Abstract:
A coal feed system to feed pulverized low rank coals containing up to 25 wt % moisture to gasifiers operating up to 1000 psig pressure is described. The system includes gas distributor and collector gas permeable pipes imbedded in the lock vessel. Different methods of operation of the feed system are disclosed to minimize feed problems associated with bridging and packing of the pulverized coal. The method of maintaining the feed system and feeder device exit pressures using gas addition or extraction with the pressure control device is also described.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 61/288,534, filed Dec. 21, 2009, entitled “A HIGH PRESSURE FEEDER AND METHOD OF OPERATION TO FEED GRANULAR OR FINE MATERIALS,” as well as U.S. patent application Ser. No. 12/970,006, filed Dec. 16, 2010, entitled “A HIGH PRESSURE FEEDER AND METHOD OF OPERATION TO FEED GRANULAR OR FINE MATERIALS,” both of which is hereby incorporated 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 disclosure relates to a device and the method of operation to feed a mixed size of granular or fine materials to a high pressure vessel. 
       BACKGROUND 
       [0004]    One difficulty in feeding granular or fine coal with moderate amounts of moisture into a high pressure gasifier includes devising a reliable way to increase the pressure of the coal stream from atmospheric pressure to the operating pressure while accurately metering the coal feed rate. Essentially, three methods have been practiced: slurry coal feed systems, dry powder feed systems with a lock vessel and rotating metering devices, and coal pumps. 
         [0005]    For the slurry coal feeder, the slurry includes 32-40 wt % water added to the powder coal to make the coal slurry viscosity low enough for pumping into a high pressure gasifier. A fundamental issue with this approach is that the cost of drying the low rank coals to a nearly moisture-free condition is very high because the drying process can be quite lengthy. For example, U.S. Pat. No. 6,162,265 discusses drying and preparing a slurry with low rank coals. The process is quite complicated and increases the capital, operating and maintenance costs to a level where the gasification facility becomes economically unattractive. It is relatively easy to remove the surface moisture from the coal; however, the drying facility required to remove nearly all the moisture including inherent moisture from coal is quite large. After removing nearly all the moisture from the coal, water is added again to make the slurry. In addition to the high cost of drying the coal, the thermal efficiency of the gasification process will be low because the water added to slurry the nearly dry coal needs to be evaporated in the gasifier. Thus, in the combined process of coal drying and slurry feeding, the moisture in and with the coal undergoes evaporation twice. The energy required to evaporate the water in the gasifier increases the oxygen consumption as well as the associated capital and operating costs. If the coal is not dried before slurrying, then the amount of water that needs to be evaporated in the gasifier nearly doubles as the low rank coals contain significantly high levels of moisture, which can be in the range 30 to 45 percent by weight of the coal. The double duty moisture evaporation in the gasifier significantly lowers process efficiency and useful syngas yield and increases operational costs with high oxygen consumption and higher cooling duties to condense moisture from the syngas. Therefore, using the slurry method to feed low rank coals to a high pressure gasifier is economically unfeasible. This may be the fundamental reason why no known commercial practice exists. 
         [0006]    Using a lock vessel, as has been operated in the prior art, to feed the granular coal and metering the coal with a rotating device such as an auger or a screw feeder encounters problems too. The lock vessel is a means to increase the pressure as the coal moves from the atmospheric pressure in the storage bin to the operating pressure in the feed vessels. The lock vessel method to increase the pressure is based on swinging the pressure between the atmospheric and operating pressures cyclically. The lock vessel has two valves: one connecting the atmospheric pressure storage bin from which coal is fed to the lock vessel and the other connecting to a coal feed vessel which receives coal from the lock vessel and is maintained at the operating pressure of the gasifier. In cyclic operation, when the lock vessel is ready to receive coal from the atmospheric vessel, the inlet valve opens. Once the coal reaches a predetermined level, the inlet valve closes. Then the lock vessel is pressurized using nitrogen or any other relatively inert gas such as CO 2 . When the lock vessel pressure equals to the feed vessel pressure, the outlet valve opens. The coal in the lock vessel is supposed to flow into the feed vessel under gravity. And most times, it does. However, when the coal moisture increases to above 5%, finely ground coal particles tend to pack in the lock vessel when the lock vessel is pressurized. The packed coal in the lock vessel will not dislodge from the lock vessel into the pressurized feed vessel. The feed vessel will gradually empty out, eventually interrupting the coal feed to the gasifier. The loss of coal feed affects the gasifier operation because it has to be shut down and restarted, thereby increasing operational costs and causing a loss in production. 
