Landesanstalt für Agrartechnik und Bioenergie

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Entwicklung und Erprobung eines Online-Messsystems für Biogasanlagen auf Basis der Nah-Infrarot-Reflexionsspektroskopie (NIRS)
(2013) Stockl, Andrea; Jungbluth, Thomas
Due to the EU?s and Germany?s political goals of expanding the use of renewable energy sources, the utilization of biomass for energy supply is expected to continue growing in the coming years. Consequently, the efficiency of biogas plants will have to be improved further. This applies both to raising the energy yield from the input materials and exploiting the full potential of the technical installations. During the four phases of the anaerobic digestion (AD) process, volatile fatty acids such as acetic and propionic acids are produced as intermediates. These compounds can be used as indicators of the function and stability of the digestion process. So far, volatile fatty acids have to be determined by sampling the digester content and analyzing the sample in the laboratory (e.g., by gas chromatography). It is thought that by using near-infrared-reflection spectroscopy (NIRS) for online measuring, the management and control of biogas plants could be facilitated, considerably. This was to be investigated in a project funded by the Baden-Württemberg Ministry of Rural Areas and Consumer Protection within the ?research platform on bioenergy? Baden-Württemberg?. In this study, a NIR-measurement system was calibrated for determining the concentration of volatile fatty acids in two semi-continuously operated, bench-scale digesters at mesophilic and thermophilic temperature level. For each of the two digesters, one NIR-sensor was calibrated for acetic and propionic acid, and a second one for total acid equivalents. The experimental studies were divided into three stages. The chapters of this research work consist of three peer-reviewed papers that describe these experiments.
Untersuchungen zur zweiphasigen Vergärung von Grassilage
(2013) Zielonka, Simon
The anaerobic digestion of grass silage as a single substrate, which is a problematic substrate in CSTR digesters, was researched at the State Institute of Agricultural Engineering and Bioenergy at the University of Hohenheim. Lab research was conducted at a two-phase biogas plant consisting of a leach bed reactor and an anaerobic filter. The research goal was to identify and to optimize relevant process parameters, to be able to judge if this biogas process is a possible alternative to biogas processes that are in use in praxis. At the test biogas plant, which consists of five pairs of reactors, we observed the effects of the varied parameters on the methane yield and its distribution to the two phases, the degree of degradation and the amount of leached COD from the leach bed reactor. The results show that the separation of the phases could be improved by frequent exchange of the leachate. The research into the leach bed reactor temperature shows that the fastest and highest methane production of the two-phase biogas plant occurs at a temperature of 55°C. The use of different substrates result in individual digestion characteristics that majorly affect the quality of the phase separation. In total, a stable digestion process was observed in all experiments, and at optimal parameters of the test plant, methane yields comparable to one phase reference systems were reached. However, the retention time of one kilogram of organic dry matter was only 25 days. The experiments showed the efficiency of the discontinuous two-phase anaerobic digestion process with a leach bed reactor and an anaerobic filter. Relevant aspects for the operation and the optimisation of the process were discovered.
A full-scale study on efficiency and emissions of an agricultural biogas plant
(2013) Nägele, Hans-Joachim; Jungbluth, Thomas
In this study we focused on process engineering for the conversion of biomass, and utilization of the gas obtained by fermentation. Several topics regarding efficiency and emissions have been addressed by conducting intensive and long-term measurements. In detail, our objectives were (1) to conduct long-term measurements of the electric energy consumption of the biogas plant and its individual components and examination of energy-saving potentials; (2) to develop a method to measure mixing quality in the digester and to examine the mixing quality by measuring nutrient distribution in the digester with different agitator setups; (3) measure the influence of maintenance strategies on efficiency and emissions at long-term operation in practical application; (4) examine the efficiency of an external biological desulfurization plant under practical conditions to enhance biogas fuel quality. The results of electric energy measurement over a period of two years showed that a percentage of 8.5% (in 2010) and 8.7% (in 2011) of the produced electric energy was required to operate the biogas plant. The consumer unit agitators with 4.3% (in 2010) and 4.0% (in 2011) and the CHP unit with 2.5% (in 2010 and 2011) accounted for the highest electrical power demand, in relation to the electric energy produced by the CHP unit. Calculations show that the agitators consumed 51% (in 2010) and 46% (in 2011) of the total electric energy demand. The results stress the need for further research in the fields of substrate homogenization in biogas plants in order to