April 07, 2015
In fossil-fuelled power plants, efficiency can be increased by raising the steam temperature to 700 °C and steam pressure to 350 bar. At the same time, CO2 and other emissions are reduced. Nickel-containing materials are playing a crucial role in such high temperature, high pressure environments.
Higher and variable temperatures
The World Energy Outlook 2014, published by the International Energy Agency, forecasts a rise in primary energy demand of 37% by 2040. Although energy from renewable sources is increasing rapidly, conventional technologies, including highly efficient coal-fired power plants, will continue to make important contributions.
Currently, the average efficiency of a coal-fired power plant is about 33% worldwide and 38% in the European Union. Since the efficiency of any power plant is a function of steam temperature, efforts are underway to increase the operating temperature from today’s maximum of 620 °C to 700 °C and above and, at the same time, increase steam pressure from 250 to 350 bar. This would bring operating
efficiency up to 50%—a 30% improvement over the current best performance.
Sustained high temperatures are not the only challenge. The growing use of renewable energy means that coal-fired power plants will need to work flexibly to be able to balance the fluctuating feed of wind and solar energy. Boiler cycling (start-up and cool down) introduces additional material and operational challenges. Up to 200 cycles per year with about 4,500 operation hours require thin walled parts that can be heated and cooled quickly. Baseload range operation with less or no flexibility (approximately 7,500 operating hours) can be achieved with thick walled parts in the boiler and run with maximum pressure.
Higher nickel alloys needed
Until now and depending on the operating temperature, the boilers of coal-fired power plants are made of structural ferritic, bainitic or martensitic steels such as P91 (UNS K90901) and P92 (K92460), stainless steels such as Type 314 (S31400) or nickel alloys.
About ten years ago the ~50% Ni Alloy 617 (N06617) was selected for manufacturing the first boilers in the European ultra-supercritical 700 °C boiler projects. This nickel alloy is widely used for industrial gas turbines and industrial furnaces due to its high creep resistance in combination with good workability and weldability. With operational experience, a modified version with tighter alloying element tolerances and lower limits for boron was created. This modified alloy, known as VDM® Alloy 617 B, shows an increase in creep rupture strength of about 25% at 700 °C.
Field testing at Mannheim power station
Field trials are important to investigate how materials and finished components behave under real conditions in the power plant. In 2011, a project called HWT II was launched (Hochtemperatur-Werkstoff-Teststrecke/High temperature material test track) to examine the operating and failure performance of thick-walled components for highly efficient power plants. For this purpose, a 725 °C test track with thick-walled pipelines and pipe fittings was set up in the Grosskraftwerk Mannheim GKM Power Plant in Germany, and came on stream successfully in 2012.
In addition to the on-going high temperature base load trial, a part of the HWT II test track is run with temperature cycles between 725 °C and 400 °C. This simulates the start of the boiler when, for example, there is no wind or solar energy available and a shutdown of the boiler when sufficient wind or solar energy is available.
The steam flow leading to the test track is taken from the main boiler and led into separate superheater tubes, in this case thin-walled tubes made of Alloy 617 B and VDM® Alloy C-263 (UNS N07263), to the boiler’s hottest parts. In the boiler, steam is heated in the tubes from 530 °C to the desired 725 °C. It is then fed into the HWT II test track. It is also possible to cool the steam down to 400 °C by introducing cold steam and water into the test track so the effects of repeated thermal cycling on materials can be observed.
Material preparation and performance
VDM delivered a total of almost 20 tonnes of Alloy 617 B and the precipitation-hardening Alloy C-263 for the HWT II test tracks. Both materials were selected because of their good resistance (100,000 hours creep rupture strength) under the specified operating conditions. Due to the high requirements for purity, the alloys were melted and cast under vacuum via Vacuum Induction Melting (VIM), and then remelted via Electroslag Remelting (ESR) and Vacuum Arc Remelting (VAR) respectively, to avoid inclusions as far as possible. Different dimensions were produced, for example 60mm diameter valve parts in Alloy 617 B and thick-walled pipes in Alloy C-263 up to 220mm diameter.
More than 9,900 operating hours at 725 °C and more than 2,600 temperature cycles of 725 °C–520 °C–400 °C–725 °C have been achieved. An inspection has shown no material problems or indications of cracking in the components.
Advanced ultra supercritical steam in China
China has for many years been working to increase the efficiency of coal-fired power plants. For example, the four 1000MW ultra-supercritical (USC) boilers at Yuhuan, in Zhejiang province, operate at an efficiency of around 45% by using 605 °C steam. The first of these units came on-line in 2006, the final one in 2007. But the goal is for even higher steam temperatures, 700 °C or higher, which can lead to efficiencies of over 50%. This is called Advanced USC technology. Nickel-containing austenitic stainless steels are needed at these temperatures for reasons of creep strength, but modifications to existing alloys are needed for cost-effectiveness. Three different alloys are currently being tested, but the most promising is called Super304H. By introducing 3% copper, precipitates will form that allow for high strength at 700 °C while maintaining adequate ductility for use as superheater and reheater tubes. The alloy has also small amounts of niobium, nitrogen and boron as well as a high carbon content. The alloy can be classified as a precipitation hardenable alloy. It is believed that this alloy is one of the keys to achieving 50% efficiency on a commercial scale.