Field of invention

The present invention relates to a method and apparatus for recovering heat energy from the underground gasification of fossil fuel and in particular, although not exclusively to apparatus and method for generating electricity from underground gasification.

Background

Underground gasification is an established process for utilising deposits of a fossil fuel such as coal, oil and gas to produce synthetic gas (syngas) including carbon monoxide, hydrogen and carbon dioxide. The process involves drilling an injector well from the surface to for example a coal or gas seam, supplying oxygen or an oxygen enriched gas to the fossil fuel source and initiating an exothermic reaction. An process or extraction well connects the underground combustion site to a surface plant for recovering the product gases generated. US 1,913,395, US 3,563,606 and US 3,775,073 describe example underground gasification of coal seams. However, there is a continuing need to provide more efficient energy extraction systems.

Summary of the Invention

It is an objective of the present invention to provide method and apparatus for recovering heat energy from the underground gasification of fossil fuel. It is a further specific objective to provide method and apparatus for converting heat energy resultant from underground fossil fuel gasification to electricity.

The objectives are achieved according to aspects of the present invention via an artificial geothermal power plant utilising a heat recovery system. The heat recovery system enables a working fluid to be circulated at or proximate to the underground region of the fossil fuel and then to recover the heat energy for subsequent storage, utilisation and in particular conversion to electrical energy.

Advantageously, the subject invention improves efficiency of power generation from fossil fuels and geothermal sources whilst dramatically reducing or eliminating green-house gas emissions from such processes. In particular, the subject invention according to specific implementations is adaptable to use a source of fossil fuels to provide a carbon neutral power source.

According to a first aspect of the present invention there is provided a method of recovering heat energy from underground gasification of a fossil fuel, the method comprising: injecting oxygen or an oxygen-containing gas into an injectional well or borehole extending from a surface an underground source of a fossil fuel; initiating an exothermic reaction of the fossil fuel at the source; pumping a working fluid through an underground looped conduit having a region extending proximate to the source of the fossil fuel to allow the exothermic reaction of the fossil fuel to heat the working fluid; recovering heat energy from the working fluid via at least one heat exchanger medium, apparatus or device. Reference within this specification to ‘an exothermic reaction ’ encompass the combustion or partial combustion of a fossil fuel, such as coal, oil and/or gas. The exothermic reaction may occur at or up a maximum reaction temperature of around 700°C. The exothermic reaction and/or combustion of the fossil fuel is controllable via controlling a supple rate of the oxygen or the oxygen-rich gas to the source of the fossil fuel.

The source of the fossil fuel may be localised at a subterranean region or may be dispersed. For example, the fossil fuel may be a coal seam, an oil reservoir or a natural gas reservoir or field. Accordingly, the exothermic reaction and/or combustion of the fossil fuel may be localised at a seam or reservoir or may be dispersed at a large subterranean region.

Preferably, the method further comprises extracting a gas produced by the exothermic reaction of the fossil fuel via an extraction well or borehole extending from the source of the fossil fuel to the surface.

Optionally, prior to the step of injecting oxygen or the oxygen-containing gas: drilling the injection well or borehole; and/or drilling the extraction well or borehole. Such drilling may utilise conventional drilling rigs and apparatus according to conventional techniques and methods.

Optionally, the method further comprises installing a geothermal heat extraction and power production facility at the surface and at a location proximate/close to the injection well or borehole. The geothermal heat extraction and power production facility comprises apparatus, devices and components familiar to those skilled in the art including for example air compressors, pumps, fluid circulation conduits, drill rigs, boring heads, heat exchangers, power cycle apparatus and components, steam generation units and associated components, electricity turbines, cooling towers and associated infrastructure. Such power production facilities may be permanent, semi-permanent or mobile facilities.

Optionally, the method further comprises installing an air separation unit at the geothermal heat extraction and power production facility to extract oxygen from the air and installing an oxygen injection system at the surface location of the injection well or borehole. Optionally, the step of pumping the working fluid comprises supplying the working fluid to the source of the fossil fuel at a first temperature and driving the working fluid proximate to the source of the fossil fuel to the surface, the working fluid that is driven from the source being at a second temperature greater than the first temperature.

According to a further aspect of the present invention there is provided a process for converting heat energy to electricity comprising: recovering heat energy from the underground gasification of a fossil fuel according to the method as described and claimed herein; and transferring heat energy to a turbine to convert the heat energy to electricity.

Preferably, prior to the step of transferring the heat energy, storing the heat energy at a heat storage medium, apparatus, system or storage reservoir.

According to a further aspect of the present invention there is provided apparatus for recovering heat energy from underground gasification of a fossil fuel, the apparatus comprising: an injection well or borehole and an extraction well or borehole extending between a surface and an underground source of a fossil fuel; an injection system to supply oxygen or an oxygen-containing gas via the injection well or borehole from the surface to the source of the fossil fuel; a gas capture system to capture gas produced by an exothermic reaction of the fossil fuel; and a heat recovery system to circulate a working fluid between the surface and a region proximate to the source of the fossil fuel, the working fluid capable of being heated by the exothermic reaction of the fossil fuel at the source.

According to a further aspect of the present invention there is provided apparatus for converting heat energy to electricity comprising: the apparatus for recovering heat energy from the underground gasification of a fossil fuel as described and claimed herein; and a turbine coupled to the heat recovery system to convert the heat energy received from the heat recovery system to electricity.

Optionally, the heat recovery system comprises at least one conduit extending underground from the surface to a region proximate to the source of the fossil fuel. Preferably, the heat recovery system further comprises a heat exchanger to transfer the heat from the working fluid to a heat transfer medium. Preferably, the heat recovery system comprises fluid circulation pumps, values, conduits, fluid flow control apparatus and components to provide a circulating flow of the working fluid to and from the region of the underground exothermic reaction.

