Mercury speciation and its emissions from a 220 MW pulverized coal-fired boiler power plant in flue gas

Distributions of mercury speciation of Hg⁰, Hg²⁺ and Hg ^P in flue gas and fly ash were sampled by using the Ontario Hydro Method in a 220 MW pulverized coal-fired boiler power plant in China. The mercury speciation was varied greatly when flue gas going through the electrostatic precipitator (ESP). The mercury adsorbed on fly ashes was found strongly dependent on unburnt carbon content in fly ash and slightly on the particle sizes, which implies that the physical and chemical features of some elemental substances enriched to fly ash surface also have a non-ignored effect on the mercury adsorption. The concentration of chlorine in coal, oxyge nand NO _x in flue gas has a positive correlation with the formation of the oxidized mercury, but the sulfur in coal has a positive influence on the formation of elemental mercury. This work was presented at the 6 ^th Korea-China Workshop on Clean Energy Technology held at Busan, Korea, July 4–7, 2006.     

Activated carbon injection for mercury control: Overview

Full-scale evaluations of the commercial feasibility of activated carbon injection (ACI) for mercury control in coal-fired power plants have been underway in North America since 2001 through DOE, EPRI and industry-funded projects. Commercial injection systems began to be sold to the power generation industry in 2005 and ACI is now considered the most robust technology for mercury control at many coal-fired units. Successful widespread implementation of this technology throughout this industry will require continued development efforts including: (1) understanding the impacts of technologies to control other pollutants, such as SO₃, for the enhancement of particulate control or selective catalytic reduction (SCR) for NO_x control, (2) options to continue using ash containing activated carbon in concrete, (3) techniques to assure the quality of delivered carbon, (4) techniques to improve the effectiveness of activated carbon, and (5) facilities to produce additional carbon supply. An overview of activated carbon injection for mercury control will be presented including the range of expected control levels, costs, balance-of-plant issues, recent developments to reduce overall control costs for many common air pollution control configurations, and developments to overcome complications caused by some new control configurations. An update on carbon supply and progress on ADA’s activated carbon manufacturing facility will also be provided.     

Fabric filter bag investigation following field testing of sorbent injection for mercury control at TXU's Big Brown Station

Field testing of mercury control sorbent injection options with a TOXECON™ configuration has been completed at TXU's Big Brown Station. Mercury control results at Big Brown were promising and have been previously reported. However, the high air-to-cloth ratio of operations at this unit results in significant differential pressure, and thus there was little operating margin before differential pressure limits were encountered, especially at high loads. This limited the use of sorbent injection as the added material contributes to the overall differential pressure. After field testing, the residual differential pressure across the test fabric filter module had increased relative to baseline conditions to the point that the plant performed a filter change of the test module several months ahead of schedule. An investigation was conducted on pre- and posttest filter samples from the test module and a parallel nontest module to examine the effect of activated carbon injection. Analysis of the samples indicates an increase in residual dust embedded in the filters which appears to explain the low fabric permeabilities. The long-term increase in differential pressure did not appear to be associated with activated carbon injection, but instead was due to a gradual buildup of embedded material on the filters that was not cleaned away by the pulse cleaning system. The injected activated carbon appeared to behave like additional fly ash in terms of baghouse differential pressure but did not appear to accelerate the buildup of residual material.     

Mercury control technologies for coal combustion and gasification systems

Development and testing of mercury control technologies have largely focused on coal-fired combustion systems, with less emphasis on advanced power systems. Mercury control is influenced by coal properties and chemistry, plant configuration, pollution control devices, flue gas conditions, and plant operations, which differ between combustion and gasification systems. Sorbents such as treated activated carbons have shown promising results in low-temperature environments; however, elevated temperature and reducing environments of many advanced systems remain challenging, requiring research and development to obtain acceptable mercury control levels. Concurrent pollutant/multipollutant control strategies that include CO₂ control are critically needed for both conventional and advanced power systems.     

