13th International Colloquium Fuels - September 2021 101 Production of renewable biomethane using bioresources and hydrogen Kati Görsch DBFZ Deutsches Biomasseforschungszentrum gemeinnützige GmbH, Leipzig, Germany kati.goersch@dbfz.de Maria Braune DBFZ Deutsches Biomasseforschungszentrum gemeinnützige GmbH, Leipzig, Germany Philipp Knötig DBFZ Deutsches Biomasseforschungszentrum gemeinnützige GmbH, Leipzig, Germany Summary The Pilot-SBG project (SBG = synthetic biogas) is a research and demonstration project for the production of biomethane, which is to be used as a climate-friendly and renewable fuel in the transport sector. Therefore, the DBFZ is planing and implementing the construction and operation of a pilot plant on a technical scale. In the pilot plant previously unused biogenic residues, by-products, and wastes are to be converted to biomethane. The plant concept combines anaerobic digestion with innovative preand post-treatment processes such as hydrothermal processes, digestate processing, and catalytic methanation. The main objectives are to increase the biomethane yield using the carbon dioxide from the biogas process and externally supplied hydrogen as well as to enhance the product portfolio of the whole plant by separating valuable by-products. The project is completed by accompanying investigations, including feedstock potential analysis, assessments as well as a comprehensive feasibility study for a plant on a commercial scale. This paper addresses the overall research and demonstration project, plant specifications, and challenges. 1. Introduction In 2018, the Greenhouse gas (GHG) emissions in the transport sector in Germany were higher than in 1990 (164 versus 162 Mio. t carbon dioxide (CO 2 ) equivalent [1]). The target set by the former climate protection law required a reduction of 52 to 55 Mio. t CO 2 equivalents and climate neutrality until 2050. In the new climate protection law an additional reduction of 10 Mio. t CO 2 equivalents until 2030. Climate neutrality is to be achieved in 2045. Concerning different scenarios [2-5], these targets for the contribution of the transport sector are very ambitious and will not be achieved by substituting fossil fuels solely. In summary, the transport sector is faced with enormous challenges in enabling sustainable and climate-friendly mobility. In addition to reducing final energy consumption by avoiding and shifting traffic and increasing the efficiency of drives used in the modes of transport, the use of renewable energy sources is of great importance. Biomethane may be a climate-friendly solution as an advanced fuel for the transport sector, especially. The requirement for advanced biomethane is that the raw materials have to comply with RED II [6], annex IX Part A, for the production of biogas/ biomethane from raw materials that often can only be processed with advanced technologies. 2. Research and demonstration project Pilot-SBG In the Pilot-SBG project previously unused biogenic residues, by-products, and wastes, that are listed in the RED II, are to be converted to biomethane used as compressed natural gas (CNG). The DBFZ is therefore planing and implementing the construction and operation of a pilot plant on a technical scale, in which technologies established at the DBFZ will be used in one process chain. The activities concerning the pilot plant (preliminary tests, commissioning, operation) are flanked by accompanying investigations and assessments as well as a comprehensive feasibility studies for a commercial-scale plant. 2.1 Plant concept For such a complex pilot plant, extensive planning was necessary. A central process of the pilot plant is the anae- 102 13th International Colloquium Fuels - September 2021 Production of renewable biomethane using bioresources and hydrogen robic digestion to biogas. To improve the digestibility, the organic waste and residues are first pretreated depending on their composition, e.g. shredded, mixed, and hydrothermal processed (HTP) (see figure 1). Biogas consists primarily of methane and carbon dioxide (CO 2 ). With the aim to increase the overall methane yield, the biogenic CO 2 is converted to methane This is done via catalytic methanation of the biogas with externally supplied hydrogen (H 2 ) (in a conventional process, CO 2 is separated from biogas and fed to the methanation reactor this separation does not take place here). After the catalytic methanation step, the product gas is expected