NOx Emission Control Research Articles

Identifying Driving Factors of Atmospheric N2O5 with Machine Learning.

Dinitrogen pentoxide (N2O5) plays an essential role in tropospheric chemistry, serving as a nocturnal reservoir of reactive nitrogen and significantly promoting nitrate formations. However, identifying key environmental drivers of N2O5 formation remains challenging using traditional statistical methods, impeding effective emission control measures to mitigate NOx-induced air pollution. Here, we adopted machine learning assisted by steady-state analysis to elucidate the driving factors of N2O5 before and during the 2022 Winter Olympics (WO) in Beijing. Higher N2O5 concentrations were observed during the WO period compared to the Pre-Winter-Olympics (Pre-WO) period. The machine learning model accurately reproduced ambient N2O5 concentrations and showed that ozone (O3), nitrogen dioxide (NO2), and relative humidity (RH) were the most important driving factors of N2O5. Compared to the Pre-WO period, the variation in trace gases (i.e., NO2 and O3) along with the reduced N2O5 uptake coefficient was the main reason for higher N2O5 levels during the WO period. By predicting N2O5 under various control scenarios of NOx and calculating the nitrate formation potential from N2O5 uptake, we found that the progressive reduction of nitrogen oxides initially increases the nitrate formation potential before further decreasing it. The threshold of NOx was approximately 13 ppbv, below which NOx reduction effectively reduced the level of night-time nitrate formations. These results demonstrate the capacity of machine learning to provide insights into understanding atmospheric nitrogen chemistry and highlight the necessity of more stringent emission control of NOx to mitigate haze pollution.

Experimental investigation of NOx emission control using carbon nanotube additives and EGR configuration on CRDI engine fueled with ternary fuel

This study aims to investigate the impact of the combined effects of carbon nanotubes and exhaust gas recirculation (EGR) of a common rail direct injection (CRDI) engine fueled with ternary fuels. Three rates of EGR (0%, 10%, and 20%) were used at a standard injection timing of 23° before Top Dead Centre (bTDC). This study involves substituting diesel with biofuels, reducing oxides of nitrogen and soot emissions by 30–40%. The experimental investigation involves the exploration of combustion, exhaust emissions, and performance parameters for a single-cylinder CRDI engine using ternary fuels, which consist of diesel, biodiesel, and bioethanol by volume %. At full load with 20% EGR, the experimental analysis indicated a 2% decrease in BTE, a 3.33% increase in BSFC, and a 3.16% increase in BSEC across the blends in performance. In terms of combustion analysis, results show a 5.25 bar in-cylinder pressure drop, 4.14% reduction in HRR, and 12.62% drop in MGT. However, in terms of emissions, 20% EGR yielded the most favorable results, showing a 62.94% reduction in NOx emissions across all blends, albeit accompanied by a 13.89% increase in hydrocarbons (HCs) and 5% rise in carbon monoxide at full load. The 20% EGR rate effectively minimized NOx emissions while slightly impacting HC and CO emissions.

Economic and carbon emission analysis of two typical denitrification systems for sintering process in the iron and steel industry

The transition of the Chinese iron and steel industry to ultralow emissions has accelerated the development of denitrification technologies. Considering the existing dual carbon targets, carbon emissions must be considered as a critical indicator when comparing denitrification systems. Consequently, this study provided a comprehensive cost-benefit model for denitrification in the steel industry, encompassing additional carbon emissions resulting from the implementation of denitrification systems. Activated-carbon adsorption and selective catalytic reduction (SCR) systems are two efficient techniques for controlling NOx emissions during sintering. Based on this model, a cost-benefit analysis of these two typical systems was conducted, and the results indicated that the unit flue-gas abatement costs of SCR and activated-carbon adsorption systems were 0.00275 and 0.0126 CNY/m3, and the unit flue-gas abatement benefits were 0.0072 and 0.0179 CNY/m3, respectively. Additionally, the effect of operational characteristics on operating costs, including duration and material prices, was analyzed. When treating the flue gas, the two systems released 0.0020 and 0.0060 kg/m3 of carbon dioxide, respectively. The primary sources of carbon emissions from the SCR and activated-carbon adsorption systems are the production of reducing agents and system operations, respectively. Furthermore, considering the features of the activated carbon adsorption system for simultaneous desulfurization, a SCR- wet flue gas desulfurization (WFGD) technology route was developed for comparison with the activated carbon adsorption system.

