Patent Publication Number: US-2019194560-A1

Title: Cooling screen with variable tube diameter for high gasifier power

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is the US National Stage of International Application No. PCT/EP2017/071574 filed Aug. 28, 2017, and claims the benefit thereof. The International Application claims the benefit of German Application No. DE 10 2016 216 453.8 filed Aug. 31, 2016. All of the applications are incorporated by reference herein in their entirety. 
    
    
     FIELD OF INVENTION 
     The invention relates to an entrained-flow gasifier for the gasification of solid and liquid fuels at temperatures between 1200 and 1900° C. and pressures between atmospheric pressure and 10 MPa (100 bar), wherein solid fuels are coals of different rank which are ground to fine dust, petroleum cokes or other solid carbon-containing materials, and liquid fuels can be oils or oil-solids suspensions or water-solids suspensions, using a free oxygen-containing oxidation agent, in which gasifier a cooling screen  8  arranged in a pressure shell  15  delimits a reaction chamber  9 . 
     BACKGROUND OF INVENTION 
     In entrained-flow gasifiers, the thermally highly loaded reaction chamber  9  is formed by a cooled tube construction. This construction, the so-called cooling screen  8 , as a whole is pressure-stable only to a limited degree, with the tubes per se being configured to be pressure-resistant. The cooling screen  8  is positioned in a pressure vessel  15 . For reasons of thermal stability of the pressure container, a certain distance between the pressure container and cooling screen is necessary. The thus resulting backspace  10  (also referred to as cooling screen gap) is flushed with an inert gas and has pressure equalization in relation to the reaction chamber, with the result that, in normal operation, equal pressure prevails in the reaction chamber and in the backspace. 
     Since pressure changes in part constitute highly dynamic processes, it must be ensured that pressure equalization can occur in each operating state and that, as a result of a flow directed into the reaction chamber, the penetration of reaction gas and dust into the cooling screen gap  10  is limited. In addition, the cooling screen as a whole must have a certain minimum resistance to pressure differences over its wall. This minimum resistance to pressure differences decreases with an increasing cooling screen diameter and cooling screen height, and therefore this problem is intensified with an increasing gasifier power. Furthermore, the cooling screen is exposed to a high thermal loading and, in order to avoid damage, good heat transfer from the reaction chamber into the cooling water is required. This requirement can be achieved by small tube wall thicknesses, this in turn counteracting the differential pressure resistance of the cooling screen. 
     The prior art presents gasifier values of 500 MW, as described, for example, in DE 197 181 31 A1. In the design described therein, a cooling screen which consists of cooling tubes welded in a gastight manner is situated within a pressure vessel. This cooling screen is supported on an intermediate base and can freely expand upward. This ensures that, upon the occurrence of different temperatures on the basis of starting-up and shutdown operations and resultant change in length, no mechanical stresses occur which could possibly lead to destruction. To achieve this, there is no fixed connection at the upper end of the cooling screen but rather an annular gap between the cooling screen collar and the burner holder flange that ensures free movability and is filled with elastic, thermally resistant fiber mats. These mats are not configured to be gastight and thus allow a dry, condensate-free and oxygen-free gas to flow behind the cooling screen gap. This flushing is intended to prevent hot gasification gas from flowing back into the cooling screen gap upon pressure fluctuations. A disadvantage with this configuration is that these mats are positioned in the annular gap only in a form-fitting manner and can be forced out of the guide under relatively high differential pressures. Consequently, the mats no longer perform their function of limiting the dust transfer from the reaction chamber into the backspace, which ultimately means that reaction gas and dust pass into the cooling screen gap  10  in spite of oppositely directed flushing. The dust and gasification gas transfer into the backspace results, on the one hand, in corrosion occurring on the rear side of the cooling screen or of the pressure shell, which can lead to destruction in the long term, and, on the other hand, the entry of dust into the cooling screen gap  10  also causes an increased CO concentration, after switching off the gasifier, within the reaction chamber and the gas-channeling downstream systems. Inspection and possible repair is thus greatly delayed for safety-related reasons. 
     Alternatively, the gap, as described in DE10 2007 045 321 and DE10 2009 005 856, can be closed by means of a corrugated tube compensator. In this configuration, the flushing gas is channeled from the cooling screen gap  10  into the reaction chamber via an additional pressure-equalizing line connected to the combination burner, in order thereby to ensure the necessary pressure equalization between the cooling screen gap and reaction chamber. A disadvantage with this solution is the high price of compensators of relatively large diameter and the additional amount of tubing required for the pressure-equalizing line. 
     In order to protect the cooling screen at high gasification temperatures and to limit the thermal loading, the cooling screen design described in DE 197 181 31 requires a sufficient layer consisting of liquid and solid slag on the cooling screen. It has been found in practice that this slag layer can form a different thickness depending on the coal used or its ash. As a result thereof, the input of heat into the cooling screen and the amount of heat to be removed therefrom can greatly increase and lead to wall temperatures above the admissible material values and to relatively high thermal wear. 
     In order to avoid damage to the cooling screen in these cases, what is required is a smaller tube wall thickness, but this, on the other hand, leads to smaller admissible pressure differences over the cooling screen wall. This admissible pressure difference is decreased further with an increasing gasifier power, since here the reaction chamber diameter and, associated therewith, the cooling screen surfaces are also increased and result in lower strength values. A remedy to this is provided by a larger tube wall thickness, but this counteracts the goal of a lower wall thickness, reduces the heat transfer and reduces the amount of heat which can be removed. An increased tube wall thickness causes greater temperature differences between the tube inner side and tube outer side, with the result that additional stresses are induced in the tube wall. Both aspects, higher stresses and higher thermal wear, lead to potentially shorter service times of the cooling screen. Consequently, owing to the contrary effect of a changed tube wall thickness, the area of application and the performance of the cooling screen are limited to strength of the cooling screen versus amount of heat which can be removed. 
     SUMMARY OF INVENTION 
     The problem on which the invention is based is to specify a technical solution for the discussed, mutually conflicting requirements. 
     The problem is solved by an object having the features of the claims. 
     The invention makes use of the finding that, through a corresponding burner configuration, the temperature release can be set such that a lower thermal loading can be realized in the conical regions of the cooling screen. 
     The solution according to the invention to the problem lies in a cooling screen configuration with sufficient strengths under high pressure difference over the cooling screen wall and a tube wall thickness which ensures a reliable operation of the cooling screen and a high heat transfer; also provided is a pressure equalization between the cooling screen gap  10  and reaction chamber  9  in all operating states. 
     Advantageous developments of the invention are specified in the subclaims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be explained in more detail below by way of figures as an exemplary embodiment to an extent that is required for understanding. In the figures: 
         FIG. 1  shows an 8-flight cooling screen according to the invention having 4 feet distributed uniformly over the circumference, and 
         FIG. 2  shows a configuration according to the invention with 8 bearing plates and 32 flushing and pressure-equalizing tubes distributed thereon. 
     
