Apparatus for Producing Combustible Gases From Carbonaceous Materials

Recently because of the high cost of oil, gas and electricity the use of bio-mass materials such as wood chips, bark, sawdust and the like for heating and generation of power is being seriously considered as a supplement to and even a replacement for oil, gas and electricity. Such apparatus as is available generates so much air borne ash that it is objectionable from the standpoint of pollution and is rapidly rendered ineffective by deposits of tar and resin on the grates and in the flues. The apparatus of this invention is designed to convert bio-mass materials such as wood, wood chips, sawdust, bark and the like to a combustible gas without generation of pollutants either in the form of solid particles such as ash or condensates such as tars and resins and to obtain a maximum BTU value per unit of wood consumed.

As herein illustrated the apparatus for generating combustible gas from bio-mass materials comprises a closed retort within which there is supported a downwardly and forwardly extending grate which divides the retort into a plenum chamber at the downwardly facing side of the grate and a mixing chamber at the upwardly facing side of the grate. There is a conductor in communication with the interior of the retort at the top for supplying the wood in the form of chips to the grate, a port at the bottom of the retort in communication with the interior thru which primary air can be induced to the downwardly facing side of the grate and a discharge port at the forward side of the mixing chamber thru which the gaseous products can be delivered to the fire box of a furnace or other heating unit in the form of a gaseous combustible fuel for combustion therein. A conveyor such as a screw is arranged to deliver wood chips to the conductor at the top at a rate to maintain a continuous supply to the grate and a blower is provided for introducing primary air to the port at the bottom of the plenum chamber at a rate to produce maximum efficiency in the conversion without carrying solid particles forwardly from the grate into the mixing chamber and at a controlled rate which determines the rate of distillation and C .fwdarw. CO conversion. A pit at the lower end of the grate is arranged to gravitationally receive the ash below the path of flow of the primary air and convection currents so as to isolate the ash and there is a conveyor such as a screw in the pit for removing the ash, operation of the latter being controlled so as to remove ash only when the temperature in the pit is below that of the burning coals at the lower end of the grate. A conductor may be employed to connect the stack of the furnace or heating unit to which the mixed gases are supplied as a fuel to the conveyor by means of which the wood chips are delivered to the grate to dry the chips and preheat the same and when such a conductor is employed an induction fan is also employed to maintain the necessary draft in the furnace or heating unit and to deliver the hot gases of combustion from the furnace or heating unit to the conveyor. The retort comprises a base from which rises a cupola and the grate which may be in the form of horizontally arranged louver-like bars or spaced grids comprising transversely spaced parallel bars divides the retort from the top to bottom into said plenum and mixing chamber which are narrower at the top than at the bottom. A vibrator may be employed to effect vibration of the grate and a refractory lining may be applied to a part or all of the interior of the retort.

The method of generating gas from bio-mass material according to the invention as carried out by the apparatus described above gravitationally moving a layer of wood chips downwardly and forwardly on the upwardly facing side of an inclined grate within a confining retort such as to form a bed of chips of more or less uniform thickness, establishing a zone of combustion near the lower end of the grate, supplying primary air to the rear side of the bed in the zone of combustion to effect reduction of the chips in such zone to CO and ash, heating of the chips above the zone of combustion to a temperature to effect destructive distillation of the chips and the evolution of the gaseous medium, collecting the ash at the lower end of said zone of combustion below the path of flow of the primary air and convection currents, removing the collected ash so as to promote progressive gravitational descent of the distilled chips above the zone of combustion into the zone of combustion and combining the mixture of evolved gaseous medium and CO at the forward side of the bed. Removal of the ash at the lower end of the combustion zone is effective only when the temperature at the lower end is lower than the burning charcoal. The chips are supplied to the upper end of the bed at a rate to maintain a continuous layer of chips and the primary air is supplied at a rate to maintain combustion without causing the solid particles and ash to become air borne. The chips may be dried and preheated prior to delivery to the zone of distillation.

The invention will now be described in greater detail with reference to the accompanying drawings wherein:

FIG. 1 is a vertical section illustrating one form of the apparatus wherein the grate is comprised of spaced grids;

FIG. 2 is a rear elevation of FIG. 1;

FIG. 3 is a perspective of one of the grids of the grate used in the unit shown in FIG. 1;

FIG. 4 is a vertical section illustrating another form of the apparatus wherein the grate is comprised of a single grid;

FIG. 5 is a rear elevation of FIG. 4, and

FIG. 6 is a flow chart diagrammatically illustrating the progress of the chips going through the apparatus for the generation of wood gas.

