Abstract:
A method of manufacturing a grate for support of combustible solids comprises mixing a kaolin-type clay with water and a small percentage of stainless steel needles, pouring the mixture into a prepared mold, allowing the mixture to harden, drilling holes through the hardened mixture for the purpose of allowing air to pass therethrough, and curing the hardened mixture at an elevated temperature. The curing of the hardened mixture is preferably accomplished by increasing the temperature gradually to a maximum of 2,000° F. and slowly decreasing the temperature to an ambient temperature.

Description:
This is a division, of application Ser. No. 961,925, filed Nov. 20, 1978 now U.S. Pat. No. 4,233,024 issued Nov. 11, 1980. 
    
    
     BACKGROUND OF THE INVENTION 
     It is well known that an imbalance in current rates of energy production and consumption could lead to an impending national energy crisis of severe proportions, bringing about serious shortages and higher prices of fuels, including those conventionally used for heating lumber dry kilns. A readily available supply of wood fiber material of low commercial value has forced lumber producers to seek a satisfactory manner of utilizing normally wasted wood fiber as a means of producing heat for the drying of green lumber. Methods now available require at economically prohibitive expense preparation of the wood fiber prior to burning, where expensive equipment of high cap tal construction cost, particularly the high horsepower electrical motors necessary, offsetting much of the savings possible from use of waste wood fiber as a fuel for heating lumber dry kilns. Moreover, methods now available produce residue deposit on lumber, such deposits presenting quality control problems, as well as unpleasant and unhealthful conditions for personnel charged with final machining of kiln dried lumber. Such methods also produce a pollution hazard with respect to atmospheric emission of pollutants. In addition, known methods present mechanical problems which produce prohibitive maintenance expense and loss of production time. 
     Previous examples of equipment for destroying cellulosic materials are known, including U.S. Pat. No. 3,886,873, issued June 3, 1975 to Sundqvist et al showing a method of destroying waste by gasification and subsequent combustion of the gaseous products, but using a rotary grate with a separate fan for introduction of combustion air. U.S. Pat. No. 2,165,802, issued July 11, 1939 to Longtin, discloses a sawdust burning furnace with a horizontal grate having parallel slots, and the Doherty patent (U.S. Pat. No. 1,154,910), patented Sept. 28, 1915, shows a furnace with an inclined grate on which kindling is ignited, but the grate so described fails to with stand the temperatures and abrasion which results from normal use. U.S. Pat. No. 3,831,535 issued Aug. 27, 1974 to Baardson discloses a burner system in which waste wood is burned as a source of heat while reducing the volume of pollutant fumes. However, it is necessary in the Baardson system to grind up raw waste wood to a fine powdery form for introduction into a combustion chamber. Caughery in U.S. Pat. No. 4,030,895, issued June 21, 1977, shows apparatus for generating combustible fuel gases having an inclined grate dividing the chamber into two parts. 
     Other patents showing generally the carbonizing, gasification, destruction, or firing of various cellulosic materials include the following: 
     
         ______________________________________3,398,058     Campbell    Aug. 20, 19683,865,053     Kolze       Feb. 11, 19753,746,521     Giddings    July 17, 19733,729,298     Anderson    April 24, 19732,086,033     Huttner     July 6, 19372,071,678     Bellay      Feb. 23, 19371,917,196     Perry       July 4, 1933  353,966     Rathbun     Dec. 7, 1886______________________________________ 
    
     A drawback associated with each of the patents listed above is their failure to teach a grating or support material which is durable in extended use. Refractory materials have been manufactured from a variety of components, including metals in particulate form. U.S. Pat. No. 2,881,083, patented Apr. 7, 1959 by Veale discloses addition of iron powder to a kaolin-type clay, in order to produce a refractory brick composition. U.S. Pat. No. 3,382,082 to Eubanks et al, patented May 7, 1968, teaches introduction of a metal powder into a ceramic refractory composition, and U.S. Pat. No. 220,715, issued Oct. 21, 1879 to Kinklein, discloses addition of iron filings to clay to produce a fire clay which makes tight stove joints. Disclosure of incorporation of aluminum metal in refractory composition can be found in U.S. Pat. No. 3,649,315, issued Mar. 14, 1972 to Booth, and in U.S. Pat. No. 2,613,153, patented Oct. 7, 1952 by Stafford, with the latter patent mentioning kaolin specifically. 
