an emerging air quality concern, raising a new challenge for emission control measures in China. In this study, we explore the source contributions to surface daily maximum 8 h average (MDA8) ozone over China in 2016 and 2017, the 2 years with the highest surface ozone averaged over Chinese cities in record. We estimate the contributions of anthropogenic, background, and individual natural sources to surface ozone over China using the GEOS-Chem chemical transport model at 0.25 ∘ × 0.3125 ∘ horizontal resolution with the most up-to-date Chinese anthropogenic emission inventory. Model results are evaluated with concurrent surface ozone measurements at 169 cities over China and show generally good agreement. We find that background ozone (defined as ozone that would be present in the absence of all Chinese anthropogenic emissions) accounts for 90 % (49.4 ppbv) of the national March–April mean surface MDA8 ozone over China and 80 % (44.5 ppbv) for May–August. It includes large contributions from natural sources (80 % in March–April and 72 % in May–August). Among them, biogenic volatile organic compound (BVOC) emissions enhance MDA8 ozone by more than 15 ppbv in eastern China during July–August, while lightning NOx emissions and ozone transport from the stratosphere both lead to ozone enhancements of over 20 ppbv in western China during March–April. Over major Chinese city clusters, domestic anthropogenic sources account for about 30 % of the May–August mean surface MDA8 ozone and reach 39–73 ppbv (38 %–69 %) for days with simulated MDA8 ozone > 100 ppbv in the North China Plain, Fenwei Plain, Yangtze River Delta, and Pearl River Delta city clusters. These high ozone episodes are usually associated with high temperatures, which induce large BVOC emissions and enhance ozone chemical production. Our results indicate that there would be no days with MDA8 ozone > 80 ppbv in these major Chinese cities in the absence of domestic anthropogenic emissions. We find that the 2017 ozone increases relative to 2016 are largely due to higher background ozone driven by hotter and drier weather conditions, while changes in domestic anthropogenic emissions alone would have led to ozone decreases in 2017. Meteorological conditions in 2017 favor natural source contributions (particularly soil NOx and BVOC ozone enhancements) and ozone chemical production, increase the thermal decomposition of peroxyacetyl nitrate (PAN), and further decrease ozone dry deposition velocity. More stringent emission control measures are thus required to offset the adverse effects of unfavorable meteorology, such as high temperature, on surface ozone air quality.

Towards the development of an efficient low-NOx ammonia combustor for a micro gas turbine

Abstract: Recent studies have demonstrated stable generation of power from pure ammonia combustion in a micro gas turbine (MGT) with a high combustion efficiency, thus overcoming some of the challenges that discouraged such applications of ammonia in the past. However, achievement of low NOx emission from ammonia combustors remains an important challenge. In this study, combustion techniques and combustor design for efficient combustion and low NOx emission from an ammonia MGT swirl combustor are proposed. The effects of fuel injection angle, combustor inlet temperature, equivalence ratio, and ambient pressure on flame stabilization and emissions were investigated in a laboratory high pressure combustion chamber. An FTIR gas analyser was employed in analysing the exhaust gases. Numerical modeling using OpenFOAM was done to better understand the dependence of NO emissions on the equivalence ratio. The result show that inclined fuel injection as opposed to vertical injection along the combustor central axis resulted to improved flame stability, and lower NH3 and NOx emissions. Numerical and experimental results showed that a control of the equivalence ratio upstream of the combustor is critical for low NOx emission in a rich-lean ammonia combustor. NO emission had a minimum value at an upstream equivalence ratio of 1.10 in the experiments. Furthermore, NO emission was found to decrease with ambient pressure, especially for premixed combustion. For the rich-lean combustion strategy employed in this study, lower NOx emission was recorded in premixed combustion than in non-premixed combustion indicating the importance of mixture uniformity for low NOx emission from ammonia combustion. A prototype liner developed to enhance the control and uniformity of the equivalence ratio upstream of the combustor further improved ammonia combustion. With the proposed liner design, NOx emission of 42 ppmv and ammonia combustion efficiency of 99.5% were achieved at 0.3 MPa for fuel input power of 31.44 kW.

