Separation Anxiety

Utilization of municipal solid waste sorting technologies could speed up the development of anaerobic digestion in the U.S.
By Lisa Gibson | September 20, 2011

Northern Spain’s 321,000-person province of La Rioja produces 2.13 megawatts of electricity for the grid, using the organic portion of its own municipal solid waste (MSW). The operation has been a point of pride for the region for years, but recently increased its efficiency with the installation of a separation technology that decreases the amount of organics lost, while increasing the amount of inerts ejected from the process.

The anaerobic digestor at the MSW facility processes about 130,000 metric tons (143,300 tons) per year and is one of nine biomethanization operations in Spain. After trying a number of sorting technologies, the plant became the premier organic separation installation for an X-ray-based sorting technology previously known only for its work in other sectors.

“We’ve now entered this organics segment,” says Alexander Wolf, sales engineer for TITECH Group, a global developer of the X-ray sorter, dubbed TITECH x-tract, along with other sorting technologies. Sufi S.A., which owns and operates the La Rioja anaerobic digester, needed a better solution for separating organics at the facility, so TITECH stepped up to the plate with its existing x-tract technology, Wolf explains.

While Spain’s developing waste-to-energy market has stalled, the U.S. still lags behind it and other European markets, attributed in part to a lack of source-separated waste. Although separation is still required in Spanish facilities, recycling sorting in Spain seems to have set the stage for its organic waste-to-energy development.

Source Separated Superiority

“There’s a lot of MSW processing going on for many years in Spain,” Wolf says. When the country joined the European Union, it had to work to achieve recycling goals set for EU member countries, he explains. So Spain implemented a source collection system for recyclables, he continues.

Unfortunately, the country still couldn’t meet those goals so it had to recover recyclables out of MSW, which then put it far above the EU regulations. “They basically sorted their own residential waste and that has a lot of organics in it, so they said, ‘OK, we’ve got to do something with these organics.’” One of the solutions was wet anaerobic digestion (AD), which made the country a perfect market for TITECH’s sorting technologies. In fact, TITECH has spread its target market to areas within Portugal as well, with three projects in the pipeline waiting for commissioning, according to Judit Jansana, head of TITECH technical sales in Spain, Portugal and Latin America. 

Still, Jansana says the Spanish organics market has experienced problems at existing facilities, leaving it behind Germany’s, due partially to much more sophisticated source separation. “The Spanish market is not so [advanced] in technology as the German country,” she says. “Their recycling quota is higher than the Spanish quota.”

Subsequently, Germany has a better market and, in fact, is a primary target for BIOFerm Energy Systems’ dry anaerobic fermentation digestion technology, according to Caroline Chappell, BIOFerm application engineer. The system uses concrete chambers to digest a more solid and dry feedstock than traditional AD can handle. Germany-based BIOFerm has a few applications in Germany using the organic portions of MSW, all source separated and delivered ready to process.

Anaerobic digestion is much more attractive to customers who don’t have to worry about separating out the organic portions of waste before feeding it into the digesters, Chappell says, adding that BIOFerm is working to install systems in North America, the U.K., Italy and Japan, as well. “We know it’s not going to be perfect, but it’s definitely better if it’s separated to the best extent possible,” she says. The first U.S. application of the system is set to begin operating this fall at the University of Wisconsin Oshkosh, running on a mix of feedstock that will include unsalable food products from supermarkets, delivered unpackaged and ready for the digesters.

But when using organics from MSW, that ideal AD feedstock suitability is uncommon in the U.S., both Wolf and Chappell agree. San Fransisco is the lone U.S. city they cite where waste is source separated. “The challenge is, when you look at the U.S., there are very [few] areas that have source-separated organics,” Wolf says. “Everything that doesn’t go into the recycling bin goes into the garbage bin, and so do the organics.” The organic content in residential waste is generally around 30 to 40 percent, but can be as high as 50 percent, he adds. “The way to recover organics from residential garbage sounds quite simple: you basically screen that out.”

Traditional screening, however, can leave behind objects such as glass, batteries and plastic in the concentrated organics. “There are a lot of foreign materials that, of course, are not desirable with the organics,” Wolf says.

But not to worry: The U.S. can still develop an organic-waste-to-energy sector sans source separation, using instead a plethora of more advanced technologies than screening that allow a more refined separation from the remaining fractions of MSW.

