1. Calculating
Realistic
PMro
Emissions Cooling
from Towers
JoelReismon
ond
Gordon
Frisbie
Greystone
Environmental
Consultants, 10470 Placerville Suite Sacramento,
Inc,, Old Road, 110, CA95827
air passing through the tower, and as part of normal
Emissionsof particulate matter less tban 10 micrometersin operati on, a very smal l amount of the ci rculat ing
diameter (PMr) from wet cooling towersrnay be calculated water may be entrained in the air stream and be car
using tbe methodologlt presented in EPA'sAP-42 [1], wbicb ried out of the tower as "drift" droplets. Because the
assumestbat all total dissoluedsolids (TDS) emitted in "drift" drift droplets contain the same chemical impurities as
particles (liquid water entra.inedin the air stream and can'ied the water circulating through the tower, the particulate
out of tbe tower tbrougb tbe induced drafifan stack) are PMro. matter within the drift droplets may be classified as an
Howner, tbis assurnptionis ouerlyconservatiue because does
it emission. The magnitude of the drift loss is influenced
not considerthat, upon euaporationof tbe drift, many of tbe by the number and size of droplets produced within
solid particles tbat remain are larger tban PMro.Particles the tower, which are determined by fill design, tower
larger than 10 micrometers not represent bealtb bazard
do a design, the air and water patterns, and design of the
and are not regulatedunder current air quality regulations. drift eliminators.
For example,for wet cooling towerswitb medium to higb TDS
leuels,tbe AP-42 metbodologjt predicts significantly higber
AP.42 METHODOFCATCUTATING
DRIFI
PARTICUTATE
PM,o emissionstban would actually occur, euen for towers EPA's AP-421.provides available particulate emis-
equippedwitb uerybigb fficiency drift eliminators (e.g., sion factors for wet cooling towers, however, these
0.00060/o drift rate). Sucb ouer-prediction may result in unre- values only have an emission factor rating of "E" (the
alktically bigb PMromodeledconcentra.tions and./ortbe need lowest level of confidence acceptable). They are also
topurcbase expensiue ErnissionReduction Credits(ERCs) in rather high, compared to typical present-day manufac-
PMronon-attainmmt areas, Sincetbese towers hauefairly low turers' guaranteed drift rates, whiqh are on the order
emissionpoints(10 to 15 m aboueground), ouer-predicting of 0.0006%.(Drift emissions are typically expressed as
PMro emission rates can also result in exceedingfederal Pre- a percentage of the cooling tower water circulation
uentionof SignfficantDete4oration (PSD)significanceleuels nte). AP-42 states that "a consen)Atiuely bigb PMro
at a project'sfenceline. Tltispaper presentsa metbodfor com- emission factor can be obtained by: (a) multiplying
puting realistic PM,o emissions from cooling towerswith medi- the total liquid drift factor by the TDS fraction in the
um to bigb TDSleuels enabling tbe engineerto determine
by ci rcul ati ng w ater, and (b) assumi ng that once t he
tbePMromassfractionof the total amount ofparticulate emit- water evaporates, all remaining solid particles are
ted by a coolingtower, within the PM10 range." (Italics per EPA).
If TDS data for the cooling tower are not available,
INTRODUCTION a source-specific content can be estimated by obtain-
Cooling towers are heat exchangers used to dissi- ing the TDS for the make-up water and multiplying it
pate Iarge heat loads to the atmosphere. 'Wet, or evap- by the cooling tower cycles of concentration. (The
orative, cooling towers rely on the latent heat of water cycles of concentration is the ratio of a measured
evaporation to exchange thermal energy between the parametet for the cooling tower water [such as con-
process and the air passing through the cooling tower. ductivity, calcium, chlorides, or phosphatel to that
T he c ooling w a te r m a y b e a n i n te g ra l p art of the patametet for the make-up water.)
