Landesanstalt für Agrartechnik und Bioenergie

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    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.