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Improving Fired Heater Efficiency
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To reduce the amount of purchased fuels, more and more plants are making use of in-plant generated sources of gases. The availability of such waste gases is usually not constant, thus the total plant's mixed gas heating value swings accordingly. When this mixed gas is fed to the furnaces or boilers that are on strictly volumetric flow control loops, it causes large upsets in the heat release rate, even at a constant flow. These upsets, in turn, cause changes in process temperatures and/or pressures, resulting in off specification products and air pollution violations, and may even cause damage to equipment. The tendency is to operate routinely at a high excess air level, resulting in a much lower overall efficiency, or to even flare the gas. For many fuel gases, the heating value per standard unit volume varies linearly with the mass per standard unit volume. Perhaps a lesser realized fact it that the air required for combustion of 1000 Btu (net) heat release is almost independent of fuel molecular weight. This paper will describe the details of a technique developed by the Foxboro Company to measure the incoming fuel gas total Btu content by using a differential pressure transmitter and vortex flowmeter in series, generate a feed forward signal to maximize overall efficiency through transients, and to do it without compromising any of the safety constraints of the furnace.
Shriver, J. E. (1983). Improving Fired Heater Efficiency. Energy Systems Laboratory (http://esl.tamu.edu); Texas A&M University (http://www.tamu.edu). Available electronically from