Industry attributes
Other attributes
Waste heat recovery is the process of reusing heat energy that would otherwise be disposed of or released into the environment. Waste heat recovery systems or units are the devices used to recover this energy for future use. These systems can be used for both industrial and domestic applications. Waste heat that is generated and not put to practical use is lost to the environment. Various waste heat recovery technologies can be implemented to recover this energy, providing a valuable additional energy source and reducing energy consumption, energy costs, and CO2 emissions while simultaneously increasing energy efficiency.
In theory, any industrial process that produces waste heat can benefit from a waste heat recovery system. The following are common sources of waste heat that can be readily recovered and reused:
- High-temperature exhaust gas streams from furnaces, kilns, ovens, and dryers
- Boiler flue gases
- Boiler blowdown
- Air compressors
- Refrigeration plant
- Hot liquid effluents
- Power generation plant
- Process plant cooling systems
- Turbocharged cars
Waste heat recovery methods include capturing and transferring the waste heat from a process with a gas or liquid back to the system as an additional energy source that can be used to create additional heat or to generate electrical and mechanical power. Waste heat can be rejected at any temperature; however, the higher the temperature, the more efficient the waste heat recovery process becomes. The quantity or amount of available waste heat can be calculated using the equation shown below:
Where:
- is the heat content (J)
- is the flow rate of the substance (m3/s),
- is the density of the flue gas (kg/m3),
- is the specific heat of the substance (J/kg*K) and
- is the difference in substance temperature (K) between the final highest temperature in the outlet (Tout) and the initial temperature in the inlet (Tin) of the system.
High-temperature exhaust gas is passed through a heat recovery device, with the recovered heat in the form of steam, hot water, or thermal fluid being returned to the process. Examples include a firetube steam boiler or a helical tube fluid heater.
A common way of recovering energy is preheating fresh air using exhaust gas or discharging air from the process. This can take the form of an air to air exchanger downstream of a primary heat recovery device as described above (Air/Fluid), or as a way of recovering relatively low-grade heat from exhaust gas. In either case, exhaust gas from an industrial process is routed through a heat recovery device where the heat energy is extracted before the cooled air is passed to the atmosphere. At the same time, clean, fresh air is drawn in on the other side of the exchanger, picking up the extracted heat. The pre-warmed air can be fed back into the process—meaning less heat is used. The two air streams need not mix directly to allow the transfer of heat. There are a variety of methods for doing this, including sophisticated heat exchangers that can capture up to 95% of the waste heat.
- Regenerative and recuperative burners—optimize energy efficiency by using heat exchanger surfaces to capture and use waste heat from hot flue gas
- Economizers—finned tube heat exchangers capable of recovering low/medium waste heat
- Waste heat boilers—the use of water tubes placed in parallel to each other and in the direction of the heat leaving the system
- Air preheaters—primarily used for exhaust to air heat recovery during low or medium temperature applications. Can be based on two designs plate type or heat pipe.
- Plate heat exchanger—transfers heat between fluids while avoiding cross-contamination by using several thin metal plates stacked or brazed in parallel forming a hollow metallic shell
- Heat pipe systems—transfer heat using the condensation and vaporization of a working fluid such as water, acetone, methanol, ammonia, or sodium in equilibrium with its own vapour
- Heat recovery steam generator (HRSG)—a complex system consisting of an evaporator, superheater, economizer, and steam drum to recover heat from the exhaust of a power generation plant
- Heat pumps—transfer heat from the source to a heat sink
- Direct electrical conversion devices—electricity is produced directly from waste heat using technologies such as thermoelectric, piezoelectric, thermionic, and thermophotovoltaic devices