reduce the demand for electric energy. Based on the results of electric energy consumption, follow-up studies have been conducted on nutrient distribution, which depends on agitator type and agitator regime. The investigation showed that significant differences in local concentrations of organic acids, which are not correlated to DM content, are found in dependence on agitator type and agitation regime. Measurements on electric energy consumption of the different agitator types verified that, depending on the agitator type, the saving potential rises up to 70%. The results for emissions and efficiency of the CHP unit confirm the fact that after readjustment of the air-fuel ratio (Lambda value), the emission values for NOx decline while CO increases. However, the emission-optimized operation mode leads to lower engine efficiency. The permanent measurements proved their legitimacy showing various emission deviations from the limiting values prior and post maintenance. In addition, the results show that by monitoring the lubricating oil quality, the oil change intervals can be maximized, while ensuring that engine performance is not endangered. This allows the operator to reduce maintenance expenditures while minimizing wear. To increase engine efficiency, the reduction of the lambda value combined with exhaust gas scrubbing and exhaust gas power generation is a promising approach. However, that would presuppose a permanent and almost total removal of H2S from the biogas. The fourth part of the study examined the technical and economic feasibility of a Fixed Bed Trickling Bioreactor (FBTB) for external biological desulfurization of biogas. In contrast to well-established biological methods to oxidize H2S, the FBTB allows removal of these from the biogas process, thus ensuring a constant low H2S concentration in the biogas. The FBTB showed H2S removal efficiencies (RE) of 98% at temperatures between 30-40°C. A major decline in RE in a range of 21-45% was observed when temperature in the FBTB dropped to a range of 5-25°C. The results revealed that different pH values of the percolation fluid and air ratios have little effect on RE. The practical use of the investigated FBTB system is an interesting technological alternative as disadvantages of internal biological desulfurization methods are being avoided. Due to high expenditures for operation resources and maintenance for FBTB operation during the research, a technical optimization is necessary to ensure economical operation. The results presented in this thesis show that the scientific instrument ?research biogas plant? is the ideal supplement to methods such as laboratory scale research and measuring programs. Research at full scale offers an entirely new opportunity to determine the interaction of process technique and process biology and to conduct long-term studies of gas utilization. Compared to measuring programs at commercial biogas plants, the research biogas plant has the advantage of being significantly better equipped with measurement technologies and that economic success is not the overall goal.
Einsatz von Spurenelementen bei der Vergärung von nachwachsenden Rohstoffen in Biogasanlagen
(2014) Vintiloiu, Anca; Jungbluth, Thomas
The operational agricultural biogas plants in Germany are fed mainly with renewable raw materials. During substrate addition, several micro and macro elements enter the digester. These elements are essential nutrients for the methanogens. If their concentration is too low, the production of biogas can be disrupted. A large number of agricultural biogas plants use therefore commercially available trace element solutions to optimize the process and to achieve higher methane yields. When the fermentation is complete, the digestate containing these trace elements (mostly heavy metals) is spread on fields as fertilizer. The amounts added to the biogas process should be kept as low as possible in order to minimize the environmental damage. The purpose of this study was to investigate the cause of trace elements deficiency in renewable raw materials fed biogas plants. It was also tested whether the chelation of the nutrients could increase their bioavailability for microorganisms and thus lead to a reduction of the amounts needed for the stabilization of the fermentation process. The effect of the complexing agent ethylenediaminetetraacetic acid (EDTA) on the bioavailability of metal ions was tested. The sole addition of EDTA to an undersupplied substrate increased the methane yield by up to 32 %. When trace elements were also added, their amounts could be reduced by up to 75 % with no negative consequences for the fermentation process. EDTA is a persistent chelating agent and so it was further tested, whether readily biodegradable chelating agents (ethylenediaminedisuccinic acid (EDDS) and iminodisuccinic acid (IDS)) could have the same effect. During the investigation, IDS had a high statistically significant positive effect on the bioavailability of the metal ions, which exceeded the effect of EDTA. IDS represents therefore a good alternative to EDTA. The bioavailability of the metal ions in the digester was increased by the use of complexing agents, which made the reduction of the trace elements amounts needed to compensate for substrate-related deficiency symptoms possible. This reduces the pollution on the agricultural land on which the digestate is used as fertilizer.
