Method and apparatus for monitoring mercury emissions

A mercury monitoring device that continuously monitors the total mercury concentration in a gas. The device uses the same chamber for converting speciated mercury into elemental mercury and for measurement of the mercury in the chamber by radiation absorption techniques. The interior of the chamber is resistant to the absorption of speciated and elemental mercury at the operating temperature of the chamber.

FIELD OF THE INVENTION 
The present invention generally relates to the monitoring of mercury in a 
gas stream and more specifically to the continuous monitoring of speciated 
and/or elemental mercury in a gas stream. 
BACKGROUND OF THE INVENTION 
Emissions of elemental mercury (e.g., nonspeciated mercury) and many 
mercury-containing compounds (e.g., speciated mercury) are strictly 
regulated in many countries because of significant environmental hazards. 
Major sources of mercury emissions are waste gases from fossil fuel 
combustion facilities and municipal solid waste incinerators. 
Atomic absorption of ultraviolet radiation is a common method of monitoring 
mercury levels in waste gases to insure compliance with existing 
environmental regulations. This technique, however, is only capable of 
detecting elemental mercury. It is incapable of detecting speciated 
mercury, which is often a contaminant in industrial waste gases. Unlike 
elemental mercury, speciated mercury absorbs radiation at a broad range of 
wavelengths and is therefore, indistinguishable from other nonmercury 
compounds in the gases. Speciated mercury also does not absorb radiation 
having a wavelength of 2537 angstroms, which is the primary wavelength at 
which elemental mercury absorbs radiation. 
Before the total mercury concentration in waste gases can be effectively 
measured by atomic absorption techniques, the speciated mercury is first 
converted into elemental mercury. After conversion of speciated into 
elemental mercury but before mercury measurement, the gas is cooled to 
condense and remove water vapor and the cooled gas is introduced into an 
atomic absorption instrument for elemental mercury detection and/or 
measurement. 
Four methods are commonly employed to convert speciated into elemental 
mercury for mercury detection by ultraviolet radiation absorption 
techniques. In one method, the waste gas is contacted with a noble metal, 
such as gold or silver, to form an amalgam containing elemental mercury. 
After removal of the waste gas, the noble metal is heated to above 
350.degree. C. to release the elemental mercury as a vapor. This method is 
a batch sampling technique and therefore is unable to provide real-time 
mercury measurements for emissions control. The method can yield 
inaccurate measurements, especially at the low mercury (e.g., 1 ppb) 
concentrations common in most waste gases. Such inaccuracy can result from 
the failure of speciated and/or elemental mercury in the waste gas to 
contact the noble metal. In another method, the gas is heated to a 
temperature in excess of 900.degree. C. in the presence of a reducing 
agent to break the molecular bonds between the mercury atoms and the other 
elements in the speciated mercury. A reducing agent is any substance that 
will form a compound with the released nonmercury elements to prevent them 
from recombining with the elemental mercury. The reducing agent can be a 
liquid, such as stannous chloride, or sulfuric acid, or solid, such as 
activated carbon in the presence of hydrogen chloride. Typical reducing 
agents are metals from Groups I (e.g., copper, silver, and gold), II 
(e.g., zinc and cadmium) and IV (e.g., lead and tin) of the Periodic Table 
of Elements. At temperatures above 900.degree. C., noble metals such as 
gold and silver will not form amalgams with mercury but act as reducing 
agents. The use of a reducing agent to remove elements that can combine 
with elemental mercury to form speciated mercury is especially important 
when the gas is cooled to condense and remove water vapor before mercury 
measurement. In yet another method, a solid is used to absorb the mercury 
in a gas sample, the solid is decomposed chemically to release the mercury 
as a vapor, and the mercury vapor is measured. The mercury emission is 
calculated from the amount of mercury vapor, with the gas sample volume 
being known. Iodized activated carbon is particularly suitable for use as 
the solid. Finally, in the last method, the speciated mercury is thermally 
decomposed at high temperatures of around 800.degree. C. A reverse 
reaction of the elemental mercury with hydrogen chloride in the waste gas 
to form speciated mercury must be excluded by first removing the mercury 
chloride from the waste gas. The hydrogen chloride is removed by an 
adsorption reaction with lime or silica gel before thermal decomposition. 
All of the methods for converting speciated into elemental mercury require 
periodic maintenance and/or replacement of components to provide as 
reliable mercury measurements as possible. For example, in the absence of 
such maintenance and/or replacement, the noble metal or activated carbon 
will degrade over time and lose the ability to adsorb mercury, the 
reducing agent will lose the ability to adsorb the released nonmercury 
elements, and the lime or silica gel will lose the ability to adsorb the 
hydrogen chloride. The periodic maintenance and/or replacement of the 
noble metal, activated carbon, reducing agent, and lime or silica gel can 
significantly increase operating costs through down time and material 
replacement costs. 
