Ammonia-based Wet Flue Gas Desulfurization Research Articles

Enhancing aerosol emission reduction in an ammonia-based WFGD system with tray implementation

Operating parameter optimization is a viable strategy to reduce aerosol emissions from an ammonia-based wet flue gas desulfurization (ammonia WFGD) system. But the optimization techniques typically cause SO2 removal efficiency reduction. Multi-hole trays can be installed in the scrubber to balance the aerosol emission control and the desulfurization efficiency. The experimental results show that the tray installation alone can barely enhance aerosol emission control. However, the tray installation can improve the SO2 removal efficiency to spare more space for operating parameter optimization. The smaller hole size favors the SO2 removal with a particular hole ratio. Optimizing the liquid-to-gas ratio (L/G) works well with tray installation to realize aerosol emission reduction, with the aerosol reduction rate over 50 %. The improvement brought about by the tray installation only works within a specific L/G range, beyond which the tray cannot make up for the desulfurization efficiency reduction caused by L/G optimization. The best aerosol emission reduction rate was achieved at L/G = 8 L/m3 with the tray installation of 35 % hole ratio and 10 mm diameter, with the desulfurization efficiency maintained above 90 %. The tray with 35 % hole ratio and 20 mm diameter can be employed at L/G = 8 L/m3 for better aerosol emission if the SO2 removal efficiency below 90 % is acceptable. The choice is due to desulfurization efficiency and aerosol emission control demand.

Structure Optimization Research Based on Numerical Simulation of Flow Field in Ammonia-Based Wet Sintering Flue Gas Desulfurization

Although more and more desulfurization equipment has been put into use in sintering plants, how to effectively remove sulfur dioxide from sintering flue gas in a desulfurization tower is still a great challenge in China. The desulfurization tower, as a critical part, needs further improvement and optimization. Therefore, based on the numerical simulation of the flow field of the ammonia-based wet desulfurization tower for sintering flue gas, ANSYS CFX is applied to conduct structural optimization research. Comparing the flow field distribution under different structure conditions, the results show that the flue gas distribution of the dual inlet tower is more uniform than that of the single inlet tower. The designed baffle not only effectively blocks the entry of the spray slurry, but also improves the flue gas distribution; the deflector with its simple structure, convenient operation, and stepped distribution installed in the entrance section can improve the uniformity of the flow field distribution. Based on the comprehensive analysis, these optimized structures are recommended in the design of an ammonia-based wet sintering flue gas desulfurization tower. This work not only develops a simulation of the desulfurization tower but also provides practical structures.

Non-Negligible Stack Emissions of Noncriteria Air Pollutants from Coal-Fired Power Plants in China: Condensable Particulate Matter and Sulfur Trioxide.

In this study, we investigated the emission characteristics of condensable particulate matter (CPM) and sulfur trioxide (SO3) simultaneously through ammonia-based/limestone-based wet flue gas desulfurization (WFGD) from four typical coal-fired power plants (CFPPs) by conducting field measurements. Stack emissions of filterable particulate matter (FPM) all meet the Chinese ultralow emission (ULE) standards, whereas CPM concentrations are prominent (even exceed 10 mg/Nm3 from two CFPPs). We find that NH4+ and Cl- increase markedly through the ammonia-based WFGD, and SO42- is generally the main ionic component, both in CPM and FPM. Notably, the occurrence of elemental Se in FPM and CPM is significantly affected by WFGD. Furthermore, the established chemical profiles in FPM and CPM show a distinct discrepancy. In CPM, the elemental S mainly exists as a sulfate, and the metallic elements of Na, K, Mg, and Ca mainly exist as ionic species. Our results may indicate that not all SO3 are included in CPM and they co-exist in stack plume. With the substantial reduction of sulfur dioxide (SO2), S distributed in SO3, CPM, and FPM becomes non-negligible. Finally, the emission factors of CPM and SO3 under typical ULE technical routes fall in the ranges of 74.33-167.83 and 48.76-86.30 g/(t of coal) accordingly.

Prediction of SO2 removal efficiency for ammonia-based wet flue gas desulfurization in a packed tower

Wet flue gas desulfurization is widely used in power plants because of its high desulfurization efficiency. The reason why it is difficult to predict sulfur dioxide removal efficiency in flue gas is that it is related to many factors. In this paper, the SO2 was removed by absorbing with ammonia solution in a packed tower. Experimental studies have shown that the operating conditions affecting the removal of sulfur dioxide include pH, liquid-to-gas, the concentration of SO2 and inlet velocity. Based on the two-film theory, the mass transfer rate equation of SO2 in the ammonia desulphurization process was studied, and the prediction of SO2 removal efficiency based on certain assumptions was developed. In order to prove the responsibility of the model, significant factors such as pH, liquid-to-gas, the concentration of SO2, and inlet velocity were studied in a packed tower. The experimental results show that it is possible to improve desulphurization efficiency by increasing the liquid-to-gas ratio and pH value, lowering the concentration of SO2 and inlet velocity. Experimental results show that the experimental value is consistent with the predicted value.

