Abstract:
Ceramic cowlings that are used as the connection between a hot gas source and a Stirling engine or a turbine. The ceramic cowling is designed and fabricated with non-dusting, high temperature, dense, low thermal expansion ceramic. It must also be highly resistant to thermal shock. Also, a combination of a ceramic cowling and a shroud for covering and holding the ceramic cowling on a Stirling engine or turbine such that hot gases can flow through the ceramic cowling and into the heat exchanger coil of the Stirling engine and exhaust in a controllable manner. A method of enhancing the power efficiency of a Stirling engine and with systems including the use of at least one enhanced power Stirling engine.

Description:
[0001]    This application claims priority from U.S. Provisional Application No. 60/858,973 filed Nov. 14, 2006 and U.S. Provisional Application No. 60/906,796 filed Mar. 13, 2007. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to a ceramic cowling for connecting a hot gas source to a Stirling engine or a turbine, the use of the cowling while housed in an insulated shroud, and systems that use the ceramic cowling/shroud combination to provide hot gas to a Stirling engine or a turbine to produce electrical power, but also to the provision of alternative sources of energy, such as steam or hot air. 
       BACKGROUND OF THE INVENTION 
       [0003]    This invention deals with a ceramic cowling that is used as the connection between a hot gas source and a Stirling engine or a turbine. The ceramic cowling is designed and fabricated with non-dusting, high temperature, dense, low thermal expansion ceramic. It must also be highly resistant to thermal shock. This invention also deals with a combination of a ceramic cowling just described Supra, and a shroud for covering and holding the ceramic cowling on a Stirling engine or a turbine such that the hot gases can flow through the ceramic cowling and into the heat exchanger coil of the Stirling engine or the turbine and exhaust in a controllable manner. This invention further deals with a method of enhancing the power efficiency of a Stirling engine or a turbine and with systems including the use of an enhanced power, Stirling engine or turbine. 
         [0004]    Stirling engines have been known and used for at least a decade. These engines work by supplying to them a fixed quantity of gaseous working medium that is contained and enclosed within each cylinder of the engine. A portion of the engine is maintained at a constant high temperature by burning any of a wide variety of fuels in the combustor and transferring heat to the gas via heater tubes. The other portion of the engine is maintained at a constant low temperature by circulating the gas through coolers. The working gas is transferred back and forth between the hot and cold portions of the engine and alternately expanded and compressed by the movement of the engine&#39;s pistons. The reciprocating motion of the pistons is converted to rotary motion via a swash plate drive which powers the generator. In each cylinder, gas passing through the heater tubes absorbs heat from the combustion and expands, pushing the piston down and thereby doing work in the swash plate. As the piston comes back up, it forces the gas out of the cylinder and through a regenerator, which absorbs heat from the gas passing through it, and stores it temporarily. The gas then passes through the tubes of a cooler and rejects heat to the coolant passing through it. The cooled gas then enters the compression space below the adjacent piston, and as this piston comes down, it is compressed and pushed back up through the regenerator, where it picks up the heat previously stored there, passes through the heater tubes, and the cycle begins again. An example of one such Stirling engine can be fund in U.S. Pat. No. 5,074,114 that issued on Dec. 24, 1991 to Meijer, et al Likewise, the use of turbines to produce energy from syngas has been known for a long time. These engines are mounted to a combustor of some sort that is capable of burning a wide variety of conventional fuels such as natural gas, hydrogen, and propane gas, and to less conventionally used fuels, if they are cleaned, such as scrap wood, forest products and waste, corn and other biomasses to supply the heated gas. 
         [0005]    These engines also work well with resource recovery fuels such as flare gas and coal bed methane gas and renewable biogas fuels from landfills or anaerobic digesters such as from sewage or agricultural waste. This results in the conversion of a wide variety of fuels into valuable electrical power and hot water for commercial, industrial and residential applications. 
         [0006]    The combustion gas is brought into a cowling or combustion chamber and there is some cooling effect that protects the metal enclosure. A certain amount of heat has to be transferred with a floor of about 1500 dF exit gas temperature. For example, a 55 kw Stirling engine has to transfer 550,000 Btu. At 1650 dF inlet temperature, one would need 6,600 pounds of mass. When one raises the inlet temperature to 2000 dF, one needs only about 4,200 pounds of mass. The pressure drop across the internal heat exchanger coil is 13.2 inches w.c. at 1650 dF, and only 4 inches w.c. at 2000 dF. The higher this mass and pressure drop, the more expensive the capital equipment, that is larger ducts, bigger fans, and increased operating costs, primarily for the fan horsepower to move the mass in and out of the engine. 
         [0007]    The limiting factor for temperatures being achieved above 1650 dF, that is, combustion inside the cowling, has been the metal cowling. If one cools the metal with water or air to preserve the cowling, one can drain away essential heat needed to produce power. Much experimentation has been done in bringing clean process flue gas, ranging in temperatures from 1600 dF to 2000 dF, directly into the engine. It was discovered that without exception, even exotic metals failed when the inlet temperature was above about 1600 dF. It should also be noted that one still needs the 1500 dF exit temperature, so that the thermal head is small when the inlet temperature is 1600 dF, and the amount of mass needed to carry 550,000 Btu becomes inordinately high, so one starts reducing power production as inlet air temperature decreases. 
         [0008]    A review of Table I will quickly make clear to those skilled in the art why it is a major advantage to be able to increase the temperature to a Stirling engine. At about 1652° F., it is noted, that there is a sharp drop off in engine pressure dropping about three inches w.c. The drop in engine pressure is even more significant as the temperature increases. 
         [0000]    
       
