The emissions of heavy metals and persistent organic pollutants from modern coal-fired power stations

Extensive research for establishing the emissions of heavy metals from coal-fired power stations is performed in the Netherlands for the past 25 years. In the Netherlands coal is fired from all over the world. This means that the emissions are established for coal of various origins. In the eighties, the emissions of installations equipped with ESPs (electrostatic precipitators) were measured. In the nineties, the influence of wet FGD (flue gas desulphurisation) on the emissions was studied. The effect of co-combustion of biomass and other secondary fuels is the main item for the last 10 years.Fifty-five elements were measured in the solid state and eight elements in the gaseous phase. It appeared that at low particulate concentration the influence of calcium containing evaporated water droplets downstream the wet FGD on the emissions of heavy metals is bigger than the composition of the coal. Also it appeared that at modern coal-fired power stations the emissions are hardly influenced by co-combustion of biomass. All the results are used for modelling, resulting in the KEMA TRACE MODEL^®, by which the emissions can be predicted. The established emission factors are for most elements in good agreement with literature values for comparable modern installations.Persistence organic pollutants (POPs) that were detected in the flue gases of coal-fired power stations are polycyclic aromatic hydrocarbons (PAH) and dioxins/furans. Measurements during full coal-firing and during co-firing of biomass have indicated that these emissions are negligible.     

The impact of wet flue gas desulfurization scrubbing on mercury emissions from coal-fired power stations

This article introduces a predictive capability for Hg retention in any Ca-based wet flue gas desulfurization (FGD) scrubber, given mercury (Hg) speciation at the FGD inlet, the flue gas composition, and the sulphur dioxide (SO2) capture efficiency. A preliminary statistical analysis of data from 17 full-scale wet FGDs connects flue gas compositions, the extents of Hg oxidation at FGD inlets, and Hg retention efficiencies. These connections clearly signal that solution chemistry within the FGD determines Hg retention. A more thorough analysis based on thermochemical equilibrium yields highly accurate predictions for total Hg retention with no parameter adjustments. For the most reliable data, the predictions were within measurement uncertainties for both limestone and Mg/lime systems operating in both forced and natural oxidation mode. With the U.S. Environmental Protection Agency's (EPA) Information Collection Request (ICR) database, the quantitative performance was almost as good for the most modern FGDs, which probably conform to the very high SO2 absorption efficiencies assumed in the calculations. The large discrepancies for older FGDs are tentatively attributed to the unspecified SO2 capture efficiencies and operating temperatures and to the possible elimination of HCl in prescrubbers. The equilibrium calculations suggest that Hg retention is most sensitive to inlet HCl and O2 levels and the FGD temperature; weakly dependent on SO2 capture efficiency; and insensitive to HgCl2, NO, CA:S ratio, slurry dilution level in limestone FGDs, and MgSO3 levels in Mg/lime systems. Consequently, systems with prescrubbers to eliminate HCl probably retain less Hg than fully integrated FGDs. The analysis also predicts re-emission of Hg(O) but only for inlet O2 levels that are much lower than those in full-scale FGDs.     

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.     

The accumulation by plants of emissions from a coal-fired power plant

Foliar concentrations of Ca, Cl, F, Mg, Mn, P, S and Zn were measured in Eucalyptus crebra and E. moluccana growing at distances up to 28 km from a 2000 MW coal-fired power plant. The results showed that concentrations of F, Mn and Zn in leaves of E. moluccana were negatively correlated with distance from the emission source (P < 0.01), the foliar concentration of Zn was positively correlated with F and Mn concentrations (P < 0.01) and that S concentration was positively correlated with P and Cl concentrations (P < 0.01). In contrast, no elements in E. crebra leaves were correlated with distance from the power plant but significant positive correlations were found between Mg and F and between S and Ca concentrations. Concentrations of Mn in both species and F in E. crebra exceeded those measured in Eucalyptus species growing in areas of Australia remote from industrial emissions, but no symptoms of air pollution-induced injury were found. All other foliar element concentrations were comparable with background concentrations in Eucalyptus species in Australia.     