Optimierung der Konservierung und der anaeroben Konversion von Zuckerrüben zur Nutzung in flexiblen Biogassystemen
(2019) Kumanowska, Elzbieta Joanna; Jungbluth, Thomas
Biogas production is well suited to balance the fluctuating electricity production from the renewable energy sources sun and wind. Due to the currently unfavorable conditions in the renewable energy supply policy in Germany, time is spent looking for alternatives for electricity production from biogas. The preparation for natural gas quality for fuel production or for natural gas grid injection would be such an alternative but requires process improvements to reduce costs. One approach would be to use two-stage biogas production, as there is a high methane content in the produced biogas, thus reducing the cost of processing to natural gas quality. A suitable substrate for both applications would be sugar beet, due to its fast biodegradability and good methane yields. The preservation of sugar beets for year-round provision has so far been problematic because it can cause high losses. In addition, it can cause process biological problems, if it is used in high proportions. In the context of this work, the use of sugar beets for biogas production was tested using these promising methods. For this purpose, storage experiments were carried out and new storage methods for the practice were developed and tested, all of which are primarily aimed at the use of sugar beet silage effluent. Practice-based point-feeding experiments were used to test its suitability for demand-oriented biogas production. Furthermore, the optimization of the two-stage biogas production from sugar beet was carried out. For this purpose, an experiment was conducted in the biogas laboratory to determine the optimum hydrolysis pH during the fermentation of sugar beet silage. In order to develop a new, optimal method for the storage of sugar beets, further knowledge regarding the process of ensiling sugar beets, the silage effluent formation and the influencing parameters was required. Therefore, mass balances were carried out in the column experiments in the laboratory of the State Institute of Agricultural Engineering and Bioenergy to determine the influence of the parameters stack height and sugar beet chips size on the silage effluent formation during the ensiling process of the chopped sugar beets. Silage effluent was produced in amount about 50% of the stored mass. About half of the silage effluent production took place during the first three weeks of storage. The produced silage effluent was characterized over the entire storage time by extremely high COD-values of 250 g l-1. The parameters stack height and particle size had no significant influence on the mass balance. On the basis of the results of the column experiments, a mobile and a stationary method on a technical scale for the storage of sugar beets were investigated. In the mobile variant, the flexible tanks, washed, chopped sugar beet was ensiled. Considering the goal to maximize silage effluent yield, the ensiling of chopped sugar beet was superior to the ensiling of whole beet. Also, soil removal is advantageous for silage effluent production as well as for silage quality. Storage in the stationary pit silos proved to be technically advantageous, and it promises to be well suited for the intended applications when in combination with washed and chopped beets. The application of produced silage effluent for demand-oriented biogas production was carried out at the research biogas plant of the University of Hohenheim. The system’s response observed as an increase in biogas production took place a few minutes after the point feeding with sugar beet silage effluent. As a result of the point feeding, the produced volumetric biogas flow rate was doubled without endangering the stable biogas plant operation. The maximum gas production was reached after about 1:45 h. In this work, a concept for the use of sugar beet for the production of high calorific biogas was tested, based on the two-stage anaerobic digestion. The experimental plants consisted of a horizontal stirred tank reactor for hydrolysis and two combined fixed bed reactors used as a methane reactor. The influence of the pH value in the hydrolysis stage on the anaerobic digestion of sugar beet silage was tested. High degradation rates and methane yields demonstrated the overall suitability of this system for sugar beet silage digestion. The best compromise of the process parameters degradation rate in complete system and methane yield was achieved at a pH value of 6. The investigation carried out for this work shows, that the concept of a new sugar beet storage method, with a focus on sugar beet silage effluent production, is well suited for demandoriented biogas production as well as for the production of a high calorific biogas by means of the two-stage biogas process.
