In this paper a new approach for 3D flame structure diagnostics using tomographic laser-induced fluorescence (Tomo-LIF) of the OH radical was evaluated. The approach combined volumetric illumination with a multi-camera detection system of eight views. Single-shot measurements were performed in a methane/air premixed laminar flame and in a non-premixed turbulent methane jet flame. 3D OH fluorescence distributions in the flames were reconstructed using the simultaneous multiplicative algebraic reconstruction technique. The tomographic measurements were compared and validated against results of OH-PLIF in the laminar flame. The effects of the experimental setup of the detection system and the size of the volumetric illumination on the quality of the tomographic reconstructions were evaluated. Results revealed that the Tomo-LIF is suitable for volumetric reconstruction of flame structures with acceptable spatial resolution and uncertainty. It was found that the number of views and their angular orientation have a strong influence on the quality and accuracy of the tomographic reconstruction while the illumination volume thickness influences mainly the spatial resolution.

Chemical energy carriers are crucial for addressing challenges that arise from time lag, large distances, and temporal fluctuations in renewable energy production, which lead to unbalanced energy production and demand. The thermochemical utilization of chemical energy carriers such as solid fuels must be decarbonized to achieve a climate-neutral circular economy as soon as possible, while remaining important for reliable electricity generation and stable economics. To accomplish this, extensive fundamental research is required to understand the underlying chemical and physical processes that can potentially be realized at an industrial scale. This paper reviews optical diagnostics used for particle-level combustion studies for clean power generation applications. The focus is on particle-resolved optical experiments for oxy-fuel coal combustion, biomass combustion, and utilization of iron in regenerative oxidation–reduction schemes. Previous studies are categorized by fuel and reactor types, investigated parameters, and experimental methodology. Phenomenological aspects of the multi-stage combustion process at the particle level are outlined using examples of bituminous coal and iron particle burning in hot gas. A selection of experimental studies is highlighted, with a particular methodological focus on measuring quantities at the particle level. These representative examples address relevant parameters, including particle number density, particle size and shape, surface temperature, ignition and combustion time, gas flame structure, gas temperature and species, nanoparticle formation, gas velocity, and particle dynamics. Finally, issues and problems that require further effort to improve diagnostics for solid fuel combustion studies are discussed.