Environmental sustainability of negative emissions technologies: A review

Negative emissions technologies (NETs) are expected to play a significant role in mitigating climate change. However, there are also concerns that a large scale deployment of NETs may cause various environmental impacts due to the use of land, water and energy resources. A number of studies have assessed the environmental performance of various NETs; however, a comprehensive review comparing a range of different NETs is not available in the literature. To address this research gap, this paper compares life cycle assessment (LCA) studies of the following options for which the data were available in the literature: bioenergy with carbon capture and storage (BECCS), biochar incorporation into soil, afforestation and reforestation, soil carbon sequestration, building with biomass, direct air carbon capture and storage (DACCS), enhanced weathering and mineral carbonation. It is evident from this review that these technologies can have net negative life cycle GHG emissions, ranging from −603 kg CO2 eq./t CO2 removed for building with biomass to −1173 kg CO2 eq./t CO2 removed for biochar incorporation into soil. However, the estimates of GHG removal potentials vary widely among the studies for each technology as well as among the NETs owing to technological differences, methodological choices and differing assumptions. For example, the net global warming potential (GWP) of biochar varies among the reviewed studies between a net positive impact of 1710 to a net negative GWP of 3300 kg CO2 eq./t CO2 removed, depending upon the feedstock, pyrolysis technology and the assumptions for credits for co-products and co-benefits. Overall, biochar used as soil amendment has the lowest GWP per tonne of CO2 removed, followed by soil carbon sequestration, while building with biomass ranks last. The review also reveals that the removal of CO2 by these technologies could lead to a significant increase in other environmental impacts. Especially, the use of energy in non-bio NETs (DACCS, enhanced weathering and mineral carbonation) leads to relatively high fossil depletion, acidification and human toxicity. These impacts can be reduced if the energy demand of NETs is met by renewables instead of fossil fuels. The paper also identifies several methodological issues and challenges in conducting LCA of NETs and provides recommendations to address them.