Abstract:
A method of constructing an inner thermal lining or jacket of a jacketed gasifier having an outer shell with an opening allowing access to an interior of the gasifier, includes inserting jacket wall segments ( 40 ) into the gasifier interior through the opening, the jacket wall segments ( 40 ) each comprising an elongate jacket plate ( 42 ) with an annulus face ( 44 ) and a plurality of transversely extending longitudinally spaced stiffener formations ( 46 ) standing proud of the annulus face ( 44 ). The jacket wall segments ( 40 ) are arranged side by side leaving an aperture or space between adjacent jacket plates. The stiffener formations ( 46 ) of adjacent spaced jacket wall segments are welded together through the aperture or space between said adjacent spaced jacket wall segments, and the apertures or spaces are closed with window plates.

Description:
TECHNICAL FIELD 
   THIS INVENTION relates to gasifiers. In particular, the invention relates to jacketed gasifiers and to a method of constructing an inner thermal lining of a jacketed gasifier. 
   BACKGROUND OF THE INVENTION 
   Fixed bed gasifiers, such as fixed bed dry bottom gasifiers, are also known as moving bed gasifiers or moving bed dry ash gasifiers. 
   Fixed bed jacketed gasifiers, such as Sasol-Lurgi fixed bed dry bottom gasifiers are being used commercially to gasify carbonaceous material such as coal to produce raw synthesis gas. Such a jacketed gasifier comprises an outer shell or pressure vessel and an inner thermal lining or jacket, which between them define an annulus or jacket space. In use, boiler feed water circulates through the annulus or space or cavity between the jacket and the outer shell by thermosyphon effect, producing saturated steam as a result of heat transfer through the jacket driven by the heat generated through the gasification process occurring inside the gasifier. The inner thermal lining or jacket is subjected to loading and stress as a result of external pressure (in use, the pressure in the annulus or jacket space is typically higher than the gasifier operating pressure), residual installation stresses, thermal fatigue, thermal expansion, clinker crushing and localised hot spots. As a result, the outer shell typically outlasts the jacket and it becomes necessary from time to time to replace the jacket, or at least a cylindrical wall thereof between top and bottom end components. Typically, the annulus or jacket space is inaccessible from outside the gasifier, i.e. welding of jacket components is only possible from inside the gasifier. This invention thus provides, inter alia, a method of constructing an inner thermal lining of a jacketed gasifier, which method can be used to replace the jacket, or portions of the jacket, of a jacketed gasifier. 
   SUMMARY OF THE INVENTION 
   According to one aspect of the invention, there is provided a method of constructing an inner thermal lining or jacket of a jacketed gasifier which includes an outer shell or pressure vessel with an opening allowing access to an interior of the gasifier, the method including 
   inserting jacket wall segments into the gasifier interior through the opening, the jacket wall segments each comprising an elongate jacket plate with an annulus face and a plurality of transversely extending longitudinally spaced stiffener formations standing proud of the annulus face; 
   arranging the jacket wall segments side by side leaving an aperture or space between adjacent jacket plates; 
   welding the stiffener formations of adjacent spaced jacket wall segments together through the aperture or space between said adjacent spaced jacket wall segments; 
   closing the apertures or spaces with window plates and welding longitudinally extending edges of adjacent spaced jacket plates to the intervening window plates to form part of a cylindrical jacket wall; and 
   welding the jacket wall to top and bottom jacket end components. 
   Preferably, the jacket plates have a thickness of less than 25 mm, more preferably less than 18 mm, even more preferably less than 15 mm, e.g. about 12 mm. Advantageously, the thinner the jacket plates the less the thermal stresses are to which the jacket plates are subjected in use, but the jacket plates must be thick enough to form a jacket wall which can withstand the differential pressure across the wall. Using the transversely extending longitudinally spaced stiffener formations, the inventors have surprisingly found that jacket plates as thin as 13 mm or 12 mm can be used, in contrast to conventional 32 mm or 25 mm thick jacket plates. 
   Said annulus faces are typically convexly curved and the stiffener formations are typically part annular, comprising a radial flange on the annulus face of a jacket plate and an end flange arranged perpendicularly to the radial flange, i.e. concentric with and facing the curved annulus face of the jacket plate. 
   The radial flange may be apertured to allow passage through the radial flange of a coolant flowing through the annulus or jacket space. 
   Preferably at least 25%, more preferably at least 35%, most preferably at least 40%, e.g. 50%, of each radial flange is void. 
   The stiffener formations may be welded together with full penetration welds and may be welded together without backing strips. 
