Patent Application
20220411828
This application claims the benefit of foreign priority to European Patent Application No. EP21181020.5, filed Jun. 23, 2021, which is incorporated by reference in its entirety for all purposes.
The conversion (methanation) of carbon dioxide (CO2) and hydrogen (H2) to methane (CH4) can be catalytic, usually nickel-based, or biological, i.e., by means of microorganisms. In both cases, methanation takes place after the following overall reaction:
CO2+4H2→CH4+2H2O (1)
In the case of biological conversion, CO2 and H2 are also used by the microorganisms involved for their anabolism, e.g., for the synthesis of enzymes. However, the conversion of CO2 and H2 in the anabolism is low compared to the conversion to methane. Biological methanation has the advantage that it is less susceptible to catalyst toxins such as hydrogen sulfide (H2S), which can be present in CO2 flows from biogas plants, for example.
The biological methanation of CO2 and H2 is divided into the two concepts ex-situ and in-situ. In the in-situ concept, H2 is fed to a bioreactor in addition to organic substrate. The organic substrate is fermented to CH4 and CO2, among others. This CO2 is in turn converted to CH4 using H2. In the ex-situ concept, only CO2 and H2 is converted in the bioreactor, but no organic substrate. The main advantage of the in-situ concept is that existing bioreactors, which have so far been fed exclusively with organic substrate, can be used, whereas an ex-situ concept requires the cost-intensive construction of new bioreactors.
Using the in-situ concept, CH4 production rates from fed H2 of 0.016 cubic meters of CH4 per cubic meter of bioreactor volume per hour (m3/m3/h) could be achieved under thermophilic temperature control (approx. 55° C.) (EP2771472B1, Example 1, Table 3). However, in the in-situ method with thermophilic temperature control taught in patent EP2771472B1, the feeding of acidic substrate is necessary. This is disadvantageous, since acid substrates are not available everywhere and thus have to be transported to the plant over long distances, entailing high costs. Moreover, the embodiments also suggest that the method can only be used for a low organic loading rate of less than 1.7 kg of organic dry substance (VS) in the form of organic substrate per m3 of bioreactor volume per day. The use of such low organic loading rates is also rendered obvious since it is known that processes with thermophilic temperature control exhibit low process stability.
The problem of low process stability with thermophilic temperature control is addressed in patent EP2586868B1 by the fact that H2 is only used in an existing bioreactor cascade. In this cascade, the first bioreactor is fed with organic substrate, but not with H2. The effluent from the first bioreactor is fed to a second bioreactor that, in turn, is loaded with H2. The second bioreactor is only exposed to a small amount of organic substrate and thus to a low organic loading rate, since a large part of the organic substrate has already been fermented in the first bioreactor. However, the disadvantages are that a bioreactor cascade must be available in the first place, i.e., this method cannot be applied to single-stage plants, and the first bioreactor of such a cascade is not available for H2 conversion.
In addition, a fundamental problem with thermophilic temperature control is the high energy input required to maintain the elevated temperature. Using an in-situ concept for mesophilic temperature control (approx. 37° C.) is therefore preferable.
However, the in-situ concept under mesophilic temperature control has so far only achieved CH4 production rates from CO2 and H2 of about 0.0017 cubic meters of CH4 under standard conditions (273.15 K and 1.01325 bar) per m3 bioreactor volume per hour (Nm3/m3/h) (DE102012112889A1, [0075]). Excess sludge from a sewage treatment plant with an VS content between 2.2 to 2.8% was fed as organic substrate. Despite mesophilic temperature control, an organic loading rate of only 1.25 kg VS/m3/d was achieved.
The invention disclosed in claim 1 is based on the problem that, in the current state of the art in a bioreactor with the in-situ concept under mesophilic temperature control and with high organic loading rate of organic substrate, it was not possible to increase the CH4 production rate sufficiently by feeding H2.
