Climate change mitigation from biochar production at farm level : A time dynamic LCA study of wheat

Sammanfattning: Agricultural production in Sweden account for a large contribution of the territorial GHG emissions. System optimization, fossil fuel removal and increased circularity is therefore of great importance in order to reach the national net zero emission target by 2045. Biochar production from biomass side flows is a cost-efficient method for carbon dioxide removal which could help to reduce the climate impact of agricultural systems. This study aimed to investigate the potential for climate change mitigation by implementing biochar production from wheat straw at farm level in Sweden. A life cycle perspective was used to assess the climate change impact for production of 1 Mg wheat, with three scenarios for straw management 1) biochar production and application on fields 2) straw incorporation into soil and 3) district heat production. A time distributed LCI was used to include the time dynamics of soil processes. Climate impact was assessed using two metrics: Total GWP100 impact(static) and global surface temperature change (time dynamic). Excess thermal energy from the pyrolysis process was assumed to be used for drying of grains and heating buildings at the farm. The results showed a total GWP100 impact of 214 kg CO2-eq/Mg wheat in the scenario with biochar production, which compare to the impact of 425-429 kg CO2-eq/Mg wheat for the scenarios with conventional straw management practices. The temperature response was ca 50 % lower all throughout the analysed time period, compared to the scenarios with straw incorporation or district heat production. The largest contribution to the impact reduction was achieved from carbon sequestration from biochar application to soil amounting to 223 kg CO2-eq/Mg. A sensitivity analysis of the biochar yield (kg biochar produced per kg of dry mass feedstock) for pyrolysis of straw confirmed that biochar production was preferable over other straw management practices for lower biochar yields as well. In conclusion, utilizing straw for biochar production could have a large potential for reducing the climate impact from wheat production in Sweden. However, a combination of measures for climate change mitigation is needed to reach net zero emissions of wheat production.