EP0130210A1

EP0130210A1 - Apparatus for the production of methane

Info

Publication number: EP0130210A1
Application number: EP84900407A
Authority: EP
Inventors: Ove Thomsen; Peter RÖNNOW
Original assignee: BIOSYSTEM E AB
Current assignee: BIOSYSTEM E AB
Priority date: 1983-01-03
Filing date: 1983-12-23
Publication date: 1985-01-09
Also published as: AU2430484A; FI843452A0; WO1984002698A1; FI843452A; DK421284D0; CA1221478A; AU567914B2; JPS60501591A; DK421284A; NO843465L

Abstract

Appareil de production anaérobie de méthane à partir d'un substrat. La production s'effectue dans deux étages, et l'appareil comprend: un digesteur (104) dans le second étage, adapté pour enfermer dans des conditions anaérobies un substrat et pour en extraire du méthane gazeux; des moyens (106) dans la partie supérieure du digesteur pour l'extraction de gaz; des moyens (112, 114) permettant de transférer le substrat du premier étage au digesteur; un premier étage comprenant une antichambre (130) destinée au traitement du substrat frais avant l'introduction du substrat dans le digesteur (104), cette antichambre (130) étant séparée du digesteur (104) et étant reliée au digesteur (104) par plusieurs organes de transport; ce premier étage comprenant des moyens d'accumulation du substrat au moins partiellement dans l'antichambre (130), le rapport entre le volume du dispositif d'accumulation et le volume du digesteur étant inférieur à un sur cent; le premier étage comprend un dispositif de pompage (134); le dispositif de pompage (134) comporte des premiers moyens permettant au dispositif de pompage de pomper du substrat de ladite antichambre et de le renvoyer à l'antichambre tout en mélangeant et en chauffant le substrat jusqu'à atteindre une température comprise entre 15o C et 70o C; le dispositif de pompage comprenant des seconds moyens lui permettant de pomper le substrat à partir du premier étage dans le digesteur après que la température du substrat a augmenté et après avoir été mélangé dans ce premier étage.Apparatus for anaerobic production of methane from a substrate. Production takes place in two stages, and the apparatus comprises: a digester (104) in the second stage, adapted to enclose a substrate under anaerobic conditions and to extract methane gas therefrom; means (106) in the upper part of the digester for extracting gas; means (112, 114) for transferring the substrate from the first stage to the digester; a first stage comprising an anteroom (130) intended for the treatment of the fresh substrate before the introduction of the substrate into the digester (104), this anteroom (130) being separated from the digester (104) and being connected to the digester (104) by several transport components; this first stage comprising means for accumulating the substrate at least partially in the anteroom (130), the ratio between the volume of the accumulation device and the volume of the digester being less than one in a hundred; the first stage includes a pumping device (134); the pumping device (134) comprises first means allowing the pumping device to pump from the substrate of said anteroom and to return it to the anteroom while mixing and heating the substrate until reaching a temperature of between 15 ° C. and 70o C; the pumping device comprising second means enabling it to pump the substrate from the first stage into the digester after the temperature of the substrate has increased and after having been mixed in this first stage.

Description

APPARATUS FOR THE PRODUCTION OF METHANE

Background of the invention

This invention relates to an apparatus for the production of methane from a substrate comprising an organic material such as manure by anaerobic digestion. In one class of anaerobic digester, a digester vessel encloses a substrate containing a culture of microorganisms and has means for removing methane gas in the upper portion of the vessel. In this class of anaerobic digester, biological material such as, for example, manure from cowsheds or swineries, waste from farms and food-production industries and sewer sludge can be used as the substrate.

To obtain a high yield of gas in a reasonable pro¬ cessing time, this known process has the characteristics of: (1) being sufficiently continuous so that the cul¬ ture of the microorganisms can be maintained for an uninterrupted gas production; (2) supplying and removing biological material sufficiently often so that the capa¬ city of the plant is efficiently utilized; (3) processing the substrate at a temperature that provides a high gas yield; and (4) handling the substrate automatically.