The invention relates to a method for the biological in-situ methanation of CO2 and H2 in a bioreactor. The method comprises feeding an organic substrate to the bioreactor wherein at least part of the organic substrate is converted to biogas by means of microorganisms, wherein the organic substrate includes crude fiber and the organic substrate is fed into the bioreactor at a rate where it includes at least 0.15 kg of crude fiber per m3 bioreactor volume per day, and operating the bioreactor is at 20-45° C. H2 is fed to the bioreactor and at least part of the H2 together with CO2 is converted to methane by means of microorganisms.
Surprisingly, it was found that even bioreactors with high organic loading rate have high potential to convert H2. However, high CH4 production rates from fed H2 are only achievable with a high supply of crude fiber. This is unlike the typical operation of biogas reactors as, normally, crude fibers, which consist mainly of biomass that is difficult to degrade, should be avoided in order to prevent a high and difficult-to-handle viscosity from developing, as well as insufficient utilization of the available bioreactor volume. Surprisingly, however, feeding a high amount of crude fiber has been shown to be helpful with respect to an increased CH4 production rate from fed H2. This is based, without any claim to completeness and with the restriction to a purely theoretical approach, on the increase of the retention time of hydrogen bubbles, as well as on the increase of the culture surface of microorganisms.
In the context of the present invention, the term “in-situ” refers to the biological methanation of fed CO2 and H2 in a bioreactor in which organic substrate is also converted to biogas.
In the context of the present invention, the dry substance (TS) refers to the solid residue obtained after removing the solvent (e.g., water) from a suspension (e.g., from a stillage) or from a solution. That is, the solid residue refers to the totality of all previously dissolved or suspended solids (e.g., crude fibers and salts).
In the context of the present invention, organic dry substance (VS) refers to the part of the TS that does not remain as ash when the TS is incinerated.
In the context of the present invention, the organic loading rate of a bioreactor is defined as the average mass flow of VS fed to the bioreactor within one day, based on the bioreactor volume. This results in the unit kg VS/m3/d.
In the context of the present invention, the term “organic substrate” refers to liquid or solid organic material flows containing VS which are fed to the bioreactor. This includes, but is not limited to, by-products of industries processing biomass, such as stillage from a bioethanol plant or potato pulp from starch production, as well as residues from agriculture and forestry, such as straw, oat husks, rice hulls or sawdust. The term “organic substrate” does not encompass the gases CO2 and H2.
In the context of the present invention, the term “microorganisms” refers to an undefined mixed culture of bacteria and/or archaea that may be found in typical biogas plants. “Microorganisms” does not imply a pure culture of a particular species.
In the context of the present invention, the measurand of the steam-volatile ammonium nitrogen (NH4—N) is used to assess the ammonium content. To determine this, ammonia is expelled from the sample with the aid of steam and collected in a boric acid solution, followed by titration using hydrochloric acid. The concentration of ammonium in the sample can be inferred from the consumption of hydrochloric acid.
In the context of the present invention, the term bioreactor volume refers to the volume of liquid located in the bioreactor. This also refers to all standardized quantities based on the bioreactor volume, such as CH4 production rate, H2 feed rate and CO2 feed rate. This bioreactor volume represents the place where biochemical reactions occur.
In the context of the present invention, hydrogen (H2) refers to hydrogen in its molecular form.
In the context of the present invention, the H2 feed rate and the CO2 feed rate refer to the volume of H2 or CO2 under standard conditions (N for short, 273.15 K and 1.01325 bar) in m3 per m3 of bioreactor volume, which is fed to the bioreactor on average every hour. This results in the unit Nm3/m3/h.
In the context of the present invention, the CH4 production rate refers to the volume of CH4 under standard conditions (N for short, 273.15 K and 1.01325 bar) in m3 per m3 bioreactor volume, which is formed in the bioreactor on average every hour. This results in the unit Nm3/m3/h. In the present invention, a distinction is made between CH4 production rate from VS and CH4 production rate from H2 fed to the bioreactor. The CH4 production rate from VS corresponds to the CH4 production rate produced in the bioreactor without feeding H2. The CH4 production rate from fed H2 is defined in equation 2
The person skilled in the art is aware that H2 can be formed in the intermediate steps of the fermentation of VS, acidogenesis and acetogenesis; the H2 together with CO2 can be converted to CH4. This H2 is not part of the H2 fed into the bioreactor (H2 feed rate). Equation (2) neglects the part of hydrogen that is not converted to CH4 in catabolism by the microorganisms involved in the H2 conversion but is used in anabolism to synthesize molecules such as enzymes.
