Flue Gas Desulfurization System Research Articles

Limestone-based dual-loop wet flue gas desulfurization under oxygen-enriched combustion

Oxygen-enriched combustion is the most promising CO2 emission reduction technology for coal burning. However, the sharp increase in CO2 and SO2 concentrations in flue gas after oxygen-enriched combustion poses a new challenge to the subsequent wet flue gas desulfurization (WFGD) system. In this study, limestone-based dual-loop WFGD technology was used to solve this problem. Using a small dual-loop falling-film WFGD experimental system, the effect of CO2 concentration on limestone dissolution, SO2 absorption, SIV oxidation, and CaSO4 crystallization during limestone-based dual-loop WFGD was investigated in detail and compared with single-loop WFGD. The results show that, although an increase in CO2 concentration is beneficial to SO2 absorption in the single-loop WFGD system, it is not conducive to limestone dissolution, SIV oxidation, and CaSO4 crystallization. Compared with single-loop WFGD, in the low pH region of dual-loop WFGD, increasing the CO2 concentration can also improve the SO2 absorption efficiency. However, it had no obvious effect on limestone dissolution, SIV oxidation, and CaSO4 crystallization. In the high pH region of the dual-loop WFGD, although an increase in CO2 concentration is not conducive to SO2 absorption, the SO2 absorption efficiency is still higher than that of single-loop WFGD. The negative effect of increasing the CO2 concentration on limestone dissolution, SIV oxidation, and CaSO4 crystallization can be effectively solved by introducing slurry from the high pH region into the low pH region. Therefore, compared with single-loop WFGD technology, dual-loop WFGD is more suitable for flue gas desulfurization after oxygen-enriched combustion.

Deep neural network-based optimal selection and blending ratio of waste seashells as an alternative to high-grade limestone depletion for SOX capture and utilization

• Wet flue gas desulfurization process was optimized using waste oyster shell. • Deep neural network–based surrogate model is used to cost optimization. • Seashells with low-grade limestone could fully replace high-grade limestone. • At an optimal blending ratio, total annualized cost could be reduced by US$788,469. • The results facilitate cost-effective and eco-friendly SO x capture and utilization. In wet flue gas desulfurization system, the resource depletion of high-grade limestone, used as conventional SO x absorbent, is becoming serious for SO x capture and utilization. This paper proposes optimal selection and blending ratio of waste seashells as an alternative to high-grade limestone depletion using a deep neural network (DNN)-based surrogate model. Cost optimization proceeds as follows: data generation, data preprocessing, development of DNN-based surrogate model, and derivation of cost optimal selection and blending ratio. First, a process model is developed to generate the datasets, which are gypsum purity according to selection and blending ratio of each seashell and limestone. In addition, a mathematical model is proposed to calculate the total annualized cost (TAC) considering the pretreatment cost of seashell, and the TAC is added to the datasets to predict the gypsum purity as well as TAC. Second, the generated datasets are preprocessed to intensify prediction performance of the DNN-based surrogate model using the z-score normalization method. Third, a DNN-based surrogate model is developed to predict the gypsum purity and TAC according to the selection and blending ratio. Finally, the cost optimal selection and blending ratio are derived from 2.4 billion data generated by the developed DNN-based surrogate model under two constraints: gypsum purity and total SO x absorbent consumption. As a result, the derived selection and optimal blending ratios are low-grade limestone (80.86%), oyster shells (10.78%), scallop shells (0.216%), cockle shells (0.323%), clam shells (2.426%), and mussel shells (5.391%), reducing the TAC by US$788,469.