         [0007]    The metering devices are generally rotating devices that require an appropriate seal to prevent the coal fines from leaking to the atmosphere. It is difficult to develop a long lasting reliable seal mechanically for the high pressure differences that exist between the feeder and the atmosphere because of the presence of coal dust and constant rotation of the device. Further, the accuracy of the metering becomes difficult because of the changes in coal bulk density. Erosion of the rotating device and housing by the coal particles is a significant issue for certain types of coal feeders. 
         [0008]    Using a coal pump to directly feed coal to a high pressure gasifier has been in development for many years; but, the device has not been developed sufficiently enough for commercial applications. The fundamental issue is the wide variance in the properties of the coal powder or granular material. As the pressure seals are designed for certain narrow range of coal particles, it is difficult to seal at high pressures all the time for naturally varying feed characteristics of ground coal. The ground coal properties vary due to variations in raw coal, variations in crushing and milling operations and segregation of coal particles during storage and conveying. Any interruption of feed or blowback of hot ignitable process gases and gasifier bed materials can be highly unsafe for the coal pump system. 
       SUMMARY 
       [0009]    The present disclosure describes an apparatus and methods of operation to feed granular or fine low rank coals with high moisture content, which can be as high as 25%, to a gasifier with operating pressure in the range of about 5 to 1000 psig. 
         [0010]    The present disclosure relates to a solids feed system that provides an apparatus and method of operation to feed prepared coal or carbonaceous materials to a high pressure vessel. The preferred coal moisture is in the range of about 1-25%. The coal particle size can be in the range of about 0-6 mm. It should be appreciated that there are various ways to define irregularly shaped particles. Generally, some coal particles can have a sphericity around 0.7—sphericity refers to diameter of an equivalent spherical volume. In the context of this disclosure, a 6 mm particle, for example, refers to particles that can pass through a 6 mm sieve screen. Normally, it is understood that this is the case (passing through a sieve screen) unless stated otherwise. Even though the feed system can handle many different solids, the system is described using coal as the solid particles to feed as coal is one of the most difficult materials to handle because of its heterogeneity and differing characteristics. 
         [0011]    The processed coal is stored in a coal storage bin and is fed to the lock vessel whenever a cycle demand signal is received from the control system. The present disclosure provides two methods to ensure that the solids particles dislodge and fall out of the lock vessel and into the feed vessel as desired by the operational sequence. 
         [0012]    The first method, according to the present disclosure, is termed as a micro-fluidization method. During operation of the lock vessel, the lock vessel may be pressurized directly to the feed vessel pressure at a predetermined pressurization profile and rate before the lock vessel exit valve opens to the feed vessel. If the solid particles pack in the lock vessel, the operation fails as the feed vessel does not receive additional solids and the solids feeding process is interrupted. In one embodiment of the present disclosure, if the system operating pressure is greater than 50 psia, the lock vessel is pressurized to a pressure 3-10% higher than the system operating pressure. The pressure is then reduced to the feed vessel operating pressure by venting the gas out of the lock vessel to the coal surge bin. The gas venting rate or the orifice in the venting line is designed to ensure the gas superficial velocity in the lock vessel is about twice the minimum fluidization velocity of the mean particles in the lock vessel. 
         [0013]    To ensure that substantially all the materials in the lock vessel are fluidized during the micro-fluidization process, gas venting elements are installed in large lock vessels. The structure of these venting elements includes steel pipes with many holes drilled through the pipe wall and all the holes are covered with gas permeable material that is impermeable to solids. One of these materials is sintered metals. Other materials and arrangements are equally feasible. 
         [0014]    Further, to aid the fluidization process, additional gas can be added to the lock vessel through a gas distribution membrane or through gas addition pipes inserted into the lock vessel. The added gas will travel to the vent pipes and in the process to fluidize the bed materials. 
         [0015]    For solid particles that do not have the strong tendency to pack throughout the lock vessel, the particles may still tend to pack mainly at the bottom of the lock vessel near the solids exit. The particle packing can be broken down by over-pressurizing or under-pressurizing the lock vessel to about 2-15 pounds per square inch (psi) higher or lower than the operating pressure of the feed vessel. When the valve between the lock and feed vessels is opened, the pressure difference will cause rapid gas exchanges between the lock and feed vessels. As a result, the bridge or particle packing near the lock vessel bottom will be broken down. 
         [0016]    Another objective of the present disclosure is to have a pressure control device in the downcomer below the feed vessel to maintain a constant pressure difference between the gasifier or reactor vessel and the solids exit at the feeder. The device allows the system to rapidly inject or extract gas out of the feed system, maintaining a desired constant feed system pressure. 
