Abstract:
The improvements proposed in this invention provide a reliable apparatus and method to gasify low rank coals in a class of pressurized circulating fluidized bed reactors termed “transport gasifier.” The embodiments overcome a number of operability and reliability problems with existing gasifiers. The systems and methods address issues related to distribution of gasification agent without the use of internals, management of heat release to avoid any agglomeration and clinker formation, specific design of bends to withstand the highly erosive environment due to high solid particles circulation rates, design of a standpipe cyclone to withstand high temperature gasification environment, compact design of seal-leg that can handle high mass solids flux, design of nozzles that eliminate plugging, uniform aeration of large diameter Standpipe, oxidant injection at the cyclone exits to effectively modulate gasifier exit temperature and reduction in overall height of the gasifier with a modified non-mechanical valve.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 61/288,533, filed Dec. 21, 2009, which is hereby incorporated herein by reference. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    This invention was made with Government support under Cooperative Agreement Number DE-FC21-90MC25140 awarded by the United States Department of Energy. The United States government has certain rights in the invention. 
     
    
     FIELD OF INVENTION 
       [0003]    This invention relates to pressurized circulating fluidized bed transport reactors in general and, more specifically, various components in a transport gasifier loop. 
       BACKGROUND 
       [0004]    Among various gasification technologies reviewed in a book by Higman and van de Burgt ( Gasification,  2003, Elsevier), it becomes clear that new technologies are desired to improve the economics of gasifying low rank coals, particularly coals with high moisture and/or high ash content such as lignite or subbituminous coals. 
         [0005]    The entrained flow gasifiers feed coals with particle sizes less than 75 microns either with a dry or slurry feed system. For dry feed methods, the coal moisture has to be less than 5% to prevent coal particles from forming cakes and bridging in the feed system, particularly in the feed system&#39;s lock vessel. For the slurry feed system, about 35 wt % water have to be added to make the coal slurry. It is necessary to dry the coal to very low moisture levels before making slurry to avoid more than 40% total water in the slurry. In general, low rank coals contain more than 30% moisture; to dry the coals to less than 5% moisture requires an expensive drying facility with large operating costs, decreasing the overall process efficiency. It is highly desirable to reduce drying duties and operational concerns in processing low rank coals. 
         [0006]    U.S. Pat. No. 6,631,698 discloses a circulating fluidized bed reactor which can be used for gasifying low rank coals. However, the apparatus can only be used in an atmospheric application and requires a large foot-print to produce the large amount of synfuel required by a modern chemical or power plant where coal gasifiers are installed. 
         [0007]    U.S. Pat. No. 5,560,900 discloses a process based on a pressurized circulating fluidized bed reactor which is also intended to partially oxidize the low rank coals. The concept proposed to process coal is based on more than five decades of experience with low pressure Fluid Catalytic Cracking (FCC) process in the petroleum industry. As a result, the reactor system, as has been disclosed, uses the Riser as a pyrolyzer with the large amount of heat necessary for the pyrolysis reaction is carried by finely divided refractory materials circulating around the reactor loop. How to separate the coal ash generated in the process from the heat carrying materials is one of the issues that are very difficult to deal with and the patent avoids the issue. Further, this reactor has a Mixing zone below the Pyrolysis zone whose diameter is much larger than the Riser diameter to ensure sufficient residence time to heat-up the coal particles fed into the Mixing zone. The minimum gas velocities necessary to entrain circulating solid particles from the Mixing zone causes the gas velocities to be unusually high in the Transport riser, leading to rapid erosion of any internals such as thermowells and erosion of the cyclone wall. Further, as the pyrolysis reaction needs a much longer residence time for completion and prevent tar formation in the product synthesis gas (syngas), the Riser has to be impractically tall in a commercial process for the reactor proposed in the patent. Also, this process does not teach how to distribute the gas (steam and air or oxygen) appropriately across the cross-section of a commercial size gasifier. 
         [0008]    Moving bed gasifiers have been used to gasify low rank coals for more than 100 years. The Lurgi gasifier, in particular, has been widely used to produce syngas for chemical synthesis. However, the moving bed gasifier requires lump coal as feed and cannot utilize the coal fines which are in abundance that often has few users nearby. Another drawback of this technology is that much of the coal is converted into tar rather than useful syngas. 
         [0009]    Moreover, all these gasifiers have complicated internals. The moving bed gasifiers have an elaborate rotating grid system and a stirring mechanism as internals for caking coals. The fluidized bed gasifiers have various types of complicated internal gasification agent distributors which are made of exotic alloys to tolerate the gasifier operating temperatures up to 1100° C. In spite of considerable efforts in designing the distribution grids and selecting expensive high temperature alloy materials, failures of these grids still occur which is commercially unacceptable. In the case of entrained flow gasifiers, the most problematic internal is the coal burner which is one of the most maintenance intensive items in the process. 
         [0010]    This invention provides an improved apparatus for a wide range of circulating fluid bed applications including gasification of low rank coals such as lignite and subbituminous coals. 
       SUMMARY OF THE INVENTION 
