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[ EN ]


Related Application
This application claims the benefit of U.S. Provisional Application No.
60/020,325, filed June 24, 1996.

Field of the Invention
The invention relates generally to apparatuses and methods for measuring chemical concentrations in gas stream. In particular, the invention relates to an apparatus for measuring the concentration of gases in a gas stream using a heated gas extraction probe and closely coupled measurement chamber.
Background of the Invention
Chemical processes require devices that can measure the concentrations of certain gases entrained in a mixture of gases (i.e., a process gas stream). Often the process gas stream is hot or laden with particles, which prevents insertion of a measurement device directly into the stream. In these situations, it is known to extract and transport a sample of the process gas stream to a remote measurement site. A problem with this technique is that the sampled gases may cool prior to performing the requisite measurement, creating the risk of precipitating chemical reactions that alter the concentrations of the chemicals to be measured. Also, the by-products of these reactions may be solids or liquids that can inhibit proper operation of the measurement device. To circumvent these difficulties, it is known to dilute the sampled gases by mixing them with a non-reactive carrier and transport the diluted mixture to the measurement site through a length of tubing that is heated to a temperature where the detrimental reactions within the diluted mixture no longer occur. Expensive heated pumps are typically used to draw the gases in such installations. However, the maximum practical temperature at which these heated sample lines and pumps can operate is approximately 400°F.

Summarv of the Invention
The present invention features an apparatus comprising an extraction probe and measurement chamber which, in combination, permits gas concentration measurements to be performed directly at the gas extraction site without diluting the sampled gases, while keeping the gas heated to a temperature above 500°F. In one detailed embodiment, the apparatus has been specifically designed to operate with laser-based gas analysis equipment for monitoring trace concentrations of reactive chemicals in utility boiler post-combustion flue gases. However, the apparatus can accommodate any number of other measurement devices that are configured mechanically to attach to its measurement chamber, and can be used in any application where continuous gas extraction is required. The apparatus can be equipped with capabilities to periodically discharge accumulated particles that have been removed from the sampled gas stream and to periodically inject calibration gases into the measurement chamber. Also, the apparatus is designed and assembled in a fashion which facilitates both installation and servicing at the measurement site.
In one embodiment, the invention features an apparatus for measuring
concentrations of gases within a high temperature gas composition. The apparatus includes a probe and a heated measurement chamber. The probe is insertible into a stream of a high temperature gas composition and includes a tip section for receiving an extracted sample and a heated extraction section extending therefrom.
The tip section includes a filter for removing particles from the sample. The filter can formed of one or more materials capable of withstanding the temperature and chemical properties of the extracted sample. The heated extraction section can include blow-back tubing for passing pressurized gas received from a gas source to the tip section for dislodging accumulated particles from the filter.
The heated measurement chamber is coupled to the proximal end of the probe for receiving the extracted sample from the heated extraction section. The heated measurement chamber includes a measurement device for measuring concentrations of gases within the sample. A condensation trap can be coupled to the measurement chamber for cooling the sample received therefrom.

The apparatus can also include a pump in fluid communication with the condensation trap, the measurement chamber and the probe. The pump provides a pressure differential for extracting the sample from the high temperature gas composition and drawing the sample through the probe, the measurement chamber and the
condensation trap. The pump is thermally isolated from the stream of the high
temperature gas composition, the probe and the measurement chamber.
In one detailed embodiment, the measurement device comprises an athermal optical multipass cell (e.g., a Herriot cell) for receiving the extracted sample The cell includes first and second concave mirrors and a mounting structure The mirrors are oppositely disposed along a common axis and separated by a selected distance The mounting structure supports the mirrors at the selected distance The first mirror defines an orifice through which an input beam of light from a light source (e.g., a laser) enters the cell. The beam is repeatedly reflected between the mirrors before exiting the cell through the orifice as an output beam. An optical device is positioned to receive the output beam exiting the cell, and a detector is coupled to the optical device for receiving the output beam to measure concentrations of gases within the high temperature gas composition
The mirrors and the mounting structure can have the same (or substantially the same) coefficient of thermal expansion. Also, the light source, the optical imaging device and the detector can be thermally isolated from the high temperature gas composition disposed in the cell
In another embodiment, the invention features a method for measuring
concentrations of gases within a high temperature gas composition. A probe is inserted into a stream of a high temperature gas composition. A sample is extracted from the high temperature gas composition at a distal end of the probe Particulates from the sample are filtered by passing the sample through a filter disposed in the distal end of the probe. The filtered sample is then passed through a heated extraction section within the probe to a heated measurement chamber. Concentrations of gases within the sample are measured in the measurement chamber. Optionally, pressurized gas received from a gas source can be provided to the distal end of the probe for dislodging accumulated particles from the filter In one detailed embodiment, the sample can be passed into an athermal optical multipass cell disposed in the measurement chamber The cell includes a pair of mirrors and a mounting structure An input beam of light can be directed into the cell through an oπfice formed in the first mirror light The input beam is repeatedly reflected between the mirrors before exiting the cell through the orifice as an output beam The output beam is directed onto a detector to measure concentrations of gases within the sample
Bπef Description of the Drawings

Figure 1 is an illustration of a gas monitoring apparatus including an extraction probe and a measurement chamber
Figure 2 is an illustration of a heated extraction section of the gas monitonng apparatus of Figure 1
Figure 3 is an illustration of an interior view of the measurement chamber and accessory enclosure, which includes valves, a manifold and plumbing components
Figure 4 is a side view of a spectroscopic device including an athermal optical multipass cell and associated optical components
Figure 5 is an illustration of an open-path monitor field configuration for a commercial embodiment of the invention
Figure 6 an illustration of a control module of a commercial embodiment of the invention
Figure 7 is a graphical representation of laser power returned to transceivers of two of the open path monitors shown in Figure 5

Figure 8 is a graphical representation of signals of the internal control module of

Figure 6 indicative of normal system operation
Figure 9 is a graphical representation of relative HF concentrations measured along the paths of two of the open path monitors shown in Figure 5

