Contact Section - Stacks
March 19, 2010

Introduction Types of Stack Opacity Sample Points Stack Cleaning Stack Drains Materials of Construction         Barrel         Tip         Structure

Associated Links Stack Troubleshooting Erection Stack Spitting

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Introduction

Stacks are used to reduce the ground level concentration of a pollutant by emitting the process gas at great heights.  The ground level concentration depends on the pollutant emitted, the gas temperature and velocity of emission, the actual stack height, wind velocity, the terrain and the atmospheric conditions.  At low wind velocities, the rise of the gas due to its temperature is likely to carry the gas to great heights.  At high wind velocities, turbulence quickly disperses the gas.  Thus at some moderate wind speed the highest ground level concentrations are likely to occur.

Types of Stacks

To achieve the requirements of gas dispersion stacks come in a variety of types and materials of construction. 

The stacks can be either self-supporting or supported with the aid of steel frames or guy wires.  Self-supporting stacks are the simplest to deal with from the point of view of plant layout.  They will occupy the minimum plot area and can be placed virtually anywhere on the site.  Stacks designed to be supported by steel structures or guy wires are more difficult to locate because of the greater area required for the supporting structure and the interference presented by the guy wires.   However, foundation area for tall self-supporting stacks ca be large because of the turning moment on the foundation from wind loads.

Where the stack height is not excessive, the stack can be fitted on top of the absorber tower to take advantage of the already high elevation of the top of the absorber tower.

Opacity

Visible Stack Plume

There is a definite distinction between stack opacity and its appearance.  Opacity is considered by the EPA and others to be the 'degree to which emissions reduce the transmission of light and obscure the view of an object in the background'.  The stack appearance is dependent on the amount of light scattered by the plume from the sun and its surroundings toward the viewer's eye relative to the luminance of the background and the opacity of the plume.

The opacity of a plume is an intrinsic physical property of the particulate matter being emitted.  The opacity can be measured by transissometer, lidar, etc. or by trained observers. The EPA has developed Reference Method 9 for determining the opacity of the plume by trained observers.

Visual observation of the plume depends on a number of factors such as location of the observer relative to the sun, environmental lighting and background contrast.  Factors such as geographical location, time of day and other illumination variables do not significantly affect the ability of trained observers to accurately evaluate plume opacities.

There is often a requirement to design a plant to meet an effluent which complies with both a weight limitation and a stack opacity requirement.  The process weight limitation is easy to design for since it is dependent on the design of the absorption tower and mist eliminator.  Designing to meet the opacity requirement is more difficult because it relies more on experience to determine the relationship between stack opacity and particulate concentration.  A relationship between opacity and particulate concentration was developed by Ensor and Pilat but relies on actual data to accurately determine the value of a proportionality constant.  The working equation is as follows:

            W = - [K p ln(I/Io)] / L

      where

W       - particulate mass concentration at the view point (g/m³)
p        - true mean particle density (g/cm³)
I/Io      - ratio of light transmitted through the plume to that which would be transmitted if there were no obstructing particles
            (opacity = 1 - I/Io)
L        - path length through which the light is transmitted (m).  'L' is essentially equal to the plume width in viewing direction.
K        - proportionality constant (cm³/m²)

The threshold of a visible stack corresponds to an opacity of approximately 5%.  The corresponding mist concentration is approximately 80 ppm SO3 or 0.087 grains H2SO4/(actual) ft³.  These values should only be used as a guideline as actual values may differ.

Sample Points

The sample points for determination of plant emissions (SO2 and acid mist) are generally located on the stack.  The sample point must be located a minimum of 2 stack diameters from the top of the stack and a minimum of 8 stack diameters from the stack breach.  These criteria ensure that the gas flow past the sample points is uniform.  A minimum of two (2) sample nozzles are required and should be located at 90° to each other.  The nozzles should be at least 4" diameter (full bore) and blinded when not in use.

A means for the lifting of equipment up to the sample platform must be provided.  As well, an electrical power supply should also be provided at the sample platform for use during stack sampling.

Stack Cleaning

The most common problem encountered with stacks is the emission of sulphate deposits.  During a prolonged stoppage the final inspection and cleaning of the stack base should be left as late as practically possible before starting up.

Some plant operators have found that cleaning the stack can be achieved by introducing steam into the base during a plant stoppage.  The steam is left on for several hours and as it condenses it washes the sulphate off the walls. 

Some plants wash their stacks on a regular basis every 6 months.   Washing the stack will tend to shorten the life of the stack.

Stack Drains

Liquid will be present in the stack due to condensation of acid from the process gas or from precipitation.  The base of the stack must be equipped with a sloped bottom and a low point drain to remove the liquid.  Sulphate will also form in the stack which will be present as a sludge at the bottom of the stack.  An inadequately sized drain will plug resulting in the accumulation of liquid at the bottom of the stack.  Provision must be made to adequately drain the stack and unplug the drain line if required.

Materials of Construction 

Barrel

The barrel of the stack or the lining (ie. portion of stack in direct contact with the process gas) is generally carbon steel.  FRP stacks are not recommended because the material will be attacked by the strong acid that condenses from the process gas.

Tip

The top 5 to 6 m of the stack is generally 316L SS.  Sulphates do not form on stainless steel so the chance of stack spitting is minimized in the region of gas higher velocities.

Structure

The structural portion of the stack whether or not the stack is self-supporting or not can be made of carbon steel or concrete.