ty of the determination of nitrogen in the coke. In summary, the results seem consistent with previous work done in this area. T o volatilize a major portion of the fuel nitrogen would require temperatures much higher than those used in this experiment. Although this investigation has determined what fraction of bound nitrogen does disappear, the gaseous form of the nitrogen molecules could not be determined. If all or most of the nitrogen had gone to ammonia as originally suspected, the chromatograph would have detected it. If as little as 5% of what had volatilized had gone t o NO, the chemiluminescent analyzer would have picked that up. When spot checks were made, less than 20 ppm NO, above background levels were ever detected. The most likely possibility is that the released nitrogen was reduced to molecular nitrogen. The quantity involved, however, was so small compared to the volume entering with the combustion air that it could not be detected. A second possibility is that the fuel nitrogen formed many small gas molecules [HCN, “3, N2 (CN)2, etc.], each in such small quantities that they cannot be detected by the chromatograph. Work done by others indicates that a 15% reduction in bound nitrogen content of the coal before it enters the furnace does not necessarily mean that NO emissions will be reduced by 15%. Very small amounts (0.1 wt %) of bound nitrogen are nearly 80% converted to NO. When the concentration is increased t o 1% by weight, only about 40% of the nitrogen goes to NO. It is thus possible that even with the maximum 15% volatilization seen in this study, NO, emissions would not be reduced significantly.
Acknowledgment Coal was supplied by Pacific Power and Light Co. of Centralia, Wash. Literature Cited (1) Bagwell, F. A., Rosenthal, K., Breen, B. P., Bell, A. W., “13th International Symposium on Combustion,’’ 391-400, The Combustion Institute, Pittsburgh, Pa, 1974. (2) Barrett, R. E., “Factors Influencing Air Pollution Emissions from Stationary Combustion Sources,” presented at Seminar on
New Developments for Combustion Engineering, Penn State University, July 26-30, 1971. (3) First, M. W., Viles, F. J., J. Air Pollut. Control Assoc., 21 (3), 122-7 (1971). (4) U.S. Department of Health, Education and Welfare, NAPCA Publication No. AP-67, “Control Techniques for Nitrogen Oxide Emissions,” 1970. (5) Weidersum, G, D., Chem. Eng. Prog., 66 (ll),49-55 (1970). (6) Bartok, W., Crawford, A. R., Skopp, A., ibid, 67 (2), 64-72 (1971). (7) Bartok, W. Crawford, A. R., Skopp, A., “Systematic Investigation of Nitrogen Oxide Emissions and Control Methods for Power Plant Boilers,” presented at 70th Annual Meeting AIChE, Atlantic City, N.J., August 1971. (8) Bartok, W., Engleman, V. S., “Basic Kinetic Studies and Modelling of Nitrogen Oxide Formation in Combustion Processes,” ibid.
(9) Blakeslee, C. E., Qurbach, H. E., J. Air Pollut. Control Assoc., 23 (l), 37-42 (1973). (10) Martin, G. B., Berkau, E. E., “Preliminary Evaluation of Flue Gas Recirculation as a Control Method for Thermal and Fuel Related Nitric Oxide Emissions,” presented at Western States Section/Combustion Institute Fall Meeting, University of California, Irvine, Calif., October 1971. (11) Shaw, J. T., J. Inst. ojFuel, 46 (3851,170-8 (1973). (12) Wasser, J. H., Hangebrauck, R. P., Schwartz, A. J., J.Air Pollut. Control Assoc., 18 (5), 332-337 (1968). (13) Martin, G. B., Berkau, E. E., “An Investigation of the Conversion of Various Fuel Nitrogen Compounds to Nitrogen Oxides in Oil Combustion,’’ presented at 70th Annual Meeting AIChE, Atlantic City, N.J., August 1971. (14) Turner, D. W., Andrews, R. L., Siegund, C. W., Combustion, 44 (2), 21-30 (1972). (15) Lowry, H. H., ed., “Chemistry of Coal Utilization,” 2nd printing, Chap. 22, John Wiley and Sons, New York, N.Y., 1947. (16) Selvig, W. A., Ode, W. E., “Low-Temperature Carbonization Assays of North American Coals,” U.S. Bur. Mines Bull. 571, US. Govt. Printing Office, Washington, DC, 1957. (17) Field, M. A., Gill, D. W., Morgan, B. B., Hawksley, P. G. W., “Combustion of Pulverised Coal,” Chap. 4, The British Coal Utilisation Research Association, Leatherhead, England, 1967. (18) Essenhigh, R. H., Howard, 3. B., “Combustion Phenomena in Coal Dusts and the Two-Component Hypothesis of Coal Constitution,’’ Penn State University, 1971. (19) Williams, A. W., “Coal Manual for Industry,” p 15, ConoverMast Pub., New York, N.Y., 1952. Received for review September 26, 1974. Accepted April 28, 1975. Work supported by Western Energy Supply and Transmission group.
