BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a method and system for controlling combustion-based emissions from solid-, liquid- or gaseous-fuel fired combustors. More particularly, this invention relates to combustors having multiple fuel inputs such as grate-fired spreader-stokers, which typically employ multiple solid fuel feeders, and furnaces having multiple liquid- and/or gaseous-fuel fired burners.
2. Description of Related Art
Furnaces having multiple primary fuel inputs invariably suffer from unbalanced primary fuel distribution. This can lead to non-uniform stoichiometry in the primary combustion regions or zones, which, in turn, results in increases in the formation of NOx, CO and, in the case of solid fuel combustors, particulates. In addition, uneven primary fuel distribution in stoker systems leads to grate problems including piling and rat-holing.
There are a substantial number of control schemes for controlling and optimizing combustion systems known to those skilled in the art. Exemplary of such control schemes is taught by U.S. Pat. No. 4,592,289 to Pershing et al. in which pollutant emissions, including particulate emissions, from spreader-stoker-fired furnaces are reduced by controlling the stoichiometric ratio of oxygen to combustible material in different regions of the furnace, which control is accomplished by controlling the amount of air injected into different regions of the furnace. Indeed, controlling the combustion airflows within the furnace is a primary element of known methods and devices for reducing emissions from furnaces.
Numerous methods and apparatuses also are known for controlling NOx formation in stokers, including fuel reburn, flue gas recirculation and amine enhanced fuel reburn. U.S. Pat. No. 5,937,772 to Khinkis et al. teaches a process for combustion in which a combustible material such as coal, municipal solids wastes, wood, wood waste, refuse derived fuels, biomass and sludge is introduced into a stoker and burned, creating a primary combustion zone or region. A mixture of flue gases and flyash having an organic content is injected into the stoker downstream of the primary combustion region to create an oxygen-deficient reburn zone, thereby reducing the NOx emissions from the stoker. See also U.S. Pat. No. 5,205,227 to Khinkis et al. which teaches the injection of a gaseous fuel, e.g. natural gas, into a stoker downstream of the primary combustion region resulting in a reduction in NOx emissions compared to conventional stokers not employing fuel reburn.
- SUMMARY OF THE INVENTION
Notwithstanding the substantial strides that have been made toward reducing flue gas emissions from stokers and other fossil fuel fired furnaces, non-uniformity of the combustion, particularly in stokers and furnaces employing multiple primary fuel inputs, which non-uniformity can result in increased emissions, remains a concern.
Accordingly, it is one object of this invention to provide a method and apparatus for controlling pollutant emissions from solid-, liquid-, or gaseous-fuel fired combustors.
It is one object of this invention to provide a method and apparatus for controlling pollutant emissions from solid-, liquid-, or gaseous-fuel fired combustors having multiple primary fuel inputs, such as multi-feeder stokers.
It is yet another object of this invention to provide a method and apparatus for improving the uniformity of combustion within solid-, liquid-, or gaseous-fuel fired combustors.
It is still a further object of this invention to provide a method and apparatus for controlling pollutant emissions from solid-, liquid-, or gaseous-fuel fired combustors having multiple primary fuel inputs by controlling individually the amount of primary fuel introduced into the combustors through each of the primary fuel inputs so as to regulate primary fuel distribution within the combustor.
These and other objects are addressed in a combustor having multiple primary fuel inputs by a method and apparatus in which the occurrence of non-uniform combustion is detected, the location of the non-uniform combustion is determined and the fuel distribution is automatically manipulated to correct the combustion irregularity. Using the method and apparatus of this invention results in primary combustion stoichiometry in the combustor that is more uniform, more stable and at closer to optimum value than is possible with conventional combustors with multiple primary fuel inputs, which, in turn, results in the reduction of pollutant emissions from the combustor. In addition, when employed in combination with conventional emission control technology, such as low-NOx burners and reburn technology, greater reductions in NOx, CO and particulates (PM10 and PM2.5) are achieved than are possible with conventional technology alone.
