Encyclopedia of Environmental Science and Engineering, Volume I and II

820 PARTICULATE EMISSIONS

quantitative procedure for design of equipment to produce

complying plumes. Equipment vendors will usually guar-

antee collection efficiency and emission concentrations by

weight, but they will not give a guarantee to meet a specified

opacity. This is indeed a serious problem at a time when a

large precipitator installation can cost several million dollars

and take twenty months to fabricate and install. Overdesign

by a very conceivable factor of two can be very expensive in

unneeded equipment. Underdesign can mean years of delay

or operation under variance or with penalty payments.

Some progress has been made in applying classical theo-

ries of light scattering and transmission to the problem of

predicting opacity. This effort has been greatly hampered by

paucity of data giving simultaneous values of light attenua-

tion, particle size distribution, and particle concentration in

a stack. Perhaps the most comprehensive work to date has

been that of Ensor and Pilat.^16

Weight Limits on Particulates

Perhaps the least equivocal method of characterizing and

specifying limits on particulate emissions is according to

weight, either in terms of a rate (weight of emissions per unit

time) or in terms of concentration (weight per unit volume).

Measurement of emission weights must be done by iso-

kinetic sampling of the gas stream, as outlined in the follow-

ing section on measurement. Although the principles of such

measurement are simple, they are difficult and time consum-

ing when applied with accurate methodology to commer-

cial installations. For this reason, such measurements have

not previously been required in many jurisdictions and are

almost never used as a continual monitoring technique.

Limits on weight rate of emissions are usually dependent

on process size. Los Angeles, for instance, permits emissions

to be proportional to process weight, up to 40 lbs/hr particu-

lates for a plant processing 60,000 lbs/hr of material. Larger

plants are limited to 40 lbs/hr. For furnaces, the determining

factor is often heat input in BTU/hr rather than process weight.

In cases where a particular plant location may have several

independent units carrying out the same or similar processes,

regulations often require that the capacities be combined for

the purposes of calculating combined emissions.

Concentration limits are usually independent of process

size. For instance, the EPA specifies incinerator emission of

0.08 grains particulates per standard cubic foot of flue gas

(0.18 gm/NM^3 ) Dilution of the flue gas with excess air is usu-

ally prohibited, or else correction must be made to standard

excess air or CO 2.

Ground Level Concentrations of

Suspended Particulates

A limit on ground level concentration of particulates is an

attempt to regulate emissions in accordance with their impact

on population. A smoke stack acts as a dispersing device,

and such regulations give incentive to build taller stacks in

optimum locations.

In theory, ground level concentrations can be measured

directly. Usually, however, emissions are measured in the

stack, and plume dispersion equations are then used to cal-

culate concentration profiles. Plume dispersion depends on

stack height, plume buoyancy (i.e. density relative to ambi-

ent air), and wind velocity, and wind patterns. In addition,

plumes are never stationary but tend to meander; and cor-

rection factors are usually applied to adjust for the sampling

time at a fixed location. Dispersion calculations are usually

easier than direct ground level measurements; and in cases

where many different sources are present, calculation offers

the only practical way to assess the contributions of a spe-

cific source. A recent evaluation of plume dispersion models

is given by Carpenter et al.^15

In some states, a plume dispersion model is incorporated

into a chart which gives an allowable weight rate of emissions

as a function of effective stack height and distance from prop-

erty lines. An example of this approach is shown in Figure 3.

FIGURE 3 Emission requirements for fine particles

based on plume dispersion model (New Jersey Air Pollution

Code).

FIGURE 2 Stack mounted opacity meter (Bailey Meter

Co.).

SPOTLAMP

LIGHT SOURCE SPACED FLANGES

FOR AIR INLET SMOKE OR DUST PASSAGE

BOLOMETER

SPACED FLANGESFOR AIR INLET

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