Anaerobic digestion agitation system

Abstract

An anaerobic digestion agitation system, and / or a method of using the same, where a plurality of agitators for agitating slurry in a digester is provided. Each agitator in the plurality comprises a rotatable shaft that is substantially vertical, a drive unit adapted to rotate the rotatable shaft, and an axial flow impeller coupled to the rotatable shaft. In operation, the plurality of agitators comprises first and second agitators that are counter-rotating and nearest neighbors.

Description

ANAEROBIC DIGESTION AGITATION SYSTEMTECHNICAL FIELD

[0001] The present disclosure relates generally to agitation systems for anaerobic digestion, and more specifically to agitations systems having multiple agitators and / or methods of using such agitation systems.BACKGROUND

[0002] Anaerobic digestion, which refers to the breakdown of organic matter by microorganisms under low oxygen conditions, produces biogas - a promising renewable source of energy.

[0003] The production of biogas via anaerobic digestion is often conducted in one or more mixed reactors (e.g., which can operate as a continuous stirred tank reactor or CSTR). Mixed reactors used for anaerobic digestion are often vertical cylindrical tanks (e.g., fabricated from steel and / or concrete) that are mixed (e.g., continuously or periodically) using a plurality of side-mounted agitators (e.g., side-entry or submersible) and / or a centrally disposed topmounted agitator (e.g., having a single co-centric shaft and one or more impellers).

[0004] Unfortunately, such agitation systems can limit the size of tanks used for anaerobic digestion (e.g., the use of very large tanks can be associated with poor agitation and / or relatively high operating costs as a result of a relatively high power consumption). Accordingly, when constructing large scale biogas production plants (e.g., producing about 1,000,000 MMBTU / year), it is common to reach the desired capacity of the plant by providing more tanks rather than constructing a single larger tank. Unfortunately, providing more tanks is associated with a relatively high capital cost (e.g., requires more material).

[0005] It would be advantageous to provide effective mixing and / or agitation while reducing costs (e.g., operating and / or capital) when scaling-up biogas production.SUMMARY

[0006] The present disclosure describes one or more agitation systems that can be used with relatively large tanks, or other types of digesters having large footprints such as lagoon-type digesters, and thus can facilitate relatively large scale anaerobic digestion using a reduced number of digesters (e.g., a single digester). Advantageously, reducing the number of digesters can reduce capital costs.

[0007] In particular, the present disclosure relates to agitation systems for anaerobic digestion that include multiple agitators, where the agitators are spaced throughout the digester in an arrangement and / or with a configuration that can provide more efficient agitation and / or can reduce the energy required for agitation (e.g., reduce operating costs).

[0008] In accordance with one aspect of the instant invention there is provided an anaerobic digestion agitation system comprising: a plurality of agitators for agitating slurry in a digester, each agitator in the plurality comprising a rotatable shaft that is substantially vertical, a drive unit adapted to rotate the rotatable shaft, and an axial flow impeller coupled to the rotatable shaft, wherein, in operation, the plurality of agitators comprises first and second agitators that are counter-rotating and nearest neighbors.

[0009] In accordance with one aspect of the instant invention there is provided an anaerobic digestion agitation system comprising: a plurality of agitators for agitating slurry in a digester, each agitator in the plurality comprising a rotatable shaft that is substantially vertical, a drive unit adapted to rotate the rotatable shaft, and an axial flow impeller coupled to the rotatable shaft, wherein the plurality of agitators is arranged in a substantially regular pattern such the plurality of agitators comprises a first plurality of agitators forming a first row of agitators and a second plurality of agitators forming a second row of agitators, the first and second rows of agitators being parallel, and wherein, in operation, the agitators in the first plurality rotate clockwise, the agitators in the second plurality rotate counterclockwise, and the first and second rows are adjacent rows.

[0010] In accordance with one aspect of the instant invention there is provided a method of converting lignocellulosic material to biogas, the method comprising: conducting ananaerobic digestion, wherein feedstock for the anaerobic digestion comprises lignocellulosic feedstock, and wherein the anaerobic digestion is conducted in at least one digester containing slurry having an undissolved solids between 2% and 12%; and agitating the slurry with a plurality of agitators, each agitator in the plurality comprising a rotatable shaft that is substantially vertical, a drive unit adapted to rotate the rotatable shaft, and at least one axial flow impeller coupled to the rotatable shaft such that it rotates within the slurry, wherein, in operation, the plurality of agitators comprises first and second agitators that are counterrotating and nearest neighbors.

[0011] In accordance with one aspect of the instant invention there is provided an anaerobic digestion agitation system comprising: a plurality of agitators for agitating slurry in a digester, each agitator in the plurality comprising a rotatable shaft that is substantially vertical, a drive unit adapted to rotate the rotatable shaft, and an axial flow impeller coupled to the rotatable shaft, wherein the plurality of agitators comprises at least one clockwise agitator and at least one counterclockwise agitator, and wherein the plurality of agitators is arranged with each clockwise agitator having at least one closest neighbor that is a counterclockwise agitator such that, in operation, there is co-current flow provided therebetween.BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Further features and advantages of the present disclosure will become apparent from the following detailed description, taken in combination with the appended drawings, in which like features are identified by like reference numerals, and in which:

[0013] FIG. la is a schematic side view of an embodiment of a lagoon-type digester;

[0014] FIG. lb is a schematic side view of an embodiment of a vertical cylindrical tank digester;

[0015] FIG. 2a is a schematic side view of a digester showing a flow pattern of an axial flow impeller;

[0016] FIG. 2b is a schematic side view of a digester showing a flow pattern of a radial flow impeller;

[0017] FIG. 3a is a perspective view of a left-handed hydrofoil impeller, where the impeller has three blades;

[0018] FIG. 3b is a schematic plan view of two counter-rotating agitators, where each impeller has three blades;

[0019] FIG. 3c is a schematic diagram showing the path the tips of the blades of the impellers can make when rotated;

[0020] FIG. 3d is a schematic diagram showing counter-current flow between two agitators rotated in the same direction;

[0021] FIG. 3e is a schematic diagram showing co-current flow between two counter-rotating agitators;

[0022] FIG. 4a is a schematic side view of an agitation system in accordance with an embodiment of the instant disclosure having three agitators;

[0023] FIG. 4b is a schematic top view of the agitation system in Fig. 4a;

[0024] FIG. 4c is a schematic side view of an agitation system in accordance with an embodiment of the instant disclosure having three agitators and fillets;

[0025] FIG. 4d is a schematic top view of the agitation system in Fig. 4c;

[0026] FIG. 5a is a schematic top view of an agitation system in accordance with an embodiment of the instant disclosure, wherein the agitators are arranged based on a square grid, wherein each agitator has a nearest neighbor that is rotated in an opposing direction, and wherein the digester has a circular cross section (e.g., a vertical cylindrical tank);

[0027] FIG. 5b is a schematic top view of an agitation system in accordance with an embodiment of the instant disclosure, wherein the agitators are arranged based on a squaregrid, wherein each agitator has a nearest neighbor that is rotated in an opposing direction, and wherein the digester has a square cross section;

[0028] FIG. 6a is a schematic top view of an agitation system in accordance with an embodiment of the instant disclosure based on a rhomboid grid, wherein each agitator has a nearest neighbor that is rotated in an opposing direction, and wherein the digester has a circular cross section;

[0029] FIG. 6b is a schematic top view of an agitation system in accordance with an embodiment of the instant disclosure based on a rhomboid grid, wherein each agitator has a nearest neighbor that is rotated in an opposing direction, and wherein the digester has a rectangular cross section;

[0030] FIG. 7a is a schematic top view of an agitation system in accordance with an embodiment of the instant disclosure based on a hexagonal grid, wherein a central agitator has six nearest neighbors that are rotated in an opposing direction, and wherein the digester has a circular cross section;

[0031] FIG. 7b is a schematic top view of an agitation system in accordance with an embodiment of the instant disclosure based on a hexagonal grid, wherein a central agitator has six nearest neighbors that are rotated in an opposing direction, and where the digester has a polygonal cross section;

[0032] FIG. 8a is a schematic top view of an agitation system in accordance with an embodiment of the instant disclosure based on a hexagonal grid, wherein the central agitator has two nearest neighbors that are rotated in an opposing direction and four nearest neighbors that are rotated in the same direction, and wherein the digester has a circular cross section;

[0033] FIG. 8b is a schematic top view of an agitation system shown in Fig. 8a showing alternating rows of agitators that rotate clockwise and agitators that rotate counterclockwise, wherein the digester has a polygonal cross section;

[0034] FIG. 9a is a schematic top view of an agitation system in accordance with an embodiment of the instant disclosure having nineteen agitators and / or full cells;

[0035] FIG. 9b is a schematic top view of an agitation system in accordance with an embodiment of the instant disclosure having twenty agitators, and

[0036] FIG. 10 is a schematic top view of an agitation system wherein the agitators are positioned and / or configured to improve agitation throughout the digester.DETAILED DESCRIPTION

[0037] Anaerobic digestion is a biological process that involves the degradation of organic matter by microorganisms (e.g., bacteria, fungi, and / or archaea) to produce a biogas that is largely composed of methane (CEL) and carbon dioxide (CO2). This biogas can be used directly as a fuel (e.g., to generate heat and / or power) or can be upgraded to produce gas that is primarily methane (e.g., >95% CH4). If of sufficient purity, upgraded biogas can be used interchangeably with natural gas and can be referred to as renewable natural gas (RNG).

