WO1981003216A1

WO1981003216A1 - Incinerator for combustible refuse

Info

Publication number: WO1981003216A1
Application number: PCT/US1981/000620
Authority: WO
Inventors: G Tucker; R Tyer; R Fitch
Original assignee: Boeing Co
Priority date: 1980-05-09
Filing date: 1981-05-08
Publication date: 1981-11-12

Abstract

An object of the invention is to provide an incinerator for combustible refuse having a feeding system which enables batch feeding preselected weights of fuel into a combustion chamber (115) of a starved-air combustor, while blocking entry of ambient air into the combustion chamber during fuel feed and controlling fuel feed in response to a fuel-conveying auger (121) in the combustion chamber. Another object of the invention is to provide a control system for a starved-air combustor having a combustion chamber (115) divided into plural combustion chamber zones (Z1, Z2, Z3) with separate overfire and underfire airflows individually provided for each zone. Fuel is fed to the combustor in selectable constant weight batches with a supply of underfire air proportional to the rate at which the auger (121) rotates to convey the fuel. Overfire air is supplied to each combustion zone in an inverse relationship to the variance of a sensed temperature within the zone from a predetermined temperature. Another object of the invention is to provide an exit gas control apparatus for name stabilization and performance tuning of a starved-air combustor including a first air conduit (201) communicating with the combustion chamber near the fuel inlet and a second air conduit (209) communicating with the combustion chamber near the residue outlet. A damper (203) controls the discharge of combustion gases entirely through the first conduit, entirely through the second conduit, or proportionately through the first and second conduits to enable the starved-air combustor to operate in full countercurrent, full co-current, or partial co-current and countercount modes, respectively.

Description

INCINERATOR FOR COMBUSTIBLE REFUSE

Technical Field

In the last century, much of the world's energy needs have been fulfilled by hydrocarbon fuels which provided a convenient, plentiful, and inexpensive energy source. The current rising costs of such fuels and concerns over the adequacy of their supply in the future has made them a less desirable energy source and has led to an intense investigation of alternative sources of energy. The ideal alternative energy source is a fuel which is renewable, inexpensive, and plenti- ful, with' examples of such fuels being the byproducts of wood, pulp, and paper mills, and household and commercial refuse.

The use of alternative energy sources is not problem-free, however, since there is a concern over the contents of the emissions from the combination of such fuels as well as the environmental ramifications of acquiring and transporting the fuel and disposing of the residue of combustion.

One promising prior art device for using such alternative energy sources, while maintaining a high degreee of environmental quality, is the starved-air combustor wherein the air supplied for combustion is

CWPI controlled in order to control temperature conditions and the rates of combustion are controlled to consume the fuel entirely. Such starved-air combustors are capable of burning various types of fuel and producing significant amounts of heat which can be employed for any numberof purposes including the production of process steam for use in manufacturing and in the generation of electricity.

Starved-air combustors, as previously known and operated, have not been entirely satisfactory in both entirely consuming the combustible elements of the fuel at high throughput while not producing noxious emissions. This problem results, in part, from the use of such starved-air combustors to burn a wide variety of fuels some of which may be non-homogeneous, e.g., household or commerical refuse. It has not been possible in the previously known starved-air combustors to tailor in a real time manner the com¬ bustion processes to the type of fuel being combusted in order to maximize the efficiency of the combustor while minimizing the generation of air pollutants. While the pollution problem can be solved to a degree by the utilization of scrubbers and other antipollution devices, such mechanisms are very expensive and their cost may militate against the use of alternative energy sources. Background Art

U.S. Patent No. 4,009,667 issued to Robert C. Tyer et al on March 1, 1977, illustrates an appropriate embodi¬ ment for a rotatably-driven auger comprised of a rotatable, water-cooled horizontal shaft supporting a spiral flight of decreasing pitch from the input end of the auger to the output end.

Disclosure of Invention

It is an object of the present invention to provide a starved-air combustor capable of efficiently utilizing many different types and quantities of fuel.

Another object of this invention is to provide a starved-air combustor which does not release noxious pollutants into the atmosphere.

A further object of this invention is to provide a starved-air combustor which is capable of combusting to a very high degree the percentage of all combustible materials provided to it as fuel.

A still further object of this invention is to provide a starved-air combustor including a control system for selectively controlling the quantity of hot combustion gases produced thereby in accordance with the demand for heat produced by the starved -air com¬ bustor.

OMPI Yet another object of this invention is to provide a starved-air combustor including means for selectively exhausting combustion gases from the combustion chamber in a direction co-current with the flow of fuel through the combustion chamber or countercurrent to the flow of fuel through the combustion chamber.

Yet another object of this invention is to provide a starved-air combustor including means for selectively feeding predetermined weights of fuel into the co bus- tion chamber of the starved-air combustor.

Yet another object of this invention is to provide a starved-air combustor wherein the combustion chamber is divided into a plurality of combustion zones and includes a control system which controls independently the injection of air into each of the combustion zones.

Another object of this invention is to provide a starved-air combustor wherein the air supplied to each combustion zone includes overfire air supplied above the fuel in the combustion zone and underfire air supplied beneath the fuel in the combustion zone and wherein the amount of overfire air supplied is dependent upon the temperature in the combustion zone and the amount of underfire air supplied is dependent upon the rate that fuel is being conveyed through the combustion chamber.

OMPI To achieve these objects, and in accordance with the purpose of the invention, as embodied and broadly described herein, the starved-air combustor comprises combustion chamber means having an inlet end for receiving fuel, the combustion chamber means for combusting the received fuel to produce a quantity of heat (hot com¬ bustion gases) related to the rate of combustion and combustion residue, the combustion chamber means including an outlet end for discharging the combustion residue and an outlet port for discharging the com¬ bustion gases, means in the combustion chamber means for conveying the received fuel from the inlet end toward the outlet end,

means for conveying the fuel through the com- bustion chamber at a variable rate; means for supplying a variable airflow to the combustion chamber; and means for controlling the rate of the conveying means and the quantity of air supplied by the supplying means to increase the quantity of hot combustion gases produced the system responsive to an increase in the heat demand and to decrease the quantity of hot combustion gases produced by the system responsive to a decrease in the heat demand, first means for communicating with the combustion chamber proximate the inlet end of the combustion chamber

OMPI for exhausting hot, evolved gases from the combustion chamber, second means communicating with the combustion chamber means proximate the outlet end of the combustion chamber means for exhausting hot, evolved combustion gases from the combustion chamber and means for con¬ trolling the exhausting of the combustion gases from the combustion chamber to exhaust selectively the evolved gases entirely through the second means or proportionately through the first means and the second means,

and means for selectively feeding predetermined weights of the fuel into the inlet end of the com¬ bustion chamber.