SUMMARY OF THE INVENTION 
It is an objective of the present invention to provide a low maintenance 
system for accurate and reliable monitoring of speciated and/or elemental 
mercury concentrations in a gas. 
It is a further objective to provide a method and apparatus for 
continuously monitoring the presence of speciated and/or elemental mercury 
in a gas. It is a related objective to provide real-time speciated and/or 
elemental mercury measurements for a gas to enable more effective control 
of mercury emissions and compliance with environmental regulations. 
It is a further objective to provide a mercury monitoring method and 
apparatus that does not have a component, such as a noble metal or 
reducing agent, that requires periodic replacement. It is a related 
objective to provide a mercury monitoring method and apparatus that does 
not employ an amalgam or reducing agent. 
The present invention addresses these and other objectives in a device for 
monitoring the presence of mercury in a gas containing speciated and/or 
elemental mercury. The device includes a gas handling system to collect a 
sample of the gas (e.g., provide a gas stream) for mercury monitoring and 
a heating means positioned to heat the gas handling system. The heating 
means not only increases the gas stream temperature to an operating 
temperature sufficient to convert the speciated mercury in the gas stream 
into elemental mercury but also maintains the gas stream at a sufficiently 
high temperature to prevent the elemental mercury from returning to the 
speciated form. The operating temperature is preferably no less than about 
700.degree. C. and more preferably ranges from about 900.degree. C. to 
about 1000.degree. C. The gas handling system preferably includes separate 
inlet and body portions for the conversion of speciated into elemental 
mercury, a containment portion for the passage of radiation through the 
gas stream, and an outlet portion for discharging the irradiated portion 
of the gas stream. Radiation is preferably passed through the gas stream 
only in the containment portion and not in the inlet and body portions of 
the gas handling system where conversion of speciated mercury into 
elemental mercury is occurring. The inlet, body and containment portions 
of the gas handling system are substantially resistant to speciated and 
elemental mercury absorption at temperatures above the gas temperature 
(e.g., which is preferably no less than about 900.degree. C.) and the 
operating temperature to inhibit mercury collection. Accordingly, the 
interiors of the inlet, body and containment portions are substantially 
free of noble metals, such as gold and silver, and mixtures thereof and 
other mercury adsorbing compounds, such as activated carbon. 
The containment portion of the gas handling system can be housed within the 
body portion. In that event, the radiation means is positioned to transmit 
radiation through the containment portion but not the inlet and body 
portions. The gas stream flows through an area between the body and 
containment portions (where speciated mercury is converted into elemental 
mercury) and then into the containment portion for mercury measurement. 
For mercury detection, the device includes radiation means for transmitting 
radiation through the gas stream in the containment portion of the gas 
handling system and radiation sensitive means for receiving the unabsorbed 
portion of the radiation and providing a signal for use in detecting 
mercury in the gas stream. The radiation means includes a light source 
that can emit radiation having a wavelength of 2537 angstroms. The light 
source can be a mercury vapor lamp. 
Where the gas contains mercury and radiation absorbing materials, such as 
nitrous oxides and sulfur oxides, the device can include a means for 
forming a magnetic field along the path of the radiation to provide a 
plurality of radiation wavelengths and/or means for selecting a radiation 
wavelength to determine only that amount of radiation absorbed by the 
elemental mercury and not by other radiation absorbing materials. The 
magnetic means produces a first portion of the radiation having a first 
wavelength at which elemental mercury and the radiation absorbing 
materials absorb radiation and a second portion having a second wavelength 
at which the radiation absorbing materials but not elemental mercury 
absorbs radiation. The wavelength selecting means passes at least one of 
these wavelengths through the containment portion for absorption by the 
gas stream components. The wavelength selecting means can include a 
rotating body having one or more openings capable of selectively passing 
only the selected radiation wavelength(s) into the gas stream. 
Where the gas to be sampled contains both speciated and elemental mercury, 
the device can include a second gas handling system for mercury 
measurement to determine the relative concentrations of speciated and 
elemental mercury in the gas before conversion of speciated into elemental 
mercury. The second gas handling system includes a second inlet portion 
and second containment portion for a second gas stream (taken from the gas 
to be sampled) and is at a temperature less than the converter operating 
temperature. A beam splitting means is used to direct a first radiation 
portion from the radiation means through the containment portion and a 
second radiation portion through the second containment portion. A second 
radiation sensitive means receives the unabsorbed second radiation portion 
and provides a second signal for use in determining the amount of 
speciated or elemental mercury in the gas. 