[0038] Anaerobic digestion is often conducted in one or more digesters. The term “digester”, as used herein, refers to any receptacle (e.g., vessel and / or space) in which at least part of the anaerobic digestion occurs. If more than one digester is used, the digesters can be connected in series and / or in parallel. In general, each digester and / or the combination of digesters can be designed and engineered to operate using a number of different configurations and / or a number of different operating conditions, including but not limited to, single-stage versus multi-stage, batch versus semi-continuous mode or continuous mode, mixed versus unmixed, and / or mesophilic versus thermophilic.

[0039] In general, it can be advantageous when at least one digester is a mixed digester (e.g., where agitation is provided continuously or periodically). While providing agitation can require a more complex digester design, can limit the size of the digester, and / or can increase operating costs (e.g., relative to non-mixed systems), the increased biogas yield and / or increased stability associated with the agitation can often offset the increased costs. For example, effective agitation can increase contact of the microorganisms with the substrate (e.g., the lignocellulosic feedstock), prevent the formation of layers (e.g., including floating layers and sediment layers), avoid forming scum at the surface, provide even heatdistribution, and / or prevent the formation of dead zones (e.g., pockets where the substrate is no longer being digested and / or where potentially toxic compounds are being concentrated).

[0040] The instant disclosure relates to agitation systems that can reduce agitation costs and / or improve agitation for large digesters (e.g., can help scale up biogas production with reduced costs). Such agitation systems include multiple agitators that are provided for agitating slurry in a digester. The instant disclosure also relates to methods of using such agitation systems for producing biogas (e.g., from lignocellulosic feedstock).Digester

[0041] In general, the digester can be any type of digester suitable for at least part of the anaerobic digestion (e.g., a digester designed to facilitate the breakdown of organic matter by microorganisms under anaerobic or low oxygen conditions and the collection of biogas). In general, the digester will have a body adapted to hold the slurry (e.g., a body having one or more walls and a floor) and at least one cover designed to substantially prevent air from entering the digester and / or biogas from escaping to the atmosphere (e.g., the cover can be integrated with, coupled to, and / or adapted to form a seal with the body).

[0042] The body can be formed from natural materials (e.g., earthen berms) and / or artificial materials (e.g., concrete, steel, etc.), can be lined with natural material (e.g., mud, clay, bentonite, etc.) and / or artificial material (e.g., plastic, rubber, concrete, epoxy, protective coating, etc.), can be of any shape (e.g., circular, square, rectangular, egg-shaped, polygonal, etc.), and / or can be constructed above ground, partially above ground and partially below ground, or fully below ground level. In some embodiments, the body is a lagoon type structure (e.g., pit or earthen basin). For example, in some embodiments, the body is an earthen basin constructed by excavating part of the required volume from below ground level and using the excavated earth to build embankments (i.e., berms) to provide the remaining digester volume, as for example, illustrated in Fig. la. In some embodiments, the body is a tank (e.g., cylindrical, square, or rectangular tank). For example, in some embodiments, the body is a vertical cylindrical tank (i.e., cylindrical in shape having a longitudinal axis in a substantially vertical direction, as illustrated in Fig. lb). In some embodiments, the body is a polygonal tank (i.e., a tank having flat walls that form a polygon, such as a hexagon, whenviewed from the top). In some embodiments, the body is a scalloped tank (i.e., a tank having walls that form a shape having scalloped edges when viewed from the top). In some embodiments, the tank is constructed with concrete and / or or steel walls. For example, in some embodiments, the tank is constructed with glass-coated steel or epoxy-coated steel. In some embodiments, the tank is constructed with concrete walls (e.g., flat or curved).

[0043] The cover, which can be formed using any suitable material (e.g., rigid and / or flexible), may or may not be removable. For example, the cover can be a fixed roof, a floating cover, and / or a gas holding cover. In some embodiments, the body is a tank and the cover is a fixed roof cover (e.g., a hard flat or dome shaped roof fixed to the top of the tank). In some embodiments, the body is a tank and the cover is a floating cover (e.g., a metal roof structure that floats on the slurry and moves up and down via vertical guide members). In some embodiments, the body is a lagoon-type structure and the cover is floating cover (e.g., a geomembrane cover that floats on the surface of the slurry). In some embodiments, the body is a lagoon-type structure and the cover is a taut geomembrane cover, which is suspended above the surface of the slurry. In some embodiments, the body is a lagoon type structure or a tank, and the cover is a gas holding cover. Gas holding covers, which for example can be dual membrane covers or gas bladders, are often inflatable. For example, in a dual membrane cover, the outer membrane often remains inflated, while the inner membrane inflates and deflates in proportion to the biogas production, and thus may be suitable for pressurized applications. In some embodiments, more than one cover is used to seal the body, or one cover is used to seal two bodies (e.g., side by side lagoons). In some embodiments, the body and cover are integrated and / or monolithic. In some embodiments, the cover and / or body is insulated (e.g., for a thermophilic anaerobic part of the anaerobic digestion).

[0044] While the digester can be of any type and / or size, the agitation system is advantageous for digesters having a relatively large footprint (e.g., having a body where the width is greater than the depth). In some embodiments, the digester has a cylindrical body having a diameter of at least about 20 meters, at least about 30 meters, at least about 40 meters, at least about 50 meters, at least about 60 meters, at least about 70 meters, at least about 80 meters, at least about 90 meters, or at least about 100 meters. In some embodiments, the digester has a square or rectangular body having a width and / or length of at least about 20 meters, at leastabout 30 meters, at least about 40 meters, at least about 50 meters, at least about 60 meters, at least about 70 meters, at least about 80 meters, at least about 90 meters, or at least about 100 meters. In some embodiments, the body has at least one dimension (e.g., diameter, length, and / or width) that is at least about 20 meters, at least about 30 meters, at least about 40 meters, at least about 50 meters, or at least about 60 meters, and a height that is between about 1.5 meters and about 20 meters, between about 2 meters and about 15 meters, or between about 2.5 meters and about 10 meters. In some embodiments the body can hold slurry at a depth between about 5 meters and about 15 meters (e.g., about 10 meters). In some embodiments, the body can hold at least about 10,000 m3, at least about 20,000 m3, at least about 30,000 m3, at least about 40,000 m3, at least about 50,000 m3, at least about 60,000 m3, at least about 70,000 m3, at least about 80,000 m3, at least about 90,000 m3, or at least about 100,000 m3of slurry. In some embodiments, the body can hold between about 100,000 m3and about 200,000 m3of slurry, or more.

[0045] While the walls and / or floor of the body of the digester can have any design (e.g., vertical, slanted, or bowed walls, or flat or contoured floor), when some or all of the agitators in the agitation system are adapted to provide downward flow, it can be advantageous for the floor surface of the digester to be substantially level and / or have one or more substantially level sections. The term “substantially level”, as used herein, refers to being generally parallel to the plane of the horizon or forming an angle to the level of the horizon that is less than 30°.Agitation System

[0046] The agitation system includes a plurality of agitators, each of which has a rotatable shaft, a drive unit for rotating the rotatable shaft, and at least an axial flow impeller coupled to the rotatable shaft.

[0047] Each rotatable shaft has a longitudinal axis about which it rotates (i.e., its rotational axis). Preferably, each rotatable shaft is a substantially vertical shaft. The term “substantially vertical”, as used herein, refers to an orientation ranging from a vertical orientation with respect to the plane of the horizon to 30° in any direction from such vertical orientation. Inoperation, each rotatable shaft is positioned such that the axial flow impeller coupled thereto is below the surface of the slurry.

[0048] Each drive unit is adapted for rotating one of the rotatable shafts. Drive units, which are well known, typically include a motor (e.g., electric motor or hydraulic motor) and a speed adjuster (e.g., gear box, belt drive, transmission). While each drive unit can be any type suitable for the agitation (e.g., submersible, top-mounted), it is often advantageous and / or more practical for the drive units to be provided above the level of the slurry (e.g., top-mounted). In some embodiments, some or all of the agitators are top-mounted agitators having a gearbox that supports the rotatable shaft as well as rotating the rotatable shaft and impeller(s). In some embodiments, the drive unit includes an external motor (e.g., external electric motor). In such embodiments, the drive unit is located above the cover, while the rotatable shaft extends through the cover and into the slurry (e.g., with appropriate sealings). In such embodiments, the drive units can be mounted directly to the cover (e.g., to a fixed cover) or can be mounted to one or more support structures provided above the cover (e.g., to platforms and / or bridges provided above a flexible cover).