Brief Description of Drawings

Figure 1 is an illustration of the starved-air combustor system of the instant invention connected between a fuel supply system and a system which produces process steam from the heat produced by the starved-air combustor system; Figure 2 is a graph illustrating the relationship between temperature in the combustion chamber and the afterburner of the starved-air com¬ bustor system as related to the amount of air supplied to the combustion chamber and to the afterburner; Figure 3 is a graph illustrating the control of the fuel flow for three given weights of fuel and a range of auger rotation rate; Figure 4 is a cross-sectional view taken along line 4-4 of a means for feeding variable quantities of fuel to the combustion chamber in a batch mode illustrated in Figure 1; Figure 5 is a timing diagram explaining the operation of the feeding means of Figure 4; Figure 6 is a longitudinal cross-sectional view of the combustion chamber of the starved-air combustor system of Figure 1 taken along the line 6-6; Figure 7 is a transverse cross-sectional view of the combustion chamber of the starved-air combustor system of Figure

1 and 6 taken along the lines 7-7; Figure 8 is a schematic logic circuit diagram illustrating the control system for supplying overfire air and underfire air to the combustion chamber and air to the afterburner of the starved-air combustor system; Figure 9 schematically illustrates the logic of the control circuit for relating the angular rate of the auger to the quantity of fuel conveyed through the combustion chamber of the starved-air combustor system; Figure 10 schematically illustrates the control circuit for controlling the quantity of underfire air supplied to the combustion chamber of the starved-air combustor system; Figure 11 schematically illustrates the control circuit for controlling the overfire air supply to the combustion zones in the combustion chamber of the starved-air com¬ bustor system; Figure 12 is an enlarged cross-sectional,

OMPl schematic view taken also taken along lines 6-6 of Figure 1 illustrating the means for exhausting combustion gases from the combustion chamber; and Figure 13 is a schematic view of an alternate embodiment of the means for exhausting combustion gases from the com¬ bustion chamber.

Best Mode for Carrying Out the Invention

Figure 1 illustrates an embodiment of a starved- air combustor, according to the present invention, coupled between a refuse feeder system and a steam generation system. As embodied herein, the refuse supply system comprises a supply conveyor 101 for conveying fuel, in this instance refuse, from a receiving building (not shown) and one or more storage silos (not shown) . The receiving building and storage silos are to insure that an adequate supply of fuel can be supplied to the combustor in order to permit the combustor to run at peak efficiency. In the illustrated embodiment, it is contemplated that the supply conveyor 101 would supply fuel to the fuel surge and recirculation bin 103 at a rate of at least fifteen tons per hour and that the capacity of the combustor system would range from 150 to 500 pounds per minute.

OMPl - 9 -

The fuel surge and reσirculation bin 103 comprises an additional means for insuring that a constant and adequate supply of fuel . is available to the combustor. The bin 103 could, for example, contain at least 10 minutes capacity of fuel, i.e., approximately 2.5 tons, which is received at the top of the bin 103 and supplied through the bottom of the bin 103 to the feed conveyor 105. Feed conveyor 105 supplies the fuel to a splitter valve 107 which may either direct the fuel into the feed and weigh bin 109 or, when the feed and weigh bin 109 is filled to capacity, to the return conveyor 111 for return to the fuel surge and recirculation bin 103. The feed and weigh bin 109 is calibrated to supply a preset weight of fuel at the inlet end 113 of a refractory-lined combustor 115 at such time that the first flight of an auger or screw-conveyor 121 within the chamber 115 has been rotated into a fuel receiving position. Within the starved-air combustor 115 there is provided a well-known oil igniter (not shown) in the input end of the combustion chamber 115 serve as a means for initially igniting the fuel upon start up of the starved air combustor supplied through the bottom of the bin 103 to the feed conveyor 105. Feed conveyor 105 supplies the fuel to a splitter valve 107 which may either direct the fuel into the feed and weigh bin 109 or, when the feed and weigh bin 109 is filled to capacity,

OMPl - 10 -

to the return conveyor 111 for return to the fuel surge and recirculation bin 103. The feed and weigh.bin 109 is calibrated to supply a constant weight of fuel at the inlet end 113 of a refractory lined combustor 115 at such time that the first flight of an auger 121 within the chamber 115 has been rotated into a fuel-receiving position. Within the starved-air combustor 115 there is provided a well-known oil igniter (not shown) in the input end 113 of the combustion chamber 115 to serve as a means for initially igniting the fuel upon start up of the starved-air combustor.

It is contemplated in the instant system that the speed of the auger would range from .3 to 1 rpm. An appropriate oil igniter would comprise an oil burner having its flame extending into the input end of the combustor

115 to heat and to ignite the initial load of fuel supplied by the feed and weigh bin 109. It is contemplated that such an oil igniter would be capable of burning oil fuel at a rate.of approximately six gallons per hour at two pounds per square inch pressure.

The combustor 115 has an output end 117 connected to a conduit 119 which feeds the top of an afterburner 129. The combustor 115 includes air supply means 123 for supplying underfire air and conduits 125 for supplying overfire air. This air is provided by a fan 126 (shown in phanton) which also supplies air through conduits 127 to the afterburner 129.. Alternately, a separate fan or fans may be provided to supply underfire air, overfire air, and air to the afterburner 129. A small air distributor 130 is connected to the upper conduit 127 to supply air into the afterburner 129 through special injectors located both at and below the midpoint of the afterburner 129.

Afterburner 129 is provided, in part, as a secondary combustor chamber which mixes the air supplied by the conduits 127 with the gaseous and entrained solid particle output of the combustor from the outlet end 117 to combust all combustible material in the gaseous output and, in part, to separate suspended ash and non-combustible solids from the hot non-combustible gas. Both the non- combustible material from the afterburner 129 and the combustion residue from the combustor 115 are fed through conduit 131 to an ash collector 135. The hot non-combustible gas is discharged into a superheater 137 from which it is supplied to a waste heat boiler 139 to produce, in this case, process steam. An electrostatic precipitator 141 removes any additional solids from the now cooler non- combustible gas exiting from the waste heat boiler 139 through an economizer 140 and the solid material is conveyed to an ash cart 135. From the precipitator 141, the non-combustible gas is drawn by a fan 143 and expelled from stack 145. Upon entering into the fan the tempera- ture of the gas is approximately 300 to 400 degress Fahrenheit and the fan 143 is of sufficient strength to exert a negative pressure in the system within the combustor 115, the afterburner 129, superheater 137, waste heat boiler 139, economizer 140, and precipitator 141.

One of the principal advantages of a starved-air combustion system is that gasification of partial oxida¬ tion of solid fuels can be made to occur at moderate ■ temperatures (1300° - 1800°F) . The significant bene¬ ficial effects of this include elimination of slagging or f sing of the fuel and ash particles, exposing the combustor structure to only moderate temperature in non-oxidizing conditions, and reducing the formation of nitrogen oxides.

The principal control difficulty in the prior art starved-air combustor systems lies in maintaining tempera¬ ture levels throughout the combustor, i.e., within the pile of fuel material and the gas space above it in the combustion chamber, while also optimizing the performance of the combustor system, i.e., mass of solid material gasified per unit of time and unit of area of grate surface. Temperature control is achieved by regulating the airflow into the combustion chamber to achieve the proper air/fuel ratio.

