The 5,000MW Waigaoqiao coal-fired power station in Shanghai, China. Photo: Siemens press picture.
Led by Mr Feng Weizhong, the engineers at Waigaoqiao 3 coal-fired power plant have been working hard continuously on a series of innovative ‘5E technologies’: Energy saving, Efficiency preservation, Environmental protection, Ensuring safety, and Elevated T-G unit. Apart from the elevated T-G unit technology, the other four have been applied successfully at Waigaoqiao 3, making it one of the most state-of-the-art high efficiency power plants in the world.
Currently, they are applying the 5E technologies, especially elevated T-G unit technology, at the new Pingshan Phase 2 power plant. On 19 October 2018, accompanied by Mr Li Li and Mr Zhang Jiancheng, I had the pleasure of visiting the construction site of Pingshan Phase 2 power plant.
The elevated T-G unit technology (described below) was developed by Feng’s team to address the problems with the conventional double reheat system design for ultrasupercritical (USC) units. Engineering studies for this technology indicated its feasibility and economic advantages. Patents for it have been granted in China, the USA and the EU. The National Energy Administration (NEA) of China approved its technical evaluation and authorised a national demonstration project for the technology in 2015. It was initially planned to build the project next to Waigaoqiao 3 power plant. However, the Shanghai government did not approve it due to its coal-control policy, so it landed in Pingshan town in Anhui province. The project passed its overall final approval on 28 December 2016 and construction started in 2017. Apart from the elevated T-G unit technology, the further improved other 4E technologies will also be applied in the new single 1350 MW Pingshan Phase 2 USC unit.
Upon commissioning by the end of August, 2020, the Pingshan Phase 2 will be the most efficient and cleanest plant in the world.
1 Elevated T-G unit for double reheat USC
Until the 1950s, pulverised coal combustion (PCC) power plants operated at subcritical steam conditions and employed a single circuit of steam through a boiler and turbine. The single steam reheat loop and second reheat loop were introduced which increased the electrical efficiency. However, the reheat loops require additional boiler and turbine components, which raised capital costs. For a 1000 MW single reheat unit, the pipelines can be up to 200 metres long. In the double reheat system, the steam has to shuttle between the boiler and turbine twice, which further increases the flow resistance and heat losses for the second reheat (see Figure 1). The steam in the pipe increases the thermal inertia which slows the turbine load adjustment. Arranging these large diameter thick wall pipes within the plant building is also quite difficult.
Figure 1 Steam flow for a conventional double reheat unit (Feng and Li, 2017)
Although the savings made in fuel consumption and the increased electrical efficiency can offset the problems described, Feng’s team decided to renovate the system to reduce the heat lost and further improve efficiency. This re-design is known as the cross compound with elevated and conventional layout, referred to as Elevated T-G unit. In this arrangement the turbines are split into two trains. The front train, consisting of the high pressure turbine (HP) and intermediate pressure turbine (IP1) coaxial with one generator as the front unit, is mounted on top of a two-pass boiler or near the outlets of the tower type boiler steam headers, which is around 80-85 m above ground level. The rear train, which consists of the IP2 and the two LP turbines coaxial with another generator as the rear unit, remains in the conventional position, roughly 17 m above ground level. As shown in Figure 2, by raising the HP turbine to the level of the boiler steam header, the main steam pipe ① and cold reheat steam pipe ② in Figure 1 are minimised. Raising the IP1 casing with HP casing, the hot reheat pipe I ③ and the cold reheat steam pipe II ④ are also minimised. The remaining hot reheat pipe II ⑤ can also be shortened if needed. The shorter pipework reduces the pressure drop and temperature loss of steam from the boiler, which increases efficiency. The cost of the piping is significantly cut.
This idea will be implemented for the first time at Pingshan Phase 2 power plant. The layout plan is as Figure 3. The main design parameters of Pingshan Phase 2 are in Table 1.
Figure 2 Steam flow for a cross compound double reheat unit with the elevated and conventional layout (Feng and Li, 2017)
Figure 3 Layout plan of Pingshan Phase 2 double reheat cross compound unit (Mao and Feng, 2017)
|Table 1 Main parameters of Pingshan Phase 2 1350 MW turbine (Latest version, 2018)|
|Turbine Parameter||Design value (THA)|
|Mass flow rate of the main stream||3192 t/h|
|Main/first reheat/second reheat pressure||31.1 MPa/8.99 MPa/2.29 MPa|
|Main/first reheat/second reheat temperature||610°C/630°C/623°C|
|Condensate temperature/pressure||19°C/4.0 kPa|
|Heat rate||6897 kJ/kWh|
2 Further optimisation with the other 4E technologies
The other 4E techniques developed and used at Waigaoqiao 3 are also exploited at this single 1350 MW unit. Generalised regeneration technology is used to expand the regenerative cycle from a single feedwater-based system to the whole unit, including water, air and coal to reduce turbine exhaust losses. Some renovations are made to narrow the gap between design and operational unit net efficiency:
- Low load condition: To ensure steady combustion under low load, on the turbine side, the feedwater temperature is kept at the rated level by adding an adjustable valve on the extraction pipe to keep the outlet pressure unchanged. On the boiler side, low oxygen and low NOx combustion technologies are used over the whole load range. For auxiliary facilities, the generalised variable frequency power system will be applied.
- Load changing condition: In order to respond fast to requests from the power grid, the governor valve of the turbine is generally throttled to retain a certain degree of thermal storage capacity. Changing the turbine load by a combination of condensate water frequency control and adjustable LP and HP steam extraction frequency control can eliminate throttling losses.
- Seasonally adapted operation: A technology which switches off the LP turbine in the summer has been developed but will not be used at Pingshan Phase 2 due to the late application.
- Solid particle erosion (SPE) problem: the successful solution from Waogaoqiao 3 has been updated and used to solve the SPE problem.
Although the double reheat system is built with existing mature 600°C materials and equipment, the Pingshan Phase 2 1350 MW unit is on its way to becoming the most efficient coal-fired power plant in the world at 49.8% (net, LHV, THA condition).
Feng W; Li L (2017) China’s National Demonstration Coal Power Project Achieves Around 50% Net Efficiency with a 600°C Class Material. In 8th IEA CCC Conference on Clean Coal Technologies, 8 – 11 May 2017, Cagliari, Sardinia, Italy. 37 pp (May 2017)
Mao J; Feng W (2016) Innovative technology for double reheat USC. In EPRI Asia Coal Power Technology Seminar, 6 Sep 2016, Tokyo, Japan. 19 pp (Sep 2016)