Patent Publication Number: US-2012023822-A1

Title: Method and System for Controlled Gasification

Description:
BACKGROUND OF THE INVENTION 
     The present invention is directed to gasification. More specifically, the present invention relates to a method and system for controlled gasification in a system having product gas recycle. 
     Most solid fuel gasification systems contain substantial thermal inertia relative to comparable liquid and gaseous fuel systems. Further, gasification of the solid fuel is preceded by heating of the fuel, and this heating requires a significant amount of time, the exact amount depending upon the fuel feed size, porosity, and density, as well as the gasification system design and operating conditions. Moreover, many solid fuel gasification systems, such as fluidized or non-fluidized bed designs, contain large amounts of stored mass (for example, from fuel and ballast material such as sand). The response of solid fuel gasification systems is often sluggish or slow, particularly when a low energy density feedstock with relatively large particle feedstock is employed. Such is often the case, for example, with feedstocks that include biomass, which is rarely amenable to fine pulverization, and frequently has a heating value of less than 10,000 Btu/lb. This sluggishness hampers the ability of solid fuel gasification systems to adequately satisfy the time-varying demand needs of many industrial processes. 
     Moreover, the mass transport and mixing processes in solid fuel gasification systems are relatively slow and generally more sensitive to changes in flow rate (occurring with changes in operating conditions) than in gaseous-based gasifiers (partial oxidizers) and liquid-based gasifiers. For example, transport gas (or liquid) flows strongly affect solid fuel mixing dynamics in entrained flow gasifiers, and total gas throughput has a major influence on mixing in bed-type solid fuel gasifiers. 
     One consequence of such mixing limitations is that solid fuel gasification systems can be highly constrained in the degree of turndown that can be achieved in system output (i.e., Btu/hr of syngas) without causing serious degradation and/or instability in gasifier performance. Further, due to the large inertia of solid fuel gasification systems, the true effect of attempting turndown in such systems is typically delayed, sometimes by several minutes or more. This can create a false sense of safety and stability in the mind of system operators, which can ultimately lead to more catastrophic effects. In slagging gasifiers, for example, time-delayed slag solidification can occur on turndown, creating a bottleneck in reactor flows that often results in a complete system shutdown. 
     The aforementioned limitations on transient response and turndown of solid fuel systems represent technical hurdles in the application of solid fuel gasification systems to dedicated processes, such as boilers and furnaces that must frequently respond to time-varying energy or production demands. 
     Conventional approaches for addressing these limitations include running the gasifier at constant output in excess of that which is needed for the process and flaring (i.e., waste burning) of surplus syngas. Another approach is to use multiple parallel reactors, each of which can be separately controlled, and yet another approach is the use of back-up fuels such as natural gas or fuel oil that can augment reactor performance during turndown. Shortcomings in these approaches include lower efficiency associated with flaring unused syngas, additional cost and complexity associated with multiple reactors, and relatively high operating cost and/or lack of availability of back-up fuels. 
     U.S. Pat. No. 4,489,562, which is incorporated by reference in its entirety, describes a method and apparatus for controlling a gasifier that delivers syngas to a boiler for power generation. The method and apparatus fail to address the above-described drawbacks. 
     What is needed is a system and method capable of dynamic response and turndown of solid fuel gasification that does not suffer from the aforementioned drawbacks. 
     BRIEF SUMMARY OF THE INVENTION 
     One aspect of the present disclosure includes a method of controlled gasification. The method includes introducing a first fuel to a gasifier in a system, introducing a first fuel to a gasifier in a system, generating a product gas by partially oxidizing the first fuel with an oxidizer including oxygen, directing a first portion of the product gas to a process chamber, and selectively introducing a recycled portion of the product gas to the gasifier. A quality parameter of the product gas is analyzed, introduction of the oxidizer and the selective introduction of the recycled portion of the product gas is based upon the quality parameter of the product gas stream, and the first fuel includes a solid fuel. 
     Another aspect of the present disclosure includes a system for controlled gasification. The system includes a gasifier configured to receive a first fuel and generate a product gas by partially oxidizing the first fuel with an oxidizer including oxygen, an analyzer configured to analyze a quality parameter of the product gas, and a controller configured to receive a product gas quality signal and to respond to the product gas quality signal by initiating selective adjustment of one or more of an oxygen flow rate being introduced to the gasifier, a recycled product gas flow rate being introduced to the gasifier, and a flow rate of the first fuel. The system is configured to recycle at least a portion of the product gas to the gasifier, and the first fuel includes solid fuel. 
     Another aspect of the present disclosure includes a system for controlled gasification. The system includes a gasifier configured to receive a first fuel and generate a product gas by partially oxidizing the first fuel with an oxidizer including oxygen and a controller configured to receive an energy value signal and to initiate an adjustment in a flow rate of the product gas to a process chamber in response to the energy value signal. The system is configured to recycle at least a portion of the product gas to the gasifier, and the first fuel includes solid fuel. 
     Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the invention. 
    
    
     
       BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1  shows a schematic view of a configuration for a system according to an exemplary embodiment of the disclosure. 
         FIG. 2  shows a schematic view of a configuration for a system according to an exemplary embodiment of the disclosure. 
         FIG. 3  shows a diagrammatic view of a control system for controlling a system according to an exemplary embodiment of the disclosure. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Provided are methods and systems for controlled gasification in response to a quality parameter of a product gas formed by the gasification. Embodiments of the methods and systems, can dynamically adjust to accommodate changes in fuel quality, and/or are not solely dependent upon adjusting secondary fuel sources for responding to changes in furnace demand. Additionally or alternatively, embodiments of the present disclosure are capable of resolving the aforementioned drawbacks. 
       FIG. 1  shows an exemplary embodiment of a system  100  having a configuration  303  according to the disclosure. The configuration  303  includes a gasifier  300  capable of providing a product gas or synthesis gas (syngas) to a process chamber (for example, a process furnace  102 ). The process furnace  102  can be any suitable process furnace (for example, a boiler, melting furnace, reheat furnace, gas turbine, reformer, chemical process reactor, or any other suitable process requiring energy input in the form of fuel). 
     A quality parameter of the product gas is analyzed. The quality parameter can be (or be representative of) composition of the product gas, temperature of the product gas, heating value of the product gas, gasifier temperature, or any other suitable parameter. In one embodiment, product gas composition, heating value, or a combination thereof of the product gas exiting the gasifier  300  is measured in a gas analyzer  317 . The gas analyzer  317  can also measure the temperature of the product gas and/or a representative temperature of the gasifier  300  (for example, refractory or slag temperature). Product gas is delivered to the furnace  102  through a flow control valve  307  (or furnace control valve). A portion of product gas (for example, a slip stream) is recycled as recycled product gas (or recycled syngas) to the gasifier  300  upstream of the flow control valve  307 . Motive force for the recycled product gas is provided by any suitable mechanism (for example, by a blower  315 ). In one embodiment, an additional flow control valve  319  (or recycle control valve) is also provided for additional control precision for recycling the recycled product gas. 
     The gasifier  300  is configured to receive the recycled product gas, a fuel containing solid fuel (for example, a purely solid fuel, a slurry including solid fuel, or any other suitable fuel containing a solid fuel), and an oxidizer containing oxygen. As used herein, except where specified otherwise, the term “oxygen” refers to an O 2  content of at least about 30% by volume. Depending upon process parameters of the furnace  102 , design of the gasifier  300 , and composition of the fuel, additional inputs may include a secondary liquid or gaseous fuel, air, water, or steam, and/or additives (for example, fly ash, compounds containing calcium, compounds containing magnesium, compounds containing silicon, compounds containing aluminum, compounds containing iron, and any combination thereof). The flow rate of the solid fuel is controlled by a fuel control valve  309 . The fuel control valve  309  can be any suitable valve for controlling fuel. For example, the fuel control valve  309  can be a rotary valve, a screw, a conveyor, a lock hopper or the like, or combinations thereof. In one embodiment, the oxidant containing oxygen is divided into two or more streams, each with separate control valves  311 ,  313 . For example, in this embodiment, the totality of gasifier oxygen (and/or oxidizer) can be introduced through oxygen control valve  311 , and then a portion of the oxygen can be split to enter the gasifier  300  through flow control valve  313  (or diverter control valve). In this embodiment, greater control over the beneficial effect of the recycled product gas operation and control of the gasifier  300  may be achieved. 
     Similarly,  FIG. 2  shows an exemplary embodiment of the system  100  having a configuration  403  according to the disclosure. The configuration  403  includes gasifier  300 , process furnace  102 , gas analyzer  317 , flow control valve  307 , blower  315 , and additional flow control valve  319 . In addition, configuration  403  shown in  FIG. 2  shows an oxidant flow control valve  321 . In one embodiment, oxygen enrichment in the furnace  102  is included and adjusted by oxidant flow control valve  321 . For example, when the quality of the product gas is below a predetermined parameter, oxidant containing oxygen can be supplied to the process furnace  102 . This additional supply of oxygen can improve stability, responsiveness, and control of the process furnace  102  operation. In one embodiment, when the solid fuel input to the gasifier  300  has a lower-than-desired heating value, a predetermined amount of oxidant containing oxygen is introduced to the process furnace  102 . The introduction of the predetermined amount of oxidant containing oxygen can maintain a flame temperature in the furnace  102  above a predetermined temperature. That is, without the introduction of the predetermined amount of oxidant to the process furnace  102  to aid in combustion of the product gas, the flame temperature and/or stability of combustion within the process furnace  102  is insufficient to meet a desired energy value and/or desired safety standards. 
     The system  100  disclosed herein is controlled through a control system  305 , the functioning of which is based upon the methods, algorithms, and equations described hereinafter. Likewise, the control system  305  can be used for controlling systems having gasifier design including, but not limited to entrained flow or bed-type designs, slagging or non-slagging designs, and/or fluidized or non-fluidized bed designs. The control system  305  can include functionality and system measurement capabilities corresponding to the relationships identified in relation to the process variables as set forth below. 
     Referring to  FIG. 1 , the following mass balance for the gasifier  300  is given as: 
         M   O2-G   +M   SF   +M   RSG   =M   SG   (1)
 
     Where M O2-G  represents mass flow of oxygen in the oxidant, M SF  represents mass flow of solid fuel in the fuel, M RSG  represents mass flow of recycled syngas, and M SG  represents mass flow of syngas generated. It will be appreciated by those skilled in the art that the mass flow of oxygen M O2-G  in the oxidant, the mass flow of solid fuel M SF  in the fuel, the mass flow of syngas M SG  in the product gas, the mass flow of recycled syngas M RSG  in the recycled product gas, and the mass flow of syngas delivered to the furnace in the product gas delivered to the furnace M F  (further described below) each can represent a portion or all of the mass flow. For the purposes of this disclosure, such mass flows are assumed to be the entire mass flow. However, it will be further appreciated by those skilled in the art that the methods disclosed herein and operated in conjunction with control system  305  can be applied to systems without mass flows being assumed to be the entire mass flow. Based upon these assumptions, the mass balance among the syngas generated, the recycled syngas, and syngas delivered to the furnace M F  is: 
         M   SG   −M   RSG   =M   F   (2)
 
     Combining equations (1) and (2), after rearranging: 
         M   O2-G   +M   SF   =M   F   (3)
 
     The energy balance for the gasifier  300  is (based upon the assumption of steady flow and no change between specific enthalpy of the recycled syngas entering the gasifier and the product gas): 
         M   SF   *H   SF   +M   O2-G   *H   O2   +M   RSG   *H   SG   =M   SG   *H   SG   +Q   L   (4)
 
     Where H represents specific enthalpy (enthalpy per unit mass), H O2-G  represents specific enthalpy of oxygen in the oxidant, H SF  represents specific enthalpy of solid fuel in the fuel, H RSG  represents specific enthalpy of recycled syngas, H SG  represents specific enthalpy of syngas generated, and Q L  is the energy lost from the gasifier  300 . The energy balance among syngas generated, the recycled syngas, and the syngas delivered to the furnace is: 
         M   SG   *H   SG   −M   RSG   *H   SG   =M   F   *H   SG   (5)
 
     The instantaneous energy input to the process furnace, Q F , is the product of the flow rate of the syngas delivered to the furnace and the heating value. That is: 
         M   F   *H   SG   =Q   F   (6)
 
     Substituting equation (6) into equation (5): 
         M   SG   *H   SG   −M   RSG   *H   SG   =Q   F   (7)
 
     By assuming that the oxygen enthalpy H O2  is equal to zero (which generally indicates that the oxidizer is introduced at a reference temperature, such as 68° F. (20° C.)) and by substituting the relation M RSG /M SG =β into equation (4) and rearranging: 
         M   SF   *H   SF   =M   SG   *H   SG *(1−β)+ Q   L   (8)
 
     Invoking a similar substitution into equation (7) yields, after rearranging: 
         Q   F   =M   SG   *H   SG *(1−β)  (9)
 
     Substitution of equation (9) into equation (8) yields the following equality: 
         M   SF   *H   SF   =Q   F   +Q   L   (10)
 
     Finally, combining equations (6) and (10) and eliminating Q F , yields: 
         M   SF   *H   SF   =M   F   *H   SG   +Q   L   (11)
 
     Equation (11) indicates that, under steady flow conditions and assuming constant gasifier energy loss, the energy delivered to the furnace from syngas correlates to the solid fuel energy input to the gasifier. Hence, although the instantaneous energy demand to the furnace  102  can be satisfied by adjustment to the ratio of mass flow rate of recycled syngas to mass flow rate of syngas generated (hence, adjustment of β) as indicated in equation (9), the equality of (11) is eventually established for steady operation by the control system  305 . 
     In one embodiment, the solid fuel in the fuel is assumed to at least consist essentially of carbon, hydrogen, oxygen, nitrogen and water: 
       Solid Fuel=CH a1 O a2 N a3   +a   4 H 2 O  (12)
 
     Where a 1 , a 2 , and a 3  are the ratios of elemental hydrogen, oxygen, and nitrogen to elemental carbon in the solid fuel, and a 4  is the molar ratio of water to fuel carbon. Similarly, the recycled syngas composition (which is assumed to be identical to the product syngas composition as described above) is assumed to consist of carbon monoxide (CO), hydrogen (H 2 ), methane (CH 4 ), water vapor (H 2 O), carbon dioxide (CO 2 ) and nitrogen (N 2 ) according to the following formula: 
       Syngas= a   5 CO+ a   6 CH 4   +a   7 H 2   +a   8 N 2   +a   9 H 2 O+ a   10 CO 2   (13)
 
