Burning Issues #2: Smokeless Flaring with ASHCOR’s Steam-Assisted Flare System
By Jamal Ghazala
Introducing ASHCOR’s Steam-Assisted Flare Systems
The primary function of a flare is to safely dispose of toxic corrosive or flammable vapors. Toxic limits are the greatest concentration of a poisonous substance that can be tolerated in the air for a length of time without danger. Most emergencies causing over-pressure on safety relief valves can be controlled within 5 to 10 minutes by shutting down a pump or a compressor. A period of 10 to 30 minutes should be sufficient to control any emergency situation without a catastrophe.
Atmospheric discharge of hydrocarbons or other flammables should be designed to avoid the formation mixtures and exposure of personnel to toxic or corrosive vapors at grade or on elevated structures. Elevated flares can successfully accomplish this objective when properly designed. Designs should be based on the characteristics of the waste gas, heat radiation, noise level, smoke and atmospheric dispersion.
Elevated flares are used primarily to dispose safely of combustible gases during emergency or upset conditions, frequently or infrequently. However, the main focus is based on continuous low flaring conditions. The flared gases are injected into the atmosphere using a flare tip designed to provide a stable flame in high crosswinds with a very high turndown.
Our steam assisted flare system offers smokeless combustion achieved by using steam to entrain primary air and to provide turbulence and mix into the flame for proper combustion of the gas/air-steam mixture, resulting in a smokeless flame. Secondary air is drawn into the combustion zone through aspiration and thermal draft. Timing and thorough mixing are required for smokeless operation. Steam acts as a catalyst and also lowers the temperature of combustion by dilution and turbulence, prolonging the oxidation process and minimizing the decomposition of hydrogen and carbon. Decomposition is a function of the hydrocarbon type and increases with the C/H ratio. The higher the molecular weight of the hydrocarbon the greater the tendency to smoke. When steam injection is used at a rate of about 0.3 pounds of steam per pounds of gas, then a fraction of heat radiation is decreased. When the smoke has been eliminated, the flame is luminous.
The steam rates required to combat smoke for paraffin and olefin hydrocarbons are given by the following equations:
W(steam) = W(Paraffin) (0.49-10.8/Mw)
W(steam) = W(Olefin) (0.79-10.8/Mw)
An accurate figure of the steam/gas ratio can be calculated only if detailed waste gas composition is provided. With compressed air, 10% extra mass flow rate is required to produce the same effect of the steam. With high pressure gas, 100% extra mass flow rate is required to produce the same effect as the steam. As for water injection, one pound of water per one pound of flare gas that smokes.
With steam flares automatic control is the best way to minimize steam consumption where only the required amount is supplied to keep the flame smokeless. Excessive steam can create burn back as well as increase noise levels while wasting valuable energy. An optical unit is used to control the steam injection by monitoring the flame appearance. This optical unit is mounted at grade.
There are three types of flared gases:
- Low Heating Value Gases
- Medium heating value gases (typically 400 to 700 BTU/SCF)
- High heating value gases (> 700 BTU/SCF).
The method of air entrainment changes the smoke behavior of the flare combustion. A Flare without smoke suppression systems are known as non-smokeless. Smokeless flares use smoke suppression systems, such as steam injection, forced draft air fans, high pressure gas injection to reduce or eliminate smoke of certain fuels.
Steam injection is the most common technique with LP flare applications. When steam is not available, air assisted flares are used. When designing a flare, attention should be given to safety and high turndown. In addition, properly designed stack height should prevent excessive heat radiation and concentration of hazardous elements at grade level. The gases should be within the flammability limits to burn and the heating value must be high enough (>200 BTU/SCF for non-assisted flares, See EPA-CFR-40, section 60.18 for details) to generate flame stability and low emission.
The flame of a conventional flare is a diffusion type, that occurs on ignition of fuel jet. Smoke is generated during combustion of hydrocarbons only when the mixture is fuel rich. Smoke formation can be reduced by a reaction that will consume hydrogen. Steam has the power to separate hydrogen molecules, minimizing polymerization, forming oxygen compounds that burn at a reduced temperature. Also, steam can react with carbon forming carbon dioxide, carbon monoxide, and hydrogen, removing the carbon before it cools down and forms smoke.
ASHCOR is a world leader in the flare and combustion industry. ASHCOR has built its reputation on customer service and quality products. Our Unique Flare Burners, Enclosed Combustors, provide reliable ignition (from purge through sonic) and improved performance compared to conventional design systems. Call us for any flaring concerns, projects, existing or developing. Our experienced engineers will assist you with engineering calculations, design, emission and radiation studies, based on industrial codes and standards.
This article is meant as a guide and for general informational purposes only. For further information please contact: Jamal Ghazala