Battle of the bulk

Alternative energy is a concept tossed around very regularly these days. With so much negative focus on fossil fuel-derived power, attention is very often directed to wind or solar resources as a means of generating a sustainable and renewable energy supply.

Clearly, no future energy strategy can do without these important options; but another power source is often overlooked, and somehow seems to continually fly under the general public’s radar: energy derived from waste (EFW).

Even with a highly efficient diversion of recyclable materials from our waste stream, the U.S. still produces mountains of garbage, and that garbage has a significant amount of potential energy trapped in it.

Detroit Renewable Power (DRP), a subsidiary of the Detroit Renewable Energy consortium, is one of the hundreds of facilities around the country that are transforming our trash into power every day.

“Our EFW facility processes up to 3,300 tons of municipal solid waste (MSW) every day,” explains John O’Sullivan, president of DRP, based in Detroit. “That trash becomes a refused-derived fuel (RDF) which is burned to create more than 700,000 pounds of steam every hour.”

A portion of that steam is then directed through the facility’s turbines, generating nearly 70 megawatts (MW) of electricity; the remaining steam is fed into the city’s steam loop, helping to heat and cool more than 140 buildings in the downtown and midtown Detroit areas.
 

An efficient process

Producing fuel from refuse is quite an exacting process. At DRP, all the source material is brought in from direct-haul trucks and dumped on-site. Loader operators then perform a quick visual inspection and remove any oversized bulky waste that could create problems for the primary shredders.

The waste stream then passes across a picking station where grapples are used to remove additional material identified to be too large for the reduction process. The feed line is then directed to one of three flail mill shredders for primary reduction, passed over a ferrous magnet to remove metals, processed to a 10-inch minus specification in a secondary shredder and compacted for storage in a RDF barn.

The compressed fuel is then routed to one of three feed bins that directly supply the boiler. After burning, the remaining ash is passed over another ferrous magnet, through an eddy current for nonferrous metal removal, and finally moved on to the landfill to be buried. In the end, the landfilled ash represents only about 2 to 3 percent of the original volume of material that was taken in—a highly efficient process for waste processing.
 

Battle of the bulk

In 2012, O’Sullivan recognized an opportunity to recover still more energy from DRP’s incoming waste streams and turned to equipment manufacturer Doppstadt US, Avon, Ohio, for a solution to this challenge.

“During our fuel production process, we divert roughly 1 percent of our incoming stream as oversize bulky waste,” says O’Sullivan. “Because these items are too difficult for our primary shredders to reduce, we were simply removing them from our feed lines and sending them to the landfill.”

For many operations, fussing over 1 percent may seem rather insignificant, but at 3,300 tons per day, that 1 percent can add up to a meaningful amount of additional fuel, not to mention the landfill costs recovered by not having to dispose of that extra leftover volume.

“We had been looking for an OBW (oversized bulky waste) machine for quite some time, and considered a number of options,” O’Sullivan says. “We brought in a DW 3080K slow-speed shredder on lease for about a year until we realized we were clearly benefitting by having that machine integrated into our processes, so this year we purchased it.”

The DW-series slow-speed shredder is a single-shaft, tooth-and-comb design which is engineered to reduce bulky or contaminated infeed to a 6-inch to 10-inch product. Doppstadt US says the slow-speed, high-torque function helps reduce wear costs and fuel requirements. All DW-series shredders come standard equipped with nitrogen accumulators which hydraulically activate the breakaway comb to allow harmless passage of tramp metal and other unshreddable material without causing damage to the machine, the company adds. It is precisely this type of material that can cause problems for DRP’s inline primary shredders, which makes the addition of the DW 3080K a true value-added process.
 

Detail oriented

Now, DRP takes all of the OBW that is diverted from both the initial tipping floor visual separation and the grapple-equipped picking station, and stockpiles it for independent reduction through its DW 3080K shredder. Any material that fails to process in the Doppstadt shredder does then get diverted to the landfill, but O’Sullivan estimates that about 90 percent of the OBW material gets processed and rerouted back into the fuel stream. “It may be a small portion of our overall volume, but we are able to recover a significant amount of that portion, and economically it easily makes the effort justifiable,” he says.

Over the course of a 365-day year, that recovered OBW represents more than three full days of fuel for the burn plant, which translates into more power to sell, creating additional revenue from an expense that was previously on the opposite side of the balance sheet. “We’ve not only eliminated most of the expense of transporting and landfilling that diverted material, we’ve managed to generate additional income from it as well. The Doppstadt has truly provided us with a double cost benefit.”

DRP has proven that obsessing over the details is what separates a good operation from a great one. Running a highly efficient process requires continuous evaluation and a constant drive to find new opportunities for fine-tuning the system.

By identifying the potential to further recover value from a discarded 1 percent of its incoming material stream, DRP has eliminated a cost and added new revenue to the process.

The article was submitted by Doppstadt US, based in Avon, Ohio. More information is available at www.doppstadtus.com.