The reduction of dioxin emissions from the processes of heat and power generation

The first reports that it is possible to emit dioxins from the heat and power generation sector are from the beginning of the 1980s. Detailed research proved that the emission of dioxins might occur during combustion of hard coal, brown coal, and furnace oil as well as coke-oven gas. The emission of dioxins occurs in wood incineration; wood that is clean and understood as biomass; or, in particular, wood waste (polluted). This paper thoroughly discusses the mechanism of dioxin formation in thermal processes, first and foremost in combustion processes. The parameters influencing the quantity of dioxins formed and the dependence of their quantity on the conditions of combustion are highlighted. Furthermore, the methods of reducing dioxin emissions from combustion processes (primary and secondary) are discussed. The most efficacious methods that may find application in the heat and power generation sector are proposed; this is relevant from the point of view of the implementation of the Stockholm Convention resolutions in Poland with regard to persistent organic pollutants.     

Elemental particle-size emissions from coal-fired power plants: Use of an inertial cascade impactor

Coal-fly-ash particles collected on coated and uncoated impaction substrates were analyzed by scanning electron microscope (SEM) techniques in combination with instrumental neutron activation analysis (INAA) to verify the sizes of particles collected and their elemental composition to estimate the significance of bounce-off and reentrainment onto back-up filters and to evaluate wall and interstage losses for the University of Washington MK III Source Test Cascade Impactor. Particles were analyzed for a total of 39 elements. In samples collected downstream from an electrostatic precipitator (ESP), particles on upper impactor stages were much smaller than indicated by impactor 50% cut-off diameters given for the impactor, which resulted in significant differences in the measured aerosol-distribution parameters of the total particulate mass and of the mass of constituent elements. On the back-up filters, large particles (i.e. aerodynamic diameter ⪢ 2 μm) including those from bounce-off and reentrainment accounted for more than 90% of the total mass, 20–30% of the volatile species of Mo, As, Sb and Se, and all of the refractory elements. Wall and interstage losses of most elements were estimated to be about 40% by mass. Bounce-off and reentrainment onto back-up filters was only slightly reduced for small (aerodynamic diameter <2μm), wet particles collected after a wet scrubber and by use of coated impaction substrates. No significant wall and interstage losses were observed for particles collected downstream from the scrubber.     

Distribution of volatile organic compounds during the combustion process in coal-fired power stations

This study concerns the distribution of volatile organic compounds (VOC) emitted during combustion processes in coal-fired power stations. Thermal desorption technique was employed to analyse VOC concentrations in gaseous emissions (trapped onto Carbotrap B sorbent) and solid samples (coal, fly ash and slag). An empirical parameter (Y) was employed to evaluate the relationships between the compounds emitted during a real combustion process and those obtained in coal analysis by thermal desorption. The Y calculation does not provide any information about VOC emissions through solid effluents as fly ash or slag. A comparison method was created for this purpose. The results obtained using this method confirm that combustion is almost complete for aromatic and aliphatic hydrocarbons. Chlorinated compounds are mainly formed during the combustion.     

Investigation of aerosol and gas emissions from a coal-fired power plant under various operating conditions

