Mesoscale behavior study of collector aggregations in a wet dust scrubber

In order to address the bottleneck problem of low fine-particle removal efficiency of self-excited dust scrubbers, this paper is focused on the influence of the intermittent gas-liquid two-phase flow on the mesoscale behavior of collector aggregations. The latter is investigated by the application of high-speed dynamic image technology to the self-excited dust scrubber experimental setup. The real-time-scale monitoring of the dust removal process is provided to clarify its operating mechanism at the mesoscale level. The results obtained show that particulate capturing in self-excited dust scrubber is provided by liquid droplets, liquid films/curtains, bubbles, and their aggregations. Complex spatial and temporal structures are intrinsic to each kind of collector morphology, and these are considered as the major factors controlling the dust removal mechanism of self-excited dust scrubbers. For the specific parameters of gas-liquid two-phase flow under study, the evolution patterns of particular collectors reflect the intrinsic, intermittent, and complex characteristics of the temporal structure. The intermittent initiation of the collector and the air hole formation-collapse cyclic processes provide time and space for the fine dust to escape from being trapped by the collectors. The above mesoscale experimental data provide more insight into the factors reducing the dust removal efficiency of self-excited dust scrubbers.

Implications: This paper focuses on the reconsideration of the capturer aggregations of self-excited dust scrubbers from the mesoscale. Complex structures in time and space scales exist in each kind of capturer morphology. With changes of operating parameters, the morphology and spatial distributions of capturers diversely change. The change of the capturer over time presents remarkable, intermittent, and complex characteristics of the temporal structure.     

PCDD/DF concentrations at the inlets and outlets of wet scrubbers in Korean waste incinerators

To further understand the effects of wet scrubbers on PCDD/DF levels, it was measured the concentrations of PCDD/DF, dust, and other gaseous pollutants at both the inlets and the outlets of seven wet scrubbers. As a result, the concentrations of PCDD/DF at the inlets and outlets of the wet scrubbers ranged from 0.2 to 37.4, and 0.8 to 6.0 ng TEQ N m-3, respectively. With the exceptions of wet scrubbers F and G, the PCDD/DF levels decreased by and large in most wet scrubbers. It was thought that their relatively high removal efficiencies were more increased with heavier loads of dust and particle-bound PCDD/DF. On the other hand, it was also surveyed the increase of gaseous PCDD/DF in wet scrubber, where the total level of PCDD/DF was decreased. However, it was not sure whether it had been resulted from the thermal adsorption/desorption phenomenon between packing materials and emission gases or not. At the very least, however, although there still remains an unexplained aspect for the increase of gaseous PCDD/DF, it is clear that wet scrubbers can be sufficiently applied to remove PCDD/DF to a certain extent, if only removal efficiencies for the particle loads are high, and if a significant part of the PCDD/DF at the inlets is particle associated.     

平煤集团坑口电厂除尘设施的技术改造

平煤集团坑口电厂原有文丘里水膜除尘器不能使排放烟尘达标排放 ,在保留原有两台文丘里水膜除尘器基础上 ,并在其前串联两台单室二电场静电除尘器 ,经改造后 ,电除尘 文丘里水膜除尘串联组合技术总除尘效率达 99 9% ,排放浓度为 10 3mg/Nm3,实现了达标排放     

Effect of pH and temperature on the kinetics of odor oxidation using chlorine dioxide

Increasing public concerns over odors and air regulations in nonattainment zones necessitate the remediation of a wide range of volatile organic compounds (VOCs) generated in the poultry-rendering industry. Currently, wet scrubbers using oxidizing chemicals such as chlorine dioxide (ClO2) are utilized to treat VOCs. However, little information is available on the kinetics of ClO2 reaction with rendering air pollutants, limiting wet scrubber design and optimization. Kinetic analysis indicated that ClO2 does not react with hexanal and 2-methylbutanal regardless of pH and temperature and implied that aldehyde removal occurs primarily via mass transfer. Contrary to the aldehydes, ethanethiol or ethyl mercaptan (a model compound for methanethiol or methyl mercaptan) and dimethyl disulfide (DMDS) rapidly reacted with ClO2. The overall reaction was found to be second and third order for ethanethiol and DMDS, respectively. Moreover, an increase in pH from 3.6 to 5.1 exponentially increased the reaction rate of ethanethiol (e.g., k2 = 25-4200 L/mol/sec from pH 3.6 to 5.1) and significantly increased the reaction rate of DMDS if increased to pH 9 (k3 = 1.4 x 10(6) L2/mol2/sec). Thus, a small increase in pH could significantly improve wet scrubber operations for removal of odor-causing compounds. However, an increase in pH did not improve aldehyde removal. The results explain why aldehyde removal efficiencies are much lower than methanethiol and DMDS in wet scrubbers using ClO2.     

