Abstract:
Methods and systems for measuring and controlling the percent stoichiometric oxidant in the pyrolyzing section of incinerators are provided. The methods and systems rely on measurements of the oxygen concentration and temperature of the gases within the pyrolysis section and mathematical relationships between these values and the percent stoichiometric oxidant.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    This invention relates generally to combustion processes and more particularly to methods and devices for measuring the percent stoichiometric oxidant in the pyrolysis section of incinerators.  
           [0003]    2. Description of the Prior Art  
           [0004]    In incineration applications, it is common practice to employ two stages of combustion. In the first stage, combustion air is supplied at a rate less than the stoichiometric air requirement. The stoichiometric air requirement is defined as the air flow rate required for complete combustion of the fuel and waste streams. Complete combustion means that the products of combustion are stable compounds such as CO 2 , H 2 O, N 2  and He (if existing).  
           [0005]    Thus, in the first stage the wastes are commonly pyrolyzed in an oxygen-deficient atmosphere. This furnace, or portion of the furnace, is commonly referred to as a reduction, primary combustion, oxygen-deficient, or pyrolyzing furnace or chamber. Additional combustion air is then supplied at a subsequent section to destroy any products of incomplete combustion. This secondary section is typically referred to as a re-oxidation section or afterburner.  
           [0006]    Pollutant emissions are strongly influenced by the amounts of combustion air supplied to the pyrolyzing section and the afterburner. Therefore, it is highly desirable to be able to measure and control the air supply to both sections. The air supply to the afterburner is typically regulated to achieve a certain level of excess oxygen in the stack exhaust gases, or in some cases to achieve a target temperature. The air, or oxidant, supply to the pyrolyzing section is more difficult to control. It is desirable to measure and control the oxidant supply to the pyrolyzing section as a percent stoichiometric oxidant, or “PSO.” The PSO is equal to the actual oxidant supply divided by the stoichiometric oxidant supply expressed as a percent. Although oxidants include compounds such as NO and NO 2 , in practice the main source of oxidant for incinerators is generally air. Therefore the term “PSA” (percent stoichiometric air) is often used in place of PSO.  
           [0007]    The PSO can also be related to an equivalence ratio. The equivalence ratio is defined as the actual fuel-to-air ratio divided by the stoichiometric fuel-to-air ratio. The equivalence ratio is related to PSO in that the equivalence ratio is simply 100/PSO. Where fuel and air are supplied to achieve complete combustion, the reaction is said to be stoichiometric, the PSO is equal to 100% and the equivalence ratio is equal to 1.  
           [0008]    One common means of directly regulating the air supply to the pyrolyzing furnace is to measure the flow rates of fuel, waste, and air; calculate the PSO; and then control the PSO to a certain value by changing the air supply. Waste compositions often vary with time, or are simply unknown. In practice, because of the difficulties associated with the uncertainties and fluctuations in waste compositions, the waste is often excluded from the stoichiometric air requirement calculation. Because of this exclusion, the method cannot accurately reflect the correct air requirement.  
           [0009]    Other common methods for controlling the air supply are either measuring and controlling the combustible level in the pyrolyzing furnace or measuring the temperature change due to addition of afterburner air. These methods are indirect ways of controlling the PSO.  
           [0010]    Oxygen sensors have been used to measure the air/fuel ratio, or equivalence ratio, in internal combustion engines and such devices have been widely used in automobiles. These sensors do not take into account the dependency of equivalence ratio on oxygen level and temperature and therefore cannot operate in wide ranges of temperatures. However, such devices are able to neglect the effect of temperature on predictions of the equivalence ratio because the exhaust gas temperatures are normally regulated within a relatively narrow range.  
           [0011]    Other devices that have recognized the need to take into account the effects of temperature utilize semiconductor chips processed to exhibit a rapid change in electrical resistance responsive to differences in exhaust gas temperature. The temperature-dependent electrical resistance is used to compensate the signal from the oxygen sensor to produce a more accurate prediction of the PSO. Due to the mechanical and electrical characteristics of the materials used in the temperature-compensating chips, such devices cannot be operated in the high temperatures (1400° to 3200° F.) commonly seen in the pyrolyzing sections of incinerators.  
           [0012]    Thus, there are needs for methods to directly measure the PSO in pyrolosis sections of incinerators that avoid the problems described above.  
         SUMMARY OF THE INVENTION  
         [0013]    By the present invention, methods of measuring and controlling the percent stoichiometric oxidant, “PSO,” in the pyrolyzing section of an incinerator, and systems for use in measuring and controlling the PSO are provided which meet the above-described needs and overcome the deficiencies of the prior art. The methods for measuring the PSO in the pyrolyzing section of an incinerator are basically comprised of the following steps. An electrical signal corresponding to oxygen concentration is generated utilizing an oxygen sensor positioned to sense oxygen concentration or partial pressure in the gases within the pyrolyzing section. An electrical signal corresponding to temperature is generated using a temperature sensor positioned to sense the temperature of the gases within the pyrolyzing section. The electrical signals are then conducted to a processor for converting the electrical signals from the oxygen sensor and the temperature sensor to an estimate of the PSO using a mathematical relationship between the electrical signals and the PSO.  
