Waste is delivered
Garbage trucks deliver waste from Minneapolis and surrounding communities to HERC. About 75 percent of the waste delivered to HERC comes from Minneapolis; the remaining 25 percent comes from suburban Hennepin County.
Waste is burned
The waste is pushed into the waste pit with a loader. A crane picks the waste up from the pit and feeds it to the boilers. Operators look for and pull out hazardous and problematic wastes, such as appliances, televisions and bulky items, so they can be disposed of properly.
Heat generates steam
The waste is burned in boilers lined with water-filled tubes. The heat of combustion converts the water in the tubes to steam that turns a turbine to generate electricity. HERC produces enough electricity to power 25,000 homes. The electricity is sold to Xcel Energy.
A portion of the steam produced is extracted after going through the second stage of the turbine and sent to the steam line. This steam provides heating and cooling to the downtown Minneapolis district energy system and Target Field. District energy is a network of pipes that aggregates the heating and cooling needs for 100 downtown buildings. District energy systems are more efficient than buildings operating their own boilers and chillers.
The steam is then condensed back to water and circulated back to the boiler, completing a closed-loop.
Controlling air emissions, recycling metals and disposing of ash
Air emissions are cleaned and treated so that emissions are consistently below the Minnesota Pollution Control Agency permitted levels.
The combustion process reduces the volume of waste by 90 percent. The material remaining after combustion is non-hazardous ash that is disposed of in a landfill.
Metals are removed and recycled before the ash is sent to a landfill. HERC recovers more than 11,000 tons of scrap metal each year, which is more than double the amount of metal collected in curbside recycling programs in the county.
See a diagram of how HERC converts waste to energy (JPG).