The method and apparatus of this invention are based upon the discovery that there is a direct correlation between the concentration of pollutants at different locations in the flue gases proximate to the flue gas exhaust from the combustor and the distribution of primary fuel input to the combustor. Tests conducted on a stoker boiler having multiple solid fuel feeders established the existence of a correlation between local grate combustion conditions and respective localized carbon monoxide and/or combustible levels in the flue gases exhausted from the combustor. The method and apparatus of this invention utilizes this correlation in a regulatory control scheme in which composition analysis of the flue gas is used as feedback for obtaining uniform combustion in the primary combustion regions of the stoker. Although other components in the flue gases could possibly be used, for example, elements introduced for this purpose through the individual primary fuel inputs, carbon monoxide or combustible analysis in the flue gas provides an acceptable feedback signal. Flue gas oxygen analysis may be used for monitoring purposes.
In accordance with the method of this invention for controlling combustion of a primary fuel in a combustion chamber having a plurality of primary combustion regions corresponding to multiple primary fuel inputs and a flue gas exhaust, a concentration of at least one flue gas component that is indicative of fuel/oxidant ratio in the flue gases generated by the combustion is measured at a plurality of locations proximate the flue gas exhaust, resulting in a plurality of measured concentrations. Each of the locations at which the measurements are made corresponds to one of the primary combustion regions. To dampen measurement noise and reduce unnecessary control fluctuations, the real-time measurements of the at least one flue gas component are processed into a localized rolling average signal. An overall average concentration of the at least one flue gas component is determined from the plurality of real-time measured concentrations. Thereafter, a delta value for the at least one flue gas component at each of the locations is determined. Delta values are the result of the difference between the at least one flue gas component concentration at each location and the overall average concentration of the at least one flue gas component. Positive delta values indicate excess fuel while negative delta values indicate a shortage of fuel. Based upon the determination of delta values, the fuel input rate for each of the primary combustion regions is adjusted, as necessary, either by increasing or decreasing the fuel input rate, until the delta value is reduced or increased to zero.
BRIEF DESCRIPTION OF THE DRAWINGS
The apparatus of this invention, in accordance with one embodiment, for controlling the combustion in a combustion apparatus comprising a combustion chamber having a plurality of primary combustion regions and a flue gas exhaust, where each of the primary combustion regions has a corresponding primary fuel input, comprises a combustion control system comprising a plurality of flue gas sensors disposed within the combustion chamber at a plurality of sensor locations proximate the flue gas exhaust. Each of the flue gas sensors is adapted to measure an amount of at least one flue gas component that is indicative of fuel/oxidant ratio. A data processor is operably connected to the flue gas sensors and is adapted to determine an average amount of the at least one flue gas component measured by the plurality of flue gas sensors. The data processor comprises a plurality of comparator blocks adapted to generate a delta value for the at least one flue gas component for each of the sensor locations. At least one fuel input controller is operably connected to the processor and to a fuel supply to each of the primary combustion regions. The at least one fuel input is adapted to control fuel input to each of the primary combustion regions based upon the delta values.
These and other objects and features of this invention will be better understood from the following detailed description taken in conjunction with the drawings wherein:
FIG. 1 is a cross-sectional view of a grate-fired spreader-stoker system with a combustion-based emissions reduction system in accordance with one embodiment of this invention;
FIG. 2 is a cross-sectional view of the grate-fired spreader-stoker shown in FIG. 1 taken along the line II-II;
FIG. 3 is a diagram showing the effect of changes in stoker feed conditions on combustibles exiting from the stoker; and
- DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS
FIG. 4 is a detailed schematic diagram of the control system in accordance with one embodiment of this invention.
The invention claimed herein is a method and system for combustion-based emissions reduction from spreader-stokers, grate fired combustors and furnaces all of which employ multiple primary fuel input means. In the case of stokers, the primary fuel input means are in the form of multiple feeders. Other furnaces or combustors to which the method and system of this invention may be applied are furnaces and combustors having multiple solid-, liquid- and/or gaseous-fired primary fuel burners. Although described in the context of a grate-fired stoker, the method and system of this invention are applicable to any furnace system employing multiple primary fuel inputs and no intention to limit the scope of this invention to grate-fired stokers should be inferred.