In a prior art type of digester of this class, the substrate is inserted into a sump and processed before being pumped into the main digester. The pro- cessing consists of mechanical agitation to create a slurry and is accompanied by the addition of water to create the proper consistency.

The prior art digesters have several disadvantages such as: (1) they do not provide maximum methane gene- ration, because some new substrate is inserted at a temperature too cold while other substrate is at a tem¬ perature that is too hot; (2) the mixing is inefficient and localized; (3) the tendency to build a crust is excessive; and (4) labor use is high, because attention is required during long periods of time. Summary of the invention

It is an object of the invention to provide an appa¬ ratus for anaerobic digestion in which the temperature of the substrate is maintained relatively uniform; It is a still further object of the invention to provide a novel anaerobic digesting system in which new substrate is mixed and heated prior to being in¬ serted into the main digester vessel;

It is a still further object of the invention to provide a novel anaerobic digester system in which mix¬ ing is accomplished by transporting substrate from one location to another through conduits;

It is a still further object of the invention to provide a novel anaerobic digester system in which the substrate is mixed sufficiently to provide a relatively uniform consistency without reducing the microbial ac¬ tivity by excessive mixing.

In accordance with the above and further objects of the invention, an anaerobic digester includes two stages separated in location, with the first of the two stages receiving new substrate and the second of the two stages serving to hold the substrate with mild turbulence while methane is generated by microbes. The first stage in¬ cludes a compartment that serves as an antechamber for the second stage, which is the main digester vessel.

The new substrate in the first stage is mixed by transporting it through conduits from location to loca¬ tion and is heated to a temperature suitable for anae¬ robic digestion or slightly higher before it is trans- ported into the second stage. Material in one embodi¬ ment may be transferred from the second stage to the first stage and back to the second stage to provide further mixing and heating if desired.

The mixing of the substrate within the first stage is accomplished for no more than fifteen minutes out of each hour and the temperature is controlled within the range of twenty-five degrees centigrade and eighty degrees centigrade during mixing. The first stage has a volume which is at least a hundred-fiftieth the vo¬ lume of the second stage. The volume of the substrate within the first stage is no greater than one third the volume of the second stage. The first stage must be sufficiently large to hold enough substrate for continuous operation of the digester Continuous operation in this description means that nu¬ trients are added sufficiently often to preserve the microbes. The time of heating divided into one time pe- riod, such as a day, and multiplied by the capacity of the antechamber must be greater than the load which must be processed in one day by the digester vessel for con¬ tinuous operation.

The capacity of the digester vessel divided by the capacity of the first stage is equal to the number of times the first stage should be emptied into the main digester vessel during the residence time for ef¬ ficient operation. Residence time is the time a load of substrate remains in the digester and should be equal to the amount of time necessary to digest a load within the digester vessel. For efficient utilization of a source of substrate, it must be possible to load the antechamber with substrate, to heat the substrate and to insert the substrate into the digester vessel a suf- ficient number of times to utilize the desired waste material from the source during the residence time of the substrate.

Material from the antechamber should be heated and inserted into the vessel sufficiently often and in a manner that creates sufficient turbulence in the vessel to break the crust which tends to form at the top of the substrate. If the substrate is mixed too often, the bacteria are reduced in efficiency, and the mixing should not take place for more than half an hour at a time. New substrate is inserted into the digester vessel at least once every eight hours to be sure a crust does not form. The amount of time must be suf¬ ficient so that the top layer in the vessel never be-

O PI comes so hard that gas bubbles cannot penetrate through it or rising warm substrate cannot break the surface.

The first stage includes conduits forming part of a heat exchanger and material must not stand within the conduits of the heat exchange unit so long as to plug them. Since this is a function of the temperature of water that surrounds the conduits within the heat exchanger, substrate should not stand in the pumps longer than fifteen minutes and, at optimum, seven minutes, with water set to heat the substrate to above sixty degrees centigrade. To maintain the efficiency of the bacteria, the mixing should be less than fifteen minutes in one hour. The maximum number of times the first stage can be emptied of substrate without a reduction in ef- ficienty is equal to the residence time divided by the time necessary to efficiently load and heat the 'sub¬ strate material in the first stage.