SO2-Tolerant Selective Catalytic Reduction of NOx over Meso-TiO2@Fe2O3@Al2O3 Metal-Based Monolith Catalysts

Abstract: It is an intractable issue to improve the low-temperature SO2-tolerant selective catalytic reduction (SCR) of NOx with NH3 because deposited sulfates are difficult to decompose below 300 °C. Herein, we established a low-temperature self-prevention mechanism of mesoporous-TiO2@Fe2O3 core–shell composites against sulfate deposition using experiments and density functional theory. The mesoporous TiO2-shell effectively restrained the deposition of FeSO4 and NH4HSO4 because of weak SO2 adsorption and promoted NH4HSO4 decomposition on the mesoporous-TiO2. The electron transfer at the Fe2O3 (core)-TiO2 (shell) interface accelerated the redox cycle, launching the “Fast SCR” reaction, which broadened the low-temperature window. Engineered from the nano- to macro-scale, we achieved one-pot self-installation of mesoporous-TiO2@Fe2O3 composites on the self-tailored AlOOH@Al-mesh monoliths. After the thermal treatment, the mesoporous-TiO2@Fe2O3@Al2O3 monolith catalyst delivered a broad window of 220–420 °C with NO con...

Vehicle emissions trapping materials: Successes, challenges, and the path forward

Abstract: The modern three-way catalyst (TWC) is very effective for treating the hydrocarbons (HCs), carbon monoxide (CO), and nitrogen oxides (NOx) from stoichiometric gasoline engines once the TWC has achieved its minimum operating temperature (e.g., 250 to 400 °C, depending on the gas species). Likewise, the diesel oxidation catalyst (DOC), selective catalytic reduction (SCR) catalyst with urea injection, and the diesel particulate filter (DPF) are effective for treating the HCs, CO, NOx, and particulate matter (PM) emissions from diesel engines once the catalysts are warmed up, although this can require a significant length of time (e.g., 1 to 3 min) because of the relatively low exhaust temperatures from diesel engines. For both types of engines, excess fueling is often used to accelerate the heating of the catalyst system after a cold start, although this decreases the fuel economy of the vehicle. Even with excess fueling, a high portion (up to 80%) of the total vehicle emissions is emitted during the cold start period (i.e., the period before the catalysts are functional). To treat the HC emissions during this cold start period, one approach is to employ a HC trap (HCT) that can adsorb the HC emissions at low temperatures and then oxidize the stored HCs to carbon dioxide (CO2) and water (H2O) at higher temperatures. To treat the NOx emissions during the cold start period, a passive NOx adsorber (PNA) can adsorb the NOx at low temperatures. For stoichiometric gasoline applications, the PNA can then reduce the stored NOx to nitrogen (N2) at higher temperatures. On diesel engines, the PNA can release the stored NOx back into the exhaust once the downstream urea/SCR system is operational. Some adsorber technologies have the capability of adsorbing HCs and NOx simultaneously. In this review, the HC trapping and passive NOx adsorbing technologies will be discussed in separate sections. This review will describe how the current trapping technologies can be applied in vehicle exhaust systems, the material properties required for efficient HCTs and PNAs, and the exhaust conditions that can inhibit/enhance their trapping properties. First, the performance of HCTs will be discussed in terms of their physical properties (e.g., pore size, acidity, presence of metal ions) and the trapping conditions (e.g., storage temperature, space velocity, and the presence of other exhaust species such as H2O and CO2). This will be followed by in-depth coverage of the reactions occurring during HC desorption. The second part of this review will focus on the composition of various PNA formulations, the effects of the trapping conditions (e.g., temperature, space velocity, the presence of other exhaust species such as CO2, H2O, CO, and C2H4), and the effects of sulfur poisoning on their trapping performance. The effect of hydrothermal aging and the regenerability of HCTs and PNAs will also be discussed. A significant amount of literature has emerged recently regarding HCTs and PNAs; this review is primarily focused on summarizing this literature and reconciling the differences presented.

