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01 March 2003

Beat the heat

By John Hyde and Ramon Biarnes

Gas velocity measurement via infrared aids cement maker.

Gas velocity measurements in very high temperature erosive and/or corrosive applications have always proved to be extremely difficult, and all industries are looking to solve this dilemma. One way to get around that problem is to use the infrared (IR) absorption cross-correlation method.

One industry that used that method is cement manufacturing, which involves heating large amounts of raw material at up to 1,400°C within kilns. The process goes through stages, using preheaters, followed by the 3 Cs: calcining, clinkering, and cooling. This new method applies to the calcining process and the use of IR velocity measurement to monitor the production of combustible gases from tire gasification.

While this application covers the cement industry, it also works in areas such as an incineration plant, gas burner outlets, and air preheaters.

A typical cement plant will use coal, oil, or both as the primary fuel for the main kiln and/or the calciner. The company FLS miljø, a member of the FLS Industries Group in Denmark, developed a system for supplementing the primary fuel (coal) with gases generated by a tire gasification process, saving four tons of coal per hour. The gas produced by this process is primarily CO, but it may also produce significant quantities of carbon particles, depending on the control of the gasification process.

At high temperature, the gasification process creates a hot combustible gas, and because its primary use is for heating the raw material to about 900°C, the gas remains hot while traveling from the gasifier to the calciner.

A number of problems occur in this system, one significant problem being the variable amount of gas produced by the gasification process. As the process starts up and closes down, the quantity and temperature of the produced gas changes. On the FLS site, the velocity of gas in the duct ranged from < 3 meters/second under start-up conditions to > 15 meters/second at full load and temperature. Temperatures ranging from 200° to 1,200°C accompanied this.

In order to monitor the mass of produced gas, FLS brought in Epsilon Technologies' IR4 IR velocity monitor. This velocity measurement tracked the gas velocity from start-up to full load with gas temperature of 1,200°C. By monitoring velocity, temperature, and pressure, the instrument reported the mass of gas.

Time of flight

The principal of time of flight velocity measurement has been in common use for many years, and as digital signal processing is becoming more cost-effective, new applications are starting to appear.

The technique involves measuring the time it takes a known event to move through a known distance, allowing a user to calculate the velocity. Depending on the situation, the event could be a vehicle passing over pressure pads on the road or submicron particles passing through laser beams.

The problem of measuring time of flight is identifying a characteristic of the target and measuring its movement with time. You can achieve this in moving gas streams by measuring solids suspended in the gas stream or hot spots in the gas stream. If neither is present, the techniques fail.

In combustion processes, a change in the molecular makeup of the gas stream takes place. The combustion process generally involves ambient air, which is a homogeneous mix of oxygen, nitrogen, water, and very low levels of other gases, which then mixes with the fuel. This fuel is generally a hydrocarbon-based fuel but could be carbon or hydrogen.

As the fuel and air mix in the combustion zone, combustion of the fuel takes place in a turbulent manner. As a consequence of this combustion process, a number of products and by-products come about. In the case of a hydrocarbon-based fuel, the main products are heat; water and carbon dioxide; and a number of by-products generated as a consequence of imperfect combustion—namely, carbon monoxide and nitrogen dioxide. The carbon monoxide generates after incomplete combustion of the fuel due to localized oxygen deficiencies, while nitrogen dioxide comes about through the combination of atmospheric nitrogen and atmospheric oxygen in localized hot spots where excess oxygen is present.

These reactions are generally not homogeneous, due to many factors such as turbulence and fuel variation. The effect of this process is to create localized pockets or clouds of these gases. The resulting gas stream comprises a range of products such as CO₂, CO, NO, N₂, O₂, H₂O, and others. The concentration of these gases will vary with time, even on a very short time scale of a few milliseconds.

If you view the above in the IR spectrum, they absorb IR radiation by varying amounts according to the wavelength of light considered, the type of gas present, and the temperature and pressure of the gas. An analogy is the process of generating clouds of different colored gas with differing levels of translucence. As these clouds are of a nonhomogeneous nature, you can track and time them.

As you determine the distance between the detectors, you can then calculate the velocity. The significant advantage of this approach is that it measures a wide range of gas velocities with a wide range of temperatures without contact with the gas. This allows maintenance-free operation, even in high-temperature corrosive applications. It also has the advantage that with modern digital electronics, calculating the time difference is very precise and determined by the accuracy of the quartz clock. The only calibration requirement is the accurate measurement of the distance between the detectors, which should remain constant for the lifetime of the instrument.

Going optical

Considering the optical components, the cross-duct bioptic system has some distinct advantages in regard to misalignment.

With any optical system, the viewing region changes shape with distance, usually ending in a conical shape. This is unavoidable in real optical systems with aberrations, even if you focus the image of the detector at infinity. As the time of flight is between two optical viewing regions, the change in shape results in a significant change in the information received vs. distance.

The geometry of the design is such that two IR beams transmit across the full duct. This results in a full cross-stack measurement, not a side wall measurement. This also means that parallel viewing regions occur where the spacing of the transmitter and receiver window spacing sets the distance between the viewing regions. Alignment of the optics affects the signal-to-noise and not the velocity reading.

In practice, the parallel viewing regions are not achievable without a massive reduction in sensitivity—a problem when the signal is already very low.

Cross-duct cross correlation

The net result of this cross-duct, cross-correlation technique is the velocity of the bulk of the gas is measured and not the average of the path length or the point closest to the receiver. This has the effect of measuring the bulk flow of the gas within the duct and rejecting the slow-moving gas at the duct wall. In addition, the cross-correlation function provides a confidence check on the data by calculating the ratio of correlation peak height to background noise height.

In applications where turbulent flow is present, the correlation level will degenerate because the cross correlation results in multiple answers. In systems where the cross correlation performs over short periods, the result can be a very noisy output.

To overcome these problems, cross correlation's design allows for it to correlate large amounts of data at high speed. The maximum data input period for the instrument is 10 seconds, with data rates a little below 50,000 samples per second. The cross correlation (pattern match) then performs on the two data sets of 500,000 samples per channel. With modern digital signal processing, this occurs in real time and results in a continuous output from the instrument. The output gets time delayed by up to 10 seconds but then updates many times per second. FT

Behind the byline

John Hyde is founder of Yortech Instruments, and Ramon Biarnes is president of Epsilon Technologies.