6.2 Emission Estimation & Continuous Monitoring

6.2 Emission Estimation & Continuous Monitoring

Environmental engineers are frequently tasked with quantifying air pollutant emissions from various industrial processes. These emission estimates are critical for regulatory compliance, obtaining air permits, designing pollution control equipment, and conducting environmental impact assessments. The U.S. EPA and other regulatory bodies recognize several methodologies for estimating emissions, ranging from broad, generic factors to highly specific, direct measurements. The choice of methodology depends on the available data, the required accuracy, the regulatory context, and the cost of the assessment. The primary methods include emission factors, material balance calculations, engineering estimates, and direct stack sampling/continuous monitoring.

EPA AP-42 Emission Factors

The most widely used approach for estimating emissions is the application of emission factors, primarily found in the EPA's AP-42, Compilation of Air Pollutant Emission Factors. An emission factor is a representative value that attempts to relate the quantity of a pollutant released to the atmosphere with an activity associated with the release of that pollutant. These factors are usually expressed as the weight of pollutant divided by a unit weight, volume, distance, or duration of the activity emitting the pollutant (e.g., pounds of particulate matter emitted per ton of coal combusted).

The general equation for estimating emissions using an emission factor is: $E = A \times EF \times \left(1 - \frac{ER}{100}\right)$ Where:

• $E$ = Emissions (e.g., lb/hr or tons/yr)
• $A$ = Activity rate (e.g., tons of material processed per hour)
• $EF$ = Emission Factor (e.g., lb of pollutant per ton of material)
• $ER$ = Overall emission reduction efficiency, % (if a control device is used)

Example Calculation: A coal-fired boiler burns 500 tons of coal per day. The AP-42 emission factor for sulfur dioxide ($SO_2$) for this specific boiler type and coal grade is 30 lb/ton. The boiler is equipped with a flue gas desulfurization (FGD) scrubber with a guaranteed control efficiency of 95%. The daily $SO_2$ emissions are: $E = 500 \text{ ton/day} \times 30 \text{ lb/ton} \times (1 - 95/100) = 15,000 \text{ lb/day (uncontrolled)} \times 0.05 = 750 \text{ lb/day (controlled)}$

While emission factors are convenient, they represent population averages and may not accurately reflect the emissions of a specific, individual source.

Material Balance Emission Calculations

A material balance (or mass balance) approach involves quantifying the flow of a material into and out of a process. This method is based on the law of conservation of mass: what enters a system must either accumulate, be transformed, or leave the system. In environmental engineering, material balances are often used to estimate emissions of volatile organic compounds (VOCs) from surface coating or solvent evaporation operations where it is assumed that all the solvent used in the process evaporates into the air.

The general material balance equation is: $\text{Input} = \text{Output} + \text{Accumulation} + \text{Destruction}$ If a process uses 10,000 gallons of a coating containing 4 pounds of VOC per gallon, and there is no control device or accumulation, the VOC emissions are simply $10,000 \times 4 = 40,000$ pounds. Material balances are highly accurate for these types of processes but are less useful for processes where chemical reactions generate pollutants (like combustion generating $NO_x$).

Stack Sampling (EPA Methods 1-5)

Direct measurement of emissions through stack sampling is the most accurate method for quantifying air pollutants. The EPA has developed specific test methods that must be meticulously followed to ensure accurate and legally defensible results. These are often required for initial compliance demonstrations and periodic testing.

• Method 1 (Sample and Velocity Traverses): Determines the optimal location for sampling ports and the number of traverse points in a stack to obtain a representative sample of the gas stream, avoiding areas of turbulent flow.
• Method 2 (Velocity and Volumetric Flow Rate): Uses a Type S Pitot tube and a draft gauge to measure the velocity head and temperature of the gas stream, calculating the volumetric flow rate.
• Method 3 (Gas Analysis for $CO_2$, $O_2$, Excess Air, and Dry Molecular Weight): Extracts a gas sample to determine the concentration of oxygen and carbon dioxide, which is necessary for molecular weight and emission rate calculations.
• Method 4 (Moisture Content): Determines the moisture content of the stack gas by extracting a sample at a constant rate and condensing the moisture in impingers.
• Method 5 (Particulate Matter): The benchmark method for determining particulate matter (PM) emissions. It involves extracting a sample isokinetically (where the velocity of the gas entering the sampling nozzle equals the velocity of the undisturbed gas stream) and collecting the PM on a glass fiber filter.

Continuous Emission Monitoring Systems (CEMS)

For large emission sources, such as power plants and major industrial boilers, regulations often require the installation of Continuous Emission Monitoring Systems (CEMS). A CEMS is a comprehensive package of equipment that continuously analyzes the gas stream for pollutants like $NO_x$, $SO_2$, $CO$, and opacity.

To ensure accuracy, CEMS must undergo rigorous calibration and quality assurance procedures. The most critical of these is the Relative Accuracy Test Audit (RATA). A RATA involves comparing the CEMS readings to a reference method (like the EPA Methods mentioned above) conducted simultaneously by an independent testing firm. The CEMS must meet specific relative accuracy criteria (typically within 10% or 20% of the reference method) to pass the audit.

Gas Concentration Corrections

Emissions regulations often specify concentration limits that are corrected to a standard dilution level, typically a specific percentage of Oxygen ($O_2$) or Carbon Dioxide ($CO_2$). This prevents facilities from diluting their exhaust gas with excess air to artificially lower the pollutant concentration and pass compliance tests.

The formula to correct a measured pollutant concentration ($C_{meas}$) to a reference oxygen level ($O_{2,ref}$) is: $C_{corr} = C_{meas} \times \frac{21 - O_{2,ref}}{21 - O_{2,meas}}$ Where:

• $C_{corr}$ is the corrected concentration.
• $C_{meas}$ is the measured pollutant concentration.
• $O_{2,ref}$ is the reference oxygen percentage (e.g., 7% for incinerators, 15% for gas turbines).
• $O_{2,meas}$ is the measured oxygen percentage in the stack gas.
• 21 represents the approximate percentage of oxygen in ambient air.

Example Calculation: A stack test measures a $CO$ concentration of 150 ppm and an $O_2$ concentration of 10%. The regulatory standard is corrected to 7% $O_2$. $C_{corr} = 150 \times \frac{21 - 7}{21 - 10} = 150 \times \frac{14}{11} = 150 \times 1.273 = 190.9 \text{ ppm}$ By correcting to a standard oxygen level, regulators ensure a level playing field and true evaluation of the emission control technology's performance.