to contain only traces of CO 2 and H 2 . The aim is to separate residual CO 2 with a downstream sodium hydroxide scrubber to get almost pure methane with a hydrogen content within the legal requirements for use as a fuel in the transport sector. After anaerobic digestion, the digestate is further treated in different processes depending on the feedstock and desired product. The solid/ liquid separation is realised with a screw press separator, a decanter centrifuge resp. a chamber filter press. Liquid phase filtrations are done with ultrafiltration and reverse osmosis modules. For solid product generation hydrothermal carbonization and thermal drying steps may be applied. The liquid phase is used to recycle process water into upstream process steps for adjusting the dry matter (dm) content and separating valuable by-products. These products expand the plant’s product portfolio may include HTC char from waste-based feedstocks, solid and liquid fertilisers, as well as digestate that can be returned to the field. The concept will also consider the extent to which a demand side management can be applied to the corresponding processes. Therefore, data for a plant in a commercial scale will be generated on a pilot scale basis. The pilot plant is followed by a tank facility so that the produced methane can be used as CNG in a companyown vehicle. A scheme of the process chain respectively a three-dimensional model of the plant can be found in fig. 1 respectively fig. 2. Fig.1: Process steps of the Pilot-SBG plant, exemplary for the rural scenario. 2.2 Operating scenarios of the pilot plant Biogenic residues and waste materials are used in the pilot plant, for which two scenarios were defined. One is a rural scenario with straw and cattle manure as feedstocks, and the other is an urban scenario with organic waste and green cuttings as raw materials. The scale of the pilot plant was selected with the background of being able to present the complex process chain, to demonstrate the feasibility of the implementation in principle and to apply scalable equipment as well as industrial standard equipment. As an example, the approximate mass balance for the rural scenario is given here: 7.3 t a -1 of cattle manure with 8% dry matter (% dm ) are fermented with 2.0 t a -1 of cereal straw with 85% dm . With the addition of 0.1 t a -1 (under standard conditions) hydrogen, 0.6 t a -1 (under standard conditions) methane is obtained. About 1.7 t a -1 of solid fertiliser and minerals with 65% dm and 3.2 t a -1 liquid fertiliser with 4% dm can be separated as by-products. 2.3 Scale-Up and technical challenges Feedstock. The scope of the anaerobic digestion is an optimised biogas yield. By using optimal aligned feedstocks as mixtures the anaerobic digestion process is expected to be stabilised. Lignocellulosic feedstocks such as straw and green cuttings are challenging materials for this conversion step. Therefore, the influence of hydrothermal pre-treatment of the lignocellulosic substrates on the anaerobic digestion is investigated. From a technical point of view, the hydrothermal pre-treatment of straw could also lead to a prevention of floating layers, which has to be verified in practice. Straw is a very challenging feedstock. It has a very low bulk density when loosened, which leads to difficulties in efficient processing. Other challenges include the high viscosity of the straw-manure feedstock mixture and the habit of straws to wrap around agitators, which may lead to poor agitatability or the tendency of this mixture to block the outlet of the reactor. Therefore, it was neces- 13th International Colloquium Fuels - September 2021 103 Production of renewable biomethane using bioresources and hydrogen sary to adapt espescially the design and configuration of the HTP reactor. The developed HTP reactor has a volume of 750 l, which is unique in the German research community. It is filled via a screw conveyor and emptied via a bottom outlet. An acceptable heating rate is realised via direct steam insertion and heat transfer through the reactor shell using a thermostate operating with heat transfer oil. This measures considerably shorten the duration of the complete hydrothermal process (from filling to emptying). Sufficient mixing of this difficult feedstock is realised via an adjustment of the agitator design. The new design ensures that the complex reactor content is permanently lifted, so that compaction on the reactor bottom