Abstract:
A method and an apparatus for monitoring fuel combustion status in a burner such as a boiler and a gasifier with high accuracy, high reliability and fast response are disclosed. The apparatus comprises a series of fiber optic flame monitors that are installed next to each nozzle inside said burner to determine temperature, flame flash frequency and the burned fuel particle density. In terms of a master controller and a group of on-line controllers, the optimized combustion of the burner is approached by monitoring the combustion status of each nozzle and regulating the discharges of air or oxygen and fuel to each nozzle, in accordance with the comparison of the data detected by flame monitors and optimal data.

Description:
FIELD OF INVENTION 
     The present invention relates to a fuel combustion control apparatus and method in general, and more particularly to a method and apparatus for the on-line fuel combustion status monitoring of boilers or burners used in power plants and other industries. 
     BACKGROUND 
     A large boiler or burner comprises a plurality of nozzles used to inject a reactive mixture of hydrocarbon fuel (i.e. coal or oil or gas) and air or oxygen into a combustion chamber where heat or syngas is produced. Heretofore, three methods have been available for monitoring the combustion status of large boilers. In one method known in the art, the volume of air and the volume of coal supplied to the combustion chamber are controlled in accordance with the temperatures inside the furnace, as disclosed in U.S. Pat. No. 5,049,063 entitled &#34;Combustion control apparatus for burner.&#34; Since the boiler is equipped with as many as 36 or more nozzles, it is impossible to determine the combustion status of the entire system and to discriminate the abnormal combustion status caused by a single nozzle or by a group of nozzles based on a localized temperature measurement inside the furnace. In another method known in the art, each nozzle is equipped with a flame detector. However, a flame detector only has a function to discriminate &#34;fire on or off,&#34; and does not possess the function of combustion status monitoring, this method often causes an excess amount of fuel to accumulate, even to a point where there is the danger of having an uncontrolled explosion within the combustion chamber. In still another method known in the art, a combustion status monitoring system may be used comprising a CCD scan camera, a monitor and an automatic control unit. The CCD camera is used to scan the flame color of each nozzle, and the combustion status is observed by the monitor to thereby optimize the volume of supplied air and fuel. Since the CCD camera cannot be installed inside the combustion chamber due to the high temperature, the small view field of the CCD camera makes it impossible to scan the entire relevant target area inside the large chamber. On the other hand, the camera cannot distinguish the flame locations, therefore, the similar signature of the background and nearby flames often cause such systems to produce unacceptable errors and incorrect results. 
     It is an objective of the present invention to provide a novel combustion status monitoring system and method based on not only the measurements of temperature, but also on the flame flash frequencies and the burned fuel particle densities inside the entire combustion chamber. It is another objective of this invention to provide a relatively simple, low cost, yet highly effective and accurate combustion status monitoring system capable of monitoring the combustion status of the entire boiler by monitoring the combustion status of each nozzle. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a schematic block diagram of the on-line combustion status monitoring apparatus. In one embodiment, if the height of the boiler is about 8 stories high, four flame monitors used in each story are connected to one on-line controller, and 32 flame monitors employed for the entire boiler are connected to 8 on-line controllers. All the on-line controllers are terminated in a master controller. 
     FIG. 2 is a graphical representation of a flame monitor. 
     
         ______________________________________REFERENCE NUMERALS IN DRAWINGS______________________________________    1.  Sight glass    2.  Optical lens    3.  Spatial filter    4.  Flame monitor housing    5.  Objective lens    6.  Optical fiber cable    7.  Optical path splitter    8.  Amplifier    9.  Optical filter    10. On-line controller    11. Master controller    12. Air and fuel flow monitor    13. Air discharge    14. Fuel discharge    15. Housing of purge air unit    16. Purge air inlet______________________________________ 
    
    
    