Figure 10 is a graphical representation of ammonia concentration recorded by the commercial embodiment of the invention over a 24 hour period m a utility boiler employing a SNCR NOx reduction system

Detailed Descπption Figure 1 shows a gas monitoring apparatus 10 including an extraction probe 12 and a measurement section 14. The apparatus has distinct functional sections and a number of accessories. The functional sections are: (i) the probe tip filter 16; (ii) the heated extraction section 18; (iii) the heated detachable measurement chamber 15; (iv) the condensation trap 20; and (v) the pumping station 22. The accessories (not shown) include: (i) a measurement chamber pressure sensor; (ii) a gas blow-back tube; (iii) valves for control of blow-back and calibration gases; (iv) a measurement chamber pressure regulator; and (v) precision temperature controllers. The functional sections and accessories, as well as their interrelationships, are described in detail hereinafter.
As shown , the probe tip 17 and extraction section 18 are mounted on one side of a mounting flange 24. The measurement section 14 and condensation trap 20 are mounted on the opposite side of the mounting flange 24 and are detachable therefrom. To install the apparatus, the probe tip 17 and extraction section 18 are inserted into the process gas stream 26 and secured in a fixed position by attaching the mounting flange 24 to a mating flange attached to the outer wall 25 of the process containment. The measurement section 14 and the trap section 20 are then attached to the opposite side of the mounting flange 24, and the pump inlet is attached to the outlet of the trap. A measurement device 70 (Figure 4) is inserted into the measurement chamber 15 and secured in a fixed position by attaching its support flange to a mating flange at the mouth of the measurement chamber.

To activate the apparatus 10, the extraction section 18 and measurement chamber 15, both of which are encased in electrically-heated ovens are first heated to their respective operating temperatures. The pump 22 is then energized, decreasing the gas pressure at its inlet and extracting a sample of gas from the process gas stream 26. The extracted sample gas is drawn through the probe tip filter 16, through the extraction section 18, through the measurement chamber 15, through the trap 20, and into the pump 22. Particles are removed from the sample gas by the filter tip 16. As the sample gas flows, its temperature rapidly equilibrates with the temperature of the extraction section 18, which is normally the same temperature as measurement chamber 14 and is normally close to the process temperature Also, as the sample gas flows through the extraction section 18, its pressure diminishes By using the pressure sensor and pressure regulator accessoπes, the gas flow rate may be adjusted so that the pressure within the measurement chamber 15 is maintained at a predetermined value Upon exiting the heated measurement chamber 14, the gas enters the unheated condensation trap 20, where it cools rapidly Solids or liquids may form due to a reduction in gas temperature and are collected permanently in this voluminous region As a result, tubing that carries the gas from the trap 20 to the pump can 22 be used, without the requirement of additional heating to prevent plugging
Once activated, the apparatus 10 operates continuously Depending on the specific mixture of the gas, the trap 20 may require periodic servicing to remove accumulated matter Also, depending on the specific application, additional pumps requiπng periodic servicing may be installed In these applications, parallel redundant pumping stations may be used to maintain continuos operation while one of the pumps is being serviced
The probe tip filter 16 removes particles from the extracted sample gas The filter

16 may be configured as a πgid porous cylinder (e g , six inches in length with a one inch outer diameter) capped at one end and fitted with a threaded or other suitable opening at the other end Alternatively, the filter 16 may be a flexible fabric-type filter bag attached to a threaded opening The threaded opening attaches to a pipe 28 (Figure 2) which conducts the extracted gas into the extraction probe section 18 The porosity of the cylinder (or bag) is selected to remove all particles entrained in the extracted gas that may either plug sections of the apparatus 10 or interfere with the measurement device The filtered particles preferably remain on the surface of the filter 16 so that they can be easily dislodged by reversing the direction of gas flow The filter material is selected to withstand the temperature and chemical nature of the stream of extracted sample gas In the detailed embodiment directed to sampling boiler post-combustion flue gases, the probe tip filter 16 is made of a sintered material having average pore dimensions of 0.5 μm This material can be 316 stainless steel, or other materials may be used for specific applications including sintered metals, glasses, or ceramics (e.g., alumina or silicon carbide). When equipped with the blow-back tube accessory 32, high-pressure air is periodically (e.g., once per hour) injected inside of the porous cylinder, thereby momentarily (e.g., five seconds) reversing the gas flow and clearing the filter surface of accumulated particles.
The heated extraction section 18 serves four primary purposes: (i) it supports the probe tip 17 within the process gas stream 26 at a predetermined depth from the process containment wall 25; (ii) it contains tubing that conducts the extracted sample gas from the probe tip to the measurement chamber 14 and helps regulate the pressure in the chamber; (iii) it contains tubing that brings preheated blow-back air and calibration gases to the probe tip 17; and (iv) it contains heaters and temperature sensors that maintain the extracted sample gas at a predetermined temperature.
Figure 2 is an illustration of a heated extraction section 18 of the gas monitoring apparatus 10. The probe tip 17 is attached to the extraction section 18 via a threaded pipe 28 or similar attachment. Both the tubing 30 that conducts the extracted sample gas, and the blow-back air tube 32 have open ends within the probe tip 17. The tubes are typically made of stainless steel, but may be coated internally with or substituted by another material (e.g., quartz or Teflon) to preclude reaction with certain gases in the extracted gas stream. Both tubes 30, 32 travel the length of the extraction section 18. The gas conduction tube 30 terminates at the opposite end of the extraction section with a fitting that easily connects the tube to the measurement chamber 15. This fitting is typically of the compression type, such as that manufactured by Swagelok, although other fittings can be used. The blow-back tube 32 terminates with a similar fitting that attaches to the accessory control valves located external to the main sections of the apparatus 10.

