Abstract:
An integrated reforming and power generation process is provided. This process employs a steam methane reformer to provide a hot process gas stream and a flue gas stream, utilizes the hot process gas stream to provide heat to produce a total steam stream comprising a process steam stream and an excess steam stream, and utilizes the flue gas stream to provide heat to at least a pre-reformer mixture stream, a reformer feed stream, the process steam stream and a pre-reformer steam stream The flue gas stream also provides heat to an integrated power generation process, and the excess steam stream is less than 15% of the total steam stream.

Description:
BACKGROUND 
       [0001]    In the interest of maximizing thermal efficiency in a standard Steam Methane Reformer (SMR) plant, steam is typically generated from two sources: flue gas waste heat and process heat. This inevitably leads to excess steam generation, more than required internally for the reforming process. In the absence of a steam customer, this results in the last resort measure of installing a steam turbine for realizing economic value. 
         [0002]    The concept of supercritical carbon dioxide (S-CO2), as a promising heat extraction working fluid for cool down of nuclear reactors, has been in existence for more than a decade. Most of the technological developments in this area have occurred from a nuclear power perspective. The proof of concept has been well established experimentally. Under the DOE GEN-IV nuclear program, Sandia National lab has developed two small S-CO2 loops (˜1 MW): Compression loop (at Sandia) and Brayton loop (at Barber Nichols). In the past few years, the idea of using S-CO2 cycle for non-nuclear applications has gained traction. Because of a lesser footprint, lower operating and capital costs, it has been proposed to be integrated in solar plants, molten carbonate fuel cells and as first bottoming cycle in combined cycle plants followed by steam as second bottoming cycle. Under the DOE Sunshot initiative (for solar applications), a 10 MWe scale up is currently under development along with industry partners. It is to be noted that for a standard SMR (120 MMSCFD), power generation is ˜19 MW. 
         [0003]    In SMR&#39;s, for good thermal efficiency purposes, the following ideas have been proposed/implemented. As discussed earlier, installation of a steam turbine to realize economic value out of excess steam. Multiple pre-reformers may be implemented to minimize excess steam. Helical Tube Reactor (HTR) technology has been developed to lower the temperature out of the reformer on the process side. 
         [0004]    To date, no prior art exists which advocates the integration of S-CO2 in an SMR in the configuration as proposed herein for significant reduction or, possibly, an elimination of export steam. 
       SUMMARY 
       [0005]    One embodiment of a closed loop supercritical carbon dioxide power generation process is disclosed. An integrated reforming and power generation process is provided. This process employs a steam methane reformer to provide a hot process gas stream and a flue gas stream, utilizes the hot process gas stream to provide heat to produce a total steam stream comprising a process steam stream and an excess steam stream, and utilizes the flue gas stream to provide heat to at least a pre-reformer mixture stream, a reformer feed stream, the process steam stream and a pre-reformer steam stream The flue gas stream also provides heat to an integrated power generation process, and the excess steam stream is less than 15% of the total steam stream. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0006]      FIG. 1  is a schematic representation of one embodiment of the present invention. 
           [0007]      FIG. 2  is an illustration of the cycle efficiency of the various cycles as a function of source temperature. 
       
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0008]    Illustrative embodiments of the invention are described below. While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and are herein described in detail. It should be understood, however, that the description herein of specific embodiments is not intended to limit the invention to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims. 
         [0009]    It will of course be appreciated that in the development of any such actual embodiment, numerous implementation-specific decisions must be made to achieve the developer&#39;s specific goals, such as compliance with system-related and business-related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure. 
         [0010]    In the present innovation, it is proposed to use the S-CO2 closed power loop for better exploiting the waste heat in the SMR flue gas section by splitting the flue gas outlet (T-750 deg C.) of the pre-reformer super-heater in the following fashion: 
         [0011]    Since the integration is deliberately done in the 450-700° C. range, all the key SMR process parameters (i.e. pre-reformer S/C, pre-reformer inlet T, reformer S/C, reformer inlet T and WGS inlet T), input natural gas feed and hydrogen production have not been effected. It is anticipated that excess steam may still be generated, but this excess steam will be 12% of the original stream production, or less. The 12% excess steam can either be directly sold or used for electricity generation by installing a small steam turbine (which will be ⅛ th  size of the steam turbine in a typical standard SMR). 
