Abstract:
A system and apparatus for analyzing stack gas for total reduced sulfur. The gas is withdrawn from a stack through a probe, filtered and regulated to a known temperature. The gas is then passed through a scrubbing column to remove sulfur dioxide and split into first and second portions. The first portion is oxidized to covert total reduced sulfur compounds to SO 2 , and the gas is then passed to an electrochemical sensor for SO 2  which is maintained at a temperature at least equal to the temperature of the regulated gas. The second portion of the scrubbed gas is passed through an electrochemical sensor for oxygen which is maintained at substantially the same temperature as the sensor for SO 2 .

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a process and apparatus for measuring the gaseous content of industrial process gas streams, in which a portion of the process gas stream is extracted, conditioned for analysis, and analyzed on a gas analysis system using electrochemical gas sensors. 
     The invention relates more specifically to a gas analysis system for measuring total reduced sulfur (TRS) and other gases, which would meet or exceed the requirements of the U.S. Environmental Protection Agency (EPA), while greatly reducing cost and maintenance requirements of existing systems. 
     2. Description of Related Art 
     Total reduced sulfur, is defined by the EPA, to be the sum of hydrogen sulfide methyl mercaptan, dimethyl sulfide and dimethyl disulfide. SO 2  is not included. Total reduced sulfur compounds are measured by removing any SO 2  that coexists with the TRS gases using an SO 2  scrubber, passing the remaining gases through a thermal oxidizer to convert the remaining TRS compounds to SO 2 , and then analyzing the SO 2  using a fluorescence type analyzer. 
     Existing TRS analyzer systems are very expensive to manufacture, install and maintain. A typical TRS analyzer system contains the following components: 
     1. A sampling probe/conditioning system that is installed on the boiler or kiln stack; 
     2. A tubing umbilical to transport the gases to the analyzer location; 
     3. An air-conditioned shelter for the TRS analysis system; 
     4. A fluorescence type SO 2  analyzer; 
     5. An SO 2  scrubber; 
     6. A thermal oxidizer; 
     7. Stainless steel sample pump; and 
     8. A regenerative air cleanup system. 
     Existing TRS analyzer systems may cost as much as $50,000.00 and the installation cost can easily double or triple that amount for analyzer shelters and umbilical tubing installation. Maintenance on these systems is complicated because the system may be spread out over 500 feet or more between the sample probe location and the analyzer location. These systems are permanently installed and must be serviced by field technicians. Due to analyzer response time requirements of the EPA and the flow required by the fluorescence type SO 2  analyzer, relatively large flows (3000 cc/min) must be pulled from the stack and 400 to 600 cc/min must be supplied to the analyzer. These gas flow rates set the size of the SO 2  scrubber, the thermal oxidizer and other system components. These systems have been highly developed over the years and little more can be done to reduce the cost of this type of system. 
     Attempts to lower the cost of the system by replacing the fluorescence type SO 2  analyzer with electrochemical sensors have had limited success. To transport the TRS gases over long distances without sample loss requires the use of a dilution type probe that provides a very dilute (50:1 dilution) gas to the analyzer. Electrochemical SO 2  sensors generally have trouble measuring these low concentrations. Another problem is that the electrochemical sensors are damaged by the extremely dry gas supplied by the dilution sampling system. Electrochemical sensors may also be damaged by too much moisture; too little and the sensor dries out, too much and condensation may form inside the sensor causing the sensor to fail. The extremely dry gas also dries out the SO 2  scrubber media, which must be back flushed with humidified air every fifteen minutes. The SO 2  scrubber distilled water reservoir must be filled at least once each week. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the invention to provide a system for monitoring TRS of relatively lesser size and cost than prior art system. 
     It is a further object of the invention to provide a system for monitoring TRS utilizing a low-cost electrochemical sensor for SO 2 . 