[0049] Each axial flow impeller, which is coupled to one of the rotatable shafts so that it can rotate when the rotatable shaft is rotated, includes multiple blades that extend away from the rotatable shaft (e.g., equally spaced within a plane perpendicular to the rotational axis). While the blades can be coupled directly to the rotatable shaft, it can be advantageous and / or more practical for the blades to be coupled to a central hub that can be fixedly coupled to the rotatable shaft (e.g., set-screwed, keyed, or welded). In general, each axial flow impeller can have any suitable number of blades (e.g., 3, 4, 5, or 6) and any suitable pitch, diameter, solidity ratio, and / or wing design. In some embodiments, the diameter of one or more of the axial flow impellers is about 2 meters, about 3 meters, about 4 meters, about 5 meters, about 6 meters, about 7 meters, about 8 meters, about 9 meters, about 10 meters, or larger. In some embodiments, the diameter of one or more of the axial flow impellers is between about 5 and about 7.5 meters (e.g., about 6 or about 7 meters). In some embodiments, the diameter of one or more of the axial flow impellers is between about 6 and about 8 meters). In some embodiments, the diameter of one or more of the axial flow impellers is between about 7.5 and about 10.5 meters. In digesters having a single centrally mounted axial flow impeller, asthe digester becomes larger, the diameter of the axial impeller must also increase in order to provide effective agitation. As will be appreciated by those skilled in the art, as the diameter of an impeller increases, the required torque can increase, which can correspond to an increase in power consumption. One advantage of various embodiments described herein is that the trade-off between impeller diameter and effective agitation is not as significant (e.g., a plurality of relatively small diameter axial flow impellers can be used).

[0050] In general, “axial flow impellers” have blades that are shaped and / or angled to promote flow substantially parallel to the axis of rotation (e.g., in contrast to radial flow impellers that promote flow substantially perpendicular to the axis of rotation). For example, consider the agitators shown in Fig. 2a and 2b. Referring to Fig. 2a, the agitator includes an axial flow impeller 18 having blades that make an angle of less than 90° to the plane of rotation, such as about 30°, 40°, 45°, 50°, or 60° (not shown). As the drive unit 14 rotates the shaft 16, the angled blades push the slurry in the digester 12 downwards towards the floor of the digester. The downward driven slurry is deflected from the floor of the digester and spreads out over the floor and flows up along the wall of the digester before being drawn back to the axial flow impeller 18 (e.g., forming a single recirculation loop). In contrast, the agitator in Fig. 2b includes a radial flow impeller 20 having blades that make an angle of about 90° to the plane of rotation (not shown). As the drive unit 14 rotates the shaft 16, the vertical blades push the slurry into the walls of the digester 12. This sideways driven slurry is deflected from the walls and flows up and down the walls (e.g., forming a double recirculation loop).

[0051] In Fig. 2a, the axial flow impeller 18 provides downward flow. However, in theory, axial flow impellers can provide axial flow in either an upward or downward direction. More specifically, the orientation of the impeller (i.e., whether it is right-handed or left-handed) and the direction in which it is rotated (i.e., clockwise or counterclockwise) can determine whether the flow is in an upward or downward direction. For example, a right-handed impeller will push the slurry in an upward direction when rotated clockwise (as viewed from the top) and will push the slurry in a downward direction when rotated counterclockwise. A left-handed impeller will push the slurry in a downwards direction when rotated clockwise-l i(as viewed from the top) and will push the slurry in an upwards direction when rotated countercl ockwi se .

[0052] The orientation of the impeller can be determined by viewing the impeller from the side (i.e., when the rotational plane of the impeller is horizontal). If the blade tip is generally angled upward to the left (e.g., like a backslash \ ), then the impeller is a left-handed impeller. If the blade tip is generally angled upward to the right (e.g., like a forward slash / ), then the impeller is a right-handed impeller. For example, Fig. 3a shows one embodiment of a lefthanded impeller. In practice, axial flow impellers manufactured for industrial agitation applications are often predominantly of one orientation (e.g., left-handed impellers) and the configuration of drive units for left-handed and right-handed impellers can be different.

[0053] Each of the axial flow impellers can be any suitable type of axial flow impeller (e.g., propeller, pitched blade, or hydrofoil). In some embodiments, each of the axial flow impellers is a hydrofoil. Hydrofoil impellers have blades angled and / or shaped to maximize the axial flow in a certain direction (e.g., in the downwards direction). For example, hydrofoil impellers, which are well known, often have tapered and cambered blades with the blade angle increasing from the tip to the hub (e.g., see Fig. 3a). Hydrofoil impellers can be beneficial for agitating digesters because they can provide relatively high flow and because they can be more efficient (e.g., can require less power and torque than a pitched blade impeller) and / or reduce shear. Providing low shear can be beneficial for the microorganisms associated with anaerobic digestion.

[0054] In some embodiments, most or all of the agitators provide downward flow (e.g., at least about 80%). In some embodiments, most or all of the agitators provide upward flow (e.g., at least about 80%). In some embodiments, some of the agitators provide upward flow while other agitators (e.g., one or more adjacent agitators) provide downward flow. Providing downward flow can be beneficial for applications where solids suspension and / or or stratification is a challenge (e.g., in digesters). For example, well positioned and designed axial flow impellers adapted to provide downward flow can discourage settling at the bottom of the digester. In order for the axial flow impellers to provide downward flow, the drive unit of agitators having a right-handed impeller will have to rotate the shaft counterclockwise (asviewed from the top), while the drive unit of agitators having a left-handed impeller will have to rotate the shaft clockwise (as viewed from the top).

[0055] In some embodiments, one or more of the agitators have a right-handed impeller, while one or more other agitators have a left-handed impeller. In some embodiments, one or more of the agitators have a right-handed impeller, while one or more other agitators have a left-handed impeller, and each of the axial flow impellers is adapted to provide downward flow. For purposes herein, an agitator having a left-handed impeller and a drive unit adapted to rotate the shaft clockwise (as viewed from above) is referred to as a “clockwise agitator”, whereas an agitator having a right-handed impeller and a drive unit adapted to rotate the shaft counterclockwise (as viewed from above) is referred to as a “counterclockwise agitator.”

[0056] In some embodiments, one or more of the agitators has more than one impeller coupled to the rotatable shaft (e.g., the axial flow impeller and another impeller, such as a radial flow impeller). Embodiments where one or more of the agitators has multiple impellers on the rotatable shaft can be advantageous when the digester is relatively tall. In some embodiments, one or more of the agitators includes multiple axial flow impellers, where each impeller is of the same orientation (e.g., two clockwise impellers). In some embodiments, one or more of the agitators includes multiple axial flow impellers, including a first one that is one orientation (e.g., left-handed) and one that is of the opposite orientation (e.g., right- handed). If an agitator includes more than one axial flow impeller (e.g., a right-handed axial flow impeller and a left-handed axial flow impeller), then the lowest axial flow impeller (e.g., closest to the floor of the digester) is determinative of whether the agitator is a “clockwise agitator” or a “counterclockwise agitator.” In some embodiments, one or more of the agitators includes two axial flow impellers, including a first one that is one orientation (e.g., left-handed) and one that is of the opposite orientation (e.g., right-handed), wherein the two axial flow impellers are positioned on the rotatable shaft such that, in operation, the lower impeller provides downward flow and the upper impeller provides upward flow.

[0057] In some embodiments, the plurality of agitators includes one or more clockwise agitators (e.g., a plurality of clockwise agitators) and one or more counterclockwise agitators (e.g., a plurality of counterclockwise agitators). In some embodiments, all or most of theagitators are clockwise agitators (e.g., at least about 80%). In some embodiments, all or most of the agitators are counterclockwise agitators (e.g., at least about 80%).

[0058] In general, the plurality of agitators will include at least two counter-rotating agitators. The term “two counter-rotating agitators”, as used herein, refers to two agitators where, in operation, the impellers rotate in opposite directions (i.e., clockwise versus counterclockwise) about distinct rotational axes. For example, consider the two counterrotating agitators illustrated in Fig. 3b. Each agitator has a distinct rotational axis (i.e., perpendicular to the plane of the figure) around which its respective axial flow impeller 18a / l 8b rotates. Axial flow impeller 18a rotates clockwise, while axial flow impeller 18b rotates counterclockwise. Referring to Fig. 3c, as each axial flow impeller 18a / l 8b rotates, the tips of its blades trace a circle of a given radius within a plane perpendicular to the rotational axis. Although axial flow impellers are generally designed to provide flow in a direction parallel to the rotation axis (e.g., upwards or downwards when the rotatable shaft is substantially vertical), the rotation of the impeller can also induce a flow pattern having a circular component (e.g., axial flow impellers that are not used with baffles can produce a vortex). It has now been recognized that it can be advantageous to provide at least two counter-rotating agitators, wherein the opposing circular components of the flows can help reduce agitation costs and / or improve agitation (e.g., for digesters of a number of different shapes and sizes). For example, consider the agitators in Fig. 3d and 3e.