     SUMMARY OF THE INVENTION 
     In order to provide a practical, workable gas generator for producing combustible gases from wood fiber material of low commercial value, a refractory gas production chamber is furnished with a continuous support channel extending from its top to bottom for conveyance of incoming cellulosic material. The support channel has a continuous grate on its lower surface, beneath which a parallel air box conveys a controlled flow of air through the grate for supporting partial combustion of wood fiber, for propelling the wood fiber material downwardly along the grate, and for raising the cellulosic material off the surface of the grate and performing a preheating function. The invention contemplates withdrawal of the combustible gases produced in the refractory gas production chamber, introduction of secondary air into the stream of combustible gas, ignition of the blended mixture in a combustion chamber combustion thereof, purging of particulate matter, and use of the heated gas to produced, such as for kiln drying of lumber. 
     Also contemplated for use in the gas production chamber is a grate constructed from a kaolin-based material interlaced with needles of metal, particularly stainless steel. 
     Accordingly, it is an object of the present invention to provide an apparatus for generating combustible gas from cellulosic material, blending the secondary source of air with the gas so generated, igniting the mixture and removing particulate material in the combustion products for conveying to a point of use, such as a lumber drying kiln. 
     Another object of the invention is to provide such an apparatus with a refractory gas production chamber having a support chute for introducing cellulosic material, the chute having a grate of a composition which is durable during the intended use. 
     A further object of the invention is to provide a grate having alternately angled ports for introduction of a controlled amount of air for progressive destruction of the cellulosic material, for raising the incoming material from the surface of the grate and performing a preheating function, and for progressively propelling the cellulosic material downwardly as it undergoes progressive decomposition. 
     Still another object of the invention is to provide apparatus for using the combustion products of wood fiber material to low commercial value for kiln drying of lumber without residue deposition on the dried lumber. 
     Yet another object of the invention is to avoid the expense of the preparation of wood fiber to be burned. 
     A further object of the invention is to reduce hazards associated with prior burning equipment, such as air pollution hazards associated with burning of cellulosic fibers in prior art devices. 
     Yet a further object is to provide a gas production chamber which runs at a temperature approximately 600 degrees F. lower than in existing devices, and thereby avoid or minimize slagging problems. 
     A still further object is to avoid placing shelf areas in the gas production chamber which could hinder the free flow of cellulosic material, such as wood chips. 
     These together with other objects and advantages which will become subsequently apparent reside in the details of construction and operation as more fully hereinafter described and claimed, reference being had to the accompanying drawings forming a part hereof, wherein like numerals refer to like parts throughout. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a longitudinal sectional view of an arrangement of the present invention, showing blowers for introduction of primary air and secondary air, a refractory gas production chamber, a low velocity air canister, and a combustion chamber for the final stage of complete combustion. 
     FIG. 2 is a continuation of the combustion chamber of FIG. 1, showing a recirculation blower in association therewith. 
     FIG. 3 is a sectional view of the recirculation blower of FIG. 2, taken substantially upon a plane passing along section line 3--3 on FIG. 2, showing in addition a low velocity air box attached to the blower. 
     FIG. 4 is a top plan view of the grate portion of the support chute in the refractory gas production chamber, taken in a direction perpendicular to the plane of the support chute. 
     FIG. 5 is an enlarged, fragmentary sectional view of the grate of FIG. 4, taken substantially upon a plane passing along section line 5--5 on FIG. 4. 
     FIG. 6 is a greatly enlarged perspective view of a stainless steel needle interlaced within the support chute material shown in FIG. 5. 
     FIG. 7 is a fragmentary sectional view of a refractory nozzle for introducing secondary air into the stream of combustion gas, showing the refractory nozzle partially broken away to disclose inner details thereof. 
     FIG. 8 is a top sectional view of the refractory nozzle of FIG. 7, taken substantially upon a plane passing along section line 8--8 on FIG. 7, showing the circulation pattern of secondary air issuing therefrom. 
