Oxidizer Manufacturers

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About Oxidizers

Oxidizers are units that are used to process gas emissions from industrial plants. They break down the hot, hazardous gases by destroying the pollutants within, before discharging them into the atmosphere. Essentially, there are two types of oxidizers used in industries, the thermal oxidizer and the catalytic oxidizer. Thermal oxidizers work on the principle of combustion and are used for the efficient destruction of hydrocarbon-based waste materials by burning the exhaust gases before they are released into the air. Catalytic oxidizers, on the other hand, utilize a catalyst to increase the rate of reaction; oxidization occurs at lower temperatures as compared to thermal oxidization.

Oxidizers are prevalent in almost all industries, especially since most countries and states have instituted regulations to control the release of noxious gases into the atmosphere. Some applications where the use of oxidizers is significant include petroleum refining, paper printing, and others with noxious outputs. Noxious gases and pollutants that oxidizers are commonly used to remove are:

• Volatile organic compounds (VOC)
• Nitrogen Oxides (Nox)
• Carbon Monoxide (CO)
• Hazardous air pollutants (HAP)
• Organic particulate matter (PM)

The pollutant removal capability of an oxidizer, be it thermal or catalytic, is widely known as its destruction removal efficiency, which normally ranges from 90% to 99%. It is to be noted that the destruction removal efficiency of an oxidizer is different from its thermal efficiency. The former is the amount of volatile organic compounds (VOCs), in milligrams per cubic meter, destroyed by the oxidizer, while the latter is the amount of heat recovered from the exiting clean air. High destruction removal efficiency indicates that fewer pollutants are being discharged into the atmosphere by that particular oxidizer.

Thermal Oxidizers

Thermal oxidizers work on the principle of combustion, and in most cases, consist of an oxidization chamber, a blower for drawing air through the oxidizer, and a burner. They supply a stream of hazardous waste gases into the combustion chamber along with air and fuel, in which they are burnt at very high temperatures to remove the hazardous elements in the gases. The flow velocity of the gases is kept sufficiently fast to prevent the particulates present in the gases from settling down.

Thermal oxidizers are built from materials that can withstand quite high temperatures, and are normally insulated. Though the design of most thermal oxidizers is determined by various factors, such as flow rate of discharge, waste composition, as well as economic considerations, all thermal oxidizers have some components that are necessary for their safe and efficient operation. One such component that is very important to the operation of thermal oxidizers is the integrated heat recovery system, which recovers and reuses the heat that is generated during the combustion process. In fact, the most significant difference between the types of thermal oxidizers is the type of heat recovery system that is incorporated into them. Based on the type of heat recovery system that thermal oxidizers have, they can be categorized into either recuperative or regenerative thermal oxidizers.

Besides the heat recovery systems, various other components are also incorporated in thermal oxidizers to facilitate their operation. The control system is such a component, which is integrated into the thermal oxidizers to automatically check and maintain all process parameters for efficient operation. In addition, emission control components, such as filters or scrubbers, are integrated as well, to treat the flue gas, created by the oxidation process, prior to its discharge.

Recuperative Thermal Oxidizers

The recuperative thermal oxidizer consists of a primary and in some cases, a secondary heat exchanger within the system. These heat exchangers are used to recover heat from the clean gases being discharged and heat the incoming stream of exhaust gases. Consequently, in some cases when the clean gases enter the secondary heat exchanger, it is used as an alternate source of thermal energy for other processes in the plant. These are conventional heat exchangers made from metal, commonly the shell and tube type.

The primary heat exchanger is used to preheat the incoming exhaust gases and is fitted immediately after the oxidization chamber and the thermal energy for preheating is recuperated from the exiting clean air. As the exhaust gases enter the heat exchanger and flow on one side of the tube or plate, the hot clean air from the oxidation chamber flows on the other side of the plate or tube. Thermal energy is transferred to the exhaust gases by conduction with the metal acting as the medium for heat transfer.