O Pl

^,Z.ι Figure 2 is a plot of temperature after reaction of fuel and air at different proportions and, as the terminilogy suggests, a starved-air combustor chamber operates at a negative percentage of excess air compared to the chemically correct amount in the temperature region indicated in Figure 2. Thus, to increase the operating temperature within the combustion chamber of a starved-air combustor, it is necessary to increase the airflow into the combustion chamber.

Also evident from Figure 2 is that the temperature within the afterburner responds to an increased airflow in the opposite manner as does the combustion chamber. Thus, to increase the temperature in the afterburner it is necessary to reduce the air supply thereto.

One problem in regulating the temperature within the combustion chamber is that the fuel bed and the injection of air into it are not necessarily homogeneous and the schedule of events leading to complete gasification or oxidation is not uniformly identical for all particles of this fuel. Local air/fuel ratio increases from average can cause radical temperature increases within the com¬ bustion chambers. Some of these pertubations in tempera¬ ture are unavoidable because, as will hereinafter be explained, the air is injected into the combustion chamber from discrete parts through the refractory lining

OMP1 of the chamber and the fuel particles are obviously discrete solid particles thereby causing non-homogeneous air/fuel mixtures where the injected air directly impacts upon the fuel particles. These conditions are only temporary, however, because the auger within the combustion chamber removes and tumbles fuel so that the non-homogeneous conditions do not last long enough to cause slagging of the non-combustible material. The major difficulty arises in correctly relating the volumes the underfire air (the air supp¬ lied beneath the fuel in the combustion chamber) and the overfire air, (air supplied above the fuel in the combustion chamber) used in partially combusting or gasifying the solid fuel and the fuel gas to the rate of fuel flow within the combustion chamber.

The prior art attempted to solve the slagging and temperature control problems by different types of control systems. One type of control system is illustrated in the above-mentioned Tyer et al patent wherein the underfire air was uniformly supplied beneath the bed fuel in the combustion chamber and the overfire air was supplied in multiple zones above the fuel fed to the combustion chamber and all of the controls of the underfire air and the overfire air were manual. Some of the drawbacks of this arrangement

OMPI were that it did not provide for altered zoning of the underfire air to accommodate changes in fuel moisture content and reactivity, and the possibility that uncontrolled variable fuel feed could lead to undesired oscillations in the operating properties in the combustion chamber.

It has also been attempted, in the prior art, to provide a combustion chamber wherein no overfire air is provided but where the underfire air was separated into three different zones with independent control of the airflow into each zone. This approach suffers from the inability to balance properly the reaction sequence in the fuel bed and the reaction sequence in the evolved combustion gases especially during changes of fuel feed rate. When the fuel feed interrupted temporarily or when the rate of fuel feed was decreased because of a reduction of fuel density, the severe local temperature aberrations occurred because of an increase in the air/fuel ratio. When the airflow rate was decreased, the gasification rate and efficiency of the starved-air combustor decreased.

The present invention, as will hereinafter be^* described, avoids the problems of the prior art starved- air combustor systems and provides a starved-air combustor of greater efficiency by providing a feed system for feeding fuel into the combustion chamber in constant weight batches, an air supply system for feeding both underfire and overfire air in a zoned manner, and a control- system for regulating underfire airflow in accordance with the rate of fuel flow through the combustion chamber and overfire airflow in accordance with the temperatures in the combustion zones. The feed system charges a fixed (operator-set) weight of fuel (M) for each rotation or partial rotation of the auger within the combustion chamber. This means that the fuel weight flow rate (m_f) is adjusted by changing the auger rotation rate (Wa_,ll,y_) . The proportion of underfire air (m ) to fuel weight flow rate is operator-set, as a function of auger rotation rate

Waug,' while overfire flow rates (mo) are controlled according to the gas temperature of the combustion zones Ti.. Thus, fuel flow and underfire air are key ^Λed to the au

Waug so t.h,at._

after M and the constants for each zone . are set, then underfire air/fuel flow ratios are constant. If auger speed drops, m is automatically decreased in proportion. Similarly, if M is decreased, m is also decreased. Thus, the response of the starved- air combustor system to changes in heat demand is through auger speed.

This approach insures that the flow of^* fuel through the fuel bed in the combustion chamber is of

OMPI ^W,P0 uniform size and is provided with the same air/fuel ratio for each batch of fuel that is fed into the combustion chamber.

Figure 3 is a graph relating fuel feed rates to auger speed. If, for example, the nominal maximum fuel feed rate and auger speed is 15 tons of fuel per minute and .9 revolutions of the auger per minute, then the line A relates decreases in the fuel feed rate to decreases in the auger speed for a constant fuel feed rate to decreases in ihe auger speed for a constant fuel batch weight Ml. It has been determined that if the auger is rotated at too slow a rate, e.g., less than .4 rpm, clinkering and slagging may occur within the combustion chamber. Thus, as the auger spe'ed approaches .4 rpm with the fuel batch weight of Ml, or the fuel feed rate is 8 tons per hour or less, then it is more efficient to Operate along performance curve B with a fuel batch weight of M2 less than Ml than performance curve A. Similarly, change to operating curve C with a fuel batch weight of M3 less than M2 should be effected when fuel is being fed at a rate of 5 or fewer tons per hour or auger speed approaches .4 rpm.

The present invention is also concerned with an apparatus for selectively feeding predetermined weights of fuel into the inlet end of the combustion chamber of the starved-air combustor. By feeding only preselected

OMPI weights of fuel, the present invention avoids a serious problem in the prior art starved-air combustors which greatly reduced the efficiency of such combustors. This inefficiency resulted from a varying fuel-to-air ratio in the combustion chamber 115. As discussed in the previously referenced patent to Tyer et al, the com¬ bustion chamber 115 is provided with underfire air which is air introduced through the walls of the combustion chamber 115 underneath the fuel in the chamber. Similarly, overfire air is injected through the walls of the combustion chamber above the fuel to aid com¬ bustion . Assuming that a constant volume of air is injected into the combustion chamber as underfire and overfire air, then the air-tc-fuel^* ratio will be deter- mined by the amount of fuel fed into the combustor. The prior art starved-air combustors did not regulate the amount of fuel fed thereto and could not establish proper air-to-fuel ratio. Also, it is advantageous to set different air-to-fuel ratios for different types of fuel consumed in the combustor in order to maximize the efficiency of combustion and to minimize the pollutants in the exhaust gas, but if the quantity of fuel fed into the combustion chamber cannot be regulated, then no definite ait-to-fuel ratio can be maintained.

The instant invention enables the air-to-fuel ratio to be selected by feeding predetermined weights of fuel into the inlet end of the combustion chamber in a batch mode. The weight of a particular fuel charge or batch could, for example, be selected to range from 150 to 500 pounds depending upon the combustability of the fuel currently being fed to the combustor. Thus, if a particular air-to-fuel ratio is desired then it can be accomplished merely by selecting a particular weight for each charge or batch of fuel and specific airflow for that feed rate.

