INTRODUCTION

Emissions that are not released through a stack, vent, duct pipes or other confined air stream are termed as fugitive sources. These emissions include equipment leaks and area emissions. It is very difficult and expensive to estimate such 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.

Although fugitive emissions were known to the regulatory agencies no estimate or standard was developed until early 80`s. Regulations during 80`s required facilities to estimate and control fugitive emission estimates. The EPA began to develop a series of National Emission Standards for Hazardous Air Pollutants (NESHAPs) which 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 plant are required to estimate fugitive emissions every six months to determine the facility's compliance with operating permits according to the Clean Air Act Amendment (CAAA) of 1990.
 
 

SOURCES  OF  FUGITIVE  EMISSIONS

The following are the general sources of fugitive emissions.
 

• Valves
• Flanges
• Seals
• Sample connections
• Open-ended line
• Pressure relief devices
• Screwed fittings

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.

CALCULATION OF FUGITIVE EMISSIONS

Several methods have been developed to estimate the fugitive emissions. Widely used methods are given below:

• 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 USEPA
• Bagging
• New Process Specific Equations

 

This is one of the simplest methods used for calculating fugitive releases. This method was developed by the SOCMI based on field studies. Here the fugitive source is identified and multiplied by the emission factor to obtain the fugitive release for the time of operation. This method is one of the simplest ones and the procedure generally overestimates the emissions. The SOCMI factors are shown below:

This method 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. Components whose reading show above 10,000 ppmv are multiplied by a leaking emission factor and which are less are multiplied by a non-leaking emission factor. The equation for estimating fugitive emission is given by:

Total Emission = [(Number of Components)₁ * (Emission Factor)₁] + [(Number of Components)₂* (Emission Factor)₂]

Stratified Emission Factors

This method is similar to the leak/non leak method. The components are tested for VOC leaks using the EPA Mehod 21. The components are put into three groups and thus offers a slightly more accurate method of calculating fugitive releases than the leak/non-leak method.

The screening value ranges are

• 0 - 1000 ppmv
• 1001 - 10000 ppmv
• Over 10000 ppmv

U.S. EPA Equations

The EPA conducted field tests to create curves which 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.

The component / service categories are:

• Valves in Light Liquid
• Valves in Gas / Vapor Service
• Pumps in light liquid Service

• Bachrach TLV Sniffer
• Century System OVA Instrument

OVA = Instrument reading * Response Factor

R.F. = ( Actual Concentration ) / (Observed Concentration from Detector )

Bagging

In this method 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 exhaust air is passed through an analyzer to measure VOC concentration which helps to obtain the mass emission rate. The flow rate of air is kept constant during bagging. The sampling of gas is done by using

• Vacuum Method or
• Blow Through Technique.

Method 21:

The Method 21 was developed by the EPA as a standard to measure VOC components for leaks. Different components have to be screened differently in slightly different ways to get the best results. Some of the interference's while screening are

• wind
• temperature fluctuations
• distance of component from screening

• open ended lines
• inaccurate components counts
• data entry errors
• missing plugs

• when the wind is calm and is less than 3 mi/hr
• as close as possible from the component
• on a warm day so that the material has a lower vapor pressure
• using different instruments to read different materials

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. Two software programs are available from EPA to measure the emissions from area sources. They are CHEMDAT7 and SIMS. Apart from these software there are other programs available too in the market. There are several methods to estimate emission data from area sources but they cannot be applied to all the sources or under all conditions.

   REFERENCES
 

• Hazardous Waste Treatment, Storage and Disposal Facilities - Air Emission Models. ( U.S. EPA)
• Chemical Manufacturers Association - Fugitive Emission Database.