Optimization of biochar quality and yield from tropical timber industry wastes

The conversion of timber industry waste to biochar was investigated, based on residues from local tropical hardwoods in Cameroon. Closed and flow reactors, loaded with few grams of the sample and fluxed with different inert gases, were used with the aim of converting these wood residues to a safe and efficient solid fuel that can be exploited by the local community for cooking purposes. The pyrolysis temperature had the highest impact on the process up to approximately 700 °C as biochar yields decreased from 87 to 23.5% while increasing the temperature. A significant concentration of carbon required temperatures greater than 500 °C for the biochar to become a good solid fuel, approaching the heating value of coal. The role of heating rate appeared marginal even in a broad range: 0.1 to 70 °C/min. The dwell time (in the range 0.5 to 5 h) at maximum temperature revealed to be of little influence on the charring which occurred mostly during the first few minutes of the isothermal phase. Thus, actual production time can be <1 h/batch (up to 500 °C at 10 °C/min). Stepwise heating revealed a sequence of devolatilization reactions following their activation energies. Use of CO2 as inert gas increased the char yield. The inert gas flow rate had two contrasting effects on the biochar yield, in relation to volatile components residence time and the biomass temperature. The reactivity of biochar in combustion suggests a trade-off between heating content and reactivity. Charring in a closed, pressurized reactor produced biochar of low value as fuel, with tars absorbed in the porous solid residue.