       [0011]    This invention provides a reliable apparatus and a method to gasify low rank coals in a class of pressurized circulating fluidized bed reactors termed as a transport gasifer. Embodiments of the invention overcome the problems mentioned above with prior art gasifiers. The transport gasifier loop includes a gasification agent distribution system, a mixing zone, a riser, a first stage cyclone called presalter cyclone, a secondary cyclone called standpipe cyclone, a seal-leg to return the solids collected in the presalter cyclone to a standpipe, and a non-mechanical valve to move the solids from standpipe to mixing zone and at the same time substantially reduce or prevent reverse flow of gases. 
         [0012]    The gasification agent feed systems according to embodiments of the disclosure can be implemented with few or no internals. The distribution system can substantially reduce or prevent hot solids back flow. The uniform distribution of the gasification agent across the cross-section of the gasifier is facilitated by the solids particle movement inside the gasifier. 
         [0013]    Embodiments of the transport gasifier can also include nozzles that feed the gas into the gasifier with mechanisms to substantially reduce or prevent the nozzles from being plugged when the gasifier is suddenly shutdown for process or safety reasons. The solids settled in the nozzles during shutdown can simply be blown back into the gasifier upon restart of gas flow to the nozzles. Accordingly, the transport gasifier nozzles can avoid being plugged. 
         [0014]    A transport gasifier according to embodiments of the disclosure can also employ a first stage presalter cyclone that can separate high loading of solids in the carrying gas as well as substantially reduce or prevent the cyclone wall from being eroded by such high solids loadings. Also, in contrast to prior art cyclones, the presalter cyclone according to embodiments of the disclosure in the first stage can be employed with no vortex finder as well as with no roof. These concepts can reduce reliability problems encountered in a commercial high pressure, high temperature first stage cyclone design, operation and long-term performance. 
         [0015]    The solids collected by the presalter cyclone can flow to the standpipe through a seal-leg. The position and design of the seal-leg in various embodiments of the transport gasifier take advantage of natural pressure gradients in order to minimize gas addition to the seal-leg and gasifier loop. 
         [0016]    A standpipe cyclone can collect particles from the gas stream and the fine solids collected by the standpipe cyclone can subsequently merge with solids collected by the presalter cyclone and return to the riser through the standpipe. Embodiments of a transport gasifier can reduce or avoid flowability problems related to fine solids as the fine solids flow down the standpipe. Also, this invention facilitates mixing of finer solids with coarser solids collected by the presalter cyclone as the coarser solids flow to the standpipe through the seal-leg. 
         [0017]    In various embodiments of the disclosure, some of the gasification agent, oxygen and/or air along with steam, can be injected at the exit of the first stage or the second stage cyclone in order to increase the gas outlet temperatures and reduce the carbon content in the fly ash. The gasification agent injection can also reduce the methane content and increase the carbon monoxide and hydrogen content in the product gas. 
         [0018]    A transport gasifier according to embodiments of the disclosure also allows for a more optimal configuration of the locations of aeration nozzles in the standpipe relative to the prior art. An aeration gas can be employed in or near the bottom of large diameter standpipes for large scale commercial applications, so that the aeration gas can be distributed and facilitate the solids flow from the standpipe to the riser through a non-mechanical valve. 
         [0019]    In one embodiment, the bulk density in the riser can be in the range of about 5-20 lb/ft3. In one embodiment, the mass ratio of the circulating solids to the feedstock can be between about 50 to 200. The wide range is beneficial to optimize design and operations for feedstocks with different coal characteristics. 
         [0020]    The transport gasifier provides methods for controlling the solids level in the standpipe and operating the gasifier with high desired solids fluxes and riser densities as well as at sustained high solids circulation rates in the loop, resulting in significant improvement to conversion of coal to syngas with maximum syngas production rates. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]      FIG. 1  is an illustration of the transport gasifier loop. 
           [0022]      FIG. 2  is a sketch of the gas distributor as part of the lower mixing zone through which a portion of the gasification agent is introduced into the gasifier. 
           [0023]      FIG. 3  is an illustration of the lower mixing zone, the upper mixing zone, the lower portion of the riser and non-mechanical valve entrance to the gasifier mixing zone along with coal injection nozzles and feed gas distribution to manage heat release and attain uniform and rapid heat-up of coal particles. 
           [0024]      FIG. 4  is an illustration of the riser bend connecting the riser and inclined crossover through which the salted out solids in the gas stream tangentially enter the presalter cyclone. 
           [0025]      FIG. 5  is a sketch of the Seal-leg connecting the presalter cyclone to standpipe. 
           [0026]      FIG. 6  is an illustration of a typical aeration nozzle design for gas streams that flow into the transport gasifier in upward direction. 
           [0027]      FIG. 7  is an illustration of standpipe cyclone design to withstand high pressure, high temperature and erosive environment of transport gasifier. 
           [0028]      FIG. 8  is an illustration of oxidant injection at the cyclone exits to effectively modulate gasifier exit temperatures and slightly improve overall carbon conversion. 
           [0029]      FIG. 9  is an illustration of aeration distributor for large diameter transport gasifier Standpipes. 