Trace Element Behavior in Coal-Fired Power Plant John W. Kaakinen” Department of Chemical Engineering, University of Colorado, Boulder, Colo. 80302
Roger M. Jorden’ Department of Civil and Environmental Engineering, University of Colorado, Boulder, Colo. 80302
Mohammed H. Lawasani and Ronald E. West Department of Chemical Engineering, University of Colorado, Boulder, Colo. 80302
Environmental control agencies and researchers have become increasingly concerned with the mobilization of trace elements to the environment from fossil fuel burning ( I ) . Natusch e t al. ( 2 ) have reported that preferential concentration of the trace elements As, Sb, Cd, Cr, Pb, Ni, Se, T1, and Zn occurs in the smallest particles emitted from coalfired power plants, which most easily pass through conventional particulate control devices. Billings e t al. ( 3 )indicate that about 90% of the mercury in coal burned in a pulverized coal furnace appears as vapor in the flue gas. Bolton et 1 Present address, Water Purification Technology, Inc., Grand Junction, Colo. 862
Environmental Science & Technology
al. ( 4 , 5 ) and Gordon e t al. (6) report several such elements occurring in greater concentrations in fly ash as compared to bottom ash. Thus, the process of coal combustion releases trace elements to the atmosphere as vapors and particles, and these particles have relatively greater concentrations of certain trace elements than the feed coal or the collected fly ash. Since it appears that the use of coal-fired boilers for the generation of electricity will continue to rise in the future, elucidation of the fate of trace elements in coal combustion is needed. The work reported herein was designed to study the character of trace element partitioning within a single pul-
H Measurements of the amounts of 17 elements in the inlets and outlets of a pulverized-coal-fired power plant indicate that Al, Fe, Rb, Sr, Y, and Nb were a t essentially constant concentrations in all outlet ashes, and that concentrations of Cu, Zn, As, Mo, Sb, Pb, 210Po,and Se were generally lowest in bottom ash and increased progressively in fly ashes collected downstream toward the stack. A model was developed for the enrichment behavior in downstream
ashes of this latter group of elements assuming that: these elements vaporize in the furnace and then condense or absorb onto fly ash, smaller particles have larger specific surface areas and thus will have greater concentrations of these elements, an increasing proportion of smaller particles in downstream fly ashes results in enrichment of these elements. Experimental enrichment values for Zn, As, Mo, and S b agree with values predicted by this model.
verized-coal-fired power plant and to generalize the resulting data to other situations, if possible. Since composition of trace elements in coal can vary greatly between mines and even within a single coal bed, compositions of combustion products from a single power plant can also vary. Thus, in order to be able to generalize the results of a limited number of tests, all potentially significant inlet and outlet streams of trace elements to the power plant were sampled a t about the same time, their total mass flow rates were calculated, and chemical analyses of the various samples were made. Calculated mass flow rates of each element measured in the various inlet and outlet streams were then related to one another to obtain mass balances and “enrichment ratios” for the various elements. These results were studied to determine if common patterns of trace element behavior could be ascertained and then related to physicochemical properties of the trace elements in the coal and combustion products and to the nature of processes occurring within the power plant. This paper summarizes the experimental methodology, results, and discussion of this work; greater detail of this work is available elsewhere (7-9).
