Abstract:
A mixed potential sensor device and methods for measuring total ammonia (NH 3 ) concentration in a gas is provided. The gas is first partitioned into two streams directed into two sensing chambers. Each gas stream is conditioned by a specific catalyst system. In one chamber, in some instances at a temperature of at least about 600° C., the gas is treated such that almost all of the ammonia is converted to NO x , and a steady state equilibrium concentration of NO to NO 2  is established. In the second chamber, the gas is treated with a catalyst at a lower temperature, preferably less than 450° C. such that most of the ammonia is converted to nitrogen (N 2 ) and steam (H 2 O). Each gas is passed over a sensing electrode in a mixed potential sensor system that is sensitive to NO x . The difference in the readings of the two gas sensors can provide a measurement of total NH 3  concentration in the exhaust gas. The catalyst system also functions to oxidize any unburned hydrocarbons such as CH 4 , CO, etc., in the gas, and to remove partial contaminants such as SO 2 .

Description:
RELATED APPLICATIONS 
       [0001]    This application is a continuation of and claims the benefit of U.S. patent application Ser. No. 11/317,190 filed on Dec. 22, 2005 and entitled “Ammonia Gas Sensor Method and Device” which is related to and claims the benefit of U.S. Provisional Patent Application Ser. No. 60/593,250, of Balakrishnan Nair and Jesse Nachlas filed on Dec. 28, 2004, and entitled “Ammonia Gas Sensor Method and Device.” These applications are incorporated herein by this reference in their entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates in general to the measurement of ammonia gases in gases or gas streams. In some embodiments, the invention relates to ammonia measurement in streams from both mobile sources such as automobiles and trucks and stationary sources such as power plants, the residue of gaseous ammonia or urea that is originally added to mitigate NO x  emissions in processes such as selective catalytic reduction. 
       BACKGROUND OF THE INVENTION 
       [0003]    There is a need in the art for ammonia sensors that can detect and measure NH 3  at temperatures higher than 500° C. for emissions control systems. Typically, for control applications, the accuracy of the measurement needs to be ±1 ppm, and the detection limit needs to be as low as 1 ppm. A review of pertinent patent and other literature revealed that currently known and used ammonia sensors are incapable of proper function at temperatures higher than 500° C. while providing a detection limit of 1 ppm. Techniques proposed for improving gas selectivity and sensitivity include the use of a polymer molecular sieve. These techniques inherently preclude use at high temperatures, since polymers are not stable chemically at such temperatures. 
         [0004]    Optical sensors for the detection of NH 3  include IR detectors and optic-fiber-based sensors. Optical sensors can generally provide accurate gas measurement with little cross-sensitivity to other gas constituents. For optical systems, however, the gas inputs must be transferred to an analysis chamber, resulting in long lag times. Further, the associated equipment for such optical sensors is generally bulky and highly expensive. In addition, the use of polymer/volatile sensing materials necessitates relatively cool gas temperatures (i.e., generally &lt;100° C.). 
         [0005]    Semiconductor sensors are one variety of currently-used sensors that are typically based on semiconductors such as metal oxides or polymers, and measure the change in resistance or capacitance of the coating as a function of adsorbed species. The primary problem with semi-conductor oxides in general is that they measure bulk properties based on adsorption of gases, and there is a significant issue of cross-contamination as all gases tend to adsorb on high-surface area ceramic substrates to some extent, resulting in significant errors in measurement. The main problem for ammonia measurements in engine exhaust streams is cross-contamination with carbon monoxide (CO), and oxides of nitrogen (NO x ). To overcome this problem, one approach that has been tried is to use an “electronic nose” based on a number of semiconductor sensors operating in parallel that generate a series of responses in the presence of a mixture of gases. This results in the requirement for a very complex electronics package to calculate out the NH 3  concentration, which is undesirable and cost ineffective. 
         [0006]    Another problem faced in semiconductor sensors is that they have a low maximum temperature for use. Polymer-based sensors are useful only at temperatures below which the polymers are chemically stable (generally lower than 150° C.). Metal oxide semi-conductor sensors are typically most sensitive around 300° C., and they generally lose their sensitivity above 450° C., since the adsorption of most gases tails off above that temperature. Further, it has been observed that in many circumstances, semiconductor sensors typically have a long response time to fluctuations in ammonia concentration since they are kinetically limited by gas adsorption. The sensor responses of the series of sensors can then be analyzed to extract out information about the various gas species. 
