Gravimetric and Volumetric Coal Feed Systems for Boilers

    Why Gravimetric Feeding?

    Coal properties constantly vary due to sizing differences and the adverse impact of moisture. These affect heating value, flow characteristics and density. The effect of moisture on coal has been studied for many years by Stock, and the resultant density changes are measured by the Gravimetric Feeder. The gravimetric feeder compensates for the variation in bulk density and feeds a fixed weight of coal in response to a boiler fuel demand. This ability to accurately weigh the coal provides significant improvement over volumetric types in terms of matching the fuel delivered by the feeder to the actual process energy required on coal fired units. Stock’s gravimetric feeders have become the industry standard in coal fired power plants, providing these advantages:

    • Fuel Savings through Improved Boiler Efficiency
    • Improved Combustion Efficiency/LOI
    • Less Slagging and Fouling, leading to Less Corrosion
    • Less NOx through Better Control of Excess Air
    • Stability and Improved Response of Combustion Controls
    • Improved Pulverizer/Cyclone/Combustor Performance
    • Reduced Safety Concerns

    How It Works

    The STOCK® gravimetric feeder weighs material on a belt between two fixed points (span) precisely located in the feeder body. A roller, located midway in the span and supported at each end by precision load cells, supports half the weight on the span. The load cells generate an electrical signal directly proportional to the weight supported. The microprocessor takes samples of the output of each load cell many times a second. If the load cell outputs are within the expected range, they are added to obtain total weight per unit of belt length. Belt speed is determined by sampling the output of a tachometer attached to the motor shaft, multiplied by a calibration factor to convert it into a belt speed signal. The microprocessor multiplies the speed and weight signal for the feeder rate of fuel output. Finally, the microprocessor matches the feeder output to the output required by the boiler control, by adjusting the feeder motor speed. Feeders provide an accuracy of +/- ½ of one percent of totalized weight when calibrated and maintained in accordance with Stock Equipment Company’s recommended procedures.

    Advantages

    Close matching of fuel demand to fuel delivered
    The bulk density of coal will range from 45 to 55 lb/ft3 (720-880 kg/m3). This density change is primarily the result of changes in moisture and sizing of the coal. Density variations are directly related to the BTU input to the firing process. Stock Gravimetric Feeders monitor the weight of the coal and raise or lower the belt speed to instantaneously compensate for these changes in density. This improvement in the feed rate consistency, and thus the actual weight of fuel delivery, allows a closer matching of fuel to air to minimize unburned carbon losses and stack losses.

    Accurate accountability of coal on an “as fired” basis
    The present emphasis on emissions monitoring and control has made accurate fuel weights even more critical. Gravimetric feeders provide verification of the “as used” fuel to assist in the compliance with established standards. Accurate weights are especially useful in performance testing. Before-and-after test results can be easily compared to determine the effects of boiler modifications and to determine if additional modifications are justified.

    Elimination of reliability disruptions
    Stock gravimetric feeders continuously monitor the fuel flow. In the event a feed rate error occurs or a component malfunctions, an alarm signal is provided along with an error message. This allows correction of a problem before serious damage occurs and often prevents unscheduled outages. A weighing system malfunction will automatically convert the feeder to a volumetric mode of operation in which an average density is used to continue feeding while separate totals are maintained.

    Volumetric feeding

    Stock can also offer volumetric feeders for applications which do not benefit from gravimetric feeding or physical space limitations prohibit the installation of gravimetric feeders. Stock Volumetric Feeders, bring the reliability and maintainability of the Stock Gravimetric Feeder into the rare plant that cannot accept fuel control by weight. All other necessary control system interfaces, alarms, and permissives can be supported. If the system arrangement requires a very short feeder, with no reasonable possibility of placing a single belt, dual belt, compact or composite gravimetric feeder, the Stock Volumetric Feeder product is the most appropriate choice.

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