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We study the role of solar heat in decarbonization of a Nordic district heating (DH) network, where most of the annual heat demand is satisfied with bioenergy. We use actual data from a Finnish municipality to create a dynamic model of the heating system with Apros® simulation software. With the help of modelling, we examine various decarbonization scenarios for the existing heating system, using different combinations solar thermal collectors, thermal energy storage (TES) and limitations on how and when solar heat can access the system. According to results, zero emissions during the summer can be achieved with annual solar share of 13.2% and at 44 €/MWh levelized cost of heat (LCoH), if the integration is supported by TES and a careful planning of solar heat integration. Our results show that a simple approach of pursuing for a maximal solar share does not necessarily lead to a reduction in carbon emission or in LCoH. In fact, aiming at higher solar shares of 15–25% in our case system, actually increase greenhouse gas emissions compared to the base case. This highlights the importance of focusing on emissions reductions instead of simple addition of renewable energy when DH utilities plan for solar heat investments.

Reforming is a key enabling technology for the production of tar and hydrocarbon free synthesis gas from biomass. In this work, two different reforming concepts, steam and autothermal reforming, were studied for cleaning of biomass-derived gasification gas. Long-term laboratory scale experiments (around 500 h) were carried out with two model biomass gasification gas compositions with low and medium hydrocarbon loads. The experiments were made using nickel and precious metal catalysts at atmospheric pressure and at temperatures around 900–950 °C. The deactivation of the catalysts was followed. Gas with low hydrocarbon content could be steam reformed with nickel and precious metal catalyst. Both autothermal and steam reforming modes were studied for gas with medium hydrocarbon content. In steam reforming mode, the catalysts deactivated more than in autothermal mode. Based on the experiments H 2 O/C REF molar ratio above 4 and O/C REF molar ratio above 8 are recommended. A concept assessment was carried out to examine plant level impacts of the reforming approaches to synthesis gas production. The results showed that the choice of reforming concept has only limited impact to the overall efficiency of synthetic biofuel production.

The biomass gasification process is modelled by utilising constrained thermodynamic equilibrium. The formation of char, tar, ammonia and light hydrocarbons and related syngas composition were described by extending the conventional chemical system with additional immaterial constraints and by defining process-dependent values for these constraints. Six different model structures were evaluated from global thermodynamic equilibrium to fully constrained local equilibrium. When models were validated against gasification setups, it was not necessary to fully constrain the system, as sufficient results were obtained by implementing constraints for char, tar, ammonia, CH4 formation as well as for the amount of carbon in light hydrocarbons. The method was shown to be versatile when it was validated against other gasification setups: by altering the models defining the constraints a new gasification conditions could be simulated. A clear benefit of the proposed method is that the gasification process can be resolved as a restricted partial equilibrium with a single calculation step. Another benefit is that chemical reactions, gasification enthalpy and the states of the system are estimated concurrently.

This study examines the impact of biomass drying on the overall efficiency of a Biomass-to-Liquids (BTL) process co-producing Fischer-Tropsch (FT) syncrude and heat. The proposed BTL process is based on steam-blown dual fluidised-bed gasification of biomass, simplified gas clean-up and FT synthesis. Biomass drying was found to have a significant effect on both the FT product yield and the overall thermal efficiency of the BTL plant. In the considered concept, FT off-gases are used as supplementary feedstock in the oxidiser, alongside with char, to satisfy the heat demand in gasification. Drying of the biomass feedstock leads to reduced need of water evaporation during gasification. This in turn reduces the demand for off-gases in the oxidiser, and allows to produce more syncrude (and less off-gases) in the FT unit. The simulations indicated that more extensive drying could result in a significant increase in FT syncrude yield: the thermal efficiency of FT syncryde production increased from 47.7% to 54.6% as the moisture mass fraction of the biomass feed was reduced from 30% to 8%. The highest overall thermal efficiencies of 85–88% were obtained with a two-stage drying concept that combines a belt dryer and an indirectly-heated steam dryer.