In the context of the present invention, the term “crude fiber content” refers to the fraction of an organic substrate referred to as “crude fiber” in food and feed analysis according to Weender. The measuring method for determining the crude fiber content is described in the following source, which is incorporated by reference:
Association of German Agricultural Testing and Research Institutes (VDLUFA [Verband Deutscher Landwirtschaftlicher Untersuchungs—und Forschungsanstalten]) (publisher), 1993: Methode 6.1.1. Bestimmung der Rohfaser (WEENDER-Verfahren) [Method 6.1.1 Determination of crude fiber (WEENDER method)]. Handbuch der Landwirtschaftlichen Versuchs—und Untersuchungsmethodik [Handbook of Agricultural Experimental and Investigative Methodology] (VDLUFA-Methodenbuch [VDLUFA method booklet]), Volume III, Die Untersuchung von Futtermitteln [The examination of food and feed], 3rd Edition, VDLUFA Verlag [Publisher], Darmstadt.
The feeding of crude fiber to the bioreactor is expressed as kg crude fiber/m3/d and represents the mass of crude fiber in kg per m3 bioreactor volume supplied to the bioreactor on average in one day.
In the context of the present invention, the term “product gas” refers to the gas leaving the bioreactor in gaseous form. Product gas may comprise a mixture of gaseous substances, in particular CH4, CO2, H2, H2S and H2O vapor. In the context of the present invention, biogas refers to a mixture of gases produced during the anaerobic decomposition of organic substrate and consisting mainly of CH4 and CO2.
In the context of the present invention, the term “product gas treatment” refers to the partial separation of gas components from the product gas.
In the present invention, organic substrate is mainly converted to CH4 and CO2 in a bioreactor at temperatures between 20-45° C. At the same time, H2 is fed into the bioreactor with the result that a biological conversion of CO2 and H2 to CH4 occurs.
Surprisingly, it was found that when using the mesophilic temperature range (20-45° C.) instead of the thermophilic temperature range as disclosed in the prior art (EP2771472B1, EP2586868B1), the advantages outweigh the disadvantages and even higher CH4 production rates can be achieved from fed H2. So far, the prior art has suggested that thermophilic temperatures are preferable because it is known that microbial activity is higher at thermophilic temperatures than at mesophilic temperatures. Mesophilic temperatures have the advantage of higher process stability and energy efficiency as less thermal energy needs to be provided.
Moreover, in the present invention, a high amount of crude fiber is fed into the bioreactor. In the prior art, substrates low in crude fiber such as whey (EP2771472B1) or sewage sludge (EP2586868B1) have been preferred. According to Weender's food and feed analysis, crude fiber content is the part of an organic substrate that remains after treatment with dilute acids and alkalis, i.e., it is not easily “digestible.” Feeding substrate with a high crude fiber content into a bioreactor is, therefore, generally regarded as a disadvantage since the poorly digestible components it contains can accumulate in the bioreactor, resulting in a high viscosity that is difficult to handle, thus reducing the usable bioreactor volume. The consequence can be problems with floating layers, stirring and pumping issues.
Surprisingly, with respect to feeding H2 into a bioreactor, it was found that the advantages of feeding one or more organic substrates with high crude fiber content outweigh the disadvantages described above. While not claiming to be exhaustive and limited to a purely theoretical approach, this is based on increasing the retention time of hydrogen bubbles, which improves their transfer to the reactor liquid and their microbial conversion, as well as on increasing the culture surfaces for microorganisms. Based on experimental results, it was determined that at least 0.15 kg crude fiber/m3/d should be fed to achieve high CH4 production rates as a result of feeding H2. For comparison, the prior art in EP2771472B1 recommends the feeding of whey, which contains virtually no crude fiber (source: https://www.ingredients101.com/whey.htm, retrieved Apr. 5, 2021). In a preferred embodiment, the method is designed such that 0.15 kg crude fiber/m3/d, preferably 0.2 kg crude fiber/m3/d, particularly preferably 1.0 kg crude fiber/m3/d is fed into the bioreactor.