     Where a 5 , a 6 , a 7 , a 8 , a 9  and a 10  are the molar ratios of CO, CH 4 , H 2 , N 2 , H 2 O and CO 2  to carbon in the solid fuel. 
     If it is assumed that complete combustion of the solid/gaseous fuel mixture results in the formation of carbon dioxide (CO 2 ) and water (H 2 O) then the molar ratio of oxygen added to carbon in the solid fuel ω stoich  for stoichiometric (complete, no excess O 2 ) combustion of the solid fuel/recycled syngas mixture is given as follows: 
       ω stoich   =n   O2   /n   C =1+0.25 *a   1 −0.5 *a   2 +0.5 *a   5 +2 *a   6 +0.5 *a   7   (14)
 
     Where n O2  and n C  are the molar flow rates of oxygen and solid fuel carbon, respectively. The stoichiometric ratio φ is defined herein as the ratio of actual to stoichiometric oxygen injected into the gasifier. While the value of φ will vary depending upon process conditions, gasifier design, and fuel properties. In one embodiment, the range of φ is 0.1 to 0.5. 
     The actual oxygen flow rate to the gasifier M O2-G , is equal to: 
         M   O2-G   =M   C *ω stoich *φ*( MW   O2   /MW   c )  (15)
 
     Where M C  is the mass flow rate of solid fuel carbon to the gasifier, and MW O2  and MW C  are the molecular weights of oxygen and carbon, respectively. 
     Although the formulae given in equations (1) through (15) assume both compositions and flow rates of solid fuel and recycled syngas are known, measurement of solid fuel composition is an optional input, and can be approximated (for example with periodically obtained data) or eliminated. The latter case is presented in a later example. 
     In one embodiment, the control system  305  can be operated according to the process set forth in  FIG. 3 . A difference between a desired energy value D and an output energy value Y (shown as a first comparator step  702 ) gives rise to an error function c such that: 
       ε= D−Y   (16)
 
     The output energy value Y may include any one or multiple process furnace variables that may be communicated to the controller as an energy value signal whose values or representatives of values are inputted to the first comparator step  702 , for example steam temperature, pressure or flow rate, electrical generator output, process furnace temperature, gas flow rate, or any other suitable performance variables, the availability and significance of which depend upon the specific nature of the process furnace (for example, boiler, melting furnace, gas turbine, etc.). The first adjustment step  730  includes such adjustments. For example, based upon the first comparator step  702 , the control system  305  may adjust syngas flow (box  704 ), solid fuel flow (box  706 ), and/or gasifier oxygen flow (box  708 ). In one embodiment, a predetermined combination of syngas flow (box  704 ), solid fuel flow (box  706 ), and/or gasifier oxygen flow (box  708 ) is adjusted when the absolute value of E exceeds a predetermined threshold, the magnitude of which is specific to the dynamics and degree of control precision required of the system. The first response of the control system is to the furnace process deviation in order to satisfy the immediate need to reduce the value of the error function ε expediently. This is accomplished through adjustment of the syngas flow rate (box  704 ) to the process furnace  102 , which is enabled via modulation of control valve  307 . 
     In order to re-attain steady operation of the system that is upset by adjusting in syngas flow (box  704 ), the solid fuel flow rate to the gasifier  300  is adjusted (box  706 ) by an amount that corresponds to the change in syngas energy flow to the furnace  102 . Since adjustments to syngas flow rate (box  704 ) and adjustments to solid fuel flow rate (box  706 ) result in a modification to the balance of fuel input to the gasifier, the stoichiometric oxygen flow rate M O2,st  and hence the gasifier oxygen flow set point M O2,sp  are also affected. As such, adjustment of the gasifier oxygen flow M O2-G  (box  708 ) is initiated. 
     While the first comparator step  702  monitors and reacts to process furnace variables, the gasifier comparator step  732  performs an analogous function for the gasifier  300 . That is, gasifier temperature is measured (box  710 ) and syngas quality is measured (box  712 ) and inputted to the gasifier comparator step  732 . 
     Temperature measurement (syngas and/or gasifier internal) permits enhanced process control, operability, and durability. Temperatures below a lower predetermined temperature, which can be defined by the gasifier type and fuel composition, result in insufficient syngas output and quality and gasifier efficiency below a desired level, due principally to slower reaction kinetics. Moreover, in slagging gasifiers, sufficiently high temperature is sustained to maintain slag (molten ash) in a molten flowing condition. Temperatures below a predetermined temperature within the gasifier, which depend substantially on ash composition, will result in local slag solidification, and will eventually deteriorate into a loss of fuel throughput condition and subsequent system shutdown. Excessively high temperature is also deleterious to the gasification process in that it leads to more rapid degradation (corrosion, erosion, and/or thermal stress) of high temperature components such as refractory, heat transfer surfaces, syngas transport pipe, and flow control devices. Further, over-temperature conditions in slagging gasifiers lead to slag viscosities that are too low, causing the slag to drain too rapidly from the gasifier and thereby withdrawing the natural protection that the slag affords the underlying refractory surface, whereas over-temperature conditions in non-slagging gasifiers, such as bed-type gasifiers, can lead to gasifier plugging and subsequent mal-distribution of reactants. 
     In heating and power applications, measurement of syngas composition can be used to derive the syngas heating value H sg  as well as to predict the stability and temperature of the syngas flame in the process furnace. That is, knowledge of the syngas heating value, in addition to aiding in the quantification of the process furnace energy balance, permits assessment of the quality of energy delivered to the furnace  102 . For example, when the syngas is used as a fuel input to a boiler originally designed to fire a conventional fossil fuel such as coal, fuel oil or natural gas, the syngas heating value is greater than or equal to a minimum value H sg,min , in order to maintain parity with the performance of the furnace using the baseline fuel. The specific furnace performance considerations extend to heat transfer rate and distribution, process temperatures, original flow control and delivery equipment (e.g., pipes, valves, burners), and original emissions control equipment. Syngas hydrogen (H 2 ) concentration is another component of syngas composition. While hydrogen and carbon monoxide (CO), for example, have similar heating values (molar basis) and burn at similar temperatures, hydrogen is a much more highly reactive gas that diffuses and combusts very rapidly. As such, the concentration of H 2  in the syngas will substantially influence the flame stability. Quantification of the effect of H 2  concentration on flame stability S L  can be seen in its influence on the laminar flame speed, which is a combustion parameter that directly relates to reaction rate and flame stability. The higher the laminar flame speed, the higher the reaction rate and the more robust the flame stability. Prior art measurements of the peak value of S L  for combustion of H 2 , CO and CH 4  in air are summarized in Table 1. Note that the peak value of S L  for H 2  is roughly an order of magnitude greater than that for either CO or CH 4 . Hence, even small concentrations of hydrogen added in a fuel gas mixture can substantially increase flame speed and improve flame stability. For a given solid fuel feed and gasifier design, hydrogen concentration in the products syngas can be increased, for example, by strategic injection of water or steam into the gasifier. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Data taken from Lewis and Von Elbe, Combustion, Flame and Explosion 
               
               
                 of Gases, 3 rd  Edition, Academic Press, New York, 1987. 
               
