Often referred to as incineration or combustion-type systems, thermal oxidation systems are the most commonly used method to control the emission of organic compounds. Thermal oxidation systems can have very high destruction rate efficiencies (DRE) — up to 99.99 percent. They operate on the simple principle that at sufficiently high temperatures — typically between 1200 to 1600°F (649 to 871°C) — any hydrocarbon can be essentially oxidized into carbon dioxide (CO2) and water (H2O). Another unique property of these oxidization systems is the ability to destroy both small particulate matter as well as airborne volatile organic compounds (VOCs). These systems also can reduce NOX, particulate matter and acid gas when combined with other methods of VOC abatement such as scrubbers, baghouse filters and selective catalytic reduction (SCR) systems.
Evaluating the appropriate oxidation system for a specific process stream requires in-depth analysis of the specific process gas and operating parameters because there are many configurations from which to choose. From basic to advanced, here is a brief overview.
Direct-Fired Afterburners. Sometimes called an incinerator, a direct-fired afterburner is the most elementary system. It contains no heat recovery system. This type of system is suited for high VOC levels and fluctuating operating conditions.
Thermal Oxidizers. Thermal oxidizers are more sophisticated than direct-fired afterburners and typically are supplied with a variety of heat recovery systems and configurations. Thermal oxidizers often are used for larger process airflows.
The three most important components of a thermal oxidizer are the combustion chamber, burner and blower. The combustion chamber is the reaction zone of the unit. It provides a place for the process gas to mix with oxygen at elevated temperatures in order to complete the combustion process. The burner performs three functions. It ignites the fuel (typically, natural gas or propane), supplements the process gas flow and maintains the appropriate combustion temperature. The blower is used to draw air through the incinerator and maintain the system’s pressure levels.
The operation of a thermal oxidizer is based on the principle of combustion, and incineration-type oxidizers work by destroying organic compounds within the combustion chamber. The amount of time that pollutants stay inside the combustion chamber is called residence time. The residence time of a specific system is dictated by temperature and air velocity, which can range between 10 and 50 ft/sec within the chamber. In general, thermal oxidizers have a retention time between 0.5 and 2 sec.
Because thermal oxidizers are designed to operate within a specific temperature window, careful consideration of insulation is a key factor. Insulation helps reduce heat losses by preventing the heat from being conducted to the outer walls. The capacity of insulation to prevent heat loss varies depending upon the width of insulation, chosen material and density. The typical width of insulation is 7” with a density of 10 lb/ft3. Proper insulation also ensures that the equipment exterior surfaces operate under safe temperature limits.
While incineration-type destruction devices can be used over a fairly wide range of organic vapor concentrations, there are potential hazards with these systems. The primary concerns are overheating, flashbacks, fires or explosions. To mitigate these risks, flame-safety detection devices, flame arrestors and control valves are added in pollutant lines, and the system is always maintained under negative pressure. In addition, the concentrations of the organics in the air must be substantially below the lower flammable level to enhance system safety. Typically, this means operating at 25 percent or below the lower explosive limit (LEL) for oxidizers with heat recovery systems and up to 49 percent or below the LEL for direct-fired afterburners.
Another hazard this type of system poses is incomplete combustion of the pollutant gases, which can result in increasingly toxic byproducts. Engineers must follow good design practices to avoid these problems. In most incinerators, a portion of the waste stream may bypass the flame and be mixed at some point downstream of the burner. For complete combustion to occur, every particle of waste and fuel must come in contact with air (oxygen), and this is where air movement or turbulence is introduced to the system. A number of methods can be used to improve mixing of 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 efficiency. Controlling turbulence — the process of mixing flame and waste stream — to obtain a uniform temperature is the most difficult part in the design of an oxidizer.
Catalytic Oxidizers. Some process streams benefit from the addition of a catalyst to affect the temperature, pressure, rate of reaction, and required reactor volume. A catalytic oxidizer typically has lower operating requirements and can greatly reduce the combustion fuel requirements and associated operating cost. Despite these benefits, catalytic oxidizers remain less common due to the deterring fact that the initial and replacement cost of the actual catalyst material is often high. The catalyst needs to be replaced regularly because processes with halogenated compounds have a detrimental effect on precious metal catalyst. This also forces manufacturers to consider the restrictions for handling and disposing of the eroded catalyst because it can be toxic to both handlers and the environment.
Oxidizers with Waste Heat Recovery
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The price of the fuel necessary to run oxidizers can be exorbitant. In order to lower operating costs, a heat exchanger can be used to transfer heat — allowing industrial processors to utilize heat that would otherwise be wasted.
Heat exchange methods are a defining feature of a thermal oxidizer and are designed to achieve a predetermined amount of effectiveness (destruction rate efficiency). The design parameters must be calculated meticulously to ensure maximum process efficiency. Both primary and secondary heat exchange methods are used.
Primary Heat Exchange Methods. A primary heat exchange is the preheating of the process gas (air) as it enters the thermal oxidizer. The heat is transferred from the hot gas leaving the combustion process to the cold air entering it, thereby reducing the amount of energy (heating) needed to bring the process gas to an oxidizing temperature. The two design approaches for primary heat exchangers are recuperative and regenerative.
Also known as shell-and-tube heat exchangers, recuperative systems are the most commonly used systems for thermal oxidizers with a low-to-medium process flow rate. Recuperative thermal oxidizers can provide up to 80 percent thermal energy recovery efficiency. Typically, they are air-to-air exchange systems: The hot, clean air passes over a series of stainless steel tubes, transferring heat energy via conduction and convection to the incoming colder air passing through the tubes. Shell-and-tube heat exchangers used for recuperative thermal oxidizer systems are further divided on the basis of their functionality and configuration. They include parallel, counter-type and series-type heat exchangers.
By contrast, regenerative thermal oxidizers forego a shell-and-tube heat exchanger and instead use media to absorb and transfer heat. In addition, they are not steady-state. They rely on a cycling gas that flows between at least two fixed packed beds. During operation, at least one bed acts as an inlet while the other serves as an outlet bed. A common combustion/retention chamber connects the two. The hot air from the combustion chamber flows through one of the beds, heating it up, while the other discharges, alternating in a continuous cycle of retention and purging. Regenerative systems can handle a range of process flow rates and typically yield a thermal energy recovery efficiency between 80 and 95 percent.
Secondary Heat Exchange Methods. Finally, while primary heat exchangers are used only for treating the immediate process streams, secondary heat exchangers are utilized to provide process heat to be used in various capacities in manufacturing facilities. Secondary heat exchange systems use the waste heat from the combustion process to run other parallel production processes and equipment throughout the manufacturing facility such as ovens furnaces and general comfort heating.
Often, both primary and secondary heat exchangers can be incorporated into a unique system application. In these combination units, the gas leaving the oxidizer will generally heat a primary or preheat exchanger; then, it will continue on to the secondary heat exchanger. The remaining heat will be used in the secondary exchanger in a manner specified to the particular external device, and then will release the air to the atmosphere.