    
    
     In the figures, identical designations designate identical elements. 
     DETAILED DESCRIPTION OF INVENTION 
     According to the invention, thin-walled tubes  5  are used in the region of the highest temperature loading, that is to say the cylindrical part of the cooling screen, and, in order to ensure the mechanical strength, thick-walled tubes  3  are used in the conical regions of the cooling screen (at the top and bottom), in particular for the purpose of taking up the bending moments through intrinsic load and under differential pressures which occur. 
     The tubes are furthermore chosen such that the tube outside diameter is kept constant over the entire cooling screen height  8  and the tube wall thickness is varied only over the tube inside diameter. The transition from the smaller to the larger tube inside diameters is here configured to be smooth via a gradual diameter increase  4  in order thus to avoid the creation of “wake areas” in which, owing to discontinuous flow conditions, sufficient cooling cannot be ensured. The maintenance of a uniform outside diameter of the cooling screen tubes leads to a homogeneous configuration of the cooling screen. In addition to production-related advantages (for example automatic welding), the thus ensured uniform tampability of refractory material is particularly advantageous. 
     To further increase the mechanical strength of the cooling screen, the mechanical load of the cooling screen is dissipated into the enclosing pressure shell  15  via feet  1 , thereby further reducing the bending moments overall and thus fundamentally increasing the admissible pressure difference. However, the feet  1  provided simultaneously cause local stress peaks. For this reason, the wall thickness transitions  4  described are, as far as possible, positioned for outside the disturbance region of the feet (region in which local stress peaks can occur by the feet in the presence of mechanical loading). While maintaining a region of thinner wall thicknesses  5  that is, as far as possible, large, the wall thickness transitions  4  are arranged vertically above the feet and, as viewed tangentially, centrally between the feet  1 . 
     With a symmetrical configuration of feet, the following formula for the horizontal arrangement of the wall thickness transitions can be used for positioning the wall thickness transitions  4  in the lower cylindrical region: 
     
       
         
           
             γ 
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                 k 
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              
             
               indicates text missing or illegible when filed 
             
           
         
       
     
     where 
     γ=angle between the center of the foot and the wall thickness transition ( 7 ), 
     n P =number of feet and 
     
       
         
           
             
               k 
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                         n 
                         R 
                       
                     
                   
                   
                     
