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Researchers create system to convert waste to energy

  • Published
  • By Mindy Cooper
  • Air Force Research Laboratory
Researchers with the Air Force Research Laboratory's Materials and Manufacturing Directorate here are developing a transportable waste-to-energy system to produce electricity at forward military operating locations.

The Department of Energy's Federal Energy Management Program supports this initiative. Executive Order 13423, signed in January 2007, requires that federal agencies increase their domestic use of renewable energy and produce it in an environmentally and economically sound, sustainable manner. The order suggests implementation at facilities not within the United States.

According to Walt Waltz, the Robotic Group lead, the system would reduce the amount of fossil fuel consumed at each forward military operating location where it would be installed.

"Forward operating bases currently use MEP-12 generators to supply electricity," Mr. Waltz said. "The system could also be implemented at domestic military bases."

The project has two stages. Stage 1 is complete. AFRL technicians built a transportable furnace system that shreds and burns solid waste, such as wood, paper and plastic.  No additional fuels are required to sustain combustion. It also burns waste oils and contaminated JP-8 or diesel fuels. 

This full-scale prototype is mounted on a 48-foot flatbed semi-trailer. The system comprises a furnace which is 9 feet tall and 6 1/2 feet in diameter, a liquid fuel burner, an 8-foot-tall shredder, two hoppers to hold the shredded solid fuel and two air blowers.

"Before the furnace can burn solid waste, it is preheated with a Beckett fuel burner, using either diesel or JP-8 fuel at a rate of around five gallons per hour," Mr. Waltz said. "This type of burner is commonly used to heat buildings and for industrial processes. After 30 minutes of pre-heating, the burner is removed and its port is sealed. No more liquid fuel is required."

Bulky solid waste items like wood, pallets, paper, plastics and yard clippings are dropped into a hopper on the shredder. Material is shredded down to less than 1/2 inch in all three dimensions. The shredded waste material falls from the shredder into a box, and from there the material is vacuumed through a large flexible hose and into the system's second hopper. 

From the second hopper the material drops through a rotary feeder into a pipe with air blowing through it. The rotary feeder helps measure the amount and speed at which the shredded waste is pneumatically conveyed into the furnace. 

The shredded waste flows steadily from the pipe into the furnace and is now fuel. Because the furnace is pre-heated, this shredded fuel heats and bursts into flame.

Mr. Waltz said that the energy from the burning fuel maintains the furnace temperature, sustaining the combustion process for the additional shredded waste that continues to enter. The burning waste swirls through the furnace, and the exhaust rises through the exhaust stack. During testing, the transportable furnace system burned wood at a rate of 150 pounds per hour.

So far researchers have completed 15 burn tests. During these tests, the system operated continuously for up to six hours. No additional liquid fuel or gas was used to burn the solids. After initial adjustments, the system performed steadily and burned well. Stack emissions were clear with no visible smoke, and an inspection inside the furnace confirmed no residual soot.

The second stage of this project, designing the energy recovery component, is under way. With a series of heat exchangers, this component will generate steam using the heat released from the burning waste. 

This steam energy potentially can be used for heating, absorption air conditioning and/or electricity generation at the forward operating bases or rural domestic bases. 

The prototype system will generate electricity, functioning as a small power plant. When completed, the transportable waste-to-energy system will burn up to 500 pounds of waste per hour.

"The transportable waste-to-energy system will reduce costs and lessen the environmental impact associated with the generation of electricity by reducing the amount of fossil fuels required at military installations," Mr. Waltz said, "By consuming combustible waste, it will also reduce the need for outside contractors to dispose of waste created at those locations and help the Air Force comply with important environmental goals and standards."

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