Wet 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.

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.

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.

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...

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.

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.

Removal characteristics of sulfuric acid aerosols from coal-fired power plants

ABSTRACTWith increasing attention on sulfuric acid emission, investigations on the removal characteristics of sulfuric acid aerosols by the limestone gypsum wet flue gas desulfurization (WFGD) system and the wet electrostatic precipitator (WESP) were carried out in two coal-fired power plants, and the effects of the WFGD scrubber type and the flue gas characteristics were discussed. The results showed that it was necessary to install the WESP device after desulfurization, as the WFGD system was inefficient to remove sulfuric acid aerosols from the flue gas. The removal efficiency of sulfuric acid aerosols in the WFGD system with double scrubbers ranged from 50% to 65%, which was higher than that with a single scrubber, ranging from 30% to 40%. Furthermore, the removal efficiency of WESP on the sulfuric acid aerosols was from 47.9% to 52.4%. With increased concentrations of SO3 and particles in the flue gas, the removal efficiencies of the WFGD and the WESP on the sulfuric acid aerosols were increased.Implications: Investigations on removal of sulfuric acid aerosols by the WFGD and the WESP in the power plants were aimed at the control of sulfuric acid emission. The results showed that the improvement of the WFGD system was beneficial for the reduction of sulfuric acid emission, while the WESP system was essential to control the final sulfuric acid aerosol concentration.

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...

Two-Phase Flow Field Numerical Simulation in Scrubber for Exhaust Gas Desulfurization

Calculation fluid dynamics software Fluent was used to conduct three-dimensional numerical simulation on gas-liquid two-phase flow field in a wet flue gas desulfurization scrubber. The k-ε model and SIMPLE computing were adopted in the analysis. The numerical simulation results show that the different gas entrance angles lead to internal changes of gas-liquid two-phase flow field, which provides references for reasonable parameter design of entrance angle in the scrubber.

Behaviour of Boiler Steel Exposed to Combustion Gases Containing Bromide and Chloride Additives

Mercury emissions from coal-fired power plants need to be reduced. In a coal-fired power plant, mercury enters the system primarily with the coal, and exits the system as bound particle compounds, soluble mercury or vapor-phase mercury. Oxidized mercury is effectively removed in wet flue gas desulfurization scrubbers – WFGD.one of the options for enhancing the process of mercury absorption is adding oxidizing agents such as bromide/bromine or chloride/chlorine. The present work describes simulation tests performed in order to evaluate the effect of bromide/chloride additions on the behavior of various steels under a combustion environment in a diesel fed steam boiler. The tested samples A209-T1A, A213-T11, A213-T22 and AISI 1020 were exposed at two locations in the boiler system: inside the flame chamber near the wall and in the middle of the stack at a temperature of 250- 300° C for 3 months. XRD and SEM techniques were used to analyze and to inspect the crystallographic structure. The results clearly show that high temperature interaction between the metal surface and the fire combustion products lead to the deposition of a protective layer composed mainly of CaSO4, FeSO4 and Fe3O4. Negligible weight loss was detected in all the tested cases. No harmful effect was detected in the presence of bromide, added as CaBr2, or chloride, added as CaCl2, to diesel fuel, up to a level of 1000 ppm.

Modeling limestone reactivity and sizing the dissolution tank in wet flue gas desulfurization scrubbers

Sulfur dioxide (SO2) is one of the pollutant gases that result from energy conversion by coal and oil combustion. Limestone slurries are widely utilized in wet flue gas desulfurization (WFGD) processes. The evaluation of the reagent's reactivity is fundamental for process design and plant operation. The comparison of different limestone and dolomite rocks through dissolution experiments was realized by applying a model for second‐order kinetics. And a total of 12 different samples were tested, at three different size fractions. A simulation model was developed to identify each sample's impact on sizing the dissolution tank of a WFGD scrubber. A second‐order model was applied within the scrubber's dissolution tank range of operation at pH values between 4 and 5. The regression coefficients for the second‐order model were higher than 0.98 in all cases. The dissolution tank volume estimation was done based on the reagent's overall chemical reaction constant in dissolution experiments. The limestone rocks, where the prevailing mineral was calcite (CaCO3), demonstrated higher chemical reactivity yielding smaller dissolution tank volumes than the dolomite rocks, where the prevailing mineral was dolomite (CaMg(CO3)2). The CaO and MgO content of a sample can be used to predict the dissolution rate; nevertheless more factors influence the rate and the tank sizing. The knowledge of the sample's composition and its reactivity is necessary for the design of a desulfurization scrubber; the reagent used is determinant for the economy and optimal operation of such scrubbers. © 2012 American Institute of Chemical Engineers Environ Prog, 32: 663–672, 2013

