Se In Coal Research Articles

Distribution of Se in Floating Coals with Densimetric Fractions in China

Scholars at home and abroad have studied the concentrations of Se in coal and the relationship between Se and sulfur assignment. However, little attention has been paid to the study of Se in floating coal, especially the distribution pattern of Se in floating coal of densimetric fractions and the influencing factors. A total of 138 coal samples were collected from 14 provinces and cities in China to test the Se and sulfur concentrations in coal, and we carried out the comparison of Se with forms of sulfur with 81 of them. A total of 10 coal samples were selected for float-and-sink analyses to investigate the distribution pattern of Se in floating coal. The results showed that the average Se concentration of Chinese coal was 2.26 μg/g, which is identical with the results of previous research studies, which found an average Se concentration of 2~3 μg/g. Selenium was not uniformly distributed in floating coals of densimetric fractions but it had some regularity. The main controlling factor was the sulfur content in the coal. The Se concentrations of the high-sulfur coal showed a “linear growth” distribution with an increasing density, the Se concentrations of the floating coal in each densimetric fraction increased slowly and the Se concentrations of the floating coal of a densimetric fraction > 2.00 g/cm3 increased sharply. The other controlling factor was the ash yield of the coal. The Se concentrations of the low–medium-sulfur floating coal showed a “parabolic-like” distribution, i.e., low Se concentrations at both ends and high Se concentrations at the middle densimetric fraction. With the increase in the ash content of the coal from 18.23% to 51.92%, the densimetric fraction with the highest Se concentration gradually progressed from 1.40 g/cm3 to 1.50 g/cm3, 1.60 g/cm3 and 1.80 g/cm3 in order. The correlation between the selenium of the raw coal from different regions and the total sulfur, pyrite sulfur and organic sulfur was not significant. However, there existed a significant correlation between Se, total sulfur, pyrite sulfur and organic sulfur in floating coals from the same mining area and depositional environment, among which pyrite sulfur was the most correlated with Se, followed by total sulfur and organic sulfur.

Investigation Into Environmental Selenium and Arsenic Levels and Arseniasis Prevalence in an Arsenic-Affected Coal-Burning Area.

This study aims to explore whether selenium (Se) concentration correlates with arseniasis in a high-arsenic coal area in the southern Shaanxi Province, China. Herein, an epidemiological investigation was conducted among 100 arsenic (As)-poisoned patients in Ziyang County, an area with high soil As and Se levels. Fifty healthy subjects were selected from areas without endemic As poisoning. The subjects in the high-As coal area were diagnosed with either normal, suspicious, mild, moderate, or severe As poisoning. Local coal, water, soil, corn, and pepper samples, as well as hair, blood, and urine samples of subjects and patients were collected and analyzed for their As and Se contents. The contents of As and Se in coal, soil, corn, pepper, and hair samples from Ziyang County were significantly higher than those in the control area. The As content of hair in Ziyang County positively correlated with As poisoning, whereas the Se content of hair and urine negatively correlated with As poisoning. The Se content in the body was negatively correlated with the degree of As poisoning, indicating that Se may accelerate the metabolism and decumulation of As and antagonize As toxicity.

Arsenic and selenium distribution and speciation in coal and coal combustion by-products from coal-fired power plants

This study investigated arsenic (As) and selenium (Se) distribution and speciation in coal and the burning by-products (fly ash and gypsum) in five coal-fired power plants in China. Total concentrations of As and Se in coal varied significantly among different power plants, but most of them were below the average As and Se content in Chinese coal. The concentrations of As and Se in fly ash were much higher than that in coal. Total As contents in four gypsum samples were lower than that in coal, while Se contents in four gypsum samples were much higher than that in coal. The species of inorganic As (AsIII and AsV) and Se (SeIV and SeVI) were determined by high-performance liquid chromatography coupled to inductively coupled plasma mass spectrometry after solvent extraction. The extraction efficiencies of As and Se in coal were low, but a considerable amount of As and Se in fly ash and gypsum can be extracted, showing that elements As and Se were easily leached from the burning by-products. Speciation analysis indicated that Se was mostly present as SeIV in all the fly ash and gypsum extracts, and AsV was the dominating species in coal and fly ash extracts. In contrast, AsIII, much more toxic than AsV, was mainly detected in gypsum. Large amounts of coal-fired by-products are produced every year, most of which end up in landfills or stockpiled in the open areas. Our results suggest that the proper disposal of coal-fired solid wastes is urgently demanded.

