Pulverised-coal combustion is the most widely used technology in coal-fired power plants and is based on decades of experience. The main developments have involved increasing plant thermal efficiencies by raising the steam pressure and temperature at the boiler outlet/steam turbine inlet whilst ensuring that the units can operate reliably and load-follow satisfactorily. The majority of existing coal-fired power plants are based on subcritical technology. Supercritical pulverised-coal power plants first came into operation in the early 1960s. More recently, in the 1990s, ultrasupercritical facilities have been constructed and operated successfully. Raising the steam pressure and temperature results in higher efficiencies (see table above) and thus lower total plant emissions, including CO2. A potential 50% efficiency (LHV basis) is foreseen for ultrasupercritical technology (depending on plant location).

At an operating pressure above 22.1 MPa (supercritical) in the evaporator part of the boiler, the cycle medium is a single phase fluid with homogeneous properties and there is no need to separate steam from water in a drum. Hence, once-through boilers are favoured in supercritical cycles. These do not require a boiler blowdown which has a positive effect on the water balance of the plant, with less condensate needing to be fed into the water steam cycle and less waste water to be treated and disposed of. However, they usually require a condensate polisher and use oxygenated water chemistry to maintain steam purity. Once-through boilers can be used with pressures of more than 30 MPa without any changes in process engineering. Wall thicknesses of the tubes and headers however need to be designed to match the planned pressure level. In the convective sections are the superheater and reheater sections and an economiser. The final stage of heat recovery is the air heater, which generally takes the flue gas temperature down from an economiser exit temperature of 350–400ºC to 120–150ºC.

Today’s supercritical pulverised-coal power plants provide reliable, cost-effective power on a continuous basis, good load cycling capability, fast daily start-ups and fast, sustained load response. Because total plant costs vary considerably depending on project scope and specifications, it is difficult to make a direct comparison between subcritical and supercritical capital costs. However, for a comparably sized plant the engineering, procurement and construction (EPC) cost for a supercritical unit is 2–5% higher than a subcritical unit. Operation and maintenance costs are about the same for the two designs. Supercritical units have additional costs for the condensate polisher and more sophisticated maintenance requirements, but lower costs for consumables such as limestone, ammonia and coal due to higher efficiencies.

Current materials based on ferritic/martensitic alloys permit steam temperatures up to around 600°C in state-of-the-art supercritical plants. Although iron-based alloys could be further developed to achieve even higher conditions, it was recognised during the early 1990s that there would be greater scope for advancement by exploiting alloys based on nickel. The increased steam conditions affects primarily the waterwalls, final superheater and reheater tubing and the thick-walled components (mainly the high pressure outlet headers and the piping to the turbine).

High efficiency supercritical and ultrasupercritical pulverised-coal power plants can satisfy the growing demand for electricity worldwide and ease increasing reliance on natural gas to fuel power plants. These technologies combined with the installation of best available air pollution control equipment will reduce total emissions including CO2 by burning less coal per megawatt-hour (MWh) produced, thus capturing most of the pollutants prior to release. The application of carbon capture and storage (CCS) will be key to the future acceptability of pulverised coal fired plants.

Performance of advanced coal-fired power plants
Herminé Nalbandian
CCC/135, ISBN 978-92-9029-454-2, May 2008
£255 non-member countries
£85 member countries
£42.50 educational price