   The welding of the longitudinally extending edges of adjacent spaced jacket plates to the intervening window plate may be effected without backing strips. In other words, all vertical welds may be effected without backing strips, advantageously reducing longitudinal seam welding compared to conventional methods of which the inventors are aware. 
   Each jacket wall segment may include top and bottom transition plates or portions to facilitate welding of the jacket wall to the top and bottom jacket end components. These transition plates or portions are typically thicker than the jacket plates, as are the top and bottom jacket end components, e.g. 32 mm or 40 mm. The transition plates or portions may be wider than the jacket plates. 
   The invention extends to a jacketed gasifier with an inner thermal lining or jacket constructed in accordance with the method as hereinbefore described and including jacket wall segments welded to intervening window plates and including adjacent stiffener formation segments which are joined together to form stiffener formations inside a jacket space. 
   According to another aspect of the invention, there is provided a jacketed gasifier which includes 
   an outer shell or pressure vessel and an inner thermal lining or jacket defining a gasification zone, an annulus or jacket space for a coolant being defined between the outer shell and the jacket; and 
   circumferentially extending vertically spaced stiffener formations mounted to the jacket, in the annulus or jacket space. 
   The gasifier may be a fixed bed gasifier and may thus include a carbonaceous material inlet, an ash outlet, a raw synthesis gas outlet and a gasification agent inlet in communication with the gasification zone. The gasifier may also include a rotatable grate above the ash outlet. 
   Typically, the coolant is boiler feed water, with the gasifier in use producing saturated steam in the annulus or jacket space. The gasifier thus typically includes a boiler feed water inlet and a steam outlet in communication with the annulus or jacket space. 
   The inner thermal lining or jacket may include a cylindrical jacket wall comprising jacket wall segments and jacket end components, as hereinbefore described. The jacket wall may have a minimum thickness of less than 25 mm, more preferably less than 18 mm, even more preferably less than 15 mm, e.g. about 12 mm or 13 mm. 
   The stiffener formations may be as hereinbefore described. 
   These and other features of the present application will become apparent to one of ordinary skill in the art upon review of the following detailed description when taken in conjunction with the drawings and the appended claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described, by way of example, with reference to the accompanying diagrammatic drawings in which 
       FIG. 1  shows a schematic vertical section of a jacketed fixed bed gasifier with parts omitted for clarity; 
       FIG. 2  shows a typical temperature footprint of a jacketed fixed bed gasifier; 
       FIG. 3  shows a three-dimensional view of a jacket wall segment and a window plate used in the method of the invention to construct an inner thermal lining of a jacketed gasifier; 
       FIG. 4  shows a top plan view of the jacket wall segment of  FIG. 3 , with a top transition plate omitted for clarity, fitted next to a similar jacket wall segment; and 
       FIG. 5  shows a longitudinal section through the jacket wall segment of  FIG. 3 , taken at V-V in  FIG. 4 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring to  FIG. 1  of the drawings, reference numeral  10  refers generally to a pressurised fixed bed dry bottom jacketed gasifier with many components or parts omitted for clarity. The gasifier  10  comprises an outer shell or pressure vessel  12  and an inner thermal lining or jacket  14  located inside the outer shell  12 . Between the outer shell  12  and the jacket  14 , an annulus or jacket space  16  is defined. The jacket  14  defines a gasification zone  18  in which coal can be gasified. A coal inlet  20  and an ash outlet  22  are provided for feeding coal into the gasification zone  18  and for removing ash from the gasification zone  18 . Typically, the coal is fed through a coal lock (not shown) and the ash is removed by means of a rotatable grate (not shown in  FIG. 1  but illustrated as  19  in  FIG. 2  which also shows a coal distributor  21 ) and an ash lock (not shown). 
   The jacket  14  comprises a top jacket end component  24 , a bottom jacket end component  26  and a circular cylindrical jacket wall  28  extending vertically between the top jacket end component  24  and the bottom jacket end component  26 . In  FIG. 1 , the cylindrical jacket wall  28  is shown with bold lines for clarity. The bottom jacket end component  26  defines a bottom knuckle  30  and the top jacket end component  24  defines a top knuckle  32 . 
