Abstract:
A process and system for removing tars from synthesis gas uses glycerol produced as a byproduct of biodiesel manufacture. The biodiesel may be made from various oil feedstocks such as canola, rapeseed, or soybean oils. Associated with the harvesting of these crops may be the ready availability of byproduct biomass useful as feedstock for gasification. In addition, methanol may be sourced from the gasification of biomass to exploit a potential synergy between biodiesel manufacture and biomass gasification. The present invention develops those synergies further by making use of a byproduct stream from the manufacture of biodiesel to remove tars from the gasifier synthesis gas and to provide a useful end use for the byproduct.

Description:
BACKGROUND OF THE INVENTION 
     I. Field of the Invention 
     This invention relates to the field of biofuel production, and in particular to the removal of tars from synthesis gas produced from biomass feedstock. 
     II. Description of the Related Art 
     Tars are generated in the gasification of biomass and other carbonaceous feedstocks to produce synthesis gas. Tars are a significant impediment to the utilization of synthesis gas due to their tendency to condense and foul downstream equipment and deactivate catalysts. 
     Conventional aqueous scrubbing techniques can remove many of the tars from the synthesis gas, but generate a waste water stream, which requires significant downstream treatment, and the heavy polyaromatic hydrocarbons (PAH) tars can precipitate and foul the scrubbing equipment. In addition, synthesis gas produced by aqueous scrubbing is not suitable for some power and all chemical applications. 
     Rising fossil fuel prices have also generated increased interest in biodiesel. However, current processes for biodiesel production utilize methanol derived from relatively expensive fossil fuel sources, such as natural gas, and generate waste byproducts of lower value such as glycerol. Biodiesel and glycerol are produced from the transesterification of vegetable oils and fats with alcohol (methanol) in the presence of a catalyst (NaOH or KOH). About 10 wt % of the vegetable oil is converted into glycerol during the transesterification process. Crude glycerol has the following typical composition: 75 wt. % glycerol, 10 wt. % ash, 10 wt. % water and 5 wt. % other organic matter. 
     Glycerol markets are currently limited and an increase in biodiesel production would cause glycerol prices to decline further. At these low prices it would become less attractive to recover high purity glycerol from crude glycerol. Generally, purification of crude glycerol is energy intensive, such as purification via vacuum distillation. 
     Further improvements are needed to reduce cost and increase efficiency of biofuel production processes. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to overcome one or more of the deficiencies of the prior art. For example, one aspect of the invention adds value to the crude glycerol byproduct of a biodiesel process by using the glycerol as a gasification or pyrolysis feedstock. In another aspect of the invention, crude glycerol is used to treat synthesis gas in a process that generates methanol as a byproduct, which is subsequently used for production of biodiesel. 
     In accordance with one embodiment, the present invention constitutes a process for removing tars from synthesis gas, comprising the treatment of the synthesis gas, containing a first amount of tars that impede utilization, with glycerol to produce a first stream of synthesis gas containing a second amount of tars less than the first amount and a second stream containing a rich glycerol and tars solution. The synthesis gas is obtained by gasifying a biomass feedstock to produce synthesis gas containing a first amount of tars that impede utilization. In an embodiment, the glycerol used in the treatment step comprises glycerol obtained by conditioning or recycling crude glycerol from a biodiesel process. 
     In a second embodiment, the glycerol used in the treatment step is obtained by combining the crude glycerol from a biodiesel process and the rich glycerol and tars solution from the treatment step to make a combined solution and subsequently conditioning the combined solution to obtain conditioned glycerol. The conditioning step preferably includes stripping the glycerol and tars from the combined solution to produce a conditioned glycerol stream. Subsequently, a small amount of heavy tars and glycerol can be purged from the conditioned glycerol stream and the remainder of the conditioned glycerol stream can be sent to the scrubber column. 
     In this second embodiment, the treatment step comprises scrubbing the synthesis gas with the conditioned glycerol. The scrubbing step includes feeding the synthesis gas, obtained from the gasifying step, containing a first amount of tars that impede utilization, and the conditioned glycerol into a scrubber column to produce an overhead stream of synthesis gas containing a second amount of tars less than the first amount and a bottom stream containing a rich glycerol and tars solution. In another embodiment, the overhead stream may be further refined by feeding it into an aqueous scrubber. 
