Demand-driven biogas production in anaerobic filters

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.Fluktuierende Energiequellen, vornehmlich in Form von Windturbinen und Photovoltaik-Anlagen werden den Großteil des Wachstums an Erneuerbaren Energien ausmachen. Die unstetige Energieversorgung dieser Quellen stellt eine Herausforderung für die Elektrizitätsnetze dar und muss mit entsprechender Regelenergie ausgeglichen werden.Eine bedarfsgerechte Energieversorgung durch wetterunabhängige Biomasse-Konversion kann diese Netzdienstleistungen erbringen. Flexible Energiebereitstellung durch Biogas wurde als wichtiger Ansatz erkannt, um Netze mit positiver und negativer Regelenergie zu stabilisieren. Die zwei-stufige anaerobe Vergärung, die aufgrund der hohen biologischen Stabilität des Anaerobfilters gegenüber Lastwechseln besonders für bedarfsgerechte Biogasproduktion geeignet ist, wird charakterisiert durch die räumlich-, zeitliche Trennung der Versäuerung von der Methanproduktion. Aus der zugeführten Biomasse wird in der ersten Stufe eine Prozessflüssigkeit, reich an organischen Säuren, Zuckern und Alkoholenproduziert, die anschließend in der zweiten Stufe zu Biogas umgewandelt wird.Die Methan-Stufe, als maßgeblich gasproduzierende Einheit steht dabei im Fokus dieser Arbeit. Im Rahmen dieser Arbeit sollte die Machbarkeit, Reproduzierbarkeit und das Ausmaß der bedarfsgerechten Biogasproduktion im Anaerobfilter begutachtet werden, auch im Hinblick auf den Einfluss der Substratzusammensetzung. Die Beurteilung der Prozesseffzienz soll anhand einer Kohlenstoffbilanz erfolgen um eventuelle Effekte der sich ändernden Betriebsparameter aufzudecken. Mit einer neuen Methode, eingeführt in der Publikation "Kinetics of Biogas Production in Anaerobic Filters", konnten kinetische Parameter der Methanbildung einzeler Fettsäuren bestimmt werden. Die Bandbreite der getesteten Intermediate wurde mit der zweiten Publikation "Intrinsic Gas Production Kinetics of Selected Intermediates in AnaerobicFilters for Demand Orientated Energy Supply" erweitert. Anhand der "Halbwertszeiten der Methan Produktion" konnte eine Reihenfolge der Gasbildungsgeschwindigkeiten etabliert werden. Die Reihenfolge der getesteten Intermediate, beginnend mit dem schnellsten,wurde wie folgt bestimmt: Essigsäure> Ethanol> Buttersäure> iso-Buttersäure> Valeriansäure> Propioinsäure> Milchsäure. Am schnellsten wurde Methan jedoch nach der Zugabe eines natürlich produzierten Hydrolysates, also eine Mischung aller einzelnen Intermediate, erzielt. Unterschiede in den absoluten Werten der Kinetik-Parameter zwischen den beiden Experimenten können dem Einfluss der angelegten Raumbelastung zugeordnet werden, da Abbauraten spezifischer Substrate im Allgemeinen von der Substratkonzentration abhängig sind. Andere Faktoren, die für die absoluten Raten der Methanproduktion verantwortlich stehen, sind Produktkonzentrationen, unionisierte Substratkonzentration, pH-Wert, Nährstoffverfügbarkeit, Temperatur, Massetransfer und die mikrobielle Population. In der dritten Publikation "Demand-Driven Biogas Production in Anaerobic Filters" wurden die vorherigen Erkenntnisse überprüft, indem im laufe eines Tages wechselnde Raumbelastungen angelegt wurden und Methanproduktionsraten im Hinblick auf Substratzusammensetzung untersucht wurden. Die Gasproduktion folgte jeder Änderung der Raumbelastung mit einer sehr kurzen zeitlichen Verzögerung. Der Prozess zeichnet sich durch eine gute Voraussagbarkeit innerhalb der Grenzen des stabilen Betriebs der anaeroben Vergärung aus. Das Einbeziehen dreier Reaktoren in die Analyse unterstreicht die gute Reproduzierbarkeit und die damit einhergehenden Vorhersagemöglichkeiten eines solchen Ansatzes des Anlagenbetriebs. Die Machbarkeit und Reproduzierarkeit konnten demnach demonstriert werden. Die Substratzusammensetzung scheint in diesem Zusammenhang keinen signifikanten Einfluss auf die Gasbildungskinetik zu haben, da der Geschwindigkeitslimitierende Faktor die acetoklastische Methanogenese ist. Die Prozesskontrolle der Hydrolyse sollte daher in Richtung des Gesamtabbaugrades optimiert werden, anstatt gezielte Intermediate zu produzieren. Die Produktion von gelösten mikrobiellen Produkten (SMP) und extrazellulären polymeren Substanzen (EPS) ist möglicherweise beeinflusst oder sogar verstärkt durch sich ständig ändernde Raumbelastung und Verweilzeit. Weitergehende Untersuchungen sind nötig um die Grenzen der sicheren Betriebsweise festzulegen, da in der Praxis durchaus extremere Szenarios denkbar sind.Ein Schlüsselelement um starke Schwankungen in der Gaszusammensetzung zu vermeiden ist die Pufferkapazität der Flüssigkeiten im Fermenter. Insgesamt eignen sich Anaerobfilter oder Reaktoren mit entsprechendem Biomasserückhalt besser als klassische volldurchmischte Systeme, da sie eine deutlich bessere Wiederstandsfähigkeit gegenüber Stoßbelastungen aufzeigen und sich schnell von Überlastungen erholen. Sie sind daher besonders für die bedarfsgerechte Biogasproduktion geeignet