which include coal; bottom ash (BA); ash from the mechanical collector hopper (MA) and electrostatic precipitator hopper (PA); fly ash from flue gas a t the scrubber inlet (SI), scrubber outlet (SO), and electrostatic precipitator outlet (PO); scrubber slurry (SS); and scrubber makeup water (MW). No gaseous trace elements in flue gas were successfully sampled. Whole coal and SS samples were obtained each day as composites of grab samples collected every half hour during each 4-hr run. Portions of MA and PA were obtained from their respective hoppers a t the end of each test. SI and PO samples were collected isokinetically from flue gas in bag filters, and SO was sampled isokinetically with alundum thimbles backed up by wet impingers. Only one grab sample of BA was obtained because of limited access for sampling this stream. The handling and preparation of the various samples for trace element analysis were carried out using a set of procedures designed to ensure representative sampling, the details of which are available elsewhere (7,8). Total mass flow rates shown in Figure 1 are the solid andfor liquid flow at each point averaged for the three runs. Average flow values were used since their standard error was less than 5%. Total mass flow rates of SI, SO, PO, PA, SS, and MW were calculated directly from test data. Total ash flow of MA was calculated from downstream rates using a previously measured fly-ash-removal efficiency of 86% for the mechanical dust collector. BA flow was estimated from the calculated ash flow to the mechanical collector using an assumed 19% (IO) for the total fraction of ash retained in the boiler bottom; a sensitivity analysis indicated that a more precise BA flow rate was not critical for a successful mass balance (8).The coal flow rate was calculated by PSCo from test data independent on the above ash flow rates. Element concentrations were determined by the following analytical methods: conventional atomic absorption
Experimental Methods Most samples were collected during three days of wet scrubber-electrostatic precipitator performance tests on the 180-MW (net) Unit No. 5 of the Public Service Co. of Colorado’s (PSCo) Valmont Power Station near Boulder, Colo. Operation was a t full-load, steady state conditions. Pulverized coal from a single mine with about 0.6% sulfur and 6% ash was burned. A flow sheet of Unit No. 5 , given in Figure 1, indicates its unique particulate control scheme, which consists of a high-efficiency mechanical dust collector followed by an electrostatic precipitator and a wet scrubber (turbulent contact absorber) in parallel. Samples were collected from the streams indicated in Figure 1, LEGEND
@ SAMPLING LOCATION - - - 8 O U N O A R Y O F M A S S BALANCE 55
Figure 1. Flow sheet of
T O T A L M A S S FLOW R A T E S I N K G l M l N AS S O L I 0 O R L I Q U I D
ATMOSPHERE
Valmont Unit No. 5 Volume 9, Number 9, September 1975 863
Table I . Trace Elements in Power Plant Samples Samples analyzed: whole coal, bottom ash (BA), mechanical collector hopper ash (MA), electrostatic-precipitatorhopper ash (PA), scrubber-inlet fly ash (SI), electrostatic-precipitator-outlet fly ash (PO), scrubber-outlet fly ash (SO), scrubber slurry SS . Measurement methods: conventional atomic absorption spectrophotometry (AA), X-ray fluorescence (XkFj, wet chemistry (WC), flameless atomic absorption (FAA), radiochemical analysis (RA), Brunauer-Emmett-Teller method (BET), aerodynamic particle size (APS) Sample
Coal BA MA PA SI PO
so ss
Analytical method:
Concentration, pg/g
AI. % by' wt
Fe. % by'wt
cu
Zn
As
0.49 8.8 9.6 10.2 9.0 9.2 7.4 0.10
0.37 6.6 7.0 6.9 7.4 7.4 4.9 0.063
9.6 82 150 23 0 28 0 320 290 2.4
7.3 58 100 250 360 370 600 2.2
15 44 120 130 150 280 1.1
AA
AA
XRF
XRF
XRF
Sr
Y
Nb
Zr
Mo
0.50
120 1800 2400 2500 2200 2500 2500 21
3.0 44 61 68 52 60 31 0.49
0.76 12 16 19 17 19 18 0.49
13 220 260 2 10 160 190 80 1.8
0.99 3.5 12 41 54 60 110 0.53
XRF
XRF
XRF
XRF
XRF
XRF-WC
Rb
2.9 48 50 73 51 56 28
Specific activity disintegrations per min/g
Concentration, pglg
Coal BA MA PA SI PO
so
ss
Analytical method:
Sb
Pb
Se
Hg
'lOPb
'loPo
'"Ra
2.8 4.7 14 14 18 22 0.10
