An anaerobic digester including an active heat pump system

Abstract

An anaerobic digester including an active heat pump system. The active heat pump system is arranged to absorb thermal energy from effluent of the digester and return at least a portion of that thermal energy to one or more of the influent, the digestate recirculation, and fluid within the tank. In an embodiment, the active heat pump system includes a heat pump for the influent and / or the recirculating digestate. In another embodiment, the active heat pump system includes an exchanger within the digester tank to transfer thermal energy to the fluid within the tank. Each heat pump is configured to absorb thermal energy of the effluent and to transfer thermal energy to the influent, the digestate, and / or the fluid within the digester. The active heat pump system may include the influent heat pump, the recirculating digestate heat pump and / or the tank fluid heat pump.

Description

Attorney Docket No. PURPOSE-002PCT AN ANAEROBIC DIGESTER INCLUDING AN ACTIVE HEAT PUMP SYSTEM BACKGROUND OF THE INVENTION 1. Field of the Invention.

[0001] The present invention relates to systems for anaerobic digestion of organic materials. More particularly, the present invention relates to the mechanisms for thermal exchange to regulate the temperature of one or more streams of material to enhance the efficiency of the anaerobic digestion process. Further, the present invention relates to an improvement of the way in which the thermal exchange is carried out. 2. Description on the Prior Art.

[0002] Organic substrates may include material obtained from waste streams, such as sewage, sewage sludge, chemical wastes, food processing wastes, agricultural wastes, animal wastes including manure, manufacturing residuals, and other organic waste and materials. Organic substrates, collectively referred to herein as feedstock material, when broken down, such as through anaerobic digestion, may be used as a source of hydrocarbon, such as methane and / or other biogases, biosolids, and other biofuels or bioproducts. Biosolids may also serve as a source of organic fertilizer.

[0003] Anaerobic digestion is a sequence of processes by which microorganisms, usually bacteria, break down the feedstock material in the absence of oxygen. The feedstock material entering the digester is referred to as the “influent” and generally is a mix of solids and liquids. The “effluent” is the resultant product of the anaerobic digestion process, along with gases such as methane. The effluent is also a liquid / solids mix, with the solids component referred to as the digestate. At least some of the digestate may be returned to the digester for further processing. Anaerobic digestion processes to generate the hydrocarbon, biogas, biosolid, and other bioproducts are complicated, costly, and difficult to control.

[0004] Aspects of the costliness and complications associated with anaerobic digestion depend largely on effective control of temperature throughout the digestion process. Specifically, the microorganisms used to digest the feedstock materials are most effective when in contact with the feedstock materials at temperatures optimal for microorganism activity. Anaerobic digesters must have a controlled temperature because the anaerobic bacteria perform well in only threeAttorney Docket No. PURPOSE-002PCT temperature regimes: cryophilic, mesophilic, and thermophilic. That is, some bacteria perform well at a relatively low temperature (cryophilic, e.g., about -20degC to about +20degC), some at moderate temperature (mesophilic, e.g., about +30degC to about +42degC), and some at relatively higher temperature (thermophilic, e.g., about +43degC to about +55degC). If the temperature strays from these temperature ranges, the metabolic processes of the bacteria are retarded, impaired, or completely stopped, leading to failed or incomplete digestion.

[0005] In an attempt to avoid such temperature deviations, it has become common in the field of anaerobic digesters to use heat exchangers to transfer heat from fossil fuels to the digester. Examples include internal and external heat exchangers that have a hot water or water / glycol mixture on one side and digestate on the other side. The hot water / glycol can be heated with a boiler and / or a heat recovery skid in a cogeneration system. For the case of the internal heat exchangers, the hot water / glycol can be pumped through pipes in the digester where heat is transferred to the digestate. Others have embedded radiant heating loops within the walls of concrete bioreactors. For the case of the external heat exchangers, a digestate recirculation loop can be exposed to the hot water / glycol, for example, in a tube-in-shell heat exchanger where heat is transferred from the hot water / glycol to the digestate. An example of a digester with an internal heat exchanger is shown in FIG.1. An example of a tube-in-shell external heat exchanger is shown in FIG.2.

