Emissions that are not released through a stack, vent, duct pipes or other confined air stream are termed fugitive sources. These emissions include equipment leaks and area emissions. It is advisable to calculate the fugitive emissions with the help of data available from direct measurement. Industries are free to use any method to estimate the fugitive emission. The emission factors given by the Synthetic Organic Chemical Manufacturing Industry (SOCMI) are widely used to perform the calculations.
The EPA began to develop a series of National Emission Standards for Hazardous Air Pollutants (NESHAPs) that established emission standards for industries. NESHAP includes the leak detection and repair (LDAR) program to detect, estimate and reduce fugitive emissions. In 1986, Congress passed the Superfund Amendments and Reauthorization Act (SARA), which requires industries to quantify the levels of certain chemicals to waste as well as the loss of those chemicals to fugitive emissions. The Clean Air Act Amendment (CAAA) of 1990 requires plants to measure every six months.
General sources of fugitive emissions include process equipment leaks, valves, flanges, seals, sample connections, open-ended line, pressure relief devices, screwed fittings, loading and uploading losses, evaporation from spills and retention ponds, uncaptured process emissions and waste treatment facilities. For uncaptured process emissions, the data on air pollutant concentration can provide a basis for determining fugitive emissions. Actual measurements taken to ensure compliance with standards should be used. In case of leaks from pipes, flanges, valves, etc., emission factors are used to determine the releases.
Several methods have been developed to estimate the fugitive emissions. Widely used methods include:
- Use of “average” SOCMI emission factors.
- Use of leak/no-leak emission factors.
- Use of stratified emission factors.
- Use of screening concentration and correlation equations by EPA.
- Bagging.
- New process-specific equations.
“Average” SOCMI emission factors is one of the simplest methods, and the procedure generally overestimates the emissions. The SOCMI factors are shown in Figure 1.
Leaking/non-leaking emission factors offers a more accurate method for measuring fugitive emissions. Here the components are screened using the EPA Method 21 for VOC leaks. This method sorts out the major contributors of fugitive sources.
Stratified emission factors is similar to the leak/non-leak method. The components are tested for VOC leaks using the EPA Method 21. The components are put into three groups and thus offer a slightly more accurate method of calculating fugitive releases than the leak/non-leak method.
For EPA equations, the EPA conducted field tests to create curves that relate the Method 21 screenings to a mass emission rate. The equations are then derived from this data. The components are screened, and the results are applied to the correlation curves to obtain mass emission rate.
In bagging, the component is enclosed in a non-leaking material. Two openings are provided, one to let in fresh air and the other to test the exhaust air.
The Method 21 was developed by EPA as a standard to measure VOC components for leaks.
With area sources, if there are a number of small sources in a particular process, then the whole process can be taken as an area and the fugitive emission can be estimated accordingly. Software programs are available from EPA to measure the emissions from area sources. They are CHEMDAT7 and SIMS.
For more information on SPED, visit www.spedweb.com or call (832) 286-3404.
References
- Hazardous Waste Treatment, Storage and Disposal Facilities — Air Emission Models. (U.S. EPA)
- Chemical Manufacturers Association — Fugitive Emission Database.