Transformation of selenium during coal thermal conversion: Effects of atmosphere and inorganic content

Study on selenium (Se) transformation during coal combustion will facilitate the development of emission–control technologies. A fixed–bed reactor was used to conduct pyrolysis and combustion experiments involving three types of coal at 600–1000 °C. In an inert atmosphere, the release ratios of Se increased slowly as temperature increased, varying between 56% and 74% for different coals at 1000 °C. All forms of Se in coal were thermally unstable and the conversion of Se in other forms into residual and organic–bound Se occurred. In an oxidizing atmosphere, the release ratios of Se differed significantly for different coals at low temperatures, but approached 100% at 1000 °C. Each form of Se in coal was released in different ratios. The water–soluble and ion–exchangeable Se, and carbonate– and oxide–bound Se were produced in the solid products. The oxidizing atmosphere was more conducive to the release of Se because the sulfide– and organic–bound Se was fully oxidized. Se could be converted into water–soluble and ion–exchangeable form and released less in Ca–rich coal. Sulfur (S) prohibited Ca from adsorbing Se by forming CaSO4, promoting the release of Se in S–rich coal.

Determination of As and Se in coal and coal combustion products using closed vessel microwave digestion and collision/reaction cell technology (CCT) of inductively coupled plasma mass spectrometry (ICP-MS)

Toxic elements arsenic and selenium in coal are of great concern mainly because of their adverse effects on human health during coal combustion. This paper describes the concentration determination of As and Se in coal and coal combustion products (CCPs), performed by collision/reaction cell technology (CCT) of inductively coupled plasma mass spectrometry (ICP-MS; collectively ICP-CCT-MS) after closed vessel microwave digestion. The reagents for 50-mg coal sample digestion are 2-ml 40% (v/v) HF and 5-ml 65% (v/v) HNO3 but for the CCP samples, the reagents include 5-ml 40% HF and 2-ml 65% HNO3. To significantly diminish the argon-based interferences at mass to charge ratios (m/z) 75 (40Ar35Cl) and 78 (40Ar38Ar), a helium and hydrogen mixture was used in the optimized hexapole collision cell. The results showed that CCT technology can effectively diminish the spectral interferences of the Ar-based polyatomic ions 40Ar35Cl and 40Ar38Ar to 75As and 78Se, respectively. The method detection limit of As and Se is 0.024 and 0.095μg/l, respectively, and their linearity of the calibration curves in the range 0–100μg/l has a determination coefficient r2>0.9999. The determination of As and Se in NIST standard reference materials of coal and fly ash samples showed that ICP-CCT-MS plus closed vessel microwave digestion is a reliable method for concentration determination of the two elements in coal and CCPs.

Distribution and fate of environmentally sensitive elements (arsenic, mercury, stibium and selenium) in coal-fired power plants at Huainan, Anhui, China

The present study has investigated the distribution of arsenic (As), mercury (Hg), stibium (Sb) and selenium (Se) in feed coal and its combustion by-products in two pulverized coal-fired power plants at Huainan city, Anhui province, China, both of which burned the bituminous coals. Experimental analysis of simultaneously sampled coal, bottom ash, fly ash and FGD products showed that the concentrations of As, Hg, Sb and Se in coal were in the ranges of typical Chinese power plants reported by others publications. Mercury was found to be the most volatile, whereas Sb was the least volatile. The overall material balances of As, Hg, Sb and Se were established based on the operation parameters of their respective boiler. The average removal efficiencies of As, Hg, Sb and Se by the ESP unit were 83%, 16%, 100%, and 72%, respectively, whereas those by the FGD process were 61%, 80%, 0% and 55%, respectively. The stack emission proportions of As, Hg and Se were 6%, 17% and 13%, respectively. And the total annual emissions of As, Hg and Se from two coal-fired power plants were estimated at 0.46t, 0.04t and 2.27t, respectively.

