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Highlights • SO 3 formation and phase transformation mechanism in coal-fired plants are studied. • The adverse effects of SO 3 on the utility operations and environment are analyzed. • Different SO 3 test standards and measurement methods are compared. • Various SO 3 control technologies are reviewed and compared. • Future trends of SO 3 emission limits and control strategies are prospected. Abstract The formation and emission of sulfur trioxide (SO 3) in coal-fired power plants has received increasing attention due to its adverse effects on the operation of plant and environment. With the wide application of selective catalytic reduction (SCR) systems, the problem caused by SO 3 has become severe, especially when high sulfur coal is burned. Emission regulations of SO 3 for coal-fired power plants, which promote the development of SO 3 measurement and control technologies, have been set in some countries and regions. In this paper, recent advances in the formation, transformation, measurement, and control mechanism and technologies of SO 3 in coal-fired power plants were summarized. The formation mechanisms of SO 3 in boiler and SCR systems and its form transition and corresponding effects on the performance of power plants were analyzed. Different SO 3 test standards and methods were compared, and online SO 3 monitor based on isopropanol absorption method were developed. Various SO 3 control technologies, including simultaneous and specific removal technology, were summarized. Low-low temperature and wet electrostatic precipitators could remove up to 90% SO 3 /H 2 SO 4 aerosol, and the removal efficiency of SO 3 in wet flue gas desulfurization can be enhanced to 80% by absorber optimization and inlet flue gas temperature decrease. For power plants that burn high-sulfur coal, the injection of alkaline sorbent before the SCR or air preheater system can remove more than 90% of SO 3 to decrease the formation of NH 4 HSO 4 and the pressure drop in the air preheater. High SO 3 removal technology and strategies for different scenarios were proposed to meet different SO 3 emission requirements in the future. [ABSTRACT FROM AUTHOR]

Wet electrostatic precipitators (WESP) have been proven to be a promising technology for the removal of sulfuric acid mist from flue gases. Recent studies indicate that WESPs, when mitigating sulfuric acid mist, can be less effective on systems that burn high sulfur content fuel. This paper reports on a pilot-scale WESP used to investigate SO 3 removal with high SO 3 concentration. Both mass concentration of SO 3 and number concentration of sulfuric acid mist were measured. Two key parameters, i.e., electrical characteristics and gas loading, were studied to evaluate their effects on SO 3 removal efficiency under severe corona suppression conditions. Results showed that the maximum corona current was reduced by 83.1% when the WESP inlet SO 3 concentration increased from 0 to 5 c 0 , and the corresponding SO 3 removal efficiency decreased from 74.5% to 54.2%. SO 3 removal efficiency can be improved by increasing corona power and reducing gas velocity. A comprehensive method was proposed to enhance SO 3 removal efficiency, with removal efficiencies greater than 90% using proper electrode configurations and lower velocity under a pilot plant condition. In addition, other challenges including new ultra-fine mist generation, material corrosion and insulator failure are summarized. [ABSTRACT FROM AUTHOR]

Resistivity is a key factor in the efficient and stable operation of electrostatic precipitators. Sixty types of typical industrial fly ashes were collected for this study. Subsequently, fly ash resistivity at the temperature range of 303–1073 K, and the chemical compositions, micrographs, and size distributions of the samples were measured. The joint influence of chemical composition and temperature on resistivity was also investigated. The main components of the fly ash samples were Fe (0.8–5.0%), K + Na + Li (0.3–5.1%), Ca + Mg (0.5–4.0%), and Al + Si (10–34%), respectively. Fe, K, and Na were highly sensitive to fly ash resistivity. Resistivity decreased with the increase in Fe, K, Na, and Li contents; by contrast, resistivity increased with Ca and Mg contents. The effects of Si and Al on fly ash resistivity were weak. Resistivity initially increased first and then decreased with the increase in temperature. Maximum resistivity was observed at 373–473 K. Based on the experimental data, a prediction model for fly ash resistivity over a wide range of temperature (303–1073 K) was established. The resistivity diagrams generated specifically for this study suggest that typical fly ash samples from different industries can be estimated using chemical composition and temperature data. [ABSTRACT FROM AUTHOR]

