EP0045420A1

EP0045420A1 - In situ coal gasification

Info

Publication number: EP0045420A1
Application number: EP81105639A
Authority: EP
Inventors: Joseph Germano Santangelo; John Matthew Fernbacher
Original assignee: Air Products and Chemicals Inc
Current assignee: Air Products and Chemicals Inc
Priority date: 1980-08-01
Filing date: 1981-07-17
Publication date: 1982-02-10
Also published as: ZA815182B; JPS5753592A; BR8104962A; AU7317981A

Abstract

In an injection well for underground gasification of carbonaceous materials, either liquids or solids, by partial combustion with oxygen-rich gas in the presence of a moderating fluid such as steam, air, C0₂ or the like, in which the moderating fluid is introduced through an annular path (16) surrounding the injection tube (14) through which the oxygen-rich gas is injected, back flow of gasification products from the well is prevented by providing a flow restriction (18) in the annular path (16) to increase the linear flow velocity of the moderating fluid while maintaining the designed mass flow rate of said moderating fluid. The flow restriction (18) is so designed that the downward flow velocity of said moderating fluid downstream of the restriction corresponds to the formula:

wherein g is the gravity constant, D is the diameter of the well casing (12) and y is greater than one.

Description

BACKGROUND OF THE INVENTION

The present invention relates to-the gasification of coal or other carbonaceous materials, either liquids or solids, in an underground location, by in situ conversion including partial combustion and distillation of volatiles.
The production of gaseous products by reacting coal in subterranean deposits with steam and oxygen-containing gas is amply described in issued patents and in technical literature. In a typical operation spaced apart wells are drilled through the overburden to the coal seam, one to serve as an injection well and the other as a production well. By the various methods well known in the art, an underground linking channel is established for gas flow communication from the injection well to the production well. By introduction of air or other oxygen-containing gas and steam through the injection well at elevated temperature, various reactions may ensue, depending on conditions employed, giving rise to vaporization of liquid hydrocarbons and to the production of hydrogen, carbon monoxide, carbon dioxide and possibly methane, as exemplified by the following type reactions:
The composition of the gas product obtained as a result of these competing chemical reactions will depend largely upon prevailing temperature at the site of the reaction and to the relative quantities of H₂0 and O - there available. Different modes of operation have been proposed as to the injection of the reactant fluids; thus while some prefer to inject oxygen-containing gas and steam simultaneously into the coal strata, others advocate that the injection of the steam and oxygen-containing gas be alternated. Some of the available alternatives in coal gasification are discussed in U.S. Patent No. 3,978,920.
Instead of spaced apart wells for injection and production respectively, it is also known to employ a single well, wherein an injection tube is provided concentric to an outer casing or bore pipe, as seen, for example in U.S. Patents Numbers 3,298,434 and 3,856,084. Reactant fluids are introduced to the coal strata through the injection tube and the gaseous reaction products withdrawn in the annulus between the inner pipe and the bore pipe.
In some instances, as seen for example in U.S. Patent No. 3,999,607, although separate spaced apart wells are employed for injection and production respectively, each of these wells or the injection well alone may be provided with an outer bore pipe or casing and an inner concentric injection tube. In such arrangement oxygen-rich gas may be injected through the inner tube and a moderating fluid such as steam injected into the concentric annulus formed between the inner tube and the outer casing or well wall. The moderating fluid may be injected simultaneously or intermittently with the oxygen to reduce oxidation reaction temperature and may comprise steam, water, N₂ or C0₂ (U.S. Patent No. 4,026,357).
Other U.S. patents of interest relating to underground coal gasification include: Numbers 3,734,184; 3,770,398; and 4,099,567.
In installations wherein an injection well is employed in which oxygen is introduced into the coal strata through an inner tube and the moderating fluid flows down the annulus between the inner tube and the casing, there is the danger of back flow of combustible gas into the annular space with the possible formation of a potentially explosive mixture. Such flow of combustible gas into the annulus could be prevented if the flow rate of the moderating fluid is sufficiently high. The relative flow rates of steam or other moderating fluid to the oxygen flow rate must be set to foster the desired reactions in the combustion zone. Thus, depending upon the relative geometry of the annulus and the injection tube, rates of downward flow of moderating fluid large enough to purge the annulus properly may be too high relative to the coal gasification reaction requirements. The same techniques and problem also exist regarding liquid carbonaceous deposits. This problem is overcome by the present invention, which . allows the introduction of moderating gas in sufficient quantities to satisfy the annular purge requirements while at the same time satisfying the requirements of the desired gasification reaction.

