Abstract:
The burning of fuel (e.g., coal) generates an exhaust flow containing particles. The flow is passed through a pulsed combustion agglomerator. The agglomerator is cyclically operated by introducing a fuel and oxidizer charge to at least one conduit, initiating combustion of the charge. The combustion generates a shock wave to which the flow is exposed causing partial agglomeration of the particles. The flow is passed through a particle removal system and exhausted.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The invention relates to coal-fired industrial equipment. More particularly, the invention relates to the control of particulate emissions from coal-fired industrial equipment such as pulverized coal-fired utility boilers.  
         [0002]     Particulate emissions from coal-fired industrial equipment have plagued industry for centuries. Among well developed technologies for removing particulate are bag houses, cyclonic separators, and electrostatic precipitators. One area of recent attention is acoustic agglomeration. U.S. Pat. No. 6,749,666 of Meegan, Jr. identifies use of a modulated acoustic field to induce particulate agglomeration. A device is used to generate a modulated acoustic field of a desired sound pressure level and frequency. The sound waves cause small particulate to agglomerate to form larger particulate that is easier to capture with conventional equipment.  
       SUMMARY OF THE INVENTION  
       [0003]     The burning of fuel (e.g., coal) in industrial equipment generates an exhaust flow containing airborne particulate. The flow is passed through a pulsed detonation particulate agglomerator. The agglomerator is cyclically operated by introducing a fuel and oxidizer charge to at least one conduit and initiating combustion of the charge. The combustion generates a shock wave to which the flow is exposed causing agglomeration of the particulate. The flow is processed through conventional particulate removal equipment and exhausted.  
         [0004]     The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0005]      FIG. 1  is a partially schematic view of a coal-fired boiler system.  
         [0006]      FIG. 2  is a transverse sectional view of a particulate agglomeration system of the boiler system of  FIG. 1 .  
         [0007]      FIG. 3  is a longitudinal sectional view of the particulate agglomeration system of  FIG. 2 .  
         [0008]      FIG. 4  is a schematic view of particulate before agglomeration.  
         [0009]      FIG. 5  is a view of particulate during agglomeration.  
         [0010]      FIG. 6  is a view of particulate after agglomeration. 
     
    
       [0011]     Like reference numbers and designations in the various drawings indicate like elements.  
       DETAILED DESCRIPTION  
       [0012]      FIG. 1  shows a schematic view of a pulverized coal-fired electric power plant  20 . The exemplary plant may be an electrical power plant having a steam generator  22  providing steam to a steam turbine electrical generator unit  24 . Along a combustion flowpath, the steam generator  22  has an upstream radiant (furnace) zone  26  followed by a downstream convective (backpass) zone  28 . The steam generator  22  receives input flows of coal  30 , air  32 , and water  34 .  
         [0013]     The coal  30  passes through a pulverizer system  40 . The air flow  32  passes through an air heater  50  (discussed below) at a downstream end of the backpass  28 . The backpass heat exchangers may comprise vertical/streamwise or horizontal/transverse tube arrays. The air enters the furnace  42  as a preheated flow  52  partially including entrained pulverized coal  44 . The furnace serves as a combustor combusting the coal and air mixture. A combustion flow  54  passes downstream along the combustion flowpath.  
         [0014]     The water flow  34  enters the convective zone  28  where it is preheated in an economizer  56  before entering the vertical walls (water walls-typically vertically extending tube arrays)  58  of the furnace  42 . Heat exchange from the combustion products  54  boils the water to produce steam. Downstream along both the gas/combustion products flowpath and water/steam flowpath, the steam is superheated to high temperature and, in turn, delivered to a high pressure turbine  60 . Exemplary superheating occurs in a two-stage process, first in a primary superheater  62  across the convective zone upstream of the economizer  56  and then in a pendant secondary superheater  64  on the radiant zone. In the radiant zone  26 , flow is primarily upward and, in the convective zone, primarily downward. The two zones are separated by a bull nose  66  adjacent the pendant heat exchanger(s).  
         [0015]     Steam from the high pressure turbine  60  continues along the water/steam flowpath and returns to the boiler to be reheated. Exemplary reheating is in a two-stage process, with a primary reheating (e.g., in a heat exchanger  70  across the convective zone between the primary superheater  62  and economizer  56 ) and a secondary reheating (e.g., in a pendant reheater  72  spanning the radiant and convective zones). Thereafter, the re-heated steam is delivered to an intermediate pressure turbine  80 .  
         [0016]     Steam exiting the intermediate pressure turbine  80  is directed to a low pressure turbine  82 . Steam (and optionally water) exiting low pressure turbine  82  may proceed to a condenser  84  for correction and processing (e.g., to return as the stream  34 ). Energy extracted by the turbines drives an electrical generator  90  to produce electrical power.  
         [0017]     After heating the water in the backpass region, the flow  54  heats the incoming air in the air heater  50  and then may proceed to a pollution control system  100 . The exemplary system  100  includes an upstream chemical scrubber  102  and a downstream particulate removal device  104  (e.g., a bag house or electrostatic precipitator). Thereafter, the combustion products may pass through a stack  110  for discharge to atmosphere.  
         [0018]     As so-far described, the system is illustrative of just one of a variety of plant configurations to which the present invention may be applied. According to the present invention, one or more particulate agglomeration systems  120  may be located along the air/combustion products flowpath. The system(s)  120  may advantageously be located within the backpass region, within the pollution control system  100 , or in between those two.  
