Biochar as a carbon dioxide removal solution: An assessment of carbon stability and carbon dioxide removal potential in Sweden [Master thesis]

By Alessandro Corbo

KTH, School of Architecture and the Built Environment (ABE), Sustainable development, Environmental science and Engineering.

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Biochar is increasingly gaining momentum in the context of climate change mitigation and its production in Sweden could potentially become a large-scale system. Carbon stability in biochar is a crucial factor to assess its the carbon sequestration potential. Currently specific methodologies to assess biochar carbon site-specific stability are missing. This work aims at filling in part this knowledge gap assessing stability for Sweden specific soil conditions. Moreover, this work aims at assessing biomass feedstock availability for biochar production from a system perspective and aims at estimating biochar production and carbon dioxide removal potentials in Sweden. Preliminary carbon stability specific thresholds are provided for soils at 10°C temperature and, thus, representative for Sweden conditions. Carbon dioxide removal functions are obtained for different feedstock categories (woody, herbaceous, biosolids and animal waste) dependent on pyrolysis conditions (Highest Treatment Temperature), and conditions for maximum carbon removal are assessed. The need for future analysis in order to validate the presented results is highlighted. Future work should focus on collecting new experimental results of biochar mineralisation based on the requirements presented in this work. An opportunity mapping for biochar production system is provided, focusing on some aspects of the interaction of the former with existing systems (agricultural, energy production and waste management). From the results of the opportunity mapping, an inventory of the available feedstock for biochar production is presented including woody residues, sewage sludge, manure, garden waste and straw. From the available feedstocks, biochar production and carbon dioxide removal potentials are estimated to range respectively between 0.9 and 1.7 million tbiochar/year and between 2 and 4.2 million tons CO2 sequestered per year (in a 100 years perspective). In terms of carbon dioxide removal potential, biochar production can significantly contribute to the goals set by Sweden in terms of climate change mitigation and emission offsetting for 2030 and 2045, potentially covering all the measures needed from carbon sinks from forest and land. It was found that the most significant contribution derives from the availability of woody residues in Sweden, whose analysis should be prioritised for future assessment of feasibility of biochar large scale production.