Abstract:
A particulate separation system for accurate measurement of vapor-phase mercury in a flue gas stream uses an inertial gas sampling filter in which the skin temperature of the filter element is controlled to allow vapor-phase mercury measurements while minimizing measurement artifacts caused by: (1) mercury thermally desorbing off particulates into the gas stream; and (2) mercury being removed from the vapor phase by collection on particulate matter at the gas/particle separation interface.

Description:
RELATED APPLICATION 
     The present application is based on, and claims priority to U.S. Provisional Patent Application Ser. No. 60/340,799, filed on Dec. 14, 2001. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to the field of systems for measuring mercury in flue gas streams. More specifically, the present invention discloses a particulate separation system for mercury analysis in flue gas streams. 
     2. Statement of the Problem 
     Several approaches are currently employed to measure vapor-phase mercury in flue gas streams containing particulate matter. However, these techniques introduce sampling artifacts that are not adequately addressed by the current methods. Many particulates present in flue gas streams have an affinity for mercury and may remove vapor-phase mercury from the gas stream or convert mercury from one vapor-phase species to another. For example, fly ash particles have been shown to both remove mercury and oxidize mercury. Many current measurement techniques use a filter to separate the particulate matter from the gas stream. However, if the particulate matter separated from the gas stream is not inert, the vapor-phase mercury measured downstream from the filter will not accurately represent the mercury upstream from the filter. 
     An example of an inertial filter in the conditioning assembly for continuous stack monitoring was disclosed by the Bendix Corporation in U.S. Pat. No. 4,161,883 (Laird et al.). Mott Metallurgical Corporation also offers an inertial gas sampling filter similar to the Bendix inertial filter. The Mott filter effectively separates the majority of the particulate matter from the gas stream and filters the remaining particulate matter. If the gas stream contains particulates with a low affinity for mercury and a large fraction of particles that can be efficiently separated by means of an inertial filter (i.e., particles having a relatively large aerodynamic diameter), the Mott system should work well to provide a particulate-free gas stream for measurement of vapor-phase mercury without altering the species of mercury (i.e., particulate, oxidized vapor, or elemental vapor). However, for gas streams containing particles with an affinity for mercury or gas streams with a large fraction of small particles, the Mott system can introduce significant sampling artifacts from the thin layer of particulate matter collected on the filter surface. 
     3. Solution to the Problem 
     The present invention is a modified inertial filter that addresses the shortcomings associated with the prior art by including a heater and temperature controller that maintain the inertial filter within a predetermined temperature range. This enables accurate measurement of vapor-phase mercury without biasing the measurement due to desorption of particulate-phase mercury or absorption of mercury vapor onto fine particles that have an affinity for mercury. 
     SUMMARY OF THE INVENTION 
     This invention provides a particulate separation system for accurate measurement of vapor-phase mercury in a flue gas stream. The process uses an inertial gas sampling filter in which the skin temperature of the filter element is controlled to allow vapor-phase mercury measurements while minimizing measurement artifacts caused by: (1) mercury thermally desorbing off particulates into the gas stream; and (2) mercury being removed from the vapor phase by collection on particulate matter at the gas/particle separation interface. 
     These and other advantages, features, and objects of the present invention will be more readily understood in view of the following detailed description and the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention can be more readily understood in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a diagram of an installation of the present invention to filter particulates from flue gas samples. 
     FIG. 2 is a side cross-sectional view of the inertial gas separation filter  40  and heater assembly  35 . 
     FIG. 3 is a cross-sectional view of the inertial gas separation filter  40  taken along the plane of one of the sample extraction ports  48 ,  49  in FIG.  2 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Turning to FIG. 1, a diagram is provided showing a typical installation of the present invention. The portion of FIG. 1 to the left of the duct wall  10  represents the exhaust stack for the flue gases to be sampled. An inlet probe  20  extends through the duct wall  10  into the flue gases. Flue gases are drawn through the distal opening  22  of the inlet probe  20  by a blower  26  and pass through an inertial gas separation filter  40 , at which point gas samples can be withdrawn for analysis. The flow meter  24  measures the flow rate of gas withdrawn from the exhaust stack. This flow rate can be adjusted by means of a flow control valve  25 . The remaining gas exiting the blower  26  returns to the exhaust stack via an exhaust probe  28  that extends through the duct wall  10 . Alternatively, an eductor pump can be used in place of a blower  26  to draw gas from the exhaust stack. The system is also equipped with an inlet isolation valve  23  and an outlet isolation valve  27  so that the inertial gas separation filter  40 , flow meter  24  and blower  26  can be isolated from the exhaust stack for service, cleaning, or replacement. If desired, the position of the opening  22  at the distal end of the inlet probe  20  can be adjusted to sample flue gases at a variety of points across the exhaust stack. For example, this can be accomplished by constructing the inlet probe from a series of tubular segments that be attached together with connectors to form a desired length. A telescoping series of tubular segments could also be substitute for this purpose. 
