METHOD AND DEVICE FOR SINGLE WELL UNDERGROUND
GASIFICATION OF FOSSIL FUELS
FIELD OF THE INVENTION
The invention relates to a method and device for single well underground gasification of fossil fuels and finds application in the energy sector.
BACKGROUND OF THE INVENTION
A method is known of underground coal gasification by drilling two wells, in one of them the oxidizing gas is supplied - air, oxygen, oxygen- enriched air. The other well is used for pumping out the obtained synthesis fuel gas. The difference in the technology is in the placement of the wells in the coalfield. They are either a pair at a greater distance or a group of several, such as in one of them the oxidizing gas is fed, while from the rest the synthesis gas is removed. These methods work well with good gas permeability of coal seams. The greater it is, the farther the wells will be, which leads to reduction in the investments costs (US3856084).
A device is also known for underground coal gasification, comprising a casting pipe in the upper part of which an outlet pipe is arranged and a movable pipeline for oxidizing gas is axially fitted in the pipe. In the bottom of the casing pipe an ignition means is coupled (US3856084). In the course of the gasification large single cavities in the coal seam are formed. The large size of these cavities prevents the control of the process and creates preconditions for serious landslide processes.
TECHNICAL SUMMARY
The object of the present invention is to provide greater controllability of the process of underground gasification and operation at low gas permeability of coal seams.
This problem is solved by a method of single well underground gasification of fossil fuels, according to the present invention, comprising supply of the oxidizing gas and removal the resulting synthesis gas through one and the same well. The oxidizing gas is fed into the bottom of the well, reaches the surface of the underground layer without passing through it. There, after ignition, the process of gasification takes place and the resulting syngas is removed through a point above the point of oxidizing gas delivery. Between the supplied oxidizing gas and the resulting syngas heat exchange takes place.
When the concentration of the synthesis gas and its temperature simultaneously increase, the supplied oxidizing gas is reduced.
Delay in the process is achieved by simultaneously reducing the flow of the oxidizing gas and removing the synthesis gas.
The object is also achieved with a device embodying the method of single well underground gasification of fossil fuels, comprising a casing in the upper part of which an outlet pipe is arranged, in the pipe a movable pipeline for the oxidizing gas is fitted axially and in the lower part of the casing pipe an ignition means is coupled. According to the invention the casing pipe is provided with perforations and the perforated zone of the pipe is equal to the height of the coal seam. In the pipeline a pipeline for firing gas is fixed axially, bearing a burner, and the ignition means is designed as a chamber, rigidly fixed to the lower part of the pipeline for oxidizing gas and closed by transverse baffle.
In an embodiment the burner is provided with an ignition device.
The upper part of the pipeline above the casing pipe is formed as a heating chamber.
Axially into the heating chamber the pipeline for firing gas with the burner is located.
The advantages of the method and the device are the following:
- A single well is drilled through which both the oxidant and the resulting synthesis gas pass
Easy control of the gasification process, including its interruption
- An opportunity for gasification of gas-tight reserves
An opportunity to fortify the fossil layer during gasification without interrupting it
- An opportunity to fortify the fossil layer after the end of the gasification
- An opportunity of maximum (full) utilization of the fossil layer through gasification
- Better energy efficiency due to oxidizing gas heating at the expense of the resulting syngas - An opportunity of underground storage of slag, ash and waste from desulphurization installations of coal thermal power plants
- The oxidizing gas is supplied at lower pressure, as it is not necessary for it to pass through a layer of fossil fuel to reach the outlet well.
- An opportunity of underground storage of carbon dioxide, with considerably greater capacity than the methods used so far
- An opportunity of gasification of gas-tight underground deposits because it is not necessary for the oxidizing gas to pass through the underground layer, so that the synthesis gas can be released in another point.
- The gasification takes place on the interface between the underground layer and the well.
DESCRIPTIONS OF THE FIGURES ENCLOSED
Figure 1 is a well with firing burner;
Figure 2 - A well with external heating of the oxidizing gas for firing and liquid firing fuel;
Figure 3 - A well with external heating of the oxidizing gas for firing without liquid firing fuel;
Figure 4 - Positioning of the wells in a cross section of two adjacent wells with the free volume in the fossil layer formed around them;
Figure 5 - View of the field with positioning of the wells in a horizontal section of the fossil layer with distribution of the wells;
Figure 6 - Horizontal section of partially fortified fossil layer,
Figure 6A - Cross section of the situation in Figure 6; Figure 7 - Horizontal section of gasified fossil layer, merging of all free volumes around the operating wells;
Figure 7A - Cross section of Figure 7;
Figure 8 - Gasification scheme of final utilization of the fossil layer.
