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. ... mehrReaction 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.