Multicomponent Infrared Gas Analyzers

Overview Advantages MC3 Analyzer MC3 Description Technical Specification

Overview

 The underlying advantages were of a multicomponent analyzer are fairly obvious. If just a single instrument could measure all of the gases, using just one source lamp, sample cell and detector, then it could replace a room full of analyzers and attendant complexity. Indeed, if one has four gas analyzers, each must have its own sample cell, sources, detectors, electronics, and ancillary equipment. Then, if it could be an infrared photometer, it could greatly reduce the components in contact with flue gas, perhaps to less complexity than any one of the four it replaced.

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General Advantages of Multicomponent IR Analyzers

1. Real-Time Correction for Quantifiable Interfering Variables

The fact is that all gas analyzers suffer interference from other common stack gas constituents. This is as true for Chemilumenescent NOx and Pulsed Fluorescent SO2 analyzers as it is for Infrared devices. In most cases the effect is manageable but still, it detracts from the overall accuracy and none of those discrete devices are capable of performing corrections for interferences. Given that a multi component analyzer can easily measure those interfering compounds, it is just as easy to continuously correct the measured variable in real time. And with recent advances in computing power and advanced iterative techniques, it is possible to be extremely accurate. 

2. Direct Measurement of All Gases.

Incredibly, many analytical techniques are incapable of directly measuring the gases they are intended for—they must use converters or indirect methods, which always inject inaccuracies into the measurement. This is especially true for the chemilumenescent reaction, which is commonly used by NOx analyzers. In that reaction only the NO fraction results in chemilumenescence, the NO2 fraction is ignored. Thus to claim the ability to measure NOx, chemilumenescent analyzers must include a catalytic converter which adds cost, complexity and are of dubious efficiency. In many combustion sources the NO2 component is very small and even with malfunctioning NO2 converters can fall within the wide accuracy requirements of the regulatory authorities. However, with sources rich in NO2 such as Gas Turbines an inefficient converter can cause serious accuracy problems.

As more SCR systems are coming on line, another gas of importance is ammonia. One technique used in the past was dual NO analyzers employing variable converters. Thoroughly discredited in practice, yet for many users there had been no alternative. Incredibly maintenance intensive, those users have experienced huge costs to purchase and maintain such equipment.

With multi component IR analyzers those gases are easy to measure. That is, both NO and NO2 can be measured separately and added arithmetically. No conversion errors possible. With the use of the Hot/Wet sampling technology and a high temperature sample cell the multi component IR analyzer measures ammonia at very low levels and accurately.

Direct measurement is superior in accuracy, and with multi component analyzers it serves to reduce cost and complexity.

3. Reduced Maintenance Effort

By having one analyzer make all measurements, the parts count alone is drastically reduced. That is bound to reduce the number of source lamps that can expire, the capillaries that can plug, the sample cells that can become contaminated, and the control systems that may crash. One analyzer also means that plant technicians do not have to be trained and proficient in multiple analytical techniques, nor do they have to stock parts for a room full of different instruments.

Finally, because of the economics of instrument manufacturing, multi component analyzers can support more quality to provide long service life. The economics are not hard to figure out—if you can spread the cost over three or four measurements you can then afford to engineer in more durability and reliability. That ultimately results in the lowest long-term costs.

4. Purchase Costs

For two measured components the cost for the MC3 multi component analyzers is roughly equivalent to the use of discrete analyzers if one considers the additional system integration cost of experienced with multiple analyzers, and the need for independent system controllers. A strong economic case can be made when there are three or more measured variables.

Multi-Component IR based systems are now in operation at hundreds of North American installations. The equipment has been referred to as “bulletproof” and most are convinced that such multi-component analyzers will present the lowest long-term cost of any possible alternative. Every aspect of the device is engineered for ultra long-term operation with little or no maintenance. Numerous references can be found that experience near 100% availability, and that means little or no maintenance costs.

