Development and Construction of  World Largest 360MW PFBC Having Hexagon Shaped Furnace

Hironori Hatazaki, Thermal Power Department, Kyushu Electric Power Co., Inc, Hidekatsu Takayama, Nishinippon Environmental Power Co., Inc, Takeshi Matsuo, Kyushu Electric Power Co., Inc, Karita Thermal Power Station Ishikawajima-Harima Heavy Industries Co., Ltd, Jyunichi Hamanaka, Ishikawajima-Harima Heavy Industries Co., Ltd and Sakae Ando, Ishikawajima-Harima Heavy Industries Co., Ltd

Pressurized Fluidized Bed Combustion Combined Cycle Power Generation, namely, PFBC is the clean coal technology, utilizing gas turbine and steam turbine, which is high efficient and friendly to earth. In early 90's, 70 MW class PFBCs had started demonstration and commercial operation all over the world. Kyushu Electric Power Company decided to apply this technology as the real commercial unit, the world largest capacity 360MW, and put into commercial operation in July 2001.
To apply PFBC to the large scale commercial plant, it is essential to demonstrate the higher efficiency than any other conventional coal firing units. In order to achieve this, the gas turbine with higher operation pressure and advanced supercritical steam condition for steam turbine were applied and achieved higher efficiency comparing to conventional units. The reduction by size and weight of the equipment is the vital must to realize large scale PFBC, as 360MW unit. To reduce the pressure vessel size, the unique design of hexagon furnace was applied to install it efficiently in smaller vessel.

The process of PFBC is shown in Figure 1 and Figure 2 denotes the outlook of the plant. In this process, coal is fed to the fluidized bed boiler as the paste, mixing with water and limestone utilized as the sorbent for desulfurization in the bed. The boiler is located inside of the pressure vessel and superheated steam is generated in the heating tubes of the boiler, being installed in the bed. Hot and high pressure flue gas at 1.3 MPa and 850 deg C, leaving the boiler, enters the gas turbine after passing cyclones for gas cleaning.
When engineering, the elevation of the plant efficiency is the first task to realize the large scale PFBC as a commercial unit. To rise the efficiency more than existing ones, the gas turbine with higher operation pressure, developed newly for this capacity of PFBC, was applied and the advanced supercritical steam condition, 24.1 MPa. and 566 / 569 deg. C, was utilized for steam turbine. At the performance test, it demonstrated the high value, approx. 42% as net efficiency on high heating value basis, which is the highest level among the existing coal fired power generation units. It is also confirmed that the PFBC has better environmental performance and dynamic characteristics in comparison with conventional units.
For the realization of the large commercial use PFBC, it is essential to reduce the size and weight to meet the practical requirement. In PFBC system, equipment such as boiler, cyclones and etc. are located in the pressure vessel to avoid the loading of excessive pressure on them. So, the shape of each equipment and arrangement of them are very significant for the reduction of pressure vessel size. As the shape of the boiler, the unique design of hexagon furnace was applied since it can be put efficiently in the round shaped vessel. Various verification of the soundness of this hexagon design was carried out , including fluidization test using cold model, because this design had never applied before. Careful study on configuration and construction of the hexagon furnace was also executed at engineering stage and it has a number of patent application. Furthermore, the engineering model was made as its scale 1/20 to investigate the soundness of construction and maintenance ability. As a result of those studies, this hexagon design can reduce the vessel diameter by 20% and its weight by 800 ton.
To reduce the weight of the system, new high tensile steel was developed through cooperation with steel makers. For the pressure vessel, the new material for thick steel plate, SPV 490, was developed, which has higher strength by 10%, comparing to the material used for previous PFBC. The new stainless steel for high temperature use was also developed, having higher tensile at high temperature and good weldability, for high temperature equipment such as cyclones.
Owing to the reduction in size and weight through works written above, 360MW PFBC unit could be assembled into one module at workshop and transferred to the site as one package, i.e. one pressure vessel with boiler and auxiliaries inside. It contributed very much to make construction time short and installation cost less.

Owing to the careful study from various views of performance, configuration, material and so on, the world largest 360MW PFBC could be completed as a real commercial unit. Its high efficiency and smooth operation have been verified through the commercial operation started from July '2001. It has opened the new era of coal fired power generation.

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