Modulating the Microstructure and Surface Acidity of MnO2 by Doping-Induced Phase Transition for Simultaneous Removal of Toluene and NOx at Low Temperature.

It is a great challenge to remove VOCs and NOx simultaneously from flue gas in nonelectric industries. This study focuses on the construction of Fe-MnO2 catalysts that perform well in the simultaneous removal of toluene and NOx at low temperatures. Utilizing the Fe-induced phase transition of MnO2, Fe-MnO2-F&R catalysts with a composite morphology of nanoflowers and nanorods were successfully prepared that provided an abundant microporous structure to facilitate the diffusion of molecules of different sizes. Through in-depth investigation of the active sites and reaction mechanism, we discovered that Fe-induced phase transition could modulate the surface acidity of Fe-MnO2-F&R. The higher concentration of surface Mn4+ provided numerous Brønsted acid sites, which effectively promoted the activation of toluene to reactive intermediates, such as benzyl alcohol/benzoate/maleic acid. Simultaneously, Fe provided a large number of Lewis acid sites that anchor and activate NH3 species, thereby inhibiting NH3 nonselective oxidation. Furthermore, additional Brønsted acid sites were generated during the simultaneous reaction process, enhancing toluene activation. Consequently, the simultaneous removal of toluene and NOx was achieved through regulation of the physical structure and the concentration of acidic sites. The present work provides new insights into the rational design of bifunctional catalysts for the synergistic control of VOCs and NOx emissions.

Spatially and Temporally Differentiated NOx and VOCs Emission Abatement Could Effectively Gain O3-Related Health Benefits.

The increasing level of O3 pollution in China significantly exacerbates the long-term O3 health damage, and an optimized health-oriented strategy for NOx and VOCs emission abatement is needed. Here, we developed an integrated evaluation and optimization system for the O3 control strategy by merging a response surface model for the O3-related mortality and an optimization module. Applying this system to the Yangtze River Delta (YRD), we evaluated driving factors for mortality changes from 2013 to 2017, quantified spatial and temporal O3-related mortality responses to precursor emission abatement, and optimized a health-oriented control strategy. Results indicate that insufficient NOx emission abatement combined with deficient VOCs control from 2013 to 2017 aggravated O3-related mortality, particularly during spring and autumn. Northern YRD should promote VOCs control due to higher VOC-limited characteristics, whereas fastening NOx emission abatement is more favorable in southern YRD. Moreover, promotion of NOx mitigation in late spring and summer and facilitating VOCs control in spring and autumn could further reduce O3-related mortality by nearly 10% compared to the control strategy without seasonal differences. These findings highlight that a spatially and temporally differentiated NOx and VOCs emission control strategy could gain more O3-related health benefits, offering valuable insights to regions with severe ozone pollution all over the world.

Evolution of ozone-precursor sensitivity in the Guangdong-Hong Kong-Macao Greater Bay Area of China under the backdrop of climate change

China is facing the dual challenges of worsening ozone (O3) pollution and climate change. Better understanding the cause of O3 pollution and its response to climate change is crucial for developing an effective O3 control strategy in the coming years. In this study, a localized three-dimensional numerical modeling system was applied to investigate the long-term evolution of ozone-precursor sensitivity (OPS) over the Guangdong-Hong Kong-Macao Greater Bay Area (GBA) during September 2030, 2040 and 2050 under different climate scenarios, including four Representative Concentration Pathway (RCP), the Nationally Determined Contribution (NDC), and carbon neutrality (NEU). RCP emissions were corrected both temporally and spatially by bounding with the actual emissions in 2020 to reduce the uncertainty of future emission estimation. The atmospheric oxidative capacity-based HO2/OH indicator was used to quantify the variations of OPS. The percentage of NOx-limited area over the GBA will increase by 9.8%, 14.8% and 16.9% under RCP2.6, RCP4.5 and RCP8.5 in 2050, comparing with those in 2020. NOx-limited area under NEU showed a greater increase by 23.1%, as a result of larger reduction degree of NOx than VOCs. This highlights that deep NOx emission reduction, mostly driven by the structural adjustments in industry, energy, and transportation under the “2060 carbon neutrality” strategy, will accelerate OPS transition into NOx-limited and ensure the gradual improvement of O3 pollution. NOx emission control in some urban areas still in the VOCs-limited OPS in 2050 under NEU, e.g., Guangzhou and Hong Kong, need to be strengthened to speed up O3 mitigation. The findings of this study would improve our understanding on the long-term trends of OPS in the GBA under the backdrop of climate change, and provide guidance for the synergistic reduction of greenhouse gases and air pollutants in the future.

