On the Mathematical Modelling of a Moving-Bed Counter-Current Gasifier Fuelled with Wood-Pellets

Abstract

The subject of this work is the mathematical modelling of a counter-current moving-bed gasifier fuelled by wood-pellets. Two versions of the model have been developed: the one-dimensional (1D) version-solving a set of Ordinary Differential Equations along the gasifier height-and the three-dimensional (3D) version where the balanced equations are solved using Computational Fluid Dynamics. Unique procedures have been developed to provide unconditionally stable solutions and remove difficulties occurring by using conventional numerical methods for modelling counter-current reactors.The procedures reduce the uncertainties introduced by other mathematical approaches, and they open up the possibility of straightforward application to more complex software, including commercial CFD packages. Previous models of Hobbs et al., Di Blasi and Mandl et al. used a correction factor to tune calculated temperatures to measured values. In this work, the factor is not required. Using the 1D model, the Mandl et al. 16.6 kW gasifier was scaled to 9.5 MW input; the 89% cold-gas efficiency, observed at 16.6 kW input, decreases only slightly to 84% at the 9.5 MW scale.

On the Thermal Stability of a Counter-Current Fixed-Bed Gasifier

“…The throughput for this type of reactor is relatively low, while the thermal efficiency is high as the temperatures in the gas exit are relatively low. However, as a consequence, tar and methane production is significant in typical operation conditions, so the product gas must be extensively cleaned before use [29][30][31][32][33][34][35][36].…”

“…The model was developed with the aim of investigating the advantages and limits in the utilization of feedstock different from woodchips, such as hydro-char derived from the hydrothermal carbonization of green waste, or a mix of olive pomace and sawdust. Recently Schwabauer et al [30] developed a 1D and 3D model for the gasifier configuration described by Mandl et al [51] in which, together with experiments, a model is also presented. All the aforementioned models are comprehensive of the main chemical and physical processes, including moisture evaporation/condensation, finite-rate kinetics of wood devolatilization and tar degradation, heterogeneous gasification (steam, carbon dioxide, and hydrogen) and combustion of char, combustion of volatile species, finite-rate gas-phase water-gas shift, extraparticle mass transfer resistances, heat and mass transfer across the bed resulting from macroscopic (convection) and molecular (diffusion and conduction) exchanges, solidand gas-phase heat transfer with the reactor walls, and radiative heat transfer through the porous bed.…”

“…Table 1. Values of the non-dimensional variables used for the simulation of the MA [30] and EL process [41]. From the data presented in the table, it is essential to note that in the case of the EL process, the parameter δ is smaller than unity, and, as already pointed out in the report [41], the convective processes largely determine the heat transfer in this reactor.…”

“…The equations are formulated for a 1D domain in conservative form for an unsteady process (see, for example [30,48]). The continuity equations for the gaseous and the solid phase are, respectively:…”

“…In Table 1, values for the non-dimensional parameters are reported in the case of the Ecoloop process (labeled as EL) and in the case of the experiments carried out by Mandl (labeled as MA). The values of the parameters that describe the pyrolysis and the oxidation kinetics are derived from the analysis performed in [30]. In the case of the EL process, thermogravimetry data presented in [41] are used.…”