Patent Publication Number: US-9428702-B2

Title: Agglomerator with ceramic matrix composite obstacles

Description:
BACKGROUND 
     Gasification is one method for extracting energy from organic materials. Gasification is a process that converts carbonaceous materials, such as coal, petroleum, biofuel or biomass, into carbon monoxide and hydrogen by reacting a raw material at high temperature with a controlled amount of oxygen and/or steam. The resulting gas mixture is called syngas. 
     One of the byproducts of gasification is ash (e.g., fly ash). Ash is one of the residues generated during the combustion of char in a gasifier. Fly ash includes the fine particles that rise with flue gases. Ash which does not rise is termed bottom ash. Ash material must be removed from the syngas before it can be used as a fuel. Gasification systems typically use one or more separation methods to remove ash from syngas. 
     Cyclone separators (cyclones) are used to remove particulates from an air, gas or liquid stream, without the use of filters, through vortex separation. Cyclones can be used to remove some of the ash material from the syngas. However, ash particles having particle diameters less than about 10 μm are not easily removed from a gas stream using cyclones. Due to the small particle size of the ash, the ash is not easily separated from the gas stream and much of the ash exits the cyclone with the gas stream. In gasification systems producing ash particles with diameters less than about 10 μm, additional separation steps are needed. 
     These gasification systems often employ candle filters. Candle filters are often metallic or ceramic, and each has drawbacks. Metallic candle filters are vulnerable to acid gas corrosion. Sulfur and alkali metal oxy-hydroxides within the syngas stream can form acid gas, which corrodes metal candle filters leading to reduced filter life and frequent filter replacement. Ceramic candle filters are fragile and also susceptible to corrosion. Ceramic candle filters are subjected to high temperatures during separation. These high temperatures can lead to cracks in the ceramic candle filter. Constituents of the syngas stream can also corrode or oxidize the ceramics. Both metal and ceramic candle filters are also vulnerable to inter-pore plugging and failure when the ash particles are submicron (diameters less than 1 μm). Additionally, metal and ceramic candle filters are large and expensive to install, operate and maintain. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a simplified schematic of a prior art gasification system having cyclones and candle filters. 
         FIG. 2  is a simplified schematic of a gasification system having a slag agglomerator. 
         FIG. 3  illustrates a cross-section of the inlet of a slag agglomerator. 
         FIG. 4  illustrates a horizontal cross-section of the slag agglomerator of  FIG. 3 . 
         FIG. 5  illustrates a cross-section through one embodiment of an agglomerator cylinder (obstacle). 
         FIG. 6  illustrates a cross-section through another embodiment of an agglomerator cylinder (obstacle). 
         FIG. 7  illustrates a slag agglomerator having a slag outlet. 
         FIG. 8  is a simplified flow diagram of a method for removing slag from a gas. 
     
    
    
     DETAILED DESCRIPTION 
     The slag agglomerator described herein has ceramic matrix composite obstacles (agglomeration cylinders or tubes). The slag agglomerator groups small slag droplets present in the syngas together to facilitate downstream slag removal that does not require candle filters. The small slag droplets impinge on the obstacles, ultimately forming larger slag particles that can be separated from the gas stream using a cyclone. 
       FIG. 1  illustrates a portion of a typical gasification system that produces fly ash with small particle diameters. Gasification system  100  includes gasifier  102 , quench station  104 , cyclone  106  and candle filters  108 . During operation of gasification system  100 , a hot gas stream exits gasifier  102 . The hot gas stream includes syngas. Fly ash is also carried by the gas stream. The gas stream exits gasifier  102  at a temperature of about 1370° C. (2500° F.). The gas stream enters quench station  104  and is cooled to about 370° C. (700° F.). The cooled gas stream then enters cyclone  106  where fly ash is separated from the gas stream by vortex separation. In gasification systems that produce fly ash having small particle diameters (less than about 10 μm), cyclone  106  is unable to remove sufficient amounts of fly ash from the gas stream. In these cases, candle filters  108  are required to complete the separation of fly ash from the gas stream. The gas stream exits cyclone  106  and is passed through one or more candle filters  108  to remove sufficient quantities of fly ash from the gas stream. Upstream quench station  104  must cool the gas stream to a relatively low temperature to prevent damage to candle filters  108 . 
