ANAEROBIC BIOGAS PLANT AND DIGESTION METHOD

Abstract

An anaerobic biogas plant for the digestion of fresh organic digestion substrate. The digester having an entry section with an inlet at a first end, a rear section at a second, opposite end, and a middle section arranged between the entry section and the rear section. The digester further includes a stirring unit comprising a shaft extending in the direction from the first end to the second end and having at least one agitating arm for mixing the substrate within the digester. The middle section of the digester includes a main outlet and the rear section includes an additional outlet. The digester further includes a separate return line leading from the additional outlet back to the entry section of the digester for allowing a direct return of a minor fraction of digested substrate removed from the rear section back into the entry section of the digester.

Claims

1. Anaerobic biogas plant comprising a digester for the digestion of fresh organic digestion substrate, said digester having an entry section with a main inlet at a first end, a rear section at a second opposite end, a middle section arranged between the entry section and the rear section, and a stirring unit comprising a shaft extending in the direction from the first end to the second end and having at least one agitating arm for mixing the digestion substrate within the digester, wherein the middle section of the digester includes a main outlet for removal of a major fraction of digestion substrate after a shortened retention time t, the rear section includes an additional outlet for removal of a minor fraction of digestion substrate after an inoculum retention time T, and the digester includes a separate return line leading from the additional outlet back to the entry section of the digester for allowing a direct return of the minor fraction of digestion substrate removed from the rear section back into the entry section of the digester.

2. Anaerobic biogas plant according to claim 1, wherein the digester is an elongated.

3. Anaerobic biogas plant according to claim 1, wherein the stirring unit extends from the entry section through the middle section into the rear section of the digester.

4. Anaerobic biogas plant according to claim 1, wherein the shaft of the stirring unit has a multitude of agitating arms, which are provided with blades at their free ends.

5. Anaerobic biogas plant according to claim 1, wherein the digester is operated in the plug-flow mode.

6. Anaerobic biogas plant according to claim 1, wherein the separate return line exclusively connects the rear section with the entry section.

7. Anaerobic biogas plant including two or more digesters, all of which being of the type as defined in claim 1.

8. Digestion method for use in an anaerobic biogas plant having at least one digester, said digester having an entry section with a main inlet at a first end, a rear section at a second, opposite end, a middle section arranged between the entry section and the rear section, a main outlet from the middle section, an additional outlet from the rear section, a section of the digester, and a stirring unit comprising at least one agitating arm for mixing the digestion substrate within the digester; the method comprising the steps of: a) supplying fresh digestion substrate through the main inlet into the digester; b) removing a minor fraction of digestion substrate via the additional outlet from the rear section of the digester after an inoculum retention time T, c) returning the minor fraction of digestion substrate via the return line directly back into the entry section of the digester to act as inoculation substrate, d) mixing the inoculation substrate with fresh digestion substrate to provide a combined digestion substrate, and e) removing a major fraction of combined digestion substrate via the main outlet from the middle section of the digester after a shortened retention time t; wherein the major fraction of combined digestion substrate is larger than the minor fraction of combined digestion substrate that is returned as inoculation substrate, and the shortened retention time t is shorter than the inoculum retention time T.

9. Digestion method according to claim 8, wherein the minor fraction of digestion substrate removed from the rear section is directly returned back into the entry section of the digester without further processing.

10. Digestion method according to claim 8, wherein the minor fraction of digestion substrate removed from the rear section is less than 50% of the amount of fresh digestion substrate fed into the digester.

11. Digestion method according to claim 1, wherein the shortened retention time t is less than 50% of the inoculum retention time T.

12. Digestion method according to claim 1, wherein the shortened retention time t is at least 2 days.

13. Digestion method according to claim 1, wherein the inoculum retention time T is at least 7 days.

14. Digestion method according to claim 10, wherein the removal and return of the minor fraction of digestion substrate from and back into the digester occurs in a continuous fashion or in regular time intervals.