Experimental study on migration and transformation of selenium species and DFT calculation in WFGD simulated slurry

Selenium is a harmful trace element emitted during the combustion process of coal-fired power plants. Most of the selenium is enriched in the downstream wet flue gas desulfurization system (WFGD). This study found that S2O82- is the most important factor affecting the oxidation of selenite. In the process of simulating slurry oxidation of selenite to selenate, the presence of S2O82- can increase the oxidation rate of selenate from about 50% to about 80%. At the same time, this study also considered the influence of selenium concentration, initial pH, SO2 concentration and temperature etc. on the transformation of selenium species. It was believed that the application of ORP to the indication of selenite oxidation status needs further demonstration. Energy Dispersive X-Ray Spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS) were used to confirm the migration of selenium in the desulfurization slurry to gypsum. In addition, DFT calculation was performed to obtain theoretical parameters for the adsorption of selenite and selenate by gypsum. The results of this study are helpful to understand the transformation and migration of selenium in the desulfurization system, which provide theoretical support for the realization of selenium in-situ control.

Study on characteristics of organic components in condensable particulate matter before and after wet flue gas desulfurization system of coal-fired power plants

Wet flue gas desulfurization (WFGD) in coal-fired power plants has a great impact on the emission of particulate matter, including filterable particulate matter (FPM) and condensable particulate matter (CPM). In this paper, CPM and FPM in flue gas before and after WFGD in coal-fired power plants were sampled in parallel. FPM was tested according to ISO standard 23210–2009, and CPM was tested according to U.S. EPA Method 202. A method for quantitatively analyzing fatty acid methyl esters in CPM was established, and the removal capacity of fatty acid methyl esters and phthalate esters by WFGD in a typical coal-fired unit was compared. Results show that WFGD has a significant effect on particle size distribution, concentration, and chemical composition. WFGD has a high removal efficiency of inorganic components in CPM, up to 54.74%. CPM contains a variety of organic compounds, including hydrocarbons, esters, siloxanes, halogenated hydrocarbons, and so on. In particular, esters are an important component in CPM, whose concentration tends to decrease after WFGD. Furthermore, a total of 11 fatty acid methyl esters and 5 phthalate esters were detected in CPM before and after WFGD. Noted that fatty acid methyl esters account for 13.38% of CPM, which make a higher contribution to the concentration of particulate matter than phthalate esters, while WFGD has a stronger control effect on the removal of phthalates.

Coal-Fired Boiler Flue Gas Desulfurization System Based on Slurry Waste Heat Recovery in Severe Cold Areas.

To reduce operating costs on the basis of ensuring the desulfurization efficiency in a wet flue gas desulfurization system, a theoretical model was put forward, and a calculation method was set up. Correlations between reaction zone height, flue gas inlet temperature, slurry inlet temperature, gas–liquid ratio and desulfurization efficiency were found. Based on the heat and mass transfer model of the spray tower, the integrated system of desulfurization tower and open slurry pool and the flue gas desulfurization-waste heat recovery system were established. Additionally, the effect of outdoor wind speed, heat dissipation area and ambient temperature on the slurry equilibrium temperature in the integrated system were analyzed. The results show the slurry equilibrium temperature of the desulfurization system is negatively correlated with outdoor wind speed and heat dissipation area, and positively related to ambient temperature. The slurry temperature is the main factor that affects the performance of the wet flue gas desulfurization system. Finally, based on the Harbin heating group Hua Hui hotspot energy-saving reconstruction project, a case analysis was conducted, which proves the flue gas desulfurization-waste heat recovery system is profitable, energy saving and a suitable investment project.

Research on Control Method of Wet Desulfurization System for Coal-fired Boiler

In wet flue gas desulfurization technology, the key factor affecting flue gas SO2 absorption is the control of slurry pH value in the absorption tower. However, most of the commonly used pH value control methods have some defects and cannot fully meet the system control requirements. On the basis of the analysis for the problems in the control process, this paper discussed a control strategy based on internal model control algorithm. The system simulation test showed that this control method had good dynamic performance, robustness and anti-interference, which could realize the relatively accurate control of pH value for wet flue gas desulfurization system, and achieve the control goal.

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.