     To achieve these and other objects the invention provides a TRS analyzer and gas conditioning system that is advantageously contained in one enclosure at the stack location. The system includes a sampling probe for withdrawing stack gas, a filter for removing particulate matter from withdrawn stack gas, a heat exchanger for precisely regulating the temperature of the filtered gas, a scrubbing column for removing SO 2  from the temperature regulated gas, splitting means for splitting the scrubbed gas into two portions, an oxidizer for oxidizing a first portion of the scrubbed gas to covert total reduced sulfur compounds to SO 2 , a first electrochemical sensor for determining SO 2  in the converted gas, a second electrochemical sensor for determining oxygen in a second gas portion, and means for regulating the temperature of the first and second sensors to a value substantially the same and which is at least equal to the temperature of the heat exchanger. This system does not require a tubing umbilical, air-conditioned shelters, sample pumps or regenerative air dryers. The entire system can be contained in a 24″W×30″W×8″D enclosure and weigh 65 pounds. This TRS analyzer can be manufactured at a much lower cost and requires less maintenance. 
     The TRS system of the invention was designed to solve the problems associated with using electrochemical sensors to measure TRS and O 2 . This system uses low flow rates to allow for long sample filter life and to allow all gas conditioning components to be miniaturized to reduce cost. Installing the analyzers at the stack location eliminates the need for a tubing umbilical. Locating the analyzers close to the source also allows the analyzers to respond quickly even with low sample flow rates. 
     The invention also provides a method for analyzing gas flowing through a stack for total reduced sulfur comprising the steps of withdrawing a portion of the gas flowing through the stack, filtering the withdrawn gas, regulating the temperature of the filtered gas to a predetermined value, scrubbing SO 2  from the temperature regulated gas in a column at substantially the same temperature as the regulated gas, splitting the scrubbed gas into first and second portions, oxidizing a first a first portion of the scrubbed gas to convert total reduced sulfur compounds to SO 2 , analyzing the converted gas for SO 2  with an electrochemical sensor maintained at a temperature at least equal to the regulated gas, and analyzing the second portion of the scrubbed gas for oxygen utilizing a second electrochemical sensor maintained at a temperature at least equal to the regulated gas and which is substantially the same as the first sensor. 
     By measuring the stack gas directly, without dilution, and installing the SO 2  scrubber into the same temperature controlled zone as the heat exchanger, the need for water addition to the scrubber has also been eliminated. By placing the electrochemical sensors into a separate temperature controlled zone and controlling the temperature difference between the gas heat exchanger and electrochemical sensors, precise control over the humidity level of the gas can be maintained. This allows the electrochemical sensors to operate long term with minimum drift. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of the TRS monitoring system of the invention; 
     FIG. 2 is a cross-sectional view of the heated sample filter shown in FIG. 1; 
     FIG. 2A is an end view of the sample filter of FIG. 2; 
     FIG. 3 is a cross-sectional view of a combination heat exchanger SO 2  scrubber shown in FIG. 1; 
     FIG. 3A is a top view of the heat exchanger—SO 2  scrubber shown in FIG. 3; 
     FIG. 4 is a cross-sectional view of a thermal oxidizer shown in FIG. 1; 
     FIG. 5 is a top view of a thermoelectric sensor heating block shown in FIG. 1; and 
     FIG. 5A is a top view of a housing manifold for the sensor block of FIG.  5 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As shown in FIG. 1, the system  10  of the invention includes a probe  11  having a probe tip  12  for insertion in the stack containing the gas to be monitored. The probe  11  may be fabricated from any material compatible with the stack gases, but is usually Hastelloy C-276 for recovery boilers and Hastelloy C-276 with a teflon liner for lime kilns. At the rear end of the probe is a transition block  14  constructed of Torlon which functions to connect the sample probe with heated filter  16  or with blowback valve  17 . A Torlon nipple  18  connects the transition block to the heated filter  16 , which is shown in greater detail in FIGS. 2 and 2A. Heated filter  16  includes a sample entrance  100  directing gas to a glass fiber filter element  102  and through sample exit port  104  in end cap  103 , also containing port  105 . The filter body  106  is manufactured from TFE teflon, and end cap  103  has a square mandrel  107  that reduces the internal volume of the filter. Square mandrel  107  only touches the filter element  102  at the corners and does not appreciably reduce the available filter surface area. Filter dead volume is further reduced by making the gap  108  between the filter element and the filter body as small as possible, gap  108  being approximately 0.050 inches. Reducing the volume of the filter in this manner improves the response time of the sampling system by reducing dead volume. The filter body  106  and end cap  103  are enclosed in a heated filter housing  110  manufactured from aluminum and heated by a cartridge heater  112  with a temperature sensed by a temperature sensor  113 , both located in the heated filter housing  110 . 