and walls is avoided. High viscosities are tackled with a very high agitator torque. In addition, the rotational direction of the agitator can be reversed. In that case the agitator works as a conveyer which supports the full depletion of the reactor. Another point for ensuring routine operation is the availability of feedstocks in the required quantity. The DBFZ is not a bulk buyer, so sometimes it is not even worth setting up a contract, but continuous production depends on regular deliveries of the feedstocks. Plant dimensions and scale-up. The pilot plant will be built on a technical scale. Due to the relatively low throughput, it was necessary to make compromises in some parts, e.g. with regard to - upscaling strategies from laboratory to pilot scale: Which degree of effort is reasonable? - technical options available on the market: What is possible in this scale? - the degree of automation required for parts of such a complex plant: What is necessary with regard to plant safety and ensuring weekend operation? For example, in a commercial scale it would not be efficient to shred straw to such a degree as in the laboratory scale. This will always be a decision between optimisation of digestibility and therefore increase of the biogas yield on the one hand and time and costs for shredding on the other hand.But at pilot scale and even more on a laboratory scale, shredding and representative sampling of the feedstock material is much more important. Since anaerobic digestion also depends, among other things, on the available surface area of the feedstock materials, the use of larger straw particles at pilot scale (compared to laboratory scale) may have an impact on microbial conversion. That needs to be observed in the routine operation of the pilot plant. For ensuring routine operation it is necessary to feed the system also on weekends from automated supply tanks. Furthermore, storage facilities must be created and it must be ensured that the feedstocks do not degrade during storage (in the refridgeration unit and in the supply tank) or change in such a way that the organic components, which are necessary for the anaerobic process, have already degraded by the time they are used. This anaerobic degradation during storage had to be included in safety considerations - therefore all storage vessels are equipped with safety valves and the storage unit is monitored with gas quality sensors. After the plant has been commissioned and completion of the test phase, it is to go into continuous operation. The entire process chain is to be represented over planned campaigns, each with a duration of about six months. For smooth continuous operation, plant parameters should not be altered frequently. This requires preliminary tests for the individual modules. One example is the composition of the biogas from the anaerobic digestion step, which depends on the used feedstock. In the case of methanation, this has an impact on the upstream biogas purification, the selection of the catalyst, the operating points, the stability of the whole synthesis process, and the conversion rates. Therefore, suitable biogas plants had to be found, possible gas sampling points identified, and gas sampling carried out. After these analyses, it was technically possible to mix a model biogas with the most important corresponding trace gases (hydrogen sulphide, siloxanes) for further experiments concerning the methanation step. For the investigation of separation processes, digestate is needed in an appropriate quantity and quality, which is based on similar feedstocks as those to be used in the scenarios. Unfortunately, digestate is a very complex mixture containing e.g. extracellular polymeric substances and a wide mixture of organic substances which is not easy to reproduce. For solid-liquid separation, for example, effective flocculants and flocculation aids need to be identified and the challenges of that are not to be underestimated. Fig. 2: Three-dimensional model of the pilot plant with (1) possibilities for storage and cooling, (2) equipment for shredding and mixing, (3) hydrothermal treatment, (4) anaerobic digestion in two reactor lines (continuous stirred tank reactor line and plug flow reactor line), (5) biogas purification and methanation, (6) process water recycling and separation of valuable by-products, (7) biomethane storage tanks, and (8) a CNG tank facility (not visible). 