We describe the ignition and control of a flash fire at elevated pressure (≈1 MPa) within a packed bed of biomass. The fire moves upward through the bed, against the downward flow of air, triggering the transformation of biomass into gas at elevated pressure, and charcoal with fixedcarbon yields that reach the thermochemical equilibrium "limit" in <30 min of reaction time. In a typical experiment 65% of the moist feedstock mass is converted into a combustible gas at 1 MPa, and 40% of the dry mass remains as charcoal, retaining as much as 66% of the energy content of the feedstock. Biomass feedstocks included woods (Leucaena and oak) and agricultural byproducts (macadamia nut shells and corncob). In the case of corncob, the fixed-carbon yield attained the theoretical limit (28%), and the bed was fully carbonized after 20 min. Some of these findings were unexpected. For example, thermochemical equilibrium calculations predicted a negligible influence of pressure on charcoal yields, but we observed considerable improvements at modest elevated pressures (e.g., 1 MPa). Furthermore, the fixed-carbon yields for oak wood and corncob in this work exceeded the yields obtained at similar elevated pressures in the absence of air within an electrically heated autoclave. Some qualitative kinetic explanations are offered to explain these unexpected results.