Method of underground coal gasification
By monitoring and adjusting oxidant parameters using predictive algorithms, the method addresses the inefficiencies in maintaining syngas quality during underground coal gasification, ensuring consistent composition and reducing logistical efforts.
Patent Information
- Authority / Receiving Office
- AU · AU
- Patent Type
- Applications
- Current Assignee / Owner
- NEURIZER LTD
- Filing Date
- 2022-11-15
- Publication Date
- 2026-07-09
AI Technical Summary
Existing methods for maintaining syngas quality in underground coal gasification suffer from inefficiencies due to the movement of the reaction zone away from the oxidant injection point, leading to inconsistent syngas composition and requiring significant logistical efforts to reposition the injection point, which is costly and time-consuming.
A method involving the use of measurement devices to monitor methane and hydrogen content in syngas, and adjusting the pressure, flow rate, and composition of gaseous oxidants injected into the coal seam based on predictive algorithms to maintain desired gas concentrations, using historical data models and process variables to control syngas quality.
This approach ensures consistent syngas quality by automating the adjustment of oxidant parameters, improving efficiency and reducing the need for manual repositioning, thereby stabilizing the syngas composition and facilitating easier processing in above-ground plants.
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Abstract
Description
TECHNICAL FIELD
[001] The present invention relates generally to in situ coal gasification ("ISG") and, in particular to maintaining constant, predictable syngas quality (composition) produced by the process. BACKGROUND
[002] Any references to methods, apparatus or documents of the prior art are not to be taken as constituting any evidence or admission that they formed, or form part of the common general knowledge.
[003] It is well known that underground coal may be gasified and that gasification of coal (via ISG process) produces syngas. This process involves the operation of a gasification reactor cavity (the reactor or gasifier) between parallel horizontal boreholes within a coal seam that is fed with an oxidant gas, typically compressed air, oxygen, steam or combinations of these gases, through a borehole (inlet well). After initial ignition of the coal seam (initiation), gasification reactions between the coal and injected oxidant gases form syngas (a mixture of CO, CO2, H2, CH4, and other gases). The raw syngas produced by the gasification reaction is removed via the second borehole (production well).
[004] In the coal gasification process, there are a number of reactions which generate syngas. These reactions include: 2022271406 15 Nov 2022 C+ H2O —► H2 + CO (Heterogeneous gasification reaction) CO+ H2O —► H2 + CO2 (Water Shift Conversion) CO + 3 H2 —>CH4 + H2O (Methanation) C+2H2—>CH4 (Hydrogenating gasification) C+ 1 / 2 O2—► CO (Partial oxidation) C+ O2 —► CO2 (Oxidation) C+ CO2—>2CO (Boudouard reaction)
[005] As coal is consumed by the gasification process, the gasifier cavity grows in size and the coal face gradually migrates, as coal is removed by hot gases flowing across the face. When oxidant (air) is fed into the gasifier, the oxidant injection point is fixed at the end of the inlet well tubing.
[006] As the gasifier grows, the reaction zone of gasification tends to move away from the injection point of the oxidant gases, reducing the efficiency of the gasification process, resulting in a noticeable decline in syngas quality. When this occurs, the oxidant injection point (coiled tube) is retracted to expose fresh coal to the oxidant.
[007] The current method for maintaining syngas quality is to move the injection point to match the movement of the coal gasification face, allowing access to fresh coal. The movement of the injection point i.e. the inlet well tubing is achieved by either shortening the tubing by cutting off a section or retracting the inlet well tubing up the inlet well. 2022271406 15 Nov 2022
[008] Both of the methods described in
[007] result in the repositioning of the injection point. However, the retraction method also requires significant logistics, operations and specialized equipment operated from the surface, to achieve the objectives while the tubing shortening method requires access to additional patents for automatically shortening the casing. While to date this method has been common practice, it should also be noted that the gas composition will change between the inlet well tubing retraction events and will require significant trials to achieve consistent syngas compositions over the longer term. SUMMARY OF INVENTION
[009] The invention provides a method of in situ coal gasification (ISG) in a coal seam between linked inlet and production wells comprising: • ignition of coal located between the wells by injecting one (or more) gaseous oxidants at a measured flow rate into the coal seam through the inlet well to maintain combustion between the wells, producing a gas mixture (syngas), • measurement of methane (CH4) and hydrogen (H2) content relative to other gases in the syngas mixture flowing out of the production well using one or more measurement devices; • varying pressure and / or flow rate and / or composition of the gaseous oxidants being injected into the injection well in response to measuring the content of methane (CH4) and hydrogen (H2) to control the content of methane (CH4) and hydrogen (H2) within a predetermined range. 2022271406 15 Nov 2022
[0010] In an embodiment, wherein the step of varying the pressure and / or flow rate and / or composition of the gaseous oxidants being injected is carried out by a process control system that is operably coupled with the one or more measurement devices
[0011] In an embodiment, the method further comprises the step of varying the oxidant flowrate and or composition of the gaseous oxidants being injected into the inlet well in response to changes in one or more process variables such as one or more of gasifier depth, coal quality, operating pressure, inlet well length, production well length and gasifier length.
