Concentration and Size of Trace Metal Emissions from a Power Plant, a Steel Plant, and a Cotton Gin Robert E. Lee, Jr.,* Howard L. Crist, Allan E. Riley, and Kathryn E. MacLeod U.S. Environmental Protection Agency, National Environmental Research Center, Research Triangle Park, N.C. 277 1 1
An in-stack cascade impactor was used to determine the size distribution of particles, before and after emission control systems, of a coal-fired power plant, a steel plant, and a cotton gin. Samples were analyzed for total particulate matter and elemental metal content by graphite-furnace atomic absorption; Se was analyzed by neutron activation. A sharp decrease in the concentration and particle size was observed after the particulates passed through the control systems. Pronounced metal concentration and particle size relationships were found in the power plant and steel plant samples. The power plant outlet samples showed an increased concentration of Cr, Pb, Sb, Zn, and Se in the particle size range of 0-1 pm diameter. The particle removal efficiency of the baghouse and electrostatic precipitator control devices in this study was a t least 98% for submicrometer sized particles.
Stationary emission sources can contribute significantly to the amount of primary particulate matter in air of anthropogenic origin, especially trace metals ( I , 2 ) . The effectiveness of particle control devices depends in large measure on the particle size of stack aerosols, yet, very little published information is available on the size distribution of particulates associated with stationary sources (3, 4 ) . Particle size fractionating devices which can operate in a harsh stack environment have become available and should lead to a better understanding of particulate emissions (57). By applying methods similar to those used in determining the weight-size distribution of particles suspended in ambient air ( 8 ) ,we have used a Pilat impactor ( 5 ) to determine the weight size distribution of total particulate matter and trace metal components in several stationary emission sources. Samples were collected both before and after the emission control devices to assess the aerosol removal efficiency. The sources tested included a coal-fired power plant equipped with an electrostatic precipitator, a cotton gin equipped with a wet scrubber, and an electric arc furnace ferro plant equipped with a baghouse control system. These studies were carried out as part of a source testing program on best controlled industries for developing emission standards on new stationary sources.
Experimental Methods Description of the Cascade Impactor. A University of Washington Mark I11 source test cascade impactor was used aerodynamically to size and collect aerosols within stacks of selected stationary sources (9). The impactor consists of seven stages and a backup filter to collect unimpacted particles, all of which are housed in a stainless steel cylinder that can be inserted into a stack or duct through a 10.2-cm (4-in.) port. The Mark I11 version differs somewhat from previous designs (5, 1 0 ) in that the airstream passes through the center of “donut” shaped collection surfaces
rather than around openings near the walls to minimize wall losses. The sampler fractionates particles in the size range of about 50-0.5-wm diameter, expressed as spheres of unit density, depending on the flow rate that can be as high as 0.006 m3/min (2.5 ft7/min). The calibration curves used for each stage as a function of temperature and flow rate were determined by the manufacturer with polystyrene latex spheres of known dimension (9). Aluminum disks cut to fit on top of each stage were weighed to the nearest microgram with a Mettler M-5 microanalytical balance; backup filters were similarly weighed after conditioning 24 hr below 50% relative humidity and room temperature. Because of high stack temperatures, glass fiber backup filters were used with the power plant and steel mill. Both membrane and glass fiber filters were used with the cotton gin although no difference in collection efficiency was observed. Particulate matter was impacted directly on the tared aluminum surfaces removed after sampling, folded, and returned to the laboratory for gravimetric and chemical analysis in much the same fashion as the ambient air studies conducted previously (8). Field Operation. In the field, the sampler equipped with an appropriate vacuum source and flow measuring device (9) was inserted inside the stack and allowed to temperature equilibrate for 15 min with the sampling nozzle facing away from the air stream. After temperature equilibration, the impactor was pointed toward the gas stream flow and a sample collected isokinetically a t a point of average flue gas velocity for periods as short as 10 min or as long as 2 hr depending on the