Performance of Gas-Atomized Spray Scrubbers at High Pressure

A large number of pressurized coal gasification processes being developed propose to use venturi scrubbers for particulate removal at high pressures. Theoretical predictions based on venturi scrubber performance models indicate that particle collection efficiency will decrease severely in these high gas pressure applications.

An exploratory theoretical and experimental program was performed to study the effect of gas pressure on venturi scrubber performance. Experiments were done on a 0.47 m³/s (1000 acfm) pilot scale venturi scrubber. Particle collection performance was determined as a function of scrubber pressure drop for venturi scrubbers operating In the range of 1-10 atm total pressure. Experimental results confirmed that the particle collection efficiency of venturi scrubbers decreases for a given scrubber pressure drop as total gas pressure Is increased. To achieve the same particle collection efficiency, the pressure drop across a venturi scrubber operated at 10 atm Is about 10 times that of the same scrubber operated at 1 atm pressure.     

Information Required for the Selection and Performance Evaluation of Wet Scrubbers

The term “wet scrubber” or simply “scrubber,” for the purpose of this report, is intended to include any device using liquid to effect the removal of solid or liquid particles which are entrained in process air or gas streams. This guide is intended to provide information required for the selection and performance evaluation of all types of scrubbers installed for the primary purpose of removing such particulates from any process gas stream. It is not intended to cover scrubbers for the collection of gaseous and/or vapor constituents which involve gas absorption mechanisms.     

Technical Aspects of Lime/Limestone Scrubbers for Coal-Fired Power Plants

This 2-part article deals with the technical aspects of lime/limestone scrubbers for coal-fired power plants. Part I covers process chemistry and scrubber systems. Part II (next month) will cover instrumentation, particulate removal, and sludge disposal.     

Technical Aspects of Lime/Limestone Scrubbers for Coal-Fired Power Plants

This is the conclusion of a 2-part article dealing with the technical aspects of lime/limestone scrubbers for coal-fired power plants, it covers instrumentation, participate removal and sludge disposal. Part I (June JAPCA) covered process chemistry and scrubber systems     

Destruction of PCDD/Fs by SCR from flue gases of municipal waste incinerator and metal smelting plant

Partitioning of PCDD/F congeners between vapor/solid phases and removal and destruction efficiencies achieved with selective catalytic reduction (SCR) system for PCDD/Fs at an existing municipal waste incinerator (MWI) and metal smelting plant (MSP) in Taiwan are evaluated via stack sampling and analysis. The MWI investigated is equipped with electrostatic precipitators (EP, operating temperature: 230 degrees C), wet scrubbers (WS, operating temperature: 70 degrees C) and SCR (operating temperature: 220 degrees C) as major air pollution control devices (APCDs). PCDD/F concentration measured at stack gas of the MWI investigated is 0.728 ng-TEQ/Nm(3). The removal efficiency of WS+SCR system for PCDD/Fs reaches 93% in the MWI investigated. The MSP investigated is equipped with EP (operating temperature: 240 degrees C) and SCR (operating temperature: 290 degrees C) as APCDs. The flue gas sampling results also indicate that PCDD/F concentration treated with SCR is 1.35 ng-TEQ/Nm(3). The SCR system adopted in MSP can remove 52.3% PCDD/Fs from flue gases (SCR operating temperature: 290 degrees C, Gas flow rate: 660 kN m(3)/h). In addition, the distributions of PCDD/F congeners observed in the flue gases of the MWI and MSP investigated are significantly different. This study also indicates that the PCDD/F congeners measured in the flue gases of those two facilities are mostly distributed in vapor phase prior to the SCR system and shift to solid phase (vapor-phase PCDD/Fs are effectively decomposed) after being treated with catalyst. Besides, the results also indicate that with SCR highly chlorinated PCDD/F congeners can be transformed to lowly chlorinated PCDD/F congeners probably by dechlorination, while the removal efficiencies of vapor-phase PCDD/Fs increase with increasing chlorination.     