The importance of the location of sodium chlorite application in a multipollutant flue gas cleaning system

In this study, removing sulfur dioxide (SO2), nitrogen oxides (NOx), and mercury (Hg) from simulated flue gas was investigated in two laboratory-sized bubbling reactors that simulated an oxidizing reactor (where the NO and Hg0 oxidation reactions are expected to occur) and a wet limestone scrubber, respectively. A sodium chlorite solution was used as the oxidizing agent. The sodium chlorite solution was an effective additive that enhanced the NOx, Hg, and SO2 capture from the flue gas. Furthermore, it was discovered that the location of the sodium chlorite application (before, in, or after the wet scrubber) greatly influences which pollutants are removed and the amount removed. This effect is related to the chemical conditions (pH, absence/presence of particular gases) that are present at different positions throughout the flue gas cleaning system profile. The research results indicated that there is a potential to achieve nearly zero SO2, NOx, and Hg emissions (complete SO2, NO, and Hg removals and ∼90% of NOx absorption from initial values of 1500 ppmv of SO2, 200 ppmv of NOx, and 206 μg/m3 of Hg0) from the flue gas when sodium chlorite was applied before the wet limestone scrubber. However, applying the oxidizer after the wet limestone scrubber was the most effective configuration for Hg and NOx control for extremely low chlorite concentrations (below 0.002 M) and therefore appears to be the best configuration for Hg control or as an additional step in NOx recleaning (after other NOx control facilities). The multipollutant scrubber, into which the chlorite was injected simultaneously with the calcium carbonate slurry, appeared to be the least expensive solution (when consider only capital cost), but exhibited the lowest NOx absorption at ∼50%. The bench-scale test results presented can be used to develop performance predictions for a full- or pilot-scale multipollutant flue gas cleaning system equipped with wet flue gas desulfurization scrubber. Implications The control of NOx, Hg, and SO2 emissions from coal-fired combustors is an important, worldwide concern affecting environmental pollution. In both the European Union and the United States, NOx and SO2 emissions are subject to newly tightened emission limits. This paper investigated the NOx, Hg, and SO2 removals in a flue gas cleaning system equipped with a classical wet limestone scrubber, enhanced by a solution of sodium chlorite. This paper demonstrated that the location at which the sodium chlorite is applied in a flue gas cleaning system affects the control of these pollutants and discusses how possible solutions could be practically implemented.

Studies of the Fate of Sulfur Trioxide in Coal-Fired Utility Boilers Based on Modified Selected Condensation Methods

The formation of sulfur trioxide (SO(3)) in coal-fired utility boilers can have negative effects on boiler performance and operation, such as fouling and corrosion of equipment, efficiency loss in the air preheater (APH), increase in stack opacity, and the formation of PM(2.5). Sulfur trioxide can also compete with mercury when bonding with injected activated carbons. Tests in a lab-scale reactor confirmed there are major interferences between fly ash and SO(3) during SO(3) sampling. A modified SO(3) procedure to maximize the elimination of measurement biases, based on the inertial-filter-sampling and the selective-condensation-collecting of SO(3), was applied in SO(3) tests in three full-scale utility boilers. For the two units burning bituminous coal, SO(3) levels starting at 20 to 25 ppmv at the inlet to the selective catalytic reduction (SCR), increased slightly across the SCR, owing to catalytic conversion of SO(2) to SO(3,) and then declined in other air pollutant control device (APCD) modules downstream to approximately 5 ppmv and 15 ppmv at the two sites, respectively. In the unit burning sub-bituminous coal, the much lower initial concentration of SO(3) estimated to be approximately 1.5 ppmv at the inlet to the SCR was reduced to about 0.8 ppmv across the SCR and to about 0.3 ppmv at the exit of the wet flue gas desulfurization (WFGD). The SO(3) removal efficiency across the WFGD scrubbers at the three sites was generally 35% or less. Reductions in SO(3) across either the APH or the dry electrostatic precipitator (ESP) in units burning high-sulfur bituminous coal were attributed to operating temperatures being below the dew point of SO(3).

