Biochar is the solid residue of biomass pyrolysis, i.e. a pyrogenic carbonaceous material resulting from the thermochemical conversion of biomass at elevated temperatures between 400-1200 °C in an oxygen-free or oxygen-limited atmosphere.
Its carbon sequestration potential is based on impeding the natural decay of biomass by conversion into a recalcitrant carbon-rich solid. As carbon stored in biomass is of atmospheric origin, stabilisation of this carbon prevents its release back into the atmosphere and biochar is therefore considered carbon-negative.
Today’s interest in biochar stems from the discovery of fertile soil patches in the Amazon Basin (Terra Preta de Indio), created by the incorporation of charcoal several millennia ago. Based on these ancient soil amendments, biochar was identified as a promising way to sequester carbon in soil over thousands of years while simultaneously improving soil fertility.
The production process of biochar represents an efficient biomass conversion option, transforming biomass into renewable energy (bio-oil and syngas) and a solid product (biochar).
While the produced energy can reduce demands for fossil fuel the main advantage of biomass pyrolysis is the retention of most of the feedstock carbon in the solid recalcitrant material, i.e. biochar, resulting in an average global warming potential of – 0.9 kg CO2-eq kg-1 of biochar. Unlike most other carbon-negative emission technologies, such as bioenergy with carbon capture and storage (BECCS) or direct air capture (DACCS), biochar acts as a stable carbon sink which can be further utilised without diminishing its carbon sequestration potential. A wide range of properties such as a high surface area, cation exchange capacity and water holding capacity promote the use of biochar in several commercial applications such as agriculture, as an additive in cement and concrete production, in bioplastics, batteries, or even as animal feed.
Since its creation in 2009, the UK Biochar Research Centre at the University of Edinburgh is focused on all aspects of biochar, from production technologies and feedstock selection to the discovery and development of novel applications, and modelling of its carbon sequestration potential.
Figure 1: Pyrolysis pilot plant at the UK Biochar Research Centre.
The biochar production technology at Ibero Massa Florestal uses a slow pyrolysis process, which consists of three phases:
Phase 1 – Wood dehydration – occurs inside the reactor, already inside the furnace.
Phase 2 – Oxidation/thermal degradation of the wood – occurs inside the reactor, caused by the heat from the gases released from the wood, which is burned inside the furnace.
Phase 3 – Pyrolysis, caused by the isothermal energy of the wood, is generated inside the reactor. The isothermal energy of the wood will continue the process, without any other external heat source.
Figures 2 and 3: Biochar production process at Ibero Massa Florestal.