Biocarbon Emissions and Risks Assessment in Pyrolysis and Combustion

Decarbonization of society requires industry to urgently reduce carbon-based and toxic gas emissions, such as polycyclic aromatic hydrocarbons (PAH), to mitigate their environmental impact and associated health risks. Biocarbon (Biochar), due to its properties closer to those of coal and the biogenic nature of its emissions, is a renewable alternative to replace fossil fuels in industrial processes. In this context, the objective of this work is to determine gaseous emissions fate and toxicity in pyrolysis and combustion for biocarbon from solid refuse fuels (SRF, named BC1) and 2 biocarbons from wood with different H/C ratios and volatile matter (VM) content (WBC1, WBC2). The obtained results were correlated to resource characteristics and compared with standard values reported for fossil fuels. Emission factors (EF, g_gas/g_fuel) were calculated for the main permanent gases (CO, CO₂, H₂, CH₄), as well as for PAH. Biocarbons EF were directly related to their physicochemical characteristics, namely their VM content and, to a lesser extent, their inorganic composition, catalyzing or inhibiting thermochemical conversion reactions. High VM content biocarbon (WBC1, 18.5 wt%) was associated with higher EF values (i.e., 3696 g_CO2/kg_fuel). However, EF-(CH₄+H₂) values at pilot scale met EN ISO 562 standard (<4 wt%) both in pyrolysis (2.8 wt% yield for WBC1) and combustion (0.07 and 0.3 wt% for WBC2 and BC1, respectively). Although PAH released by high VM content biocarbon (WBC1) were less toxic due to their lower number of aromatic rings (<3), they presented a higher EF (46.3 mg_PAH/kg_fuel) than that reported for fossil fuels (0.002-64.0 mg_PAH/kg_fuel). As a result, the toxicity equivalent factor (TEQ), associated with the carcinogenic potential of a PAH and its yield, was higher for biocarbon (WBC1, 0.132) compared with fossil fuels (0.005-0.03). Most importantly, reduce condensable species production, and, consequently, lower EF values for permanent gases were measured (2000-2980 g_CO2/kg_fuel) in combustion, and no PAH was detected, resulting in a lower TEQ.