FIGS. 1 and 2 show a grate-fired spreader stoker comprising a combustion-based emissions reduction system in accordance with one embodiment of this invention. Stoker 10 comprises a stoker grate 11 disposed in the bottom portion on top of which is disposed a solid fuel 13, which is introduced into the combustion chamber 30 of stoker 10 through solid fuel feeders 12. Primary combustion air for combustion of the solid fuel 13 is provided to the stoker grate 11 as undergrate air 15. The rate of solid fuel introduced through solid fuel feeders 12 is controlled by fuel rate controller 31, which comprises a variable speed motor 28 and a variable speed driver 29 operably connected to variable speed motor 28. Stoker 10 further comprises a flue gas exhaust 21 through which the flue gases generated by the combustion of the solid fuel 13 are exhausted from stoker 10. Ash which is generated as the result of the combustion of the solid fuel 13 is discharged from stoker 10 through ash discharge 14.
As shown in FIG. 2, combustion chamber 30 comprises a plurality of primary combustion regions 18, 19 and 20 disposed proximate stoker grate 11. Each primary combustion region 18, 19 and 20 corresponds to one of the solid fuel feeders 12. As a result, the combustion of the solid fuel 13 within each of the primary combustion regions 18, 19 and 20 can be modified by altering the fuel input rate of solid fuel 13 introduced through corresponding solid fuel feeders 12.
Combustion-based emissions reductions are performed by an advanced control logic system in accordance with one embodiment of this invention comprising process instrumentation in the form of a plurality of flue gas component sensors 26 operably connected to flue gas component analyzers 25 suitable for analyzing flue gas composition. Flue gas component sensors 26 are disposed at a plurality of flue gas component locations 22, 23, 24, which locations are proximate to flue gas exhaust 21. Localized measurements are made of flue gas components which, based upon the amounts present in the flue gas, can be used as indicators of the fuel/air ratio of the primary fuel and air employed in the combustion occurring in the primary combustion regions. In accordance with one embodiment of this invention, localized measurements of flue gas oxygen (O2), carbon monoxide (CO), nitrogen oxides (NOx) and/or combustibles are obtained in real time and serve as the primary inputs to the control logic system.
In addition to process instrumentation, the advanced control logic system of this invention comprises a data processor, identified in FIG. 1 by elements labeled “AY”. The data processor processes the oxygen, carbon monoxide and combustibles real time measurements into a rolling average signal (AVG in FIG. 1), which dampens measurement noise and reduces unnecessary control fluctuations. The localized rolling average signals for the different gas components are compiled and an overall arithmetic average of each flue gas component is calculated.
Using dedicated comparator blocks, the advanced control logic system generates delta values for each or selected flue gas components. Delta values represent the difference between the localized measurements of concentration levels for the flue gas components and the overall, arithmetically derived, concentration level for the flue gas components. Given a system utilizing a combustible concentration component, positive comparator delta values indicate excess fuel while negative comparator values indicate a shortage of fuel within each of the primary combustion regions. If the delta value from each respective comparator block is equal to zero, then combustion stoichiometry for the primary fuel and air is considered to be uniform. If, however, a delta value for a comparator block is not zero, the advanced control logic system selectively adjusts the local rate at which fuel is supplied to the different primary combustion regions 18, 19 and 20 such that comparator delta values of about zero are restored. The advanced control logic system of this invention is enabled only during periods when the delta values contain at least one negative value or one positive value. If all the delta values are positive, then greater combustion air flow is required; likewise, if all the delta values are negative, less combustion air flow is required. Under these latter two scenarios, the advanced control logic system is disabled.
Fuel redistribution is implemented through a control bias scheme applied to a set point control signal for each fuel input, in the case of a stoker, each of the feeders. A positive set point bias increases feeder speed while a negative set point bias decreases feeder speed. Feeder speed is varied by way of variable speed driver 29, which is operably connected to a motor 28, which, in turn, is operably connected to a feed rate controller 31. The amount of set point bias is dependent upon the delta value, the feeder's correlation factor, K, and tuning parameters. This K factor allows for fine-tuning the control response and is empirically determined through tests which characterize a specific feeder's sensitivity on local stoichiometry. A proportional-integral (PI) controller is also included in the loop for additional tuning of the control response. As a control precaution, the advanced control logic system of this invention includes bias limits to safeguard against unexpected control action that might swing the boiler into an excessive abnormal condition.
