Ignition and flame stabilization system for coal-air furnace

Abstract

A steam generating furnace with multiple coal-air burners has a corresponding number of supplemental oxygen-fuel burners, each mounted centrally of a coal-air burner respectively, for directing an oxy-fuel flame into a combustible mixture of powdered coal and air; the oxy-fuel burners are controlled to operate at a standby low-head setting during normal coal-air combustion, and are automatically controlled by flame sensing means during transitory coal-air flame deficiency and instability to operate at maximum heat for maintaining continuous furnace combustion. A manual over-ride switch operates the oxy-fuel burners at maximum heat such as for furnace starting, etc.

Description

BACKGROUND OF THE INVENTION

Central station steam generating boilers wherein multiple burners are arranged in banks or tiers to project a mixture of air and solid fuel such as powdered coal, into the furnace combustion chamber have long been in common use. In furnaces of this type, the powdered coal and pressurized air are blown into the combustion chamber as a combustible mixture and ignited; the resulting coal-air flames extend a material distance into the chamber, the walls of which unlike those of a kiln, are kept comparatively cool by heat transfer to water tubes lining the chamber. Thus, the chamber does not tend to "run hot" for ensuring reignition or stable combustion in case of momentary flame dieout or instability.

Accordingly, transient conditions such as brief clogging of the coal feeder, pulverizer discontinuity, decrease of blower air, etc. that reduce or briefly cut off the coal or air supply tend to cause flame instability and possible flame-out, which in turn can result in shut-down of the furnace with loss of steam generating capacity, production time, etc. Restarting the furnace after flame-out and shut-down is complicated by several factors including the need for first cleaning the combustion chamber of all gases, combustion products and unburned mixtures before reignition. This not only consumes valuable production time but also results in objectionable stack emissions tending to foul the plant pollution control equipment.

Although various flame sensing and ignition schemes have been proposed for improving furnace performance and safety, such as high voltage electrodes for igniting a coal-air mixture, stand-by pilot burners interlocked with the main burners, flame-out detectors that signal for furnace shut-down, etc. such proposals have not been altogether satisfactory. In general, they either fail to provide for reignition at flame-out, or fail to provide sufficient heat for continuous furnace operation under severe transient conditions that ordinarily cause flame instability or flame-out. In some plants, the old established practice of restarting the furnace with an oil-soaked rag torch is still in use because of complexities and possible explosion hazards in prior art automatic ignition systems.

In brief, prior art furnace combustion control systems as presently understood do not solve economically and efficiently the main problem of furnace down-time due to flame instability, nor the related problem of stack emissions incident to furnace ignition and reignition. The present invention is therefore concerned with an improved furnace ignition and flame stabilization system that overcomes the problems described above.

SUMMARY OF THE INVENTION

In accordance with the invention, the main coal-air burners of a central station steam generating furnace for example, are each equipped with a supplemental rocket type oxygen-fuel burner that is adapted to project on demand intense "oxy-fuel" flames into a coal-air mixture to ignite the mixture as it is projected into the furnace combustion chamber. Thus, the oxy-fuel flame serves a dual purpose, i.e., (1) to ignite the combustible mixture for fast, clean furnace starting, and (2) to stabilize on demand the coal-air flames should brief fluctuations or interruptions in the coal or air supplies occur during furnace operation. For performing the latter function, the oxy-fuel burners are initially set for standby operation at economical low-heat for normal furnace operation. In the event of flame instability, i.e., potential flame-out, as evidenced by material pulling back or shortening of the coal-air flames, such as due to feeder clogging, etc., narrow-angle flame detectors or radiation sensors automatically cause the oxy-fuel burners to fire at high-heat, thereby projecting intense, high velocity, oxy-fuel flames into the weakening coal-air flames. Thus, the required high temperature for maintaining furnace heat and for igniting any unburned coal-air mixture is provided; accordingly, the furnace resumes normal operation without interruption and the oxy-fuel burners automatically resume economical standby operation when the transient condition has cleared.

