Patent Publication Number: US-2011062056-A1

Title: Excess Air Control For Cracker Furnace Burners

Description:
BACKGROUND OF THE INVENTION 
     The instant invention is in the field of methods for the control of excess air in cracker furnace burners. The production of olefins by thermally cracking a hydrocarbon material, such as petroleum naphtha, is one of the most important processes in the chemical process industry. For example, ABB Corporation reportedly constructed a cracking plant in Port Arthur Tex. having a capacity to produce over a million tons of ethylene and propylene per year. The cracking process is conducted in a “cracker”. A cracker usually comprises an enclosure containing tubes and a burner. Heat generated by burning a fuel heats the hydrocarbon material flowing in the tubes so that the hydrocarbon material is thermally cracked to produce, among other things, ethylene and propylene. 
     Ordinarily, a cracker is comprised of a radiant section and a convection section. The burner is positioned in the radiant section so that the tubes positioned in the radiant section are heated primarily by radiant heat emitted from the walls adjacent to the burner. The combustion gas from the radiant section is then directed to the convection section where heat from the combustion gas is recovered to heat tubes positioned in the convection section. An oxygen sensor, such as a zirconium oxide oxygen sensor, is ordinarily positioned in the cracker between the radiant section and the convection section to facilitate of control the air/fuel ratio of the burner. The overall efficiency of the cracker is primarily a function of the amount of excess air present in the firebox and the temperature of the exhaust gas from the cracker. It can be beneficial from an efficiency viewpoint to control the amount of air in the furnace. Carbon monoxide and smoke emissions from the cracker tend to increase when the amount of air used in the burner is reduced below the stoichiometric ratio of air-to-fuel. On the other hand, too much excess air can reduce the overall efficiency of the cracker and can result in excessive emissions of oxides of nitrogen. Therefore, accurate control of the amount of excess air used in the cracker furnace is necessary for an optimum balancing of efficiency and for the control of emissions. 
     The oxygen sensor of a conventional cracker is a “point measurement device”, i.e., it measures oxygen at the position where the sensor is located. Such a measurement is not representative of the oxygen concentration in the cracker as a whole. It would be an advance in the art of the control of cracker furnaces if a system were developed that provided a more representative determination of oxygen in the cracker. Also, it is well known that conventional zirconimum oxide sensors are subject to interferences known to affect the accuracy of the O 2  measurement (such as hydrocarbons and CO gases). It would be an advance in the art of the control of cracker furnaces if a system were developed that was more immune to these interferences. 
     Section II.4.3, Sensors for Advanced Combustion Systems, Global Climate &amp; Energy Project, Stanford University, 2004, by Hanson et al., summarized the development of the tunable near-infrared diode laser and absorption spectroscopy approach for the determination of oxygen, carbon monoxide and oxides of nitrogen in the combustion gas from a coal fired utility boiler, a waste incinerator as well as from jet engines. Thompson et al., US Patent Application Publication US 2004/0191712 A1 applied such a system to combustion applications in the steelmaking industry. It would be an advance in the art if the tunable near-infrared diode laser and absorption spectroscopy approach for the determination of oxygen, carbon monoxide and oxides of nitrogen in combustion gas were applied to thermal crackers. 
     SUMMARY OF THE INVENTION 
     The instant invention is a solution, at least in part, to the above-stated problem of the need for a more reliable and representative analysis of combustion gas from a thermal cracker furnace. The instant invention is the application of the tunable near-infrared diode laser and absorption spectroscopy approach for the determination of, for example, oxygen, carbon monoxide and oxides of nitrogen in the combustion gas from a thermal cracker furnace. 
     More specifically, the instant invention is a method for control of the air/fuel ratio of the burners of a thermal cracker comprising the steps of: (a) directing a wavelength modulated beam of near infrared light from a tunable diode laser through combustion gas from the burners to a near infrared light detector to generate a detector signal; (b) analyzing the detector signal for spectroscopic absorption at wavelengths characteristic for an analyte selected from the group consisting of oxygen, carbon monoxide and nitrogen oxide to determine the concentration of the analyte in the combustion gas; and (c) adjusting the air/fuel ratio of the burners (i.e. excess air in the furnace) in response to the concentration of the analyte of step (b). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic side view of a typical thermal cracking furnace  10  for producing olefins; 
         FIG. 2  is a schematic rear view of the furnace  10  of  FIG. 1  schematic rear view of the furnace  10  of  FIG. 1 ; 
         FIG. 3  is a detailed view of a preferred tunable diode laser spectroscopy apparatus for use in the instant invention; 
         FIG. 4  is a spectra collected using the system of the instant invention showing fine structure absorbance in the wavelength region characteristic for oxygen absorbance of near infrared light generated by a tunable diode laser. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a schematic side view of a typical thermal cracking furnace  10  for producing olefins including an enclosure  11  having an air inlet  12  and an exhaust outlet  13 . An air inlet fan  14  provides forced draft through a burner  15 . An exhaust fan  16  provides an induced draft from the furnace  10 . The interior of the furnace  10  is comprised of three primary portions: the firebox portion  17 ; the bridge wall portion  18 ; and the convection portion  19 . Combustion gases from the burner  15  are first directed into the firebox portion  17  of the furnace  10 , then through the bridge wall portion  18 , then through the convection portion  19  and then out of the exhaust outlet  13 . Feed stream  20  is conducted through tubing  21  to preheat the feed. Steam  22  is introduced to the preheated feed which is then further heated by tubing  23  positioned in the convection portion  19  and then further heated by tubing  24  positioned in the firebox portion  17  to produce a product  25 . 