Separation Technologies

Without the proper extraction of inerts from the organic AD feedstock, glass and other intruders can bake together and form a concrete-like material that requires frequent cleaning, rapidly decreasing efficiency, Wolf explains. X-tract, however, uses a sensor to measure silicone density, which is not present in organic material. “That is how we can recognize inerts,” he says. “They have higher density.”

The material is fed into the system on a high-speed vibrating conveyor, and inorganic materials the system detects are ejected to another conveyor through air jets. “The ejects still have organics in it because it’s not possible to singulate all the organics on the x-tract conveyor,” Wolf says. “It’s all mingled together and organics are very stringy so you can never make a perfect singulation on it. There will always be some organic losses, but you do recover a good portion of those organics.”

Before Sufi S.A. adopted the x-tract system, it used traditional ballistic separation, which takes advantage of both density and elasticity differences to separate organics from waste streams. Material is dropped on a rotating drum or spinning cone and the resulting trajectory differences bounce out glass, metal and stones. “It’s not very efficient, but it’s what we did in the plant before x-ray technology,” Jansana says.

But of the 3,000 TITECH units installed worldwide, 2,500 are not x-tract, but instead TITECH autosort, Jansana says. The technology uses a near-infrared spectroscopy detector to remove polymers, as well as ferrous and nonferrous metals. “Our main, main, main market is the near-infrared,” she says. The system is not just used to separate organics, but also paper and cardboard in other industries.

Another possible route is air separation, which has been used in the combustion industry for years. Material is inserted into an air column that blows light materials up and out, while heavier ones fall directly out of the column. Air separation will likely require more sorting in order to achieve the proper organic concentration.

Green Power Technologies Inc. has combined a few different options into its separation system, which is more suitable for autoclave processing than for AD, but could be used for both, according to Steve Gilchrist, vice president of the company’s U.S. branch. The use of autoclaves on organic MSW to produce a dryer, densified biomass energy product is common in Britain, he says, but Green Power has reversed and improved the traditional process. “We’ve taken an opposite strategy,” he says.

In Britain, the material goes through an autoclave first, where injection of high pressure and high temperature steam sterilizes it and also shrinks the volume by about 60 percent. Next, the plastics, metals and glass are separated from the organics. That separation is made more difficult, though, by the fact that the autoclave deforms plastics, trapping some organic materials within them.

So, Green Power’s process first shreds all the material, thereby increasing the efficiency of the magnets in the next step that extract ferrous metals. Then, eddy current separation, an electromagnetic process, goes to work removing nonferrous metals, followed by lasers that work similarly to the X-ray technology, detecting density to extract plastics. “It not only pulls out the plastic,” Gilchrist explains. “It separates type 1 and type 2 plastics from all the rest, which significantly increases the profitability.” Glass can be removed manually or through an automated system, he continues, leaving only organics behind. If the organic material is destined for an autoclave, this is the point it will enter. Afterwards, the material passes through yet another magnet to remove small metal pieces, such as staples, Gilchrist says.

Green Power is focused on autoclaving uses for its technology, but the organic material could be a great candidate for AD, as well. “It’s a function of time,” Gilchrist says. “An anaerobic digester takes a lot longer than autoclaving, and so depending on the volume you’re dealing with, you might have a throughput issue.”

Organic autoclaving results in a product perfectly suited for cofiring with coal, representing an enormous market in the U.S. for Green Power’s specific organic separation technology. “There’s a tremendous potential in the U.S.,” Gilchrist says, citing the U.S. EPA’s new emissions regulations and the cessation in production of thousands of megawatts at shuttered coal plants.  

Green Power will have its first autoclave facility running in Hamtramck, Mich., in late 2012 or early 2013. The company has financing in place, as well as waste supply contracts with Detroit suburbs. 
Available Everywhere

Whether using an anaerobic digester to produce biogas, or using an autoclave to manufacture a densified biomass fuel, organic MSW represents an always-available and viable source. It presents otherwise nonexistent opportunities in areas that don’t have a steady, sustainable supply of woody biomass or other renewables. 

“Every state has municipal solid waste,” Gilchrist says. “The only thing I can guarantee you’ll find in any market that has a demand for electricity is people producing solid waste. It is the only potential form of biomass where you can have an almost perfect correlation between population, availability of the fuel and the demand for the energy.”

With innovative technologies in place to help advance the separation of organics from that ever-ready waste stream, the U.S. and the rest of the world can create a solid power-producing sector from its own trash.

Author: Lisa Gibson
Associate Editor, Biomass Power & Thermal
(701) 738-4952