GASIFICATION SYSTEM AND METHOD

Abstract

A gasification system and a method for gasifying a particulate carbonaceous fuel are disclosed. The gasification system has a gasification chamber with an upper section and a lower section with a fuel inlet for injecting a particulate carbonaceous fuel and oxidant into the upper section whereby, in a thermo-chemical reaction, synthesis gas and residual char is generated. The gasification system further includes a separator configured to receive the synthesis gas and to separate residual tar form the synthesis gas. Further, there is a char bed disposed in the lower section formed by residual char generated in the thermo-chemical reaction and a gas-inlet at a bottom portion of the lower section for injecting gas into the char bed. The residual tar is injected into the char bed whereby, in a thermal cracking process, the residual tar is converted into synthesis gas. Hereby, it is possible to utilize the otherwise lost energy contained in the residual tar, and thereby achieve better efficiency in a gasification system, in a cost-effective and simple manner.

Claims

1. A gasification system comprising: a gasification chamber having an upper section and a lower section; at least one fuel-inlet for injecting carbonaceous fuel and oxidant into said upper section whereby, in a thermo-chemical reaction, synthesis gas and residual char is generated; a separator in fluid connection with the upper section via an outlet, said separator being configured to receive said synthesis gas and to separate residual tar from said synthesis gas; a char bed disposed in said lower section, said char bed being formed by residual char generated in said thermo-chemical reaction and allowed to travel downwards within said gasification chamber to the char bed; at least one gas-inlet at a bottom portion of said lower section for injecting gas into said char bed; and at least one tar inlet arranged to inject said residual tar from said separator into said char bed whereby, in a cracking process, said residual tar is converted into synthesis gas.

2. The gasification system according to claim 1, wherein the injected gas through said at least one gas-inlet is arranged such that an injection velocity is controlled such that a fluidization of said char bed does not disrupt a balance between the downwardly directed travelling of residual char from said upper section and upwardly directed flow of gas.

3. The gasification system according to claim 1, wherein the gas injection through said at least one gas-inlet is arranged such that a gas velocity of said upwardly flowing gas within the gasification chamber is in the range from 0.1 m/s to 2.0 m/s whereby a fluidization of said char bed does not disrupt a balance between the downwardly directed travelling of residual char from said upper section and upwardly directed flow gas.

4. The gasification system according to claim 1, wherein said carbonaceous fuel is a solid particulate carbonaceous fuel, and wherein said upper section of the gasification chamber has a curved inner surface, and wherein said solid particulate carbonaceous fuel and oxidant is injected into said upper section tangentially such that an entrained flow of said synthesis gas is formed, whereby residual char is separated and allowed to travel from said upper section down towards said lower section in order to form the char bed.

5. The gasification system according to claim 1, wherein said gasification chamber further comprises a set of temperature control inlets spatially separated and distributed along a length, extending between the lower section and the upper section, of said gasification chamber, wherein said set of temperature control inlet(s) is/are configured to inject gas into the gasification chamber whereby a process temperature within said gasification chamber is controlled.

6. The gasification system according to claim 1, wherein said at least one fuel inlet comprises a feeding device for injecting said carbonaceous fuel into said upper section, and wherein said gasification system further comprises at least one oxidant inlet, separate from said at least one fuel inlet, for injecting oxidant into said gasification chamber.

7. The gasification system according to claim 6, wherein said gasification system comprises at least two oxidant inlets spatially separated along a length, extending between said lower section and said upper section of said gasification chamber, for injecting oxidant into said gasification chamber.

8. The gasification system according to claim 7, wherein a rate of injected oxidant into said gasification chamber from said at least two oxidant inlets, is individually controllable.

9. The gasification system according to claim 1, further comprising a perforated grate located at said bottom portion, in order to extract residual ashes.

10. A method for gasifying carbonaceous material comprising: providing a gasification chamber having an upper section and a lower section; injecting a carbonaceous fuel and oxidant into said upper section of the gasification chamber whereby, in a thermo-chemical reaction, synthesis gas and residual char is generated; extracting said synthesis gas from said upper section of the gasification chamber; separating residual tar from said synthesis gas; forming a char bed of said residual char in said lower section; and injecting said residual tar into said char bed.

11. The method according to claim 10, further comprising: maintaining a temperature at said upper section of the gasification chamber in the range of 800 C. to 1100 C.; and maintaining a temperature of said char bed at said lower section of the gasification chamber in the range of 1200 C. to 1500 C.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0062] For exemplifying purposes, the invention will be described in closer detail in the following with reference to embodiments thereof illustrated in the attached drawings, wherein:

[0063] FIG. 1 is a schematic illustration of a gasification system in accordance with an embodiment of the present invention;

[0064] FIG. 2 is a schematic illustration of a gasification chamber in accordance with an embodiment of the present invention;

[0065] FIG. 3 is a schematic flow chart illustrating a method for gasifying carbonacous material in accordance with an embodiment of the present invention.

