CA1072006A - Induction heating of coal in situ - Google Patents

Abstract

ABSTRACT
The electric induction heating in situ of a selected portion of an underground coal deposit, for the purpose of facilitating extraction of gases, liquids, solids and energy from the deposit. The heating is conveniently effected by passing a time-varying electrical current through a conductor encompassing the selected portion. The conductive path is preferably a toroid, quasi-toroid, helix, or simulated toroid, quasi-toroid or helix, created by a drilling and passing one or more conductors through the drill holes.

Description

10~2~06 , FIELD TO WHICH THE INVENTION RELATES
_ The present invention relates to a method of heating an underground deposit of coal by electric induction heating, for the purpose of facilitating extraction of useful energy or matter from the deposit~
BACKGROUND OF THE INVENTI ON
Roughly one half of the world's known coal deposits are located in North America and coal is the major fossil fuel resource of both North America and the world. There are two well known methods of mining coal. Coal deposits of great thickness at or near the surface are exploited by strip mining, and such deposits may account for five to ten percent of the known reserves. Strip mining generally causes serious environ- ~ `
mental degradation, in that the top soil is removed and covered, the surface and under-surface drainage of the land is seriously ~`~
disturbed, and strongly acidic compounds are commonly leached out of the material exposed after the overburden and coal are removed. Ecological restoration of the land is very expensive, and is rarely fully successful. The tendency is to regard the area mined as a sacrifice to economic necessity because of the large time lag, the high cost and doubtful result of land ~ ;
restoration after strip mining. Strip mining represents a destruction of the environment which is increasingly regarded ~
as unacceptable. Secondly, where coal deposits are at a -considerable depth, one thousand feet being typical, conventional deep mining techniques must be resorted to. The mining of -coal deposits too deep to be stripped of overburden is costly and requires a large amount of manual labour. Coal mining is inevitably accompanied by a high incidence of accidents, caused largely by rock falls and gas explosions. In addition, the coal dust in the mine atmosphere causes severe lung problems, - "~ and it is well known that many coal miners are afflicted by ~;

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' ' lV~7Z~(36 black-lung aisease. Furthermore, deep mining of coal is inefficient in that about only half of the coal is extracted, and that most of it is not mined a~ all, the seams being either too thin or too aeep to permit economic working. There is also severe ecological degradation associated with deep mining. This is principally due to the amount of rock brought to the surface with the coal, and coal dust or other fumes.
In summary, present coal-exploitation methods are costly, dangerous, cause severe environmental damage, and extract only a small percentage of the total deposits. It is ~- -o~ greatest importance that coal be efficiently utilized but this is not possible with present methods of mining.
SUMM~RY OF THE INVENTION ~ ~:
The present invention is the electric induction heating of selected portion of an underground coal deposit.
Electric induction heating of the selected portion of the underground coal deposit may be effected by passing a selected time-varying electric current through an underground conductor ..
or plurality of conductors whose path or paths are chosen to ` -~
substantially encompass the volume of the coal deposit intended to be treated. By "substantially encompassing" is meant the surrounding of the volume by the conductive path so as to generate, when a selected time-varying electric current is passed therethrough, an electromagnetic field sufficiently strong throughout at least a substantial portion of the encom-passed volume to enable it to be heated satisfactorily by induction to a desired temperature. If the location and shape of the conductive pa~h are appropriately chosen, heat will be ~;
generated within substantially the entire mass of the encompassed volume of the coal deposit, and thus the temperature of sub-

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10~20~6 stantially the entire mass o~ the deposit portion being treated can eventually be raised to a level sufficient to enable at least an economically significant portion of the gases, liquids, solids and energy which are generated by the heating of coal to be extracted. Once the temperature of the underground coal deposit has reached the desired level, the gases, liquids, solids and energy may then be extracted using extraction technology already known or yet to be developed. The present invention, however, is not directed to the extraction process which follows the heating of underground coal deposits; the present invention is confined to the induction heating technique per se, which will then be followed or accompanied by a suitable extraction process ~it is contemplated that the heating by induction may continue during at least some portion of the time required for extraction of the products and energy resulting from the heating of coal). ~ ~
Drilling techniques are known whereby other than ~-straight vertical drill holes may be formed in the earth. Such known drilling techniques may be utilized to create an appropriate underground path for one ox more conductors used to carry the -selected time varying electrical current to effect the induction heating of a portion of an underground coal deposit substantially encompassed by the conductor or conductors. In many conventional electric induction heating applications, a helical coil or wire is used, and the contents of the volume substantially encompassed `~
by the helix are then heated by induction for the particular purpose which the designer has in mind. Ideaily, a toroid-shaped conductor coil configuration would be utilized, since a toroidal coil avoids the end losses associated with a helix. If a helix is used, then to avoid the difficulty and expense of drilling continuously curved paths, it is possible to simulate : .

lO~Z006 a helical path underground by means of interconnected straight line drill holes at appropriate angles to the vertical and meeting the surface at various preselected points, through which drilled passages a condu~tor or plurality of conductors may be fed and joined together by conventional techniques so as to create a continuous conductive path which will surround an economically significant volume o a selected underground coal deposit. A selected time~-varying current caused to flow through this conductive path will then heat by induction the coal located within the volume substantially encompassed by the conductive path. In a similar matter, passages may be drilled to accommodate a toroidal, quasi-toroidal or simulated toroidal or quasi toroidal conductor path within the underground coal deposit. The selected time-varying voltage and current and the time during which they are applied are selected to raise the temperature of the mass of coal substantially encompassed by the conductive path to a desired temperature sufficient to permit the extraction of the gases, liquids and energy produced by the heating of coal.
As mentioned above, electrical induction heating of ~ ~ -coal may also be effected by the use of a quasi-toroidal con- ~
figuration of conductor turns. The following discussion is ~ `
intended to fully describe a quasi~toroidal coil. -- `
A surface of revolution is surface generated by `
revolving a plane curve about a fixed line called the axis of the surface of revolution.
A conventional torus is a surface of revolution gener- ;
ated by a circle offset from the axis, which circle, when it moves about the axis through 360, defines the toroidal surface.
The section of the torus is the circle which generated it. ;;~