Selenium in coal: A review

Abstract The World average Se content in coals (coal Clarke of Se) for hard coals and brown coals are respectively 1.6 ± 0.1 and 1.0 ± 0.15 ppm. On an ash basis, these contents are greatly increased and are 9.9 ± 0.7 and 7.6 ± 0.6 ppm, respectively. Therefore, Se is a very coalphile element: it has strong affinity to coal matter — organic and (or) inorganic but is certainly authigenic. The “coal affinity” of Se is like that for Ge and S. Both organic (Seorg) and inorganic selenium (Semin) can exist in coal. In addition, Se can occur not only as a chemical-bound form, but also in sorbed (acid leachable) selenate form in the oxidized coals. The sulfidic form of Se is represented by isomorph Se in pyrite and some more rare sulfides, and the selenidic form by clausthalite PbSe. “Organic” Se may be present as both Se-organic compounds and elemental Se° dissiminated through organic matter. There are some preliminary data that cleaning of high-sulfur coals, very effective for S, is less effective for Se due to enhanced contribution of the Seorg form (vs. Sorg). There are two genetic types of the Se-accumulations in coal: “reducing” and “oxidizing”. In the first type, Se is enriched in high-sulfur coals, concentrating in sulfide phases. These accumulations are, in general, syngenetic, and may be epigenetic only if there are abundant hydrothermal sulfides of Fe, Cu, As, Pb. In coals of the second type, Se is enriched in the bed oxidation zones. Such coals are (or were) located in the areas with arid climate and enhanced Se content in water. Se concentrates in coal as a reduction or sorption geochemical barrier, probably, mostly as Se° in oxidized organic matter and partly pyrite. This type is characterized by oxidizing zonality and close paragenesis of Se with U, Fe, Mo, V, and Pb. These Se accumulations are mostly epigenetic. A study of Se in coal is evidently insufficient, and further detailed studies are needed, especially regarding Se forms (modes of occurrence).

Clausthalite in coal

A lead selenide mineral, tentatively identified as clausthalite (PbSe) based on optical microscopy, scanning electron microscopy with energy-dispersive X-ray (EDAX), electron microprobe, and scanning proton microprobe (SPM), has been described in a number of coals. While clausthalite has been mentioned as a possible source of Se in coal, it is present in such small quantities and sizes, that the mineral identification has not been absolutely confirmed. The mineral specimens examined in this study would contribute, not only to the Pb and Se concentrations in the coal, but also, in at least one case, to Hg concentrations.

Determination of As, Mn, Pb and Se in coal by slurry electrothermal vaporization inductively coupled plasma mass spectrometry

A method for the determination of trace elements in coal by electrothermal vaporization inductively coupled plasma mass spectrometry (ETV-ICP-MS), introducing the sample as a slurry, is proposed. The slurry was prepared by mixing the powdered coal (<45 µm) with aqueous 5% v/v nitric acid solution in an ultrasonic bath. An ultrasonic probe was used to homogenize the slurry in the autosampler cup just before its introduction into the graphite tube. The best compromise conditions for the furnace temperature program, the amount of palladium used as modifier, the carrier gas flow rate, etc., were determined. By introducing 10 µL of a 4.0 mg mL−1 coal slurry, using 3 µg of Pd as a modifier, and a pyrolysis temperature of 600°C, most of the obtained values for four coal reference materials were well within the certified range, using external calibration with aqueous solutions. No carrier had to be added in addition to the palladium modifier. Using a lower slurry concentration of 1.3 mg mL−1 and 2 µg of Pd as the modifier, the values obtained for lead were significantly lower than the certified ones. This problem could be solved in part by adding NaCl as a physical carrier, in addition to the palladium, showing that this carrier reduces the differences in the analyte transport from the sample and from the calibration solutions. However, results for most elements, except for arsenic, tended to be a little low even under these conditions, hence, preference was given to the higher slurry concentration.

Arsenic Determination in Certifiable Color Additives by Dry Ashing Followed by Hydride Generation Atomic Absorption Spectrometry

The preparations of digested samples of certifiable color additives by dry ashing and wet digestion for arsenic analysis by hydride generation atomic absorption spectrometry (AAS) were compared. The dry ashing technique was based on the preparation used in ASTM D4606-86 for determination of As and Se in coal. The acid digestion method used nitric and sulfuric acids heated by microwaves in sealed vessels. The digested color additives were analyzed for As by using hydride generated from sodium borohydride mixed with the acidified solution on a flow injection system leading to an atomic absorption spectrometer. Dry ashing was preferable to wet digestion because wet digestion yielded poor recoveries of added As. Dry ashing followed by hydride generation AAS gave determination limits of 0.5 ppm As in the color additives. At a specification level of 3 ppm As, the precision of the method using dry ashing was +/- 0.4 ppm (95% confidence interval).