15. Digestion method according to claim 10, wherein the removal and return of the minor fraction of digestion substrate from and back into the digester occurs in dependence of at least one condition parameter measured within the digester, said at least one condition parameter is selected from the group consisting of pH, volume of substrate, temperature, content of volatile fatty acids, free ammonia concentration and partial gas pressure.

Description

FIGURES

[0047] FIG. 1 a schematic view of a biogas plant according to the state in the art without a separate inoculation digester, but having only one single digester which is supplied with substrate;

[0048] FIG. 2 a schematic view of an alternative biogas plant according to the state in the art, the biogas plant having a separate inoculation digester and a fast digester, which are coupled to each other and are supplied with substrate; and

[0049] FIG. 3 a schematic view of an anaerobic biogas plant having a single digester carrying out both functions of an inoculation digester and a fast digester in accordance with the present invention.

[0050] FIG. 1 shows a schematic representation of the digestion process in an anaerobic biogas plant according to the state of the art. Said biogas plant includes a single digester 10 with an inlet 11 at one end, an outlet 12 at an opposite end and a return path 13 connecting the outlet 12 with the inlet 11. Fresh digestion substrate 14 is introduced into the digester 10 and digested therein for a total standard retention time T, before being removed as (essentially fully) digested substrate 15. The term (essentially fully) digested substrate thereby refers to a substrate that has passed the four main stages of digestion, i.e. hydrolysis, acidogenesis, acetogenesis, and methanogenesis. Therefore, pH, methane yield and the concentration of volatile fatty acids (VFAs) in the digestion substrate are indicators of how far the digestion has progressed. A minor fraction 16 of the digested substrate 15 removed from the digester 10 is returned through the return path 13 back into the digester 10 to act as “self-inoculation substrate” that accelerates the digestion process.

[0051] The biogas plant shown in FIG. 2 is operated in accordance with the method described in EP 2 948 537. In this biogas plant, an inoculum digester 20 is coupled with at least one further fast digester 21. Fresh digestion substrate 22 is introduced into the inoculum digester 20 and digested for a time T before leaving the inoculum digester 20 as inoculating substrate 23. A minor fraction 24 of the inoculating substrate 23 from the inoculum digester 20 is returned to the inoculum digester 20 by means of a return path 26 to act as self-inoculate and a major fraction 25 of the inoculating substrate 23 is supplied to the fast digester 21 by means of a fast inoculation path 27. The major fraction 25 of inoculating substrate 23 is mixed in the fast digester 21 with additional fresh digestion substrate 22. Because the inoculum digester 20 produces inoculating substrate 23 required for both digesters 20, 21, the retention time in the fast digester 21 can be reduced. This allows for increasing the overall throughput of digestion substrate through the biogas plant.

[0052] The anaerobic biogas plant shown in FIG. 3 is operated in accordance with the present invention and includes a digester 30 with an inlet 31 in an entry section 32 at one end, a main outlet 33 in a middle section 34, an additional outlet 35 in a rear section 36 at an opposite end and a return line 37 connecting the additional outlet 35 with the inlet 31 for returning some digested substrate back into the digester 30 as self-inoculation substrate. Via the inlet 31 in the entry section 32, the digester 30 is supplied with fresh digestion substrate 40, which is transported in the plug-flow mode through the middle section 34 and towards the rear section 36 of the digester 30. A minor fraction 41 of digestion substrate is digested in the digester 30 for an inoculum retention time T. Then said minor fraction 41 of digestion substrate is removed via the additional outlet 35 and is returned back into the digester 30 to act as inoculation substrate 42. The inoculation substrate 42 is mixed with fresh digestion substrate 40 to obtain a combined digestion substrate 44. The combined digestion substrate 44 is subsequently digested upon passing the digester 30 in the plug-flow mode. A major fraction 45 of combined digestion substrate 44 is not transported to the rear section 36 of the digester 30 but is removed after a shortened retention time t via the main outlet 33 from the middle section 34 of the digester 30. Only the minor fraction 41 of the combined digestion substrate 44 is passed on and retrieved from the rear section 36 of the digester 30 via the additional outlet 35—again, as described above, to be returned back into the digester 30 and act as inoculation substrate 42.

