Device for drying fermentation residues

Abstract

Device for drying fermentation residues comprising ● at least one digestate dryer (GTn), comprising - an evaporator boiler (Vn) with - - a digestate inflow (ZGn) and a digestate outflow (AGn), - - a steam outlet (DAn), - - a heating device (HVn) this comprehensive - - - at least one double-walled heat exchanger plate (DPn) arranged concentrically around the central axis of the evaporator boiler (Vn), wherein the double-walled heat exchanger plate (DPn) has a steam or hot water supply line (ZHVn) at the upper end and a condensate or hot water outlet (KAn) at the lower end, and - - - a cleaning device (Bn, BTn) for cleaning the outer surfaces of the at least one double-walled heat exchanger plate (DPn), and ● a fermenter (F), wherein the digestate inflow (ZG1) of the first digestate dryer (GT1) is connected to an outlet (AF) of the fermenter (F) and the digestate outflow (AGz) of the last digestate dryer (GT1, GTz) is connected to a digestate inlet of the fermenter (F), where n represents an integer from 1 to the total number z of digestate dryers.

Description

[0001] The invention relates to a device for drying fermentation residues.

[0002] For the growing number of biogas plants, the disposal of digestate is becoming a key issue. For this, practical and cost-effective concepts must be found. Depending on the source digestate and the desired products, mechanical separation, drying, evaporation, and wastewater management processes are used. These processes, operated individually or in combination, sometimes require considerable investment and the use of thermal and electrical energy. On the other hand, storage and application costs can be saved, and revenue can sometimes be generated from the digestate products. In large-scale plants, digestate processing is almost unavoidable, and the question of its economic viability arises in the context of the entire plant (cf. LOOTSMA, A., RAUSSEN, T. (2008): Current processes for the processing and utilization of digestate. In: Bio- and secondary raw material utilization. Material - energetic III.Witzenhausen Institute - News from Research and Practice. Kassel: K. Wiemer, M. Kern (eds.). pp. 559-576.

[0003] The cited reference provides a good overview of the state of the art in drying processes. Combinations of drying methods, including mechanical separation and thermal processes, are often used, using belt, drum, pusher-turn, fluidized-bed, or superheated steam dryers. Falling-film and forced-circulation evaporators or multi-stage evaporator boilers with mechanical vapor recompression are used for evaporation or thickening of liquid digestate. Evaporation systems with vapor recompression are also used.

[0004] In belt dryer systems currently on the market, the digestate is dried using heated air that is blown over a large belt loaded with digestate. The operating principle of these systems is essentially based on evaporation. High air flow speeds are required to achieve high drying performance. Drying is additionally supported by heating the drying air. However, these drying processes require large air flows that must be generated electrically. This reduces the electrical efficiency of the entire biogas plant. Furthermore, the drying efficiency is not sufficient to process 100% of the digestate produced in the biogas plant. In addition, such systems are not hermetically sealed, which leads to odor pollution, and the exhaust air is also contaminated with ammonia and particulate matter.

[0005] Most digestate drying processes require pretreatment of the digestate, for example, by drying it to a dry matter content of 25 percent using a separator; or by mixing in a small portion of the dried material. These process steps represent additional effort that reduces the efficiency of the drying process.

[0006] German patent application DE 41 14 667 A1 discloses a method and device for processing animal waste products in the form of liquid manure. This method involves first storing the waste products in a container to generate methane gas, continuously feeding them from there to a centrifuge, and then drying the resulting solid component. The liquid component separated in the centrifuge is fed into an evaporator vessel, where the water it contains is evaporated by negative pressure, subsequently liquefied in a condenser, and returned to the natural water cycle. The waste heat from the condenser is raised by a heat pump and then fed to the evaporator vessel.In this case, as in all known condensers, the steam condensation takes place through conventional heat exchangers, i.e. with two separate media, which, among other disadvantages, results in a temperature loss and the heat exchangers quickly become dirty due to the contaminated vapors.

[0007] From DE 10 2005 048 116 A1 a method and a device for producing fertilizer moldings are known, wherein fermentation residues from a fermenter are dried in an evaporation agent.

[0008] DE 10 2007 004 892 A1 teaches a process for the treatment of organic residues in biogas plants. Korean patent application KR 2007 0 053 178 A shows a cylindrical heat exchanger assembly with a housing housing a low-temperature heat exchanger, a high-temperature heat exchanger, and a condenser. These components are interconnected by various piping. However, the housing itself, which could be considered an evaporator vessel, has no digestate inlet or outlet and therefore does not receive any digestate.

