title: Advanced optical laser diagnostics for oxy-fuel biomass combustion
Creators: name: Schneider, Henrik
Publisher: name: UNSPECIFIED
Year: 2024

description: Biomass energy with carbon capture and storage (BECCS) is a key technology for removing CO2 from the atmosphere and is an essential part of climate scenarios to prevent large temperature increases caused by the anthropogenic climate change. Combustion of pulverized biomass in an oxy-fuel atmosphere consisting mainly of CO2, O2, and H2O is a promising approach for subsequent efficient carbon capture and storage (CCS). The changed combustion atmosphere under oxy-fuel conditions results in significant changes in combustion behavior and flame stability. Although oxy-fuel combustion has been widely studied in the recent decades, the fundamentals of the complex fluid-mechanical, particle-dynamical and chemical processes and their interactions in the combustion of pulverized solid fuels are not fully understood.

In this thesis, a solid fuel combustor (SFC) has been investigated that operates gas-assisted solid fuel flames in the power range up to 70 kWth. The SFC stabilizes a swirl flame and exhibits important characteristics of modern combustors while providing large optical access. The focus of several studies is on the near-burner region where the flame is stabilized and several important combustion processes such as particle heating, volatile release, volatile combustion, onset of char combustion, and particle-fluid mixing take place.

Multi-parameter studies using advanced optical laser diagnostics were performed to improve our understanding of pulverized solid fuel combustion under oxy-fuel conditions. The advanced optical laser diagnostics applied allow in-situ measurements of flow and scalar fields with high spatial and temporal resolution while being non-invasive or minimally invasive.

Flow dynamics and particle temperatures in the near-burner region are analyzed to support our understanding of flame stabilization, burnout behavior, and burnout rate. The flow field, fluid-particle mixing, particle trajectories, and gas and particle residence times are investigated using particle image velocimetry (PIV) and high-speed particle tracking velocimetry (PTV). Particle temperatures at different stages of the combustion are evaluated using two-color pyrometry.

The influence of oxygen concentration and solid fuel type on the combustion process of fuel particles and the effects on flame stabilization and pollutant formation are analyzed. Flame stabilization is investigated using information on CH* chemiluminescence, gas flow velocities, particle velocities, and gas temperatures determined by CH* chemiluminescence imaging, a two-phase PIV/PTV approach, and tomographic absorption spectroscopy (TAS). The combustion process of biomass particles and the formation of pollutants are analyzed using qualitative soot and polycyclic aromatic hydrocarbon (PAH) measurement data, obtained using a quasi-simultaneous laser-induced fluorescence/laser-induced incandescence (LIF/LII) configuration, in combination with information on the flow field and gas temperature distribution.

A novel O2-CO2-coherent anti-Stokes Raman spectroscopy (CARS) approach is developed, experimentally validated, and successfully applied to pulverized oxy-fuel biomass combustion to study the thermochemical state at different flame positions within the combustion process. These investigations are the first CARS measurements in particle-laden reactive flows beyond demonstration. Using the O2-CO2-CARS setup, radial gas temperature and mole fraction profiles of O2 and CO2 are determined and analyzed for different oxygen concentrations in oxy-fuel conditions.

The results presented provide novel insights into the solid fuel combustion process under oxy-fuel atmospheres that go beyond the state of research and provide a unique comprehensive data set for validation of numerical simulations.