Feeding fuel in a charge or a batch mode into the inlet of the combustion chamber provides a further advan¬ tage over the prior art starved-air combustors by enabling the combustor to achieve a maximum throughput. If, for example, unregulated amounts of fuel are supplied to the combustor as exhibited in U.S. Patent No. 3,942,455 issued to Wallis on March 9, 1976, then there exists the probability that the auger within the combustion chamber will be conveying either too little or too much fuel through the combustion chamber at any one time. Feeding the fuel in predetermined batch weights, as is done by"the ^"present invention, permits control over the combustion processes and a level of efficiency in the manner not previously attainable.

Referring to Figure 4, the starved-air combustor comprises a combustion chamber 115 including an inlet end 113. As embodied herein, the combustion chamber

OMPI comprises a refractory-lined horizontal cylindrical chamber extending from the inlet end 113 to the outlet end 117. Within the chamber, there resides means for conveying fuel from the inlet end to the outlet end. As herein embodied, the conveying means comprises the screw conveyor or auger 121 formed with a rotatable cylindrical axis within the cylindrical com¬ bustion chamber with a spiral flight concentrically connected to the axis. As is well-known in the art, the spiral flight in cooperation with the axis forms an auger and provides a plurality of spaces 151 and 153 defined by the walls of the combustion chamber 115 beneath the inlet end 113 illustrates the correct time for feeding fuel into the combustion chamber. This orientation causes area 151 to have its largest volume but, if desired, different orientations of the auger to present an area 151 of different volume could also be designated the feed position or. positions.

The instant invention also includes means foe selec- tively feeding predetermined weights of the fuel into the inlet end 113 of the combustion chamber 115. As embodied herein, the feeding means comprises means for receiving and containing the fuel, means for weighing the received the contained fuel, and means for selec- tively discharing the received and contained fuel respon¬ sive to the positioning of the auger 121 into the feeding orientation and to the accumulation of a preselected weight of fuel in the receiving and containing means.

A suitable embodiment for the receiving and containing means comprises a chute 201 positioned beneath the fuel feed conveyer 105 such that the fuel conveyed by the conveyor 105 drops off of the conveyor 105 into the chute 201. From the chute 201, the fuel can either pass into the combustor feed path 203 of the feeding and receiving means or through path 205 to the return conveyor 111 for return to the surge bin 103 as pre- viously explained. A splitter 207 is rotatable in the neck of the chute 201 to guide the received fuel to the combustor feed path 203 or the return path 205.

The receiving and containing means further includes a chute 209 for guiding the fuel directed to the combustor 115 into a weigh bin 211. A cover valve 213 is provided at the inlet of the chute 209 and is rotatable either to permit the fuel to pass into the chute 209 and the weigh bin 211 when the ocver valve is in an open, or downward, position or to prevent additional fuel from entering chute 209 and weigh bin 211 when the cover valve 213 is in a closed, i.e., as illustrated in Figure 4, a horizontal position. The cover valve 213 provides an airtight seal with the sides of the chute 209 such that when the cover valve 213 is closed, outside air is prevented from entering chute 209 and weigh bin 211.

"BUKiL-so- O PI The cover, valve 213 could, alternatively, be a. slidable . valve having an- inward^* (closed) position and an outward ^* (open) position.

The weigh bin 211 is connected to chute 209. via . a flexible coupling 215. so that the weigh bin 211 and any fuel- contained therein is not. supported by the chute 209, but as will be hereinafter explained,, is . supported by means of one or more weigh- cells 223 •connected to a. stationary, support^* member 221 and to support arms 225 on the exterior of the weigh bin 211.

As embodied herein, the discharing means comprises a release, valve' 217 shown in Figure 4 in its closed position. As will also hereinafter be explained, the release, valve 217 will not be opened, i.e., rotated to extend into the' lower chute portion 219 of the feeding means, until the weighing means indicates that a pre¬ determined weight of fuel has been accumulated in the weigh bin 211 and that an auger position, sensor 227 has determined that the auger 122 has been rotated into the proper feed orientation. The lower chute portion 219 is coupled to the weigh bin 211 by means of a flexible, airtight, seal 218. so as not to support the weigh bin 211 but only to guide the fuel into the inlet end 113 of the combustion chamber 115. while simultaneously preventing ambient air from entering the combustion chamber. The weighing means, as embodied herein, comprises one or more weigh cells 223 coupled, as above-described, between, stationary support members 221 and exterior arms 225 connected to the weigh bin 211. One. skilled in the art will readily recognize that each weigh cell 223 com¬ prises any one of. a number of means whereby a particular weight can be. selected, the. weight of the weigh bin including fuel received and contained therein deter¬ mined, and an output signal generated when the measured weight of the weigh bin exceeds a. selected weight. As one example, the weigh cell 223 could comprise a. variable resistor providing a. voltage output indicative of the weight of fuel in weigh bin 211. A. voltage detector senses the. voltage output of the. variable resistor and actuates a microswitch when the sensed, voltage exceeds a threshold, voltage corresponding to a selected weight. The output of the microswitch is then employed within suitable logic circuitry, as will be hereinafter explained, to actuate the splitter 207, cover valve 213, and release valve 217 to feed the conveyor with fuel in a proper manner.

Figure 4 also illustrates, in block diagram form, functional logic circuits that are needed to control the feeding means to feed fuel either into the combustor 115 or to the return conveyor 111. Figure^' 5 is a timing diagram to be read in conjunction with the block diagram

\_] ii -._J. OMPI

^ of Figure 4 for a complete understanding of the opera¬ tion of the logic circuits.

In normal operation, during the combustor feed mode, the splitter, valve 207 will be positioned as indicated by the solid lines in Figure 4. The cover valve 213 will be in its opened, or downward position, and the release valve 217 will be in the closed posi¬ tion as shown in Figure 4. Fuel will drop from feed conveyor 105 through feed path 203 and upper chute 209 into the weigh bin 211 and when the preselected weight of a batch of charge of fuel has been accumu¬ lated in the weigh bin 211 then the weigh cells 223 will cause a bin full signal to be supplied from the weigh cells 223 to feed control circuit 231.to change from a^* low value to a high value as shown in Figure 5.

After the preselected weight has been accumulated in the weigh bin 211, it is. necessary to rotate the splitter valve 207 into the orientation shown by the dotted lines in Figure 4 and to close the cover valve 213. This is performed under the control of feed control circuit 231 by supplying the appropriate output to cover valve control 233 and to splitter valve control 235. Once the cover valve 213 has been rotated into its closed air-sealing position, then the feeding means will not change state until the auger position sensor 227 determines that the auger 121 has been rotated into

O "" an orientation such that the first area 151 is of its proper volume. When this orientation of the auger 121 is reached, the auger position sensor 227 supplied a pulse, as shown in Figure 5, to the feed control circuit 231.

There are many ways of implementing the auger position sensor 227 but one would be to attach a small magnetic flux producing element to the auger such that it would be presented in alignment with a flux sensor when the suger has been rotated into the feed orienta¬ tion.

