The combustion of solid matter is a ubiquitous phenomenon governed by multiple factors, including the properties of the solid phase, the surrounding flow field, and the reactions involved in thermochemical conversion. This thesis addresses various aspects of this complex solid-flow-chemistry interaction, driven by two primary motivations: First, the mitigation of anthropogenic climate change necessitates solid fuel combustion technologies capable of carbon capture and storage. The application of oxy-fuel combustion and the subsequent substitution of coal for biomass offers pathways to achieve negative carbon dioxide emissions. Second, fire safety for polymers can be enhanced by the development and utilization of effective flame retardants. Rigorous and reliable testing procedures are indispensable for assessing the efficacy of such additives. The core focus of this thesis revolves around the comprehensive exploration of these interconnected phenomena, facilitated by the application of minimally intrusive optical diagnostics. Through these advanced measurement techniques, the underlying physicochemical phenomena governing solid-flow-chemistry interactions are scrutinized across a spectrum of experimental scenarios. By systematically varying specific elements of the solid-flow-chemistry interaction, the impacts of individual parameters and processes are investigated, accompanied by methodological advancements needed to experimentally address these intricate phenomena. The initial segment of this work discusses aspherically shaped biomass particles dispersed within a turbulent round jet. Employing an ultra-high repetition rate fiber laser system, which operates at frequencies exceeding several 100 kHz, facilitates the measurement of key parameters characterizing turbulent fluid flows. The laser enables time-resolved flow velocimetry while achieving unparalleled temporal dynamic ranges. Furthermore, an extension of the laser system is showcased, featuring adaptable pulse-picking devices. This addition allows for remarkable flexibility in terms of repetition rates and pulsing strategies, which can be used to perform both statistical and time-resolved fluid flow measurements. In the context of a biomass-laden jet, the system is applied to perform two-phase velocimetry and simultaneous diffuse back-illumination measurements. This multi-parameter measurement setup allows for the continuous tracking of the slip velocity field and particle size evolution over time, consequently enabling the computation of the particle Reynolds number at a repetition rate of 200 kHz. The second part of this dissertation focuses on the impact of oxidizer content and diluent composition on the transition from single particle to particle group combustion under laminar conditions. Bituminous coal is studied using a multi-parameter optical diagnostics setup to determine particle number density, three-dimensional volatile flame topology, and soot formation characteristics simultaneously. An imaging simulation tool is developed and applied to the specific camera settings to assess the measurement errors associated with particle number density determination arising from the line-ofsight nature of extinction imaging methods. Decreased formation of soot and heightened reactivity of the volatile flame is observed for increasing oxygen contents, while the substitution of nitrogen with carbon dioxide in oxy-fuel conditions shows a negligible effect on combustion behavior. Finally, optical diagnostics including laser-induced fluorescence of the OH radical (OH-LIF) are utilized to explore flame retarded polypropylene. Various flame retardants, each with distinct mechanisms, are studied in micrometer-sized polymer particles and stick-shaped specimens akin to solid fuel particle investigations. Complementary thermal decomposition analyses provide insights into pyrolysis products that influence flame retardation. Remarkably, these investigations unveil a noticeable OH-LIF signal decrease for gas-phase active flame retardants that release radical scavengers. Furthermore, altered flame topology and extinction behavior during and after stick-shaped specimen interaction with a premixed methane flame are observed, showcasing the potential of the methodological approach. The presented results provide both methodological advancements in the use of optical diagnostics for reactive multi-phase flows and new insights into the phenomena that occur during the combustion of solid fuels and flame retarded polymers.