A principal object of the invention therefore is an improved ignition and combustion control system for solid fuel and air furnaces as described above, wherein the main burner flames upon detected instability are automatically supplemented for sustained heat generation by high velocity, high-heat oxygen-fuel flames.

Another object of the invention is an improved system of the character described above, wherein oxy-fuel burners at corresponding main burners are automatically operated, either at economical standby heat or at maximum heat settings, according to sensed stability or instability respectively, of the main furnace flames.

A related object of the invention is an improved furnace starting control system wherein a rocket type oxy-fuel burner is integrally related to a main coal-air burner and projects on demand intense oxy-fuel flames into the combustible mixture from the main burner for quickly and safely starting the furnace and for stabilizing furnace combustion with minimal stack emission and pollution.

Other objects, features and advantages will appear from the following description with reference to the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a partial plan view in section of the combustion chamber of a steam generating furnace showing one row of multiple coal-air burners with supplemental oxy-fuel burners according to the invention;

FIG. 2 is an enlarged sectional view of a combined coal-air and oxy-fuel burner unit of FIG. 1;

FIG. 3 is a generally schematic illustration of the oxygen and natural gas distribution and control systems for the oxy-fuel burners of FIGS. 1 and 2, and

FIG. 4 is a diagram of network circuitry illustrating the valve controls for the oxy-fuel distribution system of FIG. 3.

DESCRIPTION OF PREFERRED EMBODIMENT

As generally shown in FIG. 1, a furnace 10 has an array of burners 12 that are mounted in the front wall 14 of the furnace for projecting fuel-air flames 16 into the combustion chamber 30. The burners in the present instance are arranged in two tiers or banks of four burners each (the lower tier not showing), and are of known type using a combustible mixture of solid fuel and air. Details of the fuel-air supply to the main burners are not required for an understanding of this invention, and are not shown in FIG. 1. Where the main fuel is coal, it is pulverized and conveyed as a powder to the respective burners where it is ejected, mixed with air at normal blower pressure and blown into the furnace. The resulting combustible coal-air mixture upon ignition maintains the furnace flames. Although a "coal-air" mixture is referred to herein for brevity, it will be understood that any suitable solid fuel such as lignite, solid waste, etc. may also be used in practising the invention.

For greatly facilitating the furnace "light off" or ignition process, and for stabilizing as required the main furnace flames, the coal-air burners have incorporated therein means for injecting on demand a very hot, high velocity flame into the center of the coal-air mixture. To this end, the main cylinder-like coal-air burners 12 are open at the firing ends 18 respectively, for ejecting the coal-air mixture into the furnace, and each main burner has a rocket type oxy-fuel burner 20 mounted within and concentrically of the longitudinal axis thereof. The oxy-fuel rocket burners 20 are each supplied with commercially pure oxygen and a fuel gas such as natural gas, by mains 22 and 24 respectively, and the mains in turn feed branch lines 22' and 24' leading to the individual burners. The oxygen and gas connections to the rocket burners are controlled and regulated by means hereinafter described for operating these burners at an economical standby low-heat setting when the coal-air combustion flames are normal and stable. For furnace starting, and in case of flame instability during furnace operation, the rocket burners on demand are quickly brought up to a maximum high heat output.

Referring to FIG. 2, each coal-air burner 12 comprises essentially a plurality of spaced concentric cylinders and shells including a cylinder 11 within the rocket burner 20 is centrally mounted on supporting spiders 19 or the like, a larger concentrically spaced cylinder 13, and outer positioned shells 15 and 17. An annular cooling space 46 is formed between the cylinder 11 and rocket burner 20, and the shell 15 forms with the cylinder 13 an annular passage 35 through which powdered coal is fed to the furnace from a circumferential extension passage 15'. The extension 15' is connected to conventional coal conveyor and pulverizing means, not shown. The outer shell 17 forms with the shell 15 an annular passage 37 for blower air that is forced into the extension passage at 17'. The coal and air passages 35 and 37 converge somewhat at the burner annular exhaust 18 so that the surrounding blower air stream 37' is directed at an angle into the coal particles 35' for good mixing and for projecting the resulting mixture into the furnace. At exhaust, the coal particles carried by the low pressure blower air surround the rocket exhaust in an annular stream and therefore are drawn into the low pressure center by the high velocity rocket flames; thus maximum contact is made between the coal particles and hot oxy-fuel flames. Normally, the axially positioned rocket burner 20 has simply standby function, i.e., low-heat anchor flames originating within the burner, keep the burner in readiness for maximum heat output on demand.