In addition to being a conduit for the extracted sample gas, the gas conduction tube 30 also regulates the pressure in the measurement chamber 15. That pressure is determined by a balance between the vacuum force created by the pump 22 and its accessory pressure regulator, the pressure in the process gas stream, and the pressure drop through the extraction section conduction tube 30 The latter is determined by the diameter of the tube, the flow rate through the tube, and the viscosity of the extracted gas For the detailed embodiment, the tube diameter is about 1/8 inch, the pressure in the process gas stream is about 1 atm, the flow rate is about one liter per minute (at atmospheric pressure) and the pressure in the measurement chamber is about 1/4 atm
The two tubes 30, 32 are encased in an aluminum oven 34 formed from two half-cylinders that close around the heated components Slots are machined out of the interior of the half-cylinders to accommodate the internal components An electric heating element 36 and a thermostat-type device 38 to control the temperature are also encased in the oven 34 The aluminum oven 34 distributes the heat generated by the heating element 36 substantially uniformly throughout the length of the extraction probe section 14, thereby regulating the temperature of the extracted gas
When closed, the aluminum oven 34 forms a cylinder with an outer diameter of approximately 1 5 inches This cylinder is wrapped with insulation 39, such as that manufactured by Fibrafax, and inserted into a steel pipe The pipe 28 is attached, via a secondary flange 40, to the rear of the support flange Thus, the steel pipe 28 supports the extraction probe oven 34, its internal components, and the probe tip 17 A portion of the oven and the encased components 42 project through an opening in the flange 40 These portions 42 mate with the measurement chamber section 14, as described below
One notable feature of this mounting arrangement is that, when the measurement chamber section 14 is removed, the entire extraction probe section 18 can be withdrawn from the process by simply detaching its secondary support flange 40 This procedure does not require removal of the main support flange 24, which is used to also support the measurement chamber 14 and the accessones The supports for these components permit them to slide or swing out of the path of the extraction section without having to be dismounted from the support flange, specifically to facilitate servicing of the extraction section
For applications demanding precise control of the probe temperature, a precision temperature measurement device (e g., an RTD) is installed in the oven 34 Duπng heated operation, the RTD probe senses the oven temperature and transmits that information to a remotely located temperature control accessory (e g , a PID controller) The control accessory regulates the current supplied to the heating element to keep the temperature constant to within 1°F RTDs are particularly useful temperature sensors as compared to thermocouples, for example, because their signals can be transmitted accurately over long distances using low cost wires Thus, the PID controller can be located remotely from the probe installation site and still maintain accurate temperature control The control signal from the PLD is transmitted back to the probe using similar low-cost wires To this end, the a low voltage control signal is provided to a relay, which can be of the solid-state vaπety, collocated with the probe The relay, in turn, controls the relatively high voltage electrical current that powers the heater This power is provided by a standard local AC wall outlet or similar supply

Figure 3 shows the accessory enclosure 50, which includes valves, a manifold 52 and plumbing components 54 The blow-back air or calibration gases that can be supplied to the probe tip 17 may be introduced using the valve and manifold arrangement shown in Figure 3 For example, high-pressure (nominally 30 psig) air can be supplied via 1/4 inch (outer diameter) tubing to the inlet of one of three electrically-operated valves
Calibration gas (e g , from a compressed gas tank fitted with a regulator adjusted to nominally 3 psig) is attached to a second valve, and zero gas (e g , dry nitrogen from another compressed gas tank) is attached to a third valve An accumulator 56 is attached in parallel to the inlet of the blow-back air valve to eliminate pressure drops in its supply tube The pressures of the calibration and zero gases are set so that, when flowing, they slightly exceed the pressure of the process gas at the probe tip 17 Thus, activation of the calibration or zero gases stops the flow of process gas stream, and only the calibration or zero gas is drawn mto the measurement chamber 15
The outlets of the three valves are attached to the blow-back tube 32 within the extraction probe The valves are configured electrically so that only one can be activated at a time In one embodiment, the valves are activated by closing a switch at a local control panel, or by a low-voltage signal transmitted from a remote location In the latter case, the low- voltage signal activates a relay which controls the local AC current that activates the valve For some applications, the measurement device can be damaged if the sample gas is drawn through the measurement chamber 15 without proper temperature control. To minimize the potential for damage, the blow-back air valve may be configured to activate automatically in the event of a failure of the heaters or the temperature controllers. To accomplish this, the temperature controller is equipped with an alarm relay that de-energizes when the measured temperature is outside of a preset range, or when power to the controller is lost. The remote signal used to activate the blow-back valve is transmitted through this relay. In addition, the blow-back valve is selected to be of the "normally open" type, meaning that blow-back air is transmitted when the valve is de-energized. Under normal operation, i.e., when the probe temperature is within the preset range and blow-back air is not required, the valve is continually energized for blocking the flow of blow-back air. If a temperature alarm is activated, or if power is lost either at the remote location of the temperature controller or at the probe installation site, or if blow-back air is commanded by the system controller, then the valve is de-energized permitting air to pass. In this manner, any electrical failure in the system that permits loss of probe temperature regulation causes blow-back air to pass into the probe tip 17, thereby blocking the extraction of the potentially damaging process gas and protecting the measurement advice.

In one embodiment, the measurement chamber 15 is a 2.5 inch (outer diameter) stainless steel cylinder about 12 inches long. The forward end 58 is capped with a circular section of stainless steel welded to the cylinder. Fittings for bringing the extracted sample gas into the chamber 15 and for exhausting the gas from the chamber are welded or screwed into the cap (see Figure 3). A flange is welded onto the rear end of the chamber. The measurement device slides into the chamber from the rear opening 60 and is held in place with a mating flange. The flanges are sealed with an O-ring. For application requiring temperatures that exceed 500°F, the O-ring can be made of Kalrez Other materials may be used as required depending on the specific application.
The entire chamber 15, plus a nominal 5 inch forward region, is surrounded and supported by an aluminum oven having a 2.5 inch inner diameter and a 3.5 inch outer diameter. The forward 5 inch region has a removable clamshell style cover that permits access to the mtenor The region also has a small slotted opening for passing wires and tubing to carry blow-back and calibration gases into the oven, as described below, and another opening for the gas exhaust tube The entire oven is suπounded by insulation