           [0030]      FIG. 10  is an illustration of L+J non-mechanical valve concept to decrease the overall height of the transport gasifier. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0031]    Various embodiments and illustrations of a transport gasifier loop according to embodiments of the disclosure are described by examples and illustrations.  FIG. 1  illustrates the transport gasifier loop  100 . The gasifier vessel wall can be made of carbon steel and the shell can also constitute the pressure boundary of the gasifier. The gasifier loop  100  can be operated at pressures between about 100-1000 psia depending on the process requirement of units utilizing resultant syngas downstream. Inside the shell of the gasifier loop  100 , there can be two layers of refractory lining. An inner layer, in contact with the circulating bed of solids, can include a layer of erosion resistant refractory to protect the soft insulation refractory and vessel wall. The outer insulating layer can be in contact with the shell of the gasifier loop  100  on one side and erosion resistant refractory on the other side. The insulation refractory protects the shell from overheating. One embodiment of the transport gasifier loop  100  can include a gas distributor near the bottom of the gasifier, a lower mixing zone, an upper mixing zone, a riser, an inclined crossover, a first stage (presalter) cyclone, a second stage standpipe cyclone, a standpipe, a seal-leg connecting the presalter cyclone and the standpipe, and a non-mechanical valve that connects the standpipe and the mixing zone, which will be described in further detail herein. 
         [0032]      FIG. 2  is an illustration of the lower mixing zone (LMZ)  200  of a transport gasifier loop  100  according to an embodiment of the disclosure, through which about 25-100% of the gas (e.g., air, oxygen and/or steam) used for the gasification reactions is injected. The characteristics of the feedstock dictate the amount of gas that needs to be injected in the LMZ, and the remainder can be distributed along the height of the mixing zone (e.g., both lower and upper mixing zones). In the embodiment shown in  FIG. 2 , the LMZ  200  consists of a jetting gas distributor or the gas inlet section  225 . About 70-95% of the gas entering the LMZ can be injected through the nozzle inlet  210  in the distributor section. The remaining 5 to 30% of gas injected into the LMZ can be fed through a plurality of nozzles  270  located along the cone section  240  of the distributor at various elevations. The number of nozzles, orientations and elevations can vary depending upon the type of feedstock and gasifier size as can be appreciated. Together, the gas flowing through the gas inlet section  225  and nozzles  270  provide a way to introduce and distribute the gasification agent across the entire cross-section of the gasifier without any internals. 
         [0033]    The nozzle inlet  210  demarcates the gas distributor&#39;s refractory lined pipe with metal pipe that conveys gas from its source. The U-shaped refractory lined pipe in the embodiment shown in  FIG. 2  has a vertical section  215  through which the gas flows downward and is connected to the horizontal section  220  and another vertical section  230  through which the gas flows upward towards the throat before entering the cone section of the distributor. All these sections can be made of a refractory lined pipe. When for a safety or process reason, the gasifier trips, the solids held-up in the gasifier mixing zone and/or riser sections will fall and settle in lower portion of the gasifier, filling the vertical section  230  and a portion of the horizontal section  220 . The design of the horizontal section  220  is such that the solids will not reach the vertical section  215 . This design safely protects the connecting metal pipe at inlet  210  from hot settling solids whose temperature can range up to 2000° F. Also the length to diameter ratio of at least four for the horizontal section  220  can be employed so that that the solids settled can be blown back into the gasifier upon resumption of gasification operations. Operability and safety concerns are greatly reduced as plugging of main gasification agent feed line is reduced and/or eliminated. 
         [0034]    The gasification agent flowing through the inlet section  225  enters the cone section  240  of the LMZ  200 . The superficial gas velocity at the throat in the inlet  225  can be between about 50-300 ft/s. The wide range of velocities with which the gasification agent can be introduced into the gasifier increases the flexibility of operations by providing a method to introduce and distribute the gasification agent from start-up to full load. The gasification agent can mix with the solids refluxing and falling into the bottom of the LMZ  200 . The char carbon in the refluxing solids is combusted by the oxidant in the gasification agent. In operating the apparatus, the indication for whether the solids are refluxing and mixing well, especially all the way to the very bottom of the cone section, can be inferred from a set of temperature indications  280 . If a sufficient amount of solids has refluxed into the lower bottom section of the LMZ, the temperature indication  280  will be nearly the same as other temperature indications inside the gasifier. If the hot solids refluxing into the bottom of LMZ is less than desired, the solids level in the standpipe can be increased by reducing the rate of ash discharge or by adding more inert solids to the gasifier. This increases the rate of recycled solids flow from standpipe into the mixing zone, increasing the density of solids in the LMZ  200  which increases the rate of hot solids refluxing throughout the LMZ  200 . 
         [0035]    The gasification agent flows into the cylindrical section  250  of the LMZ  200  and the remaining oxygen in the gasification agent will be consumed by the char carbon in the circulating solids. In a properly operated gasifier, the set of temperature indications  285  and  290  will be nearly the same as the set of temperature indications  280 . The gas streams entering the LMZ  200  and the gas generated due to combustion and gasification reactions flows out of the LMZ at outlet  260 . The superficial gas velocity leaving LMZ at outlet  260  can be in the range of about 5 to 15 ft/s which is sufficient to entrain significant amount of solids out of LMZ by the gas. This allows fresh recycled solids flowing from the standpipe to reflux throughout the LMZ. As char is continuously introduced into the LMZ with fresh solids refluxing down, the temperature profile in the LMZ is maintained through combustion and gasification reactions. The bed density in the LMZ can be between 15-40 pounds per cubic feet. Such a bed density in the LMZ can be attained by adjusting the solids level and aeration rate in the standpipe (affecting the solids rate from the standpipe to mixing zone) as well as by adjusting the distribution of the gas between the LMZ and upper mixing zone  300  (affecting the superficial gas velocity in the LMZ). 
