US0071288.

18B2

(12) United States Patent (10) Patent No.: US 7,128,818 B2

Khesin et al. (45) Date of Patent: Oct. 31, 2006

(54) METHOD AND APPARATUS FOR 4,639,717 A 1/1987 De MeirSman

MONITORING GASES IN A COMBUSTION 4,709,155. A 1 1/1987 Yamaguchi
SYSTEM 4,828,673 A 5/1989 Maeda ....................... 2O4/427
4.866,420 A 9/1989 Meyer, Jr.
(75) Inventors: Mark
Pa
Khesin,
Hud
Hudson, OH (US); Carl
OH (US) D
4.885,573 A
4,901.247 A
12,2f1990
1989 Wakimoto
Fry
almer, Hudson, OH (US); Don
Schneider, Lakewood, OH (US); Doug
4,923,117. A 5/1990 Adams
Byrd, Copley, OH (US); Lars (Continued)
Andersson, Cleveland, OH (US) FOREIGN PATENT DOCUMENTS
(73) Assignee: General Electric Company, DE 4239292 A 5, 1994
Schenectady, NY (US)
- 0 (Continued)
(*) Notice: Subject to any disclaimer, the term of this
patent is extended or adjusted under 35 OTHER PUBLICATIONS
U.S.C. 154(b) by 214 days. Kimura et al. Principles and Development of a Thick-Film Zirco
nium Oxide Oxygen Sensor, pp. 101-120 from ACS Symposium
(21) Appl. No.: 10/040,917 Series 309, 1986.*
(22) Filed: Jan. 9, 2002 (Continued)
(65) Prior Publication Data Primary Examiner Kaj K. Olsen
(74) Attorney, Agent, or Firm—Hunton & Williams
US 2003/O127325 A1 Jul. 10, 2003
(57) ABSTRACT
(51) Int. Cl.
GOIN 27/409 (2006.01) A method and apparatus for monitoring and measuring gas
(52) U.S. Cl. ....................................... 204/424; 204/427 concentrations in combustor applications is provided,
(58) Field of Classification Search ................ 204/424, wherein the apparatus is a gas sensor having a plurality of
204/427, 409; 205/784.5; 73/23.31, 23.32 electrodes cooperating with a single electrolyte cell for
See application file for complete search history. detecting the presence and concentration of gaseous com
ponents of a flue gas. A voltage is generated based on the
(56) References Cited flow of ions caused by differing gas concentrations as
U.S. PATENT DOCUMENTS detected by electrodes across the electrolyte. The change in
Voltage is correlated and is used to determine the concen
3,936,648 A 2f1976 Cormault et al. tration of detected gases, such as combustible gases, nitric
4,039,844 A 8, 1977 MacDonald
4,101,403 A 7, 1978 Kita et al.
oxides, carbon monoxide, etc., contained in the flue gas. The
4,177, 112 A 12/1979 Suzuki et al.
combustor operation may then be optimized to enhance
4,253.404 A 3, 1981 Leonard efficiency and minimize undesired gas concentrations in the
4,260,363 A 4, 1981 Cratin, Jr. flue gas in a desired fashion. A calibration gas may be
4,296,727 A 10/1981 Bryan introduced to calibrate the apparatus and a reference gas
4,370,557 A 1/1983 Axmark may be provided to an electrode as a basis for correlating the
4.419, 190 A * 12/1983 Dietz et al. ................. 205,785 concentrations of the gases.
4,430, 192 A 2, 1984 Maeda ....................... 204,410
4,562.529 A 12/1985 Drummond 33 Claims, 6 Drawing Sheets

200
 US 7,128,818 B2
Page 2

U.S. PATENT DOCUMENTS Forney Corporation, "OptiFlame Burner Diagnostic System”, 1996
Month N.A.
4,944,861 A 7, 1990 Reber ......................... 204,428 M.J. Khesin, et al., “Demonstration of New Frequency-Based
5,073,769 A 12/1991 Kompelien Flame Monitoring System”, American Power Conference, Chicago,
5,076.780 A 12/1991 Erdman Apr. 1996.
5,077.550 A 12/1991 Cormier M.J. Khesin, et al., “Application of a Flame Spectra Analyzer for
5,107,128 A 4, 1992 Daval1 burner Balancing, Sixth International Joint ISA POWID/EPRI
5, 191220 A 3, 1993 Innes Controls and Instrumentation Conference, Baltimore Jun. 1996.
5,249,954 A 10, 1993 Allen M.J. Khesin, et al., “Demonstration of New Flame Monitoring
5,257.496 A 11/1993 Brown System at a Pilot-Scale Gas-Fired Combustion Test Facility”.
5,280,756 A 1/1994 Labbe American Flame Research Committee, International Symposium,
5,296,112 A 3/1994 Seger et al. Baltimore, Md, Sep. 1996.
5,332,386 A 7, 1994 Hosome MK Engineering, Inc., 'System may boost combustion efficiency'.
5,496.450 A 3, 1996 Blumenthal Industry Watch, Sep. 1996.
5,501,159 A 3, 1996 Stevers M.J. Khesin, et al., “Demonstration Tests of New Burner Diagnostic
5,599,179 A 2f1997 Lindner System on a 650 MW Coal-Fired Utility Boiler', presented at the
5,705,129 A * 1/1998 Takahashi et al. ............ 422/90 American Power Conference, Chicago, Apr. 1997.
5,756,059 A 5/1998 Zamansky et al. M.J. Khesin, et al., “Application of a New Burner Diagnostic
5,796,342 A 8, 1998 Panov System for Coal-Fired Utility Boilers', presented to the Joint
5,798.946 A 8, 1998 Khesin ISA/EPRI Symposium, Jun. 1997, Knoxville, TN.
5,827,415 A 10, 1998 Gur et al. MK Engineering, Inc., "Combustion Diagnostic System”, illus
6,067,843. A 5, 2000 Hafele et al. trated brochure distributed Jan. 1998.
6,103,098 A 8, 2000 Omara MK Engineering, Inc., “Application of MPV-1 Combustion Diag
6,171.470 B1 1/2001 Patrick nostic System—A Case Study, Application on a 650 MW Coal
6,254,749 B1* 7/2001 Yokota et al. .............. 204,424 Fired Unit Jan. 1998.
6,277,268 B1 8, 2001 Khesin et al. MK Engineering, Inc. “MPV-1 Combustion Diagnostic System for
6,341,519 B1 1/2002 Khesin Tangential Boilers', Jan. 1998.
FOREIGN PATENT DOCUMENTS MK Engineering, Inc., “MPV-1 Combustion Diagnostic System'.
distributed Feb. 1998.
EP O089630 A 9, 1983 "Algorithms convert chaos into efficiency', text as printed in
EP O 120 423 A1 * 10, 1984 Personal Engineering and Instrumentation, Apr. 1998.
EP O 476 601 3, 1992 M.J. Khesin et al., “Combustion Control—New Environmental
EP O 581 451 2, 1994 Dimension': pp. 1262-1266; Proceedings of the American Power
GB 2O29578 A 3, 1980 Conference, date unknown.
WO WO 97.24560 7/1997 M.J. Khesin et al. MPV Combustion Diagnostic and Optimization
WO 974.2495 A 11, 1997 System. The Mega Symposium; EPRI-DOE-EPA Combined Utility
Air Pollutant Control Symposium; Aug. 1999.
OTHER PUBLICATIONS GE Brochure "MK Combustion Optimization System.” 2001.
Panametrics, Inc. (brochure), In-situ oxygen analyzer FGA411, Sep.
Khesin, M. J., Ivantotov, A. A., “Fluctuations of Flue Gas Oxygen 1999.
as Indicator of Combustibles.” Teploenetgetika, 1978. 25(5) 60-63. Nicholas Szabo et al., “Microporous zeolite modified yttria stabi
Brochure for Miracle Sensor, MPV-2 Combustion Diagnostic Sys lized Zirconia sensors for nitric oxide determination in harsh envi
tem/CO Monitor, Nov. 1997. ronments.” The Ohio State University. 2001.
M.J. Khesin, et al., “Smart Flame Scanners—Myth or Reality?”. Eric Wachsman et al. “Selective detection of NOx by differential
American Power Conference, Chicago, Apr. 1995. electrode equilibria'. Solid State Ionic Devices II Ceramic Sen
M.J. Khesin. “Combustion Diagnostics based on Frequency Spectra sors, Electrochem Soc. Ed 2000-32, 298 304 (2001).
Analysis'. American Flame Research Committee. Montery, CA,
Oct. 1995. * cited by examiner
U.S. Patent Oct. 31, 2006 Sheet 1 of 6 US 7,128,818 B2

|
|

Fig. 1A
U.S. Patent Oct. 31, 2006 Sheet 2 of 6 US 7,128,818 B2

FLUE GAS
OOO-)5OOF O2

SENSOR
U.S. Patent Oct. 31, 2006 Sheet 3 of 6 US 7,128,818 B2

2OO

24 23O 2O8 204 216 202 218 220

U.S. Patent Oct. 31, 2006 Sheet 4 of 6 US 7,128,818 B2

3OO
Y.
34 32 326 32O 3O2 322 3O4

1- , --,
t
REF, ZXZZZZZZ AZK (AZAZZZZZZZZZZZZZZZZ TZZZZZZZ -
-, -es
, /
AIR
-ups-
\ -- W
N - L. A.
-- - - --- - - - A SX --