The co-axial arrangement of the main and rocket burner shown in the preferred embodiment can be modified for offset or tangential rocket firing into the coal-air mixture, the main requirement being that the oxy-fuel flames be brought into good heating contact with the coal-air mixture.

In the furnace arrangement of FIG. 1 which shows only the upper tier of coal-air burners, all the burners are represented as normally projecting the coal-air flames 16 into the combustion chamber 30 for a distance such as d.sub.1 which would correspond to normal coal-air combustion for the current boiler load of the furnace. Heat is transferred from the flames to watersteam pipes 32 that line the walls of the chamber. Suitable insulating material 44 separates the pipes from the walls. Thus, the combustion chamber tends to run "cool" due to the large amounts of heat absorbed by the water tubes for steam generation; combustion of the coal-air mixture therefore can be properly maintained only by continuously supplying coal and air at the required rate. When this rate is significantly decreased, as where the flames withdraw to a distance such as d.sub.2 for example, the rate of heat transfer from the flames becomes excessive, i.e., the flames tend to cool and become unstable. During such instability, flame-out may occur, ordinarily requiring shut-down of the furnace as described above.

For avoiding shut-down under such conditions, the unstable or short-flame condition is detected by small-angle flame radiation sensors or the like, 34 and 36 which are sensitive to the visible flame and produce a signal for quickly bringing the rocket burners 20 up to maximum heat. At this setting, the long, centrally projected oxy-fuel flames re-establish proper combustion conditions at the powdered coal exhasut conduit 35. The flame detectors 34 and 36 which can conveniently be mounted in the sidewall inspection doors 40 and 42 opposite the upper and lower burner tiers respectively, have narrow-angle sensing as indicated at angle a for the upper tier detector 34. The detector at each level is positioned so that normally the full combustion flames from the corresponding tier burners are positively sensed in the range beyond distance d.sub.2 to keep the rocket burner control at the standby low-heat setting; retraction of the coal-air flames to the unstable limit represented, for example, by distance d.sub.2 is negatively sensed to actuate controls for firing the rocket burner at maximum high heat. Optimum positioning of the flame detectors depends on the furnace characteristics; that is, it may be preferable to focus the detector on the outer edges of the normal coal-air flames 30 so that any shortening of the flames produces a demand signal for maximum rocket heat.

The so-called "rocket" oxygen-fuel burner used in the invention is preferably of the type disclosed in U.S. Pat. Nos. 3,092,166 and 3,135,626 which are assigned to the same assignee as the present invention. This type burner has characteristics such as large turn-down ratio for example, whereby high temperature, oxy-fuel flames can be supplied over a minimum-to-maximum heat range within a short time. This feature is especially advantageous in practicing the invention, both for clean, fast furnace starting and for supplementing the coal-air flames as needed.

The burner has inherent stability which is achieved by permanent anchor flames; this ensures reliable and stable flame output notwithstanding wide variations of fuel supply within a comparatively brief time. The rocket burner flame is projected at velocities up to the speed of sound, and at very high temperatures. By valve adjustments of the oxygen and fuel gas lines, the burner flame length can be varied as required from about 1 foot to as much as 10 feet, and the heat output can be varied from about 20,000 Btu/hr. to approximately 10 million Btu/hr. for a comparatively small size burner; also valve adjustment serves to control the flame temperature and its oxidizing effect by varying the oxygen-fuel ratio. Thus, the ratio control can be varied if desired, to provide excess amounts of oxygen in situations such as temporary reduction of furnace blower air where additional oxygen is required.