The apparatus of this invention is designed to convert bio-mass materials such as wood, wood chips, sawdust and other woody materials to a combustible gas and deliver it to the fire box of a domestic heating unit or commercial boiler and in one form as shown in FIGS. 1, 2 and 3, comprises a sheet metal box 10 having top and bottom walls 12 and 14, front and back walls 16 and 18 and side walls 20-20 which define an interior retort. Intermediate the front and back walls extending transversely throughout the entire width of the box from side wall to side wall there is a grate comprising downwardly and forwardly inclined grids 24 and 26 which in conjunction with a fire wall 28 perpendicular to the bottom wall 14 divide the interior of the retort into a plenum chamber 30 and a mixing chamber 32.

The grids 24 and 26 are of identical construction each comprising, as shown in FIG. 3, a rectangular frame having spaced parallel top and bottom frame members 34-34, spaced parallel side frame members 36-36 and intermediate the latter spaced parallel bars 38 which are parallel to the side frame members 36-36. The grids are made of steel with a spacing of 1/8 to 1/2 inch between the bars depending upon the size of the chips or other particulate material to be processed. A typical grid would be made of 1/4 inch by 11/4 inch bars spaced on 3/4 inch centers with the bars as uniformly parallel as possible and finished so as to be free of any obstructions on their surfaces. As illustrated the grids diverge slightly from top to bottom to facilitate downward flow of the fuel in the space between them as it is consumed, however, if the fuel flows readily without such divergence the grids may be parallel. The grate is inclined with respect to the vertical at an angle of approximately 30.degree., however, the inclination may be varied to increase the angle of inclination or decrease it according to the flowability of the material being processed. If the fuel is sufficiently resistant to free flow the forward grid 24 may be omitted.

At the top of the combustion chamber there is an opening 40 in the top wall 12 which provides communication with the interior of the retort at the upper end of the grate for receiving the lower end of a conductor 42 by means of which the material to be burned is delivered to the space between the grids. The conductor 42 is connected at its upper end to a conveyor 44 which is herein shown as a screw 46 rotatably supported within a manifold 48. The conveyor is driven by a suitable motor so as to transfer the chips from a hopper, not shown, to the upper end of the conductor 42.

There is a port 50 in the rear wall of the retort near the bottom through which primary air is delivered to the plenum chamber 30 at the downwardly facing side of the grate. A blower F of conventional construction is coupled to a flange pipe 52 surrounding the port 50 for supplying the primary air to the plenum chamber 30. There is also a port 52 in the forward wall of the retort near the bottom designed to be connected to an opening 56 in communication with the fire box of a domestic heater or boiler so as to transmit the gases generated in the retort directly to the fire box. Secondary air is introduced to the mixed combustible gases as they enter the heater thru openings 55 provided at the junction of the port 54 with the opening 56 as indicated diagrammatically by the arrows. The interior surface of the walls 12, 14, 16 and 20 defining the mixing chamber 32 are refractory lined. The corresponding portions of the walls defining the plenum chamber 30 may or may not be so lined.

At the lower end of the grate and behind the fire wall 28 there is an ash pit in the form of a trough 58 which is spaced from the bottom and extends transversely of the plenum chamber from side wall to side wall into which the ash gravitates and within the pit there is mounted a conveyor in the form of a screw 60 which is rotated periodically as will be described hereinafter to remove the ash.

A chip level switch S1 which controls the operation of the conveyor 44 is located in the conductor 42 to maintain the bed of chips between the grids, a switch S2 is located in the bin from which the chips are supplied to maintain an adequate supply to the conveyor and a switch S3 is provided for controlling the rate of discharge of the conveyor 60, operation of the latter being controlled by a thermostat T mounted in the pit 58 adjacent the lower end of the grate.

The operation is started by feeding chips thru the manifold 48 to the upper end of the conductor 42 wherein they gravitationally descend into the open space between the grids forming a relatively thick layer or bed of predetermined uniform thickness. When the space between the grids becomes filled the chips are ignited at the downwardly facing side of the grate at approximately the region C while primary air is supplied to support combustion thru the port 50. The primary combustion will proceed upwardly toward the region B. As the chips descend from the region A in the conductor 42 toward the region B they will be progressively heated to a point where distillation will begin at a temperature slightly in excess of 450.degree. F. so that in the region between zones A and B a gaseous distillate is evolved which enters the upper part of the mixing chamber 32. The conductor 42 above the grates is solid imperforate and since there is no draft at this point distillation at the zone A will not occur. As the distillation progresses the size of the wood chips shrink until at the region C only charcoal remains. In the region C the only reaction will be the conversion of the carbon charcoal to carbon monoxide which will be drawn into the mixing chamber and become mixed with the gaseous products of distillation. As this conversion takes place only a small amount of ash will remain and this will drop down into the pit 58 where it will be removed by means of the screw 60. The pit as illustrated is located at the very lower end of the grate behind the fire wall 28 so that ash that drops into it becomes substantially isolated and out of the path of flow of the primary air so that it does not become air borne and carried over into the mixing chamber. In order to insure that the screw 60 removes only ash a thermostat T is provided to control the switch S3 for driving the screw only when the temperature over the screw is less than that of the burning charcoal.

