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Our Portfolio: Monitoring Projects
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Continuous Emissions Monitor of Dioxins
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Information
Resources
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TTP#:
FT0-6-IP01; Tech ID#: 2305
Project Overview
The objective of this project is to develop a laboratory instrument which can provide
continuous monitoring of the emission levels of polychlorinated dibenzo-p-dioxins
(PCDDs) and polychlorinated dibenzofurans (PCDFs) generated by incineration
equipment at DOE sites. Once developed this instrument will be used to
systematically study the emission levels of key dioxins and furans that contribute
to Toxic Equivalence (TEQ). This information, combined with mechanistic modeling
studies being undertaken elsewhere, will lead to the design specifications for a
real-time, autonomous dioxin Continuous Emissions Monitor (CEM) that can be
used for compliance monitoring at DOE incinerators. Experts in this area suggest
that a dioxin CEM should first be used as a research tool in laboratories studying
dioxin formation and control. As such, the instrument must make rapid, accurate
measurements of dioxins but at concentrations much higher than needed for a
compliance CEM. This type of instrument will greatly accelerate our understanding
of dioxin formation and the availability of prevention and control techniques. A
real-time CEM will provide immediate feedback on how variations in combustion
operating parameters affect dioxin formation and/or destruction, thus allowing
more accurate correlations and much more comprehensive data analysis. Although
210 different dioxins can be produced during combustion, fewer than 20 are toxic
enough to warrant monitoring. Once developed, the proposed instrument will be
used to study the emission levels of these key dioxins, leading eventually to an
improved understanding of the formation of these molecules and to improved
means of monitoring and control. As our understanding of dioxin formation
improves, we will build a database using emissions from actual waste treatment
processes to correlate operating conditions with dioxin formation. This database
can also be used to identify surrogates or indicators that can be monitored more
easily and cheaper than the dioxins themselves, leading to less expensive, more
widely implemented, compliance and control strategies. CEMs also provide data
important for stakeholders' assurance that the combustion processses are
operating safely. Stakeholders such as public interest groups, permit writers, and
local citizens groups, can play a major role in permitting waste treatment facilities.
Real-time emissions data may accelerate their acceptance, saving time and
money during the permitting process.
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Technology Description
SRI International is developing a real-time, Continuous Emission Monitor capable of
detecting selected dioxin and furan congeners at levels appropriate for studying
formation mechanisms and control strategies. Once developed, the proposed
instrument will be used to study the emission levels of these key dioxins, leading
eventually to an improved understanding of the formation of these molecules and to
improved means of monitoring and control. The approach for an instrument with
ultra-trace sensitivity combines optical spectroscopy in the form of resonance
enhanced multi-photon ionization (REMPI) enhanced with a pulsed gas jet, and
time-of-flight mass spectroscopy (TOFMS). It is estimated that a minimum
detectability of approximately 0.5 ppt (1 ng/m3)
can be realized for dichlorinated
dioxins. The phenomenal sensitivity and selectivity afforded by jet-REMPI is
directly attributable to the combination of three main components; a pulsed
gas-jet, resonance enhanced multi-photon ionization, and a mass spectrometer.
All molecules could theoretically exhibit a unique optical absorption signature
associated with the specific nature of their electronic, vibrational, and rotational
energy levels. Under normal room temperature conditions, however, much of this
uniqueness is lost due overlapping spectroscopic structure, and absorption spectra
alone cannot be used to provide chemical discrimination. In the jet-REMPI
instrument, this limitation is overcome by cooling the molecules in a free jet
expansion to within a few degrees of absolute zero. As a result, a narrow bandwidth
tunable laser source can yield very high selectivity while simultaneously producing
positively-charged ions whose molecular weight can be measured by mass
spectrometry. In the event that molecular species other than those of interest
coincidentally absorb the laser radiation and become ionized, they will almost
certainly have a different molecular weight, and hence be separable by the mass
spectrometer. This ultra-sensitive chemical detector system uses as pulsed gas
valve to introduce sample vapors into the analyzer. Pulsed gas valves provide a
number of advantages over continuous gas inlets, including reduced gas flow and
hence smaller vacuum pumps, higher local gas densities, well-defined spatial
distribution, significantly reduced translational energy distribution orthogonal
to the propagation direction, and reduced internal (translational and rotational)
temperatures leading to enhanced spectroscopic resolution. Pulsed valves can
operate routinely at temperatures above 250°C to accommodate the low
volatility of many of the compounds of interest, such as the heavier dioxin
congeners and explosives. REMPI is used as an efficient and highly selective
ionization method. In the REMPI process, a molecule is raised from its ground state
to its first excited state by one photon, and subsequently ionized by a second
photon. Selectivity is provided primarily by the resonance of the first photon with
the excited state. Time-of-flight mass spectrometry is the current method of
choice for use with REMPI because it is an inherently pulsed technique that
matches well with both the pulsed gas inlet, and the pulsed laser ionization. The
mass spectrometric requirements are modest, and can readily be achieved with
either commercially available devices, or simple, custom components designed to
reduce size and weight for a transportable field instrument.
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