7-7- - * * - / / /

EVEN
YYZZZZZZZZZZZZZZZZZ - 7
I ZZ 7- --- -,;
d
f || \
?
3O8 324 31 O
l
3O6
N316
/38
3B
U.S. Patent Oct. 31, 2006 Sheet 5 of 6 US 7,128,818 B2

412

y y y
"N,
44 428 404 416 4O2 48 420

-
\ O O
| /
REF,
AR
A
t ;:
C
Y E S R A
Sesares
N X-Fes
is
is
- 1 C
/ I y
428 424 408 40 422 4O6 426
> 4C >48
Fig. 4A
U.S. Patent Oct. 31, 2006 Sheet 6 of 6 US 7,128,818 B2

500 MOUNTING FLANGE

STANLESS STEEL
FLEXBLE JACKET SENSOR
5O2
504

Fig. 5
 US 7,128,818 B2
1. 2
METHOD AND APPARATUS FOR Fossil Combustors' (referred to herein as the 268 patent).
MONITORING GASES IN A COMBUSTION These sensors are relatively simple in design and provide an
SYSTEM immediate response. An example of an existing sensor in
production and available on the market is the MK CO sensor
BACKGROUND OF THE INVENTION manufactured by the General Electric Reuter-Stokes Com
pany of Twinsburg, Ohio.
The present invention relates to monitoring and measur The 268 patent discloses, among other things, an appa
ing gases, Such as constituent gases in flue gases in com ratus for monitoring changes in a concentration of gas
bustion systems, including systems having sensors used in molecules of at least a first type in an environment. The
combustion applications, such as boiler, furnace, combus 10 apparatus includes a mass of material and first and second
tion gas turbine or fossil combustor applications. electrodes. The mass of material is permeable to ions formed
To understand the nature and operation of the invention it when gas molecules of the first type are ionized. The first
may be helpful to consider Some exemplary applications. and second electrodes are arranged on the mass of material
For instance, in numerous industrial facilities or environ Such that, when a concentration of the gas molecules of the
ments a hydrocarbon fuel is burned in a combustor (e.g., a 15
first type at the first electrode is different than a concentra
boiler or furnace) to produce heat to raise the temperature of tion of the gas molecules of the first type at the second
a fluid. For the combustor to operate efficiently and to electrode, gas molecules of the first type are ionized at the
produce an acceptably complete combustion having byprod first electrode to form ions which flow from the first elec
ucts/products of combustion that fall within the limits trode to the second electrode via the mass of material and are
imposed by environmental regulations and design con recombined at the second electrode to form gas molecules of
straints, all of the individual burners should be operating the first type, thereby generating a signal between the first
cleanly and efficiently, and all post-flame combustion con and second electrodes. Each of the first and second elec
trol systems should be properly balanced and adjusted. trodes is in fluid communication with the environment.
Emissions of unburned carbon, nitric oxides (NO, NO, More specifically, the 268 patent discloses a system for
NOx), carbon monoxide or other byproducts commonly are 25
monitoring changes in a concentration of oxygen present in
monitored to ensure compliance with environmental regu an environment, the system includes at least one Nernstian
lations. As used herein and in the claims, the term nitric type gas sensor. The sensor includes a mass of Solid
oxides shall include nitric oxide (NO), nitrogen dioxide electrolyte material and first and second electrodes. The first
(NO), and nitrogen oxide (NOx, where NOx is the sum of and second electrodes are disposed on the mass of Solid
NO and NO). The monitoring of emissions heretofore has 30
electrolyte material to generate a signal therebetween indica
been done, by necessity, on the aggregate emissions from the tive of a difference between an oxygen concentration at the
combustor (i.e., the entire burner array taken as a whole). first electrode and an oxygen concentration at the second
Some emissions, such as the concentration of gaseous com electrode. Each of the first and second electrodes is in fluid
bustibles in hot flue gases, are difficult and/or expensive to communication with the environment. However, the Nerns
monitor on-line and continuously. These emissions are typi 35
tian-type gas sensor may be used to monitor the concentra
cally measured on a periodic or occasional basis. When a tion of any number of gases. The mass of material included
particular combustion byproduct is found to be produced at in the sensor is permeable to ions formed when gas mol
unacceptably high concentrations, the combustor should be ecules of a first type are ionized. A signal is generated
adjusted to restore proper operations. However, measure between the at least first and second electrodes in response
ment of aggregate emissions, or measurement of emissions 40
to changes in the concentration of the gas molecules of the
on a periodic or occasional basis, provides little, if any, first type in the environment. Further, the sensor may be free
useful information regarding what particular combustor of a temperature control device.
parameters should be changed to effect Such an adjustment.
Three main combustion variables, namely O, CO and The 268 patent further discloses a method for calibrating
NOx should be continuously monitored to optimize the 45 a gas sensor, the method including Supplying each of a first
combustion process and to achieve a goal of providing gas having a first profile and a second gas having a second
maximum efficiency at the minimum level of emissions. profile, which is different that the first profile, to the gas
Solid electrolyte (e.g., Zirconia) based combustion sensors sensor in a predetermined sequence. The gas sensor or a
are well known and commonly used in fossil combustors to signal analyzer associated therewith is adjusted based upon
measure oxygen and combustibles (commercial Suppliers 50 an output signal of the gas sensor to calibrate the gas sensor.
include Rosemount Analytical, Ametek Thermox, and An apparatus for calibrating a gas sensor includes a Switch
Yokogawa). These sensors are usually used with reference ing system and a sequencer. The Switching system is in fluid
air applied to one of two electrodes. In most cases the communication with each of a first tank having a first
existing sensors are extractive and require high mainte predetermined gas profile and a second tank having a second
aCC. 55 predetermined gas profile. The sequencer causes the Switch
Recently some Suppliers have introduced oxygen sensors ing system to Supply gas from each of the first and second
that do not utilize a continuous Supply of reference gas. tanks to the gas sensor in a predetermined sequence.
Instead, these sensors have a sealed internal electrode filled The 268 patent further discloses, such as at FIGS. 8A and
with a mixture of metal/metal oxide that generates a constant 8B, a multi-piece pipe and connector arrangement for
partial pressure of O2 inside the sealed volume. 60 assembly and installation of the sensor in a field combustor
There are a number of methods to measure flue gas application.
combustibles (primarily CO) using solid electrolytes. One of There are a number of methods to measure NOx in flue
the methods is based on using a fluctuating signal in an gas using Nernstian Solid electrolyte sensors in the mixed
in-situ potentiometric solid electrolyte cell directly posi potential potentiometric mode. In Such designs, the analyzed
tioned in the high temperature flue gas stream, as described 65 gas, prior to reaching the measuring electrode, passes
in U.S. Pat. No. 6,277.268 (Khesin et al.), entitled “System through a porous filter that enhances its sensitivity to NO or
And Method For Monitoring Gaseous Combustibles In NOx, the sum of NO and NO. The practical use of such
 US 7,128,818 B2
3 4
“filtered NOx sensors is difficult due to the effects of other inside a protective shell to facilitate its calibration, to reduce
components, primarily CO and O. the effect of flue gas velocity and to protect its surface from
ash deposits.
BRIEF SUMMARY OF THE INVENTION According to another aspect of the invention, the com
bustibles sensor is configured or converted into a combined
The present invention overcomes the problems noted O+CO sensor, as described above, and additionally con
above, and offers additional advantages, by providing an figured in combination with a sealed O sensor (sealed
improved method and system for monitoring gases in com O+CO sensor). In this arrangement, a continuous Supply of
reference gas is not required.
bustion systems. The invention may be used in a number of 10 According to another aspect of the invention, the com
applications, including power boilers and fossil combustors. bustibles sensor is configured or converted into a combined
In one manner, the invention provides simultaneous moni potentiometric CO+NOx sensor by using it in combination
toring and/or measuring of key combustion components, with a “filtered NOx sensor. The sensor has one common
Such as oxygen, NOx and CO, using one solid electrolyte solid electrolyte cell with two measuring electrodes and one
based in-situ potentiometric sensor. Such sensors can be 15 common reference electrode. A porous thin filter made of a
grouped together to provide necessary profiling and map material, capable to oxidize CO into CO, and eliminate the
ping of combustion variables as an effective tool of com effect of CO, is placed over one of the measuring electrodes.
bustion optimization. Techniques and materials for filtering CO. This electrode
According to one aspect of the present invention, a operates in the mixed potential mode and is used to measure
fluctuational combustibles sensor provides a combined NOX. Another measuring electrode operates as a fluctua