A Decadal Change in Atmospheric Nitrogen Deposition at a Rural Site in Southern China

Elevated atmospheric reactive nitrogen (Nr) emissions and the subsequent nitrogen (N) deposition have negatively impacted the global environment, particularly in China. In order to assess the long-term trends in atmospheric N deposition in the south of China, Taojiang County in Hunan Province was selected as a representative rural area for study. We analyzed interannual variation in atmospheric Nr, including gaseous ammonia (NH3), nitrogen dioxide (NO2), nitrate acid (HNO3) vapor, particulate ammonium (NH4+), and nitrate (NO3−) in air and NH4+-N and NO3−-N in precipitation from 2011 to 2020. The 10-year average atmospheric wet-plus-dry N deposition was 41.9 kg N ha−1 yr−1, which decreased by approximately 24% after 2012, indicating that NH3 and NOx emissions were effectively reduced by emission controls introduced in 2013. Wet deposition accounted for approximately 74% of the total N deposition and was significantly influenced by annual precipitation amount. Reduced N (NH3, pNH4+, and NH4+ in rainwater) was the dominant form, comprising approximately 58% of the total N deposition, while oxidized N (pNO3−, NO2, HNO3, and NO3− in rainwater) accounted for 42% of the total N deposition. Atmospheric HNO3, NO2, and NH3 concentrations and deposition declined by 30–80% over the decade, while particulate NH4+ and NO3− concentrations and deposition remained at relatively stable levels, which suggests that ongoing research and policy should focus on rural particulate pollution. Future strategies must concentrate on the integrated control of NH3 and NOx emissions to mitigate air pollution and protect human health, particularly in rural areas because current abatement efforts are primarily directed toward urban areas and the industrial sector, whereas non-point source NH3 pollution, influenced mainly by agricultural activities, dominates in rural regions.

Simultaneous measurement of NH3 and NO by mid-infrared tunable diode laser absorption spectroscopy based on machine-learning algorithms

Accurate measurements of NH3 and NO are essential for controlling NOx emissions from coal-fired power plants, reducing ammonia slip, and improving the efficiency of selective catalytic reduction (SCR) operation. We propose a method for measuring NH3 and NO concentrations based on machine learning algorithms to address the challenges posed by overlapping interference of CO2 and H2O spectral lines in flue gas. Our approach introduces a novel method for acquiring mixture spectra for the model. Initially, we measure the spectra of individual components under different concentrations and temperatures. Subsequently, mixed spectral samples are generated by combining the measured spectra of the individual components. This approach simplifies the spectral measurement process while preserving accuracy. The particle swarm optimization support vector machine (PSO-SVM) algorithm is leveraged, providing a reliable foundation for the continuous and synchronous measurement of NH3 and NO. Upon testing, the PSO-SVM demonstrates average relative errors of 4.0 % and 0.8 % for NH3 and NO concentrations, respectively. The corresponding measurement precision is 0.05 ppm for NH3 and 0.42 ppm for NO, better than the conventional integral absorbance method. The minimum detection limit (MDL) for NH3 is 16.1 ppb at an average time of 50 s, while for NO, it is 38.4 ppb at an average time of 71 s. The methodology of this paper is expected to play an important role in reducing the influence of interfering components and improving the accuracy of field measurements.

