The boiler is the most important component of most coal-fired power plants. Virtually every power plant now operating or proposed incorporates a steam turbine in the electricity generating process. The boiler heats water until it becomes steam. When the steam enters a turbine and expands, some of energy within it is lost as the turbine spins. If the steam condenses (i.e., if water droplets form) inside the turbine, it can cause damage to the turbine blades. To prevent this, when the steam is produced, additional heat is added to superheat it and raise the temperature to 1000ºF at 2400psi. This allows the temperature of the steam to drop without forming water droplets. Depending on the configuration of the plant, the steam is either recycled and reheated or sent to a cooling tower.
Boilers of this type are called subcritical boilers because they operate below the critical point of water formation. The critical point of a substance is the temperature and pressure above which the liquid and vapor state are no longer distinguishable. As a liquid is heated, its density decreases while the pressure and density of the vapor being formed increases. As temperature and pressure increase, the liquid and vapor densities become closer and closer to each other until they are the same. At that point, the critical temperature, the 2 densities are equal and the liquid becomes a supercritical fluid.
Under supercritical conditions, the water does not actually boil; it simply decreases in density until it is a vapor. It takes very high temperatures and pressures for most substances to reach this state. The critical point of water occurs at 705ºF and 3212psi (refer to the chart at the right). Supercritical boilers operate at temperatures and pressures above those conditions. As the supercritical steam turns the high-pressure steam turbine, it passes below the critical point and then enters a condenser. The thermodynamic efficiency of a plant using supercritical steam is higher, 40-42%, than that of a similar subcritical plant, 36-38%. The term ultra-supercritical (USC) applies to boilers that operate using pressures over 4400psi. These advanced boilers take advantage of further increases in thermodynamic efficiency and 2-stage reheating to reach a thermodynamic efficiency of 48%.
Supercritical boilers were first developed in the U.S. in the 1950s, but they were unreliable and not operationally flexible. Today, time, experience, and the continuing development of high pressure / temperature materials have made them more robust and flexible. Supercritical boilers are used for all large capacity boiler operations in Japan as well as most European and Asian countries. There are more than 400 supercritical boiler plants in operation worldwide. In the U.S., power companies have been slower to adopt supercritical boilers because most of the plants operating today are very old. But the increasing demand for electricity in the U.S. will inevitably lead to an expansion in the number of new coal-fired power plants. Continuing technology advances and strict federal emission standards for new plants will compel companies to adopt the most efficient technologies possible. The Department of Energy funds several programs with industry that have resulted in cost-effective, efficient, low emission designs for new plants that use supercritical technology. Supercritical technology may become the standard for new plants and possibly for plants that are ready for repowering.
It is important to note that the boiler technology for a given plant is virtually independent of the combustion unit for that plant. Supercritical technology has proven to be effective with virtually every type, configuration, and size of combustor. This means that almost any plant, whether old or new, can be upgraded to a supercritical boiler to increase the overall plant efficiency. There is a high level of confidence in the technology, and material costs are only 2% higher than for a similar subcritical design. And new developments in high temperature materials are paving the way for the adoption of USC boilers. In the boilers, as the temperature and pressure used for steam increases, the efficiency of the boiler increases as well. The main factor limiting the temperature that can be used is the material used in the piping and fittings. So, to compliment the supercritical technology, the Department of Energy funds research to develop high temperature materials and equipment to work with USC systems.