     As noted above, candle filters  108  are susceptible to corrosion, can be fragile, possess a large footprint and require significant installation, operation and maintenance costs. Eliminating candle filters from gasification systems can decrease system cost and increase overall system efficiency. Simply removing candle filters is not an option for gasification systems that produce small particle fly ash, however. The slag agglomerator described herein employs ceramic matrix composite obstacles to increase the particle size of slag (molten fly ash) so that cyclone separation is sufficient to remove fly ash from the gas stream. 
       FIG. 2  illustrates a gasification system with a slag agglomerator. Gasification system  10  includes gasifier  12 , slag agglomerator  14 , quench station  16  and cyclone  18 . A gas stream containing syngas and fly ash is produced in gasifier  12 . In some cases, gasifier  12  produces fly ash having particle diameters less than about 10 μm. Gasifier  12  can also produce fly ash having submicron (less than 1 μm) particle diameters. In order for fly ash particles of this size to be removed from the gas stream using only a cyclone (i.e. no candle filters), the particle size of the fly ash must be increased before the particles reach cyclone  18 . Slag agglomerator  14  increases the particle size of the fly ash. 
     At high temperature, fly ash particles melt and form liquid slag droplets. Slag agglomerator  14  increases the particle size of the slag droplets by causing them to agglomerate and grow in size.  FIGS. 3 and 4  illustrate cross-sections of one embodiment of slag agglomerator  14 .  FIG. 3  illustrates a cross-section of the inlet of slag agglomerator  14  while  FIG. 4  illustrates a horizontal cross-section of slag agglomerator  14 . In one embodiment of gasification system  10 , the gas stream from gasifier  12  arrives at slag agglomerator  14  having a temperature between about 1260° C. (2300° F.) and about 1480° C. (2700° F.). Within this temperature range, fly ash particles form slag droplets. 
     Slag agglomerator  14  contains a plurality of obstacles  20  (agglomeration cylinders or tubes) and includes inlet  22  and outlet  24 . Obstacles  20  are positioned oblique or perpendicular to the flow of the gas stream. As shown in  FIG. 3 , obstacles are positioned vertically in exemplary embodiments. As the gas stream flows through slag agglomerator  14 , slag droplets present in the gas stream impinge upon and adhere to obstacles  20 . Obstacles  20  are arranged within slag agglomerator  14  so that substantially all slag droplets above 0.1 μm in diameter and below 10 μm in diameter impinge upon at least one obstacle  20  before reaching outlet  24 . Obstacles  20  can be arranged within slag agglomerator  14  in rows as shown in  FIG. 4 . As the gas stream flows through slag agglomerator  14 , the presence of obstacles  20  forces the gas stream to take a tortuous path through slag agglomerator  14 . Due to the gas stream velocity and curvatures, slag droplets within the gas stream impinge on obstacles  20 . The dimensions, number, number of rows and placement of obstacles  20 , are determined for slag agglomerator  14  depending on the characteristics of gasification system  10 . A detailed particle impact analysis is performed to determine the appropriate layout of obstacles  20 . Factors considered in such an analysis include the gas stream velocity, fly ash/slag particle size, obstacle dimensions and pressure drop, among others. 