         
               
               
               
               
             
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                 TABLE I 
               
             
             
               
                   
               
               
                   
                   
                   
                 ENGINE 
               
               
                 INLET AIR 
                 AIR MASS 
                 OUTLET AIR 
                 PRESSURE 
               
               
                 TEMPERATURE 
                 FLOW 
                 TEMPERATURE 
                 DROP 
               
             
          
           
               
                 ° C. 
                 ° F. 
                 G/Sec. 
                 Lb/Hr 
                 ° C. 
                 ° F. 
                 kPa 
                 in. wc 
               
               
                   
               
             
          
           
               
                 800 1   
                 1472 
                 1000 
                 7937 
                 677 
                 1251 
                 3.11 
                 12.5 
               
               
                 825 2   
                 1517 
                 1000 
                 7937 
                 699 
                 1290 
                 3.18 
                 12.79 
               
               
                 850 3   
                 1562 
                 1000 
                 7937 
                 722 
                 1332 
                 3.25 
                 13.07 
               
               
                 875 4   
                 1607 
                 1000 
                 7937 
                 744 
                 1371 
                 3.32 
                 13.35 
               
               
                 900 5   
                 1652 
                 1000 
                 7937 
                 767 
                 1413 
                 3.39 
                 13.63 
               
               
                 925 5   
                 1697 
                 855 
                 6786 
                 770 
                 1418 
                 2.55 
                 10.25 
               
               
                 950 5   
                 1742 
                 770 
                 6111 
                 778 
                 1432 
                 2.12 
                 8.52 
               
               
                 975 5   
                 1787 
                 705 
                 5595 
                 787 
                 1449 
                 1.82 
                 7.32 
               
               
                 1000 5   
                 1832 
                 633 
                 5024 
                 791 
                 1456 
                 1.50 
                 6.03 
               
               
                 1025 5   
                 1877 
                 575 
                 4566 
                 794 
                 1462 
                 1.26 
                 5.07* 
               
               
                 1050 5   
                 1922 
                 533 
                 4229 
                 803 
                 1478 
                 1.10 
                 4.42* 
               
               
                 1075 5   
                 1967 
                 488 
                 3872 
                 806 
                 1483 
                 0.94 
                 3.78* 
               
               
                 1100 5   
                 2012 
                 451 
                 3576 
                 810 
                 1490 
                 0.80 
                 3.22* 
               
               
                   
               
               
                 *Calculated 
               
               
                   1 = 46 kw 
               
               
                   2 = 48 kw 
               
               
                   3 = 51 kw 
               
               
                   4 = 53 kw 
               
               
                   5 = 55 kw 
               
             
          
         
       
