Y Yousef Haseli

A detailed one-dimensional model of combustion of a woody biomass particle

A detailed one-dimensional model for combustion of a single biomass particle is presented. It accounts for particle heating up, pyrolysis, char gasification and oxidation and gas phase reactions within and in the vicinity of the particle. The biomass pyrolysis is assumed to take place through three competing reactions yielding char, light gas and tar. The model is validated using different sets of experiments reported in the literature. Special emphasis is placed on examination of the effects of pyrolysis kinetic constants and gas phase reactions on the combustion process which have not been thoroughly discussed in previous works. It is shown that depending on the process condition and reactor temperature, correct selection of the pyrolysis kinetic data is a necessary step for simulation of biomass particle conversion. The computer program developed for the purpose of this study enables one to get a deeper insight into the biomass particle combustion process.

Reduced model for combustion of a small biomass particle at high operating temperatures

The aim of this work was to demonstrate a model for a spherical biomass particle combusting at high temperatures with reduced number of variables. The model is based on the observation that combustion of a small particle includes three main phases: heating up, pyrolysis, and char conversion. It is assumed that the pyrolysis begins as soon as the particle surface attains a pyrolysis temperature, yielding a char front, moving towards the center of particle as time passes. The formulation of the heating up and pyrolysis phases is based on an integral method which allows describing the energy conservation with an ordinary differential equation. The char combustion model is according to the shrinking core approximation. Model validation is carried out by comparing the predictions with experiments of sawdust particles taken from the literature, and with computations of partial differential equation-based models. Satisfactory agreement is achieved between the predictions and experimental data.

A Quasisteady Analysis of Oxy-Fuel Combustion of a Wood Char Particle

A quasisteady model for single char particle combusting at oxy-fuel conditions is presented. The model is validated against predictions of a detailed numerical model and experimental data reported in the literature. The main idea is to study burning characteristics of small char particles obtained from wood pyrolysis at the conditions corresponding to those found in industrial biomass furnaces (high heating conditions, larger and less dense particles compared to coal char particles). The results indicate that the particle temperature is considerably lower in O2/CO2 mixtures (due to the endothermicity of the gasification reactions) than that in an O2/N2 environment. For an O2 mass fraction of 0.3, the burnout time is found to be the same in both gasifying environments. However, at lower O2 mass fractions, the burnout time of a particle burning in the oxy-fuel conditions is shorter than that in an O2/N2 mixture.

A simple model for prediction of preheating and pyrolysis time of a thermally thin charring particle

The aim of this paper is to present a simple model, based on a time and space integral method, for prediction of preheating and conversion time of a charring solid particle exposed to a non-oxidative hot environment. The main assumptions are 1) thermo-physical properties remain constant throughout the process; 2) temperature profile within the particle is assumed to obey a quadratic function with respect to the space coordinate; 3) pyrolysis initiates when the surface temperature reaches a characteristic pyrolysis temperature; 4) decomposition of virgin material occurs at an infinitesimal thin layer dividing the particle into char and virgin material regions; 5) the volume of the particle remains unaltered; 6) volatiles escape through the pores immediately after formation.Employing assumption (2) allows one to convert the energy conservation equation of the particle, which is basically described in the form of a partial differential equation (PDE), into an ordinary differential equation (ODE) by performing space integration. Next, by applying approximate time integration the ODE is transformed into an algebraic equation. Applying this approach to the preheating and pyrolysis stages of a thermally thin charring solid particle leads to a set of algebraic equations which provides reactor designers with a convenient means for computation of the heating up time, mass loss history and total conversion of particle. The accuracy of the simple model is assessed by comparing its prediction with that of a one-dimensional detailed pyrolysis model. Overall, good agreement is achieved indicating that this new model can be used for engineering and design purposes.Copyright

Mathematical Modeling of Heat and Mass Transfer Processes During Pyrolysis and Combustion of a Single Biomass Particle

A detailed mathematical model is developed for simulation of heat and mass transfer processes during the pyrolysis and combustion of a single biomass particle. The kinetic scheme of Shafizadeh and Chin is employed to describe the pyrolysis process. The light gases formed during the biomass pyrolysis is assumed to consist of methane, carbon dioxide, carbon monoxide, hydrogen and water vapor with given mass fractions relevant to those found in the experiments of high heating conditions. The combustion model takes into account the reactions of oxygen with methane, hydrogen, carbon monoxide, tar and char as well as gasification of char with water vapor and carbon dioxide. Appropriate correlations taken from past studies are used for computation of the rate of these reactions.The model allows calculation of time and space evolution of various parameters including biomass and char densities, gaseous species and temperature. Different experimental data reported in the literature are employed to validate the pyrolysis and combustion models. The reasonable agreement obtained between the predictions and measured data reveals that the presented model is capable of successfully capturing various experiments of wood particle undergoing a pyrolysis or combustion process. In particular, the role of gas phase reactions within and adjacent to particle on the combustion process is examined. The results indicate that for the case of small particles in the order of millimeter size and less, one may neglect any effects of gas phase reactions. However, for larger particles, a combustion model may need to include hydrogen oxidation and even carbon monoxide combustion reactions.Copyright