Situ Coal Bed Gasification

Abstract

Deeply buried relatively thick coal bed formations are fractured explosively. Reactant input conduits communicating with upper portions of the fragmented coal zone and product withdrawal conduits communicating with lower portions thereof are provided. The uppermost layer of the fragmented zone is ignited as by injection of oxygen and fuel gas from a surface plant while the product conduits are closed off to raise the operating pressure in the fragmented zone to balance hydrostatic pressure so that the fragmented zone comprises effectively a pressurized reaction vessel. Water or steam together with regulated amounts of oxygen are then introduced while reaction products are withdrawn at a rate at which operating pressure is maintained so that a relatively higher temperature reaction zone layer is reacted in the upper layer to travel progressively downward. A graduated lower temperature region precedes the higher temperature zone. Various gasification reactions occur in the reaction zones with the net overall products being methane and CO.sub.2 with relatively little up to varying amounts of carbon monoxide and hydrogen appearing in the reaction product gas. Processing to remove CO.sub.2 and react the carbon monoxide and hydrogen if deemed necessary is done at the surface yielding a high caloric value fuel gas suitable for pipeline or for synthesis uses.

Description

BACKGROUND OF THE INVENTION

This invention was made under or in the course of Contract No. W-7405-ENG-48 with the United States Atomic Energy Commission.

The domestic economy is faced with an ever growing deficit in energy reserves and particularly in domestic supplies of liquid and gaseous petroleum fuels. The shortage of natural gas is becoming acute so that serious efforts are being directed toward obtaining imports or to produce synthetic natural gas (SNG) in large surface plants using mined coal or a variety of imported petroleum fractions. In any case the cost of such supplies will be several times the cost of natural gas which has existed in the past. Major portions of the cost of the gas produced by surface coal gasification plants are represented by mining costs and the cost of the surface installation itself. Mining costs increase drastically when deep lying coal deposits are used instead of shallow deposits which can be strip mined. Various environmental problems accompany operation of such plants as involved in strip mining and shipping as well as sulfur removal, pollution from fly ash, waste disposal and others.

Attempts have been made since the mid-nineteenth century to produce fuel gas by in-situ coal bed gasification techniques (c.f. Homer Lowery (Editor), The Chemistry of Coal Utilization, Supplemental Volume 1968, J. Wylie Press, chapter 21). Most of these attempts have been directed to shallow deposits of subbituminous coals in eastern Germany, in Russia around Moscow and in Alabama by the U. S. Bureau of Mines. In these procedures air at about atmospheric pressure is pumped down a hole or shaft, directed across one or more burning coal beds and collected in another shaft or drill hole. There have been many variations in geometry of the holes but in every case most of the coal is converted to CO.sub.2 with just enough H.sub.2 and CO to make a very low quality heating gas. These product gases typically have heating values of 100-300 BTU/ft.sup.3 while pure methane (natural gas) has a heating value of nearly 1000 BTU/ft.sup.3 and is suited to economical pipeline distribution. Analyses of the burning fronts carried out through tunnels driven parallel to the burning galleries and connected to them with small horizontal drill holes show that most of the CH.sub.4 and much of the H.sub.2 and CO is burned near the exit region. This occurs since inlet air bypasses the hot front of combustion and combines with the exit gases burning most of the gas as it combines. Even so, several plants have been operated with this process continuously over the past forty years producing low quality gas from coal which is otherwise unsuitable for use because of its high ash content.

Similar techniques are not practical for use with deeply buried coal deposits since the cost of sinking shafts and driving the necessary tunnels would be prohibitive. Also, the gas would be of such low quality and caloric content that transmission by pipeline would not be economically feasible. Accordingly, it may be seen that a need exists for a procedure with which such deposits can be economically converted into a high quality fuel gas, i.e., synthetic natural gas.

SUMMARY OF THE INVENTION

The invention relates generally to coal gasification and, more particularly, to an improved process for use in the in-situ gasification of deeply buried thick coal beds to provide high quality synthetic fuel gas.