Electric agglomeration is a useful method to improve fly-ash particle collection efficiency in traditional electrostatic precipitators (ESPs) by increasing particle diameter and achieving particle pre-charging. In this study, a laboratory agglomerator, which consisted of a pre-charging region and a static mixer, was designed and tested for fine particle agglomeration. The pre-charging provided both positive and negative discharges in parallel channels. Fine water droplets were used to enhance particle agglomeration and collection efficiency in a downstream small ESP. The operating effects were evaluated under various applied voltages and droplet concentrations. Experimental results showed that the presence of water droplets effectively improved coal-fired particle agglomeration, pre-charging, and collection in the ESP. The total number collection efficiency improved from 53.91% to 86.02% compared with the condition without the agglomerator. In addition, a novel sampling method was used to observe agglomerated particles using scanning electron microscopy, and theoretical adhesive force analysis supported the proposal of a series of power-plant particle electric agglomeration modes. [ABSTRACT FROM AUTHOR]

[Display omitted] • A field-scale demonstration setup was used to investigate particle growth and removal. • Particle size in actual flue gas was analyzed. • 80.05% particle removal efficiency could be achieved by heat exchanger. • The strategy of temperature management to control the pollutants was obtained. Flue gas generated from coal combustion is the primary source of air pollution. Temperature is an essential parameter for the treatment process of flue gas pollutants. In this study, a field-scale demonstration setup was used to investigate the effect of temperature on particle growth and removal by using actual flue gas. The ionic migration in heat exchanger was also studied, and a temperature management method was established. Results showed that the temperature had a substantial influence on particle growth and removal, the outlet particle median diameter increased from 0.116 µm to 0.155 µm, and the outlet particle mass concentration decreased from 7.05 mg/Nm3 to 4.49 mg/Nm3 when the flue gas temperature difference was 2.9–4.6 K. Changing the boiler load from 100% to 65% led to the increase in the particle removal efficiency from 78.06% to 80.33%. This increment was mainly related to the changes in temperature. In addition, when the temperature difference increased from 2.9 K to 4.6 K, the concentration of the ions in the sample significantly increased, and the removal amount of mercury was enhanced by 2.57 times. The pollutant in the flue gas was effectively removed by the temperature management installation between the wet flue gas desulfurization and wet electrostatic precipitator. [ABSTRACT FROM AUTHOR]

• A temperature-controlled experimental system was designed to study dust layer. • The role of dust layer on discharge characteristics was studied. • Criterions for migration velocity deterioration was first proposed. • Strategies to enhance discharge stability and particle migration were proposed. Charged particles were deposited on collection plates to form a dust layer in electrostatic precipitator (ESP). In turn, the dust layer could affect the performance of ESP strongly. In this work, the effect of the dust layer on the discharge characteristics and space electric strength was investigated under the different discharge gaps, thickness, and resistivity of the dust layer at temperatures that ranges of 50 °C to 400 °C. Results indicate that an abnormal discharge called back corona was observed as the voltage increased, and its onset voltage would decrease under thick dust layer, short discharge gap, and high temperature and resistivity of the dust layer. When voltage was lower than the onset voltage of the back corona, the discharge current of the stable discharge would increase with temperature but decrease as discharge gap, resistivity, and thickness of dust layer increased. Moreover, the space electric strength of stable discharge was significantly higher than that of the back corona. Space electric strength was strongly correlated with the resistivity of the dust layer. When resistivity increased, a region where the space electric strength reduced rapidly was observed. When resistivity increased from 6.27 × 109 Ω·cm to 8.45 × 1010 Ω·cm, the space electric strength decreased by 33.8% at 50 °C under voltage between electrodes of 22 kV. These variations in space electric strength support the possible decrease in migration velocity as resistivity increased, especially at high resistivity. The criteria for migration velocity deterioration were proposed for instructing ESP design and operation. [ABSTRACT FROM AUTHOR]

