Ammonia Slip in a DeNOX Process by UV Diode Array Spectroscopy

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Ammonia Slip in a DeNOX Process by UV Diode Array
Spectroscopy

• Yoav Barshad
• Applied Analytics, Inc.

Http//www.a-a-inc.com
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To comply with the Clean Air Amendment of 1990
the reduction in the amounts of certain chemicals
released to the atmosphere as a result of a
combustion process, is required. Among the
specified components are the nitrogen oxides,
found to be related to the formation of acid rain
and ground level ozone (smog).
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The Process

• Several Different Processes Have Been Developed
  and Applied in Worldwide Locations Ranging in
  Process Efficiency Levels, Temperatures, Reducing
  Reagents, Etc.
• However, the Common Factor in All Is the Need to
  Control the Ammonia Slip Below the Low ppm
  Levels.

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Selective Catalytic Reduction (SCR)
Involves injection of ammonia upstream from a
catalyst located in the flue gas stream.
Nitrogen oxides are reduced to nitrogen and
water, effective reduction levels are in the
range of 80-90. Ammonia is used as the reducing
agent. After vaporization into a carrier gas, a
controlled amount of ammonia is injected into the
flue gas. The NOx and NH3 reaction takes place
on a catalytic bed.
The reaction 4NO 4NH3 O2 4N2 6
H2O
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SCR
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SCR
Cyclone Coal fueled, Ammonia slip lt5ppmvd
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SNCR Selective Non Catalytic Reduction
Aqueous urea solution is used as reducing agent.
The reaction is a homogeneous gas phase reaction
that does not involve catalysts. NOx reduction
beyond 70 depending on combustion process. Key
parameters for NOx reduction are temperature
residence time, unit configuration and limitation
for ammonia slip.
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Controlling the Ammonia Slip

• Excessive ammonia concentration pollutes the flue
  gas.
• Plugging of air pre-heaters through the formation
  of ammonia bisulfate.
• Contamination of fly ash and flue gas
  desulfurization waste water.
• Price of the ammonia.
• The major concern - formation of ammonium salt.

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Ammonia Slip

• The formation of the salt is a function of NH3,
  slip, SO3 concentration and temperature.
• Two main forms of ammonia salts sulfate
  ((NH4)2SO4) and bisulfate ((NH4)HSO4).

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Ammonia Slip

• Ammonia bisulfate adheres to heat transfer
  surfaces resulting in fouling and reduced thermal
  performance.
• Ammonium sulfate is a powdery substance which is
  a particulate matter emitted from the stack.

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Ideal Analyzer

• Continuous on-line analysis
• Fast
• In situ - no sample lines
• Reliable measurements
• No down time due to sampling problems

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Requirementsfor Monitoring the Low Ammonia
Levels

• Very low noise levels.
• High reproducibility.
• High resolution. The high resolution is
  particularly important for this application
  since the absorbance features of ammonia are
  characteristically rather sharp in the low UV .

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UV SpectroscopyWhy?

• Robust
• Selective
• Many chemical compounds have a distinct UV
  spectrum
• Fast
• Low maintenance
• Dynamic range (ppm)

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The Spectrophotometer
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Principle of Operation
The electromagnetic spectrum

Frequency (Hz)

Frequency (Hz)

Wavelength (nm)
Wavelength (nm)




Cosmic
UV

-
4

22

200

Gamma
10

10

X
-
rays



UV/Visible light
Visible

Infrared


UHF
Radio


6

3

10

10

800

I
Sonic


Infrared
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Absorbance
I Transmitted radiation
I0 Incident radiation
sample
path length
Absorbance log (I0 / I)
Transmittance 100 ( I/ I 0 )
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Schematic Diagram - Spectrometers
slit
monochromator
sample
detector
diode array
sample
polychromator
Lamp
Lamp
Filter instrument
Diode array instrument
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Optical System

• Radiation source deuterium-discharge lamp.
• Source lens forms a single, collimated beam of
  light.
• The shutter/stray-light correction filter.

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Optical System

• Flow cell.
• Spectrograph lens and slit.
• In the spectrograph light is dispersed onto the
  diode array by a holographic grating. The result
  is a fundamental increase in the rate at which
  spectra can be acquired.

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GC-UV Why and When?

• The absorbance value at each wavelength is
  composed of the absorbance of the individual
  components. There are situation where the direct
  UV method fails, in particular when one of the
  high absorbing components concentration is
  significantly higher then the others in the
  sample.

