Status of PFBC and experience with materials performance during over 75 000 hours of operation in ABB'S PFBC plants

Abstract

The key components of ABB's pressurized fluidized bed combined cycle (PFBC) plants are the specially designed gas turbine, which we refer to as the PFBC machine, and the pressurized fluidized bed boiler used to generate and superheat steam for expansion in a steam turbine. ABB Stal's 17 MWe GT35P and 70 MWe GT140P PFBC machines, respectively, are used in ABB's 100 MWe P200 and 425 MWe P800 modules. Particulate cleanup before expansion in the turbine sections is with cyclones. So far, over 75 000 hours of operation has been accumulated on P200 modules in the world's first PFBC plants, demonstrating that PFBC meets expectations. The GT35P machines have been found to perform as expected, although high cycle fatigue failures of the variable speed low pressure turbine blades have been experienced in the Tidd and Värtan plants, leading to minor modifications of the blades and also to a change of blade material. Following the observation of some wear of turbine blading in the GT35P machines, and as originally planned while these machines were being designed, erosion protective coatings have been applied at locations where secondary flows and the formation of vortices causes enhanced interaction between particulates and the turbine components. Wear on boiler tubing and membrane walls has been very limited, certainly no more severe than is typical for AFBC units, and found to be controllable through use of coatings, in the case of evaporator and other low temperature component surfaces, or by the application of sleeves at locations with enhanced tube—particle interaction. The flame sprayed Metcoloy-2 coating originally applied to all evaporator tubing has performed very well in the Tidd plant, whereas some very local flaking of this coating has been noted in the Escatron plant. By comparison, extensive flaking of this coating has occurred in the Värtan plant. This was found to involve oxidation of the bonding layer under the micro porous flame-sprayed coating, and embrittlement of the Metcoloy-2 layer. It is believed that differences in operating conditions, i.e., in surface temperature and corrosive environment, may explain these differences in performance. During the summer of 1996, the tube bundle in one of the two boilers at Värtan was replaced with a new tube bundle, in which the evaporator tubing has a sprayed and fused coating, which should be impervious to the combustion gases. Together, the experiences obtained in the different plants provide a very significant database which is applied for the continued improvement of existing plants and also in the design of new plants. The next P200 plant will be built in Germany for operation on brown coal. The first P800 plant will be built at the Karita site of Kyushu Electric Power Company, Japan. The GT140P machine for that plant has already been manufactured and is now being shop tested at the ABB Stal workshops in Finspong, before shipment to Japan. The new P800 boiler is being manufactured at IHI's Aioi factories in Japan. Future development of ABB's PFBC plants is foreseen to include raising the turbine inlet temperature through combustion of a topping fuel in order to reach thermal efficiencies which ultimately may be in the range of 50–53% (LHV). Otherwise, the main attention is focussed on improved availability, and further enhanced performance and fuel flexibility.

Keywords:

• PFBC
• materials
• boiler tubing
• gas turbine
• coatings