            
           
           
               
               
               
            
               
                   
                   
                 Peak Laminar Flame Speed 
               
               
                   
                 Fuel 
                 (cm/sec) 
               
               
                   
                   
               
            
           
           
               
               
               
            
               
                   
                 H 2   
                 320 
               
               
                   
                 CH 4   
                 35 
               
               
                   
                 CO 
                 45 
               
               
                   
                   
               
            
           
         
       
     
     Measurement of syngas composition is also used for applications wherein syngas is used in the production of chemicals. In such cases, measurement of syngas composition may be utilized, for example, to maintain a fixed CO/H 2  ratio or control CH 4  production within specified limits. 
     While syngas heating value and composition do not possess a unique relationship to one another (i.e., several different gas compositions can and do yield the same gas heating value), repeatable and predictable relationships will occur between the composition and heating value for particular fuels gasified in a fixed gasifier. Therefore, for a given solid fuel/gasifier combination, measurement of gaseous species that will adequately satisfy the calculation of heating value and assessment of syngas quality include, but are not limited to, carbon monoxide (CO), hydrogen (H 2 ), methane (CH 4 ) and carbon dioxide (CO 2 ), alone or in combination. 
     Returning to the diagrammatic view of the control system  305  in  FIG. 3 , if the measured gasifier temperature and syngas composition are within a predetermined range, then the control system  305  bypasses the gasifier controls and returns to its gasifier monitoring/comparing function (see return step  734 ) while, in parallel, also proceeding back to the first comparator step  702  (see return step  728 ). This parallel functionality enables the multiple gasifier controls to be activated and executed simultaneously. If, however, either of the gasifier variables possess deviations from the predetermined range, the control system  305  initiates a gasifier control step  720 . The gasifier control step  720  includes one or more of the following: adjusting the solid fuel flow rate (box  724 ), adjusting the recycled syngas flow rate (box  714 ), and adjusting the gasifier oxygen flow rate (box  716 ). These adjustments are based upon the measured gasifier temperature (box  710 ) and/or the measured syngas composition and/or heating value (box  712 ). 
     Adjusting the recycled syngas flow rate (box  714 ) controls the supplemental fuel input (for example, fuel provided in addition to solid fuel) to the gasifier  300 . As such, it affects gasifier temperature. That is, higher rates of supplemental energy input from the recycled syngas generally lead to higher gasifier temperature. Conversely, lower rates of supplemental energy input from the recycled syngas generally lead to lower gasifier temperature. When, however, the energy density and/or the quality of the recycled syngas is too low, increases in the rate of recycled syngas can lead to a lowering of the gasifier temperature. Analysis of the product gas quality and measurement of the gasifier temperature are used to assess the effect that an incremental decrease or increase of recycled syngas will have on gasifier temperature. One method of quantifying this assessment is through the calculation of adiabatic flame temperature of the recycled syngas. When the recycled syngas flame temperature is higher than the flame temperature of the solid fuel within the gasifier, then incremental addition of recycled syngas will generally increase the gasifier temperature. However, when the flame temperature of the recycled syngas is lower than that of the solid fuel within the gasifier, then incremental addition of recycled syngas will generally lead to a reduction in gasifier temperature. In addition to the preceding considerations, the ratio of recycled syngas to solid fuel input determines the combined input fuel composition, which directly affects syngas quality. Adjusting the gasifier oxygen flow rate (box  716 ) controls the degree of partial oxidation within the reactor, which affects syngas composition, and also temperature through the chemical energy release of the fuel (i.e., more oxidation leads to higher chemical energy release and higher gasifier temperature, etc.). Adjusting the oxygen flow rate (box  716 ) is optionally followed in series by adjusting oxygen flow rate with recycled syngas (box  718 ) which controls the proportion of gasifier oxygen introduced with, or adjacent to, recycled syngas. That is, at fixed total gasifier oxygen flow rate M O2-G , a portion M O2-G2  of this total can be introduced adjacent to the recycled syngas stream as indicated in  FIGS. 1 and 2 . The advantages of such a technique relate in part to reducing or eliminating the dilution between this latter oxygen stream and the recycled syngas, which leads to higher recycled syngas flame temperature and, hence, higher rates of heat transfer from the recycled syngas flame to its surroundings within the gasifier. There are other advantages, such as those disclosed in U.S. patent application Ser. No. ______, titled “Method for Gasification and Gasifier,” filed concurrently with the present disclosure and incorporated by reference in its entirety. 
     The relationship between solid fuel flow rate and gasifier temperature is similar to that between the recycled syngas flow rate and temperature already disclosed. There are, however, differences between the adjusting of the solid fuel flow rate (box  724 ) and adjusting the recycled syngas flow rate (box  714 ). First, since the solid fuel energy input to the gasifier controls the energy input to the process furnace (see equation 11), under conditions of constant solid fuel quality, the solid fuel flow rate will not be altered unless the process furnace  102  demand changes (see box  706 ). This is different from the logic of recycled syngas flow control, which is invoked to modulate gasifier temperature at fixed solid fuel quality and process furnace  102  demand. When however, solid fuel quality varies, then a change in solid fuel flow rate to the gasifier  300  may be required to maintain steady process conditions within the process furnace. That is, higher solid fuel energy density (e.g., HHV) would require a lower solid fuel flow rate and lower solid fuel energy density would require a higher flow rate. 
     Upon completion of the gasifier control step  720 , the control system  305  initiates a return step  726  that initiates measurement of the gasifier temperature (box  710 ) and/or measurement of the syngas composition and/or heating value (box  712 ). This loop is repeated until convergence with gasifier  300  set point parameters is attained, and the gasifier comparator step  732  directs the control system  305  to initiate the first comparator step  702 . Note that there is a possible scenario in which gasifier convergence cannot be attained due, for example, to low solid fuel quality preventing the attainment of sufficiently high quality syngas to satisfy the process furnace  102  operating requirements. In such circumstances, the control system  305  has the option of introducing oxygen into the process furnace  102  (see M O2-F  on  FIG. 2 ) in order to enhance the syngas flame characteristics in the furnace. Thus, modulation of oxygen (box  722 ) is initiated by adjusting control valve  321  to regulate the process furnace  102  oxygen flow in accordance with the process furnace  102  performance parameters. 
     The control system  305  addresses the drawbacks of the prior art by employing recycle of syngas generated in the gasifier  300  to enable a rapid and dynamic response to changes in process furnace  102  demand while maintaining stable operation of the system  100 . In one embodiment, the recycled syngas generates a flame temperature that is higher than the flame temperature of the primary solid fuel within the gasifier  300 . This is achieved by elevating flame temperature of the recycled syngas by burning the recycled syngas stream with oxygen. The benefits that can be derived by burning a secondary fuel with oxygen in a solid fuel reactor include, but are not limited to, improved reactor turndown, improved product gas consistency and ability to gasify fuels with a broad range of slagging properties. The increases in the flame temperature afforded by oxygen combustion are illustrated in Table 2, which shows the adiabatic flame temperatures of a typical syngas in both stoichiometric (zero excess oxidizer) air and oxygen combustion. It is instructive to compare these values to the typical adiabatic flame temperature of solid fuel, for example, bituminous coal which has an adiabatic flame temperature of about 3600° F. (4060 R). As radiant heat transfer is proportional to (T high   4 −T low   4 ), where T is the absolute temperature (either Rankine, R or Kelvin, K), it is clear that the heat transfer from an oxy/syngas flame to coal in a gasifier will transfer substantially more heat than would occur from an air/syngas flame. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 Constituent 
                 Syngas 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Carbon Monoxide 
                 58 
                 mol % 
               
               
                   
                 Hydrogen 
                 21 
                 mol % 
               
               
                   
                 Methane 
                 8 
                 mol % 
               
               
                   
                 Carbon Dioxide 
                 5 
                 mol % 
               
               
                   
                 Water Vapor 
                 4 
                 mol % 
               
               
                   
