Gas Sensors for Fossil Energy Applications

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Gas Sensors for Fossil Energy Applications

• T. Armstrong
• F. Montgomery
• D. West
• 20th Annual Fossil Energy Conference
• Knoxville, TN
• June 13, 2006

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Sensor Needs

• Sensor and control needs for advanced combustion
  and gasification
• Emission, toxic contaminant sensors (NOx, SO2,
  Hg)
• Sensor and control needs for environmental
  controls
• Specific areas of interest - Hg, SOx, NOx, CO,
  CO2, H2, and HCl

DOE/NETL-2002/1162
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"Rack-mounted" techniques for detecting pollutants

• Portable electrochemical units also used
• All of these techniques require the test gas to
  be cooled to near room temperature

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Types of electro-ceramic sensors I(Typical
Toper. 400-800 oC, heating circuits omitted for
clarity)

• Potentiometric (e.g. ? sensor)
• Voltage developed due to concentration (activity)
  differences
• Physical barrier usually required between
  electrodes
• Typically requires known concentration at one
  electrode
• Mixed-potential (non-Nernstian)
• Electrochemical activity of electrode material(s)
  alters reactivity
• Both cathodic and anodic reactions at active
  electrode
• Output (V) function of electrode materials and
  atmosphere

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Types of electro-ceramic sensors II(Typical
Toper. 400-800 oC, heating circuits omitted for
clarity)

• Conductimetric (e.g. SnO2 sensor)
• Absorption / interaction with surface alters
  resistivity
• Can use multiple pads with pattern recognition
  (req. calibration)
• Typical operating T 600 oC
• Amperometric (limiting current)
• Vapp. drives electrochemical reaction involving
  gas to be sensed
• imeas. ? concentration
• Vapp., electrode materials can afford selectivity

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Our focus - Sensing methods operative at high
temperature

• Can operate closer to source
• Avoid difficulties with condensation
• Classic example of such a sensor is the zirconia
  O2 sensor used for auto exhaust
• Electrochemical (Zr1-xYxO2-2x electrolyte)
  Operates above exhaust T to avoid condensation

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High T electrochemical sensor programs at ORNL

• NOx
• CRADA with Ford
• Target market is diesel, lean burn exhausts, DG
  gensets
• NH3
• Currently funded internally
• Target market is urea SCR monitoring
• SOx
• Most recent start-up
• Target market is FE power plants

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NOx sensing Complicated by "equilibrium" between
NO and NO2

• Stable form of NOx function of T, O2
• NONO2 in exhaust cannot be assumed or
  inferred
• Either convert before sensing or have selective
  sensor(s)
• High T form is NO

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NO2 is "easy" to detect at high T

• Measure V ("mixed potential") between oxide and
  Pt.

• V ? AlnNO2/O2 agrees with theory

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Applied electrical stimulus can enhance NO
response

• IDC driven from oxide to Pt enhances NO response
• See analogous effect if constant DC voltage
  maintained.
• Best results with oxide based on chrome
• Background tends to drift

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ORNL has developed a "total NOx" sensing element

• IDC driven between oxide electrodes w/ same
  composition

• US and intl. patent apps. filed 2005

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NOx Sensor Development - NO specific Sensor

• ORNL is developing sensing elements suitable for
  exhaust NOx sensors
• NO sensor demonstrated with simulated exhaust gas
  on bench top
• Next step - evaluate and enhance performance and
  stability in real and simulated exhaust

No NO2 response
NO sensor performance in simulated exhaust gas
OAK RIDGE NATIONAL LABORATORY U. S. DEPARTMENT OF
ENERGY
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Sensing Approach Biased Electrode

• Bias current driven between oxide/Pt electrodes,
  monitor voltage (Vmeas).
• Presence of NOx causes change in Vmeas (DV).

OAK RIDGE NATIONAL LABORATORY U. S. DEPARTMENT OF
ENERGY
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Stable background and low cross-sensitivity
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SO2 release to the environment needs to be
minimized
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SNOx sensing Complicated by "equilibrium"
between SO3 and SO2

• Stable form of SOx function of T, O2
• SO3SO2 in exhaust cannot be assumed or
  inferred
• Either convert before sensing or have selective
  sensor(s)
• High T form is SO2

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Technical Approach

• Investigate use of DC electrical biasing to
  enhance the response of solid electrolyte-based
  SOx sensing elements
• Co-planar sensing elements based on YSZ
• Survey electrode materials, look for
  commonalities
• Vary electrode geometry and T, study effect(s) on
  response magnitude and speed

• Substrates fabricated in-house
• Screen-print fire electrodes
• Furnace testing simulates high temperature
• Use electrical stimulus ("bias")
• Sensing signal voltage developed between
  electrodes
• Measure SOx response
• NO2
• Study effect of varying O2, H2O, impurities
• Thermodynamics

Miniaturized sensor
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Element and test fixture geometry
Schematic of test fixture
Sensing element geometry

• YSZ substrate produced in-house at ORNL by tape
  casting.
• Electrodes deposited by screen-printing, dried
  and fired t 20 mm.

