Emerging Waste-to-Energy Technologies
Solid Waste Solution or Dead End?
Three emerging thermal waste-to-energy technologies seek to turn municipal
solid waste from a burden to an asset. Adherents of these technologies say they produce fewer toxic emissions and virtually eliminate landfilling. But none of the technologies have yet been proven on a commercial scale .
at least where trash is involved. We’ve been burning municipal solid waste (MSW) since the 1880s. But the dawning of the environmental movement eight decades later cast new light on the nitrous oxides, dioxins, and other chemicals emitted from as many as 600 mass-burn incinerators nationwide, which meanwhile had also grown in size.1,2 The ecological merits of resource conservation and recycling became another area of growing interest.
Now three new approaches to converting trash into energy so-called waste-to-energy (WTE) technologies—look to leave mass-burn incineration behind by transforming how we
think about MSW in the United States. Adherents of these emerging approaches—gasification, plasma gasification, and
pyrolysis—promise cleaner emissions and more flexibility in terms of energy output, plus in some cases the virtual elimination of landfilling through a complex two-stage treatment process.
Today, 70 mass-burn plants in 21 states8 consume about 13% of the nation’s trash, down from a peak of 14.5% in 1990.9,10 Cumulatively they offer roughly 2.5 gigawatts of power in return,11,12 less than a tenth of what the U.S. solar industry produces.13 The most recent inventory available from the U.S. Environmental Protection Agency shows that MSW incinerators released about 1% of the quantity of carcinogenic and highly toxic dioxin-like compounds in 2000 that they did just 13 years earlier.14
How Do the Technologies Work?
Those advocates may as well start by getting their facts straight, believes veteran waste-industry consultant and gasification expert Steve Jenkins. Given the technologies’ novelty, developers are prone to misrepresent their projects to the public and to regulatory agencies, he says, often leaving them painted with the same brush as mass-burn incinerators.
Gasification, plasma gasification, and pyrolysis are closely related and for the purposes of this article are referred to collectively as “conversion technologies” (the term typically encompasses other non combustion technologies as well). They involve the super-heating of a feedstock—be it MSW, coal, or agricultural residues—in an oxygen-controlled environment to avoid combustion. The primary differences among them relate to heat source, oxygen level, and temperature, from as low as about 600°F (300°C) for pyrolysis to as high as 20,000°F (11,000°C) for plasma gasification.18
In these low-oxygen environments the production of dioxins and furans from waste can be significantly reduced compared with incineration,19,20,21 with emissions potentially falling even below detection limits, Jenkins says. (In one well- publicized exception, a gasification plant in Dumfries, Scotland, repeatedly failed to meet expectations. The plant ultimately closed in 2013 after exceeding emissions limits for dioxins and other pollutants as well as producing far less energy than expected. The Scottish Environment Protection Agency cited “persistent non-compliance with the requirements of the permit” in revoking its license.22,23)
Conversion technologies are further distinguished from conventional MSW incineration by the production of synthesis gas (or syngas) composed mainly of hydrogen and carbon monoxide, a product of the thermal reactions that take place during the processes. The syngas can then be burned in a boiler system to generate electricity. It can also be processed into fuel for an efficient, low-emissions natural gas generator or refined into other valuable products.24
Landfilling versus Conversion Technologies
Even if the newer conversion technologies have yet to make an unequivocal case that they’re better than their mass-burn predecessors, some argue there’s another com- parison that may be more relevant—and ultimately more convincing.
In the United States today, landfills have a big advantage when it comes to economics. Sending trash to the dump is almost always cheaper than burning it, with tipping fees paid by haulers averaging about 33% less at landfills than at existing incinerators, according to one analysis.30 That discount would likely be even greater over costly new conversion plants.
But that’s not necessarily a deal-breaker in areas with limited landfill space, such as Los Angeles. “Our focus is to develop an alternative to landfills,” says Coby Skye, a senior civil engineer for the County of Los Angeles Department of Public Works.31 The county is running short on landfill space and reluctant to export its trash else- where, he says. “We wanted to find some- thing that’s more sustainable.”
Mass-burn combustion of MSW occurs in an oxygen-rich setting with minimal prior sorting or preparation. The resulting heat is used to produce steam and electricity.
Municipal solid waste (MSW)
MSW is the term for common mixed trash collected from homes, businesses, and institutions, including packaging, food waste, yard waste, and both durable and nondurable goods.
