Landesanstalt für Agrartechnik und Bioenergie

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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.
Effects of harvest date and ensiling additives on the optimized ensiling of Silphium perfoliatum to prevent faulty fermentation
(2024) Baumgart, Marian; Hülsemann, Benedikt; Sailer, Gregor; Oechsner, Hans; Müller, Joachim; Hu, Wei; Zhou, Zhiguo; Zhao, Wenqing
Silphium perfoliatum , an energy crop with a high fiber content but low concentrations of fermentable carbohydrates, presents challenges for complete fermentation in biogas production. To overcome this, a bioeconomic approach proposes the use of the fibers for paper and board production, which requires high-quality silage with minimal butyric acid, which affects the marketability of the fibers. This study aims to optimize the silaging process of Silphium perfoliatum by investigating the effects of harvest date, bacterial cultures and additives on fermentation results. Laboratory experiments were conducted to evaluate the effect of three harvest dates on fermentation acid composition, with a focus on increasing lactic acid production to inhibit butyric acid formation. Results indicate that an early harvest date (early September) is critical for achieving stable fermentation and minimizing ensiling losses. The addition of sugar-rich additives, such as syrup, was found to be essential, especially for later harvest dates. Despite these interventions, a late harvest (early November) consistently resulted in suboptimal fermentation. The results suggest that optimizing harvest timing and incorporating appropriate additives are key strategies for producing high quality silage and ensuring the suitability of Silphium perfoliatum fibers for industrial applications.
Investigation of phosphorus recovery from biogas digestate: low-technology approaches for enhanced solid-liquid separation
(2025) Uppuluri, Naga Sai Tejaswi; Müller, Joachim
Phosphorus (P) is a critical nutrient for agriculture, essential for plant growth and food security. However, the global dependence on finite P reserves, primarily located in northern Africa and China, presents significant challenges, including supply chain vulnerabilities and rising fertilizer costs. Additionally, excessive P runoff into water bodies contributes to environmental issues, such as eutrophication. Addressing these challenges requires sustainable P management strategies, including P recovery from alternative sources like biogas digestate. To tackle these challenges, regulatory measures and innovative recovery techniques are being explored to improve P sustainability. In Germany, regulations like the Fertilizer Ordinance and the Sewage Sludge Ordinance have introduced strict limits on P application and mandated the recovery of P from waste streams, promoting environmentally friendly nutrient management practices. These regulations aim to reduce environmental impacts while ensuring the efficient use of P resources. A primary objective of this research is to develop a cost-effective, practically feasible method for P recovery that can be easily integrated into existing biogas plants without requiring substantial infrastructure upgrades. This study investigated the recovery of P from biogas digestate using additives such as kieserite (MgSO₄∙H₂O), straw flour, and biochar to improve separation efficiency and nutrient recovery. The additives enhanced the P content in the solid phase, making it easier to recycle P back into agricultural systems. The laboratory-scale separation trials were conducted to evaluate the effectiveness of three additives categorized into reactive (kieserite) and non-reactive (straw flour and biochar) groups. The separations were carried out using a hydraulic tincture press, with a pressure of 5 MPa applied for 120 seconds. The trials tested five different treatment times: 0 h, 1 h, 2 h, 8 h, and 20 h. Kieserite was tested at 25 °C and 50 °C, while straw flour and biochar were tested only at 25 °C. The results revealed that longer treatment durations with kieserite were more effective, with 61% of P shifting into the solid phase after 20 h. In contrast, treatment duration had little impact on the effectiveness of straw flour and biochar. Kieserite treatment increased NaOH-P and HCl-P, indicating the formation of more stable, non-labile P fractions due to the interaction between (Mg²⁺) ions from kieserite and phosphate (PO₄³⁻) ions. These findings concluded that kieserite, as a reactive additive, is more effective at enhancing P recovery by converting labile P into more stable, non-labile forms. Based on the conclusions from laboratory-scale separation