Industrial Waste-Heat Recovery: Benefits and Recent Advancements in Technology and Applications (2007)

A substantial amount of energy used by industry is wasted as heat in the form of exhaust gases, air streams, and liquids leaving industrial facilities. Although it is not technically and economically feasible to recover all waste heat, a gross estimate is that waste-heat recovery could substitute for 9% of total energy used by US industry—or 1.4 quadrillion BTU—which would ultimately help improve the global competitiveness of the US (Energetics and E3M 2004). An increased use of waste-heat recovery technologies by industry would also serve to mitigate greenhouse gas (GHG) emissions. The primary sources of waste heat in industrial facilities include exhaust gases from fossil fuel-fired furnaces, boilers, and process heating equipment. These types of high-grade waste-heat sources can readily be used to preheat combustion air, boiler feedwater, and process loads. Waste-heat recovery from lower temperature sources, such as cooling water from machines and condensers, is generally somewhat more problematic, and typically involves the use of heat pumps to increase the temperature to a suitable temperature for distillation, evaporation, water heating, and space heating. This paper summarizes the results of numerous studies conducted by the authors and/or their associates to identify opportunities for waste-heat recovery in industrial facilities. It also describes recent advancements and applications in waste-heat recovery technology. Typical “energy audits” identify annual energy cost savings of about 5%. However, this paper confirms that systematic waste-heat recovery projects based on sound thermodynamic principles can yield annual energy cost savings of 10% to 20% with paybacks of 6 to 18 months for industrial facilities. Recent advancements in heat recovery technology may increase the energy savings by an additional 5% to 10%. Since only 5% of US manufacturing facilities currently use waste-heat recovery, there is tremendous potential for energy savings in the industrial sector (EIA 2002, Table 8.2). Definition of Waste Heat for This Paper Waste heat is the energy associated with waste streams of air, exhaust gases, and/or liquids that leave the boundaries of an industrial facility and enter the environment. In the definition of waste heat, it is implicit that the waste streams eventually mix with atmospheric air or groundwater and that the energy contained within them becomes unavailable as useful energy. The absorption of waste energy by the environment is often termed thermal pollution. In a more restricted definition, waste heat is the energy that is rejected from a process at a temperature high enough to permit the recovery of some fraction of the energy for useful purposes in an economic manner. Usually the energy being transferred is the sensible energy (or internal thermal energy) of the fluid, but it can also include the transfer of the latent energy of the fluid. Latent heatexchange is typically associated with a phase change between the vapor and liquid states of the fluid, such as condensation and boiling. For example, the recovery of waste heat from hot and moist air used in the lumber drying process involves the recovery of both sensible and latent