     
         [0009]    The only materials available that can drive an engine with high temperatures are ceramics. Thus, any type of industrial process that generates a high temperature waste flue gas, that is relatively clean and containing few or no particulates and low acid content, can be sent directly to the engine. If the process generates a medium temperature flue gas, say about 1200 dF, the flue gas can be supplemented with natural gas to raise the flue gas to 2000 dF. 
         [0010]    It should be noted that it is at this point that the instant invention differs markedly from the prior art systems. One of the biggest disadvantages of direct-firing a Stirling engine or a turbine with a waste gas flow is that every combustion process, no matter how clean, such as natural gas, has some particulate and some acid. Many, in fact most industrial processes, will have some contaminates. When one adds air for combustion and tempering purposes to a waste combustion gas, one has a flue gas that is high in energy but has no other purpose than to transfer energy. It has to be exhausted after the energy is removed as a contaminated combustion product. This is why Stirling engines and turbines are most popular when one can use both power and, downstream of the engine, a heat recovery device, such as a boiler or hot water heater, for co-generation. The products exiting from the direct-fired Stirling or a turbine in current systems are dirty. 
         [0011]    Secondly, the chance for fouling and deterioration are magnified when one uses a process gas, and they are susceptible to upset. For example, if one uses a clean syngas from a wood-fired gasifier and there was a blip that set sent unburned carbon or ash particles directly to the engine, this could completely destroy the heat recovery coil in the engine. 
         [0012]    The ceramic cowling and the connecting ductwork to that cowling have to be designed and fabricated with non-dusting, high temperature, dense, low thermal expansion ceramic. It must be highly resistant to thermal shock. Tests have been done using ceramics that can take the temperatures, but they cracked within a matter of hours because they could not handle thermal shock and, in some cases, the ceramic dusted and literally sand blasted the internal of the engine. 
         [0013]    The ceramics used in the cowling of this invention are non-dusting, high temperature, dense, low thermal expansion ceramics. Ceramics that are capable of these properties are, for example, Metal Rock 70M from Allied Mineral Products, Inc. Columbus, Ohio, USA and Thermo-Sil® fused silica ceramics from Ceradyne, Inc. Scottdale, Ga., USA. Such materials have bulk densities from about 1.8 to about 2.12 g/cc, compressive strengths of about 27 to 240 MPa (ASTM C-133), linear shrinkage at 1100° C. of zero to about 0.4%, flexural strengths of about 6.9 to 58 MPa, thermal conductivity of abut 0.6 to about 0.8 W/m° C., coefficient of thermal expansion from about 0.5 to about 1.7 10 −6 /° C. and a volume percent apparent porosity of from about 7 to about 15 (ASTM C-20). 
         [0014]    In summary, one can now fire a Stirling engine or a turbine at higher than normal temperatures with clean, hot air into a ceramic cowling that will take those temperatures, at a reduced mass flow, and lower pressure drop, than can be obtained with even the best combustion process inside a metal cowling to provide enhanced efficiency of the Stirling engine or the turbine. 
         [0015]    Another long-term benefit is that an air-fired engine will definitely live longer than a flue gas-fired engine. The ability of the ceramic exchanger to handle corrosive, particulate-laden process gas opens up a plethora of markets, heretofore unavailable. For example, one can now fire coal tailings, poultry litter, and forest products. One can even use hazardous wastes. 
         [0016]    In the instant invention, in every case, the indirect-fired Stirling engine or the turbine exits clean, hot air at 1500 dF. This hot air can be returned to the combustion process into either the primary or secondary chamber and used as preheated combustion air. This substantially reduces the amount of fuel need to operate the system. For example, a direct-fired Stirling engine that generates 110 kw would need 1,100 pounds of waste wood per hour. An indirect-fired engine would require only 800 pounds of wood per hour. 
         [0017]    A co-generation plant can give one a productive side effect assuming the customer needs steam in the process. Assume the customer wants to fire a conventional boiler with waste wood. The higher the temperature, the more efficient the process, however, slagging at temperatures between 1800 dF and 2200 dF is a real problem. The optimum waste wood-fired boiler would have a flue inlet temperature of about 1600 dF. If one fires a ceramic heat exchanger, as in this invention, at 2200 dF, and drops the flue gas temperature to 1600 dF, and then takes the balance of the heat out with a boiler, one ends up with the best of both worlds. Slagging is no longer a problem, the boiler will have long life, and one can remove heat with the Stirling engine or a turbine at its optimum temperature levels. One can reduce the amount of fuel by providing 100% of the combustion air as preheated air, and the down stream boiler economizer can be sized to drop the stack temperature to between 300 dF and 350 dF. 
