Coal Gasification
Is this a new technology?
Coal gasification is a well-proven technology dating back to the 18th century, although its uses have evolved significantly since then. Interest in coal gasification has wavered in the U.S. during times when the price and availability of competing fuel sources-oil and natural gas-were low. However, recent advancements in gasification technology, increasing costs of oil and gas, growing concerns about energy security, and a heightened awareness of climate change, have all led to a renewed interest in coal gasification for electric power generation in the U.S. and many other countries.
Worldwide there are just four integrated gasification combined cycle (IGCC) plants running on coal today: one in Puertollano, Spain, one in Buggenum, Netherlands, one near Terre Haute, Indiana, and one in Polk County, Florida. So while the technology is not new, our experience with commercial-scale IGCC plants is limited. FutureGen will also be advancing the technology. There is still much to learn, particularly about the economics of operating a commercial-scale plant.
FutureGen will be an IGCC power plant. IGCC is an innovative technology that combines modern coal gasification with a gas turbine and a steam turbine to produce electric power. It is one of the most promising technologies available today for reducing the environmental impacts associated with the use of coal for electricity production.
What are the advantages of IGCC technology?
Coal-fueled IGCC technology offers a number of potential benefits over conventional pulverized coal plants. Depending on the final configuration of the IGCC plant, these can include:
- Higher efficiency - The use of two turbines—a gas turbine and a steam turbine—leads to higher system efficiencies
- Lower emissions - The gasification process enables improved removal of naturally-occurring pollutants in coal, such as sulfur and mercury, resulting in lower emission than conventional coal based power plants.
- Carbon sequestration potential - The IGCC process makes it easier to capture carbon dioxide for carbon sequestration.
- Marketable byproducts - The byproducts associated with the gasification and gas clean-up process may have commercial value in nearby industries.
- Hydrogen as an alternative fuel source - Hydrogen is gaining popularity as a potential clean-burning fuel source of the future for vehicles and other industries. The ability to produce hydrogen from coal for such future applications could prove to be an important benefit of IGCC technology.
How do coal-based IGCC power plants work?
As illustrated in the figure below, IGCC power plants involve a complex chain of activities that start with a carbon-based material—in the case of FutureGen, coal—and result in electricity that powers our homes and businesses.
- The coal gasification process begins with a controlled mixture of coal, oxygen, and steam in a gasifier. An air separation unit separates air into its component parts to supply the gasifier with a stream of oxygen.
- Using a combination of heat and high pressure, the gasifier converts the constituents of coal into a synthetic gas, or "syngas". This syngas is comprised of mostly hydrogen (H2) and carbon monoxide (CO).
- Byproducts captured in the gasifier could have commercial value, depending on local market conditions. For example, the FutureGen plant could produce an ash material similar to what comes from a traditional coal plant. This ash may be used as a filler material in construction projects and building products. Alternatively, FutureGen may produce a glass-like material, known as "slag", which falls to the bottom of the gasifier. This slag may be used in road gravel.
- The syngas is then passed through a water gas shift reactor and reacted over a catalyst with added steam to convert the majority of the CO into carbon dioxide (CO2) and additional H2.
- The syngas will also have small amounts of other impurities (e.g. hydrogen sulfide) which are removed during the gas clean-up process.
- Hydrogen sulfide will be separated from the syngas and converted to elemental sulfur or possibly sulfuric acid. The sulfur byproducts may also have commercial value in a variety of products (e.g. fertilizer), depending on local market opportunities.
- Most of the CO2 is removed from the syngas leaving behind H2-rich syngas.
- One of the things that makes IGCC plants more efficient is the combined use of a gas turbine and steam turbine to produce electricity. The hydrogen-rich syngas is first fed into a gas turbine to generate electricity. The waste heat from the gas turbine is used to power a steam turbine, which in turn creates more electricity.
- Finally, much of the water used in this process will be recycled in the plant while some will be evaporated in a cooling tower.
