Influence of flue gas desulfurization (FGD) installations on emission characteristics of PM2.5 from coal-fired power plants equipped with selective catalytic reduction (SCR)

Dynamic evolution of particulate and gaseous emissions for typical residential coal combustion

Residential coal combustion still accounts for half of the heating energy consumption in many developing countries. The dynamic variation during the combustion process importantly determines the combustion facility design and appropriate air quality assessment, which was omitted in conventional studies. This study investigated the emissions of particulate and gaseous pollutants during the combustion process for typical coal types using online monitoring. During the first pyrolysis stage with temperature climbing, the organic aerosols (OA) and gases reached peak concentration. The second fierce combustion stage had the highest temperature and produced the highest cumulative emissions, particularly a substantial amount of black carbon for coals with higher volatile content. Using higher-quality coals will undoubtedly reduce PM emissions, by a factor of 10 from bituminous to anthracite coal. However, more ultrafine particles (d < 0.1 μm) from cleaner coal may pose additional health risks. Anthracite and honeycomb coal had approximately twice the energy content and emitted more CO2 per unit mass of fuel and had more persistent SO2 emissions throughout the burnout stage. The oxygenation of OA and organic gases remained increased during combustion, suggesting the pyrolysis products underwent oxidation before being emitted. The investigation of the coal combustion process suggests the importance of reducing volatiles to control PM emissions, but the potential negative synergistic effects between PM reduction and increased carbon emissions should also be considered.

Emission characteristics of lead and particulate matter from lead and zinc smelters in China

Understanding the emission characteristics of particulate matter and associated heavy metals is essential for assessing their environmental and health impacts post-emission, as well as for identifying potential control technologies for the sources. Here, a field test was conducted at two advanced smelting plants equipped with comprehensive air pollution control devices. The particles emitted from different stages of lead and zinc smelting exhibited bi-modal size distributions, with peaks observed in PM0.1-1.0 and PM2.5-10, respectively. Particulate-bound Pb was identified as the predominant Pb species in the flue gas, primarily originating from ore crushing. Consequently, over 80 % of Pb was emitted in the form of coarse particles, a marked contrast to coal-fired power plants where Pb concentrated on fine particles. High efficiencies in Pb removal were achieved by dust collectors, flue gas purification systems, and acid plants with desulfurization systems, resulting in overall Pb emission factors in lead and zinc smelting were only 89.3 and 2.60 g t-1 (of metal production), respectively. Importantly, the contribution of gas-phase Pb, which accounts for approximately 16.6 % of total emissions, must not be neglected in future emission monitoring and control efforts.

Removal and emission characteristics of hazardous trace elements in total and graded particulate matters: A case study of a typical ultra-low emission coal-fired power plant

Particulate matters (PMs) and hazardous trace elements (HTEs) emitted from coal-fired power plants (CFPPs) have raised serious environmental and human issues. Herein, total PMs and graded PMs including PM<1, PM1-2.5 and PM2.5-10 at the inlet/outlet of air pollution control devices (APCDs) were collected from a representative ultra-low emission (ULE) CFPP in China. The removal efficiencies of total PMs by selective catalytic reduction (SCR), electrostatic precipitator (ESP), wet flue gas desulfurization (WFGD) and wet electrostatic precipitator (WESP) were 0.40 %, 99.9 %, 38.1 % and 85.3 %, respectively. PM2.5-10 was robustly removed by WFGD, while PM<1 and PM1-2.5 were readily removed by WESP. The removal efficiencies of As, Cd, Cr and Pb in total PMs by APCDs followed an order: ESP > WESP > WFGD > SCR. SCR significantly decreased Se concentration by 42.8 %, contrasting to the removal of As, Cd, Cr and Pb (10.8-20.8 %). As, Cd, Cr, Pb and Se concentrations in graded PMs at the outlets of ESP, WFGD and WESP decreased with particle size increasing. All As, Cd, Cr, and Pb contents in PM<1, PM1-2.5 and PM2.5-10 at WFGD outlet increased, surpassing their analogues at ESP and WESP outlets. However, the concentration of Se declined in PM<1 at WFGD outlet. The atmospheric emission factors (EFs) of As, Cd, Cr, Pb and Se in the studied ULE CFPP were respectively 7.32, 1.27, 6.05, 122.5 and 6.42 mg/t, in line with Monte Carlo simulations. This study would not only provide a basis for emission control of PMs and HTEs in CFPPs, but also promote the improvement of respective environmental policy.