                 Nitrogen 
                 4 
                 mol % 
               
               
                   
                 Higher Heating Value 
                 337 
                 Btu/scf 
               
               
                   
                 Adiabatic Flame 
                 3683° 
                 F. (4143 R) 
               
               
                   
                 Temperature in Air 
               
               
                   
                 Adiabatic Flame 
                 5087° 
                 F. (5547 R) 
               
               
                   
                 Temperature in 
               
               
                   
                 Oxygen 
               
               
                   
                   
               
            
           
         
       
     
     In one embodiment, rapid and dynamic response to process furnace  102  energy demand changes is achieved by diverting a variable portion of the syngas product back to the gasifier in a recycle stream. That is, when process energy demand is reduced, syngas product is diverted away from the process to the recycle line. Conversely, when process energy demand is increased, syngas flow to the process is increased by diverting flow from the recycle line back to the process. 
     In another embodiment, a safe and stable increase in turndown of the gasifier  300  and, consequently, a larger system (gasifier  300  plus dedicated process furnace  102 ), is achieved by reducing or eliminating gasifier  300  temperature and syngas quality variations during turndown. For example, the control system  305  achieves temperature stabilization during turndown by off-setting the reduction in gasifier temperature that a reduction in solid fuel energy input to the gasifier would otherwise bring, by combining a portion of the recycled syngas with a portion of the oxygen, thereby enhancing heat transfer from the recycled syngas flame via the increase in recycled syngas flame temperature that oxygen affords. The syngas quality stabilization derives both from the temperature stability and the use of feedback from syngas quality and gasifier temperature measurements as input to the control system  305  to adjust oxygen and recycled syngas rates. For example, the gasifier  300  temperature may require additional syngas recycle flow as solid fuel feed is reduced, while syngas stabilizing heating value may be affected by reducing or increasing oxygen introduced to the gasifier  300 . Increasing oxygen flow, for example, increases fuel oxidation which may decrease (or prevent an increase) in syngas heating value. Decreasing oxygen flow decreases oxidation which may, for example, increase syngas heating value. 
     Further details describing aspects of the functionality of the control system  305  are presented in the following, non-limiting examples. 
     EXAMPLE 
     The Examples described herein are intended to further describe the functionality of the control system  305  and to illustrate prophetic aspects of the application of the control system  305 . Although assumptions are made for the purposes of simplicity and clarity, those skilled in the art will appreciate that the assumptions can (and often will) be removed and replaced with more specific and more precise calculations and modeling based upon the operational considerations of the system controlled by the control system  305 . 
     Each of the Examples relates to a gasifier burning agricultural waste with oxygen to produce syngas that is delivered to a boiler where it is combusted with air to raise steam and generate electricity. Chemical properties of the agricultural waste (solid fuel) are listed in Table 3. 
     
       
         
           
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                 Solid Fuel Properties 
               
            
           
           
               
               
               
            
               
                   
                 Agricultural Waste 
                   
               
               
                   
                 Constituent 
                 Concentration (wt %) 
               
               
                   
                   
               
            
           
           
               
               
               
            
               
                   
                 Carbon 
                 40.90 
               
               
                   
                 Hydrogen 
                 5.15 
               
               
                   
                 Oxygen 
                 36.62 
               
               
                   
                 Nitrogen 
                 0.92 
               
               
                   
                 Water 
                 11.27 
               
               
                   
                 Ash 
                 5.15 
               
               
                   
                 Higher Heating Value 
                 6959 
               
               
                   
                 (Btu/lb, as recv&#39;d) 
               
               
                   
                   
               
            
           
         
       
     
     The baseline fuel energy delivered to the boiler in the syngas is equal to 200 MMBtu/hr, with reference to the higher heating value of the syngas. The overall efficiency of the gasifier is 85%. That is, 15% of the energy entering the gasifier is lost and not thereafter recovered. Ten percent of the syngas generated in the gasifier is recycled back to the gasifier input for supplemental heat. Oxygen is introduced into the gasifier at 25% of the stoichiometric amount. The syngas Higher Heating Value (HHV) is maintained above 300 Btu/scf for boiler combustion stability, and the gasifier internal temperature is greater than 2000° F. to ensure slagging conditions, but less than 2200° F. to prevent excessive corrosion and refractory degradation within the gasifier. It is assumed that the recycled syngas flame temperature is greater than the solid fuel flame temperature within the gasifier. The baseline syngas composition is given in Table 4. 
     
       
         
           
               
             
               
                 TABLE 4 
               
             
            
               
                   
               
               
                 Baseline Syngas Properties 
               
            
           
           
               
               
               
            
               
                   
                   
                 Concentration 
               
               
                   
                 Syngas Component 
                 (mol %) 
               
               
                   
                   
               
            
           
           
               
               
               
            
               
                   
                 CO 
                 55 
               
               
                   
                 H2 
                 25 
               
               
                   
                 CH4 
                 5 
               
               
                   
                 CO2 
                 5 
               
               
                   
                 N2 
                 5 
               
               
                   
                 H2O 
                 5 
               
               
                   
                 Higher Heating Value 
                 310.5 Btu/scf 
               
               
                   
                   
               
            
           
         
       
     
     Energy flows around the system for baseline and subsequent the first adjustment step  730  are summarized in Table 5. Mass flows of solid fuel and oxygen to the gasifier for the same conditions are listed in Table 6. Details regarding the exemplary baseline calculations and first adjustment step  730  actions are subsequently provided herein. 
     
       
         
           
               
             
               
                 TABLE 5 
               
             
            
               
                   
               
               
                 Energy Flows (MMBtu/hr) Following 
               
               
                 Primary Control Block Actions 
               
            
           
           
               
               
               
               
               
            
               
                   
                   
                 Control Step 
                 Control Step 
                   
               
               
                   
                   
                 704: Adjust 
                 706: Adjust 
                 Control Step 
               
               
                   
                   
                 Recycle 
                 Solid Fuel 
                 708: Adjust 
               
               
                   
                 Baseline 
                 Syngas Flow 
                 Flow 
                 O2 Flow 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
            
               
                 Boiler In 
                 200 
                 190 
                 190 
                 190 
               
               
                 Gasifier 
                 222.2 
                 232.2 
                 222.2 
                 222.2 
               
               
                 Out 
               
               
                 Syngas 
                 22.2 
                 42.2 
                 32.2 
                 32.2 
               
               
                 Recycle 
               
               
                 Solid Fuel 
                 239.2 
                 239.2 
                 229.2 
                 229.2 
               
               
                 to 
               
               
                 Gasifier 
               
               
                 Gasifier 
                 39.2 
                 39.2 
                 39.2 
                 39.2 
               
               
                 Energy 
               
               
                 Loss 
               
            
           
           
               
            
               
                 Syngas Recycle Ratio, β 
               
            
           
           
               
               
               
               
               
            
               
                   
                 0.10 
                 0.182 
                 0.145 
                 0.145 
               
               
                   
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 6 
               
             
            
               
                   
               
               
                 Mass Flows (lb/hr) of Solid Fuel and Oxygen to Gasifier 
               
               
                 Following Primary Control Block Actions 
               
            
           
           
               
               
               
               
               
            
               
                   
                   
                 Control Step 
                 Control Step 
                   
               
               
                   
                   
                 704: Adjust 
                 706: Adjust 
                 Control Step 
               
               
                   
                   
                 Recycle 
                 Solid Fuel 
                 708: Adjust 
               
               
                   