In practicing the present invention there is selected a relatively thick deeply buried coal deposit having a coal bearing interval with a thickness or composite thickness of at least about 50 feet and situated far below the water table. A large volume of the deposit is fractured by any suitable means. From the standpoint of economy explosive fracturing is generally preferred since the explosive may be emplaced by means of drill holes appropriately spaced. Moreover, selected drill holes may be provided with suitable casings to be used as reactant input conduits. Product output conduits may be provided as drill holes formed by slant drilling methods extending from the surface to communicate with the lowermost fractured portions of the fractured material. Suitable casing, e.g., with perforated lower ends may then be used to conduct the product gases to a surface processing facility. In some instances, the output conduit might be provided by a strategically placed shaft with radiating galleries.

In commencing operations a mixture of oxygen and fuel gas is introduced through the reactant input conduits and is ignited so as to heat the upper layer of fragmented coal to reaction, i.e., ignition temperature. Generally, the output conduits are closed off at this time so that the pressure in the cavity defined by the fractured volume of material is built up to an operating range of the order of 500 to 1000 psi. The operating pressure may be selected so as to balance the hydrostatic pressure of the ground water thereby preventing ingress of water while the hydrostatic head may serve to eliminate or minimize leakage from the cavity. Moreover, operation at such an elevated pressure promotes methanation reactions in the fractured coal bed so as to yield a high caloric value product gas.

When the desired operating pressure is attained the product output conduits are opened and reactant water in an appropriate form together with oxygen are introduced to contact the ignited upper coal layer. Withdrawal of product gas is then correlated with reactant input to maintain operating pressure. Thereupon, the ignited layer spreads downward with a temperature in the range of about 600.degree. to 1,500.degree.K and preferably in a range of about 650.degree. to 1,100.degree.K, developing therein as the water and oxygen react with the upper coal layer. Heat carried by the flowing reaction gases heats up the bed downwardly from the hot layer zone to a lower temperature than exists in the hot zone and further reactions productive of methane occur therein. It is to be noted that use of a downwardly progressing reaction assures a stable burning or reaction zone which eliminates by passing. A product gas comprising methane, water vapor, possibly with some CO and H.sub.2, as well as CO.sub.2 is formed as a result of the foregoing reactions. The latter gas may be removed and the water vapor condensed in a surface facility and the CO and H.sub.2 may be reacted catalytically if desired to produce methane. In any event there is obtained a high caloric content gas suitable for fuel or for use in chemical syntheses.

The process has several advantageous features in that no ash or mining debris is produced at the surface. Also mining costs are eliminated and the cost of surface facilities is drastically reduced since expensive large-scale reactors or converters are not needed.

Accordingly, it is an object of the invention to provide procedure for in-situ gasification of a coal deposit.

Still another object of the invention is to provide in-situ gasification of a deeply buried coal deposit using elevated pressures and temperatures.

Another object of the invention is to provide for the economical in-situ gasification of a coal deposit to yield a high caloric value fuel gas.

Other objects and advantageous features of the invention will be apparent in the following description and accompanying drawing, of which:

FIG. 1 is a vertical sectional view of a subterranean formation having a relatively thick coal bearing interval suitable for practice of the invention;

FIG. 2 is an illustrative blasting hole pattern for emplacement of explosive for shattering the coal bearing interval shown in FIG. 1;

FIG. 3 is a schematic illustration of a plant arrangement for conducting gasification of coal shattered by detonation of explosives in the interval of FIG. 1; and

FIG. 4 is an enlarged view of the shattered coal interval shown in FIG. 3 together with a temperature profile and corresponding reactions which occur in various portions of the reaction zone.

DESCRIPTION OF THE INVENTION

Coal deposits are widely distributed in the North American continent as well as elsewhere throughout various regions of the earth. For purposes of the invention those found between about 600 and 3000 feet below the ground surface are of particular interest. The Western United States is particularly well endowed with coal deposits which have appropriate physical and chemical characteristics. An estimated 1.5.times.10.sup.12 tons of coal reserves exist therein (c.f. The Economy, Energy and the Environment, Joint Economic Committee of Congress of the United States, Sept. 1, 1970). Processing of only 30 percent of these coals would yield the equivalent of about 10,000 trillion cubic feet of gas or about 30 times presently known producible reserves.