         [0017]    The feed system has a feed device downstream of the pressure control device. The feed device consists of a downcomer, a short riser and aeration nozzles. The solid particles in the lower part of the riser flow as a moving, expanded bed. The control gas from the feed vessel and feed device flowing into the downcomer and through the riser keeps the bed expanded. One of the objectives of this disclosure is to control the solid particles feed rate by controlling the bed moving speed in the riser with the total control gas that flows through the feed device. 
         [0018]    Another objective of the disclosure is to reduce or prevent transport gas from contacting solid particles in the feed vessel and the downcomer. The transport gas is added to the top of the riser and conveying line to ensure sufficient conveying velocity is maintained. This facilitates the use of small amount of inert control gas in the feed vessel with large amount of reactive transport gas in the conveying line, preventing any reactions between the transport gas and solid particles in the feed vessel. 
         [0019]    Another objective of the current disclosure is solids feed rate control through a combination of both control gas to the feed system and feed system pressure. For a given characteristic of the solids, the control of the feed rate is based on the gas flow rate to the discharge section of the feeder and the gas flow rate to the feed vessel. For the constant feed rate operation, the gas feed to the feed vessel is slightly higher than the solids volumetric flow rate out of the feed vessel. If the characteristics of the solids changes during operation, mass flow rate of coal or solids to the gasifier or reactor can be maintained by maintaining a constant feed system pressure. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]    Aspects of the present disclosure can be better understood by illustrations and will be described by reference to the following drawings. 
           [0021]      FIG. 1  is a schematic of a feeder assembly including a feed storage bin, a lock vessel, a feed vessel, a pressure control device and a feeder device in accordance with various embodiments of the present disclosure. 
           [0022]      FIG. 2  is a schematic of the pressure control device of  FIG. 1  in accordance with various embodiments of the present disclosure. 
           [0023]      FIG. 3  is a schematic of the lock vessel of  FIG. 1  with internal gas distribution and venting arrangements in accordance with various embodiments of the present disclosure. 
           [0024]      FIG. 4  is a schematic of a lock vessel gas injection and release device in accordance with various embodiments of the present disclosure. 
           [0025]      FIG. 5  is a schematic of the feeder device of  FIG. 1  in accordance with various embodiments of the present disclosure. 
           [0026]      FIG. 6  is an experimental trend of lock and feed vessel pressures illustrating micro-fluidization. 
           [0027]      FIG. 7  is an experimental trend of coal feed rate versus total Control nitrogen flow rate through the PDAC feeder device. 
       
    
    
     DETAILED DESCRIPTION 
       [0028]    As shown in  FIG. 1 , a pressure decoupled advanced coal feeder (PDAC)  100  according to one embodiment of the disclosure, comprises an assembly of vessels and feed and pressure control devices. It includes a coal surge bin  102 , configured to temporarily store processed coal in transit from a coal preparation system to a gasifier or reactor  150 . The coal surge bin  102  also accepts vent gas from a lock vessel  108 . A plurality of filter bags  101  are installed in the surge bin  102  to filter out coal dust. Also, a screen  104  with an opening is installed to prevent failed bags that fall from interrupting the coal feeding process. In some embodiments, the openings of the screen  104  can be sized in the range of about two by about two inches. Also included are a feed vessel  114 , a pressure control device  117 , a coal feed rate control device  120  and a coal density measurement instrument  130  on the coal conveying line  132  that conveys coal to the gasifier or reactor  150 . 
         [0029]    At least one valve  106  is installed between the lock vessel  108  and the surge bin  102 . In many embodiments, two valves can be employed between the lock vessel  108  and surge bin  102 . The first valve below the surge bin  102  isolates the surge bin from the lock vessel  108  when the feeder system goes through an outage. The second valve can be located between the lock vessel  108  and the first valve below the surge bin  102  and acts as a pressure boundary between the lock vessel and the surge bin. The second valve closes in a relatively clean environment as it is timed to close at least a few seconds after the first valve closes. 
