Heat Recovery in the Metal Casting Industry
About
Metal casting involves pouring molten metal into molds to produce consumer goods such as engine blocks, suspension parts for motor vehicles, structural and metal fittings for appliances, and pipes and valves.
Metal casting relies on high temperature, and often inefficient, furnaces for heating and melting metals. The inefficiencies in these furnaces are typically the cause of significant amounts of waste heat that can be utilized more efficiently. And the energy used in the melting processes is also significant… Approximately 55% of the industry’s energy costs are for melting processes. (DOE ITP).
Much of the heat in melting operations is lost to atmosphere. When this waste energy is re-used, it may save up to approximately 20% of a facility’s energy cost and, in some cases, reduce emissions. Waste-heat-recovery devices (recuperators, regenerators, shaft/stack melters etc.) simply transfer thermal energy from the high-temperature effluent stream to a lower temperature input stream (make-up air or metal charge). Waste-heat-conversion methods, such as absorption refrigeration, thermoelectrics and thermionics, utilize the elevated flue gas temperatures to drive energy conversion devices. (Advanced Melting Technologies)
A considerable portion of the energy costs can be reduced through heat recovery at casting facilities. The most common types of casting facilities for heat recovery are aluminum and iron, but opportunities are present at many other casting facilities.
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Common heat recovery options
The most common uses for waste heat in the metal casting industry are preheating charge material and preheating combustion air. Another good use for recovered heat is space heating, especially in colder climates, where space heating bills can contribute as much as half of the total energy bill in the winter.
In other cases, more creative options have been found; for example, one metal casting facility has installed a system for using waste heat to evaporate wastewater. Initially the facility had to spend about $22,000 per year to dispose of 48,000 gallons of wastewater consisting of 90% water and 10% oil. It was discovered that exhaust gases from a reverberatory furnace could be used to evaporate the water, thereby significantly reducing waste disposal costs.
Aluminum Casting
Melting furnaces in the industry include reverberatory furnaces, stack melters, crucible furnaces, and induction furnaces.
Reverberatory furnaces are the most commonly used melting furnaces among high volume aluminum foundries and account for melting 90% of aluminum produced in the United States. Aluminum reverberatory furnaces have exhaust temperatures of about 2,000 - 2,400°F (1,090°C - 1,316°C) and thermal efficiencies around 30-35%. This high temperature exhaust is suitable for a variety of uses. The most efficient option is recuperators that pre-heat combustion air, which can save as much as 30% of energy consumption. Incoming material can also be pre-heated, which is similar to what stack smelters do. The exhaust is also suitable for generating electricity with a steam or organic rankine cycle, and generating cooling if air conditioning is required for much of the year at the facility.
Stack smelters have better seals than reverberatory furnaces and use the hot flue gases to preheat the metal charge, enabling efficiencies of 40-45%. As such, the equipment is already doing heat recovery. The temperature of exhaust gases leaving stack melters ranges from about 250 to 400ºF (120-204°C). This lower temperature limits significant heat recovery options.
For an interesting article comparing stack smelters to reverberatory furnaces, click this study. It discusses economics, casting quality, environment & safety.
Iron Casting
Melting furnaces in the Iron casting industry include induction furnaces, electric arc furnaces, and cupola furnaces.
Cupola melting furnaces may be operated with coke or natural gas, but coke is most often used in production cupola operations. Accordingly, the combustion exhaust from the furnaces is often filled with particulates which may pose special considerations for heat recovery equipment.
According to an analysis of cupola energy efficiencies by Kuttner, LLC of Port Washington, typical “low efficiency” cupolas lose about 50% of their heat in flue gases. However, newer, “high efficiency” cupolas incorporate a recuperative unit for preheating air, reducing stack losses to only 37%. The exhaust gas temperature from a cupola furnace can range from about 1,500 - 1,800°F (816 - 982°C), whereas the temperature leaving a recuperative unit is approximately 400°F (204°C).
Accordingly, older, less efficient cupola furnaces pose options for heat recovery by installing retrofit air preheaters with recuperators, essentially to mimic the newer ones.
Many foundries have already recognized the value of combusting the relatively cool (500°F 850°F) but dirty exhaust to generate a high temperature (1,600°F) gas stream and then use the heat to preheat cupola blast air or supplement heat for other operations. Once the useful heat
has been recovered, further non-beneficial cooling of the exhaust is performed before undesirable constituents are removed using a baghouse or other control technology. link
Electrical Induction Furnaces.
Electric Arc Furnaces often require water to maintain the coils at the appropriate operating temperatures. The water (sometimes water/gylcol mix) is heated in the process, absorbing 20-30% of the energy in the system to <200F (93C). This heat must be discharged by air coolers outside of the facility, which require electricity to operate. The hot water could be used to heat the building for a portion of the year. For example, a 500 kW induction furnace operating with 20% energy losses would generate 100 kW of heat. Typically, the only addition required at the facility is ductwork to redirect the warm air into the facility, or vent air to the outside during summer.
Steel Casting Heat Treating
The exhaust from steel casting heat treating processes is assumed to have the composition of typical combustion gas, but at a higher temperature than the exhaust from the aluminum stack melter. Steel casting heat treating operations typically operate at two temperature levels but at different times in the production cycle. The temperature levels in this case are 1,000°F and 1,750°F.
Further Reading
Combustion air preheating