The above-noted device is capable of continuous monitoring of the mercury 
levels in the gas stream. Relative to prior art mercury detection devices, 
the abilities of the present invention to continuously provide real-time 
feedback of mercury levels in the gas stream enables more efficient 
control of mercury removal devices and therefore facilitates compliance 
with environmental regulations. 
The device's conversion of speciated mercury into elemental mercury in the 
gas handling system and maintenance of the gas handling system at a high 
enough temperature to prevent speciated mercury from reforming eliminates 
the need for a mercury absorbing material or a reducing agent. The 
elimination of these materials enhances the long-term performance of the 
device and significantly reduces operating costs relative to prior art 
mercury detection devices. Unlike prior art mercury detection devices, 
there is no need to periodically replace such materials to maintain 
detector efficiency. 
Unlike prior art detectors, the device does not cool the gas before mercury 
measurement as cooling of the gas can cause elemental mercury to condense 
or collect onto the walls of the device. Such mercury losses can 
significantly alter the mercury measurement results. 
The present invention further provides a method for monitoring the presence 
of mercury in a gas containing speciated mercury. The method includes the 
steps: (i) continuously introducing a gas stream into the gas handling 
system during a selected time interval; (ii) maintaining, during said time 
interval, the inlet and containment portions of the gas handling system at 
an operating temperature sufficient to convert the speciated mercury into 
elemental mercury while the speciated mercury is in the gas stream and 
maintain the mercury in the elemental state; (iii) transmitting radiation 
through the containment portion during at least a portion of the time 
interval, wherein the elemental mercury absorbs a portion of the 
radiation; (iv) receiving the unabsorbed portion of the radiation with a 
radiation sensitive device; and (v) using the received radiation to 
generate a signal for use in monitoring the presence of mercury in the gas 
stream. 
In the introducing step, the gas stream preferably flows through the 
chamber at a substantially constant rate during the time interval. The 
average residence time of the gas stream in the inlet portion is no less 
than about 2 seconds to ensure complete conversion of speciated into 
elemental mercury before the gas stream enters the containment portion. 
To account for the presence of radiation absorbing materials other than 
elemental mercury in the gas stream, the transmitting step can include the 
substeps: (i) passing first radiation having a wavelength of 2537 
angstroms through the heated gas stream, wherein the radiation absorbing 
material and the elemental mercury collectively absorb a first portion of 
the first radiation and (ii) passing second radiation having a wavelength 
different from 2537 angstroms through the heated gas stream, wherein the 
radiation absorbing material absorbs a second portion of the second 
radiation. The first portion is compared with the second portion to 
determine the amount of radiation absorbed by the elemental mercury. The 
first passing and second passing steps are repeated over successive time 
periods during the time interval to generate a plurality of signals 
representing alternatively the unabsorbed portions of the first radiation 
or second radiation received during each time period. The appropriate 
unabsorbed portions are used to determine the amounts of elemental mercury 
in the gas stream. 
To control a mercury removal device, the method can include the step of 
comparing the signal to a predetermined value for the signal. Based 
thereon, an appropriate control signal is provided to the mercury removal 
device. For example, the signal could correspond to a specific mercury 
concentration in the gas stream and the predetermined value would 
correspond to an appropriate amount of mercury sorbent to be released into 
the gas for different mercury concentrations.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
A mercury monitoring device is provided that can continuously determine the 
concentrations of speciated and/or elemental mercury in a gas. The mercury 
monitoring device is particularly useful in monitoring the presence of 
mercury in waste gases, such as from fossil fuel combustion facilities and 
municipal solid waste incinerators. The mercury monitoring device has a 
mercury measurement sensitivity below about 1 .mu.g/m.sup.3 (less than 
about 0.1 ppb v/v). The device has a linear response to mercury 
concentrations greater than about 100 .mu.g/m.sup.3. 
FIG. 1 illustrates the various components of a preferred embodiment of the 
mercury monitoring device. The device includes a gas handling system 
having an inlet portion 10, body portion 12, containment portion 18, and 
outlet portion 14 for a gas stream (which is typically a split of the gas 
to be monitored) and a substantially cylindrical containment portion 18 in 
communication therewith. The containment portion 18 is contained within 
the body portion 12 for improved conversion of speciated to elemental 
mercury before measurement. Heating means 22 surrounds the inlet, body, 
and containment portions and heats them to a sufficient temperature to 
convert speciated into elemental mercury and maintain the mercury in the 
elemental state. Radiation means 26, magnetic device 54, collimator 42, 
and means 46 for selecting the wavelength of the radiation are at one end 
of the containment portion 18 and cooperate to transmit radiation 32 of a 
selected wavelength through the containment portion 18 for mercury 
detection. The radiation 32 is substantially parallel to the longitudinal 
axis of the containment portion 18. The portion of the radiation not 
absorbed by the various gas stream components is received by a radiation 
sensitive means 34 positioned at the other end of the containment portion 
18. The radiation sensitive means 34 provides a signal for use in 
detecting the presence of mercury in the gas stream and/or in determining 
the concentration of at least one of speciated mercury, elemental mercury, 
or both in the gas stream. 