         [0007]    This approach has two challenges: (1) the limited temperature capability of semiconductor based sensors (generally less than 450° C.) and (2) the complexity of accompanying electronics required to extract out meaningful gas concentrations from the signals of various sensing elements. Generally, these types of sensors are more suitable for air quality monitoring rather than for engine control. 
         [0008]    An attractive alternative is for exhaust gas hydrocarbon monitoring are solid-state electrochemical ceramic sensors. These devices can be broadly categorized into potentiometric and amperometric sensors, based on whether the monitored parameter is electrochemical potential or the current through the device at a fixed applied potential, respectively. Potentiometric sensors can be further categorized into equilibrium-potential-based devices and mixed-potential-based devices. There are three main categories of equilibrium-potential-based sensors, originally categorized by Weppner as Type I, Type II, and Type III sensors. The classification is relative to the nature of the electrochemical potential, based on the interaction of the target gas with the device. Type I sensors generate a potential due to the interaction of the target gas with mobile ions in a solid electrolyte (e.g. O 2  sensors with yttria-stabilized zirconia-YSZ, an O 2   −  ion conductor), whereas Type II sensors generate a potential due to the interaction of a target gas with immobile ions in a solid electrolyte (e.g. sensors based on CO 2 -K +  ion interaction). Type III sensors show no such direct relationship without the assistance of an auxiliary phase. Type II and Type III sensors are clearly unsuitable for high-temperature applications due to the nature of the materials used, generally nitrates, which are unstable and sometimes explosive at high temperatures. Type I sensors for NH 3  sensing are feasible, but impractical. Due to the presence of oxygen in the exhaust stream, which would interfere with the measurement, elaborate pumping cells are required for removing the oxygen prior to gas sensing. This makes the device complex and increases operating costs to the point where it is not an attractive option. The same problem of initial oxygen removal exists for amperometric devices for gas sensing. 
         [0009]    Amongst electrochemical sensors, the best option for exhaust gas monitoring to date has been mixed-potential based ceramic sensors. While this patent is directed at ammonia sensing and the key elements are the use of the catalyst system, the eventual species detected is NO x  and the discussion of mixed potential sensors for NO x  detection is relevant. Early work was performed by a Japanese group headed by Yamazoe and Miura on mixed potential sensors primarily for detection of NO x . Mixed potential sensors, which consist of metal, metal oxide or perovskite sensing electrodes on an oxygen ion conducting membrane, have a number of properties that make them very attractive for use as exhaust gas NO x  sensors. They can operate effectively at temperatures as high as 650° C. Further, they do not require elaborate pumping cells for removal of oxygen and can be fabricated in very compact shapes using relatively easy and cost-effective conventional ceramic processing techniques such as isostatic pressing, sintering, ink-processing, electrode application and post-firing. 
         [0010]    Thus, it would be an improvement in the art to provide methods and alternative configurations for ammonia-sensing systems designed to address these and other considerations. Such methods and devices are provided herein. 
       BRIEF SUMMARY OF THE INVENTION 
       [0011]    The present invention is directed to a method and design for measuring ammonia gases in exhaust streams such as, without limitation, mobile exhaust sources (including automobiles and trucks) and stationary exhaust sources (including power plants) to be used at high temperatures and to provide a gas sensor useful for measuring total NH 3  concentration in a gas stream. This method may be used to detect residue of gaseous ammonia or urea that is added in some instances to such exhaust streams to mitigate NO x  emissions in processes such as selective catalytic reduction. 
         [0012]    Thus, in some embodiments, the present invention provides ammonia sensors suitable for high-temperature use and/or sensors that measure total NH 3  concentration in an exhaust gas stream. In some configurations, the sensor and methods of the present invention include the use of two sensing chambers where the gas is treated with different catalyst systems to provide a clear difference in total NO x  concentration between the two chambers. In some embodiments, the sensors of the present invention may be capable of measuring NH 3  concentration as low as 1 ppm. 
         [0013]    In some embodiments, the present invention may further incorporate a NO x  and/or an oxygen sensor within the body of the NH 3  sensor so that oxygen and NO x  concentration can be measured simultaneously with NH 3 , thereby allowing the accurate determination of the total NH 3  concentration based on a signal which is a function of the oxygen and NO x  concentration. 
