Combustion Control and Safety A comparison between Zirconium Oxide/Catalytic and Tunable Diode (TDL) Technologies Dr Stephen Firth / Rhys Jenkins

Transcription

1 Combustion Control and Safety A comparison between Zirconium Oxide/Catalytic and Tunable Diode (TDL) Technologies Dr Stephen Firth / Rhys Jenkins

2 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

3 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 SAFETY Excess O2 is wasted energy - heating up excess Air (mainly N2), which cools the products of combustion and drops efficiency Excess O2 increases NOx 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 EFFICIENCY O2 & CO monitoring: Highlights inefficient control Highlights inefficient burner setup Highlight inefficient operation Improves Process Efficiency

4 Process Gas Analysis Monitoring Geometries In Situ Cross Process (Laser) In Situ Probe (Zirconia) DCS Control ma / Relays + AMCS Analysis Ethernet/Modbus Process Flow Variables: Flow Velocity Temperature Pressure Dust Sample Conditioning Analyzer Shelter Gas Analyzer Extractive (Paramagnetic Infrared Electrochemical)

5 Zirconia / Combustibles

6 Zirconia Complete sample measured by each sensor in turn Core T 90 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 During calibration the flow of sample gas through the analyser & transducer remains unchanged Aspirator & Sample Outlet 1.7ltr/min typical Flame arrestors for safety Probe During calibration the sensor head is flooded with calibration gas to prevent process sample from interfering Model C version dual sensor shown Aspirator Air 1.5ltr/min typical Aspirator 300 ml/min Internal Filter Cal Gas Inlet 600ml/min Flame Trap 200ml/min Flame Trap Heated Enclosure Comb Cell Temperature Interlocked Solenoid Valve 100 ml/min O 2 Cell Flow alarm Heated to prevent Condensation / corrosion Aux Air Rest. Auxiliary air to ensure Comb reading Confidence in measurement

7 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

8 Tuneable Diode Laser Spectroscopy

9 Building Blocks of a TDL Analyser Process Windows Transmitter Optics Receiver Optics Receiver 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 Laser + Temp. Control Electronics & Signal Processing

10 Line Lock Reference Techniques 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 Cuvette Gases: NH3, CO, O2

11 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 l/min Instrument Air 24V dc

12 Combustion

13 How to make you cracker furnace more efficient

14 Cross Path Average versus Spot Measurement CO =100 ppm O2=2% T= 900 C 2700 Zirconia O2 Thick film COe CO =1500 ppm O2=1.0% T= 1000 C 7930 O2 Laser transmitter 7930 O2 receiver 7930 CO/CH4 Laser transmitter Furnace cross section 7930 CO/CH4 receiver 10 m - 40 m

15 Real Life Examples Zr vs TDL Long Pathlength Furnace Laser O2 Laser CO Faster Response Better Control 4 min Zr O2 Tfx CO

16 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

17 In Situ vs Extractive Key Considerations Oxygen Sample Conditioning Measurement influences Extractive (Zirconia Based) Required: control of moisture, cooling, pressure Largely Independent of process conditions Temperature, pressure, dust, etc In Situ (Cross Stack TDL) None Good for toxic and corrosive samples 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 Maintenance Requirements Ambient Temperature Utilities Slow (10-30 sec) Flow and system dependent Medium flow alarm required for high integrity Hot Ambients can effect electronics But Extractive Zirconia Solutions Minimal. Instrument Air 1.5 l/min Mains Power. Calibration Gases Fast (<5 secs) Low minimum system components (Alignment issues esp. long pathlengths) Hot Ambients can effect electronics Up to 100 l/min of Air or Nitrogen for window purge Mains power or 24V

18 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 Measurement influences Largely Independent of process conditions Temperature, pressure, dust, etc Specific to CO and CH4 Photometric accurate Requires second analyser (expensive) 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 Utilities Hot Ambients can effect electronics But Extractive Zirconia Solutions Minimal. Instrument Air 1.5 l/min Mains Power. Calibration Gases Hot Ambients can effect electronics Up to 100 l/min of Air or Nitrogen for window purge Mains power or 24V

19 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

20 Thank You