Abstract:
A steam generator used to contain and cool the synthesis gas produced by coal gasification processes employs radiant and convection surfaces, and an integral gasifier, in a specific arrangement to achieve a cost-effective, compact design.

Description:
FIELD AND BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates generally to the field of coal gasification and, in particular, to a steam generator used to contain and cool the synthesis gas produced by coal gasification processes. 
         [0002]    Integrated gasification combined cycle (IGCC) power plants firing solid fuels have traditionally been higher capital cost and have had lower operating availability and reliability than competing solid fuel technologies such as pulverized coal combustion Rankine cycles. Primary components to be improved upon to make IGCC more competitive include uncooled gasifiers and radiant and convective synthesis gas coolers. Radiant synthesis gas cooler designs have a practical limitation of overall outside diameter due to the economics of pressure vessel containment and shipping size limitations to most power plant sites. Within these limits to vessel diameter, there is a need to maximize the compactness of the radiant heat transfer steam generating surface used to cool the gas to minimize the overall height of the radiant synthesis gas cooler. 
         [0003]    U.S. Pat. No. 4,768,470 to Ziegler utilizes coaxial flues constructed of steam generating wall surface to shorten overall cooler height. This design provides for separate flues with independent water circuits to provide for individual lifting, removal and inspection of the inner and outer flues. Another design approach developed by The Babcock &amp; Wilcox Company ca. 1992 utilizes a single flue of steam generating wall surface with additional steam generating surfaces (“wing walls”) suspended inside the flue to maximize surface area and shorten cooler height. Other companies, such as GHH Mann employ similar designs. 
         [0004]    Existing solutions still have not reduced the cost of this component to a competitive level. Single radiant cooler heights to cool synthesis gas for power plants using the largest commercial gas turbines can exceed 150 feet tall. Some plant designs have utilized two coolers, reducing overall height but further increasing costs. Additionally, redundant gasifiers, radiant coolers and convective coolers have been included in plant designs to improve plant operating availability, at a substantially higher cost. 
         [0005]    Existing solutions for convective synthesis gas coolers require a separate component from the radiant cooler, with a cooled flue connecting the two components. Convective coolers designs include both water and steam tube designs (water or steam inside the tubes, gas outside) (Shell Oil Company) and fire tube designs (gas inside the tubes, water outside) (Steinmueller, others). Both of these designs require a pressure vessel enclosure and water/steam system, separate from the radiant cooler. Turbulence created in turns in the gas flue and at the inlet to the convective cooler have created a source of fuel ash fouling that can be difficult to manage. 
         [0006]    Existing solutions for gasifiers include uncooled and cooled refractory enclosures. Uncooled enclosures (General Electric, Conoco, others) have experienced premature failures and frequent replacement. High availability with these designs typically requires a spare gasifier train, and/or firing the gas turbine on oil or gas at higher cost during repair time for the gasifier. Slow heat up and cool down times for thick refractory uncooled designs extend time during outages to repair or replace refractory. Existing cooled gasifier designs (Shell Oil Company, Future Energy) utilize separate water or steam generating circuits with a refractory coating to enclose and contain the gasifier gases. Some of these systems use low pressure, forced circulation cooling water systems that reject the heat outside of the power plant steam/water system, reducing efficiency. Prior art for containing hot solid fuel gases with molten slag in a combustion environment similar to this environment using steam generating surface integral with the downstream cooling circuitry includes Cyclone™ fired boilers (The Babcock &amp; Wilcox Company). 
         [0007]    It is thus clear that development of an economical, compact, reliable and robust synthesis gas cooler is critical to the future of IGCC systems at a commercial scale. 
       SUMMARY OF THE INVENTION 
       [0008]    Accordingly, one aspect of the present invention is drawn to a synthesis gas cooler for extracting heat from synthesis gas produced by a gasification process. The synthesis gas cooler comprises a shell having an inlet and an outlet; a fluid-cooled inner flue contained within the shell for receiving the synthesis gas; a fluid-cooled outer flue contained within the shell for receiving the synthesis gas from the inner flue; radiant heat transfer surface located within the inner flue for cooling the synthesis gas; and means for conveying the synthesis gas from the outer flue to the outlet. 
         [0009]    Another aspect of the present invention is drawn to a synthesis gas cooler as described above which employs an arrangement of convection heating surface, located within the outer flue, for further cooling the synthesis gas. 
         [0010]    Yet another aspect of he present invention is drawn to a synthesis gas cooler which employs not only radiant and convective heating surface within the same shell, but also incorporates an integral gasifier. 
