PRINCIPLE OF COMBUSTION
The primary function of the Thermal Oxidizer is to destroy the contaminants in the exhaust coming out of a process. The operation of the Thermal Oxidizer is based on the principle of combustion. The process of combustion is the most commonly used method to control emissions of organic compounds.
Combustion based systems are always simple systems capable of having very high destruction efficiency. These systems typically consist of burners, which ignite the fuel and pollutants, and a chamber, which provides the appropriate residence time for the combustion to take place. Combustion is a chemical process arising from the rapid combination of oxygen with various elements or chemical compounds resulting in release of heat. The process of combustion has also been referred to as oxidation or incineration.
It is required to achieve complete combustion of the fuel gas so that no further air pollutants are added. To achieve complete combustion once the contaminated air and fuel have been brought into contact, the following conditions must be provided: a temperature high enough to ignite the waste-fuel mixture, turbulent mixing of the air and waste-fuel mixture, and sufficient residence time for the reaction to occur. These three conditions are referred to as the 'three T's of combustion'. The rate at which a combustible product is oxidized is greatly affected by temperature. The higher the temperature, the faster the oxidation reaction will proceed.
The process of ignition depends on the following factors:
1. Concentration of combustibles in the waste stream.
2. Inlet temperature of the waste stream.
3. Rate of heat loss from the combustion chamber.
4. Residence time and flow pattern of the waste stream.
5. Combustion chamber geometry and materials of construction.
RETENTION CHAMBER DESIGN
Thermal destruction of most organic compounds occurs between 590°F and 650°F. However, most hazardous waste incinerators are operated at 1400°F. The time for which the pollutants stay in the incinerator is called residence time. The higher the residence time, the lower the temperature can be for the combustion chamber.
The residence time of gases in the combustion chamber is calculated by
t = V / Q
t = residence time, seconds
V = chamber volume, ft3
Q = gas volumetric flow rate at combustion ft3/s.
Adjustments to flow rates must be made for the extra combustion air added. For complete combustion to occur, every particle of waste and fuel must come in contact with air (oxygen). If this does not happen, unreacted waste and fuel will be exhausted from the stack. Second, not the entire fuel or waste stream is able to be in direct contact with the burner flame.
In most incinerators, a portion of the waste stream may bypass the flame and be mixed at some point downstream of the burner with the hot products of combustion. A number of methods are used to improve mixing the air and waste streams, including the use of refractory baffles, swirl-fired burners, and baffle plates. Unless properly designed, many of these mixing devices may create 'dead spots' and reduce operating temperatures.
The process of mixing flame and waste stream to obtain a uniform temperature for the decomposition of wastes is the most difficult part in the design of an incinerator. A Thermal Oxidizer must be designed very carefully and with proven methods to achieve maximum mixing of airflows and to avoid dead spots.
THERMAL OXIDIZER OPERATION
A Thermal Oxidizer consists of a combustion chamber, a burner, and a blower to draw air through the complete oxidizer. Along with the contaminant-laden gas stream, air and fuel are continuously delivered to the combustion chamber where the fuel is combusted.
The products of combustion and the unreacted feed stream enter the reaction zone of the unit. The pollutants in the process air are then reacted at elevated temperature. The average gas velocity can range from 10 fps to 50 fps. These high velocities are useful in preventing the particulates from settling down. The energy liberated by the reaction may be directly recovered from process or indirectly recovered by using a heat exchanger.
The Thermal Oxidizer should be constructed of material which can withstand high temperatures and the walls of the equipment are insulated to avoid overheating of the outside walls of the unit. These units are usually provided with sophisticated flame detection devices. The layer of insulation exposed in the Combustion Chamber is typically ceramic block that is 7” thick and a density of 10 lbs./ft3.