Patent Publication Number: US-6659026-B1

Title: Control system for reducing NOx emissions from a multiple-intertube pulverized-coal burner using true delivery pipe fuel flow measurement

Description:
BACKGROUND AND SUMMARY OF THE INVENTION 
     The present invention relates to a control system and method for reducing the NO x  emissions from a multiple-intertube pulverized-coal burner. More specifically, the present invention relates to a control system and method for use with the combination of a known NO x  emission reduction system and an improved means of detecting and monitoring the true mass flow of pulverized coal through each burner delivery pipe. The control system and method allows for better control of the burner stoichiometry and, therefore, a further reduction of NO x  emissions from such a burner. 
     NO x  refers to the combination of nitric oxide (NO) and nitrogen dioxide (NO 2 ) gases, which may be produced during the burning of coal when nitrogen is released from coal particles in the presence of excess oxygen. Both NO and NO 2  are classified as pollutants under the Clean Air Act and, therefore, a reduction in the emission thereof is highly desirable. 
     In an electric power plant, multiple-intertube pulverized-coal burners commonly form part of a roof-fired boiler that is used to generate steam for driving electrical energy-producing generators. Such a roof-fired boiler will typically have more than one multiple-intertube pulverized-coal burner. The multiple-intertube pulverized-coal burners typically utilize a series of coal pulverizing mills that pulverize larger pieces of coal into much smaller particles. These coal particles are then carried by a primary air supply through a plurality of coal supply pipes to the multiple-intertube pulverized-coal burners for subsequent combustion within a combustion chamber. As mentioned previously, during the combustion process NO and NO 2 , as well as other undesirable gases, may be produced. However, it has been found that the amount of these gases produced can be reduced by better controlling the stoichiometry of the combustion process. 
     A method and apparatus that may be retrofitted to existing multiple-intertube pulverized-coal burners to provide such a reduction through improved stoichiometry has been previously disclosed in U.S. Pat. Nos. 5,771,823, 5,960,723, and 6,155,183, all of which are incorporated by reference herein. In U.S. Pat. Nos. 5,771,823, 5,960,723, and 6,155,183, the method and apparatus controls the amount of both secondary and interjectory air to better regulate the combustion process. 
     The method and apparatus disclosed in U.S. Pat. Nos. 5,771,823, 5,960,723, and 6,155,183 controls the amount of secondary air and supplies the secondary air to the burners in an internal two-stage process. The first stage includes secondary air dampers and air flow stations that regulate the amount of secondary air to the burners. A portion or balance of the required secondary air is directed through hot air ducts to interjectory air plenums located along the furnace front wall. The secondary air flowing directly to the burners is baffled to provide a low velocity, fuel-rich central core for combustion of the fuel&#39;s volatile component in a reducing environment. The periphery of the burner maintains an oxygen-rich boundary layer that protects against reducing environments along waterfalls and corrosion potentials, and provides sustained combustion of the fixed carbon. The second stage of the process then uses one modulating interjectory air port per burner to provide the balance of the required total combustion air and sufficient turbulence to complete the combustion process. This two-stage process provides for a precise measurement of both secondary and interjectory air to the burners at all times, allowing enough combustion air to support both the burning of the fuel&#39;s volatile component and the fixed carbon, while limiting the supply of excess oxygen to reduce the potential of the fuel-bound nitrogen released with the burning of the volatile component and atmospheric nitrogen from being converted to NO x . 
     In order to attain the most ideal possible burner stoichiometry by the above apparatus and method, it is necessary to accurately control the air/fuel ratio in each individual burner. Unfortunately, because a single coal pulverizing mill may serve multiple burners through multiple supply pipes, it has been difficult in the past to obtain the true mass flow of coal to a given burner and, thus, an accurate air/fuel ratio. One method of determining the mass flow of coal has been to measure the amount of bulk coal entering each pulverizer and to divide that amount by the number of delivery pipes connected thereto. Alternatively, a flow sensor may be employed to measure the mass flow leaving the pulverizer and the measurement thus obtained may be divided by the number of delivery pipes to calculate a theoretical mass flow through each pipe. However, due to dimensional differences from pipe-to-pipe, clogging or build-up in certain pipes but not others, and because of numerous other variants that can cause the flow of coal through one supply pipe to differ from that of the next, the assumption of equivalent mass flow of coal to each burner is erroneous and has often led to a less than optimum air/fuel ratio within a given burner. 
     The proper combustion process for coal requires approximately 7.5 pounds of air for every 10,000 BTU of coal. Although the BTU rating for coal used in a typical multiple-intertube pulverized-coal burner may vary from approximately 9,500 BTU/pound to approximately 12,500 BTU/pound, the actual BTU rating of the coal used at any particular location, at any given time, is generally known from testing. Thus, by accurately determining the mass flow rate of pulverized coal through a particular supply pipe, a proper amount of combustion air can be supplied to the burner to which the supply pipe is connected, and the burning of the coal can be optimized. 
