Abstract:
An apparatus for cleaning a continuous emissions monitor system that is in fluid communication with a flue stack conducting exhaust gas from a combustion source. The apparatus comprises a housing and a probe mounted in the housing. The probe is tubular and in fluid communication with the flue stack to acquire a sample of gas from the flue stack. The probe tends to have deposits from the exhaust gas accumulate on the inner walls of the probe. A device imparts cleaning energy to the probe for dislodging accumulated deposits from the inner walls of the probe.

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates generally to continuous emissions monitoring of exhaust flue gas streams. More specifically, the present invention relates to cleaning components of continuous emissions monitoring equipment. 
     The United States Environmental Protection Agency (EPA) identifies sources of mercury emissions in the U.S. to be utility boilers, waste incinerators which burn mercury-containing wastes (municipal and medical), coal-fired industrial boilers and cement kilns that burn coal-based fuels. A particularly significant source of mercury emissions is coal-fired power plants. 
     To quantify the emissions from a source, a mercury continuous emissions monitoring system (CEMS) is employed. There are basically three forms of mercury in exhaust flue gas stream of a coal fired power plant that may be monitored by a CEMS. These forms are gaseous elemental mercury, gaseous oxidized mercury and particulate bound mercury of either form, at stack gas temperatures in excess of 200° F. 
     Mercury in the gaseous forms is relatively sticky and has a strong affinity to attach to a wide variety of interior surfaces of a CEMS components. Such gaseous mercury is extremely difficult to handle and transport through an extractive gas sampling system to a gas analyzer for measurement. Furthermore, particulate present in coal fired power plants exhaust flue streams tends to absorb gaseous mercury especially when it accumulates in the CEMS sample transport system and probe. Since exhaust flue gases usually contain relatively low levels of gaseous mercury that must be detected, the small amount of gaseous mercury present that readily attaches to surfaces of the CEMS renders any measurement made on the sample not truly representative of what is conducted in the exhaust stack. 
     More restrictive controls on mercury mandated by the EPA will likely result in higher operational costs to flue gas generators, such as coal-fired plant owners. Accordingly, there exists a real and eminent need for the development of a durable, low cost, accurate technology capable of measuring mercury emitted in an exhaust flue gas stream in real-time. A total mercury measurement is required for regulatory monitoring, whereas the evaluation of mercury control technologies and manufacturing processes requires accurate measurements of gaseous mercury. 
     SUMMARY OF THE INVENTION 
     One aspect of the present invention is directed to an apparatus for cleaning a continuous emissions monitor system (CEMS) that is in fluid communication with a flue stack conducting exhaust gas from a combustion source. The apparatus comprises a housing and a probe mounted in the housing. The probe and other components in fluid communication with the flue stack to acquire a sample of gas from the flue stack are tubular. The probe and other components tend to have deposits from the exhaust gas accumulate on the inner walls of the probe and components. A device is provided according to aspects of the invention for imparting cleaning energy to the probe and components for dislodging accumulated deposits from the inner walls of the probe and components. 
     Another aspect of the present invention is directed to a method of cleaning a continuous emissions monitor system that is in fluid communication with a flue stack conducting exhaust gas from a combustion source. The method comprises the steps of providing a tubular probe in fluid communication with the flue stack to acquire a sample of gas from the flue stack. The probe tends to have deposits from the exhaust gas accumulate on the inner walls of the probe. Cleaning energy is imparted to the probe for dislodging accumulated deposits from the inner walls of the probe. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic illustration, partly in section, of a cleaning system, according to one aspect of the invention, for a continuous emissions monitor; 
         FIG. 2  is an enlarged view of a portion the cleaning system of  FIG. 1 ; and 
         FIG. 3  is a schematic illustration, partly in section, of a cleaning system, according to another aspect of the invention, for a continuous emissions monitor. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A mercury continuous emissions monitoring system (CEMS) normally consists of a tubular probe assembly located in fluid communication with a flue stack for acquiring an exhaust gaseous sample. The CEMS also includes instrumentation located some distance away from the probe assembly to analyze the acquired sample for the presence of mercury. The amount of mercury present in the exhaust gas stream is continuously measured and recorded. Over time, the total amount of mercury emitted is established. A critical component of the mercury CEMS is the tubular probe assembly located in fluid communication with the stack for taking the sample. 
