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Trash-to-treasure: Turning nonrecycled waste into low-carbon fuel

Creating fuels out of trash could significantly reduce the amount of trash that goes into landfills and could offset fossil fuel use.


© Clark

Pre-consumer industrial recycling streams at an SRF facility. 


John R. Fyffe

Some materials recovery facilities use automated sorting systems that use a variety of mechanical, optical and magnetic sorting techniques.  


Texas Disposal Systems

Paper and cardboard commodity streams are part of the output of a materials recovery facility.


John R. Fyffe

Americans produce more than four pounds of trash per person per day, amounting to 20 percent of the world’s waste. Although recycling rates have increased over the past few decades — out of the 4.4 pounds of trash (per capita) that we produce in the U.S. each day, we compost or recycle about 1.5 pounds and incinerate another 0.5 pounds — more than 50 percent of our waste still ends up buried in landfills. Now, through the use of a novel energy recovery technique, we could reduce the amount that is sent to landfills by about 10 percent and produce a fuel that is relatively clean and more energy dense than coal. Materials that were considered garbage for generations are now being recognized for what they still offer after their useful life — valuable energy resources capable of solving multiple problems at once. Trash can become treasure.

Producing energy from trash is known as a “waste-to-energy” option. Several such options have existed for many years and are in extensive use throughout Europe and limited use in the United States. One of the more exciting options that has been proposed within the last decade is to convert waste into solid recovered fuels, or SRFs. SRFs are engineered blends of nonrecycled waste condensed into fuel pellets or cubes.

This opportunity is particularly appealing for plastics that are hard to recycle, decompose slowly in landfills, and have higher energy density than coal — baby diapers, for example. Although diapers serve an important purpose during the normal product cycle, once they have been used they are too complex to economically recycle, and as a consequence they are typically discarded, where they remain for what we expect will be thousands of years. Although they are almost certain to be an interesting archaeological find many centuries from now, today they could make a great fuel. That invites the broader question of how many other nonrecycled plastics could be turned into fuels instead of wasted in landfills.

It’s a question our team at the University of Texas at Austin decided to examine. Wanting to see the real-world capabilities of SRFs, we worked with partners in the plastics, recycling, pelletizing and cement industries to conduct an experiment in which nonrecycled waste was processed into SRF pellets. This SRF was then burned in a cement kiln, a process that normally uses a lot of coal and other fossil fuels; the SRFs replaced a fraction of these fuels. The experiment’s results demonstrate the potential for SRFs to displace fossil fuels for energy, but they also reveal some initial hurdles that must be overcome before SRFs can become a large-scale reality.

The Hierarchy of Trash

The U.S. Environmental Protection Agency has created a waste management hierarchy to highlight the ideal method of dealing with solid waste. The old adage of “reduce, reuse, recycle” still holds true as the most preferred path to manage our waste, but not all recyclables are clean enough or well-enough separated to be economically recovered.

For example, many regions in the U.S. have started switching to single-stream recycling where all recyclable materials are mixed in a single bin at home. This trend made it easier for homeowners and businesses to recycle their waste. The commingled recyclables are taken to a materials recovery facility where the commodities are separated and sold to recycling facilities or sent to landfills. Although single-stream recycling has contributed to the increase in overall recycling rates in the U.S., it unfortunately makes it easier for paper or plastics to become contaminated by other materials like food and liquids in the recycling stream, making them unfit for sale to recyclers.

Because of contamination and imperfect sorting, between 5 and 25 percent of a materials recovery facility’s incoming recyclables are discarded and sent to landfills. The waste, called “residue” in the waste management business, is a valuable mixture of paper and plastics that is currently lost to the ground. And this is where SRFs come in, taking advantage of an energy-dense waste stream that can be recovered to provide economic, environmental and resource conservation benefits.

Our Waste-to-Energy Options

Currently, there are several viable waste-to-energy options, ranging from low-tech to modern.

Mass incineration — the simple process of burning trash to make heat or electricity — has been used for decades. Furthermore, this technology has made progress recently to alleviate many of the environmental concerns over the emissions from burning trash. Unfortunately, incineration is not always complementary to recycling because they sometimes draw on some of the same materials. It is also very expensive to build new highly specialized incinerators with the necessary advanced emissions control systems required by environmental regulations. Therefore its prospects for expanded operation are hard to predict.