The size of the antechamber, the volume of the conduits within the heat exchanger and the temperature within the heat exchanger are proportioned so that: (1) the substrate may be continuously processed; (2) the substrate in the antechamber receives heat from the heat exchanger faster than it loses heat to the atmosphere under cold conditions; and (3) the antecha - ber is sufficiently large in proportion to the digester to not require loading of the digester more than ten times, and preferably twice, a day while maintaining continuous operation of the digester.

The temperature of the water in the heat exchanger is less than one hundred degrees centigrade to prevent pasteurization and the substrate should be stationary in it for less than ninety minutes. The material should be mixed for less than thirty minutes at a time and yet be able to increase in temperature to approximately sixty-five degrees centigrade.

From the above description, it can be seen that the digester system of this invention has several advan-

JUSI O PI tages such as: (1) it permits the conservation of labor by concentrating the mixing time in certain periods of a day; (2) it conserves energy; (3) it increases methane output by controlling the temperature at which new waste material is inserted into the principal por¬ tion of a digester vessel; and (4) it provides for ef¬ ficient mixing. Summary of the drawings

The above-noted and other features of the inven- tin will be better understood from the following de¬ tailed description when considered with reference to the accompanying drawings in which:

Fig. 1 is a fragmentary schematic view of an embo¬ diment of the invention; Fig. 2 is a simplified perspective view of the embodiment of Fig. 1; and

Fig. 3 is a schematic drawing of a portion of the embodiment of Fig. 1. Detailed description The manure for the process, which is supplied from a mechanized installation for manure feed, should have a proportion of dry substance of six percent to twelve percent, so-called liquid manure. Even if the process is continuous, which means that the bacteriological process never is interrupted and an adequate quantity of material always is in the processing stage, this does not mean that the supply and also the removal of material can be discontinuous so long as at each oppor¬ tunity only small-part quantities of the whole mass are handled. Thus, the feed of the manuare can be pe¬ riodic such as, for example, a few times per day and the manure is then fed into the process.

Heat supply, supply of material and removal of the same are adjusted in such a way that the material has a temperature of fifty degrees centigrade to sixty degrees σentrigrade before it is removed from the antechamber. Removal of material from the process-

ϊ J E

OMPI ing chamber is adjusted relative to supply so that the processing chamber contains a considerable quantity of material, preferably not less than three fourths of the available volume. The anaerobic microorganisms used for gas production are in the mass in the process¬ ing chamber and in new mass. All supplied mass is succes¬ sively subjected to the anaerobic microorganisms and thereby participates in gas production.

In the embodiments of Figs. 1-3, gas production advantageously takes place at the temperature of fifty degrees centigrade to sixty degrees centigrade and this temperature is maintained in the freshly supplied mass in the processing chamber by good insulation and a certain quantity of heat supplied by heat transfer from the mass in the antechamber. To still further compensate for any escape of heat, which can vary according to the season, more or less heat can be supplied by means of a heating coil (see 118 in Fig. 1). In this connec¬ tion, a regulating system of known type can be used with thermal relays, which register the temperature of the mass and regulate the same.

The material in the processing chamber 104 is de¬ composed by the bacterial culture, whereby methane is generated. This gas is taken out in the upper portion of the chamber through the pipe 106 and is conducted to either one kind or several kinds of consumption such as heat boilers or gas-driven engines and/or to storage spaces for later consumption. The gas can be stored at atmospheric pressure or be compressed and stored in pressure vessels.

Mixing of the substrate causes increased gas pro¬ duction. Such a mixing effect is obtained by the stirring action obtained by the circulation of the material by the pumping action described. Supply and removal of material are adjusted so that the average time that the substrate spends in the process is approximately five to ten days.

