Patent Publication Number: US-10767858-B2

Title: Cooling device for a burner of a gasification reactor

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This is a national stage application of International application No. PCT/EP2016/074152, filed 10 Oct. 2016, which claims benefit of priority of European application No. 15189436.7, filed 12 Oct. 2015. 
     The invention relates to a cooling device for a burner of a gasification reactor. The invention also relates to a gasification reactor provided with the cooling device. 
     The cooling device, also referred to as burner muffle, is applicable to cool and otherwise protect the reactor facing end of a burner for a gasification reactor. 
     Gasification is a process for the production of synthesis gas by partial combustion of a carbonaceous feed. The carbonaceous feed may, for instance, comprise pulverized coal, biomass, oil, crude oil residue, bio-oil, hydrocarbon gas or any other type of carbonaceous feed or any mixture thereof. The gasification reaction produces synthesis gas, which is a gas comprising, at least, carbon monoxide and hydrogen. Synthesis gas may be used, for instance, as a fuel gas or as a feedstock for chemical processes. The synthesis gas can be processed, for instance, to make predetermined types of hydrocarbon products, such as, but not limited to, methanol, synthetic natural gas, gasoline, diesel, wax, lubricant, etc. 
     U.S. Pat. No. 4,818,252 describes an arrangement for gasifying finely divided, particularly solid fuel under increased pressure with a multi-pipe wall having a plurality of pipes arranged to be supplied with a cooling medium, the multi-pipe wall limiting a gas-collecting chamber and also limiting a plurality of recesses which form combustion chambers. A burner extends into each recess. Each of the recesses has a plurality of parameters including a depth, a width and an angle of inclination of a peripheral wall, such that at least one of the parameters is changeable. For operation of the gasifying arrangement, the size of the recess may be changed in dependence upon the fuel, the speed of gasification, the temperature of the gasification, or the composition of gases as examples of operating parameters. This can be achieved in an advantageous manner by recess inserts which can change the depth of the recess. The multi-pipe wall structure may hold the recess wall releasably from the multi-pipe wall structure of the gas collecting chamber and may have an independent cooling system. For protecting of the burners, it is recommended to provide a slag-collecting protecting shield. This protecting shield can be formed advantageously from a tubular piece projecting from the cover plate and preferably coated with a layer of a fire resistant (refractory) material. 
     The recess of U.S. Pat. No. 4,818,252 is vulnerable to slag ingress, when the gasification reaction is conducted under conditions wherein a thick layer of viscous liquid slag forms on the inside of the multi-pipe wall. In such a situation the slag will flow in front of the burner head and disturb the combustion. The protecting shield is not adequate to cope with relatively thick layers of slag. 
     U.S. Pat. No. 8,628,595 discloses a gasification reactor comprising a pressure shell, a reaction zone partly bounded by a vertically oriented tubular membrane wall, and a horizontally directed burner having a burner head. The burner protrudes through the membrane wall via a cone-shaped burner muffle, comprising several vertically oriented, concentric and interconnected rings. Successive rings have an increasing diameter relative to preceding neighbouring rings so that the burner muffle has a muffle opening for the burner head at one end and a larger opening at its other flame discharge-end. The rings comprise a conduit having an inlet end for a cooling medium and an outlet for used cooling medium. The muffle opening for the burner head is located between the pressure shell and the membrane wall. At least one ring of the burner muffle protrudes into the reaction zone, to prevent slag from entering the burner muffle and from depositing on the surface of the muffle. The burner muffle of U.S. Pat. No. 8,628,595 enables to cool the surfaces of the burner muffle, resulting in a robust design which can operate at relatively high gasification pressures, exceeding, for instance, 30 bar. 
     The present invention aims to provide an improved burner muffle, having an increased lifespan. 
     The present invention provides a cooling device for a burner of a gasification reactor, the cooling device comprising: 
     several concentric rings of increasing diameter, forming a truncated cone shape having a largest diameter opening for facing the reaction zone of the gasification reactor and a smallest diameter opening for facing a burner head of the burner, each ring being a conduit having an inlet and an outlet for a cooling medium, 
     the cooling device comprising at least one part-circular outer ring having an interruption. 
     In an embodiment, the interruption extends over a predetermined radial angle. 
     The cooling device may comprise two or more part-circular outer rings. 
     The cooling device may comprise one or more first outer rings extending over a first radial angle α, being interrupted over a first angle β, and one or more subsequent outer rings extending over a second radial angle γ, the second radial angle exceeding the first radial angle, being interrupted over a second angle δ. The first radial angle may be about 240°. The second radial angle may be about 260°. 
