Producing power is by nature a wasteful enterprise. Whenever you convert one energy source into another there is inevitably some energy lost to the process. Typical power generation plants use a stable yet highly energy dense fuel to supply the process. With all fuel types there is an initial energy investment in processing the fuel into a state that is unstable and therefore capable of releasing its stored energy. With combined cycle power plants this initial energy investment is reduced by the capture of waste energy from the process.
Combined cycle power plants are capable of using multiple petroleum and natural gas derivatives in a combustion chamber. The heat from combustion converts a liquid substrate (usually water) into steam which then rotates a generator thus creating electrical power. After a given volume of steam has rotated the generator it is then directed to a cooling tower where the steam is converted back into liquid form for reuse; in traditional power plants the excess heat that is in the steam after it has moved the generator is lost to the atmosphere as the steam condenses. In the combined cycle power plant system the additional heat from the condensing steam is captured and used to pre-heat the liquid for a second reactor. This means that the second reactor requires less fuel to generate steam for its cycle.
Combined cycle power plants are more efficient that traditional power plants. They are also typically more versatile in their choices of fuel stock. In combined cycle power plants the dominate fuel for the immediate future is compressed natural gas. CNG has fewer waste emission components than coal, diesel, or synthetic fuels. Additionally the US has large untapped deposits of this fuel source from which to draw.
Here is a link to the basics of combined cycle plants.
Here is an interview with an engineer in Sri Lanka who is the project manager for a large combined cycle power plant there.
Here is a link for a combined cycle power plant that uses coal derived synthetic gas; the IGCC combined cycle power plant.
by Ben Wutzke