Consequential Life Cycle Assessment of Biochar: Comparing Different Biochar Production and Application Pathways

Abstract

Aim and approach used: Biochar, the solid residue of biomass pyrolysis, is considered an important option for climate change mitigation because of the high content of stable carbon. In addition, biochar can be made from residual biomass and thereby help manage these waste streams. Even though biochar is often spoken of as one product, it is very diverse and versatile. It is diverse because many different types of biochar can be produced by changing the feedstock and production conditions. Its versatility refers to the fact that biochar can be used in many applications. Considering this diversity and versatility, this research investigates two types of biochar, one wood-based and one manure-based, in two applications. The first application is the direct field application of biochar, the most common and most studied application. The second application is a cascading use of biochar, where it is first used in anaerobic digestion and, afterward – mixed in the digestate – applied to land. This research aims to determine whether using the biomass resources for biochar production has a positive environmental impact compared to the current use, and if yes, to identify in which scenario (combination of biochar and application) the biggest environmental gain can be achieved. A consequential life cycle assessment (LCA) is used to this end. The LCA encompasses six systems: two reference systems (one for each feedstock) and four pyrolysis systems (two for each feedstock, assuming direct or cascading use of biochar). Because the data are very uncertain, extensive sensitivity and uncertainty analyses are performed.

Scientific innovation and relevance: LCAs of biochar exist, but there are severe research and knowledge gaps, some of which are bridged in the current study. To our knowledge, this is the first LCA of biochar that compares feedstocks and applications. Regarding the application, most biochar LCAs deal with biochar as a soil amendment, whereas other applications are ignored. In this research, we compare the use of biochar as a soil amendment to its use in anaerobic digestion (where after it is applied to soil). Also, unlike many previous LCAs of biochar, we do not only consider the global warming potential but also include other impact categories. Finally, even though there are consequential LCAs of biochar, their number is limited. As mentioned before, biochar is a promising means for climate change mitigation. The IPCC added it to the negative emissions technologies, which are crucial for climate scenarios that keep global warming within the limits of the Paris Agreement. Biochar is particularly interesting as it is a relatively low-cost option. However, biochar deployment is slow; with this research, we want to contribute to its acceleration. Another promise of biochar use is the opportunity for residual biomass valorization. The biochar production feedstocks investigated here are particularly relevant for the concerned region in Belgium. Still, we believe the results also stand as a more general comparison of wood-based and manure-based biochar. The investigated applications are relevant because we compare the usual application of biochar to a more novel, cascading use.

Preliminary results and conclusions: The fundamental iterations of the LCA have been conducted. Still, the models will be strengthened in the next iterations over the coming months. Elaborations on the interpretation of results and sensitivity assessment follow in parallel. Based on the preliminary results, we hypothesize that using the biomass resources to produce biochar provides an environmental gain compared to the current use. Furthermore, we hypothesize that the cascading use of biochar will provide a greater environmental gain than the direct field application. In addition, it is expected that the sensitivity and uncertainty analyses will provide new insights into the dynamics of the environmental impact of the different systems and contribute to the development of robust guidelines for decision-making related to further development and near-future deployment of the investigated pyrolysis systems.