   In use, coarse coal is fed through a coal lock or a lock hopper (not shown) into the gasification zone  18 , with steam and oxygen (gasification agent) being fed along a steam and oxygen feed line (not shown) and typically distributed through the rotatable grate. Oxygen is required to combust some of the coal to supply energy for the endothermic gasification reactions. During gasification of the coal, steam is produced in the jacket space  16  due to heat transfer through the jacket wall  28 . This steam is removed from the jacket space  16  by means of a steam outlet (not shown) at a position above the top knuckle  32 . Water which is not converted into steam is carried over into a dam region located above the top knuckle  32 . This carried-over water is then fed back to the bottom of the gasifier via three 3 inch downcomer pipes (not shown), and re-enters the jacket space  16  between the bottom knuckle  30  and the outer shell  12 . The water converted into steam (and removed from the system) is replaced by boiler feed water which is added at the top of each downcomer pipe. The boiler feed water is at a temperature of approximately 105° C. The mixture of re-circulated water and boiler feed water enters the gasifier at a temperature of approximately 215° C. This mixture then flows up through the jacket space  16  by means of thermosyphon effect and is heated to approximately 235° C. producing saturated steam at a pressure of about 2970 kPa. Typically, part of the steam that is generated is returned to the gasifier  10  as gasification agent. 
   In the gasification zone  18 , different reaction zones are distinguishable from top to bottom, namely a drying zone where moisture is released, a devolatilisation zone where pyrolysis takes place, a reduction zone where mainly the endothermic reactions occur, an exothermic oxidation or combustion zone, and an ash bed at the bottom of the gasification zone  18 . As a result of the counter-current mode of operation, hot ash exchanges heat with cold incoming reagents, such as steam and oxygen or air, while at the same time hot raw synthesis gas exchanges heat with cold incoming coal. This results in the ash and raw gas, respectively leaving the gasification zone at relatively low temperatures compared to other types of gasifiers, which improves the thermal efficiency and lowers the steam and oxygen consumption of the gasifier. 
   The temperature profile in the gasifier  10  varies as the coal moves through the different reaction zones in the gasification zone  18 , first as a coal bed and then as an ash bed. With reference to  FIG. 2  of the drawings, a typical gasifier temperature footprint is shown. In a zone  64  (inside the coal distributor  21 ) temperatures of less that 200° C. are experienced. In zones  66  and  68  temperatures vary respectively between about 200° C. and 400° C. and 400° C. and 600° C. In zones  70 , temperatures between about 600° C. and 800° C. are experienced. Temperatures of between about 800° C. and 1000° C. are experienced in a zone  72 , with temperatures in excess of 1000° C. being experienced in a zone  74 . Reference numeral  76  indicates an ash bed. 
   The zone  74  represents a fire-bed. As can be clearly seen in  FIG. 2 , the fire-bed has a varying thickness or depth and a roughly W-shaped profile. Peripheral zones of the gasification zone  18  are typically unstable and very sensitive to changes in grate speed, gasification agent flow and gasification agent ratio. In contrast, a central zone of the gasification zone  18  is typically observed as a relatively stable zone, keeping its position as shown after the gasifier  10  has reached an equilibrium. This zone is not sensitive to changes in grate speed, gasification agent flow and gasification agent ratio. 
   The 600° C. to 800° C. hot spot represented by the zone  70  in the lower right-hand portion of the gasification zone  18  appears randomly in the ash bed  76 . It is believed that this hot spot can be attributed to coal which did not completely react in the fire-bed zone  74  and then reacts upon contact with oxygen at this random position in the ash bed  76 . 
   The temperature of the fire-bed  74  is approximately 1400° C. to 1450° C. depending on ash fusion temperature and steam fraction in the gasification agent feed. The fire-bed height is estimated to be maximum 0.5 m in thickness or depth and fluctuates under typical local channeling conditions. The raw synthesis gas disengages the coal bed at between about 450° C. and 550° C. whereas ash leaves the gasifier  10  at a temperature of between about 300° C. and 380° C. 
   The gasifier  10  typically operates at an operating pressure of about 2900 kPa. The differential pressure across the jacket  14  is thus typically about 70 kPa. The jacket  14  is exposed to material at the temperatures of the zones as shown in  FIG. 2 . Actual metal temperatures of the jacket  14  are however not only determined by temperatures inside the gasification zone  18 , but also by the cooling effect of the boiling water in the jacket space  16 . Typically, actual metal temperatures vary between about 200° C. and 400° C., although the inventors&#39; understanding of high heat flux densities and cooling system limitations, as well as actual thermocouple measurements have indicated that peak metal temperatures exceed 750° C. from time to time. 
   As will be appreciated, as a result of the varying temperature footprint inside the gasification zone  18 , external pressure, residual installation stresses, thermal fatigue, etc., the jacket  14  is subjected to loading and stress. The jacket  14 , and in particular the cylindrical jacket wall  28 , thus warps and buckles over time and it becomes necessary from time to time to replace the jacket  14 , or at least the cylindrical jacket wall  28 . Access to the interior of the gasifier  10 , i.e. the gasification zone  18 , can be obtained through the coal inlet  20 . Welding and other work can thus be carried out on the jacket  14  from inside the gasification zone  18 . It is however not practical to perform work on the jacket  14  from outside the gasifier  10  as the jacket  14  is protected by the outer shell  12 . 
