Flue Gas Desulfurization Scrubber Research Articles

Removal of multiple pollutants in air pollution control devices of a coal-fired boiler installed with a flue gas condensation scrubber

This work was conducted through field tests on a coal-fired circulating fluidized bed boiler with a condensation scrubber (CS) for flue gas waste heat recovery installed behind the wet flue gas desulfurization (WFGD) scrubber. Levels of multiple pollutants were investigated along various flue gas devices of the boiler, with the CS being the main focus. The CS demonstrated an excellent effect in removing filterable particulate matter (FPM), reducing it from 10.63–14.00 mg/Nm3 to less than 1 mg/Nm3, providing an alternative solution for ultra-low emission. The CS had the ability to remove SO2, reducing it from 211.8–272.0 mg/Nm3 to 11.1–13.0 mg/Nm3, which could be an aid to the WFGD scrubber. The CS showed an effect on SO3 removal, reducing it from 17.49–17.53 mg/Nm3 to 1.07–1.13 mg/Nm3, which might address the discharge of sulphate aerosols. The CS exhibited an effect in removing condensable particulate matter (CPM), reducing it from 29.68–38.48 mg/Nm3 to 12.18–22.80 mg/Nm3, providing a potential solution for CPM emission control. In addition, the escape of fine desulfurization slurry droplets from the WFGD scrubber could also be mitigated by the CS with the reduction efficiency of 88.30–90.51%. Overall, the synergistic removal of multiple pollutants including FPM, SO2, SO3, and CPM by the CS was verified in this case study. Considering its original function of waste heat recovery, the adoption of the CS is a promising measure for energy conservation and environmental protection.

Comprehensive Effect of Oxidant Addition in an FGD Slurry on the Removal and Distribution of Selenium: A Field Study.

Flue gas desulfurization (FGD) scrubbers capture selenium in coal-fired power plants, leading to a high concentration of selenium in the slurry. This research proves that SO32- is preferentially oxidized compared to SeO32- by S2O82-. With the increase in the oxidation-reduction potential (ORP) caused by S2O82- addition, the conversion rate of SO32- increased and the size of gypsum grains grew from 31.2 to 34.6 μm. SeO32- migrates into gypsum grains during the growth of CaSO4·2H2O, leading to selenium fixation in gypsum. In a field study of a 350 MW unit, the ORP increased from 142 to 450 mV when Na2S2O8 was fed into the FGD slurry. With the addition of the oxidant, 65.1% of selenium in the liquid phase migrated into gypsum. The concentration of selenium in the leachate of gypsum after oxidant addition decreased by 68.0%. A 2.34% increase in the selenium removal rate was observed in the scrubber. This study focuses on the migration and conversion of selenium in an actual FGD slurry via a field test. The results found in the 350 MW unit are consistent with laboratory results. The change in ORP has been proven to be effective in adjusting the selenium distribution in the FGD slurry.

CFD modeling for NOx absorption accompanying with SO2 in wet flue gas desulfurization scrubber based on gas-phase ozone oxidation

NOx removal performance accompanying with SO2 is investigated in a two-step process containing O3 pre-oxidation of NO and NOx post-absorption in the scrubber of wet flue gas desulfurization (WFGD) unit. Based on EulerianLagrangian framework, a comprehensive CFD model is established to describe the reactions of NO2 and N2O5 with S(IV) in droplets coupled with NOx transfer process and hydrodynamics in the scrubber. Taking the WFGD scrubber of 330 MW coal-fired power unit as case study, the results indicate NOx absorption efficiency in scrubber is increased by increasing O3/NO molar ratio from 1.0 to 2.2 and decreasing reaction temperature from 423 K to 363 K in O3 pre-oxidation process. The obtained highest removal efficiency is 85.79% for pH of 5.5 or 96.32% for pH of 9.0 in droplets. NO2 absorption is much slower than N2O5 and more sensitive to chemical compositions in droplets or hydrodynamics in scrubber. The overall mass transfer coefficient (KG) for NO2 is mainly determined by pH and S(IV) concentration in droplets, while the specific interfacial area (a) is largely influenced by gasdroplet hydrodynamics in scrubber. Finally, a correlation is proposed to predict the volumetric mass transfer coefficient (KGa) for NO2 absorption accompanying with SO2 in WFGD scrubber.

