New Technologies for Coal-Based Electricity Generation

 | Sep 07, 2012 | 5:20 PM EDT
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"Clean coal" sounds like an oxymoron. But new technologies are ushering in a fleet of power plants that are generating electricity from coal with greater efficiency and arguably reduced emissions.

Coal, like oil and gas, is a hydrocarbon that possesses interesting properties and challenges. When oil and gas are extracted from earth, the chemical soup that pours from the wellhead is chemically challenging and economically useless. After the soup is preprocessed, oil products are transported to refineries, wet gas products are sent to processing facilities and initial wastes, sludge and contaminants are sent for disposal.

When crude oil arrives and processed at refineries, more wastes are removed. The remaining product is separated into components, such as gasoline, diesel, jet fuel and other products. When wet gas arrives at processing facilities, wastes are removed and gases are separated. Depending on the source, wet gas contains mostly methane ("natural gas") and varying amounts of ethane, propane, butane, carbon dioxide, oxygen, nitrogen, hydrogen sulfide and trace amounts of rare gases. After separation, each product is delivered to individual markets for further processing.

Like the soup flowing from oil and gas wells, coal also contains wet gas and other economically valuable hydrocarbons, along with sludge and other wastes. For decades, companies have been extracting and producing coal gas (also called town gas). Over a century ago, town gas was used for lighting and local heating.

Today, coal gas is treated in the same manner as wet gas to separate out valuable products. Coal gas produces various amounts of hydrogen, methane (dry natural gas), carbon monoxide, ethylene and other hydrocarbons. Most coal gas contains more hydrogen than any other product. (Hydrogen is a highly effective fuel for fuel cells, but that is another subject.)

As companies develop new processes to separate out the valuable products, coal is being increasingly regarded as another form of black gold. With the ability to extract hydrogen and natural gas from coal, the industry already achieved some important milestones. But more progress can be achieved through clean coal technologies. (These technologies are sometimes used in tandem with carbon sequestration techniques to contain the byproducts.)

Electric utilities based on the older technologies burn coal in large boilers in order to generate steam energy. Steam energy is then used to move large turbines and generators, converting mechanical energy into electrical energy. The process releases tons of greenhouse gases and is terribly inefficient in terms of energy produced. Approximately 75% of the energy naturally found in coal is wasted in the energy conversion processes required to create electricity from coal. That is an awful number.

For decades, engineers have attempted to improve coal's energy conversion efficiencies and succeeded only incrementally. Then they looked at electricity production from natural gas, and decided to throw out the boiler and replace it with the gas turbine (think of a jet engine). These gas turbines reached efficiencies of approximately 40%. Then companies like General Electric (GE) and Siemens (SI) took waste heat from the gas turbines and created even more electricity. Today, the combined-cycle gas turbine (CCGT) can achieve more than 60% efficiency in transforming coal into electrical energy.

CCGTs are not cheap. But their high capital costs are offset by low production costs. Some CCGTs can economically outperform most coal-fired power plants.

Learning from the CCGT experience, the U.S. Department of Energy (DOE) teamed with companies like GE Energy to develop new methods of using coal as fuel. Using a new technology called integrated gasification combined cycle (IGCC), they avoided the inefficient steam boilers (and smokestacks), decreased air pollution and increased efficiency. This technology turns coal and other hydrocarbons into a synthetic gas and removes impurities before the gas is combusted. Some of these byproducts can be reused.

TECO Energy (TE) joined the DOE/GE team in the 1990s to build a first-generation ICGG facility. TECO has been operating its 250-megawatt facility ever since. Now Southern (SO) is attempting to build the next-generation IGCC. In a technological twist, 45% of Southern's IGCC fuel mix will be natural gas; the balance will be coal. According to EnergyBiz, a trade publication, IGCC construction at Southern's 582-megawatt facility is two-fifths complete; it should enter commercial operation in two years.

Coal, like all forms of energy, remains controversial. But it is not going away any time soon. The U.S Dept. of Energy projects that coal will remain the primary source of electricity generation for several decades. Coal is projected to fuel 38% of total generation in 2035, down from 45% in 2010. But the volume of electricity it generates will rise 2% over the period. (Natural gas and renewable energy are seen as accounting for a greater share of electricity generation.)

Thus coal will likely remain as the nation's primary fuel to produce safe, economic and reliable power. Policymakers and coal companies understand that maximizing its efficiency is in the national interest, and new generations of technology like IGCC are likely to gain in significance.

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