Demand-driven biogas production in anaerobic filters
Introduction
As a result of greenhouse gas (GHG) emission reduction policies, the share of renewable energy (RE) in energy production has seen a considerable growth in the last decade. Within the EU-28, the share had increased by 84.4% between 2003 and 2013. Electricity generated from renewable energy sources contributed to more than one quarter (25.4%) of the EU-28’s gross electricity consumption. The growth in electricity generated from renewable energy sources during the period 2003–2013 largely reflects an expansion in three renewable energy sources, namely, wind turbines, solar power and biomass [1].
Scenarios developed by the EU predict a significant share of 55–97% renewable energy supply in gross energy production by the year 2050 [2]. As outlined by Steinke et al. [3], the increase will mainly be provided by solar and wind, thereby posing challenges for grid stability and the need to counter balance the intermittent power supply by these sources.
Biogas technology not only reduces GHG emissions when compared to fossil fuel electricity production [4], but can offer grid services by making electricity production of the combined heat and power units (CHP) more flexible. With enhanced flexibility in energy production the biogas technology could play a significant role in smart grid applications such as the models presented in [5], [6], [7], and helps to increase the revenues for future virtual power plants or similar applications. Shifting the electricity output into times of higher demand can be achieved by several adjustments to biogas technology. One frequently discussed option is to adapt the biogas production to the electricity demand, thereby opening new marketing options for plant operators [8], [9], [10], [11].
Existing biogas plants in Germany are currently increasing gas storage volumes and expanding CHP capacity to achieve higher flexibility [12]. Current reaction times are approximately one day and weekend shutdowns are not the usual practice at the moment. Improvements in flexibility could be achieved by altering the feeding regime to adapt gas production within several hours. Mauky et al. [13] not only demonstrated the general feasibility but also determined savings in biogas storage capacity of 42–45% and found a more stable overall performance for continuously stirred tank reactors (CSTR) when operated in a flexible feeding regime.
Innovative plant designs such as two-staged anaerobic digestion (AD) with separated hydrolysis/acidogenesis and acetogenesis/methanogenesis could possibly adapt to imbalances in the electricity grid in even shorter time frames. Two-staged AD systems commonly consist of a CSTR or leach-bed reactor to produce a liquid that is rich in volatile fatty acids and alcohols. The acidification reactor (AR) is coupled to a methanation reactor (MR) where the soluble organics can be quickly degraded to yield biogas. The second stage is often put into practice as an anaerobic filter (AF), upflow sludge blanket reactor (UASB) or more sophisticated versions thereof [14], [15], [16], [17], [18], [19]. The advantages of two-staged setups found in literature are:
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Providing optimal conditions for the microorganism consortia taking part in the respective step of AD [20], [21],
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thereby increasing turnover rates and enabling a reduction in total reactor volume [21],
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disposal of sludge from the AR without loss of slowly growing methanogens [21],
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fractionating the produced biogas [22] and enabling high methane concentrations in the MR and
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higher overall methane yields for specific substrates [23].
Further advantages of two-staged AD over single-staged AD, potentially improving demand-driven gas production, are that the hydrolysis as rate limiting step [24] is decoupled from methanogenesis and thereby enables a shift of the gas production into times of higher demand. It could furthermore be shown that selectively producing certain intermediates in the AR is possible by controlling pH and redox-potential [14], [25], [26], which may be used for a targeted production of a hydrolysate with a high methane production rate. It is also known that attached biomass reactors have a generally higher process stability and ruggedness towards shock loads and can operate at much higher organic loading rates (OLR) than CSTRs [27]. Moreover, maintaining dormancy in the MR is also possible [18].
The scope of this research paper was to demonstrate the feasibility of operating an anaerobic filter for highly flexible gas production. The replicability of this type of operation was examined to demonstrate its predictability. Based on gas production profiles, a measure of responsiveness was introduced to determine whether and how rapidly adaptations to the production process are possible. Furthermore, the influence of substrate composition was tested and finally a carbon balance was derived to evaluate operation performance.
Section snippets
Experimental setup
The experiments were executed in three anaerobic filters of identical construction, each comprising three compartments: the inlet, the packed bed and the headspace. The packed bed consisted of type HX-9 (Christian Stöhr GmbH & Co.KG) with a surface area of 940 m2 m−3 and a porosity of 85%, left the packed bed with a net void volume of 2070 ± 15 mL. The total internal free volume was 2801 ± 16 mL, measured before inoculation. A schematic representation is depicted in Fig. 1.
The reactors were operated
Flexible gas production
The first OLR-mode of operation, i.e. phases ‘demand.A’ and ‘demand.B’ are depicted in Fig. 2, showing individual methane as well as total biogas production. Notably, the gas production followed the changes in OLR immediately and every increase or decrease in OLR found its expression in the gas production profile. The two instances per day, when the OLRCOD had increased to its highest value of ≈20 g L−1 d−1, are worth taking a closer look. Although the applied OLR was the same for both instances,
Conclusion
In this experiment, the suitability of anaerobic filters for demand-driven gas production was examined. The results indicate that anaerobic filters are well suited for highly flexible gas production. All three reactors showed degradation degrees over 90% with no significant accumulation of intermediates. The replicability and therefore its predictability were evaluated which led to the finding that an excellent degree of replication can be achieved under the presented conditions. Even by
Acknowledgments
This study originates from the project “Methanoquant” (FKZ 03SF0423B) and is supported by the German Federal Ministry of Education and Research (BMBF) and the Projekt-träger Jülich (PtJ).
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