Microbial conversion of organic residues into acid rich process liquids and their use in bio-electrochemical systems
(2020) Ravi, Padma Priya; Lemmer, Andreas
In 2016, 2.01 billion tonnes of solid waste were generated worldwide. The volume of waste is expected to grow to 3.40 billion tonnes by 2050. Worldwide, most solid waste is disposed of in landfills or dumps. Due to improper treatment and disposal of solid waste, nearly 1.6 billion tonnes of CO2 equivalents of greenhouse gas emissions were generated worldwide in 2016. This amount is expected to rise to 2.6 billion tonnes of CO2 equivalents per year by 2050. It will therefore become increasingly important in the future not only to treat waste sustainably, but also to use it as an alternative to fossil fuels. Different waste-to-energy concepts are used, particularly for the treatment of OFMSW. As an alternative to the previously dominant biogas production, intensive research is currently being carried out into technologies for the recycling of organic residual materials, including so-called bio-electric systems (BES). In contrast to biogas production, this technology enables the treatment of a wide range of wastes to produce different end products, e.g. electrical energy, hydrogen or methane, can be preferred in BES depending on the selected process parameters. Despite numerous advances in research, considerable additional optimization is still required in order to be able to use the systems in large-scale power generation. In order to use solid organic waste in BES systems, fermentative digestion is required to convert the organic components into dissolved short-chain organic acids (Volatile Fatty Acids (VFA)) and alcohols. In the course of the investigations, the solid waste residues were first digested to acid-rich hydrolysate in a hydrolysis reactor at pH-values of 5.5 and 6.0. However, this hydrolysate also contains particles that are inert to a subsequent degradation step leading to technical process disturbances. These inert particles can be removed by means of a membrane filtration step; a particle-free permeate is produced, which can be fed to the BES reactors. Within the scope of the present work, the basics of the utilization of OFMSW via microbial digestion, membrane filtration and utilization in BES should be investigated. Lab-scale BES reactors were developed and batch tests were carried out. The vegetable waste residues from hydrolysis could be efficiently converted into hydrolysate. At a pH value of 6.0, higher organic acid concentrations were achieved than at pH 5.5. At pH 6.0, based on the added organic dry matter, these were approx. 350 g kg-1 (oDMadded) and at pH 5.5 approx. 215 g kg-1 oDMadded. Likewise, the concentration of chemical oxygen demand (COD) of the hydrolysate at pH 6.0 was 21.85 % higher than at pH 5.5. However, the COD degradation rates in the AF used were insufficient because the inert particles present in the hydrolysate could not be completely microbially degraded. The subsequent integration of ceramic cross-flow membrane filtration into the two-stage system produced a particle-free permeate and significantly the increased microbial degradability. Clear differences could be shown depending on the substrate used (plant waste and grass/maize silage). The filtration step resulted in a significant improvement of the specific methane yield of permeate by 40% (vegetable waste) and 24.5% (grass/maize silage) compared to hydrolysate; proof that inert particles were separated efficiently. Finally, the process liquids hydrolysate and permeate produced by the hydrolysis of maize silage and the subsequent membrane filtration were fed to the anode chamber of two mixed-culture BES reactors. The investigations showed that all organic acids in both process liquids could be completely degraded in the BES. The highest COD (87%) and TOC degradation rates (88%) were achieved with permeate. However, the hydrolysate with added acetic acid yielded the highest current density of 470 µA/cm². Increasing the pH-value of the process liquids from 5.75 to 6.8 also significantly improved the current production and degradation rates. In this batch studies, relatively low Coulomb efficiencies of less than 10% were achieved due to the use of a mixed cultures. The promising results show that at high pH-values (pH 6.0) in hydrolysis organic residues can be efficiently converted into a hydrolysate with high concentrations of organic acids and that the system can be further optimized by coupling membrane filtration. The utilization of the permeate in BES enables, a sustainable production of bioenergy and platform chemicals with permeate enables, depending on the BES reactor configuration. In summary, it was described for the first time that the combination of the fermentative biomass degradation process with filtration via ceramic membranes and the use of permeate in BES systems is possible.
Characterisation of biogas digestate as raw material for fibre composites
(2022) Gebhardt, Marion; Lemmer, Andreas
Various synthetic fibres and natural fibres are used as reinforcing fibres in the fibre composite industry. Efforts towards sustainable products and the avoidance of land-use competition are increasingly driving the search for alternatives. Biorefineries are one possible solution. Biogas plants process structurally rich plant-based biomass. The resulting digestates have already been partially degraded. Natural reinforcement fibres are extracted chemically or biologically from plants, the use of digestates is obvious. This paper deals with the question whether biogas digestate can be used as a fibre raw material for composite materials. Digestates from four different commercially operated biogas plants in Germany are considered. Besides three biogas plants that utilize an average mix of animal excrement and plants one plant is operating solely plant-based. The solid portions of the fermentation residues were first examined regarding fibre quality. For this purpose, the fibre dimensions (fibre length and degree of slenderness) and the density of the fermentation residues were determined. Utilizing a feed analysis according to van Soest, the proportions of cell wall components were examined. The results of the investigations were compared with common fibres such as flax and wood. In addition to the properties, the possible fibre yield from the various fermentation residues was also considered. In this study, the influence of the starting substrates was considered in detail. For this purpose, the distribution of different size classes was first determined utilizing wet sieve analysis. These results were combined with the dry matter content. Digestates from a plant using exclusively plants as substrates, with a high proportion of hop vines, are considered separately. It is considered whether the additional washing of the fermentation residues brings an advantage for the fibre quality. To prove this thesis, the most common fibre properties are also examined and compared with wood fibres. These digestates from this biogas plant are used for the following investigations. Composite materials are often produced with a textile as reinforcement. Therefore, the digestate is first processed into a nonwoven. The wet laying technology is used, as it is suitable for various types of fibres. Only cellulose is used as a binding material so that the nonwoven is completely bio-based. The hot-press technology