         [0014]    Other advantages and aspects of the present invention will become apparent upon reading the following description of the drawings and detailed description of the invention. These and other features and advantages of the present invention will become more fully apparent from the following figures, description, and appended claims, or may be learned by the practice of the invention as set forth hereinafter. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0015]    In order that the manner in which the above-recited and other features and advantages of the invention are obtained will be readily understood, a more particular description of the invention briefly described above will be rendered by reference to specific embodiments thereof that are illustrated in the appended drawings. Understanding that these drawings depict only typical embodiments of the invention and are not therefore to be considered to be limiting of its scope, the invention will be described and explained with additional specificity and detail through the use of the accompanying drawings in which: 
           [0016]      FIG. 1  is a schematic view of an apparatus used to demonstrate and practice the methods of the present invention; 
           [0017]      FIG. 2  is another schematic view of an alternate embodiment of the apparatus used to demonstrate and practice the methods of the present invention; 
           [0018]      FIG. 3  is a chart illustrating the voltage response as a function of NH 3  concentration for a RuO 2  catalyst at two different temperatures; and 
           [0019]      FIG. 4  is a chart illustrating the voltage response as a function of NH 3  concentration for a NiAl 2 O 4  catalyst at two different temperatures. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0020]    The presently preferred embodiments of the present invention will be best understood by reference to the drawings, wherein like parts are designated by like numerals throughout. It will be readily understood that the components of the present invention, as generally described and illustrated in the figures herein, could be arranged and designed in a wide variety of different configurations. Thus, the following more detailed description of the embodiments of the ammonia sensor and methods of the present invention, as represented in  FIGS. 1 through 4 , is not intended to limit the scope of the invention, as claimed, but is merely representative of presently preferred embodiments of the invention. 
         [0021]    One embodiment of the present invention is an NH 3  sensor illustrated schematically in  FIG. 1 . Thus, referring first to  FIG. 1 , a schematic view of an ammonia sensor  10  of the present invention is shown which illustrates the basic features that are required to achieve the accurate measurement of NH 3  concentration is a gas stream  12 . In a first step, which in some embodiments of the method and device of the present invention is optional, the gas stream  12  undergoes desulfurization. This desulfurization stage  20  of the device and/or methods of the invention may, in some embodiments, consist of an absorbent material such as CaO, MgO, or a compound from the perovskite group of materials that serves the function of removing SO 2  from the gas stream  12 . This could be in the form of a packed pellet or infiltrated support that can be periodically replaced during servicing without disassembling the rest of the sensor package. Other suitable configurations will be known to one of ordinary skill in the art. 
         [0022]    The decision to include or omit a desulfurizer  20  from the process and/or devices of the present invention is primarily dependent upon the sulfur content of the fuel that generates the gas stream  12 . The size and volume of any desulfurizer  20  used with the apparatus and methods of the present invention will be determined by the particular application. In some instances, it is thought that if the exhaust gas  12  has less than 15 ppm sulfur dioxide in the exhaust, a desulfurizer  20  may not be required. 
         [0023]    A next step or component in the devices and methods of the present invention is for the gas sample to be split into first and second streams, Stream  1  ( 30   a ) and Stream  2  ( 30   b ) as shown in  FIG. 1 . This may be accomplished using a wide variety of structural means known to one of ordinary skill in the art, including, without limitation, a split passage to separate input gas  12  into streams  30   a  and  30   b ; and an output line for drawing away a portion of the original inflow stream  12 . 
         [0024]    A first stream, Stream  1  ( 30   a ) may then be treated with a first catalyst stage  40  at a low temperature (generally from about 300° C. to about 500° C.) such that a majority of the gas of the first stream  30   a  is converted to N 2  and H 2 O. The reaction generally proceeds thus: 
         [0000]      2NH 3 +1.5O 2 →N 2 +3H 2 O 
         [0025]    Suitable oxidation catalysts include, in some configurations, nickel aluminate(NiAl 2 O 4 ), vanadium pentoxide (V 2 O 5 ), Molybdenum Oxide (MoO 3 ), tungsten oxide (WO 3 ), iron oxide (FeO, Fe 2 O 3 , Fe 3 O 4 ), cerium oxide (CeO 2 ), copper oxide (CuO), manganese oxide (MnO 2 ), ruthenium oxide (RuO 2 ), silver (Ag), platinum (Pt) and copper (Cu), as well as various mixtures and composites containing these ingredients. Other catalysts for the low temperature oxidation of NH 3  to N 2  and H 2 O will be known to one of ordinary skill in the art and are within the scope of the present invention. 