[0059] In Fig. 3d, two axial flow agitators 19 that rotate in the same direction (i.e., both counterclockwise) are provided. When adjacent axial flow agitators rotate in the same direction (e.g., both counterclockwise), there can be counter-current flow in the region between the two agitators (e.g., as shown by the straight lines with arrows in Fig. 3d). Such counter-current flow can create turbulence and / or increase the energy required for agitation (e.g., some energy is wasted due to the counter-current flows).

[0060] In Fig. 3e, the two axial flow agitators 18a / l 8b rotate in opposite directions (i.e., are counter-rotating). When adjacent axial flow agitators rotate in opposite directions, there can be co-current flow in the region between the two agitators (e.g., as shown by the straight lines with arrows in Fig. 3e). Such co-current flow can decrease the energy required foragitation (i.e., relative to counter-current flow) and / or can encourage flow in certain directions. The term “co-current flow”, as used herein, refers to flow induced by two distinct agitators being substantially parallel and in the same direction. For example, in Fig. 3e the co-current flow occurs in the space between where the circles traced by the axial impellers 18a / l 8b are closest (e.g., the two flows in the tangential direction).

[0061] In general, the counter-rotating agitators can be synchronized (e.g., such that blades of the two impellers substantially align at some point during every rotation) or not synchronized. In some embodiments, some or all of the counter-rotating agitators are synchronized (e.g., at least about 80%). In some embodiments, some or all of the counterrotating agitators are not synchronized (e.g., at least about 80%).

[0062] In some embodiments, some or all of the counter-rotating agitators are formed from two agitators having impellers of the same orientation (e.g., both agitators having left-handed axial flow impellers or both agitators having right-handed axial flow impellers, such that one of the counter-rotating agitators provides upward flow and the other of the counter-rotating agitators provides downward flow).

[0063] In some embodiments, some or all of the counter-rotating agitators are formed from two agitators having impellers with different orientations (e.g., at least about 80%). For example, in some embodiments, the plurality of agitators includes one or more clockwise agitators (e.g., a plurality of clockwise agitators) and one or more counterclockwise agitators (e.g., a plurality of counterclockwise agitators), wherein at least one of the clockwise agitators is adjacent to one or more counterclockwise agitators such that there is co-current flow therebetween.

[0064] The agitators are generally dispersed throughout the digester at spaced apart locations (i.e., regularly and / or irregularly spaced, but positioned such that the circles traced by the blades of the axial flow impellers do not overlap). In general, the agitators can be positioned such that each agitator is associated with and / or primarily responsible for local agitation within a certain region of the digester (e.g., a so-called “cell”). For example, consider the embodiment illustrated in Figs. 4a and 4b.

[0065] Referring to Figs. 4a and 4b, the agitator system includes three agitators 10a, 10b, 10c provided for agitating digester 12. Each agitator includes a drive unit 14, a rotatable shaft 16, and an axial flow impeller 18. The outer agitators 10a and 10c are clockwise agitators. The central agitator 10b is a counterclockwise agitator. Since all adjacent agitators are counterrotating agitators, less energy can be required for the agitation (i.e., relative to if all agitators rotated in the same direction).

[0066] As illustrated, the digester 12 has been notionally divided into a plurality of regions or cells (e.g., 12a, 12b, 12c, which are illustrated with dashed lines), where the center of each cell generally corresponds to the rotational axis of one of the agitators. In general, the width of each cell (W) can be dependent on the diameter of the axial flow impeller (D). Conceptually, it can be helpful to view each cell as a distinct vessel in order determine suitable cell sizes (e.g., W), impeller sizes (e.g., D), and / or distances between agitators (e.g., S). For example, in centrally mounted agitator systems such as that illustrated in Fig. 2a, the diameter of the axial flow impeller is typically between about 30% and about 80% of the width of the digester, and often between about 40% and about 70% of the width of the vessel, in order to provide effective agitation through the digester.

[0067] In some embodiments, the agitators are positioned such that the digester can be notionally divided into a plurality of cells (e.g., at least some of which have a centrally positioned agitator), where each cell has a width that is at least about 1.3D up to about 2.8D, where D is the diameter of the axial flow impeller. In some embodiments, the agitators are positioned such that the digester can be notionally divided into a plurality of cells (e.g., at least some of which have a centrally positioned agitator), where each cell has a width that is at least about 1.5D up to about 2.5D. In some embodiments, the agitators are positioned such that the digester can be notionally divided into a plurality of cells (e.g., at least some of which have a centrally positioned agitator), where each cell has a width that is at least about 1.25D up to about 2.2D. In some embodiments, the agitators are positioned such that the digester can be notionally divided into a plurality of cells (e.g., at least some of which have a centrally positioned agitator), where each cell has a width that is about 1.5D.

[0068] In some embodiments, the agitators are positioned such that the digester can be notionally divided into a plurality of cells (e.g., at least some of which have a centrally positioned agitator), where each cell has a width that is between about 3 meters and about 20 meters (e.g., about 4 meters, about 5 meters, about 6 meters, about 7 meters, about 8 meters, about 9 meters, about 10 meters, about 11 meters, about 12 meters, about 13 meters, about 14 meters, about 15 meters, about 16 meters, or about 17 meters). In some embodiments, the agitators are positioned such that the digester can be notionally divided into a plurality of cells (e.g., at least some of which have a centrally positioned agitator), where each cell has a width that is between about 8 meters and about 30 meters (e.g., about 10 meters, about 15 meters, about 20 meters, about 25 meters, or about 30 meters).

[0069] In some embodiments, each cell contains between about 100 m3and about 7,000 m3of slurry, between about 500 m3and about 5,000 m3, or between about 1500 m3and about 4,000 m3of slurry. In some embodiments, each cell contains between about 250 m3and about 5,000 m3of slurry, between about 500 m3and about 3000 m3of slurry, between about 750 m3and about 2,500 m3of slurry, or between about 1000 m3and about 2,000 m3of slurry. In some embodiments, each cell contains between about 500 m3and about 2000 m3of slurry, when the axial flow impellers have a diameter between about 5 meters and about 7.5 meters. In some embodiments, each cell contains between about 2000 m3and about 3,000 m3of slurry. Combining the use of cells that contain between about 1500 m3and about 3,500 m3of slurry with the use of axial flow impellers having a diameter between about 8 meters and about 10 meters can provide good local agitation.

[0070] In some embodiments, all or most of the agitators (e.g., at least about 80%) are spaced at a distance S from another agitator, where S is at least about 1.3D up to about 2.8D. In some embodiments, all or most of the agitators (e.g., at least about 80%) are spaced at a distance S from another agitator, where S is at least about 1.8D up to about 2.5D. In some embodiments, all or most of the agitators (e.g., at least about 80%) are spaced at a distance S from another agitator, where S is about 2D.

[0071] In some embodiments, the agitators are positioned and / or adapted such that the flow pattern within at least some of the cells is similar to that shown in Fig. 2a. In someembodiments, the digester is provided with one or more fillets 13a (e.g., shown in Figs. 4c and 4d), which can divert deflected downflow upwards for one or more cells.

[0072] In some embodiments, the plurality of agitators is arranged in a substantially regular pattern. The term “substantially regular pattern”, as used herein with reference to agitators, means that the general position and / or type (e.g., clockwise versus counterclockwise, or upflow versus downflow) of each agitator follows an ordering rule and / or is repeated in a predictable manner. For example, without being limiting in any way, agitators can be arranged in a substantially regular pattern when they are consistently spaced from one another in one or more directions, when they are placed according to some geometric pattern, when they are placed at various points on a grid (e.g., at some or all of the grid points), when they are arranged to have a specific number of nearest neighbors, and / or when the agitators alternate between clockwise and counterclockwise orientations in one or more directions.

[0073] In some embodiments, the plurality of agitators is arranged in a substantially regular pattern that is at least based on the general position of the agitators (e.g., where at least some of the agitators are consistently spaced from one another in one or more directions, are placed according to some geometric pattern, are placed at one or more points on a grid, and / or are arranged to have a specific number of nearest neighbors). Such embodiments can be advantageous when the digester is notionally divided into a plurality of cells, where one or more of the cells includes a centrally located agitator.