       THE INVENTION 
       [0018]    What is thus described and claimed in this invention is, in one embodiment, is a cowling for connecting a hot gas source to a Stirling engine or a turbine. The cowling has a first portion, a second portion and a third portion that form an integral configuration wherein the first portion is a front, hollow hub of a pre-determined size. The first portion has a front edge and a back end. The second portion is a partial hollow hub having a size larger than the first portion. The second portion has a front end and an open back end and an outside surface. The second portion is integrally attached at the front end with the back end of the first portion such that gas can flow through the first portion into a Stirling engine heat exchanger coil or a turbine, and exit through the second portion. 
         [0019]    The third portion is rectangular in shape and has a bottom end and a top edge. The third portion is integrally attached at the bottom end to a portion of the outside surface of the second portion such that gas can exit through the third portion. 
         [0020]    There is integrally attached to the back end of the second portion, a fourth portion that is a circular hub wherein the circular hub has a set-off distal edge wherein the set-off distal edge has a flat surface. The set off distal edge has a means for attachment to the support of a Stirling engine or a turbine. 
         [0021]    The ceramic cowling has the capability of withstanding high temperatures for prolonged periods of time. By this, it is meant that the ceramic cowling can withstand up to 2400° F. for at least one year. Preferably the duration at the higher temperatures is between 2000° F. and 2200° F. at least two years, and more preferably, the duration at the higher temperatures is at least several months, that is, at least several years. 
         [0022]    In another embodiment of this invention, there is in combination, the cowling as set forth just Supra and an insulated shroud that essentially covers the cowling. The shroud has a front, four side walls, and a back. The shroud is fabricated from a metal, and has a first opening through the front for the first portion front edge of the cowling. There is a second opening through one side wall for the top edge of the second portion of the cowling and a third opening in the back to allow the pass through of gas from the second portion of the cowling into a Stirling engine or turbine heat exchanger coil. The shroud has insulation between the cowling and the shroud and the shroud has a means for attaching to a Stirling engine or turbine support structure and a means for attaching the cowling to the shroud. 
         [0023]    In yet another embodiment of this invention, there is a method of enhancing the power performance of a Stirling engine or a turbine, the method comprising equipping a Stirling engine or turbine with a cowling and shroud combination as set forth just Supra, and operating the Stirling engine or the turbine with a hot gas temperature in excess of 1652° F. 
         [0024]    In still another embodiment of this invention there is a method of powering a Stirling engine or a turbine and providing alternative non-electric power, said Stirling engine or turbine having a heat exchanger coil that has a longitudinal axis. 
         [0025]    A further embodiment of this invention is a system for powering a Stirling engine or a turbine, said system comprising in combination a gasifier having a feed mechanism for combustible materials and an ash removal system, a low NOx oxidizer, a metal heat exchanger, a ceramic heat exchanger, at least one Stirling engine, or at lease one turbine and controls for the combination, wherein any Stirling engine or turbine in the combination is fitted with a ceramic cowling in combination with a shroud for the cowling. 
         [0026]    Additionally, there is an embodiment of this invention that is a system for providing power and alternative energy, said system comprising in combination a gasifier having a feed mechanism for combustible materials and an ash removal system, a low NOx oxidizer, a metal heat exchanger, a ceramic heat exchanger, at least one Stirling engine or turbine, at least one firetube boiler, and controls for the combination, wherein any Stirling engine or turbine in the combination is fitted with a ceramic cowling in combination with a shroud for the cowling. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0027]      FIG. 1  is a view in perspective of a ceramic cowling of this invention. 
