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Fluidized-bed upgrade improves efficiency and reliability at university power plant

Issue 4 and Volume 104.

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THE UNIVERSITY OF IOWA’S power plant is the sole source of steam to the main campus and the University of Iowa’s hospitals and clinics. Year round, the cogeneration plant must supply steam for heating, chilled and hot water production, and sterilization. In addition, the plant supplies 20-30 percent of the university’s electric load requirements.

When constructed in 1920, the Iowa facility produced 250,000 lb/hr of heating and process steam. Since that time, the load requirements have increased dramatically, and today the cogeneration plant has a capacity of 730,000 lb/hr of steam and 21 MW of electric power. The power plant has two solid fuel units-a stoker-fired traveling grate boiler and a circulating fluidized-bed boiler, each with a 170,000 lb/hr capacity-and three gas/oil-fired boilers-two with 120,000 lb/hr capacities and one with a 150,000 lb/hr capacity. The 150,000 lb/hr boiler is only used for peaking and backup service.

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The fluidized-bed unit was installed in the late 1980s to supply additional steam to the university. After start-up, two older low-pressure, stoker-fired units were removed from service. A major reason for selecting the fluidized-bed unit was its ability to burn a wide variety of coals while reducing emissions of sulfur dioxide, nitrogen oxide and particulates.

Unfortunately, over the years the fluidized-bed’s reliability and efficiency had deteriorated. By 1994, the boiler required an upgrade to improve its overall performance and reliability.

The original fluidized-bed design utilized a combination of natural and forced circulation. The external heat exchanger, the major source of heat transfer from the combustion process to the steam generator, used forced circulation. With the modifications, only natural circulation is used, eliminating the need for two auxiliary water circulation pumps.

To compensate for the loss of surface area in the external heat exchanger, the university added surface area to the combustor walls, water-cooled cyclones and loop seals. In addition, using water to cool the cyclones and loop seals eliminated the need for expansion joints between the cyclone outlet and the combustor.

Another upgrade entailed the conversion of the superheater outlet valve to an automatic control valve. This valve serves as a muffled vent, but is also used during unit start-up to ensure there is an adequate flow of steam through the superheater, and can be used to automatically trim the boiler pressure as well. During severe transients, the valve minimizes the safety valve’s operation by maintaining superheater outlet pressure below the first safety valve setting.

Further modifications included the elimination of the external heat exchanger and its associated hot and cold solid material recycle system. In the upgraded unit, material flow from the cyclone outlet, back to the combustor, is through a loop seal. Because the secondary air requirements for the modified process were greater than that required by the old fluidized-bed unit, a larger secondary air fan was added.

The university also modified and upgraded the plant’s ash and coal handling systems. Although the fly-ash hoppers on the bottom of the baghouse and air heater were reused, the bottom ash collection system was modified by the addition of two water-cooled screw coolers. The external heat exchanger ash bin was relocated to serve as the combustor bottom ash bin. Similarly, the old dense-phase coal transport system, from the silos to the boiler mini-bunker, was replaced with a drag-chain conveyor system.

Before the upgrade, the fluidized-bed unit could only burn coal. With the modifications, the upgraded fluidized-bed unit now burns petroleum coke, other opportunity fuels, or a blend of two different fuels. The upgraded fuel system incorporates new gravimetric feeders on the outlets of two adjacent silos. Fuel leaving the gravimetric feeders is fed via a common drag chain conveyor to a mini bunker ahead of the boiler. Coal crushers were installed between the outlet of the drag chain conveyors and the boiler mini bunker.

Typically, the unit is operated at approximately 70 percent load. Since the modifications, the unit’s capacity factor has increased substantially. Because the fluidized-bed replaced steam that was more expensive to produce, the plant has reduced its operating costs. Figure 1 shows the improvement in capacity factor after upgrading the fluidized-bed unit.