     Obstacles  20  have exterior surfaces  26  containing a ceramic matrix composite (CMC). As described in further detail below, obstacles  20  are actively cooled to solidify a portion of the slag droplets that impinge on obstacles  20 . These frozen slag droplets will stick to the CMC forming a protective coating that prevents detrimental CMC corrosion and erosion. The heat flux through cooled obstacles  20  is maintained so that most of the slag droplets striking obstacles  20  remain molten and either flow down obstacles  20  to be removed from the bottom of slag agglomerator  14  or are re-entrained into the gas flow from the downstream side of obstacles  20  having larger drop sizes. Ceramic matrix composites can tolerate significant tensile stress and thermal shocks without cracking or breaking, making them resistant to the temperatures and forces of the gas stream and slag droplets flowing through slag agglomerator  14 . Ceramic matrix composites can provide much more strength than monolithic ceramic materials. Ceramic matrix composites constitute ceramic fibers embedded in a ceramic matrix. The CMC of obstacles  20  includes a matrix component and reinforcing fibers. In one exemplary embodiment, the matrix component of obstacles  20  is silicon carbide (SiC). Silicon carbide has high thermal conductivity properties. Additionally, silicon carbide present on exterior surfaces  26  of obstacles  20  chemically reacts with molten slag droplets. The molten slag droplets react with silicon carbide to form frozen iron silicide (Fe 3 Si). As slag droplets impinge on exterior surface  26  of obstacle  20 , the formed iron silicide produces a bonding layer on exterior surface  26 . This bonding layer allows additional slag droplets to solidify and adhere to exterior surface  26  of obstacle  20 . The bonding layer eventually covers the upstream side of exterior surface  26 , providing additional protection to obstacle  20 . In alternative embodiments, the matrix component of obstacles  20  is selected from alumina, silica, chromia, mullite and combinations thereof. 
     The reinforcing fibers of the CMC used in obstacles  20  provide support to the matrix component. Suitable materials for the reinforcing fibers include carbon, silicon carbide, alumina, mullite and combinations thereof. Silicon carbide fibers include those sold under the trade name Nicalon™ (Nippon Carbon Company). Alumina fibers include those sold under the trade name Nextel 610™ (3M). Mullite fibers include those sold under the trade name Nextel 720™ (3M). Ceramic matrix composites made from the combinations of matrix components and reinforcing fibers described above offer good resistance to thermal shock. 
     Obstacles  20  are actively cooled so that a portion of the slag droplets that impinges on exterior surfaces  26  of obstacles  20  solidify. As noted above, the gas stream entering slag agglomerator  14  can have a temperature between about 1260° C. (2300° F.) and about 1480° C. (2700° F.). At this temperature, the fly ash present in the gas stream is in a liquid state and forms slag droplets. As a slag droplet impinges on exterior surface  26 , heat is transferred from the slag droplet to exterior surface  26 , thereby reducing the temperature of the slag droplet. By reducing the temperature of the slag droplet, the droplet transitions from the liquid phase to the solid phase and solidifies on exterior surface  26 . Obstacles  20  are cooled to a temperature that causes slag droplets impinging on exterior surface  26  to solidify. In exemplary embodiments, obstacles  20  are cooled so that exterior surfaces  26  have a temperature between about 760° C. (1400° F.) and about 925° C. (1700° F.). This temperature range is below the slag solidus temperature, which is between about 1090° C. (2000° F.) and about 1260° C. (2300° F.). In one embodiment of gasification system  10 , obstacles  20  are water cooled. Water used to cool obstacles  20  can be liquid water, steam, superheated steam and combinations thereof. Other coolants such as gaseous nitrogen, argon, carbon dioxide and their combinations can also be used. 
     As the solid slag coating is formed over obstacles  20 , steady-state conditions are reached whereby the heat transferred to the coolant of obstacles  20  is equal to the convective heat transferred to obstacles  20  from the gas stream. At this steady-state condition, no additional freezing of the slag droplets occurs when they impact obstacles  20 . Instead, the slag droplets either flow down obstacles  20  to be collected and drained from the bottom of slag agglomerator  14  or the slag droplets are re-entrained into the gas stream from the back-side of obstacles  20  as a droplet with a significantly increased diameter. 
