An increase in maximum superheated steam, from 600-650°C, corresponds with a rise in net electrical efficiency of 1-3% points (LHV).
Every increase in efficiency reduces the demand for coal and reduces all emissions from coal-based power production. But the maximum temperature is limited by the types of steel that must operate in these conditions for 40 years or more.
The introduction of martensitic steels means that the superheated steam temperature in supercritical (SC) plant is up to 580°C, reaching 40% net electrical efficiency (LHV).
In state-of-the-art ultra-supercritical (USC) plant, the superheated steam temperature is up to 605°C using improved martensitic steels and austenitic steels, and the efficiency can reach 47%. Currently, steels that can tolerate 650°C are in development.
Using 650°C steels and new power plant components, it may be possible to get close to 50% efficiency without exotic nickel alloys.
There is real demand for steels that can tolerate high temperatures, according to the author of the report, High temperature steel in pulverised coal technology, Kyle Nicol.
“But it takes more than a decade of mechanical and chemical testing to develop new steels,” he said.
“Then the long-term performance in service is modelled. Despite this research, new high temperature steels in commercial service can fail unexpectedly. This leads to reduced availability and reliability, and means increased maintenance costs and reduced operational profit.
“So, new steels are a risky, expensive and long-term investment. However, the goal of increased electrical efficiency makes this effort worthwhile.”
Nicol reviews and assesses in detail the performance, problems, solutions and research efforts for steels in PCC plants in the report.
He found that ferritic steels were low-cost and well-proven, but seemed to have reached their limits. Stress corrosion cracking (SCC) of the new T24 has been found in recently commissioned water walls. But after preventative measures were introduced no SCC was detected.
Martensitic steels are used widely in SC and USC plant. They have high creep and fatigue resistance, low coefficient of thermal expansion, high thermal conductivity and good fabricability.
However, they require expensive and time-consuming post-weld heat treatment, suffer from long-term weakening mechanisms and are susceptible to Type IV cracking.
Austenitic steels are used in the superheater and reheaters of USC plant and have excellent performance up to 665°C. But due to their high coefficient of thermal expansion, cyclic operation can result in dissimilar metal welds (DMW) failures.
More PCC power plants now operate in cyclic mode despite being designed for base load operation. This is because they have to respond to the intermittency of renewable energy on the grid, as well as market forces.
Severe cyclic operation, beyond design criteria, damages all steels. New methods can accurately estimate when failures may occur, so that preventative action can be taken to minimise financial losses.