process or may provide cooling viaheat exchangers, Using AP-42 guidance, the total particulate emis-
for example, steam condensers. Wet cooling towers sions (PM), after the pure water has evaporated, can
provide direct contact befween the cooling water and be expressed as:
EnvironmentalProgress(Vo1.21,
No.2) July 2002 1,27

2. TDS, the density of the solids, and the initial drift
PM : WaterCirculation Rare x Drift Rate x TDS (1) droplet diameter, Do:
For example, for a typical power plant wet cooling Volume of drift droplet = (4/S;Sn(OutZ)t Q)
tower with a water circulation rate of 746,000 gallons
per minute (gpm), drift rate of 0.0006%, and TDS of Mass of solids in drift droplet : (TDSXp,
7,700 parts per million by weight (ppmw), (Volume of drift droplet) (3)
PM : 145,000gptn x 8.34lb water/gal x 0.0006/100x substituting,
7,700lb solids/10o lb water x 60 min/hr = 3.38Lb/hr
Mass of solids in drifr : (TDS)(p)(4/Dn(Dut2)3 (4)
On an annual basis, this is equivalent to almost 1.5
tons per year (tpy). Even for a state-of-the-art drift elimi- Assuming the solids remain and coalesce after the
nator system, this is not a small number, especially if w ater evaporates, the mass of sol i ds can also be
assumed to all be equal to PM10,a regulated criteria pof exDressedas:
lutant. However, as the following analysis demonstrates,
only a very small fraction is actually PMro.
Mass of solids = (pros) (solid particle volume) :
QroJ G/)n(Dr/z): (5)
coMPUT|NG THE FMCT|oN
PMro
Equations 4 and 5 are equivalent:
Based on a representative drift droplet size distribu-
tion and TDS in the water, the amount of solid mass in
(pros) (4/)n(Do/2)3 = (TDS) (p)(4/in(Du/2)3 (6)
each drop size can be calculated. That is, for a given ini-
tial droplet size, assuming that the mass of dissolved
Solving for Do:
solids condenses to a spherical particle after all the water
evaporates, and assuming the density of the TDS is
equivalent to a representative salt (e.g., sodium chlo- Dp = Dat(TDsxp,/pror)lt6 Q)
ride), the diameter of the final solid particle can be calcu- rilflhere:
Iated. Thus, using the drift droplet size distribution, the
percentage of drift mass containing particles small TDS is in units of ppmw
enough to produce PMro can be calculated. This method Do = diameter of solid particle, micrometers (pm)
is conservative as the final particle is assumed to be per- D6 = diameter of drift droplet, pm
fectly spherical, hence, as small aparticle as can exist.
The droplet size distribution of the drift emited from Using Formulas 2 through 7 and the particle size
the tower is critical to performing the analysis. Brent- distribution test data, TabIe l can be constructed for
wood Industries, a drift eliminator manufacturer, was drift from a wet cooling tower having the same char-
contacted and agreed to provide drift eliminator test data acteristics as our example: 7,700 ppmw TDS and a
from a test conducted by Environmental Systems Corpo- 0.00060/o drift rate. The first and last columns of this
ration (ESC) at the Electric Power ResearchInstitute table are the particle size distribution derived from test
(EPRI) test facility in Houston, Texas, in 1988. The parti- resul ts provi ded by B rentw ood Industri es. Using
cle size distribution is included in the first and last straight-line interpolation for a solid particle size 10 pm
columns of Tables 1, and 2. The data consist of water in diameteq we conclude that approximately I4.9o7o o1
droplet size distributions for a drift eliminator that the mass emi ssi ons are equal to, or smal l e r t han,
achieved a tested drift rate of 0.00030/0. we are using a
As PMro. The balance of the solid mriterial are particu-
0.00060/o drift rate, it is reasonable to expect that the lates greater than 10 pm. Hence, PMro emissions from
0.00030/o drift rate would produce smaller droplets, there- this tower would be equal to PM emissions x 0.149, or
fore, this size distribution data can be assumed to be =
3,38lb/hr x 0.1.49 0.50 lblhr. The process is repeat-
conservatiue for predicting the fraction of PMro in the ed in Table 2, with alI parameters equal except that
total cooling tower PM emissions. the TDS is 11,000 ppmw. The result is that approxi-
I n c alc ulat in g P M ro e m i s s i o n s , th e fo l l ow i ng mately 5.t10/o are smaller at 11,000ppm. Thus, while
assumptions were made: total PM emissions are Iarger by virtue of a higher TDS,
r Each water droplet was assumed to evaporate overall PMro emissions are actually louer, because more
shortly after being emitted into ambient air, into a of the solid particles arc larger than 10 pm.