[0009] German patent application DE 10 2007 034 642 A1 describes a process and a plant for treating organically contaminated waste. The technical features of an evaporator boiler equipped with heat exchangers are not disclosed.

[0010] US 2009 / 0 293 347 A1 discloses a device for producing an ethanol / water mixture. Although the device comprises a fermenter and a falling-film evaporator, no fermentation residues are dried. Instead, fermentable raw materials consisting of sugars or hydrolyzable raw materials are introduced into the reactor or fermenter as starting material, which can be fermented into sugars, ultimately yielding a fermentation solution containing ethanol / bioethanol.

[0011] The object of the invention is therefore to create a digestate dryer that overcomes the aforementioned disadvantages.

[0012] This object is achieved with a device for drying fermentation residues according to claim 1.

[0013] According to an advantageous embodiment of the invention, the fermenter has a heating device connected to the steam outlet of the first digestate dryer. This enables energy-saving heating.

[0014] An advantageous device for drying fermentation residues consists of at least two fermentation residue dryers arranged in series, with a secondary fermenter being arranged at least between the first fermentation residue dryer and the second fermentation residue dryer.

[0015] A solid / liquid separation device for the digestate can be advantageously installed downstream of the secondary fermenter. If the proportion of solid components in the digestate is too high for the drying process or during the drying process, mechanical separation of the digestate can be advantageous.

[0016] An advantageous device for drying digestate consists of at least two digestate dryers arranged in series, each of which is connected to one another via its digestate inlets and outlets, wherein the heating device of the upstream digestate dryer is connected to the steam outlet of the downstream digestate dryer, wherein the condensate accumulating in the heating device of the upstream digestate dryer can be fed to a condensate collection tank.

[0017] The resulting condensate is transported by one or more vacuum pumps. It is advantageous to cool the condensate escaping from the heaters on its way to the vacuum pump(s). This binds more ammonia in the condensate, allowing a larger volume flow to be transported, or smaller-diameter pipes to be used. This measure significantly increases the system's efficiency.

[0018] According to a further advantageous embodiment of the device for drying fermentation residues, at least one solid / liquid separation device for the fermentation residues is arranged between the fermentation residue inlets and outlets.

[0019] Depending on the desired or achieved dry substrate content, it may be advisable to separate liquid and solid components, particularly using mechanical separation devices. This separation after the secondary fermenter has the advantage that the highly volatile ammonia has already been expelled from the digestate / substrate. This results in significantly lower odor pollution and a reduction in ammonia emissions. Overall, the improved substrate digestion results in higher gas yields, lower emissions, and a reduction in construction costs. Depending on the desired result, a secondary fermenter, a solid / liquid separation device, or a secondary fermenter and a solid / liquid separation device can be advantageously arranged between two evaporators.

[0020] The device for drying digestate is described in more detail below using the figures. They show: Fig. 1 a schematic representation of a digestate dryer GT, as used in the device according to the invention for drying digestate, in three-dimensional view and side view with partially cut-away shell of the evaporator vessel V Fig. 2 a schematic representation of a device not according to the invention for drying digestate with three digestate dryers GT1 to GT3 connected in series Fig. 3 a schematic representation of a device not according to the invention for drying digestate with a secondary digester NG and a separating device QP between two digestate dryers GT1 and GT2 Fig. 4 a schematic representation of the device according to the invention for drying digestate with two digestate dryers GT1 and GT2, which are connected in a circuit with a fermenter F

[0021] In Fig. 1 the reference symbols are used without the index n. Fig. Figure 1 shows a digestate dryer GT as used in the device according to the invention. This consists of an evaporator vessel V, which is Fig. 1 is shown as open at the top, but is actually closed at the top. Concentrically arranged, double-walled heat exchanger plates DP serve as heating devices HV. Each of these double-walled plates DP is connected at the top end to a supply line ZHV and at the bottom end to a drain pipe KA. Hot water or steam is conducted through the plates DP via the supply and drain lines ZHV and KA.

[0022] It has been demonstrated that such heat exchangers can be heated directly with steam. Due to the negative pressure created during condensation (and the vacuum maintenance lines and condensate extraction described below), the steam is drawn into the heat exchanger plates at high velocity. This allows the diameter of the steam lines to be reduced, for example, from 400 mm to 150 mm.