SUMMARY OF THE INVENTION

In accordance with the present invention a bored injection well is provided with an outer casing and an inner injection tube extending through the well casing to the locus of the gasification area containing the material to be gasified or subjected to in-situ combustion, e.g. oil or coal. Oxygen-rich gas is injected downwardly through the inner tube while steam or other moderating fluid (such as C0₂, N₂, air) is introduced to flow down the annulus surrounding the inner tube at a designed mass flow rate to satisfy the requirements of the gasification reactions to obtain the desired produced gas composition. Back flow of gaseous products into the annulus is prevented by restricting the cross-sectional flow area within said annulus.at a location near the bottom of the annulus, thereby increasing the linear flow velocity of the gas flowing beyond said restriction to a predesigned rate such that backflow of combustible gas does not occur. In the manner hereinafter described the minimum linear flow velocity of the purge gas required to prevent upward flow of combustible gas into the annulus can be determined and suitable safety factors, as desired, incorporated in the design. The invention is applicable in installations wherein reaction steam is introduced through the annulus during oxygen injection as well as in operations wherein introduction of steam is in alternating sequence with that of oxygen introduction.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a diagrammatic vertical section of a typical injection well adapted for practice of the invention.
Figure 2 is an enlarged cross-sectional view, showing one form of restriction that can be employed to reduce the cross-sectional flow area of the annulus between the inner tube and the outer well casing.
Figure 3 is an enlarged partial vertical section of an alternative embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The invention is concerned with underground gasification systems wherein an injection well is spaced from a production well and an underground gas flow channel is provided there between.
Referring to Figure 1 of the accompanying drawings, there is shown a well bored through the overburden 10 down to a seam of coal 11 and a casing 12 arranged in the bore hole in well known manner. Concentrically arranged within the casing 12 is an injection tube 14._.also extending into the coal seam. Both the casing 12 and the tube 14 extend above the surface of the earth. Tube 14 is of considerably smaller diameter than casing 12. For example, the casing may have an inner diameter in the order of about six to eight or more inches and the tube may have an outer diameter in the order of about two or three inches. Through suitable control valves (not shown) oxygen can be admitted through tube 14 while a purge gas, (which may be inert or reactive) admitted through inlet 15, is flowed down the annulus 16 formed between the outer periphery of tube 14 and inner periphery of casing 12.
Within the annulus 16 and near the bottom thereof, a restricting disc or ring 18 is provided. In the embodiment illustrated in Figures 1 and 2 ring 18 fits tightly on tube 14 and extends outwardly therefrom for a distance short of reaching the inner periphery of casing 12 and thereby forming a purge fluid flow annulus 19. The determination of the design dimensions of the restricted flow path 19 is an important feature of the invention.
The significance of the relative flow areas will be appreciated from the following example. Assuming that the casing 12 has an I.D. of 8 inches, ignoring the presence therein of the tube 14, the cross-sectional area of casing 12 would be:
The cross-sectional area occupied by tube 14 is:
The area of the annular space 16 is:
50.26 -3.14 = 47.1₂ i_n.²
or about 94% of the total cross-sectional area of casing 12. So that for the purpose of flow rate design, where the diameter of the inner tube is relatively small as compared to the diameter of the casing, the inner diameter of the casing may be used as a basis ignoring the space occupied by the tube 14.
Assuming now in the embodiment illustrated in Figures 1 and 2 that the ring 18 has an outer diameter of 7.5 inches. The cross-sectional area of the ring would be:
and the restricted flow area 19 would be only:
In designing an installation for operation of a system of the type described, the following criteria must be taken into consideration to estimate the required linear flow velocity to prevent back flow of combustible gas ascending into the annulus. Of the combustible gases produced in the underground gasification of coal, the one presenting the greatest danger with respect to back flow into the annulus, is hydrogen, not only because of its inherent ready combustibility but because of its low density. The ascension of one gas counter to a downwardly flowing stream of another gas results from the buoyancy of the lighter gas in the heavier descending stream. By considering the possible gas subject to back flow as hydrogen, a conservative safety criterion is had.
The phenomena involved in counter flow of fluids with respect to one another is the subject of extensive study by Wallis, G. B., "One-Dimensional Two-Phase Flow", McGraw-Hill, (1967) particularly at pages 339-357. The correlation as originally developed and presented in the Wallis text was for gas-liquid two-phase flow where there was a large density difference between the phases. The correlation was found to be applicable for liquid-liquid as well as for liquid-gas flow. In the present instance the criteria of Wallis are employed, with certain modifications and assumptions, in the^;case of a gas (such as air or steam) flowing downward_'.in an annulus and a second gas (such as hydrogen) trying to ascend through the downwardly flowing gas stream by its buoyancy. The key problem is to determine at what downward gas velocity (say of air) will a "bubble" of hydrogen be prevented from moving upward through the annulus.
The correlation employed in designing a suitable arrangement for the purpose of the present invention, to estimate the needed purge gas downward velocity to prevent buoyant upflow of hydrogen, is:
wherein j* is a dimensionless variable as defined by the above mathematical expression (I);

V_p is the linear flow velocity of the purge gas in ft/sec;
g is the gravity constant, 32.17 ft/sec2
D is the internal diameter of the casing 12; = gas density in pounds/ft³ _;
subscript p refers to the purge gas and subscript H refers to hydrogen.

