Local deposition of mercury in topsoils around coal-fired power plants: is it always true?

Mercury (Hg) is a toxic element that is emitted to the atmosphere through human activities, mainly fossil fuel combustion. Hg accumulations in soil are associated with atmospheric deposition, while coal-burning power plants remain the most important source of anthropogenic mercury emissions. In this study, we analyzed the Hg concentration in the topsoil of the Kozani–Ptolemais basin where four coal-fired power plants (4,065 MW) run to provide 50 % of electricity in Greece. The study aimed to investigate the extent of soil contamination by Hg using geostatistical techniques to evaluate the presumed Hg enrichment around the four power plants. Hg variability in agricultural soils was evaluated using 276 soil samples from 92 locations covering an area of 1,000 km². We were surprised to find a low Hg content in soil (range 1–59 μg kg^?1) and 50 % of samples with a concentration lower than 6 μg kg^?1. The influence of mercury emissions from the four coal-fired power plants on soil was poor or virtually nil. We associate this effect with low Hg contents in the coal (1.5–24.5 μg kg^?1) used in the combustion of these power plants (one of the most Hg-poor in the world). Despite anthropic activity in the area, we conclude that Hg content in the agricultural soils of the Kozani–Ptolemais basin is present in low concentrations.     

Performance of Copper Chloride-Impregnated Sorbents on Mercury Vapor Control in an Entrained-Flow Reactor System

Abstract

An entrained-flow system has been designed and constructed to simulate in-flight mercury (Hg) capture by sorbent injection in ducts of coal-fired utility plants. The test conditions of 1.2-sec residence time, 140 °C gas temperature, 6.7 m/sec (22 ft/sec) gas velocity, and 0–0.24 g/m³ (0–15 lbs of sorbent per 1 million actual cubic feet of flue gas [lb/MMacf]) sorbent injection rates were chosen to simulate conditions in the ducts. Four kinds of sorbents were used in this study. Darco Hg-LH served as a benchmark sorbent with which Hg control capability of other sorbents could be compared. Also, Darco-FGD was used as a representative raw activated carbon sorbent. Two different copper chloride-impregnated sorbents were developed in our laboratory and tested in the entrained-flow system to examine the possibility of using these sorbents at coal-fired power plants. The test results showed that one of the copper chloride sorbents has remarkable elemental mercury (Hg⁰) oxidation capability, and the other sorbent demonstrated a better performance in Hg removal than Darco Hg-LH.     

Distribution of mercury in the combustion products from coal-fired power plants in Guizhou,southwest China

Method 30B and the Ontario Hydro Method (OHM) were used to sample the mercury in the flue gas discharged from the seven power plants in Guizhou Province, southwest China. In order to investigate the mercury migration and transformation during coal combustion and pollution control process, the contents of mercury in coal samples, bottom ash, fly ash, and gypsum were measured. The mercury in the flue gas released into the atmosphere mainly existed in the form of Hg°. The precipitator shows a superior ability to remove Hg^p (particulate mercury) from flue gas. The removal efficiency of Hg²⁺ by wet flue gas desulfurization (WFGD) was significantly higher than that for the other two forms of mercury. The synergistic removal efficiency of mercury by the air pollution control devices (APCDs) installed in the studied power plants is 66.69–97.56%. The Hg mass balance for the tested seven coal-fired power plants varied from 72.87% to 109.67% during the sampling time. After flue gas flowing through APCDs, most of the mercury in coal was enriched in fly ash and gypsum, with only a small portion released into the atmosphere with the flue gas. The maximum discharge source of Hg for power plants was fly ash and gypsum instead of Hg emitted with flue gas through the chimney into the atmosphere. With the continuous upgrading of APCDs, more and more mercury will be enriched in fly ash and gypsum. Extra attention should be paid to the re-release of mercury from the reutilization of by-products from APCDs.

Implications: Method 30B and the Ontario Hydro Method (OHM) were used to test the mercury concentration in the flue gas discharged from seven power plants in Guizhou Province, China. The concentrations of mercury in coal samples, bottom ash, fly ash, and gypsum were also measured. By comparison of the mercury content of different products, we found that the maximum discharge source of Hg for power plants was fly ash and gypsum, instead of Hg emitted with flue gas through the chimney into the atmosphere. With the continuous upgrading of APCDs, more and more mercury will be enriched in fly ash and gypsum. Extra attention should be paid to the re-release of mercury from the reutilization of by-products from APCDs.     

