Methods for determining emissions

Four different ways typically are used to determine emissions:

Direct measurement

Concentrations and flow rate can be measured directly from point sources (i.e. stacks, vents, flues) and leak rates can be measured around non-point sources ("fugitive emissions" or leaks from equipment such as flanges, valves, pump seals). Although direct measurement is more costly than other methods, this technique normally provides the most accurate results. The expense may be worthwhile if regulatory/operational needs require accurate determinations and other techniques are believed to overestimate emissions.

Such measurements are performed normally on a one-time or periodic basis (e.g. annually), and samples are typically collected over a few hours while the process is operating. Instruments are used to obtain a sample for subsequent analyses in a laboratory, or sampling and analysis can be conducted simultaneously with a continuous emissions monitor (CEM). Care must be taken when extrapolating short term measurements to long term (i.e. annual) emission estimates. The process generally should be sampled at typical or normal operating conditions unless emission information about an unusual or upset condition is required.

Emission factors

Factors relate the amount of a substance emitted to process/equipment variables. Emission factors developed for a specific site and process may yield accurate results if properly done. Published emissions factors are often developed using averaged data for several facilities and thus may not be representative of your specific process. In some cases, published factors may be the only practical way to generate estimates.

Engineering calculation

Chemical/physical properties (vapor pressure, temperature, etc.) and principles (ideal gas law, Raoult's law, etc.) can be used to estimate emissions based on process data. Calculations can provide reasonable estimates when assumptions used for such equations are justified based on your particular operation.

Mass balance

This method can be useful for processes where there is no chemical reaction, or no combustion/destruction of chemicals by heating. Since most manufacturing processes using isocyanates involve chemical reaction (moles/mass of pure compound input / output), this technique may not be appropriate for such applications and is not covered in this section.