         [0026]    In this method of the present invention and devices embodying it, Stream  2  ( 30   b ) is not treated with a low temperature catalyst  40  according to the methods and in the devices of the present invention. Instead, Stream  2  ( 30   b ) is treated by a catalyst selected from the group of nickel aluminate (NiAl 2 O 4 ), vanadium pentoxide (V 2 O 5 ), Molybdenum Oxide (MoO 3 ), tungsten oxide (WO 3 ), iron oxide (FeO, Fe 2 O 3 , Fe 3 O 4 ), cerium oxide (CeO 2 ), copper oxide (CuO), manganese oxide (MnO 2 ), ruthenium oxide (RuO 2 ), silver (Ag), platinum (Pt) and copper (Cu), and any mixture or composites thereof at a high temperature to drive formation of NO. In this step, the temperature may be greater than about 600° C., and in some instances, greater than about 650° C. to cause the following reaction: 
         [0000]      2NH 3 +2.5O 2 →2NO+3H 2 O 
         [0027]    Following this, each stream will then be passed through a next catalyst  50  at a high-temperature, preferably higher than about 700° C. This stage of the catalyst  50  consists of an oxidation catalyst such as RuO 2  or CoO 2 , or a metal such as silver or platinum which functions to oxidize unburned hydrocarbons and convert CO to CO 2 . This stage  50  of the catalyst also acts to establish a steady state concentration ratio between NO and NO 2  whereby the NO 2  percentage of the total NO x  gas present is in the range of from about 1 to about 5% optimally, and at least within the range of from about 0.5 to about 10%. In Stream  2  ( 30   b ), the NH 3  will also be oxidized almost completely to NO at this higher temperature. 
         [0028]    After the gas in each stream has been conditioned by the catalyst system it passes into separate sensor cavities  60   a ,  60   b , where two separate voltage signals are generated that are proportional to the concentration of the total NO x  present in each gas stream, i.e. Stream  1  ( 30   a ) and Stream  2  ( 30   b ). The difference between the two signals corresponding to the NO x  concentrations in each stream is a measure of the NH 3  concentration in the exhaust gas. 
         [0029]    In another embodiment the catalyst/sensor system of the present invention may be miniaturized and combined into a single housing. In this configuration the outer shell of the housing may be designed to split the gas into at least two flows and then to guide each stream through the catalyst systems and then through the sensor electrodes to exit the housing. In some embodiments, the housing is metal. In this way the gas is conditioned by the respective catalyst system prior to contacting the sensor electrode thereby enabling accurate measurement of total NO x  concentration. Various temperature zones in the device can be achieved by integrating separate heaters into the device to heat each stage of the catalyst. It is also envisioned that in addition to being an ammonia sensor, the device can also provide a measurement of the NO x  concentration of the gas. 
         [0030]    In another preferred embodiment the catalyst/sensor system and method  10  illustrated schematically in  FIG. 1  may be modified to incorporate an oxygen sensor within the housing body resulting in a sensor system that is capable of performing in gas environments with rapidly changing oxygen concentrations. In this configuration an oxygen ion-conducting electrolyte membrane may be used for both the oxygen sensor and the NH 3  sensor. 
         [0031]    It is understood that the embodiments shown and discussed herein may also be extended to other design components such as a flat plate ceramic multilayer package design, a single electrolyte disk type design and so forth. 
         [0032]    Another embodiment of the systems and methods of the present invention is shown in  FIG. 2 . This embodiment differs from the first in that in the low temperature catalytic oxidation  140  of Stream  1  ( 130   a ), instead of NO reacting with O 2 , selective catalytic reduction catalysts may be used to oxidize the NH 3  by reaction with NO to form N 2  and H 2 O. This may provide a lower NO x  concentration due to the NO consumed in the reaction according to the following equation: 
         [0000]      4NH 3 +6NO→5N 2 +6H 2 O 
         [0033]    Electronic compensation may be required due to consumption of NO x . 
         [0034]    Several examples are provided below which discuss the construction, use, and testing of specific embodiments of the present invention. These embodiments are exemplary in nature and should not be construed to limit the scope of the invention in any way. 
