Fast Pyrolysis of Low-Grade Biomasses in Fluidized Bed Reactors─Impact of Mineral Matter

Low-grade biomasses are potential feedstocks for the fast pyrolysis process to produce bio-oil for renewable fuels and chemicals, yet their high alkali and alkaline earth metal (AAEM) contents pose challenges. This study investigates the impact of AAEMs (K, Na, Ca, Mg) on fast pyrolysis product yields and bio-oil composition using four low-grade biomasses─reed, forest residues, sunflower husks, and wheat straw─pine wood being a low-AAEM reference. Both untreated and demineralized (mildly leached with HNO3 to remove AAEMs) samples of these biomasses were pyrolyzed in a bench-scale bubbling fluidized bed (BFB) reactor operated at 0.6 g/min and 350–500 °C. These results were compared with those from larger bench-scale (1 kg/h) BFB and pilot-scale (20 kg/h) circulating fluidized bed units. Demineralization increased organic liquid yields from 40–45 to 50–60 wt % of biomass, while reducing biochar, CO, and CO2 yields; pine wood showed minimal response due to its low AAEM content. Organic liquid yields were relatively insensitive to temperature between 350 and 500 °C. However, the pyrolysis temperature and AAEMs showed a synergistic effect: higher temperatures and AAEM concentrations improved bio-oil fuel quality through deoxygenation, lowering oxygen content by 15–35%, whereas demineralization alone increased oxygen content by 5–25%. With increasing individual biomass AAEM levels, the oxygen contents decreased to a minimum and then leveled off, especially at higher temperatures, while the carbon contents showed the opposite trend, indicating saturation of AAEM-catalyzed reactions beyond critical thresholds. Consistent with previous findings, demineralization increased the fraction of anhydrosugars and decreased that of light volatiles in the bio-oils. These findings highlight the importance of assessing the trade-offs between untreated and demineralized biomass as well as between high (500 °C) and low (350 °C) temperatures when designing and optimizing fast pyrolysis processes for low-grade biomasses. Differences in product yields across the three reactor scales were attributed to design and operational factors.