Flue gas desulfurization: the state of the art

Coal-fired electricity-generating plants may use SO2 scrubbers to meet the requirements of Phase II of the Acid Rain SO2 Reduction Program. Additionally, the use of scrubbers can result in reduction of Hg and other emissions from combustion sources. It is timely, therefore, to examine the current status of SO2 scrubbing technologies. This paper presents a comprehensive review of the state of the art in flue gas desulfurization (FGD) technologies for coal-fired boilers. Data on worldwide FGD applications reveal that wet FGD technologies, and specifically wet limestone FGD, have been predominantly selected over other FGD technologies. However, lime spray drying (LSD) is being used at the majority of the plants employing dry FGD technologies. Additional review of the U.S. FGD technology applications that began operation in 1991 through 1995 reveals that FGD processes of choice recently in the United States have been wet limestone FGD, magnesium-enhanced lime (MEL), and LSD. Further, of the wet limestone processes, limestone forced oxidation (LSFO) has been used most often in recent applications. The SO2 removal performance of scrubbers has been reviewed. Data reflect that most wet limestone and LSD installations appear to be capable of approximately 90% SO2 removal. Advanced, state-of-the-art wet scrubbers can provide SO2 removal in excess of 95%. Costs associated with state-of-the-art applications of LSFO, MEL, and LSD technologies have been analyzed with appropriate cost models. Analyses indicate that the capital cost of an LSD system is lower than those of same capacity LSFO and MEL systems, reflective of the relatively less complex hardware used in LSD. Analyses also reflect that, based on total annualized cost and SO2 removal requirements: (1) plants up to approximately 250 MWe in size and firing low- to medium-sulfur coals (i.e., coals with a sulfur content of 2% or lower) may use LSD; and (2) plants larger than 250 MWe and firing medium- to high-sulfur coals (i.e., coals with a sulfur content of 2% or higher) may use either LSFO or MEL.     

复合分子筛去除水中氟化物的研究

通过使用复合分子筛对模拟含氟水进行静态吸附实验,研究了复合分子筛对水中氟化物的吸附性能,并分析了吸附过程中温度、pH值和停留时间等因素对分子筛除氟效率的影响。结果表明,用80 g粒径0.9 mm的分子筛处理150 mL浓度为5 mg/L的模拟含氟水,分子筛的除氟效率达到90%以上,而同样条件下天然沸石和改性涂铁沸石的去除率分别为23.35%和80.69%。分子筛的除氟效率随着温度呈现先上升后下降的趋势,50℃时去除效率达到最高,为92.7%;在弱酸性条件下的去除率比碱性条件要高,在pH=4时去除率最大,为91.8%;停留时间初始阶段与去除率保持正相关,在45 min后反应达到平衡。分子筛的吸附速率符合二级动力学方程dqe/dt=k(qe-qt)2,吸附等温线符合Langmuir方程。     

Assessment of emissions and removal of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) at start-up periods in a hazardous waste incinerator

A study was conducted to observe the changes in polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/F) levels and congener profiles in the flue gas of a hazardous waste incinerator during two start-up periods. Flue gas samplings were performed simultaneously through Air Pollution Control Devices (APCDs) (including boiler outlet, electrostatic precipitator (ESP) outlet, wet scrubbers (WS) outlet, and activated carbon (AC) filter outlet) in different combustion temperatures during a planned cold (long) start-up and an unplanned warm (short) start-up. The results showed that PCDD/F concentrations could be elevated during the start-up periods up to levels 3–4 times higher than those observed in the normal operation. Especially lower combustion temperatures in the short start-ups may cause high PCDD/F concentrations in the raw flue gas. Assessment of combustion temperatures and Furans/Dioxins values indicated that surface-catalyzed de novo synthesis was the dominant pathway in the formation of PCDD/Fs in the combustion units. PCDD/F removal efficiencies of Air Pollution Control Devices suggested that formation by de novo synthesis existed in ESP also when in operation, leading to increase of gaseous phase PCDD/Fs in ESP. Particle-bound PCDD/Fs were removed mainly by ESP and WS, while gaseous phase PCDD/Fs were removed by WS, and more efficiently by AC filter.

Implications: This paper evaluates PCDD/F emissions and removal performances of APCDs (ESP, wet scrubbers, and activated carbon) during two start-up periods in an incinerator. The main implications are the following: (1) start-up periods increase PCDD/F emissions up to 2–3 times in the incinerator; (2) low combustion temperatures in start-ups cause high PCDD/F emissions in raw gas; (3) formation of PCDD/Fs by de novo synthesis occurs in ESP; (4) AC is efficient in removing gaseous PCDD/Fs, but may increase particle-bound ones; and (5) scrubbers remove both gaseous and particle-bound PCDD/Fs efficiently.     