A specific implementation of the present invention will now be described, by way of example only, and with reference to the accompanying drawings in which:

Figure 1 is a schematic illustration of a geothermal power plant for the recovery of heat energy from the subterranean gasification of a fossil fuel.

The current apparatus and method provides a geothermal power plant to generate power from a fossil fuel and/or geothermal source whilst minimising green-house gas emission to atmosphere.

Referring to figure 1, the present artificial geothermal power plant (AGPP) comprises an air separation unit 2 that in turn comprises an air compressor 1 and an oxygen pump 3.

The power plant further comprises a power cycle 4 connected to one or more cooling towers 5 via suitable conduits 10 and 11. Further conduits are also provided connecting the various components with one or more boreholes and wells extending underground. In particular, the plant comprises one or more subterranean wells or boreholes indicated generally by reference 12a, 12b. The wells or boreholes 12a, 12b may be provided with internal conduits, pipework etc that may be divided into separate injection wells and extraction wells to allow transfer of gases from a surface 16 to an underground source of a fossil fuel 13. In particular, an injection well or borehole extends from surface 16 to the region of the fossil fuel source 13 (via conduit 7, pump 3 and borehole 12a) to provide delivery of oxygen or an oxygen-rich gas to the source 13 from the air separation unit 2. A heat recovery system comprises power cycle 4 and a suitable working fluid conduit network comprising conduits 8, 9, 14 and 15. The heat recovery system provides as a circulating or looped arrangement in which a working fluid is capable of flowing from surface 16 to and from the fossil fuel source 13 via well or borehole 12a. In particular, conduit 9 is adapted for supplying a working fluid such as water, vapour and/or air at a first temperature from the power cycle 4 into the conduit 14 extending underground from surface 16 towards the fossil fuel source 13. A further conduit 15 extends within well 12a between fossil fuel source 13 and surface 16 and is coupled to conduit 8 for the return flow of the working fluid at a second temperature from the fossil fuel source 13 to the power cycle 4.

The subject invention is described referring to an underground natural gas reservoir of typical composition 87.1% CLL, 7.8% C2 ¾, 2.9% C3H8, 1.5% N2, and the balance of CO2. In summary, atmospheric air is drawn into the air separation unit 2 via inlet 6 and air compressor 1. Oxygen or the oxygen-enriched gas is then supplied into the borehole 12a via conduit 7 and pump 3. An exothermic reaction of the carbon source 13 is then initiated. During the course of the exothermic reaction/combustion of the fuel source, the working fluid is circulated from power cycle 4 via conduits 8, 9, 14, 15 from surface 16 into borehole 12a and proximate to the location or region of the exothermic reaction of the fossil fuel 13.

A theoretical calculation of the operating parameters, reaction conditions and output performance of the present geothermal power plant are described by way of example only based on the above natural gas composition. The lower heating value is 47.5 MJ/kg. The reactor temperature and pressure are 135°C and 100 bar, respectively. The single train air separation unit 2 supplies 100 kg/s of O2 at a purity of 95% (balance N2 and Ar) and specific energy requirement of 240 kWh/tCk. The ratio between the oxygen and fuel volumetric flow rates at the standard conditions is kept at 2.2. The liquid O2 may be pumped to a pressure 10 bar higher than a reactor pressure in a pump with isentropic efficiency of 85% and drive efficiency of 99.6%. It is assumed that the maximum combustion temperature at the fuel source 13 is 700°C. Combustion under such conditions will result in 1.8% _voi O2 excess in the vicinity of the reaction core. It is also assumed that 50% of heat can be recovered and that the temperature in the plume surrounding the reaction core is 300°C. The present power plant may operate at higher temperatures (>400°C) than the maximum temperatures in conventional geothermal plants (<400°C).

Accordingly, a potential exists to utilise more efficient power cycles for power generation, such as a conventional steam cycle (35-40%), Kalina cycle (45-58.8%) and/or a supercritical CO2 cycle (42-52%). A thermal efficiency of 40% may be assumed for further calculations.

The amount of energy required to run the O2 pump and air separation unit 2 are 1.1 and 72 MW, respectively. The amount of energy liberated from oxy-combustion of the natural gas source 13 is 592 MW. Considering the effectiveness of the heat exchanger (50%) and the thermal efficiency of the power cycle (40%), the net power output is calculated as 164 MW. Accordingly, the net thermal efficiency of the power cycle is estimated at 27.7%.

The present artificial geothermal power plant is advantageous in a number of respects including in particular the avoidance of a need to extract the fossil fuel from an underground source location. In particular, CO ₂ produced from combustion of fossil fuel may/will remain trapped in the reservoir, reducing the environmental impact. Alternatively, CO2 may be collected at the well head and reinjected (and thus trapped) into the reservoir increasing the efficiency, productivity and longevity of the field. The process of energy production presented herein therefore has potential for carbon neutrality. Advantageously, the exothermic reaction and/or combustion of the fossil fuel source will increase the temperature the reservoir which in turn provides a more energy efficient process compared to conventional geothermal power plants. Moreover, hydrogen is produced during the reaction as a by-product but at potentially considerable volumes. This can be extracted (and if necessary purified), to be used as a fuel locally, for instance to power the air purifier and increase overall efficiency, or exported as a commercial product.

Advantageously, all chemical reactions occur within the reservoir and are predicated and/or regulated by the availability of oxygen in that without oxygen, the process terminates. Thus the reaction and power output can be controlled, started and stopped via a single control factor, making the present methods and apparatus safe, highly manageable, efficient and effective.