Comparison of PMtotal, PM10, PM2.5, NOx, and SO2 Emission Factors from Coal-fired Power Plants per Load Change

For two bituminous coal-fired power plants with 500 MW and pulverized coal combustion type, the concentration of PMtotal, >PM10, PM2.5-10, PM2.5, NOx, and SO2 was measured, and their emission factors were calculated through field measurement. The measurement points started from the boiler downstream and continued to the air pollution control devices (APCDs) that are installed in series, namely, the selected catalytic reduction system (SCR), air preheater (APH), electrostatic precipitator (ESP) and wet flue gas desulfurization system (WFGD). The measurement was performed at one point for more than three times by using the Korean standard method for air pollutants. However, all measurement points, except for the stacks, were not representative of the standard test method. In addition, the PM concentration was too high to reduce the collection time due to isokinetic sampling. There is a limitation of how representative the measurement results can be. During the field measurement period, the power production rate of the two coal-fired power plants was 91.6% and 79.2% in the P-1 and P-2, respectively. Moreover, in the P-2, with a low power production rate, the concentration of PMtotal, PM10, PM2.5, and NOx was found to be low, and the emission factor calculated by dividing the measured concentration value by the fuel usage was also estimated to be low. Such results are due to the coal combustion chamber and various types of APCD being operated at a lower-load condition than the design capacity. In turn, the number of pollutants generated was less, and the removal efficiency of the pollutant became high. However, it was found that the concentration of SO2 generated and the emission factor are more significantly affected by the sulfur content of the coal than the load factor change. To this end, reducing the operation load of the coal-fired power plant improves the combustion efficiency and APCDs performance and decreases the emission factor, resulting in more reduction of the air pollutants than that based on the simple calculation.

Optimization of wet flue gas desulfurization system using recycled waste oyster shell as high-grade limestone substitutes

Wet flue gas desulfurization, which is performed in many thermal power plants, has a high desulfurization efficiency and produces desulfurized gypsum as a by-product. Currently, high-grade limestone with a CaCO3 content of 94 wt% or more is used to produce desulfurized gypsum with a purity of 93 wt% or more. However, high-grade limestone resources are depleting, so new substitutes to this are required. The objective of this work is to optimize the wet flue gas desulfurization system using recycled waste oyster shells as high-grade limestone substitutes. The process model was developed for predicting the optimal blending ratio of waste oyster shells to limestone with constraints of desulfurization efficiency and purity of desulfurized gypsum. A mathematical model was addressed for optimized process costs, which was a nonlinear programming problem that minimizes the total annualized cost (TAC) by considering the total product cost (TPC) and equivalent annual cost; these vary according to the blending ratio. The optimal blending ratio of waste oyster shells is 16.160 wt% or 599.536 kg/h, TPC and TAC are reduced by $ 840,721 and $ 760,543, respectively. The waste oyster shell can be utilized 4,901 t/y, which is about 20.66 wt% of the total landfilled waste oyster shell. The results of this study suggest waste oyster shells, which pose landfill and odor problems, can be used as excellent substitutes for high-grade limestone for high economic and environmental benefits.

Effect of Gas-Liquid Contact Intensification on Heat and Mass Transfer in Deflector and Rod Bank Desulfurization Spray Tower

The deflector and the rod bank are commonly used to optimize flue gas distribution in the original spray tower (OST) of a wet flue gas desulfurization system (WFGD). In this paper, the internal optimization mechanism of the deflector desulfurization spray tower (DST) and the rod bank desulfurization spray tower (RBST) are studied. Based on the Euler–Lagrange method, the standard k-ε turbulence model, an SO2 absorption model and a porous media model, the numerical simulation of the desulfurization spray tower is carried out with the verification of the model rationality. The results show that there are gas-liquid contact intensification effects in DST and RBST. Compared with OST, gas-liquid contact intensification enhances the heat and mass transfer effects of DST and RBST. The temperature difference between inlet and outlet of flue gas increased by 3.3 K and the desulfurization efficiency of DST increased by 1.8%; the pressure drop decreased by 37 Pa. In RBST, the temperature difference between the flue gas inlet and outlet increased by 5.3 K and the desulfurization efficiency increased by 3.6%; the pressure drop increased by 33 Pa.