TOXECON clean coal demonstration for mercury and multi-pollutant control at the Presque Isle Power Plant

We Energies and DOE, under a Clean Coal Power Initiative program, are working together to design, install, evaluate and demonstrate the EPRI-patented TOXECON^™ air pollution control process as an integrated emissions control system for mercury and particulate matter from three 90 MW units at the Presque Isle Power Plant located in Marquette, Michigan.     

An interpretation of flue-gas mercury speciation data from a kinetic point of view

Mercury emission from coal-fired power plants is causing great concern. Thermodynamic equilibrium calculations are commonly used for prediction of Hg speciation in the flue-gas. According to representative calculations, at temperatures below 725 K all Hg should exist in oxidized forms. However, Hg speciation measurements for flue-gas at much lower temperature showed a wide range of oxidized Hg fraction from 30 to 95%. A hypothesis that Hg equilibrium is frozen as the flue-gas cools could describe the data. However, a different interpretation of the measured results can be given based on kinetic considerations. Here, the oxidized Hg fraction is limited by the reaction rate of chlorine with elemental Hg in the flue-gas, while the differences in the degree of oxidation can largely be attributed to different time-temperature histories for the flue-gases. The residence time of flue-gas is likely to be a governing factor for the Hg speciation.     

燃煤锅炉煤中汞的迁移和分配规律研究

建立了固体、液体中汞的直接测定方法。采用直接测汞法,样品无需任何消解,无论固体样品还是液体样品均可用汞原子蒸气在253.65 nm处的特征吸收进行测定,方法灵敏度高、简便、快速、准确。通过对某燃煤锅炉装置所有排放污染物中汞的分析,确定了燃煤锅炉煤中汞的迁移和分配规律,为汞污染治理提供了技术支持。     

Elemental mercury vapor capture by powdered activated carbon in a fluidized bed reactor

A bubbling fluidized bed of inert material was used to increase the activated carbon residence time in the reaction zone and to improve its performance for mercury vapor capture. Elemental mercury capture experiments were conducted at 100 °C in a purposely designed 65 mm ID lab-scale pyrex reactor, that could be operated both in the fluidized bed and in the entrained bed configurations. Commercial powdered activated carbon was pneumatically injected in the reactor and mercury concentration at the outlet was monitored continuously. Experiments were carried out at different inert particle sizes, bed masses, fluidization velocities and carbon feed rates. Experimental results showed that the presence of a bubbling fluidized bed led to an increase of the mercury capture efficiency and, in turn, of the activated carbon utilization. This was explained by the enhanced activated carbon loading and gas-solid contact time that establishes in the reaction zone, because of the large surface area available for activated carbon adhesion/deposition in the fluidized bed. Transient mercury concentration profiles at the bed outlet during the runs were used to discriminate between the controlling phenomena in the process. Experimental data have been analyzed in the light of a phenomenological framework that takes into account the presence of both free and adhered carbon in the reactor as well as mercury saturation of the adsorbent.     

Mercury emissions from six coal-fired power plants in China

Mercury emission field measurements based on the Ontario Hydro Method (OHM) were conducted for six coal-fired power plants in China. The mercury mass balances for the six power plants varied from 100.3% to 139.5% of the input coal mercury for the whole system. About 0.02%–1.2% of the mercury remained in the bottom ash. In the first five power plants equipped with pulverized coal boiler, most of the mercury was emitted from the stack to the atmosphere. The plants with Electrostatic Precipitator (ESP) system emitted more Hg⁰ than Hg²⁺, while the plants with the Fabric Filter (FF) emitted less Hg⁰ than Hg²⁺. Virtually all of the Hg^P enter the ESP or the FF was removed. The FF systems had better Hg⁰ and Hg²⁺ removal efficiencies than the ESP systems. The flue gas desulfurization (FGD) system removed up to 78.0% of Hg²⁺ and only 3.14% of Hg⁰ in the flue gas, while 8.94% of the original mercury in the coal was removed by the FGD system. The average mercury removal efficiencies of the ESP systems was 11.5%, that of the FF systems was 52.3% and that of the combined ESP + FGD system was 13.7%, much lower than the average removal efficiencies of pollution control device systems in US plants which have been used in previous studies of Chinese mercury emission inventory. Hg⁰, rather than Hg²⁺ as assumed in previous estimates, has been found to be the dominant species emitted in the atmosphere. The average emission factor was found to be 4.70 g/TJ (10.92 bl/Tbtu), which is much higher than for US plants burning bituminous coals due to the high mercury content in the Chinese coal and the low mercury removal efficiency of air pollution control devices of power plants.     