   The jacket wall  28  typically comprises a plurality of elongate vertically extending wall segments and by replacing the jacket wall segments the cylindrical jacket wall  28  can be replaced, thereby extending the useful operating life of the gasifier  10 . In accordance with the invention, the jacket wall segments are replaced with jacket wall segments  40  as shown in  FIG. 3  of the drawings. Each jacket wall segment  40  comprises an elongate curved jacket plate  42  with a convexly curved annulus face  44  to which eight transversely extending longitudinally spaced T-shaped stiffener formations  46  have been welded. The stiffener formations  46  stand proud of the annulus face  44 . Each jacket wall segment  40  further comprises a top transition plate  48  and a bottom transition plate  50  welded to ends of the jacket plate  42 . 
   The jacket plate  42  has a thickness of about 12 mm. The top transition plate  48  gradually increases in thickness from where it is welded to the jacket plate  42  to reach a thickness of about 32 mm. The bottom transition plate  50  gradually increases in thickness from where it is welded to the jacket plate  42  to reach a thickness of about 40 mm. The top transition plate  48  thus has the same material thickness as the top jacket end component  24  and the bottom transition plate  50  has the same material thickness as the bottom jacket end component  26 . The top and bottom transition plates  48 ,  50  are wider than the jacket plate  42  and the jacket plate  42  and top and bottom transition plates  48 ,  50  thus define an I when seen in front or rear view. 
   Each stiffener formation  46  comprises a part annular radial flange  52  welded to the annulus face  44  of the jacket plate  42  and an end flange  54  arranged at right angles to the radial flange  52  and welded to the radial flange  52 . A plurality of apertures or slots  53  are provided in the radial flange  52  so that about 50% of the radial flange  52  is void. The stiffener formations  46  end in line with sides of the top and bottom transition plates  48 ,  50 . Thus, when two jacket wall segments  40  are placed adjacent to one another, the top transition plates  48 , bottom transition plates  50  and stiffener formations  46  of the adjacent jacket wall segments  40  are in contact but an aperture or window is defined between the adjacent jacket plates  42 . This aperture or window is indicated by reference numeral  56  in  FIG. 4  of the drawings. As will be appreciated, it is thus possible to obtain access to the stiffener formations  46  through the aperture  56  and a person working inside the gasification zone  18  can weld the stiffener formations  46  of adjacent jacket wall segments  40  together, via the aperture  56 . Typically, full penetration welds are used to weld the stiffener formations  46  of adjacent jacket wall segments  40  together, without making use of any backing strips. 
   Once all the stiffener formations  46  of adjacent jacket wall segments  40  have been welded together, the aperture  56  is closed by means of a window plate  58  as shown in  FIG. 3  of the drawings. Typically, the window plate  58  has a width of about 135 mm. Longitudinally extending edges of adjacent spaced jacket plates  42  are welded to longitudinally extending edges of the window plate  58  and the top and bottom transition plates  48 ,  50  of adjacent jacket wall segments  40  are welded together, thereby forming the cylindrical jacket wall  28 . Typically, the welding of the longitudinally extending edges of adjacent spaced jacket plates  42  to the intervening window plate  58  is effected without backing strips. The top and bottom transition plates  48 ,  50  of all the jacket wall segments  40  are welded respectively to the top jacket end component  24  and the bottom jacket end component  26  to complete the jacket  14 . Typically, this welding involves backing strips. Smooth grinding of all welds is done and the refurbished gasifier  10  is then subjected to pressure testing. 
   The construction method of the invention, as illustrated, effectively allows thinner jacket plates  42  to be used while still producing a jacket  14  which has sufficient strength to prevent buckling under the design operating differential pressure of the jacket  14 . Boiler feed water circulation is not adversely affected as a result of the apertures  53  in the radial flanges  52 . Heat flux through the jacket  14  is improved. The method of the invention drastically reduces field welding requirements relating to reduced residual stresses and in situ out of roundness deformation. The stiffener formations  46  do not introduce any dead zones into the jacket space  16  and no static build-up of steam around the apertures  53  in the radial flanges  52  is expected. Longitudinal seam welding is reduced by up to 55% compared to conventional jacket construction methods of which the inventors are aware, translating into improved refurbishment time. 
   It should be apparent that the foregoing relates only to the preferred embodiments of the present application and that numerous changes and modifications may be made herein without departing from the general spirit and scope of the invention as defined by the following claims and the equivalents thereof.