Fate of selenium in coal combustion: volatilization and speciation in the flue gas

In light of Title I of the Clean Air Act Amendments of 1990, selenium will most probably be considered for regulation in the electric power industry. This has generated interest for removing this element from fossil-fired flue gas. This study deals with coal combustion: selenium volatilization and its speciation in the cooled flue gas were investigated to better understand its chemical behavior to validate the thermodynamic approach to such complex systems and to begin developing emission control strategies. Se volatility is influenced by several factors such as temperature, residence time, fuel type, particle size, and Se speciation of the fuels, as well as the forms of the Se inthe spiked coal/coke. Spiked coke and coal samples were burned in a thermobalance, and atomic Se and its dioxide were identified in the cooled combustion flue gas by X-ray photoelectron spectroscopy (XPS). A thermodynamic calculation was applied to a complex system including 54 elements and 3,200 species that describes the coal combustion. Several theoretical predictions concerning Se behavior, such as its speciation in flue gas, agreed well with experiments, which supports using thermodynamics for predicting trace element chemistry in combustion systems.     

Volatile metal species in coal combustion flue gas

Metals are released in effluents of most of combustion processes and are under intensive regulations. To improve our knowledge of combustion process and their resulting emission of metal to the atmosphere, we have developed an approach allowing usto distinguish between gaseous and particulate state of the elements emitted. This study was conducted on the emission of volatile metallic species emitted from a coal combustion plant where low/medium volatile coal (high-grade ash) was burnt. The occurrence of volatile metal species emission was investigated by cryofocusing sampling procedure and detection using low-temperature packed-column gas chromatography coupled with inductively coupled plasma-mass spectrometry as multielement detector (LT-GC/ICP-MS). Samples were collected in the stack through the routine heated sampling line of the plant downstream from the electrostatic precipitator. The gaseous samples were trapped with a cryogenic device and analyzed by LT-GC/ICP-MS. During the combustion process, seven volatile metal species were detected: three for Se, one for Sn, two for Hg, and one for Cu. Thermodynamic calculations and experimental metal species spiking experiments suggest that the following volatile metal species are present in the flue gas during the combustion process: COSe, CSSe, CSe2, SeCl2, Hg0, HgCl2, CuO-CuSO4 or CuSO4 x H2O, and SnO2 or SnCl2. The quantification of volatile species was compared to results traditionally obtained by standardized impinger-based sampling and analysis techniques recommended for flue gas combustion characterization. Results showed that concentrations obtained with the standard impinger approach are at least 10 times higher than obtained with cryogenic sampling, suggesting the trapping microaerosols in the traditional methods. Total metal concentrations in particles are also reported and discussed.     

Development of a mercury transformation model in coal combustion flue gas

A bench-scale entrained-flow reactor was used to extract flue gas produced by burning a subbituminous Belle Ayr coal in a 580-MJ/h combustion system. The reactor was operated at 400 degrees, 275 degrees, and 150 degrees C with a flow rate corresponding to residence times of 0-7 s. Transformations of elemental mercury (Hg0) and total gas mercury (Hg(gas)) in the reactor were evaluated as functions of temperature and residence time. The most significant mercury transformations (Hg0 to Hg(p) and Hg0 to Hg2+) occurred at 150 degrees C, while virtually no obvious mercury transformations were observed at 275 degrees and 400 degrees C. Approximately 30% of total mercury has been oxidized at temperatures higher than 400 degrees C. A mass transfer-capacity limit model was developed to quantify in-flight mercury sorption on fly ash in flue gas at different temperatures. A more sophisticated model was developed to demonstrate not only the temperature and residence time effects but also to consider the effective surface area of fly ash and dependence of mercury vapor concentration on mercury transformations in flue gas. The reaction orders were 0.02 and 0.55 for Hg0 and Hg(gas), respectively. Only a few percent of the total surface area of the fly ash, in the range of 1%-3%, can effectively adsorb mercury vapor.     

As, Hg, and Se flue gas sampling in a coal-fired power plant and their fate during coal combustion

