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When power plants begin capturing their carbon emissions to reduce
greenhouse gases—and to most in the electric power industry, it’s a question of
when, not if—it will be an expensive undertaking.
Current technologies would use about one-third of the energy generated by
the plants—what’s called “parasitic energy”—and, as a result, substantially
drive up the price of electricity.
But a new computer model developed by University
of California, Berkeley chemists shows that less expensive
technologies are on the horizon. They will use new solid materials like
zeolites and metal oxide frameworks (MOFs) that more efficiently capture carbon
dioxide so that it can be sequestered underground.
“The current on-the-shelf process of carbon capture has problems, including
environmental ones, if you do it on a large scale,” said Berend Smit,
Chancellor’s Professor in the Departments of Chemical and Biomolecular Engineering and of Chemistry at UC Berkeley and a faculty senior scientist in
the Materials Sciences Division at Lawrence Berkeley National Laboratory
(LBNL). “Our calculations show that we can reduce the parasitic energy costs of
carbon capture by 30% with these types of materials, which should encourage the
industry and academics to look at them.”
Smit and his colleagues at UC Berkeley, LBNL, Rice
University, and the Electric Power
Research Institute (EPRI) in Palo Alto,
Calif., who will publish their
results online in Nature Materials, already are integrating their
database of materials into power plant design software.
“Our database of carbon capture materials is going to be coupled to a model
of a full plant design, so if we have a new material, we can immediately see
whether this material makes sense for an actual design,” Smit said.
Guiding new materials research
There are potentially millions of materials that can capture
carbon dioxide, but it’s physically and economically impossible for scientists and
engineers to synthesize and test them all, Smit said. Now, a researcher can
upload the structure of a proposed material to Smit’s Website, and the new
computer model will calculate whether it offers improved performance over the
energy consumption figures of today’s best technology for removing carbon.
“What is unique about this model is that, for the first time, we are able to
guide the direction for materials research and say, ‘here are the properties we
want, even if we don’t know what the ultimate material will look like,'” said
Abhoyjit Bhown, a coauthor of the study and a technical executive at EPRI,
which conducts research and development for the electric power industry and the
public. “Before, people were trying to figure out what materials they should
shoot for, and that question was unanswered until now.”
Fossil fuel-burning power plants, in particular coal-burning units, are a
major source of the carbon dioxide that is rapidly warming the planet and
altering the climate in ways that could impact crops and water supplies, raise
sea level and lead to weather extremes. Even with the move toward alternative,
sustainable, and low-carbon sources of energy, ranging from solar and wind to
hydrothermal, coal- and natural gas-burning power plants are being built at an
increasing rate around the world. At some point, Smit said, carbon capture will
be the only way to reduce carbon emissions sufficiently to stave off the worst
consequences of climate change.
Although no commercial power plants currently capture carbon dioxide on a
large scale, a few small-scale and pilot plants do, using today’s best
technology: funneling emissions through a bath of nitrogen-based amines, which
grab carbon dioxide from the flue gases. The amines are then boiled to release
the carbon dioxide. Additional energy is required to compress the carbon
dioxide so that it can be pumped underground.
The energy needed for this process decreases the amount that can go into
making electricity. Calculations show that for a coal-fired power plant, that
could amount to approximately 30% of total energy generated.
Solid materials should be inherently more energy-efficient than amine
scrubbing, because the carbon dioxide can be driven off at lower temperatures.
But materials differ significantly in how tightly they grab carbon dioxide and
how easily they release it. The best process will be a balance between the two,
Smit said.
Smit and his UC Berkeley group worked with Bhown and EPRI scientists to
establish the best criteria for a good carbon capture material, focusing on the
energy costs of capture, release and compression, and then developed a computer
model to calculate this energy consumption for any material. Smit then obtained
a database of 4 million zeolite structures compiled by Rice University
scientists and ran the structures through his model. Zeolites are porous
materials made of silicon dioxide, the same composition as quartz.
The team also computed the energy efficiency of 10,000 MOF structures, which
are composites of metals like iron with organic compounds that, together, form
a porous structure. That structure has been touted as a way to store hydrogen
for fuel or to separate gases during petroleum refining.
“The surprise was that we found many materials, some already known but
others hypothetical, that could be synthesized” and work more energy
efficiently than amines, Smit said. The best materials used 30% less energy
than the amine process, though future materials may work even better. The
computer model will work for structures other than zeolites and MOFs, Smit
said.
Bhown said that the theoretically best material will probably have a
parasitic energy cost of about 10%, so processes that use 20% or less are more
attractive.
GPUs dramatically speed calculations
Key to the team’s success was using graphics processing units (GPUs) instead of
standard computer central processing units (CPUs), GPUs reduced each structure’s
calculation, which involves complex quantum chemistry, from 10 days to 2 sec.
Bhown noted that most people believe that some economic incentives or
regulatory frameworks are needed to implement carbon capture, and the EPRI’s
goal is to help the industry identify the best technologies for doing so. A
survey that EPRI conducted recently suggested that developing any new
technology would take 10 to 15 years even with adequate funding.
“The collaboration between different parts of the Department of Energy
illustrates what can be achieved if researchers working on the most fundamental
aspects of carbon capture collaborate with their industry counterparts” says
Karma Sawyer, DOE program director. “This study shows how engineering and
fundamental science can speed up the process of discovery and implementation of
promising materials ready to test in the field.”
“The hope is that there is a system set up such that, when someone comes up
with a promising material, we can rapidly test it and get it to a readiness
level pretty quickly,” he said. “We are all excited by this work and look
forward to pursuing it further.”
