To replace fossil fuels with second generation biofuels, it is crucial that biomass material is made available. However, there is also the aspect of best application of the existing biomass. While high-grade feedstock is already used for biofuel, as well as for other prod­ucts, low-grade feedstock, such as residues from forestry, agriculture, industry and households, remains to be fully evaluated for this purpose.

A newly finished project within the f3 and Swedish Energy Agency collaborative research program Renewable transportation fuels and systems, has investigated the techno-economic potentials of more effective utilisation of biogas feedstock by using digestate as raw material for thermochemical conversion for the production of biofuels and/or biochemicals. Residues may be more difficult to process but are generally much cheaper and could, individually or in combination with other raw materials, be­come an economic feedstock for biofuel production. In addition, a wider biomass material basis would lead to the contribution to an increased biofuel production from several sectors in the society while waste issues are being solved.

It is interesting to do this kind of study for several reasons. First of all, the digestate contains an unneglectable amount of chemically bound energy that potentially can be converted into biofuels/biochemicals, and thus increase the biofuel yield from the same amount of biomass. Second, the suggested value chain could simplify and/or enable safer handling, storage, transport and use of the residue. Finally, the value chain could be motivated when the digestate is not allowed or is less suitable to spread as fertilizer on farmland because of too high concentrations of toxic metals, hormones and/or different pathogens.

The project, titled Knowledge synthesis on new value chains by thermochemical conversion of digestate for increased biofuel production in Sweden has been lead by Anna-Karin Jannasch, RISE, with participants from RISE and Lund University. To analyse and illustrate the effect of utilized feedstock type (digestate from WWTP or co-digestion plants), scale of operation, transportation distance and local conditions such as available heat sources, a number of case studies were carried out, where the situation as of today at the different locations was used as references. As one of the conclusions from the project, the economic analysis of variable costs and revenues shows that there is a significant room for investments for all three investigated thermochemical techniques: pyrolysis, gasification and HTL/G.

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