Mechanisms for NOx emission control and ash deposition mitigation in sludge-coal blend combustion

This paper introduces the coupled treatment of alkali and alkaline earth metals (AAEMs)-rich coal with ferric sludge as a method to efficiently and economically control sludge NOx emissions and mitigate coal ash deposition through fuel interactions. We utilized a pilot-scale drop tube furnace to quantify NOx emissions and ash deposition, supported by spectral and energy dispersive analyses. The results demonstrated co-combustion controls NOx emissions by catalyzing the reaction between char and NO and the transformation into more stable N-containing functional groups. It also facilitates the formation of harmless N2 instead of NOx by enabling Ca/Fe-based minerals to react with protein and pyrrolic nitrogen. At 950 °C, blending 10% ferric sludge mitigated ash deposition, thanks to the capture and dilution effect of Si/Al/Fe-based minerals in ferric sludge. However, blending ferric sludge aggravated ash deposition of AAEMs-rich coal at 1250 °C, due to the formation of eutectics by Fe-based minerals and Ca/Na-based feldspars. Besides, the control of NOx was reduced at 1250 °C. Therefore, the co-combustion of AAEMs-rich coal with 10% ferric sludge at 950 °C exhibited excellent performance in both emission control and ash deposition mitigation. This study, grounded in the characteristics of minerals, provides a theoretical foundation for the efficient and economical in-situ control of NOx emissions and ash deposition. It advances the development of low-carbon, environmentally friendly technologies for coupled sludge and coal combustion.

Combined control of PM and NOx emissions by corona discharge

This work investigates the potential of corona discharge to control particulate matter and NOx emissions from small-scale biomass combustion. The electrostatic precipitator (ESP) was designed and used to suppress emissions from a 15-kW heating unit. The ESP was operated at different power modes at both polarities and demonstrated the capability to reduce NOx emissions with a removal efficiency of 78 % while PM concentration was reduced to a magnitude of particle content in ambient air. The efficiency of ESP was evaluated considering the technological parameters, namely the reduced electric field, Nt-product, and specific input energy. The growth in NOx abatement was observed when the energy input exceeded 1 J/L for both polarities and Nt-product exceeded 4.5 × 108 s/cm3/1 × 109 s/cm3 for negative/positive corona. The results of this work may contribute to understanding the processes in corona discharge and optimising corona discharge systems for various applications.

Impacts of global biogenic isoprene emissions on surface ozone during 2000–2019

Biogenic isoprene is an important precursor of tropospheric ozone (O3). Here, a coupled chemistry–vegetation model was used to quantify the contributions of isoprene emissions to surface O3 pollution on the global scale during 2000–2019. The biogenic isoprene emissions showed high values in mid–low latitudes and seasonal peaks in the summer hemispheres. They promote global surface O3 concentrations by 1.75 ppbv annually with regional hotspots of 4.39 ppbv (8.8%) in China and 5.36 ppbv (11.1%) in the U.S. in boreal summer. In the past two decades, isoprene emissions increased by 1.32 TgC yr−1 (0.67% yr−1) in the Northern Hemisphere but decreased by 0.71 TgC yr−1 (0.44% yr−1) in the Southern Hemisphere. Such changes of isoprene made opposite contributions to the surface O3 trend, with 0.26 ppbv yr−1 in eastern China but −0.32 ppbv yr−1 in the southeastern U.S. due to the changes in the background regime of chemical reactions. The impact of anthropogenic changes on the O3 trend is consistent with that of biogenic isoprene, but two to four times stronger in magnitude. This study revealed that the effective control of anthropogenic NOx emissions could mitigate regional O3 pollution even with the increased isoprene emissions under global warming.

Increase in Agricultural-Derived NHx and Decrease in Coal Combustion-Derived NOx Result in Atmospheric Particulate N-NH4+ Surpassing N-NO3- in the South China Sea.

The atmospheric deposition of anthropogenic active nitrogen significantly influences marine primary productivity and contributes to eutrophication. The form of nitrogen deposition has been evolving annually, alongside changes in human activities. A disparity arises between observation results and simulation conclusions due to the limited field observation and research in the ocean. To address this gap, our study undertook three field cruises in the South China Sea in 2021, the largest marginal sea of China. The objective was to investigate the latest atmospheric particulate inorganic nitrogen deposition pattern and changes in nitrogen sources, employing nitrogen-stable isotopes of nitrate (δ15N-NO3-) and ammonia (δ15N-NH4+) linked to a mixing model. The findings reveal that the N-NH4+ deposition generally surpasses N-NO3- deposition, attributed to a decline in the level of NOx emission from coal combustion and an upswing in the level of NHx emission from agricultural sources. The disparity in deposition between N-NH4+ and N-NO3- intensifies from the coast to the offshore, establishing N-NH4+ as the primary contributor to oceanic nitrogen deposition, particularly in ocean background regions. Fertilizer (33 ± 21%) and livestock (20 ± 6%) emerge as the primary sources of N-NH4+. While coal combustion continues to be a significant contributor to marine atmospheric N-NO3-, its proportion has diminished to 22 (Northern Coast)-35% (background area) due to effective NOx emission controls by the countries surrounding the South China Sea, especially the Chinese Government. As coal combustion's contribution dwindles, the significance of vessel and marine biogenic emissions grows. The daytime higher atmospheric N-NO3- concentration and lower δ15N-NO3- compared with nighttime further underscore the substantial role of marine biogenic emissions.

