Renewable gas in a Swedish context

Sammanfattning: In order to increase the use of sustainable energy in Sweden, it is necessary to substitute fossil energy carriers with renewable counterparts. Synthetic natural gas (SNG) produced from renewable resources via biomass gasification and methanation could replace a proportion of the natural gas used in Sweden. In order to facilitate the implementation and expansion of SNG production and use in Sweden, development is necessary at multiple levels. In this thesis I have dentified and investigated possible improvements for three different levels: national distribution, industrial production and specific plant improvements. There is a clear advantage in using the existing Swedish gas grid for the transportation and distribution of renewable gas. However, in order to inject renewable gas into the gas grid, strategies are required for handling gases of mixed qualities. An industry-wide task group was formed to develop guidelines for mixed quality gas for the Swedish gas grid, and Lund University and E.ON were assigned the task of drawing upon experience from other European countries to identify suitable strategies. Two strategies were identified: the flow-weighted heating value method and the state reconstruction tool. As a result of the work performed by the task group, the Swedish gas industry standard has been updated to include these two methods. By integrating biomass gasification and the SNG production process with an existing host plant, it is possible to reduce the investment and operating costs, compared to stand-alone production, while simultaneously exploiting synergistic effects by exchanging material and energy streams with the host plant. The majority of the integrated systems studied showed a system efficiency equal to those of stand alone plants. The estimated SNG production cost for integrated cases is sufficiently low to compete with the cost of natural gas to household consumers, but not with the cost to commercial customers. It is imperative that SNG can be produced free from impurities in order to be approved for injection into the grid. Different aspects of gas cleaning technologies for tar and ammonia removal were studied. First, a proof-of-concept regenerative reverse flow reactor was constructed for thermal cracking of tars. This method showed the ability to crack tar compounds and has the potential to be a highly efficient process step in a biomass gasification gas-cleaning train. Secondly, the possibility of regenerating the spent scrubber liquid used for tar removal in gasification pants, via centrifugation, was studied in order to improve the process economy. The results showed that centrifugation is a suitable method for removing heavy insoluble tars from the scrubber liquid, thus extending its lifetime considerably. Lastly, the possibility of removing ammonia by reaction with nitrous oxides over a vanadium-based or zeolite catalyst was investigated. Neither of these catalysts was deemed suitable due to either poor conversion or deactivation of the catalyst over time. From the results of these studies, it was concluded that the current outlook for Swedish SNG production by biomass gasification is not very promising. As natural gas in Sweden is used mainly as a fuel for industrial heat and power production, producing renewable SNG from biomass would appear to be a wasteful detour compared to the direct combustion of the biomass. This is especially true as the production of SNG is currently not sufficiently efficient for SNG to be produced at a cost that is competitive with that of natural gas. Transforming biomass into a useful vehicle fuel would be a far better use of renewable resources. However, clear regulations regarding the end use of renewable SNG would then have to be developed.