Double-reheat cycles improve efficiency of coal-fired power units, so what's happening to them?
Posted: 27 August 2015 By: Debo Adams
Although double-reheat cycles can significantly improve the electrical efficiency of pulverised coal combustion (PCC) plants, most use single-reheat cycles. Surprisingly, no double-reheat units have been commissioned since the 1990s. Now, as primary energy costs rise, more stringent emission limits are put in place and advances are made in thermal power engineering, double-reheat cycles are coming back into the spotlight, especially in China.
History of double-reheat units
Until the 1950s, a single circuit of steam passed through the boiler and turbine of PCC power plant. Then a single steam reheat loop was introduced which increased the electrical efficiency by a few percentage points. The reheat loop requires additional boiler and turbine components, which raises capital costs, but savings are made in fuel consumption. In the late 1950s, a second reheat loop was introduced. This double-reheat has an electrical efficiency one to two percentage points greater than a single-reheat system.
Most double-reheat units were built in the 1960s and 1970s in Germany, the USA and Japan. They fell out of favour in the 1970s as other fuels became more widely used. By 1998, 37 coal-fired double-reheat units, amounting to 17.7 GWe, had been commissioned worldwide; just 0.4% of the total global fleet.
The peak performance of double-reheat technology was set at Nordjylland unit 3 in 1998, with ultra-supercritical (USC) steam conditions of 582°C/580°C/580°C/29 MPa, and a world record in electrical efficiency of 47.2% (net, LHV).
Prospects for new build double-reheat units
The Chinese growing demand for electricity, has led to the planned commissioning of 10 GWe of USC double-reheat units in 2015 alone. As of mid-2015, Guodian Taizhou power plant has two double-reheat units with USC steam conditions of 600°C /620°C/620°C/29 MPa, which are the largest double-reheat unit size in the world at 1000 MWe each and have set a new world record in electrical efficiency at 47.94% (net, LHV).
Improved double-reheat steam cycles, such as the Master Cycle, the Echelon Cycle and the Regenerative Turbine, employ a separate turbine module for feedwater preheating. Such cycles improve the electrical efficiency of double-reheat units up to 50% net electrical efficiency (LHV), and reduce capital costs.
Improved plant configuration to reduce the length of pipework can also increase unit efficiency and reduce capital costs. Waigaoqiao unit 9 (China) plans to mount the higher pressure steam turbine modules at the same level as the boiler steam headers outlets and the lower pressure turbine modules in the conventional turbine house to reach roughly 48.9% net electrical efficiency (LHV).
Advanced ultra-supercritical (AUSC) technology, which uses nickel alloy components to achieve higher steam temperatures, may become commercial in the early 2030s. A 600 MWe demonstration unit, using double-reheat at steam conditions 700°C/720°C/720°C/35 MPa achieving up to 50% net electrical efficiency (LHV), should be operational in Japan in 2021. From 2031, commercial AUSC double-reheat units are expected to reach 54% net electrical efficiency (LHV).
As Kyle Nicol concludes in his latest report for the IEA Clean Coal Centre, (Application and development prospects of double-reheat coal-fired power units, CCC/255): “Double-reheat technology is a proven technology for high efficiency and low emissions (HELE) power generation. When designing a new a pulverised coal fired power plant, the decision to opt for double- or single-reheat is based on an economic trade-off between capital cost and fuel cost”.
The figure below shows a scatter graph of the efficiency and unit size of all double-reheat units, operating, decommissioned, under construction and the planned AUSC units.