As long as the fuel bed is maintained between the grids the rate of distillation and thus the ultimate heat output of the system can be controlled by regulation of the flow of primary air. This air flow will determine the rate in which the incoming fuel can be elevated to distillation temperature and also the rate of conversion of charcoal to carbon monoxide. The mixed gaseous products of distillation and CO combine in the mixing chamber and are delivered to the fire box and the boiler together with secondary air, if required.

It may be desirable to dry the chips to remove excessive moisture and preheat them to a temperature somewhat near the temperature of distillation as they enter the space between the upper ends of the grids and so there is provided means for heating the chips in the hopper or on their way from the storage hopper thru the manifold 48 to the grate. The means employed may, for example, be the hot gases of combustion emitted from the heating unit or boiler delivered to the hopper or manifold thru a suitable conductor 62 including an induction draft fan 64. The hot gases from the unit or boiler would normally be lost to the atmosphere but by conducting them to the hopper the chips may be heated to a temperature of approximately 120.degree. F. to 125.degree. F. By following this procedure green chips just as they are received from the wood chipper may be used with an efficient evolution of wood gas.

It is also possible to recycle a portion of the hot gases produced within the unit by conducting some of these gases into and thru the manifold 48 so that the chips can actually be delivered to the upper end of the grate at a temperature close to the point of incipient destructive distillation, for example 325.degree. F., and by so doing substantially increased distillation rate may be maintained.

It is also possible to conduct a portion of the burning charcoal from the region C to the region A where it can be admixed with the fresh incoming chips so as to substantially increase the rate at which the incoming fuel mass is raised to the distillation temperature. The admixture of a portion of the hot coals with the fresh chips can increase the capacity of the unit by two to three times which would be a considerable factor in determining the size of the unit for the particular installation desired. Desirably, the hot coals should be removed before complete distillation so that it still contains a controlled residual of volatiles.

A modification of the apparatus is shown in FIGS. 4 and 5 wherein the retort comprises a sheet metal box 10.sup.1 having top and bottom walls 12.sup.1 and 14.sup.1, front and back walls 16.sup.1 and 18.sup.1 and side walls 20.sup.1 -20.sup.1 which define an interior combustion chamber. Intermediate the front and back walls extending transversely throughout the entire width of the box from side wall to side wall there is a grate comprising horizontally arranged grid bars 24.sup.1 which in conjunction with a riser 28.sup.1 perpendicular to the bottomwall 14.sup.1 divides the combustion chamber into a plenum chamber 30.sup.1 and a mixing chamber 32.sup.1.

The grid bars 24.sup.1 of the grate are of L-shaped cross section and are fastened in spaced parallel relation between spaced parallel frame members 36.sup.1 -36.sup.1 so as to overlap in louver-like form and so that overlapping portions defined a downwardly and forwardly sloping fuel supporting surface. In this form of the apparatus the top of the grid is arranged so that it is adjacent to the rear wall and the bottom is approximately halfway to two-thirds the way between the back and front walls. The upwardly facing side of the grid is nearly parallel to the front wall. The overlapping bars permits the use of finely dried fuels, such as wood shaving, sawdust, etc. without the danger of unburned fuel falling thru the grate. Also, the inclined bars help to reduce carry over of particles. With this grate system, using dry hardwood chips, heat outputs have been developed of up to 500,000 BTU/hrs per square foot of grate use.

At the top of the retort there is an opening 40.sup.1 in the top wall 12.sup.1 which provides communication with the interior of the mixing chamber at the upper end of the grate for receiving the lower end of a conductor 42.sup.1 by means of which the material to be burned is delivered to the surface of the grate. The conductor 42.sup.1 is connected at its upper end to a conveyor, not shown, the latter being driven by a suitable motor so as to transfer chips from a hopper to the upper end of the conductor 42.sup.1.