Simulation study on nitrogen transformation characteristics of NH3/Coal Co-firing under deeply air-staged condition

Carbon emissions have become a prominent global concern, with the utilization of fossil fuels serving as the primary contributor. Ammonia, being a hydrogen carrier without carbon, has the potential to be utilized through combustion, resulting in the production of water and nitrogen as ideal byproducts, which are environmentally benign. However, individual ammonia is difficult to combust due to its unfavorable combustion characteristics. Hence, ammonia needs to be blended with other fuels that possess superior combustion characteristics. Additionally, NH3 inherently contains a significant amount of nitrogen, a fuel element that can contribute to the increased release of NOx emissions. In this work, a simulation was conducted using CHEMKIN to investigate the nitrogen transformation characteristics during ammonia/coal co-firing under deeply air-staged combustion. Experimental studies primarily focus on macroscopic factors, such as temperature and pressure, while often neglecting the analysis of reaction mechanisms. The utilization of the chemical reaction kinetics simulation method can provide valuable insights into the reactions and substances that exert the most significant impact on the formation of NOx, as well as elucidate the pathway of fuel-nitrogen conversion during combustion. Simulation results demonstrate that as the NH3 co-firing ratio (CR-NH3) increases, the volume content of NOx initially rises, reaching its peak at approximately 10% CR-NH3, and subsequently declines. The conversion rate of NOx increases as the combustion temperature in the primary combustion area rises. The primary reactions have a greater influence on the formation of NO at low-temperature conditions. However, it is important to note that low temperature also decreases the combustion efficiency. The incorporation of overfire air (OFA) in advance has the potential to mitigate nitrogen reduction in the primary combustion area, while simultaneously improving NOx emissions. With the oxygen concentration of primary air increased, the conversion rate of NOx gradually decreases. It undergoes a sharp change before reaching 21% oxygen concentration, after which it levels off. In addition, high oxygen concentration increases the pathway of NO consumption. When the excess air coefficient of OFA falls within the range of 0.35–0.45, the NOx conversion rate can be minimized. The present study aims to offer technical assistance and guidance for effectively controlling NOx emissions in ammonia/coal co-firing boilers.

Multi-objective optimization of Fe-based SCR catalyst on the NOx conversion efficiency for a diesel engine based on FGRA-ANN/RF

With the development of industrial level and the increasingly strict emission regulations, selective catalytic reduction (SCR) system is one of the important ways to control NOx emissions from diesel engines. In this study, the effects of intake pressure on pressure and NOx conversion efficiency before and after catalyst were investigated at different loads. In order to improve the SCR efficiency, a multi-objective prediction method based on FGRA-ANN/RF was developed for SCR systems, where the predicted output parameters (the front and back section temperatures of the catalyst, the pressure difference, and the instantaneous catalytic efficiency) and the decision variables (the engine operating parameters). Firstly, the sensitivity analysis of the multi-objective inputs obtained from the ETC condition experiments is performed by the fuzzy grey relation method (FGRA) to select the high sensitivity parameters. Then, the prediction was carried out by artificial neural network (ANN) as well as the improved random forest (RF). The experimental results showed that the evaluation index R2 of RF was generally greater than 0.92 and the improved random forest method had high accuracy and robustness. It is valuable for solving SCR industrial simulation and reducing the hysteresis of ammonia injection response.