                 Baseline 
                 Syngas Flow 
                 Flow 
                 O2 Flow 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
            
               
                 Solid 
                 34,375 
                 34,375 
                 32,938 
                 32,938 
               
               
                 Fuel 
               
               
                 Oxygen 
                 10,510 
                 10,510 
                 10,510 
                 10,438 
               
            
           
           
               
            
               
                 Stoichiometric Ratio, φ 
               
            
           
           
               
               
               
               
               
            
               
                   
                 0.250 
                 0.235 
                 0.252 
                 0.250 
               
               
                   
                   
               
            
           
         
       
     
     After a period of steady operation at these baseline conditions, the boiler energy demand is reduced to 190 MMBtu/hr. To rapidly facilitate the reduction in demand, the adjustments to syngas flow rate (box  704 ) by the control system  305  reduce the flow of syngas to the boiler by adjusting the position of syngas control valve  307 . The speed of recycled syngas blower  315  and/or the position of the recycled syngas control valve  319  can also be adjusted to redirect that portion of the boiler syngas that has been reduced back to the gasifier through the recycle line. This initial control response occurs without altering either the gasifier oxygen or solid fuel flows. Moreover, since it occurs rapidly, overall energy loss from the gasifier  300  is maintained at the baseline level. It is assumed for the purpose of this example that the syngas composition remains constant during the first adjustment step  730 . Variations to syngas quality are addressed subsequently during the gasifier control step  720 . 
     Note that after the initial adjustments to syngas flow rate (box  704 ), the recycle ratio, β (included in Table 5), increases from 0.10 to 0.182, while the oxygen stoichiometric ratio, φ (included in Table 6), drops from 0.250 to 0.235 due to the higher syngas recycle flow to the gasifier at fixed oxygen flow rate. When the solid fuel flow is reduced to the new boiler demand by adjustments to solid fuel flow rate (box  706 ), the syngas recycle flow is also lowered from the rapid response adjustments to syngas flow rate (box  704 ) to a value that returns the total (recycled syngas plus solid fuel) gasifier energy input to the baseline value. While this particular reduction is only one of many specific allowable control actions controlled by the control system  305 , it is beneficial in that it provides gasifier energy input and, hence, temperature stability during boiler load fluctuations. While the oxygen flow to the gasifier remains constant during adjustments to solid fuel flow rate (box  706 ), it is of interest that the stoichiometric ratio φ increases due to the reduction in fuel flow. Ultimately, the gasifier oxygen flow is adjusted (box  708 ). It is noteworthy that, while the stoichiometric ratio set point is equal to its baseline value of 0.25, the corresponding oxygen flow rate is somewhat reduced from its baseline level due to the higher proportion of syngas to solid fuel entering the gasifier (syngas requiring slightly less oxygen per Btu than the solid fuel). Retaining a constant stoichiometric ratio is, as with total gasifier energy input, one of many specific control formulae that can be applied. In this case, the rationale is that maintaining a constant stoichiometric ratio will, at first approximation, provide a reduced or eliminated disruption to syngas quality during the control adjustment process. 
     If the first adjustment step  730  maintains stability of gasifier temperature and syngas quality during the period of transitional boiler demand, then the measurement of the gasifier temperature (box  710 ) and/or measurement of the syngas composition and/or heating value (box  712 ) will confirm that no further control action is required. Nevertheless, for the purpose of documenting in a logical manner the method of operation of the control system  305 , four prophetic Examples involving disruptions to steady gasifier operation were considered by applying the above parameters and assumptions. 
     Example 1 
     Low Syngas Heating Value 
     In a first Example, measurement of the syngas composition reveals that the CO concentration decreases by 5 mol % while the CO 2  concentration increases by the same percentage as a result of the first adjustment step  730 , lowering the syngas HHV to 294.5 Btu/scf (see Table 7 for a breakdown of the system  100  parameters, including syngas properties, energy and mass flows before and after the gasifier control step  720  actions). 
     
       
         
           
               
             
               
                 TABLE 7 
               
             
            
               
                   
               
               
                 Key System Parameters for Example 1: Low Syngas HHV 
               
            
           
           
               
               
               
            
               
                   
                 State 1a: Following First 
                   
               
               
                   
                 Adjustment Step 730 
               
            
           
           
               
               
               
            
               
                   
                 but Prior to Gasifier 
                 State 1b: Following 
               
               
                 Syngas Component 
                 Control Step 720 
                 Gasifier Control Step 720 
               
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 CO 
                 50 
                 mol % 
                 55 
                 mol % 
               
               
                 H 2   
                 25 
                 mol % 
                 20 
                 mol % 
               
               
                 CH 4   
                 5 
                 mol % 
                 10 
                 mol % 
               
               
                 CO 2   
                 10 
                 mol % 
                 5 
                 mol % 
               
               
                 N 2   
                 5 
                 mol % 
                 5 
                 mol % 
               
               
                 H 2 O 
                 5 
                 mol % 
                 5 
                 mol % 
               
               
                 Syngas HHV 
                 294.5 
                 Btu/scf 
                 344.8 
                 Btu/scf 
               
               
                 Gasifier 
                 2080° 
                 F. 
                 2050° 
                 F. 
               
               
                 Temperature 
               
               
                 Energy to Boiler 
                 190 
                 MMBtu/hr 
                 190 
                 MMBtu/hr 
               
               
                 Energy out of 
                 222.2 
                 MMBtu/hr 
                 222.2 
                 MMBtu/hr 
               
               
                 Gasifier 
               
               
                 Syngas Recycle 
                 32.2 
                 MMBtu/hr 
                 32.2 
                 MMBtu/hr 
               
               
                 Energy 
               
               
                 Solid Fuel Energy 
                 229.2 
                 MMBtu/hr 
                 229.2 
                 MMBtu/hr 
               
               
                 Gasifier Energy 
                 39.2 
                 MMBtu/hr 
                 39.2 
                 MMBtu/hr 
               
               
                 Loss 
               
            
           
           
               
               
               
            
               
                 Syngas Recycle 
                 0.145 
                 0.145 
               
            
           
           
               
               
               
               
               
            
               
                 Ratio, β 
                   
                   
                   
                   
               
               
                 Solid Fuel Flow 
                 32938 
                 lb/hr 
                 32938 
                 lb/hr 
               
               
                 Rate 
               
               
                 Oxygen Flow Rate 
                 10441 
                 lb/hr 
                 9639 
                 lb/hr 
               
            
           
           
               
               
               
            
               
                 Stoichiometric 
                 0.250 
                 0.230 
               
               
                 Ratio, φ 
               
               
                   
               
            
           
         
       
     
     Since the gasifier temperature is within the predetermined range, the oxygen flow rate is reduced in order to prevent the amount of fuel oxidation taking place from exceeding a predetermined degree. As such, the oxygen flow rate is adjusted (box  716 ) and the oxygen flow is incrementally reduced until the syngas heating value is raised once again above 300 Btu/scf. The final value for the stoichiometric ratio is 0.230. The gasifier temperature remains within acceptable limits, dropping from 2080° F. to 2050° F. due to the lower degree of partial oxidation, and hence chemical energy release, within the gasifier  300 . 
     Example 2 
     Low Gasifier Temperature 
     In a second Example, the syngas heating value remains within the predetermined range, but the gasifier temperature is lowered to 1980° F., which is lower than a desired range based on slagging considerations. This can happen for several reasons, such as lower feedstock quality or higher gasifier dilution resulting from the higher ratio of syngas to solid feedstock energy input relative to baseline conditions. The oxygen flow rate is adjusted (box  716 ) to increase the gasifier oxygen flow rate, thereby oxidizing more of the fuel inside the gasifier and releasing more thermal energy. The stoichiometric ratio φ thus incrementally increases until the gasifier temperature rises back above 2000° F. A value of φ equal to 0.27 results in a gasifier temperature of 2020° F. while maintaining the syngas HHV above 300 Btu/scf. System  100  parameters of the gasifier  300  for the second Example are provided in Table 8. 
     