For purposes of describing the invention reference will be made to a particular deposit existing in the Central Powder River Basin of eastern Wyoming about 20 miles west of Gillette. In this formation, there generally exists five separate coal beds each averaging about 50 feet in thickness although one portion has a continuous 205 foot thick seam of coal. In one area therein coal sufficient to produce a potential of 700 trillion cubic feet of gas exists in an area of 9 by 18 square miles. These coals are too deep to mine economically so that in situ gasification would utilize resources not otherwise available.

A typical section of such a deposit is shown in FIG. 1 of the drawing wherein coal seams 11 interspersed with shale layers 12 in a relatively closely spaced formation interval is shown. The indicated interval is overlain by overburden 13 comprising interspersed layers of sandstone and shale in which the water table 14 exists some level beneath the ground surface. A formation is generally selected for which the uppermost coal layer is at a depth beneath the water table sufficient to provide a hydrostatic pressure in the range of about 500 to about 1,000 psi or somewhat more. In this respect hydrostatic pressure is about 435 psi for each 1,000 feet below the water table. It is considered that the interval selected should average at least about a 20 percent coal content in order that a satisfactory reaction rate be sustained. The interval may comprise a continuous bed or interspersed coal-country rock layers.

To prepare the selected coal bearing interval, the coal seams and interspersed shale layers, if present, are shattered with explosives. The area extent is selected to encompass sufficient coal to provide more or less continuous operation for a considerable time, e.g., one year or more. It is preferred to shatter the coal in as large a unit as can be processed at one time so as to minimize the amount of coal left between the areas to be processed. One half square mile area or more may be processed at one time. Explosives such as the ammonium nitrate-aluminum-diesel or stove oil mixtures or ammonium nitrate fuel oil mixture (ANFO-Al or ANFO) widely used in the mining and construction industries may be used as may nuclear explosives developed for plowshare applications. Conventional blasting explosives may be emplaced by means of, for example, 24 inch drill holes 19 shown in FIG. 1 and, for example, with about a 60 foot spacing in a concentric hexagonal pattern as shown in FIG. 2 of the drawing. Those drill holes to be used for injecting reactants may be cased with steel tubing prior to blasting and stemmed, e.g., with drillable plugs or with removable packers to a sufficient height to contain the detonation. Casing may be used in the remaining drill holes to prevent ingress of water if needed or may simply be stemmed with water tight material. The explosives may be emplaced and detonated sequentially, e.g., from the central hole outwardly to minimize possibility of fracturing overlying strata. The explosive charge size is selected to give adequate breakage but less than enough to give a "lifting" type detonation which might unduly disturb the formation. Conventional loading and firing systems may be used. Breakage of the order of 600 tons of coal per ton of explosive may be obtained. A lesser number of nuclear devices may be used for which the spacing and device size may be determined using published information (c.f. UCRL-50929, "Aids for Estimating Effects of Underground Nuclear Explosions," T. R. Butkovich et al., Sept. 8, 1970).

Once the desired quantity of coal has been broken, the detonated area may be arranged, as shown schematically in FIG. 3, for carrying out the gasification process. The shattered or broken coal bed 21 may be considered to be disposed in a closed vessel or cavity defined by surrounding undisturbed portions of the original formation. One or more of the cased drill holes 17 (one only shown) may be drilled out to serve as reactant input conduits. Conduit holes 17 may be connected to an oxygen supply plant 22 and to receive water from a water processing plant 23. A sufficient number of drill holes are utilized, or other means, e.g., spray devices can be used to assure reasonably uniform distribution of the water to uppermost layer of the bed 21 of broken coal. Oxygen will be distributed merely by injection. At least one product output conduit 24 (one only shown) is provided to communicate with the bottom of the broken coal bed 21 for conducting product gases (CH.sub.4, CO, CO.sub.2, H.sub.2, H.sub.2 O, etc.) to a surface gas purification plant 26. It is conceivable that the outlet conduit could also be provided as a lined shaft with galleries (not shown) being mined beneath the broken coal bed and lined. A centrally located shaft could be used with several satellite gasification chambers. The gas purification plant 26 may be of conventional design similar to those used in surface gasification plant technology. In the event that the product gas comprises principally methane and CO.sub.2 as occurs with certain modes of operation the plant 26 may comprise merely a CO.sub.2 removal unit using, for example, absorption of the CO.sub.2 in water and a water condensation unit to yield gas suitable for pipeline delivery. Excess water which may accummulate or condense in lower portions of the cavity may also be withdrawn through a conduit 24 using a pump (not shown). Under other operating conditions, as when the gasification reaction hot zone approaches lower portions of the coal bed, methanation may not be complete and CO and H.sub.2 may appear in the product gases. In this event an auxiliary catalytic methanation circuit of conventional design may be included in the gas purification plant.

