Original Date: 02/23/1998
Revision Date: 01/18/2007
Best Practice : Emission Factor Refinement
Accurate emission factors are critical to determine the exact amount of total emissions released into the environment and the expected level of exposure. Instead of relying on generally accepted emission factors from various published sources, Northrop Grumman developed a source testing process which refines the emission factors. The process significantly reduces the estimated toxic exposure risk, and minimizes the requirements to notify surrounding communities of Proposition 65 hazards.
In 1986, the State of California passed Proposition 65 which required industrial manufacturers that generate toxic air emissions to notify the public in the immediate area that there may be a risk of exposure to known carcinogenic substances. Northrop Grumman’s primary toxic emission is hexavalent chromium (HexCr), generated from the production spray booths and touch-up spray operations on its manufacturing and assembly lines. Accurate emission factors are critical to determine the exact amount of total emissions released into the environment and the expected level of exposure. Instead of relying on generally accepted emission factors from various published sources, Northrop Grumman developed a source testing process which refined the emission factors. The process significantly reduced the estimated emission rate and toxic exposure risk, thereby eliminating the need to notify the general public of a toxic substance risk.
Emission factors are mathematical equations used to estimate the total amount of airborne substance emissions from a given substance and process. In the case of spray booth operations, the equation for estimated total emissions would be:
EMISSIONS = (POUNDS PROCESSED) * (1-TE) * (1-DO) * (1-CE), where TE is the transfer efficiency; DO is the dropout percentage; and CE is the control efficiency.
Typical emission factor sources are: supplier equipment specifications; Air Pollutant 40 and 42 Published Data; state and local published and/or customary and accepted values; or the California Air Resources Board Published Toxics Data. In the case of agency-approved spray booth factors using typical emission factor sources, the calculated values are:
TE for the high volume, low pressure (HVLP) spray guns is 0.65; DO of material not airborne is 0.22;
CE using dry filters is 0.97;
CE for water wash spray booths is 0.90; and
CE for High Efficiency Particulate Arrester (HEPA) filters is 0.9997.
For the West Complex spray booths (Buildings 915 and 923) and the touch-up spray operations (Building 902), these calculations translate to a combined 0.351 pounds of HexCr emissions. However, Northrop Grumman’s source testing process (which uses actual mass balance) calculates the CEs more accurately for inclusion in the emission factor equation, and reduced the combined amount of HexCr emissions by 92%. These values are then used to determine the Maximum Individual Cancer Risk (MICR) from the emissions exposure. By using source testing values, the MICR is reduced from 189 per million with typical emission factors to nine per million (a MICR of ten per million is considered significant).
Northrop Grumman’s method of calculating emission factors greatly reduces the estimated toxic exposure risk; ensures compliance with state and local air quality regulations; controls costs; and increases operational flexibility. As a result, the company enhances its image and relationship with local neighbors.
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