After the feed control circuit has received the auger position pulse and is still receiving the bin full signal at a high level, it will signal the release valve control 229 to rotate the release valve 217 to its downward orientation in order to permit the fuel contained within the weigh bin 211 to pass through lower chute 219 and into the first area 151 of the combustion chamber 115. The feed control circuit 231 will produce, after a suitable delay to provide time for the fuel to be discharged from the weigh bin 211, a restore pulse that is supplied to the auger position sensor 227, release valve control 229, cover valve control 233, and splitter valve control 235 to control feeding means in a manner to permit the accumulation of a subsequent charge or batch of fuel in the weigh bin 211. As explained above, this feeding orientation comprises: first, closing release valve 217; second, opening the cover valve 213; and third, rotating splitter valve 207 into the orientation illustrated by the solid lines in Figure 4. The weigh cells 223 will automatically reset the microswitch because, after the discharge of the fuel from weigh bin 211, the weigh cells 223 will no longer indicate that the preselected fuel weight has been accumulated in weigh bin 211.

Since one of the purposes of this instant invention is to provide preselected weights of fuel to the com- bustor 115, the feed control circuit 231 will generate an error signal if auger position sensor 227 determines that the auger 121 is in the feed orientation and pro¬ vides a pulse to feed control circuit 231, while at the same time the weigh cells 223 have not supplied signals to feed control circuit 231 indicating that that predetermined weight of fuel had been accumulated in weigh bin 211. If such a situation occurs, the starved-air combustor could either be shut down tempor¬ arily, the feed conveyor means 105 accelerated to supply greater volumes of fuel per unit time, or an

_ "_! i< il.A O PI 5 ?N_ATl - 27 -

appropriate alarm actuated to indicate that the starved-air combustor is operating at less than optimum capacity because insufficient fuel is being provided or, alternatively, any combination of these actions could be taken.

Figure 6 illustrated an embodiment of the com¬ bustion chamber 115 of the starved-air combustor system. As shown in Figure 6, the starved-air combustor system includes means for conveying the fuel through a com- bustion chamber at a variable rate. As embodied herein, the conveying means comprises screw conveyor or. auger 121 extending the length of the combustion chamber and being rotated by the auger motor and speed control 251. The auger motor and speed control 251 is capable of rotating the auger at rates of, for example, from .3 to 1.0 rpm under manual control

The fuel bed 253 is of its greatest depth at the inlet end 113 of the combustion chamber and is conveyed from the inlet end 113 to.the outlet end 117. During its travel through the combustion chamber, the fuel bed 253 gradually decreases in size as its contents are combusted and combustion gases evolved. The auger 121 is positioned off-center within the combustion chamber 115 in order to provide a gas mixing zone above the fuel bed 253. In the mixing zone, the evolved

O Pl gases are mixed with overfire air supplied by air supply means 125 (Figure 1) for further combustion. Conduits 123 supply underfire air to the combustion chamber beneath the bed of fuel 253 such that the underfire air, when at an elevated temperature, contributes to the ignition of the fuel in the fuel bed 253 by heating and drying the fuel.

The starved-air combustor system further comprises means for supplying a variable airflow to the combustion chamber 115. The physical structure for accomplishing this is illustrated in Figure 6 and, as embodied therein, the walls of the combustion chamber 115 include underfire air plenums 255 each coupled to one of the air suppply conduits 123. Air passes from the plenums 255 through pipes 256 (Figure 7) embedded in a refractory layer 257 and terminating in a plurality of ports or injectors 259 communicating with the com¬ bustion chamber 115 beneath the bed of fuel 253. The plenums 255 are separated from each other by stops or gaskets 261 to define multiple underfire combustion zones Zl, Z2, and Z3.

Similarly, overfire air is supplied to the combustion chamber 115 by means of plenums 263 (Figure 7) communicating with the overfire air supply means 125. The plenums are divided into a plurality of zones (in this case, three) and the air within each zone is injected into the combustion chamber 115 through ports or injectors 267 which extend through the layer of refractory material 257 lining the interior surface of the combustion chamber 115. As illustrated in Figure 6, the zones of the overfire iar and the zones of the underfire air may coincide and form combustion zones Zl, Z2, and Z3. A temperature sensor 271 (Figure- 7) is inserted through the refractory material 257 into the gas phase flame areas of each of the temperature zones to sense the temperature in the overfire area of the zone.

With reference t.o Figure- 7, the rotation of the auger (not shown) within combustion chamber 115 results in the fuel bed 253 being oriented as shown. Underfire air from supply 123 is supplied to plenum 255 from which it is ignited beneath the fuel bed 253 by means of pipes 256 terminating in injectors 259. The pipes 256 are embedded in the refractory lining 257 of the combustion chamber 115.

Underfire air recived by one of the plenums 263 from supply 125 is injected above the fuel bed 253 through ports 267. The temperature sensor 271 for one of the overfire air zones is provided above the

OΛ-PI fuel bed 253 and it is contemplated that a thermocouple capable of withstanding the high combustion chamber temperatures could be employed as sensor 271.

The starved-air combustor of the instant invention further comprises means for controlling the rate of the fuel conveying means or auger 121 and the volume of the airflow supplied into the zones Zl, Z2, and Z3 to increase or to decrease the quantity of heat produced in the form of hot combustion gases. The means for controlling the rate of the conveying means and the airflow supplied by the supplying means to increase the quantity of hot, combustion gases (heat) produced by the system responsive to an increase in the heat demand and to decrease the quantity of hot, combustion gases (heat) produced by the system responsive to a decrease in the heat demand, as embodied herein, is illustrated in Figure 8 as comprising an underfire air system, an overfire air system, and an afterburner air system. The afterburner air system is not a feature of the present invention and will not be discussed in detail.

As illustrated in Figure 8, the combustor 115 receives underfire air in three zones: primary (p) corresponding to Zone 1, secondary (s) corresponding

OMPI ^SNAT1 - 31 -

to Zone 2, and tertiary (t) corresponding to Zone 3. Controllers 301, 303, and 305 control the injection of underfire air from air supply line 307 into the p, s, and t zones. These three zones are set to initial values to apportion the air supplied by the air supply line 307 to the previously discussed supplier 123, but as explained above, if there is a change in heat demand then the speed of the auger will be changed necessitating corresponding changes in the supply of air to the p, s, and t zones by the controllers 301, 303, and 305 respectively. The change in auger speed as determined by the auger motor and speed control 251 (Figure 6) are supplied to multiplier 309 along with a signal indicating the weight of each batch of fuel supplied to the com- bustion chamber. This weight is represented by the quantity M and could, for example, be an output of the previosuly explained weigh cells 223. The output of multiplier 309 is a signal K which is supplied as an input to each of the controllers 301, 303, and 305 to alter the airflow into their associated underfire zones.

The overfire air system is, as previously explained, temperature dependent and thus the signal T is'an output of temperature sensor 271 (Figure 7) which monitors

BUKt_AT- O PI b _ WJPO S . the temperature in combustion zone Zl or the primary zone. The controller 313 compares the instantaneous temperature within the primary zone to a desired temperature and properly alters the airflow from air supply line 319 to the primary zone in the combustion chamber. Similarly, controllers 315, and 317 receive the temperature indica¬ tions T and T , respectively, from the temperature sensors to 271 in their associated combustion zones. Any change in the temperatures in their associated zone from the desired temperature will cause the controllers 315 and 317 to alter the airflow from air supply line 319 into the secondary and tertiary zones in the manner illustrated in Figure 2.

