Usually, waste to energy means burning trash and using the heat to operate an electric generator. That is, instead of burning fossil fuels to make steam, waste to energy burns trash.
Trash incineration is controversial in the US. There are other options.
The US produced about 268 million tons of municipal solid waste in 2017. More than half went to landfills. About 25% was recycled and another 10% composted. Just under 13% was burned.
That’s only household trash. Industry produces its own waste. Waste from poultry and meat producers includes manure, which too often winds up on open-air lagoons as well as billions of pounds of feathers. In addition, wastewater treatment plants produce tons of sludge.
There are lots of ways to turn our wastes from a problem to a resource. Here are just a few, some of which are becoming commonplace in other industrialized countries:
Waste-to-energy plants in the US
Few American cities burn municipal waste. Most trash incinerators are in the northeast or mid-Atlantic region. Urban concentration makes siting landfills there difficult. Incineration reduces the volume of waste by about 90% and its weight by about 75%.
Even fewer waste-to-energy plants exist in the US, although they are common in Europe. The EPA, however, considers waste a renewable energy resource. After all, its supply is inexhaustible.
In 2019, 75 waste-to-energy incinerators operated in the US, compared to 87 in 2009. Two of them were scheduled to close. But a plant started to operate in Palm Beach County, Florida in 2015. It was the first new waste-to-energy project in the US in more than 15 years. It generates about 96 megawatts of electricity. That much can power 40,000 or so homes and businesses.
Twenty-four states offer renewable energy credits that provide incentives to the waste-to-energy industry.
Other states, however, oppose burning trash. They also fight waste-to-energy incinerators. Burning waste, whether waste to energy or not, remains controversial in the US. Its smoke adds not only greenhouse gas to the atmosphere but also harmful particulate matter and toxins such as dioxins and furans.
Coventa, a major player in the industry, has started to retrofit some of its 44 plants. It wants to recover heavy metals and other valuable materials from the ash. It is also working on catalytic reduction systems to reduce nitrous oxide and other greenhouse gases.
Will the new plant in Florida open the door to more new waste-to-energy plants? Or is it just an anomaly in their general decline in this country?
Several waste-to-energy techniques exist for making electricity:
Direct combustion is the simplest of waste-to-energy technologies. An ordinary incinerator burns everything. All the organic material. All the plastic. Everything that can burn goes up in smoke. It leaves ash behind.
A direct combustion waste-to-energy incinerator uses the heat from burning to operate a boiler. Steam from the boiler runs an electric generator. In a sense, then, these plants work the same way as coal or gas plants. They just use different fuel.
That fuel, unfortunately, includes recyclable materials. And also plastics that release toxins as they burn. But now, a newer technology called refuse-derived fuel has come along. It removes all the recyclables and everything that can’t burn from the waste. My source appeared in 2014. At that time, the US had only 19 refuse-derived-fuel waste-to-energy.
A failed recycling method could let any landfill produce refuse-derived fuel. Montgomery, Alabama decided to make its recycling rate 100%. It stopped requiring its citizens to separate recycling from trash. Its new material recovery facility to separate recycling from unsorted trash opened in April 2014. And then the operator went out of business and closed it in October 2015.
It seems to me that landfills could use similar technology. They could extract plastic and metals and send them to the recycling plant. Then, they could compost the wet garbage. Or incinerate it as refuse-derived fuel.
I could not find the temperature range of direct combustion. I did, however, learn that the earliest American incinerators weren’t hot enough to prevent unpleasant odors from escaping into the surrounding neighborhood. Trash burns more completely at higher temperatures.
More advanced waste-to-energy techniques
Pyrolysis takes place at high temperatures—more than 750ºF––without oxygen. Trash doesn’t burn. It changes chemical composition. Therefore, it doesn’t emit as much greenhouse gas. It produces a synthetic gas that can either be used in place of natural gas or converted to biodiesel or other liquid fuels.
Conventional gasification operates at even higher temperatures, from 1,000-2,800ºF. It uses some oxygen, but much less than combustion. It produces low to medium BTU synthetic gas. Most existing gasification plants use coal as feedstock. Still, it is a very useful and mature technology for burning waste instead. Especially refuse-derived fuel.
Plasma arc gasification operates at temperatures from 4,000-20,000ºF and produces a superior synthetic gas. It is the most recent of these technologies. The first plant came online in 1985. This technology has long been used to destroy hazardous wastes, including munitions.
Such high temperatures produce no ash. Instead, the residue resembles glass. It encapsulates any remaining toxins. Since existing plants have been built for highly specialized use, they are small but scalable. New plants can have a large capacity from the start.
These last three processes produce gas and liquid biofuels, not electricity. But in each case, they could operate a boiler and therefore a generator. After all, as much as 70% of all the energy produced worldwide becomes waste heat.
According to a study in Yale Environment 360, 60% of waste heat is cooler than boiling water. The lower the temperature, the more difficult it becomes to use it. But not impossible. The very high heat from these three waste reduction methods should be easy to use for waste-to-energy projects.
Biofuels from waste
Burning the synthetic gas from gasification to make electricity could release hazardous substances into the air. Instead, various chemical processes can turn it into alcohol––either ethanol or methanol.
North America produces 468 metric tons of trash every year. That’s enough to make more than 12 billion gallons of ethanol. The US makes about that much from corn every year for a gasoline additive.
So far, however, the industry hasn’t overcome the technological and economic barriers to finding substitutes for corn. Using municipal solid waste may be easier than using switchgrass or plant-based agricultural wastes. All those feathers and all that manure can already make biofuel more easily.
So can sewage sludge. It contains a variety of organic and inorganic substances and microbes. Various processes exist to use the microbes to make ethanol.
Technology for making ethanol from sewage sludge is a fairly recent development. So it hasn’t been scaled up for commercial use. The term “microdiesel” was coined in 2006 to describe diesel bacteria make from glucose and oleic acid.
These carbon sources cost too much for commercial production of microdiesel. But sewage sludge is cheap and abundant. It could make cheaper microdiesel.
Sooner or later, scientists will find a technically and economically feasible way to make ethanol from either solid waste or sewage sludge. But what seems like rapid progress for a research team can seem glacially slow for anyone waiting for new technology to come online.
Mature waste-to-energy technology exists to make electricity and/or synthetic gas. Barriers seem to be mostly political and economic.
Biomass explained: waste-to-energy (municipal solid waste) / U.S. Energy Information Administration. December 4, 2019
Garbage in, energy out: turning trash into biofuel / David Biello, Scientific American. August 10, 2011
An independent engineering evaluation of waste-to-energy technologies / Thomas Stringfellow, Renewable Energy World. January 13, 2014
Microbial production of ethanol from sludge derived from an urban wastewater treatment plant / Patricia Godoy et al., Frontiers in Microbiology. November 1, 2018.
The state of waste-to-energy in the US / Arlene Karidis, Waste 360. March 26, 2019
Waste plant model. Public domain from Wikimedia Commons
Waste to energy plant in Sweden. Public domain from Wikimedia Commons
Burning sludge for electricity. Some rights reserved by David Hawgood
Pyrolysis. Source unknown
Biodiesel fuel pump. Some rights reserved by The Master Shake Signal.