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Show AN INFRARED BASED FUEL/AIR CONTROL FOR BOILERS FIRED WITH NATURAL GAS Martin F. Zabielski, Louis J. L. Daigle United Technologies Research Center East Hartford, Connecticut, USA .(\BSTRACT The intensity of infrared radiation emitted from the post flame front gases in natural gas flames is related to the temperature of these gases through equilibrium theory. For fully premixed natural gas flames, this radiation goes through a maximum at a stoichiometry that is slightly lean (ct> - 0 . 96). For this type of flame, the stoichiometry at which peak emission occurs is independent of fuel flow rate and is not sensitive to viewing location. For partially premixed and diffusion flames, peaked behavior is also observed. The maximum infrared emission and, hence, maximum temperature in these flames are related to not only the fuel/air ratio but also the mixing characteristics of the burner. The peaked behavior of the emission, even with the presence of soot, is ideal for a "hill-climbing" control scheme . A fuel/air controller, based on results obtained with a laboratory scale controller, has been designed and constructed to trim the fuel/air ratio of small packaged boilers (1.0-5.0 MBTU/HR) . The principal elements of the controller are: a MACSYM 120 process control computer, an interface chassis, a lead selenide infrared detector, a variable length control rod, and a fuel flow modulator . The fuel flow modulator is used to introduce a small variation in the flame so that phase sensitive detection can be performed on the infrared signal. Phase sensitive detection is used to distinguish the flame emission from the background radiation. The infrared detector is a lead selenide type which senses the radiation from CO2 in the 4.3 micron 199 region. A variable length control rod is used to trim the primary air flow. All tasks associated with open and closed loop operation are performed by the computer through a general purpose interface chassis. The software includes a routine that does phase sensitive processing with automatic phase tracking . A description of the controller elements is given along with preliminary results obtained in a field test of the controller . WITH THE AVAILABILITY OF low-cost infrared sensors and microprocessors, a fuel/air (F/A) control system based on the infrared flame emission is not only technically feasible but also economically attractive for gas fired systems. The purpose of this paper is to present the technical foundation for such a control, to describe a prototype controller for industrial applications, and discuss preliminary boiler results. The data shown in Figure 1 (Strehlow , 1968) are the adiabatic flame temperatures as a function of stoichiometry for several fuels including methane. The data show that the peak temperature for methane is reached at a stoichiometry of just slightly less than one . The peaked shape of the methane curve is ideal for a "hill-climbing" or "peak-seeking" control (Lefkowitz, 1961) . The shape of a typical natural gas shows the same behavior even though higher molecular weight hydrocarbons are present. Since the post flame gases are in equilibrium , equilibrium theory requires that the infrared emission be directly related to the temperature ; hence, the intensity of the infrared emission will vary with stoichiometry in |