Cross-administrative and downscaling environmental spatial management and control system: A zoning experiment in the Yangtze River Delta, China

Promoting the downscaling and integration of zonal management and control of various environmental pollution sources is an effective way to systematically deal with the current high-intensity and complex environmental problems. Through single-factor and comprehensive pollutant emission intensity evaluation and cluster analysis, we built a full-coverage and cross-scale environmental spatial management and control system for pollution sources, then proposed environmental zoning patterns and pollution control strategies at three scales in the Yangtze River Delta (YRD), China. At the grid scale, the reclassified 7 types of pollution source spaces can be divided into 5 levels based on pollution emission intensity, and the most urgent environmental control subjects can be determined accordingly. Up to the county scale, combined with emission intensity and regional functions, 305 counties can be divided into 5 control intensity zones, which directly correspond to different environmental control intensity, requirements and policies. Finally, at the city scale, 41 cities can be clustered into 7 pollution control zones, which are classified and named as the three-level form of geographic location, development orientation and pollution source characteristics. Fully using the zoning units at different scales of cities, counties and grids can break the limitation of inherent administrative boundaries and allow environmental integration policies to be implemented across departments and regions, also let differentiated policies be more accurately implemented to different administrative levels and pollution source, and then truly improve the efficiency of environmental management.

On-road emissions of fine particles and associated chemical components from motor vehicles in Wuhan, China

Vehicle emission is an important contributor to urban air pollution with the increasing number of motor vehicles. Ten typical vehicles were selected in Wuhan to study the emissions of fine particular matters (PM_2.5) and associated chemical components by on-road tests through a Portable Emission Monitoring System (PEMS). The emission factors of PM_2.5 and the compositions of it from different types of vehicle were obtained. Results showed that the average emission factors of PM_2.5 from gasoline and diesel vehicles were 1.266 and 16.589 mg/km. As the emission standard of vehicles increased from China III to China V, PM_2.5 emission factor gradually decreased from 17.385 to 1.520 mg/km. Emission rate of PM_2.5 was 0.0384 mg/s under low speed, and it increased to 0.0775 and 0.0964 mg/s under the medium and high speeds. The ratio of organic carbon versus elemental carbon (OC/EC) in PM_2.5 from gasoline vehicles was 6.89, which was greater than that of diesel vehicles as 3.12. Because gasoline was made of small molecules and the compression ratio of gasoline engine was relatively low, some OC remained in the area where the ignition failed in the cylinder. The top four water-soluble ions with high emission factors were Cl^-, SO₄^2-, Ca²⁺, and Na⁺, while K, Na, Ca and Mg had a larger emission factors in the 21 tested inorganic elements. These water-soluble ions and inorganic elements mainly came from the oil burning, fuel additives and engines wear. Results of PM_2.5 emission characteristics would help to improve the air quality in Wuhan.

Co-Benefits of Pollutant Removal, Water, and Heat Recovery from Flue Gas through Phase Transition Enhanced by Corona Discharge

Pollutant removal and resource recovery from high-humidity flue gas after desulfurization in a thermal power plant are crucial for improving air quality and saving energy. This study developed a flue gas treatment method involving phase transition enhanced by corona discharge based on laboratory research and established a field-scale unit for demonstration. The results indicate that an adequate increase in size will improve the ease of particle capture. A wet electrostatic precipitator is applied before the condensing heat exchangers to enhance the particle growth and capture processes. This results in an increase of 58% in the particle median diameter in the heat exchanger and an emission concentration below 1 mg/m³. Other pollutants, such as SO₃ and Hg, can also be removed with emission concentrations of 0.13 mg/m³ and 1.10 μg/m³, respectively. Under the condensation enhancement of the method, it is possible to recover up to 3.26 t/h of water from 200 000 m³/h saturated flue gas (323 K), and the quality of the recovered water meets the standards stipulated in China. Additionally, charge-induced condensation is shown to improve heat recovery, resulting in the recovery of more than 43.34 kJ/h·m³ of heat from the flue gas. This method is expected to save 2628 t of standard coal and reduce carbon dioxide emission by 2% annually, contributing to environmental protection and global-warming mitigation.

Does Ambient Secondary Conversion or the Prolonged Fast Conversion in Combustion Plumes Cause Severe PM2.5 Air Pollution in China?