Mechanism of SO3/H2SO4 transformation and reduction in wet flue gas desulfurization systems

In this study, a unique sampling unit combining quartz wool filtration and salt adsorption, which can measure aerosol and gaseous H2SO4 simultaneously, was adopted to reveal the transformation and reduction of SO3/H2SO4 in a lab-scale wet flue gas desulfurization (WFGD) scrubber. The results showed that H2SO4 in flue gas at the inlet of the WFGD scrubber is in both gaseous and aerosol states, and the gaseous state constitutes a larger proportion, differing from the conventional realization that all H2SO4 at WFGD scrubber inlets is gaseous. The experiments showed that the amount of H2SO4 aerosols increased, and that of H2SO4 vapor decreased in the scrubber, indicating H2SO4 transformation from a gaseous to an aerosol state after quenching. However, there is always a small portion of H2SO4 vapor that could be detected at the outlet, suggesting that H2SO4 cannot be completely converted to aerosols in the scrubber, differing from traditional assumption. We also found that spraying droplets could reduce H2SO4 but with low efficiency. The demister installed at the scrubber outlet is more efficient at reducing H2SO4 than the droplets, which inspires a new understanding of the mechanism of H2SO4 reduction in WFGD processes.

Mass transfer process intensification for SO2 absorption in a commercial-scale wet flue gas desulfurization scrubber

To intensify SO2 absorption process in wet flue gas desulfurization (WFGD) spray scrubber, a comprehensive CFD model coupled transfer process with chemical reactions is established by Eulerian‒Lagrangian method and verified in a commercial-scale device of 330 MW coal-fired power unit. The desulfurization performance is significantly affected by both the property of each droplet and the dispersion of droplet swarm, which are determined by the spray behaviors. Three dominant factors, including droplet diameter, injection direction and spray morphology, are explored to optimize the spray conditions for intensifying SO2 absorption process. Differing from the ideal plug flow, the optimal diameter of droplet in non-ideal flow is obtained by considering the synergistic effect of the uniformity of gas–slurry distribution and the dynamics of SO2 mass transfer. Compared with the single-direction spray with downward or upward injection, the dual-direction spray reaches a great balance between the competitive effects of the uniformity of gas‒slurry flow and the interfacial area for mass transfer. The influence of spray morphology on mass transfer process is essentially the difference of dispersion characteristics of droplets in the middle part of scrubber. A design scheme combining full-cone and hollow-cone nozzles is suggested for improving the overall SO2 removal efficiency.

Process optimization of S (IV) oxidation in flue gas desulfurization scrubbers

At present, the control of oxidation process is imprecise in the wet flue gas desulfurization system. In this study, a modified oxidation model is established to investigate convective mass transfer of O2 and the oxidation of S (IV) under natural and forced oxidation conditions. By comparison, the model results are in good agreement with experimental results. Based on the motion of droplet in the scrubber and bubble in the slurry pool, natural oxidation rate is calculated to provide guidance for forced oxidation and recommended oxidation air flow rate. Droplet accelerates until it reaches terminal velocity. Mass transfer resistance mainly occurs in the liquid film. The mass transfer flux of the droplet with diameter of 1.5 mm increases continuously until it remains stable. The natural oxidation rate increases with the increasing of inlet O2 concentration while the forced oxidation rate increases with the increasing of oxidation air flow rate. Several gas flow rates are taken into consideration to obtain the optimal oxidation air flow rate, showing that smaller drop diameter has positive impact on oxidation process in flue gas desulfurization scrubbers.