     In accordance with a third embodiment of the present invention, the conditioning of the combined solution comprises stripping the glycerol and tars from the combined solution to produce a first stream containing steam and tars and a second stream containing conditioned glycerol. In this embodiment, the stripping is preferably achieved by feeding the combined solution into a stripper column to produce an overhead stream containing steam and tars and a bottom stream comprising the conditioned glycerol stream. The process preferably further comprises purging a small amount of heavy tars and glycerol from the conditioned glycerol stream. In this embodiment of the invention, the overhead stream from the stripper column can be sent to a gasifier to serve as a moderating stream. In another embodiment, alternatively, the overhead stream from the stripper column can be condensed and treated to recover saleable product. In accordance with the present invention, the remainder of the conditioned glycerol solution obtained from the stripper column bottoms, which is not purged, is recirculated to the scrubbing column. The purged amount of the conditioned glycerol stream can be sent to a gasifier. In another embodiment, the purged amount of the conditioned glycerol stream can be used as a liquid fuel, and in yet another embodiment, the purged amount of the conditioned glycerol stream can be treated to recover saleable product. 
     In a fourth embodiment, the present invention comprises a process for producing biodiesel from seed oils wherein the crude glycerol conditioned for use in scrubbing the synthesis gas is a byproduct of the biodiesel producing step. In this embodiment the biomass gasified in a biomass gasifier to produce synthesis gas is obtained from the biomass byproduct resulting from the harvesting of the seed oils used in the biodiesel producing step. In this embodiment, the invention further comprises a process wherein the gasifying step produces methanol and wherein the biodiesel producing step includes using the methanol to produce biodiesel. 
     In a fifth embodiment, the present invention comprises a system for removing tar from synthesis gas comprising scrubbing the synthesis gas containing a first amount of tars that impede utilization in a scrubber column with glycerol to produce an overhead stream of synthesis gas containing a second amount of tars less than the first amount and a bottoms stream containing a rich glycerol and tars solution. In this embodiment, the system comprises a source of synthesis gas, a source of glycerol, and a scrubber column. In this embodiment, synthesis gas is produced by gasification of a biomass feedstock and is fed into the lower portion of the scrubber column. Furthermore, glycerol is fed into the upper portion of the scrubber column to remove tars from the synthesis gas containing a first amount of tars that impede utilization. In this embodiment, the system further comprises a stripping column wherein crude glycerol obtained from a biodiesel process and the scrubber bottoms stream containing a rich glycerol and tars stream are conditioned to remove tars and produce a conditioned glycerol stream. 
     In a sixth embodiment, the present invention comprises a system for co-producing biodiesel and synthesis gas by synergistically operating a biodiesel plant and biomass gasifier. In this embodiment, the invention comprises: a biodiesel plant in which production comprises biodiesel and crude glycerol; a biomass gasifier in which production comprises synthesis gas; a synthesis gas treatment plant; seed oil extractor wherein seed oil is extracted from seeds and plant material; a power plant in which production comprises power and steam; and a chemical plant in which production comprises methanol and other chemicals. In this embodiment seed oil is extracted from seeds and plant material and used for biodiesel production in the biodiesel plant. The biomass byproduct from extracting the seed oil is used to fuel the biomass gasifier. 
     Still referring to this embodiment of the present invention, the system comprises treating the synthesis gas obtained from the biomass gasifier with crude glycerol obtained from the biodiesel production plant wherein the crude glycerol obtained from the biodiesel production plant may be conditioned prior to treating the synthesis gas obtained from the biomass gasifier. Furthermore, in this embodiment, a power plant is operated with the synthesis gas and used to produce power and steam to drive the biomass gasifier and biodiesel production plant. In another embodiment, alternatively, the power plant is used to produce power and steam for commercial sale. Moreover, in this embodiment of the present invention, the synthesis gas is used for operating a chemical plant which produces methanol for use in the biodiesel plant. In yet another embodiment of the present invention, the synthesis gas is used to operate a chemical plant to produce other chemicals and fuels for commercial sale. 