     The tubular probe assembly experiences multiple problems. Particulate matter and moisture, which are always present in the in the exhaust stack gas stream, tend to accumulate on the inner wall of the tubular probe assembly which can clog components of the probe assembly. A clogged probe assembly reduces the accuracy of the mercury measurement or ceases measurement completely. Clogging of the probe assembly can result in a reduction of the amount of time the mercury CEMS is accurately measuring emissions in the exhaust gas stream that is mandated by governmental regulation. 
     The probe assembly is generally U-shaped with an inlet and outlet. An initial filter may or may not be located near the inlet of the probe assembly from which gas samples are drawn from. A venturi eductor is located near the outlet and is supplied by a source of clean heated air which exits from the gas outlet into the exhaust stack gas stream. 
     This flow of eductor air generates a high velocity (70-100 feet per second) gas flow through the probe assembly, creating a vacuum at the gas inlet. This vacuum at the gas inlet draws the sample stack gas into the probe assembly. Experience has shown that despite the high flow rate, particulate matter does deposit within the probe assembly, especially in the presence of high moisture content in the stack gas. This causes inaccuracies of the measurement of mercury in the exhaust gas stream, increasing maintenance and down time. 
     A gas sample acquisition apparatus  20  is illustrated in  FIG. 1 , and includes structure according to one aspect of the invention to clean at least some of its components. The gas sample acquisition apparatus  20  is part of a continuous emissions monitoring system (CEMS) and is operatively connected with a known gas analyzer. Such a gas sample acquisition apparatus  20  and CEMS is suitable for sampling desired pollutants, such as mercury, that is transported in a flue gas stream flowing in an exhaust stack  22  from a combustion source. 
     The gas sample acquisition apparatus  20  includes a housing  24 . The housing  24  is made to comply with NEMA standards and is insulated. The housing  24  is attached to the exhaust stack  22  by a tubular connector  26 . 
     Since the tubular probe assembly is mounted on the exhaust stack, access to the probe and therefore maintenance of the probe assembly is difficult and time consuming. It is desirable that the probe assembly be as reliable and maintenance-free as possible. 
     The gas sample acquisition apparatus  20  also includes a probe assembly  40  mounted in the housing  24 . Components of the probe assembly  40  are tubular. The probe assembly  40  includes an inlet or probe tip  42  that is in fluid communication with the flue gas stream in the exhaust stack  22 . The probe tip  42  is connected to an inertial filter  44  of the probe assembly  40 . The inertial filter  44  is attached to a generally U-shaped stainless steel return pipe  46 . The stainless steel return pipe  46  is attached to a venturi flow meter  48 . The venturi flow meter  48  is connected to an outlet or eductor  62  that is open to the flue gas flow. The temperature of the gas sample within the components of the probe assembly  40  located in the housing  24  is maintained via a block or jacket heater  64 . 
     The probe tip  42  extends into the exhaust stack  22  through flexible thermal insulation  82 . The probe tip  42  draws a sample from the exhaust flue gas flow. The gas sample is transported into the inertial filter  44 . The gas sample leaves the inertial filter  44  via the stainless steel return pipe  46 . The gas sample then passes through the venturi flow meter  48 . Finally, the gas sample leaves the component housing  24  by passing through the eductor  62 . The gas sample is extracted from the gas sample acquisition apparatus  20  via a sample pump (not shown) and a valve (not shown). 
     During the circulation of the gas sample through the components of the probe assembly  40 , a representative sub-sample is drawn from the inertial filter  44  at tap  84 . The sub-sample is conducted out of the housing  24  in line  86  extending through port  88 . The sub-sample is conducted to a gas analyzer for analysis in a known manner. Suitable gas analyzers are well known in the art and include, without limitation, UV atomic absorption and atomic fluorescence detectors. 
     It is desirable, but not required, to keep the components of the probe assembly  40  at around 200° C. to ensure optimum accuracy in the measurement of total gaseous mercury concentration. The entire flow path throughout the tubular components of the probe assembly  40  is relatively smooth, with no gaps in the tubing of the assembly where particulate material might collect. Accordingly, the components provide, a consistently laminar flow of the sample through the tubular components of the probe assembly  40  in contact with the flue gas sample. The size and porosity of the inertial filter  44  and other components are selected to provide the desired flow of the gas sample through the components of the probe assembly  40 . 
     To minimize particulate matter from accumulating and depositing within the components of the probe assembly  40  of the gas sample acquisition apparatus  20  is the addition of a cleaning device  102  to periodically or continually shake or vibrate the components. The cleaning device  102  is mounted to the housing  24  and operatively attached to the component of the probe assembly  40 . The force applied by the cleaning device at the proper frequency and magnitude causes any agglomerated particulate material to dislodge from the interior walls of the tubular components of the probe assembly  40 . The dislodged particulate material will break into finer particles which will be discharged into the stack&#39;s gas flow, thus yielding a clean probe assembly  40 . Thus, the probe assembly  40  is relatively maintenance free and provides a representative sample from the exhaust flue gas flow. 
