Still, the Portland metropolitan area still generates more than a million tons of garbage per year. That garbage has to go somewhere, and it is Metro’s responsibility to manage it in a way that protects the environment, protects people’s health, and gets good value for the public’s money.
As the Metro Council considers where to send the region’s garbage starting in 2020, landfills remain the most readily available, and likely the least expensive, option for getting rid of trash. But there will be other methods on the table. One method the Metro Council may consider is gasification, a process where, with very little oxygen and a very high temperature, most of the solid waste is transformed into a gas that can be used to make electricity or heat.
At a gasification facility, garbage collected from homes and businesses is deposited in a gasifier, a low-oxygen chamber that heats it to a very high temperature. The waste never burns, but is turned to ash, hydrogen and carbon monoxide gas. This gas is called syngas, and can be used to make electricity, heat, steam and synthetic chemicals. Each gasification plant uses its syngas differently.
While gasification of coal and peat has been around since the 1800s – the resulting gas was used for lighting and cooking – gasification of garbage is a newer process. Gasification of a mix of materials, such as the mix found in the average trash can, is more complicated primarily because the resulting gas has impurities.
"Over the past 10 years, until they figure out the best ways to clean the syngas, they're taking that syngas and burning it to get the energy," said Marco Castaldi, associate professor in the chemical engineering department at The City College of New York / City University of New York. "Any time you burn a hydrocarbon, you will get emissions. The question is, how much?"
Castaldi studies combustion and gasification of solid waste. He said gasification is a promising newer technology but not all the kinks have been worked out yet.
"If you do the calculations, you can theoretically show that a gasification system that makes syngas and converts it to fuel has the possibility to be more efficient than combustion," Castaldi said.
Steven Weber, vice president of business development at Covanta, a solid waste and energy company that provides gasification technology, said he is passionate about gasification.
"When you're putting material in landfills," Weber says, "you are wasting a valuable resource."
Covanta, a leader in solid waste combustion, has been developing a gasification technology for several years.
"The market was clamoring for non-combustion types of systems," Weber said. "More than 90 percent of the technologies we looked at for gasification required sorting through the trash. So we took mass burn proven technology and evolved it. We made incremental and meaningful improvements in existing technologies."
Weber said that because gasification relies on processes that use less oxygen, Covanta can use less space and build smaller facilities than traditional combustion facilities.
The Cleergas facility in China converts garbage to electricity. Courtesy Covanta
With gasification you get approximately a 90 percent reduction in the volume of solid waste and 75 percent reduction in mass, meaning 100 tons of waste turn into 25 tons of ash, Weber said.
At Covanta's demonstration gasification plant in Tulsa, Okla., the materials leftover as ash have been deemed non-hazardous by the Environmental Protection Agency. Weber said the ash is used for daily cover at landfills.
David Garcia de Herreros, North American representative for Urbaser, a solid waste management company, said gasification is a method his company recommends to clients on occasion. An advantage of gasification, as de Herreros sees it, is that "you may be able to get a higher decomposition rate and less product at the end."
However, he said that because gasifying solid waste is a newer technology, there is a lack of historical data on gasification and not many facilities in the world that do it.
Castaldi, whose scientific research has shown that emissions from combustion and gasification are very similar, said current research shows that combustion is the most reliable and efficient method of converting trash to energy.
But, he said he understands the interest in gasification. "My opinion of why people are pushing this is combustion has a stigma associated with it. Gasification groups can say, 'Hey, we're not incineration.'"
Castaldi said there is great potential in gasification, especially with additional scientific discoveries.
"The holy grail of syngas is called hot gas cleanup. You can clean syngas when it's cool, but to cool it down, you have to throw away all that energy," Castaldi said. "There is a huge effort to find technologies for that that's been going on for 30 years."