METHOD AND GAS REFINING INSTALLATION FOR PROVIDING A GAS STREAM HAVING A WOBBE INDEX WITHIN A REQUIRED RANGE

Abstract

 The invention relates to a method an gas refining installation for providing a gas stream having a Wobbe index within a required range. The method and gas refining installation both make use of refining an unrefined gas stream into a refined gas stream with a Wobbe index above said required range, and bypassing or feeding an unrefined gas stream to said refined gas stream, thereby obtaining a combined gas stream having a Wobbe index within said required range. Use can be made of either a feedback loop or a feedforward loop.

Description

[0001] The invention relates to a method and gas refining installation for providing a gas stream having a Wobbe index within a required range.

[0002] Such a method and gas refining installation for providing a gas stream having a Wobbe index within a required range are known per se. In such a method and installation an unrefined gas stream is refined by a gas refining means that is set such that the resulting refined gas stream has a Wobbe index that is within the required range. Because the composition of the unrefined gas stream may vary continuously, as may be the case when the gas stream is a biogas stream, the gas refining means may need constant resetting or adjustment to refine said gas stream such that its Wobbe index is in said required range. Adjustment or resetting of the gas refining means is however relatively slow, such that the refined gas stream may sometimes have a Wobbe index outside said required range.

[0003] It may be an object of the invention to at least partially overcome said above problem. In particular it may be an object of the invention to provide an improved method and gas refining installation for providing a gas stream having a Wobbe index within a required range.

[0004] In a first embodiment of the invention this object is achieved by a method for providing a gas stream having a Wobbe index within a required range, which method comprises the following steps, to be performed in any suitable order, of:

(a) feeding an unrefined gas stream to a gas refining means for refining said unrefined gas stream into a refined gas stream with a Wobbe index above said required range;

(a) feeding an unrefined gas stream to said refined gas stream, thereby obtaining a combined gas stream;

(c) measuring the Wobbe index of said combined gas stream;

(d) comparing the measured Wobbe index with a predetermined value or range, and

(e) controlling the flow rate of the unrefined gas stream to be fed to said refined gas stream based on said comparison in order to obtain a said combined gas stream having a Wobbe index within said required range.

[0005] In accordance with the invention the flow rate of the unrefined gas stream that is fed to the refined gas stream is controlled using a feedback loop, such that the Wobbe index of the combined gas stream is controlled using said feedback loop. Controlling the Wobbe index of the combined gas stream by using a feedback loop is relatively fast, such that the Wobbe index of the combined gas stream may be maintained within said required range, or is at least outside of said required range for relatively short time periods.

[0006] In step (d) the measured Wobbe index of the combined gas stream is compared to a predetermined value or setpoint. Said predetermined value or setpoint is chosen to be within said required range, preferably close or equal to a middles value of said required range. If the measured Wobbe index of the combined gas stream is above said predetermined value or setpoint, the flow rate of the unrefined gas stream that is fed to the refined gas stream is increased. If the measured Wobbe index of the combined gas stream is below said predetermined value or setpoint, the flow rate of the unrefined gas stream that is fed to the refined gas stream is decreased. If the measured Wobbe index of the combined gas stream is equal to said predetermined value or setpoint, the flow rate of the unrefined gas stream that is fed to the refined gas stream is maintained. The flow rate of the unrefined gas stream that is fed to said refined gas stream may thus continuously be adjusted or maintained in accordance with said comparison, such that the Wobbe index of the combined gas stream may be maintained within said required range.

[0007] In step (d) the measured Wobbe index of the combined gas stream may alternatively be compared to a predetermined range. The flow rate of the unrefined gas stream that is fed to the refined gas stream is in that case increased if the measured Wobbe index is above the predetermined range, decreased if the measured Wobbe index is below the predetermined range, or maintained if the wobbe index falls within the predetermined range. The flow rate of the unrefined gas stream that is fed to said refined gas stream may thus continuously be adjusted or maintained in accordance with said comparison, such that the Wobbe index of the combined gas stream may be maintained within said required range. Said predetermined range may be said required range, or may be a subrange falling within said required range.