potentiometric O+CO sensor. In this embodiment, a refer tional CO sensor. As a result, two signals CO+NOx are
ence gas (air) is Supplied to one electrode (the reference generated in one potentiometric Solid-electrolyte sensor.
electrode) of the combustibles sensor such that this reference This sensor arrangement does not require continuous refer
gas flows through the sensor. This may be referred to as a ence gas Supply and is essentially as shown in FIG. 4A,
flow-thru O+CO sensor. The O. component of the sensor 25 except without the reference gas Supply. Accordingly, in this
operates like a traditional Nernstian-type sensor in that it is embodiment there is no DC component from which to
operated in accordance with the Nernst equation. However, determine O. concentration.
the term Nernstian is often used generically to refer to According to another aspect of the invention, the com
sensors that are a solid electrolyte Zirconia-based sensor and bined O-CO sensor, as described above, can be used in
30 combination with the “filtered NOx sensor to form a
that do not operate in accordance with the Nernst equation.
The CO and NOx aspects of the sensor are not “Nernstian' combined O+NOX-CO sensor (FIG. 4A). In this configu
in the technical sense. Rather, the CO and NOx sensor ration, the sensor has one common solid electrolyte cell with
configurations of the present invention operate in a mixed two measuring electrodes and one common reference elec
potential mode, in that the processing associated with deter trode. One set of electrodes, e.g., the CO electrode in
mining concentration of CO and NOx deviates from the 35 combination with the reference electrode, operates as a
Nernst equation based on a number of factors, such as Nernstian sensor and is used as a combined O+CO sensor
temperature, materials used, etc. as described above having DC and fluctuating AC compo
In one manner, the present invention may be used to nents. Another set of electrodes, e.g., the NOx electrode in
combination with the reference electrode and is used to
convert existing sensors or sensor designs, such as the MK 40 measure NOx, also as described above. As a result, three gas
CO sensor, into a combined potentiometric O+CO sensor. concentrations, O.--NOX--CO, are monitored and measured
This approach offers a less complicated sensor design, where in one potentiometric Solid-electrolyte sensor.
the sensor has a solid electrolyte cell with two measuring According to yet another aspect of the invention, the
electrodes. The in-situ potentiometric sensor generates an in-situ Solid-electrolyte sensor, as described above, is
output signal which consists of two components, DC and 45 equipped with a flexible stainless hose or conduit to facili
AC. Historically, the DC component has been used to tate its packaging, assembly, installation and maintenance in
calculate O. using the Nernst equation and the AC compo a boiler. The actual gas measuring probes could be of
nent filtered out of the signal. More recently, the fluctuating significant length, 20–30 feet or more. When the length of
AC component has been used to determine concentrations of the gas measuring probes is significant (exceeds 6-8 feet),
carbon monoxide (CO), nitric oxide (NOx), or other gaseous 50 the probes have to be assembled onsite, thereby complicat
combustibles, as described in U.S. Pat. No. 6,277,268. ing assembly, transportation, insertion and retraction proce
FIGS. 2A and 3A illustrate two exemplary versions of dures. Using the flexible hose affords greater flexibility
combined O+CO sensors: a sensor having a solid electro associated with onsite installation, in conjunction with a
lyte cell with both ends open (FIG. 2A) and a sensor having Support conduit mounted in the post-flame Zone, and allows
a solid electrolyte cell with one end closed (FIG. 3A). 55 fabrication of the whole probe to take place at the factory.
When a combustor operates in the balanced-draft mode The unit is then shipped to the site fully assembled and the
(under negative pressure), the natural draft can be used as a insertion and retraction of the sensor probe unit is greatly
driving force for a reference air supply. The reference air simplified, especially in congested plant environments. FIG.
supply line can also be used for periodic calibration of both 4A illustrates a gas sensing probe with a flexible hose.
O and CO sensors by Supplying calibration gases to a 60 Accordingly, one objective of the present invention is to
reference electrode. provide simultaneous and immediate measurements of sev
Instead of continuous sensor heating and temperature eral key combustion variables, such as the concentration of
control, the sensor is positioned in the flue gas Zone at proper oxygen, nitric oxides (to include NO., NO, NOx), and other
temperature window, for example in many boiler/furnace gaseous combustibles (such as CO), using one solid elec
applications approximately between 900–1500°F. flue gas 65 trolyte-based in-situ potentiometric sensor. Existing in-situ,
temperature. The temperature is continuously measured and Solid electrolyte potentiometric combustion sensors allow
used to provide compensation. The sensor head is placed the measurement of only one combustion variable and have
 US 7,128,818 B2
5 6
essential operating difficulties. A combustion sensor charac together to provide necessary profiling and mapping of
terizing several key combustion variables simultaneously combustion variables as an effective tool of combustion
and immediately offers significant benefits for successful optimization.
on-line combustion diagnostics and optimization. To achieve the goal of stable and efficient operation of any
In one manner, the invention improves over known meth combustion apparatus, it is useful to achieve continuous,
ods by providing enhanced capabilities to simultaneously on-line monitoring of various combustion variables and their
monitor various gases, for example O, CO and NOx. One distribution profiles in different combustion Zones. When
embodiment of the invention provides a supply of reference Such monitoring is accomplished effectively, individual
air to one electrode, an external filter to another electrode, burners as well as post-flame combustion controls may be
external housing for calibration and flexible hose, in one 10 adjusted to achieve optimum relationships between the fuel
embodiment made of stainless steel, for more effective and airflows, an optimum distribution of individual air flows
assembly, installation and maintenance in combustion appli and reburning fuel flows, and an optimization of other boiler
cations and environments. adjustments, thereby increasing the efficiency of the com
Another object of the invention is to enhance the capa bustor significantly.
bilities of combustibles sensors by combining measurements 15 It is known to employ an in-situ oxygen sensor to monitor
of several key combustion variables in one in-situ potentio the concentration of oxygen in a combustor. Typically, Such
metric solid electrolyte sensor. a sensor employs a pair of porous metal (e.g., platinum)
Another objective is to convert existing fluctuational electrodes disposed adjacent one another on opposite sides
combustibles sensor into a combined (O+CO) sensor either of a solid electrolyte (e.g., yttria (YO) stabilized zirconia
by Supplying a reference gas (air) flow to one electrode of (ZrO) (YSZ)) element, with one of the electrodes (a refer
the combustibles sensor (reference air sensor) or by using it ence electrode) being Surrounded by a gas having a prede
in combination with a sealed O. Sensor. termined oxygen concentration, and the other electrode (a
Yet another objective of the present invention is to pro sensing electrode) being exposed to the gas being moni
vide a sensor that can be converted into a combined (NOX tored. In these sensors, when the solid electrolyte element is
CO) sensor by using it in combination with a filtered NO 25 heated to a sufficient temperature (e.g., above 600° C.), it
sensor. Further, the sensor can be converted into a combined becomes permeable to oxygen ions. Therefore, when the
(O+NOX--CO) sensor by using it as a flow-thru (or sealed) concentration of oxygen molecules is greater at one of the
O. sensor in combination with a filtered NOx sensor. electrodes than at the other, oxygen ions will migrate from
It is a further object of the invention to provide an one of the electrodes to the other, with the electrodes serving
apparatus and system for simultaneously monitoring and 30 as catalytic Surfaces that enable oxygen molecules to
measuring concentration levels of flue gas components to become oxygen ions. The electron imbalance resulting from
minimize undesirable emissions and to optimize combustor this flow of oxygen ions and the ionization/deionization
operation. occurring at the respective electrodes generates a voltage
between the electrodes that is a function of the ratio of the
BRIEF DESCRIPTION OF THE DRAWINGS 35 partial pressures of oxygen at the two electrodes, as well as
the temperature of the solid electrolyte material. The voltage
The present invention can be understood more completely generated between the two electrodes is defined by the
by reading the following Detailed Description of exemplary so-called “Nernst' equation, as follows:
embodiments, in conjunction with the accompanying draw
ings, in which: 40
FIGS. 1A and 1B illustrate an example of a boiler having wherein:
Solid-electrolyte sensors embodying the present invention
and positioned to produce signals indicative of levels of E=the voltage out:
gaseous combustibles; T=the absolute temperature of the sensor:
FIGS. 2A and 2B are sectional views of a gas-sensing 45 R=the Universal Gas Constant;