Figure 9 illustrates, in greater detail, the circuit for controlling the flow of fuel into the combustor 115. The mass of each fuel batch or charge is supplied to the multiplier 309 where it is multiplied by the change in auger rotation rate W . The output of the multiplier 309 is the change in fuel feed π which must be accommodated by the underfire air control system.

Figure 10 illustrates, in greater detail, the under¬ fire air control system. The controllers 301, 303, and 305 are initially set with a constant indicating the air distribution into the primary, secondary, and tertiary zones. The controllers 301, 303, and 305 each receive, as an input, the change in fuel flow through the com- bustion chamber and each generate output signals to adjust accordingly the airflow into the primary, secondary, and tertiary zones. As an example, the output of controller 301 is a signal corresponding to the new airflow into the primary zones of the combustion chamber. This is supplied to an adder 321 which receives as its other input the output of flow trans¬ mitter^' 323 indicating the amount of air currently flowing into the primary zone from the air suppl line 307. If there is a difference between the newly deter¬ mined amount and the current airflow into the primary zone then a signal representing that difference is supplied to. valve control circuit 325 to open or close a flow control device 327, e.g., a. valve. The output of the flow control device 327 is the air supplied to the primary zone (Zl in Figure 6) through the appro¬ priate air conduit 123 (Figure 6) . If the heat demand is increased, then the flow control device 327 will cause a greater airflow into the primary zone of the combustion chamber. Conversely, if the heat demand is decreased, the output of the adder^' 321 will be a negative difference and will cause valve control circuit 325 to control the flow control device 327 in a manner to restrict airflow into the primary zone of the combustion chamber. Figure 10 also illustrates

' OMPI

^ πSS^' the circuits required to control airflow into the secondary and tertiary zones in the combustion chamber but these will not be explained since they operate in the same manner as the circuit for controlling airflow into the primary zone.

Figure 11 illustrates an embodiment of a circuit for controlling the flow of overfire air into the primary, secondary, and tertiary zones. Initially, the controllers^' 313, 315, and 317, are set to values corresponding to the desired temperature in the primary, secondary, and tertiary zones, respectively, within the combustion chamber. The controller 313, as explained above, receives a signal T corresponding to the actual temperature within the primary zone and will generate an appropriate output signal representing the difference between the desired primary zone temperature and the actual primary zone temperature. This is supplied to the adder circuit 329 which receives as another input a signal corresponding to the current flow of overfire air into the primary zone. The difference between the two signals is determined and passed to valve control circuit 333 which appropriately opens or closes the flow control device, such as valve 335, to either increase or to decrease the temperature within the primary zone. This will cause a change in the temperature in the primary

OMPl ^,po ^NATl zone which will be supplied to the controller 313. When the proper temperature has been reached in the primary zone, then the adding circuit 329 will not longer signal the valve control circuit 333 to adjust the flow control device 335.

Any number of embodiments for flow transmitters, summation circuits, valve control devices and flow control devices are known in the art and, one of ordinary skill in that art may select such devices according to the above teachings.

This application is also concerned with an apparatus for selectively exhausting combustion gases from the combustion chamber through a first exhaust port located proximate the inlet end of the combustion chamber and a second exhaust port located proximate the outlet end of the combustion chamber. When the combustion gases are exhausted from the port located near the outlet end of the combustion chamber, the starved-air combustor is said to be operating in the co-current mode meaning that the exhaust gases are traveling in the same direction as the fuel within the combustion chamber. Conversely, when the combustion gases are exhausted through the exhaust port located near the inlet end of the combustion chamber, the starved-air combustor is said to be operating in the countercurrent mode meaning that the exhaust gases are traveling against the direction of flow of fuel through the combustion chamber.

It is possible, by controlling the co-current and countercurrent exhaustion rates to both position and stabilize the flame front in the fuel bed^' within the combustion chamber 115. Full co-current exhaustion tends to establish the flame front closer to the outlet end 117 of the combustor 115 wheras full countercurrent exhaustion tends to establish the flame front proximate the inlet end 113. The ability to establish and stabilize the flame front is an important feature of the present invention because it is desirable to tailor the length of the flame bed to the type and condition of the fuel being combusted. For example, if the fuel is quite wet, it may be desirable to permit a greater drying and heating distance for the fuel to travel before reaching the flame front.

As illustrated in the previously cited Tyer et al patent, the combustion chamber is supplied with overfire air, i.e., air injected into the combustion chamber above the fuel, and underfire air which is air injected into the combustion chamber from beneath the fuel bed in the combustion chamber. The progression of events trans¬ piring between the entry of the fuel air at the inlet end of the combustion chamber, the travel of the fuel through the combustion chamber while it is being combusted, and the evolution of combustion gases and combustion residue is well-known in the art of starved- air combustors. Generally, the water and the fuel are first evaporated and then, before the fuel reaches the ignition point, the cellulosic, plastic, and rubber materials begin to decompose as their temperatures increase and evolve volatile gases including heavy tars and acids. After the volatile gases are evolved, carbon particles begin to be produced and the presence of the overfire air causes the carbon particles and the tars to be combusted in the combustion chamber.

These processes all occur as the fuel travels from the inlet end of the combustion chamber toward the outlet end of the combustion chamber and, therefore, the gases near the inlet end of the combustion chamber contain a higher concentration of water, tars, and acids since they have not yet passed over the entire flame bed within the combustion chamber. Conversely, the gases present near the outlet end of the combustion chamber had a longer period of time to be mixed with the overfire air to combust further any combustible materials therein. As a result, the combustion gases near the outlet end of the combustion chamber include an increased concentration of carbon monoxide, carbon dioxide, and hydrogen and a decreased concentration of unreacted fuel chemical fragements,

Similarly, combustion gases exhausted near the inlet end of the combustion chamber will be at a lower temperature

IJU^KEATT

OMP1 than combustion gases exhausted near the outlet end of the combustion chamber. This is because much of the heat in the combustion gases is absorbed by the commonly cool and wet fuel which is received at the inlet end. The passage of the hot exhaust gases through the fuel bed causes the fuel to evolve tars, acids, and water vapor and thus contain a higher concentration of combustible materials and other pollutants.

Gases that are exhausted near the outlet end of the combustion chamber will be significantly higher in temperature since the last process to which they are subject is mixture with overfire air and further combustion of the combustible materials.

The present invention is directed to a means for selectively enabling the starved-air combustor to operate in a full co-current mode, a full countercurrent mode, or proportionally in both a co-current and countercurrent mode.

As illustrated in Figure 12, the starved-air com¬ bustor comprises a combustion chamber formed, for example, from a cylindrical combustion chamber 115 having an inlet end 113 and an outlet end 117. Within the combustion chamber 115, a bed of fuel 205 is conveyed by a conveying means from the inlet end toward the outlet end. As embodied herein, the conveying means comprises a rotatable auger 121 extending eccentrically through the cylindrical combustion chamber 115 to provide a space at the top for the mixing of overfire air 123 and combination gases. A flame front 207 illustrates an example of where the ignition point of the fuel bed 205 is within the combustion chamber 115. The fuel from the flame front 207 toward the outlet end 117 is at. a temperature at or above the ignition point of the fuel in the bed 205.