[0023] The evaporator boiler V is heated by means of a - in Fig. 1 not shown - pump via a digestate inflow ZG with digestate M, which for example comes from a fermenter F ( Fig. 3 and Fig. 4) or a secondary fermenter NG or an upstream digestate dryer GTn ( Fig. 2 and Fig. 3). The digestate M is then located between the heat exchanger plates DP. The partially dried digestate M is discharged via the digestate outlet AG and further processed as shown below.

[0024] A three-armed holding device T is attached at the top and bottom of the evaporator vessel V. This supports the shaft (drive axis A) of a brush holder BT, which also has three arms. Attached to the brush holder BT are brushes B, which extend parallel to the vertical axis (drive axis A) of the evaporator vessel V over the length of the heat exchanger plates DP of the heating device HV. The brush holder BT is moved in reverse via the drive axis A, so that the brushes B attached to the three arms each clean a circular segment of the plates DP. The brushes B are arranged so that the front and back sides of the plates DP are cleaned.

[0025] The digestate dryer GT thus functions like a heat exchanger. The heated plates DP heat the digestate M. The resulting steam is discharged via a steam outlet pipe located in the lid of the evaporator vessel V (not shown here for clarity) and fed to a further use described below. The heating device HV can be connected via its inlet ZHV and its outlet KA to a hot water circulation device, for example, that of a combined heat and power plant. If it is heated with steam, the condensate produced by the heat extraction is discharged via the outlet KA.

[0026] Fig. Figure 2 shows a schematic representation of a device for drying digestate with three digestate dryers GT3 to GT1 connected in series (from left to right). The heating device HV3 is supplied via an external heat source, in this case via the flow and return lines VL, RL of the hot water circulation system of a combined heat and power (CHP) plant. The steam generated in the evaporator boiler V3 is fed to the second heating device HV2 via the steam outlet pipe DA3. Accordingly, the first heating device HV1 is supplied with steam from the digestate dryer GT2. In this way, a cascaded supply of the digestate dryers GT3 to GT1 with the heat originally generated by the combined heat and power (CHP) plant is achieved.

[0027] The digestate M is pumped successively through the digestate dryers GT1 to GT3 (from right to left in the figure) against the flow of steam, where it is progressively dried. Before being pumped from the secondary digester NG into the digestate dryer GT1, the digestate M can be separated into solid and more or less liquid components using a solid / liquid separation device QP, such as a mechanical screw separator. The dry fraction is then discharged via the outlet AT of the separation device QP. The moisture content is fed as digestate M to the evaporator vessel V1 of the digestate dryer GT1 via the outlet AF.

[0028] The condensate accumulating in the heating devices HV1 and HV2 is fed into the condensate collection tanks KST1 and KST2 and from there pumped into the condensate line KL.

[0029] The steam generated in the evaporator boiler V1 is fed via the steam outlet DA1 and the line connection 1.1 to a conventional heat exchanger, in this case a steam condenser K (outlined in a dotted line). The condensate accumulating in the condensate collection tank KST3 is also pumped into the line KL. The heat recovered in the steam condenser K can be used for further purposes.

[0030] Steam can be diverted via line connections 1.2 and 1.3 of steam outlets DA2 and DA3 and fed to the steam condenser K or a separate heat exchanger, thus generating hot water. For example, if steam is diverted from the evaporator boiler V3, hot water can be generated at a temperature of approximately 70 °C; 55 °C downstream of the evaporator boiler V2 and 40 °C downstream of the evaporator boiler V1.

[0031] A vacuum line UA1, UA2, and UA3 is connected to each of the condensate collection tanks KST1 to KST3, each of which leads into the condensate line KL. As will be shown later, the condensate line KL is connected to a vacuum pump VP, which, on the one hand, pumps out the condensate and, on the other, maintains the desired vacuum.

[0032] Condensation in the heat exchanger plates DPn creates a negative pressure, which flows into the respective evaporator boilers Vn via the steam outlets DAn.

[0033] Via the vacuum connections UA1 to UA3, the evaporation process in the evaporators V1 to V3 is controlled by generating and / or maintaining a preset vacuum with the help of valves (not shown here for the sake of clarity) and, if necessary, pumps.

[0034] The condensate accumulating in the condensate collection tanks KST1 to KST3 is pumped into the condensate line KL or extracted by the vacuum pump VP and subjected to further processing, particularly ammonia separation. To generate the vacuum, a vacuum pump P can be assigned to each vacuum line UA1 to UA3, but this does not necessarily have to be the case. The various vacuum levels can also be set, for example, using just one vacuum pump P and the corresponding valves. These measures are within the skill of the specialist and are therefore not described or explained further here.

