June 25, 2018
University of California -
Santa Cruz
Limiting global warming to 2
degrees Celsius will require not only reducing emissions of carbon dioxide, but
also active removal of carbon dioxide from the atmosphere. This has prompted
heightened interest in 'negative emissions technologies.' A new study evaluates
the potential for recently described methods that capture carbon dioxide from
the atmosphere through an 'electrogeochemical' process that also generates
hydrogen gas for use as fuel and creates by-products that can help counteract
ocean acidification.
Limiting global warming to 2
degrees Celsius will require not only reducing emissions of carbon dioxide, but
also active removal of carbon dioxide from the atmosphere. This conclusion from
the Intergovernmental Panel on Climate Change has prompted heightened interest
in "negative emissions technologies."
A new study published June 25
in Nature Climate Change evaluates the potential for recently
described methods that capture carbon dioxide from the atmosphere through an
"electrogeochemical" process that also generates hydrogen gas for use
as fuel and creates by-products that can help counteract ocean acidification.
First author Greg Rau, a
researcher in the Institute of Marine Sciences at UC Santa Cruz and visiting
scientist at Lawrence Livermore National Laboratory, said this technology
significantly expands the options for negative emissions energy production.
The process uses electricity
from a renewable energy source for electrolysis of saline water to generate
hydrogen and oxygen, coupled with reactions involving globally abundant
minerals to produce a solution that strongly absorbs and retains carbon dioxide
from the atmosphere. Rau and other researchers have developed several related
methods, all of which involve electrochemistry, saline water, and carbonate or
silicate minerals.
"It not only reduces
atmospheric carbon dioxide, it also adds alkalinity to the ocean, so it's a
two-pronged benefit," Rau said. "The process simply converts carbon
dioxide into a dissolved mineral bicarbonate, which is already abundant in the
ocean and helps counter acidification."
The negative emissions
approach that has received the most attention so far is known as "biomass
energy plus carbon capture and storage" (BECCS). This involves growing
trees or other bioenergy crops (which absorb carbon dioxide as they grow), burning
the biomass as fuel for power plants, capturing the emissions, and burying the
concentrated carbon dioxide underground.
"BECCS is expensive and
energetically costly. We think this electrochemical process of hydrogen
generation provides a more efficient and higher capacity way of generating
energy with negative emissions," Rau said.
He and his coauthors estimated
that electrogeochemical methods could, on average, increase energy generation
and carbon removal by more than 50 times relative to BECCS, at equivalent or
lower cost. He acknowledged that BECCS is farther along in terms of
implementation, with some biomass energy plants already in operation. Also,
BECCS produces electricity rather than less widely used hydrogen.
"The issues are how to
supply enough biomass and the cost and risk associated with putting
concentrated carbon dioxide in the ground and hoping it stays there," Rau
said.
The electrogeochemical methods
have been demonstrated in the laboratory, but more research is needed to scale
them up. The technology would probably be limited to sites on the coast or
offshore with access to saltwater, abundant renewable energy, and minerals.
Coauthor Heather Willauer at the U.S. Naval Research Laboratory leads the most
advanced project of this type, an electrolytic-cation exchange module designed
to produce hydrogen and remove carbon dioxide through electrolysis of seawater.
Instead of then combining the carbon dioxide and hydrogen to make hydrocarbon
fuels (the Navy's primary interest), the process could be modified to transform
and store the carbon dioxide as ocean bicarbonate, thus achieving negative
emissions.
"It's early days in
negative emissions technology, and we need to keep an open mind about what
options might emerge," Rau said. "We also need policies that will
foster the emergence of these technologies."
Story Source:
Materials provided
by University of California
- Santa Cruz. Original written by Tim Stephens. Note: Content may be
edited for style and length.
Journal Reference:
Greg H. Rau, Heather D.
Willauer, Zhiyong Jason Ren. The global potential for converting renewable
electricity to negative-CO2-emissions hydrogen. Nature Climate Change,
2018; DOI: 10.1038/s41558-018-0203-0
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