Thermal oxidation is the rapid reaction of organic chemicals — generally hydrocarbon-based — with air at an elevated temperature, resulting in the formation of primarily carbon dioxide and water vapor. The degree of destruction or reduction efficiency (DRE) in the oxidizer is largely controlled by three design parameters: time, temperature, and turbulence.
REGENERATIVE THERMAL OXIDIZERS
A regenerative thermal oxidizer (RTO) converts volatile organic compounds (VOCs) and hazardous air pollutants (HAPs) to carbon dioxide and water vapor through thermal oxidation.
RTOs use regenerative heat transfer to achieve very high thermal efficiencies of 85 to 96 percent, which result in very low operating fuel costs — in some cases, zero. That makes them particularly economical for treating low-VOC-concentration process exhaust streams.
RTOs are robust, have few moving parts, and are fabricated from materials that are chemical- and heat-resistant. They employ rugged industrial components from nationally recognized manufacturers.
CATALYTIC THERMAL OXIDIZERS
This type of oxidizers use both precious and base metal catalysts, depending upon the application. For systems using a base metal catalyst coated onto a ceramic bead substrate, our design utilizes a tray arrangement with a gas hourly space velocity of around 14,000 h-1.
The catalyst bed is designed into a recuperative oxidizer located between the inlet pass outlet of the heat exchanger and the combustion chamber. For systems that require a precious metal catalyst, NESTEC uses a coated ceramic monolith as provided by SŰD Chemie-Clariant.
This system can be cleaned periodically by removing the beads and tumble-cleaning them before returning them to the system. A small amount of make-up catalyst is typically required after cleaning. During system inspections, NESTEC will coordinate the testing of the catalyst for activity.
RECUPERATIVE THERMAL OXIDIZERS
The Recuperative Thermal Oxidizer converts VOCs and HAPs to carbon dioxide and water vapor through high-temperature oxidation. A recuperative heat exchanger (either a plate type or shell-and-tube type) is employed to recover thermal energy from the oxidation process and thereby lower fuel costs. Operating temperatures are around 1,500°F.
While high-temperature-rated metal alloys are used to fabricate the recuperative exchangers, they may still experience thermal failures due to hot spots caused by poor airflow design, to particulate deposits, to corrosion/metal oxidation, or to high VOC concentrations causing extreme auto-ignition heat releases in parts of the exchanger.
Recuperative thermal oxidizers are less expensive than RTOs from a capital cost standpoint because they utilize lower-cost materials, and because they are smaller, using less material. Their design is simpler than an RTO, with fewer moving parts. However, thermal efficiencies can only go as high as 65 percent. Therefore, fuel costs may be as high as six to seven times as much as an RTO.
DIRECT-FIRED THERMAL OXIDIZERS
The simplest technology of thermal oxidation is a direct-fired thermal oxidizer. The process exhaust stream of VOCs and HAPs is introduced into a firing chamber through/near a burner, and enough residence time is provided to achieve the desired destruction removal efficiency (DRE) of the VOCs. Most direct-fired thermal oxidizers operate at temperature levels between 1,800°F and 2,200°F, with air flow rates from 500 SCFM to 50,000 SCFM.
These oxidizers are best applied where there is a very high concentration of VOCs to act as the fuel source (instead of natural gas or oil burner fuel) for complete combustion at the targeted operating temperature.
Typical applications include treatment of flare gas or other streams having a BTU/SCF value of 500 or greater, such as landfill methane extraction offgas; or systems that require the mixing of upper flammability limit (UFL) gases with air in order to create an oxidizable VOC concentration.