10~2~06 The inner radius of the torus is the distance between the axis and the nearest point of the circle to the axis, and the outer radius of the torus is the distance between the axis and that point on the circle most remote from the central axis.
When a coil of wire is formed having the overall shape of torus, the coil is said to form a "toroidal conductive envelope", since it envelopes a generally toroidal space. Toroidal inductor coils are well known in electrical engineering. Con- ;
ventionally, a continuous coil of wire is formed into a torus thereby forming a toroidal envelope having a circular section.
Since the coil is a continuous conductor, it follows that the turns of which the toroidal coil are formed are series connected.
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Such a toroidal coil has a desirable property that its elec~
tromagnetic field is substantially confined to the interior of the torus. The quasi-toroidal embodiments of the present invention are not concerned with true toroidal envelopes but rather with quasi-toroidal envelopes formed by a plurality of discrete interrupted turns lying at different angles so as to approximately surround the volume lying within the envelope.
By "interrupted turn" it is meant a turn having a discrete discontinuity small with respect to the length of the turn.
A first distinction be-tween a quasi-toroidal envelope and a toroidal envelope is that the turns of the quasi-toroidal ; ~;
envelope do not necessarily form a complete closed curve as is the case (except for the terminals) in a toroidal envelope, but instead each takes the form of an interrupted turn - i.e.
a curve which includes a discontinuity (there must necessarily ~
be an electrical discontinuity in order that electric current .

may be passed through the quasi-toroidal envelope from one side of the discontinuity to the other).

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A further point of distinction is that a quasi-toroidal envelope need not be a surface of revolution, nor does its section have to approximate a circle. A quasi toroidal surface includes not only surfaces of revolution formed or approximated by rotation of an interrupted circle about an axis but also any practicable topological equivalent thereof, such as a surface of revolution generated by an interrupted rectangle, or such surface "stretched" generally perpendicular ~o the axis so that an oblong or slab shaped surface results. Because of the dif~iculty of drilling curved tunnels underground, a rectangular coil configura-tion is preferred, comprising only substantially only horizontal and vertical conductive elements. (The "horizontal" conductors may depart from the horizontal to follow the upper and lower boundaries respectively of the coal deposit.) A characteristic of a quasi-toroidal conductor config-uration (and indeed also of a toroidal inductor) is that the electromagnetic field is highest near the inner radius of the quasi torus and therefore the coal may be expected to heat more quickly at the inner radius than at the outer radius. This implies that an increasing current will be required in the quasi-toroidal coil to maintain the field strength sufficient to heat at constant power the coal lying towards the outer radius of the quasi-toroid. Eventually the required current may become ;
intolerable, and in the absence of corrective measures, the operation would have to come to a halt.
It is accordingly further proposed in the quasi-toroidal ;~
embodiment of the present invention that progressive extension of the quasi-toroidal conductor configuration to quasi-toroidal structures of increasing radius be utilized to facilitate extraction of the products and energy generated by the heating - 7 ~

~0~%~06 O~ coal from lar~e underground volumes. If the conductors are arranged initially in a twelve sided array, this configuration !~
can continue to be maintained as the quasi-toroidal radius is increased up to some convenient maximum radius.
In a preferred embodiment of the invention, a central vertical shaft is excavated from the ground surface to the bottom of the underground deposit or some other convenient point . :~, .
within the coal deposit. Vertical shafts or drill holes are also sunk at locations corresponding generally to the apexes lying on a circumscribing circle of a twelve-sided figure whose centre is located generally at the centre of the central vertical shaft. From a point within the central shaft located at or near the top of the underground coal layer, horizontal tunnels are excavated radially outward towards each of the vertical shafts.
These horizontal tunnels can be continued to a radius to be a suitable maximum.
If a twelve-turn configuration is to be used, the angle between adjacent horizontal tunnels will be 30. Twelve vertical shafts or drill holes are arranged at about 20-30 feet from the central vertical shaft. This would enable the vertical and horizontal conductive elements placed in the central shaft, in the vertical drill holes and in the horizontal tunnels, to encompass an annular quasi-toroidal portion of the deposit lying .
between the central shaft and the spaced drill holes, and lying between the upper and lower tunnels, which latter as indicated ~ ;
above are suitably placed prespectively at the upper and lower extremities of the coal deposit.
If it is assumed that the innermost quasi-toroid is defined by the central shaft of radius about 5 feet and a twelve-sided array of vertical drill holes at about 20-30 feet from the `~ -....

