engleski

Modelling the syngas composition in a pilot-scale biomass updraft gasifier

Considering the climate changes caused by the combustion of fossil fuels, renewable energy sources are recognized as future resources for satisfying global energy demands. Conversion of biomass into gas suitable for further exploitation is one of the valuable renewable energy pathways due to the wide distribution and availability of raw materials. Biomass gasification is a thermochemical process of partial combustion in a reduced oxygen environment that aims to produce a hydrogen- enriched syngas. Updraft gasifier design with its advantages of high efficiency and capability to use biomass with a high moisture content produces syngas with higher hydrogen yield compared to other gasifier designs. The main flaw of the updraft gasifier is the high yield of tars as final product that decreases the lower heating value of syngas and raises the necessity for its subsequent cleaning procedure. Recently, significant research efforts have been focused on the optimization of operating conditions, increase of the syngas purity and overall gasification efficiency, especially by developing numerical models as a complementary approach to experiments. The simplest modelling approach for predicting biomass gasification behavior is using a thermodynamic equilibrium model (TEM). When describing the behavior of an updraft gasifier, special focus needs to be given to the pyrolysis process, since in this type of reactor configuration the release of the gaseous products directly outflows from the gasifier. Modelling the pyrolysis process using TEM is mostly based on correlations provided from experimental data, which can be highly dependent on biomass composition and moderately accurate for general cases. In this work, a pilot-scale biomass gasifier was modelled using a combination of a pyrolysis kinetic model whit a TEM. To describe the pyrolysis behavior and correspondent secondary gas-phase reactions, the Bio-PoliMi and two different tar cracking mechanisms, with distinct levels of complexity, were implemented. The gasification and oxidation of char were modelled using a TEM, through the minimization of Gibbs free energy approach. The predicted results of dry and clean syngas were compared to the experimental data considering four different operating conditions. The model combination that used the reduced, however detailed, tar cracking mechanism achieved lower average prediction errors in all cases, when compared to the simple tar cracking mechanism. The obtained prediction errors were generally lower when the detailed tar cracking mechanism was used. Although some discrepancies were observed in the predictions, these preliminary results show that the model approach considered in this study represents a good basis and can be considered for future development of the model.

Biomass updraft gasification, pyrolysis kinetic model, tar cracking model, agricultural residue

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