It
may seem counterintuitive, but one way to reduce carbon dioxide emissions to
the atmosphere may be to produce pure carbon dioxide in power plants 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 power plant’s byproducts is now an expensive process
requiring huge amounts of energy, special chemicals, and extra hardware.
Now
researchers at Massachusetts Institute of Technology (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 laboratory to test the
membrane technology, and have begun establishing parameters for operating the
membranes under the extreme conditions found inside a conventional power plant.
The group’s results will appear in the Journal of Membrane Sciences.
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 carbon
dioxide 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 power plants. 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%) and oxygen (21%). 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 power plants 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 power plant. 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.
“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 power plant. 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 power plant, then it’s doable,” Ghoniem says. “These are
obviously more complicated power plants, 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
laboratory work and the models developed in Ghoniem’s group will enable the
design of larger combustion systems for megawatt plants.
