Researchers at MIT have come up with a plan 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 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.
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). Illustration: Jeffrey Hanna
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 efficiencies as high as 55 to 60 percent, Ong says, according to the simulations.
Advanced ultracritical coal plants can get to 52% efficiency
Upgrading existing plants and building new high-efficiency, low-emissions (HELE) coal-fired power plants addresses climate change concerns in two important ways. In the near term, emissions can be reduced by upgrading existing plants or building new HELE plants. Such plants emit almost 20% less CO2 than a subcritical unit operating at a similar load. Over the longer term, HELE plants can further facilitate emission reductions because coal-fired plants operating at the highest efficiencies are also the most appropriate option for CCS retrofit.
The best new coal plants in China are in the Ultra-supercritical 44-46% efficiency range.
Developments in AUSC steam cycles are expected to continue this trend. AUSC coal-fired plants are designed with an inlet steam temperature to the turbine of 700–760°C. Average metal temperatures of the final superheater and final reheater could be higher, up to about 815°C. Nickel-based alloy materials are needed to meet this demanding requirement. Various research programs are underway to develop AUSC plants. If successful, a commercial AUSC-based plant would be expected to achieve efficiencies in the range of 45–52% (LHV [net], hard coal). A plant operating at 48% efficiency (HHV) would emit up to 28% less CO2 than a subcritical plant, and up to 10% less than a corresponding USC plant. Commercial AUSC plants could be widely available by 2025, with the first units coming online in the near future.
Ultracritical coal reactor turbines in China