     Port  105  is connected to a valve  19 , and then to a source of calibration gas  20 . Exit port  104  in the heated filter is connected by way of line  22  to a heat exchanger  24  formed as an assembly with an SO 2  scrubber  50 , both shown in greater detail in FIGS. 3 and 3A. The heat exchanger includes a sample entrance port  26  and a teflon exchanger tube  28  with a reservoir section  29  at the bottom thereof. The reservoir section  29  collects water condensed from the sample gas. Any water collected flows out through port  30  and through a drain valve  32 . The sample flows upwardly through teflon tube  34  and out of the heat exchanger through port  36 . The temperature of the heat exchanger is controlled by the TRS electronics control  38 , and may be set between 35 and 90° F., preferably at 50° F. 
     The filtered and dried stack gas exiting through port  36  is then split into two portions at splitter  40 . A first flow proceeds through line  42  to oxygen analyzer  44 . The remainder of the flow is directed to parallel scrubber valves  46  and  47 . The output of valve  46  is connected to input  48  of SO 2  scrubber  50  while the output of valve  47  is connected to input  49  of the SO 2  scrubber. SO 2  scrubber  50  is formed as a unit with the heat exchanger and includes parallel scrubber columns  51  and  52 . Columns  51  and  52  are teflon tubes containing the SO 2  scrubber media; forming the scrubber units in the same housing as the heat exchanger causing the sample gas flowing through the SO 2  scrubber media to be at the same relative humidity and temperature as the gas leaving the heat exchanger. The high relative humidity in the SO 2  scrubber columns prevents the scrubber material from drying out and thereby affecting the response time of the TRS gases flowing through the scrubber. 
     The unit is cooled by thermoelectric cooler  59  and heat is dissipated from the heat exchanger by heat sink  59   a.    
     Scrubbed sample gas flowing outwardly through column  51  exits at port  53  and is directed to a valve  54 ; scrubbed sample gas exits column  52  at port  56  and flows to a valve  57 . By appropriately energizing the valves  46 ,  47 ,  54  and  57 , it is possible to utilize one column for scrubbing SO 2  while the other column is scrubbed of absorbed sulfur containing compounds. 
     Scrubbed sample gas is directed through line  60  to thermal oxidizer  62 , shown in greater detail in FIG.  4 . The thermal oxidizer includes a quartz tube  120  through which the sample flows from an inlet port  122  to an outlet port  124 . The quartz tube is heated to 1200° F. by a low voltage Tophet heater  125 . Heater  125  is protected by quartz protection sleeve  126  and insulated with a ceramic fiber blanket  127 . The internal heated volume of the oxidizer is 0.7 cc. Oxidizer temperature is sensed by thermocouple  128  which is connected to TRS electronics control  38 . 
     Oxidized sample gas passes through line  64  to the SO 2  electrochemical sensor  44  installed into a teflon manifold embedded into aluminum housing  68 , which also contains oxygen sensor  66 . The housing is shown in detail in FIGS. 5 and 5A, and includes a thermoelectric cooler  130  and heat sink  132  to provide a thermal path from the cooler to the outside ambient air. 
     Sensor housing  68  sits on top of a manifold  134  formed from TFE teflon and designed to reduce dead volume to a minimum. Sample gas enters the O 2  cell through a port  136  and exits through a port  137 , while sample gas enters the SO 2  cell at port  138  and exits through port  139 . 
     The sensor housing includes a temperature sensor which is connected to TRS electronics control  38 . 