At the low-heat standby setting, the rocket burner 20 can be economically operated at about 50 scf/hr. natural gas (NG) and 75 scf/hr. oxygen (O.sub.2); at the high firing rate (which is ordinarily used only for starting and during transient flame instability) the burner can utilize approximately 1000 scf/hr. NG and 1500 scf/hr. O.sub.2 for achieving very high heat outputs. Although the rocket burner is described herein as using NG fuel, it can where preferred use a suitable liquid fuel, or a combination of liquid and gas fuels such as described in U.S. Pat. No. 3,092,166 above.

As shown in FIG. 2, the O.sub.2 and NG are fed through the branch supply lines 22' and 24' respectively, to a housing 21 at the breech end of the rocket barrel 20'. The barrel has a water jacket through which cooling water is circulated from supply line 23 to a drain line 23'. The rocket barrel near its firing end has a partition member 25 with passages for the NG and O.sub.2 respectively. The NG supply line 24' connects with a tube 28 that extends centrally of the rocket barrel to a stream divider 31, from which the NG flows in separate tubes 33 through the partition 25 as jets to the rocket exhaust. The O.sub.2 supply line 22' connects with the annular chamber 26 in the barrel, from which O.sub.2 flows through multiple passages 26' in the partition 25 to mix with the surrounding NG jets at the rocket exhaust 38.

Separate electrical starting means for each rocket burner comprises a high voltage ignition transformer 27, FIG. 2, that is connected by lead 27' and insulated conductor 29 to an electrode 29', the latter forming a spark gap at the burner exhaust with the rocket barrel functioning as return conductor. When ignition is turned on, a 120 volt primary winding (not shown) is energized to induce in the transformer secondary 6000 volts for high voltage sparking. The transformer circuitry, which can be energized at ignition through a standard relay (as indicated in FIG. 4) from a 120 volt supply outlet, is well-known and therefore not shown.

The gas distribution system shown in FIG. 3 comprises valve-controlled by-pass lines 22B and 24B that are connected in shunt with corresponding valve controlled sections of the O.sub.2 and NG conduit mains 22 and 24 respectively, for supplying the rocket burners 20 when at the standby or low-heat "idle" settings. The by-pass lines remain open during both "idle" and maximum heat settings. In particular, the NG by-pass line 24B comprises a flow-rate meter 50 in series with a manually operated valve 52 that is pre-set for minimum gas flow, and an on-off solenoid valve 54, the coil 56 of which is controlled by the circuitry of FIG. 4. SImilarly, the O.sub.2 by-pass line 22B has a flow-rate meter 58, manual flow-adjustment valve 60 and an on-off solenoid valve 62 with an operating coil 64 also controlled by the FIG. 4 circuitry.

The main NG supply conduit 24 has in series a main shut-off valve 66, and in the shunted section thereof, a flow controller 68 of the plate orifice type, a manual valve 70 that is preset according to orifice flow, and an on-off solenoid valve 72 with operating coil 74. The flow controller is connected across the orifice plate to a flow meter 76 and at the high pressure side to a pressure gauge 78. SImilarly, the main O.sub.2 supply conduit 22 has a main shut-off valve 80, orifice-plate flow controller 82 with flow meter 84 and pressure gauge 86, manually preset valve 88 and solenoid valve 90 with operating coil 92. The solenoid coils 74 and 92 are controlled from FIG. 4 as indicated. For each main supply conduit, the corresponding by-pass is in shunt with the section having the flow controller, preset valve and solenoid valve. Accordingly, it will be seen from FIG. 3 that the rocket burners in each tier of main burners (the upper tier only being shown) can be supplied when in use with O.sub.2 and NG either through the by-pass connections 22B and 24B solely, at the standby rates set in the respective by-passes (the shunted sections of the main conduits being closed), or through the main conduits for the maximum rates set at the respective flow controllers 82 and 68 when the main solenoid valves 90 and 72 are open.