OMPI However, in the antechamber, the time the mass is under treatment is not so important if it reaches the desired temperature. It is most advantageous to work the mass to achieve a uniform composition and tem- perature. The best way to achieve this is by mixing. To secure a uniform temperature, the supply and removal of the material and its circulation by pumping action can take place according to a predetermined cycle, viz., first the supply and subsequently the circulation by pumping action for a certain period of time before it is taken out, so that the material supplied will have time to acquire the desired temperature.

When the supplied material is manure, supply usual¬ ly takes place once or twice in twenty-four hours. This material is thus worked by circulation by pumping action in the chamber 104 to provide a uniform temperature and a homogeneous consistency. During the warming up and the simultaneous working action, certain gases such as carbon dioxide may escape, depending upon the charac- ter of the material. It can be of advantage to prevent these gases from being mixed with the methane produced. The chamber 104 is closed, whereby the gas generated in the chamber can be exhausted easily.

According to the invention, the heating of the mass in the antechamber serves to create the intended temperature of the material being processed in the pro¬ cessing vessel 104. However, the heating also has a pasteurizing effect. This effect can be stili further increased if the temperature is raised to and possibly above fifty-five degrees centigrade, at which tempera¬ ture pathogenic bacteria of the Salmonella family are quickly killed.

However, the material supplied to the gas-process¬ ing stage should not have a temperature above fifty-five degrees centigrade and, if pasteurizing is desired, a temperature reduction to approximately fifty degrees centigrade is necessary. This can be effectuated by letting the mass pass through a cooling loop.before its infeed and, in this connection, the emitted heat can be used for preheating freshly supplied material.

In the prior art it has been assumed that the tem- perature should be in the range of thirty degrees cen¬ tigrade .to forty degrees centigrade in bacteriological processes with cultures of anaerobes, to produce methane. In connection with the present invention, however, it has been proved that production is quite feasible at temperatures of between forty degrees centigrade and fifty degrees centigrade. At such a temperature, the process is more rapid and, therefore, the installation can be better utilized than at lower temperatures. How¬ ever, in connection with the higher temperature, it is important that the temperature be very carefully regulated so that it does not reach the range of tempe¬ rature of the coagulation of protein, viz. sixty-five degrees centigrade, which could injure the bacterial culture. The warming to a pasteurizing temperature is an advantageous phase under some circumstances, because the manure can be used without the restrictions necessi¬ tated by the use of unpasteurized manure. Thus, the manure can be spread during all seasons and can be used also for horticultural purposes.

The material that has undergone treatment in the process is discharged from the processing chamber and transported to the surrounding manure yard and a sepa¬ rate manure deposit, respectively, for subsequent use. The material treated in this manner is suitable for use as fertilizer.

In the foregoing it has been described how the plant functions in continuous operation. When starting up the process, special measures are, however, required for instructions of the culture of microorganisms to the mass in the processing chamber. For this purpose there are known special methods and additives to achieve a sure and quick start of the bacteriological process.

O PI There is a risk that the bacteriological process will cease and this can occur when the pH is incorrect (it should be approximately 6.8), when the temperature is unsuitable (outside the range of forty degrees cen- tigrade to fifty degrees centigrade) , when the mass is exposed to air, when the culture is overloaded (too rapid a supply of fresh material) , when too much ma¬ terial is removed in each discharge or when unsuitable ingredients (such as toxic chemicals) are supplied to the biological mass. When the process ceases under such circumstances, a new start-up operation is necessary and this can take place in about the same way as the original one, after correction of the sources of faults. According to the invention the antechamber and the processing chamber are located separately from each other and/or on separate levels, in which cases they are connected by a number of transportation means for transfer of the substrate. By such arrangements the plant can be adapted to different types of space. Di- stance between the chambers results in the advantage that the influence of one chamber's temperature upon the other's is minimized.

It is not necessary that the antechamber be shaped as a proper receptacle; it can, for example, comprise a pipe loop or a tubular battery where the substrate is heated for the purpose indicated. The space for pas¬ sage through the pipe or pipes in such a case thus con¬ stitutes the antechamber.