Cause Analysis and Treatment of Slurry Foaming in Desulfurization Absorption Tower of 300MW Unit

In view of the serious slurry foaming and overflow problems in the flue gas desulfurization system of a company in Shandong, the causes were analyzed and improvements were made on the basis of the original process. The results show that the inadequate combustion of the intermediate ash of the boiler and the high concentration of organic matter in the desulfurization process water are the main reasons for the slurry foaming; after improving the boiler operating conditions and adding defoamers, the boiler fly ash contains carbon The circulating pump current returns to normal levels, the desulfurization efficiency is increased from 97.2% to 99%, the power consumption of the circulating pump is reduced from 380 kWh to 316 kWh, and the moisture content of desulfurized gypsum is reduced from 16% to 9.5%.

Identification of Microbiological Activities in Wet Flue Gas Desulfurization Systems

Thermoelectric power generation from coal requires large amounts of water, much of which is used for wet flue gas desulfurization (wFGD) systems that minimize sulfur emissions, and consequently, acid rain. The microbial communities in wFGDs and throughout thermoelectric power plants can influence system performance, waste processing, and the long term stewardship of residual wastes. Any microorganisms that survive in wFGD slurries must tolerate high total dissolved solids concentrations (TDS) and temperatures (50–60°C), but the inocula for wFGDs are typically from fresh surface waters (e.g., lakes or rivers) of low TDS and temperatures, and whose activity might be limited under the physicochemically extreme conditions of the wFGD. To determine the extents of microbiological activities in wFGDs, we examined the microbial activities and communities associated with three wFGDs. O2 consumption rates of three wFGD slurries were optimal at 55°C, and living cells could be detected microscopically, indicating that living and active communities of organisms were present in the wFGD and could metabolize at the high temperature of the wFGD. A 16S rRNA gene-based survey revealed that the wFGD-associated microbial communities included taxa attributable to both thermophilic and mesophilic lineages. Metatranscriptomic analysis of one of the wFGDs indicated an abundance of active Burholderiaceae and several Gammaproteobacteria, and production of transcripts associated with carbohydrate metabolism, osmotic stress response, as well as phage, prophages, and transposable elements. These results illustrate that microbial activities can be sustained in physicochemically extreme wFGDs, and these activities may influence the performance and environmental impacts of thermoelectric power plants.

Simulation of SO2 absorption and performance enhancement of wet flue gas desulfurization system

At the present moment wet flue gas desulfurization systems (WFGD) are widely used in the SO2 emitting industries. With the improvement of the SO2 emission standards and the use of fuel with high sulfur content, there is an urgent need to develop advanced modelling tools to explore the enhancing method of SO2 absorption process on the condition of flue gas with high SO2 concentration. In this study, a detailed mathematical model of tray scrubbers, which are adopted in several China’s power plants in order to treat SO2 rich flue gas, is developed to investigate the influencing factors of the SO2 absorption process. The model is validated by an experiment using pilot-scale WFGD experimental system and can describe the SO2 absorption process in WFGD to a satisfactory level. The influence factors of SO2 absorption of droplets and in spray tower are studied through the model. The effects of pH and tray on desulfurization efficiency are analyzed. Using the tray scrubber is better in order to realize the desulfurization of flue gas with high SO2 concentration rather than adjusting the pH of the slurry. This model can be regarded as a useful tool for equipment selection and operation optimization of WFGD system.

Effects of metal ions on Hg0 re-emission under air and oxy-fuel combustion atmospheres in a WFGD system

