Influence of pressure on the gasification kinetics of two high-temperature beech wood chars with CO$_{2}$, H$_{2}$O and its mixture

This paper presents experimental data and modeling approaches to describe the influence of CO$_{2}$ and H$_{2}$O partial pressure as well as absolute pressure on the gasification kinetics of two different beech wood chars. The chars were produced at 1400 °C (P1400) and 1600 °C (P1600) at high-heating rates and short residence times in a drop-tube reactor. The gasification experiments were conducted in a single-particle reactor with forced flow-through conditions reducing diffusional effects to a minimum. The interpretation of the experimentally determined reaction rates during gasification with CO$_{2}$, H$_{2}$O and its mixture is based on the char properties (graphitization, ash dispersion and morphology) presented in a previous publication. During gasification with CO$_{2}$, P1600 shows higher reactivity as compared to P1400 for all CO$_{2}$ partial pressures and temperatures applied. The higher reactivity of P1600 during CO$_{2}$ gasification may be explained by a CaO film on the char surface catalyzing the char-CO$_{2}$ gasification reaction. On the other hand, P1400 shows higher reactivity towards H$_{2}$O which may be evoked by the lower graphitization degree and higher specific surface area. Reaction kinetic modeling for single atmosphere gasification was successfully carried out using a power law approach. The Langmuir-Hinshelwood model, however, only gave good results where a possible saturation of the char surface at high pressure was observed. Increasing the CO$_{2}$ partial pressure during gasification in mixed CO$_{2}$/H$_{2}$O atmospheres leads to higher reactivity for both chars. The reaction rate r$_{mix}$ can be expressed by addition of the single atmosphere reaction rates in the low pressure area suggesting a separate active site mechanism. Catalytic activity of CaO increases the P1600 reactivity distinctively for lower H$_{2}$O and CO$_{2}$ partial pressures. For higher H$_{2}$O and CO$_{2}$ partial pressures, P1600 reactivity stagnates due to lower specific surface area and higher graphitization degree. Here, a common active sites mechanism can be assumed