The vision was simple and elegant: By shining a beam of light at a process unit or across an entire facility, a near-instantaneous measurement of gas emissions could be obtained that was accurate to parts-per-million. The instrument could replace dozens of sensors, and didn’;t incur the cost or difficulties associated with collecting a sample, taking it to a laboratory and performing a complex analysis.
In the early 1990s, that was the vision for what is now called optical remote sensing (ORS). This technology has become simpler, more reliable and more accurate, but its popularity for monitoring emissions has not grown. A new look at this technology, however, is worthwhile.
The science of ORS — as opposed to the technical applications — is well established and understood. A light beam is shined across a distance, which could be as short as from one side of an exhaust stack to another. Molecular species in the intervening space absorb light, producing a spectrograph. The original technology was based mostly on nondispersive infrared (NDIR), which simply shines IR light through a space.
One of the more popular versions nowadays is Fourier Transform Infrared (FTIR), which employs infrared light in a specific wavelength range, and mathematically manipulates the signal to first separate the molecular species that are present, and then to block out noncritical species, such as water or nitrogen.
There are various ways to set up ORS equipment. The simplest requires a transmitter on one side of the space and a receiver on the other: the “bistatic” configuration. Across long distances (up to a few hundred meters), telescopes can be used both to send and receive the signal, providing better light-gathering power. An alternative is to place a mirror on the far side, which sends the signal back to its origin, where a combined transmitter/receiver (transceiver) is located: the “monostatic” configuration. Passing the light beam twice through the area of interest improves signal resolution.
Other variations that have been developed over the years include “extractive” analysis, in which a sample is drawn from a stream and conveyed to a temperature-controlled cell. Within the cell, a light beam can be passed back and forth through the sample multiple times to obtain a reading, which improves the accuracy of the measurement.
Extractive methods, however, approach the conventional use of FTIR instruments in laboratories, and limit two of the key advantage of ORS — the ability to sense chemicals at a distance and to provide area, as opposed to point, measurements. ORS proponents talk about “fenceline” measurement — sensing whatever chemical species cross the fenceline of a plant or site.
“We’;ve used ORS technology in the past, but we decided to go away from it and instead invest in better leak-prevention equipment,” says J.D. Tate, project leader at Dow Chemical Co.’;s Freeport, Texas, facility. He goes on to say that Dow has used the technology in experimental work, and is considering its use for measuring lower-explosive-limit (LEL) concentrations in work areas and along pipelines.
ORS proponents concede that the technology has had problems being accepted. “Cost, premature commercialization, complexity of operation and data interpretation, interferences and false positives, and misuse [right instrument for the wrong application] are only several reasons for this reduced interest in ORS technologies,” Ram Hashmonay, a team leader at Arcadis, Research Triangle Park, N.C., told attendees at the NPRA Environmental and Safety Conference this fall (San Antonio, Sept. 26-28). But he notes that new technologies promise to turn this view around.
“I guess I fall in the middle of views for or against this technology,” says a senior chemist at a major oil company in Texas. “I believe that it has more applications than is currently practiced, and more companies seem to be getting interested in it.” This chemist says his company used ORS to monitor a unit emitting VOCs and to monitor combustible vapor composition in the head space of a storage vessel.
Optical remote sensing equipment
ORS equipment (left) can be used to take readings directly from the flame at the top of a flare stack. Source: IMACC
Remediation takes the lead
The bread-and-butter business for many companies developing ORS technology has been site-remediation projects, often under the Superfund program. The contractor doing the remediation needs to ensure that hazardous gas emissions occurring during the site excavation do not exceed safety limits, or that the VOC-control system at the site is capturing sufficient emissions. This application plays to the strengths of FTIR ORS systems in that the sensor provides a near-real-time response, can watch for a multitude of chemical species, and is cost-effective when true fenceline monitoring is desired. (The remediation plans for some sites only require point monitors at the corners of the site.)
Atmos Energy Corp., Dallas, is involved in such a remediation job. “Site monitoring could have been done with point monitors, but we felt that ORS was the better way to go,” says Stuart Schulz, environmental specialist for Atmos. Atmos has hired Minnich & Scotto Inc., Freehold, N.J., to perform the site monitoring. “We’;ll be monitoring for 14 specific compounds, plus particulate levels, during several weeks of excavation work,” says Timothy Minnich, partner in the company. “We’;ll be able to provide more comprehensive monitoring and faster response than point monitors, and at a lower price.” A key element of this is to be able to handle plume movements. Stationary monitors could detect a plume only if it came over their location.
According to Minnich and his partner, Robert Scotto, his company and others have been doing such site assessments for years, and have proved the technology. But it is not required for site remediation, and managers of such projects are leaving themselves open to liability threats by not employing the technology more routinely. The same holds true for many industrial applications. Companies could perform comprehensive, ongoing monitoring of their facilities, but only do so when required by a state or local regulatory authority. “We’;ve heard that some insurance companies will consider reducing liability premiums if this technology is in place, so it can literally pay for itself,” Minnich says.
Minnich and others point to the situation in Europe, where ORS technology has been applied at numerous industrial sites and where regulatory authorities require ongoing monitoring of emissions, especially near communities.
Opsis AB, Furuland, Sweden, which was founded in 1985, is a leading developer of ORS technology, called differential optical absorption spectroscopy (DOAS), which uses a range of light wavelengths and a variety of spectroscopic techniques in one unit. The European ORS scene is benefiting from a European Union effort to standardize technologies and performance specifications for ORS equipment. The lead contractor in this effort is Sira Technology Ltd., Chislehurst, England, which is managing the remote optical sensing evaluation (ROSE) program. The company is testing both types of ORS equipment and suitable applications.
U.S.-based technology development is also moving ahead. Arcadis’; Hashmonay has taken research developed at the University of Washington and, with EPA sponsorship, has developed a technology called radial plume mapping (RPM). With this technique, one ORS system can take readings along multiple paths that have a mirror at the far end of the measuring range. With a suitable tower in the vicinity, readings along different heights can also be taken. With software developed by Arcadis, a 3-D map of a plume can be produced. Hashmonay, who is also chair of the Optical Remote Sensing Division of the Air and Waste Management Association, says the technology could provide multiple benefits for leak detection, routine monitoring or site remediation for industry. “The problem with many industrial sites is that the monitors are only located where emissions are suspected; if you’;re not monitoring the entire facility, you don’;t know about unpredicted leaks or emissions,” he says.
Robert Spellicy, president of the Industrial Monitor and Control Corp., Round Rock, Texas, is pursuing work in monitoring flare systems at chemical and refining sites. Preliminary studies have shown that flares sometimes emit VOCs when not properly operated. For example, steam is injected into the flare to control soot generation, but too much steam will lower the efficiency and cause only a fraction of the gases to combust. The first phase of a study was performed last year in cooperation with URS Corp., San Francisco, and John Zink, Tulsa, Okla., but additional studies are needed to fully develop the flare application. One of the inherent advantages of ORS in this application, Spellicy says, is that it depends on the spectral emissions of the burning gases in the flare themselves; an IR or other light emitter is not needed. This allows for totally remote measurement.
A relatively new entrant on the ORS scene is Avir Remote Sensing, Charlotteville, Va., which is commercializing technology developed at the University of Virginia, in combination with U.S. Department of Defense support. The technique, called differential absorption radiometry, is a passive technique, using a receiver to sense spectral emissions by chemical agents in an area. The company is proposing its technology as a resource for first responders and homeland security applications.
Nick Basta is editor at large for Chemical Processing magazine. E-mail him at firstname.lastname@example.org.