Mercury transformations in the exhausts from lab-scale coal flames

A laboratory-scale pulverized coal flame generated exhausts from five coals which were processed at realistic quench rates and residence times with typical flyash loadings. Mercury levels were monitored in the coal feed, all particulate streams from the furnace, and in the gaseous effluent. Contributions from elemental and oxidized species to the mercury vapors were also determined. Reported extents of Hg oxidation were not proportional to coal-Cl levels. Only the coal with excessive Cl generated an abundance of oxidized Hg species. Extents of Hg oxidation did not increase for progressively longer residence times in the exhaust system, but were affected by the level of unburned carbon, suggesting an essential role for heterogeneous chemistry. The split between particulate and vapor Hg species shifted toward particulate-Hg for progressively cooler temperatures at the exhaust outlet. The levels of particulate-Hg were generally higher for the coals that generated less oxidized Hg vapors. Appreciable Hg sorption was observed at temperatures as hot as 500 °C.     

Mercury speciation in gypsums produced from flue gas desulfurization by temperature programmed decomposition

Temperature programmed decomposition was used to identify mercury (Hg) species in gypsum samples produced from flue gas desulfurization in two Spanish power stations (A and B). As stricter emission control/reduction policies, particularly those focusing on Hg, are being implemented, wet flue gas desulfurization (FGD) technologies used for the removal of SO₂ can result in the co-removal of highly-soluble oxidized Hg. The amount of Hg retained in FGD products may increase in the future if these units are optimized for co-capture. For this reason, it is important to identify the mercury species in FGD products not only to determine the potential risk when the wastes are finally disposed of, but also to understand the behaviour of mercury during combustion and therefore to improve the technologies for mercury removal. Different mercury species were identified in the gypsum samples. In power station A, HgS were the most probable Hg species, whereas in power station B the main compound was Hg halogenated compounds.     

Simulation of mercury emission control by activated carbon under confined-bed operations

Mercury emissions from coal-fired power plants have been a great environmental and regulatory concern due to the toxic nature of mercury and the significant amount of emissions from these plants. An effective method for controlling mercury emission is to employ activated carbon to adsorb mercury from the combustion flue gas. In this study, an activated carbon mercury sorption model was applied to simulate a confined-bed mercury emission control process. Model simulations were performed to generate dynamic mercury concentration profiles and the corresponding profiles of mercury uptake by activated carbon at various bed locations under various process conditions. The simulation parameters included flue gas flow rate, inlet mercury concentration, and adsorption bed temperature. The study has demonstrated the applicability of the model for simulating the process and provided insights into the mercury control process especially the effects of flue gas flow rate, inlet mercury concentration, and activated carbon bed temperature on the process. Such information is critically needed in the design and operation of a mercury emission control process involving activated carbon adsorption.     

