Modeling Combustion of Single Biomass Particle
Coal fired power plants contribute significantly to greenhouse gases emission, notably CO2. A novel method to effectively reduce the amount of CO2 emission is to co-fire a high fraction of biomass and coal at oxygen enriched environments. However, this technique has not been demonstrated on a large-scale yet. The objective of this research is to perform a detailed modeling study of combustion of a single biomass particle and to establish reduced models which can be used for describing the main characteristics of pyrolyzing and combusting single biomass particles to be employed in design codes of industrial furnaces. To accomplish the goals of this thesis, the research has been carried out in two main stages.
First, the sub-processes involved in the biomass combustion are identified and described with a one-dimensional mathematical model based on conservation of mass, energy and momentum. The model encompasses the kinetics of biomass pyrolysis, homogeneous reactions and heterogeneous char oxidation and gasification reactions, coupled with transient transport equations. Subsequently, this model is implemented in an in-house code and a comprehensive numerical study on pyrolysis and combustion of single biomass particles is conducted. The accuracy of the model is examined by comparing its predictions with several experimental data obtained from the literature on pyrolysis and combustion of various types of single biomass particles. The computer code based on the detailed model allows one to observe time and space evolution of several parameters including biomass and char densities, gaseous species mass fractions, porosity, internal pressure, mass flux of volatiles within the pores of the solid matrix, temperature. The model is used for simulation of combustion of particles of three common shapes; i.e. slab, cylinder and sphere. The results of the detailed modeling study reveal that the combustion of a single biomass particle at the conditions of industrial furnaces (small particles and high heating conditions) consists of three main sub-processes: preheating, pyrolysis, and char oxidation. Therefore, a simplified/reduced particle model should account for these three processes.
In the next stage of the project, simplified models are developed to predict the main characteristics of pyrolyzing and combusting single biomass particles. Initially, the preheating stage is modelled using a time and space integral method, which allows one to convert the partial differential form of the heat transfer equation into an algebraic equation. This treatment is then applied to model the pyrolysis process. Two possible regimes are identified: thermally thin and thermally thick particles. A model is established for both regimes, which consists of a set of algebraic equations. This treatment highly simplifies the pyrolysis model so that it can be used in practical applications, which may involve thousands or even millions of particles. The validation of the simplified preheating and pyrolysis models is carried out using various experiments and the results of the detailed model based on partial differential equations (PDEs).
The char particle oxidation and gasification, as the last stage of the particle combustion process, is modelled using the shrinking core approximation. The accuracy of this model is assessed using the experiments reported in past studies. The model is used to study the dynamics of biomass char combustion at oxy-fuel conditions. The effects of the main process parameters on the maximum particle temperature and burnout time are examined. It is found that oxy-fuel combustion with an oxygen mass fraction of 0.3 and higher may lead to a considerable reduction in particle temperature and burnout time compared to the conventional operating case with air as the gasifying agent. In the last stage of this research, the simplified models of biomass particle pyrolysis and char combustion are combined to establish a reduced model for combustion of a single biomass particle. The accuracy of this simplified combustion model is examined using the measured data available in the literature as well as the results of the detailed model. As a conclusion, the simplified models developed in this thesis for pyrolysis and combustion of single biomass particles are efficient enough to capture the main process parameters, and computationally cheaper than the PDE-based models so that they can be used in the design codes of biomass furnaces.