104 13th International Colloquium Fuels - September 2021 Production of renewable biomethane using bioresources and hydrogen Coupling of technologies. In the pilot plant, the interfaces between individual process steps must also be coordinated with regard to the composition of the substances, so that the required quality is ensured for the subsequent process step. Therefore, gases are monitored and measures for the separation of disturbing components are inplemented: Adding oxygen to the biogas after the anaerobic digestion to separate hydrogen sulphide as well as water removal are both measures with the aim to protect the downstream methanation process. It is also important to coordinate the material flow quantities from the individual modules. Therefore, buffer vessels are planned at some interfaces. In addition, coupling this number of processes each with specific safety needs demands a superior safety concept. Now small malfunctions in one process can have an impact throughout the entire pilot plant. A comprehensive hazard and operability analysis (HAZOP) had to be performed in order to adress and counteract safety issues. As a result, a combination of technological measures (gas detection, air ventilation, flair system etc.) and organisational measures (personal protection equipment, standard operating procedures etc.) were incorporated in the engineering documentation. Quality management of the product also plays a major role since it will be used to safely fuel a CNG vehicle of the DBFZ car pool. One parameter in the present fuel standard is the hydrogen content with a maximum of 2% (based on the volume). Therefore, technological adaptions had to be made after methanation, e.g. water separation or hydrogen detection, which ensure to generate a product that can be refuelled. In the case of inadequate product mixtures, these gas flow is lead to the flair system for safe combustion. Last but not least, it was necessary to construct a new building, due to the fact, that there was no suitable available fulfilling the requirements. The new facility is adapted to the safety requirements, the scale of the pilot plant and the used materials flows. It has an own wastewater system, so that process water can be treated and recycled if required. 3. Accompanying investigations 3.1 Feedstock potentials The suitable raw materials were identified in a feedstock potential analysis and evaluated with their available quantities. The theoretical biomass potential of digestible biomasses in Germany amounted to 145.2 Mio. tonnes of dry matter (t dm ) in 2015. Of this, an amount of 38.5 Mio. t dm has already been used as technical potential of which, in turn, almost the entire quantitiy can be optimised in its use. The amount of 22.9 Mio. t dm of digestible biomass was still mobilisable in Germany in 2015. [7] The ranking of biomasses in terms of their mobilisability potential results in the largest quantities for cereal straw, cattle manure and green waste. The quantities of these three biomasses account for nearly 19 Mio. t dm (average value, annual fluctuations possible due to climatic conditions), i.e. 80% of the considered digestible biomasses that can be mobilised [8]. All data can be accessed via the DBFZ resource database, a publicly available online tool for biomass monitoring [9]. The mentioned biomass quantities result in an energy quantity of 97 to 279 petajoules (PJ) of biomethane from mobilisable biogenic digestible residues. This amount of energy can be converted into the so-called substitution potential, which describes the proportion of energy currently supplied by fossil fuels that can be replaced by the product biomethane. Based on the energy demand of 2015, this amount of energy corresponds to a total substitution potential of 4% to 11% in the transport sector. The impact for individual transportation options that are difficult to electrify is, e.g., 13% to 40% for CNG and LNG trucks, respectively, and more than 87% for the bunkering of seagoing vessels. [8]. 