Characteristics of Mercury Re-emission and Migration in a Lab-Scale Wet Flue Gas Desulfurization Scrubber under Simulated Air and Oxy-Fuel Combustion Atmospheres

Based on a lab-scale wet flue gas desulfurization scrubber, mercury re-emission and migration during the desulfurization process were experimentally investigated under simulated air and oxy-fuel co...

Reduction of oxidized mercury over NOx selective catalytic reduction catalysts: A review

• The Hg 2+ reduction process over SCR catalysts were reviewed for the first time. • The effects of typical flue gases on Hg 2+ reduction were presented. • The involved reaction mechanisms for Hg 2+ reduction were discussed in details. • Suggestions to alleviate the reduction of Hg 2+ over SCR catalysts were proposed. The control of mercury emission from flue gas has long been a serious task for the global environment and human health. Utilization of selective catalytic reduction (SCR) catalysts to oxidize Hg 0 to form Hg 2+ and subsequent capture the Hg 2+ through a wet flue gas desulfurization (WFGD) scrubber is an efficient and economical technology for Hg 0 removal in coal-fired power plants. Extensive studies have been conducted with regard to Hg 0 oxidation over SCR catalysts. However, recent studies found that the reduction of Hg 2+ , an inverse process to Hg 0 oxidation, also occurred even predominated under some SCR conditions. This new observation overturns the general knowledge that SCR catalysts always enhance Hg 0 oxidation. To avoid the neglect of Hg 2+ reduction and overcome the shortages of existing studies, a critical review of Hg 2+ reduction over SCR catalysts was undertaken. The Hg 2+ reduction behaviors under various SCR conditions were summarized and the effects of typical flue gas components like hydrogen chloride (HCl), sulfur dioxide (SO 2 ), water vapor (H 2 O), volatile organic compounds (VOCs), and carbon monoxide (CO) on Hg 2+ reduction were also presented. The involved reaction mechanisms for Hg 2+ reduction were discussed in details. Moreover, feasible measures to suppress the reduction of Hg 2+ in the SCR system were proposed. We expect to clearly explore the mechanism of the Hg 0 redox process on SCR catalysts in this review to avoid the neglect of Hg 2+ reduction, hence providing guidance for the design of SCR catalysts to achieve an efficient Hg 0 oxidation.

Synergetic process of condensing heat exchanger and absorption heat pump for waste heat and water recovery from flue gas

This paper presents a process for recovering waste heat and water from high-humidity flue gas after the wet flue gas desulfurization scrubber. A condensing heat exchanger and an absorption heat pump are integrated into the condenser cooling water system by using a portion of the condenser cooling water as the working fluid. Two different arrangements of a reference power plant (1000 MW) are studied for the heating and non-heating seasons. The condensing heat exchanger performances in terms of heat transfer surface area, working fluid flow rate and heat transfer rate are evaluated, the influence of the coal type on them is specially examined. Besides, a technical-economic analysis of the process is also performed. The results indicate that the optimal logarithmic mean temperature difference range of the condensing heat exchanger is 15–30 °C considering the cost of heat transfer surface areas. The presented process is used to determine the best low-temperature working fluid for the condensing heat exchanger while the cooling tower efficiency and water balance are not affected. For bituminous coal, reducing the high-humidity flue gas temperature by 5 °C, 81.4 t/h of water is recovered and 44.8% of water consumption in the scrubber can be saved. During the heating season, 125.8 MW of waste heat is recovered with 50.3% from the condenser cooling water and 49.7% from the high-humidity flue gas. During the non-heating season, 77.9 MW of waste heat is recovered with all from the condenser cooling water. This process is economic, especially for burning high water content coal.

Field test of SO3 removal in ultra-low emission coal-fired power plants.