Another potential solution for recovering energy from waste is pyrolysis, in which plastics are thermally processed to molecularly break them down into the building blocks of fuels that can then be processed into gases, oils or even high-quality liquid fuels that could be used in place of gasoline. There is a strong desire for alternative energy sources to displace petroleum in liquid fuels markets, making pyrolysis an appealing option. Several companies in the U.S. and Europe have recently commercialized pyrolysis techniques, and many more are in the initial testing phases. However, the incoming material typically must be high-quality homogeneous streams of plastics, making it an unlikely solution (with today’s technology) for the mixed residue stream coming from materials recovery and recycling facilities.

Because of these challenges, SRFs — which integrate the energy recovery piece of the puzzle with the reuse and recycle pieces and enable us to alleviate environmental problems while still recovering the recyclable materials — look to be like a pretty good bet. Producing SRFs from waste is not entirely new: Several American and European companies are already producing quality pelletized fuel from trash. The novel idea that we pursued was to use the materials from materials recovery facilities that cannot be economically recycled as a feedstock to produce SRFs.

The Fuel

SRFs can be created by selectively mixing and shredding a blend of plastic and paper materials and then densifying that blend into a solid form. The blending proportions and densification process are engineered to produce SRFs with consistent and predictable combustion and handling characteristics. The SRFs can be tailored to specific consumers or produced on a mass scale as a fuel for many end-uses. In general, the fuels end up having relatively high energy densities: Depending on the raw materials used, the energy content of SRFs can be significantly higher than most types of coal. Consequently, SRFs can potentially be integrated into processes that consume large amounts of coal, such as cement manufacturing, or can be co-fired at current coal-fired power plants without distorting the overall energy balances.

What makes residue-derived SRFs such adaptable and promising fuels is largely that the paper and plastic content have relatively high energy densities and are abundant in the waste stream. And for the most part, combustion of plastics is cleaner than many people think. Most plastics produced in the U.S. are created by tying together building blocks of hydrocarbon polymers, which are composed of hydrogen and carbon atoms, much like the fossil fuels from which they are derived. Most plastics found in consumer products, ranging from deli wrappers to diapers, can burn as clean as, and sometimes cleaner than, natural gas.

Of course, not all plastics are well suited to combustion. Some additives used in the production of plastic or mixed in the final product (such as chlorine) can be harmful to the environment if improperly combusted or emitted without scrubbing. Thus, attention must be paid to ensure that the incoming materials used to produce SRFs are of suitable quality, which in most cases means nothing more than sorting out problematic materials before the production process begins.

Could it Replace Coal?

Coal is SRFs’ closest fossil fuel analog in terms of fuel content and handling characteristics. U.S. consumers use nearly a billion tons of this nonrenewable fossil fuel every year for power production, resulting in 2-plus billion megawatt-hours of power — and 2-plus billion metric tons of carbon dioxide emissions per year. Another large end-user is the cement industry, which consumes 10 million tons of coal annually and creates more than 80 million tons of carbon dioxide from the combustion of fossil fuels and the chemical reactions used to make cement. At the same time, the U.S. generates 250 million tons of municipal solid waste annually, 102 million tons of which are nonrecycled plastics and paper products.

America’s appetite for energy and SRFs’ ability to displace coal create a synergistic solution. SRFs derived from the residue of materials recovery facilities can conserve finite fossil fuel resources by creating a domestic alternative to coal, all the while diverting more waste from landfills and reducing greenhouse gas production. Meanwhile, using SRFs instead of coal would also eliminate some of the other undesirable byproducts of coal production and consumption, including indirect emissions from transporting the fuel across the country (waste streams are usually transported much shorter distances), water risks caused by runoff from mines, and land disturbances caused by surface mining.

SRFs in the Real World

To test the real-world capabilities of residue-derived SRFs, our research team at the University of Texas (together with partners in the waste and cement sectors) conducted a large-scale test burn and analysis of SRFs co-fired with fossil fuels in a cement kiln.

Seventy-five tons of residue from a materials recovery facility in Virginia were gathered and combined with post-industrial waste products such as scrap plastics from a manufacturing plant. The final product, a blend of residue and post-industrial waste in a 60:40 ratio, contained mostly plastics and paper products. A fuel processing facility in Arkansas used this waste to create 130 tons of SRFs in the form of pellets, which we then burned in the precalciner portion of a cement kiln in Texas. The precalciner is a special combustion chamber of a cement kiln that serves to pre-heat and de-carbonate raw materials before entering the main kiln.

