Production of high calorific value biochars by low temperature pyrolysis of lipid wastes and lignocellulosic biomass

Abstract

Low quality waste oils and fats, with elevated levels of acidity, water and other contaminants are not appropriate for biodiesel production or other material recycling processes. Nevertheless, they can be converted to energy-dense bio-oils by pyrolysis, in an oxygen deficient atmosphere. These bio-oils may be used in combustion or upgraded to yield liquid biofuels for internal combustion engines although their chemical stability still creates some limitations during storage. This work aimed at the evaluation of biochar production by pyrolysis of mixtures of lipid wastes and pine sawdust. For this purpose, thermal conversion was performed with initial vacuum, at temperatures between 573.15 and 673.15 K and a reaction time of 60 min. Biomass incorporation varied between 0 and 38% (w/w). Biochar formation was favored by biomass incorporation and by the increase in temperature. The biochar yield varied from 1–28% and its high calorific value ranged between 24.3 to 36.6 MJ/kg. For incorporations of pine sawdust higher than 23% the carbon content and the calorific value of the biochar decreased due to a higher oxygen content in the raw materials. Highly calorific bio-oils were also produced in these conditions with yields from 24 to 83% (w/w), and high calorific values from 37.8 to 43.4 MJ/kg. These bio-oils have a carbon content higher than 75% and contain high molecular weight components. As such, they could be used in direct combustion in boilers, or as pellet additives. This approach contributes to the implementation of the renewed hierarchy for wastes as defined in the Directive 2008/98/EC namely by identifying alternatives to the deposition of the used cooking oils in landfills.