       FIG. 5  illustrates one embodiment of obstacle cooling. Disposed within obstacle  20  is coolant tube  28 . Coolant tube  28  is located inside and surrounded by the CMC material that makes up CMC shell  30 . Coolant tube  28  extends the length of obstacle  20  connecting each longitudinal end of obstacle  20  to a coolant manifold. One such manifold, coolant manifold  32  is illustrated in  FIG. 5 . Ceramic matrix composite liner  34 , metal containment shell  36  and coolant tube closeout ring  38  separate coolant manifold  32  from the portion of slag agglomerator  14  that contains the hot gas stream and agglomerator cylinders  20 . Ring seal  40  prevents coolant from entering the portion of slag agglomerator  14  that contains the hot gas stream and prevents the hot gas stream from entering coolant manifold  32 . 
     In exemplary embodiments, coolant tube  28  is metal, such as stainless steel. A coolant (cooling water, steam, etc.) is delivered from coolant manifold  32  to coolant tube  28  to cool obstacle  20  so that CMC shell  30  and exterior surface  26  is cool enough to cause a fraction of the impinging slag droplets to solidify on exterior surface  26 . In exemplary embodiments, the coolant delivered to coolant tubes  28  has a temperature between about 315° C. (600° F.) and about 425° C. (800° F.). The heat absorbed by the coolant in coolant tube  28  can be recovered and used for other purposes (e.g., drive steam turbines, reuse as steam in gasification process). 
       FIG. 6  illustrates another embodiment of obstacle cooling. Instead of having coolant tube  28  disposed within obstacle  20 , the coolant directly cools the CMC material of CMC shell  30 . Coupling (nipple)  42  connects obstacle  20  to metal tube  44 . Metal tube  44  extends from coolant manifold  32 . Ceramic matrix composite liner  34 , metal containment shell  36  and coolant tube closeout ring  38  separate coolant manifold  32  from the portion of slag agglomerator  14  that contains the hot gas stream and agglomerator cylinders  20 . Ring seal  40  prevents coolant from entering the portion of slag agglomerator  14  that contains the hot gas stream and prevents the hot gas stream from entering coolant manifold  32 . 
     In exemplary embodiments, coupling  42  is constructed of silicon nitride. This embodiment provides increased thermal efficiency compared to the embodiment illustrated in  FIG. 5 . In exemplary embodiments, the coolant delivered to obstacles  20  has a temperature between about 315° C. (600° F.) and about 650° C. (1200° F.). The heat absorbed by the coolant traveling inside CMC shell  30  can be recovered and used for other purposes (e.g., drive steam turbines, reuse as steam in gasification process). 
     However, where a CMC having silicon carbide is used for CMC shell  30 , the cooling water must be substantially free of oxygen and have a temperature lower than about 370° C. (700° F.). Oxygen present in the cooling water will react with silicon carbide present in CMC shell  30  to form silica (SiO 2 ). Water will further react with the silica to form silica oxy-hydroxides. These reactions corrode CMC shell  30  and will cause deterioration of obstacle  20 . To avoid these reactions, the cooling water used must be substantially free of oxygen. In embodiments where superheated steam is used as the cooling water, the temperature of the superheated steam must be maintained below about 650° C. (1200° F.). Temperatures above this limit can cause the steam to dissociate into molecular hydrogen and molecular oxygen, resulting in the presence of oxygen within the cooling water and subsequent corrosion of obstacle  20 . 
