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A waste heat recovery boiler recycles energy from a process to create steam, reducing fossil fuel consumption and greenhouse gas emissions. There are two types: fire-tube and water-tube boilers. Water-tube boilers can handle higher pressures and respond quickly to heat input. Waste heat boilers can be used in cogeneration plants to improve efficiency.
A heat recovery boiler takes the heat formed as a by-product of another process, heat that would normally be wasted, and uses it to create steam. The steam can be used to drive turbines that produce electricity. Alternatively, the boiler can simply be used to heat water or other types of fluid. Because it recycles some of the energy it uses, a waste heat boiler or waste heat recovery boiler can reduce the fossil fuel consumption and financial running costs of a system. This also means that fewer greenhouse gases are released into the atmosphere.
Waste heat boiler design includes two main types: fire-tube boilers or shell-and-tube boilers and water-tube boilers. In fire-tube boilers, a steel casing encloses a water-filled space with metal tubes inside. Hot gases produced by a combustion process, such as inside a furnace, pass back and forth through the pipes transferring heat to the surrounding water.
Fire tube heat recovery boilers have the advantage of being relatively simple to build, install and maintain. The thermal energy stored in the water can be used to meet short-term extra demand, although using all the heat has the disadvantage that it will take a long time to replenish. Another limitation of this type of equipment is that it cannot operate at the higher pressures of a water-tube boiler.
A heat recovery boiler of the water-tube design is capable of coping with much higher steam pressures than a fire-tube boiler, but is more difficult to build and install. Inside this type of boiler, there are narrower tubes than inside a fire tube boiler and the tubes contain water instead of hot gases. In a system inversion within a fire-tube boiler, waste heat, in the form of hot gases or furnace flames, surrounds the water-filled tubes. Insulating materials are used to protect boiler tubes from flame damage. In addition to tolerating high pressures, a water-tube waste heat boiler can respond quickly to changes in heat input.
Waste heat boilers can be used in what are known as combined heat and power plants or cogeneration plants. These are power plants where the heat that is normally created as a byproduct of electricity generation is used, improving efficiency from about 40 percent to about 70 percent. Maximum efficiency is achieved when the heat is used on or in close proximity to the plant site.
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