Under the extensive implementation of ultra-low emission (ULE) facilities in coal-fired power plants of China, sulfur trioxide (SO3) has received increasing attention due to its impact on human health and operation safety of power plants. However, systematic research and evaluation for controlling SO3 emission in various ULE facilities are still lacking. Here, a systematic study was conducted based on 378 in situ performance evaluation tests carried out in 148 coal-fired power plants. The results illustrate that the SO2/SO3 conversion rate of the selective catalytic reduction devices can be controlled within 1% before and after ULE retrofit. Also, the synergistic removal efficiency of SO3 in the low-low-temperature electrostatic precipitator and the wet electrostatic precipitator can be higher than 70%. The removal efficiency of SO3 in the wet limestone-gypsum flue gas desulfurization scrubber is 33-64% before ULE and 31-81% after, and the average efficiency of the double scrubbers is 8.7% higher than that of the single scrubber. Due to the different SO3 removing abilities of various technologies, the overall efficiency of SO3 removal is in the range between 27 and 95% adopting different ULE technical routes. Average concentration of SO3 emission can be decreased by 51.8% after ULE application.

A 1-D model of spraying performance for wet flue gas desulfurization scrubber based on predicted slurry temperature

To predict the slurry temperature in wet flue gas desulfurization system, a theoretical model was put forward and the calculation method was set up, the correlation between the parameters of inlet flue gas and the slurry temperature was first found. Based on the predicted slurry temperature, a 1-D coupled model of droplets motion, heat and mass transfer and pressureloss of liquid-gas two phases was established, and the numerical solutions were acquired by Runge-Kutta method, meanwhile, field tests were made to validate its reliability. Additionally, the distribution characteristics of droplet velocity, temperature and relative humidity of flue gas, and pressure loss over the height of spraying zone were discussed. The effect of drop diameter and other factors on the performance of spraying process were analyzed. The results show that the slurry temperature and pressure loss agree well with the operation values with errors of less than 5% and 7%. Outlet flue gas is near saturation state under discussed range of parameters. The drop diameter is the main factor that affects the performance of wet flue gas desulfurization system.

Synergistic capture of fine particles in wet flue gas through cooling and condensation

Coal-fired boilers tend to be equipped with considerable pollutant control equipment given the increase in environmental protection standard, thereby continuously increasing operating cost and decreasing the output rate of coal resources. Synergistic capture of pollutants may be a potential solution. In this work, a method for installing a cooling heat exchanger at the outlet of wet flue gas desulfurization scrubber is proposed to recover waste heat and capture fine particles. The cooling process of this method uses wet atmosphere, thereby indicating that this work differs from previous relevant works that are mainly based on dry deposition. Accordingly, the activity of particle reduction in the exchanger is also considered different. Thus, relevant mechanisms are recently explored on the basis of wet deposition through experiments. In addition to diffusiophoretic and thermophoretic depositions, particle growth or agglomeration is considered a dominant factor. Moreover, condensed water is crucial for cleaning, which may resolve the risk of fouling. Variable analysis of the influences of flue gas temperature drop, trapping surface area, initial particle concentration, and flue gas flow rate are also addressed in this work to provide references for engineering applications. Technical feasibility and economic evaluation of this method are discussed, through which possible measures for improvement are proposed. Furthermore, an applicable mode for heat recovery and a synergistic particle capture are presented, and an acceptable economic performance is estimated.

Mass Flow Analysis of Mercury Transformation and Effect of Seawater Flue Gas Desulfurization on Mercury Removal in a Full-Scale Coal-Fired Power Plant

To understand the mercury speciation and mass flow in a full-scale bituminous coal-fired power plant equipped with a selective catalytic reduction (SCR) unit, an electrostatic precipitator (ESP), and a seawater flue gas desulfurization (SWFGD) scrubber, the evolution and emission of mercury was systematically measured and analyzed. The results showed that the elemental mercury (Hg0) was oxidized mainly through heterogeneous oxidation, especially in SCR, which transformed 57–64% of elemental mercury (Hg0) to oxidized mercury (Hg2+). It changed the concentrations of different mercury species and then increased the Hg2+ removal efficiencies of SWFGD, which ranged from 67 to 82%. A total of 8.24–11.54% of mercury was adsorbed by fly ash to form particulate-bound mercury (HgP) and subsequently removed by the ESP. Mass flow and mass distribution of mercury indicated that most mercury was removed and retained in the SWFGD. The heterogeneous oxidation of Hg0 in SCR and then absorption of Hg2+ by SWFGD significant...