         [0019]      FIG. 2  shows an exemplary agglomeration system  120  positioned along the combustion products flowpath away from boiler tubes or other heat exchange elements. The exemplary system  120  includes a plurality of pulsed combustion devices  122 . Each device  122  has a conduit  124  having an outlet  126  at one end in communication with the interior  128  of the flue duct  130 . The conduit  124  may include one or more inlets for receiving fuel and oxidizer.  FIG. 2  shows exemplary fuel and oxidizer lines  140  and  142  coupled to common fuel and oxidizer sources  144  and  146 . The exemplary devices  122  further include control modules  150  which may be connected to a central control system  152 . Additional structural and operational details may be similar to those of pulsed combustion cleaning apparatus such as shown in US Pregrant Patent Publications 2005-0112516 and US 2005-0199743, the disclosures of which are incorporated by reference herein as if set forth at length. Unlike the cleaning apparatus, operation as a particulate agglomeration apparatus would be essentially continuous rather than intermittent.  
         [0020]     The exemplary duct  130  is rectangular in transverse section. Along each of the two longer sides is an associated group  160  and  162  of the devices  122 .  FIG. 3  shows the two groups at a common streamwise position along the duct  130 . Other configurations are possible.  
         [0021]     The control system  152  may operate the devices  122  to repeatedly combust charges of the fuel and oxidizer. Exemplary combustion includes detonation producing associated shock waves  170 .  FIG. 2  shows the two groups  160  and  162  as being out of phase to maximize effective coverage.  
         [0022]     As the shock waves propagate through the flue gas, they induce particle agglomeration, reducing the number density of smaller particulate and thereby increasing the effectiveness of downstream particulate removal equipment  104 .  FIGS. 4-6  show the effect of pressure waves on particle distribution.  FIG. 4  shows an exemplary distribution including relatively large and relatively small particles, prior to arrival of the shock wave. As the wave passes, smaller particles are preferentially accelerated ( FIG. 5 ) due to their low mass to cross-sectional area ratio. This may cause these particles to collide with relatively large particles or other relatively small particles. Some of these collisions result in agglomeration of particles, thereby forming yet larger particles.  
         [0023]     The formation of larger particles may be by any of several mechanisms. It may include purely mechanical bonding of solid particles to each other (e.g., by the interlinking of the irregular surfaces of adjacent particles). It may include liquid-solid bonding (e.g., where a liquid droplet sticks to a solid particle such as by surface tension/wetting). It may include liquid-liquid bonding (e.g., where the droplets merge to relax surface tension and, optionally, internally mix to form a uniform structure).  
         [0024]      FIG. 6  shows a resultant distribution wherein characteristic particle size has been substantially increased by the above mechanisms and will facilitate removal in the pollution control system  100 .  
         [0025]     The exemplary devices  122  may be fired simultaneously (e.g., repetitively and without interruption while the furnace is in operation or sequentially). A given cycle will induce particulate agglomeration within a slug of the flue gas passing through the system  120 . The cycle timing may be selected to just allow a slug refresh or allow only a partial refresh. Exemplary timing is in excess of 1 Hz, more particularly in the 1.5-10 or 2-6 Hz ranges. Alternatively, there may be groups of devices  122  at multiple streamwise positions along the combustion flowpath (either adjacent positions or positions separated by other components). The timing of the firing of the groups at different streamwise positions could be selected so that a given slug is exposed to at least one set of shock waves from such devices. For example, this may be required if needed refresh times of the devices  122  exceed the desired cycle interval for a single streamwise location. Alternatively such high device minimum refresh times might be accommodated by replacing the individual devices with pairs or clusters wherein the individual devices of each pair or cluster are sequentially fired.  
         [0026]     Particular physical and operational parameters will depend on the desired characteristics of emissions control. For coal-powered plants, this may partially be influenced by the nature of the particular coal being burned. For example, it may be desirable to remove particulate in general, on the one hand while, on the other hand, it may be desirable to remove particulate as a tool to remove one or more specific chemicals in the particulate. For example, some contaminants such as mercury tend to adsorb onto or amalgamate with other particles (e.g., fly ash). Specific adsorption (mass of pollutant adsorbed relative to particle mass) will be highest on smaller particles which have higher specific surface areas. It may be desirable to position the system  120  relatively downstream to maximize the available time for such adsorption. This may be balanced against effectiveness of agglomeration. Agglomeration effectiveness might be higher upstream where particulate is warmer and, therefore, more likely to be softer, if not actually liquid. The wave intensity may be optimized in view of the desired removal, to have sufficient strength to cause desired agglomeration, while not being so strong as to cause undesired breakup of existing particulate. Exemplary intensity is at least about 0.3 pounds per square inch differential (PSID) or 160 db overpressure at extent of range (e.g., at an opposite duct surface (e.g., about 2.5-5 m away) and/or at a location where the peak overpressure from another conduit is equal). Intensity at the conduit outlet will tend to be higher.  
         [0027]     One or more embodiments of the present invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. For example, when implemented in a reengineering or upgrade of an existing system configuration or system, details of the existing configuration may influence details of any particular implementation. Although illustrated with respect to a coal-burning plant, the invention applies to other heat transfer facilities that produce particulate. Some prime examples would be trash incinerators and biomass/wood burners. Trash incinerators may be particularly relevant due to the possible capture of a variety of toxic trace elements. Accordingly, other embodiments are within the scope of the following claims.