Thermal Oxidation Provides One Answer to VOC Emission Problems

Incineration is one practical solution for dealing with volatile organic compound (VOC) emissions. Unless there are inorganic or halogen compounds present the operation is simple and efficient. The final products are carbon monoxide (CO) and water. Vapor rates from a few hundred cubic feet per minute (cfm) to as large as 300,000 cfm with lower explosive limit (LEL) levels ranging from a few percent to 50 percent can be oxidized safely and economically.

Thermal oxidizer design involves selecting general equipment characteristics, establishing design values for temperatures and gas volumes, and determining the fuel-firing rate and combustion chamber volume. Once the process has been fully defined, the physical make-up of the system can be determined.

As an example consider the design of one thermal oxidizing system. The simplest case is a fume incinerator without heat recovery. However, where possible the designer should consider incorporating heat recovery through the use of fume preheat or a waste heat boiler. The fume process design calculations require determination of the following information:

• Inlet gas flow rate (scfm)
• Inlet gas temperature (F)
• Solvent type and vapor concentration range (% or ppm)

The desired gas temperature within the incinerator must be specified. Frequently, air pollution regulations require the gas temperature to be above a certain minimum. This may vary from about 1250 F for easily oxidized solvents to 1600 F for more difficult vapors. Where carbon monoxide formation must be prevented, a minimum design temperature of 1450 F is recommended. The desired oxidized gas temperature should be slightly higher than the required minimum.

Residence times of 0.3 to 0.5 seconds are minimum values for systems burning hydrocarbon solvents without significant objectionable impurities. Many units in existence are operating satisfactorily at 0.3 seconds residence time, but only where extremely good mixing is achieved. For carbon monoxide removal, the higher residence time of 0.5 seconds generally should be used. These temperatures normally will provide around 95 percent destruction efficiency. Higher temperatures and/or longer residence times are required to achieve higher destruction efficiencies.

Before the design of the oxidizer is tackled, a burner type and suitable fuel should be selected. These elements should be compatible with the vapor oxygen content to the extent that this is known. If the contaminated vapor will provide the oxygen, the oxidizer size and heat requirements can be reduced since it will not be necessary to accommodate outside air in the system. Natural gas and propane-fired units use contaminated air almost exclusively. Oil fired burners may be set up to use contaminated air, but frequently use outside air to avoid plugging and reduce maintenance attention.

Fuel Requirements
Auxiliary fuel requirements depend upon vapor rate, composition, incoming waste gas temperature, and fuel. Any solvent vapor heating value in the waste vapor stream will further reduce fuel input.

Combustion Chamber Size
The size of the combustion chamber is determined by both the volumetric flow rate of the waste gas stream and the combustion products formed at the design temperature and the design retention time. Since the combustion chamber should be considered as only that zone in which the design combustion temperature is attained, oxidizers often include a mixing zone before the combustion zone. Chamber volume is determined by the formula:

• Volume (acfs) ÷ Residence Time + Mixing Volume

Once the total volume has been calculated the actual size may be determined by selecting dimensions with a minimum 3:1 length/diameter ratio. Mixing devices such as baffles and rings improve the turbulence in the chamber. With proper mixing the efficiency can be increased. Assuming the vapor is oxygen rich (minimum 16 percent 0₂) the vapor may be used as combustion air. Fuel requirement is calculated as follows:

• Vapor Mass x Air Specific Heat x (Oxidizer Temperature less Inlet Temperature)

To this add the fuel heat requirement, which amounts to approximately 10 percent additional net fuel.

Vapor heat content and any vapor preheat will reduce the quantity of auxiliary fuel required. The actual configuration of the system may be either horizontal or vertical, whichever meets the individual plant space requirements or limitations.

In summary, simple oxidizers can be designed by following the above procedure. Vapors containing inorganic constituents or low oxygen content require a more complex design to ensure the construction of cost effective equipment. It is wise to contact a consulting engineer or oxidizer manufacturer with experience to avoid or minimize design, construction or operating problems.

Edited by Ian Lisk