Patent Publication Number: US-2013245353-A1

Title: Dry ash collector

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 13/040,076, filed Mar. 3, 2011. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a dry ash collector which is a component of a commercial waste processing unit. In these units, municipal waste is combusted at high temperatures to oxidize the organic content of the waste, leaving the inorganic content of the waste, also known as the ash content, to be collected and removed from the process for downstream processing or disposal. One or more waste combustion furnaces are used to combust the waste. Aspects of the present invention relate to an ash collector which can be connected to the furnace to receive the ash content of the waste following combustion. In conventional ash collectors (wet ash collectors), the hot ash is immersed in a bath of water to cool the ash (it initially enters the ash collector at temperatures between 300° and 800° F.). Because of the high heat capacity of the ash, other cooling approaches, such as direct contact with cool air, or indirect heat exchange with cooling water, are impractical and uneconomical. A second reason a water bath is used to collect ash from waste combustion furnaces is to provide a water seal for the furnace to prevent uncontrolled air from entering the furnace. Thus prior art ash collectors utilize immersion cooling (placing the ash in a bath of water) to cool off the ash. The problem with immersion cooling of the ash is the ash becomes saturated with water. This of course makes the recovered ash stream much heavier (can be as much as 30% heavier) due to the residual water content that remains with the ash. Since the price the municipal waste facilities need to pay to dispose of the recovered ash is often determined (at least in part) by the weight of the ash, reducing the weight of the ash could generate cost savings to the municipal waste facility. 
     RELATED PRIOR ART 
     AshTech Corporation of Cleveland, Ohio and Clyde Bergemann of South Yorkshire, UK offer a commercial submerged chain conveyor system for removing bottom ash from stoker/grate furnaces that handle municipal solid waste. These systems consist of a water filled trough equipped with a submerged chain conveyor. The conveyor flights drag ash along an inclined trough wherein water from the ash is drained back into the trough and dewatered ash is dropped off for disposal. The company&#39;s web site: http://www.ashtechcorp.com/SubmergedChainConveyorSystems.htm claims that it can provide systems with low water use with zero water discharge. However, embodiments of the present invention feature lower water usage than the Ashtec system, because the Ashtec system still relies upon waste immersion system, and while drying the bottom ash in the inclined trough may help remove some of the excess moisture from the refuse, the Ashtec system does not utilize the novel spraying or sensing technology of the present invention. The Integrated Pollution Prevention and Control, “Reference Document on the Best Available Techniques for Waste Incineration” August 2006 also discloses and describes a prior art water-sealed based ash collection. See FIG. 2.5 for example. 
     In most plants in the United States, Europe and Japan the (bottom) ash is quenched in a water trough at the discharge end of the grate. Walter R. Niessen, “Combustion and Incineration Processes—Applications in Environmental Engineering”, Fourth Edition, CRC Press, 2010. As explained by Stockholm Convention on Persistent Organic Pollutants (POPS), mass burn water wall incinerators are the dominant form of incinerator found at large municipal waste combustion facilities. POPS illustrates a typical schematic of facility using water quench tank for bottom ash http://www.pops.int/documents/meetings/bat_bep/2nd_session/inf10/EGB2_INF10_munwaste_i ncineration.pdf Solid Waste Management, 2005, describes some of the problems with current incineration technology. For example, the book explains that ash produced by incineration is hot and must be cooled prior to disposal. The normal method of cooling is quenching in water. After quenching, the ash is dewatered to facilitate storage or landfilling on the incinerator site or transport to a remote disposal site. See Part II, Chapter 13, as well as the title page: http://www.unep.or.jp/ietc/publications/spc/solid_waste_management/Vol_I/19-Chapter13.pdf and http://www.unep.or.jp/ietc/publications/spc/solid_waste_management/. The problems associated with conventional water based cooling systems have been known in the art for some time, and until Applicant&#39;s proposed solution has eluded inventors. For example, a paper by Cappola and Sunk describes that in a conventional Waste-To-Energy (WTE) power plant, bottom ash is typically discharged into a water quenching tank. The water level provides a seal and prevents ambient air from entering the combustion chamber. Also quenching the bottom ash with water stops combustion immediately and prevents fugitive emissions. However, one of the disadvantages of quenching are the high concentration of water in the ash (up to 30-40%) leading to unnecessary costs of transporting and landfilling water. Furthermore, the wet ash tends to bind like cement and form accretions that adhere on metals thus lowering the value of WTE metals and resulting in the loss of small ferrous and non-ferrous metal pieces. http://www.seas.columbia.edu/earth/wtert/sofos/nawtec/nawtec15/nawtec15-3202.pdf. 