particulate loading. Sampling under isokinetic conditions was achieved by using an appropriate restriction a t the inlet end of the impactor and by adjusting the flow rate with a variable speed vacuum pump. The velocity of the flue gas was monitored during sampling with a Pitot tube placed next to the impactor ( 2 1 ) . The average sampling velocity was determined from the total volume of air sampled, corrected to dry standard volume units, and the total sampling time. By use of the size calibration curve provided by the manufacturer ( 9 ) , the average stage constants were determined from the average flow rate and the approximate temperature of the flue gas. A particle size distribution curve was constructed on log-probability paper from the average stage constants and the cumulative percent mass ( 5 , 9). Chemical Analysis. In addition to gravimetric analysis of the collected particulate matter, metal constituents were determined on one set of samples collected before and after the control system by flameless atomic absorption spectrophotometry; neutron activation analysis was used to determine selenium in the power plant samples. For analysis by flameless atomic absorption spectrophotometry, the aluminum collection surfaces and the filters were cut into strips, placed in individual Pyrex boats, and ashed in a Tracer Lab LTA 600 low-temperature asher for 30 min a t 250 W with an oxygen flow of 80 cm/min. After it Volume 9, Number 7, July 1975
643
INLET ( 2 SAMPLES) M M D = 17 p m
0.1 0.01 0.1 0.5
2
5 10 20
OUTLET (2 SAMPLES MMD-=0.36 pm
40
60
80 90 95 98
C U M U L A T l V E % M A S S Z PARTICLE S I Z E Figure 1. Composite mass size distribution of particulates collected before and after an electrostatic precipitator at coal-fired power plant
was ashed, each sample was removed and placed in a 20-ml beaker, 5 ml of 1:l v/v " 0 3 added, and sonicated for 15 min a t 50°C in a sonic cleaner. The supernatant was decanted into a 10-ml volumetric flask; the aluminum collection surfaces and filter strips were rinsed with distilled water that was also added to the volumetric flask. A final sample extract of 10 ml was used in the analysis. The following metals were analyzed using a Techtron AA-5 atomic absorption spectrophotometer equipped with a graphite furnace: Fe, V, Cd, Cr, Co, Ni, Mn, Cu, Pb, Sb, and Zn. Stationary Source Tested. The sources tested included a coal-fired power plant in Illinois, a cotton gin in California, and an electric arc ferro plant in Pennsylvania. A t least two samples were collected both before and after the control devices. The power plant fired 60 tons (54,432 kg) of coal per hour for a unit generation of 105 MW. The stack gas flow rate was 257,000 dry standard cubic feet per minute (DSCFM), equivalent to 7273 m3/min. The control device was a 64-plate electrostatic precipitator with a ratio of collecting plate area to gas flow of 391. The design specifications for the precipitator were 1.5 grain (97.2 mg) per DSCFM on the inlet and 0.005 grain (0.3 mg) per DSCFM on the outlet for an efficiency of 99.7%. The ferro plant operation consisted of two electric arc furnaces with capacities of 75 and 50 tons (68,040 and 45,360 kg) per day. The batch process involved meltdown, oxidizing, slagging, and refining steps with each cycle consuming 6-8 hr. Two 900-hp fans removed the effluent gases from the shop area through adjoining openings in the shop roof and through a single exhaust duct and to a 12-compartment baghouse. The volumetric flow rate a t the outlet stack tested was 78,000 DSCFM (2207 m,i/min). The total average gas flow rate through all six outlet stacks was 454,000 DSCFM (12,848 m3/min). The effluents are ex644
Environmental Science & Technology
0.1 0.01 0.1 0.5
2 5 10 20
40 60
CUMULATIVE % MASS
80 90 95 9899
PARTICLE S I Z E
Figure 2. Composite mass size distribution of particulates collected before and after a baghouse at electric arc furnace steel plant
hausted to the atmosphere from the baghouse through six 9-ft (2.74 M) diameter stacks. During sampling, the shake cycle on the baghouse control device was ignored. The cotton gin operated continuously producing an average of 600 bales per day, each weighing approximately 500 lb (227 kg). Mechanical unloading of the cotton was made directly from wagons brought in from the field. The cotton was sent to driers for moisture adjustment and to cleaners for elimination of field debris. Successive stages of operation removed seeds and lint. Finally, the cotton was processed through the battery condenser and then the baling press. The sampling site chosen was the inlet and outlet of the wet scrubber that received the effluent from the battery condenser. The outlet flow through the scrubber duct was 16,000 DSCFM (453 mCi/min); the air from the scrubber was piped back into the plant area.