           [0028]      FIG. 2  is a view of a cross section of the ceramic cowling of  FIG. 1  through line A-A in a set off from a full side view of a Stirling engine. 
           [0029]      FIG. 3  is a full side view of a ceramic cowling and shroud of this invention. 
           [0030]      FIG. 4  is a cross sectional view of  FIG. 3  through line B-B. 
           [0031]      FIG. 5  is a view of  FIG. 4  with full side view of a Stirling engines mounted therein. 
           [0032]      FIG. 6  is a schematic drawing of a wood fired power plant of this invention utilizing two Stirling engines or two turbines. 
           [0033]      FIG. 7  is a schematic drawing of a wood fired power and steam plant utilizing two Stirling engines or two turbines. 
           [0034]      FIG. 8  is a schematic drawing of a system for providing hot air from a Stirling engine or turbine, to a conventional biomass dryer. 
           [0035]      FIG. 9  is a schematic drawing of a system for providing hot air to a wood drying kiln. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0036]    Now, with more specificity, and turning to  FIG. 1 , there is shown a ceramic cowling  100  of this invention. The ceramic cowling  100  is an integral unit comprised of four portions, that is, a first portion  1  comprising a hollow hub  5  of a pre-determined size. The hollow hub  5  can be any size desired by the user, but it is generally sized according to the size of the heat exchanger coil of the Stirling engine that it is to be used on (Stirling engines are described infra), it being sized such that the front opening  6  of the hub  5  is the same size as the diameter of the heat exchanger coil of the Stirling engine. 
         [0037]    The engines of the prior art have the inlet and outlet ducts on the same vertical face, or nearly so, and with heat driven engines with metal cowlings, there was considerable difficulty in insulating the inlet duct and the outlet duct because they were just a few inches apart from each other. The reduced diameter of each duct to fit it in this arrangement, also increased the pressure drop in the engine. The outlet duct had to make a ninety degree turn related to the flow through the engine heat exchanger coil and this meant that the pressure drop across that coil was not uniform and there was a reduction in the coil&#39;s heat exchange efficiency. Therefore, the preferred arrangement of the ceramic cowling  100  of this invention is to have the inlet duct (the first portion  1 ) directly in line with the heat exchanger coil and to have the first portion  1  of the ceramic cowling  100  to be at least as large as the coil of the heat exchanger on the Stirling engine. 
         [0038]    The first portion  1  has a front edge  7  and a back end  8  with a back edge  15  (see  FIG. 2 ), the significance of which is set forth Infra. 
         [0039]    The second portion  2  is a partial hollow hub  9  having a circumference size larger than the first portion  1 . The reason for a larger circumference than the hub  5  is that this portion of the ceramic cowling  100  is the exhaust part of the ceramic cowling  100 . This is also the portion of the ceramic cowling  100  that surrounds the heat exchanger coil of the Stirling engine, and there must be room for the hot gases to exhaust past the heat exchanger of the Stirling engine without severely impeding the flow thereof. The second portion  2  has a front end  10  and an open back end  11  (see  FIG. 2 ) and an outside surface  12 . The second portion  2  is integrally attached at the front end  10  to the back end  7  of the first portion  1  such that any hot gas provided to the ceramic cowling  100  can flow through the first portion  1  (indicated by the arrow Q) and into the Stirling engine heat exchanger coil, and exit through the second portion  2  and exit (indicated by arrow X) out of the third portion  3 . 
         [0040]    Now, the third portion  3  is rectangular in shape and has a bottom end  13  and a top edge  14 . The third portion  3  is integrally attached at the bottom end  13  to a portion of the outside surface  12  of the second portion  2 . As can be observed from  FIGS. 1 and 2 , the reason that the bottom end  13  is attached to a portion of the outside surface  12  of the second portion  2  is so that there is a curved inside surface  16  and an open air channel  17  for the expedient exhausting of the hot gases (see  FIG. 2 ). 
         [0041]    There is a fourth portion  4  that is integrally attached to the back surface  18  of the second portion  2 . This fourth portion  4  is a circular hub  19  that has a set-off distal edge  20 . The set-off distal edge  20  has a flat surface  21  that is used for interfacing with a seal (not shown) for the ceramic cowling  100 , to the Stirling engine support  22 . The ceramic cowling  100  has a means of attachment (in this example, a bolt  23 ) to the support  22  for the Stirling engine. 
