Ash Cooling Screws: A Retrospective and Looking Ahead

The first commercial screw cooler on record for cooling bottom ash from fluidized bed combustion (FBC) boilers was installed in 1979. Since then, over 300 ash screw cooler (ASC) machines have been used for this duty worldwide. The relatively quick upsurge in developing and building FBC boilers in the early 1980’s forced the industry to select existing cooling screw designs, typically used in less severe services in the chemical, grain, plastics and food industry. At the time, these were the only reasonable and readily available machines thought capable of handling abrasive, corrosive and extremely hot bottom ash. The extreme difficulties of operating and maintaining these units in this service quickly became apparent, but only after quite a number of initial designs had already been designed, purchased and installed. Today, 23 years later, both the manufacturers and users of ASC’s can look back and point to installations where, thanks to communication and cooperation between operators and the manufacturer, operating units and their installations have been successfully modified, resulting in smoother operation and much less frequent maintenance. New units can incorporate a range of features, depending upon the anticipated ash quality. As a result, ASC are now reportedly ranked low on the list of FBC operator’s and manager’s concerns. This paper will address what has been learned by both manufacturers and operators. The results of a survey of the operating and maintenance experience as well as maintenance costs of a wide sample of ASC users will be presented. A look ahead as to where ash screw cooler technology is moving is addressed, with help and continued input from current users of the equipment.