Extending the Operation of Existing Biogas Plants: Which Follow-Up Concepts and Plants Will Prevail?

Abstract

For the existing biogas plants (BGP) in Germany, the period of the public support scheme begins to end in 2021. From a technical point of view, essential components have an operational life span of more than 20 years and allow for an extended operation. However, a profitable extension would require suitable follow-up concepts and depends on the underlying plant-specific setup, the regional conditions, as well as the regulatory and economic framework. Based on an expert evaluation, four promising follow-up concepts were identified in a multistage process consisting of expert interviews, workshops, and an online survey. These follow-up concepts are “Basic flexibilization,” “Substrate change,” “Seasonal flexibilization,” and “Biomethane upgrading.” They were assessed with a plant-specific biogas repowering model for a heterogeneous data set of 2,508 BGPs and were compared in three scenario frameworks to derive robust development paths. To capture the heterogeneity of the existing BGPs in Germany, the model was developed further regarding regional parameters such as power output, substrate mix, and emission factors. Across all the scenarios, “Seasonal flexibilization” proves to be the most promising follow-up concept for more than 50% of the BGPs. This is followed by “Substrate change,” which is particularly suitable for larger BGPs with high shares of energy crops and no heat utilization. Biomethane upgrading is usually the second choice compared to participation in extended public support schemes for electricity production. However, it is the only concept that is profitable under current market conditions due to the high CO2-quota prices in the German fuel sector. The development pathways also show a significant potential to increase the net GHG reduction, which on BPG average can be nearly doubled. Our approach shows that the interplay of the heterogeneous BGP structure, the applied economic decision variable, and the nonuniform framework conditions in different energy markets is of crucial importance when assessing similar biogas markets, identifying robust follow-up pathways, or designing framework policies.

Trade-offs in the substrate mix optimization of biogas plants considering biomethane market conditions

“…The current study focuses on module 3, which contains the substrate mix optimization model. In its previous version [25], the model could only optimize cost while satisfying an individual market's technical and regulatory requirements. Details of the objective function, constraints, and related inputs of the substrate mix optimization are given in this section to show the new extensions regarding the market revenues and restrictions.…”

“…We use an existing model framework for the assessment of biogas repowering to apply the substrate mix optimization model to a wide range of BGPs and derive robust results. Initially described in [44], the model has been further developed [25] and [45] concerning the integration of additional repowering concepts, regions, and extended scenario framework capabilities. It is referred to as BGP-RepoMod in this study.…”

“…However, it might not be the best option from an environmental standpoint [24]. [25] High CO2 price levels strongly favor the profitability of biomethane or other green gases such as hydrogen. Such high CO2 price levels in all sectors are necessary to meet climate targets [26] and can induce a high price range (100 €/MWhHHV) for green gases and an overall market share of 50% [27].…”

“…In the evaluated repowering concept, biomethane is upgraded via membrane technology (as applied and described in [25]). It is assumed that the total raw gas capacity is upgraded and the CHP unit is replaced by a two-stage membrane upgrading unit.…”

“…The characteristic parameters are the rated power output, the substrate mix in the form of manure share, and the digester volume. Each parameter refers to the reference state, i.e., the output of BGP-RepoMod modules 1 and 2, with the two former having already been used previously [25]. The digester volume has been added because it is a determining parameter in calculating several constraints, such as the OLR.…”