Our experimental results showed that the SRFs had a predictable energy content of about 12,500 British thermal units per pound (25 million Btu per ton). Bituminous coal, the type normally used at this particular cement kiln, has almost exactly the same energy density, leading to a nearly one-to-one displacement opportunity. The SRFs produced for our experiment were also 40 percent more energy dense than sub-bituminous coals and 80 percent more so than lignite.

When the whole production, transportation and combustion life cycle of the SRFs is considered, large fossil fuel energy savings can be realized. Our experiment lasted a few days. Extrapolating the fossil fuel displacement rate of one ton per hour that we used in our experimental demonstration over an entire year, SRFs would reduce total fossil fuel energy use by 6 percent annually in the cement kiln. This reduction equates to about 9,000 tons of coal, enough to provide electricity to 1,500 average U.S. homes for a year. Likewise, under this scenario, carbon dioxide reductions of 14,000 tons per year are possible, largely from reduced landfill gas production. This decrease is comparable to removing 2,800 cars from the road. And that’s just one cement kiln.

The U.S. Environmental Protection Agency estimates that 85 million tons of waste flows through U.S. materials recovery facilities each day. When the magnitude of the resultant residue stream is considered, the potential for energy savings and greenhouse gas reductions is immense. The amount of SRF production that could be realized is enough to power nearly a million homes and reduce carbon dioxide emissions by 5 million tons, equivalent to removing 1.3 million cars from the road.

The Long Road Ahead

As successful as our experiment was at demonstrating the benefit of SRFs, it also revealed how much remains to be done before the full potential of residue-derived SRFs can be achieved.

First, new procedures must be established to thoroughly examine residue streams prior to use. Residue from materials recovery facilities is a heterogeneous and relatively unpredictable mixture of waste. Although most materials that end up in the residue stream are suitable for combustion, others can compromise the quality of the SRFs by reducing the handling characteristics, decreasing energy density, or producing undesired emissions when combusted.

Second, more large-scale and long-term tests need to be conducted to develop a full understanding of the challenges and cost-benefit comparison of producing SRFs from materials recovery facility residue. For example, regional and seasonal variation in materials recovery facility residue composition and availability can impact the economics and technology used to produce SRFs, so they need to be well understood before bringing this technology to market. In addition, higher feed rates should be tested to determine the technical limits and further assess the potential for fossil fuel displacement.

Third, some regulatory policies should be reconsidered before SRFs become big business. Currently there is no consensus on how to handle SRFs in the regulatory realm. In a policy context, the term “waste-to-energy” is used as a catchall for many such technologies, including SRFs. And across the U.S., waste-to-energy techniques have mixed support among states. Some states support the technologies as a means of renewable energy generation, whereas others reject them and most do not address them at all. Illinois, for example, specifically rejects any solid waste-derived resource whereas others such as Montana, New Mexico and Virginia have general incentive policies in place to promote municipal solid waste as a renewable resource. Only Wisconsin currently addresses densified fuel pellets — a term that encompasses SRFs and other less refined waste-derived fuel pellets — directly in the state’s renewable portfolio standard.

This heterogeneous policy landscape hampers interstate business and can be a road-block to investors trying to seize on new waste-to-energy opportunities. The lack of policies specifically addressing emerging technologies such as SRFs and plastics-to-fuels suggests an information gap between technology developers and policymakers. Fortunately, trends in updating renewable portfolio standards to include waste-to-energy facilities and alternative conversion processes as renewable technologies will lead to a better business environment for companies pursuing energy recovery from solid waste.

Despite technical, social, political and economic hurdles, harnessing the energy content of nonrecycled plastics and papers derived from materials recovery facility residue provides many benefits while complementing regional recycling efforts. Displaced fossil fuels, landfill avoidance, and reduced greenhouse gas emissions are just some of the advantages offered by SRF production. As recycling rates continue to increase and SRF production techniques are further refined, residue-derived SRFs will be an important resource to consider as one solution to concerns about America’s long-term energy usage, resource conservation and waste management. Now, the materials that even the recycling industry once considered trash are quickly becoming a national treasure.

Alex C. Breckel, John R. Fyffe and Michael E. Webber

Breckel and Fyffe are both recent graduates in mechanical engineering at the University of Texas at Austin. Breckel is an incoming graduate student at MIT’s Technology and Policy Program, and Fyffe is a graduate student in Stanford’s Mechanical Engineering Program. Webber is co-director of the Clean Energy Incubator and associate director of the Center for International Energy and Environmental Policy at the University of Texas at Austin. The views expressed are their own.

Tuesday, July 31, 2012 - 12:00