probe incorporating a first embodiment of the present inven F-Faraday's Constant;
tion, specifically an oxygen/combustibles sensor with one P1 the partial pressure of oxygen in the reference gas;
set of electrodes; P2 the partial pressure of oxygen in the monitored gas;
FIGS. 3A and 3B are sectional views illustrating a second C- a constant for each individual sensor, and
embodiment of the invention, specifically an oxygen/com 50 Ln(P1/P2) is the natural logarithm of the ratio P1/P2.
bustibles sensor with one set of electrodes (one end closed); As can be noted, the only variables in the Nernst equation
FIGS. 4A, 4B and 4C are sectional views illustrating a are E. T. P1, and P2. When the partial pressure of oxygen in
third embodiment of the present invention, specifically a the reference gas (P1) is held constant, the signal E output
combined O+NOx+CO sensor with two sets of electrodes; by such a prior art sensor is therefore affected only by: (1)
and 55 changes in the partial pressure of oxygen in the measured
FIG. 5 illustrates an inventive gas sensing probe having a gas P2, and (2) changes in the temperature T of the sensor.
flexible component for enhanced assembly and installation. By eliminating the effect of the sensor's temperature T on
the value of the voltage E, the voltage E output by such a
DETAILED DESCRIPTION OF THE sensor responds only to changes in the value P2 and can
INVENTION 60 therefore be used as an accurate indicator of the concentra
tion of oxygen in the measured gas (i.e., E=f(P2)). The effect
The invention may be employed in a number of combus of a Nernstian-type gas sensor's temperature T on the value
tion applications, including power boilers and fossil com of the voltage E output therefrom is typically eliminated
bustors. In one manner, the invention provides simultaneous using one of two techniques. According to one technique, a
monitoring and/or measuring of oxygen, NOx and gaseous 65 heater is provided within the sensor, and is the heater
combustibles using one common in-situ sensor operating in activated selectively to maintain the sensor at a constant
the potentiometric mode. Such sensors can be grouped temperature T. In accordance with another technique, a
 US 7,128,818 B2
7 8
thermocouple is disposed within the sensor to measure the Khesin, M. J., Ivantotov, A. A., “Fluctuations of Flue Gas
sensor's temperature T, and the Voltage E is adjusted to Oxygen as Indicator of Combustibles. Teploenetgetika,
compensate for changes in the temperature T. As used 1978, 5, each of which is hereby incorporated herein by
herein, the term “temperature control device' refers to any reference. As discussed in these articles, an output signal
device, circuitry, hardware, Software, or any combination generated by a solid-electrolyte, in-situ oxygen sensor can
thereof, that is employed to eliminate the effect of the be used to monitor gaseous combustibles by correlating the
temperature T of a Nernstian-type gas sensor on the Voltage fluctuating AC component of Such a signal with gaseous
E output thereby, using either of the two above-described combustibles.
techniques. In order to exploit the phenomenon described in refer
With Nernstian-type gas sensors that employ at least one 10 ences (1) and (2) referenced immediately above in a prac
porous catalytic electrode (e.g., a porous platinum elec tical and useful manner, however, serious technical difficul
trode), when gaseous combustibles come into contact with ties needed to be overcome. These difficulties include high
the catalytic electrode under proper conditions, they are operating temperatures (e.g., above eight-hundred degrees
caused to combine chemically with oxygen in a combustion Celsius (°C.)), gradual reduction of the catalytic capacity of
type reaction to form non-combustible by-products. For 15 sensor electrodes, inconsistency of results, and uncertainty
example, two carbon monoxide molecules (2CO) may com of signal processing algorithms used to obtain such results.
bine with one oxygen molecule (O) to form two carbon The present invention overcomes these difficulties by pro
dioxide molecules (2CO) (i.e., 2CO+O=2CO), or two viding an improved and more versatile sensor design, and an
hydrogen molecules (2H) may combine with one oxygen effective and universal method and system for monitoring
molecule (O) at the electrode to form two water molecules gaseous combustibles in a combustor.
(2HO) (i.e., 2H+O-2H2O). As used herein, the term In one application of the present invention, one or more
"gaseous combustible” refers to any gaseous molecule that solid-electrolyte gas sensors 102 are positioned in the flue
is capable of being combined chemically with oxygen in a gas flow in the post-flame Zone (described below in con
combustion-type reaction. Because of this chemical reaction nection with FIG. 1A) of a combustor 100 to measure
between gaseous combustibles and oxygen at the catalytic 25 fluctuations in the oxygen concentration of the flue gas. The
electrode, a rise in the level of gaseous combustibles causes fluctuations measured by these sensors may be used to
additional oxygen molecules near the electrode to be con calculate values which correlate with real-time levels of
Sumed, thereby decreasing the concentration of oxygen at gaseous combustibles.
the electrode and correspondingly changing the Voltage In one embodiment of the invention, each sensor includes
output by the sensor. Similarly, a decrease in the level of 30
a solid-electrolyte (e.g., YSZ) element and at least two
gaseous combustibles near the electrode causes fewer oxy metal, preferably porous, (e.g., platinum) electrodes associ
gen molecules near the electrode to be consumed, thereby ated therewith. In accordance with an aspect of the present
increasing the concentration of oxygen at the electrode and invention, at least one of the electrodes is in fluid commu
correspondingly changing the Voltage output by the sensor. nication with a flue gas for monitoring constituent gas
In the flue gas in the post flame Zone (explained below) of 35
molecules in the flue gas. At least one other electrode is
a combustor, carbon monoxide (CO) is typically the most isolated so as not to be in direct fluid contact with the flue
prevalent gaseous combustible present. In fact, carbon mon gas and may be immersed in a reference gas. The gas to be
oxide typically accounts for more than ninety-five percent of monitored may be for instance oxygen, CO, NOx, or other
the gaseous combustibles present in the flue gas. Therefore, combustible gases. By way of example, the electrode iso
the output signal from a Nernstian-type gas sensor sensing 40
lated from the flue gas is immersed in a reference gas, e.g.,
the flue gas of combustor may serve as a reliable indicator air. And say that the other electrode is in communication
of the level of CO present therein. with the flue gas and is connected to a system for monitoring
Signals from Nernstian-type gas sensors include two the concentration of oxygen or determining the concentra
components: (1) intensity (“the DC component'), and (2) tion of some other gas based on the concentration of oxygen.
fluctuating frequency (“the AC component'). The DC com 45
In this manner, when the oxygen concentration in the flue
ponent, according to the Nernst equation, is a function of gas changes from a first level to a second level, the rate at
several parameters, including sensor temperature and oxy which the oxygen concentration at the flue gas electrode
gen concentration in the analyzed and reference gases. The changes from the first level to the second level is different
DC component has been the component of interest in than the rate, if it changes at all, at which the oxygen
systems employing these sensors for determining O con 50
concentration at the isolated electrode changes from the first
centration. The fluctuating AC component is commonly level to the second level. In other words, each of the
filtered from the output signal of an oxygen sensor because electrodes may be configured and arranged so that there is a
it is considered to be useless noise. In non-in-situ Sensors, time constant associated therewith that determines how
Such as extractive arrangements in which the sensor element quickly the oxygen concentration level at that electrode rises
is external to the post-flame Zone area and sample flue gas 55
or falls to a new oxygen concentration level in the flue gas
is extracted and delivered to the external sensor, delays are and isolated reference environments.
introduced and the accuracy of the fluctuating AC compo Any of a number of different relationships involving a
nent is significantly impaired and in many instances effec time constant may exist between the oxygen concentration at
tively lost. Accordingly, the fluctuating component has been each electrode and the oxygen concentration in the respec
typically considered of little use in Such systems. 60
tive environments, and the invention is not limited to any
Experimental testing of boilers, supported by theoretical particular type of relationship. One example of a relation
analysis, has demonstrated that the fluctuational AC com ship between the oxygen concentration at an electrode and
ponent of an in-situ oxygen sensor may be used as an the oxygen concentration in the respective environment is an
indicator of combustion efficiency. This topic is discussed, exponential relationship involving a time constant Tc, Such
for example, in two articles: (1) Khesin, M. J., Johnson A. 65
aS
J., “Combustion Control: New Environmental Dimension,”
American Power Conference, Chicago, 1993; and (2)
 US 7,128,818 B2
9 10
