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Gebhard showed early on an interest in renewable energies. Since his youth he e.g. actively assists in the Förderverein Wind- und Wasserkraft Ostalb e.V. (an association dedicated to the promotion of renewables in the district of Ostalb). During the internship term of his bachelor's degree, he gained an insight into nuclear fusion research as an alternative energy source at the Max Planck Institute for Plasma Physics (IPP). Within his bachelor's thesis, he contributed to the development of a waste heat recovery system called SteamTrac at Voith Turbo. In line with his interests, he completed a master's degree in Sustainable Energy Technologies at the University of Southampton (UK). As part of his master's thesis in 2011, Gebhard developed and analysed the "WaveGyro" concept, an innovative idea for ocean wave energy capture. After graduating, he worked on the family farm, an ecologically cultivated agriculture and forestry.
Made possible by the doctoral scholarship from the Reiner Lemoine Stiftung, Gebhard has been working at the University of Stuttgart since 2014 on the research project as outlined followingly.
"Development of a Methanation-Assisted Gasification Processes for Improved Generation of Bio-SNG"
The Objective: This research aims to develop a novel thermochemical process which generates a synthetic natural gas (SNG), respectively methane, from ligneous biomass. A significant improvement compared to present processes is to be attained by utilising the enthalpy of reaction released in the highly exothermic methanation directly inside the endothermic gasification.
The Basis: Any thermochemical generation of Bio-SNG requires the gasification of biomass into a syngas (mainly composed of: H2, CO, CO2) and its subsequent synthesis to methane (CH4). While gasification and pyrolysis, i.e. the decomposition of long-chained organic molecules, are endothermic and thus require heat input, the exothermic methanation requires heat dissipation. Alongside with that dissipated waste heat up to 20% of the biomass's initial energy content is lost. The process to be developed in this research project endeavours to utilise the heat from the methanation to drive the pyrolysis and gasification, thereby achieving a considerably improved cold gas efficiency (calorific energy ratio of SNG to biomass).
The Challenge: The most significant difficulty in the proposed project is given by the vastly different temperature levels of these two sub-processes. Methanation commonly takes place at considerably lower temperatures than the gasification of biomass.
The methanation can be described quite accurately by reaction equations. The derived reaction equilibrium leads to conclusions about the reaction's conversion rate, respectively the reachable methane yield, in dependency of the reaction temperature. Though a first methanation (< 10 Vol.-% CH4; at 1bar) already occurs at temperatures of ca. 700°C, a complete conversion of syngas to methane (> 90 Vol.-% CH4; at 1bar) requires below 300°C.
Unlike the methanation, the complex dissociation of biomass cannot easily be described by distinct reaction equations. However, empirical findings show that a first gas release begins at above 100°C and a complete dissociation can require almost 1000°C. Common for gasification processes are 700 to 900°C. Hence the temperature at which the reaction heat of methanation is released is several hundred degrees below the temperature at which prevailing gasification processes require their heat input.
The Subject: The goal of this research is to set up, analyse and refine various functional principles that allow a methanation-assisted Bio-SNG-Process. Approaches to influence the unfavourable reaction equilibrium include e.g. high process pressures, catalytic exploitation of differing reaction kinetics or in-situ methane extraction. Other particularly promising principles are thermally staged constructions of the methanation as well as gasification reactor. By applying the thermally staged principles, the methanation's heat of reaction would accrue stepwise at different temperature levels (dependent on the equilibrium yield) and could thus be transferred into each thermally corresponding but spatial separated gasification stage.
The Methodology: Having set up functional principles, upon them concrete concepts for processes shall be elaborated. These will subsequently undergo a feasibility study. Promising concepts will be expanded up and improved by process simulation wherein experimental data and findings shall be incorporated continuously. The Institute of Combustion and Power Plant Technology (IFK) has a large variety of laboratory and measuring equipment available for this purpose, in particular pilot scale test facilities for analyses in realistic process conditions. Some critical process components may also require the development of specific experimental set-ups.
The Context: The motivation which gave rise to this research project came from the thereby intended contribution to the energy transition towards renewables ('Energiewende'). The production of Bio-SNG from woody biomass such as residual and waste materials which are not in competition with croplands has significant potential, both economically and ecologically. With the existing area-wide national gas grid Bio-SNG can spatially distributed and even seasonally stored. Bio-SNG can be used as fuel for natural gas vehicles but can as well be very efficiently converted to electric power by domestic combined heating, upcoming methane fuel cells or combined cycle gas turbines. Particularly the storage and on-demand electricity generation capabilities are essential for increasing renewable energy penetration, in particular wind power and photovoltaic that have highly fluctuating and only short-term predictable outputs. Consequently, improvements of the generation of Bio-SNG might be of significant economical, environmental and societal value.