Evaluating the performance of a turbulent wet scrubber for scrubbing particulate matter

A turbulent wet scrubber was designed and developed to scrub particulate matter (PM) at micrometer and submicrometer levels from the effluent gas stream of an industrial coal furnace. Experiments were conducted to estimate the particle removal efficiency of the turbulent scrubber with different gas flow rates and liquid heads above the nozzle. Particles larger than 1 µm were removed very efficiently, at nearly 100%, depending upon the flow rate, the concentration of the dust-laden air stream, and the water level in the reservoir. Particles smaller than 1 µm were also removed to a greater extent at higher gas flow rates and for greater liquid heads. Pressure-drop studies were also carried out to estimate the energy consumed by the scrubber for the entire range of particle sizes distributed in the carrier gas. A maximum pressure drop of 217 mm H₂O was observed for a liquid head of 36 cm and a gas flow rate of 7 m³/min. The number of transfer units (NTU) analysis for the efficiencies achieved by the turbulent scrubber over the range of particles also reveals that the contacting power achieved by the scrubber is better except for smaller particles. The turbulent scrubber is more competent for scrubbing particulate matter, in particular PM_2.5, than other higher energy or conventional scrubbers, and is comparable to other wet scrubbers of its kind for the amount of energy spent.

Implications: The evaluation of the turbulent scrubber is done to add a novel scrubber in the list of wet scrubbers for industrial applications, yet simple in design, easy to operate, with better compactness, and with high efficiencies at lower energy consumption. Hence the turbulent scrubber can be used to combat particulate from industrial gaseous effluents and also has a scope to absorb gaseous pollutants if the gases are soluble in the medium used for particles capture.     

Air Pollution Control Expenditures in the Chemical Industry

As industry today analyzes its emissions with more sophisticated sampling and testing techniques, it is finding more often that the problems of gaseous and particulate contaminants occur simultaneously. Current legislation is rapidly becoming more stringent and where a particular industry formerly thought it primarily had a single contaminant problem, it now finds itself faced with the necessity to comply with multiple contaminant emission level requirements. Extensive field experience in a wide diversity of industries has shown that the simultaneous removal of both the gaseous and particulate contaminants can be effectively met by crossflow scrubbers utilizing filamentous-type packings. High efficiency collection can be achieved at low operating cost in a single piece of control equipment with reliable operating results.     

TS-1分子筛催化O3/H2O2氧化乙酸

研究了酸性条件下TS-1分子筛催化O3/H2O2体系(O3/H2O2/TS-1)对降解水中乙酸效率的影响,优化了相关工艺参数,并对其作用机理进行了分析。结果表明,在pH为2.8时,TS-1的加入能显著提高臭氧化的降解效率。优化工艺参数表明,当过氧化氢投加量为3 g/L,TS-1投加量为5 g/L时,O3/H2O2/TS-1体系对乙酸具有较高的降解率,60 min后O3/H2 O2/TS-1体系对乙酸(初始浓度为100 mg/L)的去除率达到了58.7%。当pH为0.8时,O3/H2 O2/TS-1体系对乙酸的去除率仅为19.8%,降解效果较差。定量化计算表明,O3/H2O2和O3/H2O2/TS-1的Rct分别为1.62×10-8和8.67×10-7。通过测定乙酸降解过程水样中过氧化氢和液相臭氧的浓度变化,推测了具体反应机理。由于此体系在酸性条件下对乙酸有较好的降解效果,拓宽了现有O3/H2O2体系的应用范围。     

TS-1分子筛催化O_3/H_2O_2氧化乙酸

研究了酸性条件下TS-1分子筛催化O3/H2O2体系（O3/H2O2/TS-1）对降解水中乙酸效率的影响,优化了相关工艺参数,并对其作用机理进行了分析。结果表明,在pH为2.8时,TS-1的加入能显著提高臭氧化的降解效率。优化工艺参数表明,当过氧化氢投加量为3 g/L,TS-1投加量为5 g/L时,O3/H2O2/TS-1体系对乙酸具有较高的降解率,60 min后O3/H2 O2/TS-1体系对乙酸（初始浓度为100 mg/L）的去除率达到了58.7%。当pH为0.8时,O3/H2 O2/TS-1体系对乙酸的去除率仅为19.8%,降解效果较差。定量化计算表明,O3/H2O2和O3/H2O2/TS-1的Rct分别为1.62×10-8和8.67×10-7。通过测定乙酸降解过程水样中过氧化氢和液相臭氧的浓度变化,推测了具体反应机理。由于此体系在酸性条件下对乙酸有较好的降解效果,拓宽了现有O3/H2O2体系的应用范围。     