wherein: offsite, to "tweak” or otherwise bring a particular sensor into
C, the concentration of oxygen at the electrode, compliance with the design response curve. Although some
C, the concentration of oxygen in the environment, examples of gas concentration ranges are provided, the
AC, the change in concentration of oxygen in the environ ranges are largely dependent upon the particular combustor
ment, application or design, mode of operation and the particular
e the exponential operator, fuel(s) used.
t the time elapsed since the change in oxygen concentration In one embodiment, the output signal is processed in the
occurred, and frequency domain by using a frequency domain amplitude
Tc is a time constant specific to the electrode. spectrum of the signal to generate an extremum function (as
The electrodes are in fluid communication with their 10 described below), and one or more combustion parameters
respective environments by different “degrees” when the are calculated based upon one or more characteristics of the
time constants T of the two electrodes are different. The extremum function so generated. In another embodiment,
electrodes may be configured and/or arranged in any of the signal is processed in the time domain (as described
numerous ways so that their time constants T, differ from below) by analyzing one or more characteristics of a time
one another, and the invention is not limited to any particular 15 domain representation of the signal during a selected time
technique for accomplishing the same. In various illustrative interval. In still another embodiment, the signal is processed
embodiments, for example, this goal may be achieved both in the frequency and time domains, and the results of
simply by employing electrodes that differ in their design, calculations in each domain are combined to yield one or
material and/or characteristics. For example, the electrodes more combustion parameters. The levels of the gaseous
may have different geometries, may be coated by materials combustibles may then be estimated using a combination of
having different porosities, may be coated by different these calculated combustion parameters, along with limiting
materials, and/or may be coated by different amounts of a conditions which may depend, for example, on the tempera
material, e.g., a porous, high-temperature epoxy. ture, level of oxygen, and/or combustibles in the controlled
When the electrodes are configured and arranged so as to gas. These limiting conditions may, for example, be deter
have different time constants, a measured potential between 25 mined from the DC component of the sensor signal. It
the electrodes represents primarily the fluctuational AC should be appreciated that this aspect of the invention
component of the oxygen concentration in the measured gas, relating to novel techniques for processing signal(s) from
rather than representing both the AC and DC components, or oxygen sensor(s) in the frequency and/or time domains to
primarily the DC component, as was done with the prior art yield combustion parameters may be employed either with
sensors described above in which one of a pair of sensors 30 the prior art oxygen sensors described above which surround
was Surrounded by a gas having a predetermined oxygen one electrode with a reference gas, with the oxygen sensors
concentration. What constitutes a suitable difference described above in which at least two electrodes are each in
between the time constants of the electrodes may vary from fluid communication with a common gaseous environment,
application to application, and the invention is not limited to or with any other type of sensor which generates a signal that
any particular difference between the time constants. In 35 includes a fluctuational AC component representing a con
various embodiments, for example, the time constants of the centration of a gas (e.g., oxygen) or other fluid.
electrodes may differ from one another by some value When a single sensor is used, it generates a signal
between a few (e.g., two) milliseconds and several (e.g., ten) indicative of the level of gaseous combustibles at the par
minutes. ticular point where the analyzed gas comes in contact with
It should be appreciated that the novel sensor configura 40 the sensor. The signal from Such a single sensor may provide
tion described herein is not limited to applications wherein a sufficient amount of information to permit the operation of
the concentration of oxygen is monitored, as this sensor may a small, single-burner industrial combustor to be optimized.
also find applications in sensing the concentration of numer When several sensors are inserted into the flue gas flow (e.g.,
ous other types of gases, e.g., carbon monoxide (CO), nitric across the width) of a combustor, the outputs of the sensors
oxide (NOx), etc., as well. 45 represent a distribution profile of the gaseous combustibles
In one embodiment of the invention, the output signal within the combustor. Such a profile can be utilized for
from an in-situ oxygen sensor is fed to a signal analyzer, e.g., combustor balancing and optimization. For example, indi
a programmed computer, where it is analyzed and used to vidual burners and/or post-flame combustion systems can be
generate one or more combustion parameters that are cor adjusted to alter the generated profile until it reflects that
related with combustion conditions. 50 optimal and balanced combustion conditions have been
In one manner, the sensor output signals are analyzed to achieved. An understanding of (1) how the profile should
correlate an output range with a known gas concentration. appear when Such optimal and balanced combustion condi
For example, in a particular application and particular fuel, tions have been achieved, and (2) how individual burners
the NOx range of particular interest may be from 0–500 ppm and/or post-flame combustion systems affect different
(parts per million) NOX. From this, a response curve can be 55 aspects of the profile may be obtained through empirical
established by exposing the sensor to known quantities of measurements. This boiler balancing and optimization may
NOX and mapping the measured output Voltage response be particularly useful for larger, multi-burner combustion
(such as in mVolts) with the known NOx concentration. systems.
Similarly, for a range of oxygen of 0–10%, the sensor may With reference to FIGS. 1A and 1B, there is shown a
be exposed to known concentrations of oxygen and the 60 cross-sectional illustration of a combustor 100 and typical
resulting Voltage response curve is applied in processing the sighting of several in-situ oxygen sensors 102 positioned
signals received in the intended application. Likewise, for a across the width of a post-flame, flue gas duct 104 of the
CO range of 0–1000 ppm, a voltage response curve is combustor 100 to monitor the stream of hot flue gases
established and applied in processing measured concentra flowing therethrough in a post-flame Zone. The sensors 102
tions in intended applications. Although preferably sensors 65 may, for example, be solid-electrolyte sensors which mea
made pursuant to a given design will behave essentially the Sure the concentration of (and/or changes in the concentra
same, Some adjustment or offset may be required, onsite or tion of) oxygen in the flue gases, or any other sensors
 US 7,128,818 B2
11 12
capable of generating a signal indicative of the concentration at the edges of the fuel and air jets. The eddies are trans
of (and/or changes in the concentration of) one or more other formed in the combustion process, and move in the general
types of gases present in the flue gases. In practice, any direction of the furnace exit 112. The overall combustion
number of sensors 102 may be installed (preferably in a row) turbulence reflects the process of energy transfer from
across the width of the flue gas duct 104. The sensors may large-scale eddies to Smaller and Smaller eddies, down to the
also be arranged in a vertically-oriented row, or in a grid-like molecular level. The rate of the mixing process and the
manner or other effective pattern and may extend varying resulting intensity of these turbulent activities determines
depths into the duct to monitor the distribution profile of combustion stability and directly relates to the processes of
gaseous combustibles. formation and destruction of gaseous combustibles. Most of
In some embodiments, the combustor 100 may be more 10 these chaotic, turbulent activities begin and occur in the
than one, two or even three hundred feet tall. As shown in flame envelope 108.
FIG. 1A, the combustor 100 may include a plurality of Some turbulent activities do take place in the flue gas flow
combustion devices (e.g., combustion device 106) which of the post-flame Zone 110. However, small eddies associ
mix fuel and air to generate flame in a flame envelope 108 ated with combustion kinetics (i.e., Small-scale, high-fre
within the combustor 100. The combustion devices may be 15 quency turbulence) tend to dissipate quickly and generally
any of numerous types of flame-producing devices, and the do not reach the post-flame Zone 110. Typically, only large
invention is not limited to a particular type of combustion eddies (i.e., large-scale, low frequency turbulence) are
device. According to one embodiment, for example, the present in the post-flame Zone 110. This low-frequency
combustion devices may include burners (e.g., gas-fired turbulence reflects combustion variables (e.g., an amount of
burners, coal-fired burners, oil-fired burners, etc.). In such an unburned carbon and other combustibles), particularly those
embodiment, the burners may be arranged in any manner, associated with the secondary combustion processes that are