     FIG. 2 is a side cross-sectional view of the inertial gas separation filter  40  and heater assembly  35 . FIG. 3 is an orthogonal cross-sectional view of the inertial gas separation filter  40 . The inertial gas separation filter  40  includes an inlet tube  41  leading from the inlet probe  20  and an outlet tube  42  that connects to the flow meter  24  and blower  26 . Exhaust gases flow from the inlet tube  41  through an inner sintered tube  44  that is surrounded by an outer tube  43 . The outer tube  43  is made of stainless steel tubing. The inner tube  44  serves as a filter element and is made of a sintered porous metal tube. The vast majority of the particles entrained in the gas stream are prevented from depositing on or penetrating into the porous sintered tube  44  by the high-speed in-line gas flow and particle inertia. The finer particles form a permeable subsurface on the wall of the sintered tube  44 . The sintered tube  44  has an average pore size selected to prevent passage of particulates greater than a predetermined minimal size. The exhaust gases passing through the sintered tube  44  are contained within the outer tube  43  and can be withdrawn for analysis via one or more sample extraction ports  48 ,  49 . 
     The present system is also equipped with at least two temperature sensors, e.g., thermocouples  36  and  37 . The first temperature sensor  36  is used to measure the temperature of flue gases in the exhaust stack adjacent to the inlet probe  20 . For example, the first temperature sensor  36  can be mounted to a thin support rod extending through the duct wall  10  to a position upstream from, but adjacent to the distal opening  22  of the inlet probe  20  to maximize its thermal isolation. Alternatively, the first temperature sensor  36  could be attached to the inlet probe  20 . The second temperature sensor  37  measures the temperature of the inner sintered tube  44 , as illustrated in FIG. 2, via a thermocouple port  47  in the outer tube  43 . 
     Both temperature sensors  36 ,  37  are monitored by a controller  30  (e.g., a computer processor or PID controller) to determine the difference in temperatures between the gas stream entering the inlet probe  20  and the sintered tube  44 . A heater  35  regulated by the controller  30  maintains the temperature of the sintered tube within a small predetermined range of the temperature of the gas stream entering the inlet probe  20 . For example, the heater  35  could be an enclosure with an electrical heating element surrounding the outer tube  43  and the inner sintered tube  44 . 
     During operation of the filter  40 , the temperature of the sintered tube  44  is maintained by the heater  35  at a temperature such that the mercury adsorption capacity of the particulate matter collecting on the sintered tube  44  removes only minimal additional mercury from the gas stream. However, to prevent mercury from desorbing off particles that have already be separated from the gas flow or desorbing off particles in the gas flow, minimal changes in the temperature of the gas flow are allowable. In other words, the filter  40  is maintained at a temperature high enough to significantly lower the mercury adsorption capacity of any particulates collecting on the filter element  44 , while minimizing any temperature increase in the gas stream which could cause mercury to desorb from the particulate matter in the gas stream. This process efficiently separates particulate matter from a gas stream with minimal change to the speciation of the mercury (i.e., particulate mercury, oxidized vapor-phase mercury, or elemental vapor-phase mercury). The particulate-free gas can then be delivered to a mercury analyzer or manual mercury measurement system. 
     The fabrication process for the filter  40  can also include sufficiently preheating the outer tube  43  versus the inner sintered tube  44  to compensate for the high thermal differential that can exist between the filter element  44  and the outer tube  43 . This fabrication process helps to prevent fractures or cracking of the sintered tube  44  due to thermal stresses. 
     The above disclosure sets forth a number of embodiments of the present invention. Other arrangements or embodiments, not precisely set forth, could be practiced under the teachings of the present invention and as set forth in the following claims.