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COMBUSTION OF FLAMMABLE GASES

SMOKE SUPPRESSION IN FLARES

In conventional combustion processes, most fuels burn in the gaseous form. In order to burn, the combustible must be raised to a temperature at which it becomes intrinsically unstable and begins to dissociate into component radicals. The radicals formed by the heat-induced instability, or pyrolysis, form a "soup" of intermediate forms and compounds, the exact nature of the mixture being a function of the concentration, temperature and reaction times of the main constituents. At any given instant, the mixture is trying to achieve its most stable form. If the correct temperature conditions are met, and sufficient oxygen is present in the "soup", this results in the production of oxides which are the final products of combustion.

In a flare diffusion flame, raw gas emerges as a jet, from a nozzle or opening, into the normal atmosphere where it mixes with ambient air. The required mixture of air and gas to permit combustion can only exist at a fairly narrow interface or diffusion zone and the entire combustion process must take place in this zone, forming a flame envelope around the emerging raw gas stream. The thickness of the flame envelope is a function of the degree of mixing between the gas and the air and is significantly influenced by natural or induced turbulence and by the momentum and buoyancy forces generated by the gas and air flows and the heat release of the flame.

One of the early, intermediate, radicals formed in a hydrocarbon flame is free, atomic carbon. These carbon atoms are readily available for conversion to CO₂ but, if starved of oxygen in the core of the flame, will quickly combine into carbon spheres, cenospheres and chains which behave as suspended solids (soot) in the flame zone. When these soot particles eventually reach the oxygen envelope around the diffusion flame the residence time may be insufficient to permit total decay and conversion to CO₂, in which case the flame makes smoke.

Both the characteristic red/orange color of the flame and a high proportion of the flame’s radiant heat are caused by these transient solids glowing red hot due to the high flame temperature.

STEAM ADDITION

Steam addition into flares has been used for some time to reduce smoking by hydrocarbon flames and has a multiple effect in a flare flame in its qualities for smoke suppression.

Primarily, steam acts as a "heat-sink" and reduces the flame temperature to a point which adequately suppresses the pyrolysis reaction for long enough to reduce the amount of free carbon available to make smoke. This function may be fulfilled satisfactorily by any other inert medium supplying the same "heat-sink" effect.

Secondly, dissociation of steam at flame temperatures produces beneficial H-and -OH radicals which help to "sweep" the carbon into CH2- and -CHO radicals which act to hold the carbon in the gaseous phase and inhibit solid carbon production. This function will not be similarly fulfilled by other inert gases.

Additionally, the jet energy of the steam injection process may be harnessed and utilized to entrain and mix air into the flame and produce turbulence which speeds up the combustion and minimizes the time available for formation of carbon chains. This function may also be satisfactorily fulfilled by any other fluid injected in a like manner. The specific methods by which the jet energy is harnessed define the differences between almost all commercial designs of flare tip.

INERT GASES

As an acceptable substitute for steam, any other inert medium may be used. Nitrogen or Carbon Dioxide for example, will substitute for steam in the approximate ratio of 2 lb N2(or CO2) = 1 lb Steam (H2O)

Some Flare manufacturers have used Nitrogen and Carbon Dioxide injection alone, in flare tips with external mixing nozzles, for smoke suppression.

A number of differing techniques are used for steam, or inert gas introduction into the combustion zone. All of the following techniques may be applied independently or in some combination

In the simplest form, steam or inert gas addition directly into the gas steam can be beneficial for relatively light gases or low smokeless rates. Relief Stream inclusion limits the effect to "heat-sink" only, which is the least efficient solution. Utility consumption rates vary according to the compositions of both the flammable and non-flammable portions of the mixture. An approximate generalization is to assume that a requirement for the combined mixture is a mixture heating value of 650 Btu/scf (24250 kJ/m³) or less. This technique is preferred with inert gas inclusion but not with steam, which requires secondary monitoring with regard to system temperature and the possibility of sudden condensation, as well as raising corrosion concerns.

Simple steam addition usually uses a steam pipe injector in the center of the flare tip. This technique functions in the same manner as Relief Stream inclusion but avoids system-wide condensation concerns. Careful positioning and design of the steam nozzle can also generate additional flame velocity and turbulence to aid ambient air entrainment into the base of the flame.

Steam consumption rates will vary from 0.5 wts steam per wt of light hydrocarbon to as much as 3 wts steam per wt of heavy hydrocarbon.

A center nozzle may also be used to provide a low base flow of steam to function as a purge and resist internal burning in the flare tip.

Steam Injection into the base of the flame, from a ring of external nozzles, is the most common form of assist.

Using the external jets aids in air entrainment into the combustion zone and generates turbulent mixing which improves the efficiency of steam usage cutting typical steam rates to 0.35 wts steam per wt of light hydrocarbon to 1 wts steam per wt of heavy hydrocarbon.

A refinement of the external jet design utilizes venturis or venturi-like devices, fitted through the side of the flare tip, to aspirate some air into the center of the waste gas stream and promote a faster conversion of randomly formed carbon in the flame zone, thus further inhibiting the growth of soot particles in the flame.