       
         
           
               
             
               
                 TABLE 8 
               
             
            
               
                   
               
               
                 Key System Parameters for Example 2: Low Gasifier Temperature 
               
            
           
           
               
               
               
            
               
                   
                 State 2a: Following First 
                   
               
               
                   
                 Adjustment Step 730 
               
            
           
           
               
               
               
            
               
                   
                 but Prior to Gasifier 
                 State 2b: Following 
               
               
                 Syngas Component 
                 Control Step 720 
                 Gasifier Control Step 720 
               
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 CO 
                 55 
                 mol % 
                 53 
                 mol % 
               
               
                 H 2   
                 25 
                 mol % 
                 25 
                 mol % 
               
               
                 CH 4   
                 5 
                 mol % 
                 5 
                 mol % 
               
               
                 CO 2   
                 5 
                 mol % 
                 7 
                 mol % 
               
               
                 N 2   
                 5 
                 mol % 
                 5 
                 mol % 
               
               
                 H 2 O 
                 5 
                 mol % 
                 5 
                 mol % 
               
               
                 Syngas HHV 
                 310.5 
                 Btu/scf 
                 304.1 
                 Btu/scf 
               
               
                 Gasifier 
                 1980° 
                 F. 
                 2020° 
                 F. 
               
               
                 Temperature 
               
               
                 Energy to Boiler 
                 190 
                 MMBtu/hr 
                 190 
                 MMBtu/hr 
               
               
                 Energy out of 
                 222.2 
                 MMBtu/hr 
                 222.2 
                 MMBtu/hr 
               
               
                 Gasifier 
               
               
                 Syngas Recycle 
                 32.2 
                 MMBtu/hr 
                 32.2 
                 MMBtu/hr 
               
               
                 Energy 
               
               
                 Solid Fuel Energy 
                 229.2 
                 MMBtu/hr 
                 229.2 
                 MMBtu/hr 
               
               
                 Gasifier Energy 
                 39.2 
                 MMBtu/hr 
                 39.2 
                 MMBtu/hr 
               
               
                 Loss 
               
            
           
           
               
               
               
            
               
                 Syngas Recycle 
                 0.145 
                 0.145 
               
               
                 Ratio, β 
               
            
           
           
               
               
               
               
               
            
               
                 Solid Fuel Flow 
                 32938 
                 lb/hr 
                 32938 
                 lb/hr 
               
               
                 Rate 
               
               
                 Oxygen Flow Rate 
                 10438 
                 lb/hr 
                 11275 
                 lb/hr 
               
            
           
           
               
               
               
            
               
                 Stoichiometric 
                 0.250 
                 0.270 
               
               
                 Ratio, φ 
               
               
                   
               
            
           
         
       
     
     Example 3 
     High Gasifier Temperature 
     In a third Example, high gasifier temperature results from first adjustment step  730  actions. Measurement of the gasifier temperature (box  710 ) detects the high temperature condition. The control system  305  responds by reducing the syngas recycle flow rate (box  714 ), since this lowers the total energy input to the gasifier  300  without affecting the energy delivered to the boiler. System  100  parameters corresponding to the initial and final condition for this case are summarized in Table 9. 
     
       
         
           
               
             
               
                 TABLE 9 
               
             
            
               
                   
               
               
                 Key System Parameters for Example 3: High Gasifier Temperature 
               
            
           
           
               
               
               
            
               
                   
                 State 3a: Following First 
                   
               
               
                   
                 Adjustment Step 730 
               
            
           
           
               
               
               
            
               
                 Syngas 
                 but Prior to Gasifier 
                 State 3b: Following 
               
               
                 Component 
                 Control Step 720 
                 Gasifier Control Step 720 
               
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 CO 
                 55 
                 mol % 
                 55 
                 mol % 
               
               
                 H 2   
                 25 
                 mol % 
                 25 
                 mol % 
               
               
                 CH 4   
                 5 
                 mol % 
                 5 
                 mol % 
               
               
                 CO 2   
                 5 
                 mol % 
                 5 
                 mol % 
               
               
                 N 2   
                 5 
                 mol % 
                 5 
                 mol % 
               
               
                 H 2 O 
                 5 
                 mol % 
                 5 
                 mol % 
               
               
                 Syngas HHV 
                 310.5 
                 Btu/scf 
                 310.5 
                 Btu/scf 
               
               
                 Gasifier 
                 2230° 
                 F. 
                 2180° 
                 F. 
               
               
                 Temperature 
               
               
                 Energy to Boiler 
                 190 
                 MMBtu/hr 
                 190 
                 MMBtu/hr 
               
               
                 Energy out of 
                 222.2 
                 MMBtu/hr 
                 215.9 
                 MMBtu/hr 
               
               
                 Gasifier 
               
               
                 Syngas Recycle 
                 32.2 
                 MMBtu/hr 
                 25.9 
                 MMBtu/hr 
               
               
                 Energy 
               
               
                 Solid Fuel Energy 
                 229.2 
                 MMBtu/hr 
                 228.1 
                 MMBtu/hr 
               
               
                 Gasifier Energy 
                 39.2 
                 MMBtu/hr 
                 38.1 
                 MMBtu/hr 
               
               
                 Loss 
               
            
           
           
               
               
               
            
               
                 Syngas Recycle 
                 0.145 
                 0.12  
               
               
                 Ratio, β 
               
            
           
           
               
               
               
               
               
            
               
                 Solid Fuel Flow 
                 32938 
                 lb/hr 
                 32778 
                 lb/hr 
               
               
                 Rate 
               
               
                 Oxygen Flow Rate 
                 10438 
                 lb/hr 
                 10180 
                 lb/hr 
               
            
           
           
               
               
               
            
               
                 Stoichiometric 
                 0.250 
                 0.250 
               
               
                 Ratio, φ 
               
               
                   
               
            
           
         
       
     
     The solid fuel flow rate to the gasifier drops slightly since the energy loss, as a fraction of the total gasifier energy input, remained constant while the total energy input was lowered due to the reduced syngas recycle flow. Moreover, the oxygen flow rate to the gasifier significantly reduces since the stoichiometric ratio remains constant while the gasifier fuel input (solid plus recycled syngas) is lowered. 
     Example 4 
     High Syngas Heating Value Due to Variation in Solid Fuel Feedstock Properties 
     A fourth Example shows that although an increase in syngas heating value is not detrimental to the operation of either the gasifier  300  or boiler, without control system  305  intervention is nevertheless still desired to maintain substantially constant energy flow to the furnace. For example, an increase of syngas HHV from 310.5 to 376.0 Btu/scf occurring as a result of an increase in the quality of solid fuel feedstock from the original fuel described in Table 3 to a higher carbon, lower moisture fuel was analyzed. The upgraded solid fuel composition is provided in Table 10. System  100  parameters following the initial boiler load reduction (original solid fuel), and following the solid fuel/syngas HHV variation, respectively, are summarized in Table 11. 
     In this example, as the energy density of the syngas increases, the rate of energy supplied to the furnace  102 , at fixed fuel mass flow rates would increase proportionally. To maintain substantially constant energy flow to the furnace  102 , the control system  305  responds to an increase in syngas HHV by executing a reduction in syngas mass flow rate (box  704 ) through control valve  307 . If it is assumed that the control system  305  has also received data indicating an increase in the HHV of the solid fuel, then there would also be a (feed-forward) reduction in solid fuel flow rate (see box  706 ) to a value that would maintain a substantially constant solid fuel energy input rate to the gasifier  300 . This would then be followed by an adjustment (box  708 ) to oxygen flow to account for the different balance of solid fuel and recycled syngas to the gasifier  300 . In one embodiment, all of these primary control block actions can be executed without actions of gasifier control step  720 . 
     