Effect of temperature and particle size on the thermal desorption of PCBs from contaminated soil

Thermal desorption is widely used for remediation of soil contaminated with volatiles, such as solvents and distillates. In this study, a soil contaminated with semivolatile polychlorinated biphenyls (PCBs) was sampled at an interim storage point for waste PCB transformers and heated to temperatures from 300 to 600 °C in a flow of nitrogen to investigate the effect of temperature and particle size on thermal desorption. Two size fractions were tested: coarse soil of 420–841 μm and fine soil with particles <250 μm. A PCB removal efficiency of 98.0 % was attained after 1 h of thermal treatment at 600 °C. The residual amount of PCBs in this soil decreased with rising thermal treatment temperature while the amount transferred to the gas phase increased up to 550 °C; at 600 °C, destruction of PCBs became more obvious. At low temperature, the thermally treated soil still had a similar PCB homologue distribution as raw soil, indicating thermal desorption as a main mechanism in removal. Dechlorination and decomposition increasingly occurred at high temperature, since shifts in average chlorination level were observed, from 3.34 in the raw soil to 2.75 in soil treated at 600 °C. Fine soil particles showed higher removal efficiency and destruction efficiency than coarse particles, suggesting that desorption from coarse particles is influenced by mass transfer.     

Release of Mercury Vapor from Coal Combustion Ash

Abstract

The long-term stability of Hg in coal combustion byproducts (CCBs) was evaluated at ambient and near-ambient temperatures. Six CCB samples with atypically high levels of total Hg were selected for study assuming a greater potential for release of measurable amounts of Hg vapor. The samples selected included two fly ash samples from U.S. eastern bituminous coal, two fly ash samples from South African low-rank coal, one fly ash from Powder River Basin (PRB) subbituminous coal blended with petroleum coke, and one PRB subbituminous coal fly ash incorporated with flue gas desulfurization material.

Air scrubbed of Hg was passed through compacted 100-g aliquots of each sample at 1 mL/min and vented to a gold-coated quartz trap to collect released Hg vapor. The samples were maintained at ambient and near-ambient (37 °C) temperatures. All samples released low-picogram levels of Hg after 90 days. No pattern was evident to link the total Hg content to the rate of release of Hg vapor. An average of 0.030 pg Hg/g CCB/day was released from the samples, which equates to 2.2 x 10^-8 lb Hg/ton CCB/year. If this were applied to a coal-fired power plant production of 200,000 tons of fly ash per year, there would be a maximum potential release of 0.0044 lb, or 2.00 g, of Hg per year. Experiments are continuing to determine long-term vapor release of Hg from CCBs. All samples have been set up in duplicate at ambient temperature with an improved apparatus to reevalu-ate results reported in this article.     

Effects of remediation train sequence on decontamination of heavy metal-contaminated soil containing mercury

When a contaminated site contains pollutants including both nonvolatile metals and Hg, one single remediation technology may not satisfactorily remove all contaminants. Therefore, in this study, chemical extraction and thermal treatment were combined as a remediation train to remove heavy metals, including Hg, from contaminated soil. A 0.2 M solution of ethylenediamine tetraacetic acid (EDTA) was shown to be the most effective reagent for extraction of considerable amounts of Cu, Pb, and Zn (>50%). Hg removal was ineffective using 0.2 M EDTA, but thermogravimetric analysis suggested that heating to 550°C with a heating rate of 5°C/min for a duration of 1 hr appeared to be an effective approach for Hg removal. With the employment of thermal treatment, up to 99% of Hg could be removed. However, executing thermal treatment prior to chemical extraction reduced the effectiveness of the subsequent EDTA extraction because nonvolatile heavy metals were immobilized in soil aggregates after the 550°C treatment. The remediation train of chemical extraction followed by thermal treatment appears to remediate soils that have been contaminated by many nonvolatile heavy metals and Hg.