         [0030]    The valve  106  can open on demand and coal from the surge bin  102  fills the lock vessel  108  to a desired level. An initiation signal for adding coal to the lock vessel  108  can be a sequential action signal. The initiation signal can be based at least upon a coal level measurement in the coal feed vessel  114 . For illustration purposes, one can use a level probe  115  as a starting point for the cycle of coal transfer. When the level probe  115  has detected that the coal level has reached a preset level, a sequence starts with the opening of the lock vessel inlet valve  106  to fill the lock vessel  108  to a pre-determined level  107 . The level measurement for the lock vessel  108  can be any of the various level measurement techniques, such as gamma ray measurement, a conductance level probe, a capacitance level probe, a vibratory probe, ultrasonic level measurement, and other level measurement techniques. Once the level measurement has detected the solids level, the lock vessel solids inlet valve  106  is closed and the lock vessel  108  starts the pressurization process. In prior art processes, the lock vessel  108  is pressurized to a pressure equal to the feed vessel pressure and the valve  110  on the balance line  111  will open. At substantially the same time, the valve  112  at the outlet of the lock vessel  108  can open. The solids can flow out the lock vessel  108  by gravity into the feed vessel  114 . However, when the solids such as coal in the lock vessel  108  contain a substantial amount of particles less than, for example, 45 microns in size, as well as particles having a relatively high moisture content, the particles may tend to pack and cannot flow freely under gravity. Solid particles such as coal, when dried and pulverized, may not be uniform in size and moisture characteristics may change over a period of time as short as a day. When size and moisture characteristics are out of bounds even for short duration during the day, it can lead to loss of coal feed to pressurized units such as a gasifier or reactor  150 . By one of the following three methods, embodiments of the disclosure can maintain feed even if characteristics of feed particles vary. 
         [0031]    In one embodiment, the lock vessel  108  can be over-pressurized about 3-15 psi higher than the pressure in the feed vessel. In different lock vessel operational methods described herein, the pressurization sequence includes a slow pressurization rate (e.g., for up to about 60 seconds), followed by a fast pressurization rate until the pressure is within 5% of the desired final pressure, and finally a slow pressurization rate until the desired final pressure is reached. The timing of each pressurization step can change depending on the characteristics of the coal source. Packing of the particles may occur at the start of pressurization and the initial slow pressurization step is helpful to minimize the effects of packing. The final slow pressurization step can ensure that the desired pressure is reached with minimum overshoot. 
         [0032]    When the higher preset pressure of the lock vessel  108  has been reached, the valve  110  on balance line  111  can open first because the pressure is higher in the lock vessel  108  than in the feed vessel  114 , and gas will flow from the lock vessel  108  to the feed vessel  114 . The gas release from the lock vessel  108  may have a tendency to fluidize the solids in the lock vessel or break any minor bridging in the lock vessel  108 . After a short time delay (e.g., between about 500 milliseconds to about 1 second), the lock vessel exit valve  112  can be opened. As the lock vessel  108  is still slightly under positive pressure, the minor bridging of the solids in the lock vessel exit can break and fall under gravity as well as with a push from the positive pressure in the lock vessel  108 . By minor bridging, it is understood that the solids will normally flow and occasionally, (e.g., about every four to eight hours or about 30-60 cycles), there occurs a weak lock vessel bridging at the exit during which the solids fail to fall into the feed vessel  114 . Whenever such a bridging occurs, the above procedure will break the bridge and the feed process will continue. This operation method is termed as an over-pressurization mode. 
         [0033]    To maintain the pressure difference between the PDAC feeder device exit  123  and the reactor  150  substantially constant, the additional gas from over-pressurization that causes the higher pressure in the feed vessel  114  and coal feeder exit must be properly released. In one embodiment, in the present disclosure, the gas release is accomplished by a pressure control device. US Patent Publication No. 2010/0263342, which is incorporated herein by reference, describes a pressure control device (known therein as a pressure let-down device) and operating such a device in a different application where the device facilitates let-down of the solid streams pressure from an operating system pressure to substantially atmospheric pressure. 
         [0034]    In the embodiment, a pressure control device  117  is presented in  FIG. 2  for coal feeder application, where the pressure control device is reconfigured for use as a means to add gas through valve  119  to the solids stream  214 . The gas chamber  206  is isolated from the process solids stream  214  with a bed of inert granular solids  210  held in place with an inner gas permeable membrane  212  and an outer gas permeable membrane  208 . Gas can also be extracted through valve  118  to reduce solids stream  214  pressure. Unlike the device used in US2010/0263342, where the exit pressure of the solid stream is generally less than half of inlet pressure, in this embodiment the pressure difference across the pressure control device  117  is low, as the purpose is to control pressure by adding or venting gas in small amounts. With addition/extraction of gas from solids stream  214 , the pressure of the solids stream at the exit  220  of the pressure control device  117  can be maintained substantially constant. 