     FIG. 9 is a sectional view of the igniting means within the combustion chamber, and is taken substantially upon a plane passing along section line 9--9 on FIG. 1. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The inventive apparatus shown in FIG. 1 is made up of gas production chamber 20, having refractory lining 22 for containing combustion gases in chamber 20 generated from partially decomposed cellulosic material 24. Incoming cellulosic material 26 is introduced through hopper 28 and conveyor 30, which are designed to prevent substantial leakage of combustion gas from gas production chamber 20. Insulating layer 32 is supported by frame 34, which is conventional structural angle iron. Flame igniter 36 is provided as a pilot light to insure the limited combustion inside chamber 20 of the products of decomposition of cellulosic material 24 with the limited supply of air in chamber 20. Products of incomplete combustion, such as carbon monoxide, result from substances evolved from decomposing cellulosic material 24. Controlled amounts of gas and air are introduced through inlet lines 38 and 40 of flame igniter 36 to permit a permanent ignition flame to be present within gas production chamber 20. 
     Support chute 42 is provided with through holes 44 and 46 for passage of air through the grate portion 48 thereof. Through holes 44 are directed perpendicular to the plane of grate 48, as is best seen in FIG. 5, while through holes 46 are directed at an angle of 15 degrees downwardly from the normal, although through holes 46 can be provided at somewhat different angles instead. Through holes 44 are oriented perpendicular to the plane of grate 48 in order to admit air to preheat cellulosic material 26 and 24, and to lift partially decomposed cellulosic material 24 a short distance from the upper surface of grate 48, leaving an air blanket 50 therebetween. Through holes 46 in grate 48 are oriented downwardly at an angle from the perpendicular to the plane of grate 48 and, accordingly, impart a propelling force downwardly along the longitudinal extent of support chute 42. The purpose of the disclosed arrangement of grate 48 with through holes 44 and 46 as described is to provide cooling for grate 48, preheating of incoming air forming in air blanket 50, propulsion of partially decomposed cellulosic material 24 downwardly, lifting of the partly decomposed cellulosic material 24 to reduce wear on the upper surface of grate 48 and thereby prolong the useful life of grate 48, and to provide an aaccurately controllable flow of air into refractory gas production chamber 20 from blower 56 to support limited decomposition of cellulosic material 26 and 24 in a progressive manner as it moves downwardly in support chute 42. 
     It is contemplated that the apparatus described will be operated continuously for maximum eceonomic efficiency, with the products of decomposition of cellulosic material 26 in the form of charcoal and ash 52. Continuously operating conveyor 54 removes such non-destroyed material to an area which is separated from the interior of refractory gas production chamber 20, conveyor 54 being constructed to prevent substantial escape of gases from chamber 20 therethrough. It is advantageous to remove non-destroyed material continuously in the manner described, since neither time nor temperature devices for removal of such material has been found satisfactory, since their use affects the production rate of the entire unit in a negative manner, leading to increased damage and maintenance due to a higher temperature in the interior of gas production chamber 20. 
     Primary air blower 56 provides a controlled flow of air in the direction shown by arrows in duct 58 and in sheet metal wind box 60, which is mounted and sealed parallel to the lower surface of grate 48, and which provides a constant size air inlet means for forcing combustion, prehating of primary air and propelling of primary air through the through holes 44 and 46 of grate 48. The thickness of grate 48 is predetermined to provide close control of the rate of flow of all incoming air. The pressure of air in wind box 60 is controlled to assist in proper destruction of cellulosic material 24 at a predetermined feed rate of incoming cellulosic material 26. Material entering gas production chamber 20 through hopper 28 is distributed more evenly over the entire surface of grate 48 thereby. Inasmuch as the upper section of support chute 42 does not contain design slots or openings, the risk of unwanted gas combustion in incoming cellulosic material 26 is reduced, but conduction of heat from the lower section of grate 48, combined with heating of air resulting from the pressure maintained in wind box 60 maintains grate surface temperatures at about 500 degrees F., resulting in preheating of incoming cellulosic material 26. This speeds decomposition of incoming cellulosic material 26 as it passes downwardly toward conveyor 54 in support chute 42. This is particularly true prior to entry of cellulosic material 26 into the major area of gasification in gas production chamber 20. Through holes 44 and 46 are constructed of a size which reduces the production of particulate matter in gas production chamber 20 during decomposition of cellulosic material 26 and 24. Gases produced, including a proportion of carbon monoxide, converge on the throat 62 of outlet duct 64 as indicated by the arrows in gas production chamber 20 as seen on FIG. 1. Combustion gases in duct 64 then enter low velocity air canister 66, which is of conventional construction, for removal of a substantial portion of particulate material, such as smoke or the like. Hot combustion gases issuing from canister 66 then enter duct 68, within which is located refractory nozzle 70 for introduction of secondary air forced therethrough by secondary air blower 72. Further blending and mixing of secondary air with hot combustion gas occurs in combustion chamber 74, part of which is shown in FIG. 1, and the end of which is shown in FIG. 2. Electrode pair 76 is provided with a source of high voltage from high voltage feed line 78 to provide an ignition spark for completing the combustion of gases entering combustion chamber 74 from duct 68. The product formed in combustion chamber 74, which is now in a state of substantially complete combustion, is drawn from combustion chamber 74 by recirculation blower 80 for discharge through exit duct 82. Recirculation blower 80 is provided with a low velocity air box 84 to further remove particulate matter from the gas stream and thereby reduce such matters in the heated gas issuing from exit duct 82. Trapdoor 86 is used to remove accumulated solids as necessary, and valve 88 regulates the size of opening between blower 80 and air box 84. If heated gases so purged of solid particulate watter are introduced into the interior of a lumber drying kiln, problems of undesirable residue deposits on the lumber are avoided. 