Absorber water containing CO.sub.2 may be circulated to the water processing plant 23 wherein the CO.sub.2 may be stripped as in a stripping tower, or by bubbling air therethrough. This water and that withdrawn from the cavity may then be recirculated to react with the coal bed. The initial water supply and makeup water need not be fresh potable water but may be brackish water provided from a collection and storage system.

In commencing operation a flammable mixture, e.g., oxygen and fuel gas, e.g., natural gas, may be introduced and ignited to heat the uppermost layers of the coal bed to reaction temperature, i.e., preferably in the range of about 650.degree. to about 1100.degree.K and still more preferably in the range of about 650.degree. to about 850.degree.K. Other ignition procedures used in fire drive secondary oil field techniques may also be used. During this operation the product output conduit 24 is closed off so that the cavity pressure is raised to the operating range, i.e., about 500 to 1,000 psi or more. Thereafter, the product output conduit 24 is opened whereupon a gasification reaction zone having a temperature profile as shown in FIG. 4 of the drawing is established. The relative amounts of water and oxygen are generally regulated to maintain the peak reaction temperature in the uppermost coal bed layers at a level where water gas reactions and some methanation reactions occur. More complete methanation then occurs in the cooler zone ahead of the peak temperature region. As the reaction proceeds over a period of time the reaction zone progresses downwardly leaving heated ash and residual debris behind. Water and oxygen entering therethrough absorbs residual heat therefrom to provide a portion of the heat required in the gasification reaction.

It is to be noted that the downward gasification procedure provides for a very stable reaction front, which minimizes bypassing of unreacted coal, as compared to upward or lateral burning. Also, the reacted and unreacted shale and the coal ash are effective catalysts for the carbon monoxide-water reaction as well as being powerful scavengers of sulfur oxides, hydrogen sulfide and acid vapors such as may be created when using brackish water. The rock has low thermal conductivity so that heat loss does not occur to an appreciable extent. Likewise fly ash and heavy metal contaminants should be trapped in the long vertical column of rock and ash. A low pollution product synthetic natural gas is thereby produced.

COAL GASIFICATION PROCESS CHEMISTRY

Coal is an organic compound which contains essentially no free carbon in the natural state. It is made up of a series of molecules containing three or four six-carbon-rings in a phenanthrene-like structure. The phenantherene-like structure is partially hydrogen saturated so it is strained into "boat-like" shapes. Nitrogen and sulfur (non-pyrite) are contained within the rings while oxygen is mostly in the hydroxy form. These elements thus cross link the basic ring structures into phenolformaldehyde-like polymers. Because of the ring strain the whole structure is so loose that water molecules can occupy space between loosely parallel rings forming hydrogen bonds with the unsaturated carbon atoms giving the whole structure additional stability. This water comprises 10-30 percent of the weight of coal in place and is chemically part of the coal.

Coals are classified into a complex sequence depending on their hydrogen-carbon ratio, coking properties, ash content, behavior when heated and sulfur content. It is sufficient for present purposes to consider four broad categories; anthacites, bitumins, sub-bitumins, and lignites. In this order these classifications are characterized by generally increasing hydrogen-carbon ratios, generally decreasing heats of combustion and generally increasing oxygen content. The method of gasification described herein appears more readily applicable to those coals in the sub-bituminous and lignite classifications and which have hydrogen-carbon ratios approaching one in the newly mined coal. This type of coal has a large amount of fixed hydrocarbon and has a more favorable heat balance during in situ gasification.