The prediction of PCDD/DF levels in wet scrubbers associated with waste incinerators

Wet scrubber is one of the most conventional types of air pollutant control devices (APCDs), which is specially designed to treat dust and acidic gases in the flue gas simultaneously. In spite of its outstanding ability to control them, however, wet scrubbers have been considered as potential contaminant sources that may increase PCDD/DF concentrations in the flue gas. In this study, we investigated the change of PCDD/DF concentrations at the inlets and outlets of seven wet scrubbers, and compared them with other published data. With a multi-regression analysis of dust concentrations and temperature at the inlets and outlets of given wet scrubbers, we developed an empirical model to understand factors dominating the change of PCDD/DF concentrations. As a result, we confirmed that the changes of PCDD/DF concentrations in wet scrubbers are closely related to their concentrations at the inlets, which would usually be determined by the type of APCDs installed upstream of the wet scrubber.     

Formation and removal of PCDD/Fs in a municipal waste incinerator during different operating periods

The PCDD/F concentrations and removal efficiencies achieved with air pollution control devices (APCDs) during different operating periods (start-up, normal operation, and shut-down) at an existing municipal waste incinerator (MWI) in Taiwan are evaluated via stack sampling and analysis. The MWI investigated is equipped with electrostatic precipitators (EP), wet scrubbers (WS), and selective catalytic reduction system (SCR) as APCDs. The sampling results indicate that the PCDD/F concentrations at the EP inlet during start-up period were 15 times higher than that measured during normal operation period. The PCDD/F concentration observed at shut-down period was close to that measured at normal operation period. The CO concentration was between 400 and 1000 ppm during start-up period, which was about 50 times higher compared with the normal operation. Hence, combustion condition significantly affected the PCDD/F formation concentration during the waste incineration process. In addition, the distributions of the PCDD/F congeners were similar at different operating periods. During the normal operation and shut-down periods, the EP decreases the PCDD/F concentration (based on TEQ) by 18.4-48.6%, while the removal efficiency of PCDD/Fs achieved with SCR system reaches 99.3-99.6%. Nevertheless, the PCDD/F removal efficiency achieved with SCR was only 42% during the 19-h start-up period due to the low SCR operating temperature (195 degrees C).     

Foreword

Fine particles have come to be recognized as being much more significant air pollutants than particles larger than several microns diameter. The removal of fine particles from an effluent gas stream is, unfortunately, more difficult than that of large particles. Because wet scrubbers are one of the major types of air pollution control equipment which can collect fine particles, it is important to define and exploit their potentialities.     

Flue Gas Desulfurization: The State of the Art

ABSTRACT

Coal-fired electricity-generating plants may use SO₂ scrubbers to meet the requirements of Phase II of the Acid Rain SO₂ Reduction Program. Additionally, the use of scrubbers can result in reduction of Hg and other emissions from combustion sources. It is timely, therefore, to examine the current status of SO₂ scrubbing technologies. This paper presents a comprehensive review of the state of the art in flue gas desulfurization (FGD) technologies for coal-fired boilers.

Data on worldwide FGD applications reveal that wet FGD technologies, and specifically wet limestone FGD, have been predominantly selected over other FGD technologies. However, lime spray drying (LSD) is being used at the majority of the plants employing dry FGD technologies. Additional review of the U.S. FGD technology applications that began operation in 1991 through 1995 reveals that FGD processes of choice recently in the United States have been wet limestone FGD, magnesium-enhanced lime (MEL), and LSD. Further, of the wet limestone processes, limestone forced oxidation (LSFO) has been used most often in recent applications.

The SO₂ removal performance of scrubbers has been reviewed. Data reflect that most wet limestone and LSD installations appear to be capable of ~90% SO₂ removal. Advanced, state-of-the-art wet scrubbers can provide SO₂ removal in excess of 95%.

Costs associated with state-of-the-art applications of LSFO, MEL, and LSD technologies have been analyzed with appropriate cost models. Analyses indicate that the capital cost of an LSD system is lower than those of same capacity LSFO and MEL systems, reflective of the relatively less complex hardware used in LSD. Analyses also reflect that, based on total annualized cost and SO₂ removal requirements: (1) plants up to ~250 MW_e in size and firing low- to medium-sulfur coals (i.e., coals with a sulfur content of 2% or lower) may use LSD; and (2) plants larger than 250 MW_e and firing medium- to high-sulfur coals (i.e., coals with a sulfur content of 2% or higher) may use either LSFO or MEL.