[0012] In an embodiment, the method further comprises the step of injecting water in addition to the one or more gaseous oxidants into the inlet well.
[0013] In an embodiment, the measured flow rate of the gaseous oxidants being injected into the inlet well is compared with a calculated flow rate using a computed prediction function to maintain the methane (CH4) and / or hydrogen (H2) content within a pre-determined range. The computed prediction function is generated from one or more historical coal gasification data models based on historical coal gasification events, wherein in accordance with the model methane (CH4) and / or hydrogen (H2) content in a production stream of a coal gasification plant is mathematically related to several process variables measured during collection of historical data associated with the historical coal gasification date models. Such variables including depth, coal quality, operating pressure, inlet well length, production well length, gasifier length. 2022271406 15 Nov 2022
[0014] In an embodiment, the computed prediction model may be applied to one or more methane (CH4) and hydrogen (H2) content values to determine a recommended gasifier pressure or oxidant flow rate value. Comparing the recommended gasifier pressure or oxidant flow rate value with actual pressure and / or flow rate value of the gaseous oxidants and varying the gasifier pressure and / or flow rate value of the gaseous oxidants in response to that comparison to control and maintain the content of methane (CH4) and hydrogen (H2) within a predetermined range.
[0015] In an embodiment, the method comprises selection of a computed prediction model from several computed prediction models based upon characteristics of the coal located between the wells. BRIEF DESCRIPTION OF THE DRAWINGS
[0016] Preferred features, embodiments and variations of the invention may be discerned from the following Detailed Description which provides sufficient information for those skilled in the art to perform the invention. The Detailed Description is not to be regarded as limiting the scope of the preceding Summary of the Invention in any way. The Detailed Description will make reference to a number of drawings as follows: Figure 1 illustrates an example of a parallel underground coal gasification apparatus. Figure 2 illustrates an example of a linear underground coal gasification apparatus DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0017] As previously discussed, retraction of the inlet well tubing is necessary to maintain the desired gas quality by exposing the oxidant to fresh coal. Maintaining 2022271406 15 Nov 2022 consistent syngas quality is desirable because it allows for easier above ground plant processing.
[0018] In Figure 1, an underground coal gasification system and method 100 has been illustrated. In-situ coal gasification is carried out with respect to a coal seam 110 between linked inlet and production wells 120 and 130 respectively. Coal gasification involves igniting coal located between the wells 120 and 130 by injecting one or more gaseous oxidants (typically compressed air, oxygen, steam or combinations of these gases) at a measured flow rate into the coal seam through the inlet well 120 to maintain combustion between said wells to produce syngas. The concentration of methane (CH4) and hydrogen (H2) in the syngas flowing out of the production well is measured using conventional measurement devices such as a gas chromatograph (GC). The present method involves a novel step of varying the pressure and / or flow rate and / or composition of the gaseous oxidants being injected into the inlet well in response to measuring the relative concentration of methane (CH4) and hydrogen (H2) when the relative concentration of methane (CH4) and hydrogen (H2) is no longer within a stipulated or predetermined range. The novel step of varying the pressure and / or flow rate and / or composition of the gaseous oxidants being injected into the inlet well is carried out to maintain the relative concentration of the methane (CH4) and hydrogen (H2) in the stipulated range.
[0019] Importantly, the variation in pressure and / or flow rates of the injected oxidants or steam / water is based on a predictive algorithm that has been developed by the inventors through rigorous trials and experimental work. 2022271406 15 Nov 2022
[0020] As previously discussed, after ignition (initiation) of the coal seam, gasification reactions between the coal and injected oxidant gases form syngas, a gas mixture comprising CO, CO2, H2, CH4, and other gases. ISG reactions include: C+ H2O —► H2 + CO (Heterogeneous gasification reaction) CO+ H2O —> H2 + CO2 (Water Shift Conversion) CO + 3 H2 ^CH4 + H2O (Methanation) C+2H2—>CH4 (Hydrogenating gasification) C+ 1 / 2 O2—► CO (Partial oxidation) C+ O2 —> CO2 (Oxidation) C+ CO2—>2CO (Boudouard reaction)
[0021] The raw syngas exits the gasifier via the second borehole (production well) where it is directed to an above ground gas conditioning plant. The gas conditioning plant is fitted with an on-line Gas Chromatograph (GC) to provide continuous syngas composition analysis.
