Pulverised coal combustion (PCC) power plants dominate the power industry and will continue to do so for the foreseeable future. Increasing PCC plant electrical efficiency guarantees lower coal consumption, resulting in reduced fuel costs and sustaining valuable coal resources. A higher-electrical efficiency also reduces the amount of flue gas to be treated in the flue gas cleaning systems and assists the deployment of carbon capture and storage (CCS).
Increasing the superheated steam temperature is the single most effective way to increase the net electrical efficiency of PCC plant. The maximum superheated steam temperature is limited by materials in high temperature components that can operate at this temperature for a service lifetime of 20 – 40 years. Historically, the steam temperature has increased with the development of steels, from subcritical steam temperatures to supercritical (SC) to the present state-of-the-art ultra-supercritical (USC). However, further development of steels has been negligible – the steel barrier has been reached. Fortunately, current research aims to use nickel alloys to reach 700oC superheated steam, known as advanced ultrasupercritical (AUSC), with a corresponding net electrical efficiency at roughly 50% (LHV, black coal).
Developing new high-temperature materials is complex. Numerous parent and weld materials are available; high-temperature components vary in shape and size; there are a few fabrication and welding processes to choose from; and different high temperature components require certain material properties depending on the type of stresses and chemical attack involved. Therefore, high temperature materials research programmes will be expensive, time-consuming and risky. Consortia of energy utilities, component manufacturers and research establishments are required to combine their individual strengths and resources in order complete such programmes. Additionally, the inherent technical risk with materials translates to economical risk to investors, so financial support from government bodies is required to dampen the financial risk.
There are high temperature materials research programmes for AUSC PCC power plant in the US, the EU, Russia, Japan, China and India. This report reviews developments and status of these programmes.
The candidate materials for all programmes are similar. Low cost and proven ferritic steels are employed in components exposed to steam temperatures below 550°C. Proven state-of-the-art 9 – 12% chromium martensitic steels are used in components exposed to steam temperatures in the range 550 – 625°C. Austenitic steels can be used for superheater and reheater applications only with steam up to 625°C. The new generation of largely unqualified nickel alloys, 617, 625 and 740, are expected to be applied in thin- and thick-section components at temperatures above 630°C. To maintain economic favourability, new steels to fill in the temperature range 625 – 650°C are being developed. Improved steam cycles and alternative plant configurations have been developed for AUSC PCC power plant, but can be applied to SC and USC PCC power plants. The report expands on all these aspects.
The materials research programmes consist of three main stages, which can overlap each other by a few years, depending on technical readiness and funding availability. Stage one, known as small-scale laboratory tests, characterises and screens materials using mechanical and chemical tests over a period of 8 – 13 years. It includes mechanical tests such as tensile, hardness, toughness, fatigue tests and creep tests; long-term creep tests take 100,000 hours (11.5 years). Chemical tests include resistance to steamside oxidation (found in most components in the steam cycle), and fireside corrosion, to which most boiler materials are exposed.
Once sufficient information has been gained from small-scale laboratory tests, stage two manufactures large-scale components from candidate materials, and tests them in a components test facility (CTF). The CTF uses a slipstream from an operational power plant. Manufacturing the components and constructing the CTF takes four to five years. After three to five years of operation, the CTF is dismantled and the components are thoroughly evaluated.
Provided the large-scale components operate successfully, these materials, component design and manufacturing methods can be used to build a full-scale demonstration plant (FSDP) in stage three. Four to six years is required to build a FSDP in the 500 – 1000 MW range. Five years’ operation would then be needed to verify performance, and a further year to evaluate the materials. This process of gradually scaling-up materials is designed to minimise risk.
Programmes in the US, China, Japan and India have largely completed stage one, except for long-term creep tests. The programmes are now making progress in stage two. Due to technical difficulties, the European programme is still in stage two. The Indian programme aims to start operating its FSDP in 2018. For the other programmes, results from the CTF and long-term creep tests, by 2018, will provide enough technical information to decide progression to stage three. Operation of an FSDP in stage three is planned to start in 2021 and performance results would be ready for the year 2027. Providing the FSDP operates successfully and the time frame for a new build PCC plant is four years, then a commercial unit could be brought online in the year 2031. The commercial plant will be of similar size, as scaling up the technology is risky. The net electrical efficiency of a commercial unit is estimated to reach 52% (LHV, black coal), depending on its location and whether it has single or double number of reheat.
Assuming successful operation of an FSDP, the commercialisation of AUSC PCC power plant will rest entirely on the plant economics in 2027, which depend on variable factors, such as the future value of coal, cost of nickel alloys and a carbon tax.
The full report, ‘Status of advanced ultra-supercritical pulverised coal technology’, by Kyle Nicol, is available from the IEA Clean Coal Centre website.
Adapted to World Coal house style by Sam Dodson
Read the article online at: https://www.worldcoal.com/special-reports/07012014/status_of_advanced_usc_pulverised_coal_technology_384/