The extracted sample gas is carried to the measurement chamber 15 via the conduction tube 30 This tube 30 mates with a fitting which connects it to the chamber 15 The gas exits the chamber through an exhaust tube 62 which begins near the rear of the chamber 15 and passes through a compression fitting that, when tightened, secures the tube in a fixed position This configuration ensures that extracted gas circulates throughout the volume of the measurement chamber 15 and is properly sampled by the measurement device Outside of the measurement chamber 15, the exhaust tube 62 is bent 90° The tube 62 passes through the opemng in the front section of the oven and immediately enters the condensation trap 20 In this manner, the exhaust gas remains hot until it enters the trap, precluding condensation in, and potential plugging of, the exhaust tube 62
The oven surrounding the measurement chamber 15 has its own electric heating element, temperature sensor and temperature controller The temperature controller may be of the type descπbed above for the probe oven, including the alarm provision The oven is enclosed within a housing (e g , a NEMA-style box) to isolate it from the ambient environment This isolation both protects the oven and measurement chamber 15 from external hazards and also permits use of the oven in regions where the electrical components may present a hazard The housing is equipped with sealed doors, or removable ports, permitting access to the interior for installation and service Both the oven and housing are attached to a mounting plate The oven is attached via standoffs that permit the installation of thermal insulation between it and the mounting plate The mounting plate is suspended from a rod, or rail, along which the oven and housing assembly (including any attached accessoπes) can slide The rod is attached to the main support flange via a detachable hinge This arrangement allows the measurement chamber section 14 to be easily attached to, or removed from, the extraction probe section 12 as follows
First, the hinge that supports the rod is attached to its mate at the main support flange Then the oven is slid to the rear of the rod and rotated to be parallel to the extraction probe 12. The oven is then slid forward so that the rear of the extraction probe 12 mates with the front of the oven, thereby forming one continuous aluminum oven surrounding all components from the front of the extraction probe to the rear of the measurement chamber 14. When mated properly, the fittings that mate the gas conduction tube with the chamber inlet meet each other inside of the 5 inch section forward of the oven. Electrical wires from both the probe 12 and chamber heaters and their associated temperature sensors are brought into this section, as are the terminating fittings for the blow-back gas line 32 and the chamber exhaust tube 62. With the clamshell cover removed, the wires are directed through the oven's opening and brought to terminal strips within the electrical accessory box. A tube that carries the blow-back air or calibration gases is also directed through the opening and attached to its mate within the oven. Upon tightening all fittings and connecting al wires, the mating of the measurement chamber section to the extraction probe section is completed.
The condensation trap 20 rapidly cools the gases exhausted from the measurement chamber 15 in a region that is large enough to collect precipitates for an extended period of time without clogging. By collecting the precipitates in the trap, relatively small diameter, unheated tubing of any desired length can be used to conduct the cooled exhaust gases to the pump 22. In addition, the trap permits use of relatively low cost pumps that do not require supplemental heating or the ability to pump mixed gases and solids.
The condensation trap 20 is essentially a cylinder mounted outside of the measurement chamber oven. The trap 20 has a nominal 2.5 inch diameter, is 6 inches long, and made of stainless steel or other appropriate material. The cylinder is closed at both ends, but is fitted with ports (e.g., 3/8 inch diameter) to allow gases to enter and exit. The outer surface of the trap 20 may be equipped with fins or other components to enhance heat dissipation.
The trap 20 functions as follows. Gases exiting the measurement chamber are drawn through the gas exhaust tube 62 into the condensation trap 20, where they impinge on the walls of the trap and cool rapidly. Condensable or precipitative materials deposit on and are retained by the trap walls. The cooled gas is drawn out of the trap 20 through the exit port and into a filter section. The filter further cools the gas and removes any particuiate materials that remain in the gas stream prior to introducing the gas to the tubing. The tubing conducts the gas to the pump.
To accomplish these functions, the components are constructed and assembled in the following manner. The gas exhaust tube travels from its attachment port at the measurement chamber 15, through an opening in the measurement chamber oven, and directly into the condensation trap 20. The exhaust tube 62 can have a 3/8 inch outer diameter and is thermally insulated along its entire length to preclude internal
condensation. Directly outside of the measurement chamber oven, but within the oven's housing, the tube is suπounded by and passes through the typically fibrous insulation surrounding the oven. The tube then passes into the condensation trap 20, where the trap meets and is mounted to the oven's housing. Inside the trap 20, the tube is surrounded by fittings that both insulate the tube and seal the trap. The fittings can be manufactured from plastics (e.g., TFE Teflon or PEEK) though other materials may be used. The seals may be made by a combination of compression fittings and O-rings. At the exit of the trap, a short section of 3/8 inch outer diameter stainless steel tubing conducts the gas to the filter. The filter can be a ceramic element contained within a glass housing that seals at a plastic filter head. The filter head can be made of Kynar, though other materials may be used for each of the filter components.
Gases exiting the filter are transported via tubing to the pump 22. Depending on the requirements of the gas measurement apparatus, the tubing may be made of a flexible material (e.g., polyethylene, Teflon, or Kynar) or a rigid material (e.g., stainless steel). The tubing may have a 3/8 inch outer diameter and is connected to the filter and the pump 22 by standard compression or pipe fittings. The pump 22 may be any one of a number of variations. The only requirements are that the pump 22 (i) must not be degraded by the gases and (ii) can create the suction and flow rates required for the specific application. In one embodiment, the pump 22 is a Gast Model 523 rotary vane pump, capable of achieving 26" Hg of vacuum, or can draw 4.8 1/min at 0" vacuum. This pump 22 is normally operated at 23" vacuum and draws nominally 1 1/min through the measurement chamber 15. Other embodiments may use eductor, diaphragm, piston or other pumps.