Investigation of a Mercury Speciation Technique for Flue Gas Desulfurization Materials

Most of the synthetic gypsum generated from wet flue gas desulfurization (FGD) scrubbers is currently being used for wallboard production. Because oxidized mercury is readily captured by the wet FGD scrubber, and coal-fired power plants equipped with wet scrubbers desire to benefit from the partial mercury control that these systems provide, some mercury is likely to be bound in with the FGD gypsum and wallboard. In this study, the feasibility of identifying mercury species in the FGD gypsum and wallboard samples was investigated using a large sample size thermal desorption method. Potential candidates of pure mercury standards including mercuric chloride (HgCl2), mercurous chloride (Hg2Cl2), mercury oxide (HgO), mercury sulfide (HgS), and mercuric sulfate (HgSO4) were analyzed to compare their results with those obtained from FGD gypsum and dry wallboard samples. Although any of the thermal evolutionary curves obtained from these pure mercury standards did not exactly match with those of the FGD gypsum and wallboard samples, it was identified that Hg2Cl2 and HgCl2 could be candidates. An additional chlorine analysis from the gypsum and wallboard samples indicated that the chlorine concentrations were approximately 2 orders of magnitude higher than the mercury concentrations, suggesting possible chlorine association with mercury.

Kinetics of NO absorption in aqueous iron (II)thiochelate solutions

AbstractFerrous thiochelate solutions were used to absorb NO from flue gas. The absorption rates were studied as a function of pH with a double‐stirred reactor at 50°C. The rate constants of five ferrous thiochelates: Fe2+(dimercaptosuccinate)2, Fe2+(dithioloxalate)2, Fe2+(aminoethanethiol)2, Fe2+(cysteine)2, Fe2+(penicillamine)2, reacted with NO at pH5.5 and pH 7.0, have been determined. The implications of the kinetic data on the use of these iron thiochelate additives in wet flue gas desulfurization scrubbers: limestone, lime, and sodium, for simultaneous SO2 and NOx removal is discussed.

Integrated Tests for Removal of Nitric Oxide with Iron Thiochelate in Wet Flue Gas Desulfurization Systems

Wet flue gas desulfurization scrubbers can be retrofitted for combined removal of SO 2 and NO x from flue gas by adding a newly developed iron(II) thiochelate to limestone slurries. This additive enhances the solubility of NO in limestone slurries by binding NO to form iron nitrosyl thiochelates. The bound NO is then converted to an ammonium ion by iron metal, regenerating the active iron(II) catalyst for continued NO capture. The conditions of the chemical regeneration of iron(II) thiochelate have been studied. This chemical reduction method is efficient, simple, and cost-effective. Integrated tests of this new iron additive on a bench-scale system have been conducted. A turbulent contact absorber was employed to test simultaneous removal of NO and SO 2 with iron(II) thiochelate in a limestone slurry. The steady-state NO removal efficiency depends on the operating conditions of an absorber, the iron chelate concentration, the fraction of the flow diverted for regeneration, and the conditions used in the regeneration reactor. The bench-scale test results were used to develop a prediction of the performance of a full-scale spray absorber.

Experimental approach and techniques for the evaluation of wet flue gas desulfurization scrubber fluid mechanics

The fluid mechanics within wet flue gas desulfurization (FGD) scrubbers involve several complex two-phase gas/liquid interactions. The fluid flow directly affects scrubber pressure drop, mist eliminator water removal, and the SO 2 mass transfer/chemical reaction process. Current industrial efforts to develop cost-effective high-efficiency wet FGD scrubbers are focusing, in part, on optimizing the fluid mechanics. The development of an experimental approach and test facility for understanding and optimizing wet scrubber flow characteristics is discussed in this paper. Specifically, scaling procedures for downsizing a wet scrubber for the laboratory environment with field data comparisons are summarized. Furthermore, experimental techniques for the measurement of wet scrubber flow distribution, pressure drop, spray nozzle droplet size characteristics and wet scrubber liquid-to-gas ratio are discussed. Finally, the characteristics and capabilities of a new hydraulic test facility for wet FGD scrubbers are presented.

Experimental approach and techniques for the evaluation of wet flue gas desulfurization scrubber fluid mechanics

Experimental approach and techniques for the evaluation of wet flue gas desulfurization scrubber fluid mechanics