     As slag droplets impinge on exterior surfaces  26  of obstacles  20 , some of the slag droplets solidify and adhere to exterior surfaces  26 . Additional gas and slag droplets from gasifier  12  are delivered to slag agglomerator  14 , resulting in continued slag build up on obstacles  20  until a steady-state coating thickness of solid slag has been reached. Due to the velocity of the gas stream flow through slag agglomerator  14  and the impingement of additional slag droplets, the subsequent molten slag adhered to exterior surfaces  26  of obstacles  20  will eventually flow down exterior surfaces  26  of obstacles  20  or dislodge. The rate of slag dislodge is determined by gas stream velocity, surface tension and slag composition, temperature and viscosity. Typically, the dislodged slag will have a drop size larger than the slag droplets that entered slag agglomerator  14  in the gas stream. This larger drop of dislodged slag will be carried out of slag agglomerator  14  through outlet  24  by the gas stream. Molten slag that flows down exterior surfaces  26  of obstacles  20  to the bottom of slag agglomerator  14  are generally too large to be carried by the gas stream. In one embodiment of slag agglomerator  14 , this molten slag will be removed through slag outlet  46  in a bottom portion of slag agglomerator  14  as shown in  FIG. 7 . Dislodged slag drops carried by the gas stream may impinge on additional downstream obstacles  20  and will either be carried to outlet  24  by subsequent re-entrainment in the gas stream or discharged from slag outlet  46 . These dislodged slag drops will typically have drop sizes larger than 10 μm in diameter, allowing these drops to be subsequently cooled and separated from the gas stream in downstream cyclone  18 . 
     Once the gas stream (with slag drops) leaves slag agglomerator  14 , the gas stream is quenched at quench station  16 . Quench station  16  cools the gas stream using cooling water or other heat exchange material so that syngas and slag particles in the gas stream can be further processed downstream of slag agglomerator  14 . The heat absorbed by the cooling water in quench station  16  can be recovered and used for other purposes (e.g., drive steam turbines, reuse as steam in gasification process). Since candle filters are not used in gasification system  10 , the gas stream does not need to be cooled as much. In gasification system  100 , having candle filters  108 , quench station  104  typically needs to cool the gas stream to a temperature below about 370° C. (700° F.). As no candle filters are used in gasification system  10 , quench system  16  need only cool the gas stream to a temperature below about 925° C. (1700° F.), increasing the overall efficiency of gasification system  10 . The hotter post-quench gas stream in gasification system  10  can also be passed through a heat exchanger to 
     Cyclone  18  separates the slag particles from syngas in the gas stream. Because the slag droplets impinged on obstacles  20  and exited slag agglomerator  14  as slag having increased particle size, cyclone  18  can sufficiently separate the syngas and slag particles. One or more cyclones  18  can be employed for the separation. Cyclone  18  does not possess the installation, operation or maintenance costs associated with candle filters. Thus, the cost efficiency of gasification system  10  is improved relative to gasification system  100 . 
       FIG. 8  illustrates method  48  for removing fly ash (slag) from a gas using the above described slag agglomerator  14  and gasification system  10 . In step  50 , a plurality of obstacles are positioned within a slag agglomerator. Each obstacle has an exterior surface containing a CMC. In step  52 , a flow of gas and slag droplets are delivered to the slag agglomerator. The gas and slag droplets are delivered so that substantially all slag droplets impinge on the exterior surface of at least one obstacle before exiting the slag agglomerator. In step  54 , the obstacles are cooled with water so that slag droplets impinging on the exterior surfaces of the obstacles solidify on the exterior surfaces. In step  56 , the flow of gas and slag that has passed through the slag agglomerator is delivered to a cyclone to separate the slag from the gas. 
     A slag agglomerator having ceramic matrix composite agglomeration tubes improves the overall efficiency of a gasification system that produces fly ash particles having small diameters. The slag agglomerator increases the particle size of slag droplets present in syngas to facilitate downstream slag removal that does not require candle filters. Small slag droplets impinge on obstacles within the slag agglomerator to form larger slag particles that can be separated from the gas stream using only a cyclone. 
     While the invention has been described with reference to an exemplary embodiment(s), it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment(s) disclosed, but that the invention will include all embodiments falling within the scope of the appended claims.