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INTRODUCTION

The effective management and control of high temperature furnaces and process heaters is an integral part of processing hydrocarbons. Additionally, good !ame distribution is important to reduce coking of the furnace tubes, hence maximizing unit availability by minimizing the requirement for furnace decoking. Achieving safe operation, particularly during start up, shutdown and abnormal operating conditions is important so rapid detection of !ame out is also critical.

Optimal operation of furnace and process heaters largely depends on effective combustion control, a process which requires minimising excess air in the combustion chamber. This necessitates a fast speed of response when monitoring the !ue oxygen concentration at close to the optimum level and for the detection of carbon monoxide in the !ue gas, requiring measurement of these gases at the very high temperatures close to the !ame. Rapid response to changes in combustion conditions is particularly important where there is a variation in the composition of the fuel, for example when using re"nery gas. Additionally, for natural gas fuelled furnaces !ame out conditions may be detected by monitoring the methane concentration. It is also important that the !ue gas sample is representative of any variation across the typical 5m to 25m width of the furnace. As a consequence, more accurate and reliable instrumentation is required to support the control of the process.

Combustion control requires the use of effective and responsive gas analysis technologies, which are used to precisely determine the composition of a gas mixture so the fuel/air ratio can be adjusted to optimise combustion ef"ciency. A wide range of gas analysis technologies have been used for combustion control and these have been applied as in-situ measurements in the !ue gas, in close coupled

arrangements and extractive techniques requiring various sample systems.

Traditional Infrared (IR) based gas analyzers for example have been used with an extractive sample system for monitoring carbon monoxide concentrations using wide band pass "lters a technique which can result in low spectral resolution and the sensitivity limited by cross-interferences. A major development in gas analysis techniques has been the introduction of Tuneable Diode Laser (TDL) absorption spectroscopy. A single line monochromatic spectroscopy that offers advantages over multi-chromatic IR, TDL offers a highly stable measurement at very low concentrations, a reduction in the cross interference from other gases and a continuous, fast in-situ measurement.

TDL ABSORPTION SPECTROSCOPY

A typical TDL system consists of a tuneable diode laser light source, transmitting optics, an optically accessible absorbing medium, receiving optics and detector. TDL technologies are particularly suitable for insitu cross stack measurements, with a typical cross stack system consisting of the laser emitter module and receiver mounted across the process pipe line or !ue stack.

The measuring principle used is infrared single-line absorption spectroscopy, which is based on the fact that each gas has distinct absorption lines at speci"c wavelengths. The laser wavelength is scanned across a chosen absorption line of the gas to be measured and it is carefully selected to avoid cross interference from other (background) gases.

SERVOTOUGH LaserSP

The bene"ts of utilising TDL technology for combustion furnaces and process heaters

Combustion - Low Excess AirLow Excess Air levels:Controlling excess oxygen correctly means the greatest ef"ciency from the process, but this also has the advantage of reducing NOx emissions

Example gas !red process, actual excess oxygen levels will vary with heater size, fuel, loading and ambient conditions

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The detected light intensity varies as a function of the laser wavelength due to absorption of the targeted gas molecules in the optical path between transmitter and receiver. Being a spectroscopic absorption measurement technique, TDL absorption spectroscopy effectively counts molecules (or number density of molecules) that fall within the beam. To provide the gas concentration the density of the gas is required which is dependent on pressure and temperature according to the universal gas laws. For combustion applications there is often signi"cant variation in temperature of the gas sample so external inputs are provided from a 4-20mA temperature transmitter to the Laser monitor to enable the analyzer to derive an accurate gas concentration. Pressure variation in combustion applications is typically related to !uctuations in atmospheric pressure so typically either a "xed pressure is assumed or pressure compensation from a sensor in the laser monitor is used. If the highest levels of accuracy are required, then external inputs from a 4-20mA pressure transmitter may be used.

There are primarily two TDL technologies in use: Direct Absorption Spectroscopy (DAS) and Wavelength Modulated Spectroscopy (WMS). DAS is a simpler technique requiring less demanding signal processing, providing measurements from either integration of the area under the absorption curve or maximum peak amplitude of absorption line pro"le generated by the laser scan. The limitations of the DAS technique are due to a relatively noisy absorption signal which compromises measurement accuracy. DAS is also limited due to the broad absorption line shape, with measurement data contained within the wings of the absorption curve, hence a proportion of the absorption data is not scanned, and cross interferences from absorptions of background gases, molecular interactions and environmental !uctuations can be dif"cult to correct. While the DAS technique is adequate for simple analysis under stable conditions, the low signal to noise ratio and increased effect of cross interference limits its application in the demanding long path length ethylene furnace applications.

WMS is a sophisticated evolution of the DAS technique, which takes a measurement of the second harmonic of the absorption curve at modulation and detection frequencies of 100 -200 kHz.

This yields a very sharp absorption curve with all measurement data contained within the laser scan width and sharply de"ned turning points which are easily computed, allowing an accurate evaluation of the absorption peak height and line width.

