Patent Publication Number: US-2009221054-A1

Title: Biogas system

Description:
The present invention relates to a, biogas system with a fermenter which has a first and at least a second fermenting chamber for the fermentation of the fermenting medium. 
     The invention also relates to a method of mixing fermenting medium in a fermenter of the afore-described kind. 
     In fermenting chambers it is possible to produce energy-rich biogas from organic substances, e.g. sewage sludge, liquid manure, vegetable waste, plant clippings and other agricultural waste material. This biogas can be converted into heat and electrical energy in machines like gas engines and turbines. With the liberalisation of the Austrian gas market it became possible for biogenic gas producers to supply the public natural gas network, provided that the prescribed quality requirements were observed. A prerequisite for rapid fermentation and effective biogas production is homogeneous thorough mixing of the fermenting medium in the fermenting chambers, so that the solid material in the fermenting medium is not deposited on the floor, but continues to be in suspension. Mechanical mixing systems, e.g. slow rotating paddle mixers with a vertical axis, or fast rotating propeller mixers, amongst others, are prior art mixing systems for biogas systems. These mixing systems, in addition to high manufacturing costs, also have the drawback that they require intensive maintenance. 
     It is therefore an object of the present invention to propose a biogas system of the kind mentioned in the introduction, in which there is homogeneous thorough mixing of the fermenting medium, but wherein the use of mechanical mixers is not absolutely necessary. 
     This is achieved according to the invention in one advantageous embodiment by virtue of the fact that biogas formed in the first fermenting chamber can be introduced into a riser pipe disposed in the second fermenting chamber. 
     In this way, a means is created for the largely, anaerobic decomposition of organic substrates by utilising the gas pressure of the biogas which is produced in the first fermenting chamber and which is able to be forced into the riser pipe of the second fermenting chamber. The riser pipe—preferably extending over at least most of the maximum height of the fermenting chamber—of the second fermenting chamber rests on the basic principle of a mammoth pump where intake of biogas gives rise to a mixture of fermenting medium and biogas of significantly lower specific weight than the fermenting medium surrounding the riser pipe. In other words, the rising gas bubbles in the riser pipe of the second fermenting chamber reduce the density of the liquid in comparison with the surrounding liquid. The difference in density causes an ascending flow in the riser pipe which thus serves for circulation around the reactor. 
     In a preferred embodiment of the invention, it is provided that the riser pipe is designed in such a way that the fermenting medium reaches the riser pipe through an intake opening, and issues back out of the riser pipe through a discharge opening. In this connection, it is provided that the intake opening is displaced below the discharge opening. For an optimum fermentation process it can be advantageous if the riser pipe is disposed in the fermenting chamber substantially vertically and preferably centrally therein. 
     In order that even a relatively small gas pressure of the produced biomass is sufficient to overcome the hydrostatic pressure of the column of liquid, and thus induce an ascending flow in the riser pipe, it can be advantageous if the biogas can be introduced in the lower region, preferably in the bottom-most half, of the uppermost third of the riser pipe. 
     According to a preferred embodiment of the invention it can be provided that the riser pipe has a heating means, preferably a heat exchanger. In this way, the reactor can be heated, and as a result of the increased temperature of the liquid in the riser pipe an additional difference in density is produced in comparison with the liquid in the surrounding reactor space. In this connection, it can be advantageous if the riser pipe is designed so that it is double-walled at least regionally, wherein a heating fluid, preferably heating water, can circulate between the two walls. 
     The heating water can, for example, be supplied to the riser pipe by way of the excess heat of a block-type thermal power station. Good heat transfer can also be achieved because of the resultant improved mixing flow. 
     According to one embodiment of the invention, it can be provided that the biogas is transferred from the first fermenting chamber. into the riser pipe of the second fermenting chamber via a gas pipe which is preferably closed apart from one intake and one outlet. Of course, if necessary, gas valves can also be used which permit gas to be conveyed into the riser pipe of the second fermenting chamber when a set, or presettable, (excess)-pressure prevails in the first fermenting chamber. 
     Advantageously, it is provided that the first fermenting chamber is designed to be gas-tight, at least in the filled condition—except for the gas pipe. In this way, the necessary gas pressure can be prepared in the gas tower of the first fermenting chamber. 
     According to a further embodiment of the invention, it can be provided that a riser pipe is likewise disposed in the first fermenting chamber. Therein, the riser pipe can have all of the features which have been described for the riser pipe of the second fermenting chamber. In this connection, it can be advantageous if compressed air can be introduced into the riser pipe of the first fermenting chamber, so that advantageously the biogas can be desulfurized. 
     The method according to the invention of mixing fermenting medium in a fermenter which has a first and at least a second fermenting chamber for the fermentation of the fermenting medium is characterized in that biogas formed in the first fermenting chamber is introduced into a riser pipe disposed in the second fermenting chamber in order to produce a flow therein. In this connection, it is advantageous if the riser pipe is heated. 
    
