Rice U. helps Houston lock up CO2 in Ike waste

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Rice U. helps Houston lock up CO2 in Ike waste
Study: Turning hurricane waste into “green” biochar could earn profit

Hurricane Ike left more than 5.6 million cubic yards of fallen trees, broken branches and dead greenery in Houston, and Mayor Bill White today awarded a $10,000 cash prize to a team of Rice University students, faculty and staff who found a way for the city to simultaneously get rid of the debris, reduce greenhouse gases and turn a potential profit.
White recognized Rice’s team today at City Hall along with two other top finishers in the city’s “Recycle Ike” contest, which encouraged local residents to come up with creative and environmentally friendly ways to get rid of tons of Ike wood debris.
Rice’s team, which took first place in the contest, suggested the city convert the wood into biomass charcoal, or “biochar,” using a process that also creates energy. The conversion would take place in a special recycling chamber that operates at high temperatures.
When wood decomposes in mulch or compost, all of its carbon returns to the atmosphere as carbon dioxide, a major greenhouse gas. By converting Ike debris into biochar that is then used for soil amendment, the city will trap about half of the carbon in the debris.
“If all 5.6 million cubic yards of Ike debris are turned into biochar, it will lock up more than 1 million metric tons of carbon dioxide,” said Rice undergraduate Jeremy Caves, a senior majoring in Earth science. “That’s the equivalent of taking about 240,000 cars off of Houston’s roads for an entire year.”
Rice’s researchers say the most critical step in the biomass energy production is the last step — adding the biochar to soil as a fertilizer.
“That’s when this becomes a ‘carbon negative’ proposition,” said Bill Hockaday, a postdoctoral researcher in Earth science. “In effect, the charcoal in the soil acts like a sponge, soaking up organic matter, water and nutrients. Plants grow faster in the amended soil, drawing down more carbon from the atmosphere to create a positive feedback loop. It’s a win-win for the environment.”
Carrie Masiello, assistant professor of Earth science, said a number of studies have found that carbon is still trapped in the soil in biochar that Native Americans in the Amazon added to their gardens more than 500 years ago.
Rice’s team also found sound economic and policy reasons for Houston to go with biochar.
“If the U.S. adopts a carbon cap and trade scheme under the Obama administration, then biochar production would be especially profitable for the city,” said Kyriacos Zygourakis, Rice’s A.J. Hartsook Professor in Chemical and Biomolecular Engineering. “If U.S. carbon prices reflected those in Europe today, the city would generate more than $100 per ton of debris converted to biochar.”
Moreover, the city generates about 90,000 metric tons of “green” waste each year. These trees, branches, grass clippings and other organic wastes could be converted into biochar using the same equipment purchased to recycle the Ike debris, creating a sustained revenue stream for the city.
Biomass, like the Ike debris, is converted into energy and biochar at high temperatures — at least 750 degrees Fahrenheit — in a chamber containing very little oxygen. The technique is called slow pyrolysis, and Rice’s team plans to use a portion of their prize money to build a pyrolysis pilot reactor at Rice for further research.
“The city of Houston has already taken major steps to reduce its carbon footprint,” said team member Richard Johnson, Rice’s director of sustainability. “By adopting biochar to handle Hurricane Ike debris and future green waste, the city would become a recognized leader in the fight against global warming by actively removing carbon dioxide from the atmosphere.”

About Jade Boyd

Jade Boyd is science editor and associate director of news and media relations in Rice University's Office of Public Affairs.