The ambient formation of secondary particulate matter (ambient FSPM) is commonly recognized as the major cause of severe PM2.5 air pollution in China. We present observational evidence showing that the ambient FSPM was too weak to yield a detectable contribution to extreme PM2.5 pollution events that swept northern China between 11 and 14 January 2019. Although the Community Multiscale Air Quality (CMAQ) model (v5.2) reasonably reproduced the observations in January 2019, it largely underestimated the concentrations of the PM2.5 during the episode. We propose a novel mechanism, called the “in-fresh-stack-plume non-precipitation-cloud processing of aerosols” followed by the evaporation of semi-volatile components from the aerosols, to generate PM2.5 at extremely high concentrations because of highly concentrated gaseous precursors and large amounts of water droplets in fresh cooling combustion plumes under poor dispersion conditions, low ambient temperature, and high relative humidity. The recorded non-precipitation-cloud processing of the aerosols in fresh stack combustion plumes normally lasts 20–30 s, but it prolongs as long as 2–5 min under cold, humid, and stagnant meteorological conditions and expectedly causes severe PM2.5 pollution events. Regardless of the presence of the natural cloud in the planetary boundary layer during the extreme events, the fast conversion of air pollutants in water droplets and the generation of the PM2.5 through the non-precipitation-cloud processing of aerosols always occur in fresh combustion plumes. The processing of aerosols is detectable using a nano-scan particle sizer assembled on an unmanned aerial vehicle to monitor the particle formation in stack plumes. In-fresh-stack-plume processed aerosols under varying meteorological conditions need to be studied urgently.

Emission characteristics of heavy metals from a typical copper smelting plant

Heavy metal emissions from non-ferrous smelting plants have been a rising concern. However, their emission characteristics were still unclear. In this study, the concentrations and gas-particle partition of five major heavy metals (Cu, Pb, As, Cr and Cd) in the flue gas from a typical copper smelting plant were measured. The bi-modal distribution of both particulate matter and heavy metals indicated that the particles in super-micron mode was caused by the mechanical crushing and escaping of raw materials, whereas the formation of submicron mode was due to the evaporation and subsequent condensation of volatile substances. The excellent performance of existing air pollution control devices in the studied smelter could substantially reduce the particulate matter and heavy metal concentrations in the extraction and smelting stages by 99.2%-99.9%. The emission factors of PM_2.5, Cu, Pb, As, Cr, and Cd were only 283, 2.49, 0.97, 5.92, 0.28, and 0.06 g/t, mostly as the fugitive emission (84.2% on average). In addition, the 'unfilterable' phase of the heavy metals, including the gaseous species and solutes in the filter-penetrated droplet, accounted for averagely 45.8% of the total emissions at the outlet, which indicates the huge underestimation by particle collection only.

Mercury speciation and size-specific distribution in filterable and condensable particulate matter from coal combustion

Particle-bound mercury discharged with fine particulate matter from coal-fired power plants causes atmospheric pollution that impacts human health. In this study, the speciation and size-specific distribution of particle-bound mercury in filterable particulate matter (FPM) from an ultra-low emission power plant and condensable particulate matter (CPM) from an entrained flow reactor were analyzed. Most importantly, particle-bound mercury was enriched in fine particles smaller than 0.02 μm, whose mass fraction was several orders of magnitude higher than that in large particles. Particularly, HgBr₂, HgCl₂, and HgO were major mercury species in FPM, whereas CPM involves mostly HgCl₂ with a small portion of HgBr₂. The occurrence of these species was also confirmed by a thermodynamic equilibrium calculation. The results further revealed the effects of air pollution control devices (APCDs) on the speciation of particle-bound mercury. Specifically, an electrostatic precipitator (ESP) removed most particle-bound mercury. Similarly, wet flue gas desulfurization (WFGD) dramatically reduced particle-bound mercury for most particles, except those between 0.1 and 1 μm. At the outlet of WFGD, mercury bound with FPM₁₀ (smaller than 10 μm) is only 0.15% of the total mercury at the inlet of selective catalytic reduction (SCR). This knowledge provides insights that can be used to design and optimize the control strategy for mercury emission in power plants.

Comprehensive evaluation of flue gas desulfurization and denitrification technologies of six typical enterprises in Chengdu, China