    
     
       Further details and advantages of the present invention will be described with the aid of the following description of the drawings, wherein: 
         FIG. 1  is a diagrammatic section through a biogas system according to the invention in a top plan view, 
         FIG. 2  is a vertical section through the biogas system of  FIG. 1 , and 
         FIG. 3  is a detail, shown on a larger scale, of the fermenting chamber of  FIG. 2 . 
     
    
    
       FIG. 1  is a diagrammatic view in plan of a biogas system  1  according to the invention. This biogas system  1  comprises a fermenter  2  which is of a circular shape for reasons associated with statics, hydraulics and heating technology. The fermenter  2  contains a first fermenting chamber K 1  and a second fermenting chamber K 2  which jointly describe the form of a circle. The reference numeral  3  is used to denote an intake through which the fermenting medium, e.g. liquid manure, can be introduced into the fermenting chamber K 1  from above. The fermenter  2  further comprises two post-fermenting chambers K 3  and K 4 , wherein an outer wall which is formed by the first and second fermenting chambers is disposed essentially concentrically with respect to the outer wall of the two post-fermenting chambers K 3  and K 4 . The post-fermenting chamber K 4  has an outlet  4  for liquid. The fermenting chambers K 1  and K 2  thus form the core, and the post-fermenting chambers K 3  and K 4  form the circular periphery, wherein the individual fermenting chambers K 1 , K 2 , K 3 , K 4  are separated from each other by walls W 1 , W 2 , W 3 , preferably downflow baffles, so that in the region of the fermenter  2  close to the floor, it is possible for fermenting medium to pass from one fermenting chamber into the other, as shown in  FIG. 2 . There is a clear hydraulic division between the core and periphery (e.g. between chambers K 2  and K 3 ), where there is merely an overflow opening  5  at water level. In the embodiment shown, a riser pipe  6  is disposed in the first fermenting chamber K 1 , and a riser pipe  7  is disposed in the second fermenting chamber K 2 . Now, the invention rests on the basic concept of introducing the biogas occurring as a result of the fermenting process in the first fermenting chamber K 1  into a riser pipe  7  disposed in the second fermenting chamber K 2 , so as to induce an ascending flow therein. This ascending flow serves to provide circulation around the reactor, so that solid matter is not deposited on the bottom of the fermenter  2 , but continues to be in suspension, thereby producing homogeneous decomposition of the substrate. A riser pipe  6  with the features described within the context of this invention can likewise be disposed in the first fermenting chamber K 1 , but compressed air is forced into the riser pipe  6 , instead of biogas into the Thermo-Gas-Lift (a part of ca. 4% air has proven advantageous for sulfur-free biogas). 
       FIG. 2  shows a vertical section through the fermenter  2  with both fermenting chambers K 1  and K 2  which are separated from each other by a downflow baffle W 1 , so that the first fermenting chamber K 1 —except for the gas pipe [ FIG. 3 ] leading to the riser pipe  7  of the second fermenting chamber K 2 —is designed in such a way that it is gas-tight at the top, and, at the bottom, permits passage of the fermenting medium from one fermenting chamber into the other (or, in the opposite direction). To that end, the downflow baffle W 1  has in the bottom-most region, preferably in the bottom-most quarter, a gap  8  which extends across the width of the downflow baffle W 1 , through which gap the fermenting medium is able to flow. The downflow baffles W 2  and W 3  are designed in a similar way. The reference letter D is used to denote a cut-out detail which is shown on a larger scale in  FIG. 3 . 
       FIG. 3  is the cut-out detail D of  FIG. 2 , on a larger scale, with reference to which the operating principle of the fermenter  2  according to the invention will now be described more closely. The riser pipe  7  is designed in such a way that the fermenting medium arrives at the riser pipe  7  through an intake opening E, and issues back out of the riser pipe  7  at a discharge location A located at a higher level. 
     The purpose of the downflow baffle W 1  is to separate the two fermenting chambers K 1  and K 2 , wherein the downflow baffle W 1  has a closed wall in the upper region and a gap  8  in the lower region. As a result of the intense production of biogas in the fermenting chamber K 1 , an excess pressure builds up in the gas-tight tower of the fermenting chamber K 1 , and forces down the level of liquid  10 , and a corresponding volume of liquid is forced under the downflow baffle W 1  and through into the fermenting chamber K 2 . The maximum level  11  in the fermenting chamber K 2  is determined by the overflow opening  5  ( FIG. 1 ,  FIG. 2 ) into the post-fermenting chamber K 3 . A gas-overflow pipe  9  which goes from the fermenting chamber K 1  is forced directly into the riser pipe  7  of the second fermenting chamber K 2  at the entry location M. The height location of the entry location M defines the excess pressure in the fermenting chamber K 1  and makes costly pressure gauging- and regulating instruments superfluous. The rising gas bubbles reduce the density of the liquid in the riser pipe  7  in comparison with the surrounding liquid in the fermenting chamber K 2 , thereby producing an upwardly directed vertical flow. The riser pipe  7  should open out only slightly below the maximum level  11 . The difference in height between the entry location M of the biogas excess pressure line  9  into the riser pipe  7  and the overflow opening  5  defines the gas pressure in the first fermenting chamber K 1 . As soon as that gas pressure has built up as a result of the biological activity, the gas flows continuously, at a constant pressure, through the gas pipe  9  into the riser pipe  7  of the fermenting chamber K 2 . The rising gas bubbles accelerate the vertical flow of the liquid, and the gas which has escaped is able to overflow in almost pressure-free manner into the gas spaces of the post-fermenting chambers K 3  and K 4 , and carry on flowing towards a gas accumulator disposed outside the fermenter  2 . The effect of the vertical flow is yet further intensified by the heatable riser pipes  6  and  7 , since these latter are equipped with a heating means  11   a  and  11   b.  