Numerical Simulation of Combustion and Gasification of Biomass Particles

Abstract: Popular Abstract in English Most of us are familiar with using plant-derived material such as wood for producing heat for warming the cottages or cooking food when we are camping. Apart from that, plant-derived materials such as trees, forest residues, grasses, agricultural crops, agricultural residues and straw have great potential in producing energy in a larger scale. This group of material is referred to as biomass. Sweden has a great source of biomass as more than half of its area is covered with forest. Among the European countries, Sweden has the top share of using renewable energy sources which provide around 40% of the total energy of the country. The main reasons of recent attraction towards using renewable energy sources are the impact of fossil fuels in climate changes and concerns about permanency of the fossil fuels sources. In contrast to fossil fuels, biomass is renewable and can be CO2-natural if produced in a sustainable manner. The CO2 produced from combustion of biomass will be absorbed by the plants that will replace biomass in a shorter term. In this sense, there is much less increase in CO2 compared with the fossil fuels. As one of the green-house gases, CO2 is believed to have a great impact on climate changes and global warming and therefore it raises concerns about its environmental impacts. Although the replacement of fossil fuel such as coal with renewable sources like biomass can help in producing green energy, there are challenges and problems that should be addressed. Emission of harmful pollutants, deposition of remaining ash in the biomass combustion systems, and corrosion of these systems need to be understood in order to improve the efficiency and decrease the cost of energy production. The aim of this thesis is to improve the understanding of the underlying physics of conversion of biomass to thermal energy in order to provide suggestions and solutions to improve the energy production system. The questions we are trying to answer are (a) how a biomass particle evolves during the conversion to thermal energy, (b) how various forms of pollutant are formed and released during biomass conversion, and (c) how to mathematically model the process with a balance between the computational time and accuracy of the results. Mathematical equations representing the physical phenomena are solved using numerical methods with the help of advanced computational programs. A biomass conversion model is developed which is comprehensive and can take into account the processes such as evaporation of moisture, decomposition of biomass, combustion of gases extracts from biomass decomposition, surface reactions of char and changes in the thermo-physical properties of the particle. The contributions of this thesis are (1) development of a comprehensive biomass conversion model with a reduced number of assumptions, (2) assessment of various modeling approaches and assumptions of different sub-models by comparison with several experimental measurements, such as the different moisture evaporation models and assumptions regarding moisture diffusion and vapor re-condensation, the kinetic scheme and rate constants of biomass decomposition process and the amount of heat required in this process, (3) development of chemical kinetic mechanism for the release of alkali metals, potassium and sodium, from biomass during combustion in high CO2 environment, (4) development of a multi pore structure for gasification of biomass char, and (5) development of a semi-empirical model for fixed-bed combustion.