As, Hg, and Se are the most volatile elements in the flue gas from a coal-fired power plant. Significant amounts of these elements cause an undesired direct gaseous emission, which leads to a serious environmental health risk. The main focus of this study is to evaluate the possibility of simultaneous sampling of these volatile elements using an accurate official method for Hg (the most volatile element). A study of As, Hg, and Se emissions from a 1400 MW coal-fired power plant equipped with electrostatic precipitators (ESPs) was carried out for the combustion of a mixture of two types of coal. Simultaneous sampling of coal, bottom ash, fly ash, flue gas, and particles associated with the gas phase has been performed. Flue gas has been sampled by the Ontario Hydro Method Sampling Train, an ASTM method for Hg speciation. This sampling method was tested for As and Se sampling. As and Se determinations have been performed by HG-AAS, and Hg has been determined by CV-AAS. The results were used to examine the following: overall mass balances, relative distribution of these elements in the coal-fired power plant; As, Hg, and Se concentrations in coal and combustion residues; and predominant oxidation state for Hg in flue gas. The mass balances obtained for As, Hg, and Se were satisfactory in all cases; nevertheless, relative enrichment values in fly ash for As and Se were low; therefore, we concluded that As sampling in flue gas can be conducted by application of the Ontario Hydro Method; nevertheless Se released in the gas phase is not completely collected by this sampling train. Application of this sampling method allowed for performance of Hg speciation. The results indicated that Hg(II) was the predominant species in flue gas. It has also been proved that 24%, more than 99.8%, and 90% for As, Hg, and Se in the stack emissions, respectively, were in the gaseous phase.     

Emissions of air pollutants from household stoves: honeycomb coal versus coal cake

Domestic coal combustion can emit various air pollutants. In the present study, we measured emissions of particulate matter (PM) and gaseous pollutants from burning a specially formulated honeycomb coal (H-coal) and a coal cake (C-coal). Flue gas samples for PM2.5, PM coarse (PM2.5-10), and TSP were collected isokinetically using a cascade impactor; PM mass concentrations were determined gravimetrically. Concentrations of SO2, NOx, and ionic Cr(VI) in PM were analyzed using spectrometric methods. Fluoride concentrations were measured using a specific ion electrode method. PM elemental components were analyzed using an X-ray fluorescence technique. Total (gas and particle phase) benzo[a]pyrene (BaP) concentration was determined using an HPLC/fluorescence method. Elemental and organic carbon contents of PM were analyzed using a thermal/optical reflectance technique. The compositional and structural differences between the H-coal and C-coal resulted in different emission characteristics. In generating 1 MJ of delivered energy, the H-coal resulted in a significant reduction in emissions of SO2 (by 68%), NOx (by 47%), and TSP (by 56%) as compared to the C-coal, whereas the emissions of PM2.5 and total BaP from the H-coal combustion were 2-3-fold higher, indicating that improvements are needed to further reduce emissions of these pollutants in developing future honeycomb coals. Although the H-coal and the C-coal had similar emission factors for gas-phase fluoride, the H-coal had a particle-phase fluoride emission factor that was only half that of the C-coal. The H-coal had lower energy-based emissions of all the measured toxic elements in TSP but higher emissions of Cd and Ni in PM2.5.     

Comparison of personal, indoor, and outdoor exposures to hazardous air pollutants in three urban communities

Two-day average concentrations of 15 individual volatile organic compounds (VOCs) were measured concurrently in (a) ambient air in three urban neighborhoods, (b) air inside residences of participants, and (c) personal air near the breathing zone of 71 healthy, nonsmoking adults. The outdoor (O), indoor (I), and personal (P) samples were collected in the Minneapolis/St. Paul metropolitan area over three seasons (spring, summer, and fall) in 1999 using charcoal-based passive air samplers (3M model 3500 organic vapor monitors). A hierarchical, mixed-effects statistical model was used to estimate the mutually adjusted effects of monitor location, community, and season while accounting for within-subject and within-time-index (monitoring period) correlation. Outdoor VOC concentrations were relatively low compared to many other urban areas, and only minor seasonal differences were observed. A consistent pattern of P > I > O was observed across both communities and seasons for 13 of 15 individual VOCs (exceptions were carbon tetrachloride and chloroform). Results indicate that ambient VOC measurements at central monitoring sites can seriously underestimate actual exposures for urban residents, even when the outdoor measurements are taken in their own neighborhoods.     

Theoretically predicted rate constants for mercury oxidation by hydrogen chloride in coal combustion flue gases

In this work, theoretical rate constants are estimated for mercury oxidation reactions by hydrogen chloride that may occur in the flue gases of coal combustion. Rate constants are calculated using transition state theory at the quadratic configuration interaction (QCI) level of theory with single and double excitations, and are compared to results obtained from density functional theory, both including high level pseudopotentials for mercury. Thermodynamic and kinetic data from the literature are used to assess the accuracy of the theoretical calculations when possible. Validation of the chosen methods and basis sets is based upon previous and current research on mercury reactions involving chlorine. The present research shows that the QCISD method with the 1992 Stevens et al. basis set leads to the most accurate kinetic and thermodynamic results for the oxidation of mercury via chlorine containing molecules. Also, a comparison of the heats of reaction data for a series of mercury oxidation reactions reveals that the density functional method, B3LYP, with the 1997 Stuttgart basis set provides reasonably accurate results for these large systems.     