     According to another aspect, the invention provides a gasification reactor comprising: 
     a pressure shell; 
     a reaction zone partly bounded by a tubular membrane wall enclosed by the pressure shell; 
     at least one burner having a burner head, said burner head protruding the membrane wall; 
     at least one cooling device arranged in the membrane wall and enclosing the burner head of at least one burner, the at least one cooling device comprising several concentric rings of increasing diameter, forming a truncated cone shape having a largest diameter opening facing the reaction zone and a smallest diameter opening facing the burner head, each ring being a conduit having an inlet and an outlet for a cooling medium, the smallest diameter opening for the burner head being located between the pressure shell and the membrane wall; and 
     the cooling device comprising at least one part-circular outer ring having an interruption. 
     In an embodiment, the interruption of the at least one outer ring faces downward, in the direction of gravity. 
     In another embodiment, at least one ring of the cooling device protrudes into the reaction zone. 
    
    
     
       By way of example, embodiments of the invention will be described in detail herein below, with reference to the drawings, wherein: 
         FIG. 1  shows a schematic cross section of an exemplary embodiment of a gasification reactor; 
         FIG. 2  shows a cross section of a burner muffle according to the prior art; 
         FIG. 3  shows a cross section of another burner muffle according to the prior art; 
         FIG. 4  shows a front view of a practical example of a burner muffle according to the prior art; 
         FIG. 5  shows a perspective view of an embodiment of a burner muffle according to the present invention; 
         FIG. 6  shows a front view of an embodiment of a burner muffle according to the present invention; and 
         FIG. 7  shows a cross section of an embodiment of a burner muffle according to the present invention. 
     
    
    
       FIG. 1  shows an exemplary gasification reactor having a tubular pressure shell  1 , a membrane wall  3  and a reaction zone  2 . The reactor and the membrane wall are normally positioned vertically. Section  3   a  of the membrane wall  3  may have a tubular shape. The membrane wall  3  may be composed of conduits for guiding a cooling medium, such as water. The conduits generally extend in a vertical direction. Alternatively, spiraling conduits may be used. 
     Water may be supplied to the membrane wall via supply line  4  and a common distributor  5 . The used cooling water, typically in the form of a mixture of water and steam, may be discharged from the reactor via common header  6  and discharge line  7 . The reactor may comprise a quench gas supply  8  for cooling the produced syngas. A discharge line  9  may discharge the syngas, a mixture of hydrogen and carbon monoxide. Discharge line  10  may be provided to discharge slag. 
     The reactor is typically provided with one or more burners  13  for partial oxidation of a feedstock. Two diametrically opposed burners  13  are shown. The reactor may comprise, for example, two or more pairs of burners at the same elevation, or alternatively at different elevations. Suitable burners for a coal feed are, for example, described in U.S. Pat. Nos. 4,523,529 and 4,510,874. The invention however may relate to burners for any other type of hydrocarbon comprising feedstock as well. The feedstock may be provided to the burners via supply line  11 . Oxygen may be provided via an oxygen supply line  12 . 
       FIG. 2  shows a burner  13  protruding membrane wall  3 . The burner end  17  facing the reactor  2  is provided with a cooling device  14 , having a burner opening  16  for the burner head  17 . The cooling device or burner muffle  14  encloses the burner head. The opening may be located between the pressure shell  1  and the membrane wall  3 . In this example, the burner muffle  14  does not protrude into the reaction zone. Opening  18 , opposite the burner opening  16 , is flush with the membrane wall  3 . 
       FIG. 3  illustrates another prior art example of a burner  13  and a burner muffle  14 . Herein, the cooling device  14  protrudes into the reaction zone  2 . The protrusion prevents slag  32  from entering the burner muffle  14 . Preventing or limiting slag from depositing on the surface of the burner muffle  14  limits local heat fluxes. Due to the protruding burner muffle  14 , the slag  32  will flow around the exterior of the outer ring  30  downwards, preventing the slag from entering the conical recess formed by the cooling device  14 . 
     The cooling device or muffle  14  may protrude into the reaction zone  2  over a distance  36 . A minimum may be predetermined for the distance  36 , depending on the ash properties and ash content in the feedstock. The minimum for distance  36  may be about equal to the average outer diameter of the conduits that form the rings  15 . In a practical embodiment, the distance  36  may be set between about two to four times the average outer diameter of the conduits forming the rings  15 . The distance  36  is defined as the horizontal distance between the outer positioned ring  30  and the surface of the refractory  24  as shown. 
       FIG. 3  shows a burner muffle or cooling device  14  provided with a conduit  34  positioned at or near its upper end. The conduit  34  forms a slag gutter  35  along the upper part of the circumferential defined by opening  18  and outer ring  30 . The conduit  34  has an inlet at one end for a cooling medium and an outlet for used cooling medium at its other end (not shown). 
