Erosion is a generic problem in fluidized bed boilers. This paper identifies the erosion-prone areas in circulating and bubbling bed fluidized bed boilers, and the currently used methods for erosion protection. Recent experience with thermal spray coatings in these areas is discussed as are the complications presented by burning fuels that introduce corrosion to the already erosive environment. The paper concludes with some comments on future considelations for the protection of fluidized bed pressure parts from general wastage due to erosion and corrosion.
Circulating fluidized bed (CFB) and bubbling fluidized bed (BFB) boilers have been commercially available technologies for burning a variety of solid fuels since the mid 1980’s. These boilers are generally characterized by freeboard furnace velocities ranging from 2m/s (bubbling fluidized bed) to 6 m/s (circulating fluidized bed). In the BFB, this results in a dense bed restrained to the very lowest part of the fumace (Figure 1) where most of the erosive wear takes place. In the CFB, the bed density continuously decreases from the lower furnace to the upper furnace where the particulate leaves and enters a gas-solids separator (cyclone) where particles larger than 40 µm are removed and recirculated the fumace bottom while the smaller particulate and flue gas continues on through the backpass of the boiler (Figure 2). Erosion can occur anywhere in the BFB or CFB gas path.
The severity of the erosion experienced in the fluidized bed boiler will depend upon the type and configuration of boiler (i.e. BFB vs. CFB), the characteristics of the fuel ash, the material utilized as an inert bed material (limestone, sand or recycled ash) and several operating parameters affecting flue gas velocity and temperature. Whereas a discussion of the mechanisms for erosion in fluidized bed boilers is outside the scope of this paper, the areas prone to erosion and the erosion protection methods employed both past and present will be discussed. In particular, recent experience with the use of thermal spray coatings will be described and an attempt will be made to speculate on the future of thermal spray coatings as erosion protection in fluidized bed boilers.
EROSION AND EROSION PROTECTION IN FLUIDIZED BED BOILERS
Referring to Figures 1 and 2, erosion has generally been found in the following areas of fluidized bed boilers:
Lower furnace walls near bed
In-bed tubes
In-bed tube supports
Grid floor air distributors
Backpass areas having high velocities (lanes)
Lower furnace walls just above refractory interfaces
Waterwall near cyclone inlets
Evaporative and superheater surfaces in furnace and in external heat exchanger
Fumace roof tubes
Superheater tubeslbends
Economizer hanger tubes/bends
Tubing in backpass sootblowing lanes
Air heater tubing
BFB:CFB:In many cases, erosion-prone areas have been addressed by design changes in second and third generation boilers. However, where retrofit of new designs has not been technically or economically practical, other erosion protection methods have been employed. For example, in CFB''s, refractory and weld overlay (Figure 3) have traditionally been used to protect lower furnace areas from erosion.