ImplicationsA remediation train conjoining two or more techniques has been initialized to remove multiple metals. Better understandings of the impacts of treatment sequences, namely, which technique should be employed first on the soil properties and the decontamination efficiency, are in high demand. This study provides a strategy to remove multiple heavy metals including Hg from a contaminated soil. The interactions between thermal treatment and chemical extraction on repartitioning of heavy metals was revealed. The obtained results could offer an integrating strategy to remediate the soil contaminated with both heavy metals and volatile contaminants.     

Differential partitioning and speciation of Hg in wet FGD facilities of two Spanish PCC power plants

This paper evaluates the speciation and partitioning of mercury in two Spanish pulverised coal combustion power plants (PP1 and PP2), equipped with wet limestone-based flue gas desulphurisation facilities (FGD) operating with forced oxidation and re-circulation of FGD water streams. These plants are fed with coal (PP1) and coal/pet-coke blends (PP2) with different mercury contents. The behaviour, partitioning and speciation of Hg were found to be similar during the combustion processes but different in the FGD systems of the two power plants. A high proportion (86-88%) of Hg escaped the electrostatic precipitator in gaseous form, Hg²⁺ being the predominant mercury species (68-86%) to enter the FGD. At this point, a relatively high total Hg retention (72% and 65%) was achieved in the PP1 and PP2 (2007) FGD facilities respectively. However, during the second sampling campaign for PP2 (2008), the mercury removal achieved by the FGD was much lower (26%). Lab-scale tests point to liquid/gas ratio as the main parameter affecting oxidised mercury capture in the scrubber. The partitioning of the gaseous mercury reaching the FGD system in the wastes and by-products differed. In the low mercury input power plant (PP1) most of the mercury (67%) was associated with the FGD gypsum. Moreover in PP2 a significant proportion of the gaseous mercury reaching the FGD system remained in the aqueous phase (45%) in the 2007 sampling campaign while most of it escaped in 2008 (74%). This may be attributed to the scrubber operating conditions and the different composition and chemistry of the scrubber solution probably due to the use of an additive.     

Regulation of glutathione-dependent antioxidant defense system of grass carp Ctenopharyngodon idella under the combined stress of mercury and temperature

In this study, we investigated the combined effects of temperatures fluencies and mercury (Hg) on glutathione-dependent antioxidant system in fish, by measuring the oxidative stress indicator (LPO, lipid peroxidation) and the parameters involved in the glutathione-related antioxidant defense system (GPx, glutathione peroxidase; GR, glutathione reductase; GST, glutathione S-transferase; GSH, reduced glutathione), as well as the expression of related genes in grass carp, Ctenopharyngodon idella. Fish (45.37?±?3.58 g) were exposed to 10 test groups, e.g., 15 °C with/without Hg, 20 °C with/without Hg, 25 °C with/without Hg, 30 °C with/without Hg, 35 °C with/without Hg for 4 weeks. Three-way ANOVA was used to analyze the correlation between the measured parameters and experimental conditions (water temperature, Hg exposure, exposure time, and their interactions.). Our results show that there is no interaction between mercury and low temperature, but the combined effect at high temperature has been confirmed, which indicated the glutathione-dependent enzyme system in grass carp has a complex regulatory mechanism with temperature fluctuations. In the actual field monitoring, it is necessary to consider the impact of extreme temperature on the toxicity of pollutants in the aquatic ecosystem.

     

Bildung von 2,3,7,8-TCDD bei der Thermolyse von ausgewählten chlorierten organischen Verbindungen

Upon heating of 2,4,5-T to 600°C, 2,3,7,8-TCDD is formed with a yield of 0,2 %. At 800°C, the formation of TCDD decreases by a factor at 200. Tormona 80^® an ester at 2,4,5-T yields 200 ppm TCDD at 600°C and 3 ppm at 800°C. The highest formation rate is observed for 2,4,5-Trichlorophenol (0,5 % at 600°C). During the thermolysis of 2,4-D, γ-Hexachlorocyclohexane, 2,4,6-Trichlorophenol, Pentachlorophenol and Clophen A 40. 2,3,7,8-TCDD could not be detected.     