OAK RIDGE NATIONAL LABORATORY U. S. DEPARTMENT OF
ENERGY
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Test bench schematic
OAK RIDGE NATIONAL LABORATORY U. S. DEPARTMENT OF
ENERGY
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Sign, magnitude of applied stimulus govern
response
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Initial results for high-T SO2 sensor are
promising

• Electrochemical element based on YSZ.
• Currently exploring roles of geometry and
  materials.
• IP disclosure filed May 2006.

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Low cross-sensitivity
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Stable Phases in Exhaust Gas
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Operation with different materials and T's
appears feasible

• Element w/ two noble metal electrodes.
• Operating T 650 oC.
• Element displayed low sensitivity to varying
  O2.

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Summary and Future Direction

• 2 instrumented test stands operations for sensors
  of interest for FE applications
• Third test stand under consideration
• SO2 sensor under development and testing
• Compact, can operate at or above exhaust gas
  temperatures (gt700C)
• Dry gas tests indicate fast response/recovery 2-3
  seconds and no cross sensitivities
• Wet gas tests indicate steams reduces recovery
  time
• Current challenges
• Materials selection
• Effect of steam
• Sensor design
• Rapid screening of electrocatalysts

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Sensor Design Elements
N2, O2, SO2
Positve bias (oxide positive wrt Pt)
Air

O2-
YSZ
Vmeas
- I source
Iscr
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Acknowledgements

• Curt Maxey, Beth Armstrong Ted Bessman
• Developing collaboration with Prof. Stuart Adler
  at U. of Washington
• Questions??

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NH3 sensor is required for active NOx remediation
by urea SCR.

• Overdosing or catalyst poisoning could lead to
  release of NH3 ("ammonia slip").

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NH3 sensor for diesel exhaust is in development

• Adopted array-based approach.
• Currently selecting materials for max.
  orthogonality.
• IP disclosed 2006, US Pat. App. in progress.
• Working on NDA with Ford.

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Responses to NH3 and potential interferents are
additive
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Publications, Presentations, Patents

• Publications
• D. L. West, F. C. Montgomery, and T. R.
  Armstrong, Total NOx sensing elements with
  compositionally identical oxide electrodes,
  accepted by Journal of the Electrochemical
  Society May 2005.
• D. L. West, F. C. Montgomery, and T. R.
  Armstrong. NO-selective NOx sensing elements
  for combustion exhausts, accepted by Sensors and
  Actuators B, Feb. 2005.
• D. L. West, F. C. Montgomery, and T. R.
  Armstrong. Electrically biased NOx sensing
  elements with coplanar electrodes, Journal of
  the Electrochemical Society, 152 6, H749,
  2005.
• D. L. West, F. C. Montgomery, and T. R.
  Armstrong. Use of La0.85Sr0.15CrO3 in
  high-temperature NOx sensing elements, Sensors
  and Actuators B, 1062, pp. 758-765, (2005).
• Presentations
• D. L. West, F. C. Montgomery, and T. R.
  Armstrong, All-oxide total NOx sensing
  elements, 207th Meeting of the Electrochemical
  Society, Quebec City, Canada, 2004.
• D. L. West, F. C. Montgomery, and T. R.
  Armstrong, DC electrical-biased, all-oxide NOx
  sensing elements for use at 873 K, 29th
  International Cocoa Beach Conference on Advanced
  Ceramics and Composites, Cocoa Beach, FL, 2005.
• D. L. West, F. C. Montgomery, and T. R.
  Armstrong, High-T NOx sensing elements using
  conductive oxides and Pt, Proceedings of ICEF
  Engines for Mobile, Marine, Rail, Power
  Generation and Stationary Applications, Long
  Beach, CA, 2004.
• D. L. West, F. C. Montgomery, and T. R.
  Armstrong, NO-selective NOx sensing elements for
  combustion exhausts, Eurosensors XVIII, Rome,
  Italy, 2004.
• D. L. West, F. C. Montgomery, and T. R.
  Armstrong, Electrically biased NOx sensing
  elements with co-planar, multi-layered
  electrodes, 205th Meeting of the Electrochemical
  Society, San Antonio, TX, 2004.
• D. L. West, F. C. Montgomery, and T. R.
  Armstrong, Electrode materials for
  mixed-potential NOx sensors, 28th International
  Cocoa Beach Conference on Advanced Ceramics and
  Composites, Cocoa Beach, FL, 2004.
• Patents
• F. C. Montgomery, D. L. West, T. R. Armstrong,
  and L. C. Maxey, NOx Sensing Devices Having
  Conductive Oxide Electrodes, submitted to USPTO
  November 2004.