To prevent backflow of hydrogen j_P must be equal to or greater than unity. Since the density of hydrogen is much less than the density of the purge gas, equation (I) conservatively reduces to:
The minimum V_p to prevent upflow of hydrogen through the annulus 16 depends entirely upon the diameter of the casing, as is seen by setting j* p equal to unity:
For a further safety factor j
is preferably set at a value greater than one, as up to about 2, expressed as:
wherein y is greater than one and has a value of up to about 2.
The purge gas mass flow rate that will obtain the required velocity V_p then depends on the temperature and pressure prevailing at the bottom of the annulus. This mass flow rate can be reduced while still maintaining the required minimum flow velocity by restricting the cross-sectional flow area in the annulus.

Example

For example, assuming an installation wherein the desired underground reaction during the oxygen burn stage requires a pure oxygen injection rate of 600 SCFM employing a 90% oxygen/10% nitrogen mixture and an air rate of Q_air. The following equation must hold:
Solving equation V, Q_air ⁼ 87 SCFM
Substituting in equation IV, we get for an 8 inch diameter well:
For an injection well which at its bottom is at a pressure of 75 psig and at a temperature of 500°F (960°R), the mass air flow rate needed to provide a safe purge gas velocity (V ) can be calculated from the equation:
in which T is standard gas temperature 520°R, and P is standard gas pressure, 14.7 psia. T_I and P₁, respectively, are the temperatures and pressures prevailing at the bottom of the well.
Applying these values in equation (VII), we get:
But 650 SCF/min. is too high to satisfy the maximum allowable process air flow rate of 87 SCFM. Accordingly, the available cross-sectional area for downflow of air through the annulus must be reduced in such manner to obtain the required linear velocity of 9.3 ft/sec and yet stay within the limiting requirement of 87 SCFM air flow. This may be accomplished in accordance with the present invention by designing a restriction device of the proper type for placement in the annulus 16.
Employing a ring such as is illustrated in Figure 2, the central hole will have a diameter providing a tight fit over the two inch tube 14. The ring can be welded or otherwise firmly secured gas-tight to tube 14. The diameter of the ring is such as to leave an open circular gap between its outer periphery and the casing wall. The size of this gap is such as to provide a reduced annular flow area in the ratio of the estimated maximum process air flow to the minimum required purge air flow, 87/639, or 0.136 which will obtain the required proper flow velocity of 9.3 ft/sec.
The flow area needed to give a gas velocity of 9.3 ft/sec is readily calculated as 0.042 ft². This cross-sectional area is obtained by a gap 19 of one-half inch between the outer edge of the ring 18 and the inner wall of casing 12.
Instead of providing the restricted flow area as illustrated in Figure 2, at the outer periphery of the ring 18 and adjacent to the wall of the well casing 12, one may employ a ring 18 having an outer diameter equal to the inner diameter of the well casing and having a central hole therein of a proper diameter greater than the diameter of the inner tube 14. Thus, a restricted gas flow path will be had in the space left between the periphery of the central hole in the ring and the outer periphery of the tube. If desired, a ring 18A can be mounted in the annulus as shown in Figure 3 by appropriate supporting structure (not shown) so as to provide two concentric gas flow paths, one adjacent to the outer periphery of the inner tube 14 and the other path adjacent to the inner wall of the casing 12, designated by reference numerals 19A and 19B, respectively.

Claims

1. An injection well for underground gasification of carbonaceous materials by partial reaction with oxidizing gas in the presence of a moderating fluid, said well having an outer casing surrounding a gas injection tube within said casing for introduction of oxidizing gas into the bottom of said well, and providing a second gas flow path in said casing externally of said injection tube adapted to be used for admission of moderating fluid to the bottom of said well; gas flow restricting means in said second gas flow path designed to prevent backflow of combustible gas product formed by reaction adjacent to the bottom of said second path, said restricting means providing a restricted flow area within said second path such that the downward gas flow velocity immediately downstream of said means corresponds to the formula

wherein g is the gravity constant, D is the inner diameter of the casing, and y is greater than unity.

2. An injection well according to Claim 1 wherein said gas flow restricting means is in the form of a ring having an inner disc fitting tightly on said gas injection tube near the lower end thereof and extending outwardly therefrom for a distance short of reaching the inner periphery of said outer casing.

3. An injection well according to Claim 1 wherein said gas flow restricting device is in the form of an annular disc having an outer diameter equal to the inner diameter of said outer casing and having a central hole therein of a diameter less than that of said gas injection tube.

4. An injection well according to Claim 1 wherein said gas flow restricting device is an annular disc mounted adjacent the lower end of said gas injection tube and spaced from the outer periphery of said tube and from the inner wall of said casing.

5. In the underground gasification of carbonaceous materials in situ through a bored injection well by the method which comprises injecting oxygen-rich gas through a conduit within·said well and laterally spaced from the wall of said well while flowing a combustion moderating fluid downwardly in a concentric annular flow path externally of said conduit, said moderating fluid being flowed at a designed mass flow rate sufficient to satisfy the requirements of the gasification reaction for production of the desired gasification product composition, the improvement which comprises preventing upward flow of gaseous products into said annular pack by restricting the cross-sectional flow area of said annular path near the bottom thereof to increase the linear flow velocity of said moderating fluid to a predesigned rate such that the linear flow velocity immediately downstream of said restricting corresponds to the formula:

wherein g is the gas gravity constant, D is the inner diameter of said well bore and y is quantity greater than unity.