Landesanstalt für Agrartechnik und Bioenergie

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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.
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.
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.
Mono-digestion of 5-Hydroxymethylfurfural process-wastewater in continuously operated anaerobic filters: A cascade utilization approach
(2023) Khan, Muhammad Tahir; Krümpel, Johannes; Wüst, Dominik; Lemmer, Andreas
A proper remedy for the overexploitation of biomass and biobased materials in the bioeconomy is the valorization of biorefineries’ side streams into meaningful products. Hence, in pursuit of a cascade utilization of renewables, a unique biorefinery byproduct was investigated for its biogas potential, specifically methane, in continuously operated anaerobic filters. For this purpose, 5-Hydroxymethylfurfural process-wastewater, after supplementation of necessary nutrients, was diluted down to 10, 20, 30, 40, and 50 gCOD/L concentrations and thereafter tested individually at 43 °C and 55 °C. Maximum methane conversion efficiency at either temperature was observed for test substrates with 10 gCOD/L and 20 gCOD/L concentrations. At 43 °C, the anaerobic filters exhibited their highest biogas yields when supplied with the 30 gCOD/L feedstock. Further exposure of the mesophilic and thermophilic consortia to the ensuing 5-Hydroxymethylfurfural process-wastewater dilutions compromised the stability of the anaerobic process due to the soaring concentrations of short-chained volatile fatty acids. The supplementation of necessary nutrients to unlock the methane potential of the given recalcitrant substrate appears insufficient. Techniques like micro aeration, photolysis, or the use of activated carbon in the fixed bed might have the ability to enhance the biochemical methane conversion of such feedstock; otherwise, the introduction of trace elements alone may be adequate if aiming for platforms (volatile fatty acids) via anaerobic technologies.
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.
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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