Results and Discussion Size Distribution of Total Particulates. The size distributions of total particulate (TP)matter for each stationary emission source tested are shown in Figures 1 through 3. The distribution curves represent a composite of data from at least two Pilat impactor samples collected before and after the control devices. Although the samples were not collected simultaneously downstream and upstream from the control system, sampling was carried out during the same day and during the same process operation. In general, the size distribution curves for the three sources tested were not entirely well described by a lognormal function. The inlet samples for the power plant and the steel mill, Figures 1 and 2, exhibited some evidence of bimodality. For all three sources, the particle concentration and the median particle size were reduced after the aerosol passed through the control device. An estimate of the total quantity of particulate matter
I 1 I I 1 I I 1 1 1 I I
40.0--(11 COTTON G I N
-
20.0
INLET (4 SAMPLES)
10.0 8.0
E 1
. 6.0 W 2 4.0
i
0 + E
2
2.0
u z
a z
-
-
-
-
-
0.6 -
-
1.0 0.8
n W
a
0.4
-
0.2
-
0.1
2,300 pg/rn3 9.5pm -
AVE. CONC. = MMD =
-
I I
" ' 1 1 1
I 1 I 1 I I I
-
I
emitted from each source can be determined from the concentration per unit volume and the total volume of outlet gas given in the Experimental Methods. For example, the power plant emitted 2364 g/min, the steel plant 11 g/min, and the cotton gin 37 g/min. The size distribution curves can provide further information on the concentration of particulate emissions as a function of size. For example, particles having an aerodynamic size less than or equal to
1-wm diameter, as spheres of density one, range from 284 g/min for the power plant, 70 g/min for the steel plant, and 2 g/min for the cotton gin. I t is clear that the power plant represents the most important stationary emission source of the three tested, especialiy in view of the large amount of submicrometer size aerosol emitted. The data provided in Figures 1 through 3 can also be used to assess the efficiency of the control systems on each source. Overall efficiency for total particulate matter was 99.7% for the power plant equipped with an electrostatic precipitator, 99.6% for the steel plant equipped with a baghouse, and 86% for the cotton gin equipped with a wet scrubber. The removal efficiency as a function of particle size, can be calculated from the figures and appears to decrease with decreasing particle size. For example, for the particles less than or equal to 1-wm diameter, the removal efficiency dropped to 98.0% for the electrostatic precipitator, remained about the same, 99.3%, for the baghouse, and dropped substantially to 47.3% for the wet scrubber. The particle removal efficiency of the baghouse and the precipitator determined in this study agrees well with results presented by Craig (12). He found a removal efficiency for 1-pm diameter particles of 95% for an electrostatic precipitator and 99.7% for a baghouse. Concentration Measurements of Metal Constituents. The concentration and mass median diameter (MMD) of metal-containing constituents for both the control system inlet and outlet of each source tested is summarized in Table I. Iron represented the major constituent in both the inlet and the outlet samples from the power plant. Other metals detected included V, Cd, Cr, Ni, Pb, Sb, Zn, and Se. Mn was found in relatively high concentration in the inlet sample but not detected in the outlet. I t is likely that the large size of the Mn-containing particles accounted for its virtually complete removal by the electrostatic precipitator. With the exception of Se, TP and the metals measured decreased in median particle size after passing through the control system, probably because the larger metal-containing particles were more efficiently removed. It is especially disturbing that Se was detected in relatively large quantities representing an emission rate of approximately 1.7 g/min.
Table I. Concentration and Mass Median Diameter of Particulate Components from Stationary Emission Sources Power plant Inlet
-_____ Cornponent
Concn, pg/m.
MMD,
m
Steel mill
Cotton gin
_________~ Outlet
Concn, a/mr
Inlet
MMD, prn
Concn, 4 m r
-~__.
Outlet
-~
MMD,
m
Concn, a/rnd
Inlet
MMD, pm
Concn, rg/m3
Outlet
~
~
MMD,
m
_
_
Concn, d m r
MMD, Wrn
Sample Set A TP
Fe V Cd
Cr
co Ni Mn
cu Pb
Sb Zn
38 X lo5 3 x 105 970 8.5 300 0 395 600 189 689 162 23 X lo6 114
18 6 5 8.4
12 0 11.2 15
8700 1340 1.5 0.1 0.7 0 1.3 0
4.7 2.6 1.6 5.0
