The Impact of Thermal Oxidizer Sizing on Melt-System Capacity
It is important to understand rate-limiting factors when sizing a secondary aluminum-recovery system.
Driven by new regulations as well as consumer performance requirements, the demand for cast, extruded and machined aluminum parts has increased globally. Aluminum parts producers are looking to metals-recovery systems to control the costs of primary and secondary aluminum purchases and to mitigate operating costs by capturing value from byproducts.
Understanding the true rate-limiting factors when configuring and sizing a metals-recovery system is critical to realizing the full potential of the system. Additionally, managing energy within the system will have a large impact on the operating costs and uptime/maintainability of the system.
Historically, parts producers have focused on dryer and melt-furnace capacities as the rate-limiting factor when sizing the system. When sizing a system, the practical reality is that vent-stream emissions may be the real rate-limiting factor controlling your overall recovery system’s capacity. Figure 1 provides a high level overview of the major components found in modern metals-recovery systems.
One of the first steps when evaluating a potential metals-recovery project should be to understand the expected fluids and contaminants in the feedstock. Given these input conditions, a concurrent analysis of the thermal processing load for the drying system and the expected constituents in the vent stream can be conducted. From these calculations, an estimate of the projected emissions loading from the operation can be determined.
Based on the findings from the above emissions calculations, the appropriate air pollution control system can be sized. Thermal oxidation of these vent streams is a proven solution to mitigate the VOC emissions from these sources. In addition, many lessons have been learned on these specific applications through the years, allowing them to be better optimized.
Dealing with particulate matter on the inlet, avoiding formation of hazardous air pollutants (HAPs) at the discharge of the unit, and withstanding the abrasive atmosphere within the system are all challenges that have been overcome successfully. However, despite these system application improvements, even the most efficient drying system will produce some level of particulate-matter emissions. Choosing a design that limits the potential for emissions will reduce the demand on the downstream particulate-matter removal system. It also will reduce any subsequent loading on the downstream thermal oxidizer. Within the thermal oxidizer itself, burner placement and orientation, the combustion chamber configuration and the design of primary and secondary heat exchangers all will contribute to a system that can operate in the face of the particulate-matter loading one can realistically expect to see during standard operation.
In absolute terms, we are seeing relatively low natural gas costs today. Nonetheless, at the scale of many captive recycling operations, recovering waste heat from the process can produce a significant annual cost savings and an overall reduction in facility greenhouse gas emissions.
A perfect example of the cost-savings benefits of secondary heat recovery in aluminum scrap recycling can be found in an installation at a merchant metals-recycling operation in Arkansas. The existing system was an early design pretreatment system with rudimentary drying technology. The custom-engineered system upgrade included a thermal oxidizer equipped with a quad-downflow inlet plenum, axial-mounted down-fired burner, and primary and secondary heat exchangers (figure 2). This configuration maximized turbulence in the combustion chamber, resulting in excellent destruction efficiency. At the same time, it protected the burner from potential particulate carryover. The energy saved from the introduction of primary and secondary heat recovery resulted in a payback for the new thermal energy system (oxidizer and dryer heat source) of less than one-and-a-half years. This payback was achieved at a still relevant fuel value of $3.85/MMBTU.
Today, we are seeing advances in aluminum-scrap pretreatment drying technology that not only result in reduced particulate carryover but also a combined increase in VOC loading, a reduction in permittable emissions levels, and an uptick in demands for even higher system uptime and serviceability.
In response to these changes, thermal oxidizer designs have evolved along with the advances in pretreatment drying technology. Understanding and projecting loading and sizing remains a critical part of the system configuration. The latest systems being deployed incorporate multipass modular heat exchangers to maximize system energy efficiency as well as serviceability. Additionally, large-diameter stainless steel shell-and-tube heat exchangers provide a good balance of wear resistance, life expectancy, maintainability and initial procurement cost.
Figure 3 highlights the latest design recuperative thermal oxidizer for a global, Tier 1 aluminum parts producer. This system is part of an integrated melt system being provided by a metals-recovery systems provider. The system is sized to support pretreatment of 3 tonnes per hour of aluminum chips through a jet-drying system.
The thermal oxidizer system includes primary heat recovery to minimize energy consumption. Also, packaging features minimize the overall system footprint while at the same time easing maintenance and service. The unit’s modular, two-pass secondary shell-and-tube heat exchanger configuration eases maintenance. Strategically placed access doors and large-diameter stainless steel tubes help maximum service life. In addition, the modular design simplifies installation and allows for a quick changeout in the future when the heat exchanger is no longer serviceable.
A key design element is the insulated stack. At 65 feet tall and with a diameter of 42 inches, the design and materials of construction were critical to meeting the customer’s performance requirements as well as capital budget. An insulated design allowed the designers to optimize materials selection and reduce the overall project capital cost.
In conclusion, understanding and applying best practices in aluminum scrap-recycling systems is key to realizing the full potential of the recovery system. Recognizing that a 1 percent deviation in contaminant levels can translate to more than a 2 percent change in metals recovery highlights the value in investing in modern recovery technologies.
By focusing on properly sizing and configuring the thermal oxidizer early in the design process, operators will be able to realize the full potential of the metals-recovery system and avoid wasted melt-furnace capacity. The capacity through the system can only be as great as the emissions limits permit. Whether for a new system or a retrofit of an existing plant, focusing on addressing emissions control rate-limiting factors will help ensure overall project success.