By delivering excellent cross interference rejection, precise temperature and pressure correction and low noise measurements, the greater accuracy and stability given by the WMS measurement means it is the TDL method of choice for demanding applications. Second harmonic detection offers other signi"cant advantages; as WMS produces a true zero baseline signal, it eliminates the need to measure small differences between two large intensities, with the resultant elimination of the drift that occurs with traditional TDL and other spectroscopic techniques.

TDLbased systems are an ideal choice for in-situ cross stack measurements in process and combustion control applications. As there is no physical or mechanical interaction with the process other than molecular absorption they offer a highly stable base line measurement, with typically a 12 month calibration interval and a fast response measurement in hot, wet, corrosive and dusty process conditions.

COMBUSTION CONTROL

The cornerstones of a wellcontrolled combustion process are optimised air-to-fuel ratio and ef"cient fuel consumption. Crucial to this combustion ef"ciency is minimising excess air in the combustion chamber, a process which requires fast speed of response when monitoring the !ue oxygen concentration at close to the optimum level, and for the detection of carbon monoxide in the !ue gas. The SERVOTOUGH LaserSP monitor, a high performance TDL combustion monitor, is ideal for this task by combining a response time of less than two seconds with an in-situ measurement location that eliminates any sample system delays.

The bene"ts of utilising TDL technology for combustion control in furnaces and process heaters

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DIRECT ABSORPTION SPECTROSCOPY (DAS)

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Additionally, a LaserSP monitor with dual gas measurement of both carbon monoxide and methane may be used for both combustion control and detection of a !ame out condition in natural gas "red furnaces.

The ability to measure these gases at up to 1500C allows the LaserSP monitor to be located close to the furnace burner, delivering the fastest possible speed of response.

Additionally, the TDL analysis technique offers a representative path-averaged measurement of process gas concentrations with a sensitivity that enables it to highlight faults: for example problems with a single burner can be detected by the LaserSP, even in furnaces where more than 100 burners may be present.

When utilised for a typical measurement of 0-10% O2 and 0-1000ppm CO respectively, the LaserSP delivers a !ue gas analysis solution that enables optimisation of the combustion control process, improving process ef"ciency by lowering fuel consumption and reducing carbon dioxide emissions. TDL absorption spectroscopy delivers great advantages in the measurement of carbon monoxide, with the fast response and speci"city of the technology enabling carbon monoxide breakthrough to be monitored accurately. Carbon monoxide measurement using TDL monitors also avoids the problem of high sulphur levels which can inhibit gas analyzers using catalytic sensor technologies.

Servomexs advanced application knowledge has also enabled additional cost and process control bene"ts to be applied to the combustion control process. In traditional TDL in-situ installations, it is common practice to !ow a dry nitrogen (N2) purge gas over the windows of analyzers to protect the optics, which are exposed to the hot process gas stream; instrument air is not used as the oxygen content would interfere with the O2 combustion chamber measurement. Servomexs expert solution has been to extend the infra-red scan range of the LaserSP to include additional oxygen absorption lines which only appear at elevated temperatures, greater than 500C. By using one of these hot lines to monitor the process O2, it is possible to use instrument air as the window purge

gas as the O2 in the instrument air, being colder, is no longer seen at the process monitoring hot wavelength resulting in no interference to the measurement. The standard cold O2 absorption line is also monitored and the signal utilised for the purpose of checking the LaserSPs response to a test gas, and to provide an alarm in the event of an interruption of the !ow of window purge gas.

This unique Servomex air purge process typically saves $5000pa (Euro4000pa) compared to a traditional TDL with nitrogen purge.

Furnace wall vibration or thermal expansion can also cause measurement failure for basic TDL technologies, due to the movement of the wall on which the analyzer is mounted resulting in a reduction or variation in laser light falling on the laser detector. The LaserSP monitor enables the laser transmitter optics to diverge the laser beam, ensuring the receiver remains unaffected by thermal movements and vibrations of the process. This receiver over"lling technique is only made possible as a result of the advanced second harmonic (2f) wavelength modulated signal (WMS) processing used in the SERVOTOUGH Laser product range, which operates at a 100 kHz - 200kHz carrier signal frequency, providing an order

of magnitude improvement in signal to noise ratio compared to traditional direct absorption signal TDL spectroscopy.

In conclusion, Servomexs advanced TDL technologies offer signi"cant advantages when applied as a combustion control solution in a range of hydrcarbon processing applcations, maximising ef"ciency whilst reducing greenhouse gas emissions. The highly robust technique provides reliable measurement of combustion gases with less associated maintenance whilst the location of the measurement close to the !ame delivers the fastest speed of response, optimising combustion and minimising fuel costs.

TRANSFORM YOUR COMBUSTION EFFICIENCY: contact your local Servomex business center:

Asia Paci"c (+86(0)216 489 7570) Europe/Africa (+31(0)79 330 1580) India (+91 22 3934 2700) Latin America (+55 115 188 8166) Middle East (+971 6552 8073) North America (+1 281 295 5800)

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SERVOTOUGH Laser Transmitter

Different combustion reaction across process

CO reaction almost complete

Convection Tubing

SERVOTOUGH Laser receiver

A furnace process utilizing Servomex TDL technology

The bene"ts of utilising TDL technology for combustion control in furnaces and process heaters