The starved-air combustor further includes first means communicating with the combustion chamber proximate to the inlet end 113 for exhausting hot, combustion gases evolved from the combustion of the fuel within the combustion chamber 115. The starved-air combustor further includes second means communicating with the combustion chamber 115 proximate to the outlet end 117 for also exhausting hot combustion gases evolved from the com¬ bustion of the fuel in the combustion chamber 115.

As embodied herein, the first means comprises a first conduit 201 coupled at one end to the interior of the combustion chamber 115 and coupled at its other end to the duct 119 which leads to the afterburner 129. The second means comprises a second conduit 209 con¬ stituting the lower portion of the duct 119 which communicates with the interior of the combustion chamner 115 near the outlet end 117. The starved-air combustor further includes means for controlling the exhausting of the evolved gases from the combustion chamber to exhaust selectively the evolved gases entirely through the conduit 209 or proportionally through the conduit 201 and the conduit 209. As embodied herein, the controlling means com¬ prises a manually-positionable damper 203 located at the intersection of the conduit 201 and the duct 119 and having a length sufficient to seal completely the inter- section of the conduit 201 and the duct 119 when the damper 203 is positioned in a vertical position and to restrict partially the communication of conduit 209 with the duct 119 when the damper is rotated in the horizontal position. As illustrated in Figure 12, the damper has been positioned to permit exhaustion of the gases through both the conduit 201 and the conduit 209 to enable the selective balancing of co-current and countercurrent flow of combustion gases in the combustion chamber 115.

Figure 13 illustrates an alternate embodiment of the first and second exhausting means and the controlling means. As illustrated in Figure 13, the first means and the second means comprise first and second conduits 211 and 213, respectively, which interest at the location of the damper 215. The damper s selectively positionable to control the flow of exhaust gases through the conduit 211 or the conduit 213 into the exhaust gas collector or duct 119. As was the case with the embodiment illustrated in Figure 12, the overfire air and underfire air supplied to the combustion chamber together with the draft of the fan 143 (Figure 1) causes an inherent flow of combustion gases from the combusting fuel in the fuel bed 205 out of the combustion chamber 115.

It will be further apparent to those skilled in the art, that various modifications and variations can be made to the exhaust gas flow control means of the starved-air combustor without departing from the scope or spirit of the invention and it is intended that the present invention cover the modifications and yariations of the system provided that they come within the scope of the appended claims and their equivalents.

Claims

1. A starved-air combustor comprising: a combustion chamber having an inlet end for receiving fuel,, said combustion for combusting said fuel to produce hot combustion gases and combustion 5 residue, said combustion chamber also having an outlet end for discharging said combustion residue, and an outlet port for discharging said hot combustion gases; means in said combustion chamber for conveying said received fuel from said inlet end toward said outlet end; 10 and means for selectively feeding predetermined weights of said fuel into said inlet end of said combustion chamber.

2. A starved-air combustor according to claim 1 wherein __ said conveying means comprises a rotatable screw conveyor comprising a cylindrical axle extending along the length of said chamber and a spiral flight concentrically connecte to said axle, said spiral flight and said cylindrical axle defining a plurality of spaces within said combustion 20 chamber, a first of said spaces- located adjacent said inlet end of said combustion chamber.

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3. A starved-air combustor according to claim 2 wherein said feeding means comprises: means for receiving and containing said fuel; means for weighing said received and contained fuel; and means for selectively discharging said received and contained fuel responsive to the positioning of said screw conveyor into a predetermined orientation and to the accumulation of a preselected weight of said fuel in said receiving and containing means.

4. A starved-air combustor comprising: a cylindrical combustion chamber having an inlet end for receiving fuel and an outlet end for discharging com¬ bustion gases and combustion residue, said combustion chamber for combusting said received fuel to produce said combustion gases and said combustion residue; a rotatatable screw conveyor in said combustion chamber for conveying said fuel from said inlet end toward said outlet end, said screw conveyor comprising a cylindrical axle extending along the length of said # cham*ber and a spiral flight concentrically connected to said axle, said spiral flight and said cylindrical axle defining in said combustion chamber a plurality of spaces around said cylindrical axle, a first of said spaces located beneath said inlet end of said cylindrical combustion chamber; and means for selectively feeding predetermined weights of said fuel into said first space through said inlet end.

5. A starved-air combustor according to claim 4 further including means for sensing the orientation of said screw conveyor in said cylindrical combustion chamber and wherein said feeding of said fuel into said first space is responsiv to the rotation of said screw conveyor into a predetermined orientation in said combustion chamber.

6. A starved-air combustor according to claim 3 or claim 5 wherein a plurality of said predetermined orienta¬ tions are included in a single complete revolution of said screw conveyor.

7. A starved-air combustor according to the claim 6 wherein said feeding means comprises: means for receiving and for contained said fuel; means for weighing said received and contained fuel; and means for selectively discharging said received and contained fuel responsive to the positioning of said screw conveyor in said predetermined orientation and to the accumulation of a preselected weight of said fuel in said receiving and containing means.

8. A starved-air combustor according to the claim 7 further including a stationary support, wherein said

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OMPI weighing means comprises at least one weigh cell, and wherein said receiving and containing means comprises a fuel receptable connected to said support by said at least one weigh cell such that such weigh cell indicates the weight of said fuel in said receptacle.

9. A starved-air combustor according to claim 6 wherein said discharing means comprises a release valve connected to one end of said fuel receptacle to enable in a first valve position the discharging of said fuel from said fuel receptacle into said inlet end of said combustion chamber and to enable in a second valve position the accumulation of said fuel in said fuel receptacle.

10. A starved-air combustor according to claim 9 further including a cover valve connected to the other end of said fuel receptacle to enable in a first valve position the hermetic sealing of said fuel receptacle from ambient air and to prevent the accumulation of fuel in said fuel receptacle, and to enable in a second valve position said fuel to pass into said fuel receptacle.

11. A starved-air combustor according to claim 10 further including: a storage bin for storing said fuel; a first fuel feed conveyor for supplying fuel from said storage bin; a second fuel feed conveyor for supplying fuel to said storage bin; and c a fuel path controller for receiving said fuel supplied by said first fuel feed conveyor and in a first mode for supplying the received fuel to said fuel recepta¬ cle and in a second mode responsive to a predetermined amount of fuel being accumulated in said fuel receptacle

10 for supplying said fuel received from said first fuel feed conveyor to said second fuel feed conveyor.

12. A starved-air combustor according to claim 11 wherein said fuel path controller comprises a rotatable fuel feed valve having a. first position associated with 5 said first mode and a second position associated with said second mode.