[0074] The term “cell,” as used herein, refers to an area of the digester resulting from a notional subdivision of the digester into small repeat units (e.g., based on some geographical pattern). In Figs. 4a and 4b, the cell shape (i.e., as viewed from the top) is a rectangle. In some embodiments, the cell shape (i.e., as viewed from the top) is another polygon (e.g., square, rhombus, pentagon, or hexagon). In Figs 4a and 4b, all of the cells (e.g., 12a, 12b, 12c) are full cells (e.g., have a cross-sectional shape that corresponds to the repeat unit). In some embodiments, some of the cells are partial cells (e.g., the cross-sectional shape corresponds to only part of the repeat unit). For example, partial cells may occur near the periphery of a vertical tank digester. In some embodiments, the agitators are arranged such that at least about 50%, at least about 60%, at least about 70%, at least about 80%, or at leastabout 90% of the full cells have an agitator wherein the rotatable shaft is centrally located within the cell. In some embodiments, the agitators are arranged such that about 100% of the full cells have an agitator wherein the rotatable shaft is centrally located within the cell.

[0075] Advantageously, the placement of the agitators, and thus the size of the cells, the shape of the cells, and / or the packing of the cells (e.g., the number of adjacent or neighboring cells), can be selected to improve agitation. For example, the agitators can be positioned sufficiently close together and / or with an arrangement to prevent the formation of dead zones, positioned sufficiently far apart and / or with an arrangement as to reduce excessive interference between adjacent agitators, and / or positioned to encourage a specific flow pattern (e.g., from cell to cell).

[0076] In general, each agitator will have at least one nearest neighbor (i.e., an agitator that is closest to it) and / or each cell will have at least one nearest neighbor (i.e., a cell that it shares a common wall with). In some embodiments, at least some of the agitators and / or cells are arranged in a substantially regular pattern wherein at least one agitator and / or at least one cell has a plurality of nearest neighbors (i.e., two or more nearest neighbors). When an agitator has a plurality of “nearest neighbors”, each of the nearest neighbors in the plurality is equally distant from the agitator. In some embodiments, at least some of the agitators and / or cells (e.g., at least about 80%) are arranged in a substantially regular pattern wherein multiple agitators and / or cells have a plurality of nearest neighbors. In some embodiments, the number of nearest neighbors is four. In some embodiments, the number of nearest neighbors is six. In some embodiments, the agitators are arranged such that the agitators and / or cells are arranged according to a grid. For example, Figs. 5a / b, 6a / b, 7a / b, 8a / b, 9a / b, and 10 illustrate various embodiments wherein at least some of the agitators and / or cells have multiple nearest neighbors and / or are arranged based on a grid. For illustrative purposes, each agitator is represented by the circle traced by the blades of its axial flow impeller when rotated about its rotational axis (■). The diameter of many of these circles was selected to facilitate rapid identification of counter-rotating agitators and / or nearest neighbors, and is not intended to be representative of suitable impeller diameter (D) to cell width (W) ratios.

[0077] In some embodiments, the agitators are positioned according to a square grid, wherein each of the agitators is provided at the center of a grid square and / or such that the cell shape (i.e., as viewed from the top) is a square. For example, see the embodiments illustrated in Figs. 5a and 5b. Referring to Fig. 5a, agitators following such arrangements have two classes of neighbors; those in the cardinal directions that share a cell edge, and those in the diagonal direction that share a cell vertex. Referring to Fig. 5b, although the central agitator is surrounded by eight other agitators, it has only four nearest neighbors (i.e., the agitators in the cardinal directions). In the embodiments in Figs. 5a and 5b, all nearest neighbors are counter-rotating agitators, while diagonal neighbors (which are relatively far apart) rotate in the same direction. If the array of agitators is large enough, this arrangement of agitators will provide alternating rows of agitators that rotate clockwise (e.g., clockwise agitators) and agitators that rotate counterclockwise (e.g., counterclockwise agitators) as illustrated with the dotted lines in Fig. 5b.

[0078] In some embodiments, the agitators are positioned according to a rhomboidal grid (e.g., other than square), wherein each of the agitators is provided at the center of a grid rhombus and / or such that the cell shape (i.e., as viewed from the top) is a rhombus (e.g., diamond). For example, see the embodiments illustrated in Figs. 6a and 6b. In these embodiments, at least one of the agitators has four nearest neighbors, and all nearest neighbors are counter-rotating agitators, while the other neighbors (which are relatively far apart) rotate in the same direction. If the array of agitators is large enough, this arrangement of agitators will provide alternating rows of agitators that rotate clockwise (e.g., clockwise agitators) and agitators that rotate counterclockwise (e.g., counterclockwise agitators) as illustrated with the dotted lines in Fig. 6b.

[0079] In some embodiments, the agitators are positioned according to a hexagonal grid, wherein each of the agitators is provided at the center of a grid hexagon and / or such that the cell shape (i.e., as viewed from the top) is a hexagon. For example, see the embodiments illustrated in Figs. 7a, 7b, 8a, 8b, 9a, 9b, and 10. In these embodiments, at least one of the agitators has six nearest neighbors (i.e., six of the neighboring agitators are equally distant from the central agitator). For example, referring to Figs. 7a and 8a, at least one of the cells (e.g., central cell) has six identically shaped neighboring cells, each sharing one of the equallength sides. Advantageously, this hexagonal grid arrangement is more efficient (e.g., can provide a more consistent and / or uniform agitation and / or can reduce the risk of dead zones). In addition, it can be advantageous when using digesters having flat walls as illustrated in Fig. 7b, 8b, and 10.

[0080] Referring to Figs. 7a and 7b, the centrally positioned agitator rotates counterclockwise (e.g., can be a counterclockwise agitator), while each of its six nearest neighbors rotate clockwise (i.e., the centrally positioned agitator is counter-rotating with all of its nearest neighbors). Overall, there are six cell sides with co-current flow and six cell sides with counter-current flow.

[0081] Referring to Figs. 8a and 8b, the centrally positioned agitator, which rotates counterclockwise (e.g., can be a counterclockwise agitator), is counter-rotating with four of its nearest neighbors, and rotates in the same direction as two of its nearest neighbors. Overall, there are there are eight cell sides with co-current flow and four cell sides with counter-current flow (e.g., there may be cost savings relative to the embodiment in Figs. 7a and 7b). Referring to Fig. 8b, this arrangement of agitators provides alternating rows of agitators that rotate clockwise and counterclockwise as illustrated with the dotted lines.

[0082] In general, arrangements based on the hexagonal grid (e.g., Figs. 7a, 7b, 8a, 8b, 9a, 9b, and 10) can be advantageous when the number of agitators is relatively large (i.e., more than 7 agitators, such as at least 9 or at least 18) and / or when the footprint of the digester is relatively large and / or is of the lagoon type. For example, arrangements based on the hexagonal grid can provide relatively consistent and / or uniform agitation for a variety of digester shapes and / or sizes and / or operating modes. In particular, it allows each agitator to be positioned and / or adapted to provide both: 1) effective local agitation, and 2) effective global agitation.

[0083] Effective local agitation results in the contents of each cell being relatively well mixed (e.g., the composition of slurry throughout the cell is relatively consistent). For example, effective local agitation that provides downwards flow can reduce and / or prevent sediment build up at the bottom of each cell such that composition of slurry throughout the cell is relatively consistent (e.g., the flow pattern within each cell can be similar to that describedwith regard to Fig. 2a). An agitator arrangement based on a hexagonal grid is advantageous for effective local agitation because the cell walls meet at 120° (e.g., compared to 90° for square cells) and thus can reduce the possibility of forming dead zones within each cell. In addition, an agitator arrangement based on a hexagonal grid is advantageous in that it can provide more local agitation (e.g., the closer packing of cells can provide more agitators for a given digester footprint, impeller size, and D / W ratio). For example, if all or most of the agitators are adapted to provide downward flow, this can result in more of the floor of the digester being swept.

[0084] Effective global agitation results in the contents of the entire digester being well mixed (e.g., agitation is provided across the entire digester such that movement of slurry between cells is encouraged). For a batch anaerobic digestion (and / or some semi-continuous or continuous anaerobic digestions), effect global agitation can result in the temperature and / or composition of the slurry being relatively consistent through the digester (e.g., the composition of the slurry can be relatively similar between cells). Alternatively, or additionally, for semi-continuous or continuous anaerobic digestions, effective global agitation can result in more of the slurry residing within the digester for the selected retention time (e.g., reducing the amount of half-digested substrate from leaving the digester prematurely). For example, for a semi-continuous or continuous anaerobic digestion, effective agitation can result in most of the feed (e.g., more than about 60%) introduced into the digester residing within the digester for at least about 80% of the retention time (e.g., indicating that there is no short circuiting or limited short circuiting). The “retention time” of a semi-continuous or continuous anaerobic digestion is calculated as the volume of the tank (e.g., in m3) divided by the influent flow rate (e.g., in m3 / day)). An agitator arrangement based on a hexagonal grid is advantageous for effective global agitation because there can be more regions of co-current flow generated per agitator, which can be arranged to form one or more paths and / or move the contents of at least part of the cell to one or more other cells. An agitator arrangement based on a hexagonal grid is particularly advantageous for effective global agitation when the agitators are arranged in rows that alternate between clockwise and counterclockwise rotating agitators, since the cell walls meet at 120° (e.g., compared to 90° for square cells), and thus can facilitate movement of the slurry in a given direction. Forexample, the co-current flows illustrated with single bold lines in Fig. 9a can collectively move slurry from left to right (e.g., roughly along a line that traverses the digester).