In the embodiment shown, the heating means  11   a  and  11   b  are in the form of a heat exchanger, a heating fluid, preferably heating water from a tank  12  of heating water, being able to circulate between the two walls by virtue of the double-walled construction of the riser pipes  6  and  7 . The tank  12  of heating water thus supplies both riser pipes  6  and  7 , flow being promoted by the heat input, and heat transfer also being facilitated towards the contact surfaces around which it flows. The riser pipe  6  of the first fermenting chamber K 1  has a supply  13  of compressed air, wherein the forcing of air into the riser pipe  6  of the first fermenting chamber K 1  represents the starting point of de-sulfurized air being forced through the gas towers of all four fermenting chambers K 1  to K 4 . As a result of the small amount of oxygen, microbial oxidation of the H 2 —S-sulfur is possible on the surfaces of the tower, and by avoiding short circuit currents of biogas or air a degree of desulfurization in the pipe is ensured. Along the flow path through the gas towers of the four chambers K 1 -K 4  sufficient reaction surfaces are available for H 2 S-oxidation, and the elementary sulfur which has precipitated arrives back in the bio-liquid manure. Although the amount of compressed air is substantially less than the amount of pressurised gas produced from fermenting chamber K 1 , the introduction of compressed air can take place much lower down, i.e. at the lower opening of the riser pipe  6 . Therefore, the Thermo-Gas-Lift in the fermenting chamber K 1  attains a similar carrying capacity to that in fermenting chamber K 2 . 
     According to one embodiment of the invention it can be provided that the gas pipe  9  comprises an overflow valve—preferably capable of opening intermittently—by means of which the gas pressure in the first and second fermenting chambers K 1 , K 2  can be equalized. This can, for example, be done by a by-pass line which branches off from the gas line  9 , the gas being able to be introduced directly into the second fermenting chamber K 2 . The level of liquid of fermentation chamber K 2  is pushed down by the prevailing gas pressure, whereupon a corresponding volume of liquid, starting from the second fermenting chamber K 2 , is urged through the gap  8  in the downflow baffle W 1 , into the first fermenting chamber K 1 . As a result, the layer of sludge on the floor of the two fermenting chambers K 1 , K 2  begins to flow at increased speed, so that the substrate which is close to the floor is mobilised, at least intermittently, and solid matter is not able to become permanently deposited on the floor. 
     The proposed biogas system with its 4-chamber plan in this way gives rise to a so-called “plug flow” characteristic, i.e. contrary to a fully thorough-mixing reactor a minimum residence time of the substrate is ensured, and hydraulic short-circuits are avoided, thereby bringing about more complete decomposition (greater yield of biogas, better quality bio-liquid manure in terms of hygiene-related parameters and odorous substances). By virtue of the concentric arrangement of the four fermenting chambers (fermenting chambers K 1  and K 2  with the greatest conversion of gas in the core, post-fermenting chambers K 3  and K 4  at the periphery) and an optimum volume/surface ratio (&gt;1), heat loss is minimised, and temperature gradients are made possible between core and periphery. Furthermore, the hydraulic decoupling of core and periphery (overflow of liquid manure and gas without reflux) means that a high level of volume flexibility is obtained. The afore-described mixing system feeds seeding sludge from fermenting chamber K 2  into fermenting chamber K 1 , and the core can therefore be operated independently, i.e. fermenting chambers K 3  and K 4  can be used and emptied both in the manner of reaction volumes as well as in the manner of gas-tight end disposal units. 
     The present invention is not only limited to the embodiment shown, but encompasses or extends to all variants and technical equivalents which can come within the scope of the following claims. The positional information selected in the description, e.g. above, below, etc. referring to the conventional mounting orientation of the fermenter, or to the drawing which has been directly described and shown, can, in the event of positional changes, be applied to the new orientation accordingly. Passive mixing devices can also be provided, such as perforated grids, which are disposed in the region of the layer of scum of the fermenting medium in fermenting chambers K 1  and K 2 . If there is equalization of pressure between fermenting chambers K 1  and K 2 , as triggered by the overflow valve, the fermenting medium forces its way through the perforated grid, thereby preventing solidification of the layer of scum.