     Referring now to  FIG. 2 , therein is shown a schematic rear view of the furnace  10  of  FIG. 1  showing the exterior walls of the firebox portion  17 , the bridge wall portion  18  and the convection portion  19 . A tunable diode laser system  26  is mounted at the bridge wall portion  18  of the furnace  10  so that light from the tunable diode laser of the tunable diode laser system  26  can be shown through the combustion gas flowing through the bridge wall portion  18  to a light detector system  27 . 
     Referring now to  FIG. 3 , therein is shown a more detailed view of the diode laser system  26  and light detector system  27  shown in  FIG. 2 . The system shown in  FIG. 3  includes a laser module  37  containing the tunable diode laser. A control unit  31  contains the central processing unit programmed for signal processing (to be discussed below in greater detail) as well as the temperature and current control for the tunable diode laser and a user interface and display. The control unit may be contained in a separate unit as shown or may be included in one of the other components of the system, e.g. contron unit contained in the transmitter. Alignment plate  29  and adjustment rods  30  allow alignment of the laser beam  41 . The laser beam passes through a window or windows (e.g. fused silica windows, sapphire windows) into the furnace. The windows, such as dual sapphire windows  28  may be mounted in a four inch pipe flange  40 . The space between the windows  28  is purged with 25 Liters per minute of nitrogen at ten pounds per square inch gauge pressure. The flange  40  is mounted through the wall of the furnace. 
     Referring still to  FIG. 3 , the laser beam  41  is passed through a window or windows  33  (they may be dual sapphire or other suitable material such as fused silica) to a near infrared light detector  38 . The windows  33  may be mounted in a four inch pipe flange  39 . The space between the windows  33  is purged with 25 Liters per minute of nitrogen at ten pounds per square inch gauge pressure. The flange  39  is mounted through the wall of the furnace. Alignment plate  34  and adjustment rods  35  allow alignment of the detector optics with the laser beam  41 . Detector electronics  36  are in electrical communication with the control unit  31  by way of cable  37 . The control unit  31  is also in electrical communication with the process control system  32  for controlling the furnace  10  (by way of electrical cables  38 ). The optical path length of the laser beam  41  is about sixty feet. The system shown in  FIG. 3  is commercially available from Analytical Specialties of Houston, Tex. 
     The system shown in  FIG. 3  operates by measuring the amount of laser light that is absorbed (lost) as it travels through the combustion gas. Oxygen, carbon monoxide and nitrogen oxide each have spectral absorption that exhibits unique fine structure. The individual features of the spectra are seen at the high resolution of the tunable diode laser  37 . The tunable diode laser  37  is modulated (that is scanned or tuned from one wavelength to another) by controlling its input current from the control unit  31 . 
     Referring now to  FIG. 4 , therein is shown a spectrum in the region where oxygen absorbs the modulated beam of near infrared light from the tunable diode laser. The absorbance shown in  FIG. 4  is proportional to the concentration of oxygen in the combustion gas. A carbon monoxide absorbance line near 2333 nanometers is used to determine low parts per million concentration of carbon monoxide. A carbon monoxide absorbance line near 1570 is used to determine higher concentrations of carbon monoxide. A nitrogen oxide absorbance line near 2740 nanometers is used to determine low to sub parts per million concentration of nitrogen oxide. A nitrogen oxide absorbance line near 1800 is used to determine higher concentrations of nitrogen oxide. 
     Referring again to  FIG. 1 , the air/fuel ratio of the burners (excess air in furnace)  15  (which is controlled by the process controller  32  of  FIG. 3 ) can be controlled to optimize the oxygen, carbon monoxide and nitrogen oxide concentrations in the combustion gas in response to the tunable diode laser spectroscopic analysis of oxygen, carbon monoxide and nitrogen oxide outlined above. 
     CONCLUSION 
     While the instant invention has been described above according to its preferred embodiments, it can be modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the instant invention using the general principles disclosed herein. Further, the instant application is intended to cover such departures from the present disclosure as come within the known or customary practice in the art to which this invention pertains and which fall within the limits of the following claims.