[0066] 30

DETAILED DESCRIPTION

[0067] In the following detailed description, preferred embodiments of the present invention will be described. However, it is to be understood that features of the different embodiments are exchangeable between the embodiments and may be combined in different ways, unless anything else is specifically indicated. Even though in the following description, numerous specific details are set forth to provide a more thorough understanding of the present invention, it will be apparent to one skilled in the art that the present invention may be practiced without these specific details. In other instances, well known constructions or functions are not described in detail, so as not to obscure the present invention.

[0068] In FIG. 1a schematic illustration of a gasification system 1 is provided. The gasification system 1 includes a gasification chamber 2 having an upper section 3 and a lower section 4. The gasification chamber can for example be made of a ceramic material. The height of the gasification chamber is preferably in the range of 2000 mm to 4000 mm and the outer diameter in the range of 500 mm to 3000 mm. The gasification chamber 2 is shown in a cross-sectional view, the cross section taken in a plane including an elongated axis 101 (z-axis in a cylindrical coordinate system where the gasification chamber is approximated as cylinder). Further, the gasification chamber 2 is preferably of a cylindrical shape, but may be of any shape having an internal cavity 7 without departing from the scope of the invention. The lower section 4 has an inner wall 6b arranged in a tapered cylindrical shape, the inner diameter of the (cylindrical) gasification chamber 2 decreasing towards a bottom portion 5 of the gasification chamber 2. Thus, in other words the upper section 3 can be said to comprise an inner wall 6a that is, at least partly curved, e.g. cylindrically shaped and the lower section 4 has an inner wall 6b that is, at least partly, conically shaped.

[0069] Further, the gasification system has at least one fuel inlet 8 for injecting a solid particulate carbonaceous fuel and an oxidant (indicated by arrow 20) into the upper section 3 of the gasification chamber 2. The particulate carbonaceous fuel can for example be cellulose particles, e.g. wood particles, having a diameter of less than 3000 m, preferably less than 2000 m and most preferably less than 1000 m. The particulate carbonaceous fuel and the oxidant are then converted, by a thermo-chemical reaction, into synthesis gas and residual char at the upper section 3 within the gasification chamber 2.

[0070] The gasification system 1 further has a separator 10 (schematically illustrated) in fluid connection with the upper section 3 of the gasification chamber 2 via an outlet 9. The separator 10 is configured to separate residual tar from the synthesis gas produced in the gasification chamber 2. The separator 10 can for example be an oil scrubber arranged to direct the synthesis gas through an oil mist in order to remove the residual tar from the synthesis gas, such that a combustible gas 11 may be extracted. The combustible gas can subsequently be used in a combustion engine or a gas turbine for e.g. producing electricity. Alternatively, the separator can be a quench water circuit with a quench tower and a venture scrubber, where the quench tower cools the synthesis gas (which passes through a water mist in the quench tower) in order to condense residual tar, and the venture scrubber acts as a de-duster, removing small particulate matters.

[0071] Further, the gasification system 1 has a char bed 12 disposed in the lower section 4. The char bed is formed by residual char generated in the thermo-chemical reaction at the upper section 3, the residual char then being allowed to travel downwards within the gasification chamber 2 in order to form the char bed 12. The flow or movement of the residual char can either be controlled by injecting the particulate fuel and oxidant into the upper section such that a helical flow of the synthesis gas is formed and residual char is separated as in a cyclone separator within the gasification chamber 2. Alternatively, or additionally, an injection velocity of the injected gas (e.g. air) through or around the grate 15 disposed under the char bed 12 at the bottom section may be controlled such that a total upwardly directed gas velocity within the gasification chamber is limited so that a balance between the downwardly directed travelling of residual char from the upper section 3 and the upwardly directed gas flow is not disrupted. The gas-inlet 17 at the bottom portion of the lower section 4 is configured to inject gas (such as e.g. air) into the char bed in order to at least partly fluidize the char bed 12. In other words, the injected gas through or around the grate 15 may not have an injection velocity or gas velocity that is so high so that residual char within the gasification chamber 2 is blown upwards towards the outlet 9. The perforated grate 15 defines a supporting surface arranged at the bottom of the gasification chamber 2 in order to support a formation of the char bed 12. Stated differently, the supporting surface is arranged to allow a build-up of residual char at a bottom portion of the gasification chamber 2 in order to form a char bed. Thus, the supporting surface can be said to have a substantially planar extension with a normal vector extending generally along a vertical axis (naturally some tolerances and shape optimization are feasible). The fuel injection and the gas injection at the bottom will however be further discussed in reference to FIG. 2.

