Numerical Simulation of Combustion and Gasification of Biomass Particles

Sammanfattning: In this thesis, a numerical approach is adopted to study biomass thermochemical conversion. Detailed physical and chemical processes involved in the thermochemical conversion of biomass are considered. The aims are to improve the understanding of the physical and chemical processes involved and to develop and validate mathematical models for numerical simulation of the biomass conversion process. The main focus of the thesis work has been on large biomass particles under fixed bed conditions. The thermochemical conversion of single particle is first considered. A comprehensive detailed model is developed and evaluated; the results provide valuable insight into the underlying physics of thermochemical conversion of biomass. Based on the comprehensive model a simplified model is proposed which takes into account some of the detailed reaction and transport process inside the particle at high computational efficiency. A two-dimensional model for the fuel bed of biomass furnace is developed and validated.
Mathematical description of various sub-processes involved in the conversion process is presented taking into account the main features of biomass conversion. Various approaches and assumptions for modeling the different sub-processes are assessed by comparing the numerical results with experimental measurements. Specifically, the different moisture evaporation models and assumptions regarding moisture diffusion and vapor re-condensation are investigated. The kinetic scheme and rate constants of devolatilization process are studied. A systematical approach is presented for the evaluation of the heat of pyrolysis which ensures elemental mass and energy balance. The effects of shrinkage on the particle and the change of porosity and biomass density are considered in the mathematical modeling. The formation of ash around the particle, the ash melting at high temperature and the consequences on the particle conversion are investigated.
By means of a joint numerical study and advanced experimental measurements, a mechanism is proposed for the release of alkali metals from low chlorine biomass and the corresponding kinetic rate constants during devolatilization and char reaction stages are obtained. It is shown that the proposed model is able to predict the release behavior of the alkali metal during biomass gasification at various stages of the biomass conversion.
The heterogeneous char reactions at the regime II reaction is affected by both the intra-particle mass transfer and the chemical kinetic rates. The evolution of char porous structure can affect the conversion rate of the char. A model based on the classical capillary pores is developed taking into account different conversion rates for pores having different radii. This model is used to examine the contribution of each group of pores (micro, meso and macro-pores) in the conversion of biomass char.