     Sensors have been developed that may be used to accurately determine the mass flow rate of a substance, such as coal, through a conduit. Certain of these sensors use electrodes to measure the electrostatic charge of the particles traveling through the conduit; others monitor microwave absorption, attempt to directly measure air flow with and without particles entrained therein, measure the impact of particles, or measure the travel time of ultrasonic signals. Another type of sensor can measure the mass flow of coal particles by generating an alternating electric field within a feed pipe, measuring the attenuation of the electric field, and employing a derivation process to determine the quantity of solids in the flow. This type of sensor has proven particularly amenable to use with the present invention, and an exemplary device is disclosed in U.S. Pat. No. 6,109,097. 
     What is needed and has been heretofore unavailable, however, is a system employing a combination of the emission reduction method and apparatus disclosed in U.S. Pat. Nos. 5,771,823, 5,960,723 and 6,155,183 with an accurate means of determining the mass flow rate of pulverized coal through each burner supply pipe, such as the sensor disclosed in U.S. Pat. No. 6,109,097. In this manner, an optimum reduction in NO x  emissions from a multiple-intertube pulverized-coal burner may be realized. The present invention contemplates such a system, and more specifically a control system and method for allowing the successful operation of such a system. 
     To improve the performance of a NO x  emission reducing roof-fired boiler, the control system and method of the present invention is able to control the technology disclosed by U.S. Pat. Nos. 5,771,823, 5,960,723 and 6,155,183, while also incorporating input data from a mass flow sensor located in each coal supply pipe, preferably a sensor such as that disclosed in U.S. Pat. No. 6,109,097. The mass flow of coal through each burner supply pipe is measured and summed to determine the total mass flow of coal to the furnace. The ratio of coal flow through a given burner pipe to that of the average for all burner pipes on in service burners is also determined. The steam flow for the boiler is also known, and is used to determine the total theoretical combustion air required at each burner, including primary, secondary, and interjectory air. This combustion air demand may then be corrected based on the actual coal flow to each burner so that the stoichiometric ratio of the combustion air and fuel (coal) is properly maintained, thereby producing less pollutants. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In addition to the novel features and advantages mentioned above, other objects and advantages of the present invention will be readily apparent from the following descriptions of the drawings and exemplary embodiments, wherein like reference numerals across the several views refer to identical or equivalent features, and wherein: 
     FIG. 1 is a cutaway view of a typical pulverized-coal burning multiple-intertube roof-fired boiler of the present invention; and 
     FIG. 2 is a schematic diagram of a control system of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENT(S) 
     An overview of an embodiment of a pulverized-coal burning multiple-intertube roof-fired boiler  10  according to the present invention is illustrated in FIG.  1 . Such a boiler  10 , burns fuel, in this case coal, in order to produce steam to run electricity-producing generators. To produce steam, the coal is burned in a combustion chamber  15  containing a plurality of water-carrying pipes  20 , whereby the heat of combustion converts the water therein into steam. The combustion chamber  15  may communicate with a number of burners  25  at which the actual combustion occurs. 
     A supply of coal is typically maintained in one or more bins  30  that are connected to one or more coal pulverizing mills  35 . The coal pulverizing mills  35  convert the coal from larger chunks into finer particles that may be more easily entrained via an air stream for transport to the burners. A forced draft fan  40  is typically located near the top of the boiler  10  in order to supply a primary air stream for carrying the coal particles. The primary air is supplied to the pulverizing mills  35  via primary air ducts  45  connected thereto. A booster fan  50  may also be provided to increase the flow rate of the pulverized coal and primary air leaving the pulverizing mills  35  and entering the coal supply pipes  55  leading to the burners  25 . Typically there is a single coal supply pipe for each burner, but one pulverizing mill may supply more than one burner. The primary air and entrained coal particles travel through the coal supply pipes  55  and are eventually distributed through multiple burner tips  60  into the combustion chamber  15 , where the coal particles are ignited and burned. 
     The present invention regulates the amount of combustion air supplied to each burner  25  in order to optimize the burner stoichiometry. In contrast to known systems, the system of the present invention determines the amount of combustion air required based on the actual mass flow of coal to each burner  25 , thereby enabling a more accurate air/fuel ratio to be maintained. The combustion air is comprised of the primary air supply carrying the pulverized coal particles, as well as a regulated amount of secondary and interjectory air. A mass flow sensor  65  is provided in each coal supply pipe  55  to determine the actual mass flow of coal particles to each burner. It should be understood that a mass flow sensor  65  may be located at any of a variety of points within each coal supply pipe  55 . It may also be possible to utilize more than one mass flow sensor  65  within a single coal supply pipe, and to average the readings thereof to determine the mass flow of the coal particles traveling therethrough. 