     DETAILED DESCRIPTION 
     The present invention provides a method and an apparatus for the on-line fuel combustion status monitoring of large boilers and burners with fast response, high accuracy and reliability. The apparatus can be modified to include certain features, depending upon the characteristics of the fuel combustion. The apparatus can be economical to provide and operate, and can have an accuracy sufficient to meet existing and changing requirements in applications such as on-line fuel combustion monitoring in the energy industry and other related industries. 
     Referring to FIG. 1, every nozzle is equipped with a flame monitor, and each four flame monitors on the same story share an on-line controller unit, or all the flame monitors along a vertical direction share an on-line controller unit. All the on-line controllers are terminated in a master controller. Other information collected by prior art instrumentation such as the temperature of fuel before injecting into the burner, the pressure inside the chamber, steam temperature and flow output in the pipes outside the chamber, and fuel and air discharge, are also input into the master controller. The optical signals including temperature T, flame flash frequency f, and the burned fuel particle density d, collected by each flame monitor, are transmitted to the on-line controllers. Based upon all the data collected including air (or oxygen) and fuel discharges and air (or oxygen) to fuel ratio in each fuel discharge pipe, steam temperature and volume produced in the output pipes, pressure inside the chamber, and the temperature of fuel before injection to the combustion chamber, the master controller regulates the discharges of air (or oxygen) and fuel to achieve the optimized combustion status. 
     Referring to FIG. 2, a flame monitor includes a flame monitoring housing 4 which may be any high temperature metal, such as stainless steel. At the end of said housing 4 nearest combustion chamber, is a sight glass 1, which may be quartz or other single crystals. Two types of sight glasses, direct-view or inclined-view, may be used. For the direct-view glass, the view axis is coincident with the central axis of the housing 4. An inclined-view glass has an inclined view axis α corresponding to the central axis of the housing 4, as shown in FIG. 2. Due to space limitations inside the combustion chamber, the flame monitor in general cannot point directly into the flame area of a nozzle, therefore, the flame monitor with an inclined-view glass lens is adopted. An optical lens 2, a spatial filter 3, an objective lens 5 and a piece of optical fiber cable 6 are assembled inside housing 4 in turn. The said spatial filter 3 is used to delete the interference of the background flame and the nearby random flame. The second function of the spatial filter 3 is to provide an optical system with a large-view and long-focus point. The spatial filter 3 may be either an optical fiber plate or a crossed grating, the blocking part of said crossed grating and said bundle of ordered optical fibers may be fabricated by either black painting or polishing. The flame signals from the combustion chamber are conveyed through sight glass I and optical lens 2 in turn, then focused in the plane of the spatial filter 3. After the interference signals from the background and nearby fields are removed by the spatial filter 3, the flame signals are focused on one end of a piece of optical fiber 6 by said objective lens 5. The signals are then transmitted to an optical path splitter 7 though said piece of optical fiber cable 6. The light coming from said optical path splitter 7 is divided into two parts. One part goes through an infrared optical filter 9 and focus on a photoelectric converter. The output electrical signals provide the temperature changes, ranging from 500 to 1650° C. Another part of the light passes through another photoelectric converter, and the output is further divided into two signals: an AC signal and a DC signal. When the fuel discharged from a nozzle is ignited, it will explode and emit a flash, the flame flash frequency, ranging from 4 to 150 Hz, is related to the AC frequency signal. On the other hand, the burned fuel density distribution d can be determined by the brightness, since the more fuel particles that are ignited, the higher the brightness peak. Therefore, the DC signal component provides the information concerning the burned fuel particle density d. The three signals, temperature T, flash frequency f, and the burned fuel particle density d, are further amplified by an amplifier 8 and transmitted to the on-line controller 10. 
     The on-line controller performs data processing and automatic control functions. The following is a description of the operation of the burner combustion monitor system described above. 
     The radiant heat energy, W=εT 4  (ε=Boltzman constant), can be obtained from the temperature measured, and the quantity of heat in the solid angle detected by a flame monitor can be represented by Q=mcΔT (c is the specific heat, and m represents the burned fuel weight). The quantity of air and fuel discharged can be monitored by an air discharge gauge and a fuel discharge gauge, respectively. The radiant thermal energy W and quantity of heat Q should be equal when an optimization of combustion status is achieved. 
     Since the combustion efficiency relates to the quality of the fuel used, the temperature of air (or oxygen) and fuel prior to admission into the furnace, humidity and the ratio of air (or oxygen) to fuel (coal or oil or gas), and the three series of previously fixed optimal values of T, f and d have been installed in the master controller. When the signals of T, f and d from the flame detectors arc input into the master controller, the master controller compares the values represented by the signals with the three series of T, f and d ranges previously set therein. If the inputs deviate from the normal values, the master controller transmits a signal to the combustion air and fuel discharge control systems to adjust the air and fuel feed. For example, a) when all three parameters of flash frequency f, temperature T, and the burned fuel particle density d appear low, it indicates the extinction of fuel combustion, b) when flash frequency f and temperature T display normal, but the burned fuel particle density d appears low, it may indicate either a low fuel combustion efficiency (i.e. air feed is not enough or too much fuel has been discharged) or an overload. The master monitor will send an order to decrease the fuel feed to have the fuel fired more completely. If d increases, it means that the previous fuel discharge was overloaded. If d continues to decrease, it indicates the discharge of fuel is not enough and the fuel flow will be increased based on the comparison of temperature T and flash frequency f to obtain the optimized discharges of air and fuel, as well as the air to fuel ratio. A distinguishing feature of the present invention is that discharges and the combustion status of each nozzle can be monitored by its corresponding on-line controller, thus the combustion optimization of the entire burner is realized by the combustion optimization of each nozzle. Tests using the sample apparatus in a power plant demonstrate the following results. 
     Temperature measurement range: 500-3500° C. 
     Temperature measurement accuracy: &lt;0.5° C. 
     Flash frequency measurement range: 4-150 Hz 
     The burned coal particle density accuracy: 0.1% full scale 
     Response time: &lt;1 ms 
     Inclined-view flame detectors may be replaced by direct-view flame detectors, which are installed at an angle of less or equal to 90° with the nozzles. 
     With further regard, the flame monitor also includes purge air means, denoted generally by the reference numerals 15 and 16 in FIG. 2. The purge air means is designed to provide a means for the purpose of purging particles, to thus ensure the flame monitor remains unobscured and also serves as a cooling means. The purge air means includes a purge air housing 15 and an air inlet 16. The purge air can be compressed air, or oxygen, or some other gas. 
     For a relatively small burner, only one or several direct-view flame detectors may be used to detect the flame parameters of the burner with lower accuracy. 
     Although the present invention has been described through specific terms, it should be noted here that the described embodiments are not necessarily exclusive and that various changes and modifications may be imparted thereto without departing from the scope of the invention which is limited solely by the appended claims.