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An example of combustion air pre-heat
Aluminum industry
Background
In the aluminum industry, significant efficiency improvements can be achieved by using the recovered heat to preheat the combustion air or charge material, generate steam, heat water, or supply heat to other processes. Recuperators are generally used to recover heat from clean flue gases (as in aluminum reheating or heat treating furnaces), whereas regenerators with ceramic heat transfer media are used with flue gases containing contaminants (as in aluminum melters).
Common uses of heat
Preheat combustion air. Waste heat recovery devices (whether regenerative burners or recuperators) placed in the flue gas outlet or exhaust stack can extract a large portion of the thermal energy in flue gases and transfer it to the incoming combustion air. Processes operating above 1,400˚ F are generally good candidates for combustion air preheating, although it may also be used cost-effectively in processes with temperatures as low as 1,000˚ F.
Preheat the charge/load. Transferring heat from high-temperature exhaust gases to the relatively cool incoming load can reduce the energy required in the furnace and lower the energy that escapes in the exhaust gases. Heat from furnace flue gases can be used to dry the charge and preheat the load for melting furnaces or to preheat the load for heating and heat treating furnaces.
Cascade waste heat. The heat from exhaust gases can be used as a source of heat for lower temperature process heating equipment. For example, waste heat boilers can use the thermal energy from flue gases to generate hot water or steam. Waste heat from heat treating furnaces can also be used in aging or paint-drying ovens. To maximize benefits of the heat recovery, the downstream heating equipment must be in operation while the furnace is operating (US DOE)
Heat Sources
Aluminum manufacturing is divided between primary refining of aluminum from bauxite and secondary production of recycled scrap. Primary aluminum production relies on energy intensive electrolytic cells that account for about 15.6 kWh/kg or 60% of the energy associated with primary aluminum production. A small quantity of heat is lost via off-gases, while the majority of heat is lost through the cell sidewalls.
Secondary aluminum production requires only about one sixth of the energy required for primary production, which has contributed to the increased demand of aluminum recycling. A key step in secondary production is scrap melting in high temperature furnaces, where waste heat recovery is employed in only about one third of high capacity furnaces.
Below are some potential sources of recoverable heat in the aluminum industry.
Refining
Coke Calciner
There is an opportunity to recover heat from coke calciner afterburner waste gas streams at 1,900°–2,000°F (1,040°–1,090°C) to produce steam for use on the premises, or nearby. At such high temperatures, the heat can also be used to generate electricity, or cooling with absorption chillers.
Calciners
A large percentage of the total energy required to produce aluminum is used during the calcination process. Waste heat exhaust from calciners is a target for heat recovery. The exhaust contains considerable moisture (about 50% water by volume) and contaminants such as alumina particulates at a relatively low temperature (365°–392°F or 185°–200°C). The challenge of recovering heat from exhaust gases and hot alumina is that the low temperature and presence of particulates result in severe fouling of any type of heat exchanger. The industry is considering several technologies to recover latent heat of the water vapor and is working with several organizations to investigate other options.
Hot alumina
Heat from hot alumina is discharged at a very high temperature. Despite the higher temperature, the presence of particulates still presents a challenge. Attempts are being made to recover heat from hot alumina to produce hot water that can be used as a heat source to generate electricity or in other processes within the plant. However, there is a limit on how much hot water can be used for the plant processes.
Smelting
Electrolysis – Pots
There is good potential for heat recovery in this area. The sidewalls and endwalls represent a 15%–20% heat loss while the off gases represent about a 15% energy loss. It would be possible to attach a thermoelectric device on the wall, which is covered with a steel shell. Accessibility depends on the age of the pots and geometry. Some smelters are more accessible than others—for example, Warwick smelting pots are difficult to access. Some trials on heat recovery have been done in other countries.