         [0035]    As the lock vessel  108  is operated in the over-pressurization mode, when the valve  110  in the pressure balance line  111  and the valve  112  are open, the gas together with the solids can travel out of the feed vessel  114  faster due to a higher pressure difference created due to over-pressurization. If the gas and solids are allowed to flow from the feed vessel  114  to the reactor  150 , the pressure difference between the PDAC feeder device exit  123  and the reactor  150  may increase and as a result, the solids feed rate will also increase. As shown below, control of the pressure difference is critical to control the flow rate of the solids from the PDAC feeder device exit  123  to the reactor  150 . To avoid such a pressure increase, the excess gas due to over-pressurization of the lock vessel  108  can be released by the pressure control device  117  by opening the gas release valve  118 . 
         [0036]    The pressure control device  117  as shown in  FIG. 2  can be located in the downcomer, downstream of the feed vessel  114 , and receives gas and solids flow stream  214  from the lock vessel  108 . The connecting pipe  203  and the pressure control device  117  are in communication with the feeder vessel  114  at a first end and the PDAC feeder device  120  at an opposing end. When a pressure measurement device  127  detects a pressure that is higher than the preset value, the control valve  118  will open to rapidly reduce, if necessary, the feed vessel  114  pressure to the preset value. The gas and solids stream at the exit  220  of the pressure control device  117  is nearly at constant pressure. Note that a brief high coal feed rate spike is nearly harmless for the coal gasifier&#39;s performance and operation. Therefore, for the over-pressurization mode, the gas releases to reduce the feed vessel pressure need not be drastic and can be easily controlled. Operating the lock vessel  108  and the system under the over-pressurization mode can be advantageous because the logic of the system reduces any possibility of the gas reverse flow from the reactor  150  to the feeder which, if it occurs, is a safety concern. Under the over-pressurization mode, the worst scenario is a spike in the coal feed rate due to any potential inaccuracy in pressure control. Normally, the pressure control can function reliably and precisely and the coal feed rate will not show any spikes. 
         [0037]    A second operation mode for the lock vessel is termed under-pressurization mode. This mode can function when the packing that occurs in the lock vessel is due to the fines fraction in the coal that is higher than normal. Under this operating mode, the lock vessel  108  is pressurized to a slightly lower pressure than in the feed vessel  114 . Depending on the characteristics of the coal, the pressure differential can be in the range 3 to 15 psi. When the pressure in the lock vessel  108  reaches the preset pressure, which can be about 3 to 15 psi below the feed vessel pressure, the valve  112  can be configured to open. The higher pressure in the feed vessel  114  will force the gas from the feed vessel to flow upward into the lock vessel  108 . After valve  112  opens, the valve  110  in the balance line can also be configured to open after a short delay. As the coal starts to fall from lock to feed vessel, the space occupied by coal should be filled by a gas. As the coal falls into the feed vessel, an equivalent amount of gas is displaced. The gas displaced from the feed vessel can go through the balance line to lock vessel to occupy the volume of coal that moved to feed vessel. 
         [0038]    In the under-pressurization mode, even a small pressure difference can cause the gas from the feed vessel  114  to rush to the lock vessel  108  at high velocities. This exerts sufficient force from bottom and breaks any bridge that typically forms at the lock vessel exit. The rushing velocity can be as high as 120 ft/s even at a low 2 psi pressure difference at 250 psig operating pressure. Also, a part of the additional gas injected will also rush into the lock vessel  108 , fluidizing the materials in the lock vessel  108  and minimizing the effect of packing. 
         [0039]    The under-pressurization can cause a substantially instantaneous pressure reduction in the feeder system which, in turn, can cause a feed rate reduction if no further action is taken. But, a reverse flow situation is unlikely because the normal operating feeder pressure is designed to be at least 15 psi higher than the reactor pressure. In practice, the pressure differential during normal operation can be 50 psi. However, feed rate reduction from the feed system in supplying coal to the gasifier is undesirable. To avoid the feed rate reduction, gas is injected into the feeder system through the feed line  122  to upper (stream  126 ) and lower (stream  129 ) portions of the feed vessel  114 , the pressure control device  117  using valve  119  and the PDAC feed device  120  with stream  124 . The distribution of the gas between these streams is dependent on the characteristics of feed material and system design. It is understood that the number of nozzles in the feed vessel  114  and PDAC feed device  120  can be multiple. The amount of additional gas necessary for a given lock vessel size and the extent of under-pressurization can be calculated quite precisely. For example, if the lock vessel volume is 500 ft 3  with 50% of the volume occupied by the gas phase and the lock vessel  108  is 15 psi under pressurized, then the total amount of additional gas necessary is 17.5 lbs. With a known amount of additional gas needed and the desired time interval to restore the pressure in the feed vessel, people skilled in the art can size a buffer tank or select appropriate line sizes and control valves to inject the gas. To minimize the time lag and considering the actuation time for the valves, a feed forward control system can be used and a signal issued to the gas injection valves to ensure the pressure restoration time is less than about 500 milliseconds. 
