Selenium in Coal-Fired Steam Plant Emissions Anders W. Andren**' and David H. Klein2 Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tenn. 37830 Yair Talmi Analytical Chemistry Division, Oak Ridge National Laboratory, Oak Ridge, Tenn. 37830
Selenium was measured in coal, slag, and flue gas at the Allen Steam Plant in Memphis, Tenn. Approximately 0.3% of the coal-derived selenium was incorporated into slag. Sixty-eight percent of the Se was found on the fly ash particles while the rest was found in the vapor phase. Since the electrostatic precipitators at the plant are very efficient for fly ash removal, 93% of the emitted Se existed in the vapor phase. The major part of Se discharge from the steam plant occurs via fly ash removal to nearby slag ponds. All of the selenium in the slag and flue gas exists as elemental Se and is thus not immediately available for plant uptake. Selenium emission rates from data collected at the steam plant were used to calculate the minimum mobilization of Se from coal consumption in the US.and world. Calculations indicate that 1.5 to 2.5 times as much Se is mobilized by man through coal burning as by natural weathering.
Using literature data, Bertine and Goldberg (1) have postulated that the combustion of fossil fuels may mobilize several trace elements at rates that approach natural weathering processes. In an effort to better understand the mechanisms responsible for this mobilization, several investigators have studied the partitioning of trace elements within coal fired steam plants (2-7). Results from these investigations show that many elements are preferentially incorporated in fly ash or are emitted in the vapor phase. Calculations of trace element emissions thus need to consider these processes so that more realistic predictions of emissions can be made. In addition, the chemical and physical forms of these emissions need to be determined so that possible health hazards can be evaluated. Bolton et al. (5) have previously demonstrated that a complete mass balance of selenium cannot be achieved by measuring only the solid phases of the material flow in a coal-fired steam plant. This paper describes methods used to sample selenium in coal, slag, fly ash, and the vapor phase in the emissions of the T. A. Allen Steam Plant in Memphis, Tenn. In addition, the chemical state of selenium was determined for each phase to better understand biological and geochemical implications of these emissions. Experimental Procedures
Plant Description. The plant uses cyclone-fed boilers. Normal coal consumption is about 100 tons per hour. The coal is fed to the boilers in crushed form rather than the more commonly used pulverized form. As a consequence, the ash produced by the boilers is about 60% slag and 40% fly ash. Flue gases from each unit are cleaned by an electrostatic precipitator and finally discharged through a 122meter stack. Since a new Lodge Cottrell electrostatic precipitator recently had been installed, this plant represents
Present address, Water Chemistry Laboratory, University of Wisconsin, Madison, Wis. 53706. * Present address, Hope College, Holland, Mich. 856
Environmental Science & Technology
a system with the latest available particulate pollution abatement equipment. Sampling Procedure. Samples for coal, slag, and precipitator inlet and outlet flue gas analysis were obtained in August 1973. Coal samples were cornposited from grab samples taken at 15-min intervals during the flue gas sampling. Grab samples of slag were collected during the periodic flushing of the hopper and thus might not have been truly representative of the coal samples. The flue gas streams were sampled by a modification of the ASTM method (8). The flue gas samples were drawn through filters a t the precipitator inlet and outlet a t a predetermined isokinetic rate. The precipitator fly ash samples were composited for subsequent selenium analysis. The gaseous stream was then drawn through a heated probe (maintained a t the flue gas temperature of 150°C) to aqueous impinger trains designed to capture vapors of selenium. The impinger train consisted of two 10% NaZC03 solutions immersed in an ice bath. Seven runs were made for the precipitator inlet and four for the outlet. Sampling time for each run was 40 min a t the inlet and ranged from 2-4.5 hr a t the outlet. Composite samples were then made from the seven inlet and four outlet impinger solutions. The mass flow rate for coal, slag, fly ash, and flue gas, shown in Table I, was calculated as described by Klein et al. (7). The data indicate that electrostatic precipitator efficiency for the fly ash was better than 99.5%. Selenium Determinations. Selenium determinations were made using gas chromatography with a microwave emission spectrometric detection system (MES) (9). The analysis is based on chelating SeIV with a 5-nitro-o-phenylene diamine (PD) to form the thermally stable and volatile piaselenol complex. This is followed by extraction of the piaselenol complex into toluene, separation by a gas chromatic column, and finally, monitoring of the emission intensity of the 204-nm selenium line with the MES. The selenium detection limit is approximately 4 X gram. The validity of the coal, fly ash, and impinger solution determinations were compared with neutron activation analysis. Certified NBS coal and fly ash samples were also analyzed as an independent check. The average relative standard deviation for the determinations was 4.7% and ranged from 2.2-9.5%. Results and Discussion
Selenium Mass Balance. Sampling of flue gas is a continuous process extending over several hours; thus any selenium inhomogeneity problems should be averaged out. Since coal and slag were only sampled intermittently, an attempt was made to ascertain the homogeneity of those materials. Results in Table I1 indicate that both coal and slag were homogenous with respect to Se during the entire sampling period. Analyses of several composite samples of fly ash and impinger solutions are also presented in Table 11. A mass balance for Se in the Allen Steam Plant is presented in Figure 1. Extensive fractionation of Se takes place during the combustion of coal. Selenium is very
Table I. Mass Flow Rate for Coal, Slag, Fly Ash, and Flue Gas Calculated for T. A. Allen Steam Plant, Memphis, T p n . Outlet
Coal, glmin
1.47 x l o 6
Slag. glmin 8.14 X l o 4
Inlet fly ash, fly ash, glmin g/min
7.86 x l o 4
370
Flue gas, scf/min 6.21 X 10'
Table II. Selenium Concentrations in Allen Steam Plant Samples Se, I.rglg
Sample
Coal * l a Coal # 2 a Coal k 3 a Coal k4a Slag # l b Slag #2b Precipitator Precipitator Precipitator Precipitator Precipitator
inlet fly ash # l C inlet fly ash # 2 C inlet impinger solutiond outlet fly ash outlet impinger solutione
2.2 2.2 2.2 2.2 0.08 0.08 28.0 27.5 0.063 88.3
0.088
a Coal $1 represents coal particles 1-5 m m in diameter. Coal #2 0.1-1 m m . coal #3