A pilot study of mercury liberation and capture from coal-fired power plant fly ash

The coal-fired electric utility generation industry has been identified as the largest anthropogenic source of mercury (Hg) emissions in the United States. One of the promising techniques for Hg removal from flue gas is activated carbon injection (ACI). The aim of this project was to liberate Hg bound to fly ash and activated carbon after ACI and provide high-quality coal combustion products for use in construction materials. Both bench- and pilot-scale tests were conducted to liberate Hg using a thermal desorption process. The results indicated that up to 90% of the Hg could be liberated from the fly ash or fly-ash-and-activated-carbon mixture using a pilot-scale apparatus (air slide) at 538 degrees C with a very short retention time (less than 1 min). Scanning electron microscope (SEM) evaluation indicated no significant change in fly ash carbon particle morphology following the thermal treatment. Fly ash particles collected in the baghouse of the pilot-scale apparatus were smaller in size than those collected at the exit of the air slide. A similar trend was observed in carbon particles separated from the fly ash using froth flotation. The results of this study suggest a means for power plants to reduce the level of Hg in coal-combustion products and potentially recycle activated carbon while maintaining the resale value of fly ash. This technology is in the process of being patented.     

Control strategies of atmospheric mercury emissions from coal-fired power plants in China

Atmospheric mercury (Hg) emission from coal is one of the primary sources of anthropogenic discharge and pollution. China is one of the few countries in the world whose coal consumption constitutes about 70% of total primary energy, and over half of coals are burned directly for electricity generation. Atmospheric emissions of Hg and its speciation from coal-fired power plants are of great concern owing to their negative impacts on regional human health and ecosystem risks, as well as long-distance transport. In this paper, recent trends of atmospheric Hg emissions and its species split from coal-fired power plants in China during the period of 2000-2007 are evaluated, by integrating each plant's coal consumption and emission factors, which are classified by different subcategories of boilers, particulate matter (PM) and sulfur dioxide (SO2) control devices. Our results show that the total Hg emissions from coal-fired power plants have begun to decrease from the peak value of 139.19 t in 2005 to 134.55 t in 2007, though coal consumption growing steadily from 1213.8 to 1532.4 Mt, which can be mainly attributed to the co-benefit Hg reduction by electrostatic precipitators/fabric filters (ESPs/FFs) and wet flue gas desulfurization (WFGD), especially the sharp growth in installation of WFGD both in the new and existing power plants since 2005. In the coming 12th five-year-plan, more and more plants will be mandated to install De-NO(x) (nitrogen oxides) systems (mainly selective catalytic reduction [SCR] and selective noncatalytic reduction [SNCR]) for minimizing NO(x) emission, thus the specific Hg emission rate per ton of coal will decline further owing to the much higher co-benefit removal efficiency by the combination of SCR + ESPs/FFs + WFGD systems. Consequently, SCR + ESPs/FFs + WFGD configuration will be the main path to abate Hg discharge from coal-fired power plants in China in the near future. However advanced specific Hg removal technologies are necessary for further reduction of elemental Hg discharge in the long-term.     

Importance of fish gender as a factor in environmental monitoring of mercury

Total mercury concentrations were determined in the gonadal tissues of 15 female and 10 male European perch (Perca fluviatilis) from one location of the stream “Jevanský potok” located about 30 km from Prague (Czech Republic). Tissue samples were frozen at ?26 °C in polypropylene containers until further processing, which was carried out using an Advance Mercury Analyser (single purpose atomic absorption spectrometer). Mercury concentrations were present in all analysed gonad samples, and ranged from 2.3 to 12.7 μg/kg wet weight. However, we determined a mean Hg concentration (9.45 μg/kg) in male gonads that was 2.4 times greater than that of female gonads (3.9 μg/kg). This is an important finding when taking into account fish sex in environmental pollution monitoring (especially for mercury contamination).     

