Chloride containing solid electrolyte gas sensing apparatus

#The requirements for monitoring and controlling stack gas pollutants have resulted in the development of solid electrolyte gas sensors having electrolyte compositions uniquely responsive to gaseous pollutants such as SO.sub.2, CO.sub.2, NO.sub.2, HCl and Cl.sub.2 U.S. Pat. No. 4,377,460 (Hirayama and Lin) relates to SO.sub.2, NO.sub.2, and CO.sub.2 gas sensor apparatus utilizing K.sub.2 SO.sub.4 or Na.sub.2 SO.sub.4 solid electrolyte for SO.sub.2 gas detection, NaNO.sub.3 solid electrolyte for NO.sub.2 gas detection, and Na.sub.2 CO.sub.3 solid electrolyte for CO.sub.2 gas detection. A mullite (3Al.sub.2 O.sub.3 .multidot.2SiO.sub.2) membrane containing Na or K impurities is utilized between the electrolyte portions of two separated cells. U.S. Pat. No. 4,427,525 (Lin and Hirayama) utilized the same electrolyte materials, but in a dual solid electrolyte cell configuration consisting of two cells, to provide signals indicative of both O.sub.2 and either SO.sub.2, NO.sub.2 or CO.sub.2. A doped ZrO.sub.2 membrane containing Na or K impurities is utilized, so as to support alkali cation conductivity as well as O.sup.= ion conductivity.

#The above-referenced sensors are electrochemical concentration cells which sense the equilibrium of a gas species of interest and generate an EMF signal corresponding to the difference in partial pressure of the gas species across the solid electrolyte sensor. Typically, the solid state sensor includes an ion conductive solid electrolyte with electrodes disposed on its opposite surfaces. The stack gas, or monitored gas environment, contacts a sensing electrode while the opposite electrode serves as a reference electrode.

#Conventional solid electrolyte compositions require operating temperatures of between about 600.degree. C. and 900.degree. C. to exhibit the desired ion conductivity to generate a suitable EMF signal. The accuracy of the EMF measurement depends in part on the effective sealing, or isolation, of the reference electrode from the monitored gas environment contacting the sensing electrode of the electrochemical cell. In a different design, U.S. Pat. No. 4,492,614 (Welsh) describes a concentration cell to detect HCl or Cl.sub.2 gas, where the solid electrolyte is selected from a group consisting of gas-impermeable SnCl.sub.2, BaCl.sub.2 or PbCl.sub.2. These materials can be doped to improve ion conductivity by addition of KCl. The solid electrolyte separates a sample gas chamber and a reference gas chamber. Instead of using a constant source of Cl.sub.2 reference gas, a solid reference material can be used which exhibits a constant chlorine activity, such as chlorides of aluminum, lithium, copper or sodium. This solid material would be contained in the reference gas chamber.

#In order that the invention can be more clearly understood, convenient embodiments thereof will now be described, by way of example, with reference to the accompanying drawings, in which:

#FIG. 1 is an enlarged sectioned illustration of a gas probe assembly incorporating a solid electrolyte electrochemical cell assembly in accordance with the disclosed invention; and

#FIG. 2 is a concentration curve for the sensor plotting ppm HCl vs. mV cell voltage.

#Referring now to FIG. 1, the solid electrolyte electrochemical concentration cell assembly 20 shown, consists of identical alkali ion half cells 22 and 24 comprised of solid NaCl or KCl electrolyte element 23 and sensing, metal electrode 26, and solid NaCl or KCl electrolyte element 25 and reference, metal electrode 28 respectively. In all cases, both electrolytes will be of the same composition. In one embodiment, half cells 22 and 24 are secured to opposite surfaces of the closed end 34 of the closed-end tubular membrane 36. The sensing electrode 26, usually platinum, of the electrochemical cell assembly 20 is disposed in contact with a surface of the electrolyte element 23 opposite from the surface contacting the membrane 36. The reference electrode 28, usually platinum, is in intimate contact with the surface of the electrolyte element 25 opposite the electrolyte element surface contacting the membrane 36.

#The electrochemical cell assembly 20, consisting of identical alkali ion conductive half cells 22 and 24, is located within the tubular housing 30 having apertures 32 therein to permit the monitored gas environment 38, shown as arrows, containing either oxygen or a chlorine containing component gas, to enter the housing 30 and contact the sensing electrode 26. A reference gas 40, having a stable or known concentration of the gas constituent to be measured, is supplied by an inlet tube 42 from a remote reference gas source 44 for contact with the reference electrode 28. The EMF signal developed by the concentration cell 20 in response to a difference, or change in equilibrium, in the partial pressure of the gas constituent of interest, as measured between the sensing electrode 26 and the reference electrode 28, is manifested by the measuring circuit 46. A temperature controller 48 responds to the electrochemical cell operating temperature as measured by the temperature sensor 50 to control the heater 52 to maintain the operating temperature of the cell 20 essentially constant.

#The closed-end tubular membrane 36 can be of a solid composition, i.e. ceramic, glass, etc., exhibiting sodium ion conductivity at elevated temperatures which corresponds to the sodium ion conductivity of the NaCl or KCl electrolyte elements 23 and 25. Experimental evaluation has confirmed that most ceramics incorporate some alkali oxide, such as K.sub.2 O and Na.sub.2 O, in an amount up to a few percent. These ceramic materials become K.sup.+ and Na.sup.+ ion conductors at elevated temperatures corresponding to the operating temperature of the solid electrolyte electrochemical cell assembly 20.