Synthesis gas (syngas)
Syngas, composed mainly of hydrogen and carbon monoxide, is produced by conversion technology processes. It can be used as fuel for electricity or con- verted into other salable products such as liquid fuels.
This blanket term encompasses noncombustion processes that convert solid waste into useful products. For the purposes of this article, the term refers spe- cifically to gasification, plasma gasification, and pyrolysis, but other conversion technologies include depolymerization, anaerobic digestion, and fermentation.
Gasification is a process that converts any material containing carbon—such as coal, biomass, or MSW—into syngas. In the controlled presence of oxygen, temperatures of 900–3,000°F (480–1,650°C) break the feedstock molecules apart and recombine them into syngas.
Plasma gasification uses a plasma torch to provide supplemental heat for the gasification process. Temperatures can reach 5,000–20,000°F (2,760–11,000°C).
Pyrolysis is a form of gasification that occurs at relatively low temperatures of 600–1,400°F (300–760°C) in the absence of oxygen.
Waste-to-energy (WTE) technologies
The full suite of WTE technologies includes thermal processes like mass-burn incineration and gasification as well as nonthermal processes like anaerobic digestion and landfill-gas recovery.
How the Technologies Work
Gasification, plasma gasification, and pyrolysis all involve the super-heating of a feedstock—be it MSW, coal, or agricultural residues— in an oxygen-controlled environment to avoid combustion. The primary differences among them relate to heat source (this example shows torches like those used in plasma gasification), oxygen level, and temperature. Syngas cleaning is necessary for conversion to high-value products such as substitute natural gas but not for combustion in a boiler unit. Illustration: Jane Whitney for EHP
gasification and anaerobic digestion, leaving 136 tons per day for the landfill.
The latter is “what we would call a dream facility,” says Eugene Tseng, an environmental attorney and engineer whose consulting firm helped prepare the report. “You have to have a suite of technologies for what we call the integrated approach. You take the most appropriate technology for the type of waste that’s being generated.”
The study concluded that the landfill scenario would produce a net increase of 1.64 million metric tons of carbon dioxide equivalent emissions over the entire 125 years, while the integrated scenario would result in a net savings of 0.67 million metric tons. This difference of 2.31 mil- lion metric tons is comparable to 480,000 fewer passenger vehicles driven for one year. The reduction is achieved in two key ways: 1) through the displacement of emissions from fossil fuel combustion due to the electricity generated; and 2) through increased recycling efforts involved in the extensive pre-processing of materials required before feeding the two plants.34
One critical assumption embedded in the study is that biogenic carbon dioxide emissions resulting from the digestion, decomposition, or processing of biologically based materials are considered part
of the natural carbon cycle and therefore carbon neutral with zero net greenhouse gas emissions, in accordance with current state, national, and international standards.35,36,37
However, a growing community of scientists and others feel it is inappropriate to consider all of these emissions carbon neutral.38 Environmental organizations have called on the U.S. Environmental Protection Agency to account for carbon emitted from biomass waste on the basis that it too can have an immediate impact on climate change, even if it will theoretically one day be reabsorbed by trees and plants.39 Such a policy change would mean the landfill scenario in Los Angeles County’s analysis would fare better on greenhouse gas emissions than the state-of-the-art integrated facility.34
The deepest divide between ardent critics and defenders of conversion technologies, and the one that evokes the most passion on both sides, doesn’t have to do with dioxins or energy production or carbon accounting, but rather with philosophies about how to handle our society’s trash—and what, in fact, trash really is.
MSW is a mix of all kinds of materials: not just combustible carbon-based materials
but also glass, metals, and more. Proponents of a decades-old philosophy called “zero waste” contend that at least 80% of the typical MSW stream can be recycled or com- posted (e.g., through anaerobic digestion), and that reuse and waste prevention can reduce the remaining portion—if not all the way to zero, then close.
“The primary ecological benefits associated with recycling are in using recovered materials in a production cycle to displace virgin materials,” says , a solid waste specialist with the Natural Resources “The associated savings of energy, water, and carbon associated with that substitution are where most of the environmental benefits occur. … That’s the basis of the ‘closed-loop’ idea. Once you introduce a material into commerce, you should do all you can to keep it there.”
Supporters of conversion technologies, meanwhile, contend that recycling and composting aren’t enough to sufficiently improve landfill-diversion rates, and that some sort of thermal processing of leftovers is necessary. They employ the newer term “zero-waste-to-landfill” to allow for conversion technologies and other WTE strategies as an additional and in some cases preferred form of recycling.