[0008] Said method may optionally comprise a further step of measuring the Wobbe index of said refined gas stream. An advantage of this further step is that it may be monitored that the Wobbe index of the refined gas stream is above said required range.

[0009] In an alternative embodiment of the invention the above described object is achieved by a method for providing a gas stream having a Wobbe index within a required range, which method comprises the following steps, to be performed in any suitable order, of:

(f) feeding an unrefined gas stream to a gas refining means for refining said unrefined gas stream into a refined gas stream with a Wobbe index above said required range;

(g) measuring the Wobbe index of said refined gas stream;

(h) providing a second unrefined gas stream;

(i) measuring the Wobbe index of said second unrefined gas stream;

(j) calculating a required flow rate of said second unrefined gas stream to be fed to said refined gas stream based on the measured Wobbe indexes thereof in order to obtain a combined gas stream having said Wobbe index within said required range;

(k) feeding said required flow rate of said second unrefined gas stream to said refined gas stream, thereby obtaining a said combined gas stream having said Wobbe index within said required range.

[0010] In accordance with the invention the flow rate of the second unrefined gas stream that is fed to the refined gas stream is controlled using a feedforward loop, such that the Wobbe index of the combined gas stream is controlled using said feedforward loop. Controlling the Wobbe index of the combined gas stream by using a feedforward loop is relatively fast, such that the Wobbe index of the combined gas stream may be maintained within said required range, or is at least outside of said required range for relatively short time periods.

[0011] In both embodiments of the invention, i.e. the first embodiment using the feedback loop and the alternative embodiment using the feedforward loop, the settings of the refining means do not need to be adjusted during refining for controlling the Wobbe index of the gas stream, because maintaining the Wobbe index of the combined gas stream within the required range is controlled by feeding a certain amount or flow rate of unrefined gas stream to the refined gas stream. Because the refined gas stream is refined to a too high quality, such that the Wobbe index thereof is always above the required range, the quality of the refined gas stream may be lowered by feeding said certain amount or flow rate of unrefined gas stream thereto, such that the Wobbe index of the combined gas stream will be within the required range.

[0012] It is noted that in both embodiments of the invention, i.e. the first embodiment using the feedback loop and the alternative embodiment using the feedforward loop, said (second) unrefined gas stream that is fed to said refined gas stream may be a bypass gas stream of a main gas stream. A main unrefined gas stream is hereby divided into a first unrefined gas stream that is fed to said refining means and a bypass unrefined gas stream that is fed to said refined gas stream. In step (i) of the alternative embodiment the Wobbe index of either the main unrefined gas stream or of the bypass unrefined gas stream may be measured.

[0013] It is further noted that in both embodiments of the invention, i.e. the first embodiment using the feedback loop and the alternative embodiment using the feedforward loop, measuring the Wobbe index of any gas stream may be performed directly by using a Wobbe index meter. Alternatively a composition of any gas stream may be measured based on which the Wobbe index of that gas stream may be calculated.

[0014] It is further noted that in both embodiments of the invention, i.e. the first embodiment using the feedback loop and the alternative embodiment using the feedforward loop, only one gas refining means is required, because the refined gas stream is mixed with an unrefined gas stream.

[0015] It is further noted that in both embodiments of the invention, i.e. the first embodiment using the feedback loop and the alternative embodiment using the feedforward loop, said unrefined gas stream may in particular be a biogas stream.

[0016] It is further noted that the required range of the Wobbe Index may for example be determined by the grid operator. This may differ per grid operator and/or per country. For example, said required range may for example be chosen to be between 43,8 MJ/Nm3 and 44,4 MJ/Nm3 for the Netherlands.

[0017] The above described object may be achieved by a gas refining installation for providing a gas stream having a Wobbe index within a required range according to a first embodiment of the invention, said installation comprising:

• a gas refining means for refining an unrefined gas stream into a refined gas stream with a Wobbe index above said required range;
• a feed for feeding an unrefined gas stream to said refined gas stream, thereby obtaining a combined gas stream;
• a means for measuring the Wobbe index of said combined gas stream;
• a means for comparing the measured Wobbe index with a predetermined value or range, and
• a control means for controlling the flow rate of the unrefined gas stream to be fed to said refined gas stream based on said comparison in order to obtain a said combined gas stream having a Wobbe index within said required range.

