Method and Device for Generating Steam Suited to Oxycombustion

Abstract

The invention concerns a boiler adapted to oxycombustion (30) comprising a combustion chamber (31), a water heater (33) and a vaporizer (38, 40), wherein the combustion chamber includes at least partly the water heater (33). The invention also concerns an oxycombustion method with hot water generation, comprising heating cold water with the oxycombustion flame into a heated fluid. The inventive method is advantageously implemented in the inventive device.

Description

TECHNICAL FIELD

A subject of the invention is a method for generating steam suited to oxycombustion, i.e. to the combustion of a fossil fuel with oxygen, or air enriched with oxygen as oxidant. A subject of the invention is also a device for its implementation.

STATE OF THE ART

Combustion with oxygen or oxycombustion is currently one of the most attractive ways envisaged for continuing to use fossil fuels whilst limiting CO₂ emissions into the atmosphere. In fact, the combustion of these fuels with air leads to the formation of CO₂ highly diluted in the nitrogen of the combustion air which forms a significant ballast: in general the CO₂ represents only 10 to 15% of the combustion products. The re-injection of CO₂ is one of the techniques currently envisaged for limiting emission into the atmosphere. For combustion with air, it is then necessary, for one ton of reinjected CO₂ to produce 0.3 to 0.5 ton of additional CO₂ (according to the type of fuel) for the energy requirements for the separation or capture of the CO₂. The efficiency is approximately 50%. For combustion with oxygen, insofar as the production of oxygen clearly generates less CO₂ than necessary to separate the exhaust gases as above, the efficiency is increased by approximately 30 to 50%. Thus, in the case of oxycombustion, CO₂ represents, after condensation of the steam produced by the combustion, in general approximately 90% of the exhaust gases, the remainder comprising residual nitrogen and argon contained in the oxygen used as oxidant, excess oxygen introduced in order to obtain complete combustion of the fuel and other gases formed during combustion (NO_x, SO_x). CO₂ with a purity of 95% or more after separation of all or part of the noncondensable gases can thus be easily reinjected.

The main problem caused by combustion with pure oxygen or air highly enriched with oxygen is the very high flame temperature. This can in fact exceed 3000° C. whereas for standard combustion in air it is normally about 2000° C.

This very high flame temperature leads to high-radiation heat flows which are not compatible with the operation of a conventional boiler. In fact, in a conventional boiler, the combustion chamber is surrounded by tubes in which the vaporization of the water and/or the superheating is carried out. If the heat flows are too high, a situation can occur where water empties from the tubes. The fluid in contact with the hot wall is no more than steam, the heat capacity of which is very clearly lower than that of water and therefore with a clearly reduced cooling efficiency. Such a situation rapidly leads to the destruction of the tubes by overheating. This phenomenon is also known in the art under the name “dry out” or “burn out”.

A first solution involves diluting the flame gases with the CO₂ produced. However, such recycling requires significant equipment. A solution without CO₂ recycling is therefore sought.

The “dry-out” phenomenon is a function of the heat flow received and of the steam quality in the mixture (the more steam there is, the closer to dry-out conditions). The control of the heat flows which must be absorbed on the wall is very difficult. A solution for controlling these flows involves covering the walls of the tubes with refractory materials. However, this solution appreciably reduces the efficiency of the exchange surface areas in the combustion chamber and increases the installation cost.

It is also known that in order to avoid this “dry-out” phenomenon, high-pressure water has been used in the nuclear industry, which high-pressure prevents boiling at the level of the tube/shell interface. However, here it is a matter of managing a conduction phenomenon (on the tube side) which obeys a law proportional to the difference in temperature, with a shell temperature which is relatively low.

In a distinct manner, in the case of flame boilers, it is a matter of managing a radiation phenomenon which obeys Stefan-Bolzmann law, proportional to the absolute temperature to the power of four and involving emissivity and absorption factors. Moreover, transition from a standard combustion at 2000° C. to an oxycombustion at 3000° C. leads to a temperature difference of 1000° C., i.e. approximately 50% greater. The difference in terms of radiated heat is then multiplied by more than 4. The surface power density received by the walls in the case of oxycombustion can thus easily exceed 1000 kW/m².