The concentrations of fine particles and selected gas pollutants in the flue gas entering the stack were measured under several common operation modes in an operating coal power plant producing electricity. Particle size distributions in a diameter range from 10 nm to 20 μm were measured by a scanning mobility particle sizer (SMPS), and the flue gas temperature and concentrations of CO₂ and SO₂ were monitored by a continuous emission monitoring system (CEMS). During the test campaign, five plant operating modes were studied: soot blowing, bypass of flue-gas desulfurization (FGD), reheat burner operating at 0% (turned off), 27%, and 42% (normal condition) of its full capacity. For wet and dry aerosols, the measured mode sizes were both around 40 nm, but remarkable differences were observed in the number concentrations (#/cm³, count per square centimeter). A prototype photoionizer enhanced electrostatic precipitator (ESP) showed improved removal efficiency of wet particles at voltages above +11.0 kV. Soot blowing and FGD bypass both increased the total particle number concentration in the flue gas. The temperature was slightly increased by the FGD bypass mode and varied significantly as the rating of reheat burner changed. The variations of CO₂ and SO₂ emissions showed correlations with the trend of total particle number concentration possibly due to the transitions between gas and particle phases. The results are useful in developing coal-fired power plant operation strategies to control fine particle emissions and developing amine-based CO₂ capture technologies without operating and environmental concerns associated with volatile amine emissions.

Implications: The measurement of the fine particle size distributions in the exhaust gas under several common operating conditions of a coal-fired power plant revealed different response relations between aerosol number concentration and the operating condition. A photo-ionizer enhanced ESP was demonstrated to capture fine particles with higher efficiency compared to conventional ESPs, and the removal efficiency increased with the applied voltage. The characteristic information of aerosols and main gaseous pollutants in the exhaust gas is extremely important for developing and deploying CO₂ scrubbers, whose amine emissions and operating effectiveness depends greatly on the upstream concentrations of fine particles, SO₂, from the power plant.     

Characterization and inventory of PCDD/F emissions from coal-fired power plants and other sources in Taiwan

The objectives of the present study were to quantify (1) the emission factors of a variety of dioxin emission sources; (2) the overall dioxin emission inventory in Taiwan as well as in a major metropolitan (KC area); and (3) the contribution of power plants to the overall PCDD/F emission. To achieve these goals, a total of 95 flue gas samples were collected and analyzed for 17 PCDD/Fs from 20 sources to develop emission factors. The emission factor of PCDD/Fs from coal-fired power plants (0.62 microgI-TEQton(-1)) obtained in this study is considerably higher than the values reported from different countries including UK, USA, and Spain by a factor of 2-265. It means that the air pollution control devices in certain power plants need to be more efficient. The emission data showed that there is a total annual release to air of 6.1 and 95gI-TEQ from major sources in the KC area and Taiwan, respectively. The dominant sources of PCDD/Fs in the KC area are the coal-fired power plants, secondary aluminum smelting, electric arc furnaces, and open burning of rice straw, which contributed for 56%, 17%, 13%, and 3.3% to the total, respectively. However, in Taiwan, the dominant sources of PCDD/Fs are the iron ore sintering, coal-fired power plants, electric arc furnaces, and open burning of rice straw, which contributed for 32%, 28%, 23%, and 8.1% to the total, respectively. The results of this study showed that coal-fired power plants are very significant sources of PCDD/Fs and also provide an important database to assist the decision makers for formulating policies to alleviate dioxin concerns.     

An intelligent emissions controller for fuel lean gas reburn in coal-fired power plants

The application of artificial intelligence techniques for performance optimization of the fuel lean gas reburn (FLGR) system is investigated. A multilayer, feedforward artificial neural network is applied to model static nonlinear relationships between the distribution of injected natural gas into the upper region of the furnace of a coal-fired boiler and the corresponding oxides of nitrogen (NOx) emissions exiting the furnace. Based on this model, optimal distributions of injected gas are determined such that the largest NOx reduction is achieved for each value of total injected gas. This optimization is accomplished through the development of a new optimization method based on neural networks. This new optimal control algorithm, which can be used as an alternative generic tool for solving multidimensional nonlinear constrained optimization problems, is described and its results are successfully validated against an off-the-shelf tool for solving mathematical programming problems. Encouraging results obtained using plant data from one of Commonwealth Edison's coal-fired electric power plants demonstrate the feasibility of the overall approach. Preliminary results show that the use of this intelligent controller will also enable the determination of the most cost-effective operating conditions of the FLGR system by considering, along with the optimal distribution of the injected gas, the cost differential between natural gas and coal and the open-market price of NOx emission credits. Further study, however, is necessary, including the construction of a more comprehensive database, needed to develop high-fidelity process models and to add carbon monoxide (CO) emissions to the model of the gas reburn system.     