[0085] In general, arrangements wherein the agitators are arranged in rows, where the agitators in one row rotate clockwise and agitators in an adjacent row rotate counterclockwise (e.g., counter-rotating rows as illustrated in Fig. 5b, 6b, 8b, 9b, and 10) can be advantageous when the number of agitators is relatively large (i.e., more than 7 agitators, such as at least 9 or at least 18) and / or when the anaerobic digestion is operated in semi- continuous mode or continuous mode. For example, such arrangements can allow each agitator to be positioned and / or configured to provide effective local agitation within the respective cell (e.g., prevent sediment build up at the bottom of each cell), while the overall arrangement (e.g., regular pattern) and / or part of the arrangement (e.g., row configuration) is designed to move the slurry along a certain path (e.g., via the co-current flows). For example, in some embodiments, the arrangement is designed to move slurry introduced at one or more inlets on one side of the digester to one or more outlets on an opposing side of the digester (e.g., along an approximately straight path). In some embodiments, the arrangement is designed to move slurry introduced at an inlet of the digester to an outlet of the digester via a certain path that substantially covers the digester. For example, in some embodiments, the co-current flow between adjacent rows directs the slurry along a path that switchbacks across the digester (e.g., as illustrated with bolded line in Fig. 9b). Referring to Fig. 9b, feedstock introduced at point A is moved across the digester via the switchback path until it exits at point B (e.g., the effect may mimic that provided from an agitated plug flow reactor, but at reduced cost). Such embodiments can reduce the amount of half-digested substrate leaving the digester prematurely and / or can increase biogas yield. For example, providing a defined path of flow can reduce the residence time distribution.

[0086] In some embodiments, at least about 60% of the nearest neighbors of each of the agitators are counter-rotating. In some embodiments, at least about 60% of the nearest neighbors of at least about 50% of the agitators are counter-rotating. In some embodiments, at least about 100% of the nearest neighbors of each of the agitators are counter-rotating. In some embodiments, at least about 100% of the nearest neighbors of at least about 50% of the agitators are counter-rotating.

[0087] In some embodiments, each agitator has at least two nearest neighbors that are counter-rotating therewith. In some embodiments, at least about 60% of the agitators have at least two nearest neighbors that are counter-rotating therewith. In some embodiments, each internal agitator (i.e., an agitator that is not adjacent to the digester walls) has at least four nearest neighbors that are counter-rotating therewith. In some embodiments, at least about 25% or about 33% of the agitators have at least four nearest neighbors that are counterrotating therewith. In some embodiments, each internal cell (i.e., a cell that is not adjacent to the digester walls) has at least four nearest neighbors that rotate in an opposing direction to the agitator in the respective internal cell.

[0088] In some embodiments, the substantially regular pattern is provided throughout the entire digester (e.g., with possible disruptions close to the walls). In some embodiments, one or more cells formed from the substantially regular pattern contains a centrally positioned column (e.g., round, square, or hexagonal cross-section) provided for supporting a fixed roof. In some embodiments, fixed roof supports are provided at some or all of the points coinciding with common vertices of the grid shape (e.g., within one or more interstitial areas between circles traced by the rotation of the axial impeller blades). In some embodiments, the fixed roof supports (not shown) are shaped so as to function as baffles.

[0089] In some embodiments, the plurality of agitators includes one or more clockwise agitators (e.g., a plurality of clockwise agitators) and one or more counterclockwise agitators (e.g., a plurality of counterclockwise agitators), and all of the clockwise agitators have a same shape and size and all of the counterclockwise agitators have a same shape and size. In some embodiments, each of the axial flow impellers has a same general shape (e.g., mirrored shapes), size (e.g., diameter), and / or is positioned a same distance from the digester floor. In some embodiments, where the height of the slurry is H, some or all of the axial impellers are positioned between about 0.1H to about 0.7H from the floor. In some embodiments, some or all of the axial impellers are positioned between about 0.15 to about 0.5H from the floor. In some embodiments, some or all of the axial impellers are positioned between about 0.2 to about 0.4H from the floor. In some embodiments, some or all of the axial impellers are positioned between about 0.3D to about 1.5D from the floor, where D is the diameter of theaxial flow impellers. In some embodiments, the rotatable shaft extends to the floor, where it is secured with a bottom bearing.

[0090] In some embodiments, in addition to the plurality of agitators described above (e.g., the clockwise and counterclockwise agitators), the agitation system includes one or more other agitators and / or mixers (e.g., mechanical mixing and / or hydraulic mixing). For example, in some embodiments, in addition to the plurality of agitators described (e.g., the clockwise and counterclockwise agitators), the agitation system includes one or more side- mounted agitators (e.g., side-entry or submersible), one or more other top-mounted agitators, a gas recirculation system, a liquid / slurry recirculation system, and / or a mechanical pumping system, adapted to augment the agitation provided by the plurality of agitators (e.g., clockwise and counterclockwise agitators) and / or to compensate for deficiencies of the plurality of agitators (e.g., clockwise and counterclockwise agitators). For example, in some embodiments, additional mixing and / or agitation is provided in areas that have insufficient agitation and / or where solids may build up (e.g., in partial cells and / or near the walls of the digester).

[0091] In some embodiments, the digester shape and / or the walls of the digester is / are adapted to reduce and / or minimize the volume of the digester occupied by partial cells and / or that are associated with insufficient agitation. For example, in some embodiments, the walls are adapted to contour with the periphery of a plurality of cells (e.g., Fig. 7b). In some embodiments, the walls are adapted and / or positioned to reduce and / or minimize areas of the digester that are relatively far from an agitator. In some embodiments, the walls are adapted and / or positioned to reduce and / or minimize the volume of the digester occupied by partial cells that do not contain an agitator (e.g., Fig. 10).Anaerobic Digestion

[0092] The digester and agitation system are used for at least part of an anaerobic digestion. In some embodiments, the anaerobic digestion is conducted in a single digester (e.g., is a single stage anaerobic digestion). In some embodiments, the anaerobic digestion is a multiple stage anaerobic digestion, and the agitation system is provided for a digester in which the first stage is conducted. In some embodiments, the anaerobic digestion is a multiple stageanaerobic digestion, and the agitation system is provided for a digester in which the second stage is conducted. In some embodiments, the agitation system is provided for multiple digesters in which at least part of the anaerobic digestion is conducted.

[0093] In some embodiments, the agitator system is used for a digester operated in batch mode. In batch mode, the feed (e.g., feedstock) is added to the digester at the start of the process, and digestate is removed only once that part of the anaerobic digestion is complete. In some embodiments, the agitator system is used for a digester operated in semi-continuous or continuous mode. In continuous mode or semi-continuous mode, feedstock is introduced into the digester throughout the anaerobic digestion (e.g., continuously or periodically), while the digestate is also removed throughout the anaerobic digestion (e.g., continuously or periodically). In each mode (e.g., batch, continuous mode, or semi-continuous mode), the biogas may be removed from the digester and / or nutrients may be added throughout the process. For example, in some embodiments, the agitation system is provided for a digester operated in semi-continuous mode, wherein effluent (digestate) is removed and feedstock is added periodically (e.g., once a day, twice a day, three times a day, four times a day, five times a day, or six times a day), while biogas is removed substantially continuously.

[0094] In general, the anaerobic digestion can be conducted using any suitable feedstock and / or conditions. Some conditions and / or factors that can affect anaerobic digestion and / or biogas production include temperature, pH, carbon to nitrogen ratio (C / N), salinity, solids content, substrate to inoculum ratio, organic loading rate, retention time, etc. Those skilled in the art will appreciate that each condition can be selected based on the type of feedstock(s) and / or the other conditions. For example, anaerobic digestion of lignocellulosic feedstock is typically conducted at a pH within the range between about 6 and about 8.5 (and often between about 6.5 and about 7.8), at a temperature in the range between about 20°C and about 70°C (e.g., with an optimum often about 35°C for a mesophilic system and about 55°C for a thermophilic system), with a C / N ratio in the range between about 20 and about 40 (e.g., with an optimum often between about 20 and about 30), and / or with a salinity in the range between about 0 and about 8%.

