Structural analysis of biomass pyrolysis and oxidation using in-situ X-ray computed tomography

Abstract

Detailed measurements are performed to examine the heterogeneous processes that occur within the solid fuel during the combustion of biomass. Specifically, the pyrolysis, char oxidation, and volume shrinkage are examined using X-ray computed tomography (XCT). Simultaneous 3D in-situ measurements of solid density and gas-phase temperature are acquired at a spatial resolution of 300 μm for 1.6-cm size samples of birch, douglas fir, oak and walnut. To simultaneously measure the heterogeneous combustion processes and the gas-phase temperature, the flow is doped with Kr, a high X-ray contrast agent. By varying the molar oxygen concentration from 0% to 21%, regimes of pyrolysis, smoldering, and flaming are identified for different types of biomass and grain orientations. Results from these multidimensional XCT measurements provide new insights about the fuel volume reduction, from which shrinkage parameters are experimentally determined. The simultaneous acquisition of gas-phase temperature measurements also highlights the direct dependence of flame anchoring upon grain orientation. In addition, the char oxidation is found to be limited to a narrow region with 2 mm thickness at the smoldering surface, while the propagation of the pyrolysis front is measured up to 1 cm within the sample. For some biomass materials, significant density fluctuations are measured at the millimeter scale, indicating non-homogeneous reaction rates.