Thermodynamic analysis and kinetic modelling of dioxin formation and emissions from power boilers firing salt-laden hog fuel

Both organic chlorine (e.g. PVC) and inorganic chlorides (e.g. NaCl) can be significant chlorine sources for dioxin and furan (PCDD/F) formation in combustion processes. This paper presents a thermodynamic analysis of high temperature salt chemistry. Its influence on PCDD/F formation in power boilers burning salt-laden wood waste is examined through the relationships between Cl2, HCl, NaCl(g) and NaCl(c). These analyses show that while HCl is a product of combustion of PVC-laden municipal solid waste, NaCl can be converted to HCl in hog fuel boilers by reactions with SO2 or alumino-silicate materials. Cl2 is a strong chlorinating agent for PCDD/F formation. HCl can be oxidized to Cl2 by O2, and Cl2 can be reduced back to HCl by SO2. The presence of sulphur at low concentrations thus enhances PCDD/F formation by increasing HCl concentrations. At high concentrations, sulphur inhibits de novo formation of PCDD/Fs through Cl2 reduction by excess SO2. The effect of NH3, CO and NOx on PCDD/F formation is also discussed. A semi-empirical kinetic model is proposed. This model considers both precursor and de novo formation mechanisms. A simplified version is used as a stack emission model. The kinetic model indicates that stack dioxin emissions will increase linearly with decreasing electrostatic precipitator (ESP) efficiency and exponentially with increasing ESP temperature.     

中国燃煤电厂脱硝成本核算与影响因素分析

脱硝电价政策是激励中国燃煤电厂开展NOX减排工作的重要政策工具,制定补贴标准是脱硝电价政策的重要内容。脱硝成本影响脱硝电价,科学核算脱硝成本是脱硝电价政策设计的前提。依据脱硝成本与脱硝电价的关系,综合考虑燃煤电厂进行脱硝设施改造产生的建设成本、运行成本、财务成本以及环境损失成本,构建了脱硝成本核算模型,并结合电厂调研数据,核算出中国燃煤电厂脱硝成本。结果表明,燃煤机组脱硝成本均在1 480万~4 475万元;建设成本与运行成本是脱硝成本的重要组成部分,分别占脱硝成本总量的31%、51%;装机容量、建设类型、脱硝工艺是导致不同机组脱硝成本存在差异的主要原因;燃煤电厂脱硝电价政策的改革在于综合考虑所在区域、装机容量、脱硝工艺、建设类型、脱硝效率等因素的基础上,实行脱硝电价差别补贴政策。     

Mercury emission from coal-fired power plants in Poland

The paper reviews the current state of knowledge regarding sources of mercury emission in Poland. Due to the large quantities of coal burned at present, as well as taking into account existing reserves, coal remains the main energy source of energy in Poland. The data on coal consumption in Poland in the past, at present and in the future are discussed in the paper. Information on the content of mercury in Polish coals is presented.Coal combustion processes for electricity and heat production are the main source of anthropogenic mercury emission in Poland. It is expected that the current emissions will decrease in the future due to implementation of efficient control measures. These measures for emission reduction are described in the paper. Results of estimated mercury emission from coal-fired power station situated in the Upper Silesia Region, Poland are investigated. A relationship between mercury emission to the air and the mercury content in the consumed coal in power station equipped with the electrostatic precipitators (ESPs) is discussed.     