~ ~7,~06 ccntral shaft, thc ne~t step is to arrange a further pattern of drill holes to intercept the continuation of the horizontal tunnels at a further distance from the central shaft than were the first set of drill holes. The next set of vertical drill holes, for example, might be located at a distance of say 150 200 feet from the centre of the central shaft. If a further set of turns beyond the 150-200 feet distance is to be provided, the next succeeding set of drill holes might be located at, for example, 1000-1200 feet from the central shaft. At that dis- ~ ;
tance from the central shaft, the working of an underground deposit would be expected to take several years.
The reason for the foregoing spacing of vertical drill holes is this. In a toroidal or quasi-toroidal conduckor configuration, the electromagnetic field strength is highest near the inner extremities of the turns of the coil and lowest near the outer extremities of the turns of the coil. As a consequence, the coal near the inner coil extremities will be ~ ~-heated first, and heating will occur progressively outwardly `
from the innermost coils to a point at which further economic recovery from the deposit becomes impracticable. As coal is heated, in say the inner quasi-toroidal envelope region, the current required to heat the coil becomes increasingly high since the amount of conductive material lying within the electro-magnetic field generated by the conductive turns becomes increasingly small. Eventually a point is reached at which the coils become too hot or the current becomes too high to permit any further heating of coal. This point is determined in part by the ratio of the diameter of the inner set of con-ductor coil segments to the diameter of the outer conductive coil segments. Another reason for the necessity of increasing the '~- ' :

~0~006 effective inner and outer radius of the quasi-toroidal coil being utilized is that after the generation of the gases -from the heated coal deposit, the residue consists of coke.
Further heating of the coke serves no purpose and the presence of the coke serves to diminish the penetration of the magnetic field into the, as yet, unprocessed coal deposits lying at greater distances from the central shaft than the coke residue.
The simplest method tb achieve an adequa~e magnetic field intersection with unprocessed portions of the coal deposit is to step up to a larger quasi-toroidal coil having increased inner and outer radius so as to envelope the unprocessed coal regions.
Studies performed on mathematical models indicated that at least for some significant underground western coal deposits, the ratio of the outer envelope radius to inner envelope radius for the quasi~toroidal envelope should never exceed about ten, with a ratio nearer to 5:1 or 6:1 being preferred. For example, this means that if the radius of the central shaft is substantially the inner radius of the inner~
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most quasi-toroidal envelope, then the innermost~quasi-toroidal envelope should have an outer radius of the order of five or six times that of the radius of the central shaft. The next adjacent toroidal envelope may have an inner radius of say five or six times the central shaft radius and an outer radius of say 25 to 36 times the central shaft radius, and so on progres-sively outwards until some maximum radius is reached representing the economical upper limit for the working of the particular coal deposit in question.

It will be seen from the foregoing that if as few as twelve turns are used, the effect of the electromagnetic field -- 10 - , " lO~ZO~)6 produced by the coil necessarily deviates from ~he field that would be produced if a much larger number of turns were used to define the envelope. The term "quasi-toroidal" used in the specification is intended to embrace the approximation to a true annular volume or envelope within the electromagnetic field generated by a coil consisting of a relatively small number of conductive turns, usually fewer than twenty and in the examples to be considered, twelve, permeates.
The progressive heating proposal accordin~ to the invention i.e. the progressive utilization of quasi-toroidal envelopes of increasingly large radii, results in a saving in drilling and in conductor utilization, since at least some of the innermost vertical conductor elements of an outer quasi-toroidal envelope can conveniently be the outermost vertical conductive elements of the next adjacent inner quasi-toroidal envelope. Furthermore, the horizontal tunnelling can be relatively easily accomplished at the outset for the entire set of horizontal tunnelsl because the horizontal conductive elements of the outer quasi-toroidal envelope, or at least some of them, are conveniently formed in alignment with the horizontal con-ductive elements of the inner quasi-toroidal envelope, thus enabling the same horizontal tunnelling to be used to place the conductors. (In some circumstances, it may be desirable to increase the number of turns as the outer radius o~ the quasi-toroid increases.) The literature reports that the resistivity of coal drops rapidly as the temperature of the coal increases. Assuming this to be the case, it is further proposed according to this invention that oxygen, air or some other suitable material be ~ ;
injected into the vertical drill holes which are located on the .
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~06 inner radius of the quasi-toroidal coil or into the central vertical shaft o~ the quasi-toroidal coil and ignited. The ensuing combustion quickly raises the temperature of a thin layer of coal at the central vertical shaft or at the drill holes and the resistivity of the coal decreases. A lower resistivity permits larger induction currents to flow at the drill holes or central shaft. The induction heating will spread from these areas to form a continuous cylindrical shell.

SUMMARY OF THE DRAWINGS
Figure 1 is a schematic drawing of the electrical circuitry used for the input of the induction heating coil and :::
the control system. ~ `
Figure 2 is a schematic elevation view lllustrating a conductive path and associated surface electrical equipment : .
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foruse in heating by induct.ion of a selected portion o~ a coal deposit, wherein a helical coil is employed.
Figure 3 is a schematic pl~n view of the conductive path and sur~ace connections therefor illustrate~ in Figure 2 Figure 4 is a schematic view illus~rating a pattern of straight line drill holes so locate~ as to ena~le the simulation : of the conductive path of Figure 2. Figures 5 and 6 are ` schematic perspective views of alternative underground conductive paths for the induction heating of a selected portion of a coal. .
depos;t in accordance with the principles of the present invention.
. Flgure 7 is a schematic perspective view ~f a typical conductive path and surface connection wherein a quasi-toroi~al ~ ;
i3 : -conductor path is employed.
. ~igure 8 is a sche~atic diagram illustrating six optional schematic interconnection arrangeme~ts of the conductive paths of Figure 7.
_. ~ Figure 9 is a schematic elevation view o ~wo turn~
of a quasi-toroidal underground coil, with the connections to ~he sur~ace sited components of the system. .
Fisure 10 i~ a schematic elevation view of a typî~cal ~uasi-toroi~al conductor path where the heat;ng is carried out in four successive stages.
Figure 11 is a schematic plan view of a typical quasi-toroidal conductor path of Figure 10, showin~ the disposi~
tion of the conductors underground in the shaft, tunnels r and drill holes, fox the heatin~ of the coal deposit.
Figure 12 is a schematic elevation view of the -`.
con~iguration of Figure 11. -Fi~ure 13 schematically illust~ates a grid arrangement on the surface`o~ the earth for the practice of a preferre~ :
heating technique according to the invention. ~ ~

