Waste-to-energy technology treats non-recyclable waste as a resource instead of a disposal problem. Whether solid, liquid, or even gas, waste has recoverable energy. It can provide heat, fuel, or electricity.

Germany, the Netherlands, and the Scandinavian countries direct so much of their waste stream to recycling, composting, or waste-to-energy that only 1% of it reaches landfills.

We can conveniently divide waste-to-energy technology into two broad categories: thermal and biochemical. One thermal treatment, incineration, is the oldest and least desirable of them.

Thermal technologies for waste-to-energy

Incineration

The first waste incinerator in the US was installed in 1885 on Governors Island in New York City. Hundreds of waste incinerators existed by the middle of the 20^th century. No one knew much about their environmental impact.

The Clean Air Act of 1970 required them either to install technology to deal with particulate matter and other problems or close. The EPA’s Maximum Achievable Control Technology regulations in the 1990s further required them to deal with mercury and dioxin emissions.

It is remarkably difficult to find when the idea of using incinerators to generate electricity—waste-to-energy—first appeared. By the early 1990s, incinerators treated about 15% of all municipal solid waste. Most of them generated electricity. As of March 2022, 75 waste-to-energy facilities existed in 25 states. Northeastern states had most of them. Most of them were aging. But Palm Beach County, Florida, installed a new one in 2015.

Incineration poses many environmental problems. For example, burning anything emits greenhouse gases and particulate matter. It leaves behind ash that may include toxic substances.

For these reasons, any company that wants to build a new waste-to-energy incinerator will face stiff opposition. Fortunately, plenty of better alternatives exist.

Another problem: incineration destroys materials that could have been recycled. A newer concept called refuse-derived fuel sorts out anything recyclable and everything non-combustible before burning the rest.

Gasification

Most waste-to-energy technology uses waste as a feedstock, not as a fuel. The resulting gases, if any, become useful byproducts, not emissions. At the very least, these gases can supply the fuel needed to run the system

Gasification subjects waste, including non-recyclable plastics, to high temperatures (1,000-2,800º Fahrenheit) to convert them to syngas. It produces much less pollution than incineration. And the gas can provide fuel for transportation, as an alternative to natural gas, or for making fertilizer.

Similar to refuse-derived fuel, gasification requires careful sorting and pre-processing to remove anything unsuitable.

Plasma gasification

Plasma gasification, a fairly recent waste-to-energy innovation, uses even higher temperatures. It passes an inert gas through an electrical arc. It vaporizes all organic wastes and forms hydrogen, carbon monoxide, and other useful gases. And it produces no air pollution. When any remaining inorganic wastes (including heavy metals) cool, they form a glass-like substance. This, in turn, can be used as aggregate or other construction material.

Plasma gasification has been used to destroy chemical weapons and other toxic materials. Nothing toxic remains. So far, it is too expensive for large-scale use on ordinary trash.

Pyrolysis

Pyrolysis is widely used in the chemical industry but less as a waste-to-energy technology until recently. It subjects organic material to high heat in the absence of oxygen. That is, nothing can burn.

Possible feedstock includes biomass, plastics, and old tires. In the extreme heat, complex molecules break down into smaller ones. Various catalysts can improve the efficiency of pyrolysis.

What remains at the end of the process is a solid residue called biochar, a liquid comparable to crude oil, and various useful gases.

The biochar has numerous uses, including compost feedstock, water filtration, and sanitation of biowastes. Pyrolysis oil also has several uses. After additional treatment, it can even replace crude oil. The gases (including methane, hydrogen, carbon monoxide, and carbon dioxide) remain in the chamber until they are removed for use. In other words, they include potent greenhouse gases, but they are not emitted into the atmosphere.

Hydrothermal carbonization

Hydrothermal carbonization, a recent waste-to-energy innovation, subjects feedstock not only to heat but high pressure in the presence of an acid catalyst. It uses a much lower temperature than either gasification process—about 400º Fahrenheit. The process also requires feedstock with a much higher moisture content than other thermal technologies. Food waste makes an ideal feedstock.

At the end of the process, the waste has become a solid comparable to coal. This hydrochar can be burned like coal, but left alone, it sequesters carbon. The process leaves only non-toxic water as a byproduct. It produces the smallest amount of greenhouse gases of any of these waste-to-energy technologies.

Biochemical technologies for waste-to-energy

Anaerobic digestion

Anaerobic digestion uses micro-organisms to break down organic matter in the absence of air. As they consume organic waste, they emit methane-rich biogas. Remaining solids make excellent organic compost. It typically takes 15-30 days to complete a cycle.

Wastewater treatment plants use it extensively. It also works well as a waste-to-energy technology with any other organic waste, including manure, commercial food waste, and municipal solid waste.

The main technological difficulty is connecting various waste streams to anaerobic digestion facilities and cleaning them up (including screening out inorganic wastes) to supply feedstock.

Anaerobic digestion systems operate in various ways, depending on how hot the chambers are kept, how wet or dry the feedstock is, whether it operates in batches or by continuous flow, etc.

Dendro liquid energy

Dendro liquid energy is the latest waste-to-energy innovation. Developed in Germany, it is so new that I haven’t located any articles devoted specifically to describing it.

It can process mixed wastes that include plastic. It is up to four times as efficient as standard anaerobic digestion and doesn’t leave behind any waste or emit any nuisance gases. Its solid residue is inert and useful for aggregate.

It also uses small, relatively inexpensive decentralized units. Therefore, poor countries can use it for waste management and power generation.

Most of the newer technologies described in this post, however, are expensive. They require extensive pretreatment. The gas output often includes not only useful syngas but hydrogen sulfide, which presents various problems. It’s not clear if all of them are economically viable. Nonetheless, with landfills reaching capacity and the difficulty of siting new ones, we need to develop waste-to-energy technology—as well as learning not to produce so much waste in the first place.

Sources:

Energy recovery from the combustion of municipal solid waste (MSW) / US Environmental Protection Agency. March 16, 2022
The Top Innovations in Waste-To-Energy Technology / Hana Korneti, Valuer. September 7, 2021
World Energy Resources: Waste to Energy 2016 / World Energy Council<

Related posts:

Landfill? Incineration? Alternative waste management methods
Waste to energy: using an inexhaustible resource<