           [0014]    Methods of this invention for controlling the PSO in the pyrolyzing section of an incinerator basically comprise the following steps. An electrical signal is generated corresponding to oxygen concentration in the gases within the pyrolyzing section. An electrical signal is generated corresponding to the temperature of the gases within the pyrolyzing section. The electrical signals corresponding to oxygen concentration and temperature are conducted to a processor for converting the electrical signals to an estimate of the PSO using a mathematical relationship between the electrical signals and the PSO. The PSO estimate is relayed to a feedback controller for generating a combustion air blower, oxidant or fuel flow control signal to adjust the combustion air, oxidant or fuel flow based on the PSO estimate and a pre-selected PSO value. The control signal is then relayed to the combustion air blower, oxidant or fuel control device.  
           [0015]    The systems for use in measuring the PSO in the pyrolyzing section of an incinerator basically comprise the following: a means for generating an electrical signal corresponding to oxygen concentration in the gases within the pyrolyzing section, a means for generating an electrical signal corresponding to the temperature of the gases within the pyrolyzing section, and a device for converting the electrical signals corresponding to oxygen concentration and temperature to an estimate of the PSO using a mathematical relationship between the electrical signals and the PSO.  
           [0016]    The systems for use in controlling the PSO in the pyrolyzing section of an incinerator basically comprise the following: a means for generating an electrical signal corresponding to oxygen concentration in the gases within the pyrolyzing section, a means for generating an electrical signal corresponding to the temperature of the gases within the pyrolyzing section, a controller for controlling the amount of combustion air, oxidant or fuel to the pyrolyzing section of the incinerator, a device to convert the electrical signals corresponding to oxygen concentration and temperature to an estimate of the PSO using a mathematical relationship between the electrical signals and the PSO, and a means for generating a control signal for the combustion air control device based on the PSO estimate and a pre-selected PSO value. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]    [0017]FIG. 1 shows a typical incinerator with the inventive system for measuring the PSO in the pyrolyzing section operation.  
         [0018]    [0018]FIG. 2 shows a typical incinerator with the inventive system for controlling the flow rate of combustion air to the pyrolyzing section. 
     
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0019]    Preferred methods of this invention for measuring the PSO in the pyrolyzing section of an incinerator basically comprise the following steps. An electrical signal corresponding to oxygen concentration is generated utilizing an oxygen sensor positioned to sense oxygen concentration or partial pressure in the gases within the pyrolyzing section. An electrical signal corresponding to temperature is generated using a temperature sensor positioned to sense the temperature of the gases within the pyrolyzing section. The electrical signals are then conducted to a processor for converting the electrical signals from the oxygen sensor and the temperature sensor to an estimate of the PSO using a mathematical relationship between the electrical signals and the PSO. The general method is shown in FIG. 1.  
         [0020]    Suitable oxygen sensors that can be used in this invention for generating an electrical signal corresponding to oxygen concentration include, but are not limited to, zirconia-based oxygen sensors, electrochemical sensors, micro-fuel sensors, and paramagnetic sensors. Of these, zirconia-based sensors are preferred. A particularly suitable oxygen sensor is commercially available under the trade designation “Oxyfire™” from Marathon Sensors, Inc. of Cincinnati, Ohio. The sensor should be positioned to sense the oxygen concentration or partial pressure in the gases just within the pyrolyzing section of the incinerator.  
         [0021]    Suitable temperature sensors that can be used in this invention for generating an electrical signal corresponding to temperature include, but are not limited to, thermocouples, resistance temperature detectors, pyrometers and remote temperature devices. Of these, thermocouples are preferred. Particularly suitable thermocouples are commercially available as Type B or Type R integral thermocouple probes available from Marathon Sensors, Inc. of Cincinnati, Ohio. The sensor should be positioned to sense the temperature of the gases just within the pyrolyzing section of the incinerator and as close as possible to the oxygen sensor.  
         [0022]    Signals from the oxygen and temperature sensors are conducted to a processor to calculate an estimate of the PSO. A particularly suitable processor is commercially available as a “Series F4” unit from Watlow Electric Manufacturing Company of St. Louis, Mo.  
         [0023]    The processor calculates an estimate of the PSO using a mathematical relationship developed from equilibrium calculations. This method is based on the initial assumption that the pyrolyzing section has a residence time long enough to allow the oxygen concentration to reach close to its equilibrium value. Adjustments for actual non-equilibrium operating conditions can generally be made once the unit is in operation.  
         [0024]    PSO can be expressed as a function of oxygen concentration and temperature in a plurality of different forms. Among these forms, two are found to be most suitable. The first form is:  
           PSO=a+b/[ 1+(( x+eT )/ c ) d ] 
         [0025]    where x is the oxygen sensor output in millivolts, T is the temperature in ° F., and a through e are empirical constants.  