The vacuum created by the pump 22 within the measurement chamber 15 may be regulated by the use of the back-pressure regulator accessory which is installed at the pump inlet. The regulator mixes room air with the extracted gases as required to maintain a constant, preset pressure at the regulator outlet. The room air may be introduced through a filter to remove airborne particles.
The pressure at the both the measurement chamber outlet and the pump inlet may be monitored via pressure gauge accessories. The pressure at the measurement chamber outlet is measured with an absolute pressure electrical transducer gauge that transmits a signal representative of the measured pressure to a monitoring and recording device. In this manner, applications that require precise knowledge or control of the chamber pressure can determine if the pressure is within the desired operating range and take appropriate action if necessary. The pressure at the pump 22 can be monitored with a mechanical gauge having a local dial indicator, permitting easy adjustment of the backpressure regulator.
In applications requiring high reliability, dual redundant pumps may be used. In this configuration, the condensation trap is fitted with two outlets, and parallel pumping systems are installed down stream of these outlets. One pumping system is designated the primary system and operates in the manner described above. If the measurement chamber pressure created by the primary pumping system goes out of range due to degradation of the pump, plugging of the filter, or other abnormal event, the primary pump is deactivated and the secondary pump activated. An indication is transmitted to the operator that such an event has occurred, indicating the need to service the primary pumping system.

In one embodiment, the gas measurement apparatus 70 is used in conjunction with a laser-based measurement system to continuously monitor trace concentrations (e.g., one part per million) of ammonia in gases extracted from coal-fired utility boilers. The specific measurement technique is based on laser spectroscopy. The measurement device used is a Herriot cell 72. The cell 72 is inserted into the measurement chamber 15 and includes a pair of opposing concave mirrors (74, 76), separated from each other by about 25 cm and supported from a single flange 78. The supporting flange 78, which mounts to and seals the measurement chamber 15, comprises a window 80 that transmits a laser beam into the Herπot cell 72 The laser beam originates at a remote source 81, I e , the system console, and is brought to the measurement site via an optical fiber 82 Within the Herπot cell 72, the laser beam is reflected numerous times between the mirrors and exits the cell through the same window 80 The exit beam is directed via an imaging device 84 onto a photodetector 86 The support flange 78 is made of the same material as the measurement chamber 15 (e g 316 stainless steel) An insulated aluminum cover is mounted outside of the flange The cover mates to the cylindrical oven surrounding the measurement chamber 15, thereby ensuring uniform heating of the entire measurement chamber including the installed optical components
In this embodiment, the sample gas is extracted from a section of the boiler having a temperature in the range of 600-800°F If the temperature of the extracted gas is permitted to fall below approximately 450-500°F, the ammonia in the gas reacts with any SO^ m the gas stream to form ammonia-sulfur salts, which are solid precipitates If these solids are permitted to form in the measurement chamber, they would contaminate the optical surfaces and degrade the measurement signal Therefore, precise temperature control of the extracted gas, using the methodology described above, is necessary for this embodiment The solids then form within the condensation trap 20, after the extracted gas has passed through the measurement chamber 15 The temperature controllers are located within the same remote system console as the laser source
For reasons associated with the use of optical spectroscopy to perform these measurements, the pressure within the measurement chamber should be maintained between 0 22 and 0 28 atm Thus, the pressure transducer is installed at the chamber outlet and its signal is transmitted to and monitored continuously by the system console Any deviation from the preferred pressure is reported to the system operator The pump 22 is configured to draw approximately one liter per mmute (coπected to atmospheπc pressure and temperature) through the chamber 15 Since the chamber has a volume of approximately one liter and its pressure maintained at approximately 1/4 atm the gas exchange time within the chamber 15 is approximately 15 seconds
Blow-back, calibration, and zero gases are all used in this embodiment The blow-back gas is typically supplied through the facility's compressed air system It is usually filtered to remove water and oil before delivery to the extraction probe Calibration and zero gases are provided from standing cylinders of compressed gases Control of all three gases is provided both locally at the probe site and remotely from the system console The system console automatically activates the blow-back gas for 5 seconds each hour
Calibration and zero gases are activated from the system console either by manual commands issued by the operator, or automatically at preprogrammed intervals
In recent years, a body of literature has evolved that describes how spectroscopic instruments based on tunable diode lasers can be used to selectively measure trace concentrations of specific dangerous or undesirable gases that are entrained in a mixture of other gases. Indeed, several research groups have built instruments which demonstrated the capability to perform the requisite field measurements Though successfully illustrating the potential measurement power of this moderate-cost technology, the demonstration instruments were generally "one-of-a-kind" or at best, limited production devices requiring considerable user interaction to operate and maintain As such, the devices could not be made available for widespread use
The following report describes the engineering and manufacturing processes that were needed to develop these tunable diode laser spectrometers into commercially available systems A key aspect of the commercialization process has been to configure the instruments in a fashion that makes them easy to manufacture, install permanently in a wide vaπety of applications, use continuously and reliably, and service periodically To those ends, a family of modular instruments (called SpectraScan®) has been developed that can be rapidly configured from interchangeable components as needed to service a specific application These instruments continuously monitor, record, and report with rapid response time low levels of gas concentrations along open lines of sight or within closed vessels through which gases are drawn They are designed for installation in harsh and hazardous industrial or utility environments, and they incorporate automatic alarm, calibration, and self-health monitoring features that are critical for acceptance by industrial users In addition, standard interfaces are available for remote communications with other plan monitoring or control equipment
The data described herein shows the instruments to be robust and reliable in field operations Measurement capabilities include (1) monitoring ammonia slip in utility boilers that use NOx control processes and incorporating the measurements into a process-control feedback loop, (ii) monitoring water contamination of sensitive chemical processes, and (iii) open-path sensing of hazardous chemicals such as HF and H2S