[0018] Such a gas refining installation makes use of a feedback loop as described above.

[0019] Alternatively the above described object may be achieved by a gas refining installation for providing a gas stream having a Wobbe index within a required range according to an alternative embodiment of the invention, said installation comprising:

• a gas refining means for refining an unrefined gas stream into a refined gas stream with a Wobbe index above said required range;
• a means for measuring the Wobbe index of said refined gas stream;
• a means for providing a second unrefined gas stream;
• a means for measuring the Wobbe index of said second unrefined gas stream;
• a means for calculating a required flow rate of said second unrefined gas stream to be fed to said refined gas stream based on the measured Wobbe indexes thereof in order to obtain a combined gas stream having said Wobbe index within said required range;
• a feed for feeding the required flow rate of said second unrefined gas stream to said refined gas stream, thereby obtaining a said combined gas stream having said Wobbe index within said required range.

[0020] Such a gas refining installation makes use of a feedforward loop as described above.

[0021] In an embodiment of the gas refining installation, either using a feedback loop or a feedforward loop, said installation comprises a main feed for feeding a main unrefined gas stream to the installation, said main feed comprising a first branch for feeding a first part of the unrefined gas stream to the gas refining means, and said main feed comprising a second brand for feeding another part of the unrefined gas stream to the refined gas stream.

[0022] An advantage of this embodiment is that one unrefined gas stream may be fed to the installation of which said another part is bypassed from the gas refining means and fed to the refined gas stream. The flow rate of said bypassed another part of the unrefined gas stream is controlled or calculated using the feedback loop or feedforward loop, respectively. In the installation that makes use of the feedforward loop, the Wobbe index of either said another part of the unrefined gas stream that is bypassed may be measured or the Wobbe index of said one unrefined gas stream or main unrefined gas stream may be measured.

[0023] In another embodiment of the gas refining installation, a said means for measuring the Wobbe index of a gas stream is a Wobbe index meter. An advantage thereof is that the Wobbe index may directly be measured.

[0024] Alternatively a said means for measuring the Wobbe index of a gas stream is a means for measuring the composition of that gas stream from which composition the Wobbe index of that gas stream may be determined.

[0025] Said gas refining means may be any suitable type of gas refining means. In an embodiment said gas refining means comprises a membrane.

[0026] In another embodiment of the gas refining installation, said installation comprises a mass flow controller for controlling the flow rate of said (second) unrefined gas stream to be fed to said refined gas stream.

[0027] Said mass flow controller may for example be controlled by means of a controller of the PID type.

[0028] Preferred embodiments of the present invention are further elucidated in the following description with reference to the drawing, in which:

Figure 1 is a schematic representation of the method and installation according to a first embodiment of the invention, and

Figure 2 is a schematic representation of the method and installation according to an alternative embodiment of the invention.

[0029] Figure 1 shows a first embodiment of a method and installation for providing a gas stream having a Wobbe index within a required range. A feed feeds a main unrefined biogas stream 1 to a compressor 2, which compressor 2 pressurizes said main biogas stream 1 up to a certain pressure above atmospheric pressure. Said certain pressure may for example be chosen between 500 and 1500 kPa. The feed then splits into a first feed for feeding a first part 3 of the unrefined gas stream to a gas refining means 4 and a second feed, also called a bypass, for feeding or bypassing another part 5 of the unrefined gas stream to the refined gas stream 6 that exits said refining means 4. A gas composition meter 7 is provided for measuring the composition of the refined gas stream 6, from which composition the Wobbe index W (in MJ/Nm³) of the refined gas stream 6 may be calculated by using the formula:

in which

qr is the relative density (in kg/Nm³), and

H_s is the higher caloric value (in MJ/Nm³) calculated by using the formula:

in which:

H_s is the higher caloric value (MJ/Nm³)

H_s,i is the higher caloric value of component i (kJ/mol)

x, x_i is the concentration of component i (mol%)

m is the number of components

z_n is the compressibility under normal conditions.