[0095] In some embodiments, the feedstock for the anaerobic digestion is or contains lignocellulosic feedstock (e.g., agricultural residue, such as straw). The term “lignocellulosic feedstock”, as used herein, refers to any type of plant biomass or feedstock derived from plant biomass that contains cellulose, hemicellulose, and lignin. For example, some examples of different types of lignocellulosic feedstock include, but are not limited to, agricultural crop residues, energy crops, forestry residues, etc. Using agricultural crop residue, such as straw, as feedstock for biogas production has the potential to significantly increase the global supply of RNG (e.g., since agricultural crop residue is generally abundant). Unfortunately, agricultural residue can have a recalcitrant structure that results in a reduced biogas yield. Accordingly, the anaerobic digestion of agricultural residue can be associated with relatively long retention times (e.g., and thus can require more mixing per unit of incoming feedstock). The agitation system disclosed herein can be advantageous for the anaerobic digestion of agricultural residue because it can reduce agitation costs and / or can provide better agitation for the feedstock (e.g., which can have a tendency to form floating layers near the surface of the slurry). In some embodiments, the agitation system is provided for the anaerobic monodigestion of agricultural residue or energy crop.

[0096] In some embodiments, the lignocellulosic feedstock is agricultural residue or energy crop that is subjected to size reduction, debris removal (e.g., screening), and / or pretreatment (e.g., thermal, mechanical, chemical, and / or biological pretreatment) upstream of the anaerobic digestion. Size reduction reduces the average size of the lignocellulosic feedstock particles, which can make the lignocellulosic feedstock easier to handle (e.g., in a conveying system and / or in downstream processing) and can improve the performance of the anaerobic digestion. For example, the size reduction of lignocellulosic feedstock, such as straw, has been shown to enhance biogas production (e.g., with biogas production typically increasing with decreasing particle size). Size reduction of the lignocellulosic feedstock can also reduce the risk of blockages (e.g., in the conveying equipment, digester(s), and / or downstream processing equipment). Size reduction can be achieved using any suitable size reduction method or combination of methods (e.g., wet and / or dry), including but not limited to, milling, grinding, cutting, agitation, shredding, chipping, compression / expansion, and / or other types of mechanical action. Size reduction by mechanical action can be performed by any type of equipment adapted for the purpose, for example, but not limited to, hammermills, choppers, shredders, tub-grinders, roll presses, refiners, and / or hydropulpers. In some embodiments, size reduction includes processing the lignocellulosic feedstock with a hammer mill. In some embodiments, the lignocellulosic feedstock is subjected to size reduction that results in at least 80% of the resultant particles having a length less than about 30 cm, less than about 20 cm, less than about 10 cm, less than about 5 cm, less than about 3 cm, less than about 2 cm, less than about 1 cm, between about 0.05 mm and about 3 cm, between about .05 mm and about 2 cm, or between about .05 mm and about 1 cm. In some embodiments, the lignocellulosic feedstock is subjected to size reduction that includes passing the particles through a 3 / 16 inch sieve, a No. 5 mesh sieve, a No. 10 mesh sieve, a No. 12 mesh sieve, a No. 14 mesh sieve, a No. 16 mesh sieve, a No. 18 mesh sieve, a No. 20 mesh sieve, or a No. 40 mesh sieve. Appropriate size reduction can allow the lignocellulosic feedstock to be mixed with liquid, and thus conveyed as a slurry.

[0097] In some embodiments, the feedstock is fed to the digester as a slurry that contains fresh or recycled water, microbial inoculum, recycled digestate (e.g., solid or liquid), chemicals (e.g., to adjust or maintain pH), and / or nutrients. In some embodiments, a feeding system is provided that introduces the feedstock or a slurry containing the feedstock into each cell (e.g., from above). In some embodiments, a feeding system is provided that introduces the feedstock or a slurry containing the feedstock into the digester at an inlet positioned to promote the movement of the introduced feedstock along a certain path so that undigested feedstock exits from a certain outlet (e.g., as shown in Fig. 9b).

[0098] In some embodiments, the anaerobic digestion is conducted with a solids content between about 1% and about 20%, between about 2% and about 20%, between about 3% and about 15%, between about 4% and 10%, or between about 5% and about 9%. The term “solids content,” as used herein, can refer to the total solids of a material or the undissolved solids of the material, unless otherwise specified. Total solids (TS) is a measurement of the total amount of solids (i.e., dissolved and undissolved) in a material. For purposes herein, the TS of a sample is measured by weighing a sample, heating the sample at 105°C to constant weight, and weighing the resulting dried solids; the TS is the weight of the dried solids to the weight of the original sample, and can be expressed as a percent. Undissolved solids (UDS) is a measurement of the amount of solids in a material that is not in solution (e.g., cannotpass through a given filter). For purposes herein, the UDS of a sample is measured by weighing the sample, separating solid particles in the sample from at least some of the liquid (i.e., using a 1.6 pm glass filter and optionally using a centrifuge if it does not filter readily), washing the solid particles, and drying the solid particles at 37°C to constant weight; the UDS is the weight of the dried solid particles to the weight of the sample, and can be expressed as a percentage. Dissolved solids (DS) is a measurement of the amount of solids that that are in solution (e.g., dissolved) in the sample. For purposes herein, the DS of a sample is determined by subtracting the UDS from the TS. When determining the solids content at which a batch anaerobic digestion is conducted, the solids content is determined for the initial phase of the anaerobic digestion (i.e., is an initial solids content as the solids content will decrease as the anaerobic digestion progresses). When determining the solids content at which a continuous or semicontinuous anaerobic digestion is conducted, the solids content can be measured from within the digester and / or from the effluent. In general, it can be advantageous when the solids content, and in particular the UDS, is at least about 2% and less than about 15%. For example, a solids content of at least about 4% can improve accessibility of substrate to the microorganisms and / or reduce water usage, whereas a solids content that is less than about 15% can facilitate mixing and / or dilution of potential toxins. A solids content between about 2% and 7% can reduce energy required for agitation. The agitation system disclosed herein can be advantageous when the total solids is between about 2% and about 12%, particularly when the agitators are arranged to provide a certain flow pattern.

[0099] In some embodiments, the retention time in the digester is between about 5 days and about 90 days, or longer. In some embodiments, the retention time in the digester is between about 10 days and about 60 days, or between about 15 days and about 40 days. The term “retention time”, as used herein, refers to the average time the substrate stays within the digester. For a batch anaerobic digestion, the retention time is the duration of the digestion. For a continuous or semi-continuous anaerobic digestion (e.g., in a complete stirred reactor), the retention time is calculated as the volume of the tank (e.g., in m3) divided by the influent flow rate (e.g., in m3 / day). While the agitation system can be beneficial for digesters associated with any retention time, it can be particularly advantageous for digesters that have longer retention times (e.g., at least about 12 days).

[0100] In general, the agitation system can be operated in any suitable mode (e.g., continuous mode and / or intermittent mode) and / or to provide any suitable agitation conditions (e.g., low, medium, or high intensity agitation). For example, all or most of the impellers (e.g., at least about 80%) can be rotated at a constant rate (e.g., 20 RPM) or the rate can change according to a predetermined cycle or real-time feedback. In some embodiments, the agitation system is operated in intermittent mode wherein the operation of each agitator follows a cycle wherein it switches between an on-state for a given time (e.g., 15 minutes) and an off-state (e.g., 45 minutes). Intermittent agitation can decrease the maintenance and energy demands associated with the agitation. In some embodiments, some of the agitators are operated in continuous mode, while some of the agitators are operated intermittent mode. For example, in some of these embodiments, the agitators that operate in continuous mode can be at every other position on a grid (e.g., alternate with agitators that operate in intermittent mode) and / or can be concentrated within certain areas of the digester (e.g., closer to the inlet(s) and / or outlet(s)). In general, it can be advantageous when agitation substantially coincides (e.g., is provided just before, after, or during) the addition of nutrients and / or feedstock (e.g., for anaerobic digestions conducted in semi-continuous mode).

[0101] In some embodiments, all of the agitators in the plurality are operated with the same speed and timing. In some embodiments, the operation of one or more of the agitators in the plurality is synchronized. In some embodiments, the agitators are operated to induce current(s) that causes the slurry to follow a path within the digester. In some embodiments, the agitators are operated to reduce the formation of eddies. In some embodiments, the agitators are operated to change the size of any eddies formed.