     Grate 48 is shown in FIG. 4 as a perforated plate made of refractory material 88 interlaced with metallic needles 90, grate 48 having through holes 44 at right angles to the plane of grate 48, and through holes 46 angled downwardly at a predetermined angle with respect to a perpendicular. The angle of holes 46 is shown in FIG. 5 as 15 degrees, which has been found to be an angle suitable for serving the purpose of assisting and propelling partially decomposed cellulosic material 24 downwardly through support chute 42. Preferably, refractory material 88 is a kaolin-based clay which has been baked into the sheet making up grate 48. Field experience with existing burner systems fitted with a metal grate of either a bar or slot configuration have a life of very short duration, even when the grate system is made of cast iron or stainless steel. Such systems depend solely on the force of gravity to provide fuel flow, and day-to-day maintenance, as well as time in which such units are out of service, is high. 
     Grate 48 of the present invention is of hard castable refractory material in which stainless steel needles are interlaced. This construction eliminates a major source of difficulty with prior art arrangements utilizing a grate made entirely of metal, such metal grates being subject to expansion and contraction of the metal sections. These problems are minimized when special care is taken in preparation of the grate composition as herein described. 
     The refractory material used is a kaolin-based material, such as that commonly sold under the trademark &#34;Plicast KL&#34;, having a fusion point of about 3205 degrees F. and a service range of about 200-3000 degrees F. This process is exact, and must be adhered to closely if proper results are to be obtained. The procedure for manufacture of grate 48 is given in Example 1 below. 
     Refractory nozzle 70 is shown in enlarged from in FIG. 7, where it is located within aperture 92 in duct 68, the nozzle 70 projecting into duct 68 from secondary air inlet pipe 94. Inlet pipe 94 is connected to secondary air blower 72 for transport of air in the direction indicated by the broken arrows in FIG. 7. Flaps 96 are cut into the projecting portion of refractory nozzle 70, and cap 98 in sealing the end of nozzle 70 forces upwardly traveling air to exit nozzle 70 through slits 100 formed by inwardly bent flaps 96. With this arrangement, mixing of secondary air from refractory nozzle 70 with combustion gases passing horizontally along duct 68 is best seen in FIG. 8. The direction of air issuing from nozzle 70 is seen to be in a generally radial direction, but with a tangential component. 
     FIG. 9 shows spark igniter 76 held in place within combustion chamber 74 by support bar 104. A mixture of combustion gas and secondary air enters combustion chamber 74 through duct 68, and a pair of electrode tips 106 supports a spark resulting from application of high voltage carried by wire 108 through insulated support rods 110. Collars 112 adjustably position support rods 110 with respect to bar 104. High voltage can be applied to tips 106 as needed to insure complete combustion of the gas mixture within combustion chamber 74. Alternatively, gaseous material introduced from duct 68 into chamber 74 can carry small tracers, which ignite the gaseous material as it is introduced to the secondary airstream. Preferably, however, a continuous spark or natural gas pilot is used, in order to reduce the possibility of danger from unignited gases in combustion chamber 74. Example 2 presents results of a commercial test on a system of the present invention. 