As a matter of preference, the method is designed in such a way that a majority of the particles of the organic substrate have a size of 10 mm or less.
Mesophilic microorganisms thrive in a temperature range between 20-45° C. (Schiraldi and De Rosa, 2014). It was found, however, that the optimum process control is within a narrower temperature range. In a preferred embodiment, the method is designed such that the bioreactor is operated at a temperature of 35-45° C., preferably at 36-41° C., particularly preferably at 37-39° C.
The microorganisms in the bioreactor are a mixed culture of bacteria and archaea. Such mixed cultures are known to the person skilled in the art in the context of biogas production. In the process steps hydrolysis and acidogenesis, bacteria convert organic substrate to acetic, propionic and butyric acid, H2 and CO2, among others. In acetogenesis, bacteria convert propionic and butyric acid to acetic acid, H2 and CO2. In methanogenesis, archaea convert H2 and CO2, acetic acid and other methyl components to methane. Depending on the substrate, CO2 is also formed in the methanogenesis step. Compared to pure cultures, as they are mainly used in ex-situ concepts and where, for instance, only one specific methanogenic species is cultivated in a bioreactor, mixed cultures have the advantage that the bioreactor does not have to be operated sterilely to avoid contamination with other microorganisms.
The person skilled in the art is familiar with how to get a biogas process started. For example, an effluent from an existing biogas plant or animal excrement that already contains a mixed culture of bacteria and archaea is used for inoculation.
In a preferred embodiment, the method is such that the hydraulic retention time (HRT) is at least 18 days, preferably at least 25 days, and particularly preferably at least 35 days.
In a preferred embodiment, the method is designed in such a way that the H2 feed is at least 0.017 Nm3/m3/h, preferably 0.125 Nm3/m3/h, particularly preferably 0.167 Nm3/m3/h. In alternative methods in the prior art, only 0.015 Nm3/m3/h (DE102012112889, [0075]) has been achieved under mesophilic temperature control and 0.118 Nm3/m3/h under thermophilic temperature control (EP2771472B1, [0102], calculated from 1.7 NI/d and 0.6 L bioreactor volume). Thus, with the help of the described invention, there is a possibility to increase the H2 supply compared to the current state of the art, thus increasing CH4 production rates and thereby improving the productivity of bioreactors.
In a preferred embodiment, the method is designed such that the crude fibers are supplied in the form of organic substrate, particularly preferably in the form of organic substrate of plant origin. This enables the use of low-cost organic substrates, e.g., residual materials such as straw.
In a preferred embodiment, the method is designed in such a way that the organic substrate is fed into the bioreactor with an organic loading rate of more than 2.5 kg VS/m3/d, particularly preferably more than 3.0 kg VS/m3/d, despite the feeding of H2. In the current prior art, H2 has been fed into a bioreactor only at low organic loading rates, e.g., 2.1 kg VS/m3/d in patent EP2586868A2 or 1.7 kg VS/m3/d in patent EP2771472B1. For patent EP2586868A2, the calculation was based on the stated maximum CH4 production rate from VS (EP2586868A2, [0034]) and a usual gas yield for sewage sludge of 300 NI/kg VS. The calculation of the organic loading rate from patent EP2771472B1 is based on the specified retention time, reactor size and the specified material data of the organic substrate (EP2771472B1, [0101] and [0102]).
In a preferred embodiment, the method is designed such that the H2 fed to the bioreactor originates from the electrolysis of water. In a particularly preferred embodiment, the method is designed such that the electricity for the electrolysis was generated from renewable sources. This has the advantage that the CH4 produced in the bioreactor from the electrolysis H2 is a sustainable, electricity-based fuel obtained independently of oil and natural gas.