• A new corrosion-resistant experimental system was designed to study SOx absorption reaction with alkaline sorbents. • The SO 3 absorption performance of the three alkaline sorbents was Na 2 CO 3 > Ca(OH) 2 ≈ Mg(OH) 2. • The influence of operating parameters on SO 3 removal had been deeply studied. • The SO 3 absorption variation above 300 °C was similar to the condition without vapor. • The external diffusion control model was established for SO 3 absorption rate. Selective Catalytic Reduction (SCR) system can efficiently remove NOx in coal-fired power plant, while raise SO 3 concentration in the flue gas. Alkaline sorbent injection is a major approach to remove SO 3. Experiments were conducted to study the SO 3 absorption performance of alkaline sorbent. The SO 3 absorption performance of the three alkaline sorbents was Na 2 CO 3 > Ca(OH) 2 ≈ Mg(OH) 2. The ratio of SO 3 absorption rate to SO 2 absorption rate for Na 2 CO 3 increased with the increasing temperature. The key parameters on the SO 3 absorption performance of Na 2 CO 3 were studied. The SO 3 absorption by Na 2 CO 3 was increased by 72.7% with increasing temperature from 150 °C to 300 °C, and the temperature had little influence on SO 3 absorption above 300 °C. The increasing SO 3 concentration and decreasing particle size of Na 2 CO 3 enhanced SO 3 absorption. However, when the average particle diameter was less than 50 μm, the SO 3 absorption was no longer significantly increased. The increasing CO 2 concentration slightly reduced the SO 3 absorption. After adding vapor into the flue gas, the SO 3 absorption was obviously increased below 300 °C. The SO 3 absorption variation above 300 °C was similar to the condition without vapor. When reaction temperature was 300 °C or above and reaction time was less than 20 min, external diffusion became the control step for SO 3 absorption. On the basis of external diffusion control model, the mass transfer coefficient slightly increased from 1.68 × 10−3 m/s to 1.75 × 10−3 m/s when the reaction temperature increased from 300 °C to 400 °C. [ABSTRACT FROM AUTHOR]

The laboratory method used for coal calorific value measurement has a significant hysteresis. In this paper, an on-site measurement method of coal calorific value based on high power laser rapid ignition is proposed. The effects of oxygen transverse passage velocity, thermal insulation crucible bracket and coal samples on the measurement time, unburned rate and calorific value measurement error are gradually investigated. The experimental results shows that the measurement error of calorific value is within 2% and unburned rate is within 2% at the oxygen transverse passage velocity of 120 mm/s of Huainan bituminous coal. Then the utilization of porous thermal insulation crucible bracket has shown a good performance in heat dissipation to shorten the measurement time without increasing the unburned rate. Then, the effects of coal sample thicknesses, species and particle sizes on calorific value measurement are investigated. Results show that under the coal thickness from 4.2 to 5.4 mm, the calorific value measurement error is within 3%, and the unburned rate is within 2% of Huainan antiminous coal. Finally, the results shows that the method has a good performance on different coal species and particle sizes which the calorific value measurement error is within 5%. This research quantifies the effects of the experimental condition parameters on the measurement of coal calorific value based on laser ignition, which lays the foundation for the subsequent development of online coal calorific value meter. • An on-site coal calorific value measurement based on laser ignition. • New structural design of coal combustion chamber and thermal insulation crucible bracket. • Analysis of influencing factors and performance of this method. • The hysteresis is significantly reduced and the measurement error meets the needs of the power plant. • Prerequisites for corresponding instrument development. [ABSTRACT FROM AUTHOR]

[Display omitted] • Se and As were mainly distributed in PM 1.0 and PM 1-2.5 with a relatively dispersed distribution, while Cu and Pb were densely distributed on particles. • When the temperature difference of HT-ESP narrowed to 23 K, the enrichment ratio in the gas phase ( ER G) of As and Se increased by 2.7 % and 18.5 %. • During the PM removal process of HT-ESP, 8.8% of copper, 7.9% of lead, 8.3% of arsenic and 45% of selenium migrated from the gas phase into the hopper particles. Recently, large amounts of particulate matter (PM) and heavy metals (HMs) have been generated in the processes of non-ferrous metal smelting (NFMS). High-temperature electrostatic precipitator (HT-ESP) was the key method to capturing PM while extensive HMs enriched. In this work, the PM collection and migration of HMs in the HT-ESP process of copper smelting were investigated through an established pilot-scale platform. The excellent performance of the pilot-scale HT-ESP could achieve a particle collection efficiency of 99.5 %. The distribution patterns of HMs in various particle sizes revealed that the volatile heavy metals, arsenic (As) and selenium (Se) were predominantly distributed in PM 1.0 and PM 1-2.5 with a moderately dispersed distribution, whereas copper (Cu) and lead (Pb) were densely concentrated on particles. When the temperature difference between the inlet and the outlet of the HT-ESP narrowed to 23 K, the original As and Se enriched in hopper particles partially volatilized and migrated into the gas phase and adhered to tiny size particles. The enrichment ratio in the gas phase ( ER G) of As and Se increased by 2.7 % and 18.5 %, respectively. The results were beneficial to reducing the enrichment of volatile HMs in copper smelting ash and provided guidance for resource utilization of the copper smelting industry. [ABSTRACT FROM AUTHOR]