The inlet and body portions preferably have dimensions sufficient to 
provide a residence time for the gas stream of at least about 2 and more 
preferably at least about 5 seconds. The diameters and lengths of the 
inlet and body portions in the heating means 22 are sufficient for 
substantially complete conversion of the speciated mercury in the gas 
stream into elemental mercury. To facilitate the conversion of speciated 
to elemental mercury in the body portion, the body portion can include a 
heat transfer means to cause an increased transfer of thermal energy to 
the gas stream. The heat transfer means can be any substance that provides 
increased surface area and adequate levels of heat transfer such as glass 
frit, glass chips, glass wool, ceramics, metal chips or wool, or other 
substances that are substantially nonreactive with mercury. 
The inlet and outlet portions 10, 14 are positioned at the same end of the 
body and containment portions to provide a desired residence time in the 
gas stream in the body portion 12. The inlet and body portions 10, 12 of 
the gas handling system preferably have a length from the inlet of the 
inlet portion to the inner inlet 16 sufficient to provide the residence 
times referred to above before mercury measurement. 
The inlet, body and containment portions are substantially resistant to the 
absorption of speciated and elemental mercury at the gas stream 
temperature and the operating temperature to provide for accurate 
measurements. Accordingly, the inlet, body, and containment portions are 
substantially free of mercury absorbing materials, such as gold, silver, 
and other noble metals. 
The heating means 22 is any suitable heating device that can heat the inlet 
portion 10 and the containment portion 18 to an operating temperature that 
is sufficient to convert the speciated mercury into elemental mercury. 
Preferably, the operating temperature is a temperature of no less than 
about 700.degree. C. and more preferably ranging from about 900.degree. C. 
to about 1000.degree. C. 
The radiation means 26 is a light source that can produce radiation having 
a wavelength of 2537 angstroms. As noted above, elemental mercury only 
absorbs radiation having this specific wavelength. Preferred light sources 
include a mercury vapor lamp. 
Where the gas stream contains elemental mercury and other radiation 
absorbing materials, such as nitrous oxides and sulfur oxides, a plurality 
of wavelengths must be transmitted through the heated gas stream to 
provide the total mercury concentration. Comparison of the amounts of 
radiation absorbed at the various radiation wavelengths provides the 
amount of radiation absorbed by the elemental mercury alone and, 
therefore, the elemental mercury concentration in the containment portion 
18. Because the strength of the signal is proportional to the amount of 
radiation absorbed by the various gas stream components, to calculate the 
total mercury concentration the strength of a first signal is determined 
for a first portion of radiation having a wavelength of 2537 angstroms (at 
which the elemental mercury and other radiation absorbing materials absorb 
the radiation), a strength of a second signal is determined for a second 
portion of radiation having a wavelength other than 2537 angstroms (at 
which the other radiation absorbing materials but not elemental mercury 
absorb radiation), and the strengths of the first and second signals are 
compared to determine the amount of the first portion of the radiation 
absorbed only by the elemental mercury. Based on this amount and the 
length of the containment portion, it is a straightforward mathematical 
computation to determine the total mercury concentration in the gas 
stream. 
To produce the first and second radiation portions, the present invention 
uses means 54 for forming a magnetic field along the path of the radiation 
30 and the wavelength selecting means 46. The magnetic means 54 provides a 
plurality of selected wavelengths in the radiation 30. As will be 
appreciated, through the Zeeman effect, the magnetic field will provide 
altered radiation 30 having three components, a middle component having 
the wavelength of the radiation emitted by the light source (i.e., 2537 
angstroms) and plane polarized to vibrate in a direction parallel with the 
magnetic field (i.e., vertical) and two side components having wavelengths 
different from the wavelength of the middle component (which will not be 
absorbed by elemental mercury) and plane polarized to vibrate in a 
direction normal to the magnetic field (i.e, horizontal). The radiation 
means 26 is located at one end of the containment portion 18 adjacent to a 
substantially transparent containment portion surface 38 for transmission 
of the radiation 32 into the containment portion 18. 
