Vapor-rich and wastewater waste streams, such as those from the rendering process, pose challenges for traditional treatment systems. Gulf Coast Environmental Systems, LLC (GCES) provides robust, economic solutions to these challenges utilizing thermal oxidizers which apply the combination of heat and time to combust pollutants into harmless CO2 and water.
Some of the harshest pollution control applications are in the rendering field of the food processing industry. The waste stream can contain odors, ammonia, fats, grease, water and other vapors, and even particulate matter, all in a wide ranging mix. GCES’ custom designs systems to handle these difficult processes based on each facilities waste output and goals. Included in our solution is the Aqueous RTO wastewater destruction system with capabilities that allow for processing of both air and water pollution simultaneously. The process is controlled through metered injection of the water into a pre-heated inlet stream into the oxidizer for complete destruction using the standard oxidation process. The following is a comparison of the Aqueous Regenerative Thermal Oxidizer (RTO) and a Vapor-Tolerant RTO for implementation in the treatment of food rendering waste streams.
Oxidizer Technology
Thermal oxidizers, or thermal incinerators, are combustion devices that control VOCs, CO, and volatile HAP emissions by combusting them to carbon dioxide (CO2) and water. Thermal oxidizers are similar to catalytic oxidizers (catalytic oxidizers use a catalyst to promote the oxidation reaction). Important design factors include temperature (a temperature high enough to ignite the organic constituents in the waste stream), residence time (sufficient time for the combustion reaction to occur), and turbulence or mixing of the combustion air with the waste gas.
Early applications of air pollution control equipment consisted of a direct-fired oxidizer (TO), also referred to as an afterburner. A properly designed TO is capable of providing reliable destruction (95% or greater) of the emissions while operating at temperatures of 1200°F or higher.
In the early 1980’s, a new technology of air pollution control was developed. The technology still revolved around the process of thermal oxidation, but used the action of heat sink regeneration to recover most of the waste heat. This technology was called Regenerative Thermal Oxidation (RTO). Early units achieved around 90% heat recovery efficiency. Our modern units regularly achieve efficiencies of 95-97%. The advancement and adoption of heat recovery media significantly enhances the ability to reduce the energy required to heat up the incoming vapor-laden and condensed wastewater streams.
Ceramic Media
Early heat recovery ceramic media used in RTO Systems was typically a 1″ saddle. These saddles are randomly packed in the energy recovery beds to a depth of about 8’. This type of media leaves many voids and areas where condensable organics and particulates can build up. Coupled with the high initial pressure drop that is an inherent property of saddles, this results in high electricity due to the large horsepower motors require to push the air through the ceramic media beds.
Modern ceramic heat recovery media is manufactured as a “structured” ceramic block, which has a higher density and greater amount of surface area per cubic foot compared to saddle media. These features result in a smaller area and lower depth (typ. 4’ – 5’) of the bed while achieving the same amount of heat transfer efficiency. Structured ceramic media also has a reduced pressure drop for the air passing through each bed, typically 4” water column (w.c.) vs. 8” w.c. or higher for saddles. This lower pressure drop reduces the size of the fan required, consequently reducing the energy used.
RTO Configurations
There are three different types of RTO system configuration: single-canister, two-canister and three-canister designs. Canister or can refers to the number of physical chambers which hold the ceramic heat recovery media. Each design has individual advantages and drawbacks. Single-canister RTOs occupy a small footprint and have the lowest initial procurement (CAPEX) cost, but have higher maintenance (OPEX) costs. Two-canister RTOs have a low CAPEX and more efficient maintenance, while generating a DRE up to 98-99%. Three-canister RTOs are better suited to bake-out processes, have a high DRE of 99% or higher, but have a larger physical footprint.
Three-can RTO systems are the best solution for vapor-tolerant and aqueous applications. The high DRE in excess of 99% insures the odor and organic material is nearly completely destroyed. In rendering applications, in order to remove the odor and constituents from the cooker air stream to a level of undetectable emissions, a DRE over 99% is required. A single-canister RTO comes close to achieving 99% DRE, but can’t remove all odors. Two-canister RTOs experience a brief “puff” of untreated process stream from the RTO during cycling of the chambers, which allows a minimal yet detectable release of odor. The third chamber in a 3-canister RTO allows for one chamber being purged with clean air during cycling, eliminating the “puff”, pushing the DRE over 99%. The additional cost of this third chamber is negligible, making the 2-chamber RTO an improbable option.
Vapor-Tolerant Systems
Typical oxidizer applications process a gaseous waste stream. Some special applications require the handling of a waste stream laden with water vapor and/or other moisture or residues and possibly particulates and other solids.