U.S. Pat. No. 6,619,041 describes a boiler with oxycoinbustion without recycling and its constituent equipment. The boiler described has a water preheater in the “cold” flue gas section and not in the furnace.

No other patent relating to oxycombustion discloses the installation of the water preheater in the furnace.

There is nothing in the state which therefore either describes or suggests the present invention.

SUMMARY OF THE INVENTION

The invention is based on the concept of reversal, relative to a conventional boiler, of the general circulation of the combustion products (hot fluid) to water and to steam (cold fluids).

The invention provides a boiler suited to oxycombustion comprising a combustion chamber, a water preheater and a vaporizer, in which the combustion chamber is at least partly made up of the water preheater.

According to an embodiment, the combustion chamber is completely made up of the preheater.

According to an embodiment, the preheater comprises a first bundle of independent tubes, according to a pitch of 2 to 3.

According to an embodiment, the preheater comprises a first bundle of tubes grooved internally.

According to a first variant, the vaporizer is a radiation vaporizer.

According to a second variant, the vaporizer comprises a radiation vaporizer and a convection vaporizer.

According to a third variant, the vaporizer is a convection vaporizer.

According to an embodiment, the radiation vaporizer comprising a second bundle of tubes is arranged concentrically around the preheater comprising a first bundle of tubes, in the combustion chamber.

According to an embodiment, the boiler also comprises a water/steam separation flask, supplied with water by the preheater, supplying the vaporizer with water and supplied by the vaporizer with steam.

According to an embodiment, the water preheater is arranged inside the combustion chamber.

According to an embodiment, the water preheater operates counter-current to the combustion products of the combustion chamber.

The invention also provides a method for generating hot water by oxycombustion, comprising the preheating of cold water by the oxycombustion flame to produce hot water (preheated fluid).

According to an embodiment, the method also comprises the stage of vaporization of the preheated fluid product.

According to a first variant, the stage of vaporization of the preheated fluid is implemented by radiation.

According to a second variant, the stage of vaporization of the preheated fluid is implemented by convection.

According to a third variant, the stage of vaporization of the preheated fluid is implemented by radiation and by convection.

According to an embodiment, the stage of preheating the cold water by the oxycombustion flame is implemented counter-current.

According to an embodiment, the temperature of the oxycombustion flame is comprised between 2000 and 3300° C., preferably between 2500 and 3000° C.

According to an embodiment, the temperature of the cold water is comprised between 105 and 170° C. and its pressure between 8 and 500 bars.

According to an embodiment, the preheated fluid comprises water and steam according to a water/steam proportion by mass varying from 100/0 to 50/50, preferably from 100/0 to 70/30, and advantageously from 95/5 to 80/20.

According to an embodiment, the temperature of the preheated fluid is comprised between 170 and 600° C. and its pressure between 8 and 500 bars.

According to an embodiment, the temperature of the steam produced is comprised between 170 and 600° C. and its pressure between 8 and 500 bars.

According to an embodiment, the method also comprises a stage of superheating the steam produced.

According to an embodiment, the temperature of the flue gas after the water preheating stage is from 1200 to 600° C.

According to an embodiment, the temperature of the flue gas after the vaporization stage is from 250 to 150° C.

The method according to the invention is advantageously implemented in the device according to the invention.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a diagrammatic representation of the circulation of the fluids in a conventional boiler;

FIG. 2 is a diagrammatic representation of the circulation of the fluids in a boiler according to the invention;

FIG. 3 is a representation of the circulation of the fluids in a boiler according to an embodiment;

FIGS. 4A and 4B are a partial cross-section representation of a boiler according to the prior art and of a boiler according to another embodiment, respectively.

DETAILED DISCLOSURE OF EMBODIMENTS OF THE INVENTION

With reference to FIG. 1, a conventional boiler 10 is described which comprises a water preheater 11 in contact with the cooled-down combustion gases, a vaporizer 12 and a superheater 13, these elements being placed in this order going towards the flame which is situated in the combustion chamber 14. The yield of the heat exchanges is thus maximized, the counter-current or co-current taking place between the fluids with the smallest difference in temperature. The superheater produces steam at a temperature ranging up to more than 600° C.