[0053] Compared to known anaerobic biogas plants having a single digester as presented in FIG. 1, the anaerobic biogas plant in accordance with the present invention allows for a significant reduction of the average retention time (i.e. dwell time) of the combined digestion substrate in the digester and thus for increasing the overall throughput of fresh digestion substrate. This will be explained further with a specific example, with process parameters as indicated in Table 1 below:

TABLE-US-00001 TABLE 1 Digester A, Digester B, Digester C, Figure 1 Figure 2 Figure 3 Fresh 100 t/day     30 t/day + 100 t/day    Input 100 t/day = 130 t/day    Digester 130 t/day for  39 t/day for 130 t/day for capacity 21 days = 21 days +  7 days + 2′730 t 130 t/day for  30 t/day for  7 days = 14 days = 1′729 t 1′330 t Output 100 t/day    130 t/day    100 t/day    Average 2′730 t/ 1′729 t/ 1′300 t/ retention 100 t/day = 130 t/day = 100 t/day = time 27 days    13 days    13 days   

[0054] Assuming that in a biogas plant according to the prior art as shown in FIG. 1, a total input of 130 tons/day of combined digestion substrate 17 is fed into the digester 10 and digested for a retention time T=21 days. Said total input consists of 100% fresh digestion substrate 14 and 30% inoculation substrate 16. After the retention time T=21 days, a major fraction 18 of 100%=100 tons/day of the digested substrate 15 can be retrieved from the anaerobic biogas plant and a minor fraction 16 of 30%=30 tons/day is returned to the digestion chamber to act as inoculation substrate. This results in a digester capacity of 2′730 tons. For the combined digestion substrate—including the re-circulated inoculation substrate—this results in an average retention time of 27 days.

[0055] In the biogas plant of the prior art shown in FIG. 2, 30 tons/day of fresh digestion substrate 22 and 9 tons/day of a self-inoculation substrate is fed into the inoculation digester 20 and digested therein for 21 days. The 39 tons/day of digested substrate 23 retrieved from the inoculation digester 20 are divided into two fractions: A minor fraction of 9 tons/day is returned back into the inoculation digester 20 and a major fraction 25 of 30 tons/day is introduced into the fast digester 21, where the latter is mixed with 100 tons/day of fresh digestion substrate 22. The fast digester 21 of FIG. 2 is thus fed with a total input of 130 tons/day and said input is digested for a shortened retention time of 7 days. For all digestion substrate together in both digesters, this results in an average retention time of 13 days.

[0056] In accordance with the present invention as shown in FIG. 3, the digester 30 of an anaerobic biogas plant is supplied with a total input of 130 tons/day of combined digestion substrate 44, which consists of 100 tons/day of fresh digestion substrate 40 and of 30 tons/day of inoculation substrate 42. A major fraction 45 of 100%=100 tons/day of the combined digestion substrate 44 is digested for a shortened retention time t of 7 days before being removed through the main outlet 33. A minor fraction 41 of 30%=30 tons/day of the combined digestion substrate 44 is digested for another 14 days, i.e. for a total prolonged retention time T of 21 days, before being removed via the additional outlet 35 and recirculated into the digester 30 as inoculation substrate 42. For all digestion substrate, i.e. the fresh digestion substrate 40 and the inoculation substrate 42 together, this results in an average retention time of 13 days.

[0057] Compared to the anaerobic biogas plant shown in FIG. 1 that requires an average retention time of 27 days, the biogas plant of the present invention has an average retention time of only 13 days (for the same amount of input material). The biogas plants shown in FIGS. 2 and 3 have the same average retention time.

[0058] Thus, assuming that all digesters have the same digester capacity/size, the overall throughput of the anaerobic biogas plants shown in FIGS. 2 and 3 is substantially increased compared to the one shown in FIG. 1.

[0059] Although the biogas plant shown in FIG. 2 and the inventive biogas plant shown in FIG. 3 have the same average retention time, the biogas plant of the present invention has the benefit that it requires only one digester, which reduces the building and maintenance costs.