and the invention is not limited to any particular arrange influenced by post-flame combustion control systems, such
ment. For example, the burners may be situated in a wall as overfire air and reburning. A turbulent stream of hot flue
fired, opposite-fired, tangential-fired, or cyclone arrange gases passing into the flue gas duct 104 carries products of
ment, and may be arranged to generate a plurality of distinct 25 incomplete combustion, including gaseous combustibles. As
flames, a common fireball, or any combination thereof. mentioned above, these gaseous combustibles travel in the
Alternatively, a combustion device called a “stoker' which turbulent flue gas flow as relatively large eddies. And such
contains a traveling or vibrating grate may be employed to eddies, containing gaseous combustibles, should contain a
generate flame within the combustor 100. very low concentration of oxygen. Each time the proper
As defined in a publication by the National Fire Protection 30 conditions occur, Such as the presence of a catalyst and a
Association (NFPA) of Quincy, Mass., entitled “NFPA 85C, high temperature (e.g., between 900 and 1500 F) near a
an American National Standard,” p. 85C-11, Aug. 6, 1991, sensor 102, the gaseous combustibles are caused to burn and
“flame” refers to “the visible or other physical evidence of the oxygen concentration near the sensor is reduced. These
the chemical process of rapidly converting fuel and air into fluctuations in the oxygen concentration near the sensors
products of combustion, and a “flame envelope” refers to 35 electrode(s) cause pulses to be generated in the signal output
“the confines (not necessarily visible) of an independent by the sensor 102. The frequency and amplitude of these
process converting fuel and air into products of combus pulses characterizes the level of gaseous combustibles
tion. present in the analyzed flue gas flow.
Referring to FIG. 1A, when the combustion devices 106 The relationship between the sensor output signal and
in the combustor 100 are actively burning fuel, two distinct 40 levels of gaseous combustibles may be affected by various
locations can be identified within the combustor 100: (1) a factors, including operating combustion parameters, physi
flame envelope 108, and (2) a so-called “post-flame' Zone cal parameters, and chemical reactions. In order to more
110, which is the Zone outside of the flame envelope 108 accurately monitor this multi-variable process, according to
spanning some distance toward the exit 112. Outside the one embodiment of the invention, two or more mathemati
flame envelope 108, hot combustion gases and combustion 45 cally different signal processing algorithms are employed
products may be turbulently thrust about. These hot com simultaneously to analyze the signal output by the sensor,
bustion gases and products, collectively called “flue gas.” and the results of the several algorithms are combined.
make their way away from the flame envelope 108 toward Examples of methods and algorithms for processing infor
an exit 112 of the combustor 100. Water or another fluid (not mation and signals received from gas sensors of the type
shown) may flow through the walls (e.g., wall 114) of the 50 described above are described, for example, in U.S. Pat. No.
combustor 100 where it may be heated, converted to steam, 6,277.268, which is hereby incorporated herein by reference
and used to generate energy, for example, to drive a turbine. in its entirety. The 268 patent discloses signal processing
In the embodiment shown, the sensors 102 are located in the systems and calculations, such as conducted in the time and
post-flame Zone 110 of the combustor 100. It should be frequency domains, that are applicable for use with the
appreciated, however, that the invention is not limited in this 55 sensor embodiments described herein. It is understood,
respect, and that the sensors 102 alternatively may be however, that the gas sensor of the present invention is not
disposed in the flame envelope 108 if constructed to with limited to use in the processing systems described herein or
stand the harsh, high-temperature environment thereof. in the 268 patent, but may be used in any system adapted
As mentioned above, in one embodiment of the invention, to realize and appreciate the information obtainable from the
a voltage difference across the sensor and reference elec 60 beneficial use of the inventive gas sensor.
trodes includes a fluctuational component that may be Referring now to the exemplary gas sensor embodiments
analyzed to measure the concentrations of gaseous combus of the figures, which are provided by way of example and
tibles. The reason for this correlation is believed to be as not limitation, FIGS. 2A and 2B illustrate in sectional view
follows. Individual burner flames comprise a multitude of a sensor element 200 incorporating the present invention for
eddies of various sizes inside and around the flame envelope 65 use in combustion systems. Specifically, a combined O+CO
108. These eddies contribute to generating the familiar flame sensor 200 is arranged as follows. A Solid electrolyte (e.g.,
flicker at various frequencies as a result of turbulent mixing zirconia) cell 202 has one set of electrodes with internal
 US 7,128,818 B2
13 14
electrode 204 and external electrode 206 and the corre Now referring to the sectional views of FIGS. 3A and 3B.
sponding leads 208 and 210. The cell is placed in the stream which illustrate a second embodiment of a combined
of the process gas 212 at the required high temperature O+CO gas sensor 300 incorporating the present invention,
condition (usually in the 900–1500 F. range) which is the solid electrolyte cell 302 has one end closed and has a
provided by the analyzed gas itself or by an additional heater similar set of electrodes: internal electrode 304 and external
or other source. In one arrangement, cell 202 is made in the electrode 306 with the corresponding leads 308 and 310. The
form of an open tube. A smaller tube 214 is connected to a cell is placed in the process gas stream 312 at the required
Supply of reference gas (e.g. air) and Supplies the reference high temperature conditions. In this arrangement reference
gas to the cell (if open ended) the reference gas conduit gas (air) is supplied through tube 314. Protective sleeve 322
passes through the cell and the reference gas (air) flows 10 may have a side opening 316 or end opening 318 to allow
through cell 202. This version is called a “flow-thru’ sensor. the process gas to reach the external electrode 306. Solid
Tube 214 is preferably made of a material having a coeffi electrolyte cell 302 is mounted and bonded to a metal
cient of thermal expansion compatible with the solid elec element 320 which is brazed to adapter 324. Mount 320 also
trolyte material of cell 202. This tube has an opening 216 provides an electrical connection with the external lead 310.
inside to provide a constant concentration Of O inside the 15 Mount 320 is made of a material with a coefficient of
sensing chamber formed in the cell. The gap between the thermal expansion compatible with the material of cell 302.
internal tube 214 and the cell 202 should be thoroughly Reference gas (air) is supplied via tube 314 to inside of cell
sealed at the ends of the cell by a high temperature seal, 302 and escapes back through the protective sleeve 322. Cell
sealing ring or sealing adhesive 218, this seal must also be 302 is mounted in the protective sleeve 322 and sealed by
compatible with regard to thermal expansion of the sensor. means of adapter 324.
When measuring a constituent gas, for example oxygen, An additional tube 328 is provided inside the assembly for
even a small leak in and out the cell may be detrimental to calibration purposes. A calibration gas with a fixed concen
the sensor operation; for instance, it may significantly distort tration of O+CO can be supplied to the external electrode
the O2 measurement results. That is why it is important to 306 of cell 302, or a sample of the analyzed process gas can
make Sure that (a) the internal tube is sealed properly; and 25 be drawn out to a reference gas analyzer. The sectional view
(b) the sensor is designed to prevent the effects of the of FIG. 3B is taken across the opening 316 and electrodes
potential leakage on measurement results. In measuring O. 304 and 306. Thermocouple 326 is positioned in close
the processing as described herein above using the Nernst proximity to the measuring cell to be used for temperature
equation is in order. For measuring CO, the general method monitoring, control or compensation.
described above and in the 268 patent may be used. The CO 30 According to another aspect of the invention, the com
concentration may be analyzed using time domain and/or bustibles sensor is configured or converted into a combined
frequency domain. It should be appreciated that the selected O+CO sensor using a sealed O sensor (sealed O+CO
form of processing the fluctuational component to determine sensor). An example of a sealed O, sensor is model FGA411
CO concentration may depend upon several factors. Such as manufactured by Panametrics, Inc. of Waltham, Mass. Such
the type of combustor, the fuel type, and the desired or 35 a sealed O sensor does not require the use of a reference
required level of accuracy. In certain situations, such as in gas. In this case the sensor design may be essentially the
the case of simplified combustors or where a high level of same as that of FIG. 3A, only without the reference air
accuracy or sensitivity in CO measurement is not required, supply and with the electrode sealed off and surrounded by