[0006] Another practice is to use a heat exchanger to recover heat from material leaving the anaerobic digester (the “effluent”) to material that is entering the digester (the “influent”). This can be effective because the largest demand for thermal energy in most anaerobic digesters is the low temperature of the influent relative to the ideal anaerobic digester temperature, which should be the temperature of the effluent departing the digester. If the thermal energy in the effluent can be efficiently transferred to the influent, and if the influent temperature can be raised at least close to the effluent temperature, this should reduce the additional thermal energy demand of the system to that which is lost to the environment through convection, conduction, and radiation.

[0007] It is challenging to efficiently transfer the effluent thermal energy to the influent because the two flow rates are not typically synchronized and the flow rates vary with time. Also, there are substantial cost and size implications for heat exchangers that transfer high percentages of thermal energy between fluids that have a relatively low difference in temperature. The temperature difference is necessarily low because the influent must be warmer than freezing andAttorney Docket No. PURPOSE-002PCT the effluent must be near one of the three ideal anaerobic digester temperature regimes (cryophilic, mesophilic, or thermophilic). Further, it can be difficult to make rapid thermal exchange adjustments in industrial scale digesters using large heat exchange components. Beyond that, heat exchangers are “passive” thermal transfer devices in that the thermal exchange effected is dependent on the temperatures of the fluids entering and exiting the digester. Any ineffectiveness or variability of thermal exchange is more a function of fluid characteristics than it is of the design of the heat exchanger.

[0008] What is needed is a mechanism for thermal exchange in an anaerobic digester system to regulate the temperature of the digestate using the thermal energy of the effluent to improve the efficiency of the anaerobic digestion process in a cost-effective manner. The thermal exchange should be less dependent on the thermal characteristics of the influent and effluent than exists using current passive heat exchange systems. SUMMARY OF THE INVENTION

[0009] It is an object of the present invention to provide a heat exchange system for an anaerobic digester that enables rapid and thorough thermal transfer from the effluent to either or both of the influent and digestate so that the process temperature in the digester is as close to optimal as possible.

[0010] The present invention accomplishes that objective. The invention is an anaerobic digester including an active heat pump system. The active heat pump system is configured to recover thermal energy from the effluent and transfer it to either the influent, an influent holding tank, or the digestate. Using one or more heat pumps, more heat can be recovered than using a passive heat exchanger between the influent and effluent because the effluent temperature can be reduced to below the influent or digestate temperature with the heat pump. This is not possible with a passive heat exchanger because the effluent temperature can only be asymptotically reduced to the influent or digestate temperature. Also, the temperature of the transfer solution of the active heat pump can be much hotter than the effluent temperature which increases heat transfer rate and decreases required heat exchanger size and cost. This higher temperature is achieved in the heat pump system by taking advantage of the liquid to vapor state change using an expansion valve and a compressor.Attorney Docket No. PURPOSE-002PCT

[0011] The present invention is an anaerobic digester with an active heat pump system that enables heat recovery from the effluent, which heat can be transferred to the influent, an influent holding tank, or digestate returning to the digester. The active heat pump system is operated external to the digester, which reduces manufacturing and maintenance expenses. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] FIG.1 is a simplified representation on an existing anaerobic digester with an internal passive heat exchanger.

[0013] FIG.2 is a representation of an external tube-in-shell passive heat exchanger that may be used in an anaerobic digester.

[0014] FIG.3 is a simplified representation of the anaerobic digester of the present invention including the active heat pump system. DETAILED DESCRIPTION OF THE INVENTION

[0015] An aerobic digester 10 of the present invention is shown in FIG.3. The digester 10 includes an influent source tank 12, an influent fluid transfer pump 14A, an influent conduit 16, a digestate recirculation conduit 18, a recirculation fluid transfer pump 14B, a digester tank 20, an effluent outlet 24, an optional effluent fluid transfer pump 14C, an effluent conduit 26, a digestate outlet 28, and an effluent product conduit 30. The digester 10 also includes an active heat pump system 32. It is noted that a mixer may optionally be used in the digester tank 20 but that is not a requirement.