[0072] Even further, the gasification system 1 has a tar inlet 18 arranged to inject the residual char that was separated from the synthesis gas in the separator 10 into the fluidized char bed, whereby, in a catalytic/thermal cracking process, the residual tar is converted into synthesis gas. The char bed may also in some embodiments be semi-fluidized. In a semi-fluidized char bed, the char bed is allowed to fluidize at a (maximum) predefined height, indicated by the broken line 23. This is to be understood as that the top surface of the fluidizing char bed is arranged to be at a predefined height of the gasification chamber, or that the char bed 12 has a predefined maximum volume. The predefined height 23 is preferably set at a level right below where the risk of bed material (e.g. residual char particles) being pulled away from the char bed due to entrainment is low or minimal. Thus, if the char bed is allowed to fluidize (i.e. have a height or upper surface) above this predefined height an undesired entrainment of bed material may occur. However, the fluidization level or height of the fluidized char bed is preferably set as close as possible, albeit below, this predefined height, since one wants to maximize the size/volume of the char bed without passing the predefined height. The height or fluidization level is controlled by controlling the injection rate/gas velocity at the gas-inlet 17.

[0073] The tar inlet 18 is in fluid connection with the separator 10. Thus, any residual tar caught in the synthesis gas generated in the thermo-chemical process at the upper section 3 is utilized to generate more synthesis gas whereby the efficiency of the complete gasification system 1 is increased. Moreover, maintenance requirements are reduced since the amount of residual tar causing pipe-clogging or unwanted build-up in other parts of the gasification system is minimized.

[0074] Thus, it can be said that the upper section 3 of the gasification chamber forms a first reaction zone (where particulate carbonaceous fuel is gasified and synthesis gas is produced) and the lower section 4 of the gasification chamber forms a second reaction zone (where residual tar is cracked and more synthesis gas is produced).

[0075] FIG. 2 shows a slightly enlarged illustration of the gasification chamber in FIG. 1. As previously discussed, the residual char generated in the thermo-chemical reaction at the upper section 3 is allowed to travel downwards within the gasification chamber in order to form the char bed 12. Thus, there is no need to handle the high temperature residual char in any process step outside the gasification chamber and the whole gasification system can be made more cost-effective.

[0076] The residual char can for example be separated by controlling the injection of particulate (carbonaceous) fuel and the oxidant into the gasification chamber 2 such that a vortex or cyclone separation is achieved. Such a gasification chamber can be referred to as an entrained flow reactor. In more detail, the particulate fuel and oxidant (sometimes referred to as mixture) can be injected with a velocity within the range of 20 m/s to 150 m/s. As mentioned, the injected flow is preferably substantially tangential with the inner surface 6a of the upper section and with a pitch, such that a downwardly spiralling swirling/helical flow of synthesis gas is created within the gasification chamber 2. Thus, along the swirling flow within the cavity 7 the gasification chamber 2 the mixture of particulate carbonaceous fuel and oxidant undergoes a thermo-chemical reaction and synthesis gas and residual char is produced. As a result of the swirling flows, the centrifugal force causes the residual char particles towards the inner walls 6a, 6b of the gasification chamber 2, allowing the residual char to be transported towards the lower section 4 and the bottom of the gasification chamber 2, where they form the char bed 12. The char bed 12 may, as mentioned, be semi-fluidized, i.e. have a maximum predefined top surface height, as indicated by the broken line 23. Furthermore, the gasification chamber 2 may be arranged with more than one fuel inlet, such that several parallel swirling flows may be created, thereby increasing the efficiency of the gasification system further, such a system is described in the currently unpublished European Patent Application No. 15163203.1 by the same application, incorporated herein by reference.

[0077] The gasification chamber 2 can for example be defined by cylindrical coordinates, i.e. the gasification chamber 2 has an extension in a radial direction p, an extension in an azimuth angle direction , and an extension in a z-direction being perpendicular to a (p, )-plane defined by the radial and azimuth angle directions. The fuel-inlet(s) 8 are then accordingly arranged to inject the particulate carbonaceus fuel (substantially) along the azimuth angle direction. Optionally, the fuel-inlet(s) may further be configured to also inject the carbonaceous fuel slightly downwards in a negative z-direction such that a downwardly spiraling flow is achieved. The spiraling flow being coaxial with respect to the exit pipe, forming the outlet 9, where the exit pipe has a central axis parallell to the z-direction.