If the fed H2 is not completely converted in the bioreactor, it leaves the bioreactor in the product gas. The method can be designed in such a way that part of the product gas is fed to the bioreactor. In a preferred embodiment, the method is designed such that the gas mixture exiting the bioreactor, namely the product gas, is fed to a product gas treatment in which at least a portion of the H2 present in the product gas is separated and returned to the bioreactor. As a result, the CH4 production rate can be increased, and the H2 content in the treated product gas can be reduced. In a particularly preferred embodiment, the method is designed such that the separation of the H2 takes place in a membrane system. In a particularly preferred embodiment, the method is designed such that the separated and recycled H2-containing gas flow contains a maximum of 90% (v/v) H2. Limiting the H2 content is advantageous because it allows the use of simple treatment plants, e.g., single-stage membrane plants, and thus saves energy compared to the separation of almost pure H2.
In a preferred embodiment, the method is designed such that CO2 is fed to the bioreactor. In a preferred embodiment, this regulates the pH to below 8.2, significantly contributing to improving process stability. In a preferred embodiment, the method is such that the CO2 supplied originates from a bioethanol plant. This is advantageous because otherwise the CH4 production rate from fed H2 would be limited beyond the amount of CO2 produced from the organic substrate in the bioreactor. The use of CO2 from a bioethanol plant is particularly advantageous because this CO2 was originally captured from the atmosphere during plant growth with the help of sunlight as renewable energy and is produced during fermentation in concentrated form as a gas with over 90% (v/v) CO2 content. To extract the CO2 by technical means from the atmosphere or combustion gases instead would cause high energy and operating costs because the CO2 is present in much lower concentrations, e.g. 0.04% in the atmosphere. In a preferred embodiment, the method is designed such that the product gas is fed to a product gas treatment in which at least a portion of the CO2 present in the product gas is separated and returned to the bioreactor.
In a preferred embodiment, the method is designed such that the concentration of CO2 in the product gas does not fall below a value of 5% (v/v), preferably 10% (v/v), particularly preferably 40% (v/v), based on the product gas without the water vapor. In the state of the art, the recommendation is to steer away from such high CO2 levels, since the goal is usually to reduce the CO2 content in the product gas as much as possible, preferably to less than 5% (v/v), in order to be able to feed the product gas directly into existing natural gas networks (e.g., EP2771472B1 [0067], EP2586868A2 [0013] and DE102012112889A1 [0041]). Surprisingly, it was found that high CO2 concentrations can improve the overall effectiveness of the process. Although the cost of gas purification is increased by the higher CO2 concentrations in the product gas, the advantage of higher bioreactor productivity outweighs this. This enables significantly higher process stability to be achieved in the bioreactor, in particular low pH fluctuations and low concentrations of organic acids. As a result, the bioreactor's organic loading rate capacity is high, which leads to an increase in the CH4 production rate and an optimal utilization of the available fermentation volume. In a preferred embodiment, the method is designed such that the concentration of CO2 in the product gas is regulated by the amount of CO2 supplied to the bioreactor. In a further embodiment, the method is designed such that the regulation of the concentration of CO2 in the departing product gas is regulated by the amount of H2 fed to the bioreactor.
In a preferred embodiment, the method is designed in such a way that at least 0.3 mol CO2, particularly preferably at least 0.5 mol CO2, is supplied to the bioreactor per mol fed H2. This has the advantage that the CO2 content in the product gas is adjusted to a high concentration. The state of the art teaches the opposite, namely no CO2 at all or no stoichiometric quantity (feeding 0.25 mol CO2 per mol H2) to achieve a product gas with the lowest possible CO2 content. However, the disadvantage of this is process instability.
In a preferred embodiment, the method is designed such that the CO2 fed to the bioreactor has a purity of at least 90%, particularly preferably 95%. This minimizes the required amount of gas supplied and simplifies the control of the pH value in the bioreactor.