SpectraScan® monitors rely on well-known spectroscopic principles coupled with

(l) recent advances in diode lasers and optical fibers developed by the telecommunications industry, and (ii) a newly-perfected detection techmque called wavelength modulated laser absorption spectroscopy All gas molecules absorb energy at specific wavelengths in the electromagnetic spectrum, thus providing a unique signature for that chemical At wavelengths slightly different than these "absorption lines" there is essentially no absorption By (i) passing a beam of light through a sample of the target gas, and (ii) tuning the beam's wavelength to one of the target gas's absorption lines, and (iii) accurately measuπng the absorption of that beam, the concentration of target gas molecules integrated over the beam's path length can be deduced This measurement is usually expressed in units of ppm-m
The tunable diode lasers (TDL's) used in SpectraScan® serve as the wavelength-selectable light source They are single chip solid state lasers, typically packaged with thermoelectπc coolers and thermistors that permit the laser temperature to be regulated with an accuracy better than 50 mK This feature, combined with accurate control of the electrical current supplied to power the laser, permits precise selection of the laser wavelength Furthermore, the laser has a newidth which is considerably narrower than molecular absorption linewidths These combined capabilities provide the mans for tuning the instrument to a specific molecular absorption line which is selected to be free of interfeπng absorptions from other molecules More specifically, not only is the laser wavelength tuned to a specific absorption line of the target chemical, the wavelength is also rapidly and repeated scanned across the selected absorption line While this scanning, or wavelength modulation, occurs, the fraction of emitted laser power that is transmitted through the gas mixture is monitored with a photodetector. When the wavelength is tuned to be off of the absorption line, the transmitted power is higher than when it is on the line. Measurement of the relative amplitudes of off-line to on-line transmission yields a precise value of the target gas concentration along the path transited by the laser beam.
In SpectraScan®, the laser wavelength is scanned across the absorption line roughly two million times per second, causing the detector to output a signal that contains a 2 MHz amplitude modulated voltage. This signal is detected using established highly sensitive radio receiver and signal-processing techniques while remaining free of interfering signals from external radio, light or heat sources. Thus, the tunable diode laser spectrometer offers a combination of sensitivity and freedom from cross-sensitivity to other molecules that, for many chemicals of interest, is not otherwise available.
SpectraScan® instruments operate in two measurement modes concurrently. In the "normal" measurement mode, the high-sensitivity capability described above is utilized to continually monitor low and fluctuating gas concentrations with a nominal one second time constant. The dynamic measurement range in the normal mode is about 1000: 1, and the minimum sensitivity is typically 1 ppm-m, depending on gas species and system configuration. In contrast, the "event" mode is less than one percent as sensitive as the normal mode, but responds more than 100 times faster. Its purpose is to assure detection of and rapid response to catastrophic events which may rapidly form a cloud that obscures the laser beam's path. For example, the event mode will detect, in milliseconds, the leading edge of an HF cloud and activate appropriate alarms within one second of detection.
The lasers used by SpectraScan® differ from those used by early developers of the wavelength modulation detection techniques. Those early lasers typically produced light at mid-to-far infrared wavelengths (>3 um), and relied on liquid nitrogen cooling for thermal stability. The liquid nitrogen needed to be replenished frequently by a human operator, thus limiting the continuous use of these devices in harsh environments. In contrast, the laser packages that are used now were developed by and are marketed primarily for the telecommunications industry, which demands continuous reliable unattended operation. These lasers generate light at near-infrared wavelengths (>2 urn) and are readily coupled to optical fibers, making possible long-distance transmission of the laser beam. The somewhat delicate lasers, along with their associated electronics and microprocessor, therefore can be located remotely from the potentially harsh or hazardous measurement areas.

Indeed, the laser beam used by each SpectraScan® monitor originates in a control console that may be located up to 1 km from the optical measurement path. The laser beam is brought via an optical fiber to the measurement path. At the beginning of the measurement path, the laser beam impinges upon a photodetector, where the information it carries is converted into an electrical signal. The electrical signal is returned to the electronics console via coaxial cable. At the console, the signal is processed and the path-integrated target gas concentration is reported. Each console can service up to four measurement paths, with may be of either the "Open-Path" or "Process Control" configuration.


A typical open-path installation is illustrated by Figure 5. The open-path instruments 90 transmit the laser beam 92 along a line of sight that can be a few hundreds of meters in length. An optical transceiver 94 is securely mounted at one-end of the path, and a hollow corner-cube type retroreflector 96 is mounted at the opposite end. The control console 98 is typically located in a control room 100 or similar benign
environment. The transceivers are certified for installation in Class 1 , Division 2, Groups B and D electrically-classified hazard environments, where hazardous concentrations of flammable gases or vapors may be present due to intermittent abnormal operating conditions. All external components are made of materials that can withstand the harshest chemical environments.
The transmitter portion of the transceiver 94 is aimed during installation to direct the eye-safe laser beam onto the retroreflector which, with little alignment required, redirects the beam back to the receiver section. The transceiver design passively separates the transmitted beam 92A from the received beam 92B, and focuses the received beam onto the photodetector.

The SpectraScan® Process Control (PC) monitors employ the same basic control console, but measures target gas concentrations within closed multipass optical cells installed in measurement chambers through which the gases of interest are continually drawn One version of this configuration incorporates a Herriot-style optical cell that is installed within a low-pressure heated chamber, as described previously In the Herriot cell, the laser beam is reflected between a pair of mirrors many times before emerging from the cell and striking the photodetector. In one embodiment, the laser beam transits the 25 cm cell length 40 times, thus providing the sensitivity of a 10 m optical path
The Herriot cell optics are optically stable throughout their entire heated range