The receptacle for waste, in which the substrate ^" is collected after having passed through the processing chamber, can be separated from the rest of the plant or integral. However, the invention is not in this re¬ spect limited to any specific embodiment, because the waste receptacle can have any shape and location or can be divided up into several spaces. If waste is di¬ rectly transported to some kind of loading means for removal, the waste receptacle can be omitted. In Fig. 1, there is shown a simplified, schematic view of an embodiment of the invention 100 having a main control unit 102 and a processing section, or di¬ gester vessel section, 104 communicating with each other through conduits and electrical connections to permit mixing, heating, gas-processing and control functions to be performed in the main control unit 102 while methane production and removal occur in the digester vessel 104. The main control unit 102 is housed in a separate compartment outside the processing section 104. More¬ over, both sections are above ground so as to permit easy portability. In the embodiment of Fig. 1: (1) the main control unit 102 provides for the mixing and heat- ing of the waste-material substrate which is to be di¬ gested in the digester vessel section 104; and (2) the main control unit in succession runs through cycles, one part of which is for mixing and heating and another part of which is inactive. The vessel 104 includes a container which can be provided in a lot of different sizes in the range of, for example, from a capacity of twenty cubic meters, to an eighty-cubic-meter capacity.

The housing for the main control unit 102 normally also includes a heating system for heating manure and the pumping and valve systems for transferring manure through conduits and for controlling the temperature of both the digester vessel 100 and the manure or other waste in the main control unit 102. The main control unit 102 communicates with the vessel section 104 through a gas-removal conduit 106, a lower waste-material-removal conduit 108, a middle waste-removal conduit 110, a long waste-material-injec¬ tion conduit 112, a short waste-material-injection con- duit 114, a temperature sensor 116 and heating conduits 118 within its interior, with the gas-removal tube 106 being inserted through the top of the digester and the the remaining conduits being below a level 120 for the waste material.

With this arrangement, substrate may be removed from the digester vessel 104 from a location near the center of its elevation through conduit 110 or near the bottom through conduit 108. Fresh substrate may enter at the length of the digester vessel 104 most removed from the main control unit 102 through conduit 112 or from a location closer to the main control unit 102 through a conduit 114. Thus, substrate may enter at a remote location of the vessel 104 and travel to a location closer to the main control unit 102 for plug- flow operation or may both enter and be removed near the main control unit 102 for a mixing type of opera- tion.

The walls of the vessel 104 are insulated and may include heating coils to aid in controlling the tempe¬ rature and heating coils 118 may be included near the bottom to provide further heat, if necessary, or cool- ing, where desired, as a further control measure. The temperature may be monitored by a temperature-sensing device 116 to determine whether more heat must be added or removed in order to obtain the best conditions within the vessel 104. In Fig. 2, there is shown a simplified fragmentary perspective view of a portion of a main control unit 102 and the digester vessel 104, with the main control unit 102 including an antechamber 130, a heat exchanger and mixing section 132, a pumping section 134 and a portion of the boiler section 136. The gas-removal sec¬ tion and the full details of the boiler section 136 are not shown except for the gas lines 106 and a general compartment 136.

The antechamber 130 receives fresh substrate, such as manure, which may be circulated through the heat exchanger and mixing section 132. The pumping section 134 includes valves for moving the substrate between

O PI antechamber 130 and the heat exchanger and mixing sec¬ tion 132 or for moving substrate between the heat ex¬ changer and mixing section 132 and the digester vessel 104. The heat exchanger and mixing section 132 is a compartment connected through conduits 140 and 142 to the boiler section 136 to cause warm water to be pumped through it and having within it a curved pipe 146 for guiding the substrate therethrough. The curved pipe 146 is formed in several loops in a horizontal plane and connected at one end to a pipe 150 and at the other end to a second pipe 152 through which the substrate flows to and from the pumping station 134.