In a preferred embodiment, the method is designed such that the gases supplied to the bioreactor are fed near the bottom of the bioreactor. The advantage of this is the fact that the gas has a longer retention time in the bioreactor volume. In another embodiment, the bioreactors are equipped with stirrers, and gases are fed near the stirrers. The advantage of this is that the contact between gas and liquid is increased.
In a preferred embodiment, the method is designed such that the bioreactor is the first bioreactor in a cascade of bioreactors. This embodiment is particularly advantageous compared to the prior art EP2586868B1, in which the first bioreactor of a bioreactor cascade is not suitable for feeding H2 due to low process stability. Surprisingly, the invention revealed that high feeding of crude fiber, feeding of low-ammonium liquid, and feeding of CO2 can address these process stability issues, making the bioreactor volume of the first bioreactor available for the conversion of H2.
In a preferred embodiment, the method is designed to lower the NH4—N concentration in the bioreactor. In a preferred embodiment, the method is designed such that the NH4—N content in the bioreactor is adjusted to a maximum of 6000 mg/kg. This reduction of the NH4—N content is advantageous, as it also reduces the concentration of ammonia (NH3) in the bioreactor, which is particularly harmful for process stability. NH3 is in acid-base equilibrium with ammonium (NH4+) and the proportion of NH3 increases in proportion to an increasing pH. If the CO2 feed to the bioreactor is insufficient, the pH in the bioreactor may increase due to the conversion of CO2 to CH4, and thus shift the NH4+/NH3 equilibrium towards the NH3 with negative consequences. In a further embodiment, the method is designed such that the adjustment of the NH4—N content is adjusted by selecting the organic substrate fed to the bioreactor. For example, a substrate poor in nitrogen can be selected to be fed to the bioreactor in addition to an organic substrate rich in nitrogen.
In a preferred embodiment, the method is designed such that the NH4—N content is lowered by the feeding of a low-ammonium liquid, for example, by feeding fresh water. This would result in costly consumption of fresh water. Therefore, in a preferred embodiment, the method is designed such that the low-ammonium liquid is provided by treating the effluent of the bioreactor, in particular by ammonium stripping. Thus, a comparatively low investment and operating cost for effluent treatment avoids the significantly higher costs associated with a fresh water supply.
In a preferred embodiment, the method is designed such that the bioreactor is a stirring tank reactor. In a particularly preferred embodiment, the method is designed such that the bioreactor is operated continuously or semi-continuously. In a preferred embodiment, the method is designed such that the bioreactor does not include any inorganic fillers or growth media.
In a preferred embodiment, there is only a low overpressure of less than 150 mbar in the gas space of the bioreactor. This ensures a cost-saving design, maintenance and a high level of occupational safety. At the same time, the effort required to generate pressure to feed the reactant into the bioreactor and to depressurize it to remove the product gas is minimized.
The invention is explained in more detail below using four embodiments and associated drawings.
In this and the following examples, the volume flow rates of H2, CO2 and H2S leaving the bioreactor are referred to as H2, CO2, and H2S output rates, respectively, and include the volume of the respective gas under standard conditions (N for short, 273.15 K and 1.01325 bar) in m3 per m3 bioreactor volume discharged from the bioreactor per hour on average. This results in the unit Nm3/m3/h. The term “stillage” refers to the residue from the distillation of a grain mash containing ethanol. The term “whole stillage” is used as a synonym of stillage. In the context of the present invention, “wet cake” refers to the solid phase separated from the stillage by solid-liquid separation.
A daily amount of 11,207 kg of whole stillage from a bioethanol plant is fed as organic substrate to a bioreactor with 1,000 m3 bioreactor volume. The TS content of the whole stillage corresponds to 0.19 kg TS per kg of whole stillage. The VS content of the whole stillage corresponds to 0.91 kg VS per kg TS of whole stillage. This results in a daily feed of whole stillage of 1,937 kg VS or, based on the bioreactor volume of the bioreactor, an organic loading rate of 1.94 kg VS/m3/d.