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~ETAILED DESC~IPTION WITH REFERE~CE TO THE DRAWINGS
... ... .. .... .
Figure 1 illustrates the surface control system circuitry common to any type of undergrouna coil conf iguration Alternating current input 15 from an AC generator or a trans~
mission line drives a frequency changer 16 and wave shaper 17 connected to the primary winding o~ a transformer 1~. Transformer 19 is a step ~own transformer intended to supply a relatively low voltage high amperage current to the underground coil configuration and is ordinarily located close to ~he surface ~ :
interconnection unit 22 o turns of the coil.
A capacitor 20 is connected to the surface inter-connection unit 22 and hence to the underground induction coil .~
(which, because ~f its shape, has apprecîa~le inductance) i~
~.
order to resonate the underground coiI 23 at the ~re~cy selected for operation. In a series resonant circuit the positive reactance of the coil is numerically equal to ~he negative reactance of the capacitor 20; and the combined impeaance i5 `~
, purely resistive, equal to the ~hmic resistance oE the o~il pius - . ......................... . .- : .
the resistance reflected into it ro~ the resistivity to eddy 2V curren~s of the portion of the coal deposit encompassed by the-induction heating coil. The resonating capacitor 20 is employed ;:
only when the current wave form appliea to the coil23 is sinusoidal . or near sinusoidal. When a square or nearly square wave form is employed, no resonating capacitor 20 is employed, and the positîve reactance of the induction heating coil 23 remains un- :
cancelled.
It is expected that with experimental testing, the inauctive heating effects in the coal deposit will be found ~o.

b~ depen~ent upon the frequency of alternating current passea through the underground coil, and also upon the shape of the ' . ~ - 13 -.
, ~ Z~6 - wave ~orm of the current (and indeed may vary with the ternpera-ture and other parameters ~s the undçrgrouna mass is heate~), For this reason, the frequency changer 16 an~ wa~e shaper unit 17 are shown in order that alternating current of the desire~
freguency an~ wave shape be supplied to the un~erground coil.
If, however, experimentation reveals that the fxequency and wave shape of the current supplied by the high ~oltage alternating current generator or transmission line 15 is satisfac~ory, the fxeguency changer 16 and wave shaper unit 17 could be omitted and the generator or transmission line 15 connected directly to the transformex 19. ~In North ~merica it woul~ ordi~arily be expected that the A~ generator or trans-mission lin~ lS would carry curren~ havin~ a frequency o~ 60 Hz and a sinusoidal wave form).
- The surface interconnection of unit 22 for the turns of the coil is further illustratea by Figure ~ and is applicable - ---; usually to the quasi-toroidal coil hereinafter discussed.
Connections 200 and 201 represent the juncti~n be~ween the in~erconnec~ed turns of the induction coil an~ the secondary .
of transformer 19 an~ capacitor 20. For the case of the helical induction coil (Figures 2-6), the interconnections are not usually made because all turns of the coil 23 are normally in series. However, parallel or series parallel connections o the t~
turns of the helical coil could be made in the manner descrîbed in Figure 8 for the quasi~toroid. Figures ~, 3, 4 ana 5 illustrate a helical coil with series connected turns so that the surface interconnec~ion ~it 22 of Figure 1 is not employed.
The helical coil of Figure 6 does employ surface interconnectiOn unit 22. -In Figure 1, the n~mber o~ connections between surface interconnection unit 22 and the underground coîl 23 ~epen~s on . .