The heat exchanger and mixing section 132 is di- rectly below the antechamber 130 and this compartment receives fresh substrate pumped through a conduit 154. The temperature of the water in the heat exchanger and mixing section 132 is automatically controlled by con¬ trolling flow of hot water through the conduits 140 and 142. For such control, a temperature-sensing device 156 is positioned within the main control unit 102. In the preferred embodiment, it is within the antecham¬ ber 130 to sense the temperature of the substrate but may be in other embodiments within the heat exchanger and mixing section 132 to sense the temperature of the water. It is connected through a conductor 158 to a valve in the boiler section 136.

To control the flow of substrate, the pumping sec¬ tion 134 includes a pump 160, a timer clock 161, a first valve 162 and a second valve 164. The pump 160 commu¬ nicates directly with the first valve 162 and with pipe 150 to pump substrate from the curved pipe 146 into the first valve 162. The time clock 161 is settable to automatically start and stop the pump 160. The first valve 162 is a two-position fluid valve which communicates through a pipe 166 with the manifold 168 in one position and in the other position communi-

gjH i OMPI cates with a pipe 170. The pipe 170 is connected for communication with the antechamber 130 to pump substrate either from the digester vessel 104 or from the ante¬ chamber 130 through the curved pipe 146 of the heat exchanger 132. The manifold 168 contains a valve to connect the flow to conduit 166 from outlet 108 or out¬ let 110 and draw substrate from either the bottom or the top of vessel 104.

To control the flow of substrate, the second valve 164 is a three-position valve which in one position connects second pipe 152 and a conduit 171 communicating with the antechamber 130 to cause fluid to flow from the curved pipe 146 in the heat exchanger and mixing section 132 to the antechamber 130. In a second position, the second valve 164 connects second pipe 152 with a conduit 112 to cause substrate to flow from the antechamber 130 to the far end of the digester vessel 104 or substrate from the manifold 168 and the interior of the digester vessel 104 to be clr- culated through the curved pipe 146 and back to the far end of the digester through conduit 112, depending upon the position of first valve 162.

In a third position, the second valve 164 causes the pipe 152 to communicate with conduit 114 either to recirculate substrate from the digester vessel 104 through the manifold 168 and the curved pipe 146 back to the digester vessel 104 through conduit 114 at the near end or to cause fluid from the antechamber 130 to be pumped through the curved pipe 146 and into the digester vessel 104, depending on the position of first valve 162.

The manifold 168 includes an effluent-removal con¬ duit 180 through which the effluent to be removed flows under pressure from within the digester. This conduit is also connected through a conduit 182 to the top of the antechamber 130 which serves as an overflow for substrate in the antechamber 130. Fluid being removed

O PI . IPO from the digester vessel 104 may flow into the top of the manifold 168 when first valve 162 is closed so that the fluid in the manifold 168 cannot flow to the curved pipe 146 and treated substrate is removed from the di- gester vessel 104 through this mechanism as new material is added for digestion.

In Fig. 3, there is shown a schematic diagram of the boiler section 136 connected to receive gas from gas lines 106 for heat and to provide hot water through conduits 118 to the digester and to the heat exchanger through conduit 142 for temperature control. The tem¬ perature-measuring device 116 within the digester and the temperature-sensing device 156 within the antechamber are both electrically connected to the boiler section 136 to provide signals thereto. A return conduit 140 returns hot water from the heat exchanger connected to the boiler section 136.

