Burning Issues #1: Air Assisted Smokeless Flaring
By Jamal Ghazala
The air-assisted flare is designed for operations where low pressure gases are required to have a smokeless burn and where no process steam is available.
ASHCOR’s Air-Assisted Smokeless Flares supply primary air for combustion via a low-pressure fan mounted at the base of the stack. The air required for smokeless flaring is delivered as a central core within the gas flame, and is designed so good mixing of the air and gas provides a stable smokeless flame. If required, the air fan can be fitted with a dual speed (or multi-speed) motor so air-flow may be altered to suit the gas flow. Below is the break-down of the process:
Air-assisted smokeless combustion is achieved by supplying primary air with a low-pressure fan-provided turbulence to mix the gas and air, and also ensuring entrainment of atmospheric air. A sufficient amount of oxygen is required to combat smoke resulting from the combustion of heavy saturates and unsaturated hydrocarbons. Primary air is mixed with the waste gas prior to combustion and secondary air is drawn into the flame above the flare tip.
Good mixing, timing and temperature are required prior to combustion; otherwise smoke will occur due to incomplete combustion. Lower combustion temperature will help in prolonging the process of oxidation and minimizing the hydrogen-carbon decomposition.
Unsaturated hydrocarbons dissociate more easily requiring more primary air – approximately 30% plus of the stoichiometric air quantity – while saturated hydrocarbons require approximately 20% of the stoichiometric air quantity.
The amount of air required for stoichiometric combustion is based on the air/gas weight ratio. Approximately, each 96 BTU net heat release of waste gas require 1 SCF of air. Therefore, the approximate stoichiometric air ratio of several components is computed as follows:
SAR = Q/96
Where SAR = Air (SCFH)
Q = Net Heat Release (BTU/hr)
The exit velocity of the air should be between 100 and 150-ft/sec (125 ft/sec for optimum performance). In addition, the length of the flame is shorter than the same “non-smokeless” flame depending on the design of the mixing head. Smokeless flames are more vertical than non-smokeless flames and are less influenced by winds.
In most cases, air assisted flares are designed to burn the most common flaring scenarios without any smoke. The emergency flow rates are not required to be smokeless since they seldom occur and can only last for a very short duration, typically, less than few minutes. Therefore, the radiation heat intensity is calculated based on the maximum design flow rates with the air blower being off.
The approximate flame length is calculated based on the following formula:
Lf = Q0.46/2283 (ft)
The following design guidelines should be followed in the process of designing air-assisted flares:
- Calculate the SAR value based on the flare gas composition
- Determine the smokeless capacity for the flare system This is usually specified by the end user. If not available, the continuous flow rate should be used as the smokeless capacity. Air flares with smokeless capacities higher than 50 MMSCFD, are commonly staged into two flares, as needed. Large air-assisted flares (54-inch and larger) tend to lose their effectiveness, particularly with heavier hydrocarbon gases.
- Determine the amount of air required for smokeless operation
- Calculate the diameter of the gas pipe and air duct required to provide a balanced exit velocities and timely premixing of air and gas.
- Calculate the overall height of the riser based on the worst flaring scenario and under loss of power, the air blower would be off.
- The mixing head design is a vendor’s proprietary information and shall be designed to accommodate the timing and mixing mechanism between the air and the gas.