[0078] Alternatively, or additionally the downwardly directed flow of residual char formed in the thermo-chemical process can be controlled by controlling the injection velocity of the gas injection through the gas-inlet 17 at the bottom portion of the lower section 4. By maintaining the upwardly directed gas velocity within the gasification chamber in the range between 0.1 m/s and 2.0 m/s the char bed can be fluidized without disrupting the downwardly directed flow of residual char (the residual char is formed by heavy particles in relation to the synthesis gas) such that the amount of residual char exiting the gasification chamber 2 through the outlet 9 is minimized/reduced and the extracted synthesis gas is kept substantially char-free. To some extent, the fluidized char bed can also be said to form an updraft gasifier where e.g. air is provided through the grate 15.

[0079] Further, bottom ash in the char bed may be evacuated from the gasification chamber 2 through a wet-ash system. The wet ash system comprises a set of injection nozzles (not shown) disposed at the bottom portion of the lower section 4 forming a water-ash mixture having a water-level at a bottom portion. The water-ash mixture can then be allowed to flow from the bottom portion, e.g. by periodically moving the grate 15 along the longitudinal axis 101 and collected in a tank (not shown) in fluid connection with the bottom portion. This wet-ash system can be used in order to control the size of the char bed or amount of residual char collected at the lower portion. The bottom grate 15 may be perforated, whereby the bottom ash may be evacuated via holes or perforations provided through the grate 15.

[0080] Even further, the gasification system can optionally comprise a feeding device 21 such as e.g. a feeding screw or feeding pump arranged to inject a carbonaceous fuel (solid or liquid) into the gasification chamber 2.

[0081] The perforated grate 15 located at the bottom portion may for example comprise a ceramic material or any other suitable material. Moreover, the gasification chamber 2 may be arranged with a set of temperature control inlets 22 or oxidant inlets 22. The temperature control inlets/oxidant inlets 22 are preferably spatially separated and distributed along a length (elongated axis 101) of the gasification chamber 2. A set in the present context can be one or more. The temperature control inlets 22 are configured to inject gas (such as e.g. air) into the gasification chamber in order to control the temperature within the gasification chamber 2 at various sections. By having a plurality of temperature control inlets 22 a temperature gradient can be formed within the gasification chamber 2, for example going from a highest temperature in the lower section 4 to a lowest temperature in the upper section 3. The temperature control inlets may also be operated as oxidant inlets for injecting oxidant (at various vertical levels) into the gasification chamber 2 in accordance with an embodiment of the invention. The oxidant inlets may accordingly also be used for temperature/process control.

[0082] FIG. 3 illustrates a schematic flow chart describing a method for gasifying carbonaceous material in accordance with an embodiment of the present invention. The method comprises a step of providing 301 a gasification chamber having an upper section and a lower section. For example, a gasification chamber as described in reference to FIGS. 1 and 2. Next, a fuel containing carbonaceous material and an oxidant is injected 302 into the upper section of the gasification chamber, whereby, in a thermo-chemical reaction, synthesis gas and residual char is generated or produced. The carbonaceous fuel and oxidant may be injected separately through at least two separate inlets or concurrently through at least one common inlet. The synthesis gas is subsequently extracted 303 from the upper section of the gasification chamber, e.g. via an outlet provided at the upper section. Continuingly, a step of separating 304 residual tar(s) from the synthesis gas is performed. The residual tar(s) can for example be separated by condensing the residual tar(s) in the synthesis gas, sedimentation, filtering or using a centrifuge. The method further includes forming 305 a char bed at the lower section of the gasification chamber. The char bed being formed 305 from the residual char collected within the gasification chamber. Subsequently, the separated tar(s) is/are injected 306 into the char bed.

[0083] The method may further comprise maintaining a temperature within the gasification chamber at the upper section in the range of 800 C.-1100 C., preferably in the range of 850 C.-1000 C. and more preferably in the range of 900 C.-950 C. Moreover, the method can also comprise a step of maintaining a temperature of the char bed at the lower section of the gasification chamber in the range of 1200 C.-1500 C. preferably in the range of 1250 C.-1400 C. and more preferably in the range of 1300 C.-1350 C. The temperature within the different sections or portions of the gasification chamber can for example be maintained (or controlled) by injection of oxidant into the gasification chamber via a set of temperature control inlets and/or the one or more gas inlets at the bottom portion.

[0084] The invention has now been described with reference to specific embodiments. However, several variations of the gasification system are feasible. For example, injections velocities may be varied within the intervals given in order to suit specific applications and carbonaceous fuel-types, as already exemplified. Such and other obvious modifications must be considered to be within the scope of the present invention, as it is defined by the appended claims. It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting to the claim. The word comprising does not exclude the presence of other elements or steps than those listed in the claim. The word a or an preceding an element does not exclude the presence of a plurality of such elements.