A recovery boiler is a power plant subsystem used to capture energy that would otherwise be lost in waste products that were not completely combusted. This energy is recycled to the primary energy generating system to produce more power. For this reason, this equipment may alternatively be referred to as a heat recovery boiler or as a waste heat recovery boiler. The main requirement for its use is that the system’s waste products have sufficient recoverable energy to drive the primary power generating equipment. Recovery boilers may be used in the power systems for a variety of industrial equipment.
Waste recovered for further combustion in a recovery boiler is usually a gas, a liquid, or a combination of the two. Processes generating these types of wastes include metal refineries, petrochemical processing plants, and other industrial plants operating at high temperatures. Solid material may also be used, though this is less common. The most notable example of solids used in recovery boilers is in papermaking.
Recovery boilers in pulp or paper mills most commonly extract additional energy from a recycled byproduct of the papermaking process known as black liquor. When breaking down wood to make pulp, a chemical compound called cellulose is separated out from other compounds for use in paper. Black liquor is the term for a combination of water and residue left behind from the pulping process. This residue includes not only wood byproducts, but also chemicals used in the extraction process. The mixture is typically concentrated in an evaporator and is then passed on to the recovery boiler for combustion.
Whether black liquor, exhaust gas, or some other waste product is recycled, the waste product is burned in the combustion chamber of a recovery boiler. Heat produced by this combustion is typically used to heat water to create steam. The steam drives equipment, such as a turbine, thereby transforming waste heat into mechanical energy. In turn, the turbine converts mechanical energy into electrical power, which can then be used to operate the facility.
By using a recovery boiler to recapture energy that would otherwise be wasted, an industrial facility can increase its energy efficiency. Fuel cost savings can be dramatic, particularly for systems with high-temperature exhaust. In such systems, as much as half the energy put into the system could be venting up the stack if not recycled. Another benefit of recovery boilers is that the reduction in fuel burned results in reduced greenhouse gas emissions and less impact on the environment.