Abstract:
Methods and systems for the generation of electrical energy through the combination of steam flows produced from different fuel sources. Steam produced from processing of a biomass fuel source is combined with steam produced from the processing of natural gas or fossil fuel and routed through a steam turbine generator to produce electrical energy. The steam is preferably reheated after partial processing in the steam turbine generator and then recirculated for further processing in the steam turbine generators. Following extraction of all available energy from the steam, the condensed wet vapor is reheated and used for processing of both energy sources.

Description:
FIELD 
     This invention relates generally to methods and systems for the generation of electrical power through the combination of a biomass combustion system and a conventional energy system such as a natural gas or other fossil fuel combustion system. More specifically, the invention is directed at methods and systems for the combination of steam outputs from a biomass combustion cycle with the steam output of a natural gas or other fossil fuel fired combined cycle plant to generate electrical power. 
     BACKGROUND 
     The use of biomass as a means to generate electrical power is well established in the pulp and paper industry. Biomass has also been used in standalone power generation facilities. One of the drawbacks to power generation technology using biomass as a fuel, based on the generation of steam, is the inherent low efficiency of its power generation cycle relative to that of a natural gas or other fossil fuel fired power plant. This lower level of efficiency for power generation using biomass fuel sources stems from two main deficiencies. First, the moisture content of the biomass fuel is usually above 40%, which decreases the combustion efficiency of the boiler. Second, the size of the biomass fuel fired power plant is usually less than 50 MW, which results in a less efficient steam cycle than a much larger natural gas or fossil fuel fired power plant. The present disclosure seeks to overcome these and other deficiencies by combining the steam generated from biomass processing with steam generated from a natural gas or other fossil fuel cycle. 
     SUMMARY 
     The present disclosure is directed to systems and methods for power generation through the combination of a biomass fuel combustion cycle and a natural gas or other fossil fuel fired cycle. In general, the combined cycle format utilizes the steam from the biomass boiler with the steam output from a traditional gas turbine. Inefficiencies of a traditional biomass power plant are overcome through this combination. 
     The process for the generation of steam from the two sources is kept separate. Steam from the biomass source is combined with steam from the natural gas or other fossil fuel cycle and flows to a steam turbine generator. An exemplary embodiment of the present disclosure will now be described. In the exemplary embodiment, the combined steam flows are passed through the high pressure section of the steam turbine generator. The heat recovery steam generator (HRSG) that is used to produce steam from the natural gas combustion cycle is also used to reheat the combined steam flow leaving the high pressure section of the steam turbine generator, if a reheat cycle is being used. The reheated steam is routed to an intermediate pressure section of the steam turbine and then expanded to the low pressure section of the steam turbine generator to generate electrical power. The wet vapor exiting the low pressure section is sent to a condenser where it is converted to a liquid water. From the condenser, the liquid is pumped through an economizer section in the HRSG and the output of this economizer is split between the HRSG and the biomass boiler. 
    
    
     
       DESCRIPTION OF THE FIGURES 
         FIG. 1  is an exemplary embodiment of the hybrid cycle of the present disclosure. 
         FIG. 2  is an exemplary embodiment of a natural gas fueled cycle of the prior art. 
     
    
    