煤矸石中汞、砷的研究进展

煤矿生产过程中伴生的大量煤矸石不仅占用土地资源,而且在露天堆放的过程中其含有的有害微量元素汞、砷等会进入自然环境中,对环境造成严重污染。通过总结,介绍了国内外学者对于煤矸石中汞、砷的研究成果。主要从煤矸石中汞、砷含量,煤矸石加热汞、砷释放,煤矸石中汞、砷淋溶析出以及煤矸石中汞、砷赋存形态四个方面进行总结。以期为更多学者对于煤矸石中汞、砷的研究工作提供一定的帮助。     

Mercury emission control in coal-fired plants: The role of wet scrubbers

When coal is combusted, the combination of the elevated temperature and the volatility of mercury and its compounds results in the presence of gaseous elemental mercury and mercury compounds in the combustion flue gas. In January 2005, the European Commission adopted a mercury strategy that envisages a number of measures to reduce mercury levels in the environment and human exposure. A number of options for mercury removal from coal-fired power plants have been investigated. However, more effort is needed to achieve an efficient and cost-effective technology. The main objective of this work is to investigate the influence of scrubber parameters on mercury removal efficiency to establish effective measures for mercury control. In order to attain these objectives, theoretical predictions based on thermodynamical equilibrium data and lab-scale experimental tests were carried out. The results obtained point to pH and slurry concentration as the most critical parameters for converting FGD (Flue Gas Desulphurization unit) into a multipollutant control technology.     

First full-scale demonstration of mercury control in Alberta

Alberta electricity companies, TransAlta, ATCO, and EPCOR, teamed with GE Energy to conduct full-scale evaluation of sorbent injection in Sundance Unit 5 operated by TransAlta. Sundance Unit 5 fires a Western Canadian sub-bituminous coal and is equipped with cold-side ESP for PM control. Goals of the program were to evaluate: (1) the ability of achieving 70% or greater mercury reduction using activated carbon injection in long-term tests (30 days), (2) the effect of sorbent injection on ESP performance and opacity in long-term testing, and (3) the effects of combustion conditions on “natural” mercury removal in fly ash.     

Mercury species, mass flows and processes in a cement plant

The aim of the study was to evaluate the behaviour of mercury in the cement clinker production process. Simultaneous measurements of mercury in all important materials and gas streams were performed in three sampling periods on about 300 solid samples and about 80 samples taken from gas streams. Mercury species in flue gases at characteristic parts of the process were measured as total Hg(t), particulate Hg(p), elemental Hg⁰(g) and reactive Hg²⁺(g) mercury. Based on the results of measurements, mercury mass flows and mass balances of the whole and in certain parts of the process were evaluated. It was shown that the process comprises many mercury cycles which are strongly dependent on the operating conditions and technological specifics. Cycling of mercury causes a significant enrichment of mercury inside the process. In the annual mercury input of about 27 kg, raw materials accounted near by 60% and fuels about 40% (i.e. petrol coke 31%, waste tyres 10% and waste oil 0.4%). The annual emission of mercury represented 40-70% of the inputs, while cement clinker only contained about 10%. The difference between inputs and outputs (11-45%) obtained in the annual mass balance could be assigned to mercury recycling and significant enrichment inside the process, as well as variability between spot measurements. The parts of the process with the highest mercury mass flows and the lowest material/gas flows were identified. Such points represent an opportunity to remove a significant amount of mercury from the process at low material flows and to improve mercury control. Mercury was mainly emitted in gaseous form with 92% (direct mode) or 89% (combined mode) as Hg(g) on average, of which about 2/3 was as Hg²⁺(g), and about 1/3 as Hg⁰(g). Only a small part (the rest) was emitted as particulate Hg(p). Shares of individual mercury species in the last sampling period were 65.7% Hg²⁺(g), 34.0% Hg⁰(g) and 0.3% Hg(p) on average. Ratios between individual mercury forms were found to be related to operating modes. The quantities of Hg(t), Hg(g) and Hg²⁺(g) emitted were higher when operating with the raw mills off (direct mode). It was seen that the efficiency of Hg removal was strongly related to the dust removal efficiency. Bag filters very efficiently removed all mercury species.     

