Abstract:
The burning of fuel (e.g., coal) in industrial equipment generates an exhaust flow containing airborne particulate. The flow is passed through a rotary heat exchanger to preheat inlet air. The heat exchanger element is subject to fouling and is cleaned by a pulsed combustion device. The device is 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 element is exposed, dislodging and/or otherwise removing the deposits.

Description:
BACKGROUND 
       [0001]    The disclosure relates to coal-fired industrial equipment. More particularly, the disclosure relates to the cleaning of from coal-fired industrial equipment such as pulverized coal-fired utility boilers. 
         [0002]    One feature of many pieces of such equipment is the use of a rotary heat exchanger to pre-heat inlet air by transferring heat from the exhaust flow. Exemplary rotary heat exchangers are found in U.S. Pat. Nos. 4,487,252 and 5,950,707. In an exemplary axial rotary heat exchanger, the exhaust and inlet flows pass along respective angular sectors of the heat exchanger. The flows pass through a rotating core of the heat exchanger. The core has plates or other features that absorb heat when in the exhaust flowpath and then lose that heat while passing through the inlet air flow. Steam or air purges may be used to clean the core plates. 
         [0003]    Within equipment such as boilers, sootblowers have been used to clean surfaces such as boiler tubes. Steam lance sootblowers have mainly been used. Detonative or pulsed combustion sootblowers have recently been proposed. An example of such a sootblower is in U.S. Pat. No. 7,011,047. 
       SUMMARY 
       [0004]    The burning of fuel (e.g., coal) in industrial equipment generates an exhaust flow containing airborne particulate. The flow is passed through a rotary heat exchanger to preheat inlet air. The heat exchanger element is subject to fouling and is cleaned by a pulsed combustion device. The device is 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 element is exposed, dislodging and/or otherwise removing the deposits. 
         [0005]    The details of one or more embodiments 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 
         [0006]      FIG. 1  is a partially schematic view of a coal-fired boiler system. 
           [0007]      FIG. 2  is a view of a boiler unit of the system of  FIG. 1 . 
           [0008]      FIG. 3  is a longitudinally cut-away view of a heat exchanger of the system of  FIG. 1  with a first detonative core cleaning system. 
           [0009]      FIG. 4  is a partially schematic streamwise view of the heat exchanger of  FIG. 3 . 
           [0010]      FIG. 5  is a longitudinally cut-away view of a heat exchanger of the system of  FIG. 1  with a second detonative core cleaning system. 
           [0011]      FIG. 6  is a partially schematic streamwise view of the heat exchanger of  FIG. 5 . 
           [0012]      FIG. 7  is a longitudinally cut-away view of a heat exchanger of the system of  FIG. 1  with a third detonative core cleaning system. 
           [0013]      FIG. 8  is a partially schematic streamwise view of the heat exchanger of  FIG. 7 . 
           [0014]      FIG. 9  is a longitudinally cut-away view of a heat exchanger of the system of  FIG. 1  with a fourth detonative core cleaning system. 
           [0015]      FIG. 10  is a partially schematic streamwise view of the heat exchanger of  FIG. 9 . 
       
    
    