         [0026]    The second expression is in the form of a polynomial:  
           PSO=a+b ( x+eT )+ c ( x+eT ) 2   +d ( x+eT ) 3    
         [0027]    where, again, x is the oxygen sensor output in millivolts, T is the temperature in ° F., and a through e are empirical constants.  
         [0028]    For example, the zirconia-based oxygen sensor is a zirconium oxide, or zirconia, electrolytic cell having a solid state electrolyte that conducts oxygen ions at temperatures above 1400° F. The ion conduction is reflected in a voltage between the two electrodes. The magnitude of the voltage depends upon the concentration of the oxygen across the cell walls (ratios of the oxygen partial pressures) and the temperature of the cell. The cell e.m.f. can be determined by the Nernst equation:  
           x=− 0.0215( T   r )Log 10 ( P   O   /P   1 )  
         [0029]    where x is the cell output voltage in millivolts; P O  is the partial pressure of oxygen in the cell in %, 20.95%; P 1  is the partial pressure of oxygen in the measured process in %; and T r  is the absolute temperature of the probe in degrees K.  
         [0030]    The partial pressure of the oxygen in the combustion gases was calculated for equilibrium conditions at various temperatures between 1400° F. and 3000° F. and for different sub-stoichiometric conditions. These values were then input into the Nernst equation to produce the cell output voltages. Then the cell output voltages (x) and the operating temperatures of the combustion gases (T) at the different sub-stoichiometric conditions were empirically evaluated to produce the necessary constants to calculate the percent stoichiometric oxidant (PSO) for any condition within the boundary limits of the data.  
         [0031]    The equivalence ratio can also be expressed in terms of the oxygen and temperature signals since the equivalence ratio is simply 100/PSO. For example, if the PSO is 80%, the equivalence ratio is 100/80 or 1.25.  
         [0032]    The methods of this invention for measuring PSO can be applied to combustion of many types of waste compounds such as NH 3 , HCN, C 2 H 3 N, C 3 H 3 N, saturated and unsaturated organic fuels such as paraffins, olefins, cycloparaffins, acetylenes and aromatic compounds with very little error. The accuracy may be affected by excessive amounts of compounds containing bound oxygen such as water (H 2 O), NO 2  and NO. Here “excessive amount” is defined as more than about one pound of bound oxygen from any stream directed into the incinerator (e.g., waste stream or quench stream) for each pound of hydrocarbon fuel where the hydrocarbon fuel can be either waste or the fuel supplied for normal operation.  
         [0033]    Preferred methods of this invention for controlling the PSO in the pyrolyzing section of an incinerator basically comprise the following steps. An electrical signal is generated corresponding to oxygen concentration in the gases within the pyrolyzing section. An electrical signal is generated corresponding to the temperature of the gases within the pyrolyzing section. The electrical signals corresponding to oxygen concentration and temperature are conducted to a processor for converting the electrical signals to an estimate of the PSO using a mathematical relationship between the electrical signals and the PSO. The PSO estimate is relayed to a feedback controller for generating a combustion air, oxidant or fuel flow control signal to adjust the combustion air, oxidant or fuel flow based on the PSO estimate and a pre-selected PSO value. The control signal is then relayed to the combustion air blower control device. The general method is shown in FIG. 2.  
         [0034]    Air is supplied to the pyrolyzing section of the incinerator by means of a blower. The air flow rate can be changed by a number of means including using a valve, changing the blower speed or changing the blower blade pitch. The present invention allows the PSO to be controlled at a pre-selected value by adjusting the blower air flow using a suitable device chosen from the group including, but not limited to, a valve, a blower speed controller or a blower blade pitch adjusting device. This is accomplished by electronically transferring the PSO estimate from the processor to a feedback controller. The feedback controller generates a combustion air blower control device signal based on the PSO estimate and a pre-selected PSO value using standard control procedures known to those skilled in the art.  
         [0035]    A preferred system for use in measuring the PSO in the pyrolyzing section of an incinerator basically comprises a means for generating an electrical signal corresponding to oxygen concentration in the gases within the pyrolyzing section, a means for generating an electrical signal corresponding to the temperature of the gases within the pyrolyzing section, and a device for converting the electrical signals corresponding to oxygen concentration and temperature to an estimate of the PSO using a mathematical relationship between the electrical signals and the PSO.  
         [0036]    A preferred system for use in controlling the PSO in the pyrolyzing section of an incinerator basically comprises a means for generating an electrical signal corresponding to oxygen concentration in the gases within the pyrolyzing section, a means for generating an electrical signal corresponding to the temperature of the gases within the pyrolyzing section, a combustion air blower, oxidant or fuel control device for controlling the amount of combustion air, oxidant or fuel to the pyrolyzing section of the incinerator, a device to convert the electrical signals corresponding to oxygen concentration and temperature to an estimate of the PSO using a mathematical relationship between the electrical signals and the PSO, and a means for generating a control signal for the combustion air control device based on the PSO estimate and a pre-selected PSO value.  
         [0037]    Thus, the present invention is well adapted to attain the objects and advantages mentioned as well as those that are inherent therein. While numerous changes may be made by those skilled in the art, such changes are encompassed within the spirit of this invention as defined by the appended claims.