     The present invention addresses a need in the art for an improved method and apparatus for removing tars from synthesis gas. Furthermore, the present invention realizes potential synergies between biomass gasification and biodiesel production. Additionally, the present invention reduces both the water treatment requirements associated with synthesis gas scrubbing and the steam requirements for biomass gasifiers. Other objects and advantages of the present invention will be apparent to those skilled in the art from the following description of the preferred embodiments. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated herein and form part of the specification, illustrate various embodiments of the present invention and, together with the description, further serve to explain the principles of the invention and to enable a person skilled in the pertinent art to make and use the invention. In the drawings, like reference numbers indicate identical or functionally similar elements. A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: 
         FIG. 1  is a schematic diagram of a system for removing tar from synthesis gas in accordance with the present invention; and 
         FIG. 2  is a schematic diagram of a system for co-producing biodiesel and synthesis gas in accordance with the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In  FIG. 1  a system  10  for removing tars from synthesis gas according to the present invention is shown. The system includes a non-aqueous scrubber column  12  and a stripper column  14 . The scrubber column  12  includes upper and lower portions  16  and  18 . The lower portion  18  of the scrubber column  12  is in communication with a source of synthesis gas  20  via a first feedline  22 . The upper portion  16  is in communication with a source of glycerol  24  via a second feedline  26  having previously passed through a heat-exchange and heater  28 . The lower portion of the scrubber column  18  includes an exiting bottoms stream  30  while the upper portion  16  includes an exiting overhead stream  32 . The exiting bottoms stream  30  is pumped  34  into a line containing a crude glycerol stream  36  to form a combined stream  38 . The exiting overhead stream  32  is sent to an aqueous scrubber. 
     Referring still to  FIG. 1 , the stripper column  14  includes upper and lower portions  40  and  42 . The upper portion of the stripper column  40  is in communication with the combined stream  38 . A bottoms stream  44  containing conditioned glycerol originates from the lower portion  42  of the stripper column. A first portion  46  of the bottoms stream  44  is heated in a reboiler  48  and a second portion  50  of the stream is pumped at  52  and subsequently purged  62  to provide a stream of conditioned glycerol  54  which is recirculated to the scrubber column  12  and a stream of conditioned glycerol  56  that can be sent to a gasifier. The conditioned glycerol stream  54  passed through heat exchanger  60  and heater  28  and is sent to the upper portion of the scrubber column  16 . The upper portion of the stripper column  40  includes an exiting overhead stream  58  and, in an embodiment of the invention, the overhead stream from the stripper column  14  is sent to a gasifier. 
     In one embodiment of the invention, synthesis gas  20  with a first amount of tars preventing utilization is fed into the lower portion of the scrubber column  18 . A second stream comprising conditioned glycerol  24  is fed into the scrubber column  12  at a level above the synthesis gas input. In the scrubber column  12 , the higher solubility of tars in glycerol relative to synthesis gas results in the glycerol solution removing tar from the synthesis gas stream. 
     Referring still to  FIG. 1 , the scrubber column  12  produces an overhead stream  32  containing synthesis gas having a second amount of tars less than the first amount and a bottoms stream  30  containing a rich scrubbing solution comprised of glycerol and tars. The overhead stream  32  containing synthesis gas with a second amount of tars less than the first is sent to an aqueous scrubber to remove water thereby producing a stream of synthesis gas suitable for use in various applications. The bottoms stream  30  containing the rich scrubbing solution is pumped  34  into, and combined with, a stream of crude glycerol  36  obtained from a biodiesel production process. The combined stream  38  of the rich scrubbing solution and the crude glycerol is measured  60  and subsequently fed into the upper portion of a stripper column  40 . 
     Referring still to  FIG. 1 , in the stripper column  14 , the lighter and more volatile tars are removed from the combined stream  38  of rich scrubbing solution and crude glycerol and are made to exit the top of the stripper column  14  as an overhead stream  58  containing steam and the lighter and more volatile tars. This overhead stream  58  is subsequently sent to a gasifier. The conditioned glycerol stream  44  exits the bottom  42  of the stripper column  14  where a reboiler  48  heats a portion  46  of the conditioned glycerol stream allowing for further removal of steam and lighter and more volatile tars via the stripper column  14 . The remaining portion  50  of the conditioned glycerol stream is pumped at  52  and a small amount of heavy tars and glycerol are subsequently purged  62  producing a first conditioned glycerol stream  54  and a second conditioned glycerol stream  56 . The second conditioned glycerol stream  56  is sent to a gasifier while the first conditioned glycerol stream  54  is cooled in heat exchange with combined stream  39  at heat exchanger  60  and subsequently reheated at  28  for use in the scrubber  12 . 
     
       
         
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Material Balance for System in FIG. 1 
               
             
          
           
               
                 Components, 
                   
                   
                   
                   
                   
                   
               
               
                 lb/hr 
                 1 
                 2 
                 3 
                 4 
                 5 
                 6 
               
               
                   
               
             
          
           
               
                 N2 
                 66 
                 66 
                 0 
                 0 
                 0 
                 0 
               
               
                 CO2 
                 27134 
                 27125 
                 0 
                 10 
                 0 
                 0 
               
               
                 CO 
                 62813 
                 62812 
                 0 
                 1 
                 0 
                 0 
               
               
                 H2 
                 3918 
                 3918 
                 0 
                 0 
                 0 
                 0 
               
               
                 Benzene 
                 1772 
                 1585 
                 0 
                 187 
                 0 
                 0 
               
               
                 Toluene 
                 940 
                 723 
                 0 
                 217 
                 0 
                 0 
               
               
                 H2S 
                 95 
                 95 
                 0 
                 0 
                 0 
                 0 
               
               
                 COS 
                 6 
                 6 
                 0 
                 0 
                 0 
                 0 
               
               
                 HCl 
                 45 
                 36 
                 0 
                 9 
                 0 
                 0 
               
               
                 H2O 
                 84141 
                 26127 
                 2000 
                 60014 
                 0 
                 192 
               
               
                 CH4 
                 6782 
                 6782 
                 0 
                 0 
                 0 
                 0 
               
               
                 C2H4 
                 6363 
                 6363 
                 0 
                 0 
                 0 
                 0 
               
               
                 C2H6 
                 2170 
                 2170 
                 0 
                 0 
                 0 
                 0 
               
               
                 Cresol 
                 368 
                 0 
                 0 
                 368 
                 0 
                 3 
               
               
                 Phenol 
                 699 
                 0 
                 0 
                 699 
                 0 
                 0 
               
               
                 Naphthalene 
                 1361 
                 45 
                 0 
                 1297 
                 19 
                 788 
               
               
                 Anthracene 
                 606 
                 13 
                 0 
                 66 
                 526 
                 21803 
               
               
                 NH3 
                 720 
                 673 
                 0 
                 47 
                 0 
                 0 
               
               
                 Glycerol 
                 0 
                 50 
                 16000 
                 2013 
                 13937 
                 577689 
               
               
                 Methanol 
                 0 
                 0 
                 2000 
                 2000 
                 0 
                 0 
               
               
                   
               
             
          
         
       
     
     In Table 1, exemplary compositions of streams are given for various points in the system detailed in  FIG. 1 . The synthesis gas input  20  to the scrubber column  12 , containing a first amount of tars preventing utilization, is shown for illustrative purposes to contain the following tar components: 368 lb/hr of cresol; 699 lb/hr phenol; 1361 lb/hr naphthalene; and 606 lb/hr anthracene. The overhead stream from the scrubber column  32  containing a second amount of tars less than the first is shown to contain the following tar components: 0 lb/hr cresol; 0 lb/hr phenol; 45 lb/hr naphthalene; and 13 lb/hr anthracene. Furthermore, the composition of the overhead stream from the stripper column  58  containing the steam and lighter and more volatile tars is shown to contain the following tar components: 368 lb/hr cresol; 699 lb/hr phenol; 1297 lb/hr naphthalene; and 66 lb/hr anthracene. In contrast, the composition of the purged portion of the conditioned glycerol stream  56  is shown for illustrative purposes to contain the following tar components: 19 lb/hr naphthalene and 526 lb/hr anthracene. The recirculated conditioned glycerol stream  54  that is sent to the scrubber column for illustrative purposes is shown to contain: 788 lb/hra naphthalene and 21,803 lb/hr anthracene. The foregoing values are provided for purposes of illustration and are not meant to be limiting. 
     From the above, it will be appreciated that the process of the present invention facilitates removal from synthesis gas of tars that impeded commercial utilization of the gas. Table 2 illustrates various applications in which synthesis gas may be utilized and approximate levels of acceptable tar concentrations for such applications. Table 2 provides a representative sample of acceptable tar concentrations for various types of applications but is not intended to be exhaustive. 
                                   TABLE 2                   Types of Utilization                    Approximate Acceptable Tar           Type of Utilization   Concentration*                       Synthesis Gas compression   &lt;500 mg/Nm 3             Internal Combustion Engine   &lt;100 mg/Nm 3             Gas Turbine   &lt;100 mg/Nm 3             Non-condensable Chemical or    &lt;0.1 mg/Nm 3             Fuel synthesis                       *mg/Nm 3  refers to milligrams per normal cubic meter, meaning normal atmospheric conditions (0° Celsius and 1.013 bar)            
The process of the present invention can be tailored to remove an amount of tar to achieve an acceptable tar concentration for a given application.
 
     In  FIG. 2  a schematic diagram demonstrates a system  62  for producing biodiesel and gasifying biomass to produce synthesis gas according to the present invention. Plants  64  are treated to extract seed oils  66  resulting in a biomass byproduct  68 . The seed oils  66  are used for producing biodiesel  70  in a biodiesel plant  72 . The biodiesel plant  72  produces the crude glycerol byproduct  74  which subsequently is used to treat synthesis gas  76  produced by gasification of the biomass byproduct in the biomass gasifier  78 . It will be appreciated that the gasifier can include a tar removal system  10  as shown and described above. The resulting synthesis gas  76  from the biomass gasifier that is treated with the crude glycerol  74  from the biodiesel plant  72  is used to produce power and steam  80  in a power plant  82  to power both the biodiesel plant  72  and biomass gasifier  78 . The synthesis gas  74  from the biomass gasifier that is treated with the crude glycerol  74  from the biodiesel plant  72  also is sent to a chemical plant  84  wherein methanol  86  is produced for use in the production of biodiesel  70  in the biodiesel plant  72 . Additionally, the resulting synthesis gas  76  from the biomass gasifier  78  that is treated with crude glycerol  74  obtained from the biodiesel plant  72  is used to both produce power and steam  80  in the power plant  82 , and chemicals and additional fuels  88  in the chemical plant  84 , all of which may be used commercially. 
     Numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that the invention may be practiced otherwise than as specifically described herein. 
     While various embodiments of the present invention have been described above, they should be understood to have been presented by way of examples only, and not limitation. Thus, the breadth and scope of the present invention should not be limited by the above described embodiments.