Efficient preheaters improve safety and energy consumption

Author:

Jean-Francois Desmeules, Vice-President Technologies and R&D

jean-francois.desmeules@dynamic-concept.ca

Dynamic Concept, 2400 Alexis-Le-Trotteur, Jonquière Québec Canada G7X 0J7

 

 

Introduction

Aluminium producers use a lot of material under the form of sows and ingots. Alloying elements also often come in ingot bundles. It is well known that sows and ingots often have hidden shrinkage cavities that communicate with the surface via cracks. Internal scrap such as cracked sheet ingot or billet can also trap moisture. According to the “Guidelines for Handling Molten Aluminum” from the Aluminum Association, section 14, “The capability to dry scrap or other charge materials such as primary sow, T-ingot and alloying agents (especially magnesium) possibly containing moisture is a critical feature of a safe operation”. Therefore, a preheating furnace is a critical component of the production line. The good news is that not only it improves the safety, but it also improve energy efficiency and cycle times. Dynamic Concept developed a modular design furnace that is cost effective and easy to implement.

Safety requirements

An explosion can occur when moist material is charged in molten aluminium. When water is trapped between a solid surface and molten aluminium, it quickly turns to steam and expands its volume approximately 1700 times, projecting molten metal at a large distance, injuring employees and damaging equipment. This is the “physical” explosion. Furthermore, the dispersed metal has a great affinity with oxygen so depending on the conditions, a second explosion can occur, the “chemical” explosion. According to thermodynamic calculations, it has been estimated that the energy release of one part of aluminium reacting with oxygen is equivalent to detonating three parts of TNT.

Thus, explosions are to be taken very seriously. Even material that appears to be dry must be considered moist. The history of a given sow between production and melting can hardly be known and one must act as if it was exposed to rain and snow. Even without exposure to elements, some parts of the world have cold night and warm, humid days, causing condensation on stored metal.

The greatest danger comes from sows with internal voids where water is trapped, but the exterior appears dry. If such a sow is submerged in molten aluminium, it may not cause an immediate explosion. It will rather be sealed with a layer of frozen aluminium while its internal temperature rise. The explosion will occur a few minutes later when no one is suspecting the danger.

To avoid these risks, the charge material must be preheated. For example, the Aluminum Association recommendation for typical LME sows is that “furnace drying for 4 hours after the internal metal temperature has reached 400°F (204°C) should adequately dry 1500 pounds (680 kg) sows”. Important terms here is “internal temperature” and “should”. Internal temperature means that for a preheating program to be valid, testing must be made with thermocouples drilled inside the material to establish the heating curves. The use of the word “should” means that the user is responsible for establishing the preheating procedure, as the nest paragraph of the Guidelines explains that individual companies should develop their practices according to their operations. Some companies have more stringent requirements.

Melting energy efficiency

While it is important to ensure the safety of the process, it is always desirable to combine this with cost reduction or added value. The aluminium melting process, although consuming much less energy than the reduction of alumina, is still requiring large amounts of energy. Typical energy efficiency using cold air burners is around 30%, with the bulk of the combustion heat exhausted in the atmosphere under the form of hot products of combustions. Indeed, the temperature of the products of combustion in the exhaust flue is roughly the same as the setpoint temperature. The hotter the exhaust gasses, the less energy transferred to the charge. For example, the available heat with 600°C furnace gas exit temperature with natural gas at 10% excess air is approximately 66% while the available heat for 1150°C exit temperature with the same excess air is only 40%. The lower setpoint uses 1.5 times more heat from the gas to heat the load.

Moreover the heat losses through walls and air infiltration are greater when the internal temperature is higher.

Therefore it is more efficient to heat metal from the room temperature to a given setpoint when the temperature inside the furnace is lower.

Charging hot metal in a melting furnace is not only safer, it is also economical since less heat have to be transferred to the charged and less time is required to melt it.

The Dynamic Concept preheating furnace

In order to provide a solution to the safety requirements as well as providing a way to improve the efficiency of the melting process, Dynamic Concept developed a modular preheating furnace that can be used with aluminium and alloying elements.

Description

The Dynamic Concept preheating furnace is a modular enclosure fitted with natural gas burners and convection fans. The convection fans are sized to provide better heat transfer and temperature uniformity inside the furnace. A stainless steel liner inside the furnace protects the insulation. The access door in the front allows total access to the interior of the furnace to deposit the charge using a forklift. A safety lock keeps the door in safe open position for the operation and maintenance.

The design is modular, allowing a tailor made design suitable the exact capacity required by the customer. The modular design also facilitates the shipping overseas in ISO containers with facilitated assembly and installation.

Automated treatment sequence manages fans speed, temperature ramps, closed loop temperature control and automated door opening.

Optional bumpers for the lining and fans, as well as stairs and maintenance access floors are available as an option.

Furnaces have been designed for capacities ranging from 12 T to 48 T but our modular system makes it easy to adapt to different capacities or load configurations. It is also possible to use electrical resistance heating.

Process qualification

Are furnaces are fully assembled and tested in our shop prior to shipping, to minimize site assembly and commissioning requirements. Of course, site specific parameters require adjustment during commissioning, such as load configuration, altitude and exhaust chimney configuration.

During commissioning a complete qualification procedure is performed. Charge material is drilled to insert thermocouples at the core. Multiple thermocouples are used at various places inside the furnace to ensure that for a given load configuration, the preheating sequence will guarantee compliance to the safety requirements.

The typical load will heat to the minimum “safe” temperature in 1 to 2 hours, followed by the required holding time. For example, a batch of sow aluminium will typically require 2 hours to reach 204°C internal temperature. With a holding time of 4 hours the total cycle time is 6 hours plus loading/unloading. Since a holding time is required, we recommend continue heating instead of holding the temperature to reduce melting time further in the process.

One of our preheating furnaces is being used to dry and preheat 48 metric tonnes of sow aluminium at temperatures up to 450°C in a cycle of 6 hours.

About Dynamic Concept

Our mission is to increase the competitiveness of light metals producers by supplying custom-fit equipment and systems adapted to the specific conditions of each process line.

We assist our clients at all stages of a project, from preliminary studies to start up, including design, engineering, fabrication, testing, erection of equipment, and the supervision of commissioning and start up.

Our success is measured by the longevity of our business relationships with our customers as well as their satisfaction.

Other furnace technologies available

Siphon transfer

Dynamic Concept developed a thorough expertise in siphon metal transfer. This system allows a safe and efficient transfer while minimizing oxidation.

Level measurement

The patent pending FLASER provides an accurate level measurement even in the presence of dross.

Temperature measurement

We developed an automated adjustable height temperature measurement system that allow measurement of the temperature at different depths in the molten metal bath. This systems works with the FLASER to determine the temperature distribution inside the metal.

Energy efficiency improvements

We can provide many custom improvements to existing furnaces, including door replacements, to improve the energy efficiency.

References:

Aluminum Association’s Guidelines for Handling Molten Aluminium, 4th edition.

North American Combustion Handbook, Volume 1, 3rd edition