Upgrading of carbon-based reductants from biomass pyrolysis under pressure

• Charcoal properties need to be upgraded for use as a substitute for top furnace coke.
• Only the pyrolysis heating rate significant affected charcoal mechanical properties.
• Eucalyptus Camaldulensis and Urophylla wood always produced hardly friable charcoal.
• Increasing pressure increased charcoal yield.
• Charcoal reactivity more sensitive to temperature than to heating rate and pressure

The main problems involved in substituting charcoal for top charged coke in blast furnaces are missing physical-mechanical properties (friability, density, and crushing strength) and the too high reactivity of charcoal, meaning substitution is only possible in mini blast furnaces. Although such furnaces make it possible to reduce greenhouse gas emissions, their use for this purpose remains marginal and more research is needed to upgrade the properties of the charcoal so it can be used in conventional blast furnaces. Recent studies have shown that increasing pressure can increase gravimetric yields, fixed carbon content and considerably shorten the carbonization cycle. The purpose of this study was to measure the impact of pyrolysis parameters – especially pressure – on the properties of charcoal, especially crushing strength, friability and CO2 gasification reactivity. The experimental study was carried out using Eucalyptus Urophylla and Eucalyptus Camadulensis wood with two carbonization temperatures (350 and 600 °C), two relative working pressure (0.2 and 0.6 MPa) and two heating rates (1 and 5 K min−1). Six response variables were analyzed using a random factorial design: charcoal yield (Ychar), fixed carbon content (Cf), bulk density (D), crushing strength (Rm), friability (F) and the reactivity (R) of charcoal. Except for the friability of charcoal, all the other properties were well correlated with the carbonization parameter. In our conditions, the best “steel” quality charcoal (i.e. the compromise required to maximize charcoal yield, fixed carbon content, bulk density and crushing strength, while minimizing the reactivity of the resulting charcoal) was obtained at a temperature above 496 °C, a pressure above 0.6 MPa and low heating rate of around 1.1 K min−1.