       FIGS. 2 and 3  further show a burner muffle  14  comprising several vertically oriented, concentric rings  15 . The rings are typically formed by conduits for cooling medium. The cooling medium can be supplied via lines  20 , and discarded via lines  22 . 
     Lines  20  may be fluidly connected to cooling medium distributor  19 . Lines  22  may be connected to a common header  21  respectively. The header  21  typically discards of a mixture of water and steam. The cooling medium, typically comprising water, as supplied via lines  20  may be from the same source as the cooling water supplied to the conduit  33  of the membrane wall  3 . It can be also from a different source, which may have a lower water temperature and/or a different pressure. The rings are preferably welded together. 
     Rings  15  have an increasing diameter relative to its neighbouring ring  15  resulting in that the burner muffle  14  has a muffle opening  16  for the burner head  17  at one end and a larger opening  18  at its other—flame discharge—end  23 . The muffle opening  16  is horizontally spaced away from the larger opening  18 . This results in the connected rings having a cone-shaped form. 
     The angle α 1  between the horizon  26  and the direct line  25   a  between the inner positioned ring  29  at the muffle opening  16  for the burner head  17  and the next ring  29   a , adjacent to the inner ring  29 , is between 15 and 60°. Preferably the angle α 2  between the horizon  26  and the direct line  25  between the inner positioned ring  29  at the muffle opening  16  for the burner head  17  and the outer positioned ring  30  at the opening  18  at the flame discharge end  23  is between 20 and 70°. The line  25  is drawn from the centre of ring  29  to the centre of ring  30  as shown in  FIG. 2 . The line  25   a  is also drawn from the centre to the centre of the ring as shown. Preferably α 1  is greater than α 2 . The outer positioned ring  30  is the ring that forms the muffle opening  16  for the burner head  17 . 
     The number of rings  15  may be between 6 and 10. The rings  15  may form a S-curve along line  25  as shown. Preferably a sealing  28  is present between the shaft of burner  13  and the burner sleeve  36 . The sealing  28  can be extended to the burner head  17  as shown. Such a sealing  28  prevents gas and fly-ash and/or slag as present in the reaction zone from entering the burner sleeve  36  as present in the space between pressure shell  1  and membrane wall  3 . By avoiding such a gas flow, local heat fluxes are further reduced. The sealing  28  may comprise a flexible sealing material which is able to accommodate local thermal expansion. Examples of suitable sealing materials are fibre-woven and or knitted wire mesh type sealing materials. 
       FIGS. 2 and 3  also show part of the membrane wall  3 . The membrane wall  3  may typically comprise several vertical conduits  33  through which a cooling medium can flow. The cooling medium may typically comprise water. The conduits  33  can be provided with supply lines and discharge lines  31  as schematically shown. The conduits  33  may be coated with refractory  24 . 
     In use, the refractory material  24  will be covered by a layer of slag  32 , as for example described in U.S. Pat. No. 4,959,080.  FIGS. 2 and 3  also show an optional refractory mass  27  enclosing the burner muffle  14 . The refractory mass  27  prevents slag from entering the rear end of the muffle  14  and from reaching the burner head  17 . 
     However, in practice, the burner muffles muffles as described above have shown corrosion after a relatively short time of operation, e.g. in the order of a few months. Corrosion was observed, for instance, on the outer rings of the burner muffle and/or at the lower part  90  of the outer rings  18  ( FIG. 4 ). Thickness of the layer of slag below the burner muffles, indicated in  FIG. 4  as reduced slag thickness area  92 , was significantly less than the thickness of the slag layer  94  covering the inner wall of the gasifier in general. Slag coverage at the top  96  and both sides  98  of the burner muffles  14  was typically similar to the slag coverage of the inner gasifier wall. Only minimal slag coverage was found below the burner  13 . 
     The slag layer  94  shields and protects the materials of the burner muffle and the membrane wall from the high temperature and corrosive environment in the gasifier. The protection provided by the reduced slag layer thickness area  92  is correspondingly limited. The corrosion will reduce the lifetime of the burner muffle tubes. Due to the reduced protection provided by the reduced thickness of the slag layer, the membrane wall and/or the burner muffle can be damaged during long time, continuous operation of the gasifier ( FIG. 4 ). 
       FIG. 5  shows a burner muffle  100  for a gasification reactor according to the invention. The upper part  102  of the burner muffle is unchanged with respect to the embodiments as described above. The upper part  102  may extend into the gasification reactor for slag deflection. 