Based on Weender's food and feed analysis, the crude fiber content of the whole stillage corresponds to 0.039 kg crude fiber per kg TS. Calculated on the basis of the TS content of the whole stillage and the mass flow of whole stillage fed, the result is a very low supply of crude fiber via the whole stillage of only 0.08 kg crude fiber/m3/d.
Surprisingly, it was found that a significantly higher crude fiber content favors the CH4 production rate from fed H2, so 4,065 kg/d of wet cake is fed to the bioreactor in addition to the whole stillage. Wet cake contains a high TS content of approx. 0.33 kg TS per kg of wet cake, and an VS content of 0.96 kg VS per kg TS and is typically sold as feed. With a high crude fiber content of 0.14 kg crude fiber per kg TS, wet cake is not a typical organic substrate for biogas production. However, in this embodiment example, it proves to be very useful to significantly increase the crude fiber supply by 0.19 kg crude fiber/m3/d.
Moreover, 6000 kg/d of purified effluent from step 3, 15.8 Nm3 CO2/h from a bioethanol plant and 25 Nm3 H2/h from an electrolysis plant are fed to the bioreactor.
Organic substrate is converted in the bioreactor to the gases CH4, CO2 and H2S, among others. The H2 supplied from the electrolysis is converted to approx. 60%. The remaining H2 leaves the bioreactor with the other gases via the product gas (see Table 2).
The supply of CO2 from the bioethanol plant results in the CO2 content in the product gas being approx. 45% (v/v) based on product gas without the water content. Without the CO2 supply from the bioethanol plant, the CO2 content in the product gas would only be approx. 37% (v/v) based on product gas without the water content, which would lead to an unwanted, higher pH value. The supply of CO2 from the bioethanol plant thus results in a more stable reaction in the bioreactor.
The temperature in the bioreactor is controlled at 37° C.
In the embodiment example described here, a CH4 production rate of 0.053 Nm3/m3/h would be achieved if no H2 and CO2 were fed. Using the invention described here, the CH4 production rate increases to 0.056 Nm3/m3/d by feeding and converting H2 and CO2. The CH4 production rate from fed H2 corresponds to 0.0038 Nm3/m3/h. This increases the utilization of the available bioreactor volume.
The effluent of the bioreactor is fed into an effluent treatment using ammonium stripping, and ammonium is largely removed. The resulting process liquid, which is low in ammonium, has an NH4—N content of 500 mg/kg. A part of this low-ammonium process liquid is fed to the bioreactor to adjust the NH4—N content, thereby adjusting the NH4—N content to below 6000 mg/kg.
Another option for process control consists of utilizing the CO2 produced from organic substrate during biogas production instead of CO2 from a bioethanol plant. For this purpose, a product gas treatment is added to the method described in embodiment 1, in which CO2 is separated from the product gas at an hourly circulation rate of 0.016 Nm3 CO2 per m3 bioreactor volume and fed back into the bioreactor. This CO2 replaces the CO2 flow from the bioethanol plant described in embodiment 1.
In this context and as used hereinafter, circulation rate refers to the volume of the respective gas under standard conditions (N for short, 273.15 K and 1.01325 bar) in m3 per m3 bioreactor volume, which is partially separated from the product gas in the product gas treatment and fed back into the bioreactor on average per hour. This results in the unit Nm3/m3/h.
As in embodiment 1, the CO2 content in the product gas is 45% (v/v) based on product gas without the water component and thus advantageously higher than it would be at 37% (v/v) based on product gas without the water component and if CO2 had not been fed.
Since the changed CO2 source has no effect on the CH4 production rate with otherwise constant operating conditions as in embodiment 1, the CH4 production rate based on the supplied H2 is also 0.0038 Nm3/m3/h.
If feeding product gas into the natural gas network is planned, certain rules stipulated by the network operator regarding the composition of the gas must be observed. As a rule, for example, the CH4 content must be greater than 95% (v/v), the H2 content must be less than 2% (v/v) and the H2S content must be approx. 0% (v/v) based on gas without the water content.