Oxidation of mercury across selective catalytic reduction catalysts in coal-fired power plants

A kinetic model for predicting the amount of mercury (Hg) oxidation across selective catalytic reduction (SCR) systems in coal-fired power plants was developed and tested. The model incorporated the effects of diffusion within the porous SCR catalyst and the competition between ammonia and Hg for active sites on the catalyst. Laboratory data on Hg oxidation in simulated flue gas and slipstream data on Hg oxidation in flue gas from power plants were modeled. The model provided good fits to the data for eight different catalysts, both plate and monolith, across a temperature range of 280-420 degrees C, with space velocities varying from 1900 to 5000 hr(-1). Space velocity, temperature, hydrochloric acid content of the flue gas, ratio of ammonia to nitric oxide, and catalyst design all affected Hg oxidation across the SCR catalyst. The model can be used to predict the impact of coal properties, catalyst design, and operating conditions on Hg oxidation across SCRs.     

Pollution due to hazardous glass waste

Pollution resulting from hazardous glass (HG) is widespread across the globe, both in terms of quantity and associated health risks. In waste cathode ray tube (CRT) and fluorescent lamp glass, mercury and lead are present as the major pollutants. The current review discusses the issues related to quantity and associated risk from the pollutant present in HG and proposes the chemical, biological, thermal, hybrid, and nanotechniques for its management. The hybrid is one of the upcoming research models involving the compatible combination of two or more techniques for better and efficient remediation. Thermal mercury desorption starts at 100 °C but for efficient removal, the temperature should be >460 °C. Involvement of solar energy for this purpose makes the research more viable and ecofriendly. Nanoparticles such as Fe, Se, Cu, Ni, Zn, Ag, and WS₂ alone or with its formulation can immobilize heavy metals present in HG by involving a redox mechanism. Straight-line equation from year-wise sale can provide future sale data in comparison with lifespan which gives future pollutant approximation. Waste compact fluorescent lamps units projected for the year 2015 is 9,300,000,000 units and can emit nearly 9,300 kg of mercury. On the other hand, CRT monitors have been continuously replaced by more improved versions like liquid crystal display and plasma display panel resulting in the production of more waste. Worldwide CRT production was 83,300,000 units in 2002 and can approximately release 83,000 metric tons of lead.     

Mercury Mass Balances: A Case Study of Two North Dakota Power Plants

ABSTRACT

The Energy & Environmental Research Center (EERC) conducted a mercury-sampling program to provide data on the quantity and forms of Hg emitted and on the Hg removal efficiency of the existing air pollution control devices at two North Dakota power plants—Milton R. Young Station and Coal Creek Station. Minnkota Power Cooperative, Great River Energy, the North Dakota Industrial Commission, and EPRI funded the project. The primary objective was to obtain accurate measurements of Hg released from each plant, as verified by a material balance. A secondary objective was to evaluate the ability of a mercury continuous emission monitor (CEM) to measure total Hg at the stack.

At both plants, speciated Hg measurements were made at the inlets and outlets of both the electrostatic precipi-tators (ESPs) and the flue gas desulfurization (FGD) systems. A Semtech Hg 2000 (Semtech Metallurgy AB) mercury CEM was used to measure the total Hg emissions at the stack in real time. Using these measurements and plant data, the measured Hg concentrations in the coal, FGD slurries, and ESP ash, a Hg mass flow rate was calculated at each sampling location. Excellent Hg mass balances were obtained (±15%). It was also found that the Hg was mostly in the elemental phase (~90%), and the small amount of oxidized Hg that was generated was removed by the FGD systems.

Insignificant amounts of particulate-bound Hg were measured at both plants. However, 10-20% of the elemental Hg measured prior to the ESP was converted to oxidized Hg across the ESP. The data show that, at these facilities, almost all of the Hg generated is being emitted into the atmosphere as elemental Hg. Local or regional deposition of the Hg emitted from these plants is not a concern. However, the Hg does become part of the global Hg burden in the atmosphere. Also, the evidence appears to indicate that elemental Hg is more difficult to remove from flue gas than oxidized Hg is.     