with a thermoset matrix is used to produce the composites. The matrix used is a partially bio-based epoxy resin system. The most suitable process parameters are determined with two test scenarios. In the first run, the proportion of added matrix is varied at constant pressure. In the second run, the pressure is varied at constant matrix content. Destructive and non-destructive material tests are carried out to check the material properties. To make a statement about a suitable application, the mechanical properties and the water absorption are of particular importance. In addition, the behaviour towards chemicals is examined to be able to assess the resistance of the material. For this purpose, the composite material is produced with the previously determined process parameters and immersed in various chemicals. Finally, the durability of the composite materials is examined. For this purpose, the composite material is also produced with the previously determined process parameters. An epoxy resin with a higher bio-based content is used as the matrix. The material is exposed to UV radiation and humid air for three months. Afterwards, the mechanical properties and water absorption are examined again. The main finding of the presented study is that the solid components of digestate can be processed into composite materials. The properties of the digestate fibres are similar to those of wood fibres. For the yield of digestate fibres, it is advantageous if only a small proportion of animal excrements is used as substrate in the biogas plant. Additional processing after fermentation leads to an increase in fibre quality. The hot-press-technology has proven to be a suitable process, as fully impregnated composites with reproducible properties can be produced. The process parameters determined are a pressure of at least 4:5MPa and a matrix addition of 60%, which corresponds to an excess of about 10%. The properties of the composites are comparable to Wood Plastic Composites. Therefore, they can be considered adequate. The durability is shown to be inadequate due to the strong reduction in mechanical properties after artificial weathering and chemical storage. The durability is mainly dependent on the matrix. Based on the results described, an application for the digestate composites as furniture material is recommended.
Development and evaluation of methods for assessing the efficiency of biogas plants
(2022) Hülsemann, Benedikt Werner; Müller, Joachim
Biogas is a renewable energy source with main advantages compared to other renewable energy sources. The advantages include the use of organic waste as a substrate, local power and heat production, rural job creation, the possibility of a flexible gas production and a product which can easily stored and transported in a gas grid or on the roof of a digester. However, the development of the biogas sector is highly dependent on the costs of producing gas, electricity and heat. The production costs are higher than the costs for other energy sources. Growth of the biogas sector is therefore only possible if there is political promotion for biogas as there was in Germany through the EEG. Nowadays, due to the reduction of bonus payments in the EEG 2017 and EEG 2021 in Germany as well as the lack of policy promotion in several other countries, lower production costs based on a higher efficiency are essential to help the biogas sector grow further. In order to achieve higher efficiency and to tap the full potential of biogas, the efficiency has to be determined, which is done in this thesis. The input methane potential is determined using 6 different methods. These methods are compared on the basis of an investigation of 33 German agricultural BPs as well as one German and one US BP using food waste as feedstock. The four methods based on the batch test show a high sensitivity. Unfortunately, they also show efficiencies greater than 100% for most BPs, clearly indicating an underestimation of the degradable potential. Only for the US BP can an efficiency less than 70% be reported. This result is probably based on the lack of heating system corresponding to the lack of promotion of heat recovery in the US. The CE according to the BMP method also reveals an average efficiency of 95% for the German BPs. The values of the two gross calorific value-based methods show efficiencies below 100%, but with low sensitivity. The results of these methods can be used to determine the further potential of a bioeconomic process and to compare the biogas process with other industrial processes. There are several impact factors that affect the accuracy of the efficiency measurements. The installed meters are not frequently calibrated at most BPs. Also, some meters are almost completely missing, as only few BPs in Germany have a gas flow meter. Thus, assumptions and calculations are required to determine the efficiency. In the developed method, the gas flow must be calculated from the amount of the power production, the calorific value, the gas quality, the CHP unit efficiency and the conversion loss at the transformer. The last two values must be assumed, even if the database is small. Another important parameter is the feeding mass. It is measured by the German BPs, but in some cases, the data quality is low. For example, different crops are mixed in the silos and measurement of each substrate is not possible. This leads to measurement errors shown by the organic dry matter mass balance, which has a residual value of up to 24%, while only 11% can be occur based on water incorporation into the ODM. Another factor having an impact is the sampling. The results of a monthly sampling throughout the year show a fluctuation in the DM/ODM values. To investigate the accuracy of the methods used to determine the SMP of the substrate, the biochemical methane potential test is examined in detail. The BMP consists of the used inoculum, the substrate, the digestion system and the calculation. The impact of the used inoculum and the digestion system is investigated by using different inocula in one digestion system as well as by using the same inoculum in multiple digestion systems. The inocula used in this thesis are well-known and have been used in interlaboratory tests for several years. Thus, outliners were excluded. A CV of 4.8% can be reported between the different inocula, which is lower than reported in most other publications before. The use of different digestion systems shows a higher CV of up to 12.8%. For the inoculum and the digestion system, the deviation varies strongly and no clear correlation can be identified. Therefore, a correction of this effect is not possible. The biological yield efficiency of 21 of the investigated BPs is in the range of 100 ± 12.8%. This reveals the need of stricter rules for the digestion system. All digestion systems used in this thesis are described in the German guideline VDI 4630. The calculations were also done according to the German guideline VDI 4630. An influence can be neglected. However, if the results of a measurement with already dried gas are compared with the results of a calculation according to VDI 4630, which is based on the measurement with wet gas, a discrepancy can be found. Although, the CV using only one digestion system and one inoculum is only 1-7%. A comparison of the efficiency of different BPs by using the same inoculum and digestion system is hence recommended.