[0016] The active heat pump system 32 includes one or more of an influent heat pump 34, a digestate recirculation heat pump 36, and an internal tank heat pump 38. Each of the heat pumps 34 / 36 / 38 includes a compressor 40, an expansion valve 42 and an optional filter drier 44. One or more sensors and controllers may be incorporated into the heat pump system 32 to check flow rates, pressures, and / or temperatures of fluids in any one or more of the conduits and to regulate heat exchange conditions to optimize the digestion process as efficiently as possible.

[0017] The heat pump system 32 shown in FIG.3 depicts all three heat pumps, wherein influent heat pump 34 is configured to exchange thermal energy of effluent product with the influent, digestate recirculation heat pump 36 is configured to exchange thermal energy of the effluent product with the digestate recirculation, and the internal tank heat pump 38 is configured toAttorney Docket No. PURPOSE-002PCT exchange thermal energy of the effluent product with fluid in the digester tank 20. It is to be understood that the active pump system 32 of the invention may comprise one or more of the three heat pumps 34 / 36 / 38. That is, active thermal exchange may occur only with the influent, only with the digestate recirculation, only the fluid within the digester tank 20, or combinations of any two of the heat pumps, or all three heat pumps.

[0018] With respect to the operation of the digester 10 to convert feedstock material into biogas and digestate, that operation occurs substantially as in any digester, with bacteria in the digester tank 20 breaking down the feedstock material into the digestate in an anaerobic environment. The digester 10 is distinct in that the addition of the active heat pump system 32 enables much better control over process temperatures than has heretofore been possible using conventional passive heat exchangers. Specifically, thermal energy of the effluent contained in the effluent conduit 26 is tapped by the active heat pump system 32 and transferred to one or more of the influent in the influent conduit 16, the return digestate in the digestate return conduit 18, and the fluid in the tank 20. Using the active heat pump system 32 enables the absorption of thermal energy from the effluent that is greater than possible when passive exchange occurs because the most thermal energy that can be tapped passively is no greater than the differential between the influent or digestate and the effluent temperatures. That is, using the active heat pump system 32, the temperature of the effluent can be brought below the temperature of the influent, digestate, and / or tank fluid so that the thermal energy delivered to the influent, digestate, and / or tank fluid is greater.

[0019] Each heat pump 34 / 36 / 38 includes the compressor 40, heat exchanger A, the optional filter drier 44, the expansion valve 42, and heat exchanger B. When in operation, the compressor 40 creates a high pressure, high temperature superheated vapor that flows to heat exchanger B. Heat exchangers A and B can be a plate and frame, tube in shell, or other suitable heat exchanger. For influent heat pump 34, influent is pumped through the other side of heat exchanger B where it receives thermal energy and increases in temperature. For digestate recirculation heat pump 36, recirculating digestate is pumped through the other side of heat exchanger B. Finally, for internal tank heat pump 38, fluid in the tank 20 contacts heat exchanger B. The high pressure, high temperature superheated vapor transfers heat to the influent / digestate / digester fluid and becomes a high pressure, slightly cooler liquid. This high pressure, slightly cooler liquid flows through the optional filter drier 44 to the expansion valve 42 where it becomes a low pressure,Attorney Docket No. PURPOSE-002PCT low temperature liquid / vapor mixture. Due to expansion, the pressure and temperature are reduced. The low pressure, low temperature liquid / vapor mixture flows through heat exchanger A. Effluent product flows through the other side of heat exchanger A where it transfers heat to the low pressure, low temperature liquid / vapor mixture and decreases in temperature. The low pressure, low temperature liquid / vapor mixture transfers heat from the effluent product and becomes a low pressure, low temperature slightly superheated vapor. The low pressure, low temperature slightly superheated vapor flows to the compressor 40 where it is again transformed into a high pressure, high temperature superheated vapor.