         [0036]    The gas injected into the LMZ flows upward to the upper mixing zone (UMZ)  300  as shown in  FIG. 3 . The unreacted oxygen in the feed gas from the LMZ can first encounter the char carbon in upper portion of the LMZ and lower portion of UMZ. The char carbon can be refractory (e.g., non-reactive from gasification perspective) in nature and is present in the circulating solids recycled from the standpipe  700  ( FIG. 1 ) through the non-mechanical valve  800 . In the embodiment presented in  FIG. 3 , the char carbon can be utilized to generate heat energy that can be necessary for highly endothermic gasification reactions that occur in riser  400 . The gasifier temperature profile is maintained as the combustion heat generated is consumed through the gasification reactions endothermicity, heat losses and sensible heat in the syngas exiting the gasifier. Due to potentially significantly high solids circulation rates (with high mass fluxes), the char carbon content in the circulating solids can be in the range of about 0.1% to 4% which is more than sufficient to consume all the oxygen in the feed gas. Since the recycled solids from the Standpipe flowing into the mixing zone can be in the range of about 1600-2000° F., the rate of char combustion reaction is nearly instantaneous. The oxygen from the lower mixing zone can be consumed rapidly upon encountering the char carbon. 
         [0037]    For some applications, additional oxidant may be necessary to consume any excess char carbon in the circulating solids. This is accomplished by adding the oxidant  1500  directly to the UMZ  300  as shown in the embodiment in  FIG. 3 . The percentage of char carbon content in the circulating solids is controlled by the solids circulation rate, coal feed rate, the overall gasifier temperature and the temperature distribution along the height of the gasifier. Distribution of oxidant in the feed gas is helpful in controlling the heat release along the lower portion of the gasifier loop. High solids circulation rates and uniform and distributed heat release prevent hot spots. Hot spots can be highly detrimental to gasifier operation as they would lead to agglomeration, slagging and clinker formation. 
         [0038]    High solid fluxes and equally high solid circulation rates facilitate uniformly high operating temperatures around the gasifier loop  100 , leading to high hot gas efficiencies with desirable gaseous products. The hot solids circulating around the gasifier loop can be thought of as a thermal flywheel in which energy is added through char combustion and consumed through gasification reactions, heat losses and sensible heat. In one complete cycle around the gasifier loop, about 5% of thermal energy is added to the thermal flywheel in the mixing zone which is eventually consumed in the Riser and other parts of the gasifier. As the energy added and consumed is only a small percent of thermal energy circulating around the gasifier loop, the gasifier temperature around the loop is nearly uniform. 
         [0039]    Coal or other carbonaceous solids from feeders  1600  can be added to the upper section of UMZ  300  as shown in the transport gasifier&#39;s feed section embodiment in  FIG. 3 . Depending on reactivity of feed solids, the mass mean diameter (MMD) of feed solids can be in the range 200 to 500 microns. The MMDs of low rank coals, which tend to be highly reactive, can be in the 350 to 500 micron range. Such large feed size reduces grinding costs and also generates ash in the size range that is suitable for maintaining high solids (ash) circulation rates in the transport gasifier loop. 
         [0040]    Since all the oxygen fed into the gasifier can be consumed in LMZ and lower sections of the UMZ by the char carbon in the circulating solids, the coal fed into the gasifier may not come in contact with any oxygen in the feed gas. For most chemical applications, the coal is conveyed to the gasifier by CO 2  or nitrogen. Without fresh coal coming in-contact with oxygen, local hot spots can be avoided and the probability of clinker formation can be eliminated. For air-blown operation of the gasifier for integrated gasification combined cycle (IGCC) applications, it can be advantageous to convey the coal with air. The amount of air used for such conveying is less than about 15% of the total air injected into the gasifier in such an application. High solids circulation rates in the gasifier loop and injection of coal at different elevations in the upper portion of UMZ  300  can disperse the oxygen in the conveying air quickly and the possibility of forming any hot spots in the gasifier is minimized. 
         [0041]    Due to high solids circulation rates in embodiments of a transport gasifier, the coal particles are heated at a high rate (e.g., at a rate of about 50,000° F./second) in the lower portion of the Riser  400  of the gasifier. Such high heat-up rates lead to evolution of large fraction of feed as volatiles and many of the thermal cracking of volatiles and gasification reactions occur in the Riser. The carbon conversion in the riser to useful gaseous products can be in the range 65-80% in the first pass through the Riser. The unreacted char carbon can be collected by the cyclone systems and returned to the mixing zone to react with oxidant fed into the lower portions of the mixing zone. The heat released with partial or complete oxidation in the mixing zone maintains the gasifier at a desired temperature. The density of inert solids that circulates around the gasifier loop can be in the range of 15 to 20 lbs/cu ft in the Riser. Such high density of solids in a riser in an embodiment of the disclosure, whose mass mean diameter can be in the range 75 to 100 microns, provides a large amount of surface area and is effective in cracking small organic molecules and other devolatalized products from coal to desired syngas components of CO and hydrogen. 
