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This study evaluates the environmental impacts associated with the conversion of garden organic waste into value-added products, namely compost or biochar, employing various processes. Three distinct scenarios are considered: composting garden organic waste followed by screening of oversized materials (CBP), pyrolysis of oversized screenings of compost into biochar AP(I), and in-situ conversion of garden organics into biochar AP(II). A comprehensive Life Cycle Assessment (LCA) was conducted using OpenLCA software and life cycle impact assessment was conducted using Recipe 2016 midpoint methodology. The environmental ramifications of each scenario were assessed, optimising transport distances in AP(II) to achieve a functional unit of one tonne of biochar produced within a cradle-to-gate system boundary. For the first time, this study offers a holistic exploration of the benefits of soil biochar application, extending its scope to climate change mitigation, incorporating the optimisation of transport distance and its influence when scaling up the technology. The results revealed that global warming was increased from 125 kgCO 2 eq during composting of garden waste to 232 kgCO 2 eq where oversized screenings of compost is converted to biochar at an off-site facility. However, direct conversion of the oversized organic waste to biochar, without composting, showed reduced global warming impact of 56 kgCO 2 eq, and is thus the most favourable scenario to limit climate impacts of this fraction of organic garden waste. However, among 18 environmental impact indicators studied, eight indicators were either not influenced or did not significantly increase by transport distance to an off-site pyrolysis facility, while the magnitude of 10 impact indicators increased with transport distance. The insights and methodologies presented in this study hold global relevance, based on an actual case study in regional Australia, offering valuable recommendations for sustainable waste management practices and establishing biochar as a carbon-neutral or carbon-negative solution. The findings contribute to existing waste management knowledge and provide guidance for accessible carbon dioxide removal and soil carbon sequestration technologies. [Display omitted] • A comprehensive LCA for valorisation of garden organics was conducted. • Direct conversion of garden organics to biochar showed least potential environmental impacts. • Biochar provides opportunities for avoided, reduced, and removed carbon from atmosphere. • Maximum transport distance has been identified for individual impact categories. [ABSTRACT FROM AUTHOR]

The urgent need for climate change mitigation and the potential threat of energy crisis have allowed the popularity of biomass for biofuels to increase. However, the approaches to assess the climate change impacts of biomass utilization are still questionable. To allow a more accurate assessment, we developed a new framework by integrating six impact components including (1) fossil fuel-derived greenhouse gas (GHG) emissions, (2) biogenic CO 2 emissions-loss, (3) biogenic CO 2 emissions-combustion, (4) emissions from land-use practice change, (5) regrowth for compensation, and (6) difference in carbon sequestration. To illustrate the performance of the framework, seven case studies, which covered the first-, second-, and third-generation biofuels, were conducted. The majority of the positive impacts were contributed to by the fossil fuel-derived emissions, biogenic CO 2 emissions, and regrowth for compensation. For the first-generation biofuels, the life cycle GHG emissions were much higher than those of energy-equivalent fossil fuels (144 and 218 kg CO 2 /GJ for Soybean-to-Biodiesel and Corn-to-Ethanol). The second- and third-generation biofuels had less accountable positive GHG emissions (−62 to 53 kg CO 2 /GJ) due to the offset of negative impacts from land-use practice change and difference in carbon sequestration. However, the negative effects are not determinist. High uncertainty from model-simulated carbon sequestration and decomposition is perceivable. High life cycle GHG emissions for the second- and third-generation biofuels are possible. Image 1 • A new framework was developed to estimate the GHG emissions from biomass utilization. • Different biomass feedstock and conversion technologies were studied as illustrations. • The GHG emissions from the first-generation biofuels can be higher than fossil fuels. • The second- and third-generation biofuels could have negative climate change impact. • High uncertainty is perceivable from model simulation for regrowth and decomposition. [ABSTRACT FROM AUTHOR]