NOVEMBER 2008

Air Force develops mobile waste-to-energy system
Combustible waste lowers fuel usage and costs

The Air Force’s energy system preheats a furnace with fossil fuels, but burns combustible waste once the preheating is through.
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Researchers with the United States Air Force are developing a transportable waste-to-energy system to produce electricity at forward military operations.

A prototype, mounted on a 48-foot flatbed semi-trailer, is being tested at Tyndall Air Force Base in Florida by the Air Force Research Laboratory Materials and Manufacturing Directorate, which conducts programs that enhance readiness, deployment, fire protection, peacetime training, and crash and rescue operations.

The transportable waste-to-energy system could reduce the amount of fossil fuel used at each forward military operation and could also be used at domestic bases, according to Walt Waltz, the leader of the robotics group at the directorate.

By consuming combustible waste the system “will reduce the need for outside contractors to dispose of waste created at deployed military locations,” Waltz says.

Research on the transportable waste-to-energy started in 2004. The system consists of two stages, a furnace and an energy recovery unit. Researchers have completed the first stage, referred to as a transportable furnace system. The system includes a 9 foot tall furnace and an 8 foot tall shredder. The flatbed semi-trailer for the system also holds two hoppers for the shredded material and two air blowers.

The furnace needs to be preheated with a fuel burner. After 30 minutes the burner is removed. No additional liquid fuel is needed besides for preheating the furnace.

Solid waste items are first dropped into a hopper on the shredder. The solid waste material then is shredded down to less than a half of an inch in all three dimensions.

The material falls from the shredder into a box, where it is vacuumed through a hose into the system’s second hopper. The waste material next drops through a rotary feeder into a pipe with air blowing through it and flows into the furnace and is turned into fuel. During testing, the prototype has operated continuously for up to six hours.

Researchers at the directorate are now working on the second stage of the transportable waste-to-energy system, known as the energy recovery component. This component will generate steam using the heat released from the burning waste.

The Air Force intends for the steam to eventually be used for heating, air conditioning and electricity generation at forward operating bases or rural domestic bases. The system is designed to burn up to 500 pounds of waste per hour.

Consultant Nick Patz, president of Ceres Associates in Benicia, California, expects to see more transportable waste-to-energy systems in the next few years.

Patz is working with Balboa Pacific Corp., in Del Mar, California, on a portable system that was originally designed for the hazardous and medical waste markets.

“As tipping fees and energy costs have increased over the past several years the system is now profitable for municipal solid waste,” Patz says, adding that the portable method would also work well for cleaning up after natural or manmade disasters.

The transportable unit designed by Balboa Pacific, which fits on two flat-bed trailers, has the potential to consume approximately 50 tons of solid waste per day.

Patz says it is possible to set up the waste-destruction technology in as little as eight hours. The system can be set up and start producing energy within 24 hours.

Transportable systems to incinerate hazardous waste are more common than waste-to-energy systems, says Tom Brown, project manager at the Kansas City, Missouri-based environmental consulting firm Burns & McDonnell Engineering Co., Inc. “Waste-to-energy was generally viewed as an unnecessary complication,” he says.

But with the rising cost of energy and a growing focus on renewable energy, this is changing. “Transportable waste-to-energy offers the benefits of waste-to-energy with the added benefit that the facility is not fixed and can therefore travel,” Brown says.

A military unit using a transportable waste-to-energy system, for example, could carry less conventional fuel for electricity generations in remote locations. The unit could instead carry additional quantities of other types of critical supplies, Brown says.

There are disadvantages to portable generation, however. Generation temperatures and pressures are limited with transportable waste-to-energy systems, Brown says, resulting in low efficiency in conversion to electricity and low energy outputs.

“In general, small-scale, waste-to-energy systems that generate electricity are not cost effective when compared to other methods of waste disposal or with other methods of electric energy generation,” Brown says, noting that emerging technologies, such as plasma arch, pyrolysis or anaerobic digestion may reverse the economic equations.