     The burner muffle  100  has a modified lower part. At least one, for instance two or more, of the outer rings  110  of the burner muffle is interrupted over a predetermined radial angle. The interruption  116  faces downward, in the direction of gravity. The, for instance two, interrupted outer rings will form sub-rings, as illustrated in  FIG. 6 . 
     One or more, or all of rings  15  may have individual inlets and individual outlets for cooling medium. Alternatively, two or more of the rings  15  may be interconnected, forming a spiraling ring structure. 
     In an embodiment, one or more outer rings  112  may extend over a first radial angle α, being interrupted over an angle β. One or more subsequent outer rings  114  may extend over a second radial angle γ, exceeding the first radial angle, being interrupted over an angle δ. For instance, a first interrupted outer ring  112  may extend over about 240°, being interrupted over 120°. A subsequent interrupted outer ring  114  may extend over about 260°, being interrupted over 100°. 
     The one or more interrupted rings  110 ,  112 ,  114  may be replaceably connected to the rest of the burner muffle  100 . Outer ring connections  120  may be breakable and replaceable. The connections  120  may be, for instance, welded, clamped, (crimp) fitted, bolted, or otherwise replaceably connected. 
     The interrupted outer rings  110  can be replaced separately, obviating the replacement of the entire burner muffle  100 . This is beneficial, for instance, because: a) the repair time is reduced compared to the exchange of the entire burner muffle; and b) the repair costs are significantly reduced with respect to replacing the entire cooling device  100 . 
     Using a conservative estimation, it is assumed that the entire outer ring, in use, will be covered with slag and has to be able to withstand a maximum specified heat flux of 1500 KW/m 2 . The outer ring herein may include, at least, rings  110 , and optionally also ring  34  indicated in  FIG. 3 . Tests have indicated that, in practice, the estimated maximum specified heat flux of 1500 Kw/m 2  can be exceeded. 
     A full circular ring, extending 360°, can withstand a max heat flux of 1800 KW/m 2  before departure from nucleate boiling (DNB) will occur. Departure from DNB will typically result in immediate damage to the tube of the cooling ring. 
     A part circular ring  110  can withstand an increased heat flux. A part circular ring  112 , extending over for instance 240°, may withstand a maximum heat flux of 2100 KW/m 2  before departure from nucleate boiling will occur. Herein, rings may be made of the same material, for comparison. 
     Given operational challenges in practice, especially in early stages of the process, for instance during start up of a gasification process, higher design margins for DNB in burner muffle tubes are highly recommended. 
     In addition, the interrupted rings of the cooling device of the invention improves repair possibilities. High temperature corrosion, resulting from, for instance, H2S in the syngas, will typically start at the rings closest to the gasification reactor, which are the most exposed to the syngas. 
     In prior art cooling devices, the entire muffle  14  needs to be replaced if, for instance, the outer ring shows heavy wall thinning due to corrosion. Overlay welding or local repairs are possible, but repair quality is always a concern. 
     The accessibility for repair may depend on the protrusion  36  of the muffle. For instance: —A protrusion exceeding 80 mm may allow to exchange one outer ring in situ; —A protrusion exceeding 100 mm may allow to exchange two outer rings in situ. 
     Based on practical experience, the design of the gasification reactor may be modified. For instance, the size of the gasifier has been changed to a so called “intensified” design, wherein the diameter of the gasification reactor  2  is smaller. As a result, the slag load on the gasifier wall increased correspondingly. 
     The burner muffle according to the invention reduces corrosion on the outer rings. The muffle is provided with interrupted outer rings. Also, the outer rings have larger safety factors for departure from nucleate boiling (DNB). In the burner muffle of the invention, slag will not drop from the outer rings, but flow downward on the membrane wall below the burner muffle, covering the membrane wall in the area  92  below the burner muffle, and potentially also the lower section of the burner muffle, with an even layer of slag. The layer of slag provides additional protection from the corrosive environment in the gasifier. Thus, the cooling device of the invention prevents corrosion of the outer rings thereof, limiting corrosion. Also, the device improves the protective slag layer on the membrane wall. This increases the lifespan of the burner muffle and the membrane wall. 
     In a practical application, the temperature in the reactor chamber may typically be in the range of 1500 to 1700° C. The pressure in the reactor chamber may generally be in the range of 25-60 barg. 
     The wall thickness of the conduits of the burner muffle is preferably as small as possible to optimize heat transfer and to limit the wall temperature. The minimum wall thickness will be determined by the mechanical strength of the conduit material, as required locally. The diameter of the conduits  15  may be between about 2 and 5 cm. The rings may be made from a low alloy steel with a Cr content up to 5 wt % or a high alloy steel with Cr content above 15 wt %. 
     The present invention is not limited to the above described embodiments thereof, wherein various modifications are conceivable within the scope of the appended claims.