”Waste heat recovery” is the process of “heat integration”, that is, reusing heat energy that would otherwise be disposed of or simply released into the atmosphere. By recovering waste heat, plants can reduce energy costs andCO2 emissions, while simultaneously increasing energy efficiency.
Several common consumer items recover waste heat. For example, consider turbocharged cars, which are provided by multiple car manufacturers.
In regular, non-turbocharged cars, the internal combustion gasoline engine expels hot gas through the car’s exhaust after its fuel is burned. That gas contains both heat and kinetic energy—a portion of which can be recovered. Turbocharged engines divert the hot gas to a turbine, which is used to spin an air compressor. The compressed air is routed to the engine’s combustion chamber with the vaporized gasoline, resulting in a more efficient ignition and greater power made with lower fuel consumption.
The energy benefits of industrial waste heat recovery can be similar, and some examples are examined in this article.
Stack economizers, commonly used to heat water, are among the simplest type of waste heat recovery. Plant workers may be familiar with these common devices that recover waste heat. Boiler stack economizers use heat energy from the gas expelled in the heating process into the stack to heat boiler feed water and reduce the amount of energy required to make steam. According to the US Department of Energy, installation of a boiler feed water economizer can raise the thermal efficiency of a boiler and reduce fuel consumption by 5 - 10%.
Using a principle similar to economizers, waste heat boilers recover heat generated in furnaces or exothermic chemical reactions at industrial plants. These locations may contain significant energy that should not be wasted up a stack. Instead, this energy can be captured to generate low-to-medium pressure steam in a waste heat boiler (WHB). A WHB can also be used to remove the heat from a process fluid that needs to be cooled for either transport or storage, and generate steam from that heat. The steam generated in WHB may be used for heating applications, or to drive turbines that generate electricity, compress vapors, or pump liquids. WHB steam may contain significant wetness, so it is recommended that a high efficiency separator and steam trap combination is installed to ensure that the WHB delivers optimal quality steam to the recipient process.
Many highly efficient industrial plants with cogeneration or combined cycle systems use a gas-turbine (essentially a jet engine) to generate electricity then create steam from the waste heat using a heat recovery steam generator (HRSG). This section will explain how this process works and how an HRSG comes into action.
Consider the previous example of a turbocharged car engine, but instead change the motor to a jet engine. The gas-turbine/jet engine is fired using natural gas and its exhaust contains extremely hot vapor that would simply be expelled to atmosphere if some of the heat and kinetic energy were not captured. So, how can that waste heat be used as was done with the turbocharged auto? The expelled hot gas needs to drive another turbine, so the exhaust is passed through a HRSG, which creates superheated steam that drives a downstream steam-turbine. The turbine can either drive a generator (combined cycle system) or just use the steam in process applications (cogeneration or combined heat and power (CHP)). HRSGs can have either a single steam drum (as shown in the animation below) or multiple steam drums and pressures. There are also both unfired varieties with natural circulation (shown below) and varieties with duct firing, which is additional heating. Duct firing increases steam generation and quality, and has the ability to create superheated steam and even greater power at a turbine.
Some energy efficient CHP systems may add refrigeration by incorporating absorption chillers that use steam created from waste heat.
The mechanism of absorption chillers can be broken down into the following stages: