Uncertainty in climate change impacts in Danish straw and digestate pyrolysis systems

With increasing political ambitions for biomass pyrolysis as a climate change mitigation tool in the Danish agricultural sector, a deeper understanding of the range of expected climate impacts related to deployment of the technology is necessary. Straw and manure-based digestate are currently the main feedstocks in focus for large scale Danish biomass pyrolysis. Existing climate impacts assessments of straw and digestate biomass pyrolysis are limited and no extensive assessment of uncertainty in these systems has been performed. In this study, a climate change impact assessment of straw and digestate pyrolysis in a Danish context is performed with focus on uncertainty ranges in results. Results show that systems with biomass pyrolysis potentially provide climate benefits comparted to reference management of straw and digestate in the ranges -1118 (-972 to -1264) and - 42 (-10 to -76) kg CO₂e with GWP100 per ton wet feedstock, respectively. The approach applied to assess uncertainty in this study allows for detailed understanding of the driving mechanisms of climate impacts in the modelled systems. While the relative uncertainty in straw pyrolysis net impact is low, relative digestate pyrolysis net impact uncertainty is substantial. The digestate dewatering process and storage methane emissions are the main sources of uncertainty and are the primary areas with potential for optimization. The results indicate a substantial improvement potential for full system optimization. Efficient harvesting of dry matter and carbon for pyrolysis in dewatering is critical while ensuring that benefits are not offset by increased storage emissions from the resulting liquid fraction.