         [0011]    The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and he specific benefits attained by its uses, reference is made to the accompanying drawings and descriptive matter in which preferred embodiments of the invention are illustrated. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    In the Figures: 
           [0013]      FIG. 1  is a schematic sectional side view of a first embodiment of a radiant synthesis gas cooler according to the present invention; 
           [0014]      FIG. 2  is a sectional view of  FIG. 1  viewed in the direction of arrows  2 - 2  of  FIG. 1 ; 
           [0015]      FIG. 3  is a schematic sectional side view of a second embodiment of a radiant synthesis gas cooler illustrating placement of convection heating surface according to the present invention; and 
           [0016]      FIG. 4  is a schematic sectional side view of a third embodiment of a radiant synthesis gas cooler illustrating placement of an integral gasifier according to the present invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0017]    Referring to the drawings generally, wherein like reference numerals designate the same or functionally similar elements throughout the several drawings, and to  FIG. 1  in particular, there is shown a schematic sectional side view of a first embodiment of the present invention, drawn to a radiant synthesis gas cooler, generally designated  10 . The synthesis gas cooler  10  is typically a cylindrical vessel having its longitudinal axis oriented vertically. In this embodiment, the cooler  10  is provided with hot synthesis gas  12  from a gasifier (not shown) at an inlet  14  provided at the top of the cooler  10 . The gas  12  enters an inner flue or enclosure region  16  provided within the cooler  10 . The inner flue is defined by inner enclosure wall  18 , preferably cylindrical in shape, and comprised typically of fluid-cooled tubes. The working fluid within the tubes may be water, steam or a mixture thereof. In addition to the fluid-cooled tubes forming the inner enclosure wall  18 , the inner flue  16  is also provided with a plurality of fluid-cooled, wing wall surfaces  20  which are internally suspended within the cooler  10  so that a significant portion of the wing walls  20  are exposed to the incoming synthesis gas  12 , thereby heating the working fluid (again water, steam or a mixture thereof) conveyed through the wing walls  20 . The wing walls  20  are generally constructed as a planar bank of tubes provided adjacent to one another, and are provided with inlet and outlet manifolds or headers  22  which distribute or collect the working fluid conveyed through the wing walls  20 . The number and arrangement of the wing walls  20  provided would be determined by heat transfer and other requirements. Thus, while  FIG. 2  illustrates an arrangement of six (6) wing walls  20  arranged around the vertical longitudinal axis of the cooler  10 , a greater or fewer number of wing walls  20  may be provided to suit particular heat transfer and cooling requirements. As the hot synthesis gas  12  flows down through the inner flue  16 , it is cooled by the inner enclosure wall  18  and the wing walls  20 , and at a bottom region  24  of the inner flue  16  the synthesis gas  12  turns upwardly substantially 180 degrees through one or more openings  26  provided in the inner enclosure wall  18  and then into an outer flue or enclosure region  28  defined by the inner enclosure wall  18  and a similarly constructed outer enclosure wall  30 . The outer flue  28  thus has a substantially annular shape. Manifolds or headers  32  may be provided to facilitate formation of these openings  26 , if necessary. The synthesis gas  12  is then conveyed upwardly through the outer flue  28 , through one or more openings  34 , and then out of the cooler  10  via synthesis gas outlet  36 . 
         [0018]    The distance between the inner enclosure wall  18  and the outer enclosure wall  30 , as well as the distance between the outer enclosure wall  30  and a shell  38  forming the cooler  10  will be of a size sufficient to provide access and inspection when the cooler  10  is out of service. The enclosure walls  18 ,  30  forming the inner and outer flues  16 ,  28 , respectively, will preferably be provided as independent fluid circuits to provide for individual lifting, removal and inspection. All water/steam generating surface will be arranged to provide for natural circulation, avoiding the need for a forced circulating system with circulation pumps. Solids entrained in the hot synthesis gas  12  flowing downwardly through the inner flue  16  will tend to fall out of the synthesis gas  12  at the bottom region  24  where the synthesis gas  12  makes an approximately 180 degree turn upwardly into the outer flue  28 . The solids fall into a water bath  38  provided at a lower portion of the cooler  10 , thereby permitting the solids to be cooled and removed via solids outlet  40 . Sootblowers  42  may be provided at the openings  26  provided at the bottom region  24  where the synthesis gas  12  makes the 180 degree turn into the outer flue  30 , if required to prevent pluggage from accumulated solids. 
         [0019]    The combination of the inner and outer flues  16 ,  28 , with the wing walls  20  located within the inner flue  16 , results in an overall height of the cooler  10  that is substantially less than with either construction individually. Providing independent inner and outer flues  16 ,  28  with space for lifting and removal while accommodating the wing wall headers  22  and connections thereto (not shown) inside the inner flue  16  will require a novel inner flue  16  design, particularly at the bottom of the inner flue  16 . 