Post-combustion flue gas desulfurization and denitrification technologies are essential in achieving the full compliance of fine particulate matter (PM_2.5, aerodynamic diameter less than 2.5 μm) air quality standards by 2030 in China as sulfur dioxide (SO₂) and nitrogen oxides (NO_X) are the main precursors of PM_2.5. Some studies have addressed the performance evaluation of desulfurization technology, but none included the water-soluble ions (sulfate (SO₄^2-), nitrate (NO₃^-), etc.) as an indicator nor accounted for uncertainty involved. In this study, we present a multilevel fuzzy method that integrates the analytic hierarchy process with fuzzy theory, defines SO₄^2-concentration as a new environmental indicator, and is supplemented with an uncertainly analysis and apply the method for the techno-economic and environmental evaluation of desulfurization and denitrification technologies in six typical enterprises (including two power plants and three industrial production plants and a waste incineration plant) in Chengdu, China. The evaluation shows that first, the fluctuating desulfurization rate and the dosage leads to changed ranking of the economic and technical secondary evaluation results, with the overall comprehensive evaluation ranks unchanged. Second, from the perspective of environmental protection agency and the public, if the environmental indicators are empowered, the lower the SO₄^2-concentration of an enterprise, the better its evaluation ranking will be and vice versa. Third, if we re-empower from the perspective of the enterprise, under the condition that the technical feasibility is met and the environmental indicators are basically up to standard, the low-cost removing process is more likely to be the tendency of the enterprise. In summary, the findings of the study have led to the conclusions that (1) for the power industry, the integration of desulfurization, denitrification, and dedusting technologies should be promoted rigorously; (2) the non-power industry should continue the end-of-pipe treatment and environmental protection regulatory policies of the power industry; and (3) the energy industry structure should be optimized with enhanced end-of-pipe control technologies to achieve deep reduction of air pollutants.

Probing mesoporous character, desulfurization capability and kinetic mechanism of synergistic stabilizing sorbent CaxCuyMnzOi/MAS-9 in hot coal gas

Developing structurally stable sorbents for high-temperature H₂S direct removal is recognized as a valuable energy-saving strategy for efficient utilization of hot coal gas (HCG), which depends upon their mesoporous features and desulfurization capabilities. Herein, tailored hierarchical Ca_xCu_yMn_zO_i/MAS-9 sorbents were fabricated via a facile sol-gel method using high-activity phase Ca_xCu_yMn_zO_i anchored onto versatile mesoporous MAS-9. After O/S-exchange procedure, noteworthy straight channels of MAS-9 (S_BET = 808 m² g^-1) provided enough available spaces for the storage of generative large MeS_y nanoparticles, which was better than other conventional zeolites. The probing of variables (i.e. support type, active ingredient, loadings, and sulfidation temperature) on H₂S removal revealed that 50%Ca₃Cu₁₀Mn₈₇O_i/MAS-9 shared an excellent breakthrough sulfur capacity (171.57 mg g^-1) at 800 °C, even it experienced six reusable cycles, due to synergistic stabilizing effect of Ca-Cu-Mn and high-temperature tolerance of SiOAl framework of MAS-9. Especially, CaO dopant endowed the sorbent with superficial alkalinity and high-temperature resistance. The brilliant desulfurization behavior was also described by the fast H₂S diffusion or component deactivation vs. duration time on stream according to the followed kinetic investigation. Thus, the refined Ca₃Cu₁₀Mn₈₇O_i/MAS-9 possesses the expected representative desulfurization nature and great potentiality for raw HCG in practical applications.

Air pollutant emissions from coal-fired power plants in China over the past two decades

China is the largest coal producer and consumer in the world, and coal-fired power plants are among its major sources of air pollutants. The Chinese government has implemented various stringent measures to reduce air pollutant emissions over the past two decades. National statistical data, emission inventories, and satellite observations indicate that air pollutant emissions from coal-fired power plants have been effectively controlled. Field measurements at coal-fired power plants can provide valuable information about the long-term trend of air pollutant emissions and the driving factors. In this study, we evaluated air pollutant emissions from 401 units at 308 coal-fired power plants. An appreciable reduction in air pollutant concentrations and emission factors from coal-fired power plants in China is observed over the past two decades. The drivers for this trend from the perspective of policy making, application of removal technologies, tightening of emission standards, technological improvement, monitoring systems, and economic measures are discussed. Currently, concentrations of typical air pollutants from coal-fired power plants in China are lower than those in Japan, Germany, and the US. This can be attributed to the policies and lenient emission standards for power plants in these countries. The technological improvement of air pollution control devices is the key factor that has led to reductions in air pollutant emissions in China. China has built the largest system of clean coal-fired power plants in the world.