     It should be noted that the figures are not drawn to scale and that elements of similar structures or functions are generally represented by like reference numerals for illustrative purposes throughout the figures. It also should be noted that the figures are only intended to facilitate the description of the preferred embodiments. 
     DETAILED DESCRIPTION 
     The system and methods of the present disclosure allow for more efficient energy production from biomass fuel. The enhanced efficiency of the present method is achieved by combining a typical biomass fuel cycle with a natural gas or fossil fuel cycle in a hybrid process. Steam generated from the biomass fuel cycle is combined with steam generated from the natural gas or other fossil fuel cycle and the combined steam flow is then routed through a steam turbine generator. The proposed process combusts the biomass fuel separately from the natural gas. The combusted gases are kept separate, for post combustion processing prior to discharge to the atmosphere. 
     In a preferred embodiment such as that shown in  FIG. 1 , the biomass fired boiler operates in parallel with the combined cycle heat recovery boiler (HRSG). This has several effects. First, the operating steam pressure of the biomass boiler is no longer limited by the size of the steam turbine generator. Second, the steam from the biomass boiler can now operate in a reheat (or non reheat) type Rankine cycle. Third, a separate power generation cycle for the biomass fuel is avoided thereby resulting in capital economy and avoidance of separate operation staff. Fourth, the steam produced from the biomass fuel resource can displace the use of natural gas or fossil fuel as a supplemental fuel in the HRSG. 
     Typically, biomass processes are limited to using industrial class type of steam turbine generators. The combination of cycles described in the present disclosure permits the use of utility class steam turbine generators. Because utility class steam turbine generators are markedly more efficient than industrial class steam turbine generators, the ability to use a utility class steam turbine generator provides further efficiencies to the present system and methods over traditional systems for generation of electrical power from biomass. 
     A wide variety of biomass products can be used with the invention of the present disclosure. The fuel or feedstock can comprise any of renewable solid fuels such as green tree chips, forest residues, yard clippings, wood chips, urban waste wood, construction and demolition waste wood, sugar cane fiber (bagasse), or other agricultural waste. 
     The concepts of the present disclosure can also be used with high chlorine content fuels such as municipal solid waste (MSW) or refuse derived fuels (RDF). In applications using high chlorine content fuels, the steam temperature of the cycle will be reduced as compared to low chlorine content fuels. The present disclosure provides exemplary sources of biomass energy fuels, but the scope of the present disclosure is not limited to these specific examples. To the contrary, any biomass fuel source that is capable of producing steam temperatures compatible with the natural gas or other fossil fuel cycle can be used. 
     The biomass fuel may be brought to the plant site by truck and be unloaded, stored and reclaimed in the same manner as is the current practice in biomass operations. 
     High pressure steam can be generated from biomass fuel using any method known in the art. For example, the use of stoker firing, bubbling fluid bed, circulating fluid bed technology and the like are all within the scope of this disclosure. In a preferred embodiment, the steam boiler operating pressure and temperature will be at the high end of current practice so that the steam pressure and temperature derived from the biomass fuel cycle is on the same order as the steam pressure and temperature of the natural gas or fossil fuel cycle. In general, the steam generation process for the biomass fuel portion of the combined cycle will operate in a similar manner to ongoing biomass fuel power generation projects with the exception that the steam is not directly routed to a dedicated steam turbine generator. Instead, as will be discussed further below, the steam generated from the biomass fuel portion is combined with steam generated from the natural gas or fossil fuel cycle and the combined steam flow is routed to the steam turbine generator. Steam generated from natural gas or fossil fuel can be obtained by any means known in the art and the present disclosure is not limited to any specific method. 
     An exemplary embodiment of the present disclosure is shown in  FIG. 1 .  FIG. 1  depicts an exemplary wood boiler  102  using green wood as a feedstock, but, as described above, the concepts of this disclosure are not limited to the use of a wood boiler  102  or green wood as a feedstock. The temperatures, loading values, and other characteristics discussed herein are exemplary and not intended to restrict the scope of the present disclosure. 
     In the exemplary embodiment, green wood  104  is supplied to a wood boiler. In the exemplary embodiment, the wood boiler  102  would receive 250,000 tons per year (TPY) of green wood  104  with a heat energy of 243 MMBTU/hr (LHV). Using processing methods standard in the industry and readily known to those of skill in the art, the green wood  104  is processed in the wood boiler  102  to produce steam. The output of steam  106  from the wood boiler  102  in the exemplary embodiment is 156,000 lbs/hr. 
     Natural gas  108  is processed through a gas turbine  110  to produce hot gas  112 . The natural gas  108  process also creates energy, via a generator  114 . In the exemplary embodiment, the output of energy from generator  114  is 170 MW. The hot gas  112  produced from the gas turbine  110  is output at a temperature of approximately 1144° F. at 3,369 lbs/hr. The hot gas  112  then enters a conventional heat recovery steam generator (HRSG) unit  116 . In the exemplary embodiment, the HRSG  116  is an unfired three drum HRSG. The high pressure steam output  118  from the HRSG  116  in the exemplary embodiment is about 391,000 lbs/hr. The steam output  118  from the HRSG  116  is combined with the steam output  106  from the wood boiler  102 . The combined steam flow  120  is then routed to a high pressure (HP) section  122  of the steam turbine generator  124 . In the embodiment shown in  FIG. 1 , this steam line routes steam at approximately 547,000 lbs/hr. 
     After expanding to a lower pressure in the HP section  122  of the steam turbine generator  124 , the steam  126  is returned to the HRSG and is reheated to its original temperature. The reheated steam  128  then returns to the steam turbine generator  124  to expand until all of the available work is extracted from the steam. The reheat cycle improves the performance of the steam turbine generator  124  and provides an added level of efficiency to the systems and methods disclosed herein. 
     A second steam output (intermediate pressure)  130  from the HRSG  116  is combined with the reheated steam  128  to form steam line  132  and routed to an intermediate pressure (IP) section  134  of the steam turbine generator  124 . In the embodiment shown in  FIG. 1 , the combined steam line  132  routes steam at approximately 583,000 lbs/hr. After expanding in the IP section  134 , the steam  136  is routed to a low pressure (LP) section  138  of the steam turbine generator  124 . The LP section  138  also receives a LP steam input  140  from the HRSG  116 . In the exemplary embodiment, steam  140  is taken directly from the HRSG  116  and routed to the LP section  138  from the HRSG  116  at about 72,000 lbs/hr. This combines with the steam flow  136  and expands to a condenser  142 . The condenser  142  receives the output of the LP section  138  of the steam turbine. The steam exiting the LP section  138  is condensed to liquid water and enters the condensate pump  143 . The output  144  from the condensate pump  143  is then routed back through the HRSG economizer to heat the water. The heated water  146  is then supplied back into the HRSG  116  and to the wood boiler  102 . In the exemplary embodiment, the output of energy from the steam turbine generator  148  is 112 MW. 
     The example shown does not use any supplemental firing in the HRSG. The use of supplemental firing in the HRSG can be used to replace the biomass steam source, when the biomass portion of the plant is out of service for maintenance. This allows the electrical output of the plant to be maintained, when the biomass plant is not operating. 
     The amount of steam coming from the biomass boiler is somewhat arbitrary and is tied to the amount of sustainable biomass fuel available. The case shown is based on an annual consumption of 250,000 tons of biomass per year. Large biomass plants in New England burn up to 500,000 tons per year. 
     An exemplary performance for the hybrid cycle is as follows: 
     