Health risk assessment of dioxin emissions from municipal waste incinerators: the Neerlandquarter (Wilrijk, Belgium)

Two municipal waste incinerators in the vicinity of a residential area close to the city of Antwerp caused concern to local habitants. Risk assessment was performed combining chemical, toxicological measurements and model calculations. As the first step in risk assessment an inventory was made of historic emissions from both incinerators with emphasis on dioxins. The operational atmospheric transport and deposition model for priority substances (OPS) was used to calculate the deposition of dioxins in the vicinity of incinerators. The observed soil contamination pattern did not correspond to the calculated deposition pattern, indicating that other sources may contribute at least partly to the local PCDD/PCDF contamination of the area. Dioxin exposure of people in the Neerlandquarter as a function of the food consumption behavior was calculated using a mathematical model (VLIER-HUMAAN) combined with transfer factors. According to the results of these calculations, just residing in the impact area does not result in a meaningful risk. Only if locally produced food was consumed (milk, meat and vegetables), exposure in the Neerlandquarter was enhanced compared to the average dioxin exposure estimated for the Flemish population. Exposure in 1997 was below the exposure in 1980. As a consequence of different eating habits and lower bodyweight, children are subjected to significantly higher exposure than adults. Adverse health outcomes from dioxin exposure in the past cannot be excluded. There was no evidence for enhanced exposure to genotoxicants based on a comparison of chromosomal damage to blood cells of children from the study area to those from a control group.     

Quantification of hourly variability in NO(x) emissions for baseload coal-fired power plants

The objectives of this paper are to (1) quantify variability in hourly utility oxides of nitrogen (NO(x)) emission factors, activity factors, and total emissions; (2) investigate the autocorrelation structure and evaluate cyclic effects at short and long scales of the time series of total hourly emissions; (3) compare emissions for the ozone (O3) season versus the entire year to identify seasonal differences, if any; and (4) evaluate interannual variability. Continuous emissions monitoring data were analyzed for 1995 and 1998 for 32 units from nine baseload power plants in the Charlotte, NC, airshed. Unit emissions have a strong 24-hr cycle attributable primarily to the capacity factor. Typical ranges of the coefficient of variation for emissions at a given hour of the day were from 0.2 to 0.45. Little difference was found when comparing weekend emissions with the entire week or when comparing the O3 season with the entire year. There were substantial differences in the mean and standard deviation of emissions when comparing 1995 and 1998 data, indicative of the effect of retrofits of control technology during the intervening time. The wide range of variability and its autocorrelation should be accounted for when developing probabilistic utility emission inventories for analysis of near-term future episodes.     

Memory effect on the dioxin emissions from municipal waste incinerator in Taiwan

Evaluation of the dioxin removal efficiency of the air pollution control device (APCD) at an existing municipal waste incinerator (MWI) located in Taiwan is conducted via stack sampling and analysis. The MWI investigated is equipped with cyclones, dry lime scrubbing systems and fabric filters as APCDs. Results indicate that the patterns of dioxin isomers at APCDs' inlet and stack are similar. During the first year of carbon injection, the concentrations of emitted dioxin decreased greatly. The dioxin removal efficiency increased from 26.9% to 96.6% after injecting 115 kg/day activated carbon (AC). At the second year, the dioxin removal efficiency reaches 98.7% after injecting the same rate of AC continuously. The lower efficiency achieved with activated carbon injection (ACI) during the first year can be attributed to the memory effect, i.e. the dioxin or precursor desorbs slowly to the flue gas and increases the dioxin concentration at stack, resulting in a lower dioxin removal efficiency than expected.     

