At this time of year, when many municipalities are gearing up for holiday tree recycling programs, the city of Houston is dealing with something far more monumental – more than 5.6 million cubic tons of tree waste left behind after Hurricane Ike swept through Southeast Texas in early September.
The city turned some of the debris into mulch, but launched a contest in October, Recycle Ike, to spark ideas for keeping the remaining tree waste from simply being disposed of in landfills.
The winners, announced last week, are a Rice University team of students and scientists who will create a biomass charcoal from the tree remains. The group was among more than 200 entrants from around the world that submitted ideas.
The top $10,000 prize will be used to build a pilot bioreactor on campus. The “biochar” it produces helps the environment in three ways – as a soil enrichment; as a way to stop the decaying waste from releasing carbon into the atmosphere from the decaying wood; and as a method for producing methane gas during the conversion process which can be sold to generate energy.
Using a process called pyrolysis, the bioreactor will heat the biomass tree waste to 400-500°C under low-oxygen conditions to produce the biochar, according to a report by the Rice University team.
“Approximately 50% of the carbon in the feedstock biomass is captured in the biochar, which can be used to sequester carbon in soils. Biochar is carbon-rich and chemically stable in soils, enabling carbon to be
sequestered in soils for centuries or millennia. In addition, biochar improves soil fertility by improving water holding capacity and cation exchange capacity,” the report states.
The methane gas produced during the heating process will be captured and used to generate electricity.
If the pilot project works as projected, it argues for using pyrolysis to deal with all of Hurricane Ike’s considerable waste, say the plan’s proponents. (Ike was the fourth most destructive hurricane to hit the United States.)
“Applied to the entire Hurricane Ike biomass debris, pyrolysis has the potential to sequester more than 1,000,000 metric tons of CO2 and offset an additional 280,000 metric tons of CO2 through the use of syngas [the methane] to generate electricity, removing the equivalent of 300,000 cars from Houston’s roads for a year,” according to the Rice report.
Next, pyrolysis could help the city of Houston more efficiently deal with its ongoing organic waste needs, the researchers say. By turning biomass into soil-enriching biochar and capturing the methane gas for electricity production, they estimate that the city could save carbon emissions equivalent to taking 17,000 cars off the road annually.
The Rice team is nothing if not enthusiastic, concluding: “As a leader already in energy and medicine, the city of Houston stands to become a leader in sustainability. By adopting pyrolysis to handle the Hurricane Ike debris and its annual waste stream, Houston can position itself to be at the forefront of the movement to reduce CO2 emissions and actively remove CO2 from the atmosphere…Ultimately, biochar production promises to revolutionize green waste management in Houston, combining sustainability with a global environmental outlook.”