combustion control and safety a comparison between ... 09, 2017 · transmitter 7930 co/ch4 laser...
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Combustion Control and Safety – A
comparison between Zirconium
Oxide/Catalytic and Tunable Diode (TDL)
Technologies
Dr Stephen Firth / Rhys Jenkins
Agenda
Introduction
– Combustion Theory / Practical Benefits
– Possible measurement Solutions overview
Zirconia Solutions
– Technology Overview
– Practical Advantages / Disadvantages
Laser Solutions
– Technology Overview
– Practical Advantages / Disadvantages
Practical Examples
– Zirconia vs Laser – measurement differences
– Practical Matters – Laser purge / Ambient Temperature
– Comparison
Fired Heater - “Safety and Efficiency”
■ CO is wasted fuel and money
■ Fuel Breakthrough is unsafe
■ Excess soot and sooting of tubes
means poor efficiency and high
maintenance
■ CO, and CO+CH4 monitoring:
– Highlights inefficient control
– Highlights inefficient burner setup
– Highlights burner maintenance
– Highlights Leaking Tubes
– Helps ensure safe start-up and
shut-down
– Help ensure safe operation
– Improves Site Safety
and Process Efficiency
■ Excess O2 is wasted energy -
heating up excess Air (mainly N2),
which cools the products of
combustion and drops efficiency
■ Excess O2 increases NOx
■ O2 & CO monitoring:
– Highlights inefficient
control
– Highlights inefficient
burner setup
– Highlight inefficient
operation
– Improves Process
Efficiency
SAFETY
EFFICIENCY
Process Gas Analysis Monitoring Geometries
In Situ Cross Process (Laser)
DCS Control
mA / Relays
+
AMCS Analysis
Ethernet/Modbus
Sample Conditioning
Gas Analyzer
In Situ Probe (Zirconia)
Extractive (Paramagnetic Infrared Electrochemical)
Process Flow
Variables: •Flow Velocity •Temperature
•Pressure •Dust
Analyzer Shelter
Zirconia / Combustibles
Temperature
Interlocked
Solenoid Valve
Model C version dual sensor shown
Aux
Air
Rest.
Aspirator Air
200ml/min
Aspirator &
Sample Outlet
Heated
Enclosure
Flame
Trap
Internal
Filter O2 Cell
Comb Cell
Aspirator
Flame
Trap
Probe 100 ml/min
1.5ltr/min typical
1.7ltr/min typical 300 ml/min
Cal Gas Inlet 600ml/min
During calibration the
sensor head is ‘flooded’
with calibration gas to
prevent process sample
from interfering
During calibration the flow
of sample gas through the
analyser & transducer
remains unchanged
Flow
alarm
Confidence in measurement
Auxiliary air
to ensure
Comb
reading
Heated to prevent
Condensation /
corrosion
Flame arrestors for safety
Zirconia
■ Complete sample
measured by each
sensor in turn
■ Core T90 response
kept to < 20 s
■ Temperature interlock
ensures flue gas is not
drawn into a cold
head
■ Low flow technique
fully pneumatic and
driven by instrument
air
Zirconium Oxide Technology
Performance
• Decent response time
• Unaffected by background gases
• Sample at hot / wet condition
• Historically Acceptable
Economics
• Well Know and Understood
• Very acceptable operational life
• Low maintenance requirements
• COe sensor added at modest cost
Installation
• Single Flange
• Split configuration (control unit accessible)
• Simple validation / calibration
• SIL1 Rating
Utilities
• Minimal – Instrument Air
• Mains Power
Tuneable Diode Laser Spectroscopy
Building Blocks of a TDL Analyser
Laser + Temp. Control
Receiver Optics
Transmitter Optics
Process Windows
Receiver
Electronics & Signal Processing
Comparison of light sources and laser bandwidth
Demonstrates very narrow laser bandwidth
So laser selective to the measured gas Reduces cross interference
compared to NDIR
Line Lock Reference Techniques
Cuvette Gases:
NH3, CO, O2
Line Lock Reference Cuvette
Always available, continuously scanned.