[0102] The anaerobic digestion produces what is referred to as raw biogas. Raw biogas is a gas mixture that is predominantly methane (CPU) and carbon dioxide (CO2), and that can contain water (H2O), hydrogen sulfide (H2S), and / or ammonia (NH3). The composition and / or properties of raw biogas can vary depending on the feedstock (e.g., whether it is mono-digestion or co-digestion) and / or various conditions of the anaerobic digestion (e.g., retention time). The raw biogas produced from the anaerobic digestion is collected and can be subjected to biogas processing that includes one or more purification processes (e.g., biogas cleaning and / or biogas upgrading). The term “biogas cleaning”, as used herein, refersto a process where biogas (e.g., raw biogas) is treated to remove one or more non-methane components (e.g., H2O, H2S, O2, NH3, etc.), but does not remove a significant amount of carbon dioxide and / or nitrogen (e.g., the calorific value of the biogas may not change significantly as a result of biogas cleaning). The term “biogas upgrading”, as used herein, refers to a process where biogas (e.g., raw or cleaned biogas) is treated to remove one or more components (e.g., CO2, N2, H2O, H2S, O2, NH3, etc.), wherein the treatment increases the calorific value of the biogas. For example, biogas upgrading typically includes removing carbon dioxide and / or nitrogen (e.g., if present in significant amounts). Biogas upgrading, which can include biogas cleaning, produces upgraded biogas. The term “upgraded biogas”, as used herein, refers to biogas that has been upgraded (i.e., can refer to partially purified biogas or fully upgraded biogas, such as RNG). In general, the one or more purification processes (e.g., biogas cleaning and / or biogas upgrading) can use any suitable technology or combination of technologies that can separate methane from one or more non-methane components in the biogas (e.g., from CO2, N2, H2S, H2O, NH3, and / or O2) and / or separate carbon dioxide from methane or from one or more other non-methane components. Such technologies can include, but are not limited to, absorption, adsorption, membrane, and / or cryogenic separations.

[0103] The terminology used herein is for the purpose of describing certain embodiments only and is not intended to be limiting of the invention. For example, as used herein, the singular forms "a," "an," and "the" may include plural references unless the context clearly dictates otherwise. The terms “comprises”, "comprising", “including”, and / or “includes”, as used herein, are intended to mean "including but not limited to." The term “and / or”, as used herein, is intended to refer to either or both of the elements so conjoined. In the context of describing the combining of components by the “addition” or “adding” of one component to another, or the separating of components by the “removal” or “removing” of one component from another, those skilled in the art will understand that the order of addition / removal is not critical (unless stated otherwise). The terms “remove”, “removing”, and “removal”, with reference to one or more impurities, contaminants, and / or constituents of biogas, includes partial removal. The term “associated with”, as used herein with reference to two elements, is intended to refer to the two elements being connected with each other, linked to each other, related in some way, dependent upon each other in some way, and / or in some relationshipwith each other. The terms “first”, “second”, etc., may be used to distinguish one element from another, and these elements should not be limited by these terms. The terms “upstream” and “downstream”, as used herein, refer to the disposition of a step / stage in the process with respect to the disposition of other steps / stages of the process. For example, the term upstream can be used to describe a step / stage that occurs at an earlier point of the process, whereas the term downstream can be used to describe a step / stage that occurs later in the process. Unless defined otherwise, all technical and scientific terms used herein have the same meanings as commonly understood by one of ordinary skill in the art.

[0104] 0f course, the above example and / or embodiments have been provided as examples only. It will be appreciated by those of ordinary skill in the art that various modifications, alternate configurations, and / or equivalents will be employed without departing from the scope of the invention. Accordingly, the scope of the invention is therefore intended to be limited solely by the scope of the appended claims.

Claims

Claims1. An anaerobic digestion agitation system comprising: a plurality of agitators for agitating slurry in a digester, each agitator in the plurality comprising a rotatable shaft that is substantially vertical, a drive unit adapted to rotate the rotatable shaft, and an axial flow impeller coupled to the rotatable shaft, wherein, in operation, the plurality of agitators comprises first and second agitators that are counter-rotating and nearest neighbors.

2. The anaerobic digestion agitation system according to claim 1, wherein the axial flow impeller of the first agitator is left-handed and the axial flow impeller of the second agitator is right-handed.

3. The anaerobic digestion agitation system according to claim 1 or 2, wherein the plurality of agitators comprises a plurality of clockwise agitators and a plurality of counterclockwise agitators, wherein the plurality of clockwise agitators comprises the first agitator and the plurality of counterclockwise agitators comprises the second agitator.

4. The anaerobic digestion agitation system according to any one of claims 1 to 3, wherein the plurality of agitators is arranged in a substantially regular pattern.

5. The anaerobic digestion agitation system according to claim 4, wherein the substantially regular pattern provides at least one of the agitators with six nearest neighbors.

6. The anaerobic digestion agitation system according to claim 4, wherein the substantially regular pattern provides at least one of the agitators with four nearest neighbors.

7. The anaerobic digestion agitation system according to any one of claims 4 to 6, wherein the substantially regular pattern provides a first row of at least three agitators that, in operation, rotates clockwise, and a second row of at least three agitators that, in operation, rotates counterclockwise, and wherein the first and second rows are parallel and adjacent.

8. The anaerobic digestion agitation system according to any one of claims 4 to 6, wherein the substantially regular pattern provides a first row of counterclockwise agitators and asecond row of clockwise agitators, and wherein the first and second rows are parallel and adjacent.

9. The anaerobic digestion agitation system according to any one of claims 1 to 8, wherein the plurality of agitators comprises a third other agitator that is also a nearest neighbor to the first agitator, and wherein in operation, the first and third agitators are counter-rotating.

10. The anaerobic digestion agitation system according to any one of claims 1 to 9, wherein the axial flow impeller of each of the agitators in the plurality has a same diameter and is positioned at a substantially same height within the digester.

11. The anaerobic digestion agitation system according to any one of claims 1 to 4 and 6 to 10, wherein the plurality of agitators comprises at least seven agitators.

12. The anaerobic digestion agitation system according to any one of claims 1 to 11, wherein each axial flow impeller is a hydrofoil.

13. The anaerobic digestion agitation system according to any one of claims 1 to 12, wherein the slurry has an undissolved solids that is greater than 2% and less than 12%.

14. The anaerobic digestion agitation system according to any one of claims 1 to 13, wherein the slurry contains agricultural residue.

15. The anaerobic digestion agitation system according to any one of claims 1 to 14, wherein each axial flow impeller has a diameter of D and is positioned between 1.3D and 2.8D away from its nearest neighbor.

16. The anaerobic digestion agitation system according to any one of claims 1 to 15, wherein the digester contains at least at least 10,000 m3of the slurry.

17. The anaerobic digestion agitation system according to any one of claims 1 to 16, wherein each agitator in the plurality of agitators is a top-mounted agitator wherein the drive unit is positioned above a cover of the digester.

18. The anaerobic digestion agitation system according to any one of claims 1 to 16, wherein the digester comprises a lagoon having a flexible cover, and wherein the anaerobic digestion agitation system comprises at least one support structure to which the plurality of agitators is mounted.

19. An anaerobic digestion agitation system comprising: a plurality of agitators for agitating slurry in a digester, each agitator in the plurality comprising a rotatable shaft that is substantially vertical, a drive unit adapted to rotate the rotatable shaft, and an axial flow impeller coupled to the rotatable shaft, wherein the plurality of agitators is arranged in a substantially regular pattern such the plurality of agitators comprises a first plurality of agitators forming a first row of agitators and a second plurality of agitators forming a second row of agitators, the first and second rows of agitators being parallel, and wherein, in operation, the agitators in the first plurality rotate clockwise, the agitators in the second plurality rotate counterclockwise, and the first and second rows are adjacent rows.

20. A method of converting lignocellulosic material to biogas, the method comprising: conducting an anaerobic digestion, wherein feedstock for the anaerobic digestion comprises lignocellulosic feedstock, and wherein the anaerobic digestion is conducted in at least one digester containing slurry having an undissolved solids between 2% and 12%; and agitating the slurry with a plurality of agitators, each agitator in the plurality comprising a rotatable shaft that is substantially vertical, a drive unit adapted to rotate the rotatable shaft, and at least one axial flow impeller coupled to the rotatable shaft such that it rotates within the slurry, wherein, in operation, the plurality of agitators comprises first and second agitators that are counter-rotating and nearest neighbors.

21. The method according to claim 20, wherein, in operation, each of the agitators in the plurality is rotated at a same speed.

22. The method according to claim 21, wherein, in operation, at least some of the agitators in the plurality are synchronized.

23. An anaerobic digestion agitation system comprising: a plurality of agitators for agitating slurry in a digester, each agitator in the plurality comprising a rotatable shaft that is substantially vertical, a drive unit adapted to rotate the rotatable shaft, and an axial flow impeller coupled to the rotatable shaft, wherein the plurality of agitators comprises at least one clockwise agitator and at least one counterclockwise agitator, and wherein the plurality of agitators is arranged with each clockwise agitator having at least one closest neighbor that is a counterclockwise agitator such that, in operation, there is co-current flow provided therebetween.

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