Since each coal will have a slightly different chemical makeup, a universally applicable set of reactions cannot easily be written. For present purposes gasification of coal from the Central Powder River basin as representative of widely distributed coal deposits will be considered. This coal has a heating value of about 9000 BTU/lb and chemical properties as shown in Table I. There are generally five coal beds in the area each averaging 50 ft in thickness although one drill hole (sec. 29 T49NR75W) indicated 205 ft of continuous coal in one bed. ##SPC1##

The "formula" for the coal indicated in Table 1 is CH.sub.1.16 O.sub.0.35 N.sub.0.015 S.sub.0.005 which can be simplified to CH.sub.1.16 O.sub.0.35 for the thermodynamic calculations. This coal has a gram-formal weight of 18.76; this should not be confused with its molecular weight which is likely nearer one or two thousand. In the following calculation the "mole" used is assumed to be the simple one shown above.

In order to calculate heat balances and chemical reactions it is necessary to know the enthalpy H.degree..sub.298 and the entropy S.degree..sub.298 of the coal compound. The enthalpy or heat of formation of coal can be determined from the heat of combustion and the known heats of formations of the reactant oxygen and combustion products. From this calculation, H.degree..sub.298 of this coal is found to be -30.627 K cal mole.sup.-.sup.1.

Estimating the entropy of formation is more difficult and may be attempted by making assumptions about the chemical structure of coal. By comparison with a number of organic compounds whose compositions are similar, the entropy of the coal can be estimated to be between 5 and 15 cal deg.sup.-.sup.1 mole.sup.-.sup.1. Use of the Latimer rule for estimating entropy, ##SPC2##

(3/2 R lnM.sub.i -0.94), yields a value of 10.24 cal deg.sup.-.sup.1 mole.sup.-.sup.1 for the coal in question. This very important property may be measured more accurately by conventional experimental methods. However, it will be appreciated that there may be some shift in optimum operating temperatures if the true value is much different than that assumed, however, an assumed value can be used to evaluate the heat and equilibrium reactions which may occur. Its choice does not affect the heat balance significantly. A value of 10.1 cal deg.sup.-.sup.1 mole.sup.-.sup.1 is assumed in the following.

The reactions occurring during gasification of coal can now be examined and attention is directed to four categories, i.e., combustion, pyrolysis, reaction with water, and with hydrogen as shown in Table 2. These are the compounds which contact the coal at various locations in the active reaction zone. Neither CO.sub.2 nor CO react with coal at lower temperatures and these reactions are omitted in Table 2.

The estimated .DELTA.H for reaction and the free energy, .DELTA.F, are shown in Table 2 for temperatures of 500.degree. and 1000.degree.K. Following the usual convention, a negative value of .DELTA.H means heat is released and a positive .DELTA.H means heat is consumed. Negative values of .DELTA.F imply reactions favor the product or right side of the chemical equation, while positive values of .DELTA.F indicate that the reactants or left side compounds are favored. Thus a reaction with positive .DELTA.H and negative .DELTA.F will proceed to completion while consuming heat. ##SPC3##

The thermodynamic quantities in Table 2 show that coal decomposes or reacts with water (reactions 1 and 2) only if heat is supplied at temperatures above 500.degree.K. Therefore, in this coal and in the absence of oxygen or hydrogen (reactions 3 and 4) the coal will not continue to react and no "runaway" burn can occur. The reaction which produces methane (reaction 2) directly from coal requires heat to be supplied and appears to be somewhat more favored at these temperatures than is pyrolysis (reaction 1), so that that the direct conversion of coal to methane with water is possible if enough O.sub.2 is supplied to provide the necessary heat via reaction 4. If, on the other hand, reaction 1 occurs faster, then methane is produced through reactions 1 and carbon is then reacted with water (reaction 5), a portion of the CO reacted further with water (reaction 8), to produce enough hydrogen to balance reaction 9 with the same net result. Two competitive reactions may also be considered. This second path of methane production, i.e., from carbon, can be inhibited if reaction 3 depletes all the hydrogen (although methane is produced anyway by other reactions) or if reaction 10 occurs and depletes all the CO before 8 and 9 can occur. It appears from experiments done in surface methanators that the rates of reaction of 8 and 9 are favored under some conditions while 10 is favored under others. However, if 10 does occur then, when the higher temperature zone reaches the product carbon, reaction 7 will start the whole cycle over until the product gas is H.sub.2 and CO. This can be combined at the surface to form methane with the proper catalyst as in conventional practice. The secondary reactions which occur between C, CH.sub.4, CO.sub.2, H.sub.2 and H.sub.2 O shown in Table 2 are reproduced from a similar table in Homer Lowery (Editor), The Chemistry of Coal Utilization, Supplemental Vol. 1968, J. Wylie Press, Chapter 21.

