Thermal oxidation systems can be designed to handle a wide range of exhaust streams, including organic compounds containing halogens, sulfur, and nitrogen. The components in a modular system are selected according to exhaust stream contaminants, flows, and the desired energy recovery applications within the plant.
The Direct Fired Thermal Oxidizer (DFTO) is one of the first abatement technologies to utilize combustion in purifying air pollutant emissions. Also referred to as an Afterburner or Straight Thermal, these oxidizers prevent harmful emissions from entering the atmosphere. However, they may also require a significant expenditure in supplemental fuel to achieve destruction temperatures within the system. Consequently, they are typically applied to specific processes with unique conditions, such as carbon fiber manufacturing, soil remediation, or chemical and pharmaceutical acid emission control.
In recent years, DFTO technology has evolved by incorporating features that minimize the effects of combustion with nitrogen compounds and associated nitrogen oxide (NOx) generation, Hazardous Air Pollutants (HAPs), as well as excessive energy consumption. These highly efficient and effective technologies are proving particularly useful in the carbon fiber manufacturing industry because they ensure the production of these advanced materials is environmentally compliant and maintains a competitive energy conservation profile.
A high performance modular carbon fiber system can include:
- Multi-zone DFTOs to regulate hydrogen cyanide (HCN) from the low temperature (LT) or high temperature (HT) carbonization processes with minimal NOx generation
- Baghouses to constrain silica (SiO2) generated in the high temperature oxidation process
- Multi-combustion chamber regenerative thermal oxidizers (MCC RTO) to control the emissions from the ovens
- Heat exchangers for preheated return air back to the ovens
- Multiple fans (as needed)
One typical approach
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 sub-stoichiometric operation causes hydrocarbons, hydrogen cyanide, and ammonia to dissociate, resulting in free nitrogen. The free nitrogen and combustible gases are competing for the limited oxygen available from the oxygenated hydrocarbons and combustion air from the burner. This prevents the nitrogen from being oxidized and forming harmful 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 one second residence time.
The second zone of the system takes the nitrogen, water, 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 final zone. A well selected 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. Prior to the induced draft fan and fan temperature control damper, deoxidation air is added to this section so that the outlet oxygen concentration is maintained at the discharge of the system. This optimized 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.
NESTEC has installed several modular DFTO systems with scrubbers, waste heat boilers, and various heat recovery exchangers. These applications will be covered in future newsletters.
If you have an application that requires multiple components in order to achieve the best air emission compliance and energy conservation that will maximize your Thermal Oxidation application performance and reduce your energy bill, contact NESTEC for a free process analysis.
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