trials, practical scale experiments were conducted at the research biogas facility ‘Unterer Lindenhof’ at the University of Hohenheim. In these trials, straw flour and kieserite were used as additives, with treatment durations of 4 h and 22 h. The shorter duration represented same-day processing, while the longer duration simulated overnight treatment. Results showed that extending the treatment time with kieserite significantly improved P removal efficiency (PRE), reaching 67% of P shifted to the solid phase after 22 h. Straw flour, on the other hand, achieved a 52% PRE at the same duration, with most of the P remaining in labile fractions regardless of treatment time. Kieserite treatment resulted in notable changes in the distribution of P fractions, shifting from NaHCO₃-P to more stable NaOH-P and HCl-P fractions as the treatment duration increased. These experiments provide a technical proof-of-concept for the use of additives in biogas plants for digestate treatment to enhance P recovery into the solid phase, supporting more sustainable nutrient management practices. Biochar modified with kieserite and calcium chloride (CaCl₂) was evaluated as an additive for P recovery, along with the metal salts used independently. The modification aimed to load Mg²⁺ and Ca²⁺ ions onto the biochar surface, enhancing its effectiveness. Initial separation trials established that using 5 gadditive/Ldigestate with a 22 h treatment time provided optimal conditions for solid-liquid separation. The separation trials were made in the laboratory using a hydraulic tincture press. The modification significantly increased the Mg content in kieserite-modified biochar (KIS-B) and the Ca content in CaCl₂-modified biochar (Ca-B). Both modified biochars and the metal salts increased P transfer to the solid phase, with the metal salts alone demonstrated higher PRE. KIS-B and Ca-B shifted P to non-labile fractions, while kieserite and CaCl₂ alone resulted in an even higher proportion of non-labile P fractions. Although the modified biochar showed slightly lower PRE compared to metal salts, its potential benefits in agricultural applications are noteworthy. Future studies should include a comprehensive cost-benefit analysis to evaluate the long-term financial sustainability of implementing these recovery techniques at full scale. Reducing the reliance on expensive, finite phosphorus reserves through local recovery can lower input costs for farmers, making the process economically attractive. Additionally, plant pot trials will be essential in assessing the agronomic efficiency of P recovered from biogas digestate. These trials can help determine how effectively the recovered P promotes plant growth and nutrient uptake compared to conventional fertilizers. Ultimately, refining these recovery techniques and assessing their impact on both economic viability and agricultural productivity will play a key role in advancing sustainable nutrient management practices globally.
New sustainable banana value chain: Waste valuation toward a circular bioeconomy
(2023) Krungkaew, Samatcha; Hülsemann, Benedikt; Kingphadung, Kanokwan; Mahayothee, Busarakorn; Oechsner, Hans; Müller, Joachim
According to the needs of sustainability, a new sustainable banana chip value chain, which is a combination of the traditional banana chip value chain and the banana waste value chain, was designed. Scenarios were created assuming that an anaerobic digester would be implemented to produce biogas—which can act as a substitute for liquefied petroleum gas (LPG) used in banana processing—from banana wastes. The values of banana residues throughout the value chain were determined depending on farm gate tree price, transportation cost, and the final value of LPG substitution. The value chain was optimized using two objective functions: total chain profit maximization and factory profit maximization. The tree price at the farm gate was determined and assumed to be between USD 0.067 and USD 0.093 per tree, and the transportation cost of tree transportation was assumed to be between USD 0.31 and USD 0.39 per km. Different tree prices and transportation costs affected the profits of all stakeholders throughout the chain. The scenarios that maximized total chain profits showed superior environmental performance compared to the scenarios that maximized factory profits. The proposed sustainable value chain will lead to an increase in farmers’ profits of 15.5–17.0%, while the profits gained by collectors and factory will increase between 3.5 and 8.9% when compared to business as usual.
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.

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