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107%~106 the manner o connections and on the number of turns of the coil~ ~rbitrarily; twelve connections correspondin~ ~o twelve turns of a coil have been shown~ The exac~ numher depends on the operating structure and parameters for the particular case.
In Figure 2, a coal deposit is shown located betw2en .. .. .. . . . ..
an overburden layex and a rock floor Within the coal deposît, . .
an electxical con~uctor 11 forms a genexally helical.path ~ ~:
substantially encompassing the volume ABCD within the said aeposit. (In the plan view v~ the same regicn illustrated schemat;cally in Figur~ 3,~ the same volume is identified by the letters ABEF.) At each ~nd of the helix, the con~uctor 11 -. .
extends vertically upwaras to the surface of the ground along --paths llaj llb respsct.ively which, at the .
surface, exten~ along surface paths llc, 11~ respectively to the control system circuitry of ~igure.l, at 200, 201..
.~ A cylindrical helical coil confisuration is frequently found in industrial induction heating apparatus because the electromagnetic field is strongest within such helix and ~ecreases in in~ensity outside the coil~ Thus i~ the material loca~ed within the volume encompassea by the helix is rela~ively :;
unifoxm, the induction heating energy can be expected to be transferred to s~bstantially all ~he material encompassed by the coil. The above is true also of a toroidal coil, and the ~:
toroi.d avoids the end losses associated with a helix. If the economics o~ the situation warrant it, a toroid (or simula~ed toroid) could be used instead of a helix. The rate of absorption of energy from the helical conductive path increases with the intensity of the electromagnetic field generated, and also increases wi~h the conductivity of the energy absorbing material located within the helix. The rate of absorp~ion o~ energy also 2~0 increases with increasing frequency, within certain limits.
There may also be an optimum frequency for energy absorption of any given condition, which optimum frequency may con-ceivably vary over the duration of the heating and extraction processes.
A helix oriented in a direction perpendicular to the orientation of the helix of Figures 2 and 3 might perhaps be more easily formed than that of Figures 2 and 3. Figure 5 illustrates such a helical path substantially encompassing and intended to heat by induction the volume ~HIJ.
In any event, the helix of Figures 2 and 3 may be simulated by a number of interconnected straight line conductive ;
paths which can be formed in the manner illustrated by Figure 4.
The conductive paths of Figure 4 are formed in interconnected straight line drill holes. Vertical drill holes 31 and 71 are formed. Drill holes 33, 35, 37, 39, 41, 43, 45, 46, 47, 49, 51, 53, 55, 57, 59, 61, 63, 65, 67 and 69 are formed at appropriate ~ -angles to the surface to enable these drill holes to intersect one another and with holes 31 and 37 at points 73, 75, 77, 79, .~r 81, 83, 85, ~7, 89, 91, 93, 95, 97, 99, 101, 103, 105, 107, 109, 111 and 113, thereby forming the simulated helical path commencing - ~
at point 73 and ending at point 113. Conductors may be located ; `
along the appropriate portions ~viz. between points of inter-section and between the surface points 73, 113) of the aforementioned drill holes and interconnected at the afore- ;-`
mentioned points of intersection so as to form a continuous ;
conductive path beginning with vertical segment 31 and ending `~
with vertical segment 71.
Alternatively a series of generally rectangular conductive loops may be formed, each loop located within a ~`

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l~qZ~86 plane, the planes of the loops being parallel to one another, so as to define an encompassea volume KL~OP, as illus-trated schematically in Figure 6. These rectangular loops of course will remain open at some point, e.g. at a corner, s~ as to enable current to flow around the loop. The loops are then connected at the surface interconnec~ion 22 in the manner illus-- . . . . .
trated in Figure 6 to form a continuous circuit from terminal 200 to terminal 201. Other possible arrangements of inter- ~ .

. connectea series or parallel connected loops will xeàdily occur to those skilled in the art~
.
In each of Figures ~ through 6, the junctions 200, 201 represent the connection points be-tween the underground induction coil and the circuitr~ of Figure 1.
Alternatively, a quasi-toroidal coil configuration may be utilized for the induction heating of an undergrouna coal deposit. .

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-. ~........... Figure 7 iliustrates schematically an embo~iment o~
c~n inner quasi-toroidal envelope constructed in accoraance with the presen~ invention Within a coal deposit, inner vertical conauctor segments 1 are connected by upper hoxizontal conduc~or segments 3 ana lower horizonta~ conductor segments 4 to outer vertical conductor segments 2 and 5. U~ horizontal oonductor sesments 3 are connected to vertica~ ronductor segments 5. In Figure 7, by.way of example, twelve turns are illustrated, each turn being composed of three vertical conauctor segments 1, 2 and 5 ana two horizontal conductor ~gments 3 an~ 4 so as to form a substantially rectangular turn. The turns are arranged at angles of about 30 to one another. It ~Jill be noted that the turns do not comprise comp~ete turns. There is a discontinuity present at the outer upper corner o~ each ~B
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-10~2006 rectangular turn. This of course is essential in order that current flow around the parallel connected, series connected or series-parallel connected rectangular turns. The term "interrupted turn" is sometimes used herein to indicate that such a discontinuity'is present.
Vertical conductor segments 2 and 5 extend above the surface of the ground where varlous interconnections hereinater described and depicted in Figure 8 may be made in the surface interconnection unit 22 of Figure l. The dotted lines of Figure 7 illustrate the case where the turns of the coil are connected in series. The input terminals 200 and 201 ' of the coil configuration are connected to the control system circuitry of Figure l (not shown in Figure 7?.
' When alternating current is applied to ~erminals 200 and 201, an electromagnetic field is generated by the rectangular turns of the coil. The electromagnetic field tends - ~
to permeate a quasi-toroidal space which differs- from true ~ -toroidal space not only because of the drop off in field between conductive turns ~especially at their outer ,~
extremities) but also because of the interruptd rectangular coil configuration in distinction from the usual circle coil i~ "
configuration which would appear in conventional small scale ' toroidal inductors. The quasi-toroidal space has an inner radius defined by the radius of the notional circle on which the junction points of conductors l with conductors 4 lie. The - . :- .
outer radius of the quasi-toroidal space is defined by the ;~
outer vertical conductor segment 2. The upper limit of the quasi-toroidal space is defined by a notional horizontal annular surface in which the upper conductor segments 3 lie. A
similar notional annular surface in which the lower conductor segments 4 lie defines the lower boundary of the quasi~