Variation characteristics of final size-segregated PM emissions from ultralow emission coal-fired power plants in China

The ultralow emission (ULE) retrofits for Chinese coal-fired power plants (CFPPs) are nearing completion. Large-scale and rapid retrofits have resulted in significant changes in the emission level and characteristics of particulate matter (PM). To investigate the variation characteristics of final three size fractions PM (PM_2.5, PM_10-2.5, PM_>10) emissions, we conducted field tests at the outlets of wet flue gas desulfurization (WFGD) and wet electrostatic precipitator (WESP) by a pair of two-stage virtual impactors in eight representative ULE CFPPs. Our results indicate that, after WESP installations, the mass concentrations of final PM are significantly reduced and those of the final total ions and elements decrease as most individual chemical compositions are reduced. WESP presents an excellent removal performance for large particle sizes and high PM concentrations. SO₄^2- is the major ionic component at both the outlets of WFGD and WESP, and its proportion in total ions is reduced to some extent through WESP. Furthermore, the average mass contents of SO₄^2- and most elements in PM_2.5 are significantly lower than those in PM_10-2.5 and PM_>10 whether at the WFGD-outlets or WESP-outlets. By comparison, chemical profiles of PM have substantially changed after ULE retrofits, and those after WFGD (e.g., sulfate, Zn, Pb, and Cu) have also changed relative to existing data. The end-tail emission factors (EFs) of PM_2.5, PM₁₀, and PM_total under the typical ULE technical routes of WESP are calculated in time, and the corresponding EFs are in the range of 2.82-8.97, 15.7-27.6, and 38.6-61.7 g t^-1, respectively. We believe the latest detailed PM EFs and the associated chemical profiles provided in this study are more representative of the current emission situations of Chinese CFPPs.

Refined assessment of size-fractioned particulate matter (PM2.5/PM10/PMtotal) emissions from coal-fired power plants in China

Chinese coal-fired power plants (CFPPs) are experiencing large-scale and rapid retrofitting of ultralow emission (ULE), causing significant changes in emission level of particulate matter (PM) from CFPPs. In this study, based on coal ash mass balance over the whole process, an integrated emission factors (EFs) database of three size-fractioned particulate matters (PM_2.5, PM₁₀, and PM_total) for CFPPs is constructed, which covers almost all typical ULE technical routes installed in CFPPs. To verify the reliability of PM EFs established in this study, we compare those with related results based on field tests. Overall, the gaps in the EFs of PM_2.5, PM₁₀, and PM_total obtained by the two methods are not outrageous within a reasonable range. By combined with the refined size-fractioned PM EFs and unit-based activity level database, a detailed high-resolution emission inventory of PM_2.5, PM₁₀, and PM_total from Chinese CFPPs in 2017 is established, with the corresponding total emissions of 143, 207, and 267 kt, respectively. Our estimation of PM_total emission is comparable to the official statistics announced by China Electricity Council (CEC), which further demonstrates the reliability of PM EFs constructed in this study. Moreover, potential reductions of PM from CFPPs at two stages before and after 2017 are assessed under three application scenarios of major ULE technical routes. We forecast the final annual emissions of PM_2.5, PM₁₀, and PM_total until 2020 will be reduced further, which fall within the range of 86-111 kt, 120-157 kt, and 142-184 kt, respectively, if all CFPPs achieve ULE requirements under the three scenarios. We believe our integrated database of PM EFs of CFPPs has good universality, and the forecast results will be helpful for policy guidance of ULE technologies, emissions inventory compilation, and regional air quality simulation and management.

Transformation and removal of ammonium sulfate aerosols and ammonia slip from selective catalytic reduction in wet flue gas desulfurization system

Selective catalytic reduction (SCR) denitration may increase the emission of NH₄⁺ and NH₃. The removal and transformation characteristics of ammonium sulfate aerosols and ammonia slip during the wet flue gas desulfurization (WFGD) process, as well as the effect of desulfurization parameters, were investigated in an experimental system equipped with a simulated SCR flue gas generation system and a limestone-based WFGD system. The results indicate that the ammonium sulfate aerosols and ammonia slip in the flue gas from SCR can be partly removed by slurry scrubbing, while the entrainment and evaporation of desulfurization slurry with accumulated NH₄⁺ will generate new ammonium-containing particles and gaseous ammonia. The ammonium-containing particles formed by desulfurization are not only derived from the entrainment of slurry droplets, but also from the re-condensation of gaseous ammonia generated by slurry evaporation. Therefore, even if the concentration of NH₄⁺ in the desulfurization slurry is quite low, a high level of NH₄⁺ was still contained in the fine particles at the outlet of the scrubber. When the accumulated NH₄⁺ in the desulfurization slurry was high enough, the WFGD system promoted the conversion of NH₃ to NH₄⁺ and increased the additional emission of primary NH₄⁺ aerosols. With the decline of the liquid/gas ratio and flue gas temperature, the removal efficiency of ammonia sulfate aerosols increased, and the NH₄⁺ emitted from entrainment and evaporation of the desulfurization slurry decreased. In addition, the volatile ammonia concentration after the WFGD system was reduced with the decrease of the NH₄⁺ concentration and pH values of the slurry.