Emissions of sulfur trioxide from coal-fired power plants

Emissions of sulfur trioxide (SO3) are a key component of plume opacity and acid deposition. Consequently, these emissions need to be low enough to not cause opacity violations and acid deposition. Generally, a small fraction of sulfur (S) in coal is converted to SO3 in coal-fired combustion devices such as electric utility boilers. The emissions of SO3 from such a boiler depend on coal S content, combustion conditions, flue gas characteristics, and air pollution devices being used. It is well known that the catalyst used in the selective catalytic reduction (SCR) technology for nitrogen oxides control oxidizes a small fraction of sulfur dioxide in the flue gas to SO3. The extent of this oxidation depends on the catalyst formulation and SCR operating conditions. Gas-phase SO3 and sulfuric acid, on being quenched in plant equipment (e.g., air preheater and wet scrubber), result in fine acidic mist, which can cause increased plume opacity and undesirable emissions. Recently, such effects have been observed at plants firing high-S coal and equipped with SCR systems and wet scrubbers. This paper investigates the factors that affect acidic mist production in coal-fired electric utility boilers and discusses approaches for mitigating emission of this mist.     

Source profile and health risk assessment of PM2.5 from coal-fired power plants in Fuxin,China

In order to improve and establish the localized source profile of PM_2.5 in Fuxin, the ashes under dust catcher were collected from four typical coal-fired power plants in Fuxin and twenty-eight components were measured. The source profile of PM_2.5 in the soot of the four coal-fired power plants was established. SO₄^2? was the most abundant component in the PM_2.5 of the soot of the four coal-fired power plants, followed by Ca²⁺ and organic carbon (OC). The content of element components in PM_2.5 smoke ranges from 5.06 to 10.97%, the content of ionic components ranges from 36.53 to 48.59%, and the total carbon content ranges from 9.43 to 11.36%. The divergence coefficient of PM_2.5 source profile in Fuxin coal burning smoke is mostly similar to that of Fushun, whereas the divergence coefficient of Colorado reaches 0.65, indicating that Fuxin coal burning power plant smoke has no similarity to Colorado. The order of the geological accumulation index of Ni, Cu, V, Mn, and Cr was Cr (4.58) > Mn (4.42) > V (4.38) > Cu (4.09) > Ni (4.06), showing a heavy pollution level. The health risk assessment model recommended by the USEPA was used to assess the health risk of heavy metals in soot of coal-fired power plants, and the non-carcinogenic risk values of As for children and adults were 45.7 and 4.90, respectively. The carcinogenic risk values of Cr for adults and children were the highest, with values of 3.66 × 10^?5 and 2.06 × 10^?5, respectively, followed by As.

     

燃煤电厂建设项目环境影响评价中有关问题的探讨

在对燃煤电厂建设项目进行环境影响评价(以下简称环评)时,针对其工程特点和污染特征,总结和分析了环评中存在的一些问题,概况出项目选址、大气预测模式、烟囱高度、冷却塔的噪声、清洁生产等值得重视的问题,并提出相应的解决措施。     

百万燃煤机组烟气污染物超低排放改造费效评估

随着我国火电厂大气污染物排放标准的日趋严格,燃煤发电企业陆续开展环保装备升级改造工作,其中部分燃煤电厂已完成大气污染物超低排放改造工作。目前,针对超低排放改造后的成本效益,仍缺乏系统性的分析及评估。基于大量电厂运行DCS及CEMS数据,以某百万燃煤机组烟气污染物超低排放技术改造的情况为实际案例,采用费用-效益分析的方法,对其展开超低排放技术运行经济性评估及研究。同时,结合情景分析,研究负荷、含硫量及年发电时间等关键影响因素变化对污染物脱除成本的影响。结果表明：改造后,污染物（SO2、NOx及PM）脱除成本比改造前增加约13~20元·（MWh）-1,约占上网电价的2.8%~4.4%。改造后,该电厂SO2、NOx及PM排放绩效分别达到0.048、0.109及0.007 g·（kWh）-1,每年可产生环境效益约1 344万元。此外,提升机组运行负荷能显著降低污染物脱除装备运行成本从实际运行平均负荷（66%负荷）提高到满负荷运行,FGD、SCR脱硝及ESP+WESP除尘单位发电量运行成本分别可下降约30.5%、32.1%和38.1%。