Page 239 - Sustainable On-Site CHP Systems Design, Construction, and Operations

212 De s i g n

Air Dispersion Model
Both the ambient air quality impact assessment and the risk analysis are based upon
an air dispersion modeling analysis. Generally, the exhaust plume of a CHP system
will be concentrated upon exiting the stack, but disperses in a bell-shape pattern as it
travels downwind. The Gaussian algorithm can be used to estimate the dispersion of
the plume. Computer models such as U.S. EPA’s Industrial Source Complex Short
Term (ISCST) and the American Meteorological Society/EPA Regulatory Model
(AERMOD) incorporate the Gaussian algorithm to estimate the diffusion properties
of the exhaust plume, and also take into account exhaust velocity and temperature.
These models also consider meteorological and topographical characteristics that are
unique to the project location. Once the project’s plume dispersion is understood,
then the concentration of specific pollutants at specific locations surrounding the
project can be analyzed to determine the associated air quality impacts and potential
health risks.

Air Quality Impact Analysis
Table 12-1 provides a summary of the impacts of various pollutants. Regulatory agen-
cies often establish health-based standards for ambient concentrations of pollutants
such as NO , CO, SO , and PM. They may also restrict the degree to which a project can
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contribute to existing ambient concentrations of these pollutants. If the concentration of
a pollutant from a CHP project are likely to cause an ambient air quality standard to be
exceeded or if it significantly adds to what is already a violation of an ambient air
quality standard, the CHP project could be rejected by the permitting authority. By
using a dispersion model to determine downwind exhaust dispersion properties, com-
bined with the emissions inventory, it is possible to estimate the project’s contribution
to local or regional ambient air quality and determine the off-site concentrations of a
pollutant that originates from the proposed CHP project. The modeling analysis can
also demonstrate the anticipated cumulative concentration of a pollutant from both the
proposed project and existing sources.

Health Risk Assessment
An additional assessment may be required to estimate the health risks that can be
attributed to a CHP project. Health risk assessments rely upon the air dispersion model
results, combined with the hazardous pollutant emission inventory, to determine peak
short-term and average long-term pollutant concentrations at locations where people
may live and work (receptors). The concentrations are then measured against pollutant-
specific toxicology data to determine the increased health risks attributed to each
pollutant. The toxicological data are published by health agencies and generally reflect
the results of empirical studies.
Three types of health impacts are typically analyzed. The most common is an
assessment of increased cancer risk that may be expected for nearby receptors. This
risk exists due to the extended exposure to certain pollutants over a lifetime. A cancer
risk assessment may include a determination of both individual risk and the cancer
burden of an entire community (number of potential cases, based upon individual
risk). Additional analyses may also be required to identify chronic noncancer health
risks that are attributed to long-term exposure to certain pollutants. Finally, analyses
may identify acute health risk that may be attributed to short-term exposure to
certain pollutants.