         [0042]      FIG. 3  is a full cross sectional side view of the combination of the ceramic cowling  100 , the shroud  15 . 
         [0043]    Turning now to  FIG. 3 , wherein there is shown a full side view of a ceramic cowling  100  and shroud  15  combination of this invention and to  FIG. 4 , which is a cross sectional view of  FIG. 3 , there is shown in addition, support saddles  16  for the shroud  15 , alloy steel bolt rings  17  for bolts  18 , which bolts  18  are used to attach the ceramic cowling  100  to the shroud  15 . The bolts  18  are also furnished with gasketing  24 . The shroud casing  19  is fabricated from steel and the preferred material is ten gage carbon steel. The metal shell or casing  19  is full seam weld and supports the ceramic cowling  100 . The inlet (portion  1 ) and the outlet (portion  4 ) connections are gas tight and have gasketed seals. The inlet seal  20  and the outlet seal  21  are between the engine and the inlet and outlet ducts,  1  and  2  respectively. The outlet duct  1  has a very positive pressure and in some cases it could be slightly negative. The inlet duct  1 , however, has to have metal outer flange sleeve  22  that bolts up against the mating flange  23  on the steel casing  19 . This duct can contain an expansion joint, not shown. There is insulation  25  sandwiched between the steel casing  19  and the ceramic cowling  15 . 
         [0044]      FIG. 5  is a view of a full Stirling engine  90  inserted into the combination of the ceramic cowling  100  and the shroud  15 . The heat exchanger coil  91  is also shown to clarify how the engine occupies the combination. 
         [0045]    Turning now to  FIG. 6 , there is shown a schematic of a system of this invention that is a wood fired power plant utilizing two Stirling engines to generate electrical power, in which there is shown a gasifier  40 , in this case, a ram feed gasifier, a feed hopper  41  for the biomass, an ash removal system  42 , a syngas exit port  43 , and an auxiliary air fan  44 . The details of the gasifier  40 , the low NOx oxidizer  45 , the metal heat exchanger  60 , the ceramic heat exchanger  50 , boilers, and Stirling engines  70 , do not need to be defined as such components are conventional and well-known in the art. 
         [0046]    The gasifier  40  is fed biomass that is incinerated to produce hot syngas. Ambient air  49  is fed into the gasifier  40  to temper and help burn the biomass. The hot syngas produced by this burning is ducted at about 1150° F. (66) to a low NOx oxidizer  45 . The low NOx oxidizer  45  is equipped with a syngas inlet port  46 , a syngas outlet port  47 , and two additional inlet ports  48  for heated air at 1500° F., 68 from the Stirling engines. The heated gas from the Stirling engines can also be fed to the metal heat exchanger  60  at about 1500° F. at  72 . The NOx oxidizer  45  is ducted to the outlet port  43  of the gasifier  40 , and is ducted at its outlet end  47  to a ceramic heat exchanger  50 . The ceramic heat exchanger  50  has an inlet port  51  for the heated, NOx-free syngas and an outlet port  52 . The cleaned syngas is fed ( 67 ) to the ceramic heat exchanger  50  at about 2200° F. and moved into the interior of the ceramic heat exchanger  50  and flows around the lower ceramic tubes  53  and the upper ceramic tubes  62  within the heat exchanger  50 , and exits  69  at 1600° F. through the outlet port  52  and moves into an alloy metal heat exchanger  60  through an inlet port  54 . The alloy metal heat exchanger  60  also has an outlet port  55  that exhausts to an induction draft fan  56  that is interconnected to the stack  57  where exhaust exits  65  the stack  57  at approximately 575° F. to the atmosphere. The alloy metal heat exchanger  60  has an overfire air fan  58  vented into it through an inlet port  59  that brings in ambient air  71 . 
         [0047]    Turning back to the ceramic heat exchanger  50 , it should be noted that heated outside air from the alloy metal heat exchanger  60  is passed through the metal heat exchanger  60  and ducted into the ceramic heat exchanger  50  through inlet port  61 , and that this air is moved through the ceramic tubes  53  and is thereby heated by the heated syngas. The heated air travels through the lower set of ceramic tubes  53 , into the upper set of ceramic tubes  62 , and out of the ceramic heat exchanger  50  and about 1800° F. (72) and into the double set of Stirling engines  70  through an air inlet  63  in each such engine. The heated air moves through the Stirling engines  70 , powering them. 
         [0048]    In another embodiment of this invention, the preheated combustion air from the Stirling engines  70  is moved 74 at about 1500° F. to a firetube boiler  64  to provide saturated steam  76  ( FIG. 6 ). It should also be noted in  FIG. 6 , which is a schematic of a system in which the Stirling engines feed directly into a firetube boiler  64 , that the hot gas  66  from the Stirling engines do not feed into the oxidizer  45  and instead, the oxidizer is fed ambient air  74  from a fan  75 . 
