(Credit: MIT)
New York, April 5 (IANS) Scientists have proposed a new concept that could make 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.
The concept, proposed by doctoral student Katherine Ong and professor Ahmed Ghoniem at the Massachusetts Institute of Technology (MIT), combines into a single system two well-known technologies -- coal gasification and fuel cells.
Coal gasification is a way of extracting burnable gaseous fuel from pulverised coal, rather than burning the coal itself, while 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, explained in the Journal of Power Sources, 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, Ong said, 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 said.
First, the coal is pulverised 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, the MIT researchers said.
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 study, 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.
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