a simplified processing may be acceptable, for instance a media or material to effect an internal-reference sensor.
calculating the standard deviation of the signal fluctuations 40 According to another aspect of the invention, the com
(the AC component). bustibles sensor can be configured or converted into a
The in-situ sensor has several design features. In one combined CO+NOx sensor by using it in combination with
embodiment, Cell 202 is made sufficiently long (for example a “filtered NOx sensor. The sensor has one common solid
up to 3–5 inches long) and the electrodes 204 and 206 are electrolyte cell with two measuring electrodes and one
positioned in its middle section (approx. 4 of the total 45 common reference electrode. A porous thin filter made of
length). The end sections of the cell are preferably not material suitable to oxidize CO into CO to eliminate the
covered by the electrodes to provide more effective air-tight effect of CO, is placed over one of the measuring electrodes.
sealing and also to create conditions that, even if a leak This electrode operates in the mixed potential mode and is
occurs, it will be carried away by the stream of the analyzed used to measure NOX. Examples of operation in the mixed
process gas. For this purpose, the sensor is placed into an 50 potential mode and of filter devices for use in NOx analysis
external housing 220 with, in one embodiment, three open are described in the following references, which are incor
ings: central opening 222 to provide access of the process porated herein by reference: Nicholas Szabo et al.
gas to the external electrode 206 and end openings 224 and “Microporous Zeolite Modified Yttria Stabilized Zirconia
226 to create an additional draft for potential leaks. The Sensors For Nitric Oxide Determination. In Harsh Environ
sectional view of FIG. 2B is taken across the openings 216 55 ments'. The Ohio State University, 2001, and Eric Wachs
and 222 and electrodes 204 and 206. To prevent erosion and man et al., “Selective Detection Of NOx By Differential
deposits, openings 222-226 should be pointed away from Electrode Equilibria, Solid State Ionic Devices II—Ce
the incoming flue gas flow. It is also important that the ramic Sensors, Electrochem. Soc., Ed., 2000–32, 298–304
sealing material 218 does not come in contact with the (2001). Another measuring electrode operates as a fluctua
porous electrodes (for example made of platinum). 60 tional CO sensor, as described above. As a result, two
Additional tube 228 is provided inside housing 220 for signals, CO+NOX, are generated in one potentiometric Solid
calibration purposes. A calibration gas with a fixed concen electrolyte sensor. In this arrangement, the sensor does not
tration of O. can be supplied from outside to the external require a continuous reference air Supply.
electrode 206, or a sample of the analyzed gas can be drawn According to another aspect of the invention, the com
out to a reference gas analyzer. Thermocouple 230 is posi 65 bined O-CO sensor, described above, can be converted into
tioned in close proximity to the cell 202 to be used for a combined O+NOX--CO sensor by using it in combination
temperature monitoring, control and/or compensation. with a “filtered NOx sensor, described above. A “flow-thru'
 US 7,128,818 B2
15 16
version of such sensor is schematically shown in FIGS. 4A a solid electrolyte cell disposed within the outer shell;
and 4.B. Sensor 400 includes a cell 402 that has two sets of at least one seal cooperating with the electrolyte cell to
electrodes: one common reference electrode 404 and two form a sensing chamber isolated from the flue gas;
measuring electrodes 406 and 408 with the corresponding a first electrode disposed within the sensing chamber and
connecting leads A, B and C. A reference gas, e.g., air, is being isolated from the flue gas so as not to be in a
delivered to the sealed space surrounding electrode 404 via direct fluid contact with the flue gas; and
opening 416 in Supply tube 414. The space is defined in part a second electrode disposed in the outer shell and posi
by cell 402 and is insulated by high temperature seal or tioned in close proximity to the at least one opening so
sealant 418, similar to that shown in FIG. 2A. The sectional as to be in fluid contact with the flue gas, a Voltage
view of FIG. 4B is taken across electrodes 404 and 406. 10 being generated across the first and second electrodes
A porous thin filter 410 made of a material, capable to representing at least two conditions, wherein the first
oxidize and practically eliminate the effect of CO, e.g., as electrode and the second electrode generate a signal
described in the references incorporated above, is placed comprised of a DC component and a fluctuating AC
over measuring electrode 408. The sectional view of FIG. component, the two conditions comprised of the DC
4C is taken across electrodes 404, 408 and filter 410. This 15 component and a fluctuating AC component; and
electrode operates in the mixed potential mode and is used the gas sensor system further comprising a processing
to measure NOX. Another measuring electrode operates as a portion, the processing portion configured to analyze
Nernstian sensor and is used as a combined O+CO sensor, each of the DC component and a fluctuating AC
as described and illustrated in FIG. 2A and above. This component to determine gas concentrations in the flue
sensor will require a continuous reference air Supply. As a gas, and
result, all three signals O+NOx+CO are generated in one wherein the first electrode and the second electrode are
potentiometric Solid-electrolyte sensor. configured to have different time constants resulting in
For NOx determination, the sensor should be pre-cali a measured potential between the first electrode and the
brated based on the relationship between the voltage mea second electrode the processing portion configured to
sured from the filtered electrode and the known NOx con 25 analyze the measured potential to represent the fluctu
centration to establish an expected response curve, in a ating AC component.
manner such as described above, to cover a NOx concen 2. The gas sensor system of claim 1, further comprising a
tration range determined to be appropriate for the particular reference gas conduit disposed in the sensing chamber and
design or application. A particular sensor and processing adapted to Supply a reference gas to the chamber.
function may be further calibrated, onsite or offsite, by 30
3. The gas sensor system of claim 1, further comprising a
exposing the particular sensor to a known NOx concentra conduit disposed within the outer shell and adjacent the
tion and adjusting the sensor response to more closely align electrolyte cell and second electrode, the conduit being in
with the expected or design response curve. fluid communication with the flue gas.
According to yet another aspect of the invention, the 4. The gas sensor System of claim 3, wherein the conduit
in-situ solid electrolyte sensor 502 is equipped with a 35
delivers a calibration gas in close proximity to the second
flexible stainless hose or jacket 504 to facilitate its packag electrode, the second electrode being effectively calibrated
ing, assembly, installation and maintenance in a boiler, as based at least in part on the effect of the calibration gas on
illustrated in FIG. 5. Existing in-situ combustion sensors the condition sensed by the second electrode.
could be of significant length, 20–30 ft or more. These 5. The gas sensor system of claim 4, wherein the calibra
sensors have traditionally been assembled at site, and their 40
tion gas comprises an essentially fixed concentration of O.
assembly, transportation, insertion and retraction is difficult. 6. The gas sensor system of claim 3, wherein sample flue
Use of the flexible hose enables complete assembly and gas is extracted from the sensor through the conduit for
testing of the sensor at the factory. On site the sensor can be delivery to a reference gas analyzer.
easily assembled and inserted into a permanent Support tube.
The risk of damage during field assembly is eliminated, 45 7. The gas sensor system of claim 1, wherein the solid
removal and replacement is simplified. The overall weight of electrolyte cell is tubular in shape, the sensor comprising
the sensor is reduced by over thirty percent. The flexible two seals disposed substantially at respective ends of the
arrangement also provides ease in installing and retrofitting electrolyte cell to cooperate to form the sensing chamber.
sensors in locations that are difficult to access physically, 8. The gas sensor system of claim 1, further comprising a
Such as where sensors are located at confined spaces. 50 thermocouple located in close proximity to the electrolyte
While the foregoing description includes many details and cell and being adapted to monitor temperature and provide
specificities, it is to be understood that these have been a reference to adjust for varying temperature conditions in
included for purposes of explanation only, and are not to be the outer shell.
interpreted as limitations of the present invention. Many 9. The gas sensor system of claim 1, wherein the first
modifications to the embodiments described above can be 55 electrode is in fluid contact with a reference gas and a
made without departing from the spirit and scope of the Voltage signal generated across the first and second elec
invention, as is intended to be encompassed by the following trodes is analyzed to monitor the concentration of gases in
claims and their legal equivalents. the flue gas.
What is claimed is: 10. The gas sensor system of claim 9, wherein the voltage