The boiler section 136 includes a heating and,pump¬ ing system 186, a digester temperature-control system 188 and a heat-exchanger temperature-control system 190. The heating and pumping system 186 receives gas and supplies hot water to the digester temperature- control system 188 and to the heat-exchanger tempera¬ ture-control system 190 under the control of the tem- perature-measuring devices 116 and 156, respectively. The heating and pumping system 186 includes a gas boiler 192, a pump 194, an expansion tank 196, a meter 198 and a safety valve 200. The gas boiler 192 receives gas from gas lines 106 and supplies hot water to the pump 194, which pumps it to the expansion tank 196, the meter 198 and the safety valve 200 to ensure that the pressure does not exceed a predetermined limit. It also supplies hot water through a conduit 202 to the digester temperature-control system and through a conduit 204 to the heat-exchanger temperature-control system 190. Water is returned to the boiler 192 from the digester vessel 104 and from the heat-exchanging system through the conduit 140. The digester temperature-control system includes conduit 202, which flows through first and second ma¬ nual valves 206 and 208, filter 210 and automatically controlled valve 212, which is controlled by the tem- perature-measuring device 116. Return water flows through conduit 118 through the parallel manually controlled valves 214 and 216 to conduit 140. Similarly, the heat- exchanger temperature-control system 190 includes the conduit 204, which flows through the two manual valves 218 and 219, the filter 220 and the valve 222, which is controlled by the temperature-measuring device 156. Water is returned through the conduit 140.

In operation, substrate to be treated within the digester vessel 104 is periodically inserted into the antechamber 130 (Fig. 2) through the conduit 154. For example, manure may be used as the substrate and it may be inserted twice a day. For this purpose, the ante¬ chamber 130 is sufficiently large so that, with the economically convenient number of times of filling it, it will supply enough substrate to the digester vessel 104 (Fig. 1) each day to permit continuous operation of the vessel without destruction of the culture therein. To permit an amount of substrate to be stored in the antechamber sufficient to avoid an excessive number of periods of time for loading the antechamber 130, the ratio of the volume of the antechamber 130 to the volume of the digester vessel 104 should be at least one to one hundred and thus there will be fewer than ten periods of filling the antechamber 130 each day, for a maximum residence time of ten days within the digester vessel 104.

In the preferred use of the embodiment of Figs. 1-3, the ratio of the volume of the antechamber to the volume of the digester vessel is preferably in the range of from 2:5 to 1:10, and thus accommodate a relatively short digestion period in the digester vessel 104, as¬ suming that the antechamber is filled twice each day. When the antechamber 130 is filled, the valves 162 and 164 are positioned to permit substrate to flow through conduit 171 (Fig. 2) to the valve 164 to con¬ duit 152 and heat-exchange pipes 146 within the heat exchanger 132 and through the pump 160 and the valve 162. At the same time, water at a temperature within the range of fifteen degrees centigrade to ninety degrees centigrade and preferably at eighty-five degrees centigrade flows from the boiler 136 through the heat exchanger 132. The mixing is preferably for a period of less than 15 minutes because the biological activity decreases after that period of time.

The pump 160 is positioned to completely remove the substrate from the heat-exchange pipes 146 at the end of a mixing cycle or the water may be cooled or removed so that: (1) the temperature of the substrate is increased to approximately fifty-five degrees cen¬ tigrade but always within the range of ten degrees to seventy degrees; (2) the heating and mixing within the heat-exchange pipes 146 lasts not more than fifteen minutes in each hour; and (3) the substrate does not remain stationary in the pipes in the presence of the heated water for more than fifteen minutes at a time. The volume of the heat-exchange pipes 146 holding the substrate within the heat exchanger is preferably one sixty-fourth of the volume of the antechamber 130 so that only a small amount of the total volume of the antechamber is in the heat exchanger and the remainder is being removed into the antechamber, where it is mixed for heating the total amount. The volume of the ante¬ chamber should always be at least thirty percent greater than the volume of the heat-exchange pipes 146 so that there is always more substrate in the antechamber 130 than in the heat-exchange pipes to permit adequate mix- ing.

When the substrate is at a suitable temperature, the positions of the valves 162 and 164 may be changed so that.the substrate flows from the antechamber 130 (Fig. 2) through the conduit 171 into the valve 162, where it is pumped by the pump 160 through the conduit 150 and the heat-exchange pipe 146 and from there through 5 the conduit 152 through the valve 164 and into the con¬ duit 112 or the conduit 114. Both conduits 112 and 114 channel the substrate into the digester vessel 104, with one of them moving it to the far end of the ves¬ sel and the other to the near end. 10 With this procedure, substrate in the vessel is replaced by fresh substrate, causing the spent material to be removed by overflowing and passing through the manifold 168, through the conduit 180, and from there into a collection reservoir. The ratio of the sizes

-15 of the digester vessel and the antechamber is propor¬ tioned to provide an adequate digestion rate, as de¬ scribed above, so that the ratio of the antechamber 130 volume to the digester vessel 104 volume is at least one to one hundred.