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' lO~Z~ ' toroidal space. Thus the turns formed by the inner an~ outer ver~ical conductor se ~ nts 1, 2 and 5 and the upper and lower horizontal conductor segments 3 and 4 ~ogether form a quas~
toroidal envelope which substantially surrounds the quasi-toroidal ~:
space defined above. Obviously the more turns that are use~ in the envelope, the more closely the actual electromagnetic fiela will extend throughout the entir~ quasi~-toroidal space surrounde~
by the envelope. However, bearing in mind t.hat tunnelling or drilling is required for the introduction o each of the co~ductor elements into an underground coal aeposit, a ~raae of~
must be made between effic~ency of generation of the electro~ . -magnetic field within the quasi-toroidal space and the economics obtained by minimizing the number of holes or tunnels drill~d or .
excavated. In the discussion which follows i~ will be assume that the number of turns of the guasi-toroiaal coil is ~welv~
.
. H~wever, some other number of turns may be utilizea in appropriate ~:
- .~:~~- situations, and empirical eva1uation of the effectiveness o~
the number of turns initially employed will undou~tedly ~e made . .
: in particular applications to ~etermine whether a greater or 2 0 fewer number o turns may be suitable . Obviously additional tunnels and drill holes can be provided to increase the number of turns as required. Since the detailed design in no way affects.
the principles herein disclosed, the examp1es shown in the drawings must not be considered unique.
. The surface interconnection unit 22 of turns of ~he coil of Fiyure 1 is elaborated upon in Figure ~ Numerals 200 ana 201 correspond to the input to the surface interconnec~io~s 22 of Figure 1. :
Figure 8 shows ln schematic form the twelve turn coil of Figure 7, with the turns connectea in six possible ways, In ' . . . -.

~ 107~006 ~

detail A the twelve turns are connected in series, as in Figure 7; in de~ail B six series connections each of two turns in parallel are provided; in detail C, ~our series connections each of 3 turns in parallel; in detail D, 3 series connections each of 4 turns in parallel; in ~etail E, 2 series connections ëach of 6 turns in parallel; ana in detail F a single pat~.of twelve turns in parallel. me tabulation below shows that ~ese provide a relative inductance range of 144 to l, ~and therefore a relative resonating capacitance range o~ 1 to 144) and ~his.
wide range permits convenient choices of other circuit parameters -in a gr~at Yariety of coal deposits.
- : Relative Relative Max . Relative ` '-Turn Connections , Inductance Currents Resistance . ::
A .144 1 144 B 36 : 2 36 C 16 : 3 .16 :: :
~ . 9 4 9 -E 4. . 6 4 F . ~ 1 12 .Figure 9 shows a schematic elevation view o~ two ~urns - ,`
~f the coil in Figure 8 with the central vertical shaft 9, the , horizontal tunnels 10, and the vertical arill holes 11 throug~
: which the conductors~are threadedO The surface interconnec~lon unit 22, drawn from one of the options of Figure 8, is also .
shown.
: ~he resistivity of ~ry coals at 20C ranges from 10 to 101 ohm cm. However, the resistivity decreases expon- ;~
entially with temperature and reaches zbout S ohm cm at 900C~
It may be useful to take advantage of thi5 property of coal before in~uction heating is initiatea. Referring to ~igure g, oxygen or other s~itable gas or liqui~ is injected at the ~`.
inner face of the poxtion of the deposit to ~e heatea. Here, ~ .
the central shaft 9 of Figure 9 or drill ~oles 23 ~as seen in Figure 10) at the inner radius of . ~
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a quasi-toroidal coil would be so injected. Next, the coal along the inner face or drill holes is ignited. This ~ -reduces the resistivity of the coal at the ~rill hole or inner face. Thus, when induction heating is commenced, by applying current to the turns of the coil large curxents will flow more readily because of the greatl~. reduced resistivity. The induction heating will then spread out- ..
wardly from the inner face or drill holes so ignited an heated.. In Figure 9, this would be from shaft 9 out~ards.
, ~or the xeasons previously discussed, there is a practical upper limit on the ratio of the outer ra~ius o.
the.qu~si-toroidal envelope defined.by the ver~ical conductors ~. ;-.
2 of Figure 9 to the inner radius of the ~uasi-toroidal envelope defined by the location of the inner vertical conductor segments l-of Figure 9. For this reason it may be desirable to provide : :
, . - .
a further quasi-tvroidal envelope surrounding that illustrated , . . ~ .
',' :. ' ' /' '~' ~ ', ~ ,' ~

. / ' - , ~

/

/ - ~Oa -- ; ~
~.:

107Z0~6 in Figure 7 or 9. Such further quasi-toroidal envelope could utilize as its innermost vertical conductor elements the con-ductor elements 2 of Figure 7 or 9. Mathematical studies have shown that the ratio of the outer radius of the quasi-toroidal envelope to the inner radius of the quasi-toroidal envelope should not be greater than about 5 or 6 for best results. If this limit is observed, the efficiency of the induction heating process is greatly increased, since the ohmic losses in the coil conductors are kept to a low value, and the energy is principally expended in heating the coal.
Figure 10 is a schematic elevation view of the con-ductor paths which may be used for a four phase coal heating operation. A central shaft 9 of radius about five feet is sunk ~ ;
from the surface through the overburden 20, and through the coal ;~
deposit 21. Two sets of equally spaced radial hori~ontal tunnels 22 of say 40 inch diameter are drilled from the central shaft 9. -One set of radial horizontal tunnels 22 is located at the upper face of the coal deposit 21. The second set of horizontal tunnels 22 is located at the lower face of the coal deposit 21. Next/ ~-four sets of vertical drill holes 23 are sunk from the surface `~
through to the bottom of the coal deposit 21. Each set consists of twelve vertical drill holes 23 equally spaced about the circumference of a circle and located so as to intersect the upper and lower horizontal tunnels 22. Each vertical drill hole has a radius of about 16 inches. The number of sets of vertical 5:
drill holes is dependent upon the extent of the coal deposit.
For illustrative purposes, four sets have been described here.
Figure 11 is a schematic plan view of the configuration of Figure 10 illustrating the vertical drill holes 23. Four sets of vertical drill holes 23 are depicted. The inner set of twelve .:

72~6 vertical arill noles 23 lies upon the circumference of a circle of radius 20-30 feet. The second set lies on a circle of raaius 100-200 feet; the thira set on a circle of radius 500-1200 feet and the fourth set on a circle of radius about -' 250C-7200 feet. The dashed lines of Figure 11 show the horizon-tal tunnels 22. There are twelve such tunnel~ at the upper face of the coal deposit and twelve more at the lower fac~. Obviously, both sets of tunnels cannot be shown in a plan view.
Figure 12 is a s~hematic elevation view showing the con~uctors of one turn o the coil within the vertical drill - holes, central shat and horizontal tunnels, Th~ cross-ha~ched area 9 d~picts the central shaft. The solia lines }l~us~rate a conductiny element located within a horizontal tunnel, vertical drill hole or central shaft. A dashed line represents such a tunnel, drill hole or central shaft with'no conductor, '' Wi~h respect to aetail A of ~igure 12 a single tur~
~i ' of,the coil is shown. It is preferable to insta~l tha conductors for all four phases of the coal heating operation before beginning to heat the first phase In the firs. phase~ represented ~y ~' :
detail A, the inner vertical conductor segmen~ 1 is connectea ~o the lowe~ horizontal conductor segment 4. Segment 4 is connected to outer vertical conductor segment 2. Ver~ical conductor ,~ -~
segment 1 is connected to upper horizontal conauctor 3 an~ ~he latter is connected to vertical conductor segment 5. Conduc~ors .
2 and 5 are connected to the surface connection arrangemen~ of Figure 8. The inner radius of the phase 1 coil is about ~i~e ';
feet corresponding to the radius o~ the vertical central shaft 9.
The outer radius of the phase 1 coil is 20-30 feet. Pow~r 15 applied to the coil to initiate the heating of the coal.
When heating of the coal aeposlt lyin~ wi~hin ~he conductor segments 5, 3, 1, 4 and 2 has been complete~, phase 1 1,~.,!, r 2 ;~

,,.. ", . . ...

-`- 107Z006 of the coal heating operation IS inished and phase 2 shown in detail B of Figure 12 may be begun. In detail B o~ Fi~ure 12 conductor segments 2, 30, 31, 32 and 33 are connec~ea so as to form one turn of the electrical induction coil. The phase 2 coil has an inner radius of 20-30 feet and an outer radius of 150-200 eet. Note that conductorsegment 2 is used for both ~ -phase 1 and phase 2.
In a similar fashion, the necessary changes being ~:
made, phase 3 and 4 follow phases 1 and 2. Detail C and detail . D v~ Figure 12 illustrate the interconnection of the conductors . :;
for phases 3 and 4. As each phase is completea, the conductors unused in the precedi~g. stage may i desired be disconnected and ;.;~-withdrawn for use~elsewhere. It will be noted that ~he coil ~ ~:
~ connections are brought out at each secon~ ~rill hole along the i radius shown in Figure 12. The changing o~ connections between ~ .
tl successive phases is therefore facilitated. The arrangement o~ .
the installation in a concentric coni~iguration has two importan~
~ advantages: it permits the utillzation o~ ~he vertical ~rill -~
; holes and coil conductors twice, for ~he outer conductors ~f one stage ana the inner conductors of the succeedin~ stage; and heat transmittea outwardly from any phase is utilized in the :~
!,:
succeeding phase. It will be noted that no coil connections are ~. :
made at the upper end of the central shaft 9 of Figure 11~ This ~
is desirable, since this shaft among others is utilized for the ;`
eduction of the gas, and other produc~s which result ~rom the heating of coal. If necessary, other vertical drill holes coul~ ~;
be sunk to provide paths for the removal of the g~ses. :
.~.
Figure 13 is a schematic plan view of a method for - heating an extensive ~egion of an unde~ground coal ~eposi-t which involves the simultaneous, sequential,or simultaneous and , ~ ~ ~! , 2 3 ,~
.. . .
- . - . . ., - -, .- ., - - . . ., ; ", .

lO~Z~06 sequential heating of two or more portions of a deposit. By r way of example, four sets of concentric underyround coils as discussed above with reference to Figures lO, ll and 12 are shown. Each set is placed within a circular area, area 1, area 2 and area 3. Here, by way of example, a sixteen turn coil is shown. The small circles show the vertical drill holes 23 of Figure ll. The dotted straight lines depict underground horizontal tunnels 22 of Figure 11. The four annulax regions are also shown by way of example.
Within each area, heating will progress outwardly into the coal deposit by changing the coil connections found in Figure 12~ It is thus seen that the use of four sets of concentric coils permits a much larger volume of coal to be heated than would be the case if only one area at a time is processed.
For the sake of completeness, a possible extraction method for use with the invention will now be described. The particular extraction method chosen is at the discretion of the -~
user and is not a part of this invention per se. -~
2a Coal and lignite are classed as intrinsic semi~
conductors, as are the other fossil fuels-oil-sand, oil-shale, petroleum, etc., and have the electrical conductivity (or `
resistivity) variations character1stic of this class of materials.
The specific electrical resistivity of all dry coals is extremely high at 20C in the range of 10l to 1014 ohm cm, with the anthracites near the upper limit and the lignites near the lower limit. The resistivity decreases exponentially with the absolute temperature, and for all coals reaches a value of the order of 5 ohm cm at about 900C temperature. In order to heat coal effectively by electrical induction it may be useful to - 2~ -~t .