Investigation on condensable particulate matter emission characteristics in wet ammonia-based desulfurization system

Particulate matter emissions from ammonia-based wet flue gas desulfurization (Ammonia-WFGD) systems are composed of a filterable particulate matter and a condensable particulate matter (CPM) portion. However, the CPM part has been ignored for a long time, which results in an underestimation of the aerosol problems caused by Ammonia-WFGD systems. In our research, the characteristics of the CPM that emits from an Ammonia-WFGD system are investigated experimentally for the first time, with the US Environmental Protection Agency Method 202 employed as the primary measurement. The influences of some essential desulfurizing parameters are evaluated based on the experimental data. The results show that CPM contributes about 68.8% to the total particulate matter emission. CPM consists mainly of ammonium sulfates/sulfites, with the organic part accounting for less than 4%. CPM is mostly in the submicron fraction, about 71.1% of which originates from the NH3–H2O–SO2 reactions. The appropriate adjustments for the parameters of the flue gas and the desulfurizing solution can inhibit CPM formation to different extents. This indicates that the parameter optimizations are promising in solving CPM emission problems in Ammonia-WFGD systems, in which the pH adjustment alone can abate CPM emission by around 49%. The opposite variations of the parameters need attention because they can cause tremendous CPM emission increase.

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

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

Study on the ammonia emission characteristics in an ammonia-based WFGD system

Ammonia and its contribution to secondary organic and inorganic aerosols are harmful to the environment and human health. The ammonia-based wet flue gas desulfurization (ammonia-WFGD) is effective in removing SO2 from industrial flue gas but also is a potential source of ammonia emission. In this paper, experiments were done in a pilot-scale ammonia-WFGD system. The definition of the ammonia emission was first stated to include the gaseous ammonia, and the solid or liquid aerosols containing ammonia. The emitted ammonia was then distinguished into the filterable ammonia, the condensable ammonia, and the gaseous ammonia based on their behaviors during emission processes. A piecewise sampling system was built accordingly to sample them, respectively. The parameters, such as the flue gas temperature, the desulfurizing solution temperature, the solution concentration, and the pH, were adjusted to investigate their influences on the ammonia emission characteristics. Results showed that large amounts of ammonia were emitted from the ammonia-WFGD system, in which the gaseous ammonia accounted for around 19%, with the filterable ammonia and the condensable ammonia sharing the rest 81%. The flue gas temperature, the solution concentration, and the solution pH were equally important to the total ammonia emission control. Theoretically, optimizing any one of the above three parameters could lower the ammonia emission by 38%. All the parameters had impacts on the individual ammonia emissions, although to different extents. It is worthwhile to conduct further researches on parameter optimizations to achieve better ammonia emission control.

Study on the Hg emission and migration characteristics in coal-fired power plant of China with an ammonia desulfurization process

Migration and emission characteristics of mercury in coal-fired power plant with an ammonia-based wet flue gas desulfurization (Am-based WFGD) process were studied based on the Ontario Hydro Method (OHM). The temperature-programmed desorption (TPD) method was used to identify the mercury species enrichment mechanisms in fly ash and the by-products of desulfurization. The SCR has a 95.41% mercury oxidation efficiency attributed to the high chlorine content in feed coal. SCR+ESP+WFGD combination has 94.15% gaseous mercury removal efficiency. Mercury distribution analysis showed that most of incoming mercury was deposited in solid (76.46%), and only 4.94% gaseous mercury was emitted to atmosphere. The TPD results indicate that HgCl2 and HgO are the dominant mercury species in fly ash while mercury in WFGD products exists in HgCl2 form. The re-emission of mercury during ammonia desulfurization did not occur due to the presence of forced oxidation process, while mercury release from desulfurization product was monitored suggesting the mercury re-emission from the ammonium sulfate separate and drying system could not be ignored. The mercury emission factor of the coal-fired power plant is calculated as 0.319g/1012J, less than the mean value of Chinese power plant. With the equipment of Am-based WFGD, the mercury content in ammonium sulfate is 1.126mg/kg, therefore the environmental influence from ammonium should be taken into account.

