By Edwin L. Aguirre
A team of university and U.S. Army researchers, led by UML Mechanical and Industrial Engineering Assoc. Prof. Juan Pablo Trelles, has developed a way to extract hydrogen from plastic waste that can be used as fuel for transportation as well as to produce electricity in fuel cells.
Hydrogen is a clean-burning fuel, generating energy while leaving only water as a byproduct. It is currently produced mainly by reforming natural gas or by splitting water into hydrogen and oxygen through electrolysis.
According to the U.S. Energy Information Administration, global energy demand is projected to increase by 56% by 2040, driven by population growth and industrialization, particularly in developing countries. At the same time, more than 400 million tons of plastics are produced globally each year, which, combined with low recycling rates, has led to a dramatic increase in plastic waste, polluting the environment and interfering with ecosystems.
The team is using plasma technology to deal with both issues at the same time, by breaking down plastic waste – in this case, low-density polyethylene (LDPE) – into its chemical components. Hydrogen is then produced efficiently in the process, while minimizing carbon dioxide emissions, according to Trelles.
In 2019, the United States produced an estimated 3.5 million metric tons of LDPE, which is commonly used in single-use plastic bags and packaging, as well as in containers, bottles, trays, tubing and molded parts for many consumer products. Unlike other plastics, LDPE breaks down when exposed continuously to sunlight, releasing significant amounts of greenhouse gases like methane and ethylene.
The researchers say their strategy could help address the challenge of disposing plastic waste, which is often incinerated or dumped in landfills, thereby cutting down greenhouse gas emissions while simultaneously providing a source of hydrogen.
“Our plasma technology, when powered by renewable electricity from wind and solar, can lead to the sustainable upcycling of plastic waste and production of green hydrogen,” says Trelles.
The project is funded by the U.S. Army Combat Capabilities Development Command (DEVCOM) Soldier Center in Natick, Massachusetts. The team’s initial findings were published in the International Journal of Hydrogen Energy last December.
Aside from Trelles, the UML team includes Mechanical and Industrial Engineering Assoc. Prof. Hunter Mack and Chemical Engineering Assoc. Prof. Hsi-Wu Wong, as well as researchers from the Army DEVCOM Soldier Center. Energy engineering Ph.D. student Benard Tabu is the lead researcher working in Trelles’ group.
Producing Green Energy
In this project, the researchers use high-voltage electrical discharges inside a reactor chamber, which is held at atmospheric pressure conditions (no pressurization or vacuum is needed) and without external heating or cooling. The electrical discharge breaks down the air molecules inside the chamber, creating plasma – an electrically conducting gas consisting of highly energetic particles, free electrons and heavy particles (ions and neutral atoms and molecules).
“The electrons and highly reactive gas species in the plasma then collide with the LDPE, which is made up of hydrogen and carbon chains, breaking these chains and producing hydrogen and light hydrocarbons as gas products and carbon compounds as solid products,” says Tabu, who is a Fulbright scholar from the Gulu District in Uganda.
The team’s reactor is different from reactors that use thermal plasma, which feature temperatures generally exceeding 12,000 degrees Celsius (more than 21,000 degrees Fahrenheit) and are being used in waste treatment plants around the world. Thermal reactors typically have low rates of hydrogen production, low energy efficiency and limited selectivity in the chemical reactions, according to Tabu.
“We are exploiting the use of nonthermal plasma, in which the gas is held at relatively low temperatures of less than 1,000 degrees, but the free electrons remain at very high energy levels to extract hydrogen from the waste materials,” he says.
This can lead to potentially higher efficiency in the waste treatment process and can be implemented in compact, inexpensive installations, Tabu says.
With their current system, the researchers estimate that from 1 metric ton of LDPE, they can produce up to 6 kilograms of hydrogen.
In addition to LDPE, the team’s plasma technology can potentially be applied to processing other plastic wastes, as well as biomass waste from agriculture, food industry and cellulose from sawmill dust, and converting them into high-value chemicals and additives.
“We envision this technology to be the initial steps toward converting any solid organic waste into valuable products by direct use of renewable energy, leading to greater environmental and economic benefits,” says Tabu.