Retention of arsenic and selenium compounds using limestone in a coal gasification flue gas

Volatile arsenic and selenium compounds present in coals may cause environmental problems during coal combustion and gasification. A possible way to avoid such problems may be the use of solid sorbents capable of retaining these elements from flue gases in gas cleaning systems. Lime and limestone are materials that are extensively employed for the capture of sulfur during coal processing. Moreover, they have also proven to have good retention characteristics for arsenic and selenium during combustion. The aim of this work was to ascertain whether this sorbent is also useful for retaining arsenic and selenium species in gases produced in coal gasification. The study was carried out in a laboratory-scale reactor in which the sorbent was employed as a fixed bed, using synthetic gas mixtures. In these conditions, retention capacities for arsenic may reach 17 mg g(-1) in a gasification atmosphere free of H2S, whereas the presence of H2S implies a significant decrease in arsenic retention. In the case of selenium, H2S does not influence retention which may reach 65 mg g(-1). Post-retention sorbent characterization, thermal stability, and water solubility tests have shown that chemical reaction is one of the mechanisms responsible for the capture of arsenic and selenium, with Ca(AsO2)2 and CaSe being the main compounds formed.     

Removal of PCDD/Fs from flue gas by a fixed-bed activated carbon filter in a hazardous waste incinerator

The adsorption of polychlorinated dibenzodioxins and dibenzofurans (PCDD/Fs) by activated carbon (AC) was examined in a fixed-bed AC unit in a hazardous waste incinerator (IZAYDAS) in Turkey. Results showed that the removal efficiencies of PCDD/Fs decrease as the chlorination level increases, which was explained by the difference in gas/particle partitioning of the compounds. Since dioxins are tightly adsorbed by activated carbon, other flue gas constituents showed no clear effect on the dioxin removal. Adsorption kinetics indicated that the adsorption of volatile congeners and homologues fits well with Henry's law, possibly due to the higher gaseous fractions, while the correlation was lower for lowly volatile ones. PCDD/F congeners and homologues had a concentration value up to which no adsorption occurred, which could be attributed to the insufficient contact times at the low concentrations.     

Effect of near-wall air curtain on the wall deposition of droplets in a semidry flue gas desulfurization reactor

The wall deposition of droplets is an important issue affecting the desulfurization efficiency and operating stability of semidry flue gas desulfurization (FGD) reactors. Various near-wall air velocities, near-wall air flow inlet heights, and spray characteristics were analyzed numerically to investigate their effect on the gas-liquid flow and droplet deposition characteristics. The analytical results show that the near-wall air curtain effectively reduces the wall deposition of droplets in the semidry FGD reactor. The droplet deposition ratio decreased rapidly with increasing near-wall air velocity due to the increased gas flow rates and the altered gas velocity distribution. The near-wall air flow inlet height had an optimum value due to the rapid decline of the near-wall air momentum along the reactor height. The optimum distance between the near-wall air inlet height and the droplet injection height was 1.2 times that of the droplet vertical movement distance before deposition based on the linear droplet movement. For commonly used spray characteristics in the semidry FGD process, i.e., droplet diameters of 50-150 microm, spray angles of 10-70 degrees and droplet initial velocities of 20-100 m/s, the droplet deposition ratio with the addition of the near-wall air curtain varied slightly with the droplet diameter and the spray angle but increased rapidly with the initial droplet velocity. Therefore, for the semidry FGD processes, the near-wall air curtain is an effective method to reduce the wall deposition of droplets for various droplet diameters and spray angles while the initial droplet velocity should be carefully controlled to reduce the wall deposition of droplets and improve the operating stability.     

Survey of catalysts for oxidation of mercury in flue gas

Methods for removing mercury from flue gas have received increased attention because of recent limitations placed on mercury emissions from coal-fired utility boilers by the U. S. Environmental Protection Agency and various states. A promising method for mercury removal is catalytic oxidation of elemental mercury (Hg0) to oxidized mercury (Hg2+), followed by wet flue gas desulfurization (FGD). FGD cannot remove Hg0, but easily removes Hg2+ because of its solubility in water. To date, research has focused on three broad catalyst areas: selective catalytic reduction catalysts, carbon-based materials, and metals and metal oxides. We review published results for each type of catalyst and also present a discussion on the possible reaction mechanisms in each case. One of the major sources of uncertainty in understanding catalytic mercury oxidation is a lack of knowledge of the reaction mechanisms and kinetics. Thus, we propose that future research in this area should focus on two major aspects: determining the reaction mechanism and kinetics and searching for more cost-effective catalyst and support materials.