Several other conclusions can be drawn from this table. No oxygen can survive, even at modest temperatures, since reactions 4, 5 and 11 are all exothermic with strongly negative free energies. Reactions 1, 2 and 7 consume energy at high temperature while 6 and 9 or 10 produce energy at lower temperatures. This combination causes the heat to be "spread" through the reaction zone, and helps or assures that the combustion zone will "skip" across barren shale zones without external attempts at ignition as the reaction zone proceeds downwardly.

All the information in Tables I and 2 can be combined to do an overall energy balance if the initial reaction temperature is selected. In the following illustrative example it is proposed that water and oxygen are added just sufficient to maintain reaction 2 of Table 2 at 700.degree.K. The ambient temperature is assumed to be 300.degree.K and the coal composition is as shown in Table 1. Under these conditions, the heat required is that to increase the coal, ash, and water from ambient temperature to the reaction temperature, to vaporize the water and to sustain the reaction. The heat supplied must come from combustion of coal. For this calculation it is convenient to express the thermodynamic quantities on a unit weight basis as shown in Table 3.

The calculation above, while approximate since several of the heat capacities have been estimated, can be used in preliminary design of the facility. Optimized conditions can be determined with more precision using computer simulation and/or by varying operating conditions. The calculation leaves the products at 700.degree.K but a significant part of this heat will be recovered during the flow of hot gases through the cooler unreacted coal beyond the reaction zone and during the flow of input oxygen and water through the ash and spent material leading up to the reaction zone permitting use of lesser amounts of O.sub.2 once operation is established. The water use has been calculated as if all of the water remains in the coal. It is most likely some will vaporize in the hot downstream gases and appear as liquid condensate at the base of the broken zone. In this case it is pumped to the surface and re-injected as needed.

TABLE 3

1. 1 g coal-in-place = 0.936 coal + 0.064 g ash

2. 1 g coal + 1.9 g O.sub.2 .fwdarw. 2.34 g CO.sub.2 + 0.56 g H.sub.2 O (g)

.DELTA.H = -5291 cal/g coal

3. 1 g coal + 0.513 g H.sub.2 O (g) .fwdarw. 0.475 g CH.sub.4 + 1.038 g CO.sub.2

.DELTA.H = 529 cal/g coal

4. 1 g coal at 300.degree.K .fwdarw. 1 g coal at 700.degree.K

.DELTA.H = +255/cal/g

5 1 g H.sub.2 O (l) .fwdarw. 1 g H.sub.2 O (g) (T = 393.degree.K)

.DELTA.H = +575 cal/g

Let x equal the coal-in-place to be burned to provide heat and assume 1 g of coal-in-place is converted to methane. The equation for heat balance is as follows:

.936 .times. 5291 x -255x = 255 + 529 .times. .936 + .936 .times. 575 [.513 - .936 .times. .56x]

x = 0.206

Thus essentially 0.2 grams of coal-in-place must be burned for each gram of coal-in-place converted with water. Table 4 summarizes the products and reactants per ton of coal-in-place.

TABLE 4

Reactants Coal-in-place 1 metric ton Oxygen 0.304 tons Water 0.306 tons 306 liters Products Methane 0.370 ton 19.8 MCF Carbon dioxide 1.175 ton 22.9 MCF Ash 0.064 ton Nitrogen 0.0072 ton 0.22 MCF S* 0.0058 ton 0 *It is not predictable whether sulfur will appear as SO.sub.2 or H.sub.2 S, but CS.sub.2 is not found at low temperatures. Either SO.sub.2 or H.sub.2 S will likely react with the shales and be absorbed before reaching the product line so that a low pollutant content gas is produced.

