PhD example by Dr. Michael Schlüter
Optimisation of the methane yield in the heterogeneously catalysed methanation at reduced temperatures and pressures by targeted equilibrium shift
With almost 25 %, the use of biomass (status quo 2016) accounts the second largest share regarding the generation of electricity from renewable energies in Germany, right after wind power and before solar and hydropower. In addition to the direct generation of heat, biomass is used as a raw material for the synthesis of solid, liquid and gaseous secondary energy carriers. These are able to replace their fossil equivalents and to reduce the emission of greenhouse gases (GHG) [Kaltschmitt 2009]. Especially, the synthesis of of a biogenic substitute for natural gas from fossil sources, so-called Bio-SNGs (SNG – synthetic natural gas/synthetic natural gas), is a promising way to achieve this. Investigations have shown that by substituting fossil natural gas with its biogenic equivalent, taking into account the different production processes, a reduction in GHG emissions of more than 80 % can be achieved [EU 2009] [Pucker 2012] [Müller-Langer 2015]. A further advantage of the area-wide methane synthesis from biomass for the production of Bio-SNG is the increasing independence from natural gas imports due to the use of regional resources [Rönsch 2011]. Against the background of the energy system transformation, this synthesis pathway is again attracting more and more attention nowadays.
For the thermochemical generation of artificial natural gas, the use of lignocel lulosic biomass species, in particular wood and straw, is necessary. Due to their complex cell structure, they cannot be directly converted into biogas by biochem ical fermentation [Kaltschmitt 2009] [Müller-Langer 2015]. Initially, the biomass passes through one or more several pretreatment steps (e. g. crushing, drying), depending on the raw mate rial used. Subsequently, it is thermochemically converted into synthesis gas by means of sub-stoichiometric addition of an oxygen-containing gasification agent. [Kaltschmitt 2009]. The subsequent methanation is the central chemical transformation of the process. The Purified synthesis gas is transferred into a methane-rich gas by means of a catalyst, usually consisting of nickel (Ni) on a support of aluminium oxide (Al2O3), at increased temperatures and under pressure. In order to achieve natural gas quality and thus be able to feed it into the existing natural gas network [DVGW 2013], the raw product from methanation still has to be processed. This is done, for exam ple, by drying and separating by-products or the admixture of other gases (e. g. propane) to increase the calorific value [Müller-Langer 2015].
For the thermochemical process, the total energy efficiency is as follows of up to 65 % [Kopyscinski 2010]. However, the step from application-oriented research to the comprehensive production of Bio-SNG has so far failed due to the high costs and the current, comparatively low price for fossil natural gas (as of 2017). More recent studies have shown that border-crossing prices for fossil natural gas reach from 6.25 to 17.00 €ct kWh-1 depending on the raw material used and the size of the SNG production plant [Rönsch 2009] [Rönsch 2012] [Aranda 2014] [Rönsch 2014], whereas the prices in Germany were significantly lower with 1.81 €ct kWh-1 at the beginning of 2017 [BAFA 2017]. Currently, there is no plant producing Bio-SNG on a commercial scale. All in all, the commercial production of Bio-SNG is confronted with numerous problems. In order to be able to carry out this synthesis economically, it is necessary to increase the economic competitiveness of Bio-SNG. To achieve this, its production costs must be significantly reduced.
The investigations focussed on methanation as a central partial reaction of the SNG production process for the conversion of the gaseous raw materials from the upstream gasification to the methane-rich raw product. Conventional sys tems for the production of SNG or Bio-SNG operate in the field of methanation at temperatures that are in some cases significantly above 300 °C, and pres sures above 20 bar [Kopyscinski 2010] [Rönsch 2016] [Sculptor 2016]. It is to be assumed that the investment and operating costs for methanation decline due to reduced process parameters (temperature and pressure). Since methanation itself accounts for about 10–15 % of the investment costs of a Bio-SNG plant [Heyne 2014] [Rönsch 2014], a reduction in methanation costs would also mean a reduction in the costs of the entire process. The aim of the research work was to ensure that, even under these reduced conditions (temperature ≤ 300 °C, pressure ≤ 5 bar), the reaction with a commercial nickel catalyst delivers maximum methane yields.