Influence of pressure on the gasification kinetics of two high-temperature beech wood chars with CO2, H2O and its mixture

This paper presents experimental data and modeling approaches to describe the influence of CO2 and H2O partial pressure as well as absolute pressure on the gasification kinetics of two different beech wood chars. The chars were produced at 1400 degrees C (P1400) and 1600 degrees 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 CO2, H2O and its mixture is based on the char properties (graphitization, ash dispersion and morphology) presented in a previous publication. During gasification with CO2, P1600 shows higher reactivity as compared to P1400 for all CO2 partial pressures and temperatures applied. The higher reactivity of P1600 during CO2 gasification may be explained by a CaO film on the char surface catalyzing the char-CO2 gasification reaction. On the other hand, P1400 shows higher reactivity towards H2O 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 CO2 partial pressure during gasification in mixed CO2/H2O atmospheres leads to higher reactivity for both chars. The reaction rate rmix 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 H2O and CO2 partial pressures. For higher H2O and CO2 partial pressures, P1600 reactivity stagnates due to lower specific surface area and higher graphitization degree. Here, a common active sites mechanism can be assumed.

Automated summary

This study investigates the influence of CO2, H2O partial pressure, and absolute pressure on the gasification kinetics of wood chars through experimental data and modeling. The results show that chars produced under different conditions exhibit varied reactivity towards CO2 and H2O. Increasing CO2 partial pressure during gasification in mixed atmospheres leads to higher reactivity for both chars.