The heating value of the coal used is 9000 BTU per lb or 19.8 million BTU per metric ton. The heating value of the methane produced is 18.6 million BTU so the heating value of the coal has been largely conserved by this process even though only 46.4 percent of the carbon is converted to methane.

The small loss is the heat left in hot ashes, shale and gas. The shales between coal beds will use some heat, but under the assumption that all product heat is supplied from the coal and the products are left at the reaction temperature this loss does not reduce the reaction efficiency. In a very real sense, the heat balance assumed in arriving at Table 4 is belived to be overly pessimistic from the standpoint of efficiency and that an even more favorable result will be attained in practice.

Process

more particularly, starting with the broken coal system depicted in FIG. 2 and assuming methane as a product, the thermodynamics described in the previous section may be used to select best operating conditions of the coal gasification process. The process to be described is thus useful for comparison with other coal gasification schemes and for approximate economic analyses.

The schematic plant arrangement as shown in FIG. 3 may be used. The oxygen plant can be a standard cryogenic unit including a high pressure injection pump. The water processing plant may simply consist of a storage, CO.sub.2 stripping unit if recycle water is to be used, and pumping system including a high pressure injection pump. The gas purification plant may be of conventional design adapted to remove SO.sub.2 if such appears in the product. However, it seems most likely that SO.sub.2 will be formed in the ground, however, if that is the case then it will be absorbed in the shale by reaction with carbonates so that no sulfur oxide gases will be present in the produced gas. This is a very important and environmentally favorable consequence of the in situ processes as contrasted with surface coal gasification methods. Removal of CO.sub.2 can be accomplished by water scrubbing or by expansion cooling. Also simple high pressure potassium carbonate scrubbing may be an economic procedure for CO.sub.2 removal and would be effective in removing sulfur compounds as well, should they be present. The solution can be reconditioned and recycled.

The plant size depends on the quantity of gas to be produced, which, in turn, depends on pipeline proximity. For example, with 100 BCF per year as the desired rate and assuming that each broken coal unit as shown in FIG. 3 is processed in one year and contains over 5 million metric tons of coal in place, the total requirements are shown in Table 5.

TABLE 5

Annually Daily Gas Produced 100 BCF 274 MMCF Coal Consumed 5.05 million metric tons 13.8 thousand metric tons Oxygen Consumed 1.53 million metric tons 4.19 thousand metric tons Water Consumed 1.54 billion liters 4.23 million liters Drill Holes .about.240 0.6 (e.g., 24 inch with 60 foot spacing) Explosive approximately 9 kilotons (ANFO-Al)

This process requires the coal to be burned from the top downward in the explosive fractured region alone. This is required to assure maximum stability of the burning front and thus avoid one of the major problems encountered in previous in situ gasification attempts where coal was bypassed by the input gases. Subsequent units to be processed should be separated far enough so that no gas bypass can occur in a previously burned out region. However, once ground subsidence has occurred, it may be possible to later process the remnant coal. This makes it desirable to process large areas simultaneously as indicated in FIG. 2. The region shown in the lower portion of FIG. 2 is approximately to the scale of a 100 BCF per year operation.

In addition, it appears that the in situ process is conducted under very favorable kinetic conditions. FIG. 4 shows a conceptual vertical section through the reacting coal region along with the chemical reactions and approximate temperature distribution. In the inlet region water is being vaporized and heated up. As soon as the gases reach the coal, oxygen and the water react very quickly producing the high temperature peak. In the downstream region carbon monoxide and water react at lower temperature to produce carbon dioxide, methane and heat. This heat causes the extended intermediate temperature zone. Finally, at the lowest temperature water vaporization and condensation occur. The thickness of the very high temperature zone probably does not exceed 10 meters.

Assuming gas production rates from Table 5 and the area from FIG. 2 it is found that the superficial gas velocity in the broken coal is 0.1 ft/minute and with a reasonable average porosity in the broken coal the actual gas velocity should be 15 to 20 cm min.sup.-.sup.1. The reaction zone should be about 10 m thick so that the time available for reaction in the high temperature zone is thus of the order of one hour.

