US2385508A - Combustion of coal - Google Patents

Description

E. s. HAMMOND 2, 85,508

COMBUSTION OF COAL Filed Oct. 25, 1945 2 Sheets-Sheet 1 Sept. 25, 1945.

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COMBUSTION OF COAL Filed Oct. 23, 1943 2 Sheets-Sheet 2 INVENTOR Edge/'57 Ham/non 0 Patented Sept. 25, 1945 fum'rso s'r 'rss PATENT has.

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iOornpany, a corporation of New Appltcation oetoberzs, 194:, Serial Na'souu 2 cam (01. 241-) This invention relates to the combustion of coal and particularly to a method and apparatus for the combustion of pulverized coal.

Systems utilizing pulverized coal. asheretofore known and used, are open to numerous objections such as high cost, limitations. upon the kind of. fuel usable therein, and non-uniformity and general difflculty'in operation. My improved system overcomes these objections and provides a process and apparatus capable of satisfactorily burning .not only fuel which is unsuitable for firing in pulverized form by known equipment,

but even fuel of such character that it is usable only with diiliculty in any combustion system, for example, anthracite of considerable hardness .and substantial ash content.

A complete understanding of the invention .may be gained from the following detailed description of a present preferred apparatus and the explanation of its operation.

In the drawings: 1 Figure 1 is an elevation showing the wall of a boiler furnace or the like in section, with a burner extending thereinto and coal pulverizing apparatus connected to the burner;

1 Figure 2 is a section taken on the plane of line 11-11 of Figure 1 with parts in elevation and parts omitted for the sake of clearness;

- Figure 3 is a central vertical section through the injector which delivers preliminarily ground coal to a final grinding zone or impact grinder; and

Figure 4 is a central vertical section through the burner.

Referring in detail to the drawings, I have shown a burner l0, projecting into a port H in th wall l2 of a boiler furnace or other com-- bustion chamber. A hopper l3 receives preliminarily ground coal from any suitable source, e. g., a conventional mechanical grinder. The coal delivered to the hopper I3 is preferably ground to such size that it may readily be reduced to a fine powder by an impact grinder. The

other grinders, as. for example, that of Andrews Patent 2,032,827. The grinder i8 comprises a continuous ductclosed upon itself which may convenientlybe composed of lengths of pipe 2. and return bends 2| and 22 secured together by coupling flanges 22. A steam-jet injector 24 discharges tangentially into the bend 22 near one end thereof. The injector has a nozzle 25 discharging into a Venturi tube 28,.and connected by a pipe 33 to a suitable source of superheated steam. The njector also has an-inlet branch 21.

A pipe 28 extends downwardly from the end of the feeder H to the injector branch 21. The upper end of the pipe 28 projects above the feeder and is provided with a valve 29 whereby to control the amount of air entrained with the coal delivered by the feeder I 4 to the injector 24 and thereby discharged into the impact grinder 9.

Th grinder I9 is alsoprovided with nozzles 30 spaced around the bend 22 from the injector. These ,nozzles are supplied with superheated steam from a manifold 3| through connections 22. The grinder I9 is also provided with an oil- Y the larger particles traveling to the outside.

Thus only the finest particles escape through the bleeder 24, the larger ones continuing around their course for further impact grinding, and

hopper It delivers coal to a continuous feeder ll illustrated as of the horizontal screw type. The hopper and feeder are mounted on a convenient support such as an elevated platform II. The

feeder II is driven by a motor it through a reducer I I and coupling I 8, the motor and reducer being mounted on the platform as shown.

The preliminarily ground coal is delivered by the feeder I 4 to an impact grinder is which provides a final grinding zone. The grinder I! is preferably of the type shown in Kidwell et al. Patent 2,219,011 (Figure 6) I may. however, use

eventual discharge through the bleeder. The expansion of the steam as it emerges from the nozzles 25 and 30 releases suflicient energy to cause the necessary circulation of the coal through the grinder and its passage therefrom through bleeder 34 and suitable pipe connections to the burner proper.