Phosphate turnover during anaerobic digestion of chicken, pig and dairy manure
(2023) Dinkler, Konstantin; Müller, Joachim
Phosphate (P) is used extensively in agriculture. This has led to a reliance on P imports. Meanwhile, the framework for fertilization with digestate and manure in the European Union has become more stringent in recent years. Therefore, nutrients should be recovered as fertilizer to reduce dependencies, redistribute nutrient and amplify the product portfolio of biogas plants. Current nutrient recovery processes have in common that they are post digestion treatments of digestate, which neglect the phosphate behavior during digestion. It is necessary to closely evaluate P behavior during AD to optimize post digestion treatments of digestate by using digestion as a pretreatment for digestate. Therefore, it was the overall objective of this work to evaluate the turnover of P during anaerobic digestion in laboratory scale batch and continuous digestion systems. In laboratory experiments with batch reactor systems three different manures, namely pig, dairy and chicken manure were digested. Activated sludge served as inoculum. A set of 120 mL batch digesters were filled and individual bottles were opened after defined times and discarded afterwards until the last reactors were opened on day 30. The results showed that H2O-P and NaHCO3-P decreased over the digestion period by up to 40.1 %. Meanwhile, NaOH-P increased. Overall, it could be concluded that anaerobic digestion leads to a mineralization of P. The mineralization was especially profound during the first few days after the substrate was mixed with the inoculum, concluding that the ions in the inoculum played a significant role in this mineralization. In effect, AD reduces immediate plant availability but increases slow-release fertilization effects. During the batch experiments it was found that for a defined measurement wavelength for digestate the absorbance spectrum of digestate extracts needed to be analyzed and a drying temperature needed to be determined for sample treatment. For the evaluation of these two aspects samples were dried at 50°C and at 105°C and freeze dried. These samples and undried digestate were extracted by Hedley fractionation. The coloring agent was added to the extracts and the spectra between 600 nm and 1100 nm were measured. The spectral lines showed two peaks (709 nm and 889 nm). The lower wavelength proved to be more stable at low absorbance, making this the better wavelength for analysis. The analysis of the Hedley extracts showed that drying increases the H2O-P and NaHCO3-P fraction by up to 70 %. The samples were rinsed with preceding solvent to increase accuracy. Overall, the adapted method achieved higher accuracy for H2O-P, NaHCO3-P than the former method. The adapted fractionation was used for the analysis of samples during experiments in continuously stirred tank reactors. Chicken and dairy manure were each co-digested with straw and the parameters OLR and temperature were varied. The results showed that OLR had a negative correlation with H2O-P, which decreased by up to 50.49 %. Meanwhile, HCl-P increased significantly in chicken manure digestate, showing a positive correlation with OLR. It was proven that temperature has a minor effect on P transformation with a slightly higher mineralization of P under thermophilic conditions. Especially the high calcium concentration in chicken manure dominated the P turnover during the digestion, which can also be seen in the positive correlation of OLR with HCl-P as well as a high Pearson correlation coefficient above 0.85 for calcium and phosphate in chicken manure digestion. The results of this work have proven that P changes its chemical composition significantly during anaerobic digestion. The parameters of the digestion process had a decisive effect on the final composition with OLR and substrate composition being the major drivers. The results further showed that gas production and high P solubility are in conflict because for increased H2O-P OLR needs to be reduced. Future work should focus specifically on the combination of this anaerobic digestion and post-digestion treatments for cost effective recovery. This can play a key role for future profitability of biogas projects.
