Multistage Heat Recovery for Aluminum Recycling Facility

Project No 2301

The Challenge

Aluminum parts producers are looking to metals recovery systems to control costs of primary and secondary aluminum purchases and to mitigate their operating cost by capturing value from byproducts. A metals recycling operation in Arkansas needed to upgrade their existing system, which was an early design pretreatment system with rudimentary drying technology. 

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 cost and uptime/maintainability of the system. Historically, parts producers have focused on dryer and melt furnace capacities as the rate limiting factor when sizing their system.  The practical reality, when sizing a system, is that vent stream emissions may be the real rate limiting factor controlling your overall recovery system’s capacity. 

The Solution

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, and the appropriate air pollution control system can be determined and sized. 

While thermal oxidation of these vent streams is a proven solution to mitigate the VOC emissions from these sources, many lessons have been learned on these specific applications through the years, that allow for the optimization of these systems. Dealing with particulate matter on the inlet, avoiding formation of HAPs at the discharge of the unit, and withstanding the abrasive atmosphere within system, are all challenges which have been successfully overcome.

However, despite these system application improvements, even the most efficient drying system will produce some level of particulate matter emissions.  Choosing a design which limits that potential will reduce the demand on the downstream PM removal system and reduce any subsequent loading on the downstream thermal oxidizer.  Within the Thermal Oxidizer itself, burner placement/orientation, combustion chamber configuration and the design of primary and secondary heat exchangers will all contribute to a system which can operate in the face of the particulate matter loading one can realistically expect to see during standard operation.

At the scale of many captive recycling operations, recovering waste heat from the process can produce a significant annual cost savings and overall reduction in facility greenhouse gas emissions.

The custom engineered system upgrade included an innovative design of a thermal oxidizer for this application equipped with a quad downflow inlet plenum, axial mounted down-fired burner as well as primary and secondary heat exchangers . This configuration maximized turbulence in the combustion chamber, resulting in excellent destruction efficiency and protected the burner from potential particulate carryover.

The Results

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 1.5 years! This exceptional payback was achieved at a still relevant fuel value of $3.85/MMBtu.