Data from experiments conducted in the 1100.degree.-1300.degree.K range indicate the reaction kinetics are pseudo first order and follow an Arrehenious temperature dependence. Based on the observed rates at the higher temperatures, rates for complete reaction at 700.degree.K are estimated to be on the order of one hour comparable to the residence time computed herein. Accordingly, maximum operating temperatures in the present process may be decreased as compared to conventional processes where high reaction rates are required for economic operation.

An economic analysis indicates that the capital cost requirements are between 15 and 30 percent of surface plants of similar capacity. Operating costs are very comparable to surface plants so that a profitable sales price for the product is considerably smaller (from 27-92 percent less) than for gas from surface plants. Since these costs are not as dependent on interest rates, there is considerably less overall risk for the investor or investment cost for exploitation of the process.

The drilling costs which are the major operating costs have been estimated assuming purchased rigs (nine) operated by permanent crews on a year-round basis. This results in considerably lower "per foot" drilling costs. In this analysis the use of ammonium nitrate explosives was assumed. The same amount of coal could be broken by c.a. nine 100-kt nuclear explosions. In this case, explosives would be more costly and drilling lest costly. On balance and to the accuracy of these analyses, the resulting total cost might be up to 6 cents per MCF less using nuclear explosives, depending on the amount of seismic damage. Only about 9-kt of chemical explosives are required because they are emplaced so as to avoid "lifting" the ground. When underground methanation is not complete, then construction and operation of a surface methanator to complete or accomplish this function would increase the gas price by about 18 cents per MCF.

Although the invention has been hereinbefore described and illustrated in the accompanying drawing with respect to specific steps of the method thereof, it will be appreciated that various modifications and changes may be made therein without departing from the true spirit and scope of the invention, and thus it is not intended to limit the invention except by the terms of the following claims.

Claims

What I claim is:

1. A process for in-situ gasification of a subterranean coal deposit to produce synthetic natural gas comprising:

selecting a coal deposit formation having a relatively thick coal bearing interval distributed therealong and situated beneath the water table at a depth yielding a hydrostatic pressure of at least about 500 psi;

emplacing and detonating explosive charges in said coal bearing interval of said formation to provide a deep bed of broken coal disposed in a closed cavity defined by undisturbed portions of the formation;

providing at least one reactant input conduit communicating with the uppermost layers of the broken coal bed in said cavity together with at least one product withdrawal conduit communicating with lowermost portions of the coal bed in said cavity;

injecting oxygen through said input conduit and igniting upper layer portions of said coal bed while raising the operating pressure in said cavity to balance said hydrostatic pressure;

then injecting water through said input conduit to react with said ignited coal layer together with sufficient oxygen to supply heat needed in the reaction while withdrawing reaction product gas through said product withdrawal conduit to form a reaction zone comprising an upper high temperature layer region merging into a lower region having gradually decreasing temperatures therein, which reaction zone progresses downwardly through said coal bed so that a product gas containing methane is delivered to said withdrawal conduit; and

separating said methane from said product gas withdrawn from said product withdrawal conduit.

2. A process as defined in claim 1 wherein the coal in said coal bearing interval has a composite thickness of at least about 50 feet.

3. A process as defined in claim 1 wherein the temperature in the high temperature region of said reaction zone is in the range of about 600.degree.K to about 1500.degree.K.

4. A process as defined in claim 1 wherein the temperature in the high temperature region of said reaction zone is in the range of about 650.degree.K to about 1,100.degree.K.

5. A process as defined in claim 4 wherein said product gas comprises a mixture including methane and CO.sub.2.

6. A process as defined in claim 4 wherein said product gas comprises a mixture including methane, CO.sub.2, CO and H.sub.2, wherein said product gas is processed in a surface methanator to complete the methanation reaction and wherein residual CO.sub.2 is removed from the product gas to yield high quality synthetic natural gas.

7. A process as defined in claim 4 wherein the operating pressure in said cavity is in the range of about 500 to about 1,000 psi.

8. A process as defined in claim 7 wherein the temperature in the high temperature region is of the order of 700.degree.K.