lO~Z~06 take advantage of this great reduction in resistivity at an -elevated temperature. Before the electrical induction heating cycle is begun, therefore, and after the electrical conductors are in place, oxygen or other suitable material is injected through drill holes at the inner face of the annulus which is to be processed, and the coal in these drill holes is ignited.
The ensuing combustion quickly raises the temperature of a ;
thin layer of coal at each drill hole, and reduces its resis-tivity to a low value. As soon as this has been accomplished, the oxygen supply or other suitable material is discontinued, and the electrical current is applied to the turns of the underground coil. The magnetic field induces eddy currents mainly in the high-temperature low-resistivity area surrounding each drill-hole, and the induction-heating spreads from these -focus points, to form a continuous cylindrical shell.
When coal is heated to 400C and above, coal gas and coal tar are evolved. At first, in the range 400C-500C coal tar is produced. Above, 500C, coal gas is generated. The increased temperature also serves to convert the liquid coal tar to a gaseous form. The gases are led to the surface through the central shaft used in the case of the quasi-toroid and the vertical drill holes where they are collected and separated into their constituents.
When the gases have been evolved, the residual deposi-t within the ground consists primarily of coke. Upon removing the coil conductors, air or oxygen may be injected into the coke. Combustion will ensue with great quantities of carbon dioxide being formed. The carbon dioxide may be led to the surface where it may be used to drive a turbineO
A great deal of the heat produced during the combustion .

~ - 25 - ~

lOq~
process is retained underground by the overburden. Said heat may be removed by a heat exchanging process such as injecting low temperature steam into the ground and removing it as high temperature steam to drive a steam turbine.
In every case the vertical drill holes, horizontal tunnels and central shaft (for the case of a quasi-toroid~ are usea to lead the various gases to the surface.
The possible method of use delineated above optimally represents a virtually complete removal of the gaseous products and energy from a coal deposit. No mining is necessary and the entire sequence of events occurs above the site of the coal deposit.
Variations and modifications in the above-described specific techniques and configurations will occur to those skilled in the art. The present invention is not to be restricted ;
thereby but is to be afforded the full scope defined by the appended claims. ;

', ' : ':

- 26 - `

Claims (16)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method for heating a selected portion of an underground deposit of coal which comprises the step of heating said selected portion by electrical induction heating.

2. In the conditioning of a selected naturally-occurring coal deposit to facilitate extraction of hydrocarbons and other products therefrom,the improvement comprising the induction heating of a selected portion of said coal deposit in situ over a period of time so as to heat said portion to a temperature lying within a selected range of temperatures.

3. The method of Claim 2, wherein the heating is effected by means of a selected time-varying voltage and current, passed through a conductor substantially encompassing said selected portion.

4. The method of Claim 3, wherein the conductor forms loops or turns each of which substantially surrounds part of said selected portion.

5. The method of Claim 4, wherein the path of the conductor defines a helix or toroid.

6. The method of Claim 4, wherein the conductor comprises connected segments approximating a helix or toroid.

7. A method of heating In situ a selected portion of an underground coal deposit, comprising:
(a) disposing at least one electrical conductor in at least one underground path whose shape and location are chosen to form, when voltage is applied across the ends of said conductor, an - Page 1 of Claims -electric circuit substantially encompassing said portion; and (b) passing a selected time varying electric current through said conductor of a magnitude and for a time selected to heat said portion by induction to a selected temperature.

8. A method of heating a selected portion of an under-ground coal deposit in situ comprising:
(a) forming a quasi-toroidal conductor arrangement in the deposit substantially to envelope the said selected portion, and (b) applying a selected time varying current and voltage, to the conductor arrangement to heat the selected portion by induction heating to a selected temperature.

9. A method as defined in Claim 8, wherein the ratio of the outer radius to the inner radius of said quasi-toroidal conductor arrangement lies in the range 2:1 to 10:1.

10. A method as defined in Claim 8, wherein the ratio of the outer radius to the inner radius of said quasi-toroidal conductor arrangement is of the order of 5:1.

11. A method as defined in Claim 8, comprising forming within the deposit a second quasi-toroidal conductor arrangement whose inner radius is substantially the outer radius of the first-mentioned quasi-toroidal conductor arrangement, and applying a selected time varying current and voltage to the second conductor arrangement to heat the coal deposit therein to a selected temperature.

- Page 2 of Claims -

12. A method as defined in Claim 11, wherein the ratio of the outer radius to the inner radius of each said quasi-toroidal conductor arrangement lies in the range 2:1 to 10:1.

13. A method as defined in Claim 11, wherein the ratio of the outer radius to the inner radius of each said quasi-toroidal conductor arrangement is of the order of 5:1.

14. The method as defined in Claim 8, wherein the individual turns of the quasi-toroidal conductor arrangement are of interrupted rectangular configuration.

15. A method as defined in Claim 11, wherein the individual turns of each said quasi-toroidal conductor arrangement are of interrupted rectangular configuration.

16. A method as defined in Claim 8 wherein after the electrical conductor arrangement is in place and before electrical induction heating is begun, a combusting agent is injected into the portions of the coal deposit adjacent the inner conductors of said quasi-toroidal conductor arrangement and said portions are ignited to reduce the resistivity of uncombusted portions adjacent thereto.

- Page 3 of Claims -