         [0020]    Depending upon the amount of heat in the synthesis gas  12  provided to the cooler  10 , additional heating surface may be required, and a second embodiment of the present invention to accomplish this task is illustrated in  FIG. 3 . As those skilled in the art will appreciate, the second embodiment shares several design features with the first embodiment of  FIG. 1 , and in particular also provides an arrangement of convection heating surface  50  arranged within the outer flue  28  as shown. This convection heating surface  50  can be water or steam cooled, and comprised of one or more banks of tubes arranged so that the synthesis gas  12  flows over the outside of the tubes. The banks of convection heating surface  50  may be provided within the outer flue  28  anywhere around the perimeter of the cooler  10 . In one specific feature of this embodiment, the convection heating surface  50  may employ the same fluidic circuitry (an integrated cooling approach) as is employed in the steam generating surface comprising the inner and outer enclosure walls  18 ,  30 , respectively, thus eliminating the need for a separate cooling system. Alternatively, a separate fluidic circuit may be employed for the convection heating surface  50 . Synthesis gas  12 , after flowing over the convection heating surface  50 , exits the outer flue  28  via openings  34 , and exits the cooler via gas outlet  36 . Sootblowers  52  can be provided to clean the convection heating surface  50  to prevent pluggage. 
         [0021]    The convection heating surface  50  eliminates the need for a convection cooler component separate and detached from the radiant cooler  10 , as well as the otherwise attendant connecting flues with turns, pressure vessel containment for same and, in the case of the aforementioned integrated cooling approach, a separate cooling system. The synthesis gas  12  flowing from the radiant cooling section (the inner flue  16 ) upwardly over the convection heating surface  50  located within the outer flue  28  travels substantially in a straight line, minimizing gas turbulence at the inlet to the outer flue  28 . This minimizes the potential for uncontrollable ash pluggage, and permits the ability to provide sootblowers  52  adjacent the convection heating surface  50  to clean same. This design is especially advantageous as compared to the turbulence and attendant uncontrollable pluggage problems typically encountered at the abrupt entrance to tubes at the inlet tubesheet of a fire tube cooler design. 
         [0022]    If desired, a further simplification of the structures and equipment employed in a gasification system can be accomplished by means of a third embodiment of the present invention, as illustrated in  FIG. 4 . As shown therein, this arrangement extends the tubes comprising the enclosure wall  18 , and which defines the inner flue  16 , upwardly to form an integral, fluid-cooled gasifier enclosure region  60  in an upper region of the cooler  10 . The integral gasifier  60  is thus positioned within the cooler  10  to provide the synthesis gas  12  to the inner flue  16 . The tubes forming the enclosure wall  62  of the gasifier enclosure region  60  would have a refractory coating  64  to protect the surface of the tubes from molten slag and to maintain the gasifier enclosure region  60  environment at temperatures sufficient for the proper gasification reactions to occur. 
         [0023]    This gasifier enclosure region  60  according to the present invention overcomes the problems associated with uncooled, refractory gasifiers, as well as the prior art for cooled gasifiers. The present invention improves on prior cooled gasifier designs by integrating the cooling circuitry for the gasifier enclosure region  60  into the same fluid-cooled circuitry as that provided for the radiant cooler  10 , eliminating the need for a separate cooling system. This design also recovers the heat rejected from the gasifier enclosure region  60  and transfers it into the gasification plant&#39;s steam/water system, thereby improving efficiency and providing modest fuel cost savings over the life of the unit. 
         [0024]    The above discussion of each of the three design embodiments list the technical advantages of each over the prior art. From a commercial perspective, the combined inner/outer flue with wing wall design concept substantially reduces cost by significantly reducing the overall height of the radiant synthesis gas cooler. These cost reductions are obtained not just from the reduced cost of the outer vessel, but also from transportation costs, fuel piping costs, steel structure costs, and costs to construct the component on site. Providing separable inner and outer flues minimizes maintenance costs. This is important with a gasification process cooler, which experiences a more aggressive corrosion environment and requires more maintenance over time than combustion gas coolers used in a conventional pulverized coal plant. 
         [0025]    The incorporation of the convection heating surface integrally within the radiant cooler enclosures eliminates the cost of a separate component. The cost savings are substantial here as well, because in addition to saving on an extra pressure vessel there are also savings in reduced gas flue and steam/water piping costs, steel structure costs and construction costs. The savings from higher availability on solid fuel, due to reduced or eliminated convective cooler plugging, can be more than the entire capital cost of a convective cooler over the life of the unit. 
         [0026]    The incorporation of an integral, cooled gasifier provides modest cost savings over separate cooled gasifiers by eliminating the need for separate pressure vessels and some of the cooling circuitry. While it may be somewhat more expensive in capital cost as compared to an uncooled gasifier, it is believed that the higher availability using solid fuels will be substantial, greatly exceeding any capital cost difference. 
         [0027]    The cost savings from combining some or all of the three design concepts to allow elimination of a spare component train are significant. Again, these savings expand beyond just the extra components to include all the supporting equipment and steel structures surrounding the components and the construction costs associated with building it. It will thus be appreciated that an important, fundamental improvement provided by the present invention involves consolidating individual components into one integrated component to make it compact, low cost, more reliable, and more maintainable. 
         [0028]    While specific embodiments of the invention have been shown and described in detail to illustrate the application of the principles of the invention, those skilled in the art will appreciate that changes may be made in the form of the invention covered by the following claims without departing from such principles. In some embodiments of the invention, certain features of the invention may sometimes be used to advantage without a corresponding use of the other features. Accordingly, all such changes and embodiments properly fall within the scope of the following claims.