[0020] In an embodiment of the invention, the active heat pump system 32 may include one or more ground source heat pumps that may be used with or instead of one or more of the heat exchangers associated with the effluent product. Specifically, the ground source heat pump could replace or supplement the thermal energy of the effluent in the warming of either or both of the influent and the recirculated digestate.

[0021] While the present disclosure has been directed to the use of an active heat pump system to improve the operation of an anaerobic digester, the invention is not limited to improvement of that process alone. In one example, some or all of the thermal energy recoverable from the effluent effectively with the active heat pump system may be directed to another heat sink such as, for example, a residential or commercial building needing primary or supplemental heat.

[0022] In another use of the active heat pump system, the invention may be used to cool rather than heat the influent. For example, in an anaerobic digestion process, the influent can be hotter than the ideal anaerobic conditions. In that case, the effluent can be used to cool the influent before introduction to the digester tank. In that application, the fluid transfer associated with operation of the influent heat pump 34 would be reversed so that the effluent product would be used to cool the influent prior to entering the digester tank 20.

[0023] In another application of the invention, in some food waste digesters, there is a requirement to pasteurize the feedstock prior to digestion. Pasteurization requires a holding time at a certain temperature to kill potential pathogens. The higher the temperature the shorter the time and the lower the temperature the longer the time required for pasteurization. Heating to a high temperature is desirable because the batch time is shorter, but also undesirable because the energy required to heat the feedstock to a higher temperature is greater. In that process, thermal energy from the feedstock after pasteurization may be transferred to the feedstock with an activeAttorney Docket No. PURPOSE-002PCT heat pump system such as that described herein prior to pasteurization. This has commonly been done using a passive heat exchanger. Using a second stage active heat pump system would provide all the same advantages of time, control, and physical space. Further, in this case, a first stage active heat pump system can be used to transfer thermal energy from the effluent to the influent feedstock prior to the second-stage heat pump. More specifically, a first-stage active heat pump system can recover thermal energy from the digester effluent and transfer it to the influent prior to pasteurization. For a mesophilic digester, this may cool the effluent from 100F to 40F and heat the influent from 50F to 110F. The second-stage heat pump would recover thermal energy after pasteurization and transfer it to the influent after the first-stage heat pump but prior to pasteurization. For a 170F pasteurization process, this might cool the post- pasteurization influent from 170F to 100F and increase the pre-pasteurization influent from 110F to 180F. This provides for thermal loss of 10F in the system before any external energy source (other than compressor power) is required to provide thermal energy to the pasteurization system.

[0024] While the invention has been described with respect to a specific example embodiment, it is to be understood that its scope is defined by the appended claims.

Claims

Attorney Docket No. PURPOSE-002PCT What is claimed is:

1. An anaerobic digester comprising: - an influent conduit containing an influent of an influent source tank, wherein the influent is to be digested; - a digester tank configured for anaerobic digestion of the influent therein; - an effluent conduit containing effluent from the digester tank; - a digestate recirculation conduit containing digestate of the digester tank; and - an active heat pump system coupled to the effluent conduit and to one or more of the influent conduit and the digestate recirculation conduit, wherein the active heat pump system is configured to actively absorb thermal energy from the effluent and transfer that thermal energy to either or both of the influent and the digestate.

2. The anaerobic digester of Claim 1, wherein the active heat pump system includes an influent heat pump and a digestate recirculation heat pump.

3. The anaerobic digester of Claim 2, wherein the active heat pump system further includes an internal tank heat pump.

4. An anaerobic digester comprising: - an influent conduit containing an influent of an influent source tank, wherein the influent is to be digested; - a digester tank configured for anaerobic digestion of the influent therein; - an effluent conduit containing effluent from the digester tank; - a digestate recirculation conduit containing digestate of the digester tank; and - an active heat pump system coupled to the effluent conduit, wherein the active heat pump system includes a heat exchanger located within the digester tank, and wherein the active heat pump system is configured to actively absorb thermal energy from the effluent and transfer that thermal energy to the influent in the digester tank.

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