         [0012]    In a supercritical cycle the working fluid is maintained above the critical point during the compression phase of the cycle. 
         [0013]    As shown in  FIG. 2 , a simple SC CO2 Brayton cycle (comprising one turbine and one compressor) has higher thermodynamic efficiency than a steam (Rankine) cycle for temperatures greater than 450 deg C. The more complex 3t/6c (comprising three turbines and six compressors) He Brayton cycle has higher efficiencies than the simple SC CO2 Brayton cycle for temperatures greater than 700 deg C. Hence, in the temperature range 450-700 deg C., C CO2 is the optimum working fluid for heat extraction. 
         [0014]    By adding an extra compressor and, the SC CO2 cycle achieves a thermodynamic efficiency of 50% in the same temperature range. The gain in efficiency, as compared to steam, is primarily because of
       a) a significant reduction in compression work due to the liquid like density near the critical point,   b) there are no pinch limitations as encountered in steam generation, since SC CO2 behaves like a single phase fluid in supercritical region, and   c) the critical point (31 deg C.) is near the desired heat rejection temperature of 20 deg C.       
 
         [0018]    An added benefit, as compared to a steam cycle for same power output, is that the overall footprint is significantly reduced. The high pressure range (typically 70-200 bara) helps in reducing the size of the compressors, turbines and heat exchangers by orders of magnitude. Further, CO2 is a non-toxic, inexpensive, stable, inert, relatively non-corrosive, inflammable and well characterized fluid. 
         [0019]    Following are the key advantages realized from the proposed integration with an SMR:
       a) the ability to minimize or, possibly, eliminate export steam generation   b) due to the higher efficiency of SC CO2 cycle, there is approximately a 12% gain in power generation when compared with a steam cycle. This is assuming a small steam turbine (˜⅛ th  size of the steam turbine in a pure steam cycle, 80% efficiency and condensing) is installed.   c) the flue gas steam generator is eliminated and there is approximately a 35% reduction in boiler feed water requirement.   d) as previously mentioned, the overall footprint, as compared to steam cycle for the same power output, is significantly reduced.       
 
         [0024]    Turning now to  FIG. 1 , one embodiment of the present invention is presented. Hydrocarbon fuel stream  101  and steam stream  102  are combined into pre-reformer mixture stream  133  and introduced into pre-reformer preheating module  125 . Within module  125 , pre-reformer mixture stream  133  is heated against flue gas stream  124 , thereby producing heated pre-reformer stream  134  and flue gas stream  126 . 
         [0025]    Pre-reformer mixture stream  133  may have a temperature of between 275 and 350 C., preferably 310 C. Heated pre-reformer stream  134  may have a temperature of between 475 and 525 C., preferably 490 C. Flue gas stream  124  may have a temperature of between 825 and 875 C., preferably 850 F. Flue gas stream  126  thus exits module  125  with a reduced temperature of between 725 and 775 C., preferably 750 C. 
         [0026]    Heated pre-reformer stream  134  is then introduced into pre-reformer  103 , thereby producing reformer mixture  105 . Reformer mixture  105  is then combined with steam stream  104  thereby forming reformer mixture  110 . Reformer mixture  110  may have a temperature of between 575 and 625 C., preferably 600 C. 
         [0027]    Reformer mixture  110  is then introduced into reformer preheating module  109 . Within module  109  reformer mixture stream  110  is heated against flue gas stream  114 , thereby producing heated reformer stream  136  and flue gas stream  111 . 
         [0028]    Heated reformer stream  136  is then combined with steam stream  137  thereby forming reformer mixture stream  138 . Reformer mixture stream  138  is further heated in reformer pre-heating module  107 . Within module  107 , reformer mixture  138  is heated against flue gas stream  111 , thereby producing heated reformer stream  108  and flue gas stream  124 . 
         [0029]    Flue gas stream  111  may have a temperature of between 875 and 925 C., preferably 900 C. Flue gas stream  114  may have a temperature of between 1025 and 1075 C., preferably 1057 C. Heated reformer stream  108  may have a temperature of between 625 and 675 C., preferably 652 F. 
         [0030]    Heated reformer stream  108  then enters reformer  113 , wherein it is heated and catalytically produces process gas stream  115 . Fuel stream  112  and heated air stream  130  are introduced into reformer  113 , where they combust, thereby providing heat for the above catalytic reaction, and producing flue gas stream  114 . Process gas stream  115  enters heat recovery boiler  116 , wherein condensate stream  118  is heated to produce process boiler steam stream  117 , and syngas stream  119 . Process boiler steam stream may have a temperature of between 250 and 300 C., preferably 270 C. 