[0035] The hot condensate emerging from the respective condensate drain KAn of the digestate dryer GTn (e.g. emerging from the heating device HV2 at approx. 60°C to 70°C) still contains a greater or lesser proportion of steam. The vacuum pumps used as standard are equipped with a cooling system. However, it has been shown that the efficiency of the system can be significantly improved if the condensate emerging from the condensate drain KAn and fed to or contained in the condensate collection tanks KST3, KST2, KST1 is additionally cooled. This not only increases the performance of the vacuum pumps, but also means that the cooled condensate contains less carbon dioxide and, advantageously, more ammonia is bound in the condensate liquid. It is therefore advisable to install an additional cooling device, for example in or on the condensate collection tanks KST3, KST2, KST1 or on the pipes to the vacuum pump VP orto the vacuum pumps P (if several are used) and / or to additionally cool the vacuum pump(s) P themselves. Cooling the condensate by approximately 20 °C to 25 °C already results in a significant improvement in efficiency.

[0036] At the digestate outlet AG3 of the evaporator V3, digestate M with the desired dry matter content of, for example, 25% is produced and can be transported to a digestate storage facility not shown here.

[0037] In the embodiment according to Fig. Figure 3 shows a schematic representation of a device for drying digestate, in which a secondary digester NG with a downstream separation device QP is arranged between two digestate dryers, here between the digestate dryers GT1 and GT2. The evaporator vessel V1 of the digestate dryer GT1 is fed with digestate M from the fermenter F. In the evaporator vessel V1, for example, one-third of the liquid from the digestate M can then be evaporated. This results in a volume reduction of the digestate M, which can be approximately 25%, for example, so that only a 25% smaller secondary digester NG is required. This correspondingly requires less heat in the secondary digester NG. In addition, the evaporation before the secondary fermentation achieves better substrate disruption, which has a beneficial effect on the secondary fermentation.

[0038] From the secondary fermenter NG, the digestate M can be fed directly to the second digestate dryer GT2, or, as shown, to a separation device QP. This separation device QP can be, for example, a mechanical screw press. However, a centrifuge, etc., can also be used. The pressed, relatively solid components (the dry fraction) are fed via the outlet AT for further use or processing. The liquid components are fed into the evaporator vessel V2 of the digestate dryer GT2. The heating cascade functions accordingly as described above using Fig. 2: The third heating device HV3 of the third digestate dryer GT3 is heated by the waste heat of a combined heat and power plant (CHP) with hot water and the steam generated in the respective evaporator boilers V3 to V1 is used to heat the upstream evaporator boilers V2 and V1 via their heating devices HV2, HV1. Fig. 2 known capacitor K is for clarity in Fig. 3 not shown, but can also be part of the device.

[0039] It should be noted that a combination of the secondary fermenter NG and a mechanical separation device QP is not necessarily required, nor is it necessarily required at this point. The mechanical separation device QP or the secondary fermenter NG can also be arranged separately between two evaporators V1 to V3. It is also possible to use either a secondary fermenter NG or a mechanical separation device QP. The use of a secondary fermenter NG and / or a separation device QP in the cascade between the evaporator vessels V3 to V1 can be planned by a specialist depending on the desired degree of drying.

[0040] The separation of solid and liquid components by the mechanical separation device QP downstream of the secondary digester NG has the advantage that the highly volatile ammonia is already expelled from the substrate through evaporation in the digestate dryer V1, thus significantly reducing odor nuisance and ammonia emissions. In addition, the improved substrate digestion results in a higher gas yield; furthermore, the reduction in digestate and substrate volume results in lower construction costs, as the secondary digester NG can be designed smaller.

[0041] In Fig. Figure 4 shows the device according to the invention for drying digestate, wherein two digestate dryers GT1, GT2 are connected in a circuit with the digester F. Such a system is particularly suitable for drying digestate with a high liquid content, such as liquid manure. The digester F is continuously fed with liquid manure via a liquid manure pit VG (not shown in detail). From the digester F, the liquid manure passes through a circular process via the evaporator vessels V1 and V2 and is then returned to the digester F. Due to the continuous feed from the liquid manure pit VG, the dry matter content in the digester F remains constant, for example at 10%. In this system, the evaporator vessels V2, V1 are also heated as described above.

[0042] In one embodiment of the device according to the invention, the fermenter F is heated by a heating device HVF. This is fed with steam from the evaporator vessel V1, for which purpose the heating device HVF of the fermenter F is connected to the steam outlet DA1 of the first digestate dryer GT1. Here, too, it is possible (as shown in Fig. 2), to divert part of the steam via connection 1.1 and feed it to another heating device.