[0030] It is noted that instead of measuring the gas composition from which the Wobbe index may be determined, it is also possible or even preferred to use a Wobbe index meter for directly measuring the Wobbe index of the refined gas stream 6.

[0031] The gas refining means 4 is set such that the Wobbe index of the refined gas stream 6 is always above a required range, which required range may for example be dictated by a grid operator. The meter 7 may be used for monitoring whether the Wobbe index of the refined gas stream 6 is indeed above said required range. Said refining means 4 further comprises a discharge 8 for discharging a methane poor fraction of the gas. As described above, via said bypass said another part 5 of the unrefined gas stream is fed to the refined gas stream 6, resulting in a combined gas stream 12. Said another part 5 of the unrefined gas stream is fed to the refined gas stream 6 using a valve 10. A pressure reduction valve 9 is provided for controlling the pressure of gas stream 6, the gas refining means 4, and the first part 3 of the unrefined gas stream to a pressure above the pressure of said combined gas stream 12, which pressure of the combined gas stream 12 may be dictated by said grid operator. The relatively high pressure of gas stream 5, provided by the compressor 2, enables injection of gas stream 5 into refined gas stream by means of valve 10. The Wobbe index of the combined gas stream 12 is measured using a meter 11, which may either be a Wobbe index meter or a gas composition meter as described above. The Wobbe index of the combined gas stream 12 is compared to a predetermined value or setpoint. Said predetermined value or setpoint is chosen to be within said required range, preferably close or equal to a middles value of said required range. If the measured Wobbe index of the combined gas stream 12 is above said predetermined value or setpoint, the flow rate of the unrefined gas stream 5 that is fed to the refined gas stream 6 is increased. If the measured Wobbe index of the combined gas stream is below said predetermined value or setpoint, the flow rate of the unrefined gas stream 5 that is fed to the refined gas stream 6 is decreased. If the measured Wobbe index of the combined gas stream is equal to said predetermined value or setpoint, the flow rate of the unrefined gas stream 5 that is fed to the refined gas stream 6 is maintained. This way, the flow rate of the unrefined gas stream 5 that is fed to said refined gas stream 6 is thus continuously adjusted or maintained in accordance with said comparison, such that the Wobbe index of the combined gas stream may be maintained within said required range. Using this feedback loop, the quality of the combined gas stream 12, in particular the Wobbe index thereof, may be controlled easily and or fast.

[0032] Figure 2 shows a second embodiment of a method and installation for providing a gas stream having a Wobbe index within a required range. Only the differences with the first embodiment of figure 1 will be described. For a further description the reader is referred to the description of figure 1. The same reference numerals are used for the same elements.

[0033] In the embodiment of figure 2 use is made of a feedforward loop instead of a feedback loop. The Wobbe index of the refined gas stream 6 is measured using meter 7. A further meter 13, which may either be a Wobbe index meter or a gas composition meter as described above with respect to figure 1, is used for measuring the Wobbe index of the unrefined bypass gas stream 5. Based on the measured Wobbe indexes of both the refined gas stream 6 and the unrefined bypass gas stream 5 the flow rate of unrefined gas stream 5 that has to be fed to the refined gas stream 6 in order to obtain a combined gas stream having a Wobbe index that is within the required range is calculated. This calculated flow rate of the unrefined gas stream 5 is fed to the refined gas stream 6 using valve 10. Said meter 11 is not required in this second embodiment of the invention. However, if desired said meter 11 may be provided to monitor if the combined gas stream 12 has indeed a Wobbe index that lies within the required range.

[0034] It is noted that the flow rate of the unrefined gas stream 5 that is bypassed to the refined gas stream 6 may in either embodiment be controlled by a mass flow controller. This mass flow controller is known per se and not shown in the figures. The mass flow controller may be controlled using a controller of the PID type.

[0035] It is noted that the invention is not limited to the shown embodiments but also extends to variants within the scope of the appended claims.