This study was conducted to research on the size distribution and concentration measurements of slurry droplet by DHM (digital holographic microscopy). The slurry from desulfurization tower was atomized into micron droplets. The ability of DHM to detect micron droplets was studied by simulation. Micro-holographic measurement system of spray field was built, with equivalent CCD (charge-coupled device) pixels being 0.375 μm. The size distribution of droplets was measured by DHM, showed unimodal distribution from 0.53 to 5.3 μm, with the peak at about 2.5 μm. The average particle size was 2.09 μm. Number concentration of the slurry droplets was also obtained. The concentration of DHM concentrated in the range of 1.5–3 μm, about 53–1118 mm −3 . Accordingly, the concentration of PDA (phase Doppler anemometer) concentrated in the range of 0.5–5 μm, about 19–610 mm −3 . The experimental results show that method based on DHM for slurry droplets measurement is feasible. Based on the simulation and experimental research, an on-line measurement system for slurry droplets was designed. [ABSTRACT FROM AUTHOR]

[Display omitted] • A modified charging model and an experimental system were further developed to study the charging of particle-SO 3 system. • Particle charging characteristics with the presence of different concentration of SO 3 was studied. • Charging of sulfuric acid aerosol was investigated under various conditions. • The method of discharge electrode optimization was developed to enhance particle charging. • Theoretical guidance to enhance particle charging under different conditions was proposed. Particle and SO 3 emitting from the processes of fuel combustion considerably deteriorate environment. Charging of particle in electric field was widely used for achieving effective removal of particle from SO 3 -containing flue gas in various industries. This study provided an insight into particle charging in electric field with presence of SO 3 in terms of particle charging simulation and experiments under various conditions. Results illustrated that once temperature was below acid dew point, particle charge increased and then declined with the increasing concentration of SO 3. Besides, sulfuric acid aerosol transformed from gaseous SO 3 was enhanced by higher electric field strength and temperature, but this increase seemed to be limited when reached maximum. Meanwhile, electrode optimization could enhance charging by improving ion density, which charge of particle with 0.04 μm and 1.22 μm was increased by 3.53 and 1.16. Moreover, improving dielectric constant and electric field strength mainly acted on charging of larger particle, whereas improving ion density determined small particle charging. As for medium-sized particle, simultaneously combining electric field strength and ion density adjustment to enhance particle charging was effective. Find suggested that different parameter adjustment methods should be selected according to particle diameter to enhance particle charging and reduce particle emission of SO 3 -containing flue gas. [ABSTRACT FROM AUTHOR]

[Display omitted] • Steps with O and Ca at the edges are the best termination for SO 3 adsorption. • Adsorption energy has a remarkable increase at edge sites on step surface. • Coordination of O and Ca at edges affect SO 3 adsorption. • Pre-adsorption of SO 3 or SO 2 at neighboring sites increases SO 3 adsorption energy. • SO 3 adsorption is more stable than the adsorption of SO 2 and CO 2 at edge sites. With the application of high-sulfur coal and oxy-fuel combustion in power plants, the formation of SO 3 in flue gas has strongly enhanced. Hence, the removal of SO 3 has received growing attention. Within this context, SO 3 sorption using calcium-based sorbents is an interesting approach. In this study, we analyzed the steps on a CaO surface and examined their influence on the adsorption of SO 3 molecules using density function theory calculations. The results indicate that steps with O and Ca atoms at the edges are the most stable terminations and also the best terminations for SO 3 adsorption. SO 3 molecules bind to O CaO sites by S SO3 atoms and weak bonds between Ca and O SO3 are formed. At edge sites, the adsorption energy of SO 3 , SO 2 and CO 2 shows a remarkable increase compared to that on a flat surface, whereas it decreases at the top site nearby the edge. The adsorption energy of SO 3 is also affected by the O CaO that interacts with S SO3 and the Ca near the O CaO. Interestingly, the SO 3 adsorption energy can increase if an SO 3 , SO 2 or CO 2 molecule previously adsorbed at a neighboring site. SO 3 adsorption is more stable than SO 2 and CO 2 absorption on step surface and has an advantage in competitive adsorption with SO 2 or CO 2. The calculation results thus indicate that the formation of a step surface could be an effective method to enhance SO 3 sorption on calcium-based sorbents. [ABSTRACT FROM AUTHOR]