Nitrogen-rich and lignocellulosic biomass for biogas production : methane yield potentials, process stability and nutrient management
(2023) Morozova, Ievgeniia; Lemmer, Andreas
A sustainable energy supply and bio-based economic processes are of central importance for the future development of many Eastern European countries. Due to the large agricultural potentials of these countries, bioenergy systems can make a significant contribution to sustainable electricity and heat production if they are reasonably integrated into an energy supply structure based on various renewable energy sources. This requires the use of regenerative starting products and the complete utilisation of all by-products of the overall process. With such a cradle-to-cradle approach, biogas technology can be a central component of future energy systems. The focus of this study is on Ukraine. In the future, bioenergy villages can make a decentralised contribution to a sustainable energy supply in this country. This study aims to determine the methane yield potential of various energy crops from Ukraine, investigate the process stability during fermentation in biogas plants and derive concepts for optimized digestate management. Seven different crops with a total of 22 varieties were investigated for their specific biomass yields, methane yields and areal methane yields. The crops were cultivated in Ukraine. The biogas production potential of the collected crop samples was determined using the Hohenheim Biogas Test in Germany. The Ukrainian variety “Osinnii zoretsvit” of miscanthus, “Giganteus” species, from the 8th year of vegetation, harvested at the stem elongation stage, resulted in the highest areal methane yield of 7404.55 ± 199.00 m3*ha-1 and the lowest N requirement per unit methane produced (23.41 ± 7.18 gN*m-3) among all the studied crops. The maize variety "Svitanok MV" (FAO 250) had the highest value of areal methane yield of 6365.67 ± 55.49 m3*ha-1 among the annual crops when harvested at the stage of wax maturity; remarkable was its unusually high specific methane yield of 0.41 ± 0.00 m3*kg-1VS. The Ukrainian sugar sorghum variety "Favoryt", harvested at the beginning of flowering, had an areal methane yield of 5968.90 ± 82.70 m3*ha-1, making it an attractive alternative energy crop for Ukraine. In the second part of the work, experimental investigations were carried out to test how N-rich substrates influence the stability and efficiency of the biogas process. For this purpose, different variants with various N-increase rates of the input materials at two initial concentrations were evaluated in the laboratory. The continuous trials were conducted over a period of 33 weeks. The modelling procedure was applied to evaluate the effects of TAN (total ammonia nitrogen) and FAN (free ammonia nitrogen) on the degree of methane production inhibition for all scenarios studied. It was concluded that the higher the N-increase rate in the feeding regime, the more methane production is inhibited. The maximum nitrogen concentration in the digestate achieved during stable fermentation processes in this study was 11.5 g*kg-1FM, which corresponded to the values of TAN and FAN of 9.07 g*kg-1FM and 0.85 g*kg-1FM, respectively. These values are much higher than those reported up to now in the literature. At the same time, process efficiency decreased with increasing nitrogen concentrations. As a final step, the technology for nutrients recovery from digestate was developed and tested in this work. First, the digestate separation with a screw press separator was carried out as a "benchmark" at the research biogas plant "Unterer Lindenhof" on a technical scale. Subsequently, a methodology for digestate separation at laboratory scale was developed based on a tincture press, which corresponds to the technology used in practice. The effect of pretreatment of digestate with various biocoal-based additives was studied. In this study, six variants of biocoals synthesized at either 350 °C or 600 °C and partially impregnated with Mg or Ca before pyrolysis were produced. Different reaction times and conditions between the biocoals and the digestate were tested. The results on nutrient removal showed that the biocoals impregnated with Mg prior to pyrolysis had a positive effect on nutrient removal efficiency. The Mg-impregnated biocoal synthesised at 600 °C showed removal efficiencies for NH4+, P and K of 56.04%, 66.66% and 51.77%, respectively. These values were much higher than those for the control variant and much higher than the values found up to now in the literature. By using the nutrient-rich solid fraction of the digestate as fertiliser to cultivate bioenergy crops for further use in biogas production, the production cycle is closed, and the cradle-to-cradle approach is achieved.
