The Physics and Construction of Thermal Oxidizers and Afterburners

The three most important components of a thermal oxidizer are a combustion chamber, a burner, and a blower to draw air through the incinerator. Air and fuel are continuously delivered to the incinerator along with the contaminant-laden gas stream, where the fuel is combusted. The products of combustion and the unreacted feed stream are mixed and enter the reaction zone of the unit. The pollutants in the process air are then reacted at elevated temperatures. The unit requires operating temperatures in the 1200-1600oF range for combustion of most pollutants.

RESIDENCE TIME

The residence time of the system is dictated by the air velocity within the chamber, but generally retention times between 0.2 and 2.0 seconds are used. The average velocity of the gas can range between 10 fps and 50 fps. These high velocities are useful in preventing the particulates from settling down. The energy liberated by the reaction may be directly recovered in the process or indirectly recovered by using a heat ex-changer. 

CONSTRUCTION

The oxidation unit is constructed of a material that can withstand high temperatures. The walls of the equipment are insulated to avoid overheating of the outside walls of the unit. The typical width of insulation is 7 inches, consisting of different layers of ceramic board, blanket and mineral wool. Afterburners are designed to operate between airflow rates from 100 to 2000 SCFM capacity. The typical VOC Destruction Efficiency ranges from 98% to 99.99%. These units are provided with sophisticated flame detection devices to avoid the flame from entering into the contaminant lines. 

The principle of operation of thermal oxidizers is the same as that of afterburners. However, thermal oxidizers are used in the range from 2000 to 100,000 CFM. Consequently, the heat input to thermal oxidizers is also quite high. In order to reduce energy costs, most of this heat is recovered using a heat ex-changer.

The heat ex-changer does this by having the combusted, hot, clean air pass over a series of stainless steel tubes containing the cold process gas, which is laden with harmful VOC’s. When the hotter air passes over the tubing, it transfers heat energy to the incoming colder air. This transference of heat energy means that less energy is necessary to combust the process gas. Thus less fuel will be used.  

A typical layout of Thermal Oxidizer with Heat ex-changer is illustrated below.

A typical thermal oxidizer consists of an air-to-air heat ex-changer, combustion chamber, and an induced draft blower. A pre-heat recovery system transfers heat to the process gas, heating it to a temperature where less energy is required for combustion.

Factors Affecting Sizing

The size of an incinerator is affected primarily by two basic factors:

Exhaust flow incinerated

Consumption of auxiliary fuel is directly proportional to the flow rate of the exhausted process gas. The relationship between these can be expressed by:

BTUH = SCFM (1.1) (T₂-T₁)

Where BTUH = # Btu’s per Hour of natural gas required to heat the number of SCFM for T₁ to T₂

So, the lower the process exhausts flow, the less natural gas required.

Heat Recovery

The amount of heat recovery desired affects the sizing of the heat exchanger and thus affecting the entire unit. The more heat recovery desired, the larger the heat exchanger; and so the larger the entire unit.