EMBODIMENTS OF THE INVENTION
For control of the gasification process essentially two parameters are monitored - the concentration of the exiting synthesis gas and the pressure in the pipeline with the oxidizing gas. As oxidizing gases air, oxygen or oxygen- enriched air are used.
The main reaction in the gasification is:
2C8 + 02 «-→ 2CO + Qi [1]
In gasification of fossil fuels containing a mixture of hydrocarbons (e.g. oil shale) the reaction will be:
CnH2(n+i) + (n+i/2)02 «-→ nCO + (n+1 )H20 + Q2 [2]
CO + H20 «-→ C02 + (n+1)H20 + Q3 [3]
Reaction [3] takes place with the moisture present in the deposit as well.
Besides the above described reactions, reactions of complete carbon oxidation also proceed.
Cs + 02 «-→ CO + Q4 [4]
CnH2(n+i) + (11/2 n +1 )02 <-→ nC02 + (n+1)H20 + Q6 [5] In these processes the solid carbon and the moisture are converted to a gaseous mixture of products of the above reactions. This mixture is removed under the pressure of the oxidizing gas.
Information about the proceeding of the process is given by the temperature of the exiting syngas and the concentration of carbon dioxide. When the concentration of carbon dioxide and the temperature increase simultaneously, this means that the supplied oxidant is in excess and must be reduced and vice versa.
The process is delayed by simultaneously reducing the flow of oxidizing gas and removing the syngas, so as to maintain constant pressure in the underground space. In this way the temperature in the underground layer is retained for a longer time. The return of the yield to the optimal range is achieved by increasing the rate of the oxidizing gas, by maintaining constant pressure again, by removal of the synthesis gas.
In the classical method of underground gasification, where two wells are used, the energy necessary for reaction 3, for evaporation of the moisture in the fossil fuels and for heating the oxidant is provided by reactions 4 and 5. In this case, the oxidant is preheated by indirect heat exchange by the synthesis gas on its way from the surface of the coal seam. This results in improved energy efficiency of the gasification, reducing heat losses with the obtained gas.
The main element of the device for implementation of the described method of single well gasification of fossil fuels, presented in Fig. 1 , comprises a casing 1 , closed at the top and provided with perforations 5 for synthesis gas, which serves to strengthen the walls of the well and at the same time performs the function of collector of the resulting synthesis gas. The perforated area of pipe 1 is equal to the height of the coal seam 10. Axially in the casing pipe 1 a pipeline for the oxidizing gas 3 is arranged, which separates the oxidizing gas from the resulting synthesis gas. Axially in pipeline 3 a pipeline for firing gas 2 is fitted, serving o for fossil fuel firing only. The oxidizing gas is fed through pipe 4, attached to the upper part of the oxidizing gas pipeline 3.
In the lower part of the casing pipe 1 by means of transverse baffle 13 chamber 6 is formed for the oxidizing gas, the area in which the oxidant is supplied.
At the bottom of the pipeline for the firing gas 2 a burner 7 is coupled, which is used for fossil fuel firing only. Next to the burner 7 an ignition device 8 is installed for the ignition of the burner with the firing gas.
In the upper part of the casing pipe 1 an outlet pipe 9 is arranged for the removal of the resulting synthesis gas from a layer of fossil fuel.
Fig. 2 shows a well with external heating of the oxidizing gas, such as the heating chamber 12 is arranged above the casing pipe 1.
At the bottom of the well liquid fuel is loaded. The pipeline for the firing gas 2 with the burner 7 is located on the upper wall of the heating chamber 12.
It is possible liquid fuel to be supplied for firing fossil fuel only - Fig. 3. Example 1
During drilling the casing pipe 1 is lowered so that the area of perforation to be equal to the height of the coal seam. Together with the pipeline for oxidizing gas 3 another pipeline for firing gas 2 is also lowered, at whose end there is a burner 7 and an igniter 8. The oxidizing gas is fed first and then the firing gas. The igniter 8 is switched on and after the area at the bottom of the well takes fire, the flow of the firing gas is stopped, the feeding of the oxidizing gas remains - Fig. 1.