Method of flash evaporation and condensation – heat pump for deep cooling of coal-fired power plant flue gas: Latent heat and water recovery

Flue gas waste heat recovery and utilization is an efficient means to improve the energy efficiency of coal-fired power plants. At present, the surface corrosion and fouling problems of heat exchanger hinder the development of flue gas deep cooling. In this study, a novel flue gas deep cooling method that can reduce flue gas temperature below the dew point of vapor to recover latent heat and obtain clean water simultaneously is proposed to achieve improved energy efficiency. The heat transfer mode of this method is the direct contact mode, which takes the scrubber, e.g. the flue gas desulfurization (FGD) scrubber, as the deep cooling exchanger. The flash evaporation and condensation (FEC) device and heat pump (HP) are utilized to provide low-temperature medium, such as FGD slurry or water, for washing and deep cooling flue gas, to collect recovered water, and to absorb recovered waste heat. This method is called as the FEC–HP method. This paper elaborated on two optional models of the proposed method. The mechanism for recovering heat and water was also analyzed using the customized flue gas humidity chart, and the method to quantitate recovered heat and water, as well as the results of the case of a 300MW coal-fired generator set were provided. Net present value calculations showed that this method is profitable in the scenario of burning high-water-content coals. Several potential advantages of this method and suggestions for practical application were also discussed.

Selenium Partitioning and Removal Across a Wet FGD Scrubber at a Coal-Fired Power Plant.

Selenium has unique fate and transport through a coal-fired power plant because of high vapor pressures of oxide (SeO2) in flue gas. This study was done at full-scale on a 900 MW coal-fired power plant with electrostatic precipitator (ESP) and wet flue gas desulfurization (FGD) scrubber. The first objective was to quantify the partitioning of selenium between gas and condensed phases at the scrubber inlet and outlet. The second objective was to determine the effect of scrubber operation conditions (pH, mass transfer, SO2 removal) on Se removal in both particulate and vapor phases. During part of the testing, hydrated lime (calcium hydroxide) was injected upstream of the scrubber. Gas-phase selenium and particulate-bound selenium were measured as a function of particle size at the inlet and outlet of the scrubber. The total (both phases) removal of Se across the scrubber averaged 61%, and was enhanced when hydrated lime sorbent was injected. There was evidence of gas-to-particle conversion of selenium across the scrubber, based on the dependence of selenium concentration on particle diameter downstream of the scrubber and on thermodynamic calculations.

Optimization of a Wet Flue Gas Desulfurization Scrubber through Mathematical Modeling of Limestone Dissolution Experiments

Dissolution rates of two very pure limestone samples were measured experimentally by means of the pH-stat method under conditions where mechanical stirring did not affect the rates considerably. The experimental results were modeled mathematically by considering the surface areas of the particles changing dynamically through the reaction; moreover, a surface factor was introduced in order to account for the nonsphericity of the particles. The surface areas were measured by means of gas adsorption and by particle size distribution (laser diffraction). Liquid-phase concentrations were measured by inductively coupled plasma optical emission spectrometry, and surface compositions were measured by X-ray spectroscopy. Furthermore, scanning electron microscope images of the samples are presented. Subsequently, an optimization model of a scrubber was developed by using the intrinsic parameters of the samples, which were determined experimentally. The optimization results indicate that up to 34–50% of the power re...

Research on Thermal Properties of Two Phase in a Flue Gas Desulfurization Wet Scrubber

A mathematical model was established to predict heat and mass transfer in a flue gas desulfurization scrubber under stable operation condition. Thermal properties of liquid and gas phase were studied. The temperature calculation formula of liquid was presented which is in alignment with the operation parameters. The distribution of temperature and humidity of out flue gas is analyzed through MATLAB software. The results show that thermal properties of outlet flue gas are only related to temperature, humidity, and liquid-to-gas ratio of inlet flue gas. With the increase of temperature and humidity of inlet flue gas, those of out flue gas increase also; with the increase of liquid-to-gas ratio, those of out flue gas decrease. Under actual operation condition of plant power, outlet flue gas is nearly in adiabatic saturation state.