Engine combustion and emission fuelled with natural gas: A review

Abstract: Natural gas (NG) is one of the most important and successful alternative fuels for vehicles. Engine combustion and emission fuelled with natural gas have been reviewed by NG/gasoline bi-fuel engine, pure NG engine, NG/diesel dual fuel engine and HCNG engine. Compared to using gasoline, bi-fuel engine using NG exhibits higher thermal efficiency; produces lower HC, CO and PM emissions and higher NOx emission. The bi-fuel mode can not fully exert the advantages of NG. Optimization of structure design for engine chamber, injection parameters including injection timing, injection pressure and multi injection, and lean burn provides a technological route to achieve high efficiency, low emissions and balance between HC and NOx. Compared to diesel, NG/diesel dual fuel engine exhibits longer ignition delay; has lower thermal efficiency at low and partial loads and higher at medium and high loads; emits higher HC and CO emissions and lower PM and NOx emissions. The addition of hydrogen can further improve the thermal efficiency and decrease the HC, CO and PM emissions of NG engine, while significantly increase the NOx emission. In each mode, methane is the major composition of THC emission and it has great warming potential. Methane emission can be decreased by hydrogen addition and after-treatment technology.

A review on low temperature combustion engines: Performance, combustion and emission characteristics

Abstract: Low temperature combustion (LTC) is a recent engine technology that can reduce the oxides of nitrogen (NOx) and soot emissions simultaneously while maintaining higher thermal efficiency. The present review work investigates the performance, emission and combustion characteristics of LTC mode engines. Partially premixed LTC (PPLTC), homogeneous charge compression ignition (HCCI), premixed charge compression ignition (PCCI) and reactivity controlled compression ignition (RCCI) modes are researched under LTC mode. In recent decades, different engine strategies have been employed to reduce exhaust emissions and to enhance thermal efficiency. Exhaust gas recirculation, variable valve timing (VVT), advanced fuel injection technologies are adapted to achieve LTC mode in internal combustion (IC) engines to get improved outcomes. This review highlights the properties of fuels, fuel supply systems, valve actuation mechanisms, engine operating conditions and its effects on the engine characteristics. This review provides a perspective plan to the researchers for enhancing the performance, emission and combustion behavior of an engine by using LTC mode with lower NOx and soot emissions. Among LTC mode engines, RCCI mode engine operates well in 60% load, 60% premixed ratio, 35:1 air-fuel ratio and 56% brake thermal efficiency within the combustion phasing control.

A comprehensive study on emission and performance characteristics of a diesel engine fueled with nanoparticle-blended biodiesel

Abstract: The present work is aimed to analyze the performance and emission characteristics of mahua biodiesel-fueled diesel engine with copper oxide nanoparticle at various particle sizes (10 and 20 nm) and the results compared with conventional diesel fuel (BD). Experiments were conducted in a four-stroke, single-cylinder, constant speed, and naturally aspirated research diesel engine with an eddy current dynamometer. Conventional transesterification process is carried out to convert the raw mahua oil into mahua oil biodiesel (BD100). Copper oxide (CuO) was chosen as a nanoparticle; the mass fraction of 100 ppm and the particle sizes of 10 and 20 nm were blended with mahua oil methyl ester using an ultrasonicator, and the physicochemical properties were measured. The physicochemical properties of BD100 and nanoparticle-included BD100 are at par with EN14214 limits. Brake-specific fuel consumption (BSFC) of BD100 is higher than that of diesel, and brake thermal efficiency (BTE) is lower than that of diesel (D100). The inclusion of 10-nm particle size of CuO nanoparticle in BD100 improves the BSFC and BTE by 1.3 and 0.7%, respectively, when compared with BD100. The CuO nanoparticle inclusion at 20-nm nanoparticle in biodiesel further improves the performance parameters than those at 10-nm nanoparticle. Further, the BD100 promotes a lower level of smoke emissions, carbon monoxide (CO), and hydrocarbon (HC) and with a prominent increase in oxides of nitrogen (NOx) emissions. The inclusion of 10-nm particle size of CuO nanoparticle in BD100 reduces the NOx, HC, CO, and smoke emission by 3.9, 5.6, 4.9, and 2.8%, respectively, at peak load condition when compared with BD100. The addition of CuO nanoparticle at 20-nm nanoparticle in biodiesel further reduces the NOx, HC, CO, and smoke emissions.