     Combustion chamber 74 is preferably constructed of refractory board and is preferably rectangular in shape. Recirculation blower 80 produces draft to remove the gaseous products of combustion from combustion chamber 74, and the heated combustion products are then conveyed to the point of use, which is contemplated as the interior atmosphere of a lumber drying kiln. With the present invention, the support chute and grate is not adversely affected by heat generated by combustion in chamber 20 or by the chemical effects of gasification. The present invention avoids the necessity of time, temperature or electrical limiting devices or limited switches to control the introduction of fibrous material and withdrawal of ash and charcoal products. Continuous operation, besides representing a more efficient use of invested capital, also tends to reduce damage and maintenance as well. With use of the support chute and grate of the present invention, necessity for mechanical devices to vibrate or dislodge cellulosic material in order to produce even fuel flow is avoided. In fact, no moving parts are required for forcing the flow of fuel. The continuous nature of the non-destroyed discharge, independent of time or temperature limiters, provides for equal removal of non-destroyed material throughout the full width of the removal system. With use of the particulate matter removal systems disclosed in the present invention, a problem commonly found with prior art systems is avoided, namely the deposition of residue or particulate matter on lumber dried in kilns furnished with the combustion gases of prior art devices. In addition, health and environmental hazards are successfully minimized with the present invention insofar as air pollution conventionally results from particulate matter removed in the canister, air box, and combustion chamber of the present invention. 
     EXAMPLE 1 
     100 pounds of the trademarked material &#34;Plicast KL&#34; mix is added to previsely 5 quarts (10.4 pounds) of water in a cement mixer. While the the mixing proceeds 11/2 pounds of stainless steel needles are gradually added to the mixture, the needles having the approximate dimensions 0.010 inches by ×0.022 inches×1 inch, as illustrated in FIG. 6. The resulting material is mixed thoroughly in the cement mixer for 5 minutes, and then poured into prepared molds while vibrating for proper consistency. The mold is allowed to stand at room temperature for 15 hours. The cast section is then drilled to provide rows of 1/4 inch holes on 2 inch centers with alternate rows of these holes at a 15-degree diagonal. The sections are then cured in an oven at a temperature of 100 degrees F. The temperature of the oven is increased 50 degrees F. per hour until the oven temperature reaches 1000 degrees F., at which level it is maintained for 8  hours. The temperature is then increased 100 degrees F. per hour until the oven reaches 2000 degrees F. The oven temperature is then decreased gradually until the sections are cooled to an ambient temperature. 
     EXAMPLE 2 
     A commercial test was performed on a system shown in FIGS. 1 and 2 having a capacity of 10,500,000 BTU&#39;s, where grate 48 had a surface area of 35 square feet. The incoming cellulosic material 26 was southern pine planer shavings having a moisture content of about 15%. The unit produced gas for a forced air heater, firing directly into the drying chamber of a high temperature lumber drying kiln held at about 240 degrees F. 
     Tests were run in 5- to 24-hour continuous cycles, starting with 68,000 pounds of material in a storage bin. The heat required to dry the lumber for each 24-hour charge was 101,500,000 BTU&#39;s, or 507,500,000 BTU&#39;s for the 5 days of testing. At peak loading, the unit generated gas having a heat value of 10,480,000 BTU&#39;s from 1310 pounds of planer shavings per hour. To assure accuracy, the unit was attached to an existing propane fired furnace, with propane burner disconnected, and identical results were achieved using either propane or using the apparatus of the present invention. 
     The test results show a production rate of 300,000 BTU&#39;s per square foot of grate surface area, at full firing temperature, with a turndown rate of 41/2 to 1. 
     Throughout the specification and claims, parts and proportions have been given in weight percent unless otherwise specified, and temperatures are given in degrees F. unless otherwise specified. 
     It should be particularly pointed out that with use of grate 48 of the present invention, gas combustion chamber 20 is enabled to run at a temperature of approximately 600 degrees lower than most alternative devices for burning of cellulosic materials such as wood chips. The interior of gas production chamber 20 never reaches a temperature greater than 1200 degrees F., thereby avoiding slagging which occurs typically at temperatures of 1500 degrees F. or above. 
     Moreover, the structure of the gas production chamber avoids all horizontal shelf areas where material could collect and eliminate or hinder free flow thereof. 
     The foregoing is considered as illustrative only of the principles of the invention. Further, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation shown and described, and accordingly, all suitable modifications and equivalents may be resorted to, falling within the scope of the invention.