Coal-Fired Fluidized Bed Combustion Cogeneration

Abstract

The availability of an environmentally acceptable multifuel technology, such as fluidized bed combustion, has encouraged many steam producers/ users to investigate switching from oil or gas to coal. Changes in federal regulations encouraging cogeneration have further enhanced the economic incentives for primary fuel switching. However, this addition of cogeneration to the fuel conversion analysis considerably complicates the investigation. A system design for cogeneration of steam and electricity at a nominal 40,000 pound per hour capacity utilizing fluidized bed combustion is described. The basic system incorporates silo storage of coal, ash, and limestone with dense phase conveying. The system generates power utilizing either a backpressure turbine or a condensing turbine with steam extraction. Three case studies performed for specific end users are presented. The interaction among plant steam requirements, rate purchase structure, and electrical energy buy back rate is discussed. How these factors interact determine the final design and the choice of fuels is illustrated. Because the decision to switch fuel, as well as to cogenerate, is usually economically motivated, an in-depth understanding of the steam/electrical needs and interactions is critical. How these considerations are integrated in the system and the effect they have on the monetary returns are discussed. Electric rate agreements vary significantly from one state to another. Therefore, the examples selected are intended to provide, insight into this variability. For example, one rate structure encourages solid fuel cogeneration. The second is a block structure with low sell back rates making cogeneration difficult to justify. How these rate schedules affected the recommended design illustrates that the system selection is very important.