         [0049]    There are typically five arrangements that can be configured from using hot air from a ceramic heat exchanger  50  to drive Stirling engines  70  and wherein the heated air from the Stirling engines can be used in energy production as an alternative to electrical energy provided by the Stirling engines. 
         [0050]    Such heated air from Stirling engines has to be processed indirectly, such as sending it to a waste heat boiler as described just Supra. 
         [0051]    A first arrangement would be where the air is returned to the combusters, such as the gasifier  40  or the oxidizer  45 , as preheated combustion air, such as is shown in  FIG. 6 . This substantially reduces the amount of fuel required. 
         [0052]    In a second arrangement, the heated air is mixed with the flue gas between the ceramic heat exchanger  50  and the metal heat exchanger  60  as shown in  FIG. 6 . This reduces the size of the metal heat exchanger  60  because one has a higher flue gas mass to transfer heat. 
         [0053]    A third arrangement is where there is a need for steam or hot water, the heated air can be sent to the boiler or water heater as combustion air for the auxiliary natural gas and/or oil fired burner as shown in  FIG. 6 . The end user of the system normally requires turndown or peaking of these heat recovery units. Solid waster-fired systems do not have a large turndown ratio or the ability to respond readily to steam or water demands. The auxiliary burner can supply peak energy rapidly and use the engine hot air exhaust as preheated combustions air. The auxiliary burner also assists in start-up and shutdown, and is a heat source if the solid waste train is down for maintenance. 
         [0054]    In a fourth arrangement, one of the best waste fuels is wet forest products. Most waste products&#39; moisture can range as high as 60%, since it is bark, small limbs, and leaves. When one gets to about 52% moisture, one doesn&#39;t have sufficient energy available to reach a high enough entrance temperature to the ceramic heat exchanger to transfer heat to the engine air. When the forest products are in the 20% range, that is kiln dried, to 45%, that is, air dried surface moisture range, the gasifiers and oxidizers work very well. Pre-drying of the fuel makes firing of high moisture material practical. 
         [0055]    Most of the forest products in the logging industry are in the 59% range and they need power so the engine air  74  can be sent to a conventional rotary or conveyor dryer  77  located between the storage and the feed hopper  41 , and then conveyed by a rotary conveyor  79  to the feed hopper  41 . The high temperature air would be mixed with ambient air  81  from a fan  80 , and in turn would mix directly with the biomass to reduce the moisture content down to the 35% to 40% range. Partially drying wood with hot air gives one a non-polluting affluent. This is shown in  FIG. 9 . 
         [0056]    With regard to arrangement five, there are industries that need clean hot air for particular processes. For example, lumber mills require humidity controlled hot air to dry wood. The engine air  74  can be sent directly to a wood drying kiln  78  where it is mixed with humid air being recirculated, with a portion exhausted to the atmosphere. This is shown in  FIG. 9 . 
         [0057]    Also contemplated within the scope of this invention is the use of a turbine in place of a Stirling engine, or the use in combination with a Stirling engine, either singly, or in multiple units of either a Stirling engine or a turbine. 
         [0058]    Turbines, as used herein, means any conventional turbine. These have been defined as a machine for generating rotary mechanical power from the energy in a stream of fluid supplied to the turbine. “Fluid” as used herein means those fluids most commonly used in turbines such as steam, hot air, or combustion products and water. Steam raised in fossil fuel fired boilers or nuclear reactor systems is widely used in turbines for electrical power generation, ship propulsion, and mechanical drives. The combustion gas turbine has these applications in addition to important uses in aircraft propulsion. Water turbines are used for electrical power generation. 
         [0059]    Energy, originally in the form of head or pressure energy, is converted to velocity energy by passing through a system of stationary and moving blades in the turbine. Changes in the magnitude and direction of the fluid velocity are made to cause tangential forces on the rotating blades, producing mechanical power via turning rotors. Turbines effect the conversion of fluid to mechanical energy through the principles of impulse, reaction, or a mixture of the two.