Experimentelle Entwicklung einer modellbasierten prädiktiven Regelung für den flexiblen Betrieb von Biogasanlagen
(2023) Dittmer, Celina; Lemmer, Andreas
The transformation of the energy system requires controllable producers due to increasingly decentralised, fluctuating electricity generation from wind turbines and photovoltaics. Biogas plants can make a substantial contribution here by making plant operation more flexible and thus providing electricity as needed. Technical adjustments, such as the expansion of gas storage capacities and CHP output, can compensate for short-term fluctuations. However, in order to be able to shift the potential of electricity generation over longer periods of time, an adapted feed-in strategy is essential. The control of biogas production poses several challenges in practical implementation. First, the conversion of biomass into biogas is a complex process and must be considered individually for each biogas plant. Models developed so far use parameters for all characteristic process phases and influencing variables in order to be able to model anaerobic digestion. In contrast, biogas plants are often only rudimentarily equipped with measurement technology, so that corresponding parameters are not available. In this work, a model-predictive control of biogas plant operation was developed to enable demand-driven electricity generation. The aim was to develop models that are particularly well suited for practical use. Thus, for the first time, a successful application on almost all biogas plants could be possible without or with only minor adaptations to the existing measurement technology. All studies carried out in this thesis are based on a real-world laboratory, the "Unterer Lindenhof". This includes a practical biogas plant as well as an electrical consumption corresponding to that of a village with about 125 inhabitants. In a first step, forecasting models were evaluated to predict the electricity demand of the real-world laboratory over 48 hours in advance. Four models from the field of time series analysis were examined, one TBATS and three different ARIMA models. In an evaluation of 366 forecasts each, all four models performed sufficiently well to provide a set point for biogas plant operation, with average MAPE values of 13-16 %. Further investigations showed that forecasts can also be carried out over a period of up to 14 days without significant losses in forecast quality. In a further step, a model was developed to simulate biogas production. This is also based on time series analysis, or more precisely on a regression model. Thus, it differs significantly from previous developments in this field, which are mostly based on the complex ADM1. It turns out to be very advantageous that the developed simulation model uses as input parameters only historical data of the last four weeks of biogas production and the amount of solid substrates fed in, without considering their composition. The simulation of biogas production over 48 hours in advance is based on correlations resulting from these two data sets. An evaluation of the model over 366 simulations resulted in an average MAPE of 14-18 %. Data from both digesters of the biogas plant were used, which can be considered as independent systems, demonstrating the adaptability of the model. In a third step, the feeding schedule was developed for demand-based biogas production. For each 48 hours in advance, 1500 randomised feeding schedules were calculated. Some constraints were imposed, such as the maximum amount of substrate that is technically possible in the biogas plant. The biogas production expected from the feeding schedules could be calculated using the simulation model. By comparing the simulation with the desired biogas demand profile, the simulation with the least deviations could be determined and the appropriate feeding plan selected and implemented. The entire model predictive control system was used and thoroughly tested in a field trial at the real-world laboratory "Unterer Lindenhof". Over a period of 36 days, an average MAPE of less than 20 % was achieved in comparison between the real biogas production and the desired biogas demand. During the test period, the biogas demand was derived from the predicted electricity demand of the real-world laboratory. The investigations carried out show that the model-predictive control system developed enables demand-oriented electricity generation on full-scale and that, due to the models being very close to practice for the first time, adaptation to almost all biogas plants is possible.
Optimization of thermodynamic parameters of the biological hydrogen methanation in a trickle-bed reactor for the conditioning of biogas to biomethane
(2023) Holl, Elena; Oskina, Anastasia; Baier, Urs; Lemmer, Andreas
The increased demand for resources and energy that is developing with rising global consumption represents a key challenge for our generation. Biogas production can contribute to sustainable energy production and closing nutrient cycles using organic residues or as part of a utilization cascade in the case of energy crops. Compared to hydrogen (H2), biogas with a high methane (CH4) content can be fed into the gas grid without restrictions. For this purpose, the CH4 content of the biogas must be increased from 52 to 60% after anaerobic digestion to more than 96%. In this study, biological hydrogen methanation (BHM) in trickling-bed reactors (TBR) is used to upgrade biogas. Design of experiments (DoE) is used to determine the optimal process parameters. The performance of the reactors is stable under all given conditions, reaching a “low” gas grid quality of over 90%. The highest CH4 content of 95.626 ± 0.563% is achieved at 55 °C and 4 bar, with a methane formation rate (MFR) of 5.111 ± 0.167 m3/(m3·d). The process performance is highly dependent on the H2:CO2 ratio in the educts, which should be as close as possible to the stochiometric ratio of 4. In conclusion, BHM is a viable approach to upgrade biogas to biomethane quality and can contribute to a sustainable energy grid.

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