They are installed in measurement chambers of one liter volume By operating the chambers at low pressure (<0.25 atm) rather than atmospheric pressure, the gas exchange rate is increased, thereby enhancing the instrumental response time Also, the
spectroscopy is simplified by reducing absorption line widths and making more absorption lines accessible with freedom from interference by nearby absorption lines of other gas species This feature is especially crucial in the important application of continuously monitoring ammonia in boiler flue gas streams Here, relatively weak ammonia absorption lines are surrounded by lines from water and carbon dioxide Though these lines would be undetectable if the water and carbon dioxide concentrations were the same as in a normal atmosphere, their high concentrations (10 t 20%) in flue gases actually make them quite significant absorbers when compared to ammonia. Narrowing the linewidths has allowed us to make these ammonia measurements with no measurable cross-sensitivity to water, carbon dioxide, or other flue gases. The optical cell, measurement chamber, and the extraction probe to which they are intimately attached, can be heated controllably to 500°F This unique capability prevents condensation of contaminants that could plug the probe or destroy the optical quality of the internal mirrors.
The probe/cell combination mounts directly on a boiler wall at any three-inch diameter pipe port. It is equipped with valves for automatically injecting calibration zero gases into the measurement cell, and for periodically cleaning the probe filters by backflushing with purge gas
4.2 3 CONTROL MODULE The SpectraScan® control module 102 is shown in Figure 6 The control module 102 is designed for installation in a standard 19 inch rack It comprises a chassis into which a number of sub-modular components are installed Each sub-module performs a specific function, and the specific sub-modules used in any particular instrument depends on the gas being momtored and the number of measurement paths used The sub-modules constitute

• A laser emitter subsystem, which contains a laser source module and a pair of electromcs modules that synthesize the laser modulation and control signals
Industry standard optical fiber connectors transmit the laser light out of the laser source module Ports are available to supply laser power to the reference cell and as many as four measurement paths Each laser emitter module is tuned to the optimum wavelength for detection of the selected target gas The laser emitter module is designed to protect the delicate laser from damage that can result from power surges and spikes, wiπng errors, or failures of other components

• A reference cell, with is essentially a short measurement path through a sealed vessel containing a small amount of the target gas Its output signals are processed differently from normal measurement paths in order to provide a feedback signal which is used to lock the laser wavelength to the center of the target absorption line

• Analog signal processors, which contain the electronic components that extract the absorption signal information from each detector's output, and convert it to a format suitable for analysis by an embedded digital computer/controller

• A microcomputer-based signal analyzer and system controller that interprets the processed absorption signals and provides appropπately formatted outputs
This sub-module also monitors and stores system operating parameters, thereby facilitating start-up and permitting automatic identification of system failures, contains hardware and software that, combined with the output from the reference cell receiver, continually adjusts the laser's electrical inputs to keep the laser
wavelength locked onto the gas absorption line, outputs data and other operating information via serial and parallel ports, and, can be connected to a modem for remote access to facilitate servicing the instrument

• A user interface which provides visual indications of measured gas
concentrations and any abnormal conditions, be they high gas concentration
measurements or improper instrument operation This sub-module also provides a connection for an electrical diagnostic cable that accesses internal signals to
facilitate installation and troubleshooting of the instrument
In addition to the control module, three other modular components are used as needed to form a complete SpectraScan® system

• A power supply module that is configurable to convert all worldwide AC
power standards to dc voltages needed to operate the other modules This module is required for all SpectraScan® instruments
• A system interface module that houses relays indicative of system operation and terminal strips for connections of the relays, 4-20mA concentration output, and serial digital output to other equipment These signals can be used to activate alarms or to automatically initiate emergency procedures , The system response time is typically less than one second The relays are also used to activate purge, calibration, and zero gases for an extraction probe, so this module is required for process control systems.

• A temperature control module houses a pair of PID controllers for each
extraction probe This module is required for applications where the probe must be heated
These modules are each intended for installation in the same 19 inch rack as the Control Module, and the rack containing the group of modules is called the System Console The System Console rack may be housed in an NEMA 12,4 or 4x enclosure as needed to suit its installation environment Feedthroughs for field wiring are provided as necessary An air-purged enclosure is available for installations in a classified hazard area
4 3 APPLICATIONS Some of the applications for SpectraScan® monitors are described herein
Toxic chemicals, such as Hydrogen Sulfide (H2S) and Hydrogen Fluoride (HF) are used or produced as by-products at many refineries. Though normally not released to the atmosphere, refinery operators wish to continuously monitor wide areas near storage and processing sites to detect and correct small leaks of these hazardous gases before they become serious problems. Furthermore, H2S is often transported by long pipelines within the refineries. Operators wish to monitor these pipelines for small leaks that may indicate an incipient failure
SpectraScan® is used in an open path configuration to meet these needs An eye-safe laser beam is projected from one of the system's transceivers to a retroreflector located up to 200 meters away. The SpectraScan® system then reports the amount (expressed as ppm-m) of the gas species being monitored along the path The minimum sensitivity for HF measurement is about 0.2 ppm-m, and for H2S, about 5 ppm-m These units can be inteφreted as meaning that a one meter diameter cloud containing 5 ppm of H2S would be detected by SpectraScan® This detection limit can be contrasted with accepted exposure standards of 20 ppm over an 8 hour period. At concentrations of a few hundred ppm, H2S can no longer be smelled and becomes rapidly lethal Thus the sensitivity offered by SpectraScan® is more than adequate to help assure safe working conditions. Similarly, the accepted safe 8 hour exposure to HF is 3 ppm, well above SpectraScan' s® minimum sensitivity.
SpectraScan® is also useful for monitoring very low levels of ambient HF that are present during normal refinery operations. For example, the sensitivity of 0.2 ppm-m translates into a sensitivity of 2 ppb distributed uniformly over a 100 m path This exquisite sensitivity to low average concentrations is now being used to help refinery operators contain previously undetected emissions