     Any means of shaking, vibrating, or otherwise mechanically exciting the interior surfaces of the components of the probe assembly  40  are contemplated by this invention so other traditional and labor intensive means of cleaning the probe assembly (for example, brushes) would not have to be implemented. Particulate matter does not stick to the interior surfaces of the components of the probe assembly  40  upon the application of appropriate predetermined vibratory force and frequency. The vibratory force can be applied periodically, continually, or in concert with the additional cleaning air. The cleaning device  102  is connected to a controller  104 . The controller  104  establishes when the cleaning device  102  is activated, the duration of actuation, the intensity of actuation and frequency of activation. 
     The vibratory or shaking force can be supplied by either an electrical, mechanical or pneumatic cleaning device  102 . An example of the cleaning device  102  would be the addition of a silent pneumatic turbine vibrator, model number VS-160, as manufactured by Vibco of Wyoming, R.I. While the cleaning device  102  is illustrated as attached to the probe assembly  40  at the inertial filter and applying a reciprocal vertical cleaning force, it may be operatively connected with any component of the probe assembly  40  and apply any suitable force. 
     A gas sample acquisition apparatus  20  is illustrated in  FIG. 3 , and includes structure according to another aspect of the invention to clean its components. The gas sample acquisition apparatus  20  is part of a continuous emissions monitoring system (CEMS) and is operatively connected with a known gas analyzer. Such a gas sample acquisition apparatus  20  and CEMS is suitable for sampling desired pollutants, such as mercury, that is transported in a flue gas stream flowing in an exhaust stack  22 . 
     The gas sample acquisition apparatus  20  includes the housing  24 . The housing  24  is attached to the exhaust stack  22  by the tubular connector  26 . The probe assembly  40  includes the probe tip  42  connected to the inertial filter  44 . The inertial filter  44  is attached to the return pipe  46 . The return pipe  46  is attached to the venturi flow meter  48 . The venturi flow meter  48  is connected to the eductor  62  that is open to the flue gas flow. The temperature of the components of the gas sample acquisition apparatus  20  is maintained via a block or jacket heater  64 . 
     The probe tip  42  extends into the exhaust stack  22  through flexible thermal insulation  82 . The probe tip  42  draws a sample from the exhaust flue gas flow. The gas sample is transported into the inertial filter  44 . The gas sample leaves the inertial filter  44  via the stainless steel return pipe  46 . The gas sample then passes through the venturi flow meter  48 . Finally, the gas sample leaves the housing  24  by passing through an eductor  62 . The gas sample is extracted from the gas sample acquisition apparatus  20  via a sample pump (not shown) and a valve (not shown). 
     During the circulation of the gas sample through the components of the probe assembly  40 , a representative sub-sample is drawn from the inertial filter  44  at tap  84 . The sub-sample is conducted out of the housing  24  in line  86  extending through port  88 . The sub-sample is conducted to the gas analyzer for analysis in a known manner. 
     To minimize particulate matter from depositing within the components of the probe assembly  40  of the CEMS a cleaning device  202  periodically or continually applies acoustic cleaning energy to the components of the probe assembly. The cleaning device  202  is in the form of an acoustic horn, available from BHA Group, Inc. in Kansas City, Mo. Upon the application of sufficient force by the cleaning device  202  at a proper frequency, agglomerated particulate material will dislodge from the interior walls of the components of the probe assembly  40 . The dislodged particles which will be discharged into the stack&#39;s gas flow, thus yielding a clean gas sample acquisition apparatus  20 . 
     The cleaning device  202  is illustrated as mounted to the top of the housing  24  and direct sound waves downwardly through an opening in the housing at the components of the probe assembly  40 . The cleaning device  202  may be mounted anywhere on the housing  24  and in any orientation to deliver effective acoustic energy at the components of the probe assembly  40 . 
     The acoustic vibratory or shaking force can be applied periodically, continually, or in concert with the additional cleaning air. The cleaning device  202  is connected to a controller  204  and an air supply  206  for the cleaning device  202 . The controller  204  establishes when the cleaning device  202  is activated to deliver acoustical energy, the duration of actuation, the intensity of actuation and frequency of activation. 
     While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.