Mechanisms governing the fate of mercury in coal-fired power systems

This paper summarizes the results of a significant research and development investment to understand the fundamental mechanisms governing the speciation of mercury in coal-fired power systems. An extensive experimental search was conducted through all the parameter variations characterizing the majority of full-scale U.S. coal-fired power plants, in an effort to quantify the impact each parameter variation had on mercury oxidation and removal from the gas phase. As a result of this extensive investigation, the mechanisms responsible for mercury speciation differences observed for different coal types, power plants, and pollution control devices were elucidated. Specifically, unburned carbon was found to catalytically enhance mercury oxidation, while the actual differences in chlorine concentration from plant to plant were found to be of secondary importance to carbon. In addition, synergistic enhancement of mercury capture by carbon and calcium in flyash was quantitatively described for various coal-fired configurations, such as coal blending and sorbent injection, including development of contour plots of mercury removal at an ESP inlet and exit as a function of carbon and calcium concentration at the ESP inlet.     

Fate of trace element haps when applying mercury control technologies

During the past several years, and particularly since the Clean Air Mercury Rule (CAMR) was promulgated in June of 2005, the electric utility industry, product vendors, and the research community have been working diligently to develop and test Hg control strategies for a variety of coal types and plant configurations. Some of these strategies include sorbent injection and chemical additives designed to increase mercury capture efficiency in particulate control devices. These strategies have the potential to impact the fate of other inorganic hazardous air pollutants (HAPs), which typically include As, Be, Cd, Cr, Co, Mn, Ni, Pb, Se, and Sb. To evaluate this impact, flue gas samples using EPA Method 29, along with representative coal and ash samples, were collected during recent pilot-scale and field test projects that were evaluating Hg control technologies. These test programs included a range of fuel types with varying trace element concentrations, along with different combustion systems and particulate control devices. The results show that the majority of the trace element HAPs are associated with the particulate matter in the flue gas, except for Se. However, for five of the six projects, Se partitioning was shifted to the particulate phase and total emissions reduced when Hg control technologies were applied.     

Mercury oxidation and adsorption characteristics of chemically promoted activated carbon sorbents

Previous entrained-flow tests conducted under elemental mercury (Hg⁰)-laden air found that significant amounts of oxidized mercury (Hg²⁺) are not adsorbed onto cupric chloride-impregnated carbon (CuCl₂-AC) and brominated activated carbon (DARCO Hg-LH), but entrained to the gas phase. In this study, these sorbents were tested in a fixed-bed system and a filter-added entrained-flow system to further investigate Hg⁰ oxidation and adsorption characteristics of CuCl₂-AC and DARCO Hg-LH. These test results suggested that CuCl₂-AC has different sites available for Hg⁰ oxidation and Hg adsorption, and the resultant oxidized mercury generated from the reaction between Hg⁰ and CuCl₂ is re-adsorbed at the site of CuCl₂-AC available for adsorption. The resultant oxidized mercury was also found to be easily re-adsorbed onto CuCl₂-AC and DARCO Hg-LH in the filter connected to the entrained-flow reactor.     

Evidence of powdered activated carbon preferential collection and enrichment on electrostatic precipitator discharge electrodes during sorbent injection for mercury emissions control

Removal of mercury from coal-derived flue gas by injecting powdered sorbents often involves a substantial portion removed within an electrostatic precipitator (ESP). The present investigation uses a lab-scale ESP to assess the potential for injected sorbents to collect preferentially on discharge electrode wires. Such preferential collection would increase the adsorption capacity of the accumulated dust cake on the discharge electrodes, increasing their potential contribution to the total mercury removal performance of the ESP. The lab-scale results involving various fly ashes and both carbon-based and non-carbon mercury sorbents confirm that powdered activated carbon is enriched in the discharge electrode dust cake relative to its concentration in suspension in the gas flow. Other results explore the effects of applied ESP polarity, voltage, and power, percent PAC added to the fly ash, and total particulate matter loading entering the ESP on the collection behavior.