Investigation on char conversion process
by NGUYEN Hong Nam
Diversifying the biomass feedstock for gasification is crucial to ensure a long-term development of this technology. In this context, rice husk is a promising feedstock thanks to its availability worldwide. In the first part of this thesis, we established an updated state-of-the-art of rice husk gasification, from domestics and industrial applications to research studies. Gasification systems for electricity production reflect the potential and expectations of users and developers related to this technology. The current technology brings positive socio-economic impacts, as highlighted through our case-study of rice husk gasification in Cambodia. However, negative impacts on the environment and low conversion efficiency lead to a stagnation in the technology development. In addition, actual research and development emphasized the needs of obtaining a deeper knowledge on the kinetics of rice husk during gasification. In the second part, gasification of rice husk char has been specifically investigated using a macro-thermogravimetric reactor, entirely designed and set up as part of this work. A complete parametric study of char gasification in H2O and CO2 atmospheres was performed in a wide range of operating conditions, comparable to those that exist in industrial gasifiers. In the selected operating conditions, gasification of a rice husk char particle was not limited by heat and mass transfers. At a temperature of 900°C, a gas velocity of 0.08 ms-1 and under a partial pressure of 0.2atm, steam gasification of a char particle was complete after 3500 s, whereas CO2 gasification was complete after 12000 s. The temperature had the most effect on the conversion rate of rice husk char, followed by partial pressure and pyrolysis heating rate. Comparison of gasification kinetics of different rice husk char types indicated that our findings obtained with a specific rice husk could also be extrapolated to other types, despite their significant difference in ash content. Two kinetic models for C-H2O and C-CO2 reactions taking into account changes in the reactive surface were developed to predict the conversion of a particle. Results of this study could help researchers and engineers in the design, modelling or optimization of a new efficient rice husk gasifier. Future research studies are being planned in the continuation of this thesis in both two directions: academic fundamental research at a laboratory scale and applied research at a pilot scale.
KEYWORDS: biomass, rice husk char, gasification, kinetics, modelling, rice husk