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Show fuel ratio (zero excess air), while still achieving complete combustion, the more efficient the fuel use. The minimum acceptable level of excess air under ideal conditions depends on the fuel characteristics, and on the efficient mixing of fuel and air. A significant reduction in fuel consumption is possible by minimizing the excess air. In practice, because of the limitations in the present combustion control systems and maldistribution of fuel and air, the overall air-fuel ratio is significantly higher than required for complete combustion of the fuel. One of the most attractive methods of eliminating the undesirable consequences outlined above is to determine the state of operation of each individual burner and controlling its own air-fuel ratio to maintain a value as close as possible to the minimum acceptable level. Possible techniques for helping to achieve individual air-fuel ratio control for multiburner units have been under investigation for several years. Only recently, the instrumentation for on-line measurement of the air-fuel ratio of individual burners has been developed to a point where success is attainable. An instrument based on spectral emissions from the flame has been under development at Thermo Electron Corporation for assessment of the combustion conditions in individual burners in a multiburner boiler. A prototype unit has been designed, built, and tested in the MIT Conbustion Research Facility (MIT-CRF). Further tests are planned in an industrial boiler at a Polaroid facility. This paper describes the principle of operation of the Spectral Flame Analyzer (SFA) , the results of the tests carried out in the MITCRF, and the tests to be carried out in the industrial boiler at the Polaroid facility in Waltham, Massachusetts. INDIVIDUAL BURNER AIR-FUEL RATIO CONTROL Several techniques have been attempted in the past for potential application to individual burner air-fuel ratio control systems. These include acoustic characteristics of the flame, inflame probes to detect key chemical species, and electromagnetic radiation or absorption behavior of regions of the flame. Of these, in-flame probes incurred severe problems from erosion and corrosion, while acoustic techniques have proved useful only in single-burner units for flame sensing. The electromagnetic spectrum has been much more fruitful in yielding potential methods for flame monitoring and control. Included in the electromagnetic spectrum are the ultraviolet (UV), visible, and infrared (IR) emissions. The optical region of the spectrum provides the best prospects for monitoring capability that can be utilized in an automatic control system. The spectral emissions from premixed gaseous flames are known to consist essentially of band radiation which is characteristic of the chemical species present in the flame. The principal contributor to radiation in gas-, oil-, and coal-fired furnaces are the stable species such as C02 and R20 , and particulates such as soot and fly ash. In addition, there are emissions from the reaction zone (flame-front region) in which the combustion chemistry takes place. These regions contain free radicals and combustion intermediate species such as OR, CR, CN, and C2· These radicals are produced in excited electronic states as a result of the chemical process, and the region is characterized by strong, visible, and UV emission spectra. 1 In a detailed investigation of the UV, visible, and IR spectral regions and the spectrometric measurements made at the MIT-CRF, Beer2 reported that the characteristic OR emissions offered the strongest signal. Also, the OR emissions showed an overall decrease with fuel equivalence ratio, and could form the basis of a flame quality control system. 222 Consistent with these findings, Thermo Electron Corporation has under development an SFA. In this system, emissions from the OR, CR, and the C02 radicals provide input signals to monitor the combustion conditions. These signals can further be used for control of the air-fuel ratio in individual burners. DEVELOPMENT OF TRE SPECTRAL FLAME ANALYZER During the fuel combustion, the flame-front region contains free radicals and combustion intermediate species such as OR, CR, CN, and C2· This region is characterized by strong, visible, and UV emission spectra. Further downstream from the burner exit and reaction zone, the gases consist mainly of C02' N2' and water vapor, together with more or less CO and 02' depending upon the combustion efficiency. The molecules of C02' R20, and CO are thermally excited to higher vibration-rotation states. Consequently, this gas region shows strong IR emission and absorption bands at various wavelengths that can be associated with these molecules. The measurement of the UV signals (CR, OR, etc.) and the IR signals (C02' R20, etc.) would, therefore, cover the entire flame field and would have a potential for control of the air-fuel ratio in the burner. The SFA is based on the above principle and monitors the signals both in the UV and in the IR range. The SFA3 consists of two major components - an optical bench and a microprocessor unit. In the optical bench, the light emissions are directed through four separate filters (mounted on a rotating wheel), and then directed to the separate UV and IR detectors. The signals from the detectors are then sent to the microprocessor unit for demultiplexing into the four discrete signals. These signals provide functions of the OR, CR, and C02 emissions. A schematic of the optical unit is shown in Figure 1. Light from the flame is focused through a filter wheel, and onto a water-cooled PbSe IR detector and a UV detector. The filter wheel rotates at 1800 rpm, alternately positioning one of four narrow-band filters in the path of the beam. Optical pickups located on the |
