Waste-to-Energy: How It Works
Monitor and Control
The air stream rising to the stack is continuously monitored to ensure compliance with air quality standards. The entire process can be controlled to optimize efficiency in the combustion, heat and steam generation, electrical energy, and environmental control processes.
Waste material is received in an enclosed receiving area, where it is more thoroughly mixed in preparation for combustion. Negative airflow will carry dust and odor into the combustion chamber from the receiving area, along with the waste to eliminate its spread outside the facility.
Mixed waste enters the combustion chamber on a timed moving grate, which turns it over repeatedly to keep it exposed and burning—the way turning over or poking a fireplace log brightens the fire. A measured injection of oxygen and fumes drawn from the receiving area makes for a more complete burn.
Fly Ash Capture
Although fly ash is captured throughout the process, the finest airborne particulates are removed in the filter baghouse, where an induction fan draws air through fabric bags toward the stack or chimney. This process removes 96 percent of any remaining particulates. The bags are vibrated at intervals to shake loose particulates caked on their inner and outer surfaces. Captured fly ash is often returned to landfills.
Acid Gas Treatment
The acidic combustion gasses are neutralized with an injection of lime or sodium hydroxide. The chemical reaction produces gypsum. This process removes 94 percent of the hydrochloric acid.
Bottom Ash Recycling
The unburned remains of combustion—"bottom ash"—are passed by magnets and eddy current separators to remove both ferrous (steel and iron) and other metals—such as copper, brass, nickel, and aluminum—for recycling. The remaining ash can be used as aggregate for roadbeds and rail embankments. Ash is generated at a ratio of about 10 percent of the waste’s original volume and 30 percent of the waste’s original weight.
Steam Power Generation
Highly efficient superheated steam powers the steam turbine generator. The cooling steam is cycled back into water through the condensor or diverted as a heat source for buildings or desalinization plants. Cooled stream is reheated in the economizer and superheater to complete the steam cycle.
Mercury and Heavy Metal Capture
Activated carbon (charcoal treated with oxygen to increase its porosity) is injected into the hot gases to absorb and remove heavy metals, such as mercury and cadmium.
NOX Treatment Dioxins/Furans Treatment
Nitrogen oxide in the rising burn gases is neutralized by the injection of ammonia or urea. Dioxins and furans are destroyed by exposing flue gases to a sustained temperature of 1,562°F/850°C for two seconds. This process removes more than 99 percent of dioxins and furans.
Electric Power and Heat
A 1,000 ton-per-day WTE plant produces enough electricity for 15,000 households. Each ton of waste can power a household for a month. If combined with a cogeneration plant design, WTE plants can, while producing electricity, also supply heat for nearby businesses, desalination plants and other purposes.