         [0042]    For highly reactive fuels such as low rank coals, the configuration of transport gasifier can have the same or similar internal diameter for LMZ, UMZ and riser. For less reactive fuels, the LMZ internal diameter is larger than upper portion of the gasifier. As the LMZ will handle more char carbon from less reactive fuels, the LMZ&#39;s functionality changes to optimizing partial oxidation and steam gasification reactions. 
         [0043]    The unreacted char carbon and circulating inert solids travel along the riser  400  to the top and exit the riser  400  through a specially designed bend  450  that connects the riser  400  with the inclined crossover  550 . An example of the bend is illustrated in  FIG. 4 . The structure and design of the bend  450  minimizes the pressure drop and avoids erosion of the inclined crossover  550  and the bend  450 . The solid particles that constitute the high mass circulating solids in the transport gasifier can be continuously derived from ash generated in the gasifier from feed coal. They can be of irregular shape and abrasive. If the bends are not carefully designed, even erosion resistant refractory will last for only a small fraction of its intended lifetime. The solids and gas stream enter the bend  450  at 15 to 35 ft/s velocity. With a long radius bend  450  that is technically feasible based on other constraints, the circulating solids stream impinges and tend to erode the upper portion of the bend. With a tee- or cross-type bend, both upper and lower portions of the bend tend to erode. In the embodiment of  FIG. 4  used in one embodiment of a transport gasifier, a small part of the circulating stream enters the extended portion of the bend. This stream cycles around in the extended portion of the bend and enters the inclined crossover  550 , pushing the main circulating stream that enters the inclined crossover  550  away from upper portions of the crossover  550 . These actions lead to the main circulating stream directed towards lower portion of the crossover  550  and the point of contact is termed as first touch-down. Such contacts (e.g., second touch-down, etc.) may occur in an improperly designed system. 
         [0044]    In one embodiment, the extended portion of the bend  450  along with the inclined crossover  550  is designed to reduce or eliminate the erosion of upper portion of the crossover  550  refractory as well as the effects of touch-downs. 
         [0045]    The solids and gas mixture exiting the riser  400  through the bend  450  enters a first stage cyclone, the presalter cyclone  500 . As illustrated in  FIG. 4 , the crossover  550  connecting the riser bend  450  and the presalter cyclone  550  is inclined downward. The angle of inclination &lt;a&gt; can be in the range of about 15 to 60 degrees depending on the characteristics of solids circulating in the gasifier loop. The inclination will cause the solids to separate from the gas in the crossover  550 , and bulk of the salting-out solids will flow along the bottom of the crossover and directly into the barrel of the presalter cyclone  500  without much spinning along the wall; this action of solids along with other concepts described in U.S. Pat. No. 7,771,585, which is hereby incorporated herein by reference in its entirety, can decrease the possibility of erosion of the cyclone wall. The presalter cyclone is conceptualized for high solids circulation rates and mass fluxes which are critical for circulating pressurized fluidized bed gasifier performance. The presalter cyclone in  FIG. 4  can be implemented with no vortex finder as well as with no roof. These concepts reduce or eliminate numerous reliability problems encountered in a commercial high pressure, high temperature cyclone design, operation and long-term performance. 
         [0046]    The solids collected by the presalter cyclone  500  can then flow into a seal-leg  900  as illustrated in  FIG. 5  which shows the sub-loop for the cyclone system including the presalter cyclone, the seal-leg  900  and the crossover between the presalter cyclone  500  and the standpipe cyclone  600 . The seal-leg comprises a downcomer  910  on one end connecting the cone of the Presalter cyclone to the horizontal leg portion  930 , a vertical seal-leg riser section  920  and an inclined section  940  connecting the Seal-leg Riser and the gasifier standpipe  710 . The length of the horizontal leg portion  930  can be about 2-10 times the inner diameter of the horizontal leg and depends on the characteristics and the solids circulation rates in the gasifier loop. The solids exit the horizontal leg through a short vertical leg (seal-leg Riser  920 ) and flow upwards; the height of the seal-leg riser will depend on the design of other parts in the seal-leg loop. In one embodiment of a transport gasifier, the height of the seal-leg riser  920  will be such that the solids level in the seal-leg downcomer  910  can be less than about 4-10 times the downcomer diameter. The pressure difference between the presalter cyclone inlet  510  and the standpipe cyclone inlet  590  can, in some embodiments, be nearly the same as the pressure difference between  510  and seal-leg exit  990  to the standpipe. The additional flow resistance in the seal-leg downcomer is reflected in the form of a solids level  915  in the downcomer. The higher the flow resistance, the higher is the solids level in the downcomer. The design of the seal-leg riser height can be used to adjust the flow resistance in the seal-leg and the solids level in the downcomer. 
