Combined flow, temperature and soot investigation in oxy-fuel biomass combustion under varying oxygen concentrations using laser-optical diagnostics

Abstract

In pulverized oxy-fuel biomass combustion, intricate interaction of complex fluid-mechanical, particle-dynamical, and chemical processes manifests even at intermediate scales. This study performs a comprehensive analysis aimed to understand the processes of flame stabilization, pollutant formation, and combustion behavior of biomass particles, while evaluating the impact of varying oxygen concentration on combustion. The investigation is conducted with a comprehensive data set, encompassing various oxy-fuel operation conditions ranging from 27vol.% to 36vol.% in O2 concentration, along a comparable air flame. These conditions are realized in an optically accessible gas-assisted solid fuel combustor designed to generate flames up to 70kWth. Advanced laser-optical diagnostics are employed to capture the combustion process. These techniques include the use of two-phase PIV/PTV for simultaneous measurements of flow and particle velocities, the application of tomographic absorption spectroscopy to elucidate three-dimensional gas temperature distributions, and the establishment of a quasi-simultaneous LII and LIF experimental setup to capture both PAH and soot occurrences. Additionally, chemiluminescence imaging of CH∗ is incorporated. The analysis extends to both single-phase and two-phase combustion, revealing a pronounced influence of oxygen concentration on both regimes. The inner recirculation zone, characteristic of a swirl flame, decreases with increasing oxygen content, coupled with an acceleration of the main flow downwards due to heightened heat release proximate to the nozzle. In two-phase combustion, a significant combustion delay is observed compared to single-phase combustion. Furthermore, the occurrences of PAHs and soot are clearly separated, and a pronounced influence of local gas composition on soot formation is evident. Comparative examination between oxy-fuel and air combustion show that an oxy-fuel condition with an oxygen concentration just below 33vol.% closely mirrors the flame characteristics of air combustion.