3.2 Assessments and feasibility study The entire project is accompanied by comprehensive assessments (technical, economic, and ecological) with the focus on a possible commercial scale.Therefore, a potential plant dimension had to be defined first. For this purpose, a SWOT analysis was carried out for three different possible plant dimensions. From the compilation and evaluation of the SWOT analysis, the consideration of a plant size/ dimension of 1,300 m³ h -1 (under standard conditions) methane in biogas (before the methanation step) appeared to be reasonable. After catalytic methanation, a biomethane output of almost 2,400 m³ h -1 (under standard conditions) can be achieved. A feasibility study for the construction of a commercial plant is carried out in order to consider location issues, market conditions, and stakeholder interests. With regard to the questions concerning the stakeholders, it is important to determine which conditions must be fulfilled, so that they are open to participate, especially the raw material suppliers must be willing to provide the substrates for a commercial plant concept. Conducting interviews in selected regions which are particularly important in terms of raw material availability is intended to identify factors that may be conducive or inhibiting with regard to implementation. Based on the analysis of the raw material availability and the visualisation of the spatial distribution, priority districts/ regions will be identified using a geographic information system. Interviews are conducted to trace current raw material utilisation routes, among other things, in order to identify, where the Pilot-SBG concept can provide an advantage compared to the existing route. 13th International Colloquium Fuels - September 2021 105 Production of renewable biomethane using bioresources and hydrogen 4. Concluding remarks The Pilot-SBG project is aligned with current political and legal requirements and strategies. By using available biogenic residues and waste materials and recycling nutrients as well as minerals, the link to a circular economy is established. The German National Hydrogen Strategy is addressed by integrating green hydrogen to increase the methane yield through methanation of the CO 2 contained in biogas. In the planned pilot plant hydrogen will be provided via gas cylinders for reasons of economic efficiency. But in a later commercial scale a so called SynBioPTX approach is adopted with hydrogen provided by renewable sources. During economic assessment this is one of the considered aspects. Furthermore, the objectives of the Climate Protection Plan are targeted, when biomethane is provided as a climate-friendly renewable fuel for non-electrifiable transport sectors. 5. Acknowledgement The research and demonstration project Pilot-SBG (https: / / www.dbfz.de/ en/ projects/ pilot-sbg/ ) is financed by the German Federal Ministry of Transport and Digital Infrastructure within the framework of the Mobility and Fuels Strategy. References [1] Klimaschutzbericht 2019. BMU (ed). Date: 18.08.2020. https: / / www.bmu.de/ fileadmin/ Daten_ BMU/ Download_PDF/ Klimaschutz/ klimaschutzbericht_2019_kabinettsfassung_bf.pdf. [2] Öko-Institut e. V., Fraunhofer Institut für System- und Innovationsforschung (eds.), 2015. Klimaschutzszenario 2050. Studie im Auftrag des BMU. https: / / www.oeko.de/ oekodoc/ 2451/ 2015-608-de. pdf. [3] T. Bründlinger, J. E. König, O. Frank, D. Gründig, C. Jugel, P. Kraft, O. Krieger, S. Mischinger, P. Prein, H. Seidl, S. Siegemund, C. Stolte, M. Teichmann, J. Willke, M. Wolke. Deutsche Energie- Agentur GmbH (ed.). dena-Leitstudie 2018. Integrierte Energiewende - Impulse für die Gestaltung des Energiesystems bis 2050. https: / / www.dena.de/ fileadmin/ dena/ Dokumente/ Pdf/ 9261_dena-Leitstudie_Integrierte_Energiewende_lang.pdf. [4] P. Gerbert, P. Herhold, J. Burchardt, S. Schönberger, F. Rechenmacher, A. Kirchner, A. Kemmler, M., Wünsch. BDI (ed.). Klimapfade für Deutschland 2018. https: / / bdi.eu/ publikation/ news/ klimapfade-fuer-deutschland/ . [5] Verkehr in Zahlen 2019/ 2020. https: / / www.bmvi. de/ SharedDocs/ DE/ Publikationen/ G/ verkehr-inzahlen-2019-pdf.pdf? __blob=publicationFile. [6] Richtlinie (EU) 2018/ 2001 des Europäischen Parlaments und des Rates vom 11. Dezember 2018 zur Förderung der Nutzung von Energie aus erneuerbaren Quellen. [7] A. Brosowski, T. Krause, U. Mantau, B. Mahro, A. Noke, F. Richter, T. Raussen, R. Bischof, T. Hering, C. Blanke, P. Müller, D. Thrän, D.: How to measure the impact of biogenic residues, wastes and by-products. Biomass and Bioenergy, 127 (2019) dx.doi. org/ 10.1016/ j.biombioe.2019.105275. [8] A. Brosowski, T. Krause, U. Mantau, B. Mahro, A. Noke, F. Richter, T. Raussen, R. Bischof, T. Hering, C. Blanke, P. Müller, D. Thrän. Schlussbericht “Arbeitsgruppe Biomassereststoffmonitoring”, 5.06.2019. https: / / www.fnr-server.de/ ftp/ pdf/ berichte/ 22019215.pdf. [9] https: / / webapp.dbfz.de/ (always available)