Migration and Emission Characteristics of Ammonia/Ammonium through Flue Gas Cleaning Devices in Coal-Fired Power Plants of China

To investigate the up-to-date migration and emission characteristics of NH₃/NH₄⁺ in coal-fired power plants (CFPPs) after implementing ultralow emission retrofitting, typical air pollution control devices (APCDs) in CFPPs, including flue gas denitrification, dust collectors, combined wet flue gas desulfurization (WFGD), and wet precipitators are involved in field measurements. The results show that most of the excessive injected and/or unreacted ammonia from the flue gas denitrification system, whether selective catalytic reduction (SCR) or selective noncatalytic reduction (SNCR), is converted into particle-bound NH₄⁺ (>91%), and the rest (less than 9%) is carried by flue gas in the form of gaseous NH₃, with a concentration value of 0.15-0.54 mg/(N m³) at the denitrification outlet. When passing through dust collectors, particle-bound NH₄⁺ concentration decreases substantially along with the removal of particle matter. In WFGD, the dissolution and volatilization effects affect the gaseous ammonia concentration, which decreases when using limestone slurry and a 20% solution of ammonia as a desulfurization agent, while liquid ammonia solution with a high concentration (99.8%) may cause the flue gas NH₃ concentration to increase considerably by 13 times. Particle-bound NH₄⁺ concentration is mainly influenced by the relative strength of desulfurization slurry scouring and flue gas carrying effects and increases 2.84-116 times through ammonia-based WFGD. Furthermore, emission factors of NH₃ for combinations of APCDs are discussed.

Significant impact of heterogeneous reactions of reactive chlorine species on summertime atmospheric ozone and free-radical formation in north China

Heterogeneous reactions of N₂O₅, O₃, OH, ClONO₂, HOCl, ClNO₂, and NO₂, with chlorine-containing particles are incorporated in the Community Multiscale Air Quality (CMAQ) model to evaluate the impact of heterogeneous reactions of reactive chlorine species on ozone and free radicals. Changes of summertime ozone and free radical concentrations due to the additional heterogeneous reactions in north China were quantified. These heterogeneous reactions increased the O₃, OH, HO₂ and RO₂ concentrations by up to 20%, 28%, 36% and 48% for some regions in the Beijing-Tianjin-Hebei (BTH) area. These areas typically have a larger amount of NOx emissions and a lower VOC/NOx ratio. The zero-out method evaluates that the photolysis of ClNO₂ and Cl₂ are the major contributors (42.4% and 57.6%, respectively) to atmospheric Cl in the early morning hours but the photolysis of Cl₂ is the only significant contributor after 10:00 am. The results highlight that heterogeneous reactions of reactive chlorine species are important to atmospheric ozone and free-radical formation. Our study also suggests that the on-going NOx emission controls in the NCP region with a goal to reduce both O₃ and secondary nitrate can also have the co-benefit of reducing the formation Cl from ClNO₂ and Cl₂, which may also lead to lower secondary organic aerosol formation and thus the control of summertime PM_2.5 in the region.

Unexpectedly Increased Particle Emissions from the Steel Industry Determined by Wet/Semidry/Dry Flue Gas Desulfurization Technologies

"Ultralow-emission" standards have started to be implemented for steel plants in China. Flue gas desulfurization (FGD) systems integrating desulfurization and dedusting, common end-of-pipe technologies before the stacks, have been a key process for controlling the complexity of sintering flue gas to meet ultralow-emission requirements. This study reports comprehensive analysis of the influence of wet/semidry/dry FGD systems on particulate emissions via a field investigation of five typical sinter plants equipped with various FGD devices. The size distribution and mass concentration of particulate matter (PM) are adjusted to different ranges by these FGD systems. Chemical analysis of the PM compositions shows that 20-95% of the mass of inlet PM is removed by FGD systems, while it is estimated that approximately 17, 63, 59, and 71% of the outlet PMs are newly contributed by desulfurizers and their byproducts for the tested wet limestone, wet ammonia, semidry circulating fluidized bed, and activated coke FGD systems, respectively. The newly contributed compositions of PM_2.5 emitted from these FGD systems are dominated by CaSO₄, (NH₄)₂SO₄, CaSO₄ + CaO, and coke carbon, respectively. These results suggest that the deployment of FGD technology should be comprehensively considered to avoid additional negative impacts from byproducts generated in control devices on the atmosphere.