The effects of metal ions, including Fe2+, Fe3+, Mn2+, and Cu2+, on Hg0 re-emission were investigated in a simulated wet flue gas desulfurization (WFGD) system under air and oxy-fuel combustion atmospheres. The results show that in the presence of metal ions, Hg0 re-emission is enhanced under a N2 atmosphere but inhibited under N2/9%O2, air, and oxy-fuel combustion atmospheres. In the inhibition cases, Hg2+ reduction and Hg0 re-emission were accelerated in the initial stages of the experiments. Fortunately, Fe2+, Fe3+, Mn2+, and Cu2+ have a positive impact on mercury stabilization in the liquid phase, and the precipitates of Fe2+/Fe3+ can also adsorb mercury in the solid phase, thus offsetting the acceleration of Hg0 re-emission in the initial stage. Therefore, Hg0 re-emission is suppressed by metal ions under N2/9%O2, air, and oxy-fuel combustion atmospheres. With an increase in CO2 concentration, the inhibition of Fe2+, Fe3+, and Cu2+ was intensified, whereas the suppression by Mn2+ was slightly weakened. Under an oxy-fuel combustion atmosphere, increasing O2 concentration would promote the inhibition of Mn2+ on Hg0 re-emission, but hardly affect the inhibitory effect of Cu2+.

A new calculation method of efficiency for gypsum and wastewater hydrocyclones in FGD unit in a power plant

This study was carried on hydrocyclones in the Wet Flue Gas Desulfurization (WFGD) system of a local thermal power plant. In WFGD systems, hydrocyclones are used for classification in terms of PSD of limestone, dewatering the gypsum slurry and recycling the wastewater. Seperation efficiencies of hydrocyclones (waste water and gypsum) in power plant were calculated referring to each hydrocyclones’ inlet size of D25. Results obtained with Malvern Mastersizer for the samples from each exits of hydroclones were taken into consideration. Separation efficiency for waste water hydrocyclone was calculated as 4.0 % while it was calculated 77.5 % for gypsum hydrocyclone.

Effect of Fe3+ concentration and pH on arsenic removal, migration and speciation transformation in lab-scale wet flue gas desulfurization system

Wet flue gas desulfurization (WFGD) system of coal-fired power station can synergistically remove arsenic from flue gas. However, the detailed mechanism of arsenic removal by desulfurization slurry and its further migration and transformation from gas to desulfurization products still remains unclear. In this study, two important factors, the Fe3+ concentration and the pH value, were investigated for their effects on arsenic removal, redistribution and transformation in a lab-scale reactor. The results show low Fe3+ concentration (≤1 mg L-1) in slurry may lead to the decrease of particle size and the increase of Zeta potential of gypsum particles, thus promoting arsenic removal from flue gas, while more Fe3+ (10–500 mg L-1) could reduce its removal due to the increase of mass transfer resistance. Arsenic removal efficiency decreases with pH value rising. The slurry with Fe3+ is easier to remove As(III) from flue gas than As(V) due to the strong polarity of AsCl3(g). Either increasing Fe3+ concentration (pH = 5.5) or rising pH value (Fe3+ concentration = 50 mg L-1) can promote arsenic migration from desulfurization wastewater into gypsum. At any pH value, Fe-gypsum has a stronger ability to adsorb As(V) than As(III). Acid leaching model test indicates the arsenate incorporated into gypsum lattice can reach 12% without Fe3+, but the presence of Fe3+ hinders the arsenate doped into gypsum crystal, which is less than 1%. This raises the potential environmental risk of further utilization of gypsum due to the unstable arsenic speciation. The main arsenates substituting SO42- doped into gypsum lattice are HAsO42- and AsO43-.

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.

Mercury in desulfurization gypsum and its dependence on coal properties in coal-fired power plants

To disclose possible influencing factors on mercury (Hg) contents in flue gas desulfurization (FGD) gypsum of coal-fired power plants (CFPPs), 11 major CFPPs in Guizhou province of southwest China were surveyed for analyzing Hg contents in feed coal, limestone and gypsum as well as coal properties. It was found that Hg contents in the desulfurization gypsum of the 11 CFPPs varied greatly from 160 to 1482 μg kg−1, with an average of 595 ± 353 μg kg−1. The majority of Hg in gypsum was introduced from the flue gas downstream of the particulate matter control devices, while a much lesser portion (average 3.43 ± 3.85%) was stemmed from limestone. Hg content in gypsum has a strong positive correlation with chlorine content, a weak positive correlation with Hg content and a weak negative correlation with sulfur content in feed coal. Multiple regression analysis results showed that Hg and Cl contents in feed coal were the major determinants of Hg content in flue gas FGD gypsum despite the proportioning of Hg went in wet FGD system was not been evaluated. Results from the present study suggested that coal characteristics are important factors affecting Hg contents in gypsum.