         [0031]    Flue gas stream  126  splits into flue gas stream  128  and flue gas stream  127 . Flue gas stream  128  may comprise between 50 and 70%, preferably 60% of flue gas stream  126 . At least a portion  121  of process boiler steam stream  117  enters superheater module  122 , wherein it exchanges heat with flue gas stream  127 , thereby producing flue gas stream  135  and super heated steam stream  123 . Steam stream  123  is then split into at least stream  102 ,  104 , and  137 . Excess steam stream  120  may comprise less than 20% of the total process boil steam stream  117 . Excess steam stream  120  may comprise between 10 and 15%, preferably 12% of the total process boil steam stream  117 . Superheated steam stream  123  may have a temperature of between 300 and 350 C., preferably 335 C. Flue gas steam  135  may have a temperature of between 600 and 650 C., preferably 630 C. 
         [0032]    Flue gas stream  135  is recombined with flue gas stream  128 , thus producing flue gas stream  201 . Flue gas stream  201  may have a temperature of between 675 and 725 C., preferably 700 C. 
         [0033]    Flue gas stream  201  enters power cycle reheat module  202 , wherein it indirectly exchanges heat with warm supercritical carbon dioxide stream  204 , thereby producing heated supercritical carbon dioxide stream  205 , and flue gas stream  203 . Cooled combined flue gas stream  203  may further indirectly exchange heat with process streams, such as ambient air stream  114 , thereby producing hot air stream  116  and exhaust gas stream  117 . Cooled combined flue gas stream  203  may have a temperature of between 435 and 485 C., preferably 460 C. Exhaust gas stream  117  may have a temperature of between 100 and 200 C., preferably between 125 and 175 C., more preferably 150 C. 
         [0034]    Flue gas stream  203  then enters air heater module  129 , wherein it indirectly exchanges heat with inlet air stream  130 , thereby producing heated air stream  131  and stack stream  132 . Inlet air stream  130  may be ambient temperature. Inlet air stream  130  may have a temperature of between 0 and 40 C. preferably between 10 and 30, more preferably 20 C. Stack stream  132  may have a temperature of between 125 and 175 C., preferably 150 C. 
         [0035]    Turning now to  FIG. 2 , one embodiment of the present invention is presented. Hot gas stream  201  indirectly exchanges heat with warm supercritical carbon dioxide stream  204 , thereby producing heated supercritical carbon dioxide stream  205 , and cooled combined flue gas stream  203 . 
         [0036]    Heated supercritical carbon dioxide stream  205  then enters turbine  206 , wherein it is expanded, thus producing energy. The energy is mechanically introduced into shaft  223 , wherein it powers main compressor  216  and re-compressor  218 , with excess mechanical energy being converted to electricity in generator  222 . As heated supercritical carbon dioxide stream  205  is expanded, it exits turbine  206  as expanded supercritical carbon dioxide stream  207 . Expanded supercritical carbon dioxide stream  207  then enters high temperature recuperator  208 , wherein it indirectly exchanges heat with combined stream  221  (described below). 
         [0037]    This produces cooled expand supercritical carbon dioxide stream  109 , and warm supercritical carbon dioxide stream  204 . Cooled expand supercritical carbon dioxide stream  109  is then introduced into low temperature recuperator  210 , wherein it indirectly exchanges heat with compressed first stream  217  (described below). This produces heated first stream  220  and cooled, expanded supercritical carbon dioxide stream  211 . Cooled, expanded supercritical carbon dioxide stream  211  is then divided into first stream  212  and second stream  213 . The first stream  212  may comprise between 50% and 70%, preferably between 55% and 65%, more preferably 60% of cooled, expanded supercritical carbon dioxide stream  211 . 
         [0038]    First stream  212  may enter reject heat exchanger  214 , wherein it is cooled, thereby producing cooled first stream  215 . Cooled first stream  215  then enters main compressor  216 , wherein it is compressed into compressed first stream  217 . 
         [0039]    Second stream  213  enters re-compressor  218 , wherein it is compressed into compressed second stream  219 . Compressed second stream  219  is then combined with heated first stream, to produce combined stream  221 .