Synergistic removal of dust using the wet flue gas desulfurization systems

Coal is still a major energy source, mostly used in power plants. However, the coal combustion emits harmful SO₂ and fly ash. Wet flue gas desulfurization (WFGD) technology is extensively used to control SO₂ emissions in power plants. However, only limited studies have investigated the synergistic dust removal by the WFGD system. Spray scrubbers and sieve-tray spray scrubbers are often used in WFGD systems to improve the SO₂ removal efficiency. In this study, the synergistic dust removal of WFGD systems for a spray scrubber and a sieve-tray spray scrubber was investigated using the experimental and modelling approaches, respectively. For the spray scrubber, the influence of parameters, including dust particle diameters and inlet concentrations of dust particles, and the flow rates of flue gas and slurry of limestone/gypsum on the dust removal efficiency, was investigated. For the sieve-tray spray scrubber, the influence of parameters such as the pore diameter and porosity of sieve trays on the dust removal efficiency was examined. The study found that the dust removal efficiency in the sieve-tray spray scrubber was approximately 1.1-10.6% higher than that of the spray scrubber for the same experimental conditions. Based on the parameters investigated and geometric parameters of a scrubber, a novel droplets swarm model for dust removal efficiency was developed from the single droplet model. The enhanced dust removal efficiency of sieve tray was expressed by introducing a strength coefficient to an inertial collision model. The dust removal efficiency model for the sieve-tray spray scrubber was developed by combining the droplets swarm model for the spray scrubber with the modified inertial collision model for the sieve tray. The results simulated using both models are consistent with the experimental data obtained.

Characteristics of particulate matter from four coal-fired power plants with low-low temperature electrostatic precipitator in China

The performance of traditional electrostatic precipitators (ESPs) is strongly affected by the flue gas temperature. Operating under much lower temperatures (e.g., ~90 °C), the low-low temperature electrostatic precipitators (LLT-ESPs) are considered as an effective technology to improve the particulate matter (PM) removal efficiency in coal-fired power plants (CFPPs). Wet flue gas desulfurization (WFGD) can also be affected by the decrease in the operating temperature of the ESP. This study evaluates the influence of various ESP operating temperatures on ESP performance, PM_2.5 (particles with an aerodynamic diameter of ≤2.5 μm), and total dust emission characteristics at the ESP and WFGD outlets in CFPPs equipped with LLT-ESPs. PM_2.5 and total dust concentrations at the ESP and WFGD outlets in CFPPs installed with LLT-ESPs are much lower than those with traditional ESPs. The PM concentrations at both the ESP and WFGD outlets show a decreasing trend with a decrease in the operating temperature. However, the concentration of total water-soluble ions (mainly SO₄^2-, Cl^-, and NH₄⁺) in the total dust at the outlet of ESPs increases from 0.3 to 0.8 mg/m³ as the temperature decreases from > 90 °C to 80-90 °C, which is contrary to that at the WFGD outlet (decreases from 4.7 to 0.8 mg/m³). The PM_2.5 and total dust concentrations increase by 10.2-80.2% and 13.7-77.0%, respectively, through the WFGD unit due to the entrainment of a gypsum slurry. A relatively lower operating temperature of LLT-ESPs in power plants is also beneficial to decrease the incremental effect of PM emissions in the process of WFGD. The recommended operating temperature for LLT-ESPs is ~90 °C, and limited improvement on PM reduction can be achieved with a further temperature decrease from 90 to 80 °C.

Updated Hourly Emissions Factors for Chinese Power Plants Showing the Impact of Widespread Ultralow Emissions Technology Deployment

Nationwide severe air pollution has prompted China to mandate the adoption of ultralow emissions (ULE) control technologies at all of its coal-fired power plants by 2020. This process has accelerated greatly since 2014 and, combined with operational adjustments related to overcapacity, has reduced the emissions of nitrogen oxides (NO _x), sulfur dioxide (SO₂), and particulate matter (PM). Yet the quantitative understanding of ULE benefits is poor. Using detailed emissions data from 38 units at 17 power plants, corresponding to 10 combinations of ULE technologies representative of the Chinese power sector, we show that emissions factors for NO _x, SO₂, and PM are up to 1-2 orders of magnitude lower after ULE retrofitting. The effectiveness in cutting emissions shows a large spread across the various ULE technology combinations, providing an opportunity to choose the most efficient, economically viable technology (or a combination of technologies) in the future. The temporal variations in emissions at hourly resolution reveal the effects of power plant load on emissions, an increasingly important factor given that power plants are not operated at full capacity. These data will be useful in efforts to understand the evolving state of air quality in China and can also provide a basis for benchmarking state-of-the-art air pollution control equipment globally.