         [0040]    Another embodiment of the disclosure is termed micro-fluidization. The lock vessel  108  can be pressurized about 3-15% higher than the pressure in the feed vessel  114 . The high pressure gas is then released from the lock vessel  108  through valve  134  in vent line  128  until the pressure in the lock vessel  108  is essentially the same as the feed vessel  114 . If gas is released uniformly throughout the solids bed in the lock vessel  108  and the gas superficial velocity in the lock vessel  108  is substantially higher than the minimum fluidization velocity of the particles, then the solids bed in the lock vessel  108  will be fluidized. This minimizes the effects of particle packing in the lock vessel  108  and the solids will free fall into the feed vessel  114  when the valve  112  shown in  FIG. 1  opens. 
         [0041]    For most solids, the particle packing effects in the lock vessel  108  are reduced during later stages of the fast pressurization step. A variation of micro-fluidization can be effected by venting gas in the time range of about 1 to 3 seconds during the fast pressurization step and then continuing to pressurize to feed vessel pressure before opening the balance line valve  110  and the valve  112  between the lock vessel  108  and feed vessel  114 . This variation is helpful as much of the particle packing occurring during the initial pressurization stage can be dislodged and solids aerated. Solids have fewer tendencies to pack with continued pressurization after the micro-fluidization step. 
         [0042]    One aspect of micro-fluidization includes releasing the high pressure gas from the lock vessel  108  as uniformly as possible. The method can be best illustrated with the lock vessel  108  depicted in  FIG. 3 . To release gas substantially uniformly throughout the lock vessel  108 , gas release elements can be installed inside the lock vessel  108 . The gas release elements  310  and  320  are similar in structure and are further illustrated in  FIG. 4 . The gas release elements  310  and  320  comprise a ring header  410  for collection as well as for distribution of the pressurizing gas as necessary. A plurality of gas permeable pipes  412  are mounted onto the ring header  410 ; the length of the pipes  412  can vary depending upon the shape of the lock vessel  108 . 
         [0043]      FIG. 4  illustrates the structure of the gas permeable pipe  412 . The inner part  452  is a pipe with a plurality of holes drilled through its wall. The size of each hole can vary, but in one embodiment, they can be sized from about ¼inch to about ½ inch. The number of holes depends at least upon the pipe size and the amount of gas to be released. One guideline is that the total cross-sectional area of the holes can be at least 10 times the cross-sectional area of the pipe  412  based on the pipe diameter so that the holes will not provide any substantial resistance to the gas flow. The outer member  454  of the gas permeable pipe  412  can be made of any gas permeable material. However, as discussed below, the pipe  412  can be used for gas to flow both ways. Therefore, in one embodiment, the pipe  412  can be rigid. The most common material that fits this application is sintered metals which are available in various pore sizes. The sintered metal can restrict particles from entering the inner pipe and erode the venting valves. 
         [0044]    When the lock vessel  108  operates in a micro-fluidization mode, the gas release pipe  412  can also be used during pressurization to distribute the pressurizing gas  302  for materials that have low permeabilities. When these internals are used for the gas distribution, they can distribute the gas quicker and more uniformly throughout the solids bed overcoming flow restrictions due to low permeabilities. The uniform distribution of the gas throughout the solids bed in the lock vessel  108  is a challenge for those solid particles with poor permeability. Some practitioners promote a slow pressurization technique so that the gas can penetrate throughout the bed during the pressurization stage. However, often times for commercial operation, this technique becomes impractical as it requires long pressurization times. The internal arrangement embodiments of the lock vessel  108  presented in  FIGS. 3-5  render a much shorter gas flow path in the bed compared to the use of a conical membrane and/or nozzles on a side of the lock vessel  108  as the pressurization means. During the pressurization stage, a hyperbolic membrane  330  located in the cone section of the lock vessel  108  (as illustrated in  FIG. 3 ) can also be used to add gas  302 . 
