Mobile-manufactured biochar in mine closure, costly yet carbon-negative - A techno-economic and life cycle assessment of growing media value chains

This concept case explored a circular bioeconomy-based value chain to produce new high-quality growing media to be used in mine closures. Our specific interest was in the production of biochar from waste wood using the mobile pyrolysis unit and use of biochar as a supplement in mine closure growing media. A life cycle costing (LCC) and environmental impact assessment (LCA) was conducted for the production and value chain of four different growing media solutions based on mixtures of composted sewage sludge (COM), fly ash (ASH), peat, till, and waste-wood-based biochar (BC). The application of wood waste biochar to compost-till growing media (10 % by volume) increased the costs by 51–71 % (BC-COM-TILL, €28.60 m-3). The most affordable scenario was based on compost, ash and till (€16.76 m-3). Environmental impacts were acknowledged in the costs according to their carbon dioxide equivalent (CO2eq) emissions assessed in LCA (€52.56 CO2eq t-1). Accounting for the long-term carbon storage capabilities of the materials, the emissions were highest in peat and till based solution (74.7 kg CO2eq m-3), and lowest in the most expensive solution with biochar (-49.7 kg CO2eq m-3). The biochar-based solution turned into a carbon sink with negative CO2eq emissions. The study 1) highlighted the lower emissions of growing medias based on circular bioeconomy-based solutions compared to peat used in the traditional growing media solution; 2) showed that the climate emissions of biochar-based growing media were negative; 3) indicated that the price of biochar-based growing media was high, while suggestions were made to moderate the cost. The positive effects of biochar on the plant growth in mine areas have been documented elsewhere, but not acknowledged in this study’s environmental or economic results.