         [0047]    One objective of the seal-leg  900  can be to substantially ensure that the process gas flows from presalter cyclone upwards to standpipe cyclone inlet. This is achieved with a flowing column of solids in the seal-leg which prevents short-circuit of process gas flow to Standpipe. Normally, the solids flow through the seal-leg is driven by the column of solids in the downcomer. In embodiments of this disclosure, the solids flow is driven by both pressure difference between the presalter cyclone and the standpipe as well as the column of solids in the downcomer. Due to this pressure difference and/or column of solids, higher solid fluxes can be realized through the seal-leg with minimum solids level in the downcomer and with minimum requirements for solids aeration. For circulating solid loops requiring high solid circulation rates such as in the case of gasification, embodiments of the disclosure lead to feasible compact seal-leg design for large scale commercial gasifiers. Furthermore, the solids level  915  in the downcomer can also be adjusted by the resistance of the flow path from  510  through the crossover bend  520  to the standpipe cyclone entrance  590 . It may be desirable to increase the flow resistance in this part of the loop so that the solids level in the seal-leg downcomer can further be minimized to reduce the aeration rate and maximize the solids flux in the seal-leg. 
         [0048]    To ensure high solids circulation rates in the gasifier loop, it may be desirable that the solids flow through the seal-leg smoothly. This can be accomplished by injecting a minimum amount of recycle gas into the seal-leg. The recycle gas characteristics can be nearly the same as the syngas produced in the gasifier, but it has been subjected to cooling, clean-up and recompression. In this embodiment, the aeration gas  980  to the seal-leg is divided into three branches. The aeration flow to the seal-leg downcomer  910  is generally downward inclined and the superficial velocity is between 0.03-0.1 ft/s in terms of the cross-section area of the Seal-leg downcomer. 
         [0049]    Aeration gas  950  to the horizontal section  930  can be accomplished via nozzles  1100  having a design as illustrated in  FIG. 6 . This part of the transport gasifier comprises two legs and is termed as a dog-leg nozzle. The gas feed leg  1120  forms a substantially right angle with the drain/clean-up leg  1130 . This type of aeration nozzle embodiment can be used for the refractory lined pipes where, if a prior art straight nozzle is used, may have a length to diameter (L/D) ratio generally more than 20. Such high L/D ratios lead to nozzle plugging which is detrimental to operations. As illustrated in  FIG. 6 , due to high pressure, high temperature and erosive environment typical of circulating fluidized bed gasifiers, the pipe generally has two layers of refractory. The inner layer  1140  is in contact with the circulating solids through the flow channel  1110  and comprises erosion-resistant refractory. The outer layer  1150  is in contact with the shell  1160  of the pipe and comprises insulation refractory to ensure that the shell metal temperature is below 300° F. The distance between the inner channel wall  1115  and the interconnection point between clean-up leg  1130  and gas feed leg  1120  can be in the range about 4-8 times the inner diameter of the nozzle. Because of the L/D ratio of this embodiment, even if the nozzle is filled with the solids, the aeration gas from gas feed leg  1120  can push the solids out of the nozzle and into the flow channel. This embodiment is successfully used in the transport gasifier nozzles where the aeration gas and gasification agent flow direction is upward. 
         [0050]    Keeping the nozzle clean after a trip or shutdown can help ensure aeration gas flow to fluidize the solids and maintain high solids flow through the Seal-leg. The aeration rate to the horizontal leg of the seal-leg can be between about 0.03-0.1 ft/s based on the cross-sectional area of the horizontal leg and the gasifier operating pressure and temperature. Another branch of aeration gas  950  is fed to the seal-leg riser. Under normal circumstances, the aeration to the seal-leg riser is unnecessary. The only time the aeration gas may be required is when the solids flux is above about 450 lb/ft 2 s. The normal capacity for the seal-leg presented in this invention of transport gasifier with both differential pressure and downcomer solids column driven solids flow is in the range of about 200-500 lb/ft 2 s. 
         [0051]    The gas with much reduced particle loading exits from the top of the presalter cyclone  500  and enters another cyclone located on top of the standpipe  700 . A sketch of the transport gasifier&#39;s standpipe cyclone  600  is given in  FIG. 7 . In this non-limiting embodiment, the standpipe cyclone  600  has no cone and has the same diameter as the standpipe, simplifying the design and construction. It simply has a tangential inlet to the standpipe. As the standpipe cyclone  600  inlet receives a low concentration of fine particles in the gas stream, the cyclone has a vortex finder to ensure high capture efficiencies. 
         [0052]    In the high pressure (up to about 1000 psig) and high temperature (up to about 2000° F.) environment of a transport gasifier, prior art designs to support a vortex finder is inadequate. As illustrated in  FIG. 7 , embodiments of the disclosure satisfactorily operate in the gasification environment. The vortex finder pipe has a thin layer of refractory on both inside and outside to protect from erosion. The support for vortex finder is embedded inside the insulating refractory which in-turn is attached to the shell with an expansion loop. The relatively low temperature at the support location and the expansion loop ensures that the support experiences minimum additional stress due to thermal expansion. 
         [0053]    The collection efficiency of both cyclones combined can exceed 99.999%. Such high collection efficiencies facilitate high carbon conversions in the gasifier as loss of char carbon through the cyclone system is minimized. The Presalter cyclone concepts presented in U.S. Pat. No. 7,771,585 along with the embodiments of the present disclosure facilitate attainment of high collection efficiencies in harsh gasification environments while protecting the cyclone refractories from erosion and reducing serious failures that are inherent with the use of prior art cyclones. 