         [0045]    In one embodiment, one guideline for micro-fluidization is that the pressure in the lock vessel  108  should be about 3-15% higher than the feed vessel pressure depending on the operating pressure, the size of the lock vessel  108  and characteristics of the feed material. The percent of over-pressurization in the lock vessel  108  is lower with the increased operating pressure and with the increased size of the lock vessel  108 . After the lock vessel  108  has reached the preset over-pressure for micro-fluidization, the pressure release rate through the internal gas release elements  310  and  320  as well as the normal venting line  128  can be such that the gas velocity in the lock vessel  108  is larger than two times the minimum fluidization velocity to facilitate fluidization of substantially all materials in the lock vessel  108 , reducing the effect of the particle packing. In one non-limiting example, if the feed vessel is pressurized at about 500 psi and a 10% over pressure is chosen before the vent valves are open, then the ‘preset’ overpressure is 550 (500+10% of 500) psi. 
         [0046]    To ensure the coal in the lock vessel  108  is fluidized during the gas release stage for materials prone to packing, gas stream  302  (as indicated by  FIG. 3 ) can be injected for a few seconds through the membrane  330  in the cone section of the lock vessel  108 . Since the cone and exit sections of the lock vessel  108  are most likely to be packed, the gas injection during the gas release can aid the solids to be fluidized in these sections and thus reduce the effects of particle packing and bridging that may occur during pressurization. After the over-pressurized gas is released and the lock vessel pressure equals the feed vessel pressure, the solids can freely fall into the feed vessel  114 . The gas release operation during micro-fluidization can, in one embodiment, last between about 1 to about 5 seconds depending upon the operating pressure. 
         [0047]    As has been mentioned above, the pressure control device  117  can be used to control the pressure difference between the feeder and the reactor  150  either by withdrawing the gas or by adding the gas to the solids stream  214  flowing through the device  117 . The second method used to maintain the pressure difference between the coal feeder and the gasifier or reactor  150  is the PDAC feeder device  120  as shown in  FIG. 5 . The control gas added through the feed vessel  114  and pressure control device  117  flows through the PDAC feeder device  120 . Additional control gas  124  is added as necessary to control the coal feed rate through the device  120 . 
         [0048]    The coal feed rate is controlled through a combination of both control gas to the feed system that passes through the PDAC feeder device  120  and feed system pressure. For a given characteristic of the solids, the control of the feed rate is based at least upon the gas flow rate to the discharge section of the feeder and the gas flow rate to the feed vessel and the pressure control device  117 . For the constant feed rate operation, the gas feed to the feed vessel  114  is slightly higher than the solids volumetric flow rate out of the feed vessel  114 . A part of the gas entering the feed vessel  114  occupies the volume displaced as solids are continuously fed out of the vessel  114 . If the characteristics of the solids changes during operation due to changes in grind or segregation in surge bin  102 , the mass flow rate of coal or solids to the gasifier or reactor  150  can be maintained by maintaining a substantially constant feed system pressure. 
         [0049]    As shown in  FIG. 5 , the coal from the pressure control device  117  moves down a short inclined downcomer section of the pipe  140 . The coal then moves through a short vertical riser  141  before being discharged to the conveying line through feeder exit  123 . A part of the conveying gas  125  enters the top of the riser through pipe  142  and exits into the riser  141  through an inverted cone  144  coupled to the end of pipe  142 . The conveying gas as it leaves the inverted cone  144  picks-up coal particles from the top of the moving expanded bed to convey the coal to the downstream unit. Both the control gas and conveying gas can flow upward through section  143 . Depending upon the characteristics of feed material and downstream process, the superficial gas velocity in section  143  of the riser  141  can be maintained between about 3 and 10 ft/s by adjusting the conveying gas flow rate. Additional conveying gas is added downstream of the feeder device exit  123  to maintain a superficial gas velocity in the line to convey coal to the gasifier or reactor  150 . Even though one configuration of feed rate control device  117  is shown in  FIG. 5 , a person skilled in the art of handling solids should appreciate other configurations involving changes to the way conveying gas is introduced into the riser and the way the coal is conveyed out of the riser that are consistent with this disclosure. 
         [0050]    The size of the inclined leg in downcomer section  140  and riser  141  are chosen to achieve turndown and control over coal feed rate. The inclined leg is aerated by a small portion of the control gas and the coal flows through the inclined leg essentially by gravity. The diameter of the inclined leg is chosen to provide more than the maximum coal feed rate that the device is designed for in order to ensure that the feed to the riser bottom is not limited at the maximum coal feed rate through the riser. Much of the control gas added to various locations in the feed system can enter the riser  141 . The characteristics of gas and solids in the riser  141  can be described as a moving dense expanded bed. The coal feed rate is proportional to the superficial gas velocity or the volumetric gas flow rate due to the control gas that eventually flows through the riser  141 . The superficial gas velocity in this portion of the riser can be between about 0 and about 1 ft/s. The control gas can range between about 10 to 15% of the total conveying gas. 
