Ceramic membranes may reduce carbon dioxide emissions from gas and coal-fired powerplants.
It may seem counterintuitive, but one way to reduce carbon dioxide
emissions to the atmosphere may be to produce pure carbon dioxide in
powerplants that burn fossil fuels. In this way, greenhouse gases — once
isolated within a plant — could be captured and stored in natural
reservoirs, deep in the Earth’s crust.
Such “carbon-capture”
technology may significantly reduce greenhouse gas emissions from cheap
and plentiful energy sources such as coal and natural gas, and help
minimize fossil fuels’ contribution to climate change. But extracting
carbon dioxide from the rest of a powerplant’s byproducts is now an
expensive process requiring huge amounts of energy, special chemicals
and extra hardware.
Now researchers at MIT are evaluating a
system that efficiently eliminates nitrogen from the combustion process,
delivering a pure stream of carbon dioxide after removing other
combustion byproducts such as water and other gases. The centerpiece of
the system is a ceramic membrane used to separate oxygen from air.
Burning fuels in pure oxygen, as opposed to air — a process known as
oxyfuel combustion — can yield a pure stream of carbon dioxide.
The
researchers have built a small-scale reactor in their lab to test the
membrane technology, and have begun establishing parameters for
operating the membranes under the extreme conditions found inside a
conventional powerplant. The group’s results will appear in the Journal of Membrane Sciences, and will be presented at the International Symposium on Combustion in August.
Ahmed
Ghoniem, the Ronald C. Crane Professor of Engineering at MIT, says
ceramic membrane technology may be an inexpensive, energy-saving
solution for capturing carbon dioxide.
“What we’re working on is
doing this separation in a very efficient way, and hopefully for the
least price,” Ghoniem says. “The whole objective behind this technology
is to continue to use cheap and available fossil fuels, produce
electricity at low price and in a convenient way, but without emitting
as much CO2 as we have been.”
Ghoniem’s group is
working with other colleagues at MIT, along with membrane manufacturers,
to develop this technology and establish guidelines for scaling and
implementing it in future powerplants. The research is in line with the
group’s previous work, in which they demonstrated a new technology
called pressurized oxyfuel combustion that they have shown improves
conversion efficiency and reduces fuel consumption.
Streaming pure oxygen
The
air we breathe is composed mainly of nitrogen (78 percent) and oxygen
(21 percent). The typical process to separate oxygen from nitrogen
involves a cryogenic unit that cools incoming air to a temperature
sufficiently low to liquefy oxygen. While the freezing technique
produces a pure stream of oxygen, the process is expensive and bulky,
and consumes considerable energy, which may sap a plant’s power output.
Ghoniem
says ceramic membranes that supply the oxygen needed for the combustion
process may operate much more efficiently, using less energy to produce
pure oxygen and ultimately capture carbon dioxide. He envisions the
technology’s use both in new powerplants and as a retrofit to existing
plants to reduce greenhouse gas emissions.
Ceramic membranes are
selectively permeable materials through which only oxygen can flow.
These membranes, made of metal oxides such as aluminum and titanium, can
withstand extremely high temperatures — a big advantage when it comes
to operating in the harsh environment of a powerplant. Ceramic membranes
separate oxygen through a mechanism called ion transport, whereby
oxygen ions flow across a membrane, drawn to the side of the membrane
with less oxygen.
A two-in-one solution
Ghoniem
and his colleagues built a small-scale reactor with ceramic membranes
and studied the resulting oxygen flow. They observed that as air passes
through a membrane, oxygen accumulates on the opposite side, ultimately
slowing the air-separation process. To avert this buildup of oxygen, the
group built a combustion system into their model reactor. They found
that with this two-in-one system, oxygen passes through the membrane and
mixes with the fuel stream on the other side, burning it and generating
heat. The fuel burns the oxygen away, making room for more oxygen to
flow through. Ghoniem says the system is a “win-win situation,” enabling
oxygen separation from air while combustion takes place in the same
space.
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| Members of the Ghoneim lab. |
MIT
researchers are investigating ceramic membranes as a way to reduce
carbon dioxide emissions in powerplants. In their system, the air (red
dots) that’s needed for combustion passes over a ceramic membrane (red
layer). Only oxygen passes through the membrane, mixing with fuel (black
and green dots) to produce a pure stream of carbon dioxide and water.
After evaporating water, carbon dioxide can then be captured and stored.
“It turns out to be a clever way of doing things,” Ghoniem says. “The
system is more compact, because at the same place where we do
separation, we also burn. So we’re integrating everything, and we’re
reducing the complexity, the energy penalty, and the economic penalty of
burning in pure oxygen and producing a carbon dioxide stream.”
The
group is now gauging the system’s performance at various temperatures,
pressures and fuel conditions using their laboratory setup. They have
also designed a complex computational model to simulate how the system
would work at a larger scale, in a powerplant. They’ve found that the
flow of oxygen across the membrane depends on the membrane’s
temperature: The higher its temperature on the combustion side of the
system, the faster oxygen flows across the membrane, and the faster fuel
burns. They also found that although the gas temperature may exceed
what the material can tolerate, the gas flow acts to protect the
membrane.
“We are learning enough about the system that if we
want to scale it up and implement it in a powerplant, then it’s doable,”
Ghoniem says. “These are obviously more complicated powerplants,
requiring much higher-tech components, because they can much do more
than what plants do now. We have to show that the [new] designs are
durable, and then convince industry to take these ideas and use them.”
The
lab work and the models developed in Ghoniem’s group will enable the
design of larger combustion systems for megawatt plants.
Madhava
Syamlal, focus area leader for computational and basic sciences at the
National Energy Technology Laboratory, says simulations such as
Ghoniem’s will help push next-generation technologies such as
oxygen-separating membranes into powerplants. “We have seen that in
other areas, like aircraft, simulations really improve how the product
is developed,” Syamlal says. “You can use simulations and even skip some
of the intermediate testing and go directly to designing and building a
machine. In the energy industry, these are the pieces we need to
increase the scale quite rapidly.”
Ghoniem’s group includes
research scientist Patrick Kirchen and graduate students James Hong and
Anton Hunt, in collaboration with faculty at King Fahed University of
Petroleum and Minerals (KFUPM) in Saudi Arabia. The research was funded
by KFUPM and King Abdullah University of Science and Technology.
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