[0022] In a typical ISG reaction, as coal is consumed by the gasification process, the resultant cavity grows in size and the coal face gradually migrates as coal is removed by hot gases flowing across the face.
[0023] As an example, for a system with Total Gasifier System distance of 1000m through a bore size of 6” the total residence time from oxidant injection to GC at an operating pressure of 3000kPag, and oxidant injection rate of 2000Nm3 / h the travel time would be 324 seconds or 5.4 minutes. 2022271406 15 Nov 2022
[0024] The inventors have carried out various trials and acquired comprehensive data during the Pre-Commercial Demonstration Project (PCD) to develop a predictive algorithm based on inputs including, coal quality, gasifier depth, hydrostatic pressure and time delay for predicting syngas composition. While carrying out these trials, the inventors have determined that the quality of syngas, namely the relative concentrations of methane (CH4) and hydrogen (H2) varies on the following input parameters: A. Gasifier operating pressure; B. inlet (oxidant) flowrate; C. outlet (raw syngas) flowrate; D. coal quality; E. water presence; F. gasifier development stage; G. the reaction zone of gasification stage movement.
[0025] To enable steady operation of the above ground gas conditioning plant shut, the raw syngas components need to be maintained within a pre-determined range. Additionally, syngas composition may be varied (Within reasonable limits) to optimise the production of hydrogen (H2) or CH4 (methane) subject of the desired product output. This can be done by utilizing the proposed algorithm.
[0026] During the initial Pre- Commercial Demonstration Plant (PCD) stage, an operational database was compiled by recording relative concentration of methane (CH4) and hydrogen (H2) under various operating conditions during underground coal 2022271406 15 Nov 2022 gasification when the input parameters (A) to (G), as listed above, are varied. Some of the parameters such as (A) gasifier operating pressure, (B) inlet flowrate comprise a subset of the operational parameters which may be varied during operation. Other operational parameters such as coal quality and water presence are somewhat non variable or fixed and dependent on the geology of the location where the coal seam is located.
[0027] The step of compiling the database: The inventors have developed one or more prediction models for the expected concentration of methane (CH4) and hydrogen (H2) based on the operational parameters (A) to (G), resulting in the compilation of a database. This enabled the establishment of a functional relationship between the syngas quality (relative concentration of methane and hydrogen in the syngas) under different operating conditions. The inventors have also realised that more than one prediction model may be applicable depending on some of the nonvariable or fixed parameters.
[0028] The prediction model for predicting one or more of the operational parameters for the underground coal gasification unit to maintain syngas quality includes: an operational result database that relates operating conditions of the syngas produced via underground coal gasification in the past to result information on the gasification process, and that stores a plurality of information obtained by relating the operating parameters (inputs) to the result information (outputs). A similarity calculating means is also provided for calculating a similarity composed of a plurality of comparison results by comparing the operating conditions stored in the operational result database with operating conditions of a prediction target syngas quality. The invention also provides a means of constructing the one or more prediction models, wherein, when 2022271406 15 Nov 2022 creating a prediction model expressing the relationship between the operating conditions and a syngas quality result which is very close to the predetermined syngas quality target. The prediction model may be constructed by using similarity in operational parameters as a weight of an evaluation function for estimating a modelling error, and employing a physical characteristic of the syngas quality as a model construction condition to calculate one or more of the variable operating parameters such as gasifier pressure (A) or oxidant flowrate (B) as mentioned in earlier sections.
[0029] The invention may also incorporate a control device which is to be in communication with the predictive model generator to control one or more of the operational parameters of the coal gasification unit (gasifier).
[0030] The prediction models developed by the present inventors will assist in maintaining syngas quality and have improved the prediction accuracy not only when the gasification is carried out under operational parameters which fall within the scope of the accumulated database but also allow for extrapolation. As such, maintaining syngas quality during periods when the using the presently described method is more accurate and efficient when compared with currently known methods especially in transient operating periods e.g. when the reaction zone for the gasification moves away from the injection point.
[0031] The syngas quality forecasting algorithm also provides a means for maintaining the desired syngas composition during the ISG process. 2022271406 15 Nov 2022
[0032] The algorithm has been developed to be applicable to any coal seam variable i.e. depth, coal quality, operating pressure, inlet well length, production well length, gasifier length. The target algorithm output can be maintained by changes in various inputs.