Off gases from the pot are 212°F (or 100°C) after the scrubbers and only 212°–266°F (or 100°–130°C) before the scrubbers. Some companies have expressed an interest in capturing waste heat before the scrubbers because there are issues with build-up and particulate fouling. Make-up air is used to prevent fugitive emissions and the duct system needs to be open for periodic maintenance. Cleaning the gases remains a challenge. An Installation in Europe utilizes a heat exchanger on exhaust gas stream to recover heat from the aluminum smelting cells or reactors, commonly known as pots. A heat exchanger (gas to water) upstream of the scrubber is used to get hot water to supply heat to a district heating system at a location in Iceland.
Anode Baking
Anode baking processes generate hot exhaust gases from fuel combustion. These gases represent substantial heat loss. The exhaust gases contain a fair amount of volatiles and tar vapors. Dealing with fouling from tars is also an issue. Exhaust temperatures are 390°–750°F (or 200°–400°C) and contain large amounts of excess air. “Pitch Burn technology” is getting cleaner and could provide further opportunities.
Melting
On the primary cast side, holding furnaces use a cold charge to cool down the superheated metal or increase production. The furnaces operate in semi-continuous or batch mode and result in variable exhaust gas flows and temperatures. Most furnaces are gas fired; however, there are a few plants with electric furnaces. Exhaust gases may have some hydrogen fluoride—usually less than 1 ppm—but in some cases it could be 5–10 ppm. This limits the use of conventional heat exchangers to recover heat. Wall losses and opening losses could be a good percentage of the total heat input; however, these losses are reduced by using best practices in furnace design and operations. Heat loss from these furnaces can be considered a low-impact, high-complexity category.
On the secondary side, melting furnaces discharge a large amount of heat as exhaust gases. They are at a high temperature—usually greater than 1,600°F (870°C) —and can contain particulates, organic vapors, and flux material vapors and can be classified in the same category as flue gases from gas melting furnaces. Separating these contaminants at higher temperatures can be considered a cross-industry need. Another possibility that should be considered is treating the gases with absorbents to neutralize the “bad actors.” This will eliminate the need to cool the gases and creates cleaner gases that can be used for heat recovery in currently available heat recovery systems. The possibility of pre-cleaning or pre-treating the hot flue gases to remove the contaminants should be considered an R&D need.
Recycling and Secondary Melting
The following is a list of the types of waste heat sources typically found in secondary aluminum melting:
Waste heat in clean exhaust gases from reverb furnaces, gas generators, and crucible heaters+
These gases range from 1,450°F (790°C) in the crucible heaters to 2,000°F (1,090°C) in reverb furnaces, contain combustion products of natural gas, and do not contain any major contaminants. They are prime candidates for heat recovery for preheating combustion air. These gases contain as much as 60% of the total heat input for the heating systems. Possible options for heat recovery include combustion air preheating using recuperator or regenerative type units, or low-temperature power generating cycles such as an Organic Rankine Cycle based system.
Waste heat from rotary furnaces:
These gases are relatively cold and contain a large amount of contaminants. The mass flow rate is cyclic and often unpredictable. There is no industry accepted method of recovering sensible heat from these gases. Development of a low-pressure drop filter or higher temperature fabric material for baghouses can help clean up the gases so they can be used for heat recovery.
Waste heat from delacquering systems
These systems remove volatile materials from the UBCs and preheat them to about 900°F (480°C) before they are discharged from a rotary kiln. The preheated but partially cooled UBCs are charged in the reverb furnace. Delacquering systems use a gas generator that incinerates volatiles from the paint coating on UBCs. The exhaust gases from the unit are relatively clean and are discharged at about 650°F (340°C). There is no heat recovery from these gases.
Waste heat from crucible heaters
These are relatively small units used to preheat crucibles before pouring hot aluminum metal from rotary furnaces or reverb furnaces. The burners are relatively small and fired in an uncontrolled fashion. The gases are discharged at about 1,450°F (790°C) and the heat is not recovered. (ORNL)
Examples
- An in depth study of the different heat recovery options and associated economics at Aluminum facilities [link]