To pass radiation of a selected wavelength through the containment portion 
surface 38, the wavelength selecting means 46 is located between the 
radiation means 26 and containment portion 18. The wavelength selection 
means 46 includes a modulator 58, such as a Pockels cell and a polarizer 
62. The Pockels cell rotates the linearly polarized output of the 
radiation means in a strong magnetic field by 90.degree. or 0.degree., 
depending upon the bilevel input voltage to the cell. During a given time 
interval, the modulator 58 rotates the plane of polarization of the 
radiation component to be selectively passed by the polarizer 62 to a 
vertical orientation, and the polarizer 62 passes the component having the 
vertical plane of polarization. By way of example, at the lower voltage 
level, the Pockels cell will not rotate the plane of polarization and the 
polarizer 62 will pass the middle component (but not the side components) 
and at the higher voltage level the cell will rotate the plane of 
polarization by 90.degree. and the polarizer will pass the side components 
(but not the middle component). The polarizer 62 will block the other 
component(s) which vibrate in the plane normal to the vertical plane of 
polarization. Thus, the middle and side components are alternately passed 
by the polarizer 62 through the containment portion. 
A collimator 42 provides radiation 36 having a narrow range of angular 
orientations for the wavelength selection means 46. The collimator 42 is 
any suitable optical device, such as a lens, for increasing the radiation 
intensity. 
The radiation sensitive means 34 is any suitable detector capable of 
providing a signal in response to the received radiation 40 that is not 
absorbed by the various components of the gas stream. The radiation 
sensitive means 34 is positioned relative to the radiation means 26 (e.g., 
along the longitudinal axis of the containment portion 18) so as to 
receive the radiation 40 that is not absorbed by the various components of 
the gas stream. As in the case of the radiation means 26, the radiation 
sensitive means 34 is located at one end of the containment portion 18 
adjacent to another substantially transparent containment portion surface 
50 to enable the radiation sensitive means to receive the unabsorbed 
radiation 40. 
The positioning of the radiation means 26 and radiation sensitive means 34 
at opposite ends of the containment portion 18 eliminates the need for a 
reducing agent in the containment portion 18. Unlike prior art mercury 
detectors which convert speciated mercury into elemental mercury in one 
vessel and cool the gas stream as it is transported to a separate vessel 
for mercury measurement, the mercury monitoring device of the present 
invention performs the conversion and measurement in the inlet portion 10 
and body portion 12 and does not cool the gas stream before measurement. 
Because the gas stream is not cooled, the elemental mercury will not 
recombine with the released elements before measurement. Accordingly, it 
is unnecessary to remove the released elements from the gas stream through 
the use of a reducing agent. 
By eliminating the cooling of the gas prior to elemental mercury 
measurement, the present invention overcomes the need for removing 
hydrogen chloride and other chloride-containing compounds from the gas to 
prevent the reverse reaction of the elemental mercury with the hydrogen 
chloride to form speciated mercury. Referring to FIGS. 14 and 15, the 
equilibrium concentrations of mercuric chloride, elemental mercury, and 
hydrogen chloride are shown as a function of temperature. At a temperature 
above about 800.degree. C., the percentage of the total mercury in a gas 
that is mercuric chloride rapidly decreases relative to the amount that is 
elemental mercury and hydrogen chloride. FIGS. 14 and 15 may overestimate 
the amount of mercuric chloride because the figure fails to reflect 
reaction kinetics. Accordingly, the mercuric chloride concentration at the 
converter operating temperature can be significantly less than the 
concentration given in FIGS. 14 and 15. In any event, it has been 
discovered that maintaining the gas at a temperature of preferably no less 
than about 700.degree. C., and more preferably no less than about 
900.degree. C., during mercury measurement provides a high degree of 
accuracy in measuring total mercury concentration. 
FIGS. 2 and 3 illustrate the use of the above-described mercury monitoring 
device to monitor the effectiveness of and/or to control the operation of 
a mercury removal means 66. FIG. 2 illustrates the use of a mercury 
monitoring device 70 with the inlet portion 10 positioned downstream of 
the mercury removal means 66 (which typically is a device using either a 
sorbent or noble metal or wet or dry scrubber for mercury removal) to 
monitor the effectiveness of and/or to control the operation of the 
mercury removal means 66. FIG. 3 illustrates the use of a mercury 
monitoring device 70 with the inlet portion 10 positioned upstream of the 
mercury removal means 66 to control the operation of the mercury removal 
means 66. In either case, the signal represents either the presence of 
elemental or speciated mercury in the gas or the concentration of at least 
one of elemental and speciated mercury, or both, in the gas. To control 
the operation of the mercury removal means 66, the signal from the mercury 
monitoring device is received by a computer 74 which analyzes the signal 
and provides a control signal to the mercury removal means 66. To analyze 
a signal corresponding to the concentration of elemental and/or speciated 
mercury, the computer 74 can compare the signal to one or more 
predetermined values for the signal and generate an appropriate control 
signal based thereon. For example in the case of a mercury removal means 
66 that releases a mercury sorbent into the gas, the strength of the 
control signal would fluctuate depending upon the concentration of 
elemental and/or speciated mercury in the gas. In this manner, an amount 
of sorbent is released into the gas that is related to the actual 
concentration of elemental and/or speciated mercury in the gas, thereby 
reducing sorbent consumption and the operating costs of the mercury 
removal means 66. 