The food processing industry is one of these applications. Rendering is a process that converts waste animal tissue into stable, value-added materials. Rendering applications require the destruction of organic and wastewater emissions (odor, ammonia, fats, grease, and particulate) emitted from the cooker process. These are carried over as liquid and warm air with up to 100% humidity.
Due to the amount of condensable organics and wastewater being emitted from the cooker, a residual layer of fats, grease, and oils will build up on the ductwork, valves, and inlet ceramic media of an RTO unit. Over time, this buildup will create conditions of flammability in the RTO. To mitigate the risk of fire, a “bake-out” cycle is used to remove such buildup in a controlled manner during a preset schedule. A bake-out cycle revolves around varying the timing of the inlet and outlet valves and using the heat in the RTO to dry and incinerate the residue to ash.
Performing the bake-out of the residue safely and under a controlled method 
prevents undue stress and damage to the RTO system. A three-canister RTO system is ideally suited to bake-out cycle applications and handles bake-outs better than single or two-canister RTOs. In a three-canister system, minimal smoke and odor is released to atmosphere during the bake-out cycle; single or two-canister units will release visible smoke and odor. Additionally, three-canister RTOs utilize butterfly valves to control the process cycles, which are better suited to the high temperature exposure of the bake-out cycle as compared to the poppet valves of a two canister RTO or the rotor plate of a single canister RTO.
Due to the high temperature exposure of bake-out cycles and the acidic and moisture/vapor-laden nature of the incoming exhaust stream, stainless steel is used to fabricate the cold condensable areas of the RTO unit. The incoming ductwork, valves, ceramic media supports, outlet ductwork, fan, and exhaust stack are exposed to the condensable liquids and corrosive elements of the air stream. These are manufactured from stainless steel or other corrosion resistant alloys (CRA).
Instrumentation, controls and safety systems are also adjusted to accommodate the more severe, moisture laden process stream. Due to the volatility and variety of the process stream, the control system has special accommodations to maintain system performance across a wide variety of operating conditions.
The combination of these materials, components, design considerations and manufacturing techniques extend the service life of the equipment, minimize maintenance, and insure reliable performance.
Aqueous Systems
GCES developed our Aqueous Oxidizer systems to handle the unique requirements vaporizing and destroying the contaminants in wastewater. With some of the same challenges as vapor-tolerant systems, aqueous systems are actually a more controlled version. Where a vapor-tolerant system such as those in rendering applications noted above can see a wide variety of vapor content, residues and other contaminants, an aqueous oxidizer processes the water in a controlled, metered manner. The particulate that is presented in rendering applications also provides for a more challenging design compared to an aqueous system.
The basic oxidizer is the same as a normal configuration with the added fluid-handling capability. This consists of pumps, mixing and storage tanks as needed, spray/injection nozzles and associated piping and controls. This system provides controlled injection and processing of the water into the inlet stream for processing through the oxidizer, using heat from the combustion chamber to help vaporize the fluid. Automatically synchronizing air flow, water pressure and flow, process temperature and pressure provides steady-state, reliable operation.
As shown in the diagram, the process inlet to the RTO comes off the evaporated hot-air recirculation line from the RTO combustion chamber. This is a controlled injection of the moisture unlike rendering applications where water carryover cannot be controlled.
To handle potential moisture collection due to condensation in the inlet ductwork, fan and exhaust stack, condensate drains are installed to allow drainage of the liquid during maintenance. These drains for an RTO typically consist of multiple tap points with manual shut-off valves and piping routed to an appropriate drain.
The system includes the same bake-out feature as the vapor-tolerant system. The frequency of such bake-outs is dependent on the make-up of the waste water stream. If it contains organics, oils and other solids or residues, these cycles may be more frequent.
Similarly, CRA materials are used to manufacture the cold condensable areas of aqueous RTO units, including incoming ductwork, valves, ceramic media supports, outlet ductwork, fan, and exhaust stack.
Experience
GCES has installed several generations of RTO technology for vapor-tolerant, rendering applications. Through the knowledge gained with each installation, there are many design and construction details that are unique and important to properly fitting the unit to the application. GCES is referred to as the go-to supplier of RTOs for the rendering industry.
Conclusion
Thermal oxidizers are a proven, reliable method of treating air pollution. GCES has taken this to the next level for treating vapor-rich waste streams and even wastewater destruction by carefully considering and implementing critical factors such as materials, controls, media, instrumentation and system configuration. Vapor-tolerant systems are the more difficult solution due to the inconsistency in the volume and make-up of the vapor, liquid, residue, and other contaminants in the waste stream. With over half a dozen of these RTO systems reliably operating around the globe, GCES is known as the “go-to” guys in the industry for this severe application. Drawing on these experiences and principles, GCES’ Aqueous Thermal Oxidizer Systems flawlessly process wastewater streams through careful blending, heating and injection. Let GCES design and build the solution to your vapor or wastewater problem today.