The effluent pollutants emitted to the atmosphere from a source may contain many different pollutant materials. Effluent measurements and sampling procedures follow specific test methods and protocols to ensure representative and accurate emission data. Source emission testing encompasses a broad range of data collection; effluent measurements are vital data points determined during a test run.
Typical effluent measurements assessed during each test run include:
- Effluent temperature
- Absolute and gauge pressure of stack gas
- Effluent velocity
- Effluent moisture content (volume basis)
- Oxygen concentration
- Carbon dioxide concentration
- Molecular weight of stack gas
- Molecular weight of dry stack gas
- Volumetric flow rate (actual, standard conditions)
Temperature and velocity pressure measurements are made at designated locations across the diameter or depth of an enclosed duct. These locations refer to the centroid of equal areas along the sampling plane of the stack.
Velocity pressure and temperature are proportional to stack gas velocity. Temperature is typically measured with a thermocouple and velocity pressure is measured using a pitot tube connected to a pressure sensing device. The pitot tube is a simple apparatus used to measure the velocity of a fluid flowing in an open channel. It should, however, be considered and treated as a sophisticated instrument.
The effluent moisture concentration in the stack gas is determined by the condensation method which is referenced as U.S. EPA Method 4 (or Method D of Environment Canada’s Reference Method 8). This method involves pulling a known volume of stack gas through a series of cold bubblers and a drying agent. The water vapour in the sample gas stream condenses in the bubblers and the dry gas remaining is measured with a calibrated dry gas meter. The amount of condensed water recovered in the condensing system is also measured. The quantity of water vapour that was in the sample stream can be determined from the amount of aqueous water collected in the condensing system. The volume of water vapour divided by total sample volume (water vapour plus dry gas) is the water vapour fraction of the stack gas.
The molecular weight of the effluent stack gas on a dry basis is the sum of the mole fractions of the major constituents of the gas. Major constituents are oxygen, carbon dioxide, and nitrogen. Oxygen and carbon dioxide are measured and nitrogen is the remainder. In ambient air, the dry molecular weight is approximately 28.9 g/mole, or 21% oxygen and 79% nitrogen. The molecular weight of oxygen as O2 is 32 g/mole and the molecular weight of nitrogen gas as N2 is 28 g/mole. Obviously the molecular weight of the mixture will be somewhere between 28 and 32; but closer to 28 since over three quarters of the mixture consists of nitrogen. In some cases carbon monoxide and sulphur dioxide are major constituents and should be considered in the determination of molecular weight.
The molecular weight on a wet basis takes into account the mole fraction of measured water vapour in the gas. For example, if the effluent moisture concentration is determined as 10%, then the fraction of dry gas in the effluent is 90% (100% -10%). In this case, 90% of the stack gas has a molecular weight equal to the measured dry molecular weight and 10% has a molecular weight of water, which is 18 g/mole.
Therefore the molecular weight of the “wet” stack gas is calculated as follows: (90% x molecular weight of dry gas) + (10% x 18). The molecular weight on a wet basis is always less than that on a dry basis because the molecular weight of water is less than oxygen or nitrogen.
The effluent volumetric flow rate on an “Actual Basis” is the product of the stack cross-sectional area at the sampling plane and the average stack gas velocity. Flow rate on a reference or standard basis uses the universal gas laws to convert the volume rate at actual “measured” conditions to reference conditions. The volumetric flow rate at dry reference conditions refers to the flow rate of the dry gas only.
For example, if a source has a reference volumetric flow rate of 10 m³/s and a moisture content of 15%, then the dry reference flow rate of the source is 8.5 R.m³/s. Conversely, the volumetric flow rate of water vapour in this example is 1.5 R.m³/s.
For more detailed information on effluent measurements, please contact:
Source Testing (44 KB)