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MC3 Analyzer

The MC3 analyzer is a multicomponent analyzer with the following major improvements:

1. Sensitivity/Low Range Accuracy has Doubled

Sensitivity has improved by a factor of two. That means, for the most part, that the lowest full scale range of all measured gases has been cut in half without a decrease in the signal to noise ratio, or if the same range is used the accuracy is nearly doubled. That is made possible by the following enhancements:

• The source lamp generates twice as much energy and the solid-state detector noise is reduced by more than 70%.
• The mechanical and thermal design produce improved stability and reproducibility.
• The new multipath sample cell is much improved and path length can now be adjusted to best meet each application’s optimum length. This greatly reduces the effect of water interference. The sample cell has been in operation at a waste to energy plant since1999 with no problems encountered.
• The use of advanced microprocessor technology allows an algorithm with an iterative approach for correction of interfering compounds. Somewhat akin to FTIR techniques, as many as 30 iterations are used for each gas to precisely home into both additive and multiplicative corrections. 

2. Sample Cell Volume Reduction

Not only is the sample cell adjustable for optimization of path length but also the volume has been reduced by half. This means that the response time is greatly improved, even at lower flow rates. The result, added sample pump diaphragm life, twice as much time between sample filter cleanings, and half as much calibration gas usage (can be a big cost saving!).

3. Parts Count and Complexity Halved 

Advances in miniaturization and materials have resulted in an instrument that is simpler and more robust than ever before. This means that there will be even less failures, and those that occur will be easier to fix. Indeed, the mechanical and electrical design of the MC3 is a service person’s dream.

4. Reduced Manufacturing Cost

Many steps in the manufacturing process have been streamlined and along with the reduced parts count, significant savings are enjoyed. These savings are passed onto the user and costs now are, depending on application, 25-30% less for the analyzer portion of the CEMS. 

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MC3 Description

The infrared optical bench consists primarily of an infrared source with chopper, a sample cell that the infrared light pulses travel through, a motor driven filter disk that can put up to 12 different optical filters or gas filled cells in the infrared light beam, and a solid state detector to measure energy of the infrared light pulses. To understand the operation it is necessary to first understand the underlying analytical techniques.

MC3 Optical Bench

1. Gas Filter Correlation Technique

The Gas Filter Correlation technique is a well-known method to reduce cross sensitivities to gases that cause interference in infrared measurements. In the technique an optical band pass filter is used to select an infrared band and then a cell filled with 100% concentration of the gas of interest is placed in the beam, effectively blocking the spectral lines that the gas absorbs at. Alternately with the gas filled cell, a clear aperture with no absorption is placed in the beam. In essence, the gas filled cell is blocking all of the energy in a specific spectral line, creating a reference pulse that is absolutely free of variation due to the gas of interest. The other pulse coming through the clear aperture can be reduced by the gas of interest, thus will vary the ratio of the two pulses with variations of the gas of interest. The result from the IR detector is the presence of two electrical pulses that represent IR energy levels, one missing the energy absorbed by the gas filled cell and the other containing all of the energy of the infrared source. If the IR beam passes through a sample cell that has none of the gas of interest, then the two pulses will be significantly different in amplitude. If the gas of interest is present in the sample cell then the ratio of the two pulses is reduced...in theory if the sample cell concentration is high enough then the two pulses will be the same amplitude with a ratio of 1. The Gas Filter Correlation technique is applicable to any gas that presents adjacent harmonic spectral absorption lines. It is important to note that variations in optical clarity like dirt on cell windows, source strength, and other causes not related to the spectral lines selected, will have no effect on the RATIO of the two pulses, making this an extremely sensitive and selective analytical technique.

Gas Filter Correlation Technique

2. Single Beam Dual Wavelength Technique 

In the Single Beam Dual Wavelength technique ultra narrow band optical filters are used to select infrared wavelengths. One wavelength is at a spectral line that the gas of interest absorbs (measuring) at, and the other is one at which it does not (reference). Under a zero condition in the sample cell the two cells are alternately placed in the beam and the resultant electrical pulses are set as equal. If the gas of interest is present in the sample cell the measuring pulse is reduced in relation to the reference pulse and the ratio of the two is used to output a signal proportional to concentration in the sample cell. The MC3 analyzer can utilize either of these techniques, depending on the gas to be measured and the range of analysis. 

