At the end of 2016, for the first time in two decades, a new commercial hazardous waste incinerator built in the U.S. came on-line. The $120 million incinerator, operated by Clean Harbors in El Dorado, Ark., can handle industrial and laboratory chemicals, manufacturing by-products, medical waste, and other solid and liquid materials.

“As all chemists know, you can’t destroy or create matter,” says Scot Shoemaker, the company’s vice president for engineering and operations. “But you can change its form into something less toxic, and that’s something that incinerators do very effectively.”

After the U.S. passed the Resource Conservation & Recovery Act—the primary law governing disposal of hazardous waste—in 1976, commercial waste handlers spent 20 years building hazardous waste incinerators to the point of overcapacity. Demand is only now catching up to capacity, leading to the need for a new incinerator, says Shoemaker, who has a bachelor’s degree in chemistry and a master’s in chemical engineering. The U.S. market for hazardous waste management is expected to reach $13.6 billion in 2020, according to a 2016 report by the market research firm Freedonia Group.

Clean Harbors already had two incinerators at its El Dorado site, and the additional unit will increase the site’s capacity from roughly 73,000 metric tons to approximately 140,000 metric tons of material annually. The company has no plans to take any of its other incinerators off-line, Shoemaker says. The Arkansas Economic Development Commission and the El Dorado-Union County Chamber of Commerce supported the expansion.

Besides being the first U.S. hazardous waste incinerator built in the 21st century, the new facility is also the only one required to meet current air emissions standards under the Clean Air Act; older incinerators are grandfathered in under less stringent limits. “It meets air pollution control requirements that I thought were not really achievable,” says David Case, executive director of the Environmental Technology Council, a trade association of commercial environmental firms involved in waste management. For example, the new incinerator may not release more than 8.1 µg mercury/m³ dry gas from its stack, compared to 130 µg for older incinerators. The new incinerator also must limit emissions of nitrogen oxides to 40 tons/year; restrictions on older incinerators vary by state.

To meet those standards, “we used proven, reliable technologies that have been used for a number of years,” Shoemaker says. “We looked at alternatives, but to meet those very stringent emissions limits, we decided that we needed to run with what we knew.” The key instead was in how Clean Harbors designed and configured the various components of the system. In addition to controlling emissions, Clean Harbors also wanted to engineer the incinerator to stay running more than 90% of the time.

The first step to meeting both goals happens before any waste enters the incinerator. Clean Harbors staff carefully manage incoming waste to ensure that they know exactly what they’re dealing with. Customers document what is in the waste, but Clean Harbors follows up with its own analysis to ensure the documentation is accurate—whether the waste arrives “in a pail or a tanker truck,” Shoemaker says. Tests include pH measurements, various forms of chromatography for halogens and organic compounds, spectrometry for metals, and bomb calorimetry for determining heat of combustion.

“If we can’t safely handle something, then we’ll reject it and send it back to the customer,” Shoemaker says. “It doesn’t happen very often. Most customers are very responsible with their waste streams.”

Once the contents of the waste are verified, the waste is stored until the incinerator can accept it. The incinerator can accept waste in multiple forms, including bulk solid, shredded material, and liquid. Computer programs analyze the inventory of stored waste to determine what combinations to feed into the incinerator and when. “The programs take into account things like the physical characteristics and the energy, halogen, or metal content of the waste, and create a mix such that we can run our incinerator in a steady-state fashion and meet emissions compliance,” Shoemaker says.

When waste enters the incineration unit, it is burned at about 1,000 °C in a rotary kiln followed by a secondary chamber to ensure the material is completely combusted. Ash and noncombustible solids get removed and gases and airborne particulates pass through various cleanup steps: spray drying to remove particulate matter, salts, and some metals; wet scrubbing to remove sulfur dioxide, acids, and halogenated species; activated carbon treatment for dioxins, furans, and mercury; and catalytic reduction of nitrogen oxides.

Clean Harbors used fluid dynamics modeling and other approaches to design the process and determine how to configure specific components, Shoemaker says. In some cases, the company turned to special materials to get the desired performance. For example, the company used specialized stainless steel alloys to resist corrosion in the baghouse where trace acidic material collects after the wet scrubbing steps. All material that doesn’t waft out of the stack gets sent to contained landfills.

“When customers send us their waste, they expect us to ensure that we are safely and compliantly managing the stream through complete destruction and proper disposal,” Shoemaker says, noting that the U.S. now has clearer skies and cleaner water than it has in the past. “Incineration is needed technology and is a key component of proper waste management and disposal today.”