Table 4 shows the gas flows in this embodiment. The liquid flows in this example are the same as in embodiment 1 and shown in Table 1. The temperature in the bioreactor is still controlled at 37° C. and the effluent treatment from step 3 in embodiment 1 is maintained.
Recirculation of H2 increases the retention time of H2 in the bioreactor. This allows the conversion of the H2 coming from the electrolyzer to be increased from 60% to 95% (v/v). The CH4 production rate from supplied H2 thus corresponds to 0.00594 Nm3/m3/h. Due to the increased amounts of recycled gas, the increase in recycled CO2 is also necessary to keep the CO2 content in the product gas high. Thus, the advantages of a stable CO2 concentration in the product gas described in embodiment 1 can be maintained.
Embodiment 4 includes experiments on a pilot-plant scale.
Two bioreactors “A” and “B” of the stirring-tank-reactor type were fed with different organic substrates as well as H2 and CO2.
Bioreactor A was fed with whole stillage and low-ammonium process liquid from a bioethanol plant. Feeding was carried out semi-continuously, distributed in five equal intervals per day.
Bioreactor B was fed with cereal straw and low-ammonium process liquid. Feeding was done once a day.
The bioreactor volume corresponded to 75 l (bioreactor A) and 55 l (bioreactor B). Mixing was performed with central stirrers at a rotational speed of 370 rpm for both bioreactors. The temperature of the bioreactor volume was controlled using water as a heating medium at about 39° C., which flowed around the bioreactors via a double casing. The supplied quantities of H2 and CO2 were controlled separately for each gas via thermal mass flow controllers (EL-Flow Select, Bronkhorst Deutschland Nord GmbH). Product gas volume flows were determined in both bioreactors using drum gas meters (TG 0.5, Dr.-Ing. RITTER Apparatebau GmbH & Co. KG). The gas composition of the product gas was determined once a week using gas chromatography (MobilGC, ECH Elektrochemie Halle GmbH; configuration: Hayesep QS column 45° C., molecular sieve column 55° C., thermal conductivity detector, carrier gas argon). The TS content of the substrates was determined gravimetrically after drying at 105° C. until mass constancy was achieved. The ash content was determined gravimetrically after annealing at 650° C. for at least two hours. The VS content was calculated based on the difference between the dry substance content and the ash content. The crude fiber content was determined according to the VDLUFA method book (see section Definitions). The low-ammonium process liquid was neglected in the determination of the organic loading rate and the fed crude fiber.
In both bioreactors, organic substrate is converted to biogas by means of microorganisms, as indicated by a lower amount of VS in the effluent compared to the reactor feed, as well as by means of the measured CH4 production rates, which are higher than the calculated CH4 production rates from fed H2 and therefore must originate from organic substrate decomposition (data not shown).
The organic loading rate is comparable for both reactors at approx. 3 kg VS/m3/d. However, the feeding of crude fiber differs significantly. While in bioreactor A only about 0.14 kg crude fiber/m3/d was fed via the whole stillage, the amount in bioreactor B via the straw was significantly higher at about 1.2 kg crude fiber/m3/d.
Bioreactor B exhibited significantly higher performance for in-situ methanation of H2 and CO2. Even at high H2 feed rates of 0.197 Nm3/m3/h, low H2 output rates of about 0.014 Nm3/m3/h were measured. Thus, according to equation (2), the CH4 production rate from fed H2 is about 0.046 Nm3/m3/h. In bioreactor A, on the other hand, higher H2 output rates were measured than in bioreactor B of 0.048 Nm3/m3/h, despite significantly lower H2 feed rates of only 0.081 Nm3/m3/h. This corresponds to a CH4 production rate from fed H2 in bioreactor A of 0.008 Nm3/m3/h.
Schiraldi C., De Rosa M. (2014) Mesophilic Organisms. In: Drioli E., Giorno L. (eds) Encyclopedia of Membranes. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-642-40872-4_1610-2
| Number | Date | Country | Kind |
|---|---|---|---|
| 21181020.5 | Jun 2021 | EP | regional |