1. A gas sensor System for monitoring gas concentrations 60 signal represents the concentration of at least two selected
in flue gas generated by a combustor, the gas sensor System from the group consisting of oxygen, carbon monoxide, and
comprising: nitric oxide.
a gas sensor comprising: 11. The gas sensor System of claim 1 further comprising
an outer shell disposed in a stream of flue gas in a a third electrode disposed within the outer shell and being in
post-flame Zone of the combustor, the outer shall hav 65 fluid communication with the flue gas, the third electrode
ing at least one opening in a fluid communication with cooperating with one of the first and second electrodes to
the flue gas; sense the concentration of an intended gas in the flue gas, the
 US 7,128,818 B2
17 18
intended gas being one or a group consisting of oxygen, 26. The gas sensor system of claim 1, wherein the
carbon monoxide, and nitric oxides. electrolyte cell is comprised of yttria stabilized zirconia.
12. The gas sensor system of claim 11, wherein the third 27. The gas sensor system of claim 1, wherein the
electrode is at least in part covered by a filter to react with electrolyte cell is comprised of zirconia.
a second gas in the flue gas to eliminate the effect of the 28. The gas sensor system of claim 1, wherein the first
second gas so as to enhance the accuracy of the concentra electrode possesses a first associated time constant and the
tion measured of the intended gas. second electrode possesses a second associated time con
13. The gas sensor system of claim 11, wherein the first stant, the first time constant being different than the second
and second electrodes cooperate to generate a first signal time constant; and
representing the concentration of a first intended gas and the 10 wherein each of the time constants respectively associated
second and third electrodes cooperate to generate a second with the first electrode and the second electrode is
signal representing the concentration of a second intended calculated by the processing portion using a relation
gas, the first and second intended gases each being one of a ship:
group consisting of oxygen, carbon monoxide, and nitric
oxides. 15
14. The gas sensor system of claim 13, wherein one the wherein:
first and second signals may be further analyzed to deter C, the concentration of oxygen at the electrode,
mine the concentration of a third intended gas. C, the concentration of oxygen in the environment,
15. The gas sensor system of claim 1, wherein electrical AC, the change in concentration of oxygen in the envi
signals representing the gas concentrations respectively ronment,
sensed by the first and second electrodes are generated, the e the exponential operator,
signals being processed by the system in a time domain to t the time elapsed since the change in oxygen concen
yield combustion parameters, the processing portion calcu tration occurred, and
lating the standard deviation of signal fluctuation of the AC Tc is the time constant specific to the electrode:
component. 25 wherein each time constant, as calculated by the process
16. The gas sensor system of claim 1, wherein the ing portion, determines how quickly the oxygen con
combustor is one of the group consisting of a boiler, a centration level at that electrode changes; and
furnace, and a gas turbine. wherein the processing portion is configured to analyze
17. The gas sensor system of claim 1, wherein the said relationship.
combustor includes a burner that generates flue gases, the 30 29. The gas sensor System of claim 1, further comprising
burner being one selected from the group consisting of a a flexible hose connected to the gas sensor, the flexible hose
gas-tired burner, a coal-fired burner, an oil-fired burner, and for facilitating the assembly and installation of the gas
a fossil fuel-fired burner. sensor into said combustor.
18. The gas sensor system of claim 1, wherein the first and 30. An emissions monitoring system for monitoring con
second electrodes are made from a material that is porous 35 stituent concentration of flue gas components in a combus
and catalytic. tor, the monitoring system comprising:
19. The gas sensor system of claim 1, wherein the a first sampling probe comprising:
electrolyte cell has one closed end. an outer shell disposed in a stream of flue gas in a
20. The gas sensor system of claim 1, wherein the DC post-flame Zone of the combustor, the outer shell
component is processed by the processing portion in accor 40 having at least one opening for receiving a flue gas;
dance with the Nernst equation and is used to determine the at least one seal cooperating with the electrolyte cell to
O concentration. form a sensing chamber isolated from the flue gas;
21. The gas sensor system of claim 20, wherein the AC a first electrode disposed within the sensing chamber
component is processed by the processing portion to deter and being isolated from the flue gas so as not to be
mine the concentration of at least one selected from the 45 in a direct fluid contact with the flue gas;
group consisting of carbon monoxide, nitric oxides and a second electrode, disposed in the outer shell and
gaseous combustibles. positioned in close proximity to the at least one
22. The gas sensor system of claim 1, wherein the DC opening so as to be in fluid contact with the flue gas,
component is analyzed by the processing portion to deter a Voltage being generated across the first and second
mine an O concentration in the flue gas. 50 electrodes representing at least two conditions;
23. The gas sensor system of claim 22, wherein the a second sampling probe for monitoring the concentration
fluctuating AC component is analyzed by the processing of a second flue gas component, the second sampling
portion to determine a parameter representing the concen probe comprising:
tration of combustibles in the flue gas. an outer shell disposed in a stream of flue gas in a
24. The gas sensor system of claim 1, wherein the 55 post-flame Zone of the combustor, the outer shell
fluctuating AC component is analyzed by the processing having at least one opening for receiving a flue gas;
portion to determine a concentration in the flue gas of at least at least one seal cooperating with the electrolyte cell to
one selected from the group consisting of carbon monoxide, form a sensing chamber isolated from the flue gas;
and nitric oxides. a first electrode with an associated time constant dis
25. The gas sensor System of claim 1, wherein a Support 60 posed within the sensing chamber and being isolated
conduit is disposed in the post flame Zone of the combustor from the flue gas so as not to be in a direct fluid
and at one end is supported by and affixed to a wall of the contact with the flue gas;
combustor, the gas sensor being atone end attached and a second electrode with an associated time constant that
Supported by the Support conduit, electrical leads being is different from the time constant associated with
connected to the first and second electrodes and being 65 the first electrode, disposed in the outer shell and
disposed in the Support conduit at the one end of the Support positioned in close proximity to the at least one
conduit. opening so as to be in fluid contact with the flue gas,
 US 7,128,818 B2
19 20
a Voltage being generated across the first and second C, the concentration of oxygen at the electrode,
electrodes representing at least two conditions; and C, the concentration of oxygen in the environment,
at least one analyzer having inputs for monitoring the AC, the change in concentration of oxygen in the envi
receiving the Voltages generated by the first anti second ronment,
sampling probes and having a processor for analyzing e the exponential operator,
the Voltage data to determine the concentrations of the t the time elapsed since the change in oxygen concen
first and second flue gas components, the Voltage data tration occurred, and
includes a DC component and a fluctuating AC com Tc is the time constant specific to the electrode:
ponent, the analyzer analyzing each of the DC compo wherein each time constant, as calculated by the analyzer,
nent and a fluctuating AC component to determine the 10
determines how quickly the oxygen concentration level
constituent concentration of flue gas components; and at that electrode changes; and
wherein the first electrode and the second electrode are wherein the analyzer is configured to analyze said rela
configured to have different time constants resulting in tionship.
a measured potential between the first electrode and the 32. The monitoring system of claim 30, further compris
second electrode, the analyzer configured to analyze 15
ing a flexible hose connected to at least one of the first
the measured potential to represent the fluctuating AC sampling probe and the second sampling probe, the flexible
component. hose for facilitating the assembly and installation of at least
31. The monitoring system of claim 30, wherein the first one of the first sampling probe and the second sampling
electrode, of the first sampling probe, possesses a first probe into said combustor.
associated time constant and the second electrode, of the first 33. The monitoring system of claim 30, wherein the
sampling probe, possesses a second associated time con analyZer:
stant, the first time constant being different than the second analyzing the DC component to determine an O concen
time constant; and
wherein each of the time constants respectively associated tration in the flue gas; and
with the first electrode and second electrode is calcu 25 analyzing the fluctuating AC component to determine a
lated by the analyzer using a relationship: parameter representing the concentration of combus
tibles in the flue gas.
wherein: