Abstract:
Method of producing syngas in an IGCC system, comprising compressing and heating carbon dioxide-rich gas to produce heated compressed carbon dioxide-rich gas, mixing the heated compressed carbon dioxide-rich gas with oxygen and feedstock to form a feedstock mixture, subjecting the feedstock mixture to gasification to produce syngas, cooling the syngas in a radiant syngas cooler, contacting syngas cooled in the radiant syngas cooler with compressed carbon dioxide-rich gas to further cool the syngas, and removing an amount of carbon dioxide-rich gas from the product mixture and compressing the removed carbon dioxide-rich gas prior to mixing with oxygen and feedstock.

Description:
The present invention relates generally to improvements in operations of an integrated gasification combined cycle plant, and more specifically to methods of recycling supplied carbon dioxide-rich gas from syngas to a gasifier and/or a radiant syngas cooler inlet after heating. 
     BACKGROUND OF THE INVENTION 
     In at least some known integrated gasification combined cycle systems (IGCC), carbon dioxide (and, more generally, carbon dioxide-rich gas) removed from syngas is vented or is used for the production of chemicals, and is typically not recycled to the gasifier (also referred to herein as gasification reactor). In those systems wherein carbon dioxide-rich gas has been recycled back to the gasifier (i.e., in some gaseous feedstock plants and a few liquid feedstock plants), the recycling has been performed to increase the carbon monoxide to hydrogen ratio in the syngas for processes generating oxo-chemicals. However, in such processes, no benefits have been achieved with regard to reduced oxygen consumption or improved carbon conversion with a gaseous feedstock. 
     A need exists for improving IGCC efficiency with respect to processing of carbon dioxide-rich gas in an IGCC plant. Specifically, a need exists for a gasification method that has a reduced oxygen and/or hydrogen consumption, and that has an increased carbon conversion. Additionally, it would be advantageous if cooling methods could be provided that required a lower heating value as compared to conventional methods, thereby providing for a more cost-efficient and economical alternative. 
     BRIEF DESCRIPTION OF THE INVENTION 
     It has now been discovered, surprisingly, that by recycling carbon dioxide-rich gas to a gasifier in gasifying systems such as used in an IGCC plant, the oxygen from the carbon dioxide-rich gas participates in the gasification reactions, and facilitates reducing oxygen consumption and increasing carbon conversion. Both reduced oxygen consumption and higher carbon conversion facilitate increased IGCC plant efficiency. Mixing carbon dioxide-rich gas with hot syngas at the inlet of a radiant syngas cooler as described herein, favorably alters the reverse water gas shift reaction, such that the carbon dioxide reacts endothermically with hydrogen in the syngas to facilitate producing more carbon monoxide. Increased carbon monoxide facilitates reduced hydrogen consumption, and increased IGCC efficiency. 
     In a first aspect, a method of recycling from a first syngas mixture of a gasification system is provided. The method includes removing carbon dioxide-rich gas from the first syngas mixture in a separation device; compressing the carbon dioxide-rich gas; and feeding at least a first portion of the compressed carbon dioxide-rich gas to a gasifier. 
     In another aspect, a method of recycling carbon dioxide from a first syngas mixture of a gasification system is provided. The method includes removing carbon dioxide-rich gas from the first syngas mixture in a separation device; compressing the carbon dioxide-rich gas; producing a second syngas mixture in a gasifier; mixing the second syngas mixture and at least a first portion of the compressed carbon dioxide-rich gas to form a combined syngas mixture; and introducing the combined syngas mixture into a radiant syngas cooler to facilitate cooling the second syngas mixture. 
     In a further aspect, a method of recycling carbon dioxide from a first syngas mixture of a gasification system is provided. The method includes removing carbon dioxide-rich gas from the first syngas mixture in a separation device; compressing the carbon dioxide-rich gas; producing a second syngas mixture in a gasifier; mixing the second syngas mixture and at least a first portion of the compressed carbon dioxide-rich gas to form a combined syngas mixture and introducing the combined syngas mixture into a convective syngas cooler to facilitate cooling the second syngas mixture. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of an exemplary integrated gasification combined-cycle (IGCC) power generation plant; and 
         FIG. 2  is a schematic drawing that illustrates exemplary processes of the invention wherein carbon dioxide-rich gas, from the syngas in the separation unit, is recycled to one of the gasifier, radiant syngas cooler, and convective syngas cooler. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  is a schematic diagram of an exemplary integrated gasification combined-cycle (IGCC) power generation plant  100 .  FIG. 2  is a schematic diagram illustrating exemplary processes of the invention. While  FIG. 1  depicts only a portion of IGCC plant  100 , it should be understood by one skilled in the art that the methods as described herein can be used in a complete IGCC plant (including at least one steam turbine engine and an electrical generator) and/or in structurally similar IGCC plants as known in the art. 
     Furthermore, it should be understood by one skilled in the art that while described herein with an IGCC power generation plant, the present invention can be used with any known separation and/or gasification system without departing from the scope of the present invention. More particularly, systems including separation devices for providing physical and/or chemical separation, pressure-swing adsorption, temperature-swing adsorption, membrane separation, and the like, and combinations thereof can suitably be used with the methods of the present invention. 
     In the exemplary embodiment, IGCC plant  100  includes gasification system  200 . Moreover, in the exemplary embodiment, system  200  includes at least one air separation unit  202  that is coupled in flow communication with an air source (not shown) via an air conduit  204 . Such air sources may include, but are not limited to, dedicated air compressors and/or compressed air storage units (neither shown). Unit  202  separates air into oxygen (O 2 ), nitrogen (N 2 ), and other components are released via a vent (not shown). 
     System  200  includes a gasifier  208  that is coupled in flow communication with unit  202  and that receives the O 2  channeled from unit  202  via an O 2  conduit  210 . System  200  also includes a coal grinding and slurrying unit  211 . Unit  211  is coupled in flow communication with a coal source and a water source (neither shown) via a coal supply conduit  212  and a water supply conduit  213 , respectively. Unit  211  is configured to mix the coal and water to form a coal slurry reactant stream, referred to hereinafter as “feedstock” (not shown) that is channeled to gasifier  208  via a coal slurry conduit  214 . 
     Gasifier  208  receives the feedstock and O 2  via conduits  214  and  210 , respectively. Gasifier  208  facilitates the production of a hot, raw synthetic gas (syngas) stream (not shown). The raw syngas includes carbon monoxide (CO), hydrogen (H 2 ), carbon dioxide (CO 2 ), carbonyl sulfide (COS), and hydrogen sulfide (H 2 S). While CO 2 , COS, and H 2 S are typically collectively referred to as acid gases, or acid gas components of the raw syngas, from hereon, CO 2  will be discussed separately from the remaining acid gas components. Moreover, gasifier  208  also produces a hot slag stream (not shown) as a by-product of the syngas production. The slag stream is channeled to a slag handling unit  215  via a hot slag conduit  216 . Unit  215  quenches and breaks up the slag into small slag pieces wherein a slag removal stream is produced and channeled through conduit  217 . 
     Referring to  FIG. 1 , gasifier  208  is coupled in flow communication with radiant syngas cooler (RSC)  144  via a hot syngas conduit  218 . RSC  144  receives the hot, raw syngas stream and transfers at least a portion of the heat to heat recovery steam generator (HRSG)  142  via conduit  146 . Subsequently, RSC  144  produces a cooled raw syngas stream (not shown) that is channeled to convective syngas cooler (CSC)  260  via a syngas conduit  219 . CSC  260  further cools the raw syngas stream. 
     Referring again to both  FIGS. 1 and 2 , the cooled raw syngas stream is then channeled to a syngas scrubber (shown in  FIG. 2  generally at  270 ) and low temperature gas cooling (LTGC) unit  221  via a syngas conduit  220 . Unit  221  removes particulate matter entrained within the raw syngas stream and facilitates the removal of the removed matter via a fly ash conduit  222 . Unit  221  also provides additional cooling to the raw syngas stream. Moreover, unit  221  converts at least a portion of COS in the raw syngas stream to H 2 S and CO 2  via hydrolysis. 
     System  200  also includes a separation device  250  that is coupled in flow communication with unit  221  and that receives the cooled raw syngas stream via a raw syngas conduit  225 . Device  250  facilitates removing at least a portion of acid components (not shown) from the raw syngas stream as discussed in more detail below. Such acid gas components include, but are not limited to, CO 2 , COS, and H 2 S. Moreover, in one aspect, device  250  is coupled in flow communication with a sulfur reduction subsystem  275  via a conduit  223 . Subsystem  275  also receives and facilitates the separation of at least some of the acid gas components into components that include, but are not limited to, CO 2 , COS, and H 2 S. The separation and removal of such CO 2 , COS, and H 2 S via device  250  and subsystem  275  facilitates the production of a clean syngas stream (not shown) that is channeled to gas turbine  114  via a clean syngas conduit  228 . 
     Referring to both  FIGS. 1 and 2 , device  250  channels a CO 2 -rich gas stream to gasifier  208  via a CO 2 -rich gas stream conduit  224 . As used herein, “carbon dioxide-rich gas” or “CO 2 -rich gas” refers to a gas stream having over 50% (by weight) carbon dioxide. In one aspect, a final integrated acid-rich gas stream (not shown) includes the CO 2 -rich gas stream and also includes predetermined concentrations of COS, and H 2 S (not shown), which have been further separated from the raw syngas stream by sulfur reduction subsystem  275  as described above, and optionally, tail gas treatment unit (TGU)  277 . In some embodiments, as shown in  FIG. 2 , after separating COS and H 2 S, the stream containing COS and H 2 S is compressed via compressor  300  prior to being mixed with the CO 2 -rich gas stream and channeled to gasifier  208  via CO 2 -rich gas stream conduit  224 . 
     Separation device  250  removes from about 15% (by total moles carbon dioxide present in syngas) to about 50% (by total moles carbon dioxide present in syngas) carbon dioxide-rich gas from the syngas. As noted above, the CO 2  is channeled as CO 2 -rich gas stream (also referred to herein as “recycled CO 2 -rich gas stream”) or with COS and H 2 S as final integrated acid-rich gas stream to gasifier  208 . 
     Separation device  250  is coupled in flow communication with gasifier  208  via conduit  224  wherein the recycled CO 2 -rich gas stream is channeled in predetermined portions to gasifier  208 . As further shown in  FIG. 2 , CO 2 -rich gas from device  250  is compressed via CO 2  compressor  302  and is heated via CO 2  heater  304  when channeled via conduit  224 . 
     In operation, in one embodiment, O 2  and feedstock, via conduits  210  and  214 , respectively, may be mixed with CO 2 -rich gas, via conduits  224  and  402 , that has been compressed in compressor  302  and that may or may not have been heated in heater  304 . The resulting feedstock mixture is fed to inlets  306   a ,  306   b , and  306   c  of gasifier  208 , wherein gasification occurs, in accordance with conventional procedures, to produce syngas. 
     It has been found that by compressing and/or heating the CO 2 -rich gas prior to feeding a portion into a gasifier, an increased carbon conversion during gasification in gasification system  200  and subsequent processes in IGCC plant  100  is facilitated. In one embodiment, the methods of the present invention can increase carbon conversion by up to about 3% as compared to conventional gasification systems. The increased carbon conversion facilitates improving the efficiency of IGCC plant  100 . More particularly, by increasing the carbon conversion in gasifier  208 , an increased concentration of carbon monoxide (CO) is produced via the Boudouard reaction:
 
CO 2 +C→2CO
 
and the reverse water gas shift reaction:
 
CO 2 +H 2 →CO+H 2 O
 
By increasing carbon monoxide production, a reduced oxygen consumption during gasification is facilitated, which further facilitates increasing IGCC plant  100  efficiency. Specifically, when the CO 2 -rich gas is compressed, less oxygen is required during gasification as CO produced in the Boudouard and reverse water gas shift reactions provides an oxygen source. In one embodiment, the methods of the present invention can reduce oxygen consumption by up to about 2% per unit of syngas production (i.e., hydrogen and CO production) as compared to conventional gasification systems.
 
     The CO 2 -rich gas separated from the syngas mixture in a separation device  250  is typically compressed to a pressure in the range of from about 50 pounds per square inch to about 300 pounds per square inch above the pressure in gasifier  208  of a conventional IGCC plant  100 . The gasifier pressure typically ranges from about 400 pounds per square inch to about 900 pounds per square inch. 
     In another aspect, if the CO 2 -rich gas is heated, less O 2  is required during gasification to heat the syngas. Specifically, heated CO 2 -rich gas is already being added to the syngas, and thus the temperature need not be raised to the extent of conventional gasification to produce the desired hot raw syngas. As such, a more efficient IGCC gasification process is facilitated. 
     Compressed CO 2 -rich gas typically has a temperature ranging from about 200° F. (93.3° C.) to about 300° F. (148.9° C.). When heated, the compressed CO 2 -rich gas is typically heated to a temperature of from about 550° F. (278.8° C.) to about 700° F. (371.1° C.). For example, in one aspect, the compressed CO 2 -rich gas is heated to a temperature of about 650° F. (343.3° C.). 
     In addition to reduced oxygen consumption, a higher carbon conversion (produced when CO 2 -rich gas is compressed and/or heated) can lead to a reduced hydrogen consumption via the reverse water gas shift reaction described above. In particular, CO has a lower heating value as compared to H 2 . Accordingly, by substituting CO for H 2  in the gasification process, improved efficiency results. 
     Syngas produced in gasifier  208  exits the gasifier  208  at outlet  310 . As described generally above, the hot raw syngas is channeled to RSC  144  and CSC  260  wherein the syngas is cooled and is then channeled to syngas scrubber  270 , LTGC unit  221  and, finally, to separation device  250 . 
     In one aspect, the hot raw syngas may be cooled prior to being introduced into RSC  144  by mixing the hot raw syngas mixture at addition point  312  with a portion of compressed and/or heated carbon dioxide-rich gas separated by separation device  250  via conduit  404  to form a combined syngas mixture. The combined syngas mixture may then be introduced into RSC  144  via inlet  314  where it is cooled. 
     It has been found that by mixing the hot raw syngas produced in gasifier  208  with a portion of compressed and/or heated CO 2 -rich gas, IGCC plant  100  can further be run more efficiently. Specifically, as described above, the compressed and/or heated CO 2 -rich gas increases carbon conversion via the reverse water gas shift reaction, thereby reducing the hydrogen consumption. Because CO has a lower heating value as compared to H 2 , CO is also capable of being cooled more efficiently as compared to H 2 , thus resulting in a lower cost and higher efficiency IGCC plant  100 . 
     Additionally, in some plants, the separation systems further include soot blowing in radiant syngas cooler  144  to blow off deposits of ash and slag on the tubes of cooler  144 . Conventionally, nitrogen (N 2 ) is used for the soot blowing. With the present methods, however, CO 2  can be used in place of N 2  for soot blowing to provide further advantages. Specifically, as CO 2  is denser than N 2 , less CO 2  is required for soot blowing. Furthermore, it has been found that CO 2  is easier to separate from the syngas as compared to N 2 . 
     Cooled syngas exits RSC  144  at outlet  318  and may be channeled to CSC  260  for further cooling. Particularly, in one embodiment, the syngas is further cooled to a temperature of about 900° F. (482.2° C.) to about 1600° F. (871.1° C.), and more particularly, to a temperature of about 1300° F. (704.4° C.). Once cooled, the syngas may be introduced into syngas scrubber  270  and LTGC unit  221  for removal of particulate matter entrained within the raw syngas stream and then introduced into separation device  250  to facilitate separation of at least some of the acid gas components into components that include, but are not limited to, CO 2 , COS, and H 2 S. 
     In one aspect, the cooled syngas exits RSC  144  and is mixed at addition point  320  with a portion of compressed carbon dioxide-rich gas, separated by separation device  250  to form a combined syngas mixture. Typically, if compressed CO 2 -rich gas is added at addition point  320 , the CO 2 -rich gas is not heated in CO 2  heater  304 . As the compressed CO 2 -rich gas is not heated, it can allow for further cooling of the cooled syngas. 
     Once mixed, the combined syngas mixture is fed to inlet  322  of convective syngas cooler (CSC)  260  where it is subjected to further cooling. Cooled syngas exiting the CSC  260  at outlet  326  is typically at a temperature in the region of from about 400° F. (204.4° C.) to about 800° F. (426.7° C.). The cooled syngas is then fed to syngas scrubber  270 , LTGC unit  221  and separation device  250 . 
     While described herein as conducting consecutive addition points of compressed and/or heated CO 2 -rich gas into the syngas mixtures of gasification system  200 , it should be recognized that the compressed CO 2 -rich gas may be added at only one or two of the addition points described above without departing from the scope of the present invention. For example, in one aspect, syngas is first produced with only O 2  and feedstock in gasifier  208  (i.e., without the addition of compressed and/or heated CO 2 -rich gas) and the syngas mixture can then be mixed with a first portion of compressed and/or heated CO 2 -rich gas at addition point  312  and cooled in RSC  144 . In another aspect, syngas is produced from O 2  and feedstock and cooled in RSC  144 . Once the cooled syngas exits RSC  144  at outlet  318 , the cooled syngas can be mixed with a first portion of compressed and/or heated CO 2 -rich gas at addition point  320  and further cooled in CSC  260 . 
     One additional advantage of using the methods of the present invention includes not requiring the inclusion of sulfur removal subsystem  275  in IGCC plant  100 . In particular, as the compressed and/or heated carbon dioxide-rich gas is removed from separation device  250  and mixed again with syngas, the CO 2 -rich gas does not have to be purified to remove sulfur. As a result, separation device  250  can be optimized to allow the CO 2 -rich gas to contain sulfur. 
     While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.