The operation of the mercury monitoring device 70 of the present invention 
will now be described as applied to a gas containing not only speciated 
mercury but also other radiation absorbing materials, such as nitrous 
oxides and sulfur oxides, that absorb radiation over a broad spectrum of 
wavelengths including a wavelength of 2537 angstroms. 
Referring to FIGS. 1, 2 and 3, a gas stream, which is a split of a gas 78 
flowing in a conduit 82, continuously flows over a selected time interval 
through the gas handling system (i.e., the inlet portion 10, the body 
portion 12 through (i.e., the annulus between the body and containment 
portions) the containment portion 18, and the outlet portion 14) and is 
returned to the gas 78 in the conduit 82. The time interval is selected 
based on the desired period over which mercury detection is to be 
conducted. Typically, the time interval is coterminous with the duration 
of the gas flow through the conduit 82. Preferably, the gas stream flows 
into the containment portion 18 at a substantially constant rate during 
the selected time interval. 
The inlet portion 10, body portion 12, and containment portion 18 are 
maintained, during the time interval, at an operating temperature 
sufficient to convert the speciated mercury into elemental mercury and 
maintain the mercury in the elemental state. The gas stream is heated to 
the operating temperature while passing through the inlet portion 10 and 
body portion 12 and maintained at the operating temperatures in the 
containment portion 18. The speciated mercury is converted into elemental 
mercury in the gas phase in the inlet portion 10 and body portion 12. 
Radiation 32 of differing wavelengths is transmitted through the 
containment portion 18 at selected time intervals with a portion of the 
radiation being absorbed by the elemental mercury and a portion by other 
radiation absorbing materials. As noted above, to account for the other 
radiation absorbing materials present in the gas stream (besides elemental 
mercury), the transmission of the radiation 32 is done in a series of 
steps: (i) in a first time interval first radiation 32 having a wavelength 
of 2537 angstroms is passed through the heated gas stream with a first 
portion of the first radiation being absorbed by the radiation absorbing 
materials, including the elemental mercury, and (ii) in a second time 
interval second radiation 32 having a wavelength different from 2537 
angstroms is passed through the heated gas stream with a second portion of 
the second radiation being absorbed by the radiation absorbing materials 
other than the elemental mercury. For continuous mercury monitoring, the 
two steps are repeated over successive time periods. Preferably, radiation 
32 is transmitted through the containment portion 18 substantially 
continuously (i.e., switching occurs at least 300 times every second). 
The unabsorbed portion 40 of the radiation is received by the radiation 
sensitive means 34 and the received portion of the radiation is used to 
generate a signal for use in monitoring the presence and/or concentration 
of elemental and/or speciated mercury in the gas stream and therefore the 
gas 78. The strength of the signal is proportional to the amount of 
unabsorbed radiation which is in turn proportional to the amount of 
radiation absorbing materials, including elemental mercury, in the gas 
stream. The first portion referred to above generates a first signal 
having a strength proportional to the amount of radiation absorbing 
materials, including elemental mercury, in the gas stream, and the second 
portion generates a second signal having a strength proportional to the 
amount of radiation absorbing materials excluding elemental mercury. To 
determine the concentration of speciated and elemental mercury, the 
relative strengths of the first and second signals are compared as 
described above. 
The signal is used to generate a control signal to control the operation of 
the mercury removal means 66. The control signal is generated by comparing 
the signal to predetermined values for the signal that correspond to 
different speciated and/or elemental mercury concentrations. 
As will be appreciated, the mercury monitoring device of the present 
invention can alternatively be used as a batch sampling system. 
In alternative embodiments, the mercury monitoring device has other 
components and/or is in other configurations. The alternate embodiments 
are sometimes preferable in specific applications. 
In another embodiment, the mercury monitoring device does not include a 
body portion in the gas handling system. The gas handling system has the 
inlet portion 10, containment portion 18, and outlet portion 14. To 
provide a sufficient length for the inlet portion 10 to substantially 
completely convert the speciated mercury in the gas stream into elemental 
mercury, the inlet portion 10 can be coiled, as shown, around the 
containment portion 18 in the heating means 22. 
The inlet and outlet portions 10, 14 are positioned at opposite ends of the 
containment portion to provide the desired residence time of the gas 
stream in the containment portion 18. The containment portion 18 of the 
gas handling system preferably has a distance between the input and output 
portions sufficient to provide the residence times noted above in the 
containment portion 18 before mercury measurement. 