Steam Explosion Conditions Highly Influence the Biogas Yield of Rice Straw

Straws are agricultural residues that can be used to produce biomethane by anaerobic digestion. The methane yield of rice straw is lower than other straws. Steam explosion was investigated as a pretreatment to increase methane production. Pretreatment conditions with varying reaction times (12–30 min) and maximum temperatures (162–240 °C) were applied. The pretreated material was characterized for its composition and thermal and morphological properties. When the steam explosion was performed with a moderate severity parameter of S0 = 4.1 min, the methane yield was increased by 32% compared to untreated rice straw. This study shows that a harsher pretreatment at S0 > 4.3 min causes a drastic reduction of methane yield because inert condensation products are formed from hemicellulose

Modeling and Simulation of Biogas Production in Full Scale with Time Series Analysis

Future biogas plants must be able to produce biogas according to demand, which requires proactive feeding management. Therefore, the simulation of biogas production depending on the substrate supply is assumed. Most simulation models are based on the complex Anaerobic Digestion Model No. 1 (ADM1). The ADM1 includes a large number of parameters for all biochemical and physicochemical process steps, which have to be carefully adjusted to represent the conditions of a respective full-scale biogas plant. Due to a deficiency of reliable measurement technology and process monitoring, nearly none of these parameters are available for full-scale plants. The present research investigation shows a simulation model, which is based on the principle of time series analysis and uses only historical data of biogas formation and solid substrate supply, without differentiation of individual substrates. The results of an extensive evaluation of the model over 366 simulations with 48-h horizon show a mean absolute percentage error (MAPE) of 14–18%. The evaluation is based on two different digesters and demonstrated that the model is self-learning and automatically adaptable to the respective application, independent of the substrate’s composition

Modeling and Simulation of Biogas Production in Full Scale with Time Series Analysis

Future biogas plants must be able to produce biogas according to demand, which requires proactive feeding management. Therefore, the simulation of biogas production depending on the substrate supply is assumed. Most simulation models are based on the complex Anaerobic Digestion Model No. 1 (ADM1). The ADM1 includes a large number of parameters for all biochemical and physicochemical process steps, which have to be carefully adjusted to represent the conditions of a respective full-scale biogas plant. Due to a deficiency of reliable measurement technology and process monitoring, nearly none of these parameters are available for full-scale plants. The present research investigation shows a simulation model, which is based on the principle of time series analysis and uses only historical data of biogas formation and solid substrate supply, without differentiation of individual substrates. The results of an extensive evaluation of the model over 366 simulations with 48-h horizon show a mean absolute percentage error (MAPE) of 14–18%. The evaluation is based on two different digesters and demonstrated that the model is self-learning and automatically adaptable to the respective application, independent of the substrate’s composition

Anaerobic Degradation of Individual Components from 5-Hydroxymethylfurfural Process-Wastewater in Continuously Operated Fixed Bed Reactors