     SUMMARY OF THE INVENTION 
     Aspects of the present invention provide an apparatus, system, and method for adequately cooling bottom ash without immersing the bottom ash in a bath of water. In some embodiments, the ash collector may have a shallow water pool to minimize the generation of fly ash. The water pool maintains 6 inches to 12 inches of water. Preferably about 6-8 inches of water are maintained to provide fly ash suppression without saturating a majority of the ash. The water pool is typically placed in the container of the ash zone, and height of the ash zone may be 8 feet, meaning that a filled ash zone has about 6 inches of saturated ash or about 6.25 percent saturated ash and about 93.75 percent unsaturated ash. Water levels may be maintained by the controller so that they are too low to form a water seal. One way the prior art ash collectors (wet ash collectors) and the invention (dry ash collector) differ is the wet ash collectors primarily use a pool of water to cool the ash, while certain embodiments of the present invention use water sprayers. In addition, wet ash collectors use the pool of water to generate a seal to allow for the generation of negative pressure in the reception zone, whereas certain embodiments of the invention use the ash itself to create the negative pressure seal. 
     Most combustions systems (including certain embodiments of the present invention) feature a negative pressure source (such as a vacuum) in the system. The negative pressure may be created by an induction draft fan for example. This negative pressure helps keeps fly ash from exiting the system, and it generally pulls combustion gases and air born particles through the boiler and downstream conditioning equipment and out the stack. The ash collector reduces the flow of gases from the beach area to the furnace and maintains a seal in between the beach area and the furnace while removing ash from system. Even though ash is passed through the seal, the ash seal can be maintained. The stack emits heat, water vapor, carbon dioxide, oxygen and other gases into the atmosphere. Certain configurations of ash collectors (including certain embodiments of the present invention) provide an air tight seal at the ash collection zone to maintain negative pressure in the reception zone by preventing air from entering the reception zone and the furnace through the ash collector. Prior art systems accomplished this using a water seal. Mechanical valves and other similar mechanisms cannot provide constant sealing, because they would need to be opened to allow ash to fall through them (thus breaking the seal.) Prior art systems using a water seal also benefit from the water providing the second function of cooling the refuse at the cost of increasing the water content of the ash. One of the purposes of the ash seal is to prevent air flow from the beach area into the furnace. Combustion in the furnace is usually carefully controlled. If a seal is not present in the ash collector, gas may be drawn by the negative pressure source through the ash zone into the furnace disrupting the controlled combustion. 
     Rather than relying upon water to provide the seal, aspects of the present invention feature an ash seal which is used to maintain negative pressure in the reception zone and other areas of the ash collector. To form the ash seal, ash is allowed to fall and accumulate in the ash zone filling a container until the ash level meets a level specified by a controller. The controller may use an ash sensor to determine ash levels in the container. When the container is filled, air cannot pass through the ash zone because the ash blocks the air flow. In some configurations, ash will also partially fill the beach area, further blocking the passage of air through the ash collector. Because the ash blocks or substantially blocks the flow of air in the ash collector, negative pressure may be maintained in the reception zone. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a prior art wet bottom ash collector in combination with a grate and a furnace. 
         FIG. 2  is an embodiment of dry bottom ash collector in combination with a waste refuse combustion system. 