Claims

1. A method for providing a gas stream having a Wobbe index within a required range, said method comprising the steps, to be performed in any suitable order, of:

(a) feeding an unrefined gas stream to a gas refining means for refining said unrefined gas stream into a refined gas stream with a Wobbe index above said required range;

(b) feeding an unrefined gas stream to said refined gas stream, thereby obtaining a combined gas stream;

(c) measuring the Wobbe index of said combined gas stream;

(d) comparing the measured Wobbe index with a predetermined value or range, and

(e) controlling the flow rate of the unrefined gas stream to be fed to said refined gas stream based on said comparison in order to obtain a said combined gas stream having a Wobbe index within said required range.

2. A method for providing a gas stream having a Wobbe index within a required range, said method comprising the steps, to be performed in any suitable order, of:

(f) feeding an unrefined gas stream to a gas refining means for refining said unrefined gas stream into a refined gas stream with a Wobbe index above said required range;

(g) measuring the Wobbe index of said refined gas stream;

(h) providing a second unrefined gas stream;

(i) measuring the Wobbe index of said second unrefined gas stream;

(j) calculating a required flow rate of said second unrefined gas stream to be fed to said refined gas stream based on the measured Wobbe indexes thereof in order to obtain a combined gas stream having said Wobbe index within said required range;

(k) feeding said required flow rate of said second unrefined gas stream to said refined gas stream, thereby obtaining a said combined gas stream having said Wobbe index within said required range.

3. The method according to claim 1 or 2, wherein a main unrefined gas stream is provided of which a part is fed to the gas refining means in step (a), respectively step (f), and of which another part is fed to the refined gas stream in step (b), respectively step (k).

4. A gas refining installation for providing a gas stream having a Wobbe index within a required range, said installation comprising:

- a gas refining means for refining an unrefined gas stream into a refined gas stream with a Wobbe index above said required range;

- a feed for feeding an unrefined gas stream to said refined gas stream, thereby obtaining a combined gas stream;

- a means for measuring the Wobbe index of said combined gas stream;

- a means for comparing the measured Wobbe index with a predetermined value or range, and

- a control means for controlling the flow rate of the unrefined gas stream to be fed to said refined gas stream based on said comparison in order to obtain a said combined gas stream having a Wobbe index within said required range.

5. A gas refining installation for providing a gas stream having a Wobbe index within a required range, said installation comprising:

- a gas refining means for refining an unrefined gas stream into a refined gas stream with a Wobbe index above said required range;

- a means for measuring the Wobbe index of said refined gas stream;

- a means for providing a second unrefined gas stream;

- a means for measuring the Wobbe index of said second unrefined gas stream;

- a means for calculating a required flow rate of said second unrefined gas stream to be fed to said refined gas stream based on the measured Wobbe indexes thereof in order to obtain a combined gas stream having said Wobbe index within said required range;

- a feed for feeding the required flow rate of said second unrefined gas stream to said refined gas stream, thereby obtaining a said combined gas stream having said Wobbe index within said required range.

6. The gas refining installation according to claim 4 or 5, comprising a main feed for feeding a main unrefined gas stream to the installation, said main feed comprising a first branch for feeding a first part of the unrefined gas stream to the gas refining means, and said main feed comprising a second brand for feeding another part of the unrefined gas stream to the refined gas stream.

7. The gas refining installation according to any of claims 4 - 6, wherein a said means for measuring the Wobbe index of a gas stream is a Wobbe index meter.

8. The gas refining installation according to any of claims 4 - 6, wherein a said means for measuring the Wobbe index of a gas stream is a means for measuring the composition of that gas stream from which composition the Wobbe index of that gas stream may be determined.

9. The gas refining installation according to any of claims 4 - 8, wherein said gas refining means comprises a membrane.

10. The gas refining installation according to any of claims 4 - 9, comprising a mass flow controller for controlling the flow rate of said (second) unrefined gas stream to be fed to said refined gas stream.

11. The gas refining installation according to claim 10, wherein said mass flow controller is controlled by means of a controller of the PID type.

Drawing