This illustration depicts a possible
configuration for the combined system proposed by MIT researchers. At the
bottom, steam (pink arrows) passes through pulverized coal, releasing gaseous
fuel (red arrows) made up of hydrogen and carbon monoxide. This fuel goes into
a solid oxide fuel cell (disks near top), where it reacts with oxygen from the
air (blue arrows) to produce electricity (loop at right).
Credit: Jeffrey Hanna
Most
of the world's nations have agreed to make substantial reductions in their
greenhouse gas emissions, but achieving these goals is still a considerable
technological, economic, and political challenge. The International Energy
Agency has projected that, even with the new agreements in place, global
coal-fired power generation will increase over the next few decades. Finding a
cleaner way of using that coal could be a significant step toward achieving
carbon-emissions reductions while meeting the needs of a growing and
increasingly industrialized world population.
Now,
researchers at MIT have come up with a plan that could contribute to that
effort by making it possible to generate electricity from coal with much
greater efficiency -- possibly reaching as much as twice the
fuel-to-electricity efficiency of today's conventional coal plants. This would
mean, all things being equal, a 50 percent reduction in carbon dioxide
emissions for a given amount of power produced.
The
concept, proposed by MIT doctoral student Katherine Ong and Ronald C. Crane
(1972) Professor Ahmed Ghoniem, is described in their paper in the Journal
of Power Sources. The key is combining into a single system two well-known
technologies: coal gasification and fuel cells.
Coal
gasification is a way of extracting burnable gaseous fuel from pulverized coal,
rather than burning the coal itself. The technique is widely used in chemical
processing plants as a way of producing hydrogen gas. Fuel cells produce
electricity from a gaseous fuel by passing it through a battery-like system
where the fuel reacts electrochemically with oxygen from the air.
The
attraction of combining these two systems, Ong explains, is that both processes
operate at similarly high temperatures of 800 degrees Celsius or more.
Combining them in a single plant would thus allow the two components to
exchange heat with minimal energy losses. In fact, the fuel cell would generate
enough heat to sustain the gasification part of the process, she says,
eliminating the need for a separate heating system, which is usually provided
by burning a portion of the coal.
Coal
gasification, by itself, works at a lower temperature than combustion and
"is more efficient than burning," Ong says. First, the coal is
pulverized to a powder, which is then heated in a flow of hot steam, somewhat
like popcorn kernels heated in an air-popper. The heat leads to chemical
reactions that release gases from the coal particles -- mainly carbon monoxide
and hydrogen, both of which can produce electricity in a solid oxide fuel cell.
In
the combined system, these gases would then be piped from the gasifier to a
separate fuel cell stack, or ultimately, the fuel cell system could be
installed in the same chamber as the gasifier so that the hot gas flows
straight into the cell. In the fuel cell, a membrane separates the carbon
monoxide and hydrogen from the oxygen, promoting an electrochemical reaction
that generates electricity without burning the fuel.
Because
there is no burning involved, the system produces less ash and other air
pollutants than would be generated by combustion. It does produce carbon
dioxide, but this is in a pure, uncontaminated stream and not mixed with air as
in a conventional coal-burning plant. That would make it much easier to carry
out carbon capture and sequestration (CCS) -- that is, capturing the output gas
and burying it underground or disposing of it some other way -- to eliminate or
drastically reduce the greenhouse gas emissions. In conventional plants,
nitrogen from the air must be removed from the stream of gas in order to carry
out CCS.
One
of the big questions answered by this new research, which used simulations
rather than lab experiments, was whether the process would work more
efficiently using steam or carbon dioxide to react with the particles of coal.
Both methods have been widely used, but most previous attempts to study
gasification in combination with fuel cells chose the carbon dioxide option.
This new study demonstrates that the system produces two to three times as much
power output when steam is used instead.
Conventional
coal-burning power plants typically have very low efficiency; only 30 percent
of the energy contained in the fuel is actually converted to electricity. In
comparison, the proposed combined gasification and fuel cell system could
achieve efficiency as high as 55 to 60 percent, Ong says, according to the
simulations.
The
next step would be to build a small, pilot-scale plant to measure the
performance of the hybrid system in real-world conditions, Ong says. Because
the individual component technologies are all well developed, a full-scale
operational system could plausibly be built within a few years, she says.
"This system requires no new technologies" that need more time to
develop, she says. "It's just a matter of coupling these existing
technologies together well."
The
system would be more expensive than existing plants, she says, but the initial
capital investment could be paid off within several years due to the system's
state-of-the-art efficiency. And given the importance of reducing emissions,
that initial capital expense may be easy to justify, especially if new fees are
attached to the carbon dioxide emitted by fossil fuels.
"If
we're going to cut down on carbon dioxide emissions in the near term, the only
way to realistically do that is to increase the efficiency of our fossil fuel
plants," she says.
https://www.sciencedaily.com/releases/2016/04/160404111312.htm
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