     The control system  100  of the present invention is illustrated in the schematic diagram of FIG.  2 . Since the required boiler output and the boiler efficiency are known, the steam flow output  105  of the boiler  10  is used to determine the amount of coal required to be input to the boiler, and also the amount of combustion air (secondary air) required  110  to support the burning of the coal in the combustion chamber  15 . Because a typical boiler of the present invention generally has a number of individual burners  25 , it is necessary to determine the amount of combustion air required per burner by dividing  115  the total amount of combustion air required  110  by the number of burners in service. The resulting value represents the per burner average secondary air demand  120  for the system. 
     In order to accurately control the combustion process, it is also necessary to know the actual fuel (coal) flow to each burner. To this end, the flow of pulverized coal particles through each coal supply pipe  55  is measured using the mass flow sensor  65  located therein. The flow measurement from each coal supply pipe  55  is then summed  125  to obtain the total fuel mass flow to the furnace. The total mass flow of fuel to the furnace is then divided  130  by the total number of burners in service to obtain an average fuel mass flow per burner. However, as discussed above, due to differences in piping size and configuration, as well as build-up and other factors, the fuel mass flow to each burner is typically not equal. Consequently, it is preferable to determine the percentage of fuel mass flow to a given burner relative to the average fuel mass flow for all of the in-service burners. This percentage is derived by dividing the measured value of the fuel mass flow through each burner supply pipe  55  as recorded by each mass flow sensor  65 , by the average fuel mass flow per burner  135  obtained above. Thus, a given burner supply pipe  55  may have a fuel mass flow that is, for example, 100%, 90%, or 75% of the average fuel mass flow per burner. This percentage represents the ratio of mass or BTU&#39;s of fuel flowing to each burner as compared to the other in-service burners. 
     The percentage of fuel mass flow to each burner is then multiplied  140  by the per burner average secondary air demand  120  for the system, which was determined previously. This value  145  is then used to adjust the combustion air flow to each burner. Such an adjustment is necessary because secondary air is not the only contributor to the combustion process. As mentioned above, the primary air used to carry the pulverized coal particles to the burners  25  also contributes to the combustion of the coal in the combustion chamber  15 . It is, therefore, necessary to correct the calculated combustion air demand to reflect the contribution to combustion of the primary air flow. As the primary air flow rate is known, the ratio of primary air flow to combustion air demand may be determined. The value  145  obtained by multiplying  140  the percentage of fuel mass flow to each burner by the per burner average secondary air demand  120  for the system is then multiplied  150  by the primary air ratio  155  to obtain a corrected secondary air demand  160  that may be sent to a secondary air controller provided for each burner. This corrected secondary air demand  160  reflects not only the true value of secondary air required to support proper combustion, but also the true percentage of fuel mass flow to the particular burner. 
     As discussed above, combustion air demand also includes interjectory air, which is preferably provided on a per burner basis. Interjectory air is necessary to achieve char burn out and to oxidize carbon monoxide within the combustion chamber. To ensure the proper amount of interjectory air is introduced to the combustion process, the interjectory air demand is corrected  165 . This may be accomplished by measuring the amount of excess oxygen present in the furnace exhaust gases. The excess oxygen correction factor  165  is then multiplied  170  by the value  145  obtained by multiplying  140  the percentage of fuel mass flow to each burner by the per burner average secondary air demand  120 . In this manner, it is possible to accurately regulate the percentage of oxygen to promote safe combustion and to maintain the atmosphere inside the furnace within a non-reducing range. 
     Once corrected secondary and interjectory air demands  160 ,  175  are obtained, the supply of each is controlled using a proportional and integral (PI) control loop  180 ,  185 . The proportional and integral control loops  180 ,  185  compare the secondary and interjectory air demand to the actual secondary and interjectory air flow, respectively. A proportional and integral controller in communication with each control loop can then make adjustments to a secondary air damper  190  or interjectory air register  195  as necessary to ensure that the calculated amount of secondary air and interjectory air is sent to the burners. The PI control loop continually monitors the corrected air demands and makes adjustments to the air flows accordingly. 
     As can be understood from the above description, the system of the present invention is able to control the combustion process within a pulverized-coal burning multiple-intertube roof-fired boiler more accurately than has heretofore been possible. The system of the present invention incorporates the actual mass flow of fuel (coal) to each burner into the equations used to regulate the combustion process. This allows a more complete and clean burning of the fuel within the combustion chamber than is possible with known systems, wherein coal flow to each burner is generally assumed to be equivalent and equal to the amount of bulk coal entering each pulverizer divided by the number of delivery pipes connected thereto. Thus, the system of the present invention allows a pulverized-coal burning multiple-intertube roof-fired boiler to operate with a reduced level of NO x  emmissions not previously possible. 
     While certain embodiments of the present invention are described in detail above, the scope of the invention is not to be considered limited by such disclosure, and modifications are possible without departing from the spirit of the invention as evidenced by the following claims: