Dynamics of operation for flare systems
Detailed studies of problems with flaring performance led a refinery to define and implement a scheme for optimum flare performance
YAHYA AKTAÅž and Ã–ZGE Ã–ZARIK
TÃ¼praÅŸ Izmir refinery
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Flares are often used in petroleum refineries as control devices for regulated vent streams as well as to handle non-routine emissions. Flares are used to dispose of unwanted gases or liquids. Rarely, liquids are separated from the gas and burned in the liquid state or vaporised and burned as a gas. The unwanted material is generally composed of hydrocarbon gases.1
The flaring process can produce different pollutants — SO2, NOx, CO, VOCs, CH4 and CO2 — their constituents depending on two main factors: the waste gas composition sent to the flame and the combustion efficiency.
Besides waste gas composition and combustion efficiency, main factors affecting flaring availability and reliability are operating and maintenance practices. For operational safety in flaring, there should be:
• Well defined operating procedures and training
• Planned and routine maintenance of flare components.3
Smokeless flaring and combustionefficiency
The flare is usually required to be smokeless for the gas flows that are expected to arise from normal day to day operations. This is usually 15-20% of the maximum gas flow.
Various techniques are available for producing smokeless operation, most of which are based on the premise that smoke is the result of a fuel-rich condition and is eliminated by promoting uniform air distribution throughout the flames.4
One of the main challenges for flaring is initiating and maintaining ignition systems. Initiation of burning requires a pilot that can withstand strong winds, rain and inert surroundings.2
The primary function of a flare is to use combustion to convert flammable, toxic, or corrosive vapours to less objectionable compounds. Briefly, one theory suggests that steam separates the hydrocarbon molecules, thereby minimising polymerisation, and forms oxygen compounds that burn at a reduced rate and temperature and are not conducive to cracking and polymerisation. Another theory claims that water vapour reacts with the carbon particles to form carbon monoxide, carbon dioxide and hydrogen, thereby removing the carbon before it cools and forms smoke.5
Smokeless flares eliminate any noticeable smoke over a specified range of flows. Smokeless combustion is achieved by utilising air, steam, pressure, energy, or other means to create turbulence and entrain air within the flared gas stream.
Typically, smoking tendency is a function of the gas’s calorific value and of the bonding structure of the hydrocarbons. The paraffinic series of hydrocarbons has the lowest tendency to produce smoke, whereas olefinic, diolefinic and aromatic hydrocarbons have a much higher tendency to produce smoke.
Smokeless flares can be provided with a steam assist or air assist system to improve combustion. Gas system hydraulics (that is, the gas pressure drop available for the flare equipment) can influence the method chosen for smoke suppression. The pressure (kinetic energy) of the flare gas can, if sufficient, be used to make the flare operate without smoke. The smoke suppression method is dependent on the utility availability and cost of the utility.5
Steam assisted flares are single burner tips, elevated above ground level for safety reasons, which burn the vented gas in essentially a diffusion flame. They reportedly account for the majority of flares installed and are the predominant type of flare found in refineries and chemical plants.6
In TüpraÅŸ Izmir refinery, except for acid gas flares all the flares are steam assisted with a single burner tip.
The quantity of steam required for smokeless burning is a function of the gas composition, the flare burner’s size and design, the steam injector design, operating pressure and the environmental conditions. While steam assist enhances the combustion of relief gases that smoke, it adversely affects the combustion of relief gases with a high level of inerts. Relief gases with a high level of inerts, when flared from a steam assisted flare, can require a greater calorific value to sustain the required flame stability and the efficiency of hydrocarbon destruction. Steam is often injected into the relief gas discharge at the top of a flare burner. Typically, a steam ring that has a number of injection nozzles or slots is employed.
The upper steam injection functions to draw in air and to force the air mixture into the relief gas discharging from the flare burner. The steam injection pattern is intended to enhance fuel-air mixing and can add to the momentum of the relief gas discharge. The steam and air acts to dilute the hydrocarbon fuel content, which also reduces the smoking tendency. The steam vapour can also participate in the combustion kinetics, assisting in the conversion of carbon to carbon monoxide.
The effective addition of steam from an upper steam ring increases the turbulence of combustion. The overall noise level of the flare burner increases due to both the additional combustion noise and the jet noise from the steam nozzles. Steam assisted, smokeless flares can have significantly increased overall noise levels in comparison to flares with no steam assist.
In addition to the operating considerations mentioned earlier for the pipe flare, attention should be given to the rate of steam injection. If too little steam is added to the flare burner, a smoking, softer, more wind deflected flame is produced. Proper steam injection proportions the steam injection rate to the relief gas flow rate. The lowest cost operation for steam injection is to operate just above the incipient smoke point for the gas composition and flow rate.
Higher steam injection rates make the flame harder, cleaner and less wind deflected. Higher steam-injection rates also increase the noise levels. Excessive steam injection rates produce combustion instability accompanied by excessive flare noise. At the extreme, over-steaming can extinguish the flame.
Steam flows from an upper steam ring can condense and create water and blockage problems for a flare burner.
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