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In this paper, a one-dimensional model for biomass gasification in a stratified concurrent (downdraft) reactor is presented. Heat and mass transfer across the bed are coupled with moisture evaporation, biomass pyrolysis, char combustion and gasification, gas-phase combustion and thermal cracking of tars. Numerical simulation resulted in predicting the influence of model parameters, kinetic constants and operational variables on process dynamics, structure of the reaction front and quality of the producer gas. For high values of the air-to-fuel ratio and of the primary pyrolysis rate, the process is top-stabilized, resulting in a high conversion efficiency and good gas quality. As the air flow is decreased below a critical limit value, the reaction front becomes grate-stabilized. The two different configurations are largely determined by the gasphase combustion of volatile pyrolysis products. [ABSTRACT FROM AUTHOR]

Abstract: Understanding the heat and mass transfer phenomena in a coal gasifier is very useful for the assessment of gasifier performance and optimization of the design and operating parameters. In this paper, performance of an entrained flow air blown laboratory scale gasifier is numerically simulated with Fluent software. In the model, the continuous phase conservation equations are solved in an Eulerian frame, while those of particle phase are solved in a Lagrangian frame, with coupling between the two phases carried out through interactive source terms. The dispersion of the particles due to turbulence is predicted using a stochastic tracking model, in conjunction with the k–ε equations for the gas phase. The coal gasification model adopted includes devolatilization, combustion of volatiles, char combustion and gasification. The gasification performance inside the gasifier has been predicted for different air ratios as well as for different air and steam inlet temperatures. The overall temperature inside the gasifier is found to increase when the degree of air/steam pre-heating is increased, resulting in acceleration of the different reaction steps in the gasifier. The overall gasification performance indices such as carbon conversion, heating value of the exit gas and cold gas efficiency have been predicted. The predicted results show good agreement with available experimental data in literature. [Copyright &y& Elsevier]

Abstract: The paper presents the geometry of infinite regular polyhedrons that can be used to construct the heat exchange units. The basic structural component of the infinite solid consists of six hexagons, which meet in the common vertex. The mathematical model of heat and mass transfer inside the labyrinth heat exchanger is shown. The first and the second Kirchhoff’s laws are used to simulate fluid flow distribution at hydraulic network created with connected basic solids. Heat balance equations are adopted to describe forced convection heat transfer inside developed heat exchanger. Good accordance among the experimental data and calculation results over whole simulation range is obtained. The efficiency and thermal characteristic of the labyrinth heat exchanger is presented in the form of graphs. [Copyright &y& Elsevier]