With reference to FIG. 2, a boiler according to the invention suited to oxycombustion is described. In the latter, the cold fluid is brought into contact with the hottest combustion products. The term “oxycombustion” covers combustions the oxidant of which is air enriched with oxygen starting from a value greater than 22% by volume. The heat flows radiated over the tubes at the level of the combustion chamber vary between 200 and 3000 kW/m² and preferably between 300 and 1000 kW/m².

Any fuel can be used, for example gas, oil, various oil residues (heavy residues in particular) or coal.

The invention is used in numerous fields. It can be used for the generation of electricity from fossil energy, which would no longer be penalized by CO₂ emissions. It can be used for the production of heavy oils within the framework of an activation of production by injection of steam into the deposit (activation of heavy-oil fields by steam), for example according to the so-called steam-assisted gravity drainage (SAGD) technique. The invention is particularly suited to the generation of high-pressure steam for the activation of heavy-oil fields. In fact, the higher the steam pressure, the greater the enthalpy to be provided for preheating the water before vaporization, and the lower the heat of vaporization. This makes it possible to envisage for high steam pressures, combustion chambers used only for heating water.

It can also be used in the case where the CO₂ is reinjected into the wells, either in order to clear it, or within the framework of the enhanced oil recovery (EOR) technique. For operations downstream of oil production, the invention will allow the use of various oil products.

Generally, the boiler according to the invention comprises a preheater at the level of the combustion chamber and a vaporizer downstream of the preheater. The terms “downstream” and “upstream” are given with respect to the direction of the flow of the combustion products (or in other words with respect to the gradient of temperatures in the boiler). Cold water enters the preheater and a preheated fluid leaves it, with a water/steam proportion by mass which can vary from 100/0 to 50/50 at the outlet from the preheater, preferably from 100/0 to 70/30, and advantageously 95/5 to 80/20. If necessary, a superheater can be arranged in the installation for the production of superheated steam, in particular in the case of electricity generation.

In the embodiment illustrated in FIG. 2, the boiler 20 comprises a combustion chamber 21, a preheater 22 fitted along the walls of the combustion chamber. The fluid leaving the preheater 22 is sent into a flask which separates the gaseous part from the liquid part. The latter is sent to the vaporizer 23, also situated partly in the combustion chamber. The fluid is vaporized in this so-called primary vaporizer. If the quantity of steam produced is not sufficient, it is also possible to use in parallel a secondary vaporizer 24 connected to the flask. This secondary vaporizer absorbs most of the heat transmitted by convection of the hot gases unlike the primary vaporizer which essentially absorbs the heat transmitted by radiation. If the preheater produces a fluid, the steam quality of which is already very high, it is even possible to use only the secondary vaporizer 24 for the production of steam. In the case where a superheater (not shown) is required, it is generally placed at the level of the secondary vaporizer, i.e. immediately upstream of or at the same level as the latter, or optionally downstream of the primary vaporizer.

With reference to FIG. 3, according to an embodiment of the invention, a boiler is described arranged vertically with one or more flame(s), towards the bottom. The boiler 30 comprises a combustion chamber 31, equipped with burners supplied from a source 32, for example of gas or heavy oil products. The temperature of the flame in the combustion chamber is for example approximately 2000 to 3000° C. A preheater 33 is joined to the combustion chamber 31. This preheater is supplied with cold water via the pipe 34. By way of example, the cold water is at a temperature of approximately 136° C. under a pressure of approximately 180 bars. Generally, the characteristics of the cold water used in the invention are situated within the following ranges: a temperature between 105 and 170° C. and a pressure between 8 and 500 bars.

The preheated fluid leaves the preheater via the pipe 35; it is at a temperature of approximately 337° C. under a pressure of approximately 180 bars. Generally, the characteristics of the heated fluid used in the invention are situated within the following ranges: a temperature between 170 and 600° C. and a pressure between 8 and 500 bars.