Example 2
In the casing pipe 1 the oxidizing gas pipeline 3 is lowered only. Through it a certain amount of liquid fuel (e.g. oil) is supplied. Firing gas is fed then with temperature above the temperature of spontaneous combustion of the liquid fuel.
The firing is ascertained by the maintenance of high temperature of the exiting syngas. If that does not happen, the operation is repeated with a new amount of liquid fuel - Fig. 2.
Example 3
In the casing pipe 1 the oxidizing gas pipeline 3 is lowered only. Through it the oxidizing gas is fed, heated to the ignition point of the located in the deposit fossil fuel. Once the exiting gases from the space between the two pipes reach temperature above the temperature of the oxidizing gas, its heating is stopped - Fig. 3.
The resulting syngas passes through the holes of the casing pipe 1 , passes through the space between the casing pipe 1 and pipe 3 for oxidant and comes to the surface, where it is delivered for incineration or as raw material for chemical processing. USE OF THE INVENTION
To achieve maximum utilization of the coal seam, hexagonal distribution of the wells is most appropriate. In that way the distances between all neighboring wells are equal, unlike the square lattice. A section of the positioning of the wells is given in Fig. 4 and a view of the field in Fig. 5.
The opportunity to control the rate of gasification gives a significant advantage to this method. In the development of underground deposits there is always the possibility of landslides of overlying layers. The single well drilling, according to the invention, enables avoiding these landslides in two ways.
The first option is not allowing the merger of the hollow volumes formed around the wells. In this way the remaining walls between them strengthen the layers, but a large part of the deposit is not utilized.
In the second option the process in part of the wells is stopped before the merger of their volumes. Then the pipes for oxidant and firing gas are removed, the casing pipe remains only. It is pulled out to the upper end of the formed cavity and inert material is poured through it to fill the cavity - Fig. 6 and Fig. 6A.
After that the gasification in the rest of the wells continues until they all merge - Fig. 7 and Fig. 7A. In this way maximum utilization of the deposit is achieved. Finally inert material is fed through the remaining wells as well. This stabilizes the whole field and prevents future landslides. As inert materials waste slag, fly ash of coal power plants are most suitable, as well as calcium sulfite or sulfate from desulphurization installations. This avoids the need of building and maintaining slag dumping sites.
Maximum utilization of the deposit is obtained when, after merging the volumes of the individual wells, the process in part of them is stopped. Half of those operating are only used for injection of the oxidant and the rest for output of the resulting synthesis gas - Fig. 8. Thus at the end of the process the possibility of bypassing the oxidizing gas in gasification in high volumes is avoided.
After utilization of the deposit through gasification, large fortified underground cavities remain. They are the most appropriate option for storage of carbon dioxide, if the layers around them have the appropriate composition and structure. In the currently developed methods for underground storage it is necessary to make a large number of drillings in order to obtain a sufficiently large surface area for absorption of the gas.
The remaining after the gasification volumes and their respective contact surface are much larger than those obtained by the methods developed so far.
Therefore, instead of the above-described technology for final filling with inert materials, carbon dioxide can be stored, as a measure for limiting the greenhouse effect. An advantage of this method is that lower pressure is initially necessary for the storage of the dioxide, due to the large free volume. There is also a positive side effect here. Initially the remaining after the gasification gas has a high temperature, therefore low density. After the injection of the colder carbon dioxide the temperature begins to decrease, the residual synthesis gas shrinks and releases space for more dioxide without artificially increasing the pressure. This is a major advantage compared to storage in depleted oils fields, where temperatures are low and there is also residual pressure from the fluid used for the extraction of oil.
The large contact surface with the adjacent layers allows again working at lower pressure compared to purpose-built storage wells. This is due to the fact that the rate of absorption is proportional both to the pressure and the contact surface. Consequently, high speed of absorption (storage) will be provided primarily by the surface area, rather than by raising the temperature.
The injection of carbon dioxide in the released from the gasification volumes has a fortifying effect on the upper layers as well.