         [0051]    The conveying gas  125  may be different from the control gas  124  added to various locations in the PDAC feed system. Due to reactivity concerns, there may be situations where it is not desirable to have conveying gas enter the feed vessel  114 , which can contain bulk amounts of the feed material. In air blown gasification, the conveying gas is air and the control gas is typically an inert gas such as nitrogen or carbon dioxide. The PDAC feed device embodiment shown in  FIG. 5  effectively prevents air from entering the feed vessel  114  and reacting with coal. 
         [0052]    The apparatus and methods presented increases the feed system&#39;s reliability to maintain coal feed rate to the gasifier even with difficult to handle coals as well as with varying coal characteristics and preparation methods. The system&#39;s reliability is further increased as the feed device embodiment in  FIG. 5  has no moving parts, which are norm in other types of coal feeders. Rotary table feeders, auger feeders, piston feeders and rotating dry pump feeders are examples of other types of coal feeders that have moving parts that can be prone to wear and tear and other maintenance problems such as motors, shaft seal leakage, maintaining small clearances (e.g., 5 mil) between stationary and rotating parts, blow through and erosion issues. 
       Example 
       [0053]    Described below is one non-limiting example of an embodiment of the disclosure. None of the descriptions, ranges, or other information in this example should be considered to limit the scope of the present disclosure. In a fast circulating, pressurized fluidized bed gasification pilot test facility, an auger and rotary table feeder systems were tested and poor availability of these systems and other inherent scalability issues led to the development of the PDAC feeder system. The PDAC feeder system can be directly lined-up to the gasifier for on-line gasification operation or can be connected to a closed-circuit Off-line Coal Feed Test System. The Off-line system, which included a large receiver pressure vessel simulating the gasifier and a pressure let-down system, allowed us to operate substantially continuously in closed-loop and test various concepts rigorously. When on-line to the gasifier, the feeder system can feed from about 300 to 6000 lbs per hour of coal at about 100 to 280 psig pressure; while in off-line test loop, the feeder system can feed similar rates up to about 480 psig pressure. For test purposes, the mass mean diameter of the coal was varied from about 200 to 800 microns. Low rank coals were predominantly tested and the moisture content of the feed coal varied from about 18 to 25 wt %. 
         [0054]    During pressurization, tendency for coal particles to pack causes coal bridging in the lock vessel, which may negatively affect feed system performance and availability. In pilot test facility, originally, the traditional pressurization mode (equal-pressurization) was used for operation and that led to a number of outages. Lock vessel at the pilot test facility has been designed and operated in various modes described earlier: slow pressurization, under-pressurization, over-pressurization and micro-fluidization. Variety of operation modes were designed either to minimize bridging and packing potential or to break the bridging and packing once detected without losing feed to the gasifier. 
         [0055]    As the lock vessel at the pilot test facility was small, no internals as described earlier was necessary and the micro-fluidization was practiced during pressurization before the pressure reached the feed vessel pressure ( FIG. 6 ). Also, for test purposes, in the over-pressurization mode, the valve between the lock and feed vessels was opened first and after a short time delay (1 to 5 seconds), the valve on the balance line was opened. As the internal surface of the lock vessel had a smooth finish, reducing the wall friction, the intentional pressure difference for a short duration after opening the valve between the lock and feed vessel was helpful at times in assisting the solids to move to the feed vessel. A form of internal pipes with gas manifold discussed in reference to  FIG. 4  was tested. These pipes were located near the wall of the vessel and the ½ inch holes on one side of the pipe with sintered metal covering were facing the wall. After opening the valve between the lock and feed vessels, nitrogen was pulsed through the manifold and pipes to dislodge moderate packing on the side of the vessel. After experimenting with various means invented, the lock vessel operated trouble-free with a number of different coals, coal characteristics, grind sizes and moisture content. 
         [0056]    At the test facility, the pressure control device is coupled with the PDAC feeder device to control and regulate the coal feed rate. During operation transitions such as start-up, ramp-up, ramp-down and gasifier trips, the pressure control device aeration or vent were used to quickly stabilize the PDAC system and the feed rate to new conditions. In normal operation, control (gas) nitrogen flow was used to control the coal feed rate. As shown in  FIG. 7 , the PDAC feed system was designed to maintain proportionality between coal feed rate and control nitrogen flow rate in the desired operations range. The test facility also had a Trim control nitrogen flow to feeder device to slightly vary coal feed rate to address fluctuations inherent in the gasifier. Both the pressure control device and the PDAC feeder device were maintenance-free and 100% available in over 3,500 hours of testing.