13. A starved-air combustor system for producing a variable quantity of heat responsive to a demand for said heat, said combustor system comprising: 0 a combustion chamber having an inlet end for receiving said fuel, said combustion chamber for com¬ busting said fuel to produce a quantity of heat related to the rate of combustion; means for conveying said fuel through said combustion 5 chamber at a variable rate; means for supplying a variable quantity of air to said combustion chamber; and means for controlling the rate of said conveying means and the quantity of fuel supplied by said supplying means to increase the quantity of heat produced by said system responsive to an increase in said heat demand and to increase the quantity of heat produced by said system responsive to a decrease in said heat demand.

14. A starved-air combustor system according to claim 13 further including.means for feeding selectable weights of fuel into said inlet end of said conveyor in a batch mode.

15. A starved-air combustor system according to claim 14 wherein said combustor chamber is cylindrical and includes an outlet end for discharging combustion residue produced by said combustion and wherein said conveying means comprises: a rotatable screw conveyor in said combustion chamber for conveying said constant weight batches of fuel from said inlet end of said combustion chamber to said outlet end of said combustion chamber and for conveying said combustion residue toward said outlet end of said com¬ bustion chamber; and means for rotating said rotatable screw conveyor at a variable rate directly proportional to said heat demand to control the flow of said fuel through said combustion chamber.

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OMPl WIPO ^"y_>

16. A starved-air combustor system according to claim 15 wherein said combustion chamber is divided into a plurality of combustion zones and wherein said air supplying means comprises: a set of overfire air injectors associated with each of said plurality of zones for injecting air above the fuel in said associated combustion chamber zone; a temperature sensor associated with each of said combustion chamber zones for determining the temperature in said associated combustion chamber zone; and an overfire air controller for controlling the quantity of air injected into each of said combustor zones by said associated set of overfire air injectors in response to the temperature in said associated zone as determined by said associated temperature sensor.

17. A starved-air combustor system according to claim 16 wherein said air supplying means further comprises a set of underfire air injector associated with each said combustion chamber zone for injecting air into said associated zone beneath said fuel in said associated zone; and an underfire air controller for controlling the quantity of air injected into each of said associated zones by said set of underfire air injectors in direct proportion to said flow of fuel through said combustion chamber.

18. A starved-air combustor system for producing a variable quantity of heat responsive to a demand for said heat, said combustor system comprising: a combustion chamber having an inlet end for receiving fuel, said combustion chamber for combusting said received fuel to produce hot combustion gases and combustion residue related to the rate of combustion of said fuel, said combustion chamber further having an outlet end for discharging said hot combustion gases, said combustion chamber being divided into a plurality of combustion zones serially spaced from said inlet end to said outlet end; means for conveying said fuel in said combustion chamber from said inlet end toward outlet end at a ^• variable rate; means for feeding selectable weights of said fuel into said inlet end in a batch mode; a plurality of air injector means singularly associated with each of said combustion zones for independently supplying to said associated combustion zone a variable airflow; and means for controlling the rate of said conveying means and the airflow of each of said injector means to increase the quantity of heat produced by said system responsive to an increase in said heat demand and to decrease the quantity of heat produced by said system responsive to a decrease in said demand.

19. A starved-air combustor system according to claim 18 wherein each of said air injector means comprises: a set of overfire air injectors for injecting air above the fuel in said associated combustion zone; a temperature sensor associated with each of said combustion zones for determining the temperature in said associated combustion zone; and an overfire air controller for controlling the airflow of said set of overfire air injectors in an inverse relationship to determine changes in the tem¬ perature in said associated combustion zone.

20. A starved-air combustor system according to claim 19 wherein each of said plurality of air injector means further includes: a set of underfire air injectors for injecting air into said associated combustion zone beneath the fuel in said associated combustion zone; and an underfire air controller for controlling the air flow of said set of underfire air injectors in direct proportion to the rate of conveyance of said fuel in said combustion chamber.

21. A starved-air combustor comprising: a combustion chamber having an inlet end for receiving fuel, said combustion chamber for combusting

OMPI . WPO said received fuel to produce hot, combustion gases and combustion residue, said combustion chamber including an outlet end for discharging said combustion chamber including an outlet end for discharging said combustion residue; means for conveying said fuel through said com¬ bustion chamber from said inlet end toward said outlet end; first means communicating with said combustion chamber proximate said inlet end of said combustion chamber for exhausting said hot, combustion gases from said combustion chamber; second means communicating with said combustion chamber proximate said outlet end of said combustion chamber for exhausting said hot, combustion gases from said combustion chamber; and means for controlling the exhausting of said com¬ bustion gases from said combustion chamber to exhaust selectively said evolved gases entirely through said second means or proportionately through said first means and said second means.

22. A starved-air combustor according to claim 21 further including an exhaust gas collector and wherein said first means comprises a first conduit communicating with said exhaust gas collector and said second means comprises a second conduit communicating with said exhaust gas collector.

23. A starved-air combustor according to claim 22 wherein said controlling means comprises a rotatable damper in said exhaust gas collector positionable in a first orientation to completely block the exhaustion of said combustion gases through said first conduit and to permit the complete exhaustion of said com¬ bustion gases through said second conduit, a second orientation to partially block the exhaustion of said combustion gases through said second conduit and to permit the partial exhaustion of said combustion gases through said first conduit, and a plurality of third orientations intermediate said first orientation and said second orientation, said third orientations for permitting the proportional exhaustion of said com- bustion gases from said combustion chamber through siad first conduit and said second conduit.

24. A starved-air combustor according to claim 23 wherein said damper is manually positionable.

25. A starved-air combustor comprising: a cylindrical combustion chamber having an inlet end for receiving fuel, said combustion chamber for combusting said received fuel to produce combustion gases and combustion residue, said cylindrical com¬ bustion chamber further including an outlet end for discharging said combustion residue;

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OMPI ^SNATlO a screw conveyor in said combustion chamber for conveying said fuel from said inlet end toward said outlet end, said screw conveyor for also conveying said residue toward said outlet end; first means communicating with said combustion chamber proximate to said inlet end for exhausting said hot combustion gases from said combustion chamber; second means communicating with said combustion chamber proximate to said outlet end for exhausting said hot combustion gases from said combustion chamber; means for selectively directing said hot combustion gases to (1) pass over said fuel to heat and to dry said fuel and to be exhausted from said combustion chamber throug^"h said first means, (2) to pass over said combustion residue to heat and to further combust said combustion residue to be exhausted from said combustion chamber through said second means, and (3) to proportion said combustion gases into a first volume and a second volume, said first volume passing over said fuel and being exhausted through said first means and said second volume passing over said residue and being exhausted through said second means.

26. A starved-air combustor according to claim 25 further including an exhaust gas collector and wherein said first means comprises a first conduit communicating with said exhaust gas collector and said second means comprises a second conduit communicating with said exhaust gas collector.

27. A starved-air combustor according to claim 26 wherein said means for selectively directing hot combustion gases comprises a damper said exhaust gas collector positionable in a first orientation to block completely the exhaustion of said hot combustion gases through said first conduit and to permit said combustion gases to be completely exahusted through said second conduit, and a plurality of second orientations to permit the proportional exhaustion of said combustion gases through said first conduit and said second conduit.