     The gas exiting the sensors flow through orifice protection filters  70  or  72 , and then to eductor  75 , through tubes  74  or  76 . The gas flowing through the sensors is vented at  78 . The eductor utilizes instrument gas provided for the blowback circuit through source  88 , pressure regulated by regulator  89  and gauge  90 . For scrubber back flush purposes, the circuit includes a charcoal filter  82 , potassium permanganate filter  83 , particulate filter  84  and back flush orifice  85 , which is connected to the outlet valves from the scrubber columns. After cleaning the columns, the gas is vented at port  92 , which is connected to the inlet valves for the scrubber columns. 
     Other elements of the system can also be cleaned by the instrument gas in blowback mode. In particular, the instrument gas is connected to the transition block  14  through valve  17 , and the heat exchanger through valves  93  and  94 . 
     Operation of the System 
     Stack gas containing TRS compounds enters the analysis system at tip  12  of probe  11 . Stack gas is drawn through the probe at a flow rate of 50 cc/min, and from the probe the stack gas flows into the transition block  14 . From the transition block  14 , the gas flows through the Torlon nipple  18  into filter  16 , which is heated to 300° F. to prevent sample loss on the glass fiber filter element. Alternatively, calibration gas can enter the heated filter through valve  19 , permitting the gas to flow through the same path as the stack gas and detect any losses in the sampling system. Blowback valve  17  is provided periodically to clean the probe by allowing high pressure air from inlet  88  to blow accumulated particulate matter back into the stack. This air usually has a pressure of 60 to 90 psig. 
     From the heated filter, the filtered stack gas flows through line  22  into heat exchanger  24 , which removes the water from the stack gas to provide a dry basis measurement, as required by the EPA. Gas flows through the heat exchanger tubes  28  and  34  in series, connected at the bottom by a teflon liquid reservoir  29 . The heat exchanger is thermoelectrically cooled to a temperature between 35 and 90° F., and preferably 50° F., with control by the TRS electronics control  38 . Condensate collected in the heat exchanger drains into the reservoir  29  and is drained periodically through drain valve  32 . Drain valve  32  can be energized automatically every 15 minutes by the TRS electronics control. 
     Exchanger purge valves  93  and  94  are provided to force condensate out the drain valve during the probe blowback. Blowback valve  17 , exchanger purge valve  93  and exchanger purge valve  94  and drain valve  32  are all energized simultaneously for the blowback cycle, which can last approximately 5 seconds. 
     From the heat exchanger, the filtered and dried stack gas flows to point  40 , where the gas stream splits into two equal flows, typically 25 cc/min. Because the electrochemical sensors are not flow sensitive, exact flow rates are not important and the flow rate of 25 cc/min was chosen as a good compromise between filter life, SO 2  scrubber life, response time and cost to manufacture. Flows between 5 and 100 cc/min can also be used depending on the application. 
     From point  40 , a stack gas flow of 25 cc/min flows into scrubber valves  46  and  47 . With the SO 2  scrubber valves de-energized, the stack gas flows through valve  47  through scrubber column  52  and through exit valve  57  into the thermal oxidizer unit. While SO 2  scrubber column  52  is being used, scrubber column  51  is being back flushed to vent with filtered instrument air which enters the system at point  88 , and flows through pressure regulator  89  and gauge  90 . Pressure regulator  89  is used to set the correct operating pressure of 30 psig for the sample eductor  75  used to vent the gases after analysis and the SO 2  scrubber back purge orifice  85 . From the pressure regulator, the instrument air flows through the charcoal filter  82  and potassium permanganate filter  83 , provided to insure that the instrument air is scrubbed of any sulfur compounds that could conceivably contaminate the SO 2  scrubber columns. The air then flows through particulate filter  84  and scrubber back flush orifice  85 , which sets the back flush flow at 250 cc/min, or ten times the normal forward flow of 25 cc/min. Setting the flows in this manner insures that the SO 2  scrubber column will be completely regenerated before the start of each measurement cycle. After 15 minutes, the TRS electronics control  38  automatically energizes all four scrubber valves  46 ,  47 ,  54  and  57  simultaneously, causing column  51  to become the active column and column  52  to go into the back flush mode. The TRS electronics control  38  also causes a 5 second blowback/heat exchange purge cycle at the time of scrubber switching. The control holds both analyzer outputs constant for three minutes after a blowback/purge cycle. 