     
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     At a very high level, aspects of the present invention relate to an ash collector  100  for a combustion system  1  having a furnace  20 . Methods of collecting bottom ash, and using an ash collector are also presented. Starting from the left of  FIG. 2 , there is an intake  200 , a chute  205 , a combustor  210 , a grate  10 , under fire air chambers  215 - 217 , air chamber  220 , moving grate  10 , a furnace  20 , a super heater  225 , super heater fly ash collectors  230 - 232 , an economizer  235 , a scrubber  245 , a baghouse  250 , baghouse fly ash collectors  255 - 257 , a negative pressure source  165 , a stack  255 , an economizer fly ash collector  240 , an ash collector port  160 , an exhaust fan  170 , a fly ash and scrubber solids removal conveyor  260  (used for removing the fly ash and scrubber solids from baghouse  250  while maintaining the negative pressure seal), a conditioner  265  (used to moisten the fly ash with water to prevent dusting), a super heater fly ash conveyor  270 , and a dry ash collector  100 .  FIG. 1  depicts a prior art combustion system containing a grate  10 , a furnace  20 , and a wet ash collector  30 . Details concerning the grate, furnace and other components depicted are described in US Application Publication 2010/0288173, filed May 18, 2009, herein incorporated by reference in its entirety. Although the liquid used to cool the ash is often water (for reasons of cost, availability, etc) other fluids may be used in the present invention. 
     The arrows on  FIG. 2  denote the direction of gas flow. Generally speaking, all gas flows towards the negative pressure source. Municipal waste in the chute  205  and combustor  210  provide a waste seal  101  for maintaining the negative pressure from the negative pressure source  165 . An ash seal  102  is formed in the ash zone, preventing the negative pressure source from drawing air and fly ash back into the furnace. Drawing fly ash back into the furnace is undesirable because increasing the fly ash in the system  1 , increases fouling and erosion of components like the super heater  225  and economizer  235 , air inleakage from the beach area  43  negatively affects combustion control and reduces heat transfer efficiency. and increases the load on the negative pressure source. 
       FIG. 2  shows a combustion system utilizing a dry ash collector  100 . As shown in  FIG. 2 , the ash collector  100  may contain heat sensors  110 , sprayers  120 , an ash collector port  160 , a ram  150 , a pool  141 , a negative pressure source  165 , a water level detector  140 , and an ash detector  130 . The ash collector  100  may also contain several zones including a reception zone  41 , an ash zone  42 , a beach area  43 , and a collection zone  44 . A zone can be a mechanical housing formed by panels or walls to form a geometric shape having an internal volume for housing ash and system components such as the heat sensor or sprayer. Heat sensors  110  and sprayers  120  may be positioned in any of the zones, but the embodiment of  FIG. 2  shows the heat sensors  110  in the receiving zone  41  and beach area  43 , with sprayers  120  in the reception zone  41 , ash zone  42 , and collection zone  44 . As shown in the embodiment of  FIG. 2 , the ram  150 , ash detector or ash level sensor  130 , and a water sensor or water level detector  140  are in the ash zone  42 . The system may also be controlled by one or more controllers or regulators  402 , which can control and receive data from components such as the heat sensors  110 , sprayers or spray nozzles  120 , ash detector  130 , water level detector  140 , ram, or exhaust fan  170 . For example, the controller  402  may be connected to the ash detector for increasing the ash in the ash zone  42  if the ash level in the ash zone is too low to maintain the ash seal by for example increasing the speed of the grate or decreasing the speed of the ram. Controller  402  may have a wired or wireless connection to these components. Controller  402  may contain a microprocessor, computer logic, memory, and software instructions for causing the controller to receive input from components and to cause the controller  402  to send instructions to the components. In some embodiments, controller  402  can also control other elements of the system such as the speed of the grate  10  (or a second ram which pushes items on the grate), the temperature of the furnace  20 , etc. 
     Although embodiments of the ash collector  100  are referred to as a dry ash collector, the ash collector may still comprise a small amount of water at the bottom of the ash zone  42 . One purpose of having a small pool of water  141  is to trap smaller particles which have a tendency to fall through larger particles and collect at the bottom of the ash collector and may become airborne. Without this water, the flyable ash may become airborne when the ash exits the ash discharger. A water level detector  140  monitors the amount of water in the ash zone  42 , and may send information to the controller  402  regarding the water level. If the controller  402  determines water levels are too low, the controller may cause the sprayer (particularly sprayers in the ash zone  42 ) to inject more water into the ash zone  42 . The sprayers may be connected to a fluid source or water reservoir (not illustrated) for providing fluid or water to the sprayers. A drain may also be added to the ash zone to remove excess water. The water level may range from being only a few inches to about twelve inches depending on the type of waste being combusted. This lower water level helps prevent the ash from collecting excess weight. Some embodiments of the present invention may not contain a water bath  141 , but if they do, water levels are kept to a maximum depth of less than 12 inches, and preferably less than 6 inches to avoid soaking too much ash with water. 