In some cases, a secondary heat exchanger is integrated into the system and usually positioned in series after the primary heat exchanger. Heat transfer in these secondary heat exchangers, as in the primary, occurs between two gases, but the fluid being heated by the exiting clean air stream is utilized in another part of the plant, and in most cases, as part of the process.

Thermal oxidizers utilizing this type of a heat recovery system normally have thermal efficiencies ranging from 50% to 75%. While this thermal efficiency is sufficient to offer significant energy and cost saving, it is still less as compared to the newer regenerative thermal oxidizers.

Regenerative Thermal Oxidizers

In the regenerative thermal oxidizer (RTO) ceramic heat transfer beds are used to recover heat energy from the oxidization process, which is then used to heat the incoming gas stream. This type of system has a very high thermal efficiency, in the range of 90% to 95%, allowing for great fuel savings and consequently lower operating costs. Not only are the ceramic heat recovery materials significantly more thermally efficient than the conventional metal heat exchangers used in recuperative thermal oxidizers, they are also more durable at very high temperatures. This essentially aids in increasing the usage life of regenerative thermal oxidizers as compared to recuperative thermal oxidizers. Because of their high thermal efficiency, this type of thermal oxidizer is ideal for use with gas streams in which the VOC concentrations is low and the flow rates of the stream is high.

A common RTO design in use today is the two-chamber or two-canister RTO, which consists of two canisters containing beds of ceramic media that act as a heat sink. Ceramic is used to retain the heat recovered from the clean gases that are being discharged and to impart that heat to the incoming exhaust gases. Each pipe, leading to and from the combustion chamber, has one of the ceramic media canisters in them, and by using a set of valves actuated by the control system, the inlet and outlet pipes are interchangeable. Essentially, by changing the flow configurations of the pipes leading to and from the combustion chamber, the exhaust gas stream can enter through either pipe, and the clean gases can also exit from either. With this system, while one ceramic media canister is imparting heat to the exhaust gases, the other canister is recovering heat from the clean gases.

Another type of RTO that is in use today is the Regenerative Catalytic Oxidizer or the RCO. In this type of oxidizer, the ceramic media is coated with a catalyst that aids in the oxidation of the exhaust gases at lower temperatures. This results in an increased reduction in the operating costs as compared to conventional regenerative thermal oxidizers. The catalysts that are normally used are precious-metal catalysts, such as platinum or rhodium.

Direct Fired Thermal Oxidizer

Another type of thermal oxidizer is the direct fired thermal oxidizer, in which the exhaust gas stream is fed into the combustion chamber through the burner and are burnt in the chamber as long as necessary to achieve the required destruction removal efficiency (DRO) of the VOC. This type of thermal oxidizer is also known as afterburner. Though these oxidizers are the least capital intensive, the lack of a heat recovery system makes their operation quite expensive. This results in making these thermal oxidizers particularly ideal for use with exhaust gas streams that have a very high concentration of VOCs and the gas can be used as a fuel in the oxidizer.

Catalytic Oxidizer

Catalytic oxidizers, while similar in purpose to thermal oxidizers, are completely different in operation. As their name suggests catalytic oxidizers utilize a catalyst to break down the VOCs present in the exhaust gases. The oxidation in this process is basically a chemical reaction between the catalyst and the VOC hydrocarbon molecules. These types of oxidizers are commonly found as stand-alone components in the exhaust system of automobiles, to control the discharge of noxious gases produced by internal combustion engines into the atmosphere. They are also used in various industries and commonly consist of a catalyst bed, a heat exchanger, and a burner. The catalyst bed is essentially made of either honeycombed ceramic or numerous ceramic beads, both coated with the catalyst. The types of catalysts that are normally used are precious metal catalysts, such as platinum, palladium, or rhodium. The heat exchanger in the catalytic oxidizer is used in a recuperative role, for the recovery of heat produced during the oxidation process.

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