[0043] The above-described systems can be optimally controlled through the interaction of heating temperature and negative pressure. Such a control system is illustrated by the system shown in Fig. 4. The heating device HV2 is supplied with hot water at a flow temperature of 95 °C via the combined heat and power plant (CHP). At a corresponding negative pressure, the digestate then boils at approximately 70 °C. Steam at a temperature of approximately 69 °C is therefore again available for the heating device HV1. This means that the thermally generated energy of the combined heat and power plant (CHP) can be used twice. This is particularly advantageous for the fermentation of liquid manure (liquid manure is referred to as digestate for the purposes of the application), since hardly any heat is generated during its fermentation process. In the evaporator boiler V1, the digestate boils at approximately 60 °C at a corresponding negative pressure. This means that the entire thermal energy generated by the combined heat and power plant (CHP), at a temperature level of just under 60 °C, is still available to heat the fermenter F or, as described, external heating devices.

[0044] The thickened slurry is transported to a digestate storage facility GL via the outlet GLA of the fermenter F.

[0045] Representative of all other described systems is Fig. 4 shows a vacuum pump VP, which sucks the excess condensate through the condensate line KL and feeds it to a condensate treatment system KA. In the embodiment according to Fig. 4 The condensate from the fermenter F is additionally fed to the condensate treatment plant KA via the line LF.

[0046] The components required for the devices, such as valves, pumps, sensors, controls, etc., are not shown in the figures. They are within the expertise of the process engineer, who can design and implement them as needed.

Claims

[1] Device for drying fermentation residues comprising ● at least one digestate dryer (GTn), comprising - an evaporator boiler (Vn) with - - a digestate inflow (ZGn) and a digestate outflow (AGn), - - a steam outlet (DAn), - - a heating device (HVn) this comprehensive - - - at least one double-walled heat exchanger plate (DPn) arranged concentrically around the central axis of the evaporator vessel (Vn), wherein the double-walled heat exchanger plate (DPn) has a steam or hot water supply line (ZHVn) at the upper end and a condensate or hot water discharge line (KAn) at the lower end, and - - - a cleaning device (Bn, BTn) for cleaning the outer sides of the at least one double-walled heat exchanger plate (DPn), and ● a fermenter (F), wherein the digestate inflow (ZG1) of the first digestate dryer (GT1) is connected to an outlet (AF) of the fermenter (F) and the digestate outflow (AGz) of the last digestate dryer (GT1, GTz) is connected to a digestate inflow of the fermenter (F), wherein n is an integer from 1 to the total number z of digestate dryers. [2] Device for drying fermentation residues according to claim 1, wherein the fermenter (F) has a heating device (HVF) which is connected to the steam outlet (DA1) of the first fermentation residue dryer (GT1). [3] Device for drying fermentation residues according to one of the preceding claims, with at least two fermentation residue dryers (GT1, GT2, GTn .. GTz) arranged in series, wherein a secondary fermenter (NG) is arranged at least between the first fermentation residue dryer (GT1) and the second fermentation residue dryer (GT2). [4] Device for drying fermentation residues according to claim 3, wherein a solid / liquid separation device (QP) for the fermentation residues (M) is arranged downstream of the secondary fermenter (NG). [5] Device for drying fermentation residues according to one of the preceding claims, with at least two fermentation residue dryers (GT1, GT2, GTn .. GTz) arranged in series, wherein the fermentation residue dryers (GT1, GT2, GTn .. GTz) are each connected to one another via their fermentation residue inlets (ZG1, ZG2, ZGn .. ZGz) and fermentation residue outlets (AG1, AG2, AGn .. AGz), wherein the heating device (HV1 .. HVz-1) of the respectively upstream fermentation residue dryer (GT1 .. GTz-1) is connected to the steam outlet (DA2 .. DAz) of the downstream fermentation residue dryer (GT2 .. GTz), and at the outlet (KA1 .. KAz-1) of the heating device (HV1 .. HVz-1) a condensate collection tank (KST1 .. KSTz-1) is connected. [6] Device for drying fermentation residues according to claim 5, characterized bythat a cooling device is arranged to cool the condensate. [7] Device for drying fermentation residues according to one of the preceding claims, with at least two fermentation residue dryers (GT1, GT2, GTn .. GTz) arranged in series, wherein at least one solid / liquid separation device (QP) for the fermentation residues (M) is arranged between the fermentation residue inflows (ZG2 .. ZGz) and the fermentation residue outflows (AG1 .. AGz-1).

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