Transformation pathways from calcium sulfite to α-calcium sulfate hemihydrate in concentrated CaCl2 solutions

The disposal of sulfite-rich flue gas desulfurization (FGD) scrubber sludge has been a tough problem. The sludge now finds its prospective utilization way by being transformed into α-calcium sulfate hemihydrate (α-HH). At low pH, atmospheric pressure and 95.0°C, the calcium sulfite hemihydrate (CSH) is oxidized and then changed to α-HH via calcium sulfate dihydrate (DH) in 2.50–3.50m CaCl2 solutions (i.e. CSH→DH→α-HH). Direct transformation from CSH to α-HH (i.e. CSH→α-HH) occurs in a 4.00m CaCl2 solution. The formation of DH at low CaCl2 concentrations is probably attributed to faster heterogeneous nucleation. The increment in CaCl2 molality lowers the water activity, and enlarges the supersaturation gap between DH and α-HH nucleation, which makes α-HH nucleation more competitive. Higher CaCl2 concentration favors the α-HH formation, and nucleation kinetics accounts for the transformation pathway evolution. The preparation of α-HH from the sulfite-rich FGD sludge is of great sense to electric power industry and environmental conservation.

Mercury re-emission in flue gas multipollutants simultaneous absorption system.

Recently, simultaneous removal of SO2, NOx and oxidized mercury in wet flue gas desulfurization (WFGD) scrubber has become a research focus. Mercury re-emission in traditional WFGD system has been widely reported due to the reduction of oxidized mercury by sulfite ions. However, in multipollutants simultaneous absorption system, the formation of a large quantity of nitrate and nitrite ions as NOx absorption might also affect the reduction of oxidized mercury in the aqueous absorbent. As such, this paper studied the effects of nitrate and nitrite ions on mercury re-emission and its related mechanism. Experimental results revealed that the nitrate ions had neglected effect on mercury re-emission while the nitrite ions could greatly change the mercury re-emission behaviors. The nitrite ions could initially improve the Hg(0)-emission through the decomposition of HgSO3NO2(-), but with a further increase in the concentration, they would then inhibit the reduction of bivalent mercury owing to the formation of Hg-nitrite complex [Hg(NO2)x(2-x)]. In addition, the subsequent addition of Cl(-) could further suppress the Hg(0) emission, where the formation of a stable Hg-SO3-NO2-Cl complex was assumed to be the main reason for such strong inhibition effect.

New insights into synergistic effects and active species toward Hg0 emission control by Fe(VI) absorbent

With the timelines for implementation of the U.S. Environmental Protection Agency’s mercury emission control rules approaching soon, finding an innovative and cost-effective mercury removal technology has become increasingly important. The most challenging species in all the mercury forms is gaseous elemental mercury (Hg0). The objective of this work is to inspect the absorption behavior and removal of Hg0 by Fe(VI). The synergistic effects of various factors were investigated and the optimization for Hg0 removal by Fe(VI) was achieved based on the response surface methodology. The speciation of Fe(VI) and Hg reaction species was emphatically analyzed in the entire pH range in order to clarify the possible Fe(VI)-based Hg0 removal mechanism in depth. According to the thermodynamics calculation, the Gibbs free energies for the reaction between Fe(VI) and Hg0 in the weak alkaline/acid medium are determined as −120.03kJ/mol and −235.42kJ/mol, respectively, implying the oxidation of Hg0 by Fe(VI) is spontaneous and thorough. Finally, industrial feasibility analysis indicates that Fe(VI) has the practical potential to be a promising removal agent for Hg0 emission control from coal-fired flue gas, which might be integrated into the existing mainstream flue gas desulfurization scrubber to achieve the removal of multi-pollutants simultaneously, in prospect.