The combination poisoning effect of KCl and ZnCl2 on V2O5-WO3/TiO2deNOx catalyst

V2O5-WO3/TiO2 catalyst was used to NH3-SCR technology to control NOx emission from the waste incineration plants. While the flue gas always contains toxic substances, which resulting in the deactivation of catalysts. Most studies only reported the toxic effect of one single component. Actually, the flue gas contains multiple toxic components. Hence, the effect of combination poisoning of KCl and ZnCl2 on the activity and physicochemical properties of V2O5-WO3/TiO2 catalyst were analyzed to reveal the structure-activity relationship of the catalyst. The activity results showed that the co-existence of KCl and ZnCl2 would aggravate the toxic effect. The characterization results showed that K+, Zn2+, and Cl– would bond with V-OH and OV active sites, thereby destroying active acid sites (V-OH) and V5+⇄V4+ redox cycle. In which, KCl had a greater negative effect on active acid sites (V-OH), while ZnCl2 had a greater negative effect on the redox sites (OV). The active acid sites and redox sites on the surface of V2O5-WO3/TiO2 catalyst were further destroyed when KCl and ZnCl2 was co-existence. Moreover, the co-existence of KCl and ZnCl2 would promote the formation of stable and low-activity nitrates and nitro species, thereby inhibiting NOx participation in NH3-SCR reaction. Eley-Rideal (E-R) mechanism exists on V2O5-WO3/TiO2 catalyst, and the combination poisoning of KCl and ZnCl2 does not change the reaction mechanism.

Optimization of Injection Source Settings for SNCR Numerical Simulation of Low-Water Content Biomass Boilers with Blending.

In order to control NOx emissions and meet China's ultralow emission standards, a numerical simulation based on the computational fluid dynamics (CFD) approach is performed for the optimization of the reductant injection volume, number of injection sources, distribution, and injection direction for the flue gas denitrification process of a circulating fluidized bed boiler (CFB) blended with low-water content biomass in a 168 MW unit of a thermal power plant. Using the target power plant boiler entity as a template, a simplified geometric model is established, 1:1, and the mass fractions of each flue gas component set by the inlet boundary conditions are O22, H2O11.6, CO216.2%, and NO0.05%(about 134 ppm), and the reduction reactions under different optimized conditions are numerically simulated using the SNCR model in ANSYS Fluent 2021 R1. The simulation results under each condition were analyzed. The results show that the optimal ammonia-to-nitrogen ratio should be taken as NSR = 1.25, the denitrification efficiencies of 81.00, 81.63, and 82.74% at the three outlets are high, and the ammonia escapes of 1.76, 2.08, and 9.42 mg/s are within a reasonable range; increasing the number of injection sources can significantly reduce the disturbance of the flue gas flow field by reductant injection; the direction of injection is parallel to the direction of the flue gas flow, and the line of the injection source is orthogonal to the direction of the flue gas flow, which is conducive to the mixing of the reductant and flue gas; the optimized boiler denitrification efficiency reaches 74.2%, meeting the ultralow emission requirements of nitrogen oxides and ammonia escape.

Overview of mechanisms of Fe-based catalysts for the selective catalytic reduction of NOx with NH3 at low temperature.

With the increasingly stringent control of NOx emissions, NH3-SCR, one of the most effective de-NOx technologies for removing NOx, has been widely employed to eliminate NOx from automobile exhaust and industrial production. Researchers have favored iron-based catalysts for their low cost, high activity, and excellent de-NOx performance. This paper takes a new perspective to review the research progress of iron-based catalysts. The influence of the chemical form of single iron-based catalysts on their performance was investigated. In the section on composite iron-based catalysts, detailed reviews were conducted on the effects of synergistic interactions between iron and other elements on catalytic performance. Regarding loaded iron-based catalysts, the catalytic performance of iron-based catalysts on different carriers was systematically examined. In the section on iron-based catalysts with novel structures, the effects of the morphology and crystallinity of nanomaterials on catalytic performance were analyzed. Additionally, the reaction mechanism and poisoning mechanism of iron-based catalysts were elucidated. In conclusion, the paper delved into the prospects and future directions of iron-based catalysts, aiming to provide ideas for the development of iron-based catalysts with better application prospects. The comprehensive review underscores the significance of iron-based catalysts in the realm of de-NOx technologies, shedding light on their diverse forms and applications. The hope is that this paper will serve as a valuable resource, guiding future endeavors in the development of advanced iron-based catalysts.