In use, the injector, impact grinder and associated piping will be well insulated against thermal loss so that there is no opportunity for the formation of free water vapor by condensation within the system. I have omitted the insulation from the drawings in the interest of clear-.

ness. The expansion of the steam is substantially adiabatic, and the initial degree of superheat of the steam is so regulated that after ex-' pansion it is still in superheated condition, 1. e.,

there is no saturated steam formed. Since the amount of entering air is limited, the cooling effect is slight and is also controllable. I

The coal withdrawn through the bleeder 34 consists of particles of such fineness that they will remain suspended in the circulating gaseous medium so long as it is moving and will settle therefrom only at a very slow rate even when the gas is motionless. I utilize this fact to convey the fuel to the burner by the energy resulting from the expansion of the superheated steam, and am thereby enabled to maintain the coal in suspension in the steam while it travels from the impact grinder toth burner. The bleeder pipe 34 serves to conduct the suspension of finely ground coal in steam from the grinder to the burner l through pipe connections 35 and 35. The pipe 35 delivers the suspension to a centrifugal separator 31 the outlet of which is connected to the pipe 36. The separator 31 is particularly important when utilizing the harder coals and may be omitted, as explained hereinafter, in some cases. The separator operates in the known manner to remove a portion of the superheated steam from the suspension, allowing the coal and the remainder of the steam to proceed through the pipe 36 to the burner ill. The amount of steam removed by the separator may be controlled by an adjustable pressure-relief valve indicated generally at 38.

The burner II), as best shown in Figure 4, comprises an outer tub 39 and an inner tube 49. A partition 4| closes the annular space between the inner end of the tube 40 and the interior of the tube 39, and is provided with holes spaced circumferentially thereof. The burner has an outer end wall 42 through which the pipe 36 extends so that the suspension of coal in a stream of superheated steam is discharged axially through the tube 40. A pipe 43 having a control valve or damper 44 therein extends from any convenient source of air under suitable pressure to the outer end of the tube 39. By this arrangement, air for combustion is supplied through the holes in the partition 4| to the interior of the tube 39 adjacent the inner end of the tube 40. The resulting combustible mixture, when ignited, burns completely and efiiciently. The type of flame may be controlled by varying the proportions of air and fuel. The volume of air supplied may be controlled by adjusting the damper 44 while the amount of fuel supplied may be controlled by varying the feeder l 4.

Practically all the air required for combustion of the fuel is supplied to the burner through the pipe 43. A certain amount of air, it is'true, will inevitably be entrained with the coal entering the injector 24. The amount of air so entrained will be controlled by the valve 29 so that the fuelair ratio in the grinder l9 and connections therefrom to the burner will be below the explosive value. This prevents th flame from working backthrough the pipe connections between the grinder and burner, even when the velocity of the fuel is below the rate of flame propagation. The atmosphere in the grinder |9 is composed of superheated steam with a small amount of air, but the amount of air is insufiicient to make the suspension of coal combustible.

The method of my invention comprises: delivering preliminarily ground fuel to a final grinding zone, viz., that provided by th impact grinder [9; discharging the coal into this zone with a jet of superheated steam expanding therein, to subject the coal to further grinding by impact in the final grinding zone, and reduce it to pa ticles of such size that they remain suspended in th steam; conducting the suspension of coal particles in the steam to the burner I 0 by the energy resulting from expansion, of the steam; the maintenance of the coal in suspension con tinuously between the final grinding zone and burner; the introduction at the burner of the amount of air required for eilicient combustion; and the limitation of the amount of air in the suspension between the final grinding zone and burner to an amount below that which would result in a combustible mixture, or excessively cool the stream.