The heated fluid is sent to a water/steam separation flask 36. The water at the bottom of the flask 36 is sent via the pipe 37 to the primary vaporizer 38. Steam is produced in this vaporizer and leaves it via the pipe 39 in order to be sent to the flask 36. The steam is at a temperature of approximately 357° C. under a pressure of approximately 180 bars. Generally, the characteristics of the steam produced in the invention are situated within the following ranges: a temperature between 170 and 600° C. and a pressure between 8 and 500 bars.

At the outlet from the combustion chamber, i.e. at the outlet from the primary vaporizer, the temperature of the gases is then approximately 1000° C. to 1300° C. It is possible to obtain a lower outlet temperature in order to further increase the quantity of steam produced at the level of the primary vaporizer. The choice of this temperature is determined by an economic optimum which takes into account a comparison of the additional exchange surface areas on the primary and secondary vaporizers necessary in order to obtain the same production of steam.

However, the temperature of 1000° C. to 1300° C. at the outlet from the radiant zone being fixed for the economic-optimum reasons indicated above, it is possible in the present case, in order to increase the quantity of steam produced, to use a secondary vaporizer 40 which is heated essentially by convection. The water at the bottom of the flask 36 is sent via the pipe 41 to the secondary vaporizer 40. Steam is produced in this vaporizer and leaves it via the pipe 42 in order to be sent to the flask 36. The steam is at a temperature of approximately 357° C. under a pressure of approximately 180 bars. The flue gas finally leaves the boiler via the chimney 43.

Optionally, it is possible to provide a separator 44 in order to separate the exhaust gases from the water formed during the combustion, in particular by condensation. A substantially dry current of CO₂ is then extracted via the pipe 45.

Hereafter, certain elements of the boiler according to the invention are described more particularly, namely water preheater, primary radiation vaporizer, secondary convection vaporizer and superheater.

Water Preheater

As mentioned previously, the combustion chamber is constituted essentially, and more particularly around the combustion zone, by the water preheater. This water preheater generally comprises straight tubes, which are preferably smooth on the outside. These tubes are advantageously independent of one another. If the outside diameter of the tubes d, and the centre distance between the axes of the tubes p are considered, a ratio p/d is obtained, which is the “pitch” of the tubes. This pitch is for example 2 to 3. These tubes can be grooved (“corrugated”) or smooth on the inside or, as a variant, smooth with an insert. These tubes are arranged all around the combustion chamber which can be circular or rectangular in section. These tubes are supplied with cold water via the bottom from collectors. The hot water is extracted via the top. Thus, a possible local vaporization does not prevent the general movement of the fluid. The tubes constituting this preheater are preferably small in diameter, so as to limit their thickness and/or increase the internal heat transfer coefficient. Co-current operation is also possible but, in this case, the burners are arranged in the floor. A side position of the burners is also possible.

The water vaporization section comprises two separate vaporizers, a primary radiation vaporizer and the other a secondary convection vaporizer.

Primary Radiation Vaporizer

The radiation vaporizer is situated just below the water preheater if the burners are situated at the top of the combustion chamber, and just above if the burners are situated at the bottom of the combustion chamber. This vaporizer comprises straight tubes which are smooth on the outside, grooved or smooth on the inside. This vaporizer is supplied with hot water originating from the flask via amply dimensioned descending water piping and collectors. The steam produced in these tubes is returned to the flask via collectors situated close to the upper outlet from this vaporizer. The circulation of the water-steam emulsion can take place by natural circulation or optionally by forced recirculation.

Co-current operation is also possible. The diameter of the tubes is chosen in particular by optimization between a good absorption of the heat flows and a sufficient circulation of the emulsion. The water preheater and radiation-vaporizer assembly is dimensioned so that the temperature of the flue gas at the outlet from the combustion chamber is situated between 1000 and 1300° C. The distribution of the quantity of energy transmitted to each of the two exchangers is a function of the steam pressure of the boiler.

Secondary Convection Vaporizer

Complementary vaporization is carried out in a convection exchanger situated downstream of the combustion chamber. This vaporizer bundle is supplied with hot water from the flask via descending water piping independent of that which supplies the radiation vaporizer or as a variant via the same piping. This vaporizer bundle can be either vertical, or inclined relative to the horizontal and, in this case, the circulation of the water-steam emulsion can be natural; or with horizontal tubes and, in this case, the recirculation is forced with an independent pump or optionally with the same pump as that which supplies the primary radiation vaporizer. The coldest tubes can be equipped on the outside with blades if the quality of the combustion products and the virtual absence of dust allow. The flue gas allowed to enter this vaporizer at a temperature comprised between 1000 and 1300° C. is cooled down to a temperature of 10 to 20° C. above the vaporization temperature for example.

