The Covanta Hennepin facility operates two mass burn waterwall Municipal Solid Waste (MSW) combustion units, each capable of processing 606 tons per day (TPD) of MSW. The mass burn technology and W&E boiler grates allows for the immediate processing of MSW with no presorting of materials required.

• Delivery of MSW – Solid waste collection and transfer vehicles are weighed at the scale house, then enter the enclosed tipping area to discharge their load onto the tipping floor or directly into the 7,000-ton storage bunker. Unacceptable or non-processible wastes such as tires, appliances and items too large for the feed chutes are placed in roll-off containers for recycling or alternate disposal.
   
• The Combustion Process – One of two overhead cranes lifts the waste from the storage pit into the feed chute leading to the furnace. The crane operator mixes the MSW in order to provide a uniform fuel mixture. Dust and odors within the storage bunker and tipping hall area are controlled by drawing in combustion air from these areas into the boilers and utilizing automatic high speed entrance and exit doors to keep the areas under a slight negative pressure.
  
  Solid waste drops through feed chutes and is moved onto furnace grates by hydraulically operated ram feeders. The MSW moves down through the feed chute onto the horizontal moveable grates of the boiler. The high chrome/nickel, cast steel alloy grate bars transport the waste through a drying zone, an ignition zone, a combustion zone and into a post-combustion zone.
  
  A forced draft fan supplies the primary combustion air underneath the grate. In addition, secondary air is injected through the front and rear walls of the furnace to complete combustion and control emissions.
   
• Production of Steam/Electricity – Inside the steel tubes that form the furnace walls of the boiler, water is converted to steam from the heat transfer of the combustion process. The steam is superheated and directed to an extracting, condensing turbine-generator to produce electricity. Electricity produced flows to switchgear and on to an electrical transmission lines interconnected to the Xcel Energy distribution system.
   
• Ash Residue – Grate siftings, residue from the grates and ash from the boiler/economizer hoppers (termed bottom ash) are conveyed to the ash discharges which contain water to quench the hot ash. Cooled ash settles to the bottom of the discharger and is pushed by a hydraulic ram onto the main ash conveyor. The bottom ash is conveyed to a classification building where the ferrous and oversized materials are removed to a temporary storage bin.
  
  Fly ash, the particulates collected in the air pollution control system, is conveyed through an enclosed system to an ash conditioner. The fly is treated with Dolomitic Lime for pH control and combined with the remaining bottom ash in the classification building. The combined ash is conveyed to the ash storage building and loaded into trucks for delivery to a privately owned landfill with specially designed ash Monocells.
   
• Diagram of Process (Diagram)
   
• Pollution Control
   
  • Air emissions are controlled by high efficiency combustion within the furnace/boiler as well as by selective non-catalytic reduction (SNCR), spray dryer absorbers, fabric filter baghouses and an activated carbon injection system.
    • Highly efficient combustion controls potential organic pollutants as well as carbon monoxide.
    • The SNCR system injects ammonia into the furnace to control nitrogen oxide emissions;
    • Spray dryer absorbers utilize a lime/water slurry mixture to control the boiler outlet gases. The lime slurry mixture neutralizes acid gases, such as sulfur dioxide and hydrogen chloride and cools the outlet gases as well;
    • The fabric filter baghouse removes particulate matter (fly ash) and provides a secondary acid gas neutralization surface on the filtercake; and,
    • The activated carbon injection system controls mercury emissions.
       
  • Covanta Hennepin also utilizes state-of-the art continuous emissions monitoring systems (CEMS). The CEMS measures the stack gas for emissions, such as carbon monoxide, sulfur dioxide, nitrogen oxides, oxygen and opacity and allows the control room to continuously monitor the performance of each combustor unit. The CEMS data is reviewed and summarized into a report, which is routinely submitted to the Minnesota Pollution Control Agency and EPA for review.