       
         
               
               
               
             
           
               
                   
               
             
             
               
                   
                 Power from combustion turbine 
                 170 MW 
               
               
                   
                 Engine fuel consumption 
                 1,551 MMBtu/hr (LHV) 
               
               
                   
               
             
          
         
       
     
     HRSG Steam Generation: 
     
       
         
               
               
             
           
               
                   
               
             
             
               
                 HP Steam 
                 391,000 lb/hr @ 1792 psia, 1053° F. 
               
               
                 Reheat Steam 
                 583,000 lb/hr @ 420 psia, 1050° F. 
               
               
                 LP Steam 
                  72,000 lb/hr @ 76 psia, 546° F. 
               
               
                 Biomass Steam Generation 
                 156,000 lb/hr @ 1800 psia, 950° F. 
               
               
                 Biomass Fuel Consumption 
                 243 MMBtu/hr (LHV) 
               
               
                 Condenser pressure 
                 1.1 psia 
               
               
                 Power from steam turbine 
                 112.7 MW 
               
               
                 Total plant net power output 
                   275 MW 
               
               
                   
               
             
          
         
       
     
     The LHV net station heat rate for this cycle is approximately 6590 Btu/kWh, which will vary depending on the level of biomass processing required. If the same amount of biomass fuel were burned in a conventional biomass power plant approximately 17 MW would be produced. The hybrid cycle produces an increase of 21 MW of electricity attributable to the firing of biomass fuel as described above. This is a performance increase of 23% in electricity production relative to traditional methods of biomass fuel use. 
     Various combustion turbine manufacturers offer predesigned fossil fuel based combined cycle packages, with the combustion turbine(s) as the prime mover for the combined cycle. The larger the combustion turbine, the more complex the combined cycle design. The intent of this complexity is to improve the overall efficiency of the power generation cycle. 
     Using the above approach and other optimization features, combined cycle power plants are able to achieve electrical production efficiencies of 50% or more. On the other hand, traditional renewable biomass fired power plants operate in the 23-25% efficiency range. 
     The hybrid concept is not limited to any specific combined cycle arrangement. For purposes of this example, a biomass fired boiler was combined with an unfired three drum HRSG and a single, three section, condensing steam turbine generator with no steam extractions. In other circumstances, other fossil fuel fired generation technologies utilizing a steam cycle could be used. In addition, the cycle can be used in cogeneration applications where steam created in the combined cycle and hybrid cycle are used in part, for industrial purposes. Furthermore, the hybrid concept is not limited to a reheat based, combined cycle. The concept is fully compatible with smaller prime movers which would not make use of a reheat feature in the HRSG and steam turbine portions of the power generation cycle. 
       FIG. 2  depicts an exemplary standard combined cycle of the prior art that does not include a biomass fuel cycle. The process for this cycle is similar to that described above for the hybrid process, with the exception that the standard combined cycle does not include a steam input from the renewable energy cycle and does not recirculate the reheated water to the wood boiler. Using the same level of natural gas input, the standard combined cycle would have the following characteristics: 
     
       
         
               
               
               
             
           
               
                   
               
             
             
               
                   
                 Power from combustion turbine 
                 170 MW 
               
               
                   
                 Engine fuel consumption 
                 1,551 MMBtu/hr (LHV) 
               
               
                   
               
             
          
         
       
     
     HRSG Steam Generation: 
     
       
         
               
               
             
           
               
                   
               
             
             
               
                 HP Steam 
                 433,000 lb/hr @ 1454 psia, 1050° F. 
               
               
                 Reheat Steam 
                 474,000 lb/hr @ 376 psia, 1050° F. 
               
               
                 LP Steam 
                  53,000 lb/hr @ 61.5 psia, 546° F. 
               
               
                 Condenser pressure 
                 1.1 psia 
               
               
                 Power from steam turbine 
                  90.4 MW 
               
               
                 Total plant net power output 
                 253.9 MW 
               
               
                   
               
             
          
         
       
     
     The LHV net station heat rate from this cycle is 6110 Btu/kWh (49.8% efficient on a HHV basis). 
     In the foregoing specification, the invention has been described with reference to specific embodiments thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the invention. For example, each feature of one embodiment can be mixed and matched with other features shown in other embodiments. Features and processes known to those of ordinary skill may similarly be incorporated as desired. Additionally and obviously, features may be added or subtracted as desired. Accordingly, the invention is not to be restricted except in light of the attached claims and their equivalents.