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Multi-component analysis The term multi component
analysis is used to describe procedures in which
several components are determined simultaneously.
A variety of powerful mathematical tools can be
applied to the analysis of spectroscopic
generated data. Provided that linearity and
additivity of the signal are assumed.
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SO2
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Gas Chromatography

• Very sensitive
• Not selective
• Slow

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GC-UV

• Selective
• Sensitive
• Fast

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The Detector Sees the Components at Different
Times
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GC-UV Flow Cell
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FSFC - Fast Separation Flow Cell

• The fast separation flow cell is actually a crude
  chromatographic column, combined with a so called
  light pipe long path flow cell.

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FSFC - Fast Separation Flow Cell

• This detector / separation column combination
  provides most of the benefits of an hyphenated
  technique where a very selective method is
  combined with a very sensitive method.
• The entire flow cell is held at high temperature
  optimized for the best separation of the
  components of interest.

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FSFC

• The packing material is composed of a material
  similar to the kind used for preparative HPLC.
• The individual components, following the
  separation, flow through the light pipe to
  interact with light, the absorbance spectrum
  measured and reprocessed to give the
  concentrations of the individual components in
  the sample.

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UV absorbance spectra
NO
NO2
SO2
190
220
190
220
NH3
190
250
280
220
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NO2
NH3
NO
NO, NO2 NH3
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The ASA-400

• Industrial UV/VIS spectrophotometer.
• Industrial computer to control the analyzer and
  sampling system.
• A fast separation gas flow cell (GC).
• An injection system.
• A sampling system to withdraw a representative
  sample from the stack.

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Sampling Systems Main Task
To withdraw a representative sample from across
the stack corresponding to a spatial average
across the stack.
How?
The sample is drawn through 12 holes with non
equal diameter. The largest hole is located
further away from the outlet while the one closes
to the outlet is of the smallest diameter. The
sizes of the openings are calculated so that
under constant pressure drop each hole will
provide the same resistance to the sample flow.
The holes are big enough ( ca. 3/8) not to be
plugged by the particulate matter.
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Sampling Systems Function

• The sample is drawn through a ceramic filter
  (external) and through a 4 way valve (injection
  valve).
• The sample flow splits into a valve bypass and
  valve flow to optimize response times and
  pressure drops.

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Sampling Systems Function

• The bypass flow can be adjusted by adjusting the
  orifice size.
• The flow is being drawn out by the air driven
  aspirator and valve II which is open at this
  stage. At this point the injection valve is being
  continuously refreshed with fresh sample,
  constantly present in a fixed volume (tube) as
  part of the valve.

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Sampling Systems Function

• When the injection signal arrives, the injection
  valve turns around and the trapped volume of the
  sample is being brought to the sample loop.
• Where it is being carried immediately by the
  flowing carrier (nitrogen) to the fast separation
  column.

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Sampling Systems Function

• The sample reaches the column and due to
  differences in absorption and description rates
  of each of the components to the surface of the
  column it serves as a different restriction to
  each component,
• Once each component arrives at the flow cell it
  is processed individually.
• After 1 minute the process reverses itself and
  sample is being drawn again.

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Sampling Systems Function

• While the previously injected sample is being
  analyzed the sampling system goes through some
  other action.
• Immediately after injection valve II closes and
  forces the air to go through the aspirator back
  through the injection valve and the orifice.

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Sampling Systems Function

• The opening of valve I brings a larger amount of
  air through a heat exchanger, through the filter,
  and pushes all particulate matter back to the
  stack.
• Removing the particles from the filter on a
  continuous manner and preventing the formation of
  a cake on the filter is probably the biggest
  and most important factor in maintaining sample
  integrity.

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The sampling system
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The Sampling Probe
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Every minute valve III rotates and injects a
fresh sample. Immediately following the injection
back flow is initiated for a period of about 30
seconds followed by the drawing of a fresh sample
until a new injection takes place. The first raw
demonstrates the instruments output at this time
period (the concentrations). The second raw shows
that valve I opens at time 0 of each cycle. Raw 3
shows the back flow for half a cycle time
(30sec). Raw 4 shows the sample flow for half a
cycle time. Four cycles are shown.
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NO2
NH3
NO
NO, NO2 NH3
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SO2
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DeNox Ammonia slip application
SO2 gt 500ppm
SO2 lt 500ppm
ASA-400 includes OMA-517 GC fast separation
flow cell
OMA-517 no GC fast separation flow cell required
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