single, solid, spherical particle. The percentage of P MI./P M w as cal cul a t ed f or
. Drift water droplets have a density (p) of water; cooling tower TDS values from 1,000 to 12,000 ppmw
1.0 g/cmj or 1.0 * 10-6 pglpm3. and the results are plotted in Figure 1..Using these
. The solid particles were assumed to have the same data, Figure 2 presents predicted PMro emission rates
density (pror) as sodium chloride, (i.e.,2.2 g/c^3). for the 745,000 gpm example tower. As shown in this
Figure, the PM em,ission rate increases in a straight
Us ing t he f o rmu l a fo r th e v o l u me o f a sphere, line as TDS increases, however, the PMro emission
V = 4n^r J / 3,and th e d e n s i ty o f p u re w a te r, p* = 1.0 rate increases to a maximum at around a TDS of 4,000
g/.*3, the following equations can be used to derive ppmw, and then begins ta decline. The reason is that
the solid particulate diameter, Do, as a function of the at higher TDS, the drift droplets contain more solids
t28 July}oA2 Environmental Progress (Vo1.21, No.2)

3. Ioble L Resultant solid particulate size distribution (TDS = 7,700 ppmw).
solid particulatesizedistribution(TDS= 11,000
Tobfe2. Resultant ppmw),
Environmental Progress (Vo1.21, No.2) 2002 I29

4. 90
80
70
60
850
b40
o.
30
20
10
0
1000 2000 3000 4000 5000 6000 7000 8000 9000 100001100012000
TDS(ppmw)
Water
Girculating
Figure l. Percentageof drift PM that evaporates to PMro.
6.0
rate GPMand0.0006% rate,
circulation of 146,000 drift
E 5.0
ll
o 4.0
G
t PM Emission
Rat; z
C
.9 3.0
o
.9,
E 2.0
uJ
(f PM16
Emission
= 1.0
o
0.0
1000 2000 3000 4000 5000 6000 7000 8000 9000 100001100012000
CirculatingWaterTDS(ppmw)
Figule 2. PM10emission rate vs. TDS.
and therefore, upon evaporation, result in larger solid istic PMro emission rates from wet mechanical draft
particles for any given initial droplet size. cooling towers equipped with modern, high-efficiency
drift eliminators and operating at medium to high lev-
els of TDS in the circulating water.
c0NcLusl0N
The emission factors and methodology given in
EPA'sAP-42, Chapter 1.3.4 Wet Cooling Towers [1], do TITEMTURECITED
not account for the droolet size distribution of the 1. "Compilation of Air Pollutant Emission Factors,"
drift exiting the tower This is a critical factor, as more AP-42, Fifth Edition, Yolume I: Stationary Point
than 850/o the mass of particulate in the drift from
of and Area Sources, Chapter I3.4 /et Cooling Tow-
most cooling towers will result in solid particles larger ers, http:/ /www.epa.gov/ttn/chief /ap42/ , rJ.S.
than PMro once the water has evaporated. Particles Environmental Protection Agency, Office of Air
larger than PMro are no longer a regulated air pollu- Quality Planning and Standards,ResearchTriangle
tant, because their impact on human health has been Park, NC, January 1.995.
shown to be insignificant. Using reasonable, conserva-
tive assumptions and a realistic drift droplet size distri-
bution, a method is now available for calculating real-
730 July 2002 Environmental Progress(Vo1.21,No.2)