Fuel feeders in spreader-stokers comprise multiple mechanical, constant-volume screw or slat-type conveyors to ensure proper grate fuel coverage and the proper amount of fuel to meet the thermal demands of the furnace. For a stoker boiler, a boiler master controller is responsible for controlling total heat demand by regulation of fuel flow via modulation of feeder conveyor speed as measured in revolutions per minute (rpm). Feeder tests conducted on a coal-fired spreader-stoker established that a 1% change in conveyor speed produced a change of about 3.5% in the boiler flue gas combustible concentration (based upon a full scale of 5000 dppmv). During this test the flue gas oxygen was measured at 2-3%. At lower oxygen levels (about 1%), feeder changes produced a stronger response in flue gas combustibles as shown in FIG. 3.
The combustion-based emissions reduction system of this invention relies on modulation of feeder speed (rpms) through changes in the feeder's bias controller output value. Bias adjustments can either increase or decrease feeder speeds. System control logic increases the speed of selected feeders while decreasing the speed of other feeders, all the while maintaining total feeder revolutions per minute constant. This response eliminates interferences with a stoker's thermal heat input as set by the boiler master controller and steady steam production is maintained. Stoker primary combustion regions are also maintained at optimum stoichiometry to achieve the greatest reduction in NOx and other regulated emissions.
A more detailed combustion-based emissions reduction control system is shown in FIG. 4. This illustration shows a control configuration for a spreader-stoker boiler having five feeder units and three boiler exit flue gas combustible analyzers. The system comprises similar control blocks and control functions associated with combustion-based emissions reduction as previously described. In this example, additional control elements calculate simulated real-time and average gas compositions to implement bias control of #2 and #4 feeders. Direct measurements of boiler exit combustibles control the bias for the #1, #3 and #5 feeders. In this example, feeder rpm is changed through a variable-speed controller regulating the feeder drive motor rpm.
In accordance with one embodiment of this invention, the combustion-based emissions reduction system of this invention is applied to furnaces employing other advanced techniques for reducing NOx emissions. One of these techniques, as shown in FIG. 1, known as fuel reburn, involves the injection of a fuel through a reburn fuel input 16 downstream of the primary combustion regions and the injection of overfire air through overfire air input 17 downstream of the reburn fuel input 16. The use of this invention in combination with conventional reburn technology provides up to a 90% NOx emissions reduction compared to baseline emissions. Attributes which appear to account for this include the ability of the method of this invention to control stoichiometry at 0.6 to 1.0 (Control of grate combustion stoichiometry is a major variable in NOx formation); improved level of combustion uniformity and combustion stoichiometry over the entire grate; reduction in the number of non-uniform occurrences; and increased rates of reburn fuel injection, up to 15 to 25% compared to conventional uses of 5 to 10% thermal input due to improved grate combustion zone conditions.
In addition, the method and system of this invention are adaptable to solid, liquid or gas-fired combustors, furnaces and boilers. The logic employed by this invention continuously monitors, detects and manipulates localized fuel response to non-uniform stoichiometry as inferred from flue gas composition. Grate fuel distribution is controlled without interacting or interfering with furnace or boiler master demand for total thermal heat input. Application of this invention to stokers results in fewer incidences of grate problems due to piling, rat holing and hot spots, reductions in combustor particulates, reduction in unburned carbon levels in bottom ash and fly ash, boosts in boiler thermal efficiency, less maintenance due to fouling and deposition on boiler internals, improved heat release profile at the grate due to more stable and uniform combustion, and improved depth and evenness of the ash layer on the grate.
While in the foregoing specification this invention has been described in relation to certain preferred embodiments thereof, and many details have been set forth for purpose of illustration, it will be apparent to those skilled in the art that the invention is susceptible to additional embodiments and that certain of the details described herein can be varied considerably without departing from the basic principles of the invention.