ROCKET BURNER OPERATION

In FIG. 4, which shows a circuit network 100 for controlling the starting sequence of the rocket burners and subsequent settings for standby and maximum firing, a main power controlling switch S is arranged to connect the network line terminals 102 and 104 with a source of voltage E. A power-on indicating lamp 106 is connected across the line and has in parallel therewith a branch circuit including in series, a rocket ignition switch 108, a normally closed emergency stop switch 110 and a relay coil 112 that gang-operates through connections 114, a pair of switches 116 and 118 for "on-off" and "off-on" conditions, respectively. As shown, the coil 112 is de-energized so that the switch 116 is closed to light the "off" ignition signal lamp 120.

To start, the power switch S and the ignition switch 108 are closed, thereby energizing coil 112 for opening the off signal switch 116 and closing switch 118 which lights "idle" signal lamp 122 and locks in the ignition circuit from terminal 104 through "stop" switch 110. According to preferred rocket starting sequence, the fuel gas is first turned on and spark-ignited at the rocket exhaust, and after a brief delay O.sub.2 is turned on. Immediately upon closing of the ignition switch 108 (subsequently through the holding circuit switch 118) the coil 56 of the NG by-pass solenoid valve 54 is energized to open the by-pass line for supplying NG at the idle rate to the respective rocket burners 20, FIG. 3. During starting, the main NG and O.sub.2 valves 72 and 90 remain closed as the line switch 134 is open.

The spark igniter coil 130 (in shunt through a normally closed time delay switch 126 with idle relay coil 112) is also energized as the ignition switch 108 is closed, through the time delay switch 126. The energization of coil 130 causes through circuitry hereinafter referred to, energization of the high voltage ignition transformer 27, FIG. 2. Ignition sparking at 6000 volts is thereby produced at the rocket starting electrode 29' for igniting the NG fuel as soon as it is turned on.

With an idle NG flame at the rocket exhaust and the ignition transformer still energized for ensuring ignition, the rocket is in readiness for flow of O.sub.2 to the rocket exhaust. A time-delay relay coil 132 is energized through the locked-in ignition switch 118 to actuate the timing means 136 which after a five-second delay closes the line switch 134 for energizing the coil 64 of the O.sub.2 by-pass valve 62. The rocket burner is now operating normally at "idle" with permanently established oxy-fuel flames, and will continue to operate at this setting in the absence of further control. The high voltage sparking referred to above continues for ten seconds and is then automatically shut off at the switch 126 by timer 128. The time delay relay coil 124 that started operation of the timer is in parallel with idle coil 112 and was energized therewith.

The rocket system is set to operate automatically at maximum rate upon demand by momentary closing of the run switch 138 (the idle switch 140 being closed) to energize the relay coil 142 that closes through a connection 143 the automatic control switch 144. The idle switch 145 for signal light 122 is concurrently opened through relay connection 147. Closing of the automatic control switch 144 establishes a holding circuit for the relay coil 142 and also puts control potential on the open terminals of the parallel-connected manual and flame detector switches 146 and 148 respectively. Either switch when closed jointly energizes the parallel-connected solenoid coils 74 and 92 of the main NG and O.sub.2 valves 72 and 90 respectively, for maximum high-heat firing of the rocket burners.

Referring briefly to FIG. 1, the upper and lower tier flame detectors 34 and 36 can be connected to the associated control circuitry so that flame instability signals from either burner tier can turn on for high firing all the rocket burners of both tiers, thereby quickly injecting very large amounts of heat into the combustion chamber for supplementing all the coal-air flames; or simply the rocket burners of the affected tier can be fired, assuming that each tier has its separate coal conveyor and pulverizer feed system. For operating the flame detector relay switch 148 in proper sense, FIG. 4, the relay 150 is controlled in obvious manner through amplifier or intermediate relay means 149 so that the relay 150 is energized when the signal from the detector is "negative". That is, when the millivolt output of the detector drops below a set point (as where the visible flame recedes from the detector angle a), a sensitive relay is tripped which in turn causes energization of the flame relay coil 150. The flame detectors 34 and 36 are calibrated for proper sensitivity to radiation from the visible flame. This gives more reliable flame temperature indication than would be obtained by ultra-violet or infra-red detectors.

