The Direct Fired Thermal Oxidizer (DFTO) was one of the first abatement technologies to utilize combustion in purifying air pollutants. Also referred to as an Afterburner or Straight Thermal, these oxidizers do prevent harmful emissions from entering the atmosphere, however they may also require a significant amount of supplemental fuel to achieve destruction temperatures within the system.
In recent years the technology has evolved, incorporating features that minimize the effects of combustion. The highly efficient and effective technology is being uniquely applied in the Carbon Fiber industry to ensure the production of these materials is environmentally sound.
The LT and HT furnace exhausts are injected separately into the first stage of the DFTO where there is insufficient oxygen for complete combustion of the hydrocarbons. In this reducing environment, the substoichiometric operation causes the hydrocarbons, hydrogen cyanide and ammonia to dissociate resulting in free nitrogen. The free nitrogen and combustibles gases are competing for the limited oxygen available from the oxygenated hydrocarbons and combustion air from the burner. This keeps the nitrogen from being oxidized to NOx. This first stage of combustion is designed in a cylindrical refractory lined chamber capable of continuous operation at very high temperatures for a minimum of 1 second residence time.
The second zone of the system takes the N2, H2O and residual hydrocarbons leaving the first zone and cools these gases. This cylindrical refractory lined chamber uses a venturi type design to maximize the mixing of the hot gases and cooling medium. The gases leaving the first zone are cooled to the point where autoignition of the remaining combustibles will still occur in the last zone. The proper cooling medium is required to adsorb a great deal of energy without substantially increasing the outlet gas volume.
The third and final zone reintroduces air into the stream so that oxidation of the residual hydrocarbons, carbon monoxide and hydrogen can occur. Reoxidation air is added to this section so that the outlet oxygen concentration is maintained at the discharge of the system, prior to the induced draft fan and fan temperature control damper. This proper oxygen concentration and residence time at temperature ensures extremely high levels of destruction. Since NOx formation in this zone is to be prevented, the outlet temperature from this stage is limited by a predetermined temperature. The last chamber is insulated with a ceramic fiber insulation to minimize weight while still protecting the oxidizer shell from high temperatures.
Learn more about Multi-Stage Regenerative Thermal Oxidizers with this interactive diagram.