Experimental research and engineering application on the treatment of desulfurization wastewater from coal-fired power plants by spray evaporation

The spray evaporation is a promising zero discharge technology for desulphurization wastewater treatment. In this work, a pilot-scale platform for the spray evaporation of desulfurization wastewater was built, the desulphurization wastewater was atomized by a rotary atomizer and evaporate in the evaporation tower, and the wastewater atomization and evaporation characteristics were investigated systematically. The results showed that the increase of atomizer rotating speed can improve the atomization effect of desulfurization wastewater, while the increase of its flow rate and total amount of dissolved solids (TDS) was not conducive to that. For the evaporation of desulfurization wastewater, the increase of flue gas temperature, gas-liquid ratio (the ratio of flue gas volume to desulfurization wastewater volume, Nm3 m−3) and atomizer rotating speed can promote its rapid evaporation, while the increase of wastewater TDS reduced the evaporation rate of wastewater droplets. On this basis, a bypass evaporation tower system was designed and constructed in a 330 MW coal-fired power plant to treat the desulphurization wastewater from limestone–gypsum wet flue gas desulfurization (WFGD) system, and the influence of wastewater evaporation on the flue gas and particle characteristics was investigated under different boiler loads. The results show that this evaporation system can ensure the evaporation effect of desulphurization wastewater under different boiler loads, lead to the reduction of flue gas temperature and the increase of humidity, reduce the concentration of SO2 and particles in flue gas, and enlarge the particle size at the outlet of evaporation tower. This work can provide a reference for the desulphurization wastewater treatment in coal-fired power plants.

Mercury speciation and mass distribution of coal-fired power plants in Taiwan using different air pollution control processes

ABSTRACT In this study, the mercury (Hg) emission, speciation, and mass distribution of four coal-fired power plants (CFPPs) located at central, southern, and northern Taiwan with various types of air pollution control devices were investigated. Gaseous Hg in the coal-combustion flue gas was sampled by using the Ontario Hydro method, and the solid and liquid samples were collected for understanding the Hg mass balance. The experimental results showed that the total Hg concentrations in flue gases at the inlets of selective catalytic reduction (SCR) varied from 2.984 to 4.692 μg Nm−3, while the total Hg concentrations in the flue gases at the stacks ranged from 0.240 to 0.675 μg Nm−3. These four CFPPs showed similar Hg speciation results at the stacks. The average Hg removal efficiencies of Plants 1 (SCR + electrostatic precipitator [ESP] + wet flue gas desulfurization [WFGD]), 2 (SCR + ESP + WFGD), 3 (SCR + bag filter (BF) + seawater flue gas desulfurization [SWFGD]) and 4 (SCR + BF + SWFGD) were 92.4%, 90.1%, 85.9%, and 84.8%, respectively. Coal was the major raw material in Hg input of CFPPs with a mass flow rate ranging 5.87–12.05 g hr−1. Elemental Hg (Hg0), accounting for 66.4%−97.1% of the total Hg, was the dominant species emitted to the atmosphere. The Hg mass balances for the four CFPPs varied from 86.0% to 117% of the Hg input, suggesting that good mass balances were obtained from the tested CFPPs. Implications: Mercury emissions from coal-fired power plant (CFPPs) have been greatly concerned and should thus be better comprehended. The present study examined the mercury speciation and mass distribution of four CFPPs located at Taiwan. Overall, these CFPPs had similar Hg speciation results at stack and Hg0 was the dominant species emitted to the atmosphere. The selective catalytic reduction (SCR) + electrostatic precipitator (ESP) + wet flue gas desulfurization (WFGD) system had the highest Hg removal efficiency and the Hg mass balances for the four CFPPs varied from 86.0 to 117%. This study helps better understanding the Hg emission inventory of CFPPs and provides useful information for selecting adequate air pollution control devices (APCDs) for Hg control.