Single Beam Dual Wavelength Technique

3. Folded Path Sample Cell

One of the early challenges to be faced by Infrared photometers was that of the sample cell path length. That is the simple problem of trying to place enough molecules into the infrared beam to produce a usable signal. Indeed, many IR applications would have required lengths longer that 60 feet for the lower ranges of analysis, an impossible task for any practical instrument. In the late 60s sample cell developers created a technique that allows very long path lengths to be achieved in compact cells, using mirrors and a folded path technique. Simple and robust they have allowed IR photometers to routinely have paths as long as 20 meters yet remain in small envelopes. The sample cell in the MC3 is fully adjustable for optimization of each application and has reduced volume to save on maintenance and calibration gases. The standard sample cell path is 6 meter however for higher sensitivity it is possible to incorporate a 12 meter path length.

MC3 Folded Path Sample Cell

4. Construction

The MC3 is specifically designed for Continuous Emission Monitoring service and the optical bench, sample cell and control components are all integrated into one convenient, and easily serviced enclosure. In particular the integrated design allows the photometer source and detector to be housed in the same temperature controlled housing which significantly improves stability. The filter disk, holding 12 filters each is controlled by a stepper motor. The filters are near 1/2 inch in diameter and the infrared beam is less than 1/8 inch so that alignment of the filters is never critical. The solid state IR detector utilizes the exact same technology as found in the Sidewinder missile. It is located on its own subassembly but in the temperature controlled housing with the IR source and Pre-Amplifier. The MC3 analyzer is in a standard 19-inch rack and weighs 38 lbs. 

5. System PLC 

A Programmable Logic Controller (henceforth referred to as the System PLC) controls the entire EcoChem CEMS (sampling system and MC3 analyzer). A commonly used industry standard PLC is deployed. The System PLC is flexible and has to capability to accommodate various configurations. Depending upon the customers needs separate modules can be added for input/output purposes, temperature controllers, ethernet or serial communication etc. The System PLC can be easily programmed in the field using a touch-screen display. Using the System PLC we can define the set points (e.g. sampling line temperature) and other crucial parameters (e.g. time values associated with a calibration sequence) . Graphical illustrations (e.g. stripcharts, bar charts etc) of gaseous concentration levels and various system conditions are also shown on the display screen. 

6. Inputs and Outputs

The MC3 analyzer communicates with the System PLC using a serial link and Modbus protocol. The System PLC unit can be connected to the plant’s network and standard Ethernet TCP/IP can be used for communications. This is especially useful when a remote data acquisition system is employed. Additional System PLC units can be located anywhere within the plant and the only connection needed to communicate all system analog and digital signals is the plant’s existing network instead of scores of conductors and cable trays. In addition, the analyzer can be controlled via modem, Ethernet or TCP/IP. This feature can greatly simplify complex applications that translate into significant savings of installation cost. 

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Technical Specifications

Customization of the MC3 analyzer is possible by varying the sample path cell. What follow are performance specifications for what are typically the lowest ranges possible for the various gases. It is likely that any specific measurement parameter can be modified, for instance if 2% cross sensitivity can be tolerated then the lowest range can be cut in half, so please consult with EcoChem for any special needs.

Lowest Full Scale Measurement Ranges

GFC = Gas Filter Correlation      SBDW=Single Beam Dual Wavelength     ZRO=Zirconium Oxide

It should be noted that full-scale sensitivities are dependant on the water vapor content of the measured gases.  Thus when measuring with hot/wet gas and high levels of water vapor the measurement accuracy of other gases may be reduced.  It is economically and physically practical with the MC3 to use both techniques in the same CEMS.

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