       
         
           
               
             
               
                 TABLE 10 
               
             
            
               
                   
               
               
                 Adjusted Solid Fuel Properties 
               
            
           
           
               
               
               
            
               
                   
                 Solid Fuel 
                   
               
               
                   
                 Constituent 
                 Concentration (wt %) 
               
               
                   
                   
               
            
           
           
               
               
               
            
               
                   
                 Carbon 
                 53.1 
               
               
                   
                 Hydrogen 
                 6.6 
               
               
                   
                 Oxygen 
                 30.5 
               
               
                   
                 Nitrogen 
                 0.8 
               
               
                   
                 Water 
                 2.0 
               
               
                   
                 Ash 
                 7 
               
               
                   
                 Higher Heating Value 
                 8050 
               
               
                   
                 (Btu/lb, as recv&#39;d) 
               
               
                   
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 11 
               
             
            
               
                   
               
               
                 Key System Parameters for Supplemental Case: High Syngas HHV 
               
            
           
           
               
               
               
            
               
                   
                 State 4a: Following 
                 State 4b: Following State 
               
               
                   
                 Energy Demand reduction 
                 4a and after Additional 
               
               
                   
                 and After First 
                 Control Adjustments 
               
               
                   
                 Adjustment Step 730 
                 Resulting from Shift in 
               
               
                 Syngas 
                 with Solid Fuel as 
                 Solid Fuel Properties 
               
               
                 Component 
                 in Table 3 
                 from Table 3 to Table 10 
               
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 CO 
                 55 
                 mol % 
                 55 
                 mol % 
               
               
                 H 2   
                 25 
                 mol % 
                 24 
                 mol % 
               
               
                 CH 4   
                 5 
                 mol % 
                 12 
                 mol % 
               
               
                 CO 2   
                 5 
                 mol % 
                 5 
                 mol % 
               
               
                 N 2   
                 5 
                 mol % 
                 1 
                 mol % 
               
               
                 H 2 O 
                 5 
                 mol % 
                 3 
                 mol % 
               
               
                 Syngas HHV 
                 310.5 
                 Btu/scf 
                 376 
                 Btu/scf 
               
               
                 Gasifier 
                 2080° 
                 F. 
                 2080° 
                 F. 
               
               
                 Temperature 
               
               
                 Energy to Boiler 
                 190 
                 MMBtu/hr 
                 190 
                 MMBtu/hr 
               
               
                 Energy out of 
                 222.2 
                 MMBtu/hr 
                 222.2 
                 MMBtu/hr 
               
               
                 Gasifier 
               
               
                 Syngas Recycle 
                 32.2 
                 MMBtu/hr 
                 32.2 
                 MMBtu/hr 
               
               
                 Energy 
               
               
                 Solid Fuel Energy 
                 229.2 
                 MMBtu/hr 
                 229.2 
                 MMBtu/hr 
               
               
                 Gasifier Energy 
                 39.2 
                 MMBtu/hr 
                 39.2 
                 MMBtu/hr 
               
               
                 Loss 
               
            
           
           
               
               
               
            
               
                 Syngas Recycle 
                 0.145 
                 0.145 
               
               
                 Ratio, β 
               
               
                 Relative Syngas 
                 1.0  
                 0.826 
               
               
                 Molar Flow Rate 
               
               
                 to Boiler 
               
            
           
           
               
               
               
               
               
            
               
                 Solid Fuel Flow 
                 32938 
                 lb/hr 
                 28474 
                 lb/hr 
               
               
                 Rate 
               
               
                 Oxygen Flow 
                 10438 
                 lb/hr 
                 12799 
                 lb/hr 
               
               
                 Rate 
               
            
           
           
               
               
               
            
               
                 Stoichiometric 
                 0.250 
                 0.250 
               
               
                 Ratio, φ 
               
               
                   
               
            
           
         
       
     
     Reference to Table 11 shows that while the energy flow rate to the furnace  102  remains unaltered as a result of the high syngas HHV condition, the molar (i.e., volumetric) flow of syngas to the boiler decreases by 17.4% from the post-boiler load reduction/original solid fuel value, changing from a relative value of 1.0 to 0.826 (see “Relative Syngas Molar Flow Rate to Boiler” entry in Table 11). In addition, the solid fuel flow rate lowers due to the increase in solid fuel HHV and the oxygen flow rate increases significantly, even though the stoichiometric ratio remains at 0.25. This is because the stoichiometric molar ratio of oxygen added to carbon in the solid fuel to ω stoich  increases due to the change in both solid fuel and recycled syngas composition. 
     It has been assumed in the preceding Example that the change in solid fuel composition was detected in a timely manner and inputted into the control system  305 . However, the control system  305  can function effectively in the absence of this measurement. This is because, in such a case, the deviation in syngas quality will still be detected in the furnace  102  as an increase in energy input. Hence, the syngas flow rate to the furnace  102  will still be gradually reduced via adjustment to syngas flow rate (box  704 ). However, at this stage, since no solid fuel flow rate data are available, the control system will not execute an adjustment in solid fuel flow rate (see box  706 ). Instead, after executing necessary adjustments to the oxygen flow rate due to any change in recycled syngas flow that may accompany the change in positioning of control valve  307 , the control system  305  proceeds to gasifier measurements (see box  710  and box  712 ), and the gasifier comparator step  732 . Due to the increase in solid fuel HHV and constant feed rate, the gasifier  300  temperature will rise, thereby directing the control system to enter the gasifier control step  720  where the three pathways of solid fuel, recycled syngas, and oxygen adjustment are available, alone or in tandem. The simultaneous existence of high gasifier temperature and high syngas HHV will instruct the control system  305  to lower the gasifier energy input via either the solid fuel (box  724 ) or recycled syngas (box  714 ) flow reduction mechanisms. However, as a unilateral reduction in recycle flow (box  714 ) would, all other factors remaining constant, increase the syngas delivered to furnace  102  (i.e., displaced recycled syngas will go to furnace), which in this case is not desired, this would not be a suitable response. The appropriate response to this scenario is thus a reduction in solid fuel flow rate (box  724 ), just as it was when solid fuel quality data were available. Hence, the control system  305  executes this response while continuing to monitor the furnace  102  and gasifier  300  performance parameters for compliance, and invoking refining adjustments along the path to stable and efficient operation of the system  100 . 
     While the preceding example provides specific options and methods of control of the control system, it will be clear to one skilled in the art that numerous other scenarios and responses not covered herein are possible. The particular control actions delimited in this inventive disclosure are, therefore, to be adaptive to the particulars (i.e., gasifier design, furnace design, fuel properties, etc.) of the system it serves. Furthermore, while the invention has been described with reference to one or more preferred embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.