       [0016]    Like reference numbers and designations in the various drawings indicate like elements. 
       DETAILED DESCRIPTION 
       [0017]      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 . 
         [0018]    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/exhaust flowpath. 
         [0019]    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). 
         [0020]    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 . 
         [0021]    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. 
         [0022]    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. 
         [0023]      FIG. 2  shows the air heater  50  as a rotary air heater having a housing or body  120 . The housing  120  has a first portion  122  along the exhaust flowpath  124  and a second portion  126  along the inlet air flowpath  128 . A heat transfer core  130  is mounted within the housing to rotate about an axis  132 .  FIG. 3  shows the exemplary core  130  as including a hub  140  supported by an axle to be driven by an electric motor for rotation about the axis  132 . A plurality of heat transfer surfaces  142  (e.g., plates) extend radially outward from the hub to a periphery  144 . The core has a first axial surface  150  and a second axial surface  152 . In the exemplary implementation, the first axial surface  150  is upstream along the exhaust flowpath and the second axial surface  152  is downstream. Depending upon implementation, the surface will not be a single face but, rather, will be formed by discrete portions (e.g., edge portions of plates). The rotation of the core brings heat transfer portions of the core  130  sequentially through the exhaust gas flowpath and the inlet air flowpath. The exemplary heat exchanger is positioned so that the heat exchange is counterflow (i.e., the exhaust flow and air inlet flow are in opposite directions). 
         [0024]    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 detonative cleaning systems may be located along the air/combustion products flowpath and positioned to clean the element. 
         [0025]      FIGS. 3 and 4  show an exemplary cleaning system  220 . The exemplary system  220  includes a plurality of pulsed combustion devices  222  and  223 . In the exemplary implementation, two devices are shown, the first device  222  being upstream of the core along the exhaust flowpath and the second device  223  being downstream of the core along the exhaust flowpath. Each device  222 ,  223  has a conduit  224  having an outlet  226  at one end in interior  228  of the housing  120  and facing an associated core axial end  150 ,  152 . Exemplary combustion conduits have lengths of  0 . 5 - 4 m and cross-sectional areas of 20-730 cm 2 . The conduit  224  may include one or more inlets for receiving fuel and oxidizer.  FIG. 2  shows exemplary fuel and oxidizer lines  240  and  242  coupled to common fuel and oxidizer sources  244  and  246  (e.g., tank systems). Exemplary fuel consists in majority part, by mass, of fuel selected from the group consisting of hydrogen, hydrocarbon fuels, and their mixtures. Exemplary oxidizer consists essentially of oxygen (e.g., from liquid oxygen tanks). Alternative oxidizer is compressed air. Ignitors (e.g., spark plugs  248 ) may be positioned to ignite admitted fuel/oxidizer charges. 
         [0026]    The exemplary system further includes a control module  250  which may be connected to a central control system  252 . 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. 
         [0027]    The control system  252  may operate the devices  222  and  223  to repeatedly combust charges of the fuel and oxidizer. Exemplary combustion includes detonation producing associated shock waves  270 . The shockwaves may pass along the core plates, cleaning the plate surfaces. 
         [0028]    Particular physical and operational parameters will depend on the characteristics of the heat exchanger. For coal-powered plants, this may partially be influenced by the nature of the particular coal being burned. and the nature of the particular heart exchanger core. The exemplary devices  222  and  223  may be fired simultaneously (e.g., repetitively and without interruption while the furnace is in operation or sequentially). 
         [0029]    An exemplary control and firing protocol involves a series of discharges timed to provide full circumferential coverage. For example, the coverage of a single firing may be deemed effective for a relatively small sector (e.g., ˜10°, more broadly 5-20°). The firing may be synchronized to the rotation of the core so as to provide complete coverage. If the firing cycle is short enough, consecutive sectors may be progressively sequentially cleaned with the next uncleaned sector being cleaned immediately after the prior sector. If the cycle/refresh rate is not sufficient for this, an uncleaned sector may be allowed to pass unaddressed through the cleaning zone. For example, one full revolution plus the sector increment (e.g., the ˜10°) could pass between each of the firings (an exemplary thirty-six total firings, each separated from the prior firing by 370°, if the increment is 10°). 
         [0030]    Other timing variations involve redundant coverage of firings, repeat firings along a given sector, and the like. Other variations involve different delays between firings. For example, if the cycle/refresh rate is sufficient the second firing could be made before a full revolution has passed from the first firing, but sill leaving an intervening uncleaned portion. With the 10° example, the second firing could be more than 10° but less than 370° after the first, etc. For example the second firing could be 180° after the first. The third could be 190° after the second. The fourth could be 180° after the third, with subsequent alternating 190° and 180° intervals. There could be a rotation sensor  280  for detecting rotation of the core and coupled to the control system to permit the synchronization. 
         [0031]    An exemplary operation is a continuous operation with individual discharges/firings at a fixed frequency (or nearly fixed due to the synchronization with rotation noted above). An exemplary nominal frequency is 0.5-2.0 firings per minute. Alternatively, each full cleaning of the core may be initiated responsive to sensed parameters passing a predetermined first threshold and/or the passage of a predetermined interval. An exemplary interval may be up to daily. An exemplary sensed condition may involve a pressure difference across the core on one or both of the hot side and cool side (e.g., as detected by upstream pressure sensor  284  and downstream pressure sensor  286 ). The cleaning may continue until the sensed condition has passes (below for a pressure drop) a predetermined second threshold. 
         [0032]      FIGS. 5 and 6  show an alternate system configuration having respective upstream and downstream devices  320  and  322 . the devices have conduits  324  which may be similar to conduits  224  except for the outlet  326 . Relative to the outlet  226 , the outlet  326  is closer to the wall surface of the body  120 . The outlet  326 , however is directed obliquely relative to the adjacent core surface/end  150 ,  152  to compensate so that the wave  340  has adequate coverage. 
         [0033]      FIGS. 7 and 8  show an alternate system configuration having respective upstream and downstream devices  420  and  422 . the devices have conduits  424  which may be similar to conduits  224  except for having multiple outlets  426 ,  428 ,  430 , and  432  in a linear array along the side of the conduit. The array extends to a closed end  434 . The conduit  424  may thus have a greater penetration into the flowpath. The outlets, however may produce overlapping shock waves  450  which yield a more radially uniform and circumferentially concentrated net effect. 
         [0034]      FIGS. 9 and 10  show an alternate system configuration having respective upstream and downstream devices  520  and  522 . the devices have conduits  524  which may be similar to conduits  424  except for one-to-all of: a progressive (e.g., step-wise) decrease in conduit cross-sectional area along the array of outlets  526 ,  528 ,  530 , and  532 ; a progressive decrease in outlet size ; and a progressive decrease in outlet spacing. The array extends to a closed end  534 . The outlets, may produce overlapping shock waves  450 ,  452 ,  454 , and  456  which yield a more radially progressive distribution that compensates for the relatively slower speed of inboard portions of the core passing through the influence of the shock waves. The circumferential span of the effective shockwave footprint on the core may thus radially increase. 
         [0035]    One or more embodiments have been described. Nevertheless, it will be understood that various modifications may be made. 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. Although shown fixed, the conduits may be retractable (e.g., as are retractable sootblowers). Accordingly, other embodiments are within the scope of the following claims.