Recent international agreements to curb greenhouse gas emissions may be challenged by continued reliance on coal-fired power generation, a trend predicted by the International Energy Agency. Increasing the efficiency of coal combustion – only 30% of the energy contained in the fuel is converted to electricity in conventional coal power plants— offers a cleaner route to power production.

Engineers at the Massachusetts Institute of Technology (MIT) propose a hybrid system that combines coal gasification and fuel cells, yielding a 50% reduction in carbon dioxide emissions for a given amount of power produced. Both technologies operate at similarly high temperatures of 800 degrees Celsius or more, and combining them in a single plant would thus allow the two components to exchange heat with minimal energy losses.

Illustration shows steam passing through pulverized coal, releasing gas which enters the fuel cell at the top of the hybrid system. Image source: Jeffrey HannahIn fact, the fuel cell would generate enough heat to sustain the gasification part of the process, eliminating the need for a separate heating system, which is usually provided by burning a portion of the coal. The hybrid scheme also has potential to achieve efficiencies as high as 55-60%.

In coal gasification, which proceeds at a lower temperature than combustion, coal is first pulverized to a powder, which is then heated in a flow of hot steam. 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.

Simulations suggested that the system produces two to three times as much power output when steam is used instead of carbon dioxide to react with the coal particles.