The inlet portion 10 is positioned transversely to the containment portion 
18 to ensure complete conversion of the speciated mercury in the gas 
stream to elemental mercury before the gas stream is irradiated. If the 
inlet and containment portions had the same longitudinal axis, the mercury 
measurement could be inaccurate due to the presence of speciated mercury 
along the path of the radiation. 
In operation, the gas stream continuously flows through the inlet portion 
10, the containment portion 18, and the outlet portion 14, and is returned 
to the gas 78 in the conduit 82. The inlet portion 10 and containment 
portion 18 are maintained, during the time interval, at an operating 
temperature sufficient to convert the speciated mercury into elemental 
mercury and maintain the mercury in the elemental state. The gas stream is 
heated to the converter operating temperature while passing through the 
inlet portion 10 and maintained at the operating temperature in the 
containment portion 18. Radiation 32 of differing wavelengths is 
transmitted through the containment portion 18 at selected time intervals, 
as described above. 
In another alternative embodiment shown in FIG. 5, the device has a second 
gas handling system including a second inlet portion 88 and second outlet 
portion 92 communicating with a second containment portion 86 for 
determining the concentration of speciated mercury or elemental mercury in 
the gas stream. The second containment portion 86 is at a temperature less 
than the temperature of the first containment portion 18. As desired, a 
second heating means 90 can be included to maintain the temperature of the 
second containment portion 86 at the temperature of the gas in the conduit 
82. For flue gases, the second containment portion 86 is preferably 
maintained at a temperature ranging from about 200.degree. C. to about 
400.degree. C. Beam splitting means 96 directs a first radiation portion 
100 through the first containment portion 18 and a second radiation 
portion 104 through the second containment portion 86. A mirror 108 
reflects the second radiation portion 104 through the second containment 
portion 86. A second radiation sensitive means 112 receives the unabsorbed 
component of the second radiation portion and provides a second signal for 
use in determining the amount of elemental mercury in the second gas 
stream. The second signal corresponds to the elemental mercury 
concentration in the gas 78 and can be compared with the signal for the 
unabsorbed first radiation portion (which represents the total mercury 
concentration) to determine the concentration of speciated mercury in the 
gas 78. This embodiment is particularly useful where the mercury removal 
means 66 is capable only of removing speciated or elemental mercury but 
not both. If other radiation absorbing materials besides elemental mercury 
are present, the first and second radiation portions can have their 
wavelengths sequentially altered as discussed above. 
In other embodiments, the modulator 58 can be a means for changing the 
wavelength of the radiation using the modulator 200 shown in FIG. 6. The 
modulator 200 is a 1/2-waveplate 202 (i.e., a quartz plate with a special 
optical coating) centered in a suitable housing 204. When the waveplate 
optical axis is displaced 0.degree. from the vertical component of the 
magnetic field from the magnetic means 54 , no polarization rotation 
occurs. At an angle of 45.degree., the polarization is rotated by 
90.degree.. Calculations predict that the behavior at any angle of 
rotation is given by the equation y=a.sub.1 cos.sup.2 2.theta.+a.sub.2 
sin.sup.2 2.theta., where .theta. is the angle between the vertical 
component of the magnetic field and the optical axis of the waveplate and 
a.sub.1 and a.sub.2 reflect the relative magnitudes of the absorption 
coefficients for the two wavelength components. It is preferred that the 
waveplate be rotated at a rate ranging from about 350 to about 370 rpm. 
It has been discovered that the radiation output from the modulator 200 is 
a mixture of the middle and side components, depending upon the angle 
between the vertical magnetic field lines and the optical axis of the 
waveplate. The radiation output is therefore not as distinct as the 
Pockels cell referred to above, which produces radiation composed 
substantially entirely of either the side or middle components. An analog 
electronics circuit permits the elemental mercury concentration to be 
determined from the mixed wavelength radiation components that are output 
from the modulator 200 and passed through the polarizer 62. 
Thus, the rotating waveplate is the exact analog of the Pockels cell, but 
it achieves polarization rotation through mechanical rotation rather than 
electronic means as with the Pockels cell. 
In another embodiment, the wavelength selecting means can exclude the 
polarizer 62 and modulator 58. To form radiation of different wavelengths, 
the magnetic device 54 can be alternatively switched on and off. This 
embodiment has the disadvantage that it is difficult to shield the 
magnetic field produced by the magnetic device from interfering with the 
circuitry of the mercury monitoring device. 