20 Periodically, such as every eight hours, the sub¬ strate within the digester is circulated to prevent a crust from forming on its surface which is so hard as to cause substrate to accumulate under it. For this purpose, the valves 162 and 164 are adjusted to pump

25 substrate from either the outlet 110 or the outlet 108 of the digester vessel 104, through the manifold 168, the conduit 166, the valve 162, the pump 160, the conduit 150 and the heat-exchange pipes 146 and back through the valve 164 to either the digester vessel 104 or the

30 antechamber 130, depending on the position of the valve 164.

From the above description, it can be seen that the digester system of this invention has several advan¬ tages, such as: (1) it permits the conservation of labor

35 by concentrating the mixing time into certain periods of a day; (2) it conserves energy; (3) it increases methane output by controlling the temperature at which

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OMPI new substrate is inserted into the principal portion of a digester vessel; and (4) it provides for efficient mixing.

Although a preferred embodiment has been described with some particularity, many modifications and varia¬ tions may be made in the preferred embodiment without deviating from the invention. Accordingly, it is to be understood that, within the scope of the appended claims, the invention may be practiced other than as specifically described.

Claims

1. Apparatus for the anaerobic production of methane from a substrate in a first stage and a second stage c h a r a c t e r i s e d in that it comprises: a digester vessel (104) in said second stage adapt- ed to anaerobically enclose a substrate and to have methane gas removed therefrom; means (106) in the upper portion of the digester vessel for removal of gas; means (112, 114) for transferring substrate from the first stage to the digester vessel; first stage means including an antechamber (130) for processing fresh substrate before insertion of the substrate into the digester vessel means (104) , which antechamber (130) is located separately from the diges- ter vessel (104) and connected to the digester vessel (104) by a number of transportation means; said first stage means including means for accu¬ mulating substrate at least partly in said antechamber (130) having a volume that is in a ratio with the diges- ter vessel volume less than one to one hundred; said first stage means including pumping means (134); said pumping means (134) including first means for enabling said pumping means to pump substrate from said antechamber and return it to the antechamber while heating and mixing the substrate to a temperature of between fifteen degrees centigrade and seventy degrees centigrade; and said pumping means including second means for enabl- ing said pumping means to pump said substrate from said first stage means into said digester vessel after the temperature of the substrate has been increased and it has been mixed in said first stage means.

" CFREX;

O PI

2. Apparatus according to claim 1, c h a r a c ¬ t e r i s e d in that said pumping means (134) includes conduit means (146) having a diameter less than one third the diameter of said antechamber and a volume less than eighty percent the volume of the substrate contained in said first stage means.

3. Apparatus according to claim 1 or 2, c h a ¬ r a c t e r i s e d in that said pumping means (134) includes a heat exchanger (132) , with said conduit means (146) passing through said heat exchanger in a curvili¬ near path.

4. Apparatus according to claim 3, c h a r a c ¬ t e r i s e d in that said pumping means includes valve means (162, 164) for selectively moving said substrate through a first path from said antechamber through said pumping means and back to said antechamber and through a second path from said antechamber to said digester vessel.

5. Apparatus according to any of claims 1-4, c h a - r a c t e r i s e d in that said pumping means includes: heat exchanger means containing water of between fifty degrees centigrade and a ninety degrees centi¬ grade; boiler means (136) for heating said water; and means for heating said substrate in said heat ex¬ changer means.

6. Apparatus according to any of claims 1-5, c h a ¬ r a c t e r i s e d in that said pumping means includes means for removing substrate from said digester vessel means into said antechamber through said pumping means, heating and mixing said removed substrate and returning it to said digester vessel means.