Pyrolysis of switchgrass in an auger reactor for biochar production: A greenhouse gas and energy impacts assessment

A life cycle approach was used to assess greenhouse gas (GHG) emissions and energy balances of switchgrass pyrolysis in an auger reactor for biochar production, with bio-oil and syngas as co-products. The system boundaries included the cultivation of switchgrass on marginal lands, harvesting, transport, conditioning, pyrolysis, the amendment of biochar in soil to sequester carbon (C) and to reduce N2O emissions, and the valorisation of bio-oil and syngas as energy sources. Two pyrolysis scenarios were evaluated. Scenario A involves a lower pyrolysis temperature and a shorter solid residence in the reactor as compared to scenario B. A negative GHG emissions balance of −2110 and −2561 kg CO2e Mg−1 biochar was obtained for scenarios A and B, respectively. Biochar C sequestration contributed the most to the reduction of GHG emissions in scenario B due to the high C content and stability in biochar. However, scenario B resulted in a higher energy consumption (13,563 MJ Mg−1 biochar) than scenario A (2925 MJ Mg−1 biochar) due to a higher energy consumption of the pyrolysis unit. These results confirm that pyrolysis of switchgrass for biochar and bio-oil production can be a negative emission technology, but pyrolysis operating parameters should be selected carefully.