                       
                         n 
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             , 
           
         
       
     
     where n R =number of cooling screen tubes. 
     Symmetrical configuration means that always the same number of wall thickness transitions is arranged between the feet, that is to say k is an integral number. 
     The vertical distance x between the foot and the first wall thickness transition is chosen such that at least one further tube with a large wall thickness is situated between the uppermost tube connected to the foot and the tube having a wall thickness transition. Here, the foot is advantageously configured such that at least three tubes in the conical region and three tubes in the cylindrical part are fixedly connected to each foot. With an additional fastening of the foot to the upper tubes of the lower conically configured cooling screen part, the load take-up of the cooling screen can be configured to be particularly advantageous. 
     In the upper region of the cooling screen, the thick-walled tubes are used in the conical part and continued into the cylindrical part to such an extent that at least one cooling screen tube achieves half a revolution in the cylinder. A further increase in the cooling screen strength is possible by an optimization of the upper and lower conical cooling screen part in association with an increase in the setting angle  16 . However, since, on the other hand, this increase in the setting angle leads to an increase in the cooling screen gap  10 , the amount of gas to be removed upon emergency depressurization of the reactor  9  increases. With the flushing and pressure-equalizing lines  13  remaining the same, an increased amount of gas in turn increases the pressure difference over the cooling screen and counteracts an increase in the strength by a larger setting angle. Therefore, in an advantageous configuration, an angle  16  of between 35° and 60° is chosen. In the exemplary embodiment of  FIG. 2 , this angle  16  is chosen to be 45°. 
       FIG. 1  represents an exemplary embodiment with eight cooling screen tubes (8-flight cooling screen) and 4 feet distributed uniformly over the circumference. The vertical distance has been chosen to be four tube diameters and the horizontal distance has been chosen to be 22.5°. 
     In spite of the described active measure for increasing the admissible differential pressure over the cooling screen wall, the admissible differential pressure in gasifiers of relatively large output (and hence volume) is less than in the case of relatively small gasifier powers of up to, for example, 500 MW, which means that further measures are necessary in order to ensure secure operation without accumulation of coal dust in the cooling screen gap or corrosion of the pressure container  15  or of the rear side of the cooling screen  8 . It is ensured in terms of construction that there is made available, in each operating state, a sufficiently large flow-traversed area for pressure equalization, but without allowing unhindered dust and reaction gas entry into the backspace of the cooling screen. For this purpose, metal flushing and pressure-equalizing tubes  12  are positioned in the expansion gap of the cooling screen in such a way that, on the one hand, the admissible pressure difference over the cooling screen wall is not exceeded and that, on the other hand, the vertical thermal expansion of the cooling screen remains ensured. For the purpose of preventing dust transfer, the necessary gap remaining for expansion is filled with flexible, thermally stable ceramic fiber mats  11 . For the arrangement of the metal tubes over the circumference, bearing plates  13  are positioned at the upper termination of the cooling screen, with the number of these bearing plates being chosen such that they correspond to the number of cooling screen tubes. The metal tubes  12  are uniformly distributed on these bearing plates, and the remaining annular space between the cooling screen termination and pressure container is sealed by means of fiber mats  11  which are advantageously arranged above the tubes. In order to ensure a directed flow or to avoid backflows, a dry, condensate- and oxygen-free gas as flushing gas is introduced into the reaction chamber  9  via the nozzle  14  and the flushing and pressure-equalizing tubes  12 . 
       FIG. 2  illustrates an exemplary embodiment with eight bearing plates and 32 flushing and pressure-equalizing tubes distributed thereon. 
     The invention is also provided by a reactor for the gasification of solid and liquid fuels in the entrained flow at temperatures between 1200 and 1900° C. and pressures between atmospheric pressure and 10 MPa (100 bar), wherein solid fuels are coals of different rank which are ground to fine dust, petroleum cokes or other solid carbon-containing materials, and liquid fuels can be oils or oil-solids or water-solids suspensions, using a free oxygen-containing oxidation means, wherein the reactor has a cooling screen  8  and a pressure shell  15 , wherein a cooling screen  8  delimits a reaction chamber  9  in a pressure shell  15 , the cooling screen is configured with a plurality of tubes which are wound in parallel and through which a cooling liquid flows, the cooling screen tubes have wall thickness changes with a thicker wall thickness in the lower and upper region and a thinner wall thickness in the central cylindrical region, and the setting angle of the conical cooling screen region has an angle  16  of 35° to 60°. 
     The present invention has been explained in detail for illustrative purposes on the basis of specific exemplary embodiments. Here, elements of the individual exemplary embodiments may also be combined with one another. The invention is therefore not intended to be restricted to individual exemplary embodiments, but rather restricted only by the appended claims. 
     LIST OF REFERENCE SIGNS 
     Key:
       1  Foot     2  Thick-walled cooling screen tube connected to foot     3  Thick-walled cooling screen tube     4  Wall thickness transition of the cooling screen tube     5  Thin-walled cooling screen tube     6  Vertical distance between foot and wall thickness transition     7  Horizontal distance between foot and wall thickness transition     8  Cooling screen     9  Reaction chamber     10  Cooling screen gap     11  Fiber mats     12  Flushing and pressure-equalizing lines     13  Bearing plate for flushing and pressure-equalizing lines     14  Flushing connection in the pressure container     15  Pressure container     16  Setting angle of the conical cooling screen part     17  Conical cooling screen portion at the lower end of the cooling screen     18  Cylindrical cooling screen portion