The distribution and sea–air transfer of volatile mercury in waste post-desulfurization seawater discharged from a coal-fired power plant

The waste seawater discharged in coastal areas from coal-fired power plants equipped with a seawater desulfurization system might carry pollutants such as mercury from the flue gas into the adjacent seas. However, only very limited impact studies have been carried out. Taking a typical plant in Xiamen as an example, the present study targeted the distribution and sea–air transfer flux of volatile mercury in seawater, in order to trace the fate of the discharged mercury other than into the sediments. Samples from 28 sampling sites were collected in the sea area around two discharge outlets of the plant, daily and seasonally. Total mercury, dissolved gaseous mercury and dissolved total mercury in the seawater, as well as gaseous elemental mercury above the sea surface, were investigated. Mean concentrations of dissolved gaseous mercury and gaseous elemental mercury in the area were 183 and 4.48 ng m^?3 in summer and 116 and 3.92 ng m^?3 in winter, which were significantly higher than those at a reference site. Based on the flux calculation, the transfer of volatile mercury was from the sea surface into the atmosphere, and more than 4.4 kg mercury, accounting for at least 2.2 % of the total discharge amount of the coal-fired power plant in the sampling area (1 km²), was emitted to the air annually. This study strongly suggested that besides being deposited into the sediment and diluted with seawater, emission into the atmosphere was an important fate for the mercury from the waste seawater from coal-fired power plants.     

A New Method to Assess Mercury Emissions: A Study of Three Coal-Fired Electric-Generating Power Station Configurations

Abstract

U.S. Environmental Protection Agency (EPA) Method 7473 for the analysis of mercury (Hg) by thermal decomposition, amalgamation, and atomic absorption spectroscopy has proved successful for use in Hg assessment at coal-fired power stations. In an analysis time of ～5 min per sample, this instrumental methodology can directly analyze total Hg—with no discrete sample preparation—in the solid matrices associated with a coal-fired power plant, including coal, fly ash, bottom ash, and flue gas desulfurization (FGD) material. This analysis technique was used to investigate Hg capture by coal combustion byproducts (CCBs) in three different coal-fired power plant configurations. Hg capture and associated emissions were estimated by partial mass balance. The station equipped with an FGD system demonstrated 68% capture on FGD material and an emissions estimate of 18% (11 kg/yr) of total Hg input. The power plant equipped with low oxides of nitrogen burners and an electrostatic precipitator (ESP) retained 43% on the fly ash and emitted 57% (51 kg/yr). The station equipped with conventional burners and an ESP retained less than 1% on the fly ash, emitting an estimated 99% (88 kg/yr) of Hg. Estimated Hg emissions demonstrate good agreement with EPA data for the power stations investigated.     

Anthropogenic mercury emissions in China

An inventory of mercury emissions from anthropogenic activities in China is compiled for the year 1999 from official statistical data. We estimate that China's emissions were 536 (±236) t of total mercury. This value includes open biomass burning, but does not include natural sources or re-emission of previously deposited mercury. Approximately 45% of the Hg comes from non-ferrous metals smelting, 38% from coal combustion, and 17% from miscellaneous activities, of which battery and fluorescent lamp production and cement production are the largest. Emissions are heaviest in Liaoning and Guangdong Provinces, where extensive smelting occurs, and in Guizhou Province, where there is much small-scale combustion of high-Hg coal without emission control devices. Emissions are gridded at 30×30 min spatial resolution. We estimate that 56% of the Hg in China is released as Hg⁰, 32% as Hg²⁺, and 12% as Hg^p. Particulate mercury emissions are high in China due to heavy burning of coal in residential and small industrial settings without PM controls. Emissions of Hg²⁺ from coal-fired power plants are high due to the absence of flue-gas desulfurization units, which tend to dissolve the soluble divalent mercury. Metals smelting operations favor the production of elemental mercury. Much of the Hg is released from small-scale activities in rather remote areas, and therefore the activity levels are quite uncertain. Also, emissions test data for Chinese sources are lacking, causing uncertainties in Hg emission factors and removal efficiencies. Overall, we calculate an uncertainty level of ±44% (95% confidence interval) in the estimate of total emissions. We recommend field testing of coal combustors and smelters in China to improve the accuracy of these estimates.     

Mercury distribution in seawater discharged from a coal-fired power plant equipped with a seawater flue gas desulfurization system

Background and purpose  

More and more coal-fired power plants equipped with seawater flue gas desulfurization systems have been built in coastal areas. They release large amount of mercury (Hg)-containing waste seawater into the adjacent seas. However, very limited impact studies have been carried out. Our research targeted the distribution of Hg in the seawater, sediment, biota, and atmosphere, and its environmental transportation.