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