         [0054]    Some applications, such as power generation, and other applications may require precise control of the desired steam generation rate from cooling the hot syngas from gasifier and ensure the designed power output is maintained. However, there are many design uncertainties and aging of equipment that can cause the actual steam generation rates to differ from the designed rate. One of the desired features and also an effective feasible method is to modulate the gasifier exit temperature to achieve precise control of the steam generation rate. The extent to which the operating temperature of the entire gasifier loop can be changed as well as the rapidity of the change to achieve and maintain the desired gasifier exit temperature is limited. As shown in  FIG. 8 , the Transport gasifier exit temperature can easily be modulated by injecting a small fraction of the oxidant  1500  to the presalter cyclone  500  or the standpipe cyclone  600  exit, where the solids concentration is low and carbon concentration in the solids is relatively high. The fraction of oxidant  1500  injected is less than about 5% of total oxidant that enters the gasifier. In this embodiment of a transport gasifier, the oxidant injection also increases slightly the carbon conversion in the gasifier loop and reduces any aromatic hydrocarbon constituents in the syngas. 
         [0055]    The propensity for solids circulation in the gasifier loop depends upon the hydrostatic head of the solids in the standpipe. The solids in the standpipe may need to be in a fluidized state. This is achieved both by gas entrained by the solids flowing downward in the standpipe and by fluidizing with the recycle gas through nozzles and distributors in the standpipe. In coal processing, the circulating solid is ash from the coal itself and mass mean diameters of the solids can range from about 75 to 100 microns depending on ash characteristics and with the cyclones functioning efficiency. The solids in this size range naturally entrain a certain amount of gas as it flows from presalter cyclone through seal-leg to standpipe. In addition, nozzles strategically located around the standpipe and aeration grid at the bottom of large diameter standpipes as shown in the embodiment in  FIG. 9  provide sufficient fluidization and hydrostatic head to maintain high solids circulation rates around the gasifier loop. Aeration gas  1700  flows through the distribution grid which is typically located about six to eighteen inches below the non-mechanical valve  800 . The solids level in the standpipe is maintained substantially constant by withdrawing coarser ash from the bottom of seal-leg riser and finer ash downstream of the transport gasifier. 
         [0056]    A non-mechanical valve  800  connects the standpipe to the mixing zone as illustrated in  FIG. 10 . One purpose of the non-mechanical valve is to reduce or prevent reverse flow of gases from the mixing zone into the standpipe. Typical non-mechanical valves that have been used in practice are termed as J-leg, L-leg and Y-leg. With both J-leg and Y-leg, the angle of inclination varies depending upon the characteristics of circulating bed of solids. If the coal throughput is fairly low, then the gasifier size is small and the centerline distance between the riser and standpipe is relatively small. Under these conditions, a J-leg is the preferred configuration. As the centerline distance between riser and standpipe increases, then the standpipe hydrostatic necessary to overcome the J-leg resistance also increases. This necessitates an increase in the height of the gasifier and a corresponding increase in the structure height and, thus, the capital costs. For transport gasifiers requiring larger throughputs, a new configuration as shown in  FIG. 10  and termed as L+J leg, offers potential advantages. The short L-portion in the illustrated embodiment will serve as the non-mechanical valve reducing or preventing reverse flow of gases. The inclined J-portion will become part of the mixing zone/riser in the sense that oxidant  1500  and steam mixture can be introduced into the J-portion and char combustion reactions can be initiated. This way the hydrostatic head lost due to non-mechanical valve resistance is significantly reduced and thus making it feasible to reduce the height of the standpipe. Also, in this embodiment, an additional volume of the gasifier (J-portion of the L+J leg) where combustion and gasification reactions similar to that in mixing zone and riser becomes feasible. This reduces the height of the riser portion of the gasifier. Overall, the illustrated L+J embodiment can reduce the height of the gasifier which can be beneficial with large scale commercial gasifier design. 
       Example 
       [0057]    Described below is one non-limiting example of an engineering scale test unit of the transport gasifier illustrated in  FIG. 1  that was built and extensively tested. None of the descriptions, ranges, or other information in this example should be considered to limit the scope of the disclosure above. The test unit gasifier has a nominal coal feed rate between 3,000 and 6,000 lbs/hr and uses both air and oxygen as the oxidant to react with char carbon in circulating solids to provide heat for the gasification reactions. Prior to tests with coal in the engineering scale unit, various embodiments of the transport gasifier were first tested in a large cold flow test unit with similar configuration. A number of different low rank coals were tested. The start-up solids inventory comprised coarse ash drained from the gasifier from a previous test run. The material in the solids stream at the test facility occasionally comprised sand with a mean particle size of 100-120 microns. Over a period of two days, the sand was gradually replaced with ash generated from feed coal. The particle size of the coal ash slightly depended on the coal properties and was nearly independent of the feed particle size in the range tested. Table I illustrates typical particle size of circulating solids for two different feeds. The median mass diameter is about 100 microns for the subbituminous coal tested at the transport gasifier and 80 microns for the lignite tested. As data was gathered by operating the gasifier at different test conditions, the solids flux in the riser varied in the range of 75-350 lb/ft 2 s. The bulk density in the riser varied from 5 to 15 lb/ft 3 , which is substantially higher than other circulating fluidized bed risers. Because of the high bulk density in the riser, the temperature across the entire riser was nearly uniform. The superficial gas velocities tested in the riser were in the range 20-35 ft/s at operating pressures that were in the range 160-250 psig. 
         [0000]    
       
