Frequency-controlled microwave fields unlock low-temperature biochar gasification

Steam gasification of biomass-derived biochar is a promising route for green syngas production but is intrinsically constrained by a high activation barrier, typically requiring temperatures above 850 °C. Here, we demonstrate that variable-frequency microwave (VFM) fields can reshape the reaction energetics of the steam-carbon reaction. At a fixed microwave power of 100 W, efficient gasification of coconut-shell biochar occurs at 510-640 °C, a temperature range where conventionally heated fixed-bed reactors remain inactive. Under quasi-isothermal conditions at approximately 580 °C, distinct gasification rates are observed at different microwave frequencies, with 3705  MHz resulting in significantly higher H₂ and CO formation rates than 4530  MHz. At 3705  MHz, H₂ and CO production rates reach 35.01 and 32.75  mmol g_C^-1h^-1, with a carbon conversion of ∼ 44 % after 1  h, whereas 4530  MHz yields considerably lower rates of 25.58 and 21.78  mmol g_C^-1h^-1, respectively. Reactive molecular dynamics simulations reveal that at favorable frequencies, microwave fields promote interfacial water dipole alignment and H₂O dissociation, reducing the apparent activation energy from 138.05 to 72.85  kJ mol^-1. These results identify variable-frequency microwave fields as an innovative external-field paradigm for engineering reaction energy barriers and enabling low-temperature steam gasification.