Cobalt impregnated porous catalyst promoting ammonium sulfate recovery in an ammonia-based desulfurization process

Oxidation of ammonium sulfite plays an important role in the ammonia-based wet flue gas desulfurization. In this work, a porous molecular sieve (4A type) supported cobalt catalyst, named as Co-MS-4A, was developed to expedite the oxidation of ammonium sulfite. The experimental results revealed that the oxidation rate was increased by 1.7 to 6.0 times in comparison with the uncatalyzed scenario. The Co-MS-4A catalyst with a nominal Co loading of 1.2wt% exhibited the highest oxidation rate of (NH4)2SO3 among the tested catalysts. It was characterized by Brunauer–Emmett–Teller (BET), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and Energy Dispersive Spectrometer (EDS). The BET data showed that the support retained its mesoporous structure after the cobalt impregnation. Due to the low cobalt loading in the catalyst, no obvious Co diffraction peaks was observed by the XRD. The presence of the cobalt species including Co2+ and Co3+ in the catalyst was confirmed as depicted by the XPS spectra. The TEM-EDS spectra indicated that cobalt particles distributed uniformly in the tunnels of MS-4A support. Furthermore, the effects of operation parameters, such as ammonium sulfite concentration, catalyst concentration, pH, oxygen partial pressure, and temperature, on the oxidation rate of ammonium sulfite were investigated in a bubbling tank. The apparent activation energy of the ammonium sulfite oxidation catalyzed by Co-MS-4A was 17.67kJ·mol−1. The kinetic analysis implied that the catalyzed oxidation of ammonium sulfite is rapid with a Hatta number higher than ∼4.15, and that the overall reaction was controlled by the mass transfer of oxygen. The work provides a new approach for recycle of byproduct in ammonia desulfurization without transition metal consumption and pollution.

Aerosol formation by heterogeneous reactions in ammonia-based WFGD systems

Aerosol problems in the ammonia-based wet flue gas desulfurization (ab-WFGD) system are of major concern in the field of atmospheric science. Previous studies stated that the heterogeneous reactions of SO2-H2O-NH3 (the SO2 reactions) are one of the major sources of the aerosol emission from ab-WFGD systems. In this paper, the aerosol formation from the SO3 relating reactions (the SO3 reactions) was investigated experimentally. The results were compared with those from the SO2 reactions to distinguish the contributions they made to the aerosol emission. The influences of the corresponding parameters, such as the moisture content, the reaction temperature, and the NH3-to-SO2/SO3 ratio (NH3/SOx), on the aerosol formation were also explored. The results show that although the SO3 concentration is lower than the SO2 concentration, the aerosols formed by the SO3 reactions are much more than those generated from the SO2 reactions (numerical concentration). More than 90% of the aerosols are ultrafine particles. The aerosols formed from the SO2 reactions are smaller than those from the SO3 reactions. When the reaction temperatures increase above ~65°C, the SO2 reactions are hindered significantly while the SO3 reactions are barely influenced. Higher moisture content can enhance the aerosol formation from the SO2 reactions, but can hardly affect the SO3 reactions. The increment of the NH3/SOx ratios boosts both the SO2 reactions and the SO3 reactions. When the NH3/SO3 reached 2.0 and more, the aerosol formation from the SO3 reactions stops increasing while the aerosol concentration in the SO2 reactions keeps growing.Moreover, the parameter optimization methods were applied in a pilot-scale ab-WFGD system to evaluate the efficiency for aerosol emission control. The results show that decreasing the flue gas temperature before WFGD to around 80°C can reduce the aerosol emission by ~20%. Adjusting the desulfurization solution pH to about 4.5 can decrease the aerosol emission by ~45% while keeping the desulfurization efficiency above 95%.

Full-scale simulation of flow field in ammonia-based wet flue gas desulfurization double tower

With the increasingly strict environmental requirements, sintering flue gas desulfurization in double tower had been applied to improve desulfurization efficiency and reduce the “ammonia escape” and “aerosol” phenomena. In this work, the operating conditions of a sintering plant were simulated by ANSYS CFX and the flow fields without and with fluid spray of a full-scale ammonia-based wet flue gas desulfurization (WFGD) double tower were investigated. According to the results, the evaporating tower had a profound cooling effect on flue gas with spray. The gas flow distribution was non-uniform in evaporating tower, which needed further optimizations. The flow with spray was more uniform in both towers. The pressure drop mainly took place in absorption areas of desulfurization tower. The velocity of inlet region with spray was lower than that without spray in desulfurization tower. The temperature decreased along with the forward direction of gas flow due to the heat transfer with spray in desulfurization tower. The study provided useful data for further optimization in order to achieve high desulfurization efficiency.