Superheater

A superheater (not shown) is generally placed before the convection vaporizer or after the first rows of tubes of this convection vaporizer. This superheater can comprise two or three bundles. Between each of these bundles, a device for desuperheating by injection of water makes it possible to control the temperature of the superheated steam.

With reference to FIG. 4A, a section of a combustion chamber of a conventional boiler is described. It comprises an outer (watertight) enclosure 51, tubes 52a, 52b, etc., in which the water-steam emulsion circulates and which are interconnected by blades 53a, 53b, etc., so as to form a watertight enclosure. In this case, these tubes form the standard vaporizer.

With reference to FIG. 4B, a section of a combustion chamber of a boiler according to an embodiment of the invention is described. It comprises an outer watertight enclosure 51 coated with refractory materials. Tubes for the circulation of cold water 54a, 54b, etc., are placed concentrically, for example, in the direction of and around the furnace. These tubes 54a, 54b, etc., in this case, form the preheater. This arrangement allows these tubes to receive heat flows over their whole surface, the surface opposite the flames receiving the radiation re-emitted by the refractory walls.

Below this preheater, the primary vaporizer can either be designed in a manner similar to the preheater, with refractory walls behind the tubes, either in a more conventional manner with tubes with longitudinal blades integral with each other, forming a screen and ensuring the watertightness of the combustion chamber.

In FIG. 4B, the combustion chamber comprises, as an example, an outer watertight enclosure 51, tubes 52a, 52b, etc., which are interconnected by blades 53a, 53b, etc., so as to form a watertight enclosure. These tubes 52a, 52b, etc., in this case, form the vaporizer. Other cold water circulation tubes 54a, 54b, etc., are placed concentrically for example in the direction of and around the furnace. These tubes 54a, 54b, etc., in this case, form the preheater. This variant is particularly suited to cases where the heat flows remain limited. This design makes it possible to limit the direct radiation on the tubes of the vaporizer, particularly in the zone where the steam quality is already high.

This variant also allows the remodelling of certain boilers in order to convert them at less cost to boilers according to the invention, since it is sufficient to insert additional tubes acting as a water preheater, into an existing combustion chamber already equipped with vaporizer bundles.

These remodellings are favoured by the fact that the boilers concerned have initially been constructed for combustion with air as oxidant, therefore the surface areas of the walls are relatively large.

The invention offers the following advantages compared with conventional boilers:

Compared with a boiler supplied with atmospheric air.

Combustion with pure oxygen considerably reduces the nitrogen ballast which facilitates the collection of the CO₂.

This reduction in the nitrogen ballast offers the following advantages:

there is an increase in the flow radiated directly by the flame and an increase in the flow radiated by the flue gas. The first effect is due to the increase in the combustion temperature, the second is linked to the fact that with an equal gaseous layer thickness, a higher concentration of CO₂+H₂O increases the emissive power of the flue gas (in fact triatomic gases are radiant, unlike diatomic gases). There is therefore a notable reduction in the necessary exchange surface areas.

there is a reduction in the volume of the flue gas produced. This reduction leads to a reduction in the convection exchanges and, consequently, in the convection exchanger surface areas.

The oxycombustion boiler according to the invention will, with similar power and yield, have a combustion chamber smaller than the combustion chamber of a boiler using atmospheric air, and clearly less significant convection bundles. The cost and the weight are therefore reduced, the second of these characteristics being of prime importance in the case of offshore installations.

Compared with a boiler supplied with oxygen with CO₂ recycling.

In order to avoid the difficulties produced by the high temperature level of combustion with oxygen, one solution involves diluting the flame by a recycling of CO₂ originating downstream of the boiler.

The flows radiated directly from the combustion zone will thus be smaller. However, the advantage of compactness disappears since the combustion chamber surface area is then of the same order of magnitude as that of a boiler using atmospheric air.