A potential source for PM2.5: Analysis of fine particle generation mechanism in Wet Flue Gas Desulfurization System by modeling drying and breakage of slurry droplet

Aerosol particulate matter with dynamic diameter smaller than 2.5 μm (PM_2.5) is the main cause for haze pollution in China. As a dominant precursor of PM_2.5, SO₂ emitted from industrial process is now strictly controlled by using limestone/gypsum Wet Flue Gas Desulfurization (WFGD) system in China. However, a phenomenon that fine particle derived from WFGD is recently addressed, and is suggested to be a potential source of primary PM_2.5. Herein, a first investigation into the particle generation mechanism in WFGD system is conducted with a novel droplet (containing particles) drying and breakage model. The proposed model considers a random and porous crust instead of the previous regular crust assumption, and is verified by comparing the modeling results with measurements. An orthogonal test with four factors and three levels is carried out through modeling calculation, and flue gas temperature (T_g) in the inlet is found to be a governing parameter for PM_2.5 yields in WFGD. With T_g in range of 120-160 °C, PM_2.5 yields in desulfurizing tower can reach a maximum value at ∼2 × 10⁸ cm^-3 under typical WFGD condition. To avoid this situation and reduce the PM_2.5 generation, T_g is suggested to be lower than 120 °C. Additionally, a new insight of the elimination effect of gas-gas heater (GGH) on "gypsum rain" in WFGD system is provided.

Effects of Wet Flue Gas Desulfurization and Wet Electrostatic Precipitators on Emission Characteristics of Particulate Matter and Its Ionic Compositions from Four 300 MW Level Ultralow Coal-Fired Power Plants

To achieve ultralow-emission (ULE) standards, wet electrostatic precipitators (WESP) installed downstream from wet flue gas desulfurization (WFGD) have been widely used in Chinese coal-fired power plants (CFPPs). We conducted a comprehensive field test study at four 300 MW level ULE CFPPs, to explore the impact of wet clean processing (WFGD and WESP) on emission characteristics of three size fractions of particulate matter (PM: PM_2.5, PM_10-2.5, and PM_>10) and their ionic compositions. All these CFPPs are installed with limestone-based/magnesium-based WFGD and followed by WESP as the end control device. Our results indicate that particle size distribution, mass concentration of PM, and ionic compositions in flue gas change significantly after passing WFGD and WESP. PM mass concentrations through WFGD are significantly affected by the relative strength between desulfur slurry scouring and flue gas carrying effects. Concentrations of ions in PM increase greatly after passing WFGD; especially, SO₄^2- in PM_2.5, PM_10-2.5, and PM_>10 increase on average by about 1.4, 3.9, and 8.3 times, respectively. However, WESP before the stack can effectively reduce final PM emissions and their major ionic compositions. Furthermore, emission factors (kg/(t of coal)) of PM for different combinations of air pollution control devices are presented and discussed.

On-road emissions of ammonia: An underappreciated source of atmospheric nitrogen deposition

We provide updated spatial distribution and inventory data for on-road NH₃ emissions for the continental United States (U.S.) On-road NH₃ emissions were determined from on-road CO₂ emissions data and empirical NH₃:CO₂ vehicle emissions ratios. Emissions of NH₃ from on-road sources in urbanized regions are typically 0.1-1.3tkm^-2yr^-1 while NH₃ emissions in agricultural regions generally range from 0.4-5.5tkm^-2yr^-1, with a few hotspots as high as 5.5-11.2tkm^-2yr^-1. Counties with higher vehicle NH₃ emissions than from agriculture include 40% of the U.S.

POPULATION

The amount of wet inorganic N deposition as NH₄⁺ from the National Atmospheric Deposition Program (NADP) network ranged from 37 to 83% with a mean of 58.7%. Only 4% of the NADP sites across the U.S. had <45% of the N deposition as NH₄⁺ based on data from 2014 to 2016, illustrating the near-universal elevated proportions of NH₄⁺ in deposition across the U.S. Case studies of on-road NH₃ emissions in relation to N deposition include four urban sites in Oregon and Washington where the average NH₄-N:NO₃-N ratio in bulk deposition was 2.3. At urban sites in the greater Los Angeles Basin, bulk deposition of NH₄-N and NO₃-N were equivalent, while NH₄-N:NO₃-N in throughfall under shrubs ranged from 0.6 to 1.7. The NH₄-N:NO₃-N ratio at 7-10 sites in the Lake Tahoe Basin averaged 1.4 and 1.6 in bulk deposition and throughfall, and deposition of NH₄-N was strongly correlated with summertime NH₃ concentrations. On-road emissions of NH₃ should not be ignored as an important source of atmospheric NH₃, as a major contributor to particulate air pollution, and as a driver of N deposition in urban and urban-affected regions.