       EXAMPLE 1 
       [0035]    An experiment was set up to test the concept of using a catalyst at two different temperatures so that when the gas passes through the high temperature catalyst all of the NH 3  is converted to NO and when the gas passes through the low temperature catalyst the NH 3  is converted to N 2  and H 2 O. A catalyst was fabricated by chopping up some high purity Al 2 O 3  insulation felt into small chips approximately 1 mm×1 mm×1 mm. The felt chips were then impregnated with a RuCl 2  solution followed by drying at 80° C. for 1 hour. The dried impregnated chips were then installed into a test apparatus that was a ⅜″ outside diameter stainless steel tube with compression fittings attached to each end of the tube. The felt chips were held in place with a piece of nickel mesh on each side of the bed of chips to keep them properly located within the stainless steel tube and prevent them from being displaced by the flowing gas. The tube apparatus was then installed in a small tubular resistively heated furnace that had a PID temperature controller connected to the furnace. The catalyst was then heated to 600° C. in flowing air to convert the RuCl 2  to RuO 2 . To complete the experimental test setup a mixed potential type NO x  sensor was connected to the gas plumbing system so that after the gas passed through the catalyst it would go to the NO x  sensor. 
         [0036]    The catalyst and NO x  sensor were then connected to a gas mixing system using 4 MKS mass flow controllers for mixing and controlling the flow of various gas compositions. The catalyst was then heated to a temperature of 300° C. and various NH 3  concentrations were mixed and passed through the catalyst and onto the NO x  sensor. Next, the catalyst was heated to 700° C. and the same sequence of measurements was repeated. The voltage response of the NO x  sensor at the various NH 3  concentrations and the two temperatures is shown in  FIG. 3 . The results indicate that when the gas passes through the high temperature catalyst that all of the NH 3  is converted to NO whereas, when the gas passes through the catalyst at 300° C., the majority of the gas is converted to N 2  and H 2 O. It should be noted that, without being limited to any one theory, it is thought that since this catalyst did not result in 100% conversion at the low temperature to N 2  and H 2 O, a more desirable result would be achieved by the use of a catalyst that is capable of achieving about 100% conversion to N 2  and H 2 O. This would lead to a sensor with better accuracy and sensitivity. A next step was thus considered to be the study of a variety of catalysts to find more optimum oxidation performance. 
       EXAMPLE 2 
       [0037]    A second experiment was set up to test the concept of using a catalyst at two different temperatures so that when the gas passes through the high temperature catalyst all of the NH 3  is converted to NO and when the gas passes through the low temperature catalyst the NH 3  is converted to N 2  and H 2 O. A catalyst was fabricated by mixing 10 wt. % La 2 O 3 /Al 2 O 3  followed by infiltration of Nickel nitrate to produce a 15 wt. % Ni composition. This precursor powder was then dried and calcined at about 800° C. in air. The calcined powder was then installed into a test apparatus that was a ⅜″ outside diameter stainless steel tube with compression fittings attached to each end of the tube. The packed powder was held in place with a piece of nickel mesh on each side of the bed of powder to keep it properly located within the stainless steel tube and to prevent the powder from being displaced by the flowing gas. The tube apparatus was then installed in a small tubular resistively heated furnace that had a PID temperature controller connected to the furnace. To complete the experimental test setup a mixed potential type NO x  sensor was connected to the gas plumbing system so that after the gas passed through the catalyst it would go to the NO x  sensor. 
         [0038]    The catalyst and NO x  sensor were then connected to a gas mixing system using 4 MKS mass flow controllers for mixing and controlling the flow of various gas compositions. The catalyst was then heated to a temperature of about 400° C. and various NH 3  concentrations were mixed and passed through the catalyst and on to the NO x  sensor. Next, the catalyst was heated to about 700° C. and the same sequence of measurements was repeated. The voltage response of the NO x  sensor at the various NH 3  concentrations and the two temperatures is shown in  FIG. 4 . The results indicate that when the gas passes through the high temperature catalyst that all of the NH 3  is converted to NO whereas, when the gas passes through the catalyst at about 400° C. all of the gas is converted to N 2  and H 2 O. Using this catalyst appears to result in nearly 100% conversion at the low temperature to N 2  and H 2 O. Thus, this catalyst composition produces nearly 100% selective oxidation of NH 3  to N 2  and H 2 O thereby enabling the effective use of a mixed potential NO x  sensor used in conjunction with this catalyst to successfully construct an NH 3  sensor. 
         [0039]    While specific embodiments of the present invention have been illustrated and described, numerous modifications come to mind without significantly departing from the spirit of the invention, and the scope of protection is only limited by the scope of the accompanying claims.