Covanta Energy Corporation - Mass Burn Process

Mass Burn Process

Mass burn technology, the most common municipal solid waste-to-electricity technology, involves the combustion of unprocessed or minimally processed refuse. The major components of a mass burn facility include:

• Refuse receiving, handling, and storage systems
• The combustion and steam generation system (a boiler)
• A flue gas cleaning system
• The power generation equipment (steam turbine and generator)
• A condenser cooling water system
• A residue hauling and storage system
• Incoming trucks deposit the refuse into pits, where cranes then mix the refuse and remove any bulky or large non-combustible items (such as large appliances). The refuse storage area is maintained under pressure less than atmospheric in order to prevent odors from escaping. The cranes move the refuse to the combustor charging hopper to feed the boiler.

Heat from the combustion process is used to turn water into steam, with the steam then routed to a steam turbine-generator for power generation. The steam is then con-densed via traditional methods (such as wet cooling towers or once-through cooling) and routed back to the boiler. Residues produced include bottom ash (which falls to the bottom of the combustion chamber), fly ash (which exits the combustion chamber with the flue gas ), and residue (including fly ash) from the flue gas cleaning system.

The combined ash and air pollution control residue typically ranges from 20 percent to 25 percent by weight of the incoming refuse processed. This ash residue may or may not be considered a hazardous material, depending on the makeup of the municipal waste.

It may be possible to avoid the production of hazardous ash by preventing the sources which create hazardous waste from enter-ing the system. It is also possible to treat the ash. Both of these methods avoid the costs of disposal at a limited number of landfills classified as able to handle hazardous materials. Non-hazardous ash can be mixed with soils for use as land-fill cover, or can be sold (or given away) for such beneficial uses as pavement aggregate.