Study on the Emission Characteristics of Typical City Buses under Actual Road Conditions

Five typical in-use city buses in Tangshan with different emission standards, fuel power types, and emission control technology routes were selected as the research objects. PEMSs (Portable Emissions Measurement Systems) and OBD (On-Board Diagnostic) remote monitoring technologies were used to conduct research on actual road conditions and emission characteristics. The research aimed to analyze the driving characteristics of different power types of bus engines and after-treatment systems, as well as their impact on emissions. The results indicated that the actual road driving is mainly in the urban and suburban areas of the local region. Additionally, the VSP (Vehicle Specific Power) was found to be distributed in the low- and medium-speed range of mode Bin0~Bin24. The emission rates of each pollutant showed an increasing trend with the increase in VSP. THC and NOx emissions of CNG and LNG buses are significantly higher than those of gas–electric hybrid buses. However, the PM emissions are lower. Therefore, it is necessary to strengthen the NOx emission control of CNG and LNG buses. With the improvement in emission standards and the use of hybrid technology, CO2 and NOx emissions of China IV buses can be reduced by 13% and 53.5% compared with China III buses of the same tonnage. The CO2 and NOx emissions of China V buses using natural gas combustion and an electric mixture are 20% and 18.8% lower than those of China V buses of the same tonnage using only natural gas. In the actual operation of the diesel–electric hybrid bus equipped with the SCR (Selective Catalytic Reduction) system, the working time of the engine is about 35.5 ± 5%, and the working time of the SCR system only accounts for 65.5 ± 12%. The low working conditions of the SCR system and the low temperature of SCR system are the reasons for the high NOx concentration at the rear end of the SCR system when the hybrid electric bus is running at low speeds.

Thermodynamic and emission characteristics of a hydrogen-enriched natural gas-fired boiler integrated with external flue gas recirculation and waste heat recovery

Hydrogen-enriched natural gas (HENG) is a low-carbon fuel and its utilization in combustion devices has been under extensive discussion recently as it is a promising way to reduce the CO2 emission during combustion. The applicability of HENG in the existing natural gas-fired combustion equipment in terms of its efficiency and emissions attracts increasing attention, especially for industrial and domestic small-scale boilers. In this study, the thermal efficiency and pollutant emissions of a 4.2 MWth HENG-fired boiler integrated with external flue gas recirculation (FGR) were respectively evaluated based on the thermodynamic and heat transfer models and the Chemical Reaction Network model. Results suggested that NOx emission rose by ∼19% as the hydrogen volumetric fraction in the fuel increased from 0 to 0.4 at a constant excess air ratio and FGR rate. To suppress the NOx rise, the FGR rate was tuned higher while the system thermal efficiency decreased subsequently. To further improve the overall system thermal efficiency, a cascade flue gas waste heat recovery strategy including sensible heat recovery using an external economizer and latent heat recovery using dual-spray heat exchangers was proposed. The thermodynamic analysis demonstrated that the external economizer improved the overall system thermal efficiency by 0.4% - 0.7% and the dual-spray heat exchangers promoted the overall system thermal efficiency by over 5%. Thus, a comprehensive performance optimization strategy was developed for HENG-fired boilers in terms of their overall system thermal efficiency promotion, NOx emission control, and CO2 emission reduction.

Application and Characteristics of Hydrogen in Alternative Fuels for Internal Combustion Engines

Petroleum has been used as the power source for internal combustion engines for hundreds of years. Nowadays, the problems of fossil energy shortage and environmental pollution are becoming increasingly serious. In response to China's carbon neutrality strategy, it is urgent to seek alternative fuels that can replace petroleum as the power source of internal combustion engines. The challenges of alternative fuels include reducing post-combustion pollutant emissions and being able to recycle them while maintaining the original engine performance. Using hydrogen as fuel can reduce automobile exhaust emissions, promote the development of hydrogen internal combustion engines, and achieve sustainable social and economic development. This article reviews the ideality of hydrogen as an alternative fuel for internal combustion engines and the combustion characteristics of hydrogen internal combustion engines. The bottleneck problems (such as abnormal combustion, NOx emission control and power recovery) that need to be solved urgently in the development of hydrogen internal combustion engines are pointed out. It’s found that these problems can be solved by the combination of software simulation and experimental verification in practice.