[0033] Syngas composition, in particular H2 & CH4 content of the syngas (output) can be controlled by manipulation of inputs i.e. gasifier operating pressure, oxidant (air) flow and water injection in the gasification process.
[0034] It is also envisioned that retraction of the injection point may be automated based on the predictive function generated by the above described method as shown in Figure 1. EXAMPLE To enable steady operation of the above ground gas conditioning plant, the raw syngas components need to be maintained within a known range. Syngas composition may be altered (Within reasonable tolerance) to optimise hydrogen (H2) or methane (CH4) production subject to the desired product output. This can be done by utilizing the present algorithm. An example of a raw syngas composition is shown in the tables below: Hydrogen (H2) mol% 20.7 Methane (CH4) mol% 8.6 Carbon Monoxide (CO) mol% 2.5 Ethane (C2H6) mol% 0.6 2022271406 15 Nov 2022 Carbon Dioxide (CO2) mol% 21.6 Nitrogen (N2) mol% 45.3 Other mol% 0.8 Total mol% 100.0 By modifying parameters A (pressure) and / or B (oxidant flowrate) the syngas composition may be changed: Hydrogen (H2) mol% 17.95 Methane (CH4) mol% 11.06 Carbon Monoxide (CO) mol% 0.35 Ethane (C2H6) mol% 1.03 Carbon Dioxide (CO2) mol% 23.37 Nitrogen (N2) mol% 46.24 Other mol% 0.0 Total mol% 100
[0035] In compliance with the statute, the invention has been described in language more or less specific to structural or methodical features. The term “comprises” and its variations, such as “comprising” and “comprised of’ is used throughout in an inclusive sense and not to the exclusion of any additional features. 2022271406 15 Nov 2022
[0036] It is to be understood that the invention is not limited to specific features shown or described since the means herein described comprises preferred forms of putting the invention into effect.
[0037] The invention is, therefore, claimed in any of its forms or modifications within the proper scope of the appended claims appropriately interpreted by those skilled in the art.
Claims
1. A method of in situ coal gasification (ISG) in a coal seam between linked inlet and production wells comprising:• igniting coal located between the wells by injecting one or more gaseous oxidants at a measured flow rate into the coal seam through the inlet well to maintain combustion between the wells, producing a syngas gas mixture ,• measurement of methane (CH4) and hydrogen (H2) content relative to other gases in the syngas mixture flowing out of the production well using one or more measurement devices; varying pressure and / or flow rate and / or composition of the gaseous oxidants being injected into the injection well in response to measuring the content of methane (CH4) and hydrogen (H2) to control the content of methane (CH4) and hydrogen (H2) within a predetermined range.
2. A method in accordance with claim 1 wherein the step of varying the pressure and / or flow rate and / or composition of the gaseous oxidants being injected is carried out by a process control system that is operably coupled with the one or more measurement devices.
3. A method of in situ coal gasification in accordance with claim 1 or claim 2 further comprising the step of varying the flow rate and or composition of the gaseous oxidants being injected into the inlet well in response to changes in one or more coal seam variables such as one or more of depth, coal quality, operating pressure, inlet well length, production well length and gasifier length.2022271406 15 Nov 20224. A method in accordance with any one of the preceding claims further comprising the step of injecting water in addition to the one or more gaseous oxidants into the inlet well.
5. A method in accordance with any one of claims 1 to 4 wherein the measured flow rate of the gaseous oxidants being injected into the inlet well is compared with a calculated flow rate using a computed prediction function to maintain the methane (CH4) and / or hydrogen (H2) content within a pre-determined range, the computed prediction function being generated from one or more historical coal gasification data models from one or more historical coal gasification events, wherein in accordance with the model methane (CH4) and / or hydrogen (H2) content in a production stream of a coal gasification plant is mathematically related to a plurality of variables measured during collection of historical data associated with the historical coal gasification date models, said plurality of variables including depth, coal quality, operating pressure, inlet well length, production well length, gasifier length.
6. A method in accordance with claim 5 wherein the computed prediction model is applied to one or more methane (CH4) and hydrogen (H2) content values in the predetermined range to output a recommended pressure value or flow rate value for the gaseous oxidants flowing into the inlet well, comparing the recommended pressure value or flow rate value for the gaseous oxidants with an actual pressure and / or flow rate value of the gaseous oxidants and varying the actual pressure and / or flow rate value of the gaseous oxidants in response to that comparison to control and maintain the content of methane (CH4) and hydrogen (H2) within a predetermined range.2022271406 15 Nov 20227. A method in accordance with claim 5 or claim 6 wherein the method comprises selection of a computed prediction model from a plurality of computed prediction models where said selection is based upon characteristics of the coal located between the wells.