Daytime atmospheric oxidation capacity in four Chinese megacities during the photochemically polluted season: a case study based on box model simulation

Abstract: . Atmospheric oxidation capacity is the basis for converting freshly emitted substances into secondary products and is dominated by reactions involving hydroxyl radicals (OH) during daytime. In this study, we present in situ measurements of ROx radical (hydroxy OH, hydroperoxy HO2 , and organic peroxy RO2 ) precursors and products; the measurements are carried out in four Chinese megacities (Beijing, Shanghai, Guangzhou, and Chongqing) during photochemically polluted seasons. The atmospheric oxidation capacity is evaluated using an observation-based model and radical chemistry precursor measurements as input. The radical budget analysis illustrates the importance of HONO and HCHO photolysis, which account for ∼50 % of the total primary radical sources. The radical propagation is efficient due to abundant NO in urban environments. Hence, the production rate of secondary pollutants, that is, ozone (and fine-particle precursors ( H2SO4 , HNO3 , and extremely low volatility organic compounds, ELVOCs) is rapid, resulting in secondary air pollution. The ozone budget demonstrates its high production in urban areas; also, its rapid transport to downwind areas results in rapid increase in local ozone concentrations. The O3 – NOx –VOC (volatile organic compound) sensitivity tests show that ozone production is VOC-limited and that alkenes and aromatics should be mitigated first for ozone pollution control in the four studied megacities. In contrast, NOx emission control (that is, a decrease in NOx ) leads to more severe ozone pollution. With respect to fine-particle pollution, the role of the HNO3 – NO3 partitioning system is investigated using a thermal dynamic model (ISORROPIA 2). Under high relative humidity (RH) and ammonia-rich conditions, nitric acid converts into nitrates. This study highlights the efficient radical chemistry that maintains the atmospheric oxidation capacity in Chinese megacities and results in secondary pollution characterized by ozone and fine particles.

Experimental investigation of combustion, performance and emission characteristics of a diesel engine fuelled with diesel–biodiesel–alcohol blends

Abstract: The purpose of this study is to investigate the impacts of diesel–biodiesel–alcohol blends on the combustion, performance and emissions characteristics of a single-cylinder diesel engine. Tests were conducted at different engine speeds of 1750, 2250, 2750 and 3250 rpm and under full load. In this study, different fuels [called as reference diesel (D100), 20 vol% cottonseed methyl ester (D80C20), 10 vol% ethanol (D90E10) and finally the ternary type of their derivations (D70C20E10)], were used. The experimental results showed that the highest reduction values were observed on CO emission by 42%, 30% and 8% for the D90E10, D70C20E10 and D80C20 fuels, respectively. These reductions for HC emission were achieved as 40%, 31% and 23% for the D90E10, D70C20E10 and D80C20, respectively. On the other hand, the reductions of NOx and CO2 emissions were not sharp and varied between 2–7%. Besides the reductions on the exhaust emissions, biodiesel–ethanol blend presented better results in terms of HRRmax and CPmax than using biodiesel alone. Additionally, ignition delay of the biodiesel blends was longer than that of D100 fuel owing to their low cetane numbers. Combustion duration was shortened with the increment in engine speed because the turbulence increased in the combustion chamber at high engine speed. This case also improved the homogeneity of test fuels and increased the quality of the combustion process. As a consequence, this paper clearly reported that it is possible to achieve fewer emissions, the highest CPmax values with the presence of ethanol in biodiesel fuels rather than using biodiesel alone for diesel engines.

Direct observation of changing NO x lifetime in North American cities.

Abstract: NOx lifetime relates nonlinearly to its own concentration; therefore, by observing how NOx lifetime changes with changes in its concentration, inferences can be made about the dominant chemistry occurring in an urban plume. We used satellite observations of NO2 from a new high-resolution product to show that NOx lifetime in approximately 30 North American cities has changed between 2005 and 2014 in a manner consistent with our understanding of NOx chemistry.