Achieving carbon credits through biomass torrefaction and hydrothermal carbonization: A review

Different additives can capture carbon dioxide (CO₂) during biomass torrefaction. Biochar and hydrochar can potentially sequester CO₂. A comparative review of CO₂ sequestration via biochar and hydrochar and its relationship with carbon credits is inadequate. This research aims to explore CO₂ sequestration during biomass torrefaction and hydrothermal carbonization (HTC) with additives (magnesium hydroxide: Mg(OH)₂, and calcium oxide: CaO), conduct a comparative analysis of CO₂ sequestration by biochar and hydrochar, analyze the energy increment in both, and determine the prospects of carbon credit and carbon rights related to these processes. During torrefaction, Mg(OH)₂ captures up to 62 % of CO₂, while no additives are needed in hydrochar production since CO₂ is not released. CO₂ absorption by biochar and hydrochar ranges from 0.03 to 3.5 mmol g⁻¹. Torrefied biochar exhibits varying carbon contents between 50 and 70 wt%, while the ranges in hydrochar are 48–70 wt%, resembling lignite. The higher heating values (HHV) of biochar, hydrochar, and lignite are also comparable, nearly 25 MJ kg⁻¹. Biochar-based electricity production's global warming potential (GWP) is lower than coal-based production, while hydrochar-based production has a higher GWP. Hydrochar production is less efficient due to its drying and activation methods. Biomass torrefaction and HTC can earn carbon credits by reducing emissions and are tied to carbon rights through enhanced carbon sequestration on biomass-producing land. Future research directions in carbon credits and carbon rights for torrefied biochar and HTC-derived hydrochar can focus on optimizing production processes, refining conversion technologies, and maximizing carbon sequestration.