In the burning of the harder coals having a high ash content, for which the invention is particularly adapted, it may be necessary, in order to grind the coal to the desired degree of fineness. to utilize an amount of superheated steam greater than that which can be passed through the combustion chamber without causing an objectionable loss of heat therein, since the steam which enters the combustion chamber, while not condensed, will absorb a certain amount of heat therefrom as it is raised to the temperature existing within the chamber. It is for this reason that I provide the separator 31. By appropriately adjusting the valve 38, I cause the separarator to remove from the system any steam in excess of that required to maintain the flow of the suspended coal particles to the burner. It will be understood, however, that the proportion of the steam removed by the separator 31 should not be great enough to permit precipitation of the coal particles or otherwise interfere with their maintenance in suspension continuously between the impactgrinder and the burner. If the coal being processed is relatively low in hardness, less steam will be required for the impact grinding thereof and the total amount of steam employed for grinding may not be greater than that which can be passed through the combustion chamber without excessive loss of heat. In such case, the separator 31 is unnecessary and may be omitted or by-passed by suitable valves and pipe connections (not shown). While the invention is particularly adapted for processing hard coal, it may be found advantageous to mix a small percentage of bituminous coal with the hard coal, in order to produce a combustible mixture which ignites readily when supplied to the burner.

I have found that particles of solid fuel which have been powdered by mutual attrition in a gaseous conveying medium under conditions inhibiting the formation and presence of moisture are peculiarly well adapted for immediate combustion if conveyed to the combustion zone in suspension in the gaseous medium, and also that the combustion system of my invention has a convenience and flexibility in operation akin to systems using fuel gas. This is probably due in part to the fact that the particles are given no opportunity to agglomerate and consequently present a maximum surface area for combustion. This, however, is not a complete explanation and the reasons for the remarkable qualities of the fuel are not fully apparent. The fact that the particles are reduced in size by mutual impact rather than by the attrition of grinding surfaces seems to impart a different quality to them. There is advantage, too, in having a limited amount of air present with the superheated.

steam during the fine grinding and a better quality of fuel results.

My invention constitutes an important improvement in coal combustion in that it makes possible the eiiicient burning of hard coal having a high ash content, which could not be burned satisfactorily either on mechanical stokers or in pulverized form when processed by equipment previously available. The invention thus opens the industrial market to certain types of hard coal which have not previously been salable therein. The apparatus comprising the invention is simple and is largely automatic in operation so that the required personal attendance is kept at a The method and apparatus of my invention provide a flexibility of control both as to the firing rate and the Iuel-air ratio which make for eflicient operation under all load conditions.

Although I have illustrated and described but a preferred embodiment and practice of the invention, changes in the structure and procedure disclosed may be made without departing from the spirit of the invention or the scope of the appended claims.

I claim:

1. In a method of preparing and supplying coal for combustion, the steps including delivering preliminarily ground coal to a final grinding zone, discharging superheated steam into said zone so as to cause the coal to traverse a closed path repeatedLv, thereby further grinding the coal by mutual impact in said zone to reduce it to particles small enough to be capable of suspension in said steam, conveying the steam and the coal suspended therein to a burner by the energy resulting from expansion of the steam, maintaining the coal in a state of suspension in the steam continuously between the point of introduction of the steam and the burner, maintaining the fuel-air ratio below the explosive value between said point and the burner, and maintaining the steam in a superheated condition between said point and the burner.

2. In a method of preparing and supp yin coal for combustion, the steps including delivering preliminarily ground coal to a final grinding zone, discharging superheated steam into said zone so as to cause the coal repeatedly to traverse a closed path, thereby subjecting the coal to further grinding to reduce it to particles of a size capable of suspension in the steam, conducting the steam and the coal suspended therein to a burner under the pressure of the steam supplied to said zone, maintaining the coal in a state of suspension in the steam continuously between the point of introduction of the steam and the burner, and maintaining the steam in a state of superheat up to the burner.

EDGAR S. HAMMOND.

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