Aerosol Formation Characteristics during Ammonia-Based WFGD Processes

The aerosol emission from the ammonia-based wet flue gas desulfurization (WFGD) system was investigated in an industrial device and in the lab-scale experimental system. The aerosol formation processes in the WFGD system were simplified into the entrainment mechanism and the heterogeneous reaction mechanism. The aerosol formation properties were studied individually under the two mechanisms. The influences of some desulfurization parameters were also explored. The results indicated that the aerosol emission increased significantly after the ammonia-based WFGD system. The aerosols were mainly in the submicron range in terms of the numerical concentration. For the mass concentration, however, the particles were mostly micron ones. The micron particles were mainly generated by the entrainment mechanism, in which condition the aerosol formation increased with the elevation of the flue gas superficial velocity and the desulfurization solution concentration. In contrast, the heterogeneous reaction mechanism was...

Investigation on the removal of SO3 in ammonia-based WFGD system

The SO3 removal properties were investigated in the wet flue gas desulfurization (WFGD) system. The relationship of the aerosol emission properties and the SO3 concentration in the flue gas was explored experimentally based on the ammonia desulfurization system. The impacts of the operating parameters, such as the liquid-to-gas ratio (L/G), the flue gas temperature, the superficial velocity, and the particle concentration in the flue gas on the SO3 removal efficiency were studied. The results showed that when the SO3 concentration in the flue gas increased, the aerosol emission concentration was higher although the aerosol sizes became smaller within the measurement range of the electrical low pressure impactor (ELPI). When the SO3 concentration was 12mg/Nm3 and 47mg/Nm3, respectively, the aerosols emitted from the WFGD system showed the similar morphologies. Under the same operating conditions, the SO3 removal efficiencies in the ammonia-based WFGD system were higher than those in the limestone–gypsum WFGD system, both of which rose with the increase of SO3 concentrations. In the ammonia-based WFGD, the SO3 removal efficiency would be improved when the L/G and the particle concentration in the flue gas were raised and the flue gas temperature and the superficial velocity were reduced. Two SO3 removal mechanisms were proposed in the ammonia-based WFGD processes. One was the collision trap mechanism and the other was the heterogeneous reaction mechanism. The sulfuric acid aerosols formed though the quenching process of hot flue gas were measured to be mainly submicron particles. For the removal of these particles, the Brownian diffusion was proved to be the predominant removal mechanism in the collision trap processes, rather than the inertial impaction.

Emission and Formation Characteristics of Aerosols from Ammonia-Based Wet Flue Gas Desulfurization

The emission and formation characteristics of aerosols were studied during the ammonia-based wet flue gas desulfurization (WFGD) process. The particle characteristics before and after desulfurization were measured in a power plant. The aerosol formation characteristics and effects of operation parameters were investigated with an experimental system. The results indicated that the aerosol emission was related to the type of ammonia-based desulfurization technology and the particle morphology and elements changed significantly during desulfurization. Aerosols formed during desulfurization were from the heterogeneous reactions and the entrainment and evaporation of desulfurization liquid. The heterogeneous reactions made greater contribution to the aerosol formation with different components, and smaller aerosols were generated. With the decrease of gas flow velocity and the solid content of desulfurization liquid, the aerosol concentration from the entrainment and the evaporation of the desulfurization liq...

Hydrolysis of Urea for Ammonia-Based Wet Flue Gas Desulfurization

Nowadays, the ammonia-based wet flue gas desulfurization is gaining more popularity. In this study, the ammonia-based wet flue gas desulfurization obtained ammonia resource from urea hydrolysis instead of anhydrous ammonia and aqueous ammonia, which was beneficial for the development of circular economy. An electric-heating reactor was employed as the hydrolysis reactor instead of traditional ones using hot air or water vapor as the heat carrier. The effects of temperature, pressure, urea solution concentration, and time on urea hydrolysis were investigated, and the optimum hydrolysis condition was estimated to be 30 (wt, %) urea solution hydrolyzing for 4 h at 140 °C, 0.4 MPa. The desulfurization experiment was conducted in a bubbling reactor. The effects of SO2 concentration and liquid–gas ratio on desulfurization were investigated. The ammonia-based absorbent for desulfurization was prepared by the ammonia produced from the 30 (wt, %) urea solution hydrolyzing for 4 h at 140 °C, 0.4 MPa. The experiment...