EXPERIMENT 1 
A test was conducted using the dual chamber mercury detection device to 
determine the concentration of elemental and speciated mercury in a gas 
stream. Two mercury sources were used during the testing: (i) elemental 
mercury and (ii) mercuric chloride (e.g., HgCl.sub.2). A permeation tube 
was used to generate elemental mercury, and a diffusion vial was 
fabricated to generate mercuric chloride vapor. The sources were 
calibrated by periodically weighing them using an analytical balance. 
Constant permeation rates were obtained after a suitable stabilization 
period. It is widely believed that elemental mercury and mercuric chloride 
are the two most predominant species in flue gases generated from coal 
combustion. 
The flow rate of the test gas was approximately 800 cc/min. The 
concentration of the mercury sources was varied over time and ranged from 
1 to 10 ppb v/v. The target for the detector was to be capable of 
measuring mercury concentrations down to approximately 0.1 ppb v/v. The 
device was able to efficiently measure the elemental and speciated mercury 
concentrations as they were varied over time. The speciated mercury was 
efficiently converted into elemental mercury before measurement. 
EXPERIMENT 2 
In another experiment, elemental mercury was introduced into the mercury 
detection device shown in FIG. 1 at a concentration of 4.2 .mu.g/m.sup.3 
(0.7 ppb v/v). FIG. 7 depicts the device's response to the mercury 
concentration as well as the response when no gas is in the device. Based 
on the peak-to-peak noise level observed, a minimum level of detection 
(defined as 2X noise level) of 0.2 .mu.g/m.sup.3 (27 ppt v/v) was 
calculated. 
The response of the device to a gas having a mercury concentration of 10 
.mu.g/m.sup.3 and 500 ppm sulfur dioxide is depicted in FIG. 8. As can be 
seen from FIG. 8, the device eliminates the effects of interfering gases 
such as sulfur dioxide. 
The mercury concentration in the gas was varied over a concentration range 
of 0-6 ppb (v/v). The range is expected to cover most concentrations 
expected in coal-fired and municipal solid waste generated flue gases. A 
dilution probe was used on the analyzer where high concentrations of 
mercury were expected, such as when monitoring uncontrolled emissions 
ahead of an APCD. FIG. 9 shows the linear response of the analyzer over 
the concentration range. 
EXPERIMENT 3 
A mercury monitoring device different from the device shown in FIG. 1 was 
employed to determine the ability of the converter to convert speciated 
mercury into elemental mercury. The device had a separate converter and 
mercury measurement device. The converter thermally cracked the chemical 
bonds in two surrogate speciated mercury compounds--mercuric chloride and 
dimethyl mercury. The gas sample was then cooled to a temperature of about 
200.degree. C. and contacted with a fixed bed of tin to remove chlorine. 
The cooled gas sample was input into the mercury measurement device which 
irradiated the sample to measure elemental mercury concentration. A bypass 
valve was employed to permit the gas sample to bypass the converter and be 
input directly into the mercury measurement device. 
FIGS. 10 and 11 show the response of the device when the gas sample was 
alternately diverted through the converter to the mercury measurement 
device (i.e., which is reflected by the rises in mercury concentration) 
and when the sample bypassed the converter and was input directly into the 
device (i.e., which is reflected by the drops in mercury concentration). 
The bypass valve was alternately actuated and deactuated in a sequence to 
alternately pass the sample through the converter or around the converter. 
The sequence was repeated for a number of cycles to establish the fact 
that substantially all of the speciated mercury was being converted to 
elemental mercury. 
EXPERIMENT 4 
The device shown in FIG. 5 was tested at a pilot-scale thermal treatment 
facility. The treatment system was intended to reduce the volumes of 
hazardous waste being containerized and stored at the facility. The system 
included a plasma hearth chamber, a secondary combustor, an acid gas 
scrubber, a baghouse, and a bank of HEPA filters. At the time of testing, 
the facility was evaluating a unique mercury removal system and the 
analyzer was used to measure concentrations of mercury entering and 
exiting the system. FIG. 12 shows relative concentrations of total and 
elemental mercury entering and leaving the mercury scrubber over a 4-hour 
period of time. FIG. 13 shows concentrations of total and elemental 
mercury leaving the scrubber over a 4-hour period. The "spikes" in 
concentration correlate to the input of wastes to the thermal treatment 
unit. The difference in mercury readings between the total and elemental 
measurements is the concentration of speciated mercury (i.e., the 
concentration of chemically combined mercury). 
The above-described series of tests shows that the device responds rapidly 
to changes in mercury concentration, and that the device tracks the 
mercury emissions as a function of the input waste gas. 
While various embodiments of the present invention have been described in 
detail, it is apparent that modifications and adaptations of those 
embodiments will occur to those skilled in the art. However, it is to be 
expressly understood that such modifications and adaptations are within 
the scope of the present invention, as set forth in the following claims.