Process and process plant for converting feedstock comprising a carbon-containing solid fuel

Abstract

The present invention provides process for converting feedstock comprising a carbon-containing solid fuel, such as biomass and/or carbon-containing solid waste material, to synthesis gas and downstream processing of the synthesis gas, the process comprising the following steps: (a) converting the feedstock in a gasifier, the gasifier comprising a fluidized bed zone and a post-gasification zone to produce synthesis gas; (b) obtaining the synthesis gas produced in step (a) downstream of the gasifier; (c) recycling a portion of the synthesis gas of step (b) into at least one upstream process unit, such as the gasifier, being arranged upstream of a position in the process from which said portion is recycled.

1 . A process for converting feedstock comprising a carbon-containing solid fuel, such as biomass or carbon-containing solid waste material, to synthesis gas and downstream processing of the synthesis gas, the process comprising the following steps:
 (a) converting the feedstock in a gasifier, the gasifier comprising a fluidized bed zone and a post-gasification zone, to produce synthesis gas;   (b) obtaining the synthesis gas produced in step (a) downstream of the gasifier;   (c) recycling a portion of the synthesis gas of step (b) into at least one upstream process unit, such as the gasifier, the upstream process unit being arranged upstream of a position in the process from which said portion is recycled.   
     
     
         2 . The process according to  claim 1 , wherein the at least one upstream process unit comprises the gasifier. 
     
     
         3 . The process according to  claim 2 , wherein step (c) comprises at least one of the sub-steps:
 (c1) recycling the portion of the synthesis gas of step (b) into the fluidized bed zone; and   (c2) recycling the portion of the synthesis gas of step (b) into the post-gasification zone.   
     
     
         4 . The process according to  claim 1 , wherein the fluidized bed zone comprises the following subzones, starting from the lowest position of subzone A with respect to the earth's surface:
 Subzone A in which a step (a1) of contacting the feedstock with a gasification agent comprising steam and oxygen is carried out, at an average temperature in the range of between about 350-400° C. below the ash softening temperature of the feedstock, to partially oxidize the feedstock; and   Subzone B, being arranged on top of subzone A, in which a step (a2) of contacting the partially oxidized product produced in step (a1) with a gasification agent comprising steam and oxygen is carried out, at a higher average temperature than in step (a1) and at an average temperature in the range of between about 250-350° C. below the ash softening temperature of the feedstock;   
       and wherein the post-gasification zone comprises the following subzones starting from the lowest position of subzone C with respect to the earth's surface:
 Subzone C, being arranged on top the fluidized bed zone, in which step (a3) of contacting at least a portion of the product produced in the fluidized bed zone, in particular in step (a2), with a gasification agent comprising steam and oxygen is carried out, at a higher average temperature than in the fluidized bed zone, in particular than in step (a2), the average temperature being between about 200-300° C. below the ash softening temperature of the feedstock; and 
 Subzone D, being arranged on top of subzone C, in which step (a4) of contacting at least a portion of the product produced in step (a3) with a gasification agent comprising steam and oxygen is carried out at a higher average temperature than in step (a3), the average temperature being between about 100 to about 250° C. below the ash softening temperature of the feedstock, to produce the synthesis gas. 
 
     
     
         5 . The process according to  claim 4 , wherein at least a part of the portion of step (c) is recycled and introduced into subzone A. 
     
     
         6 . The process according to  claim 4 , wherein the post-gasification zone further comprises:
 Subzone E, arranged on top of subzone D, in which step (a5) of cooling at least a portion of the product produced in step (a4) to an average temperature lower than in step (a4) is carried out, the synthesis gas being cooled by about 50° C. to about 100° C. in terms of the average temperature within subzone E, optionally compared to the average temperature of subzone D; and   wherein at least a part of the synthesis gas portion in step (c) is recycled into subzone E for cooling.   
     
     
         7 . The process according to  claim 1 , wherein step (b) comprises the sub-steps of:
 (b1) cooling the synthesis gas produced in step (a) by a raw gas cooler downstream of the gasifier to obtain a cooled synthesis gas;   (b2) filtering the cooled synthesis gas obtained in step (b1) by a filter unit to obtain a filtered synthesis gas;   (b3) cleaning the filtered synthesis gas obtained in step (b2) by a washing unit to obtain a purified synthesis gas;   (b4) removing carbon dioxide from purified synthesis gas obtained in step (b3) using a CO shift and CO hydrolysis unit to obtain clean synthesis gas; and   (b5) removing further carbon dioxide and H 2 S in an acid gas removing unit to produce clean synthesis gas for further processing.   
     
     
         8 . The process according to  claim 7 , wherein the synthesis gas of which a portion is recycled in step (c) is selected from the any of the synthesis gases obtained in any of steps (b1) to (b5) or a combination thereof. 
     
     
         9 . The process according to  claim 1 , the gasifier further comprising a sedimentation zone with at least one discharge leg, the sedimentation zone being arranged below the fluidized bed zone with respect to the earth's surface, wherein
 heavy solid residues migrating in a downwards direction within the gasifier and being obtained in the fluidized bed zone are discharged from the at least discharge leg of the sedimentation zone; and   at least a part of the synthesis gas portion of step (c) is recycled to the sedimentation zone, in particular to the at least one discharge leg.   
     
     
         10 . The process according to  claim 1 , wherein a cyclone separator, in which fine solids are separated from the synthesis gas, is arranged downstream of the gasifier. 
     
     
         11 . The process according to  claim 10 , wherein the solids are recycled into the gasifier by a return line which connects said cyclone separator and the gasifier; and wherein the at least one upstream unit comprises the return line at least a part of the synthesis gas portion of step (c) is recycled and introduced into the return line. 
     
     
         12 . The process according to  claim 10 , wherein an intermediate cooling unit is arranged downstream and outside of the gasifier and upstream of the cyclone separator, wherein the at least one upstream unit comprises the intermediate cooling unit and at least another part of the synthesis gas portion of step (c) is recycled and introduced into the intermediate cooling unit; and wherein the cyclone separator is a cyclone candle filter unit. 
     
     
         13 . The process according to  claim 1 , wherein the synthesis gas portion in step (c) is at least 2 Ma.-% up to 30 Ma.-% in relation to the total mass of the synthesis gas obtained in step (b). 
     
     
         14 . The process according to  claim 1 , further comprising the step of densifying, in particular pelletizing, the feedstock prior to an introduction into the gasifier. 
     
     
         15 . A process plant for converting feedstock comprising a carbon-containing solid fuel, such as biomass or carbon-containing solid waste material, to synthesis gas and downstream processing of the synthesis gas, the plant comprising:
 a gasifier, the gasifier comprising a fluidized bed zone and a post-gasification zone and being configured to produce synthesis gas;   a downstream arrangement connected to said gasifier, the downstream arrangement being configured to obtain the synthesis gas produced in said gasifier;   a recycling arrangement arranged downstream of the gasifier and being configured to recycle at least a portion of the synthesis gas obtained in the downstream arrangement into at least one upstream process unit, such as the gasifier, the upstream process unit being arranged upstream of a position in the process from which said portion is recycled.