         
               
             
               
               
               
             
               
               
               
             
           
               
                 TABLE I 
               
             
             
               
                   
               
               
                 Particle Size in Circulating Solids 
               
             
          
           
               
                 Mass 
                 Particle size (microns) 
                 Particle size (microns) 
               
               
                 Percent 
                 Subbituminous Coal 
                 Lignite 
               
               
                   
               
             
          
           
               
                 10 
                 49 
                 35 
               
               
                 20 
                 61 
                 46 
               
               
                 30 
                 71 
                 57 
               
               
                 40 
                 83 
                 69 
               
               
                 50 
                 97 
                 84 
               
               
                 60 
                 117 
                 104 
               
               
                 70 
                 150 
                 135 
               
               
                 80 
                 235 
                 197 
               
               
                 90 
                 392 
                 347 
               
               
                 95 
                 489 
                 461 
               
               
                 100 
                 640 
                 639 
               
               
                   
               
             
          
         
       
     
         [0058]    The streams exiting the presalter and standpipe cyclones were isokinetically sampled. Results from these samples along with circulation rates were used to calculate individual cyclones and overall collection efficiencies. For tests with different fuels at various process conditions, the presalter cyclone efficiency is generally greater than 99.5%. Under most circumstances, the cyclone efficiency is between 99.6 to 99.7%. With the high solids loading and relatively larger particles at the presalter cyclone inlet, the erosion of the presalter cyclone wall had been a major concern. The cyclone wall showed little sign of erosion after more than 6300 hours of operation. This was not the case with a conventional cyclone which experienced serious erosion at a similar installation. 
         [0059]    The overall collection efficiencies achieved during various tests was around 99.95%. The gas stream exiting the gasifier was generally in the range of 1600-1800° F. and was cooled in a syngas cooler to 600-1000° F. The cooled gas stream was filtered with candle filters to remove fine dust from the syngas. The typical mean particle size of fine dust (fly ash) was between 10 to 15 microns. Nearly all the char carbon loss from the gasifier is with the fly ash in the gas stream exiting the Standpipe cyclone. For lignite and subbituminous coals, the carbon conversion in the gasifier was generally greater than 98%. The solids collected by the presalter cyclone flow into the Seal-leg downcomer; the solids flux in the seal-leg downcomer was in the range of 100 to 470 lb/ft 2 s depending upon the solids circulation rates tested in the gasifier loop. The high solids flux rates were achieved as the solids were fully aerated and were near the minimum fluidization void fraction as well as due to the favorable pressure difference across the Seal-leg. The void fraction and solids velocity in the seal-leg downcomer and standpipe have been confirmed by different measurement methods including injection of solid particle tracers and CAT scans of flowing streams. The dog-leg type aeration nozzle in the Seal-leg and at other locations around the gasifier loop had been proven to be plug-free in spite of many gasifier shutdowns and trips. The various embodiments of transport gasifier presented in the Figures have been tested at the facility with various fuels at many different process test conditions. In addition to testing the transport gasifier invention for syngas generation with various coals, the concept was also successfully tested for nearly 5,000 hours as a pressurized circulating fluidized bed combustor termed as Transport combustor. Tests with two different applications, gasification and combustion, indicate that the embodiments developed in this invention can be used as transport reactor for a number of other applications requiring exposure of reactants to large surface area of solids and circulation rates.