Abstract: Pulsed corona discharge is an efficient method on NO oxidization, and the investigation of the oxidization process is significant both for model validation and industrial application. In-situ visualization of NO and OH in pulsed corona discharge was performed by planar laser-induced fluorescence (PLIF) in this work. Two dimensional NO oxidization and OH consumption were studied under different conditions. Some significant results were obtained for mixing behaviors of NO oxidization process. The NO oxidization rate and OH consumption increased by 22% and 40%, respectively, as the Re number of additional gas flow increased from 1379 to 4138. The OH utilization ratio was defined to describe the effect of OH radicals on NO oxidization process. It is demonstrated that OH plays an important role on NO oxidization. The NO reaction and OH consumption zone has a good consistent. [Copyright &y& Elsevier]

• Established a dual-layer monolith model coupling reaction and mass transfer. • Gave suggestions on design of active layer thickness by analyzing over 175 cases. • Analyzed the influences of average pore size of catalyst on SCR and SO 2 Oxidation. Nitrogen oxides (NOx) and sulfur dioxide (SO 2) are common gaseous pollutants in the flue gas of stationary resources, such as thermal power plants. The undesired oxidation of SO 2 on the catalytic monolith for Selective Catalytic Reduction (SCR) with ammonia will cause deposition of ammonium bisulfate, corrosion and extra pressure drop to downstream devices. A dual-layer model of catalytic monolith was established in this work to thoroughly investigate the effects of active layer thickness on SCR reaction and SO 2 oxidation under various operating parameters. Besides, the influences of different sizes of nanopore in catalyst on both reactions are discussed. Thus, optimal parameters and suggestions were given for the design of SCR catalytic monolith with low SO 2 oxidation. [ABSTRACT FROM AUTHOR]

• Developed a new RuO 2 -WO 3 /Sn 0.2 Ti 0.8 O 2 catalyst formula for CVOCs catalytic removal. • Studied the effect of RuO 2 loading on catalyst's acidity and catalytic performance. • Studied the synergistic catalysis of CVOCs and multiple pollutant components. Chlorine-containing volatile organic compounds emitted from combustion boilers and chemical industries will cause great harm to the ecological environment and human health. Catalytic combustion technology is one of the commonly used technologies for the treatment of CVOCs. Poor low-temperature activity, high levels of toxic by-products, and poor resistance to chlorine poisoning are common problems faced by its catalyst development. In order to effectively remove CVOCs, we constructed a RuO 2 -WO 3 / Sn 0.2 Ti 0.8 O 2 catalyst formula in which RuO 2 and WO 3 were supported on Sn 0.2 Ti 0.8 O 2 , and tested with DCM as a model molecule. By detecting the catalytic performance of RuO 2 -WO 3 /Sn 0.2 Ti 0.8 O 2 series catalysts with different RuO 2 loadings on DCM and corresponding characterization methods, we found that 3 wt%RuO 2 -5 wt%WO 3 /Sn 0.2 Ti 0.8 O 2 catalyst had the best catalytic performance (T 90% value was 291 °C, CO 2 selectivity was close to 100% at 325 °C). The actual industrial environment often contains multiple gas pollutants at the same time, so it is important to study the ability of the catalyst to co-process multiple pollutants. The effects of other pollutants (propane, acetone, SO 2 and NO 2) in the reaction atmosphere on the catalytic oxidation performance of DCM were further studied. It was found that the RuO 2 -WO 3 /Sn 0.2 Ti 0.8 O 2 catalyst could achieve the coordinated removal of acetone, propane and dichloromethane, and the effects of SO 2 and NO 2 on catalytic oxidation performance of DCM was revealed. It is hoped that it can serve as a reference and guidance for the subsequent industrial application of the catalyst in this system. [ABSTRACT FROM AUTHOR]