A comprehensive review of the life cycle assessment of microalgal biochar applications

Microalgae are considered a renewable and promising biomass source. One of the practical energy streams from microalgae biomass is the production of biochar. Microalgal biochar can be produced through pyrolysis, hydrothermal processing, and torrefaction. Microalgal biochar is complementary for the co-firing of conventional solid biofuels as it enhances their combustion properties while significantly reducing the energy consumption. Other notable applications of microalgal biochar are found in carbon capture and sequestration, soil amendment, wastewater treatment, and energy storage. Out of these applications of microalgal biochar, energy storage has been shown to have limited studies. To assess the environmental impact of microalgal biochar through various processes and utilization, life cycle assessment (LCA) is performed. This study reviews the LCA of microalgal biochar through various thermochemical processes, numerous applications, and its disposal. Multiple processes in LCA have been identified to have a significant environmental impact, which are called hotspots. The results show that drying and the biochar conversion processes are identified as process hotspots, attributing to their high energy requirement from their high-temperature gradients relative to the other processes. The LCA studies of different microalgal biochar reveal significant differences in the goal and scope, functional unit, and system boundaries of these studies. This variation yields a wide range of values for the global warming potential impact from −1.54 kg CO₂-eq to 1.36 × 10³ kg CO₂-eq. The challenges encountered in reviewing LCA of microalgal biochar studies show the use of significantly different assumptions, system boundaries, and life cycle inventory.