No maintenance required
Self diagnostics inbuilt, an advantage for Safety Instrumented Systems
Filled with the gas of interest, rather than locking on to a process water line – better reading stability
Laser Technology
Performance
• Fast response time
• Unaffected by background gases
• Sample at hot / wet condition
• Gaining Acceptance
Economics
• Price X3 or X4 that of zirconia
• Very acceptable operational life
• Low maintenance requirements
• CO/CH4 needs a second Analyser
• Better CO measurement
Installation
• Dual Flange
• Split configuration (control unit accessible)
• Simple validation / off line calibration
• Alignment potential issue
• SIL2
• Hazardous Area available
• Effected by sample pressure and Temperature
Utilities
• 50-100 l/min Instrument Air
• 24V dc
Combustion
How to make you cracker furnace more efficient
Cross Path Average versus Spot Measurement
7930 O2 Laser transmitter
7930 CO/CH4 Laser transmitter
7930 O2 receiver
7930 CO/CH4 receiver
Furnace cross section
10 m - 40 m
2700
Zirconia O2
Thick film COe
CO =1500 ppm O2=1.0% T= 1000 C
CO =100 ppm O2=2% T= 900 C
Real Life Examples – Zr vs TDL Long Pathlength Furnace
4 min
Laser O2 Laser CO Faster Response Better Control
Zr O2 Tfx CO
Some Combustion Process….it’s Obvious which to use !
• High Sulphur Fuels on Heaters and Incinerators
• Lots of SO2/SO3/S
• Highly Corrosive
Presence of sulphur in the “fuel” the sample is corrosive and attacks the traditional zirconia sensor and metal tubes.
Lasers is none contact with the sample, so no corrosion,
hence, less maintenance.(saving $100K/year on a US plant)
No zirconia cells to change
Less frequent calibration due to laser stability
Less frequent calibration due to laser stability
In Situ vs Extractive – Key Considerations
Oxygen Extractive
(Zirconia Based)
In Situ
(Cross Stack TDL)
Sample Conditioning Required: control of moisture, cooling, pressure
None – Good for toxic and corrosive samples
Measurement influences
Largely Independent of process conditions – Temperature, pressure, dust, etc
Affected by process conditions – Temperature, Pressure, Dust, Window Purge Flow Rate, etc
Calibration Precision Calibration / Verification possible Off Line Calibration Only. Verification possible (accuracy: ~3% of span)
Response Time Slow (10 - 30 sec)
Flow and system dependent
Fast (<5 secs)
Maintenance Requirements
Medium – flow alarm required for high integrity
Low – minimum system components
(Alignment issues esp. long pathlengths)
Ambient Temperature “Hot” Ambients can effect electronics
But Extractive Zirconia Solutions
“Hot” Ambients can effect electronics
Utilities Minimal. Instrument Air 1.5 l/min
Mains Power. Calibration Gases
Up to 100 l/min of Air or Nitrogen for window purge
Mains power or 24V
In Situ vs Extractive – Key Considerations
CO / CH4 Extractive
(Electrochemical/catalyst Based)
In Situ
(Cross Stack TDL)
Measurement General combustibles Sensor (eg reacts to H2)
Sensor – relative
Low Cost Addition to Zr analyser
Specific to CO and CH4
Photometric – accurate
Requires second analyser (expensive)
Measurement influences
Largely Independent of process conditions – Temperature, pressure, dust, etc
Affected by process conditions – Temperature, Pressure, Dust, Window Purge Flow Rate, etc
Calibration Precision Calibration / Verification possible Off Line Calibration Only. Verification possible (accuracy: ~3% of span)
Response Time Slow (10 - 30 sec)
Flow and system dependent
Fast (<5 secs)
SIL Accessment SIL1 SIL2
Ambient Temperature “Hot” Ambients can effect electronics
But Extractive Zirconia Solutions
“Hot” Ambients can effect electronics
Utilities Minimal. Instrument Air 1.5 l/min
Mains Power. Calibration Gases
Up to 100 l/min of Air or Nitrogen for window purge
Mains power or 24V
Conclusions
■ Both zirconia and TDL offer great advantages when considered
as complementary techniques for combustion control
■ Zirconia offers specific point measurement with a higher level of
inherent accuracy coupled with true calibration / validation
(good for “small” <5m Furnaces)
■ TDL offers a faster, overall measurement with less associated
maintenance (good for “large” >5m Furnaces and corrosive
processes)
■ TDL offers a good measurement with significantly less
maintenance for Corrosive Processes (eg Sulphur Furnaces)
■ For CO/CH4 measurement the laser offers the better
measurement but at the price of a second analyser
Thank You
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