At the bottom of the plenum chamber 30.sup.1 there is a port 50.sup.1 thru which primary air is delivered to the plenum chamber 30.sup.1 at the downwardly facing side of the grate. A blower F.sup.1 of conventional construction is coupled to a flange 52.sup.1 surrounding the port 50.sup.1 for supplying primary air to the plenum chamber. The mixing chamber 32.sup.1 has at the bottom a port 54.sup.1 which is connected by a conductor 56.sup.1 and venturi 57.sup.1 to the fire box of a domestic heater or boiler so as to transmit the gases generated by the unit directly to the fire box. Secondary air is delivered to the venturi thru a conductor 59.sup.1. The venturi not only provides for mixing the gases but also to induce the mixture to flow into the fire box of the unit within which it is to be burnt as a fuel. The interior walls of the entire retort are lined with a refractory material 61.sup.1.

At the lower end of the grate and forwardly of the riser 28.sup.1 there is an ash pit 58.sup.1 which is below the lower level of the grate and well below the port 54.sup.1 into which the ash gravitates and within the pit there is mounted a conveyor in the form of a screw 60.sup.1 which is rotated periodically to remove the ash.

As described with reference to the apparatus shown in FIGS. 1 to 3 a chip level switch S1 controlling the feed screw is provided in the conductor 42.sup.1 to maintain a uniform bed of chips on the grate, a switch S2 is located in the supply bin from which the chips are supplied to maintain an adequate supply of chips, and a switch S3 is provided to control the rate of discharge of the conveyor 60.sup.1, operation of the latter being controlled by a thermostat T mounted in the pit 58.sup.1 adjacent the lower end of the grate.

The operation of the apparatus shown in this form of the invention is the same as that described with reference to the form illustrated in FIGS. 1 to 3 and hence need not be repeated herein.

The specific configuration of the apparatus shown in FIGS. 4 and 5 appears to be the more satisfactory for adoption as a standard commercial product and seems to be especially effective in the handling of a wide variety of fuel types. Units of this configuration have been built and tested and have developed up to 1.7 million BTU/hour.

In this form of the invention a vibrator 63.sup.1 may be employed which, as shown, is connected to the rear side of the grate thru suitable openings in the rear wall for the purpose of vibrating the grate to assist in the smooth downward movement of the fuel over the surface of the grate bars. Such vibration is used principally for light fuels.

FIG. 6 is a flow chart diagrammatically illustrating the progress of the chips from the storage hopper 66 in the form of green chips to a drier 68 which may be the manifold 48 and from thence to the wood gas generator 10 where the chips are converted to gas, the delivery of the gas to the furnace 70 for which it is the fuel and finally the delivery of the products of combustion produced in the furnace to the stack 72. The stack gases, as shown, may be conducted back thru a conduit 62 to the drier 68 together with a quantity of fresh air by means of the fan 64 and from the drier to the atmosphere. Primary air is introduced to the unit, hot gases are recirculated and secondary air is delivered to the furnace, all as diagrammatically illustrated in the flow chart.

Starting with dry approximately 5% moisture content chips of mixed origin of approximately 1/2 inch, produced for example in a conventional wood chipping machine, and supplying them to the grate to form a bed of approximately 2 inches in thickness from back to front, a mixture of gaseous products of distillation and CO were evolved at the rate of 40 cubic feet per minute having a heat value of 8000 BTU per pound of chips delivered to the grates.

Starting with the same mixed chips but in the green condition and pre-drying with the waste heat from the stack the green chips may be brought to a dry condition to give the same heat value as those in Example 1.

Throughout the performance of the apparatus tests were made of the stack gases produced and it was found that the solid matter carried over was too low to be measurable by conventional equipment used for stack gas particulates. Furthermore, daily use for a period of three months of the unit for seasoning lumber in dry kilns resulted in no noticeable deposit of tar or resins on the interior surface of the unit, the grates and/or the ducts leading to and from the unit.

Conventional boiler controls may be employed to control the primary air and thru such control effect modulation of the output. Modulation of the heat load can be had by controlling both the primary and secondary air. By regulating the feed of fuel and use of primary air to control heating of the fuel to above a temperature necessary to effect destructive distillation and controlling the rate of conversion of C to CO the rate of fuel feed may be established.

The apparatus is capable of reducing bulk material, for example, municipal waste, bark or sawdust and agriculture waste such as beet pulp, straw, rice hulks, etc.

While the grates shown in both forms of the apparatus are stationary it is within the scope of the invention to employ movable grates, that is, grates which constitute in effect conveyors for moving the fuel thru the second zone.

The terminology "bio-mass" material used herein is to generically include materials derived from vegetable matter essentially cellulosic in composition which would include not only woody material but coal as distinct from liquid or gaseous materials.

It should be understood that the present disclosure is for the purpose of illustration only and includes all modifications or improvements which fall within the scope of the appended claims.