A process called Selective Non-Catalytic Reduction (SNCR) is frequently used to reduce the amount of NOx emitted by industrial or utility boilers When by ammonia or urea is injected and mixed properly with the combustion effluent stream, it reduces the NOx to trogen and water However, if too much chemical in injected, the result is costly "ammonia slip" Not only is the unused raw chemical wasted, thereby adding unneeded expense to the NOx control process, but the excess ammoma has a number of undesirable side affects It ends up either sticking to ash particles, making them unsuited for post-combustion use, or reacts with sulfur to create ammoma sulfate deposits that frequently foul boiler heat exchange surfaces, or is emitted mto the atmosphere A single plan outage caused by fouling of heat exchange surfaces can cost an operator hundreds of thousands of dollars In some cases, plants have been found to have outages every few weeks due to excess ammoma slip In contrast, if too little chemical is injected into the process, NOx reduction is incomplete, resulting in excess NOx emission to the atmosphere
The SpectraScan® Process Control probe is used in this application As described above, the probe continuously and rapidly draws boiler flue gases, containing low concentrations (0-50 ppm) of ammonia in a mixture of other gases (mostly N2, CO2, and H20), through a small hot optical cell SpectraScan' s® laser beam passes through the cell and acquires the information needed to determine the ammoma content in the effluent The system provides feedback to a control system that is used in conjunction with other sensors, such as NO* meters and SpectraTemp™ boiler gas temperature monitors, to optimize chemical injection rates
Many chemical processes can be disrupted by small quantities of contaminants

SpectraScan® is currently being tested for an application where as little as a few tens of ppm of water in a liquid chemical process stream can destroy the process In such an event, the operator may have to dispose of millions of dollars of product that was contaminated pπor to the discovery of the excess water Furthermore, the water vapor sensors currently used in this process are frequently destroyed by concentrations exceeding a few tens of ppms and must be replaced after each contamination was a shortlived event or an ongoing problem without an operable water sensor, the process must be shut down, with lost revenues of millions of dollars per day, while the source of contamination is found and corrected
SpectraScan® is expected to valuable in this process because of its high sensitivity coupled with its rapid response and short recovery tie To monitor water contamination, a small portion of the process stream is diverted into a side path Gaseous mtrogen is sparged into this side path and some of the liquid water in the process stream is absorbed into the mtrogen The water-bearing gas is then passed through a measurement cell and the water concentration in the gas is reported with a sensitivity of about 1 ppm Alarms to automatically shut down the process when water concentration exceeds a threshold can be programmed into the instrument
The are some examples of data acquired by SpectraScan® monitors
An open-path SpectraScan® instrument has been operating continuously within the HF alkylation umt of a North American refinery since July 1995 The instrument was configured with two measurement paths, each about 80 m long, one spanning the east border of the alkylation unit, the other spanning the south border Some typical data acquired by this instrument over a 24 hour period on a late summer day are shown in Figures 7-9
Figure 7 shows a voltage proportional to the laser power received at each detector Although there are some insignificant gradual variations in the received power, due pπmaπly to small movements of the transceiver on its mounting post, this figure shows that the laser beam was transmitted and received essentially continuously throughout the recorded period The short-lived interval where the laser power on Path 1 drops to zero is called a beam clock, and is the result of refinery workers walking in the beam's path SpectraScan® automatically recognizes beam blocks and, when one occurs, it freezes the blocked path's reported concentration at the last known value, instead of updatmg the data stream It also activates a warning that communicates the blocked condition to the plan operator via illumination of an LED Indicator on the User Interface Panel and closure of a relay in the System Interface module
Figure 8 shows some of the internal system signals that are continually momtored to assure that the instrument is operating properly These signals include the laser driver current (Current), the laser temperature (Temp), a signal proportional to the output of a photodiode that monitors the laser's output (Photo), a signal proportional to the ambient temperature of the control console (Backplane), and a signal proportional to the current required to operate the thermoelectric cooler that maintains the laser temperature (TEC) This figure simply shows that these signals are all within their normal operating range and change little over the reporting period The combined data of Figures 7 and 8 shows that the instrument is operating normally
Figure 9 shows the HP concentration reported along the two paths duπng this period These data were selected from months of daily traces specifically to make this illustration The data are characterized by a typical low concentration of a few ppm-m, which translates into an average ambient concentration of a few tens of ppb, well within accepted safety limits The zero points of the two data sets have been offset from each other for claπty. HF emissions greater then usual ambient are seen for two periods in these data' First a period lasting about 90 minutes, and later a short-lived "spike" Path 1 shows higher concentrations because of wind direction The 90 minute emission was caused by maintenance work on a pump in the alkylation unit The spike was attributed to a specific plant maintenance operation that was previously believed to not cause any HF release, because the release was too small to be detected by other instruments Plan operations have since been modified to minimize releases from that type of operation

The need for continuous ammoma monitoring in utility boilers was described above Figure 10 shows 24 hours of continuous data from a SpectraScan® instrument that had been installed at one such site for several months These data are characterized by typically low ammonia concentrations (between zero and a few ppm), with intermittent periods of several hours duration when ammonia levels increase above ten ppm These periods of relatively high ammonia slip have been correlated with periods of low plant load and cooler gas temperatures When plan load was high, gas temperatures at the ammonia injection sites were fund to be high enough that much of the injected ammonia was burned before it reacted with NOx, resulting in poor NOx reduction and excess consumption of injected chemical In contrast, during peπods of low load the gas temperature was too low for the NOx reduction reaction to proceed efficiently, leading to excess chemical injection and high ammonia slip These observations have helped plant operators select the appropriate locations to inject the NOx reduction reagent to maximize the efficiency of the process.
This section has discussed just a few examples of how spectroscopic
measurements using tunable diode laser technology can be used for process monitoring and control applications. To reiterate, the main advantages of this technique over
competing optical monitoring methods are the sensitivity, immunity from interferences, and speed. In addition, the instruments are robust, reliable, and cost effective. There are no moving parts, and delicate electronic components can be located in benign
environments while being connected via rugged optical fiber to measurement paths located in harsh or hazardous areas.

Of course, all optical techniques offer significant advantages when compared to non-optical sensors in many applications. Open-path measurements like those described herein are possible only optically. Optical measurements are generally non-intrusive, that is, they do not interact with or perturb the sample being measured. For these reasons, explosive growth is expected in the use of tunable diode laser monitors for chemical
sensing, now that these instruments are available commercially.
While the invention has been particularly shown and described with reference to specific prefeπed embodiments, it should be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.