Basic Calculations for Occupational Safety and Environmental Health Professionals - Mathematics and Statistics for the Environment - page 403

Particles play an important role in atmospheric phenomena; for example, particulates provide
the nuclei upon which ice particles and cloud condensation are formed, and they are essential for
condensation to take place. The most important role airborne particulates play is in cloud formation.
Simply put, without clouds, life would be much more difficult, and the cloudbursts that eventually
erupted would cause devastation so extreme that it is difficult to imagine or contemplate.

15.5.4 p ollution e mission m easurement p arameters
Because of the gaseous and particulate emissions that can be produced, combustion sources consti-
tute a significant air quality control problem. Combustion processes can add carbon dioxide, water
vapor, and heat to the atmosphere, and can produce a residue that must be disposed of in concen-
trated form. In the past, these environmental costs were tolerated in the interest of producing useful
energy. However, it is becoming increasingly clear that the presence of these emissions in the atmo-
sphere can result indirectly in a greenhouse effect and exacerbate the problem of acid rain. Because
of the environmental impact of combustion emissions, USEPA has developed emission standards
for the combustion or incineration industry. These standards usually establish the maximum allow-
able limit, based on volume or mass flows at specified conditions of temperature and pressure, for
the discharge of specific pollutants. Emissions are measured in terms of the concentration of pol-
lutant per volume or mass of stack (flue) gas, the pollutant mass rates, or a rate applicable to a given
process. Standards fall into the following six categories:

1. Pollutant mass rate standards are based on the fixed rate of emissions (i.e., the mass of
pollutant emitted per unit time, expressed in lb/hr or kg/hr).

2. Process rate standards establish the allowable emissions in terms of either the input energy
or the raw material feed of process.

3. Concentration standards limit either the mass (weight) or volume of the pollutants in the
gas leaving the stack.

4. Ambient concentration standards address such pollutants as toxic metals, organics, and
hydrogen chloride (measured in µg/m 3 ).

5. Reduction standards are expressed as a percent reduction of the pollutants.

6. Opacity standards address the degree to which the stack emissions are visible and block
the visibility of objects in the background. Stack emissions of 100% opacity totally block
the view of background objects and indicate high pollutant levels. Zero percent opacity
provides a clear view of the background and indicates no detectable particulate matter
emissions.

15.5.5 s tandard C orreCtions
Because combustion systems always produce stack gas that is at a higher temperature and pres-
sure than those of the standards, and because actual levels of pollutants emitted can be made
to appear smaller if excess air is added to the stream, corrections for these differences must be
made. With regard to increases or decreases in gas temperature and pressure and the subsequent
effect on gas volume, the USEPA recommends using the ideal gas law. For excess air correction
calculations, various federal USEPA and state regulations give procedures for calculating percent
excess air based on dry gas (Orsat type) analyses. Based on USEPA's Method 3B, Gas Analysis
for Carbon Dioxide, Oxygen, Excess Air, and Dry Molecular Weight, the percent excess air can
be determined by any of the following three equivalent relationships:

TotalAir − Theoretical Air

×100

%ExcessAir = (15.9)

Theor etical Air