Numerical simulation research of flow field in ammonia-based wet flue gas desulfurization tower

With the increasingly strict requirements of environmental protection, the removal of sulfur dioxide from a desulfurization tower has been a major target in China. Aimed at sintering flue gas desulfurization tower, ANSYS CFX software was used to simulate gas–liquid two-phase flow field in full-scale, in which the demist area was modeled as a porous medium regions, and the mesh independence confirmation was took into consideration. Comparing the flow field distribution under different spray levels, the flow field of the combination model of 2nd and 3rd spray level was relatively uniform, gas–liquid contact time was longer, which was beneficial to heat transfer; And the flow field distributions in tower for 3 layouts were not yet perfect for desulfurization; the pressure drop took place mainly in the absorber zone. This research could provide reference for the optimization and the select of spray levels with high desulfurization efficiency later.

Modeling and Simulation of Sulfur Dioxide Abatement with Ammonia Absorbent in the Spray Scrubber

In this paper, a model of ammonia-based wet flue gas desulfurization system was developed to simulate the process of absorption of SO2 and oxidation of total sulfite. The influence of pH, liquid-gas ratio, flue gas velocity and SO2 concentration on the desulfurization process were analyzed. The calculated desulfurization efficiency of the ammonia-based WFGD system for 2×210 t•h-1 boilers unit in China were compared to that of corresponding measured ones. The results show that the simulated values agreed well with the measured values for the operating conditions of pH, liquid/gas ratio and SO2 concentration. Corresponding amount of air needed for oxidation of total sulfite formed in SO2 absorption process was also calculated.

The formation and removal characteristics of aerosols in ammonia-based wet flue gas desulfurization

The characteristics of aerosol generation were studied experimentally in an ammonia-based wet flue gas desulfurization process. Particle size distributions and concentrations, morphologies and compositions before and after desulfurization were measured using an electrical low pressure impactor and scanning electron microscopy, respectively. The results show that aerosols can be generated between ammonia and sulfur dioxide resulting in gas-phase reaction and the aerosol concentration at the outlet of scrubber is significantly higher than at the inlet. Before desulfurization the particles are primarily silica-alumina minerals including O, Al, Si and C, while after ammonia-based desulfurization aerosol particles have smooth surfaces with regular structures, such as cubic and prismatic crystals, and contain principally O, S and N. Particle sizes up to 10 μm were measured, but the majority of the aerosol particles are in the submicron range. Separation of submicron particles from flue gas is difficult by conventional desulfurization scrubber; however high removal efficiency can be achieved based on the enlargement of the particles by heterogeneous condensation. The influence of the supersaturation degree and liquid–gas ratio on the removal efficiencies of the particles are demonstrated additionally. It indicates that aerosols generated in ammonia-based desulfurization process can be deliquesced and gradually enlarged in a high humidity flue gas, but the supersaturation degree higher than 1.2 is required for heterogeneous nucleation of water vapor with ash particles from coal combustion.

Modeling of ammonia-based wet flue gas desulfurization in the spray scrubber

A mathematical model of ammonia-based wet flue gas desulfurization process was developed based on the double film theory. The calculated results of the desulfurization system for two 220 t·h−1 boilers per unit by this model were compared to that of corresponding measured data. It was found that the calculated results agree well with the measured data for the operating conditions of pH, liquid/gas ratio and SO2 concentration. This model can provide predictions of the absorption performance of an ammonia-based wet flue gas desulfurization process and appears to be helpful for designing scrubbers for SO2 absorption with ammonia absorbent.

Kinetics of NOxAbsorption into (NH4)2SO3Solution in an Ammonia-Based Wet Flue Gas Desulfurization Process

The mass transfer and kinetics of NOx absorption into (NH4)2SO3 solution, the main compound of an ammonia-based wet flue gas desulfurization process, have been investigated in a double-stirred reactor. Under the experimental conditions, the gas−liquid reaction between NOx and the (NH4)2SO3 solution without O2 coexisting is controlled mainly by the gas film because the (NH4)2SO3 concentration is higher than 0.05 mol/L. In this case, the absorption rate of NOx is found to be zero-order with respect to the (NH4)2SO3 concentration. The inlet partial pressure and the oxidation degree (Φ = NO2/NOx) have an apparent effect on the absorption rate of NOx. In this research, a simplified mathematical calculated model is applied to the simulation of the absorption process. The experimental results demonstrate that the orders of the reaction with respect to the concentration of NOx (NO2* or NO*) in the gas phase and the reaction rate constants of NOx (NO2* or NO*) with (NH4)2SO3 are all a function of the oxidation deg...