Thermodynamic model of a downdraft gasifier

This paper describes a thermodynamic model of a fixed bed downdraft gasifier, based on the Cycle-Tempo software (TU Delft, the Netherlands), used to convert a solid biomass into a syngas, which can be more efficiently burned, with respect to its direct combustion for heat and power generation, and much more easily carried, where needed. The gasification process is supposed to occur at ambient pressure using air as gasifying agent. In the gasification process, the gas produced during pyrolysis is partially burned, raising the internal temperature and favoring a partial tar decomposition. Therefore, hot gases and carbon fractions react in the reduction zone, accelerating the formation of combustible species (mainly CO and H2). In the present gasifier model, all of the main gasification processes (i.e. drying, pyrolysis, oxidation and reduction) have been separately implemented. Moreover, a relation between the air/fuel equivalence ratio, λ, the granulometry and the ash content of the biomass has been introduced, making the model more versatile. Finally, the model has been validated against several experimental data available in the literature. Results show that the model is able to assess, with a fairly good agreement, both the composition and the heating value of the syngas, derived from several types of biomass, taking also into account the impact of its moisture content.