REGENERATIVE THERMAL OXIDIZER

PRINCIPLES OF COMBUSTION

Thermal oxidation technology has proven to be industry choice for destroying Volatile Organic Compounds (VOCs). The process of combustion is the most commonly used method to control emission of organic compounds. Combustion type systems are always simple systems capable of having very high destruction efficiency.

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 air waste, 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 "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.

Thermal destruction of most organic compounds occurs between 900°F and 1,300°F. The time for which the pollutants stay in the incinerator is called residence time. The higher the residence time the lower temperature can be used for the combustion chamber. The residence time of gases in the combustion chamber may be calculated by

t = V / Q

where, t = residence time, seconds

V = chamber volume, ft3

Q = gas volumetric flow rate at combustion ft3/s.

Adjustments to flow rate must add 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 all of the 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. There are three types in which the combustion type of systems can be divided, which are as follows:

1.Thermal Recuperative Oxidizer

2.Catalytic Oxidizer

3.Regenerative Thermal Oxidizer.

All these types of Oxidizers are manufactured at Epcon. Catalytic Oxidizers use a catalyst to carry out destruction of VOCs at lower temperatures. Catalytic Oxidizers may or may not be provided with recuperative Heat Exchangers depending on the application requirements. The primary difference between the Regenerative Thermal Oxidizers and Thermal Recuperative Oxidizers is the principle of heat recovery.

TYPES OF HEAT RECOVERY

As energy costs become important, addition of heat recovery becomes essential. In both systems, hot clean combustion gases are used to preheat the incoming process exhausts. Heat Recovery takes two basic forms:

(A) Recuperative(B) Regenerative

Shell and Tube Type Heat ExchangerPacked Bed Heat Exchanger

Transfers heat from hot gases exiting the Uses the heat transfer media to store the

oxidizer chamber to cold process air heat during heat recovery cycle. Heat

entering the Thermal Oxidizer stored by the media is transferred to cold process air during preheat cycle.

Utilizes stainless heat exchanger to Uses ceramic type media that has low

transfer heat with minimum corrosionthermal conductivity but high capability

and required surface area necessary forof storing heat.

heat transfer.

Heat exchanger recovers more than 95%

Heat Exchanger recovers as high as 80%of heat in the exhaust gases.

of heat in the exhaust gases.

OPERATION SEQUENCE (TWO CHAMBER RTO)

Continued preheating of the process gas causes the ceramic media canister to cool down, at the same time the ceramic media in the next chamber is heated. The inherent nature of the Regenerative Systems involves discontinuous or cyclic operation.

CYCLE –1

CANISTER 1- preheat cycle

Cold process gases are preheated by the media in Canister 1.

CANISTER 2 - heat recovery cycle

Hot air exiting the residence chamber heats up the media in Canister 2.

The attached 3-D drawing shows the same position in the cycle.

CYCLE –2

CANISTER 2- preheat cycle

Cold process gases are preheated by the media in Canister 2.

CANISTER 1 - heat recovery cycle

Hot air exiting the residence chamber heats up the media in Canister 1.

Heat Recovery Efficiency

Selection of the correct heat recovery efficiency in an RTO is a function of the following parameters:

  • VOC loading in the exhaust stream.
  • Presence of corrosive gases in the exhaust stream.
  • Plant operation cycle for the RTO.

Heat Recovery Efficiency is defined by the following equation:

Eff. = (Preheat Temperature - Process Gas Inlet Temperature)

(Oxidizer Residence Chamber Temp. - Process Gas Inlet Temp.)

For Example:

A RTO with a Preheat Temperature of 1335°F (724°C) at an Oxidizer Residence Chamber Temperature of 1400°F (760°C) assuming an inlet temperature of 100°F (38°C), will have Heat Recovery Efficiency as follows:

Eff = (1335 - 70)

(1400 - 70)

=0.951 or approximately 95%

Valves

In Epcon’s Two Canister RTO, there are two 30” poppet valves that control the airflow. The dampers are located in a position for easy maintenance. Access doors have been provided on each damper in the event access inside the poppet box is needed. The valve shaft can be disconnected and slid apart to allow replacement of the valve plate when it becomes worn out.

Insulation

The Thermal Oxidizer is 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. The typical width of insulation is 6 inches in the heat recovery chambers and 7 inches in the combustion chamber consisting of ceramic block.

Controls and Electrical Design

This Thermal Oxidizer is designed to simplify maintenance and service. The control panel contains Honeywell Flame Management System, relays, timers, and switches. This system is responsible for the complete control of the burner start-up, from pre-purge to the ignition trial and the final flame. Another control point is the High Temperature Limit, which monitors the operating temperature. A Variable Frequency Drive (VFD) controls the main exhaust fan. This allows for variable speed control of the fan to match the desired volume through the oxidizer. Other items in the control panel are the indicating lights and push/pull buttons. The control panel should be checked and monitored periodically to ensure safe and correct operation.

Other Features

Like the control panel, the gas train contains components that require little maintenance. Adjustments to the gas train have been made at the factory. Components have been set for proper operation and control. Many components are totally enclosed to protect them from damage by dust and dirt. Should any minor adjustments for the pilot or main gas regulator be required, refer to the literature on the specific component.

In the unlikely event that the ceramic media must be removed, there is an access doorson the top of the unit to allow access. .

The Thermal Oxidizer should perform satisfactorily and give many years of trouble free service. It however, like any high quality equipment, deserves proper maintenance and care.

Please refer to the maintenance section of the manual for various schedules and types of maintenance required. For operator's and maintenance crew's convenience, a detailed Bill of Material has been prepared. Also, in the technical manuals you will find all manufacturer's literature on all the items used in the Thermal Oxidizer. Please follow there instructions explicitly.

Additionally, you will find a Spare Parts List outlining the recommended items for spares. Should you have any requirements for spare parts, please contact EPCON INDUSTRIAL SYSTEMS, INC. at (936) 273-3300, fax to (936) 273-4600, or email us at .

NOTE:This system does not contain Asbestos in any form or fashion to the best of our knowledge.

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