Boiler Control improving efficiency of boiler systems by Hjalti Kristinsson

By Hjalti Kristinsson

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TECHNICAL BACKGROUND satility. The fuel is fed into the boiler using a device that propels the fuel particles into the air above the grate. This means that the spreader can distribute the fuel evenly and to a uniform depth over the entire grate. By propelling the fuel into the air, it allows for some of the finer particles to ignite and burn in suspension while the coarser particles burn in a thin bed on the grate. Because of this suspension burning, the spreader stoker response time is better than that of the massfeed stoker.

In boilers, ultrasonic sensors can be used to monitor the fuel bed thickness on the grate. This can serve as a feedback measurement to the fuel controller to verify if the correct amount of fuel enters the burner, as well as detecting any irregularities in the fuel bed. The ultrasonic range measurement is done by emitting a sound signal and then measuring the time it takes for the signal to echo back. The speed of the sound signal depends on the transmission medium, in this case the combustion air, and the temperature.

Measuring moisture in fuel using an indirect, rather than a direct method, is easier but introduces more uncertainties since moisture in flue gases depends on more parameters than just fuel moisture content. 2 Solid Fuel Control Techniques 35 ments are made after combustion (Marklund & Schuster, 1991). 3. 2 Flame Front Positioning The motivation for flame front positioning is to achieve a more stable combustion process; less dependent on variations in fuel quality and moisture content. A more stable process would result in reduced emissions and higher efficiency.

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