A kinetic approach to the mathematical model of fixed bed gasifiers

Abstract

This work presents a comprehensive mathematical model of a fixed bed gasifier, where heat and mass transport resistances and chemical kinetics are accounted for both at the reactor and the particle scale. A multistep kinetic model of devolatilization of solid fuels, such as coals, plastics, biomasses and wastes has been employed and validated. The kinetic model of refuse derived fuels (RDF) and wastes is simply based on a linear combination of the devolatilization models of its main constituents. Ligno-cellulosic and plastic materials, together with ash and moisture, allow to account for the high heterogeneity of RDF. Successive gas phase reactions of the released species are described with a detailed kinetic scheme. Furthermore, an accurate description of heat and mass transport between gas and solid phases allows the proper characterization of combustion and gasification of the solid fuel at the particle and reactor scale. The mathematical model of a counterflow fixed bed reactor is then applied first to discuss the importance of heat transfer resistances at the particle scale, then to describe coal and biomass gasification. This work summarizes several facets of this problem with validations and examples and it allows to evaluate feasibility and limitations of the proposed approach.

Introduction

The environmental apprehension towards the combustion of fossil fuels together with the growing concern on waste materials drive the interest in gasification processes of biomasses, coals, plastics, and refuse derived fuels (RDF). Pyrolysis and gasification of solid fuels are nowadays promising alternative to direct combustion, both electric and thermal energy are viable products, together with chemicals. Gasification is a robust proven technology that can be operated either as a simple, low technology system based on a fixed-bed gasifier, or as a more sophisticated system using fluidized-bed technology (McKendry, 2002). The properties of the biomass feedstock and its preparation are key design parameters when selecting the gasifier system. Differences between fixed bed and fluidized bed gasifiers are discussed by Warnecke (2000). The mathematical description of such processes is rather difficult due to the complex phenomena involved, such as modelling solid devolatilization, gas–solid interactions and secondary gas phase reactions.

During last years, many efforts have been devoted to understand and describe gasification process (Biagini et al., 2009, Recman and Hájek, 2009, Juřena et al., 2009). Gronli and Melaaen (2000) studied wood pyrolysis using a mono-dimensional model, devoting a particular attention both to kinetics and transport resistances inside the biomass particles. Thunman and Leckner (2005) explored the influence of particle size and fuel density during oxidation process in a fixed bed reactor. In particular they showed that inter- and intra-particle resistances, more important in bigger particles, lead to different temperatures inside the particles and between solid surface and gas phase. Also Yang, Ponzio, Lucas, and Blasiak (2006) analyzed the importance of chemical and physical processes during gasification showing the influence of oxygen concentration, fuel ratio and process temperature. Di Blasi (2004) developed a comprehensive model able to describe wood gasification in a counter current flow reactor. Such a model is able to describe the dynamic behaviour of the system taking into account mass and thermal diffusion along the reactor. The density of the bed varies in the devolatilization region, while solid velocity is assumed constant. On the other side, during gasification and combustion solid velocity changes due to the reaction effect, while bed density remains constant. A one-dimensional model of countercurrent fixed-bed coal gasification has been developed and discussed by Hobbs, Radulovic, and Smoot (1993). Solid velocity is there evaluated using continuity equation, bed density is kept constant and porosity varies with conversion. Finally, Corella, Toledo, and Molina (2007) analyzed the economic feasibility of different biomass gasification processes.

It is then clear that this multiscale and multiphase problem does require a very careful attention in order to define and develop at least preliminary models. However, it is worth to emphasize the importance of these models, which could first improve the understanding of the whole process and then simplify the scale-up and the optimization of the gasifier. In this work, we proposed the methodology for solving such problems, showing the approach for the main facets involved in solid fuels gasification. Finally, an application example of a fixed bed gasifier model is provided, emphasising the thermal features of the reactor as well as the role of feedstock characterization.