The flue gas of steam generators and boilers is characterised by values of temperature and vapour water content higher than those of the surrounding ambient air. The same is true for stacks with scrubbers and for biomass sources. The flue gas emitted by a stack moves vertically due to its initial velocity and to the thermal buoyancy, and horizontally due to the ambient wind speed.
During its motion the plume dilutes due to the entrainment of ambient air caused by wind speed and atmospheric turbulence, and eventually it reaches the values of the ambient air. The dilution process interests all the scalar properties of the plume which means, for example, the concentration of pollutants, the plume temperature and the plume moisture.
Under certain meteorological conditions plumes with high moisture levels can be visible due to the fact that the entrained ambient air decreases the plume temperature causing the condensation of water vapour. If the visible plume touches the ground, it generates plume induced fogging if the ambient temperature is positive, and plume induced icing if ambient temperature is below 0 °C.
The formation of visible plumes should be avoided if possible, or at least efforts must be done to reduce their sizes. In fact visible plumes have a series of negative effects:
The assessment of visibility of moist plumes emitted by stacks requires specific simulation models for the following reasons:
Even if this post is focused on moist plumes emitted by stacks, it is worth to say that the emissions from cooling towers must be simulated with proper models, such as SACTI, which are also capable to consider the drift water. Drift water is cooling water which is captured by the air flowing within the tower and is then emitted as fine droplets with dimensions ranging from few mm to thousands of mm. Drift water is a serious environmental problem because it may contain chemical elements present within the cooling water. The plume of a cooling tower therefore contains water with two different natures: drift water which has a mechanical nature, and saturated water which has a thermodynamic nature.
It is clear that, in order to simulate the moist plume rise from stacks, it is important to choose a suitable model. Few of such models exist, and one of them is WetPlume developed by Enviroware. WetPlume has been described in a peer reviewed paper which also contains some validation tests.
Using WetPlume, we carry out stack plumes visibility studies determining:
The above variables can be evaluated varying the stack emission parameters (water content, emission temperature, emission velocity, etc.) in order to find out the best conditions to decrease the number of visible plumes and their length.
WetPlume also allows to study in detail the visible plume shape and size at specific hours as in the following figures.
Effect of ambient relative humidity on the visible plume fixed other environmental variables and the release parameters
Effect of the flue gas exit temperature on the visible plume fixed other environmental variables and the release parameters
Effect of the flue gas specific humidity on the visible plume fixed other environmental variables and the release parameters
Contact us for additional information about WetPlume or to apply it in visible plume assessment studies.