Production of bio-based materials in biorefineries is coupled with the generation of organic-rich process-wastewater that requires further management. Anaerobic technologies can be employed as a tool for the rectification of such hazardous by-products. Therefore, 5-hydroxymethylfurfural process-wastewater and its components were investigated for their biodegradability in a continuous anaerobic process. The test components included 5-hydroxymethylfurfural, furfural, levulinic acid, and the full process-wastewater. Each component was injected individually into a continuously operating anaerobic filter at a concentration of 0.5 gCOD. On the basis of large discrepancies within the replicates for each component, we classified their degradation into the categories of “delayed”, “retarded”, and “inhibitory”. Inhibitory represented the replicates for all the test components that hampered the process. For the retarded degradation, their mean methane yield per 0.5 gCOD was between 21.31 ± 13.04 mL and 28.98 ± 25.38 mL. Delayed digestion was considered adequate for further assessments in which the order of conversion to methane according to specific methane yield for each component from highest to lowest was as follows: levulinic acid > furfural > 5-hydroxymethylfurfural > process-wastewater. Disparities and inconsistencies in the degradation of process-wastewater and its components can compromise process stability as a whole. Hence, the provision of energy with such feedstock is questionable

Model Predictive Control: Demand-Orientated, Load-Flexible, Full-Scale Biogas Production

Biogas plants have the great advantage that they produce electricity according to demand and can thus compensate for fluctuating production from weather-dependent sources such as wind power and photovoltaics. A prerequisite for flexible biogas plant operation is a suitable feeding strategy for an adjusted conversion of biomass into biogas. This research work is the first to demonstrate a practical, integrated model predictive control (MPC) for load-flexible, demand-orientated biogas production and the results show promising options for practical application on almost all full-scale biogas plants with no or only minor adjustments to the standardly existing measurement technology. Over an experimental period of 36 days, the biogas production of a full-scale plant was adjusted to the predicted electricity demand of a “real-world laboratory”. Results with a mean absolute percentage error (MAPE) of less than 20% when comparing biogas demand and production were consistently obtained

Effects of Increasing Nitrogen Content on Process Stability and Reactor Performance in Anaerobic Digestion

The aim of this study was to analyse the effect of different nitrogen increase rates in feedstock on the process stability and conversion efficiency in anaerobic digestion (AD). The research was conducted in continuously stirred tank reactors (CSTR), initially filled with two different inocula: inocula #1 with low and #2 with high nitrogen (N) concentrations. Three N feeding regimes were investigated: the “0-increase” feeding regime with a constant N amount in feeding and the regimes “0.25-increase” and “0.5-increase” where the N concentrations in feedstock were raised by 0.25 and 0.5 g·kg−1, respectively, related to fresh matter (FM) every second week. The N concentration inside the reactors increased according to the feeding regimes. The levels of inhibition (Inhibition) in specific methane yields (SMY), related to the conversion efficiency of the substrates, were quantified. At the N concentration in digestate of 10.82 ± 0.52 g·kg−1 FM measured in the reactors with inoculum #2 and “0.5-increase” feeding regime, the level of inhibition was equal to 38.99% ± 14.99%. The results show that high nitrogen increase rates in feeding regime are negatively related to the efficiency of the AD process, even if low volatile fatty acid (VFA) concentrations indicate a stable process

Mono-Digestion of 5-Hydroxymethylfurfural Process-Wastewater in Continuously Operated Anaerobic Filters: A Cascade Utilization Approach

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

Steam Explosion Conditions Highly Influence the Biogas Yield of Rice Straw

Straws are agricultural residues that can be used to produce biomethane by anaerobic digestion. The methane yield of rice straw is lower than other straws. Steam explosion was investigated as a pretreatment to increase methane production. Pretreatment conditions with varying reaction times (12–30 min) and maximum temperatures (162–240 °C) were applied. The pretreated material was characterized for its composition and thermal and morphological properties. When the steam explosion was performed with a moderate severity parameter of S0 = 4.1 min, the methane yield was increased by 32% compared to untreated rice straw. This study shows that a harsher pretreatment at S0 > 4.3 min causes a drastic reduction of methane yield because inert condensation products are formed from hemicelluloses

Two-stage anaerobic digestion : state of technology and perspective roles in future energy systems

Two-stage anaerobic digestion (TSAD) systems have been studied on a laboratory scale for about 50 years. However, they have not yet reached industrial scale despite their potential for future energy systems. This review provides an analysis of the TSAD technology, including the influence of process parameters on biomass conversion rates. The most common substrate (35.2% of the 38 selected studies) used in the analysed data was in the category of rapidly hydrolysable industrial waste with an average dry matter content of 7.24%. The highest methane content of 85% was reached when digesting food waste in a combination of two mesophilic continuously stirred tank reactors with an acidic (pH 5.5) first stage and alkaline (pH 7) second stage. Therefore, the review shows the limitations of the TSAD technology, future research directions, and the effect of integration of TSAD systems into the current strategy to reduce greenhouse gas emissions