The recycling of the flue gas or the CO₂ requires a relatively significant recycling network with additional energy consumption for the recycling blower. Recycling, of whatever kind, always leads to drawbacks which are absent within the framework of the invention:

If recycled flue gas is taken directly from the outlet from the boiler, the recycling does not affect the heat yield of the boiler, on the other hand the additional energy consumption is significant due to the temperature and volumes of gas to be recycled. Moreover, account will have to be taken of sulphur corrosion if the temperature of the recycled fluid is close to its dew-point temperature.

If the CO₂ is taken up after cooling down and separation of the acid condensates, the additional energy consumption is reduced as its temperature is clearly lower, the condensation taking place at a low temperature, on the other hand the yield of the boiler is affected by the heating of this CO₂ up to the boiler-outlet temperature.

The method according to the invention can be implemented under pressure (boiler with a pressurized combustion chamber), which can offer an advantage when it is desired to reinject the CO₂ produced.

The invention is not limited to the embodiments described but is capable of numerous variations easily accessible to a person skilled in the art.

Claims

1. Boiler suited to oxycombustion comprising a combustion chamber, a water preheater and a vaporizer, in which the combustion chamber is at least partly made up of the water preheater.

2. Boiler according to claim 1, in which the combustion chamber is completely made up of the preheater.

3. Boiler according to claim 1, in which the preheater comprises a first bundle of independent tubes, according to a pitch of 2 to 3.

4. Boiler according to claim 1, in which the preheater comprises a first bundle of tubes grooved internally.

5. Boiler according to claim 1, in which the vaporizer is a radiation vaporizer.

6. Boiler according to claim 1, in which the vaporizer comprises a radiation vaporizer and a convection vaporizer.

7. Boiler according to claim 1, in which the vaporizer is a convection vaporizer.

8. Boiler according to claim 5, in which the radiation vaporizers comprising a second bundle of tubes is arranged concentrically around the preheater comprising a first bundle of tubes, in the combustion chamber.

9. Boiler according to claim 1, also comprising a water/steam separation flask, supplied with water by the preheater, supplying the vaporizer with water and supplied by the vaporizer with steam.

10. Boiler according to claim 1, in which the water preheater is arranged inside the combustion chamber.

11. Boiler according to claim 1, in which the water preheater operates counter-current to the combustion products of the combustion chamber.

12. Boiler according to claim 1, also comprising a superheater.

13. Method for generating hot water by oxycombustion, comprising the preheating of cold water by the oxycombustion flame to produce hot water.

14. Method according to claim 13, also comprising the stage of vaporization of the preheated fluid produced.

15. Method according to claim 14, in which the stage of vaporization of the preheated fluid is implemented by radiation.

16. Method according to claim 14, in which the stage of vaporization of the preheated fluid is implemented by convection.

17. Method according to claim 14, in which the stage of vaporization of the preheated fluid is implemented by radiation and by convection.

18. Method according to claim 13, in which the stage of preheating the cold water by the oxycombustion flame is implemented counter-current.

19. Method according to claim 13, in which the temperature of the oxycombustion flame is comprised between 2000 and 3300° C., preferably between 2500 and 3000° C.

20. Method according to claim 13, in which the temperature of the cold water is comprised between 105 and 170° C. and its pressure between 8 and 500 bars.

21. Method according to claim 13, in which the temperature of the preheated fluid is comprised between 170 and 600° C. and its pressure between 8 and 500 bars.

22. Method according to claim 13, in which the preheated fluid comprises water and steam according to a water/steam proportion by mass varying from 100/0 to 50/50, preferably from 100/0 to 70/30, and advantageously from 95/5 to 80/20.

23. Method according to claim 14, in which the temperature of the steam produced is comprised between 170 and 600° C. and its pressure between 8 and 500 bars.

24. Method according to claim 14, also comprising a stage of superheating the steam produced.

25. Method according to claim 13, in which the temperature of the flue gas after the water preheating stage is from 1200 to 600° C.

26. Method according to claim 13, in which the temperature of the flue gas after the vaporization stage is from 250 to 150° C.

27. Method according to claim 13, implemented in a device according to one of claims 1 to 12.