Increasing the reactivity of waste biochars during their co-gasification with carbon dioxide using catalysts and bio-oils

Highlights: • Biochar mixtures were gasified under carbon dioxide up to 1000 °C. • Pore structure, mineral matter, catalysts and bio-oil blending were examined. • Kinetics of uncatalyzed or catalyzed raw fuels and their mixtures were determined. • Conversion ranged between 73% and 100% and cold gas efficiency from 40% to 69%. • Alkali species lowered the temperature, improved the yield of CO and reduced activation energy An energy crop, an agricultural waste biochar and their blends with an industrial waste biochar were gasified under carbon dioxide up to 1000 °C, employing the thermal analysis technique. Thermal behavior, reactivity, conversion and cold gas efficiency were determined. The influence of pore structure, chemical functional groups, inherent mineral matter, external catalysts and bio-oil blending was examined. A simple independent parallel reactions model was developed to provide kinetics of uncatalyzed or catalyzed raw fuels and their mixtures. The bulk of the process occurred between 800 °C and 950 °C, conversion ranged between 73% and 100%, whereas cold gas efficiency from 40% to 69%. Gasification reactivity was highly correlated to the porous structure of char and was ranked as sawdust>helianthus>grape husks. Blending of biochars with the industrial waste increased the conversion and the cold gas efficiency of CO produced. Impregnation of biochars with alkali species lowered the reaction temperature, improved the yield of CO up to about 35% and reduced activation energy values from 201 to 273 kJ/mol to 153–180 kJ/mol. K₂CO₃ and CaO catalysts presented a better overall performance. Catalyst loading of 10% wt was enough for improving the process. Also, the effect of bio-oil addition to biochars was positive, increasing the conversion to 91–92%.