As shown, the manual switch 146 can override as desired, the flame detector switch 148 thereby providing on demand high rocket burner heat for furnace starting, or for optionally supplementing the main coal-air flames, such as in marginal situations involving possible inadequate coal and air supplies. A "run" signal lamp 152 connected across the relay coils of the main valves indicates when the rocket burners are firing at maximum rates. The high firing rate is continued while the abnormal transient conditions such as coal pulverizer discontinuity, etc. continue. When the coal-air flames have returned to normal length to indicate stable combustion conditions, the extended visible flame is detected and the resulting positive signal causes opening of the flame detector switch 148 and closing of the main NG and O.sub.2 valves 72 and 90. The burners automatically return to standby firing as the NG and O.sub.2 by-pass valves 54 and 62 have remained open during the high-rate firing. In case of failure of oxygen and fuel gas pressures or water cooling failure, the rocket burner is automatically shut down by conventional safety controls.

Where maximum firing of the rockets is no longer desirable, as where the coal-air feed is sufficiently restored and shut down is not indicated, the manually controlled idle switch 140 can be opened to de-energize relay 142 and open the line power switch 144, thereby overriding the detector switch 148 and causing closing of main valves 72 and 90 and return of the burners to idle. Manual high-fire control can be advantageously had through the override switch 146 for boiler load changes when irregular combustion may occur. Rocket firing immediately corrects this condition with practically no formation of smoke or soot during the load change; also at start, the maximum-heat firing of the oxy-fuel rockets not only establishes normal combustion of the coal-air flames in less time, but also avoids stack emissions of the black smoke and soot ordinarily observed at starting.

Multiple tiers of coal-air burners can as mentioned above, be supplemented by oxy-fuel rocket burners according, for example, to the furnace feed arrangements, i.e., where each tier has its own coal pulverizer and conveyor system, the flame detector for a specific tier may have its control limited to that tier. In such case, the control circuitry of FIG. 4 would be substantially duplicated for each tier.

Accordingly, it is seen that the manual and automatic combustion controls of the invention encompass in combination the entire range of operating conditions encountered in central stations boiler operations.

Having set forth the invention in which is considered to be the best embodiment thereof, it will be understood that changes may be made in the system and apparatus as above set forth without departing from the spirit of the invention or exceeding the scope thereof as defined in the following claims.

Claims

I claim:

1. Apparatus for maintaining flames in excess of a predetermined length in a furnace comprising, fuel-air burner means for projecting a fuel-air flame in an annular converging pattern from a furnace wall; oxy-fuel burner means disposed substantially centrally within said fuel-air burner means for projecting oxy-fuel flames into said converging fuel-air flames thereby augmenting the length and stability of said fuel-air flames; radiation responsive detection means, disposed along a side wall of said furnace adjacent to said wall from which said fuel-air and oxy-fuel flames are projected, for producing first and second signals upon said fuel-air flame failing to exceed and exceeding a predetermined length; main O.sub.2 and fuel valve means; auxiliary O.sub.2 and fuel valve means; means responsive to said first signal for closing said auxiliary O.sub.2 and fuel valve means and for opening said main O.sub.2 and fuel valve means to project a hot, intense oxy-fuel flame into said fuel-air flame thereby increasing the length of said fuel-air flame to at least said predetermined length; and means responsive to said second signal for closing said main O.sub.2 and fuel valve means and opening said auxiliary O.sub.2 and fuel valve means to maintain a low intensity, standby oxy-fuel flame as long as said fuel-air flame length exceeds said predetermined length.

2. Apparatus as defined in claim 1 further comprising manually operable switch means connected to said means responsive to said second signal such that upon operation of said switch means, said second signal is overridden thereby supplying fuel and oxygen to said oxy-fuel burner to enable projection of said hot, intense oxy-fuel flame notwithstanding an indication by said radiation responsive detection means that said fuel-air flame exceeds said predetermined length.

3. A furnace combustion system as specified in claim 1 wherein the oxygen-fuel burner means is of the rocket type for projecting oxygen-fuel flames into the fuel-air mixture at different rates of heat output.