     The SO 2  scrubber is located in the same aluminum housing as the heat exchanger such that the stack gas leaving the heat exchanger has a dew point of 50° F. or a relative humidity of 100% at 50° F. If the SO 2  scrubber were allowed to operate at prevailing ambient temperatures between −20° F. and 122° F., the scrubber columns would be getting wetter at temperatures below 50° F. and drier at temperatures above 50° F. Precise control of the SO 2  temperature and humidity is a key factor to obtaining a reliable and accurate output from the TRS analyzer. By installing the SO 2  scrubber in the same block as the heat exchanger, the SO 2  scrubber is kept under the correct moisture conditions that allow efficient scrubbing of SO 2  without losing the TRS compounds. 
     The scrubber material must be acidic to allow the passage of the TRS gases. It is desirable to operate the scrubber in the range of 50 to 100% relative humidity to maintain the correct scrubber pH and prevent the loss of TRS gases on the scrubber material. Operating the SO 2  scrubber in this manner eliminates the need to add distilled water to the analyzer on a weekly basis, and eliminating the distilled water reservoir also eliminates the need for heating the enclosure to prevent the water from freezing. Controlling the SO 2  scrubber temperature will also give much more consistent output than existing systems that allow the scrubber temperature to vary with the enclosure temperature. 
     From the SO 2  scrubber, the filtered, dried and scrubbed stack gas flows to thermal oxidizer  62 , which operates at a temperature of 1200° F. The thermal oxidizer tube  120  is made of quartz glass and is generally of dimensions 0.250 inches OD×0.150 inches ID×4 inches long. The operating temperature of 1200° F. is much lower than the temperatures of the conventional oxidizers, which operate between 1600 and 2200° F., the higher temperatures being necessary due to the higher flow rates. Operating the thermal oxidizer at 1200° F. allows the thermal oxidizer heater element to have a much longer operating life than higher temperature oxidizers and this benefit is made possible by the lower flow rates possible with electrochemical sensors. 
     From the thermal oxidizer, the converted stack gas flows to the SO 2  electrochemical sensor  66  installed in the teflon manifold embedded into the aluminum housing. The aluminum housing is thermoelectrically temperature controlled to 65° F., with the temperature being sensed and controlled by the TRS electronics control  38  which is adjustable between 35 and 90° F. Both the heat exchanger temperature control and the sensor temperature control can heat or cool as required to maintain the set temperature. Maintaining the heat exchanger at 50° F. and the sensors at 65° F. causes the stack gas flowing to the SO 2  sensor to be at a constant relative humidity of approximately 50%. This relative humidity insures that the sensors do not dry out while also insuring that the stack gas is dry enough to prevent condensation on the sensor membrane. The heat exchanger and sensor temperatures may be set to achieve any controlled humidity level between 10 and 100%, as desired. Controlling the sensor to a constant temperature also has the added benefit of eliminating the temperature related drift of the electrochemical sensor. Controlling both the electrochemical sensor temperature and humidity is the key to long term analyzer stability and low maintenance. 
     From the output of the SO 2  sensor, the stack gas flows through orifice protection filter  72  and to eductor  75  and to vent at point  78 . 
     The stack gas at point  40  flows in a parallel circuit to the oxygen sensor  44  which is also an electrochemical sensor. The relative humidity of the gas stream and temperature of the O 2  sensor is maintained and controlled in the same manner as the SO 2  sensor. From the O 2  sensor, the stack gas flows through orifice protection filter  70  and through sample eductor  75  to vent at point  78 . 
     The preceding description has described a method of sample conditioning and temperature control that allows electrochemical sensors to accurately and reliably measure TRS and O 2 . This method would also enhance the operation of electrochemical sensors measuring many gases such as H 2 S, CO, NO, NO 2 , Cl 2 , HCN, HCl and NH 3 . Due to the gas cross-sensitivity problems associated with electrochemical sensors, different gas scrubbers or filters would need to be used, but the basic method of humidity and temperature control could be applied to all the sensors mentioned above.