Browsing by Subject "Anaerobic"

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    Demand-driven biogas production in anaerobic filters
    (2017) Krümpel, Johannes; Jungbluth, Thomas
    Fluctuating energy sources, namely wind turbines and photovoltaic, will be the mayor contributors to the increase in share of renewable energies. The intermittent energy supply by these sources poses challenges for the power grid and need to be counter balanced. A demand-driven energy supply by weather independent biomass conversion can offer these grid services. Flexible energy production from biogas has been identified as a vital approach to provide the grid with positive and negative balancing power. The two-staged anaerobic digestion may be especially suitable for demand orientated gas production due to the advantages of the anaerobic FIlters to withstand high organic loading rates and shock loading. Two staged anaerobic digestion is characterized by a spatio-temporal separation of acidification and methane production. A liquid rich in soluble products, such as volatile fatty acids, alcohols and sugars is produced in the first stage and and is subsequently converted to biogas in the second stage. The methanation stage as the main gas producing unit in such a system is in the focus of this research.The ability to react to sudden changes in demand might be influenced by substrate composition and controlled hydrolysis towards certain intermediates could improve the reaction times towards increased demand. It is therefore one focus of this research work to examine intrinsic methane production kinetics of common intermediates of anaerobic digestion. Other major questions are how fast the methane production can be adapted to sudden changes in demand and to what extent these adaptions are reproducible. It was therefore of interest to demonstrate the feasibility, reproducibility and the possible extent of demand-driven biogas production in anaerobic filters, with respect to changing substrate composition. Furthermore the evaluation of the process effciency based on carbon fluxes should be examined to unfold effects resulting from changing operational conditions. With a newly developed methodology, introduced in the publication "Kinetics of Biogas Production in Anaerobic Filters" kinetic parameters of methane production for individual volatile fatty acids (VFA) could be determined. The bandwidth of tested intermediates was broadened in the second research paper "Intrinsic Gas Production Kinetics of Selected Intermediates in Anaerobic Filters for Demand Orientated Energy Supply". It has been found that intermediates could be ordered according to their half-lives of methane production. The apparent order, beginning with the fastest was acetic acid >ethanol >butyric acid >iso-butyric acid> valeric acid> propionic acid> propanediol> lactic acid. However the mixture of these individual components administered as a naturally produced hydrolysate revealed the fastest methane production kinetics. Differences in the absolute values of determined kinetic parameters between the two experiments can be attributed to variations in organic loading rate (OLR), since degradation rates of a specific substrate are determined by substrate concentration. But also other parameters influence the absolute rate at which methane is produced, such as the concentration of products or unionized substrate itself, pH, nutrient availability, bioenergetics, temperature, inhibition, mass transfer and microbial population. In the third research paper "Demand-Driven Biogas Production in Anaerobic Filters "the previous findings have been put to the test by applying changes in OLR throughout the day and examining different substrate compositions with respect to the methane production rates. As demonstrated, the gas production followed the applied OLR with a distinctive expression of each change in the OLR. That marks the process as highly predictable and defined boundaries within safe operation of AD, in terms of VFA accumulation,can possibly be satisfied by process control. The inclusion of three reactors in the analysis emphasizes the repeatability and therefore the predictability of such an approach of operation. Feasibility and reproducibility of demand-driven biogas production by anaerobic filters could thus be demonstrated. It has been found that the hydrolysate composition has no significant influence on methane production kinetics for demand orientated gas production, since the maximum rate is limited by acetoclastic methanogenesis. The control of the hydrolysis should focus on high overall degradation, rather than towards the production of specific intermediates. A key factor in order to prevent large fluctuation in gas composition is alkalinity, specifically the provision of nitrogenous compounds is vital to maintain stable conditions. Anaerobic filters or attached biomass reactors in general seem to exhibit superior performance towards shock loading and are therefore especially suited for demand orientated gas production as they recover quickly from overloading.Formation of soluble microbial products (SMP) and extracellular polymeric substances (EPS) may be influenced or exaggerated by constantly changing HRT and OLR. Further research in order to evaluate the limits of safe operation is recommended as more extreme scenarios than the ones examined in this work are imaginable in practice.