     The heat sensors  110  may be thermocouples. The heat sensors  110  can be used to determine the temperature of ash both in the receiving zone  41  and the beach area  43  (according the  FIG. 2  embodiment.) The controller  402  may change the amount, speed, and/or direction of the spray of water to reduce the temperature of the ash if it exceeds a predetermined temperature. Similarly if the temperature of the ash is below a certain level, the controller may change the amount, speed, and/or direction of the spray to avoid adding unnecessary water to the system. 
     Some controllers may alternatively employ equations which directly determine speed, volume, and angle of the ejected water by considering temperature data received by the sensors. Additionally, the controller  402  can also take into account the speed of the grate (or speed of a ram pushing refuse on the grate), temperature inside the furnace  20 , and amount of ash falling into the reception zone  41 . The amount of ash falling into the reception zone  41  per unit of time is called the ash flow rate. This value may be computed by the controller  402  by for example noting how fast ash is removed from the ash zone  42 . If the ram  150  moves the bottom ash at a constant speed, the amount of ash in the ash zone  42  (as measured by the ash detector  130 ) can be used to determine the ash flow rate entering the reception zone  41 . The ash flow rate may also be used by the controller to approximate the temperature of ash falling into the reception zone. In other embodiments, the sole or primary determination of how much and how fast to spray may be determined by the controller&#39;s analysis of the ash flow rate. Temperature information received from the temperature sensors  110  may then be analyzed by the controller to fine tune the amount or direction of the sprayers&#39; ejection of water. 
     The ash sensor  130  may use infrared radiation to detect the amount of ash in the ash zone  42 . It is important to maintain a sufficient amount of ash in the ash zone to prevent air inleakage into the furnace  20  from the ash collector  100  and assist in maintaining negative pressure in the reception zone  41  and other areas. To with, the controller  402  may increase the speed of the grate or decrease the speed or movement rate (ash moved per minute) of the ram to maintain the ash seal  102 . In the prior art system (see  FIG. 1 ), a water bath was used to maintain an air tight seal. This led to the immersion of the ash, as the ash entered the water, cooling the ash in process. Here ( FIG. 2 ), the ash itself provides the seal. Ash from the bottom of the ash zone  42  can be moved up the beach area by a ram  150 , screw, or moving grate. The ram  150  may be hydraulically or mechanically controlled and directed to push the bottom ash up the beach ramp. Some of the excess water may run back down the ramp into the ash zone  42 . Temperature sensors  110  may be provided to measure the temperature of the ash in the beach area  43 . If the controller  402  determines the ash in the beach area is too hot, it may cause the sprinklers  120  to add additional water to the ash. However, in most configurations, the primary purpose of the sprinklers in the collection zone is suppress the formation of any fly ash. Ash and gases that are not suppressed, may be drawn through the ash collector port  160 , where the gas may be distributed to the scrubber  245  and baghouse  250 . It is preferable to keep the ash in the ash collector, because increasing the fraction of fly ash passed through the scrubber and baghouse can increase costs and wear of these components. this can reduce overall system performance by increasing the rate of fouling for heat transfer surfaces and burdening the capacity of the flue gas removal equipment including the baghouse  250  and negative pressure source. 
     Ash falling through the reception zone  41  may contain uneven temperatures. The ash in the center of the reception zone may be hotter and more difficult to expose to water. To explain this in more detail, in an exemplary deployment of the disclosed technology, the reception zone may be a housing 8 feet wide by 4 feet deep by 16 feet long. If the sprayers  110  and  120  are positioned along the perimeter of the housing walls, special techniques will need to be employed to make sure that all ash is sprayed with water otherwise uneven cooling of the ash may occur. To help prevent uneven cooling, the controller  420  may change the direction of the sprayers, the pressure of the water emitted by the sprayers, spray interval, or flow rate of water exiting through the sprayers to maintain a desired temperature range of the ash in the ash zone. Various size nozzles may be used based on ash temperature.